Electronic Structures of Main-Group Carbene Analogues

Abstract

The electronic structures of fifteen Group 13-16 carbene analogues are analyzed using various quantum chemical methods and compared to the data obtained for the parent N-heterocylic carbene (NHC), imidazol-2-ylidene. The results of this study present a uniform analysis of the similarities and differences in the electronic structures of p-block main group carbene analogues. Though all systems are formally isovalent, the theoretical analyses unambiguously indicate that their electronic structures run the gamut from C=C localized (Group 13) to C=N localized (Group 16) via intermediate, more delocalized, systems. In particular, neither the stibenium ion nor any of the chalcogenium dications is a direct analogue of imidazol-2-ylidene as they all contain two lone pairs of electrons around the divalent main group center, instead of the expected one. The reason behind the gradual change in the electronic structure of main group analogues of imidazol-2-ylidene was traced to the total charge of the systems, which changes from anionic to dicationic when moving from left to right in the periodic table. Results from theoretical analyses of aromaticity show that all Group 13-16 analogues of imidazol-2-ylidene display some degree of aromatic character. The heavier Group 13 anions benefit the least from π-electron delocalization, whereas the cationic Group 15 systems are on par with the parent carbon system and display only slightly less aromatic character than cyclopentadienide, a true 6π-electron aromatic species. The σ-donor and π-acceptor ability of the different main group carbene analogues is also evaluated.

Do Extremely Bent Allenes Exist?

“…[16,18] These results can be contrasted to data reported for carbodiphosphoranes 7 which, even as isolated molecules, appear as true divalent carbon(0) compounds. [19,23,24] A comparison of 1a to data reported for imidazol-2-ylidene [16,25] and carbodicarbenes [16] reveals that, as an isolated entity, its electronic structure has more in common with the former. Hence, it can be expected that systems 1 function as two electron donors unless extremely strong electrophiles are used.…”

“…Similar behavior has already been demonstrated for the heavier group 14 analogues of NHCs which contain a bipyridyl backbone. [25,26] Surprisingly, theoretical calculations show that AlH3 forms a marginally stable (15 kJ mol −1 ) 2:1 complex even with the parent imidazol-2-ylidene. Such systems are currently under active computational investigation by our group.…”

Phospha-Fischer Carbenes: Synthesis, Structure, Bonding, and Reactions of Pd(0)− and Pt(0)−Phosphenium Complexes

“…NHC are strong -donors and weak -acceptors, whereas NHP are weak -donors and good -acceptors on account of the formal positive charge and isotropic s-(as opposed to directional sp 2 -) character of the "lone pair" orbital on phosphorus. 19 The two families should therefore show reciprocal reactivity in transition metal chemistry. Often considered "phosphine replacement" ligands, [21][22][23] NHC have triggered an explosion of research in the field of homogeneous catalysis.…”

Ligandenstabilisierte Chalkogendikationen

“…[16] Eine vergleichende Studie von Tuononen et al über die elektronische Struktur von p-Block-Elementanaloga der NHCs bestätigt noch einmal die aus Strukturdaten gewonnenen Rückschlüsse. [17] Wie in Abbildung 2 gezeigt, besteht ein besonderes Merkmal der Elemente der Gruppe 16 in einem Orbital mit p-Symmetrie für das freie Elektronenpaar (das HOMO); [18] zusätzlich dazu haben sie -analog zu den NHCs und ihren Analoga aus den -Ringe (E = S, Se) von den angelegten Kriterien ab. Eine Analyse der natürlichen Orbitalbindungsenergien ergab zwar, dass die Chalkogensysteme weniger p-stabilisiert sind als ihre Analoga der Gruppen 13-15, bei magnetischen und Molekülorbitalanalysen findet man aber charakteristische Merkmale für eine Aromatizität der Gruppe-16-Heterocyclen.…”

“…Aus den Rechnungen folgt außerdem, dass wegen der schwachen Bindung zwischen dem Diiminliganden und dem Tellurzentrum sowie der großen formalen -und berechneten -positiven Ladung am Chalkogenzentrum möglicherweise kein stabiles Telluranalogon von 1 und 2 isoliert werden kann. [17] Das gut etablierte Konzept, Kationen von elektronegativen Elementen des p-Blocks durch s-Donorliganden zu stabilisieren, lässt sich anhand der Stabilisierung des I + -Ions durch Stickstoffbasen veranschaulichen, z. B. im Bispyridinaddukt I(py) 2 + X À (X = ClO 4 , NO 3 , RCO 2 ).…”