Carbodicarbenes and Related Divalent Carbon(0) Compounds

Abstract

Quantum-chemical calculations using DFT and ab initio methods have been carried out for fourteen divalent carbon(0) compounds (carbones), in which the bonding situation at the two-coordinate carbon atom can be described in terms of donor-acceptor interactions L-->C<--L. The charge- and energy-decomposition analysis of the electronic structure of compounds 1-10 reveals divalent carbon(0) character in different degrees for all molecules. Carbone-type bonding L-->C<--L is particularly strong for the carbodicarbenes 1 and 2, for the "bent allenes" 3 a, 3 b, 4 a, and 4 b, and for the carbocarbenephosphoranes 7 a, 7 b, and 7 c. The last-named molecules have very large first and large second proton affinities. They also bind two BH(3) ligands with very high bond energies, which are large enough that the bis-adducts should be isolable in a condensed phase. The second proton affinities of the complexes 5, 6, and 8-10 bearing CO or N(2) as ligand are significantly lower than those of the other molecules. However, they give stable complexes with two BH(3) ligands and thus are twofold Lewis bases. The calculated data thus identify 1-10 as carbones L-->C<--L in which the carbon atom has two electron pairs. The chemistry of carbones is different from that of carbenes because divalent carbon(0) compounds CL(2) are pi donors and thus may serve as double Lewis bases, while divalent carbon(II) compounds are pi acceptors. The theoretical results point toward new directions for experimental research in the field of low-coordinate carbon compounds.

Carbodiphosphorane Analogues E(PPh3)2 with E=C–Pb: A Theoretical Study with Implications for Ligand Design

“…L to a wider range of donor ligands beyond L = PPh 3 . [10][11][12][13][14] A new class of carbones termed carbodicarbenes CA C H T U N G T R E N N U N G (NHC) 2 (NHC=N-heterocyclic carbene) (2 C, Scheme 1), which features unusual carbon!carbon donor-acceptor bonds between a naked carbon(0) atom in the 1 D state as an acceptor and N-heterocyclic carbene ligands as donors, was predicted to be stable enough to become isolated. [10] Several examples of carbodicarbenes with NHC and other carbene ligands have in the meantime become synthesized.…”

Lewis Base Stabilized Dicarbon: Predictions from Theory

“…[9,10] In each case, both lone pairs of electrons on carbon were predicted and confirmed to be chemically accessible. [11][12][13] Phosphine-stabilized "C 3 " has also been reported, but not structurally verified. [14] NHC-stabilized "allotropes" of "E 2 " have been reported for Si/Ge from Group 14, and P/As from Group 15.…”

“…Proton affinity (PA) has been used to measure the potential for atom centers to act as ligands for related compounds, such as 1 and 2. [11,13,24] The first (PA1) and second (PA2) proton affinities for 5 and 6 (Table 1) are very high. PA1 ranges from 1163 kJ mol À1 for 5 Me to 1256 kJ mol À1 for 6 Ph.…”

Carbodiylides C(ECp*)2 (E=B–Tl): Another Class of Theoretically Predicted Divalent Carbon(0) Compounds

“…[4] New carbodicarbenes and related compounds have recently been calculated in a theoretical study [5] that showed that other divalent carbon(0) compounds had already been previously synthesized, but the donor-acceptor bonds had not been identified. [6] It was suggested that, in the light of recent theoretical and experimental findings, there should be a rethinking regarding the bonding of carbon.…”