Cationic Tetrylene-Iron(0) Complexes: Access Points for Cooperative, Reversible Dihydrogen Activation and Open-Shell Iron(-I) Ferrato-Tetrylenes

Abstract

The oxidative addition of catalytically relevant small molecules in molecular iron complexes poses a considerable challenge in achieving ‘precious metal catalysis’ utilizing this Earth abundant metal. Here, we show that non-innocent ligands based upon cationic heavier tetrylenes, EII (E = Ge, Sn), can work in synergy with a reactive iron center for the oxidative cleavage of inert bonds. Specifically, the open-shell cationic stannylene-iron(0) complex 4 (4 = [PhiPDippSn·Fe·IPr]+; PhiPDipp = {[Ph2PCH2Si(iPr)2](Dipp)N}; Dipp = 2,6-iPr2C6H3; IPr = [(Dipp)NC(H)]2C:) cleaves dihydrogen under very mild condi-tions (1.5 bar, 298K), in forming bridging hydrido-complex 6, which features a [Sn-(μ-H)2-Fe] core. This reaction is readily reversible, with hydrogen being entirely extruded after simple freeze-thaw degassing of reaction mixtures, regenerating 4. Computational investigation of the mechanism incites the ne-cessity of both the Fe0 and SnII centers in the key H-H bond scis-sion step. The related GeII system, 3, does not activate dihydro-gen. However, one-electron reduction of this species leads to clean oxidative addition of one C-P linkage of the PhiPDipp ligand in an intermediary Fe-I complex, leading to FeI phosphide species 7. In contrast, the same one-electron reduction reaction of 4 gives facile access to the iron(-I) ferrato-stannylene, 8. This presents strong evidence for the intermediacy of such a species in the reduction of 3, and represents an example of a covalently bound formal iron(-I) compound. EPR spectroscopy, SQUID magnetometry, and supporting computational analysis strongly indicate the high localization of electron spin density at Fe-I in this unique d9-iron complex.