Heterogeneous Electron-Transfer Rate Constants for M₂(O₂CR)₄^0/+, Where M = Mo, W, Ru, or Rh and R = Alkyl or Aryl

Abstract

By the use of Nicholson's method, the heterogeneous electron-transfer rate constants (ks) for the oxidation of a series of M2(O2CR)4 complexes have been determined in benzonitrile, where the metal M = Mo, W, Ru, or Rh and R = alkyl or aryl. For R = tBu, the values of ks follow the order M = Mo > W > Ru > Rh. No simple influence of R on ks was observed, although added ligands that are known to reversibly bind to the dinuclear center were shown to influence the E1/2 values in order of their basicity and to suppress the rate of electron transfer. The reported data are compared with those obtained for Cp2Fe0/+, Cp2*Fe0/+, and Ru(bpy)2(2)+/3+ and with earlier work on dirhenium multiply bonded compounds.

Observation of 1MLCT and 3MLCT Excited States in Quadruply Bonded Mo2 and W2 Complexes

“…Owing to steric hindrance, this dihedral twist is expected to persist in solution, as is also predicted by the structural optimization. A similar increase in oxidation potential was reported for Mo 2 (O 2 C−2,6-Me 2 −C 6 H 3 ) 4 , a complex in which the 2,6-Me 2 −C 6 H 3 unit is twisted with respect to the O 2 C plane, relative to the conjugated Mo 2 (O 2 C−3,5-Me 2 −C 6 H 3 ) 4 . As reported previously, the W 2 complex 5 is significantly easier to oxidize than the corresponding Mo 2 complex 1 …”

“…The 1 δδ* transition in Mo 2 (O 2 C−R) 4 (R = H, alkyl, aryl) complexes is weak and is typically observed at ∼436 nm (ε ∼ 100 M -1 cm -1 ). ,, The broad absorption observed in the 400−600 nm region for the Mo 2 complexes in Table exhibits molar extinction coefficients of ∼10 4 M -1 cm -1 . In the related Mo 2 (O 2 C−R) 4 (R = C 6 H 5 , substituted phenyl), intense transitions in the visible region (ε ∼ 10 4 M -1 cm -1 ) have previously been assigned as spin-allowed metal-to-ligand charge transfer ( 1 MLCT) arising from Mo 2 δ → O 2 C π*. ,,,− The red-shift in the lowest energy transition along the series 1 , 2a and 2b , and 4a is consistent with this assignment (Table ); these complexes possess similar oxidation potentials, with the electron being removed from their Mo 2 δ HOMO, and reduction potentials that parallel the energy of the aryl π* LUMO. Although the anthracene ring in 3 is easier to reduce than the benzene, 1-naphthalene, and 2-naphthalene rings in 1 , 2a , and 2b , respectively, the measured oxidation potential of 3 indicates a greater stabilization of the δ HOMO, thus shifting the Mo 2 δ → O 2 C π* 1 MLCT transition to an energy similar to those observed for 2a and 2b .…”

“…For the Mo 2 complexes, with the exception of complex 3 , the oxidation potential is relatively insensitive to the aryl substituent of the carboxylate ligands and is observed in the 0.50−0.57 V vs SCE range (Table ). As previously reported, the oxidation from the HOMO Mo−Mo δ orbital is slightly more difficult for Mo 2 (O 2 C−R) 4 complexes, where R = aryl than R = alkyl ( E 1/2 ∼ 0.49 V vs SCE). , In addition, it has also been shown that the stability of the δ orbital increases with the electron-withdrawing ability of the substituent R‘ in Mo 2 (O 2 C- p -R‘-C 6 H 4 ) 4 , thus making it more difficult to remove an electron from the bimetallic core for R‘ = NO 2 than R‘ = OCH 3 . , Complex 3 is more difficult to oxidize than the other Mo 2 carboxyaryl systems listed in Table by ∼0.2 V. The crystal structure shows that the anthracene part of each ligand in 3 is twisted with respect to the Mo 2 O 2 C plane, thus decreasing the conjugation between the O 2 C unit and the anthracene. Owing to steric hindrance, this dihedral twist is expected to persist in solution, as is also predicted by the structural optimization.…”

Engineering radical polymer electrodes for electrochemical energy storage

“…Radical polymers generally exhibit rapid heterogeneous and self-exchange rate constants. The heterogeneous rate constants (or the redox reaction rates) of TEMPO and poly(4-nitronylnitroxylstyrene) are around an order of magnitude faster (10 −1 cm/s) than those for metal complexes (10 −3 -10 −2 cm/s) [220,221] and many orders of magnitude faster than for disulfides (10 −8 cm/s) [63,126,148,218]. Additionally, the electron self-exchange rate constant for TEMPO [129], poly(PROXYL-ethylene oxide) [129], and poly(TEMPO-norbornene) cross-linked by coumarin [152] are approx.…”

Quadruply Bonded Dimolybdenum Atoms Surrounded by Dendrons: Preparation, Characterization, and Electrochemistry

“…The reversibility at the electrode for the oxidation in these carboxylate systems, as indicated by the fact that ( i pc / i pa ) is close to unity for D0 − D2 (for the rate of scanning that accounts best for molecular diffusion away from the working electrode and rate of electron transfer), stands in contrast to the quasireversible waves previously reported for other dimolybdenum tetracarboxylate-bridged (DMTC) systems . Since the voltammogram for D0 obtained in THF was quasireversible ( E 1/2 /mV = 479 mV, Δ E p /mV = 274 mV), this may simply be a reflection of the stability afforded by using benzonitrile (recently shown to be a constructive solvent for these DMTC systems) as the solvent for the voltammetric measurements. Interestingly enough, the cyclic voltammogram for Mo 2 (acetate) 4 measured in benzonitrile, previously reported as quasireversible in dmf and described as similar to the quasireversible results obtained for dimolybdenum tetrabutyrate 21c which were measured in CH 3 CN, CH 2 Cl 2 , and EtOH, also consisted of a reversible oxidation, Table .…”