Carbones as Ligands in Novel Main‐Group Compounds E[C(NHC)₂]₂ (E=Be, B⁺, C²⁺, N³⁺, Mg, Al⁺, Si²⁺, P³⁺): A Theoretical Study

Abstract

Quantum chemical calculations of the main-group compounds E[C(NHC ) ] (E=Be, B , C , N , Mg, Al , Si , P ) have been carried out using density functional theory at the BP86/def2-TZVPP and BP86-D3(BJ)/def2-TZVPP levels of theory. The geometry optimization at BP86/def2-TZVPP gives equilibrium structures with two-coordinated species E and bending angles C-E-C between 152.5° (E=Be) and 110.5° (E=Al). Inclusion of dispersion forces at BP86-D3(BJ)/def2-TZVPP yields a three-coordinated beryllium compound Be[C(NHC ) ] as the only energy minimum form. Three-coordinated isomers are found besides the two-coordinated energy minima for the boron and carbon cations B[C(NHC ) ] and C[C(NHC ) ] . The three-coordinated form of the boron compound is energetically lower lying than the two-coordinated form, while the opposite trend is calculated for the carbon species. The theoretically predicted bond dissociation energies suggest that all compounds are viable species for experimental studies. The X-ray structure of the benzoannelated homologue of P[C(NHC ) ] that was recently reported by Dordevic et al. agrees quite well with the calculated geometry of the molecule. A detailed bonding analysis using charge and energy decomposition methods shows that the two-coordinated neutral compounds Be[C(NHC ) ] and Mg[C(NHC ) ] possess strongly positively charged atoms Be and Mg. The carbodicarbene groups C(NHC ) serve as acceptor ligands in the compounds and may be sketched with dative bonds (NHC ) C←E→C(NHC ) (E=Be, Mg). Dative bonds in which the carbones C(NHC ) are donor ligands are suggested for the cations (NHC ) C→E←C(NHC ) (E=B , Al ). The dications and trications possess electron-sharing bonds in which the bonding situation is best described with the formula [(NHC ) C] -E-[C(NHC ) ] (E=C, Si, N , P ).

How to Bend a Cumulene

“…The structure of 3NHC is almost linear,s oIe xpected al inear 4NHC.M oreover, the DI and E int values of the C1ÀC2 bond of the linear 4NHC are higher than the values of the bent structure,b oth indices suggesting as tronger C1ÀC2 bond in the linear configuration. So, what makes the bent structurem ore stable?F igure 4a shows the PES as af unction of q angle for as eries of [4]cumulenes with carbene terminal groups (tricarbodicarbenes, LÀC= C=CÀL) that have increasing p-accepting nature. Only 4NHC has as table bent configuration.…”

“…Contrary to classical allenes, the bent allene, 2NHC,h as two lone pairs on the same carbon that can form covalentb onds with somem etals. [7,8] Therefore, it is expected that the bent 4NHC will have some similarf eatures that distinguish it from the resto ft he [4]cumulenes.T he Laplacian of the electron density (r 2 1)s hows ac hargec oncentration (CC) in the vicinity of C2 and C4 ( Figure 5). These two CCs correspond to the lone pairs of these atoms, which have a HOMOÀ1o rbital (see the Supporting Information).…”

“…If these carbon atoms are poor p acceptors, the structure can lose its linearity. If ound that the minimum energy structure of one [4]cumulene or tricarbodicarbene is bent( 4NHC). The bentg eometry confers some uniquef eatures that are not present in classical linear cumulenes.…”

“…All the CAAC cumulenes are in a trans conformation. PESs as af unction of the angle C1-C2-C3 were calculated while forcing the other terminal angle (C2-C3-C4 for [3]cumulenes, C3-C4-C5 for [4]cumulenes, C4-C5-> C6 for [5]cumulenes, C5-C6-C7 for [6]cumulenes, and C6-C7-C8 for [7]cumulenes) to be equal to the first one. All the other coordinates were optimized during the geometry scan.…”

“…Carbones( CL 2 )h ave recently received considerable attention because of their interesting bondings cenarios. [1][2][3][4][5][6][7][8][9][10][11][12][13] The bonding in these compounds has been reported to consist of the s donation of the ligandsLto af ormally carbon(0) atom (L! C !…”