Distinguishing Carbones from Allenes by Complexation to AuCl

Abstract

Quantum chemical calculations have been performed for the dicoordinated carbon compounds C(PPh(3))(2), C(NHC(Me))(2), R(2) C=C=CR(2) (R = H, F, NMe(2)), C(3)O(2), C(CN)(2)(-) and N-methyl-substituted N-heterocyclic carbene (NHC(Me)). 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 η(1) fashion, whereas allenes bind them in η(2) 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 [(R(2)N)(2)]C=C=C[(NR(2))(2)] in which the central carbon atom is a tetravalent C(IV) species, but the reactivity suggests that TAAs should be considered as divalent C(0) compounds C{C[(NR(2))(2)]}(2), that is, as "hidden" carbones. Carbon suboxide binds one AuCl preferentially in the η(1) mode, whereas the equilibrium structures of the η(1)- and η(2)-bonded diaurated complex are energetically nearly degenerate. The doubly negatively charged isoelectronic carbone C(CN)(2)(2-) binds one and two AuCl very strongly in characteristic η(1) fashion. The N-heterocyclic carbene complex, [NHC(Me)(AuCl)], possesses a high bond dissociation energy (BDE) for the splitting off of AuCl. The diaurated NHC adduct, [NHC(Me)(AuCl)(2)], has two η(1)-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 [NHC(Me)(AuCl)(2)] is clearly smaller than the values for the second AuCl in doubly aurated carbone complexes.

Revisiting the reactivity of different carbon bases: A theoretical study

“…The use of proton affinity values and complexation energies with Lewis acids to probe the nature and extent of element (0) character in a given molecule has been pioneered by Frenking and coworkers. [12,14,15] Table 1 contains the computed natural charge at the central carbon atom obtained using NBO method, NBO occupancies of the occupied r and p symmetric frontier molecular orbitals (MO), proton affinity values and bond dissociation energies (BDE) for the dissociation of one molecule of AuCl from L-AuCl and L-(AuCl)2, (L 5 1-12). It is evident from Table 1 that introduction of ylide increases the electron density at the carbenic carbon as revealed by the increase in negative charges.…”

“…To address these questions, quantum mechanical calculations have been carried out on the following closed-shell singlet carbenes (1-7) and carbones (8)(9)(10)(11)(12) as shown in Scheme 1. This study reveals that the distinction based on the previously proposed reactivity parameters [14,15] becomes blurred for electron rich carbones. Hence, here in this report, another probable reactivity parameter based on nucleophilic addition reaction has been proposed for the first time.…”

“…[14] Very recently, Frenking et al has suggested the use of other reactivity parameter such as complexation with one and two molecules of AuCl to distinguish a carbon(0) from a carbon(II) compound. [15] These reactivity parameters have been previously proposed by Frenking and coworkers as distinguishing features between these two classes of compounds. [14,15] However, in the same study, they have also…”

“…[15] These reactivity parameters have been previously proposed by Frenking and coworkers as distinguishing features between these two classes of compounds. [14,15] However, in the same study, they have also…”

“…[15] This has raised some intriguing questions: (i) Does a high value of second proton affinity really distinguish a carbene from a carbone? (ii) Does complexation with Lewis acids provides clue to the carbon(0) or carbon(II) character of these Lewis bases?…”