Ligand dynamics and protonation preferences of Rh and Ir complexes bearing an almost, but not quite, pendent base

Abstract

Pendent nucleophiles are essential partners in the cleavage and formation of bonds with hydrogen (e.g. protonation/deprotonation), but binding of the pendent group to the metal and the potential trapping of complexes in inactive states are a significant problem. The dipyridylmethane-based ligand framework bis(2-pyridyl)-N-pyrrolidinomethane (CPy), bearing a hemilabile pyrrolidine moiety, has been synthesized and complexes of the type [(CPy)M(COD)]X (COD = 1,5-cyclooctadiene) were prepared. The solution-phase ligand dynamics and relative protonation preferences were investigated viaH NMR spectroscopy; although favorable, pendent amine binding does not kinetically inhibit pendent base protonation. Protonation at the metal (with concomitant pyrrolidine binding) has been found to be favorable for Ir, whereas N-protonation is favorable for Rh. DFT calculations predict that the Rh hydrides have much higher relative acidities than their Ir congeners (ΔpK ≃ 7-8 in CHCl), and are also more acidic than the strong acid [H(OEt)][B(CF)].

A Terminal Rh Methylidene from Activation of CH2Cl2

“…30 Despite these examples, haloalkanes are still relatively rare as precursors for non-halogenated metal alkylidenes (e.g., MCR 2 ) of synthetic interest. 31 One of the research areas of our group has focused on the generation of transition metal electrophiles for heterolytic C H activation chemistry, 32,33 most recently exploring late-metal pincer systems. In an effort to minimize the labilization of weakly binding alkane and arene intermediates, ligand platforms bearing a trans-ether have been targeted (e.g., dibenzofuran-and xanthene-based).…”

“…One of the research areas of our group has focused on the generation of transition metal electrophiles for heterolytic CH activation chemistry, , most recently exploring late-metal pincer systems. In an effort to minimize the labilization of weakly binding alkane and arene intermediates, ligand platforms bearing a trans -ether have been targeted (e.g., dibenzofuran- and xanthene-based). , Generation of strong transition metal electrophiles typically requires protonation or abstraction of X-type ligands in weakly polar nondonating solvents.…”

Hemilabile Proton Relays and Redox Activity Lead to {FeNO}x and Significant Rate Enhancements in NO2– Reduction

“…As in the case of NO 2 − reduction, many important biological reactions occur by managing the proton and electron flow at the enzyme active site. 18–20 To this end, there has been a resurgence into redox-active, 20,21 hemilabile, 23–27 and so called proton-responsive ligand scaffolds 28–40 However, studies that incorporate the triad of redox-activity, hemilability, and proton responsivity, all in a single ligand scaffold, are exceedingly limited. 41–47 Our group has been active in developing methodologies that control the movement of both protons and electrons for biologically relevant reactions by utilizing the redox-active pyridinediimine (PDI) scaffold merged with a proton-responsive secondary coordination sphere.…”

“…As in the case of NO 2 – reduction, many important biological reactions occur by managing the proton and electron flow at the enzyme active site. − To this end, there has been a resurgence into redox-active,,, hemilabile, − and so-called proton-responsive ligand scaffolds. − However, studies that incorporate the triad of redox activity, hemilability, and proton responsivity, all in a single ligand scaffold, are exceedingly limited. − Our group has been active in developing methodologies that control the movement of both protons and electrons for biologically relevant reactions by utilizing the redox-active pyridinediimine (PDI) scaffold merged with a proton-responsive secondary coordination sphere. − Those preliminary studies successfully showed (1) the PDI scaffold can be tailored to facilitate NO 2 – reduction and (2) the NO 2 – reduction is dependent on the protonation state of the secondary coordination sphere (proton responsivity). Furthermore, given the revelations of the role ligand hemilability plays in biomimicry, − we reasoned that the pendant base(s) could be utilized to introduce hemilability into the PDI, yielding ligand scaffolds that increase activity even further.…”

Rh Methylidenes from Redox Cascade Activation of Chloroalkanes

“…One of the research areas of our group has focused on the generation of transition metal electrophiles for heterolytic C-H activation chemistry, 29,30 most recently exploring late-metal pincer systems. In an effort to minimize the labilization of weaklybinding alkane and arene intermediates, ligand platforms bearing a trans-ether have been targeted (e.g.…”