Recent developments in the chemistry of non-trigonal pnictogen pincer compounds: from bonding to catalysis

Abstract

The combination of well-established meridionally coordinating, tridentate pincer ligands with group 15 elements affords geometrically constrained non-trigonal pnictogen pincer compounds. These species show remarkable activity in challenging element-hydrogen bond scission reactions, such as the activation of ammonia. The electronic structures of these compounds and the implications they have on their electrochemical properties and transition metal coordination are described. Furthermore, stoichiometric and catalytic bond forming reactions involving B-H, N-H and O-H bonds as well as carbon nucleophiles are presented.

Scheme 1. Examples of small molecule activation by transition metals and aluminium following oxidative addition (top) and cooperative pathways (middle, bottom), R = Ph, p-OMeBn, p-CF₃Bn.

Scheme 2. Selected reactions of the T-shaped phosphorus(i) compound I reported by Arduengo and the limiting resonance structures.

Fig. 1. Qualitative frontier orbital diagram of a σ³–Pn compound and the effect of non-trigonal perturbation towards a T-shaped structure (top). Bending mode of 1 resulting in C_2v-1 (bottom).

Fig. 2. (a) Definition of geometrical parameters used to index the computational structures of P(NH₂)₃ within local C_s symmetry. Hydrogens are omitted for clarity. (b, c and d) Contour maps depicting the (b) total electronic energy (E_tot), (c) LUMO energies (E_LUMO), (d) ΔE_HOMO/LUMO for P(NH₂)₃ structures with C_s-symmetry (N = 4141 discrete input structures in the range 80° ≤ θ ≤ 120° and 80° ≤ ϕ ≤ 180° at 1° increments). Energies are shown in units of Hartrees. Points corresponding to the structures of P(NMePh)₃, 1 and C_2v-1 are superimposed as black points. The red points correspond to the absolute minimum. Used with permission from K. Lee, A. Blake, A. Tanushi, S. McCarthy, D. Kim, S. Loria, C. Donahue, K. Spielvogel, J. Keith, S. Daly and A. T. Radosevich, Validating the Biphilic Hypothesis of Nontrigonal P(iii) Compounds, Angew. Chem. Int. Ed., John Wiley & Sons, 2019.

Scheme 3. Inversion mechanisms of trigonal pnictogens.

Fig. 3. Calculated structural trends within the series of pnictogen compounds 2-Pn ligated by an NNN pincer ligand.

Fig. 4. Series of geometrically constrained NCN ligated pnictogen compounds 3-Pn and their fluxionality in solution dependent on the central element, R = ^tBu, Mes, dmp; Mes = 2,4,6-Me₃–C₆H₂, dmp = 2,6-Me₂C₆H₃.

Scheme 4. Reduction of T-shaped pnictogens species towards dimeric dianions and monomeric radical anions, 2,2,2-crypt = N[CH₂CH₂OCH₂CH₂OCH₂CH₂]₃N.

Scheme 5. Reversible P-centred hydride and fluoride addition to transition metal complexes featuring geometrically constrained phosphorus ligands.

Scheme 6. Possible activation of E–H bonds by geometrically constrained pnictogen pincer compounds.

Scheme 7. Catalytic hydrogenation of azobenzene from ammonia–borane mediated by I (top) and cooperative addition to the diazadiphosphapentalene 14 (bottom).

Scheme 8. Hydrogenation of azoarenes and nitroarenes by ammonia-borane employing a Bi(i/iii) redox system.

Scheme 9. Catalytic hydroboration of imines by 1.

Scheme 10. Proposed mechanisms of oxidative addition of amines by I.

Scheme 11. Oxidative addition of ammonia towards ONO pincer platforms (top) and cooperative activation by a diazadiphosphapentalene (bottom).

Scheme 12. Cooperative N–H bond activation of primary amines mediated by 1, R = H, alkyl, aryl.

Scheme 13. Reaction products of oxidative addition of water and alcohols to geometrically constrained phosphorus compounds (top) and electrocatalytic generation of dihydrogen from acetic acid by 3-Bi.

Scheme 14. Reactions of geometrically constrained pnictogen species towards carbon-based nucleohpiles, R = Me, ^tBu, Ph, dppe = diphenylphosphinoethane.

Scheme 15. Reaction of NCN ligated As, Sb and Bi compounds towards electron deficient alkynes and maleimides, dmp = 2,6-Me₂C₆H₃.

Josh Abbenseth

Jose M. Goicoechea