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))(eta(2)-C2H4)(eta(5)-C5Me5)(2)] (1) with simple internal alkynes (RC)-C-1 equivalent to CR2 gives complexes [Ti(II)(eta(2)-(RC)-C-1 equivalent to CR2)(eta(5)-C5Me5)(2)] {R-1, R-2: Ph, Ph (3), Ph, Me (4), Me, SiMe3 (5), Ph, SiMe3 (6), t-Bu, SiMe3 (7), and SiMe3, SiMe3 (8). In contrast, alkynes with R-1 = Me and R-2 = t-Bu or i-Pr afford allene complexes [Ti(II)(eta(2)-CH2=C=CHR2)(eta(5)-C5Me5)(2)] (11) and (12), whereas for R-2 = 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] + [Me3SiC equivalent to CSiMe3] reversible arrow [8] + [C2H4] approached 1.
Compound 9 (R-1, R-2: 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 b(2) double right arrow a(1) 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.