Divalent Titanium via Reductive N-C Coupling of a Ti^IV Nitrido with π-Acids

The nitrido-ate complex [(PN)₂Ti(N){μ₂-K(OEt₂)}]₂ (1) (PN^-=(N-(2-PⁱPr₂-4-methylphenyl)-2,4,6-Me₃C₆H₂) reductively couples CO and isocyanides in the presence of DME or cryptand (Kryptofix222), to form rare, five-coordinate Ti^II complexes having a linear cumulene motif, [K(L)][(PN)₂Ti(NCE)] (E=O, L=Kryptofix222, (2); E=NAd, L=3 DME, (3); E=N^tBu, L=3 DME, (4); E=NAd, L=Kryptofix222, (5)). Oxidation of 2-5 with [Fc][OTf] afforded an isostructural Ti^III center containing a neutral cumulene, [(PN)₂Ti(NCE)] (E=O, (6); E=NAd (7), N^tBu (8)) and characterization by CW X-band EPR spectroscopy, revealed unpaired electron to be metal centric. Moreover, 1e^- reduction of 6 and 7 in the presence of Kryptofix222cleanly reformed corresponding discrete Ti^II complexes 2 and 5, which were further characterized by solution magnetization measurements and high-frequency and -field EPR (HFEPR) spectroscopy. Furthermore, oxidation of 7 with [Fc*][B(C₆F₅)₄] resulted in a ligand disproportionated Ti^IV complex having transoid carbodiimides, [(PN)₂Ti(NCNAd)₂] (9). Comparison of spectroscopic, structural, and computational data for the divalent, trivalent, and tetravalent systems, including their ¹⁵N enriched isotopomers demonstrate these cumulenes to decrease in order of backbonding as Ti^II→Ti^III→Ti^IV and increasing order of π-donation as Ti^II→Ti^III→Ti^IV, thus displaying more covalency in Ti^III species. Lastly, we show a synthetic cycle whereby complex 1 can deliver an N-atom to CO and CNAd.