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

Relationship between Transition-Metal Hydride Bond Lengths and Stretching Wavenumbers

Here it is demonstrated that there is a linear relationship between the terminal 3d metal hydride stretching wavenumber νMH and the metal hydride distance dMH reported to date: νMH ∼ (-1.05dMH + 3.35) × 103 cm-1. That is, upon moving from Group 3 to Group 10 metals in complexes and binary molecules, νMH (cm-1) increases as the metal-hydrogen distance dMH (Å) decreases as determined by X-ray diffraction, molecular spectroscopy, and, in one case, neutron diffraction. This trend contrasts with the relatively constant bond dissociation free energy (BDFE) of the diamagnetic complexes and the usually smaller BDFE (by as much as -20 kcal/mol) of paramagnetic ones, thus showing that there is no correlation between either νMH or its force constant with the BDFE. If a positive charge is added to the complex by metal-ion oxidation, νMH may increase or decrease depending on the coordination number and spin state of the complex.

Reactivity umpolung (reversal) of ligands in transition metal complexes

The success and power of homogeneous catalysis derives in large part from the wide choice of transition metal ions and their ligands. This tutorial review introduces examples where the reactivity of a ligand is completely reversed (umpolung) from Lewis basic/nucleophilic to acidic/electrophilic or vice versa on changing the metal and co-ligands. Understanding this phenomenon will assist in the rational design of catalysts and the understanding of metalloenzyme mechanisms. Labelling a metal and ligand with Seebach donor and acceptor labels helps to identify whether a reaction involving the intermolecular attack on the ligand is displaying native reactivity or reactivity umpolung. This has been done for complexes of nitriles, carbonyls, isonitriles, dinitrogen, Fischer carbenes, alkenes, alkynes, hydrides, methyls, methylidenes and alkylidenes, silylenes, oxides, imides/nitrenes, alkylidynes, methylidynes, and nitrides. The electronic influence of the metal and co-ligands is discussed in terms of the energy of (HOMO) d electrons. The energy can be related to the pKLACa (LAC is ligand acidity constant) of the theoretical hydride complexes [H-[M]-L]+ formed by the protonation of pair of valence d electrons on the metal in the [M-L] complex. Preliminary findings indicate that a negative pKLACa indicates that nucleophilic attack by a carbanion or amine on the ligand will likely occur while a positive pKLACa indicates that electrophilic attack by strong acids on the ligand will usually occur when the ligand is nitrile, carbonyl, isonitrile, alkene and η6-arene.

Proton transfer kinetics of transition metal hydride complexes and implications for fuel-forming reactions

Proton transfer reactions involving transition metal hydride complexes are prevalent in a number of catalytic fuel-forming reactions, where the proton transfer kinetics to or from the metal center can have significant impacts on the efficiency, selectivity, and stability associated with the catalytic cycle. This review correlates the often slow proton transfer rate constants of transition metal hydride complexes to their electronic and structural descriptors and provides perspective on how to exploit these parameters to control proton transfer kinetics to and from the metal center. A toolbox of techniques for experimental determination of proton transfer rate constants is discussed, and case studies where proton transfer rate constant determination informs fuel-forming reactions are highlighted. Opportunities for extending proton transfer kinetic measurements to additional systems are presented, and the importance of synergizing the thermodynamics and kinetics of proton transfer involving transition metal hydride complexes is emphasized.

Tetraphenylporphyrin electrocatalysts for the hydrogen evolution reaction: applicability of molecular volcano plots to experimental operating conditions

Recent years have seen an increasing interest in molecular electrocatalysts for the hydrogen evolution reaction (HER). Efficient hydrogen evolution would play an important role in a sustainable fuel economy, and molecular systems could serve as highly specific and tunable alternatives to traditional noble metal surface catalysts. However, molecular catalysts are currently mostly used in homogeneous setups, where quantitative evaluation of catalytic activity is non-standardized and cumbersome, in particular for multistep, multielectron processes. The molecular design community would therefore be well served by a straightforward model for prediction and comparison of the efficiency of molecular catalysts. Recent developments in this area include attempts at applying the Sabatier principle and the volcano plot concept - popular tools for comparing metal surface catalysts - to molecular catalysis. In this work, we evaluate the predictive power of these tools in the context of experimental operating conditions, by applying them to a series of tetraphenylporphyrins employed as molecular electrocatalysts of the HER. We show that the binding energy of H and the redox chemistry of the porphyrins depend solely on the electron withdrawing ability of the central metal ion, and that the thermodynamics of the catalytic cycle follow a simple linear free energy relation. We also find that the catalytic efficiency of the porphyrins is almost exclusively determined by reaction kinetics and therefore cannot be explained by thermodynamics alone. We conclude that the Sabatier principle, linear free energy relations and molecular volcano plots are insufficient tools for predicting and comparing activity of molecular catalysts, and that experimentally useful information of catalytic performance can still only be obtained through detailed knowledge of the catalytic pathway for each individual system.

Trans Ligand Determines the Stability of Paramagnetic Manganese(II) Hydrides of the Type trans-[MnH(L)(dmpe)2]+ Where L is PMe3, C2H4, or CO

Paramagnetic metal hydride (PMH) complexes play important roles in catalytic applications and bioinorganic chemistry. 3d PMH chemistry has largely focused on Ti, Mn, Fe, and Co. Various Mn^II PMHs have been proposed as intermediates in catalysis, but isolated Mn^II PMHs are limited to dimeric high-spin Mn^II structures with bridging hydrides. In this paper, a series of the first low-spin monomeric Mn^II PMH complexes are generated by chemical oxidation of their Mn^I analogues. This series is of the type trans-[MnH(L)(dmpe)₂]^+/0 where the trans ligand L is PMe₃, C₂H₄, or CO [dmpe is 1,2-bis(dimethylphosphino)ethane], and the thermal stability of the Mn^II hydride complexes was found to be strongly dependent on the identity of the trans ligand. When L is PMe₃, the complex is the first example of an isolated monomeric Mn^II hydride complex. In contrast, when L is C₂H₄ or CO, the complexes are only stable at low temperatures; upon warming to room temperature, the former decomposed to afford [Mn(dmpe)₃]⁺, accompanied by ethane and ethylene, whereas the latter eliminated H₂, generating [Mn(MeCN)(CO)(dmpe)₂]⁺ or a mixture of products including [Mn(κ¹-PF₆)(CO)(dmpe)₂], depending on the reaction conditions. All PMHs were characterized by low-temperature electron paramagnetic resonance (EPR) spectroscopy, and stable [MnH(PMe₃)(dmpe)₂]⁺ was further characterized by UV-vis and IR spectroscopy, Superconducting Quantum Interference Device magnetometry, and single-crystal X-ray diffraction. Noteworthy spectral properties are the significant EPR superhyperfine coupling to the hydride (∼85 MHz) and an increase (+33 cm^-1) in the Mn-H IR stretch upon oxidation. Density functional theory calculations were also employed to gain insights into the acidity and bond strengths of the complexes. Mn^II-H bond dissociation free energies are estimated to decrease in the series of complexes from 60 (L = PMe₃) to 47 kcal/mol (L = CO).

Large changes in hydricity as a function of charge and not metal in (PNP)M–H (de)hydrogenation catalysts that undergo metal–ligand cooperativity

Ligand pKa and metal hydricity scale with one another in (de)hydrogenation catalysts that undergo metal–ligand cooperativity, irrespective of metal or ligand identity. Anionic hydrides are significantly more hydridic than their neutral counterparts.