Carbones - A Classification on the Magnetic Criterion

Carbones (carbodiphosphoranes, bent allenes and chalcogen-stabilized carbones) bear the same resonance contributor X+ -C2- -Y+ (X+ , Y+ =PR3 + , CR2 + , SR2 + , SeR2 + , S+ R2 =NR) and exhibit unique bonding and donating properties at the central carbon atom. A classification is given on basis of both the geometry and the magnetic properties (13 C chemical shift of the central carbon atom and the spatial magnetic properties, through-space NMR shieldings (TS NMRSs), actually the anisotropy effect or the ring current effect of aromatic species). TS NMRS values have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and the results visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. The synergy of geometry (linear or bent, orthogonal or twisted structures) and NMR characteristics (extend of the high field shift of the central carbon atom, anisotropy effect of the allene-like C=C double bonds or the ball-like anisotropy effect of carbone-like central carbon atom) provides a comprehensive picture of the dominating resonance contributor.

Synthesis, Electronic Structure, and Reactivities of Two-Sulfur-Stabilized Carbones Exhibiting Four-Electron Donor Ability

Bis(sulfane)carbon(0) (BSC; Ph₂ S→C←SPh₂ (1)) is successfully synthesized by deprotonation of the corresponding protonated salt 1⋅HTfO. The diprotonated salt 1⋅(HTfO)₂ as the starting material can be also easily accessed by the deimination of iminosulfane(sulfane)carbon(0) (iSSC)⋅HBF₄ . Density functional theory calculations revealed the peculiar electronic structure of 1, which has two lone pairs of electrons at the central carbon atom. The largest proton affinities (PA(1): 297.5 kcal mol^-1 ; PA(2): 183.7 kcal mol^-1 ) and the highest energy levels of the HOMOs (HOMO: -4.89 eV; HOMO-1: -5.02 eV) for 1 among the two-sulfur-stabilized carbones clearly indicate the strong donor ability of carbon center stabilized by two S^II ligands. The donating ability of these lone pairs of electrons is demonstrated by the C-diaurated and C-proton-aurated complexes, which provide the first experimental evidence for two-sulfurstabilized carbones behaving as four-electron donors. Furthermore, the syntheses and application of Ag^I carbone complexes as carbone transfer agents are also reported.

Synthesis, Structure, and Reactivities of Iminosulfane- and Phosphane-Stabilized Carbones Exhibiting Four-Electron Donor Ability

Iminosulfane(phosphane)carbon(0) derivatives (iSPCs; Ar3 P→C←SPh2 (NMe); Ar=Ph (1), 4-MeOC6 H4 (2), 4-(Me2 N)C6 H4 (3)) have been successfully synthesized and the molecular structure of 3 characterized. Carbone 3 is the first thermally and hydrolytically stable carbone stabilized by phosphorus and sulfur ligands. DFT calculations reveal the electronic structures of 1-3, which have two lone pairs of electrons at the carbon center. First and second proton affinity values are theoretically calculated to be in the range of 286.8-301.1 and 189.6-208.3 kcal mol(-1) , respectively. Cyclic voltammetry measurements reveal that the HOMO energy levels follow the order of 3>2>1 and the HOMO of 3 is at a higher energy than those of bis(chalcogenane)carbon(0) (BChCs). The reactivities of these lone pairs of electrons are demonstrated by the C-diaurated and C-proton-aurated complexes. These results are the first experimental evidence of phosphorus- and sulfur-stabilized carbones behaving as four-electron donors. In addition, the reaction of hydrochloric salts of the carbones with Ag2 O gives the corresponding Ag(I) complexes. The resulting silver(I) carbone complexes can be used as carbone transfer agents. This synthetic protocol can also be used for moisture-sensitive carbone species.

Coordination chemistry at carbon: the patchwork family comprising (Ph3P)2C, (Ph3P)C(C2H4), and (C2H4)2C

The revitalized concept of "coordination at carbon" allows relationships between seemingly unrelated families of carbon-centered compounds to be discovered generating fascinating patchwork families of compounds. It is shown how olefins and cyclopropanes can be regarded as donors for carbon acceptors C(1), C(2), and C(3). Through this approach, hydrocarbons such as spiropentane and dicyclopropylidene are found to be counterparts of the bis-ylidic carbodiphosphoranes and the corresponding mixed mono-ylidic systems.

Stable cyclic carbenes and related species beyond diaminocarbenes

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ-donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.