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-C<INF>5</INF>H<INF>5</INF><INF>-</INF><INF>n</INF><INF></INF>Me<INF>n</INF><INF></INF>)<INF>2</INF>Ti(η²-btmse)] (n = 3−5; <BO>1b</BO>−<BO>d</BO>) with bis((trimethylsilyl)ethynyl)dimethylsilane (<BO>2a</BO>) afford the unusual 1,1-bis(η⁵-cyclopentadienyl)-4,4-dimethyl-3,5-bis(trimethylsilyl)-1-titana-4-silacyclohexa-2,5-diene complexes [(η⁵-C<INF>5</INF>H<INF>5</INF><INF>-</INF><INF>n</INF><INF></INF>Me<INF>n</INF><INF></INF>)<INF>2</INF>Ti{C&dbd;C(SiMe<INF>3</INF>)}<INF>2</INF>SiMe<INF>2</INF>] (<BO>4b</BO>−<BO>d</BO>), whereas the nonmethylated precursor [(η⁵-C<INF>5</INF>H<INF>5</INF>)<INF>2</INF>Ti(η²-btmse)] (<BO>1a</BO>) gives under similar conditions the known dinuclear, acetylide-bridged complex [{(η⁵-C<INF>5</INF>H<INF>5</INF>)<INF>2</INF>Ti(μ-η¹:η²-C&tbd1;CSiMe<INF>3</INF>)}<INF>2</INF>] (<BO>3a</BO>). In contrast, analogous reactions with bis((trimethylsilyl)ethynyl)diphenylsilane (<BO>2b</BO>) give the product of simple ligand exchange [(η⁵-C<INF>5</INF>Me<INF>5</INF>)<INF>2</INF>Ti(η²-Me<INF>3</INF>SiC&tbd1;CSiPh<INF>2</INF>C&tbd1;CSiMe<INF>3</INF>)] (<BO>6d</BO>) from <BO>1d</BO> and mixtures containing the similar complex [(η⁵-C<INF>5</INF>HMe<INF>4</INF>)<INF>2</INF>Ti(η²-Me<INF>3</INF>SiC&tbd1;CSiPh<INF>2</INF>C&tbd1;CSiMe<INF>3</INF>)] (<BO>6c</BO>) and the titanasilacyclohexadiene [Ti(η⁵-C<INF>5</INF>HMe<INF>4</INF>)<INF>2</INF>Ti{C&dbd;C(SiMe<INF>3</INF>)}<INF>2</INF>SiPh<INF>2</INF>] (<BO>5c</BO>) from <BO>1c</BO>. Hydrogenolysis of <BO>5c</BO> (1 bar/3 h) affords 1,4-bis(trimethylsilyl)-5,5-diphenyl-5-silacyclopenta-1,3-diene (<BO>7</BO>). Compounds <BO>4</BO> and <BO>5</BO> possess a surplus of two bonding electrons at the titanium-bonded carbon atoms (C^α) with paired spins, whose presence is reflected by extremely short Ti−C^α bond lengths (1.981(4)−1.998(3) Å) and C^α−C^α 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 C^α interact with the titanocene 1a<INF>1</INF> orbital, giving rise to a three-center−two-electron, Δ-shaped bond.