Low-valent ansa-dimethylsilylene-, dimethylmethylene-bis(cyclopentadienyl) titanium compounds and ansa-titanium–magnesium complexes

Reduction of ansa-titanocene dichlorides [X(η5-C5H4)2TiCl2] (X = SiMe2 (1) and CMe2 (2)) in tetrahydrofuran (THF) by preactivated magnesium in the presence of bis(trimethylsilyl)acetylene (BTMSA) yielded complexes [X(η5-C5H4)2Ti(η2-BTMSA)] (X = SiMe2 (3) and CMe2 (4)). The prolonged action of excess magnesium and BTMSA resulted in the formation of ansa-titanium-magnesium complexes [μ-X{(η5-C5H4)Ti(η2-SiMe3C≡CSiMe3)2}{(η5-C5H4)Mg(THF)}] (X = SiMe2 (5) and CMe2 (6)), which had their metals bounded via the ansa-ligand and ionic bonding between magnesium and the BTMSA ligands. Compounds 5 and 6 easily isomerized to 5a and 6a through rotation of their {(C5H4)Mg(THF)} moiety around the ansa- X–C bond. This rotation canceled the interaction of magnesium with the BTMSA ligands, which resulted in a large high-field shift of acetylenic carbon resonances. The geometry of both structures optimized by Density Functional Theory (DFT) computations together with the good correlation of computed magnetic shieldings and experimental 13C NMR shifts of 5 and its isomer 5a approved the above isomerization. The simultaneous ionic bonding of Mg2+ to the cyclopentadienyl ligand, which is η5-coordinated to the {Ti(η2-BTMSA)2} moiety in 5a got also confirmed by computational results. Reduction by magnesium was used to obtain Ti(III) dimers of ansa-titanocene acetylides [{X(η5-C5H4)2Ti (μ-η1:η2-C≡CSiMe3)}2] (X = SiMe2 (7) and X = CMe2 (8)) via redox splitting of 1,4-bis(trimethylsilyl)buta-1,3-diyne (BSD) by transient Ti(II) titanocene. The thermally stable Ti(II) compounds 3 and 4 were also shown to react with BSD yielding respectively 7 and 8. The reduction of 1 and 2 with magnesium in THF afforded Ti(III) monochloride dimers [{ansa-X(η5-C5H4)2Ti(μ-Cl)}2] (X = SiMe2 (10); CMe2 (11)), which in contrast to the non-ansa dimer [(η5-C5H5)2Ti(μ-Cl)}2] were silent in toluene glass EPR spectra. No evidence of obtaining quadruply bridged dititanium complexes capped with the ansa-ligands was encountered among crystal structures of 7–11, even though the total energy of optimized molecules 7 and 10 and their capped congeners differed by more than 45 kJ/mol, favouring the capped isomers in both cases.