Reactions of hydrogen or deuterium molecule with a rhodium(II) metalloradical: kinetic evidence for a four-centered transition state

Metalloradical Catalysis: General Approach for Controlling Reactivity and Selectivity of Homolytic Radical Reactions

Since Friedrich Wöhler's groundbreaking synthesis of urea in 1828, organic synthesis over the past two centuries has predominantly relied on the exploration and utilization of chemical reactions rooted in two-electron heterolytic ionic chemistry. While one-electron homolytic radical chemistry is both rich in fundamental reactivities and attractive with practical advantages, the synthetic application of radical reactions has been long hampered by the formidable challenges associated with the control over reactivity and selectivity of high-energy radical intermediates. To fully harness the untapped potential of radical chemistry for organic synthesis, there is a pressing need to formulate radically different concepts and broadly applicable strategies to address these outstanding issues. In pursuit of this objective, researchers have been actively developing metalloradical catalysis (MRC) as a comprehensive framework to guide the design of general approaches for controlling over reactivity and stereoselectivity of homolytic radical reactions. Essentially, MRC exploits the metal-centered radicals present in open-shell metal complexes as one-electron catalysts for homolytic activation of substrates to generate metal-entangled organic radicals as the key intermediates to govern the reaction pathway and stereochemical course of subsequent catalytic radical processes. Different from the conventional two-electron catalysis by transition metal complexes, MRC operates through one-electron chemistry utilizing stepwise radical mechanisms.

Site-Isolated Rhodium(II) Metalloradicals Catalyze Olefin Hydrofunctionalization

Rh(II) porphyrin complexes display pronounced metal-centered radical character and the ability to activate small molecules under mild conditions, but catalysis with Rh(II) porphyrins is extremely rare. In addition to facile dimerization, Rh(II) porphyrins readily engage in kinetically and thermodynamically facile reactions involving two Rh(II) centers to generate stable Rh(III)-X intermediates that obstruct turnover in thermal catalysis. Here we report site isolation of Rh(II) metalloradicals in a MOF host, which not only protects Rh(II) metalloradicals against dimerization, but also allows them to participate in thermal catalysis. Access to PCN-224 or PCN-222 in which the porphyrin linkers are fully metalated by Rh(II) in the absence of any accompanying Rh(0) nanoparticles was achieved via the first direct MOF synthesis with a linker containing a transition-metal alkyl moiety, followed by Rh(III)-C bond photolysis.

Iridium-Catalyzed Hydrogenation of a Phenoxy Radical to the Phenol: Overcoming Catalyst Deactivation with Visible Light Irradiation

Piano-stool iridium hydride complexes bearing phenylpyridine ligands are effective precatalysts for promoting the formation of element-hydrogen bonds using H2 as the stoichiometric H-atom source. Irradiation with blue light resulted in a profound enhancement of catalyst turnover for the iridium-catalyzed hydrogenation of the aryloxyl radical 2,4,6-tBu3-C6H2O• to the corresponding phenol. Monitoring the progress of the reaction revealed the formation of an iridium 3,3-dimethyl-2,3-dihydrobenzofuranyl compound arising from two C-H activation events following the proton-coupled electron transfer (PCET) step. Under thermal conditions, this compound was inactive for catalytic aryloxide hydrogenation, representing a deactivation pathway. Irradiation with blue light under H2 released the free heterocycle and regenerated the piano-stool iridium hydride precatalyst, establishing a pathway for catalyst recovery and overall enhanced turnover.

Photo-Initiated Cobalt-Catalyzed Radical Olefin Hydrogenation

Outer‐sphere radical hydrogenation of olefins proceeds via stepwise hydrogen atom transfer (HAT) from transition metal hydride species to the substrate. Typical catalysts exhibit M−H bonds that are either too weak to efficiently activate H₂ or too strong to reduce unactivated olefins. This contribution evaluates an alternative approach, that starts from a square‐planar cobalt(II) hydride complex. Photoactivation results in Co−H bond homolysis. The three‐coordinate cobalt(I) photoproduct binds H₂ to give a dihydrogen complex, which is a strong hydrogen atom donor, enabling the stepwise hydrogenation of both styrenes and unactivated aliphatic olefins with H₂ via HAT.

Proton-Coupled Group Transfer Enables Concerted Protonation Pathways Relevant to Small-Molecule Activation

The mechanistic identification of Nature's use of concerted reactions, in which all bond breaking and bond making occurs in a single step, has inspired rational designs for artificial synthetic transformations via pathways that bypass high-energy intermediates that would otherwise be thermodynamically and kinetically inaccessible. In this contribution we electrochemically activate an organometallic Ruthenium(II) complex to show that, in acetonitrile solutions, the movement of protons from weak Brønsted acids, such as water and methanol, is coupled with the transfer of its negatively charged counterpart to carbon dioxide (CO₂)─a process termed proton-coupled group transfer─to stoichiometrically produce a metal-hydride complex and a carbonate species. These previously unidentified pathways have played key roles in CO₂ and proton reduction catalysis by enabling the generation of key intermediates such as hydrides and metallocarboxylic acids, while their applicability to carbon acids may provide alternative approaches in the electrosynthesis of chemical commodities via alkylation and carboxylation reactions.

Homoleptic Rhodium Pyridine Complexes for Catalytic Hydrogen Oxidation

The homoleptic rhodium pyridine complex [Rh(py)₄]⁺ ([1]⁺) is prepared from simple precursors. Lacking good π-acceptor ligands but being sterically protected, [1]⁺ reversibly oxidizes to colorless [Rh(py)₄(thf)₂]²⁺. This monomeric S = 1/2 Rh(II) complex activates H₂ to give [HRh(py)₄L]²⁺, which can also be generated by protonation of [1]⁺. The Rh(III)-H bond is weak, being susceptible to H atom abstraction as well as deprotonation. These results underpin a novel catalytic system for the oxidation of H₂ by ferrocenium.

Metal/Ligand Proton Tautomerism Facilitates Dinuclear H2 Reductive Elimination

Using the doubly protic bis-pyrazole-pyridine ligand (N(NN^H)₂), we have synthesized an octahedral Ir^III-H [HIr(κ³-N(NN^H)(NN^-))(CO)(^tBuPy)]⁺ ([1-MH]⁺) from an Ir^I starting material. This hydride was generated by adding sufficient electron density to the metal center such that it became the thermodynamically preferred site of protonation. It was observed via UV-vis spectroscopy that [1-MH]⁺ establishes a [^tBuPy] dependent equilibrium with a ligand protonated square-planar Ir^I [Ir(N(NN^H)₂)(CO)]⁺ ([2-LH]⁺). This example of metal/ligand proton tautomerism is unusual in that the position of the equilibrium can be controlled by the concentration of exogeneous ligand (i.e., ^tBuPy). This equilibrium was shown to be key to the reactivity of the Ir^III-H; 2 equiv of [1-MH]⁺ release H₂, converting to the Ir^II dimer [[Ir(N(NN^-)(NN^H))(CO)(^tBuPy)]₂]²⁺ ([7]²⁺) under mild conditions (observable at room temperature). Mechanistic evidence is presented to support that this dinuclear reductive elimination occurs by tautomerization of the metal hydride [1-MH]⁺ to a ligand protonated species [1-LH]⁺, from which ligand dissociation is facile, generating [2-LH]⁺. Subsequent reaction of [2-LH]⁺ with [1-MH]⁺ allows for production of H₂ and the Ir^II dimer [7]²⁺. The tautomerization between the metal-hydride and the ligand protonated species provides a low energy pathway for ligand dissociation, opening the needed coordination site. The ability to control the interconversion between a metal-hydride and a ligand-protonated congener using an exogeneous ligand introduces a new strategy for catalyst design with proton responsive ligands.

Rhodium porphyrin molecule-based catalysts for the hydrogenation of biomass derived levulinic acid to biofuel additive γ-valerolactone

RhTPP and RhTCPP were prepared and utilized for the conversion of levulinic acid to γ-valerolactone and the reaction mechanism was proposed.

Zerovalent Rhodium and Iridium Silatranes Featuring Two-Center, Three-Electron Polar σ Bonds

Species with 2-center, 3-electron (2c/3e^- ) σ bonds are of interest owing to their fascinating electronic structures and potential for interesting reactivity patterns. Report here is the synthesis and characterization of a pair of zerovalent (d⁹ ) trigonal pyramidal Rh and Ir complexes that feature 2c/3e^- σ bonds to the Si atom of a tripodal tris(phosphine)silatrane ligand. X-ray diffraction, continuous wave and pulse electron paramagnetic resonance, density-functional theory calculations, and reactivity studies have been used to characterize these electronically distinctive compounds. The data available highlight a 2c/3e^- bonding framework with a σ*-SOMO of metal 4- or 5d_z ² parentage that is partially stabilized by significant mixing with Si (3p_z ) and metal (5- or 6p_z ) orbitals. Metal-ligand covalency thus buffers the expected destabilization of transition-metal (TM)-silyl σ*-orbitals by d-p mixing, affording well-characterized examples of TM-main group, and hence polar, 2c/3e^- σ "half-bonds".

Synthesis and applications of rhodium porphyrin complexes

A review on rhodium porphyrin chemistry, ranging from synthesis and properties to reactivity and application.

A T-Shaped Nickel(I) Metalloradical Species

A T-shaped Ni^I complex was synthesized using a rigid acridane-based pincer ligand to prepare a metalloradical center. Structural data displays a nickel ion is embedded in the plane of a PNP ligand. Having a sterically exposed half-filled dx2-y2 orbital, this three-coordinate Ni^I species reveals unique open-shell reactivity including the homolytic cleavage of various σ-bonds, such as H-H, N-N, and C-C.

The cobalt hydride that never was: revisiting Schrauzer's "hydridocobaloxime"

Molecular cobalt-dmg (dmg = dimethylglyoxime) complexes are an important class of electrocatalysts used heavily in mechanistic model studies of the hydrogen evolution reaction (HER). Schrauzer's early isolation of a phosphine-stabilized "[H-Co(III)(dmgH)2P(nBu)3]" complex has long provided circumstantial support for the plausible intermediacy of Co(III)-H species in HER by cobaloximes in solution. Our investigation of this complex has led to a reassignment of its structure as [Co(II)(dmgH)2P(nBu)3], a complex that contains no hydride ligand and dimerizes to form an unsupported Co-Co bond in the solid state. A paramagnetic S = 3/2 impurity that forms during the synthesis of [Co(II)(dmgH)2P(nBu)3] when exposed to adventitious oxygen has also been characterized. This impurity features a (1)H NMR resonance at -5.06 ppm that was recently but erroneously attributed to the hydride resonance of "[H-Co(III)(dmgH)2P(nBu)3]". We draw attention to this reassignment because of its relevance to cobaloxime hydrides and HER catalysis and because Schrauzer's "hydridocobaloxime" is often cited as the primary example of a bona fide hydride that can be isolated and characterized on this widely studied HER platform.

Iridium porphyrins in CD3OD: reduction of Ir(III), CD3-OD bond cleavage, Ir-D acid dissociation and alkene reactions

Methanol solutions of iridium(III) tetra(p-sulfonatophenyl)porphyrin [(TSPP)Ir(III)] form an equilibrium distribution of methanol and methoxide complexes ([(TSPP)Ir(III)(CD3OD)(2-n)(OCD3)n]((3+n)-)). Reaction of [(TSPP)Ir(III) with dihydrogen (D2) in methanol produces an iridium hydride [(TSPP)Ir(III)-D(CD3OD)](4-) in equilibrium with an iridium(I) complex ([(TSPP)Ir(I)(CD3OD)](5-)). The acid dissociation constant of the iridium hydride (Ir-D) in methanol at 298 K is 3.5 × 10(-12). The iridium(I) complex ([(TSPP)Ir(I)(CD3OD)](5-)) catalyzes reaction of [(TSPP)Ir(III)-D(CD3OD)](4-) with CD3-OD to produce an iridium methyl complex [(TSPP)Ir(III)-CD3(CD3OD)](4-) and D2O. Reactions of the iridium hydride with ethene and propene produce iridium alkyl complexes, but the Ir-D complex fails to give observable addition with acetaldehyde and carbon monoxide in methanol. Reaction of the iridium hydride with propene forms both the isopropyl and propyl complexes with free energy changes (ΔG° 298 K) of -1.3 and -0.4 kcal mol(-1) respectively. Equilibrium thermodynamics and reactivity studies are used in discussing relative Ir-D, Ir-OCD3 and Ir-CD2- bond energetics in methanol.

Evidence for formation of a Co-H bond from (H2O)2Co(dmgBF2)2 under H2: application to radical cyclizations

Under H(2), the radical cyclization of appropriate dienes can be catalyzed by cobaloximes. H• can be abstracted from an intermediate (presumably a cobalt hydride) by trityl radicals (Ar(3)C•) or by TEMPO. The rate-determining step in these reactions is the uptake of H(2), which is second order in cobalt and first order in hydrogen; the third-order rate constant is 106(3) M(-2)·s(-1).

Catalytic carbon-carbon σ-bond hydrogenation with water catalyzed by rhodium porphyrins

The catalytic carbon-carbon σ-bond activation and hydrogenation of [2.2]paracyclophane with water in a neutral reaction medium is demonstrated. The hydrogen from water is transferred to the hydrocarbon to furnish hydrogen enrichment in good yields.

New Routes to Low-Coordinate Iron Hydride Complexes: The Binuclear Oxidative Addition of H(2)

The oxidative addition and reductive elimination reactions of H(2) on unsaturated transition-metal complexes are crucial in utilizing this important molecule. Both biological and man-made iron catalysts use iron to perform H(2) transformations, and highly unsaturated iron complexes in unusual geometries (tetrahedral and trigonal planar) are anticipated to give unusual or novel reactions. In this paper, two new synthetic routes to the low-coordinate iron hydride complex [L(tBu)Fe(μ-H)](2) are reported. Et(3)SiH was used as the hydride source in one route by taking advantage of the silaphilicity of the fluoride ligand in three-coordinate L(tBu)FeF. The other synthetic method proceeded through the binuclear oxidative addition of H(2) or D(2) to a putative Fe(I) intermediate. Deuteration was verified through reduction of an alkyne and release of the deuterated alkene product. Mössbauer spectra of [L(tBu)Fe(μ-H)](2) indicate that the samples are pure, and that the iron(II) centers are high-spin.

A Chelate-Stabilized Ruthenium(σ-pyrrolato) Complex: Resolving Ambiguities in Nuclearity and Coordination Geometry through 1H PGSE and 31P Solid-State NMR Studies

Reaction of RuCl2(PPh3)3 with LiNN' (NN' = 2-[(2,6-diisopropylphenyl)imino]pyrrolide) affords a single product, with the empirical formula RuCl[(2,6-iPr2C6H3)N=CHC4H3N](PPh3)2. We identify this species as a sigma-pyrrolato complex, [Ru(NN')(PPh3)2]2(mu-Cl)2 (3b), rather than mononuclear RuCl(NN')(PPh3)2 (3a), on the basis of detailed 1D and 2D NMR characterization in solution and in the solid state. Retention of the chelating, sigma-bound iminopyrrolato unit within 3b, despite the presence of labile (dative) chloride and PPh3 donors, indicates that the chelate effect is sufficient to inhibit sigma --> pi isomerization of 3b to a piano-stool, pi-pyrrolato structure. 2D COSY, SECSY, and J-resolved solid-state 31P NMR experiments confirm that the PPh3 ligands on each metal center are magnetically and crystallographically inequivalent, and 31P CP/MAS NMR experiments reveal the largest 99Ru-31P spin-spin coupling constant (1J(99Ru,31P) = 244 +/- 20 Hz) yet measured. Finally, 31P dipolar-chemical shift spectroscopy is applied to determine benchmark phosphorus chemical shift tensors for phosphine ligands in hexacoordinate ruthenium complexes.