Discussion of Planarity of Molecular Structures Using Novel

May 11, 2012 - Department of Inorganic Chemistry, Taras Shevchenko National University of Kyiv, Volodymyrska Street 64, Kyiv 01601, Ukraine. ‡. Cent...
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Discussion of Planarity of Molecular Structures Using Novel Pentanuclear Cu/Ni Complexes as an Example Elena A. Buvaylo,† Oksana V. Nesterova,† Vladimir N. Kokozay,† Olga Yu. Vassilyeva,† Brian W. Skelton,‡ Roman Boča,§,∥ and Dmytro S. Nesterov*,† †

Department of Inorganic Chemistry, Taras Shevchenko National University of Kyiv, Volodymyrska Street 64, Kyiv 01601, Ukraine Centre for Microscopy, Characterisation and Analysis, University of Western Australia, Crawley, Western Australia 6009, Australia § Department of Chemistry, FPV, University of SS Cyril and Methodius, Trnava, Slovakia ∥ Institute of Inorganic Chemistry, FCHPT, Slovak University of Technology, 812 37 Bratislava, Slovakia ‡

S Supporting Information *

ABSTRACT: Two novel heterometallic complexes, [Cu2Ni3(NCS)4(Me2Ea)6] (1) and [Cu2Ni3Cl4(Me2Ea)6] (2) (HMe2Ea = 2-dimethylaminoethanol), have been prepared using a one-pot reaction of copper powder with nickel thiocyanate (1) or chloride (2) in an acetonitrile solution of HMe2Ea in air. The magnetic investigations disclose dominant ferromagnetic exchange interactions with S = 4 and 2 ground states for 1 and 2, respectively. A planar arrangement of five metal centers in both compounds inspired us to study the question of planarity of polynuclear coordination compounds having MaXb cores (where M = metal and X = bridging atom). The distribution, obtained by the searches via the Cambridge Structural Database, of the structures possessing a planar disposition of the metal centers, depending on their nuclearity, is discussed, and typical planar molecular structure types (MST) are identified. It is demonstrated that the relative amounts of planar structures obey exponential decay in the 3−8 nuclearity region, with the exception of heptanuclear structures. The factors that influence the disposition (planar or nonplanar) of metal centers in the MaXb molecular structure types are selected. Finally, we offer a model for general description of MSTs by the graph theory toward the design of high-nuclear coordination compounds.

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field of coordination polymers, the prediction of the crystal structures of molecular compounds remains a challenge. As well as MOFs, 1b,5 the polynuclear coordination compounds of molecular nature have a wide range of interesting properties and potential applications, such as single molecule magnetism, molecular catalysis, and many others.6 Obviously, the design of coordination polymers (MOFs) is simplified by the presence of symmetry rules and restrictions (properties of a crystal lattice), while the synthesis of molecular assemblies usually involves chemical considerations only. One could imagine that if there would exist some general rules, describing formation of polynuclear MaXb (where M = metal and X = bridging atom) aggregates, their design could undergo a revolutionary improvement. It has been shown (on examples of metal alkoxides) that the size and number of metal atoms and ligands (donor atoms) could have more influence on the final structure of the resulting polynuclear molecule MaXb than their chemical nature.7 Moreover, most of these structures can be described by a limited number of molecular structure types (MST),7a where the

INTRODUCTION The design of supramolecular architectures and polynuclear molecular assemblies is a rapidly growing field in modern coordination chemistry.1 Currently, the solid state structures of coordination compounds are commonly established by diffraction methods (mostly by single crystal X-ray analysis), that give precise information about atomic positions in the crystal structure of the complex. The expansion of these methods is illustrated by more than 250 000 structure records in the Cambridge Structural Database (CSD),2 and the size of the annual contributions to the CSD continually increases.2,3 Such a huge data array, along with the development of more powerful computers, encouraged scientists to search for correlations between the initial molecular configuration of the compound and its final solid-state structure (molecular or polymeric one), in an attempt to understand if crystal structures are predictable or not. These efforts resulted in the development of numerous concepts,4 particularly used in the design of metal−organic frameworks (MOFs), where a careful control over the geometries of molecular metal-complex particles (building blocks), combined with the symmetry rules, allows targeted synthesis of MOFs with desired topologies.1a−d,3 In contrast to striking achievements that have been done in the © 2012 American Chemical Society

Received: March 15, 2012 Revised: April 16, 2012 Published: May 11, 2012 3200

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MST is seen as a set of topologically identical M a X b combinations.8 These observations underlie the molecular structure design concept, developed by Kessler in 2003.7a Although this concept was not used in the present investigation, the idea of MSTs was found to be appealing for comparison of the structures of related compounds. Motivated by these findings, we started to investigate the properties of molecular structure types of coordination compounds we are dealing with and to compare our results with the statistical data obtained from the CSD. Despite the impressive progress in the preparation of polynuclear complexes of the d-block elements, the number of high nuclearity (five and higher metal centers) CuII/Ni compounds, and those studied magnetically in particular, is limited to a few examples.2 The synthetic routes that can provide heterometallic complexes in a controlled fashion seem to offer potential advantages over the self-assembly route in that they enable more stringent control over the course of the reaction and upon the products that form. However, in the field of high nuclearity mixed-metal complexes, the designed assembly approach has not proved to be very effective both because of synthetic difficulties in obtaining elaborate ligands and molecular synthons that can be used as ligands and because of limits of human imagination in designing molecules such as the {Ni24} cage.9 Herein, we report the direct synthesis,10 crystal structures, and magnetic properties of two novel heterometallic complexes [Cu2Ni3(NCS)4(Me2Ea)6] (1) and [Cu2Ni3Cl4(Me2Ea)6] (2) (HMe2Ea - 2-dimethylaminoethanol). The pentanuclear M5(μ3X)4(μ-X)4 cores in 1 and 2 that were found to possess a planar arrangement of the metal centers inspired us to discuss the question of planarity of polynuclear structures.

Figure 1. Molecular structure of 1, showing the atom numbering, with 20% probability displacement ellipsoids. Hydrogen atoms and one set of the disordered atoms have been omitted for clarity.

joined by μ2- and μ3-oxygen atoms from Me2Ea ligands, nitrogen atoms from μ1,1-NCS groups (1), or chloride bridges (2). In the Cu−Ni−Cu−Ni parallelogram centered on the fifth Ni(II), the bridged M···M separations range from 2.85 to 2.96 and from 2.86 to 3.06 Å for 1 and 2, respectively. One of the three nickel atoms, Ni(1), is located on the inversion center and has a distorted octahedral polyhedron (Figure 2) formed by O atoms of aminoalcohol ligands (Figure 1 and Supporting Information Figure S1). The two other Ni(II) atoms, Ni(2) and Ni(2)′, are related by the inversion center and are also octahedrally coordinated but have O3N3 (1) or O3N2Cl (2) donor sets formed by donor atoms of thiocyanate (1) or chloride (2) in addition to O and N atoms of Me2Ea ligands. All Cu(II) atoms in 1 and 2 exhibit elongated square pyramidal coordinations, being bonded to two OMe2Ea, one NMe2Ea, and one NNCS (1) or one Cl (2) atom in the basal plane and one NNCS (1) or Cl (2) atom in the apical position. NCS(20) (1) and Cl(1) (2) are the terminal ligands coordinated only to the Cu atom, whereas NCS(30) (1) and Cl(2) (2) asymmetrically link the Ni(II) and Cu(II) metal centers. Compound 1 represents a quite rare example of a coordination compound with the μ1,1-NCS bridging between two different metals. To the best of our knowledge, only two examples of such bridging based on K/Mn and Cu/Pb metal pairs exist.12 Magnetic Properties. The magnetic susceptibility of 1 is increasing on cooling over the whole temperature range (300− 2 K) (Figure 3). The effective magnetic moment at room temperature adopts a value of μeff = 6.5 μB, which is close to the value expected for two Cu(II) and three Ni(II) uncoupled spins: μeff/μB = g[2s1(s1 + 1) + 3s2(s2 + 1)]1/2 = 2.74g = 6.0 with g = 2.2. On temperature lowering, the effective magnetic moment progressively increases until μeff = 10.2 μB at T = 2 K, which is a fingerprint of the dominating ferromagnetic exchange coupling. The susceptibility data (corrected for the underlying diamagnetism) have been fitted to a model that involves the isotropic exchange according to Figure 4. The corresponding spin Hamiltonian

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RESULTS AND DISCUSSION Synthesis and IR Characterization. The pentanuclear complexes 1 and 2 were prepared in a one-pot reaction from copper powder and NiX2 [X = NCS (1), Cl (2)] in an acetonitrile solution of 2-dimethylaminoethanol, using the molar ratio Cu/NiX2 = 1:2. The reaction was initiated and brought to completion by heating and stirring. After successive additions of PriOH (1) or standing at room temperature during one week (2), microcrystals that showed analytical data accounting for the presence of Cu(II) and Ni(II) in a 2:3 stoichiometry were formed. The following reaction scheme can be envisaged: 2Cu 0 + 3NiX 2 + 8HMe 2Ea + O2 → [Cu 2Ni3X4(Me2Ea)6 ] + 2HMe2Ea·HX + 2H 2O

The IR spectra of these complexes show all the characteristic ligand peaks. For 1, in the regions characteristic for thiocyanate ligand,11 the strong bands (2100 and 2070 cm−1) are due to the ν(CN) vibrations, and the weak bands at a lower frequency (830 and 810 cm−1) are attributable to the ν(CS) absorption peaks. These data provide evidence of the presence of different types of NCS ligand, with this being subsequently confirmed by the X-ray analysis. Description of the Structures. The structural configurations of 1 and 2 are similar and reveal a planar arrangement of five metals in the molecules (Figure 1 and Supporting Information Figure S1; Table 1). The molecular structures of complexes are based on a pentanuclear Cu2Ni3 core with general formula {M5(μ3-X)4(μ-X)4} where the metal atoms are 3201

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Table 1. Selected Geometrical Parameters (distances/Å and angles/deg) for 1 and 2

Cu(1)−O(1) Cu(1)−O(2) Cu(1)−N(1) Cu(1)−X Cu(1)−Y Ni(1)−O(1) Ni(1)−O(2) Ni(1)−O(3) Ni(2)−O(1) Ni(2)−O(2) Ni(2)−O(3) Ni(2)−N(2) Ni(2)−N(2′) Ni(2)−N(3) Ni(2)−Y O(1)−Cu(1)−O(2) O(1)−Cu(1)−N(1) O(1)−Cu(1)−X O(1)−Cu(1)−Y O(2)−Cu(1)−Y N(1)−Cu(1)−O(2) X−Cu(1)−O(2) N(1)−Cu(1)−X N(1)−Cu(1)−Y X−Cu(1)−Y O(1)−Ni(1)−O(2) O(1)−Ni(1)−O(3) O(2)−Ni(1)−O(3) O(2)−Ni(2)−O(3) O(2)−Ni(2)−N(2) O(2)−Ni(2)−N(2′) O(2)−Ni(2)−N(3) O(2)−Ni(2)−Y O(2)−Ni(2)−O(1) O(3)−Ni(2)−N(2) O(3)−Ni(2)−N(2′) O(3)−Ni(2)−N(3) O(3)−Ni(2)−Y O(3)−Ni(2)−O(1) N(2)−Ni(2)−N(3) N(2)−Ni(2)−Y N(2)−Ni(2)−O(1) N(3)−Ni(2)−Y N(3)−Ni(2)−O(1) Y−Ni(2)−O(1)

1 [X = N(20), Y = N(30)]

2 [X = Cl(1), Y = Cl(2)]

1.926(4) 1.928(4) 1.986(5) 1.92(3) 2.669(8) 2.091(4) 2.063(4) 1.992(4) 2.385(4) 2.065(5) 1.968(4) 1.95(2) 2.286(19) 2.068(8) 1.996(6) 77.3(2) 87.9(2) 173.9(10) 79.7(2) 78.5(2) 164.9(2) 97.1(11) 93.8(11) 101.9(2) 97.1(11) 70.82(16) 86.44(16) 81.83(17) 82.3(2) 92.3(6) 75.5(6) 163.6(3) 93.7(2) 65.02(15) 102.7(7) 99.1(6) 85.4(2) 165.1(2) 79.3(2) 101.0(7) 91.8(7) 156.9(6) 95.4(3) 102.0(3) 86.0(2)

1.995(4) 1.948(4) 2.021(5) 2.2472(17) 2.7116(14) 2.092(3) 2.114(4) 2.020(4) 2.304(4) 2.086(3) 1.990(3) 2.157(5)

Figure 2. Polyhedral representation of the centrosymmetric core in 1 and 2. Color codes: Cu, cyan; Ni, green; O, red; N, blue.

1: Cu−O−Ni(t) (84, 93°), Cu−O−Ni(c) (95, 96°), and Ni− O−Ni (92, 87, 79°), which imply that J1 > 0, J2 ≥ 0, and J3 < 0. The fitting procedure for 1 gave J1 = +15.5 cm−1, J2 = +21.5 cm−1, J3 = −4.7 cm−1, and g = 2.291 (R = 0.011). The quality of the fit is shown in Figure 3. The dominating ferromagnetic exchange is consistent with small angles over the superexchange pathways. The ground state is S = 4. A very good fit with no zero-field splitting parameter D suggests that the D-value must be rather small, affecting the lowest temperature data only. In fact, D varies from −8 to +10 cm−1 in mononuclear Ni(II) complexes, and for the octahedral geometry it is exactly zero; in dinuclear and polynuclear systems, it loses its significance. The revised fitting procedure gave J1 = +15.2 cm−1, J2 = +26.3 cm−1, J3 = −5.0 cm−1, g = 2.287, and D = 1.9 cm−1 (R = 0.011). The result remains nearly the same when D of the central Ni(II) center with the {NiO6} chromophore is involved/ignored. Therefore, it may be concluded that the effect of the D-parameter is insignificant in the present case. The magnetic functions for compound 2 again indicate an overall exchange coupling that is of the ferromagnetic nature (Figure 5). The room-temperature value of the effective magnetic moment is almost identical with that of compound 1. On cooling, it gradually increases until 16 K and then it drops down. This turning can be caused by some zero-field splitting and/or intermolecular interactions. The fitting procedure for 2 applied to data above 16 K gave J1 = +6.1 cm−1, J2 = +15.4 cm−1, J3 = −7.6 cm−1, and g = 2.198 (R = 0.027). Again the deduced ferromagnetic exchange coupling is consistent with small angles over the superexchange pathways. However, owing to competition of the ferromagnetic and antiferromagnetic exchange, the ground state is S = 2, with close lying S = 3 and S = 4. Planarity of Known Crystal Structures. Previously we reported a heterotrimetallic complex [Cu2CoNi2Cl4(Me2Ea)6] with a 2-dimethylaminoethanol ligand.8 In this molecule the central cobalt atom is located on an inversion center. The presence of the molecular inversion center requires all five metal atoms to lie in the same plane. Complexes 1 and 2 in the present study show the same planar arrangement of the metal centers, and 2 is found to be isostructural with the published compound. In the present case, a perfect planar arrangement of the metal centers in 1 and 2 has attracted our attention. The search for polynuclear compounds (at least trinuclear) with a M−X−M linkage between metal centers resulted in ca. 10 000 hits in the CSD, that were then analyzed using the CSD and local software. The carbonyl (M−C−O) and organo-

2.132(5) 2.3993(15) 75.34(15) 86.04(18) 168.73(11) 85.11(11) 88.00(10) 160.75(19) 100.22(12) 97.17(15) 95.37(12) 105.26(6) 69.92(14) 86.51(14) 80.23(14) 81.62(13) 83.31(16) 161.68(17) 93.83(10) 66.37(14) 97.43(17) 84.94(15) 168.30(13) 81.69(15) 110.85(19) 92.72(12) 149.53(15) 96.92(12) 99.45(17) 86.60(10)

⃗ S Ni1 ⃗ + SCu2 ⃗ S Ni3 ⃗ )−J Ĥ = −JCu − Ni(t)ℏ−2(SCu1 Ni − Ni ⃗ S Ni2 ⃗ + S Ni2 ⃗ S Ni3 ⃗ )−J ℏ−2(S Ni1 ℏ−2 Cu − Ni(c) ̂ z + SCu2, ̂ z ⃗ S Ni2 ⃗ + SCu1 ⃗ S Ni2 ⃗ ) + μ Bz ℏ−1g (SCu1, (SCu2 B ̂ z + S Ni2, ̂ z + S Ni3, ̂ z) + S Ni1,

(1)

involves three coupling constants, J1 = JCu−Ni(t), J2 = JNi−Ni, and J3 = JCu−Ni(c), where the symbol (t) stands for the terminal and (c) for the central Ni atoms. On the basis of the bond angles M−O−M, three superexchange pathways can be considered for 3202

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Figure 3. Magnetic functions for 1: the temperature dependences of the effective magnetic moment (left) and magnetic susceptibility (inset); reconstructed zero-field energy levels (right). Open symbols, experimental; solid line, calculated.

the number of coordination compounds possessing an even number of metals is significantly greater than the number of those containing an odd number.13 Considering the planar metal arrangements in 1 and 2, as well as that in other complexes, previously obtained by the direct synthesis approach,8,14 it was interesting to investigate the relative number of planar MaXb configurations within known molecular structures of polynuclear coordination compounds (restricted to the array of about 10 000 hits). The term “planar” concerns metal atoms only, and planar MSTs were defined as those where a plane, passing through any four metal atoms, resulted in the maximum M-plane distance being less than 0.3 Å. It is important to notice that we find it reasonable to extend the concept of “molecular structure types”7a formulated for simpler compounds (metal alkoxides, iodocuprates, etc.) to any

Figure 4. Model of the exchange coupling in 1 and 2.

metallic (M−C−C) compounds, as well as coordination polymers and polyoxometalates, were excluded from that array. The distribution of the structures depending on their nuclearity in the range from 3 to 8 is shown in Figure 6, and obviously, it is in accordance with the previous observation that

Figure 5. Magnetic functions for 2: the temperature dependences of the effective magnetic moment (left) and magnetic susceptibility (inset); reconstructed zero-field energy levels (right). Open symbols, experimental; solid line, calculated. 3203

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coordination compound possessing a planar disposition of the metal centers. Let us discuss some details of the distribution, depicted in Figure 6. Obviously, the trinuclear compounds are always planar, independently on the MST. The tetranuclear complexes are the most common within compounds selected in the CSD for the present study. The nonplanar cubane-like structure {M4(μ3-X)4} dominates over all other MSTs, showing 1266 hits in all 3842 structures (27%), while other nonplanar configurations are less common. Among the nonplanar MSTs, one can also notice the adamantane-like {M4(μ-X)6} array, which is a part of the diamondoid network (hexagonal packing). The main planar M4Xb combinations (Figure 8) describe 95% of the planar tetranuclear complexes found in the CSD using the restrictions stated above.

Figure 6. Left: the dependence of the number of complexes on their nuclearity according to the CSD search with restrictions given in the text. The part of the structures with a planar disposition of metal centers is shown in blue. Right: the dependence of the percentage of planar structures on the nuclearity (black circles are experimental data; the solid line is the exponential fit); the red circle represents the percentage of planar heptanuclear structures. Blue and green curves demonstrate the percentages of planar graphs19 among all possible graphs and 4-graphs, respectively.

complex whose coordination core can be described by the MaXb formula. By removing all nonbringing nonmetal atoms, one can get the MST of the respective structure. The percentage of planar compounds gradually decreases with the nuclearity increase (Figure 6). The only exception in the current dependence is the heptanuclear compounds (Figure 6), with 24% of planar arrays observed compared to ca. 13% expected (calculated as a simple average of the percentages of planar hexa- and octanuclear structures). This effect can be explained by the greater stability of the heptanuclear MST [M7(μ3-X)6(μ-X)6], named the “Anderson structure”15 (the name originated from polyoxometalates chemistry), which is common for heterometallic coordination compounds with alcohol or aminoalcohol ligands (Figure 7).

Figure 8. Top: geometries of planar M4Xb coordination cores, and the most widespread tetranuclear MSTs, cubane- and adamantane-like ones. Below: the numbers of the respective structures in the CSD.

Figure 7. Geometry of the Anderson-type M7(μ3-X)6(μ-X)6 MST (left) and its embodiment in the largest planar compound [Mn19O12(L)14(HL)10]·HL (HL = HOC2H4OCH3)16 (right, only Mn and O atoms are shown).

Examples of pentanuclear planar MSTs are shown in Figure 9. The particular topology of five coplanar metal centers is limited to no more than 100 examples of molecular coordination compounds according to the CSD, and MST a appears to be the most widespread type. MST g, the type compounds 1 and 2 belong to, was previously observed in [Cu2CoNi2Cl4(Me2Ea)6] only.8 Rather surprisingly, MST f, which is built up of two incomplete cubes fused via the common vertex, shows only a few structures, while its “monomer”the incomplete cube M3(μ3-X)(μ-X)3is a quite common trinuclear array. Also, the planar pentanuclear complexes, in contrast to tetranuclear ones, show a number of

This MST, possessing 3-fold rotation symmetry, can be viewed as a finite 2D segment of hexagonal close packing. We found 43 perfectly planar polynuclear cores of this type. We presume that a polynuclear array of the Anderson type can be “propagated” by an additional line of MX fragments in the same plane to form a larger odd-nuclear MST. An example of such an MST indeed was found in the CSD in the disklike complex [Mn19O12(L)14(HL)10]·HL (HL = HOC2H4OCH3) (Figure 7).16 As far as we aware, it represents the largest known 3204

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The relative amounts of planar structures of metal complexes, obtained within the present research (Figure 6), can be fitted to the general exponential equation y = Ae−x/t + B, where x is the nuclearity and y is a percentage of planar arrangements. The fitting procedure (heptanuclear structures were excluded because of the Anderson type phenomenon) resulted in A = 2378(144), B = 10.7(0.5), and t = 0.91(0.02). The number B represents a limit for the above function when x → ∞. Although this coefficient (as well as A and t) has no direct physical meaning, the nonzero positive value of B means that there could exist a finite number (percentage) of planar arrays even for compounds of very high nuclearity. It finds confirmation in the generation of high-nuclear planar structures, such as the Mn19 core.16 A typical exponential decay of relative amounts of planar structures, together with the distinctive nonmonotonic decrease of the amounts of structures depending on their nuclearity (Figure 6, left),13 point to a conclusion that these dependences are far from the random character (as one could imagine) and obey some definite laws, whose establishment is a purpose of our studies. Possible Models for Description of MSTs. As we mentioned above, the predictable self-assembly synthesis of complex polynuclear species would require, in particular, a deep understanding of how these species are organized in terms of MaXb assemblies (MSTs), including the classification of all possible MSTs together with the evaluation of their properties as well as relative stabilities. Although general theoretical evaluation is far beyond the present study, we can presume a roadmap for the further investigations in the field. Obviously, the evaluation procedure should proceed via two major steps: (a) generation of the chemically reasonable MST and (b) evaluation of its properties (Figure 11). One can notice that this scheme is related to that for crystal structure prediction methods.4d The main aim is to find, using an existing data set, a relation between the structure of MST and its relative stability. Since the stability can be considered as a property of MST, the quantitative structure−property relationship (QSPR) concept17 could be applied. The main idea of this concept, in a few words, consists in the investigation of the P = f(β) function, where P and β are the parameters of a property and structure, respectively. This function involves a number of parameters, calculated and empirical ones, resulting in a quantitative prediction of the desired property for any novel compound formed. All these problems are rather complicated, and in the present discussion we would like to pay attention to step (a) only. From the mathematical point of view, the combination of metal (M) atoms, bridging (X) atoms, and linking edges represents a graph. The graph theory is already known to have a wide range of chemical applications,17b particularly in the field of organic chemistry. However, as far as we are aware, it has never been applied to general classification of polynuclear coordination compounds taken as discrete MaXb assemblies. Herein, we attempt to utilize this theory as a basis for description of a set of molecular structure types, exhibiting planarity as their common property. The model where both M and X centers are considered as vertexes appears the most obvious one, but for this case the set of MaXb types with constant a (nuclearity) will be described by a set of graphs with a different number of vertexes, thus complicating the model. For example, for compounds 1 and 2, this model will result in the 13-vertex graph, which is nonplanar (while the MST of 1 and 2, g (Figure 9), is planar). A planar

Figure 9. Top: geometries of planar M5Xb coordination cores. Below: numbers of respective structures in the CSD.

specific types which can be difficult to classify due to the presence either of a large distortion or of a number of supporting organic bridges, and thus only 85% of planar pentanuclear structures are covered by pentanuclear a−g types (Figure 9). The complexes of higher nuclearities (higher than 5) are difficult to classify using speculative considerations. The most typical hexanuclear MSTs, namely, wheel, chain, and grid (Figure 10), cover 67% of the hexanuclear planar arrays analyzed.

Figure 10. Left: geometries of typical planar M6Xb coordination cores. Right: the numbers of the respective structures in the CSD. 3205

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Figure 11. General scheme for evaluation of molecular structure types.

Figure 12. Left: Examples of a description of MSTs by graphs (metal centers are considered as graph vertexes). Right: the “butterfly graph” of MSTs M5(μ3-X)4(μ-X)4 showed by 1, 2, and8 and M5(μ3-X)2(μ-X)6.

1,2,3-benzotriazolate) of Td symmetry demonstrate.18 Thus, the disposition of metals, planar or nonplanar, in the structure is defined by at least three factors: (1) the properties of the graph; (2) the chemical restrictions of the MST; and (3) distortion of a real structure (sterical hindrance of ligands, etc).

graph is defined as a graph that can be drawn in a plane without any of its edges crossing each other (see below). Another model considers a metal center as the vertex and a M−X−M bridge as the edge. From this point of view, MST MaXb represents a graph G = (V, E) where V = a and the number of edges E should be further estimated (Figure 12). The main problem is how one should evaluate the contribution of μ3-X (and higher bridgings) into the overall edges number. The simplest way is to assume that metal centers, bridged by a μn-X atom, form a complete graph (Figure 13), which in such a way becomes a subgraph of the graph G. For instance, using this model the MST of 1 and 2 can be presented with a “butterfly graph”, a planar undirected graph with five vertexes and six edges (Figure 12). The criteria for planarity of the MST should be based on the analogous criteria for a respective graph. If the graph G = (V, E) is planar, then the configuration of the respective MST(s) MaXb is undefined. For example, although MSTs M4(μ4-X)(μ3-X)8, M4(μ3-X)4, and M4(μ-X)6 (Figure 7, i, j, and k, respectively) belong to one and the same complete planar graph (4, 6), the cubane- and adamantane-like MSTs cannot be represented in a planar manner from the chemical point of view, while the first MST M4(μ4-X)(μ3-X)8 can have both planar and nonplanar realizations. On the other hand, if the MST belongs to a nonplanar graph, the structure of the molecule possesses a nonplanar configuration only, which pentanuclear compounds [MZn4Cl4(L)6] (MII = Zn, Fe, Co, Ni, or Cu; L = 5,6-dimethyl-

Figure 13. Complete graphs (metal centers are considered as graph vertexes) formed by some Ma(μn-X) fragments according to the proposed model (see the text). 3206

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The realization of a planar structure requires all these factors to allow the planar configuration; that is, the probability of a planar structure formation can be described as P = PGPMSTPdist, where the PG, PMST, and Pdist parameters belong to the above factors, respectively. The construction of all possible graphs, together with the evaluation of their planarity, is not trivial.19 In the present study we used the data obtained by Rücker and Meringer,19 particularly because their investigation was chemically oriented. The percentages for the V = 1−10 range, obtained by them, are depicted in Figure 6 in two forms: for all possible graphs as well as for 4-graphs. The last ones are defined as the only graphs containing no vertex of a degree higher than four (such a set of graphs is more reasonable for evaluation of organic structures, since a carbon atom can form no more than four bonds). From the point of view of MST, the most interesting would be to limit the distribution to 6-graphs, but we were unable to find such data in the literature. One can assume that the distribution curve for 6-graphs would be located approximately between the curves for all graphs and 4-graphs (Figure 6). Finally, it is easy to see that the calculated part of planar graphs (P G component)19 is most likely a theoretical limit for a number of planar structures, while a correct fit of the observed amounts of planar structures (Figure 6) should involve other contributions (PMST and Pdist), whose quantitative estimation represents a fundamental task for future investigations.

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Article

EXPERIMENTAL SECTION

General. All chemicals were of reagent grade and used as received. All experiments were carried out in air. Elemental analyses were performed by standard titrimetric methods for anions and with a Carlo Erba Strumentazion Analyzer (for C, H, and N). Quantitative determinations of the metals were performed by atomic absorption spectroscopy. Synthesis of [Cu2Ni3(NCS)4(Me2Ea)6] (1). Copper powder (0.16 g, 2.5 mmol), Ni(NCS)2·1.5H2O (1.01 g, 5 mmol), CH3CN (20 cm3), and 2-dimethylaminoethanol (2 cm3) were heated to 50−60 °C and stirred magnetically until total dissolution of copper powder was observed (150 min). Green-blue crystals suitable for X-ray crystallography were deposited from the reaction mixture in the presence of PriOH after 6 days. The crystals were filtered off and finally dried in vacuo at room temperature. Mass collected: 0.82 g, yield 61% (on copper). Anal. Calcd for C28H60Cu2N10Ni3O6S4 (Mr = 1064.28): C, 31.60; H, 5.68; N, 13.16; Cu, 11.94; Ni, 16.55. Found: C, 31.2; H, 5.6; N, 13.5; Cu, 12.1; Ni, 16.5%. IR (KBr, cm−1): 3600− 3400(br), 3000(w), 2980(w), 2960(w), 2910(m), 2850(sh), 2820(w), 2100(sh CN), 2070(s CN), 1465(m), 1280(w), 1220(m), 1170(w), 1080(sh), 1070(m), 1020(m), 955(m), 910(w), 890(w), 830(w CS), 810(w CS), 645(m), 610(m), 530(w), 490(w), 470(w). The compound is soluble in DMF under heating and insoluble in water. It is stable in air for periods of months. Synthesis of [Cu2Ni3Cl4(Me2Ea)6] (2). This complex was obtained in a way similar to that of 1, but Ni(NCS)2·1.5H2O was replaced by NiCl2·6H2O. The resulting green solution was allowed to stand at room temperature. Yellow-green crystals suitable for X-ray crystallography were deposited after five days. They were filtered off and dried in vacuo at room temperature. Mass collected: 0.68 g, yield 56% (on copper). Anal. Calcd for C24H60Cl4Cu2N6Ni3O6 (Mr = 973.79): C, 29.60; H, 6.21; N, 8.63; Cu, 13.05; Ni, 18.09; Cl, 14.56. Found: C, 29.4; H, 6.2; N, 8.7; Cu, 12.9; Ni, 18.2; Cl, 14.8%. IR (KBr, cm−1): 3550−3400(br), 3000(w), 2980(w), 2960(w), 2910−2850(br), 2830(w), 1640(s), 1465(m), 1400(w), 1375(w), 1340(w), 1280(m), 1220(m), 1160(m), 1090(s), 1050(sh), 1020(m), 950(m), 900(w), 790(m), 720(br), 635(m), 530(w), 490(w). The compound is soluble in DMF under heating and insoluble in water. It is stable in air for periods of months. Physical Measurements. Infrared spectra were recorded as KBr discs on a UR-10 spectrophotometer in the 4000−400 cm−1 region. Magnetic susceptibility data of powdered samples were collected on a MPMS Quantum Design SQUID magnetometer (XL-5) in the temperature range 300−1.8 K and at a field of 1000 G. The output data were corrected for the experimentally determined diamagnetism of the sample holder and the diamagnetism of the sample calculated from Pascal’s constants. X-ray Structure Determinations. The crystal data were collected on a Bruker Smart diffractometer (1) and Rigaku R-Axis II Imaging Plate diffractometer (2) fitted with graphite-monochromated Mo Kα (λ = 0.71073 Å). Following multiscan absorption corrections, and solution by direct methods, the structures were refined against F2 with full-matrix least-squares using the program SHELXL-97.23 Anisotropic displacement parameters were employed for the non-hydrogen atoms. All hydrogen atoms were added at calculated positions and refined by use of a riding model with isotropic displacement parameters based on the isotropic displacement parameter of the parent atom. Crystallographic parameters are listed in Table 2 with selected geometric parameters in Table 1. Metal atom types were assigned on the basis of structure refinement, coordination geometries, and elemental analysis. For 1, the atoms of one thiocyanate and one 2-dimethylaminoethanol ligand were modeled as being disordered, each over two sets of sites with occupancies constrained to 0.5 after trial refinement. CCDC 836747 and 868771 contain the supplementary crystallographic data for this paper.

CONCLUSIONS

We successfully synthesized two heterometallic Cu/Ni pentanuclear complexes possessing ferromagnetic properties and a symmetry-imposed planar disposition of the metal centers. Our main purpose for this time is to attract attention to the problem of symmetry and the structure−properties relationship of molecular structure types (MaXb assemblies), since the development of such ideas could influence the targeted synthesis of polynuclear coordination compounds. The question of planarity, discussed in the present work, is of special interest, since planarity is a general property of MaXb assemblies and the planar or nonplanar disposition of metals could have a crucial impact on the properties of a coordination compound (e.g., magnetic ones). We have demonstrated that the relative amounts of planar structures gradually decrease with the nuclearity increase, and we compared these numbers with the theoretically calculated19 percentages of planar graphs. Analyzing these dependences, we have offered a model for a general description of MSTs by the graph theory and selected three factors that could influence the planarity of the MSTs. Work by Winpenny,20 Christou,21 and Saalfrank,22 as well as many other researchers, revealed that high-nuclear complexes can be obtained by employing a rather simple synthetic route spontaneous (or “serendipitous”) self-assembly. However, the present level of understanding of such a strategy does not allow for highly predictable preparation of species with desired nuclearity and topology. We believe that sometimes the theoretical investigation of molecular structure types and their properties will result in new approaches, where the synthetic pathway toward the desired high-nuclear molecule will be theoretically optimized before its realization, as already happened to many other fields in chemistry. 3207

dx.doi.org/10.1021/cg300353b | Cryst. Growth Des. 2012, 12, 3200−3208

Crystal Growth & Design

Article

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Table 2. Crystal Data and Structure Refinement for 1 and 2 empirical formula formula weight crystal system space group a/Å b/Å c/Å β/deg V/Å3 Z T/K μ/mm−1 Rint meas/ind reflns R1 wR2

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1

2

C28H60Cu2N10Ni3O6S4 1064.28 monoclinic P21/n 12.033(4) 14.992(4) 12.409(4) 99.436(9) 2208.3(12) 2 150(2) 2.441 0.118 19246/6013 0.058 0.1574

C24H60Cl4Cu2N6Ni3O6 973.79 orthorhombic Pbca 10.8948(4) 16.0279(6) 21.7661(5) 90.00 3800.8(2) 4 180(2) 2.884 0.0419 6504/3370 0.0285 0.0954

ASSOCIATED CONTENT

S Supporting Information *

Plot of the molecular structure of 2 and X-ray crystallographic information files (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +380 44 286 2467. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. G. A. Van Albada (LIC) and Dr. Patrick Franz (University of Bern) for collecting the magnetic susceptibility data and Professor Paul Raithby (University of Bath) for X-ray data collection for 2. The Slovak Grant Agency (VEGA 1/ 0052/11 and 1/0233/12, APVV-0014-11) and the State Fund for Fundamental Researches of Ukraine (Project 28.3/017) are acknowledged for financial support.

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REFERENCES

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dx.doi.org/10.1021/cg300353b | Cryst. Growth Des. 2012, 12, 3200−3208