Twinned versus linked organometallics - bimetallic “half-baguette” pentalenide complexes of Rh(i)

Abstract

The application of Mg[Ph₄Pn] and Li·K[Ph₄Pn] in transmetalation reactions to a range of Rh(I) precursors led to the formation of “half-baguette” anti-[Rh^I(L)_n]₂[μ:η⁵:η⁵Ph₄Pn] (L = 1,5-cyclooctadiene, norbornadiene, ethylene; n = 1, 2) and syn-[Rh^I(CO)₂]₂[μ:η⁵:η⁵Ph₄Pn] complexes as well as the related iridium complex anti-[Ir^I(COD)]₂[μ:η⁵:η⁵Ph₄Pn]. With CO exclusive syn metalation was obtained even when using mono-nuclear Rh(I) precursors, indicating an electronic preference for syn metalation. DFT analysis showed this to be the result of π overlap between the adjacent M(CO)₂ units which overcompensates for d_z² repulsion of the metals, an effect which can be overridden by steric clash of the auxiliary ligands to yield anti-configuration as seen in the larger olefin complexes. syn-[Rh^I(CO)₂]₂[μ:η⁵:η⁵Ph₄Pn] is a rare example of a twinned organometallic where the two metals are held flexibly in close proximity, but the two d⁸ Rh(I) centres did not show signs of M–M bonding interactions or exhibit Lewis basic behaviour as in some related mono-nuclear Cp complexes due to the acceptor properties of the ligands. The ligand substitution chemistry of syn-[Rh^I(CO)₂]₂[μ:η⁵:η⁵Ph₄Pn] was investigated with a series of electronically and sterically diverse donor ligands (P(OPh)₃, P(OMe)₃, PPh₃, PMe₃, dppe) yielding new mono- and bis-substituted complexes, with E-syn-[Rh^I(CO)(P{OR})₃]₂[μ:η⁵:η⁵Ph₄Pn] (R = Me, Ph) characterised by XRD.