Crystal Structure, Physical Properties, and Hydrolysis Kinetics of [(O)(tmpa)V(IV)(&mgr;-O)V(V)(tmpa)(O)](3+).

Abstract

We report the synthesis, crystal structure, physical properties and hydrolysis kinetics of [(O)(tmpa)V(IV)(&mgr;-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)](ClO(4))(3).CH(3)CN crystallizes in the triclinic, P&onemacr; (No. 2) space group with a = 10.719(2) Å, b = 11.609(1) Å, c = 19.516(2) Å, alpha = 87.876(9) degrees, beta = 79.295(9) degrees, gamma = 68.743(9) degrees, and V = 2222.6(6) Å(3); Z = 2. The two vanadium centers are crystallographically equivalent in a linear, oxo-bridged cation with V-O(b) and V=O bond lengths of 1.800(1) and 1.597(4) Å, 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 (51)V 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), CH(3)CN) whose energy is insensitive to variations in the solvent; d-d transitions are observed at 1.25 x 10(4) cm(-1) ((2)E <-- (2)B(2)) and 1.44 x 10(4) cm(-1) ((2)B(1) <-- (2)B(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, CH(3)CN, I = 0.1 M), respectively. The hydrolysis reaction: [V(2)(tmpa)(2)(O)(3)](3+) + H(2)O --> [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; DeltaH() = 71 kJ/mol, DeltaS() = -59 J/mol K). [V(4)O(5)(tmpa)(4){Fe(CN)(6)}](ClO(4))(4).H(2)O was isolated as an unexpected product from the reaction of [V(2)(tmpa)(2)(O)(3)](ClO(4))(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 (CH(3)CN solution).