No abstract

Two series of octahedral oxovanadium(IV) compounds, containing charged or neutral axial ligands, with the tetradentate amidate molecules Hcapca and H2capcah of the general formulae trans-[V(IV)OX(capca)]0/+ (where X = Cl- (1.CH2Cl2), SCN- (2), N3 (3), CH3COO- (4), PhCOO- (5), imidazole (6. CH3NO2), and eta-nBuNH2 (7)) and cis-[V(VI)OX(Hcapcah)]0/+ (where X = Cl- (8.0.5CH2Cl2), SCN (9), N3 (10.2CH3OH), and imidazole (11)), were synthesized and characterized by X-ray crystallography (1.CH3OH,8.CHCl3, 9.2CH3CN, 10.CH3CN and cis-[VO(imidazole)(Hcapcah)+) and continuous-wave electron paramagnetic resonance (cw EPR) spectroscopy. In addition to the synthesis, crystallographic and EPR studies, the optical, infrared and magnetic properties (room temperature) of these compounds are reported. Ab initio calculations were also carried out on compound 8 CHCl3 and revealed that this isomer is more stable than the trans isomer, in good agreement with the experimental data. The cw EPR studies of compounds 1-5, that is, the V(IV)O2+ species containing monoanionic axial ligands, revealed a novel phenomenon of the reduction of their A, components by about 10% relative to the N4 reference compounds ([V(IV)O-(imidazole)4]2+ and [V(IV)O(2,2-bipyridine)2]2+). In marked contrast, such a reduction is not observed in compounds 6. CH3NO2-11, which contain neutral axial ligands. Based on the spin-Hamiltonian formalism a theoretical explanation is put forward according to which the observed reduction of Az is due to a reduction of the electron - nuclear dipolar coupling (P). The present findings bear strong relevance to cw EPR studies of oxovanadium(IV) in vanadoproteins, V(IV)O2+-substituted proteins, and in V(IV)O2+ model compounds, since the hyperfine coupling constant, Az, has been extensively used as a benchmark for identification of equatorial-donor-atom sets in oxovanadium(IV) complexes.

Reaction of [V(VI)OCl(2)(thf)(2)] with a bidentate nitrogen-donor ligand (L: phen=1,10-phenanthroline, 5-mephen=5-methyl-1,10-phenanthroline, bipy=2,2'-bipyridine, 5,5'-me(2)bipy=5,5'-dimethyl-2,2'-bipy) in methyl alcohol, in the presence of triethylamine, leads to the formation of hexameric [V(2) (IV)V(4) (V)] oxo-alkoxo-vanadates of the general formula [V(6)O(12)(mu(2)-OCH(3))(4)(L)(4)].x H(2)O [L=phen (1.4 H(2)O), 5-mephen (2.6 H(2)O), bipy (3.4 H(2)O), 5,5'-me(2)bipy (4.H(2)O)]. X-ray structure analysis of 1.2 H(2)O and 4.8 CH(3)OH revealed a pair of V(3)O(13)N(4) trimeric units sharing two corners, with a centrosymmetric planar V(6)-core. In addition, a fully oxidized V(V) species [V(V) (4)O(8)(OCH(3))(2)(mu(3)-OCH(3))(2)(5,5'-me(2)bipy)(2)].3 CH(3)OH (5.3 CH(3)OH) was isolated from the reaction mixture used for the synthesis of 4.H(2)O. The crystal structure of 5.3 CH(3)OH revealed a dicubane-like framework with two missing vertices. Electron paramagnetic resonance (EPR) and variable temperature magnetic susceptibility studies for the hexamers 1.4 H(2)O and 3.4 H(2)O showed the complete localization of the single 3d electrons on the V(IV) ions and unusual ferromagnetic interaction between the two paramagnetic vanadium(IV) ions separated by a distance of about 5.1 A. Furthermore, intermolecular antiferromagnetic interactions through pi-contacts of phenyl rings were observed for these species below 8 K. The ferromagnetic exchange coupling observed in the hexanuclear compounds 1.4 H(2)O and 3.4 H(2)O is also discussed using ab initio UHF calculations on a model compound. The value of the exchange coupling constant (3.7 cm(-1)) for this model compound, calculated using the broken symmetry approach, is in good agreement, both in sign and magnitude, with the experimental J values (6.00 cm(-1) for 1.4 H(2)O and 8.54 cm(-1) for 3.4 H(2)O).

We report the synthesis, structures, and magnetic properties of twelve iron(III) phosphonate cages: [Fe(4)(mu(3)-O)Cl(PhCO(2))(3)(PhPO(3))(3)(py)(5)] 1, [Fe(4)(mu(3)-O)((t)BuCO(2))(4)(C(10)H(17)PO(3))(3)(py)(4)] 2 (C(10)H(17)PO(3)H(2) = camphylphosphonic acid), [Fe(7)(mu(3)-O)(2)(PhPO(3))(4)(MeCO(2))(9)(py)(6)] 3, [Fe(7)(mu(3)-O)(2)(PhPO(3))(4)(PhCO(2))(9)(py)(6)] 4, [Fe(7)(mu(3)-O)(2)((t)BuPO(3))(4)((t)BuCO(2))(8)(py)(8)](NO(3)) 5, [Fe(7)(mu(3)-O)(2)(PhPO(3))(4)(MeCO(2))(8)(py)(8)] 6, [Fe(9)(mu(3)-O)(2)(mu(2)-OH)(PhPO(3))(6)((t)BuCO(2))(10)(MeCN)(H(2)O)(5)] 7, [Fe(9)(mu(3)-O)(2)(mu(2)-OH)(C(10)H(17)PO(3))(6)(PhCO(2))(10)(H(2)O)(6)] 8, [Fe(6)(mu(3)-O)(2)(O(2))((t)BuCO(2))(8)(PhPO(3))(2)(H(2)O)(2)] 9, [Fe(6)(mu(3)-O)(2)(O(2))((t)BuCO(2))(8)(C(10)H(17)PO(3))(2)(H(2)O)(2)] 10, [Fe(6)(mu(3)-O)(2)(O(2))((t)BuCO(2))(8)((t)BuPO(3))(2)(py)(2)] 11, and [Fe(14)(mu(3)-O)(4)(O(2))(2)(PhPO(3))(8)((t)BuCO(2))(12)(H(2)O)(12)](NO(3))(2) 12. The results have allowed us to compare the magnetic exchange found with magneto-structural correlations found previously for iron-oxo cages.

High-frequency electron spin echo-electron nuclear double resonance (ESE-ENDOR) spectroscopy is applied to oxovanadium VO 2+ complexes of Hcapca in the form of trans-[VOX(capca)] and to H 2 capcah in the form of cis-[VOX(Hcapcah)], where X ) Clor SCN -. Nuclear quadrupolar coupling constants (nqcc), which are unobtainable by conventional continuous-wave electron paramagnetic resonance (CW-EPR), were measured and reported in terms of P | (P | ) 3e 2 qQ/84 for I ) 7 / 2 ). P | values for trans-[VOCl(capca)] and trans- [VOSCN-(capca)] were calculated to be -0.18 and -0.21 MHz, respectively. In the cases of cis-[VOCl(Hcapcah)] and cis-[VOSCN(Hcapcah), P | values were calculated to be -0.35 and -0.45 MHz, respectively. The experimental results are supported by DFT calculations of quadrupolar and hyperfine couplings for various oxovanadium compounds, including the cis and trans complexes studied by ESE-ENDOR. The charged ligands, coordinated axially trans to the oxo bond, reduce the electric field gradient along the VdO bond, thereby decreasing the observed magnitude of the nuclear quadrupolar coupling constants relative to those of the comparable cis compounds. This experimental finding is confirmed by quantum mechanical calculations. Although the absolute values of quadrupolar splittings cannot be calculated with acceptable accuracy, the observed experimental trends are very well reproduced. Thus, the complementary use of DFT and pulsed-ENDOR is a promising methodology for the study of biologically relevant vanadyl compounds.