Thermodynamics of H+/H•/H–/e– Transfer from [CpV(CO)3H]−: Comparisons to the Isoelectronic CpCr(CO)3H

Cyclopentadienyl ring activation in organometallic chemistry and catalysis

The cyclopentadienyl (Cp) ligand is a cornerstone of modern organometallic chemistry. Since the discovery of ferrocene, the Cp ligand and its various derivatives have become foundational motifs in catalysis, medicine and materials science. Although largely considered an ancillary ligand for altering the stereoelectronic properties of transition metal centres, there is mounting evidence that the core Cp ring structure also serves as a reservoir for reactive protons (H⁺), hydrides (H^-) or radical hydrogen (H^•) atoms. This Review chronicles the field of Cp ring activation, highlighting the pivotal role that Cp ligands can have in electrocatalytic H₂ production, N₂ reduction, hydride transfer reactions and proton-coupled electron transfer.

Coordination-Induced Weakening of a C(sp3)-H Bond: Homolytic and Heterolytic Bond Strength of a CH-Ni Agostic Interaction

The scission of a C(sp³)-H bond to form a new metal-alkyl bond is a fundamental step in coordination chemistry and catalysis. However, the extent of C-H bond weakening when this moiety interacts with a transition metal is poorly understood and quantifying this phenomenon could provide insights into designing more efficient C-H functionalization catalysts. We present a nickel complex with a robust adamantyl reporter ligand that enables the measurement of C-H acidity (pK_a) and bond dissociation free energy (BDFE) for a C(sp³)-H agostic interaction, showing a decrease in pK_a by dozens of orders of magnitude and BDFE decrease of about 30 kcal/mol upon coordination. X-ray crystallographic data is provided for all molecules, including a distorted square planar Ni^III metalloradical and "doubly agostic" Ni^II(κ²-CH₂) complex.

Reductions of M{N(SiMe3)2}3 (M = V, Cr, Fe): Terminal and Bridging Low-Valent First-Row Transition Metal Hydrido Complexes and "Metallo-Transamination"

The reaction of the vanadium(III) tris(silylamide) V{N(SiMe₃)₂}₃ with LiAlH₄ in diethyl ether gives the highly unstable mixed-metal polyhydride [V(μ₂-H)₆[Al{N(SiMe₃)₂}₂]₃][Li(OEt₂)₃] (1), which was structurally characterized. Alternatively, performing the same reaction in the presence of 12-crown-4 affords a rare example of a structurally verified vanadium terminal hydride complex, [VH{N(SiMe₃)₂}₃][Li(12-crown-4)₂] (2). The corresponding deuteride 2D was also prepared using LiAlD₄. In contrast, no hydride complexes were isolated by reaction of M{N(SiMe₃)₂}₃ (M = Cr, Fe) with LiAlH₄ and 12-crown-4. Instead, these reactions afforded the anionic metal(II) complexes [M{N(SiMe₃)₂}₃][Li(12-crown-4)₂] (3, M = Cr; 4, M = Fe). The reaction of the iron(III) tris(silylamide) Fe{N(SiMe₃)₂}₃ with lithium aluminum hydride without a crown ether gives the "hydrido inverse crown" complex [Fe(μ₂-H){N(SiMe₃)₂}₂(μ₂-Li)]₂ (5), while treatment of the same trisamide with alane trimethylamine complex gives the iron(II) polyhydride complex Fe(μ₂-H)₆[Al{N(SiMe₃)₂}₂]₂[Al{N(SiMe₃)₂}(NMe₃)] (6). Complexes 2-6 were characterized by X-ray crystallography, as well as by infrared, electronic, and ¹H and ¹³C (complex 6) NMR spectroscopies. Complexes 1 and 6 are apparently formed by an unusual "metallo-transamination" process.

Sensory Perception of Non-Deuterated and Deuterated Organic Compounds

The chemical background of olfactory perception has been subject of intensive research, but no available model can fully explain the sense of smell. There are also inconsistent results on the role of the isotopology of molecules. In experiments with human subjects it was found that the isotope effect is weak with acetone and D6 -acetone. In contrast, clear differences were observed in the perception of octanoic acid and D15 -octanoic acid. Furthermore, a trained sniffer dog was initially able to distinguish between these isotopologues of octanoic acid. In chromatographic measurements, the respective deuterated molecule showed weaker interaction with a non-polar liquid phase. Quantum chemical calculations give evidence that deuterated octanoic acid binds more strongly to a model receptor than non-deuterated. In contrast, the binding of the non-deuterated molecule is stronger with acetone. The isotope effect is calculated in the framework of statistical mechanics. It results from a complicated interplay between various thermostatistical contributions to the non-covalent free binding energies and it turns out to be very molecule-specific. The vibrational terms including non-classical zero-point energies play about the same role as rotational/translational contributions and are larger than bond length effects for the differential isotope perception of odor for which general rules cannot be derived.

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.

Systematic Trends in the Electrochemical Properties of Transition Metal Hydride Complexes Discovered by Using the Ligand Acidity Constant Equation

Understanding the thermodynamics of paramagnetic transition metal hydride complexes, especially of the abundant 3d metals, is important in the design of electrocatalysts and organometallic catalysts. The pK_a^MeCN([MHL_n]⁺/[ML_n) of paramagnetic hydrides in MeCN are estimated for the first time using the ligand acidity constant (LAC) equation where contributions to the pK_a^MeCN from each ligand are simply added together, with the sum corrected for effects of charge and 5d metals. The pK_a^LAC-MeCN([MHL_n]⁺/ML_n) of over 200 hydride complexes MHL_n are used, along with their electrochemical potentials from the literature, in an uncommonly applied thermochemical cycle in order to reveal systematic trends in the redox couples M^III/II and M^V/IV (M = Cr, Mo, W), Mn^II/I, Re^VI/V and Re^IV/III, M^III/II and M^IV/III (M = Fe, Ru, Os), and M^III/II and M^II/I (M = Co, Rh, and Ir) and allow the estimation of the bond dissociation free energies BDFE(MH) of the unoxidized hydrides MHL_n and the prediction of the electrochemical potential for their oxidation. Density functional theory (DFT) calculations are used to validate the pK_a^LAC-MeCN values of hydrides of W^III, Mn^II, Fe^III, Ru^III, Co^II, and Ni^III. When a pK_a^LAC-MeCN is less than zero for a given complex [MHL_n]⁺, the oxidation of MHL_n is irreversible due to proton loss from the oxidized complex to the solvent. When pK_a^LAC-MeCN ≫ 0, the oxidation is reversible when there is no gross change in the coordination geometry upon a change in the redox state. Twenty paramagnetic hydrides prepared in bulk all have pK_a^LAC-MeCN > 8.

Catalytic hydrogen atom transfer to alkenes: a roadmap for metal hydrides and radicals

Hydrogen atom transfer from metal hydrides to alkenes appears to underlie widely used catalytic methods – the mechanistic implications are fascinating.

Hydrogen atom transfer from a metal hydride (MHAT) has emerged as a powerful, if puzzling, technique in chemical synthesis. In catalytic MHAT reactions, earth-abundant metal complexes generate stabilized and unstabilized carbon-centered radicals from alkenes of various substitution patterns with robust chemoselectivity. This perspective combines organic and inorganic perspectives to outline challenges and opportunities, and to propose working models to assist further developments. We attempt to demystify the putative intermediates, the basic elementary steps, and the energetic implications, especially for cage pair formation, collapse and separation. Distinctions between catalysts with strong-field (SF) and weak-field (WF) ligand environments may explain some differences in reactivity and selectivity, and provide an organizing principle for kinetics that transcends the typical thermodynamic analysis. This blueprint should aid practitioners who hope to enter and expand this exciting area of chemistry.

H2 Evolution from a Thiolate-Bound Ni(III) Hydride

Terminal Ni^III hydrides are proposed intermediates in proton reduction catalyzed by both molecular electrocatalysts and metalloenzymes, but well-defined examples of paramagnetic nickel hydride complexes are largely limited to bridging hydrides. Herein, we report the synthesis of an S = 1/2, terminally bound thiolate-Ni^III-H complex. This species and its terminal hydride ligand in particular have been thoroughly characterized by vibrational and EPR techniques, including pulse EPR studies. Corresponding DFT calculations suggest appreciable spin leakage onto the thiolate ligand. The hyperfine coupling to the terminal hydride ligand of the thiolate-Ni^III-H species is comparable to that of the hydride ligand proposed for the Ni-C hydrogenase intermediate (Ni^III-H-Fe^II). Upon warming, the featured thiolate-Ni^III-H species undergoes bimolecular reductive elimination of H₂. Associated kinetic studies are discussed and compared with a structurally related Fe^III-H species that has also recently been reported to undergo bimolecular H-H coupling.