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

Abstract

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 Rh(esp)C( p-MeOPh) (19) and BiRh(esp)C( p-MeOPh) (20), employing a combination of UV-vis and resonance Raman spectroscopy. The results reveal that carbene chemoselectivity in MRh(L) catalysis can be modulated to a previously unrecognized extent by the distal metalloligand.

Chiral Heterobimetallic Bismuth–Rhodium Paddlewheel Catalysts: A Conceptually New Approach to Asymmetric Cyclopropanation

“…The structure of 5 a ⋅EtOAc in the solid state (Figure ) proves that the co‐crystallized EtOAc is bound to rhodium, whereas the bismuth center is unligated. This finding concurs with literature data which suggest that the Bi II atom exhibits hardly any Lewis acidity . The ligand sphere adopts the α,α,α,α‐conformation, presumably because this arrangement places the tert ‐butyl groups as the most bulky substituents at maximum distance from one another.…”

“…Specifically, combined experimental, spectroscopic and computational studies showed that formal replacement of one Rh II center in the bimetallic core by Bi II enhances the electrophilic character of the resulting carbenes to a significant extent, although the rate of metal–carbene formation is manifestly slower . More important in the present context is the fact that the bismuth site seems to lack any notable Lewis acidity and proved incapable of decomposing ethyl diazoacetate . Even though the ionic radius of bismuth is larger than that of rhodium and the bismuth center hence certainly more exposed, any deleterious background reaction should cease.…”

“…Outlined herein is a conceptually different approach to catalyst optimization which builds upon recent insights into structure and bonding in heterobimetallic paddlewheel carbene complexes (Scheme ) . Specifically, combined experimental, spectroscopic and computational studies showed that formal replacement of one Rh II center in the bimetallic core by Bi II enhances the electrophilic character of the resulting carbenes to a significant extent, although the rate of metal–carbene formation is manifestly slower .…”

Dirhodium(II,II) Paddlewheel Complexes

“… [68] Such complexes are used in catalysis and their reactivity is compared with dirhodium(II,II) analogs to study the effect of metal‐metal interactions [69] . The enhanced electrophilicity of the rhodium atom in Bi‐Rh paddlewheel complexes (due to interaction between 4 d orbitals of rhodium and 6 p orbitals of Bi, also resulting in reduced π ‐back bonding to the species bound in axial position) can lead in some cases to different reaction outcomes as demonstrated on the catalyzed insertion of carbenes into the molecule of dichloromethane [70] …”

Chiral Heterobimetallic Bismuth–Rhodium Paddlewheel Catalysts: A Conceptually New Approach to Asymmetric Cyclopropanation

“…Outlined herein is a conceptually different approach to catalyst optimization which builds upon recent insights into structure and bonding in heterobimetallic paddlewheel carbene complexes (Scheme ) . Specifically, combined experimental, spectroscopic and computational studies showed that formal replacement of one Rh II center in the bimetallic core by Bi II enhances the electrophilic character of the resulting carbenes to a significant extent, although the rate of metal–carbene formation is manifestly slower . More important in the present context is the fact that the bismuth site seems to lack any notable Lewis acidity and proved incapable of decomposing ethyl diazoacetate .…”