Physical properties and hydrolysis kinetics of (μ-oxo)bis [tris(2pyridylmethyl)aminechromium(III)] dimers with complementary sulfate or molybdate bridging ligands

Abstract The complexes [(tmpa)Cr(μ-O)(μ-EO 4 )Cr(tmpa)](ClO 4 ) 2 · n H 2 O (E = S, n = 1; E = Mo, n = 0; tmpa = tris(2-pyridylmethyl)amine) have been prepared through the displacement of water from [Cr(tmpa)(OH)]2(ClO 4 ) 4 · 4H 2 O by the tetrahedral oxyanions in CH 3 CN solution. Electronic spectra, asymmetric CrOCr stretching frequencies, half-wave Cr(III,IV/III,III) reduction potentials and 1 H NMR features of the O 2− , SO 4 2− and O 2− , MoO 4 2− -bridged dimers are essentially identical to those of [Cr(tmpa)CI] 2 O 2+ , consistent with structures having near-linear CrOCr bridging units and symmetric tmpa chelation placing both apical nitrogen atoms trans to oxyanion oxygen atoms. The title complexes undergo biphasic hydrolysis reactions in the pH 4.80–7.80 interval. The fast phase rate exhibits no pH dependence, while k slow increases with pH in a way that suggests the presence of both protonated and deprotonated reactant species. Since these findings mirror those reported previously for [Cr(tmpa)CI] 2 O 2+ hydrolysis, we propose a closely related mechanism featuring H 2 O-induced ring-opening of the bridging EO 4 2− group in the first step ( k fast ) followed by loss of EO 4 2− from two intermediates, [(EO 4 )(tmpa)CrOCr(tmpa)(H 2 O)] 2+ ( k a ) and [(EO 4 )(tmpa)CrOCr(tmpa)(OH)] + ( k b ), related by a rapid ionization equilibrium ( K a ). Reported k fast , k a , k b and p K a parameters (25.0°C, I = 0.1 M) are 1.7 × 10 −1 , 6.8 × 10 −3 , 7.98 × 10 −2 s −1 , 6.66 (SO 4 2− ) and 1.6 × 10 −1 , 6.4 × 10 −3 , 8.01 × 10 −2 s −1 , 6.76 (MoO 4 2− ), respectively. Our findings for both fast and slow steps in the hydrolysis of Cl − , SO 4 2− and MoO 4 2− are consistent with a pathway in which bond-making to the incoming water ligand and bond-breaking is promoted specifically by a cis -labilizing influence of the oxo bridge.