Bulky Borylphosphido Complexes of Iron and Cobalt with N-Heterocyclic Carbene as an Ancillary Ligand

Terminal primary phosphido metal complexes are valuable synthetic reagents and intermediates for phosphination reactions and phosphinidene species. Their tendency to aggregate to form bridging phosphido complexes, however, poses a synthetic challenge. Herein, we show that the use of sterically demanding N-heterocyclic boryl-substituted phosphido and N-heterocyclic carbene (NHC) ligands allows access to terminal borylphosphido iron and cobalt complexes. The salt metathesis reactions of [(IAd)MCl(μ-Cl)]2 (M = Fe or Co; IAd = 1,3-diadamantylimidazol-2-ylidene) with 4 equiv of K[PH(NHB)] (NHB = B(N(Dipp)CHCHN(Dipp)); Dipp = 2,6-iPr2C6H3) afford three-coordinate metal primary bis(phosphido) complexes [(IAd)M(PH(NHB))2] (M = Fe (1-Fe) or Co (1-Co)). Spectroscopic, magnetic, and computational studies indicated that 1-Fe adopts an S = 2 ground state and 1-Co features an S = 3/2 state. Reactivity studies revealed that 1-Fe and 1-Co react with 2 equiv of 4-(dimethylamino)pyridine (DMAP) to form tetrahedral complexes [(DMAP)2M(PH(NHB))2] (M = Fe (2-Fe) or Co (2-Co)), and their interaction with the 2,4,6-tri-tert-butylphenoxyl radical (SMesO•, 2 equiv) leads to P–P bond-forming reductive elimination to yield mononuclear iron(II) and cobalt(II) aryloxides [HIAd][Fe(OSMes)3] (3) and [(IAd)Co(OSMes)2] (4), along with diphosphine (NHB)HP-PH(NHB) (5).