Stepwise hapticity changes in sequential one-electron redox reactions of indenyl-molybdenum complexes: combined electrochemical, ESR, X-ray and theoretical studies

Reduction of the dication [(eta5-Ind)(Cp)Mo{P(OMe)3}2]2+(12+) and oxidation of the neutral complex(eta3-Ind)(Cp)Mo{P(OMe)3}2(1) proceed through a one-electron intermediate, 1+. The structures of 12+ and 1 have been determined by X-ray diffraction studies, which show the slip-fold distortion angle, Omega, of the indenyl ring increasing from 4.1° in 12+ to 21.7° in 1. Cyclic voltammetry and bulk electrolysis were employed to define the thermodynamics and heterogeneous charge-transfer kinetics of reactions 12+ + e- <-> 1+ and 1+ +e- <-> 1:DeltaE1/2 =113 mV in CH3CN and 219 mV in CH2Cl2/0.1 M [NBu4][PF6]; ks =0.4 cm s-1for 12+/1+ couple, 1.0 cm s-1 for 1+/1 couple in CH3CN. ESR spectra of 1+ displayed a surprisingly large hyperfine splitting (7.4 x 10-4 cm-1) from a single 1H nucleus, and spectra of the partially deuterated indenyl analogue confirmed assignment of aH to the H(2) proton of the indenyl ring. The related eta5 18-electron complexes [(eta5-Ind)(Cp)Mo(dppe)]2+(22+) (dppe)=diphenylphosphinoethane) and (eta5Ind)(Cp)Mo(CN)2(3) may also be reduced in two successive one-electron steps; ESR spectra of the radicals 2+ and 3- showed a similarly large aH(2) (8.7 x 10-4 and 6.4 x 10-4 cm-1, respectively). Molecular orbital calculations (density functional theory, DFT, and extended Huckel, EH) predict metal-indenyl bonding in 1+ that is approximately midway between that of the eta5 and eta3 hapticities (e.g., Omega = 11.4°). DFT results show that the large value of aH(2) arises from polarization of the indenyl-H(2) by both inner-sphere orbitals and the singly occupied molecular orbital (SOMO) of 1+. The measured ks values are consistent with only minor inner-sphere reorganizational energies being necessary for the electron-transfer reactions, showing that a full eta5/eta3 hapticity change may require only small inner-sphere reorganization energies when concomitant with a pair of stepwise one-electron-transfer processes. The indenyl ligand in 1+ is best described as donating approximately four pi-electrons to Mo by combining a traditional eta3 linkage with two “half-strength” Mo-C bonds.