| Abstract: | A density functional theory computational chemistry study has revealed a fundamental structural difference between Ti(Cp)3]+ and its congeners Zr(Cp)3]+ and Hf(Cp)3]+/(Cp=cyclopentadienyl). Whereas the latter two are found to contain three uniformely η5-coordinated Cp ligands (3η5-structural type), Ti(Cp)3]+ is shown to prefer a 2η5η2 structure. Ti(Cp)3]+B(C6F5)3(Me)]? ( 10 ?B(C6F5)3(Me)]?) was experimentally generated by treatment of Ti(Cp)3(Me)] ( 7a ) with B(C6F5)3 (Scheme 3). Low-temperature 1H-NMR spectroscopy in CDFCl2 (143 K, 600 MHz; Fig. 8) showed a splitting of the Cp resonance into five lines in a 2 : 5 : 2 : 5 : 1 ratio which would be in accord with the theoretically predicted 2η5η2-type structure of Ti(Cp)3]+. The precursor Ti(Cp)3(Me)] ( 7a ) exhibits two 1H-NMR Cp resonances in a 10 : 5 ratio in CD2Cl2 at 223 K. Treatment of HfCl(Cp)2(Me)] ( 6c ) with sodium cyclopentadienide gave Hf(Cp)3(Me)] ( 7c ) (Scheme 1). Its reaction with B(C6F5)3 furnished the salt Hf(Cp)3]+B(C6F5)3(Me)]? ( 8 ?B(C6F5)3(Me)]?), which reacted with tert-butyl isocyanide to give the cationic complex Hf(Cp)3(C=N?CMe3)]+ ( 9a ; with counterion B(C6F5)3(Me)]? (Scheme 2). Complex cation 9a was characterized by X-ray diffraction (Fig. 7). Its Hf(Cp3) moiety is of the 3η5-type. The structure is distorted trigonal-pyramidal with an average D?Hf?D angle of 118.8° and an average D?Hf?C(1) angle of 96.5° (D denotes the centroids of the Cp rings; Table 6). Cation 9a is a typical d0-isocyanide complex exhibiting structural parameters of the C≡N?CMe3 group (d(C(1)?N(2))=1.146 (5) Å; IR: v?(C≡N) 2211 cm?1) very similar to free uncomplexed isonitrile. Analogous treatment of 8 with carbon monoxide yielded the carbonyl (d0-group-4-metal) complex Hf(Cp)3(CO)]+ ( 9b ; with counterion B(C6F5)3(Me)]?) (Scheme 2) that was also characterized by X-ray crystal-structure analysis (Fig. 6). Complex 9b is also of the 3η5-structural type, similar to the peviously described cationic complex Zr(Cp)3(CO)]+, and exhibits properties of the CO ligand (d(C?O)=1.11 (2) Å; IR: v?(C≡O) 2137 cm?1) very similar to the free carbon monoxide molecule. |
