Reactions of Titanocene Bis(trimethylsilyl)acetylene Complexes with Carbodiimides: An Experimental and Theoretical Study of Complexation versus C–N Bond Activation

The reaction of the low valent metallocene­(II) sources
Cp′2Ti­(η2-Me3SiC2SiMe3) (Cp′ = η5-cyclopentadienyl, 1a or η5-penta­methylcyclopentadienyl, 1b) with different carbodiimide substrates RNCNR′ 2-R-R′ (R = t-Bu; R′ = Et;
R = R′ = i-Pr; t-Bu; SiMe3; 2,4,6-Me-C6H2 and 2,6-i-Pr-C6H3) was investigated to explore the frontiers
of ring strained, unusual four-membered heterometallacycles 5-R. The product complexes show dismantlement, isomerization,
or C–C coupling of the applied carbodiimide substrates, respectively,
to form unusual mono-, di-, and tetranuclear titanium­(III) complexes.
A detailed theoretical study revealed that the formation of the unusual
complexes can be attributed to the biradicaloid nature of the unusual
four-membered heterometallacycles 5-R, which presents
an intriguing situation of M–C bonding. The combined experimental
and theoretical study highlights the delicate interplay of electronic
and steric effects in the stabilization of strained four-membered
heterometallacycles, accounting for the isolation of the obtained
complexes.

本研究以低价茂金属(II)源（low valent metallocene(II) sources）Cp′₂Ti(η²-Me₃SiC₂SiMe₃)为反应前体，其中Cp′分别为η⁵-环戊二烯基（η⁵-cyclopentadienyl，对应物种1a）与η⁵-五甲基环戊二烯基（η⁵-pentamethylcyclopentadienyl，对应物种1b），考察其与一系列碳二亚胺（carbodiimide）底物RN=C=NR' 2-R-R'的反应行为，以期探索环张力异常四元杂金属环（four-membered heterometallacycle）5-R的研究边界。所选用的碳二亚胺底物包括：R = 叔丁基（t-Bu）、R' = 乙基（Et）；R = R' = 异丙基（i-Pr）、叔丁基、三甲基硅基（SiMe₃）、2,4,6-三甲基苯基（2,4,6-Me-C₆H₂）以及2,6-二异丙基苯基（2,6-i-Pr-C₆H₃）。实验结果显示，所得产物配合物可分别通过所涉碳二亚胺底物的分解、异构化或碳-碳偶联过程得到，对应生成罕见的单核、双核与四核钛(III)配合物。详细的理论研究表明，这类异常配合物的形成可归因于四元杂金属环5-R的双自由基（biradicaloid）特性，该特性呈现出独特的金属-碳（M–C）成键模式。本次结合实验与理论的研究揭示了电子效应与位阻效应在稳定环张力四元杂金属环过程中的精细协同作用，为所分离得到的各类配合物的稳定性提供了合理解释。