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]–.