Steric Effects in the Oxidative Addition of MeI to a Sulfido-Bridged ZrRh2 Early–Late Heterobimetallic Compound

Reaction of the early–late heterobimetallic compound [Cptt2Zr­(μ3-S)2{Rh­(CO)}2(μ-dppm)] (1) with MeI affords the unusual oxidative addition product [Cptt2Zr­(μ3-S)2Rh2(μ-CO)­(μ-dppm)­(I)­(COCH3)] (4), showing the presence of a bridging carbonyl and a terminal acetyl ligand. The optimized structure of 4 by DFT calculations is further substantiated by the spectroscopic data of the 13CO-labeled complex 4*. The same reaction carried out on the structurally related gem-dithiolate dinuclear complexes [Rh2(μ-S2CR2)­(CO)2(PPh3)2] (2, 3) gives the one-center oxidative addition acetyl products [Rh2(μ-S2CR2)­(COCH3)­(I)­(CO)­(PPh3)2] (R2 = −(CH2)5–, 5; R = Bn (benzyl), 6). Reaction of 3 with molecular iodine affords the dinuclear compounds [Rh2(μ-S2CBn2)­(I)2(μ-CO)­(PPh3)2] (7) and [Rh2(μ-S2CBn2)­(I)2(CO)2(PPh3)2] (8), the transannular oxidative addition product, in a 40:60 ratio. Compound 7 was also formed in the reaction of 3 with MeI after a prolonged reaction time. The molecular structures of compounds 5 and 7 have been determined by X-ray analyses. A natural bond orbital (NBO) analysis has shown an analogous bonding scheme in the “Rh2(μ-CO)­P2” subunit of complexes 4 and 7. Although both complexes can be formally described as composed of a Rh­(III)–Rh­(III) unit, assuming a ketonic character of the bridging carbonyl ligand, the natural charges on the rhodium atoms and the WBI indexes for the Rh–Rh and CO bonds points to electronically unsaturated metal centers bridged by a carbonyl ligand lacking ketonic character and a significant metal–metal interaction. In addition, DFT calculations on the reaction pathway leading to the formation of 4 evidenced the key role of the bulky “Cptt2Zr” fragment in the course of the reaction.