Enhanced Electrophilicity of Heterobimetallic Bi–Rh Paddlewheel Carbene Complexes: A Combined Experimental, Spectroscopic, and Computational Study

Dirhodium
paddlewheel complexes are indispensable tools in modern organometallic
catalysis for the controlled decomposition of diazo-compounds. Tuning
the reactivity of the thus-formed transient carbenes remains an active
and dynamic field of research. Herein, we present our findings that
the distal metal center plays an as yet underappreciated role in modulating
this reactivity. Replacement of one rhodium atom in the bimetallic
core for bismuth results in the formation of a significantly more
electrophilic carbene complex. Bismuth-rhodium catalysts thereby facilitate
previously unknown modes of reactivity for α-diazoester compounds,
including the cyclopropanation of alkenes as electron deficient as
trichloroethylene. While dirhodium paddlewheel complexes remain the
catalysts of choice for many carbene-mediated transformations, their
bismuth-rhodium analogues exhibit complementary reactivity and show
great potential for small molecule and solvent activation chemistry.
DFT calculations highlight the importance of metal–metal bonding
interactions in controlling carbene electrophilicity. The paucity
of these interactions between the 4d orbitals of rhodium and the 6p
orbitals of bismuth results in weaker π-back-bonding interactions
for bismuth-rhodium carbene complexes compared to dirhodium carbene
complexes. This leads to weakening of the rhodium-carbene bond and
to a more carbene-centered LUMO, accounting for the observed enhancement
in bismuth-rhodium carbene electrophilicity. These findings are supported
by a detailed spectroscopic study of the “donor–donor”
carbene complexes Rh2(esp)2C­(p-MeOPh)2 (19) and BiRh­(esp)2C­(p-MeOPh)2 (20), employing a combination
of UV–vis and resonance Raman spectroscopy. The results reveal
that carbene chemoselectivity in MRh­(L)4 catalysis can
be modulated to a previously unrecognized extent by the distal metalloligand.