Nonclassical Bonding in Titanasilacyclohexadiene Compounds Resulting from Highly Methyl-Substituted Titanocene−Bis(trimethylsilyl)ethyne Complexes and Bis((trimethylsilyl)ethynyl)silanes

Reactions at elevated temperatures (150 °C) of titanocene−η2-bis(trimethylsilyl)ethyne
(btmse) complexes [(η5-C5H5-nMen)2Ti(η2-btmse)] (n = 3−5; 1b−d) with bis((trimethylsilyl)ethynyl)dimethylsilane (2a) afford the unusual 1,1-bis(η5-cyclopentadienyl)-4,4-dimethyl-3,5-bis(trimethylsilyl)-1-titana-4-silacyclohexa-2,5-diene complexes [(η5-C5H5-nMen)2Ti{CC(SiMe3)}2SiMe2] (4b−d), whereas the nonmethylated precursor [(η5-C5H5)2Ti(η2-btmse)] (1a)
gives under similar conditions the known dinuclear, acetylide-bridged complex [{(η5-C5H5)2Ti(μ-η1:η2-C⋮CSiMe3)}2] (3a). In contrast, analogous reactions with bis((trimethylsilyl)ethynyl)diphenylsilane (2b) give the product of simple ligand exchange [(η5-C5Me5)2Ti(η2-Me3SiC⋮CSiPh2C⋮CSiMe3)] (6d) from 1d and mixtures containing the similar complex [(η5-C5HMe4)2Ti(η2-Me3SiC⋮CSiPh2C⋮CSiMe3)] (6c) and the titanasilacyclohexadiene [Ti(η5-C5HMe4)2Ti{CC(SiMe3)}2SiPh2] (5c) from 1c. Hydrogenolysis of 5c (1 bar/3 h) affords 1,4-bis(trimethylsilyl)-5,5-diphenyl-5-silacyclopenta-1,3-diene (7). Compounds 4 and 5 possess
a surplus of two bonding electrons at the titanium-bonded carbon atoms (Cα) with paired
spins, whose presence is reflected by extremely short Ti−Cα bond lengths (1.981(4)−1.998(3) Å) and Cα−Cα contacts (1.821(4)−1.933(2) Å), the latter excluding the presence of
a 3-silacyclopenta-1,4-diene moiety simply bonded to titanium via two σ bonds. DFT
calculations showed that the two singly occupied p orbitals at Cα interact with the titanocene
1a1 orbital, giving rise to a three-center−two-electron, Δ-shaped bond.