Alan Herbert Cowley. 29 January 1934—2 August 2020

Alan Cowley was one of the most creative main group chemists of his generation, and had a central role in what was often described as the renaissance of main group chemistry. Throughout his career Alan always had an eye for what was new. In his early years as an independent researcher, Alan made many fundamental contributions to the chemistry of phosphorus, not only in terms of the synthesis of new compounds but also in their study by employing novel analytical and computational methods. Starting in the 1980s he was at the forefront of emerging research into low-coordinate phosphorus chemistry and made seminal contributions in the areas of multiply bonded species, such as phosphenium ions and diphosphenes, as well as in the transition metal coordination chemistry of phosphinidenes. In the second half of his career, Alan also turned his attention to the study of single source precursors for important solid-state electronic materials, many of which were far superior to known examples. In all of the many areas in which Alan worked, he was a great collaborator with colleagues and researchers across the world, both in chemistry and in other disciplines. This was made all the easier by Alan's charm and easy-going nature, which was also manifest in the interactions he had with his many group members over a period of almost half a century. Alan was a gentleman in every sense and is much missed by friends, colleagues, collaborators and family.

The Different Story of π Bonds

We revisit “classical” issues in multiply bonded systems between main groups elements, namely the structural distortions that may occur at the multiple bonds and that lead, e.g., to trans-bent and bond-length alternated structures. The focus is on the role that orbital hybridization and electron correlation play in this context, here analyzed with the help of simple models for σ- and π-bonds, numerically exact solutions of Hubbard Hamiltonians and first principles (density functional theory) investigations of an extended set of systems.

Hydrazine Formation via NiIII-NH2 Radical Coupling in Ni-Mediated Ammonia Oxidation

<p>Given the diverse mechanistic possibilities for the overall 6e^-/6H⁺ transformation of ammonia to dinitrogen, identification of M(NH_x) intermediates involved in N–N bond formation is a central mechanistic challenge. In analogy to water oxidation mechanisms, which widely invoke metal oxo intermediates, metal imide and nitride intermediates have commonly been proposed for ammonia oxidation, and stoichiometric demonstration of N–N bond formation from these metal-ligand multiply bonded species is well-precedented. In contrast, while the homocoupling of M–NH₂ species to form hydrazine has been hypothesized as the key N–N bond forming step in certain molecular ammonia oxidation systems, well-defined examples of this transformation from M–NH₂ complexes are essentially without precedent. This work reports the first example of net ammonia oxidation mediated by a molecular Ni species, a transformation carried out via formal Ni^II/Ni^III oxidation states. The available data are consistent with a Ni^III–NH₂ intermediate featuring substantial spin at N undergoing N–N bond formation to generate a Ni^II₂(N₂H₄) complex. Additional and structurally unusual Ni_x(N_yH_z) species – including a Ni₂(<i>trans</i>-N₂H₂) complex – are characterized and studied as intermediates in the Ni-mediated ammonia oxidation cycle described herein.</p>

Hydrazine Formation via NiIII-NH2 Radical Coupling in Ni-Mediated Ammonia Oxidation

<p>Given the diverse mechanistic possibilities for the overall 6e^-/6H⁺ transformation of ammonia to dinitrogen, identification of M(NH_x) intermediates involved in N–N bond formation is a central mechanistic challenge. In analogy to water oxidation mechanisms, which widely invoke metal oxo intermediates, metal imide and nitride intermediates have commonly been proposed for ammonia oxidation, and stoichiometric demonstration of N–N bond formation from these metal-ligand multiply bonded species is well-precedented. In contrast, while the homocoupling of M–NH₂ species to form hydrazine has been hypothesized as the key N–N bond forming step in certain molecular ammonia oxidation systems, well-defined examples of this transformation from M–NH₂ complexes are essentially without precedent. This work reports the first example of net ammonia oxidation mediated by a molecular Ni species, a transformation carried out via formal Ni^II/Ni^III oxidation states. The available data are consistent with a Ni^III–NH₂ intermediate featuring substantial spin at N undergoing N–N bond formation to generate a Ni^II₂(N₂H₄) complex. Additional and structurally unusual Ni_x(N_yH_z) species – including a Ni₂(<i>trans</i>-N₂H₂) complex – are characterized and studied as intermediates in the Ni-mediated ammonia oxidation cycle described herein.</p>

On the Crystal Chemistry of Inorganic Nitrides: Crystal-Chemical Parameters and Opportunities in the Exploration of Their Compositional Space

The scarcity of nitrogen in Earth’s crust, combined with challenging synthesis, have made inorganic nitrides a relatively-unexplored class of compounds compared to their naturally-abundant oxide counterparts. To facilitate exploration of their compositional space via <i>a priori</i> modeling, and to help <i>a posteriori</i> structure verification not limited to inferring the oxidation state of redox-active cations, we derive a suite of bond-valence parameters and Lewis-acid strength values for 76 cations observed bonding to N^3-, and further outline a baseline statistical knowledge of bond lengths for these compounds. We examine structural and electronic effects responsible for the functional properties and anomalous bonding behavior of inorganic nitrides, and identify promising venues for exploring uncharted compositional spaces beyond the reach of high-throughput computational methods. We find that many mechanisms of bond-length variation ubiquitous to oxide and oxysalt compounds (e.g., lone-pair stereoactivity, the Jahn-Teller and pseudo Jahn-Teller effects) are similarly pervasive in inorganic nitrides, and are occasionally observed to result in greater distortion magnitude than their oxide counterparts. We identify inorganic nitrides with multiply-bonded metal ions as a promising venue in heterogeneous catalysis, e.g. in the development of a post-Haber-Bosch process proceeding at milder reaction conditions, thus representing further opportunity in the thriving exploration of the functional properties of this emerging class of materials.<br>

On the Crystal Chemistry of Inorganic Nitrides: Crystal-Chemical Parameters and Opportunities in the Exploration of Their Compositional Space

The scarcity of nitrogen in Earth’s crust, combined with challenging synthesis, have made inorganic nitrides a relatively-unexplored class of compounds compared to their naturally-abundant oxide counterparts. To facilitate exploration of their compositional space via <i>a priori</i> modeling, and to help <i>a posteriori</i> structure verification not limited to inferring the oxidation state of redox-active cations, we derive a suite of bond-valence parameters and Lewis-acid strength values for 76 cations observed bonding to N^3-, and further outline a baseline statistical knowledge of bond lengths for these compounds. We examine structural and electronic effects responsible for the functional properties and anomalous bonding behavior of inorganic nitrides, and identify promising venues for exploring uncharted compositional spaces beyond the reach of high-throughput computational methods. We find that many mechanisms of bond-length variation ubiquitous to oxide and oxysalt compounds (e.g., lone-pair stereoactivity, the Jahn-Teller and pseudo Jahn-Teller effects) are similarly pervasive in inorganic nitrides, and are occasionally observed to result in greater distortion magnitude than their oxide counterparts. We identify inorganic nitrides with multiply-bonded metal ions as a promising venue in heterogeneous catalysis, e.g. in the development of a post-Haber-Bosch process proceeding at milder reaction conditions, thus representing further opportunity in the thriving exploration of the functional properties of this emerging class of materials.<br>