Synthesis of NPN-Coordinated Tantalum Alkyl Complexes and Their Hydrogenolysis: Isolation of a Terminal Tantalum Hydride Incorporating a Doubly Cyclometalated NPN Scaffold.

The closely related benzylene-linked diaminophosphines PhP(CHCH-o-NHPh) (AH) and PhP(CH-o-CHNHXyl) (BH with Xyl = 3,5-MeCH) were employed for the synthesis of tantalum(V) alkyls, which were then studied with respect to hydrogenolysis. In the case of AH, the tantalum trimethyl complex [Ta(A)Me] (1) and the tantalum hydrocarbyl complex [Ta(A)(CHSiMe)(η-EtC≡CEt)] (2) were prepared from the ligand's dilithium salt (A)Li(diox). Upon hydrogenolysis of 1 and 2, the formation of methane and SiMe, respectively, was observed, but well-defined tantalum hydrides could not be detected. In the case of BH, the cyclometalated species [Ta(B*)(NMe)] (3 with B* = κ-N,P,N,C-(PhP(CH-o-CHNXyl)(CH-o-CHNXyl))) was isolated and converted to the corresponding diiodo species [Ta(B*)I] (4). Treatment of 4 with LiCHSiMe resulted in the isolation of the corresponding dialkyl complex [Ta(B*)(CHSiMe)] (5), which was converted to the doubly cyclometalated monoalkyl complexes [Ta(B**)(CHSiMe)(PMe)] (6 with B** = κ-C,N,P,N,C-(PhP(CH-o-CHNXyl))) and [Ta(B**)(CHSiMe)(dmpe)] (7) via reaction with PMe and dmpe, respectively. In contrast to 5 and 6, 7 was found to react cleanly with dihydrogen to afford the corresponding terminal tantalum(V) hydride [Ta(B**)(H)(dmpe)] (8). Upon reaction of 7 with D, the deuteride [Ta(d-B**)(D)(dmpe)] (9) was obtained and found to contain deuterium atoms in the methine positions of both tantalaaziridine subunits. The partially deuterated derivatives [Ta(B**)(D)(dmpe)] (10) and [Ta(d-B**)(H)(dmpe)] (11) were generated via reaction of 8 and 9 with PhSiD and PhSiH, respectively. Prior to the addition of gaseous D or H, no H/D scrambling was observed in 10 or 11, indicating that the exchange of the methine positions proceeds via addition of D or H across the tantalaaziridine Ta-C bonds.