Chloride, Alkoxide or Silicon: The Bridging Ligand Dictates the Spin-state in Dicobalt Expanded Pincer Complexes

We report the synthesis and characterization of a series of high andnlow spin dicobalt complexes of the tBuPNNP expanded pincer ligand. Reacting this dinucleating ligand in its neutral form with two equivalents of CoCl2(THF)1.5 yields a high-spin dicobalt complex featuring one Co inside and one Co outside of the dinucleating pocket. Performing the same reaction in the presence of two equivalents of KOtBu provides access to a high-spin dicobalt complex wherein both Co centers are bound within the PNNP pocket and this complex also features a bridging OtBu ligand. Reacting either of the high-spin complexes with excess diethyl silane affords a low-spin dicobalt complex containing two unusual bridging Si-based ligands. These complexes were investigated using NMR spectroscopy, XAS, single crystal X-ray structure determination and computational methods, showing that the Si-based ligands are best described as base-stabilized silylenes.