Crystal structure, physical properties, and hydrolysis kinetics of [(O)(tmpa)VIV(μ-O)Vv(tmpa)(O)]3+

We report the synthesis, crystal structure, physical properties and hydrolysis kinetics of [(O)(tmpa)V-IV-(u-O)V-V(tmpa)(O)](3+) (tmpa = tris(2-pyridylmethyl)amine), the first cation in its class for which both solid state and solution-phase characteristics are fully consistent with expectations for a Robin-Day type III mixed-valence complex. [V-2(tmpa)(2)(O)(3)](ClO4)(3) . CH3CN crystallizes in the triclinic, <P(1)over bar> (No. 2) space group with a = 10.719(2) Angstrom, b = 11.609(1) Angstrom, c = 19.516(2) Angstrom, alpha = 87.876(9)degrees, beta = 79.295(9)degrees, gamma = 68.743(9)degrees and V = 2222.6(6) Angstrom(3); Z = 2. The two vanadium centers are crystallographically equivalent in a linear, ore-bridged cation with V-O-b and V=O bond lengths of 1.800(1) and 1.597(4) Angstrom, respectively. A 15-hyperfine line EPR spectrum (g = 1.9704, A = 46.4 x 10(-4) cm(-1)) was observed for [V-2(tmpa)(2)(O)(3)](3+), consistent with coupling of a single electron to two equivalent V-51 atoms. The mixed-valence complex exhibits an intervalence transfer (IT) band at 1.05 x 10(4) cm(-1) (epsilon = 2.1 x 10(3) M-1 cm(-1), CH3CN) whose energy is insensitive to variations in the solvent; d-d transitions are observed at 1.25 x 10(4) cm(-1) (E-2 <-- B-2(2)) and 1.44 x 10(4) cm(-1) (B-2(1), B-2(2)). [V-2(tmpa)(2)(O)(3)](2+), [V-2(tmpa)(2)(O)(3)](3+), and [V-2(tmpa)(2)(O)(3)](4+) comprise a redox family related by vanadium (IV, V/IV, IV) and (V, V/IV, V) half-wave reduction potentials of 0.25 and 1.62 V vs NHE (25 degrees C, CH3CN, I = 0.1 M), respectively. The hydrolysis reaction: [V-2(tmpa)(2)(O)(3)](3+) + H2O --> [V-IV(tmpa)(O)(OH)](+) + [V-V(tmpa)(O)(2)](+) + H+ proceeds with an acid-independent rate constant of 2.37 x 10(-3) s(-1) (25 degrees C; Delta H double dagger = 71 kJ/mol, Delta S double dagger = -59 J/mol K). [V4O5(tmpa)(4){Fe(CN)(6)}](ClO4)(4) . H2O was isolated as an unexpected product from the reaction of [V-2(tmpa)(2)(O)(3)](ClO4)(2) . 2H(2)O with equimolar K-3[Fe(CN)(6)] in aqueous solution. It is proposed that pairs of V-IV = O and V-V = O units aggregate with [Fe(CN)(6)](4-) into a charge transfer complex. Hexacyanoferrate(II)-to-vanadium IT charge transfer bands are observed at 1.98 x 10(4) cm(-1) (epsilon = 5.3 x 10(3) M-1 cm(-1)) and 2.72 x 10(4) cm(-1) (epsilon = 9.3 x 10(3) M-1 cm(-1)); a 29-line EPR spectrum (g = 1.9737, A = 22.0 x 10(-4) cm(-1)) demonstrates equal delocalization of unpaired electron density over all four V atoms (CH3CN solution).