Functionalization of Rh^III–Me Bonds: Use of “Capping Arene” Ligands to Facilitate Me–X Reductive Elimination

Abstract

We show how to improve the yield of MeX from CH 4 activation catalysts from 12% to 90% through the use of "capping arene" ligands. Four (FP)Rh III (Me)(TFA) 2 {FP = "capping arene" ligands, including 8,8′-(1,2-phenylene)diquinoline (6-FP), 8,8′-(1,2naphthalene)diquinoline (6-NP FP), 1,2-bis(N-7-azaindolyl)benzene (5-FP), and 1,2-bis(N-7-azaindolyl)naphthalene (5-NP FP)} complexes. These complexes and (dpe)Rh III (Me)(TFA) 2 (dpe = 1,2-di-2-pyridylethane) were synthesized and tested for their performance in reductive elimination of MeX (X = TFA or halide). The FP ligands were used with the goal of blocking a coordination site to destabilize the Rh III complexes and facilitate MeX reductive elimination. On the basis of single-crystal X-ray diffraction studies, the 6-FP and 6-NP FP ligated Rh complexes have Rh−arene distances shorter than those of the 5-FP and 5-NP FP Rh complexes; thus, it is expected that the Rh−arene interactions are weaker for the 5-FP complexes than for the 6-FP complexes. Consistent with our hypothesis, the 5-FP and 5-NP FP Rh III complexes demonstrate improved performance (from 12% to ∼60% yield) in the reductive elimination of MeX. The reductive elimination of MeX from (FP)Rh III (Me)(TFA) 2 can be further improved by the use of chemical oxidants. For example, the addition of 2 equiv of AgOTf leads to 87(2)% yield of MeTFA and can be achieved in CD 3 CN at 90 °C using (5-FP)Rh(Me)(TFA) 2 .

Rhodium and Iridium Complexes Bearing “Capping Arene” Ligands: Synthesis and Characterization

“…The compound 5- Me FP (4,5-bis­( N -7-azaindolyl)-1,2-dimethylbenzene, 3 ) was synthesized by a Cu-catalyzed Ullmann reaction using 2.4 equiv of 7-azaindole and 1 equiv of 4,5-dibromo-1,2-dimethylbenzene as the starting materials (Scheme ). , Other proligands, 5-FP ( 1 ), 5- NP FP ( 2 ), 6-FP ( 4 ), 6- NP FP ( 5 ), 6- Me FP ( 6 ), and 6- F FP ( 7 ), as well as some of the corresponding Rh complexes, (5-FP)­Rh­(C 2 H 4 )Cl ( 1a ), (5- NP FP)­Rh­(C 2 H 4 )Cl ( 2a ), (6-FP)­Rh­(C 2 H 4 )Cl ( 4a ), and (6- NP FP)­Rh­(C 2 H 4 )Cl ( 5a ), were synthesized following published procedures. − ,, The new Rh complexes (5- Me FP)­Rh­(C 2 H 4 )Cl ( 3a ) and (6- Me FP)­Rh­(C 2 H 4 )Cl ( 6a ) were synthesized using a method similar to the synthesis of 1a …”

“…Four broad resonances are observed in the 1 H NMR spectra for coordinated C 2 H 4 of both (6-FP)­Rh­(C 2 H 4 )Cl ( 4a ) and (6- Me FP)­Rh­(C 2 H 4 )Cl ( 6a ), and the ethylene resonances of (6- NP FP)­Rh­(C 2 H 4 )Cl ( 5a ) appear as four distinct triplets with 3 J HH = 10 Hz. Coordinated ethylene resonances in 13 C­{ 1 H} spectra of 1a – 4a and 6a appear as broad peaks, while the resonance of ethylene for 5a displays a doublet splitting pattern ( 1 J CRh = 15 Hz) . Although quantitative comparisons cannot be made without variable-temperature assessments (see below), on the basis of the observations of room-temperature 1 H and 13 C­{ 1 H} NMR spectra, the relative activation barriers for C 2 H 4 rotation of the six Rh complexes can be estimated as 5a > 6a ≈ 4a > 3a ≈ 2a ≈ 1a .…”

“…Resonances due to coordinated C 2 H 4 in the 1 H NMR spectra of complexes 1a – 6a : (a) (5-FP)­Rh­(C 2 H 4 )­Cl ( 1a ); (b) (5- NP FP)­Rh­(C 2 H 4 )Cl ( 2a ); (c) (5- Me FP)­Rh­(C 2 H 4 )Cl ( 3a ); (d) (6-FP)­Rh­(C 2 H 4 )Cl ( 4a ); (e) (6- NP FP)­Rh­(C 2 H 4 )Cl ( 5a ); (f) (6- Me FP)­Rh­(C 2 H 4 )­Cl ( 6a ) …”

“…Recently, our group reported the isolation and study of a series of “capping arene” ligated Rh complexes (Scheme ). − We were drawn to the capping arene ligand motif on the basis of the possibility of modulating the metal–arene interaction through the ligand structure and, hence, influencing metal-mediated reactivity, especially for redox reactions that involve substantial changes in the metal geometry. For example, a potential leverage point of the capping arene ligand structure is the ability to control the distance between the metal center and the arene group by adjusting the N–A backbone (see Scheme ) between the coordination atom and the arene moiety.…”

“… a The dashed line in the 6- X FP structure (bottom right) shows the η 2 coordination between the metal center and arene moiety …”