Nonclassical bonding in titanasilacyclohexadiene compounds resulting from highly methyl-substituted titanocene-bis(trimethylsilyl)ethyne complexes and bis((trimethylsilyl)ethynyl)silanes
Abstract
Reactions at elevated temperatures (150 degrees C) of titanocene-eta(2)-bis(trimethylsilyl)ethyne (btmse) complexes [(eta(5)-C5H5-Me-n(n))(2)Ti(eta(2)-btmse)] (n = 3-5; 1b-d) with bis((trimethylsilyl)ethynyl)dimethylsilane (2a) afford the unusual 1,1-bis(eta(5) -cyclopentadienyl)-4,4-dimethyl-3,5-bis(trimethylsilyl)-1-titana-4-silacyclohexa-2,5-diene complexes [(eta(5) -C5H5-nMen)(2)Ti{C= C(SiMe3)}(2)SiMe2] (4b-d), whereas the nonmethylated precursor [(eta(5)-C5H5)(2)Ti(eta(2)-btmse)] (1a) gives under similar conditions the known dinuclear, acetylide-bridged complex [{(eta(5)-C5H5)(2)-Ti(mu-eta(1):eta(2)- C CSiMe3)}(2)] (3a). In contrast, analogous reactions with bis((trimethylsilyl)ethynyl)diphenylsilane (2b) give the product of simple ligand exchange [(eta(5)-C5Me5)(2)Ti(eta(Me3SiC)-Me-2 CSiPh2C=CSiMe3)] (6d) from 1d and mixtures containing the similar complex [(eta(5)-C5HMe4)(2)Ti(eta(2)-Me3SiC CSiPh2C CSiMe3)] (6c) and the titanasilacyclohexadiene [Ti(eta(C5HMe4)-C-5)(2)Ti{ C=C(SiMe3)}(2)SiPh2] (5c) from 1c.
Hydrogenolysis of 5c (1 bar/3 h) affords 1,4bis(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 (C-alpha) with paired spins, whose presence is reflected by extremely short Ti-C-alpha bond lengths (1.981(4)1.998(3) angstrom) and C-alpha-C (alpha)contacts (1.821(4)-1.933(2) angstrom), the latter excluding the presence of a 3-silacyclopenta-1,4-diene moiety simply bonded to titanium via two sigma bonds.
DFT calculations showed that the two singly occupied p orbitals at C-alpha interact with the titanocene 1a(1) orbital, giving rise to a three-center-two-electron, Delta-shaped bond.