Wrapping Rhodium in a Borane Canopy: Implications for Hydride Formation and Transfer

Abstract

The unique reactivity of metal hydrides has encouraged considerable advances in both synthesis and catalysis. In this arena, ligand optimization has played a significant role, wherein metal−ligand cooperativity can be employed to guide reactivity. Of the organometallic hydride donors, rhodium(I) bis(diphosphine) compounds are known to be among the strongest with hydricity (ΔG H − ) values near that of conventionally used reducing agents: e.g., trialkyl-substituted borohydrides (ΔG H − ≈ 26 kcal mol −1 for [HBEt 3 ]  ). Herein, we assess the thermodynamic site of hydride transfer using a rhodium(I) compound bearing a boron-rich diphosphine ligand: [Rh I (P 2 B Cy 4 ) 2 ]BPh 4 (P 2 B Cy 4 = 1,2-bis(di(3-dicyclohexylboraneyl)propylphosphino)ethane). The divergent reactivity of [Rh I (P 2 B Cy 4 ) 2 ]BPh 4 and its all-alkyl relative [Rh I (dnppe) 2 ]BPh 4 (dnppe = 1,2-bis(di-n-propylphosphino)ethane) toward hydride donors is underscored, where for the former the preferred site of hydride transfer is the ligand scaffold and not rhodium. This result is contrasted by facile generation of [Rh I (dnppe) 2 (H)]a species having a value of ΔG H − ostensibly similar to that of [Rh I (P 2 B Cy 4 ) 2 (H)] (which is not observed), underscoring a noninnocent and cooperative role for the boron-rich secondary coordination sphere of the P 2 B Cy 4 scaffold. The use of the ligand to serve as a hydride source was accordingly examined.

A guide to secondary coordination sphere editing

“…102,103 In a series of studies, the authors coordinated this ligand to several late-transition metals including nickel (Ni), 104,105 cobalt (Co), 106 and rhodium (Rh). 107 To follow, this tutorial highlights differential reactivity witnessed between such P 2 B Cy4 -ligated compounds and their ''carbon-only''-substituted analogues.…”

“…Primary efforts focused on reaction of [Rh(P 2 B Cy4 ) 2 ] + with n-BuLi, a known method to deliver Rh-H units following 1-butene eliminination. 107 Density functional theory determined that the thermodynamic site of hydride transfer was at rhodium(I) and not at boron. This contribution suggests that the boron-rich SCS is a convenient placeholder for reactive hydride equivalents.…”

Rhodium disulfur and dioxygen complexes: examination of boron secondary coordination sphere effects

“…(1) 12 (dnppe = 1,2-bis(di-n-propylphosphino)ethane) was first screened as a structural isostere devoid of borane functionality (Figure 4). Exposure of a degassed THF solution of 1 to 1 atm of dry O2 resulted in a lightening of its dark orange color.…”

“…In a recent contribution, we reported on the synthesis of a rhodium(I) bis(diphosphine) compound, [Rh I (P2B Cy 4)2]BPh4 (1) (P2B Cy 4 = 1,2-bis(di(3-dicyclohexylboraneyl)propylphosphino)ethane) that was found to be reactive toward nucleophiles (hydride and alkyl anions), undergoing functionalization at its boron-rich SCS and not at rhodium. 12 This report was particularly thought-provoking given the similarity in hydride Å. 5 Taking the interacting and non-interacting isomers, an energy difference (DE) of +0.4 kcal mol -1 was calculated with a barrier height ((DE † ) taken as the energy difference between the lowest and highest energy isomer) of only +2.4 kcal mol -1 .…”

Toggling the Z-type interaction off-on in nickel-boron dihydrogen and anionic hydride complexes

“…Within the last decade the use of a Lewis acidic borane as a supporting moiety 4 has proven to be an effective strategy in bolstering first-row transition metal-based H 2 and hydride reactivity. 5 We have found that bonding a heavy group 13 center (Al, Ga, In) to a d 10 nickel atom engenders catalytic hydrogenation reactivity. 6,8 b Despite precedence in the literature, 7 the Ni–B pairing has until now remained elusive in our double-decker ligand scaffold, [N(( o -C 6 H 4 )NCH 2 P i Pr 2 ) 3 ] 3− (abbrev.…”