Low-valent ansa-dimethylsilylene-, dimethylmethylene-bis(cyclopentadienyl) titanium compounds and ansa-titanium-magnesium complexes
Abstrakt
Reduction of ansa-titanocene dichlorides [X(eta(5)-C5H4)(2)TiCl2] (X = SiMe2 (1) and CMe2 (2)) in tetrahydrofuran (THF) by preactivated magnesium in the presence of bis(trimethylsilyl) acetylene (BTMSA) yielded complexes [X(eta(5)-C5H4)(2)Ti(eta(2)-BTMSA)] (X = SiMe2 (3) and CMe2 (4)). The prolonged action of excess magnesium and BTMSA resulted in the formation of ansa-titanium-magnesium complexes [m-X {(eta(5)-C5H4) Ti(eta(2)-SiMe3C CSiMe3)(2)}{(eta(5)-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 C-13 NMR shifts of 5 and its isomer 5a approved the above isomerization. The simultaneous ionic bonding of Mg2+ to the cyclopentadienyl ligand, which is eta(5)-coordinated to the {Ti(eta(2)-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(eta(5)-C5H4)(2)Ti (mu-eta(1): eta(2)-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(eta(5)-C5H4)(2)Ti(mu-Cl)}(2)] (X = SiMe2 (10); CMe2 (11)), which in contrast to the non-ansa dimer [(eta(5)-C5H5)(2)Ti(mu-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.