PUB - Publikationen an der Universität Bielefeld

Structure and bonding in several Stereoisomers of the recently synthesized silyl cation (Me5C5)(2)Si+H (2) and its hydrogen-substituted analogue (C5H5)(2)Si+H (3) has been investigated by quantum-mechanical methods. DFT calculations at the B3LYP/6-311G(2d,p)//B3LYP/6-31G(d)+Delta ZPVE level as well as coupled cluster calculations at the CCSD(T)/6-311G(d) level for the parent silyl cation 3 reveal that the eta (1.5):eta (1.5)-coordinated isomer 3c of C-2 symmetry is the most stable isomer. Two isomers of 3, the eta (2):eta (3)-bonded 3f (C-8 symmetry) and the eta (2):eta (2)- coordinated 3e (C-2 upsilon symmetry), are, however, very close in energy (DeltaE < 1 kcal mol(-1)). This implies that 3 is a highly fluxional molecule which undergoes a series of effectively barrierless haptotropic shifts leading to a circumambulatory migration of the SiH group about the cyclopentadienyl ring. The positive charge in the silyl cation 3 is effectively transferred to the attached cyclopentadienyl substituent, leading to an exceptional thermodynamic stability: the secondary cation 3 is more stable than Me3Si+ by 18.7 kcal mol(-1) (at B3LYP/6-311G(2d,p)//B3LYP/6-31G(d)+Delta ZPVE). Similarly, for the experimentally observed permethylated silyl cation 2, three low-energy isomers with different hapticities for the silicon have been found to be very close in energy (DeltaE < 1.5 kcal mol(-1) at B3LYP/6-311G(2d,p)//B3LYP/6-31G(d)), with the eta (2):eta (3) isomer 2f only marginally favored. This clearly indicates that also 2 is a highly fluxional molecule. Si-29 NMR chemical shift calculations at the GIAO/B3LYP/6-311G(2d,p)//B3LYP/6-31G(d) level suggest that the experimentally observed cation adopts the eta (3):eta (2)-bonded structure 2f. The unique intramolecular stabilization of the silyl cation 2 by the pentamethylcyclopentadienyl substituents, and the additional steric protection of 2 by the methyl groups results in very low association energies A(E) between the cation and solvent molecules (benzene, A(E) = -3.2 kcal mol(-1); dimethyl ether, A(E) = -6.4 kcal mol(-1)). Although these interaction energies must be corrected by,ca. -2 kcal mol(-1) due to the neglect of intermolecular dispersion energy contributions by the applied DFT method, it can be concluded that 2 is best regarded as a free silyl cation even in solvents as nucleophilic as THF.