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.