Synthesis and crystal structures of the dimetal compounds [CoRh(CO)2(PPh3)(η4-C4Me4)(η5-C2B9H11)], [Rh2(CO)2(PPh3)2(η5-C2B9H11)], and [RhIrH(µ-σ : η5-C2B9H10)(CO)3(PPh3)2]

Abstract

The salt [NEt₄][Rh(CO)(PPh₃)(η⁵-C₂B₉H₁₁)](1) reacts with the complexes [Co(CO)₂(NCMe)(η⁴-C₄Me₄)][PF₆], [Rh₂(µ-Cl)₂(CO)₄], and [IrCl(CO)₂(NH₂C₆H₄Me-4)] to afford, respectively, the dimetal compounds [CoRh(CO)₂(PPh₃)(η⁴-C₄Me₄)(η⁵-C₂B₉H₁₁)](2), [Rh₂(CO)₂(PPh₃)₂(η⁵-C₂B₉H₁₁)](3), and [RhIrH(µ-σ : η⁵-C₂B₉H₁₀)(CO)₃(PPh₃)₂](4). Complex (3) is also obtained in an unusual reaction involving addition of BH₃·thf (thf = tetrahydrofuran) to (2). Treatment of (4) with K[BH(CHMeEt)₃] in the presence of NEt₄Cl gives the salt [NEt₄][RhIr(µ-σ : η⁵-C₂B₉H₁₀)(CO)₃(PPh₃)₂], a process which can be reversed by addition of HBF₄·Et₂O to the latter. The structures of complexes (2)–(4) have been established by single-crystal X-ray diffraction studies. In (2) and (3) the metal–metal bonds [Co–Rh 2.746(3), Rh–Rh 2.692(3)Å] are bridged by the C₂B₉H₁₁ group such that the latter is η⁵co-ordinated to a rhodium atom while forming a three-centre two-electron B–H⇀M (M = Co or Rh) with the other metal centre. However, whereas in (2) it is a BH group which is α to a CH fragment in the pentagonal face of the cage which forms the B–H⇀Co bond, in (3), the B–H⇀Rh bond is formed by a BH group which is β to a CH group. In complex (3) both metal centres are co-ordinated by a CO and a PPh₃ ligand. This is also true for the rhodium atom in (2), while the cobalt atom in the latter carries the tetra-methylcyclobutadiene ring and a CO group. Compound (4) has a Rh–Ir bond [2.781(1)Å] bridged by a C₂B₉H₁₀ moiety. The latter is η⁵ co-ordinated to the rhodium atom and forms an exopolyhedral B–Ir bond using a boron atom in the pentagonal face of the ligand which is α to a CH fragment. The iridium centre is ligated by two CO groups, and by a PPh₃ and a hydrido ligand, while the rhodium atom carries a CO and a PPh₃ group. The n.m.r. spectra (¹H, ¹³C-{¹H}, ³¹P-{¹H}, and ¹¹B-{¹H}) of the new compounds are reported and discussed.