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) (<b>4Ti</b>), (<i>rac</i>-ebthi)­Ti­(η²-Ph₂PC₂PPh₂) (<b>5Ti</b>), and (<i>rac</i>-ebthi)­Zr­(η²-Ph₂PC₂PPh₂) (<b>5Zr</b>) [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 (<b>6</b>) was formed and isolated in the solid state. In solution, this complex is in equilibrium with the very spectacular structure of complex <b>7b</b> 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 <b>6</b> did not react with a second [Cp₂Zr] fragment to form a homobimetallic complex. In contrast, for (<i>rac</i>-ebthi)­Zr­(η²-Ph₂PC₂PPh₂) (<b>5Zr</b>) this reaction occurs. In the reaction of complex <b>4Ti</b> 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 (<b>11</b>). 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 <b>7b</b>, a <i>flip</i>-<i>flop</i> coordination of the phosphorus atoms was proposed. Complexes <b>4Ti</b>, <b>5Ti</b>, <b>5Zr</b>, <b>6</b>, and <b>11</b> were characterized by X-ray crystallography