Reactions of Neutral and Cationic Diamide-Supported Imido Complexes with CO2 and Other Heterocumulenes: Issues of Site Selectivity

Experimental and DFT computational studies of the reactions of the cationic diamide-pyridine-supported methyl tungsten complex [W(NPh)(N2Npy)Me]+ (2+) with the heterocumulenes CO2, CS2, RNCO, and RNCS (R = tert-butyl or aryl) are described, together with comparative studies of the group 5 compounds M(NtBu)(N2Npy)Cl(py) (M = Nb (13) or Ta (14)) and Ta(NtBu)(N2Npy)Me (N2Npy = MeC(2-C5H4N)(CH2NSiMe3)2). In all of the reactions of 2+ the heterocumulene inserted exclusively into a W−Namide bond to give unstable intermediates (isolated for certain isocyanate substrates), which subsequently underwent rearrangement via a 1,3-migration of a SiMe3 group. DFT studies of the reaction of 2+ with CO2 showed that insertion into the W−Me bond is in fact thermodynamically preferred, but that insertion into W−Namide is kinetically more facile. Cycloaddition to the WNPh bond was neither kinetically nor thermodynamically viable. The reactions between Ta(NtBu)(N2Npy)Me and CO2 or RNCO in contrast gave complex mixtures, as did those between 13 and 14 and CO2. DFT studies showed that in a neutral but otherwise identical tantalum analogue of 2+ the preference for substrate attack at the M−Namide bond is much less pronounced. Reactions of 13 and 14 with the more sterically discriminating RNCO (R = tBu or p-tolyl) did, however, exclusively afford M−Namide insertion/1,3-SiMe3 migration products, and Nb(NtBu){MeC(2-C5H4N)(CH2NSiMe3)(CH2NC(OSiMe3)NtBu)}Cl was structurally characterized. The previously reported reaction of Ti(NtBu)(N2Npy)(py) with ArNCO (Ar = 2,6-C6H3iPr2) was re-evaluated and shown to involve an analogous Ti−Namide insertion/1,3-SiMe3 migration reaction, ultimately forming Ti(NtBu){MeC(2-C5H4N)(CH2NSiMe3)(CH2NC(N(Ar)SiMe3)O)}(py)