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

Abstract

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)(η(2)-Me3SiC2SiMe3) (M = Ti, no L; M = Zr, L = pyridine; Cp' = substituted or unsubstituted bridged or unbridged η(5)-cyclopentadienyl), the expected mononuclear complexes Cp*2Ti(η(2)-Ph2PC2PPh2) (4Ti), (rac-ebthi)Ti(η(2)-Ph2PC2PPh2) (5Ti), and (rac-ebthi)Zr(η(2)-Ph2PC2PPh2) (5Zr) [ebthi = ethylenebis(tetrahydroindenyl)] were obtained. When [Cp2Zr] was used in the reaction of Cp2Zr(py)(η(2)-Me3SiC2SiMe3) with Ph2P-C≡C-PPh2, the dinuclear complex [Cp2Zr(η(2)-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(η(2)-Ph2PC2PPh2) (5Zr) this reaction occurs. In the reaction of complex 4Ti with the Ni(0) complex (Cy3P)2Ni(η(2)-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.

Recent Synthetic and Catalytic Applications of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes

“…Reactions of the steric demanding titanocene sources Cp* 2 Ti(η 2 ‐btmsa) and rac ‐(ebthi)Ti(η 2 ‐btmsa) with bis(diphenylphosphanyl)acetylene resulted in the formation of the titanacyclopropenes Cp* 2 Ti(η2‐Ph 2 PC 2 PPh 2 ) and rac ‐(ebthi)Ti(η2‐Ph 2 PC 2 PPh 2 ) (Scheme ) …”

Reactions of [Cp2Ti(η2‐Me3SiC2SiMe3)] with 1,4‐Bis(diphenylphosphanyl)but‐2‐yne: Coupling and Isomerization versus Phosphorylation

“…Recently, we investigated the reactions of symmetrically dihetero-substituted alkynes YCϵCY (Y = BR 2 , [1] NR 2 , [2] OR, PR 2 , [3] SR [4] ) with group 4 metallocenes. It became evident that heteroatoms have a significant influence on the structural parameters and reactivities of the obtained complexes as the heteroatoms of α-hetero-substituted alkynes interact directly with the alkyne triple bond and increase its electron density for donor heteroatoms (NR 2 , OR, PR 2 , SR) or decrease its electron density for acceptor heteroatoms (BR 2 ).…”

“…It became evident that heteroatoms have a significant influence on the structural parameters and reactivities of the obtained complexes as the heteroatoms of α-hetero-substituted alkynes interact directly with the alkyne triple bond and increase its electron density for donor heteroatoms (NR 2 , OR, PR 2 , SR) or decrease its electron density for acceptor heteroatoms (BR 2 ). More recently, we extended these investigations to unsymmetrically dihetero-substituted alkynes YCϵCSiMe 3 , for example, the α-donor-substituted trimethylsilylalkynes R 2 NCϵCSiMe 3 , EtOCϵCSiMe 3 , and Me 2 PCϵCSiMe 3 . [5] In these investigations we aimed to use donor heteroatoms to supply an additional coordination site directly attached to the alkyne moiety, which could promote the formation of highly strained, four-membered metallacycles of the phosphorus atoms.…”

“…[6] For the bis-phosphorus-substituted alkyne Ph 2 PCϵCPPh 2 , [3] such an interaction was proposed to be dynamic in nature with a rapid exchange in the coordination of both phosphorus atoms in a flip-flop mechanism. [3] It seemed particularly interesting to investigate how the chemistry would change if the P atoms were located not in the α, but in the β positions with respect to the triple bond, for example, in Ph 2 PCH 2 CϵCCH 2 PPh 2 (2). This ligand is typically coordinated through its phosphorus atoms in singly bridged molybdenum, iron, and cobalt complexes, [7] as well as in doubly bridged molybdenum, [8] nickel, [7] and platinum complexes.…”

Rosenthal’s Zirconocene

“…10 A four-membered heterometallacycle was observed in solution that is in equilibrium with the dinuclear complex.…”