Structures, Photophysical Properties, and Growth

crystals Article

Structures, Photophysical Properties, and Growth Patterns of Methylurea Coordinated Ni(II) and Mn(II) Complexes Wenting Wang 1,2 , Zhihuang Xu 2 , Liwang Ye 2 , Genbo Su 2 and Xinxin Zhuang 2, * 1 2

*

College of Chemistry, Fuzhou University, Fuzhou 350116, China; [email protected] Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China; [email protected] (Z.X.); [email protected] (L.Y.); [email protected] (G.S.) Correspondence: [email protected]; Tel.: +86-591-6317-3971

Academic Editor: Helmut Cölfen Received: 24 January 2017; Accepted: 23 February 2017; Published: 28 February 2017

Abstract: Four coordinated complexes Mnx Ni(1−x) (C2 H6 N2 O)6 SO4 (x=0, 0.33, 0.75, 0.90) were synthesized and characterized. Single-crystal X-ray analysis revealed that the complexes belong to the trigonal crystal family, R-3c space group. Spiral and terraced nucleus growth modes were observed by atomic force microscopy (AFM) in the crystals. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) measurements showed their excellent thermostability until 200 ◦ C. UV–vis spectra revealed that the transmission peaks of these crystals have a slight bathochromic shift compared to nickel sulfate hexahydrate (NSH), and the transmittance in the UV range increased as the proportion of Mn2+ increased. With their photophysical properties remaining similar, the much higher heat endurance is rendering these crystals better suitable for UV light filter (ULF) applications. Keywords: crystal structure; UV light filter; thermostability; methylurea

1. Introduction Nickel(II) complexes have been widely applied in many regions, for example, catalyzing the synthesis of new compounds [1,2] or being a component of rechargeable batteries [3]. The complexes are also important constituents of ultraviolet light filters (ULFs). ULFs are individual compounds or mixtures to filter most light out and letting one specific wavelength pass. Nickel sulfate hexahydrate (NiSO4 ·6H2 O, NSH) is the most commonly used crystal in ULFs, as it has a tremendous absorption from 350 nm to 430 nm in aqueous solution, with a molar absorptivity of 5.13 ± 0.03 mol−1 ·dm3 ·cm−1 at 393 nm over the range from 0.04 to 0.08 mol·cm−3 [4]. With a character of high transmission efficiency (>80%) to ultraviolet light (250–340 nm), its potential for acting as a ULF crystal has been exploited. In 1932, NSH crystal was first grown, with its crystal structure reported in detail by Beevers and Lipson [5]. After decades of development, the growth of NSH has been promoted to a larger scale with a lower cost, which leads to a wide usage of NSH in opticalphysics, chemistry, communication, and military [6]. However, NSH crystal starts to dehydrate when the temperature reaches 73 ◦ C as a result of weak coordination bonds between the central metal and the O in water ligands [7], which greatly limits its potential applications in some fields, especially under high-temperature conditions. So, efforts have been made during the past decades in order to promote its thermal stability while maintaining its optical properties. In general, the method applied can be grouped into two categories. One is to introduce another metal cation into NSH, to obtain a doped or mixed crystal. K2 Ni(SO4 )2 ·6H2 O (KNSH) crystal was reported to have a dehydration temperature of 97 ◦ C in 1998, by Northrop

Crystals 2017, 7, 68; doi:10.3390/cryst7030068

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Grumman Company [8]. Besides, a number of other crystals were reported as a follow-up, such as Cs2 Ni(SO4 )2 ·6H2 O [9], FeNi(SO4 )2 ·12H2 O [10], Rb2 Ni(SO4 )2 ·6H2 O [11], (NH4 )2 Co0.17 Ni0.83 (SO4 )2 ·6H2 O [12], K2 Co0.1 Ni0.9 (SO4 )2 ·6H2 O [13], and K2 Znx Ni1−x (SO4 )2 ·6H2 O [14]. In 2014, guanidine carbonate-doped NSH crystal, whose dehydration temperature was close to 100 ◦ C, had been investigated by Silambarasan et al. [15]. Meena utilized urea/thiourea to induce the growth of (NH4 )2 Ni(SO4 )2 ·6H2 O crystal, which can maintain its weigh until the temperature reaches up to 100 ◦ C [16]. The other method is to replace its water ligands with other molecules, which have a stronger coordinating ability with Ni2+ . As Ni2+ has an irregular electron configuration (9–17 electron configuration), the interaction between the central atom and the ligand tends to be a covalent bond. The lower the electronegativity of the coordinated atom is, the easier it is to donate an electron pair, which is more conducive to the formation of a stable coordination covalent bond. Ni(C6 H12 N4 )2 SO4 ·4H2 O crystal, having two of the six water ligands replaced by hexamethylenetetramine, with a better stability and a slight change in absorption band, has been reported recently by Shengmin Gao et al. [17]. Though many methods have been tried, there is still not a dramatic increase in dehydration temperature. Methylurea is believed to have a strong tendency to bind with divalent metal ions. In 1958, complex Ni(C2 H6 N2 O)6 SO4 (I) was synthesized by Nardelli et al. [18,19]. However, except for crystal cell parameters and physical constants, there are no detailed studies based on this complex. In this paper, three complexes—Mnx Ni(1−x) (C2 H6 N2 O)6 SO4 (x=0.33 (II), 0.75 (III), 0.90 (IV))—were synthesized on the basis of (I), and single crystals of (I), (II), (III), and (IV) with centimeter-scale sizes were obtained from aqueous solution by evaporating solvent slowly. The structures of grown crystals were determined by X-ray single-crystal diffraction, and bulk purities were confirmed by powder X-ray diffraction. Metallographic microscope and AFM (atomic force microscopy) were employed to obtain the information on micro-aspects. UV–vis spectra were obtained to study their photophysical properties. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) measurements showed their excellent stability until 200◦ C, indicating the pronounced enhancement in heat endurance compared to NSH. 2. Results and Discussion 2.1. Structures of (II), (III), and (IV) The structure of complex (I) crystallizes in the trigonal space group R-3c with a = b = 10.93(2) Å, c = 40.19(8) Å, and V = 4206.8(6) Å3 , which was reported by Nardelli et al. in 1958 [18]. Crystal data, data collection, and structure refinement details for (II), (III), and (IV) are summarized in Table 1. The structures were solved by direct methods and refined by the full matrix least-squares method using Olex2 and SHELXL [20,21]. In one repeating unit, M (Ni2+ /Mn2+ ) serves as the coordinate center, surrounded by six O atoms from the carbonyl group of methylurea. In general, these six O atoms coordinate with the center by bonds of similar length, giving an octahedral structure. As they are in a similar chemical environment, the O atoms are equal to each other. The center atom M and the two O atoms—which are opposite to each other—are localized on a line, as the angle between them is approaching 180◦ (Figures 1 and 2a). Selected bond lengths and bond angles are listed in Table 2. As shown in Figure 2a,c, the hydrogen bond N–H . . . O (between an O atom of one methylurea and N atoms of another one) pack the complex so it is more compact. The SO4 2− anion, which carries negative charge to balance the positive charge, has a tetrahedral geometry and is associated with the octahedral unit by N–H...O hydrogen bond. The hydrogen bond lengths and bond angles of (II), (III), and (IV) are listed in Tables 3–5. The unit cell is shown in Figure 2b; the C, N, and H atoms are omitted for clarity. Complexes(III) and (IV) have identical structures to (II), except the increasement of the M–O bond length and the increased proportion of Mn2+ , which has a larger ionic radius than Ni2+ . The hydrogen bonds in these three crystal structures play a crucial role in raising the decomposition temperature. Compared to NSH, whose hydrogen bonds only show up between H2 O ligands and


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the SO4 2− anion, there are associations between the H in the amino group and the O in the carbonyl group of different methylurea ligands. The complexes are packed more tightly, as a result of these intramolecular attractions. To break up the structure of these complexes, the energy for disassociation of the hydrogen bonds has to be applied, which could lead to an increasement of thermal stability. Crystals 2017, 7, x FOR PEER REVIEW 4 of 11 Crystals 2017, 7, x FOR PEER REVIEW

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2+/Mn2+) in (I), (II), (III), and (IV), shown with 30% Figure 1. The coordination environment of the 2+ /Mn2+ ) in (I), (II), (III), and (IV), shown with Figure 1. The coordination environment of M the(Ni M (Ni 2+) in (I), (II), (III), and (IV), shown with 30% Figuredisplacement 1. The coordination environment of the M been (Ni2+/Mn probability ellipsoids. H atoms have omitted for clarity. 30% probability displacement ellipsoids. H atoms have been omitted for clarity.

probability displacement ellipsoids. H atoms have been omitted for clarity.

(a) (a)

(b) (b)

(c) 2+/Mn2+in (I), (II), (III), and (IV); (b) packing (c) Figure 2. (a) The coordination polyhedral aroundNi Figure 2. (a) The coordination polyhedral aroundNi2+ /Mn2+ in (I), (II), (III), and (IV); (b) packing diagram for(I), (II), (III), and (IV); C, H, N atoms have been omitted for clarity; (c) packing diagram Figure 2. (a) coordination polyhedral aroundNi in (I), (II), (III), and (b)diagram packingfor diagram forThe (I), (II), (III), and (IV); C, H, N atoms have2+/Mn been2+omitted for clarity; (c) (IV); packing for (I), (II), (III), and (IV); viewed along [100]. diagram for(I), (II), (III), and (IV); C, H, N atoms have been omitted for clarity; (c) packing diagram (I), (II), (III), and (IV); viewed along [100]. for (I), (II), (III), and (IV); viewed along [100].


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Table 1. Experimental details. Crystal Data

(II)

(III)

(IV)

CCDC No. Chemical formula Mr Crystal system, space group Temperature (K) a, c (Å) V (Å3 ) Z Radiation type µ (mm−1 ) Crystal size (mm) Diffractometer Absorption correction Tmin , Tmax No. of measured, independent and observed[I > 2σ(I)] reflections Rint (sin θ/λ)max (Å−1 ) R[F2 > 2σ(F2 )], wR(F2 ), S Data/restraints/parameters H-atom treatment ∆$max , ∆$min (e·Å−3 )

1511167 C12 H36 Mn0.33 N12 Ni0.67 O6 ·O4 S 598.04 Trigonal, R-3c 293 10.9919 (7), 40.634 (5) 4251.7 (7) 6 Mo Kα 0.75 0.18 × 0.16 × 0.12 Bruker P4 diffractometer Multi-scan Crystal Clear 0.740, 1.000 8948, 964, 875 0.037 0.624 0.042, 0.118, 1.10 964/9/63 H-atom parameters constrained 0.74, −0.53

1511166 C12 H36 Mn0.75 N12 Ni0.25 O6 ·O4 S 596.47 Trigonal, R-3c 293 11.0402 (5), 40.696 (4) 4295.7 (6) 6 Mo Kα 0.65 0.20 × 0.20 × 0.20 Bruker P4 diffractometer Multi-scan Crystal Clear 0.752, 1.000 9215, 1090, 932 0.039 0.649 0.043, 0.120, 1.07 1090/0/63 H-atom parameters constrained 0.58, −0.47

1515437 C12 H36 Mn0.90 N12 Ni0.10 O6 ·O4 S 595.53 Trigonal, R-3c 293 11.0845(6), 40.805(4) 4341.8(7) 6 Mo Kα 0.61 0.18 × 0.16 × 0.14 Bruker P4 diffractometer Multi-scan Crystal Clear 0.840, 1.000 9929, 1090, 946 0.046 0.648 0.048, 0.139, 1.08 1090/0/63 H-atom parameters constrained 0.63, −0.42


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Table 2. Selected geometric parameters (Å, ◦ ) for (II), (III), and (IV). Bond Lengths and Bond Angles

(II)

(III)

(IV)

M–O1 M–O1 i M–O1 ii M–O1 iii M–O1 iv M–O1 v O1–C1 N1–C1 N1–C2 N2–C1 O1 ii –M–O1 v O1 i –M–O1 O1 iv –M–O1 iii

2.0921(14) 2.0923(14) 2.0923(15) 2.0923(14) 2.0921(14) 2.0921(14) 1.256(3) 1.332(3) 1.441(4) 1.328(3) 180.0 179.99(9) 179.990(1)

2.1366(13) 2.1366(13) 2.1365(13) 2.1366(13) 2.1366(13) 2.1365(13) 1.255(2) 1.329(3) 1.432(4) 1.335(3) 180.0 180.0 180.00(7)

2.1706(14) 2.1708(14) 2.1708(14) 2.1708(14) 2.1706(14) 2.1706(14) 1.258(2) 1.332(3) 1.439(4) 1.333(3) 180.0 180.0 180.0

Symmetry codes: i −x + 4/3, −y + 2/3, −z + 2/3; −z + 2/3; iv −x + y + 1, −x + 1, z; v −y + 1, x − y, z.

ii

y + 1/3, −x + y + 2/3, −z + 2/3;

iii

x −y + 1/3, x − 1/3,

Table 3. Hydrogen-bond geometry (Å, ◦ ) for (II). D–H···A vii

N1–H1···O2 N1–H1···O3 N2–H2A···O1 v N2–H2B···O2 viii N2–H2B···O2 ix

D–H

H···A

D···A

D–H···A

0.86 0.86 0.86 0.86 0.86

2.00 2.62 2.11 2.31 1.98

2.839 (4) 3.441 (3) 2.881 (3) 3.040 (6) 2.837 (4)

163 160 150 144 179

Symmetry codes: iv –x + y + 1, −x + 1, z; v –y + 1, x − y, z; vi –y + 1, x − y + 1, z; vii y−1/3, x + 1/3, −z + 5/6; –x + y, −x + 1, z; ix x – y + 2/3, −y + 4/3, −z + 5/6; x –x + 2/3, −x + y + 1/3, −z + 5/6.

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Table 4. Hydrogen-bond geometry (Å, ◦ ) for (III). D–H···A x

N1–H1···O2 N1–H1···O3 N2–H2A···O1 iv N2–H2B···O2 N2–H2B···O2 ix

D–H

H···A

D···A

D–H···A

0.86 0.86 0.86 0.86 0.86

1.99 2.63 2.14 2.31 1.98

2.831(4) 3.450(3) 2.916(2) 3.045(5) 2.835(4)

164 159 151 143 179

Symmetry codes: iii x – y + 1/3, x − 1/3, −z + 2/3; iv –x + y + 1, −x + 1, z; v –y + 1, x − y, z; vi –y + 1, x − y + 1, z; y − 1/3, x + 1/3, −z + 5/6; viii –x + y, −x + 1, z; ix x − y + 2/3, −y + 4/3, −z + 5/6; x −x+2/3, −x + y + 1/3, −z + 5/6.

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Table 5. Hydrogen-bond geometry (Å, ◦ ) for (IV). D–H···A

D–H

H···A

D···A

D–H···A

N1–H1···O2 N1–H1···O3 vii N2–H2A···O1 iv N2–H2B···O2 x N2–H2B···O2 vi

0.86 0.86 0.86 0.86 0.86

2.00 2.64 2.16 2.31 1.98

2.832(4) 3.457(3) 2.944(3) 3.039(5) 2.836(4)

164 160 151 143 179

Symmetry codes: iii x −y + 1/3, x − 1/3, −z + 2/3; iv −x + y + 1, −x + 1, z; v –y + 1, x − y, z; vi –y + 1, x − y + 1, z; y − 1/3, x + 1/3, −z + 5/6; viii –x+y, −x + 1, z; ix x −y + 2/3, −y + 4/3, −z + 5/6; x –x + 2/3, −x + y + 1/3, −z + 5/6.

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Powder X-ray diffraction (PXRD) experiments were also carried out to confirm the phase purity of the complexes (Figure 3). The positions of the peaks calculated by PXRD are fully overlapped with


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the measured ones measured by single-crystal diffraction (XRD), illustrating the synthesized the ones by single-crystal diffraction (XRD), illustrating the the factfact thatthat the synthesized sample sample is of high purity. The positions of diffraction peaks show a shift to low angles, corresponding is of high purity. The positions of diffraction peaks show a shift to low angles, corresponding with the Crystals 2017, 7, 68 6 of 11 with the fact that the cell parameters of the title crystals increased with the enhancement of 2+ fact that the cell parameters of the title crystals increased with the enhancement of proportion of Mn . 2+ proportion of Mn . the ones measured by single-crystal diffraction (XRD), illustrating the fact that the synthesized sample is of high purity. The positions of diffraction peaks show a shift to low angles, corresponding with the fact that the cell parameters of the title crystals increased with the enhancement of proportion of Mn2+.

Figure 3. Powder X-ray diffraction patterns for (I), (II), (III), and (IV).

Figure 3. Powder X-ray diffraction patterns for (I), (II), (III), and (IV).

2.2. Dislocation Etch Pit and Growth Steps

2.2. Dislocation Etch Pit and Growth Steps

Crystal plane indices were indexed according to the angles. Figure 3. Powder X-ray diffraction patterns forcrystal (I), (II), edge (III), and (IV). A schematic diagram

Crystal plane indices were indexed according the crystal edge angles. A schematic diagram of of crystal planes is shown in Figure 4. The (0 0 0 1)to planes of the crystals were corroded by ultrapure directly forPit2 and min, and were observed by planes a Leica of DM2500 metallographic microscope. The crystal planes is Etch shown in Figure 4. The (0 0 0 1) the crystals were corroded by ultrapure 2.2.water Dislocation Growth Steps offor dislocation etchwere pits observed for (I), (II),by (III), and (IV) are shown in Figure 5a–d, respectively. All waterphotos directly 2 min, and a Leica DM2500 metallographic microscope. The Crystal plane indices were indexed according to the crystal edge angles. A schematic diagram photos 3 rotation axis and three vertical symmetry planes in the etch dislocation etch pits were trigonal. A L of dislocation etch ispits for (I), (II), (III), and(0(IV) are shown in Figure respectively. All dislocation of crystal planes shown in Figure 4. The 0 0 1) planes of the crystals5a–d, were corroded by ultrapure pits could be easily observed, which is in agreement with the fact that these crystals crystallize in a 3 rotation etch pits were trigonal. A Land axis andby three vertical symmetry planes inmicroscope. the etch pits could be water directly for 2 min, were observed a Leica DM2500 metallographic The trigonal crystal system. The pits in Mn2+-containing crystals ((II), (III), and (IV)) are found to be deeper easily observed, whichetch is inpits agreement with fact that crystals crystallize in a trigonal photos of dislocation for (I), (II), (III),the and (IV) arethese shown in Figure 5a–d, respectively. All crystal and smaller than those of (I), because of the enhancement of solubility along with the rising 2+ -containing 3 rotation axis and three vertical symmetry planes in the etch dislocation etchin pits were trigonal. A Lcrystals system. The pits Mn ((II), (III), and (IV)) are found to be deeper and smaller proportion of Mn2+. The {0 1 −1 4} planes were split, and the cleavage planes of (III) were studied by pits couldofbe(I), easily observed, which is in agreement with the along fact that these crystallize in of a Mn2+ . than those because of the enhancement ofofsolubility with thecrystals rising proportion AFM, in order to have a better understanding growth steps. Growth hillocks of screw dislocation 2+ crystal system. The pits inand Mn the -containing crystals ((II), (III), and (IV)) are found be deeper Thetrigonal {0were 1 −1observed 4} planes cleavage ofthe (III) were studied bytoAFM, in order to on were {0 1 −1split, 4} planes (Figure 6a). Theplanes shape of growth hillock is elliptic, indicating and smaller than those of (I), because of the enhancement of solubility along with the rising have an a better understanding of growth steps. Growth hillocks of screw dislocation were observed anisotropic morphology in growth. Growth steps on the left side of hillock are base steps, but the proportion of Mn2+. The {0 1 −1 4} planes were split, and the cleavage planes of (III) were studied by are accumulated, which is considered to behillock causedisby soluteindicating transport effect. The on {0 right 1 −1 ones 4} planes (Figure 6a). The shape of the growth elliptic, an anisotropic AFM, in order to have a better understanding of growth steps. Growth hillocks of screw dislocation dislocation outcrop ofGrowth growth steps hillockon shownleft in Figure 6a and b are was covered by but growth layers of a are morphology in growth. steps, the right ones were observed on {0 1 −1 4} planes (Figure the 6a). The side shapeofofhillock the growthbase hillock is elliptic, indicating terraced nucleus, which is the step source of further nucleating. New nucleus formed on this basis, accumulated, which is considered to be caused by on solute transport effect.are Thebase dislocation of an anisotropic morphology in growth. Growth steps the left side of hillock steps, but outcrop the and grew layer upon layer, finally resulting in an island morphology [22–24]. As shown in Figure 6c, growth hillock shown in Figure 6a,b was covered by growth layers of a terraced nucleus, which is right ones are accumulated, which is considered to be caused by solute transport effect. The the vertical height of the steps is 0.69 nm, which approximates the interplanar spacing of the {0 1 −1 outcrop of growth hillock shown in Figureformed 6a and on b was by growth layers upon of a layer, thedislocation step source of further nucleating. New nucleus thiscovered basis, and grew layer 4} planes. As there is a sufficient growth component surrounding {0 1 −1 4} planes, it is easier to form terraced nucleus, which is themorphology step source of[22–24]. further nucleating. New nucleus formed on thisheight basis, of the finally resulting in an island As shown in Figure 6c, the vertical step here, and the area of the crystal face is the smallest with a high tolerance to impurities, resulting andisgrew layer layer, finally resulting an island morphology in FigureAs 6c,there is steps nm,upon which the in interplanar spacing of[22–24]. the {0 As 1 −shown 1 4} planes. in a0.69 relatively straightapproximates step flow. the vertical height of the steps is 0.69 nm, which approximates the interplanar spacing of the {0 1 a sufficient growth component surrounding {0 1 −1 4} planes, it is easier to form step here, and−1the area 4} planes. As there is a sufficient growth component surrounding {0 1 −1 4} planes, it is easier to form of the crystal face is the smallest with a high tolerance to impurities, resulting in a relatively straight step here, and the area of the crystal face is the smallest with a high tolerance to impurities, resulting step flow. in a relatively straight step flow.

Figure 4. Schematic diagram and crystal plane indices for (I), (II), (III), and (IV).



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(a)(a)

(b)(b)

(c)

(d)

(c)

(d) Figure 5. Dislocationetch etchpits pits for (a) (III), and (d) (d) (IV).(IV). Figure 5. Dislocation (a) (I), (I),(b)(II), (b)(II),(c)(c) (III), and Figure 5. Dislocation etch pits for (a) (I), (b)(II), (c) (III), and (d) (IV).

(a)

(b)

(a)

(b)

(c) Figure 6. (a) Atomic force microcopy (AFM) images of growth hillock of {0 1 −1 4} planes; (b) magnified AFM images of growth steps of {0 1 −1 4} planes; (c) growth steps flow curve of {0 1 −1 4} planes.

Figure 4}planes; planes;(b) (b)magnified magnified Figure6.6.(a) (a)Atomic Atomicforce forcemicrocopy microcopy(AFM) (AFM) images images of of growth growth hillock of {0 1 − −114} AFM 1 4} planes; planes; (c) (c) growth growthsteps stepsflow flowcurve curveofof{0{011−1−4} 1 4} planes. AFMimages imagesofofgrowth growthsteps stepsofof{0{011−−1 planes.

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2.3. TGA DTA Analysis 2.3. TGA andand DTA Analysis Thermalgravimetric gravimetricanalysis analysis (TGA) (TGA) and differential (DTA) measurements were Thermal differentialthermal thermalanalysis analysis (DTA) measurements ◦ C/min from 40 ◦ C to 800 ◦ C under done withwith STA449C-QMS403C instrument at a heating rate of 10of were done STA449C-QMS403C instrument at a heating rate 10 °C/min from 40 °C to 800 °C N2 . As in Figure 7, crystals (I), (II), and (IV) decompose at 208at◦ C, under N2.shown As shown in Figure 7, crystals (I),(III), (II), (III), and start (IV) to start to decompose 208202 °C,◦ C, 202195 °C,◦ C, ◦ ◦ and respectively, far higher than the decomposition temperature of NSHof(73 C).(73 All°C). theAll DTA 195 °C,183 andC, 183 °C, respectively, far higher than the decomposition temperature NSH ◦ C, respectively, indicating an excellent measurements of these three crystals show a peak above 200 the DTA measurements of these three crystals show a peak above 200 °C, respectively, indicating an thermostability. Both TGA andTGA DTAand analysis the same the replacement excellent thermostability. Both DTAreach analysis reachconclusion the same that conclusion that the of water ligands in NSH by methylurea enhanced itsenhanced heat endurance to a great extent. The higher replacement of water ligands in NSH by methylurea its heat endurance to a great extent. degradation temperature is predictable, because of the increase of binding energy between the central The higher degradation temperature is predictable, because of the increase of binding energy metal and between the ligands. central metal and ligands.

Figure Thermogravimetricanalysis analysis(TGA) (TGA)and anddifferential differential thermal thermal analysis (DTA) Figure 7. 7. Thermogravimetric (DTA)curves curvesfor for(I), (I),(II), (III), and (IV). (II), (III), and (IV).

2.4. Photophysical Properties 2.4. Photophysical Properties UV recordedwith witha PE-Lambda a PE-Lambda spectrometer by testing the UVabsorption absorptionspectra spectra were recorded 950 950 spectrometer by testing the samples samples a thickness 3 mm) infrom the 200 range fromnm. 200As toshown 1000 nm. As shown in Figure (with a (with thickness of 3 mm)of in the range to 1000 in Figure 8, compounds (I),8,(II), compounds (I), have (II), (III), andphotophysical (IV) have similar photophysical properties, showing peaks 372836 nm, (III), and (IV) similar properties, showing peaks at 372 nm, 466 nm,at and nm. 2+ 466 nm, and 836 nm. Thewhich decent absorption, covers ofcaused the spectra, by The decent absorption, covers most ofwhich the spectra, is most mainly by Ni is ,mainly leadingcaused to the result 2+, leading to the result that more light can pass through the crystals 2+ Nithat when the Compared portion of to Mn more light can pass through the crystals when the portion of Mn2+ increases. NSH increases. Compared to NSH transmittance ofbathochromic all three crystals slight bathochromic crystal, the transmittance of crystal, all threethe crystals has a slight shifthas (10anm). This phenomenon shift This phenomenon can be explained the fact electron that methylurea a relatively electron to can(10 be nm). explained by the fact that methylurea is aby relatively sufficientismolecule, compared sufficient molecule, compared to water. The two electron-donating groups, amino andwith methylamino, water. The two electron-donating groups, amino and methylamino, are associated the carbonyl aregroup, associated withthe theOcarbonyl group, making the O more electron-rich to the one in the to making more electron-rich compared to the one in thecompared water molecule, leading water molecule, of leading to a diminution the energy difference between bonding molecular orbitals a diminution the energy difference of between bonding molecular orbitals (MOs) and nonbonding (MOs) nonbonding MOs, thus, resulting in aofbathochromic shift of the transmittance spectra. MOs,and thus, resulting in a bathochromic shift the transmittance spectra. Still, the three newly Still, the three newly synthesized complexes have a veryonly low at absorption, at UVnm), range (200–395 synthesized complexes have a very low absorption, UV rangeonly (200–395 allowing UV nm), allowing light selectively. to pass through With their photophysical properties remaining light to pass UV through With selectively. their photophysical properties remaining similar, the higher similar, the highermakes heat-endurance makes these crystals better for NSH. heat-endurance these crystals better substitutions for substitutions NSH.

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Figure 8. Transmission spectra forfor (I),(I), (II),(II), (III), and (IV). Figure 8. Transmission spectra (III), and (IV).

3. Experimental 3. Experimental Synthesis andand Crystallization Synthesis Crystallization Nickel sulphate (AR), manganese sulphate (AR), and methylurea (AR) were used without Nickel sulphate (AR), manganese sulphate (AR), and methylurea (AR) were used without further purification. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) further purification. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) ◦ C/min measurements were done with STA449C-QMS403C instrument at at a heating rate ofof 1010°C/min from measurements were done with STA449C-QMS403C instrument a heating rate from ◦ ◦ 40 40 °C to 800800 °C under N2N . Absorption spectra were recorded with PE-Lambda 950 spectrometer byby C to C under . Absorption spectra were recorded with PE-Lambda 950 spectrometer 2 testing samples with a thickness of 3 mm in the range from 200 to 1000 nm. testing samples with a thickness of 3 mm in the range from 200 to 1000 nm. Complexes (I),(I), (II), (III), and (IV) were synthesized in in accordance of of thethe following reaction: Complexes (II), (III), and (IV) were synthesized accordance following reaction: (1−x)NiSO4·6H2O + xMnSO4·H2O + 6C2H6N2O → MnxNi(1-x)SO4·6C2H6N2O + H2O (1) (1−x)NiSO4 ·6H2 O + xMnSO4 ·H2 O + 6C2 H6 N2 O → Mnx Ni(1-x) SO4 ·6C2 H6 N2 O + H2 O (1) In general, stoichiometric nickel sulphate, manganese sulphate and methylurea were dissolved In general, stoichiometric manganese sulphate andfiltration methylurea dissolved in in ultrapure water, respectively,nickel and sulphate, the solutions were mixed after by awere microporous ultrapure respectively, solutions filtration by allowed a microporous filtering filtering film.water, The mixed filtrate and was the stirred for 7 hwere at 60mixed °C in aafter beaker and was to evaporate ◦ C in a crystals film. at Thethe mixed was stirred for 1week, 7 h at 60regular beaker and was allowed to evaporate slowly samefiltrate temperature. After with good optical propertiesslowly were at the same temperature. After 1week, regular crystals with good optical properties were attained. attained. The grown crystals areare shown coupled plasma (ICP) revealed thethe The grown crystals shownininFigure Figure9. 9.Inductively Inductively coupled plasma (ICP) revealed 2+ /Ni 2+ complexes 2+/Ni 2+ in proportions of of Mn (II),(II), (III),(III), andand (IV)(IV) areare 0.33:0.67, 0.75:0.25, and 0.90:0.10, proportions Mn in complexes 0.33:0.67, 0.75:0.25, and 0.90:0.10, respectively. Anal. calcd. (%) forfor C12 O12 10SNi (599.30): C, 24.05; H, 6.05; N, N, 28.04. Found (%): C, C, respectively. Anal. calcd. (%) CH HN3612N O10 SNi (599.30): C, 24.05; H, 6.05; 28.04. Found (%): 1236 24.04; H,H, 5.98; N,N, 27.75. Anal. calcd. (%)(%) forfor C12CH 36 N 12 O 10 S Ni 0.33 Mn 0.67 (598.04): C, 24.10; H, 6.07;N, 24.04; 5.98; 27.75. Anal. calcd. H N O S Ni Mn (598.04): C, 24.10; H, 6.07; 12 36 12 10 0.33 0.67 28.11. FoundFound (%): C,(%): 24.08; 6.02;H, N, 6.02; 27.75.N, Anal. calcd. (%)calcd. for C12(%) H36N 12O SNi 0.25 0.75 (596.47): C, N, 28.11. C,H, 24.08; 27.75. Anal. for C1012 H36 NMn O SNi Mn 12 10 0.25 0.75 24.16; H, C, 6.08; N, H, 28.19. Found C, (%): 24.14;C, H, 6.02; N, 27.78.Anal.calcd. (%) (%) forfor (596.47): 24.16; 6.08; N, 28.19.(%): Found 24.14; H, 6.02; N, 27.78.Anal.calcd. C12CH1236H N36 12O Mn 0.90 (595.53): C, 24.20; H, 6.09; N, 28.23. Found (%):(%): C, 24.16; H, 6.03; N, N, 27.80. N10 O100.10 SNi Mn C, 24.20; H, 6.09; N, 28.23. Found C, 24.16; H, 6.03; 27.80. 12SNi 0.10 0.90 (595.53): − 1 −1 Selected IRIR data (m), (s), 1594 1594(m), (m),1575 1575(m), (m),1416 1416(m), Selected data(KBr, (KBr,cm cm ): 3424 ): 3424 (m),3322 3322(m), (m),2937 2937(w), (w),2884 2884 (w), (w), 1650 (s), −1 −1 (m), 1369 (m), 1126(m), (m),771 771(w), (w),621(w), 621(w),528 528(w). (w). In In the IR spectrum, 1369 (m), 1126 spectrum, aa pointed pointedpeak peakoccurs occursatat1650 1650cm cm − 1 − 1 −1 and −1 areare because of of C=O stretching vibration in in the 3322 thethe because C=O stretching vibration theamide. amide.Two Twopeaks peaksatat3424 3424cm cm and 3322cm cm characteristic peaks of of N–H stretching vibrations. characteristic peaks N–H stretching vibrations.


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(a)

(b)

(c)

(d)

Figure 9. Photographs of crystals (a) (I), (b) (II), (c) (III), and (d) (IV). Figure 9. Photographs of crystals (a) (I), (b) (II), (c) (III), and (d) (IV).

4. Conclusions 4. Conclusions In summary, four complexes, MnxNi(1−x)(C2H6N2O)6SO4 (x=0, 0.33, 0.75,0.90),were synthesized In summary, four complexes, Mn Ni −x) (C2 H6 N2 O)6 SO4 (x=0, 0.33, 0.75,0.90),were synthesized and characterized. XRD revealed the xtitle(1crystals belong to the trigonal system, R-3c space group. and characterized. XRD revealed the title crystals belong to the trigonal system, R-3c space group. Extra intramolecular hydrogen bonds make the complexes more stable than NSH. Two growth Extra intramolecular hydrogen bonds make the complexes more stable than NSH. Two growth patterns, patterns, spiral or terraced nucleus, were observed by AFM on {0 1 −1 4} planes, and the height of spiral or terraced nucleus, were observed by AFM on {0 1 −1 4} planes, and the height of growth growth steps corresponds with plane spacing. TGA and DTA measurements proved that the heat steps corresponds with plane spacing. TGA and DTA measurements proved that the heat endurance endurance could be enhanced to a great extend ◦(upto 200 °C) by replacing all six water ligands of could be enhanced to a great extend (upto 200 C) by replacing all six water ligands of NSH with NSH with methylurea. UV–vis spectra revealed the transmission peaks of title crystals have a slight methylurea. UV–vis spectra revealed the transmission peaks of title crystals have a slight bathochromic bathochromic shift (10 nm), compared to NSH. With their photophysical properties remaining shift (10 nm), compared to NSH. With their photophysical properties remaining similar, the much similar, the much higher heat endurance makes these crystals suitable for ULF applications. higher heat endurance makes these crystals suitable for ULF applications. Supplementary Materials: The following are available online at www.mdpi.com/2073-4352/7/3/68/s1, Table S1: Supplementary Materials: The following are available online at www.mdpi.com/2073-4352/7/3/68/s1, Table S1: Bond Bond lengths lengths (Å) (Å) and and angles angles (°) (◦ ) for(II), for (II),Table TableS2: S2:Torsion Torsionangles angles (°) (◦ )for for(II), (II),Table Table S3:Bond S3:Bond lengths lengths (Å) (Å) and and angles angles (°) for (III), Table S4:Torsion angles (°) for(III), Table S5: Bond lengths (Å), and angles (°) for(IV), Table S6: ◦ ◦ ◦ ( ) for (III), Table S4:Torsion angles ( ) for (III), Table S5: Bond lengths (Å), and angles ( ) for (IV), Table S6:Torsion Torsion angles angles (°) (◦ ) for for (IV). (IV). WentingWang Wangconceived, conceived,designed, designed, performed experiments, analyzed the data Contributions: Wenting Author Contributions: performed thethe experiments, analyzed the data and and wrote the paper; Zhihuang XuLiwang and Liwang Ye contributed materials and analysis tools; Genbo and wrote the paper; Zhihuang Xu and Ye contributed materials and analysis tools; Genbo Su and Su Xinxin Xinxin Zhuang gave guidance and advice. Zhuang gave guidance and advice. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.

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