“Chiral-at-Metal” Hemilabile Nickel Complexes with a Latent d10-ML2 Configuration: Receiving Substrates with Open Arms

Complexes with highly reactive stereogenic metal centers are of great interest to asymmetric synthesis. Thus, by reacting [Ni­(COD)2] with 2 equiv of the P-alkene ligand (S)-5 ((S)-(+)-N-(3,5-dioxa-4-phosphacyclohepta­[2,1-a;3,4-a′]­dinaphthalen-4-yl)­dibenz­[b,f]­azepine) or (SP,SC)-6 ((2S,5S)-(-)-N-(aza-3-oxa-2-phosphabicyclo­[3.3.0]­octan-4-on-2-yl)­dibenz­[b,f]­azepine), the diastereomerically and enantiomerically pure tetrahedral complexes (Δ,S,S)-[Ni­(5-κP,η2-alkene)2] (2a) and (Δ,SP,SC,SP′,SC′)-[Ni­(6-κP,η2-alkene)2] (2b) were obtained in almost quantitative yields on multigram scales. The Ni atoms showed in both cases stable Δ configurations. Even though these Ni(0) complexes were air stable in the solid state, once dissolved, complex 2a readily activated CS2, alkynes, and enones as the formal d10-ML2 fragment [Ni­(5-κP)2] (4) to form adducts 8–11. This is possible thanks to the decoordination of the hemilabile alkene arms of the P-alkene ligands, and the X-ray crystal structures of the CS2 and 4-ethynyltoluene adducts confirmed the η2 coordination modes of the substrates and the concomitant opening up of the alkene arms of ligand 5. The coordination of α,β-unsaturated carbonyl compounds in complexes 11a–c was reversible.