Dinuclear Polyhydrides of Rhenium: Isolation and Characterization of Diamagnetic and Paramagnetic Complexes Containing the n+, (n+1)+, and (n+1)+ Cores (n = 0 or 1) Stabilized by Phosphine, Nitrile, and Isocyanide Ligands

Article abstract of DOI:10.1021/ja00313a033

The electrochemical oxidation of the dirhenium octahydrides Re2(μ-H)4H4(PR3)4 (PR3 - PPh3, PEtPh2, or PEt2Ph) and Re2(μ-H)4H4(AsPh3)4 occurs at potentials between -0.15 and -0.40V vs.SCE, to produce the corresponding paramagnetic, ESR-active monocations.This oxidation has been accomplished chemically in the case of the PPh3 derivative using Ph3C⁺PF6^- and C7H7⁺PF6^- as oxidants, dichlormethane as solvent, and a reaction temperature of 0 deg C; the resulting product is PF6.However, when the reaction with Ph3C⁺PF6^- is carried out in nitrile solvents, RCN (R = Me, Et, or Ph), then hydride abstraction occurs in preference to oxidation and the diamagnetic complexes PF6 are formed.The reaction of PF6 with t-BuNC affords the substitution product PF6.The complexes containing the ⁺ moiety have been shown by cyclic voltammetric measurements (on 0.2M Bu4NPF6-CH2Cl2 solutions) to exhibit two quasi-reversible one-electron oxidations (at ca. 0.0 and +0.70 V for the RCN derivatives and +0.19 and +0.94 V vs.SCE for the t-BuCN derivative).Oxidation to paramagnetic (PF6)2 (for L = RCN or t-BuNC) has been achieved by using NO⁺PF6^-.In contrast to the sluggish reaction of Re2(μ-H)4H4(PPh3) with t-BuNC, the oxidized monocation reacts extremely rapidly (in CH2Cl2 at 0 deg C) to give PF6.Like the other polyhydrides this complex possesses two accessible one-electron oxidations, and it may be oxidized chemically to (PF6)2 very easily by using NO⁺PF6^-.While the diamagnetic complexes PF6 and PF6 do not react significantly with excess t-BuNC within reasonable periods of time, their paramagnetic oxidized congeners do so extremely rapidly, the rhenium(I) complex PF6 and gaseous H2 being the final products of these reactions.The complexes desriebed in this report that contain the ⁺, ²⁺, and ²⁺ cores constitute very rare examples of isolable radical cations of transition-metal hydrides, the first to be prepared for dimetal polyhydrides.The highly reactive nature of these species, compared to their far less reactive diamagnetic precursors, provides an important extension of the notion that paramagnetic electron-deficient organometallic species exhibit enhanced substitution lability relative to their 18-electron analogues.