Synthesis and crystal structures of thermally stable titanocenes Lenka Lukes ˇova ´ a , Michal Hora ´c ˇek a , Petr S ˇ te ˇpnic ˇka b , Karla Fejfarova ´ b , Ro ´ bert Gyepes b ,Ivana Cı ´sar ˇova ´ b , Jir ˇı ´ Kubis ˇta a , Karel Mach a,  a J. Heyrovsky ´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejs ˇkova 3, 182 23 Prague 8, Czech Republic b Department of Inorganic Chemistry, Charles University, Hlavova 2030, 128 40 Prague 2, Czech Republic Received 28 May 2002; accepted 18 July 2002 Dedicated to Professor Pascual Royo on the occassion of his 65th birthday in recognition of his outstanding contributions to organometallic chemistry Abstract Reduction of fully substituted titanocene dichlorides [TiCl 2 (h 5 -C 5 Me 4 R) 2 ] (R /SiMe 2 CH 2 CH 2 Ph, 3; SiMe 2 Ph, 4; and SiMePh 2 , 5) with magnesium in THF proceeds via the formation of titanocene monochlorides [TiCl(h 5 -C 5 Me 4 R) 2 ] 6  /8 to afford monomeric titanocenes, [Ti(h 5 -C 5 Me 4 R) 2 ], 9  /11. Titanocene monochlorides give the expected EPR spectra in toluene solution and glass. The structure of 7 was further corroborated by single-crystal X-ray diffraction. Titanocenes 9  /11 are EPR silent down to  /196 8C but exhibit paramagnetic broadening of the signals in solution NMR spectra. The positions of the NMR signals are temperature- dependent, obeying the Curie Law in the range investigated (0  /60 8C). As revealed by X-ray crystallography, titanocenes 9 and 11 possess bent metallocene structures with the cyclopentadienyl rings tilted at an angle of 9.8(1) and 14.4(2)8, respectively. Titanocenes 9  /11 are easily oxidized with PbCl 2 to the parent dichloride complexes 3 /5. Titanocenes 9 and 10 react with bis(trimethylsil- yl)ethyne (btmse) only in large excess of the alkyne to give an equilibrium concentration of the respective [Ti(h 5 -C 5 Me 4 R) 2 (h 2 - Me 3 SiC  /CSiMe 3 )] complexes. On the other hand, titanocene 11 does not observably react with btmse. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Titanium; Titanocenes; Bent titanocenes; Silyl substituents; X-ray crystallography 1. Introduction Electron-deficient, monomeric titanocenes are ther- mally unstable due to their strong tendency to acquire more valence electrons by means of various rearrange- ments which generally result in a conversion of Ti(II) to Ti(III) or Ti(IV) [1]. Accordingly, the parent titanocene, [Ti(h 5 -C 5 H 5 ) 2 ], rearranges to green ‘dimeric titanocene’, [(m  /h 5 :h 5 -C 5 H 4 C 5 H 4 ){Ti(m-H)(h 5 -C 5 H 5 )} 2 ], very ra- pidly under ambient conditions [2]. Thermal stability of methyl-substituted titanocenes increases with the number of electron donating methyl groups on the cyclopentadienyl ligands; however, compounds analo- gous to dimeric titanocene are obtained from transiently formed titanocenes containing mono- up to trimethy- lated cyclopentadienyl ligands [3]. The coordination- stabilized titanocene [(m-N 2 ){Ti(h 5 -C 5 Me 4 H) 2 } 2 ] readily eliminates dinitrogen and, subsequently, dihydrogen to give a paramagnetic dimer with bridging Ti  /C Cp s- bonds [{Ti(h 1 :h 5 -C 5 Me 4 )(h 5 -C 5 Me 4 H)} 2 ] [4].Even the relatively stable decamethyltitanocene, prepared by an elimination of dinitrogen from [(m-N 2 ){Ti(h 5 - C 5 Me 5 ) 2 } 2 ], is inherently contaminated by hydride species [TiH(h 5 -C 5 Me 5 )(h 1 :h 5 -C 5 Me 4 CH 2 )] [5]. Thus, so far only silicon-modified fully ring-substituted tita- nocenes [Ti(h 5 -C 5 Me 4 SiMe 2 R) 2 ] (R /Bu t 1 [6] or Me 2 [7]) were found to be stable at ambient temperature. Titanocene 1 resulted from the reduction of [TiCl(h 5 - C 5 Me 4 SiMe 2 Bu t ) 2 ] with sodium amalgam [6]. An ana- logous reduction of [TiCl 2 {h 5 -C 5 Me 4 (SiMe 3 )} 2 ] with magnesium metal is complicated by side reactions [8] and, hence, compound 2 was obtained alternatively by thermolysis of bis(trimethylsilyl)ethyne (btmse) complex [Ti{h 5 -C 5 Me 4 (SiMe 3 )} 2 (h 2 -btmse)] [7]. Here, we report  Corresponding author. Tel.:  /420-2-66053735; fax:  /420-2- 86582307 E-mail address: mach@jh-inst.cas.cz (K. Mach). Journal of Organometallic Chemistry 663 (2002) 134  /144 www.elsevier.com/locate/jorganchem 0022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0022-328X(02)01726-6