Examining the Modular Synthesis of [Cp*Rh] Monohydrides Supported by Chelating Diphosphine Ligands

Abstract

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

Evidence for Charge Delocalization in Diazafluorene Ligands Supporting Low‐Valent [Cp*Rh] Complexes**

“…[36] To confirm our assignment of these resonances, we carried out multifrequency NMR experiments at 400, 500, 600, and 800 MHz that confirm the involvement of second-order effects which are visible in the spectra as "roofing". [37,38] Digital simulations of the field-dependent spectra reproduced the experimental data, confirming our assignment (Figure S26).…”

Mechanistic Roles of Metal- and Ligand-Protonated Species in Hydrogen Evolution with [Cp*Rh] Complexes

“…Eager to further interrogate this species, we repeated this protonation experiment with IR monitoring, providing unequivocal evidence from vibrational data for the formation of [Cp*Rh(H)(bpy)] + (4) based on a characteristic Rh-H stretch at 1920 cm -1 (Figure 5). Notably, this value is similar to other Rh-H stretches (1986-1936 cm -1 ) previously reported by our group for analogous [Cp*Rh] monohydride complexes ligated by bisphosphine ligands, albeit slightly lower in energy, and is in good agreement with our DFT-calculated spectrum (1920 vs 1902 cm -1 , respectively, see SI, Figure S32) (24,26). The UV-vis spectrum of 4 displays a prominent absorption at 400 nm and weaker features that tail into the visible region.…”

“…To this point, 3 exhibits vivid pigmentation, attributable to delocalization of electron density by π-backbonding into the bpy π-system (45). The properties of bpy thus strongly influence the catalytic reactivity paradigm observed, reactivity that is not echoed in systems supported by bidentate bisphosphine ligands (24,26) or even the closely related 9,9′-dimethyl-4,5diazafluorene ligand (43), as well as impart spectral uniqueness to the [Cp*Rh] system.…”

“…Despite attempts, the putative [Cp*Rh-H] intermediate has only been observed in mixtures of products that are generated under very specific reaction conditions involving, variously, photochemical reactors, buffered solutions, or low temperatures (-40°C) (12,23). On the other hand, our group has recently developed reliable synthetic procedures to access analogous phosphine-supported [Cp*Rh] monohydrides, but these species are remarkably resistant to protonolysis due to enhanced stabilization provided by phosphine ligands (24,25,26). Contrasting with these monohydrides, isolated [(Cp*H)Rh(bpy)] + reacts with even moderate acids such as anilinium ([PhNH3] + , pKa = 10.6 in MeCN) to quantitatively yield H2 and regenerate the starting Rh(III) complex (18).…”

Efficient Nicotinamide Adenine Dinucleotide Regeneration with a Rhodium–Carbene Catalyst and Isolation of a Hydride Intermediate

“…Generally, reduction of NAD + can be conducted by chemical, − photochemical, − enzymatic, − and electrochemical means. − To be specific, vast development in the field of organometallic chemistry cultivated numerous chemical regeneration catalysts including organometallic complexes of ruthenium, iridium, − and rhodium. , Rhodium complexes, in particular, feature improved 1,4-regioselectivity, high stability, and very good activity, making rhodium-incorporated organometallic catalysts the best prospects. − The main drawbacks of hitherto presented rhodium complexes are, however, their low TOFs and a consequent increase in the concentration of added catalyst. , In order to overcome the shortcomings, improved activity of the catalyst is necessary . Extensive research has been done to address the issue, and this led to isolation of the first catalytic intermediate of [Cp*Rh­(bpy)­Cl] + with a Cp*–H fragment, (Cp*H)­Rh­(bpy)­Cl. , Furthermore, recent studies on rhodium complexes with phosphine-containing ligands reported the isolation of stable rhodium hydride complexes. − However, these findings opened a new debate regarding the structure of the intermediate through which the hydride is transferred to NAD + , either directly via a Rh–H intermediate or by its equilibrium tautomer with a η 4 -Cp*H moiety. Previous studies show that, to achieve a fast intermediate generation, increased electron density on the metal center with more electron-donating ligand systems is essential. , Yet, the relationship between the reaction rate and the form of the intermediate has not been intensely scrutinized.…”