COMPARISON OF CHARACTERISTIC STRUCTURAL FEATURES AMONG THE TRIAD OF TRIS(CYCLOPENTADIENYL)(GROUP-4 METAL) COMPLEX CATIONS - A COMBINED THEORETICAL AND EXPERIMENTAL-STUDY

A density functional theory computational chemistry study has revealed a fundamental structural difference between [Ti(Cp)(3)](+) and its co ngeners [Zr(Cp)(3)](+) and [WT(Cp)(3)](+)/(Cp = cyclopentadienyl). Whe reas the latter two are found to contain three uniformely eta(5)-coord inated Cp ligands (3 eta(5)-structural type), [Ti(Cp)(3)](+) is shown to prefer a 2 eta(5)eta(2) 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 H- 1-NMR spectroscopy in CDFCl2 (143 K, 600 MHz; Fig. 8) showed a splitti ng of the Cp resonance into five lines in a 2:5:2:5:1 ratio which woul d be in accord with the theoretically predicted 2 eta(5)eta(2)-type st ructure of [Ti(Cp)(3)](+). The precursor [Ti(Cp)(3)(Me)] (7a) exhibits two H-1-NMR Cp resonances in a 10:5 ratio in CD2Cl2 at 223 K. Treatme nt 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(C p)(3)(C=N-CMe3)](+) (9a; with counterion [B(C6F5)(3)(Me)](-) (Scheme 2 ). Complex cation I)a was characterized by X-ray diffraction (Fig. 7). Its Hf(Cp-3) moiety is of the 3 eta(5)-type. The structure is distort ed trigonal-pyramidal with an average D-HI-D angle of 118.8 degrees an d an average D-Hf-C(1) angle of 96.5 degrees (D denotes the centroids of the Cp rings; Table 6). Cation 9a is a typical d(0)-isocyanide comp lex exhibiting structural parameters of the C=N-CMe3 group (d(C(1)-N(2 ))= 1.146(5) Angstrom; IR: <(nu)over bar>(C=N) 2211 cm(-1)) very simil ar to free uncomplexed isonitrile. Analogous treatment of 8 with carbo n monoxide yielded the carbonyl (d(0)-group-4-metal) complex [Hf(Cp)(3 )(CO)](+) (9b; with counterion [B(C6F5)(3)(Me)](-)) (Scheme 2) that wa s also characterized by X-ray crystal-structure analysis (Fig. 6). Com plex 9b is also of the 3 eta(5)-structural type, similar to the peviou sly described cationic complex [Zr(Cp)(3)(CO)](+), and exhibits proper ties of the CO ligand (d(C-O)=1.11(2) Angstrom; IR: <(nu)over bar>(C=O ) 2137 cm(-1)) very similar to the free carbon monoxide molecule.