Reactions of ^tBuC⋮P with Cyclooctatetraene-Supported Titanium Imido Complexes

Reaction of the pseudo two-coordinate titanium imido complexes [Ti(NtBu)(COT)] (1; COT = η8-C8H8) and [Ti(NtBu)(COT‘ ‘)] (2; COT‘ ‘ = η8-1,4-C8H6(SiMe3)2) with 2 equiv of tBuC⋮P generated the new complexes [Ti{N(tBu)PC(tBu)PC(tBu)}(COT)] (4) and [Ti{N(tBu)PC(tBu)PC(tBu)}(COT‘ ‘)] (5), respectively. Complex 4 was crystallographically characterized, and a density functional theory (DFT) study combined with photoelectron (PE) spectroscopy revealed this apparently 20-valence-electron species to contain a HOMO which is almost entirely ligand-based. In contrast, the organic compound N(Ar)P2C2tBu2 (6), which incorporates a 1,2,4-azadiphosphole ring, was the only isolated product from the reaction of the arylimido species [Ti(NAr)(COT)] (3; Ar = 2,6-iPr2C6H3) with an excess of tBuC⋮P. DFT studies indicated that the mechanisms for the formation of compounds 4−6 are similar. Initially, one molecule of tBuC⋮P undergoes a [2 + 2] cycloaddition with [Ti(NR)(COT)] to form [Ti{N(R)PC(tBu)}(COT)], which contains a Ti−C bond. Subsequently, a second molecule of tBuC⋮P reacts with [Ti{N(R)PC(tBu)}(COT)] to form [Ti{N(R)PC(tBu)PC(tBu)}(COT)]. When R = 2,6-iPr2C6H3 or a substituent which is less sterically bulky, the formation of a heterocyclic ring such as N(Ar)P2C2tBu2 (6) is favored. However, when R = tBu, it is sterically unfavorable to form such a ring and thus compound 4 is stable.