Monoclinic honeycomb-layered compound Li3Ni2SbO6: preparation, crystal structure and magnetic properties.

Two synthetic routes-ion-exchange preparation from layered Na(3)Ni(2)SbO(6) at 300 °C and direct solid-state synthesis at 1150 °C resulted in layered Li(3)Ni(2)SbO(6), a cation-ordered derivative from the rocksalt type. The Fddd form reported earlier could not be reproduced. According to the XRD Rietveld analysis, Li(3)Ni(2)SbO(6) is a pseudohexagonal monoclinic structure, C2/m, with a = 5.1828(2) Å, b = 8.9677(3) Å, c = 5.1577(2) Å, ß = 109.696(2)°. No Li/Ni mixed occupancy was detected. At high temperatures, the magnetic susceptibility follows the Curie-Weiss law with a positive value of Weiss temperature, ∼8 K, indicating a predominance of ferromagnetic interactions. However, Li(3)Ni(2)SbO(6) orders antiferromagnetically at T(N)∼ 15 K. The effective magnetic moment is 4.3 µ(B)/f.u. which satisfactorily agrees with theoretical estimations assuming high-spin configuration of Ni(2+) (S = 1). Electron spin resonance (ESR) spectra show single Lorentzian shape line attributed to Ni(2+) ion in octahedral coordination. The absorption is characterized by isotropic temperature independent effective g-factor g = 2.150 ± 0.005. In accordance with the layered honeycomb crystal structure determined for Li(3)Ni(2)SbO(6), the superexchange interaction between Ni(2+) ions through Ni-O-Ni pathways within Ni(2)SbO(6) layers are assumed to be ferromagnetic, while the dominant interaction between layers is antiferromagnetic.

A fractional bond order of 1/2 in Pd(2)(5+)--formamidinate species; the value of very high-field EPR spectra.

Reaction of Pd(2)(DAniF)(4), 1, (DAniF = di-p-anisylformamidinate) with 1 equiv of AgPF(6) in CH(2)Cl(2) at or below -10 degrees C produces the paramagnetic species [Pd(2)(DAniF)4]PF(6), 1-PF(6), that has been studied by X-ray crystallography, UV-vis spectroscopy, electrochemistry, and multifrequency (9.5, 34.5, 110, and 220 GHz) EPR spectroscopy. Upon oxidation of the precursor, the Pd-Pd distance decreases by 0.052 Angstrom from 2.6486(8) to 2.597(1) Angstrom. The EPR spectra show broad signals with line widths of about 1000 G. The spectra collected at high field show a large spread of g tensor components ( approximately 0.03), but these are masked at lower frequencies (9.5 and 34.5 GHz). A reinvestigation using high-field EPR of the p-tolyl analogue, which is the only other structurally characterized Pd(2)(5+) species (Cotton, F. A.; Matusz, M.; Poli, R.; Feng, X. J. Am. Chem. Soc. 1988, 110, 1144), shows that this species, which had been reported to give an isotropic 9.5 GHz EPR spectrum, also gives anisotropic 110 and 220 GHz EPR spectra with a similarly large spread of g tensor components consistent with the unpaired electron residing in a metal-based MO. The results of these studies and calculations using density functional theory are consistent with the oxidation being metal-based, resulting in an uncommon Pd(2)(5+) species with a Pd-Pd bond order of 1/2.

Better understanding of the species with the shortest Re2(6+) bonds and related Re2)(7+) species with tetraguanidinate paddlewheel structures.

A series of compounds has been made containing quadruply bonded Re2(hpp)4X2 species (hpp = the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2a]pyrimidine), where X is CF3SO3 (1), CF3CO2 (2), and F (3). The distances of 2.1562(7), 2.1711(5), and 2.1959(4) A for 1-3 show significant effects of the sigma and pi electron donating ability of the axial ligands on the metal-metal distance. With the weakly coordinating triflate ligand the Re-Re distance is the shortest for any quadruple bonded species known. In addition to examining the effects of axial ligands on the Re2(hpp)42+ core, our study of the Re2(hpp)43+ core is being extended beyond the preliminary results previously reported in only one compound [Re2(hpp)4Cl2]PF6 (Dalton Trans. 2003, 1218). We now report the structural characterization by both X-ray and neutron diffraction of the compound [Re2(hpp)4F](TFPB)2, 4 (TFPB = the anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), and a detailed study by EPR spectroscopy of [Re2(hpp)4Cl2]PF6 at 9.5, 34.5, and 95 GHz frequencies, using dilute fluid solutions, frozen glass, and neat powder, show that the unpaired electron in the [Re2(hpp)4Cl2]+ ion is in an MO of predominant metal character with little mixing from the guanidinate ligands.

A diamagnetic dititanium(III) paddlewheel complex with no direct metal-metal bond.

Reaction of Ti[N(But)Ar]3 (Ar = 3,5-C6H3Me2 or Ar' = C6H5) with CO2 at -40 degrees C produces diamagmetic Ti(III) paddlewheel complexes with long Ti-Ti separations (>3.4 Angstrom), thus excluding direct Ti-Ti bonding. 1H NMR spectroscopy shows that the compounds are diamagnetic in solution in the temperature range of -65 to +70 degrees C. In the solid state, the diamagnetism was found to persist between 2 and 300 K. Calculations at the density functional theory level suggest that the diamagnetism results from antiferromagnetic coupling by superexchange through the ligand pi system.

Metal-metal bonding in mixed valence Ni2(5+) complexes and spectroscopic evidence for a Ni2(6+) species.

Dinickel(II) complexes of the ligands N,N'-di-p-anisylformamidinate (DAniF) and N,N',N''-triphenylguanidinate (TPG) have been synthesized and crystallographically characterized, along with their one-electron-oxidized analogues. In both systems, the Ni-Ni distances become shorter by approximately 0.1 A upon oxidation, in accord with the proposal that the resulting Ni2(5+) complexes are appropriately described as having one electron removed from a metal-based sigma orbital and an overall Ni-Ni bond order of 1/2. Although conventional DFT calculations on the model compounds Ni2(HNCHNH)4 and [Ni2(HNCHNH)4]+ appear to predict that the lowest energy state of the latter species would have one unpaired electron in an essentially ligand-based orbital. A single-point calculation of Ni2(DAniF)4 employing the geometry of its crystal structure with the full ligand included reveals a reversal of the previously predicted order of the HOMO and HOMO-1, and suggests that the unpaired electron in [Ni2(DAniF)4]+ is in a metal-based orbital of sigma symmetry. This is verified by the axial EPR spectrum of the compound in solution. The compound Ni2(DAniF)4 shows an unexpectedly rich cyclic voltammogram with four stepwise reversible oxidation waves. Coulometric experiments show that the doubly oxidized species has a significant lifetime at -25 degrees C, and by spectroelectrochemistry, its UV-vis spectrum was recorded. We propose that this species contains a Ni2(6+) core with a single Ni-Ni sigma bond.

Searching for precursors to metal-metal bonded dipalladium species: a study of Pd2(4+) complexes.

Reactions of Pd3(OAc)6 with lithium salts of mononegative bidentate N,N-ligands, L, of various types, such as formamidinates, benzamidinates, triazinates, and guanidinates, were investigated in a search for ways to obtain Pd2(4+) compounds that could serve as precursors to paddlewheel complexes with a metal-metal bond and a Pd2(5+) core. It was found that the reactions are complex and that either square planar mononuclear or dinuclear species may be formed depending on the reaction conditions or the method of isolation. For Pd2L4 compounds, alpha and beta isomers were sometimes obtainable. In the alpha isomer, all N,N-ligands serve as bridges, whereas in the beta isomer, two ligands bridge the Pd2(4+) unit and each of the other two chelate to a metal atom. Electrochemistry shows that the paddlewheel compounds Pd2(TolNC(H)NTol)4, Pd2(PhNC(Ph)NPh)4, and Pd2(PhNC(Ph)NPh)3(OAc) and the orthometalated complex cis-Pd2[eta2-C6H4NC(Ph)N(H)Ph]2(mu-OAc)2 have reversible oxidation waves between 0.70 and 0.92 V vs Ag/AgCl, which makes them good candidates for chemical oxidation.