Displacement of ethene from the decamethyltitanocene-ethene complex with internal alkynes, substituent-dependent alkyne-to-allene rearrangement, and the electronic transition relevant to the back-bonding interaction

Abstract

The titanocene-ethene complex [Ti(II)(η²-C₂H₄)(η⁵-C₅Me₅)₂] (1) with simple internal alkynes R¹CCR² gives complexes [Ti(II)(η²-R¹CCR²)(η⁵-C₅Me₅)₂] {R¹, R²: Ph, Ph (3), Ph, Me (4), Me, SiMe₃ (5), Ph, SiMe₃ (6), t-Bu, SiMe₃ (7), and SiMe₃, SiMe₃ (8). In contrast, alkynes with R¹ = Me and R² = t-Bu or i-Pr afford allene complexes [Ti(II)(η²-CH₂CCHR²)(η⁵-C₅Me₅)₂] (11) and (12), whereas for R² = Et a mixture of alkyne complex (13A) and minor allene (13) is obtained. Crystal structures of 4, 6, 7 and 11 have been determined; the latter structure proved the back-bonding interaction of the allene terminal double bond. Only the synthesis of 8 from 1 was inefficient because the equilibrium constant for the reaction [1] + [Me₃SiCCSiMe₃] ⇌ [8] + [C₂H₄] approached 1. Compound 9 (R¹, R²: Me), not obtainable from 1, together with compounds 3–6 and 10 (R¹, R²: Et) were also prepared by alkyne exchange with 8, however this reaction did not take place in attempts to obtain 7. Compounds 1 and 3–9 display the longest-wavelength electronic absorption band in the range 670–940 nm due to the HOMO → LUMO transition. The assignment of the first excitation to be of predominantly a b₂ → a₁ transition was confirmed by DFT calculations. The calculated first excitation energies for 3–9 followed the order of hypsochromic shifts of the absorption band relative to 8 that were induced by acetylene substituents: Me > Ph ≫ SiMe₃. Computational results have also affirmed the back-bonding nature in the alkyne-to-metal coordination.