ABSTRACT
Reduction of the dication [(eta5-Ind)(Cp)Mo[P(OMe)3]2]2+ (1(2+)) 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 1(2+) 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 degrees in 1(2+) to 21.7 degrees in 1. Cyclic voltammetry and bulk electrolysis were employed to define the thermodynamics and heterogeneous charge-transfer kinetics of reactions 1(2+) + e(-) <==> 1+ and 1+ + e(-) <==> 1: DeltaE1/2 = 113 mV in CH3CN and 219 mV in CH2Cl2/0.1 M [NBu4][PF6]; k(s) = 0.4 cm x s(-1) for 1(2+)/1+ couple, 1.0 cm x s(-1) for 1+/1 couple in CH3CN. ESR spectra of 1+ displayed a surprisingly large hyperfine splitting (7.4 x 10(-4) x cm(-1)) from a single 1H nucleus, and spectra of the partially deuterated indenyl analogue confirmed assignment of a(H) to the H2 proton of the indenyl ring. The related eta5 18-electron complexes [(eta5-Ind)(Cp)Mo(dppe)]2+ (2(2+)) (dppe = diphenylphosphinoethane) and (eta5-Ind)(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 a(H2) (8.7 x 10(-4) and 6.4 x 10(-4) x cm(-1), respectively). Molecular orbital calculations (density functional theory, DFT, and extended Hückel, EH) predict metal-indenyl bonding in 1+ that is approximately midway between that of the eta5 and eta3 hapticities (e.g., Omega = 11.4 degrees ). DFT results show that the large value of a(H2) arises from polarization of the indenyl-H2 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.
ABSTRACT
A single-crystal X-ray determination of the [Li(CH(3)CN)(2)(+)](6-CH(3)-nido-5,6,9-C(3)B(7)H(9)(-)) salt has shown that the 6-CH(3)-nido-5,6,9-C(3)B(7)H(9)(-) tricarbadecaboranyl anion has a nido-cage geometry based on an octadecahedron missing the unique six-coordinate vertex. The resulting six-membered open face is puckered, with two of the cage carbons (C6 and C9) occupying the low-coordinate cage positions above the plane of the four remaining atoms (C5, B7, B8, and B10). The Li(+) ion is centered over the open face and is solvated by two acetonitrile molecules. The reactions of the 6-CH(3)-nido-5,6,9-C(3)B(7)H(9)(-) anion with various vanadium halide salts, including VCl(4), VCl(3), and VBr(2), each resulted in the isolation of the same five paramagnetic products (2-6) of composition V(CH(3)-C(3)B(7)H(9))(2). X-ray crystallographic determinations of 2-5 showed that the complexes consist of two octadecahedral VC(3)B(7) fragments sharing a common vanadium vertex and established their structures as commo-V-(1-V-4'-CH(3)-2',3',4'-C(3)B(7)H(9))(1-V-2-CH(3)-2,3,4-C(3)B(7)H(9)) (2), commo-V-(1-V-5'-CH(3)-2',3',5'-C(3)B(7)H(9))(1-V-4-CH(3)-2,3,4-C(3)B(7)H(9)) (3), commo-V-(1-V-5'-CH(3)-2',3',5'-C(3)B(7)H(9))(1-V-2-CH(3)-2,3,4-C(3)B(7)H(9)) (4), and commo-V-(1-V-2-CH(3)-2,3,4-C(3)B(7)H(9))(2) (5). These complexes can be considered as tricarbadecaboranyl analogues of vanadocene, (eta(5)-C(5)H(5))(2)V. However, unlike vanadocene, these complexes are air- and moisture-stable and have only one unpaired electron. The five complexes differ with respect to one another in that they either (1) contain different enantiomeric forms of the CH(3)-C(3)B(7)H(9) cages, (2) have a different twist orientation of the two cages, or (3) have the methyl group of the CH(3)-C(3)B(7)H(9) cage located in either the 2 or 4 position of the cage. Subsequent attempts to oxidize the compounds with reagents such as Br(2) and Ag(+) were unsuccessful, illustrating the ability of the tricarbadecaboranyl anion to stabilize metals in low oxidation states. Consistent with this, both the electrochemical oxidation and the reduction of 2 were much more positive than those of the same oxidation state changes in vanadocene. The one-electron reduction of 2 is a remarkable 2.9 V positive of that of Cp(2)V.
