transformation at discrete molecular level - ACS Publications

Review pubs.acs.org/crystal

Recent Advances in Single-Crystal-to-Single-Crystal Transformation at the Discrete Molecular Level Archana Chaudhary,† Akbar Mohammad,† and Shaikh M. Mobin*,†,‡,§ †

Discipline of Chemistry, ‡Center for Biosciences and Bio-Medical Engineering, and §Discipline of Metallurgy Engineering and Materials Science, Indian Institute of Technology Indore, Simrol, Khandwa Road, Indore 453552, India ABSTRACT: The present review focuses on the solid state structural transformations in discrete molecules. Since the molecules in the crystal lattice are closely packed, any transformation in the solid state is difficult due to restricted movement of the molecules. These transformations can be triggered by external stimuli, such as light, heat, uptake or exchange of solvent, mechanochemical force, etc. which may be observed by a change in their physical properties like color, magnetism, porosity, and optical. Single-crystal-to-single-crystal (SCSC) transformations in metal−organic frameworks and coordination polymers are well documented; however, reports on SCSC transformations at the discrete molecular level still remain scarce. In this review, we have compiled some interesting examples from the literature on SCSC transformation at the discrete molecular level using various external stimuli.

1. INTRODUCTION Solid state structural transformation in a single-crystal-to-singlecrystal (SCSC) manner is an attractive phenomenon as evinced by the increase in the number of publications.1−10 SCSC transformation is fascinating because in some cases it leads to the formation of unusual products which otherwise cannot be designed by routine synthetic routes which help to directly visualize the change in molecular structure during the transformation process.11−15 In the solution phase, molecules move freely, so they come closer in proper orientation (via reactive functional groups) for occurrence of the chemical reaction homogeneously. On the contrary, in the solid state or crystal form movement of the molecular components is confined.16,17 However, the solid state reactions can also occur with ease if the reactive functional groups are closely and properly oriented.18 In the solid state molecules are closely bound to each other with different types of bonding forces, i.e., van der Waals bonding, ionic bonding, covalent bonding, and metallic bonding. Schmidt et al. established the relationship between the structure and reactivity in solid-state reactions by exposing the solid materials to light (photochemical reactions) that provided a pathway to the fields of solid-state organic photochemistry and crystal engineering.19,20 The crystal engineering principles assist solid state chemists to conveniently design well oriented and reactive materials for solid state reactions.21−25 In a typical example, [2 + 2] photodimerization in the solid state occurs only if the reactive centers are aligned parallel and separated by less than 4.2 Å. The phenomenon of SCSC transformation has been noticed long ago,26 and since then it has drawn great attention and has been extended to many other complexes as well.27−30 In many complexes SCSC transformation is facilitated by a change in © 2017 American Chemical Society

color of the crystals which makes them a potential candidate for sensor technology.31−34 SCSC transformation is the process where a single crystal is exposed to solvent vapors, heat, and light and sometimes by applying mechanochemical forces, resulting in structurally transformed products (Figure 1).35−39

Figure 1. General schematic representation of SCSC transformation under different external stimuli.

In general, exposing single crystals to external forces results in the loss of single crystallinity.40 Thus, the major challenge in SCSC transformation is to retain crystallinity of the transformed single crystal which can be authenticated by X-ray diffraction techniques viz. single crystal X-ray diffraction,41−44 powder X-ray diffraction,45−47 and recently by synchrotron facility.48 These structural transformations are accompanied by a change in physical properties such as color, magnetism, Received: January 31, 2017 Published: March 20, 2017 2893

DOI: 10.1021/acs.cgd.7b00154 Cryst. Growth Des. 2017, 17, 2893−2910

Crystal Growth & Design

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Chart 1

porosity, luminescence, chirality, etc. as well as a change in coordination number, geometry, and dimensionality.49,50 SCSC transformations in metal−organic frameworks (MOFs) and coordination polymers (CPs) are well documented;51−54 however, SCSC transformations at the discrete molecular level are still not so common, and thus in this review, we have attempted to compile the examples of SCSC transformation in discrete molecules. The whole review has been divided into the following sections based on the origin of SCSC transformations: (I) thermally triggered SCSC transformations, (II) solvent-induced SCSC transformations, (III) SCSC transformations via ion exchange, (IV) light-induced SCSC transformations, (V) gas-induced SCSC transformations, and (VI) SCSC transformations using mechanochemical forces. Further, we discuss some examples where physical properties of the complexes are changed with the structural transformation.

environment and two octahedral Cu centers with N1O5 environment). This breaking and formation of covalent bonds restricted reversibility. The transformation of 5 to 6 is different from 3 to 4 where the uncoordinated donor O atoms inside the coordination sphere were facilitating the removal of coordinated water molecules. In this case, the donor atoms changed their binding modes without removing any small molecule from the coordination sphere for generating the tetramer 6. Sundberg et al. reported another interesting example of reversible SCSC transformation by the translational movement of two lattice nitrate anions approximately 4−6 Å away from a cobalt center in complex [{(bpbp)Co2(O2)}2(NH2bdc)](NO3)4·7H2O (7) (bpbp = 2,6-bis(N,N-bis(2-pyridylmethyl)aminomethyl)-4-tert-butylphenolato; NH2bdc = 2-amino-1,4benzenedicarboxylato).57 The oxy form 7 transformed to the deoxy form [{(bpbp)Co2(NO3)}2(NH2bdc)](NO3)2·2H2O (8) on heating for approximately 5 min on a hot-plate at 80 °C. On uptake of a stoichiometric amount of O2, 8 reverted back to the oxy form 7. However, the Co(II) ions remained hexacoordinated in both cases (Figure 2).

2. THERMALLY TRIGGERED SCSC TRANSFORMATIONS In this section, we discuss structural transformations prompted by heating the single crystals. The heating of single crystal results in structural transformations by breaking and formation of new bonds or rearrangements of atoms or change in dimensionality.55 2.1. 0D to 0D Structural Transformations. Mathur and Lahiri et al. designed a series of dinuclear Cu(II) complexes by keeping in mind two objectives: (i) complexes containing some lattice water molecules and (ii) systematic incorporation of ancillary ligands from lighter -OAc to bulkier -OnPr (OAc = acetate; OnPr = n-propionate) in a stepwise manner as shown in Chart 1 (where M = Cu). The hydrated [(OAc)Cu(μ-hep)2Cu(OAc)]·2H2O (1) [Step 1] and [(OAc)Cu(μ-hep)2Cu(OnPr)]·2H2O (2) [Step 2] showed reversible SCSC transformations.56 The packing features of 1 and 2 revealed that the two lattice H2O molecules were present as a tetrameric water cluster. On heating at 110 °C, 1 and 2 transformed to the dehydrated [(OAc)Cu(μhep)2Cu(OAc)] (3) and [(OAc)Cu(μ-hep)2Cu(OnPr)] (4), respectively, via removal of the lattice water molecules without any alteration in the core structural motifs. 1, 2, and their transformed products 3, 4 were present in a pentacoordinated square pyramidal N1O4 environment. Further, 1 and 2 were regenerated from 3 and 4, respectively, on exposure to water vapors at ambient conditions. However, in the case of step III where both the metal centers were bound to heavier OnPr, the hydrated complex [(OnPr)Cu(μ-hep)2Cu(OnPr)]·2H2O (5) converted to a unique double open cubane tetramer [Cu4(μ3-hep) 2(μ-hep) 2(μ-O nPr)2(OnPr)2] (6) either by vapor diffusion or by heating. During this structural transformation (5 → 6) one terminal bidentate OnPr− of each dimeric unit converted to a bridged propionate (μ2-OnPr−) and one μ2-hep− converted to μ3-hep−. The coordination environment of dimer 5 was similar to 1 and 2, whereas the transformed product tetramer 6 had two types of Cu centers (two square-pyramidal Cu atoms with N1O4

Figure 2. SCSC transformation between 7 and 8 by inward and outward movement of the NO3− ion.

The bridging peroxo and nitrates ligands in 7 and 8, respectively, were located on opposite sides of the plane formed by the Co4(NH2bdc) unit. During the transformation of oxy to deoxy form, a SN2 type reaction at each Co(II) ion was observed where the nitrates pushed the O2 release accompanied by the sliding of some donor atoms and movement of the 2894



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Figure 3. SCSC transformation by dehydration/rehydration. Color Code: Cyan- Metal atom; Blue- Nitrogen atom; Red- Oxygen atom; BlackCarbon atom; White- Hydrogen atom; Light Green- Halogen atom; Yellow- Sulphur/Phosphorous atom.

Figure 4. Stepwise SCSC transformation of an ionic monomer to a neural dimer to another ionic monomer by inward and outward movement of the SO42− ion. Color codes: the same as in Figure 3.

spin (LS) Fe(II)−N bond,59 whereas in 10 the average Fe−N bond lengths slightly increased suggesting the high spin (HS) Fe(II) centers (Fe1−N = 2.151 Å and Fe2−N = 2.138 Å). Luo et al. reported SCSC transformation by dehydration/ rehydration cycle for [Dy 2 (phen) 2 (L) 6 ]·2H 2 O (11)/ [Dy2(phen)2(L)6] (12) (phen = 1,10-phenanthroline; L = (−)-4,5 bis(pinene)-2,2-bipyridine).60 On heating 11 at 160 °C for 24 h under a vacuum, 12 was obtained. The color of 11 changed from yellow to colorless in 12. On immersion of 12 in water, 11 was regained. The regenerated 11 was further heated, and surprisingly the crystallinity still remained with rehydration of 12, indicating the good quality of the crystal. SCSC transformation of 11 → 12 observed changes in lattice parameters and coordination geometry of the Dy(III) atom. In 11, the Dy(III) atom acquired nine coordinated monocapped square-antiprismatic geometry with C4h symmetry. On the contrary, in dehydrated 12, the Dy(III) atom was in eightcoordinated square-antiprismatic coordination geometry with D4d symmetry. In 11, there was a regular square-antiprism which was characterized by the four ideal dihedral angles (0°, 0°, 52.4°, and 52.4°), whereas, in 12, the corresponding values were 5.3°, 5.6°, 58.2°, 50.9°, demonstrating some distortion in regular square-antiprism. Both 11 and 12 exhibited dinuclear structures, where β-naphthoic acid and phen ligands were behaving as terminal ligands. The carboxylate bridges in both

supporting ligands. The authors suggested that out of the four lattice nitrate anions two which were nearer to the metal centers moved and coordinated to the Co(II) ion in the deoxy form 8. During this process, the bond distances of the coordinated atoms and metal centers also changed along with the expansion (approximately 2%) of the crystal lattice. The average O−O bond distance was 1.423(9) Å in 7 and the distance between the O atoms of coordinated nitrate was average 2.185(7) Å in 8. In the deoxy form the Co−Co distance averaged 3.448(2) Å, which shortened to av. 3.171(2) Å in the oxygenated form. Wei et al. reported reversible SCSC transformation of tetranuclear [Fe(tpa){N(CN)2}]4·(BF4)4(H2O)2 (9) (tpa = tris(2-pyridylmethyl)amine) by guest desorption/resorption.58 In 9, the two water molecules were dehydrated by heating at 350 K generating [Fe(tpa){N(CN)2}]4·(BF4)4 (10) without altering the structural core. In asymmetric unit of 9, two distinct Fe centers were present along with two BF4− anions and one water molecule. Each Fe center was present in distorted octahedral geometry, where four coordination sites were occupied by N atoms of tpa ligand and two by N(CN)2 groups. The four iron centers were connected by μ-N(CN)2 making a square cavity of the size 11.55 and 12.20 Å. In 9, the Fe−N bond lengths were Fe1−N = 1.973 Å and Fe2−N = 1.967 Å, which were in the expected range of low 2895



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Figure 5. SCSC transformation of 18 → 19 by insertion of counter NO3− ion into the coordination sphere. Color codes: the same as in Figure 3.

Figure 6. Perspective views of 20 (entrapped acetonitrile molecules are shown in red circle) and 21 (showing robust network after solvent removal). Silver atoms are presented in the space filling representation; counterions PF6 are located at the corners of the unit cells. Reprinted with permission from ref 63. Copyright 2011 Royal Society of Chemistry.

structures were different with μ2:η1:η2 and μ2:η1:η1 binding mode in 11 and only μ2:η1:η1 coordination mode in 12 (Figure 3). Complex {K4[Zr(DBQ)4]}·9H2O (13) (DBQ = dihydroxybenzoquinonate) also underwent reversible SCSC transformation producing anhydrous K4[Zr(DBQ)4] (14), on heating at 100 °C.61 Both 13 and 14 possessed eightcoordinated zirconium atom which was surrounded by DBQ ligand in a bidentate manner. A unique two-step SCSC transformation was observed in a cobalt complex [Co(hep-H)(H2O)4]·SO4 (15), by the inward and outward movement of counter SO42− ion.62 On heating at 120 °C, the orange single crystal of monomer [Co(hepH)(H2O)4]·SO4 (15) transformed to pink dimer [(Co(hepH)(H2O)2(μ2-sulfato-O,O′))2] (16). Moreover, when a drop of 1 N HCl was added to the pink crystal of 16, it immediately transformed to a green ionic monomer [Co(H2O)6]·SO4 (17). However, no changes in cell parameter were noticed with one drop of 1 N H2SO4 over 16, but some loss in crystallinity was observed. This transformation may be because HCl was actually cleaving the hep ligand from metal center. In 15, 16, and 17, the Co(II) atom was present in a hexacoordinated environment with distorted octahedral geometry (Figure 4). The distance of the SO42− ion from Co(II) center in 15 was 5.07 Å which came closer to bind to metal centers to form the dimer 16 and further moved to 4.80 and 4.43 Å in 17. Similar to the above-mentioned example, the structural transformation was observed in [Co(hep-H)2(H2O)2]·(NO3)2 (18) by insertion of counter NO3− ion into the coordination sphere, replacing two coordinated water molecules, selectively. On heating at 110 °C, the crystal of monomer [Co(hepH)2(H2O)2]·(NO3)2 (18) transformed to another monomer [Co(hep-H)2(NO3)]·NO3 (19).62 In 18 the two coordinated

H2O molecules were cleaved on heating, and one of the two NO3− ions from crystal lattice came closer and bound to the Co(II) center in 19. The distance between lattice NO3− ion and Co(II) was 4.83 Å in 18 which decreased slightly to 4.82 Å in 19 (Figure 5). Complexes 16 and 18 are interesting examples of solid state transformation where the counteranion (SO42− or NO3−) moves into the coordination sphere and causes the structural changes without changing the coordination number and geometry of the central metal. The transformation of 18 → 19 is similar to the transformation between 7 and 8 where only one NO3− ion enters the coordination sphere without changing the coordination number of the central Co(II) atom. Dobrzańska reported reversible SCSC transformation in a dinuclear species, [Ag2(bitmb)2]·(PF6)2·2CH3CN (20) to [Ag2(bitmb)2]·(PF6)2 (21), by heating the crystals of 20 at 100 °C for 40 min (where bitmb = 1,3-bis(imidazol-1ylmethyl)-2,4,6-trimethylbenzene).63 Upon exposure of crystals of 21 to acetonitrile vapors for approximately 45 min, 20 was further obtained. In asymmetric units, 20 and 21 both contained one silver ion, one bitmb molecule, one CH3CN molecule, and two halves of counterions PF6. Here, Ag atoms formed a dinuclear, rectangular metallocyclic [Ag2bitmb2]2+ cation. During transformation of 20 to 21, some noteworthy changes occurred in the structure as well as crystallographic data. In 21 the c-axis was shortened due to the contraction of the space which was occupied by CH3CN molecules in 20, and the columns of stacked metallocycles tilted slightly in a manner that the imidazole rings (N1−C5) came to the position that was occupied before by the solvent molecules, resulting in the tight packing of the system (Figure 6). The Ag····Ag separation in 2896



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The asymmetric unit of 26 contained two independent symmetric dimer molecules. Both dimers in asymmetric unit were arranged in a different orientation with an overall presence of H2O and C2H5OH solvents in the lattice. In the surroundings of Dy1 dimer two H2O and two C2H5OH molecules were present which differentiated it from the Dy2 dimer. The coordination sites of both Dy centers were occupied by six O atoms of acid (two by terminal O and four by bridging O) and two N atoms of phen, forming square antiprismatic geometries. On desolvation, the coordination number changed from eight to nine {where another coordination site was occupied by bridging O, changing to μ2 mode, {Dy−O = 2.753(3) and 2.806(3) Å (27) vs 3.080(3) and 3.187(3) Å (26)}, with monocapped square antiprismatic geometries in 27. The intradimer Dy−Dy distances decreased to ca. 4.0 Å from ca. 4.12 Å for Dy1−Dy1A and 4.19 Å for Dy2−Dy2B in 26. The interdimer distance was ca. 11 Å in 26 which reduced to ca. 10 Å in 27. Reversible SCSC transformation was observed in two multifunctional chiral dysprosium(III) compounds on the release/reabsorption of solvent molecules along with the changes in their ferroelectric, magnetic, and NLO properties.67 [Dy(L)2(acac)2]·NO3·CH3OH·H2O (28) (L = (−)-4,5 bis(pinene)-2,2-bipyridine) underwent SCSC transformation by heat, producing 29 and by pressure, producing 30. 29 and 30 also showed reversible SCSC transformation by heating and cooling. 28 was regenerated from 29 and 30 on exposing to methanol vapors. [Dy(L)2(acac)2]·NO3·CH3OH·H2O (28′) (L = (+)-4,5 bis(pinene)-2,2-bipyridine) was a mirror image of 28 and showed a similar kind of SCSC transformation. Thus, only one example has been discussed. During structural transformations, the space group changed from polar to nonpolar (i.e., P21 for 28 and C2221 for 29). The coordination geometry of Dy(III) ion was slightly distorted square antiprism in all the three structures. In the crystal lattice of 28, one cationic Dy(III) complex, one nitrate counterion, two disordered methanol, and water molecules were present. The asymmetric unit of 29 varied from 28 by the presence of partially occupied H2O molecules, disordered NO3− anion, and absence of methanol molecule. The crystal structure of 30 differs from 29 by the presence of ordered NO3− anion (Figure 8). As 29 and 30 had almost similar structures, so physical properties only of 28 and 29 were investigated. 28 showed ferroelectric behavior, whereas 29 exhibited no ferroelectric effect, in accordance with its nonpolar space group. Both 28 and 29 exhibited slow magnetic relaxation behavior in the temperature range of 300−1.8 K, and both compounds showed second-order NLO properties. Costa et al.68 reported a very fine example of reversible SCSC transformation through solvent exchange in a low spin Fe(II) complex [Fe(bpp)(H2L)](ClO4)2·1.5C3H6O (31) (bpp = 2,6bis(pyrazol-3-yl)pyridine; H2L = 2,6-bis(5-(2-methoxyphenyl)pyrazol-3-yl)pyridine). 31 was a compact discrete, nonporous material hosting acetone solvent molecules. This low spin complex 31 extruded one-third of these acetone molecules, leading to high spin [Fe(bpp)(H2L)](ClO4)2·C3H6O (32). 32 transformed to the original 31 on reabsorption of acetone molecules. Interestingly, in 32 lattice acetone molecule could be replaced by methanol and water generating low spin [Fe(bpp)(H2L)](ClO4)2·1.25MeOH·0.5H2O (33). 33 again transformed to 31 on acetone reabsorption. Along with the spin switching at ambient conditions, this series of SCSC trans-

the cyclic units increased slightly from 7.613 Å (in 20) to 7.829 Å (in 21). Barbour et al. reported irreversible SCSC transformation upon loss of two MeOH molecules in two isostructural metallocycles [M2(L)2Cl4]·2CH3OH {M = Zn (22) and Co (22′) and L = 4,4′-bis(2-methylimidazol-1-ylmethyl)biphenyl}, on heating at ∼65 °C for several hours.64 In solvated metallocycles, both the structures (22 and 22′) were positioned about sites of 2/m symmetry, with a mirror plane passing through the metal centers. The metal centers were present in a tetracoordinated environment bound to two Cl and two N atoms of imidazole, forming a metallocycle. The desolvated structures [M2L2Cl4] (23 and 23′) were in monoclinic space group C2/c. The change of conformation also resulted in a slight elongation of the M···M distances from 15.964(2) to 16.365(3) Å for 23, and from 15.881(2) to 16.295(4) Å for 23′. The N−M−N angles also decreased slightly from 113.0(3)° to 107.3(2)° for 23 and from 115.6(2)° to 108.7(3)° for 23′. Dobrzańska et al. reported a discrete cyclic dinuclear complex [Ag2(L)2](BF4)2·2CH3CN (24), (L = 1,4-bis(2methylimidazol-1-ylmethyl)benzene) that retained its crystallinity and solvent-templated channel structure after acetonitrile molecules were removed on heating at 80 °C, yielding a porous, gas sorbing material [Ag2(L)2](BF4)2 (25).65 In 24, the discrete molecules were stacked forming 1D channels where acetonitrile molecules were entrapped (Figure 7), while the BF4− ions are

Figure 7. Space filling projection, showing the packing arrangement of rectangular [Ag2(L)2]2+ host complexes to form channels along [001]. Color code: carbon-gray; hydrogen-white; silver-light blue; nitrogendark blue. Reprinted with permission from ref 65. Copyright 2005 American Chemical Society.

situated between adjacent columns of stacked [Ag2(L)2]2+ complexes. On removal of two acetonitrile molecules, the packing arrangement of the host remained intact, yielding a porous lattice which was capable of absorbing various gases such as CO2, N2, H2, and CH4. The important aspect of this work was that all the gas absorption experiments were performed at room temperature. Zhu et al. reported reversible SCSC transformation in [Dy2(phen)2(L)6]·(H2O)0.5(C2H5OH) (26) (phen = 1,10phenanthroline; LH = 2-methylbenzoic acid), which on heating at 160 °C under a vacuum underwent SCSC transformation, producing Dy2(phen)2(L)6 (27), a solvent free phase.66 27 could also be prepared hydrothermally by the reaction of DyCl3, phen, HL, and Na2CO3. 2897



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Figure 8. Reversible SCSC transformation of 28 under different conditions. Color codes: the same as in Figure 3.

Figure 9. SCSC transformation of 31 under different conditions. Color codes: the same as in Figure 3.

formations could also be monitored by the drastic color change from dark red (31) to light orange (32) to dark red (33) showing its application in sensing technology. In all the three complexes, both bpp and H2L bound to the metal in mer fashion through their N-donor atoms, generating a distorted octahedral geometry around the Fe(II) center (Figure 9). The ligand unit was present in a syn, anti-configuration of the methoxy phenyl rings with respect to the central

coordination pocket. The remarkable change during the SCSC transformation was the alteration of Fe−N bond distances, i.e., 1.949 Å (31), 2.169 Å (32), 1.947 Å (33), which was attributed to the spin crossover (LS → HS → LS). The variable-temperature magnetic susceptibility measurements of these compounds were also recorded which were in good agreement with the results of single crystal XRD. The magnetization of 31 was studied in the temperature range of 2− 2898



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37 the COO− showed twisted dicoordinated mode {Cu−O(8) = 1.95 Å, Cu−O(7) = 2.66 Å, Cu−O(2) = 1.93 Å, Cu−O(1) = 2.53 Å) toward the Cu(II) center with an extended bond length between Cu−Cu centers, i.e., 4.70 Å. Vittal et al. reported transformation of a hydrogen-bonded 3D-network [{Zn(sala)(H2O)2}2]·2H2O into a covalently bonded framework [{Zn(sala)}n] on heating [where, H2sala = N-(2-hydroxybenzyl)-L-alanine].71 Structural transformation of a nickel monomer was reported by Cheng and Foxman, but for this transformation, single crystals of monomer and polymer were grown from solutions of monomer.72 2.3. 0D to 2D Polymer Transformation. Another example of dehydration of coordinated water molecule involved double end-on azido-bridged, centrosymmetric [Co2(8-qoac)2(N3)2(H2O)2] (38) (8-qoac = quinoline-8-oxy-acetate) complex which on heating at 150 °C under a vacuum resulted in 2D-coordination polymer [Co2(8-qoac)2(N3)2]n (39) via removal of coordinated water molecules.73 This was the first report on the SCSC transformation of a discrete dimeric metal complex to a 2D-coordination polymer. The transformation of 38 to 39 involved changes in the symmetry, lattice parameters, dimer orientations, and molecular stacking fashions. In 38, the Co(II) ion was in a highly distorted octahedral geometry surrounded by a tridentate 8qoac, two azido, and one aqua ligand. One of the oxygen of acetate group remained uncoordinated in 38 which took up the vacant site of Co(II) in 39, after removal of aqua ligand on heating. This oxygen atom bound to the metal center in a neighboring dimer, resulting in the formation of a mixed azido/ carboxylato-bridged layer in the 2D-coordination polymer 39 (Figure 12). 2.4. 0D to 3D Transformation. Liu et al. reported reversible SCSC transformation of monomer [Cu(tzbc)2(H2O)4] (40) (Htzbc = 4-(1H-1,2,4-triazol-1-yl)) to 4fold interpenetrated diamond MOF [Cu(tzbc) 2] (41), stimulated by ethanol vapor at 85 °C for 50 min or methanol vapor at 80 °C for 45 min.74 40 could be regenerated by exposing 41 to air for three months. The driving force for this structural transformation might be the solvent-driven flexible coordination of tzbc ligand and variable Jahn−Teller distortion of Cu(II) center. 41 exhibited CO2 adsorption up to 12.5 wt % at room temperature and low pressure, suggesting its further applications in the separation of industrial and automobile exhaust. In 40, the Cu(II) atom localized at the inversion center, possessing Jahn−Teller distorted octahedral geometry, the

375 K. The χMT vs T curve showed low spin state for 31, high spin state for 32, and low spin state for 33 (Figure 10).

Figure 10. Plots of χMT vs T showing the transformation of 31 into 32 and the SCO properties of the 32 (black) upon cooling and again cycling the temperature back to 375 K. The inset shows in detail the hysteresis loop of 32. Reprinted with permission from ref 68. Copyright 2014 American Chemical Society.

Shi et al. reported reversible SCSC transformation of a lowdielectric compound (HPy)2[Na(H2O)2Co(CN)6] (34) (HPy = pyridinium cation) to a high-dielectric semihydrated compound (HPy)2[Na(H2O)Co(CN)6] (35) via stepwise removal of H2O by heating at 333 K for 1 h.69 The switching property was due to the different surrounding environment of the HPy cation in 34 and 35, through the hydrogen-bonding interactions and the crystal packing which affected the dynamics of the cations. The main focus of this work was the switchable dielectric constant (SDC) with variable temperatures. 2.2. 0D to 1D Polymer Transformations. Hundal et al. reported irreversible SCSC transformation in a dimeric copper complex [Cu2(μ2-DNB)4·(H2O)2]·2CH3CN (36) (DNB = 3,5dinitro benzoate) which on heating at 50 °C transformed to a 1D-polymeric chain [Cu(μ2-DNB)4·(H2O)2]n (37).70 The coordination environment of the Cu(II) ion and binding mode of the ligand is shown in Figure 11. The COO− group in 36 showed a simple dicoordinated mode toward Cu(II) the center with a typical paddle wheel type structure (av. Cu−O bond distance 1.96 Å and Cu−Cu distance 2.62 Å); however, in

Figure 11. Structural transformation of a dimer to 1D-network. Color codes: the same as in Figure 3. 2899



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distorted octahedral coordination geometry around the metal center. The average Fe−N bond lengths were 2.144 Å and the Σ value for the Fe(II) center was 144.7°, suggesting the presence of Fe (II) ion in high spin state, whereas the average Fe−N bond lengths and Σ value were 1.952 Å and 90.5°, respectively, falling in the region of low spin Fe (II) center.59 In 42, C−H····π and C−H····O interactions were present between the complex and solvent diethyl ether molecules, whereas in 43, these interactions were absent. In both complexes, Et2O molecules were present in the voids which were constructed by the complex cations and BF4− anions. During the transformation of 42 to 43, no significant loss in crystallinity was observed which suggested the presence of the robust 3D network. Supriya and Das reported reversible SCSC transformation by exchange of solvents on one of the center of a trinuclear iron complex, [Fe3(μ3-O)(μ2-CH3COO)6(C5H5NO)2(H2O)]ClO4· 3H2O (44) (Figure 14). Red single crystal of 44 on exposure to MeOH vapors transformed to [Fe3(μ3-O)(μ2-CH3COO)6(C5H5NO)2(MeOH)]ClO4·3H2O (45), replacing the H2O molecule with MeOH at the Fe(II) center with a slight decay in crystallinity. However, the overall structure and color of tri-iron cluster remained unchanged. On leaving at ambient temperature, the red crystal of 45 showed reversible transformation back to 44.78 Besides showing reversible SCSC transformation through Fe−O(H)Me bond formation and Fe−OH2 bond breaking in a gas−solid reaction, 44 also showed selectivity toward methanol. On exposure to different alcohols viz. methanol, ethanol, 1propanol, 2-propanol, 1-butanol single crystals of 44 remained unaffected toward all alcohols except methanol. It may be suggested that both H2O and MeOH molecules were the competitors for the iron center because in the presence of water vapors compound 44 was regenerated, maintaining the cycle. 3.2. Dimer to Tetramer Structural Transformation. Later, Mathur and Lahiri et al. reported SCSC transformation in a symmetric, dimeric complex [Cu(μ2-hep)(TFA)(H2O)]2 (46) (hep-H = 2-(2-hydroxyethyl)pyridine; TFA = trifluoroacetic acid) via the removal of coordinated water molecules.79 When blue single crystals of 46 were exposed to the vapors of

Figure 12. Structural transformation of 0D to a 2D-network (magenta color arrows show the coordination of free oxygen of acetate group of neighboring molecule, and removal of H2O molecules; hydrogen atoms have been removed for clarity in 39). Color codes: the same as in Figure 3.

−COOH of tzbc remained uncoordinated which took up the vacant sites of the Cu(II) ion (due to the removal of H2O molecules) of the neighboring monomer, generating a diamondoid 3D-network in 41 (Figure 13).

3. SOLVENT-INDUCED SCSC TRANSFORMATIONS In porous materials such as MOFs and CPs there are voids which absorb guest molecules making them capable of hosting external molecules;75,76 however, in nonporous materials incorporation of the guest molecules via reorganization of internal lattice is not so common because covalent interactions are absent in the long-range, resulting in the collapse of the lattice. 3.1. Monomer to Monomer Structural Transformations. Oshio et al. discussed SCSC transformation and magnetic bistability for [Fe(dppFc)2](BF4)2·2Et2O (42) which transformed to [Fe(dppFc)2](BF4)2·Et2O (43) (dppFc = 1-ferrocenyl-2-{(2,6-bis(pyrazolyl)pyridyl)ethylene) in a SCSC transformation manner on releasing of one diethyl ether molecule, at room temperature.77 42 could be recovered on exposure of the single crystal of 43 to diethyl ether; however, the core structure did not change. The asymmetric unit of 42 consisted of half of the iron complex cation, one anion, and one Et2O molecule with

Figure 13. SCSC transformation of a monomer to a 3D-network (magenta color arrows show the coordination of −COO groups of neighboring molecules and removal of H2O molecules). Color codes: the same as in Figure 3. 2900



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Figure 14. Reversible SCSC transformation of 44 to 45 (via H2O and MeOH exchange). Color codes: the same as in Figure 3.

Figure 15. SCSC transformation of a dimer to tetramer via removal of H2O molecules. Color codes: the same as in Figure 3.

observed via a LS−HS intermediate phase. This spin crossover behavior was studied by SCXRD, magnetic studies, as well as Mössbauer spectroscopic studies. The asymmetric unit of 48 consisted of one complete Fe(tpa)(NCS)2 molecule, while that of 49 had {Fe(tpa)(NCS)2 and Fe(tpa)(NCS)2·CH3OH}, denoted as Fe1 and Fe2 centers, respectively. The transformation of 48 to 49 was encouraged by the incorporation of the methanol molecule at the Fe2 site which further involved in hydrogen bonding (S··· H−O) via one NCS− group. The hydrogen bonding was responsible for the distinct crystal packing of 49 and also caused molecular distortion at the Fe2 site. 49 showed SCO behavior where at 298 K the existence of an intermediate HS(Fe1)-LS(Fe2) phase {For (Fe1) center Fe−N = 2.164(3) Å and Σ = 88.7° and (Fe2) center Fe−N = 2.013(3) Å and Σ = 56.8°} was observed. This was a rare example of SCO behavior {[LS−LS] (120 K) to [HS−HS] (350 K)} in a monomeric complex which was authenticated by single crystal XRD data, especially all over the transition temperature regions. Later, similar to 48, Zheng et al.81 also reported SCSC transformation and spin crossover behavior for [Fe(tpa)(NCS)2]·X and [Fe(tpa)(NCS)2]2·Y (where X = n-PrOH, iPrOH, CH2Cl2, CHCl3, CH3CN, and Y = CH3OH, C2H5OH). The coordination sphere was similar to that of 48, and thus this example has not been discussed in detail. 3.4. Dimer to Dimer Structural Transformations. In another solvent induced reversible SCSC transformation,

various alcohols (methanol, ethanol, and isopropanol) at room temperature, for 24 h, it resulted in the formation of a green tetrameric complex [Cu4(μ3-hep)2(μ2-hep)2(μ2-TFA)2(TFA)2] (47). On exposure of 46 to light or heat or a vacuum, loss of single crystallinity was observed. Unlike 44, 46 responded to all the alcohol vapors. In 46 each Cu(II) ion was surrounded with two μ2-alcoholic oxygen donors, two oxygen atoms of TFA and H2O, and one pyridine nitrogen atom of hep. The packing features of 46 revealed the existence of strong hydrogen bonding between coordinated H2O molecules and pendant oxygen atoms of TFA and hep, forming a H-bonded tetrameric core (Figure 15). This hinted at the SCSC transformation of 46 to 47, where as soon as the coordinated H2O molecule was cleaved from the Cu(II) center instead of alcohols from vapor coordinated to the Cu(II) center (as reported by Das et al.) the neighboring pendant carbonyl group (-CO) of the bonded TFA (which was anti to the H2O molecule) was quick enough to bind the Cu(II) center and facilitated the formation of tetramer 47 {with two Cu−O (μ3-hep) and two Cu−O (μ2-TFA)}. 3.3. Monomer to Dimer Structural Transformation. In another example, [Fe(tpa)(NCS)2] (48) transformed to {[Fe(tpa)(NCS)2]·[Fe(tpa)(NCS)2·CH3OH]} (49) (tpa = tris(2-pyridylmethyl)amine), followed by a remarkable color change from yellow to red, on exposure of the single crystal of 48 to methanol vapor.80 49 was a spin crossover compound where a two-step spin transition (LS−LS → HS−HS) was 2901



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Figure 16. SCSC transformation of 50 → 51 → 52 → 53 followed by changing luminescence.

crystalline polymorph:-α-Au2(μ-dppe)2I2·2OCMe2 (50) with orange emission converted to β-Au2(μ-dppe)2I2·.2OCMe2 (51) with green emission upon exposure to air (dppe = 1,2bis(diphenylphosphino)-ethane).82 Interestingly, upon exposure of 51 to acetone vapor, it converted back to 50. Both 50 and 51 transformed to a microcrystalline powder 52 with green emission, upon loss of acetone molecule. Again exposing 52 to acetone vapor, it gave a desolvated phase Au2(μ-dppe)2(μ-I)2 (53) with orange emission. These conversions were characterized through X-ray diffraction studies as well as emission spectroscopy. Both polymorphs 50 and 51 contained dimeric molecules where tricoordinate Au(I) centers were present which differ from each other by Au···Au distances (i.e., 3.67 Å for 50 and 3.39 Å for 51) (Figure 16). Because of the shortening of Au··· Au distance in 51, aurophilic interactions were more significant and affected the luminescence (orange to green) of the complex. In the β-polymorph 51 Au−I distance was lengthened, causing the change in the orientations of the phenyl rings and also shifting the positions of the Me2CO molecules. The solvent molecules did not bind to the metal in any case. In all cases, gold(I) atom was tetracoordinated with highly distorted tetrahedral environment. It was an interesting case of SCSC transformations among these four types of crystals because here solvent vapors played an unusual role in stimulating structural changes within solids, without altering the composition. Tzeng and Chao reported SCSC transformation of [Au2(O5NCS2)2]·2CH3CN (54) via immersion of the crystals of 54 in tert-butylbenzene or m-xylene solution for 2 days, resulting in [Au2(O5NCS2)2]·tert-butylbenzene·H2O (55) and [Au 2 (O 5 NCS 2 ) 2 ]·0.5m-xylene (56), respectively {where O5NCS2 = (aza-[18]crown-6)dithiocarbamate)}.83 In these dinuclear complexes, Au(I) was coordinated with two sulfur atoms of different (aza-[18]crown-6)dithiocarbamate ligands forming an eight-membered ring. 54 and 55 exhibited intra and

inter Au(I)····Au(I) interactions which helped in tuning their luminescence properties which were the main focus of this work.

4. SCSC TRANSFORMATIONS VIA ION EXCHANGE SCSC transformation may also be stimulated by ion exchange (both cation and anion). For achieving the ion exchange based SCSC transformation, a single crystal is immersed in the solution of another compound with which the cation/anion is exchanged, and structural changes via ion exchange are observed. Some examples are listed below. 4.1. SCSC Transformation by Anion Exchange. Hong et al. used a C3-symmetry, semirigid, tripedal, pyridine-based ligand tppa for synthesizing an octahedral discrete, nanocage [Cu6(tppa)8(H2O)6]·(ClO4)12·(H2O)24 (57) (tppa = N,N′,N″tris(3-pyridinyl)phosphoric triamide) with a large cavity size (Cu−Cu distance between opposite centers ca. 12.786 Å and volume ca. 900 Å3), flexible windows and all PO moieties of ligands inside positions. All these features as well as free rotated phosphorylacylamide bonds resulted in the flexibility of tppa ligands where the pyridyl rings could be rotated by a certain angle, making the size of the window adjustable for anion exchange. For anion exchange, the crystals of 57 were immersed in concentrated NaCl solution, at room temperature where the Cl− ions were exchanged with ClO4−, resulting in a 1D-polymeric chain [Cu6Cl5(tppa)8(μ-Cl)]n·6Cl·22H2O (58) via bridging of consecutive ball units by Cl− ions.84 In 57, each Cu(II) center was coordinated to four N atoms of different tppa and one O atom of H2O molecule, generating tetragonal pyramid geometry. The anion exchange between Cl− and ClO4− anions took place outside the cages and with coordinated H2O molecules at some Cu(II) centers. Because of this action at some copper centers coordination geometries changed from tetragonal pyramid to octahedron where four pyridyl N and two Cl− ions were ligated to the metal center in 58 (Figure 17). 2902



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In 62 two [Zn2L(OH)]2+ units were assembled with two 4,4′-bpe, forming a tetranuclear rectangular assembly, [Zn4(L)2(OH)2(4,4′-bpe)2]4+ where each zinc center was in a square pyramidal geometry (Figure 18). In this assembly CC

Figure 17. SCSC transformation of 0D to 1D-polymer via ClO4− and Cl− exchange. Color codes: the same as in Figure 3.

4.2. SCSC Transformation by Cation Exchange. Huang et al. reported a cation-exchange-induced SCSC transformation in a nanoporous dimeric [Co(L)Cl2]2 (59) (L = N,N-bis-(3pyridyl)isophthalamide) complex.85 On immersion of single crystals of 59 in the methanolic solution of HgCl2 and CdSO4, single crystals of a macrocyclic complex [HgLCl2]2 (60) and a 2D-wavy sheet [CdLSO4(H2O)2]n (61), respectively, were obtained. The cation exchange occurred through a recurrent dissolving−exchange−crystallization process of solvent mediated mechanism accompanied by the old metal−ligand bond breaking and the new metal−ligand bond formation. 59 and 60 were isostructural, where the metal ion was connected to two Cl atoms and two pyridine N atoms of different moieties. In 61, ligand moieties bridged between consecutive Cd(II) atoms, forming an infinite 1D-polymeric chain along the a-axis. 1D-polymeric chains were connected by sulfate anions, forming a 2D-network.

Figure 18. Photoinduced SCSC transformation of 62 to 63.

bonds were parallel and separated by 3.64 Å, fulfilling the Schmidt’s criteria of [2 + 2] photoreaction, thus resulting in 4,4′-tpcb in 63. In order of accommodating 4,4′-tpcb, the Zn···· Zn spacing between the metal centers within and between the Schiff-base ligands increased and decreased slightly [Zn(1)− Zn(2) = 3.135(1) Å, Zn(1)−Zn(2′) = 13.54 Å for 62 and Zn(1)−Zn(2) = 3.182(1) Å, Zn(1)−Zn(2′) = 13.36 Å for 63]. This was the first example where a transition metal ion complex directed a [2 + 2] photodimerization in the solid state. Vittal et al. also reported SCSC transformation by the photoreaction in zinc complexes (i) [Zn2(ptol)4(4spy)2] (ptol = para-toluate and 4spy = 4-styrylpyridine) and (ii) [Zn2(ptol)4(2F-4spy)2] (2F-4spy = 2-fluoro-4′-styrylpyridine). In the latter case the transformation occurred very unusually, between phenyl-olefin double bonds and was reversible by heating.87 MacGillivray et al. reported SCSC transformation in [Ag2(Cl-pyr-pe)4(ClO3)2] (64) (Cl-pyr-pe = rctt-1,3-bis(4-Cl3-pyridyl)-2,4-bis(phenyl)-cyclobutane).88 64 polymerized producing [Ag2(Cl-pyr-pe)2(Cl-pyr-p-cb)(ClO3)2]n (65), (Cl-pyrp-cb = trans-1-(4-Cl-3-pyridyl)-2-(phenyl)ethylene) a 1Dcoordination polymer, with direct cross-linking of cyclobutane rings via [2 + 2] cycloaddition reaction under UV irradiation (broadband 450 W medium-pressure Hg-lamp). In both the structures Ag(I) ion was in distorted tetrahedral geometry. In 64 CC bonds were beyond the limit of Schmidt criteria for a photodimerization reaction (Figure 19), whereas the CC bonds of nearest-neighbor complexes involving the planar olefins were separated at 3.77 Å, in a head-to-tail manner. During the transformation of 64 → 65 there was a

5. LIGHT-INDUCED SCSC TRANSFORMATIONS The solid state structural changes observed by exposing the single crystal to the light are known as photoinduced SCSC transformation. In maximum cases, the photoinduced SCSC transformation is accompanied by [2 + 2] cycloaddition where two reactive centers are aligned parallel to each other with the separation between them being less than 4.2 Å.18 5.1. Structural Transformations via [2 + 2] Cycloaddition. Tetranuclear rectangular complex [Zn4(L)2(OH)2(4,4′-bpe) 2 ](ClO 4 ) 4 ·4H 2 O (62) {LH = 2,6-bis[N-(2pyridylethyl)formimidoyl]-4-methylphenol); 4,4′-bpe = trans1,2-bis(4-pyridyl)ethylene} on exposure to UV lamp (using either 419 nm or broad band Hg lamp) for ∼5 h underwent [2 + 2] cyclodimerization where two 4,4′-bpe reacted to give 4,4′tpcb, in [Zn4(L)2(OH)2(4,4′-tpcb)](ClO4)4·4H2O (63) (4,4′tpcb = rctt-tetrakis(4-pyridyl)cyclobutane).86 2903



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reported the transition metal complex [Ag2(4-stilbz)4][CO2CF3]2 (70) (4-stilbz = trans-1-(4-pyridyl)-2-(phenyl)ethylene) mediated photo-dimerization in the solid state where argentophilic forces (Ag···Ag interactions) played a key role for the stacking of olefins.90 70 exhibited a regiocontrolled head-tohead [2 + 2] photodimerization, producing [Ag2(4-pyr-phcb)2][(CO2CF3)2] (71) (4-pyr-ph-cb = rctt-1,2-bis(4-pyridyl)3,4-bis(phenyl)cyclobutane) in 100% yield, on exposure to UV radiation (broadband Hg lamp) for approximately 18 h. Besides the formation of carbon−carbon single bond this SCSC transformation involved breaking of Ag−Ag bond and formation of Ag···C interactions, respectively (Figure 21). In dinuclear complex 70 the coordination sphere of the Ag(I) ion adopted a T-shape geometry (Ag···Ag distance: 3.41 Å). The olefins (CC) bonds which were involved in photodimerization reaction, were approximately parallel and were separated by 3.82 Å. There was a major repositioning of the Ag(I) and carboxylate ions during this photodimerization. In 71, each Ag(I) ion moved ∼1.16 Å (Ag···Ag distance =5.73 Å) in the direction which was nearly parallel to the newly generated C−C bonds, whereas each carboxylate ion had rotated ∼90° in a way that each −CF3 group was oriented nearly perpendicular to the pyridyl groups. 5.3. Redox Reaction Resulting in 0D to Oligomer Transformation. In another example of photoinduced SCSC transformation, a yellow dinuclear oxovanadium(V) complex [VV2O2(L)2] (72) (H3L = 2,6-bis(hydroxymethyl)-p-cresol) on exposure to white light and air simultaneously transformed to a green oligomer [VIV2O2(L*)2]∞ (73).91 During this transformation process (72 → 73), each V(V) center was reduced to V(IV) with two-electron alcohol-to-aldehyde oxidation in the ligand, while the additional electron released was probably consumed by molecular oxygen generating hydrogen peroxide (Figure 22). Centrosymmetric molecule 72 consisted of two fivecoordinated vanadium(V) atoms which were connected by bridging phenoxido O atoms. On the contrary, the crystal structure of 73 revealed the presence of a hexacoordinated vanadium(IV) with distorted octahedral geometry. As a consequence of changing coordination from five to six, there was a displacement of the vanadium center from the basal O4 plane which changed from 0.336 to 0.257 Å, significantly from 72 to 73. 5.4. Stepwise Transformation of a Monomer to Dimer to 1D-Polymeric Chain. Vittal et al. reported [2 + 2] cycloaddition for two monomeric Zn(II) complexes [ZnBr2(4spy)2] (74) and [ZnBr2(2F-4spy)2] (75) (4-spy = trans-4styrylpyridine; 2F-4spy = trans-2-fluoro-4′-styrylpyridine),

Figure 19. 0D to 1D structural transformation (in 65 hydrogen atoms have been omitted for clarity). Color codes: the same as in Figure 3.

significant decrease in Ag····Ag separation across the ClO3− bridges (4.9 Å in 64 and 4.6 Å in 65) and the N−Ag−N angles (143° for 64 and 131° for 65) and slight increase in Ag····Ag separation along the 1D-chain which was 15.6 Å in 64 and 15.9 Å in 65. The bridging mode of the ClO3− anions also underwent a variation in geometry (Figure 19). Jin and co-workers reported [2 + 2] cycloaddition in two organometallic macrocycles. Tetranuclear complexes [Cp*4M4(μ-bpe)2(μ-η2-η2-C2O4)2](OTf)4 (66: M = Ir; 67: M = Rh and bpe = trans-1,2-bis(4-pyridyl)ethylene) transformed to 68 and 69, respectively, on keeping under UV irradiation using a Hg lamp for ∼25 h.89 This structural transformation was similar to 62 → 63 where 4,4′-bpe dimerized, yielding 4,4′tpcb. In 66, each Ir center was present in a hexacoordinated environment and was ligated to one pyridyl N atom of 4,4′-bpe and two O atoms of oxalato ligands, assuming that the Cp* was functioning as a three-coordinate ligand (Figure 20). In 66, two 4,4′-bpe ligands were close to each other because of a significant face-to-face π····π interaction, and the double bonds of the two 4,4′-bpe ligands were parallel with a distance of 3.23 Å. In 68, the pyridyl rings were bent toward each other with Ir····Ir separations 5.562 and 13.122 Å which resulted in the lengthening of C(1)−C(2) bond {1.87(2) Å} of cyclobutane, longer than the reported coordination complexes. 5.2. Argentophilic Forces Mediated SCSC Transformation. In another example MacGillivray and Coworker

Figure 20. SCSC transformation via [2 + 2] cycloaddition in organometallic macrocycles. 2904



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Figure 21. SCSC transformation of 70 to 71 under UV irradiation.

butane rings, generating 1D-coordination polymer 78. Transformation of 74 to 76 was reversible under thermal conditions (Figure 23). 5.5. Structural Transformations by Conformational Change and Olefin Bond Reduction. In another photoinduced SCSC transformation example, a rhenium macrocycle 79 underwent structural change, generating two different kinds of structures 80 and 81.93 In complex 79 two ethylene units which were at distance of 3.4 Å underwent conformational change from parallel to criss-cross conformation, generating 80, and photoreduction, generating 81 under UV-light irradiation. Tetranuclear rectangular structure of 79 consisted of two molecules of bpe, four anionic hydroxyl groups, and four facRe(CO)3 cores with distorted octahedral geometry around each rhenium center (Figure 24). The pyridine units were aligned face-to-face and parallel to each other with a distance ∼3.53 Å, suggesting strong π···π stacking interactions between them. In 80 and 81 the bpe units were not parallel; they were twisted with respect to each other. The distance of the C−C bond which connected two pyridine units differed in 80 and 81. The C−C bond length of 1.36 Å in 80 showed double bond character similar to the C−C bond distance 1.34 Å of 79. The corresponding C−C bond distance in 81 was 1.45 Å which was in the range of C−C single bond, indicating the reduction of the olefin bond.

Figure 22. Structural transformation of a dimer to oligomer. Color codes: the same as in Figure 3.

generating dimeric Zn(II) complexes [Zn2Br4(rctt-ppcb)(4spy)2] (76) and [Zn2Br4(rctt-F-ppcb)-(2F-4spy)2] (77), respectively.92 Complex 75 was isomorphous and isostructural to 74 so only one example has been discussed here. Irradiation of the single crystals of 74 to UV light for 24 h ensured the generation of 76. In 74 only 50% of the 4spy ligands underwent photochemical reaction, as the distance between some CC bonds were 3.73 and 3.71 Å, while the distance between some neighboring CC bonds was 5.16 Å. On prolonged irradiation under UV light (60 h) dimer 76 exhibited quantitative conversion of olefin groups to cyclo-

Figure 23. Stepwise SCSC transformation of a monomer to dimer to polymer. 2905



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Figure 24. SCSC transformation of tetranuclear 79 → 80 and 81 by photoirradiation.

Figure 25. SCSC transformation of 82 to 83 via formation of an intermediate (Representing two different type of bonding mode of − NO2− by dotted lines).

Figure 26. Structural transformations by gas absorption.

5.6. Structural Transformation Involving Change of Binding Mode of Ligand. Another nitro complex [Ni(dppe)(η1-NO2)Cl] (82) underwent SCSC transformation on exposure to the light of a UV light-emitting diode (LED; ca. 400 nm) (dppe = 1,2-bis(diphenylphosphino)-ethane).94 82 produced the photoactivated metastable nitrito complex [Ni(dppe)(η1-ONO)Cl] (83) which exhibited reversible 100% conversion. Variable temperature structural determination experiments confirmed the formation of metastable 83, in the temperature range 100−160 K. At temperature of 160 K, the metastable state structure 83 completely transformed back into nitro-(η1-NO2) conformation 82 which was thermodynamically favorable.

In 82 the nickel center was ligated with a bidentate dppe ligand, a Cl atom, and a nitro-(η1-NO2) group forming squareplanar geometry. The crystal of 82 was exposed to UV lightemitting diode at 100 K, after a period of ∼20 min the irradiation was turned off and another data set was collected. Surprisingly, in this set, a high conversion of 70% from the nitro-(η1-NO2) (N1, O1, and O2) into the nitrito-(η1-ONO) isomer (N1B, O1B, and O2B) was observed (Figure 25). This crystal was further irradiated for ∼90 min, and again data were collected which showed 100% structure conversion with nitrito (η1-ONO) conformation (Figure 25). Both structures remained in the same Cc space group with slight change in the unit cell and packing arrangement. Besides, a clear change in bonding 2906



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Figure 27. SCSC transformation of 88 to 89 (showing hydrogen bonding interactions between free carboxylate groups and coordinated NH3 molecules in 88).

membered metallomacrocycle 89 along with the change of coordination geometry of the copper atom. In 88, the copper atom was present in a square-planar geometry and was surrounded by one O atom of carboxylate group of L and three NH3 molecules, and the other carboxylic group remained uncoordinated which was involved in hydrogen-bond interactions between the coordinated ammonia molecule of the neighboring monomer (N····O distance: 2.835 Å). The distance between the copper atom in one monomer and the oxygen atom of the uncoordinated carboxylic group in the adjacent monomer was 3.796 Å, comparable to the distance for a [2 + 2] photoreaction in the solid state. In 89 the uncoordinated carboxylate group of ligand coordinated to the Cu ion of the other monomer, creating square-pyramidal geometry. Besides, slight variations in the Cu−N and Cu−O bond lengths and the average dihedral angle between the central benzene ring and the side benzene rings of ligand were also observed (Figure 27). This was the first report where dimerization of metal complex by dry grinding was documented.

mode of the -NO2 group from 82 to 83, no significant changes in the molecular parameters were noticed (Figure 25).

6. GAS-INDUCED SCSC TRANSFORMATIONS In gas-induced SCSC transformation, the single crystal is exposed to the gas to cause structural changes. In the case of MOFs and CPs the gas molecules are entrapped in the cavity and cause structural changes via host−guest mechanism, but in the case of discrete molecules gases bind to the metal center, thus structural transformation is not an easy task. Crudden et al. reported gas-induced SCSC transformation in rhodium NHC complex [{Rh(SIPr)(C2H4)Cl}2] (SIPr = N,N′(2,6-iPr2C6H3)2C3H4N2), having the saturated SIPr ligand which was able to activate small molecules, such as N2, O2, and CO and also intramolecular C−H bonds of the SIPr ligand. Dimeric complex [{Rh(SIPr)(C2H4)Cl}2] (84) on treatment with 2 equiv of SIPr in a nitrogen atmosphere produced [Rh(SIPr)2(N2)Cl] (85) where dinitrogen bound the metal in end-on manner.95 Upon exposing, the THF solution of 85 to oxygen or air, a new complex [Rh(SIPr)2(O2)Cl] (86) was generated where O2 was bound to rhodium in side-on fashion, while its exposure to CO atmosphere resulted in complex [Rh(SIPr)2(CO)Cl] (87). The X-ray structures of 84−87 suggested the presence of pseudo square-planar geometry around the rhodium center, with slight distortion arising from a subtle tilting of the two rhodium carbene bond. Reversible SCSC transformation by absorption of SO2 was observed in an organoplatinum complex [PtCl(NCN-OH)] which was monitored by P-XRD and solid state FT-IR.96 In another example absorption and desorption of HCl gas has been reported in a nonporous crystalline solid [CuCl2(3Clpy)2] in an SCSC manner. Unequivocal evidence for this transformation were obtained by PXRD patterns.97

8. OVERVIEW The present review exceptionally focuses on SCSC transformations at the discrete molecular level. To date, SCSC transformations at the polymeric level have been well documented, but SCSC transformations at the discrete level still remain less explored. This review broadly covered the various examples where discrete SCSC transformations occurred via a different mode of external stimuli such as light, heat, vapor, and solvent (Table 1). We are the first to attempt to systematically arrange the discrete molecular SCSC transformation reports in this review. We assume that prediction of SCSC transformations at the discrete molecular level is a daunting task due to breaking and formation of covalent bonds. However, it may be suggested that complexes where water/ solvent molecules are present in the crystal lattice or bound to the metal center are more prone to structural transformation. SCSC transformations of 7 → 8; 15 → 16 → 17 and 18 → 19 suggest that if counterion present in the crystal lattice is located at the distance of less than 6 Å from the metal center, it may move in and out of the crystal lattice, maintaining the crystallinity of the sample. In many examples discussed in the present review, SCSC transformation was accompanied by change in magnetic properties (28 → 29 → 30; 31 → 32 → 33; 42 → 43 and 48 → 49), colors (11 → 12; 15 → 16 → 17; 31 → 32 → 33; 46 → 47; 48 → 49; 72 → 73 and 88 → 89) and luminescence (50 → 51 → 52 → 53; 54 → 55 and 56) which in future may find some practical applications in sensing technology. Study of the packing diagrams of some SCSC

7. SCSC TRANSFORMATION BY MECHANOCHEMICAL FORCES In the literature, there is only one example of discrete molecules where the structural changes have been stimulated by mechanochemical forces (i.e., grinding) in the single crystals of a discrete molecule. In a copper monomer [Cu(NH3)3(L)]·(H2O)0.66 (88) (H2L = 2,2′-(1,2-phenylenebis(methylene))bis(sulfanediyl)-dibenzoic acid) SCSC transformation was stimulated by two different means, thermal and mechanochemical.98 Blue crystals of monomeric 88 dimerized to green color 89, due to change in the copper coordination geometry, on heating at 176 °C for 15 min or by solid state reaction under dry grinding conditions. The most interesting structural changes during this transformation were the dimerization of monomer 88 into a 302907



Review

Table 1. Summary of All the Examples compound → transformed product [(OAc)Cu(μ-hep)2Cu(OAc)]·2H2O (1) ↔ [(OAc)Cu(μhep)2Cu(OAc)] (3) [(OAc)Cu(μ-hep)2Cu(OnPr)].2H2O (2) ↔ [(OAc)Cu(μhep)2Cu(OnPr)] (4) [(OnPr)Cu(μ-hep)2Cu(OnPr)]·2H2O (5) → [Cu4(μ3hep)2(μ-hep)2(μ-OnPr)2(OnPr)2] (6) [{(bpbp)Co2(O2)}2(NH2bdc)](NO3)4·7H2O (7) ↔ [{(bpbp)Co2(NO3)}2(NH2bdc)](NO3)2·2H2O (8) [Fe(tpa){N(CN)2}]4·(BF4)4(H2O)2 (9) → Fe(tpa){N (CN)2}]4·(BF4)4 (10) [Dy2(phen)2(L)6]·2H2O (11) ↔ [Dy2(phen)2(L)6] (12) K4[Zr(DBQ)4]·9H2O (13) ↔ K4[Zr(DBQ)4] (14) [Co(hep-H)(H2O)4]·SO4 (15) → [(Co(hep-H)(H2O)2(μ2sulfato-O,O′))2] (16) [(Co(hep-H)(H2O)2(μ2-sulfato-O,O′))2] (16) → [Co (H2O)6]·SO4 (17) [Co(hep-H)2(H2O)2](NO3)2 (18) → [Co(hep-H)2(NO3)] NO3 (19) [Ag2(bitmb)2](PF6)2·2CH3CN (20) ↔ [Ag2(bitmb)2](PF6)2 (21) [M2(L)2Cl4]·2CH3OH (22) → [M2(L)2Cl4] (23) [M = Zn, Co] [Ag2(L)2](BF4)2·2CH3CN (24) → [Ag2(L)2](BF4)2 (25) [Dy2(phen)2(L)6]·(H2O)0.5(C2H5OH)] (26) ↔ Dy2(phen)2(L)6 (27) [Dy(L)2(acac)2]·NO3·CH3OH·H2O (28) ↔ [Dy (L)2(acac)2]·NO3·H2O (29) [Dy(L)2(acac)2]·NO3·CH3OH·H2O (28) ↔ [Dy (L)2(acac)2]·NO3·H2O (30) [Fe(bpp)(H2L)](ClO4)2·1.5C3H6O (31) ↔ [Fe(bpp)(H2L)] (ClO4)2·C3H6O (32) [Fe(bpp)(H2L)](ClO4)2·C3H6O (32) → [Fe(bpp)(H2L)] (ClO4)2·1.25MeOH·0.5H2O (33) (HPy)2[Na(H2O)2Co(CN)6] (34) ↔ (HPy)2[Na(H2O)Co (CN)6] (35) [Cu2(μ2-DNB)2·2H2O]·2CH3CN (36) → [Cu2(μ2DNB)2·.2H2O]n (37) [Co2(8-qoac)2(N3)2(H2O)2] (38) → [Co2(8-qoac)2(N3)2]n (39) [Cu(tzbc)2(H2O)4] (40) ↔ [Cu(tzbc)2] (41) [Fe(dppFc)2](BF4)2·2Et2O (42) ↔ [Fe(dppFc)2](BF4)2·Et2O (43) [Fe3(μ3-O)(μ2-CH3COO)6(C5H5NO)2(H2O)]ClO4·3H2O (44) ↔ [Fe3(μ3-O)(μ2CH3COO)6(C5H5NO)2(MeOH)] ClO4·3H2O (45) [Cu(μ2-hep) (TFA)(H2O)]2 (46) → [Cu4(μ3-hep)2(μ2hep)2(μ2-TFA)2(TFA)2] (47) [Fe(tpa) (NCS)2] (48) → {[Fe(tpa) (NCS)2]·[Fe(tpa) (NCS)2·CH3OH]} (49) α-Au2(μ-dppe)2I2·2OCMe2 (50) ↔ β-Au2(μdppe)2I2·2OCMe2 (51) β-Au2(μ-dppe)2I2·2OCMe2 (51) → Au2(μ-dppe)2I2 (52) Au2(μ-dppe)2I2 (52) → Au2(μ-dppe)2(μ-I)2 (53) [Au2(O5NCS2)2]·2CH3CN (54) → Au2(O5NCS2)2]·tertbutylbenzene·H2O (55) [Au2(O5NCS2)2]·2CH3CN (54) → Au2(O5NCS2)2]·0.5mxylene (56) [Cu6(tppa)8(H2O)6]·(ClO4)12.(H2O)24 (57) → [Cu6Cl5(tppa)8(μ-Cl)]n·6nCl·22nH2O (58) [Co(L)Cl2]2 (59) → [HgLCl2]2 (60) and [CdLSO4(H2O)2]n (61) Zn4(L)2(OH)2(4,4′-bpe)2](ClO4)4·4H2O (62) → [Zn4(L)2(OH)2(4,4′-tpcb)](ClO4)4·4H2O (63) [Ag2(Cl-pyr-pe)4(ClO3)2] (64) → [Ag2(Cl-pyr-pe)2(Cl-pyr-pcb) (ClO3)2]n (65) [Cp*4M4(μ-bpe)2(μ-η2-η2-C2O4)2](OTf)4 [M = Ir (66); Rh (67)] → [Cp*4M4(μ-tbcp)2(μ-η2-η2-C2O4)2](OTf)4 [(68), (69)]

external stimuli

brief description of SCSC transformation

ref

heat

removal/reabsorption of lattice water molecules

56

heat

removal/reabsorption of lattice water molecules

56

heat/vapor

change in the coordination environment of metal center as well as binding mode of ligand, causing a dimer to tetramer transformation via removal of lattice water molecules removal/reabsorption of some water molecules from lattice and oxygen from coordination sphere removal of lattice water molecules

56

heat heat heat heat heat

57 58

heat

removal/reabsorption of lattice water molecules removal/reabsorption of lattice water molecules Insertion of counter SO42− ion in coordination sphere, monomer to dimer transformation Movement of coordinated sulfato ligand outside coordination sphere, dimer to monomer transformation insertion of one counter NO3− ion in coordination sphere, which binds Co(II) in a bidentate manner removal/reabsorption of lattice solvent molecules

63

heat

removal of lattice solvent molecules

64

heat heat

removal of lattice solvent molecules removal/reabsorption of lattice solvent and water molecules

65 66

heat

removal/reabsorption of lattice solvent molecule

67

pressure


67

acetone


68

acetone

removal of lattice acetone molecule and absorption of methanol and water molecules Removal/Reabsorption of one water molecule

68

Removal of lattice solvent molecules, and transformation of discrete dimer to 1D-polymeric chain removal of coordinated water molecules causing a dimer to 2D coordination polymer transformation removal/reabsorption of coordinated water molecules causing a monomer to 3D-polymer transformation removal/reabsorption of one lattice diethyl ether molecule

70

hydrochloric acid heat

heat heat heat methanol/ethanol vapors, heat air

60 61 62 62 62

69

73 74 77

methanol vapors

replacement of water molecule by methanol in coordination sphere and vice versa

78

alcohol vapors

removal of coordinated water molecules causing dimer to tetramer transformation insertion of methanol molecule in coordination sphere causing monomer to dimer transformation change of phase

79

removal of lattice solvent molecules change of binding mode of iodine from terminal to bridging removal of two acetonitrile molecules from lattice and addition of tertbutylbenzene and water molecules removal of two acetonitrile molecules from lattice and addition of mxylene molecule transformation of a discrete nano cage to a 1D-polymeric chain

82 82 83

transformation of dimeric complex to macrocycle (60) and 2D-wavy sheet (61) [2+ 2] cycloaddition

85

methanol air air acetone tert-butylbenzene m-xylene anion exchange cation exchange UV irradiation UV irradiation

photopolymerization, generating 1D-polymeric chain from dimeric complex [2 + 2] cycloaddition

UV irradiation

2908

80 82

83 84

86 88 89



Review

Table 1. continued compound → transformed product

external stimuli

brief description of SCSC transformation

ref

[Ag2(4-stilbz)4][CO2CF3]2 (70) → [Ag2(4-pyr-ph-cb)2] [(CO2CF3)2] (71) [VV2O2(L)2] (72) → [VIV2O2(L)2]2 (73) [ZnBr2(4-spy)2] (74) ↔ [Zn2Br4(rctt-ppcb)(4spy)2] (76)

UV irradiation

[2 + 2] cycloaddition

90

white light and air UV irradiation

91 92

[ZnBr2(2F-4spy)2] (75) ↔ [Zn2Br4(rctt-F-ppcb)-(2F-4spy)2] (77) [Zn2Br4(rctt-ppcb)(4spy)2] (76) → [Zn2Br4(rctt-ppcb) (4spy)2]n (78) rhenium macrocycle (79) → 80 and 81 [Ni(dppe)(η1-NO2)Cl] (82) → [Ni(dppe)(η1-ONO)Cl] (83) [{Rh(SIPr)-(C2H4)Cl}2] (84) → [Rh(SIPr)2(N2)Cl] (85) [Rh(SIPr)2(N2)Cl] (85) → [Rh(SIPr)2(O2)Cl] (86) [Rh(SIPr)2(O2)Cl] (86) → [Rh(SIPr)2(CO)Cl] (87)

UV irradiation

redox reaction causing dimer to oligomer transformation [2 + 2] cycloaddition causing monomer to dimer transformation (reversible under thermal conditions) [2 + 2] cycloaddition causing monomer to dimer transformation (Reversible under thermal conditions) dimer to 1D-polymer transformation conformation change causing 80 and photoreduction causing 81 change of binding mode from η1-NO2 to η1-ONO

93 94

insertion of nitrogen in coordination sphere insertion of oxygen in coordination sphere insertion of carbon monoxide in coordination sphere

95 95 95

transformation of monomer to macrocycle

98

[Cu(NH3)3(L)]·(H2O)0.66 (88) → macrocycle (89)

UV irradiation UV irradiation light emitting diode (LED) nitrogen gas oxygen gas carbon monoxide gas thermal/ mechanochemical

transformation examples 22 → 23; 24 → 25 and 42 → 43 shows the existence of a robust 3D network even after removal of a guest molecule which further suggests about their future applications as gas absorbing materials. In future, there is an existing scope in designing heterometallic discrete molecules, inorganic cocrystals and exploring the possibility of SCSC transformations of these complexes to produce new molecular structures and studying their optical and magnetic properties.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shaikh M. Mobin: 0000-0003-1940-3822 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS A.C. thanks DST, New Delhi, for providing research grant (SR/ WOS-A/CS-1020/2015). S.M.M. thanks SERB-DST (Project No. EMR/2016/001113), New Delhi, for financial support.

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