Hydrosilylation induced by N→Si intramolecular coordination: Spontaneous transformation of organosilanes into 1-aza-silole-type molecules in the absence of a catalyst

Abstract

Our attempts to synthesize the N→Si intramolecularly coordinated organosilanes Ph2L1SiH (1 a), PhL1SiH 2 (2 a), Ph2L2SiH (3 a), and PhL 2SiH2 (4 a) containing a CH=N imine group (in which L 1 is the C,N-chelating ligand {2-[CH=N(C6H 3-2,6-iPr2)]C6H4}- and L2 is {2-[CH=N(tBu)]C6H4}-) yielded 1-[2,6-bis(diisopropyl)phenyl]-2,2-diphenyl-1-aza-silole (1), 1-[2,6-bis(diisopropyl)phenyl]-2-phenyl-2-hydrido-1-aza-silole (2), 1-tert-butyl-2,2-diphenyl-1-aza-silole (3), and 1-tert-butyl-2-phenyl-2-hydrido- 1-aza-silole (4), respectively. Isolated organosilicon amides 1-4 are an outcome of the spontaneous hydrosilylation of the CH=N imine moiety induced by N→Si intramolecular coordination. Compounds 1-4 were characterized by NMR spectroscopy and X-ray diffraction analysis. The geometries of organosilanes 1 a-4 a and their corresponding hydrosilylated products 1-4 were optimized and fully characterized at the B3LYP/6-31++G(d,p) level of theory. The molecular structure determination of 1-3 suggested the presence of a Si=N double bond. Natural bond orbital (NBO) analysis, however, shows a very strong donor-acceptor interaction between the lone pair of the nitrogen atom and the formal empty p orbital on the silicon and therefore, the calculations show that the Si=N bond is highly polarized pointing to a predominantly zwitterionic Si +N- bond in 1-4. Since compounds 1-4 are hydrosilylated products of 1 a-4 a, the free energies (ΔG298), enthalpies (ΔH298), and entropies (ΔH298) were computed for the hydrosilylation reaction of 1 a-4 a with both B3LYP and B3LYP-D methods. On the basis of the very negative ΔG298 values, the hydrosilylation reaction is highly exergonic and compounds 1 a-4 a are spontaneously transformed into 1-4 in the absence of a catalyst. Spontaneous transformation: 1-Aza-silole-type complexes 1-4 were prepared by spontaneous hydrosilylation of the CH=N imine moiety in N→Si intramolecularly coordinated organosilanes 1 a-4 a (see figure). The structure and reactivity of the non-isolable compounds was investigated. A strong N×××Si interaction was found in 1 a-4 a. DFT studies proved that the hydrosilylation reaction is highly exergonic. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.