Synthesis, crystal structures and magnetic properties

FULL PAPER

Dalton

Liviu Toma,a Luminita Marilena Toma,a Rodrigue Lescou¨ezec,a Donatella Armentano,b Giovanni De Munno,b Marius Andruh,c Joan Cano,d Francesc Lloreta and Miguel Julve*a a Departament de Qu´ımica Inorg`anica/Instituto de Ciencia Molecular, Facultat de Qu´ımica de la Universitat de Val`encia, Dr. Moliner 50, 46100, Burjassot (Val`encia), Spain. E-mail: [email protected] b Dipartimento di Chimica, Universit`a degli Studi dela Calabria, 87030, Arcavacata di Rende, Cosenza, Italy c Inorganic Chemistry Laboratory, Faculty of Chemistry, University of Bucarest, Str. Dumbrava Rosie nr. 23, 020464, Bucharest, Romania d Instituci´o Catalana de Recerca i Estudis Avanc¸ats (ICREA)/Departament de Qu´ımica Inorg`anica and Centre de Recerca en Qu´ımica Te´orica, Universitat de Barcelona, Diagonal 647, 08028, Barcelona, Spain

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Synthesis, crystal structures and magnetic properties of cyanideand phenolate-bridged [MIII NiII ]2 tetranuclear complexes (M = Fe and Cr)†

Received 5th January 2005, Accepted 23rd February 2005 First published as an Advance Article on the web 14th March 2005

The binuclear complex NiII 2 L(H2 O)2 (ClO4 )2 (1) and the neutral tetranuclear bimetallic compounds [{MIII (phen)(CN)4 }2 {NiII 2 L(H2 O)2 }]·2CH3 CN with M = Fe (2) and Cr (3) [H2 L = 11,23-dimethyl-3, 7,15,19-tetraazatricyclo[19.3.1.19,13 ]hexacosa-2,7,9,11,13(26),14,19,21(25),22,24-decaene-25,26-diol] have been synthesized and the structures of 2 and 3 determined by single crystal X-ray diffraction. 2 and 3 are isostructural compounds whose structure is made up of centrosymmetric binuclear cations [Ni2 (L)(H2 O)2 ]2+ and two peripheral [M(phen)(CN)4 ]− anions [M = Fe (2) and Cr (3)] acting as monodentate ligands towards the nickel atoms through one of their four cyanide nitrogen atoms. The environment of the metal atoms in 2 and 3 is six-coordinated: two phen-nitrogen and four cyanide-carbon atoms at the iron and chromium atoms and a water molecule, one cyanide-nitrogen and two phenolate-oxygens and two imine-nitrogens from the binucleating ligand L2− at the nickel atom build distorted octahedral surroundings. The values of the Fe · · · Ni and Cr · · · Ni separations through the ˚ respectively, whereas the Ni–Ni distances across the double single cyanide bridge are 5.058(1) and 5.174(2) A ˚ (3). The magnetic properties of 1–3 have been investigated in the phenolate bridge are 3.098(2) (2) and 3.101(1) A temperature range 1.9–290 K. The magnetic behaviour of 1 corresponds to that of an antiferromagnetically coupled ˆ = −J Sˆ A ·Sˆ B . An overall nickel(II) dimer with J = −61.0(1) cm−1 , the Hamiltonian being defined as H antiferromagnetic behaviour is observed for 2 and 3 with a low-lying singlet spin state. The values of the intramolecular magnetic couplings are J Fe–Ni = +17.4(1) cm−1 and J Ni–Ni(a) = −44.4(1) cm−1 for 2 and J Cr–Ni = ˆ = −J M–Ni (Sˆ M ·Sˆ Ni + Sˆ Ma ·Sˆ Nia ) −J Ni–Nia SNi SNia ]. Theoretical +11.8(1) cm−1 and J Ni–Ni(a) = −44.6(1) cm−1 for 3 [H calculations using methods based on density functional theory (DFT) have been employed on 2 in order to analyze the efficiency of the exchange pathways involved and also to substantiate the exchange coupling parameters.

Introduction

DOI: 10.1039/b500168b

The impressive variety of chemical and physical properties of the extended metal–cyanide frameworks is at the origin of the continuous interest in these compounds.1–5 Without being exhaustive, one can cite their role as molecular sieves,1,6 ion-exchange materials,7 catalysts for the production of ether polyols or polycarbonates,8 multilayered thin films on supports,9 room temperature magnets,10–12 single-molecule magnets,13 single chain magnets,14 electrochemically tunable magnets,15 photoinduced magnetism15b,16 and high spin molecules.17 The asymmetric character of the cyanide ligand makes it a very appropriate choice aimed at preparing heterometallic species. The main synthetic route for the construction of

† Electronic supplementary information (ESI) available: Fig. S1: Perspective drawing of the structure of complex 3 showing the atom numbering (the hydrogen atoms and the acetonitrile molecules are omitted for simplicity). Fig. S2: A view along the a axis showing the hydrogen bonding between the neutral tetranuclear units of complex 3. The formula connecting the energies of five calculated states and the exchange parameters. See http://www.rsc.org/suppdata/dt/b5/b500168b/ This journal is

©

heterometallic cyanide-bridged species has been the use of hexacyanometallates of the type [M(CN)6 ](6−m)− (where M is transition metal ion). Their reaction with fully solvated metal ions affords the well known family of the highly insoluble threedimensional Prussian Blue compounds.2 Interestingly, lower dimensionality systems are obtained when the outer metal ion is partially blocked with polydentate end cap ligands.3,4,17g,h A more recent approach to prepare low dimensional cyanide-bridged compounds consists of using stable six-coordinate compounds of general formula [ML(CN)x ](x + l–3)− (M = trivalent transition metal ion, L = polydentate ligand and x = 2–4) as the cyanidebearing building blocks.14a,c,d,18–26 The number of cyanide ligands, their relative arrangement around M and the overall charge of this type of complexes are dependent on the denticity and stereochemistry of the blocking ligand L. A priori, these tailored building blocks allow the preparative chemists to get a better control of the nuclearity, topology and dimensionality of the resulting cyanide-bridged compound when using them as ligands either towards fully solvated metal ions or coordinatively unsaturated preformed complexes. Some selected examples that illustrate these points are: (i) the design of 4,2-ribbon like bimetallic chains by using the [ML(CN)4 ]− unit [M = Fe(III)

The Royal Society of Chemistry 2005

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and Cr(III) and L = 1,10-phenanthroline (phen) and 2,2
bipyridine (bipy)] as a ligand, some of these chains exhibiting slow magnetic relaxation and hysteresis effects;14,21b,26a,b,f ,g (ii) high nuclearity boxes, high spin molecules and ladder-like compounds whose topologies are a consequence of the facand mer- arrangements of the three cyanide ligands in the mononuclear precursors fac-[Cr(Me3 tacn)(CN)3 ] (Me3 tacn = 1,4,7-trimethyltriazacyclononane), fac-{[Fe{HB(pz)3 }(CN)3 ]3 [HB(pz)3 = hydrotris(1-pyrazolyl)borate] and [Fe(bpca)(CN)3 ]− [bpca = bis(2-pyridylcarbonyl)amidate];25b,26d,e (iii) tetranuclear squares of general formula [Fe2 M2 (l-CN)4 (bipy)6 ]n+ (M = first row transition metal ion), the cyanidebearing precursor being the cis-[FeII/III (bipy)2 (CN)2 ]0/+ unit.18 In the context of our previous works concerning the use of the [Fe(phen)(CN)4 ]− and [Cr(phen)(CN)4 ]− building blocks as ligands, we have investigated their complex formation with the binuclear complex NiII 2 L(H2 O)2 (ClO4 )2 (1) [H2 L = 11,23-dimethyl-3,7,15,19-tetraazatricyclo[19.3.1.19,13 ]hexacosa2,7,9,11,13(26), 14,19,21(25),22,24-decaene-25,26-diol]. The neutral tetranuclear bimetallic compounds [{MIII (phen)(CN)4 }2 {NiII 2 L(H2 O)2 }]·2CH3 CN with M = Fe (2) and Cr (3) have been synthesized and their structures determined by single crystal X-ray diffraction. These results together with the magnetic study of 1–3 and the theoretical analysis of the exchange pathways involved are presented here.

All theoretical calculations were carried out with the hybrid B3LYP method,29–31 as implemented in the GAUSSIAN98 program.32 Double- and triple-f quality basis sets proposed by Ahlrichs and co-workers have been used for all atoms.33,34 The broken symmetry approach has been employed to describe the unrestricted solutions of the antiferromagnetic spin states.35–38 The geometries of the heterodinuclear M(III)–CN–Ni(II) models were built from the experimental crystal structures. A quadratic convergence method was used to determine the more stable wave functions in the SCF process. The atomic spin densities were obtained from natural bond orbital (NBO) analysis.39–41

Experimental

Crystallography

Materials

Crystals of dimensions 0.44 × 0.40 × 0.10 (2), 0.42 × 0.38 × 0.10 mm (3) were mounted on a Bruker R3m/V automatic fourcircle diffractometer and used for data collection. Diffraction data were collected at room temperature by using graphite ˚ ) with the x– monochromated Mo-Ka radiation (k = 0.71073 A 2h scan method. The unit cell parameters were determined from least-squares refinement of the setting angles of 25 reflections in the 2h range of 15–30◦ . Information concerning crystallographic data collection and structure refinements is summarized in Table 1. Examination of two standard reflections, monitored after every 98 reflections, showed no sign of crystal deterioration. Lorentz-polarization and W-scan absorption corrections42 were applied for compound 3, whereas W-scan absorption was ignored for compound 2. The maximum and minimum transmission factors were 0.744 and 0.667 for 3. The structures

Chemicals were purchased from commercial sources as reagents pure for analysis and used as received. The complexes PPh4 [Fe(phen)(CN)4 ]·2H2 O, PPh4 [Cr(phen)(CN)4 ]·H2 O·CH3 OH and [NiII 2 L(H2 O)2 Cl2 ] were prepared by following previously reported procedures.26a,g,27 Elemental analysis (C, H, N) was performed by the Microanalytical Service of the Universi´ dad Autonoma de Madrid. A value of 1 : 1 for the Fe : Ni (2) and Cr : Ni (3) molar ratio was determined by electron probe X-ray microanalysis at the Servicio Interdepartamental of the University of Valencia. Preparations Ni2 L(H2 O)2 (ClO4 )2 (1). This was prepared by the reaction of aqueous solutions of [Ni2 (L)(H2 O)2 Cl2 ] (1 mmol) and silver perchlorate (2 mmol) in the dark. After stirring during five hours, the white precipitate of silver chloride is removed by filtration and discarded. Complex 1 is obtained as a green polycrystalline powder by slow evaporation of the mother liquor at room temperature. The yield is practically quantitative. Anal. Calc. for C24 H50 Cl2 N4 Ni2 O12 (1): C, 38.20; H, 3.97; N, 7.43. Found: C, 36.48; H, 3.09; N, 6.15%. IR stretching perchlorate (KBr/cm−1 ): 1115s, 1040s and 625m. [{MIII (phen)(CN)4 }2 {NiII 2 L(H2 O)2 }]·2CH3 CN with M = Fe (2) and Cr (3). Single crystals of 2 and 3 were grown by a slow diffusion method using an H-shaped glass vessel. The starting solutions were an aqueous solution of Li[M(phen)(CN)4 ] (0.033 mmol) in one arm with M = Fe (2) and Cr (3) [generated by the metathesis reaction of PPh4 [Fe(phen)(CN)4 ]·2H2 O (2) or PPh4 [Cr(phen)(CN)4 ]·H2 O·CH3 OH (3) and LiClO4 in a 1 : 1 molar ratio in acetonitrile] and an acetonitrile solution of NiII 2 L(H2 O)2 (ClO4 )2 (0.016 mmol) in the other one. The diffusion liquid was an acetonitrile : water 1 : 1 (v/v) mixture. After a few weeks, dark (2) and pale (3) brown prisms were formed. The crystals were collected and dried on filter paper. Yield: ca. 50% for 2 and 3. Anal. Calc. for C60 H52 Fe2 N18 Ni2 O4 (2): C, 54.66; H, 3.94; N, 19.11. Found: C, 56.21; H, 2.98; N, 18.12%. Anal. Calc. for C60 H52 Cr2 N18 Ni2 O4 (3): C, 54.98; H, 3.96; N, 19.22. Found: C, 55.12; H, 4.42; N, 18.03%. IR stretching cyanide (KBr)/cm−1 ): 2151m, 2128s and 2117m (2) and 2156w and 2136s (3). 1358

Dalton Trans., 2005, 1357–1364

Physical techniques The IR spectra (KBr pellets) were performed on a Nicolet Avatar 320 FT-IR spectrophotometer. Magnetic susceptibility measurements on polycrystalline samples of 1–3 were carried out with a Quantum Design SQUID magnetometer in the temperature range 1.9–300 K and under applied magnetic fields ranging from 50 Oe to 1 T. Magnetization versus magnetic field measurements of 1–3 were carried out at 2.0 K in the field range 0–5 T. Diamagnetic corrections of the constituent atoms were estimated from Pascal constants28 as −336 × 10−6 (1), −676 × 10−6 (2) and −678 × 10−6 cm3 mol−1 (3). Computational details

Table 1 Crystallographic data for [{FeIII (phen)(CN)4 }2 {NiII 2 (L)(H2 O)2 }]·2CH3 CN (2) and [{CrIII (phen)(CN)4 }2 {NiII 2 (L)(H2 O)2 }]·2CH3 CN (3)

Empirical formula Mr Space group ˚ a/A ˚ b/A ˚ c/A a/◦ b/◦ c /◦ ˚3 V /A Z qc /g cm−3 F(000) T/K l(Mo-Ka)/cm−1 R1 a wR2 b Sc

2

3

C30 H26 FeN9 NiO2 659.16 P1¯ 9.841(2) 11.514(2) 14.595(3) 100.95(3) 103.44(3) 109.17(3) 1454.0(5) 2 1.506 678 293 1.192 0.042 0.105 0.928

C30 H26 CrNiN9 O2 655.31 P1¯ 10.000(3) 11.492(3) 14.669(4) 101.02(2) 103.63(2) 109.18(2) 1479.6(7) 2 1.471 674 293 1.048 0.055 0.145 0.977

R1 = R (|F o | − |F c |)/R |F o |. b wR2 = {R [w(F o 2 − F c 2 )2 ]/R [w(F o 2 )2 ]}1/2 and w = 1/[r2 (F o 2 ) + (aP)2 + bP] with P = [F o 2 + 2F c 2 ]/3, a = 0.0743 (2) and 0.1027 (3), and b = 0 (2, 3). c Goodness of fit = [R w(|F o | − |F c |)2 /(N o − N p )]1/2 . a

˚ ] and angles [◦ ] for 2a Table 2 Selected bond lengths [A

Table 3

˚ ] and angles [◦ ] for 3a Selected bond lengths [A

Fe(1)–N(1) Fe(1)–N(2) Fe(1)–C(13) Fe(1)–C(14) Fe(1)–C(15) Fe(1)–C(16) Ni(1)–N(6) Ni(1)–N(7)

2.001(3) 2.008(3) 1.950(3) 1.924(4) 1.908(3) 1.959(3) 2.105(3) 2.025(3)

Ni(1)–N(8) Ni(1)–O(1) Ni(1)–O(1a) Ni(1)–O(2) C(13)–N(3) C(14)–N(4) C(15)–N(5) C(16)–N(6)

1.996(3) 2.019(2) 2.035(2) 2.141(2) 1.137(4) 1.146(4) 1.154(4) 1.141(4)

Cr(1)–N(1) Cr(1)–N(2) Cr(1)–C(13) Cr(1)–C(14) Cr(1)–C(15) Cr(1)–C(16) Ni(1)–N(6) Ni(1)–N(7)

2.091(3) 2.089(4) 2.069(5) 2.058(5) 2.063(5) 2.075(4) 2.093(4) 2.022(3)

Ni(1)–N(8) Ni(1)–O(1) Ni(1)–O(1a) Ni(1)–O(2) C(13)–N(3) C(14)–N(4) C(15)–N(5) C(16)–N(6)

1.996(4) 2.026(3) 2.033(3) 2.146(3) 1.133(6) 1.132(6) 1.136(5) 1.141(5)

N(1)–Fe(1)–N(2) C(13)–Fe(1)–N(1) C(14)–Fe(1)–N(1) C(15)–Fe(1)–N(1) C(16)–Fe(1)–N(1) C(13)–Fe(1)–N(2) C(14)–Fe(1)–N(2) C(15)–Fe(1)–N(2) C(13)–Fe(1)–C(14) C(13)–Fe(1)–C(15) C(13)–Fe(1)–C(16) C(14)–Fe(1)–C(16) C(15)–Fe(1)–C(14) C(15)–Fe(1)–C(16) C(16)–Fe(1)–N(2) N(3)–C(13)–Fe(1) N(4)–C(14)–Fe(1) N(5)–C(15)–Fe(1)

81.84(11) 92.69(12) 95.33(13) 175.72(12) 89.27(11) 92.25(12) 176.89(13) 94.54(13) 89.15(14) 89.72(14) 178.04(13) 90.83(13) 88.24(14) 88.32(13) 87.87(12) 177.8(3) 178.7(4) 177.0(3)

N(6)–C(16)–Fe(1) C(16)–N(6)–Ni(1) O(1)–Ni(1)–O(1a) Ni(1)–O(1)–Ni(1a) O(1)–Ni(1)–O(2) O(1a)–Ni(1)–O(2) O(1)–Ni(1)–N(6) O(1a)–Ni(1)–N(6) O(1)–Ni(1)–N(7) O(1a)–Ni(1)–N(7) O(2)–Ni(1)–N(6) O(2)–Ni(1)–N(7) O(1)–Ni(1)–N(8) O(1a)–Ni(1)–N(8) O(2)–Ni(1)–N(8) N(6)–Ni(1)–N(7) N(6)–Ni(1)–N(8) N(7)–Ni(1)–N(8)

174.0(3) 155.8(2) 80.34(9) 99.66(9) 90.24(9) 87.69(9) 89.33(10) 95.34(9) 91.15(10) 169.89(9) 176.82(9) 86.89(10) 169.65(9) 89.55(10) 87.13(11) 89.97(10) 93.84(11) 98.70(11)

N(2)–Cr(1)–N(1) C(13)–Cr(1)–N(1) C(14)–Cr(1)–N(1) C(15)–Cr(1)–N(1) C(16)–Cr(1)–N(1) C(13)–Cr(1)–N(2) C(14)–Cr(1)–N(2) C(15)–Cr(1)–N(2) C(16)–Cr(1)–N(2) C(13)–Cr(1)–C(16) C(14)–Cr(1)–C(13) C(14)–Cr(1)–C(15) C(14)–Cr(1)–C(16) C(15)–Cr(1)–C(13) C(15)–Cr(1)–C(16) N(3)–C(13)–Cr(1) N(4)–C(14)–Cr(1) N(5)–C(15)–Cr(1)

79.40(14) 94.18(15) 95.11(16) 171.84(15) 87.82(14) 92.34(16) 174.36(16) 93.31(17) 87.10(15) 177.79(16) 89.30(17) 92.10(18) 91.47(16) 89.73(17) 88.17(16) 176.8(4) 179.3(5) 177.0(4)

O(1)–Ni(1)–O(1a) Ni(1)–O(1)–Ni(1a) O(1)–Ni(1)–O(2) O(1a)–Ni(1)–O(2) O(1)–Ni(1)–N(6) O(1a)–Ni(1)–N(6) N(6)–Ni(1)–O(2) N(7)–Ni(1)–O(1) N(7)–Ni(1)–O(1a) N(7)–Ni(1)–O(2) N(7)–Ni(1)–N(6) N(8)–Ni(1)–O(1) N(8)–Ni(1)–O(1a) N(8)–Ni(1)–O(2) N(8)–Ni(1)–N(6) N(8)–Ni(1)–N(7) N(6)–C(16)–Cr(1) C(16)–N(6)–Ni(1)

80.31(11) 99.69(11) 90.04(13) 87.15(11) 89.81(13) 95.50(12) 177.27(13) 91.11(13) 169.69(12) 87.15(13) 90.14(13) 169.62(12) 89.64(13) 86.84(15) 93.77(15) 98.61(14) 171.0(4) 160.0(3)

a

Symmetry code: (a) −x, −y, −z.

a

Symmetry code: (a) −x, −y, −z.

were solved by standard Patterson methods and subsequently completed by Fourier recycling. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms (except those of the acetonitrile and water molecules, which were refined with restraints) were set in calculated positions and refined as riding atoms with a common fixed isotropic thermal parameter. The refinement was performed on F against 5739 (2), 5858 (3) reflections. The residual maxima and minima in the final Fourier˚ −3 for 2, 1.64 and difference maps were 0.76 and −0.83 e A ˚ −3 for 3. Solutions and refinements were performed −0.49 e A with the SHELXTL PLUS system.43 The final geometrical calculations were carried out with the PARST program.44 The graphical manipulations were performed using the XP utility of the SHELXTL PLUS system. Main interatomic bond distances and angles are listed in Tables 2 (2) and 3 (3). CCDC reference numbers 259752 (2) and 259753 (3). See http://www.rsc.org/suppdata/dt/b5/b500168b/ for crystallographic data in CIF or other electronic format.

Results and discussion Description of the structures [{MIII (phen)(CN)4 }2 {NiII 2 L(H2 O)2 }]·2CH3 CN with M = Fe (2) and Cr (3). Compounds 2 and 3 are isostructural. The structure is made up of heterotetranuclear neutral [{MIII (phen)(CN)4 }2 {NiII 2 L(H2 O)2 }] [M = Fe (2) and Cr (3)] units and non-coordinated acetonitrile molecules. The tetranuclear entity (Figs. 1 and S1† for 2 and 3, respectively; identical atom numbering was used in both drawings) is constituted by a central [Ni2 (L)(H2 O)2 ]2+ dinuclear motif to which two peripheral [M(phen)(CN)4 ]− [M = Fe(2) and Cr(3)] units are connected through one of the four cyanide groups. The macrocyclic ligand L is almost flat, enclosing two distorted octahedral nickel(II) centres, bridged by phenoxide oxygen atoms. Each nickel atom has two imine-nitrogen [N(7) and N(8)] and two phenoxide-oxygen [O(1) and O(1a)] atoms forming the strictly planar N2 O2 equatorial donor set [deviations ˚ for O(1a)] whereas a water molecule not greater than 0.028(1) A [O(2)] and a cyanide-nitrogen [N(6)] fill the axial positions. ˚ (3) The nickel atom is shifted by 0.074 (2) and 0.080(1) A

Fig. 1 Perspective drawing of complex 2 showing the atom numbering. Hydrogen atoms were omitted for simplicity.

from the N2 O2 basal plane towards the axial N(6) atom. An inversion centre is located in the middle of Ni2 O2 plane. The two nickel atoms are separated by 3.098(2) (2) and 3.101(1) ˚ (3), with a Ni–O–Ni bridge angle of 99.66(9) (2) and A 99.69(11)◦ (3). The mononuclear cyano-containing units act as monodentate ligands towards the [Ni2 L(H2 O)2 ]2+ unit through Dalton Trans., 2005, 1357–1364

1359

Fig. 2 A view along the a axis showing the hydrogen bonding between the neutral tetranuclear units of complex 2.

one of its four peripheral groups. This leads to the unprecedented cyano-bridged heterometallic FeIII 2 –NiII 2 (2) and CrIII 2 –NiII 2 (3) tetranuclear units. The iron (2) and chromium (3) atoms are coordinated by two phen-nitrogen atoms and four cyanidecarbon atoms, in a distorted octahedral geometry. The values of the MIII –N(phen) bond distances [2.001(3) and 2.008(3) ˚ ] (2) and [2.091(3) and 2.089(4) A ˚ ] (3) and that of the A angle subtended by the chelating phen [81.84(11)◦ at Fe(1) (2) and 79.40(14)◦ at Cr(1) (3)] are practically the same as those observed in the mononuclear [Fe(phen)(CN)4 ]−26a and [Cr(phen)(CN)4 ]− .26g This agreement also applies to the MIII – C(cyano) bonds [values varying in the range 1.908(3)–1.959(3) ˚ at Fe(1) (2) and 2.058(5)–2.075(4) A ˚ at Cr(1) (3)]. The A values of the cyanide C–N bonds at Fe(1) in 2 [1.137(4)– ˚ ] and at Cr(1) in 3 [1.132(6)–1.141(5) A ˚ ] are in good 1.154(4) A agreement with those reported in the literature.26a,g The M(1)– C–N angles for both terminal [177.0(3), 177.8(3) and 178.7(4)◦ (2) and 177.0(4), 176.8(4) and 179.3(5)◦ (3) for M(1)–C(15)– N(5), M(1)–C(13)–N(3) and M(1)–C(14)–N(4)] and bridging [174.0(3) (2) and 171.0(4)◦ (3) for M(1)–C(16)–N(6)] cyanide ligands are somewhat below that of strict linearity. The Ni(1)– N–C unit at the bridging cyanide is significantly bent [155.8(2) (2) and 160.0(3)◦ (3) for Ni(1)–N(6)–C(16)]. The metal–metal ˚ separations via the cyano-bridges are 5.058(1) (2) and 5.174(2) A (3) whereas those between the peripheral metal ions are 10.68 (2) ˚ (3). The shortest intermolecular MIII –MIII , and NiII – and 10.90 A III ˚ (3) and 6.025(2) M distances are 7.161(3) (2) and 7.020(3) A ˚ (3) [M(1) · · · M(1b) and Ni(1) · · · M(1a); (a) = (2) and 6.118(2) A −x, −y, −z and (b) = −x, 1 − y, 1 − z]. Bond distances and angles for the macrocyclic L ligand in 2 and 3 are in agreement with those observed for this ligand in its homodinuclear metal complexes with copper(II),45 nickel(II),46 cobalt(II)47 and iron(II)48 ions. The phen molecules coordinated to Fe(1) (2) and Cr(1) (3) are in agreement with those reported for the free ligand.49 Views of the crystal packing (Figs. 2 and S2† for 2 and 3, respectively) show that the tetranuclear units are linked through a hydrogen bond involving the coordinated water molecule and one terminal cyanide ligand ˚ (3) for O(2) · · · N(5c) and 164(3) (2) [2.826(4) (2) and 2.835(5) A and 167(4)◦ (3) for O(2)–H(1w) · · · N(5c); (c) = 1 + x, y, z]. Graphite-like interactions through the phen ligand occur, the ˚ interplanar phen–phen separations being 3.51 (2) and 3.53 A (3). These interactions lead to chains growing along the c axis, the intrachain MIII · · · MIII separations being 8.853(2) (2) and ˚ (3) [M(1) · · · M(1d), (d) = −x, −y, 1 − z]. 8.971(2) A 1360

Dalton Trans., 2005, 1357–1364

Magnetic properties The temperature dependence of the vM T product of 1 [vM is the magnetic susceptibility per two nickel(II) ions] is shown in Fig. 3. vM T at 300 K is 1.80 cm3 mol−1 K, a value which is lower than that expected for two magnetically isolated S = 1 spin sates. Upon cooling, vM T continuously decreases and it practically vanishes at very low temperatures. The magnetic susceptibility versus T plot (see Fig. 3) exhibits a broad maximum centred at 80 K. These features are characteristic of an intramolecular antiferromagnetic interaction between the two single-ion triplet spin states. The magnetic data of 1 were analyzed with the ˆ = −J Sˆ A ·Sˆ B where J is the isotropic spin Hamiltonian H magnetic coupling parameter and SA = SB = 1 (local spins), vM being expressed as in eqn. (1) vM = 2Nb2 g2 /kT{[exp(J/kT) + 5 exp(3J/kT)]/[1 + 3 exp(J/kT) + 5 exp(3J/kT)]} +Na

(1)

In this expression, N, b, k and g have their usual meanings and the last term (Na) is the temperature-independent paramagnetism which is assumed to be 200 × 10−6 cm3 mol−1 for two nickel(II) ions. Although Ginsberg et al.50 had also considered the effects of the single-ion zero-field splitting (D) on the magnetic susceptibility of nickel(II) binuclear complexes, these effects are expected to be negligible in the cases of a large stabilization

Fig. 3 Thermal dependence of vM (D) and vM T (䊊) for complex 1. The solid line is the best fit curve (see text).

of the singlet ground state.51 Complex 1 is just one example of this situation. Least-squares fitting of the magnetic data through eqn. (1) leads to the following parameters: J = −61.0(1) cm−1 and g = 2.19(1). The excellent agreement between theoretical and experimental magnetic data supports the validity of the model that we have chosen. The value of the antiferromagnetic coupling in 1 (J = −61 cm−1 ) is encompassed between those reported for the complexes NiII 2 L(py)2 (BF4 )2 (py = pyridine) [J = −46 cm−1 ]48 and NiII 2 L(H2 O)2 Cl2 (J = −72 cm−1 ).52 The lack of structural knowledge for these three compounds precludes a detailed discussion about the structural factors which are at the origin of the different values of the magnetic coupling observed. However, it is clear that the binucleating ligand L being relatively rigid, the two nickel(II) ions are constrained to lie in the plane of L, as observed in the crystal structures of this ligand with other transition metal ions.45,47,48 Slight structural changes (value of the angle at the phenoxo bridge and number and nature of the axial ligands, for instance) would account for the variation of the magnetic coupling. Anyway, the magnetic study of compound 1 was performed having in mind that this entity occurs in the tetranuclear compounds 2 and 3 where additional exchange pathways (single cyanide bridge) are present. The thermal dependence of the vM T product of the complexes 2 and 3 [vM is the magnetic susceptibility per NiII 2 MIII 2 unit, M = Fe (2) and Cr (3)] is shown in Figs. 4 and 5, respectively. The values of vM T at 300 K are 3.60 (2) and 5.90 cm3 mol−1 K (3). They are as expected for a four spin system made up of two spin triplets and either two spin doublets (2) or two spin quadruplets (3). Upon cooling, vM T continuously decreases and it reaches values of 0.15 (2) and 0.6 cm3 mol−1 K (3) at 1.9 K. The susceptibility versus T plot for 2 and 3 in the low temperature range exhibit maxima at 3.5 (2) and 2.9 K (3) (see insets of Figs. 4 and 5). These features are characteristic of an overall antiferromagnetic behaviour. Keeping in mind the tetranuclear structure of 2 and 3, their magnetic data were ˆ = −J M–Ni analyzed through the isotropic spin Hamiltonian H ˆ Ni Sˆ Nia ] with M = Fe (2) and Cr (3). (Sˆ M ·Sˆ Ni + SMa ·Sˆ Nia ) −J Ni–Nia S· SNi and SM are the spin operators associated with the interacting local spins [SNi = 1 and SM = 1/2 (2) and 3/2 (3)] and J M–Ni and J Ni–Nia are the magnetic couplings through the single cyanide bridge and the double phenoxo bridge, respectively. There is no analytical expression to model the magnetic data of 2 and 3 and consequently, we used the matrix diagonalization techniques to

Fig. 4 Thermal dependence of the vM T product for complex 2: (䊊) experimental data, (—) best-fit curve (see text). The inset shows the vM versus T plot in the low temperature range.

Fig. 5 Thermal dependence of the vM T product for complex 3: (䊊) experimental data, (—) best-fit curve (see text). The inset shows the vM versus T plot in the low temperature range.

reproduce them. Best-fit parameters are: J Fe–Ni = +17.4(1) cm−1 , J Ni–Ni(a) = −44.4(1) cm−1 , gNi = 2.20(1) and gFe = 2.23(1) for 2 and J Cr–Ni = +11.8(1) cm−1 , J Ni–Ni(a) = −44.6(1) cm−1 , gNi = 2.22(2) and gCr = 1.99(1) for 3. The calculated curves match very well the magnetic data over the whole temperature range. The consideration of the magnetic interaction between the nonadjacent magnetic centers in 2 and 3 (J M–Nia and J M–Ma ) did not improve the fit and the values of these magnetic couplings were practically zero. In addition, the DFT type calculations (see below) carried out on the whole structure of 3 gave very small values of J M–Nia and J M–Ma , supporting thus the validity of our approach. Several comments are in order in the light of the above set of magnetic couplings. Firstly, the magnetic coupling between the nickel(II) ions through the double phenoxo bridge in 2 [J Ni–Nia = −44.4(1) cm−1 ] and 3 [J Ni–Nia = −44.6(1) cm−1 ] are practically identical and very close to that measured in NiII 2 L(py)2 (BF4 )2 (py = pyridine) [J = −46 cm−1 ],48 but somewhat smaller than those observed in the dinuclear species 1 and NiII 2 L(H2 O)2 Cl2 (J = −72 cm−1 ).52 This features seems to indicate that the structural parameters around the nickel(II) ion in 2 and 3 have to be closer to those of the pyridine-containing species; however, the lack of structural information of this last compound, precludes us from going further. Secondly, the most remarkable feature of the above mentioned set of magnetic parameters is the ferromagnetic nature of the magnetic coupling between nickel(II) and the low-spin iron(III) (2) and chromium(III) (3) ions through the single cyanide bridge. Simple symmetry considerations account for this ferromagnetic nature of the magnetic interaction. Under ideal Oh symmetry for the octahedral NiII (t2g 6 eg 2 ) and MIII ions [t2g 5 eg0 and t2g 3 eg 0 for low-spin FeIII and CrIII , respectively], the interacting magnetic orbitals are strictly orthogonal, and thus ferromagnetic coupling is predicted, as observed. In previous reports, compounds containing single cyanide-bridged CrIII –NiII and FeIII –NiII pairs were prepared by reacting the hexacyanometallate [Cr(CN)6 ]3− and [Fe(CN)6 ]3− units with nickel(II) ions whose coordination sphere was partially blocked with polydentate ligands.17g,h,i,25e,53,54 In all the cases, a ferromagnetic coupling between the interacting metal ions was observed, the maximum values being J = +18.5 cm−1 for AsPh4 {[Ni(dienpy2 )]Cr(CN)6 } (AsPh4 + = tetraphenylarsonium cation and dienpy2 = 1,11bis(2-pyridyl)-2,6,10-triazaundecane)17h and J = +11.0 cm−1 for [(tach)(H2 O)12 Ni4 Fe4 (CN)12 ]Br8 ·18H2 O (tach = 1,3,5triaminocyclohexane).25e Dalton Trans., 2005, 1357–1364

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Analysis of the exchange pathways in 3 The two most important exchange pathways involved in 3 are a single cyanide bridge connecting local quartet [chromium(III)] and triplet [nickel(II)] spins and a double phenoxo group linking two spin triplets [nickel(II)]. In order to substantiate the intramolecular magnetic interactions observed in 3 and to analyze the exchange pathways involved therein (the case of 2 is a much simpler case given that the outer metal ion has only one unpaired electron in the t2g orbitals), we performed DFT type calculations on the whole tetranuclear molecule of 3 and on its model dinuclear fragment (Fig. 6). In the whole molecule, we considered the four intramolecular coupling parameters, that is J Cr–Ni , J Ni–Ni(a) , J Cr–Ni(a) and J Cr–Cr(a) . Five configurations A– E defined as (MS Cr , MS Ni , MS Nia , MS Cra ) have been calculated (A: 3/2, 1, 1, 3/2; B: 3/2, −1, −1, 3/2; C: 3/2, −1, 1, 3/2; D: 3/2, 1, 1, −3/2; E: 3/2, −1, 1, −3/2). The J values obtained from the energies of these five configurations in the way explained in reference 55 are J Cr–Ni = +8.5 cm−1 , J Ni–Ni(a) = −33.5 cm−1 , J Cr–Ni(a) = −0.08 cm−1 and J Cr–Cr(a) = −0.07 cm−1 . These values give validity to the approach we used above to fit the magnetic properties of the tetranuclear species where the two last interactions were neglected. The calculated values of J Cr–Ni and J Ni–Ni(a) agree well with those obtained by the fit of the magnetic data. Although the Ni–N–C fragment if far from being linear [159.6(3)◦ for Ni(1)–N(6)–C(16)], the ferromagnetic nature of the magnetic coupling between the Cr(III) and Ni(II) ions through the bridging cyanide is confirmed by the theoretical calculations.

Fig. 7 Effect of the deviation of the linearity in the fragment Ni–N–C on the coupling constant for the model structure shown in Fig. 6.

Fig. 8 Most important contributions to the magnetic coupling in compound 3 showing the influence of the bending of the CrIII –C–N–NiII unit on them.

Fig. 6 Heterodinuclear CrIII –NiII (green and blue big circles, respectively) model fragment of 3 built from the experimental structure. Ammonia groups were used as peripheral ligands, the corresponding bond lengths being optimized to reproduce the ligand field strength of the nitrogen heterocycle, the terminal cyanide groups and the macrocyclic ligand.

In an attempt to look for the dependence of the magnetic coupling on the value of the Ni–N–C angle (a), we focused on the heterodinuclear NiII –N–C–CrIII model shown in Fig. 6 where the value of a was varied in the range 180–135◦ . The plot with the calculated J values against a is shown in Fig. 7. One can see there that the ferromagnetic interaction between the Ni(II) and Cr(III) ions through the single cyanide bridge is weakened with the bending at the cyanide bridge and it becomes antiferromagnetic for values of a ≤ 148◦ . It deserves to be noted that we have found a value of J Cr–Ni for a = 159.6◦ very close to that found through the DFT calculations on the tetranuclear molecule. The influence of the deviation from linearity of the Ni–N–C unit on the magnetic coupling NiII –N–C–CrIII can be explained on a simple orbital basis as shown in Fig. 8. Although there are six different contributions to the exchange coupling constant of the CrIII –NiII pair, the inappropriate orientation of four pairs of magnetic orbitals allow us to discard them. The two remaining ones are depicted in Fig. 8. They concern the two t2g orbitals (dyz and dxz ) of the CrIII and the dz2 magnetic orbital of the NiII which are strictly orthogonal (ferromagnetic 1362

Dalton Trans., 2005, 1357–1364

contribution) when the NiII –C–N–CrIII fragment is linear [left, Fig. 8a, b]. The bending of the C–N–Ni unit causes the following effects: (i) the ferromagnetic nature of the magnetic coupling in case (a) is kept whereas (ii) it changes from ferro- to antiferromagnetic in case (b). It deserves to be noted that this last contribution counterbalances the ferromagnetic one as far as the bending increases and the overall magnetic coupling becomes antiferromagnetic in agreement with the DFT calculations. A similar analysis can be performed for the case of compound 2 where only a t2g orbital (which is a mixture of dxz and dyz )14a is half filled [low-spin iron(III) ion].

Conclusions In this work we show how the building blocks [MIII (phen)(CN)4 ]− [M = Fe and Cr] can be used as ligands towards the preformed dinuclear [NiII 2 L]2+ (L = binucleating macrocyclic ligand) species to afford neutral tetranuclear compounds of formula [{MIII (phen)(CN)4 }2 {NiII 2 L(H2 O)2 }]·2CH3 CN with M = Fe (2) and Cr (3) where intramolecular ferromagnetic (between MIII and NiII ions through the single cyanide bridge) and antiferromagnetic interactions (between the NiII ions across the double phenoxo bridge) coexist. Complexes 2 and 3 are examples of magnetic engineering because both the mononuclear and dinuclear stable precursors are assembled in a controlled manner to get intramolecular ferro- and antiferromagnetic magnetic interactions, as previously thought. Theoretical calculations allowed us to analyze and substantiate the magnetic coupling though the cyanide bridge and foresee its variation with the bending at the cyanide bridge. The easy modification of the cyanide-bearing mononuclear precursor and the variety of metal ions that can replace the nickel(II) ion in the macrocycle, would provide a plethora of new magnetic compounds in a near future

which would exhibit magnetic interactions of different sign and tunable magnitude.

Acknowledgements ˜ de Ciencia This work was supported by the Ministerio Espanol y Tecnolog´ıa (Project PBQU2001-2928), the Italian Ministero dell’Universit`a e della Ricerca Scientifica e Tecnologica and the European Union (Project QuEMolNa, MRTN-CT-504880). One of us (L. M. Toma) thanks the Spanish Ministery of Education for a FPU predoctoral grant.

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