Rh Complexes with Pincer Carbene CNC Lutidine-Based Ligands: Reactivity Studies toward H₂Addition

Abstract

Lutidine-based NHCs pincer precursors CNC Me and CNC Mes were combined with [Rh­(acac)­(nbd)] (acac = acetylacetonate; nbd = 2,5-norbornadiene) in the presence of Cs2CO3 to yield complexes [(CNC) Me Rh­(nbd)]­PF6 (3) and [(CNC) Mes Rh­(NCMe)]­PF6 (4), respectively. While in 3, the nbd diolefin remains coordinated, in 4, the voluminous mesityl ligands induce nbd decoordination, so acetonitrile stabilizes Rh­(I) adduct 4, which in turn undergoes substitution reactions with a number of neutral ligands to produce cationic complexes [(CNC) Mes Rh­(L)]­PF6 (L = CO (5), PMe2Ph (6), PEt3 (7), C2H4 (8)). These complexes have been studied through VT NMR measurements and the molecular structures by X-ray crystallography in the case of adducts 4–7. Deprotonation reactions on cationic complexes 3–6 with hard bases yielded the corresponding neutral complexes [(CNC)* Me Rh­(nbd)] (9) and mesityl derivatives [(CNC)* Mes Rh­(L)] (L = NCCH3 (10), CO (11), PMe2Ph (12)), all of which are dearomatized complexes due to the deprotonation of one of the methylene arms, a situation confirmed by the X-ray molecular structure of carbonyl adduct 5. We studied the reactivity toward dihydrogen with neutral adducts 11 and 12. The electron rich phosphane 12 adds H2 through oxidative addition, affording the bis­(hydrido) Rh­(III) complex [(CNC)* Mes Rh­(PMe2Ph)­H2] (14). However, the carbonyl adduct 11 reacts with H2 in a different way, so that the central pyridinic ring becomes hydrogenated, breaking the aromaticity and leading to complex [(CNC-H2)* Mes Rh­(CO)] (13), isolated as a mixture of two isomers. DFT studies were carried out in order to establish the mechanisms followed on these hydrogenations, finding that, while the phosphane adduct reacts following an oxidative addition mechanism, the carbonyl complex follows a more complex base-induced profile due to the instability of hydride carbonyl species.

Efficient solventless dehydrogenation of formic acid by a CNC-based rhodium catalyst

“…However, upon addition of HCOONa (10 mol%), catalyst 1 decomposed FA (20 μL, 0.53 mmol) at 353 K in less than one minute, resulting in the production of a gaseous mixture. It is worth mentioning that the related complexes [(CNC) Mes Rh(L)]PF 6 (L = CO, NCCH 3 ), 10 bearing non-phosphine ligands, did not show any catalytic activity on FA dehydrogenation under the same conditions.…”

“…The difficulty in achieving net addition of dihydrogen to 1 is in contrast to the behaviour observed for the related deprotonated amido complex [(CNC) Mes *Rh(PMe 2 Ph)], which easily adds dihydrogen to afford the corresponding bis(hydrido) Rh III complex [(CNC) Mes *Rh(PMe 2 Ph)H 2 ]. 10…”

“…9 More specifically, we were interested in the chemistry of a pincer ligand based on a lutidine core bearing two NHC moieties in combination with rhodium, namely complex [(CNC) Mes Rh(PMe 2 Ph)]PF 6 ( 1 ) (Scheme 1). 10 Herein we report the catalytic activity of complex 1 in FA dehydrogenation under solvent-free conditions at 353 K to yield exclusively carbon dioxide and dihydrogen. The catalytic efficiency can be tuned by the addition of sodium formate and also by the presence of water.…”

Recent developments on the transformation of CO2utilising ligand cooperation and related strategies

“…Other transition metal complexes with pincer ligands containing symmetrical N-heterocyclic carbene (NHC) donors were reported. These include NHCs based on a lutidine central unit, including CNN 147,148 and CNC 149 systems. These also showed interesting reactivity in the activation of small molecules via the dearomatisation–aromatisation process.…”

“…The Casado and Polo groups further reported on the reactivity of a series of cationic rhodium complexes bearing pincer carbene CNC ligands, as outlined in Scheme 41. 149,151 Deprotonation of complex 134 was achieved with a strong base, resulting in the formation of dearomatised neutral complex 135. This complex was found to react with CO2 via a MLC activation.…”