Supporting Information PlaceholderABSTRACT: [Cp*Rh] hydride complexes are invoked as intermediates in certain catalytic cycles, but few of these species have been successfully prepared and isolated, contributing to a relative shortage of information on the properties of such species. Here, the synthesis, isolation, and characterization of two new [Cp*Rh] hydrides are reported; the hydrides are supported by the chelating diphosphine ligands bis(diphenylphosphino)methane (dppm) and 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos). In both systems, reduction of precursor Rh(III) chloride complexes with Na(Hg) results in clean formation of isolable, formally 18e -Rh(I) species, and subsequent protonation by addition of near-stoichiometric quantities of anilinium triflate to the Rh(I) species returns high yields of the desired monohydride complexes. Single-crystal X-ray diffraction data for these compounds provide evidence of direct Rh-H interactions, confirmed by complementary infrared spectra showing Rh-H stretching frequencies at 1982 cm -1 (for the dppm-supported hydride) and 1936 cm -1 (for the Xantphos-supported hydride). Findings from comprehensive multinuclear NMR experiments reveal the properties of the unique and especially rich spin systems for the dppm-supported hydride; multifrequency NMR studies in concert with spectral simulations enabled full characterization of splitting patterns attributable to couplings involving diastereotopic methylene protons for this complex. Taken together with prior reports of related monohydrides, the results show that the reduction/protonation reaction sequence is modular for preparation of [Cp*Rh] monohydrides supported by diverse diphosphine ligands spanning from four-to eight-membered rhodacycles. ASSOCIATED CONTENT Supporting InformationNMR spectra and characterization of complexes and detailed information about performed simulations, reactivity, and X-ray crystallographic data (PDF)Cartesian coordinates (XYZ) Accession CodesCCDC 2044718-2044719 and 2067363-2067366 contain the supplementary crystallographic data for compounds 1-6. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing

Half-sandwich rhodium monohydrides are often proposed as intermediates in catalysis, but little is known regarding the redox-induced reactivity accessible to these species. Here, the κ 2bis-diphenylphosphinoferrocene (dppf) ligand has been used to explore the reactivity that can be induced when a [Cp*Rh] monohydride undergoes remote (dppf-centered) oxidation by 1e -. Chemical and electrochemical studies showed that one-electron redox chemistry is accessible to Cp*Rh(dppf), including a unique quasi-reversible Rh II/I process at -0.96 V vs. ferrocenium/ferrocene (Fc +/0 ). This redox manifold was confirmed by isolation of an uncommon Rh(II) species that was characterized by EPR spectroscopy. Protonation of Cp*Rh(dppf) with anilinium triflate yielded an isolable and inert monohydride, and this species was found to undergo a quasireversible electrochemical oxidation at +0.41 V vs Fc +/0 that corresponds to iron-centered oxidation in the dppf backbone. Thermochemical analysis predicts that this dppf-centered oxidation drives a dramatic increase in acidity of the Rh-H moiety by 23 pK a units, a reactivity pattern confirmed by in situ 1 H NMR studies. Taken together, these results show that remote oxidation can effectively induce M-H activation and suggest that ligand-centered redox activity could be an attractive feature for design of new systems relying on hydride intermediates.

Chromium precatalysts typically undergo in situ or ex situ activation to enable olefin oligomerization. The activation processes significantly impact catalytic reactivity, but the conditions used often involve low concentrations of chromium, increasing the difficulty of obtaining mechanistic insights. Here, we describe investigations into the redox chemistry accessible to a family of Cr III precatalysts for ethylene oligomerization. Cyclic voltammetry (CV) studies of the Cr III complexes 1, 2, and 3 (Cr(P,N)-Cl 3 (THF), where (P,N) represents one of a series of bidentate phosphorus-and nitrogen-containing ligands and THF is innersphere tetrahydrofuran) reveal single quasi-reversible reductions for each of the complexes, all near −1.5 V versus ferrocenium/ ferrocene (Fc +/0 ). CV also indicates minor formation of heterogeneous material upon electrochemical reduction; these processes were quantified with electrochemical quartz crystal microbalance (EQCM) studies and X-ray photoelectron (XP) spectroscopy. Electron paramagnetic resonance (EPR; at X-band) studies of the paramagnetic Cr III complexes are reported as well as studies of the in situ reduction of the Cr III complexes. Treatment of the Cr III complexes with decamethylcobaltocene (Cp* 2 Co), triethylaluminum (AlEt 3 ), or modified methylaluminoxane (MAO) primarily results in production of S = 2 Cr II species. Minor quantities (ca. 1% conversion) of Cr I species are also produced. The structure of the Cr II form of 1 was obtained from single-crystal X-ray diffraction analysis and is consistent with the proposed reactivity pattern. Titration of 1 with Cp* 2 Co reveals isosbestic behavior in UV−vis spectra consistent with one-electron reduction of 1, confirming these findings. Notably, however, reduction of 1 with 2 equiv of Cp* 2 Co in the presence of AlEt 3 reveals that the system can cleanly undergo further reduction by at least two electrons. Taken together, these results provide insights into the redox chemistry accessible to Cr III precatalysts for ethylene oligomerization.

Redox-induced reactions of organometallic complexes are ubiquitous in molecular electrochemistry and electrocatalysis research. However, a detailed knowledge of the kinetic parameters associated with individual elementary steps in these reactions is often challenging to obtain, limiting an understanding of the reactivity pathways that can be used to construct new catalytic cycles. Here, the kinetics of redox processes in model [Cp*Rh] complexes have been explored with substituted bis­(2-pyridyl)­methane (dipyridylmethane, dpma) ligands. Complementing prior work with [Cp*Rh] complexes bearing 2,2′-bipyridyl ligands, we find that the redox chemistry in these species is strongly affected by the disrupted inter-ring conjugation of dpma ligand frameworks. In particular, [Cp*Rh] complexes bearing κ2-dpma ligands with varying substitution at the bridging methylene position undergo a unique electrochemical–chemical (EC) process upon reduction from Rh­(II) to Rh­(I) as observed by cyclic voltammetry; transient electrogenerated Rh­(I) species undergo a ligand rearrangement that results in facial η2 coordination of one pyridine motif on the dpma platform. Studies of a family of [Cp*Rh] complexes bearing dimethyl (Me2dpma)-, dibenzyl (Bn2dpma)-, methyl,methylpyrenyl- (MePyrdpma)-, and bis­(methylpyrenyl) (Pyr2dpma)-substituted dpma ligands reveal a uniform trend in the first-order rate constants associated with this EC process involving ligand rearrangement, providing kinetic insight into a key process that enables the stabilization of low-valent rhodium by substituted dpma-type ligands.

Half-sandwich rhodium monohydrides are often proposed as intermediates in catalysis, but little is known regarding the redox-induced reactivity accessible to these species. Here, the κ2-bis-diphenylphosphinoferrocene (dppf) ligand has been used to explore the reactivity that can be induced when a [Cp*Rh] monohydride undergoes remote (dppf-centered) oxidation by 1e–. Chemical and electrochemical studies showed that one-electron redox chemistry is accessible to Cp*Rh(dppf), including a unique quasi-reversible RhII/I process at –0.96 V vs. ferrocenium/ferrocene (Fc+/0). This redox manifold was confirmed by isolation of an uncommon Rh(II) species that was characterized by EPR spectroscopy. Protonation of Cp*Rh(dppf) with anilinium triflate yielded an isolable and inert monohydride, and this species was found to undergo a quasireversible electrochemical oxidation at +0.41 V vs Fc+/0 that corresponds to iron-centered oxidation in the dppf backbone. Thermochemical analysis predicts that this dppf-centered oxidation drives a dramatic increase in acidity of the Rh–H moiety by 23 pKa units, a reactivity pattern confirmed by in situ 1H NMR studies. Taken together, these results show that remote oxidation can effectively induce M–H activation and suggest that ligand-centered redox activity could be an attractive feature for design of new systems relying on hydride intermediates.