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One-Dimensional [Ni(O [ChemPlusChem]
[October 28, 2014]

One-Dimensional [Ni(O [ChemPlusChem]


(ChemPlusChem Via Acquire Media NewsEdge) Five nickel complexes, [Ni2 (3,4,5-tmb)4 (4,4'-bpy)] (1), [Ni2 (2- ntc)4 (4,4'-bpy)] (2), [Ni(9-atc)2 (4,4'-bpy)] (3), [Ni(3,4,5-tmb)2 (t- bpee)] (4), and [Ni2 (2-ntc)4 (t-bpee)2](5) (3,4,5-tmb= 3,4,5-trimethoxybenzoate; 2-ntc = 2-naphthalenecarboxylate; 9-atc =9-anthracenecarboxylate; 4,4'-bpy =4,4'-bipyridine; t-bpee = trans-1,2-bis(4-pyridyl)ethylene), are prepared. They are characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, and single-crystal and powder X-ray diffraction. 1 and 2 form linear chains of paddlewheel units joined by 4,4'-bpy ligands; 3 and 4 display zigzag chains formed by cis-bis(chelate) Ni(O2 CR)2 units joined by 4,4'-bpy and t-bpee, respectively; and the structure of 5 consists of double chains. The magnetic behavior of the compounds is also studied, and DFT calculations using the broken-symmetry approximation are performed to gain a better understanding of the strong antiferromagnetism found in 1.



Keywords: 1 D polymers * broken symmetry * density functional calculations * magnetic properties * nickel complexes (ProQuest: ... denotes formulae omitted.) Introduction The synthesis and design of 1 D coordination compounds have been of interest for chemists, physicists, and materials scien- tists in general, because of their possible applications in sever- al fields such as electronics, optics, or catalysis.[1] In particular, 1 D magnetic polymers are attractive because of their potential use as molecular-based ferromagnets.[2,3] Moreover, 1 D com- pounds are the simplest extended systems that can be mod- elled mathematically to understand the origin of their magnet- ic behavior.[4] One-dimensional coordination polymers can adopt several structural motifs, that is, linear, zigzag, helical, or ladder chains. Correlations between the structure, including the nature of the chemical bonds, and the magnetic properties are necessary for the design of new materials. However, in many cases, the number of variables that play a role in the synthesis and struc- ture of 1 D compounds and the poor understanding of several of them hinder the prediction of the final result.[1] One of the most important variables is the nature of the metal ion. Of the transition metals, the Ni2 + ion is especially in- teresting because of its variety of coordination environments, giving rise to a rich range of molecular structures. Thus, a large diversity of 1 D compounds containing nickel has been de- scribed.[5] Some contain halide or pseudohalide ligands,[5] which can facilitate magnetic interactions and give rise to com- plexes with ferro- and/or antiferromagnetic properties.

The choice of ligand is also crucial. Organic ligands contain- ing N or O donor atoms offer great potential for chemical and structural diversity. In particular, carboxylate ligands, which can have different bridging modes of coordination (syn-syn, anti- anti,orsyn-anti), give rise to different magnetic interactions. Thus, exchange interactions through syn-syn and anti-anti co- ordination modes are usually weak antiferromagnetic,[6] where- as the syn-anti mode can lead to either antiferro- or ferromag- netic interactions. In addition, antiferromagnetic interactions can also be very strong in paddlewheel structures, in which the metal centers are bridged by four carboxylate groups,[7, 8] and important ferromagnetic interactions are even possible across these ligands.[9] Carboxylate ligands with extended aromatic rings[10] favor the existence of intra- and/or intermolecular p···p stacking and C^H···p interactions, which play an important role in the for- mation of 1 D structures. Auxiliary ligands such as halides, pseudohalides, or bipyridines, employed to connect the car- boxylate-metal units, also have a very important influence on the arrangement of the compounds. Variations in the carboxyl- ate[11] or diamine[12] ligands have been studied previously to gain an understanding of the influence of the ligands on the structural variations in series of diamagnetic Zn2 +,Cd2+ ,or Ag + compounds.


Experimental variations were restricted to reduce some of the abovementioned variables. Thus, we selected Ni2 + as the paramagnetic center, three bulky arylmonocarboxylates as link- ers to hinder the formation of 3 D networks (3,4,5-trimethoxy- benzoate (3,4,5-tmb), 2-naphthalenecarboxylate (2-ntc), and 9- anthracenecarboxylate (9-atc)), and two bipyridines as spacer ligands to favor the formation of 1 D compounds (4,4'-bipyri- dine (4,4'-bpy) and trans-1,2-bis(4-pyridyl)ethylene (t-bpee)). Other parameters such as stoichiometry, solvent, temperature, reaction time, and crystallization process were also fixed. Sol- vothermal methods (conventional and microwave-assisted) were used for better control of the experimental parameters. With these methods, the amount of solvent, reaction time, and number of reaction steps can be reduced drastically, and simul- taneously, the yield is increased.[13] In this work we describe the preparation of [Ni2(3,4,5-tmb)4(4,4'-bpy)] (1), [Ni2(2- ntc)4(4,4'-bpy)] (2), [Ni(9-atc)2(4,4'-bpy)] (3), [Ni(3,4,5-tmb)2(t- bpee)] (4), and [Ni2(2-ntb)4(t-bpee)2](5). The compounds are characterized, and their properties and the relationship be- tween the structure and magnetic properties are analyzed.

Results and Discussion Synthesis of the compounds Slight differences in the reaction parameters can induce the formation of different isomers,[14] so we prepared 1-5 under identical experimental conditions (stoichiometry, solvent, con- centration, temperature, reaction time, and crystallization pro- cess) to keep the nature of the ligands as the sole variable (Table 1). The 1:2 :4 stoichiometry for N^N ligand/nickel salt/ carboxylic acid gave rise to 1-5. However, that stoichiometry was later adjusted in the syntheses of 3-5 to improve the yields.

All the compounds can be synthesized by using Ni(OH)2 as a starting material. However, 1 and 2 have always been ob- tained together with an impurity of insoluble Ni(OH)2 revealed by elemental analyses, IR spectra, and powder X-ray diffracto- grams of the bulk material. If Ni(NO)3·6 H2O is employed in- stead of Ni(OH)2, 1 is isolated in high yield. Unfortunately, this strategy did not work with 2, and another unidentified product was obtained. Different synthetic methods, stoichiometry, tem- peratures, reaction times, and pH values were assayed unsuc- cessfully in an attempt to avoid the presence of Ni(OH)2 as an impurity in the synthesis of 2. Nevertheless, the use of Ni(OH)2 as a starting material allowed the isolation of a few single crys- tals of 2, which were used to characterize the sample by single-crystal X-ray diffraction and IR spectroscopy.

Microwave-assisted solvothermal synthesis allowed the re- duction of the reaction time for 3,4,5-tmb derivatives (1 and 4) to just two hours. Unfortunately, 2, 3, and 5 could not be ob- tained as a single product through this synthetic procedure. The influence of the reaction conditions on the nature of the products in the nickel complexes is well illustrated by the preparation of 3, which is obtained by using solvothermal or microwave activation. However, if the reaction is performed at ambient temperature and pressure in methanol/water, [Ni(9- atc)2(4,4'-bpy)(CH3OH)2] is obtained.[10] The reaction of Ni(OH)2 or Ni(NO3)2·6 H2O, 9-Hatc, and t-bpee gave rise to an unidentified insoluble compound in all cases.

Infrared spectra The infrared spectra (see S1 in the Supporting Information) show bands ascribed to the na(COO) and ns(COO) modes in the 1634-1608 cm^1 and 1417-1391 cm^1 intervals, respectively. The energy difference na^ns depends on the coordination mode of the carboxylate group. The lowest values are expect- ed for chelating coordination, bridging coordination displays intermediate values, and the highest values are expected for unidentate carboxylate ligands.[13b, 15] A splitting of the asym- metric na(COO) was observed in 1 and 2 ,sotwo n a- n s values were calculated in each complex (238 and 220 cm^1 in 1 and 230 and 211 cm^1 in 2). Both are consistent with a bridging mode for the carboxylate ligand. The differences of 194 and 202 cm^1 were assigned in 3 and 4 to a chelating mode. Differences of 192 cm^1 and 237 cm^1 were found in 5, consistent with che- lating and bridging modes, re- spectively (Table 2).

Thermal stability The thermogravimetry curves measured in a nitrogen atmos- phere show a one-step decom- position process to form NiO for 1, 3, and 5. This process begins at 350 8C for 1 and 3 and at 3108C for 5 (Table 3 and S2 in the Supporting Information). The process is different for 4, in which there is a first weight loss at 40 8C corresponding to 2.5 noncoordinated water molecules. A second weight loss occurs at 175 8C to form the intermediate [Ni(3,4,5-tmb)2(4,4'- bpy)0.5]. Finally, NiO is formed at 320 8C. The identity of the oxide was confirmed by powder X-ray diffraction in all cases. As an example, the powder X-ray diffraction data for NiO ob- tained after the thermogravimetric measurement of 1 are shown in S3 in the Supporting Information.

Description of the structures All the compounds form 1 D coordination polymeric structures with the ditopic N,N-donor ligands joining the metal centers along the chains.

Complexes 1 and 2 display dinickel paddlewheel units, with the two metal atoms bridged by four carboxylate ligands. These dimetallic units are joined by axially coordinated 4,4'- bpy ligands, giving rise to linear chains (Figure 1) in the [010] direction in compound 1 and the [011] direction in 2. Each Ni2 + ion presents a square-pyramidal coordination environ- ment NiNO4, in which the oxygen atoms form the base of the pyramid and the nitrogen atom is at the apex (Figure 1 and S4a).

The intradimer Ni···Ni distances are 2.670 and 2.646 ^ in 1 and 2 , respectively. These distances are very similar to those found for the 1 D polymeric complexes [Ni2(O2CCH3)4(dbp)] (dpb = 2,3'-dipyridylamine) (2.6371 ^)[16a] and [Ni2(O2CCH3)4- (hmta)] (hmta = hexamethylenetetramine) (2.622 ^).[16b] They are also similar to that found in the 3 D compound [Ni2(2,6- ndc)2(4,4'-bpy)] (2,6-ndc = 2,6-naphthalenedicarboxylate) (2.654 ^),[17] and to those reported for the dimeric molecular complexes [Ni2(O2CCMe3)4(py)2] (2.603 ^),[8] [Ni2(O2CArTol)4(py)2] (ArTolCO2H = 2,6-di(p-tolyl)benzoic acid) (2.574 ^),[18] and [Ni2- (O2CPh)4(NITppy)2] (NITppy =2-(4-pyridyl)-4,4,5,5-tetramethyli- midazoline-1-oxyl-3-oxide) (2.645 ^).[19] However, these distan- ces are shorter than those found in similar discrete complexes [Ni2(O2CCMe3)4L2](L= 2,3-lutidine, 2,4-lutidine, 2,5-lutidine, 2- methylquinoline, 2-picoline, 2-ethylpyridine, Et3N) (2.7171- 2.754 ^)[8] and [Ni2(O2CPh)4L2]·2 MeCN (L=1-methyl-4,5-diphe- nylimidazole) (2.734 ^).[20] Theoretical calculations[21] performed on [Ni2(O2CR)4L2] com- plexes explain the existence of two stable conformational iso- mers with different Ni···Ni distances. In the asymmetrical con- former (AS), the spin density on each Ni atom is slightly higher than the spin densities on metal atoms in the symmetrical con- former (S). In accordance with these calculations, conformers AS, with Ni···Ni distances greater than 2.70 ^, are more stable than the more symmetrical complexes with shorter Ni···Ni dis- tances. The distortion is caused by a second-order Jahn-Teller effect. However, there must be another effect that makes the S conformer stable, which could be a bonding interaction be- tween the metal centers. That interaction should be more fa- vorable in an S isomer with a shorter Ni···Ni distance. Actually, there are experimentally more examples of S compounds, al- though this can also be explained by the better packing in the S isomers.

In accordance with the classification of Panina et al. ,[21] com- plex 1 is an S conformer. It displays the most symmetrical con- formation possible, because it is imposed by an inversion center between the metal atoms : Ni^Ni^N angle of 1808 and O^Ni^Ni^O torsion angle of 08 (see S4 b in the Supporting In- formation). Compound 2 can be considered an AS conformer because its distortion is higher than any found for an S confor- mer. Surprisingly, the Ni···Ni distances in 1 and 2 are similar de- spite the fact that S and AS conformers described in the litera- ture have very different values of this parameter.

In summary, the Ni···Ni distances seem to be a compromise of many factors such as the distortion of the dimetallic unit, the bonding interaction between the Ni atoms, and the pack- ing in the solid state. In addition, an increase in basic character of the axial ligand seems to lead to greater Ni···Ni distances.

The distance between paddlewheel units within the same chain is determined by the length of the 4,4'-bpy ligand (13.768 ^ in 1 and 13.700 ^ in 2). The chains are arranged par- allel to one another, and are separated in 1 by a distance of 6.731 ^ (c/2) in the [001] direction and 9.244 ^ (a/2) in the [100] direction. In compound 2, the distances between adja- cent chains are 5.575 ^ (b/2) in the [001] direction and approxi- mately 8.63 ^ in the [011] direction.

The existence of two types of weak interactions for the aro- matic rings in 2 is remarkable, determining the 3 D macromo- lecular structure of the compound. Two types of such bonds can be found : face-to-face interactions between 2-ntb ligands situated at 3.440 ^; and aromatic C^H/p hydrogen bonds[22] between two C^H (C4^H4) units from the 4,4'-bpy and the 2- ntb p-system at 2.798 ^ (Figure S4 c in Supporting Informa- tion). The nature of these aromatic interactions and their for- mation can be explained by considering direct electrostatic in- teractions between polarized atoms in addition to polar/p ef- fects. Solvation/desolvation processes must also be taken into account, especially if hydrogen-bonding solvents are used.[23] The sum of such weak interactions contributes to the stability of the whole system.

Very few examples of polymeric 1 D paddlewheel structures of Ni2 + have been reported to date, most of them with carbox- ylate or dithiocarboxylate ligands.[16,24] In particular, only two 1 D paddlewheel polymers with the formula [Ni2(O2CCH3)4L] (L = hexamethylenetetramine, 2,3'-dipyridylamine) are known.[16] These compounds form zigzag chains because of the nature of the linkers, whereas 1 and 2 give linear chains.

Compounds 3 and 4 do not contain dimetallic paddlewheel units, but isolated nickel centers with an octahedral coordina- tion polyhedron NiN2O4. The arrangement can be described as zigzag chains formed by cis-bis(chelate) Ni(O2CR)2 units joined by N,N'-donor ligands (4,4'-bpy and t-bpee, respectively ; Fig- ures 2, 3 (top), and S4 d). It is well known that the cis-bis- (chelate) coordination environment in octahedral centers gen- erates chirality on the metal atoms.[25] However, contiguous centers in these chains have the opposite configuration, giving rise to an achiral meso form in both compounds. Although there are very few examples of 1 D compounds of the type [Ni- (O2CR)2(N^N)] , both configurations, cis[26] and trans,[27] are known.

The structure of compound 3 contrasts with the linear 1 D arrangement observed in the complex [Ni(9-atc)2(4,4'-bpy)- (CH3OH)2][10] with a trans,trans,trans configuration, in which the 9-atc units act as monodentate ligands.

Even though compounds 3 and 4 display the same type of 1 D polymeric chains, the spatial arrangement in the two struc- tures is quite different. In compound 3, the chains are parallel to one another and follow the [001] direction (Figure 2). The packing of the chains is less dense (Dc= 1.342 g cm^3) than those found in the structures of 1 and 2 (Dc = 1.432 and 1.426 g cm^3, respectively). In 3, there are aromatic C^H/p hy- drogen bonds between the hydrogen atoms of the 4,4'-bpy ligand and the 3,4,5-tmb aromatic ring at 2.722, 2.680, 2.717, and 2.838 ^ (Figure S4 e). However, in complex 3,nop···p inter- actions between adjacent anthracene rings of the 9-atc ligands are observed, in contrast with the situation described for [Ni(9- atc)2(4,4'-bpy)(CH3OH)2].

The arrangement of the chains in 4 (Figure 3) is even less dense (Dc=1.283 g cm^3). The packing of the 1 D polymers can be described as a stacking in the [010] direction of layers of chains. Within each of the layers, the axes of the chains are parallel to one another (Figure 3, top), but the direction of the chains in the adjacent layers changes. Thus, the chains in neighboring layers follow the alternating directions [101] and [101]. Compound 4 presents face-to-face interactions (3.611 ^) between the aromatic rings of 3,4,5-tmb and t-bpee ligands, which form an angle of 12.568, and also C^H/p hydrogen bonds (2.519 ^) between the hydrogen atoms of the methoxy groups and the aromatic rings of the t-bpee ligands (see S4 f).

The structure of 5 displays ladder chains of dinickel units in which the two metal atoms are bridged by two carboxylate li- gands (2-ntb) and joined along the [110] direction by the t-bpee ligands. Each center is also coordinated to one chelat- ing carboxylate ligand (Figure 4 and S4 g). The metal atoms show an octahedral NiN2O4 coordination environment ; half the carboxylate ligands display a bridging mode, and the other half act as chelating ligands. The same situation was found in the previously reported compound [Ni2(btc)2(4,4'- bpy)2]·0.75 nt·H2O (btc = benzenetricarboxylate, nt = naphtha- lene).[28] However, this latter complex presents a 2 D polymeric structure because of the higher connectivity of the carboxylate ligands.

The packing of the chains is achieved in a similar fashion to that found in 4. There is a stacking of layers in the [001] direc- tions with chains within the same layer parallel to one another. Adjacent layers display chains that are alternately parallel to the [110] and [10] directions.

Compound 5 presents one type of face-to-face aromatic in- teraction and two types of C^H/p hydrogen bonds. The former interactions are established between two 2-ntb aromat- ic rings at 3.650 ^. The latter occur between the hydrogen atoms of the t-bpee ligands and a 2-ntb aromatic ring (2.912 ^), and between the hydrogen atoms of a 2-ntb ligand and a 2-ntb aromatic ring (2.878 ^) (see S4 h in the Supporting Information).

In summary, different structural arrangements are obtained in complexes 1-5. The use of 4,4'-bpy allows the preparation of paddlewheel linear chains with 3,4,5-tmb and 2-ntc. Howev- er, an increase in the number of aromatic rings in the arylcar- boxylate destabilizes this type of structure. This can be attrib- uted to the different number and nature of the supramolecular interactions that arise from the use of different aromatic rings attached to the carboxylic group. It is remarkable that in com- pounds containing 4,4'-bpy, the axes of the chains are always arranged in parallel, whereas the polymers containing t-bpee ligands follow different directions in alternate layers. In addi- tion, the less rigid backbone of the t-bpee spacer probably prevents the formation of paddlewheel linear chains. Unfortu- nately, we were unable to obtain crystals suitable for X-ray analysis from the reaction with benzoic acid, although the IR spectrum suggests the presence of bridging carboxylate li- gands.

DFT studies on compound 1 Density functional theory (DFT) calculations using the broken- symmetry (BS) approach[29] were performed to better under- stand the magnetic behavior of 1 (see below), which is typical for paddlewheel [Ni2(O2CR)L2] compounds.[8,30-32] A paddle- wheel unit (Figure 5) was fully optimized at the DFT/B3LYP level in the high-spin (HS) state (S = 2). The BS state guess wavefunction was generated from the quintuplet one by flip- ping spins corresponding mainly to Ni centers. Upon rerunning the self-consistent field (SCF) procedure with this initial guess and setting 1 for the multiplicity, the BS state is achieved. The BS determinant is an Sz-eigenfunction with Sz= 0, but the com- puted average spin square value is close to 2, a triplet state. Therefore, the BS wavefunction can be read as the product of two local triplets on Ni atoms with Sz values equal to + 1 and ^1. Mulliken spin densities calculated for the BS state (Figure 5) show this spin polarization on Ni atoms and also on surrounding atoms.

It is well known that isotropic interaction between paramag- netic centers is described through an exchange coupling con- stant, J, which, in the frame of the BS approach, can be esti- mated from the energy difference between the HS and BS states. For systems such as compound 1, with two magnetic centers bearing two unpaired electrons, J can be evaluated from [Equation (1)] , in which EQ and EBS are the energy values of the HS and BS states, respectively.[33] ... (1) The J value computed for compound 1 is ^356 cm^1, which indicates a very strong antiferromagnetic coupling, although the values found experimentally should be more reliable.

Magnetic properties Variable-temperature magnetic measurements were performed for complexes 1 and 3-5. Polycrystalline samples obtained from crushed single crystals were used in all cases.

The magnetic moment and susceptibility of 1 in the temper- ature range 300-2 K are depicted in Figure 6. On cooling, the magnetic susceptibility first decreases from 300 to 90 K, and then increases continuously upon lowering the temperature. The magnetic moment decreases constantly in the range 300- 2K from 2.22 to 0.31mB. In this representation (and also in the representation of the magnetic susceptibility), a slight maxi- mum is observed at about 60 K. This is caused by traces of mo- lecular dioxygen, which are difficult to remove, and is usually observed if the paramagnetic signal is weak. Taking into ac- count this correction, we should observe a continuous de- crease in magnetic moment down to very low temperatures. It is worth mentioning that meff at room temperature is half the value expected for a dinickel complex. The drop in magnetic moment in this kind of dinickel paddlewheel complex has tra- ditionally been explained as a consequence of antiferromag- netic coupling between the metal atoms through the carboxyl- ate ligands, assuming the absence of a metal-metal bond.[30-32] However, the fact that very strong antiferromagnetism can be found in paddlewheel compounds bridged by four carboxylate ligands, despite the fact that the exchange through one, two, or three carboxylates is usually very weak,[6] suggests that other factors are in play. The spin-density distribution from our DFT calculations (Figure 5) indicates that the carboxylate li- gands do not play an important role in the magnetic ex- change. More recently, a direct exchange between the metal centers through a weak M^M bond[7b] and a temperature-de- pendent spin equilibrium[7a] has been considered to explain the magnetic behavior of "M2(carboxylate)4" complexes (M = Co, Cu).

Taking into account the DFT studies, the magnetic data of 1 have been fitted with a model [Eq. (2)] based on the spin Hamiltonian H = ^2 JS1S2. Equation (2) is derived from a general expression[34] for symmetrical dinuclear compounds consider- ing the existence of three possible spin states (S= 0, 1, 2). This equation takes into account an antiferromagnetic intradimer constant (J). An antiferromagnetic interdimer constant (zJ'), a temperature-independent paramagnetism (TIP) term, and a paramagnetic impurity (P) term are also considered. For better fit parameters to be obtained, the value of the gyro- magnetic constant (g) was fixed to 2.2, a typical value found for other Ni2 + compounds.[35] ... (2) The terms N, b, k, and T have the usual meanings. The pa- rameters obtained in the fits are shown in Table 4. The para- magnetic impurity (0.05 %) is similar or lower than those calcu- lated in other nickel complexes,[36] and the TIP value (5·10^4 cm3 mol^1) is usual for nickel complexes.[37] The J value calculated from Equation (2) (^568 cm^1) is very large, which indicates very strong intramolecular antiferromag- netic coupling. This J value is larger than those calculated for some dimeric tetracarboxylato complexes [Ni2(O2CR)L2](L= 2,4- lutidine, 2,5-lutidine, 2-ethylpyridine, 2-picoline, quinoline, and PPh3)(^J =126-224 cm^1)[31] although the data were obtained from magnetic measurements in the range 80 to 300 K. Analo- gously, the antiferromagnetic coupling of the dimer complex [Ni2(O2CCMe3)(py)2](2J= ^260 cm^1)[32] is lower than that ob- served for compound 1, although both compounds show a similar variation of the magnetic moment with temperature. The more pronounced drop in the magnetic moment in 1 with respect to the complex [Ni2(O2CCMe3)(py)2] is in accordance with its stronger antiferromagnetic coupling. Analogous cou- pling constants (from ^622 to ^649 cm^1) have been observe- d[24a] in the Ni2 + ···Ni3 + dithiocarboxylato complexes [Ni2I(S2CR)4] (R = Et, nPr, and nBu), although in these compounds, the anti- ferromagnetic interaction is intermolecular and occurs through the bridging iodide ligands. The intermolecular zJ' values are highly dependent on the type of compounds, and the calculat- ed value for 1 is somewhat higher than those calculated for other Ni complexes.[32] In summary, the strong antiferromagnetic interactions in compound 1 could occur through the four carboxylate ligands as proposed for other dinickel compounds.[30-32] However, the large J value and short Ni···Ni distance suggest that a direct ex- change is more likely across a Ni···Ni interaction. Although a through-space interaction between the Ni atoms is also pos- sible, the large J value makes this hypothesis unlikely.

Complexes 3 and 4 present a very different magnetic behav- ior from that of 1 (Figure 7). In both cases, the magnetic sus- ceptibility measured at 0.5 T shows a continuous increase with decreasing temperature, which is characteristic of paramagnet- ic compounds. The magnetic moments at room temperature are 3.12 and 3.10 mB for 3 and 4, respectively, close to the values expected for noninteracting S = 1 spin per Ni atom with a small spin-orbit coupling. The magnetic moment shows almost constant values from 300 to 25 K. A sharp decrease ap- pears at lower temperatures, which could be ascribed to a zero-field splitting ( D ) and a small antiferromagnetic interac- tion between the nickel centers (zJ') of different chains. As de- picted in Figure 7 (compound 3) and Figure S5 in the Support- ing Information (compound 4), a slight maximum is observed at a low temperature in the representation of the magnetic moment. At a lower applied magnetic field (0.01 T) the maxima become sharper. The shortest Ni···Ni distances are 8.351 and 6.003 ^ for compounds 3 and 4, respectively. There- fore, a likely explanation is the existence of ferromagnetic in- teractions between chains, although the presence of a very low amount of a ferromagnetic impurity cannot be ruled out.

These magnetic data have been fitted with a model taking into consideration the existence of zero-field splitting, which is usual in the axially distorted d8 high-spin isolated Ni2+ (S = 1), according to Equation (3).[38] ... (3) However, the drop in the magnetic moment with tempera- ture may also be influenced by an additional small antiferro- magnetic interchain coupling between the Ni2 + centers. Thus, this interaction (zJ') was introduced in Equation (3) as a pertur- bation of the molecular field approximation.[38] The g, D, and J values are similar to those previously found in other Ni2+ compounds.[13b, 35] Good agreement between the experimental and calculated data is obtained with both models for compounds 3 and 4 (Table 4).

... (4) The susceptibility measurements of complex 5 show a con- tinuous increase with decreasing temperature up to a maxi- mum (N^el temperature) at 6 K (Figure 8). The magnetic moment at room temperature is 4.61 mB, which is close to that expected for two noninteracting Ni2+ ions (S = 1). The repre- sentation of the magnetic moment versus temperature dis- plays almost constant values in the interval 300-25 K.

The magnetic data of complex 5 were fitted by using the Bleaney-Bowers equation with S = 1 [Eq. (5)], which considers the existence of a weak antiferromagnetic exchange within the dimeric unit. Thus, the g value obtained from this fitting is sim- ilar to those previously obtained for 3 and 4, although J is higher in absolute value (Table 4). The use of a model[35d] that takes into consideration the zero-field splitting does not im- prove the fit.

... (5) Conclusion Solely 1 D compounds have been isolated under the same ex- perimental conditions by employing bulky carboxylate ligands and bipyridine spacers in the presence of Ni2 + ions. However, the nature of the 1 D [Ni(O2CR)2(N^N)x] compounds is very dif- ferent. Linear chains are built, linking paddlewheel dimetallic units with 4,4'-bpy ligands. In these chains, the stability of the paddlewheel fragment seems to decrease as the number of ar- omatic rings in the carboxylate increases. However, with the less rigid backbone of the t-bpee ligand, the paddlewheel structure is not formed under identical conditions.

The magnetic properties of compounds 1-5 are very depen- dent on the nature of the chains. In complexes 1 and 2, the formation of paddlewheel units favors strong intramolecular antiferromagnetic coupling and weak antiferromagnetic inter- molecular interactions. DFT calculations using the BS approxi- mation have proved to be a very useful tool for explaining the complicated magnetic behavior of compound 1. However, in the zigzag monometallic chains and ladder-like chains, only weak antiferromagnetic couplings are observed.

Experimental Section General comments and physical measurements All reactants and solvents were obtained from commercial sources and used without further purification. A Memmert Universal Oven UFE 400 and Teflon-lined stainless steel autoclaves were employed for conventional solvothermal syntheses. Solvothermal reactions under microwave radiation were performed in an ETHOS ONE mi- crowave oven using TFM Teflon closed vessels with a temperature- and pressure-control device.

FTIR spectra were registered with a PerkinElmer Spectrum 100 equipped with a universal ATR accessory in the spectral range 4000-650 cm^1. Elemental analyses were performed by the Micro- analytical Service of the Universidad Complutense of Madrid. Ther- mogravimetric analyses (TGA) were performed at a heating rate of 5 8C min^1under a nitrogen atmosphere on a PerkinElmer Pyris 1 TGA Instrument. The magnetic susceptibility data were measured on a Quantum Design MPMSXL SQUID (Superconducting Quantum Interference Device) magnetometer over the temperature range 2- 300 K. All data were corrected for the diamagnetic contribution of both the sample holder and the compound to the susceptibility. The molar diamagnetic corrections for the complexes were calcu- lated on the basis of Pascal's constants. All calculations were per- formed with GAMESS package,[39] version 2012 R2. Unrestricted DFT methodology with hybrid B3LYP functional was employed. SBKJC effective core potential[40] (ECP) as it is implemented in the package was used for Ni atoms and 6-31G(d) for the rest of atoms but not hydrogen for which 6-31G basis set was selected. Yields, el- emental analyses, and IR absorptions are collected in Table 5.

Synthesis A) Solvothermal synthesis : A mixture of the reactants in ethanol (9 mL) and water (3 mL) was sealed into a Teflon-lined stainless au- toclave. The system was heated at 150 8C for three days under au- togenous pressure, and slowly cooled afterwards to obtain green single crystals of compounds 1-5, which were washed with etha- nol (2 ^5 mL) and diethyl ether (2 ^5 mL) and dried under vacuum.

B) Microwave synthesis : An 85 mL Teflon vessel with a magnetic stirrer was charged with the same quantities of the same reactants and solvents as used in the previous method, heated in a micro- wave oven for 2 h at 150 8C, and cooled afterwards. The green crystalline solids obtained were isolated through the same proce- dure as that used in the previous method.

[Ni2(3,4,5-tmb)4(4,4'-bpy)] (1): This product was obtained by either conventional or microwave-assisted solvothermal synthesis using Ni(NO3)2·6 H2O (0.46 g, 1.6 mmol), 3,4,5-trimethoxybenzoic acid (0.48 g, 4.0 mmol), and 4,4'-bipyridine (0.12 g, 0.8 mmol) as reac- tants.

[Ni2(2-ntc)4(4,4'-bpy)] (2): A mixture of green single crystals of 2, together with insoluble unidentified products, was obtained by using either method (A or B). Ni(OH)2 (0.15 g, 1.6 mmol), 2-naph- thalenecarboxylic acid (0.69 g, 4.0 mmol), and 4,4'-bipyridine (0.12 g, 0.8 mmol) were used as reactants.

[Ni(9-atc)2(4,4'-bpy)] (3): Complex 3 was obtained by conventional solvothermal synthesis using Ni(OH)2 (0.15 g, 1.6 mmol), 9-anthra- cenecarboxylic acid (0.89 g, 4.0 mmol), and 4,4'-bipyridine (0.24 g, 1.6 mmol) as reactants. Microwave synthesis gave rise to a mixture of crystals of 3 and insoluble unidentified products.

[Ni(3,4,5-tmb)2(t-bpee)] (4): This compound was synthesized by solvothermal and microwave synthesis. The reactants employed were Ni(OH)2 (0.15 g, 1.6 mmol), 3,4,5-trimethoxybenzoic acid (0.48 g, 4.0 mmol), and trans-1,2-bis(4-pyridyl)ethylene (0.29 g, 1.6 mmol).

[Ni2(2-ntc)4(t-bpee)2] (5): Crystals of 5 were obtained through the solvothermal method. Ni(OH)2 (0.15 g, 1.6 mmol), 2-naphthalene- carboxylic acid (0.69 g, 4.0 mmol), and trans-1,2-bis(4-pyridyl)ethy- lene (0.29 g, 1.6 mmol) were employed as reactants. The micro- wave method gave rise to crystals of 5 together with impurities.

Structural analysis Suitable single crystals of compounds 1-5 were selected from the crystals produced through the different processes. These were mounted on a Bruker-Siemens Smart CCD diffractometer equipped with a normal focus 2.4 sealed-tube X-ray source (MoKa radiation, l=0.71073 ^). Crystallographic data for compounds 1-5 are listed in Table 6. CCDC 952040-952044 (1-5) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

In all cases and for all the synthetic methods, the purity of the bulk compound was also checked through powder X-ray diffrac- tion. Powder X-ray measurements were performed on an X'Pert PRO ALPHA1 diffractometer with q/2q configuration. Rietveld re- finements were performed using X'pert High Score Plus software in automatic mode (See S6 in the Supporting Information).

Acknowledgements Financial Support received from the Spanish Ministerio de Econo- m^a y Competitividad (CTQ2011-23006) and Comunidad de Madrid (S2009/MAT-1467) is gratefully acknowledged.

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Received : February 5, 2014 Published online on March 18, 2014 Miguel Cortijo,[a] Santiago Herrero,*[a] Bel^n Jerez,[a] Reyes Jimenez-Aparicio,*[a] Josefina Perles,[a] Jos^ L. Priego,[a] Javier Torroba,[a] and Jos^ Tortajada[b] [a] M. Cortijo, Dr. S. Herrero, B. Jerez, Prof. R. Jim^nez-Aparicio, Dr. J. Perles, Dr. J. L. Priego, Dr. J. Torroba Departamento de Quimica Inorg^nica Facultad de Ciencias Qu^micas Ciudad Universitaria, 28040 Madrid (Spain) Fax: (+ 34) 91-3944352 E-mail : [email protected] [email protected] Homepage : http ://www.ucm.es/-quimica_inorganica_1/qcmm.htm http://www.ucm.es [b] Dr. J. Tortajada Departamento de Quimica Fisica Facultad de Ciencias Quimicas Ciudad Universitaria, 28040 Madrid (Spain) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201402005.

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