Synthesis and reactivity of binuclear cyclopentadienylrhodium nitrosyl cations, and the X-ray structure of [Rh2(µ-CO)(µ-NO)(η-C5Me5)2][PF6]

Abstract

The complex [Rh(CO)₂(cp)](cp =η⁵-C₅H₅) reacts with [NO][PF₆] in CH₂Cl₂ to give [Rh₂(CO)₂(µ-NO)(cp)₂][PF₆](1) which is cleaved by ligands, L, to yield [Rh(NO)L(cp)][PF₆](2; L = PPh₃, cyclo-octene, or cyclohexa-1,3-diene) and [Rh(CO)₂(cp)]; the related reaction of (1) with ethylene is reversible. With RCCR, complex (1) affords [Rh₂(CO){µ-N(O)C(R)CR}(cp)₂][PF₆](3; R = Ph or C₆H₄OMe-p)via alkyne insertion into a metal–nitrosyl bond. Slow, thermal decarbonylation of (1) gives [Rh₂(µ-CO)(µ-NO)(cp)₂][PF₆](4), but [NO][PF₆] and [Rh(CO)₂(η-C₅Me₅)] directly afford [Rh₂(µ-CO)(µ-NO)(η-C₅Me₅)₂][PF₆](5), X-ray studies on which revealed a short rhodium–rhodium bond [2.577(1)Å] symmetrically bridged by the nitrosyl [Rh–N(O) 1.95(1)Å] and carbonyl [Rh–C(O) 2.00(2)Å] ligands; the' [PF₆]^–anion is disordered. Crystals of (5) are monoclinic, space group A2, and the structure has been refined to R 0.034 for 1 519 reflections [I 1.5σ(I)]. Complex (5) forms a 1: 1 adduct with PPh₃, namely [Rh₂(µ-CO)(µ-NO)(PPh₃)(η-C₅Me₅)₂][PF₆](6), and reacts with a second equivalent of [NO][PF₆] to yield [{Rh(µ-NO)(η-C₅Me₅)}₂][PF₆]₂(7). At a platinum electrode, in CH₂Cl₂, (5) undergoes reversible one-electron reduction to [Rh₂(µ-CO)(µ-NO)(η-C₅Me₅)₂]. Similarly, [{Rh(µ-NO)(cp)}₂] is reduced in tetrahydrofuran to the monoanion [{Rh(µ-NO)(cp)}₂]^–.