Oriented internal electrostatic fields: an emerging design element in coordination chemistry and catalysis

Abstract

The power of oriented electrostatic fields (ESFs) to influence chemical bonding and reactivity is a phenomenon of rapidly growing interest. The presence of strong ESFs has recently been implicated as one of the most significant contributors to the activity of select enzymes, wherein alignment of a substrate's changing dipole moment with a strong, local electrostatic field has been shown to be responsible for the majority of the enzymatic rate enhancement. Outside of enzymology, researchers have studied the impacts of "internal" electrostatic fields via the addition of ionic salts to reactions and the incorporation of charged functional groups into organic molecules (both experimentally and computationally), and "externally" via the implementation of bulk fields between electrode plates. Incorporation of charged moieties into homogeneous inorganic complexes to generate internal ESFs represents an area of high potential for novel catalyst design. This field has only begun to materialize within the past 10 years but could be an area of significant impact moving forward, since it provides a means for tuning the properties of molecular complexes via a method that is orthogonal to traditional strategies, thereby providing possibilities for improved catalytic conditions and novel reactivity. In this perspective, we highlight recent developments in this area and offer insights, obtained from our own research, on the challenges and future directions of this emerging field of research.

Fig. 1. Electric field lines for (a) point charges of opposite sign in close proximity, and (b) separated anodic and cathodic plates of a capacitor.

Fig. 2. Enolization reaction of 5-androsten-3,17-dione occurring at the active site of ketosteroid isomerase (KI). Red arrows indicate dipole moments associated with the CO and C–O bonds. 19-nortestosterone was used as a vibrational probe for measuring local ESF strengths via Stark spectroscopy.

Fig. 3. (a) Experimental set-up for the Darwish, Ciampi, Diez-Perez and Coote study of the Diels–Alder reaction between an STM tip and a gold surface. (b) Resonance structures of a proposed Diels–Alder reaction transition state; the minor resonance contributor (right) can be stabilized by a properly aligned ESF (shown with an arrow).

Fig. 4. Parallel electrode plates used to impose external ESFs during the Rh porphyrin-catalyzed rearrangement of 1-diazo-3,3-dimethyl-5-phenylhex-5-en-2-one into products a and b. Key transition states associated with the formation of a and b, which are inequivalently (de)stabilized by the presence of ESFs, are shown.

Fig. 5. Various complexes that have been used to delineate inductive vs. electrostatic effects. For (a) and (b) “none” indicates the salen complex containing methoxy group substitution at the 3-positions of the salen rings in place of the crown ether functionality.

Fig. 6. Ligand design strategy and application of a series of phosphinimine-substituted tren ligands (P₃tren) and their Cu^I coordination complexes. ESFs emanating from the phosphonium residues in the secondary coordination spheres stabilize the a₁-symmetric d_z² orbital, resulting in unique 2 : 1 : 2 d-orbital splitting patterns that lead to Jahn–Teller distortions on oxidation to Cu^II.

Fig. 7. DFT-calculated electrostatic potential maps along the canonical z-axes for various copper(i) complexes. Reproduced from ref. with permission from the Royal Society of Chemistry.

Fig. 8. Valence manifold tuning at an Fe^III amido complex supported by an SCS pincer ligand. The full [K(crown)(thf)₂]⁺ ion was found to be critical for reproducing experimental observables.

Fig. 9. (a) Iron tetraphenylporphyrin-based CO₂ reduction electrocatalysts containing either cationic (–NMe₃⁺) or anionic (–SO₃⁻) substituents at different positions on the porphyrin phenyl rings; (b) schematic of a proposed through-space coulombic interaction between cationic ammonium groups on the tetraphenylprophyrin ligand and a bound CO₂˙⁻ molecule; and (c) a series of iron tetraphenylporphyrin complexes in which inductive effects were systematically modulated through incorporation of pentafluorophenyl substituents.

Fig. 10. Examples of charge incorporation into the secondary coordination spheres of CO₂ reduction electrocatalysts.

Fig. 11. An oxygen reduction reaction (ORR) electrocatalyst with pendant –NMe₃⁺ units in the secondary coordination sphere.

Fig. 12. (a) An iron oxo complex shown to engage in fast C–H hydroxylation chemistry, and (b) an Fe^III hydroxide complex shown to engage in unprecedentedly fast HAT reactivity with C–H bonds.

Fig. 13. Oxidative addition transition states for (a) reaction without the presence of a chloride anion, (b) with the presence of a non-covalently bound chloride anion, and (c) with an artificial oriented ESF (3.6 V nm⁻¹) in place of the chloride anion.

Fig. 14. Aerobic oxygenations of cyclohexene using Yang's Fe^III salen catalyst with redox innocent metals tethered in the secondary coordination spheres.

Fig. 15. Mn^V nitride complexes that were shown to display inverse scaling relationships between Mn^V/Mn^IV redox potentials and the rate of N₂ release upon oxidation.

Fig. 16. Top: major resonance contributors to the ground state of a novel phosphine selenide architecture bearing an appended –BF₃⁻ unit. Bottom: the computational use of a point charge near a neutral varient (–Et) of the phosphine was needed to mimic experimental observables for the properties of the phosphine selenide.

Fig. 17. Au^I-catalyzed rearrangement of aryl alkynyl sulfides to dihydrobenzothiepinones. The regioselectivity of the reaction was found to be dependent on the amount and distribution of charge density accumulated within the transition state.

Fig. 18. Au^I-catalyzed hydroarylation reaction in which product regioselectivity is modulated via electrostatic interaction between the cationic Au^I complex and the SbF₆ counteranion. Substrate substitution: R = Cl, Br, I, OTf, Me, 3,4-(CH₂)₃, OMs, OMe, and OAc.

Fig. 19. Ruthenium water oxidation catalysts containing (a) both cationically and anionically charged pyridyl ligands, (b) cationically charged pyridyl ligands, and (c) anionically charged pyridyl ligands. (d) Schematic showing how various combinations of complexes shown in (a), (b), and (c) may be used to ease the formation of the pre-reactive dimer complex.

Fig. 20. Cupric superoxide complexes supported by P₃tren ligands are affected by electrosteric repulsion, whereby increasing the charge in the secondary coordination sphere enhances the thermal stability of the Cu^II : O₂˙¹⁻ unit by preventing the formation of a bridging 1,2 peroxide.

Alexander B. Weberg

Ryan P. Murphy

Neil C. Tomson