Pyridine versus acetonitrile coordination in rhodium–N-heterocyclic carbene square-planar complexes

Abstract

Experimental and theoretical studies on the factors that control the coordination chemistry of N-donor ligands in square-planar complexes of the type RhCl(NHC)L(1)L(2) (NHC = N-heterocyclic carbene) are presented. The dinuclear complexes [Rh(μ-Cl)(IPr)(η(2)-olefin)]2 {IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-carbene} have been reacted with different combinations of ligands including pyridine, acetonitrile, 2-pyridylacetonitrile, triphenylphosphine, tricyclohexylphosphine, carbon monoxide or molecular oxygen. In addition, the reactivity of RhCl(IPr)(PPh3)2 has also been studied. Pyridine preferentially coordinates trans to the carbene ligand whereas π-acceptor ligands (olefin, CO or PPh3) are prone to bind cis to IPr and trans to chlorido, unless steric bulk hinders the coordination of the ligand (PCy3). In contrast, acetonitrile is more labile than pyridine but is able to form complexes coordinated cis-to-IPr. Molecular dioxygen also displaces the labile cyclooctene ligand in RhCl(IPr)(η(2)-coe)(py) to give a square-planar dioxygen adduct which can be transformed into a peroxo derivative by additional coordination of pyridine. Charge decomposition analysis (CDA) shows that σ-donation values are similar for coordination at cis- or trans-IPr positions, whereas efficient π-backbonding is significantly observed at cis position being the favoured coordination site for π-acceptor ligands. The Rh-IPr rotational barrier in a series of square-planar complexes has been analysed. It has been found that the main contribution is the steric hindrance of the ancillary ligand. The presence of a π-donor ligand such as chlorido slows down the dynamic process.

Convenient synthetic access to fluorescent rhodacyclopentadienes via ligand exchange reactions

“…The 13 C{ 1 H} NMR spectra of both complexes 12 and 13 display a doublet of a doublet at d ¼ 174 ppm with coupling constants of J P-C ¼ 146 Hz and J Rh-C ¼ 48 Hz, which we assign to the carbene carbon atoms bound to the rhodium metal center. Similar rhodium-C(NHC) coupling constants in the range of 40e60 Hz have been reported [29e31], and 2 J phosphorus-carbene-carbon atom couplings above 100 Hz are commonly observed if the phosphine ligand is trans to the NHC ligand [32]. Respective cis P-C couplings, on the other hand, are often not larger than 20 Hz [32].…”

“…Similar rhodium-C(NHC) coupling constants in the range of 40e60 Hz have been reported [29e31], and 2 J phosphorus-carbene-carbon atom couplings above 100 Hz are commonly observed if the phosphine ligand is trans to the NHC ligand [32]. Respective cis P-C couplings, on the other hand, are often not larger than 20 Hz [32].…”

“…However, phosphines and carbenes are both considered moderate trans influence ligands [28,34] and indeed similar observations have been made by other groups. Nolan and co-workers for example observed only small elongations of the Rh-P bond length of about 0.015 Å in rhodium complexes where a trans phosphine ligand is exchanged for a NHC ligand [32].…”

Preparation of Butadienylpyridines by Iridium‐NHC‐Catalyzed Alkyne Hydroalkenylation and Quinolizine Rearrangement

“…The most noticeable feature of the 1 H NMR spectrum is the appearance of only one septuplet at around δ 3.3 ppm ( J H ‐ H ≈ 6 Hz) corresponding to the CH isopropyl protons of the IPr ligand and a shielded doublet at δ 2.9 ppm ( J H ‐ H ≈ 10 Hz) for the =CH protons of cyclooctene ligand, which is in agreement with the presence of a symmetry plane containing the O,O’ ‐bidentate ligand and bisecting the IPr and coe ligands. However, free rotation of the IPr ligand around the Ir−C bond axis is also required to render the four CH‐isopropyl protons equivalent [29] . In addition, the 13 C{ 1 H} NMR spectrum shows the characteristic resonance of the carbenic carbon atom around δ 155 ppm [30]…”

Pd-N-Heterocyclic carbene catalysed Suzuki-Miyaura coupling reactions in aqueous medium

“…The development of N-heterocyclic carbenes (NHC) and their different metal complexes has provided a new approach in homogeneous catalysis. 1 With these developments, numerous metal-NHC complexes with the inclusion of Ag, 2 Ru, 3 Ir, 4 Rh, 5 and Pd 6 have been prepared. Although there are numerous metal complexes of NHCs, Pd-NHC complexes have particular importance due to their robustness regarding air, moisture and high temperature.…”