Distinguishing carbones from allenes by complexation to AuCl

Esterhuysen C. ; Frenking G. (2011)

Article

Quantum chemical calculations have been performed for the dicoordinated carbon compounds C(PPh3)2, C(NHCMe) 2, R2C=C=CR2 (R=H, F, NMe2), C 3O2, C(CN)2- and N-methyl-substituted N-heterocyclic carbene (NHCMe). The geometries of the complexes in which the dicoordinated carbon molecules bind as ligands to one and two AuCl moieties have been optimized and the strength and nature of the metal-ligand interactions in the mono- and diaurated complexes were investigated by means of energy decomposition analysis. The goal of the study is to elucidate the differences in the chemical behavior between carbones, allenes and carbenes. The results show that carbones bind one and two AuCl species in η1 fashion, whereas allenes bind them in η2 fashion. Compounds with latent divalent carbon(0) character can coordinate in more than one way, with the dominant mode indicating the degree of carbone or allene character. The calculated structures of the mono- and diaurated tetraaminoallenes (TAAs) reveal that TAAs exhibit a chameleon-like behavior: The bonding situation in the equilibrium structure is best described as allene [(R2N)2]C=C=C[(NR 2)2] in which the central carbon atom is a tetravalent CIV species, but the reactivity suggests that TAAs should be considered as divalent C0 compounds C{C[(NR2) 2]}2, that is, as "hidden" carbones. Carbon suboxide binds one AuCl preferentially in the η1 mode, whereas the equilibrium structures of the η1- and η2-bonded diaurated complex are energetically nearly degenerate. The doubly negatively charged isoelectronic carbone C(CN) 22- binds one and two AuCl very strongly in characteristic η1 fashion. The N-heterocyclic carbene complex, [NHCMe(AuCl)], possesses a high bond dissociation energy (BDE) for the splitting off of AuCl. The diaurated NHC adduct, [NHCMe(AuCl) 2], has two η1-bonded AuCl moieties that exhibit aurophilic attraction, which yield a moderate bond strength that might be large enough for synthesizing the complex. The BDE for the second AuCl in [NHCMe(AuCl)2] is clearly smaller than the values for the second AuCl in doubly aurated carbone complexes. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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