METAL PI-COMPLEXES OF BENZENE-DERIVATIVES .34. TETRAPHENYLSILANE AS A CHELATING LIGAND - SYNTHESIS, STRUCTURAL CHARACTERIZATION, AND REACTIVITY OF THE TILTED BIS(ARENE) METAL-COMPLEXES [(C6H5)2SI(ETA-6-C6H5)2]V, [(C6H5)2SI(ETA-6-C6H5)2]CR

By means of lithiation and subsequent reaction with triphenylchlorosilane and diphenyldichlorosilane, respectively, the complexes bis((triphenylsilyl)-η6-benzene)M (6, M = Cr; 8, M = V) as well as (1-6:1’-6’-η-tetraphenylsilane)M (5, M = Cr; 7, M = V) were prepared and characterized by 1H and 13C NMR (5, 6) and EPR spectroscopies (6+, 7, 8) and by cyclic voltammetry (6-8). 5 was subjected to X-ray crystallographic analysis; the complex crystallizes in the monoclinic space group P21/n with a = 764.7 (3) pm, b = 1854.5 (8) pm, c = 1261.8 (5) pm, β = 93.60 (3)°, and Z = 4. The most pertinent features of the molecular structure of 5 are the tilting angle of the sandwich axis (165.6°), the bending of the ipso C–Si bond out of the η6-arene plane by 40.8°, and the small angle C(η-arene)–Si–C(η-arene) of 95.9°. Judging from the C–C bond lengths and13C chemical shifts, the ipso C atoms of the coordinated arenes are in a hybridization state between sp2 and sp3. The deviation of the two η-arenes from a parallel disposition exerts a significant influence on ring proton chemical shifts, equivalent protons having the smallest interannular separations being shifted farthest upfield. For the paramagnetic vanadium analogues, EPR measurements further show that tilting is accompanied by an increase in metal → ligand spin delocalization. The single-atom-bridged tilted complexes 5 and 7 are labile in solution. Whereas for the chromium species 5 protodesilylation to yield unsubstituted bis(benzene)chromium (9) dominates, the vanadium species 7 undergoes metal–ligand cleavage. Tilting exerts only a minor influence on the redox potential since E1/2 for the couple 70/- lies approximately halfway between the E1/2 values for 1-Ph3Si- and 1,1’-(Ph3Si)2-substituted bis(benzene)-vanadium. © 1990, American Chemical Society. All rights reserved.