Investigating Metal-Metal Bond Polarization in a Heteroleptic Tris-Ylide Diiron System

This article describes the synthesis, characterization, and S-atom transfer reactivity of a series of C₃-symmetric diiron complexes. The iron centers in each complex are coordinated in distinct ligand environments, with one (Fe_N) bound in a pseudo-trigonal bipyramidal geometry by three phosphinimine nitrogens in the equatorial plane, a tertiary amine, and the second metal center (Fe_C). Fe_C is coordinated, in turn, by Fe_N, three ylidic carbons in a trigonal plane, and, in certain cases, by an axial oxygen donor. The three alkyl donors at Fe_C form through the reduction of the appended N═PMe₃ arms of the monometallic parent complex. The complexes were studied crystallographically, spectroscopically (NMR, UV-vis, and Mössbauer), and computationally (DFT, CASSCF) and found to be high-spin throughout, with short Fe-Fe distances that belie weak orbital overlap between the two metals. Further, the redox nature of this series allowed for the determination that oxidation is localized to the Fe_C. S-atom transfer chemistry resulted in the formal insertion of a S atom into the Fe-Fe bond of the reduced diiron complex to form a mixture of Fe₄S and Fe₄S₂ products.