Synthesis, Characterization, and Theoretical Investigation of a Transition State Analogue for Proton Transfer during C–H Activation by a Rhodium-Pincer Complex

Abstract

A heterobimetallic rhodium-pincer complex bearing a phenylzinc ligand was synthesized and characterized by multinuclear NMR, COSY, NOESY, and X-ray crystallography. The crystal structure of this complex shows that it possesses a bridging Rh− Zn−C fragment with a geometry similar to the Rh−H−C fragment in a proposed transition state for metal to ligand proton transfer during redox-neutral C−H activation with dearomatized rhodium pincer complexes. Bonding analysis indicates that these fragments are isolobal, suggesting that the transition state analogue models not only the structure but also the bonding interactions that underlie metal−ligand cooperativity in the C−H activation transition state. The similarity of the transition state and its analogue prompted re-evaluation of the relevant rate equations to determine the relative contributions of viable proton transfer pathways. Parallel analysis of the transition state and its isolobal analogue thus serves as a bridge between theory and experiment that is rarely available in studies of bonding in transition states.

An iridium complex with an unsupported Ir—Zn bond: diiodido(η5-pentamethylcyclopentadienyl)bis(trimethylphosphane)iridiumzinc(Ir—Zn) benzene hemisolvate

“…The angles about Zn are 127.19 (5) for Ir1-Zn-I1, 126.73 (5) for Ir1-Zn1-I2, and 105.11 (4) for I1-Zn1-I2. The larger Ir-Zn-X angles and the comparably small X-Zn-X angle are consistent with what has been observed for the ZnBr 2 and ZnCl 2 fragments in the existing Rh-Zn complexes (Gair et al, 2019). These complexes had Rh1-Zn1-X1 (where X = Br or Cl) angles of 130.14 (2) and 130.26 (4) , Rh1-Zn1-X2 angles of 120.42 (2) and 120.31 (11) , and X1-Zn1-X2 angles of 109.43 (2) and 109.41 (4) .…”

“…1. The Ir1-Zn1 distance is 2.452 (1) Å , which is within the expected distance range when compared to other examples of M-Zn bonds, specifically those of unsupported Rh-Zn bonds, which were determined to be 2.4224 (6) Å for Rh-ZnCl 2 and 2.4147 (5) Å for Rh-ZnBr 2 (Gair et al, 2019). The only crystallographically characterized Ir-Zn bonds are those of a structure that contains a bridging zinc dihalide, which leads to expected longer M-Zn bond distances of 2.563 (1) and 2.566 (1) Å (Kimura et al, 2012).…”

“…For iridium, it appears, in fact, that regardless of the ligand coordination sphere there is no single example of an unsupported iridum-zinc bond. The scarcity of examples for iridium contrasts with the situation of rhodium, for which a couple of examples of unsupported Rh-ZnX 2 structures exist with a PNP 'pincer' providing the coordination environment at rhodium (Gair et al, 2019). Additionally, several Rh-Zn structures exist with Zn-Cp* and Zn-C environments (Cadenbach et al, 2009).…”

Effects of Additives on Catalytic Arene C–H Activation: Study of Rh Catalysts Supported by Bis-phosphine Pincer Ligands

“…Intermediates 10 – 12 were also formed when heating ( R PNP)­Rh I complexes 1 – 3 in HOAc/toluene- d 8 solution at 150 °C without the presence of Cu II or Ag I salt. This observation excludes the possibility of the intermediate being ( R PNP)­Rh I –M complexes (M = Cu or Ag), which could also have a similar 1 J Rh,P of approximately 120 Hz and could be formed by heating the Rh I complex in the presence of metal salts. ,, Also, heating complexes 1 – 3 to 150 °C in toluene- d 8 did not result in the formation of 10 – 12 , but intermediates 10 – 12 were observed when heating Rh III –H complexes 4 – 6 in toluene- d 8 . The 13 C­{ 1 H} NMR spectrum of intermediate 10 reveals a doublet of triplets at 194.7 ppm ( 1 J Rh,C = 70 Hz, 2 J P,C = 15 Hz) (Figure ), and the reaction solution exhibits an IR absorption at 1985 cm –1 .…”

Insight into the Scope and Mechanism for Transmetalation of Hydrocarbyl Ligands on Complexes Relevant to C–H Activation

“…Overall, our study of the scope of hydrocarbyl group transfer between transition metals reveals that a variety of organometallic species relevant to C–H activation are capable of transferring hydrocarbyl fragments to Pt­(II) electrophiles . A substituent effect study indicates that electron-rich hydrocarbyl fragments are transferred more rapidly than electron-poor fragments.…”