Between T and Y: Asymmetry in the Interaction of LAu(I) with Bipy and β‐Diiminate‐like Ligands

Abstract

The combination of an LAu(I) fragment with a potentially chelating ligand L 0 bL 0 can result in different coordination modes of L 0 bL 0 : strictly monodentate, symmetrically bidentate, or intermediate with asymmetric bidentate binding of L 0 bL 0. Density Functional calculations indicate that for π-acceptor ancillary ligands L (C 2 H 4 , CO) and bis(nitrogen) donors L 0 bL 0 (bipyridine, phenanthroline, β-diiminate) symmetric chelate structures are obtained. With primarily σ-donating ancillary ligands L (Me À , Cl À , MeCN) the asymmetric coordination mode is the norm. Phosphine ancillary ligands L are on the edge and display the highest sensitivity to ligand variation. Asymmetry increases when (a) going from anionic (β-diiminate) to neutral (bipyridine, phenanthroline) bidentates L 0 bL 0 ; (b) making L 0 bL 0 less electron-rich e. g. through having aryl instead of alkyl groups at N or through introduction of CF 3 substituents. Inversion of the asymmetry through "gold hopping" is remarkably facile (barrier mostly < 6 kcal/mol, often~1 kcal/mol). The high-temperature fluxionality reported for two (PPh 3)Au(βdiiminate) complexes is tentatively assigned to imine inversion (rather than gold hopping) as the rate-limiting step.

Asymmetric Coordination Mode of Phenanthroline-like Ligands in Gold(I) Complexes: A Case of the Antichelate Effect

“…Symmetrical chelate structures are predicted for π-accepting auxiliary ligands such as C 2 H 4 and CO, which are weak σdonors. 14 It should be noted that gold(I) is not unique in the formation of such asymmetric chelates: similar asymmetric structures are also characteristic of other metal ions with d 10 electron configurations (Cu(I), Ag(I), Hg(II)). Examples are [μ 2 -bpy(Agbpy) 2 ] 2+ , 19 [μ 2 -1,4-bpy{Cu(2,2'-bpy)} 2 ] 2+ , 2020 [(1,3 − dimethyluracil-5-yl)Hg(bpy)] 21 and [MeHg(bpy)] +22, 23 complexes with an unsymmetrically coordinated bpy ligand.…”

“…Gold­(I) complexes usually have coordination numbers of two, three, or four, , but two-coordinate complexes are by far the most common. The preference of Au­(I) for the formation of a two-coordinate (linear) environment is so pronounced that the coordination of bis­(nitrogen) bidentate ligands such as 2,2’-bipyridine (bpy) and 1,10-phenanthroline (phen) does not lead to a symmetrical three-coordinate geometry but to asymmetric structures with one shortened and another elongated Au–N bond. − The exception is the [(C 2 H 4 )­Au­(F 2 bpy)] + (F 2 bpy = 5,5’-difluoro-2,2’-bipyridine) complex, which has a symmetrical structure with two almost equal Au–N distances . The preference of gold­(I) for linear geometry in [(phosphine)­Au­( N , N -ligand)] complexes can be explained in terms of the concept of the three-center four-electron (3c/4e) bond, according to which one of the nitrogen atoms of the bidentate ligand donates a lone pair to the empty Au–P antibonding orbital, forming a 3c/4e P–Au–N bond. , In this case, the 3c/4e bonding interaction is maximized when three atoms approach the linear geometry.…”

“…These ligands include phosphines and CH 3 – . Symmetrical chelate structures are predicted for π-accepting auxiliary ligands such as C 2 H 4 and CO, which are weak σ-donors …”