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Issue 24, 2017
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Normal-to-abnormal rearrangement of an N-heterocyclic carbene with a silylene transition metal complex

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Abstract

The synthesis and characterization of the N-heterocyclic carbene (NHC) stabilized dichlorosilylene Group 6 metal complexes {(IPr)SiCl2}W(CO)5 (3-W), {(IPr)SiCl2}2Cr(CO)4 (4-Cr), and {(IPr)SiCl2}2W(CO)4 (4-W) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) are reported. Treatment of 3-W with CsOH in the presence of IPr leads to the formation of an abnormal-NHC (aNHC) metal complex (aIPrH)W(CO)5 (6-W) (aIPrH = 1,3-bis(2,6-diisopropylphenyl)imidazol-4-ylidene), unveiling an unprecedented normal-to-abnormal transformation route of an NHC. DFT calculations support the proposed mechanism that involves CsOH mediated deprotonation of the IPr-backbone of 3-W to yield a ditopic carbanionic-NHC (dcNHC) complex 5a-W. Subsequent 1,4-migration of the W(CO)5 moiety and hydrolysis of the unmasked SiCl2 rationalize the formation of 6-W. The desired H2O molecule is generated in the initial step on deprotonation of IPr with CsOH. In contrast to the literature precedents, the calculations indicate that the abnormal complex 6-W is 13.5 kcal mol−1 thermodynamically higher in energy than the normal counterpart (IPr)W(CO)5 (8-W). Interestingly, as the aNHC-compounds reported so far are more stable than their normal counterparts, this finding showcases an opposite trend. Moreover, reaction pathways to the synthesized and related complexes have been investigated by DFT calculations.

Graphical abstract: Normal-to-abnormal rearrangement of an N-heterocyclic carbene with a silylene transition metal complex

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Publication details

The article was received on 03 Apr 2017, accepted on 19 May 2017 and first published on 22 May 2017


Article type: Paper
DOI: 10.1039/C7DT01199G
Citation: Dalton Trans., 2017,46, 7791-7799
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    Normal-to-abnormal rearrangement of an N-heterocyclic carbene with a silylene transition metal complex

    R. S. Ghadwal, D. Rottschäfer, D. M. Andrada, G. Frenking, C. J. Schürmann and H. Stammler, Dalton Trans., 2017, 46, 7791
    DOI: 10.1039/C7DT01199G

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