DINUCLEAR NICKEL(II) PIVALATE WITH μ-AQUA AND DI-μ-PIVALATO BRIDGES SHOWING A FERROMAGNETIC INTERACTION

Dinuclear nickel(II) complex, [Ni2{O2CC(CH3)3}4(OH2){HO2CC(CH3)3}4] (1), was synthesized and characterized by elemental analysis, IR and UV-Vis-NIR spectroscopy, and temperature dependence of magnetic susceptibilities (4.5—300 K). Single-crystal X-ray crystallography revealed a dinuclear core with μ-aqua and di-μpivalato bridges having monodentate pivalato and monodentate pivalic acid molecules. Magnetic data analysis showed a ferromagnetic interactions between the two nickel atoms with g = 2.251, J = 2.78 cm, D = 3.75 cm, and tip = 184 x 10 cm mol; g = 2.253, J = 2.73 cm, D = –3.26 cm, and tip = 176 x 10 cm mol.

Although a lantern-type dinuclear core was also found in some adducts of nickel(II) pivalate [10][11][12], the structure of the parent complex, nickel(II) pivalate, has not been elucidated yet.Therefore, we tried to reveal the molecular structure of nickel(II) pivalate by crystallizing the parent complex using various kinds of organic solvents.In this study, we isolated a crystalline material by recrystallizing the product from hexane.The isolated complex was characterized by elemental analysis, IR and UV-Vis-NIR spectroscopy, measurement of temperature dependence of magnetic susceptibilities (4.5-300 K), and single-crystal X-ray crystallography.The X-ray structure analysis of the complex revealed a different dinuclear core from the lantern-type dinuclear copper(II) complexes.Interestingly temperature dependence of magnetic susceptibilities (4.5-300 K) of the complex showed a ferromagnetic property.Herein, we report on the crystal structure, spectral features, and magnetic property of this complex.
Nickel(II) carbonate hydroxide (0.358 g, 1.17 mmol) and pivalic acid (0.966 g, 9.45 mmol) were mixed at room temperature and then heated at 60°C with stirring for 1 hr.The resulting green product was dissolved in hexane (200 mL) and the solution was fi ltrated.The solution was concentrated by rotary evaporator to 5 mL and left in a refrigerator overnight.Green crystals that deposited were collected by fi ltration, washed with small amount of petroleum ether.Yield: 0.458 g (27.5%  Measurements: Elemental analyses for carbon, hydrogen, and nitrogen were done using a Thermo-Finnigan FLASH EA1112 series CHNO-S analyzer.Infrared spectra were measured with a JASCO MFT-2000 FT-IR Spectrometer in the 4000-600 cm -1 region.Electronic spectra were measured with a Shimadzu UV-Vis-NIR Recording Spectrophotometer (Model UV-3100).Magnetic susceptibilities were measured with a Quantum Design MPMS-XL7 SQUID susceptometer operating at a magnetic fi eld of 0.5 T over a range of 4.5-300 K.The susceptibilities were corrected for the diamagnetism of the constituent atoms using Pascal's constants.The effective magnetic moments were calculated from the equation μ eff = 2.828√χ M T, where χ M is the molar magnetic susceptibility per mole of dinuclear nickel molecule.
X-Ray crystallography: X-Ray diffraction data were collected on a Bruker SMART APEX CCD diffractometer (Mo K radiation) at 90 K and indexed using the SMART software.Crystal data and details concerning data collection are given in Table 1.The cell parameters were refi ned by full-matrix least-squares on F 2 .Integrated intensity information for each refl ections was obtained and corrected using the SAINT+ program package including the reduction program SAINT and the empirical absorption correction program SADABS.The structure was solved using the SHELXTL program.The structure was solved by direct methods, and the residual non-hydrogen atoms were located by D-Fourier synthesis.All of non-hydrogen atoms were refi ned by full-matrix least-squares on F 2 .The hydrogen atoms except for those of water molecules were inserted at their ideal positions and fi xed there.All
The diffused refl ectance spectra of 1 are shown in Figure 3. Four absorption bands appear at around 390, 664, 740 sh, and 1132 nm in solid.These bands can be attributed to d-d transitions of octahedral nickel(II) complex in origin.The fi rst band at 1132 nm may be assigned to spin-allowed 3 A 2g ( 3 F)→ 3 T 2g ( 3 F) transition, a shoulder at 740 nm to a spinforbidden 3 A 2g ( 3 F)→ 1 E g ( 1 D) transition, a band at around 664 nm to spin-allowed 3 A 2g ( 3 F)→ 3 T 1g ( 3 F) transition, and a band at 390 nm to spin-allowed 3 A 2g ( 3 F) → 3 T 1g ( 3 P) transition [15].The spectral feature is in harmony with the presence of two octahedral nickel(II) ions.
Single crystals of 1 were obtained by recrystallization of the reaction product from hexane.X-ray crystal structure analysis revealed that the present complex has a dinuclear nickel(II) core being different a little from the lantern-type dinuclear metal complexes.The structure of the dinuclear complex drawn by ORTEP is shown in Figure 4.The Ni1 atom is coordinated by two pivalato-oxygen atoms (O1 and O3) of syn-syn-bridging, one monodentate pivalato-oxygen atom (O7), two oxygen atoms (O5 and O9) of monodentate pivalic acid molecules, and bridging aqua-oxygen atom (O17) in an octahedral geometry, whereas the Ni2 atom is coordinated by two bridging pivalato-oxygen atoms (O2 and O4), one monodentate pivalato-oxygen atom (O11), two oxygen atoms (O13 and O15) of monodentate pivalic acid molecules and μ-aqua-oxygen atom (O17) in an octahedral geometry.The Ni1•••Ni2 distance is 3.3969( 7 at 230 K and is almost same as the present crystal structure taken at 90 K, whereas the Winpenny's structure at 150 K belongs to different crystal system and space group, orthorhombic Pbca, irrespective of the similar dinuclear structure. Temperature dependence of effective magnetic moment of 1 is shown in Figure 5.The magnetic moment is 4.65  B at 300 K per dinuclear nickel(II) molecule, which is signifi cantly higher than the spin-only value (4.00 μ B ).The magnetic moment gradually increases with lowering of temperature until reaching 5.36  B at 4.5 K, being typical of ferromagnetic behavior [19].The magnetic data were analyzed with magnetic susceptibility Eq.( 1) based on the Heisenberg model H = -2JS Ni1 •S Ni2 considering zero-fi eld-splitting (D) by the Ginsberg's method [20].

Figure 5 .
Figure 5. Temperature dependence of magnetic moment of 1.
Calcd for C 40 H 78 Ni 2 O 17 : C, 50.66;H, 8.29%.IR (KBr): ν(OH) 3350sh, 3140sh; ν as (CO 2 at the International Conference dedicated to the 55 th anniversary from the foundation of the Institute of Chemistry of the Academy of Sciences of Moldova ). Anal.Found: C, 51.08; H, 7.96%.Presented of the calculations were carried out on a Pentium IV Windows 2000 computer utilizing the SHELXTL software package.CCDC 997144 for 1 contains supplementary crystallographic data for this paper.The data can be obtained free of charge at www.ccdc.cam.ac.uk/ conts/retrieving.html[or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB12 1EZ, UK; fax: (internet.)+44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk].