Rh Complexes with Pincer Carbene CNC Lutidine-Based Ligands: Reactivity Studies toward H2 Addition

Lutidine-based NHCs pincer precursors CNCMe and CNCMes were combined with [Rh(acac)(nbd)] (acac = acetylacetonate; nbd = 2,5-norbornadiene) in the presence of Cs2CO3 to yield complexes [(CNC)Rh-Me(nbd)]PF6 (3) and [(CNC)Rh-Mes(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)Rh-Mes(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)*Rh-Me(nbd)] (9) and mesityl derivatives [(CNC)*Rh-Mes(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 H-2 through oxidative addition, affording the bis(hydrido) Rh(III) complex [(CNC)*Rh-Mes(PMe2Ph)H-2] (14). However, the carbonyl adduct 11 reacts with H-2 in a different way, so that the central pyridinic ring becomes hydrogenated, breaking the aromaticity and leading to complex [(CNC-H-2)*Rh-Mes(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.

Automated summary

New complexes were synthesized by reacting lutidine-based NHCs with [Rh(acac)(nbd)] under the presence of Cs2CO3, leading to the formation of a variety of cationic and neutral complexes. These complexes were characterized by VT NMR measurements and X-ray crystallography, with reactivity studies towards hydrogenation revealing different mechanisms based on the ligands involved.