Symmetrical P4 cleavage at cobalt: characterization of intermediates on the way from P4 to coordinated P2 units.

Degradation of white phosphorus (P(4)) in the coordination sphere of transition metals is commonly divided into two major pathways depending on the P(x) ligands obtained. Consecutive metal-assisted P-P bond cleavage of four bonds of the P(4) tetrahedron leads to complexes featuring two P(2) ligands (symmetric cleavage) or one P(3) and one P(1) ligand (asymmetric cleavage). A systematic investigation of the degradation of white phosphorus P(4) to coordinated μ,η(2:2)-bridging diphosphorus ligands in the coordination sphere of cobalt is presented herein as well as isolation of each of the decisive intermediates on the reaction pathway. The olefin complex [CpCo((i)Pr(2)Im)(η(2)-C(2)H(4))], 1 (Cp = η(5)-C(5)Me(5), (i)Pr(2)Im = 1,3-di-isopropylimidazolin-2-ylidene), reacts with P(4) to give [CpCo((i)Pr(2)Im)(η(2)-P(4))], 2, the insertion product of [CpCo((i)Pr(2)Im)] into one of the P-P bonds. Addition of a further equivalent of the Co(I) complex [CpCo((i)Pr(2)Im)(η(2)-C(2)H(4))], 1, induces cleavage of a second P-P bond to yield the dinuclear complex [{CpCo((i)Pr(2)Im)}(2)(μ,η(2:2)-P(4))], 3, in which a kinked cyclo-P(4)(4-) ligand bridges two cobalt atoms. Consecutive dissociation of the N-heterocyclic carbene with concomitant rearrangement of the cyclo-P(4) ligand and P-P dissociation leads to complexes [CpCo(μ,η(4:2)-P(4))Co((i)Pr(2)Im)Cp], 4, featuring a P(4) chain, and [{CpCo(μ,η(2:2)-P(2))}(2)], 5, in which two isolated P(2)(2-) ligands bridge two [CpCo] fragments. Each of these reactions is quantitative if performed on an NMR scale, and each compound can be isolated in high yields and large quantities.