SUBSTITUENT EFFECTS ON THE PHYSICAL-PROPERTIES OF (MU-OXO)(MU-CARBOXYLATO)BIS(TRIS(2-PYRIDYLMETHYL)AMINECHROMIUM(III)) DIMERS

In order to document the influence of carboxylate substituents on the physical properties of [(tmpa)Cr(mu-0)(mu-RCO2)Cr(tmpa)]3+ dimers, electronic spectra, Cr(III,IV/III,III) half-wave reduction potentials, mu-OH- ionization constants and antiferromagnetic coupling constants are reported for R=CH2Cl, CHCl2, C(CH3)3, C(C6H5)3, 1-adamantyl and Ph-4-X (X=NMe2, OH, OCH3, CH3, CF3, F, Cl, CN, NO,); tmpa=tris(2-pyridylmethyl)amine. Inductive effects of bridging benzoato para substituents are transmitted effectively to the mu-O2- group in [(tmpa)Cr(mu-0)(mu-4-X-PhCO2)Cr(tmpa)]3+ dimers. A linear free energy relationship between E1/2 Cr(III,IV/III,III) and pK(a)(Cr(mu-OH)Cr) indicates a high degree of cooperativity among bridging oxo, complementary bridging and non-bridging ligands with regard to pi-donation towards the Cr(III) centers. Trends in antiferromagnetic coupling constants are interpreted through the influence of configuration interaction between ligand- and metal-centered b1 symmetry orbitals on the HOMO(b1)-LUMO(a2) singlet-triplet energy gap.