Abstract
One of the potential challenges for transition metal catalyzed methane partial oxidation is the reductive elimination of MeX from catalytic intermediates. For example, Rh complexes are known to initiate C–H activation reactions, but reductive elimination processes from RhIII are inhibited thermodynamically and/or kinetically. Also, in general, oxidative addition and reductive elimination reactions are central steps in many catalytic processes, and controlling the energetics of reaction intermediates is key to enabling efficient catalysis. A series of oxidative addition and reductive elimination reactions using (RPNP)RhX complexes (R = tert-butyl, iso-propyl, mesityl and phenyl; X = Cl, I) was studied to deduce the impact of the size of the phosphine substituents. Using (RPNP)RhCl as starting material, oxidative addition of MeI was observed to produce (RPNP)Rh(Me)(I)Cl, which was followed by reductive elimination of MeCl to form (RPNP)RhI. The thermodynamics and kinetics vary with the identity of the substituent "R" on phosphorus of the PNP ligand. Large steric bulk (R = tert-butyl, mesityl) favors RhI compared with two smaller substituents (R = iso-propyl, phenyl). An Eyring plot for the oxidative addition of MeI to (tBuPNP)RhCl in THF-d8 is consistent with a polar two-step reaction pathway, and the formation of [(tBuPNP)Rh(Me)I]I is also consistent with this mechanism. DFT calculations show steric bulk affects the reaction energies of addition reactions that generate six-coordinate complexes by tens of kcal·mol−1. Ligand steric bulk is calculated to have a reduced effect (a few kcal·mol−1) on SN2 addition barriers, which only require access to one side of the square plane.
Apart from the steric affect based on PNP ligands, another strategy, which is blocking one coordination site of the Rh center with to destabilize higher oxidation state, is applied to facilitate the reductive elimination from Rh–Me complexes. This is realized by the design of "capping arene" ligands (FP), which is a series of ligands with arene moieties that serve to block the coordination of other ligands. Four (FP)RhIII(Me)(TFA)2 complexes were synthesized and tested for their performance in reductive elimination of MeX (X = halide or pseudohalide). Compared with 6-FP [8,8'-(1,2-phenylene)diquinoline] and 6-NPFP [8,8'-(1,2- naphthalene)diquinoline] ligated complexes, 5-FP [1,2-bis(N-7-azaindolyl)benzene] and 5-NPFP [1,2-bis(N-7-azaindolyl)naphthalene] counterparts provide longer Rh–arene distances and lead to weaker or negligible η2 coordination between Rh and arene motif, which might destabilize RhIII species. Consistent with our hypothesis, 5-FP and 5-NPFP complexes demonstrate better performance in the reductive elimination of MeX. The reductive elimination can also be induced by oxidants. With the addition of 2 equiv. of AgOTf, 87(2)% yield of MeTFA can be achieved in CD3CN at 90 °C using (5-FP)Rh(Me)(TFA)2.
To study the potential of olefin functionalization catalyzed by Rh and Ir, a series of olefin coordinated RhI and IrI complexes bearing "capping arene" ligands were synthesized and characterized. There are structural differences that are a function of both the identity of the capping arene ligand and the identity of metal. For 5-XFP ligands (1,2-bis(N-7-azaindolyl)benzene and its variations on the "capping arene" moiety), the coordination with both Rh and Ir gives rise to complexes that are best described as 16-electron square planar. But, for 6-XFP ligands (8,8'-(1,2-phenylene)diquinoline and its variations on the capping arene moiety), the structures of Rh and Ir complexes are best described as 18-electron trigonal bipyramidal due to an η2-C,C interaction between the metal center and the arene of the capping arene ligand. The position of η2 interaction on the arene is variable as a function of the substituents on the capping arene moiety. NMR spectroscopic studies of ethylene rotation demonstrated that the Ir complexes possess a higher activation barrier to rotation than Rh complexes, and the capping arene ligands tend to give increasing rotational barrier following the order of 5-XFP < 6-FP < 6-Me,FFP < 6-NPFP in (FP)M(C2H4)Cl-type complexes.
Promoting the migratory insertion step without the use of precious Ru salts has been one of the next steps to modify the industrialized process of methanol carbonylation to produce acetic. In our preliminary studies, capping arene ligated Ir complexes are shown to serve as catalyst precursors for methanol carbonylation without additives. There is a slightly different coordination mode between 5-XFP and 6-XFP Ir complexes in the η2 interaction, and this might play a role in the dissociation of iodide ligand after the oxidative addition of MeI, which is a key step in promotion of migratory insertion.
