Synthesis, Characterization and Reactivity of Group 4 Metallocene Bis(diphenylphosphino)acetylene ComplexesA Reactivity and Bonding Study

A study of the coordination chemistry of bis­(diphenylphosphino)­acetylene, Ph₂P–CC–PPh₂, with selected group 4 metallocenes is presented. By substitution of the alkyne in complexes of the type Cp′₂M­(L)­(η²-Me₃SiC₂SiMe₃) (M = Ti, no L; M = Zr, L = pyridine; Cp′ = substituted or unsubstituted bridged or unbridged η⁵-cyclopentadienyl), the expected mononuclear complexes Cp*₂Ti­(η²-Ph₂PC₂PPh₂) (4Ti), (rac-ebthi)­Ti­(η²-Ph₂PC₂PPh₂) (5Ti), and (rac-ebthi)­Zr­(η²-Ph₂PC₂PPh₂) (5Zr) [ebthi = ethylenebis­(tetrahydroindenyl)] were obtained. When [Cp₂Zr] was used in the reaction of Cp₂Zr­(py)­(η²-Me₃SiC₂SiMe₃) with Ph₂P–CC–PPh₂, the dinuclear complex [Cp₂Zr­(η²-Ph₂PC₂PPh₂)]₂ (6) was formed and isolated in the solid state. In solution, this complex is in equilibrium with the very spectacular structure of complex 7b as the first example of such a highly strained four-membered heterometallacycle of a group 4 metal, involving the rare R₂PCCR′ fragment in the cyclic unit. Both the stability and reactivity of heterodisubstituted alkynes X–CC–X (X = NR₂, PR₂, SR, SiR₃, etc.) themselves and also of their complexes are of general interest. Complex 6 did not react with a second [Cp₂Zr] fragment to form a homobimetallic complex. In contrast, for (rac-ebthi)­Zr­(η²-Ph₂PC₂PPh₂) (5Zr) this reaction occurs. In the reaction of complex 4Ti with the Ni(0) complex (Cy₃P)₂Ni­(η²-C₂H₄) (Cy = cyclohexyl), C–P bond cleavage of the alkyne ligand resulted in the formation of the isolated complex [(Cy₃P)­Ni­(μ-PPh₂)]₂ (11). The structure and bonding of the complexes were investigated by DFT analysis to compare the different possible coordination modes of the R₂P–CC–PR₂ ligand. For compound 7b, a flip-flop coordination of the phosphorus atoms was proposed. Complexes 4Ti, 5Ti, 5Zr, 6, and 11 were characterized by X-ray crystallography.