The 1,4-Diphosphabuta-1,3-diene Ligand for Coordination of Divalent Group 13 and 14 Elements:  A Density Functional Study

Abstract

The 1,4-diphosphabuta-1,3-diene (DPB) ligand as a tool for stabilizing anionic group 13 (B, Al, Ga) and neutral group 14 (C, Si, Ge) cyclic Arduengo-type carbenes is studied by quantum chemical calculations at density functional level. Accordingly, for the former group this ligand is better suited than the corresponding 1,4-diazabuta-1,3-diene (DAB) ligand. It results in larger electron affinities for the corresponding doublet states. For the latter group the DPB ligand yields essentially smaller singlet-triplet separations than the DAB ligand. An exception is the anionic boron compound with relative low singlet stability for both the DAB and DPB ligands.

Bonding aspects of P-heterocyclic carbene transition metal complexes. A computational assessment

“…2 As already mentioned, [M]-NHC complexes as well as the NHC ligands 1 and 2 adopt C 2v symmetry with planar coordination around nitrogen, whereas PHC carbenes 3 and 4 and their [M]-PHC complexes adopt C 2 symmetry, with pyramidalization of the phosphorus atoms and a slight tilting of the five-membered ring. For carbene 4, the non-planar geometry around phosphorus has been explained in the theoretical study by Schoeller and co-workers, who trace this effect to the strong tendency for pyramidalization of trigonal phosphorus, which is in part due to the fact that phosphorus has a tendency to suffer from orbital non-hybridazation [34]. Two geometric arrangements possessing pyramidal P-atoms are conceivable, one structure displaying anti-pyramidalization and adopting C 2 symmetry, the other displaying syn-pyramidalization and adopting C s symmetry.…”

“…The ongoing search for phosphorous stabilized carbenes is supported by theoretical calculations, which not only provide insight into the chemical bonding of these hitherto unknown compounds, but which also suggest design strategies and identify desirable structural elements [30][31][32][33][34]. A major step was made when stable amino(phosphino) carbenes (I) could be isolated and characterized [35].…”

On the ligand properties of the P- versus the N-heterocyclic carbene for a Grubbs catalyst in olefin metathesis

“…In the NHC carbene the replacement of the nitrogen by phosphorus atoms causes a strong decrease in the singlet-triplet (S-T) energy separation [52]. The lone pairs at P are less capable for p-donation into the vacant p-orbital at the carbene unit than those at N. Furthermore, the tendency for trigonal phosphorus to pyramidalize is per se larger than for trigonal nitrogen.…”

A Stable P‐Heterocyclic Carbene

“…Previous calculations have shown that the nitrogen centers of the parent NHC A N (Scheme 1) are in a perfectly planar environment, [19,20] whereas the phosphorus centers of the corresponding PHC A P are strongly pyramidalized, as expected. [21,22] Consequently, the singlet-triplet gap drops from 79 kcal mol À1 for A N [19] to 21 kcal mol À1 for A P , [22] although the latter is predicted to be highly unstable with respect to dimerization. [21] However, there are several ways to decrease the inversion barrier at phosphorus, as discussed recently in a comprehensive review, [23] the simplest being to use bulky substituents.…”