Crystal Structure, Physical Properties, and Hydrolysis Kinetics of [(O)(tmpa)VIV(μ-O)Vv(tmpa)(O)]3+

Robert A. Holwerda, Bruce R. Whittlesey, Mark J. Nilges

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We report the synthesis, crystal structure, physical properties and hydrolysis kinetics of [(O)(tmpa)VIV-(μ-O)VV(tmpa)(O)3+ (tmpa = tris(2-pyridylmetnyl)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. [V2(tmpa)2(O)3](ClO4) 3·CH3CN crystallizes in the triclinic, P1̄ (No. 2) space group with a = 10.719(2) Å, b = 11.609(1) Å, c = 19.516(2) Å, α = 87.876(9)°, β= 79.295(9)°, γ = 68.743(9)°, and V = 2222.6(6) Å3; Z = 2. The two vanadium centers are crystallographically equivalent in a linear, oxo-bridged cation with V-Ob 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 [V2(tmpa)2(O)3]3+, consistent with coupling of a single electron to two equivalent 51V atoms. The mixed-valence complex exhibits an intervalence transfer (IT) band at 1.05 x 104 cm-1 (∈ = 2.1 x 103 M-1 cm-1, CH3CN) whose energy is insensitive to variations in the solvent; d-d transitions are observed at 1.25 x 104 cm-1 (2E ← 2B2) and 1.44 x 104 cm-1 (2B12B2). [V2(ImPa)2(O)3J2+, [V2(tmpa)2(O)3]3+, and [V2(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 °C, CH3CN, I = 0.1 M), respectively. The hydrolysis reaction: [V2(tmpa)2(O)3]3+ + H2O → [VIV(tmpa)(O)(OH)]+ + [VV(tmpa)(O)2]+ + H+ proceeds with an acid-independent rate constant of 2.37 x 10-3 s-1 (25 °C; ΔH‡ = 71 kJ/mol, ΔS‡ = -59 J/mol K). [V4O5(tmpa)4{Fe(CN)6}](ClO 4)4·H2O was isolated as an unexpected product from the reaction of [V2(tmpa)2(O)3](ClO4) 2·2H2O with equimolar K3[Fe(CN)6] in aqueous solution. It is proposed that pairs of VIV=O and VV=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 104 cm-1 (∈ = 5.3 x 103 M-1 cm-1) and 2.72 x 104 cm-1 (∈ = 9.3 x 103 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).

Original languageEnglish
Pages (from-to)64-68
Number of pages5
JournalInorganic Chemistry
Issue number1
StatePublished - Jan 12 1998


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