The Nature of Protonated Decamethylsilicocene, (Me₅C₅)₂Si⁺H

Abstract

Structure and bonding in several stereoisomers of the recently synthesized silyl cation (Me 5 C 5 ) 2 Si + H ( 2 ) and its hydrogen-substituted analogue (C 5 H 5 ) 2 Si + H ( 3 ) has been investigated by quantum-mechanical methods. DFT calculations at the B3LYP/6-311G(2d,p)//B3LYP/6-31G(d)+ΔZPVE level as well as coupled cluster calculations at the CCSD(T)/6-311G(d) level for the parent silyl cation 3 reveal that the η 1.5:η 1.5 -coordinated isomer 3c of C 2 symmetry is the most stable isomer. Two isomers of 3, the η 2:η 3 -bonded 3f ( C s symmetry) and the η 2:η 2 -coordinated 3e ( C 2 v symmetry), are, however, very close in energy (Δ E < 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 Me 3 Si + by 18.7 kcal mol - 1 (at B3LYP/6-311G(2d,p)//B3LYP/6-31G(d)+Δ Z PVE). 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 (Δ E < 1.5 kcal mol - 1 at B3LYP/6-311G(2d,p)//B3LYP/6-31G(d)), with the η 2:η 3 isomer 2f only marginally favored. This clearly indicates that also 2 is a highly fluxional molecule. 29 Si 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 η 3:η 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.