Using internal electrostatic fields to manipulate the valence manifolds of copper complexes

Abstract

A series of tetradentate tris(phosphinimine) ligands (^R3P₃tren) was developed and bound to Cu^I to form the trigonal pyramidal, C_3v-symmetric cuprous complexes [^R3P₃tren-Cu][BAr^F₄] (1^PR3) (PR₃ = PMe₃, PMe₂Ph, PMePh₂, PPh₃, PMe₂(NEt₂), BAr^F₄ = B(C₆F₅)₄). Electrochemical studies on the Cu^I complexes were undertaken, and the permethylated analog, 1^PMe3, was found to display an unprecedentedly cathodic Cu^I/Cu^II redox potential (−780 mV vs. Fc/Fc⁺ in isobutyronitrile). Elucidation of the electronic structures of 1^PR3via density functional theory (DFT) studies revealed atypical valence manifold configurations, resulting from strongly σ-donating phosphinimine moieties in the xy-plane that destabilize 2e (d_xy/d_x²−y²) orbital sets and uniquely stabilized a₁ (d_z²) orbitals. Support is provided that the a₁ stabilizations result from intramolecular electrostatic fields (ESFs) generated from cationic character on the phosphinimine moieties in ^R3P₃tren. This view is corroborated via 1-dimensional electrostatic potential maps along the z-axes of 1^PR3 and their isostructural analogues. Experimental validation of this computational model is provided upon oxidation of 1^PMe3 to the cupric complex [^Me3P₃tren-Cu][OTf]₂ (2^PMe3), which displays a characteristic Jahn–Teller distortion in the form of a see-saw, pseudo-C_s-symmetric geometry. A systematic anodic shift in the potential of the Cu^I/Cu^II redox couple as the steric bulk in the secondary coordination sphere increases is explained through the complexes' diminishing ability to access the ideal C_s-symmetric geometry upon oxidation. The observations and calculations discussed in this work support the presence of internal electrostatic fields within the copper complexes, which subsequently influence the complexes' properties via a method orthogonal to classic ligand field tuning.

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Introduction

Frontier orbitals are at the heart of chemical reactivity. Many reactions can be simplified to an interaction between the highest occupied molecular orbital (HOMO) of one species and the lowest unoccupied molecular orbital (LUMO) of another. Deliberate control of the frontier orbital energies is therefore crucial to facilitating desirable chemistry and preventing side reactions. The development of new tools for manipulating valence manifolds may be envisioned to support the creation of unique catalysts, photochemical reagents, and separations technologies.

The active site architectures of metalloenzymes have historically provided much inspiration to this task. Current design principles such as metal choice, active-site geometries, use of pH-responsive ligands, incorporation of hydrogen-bonding moieties into secondary coordination spheres, and the use of multinuclear clusters have all received inspiration or analogy in bioinorganic chemistry. An underexplored concept from enzymology is the use of properly aligned electric fields for reaction site engineering. As described in detail below, the application of this burgeoning method to transition metal chemistry provides a promising new avenue for exerting precise control over the electronic structures and reactivity profiles of homogeneous systems. Inasmuch as orbital energies reflect the energy of a single electron experiencing the waveform and potential energy environment of the orbital, the structure of the valence manifold should be susceptible to the application of a properly aligned electrostatic field. The general importance of local electrostatic fields to enzymatic function was originally proposed by Warshel¹ and has since been studied in detail by Boxer.^2–4 Accumulated work in this area has even been used in support of the argument that electrostatic effects may be general to enzyme catalysis.^3–5

Research outside of the biochemical field has investigated the effects of electrostatic fields on the electronic structure and reactivity of various materials. This work has included (i) the use of bulk, homogeneous electric fields,^6–8 (ii) the study of anion effects on reaction selectivity,⁹ (iii) the study of electrostatic stabilization of in situ-generated radical species,^10–14 and (iv) in-depth computational studies on reaction barrier (de)stabilization of organic molecules bearing charged functional groups.^15–20 Limited examples are known of transition metal complexes that use oriented internal electrostatic fields to tune the properties of the system. The Tolman group reported cupric hydroxide compounds bound by NNN pincer-type bis(carboxamide)pyridine ligands, wherein charged residues were incorporated at the para positions of the carboxamide aryl rings.²¹ A shift of ∼0.3 V in the Cu^II/Cu^III redox couple was observed upon switching between substitution of a cationic group (–NMe₃⁺) with an anionic group (–SO₃⁻), implicating a through-space electrostatic effect on the potential of this redox couple (Fig. 1a). Similarly, using a salen-based ligand containing a crown-ether moiety, Yang and coworkers showed a systematic anodic shift in the Co^II/Co^III redox couples across a series of Co^II complexes as alkali and alkaline earth metals were tethered in the secondary coordination sphere (Fig. 1b).²² These anodic shifts correlated with the strength of electrostatic fields emanating from the redox-innocent metal, as opposed the Lewis acidity of these metals. Impressively, securing redox-innocent metals in the crown-ether pocket of this ligand was also shown to improve aerobic oxidation catalysis with Fe^III complexes²³ and promote stability across a series of Mn^V-nitride complexes.²⁴

Fig. 1 Reported homogeneous inorganic complexes containing internal electrostatic fields generated via the incorporation of charged functional groups (a, c and d) or tethered redox-innocent metals (b) into the secondary coordination sphere.

Success in this field has also been realized through incorporation of charges into the secondary coordination spheres of heme-based complexes. Savéant published an elegantly designed series of iron/porphyrin-based CO₂ electro-reduction catalysts, wherein charged functional groups were placed at different distances from the catalyst active site.²⁵ In this case, a dramatic decrease in overpotential was observed when the cationic charges were placed in closest proximity to the CO₂ binding site (Fig. 1c). Mayer reported this same complex to be a fast and efficient catalyst for the oxygen reduction reaction (ORR).²⁶ The Shaik group recently published a computational study²⁷ characterizing two Fe/porphyrin complexes that contain cationic pyridinium residues in the secondary coordination sphere. Groves, in his original report on these complexes, showed that they undergo unprecedentedly fast C–H activation kinetics (Fig. 1d for the Fe^III–OH complex).^28,29 Shaik's study was able to reproduce activation energies from the experimental data through replacement of the charged functional groups with external oriented electrostatic fields, thereby implicating the catalytic effect as being almost purely electrostatic in origin.²⁷

Beyond the electrostatic effects proposed by Tolman, Yang Savéant, and Groves/Shaik, little is known about the origin, magnitude, and impact of internal electrostatic fields on transition metal centers. Herein, we report a series of Cu^I complexes bound by novel tetradentate, tren-based tris(phosphinimine) ligands that exhibit a dramatic effect on the Cu^I/Cu^II redox potential. The major resonance contributors of the phosphinimine moieties of the ligands are the charge-separated, zwitterionic forms, in which a substantial amount of cationic charge on phosphorous results in the development of a cationic electrostatic field near an open coordination site at the metal center. The use of classical coordination chemistry, electrochemistry, and a textbook Jahn–Teller distortion allowed for an in-depth view into the effects brought by the development of strong, local electrostatic fields within this series of molecules.

Results and discussion

Ligand design and syntheses

Phosphinimines are often employed as strong, neutral, organic bases. Some are commercially available, and their pK_a values can range from 26 to 50, allowing for a multitude of applications.^30,31 These high basicities are on par with, or stronger than, the neutral bases 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU), 1,1,3,3-tetramethylguanidine (TMG), and triazabicyclodecene (TBD),³² and can be attributed to significant anionic character at the nitrogen atoms. Dyson and coworkers used DFT calculations to support the presence of significant ylidic character at the N–P bond of HN=PH₃ (δ_N = −1.28; δ_P = +1.14).³³ We calculated similar charge distributions for a series of P-substituted N-methyl phosphinimines, with the most notable change from the parent phosphinimine being an increase in the partial positive charge on phosphorus (δ_P) as alkyl/aryl/amido groups are introduced on phosphorus (Fig. 2).

Fig. 2 Important resonance structures and select partial charges of truncated phosphinimine ligands as determined by natural population analyses (NPA). These charges are consistent with those calculated by Dyson and coworkers.³³

We next sought to incorporate phosphinimine moieties into a multidentate ligand scaffold^34–41 with the intention of positioning multiple cationic phosphonium residues about a single open coordination site at a metal center. Ideally, the ligand would simultaneously take advantage of the strong σ-donation offered by the Lewis basic phosphinimine nitrogens.^42–47 A tris(2-aminoethyl)amine (tren) ligand backbone was selected due to the multitude of complexes bound by related ligands that have been described in the literature. The novel series of tren-based tris(phosphinimine)ligands (^R3P₃tren) were synthesized via the Staudinger reaction (Scheme 1), wherein the ratio of aryl:alkyl substituents bound to the phosphorous atoms in the secondary coordination sphere was systematically modulated (PR₃ = PMe₃, PMe₂Ph, PMePh₂, PPh₃). Additionally, in order to probe the impact of incorporating a strong electron-donating group bound to phosphorous, a PR₃ = PMe₂(NEt₂) substituted ligand was synthesized following the same reaction scheme. These ligands were characterized using standard analytical techniques (see ESI†). The ³¹P{¹H} NMR spectral resonances were identified at ca. 3 ppm, with a modest upfield shift as the number of aryl groups bound to phosphorous was increased. Notably, the ³¹P{¹H} NMR spectral resonance for the PR₃ = PMe₂(NEt₂) derivative was observed significantly downfield of the rest (20.79 ppm), indicative of a more electron-deficient phosphorous atom. This is consistent with the calculated δ_P values, which showed a marked increase for the PMe₂(NEt₂)-substituted phosphinimine compared to the all-alkyl/aryl-substituted analogs.

Scheme 1 Syntheses of ^R3P₃tren ligands and their Cu^I complexes.

Synthesis of [^R3P₃tren-Cu]⁺ complexes (1^PR3) and electrochemical studies

Treatment of ^R3P₃tren with 1 equiv. of CuBAr^F₄·4MeCN (BAr^F₄ = B(C₆F₅)₄⁻) in tetrahydrofuran (THF) afforded the cuprous complexes 1^PR3 as white (1^PMe3) or yellow (1^PMe2Ph, 1^PMePh2, 1^PPh3, 1^PMe2(NEt2)) solids (Scheme 1). Crystallographic analyses of 1^PMe3, 1^PMe2Ph, and 1^PPh3 revealed trigonal pyramidal complexes (Fig. 3). The Cu–N_ax distances range from 2.175–2.236 Å (1^PMe2Ph < 1^PMe3 < 1^PPh3), and the Cu–N_eq distances range from 2.017–2.081 Å (1^PMe3 < 1^PPh3 < 1^PMe2Ph). Neither trend was found to correlate with the steric bulk of the phosphorus substituents. Notably, the vacant axial coordination site of 1^PMe3 is retained in the solid state, irrespective of the identity of the anion or the presence of Lewis basic solvents (see ESI†). NMR spectra of 1^PR3 were obtained in CD₃CN and found to display C_3v symmetry on the NMR timescale; single ³¹P{¹H} resonances were located downfield (between 20 and 25 ppm) of those for the free ligands (^R3P₃tren). For 1^PMe2(NEt2), the single spectral resonance corresponding to the phosphorous environment is observed further downfield at 38.11 ppm.

Fig. 3 Solid state structures of (a) 1^PMe3, (b) 1^PMe2Ph, and (c) 1^PPh3. Ellipsoids shown at 50% probability; hydrogen atoms and anions omitted for clarity.

Electrochemical studies of 1^PR3 were undertaken in isobutyronitrile (IBN) and are reported relative to Fc/Fc⁺. As shown in Fig. 4, 1^PR3 displayed quasi-reversible E_1/2 features that we assign as the Cu^I/Cu^II redox couples. The most anodic feature was observed for 1^PPh3 at −415 mV. As the ratio of alkyl:aryl substituents bound to phosphorous increased, a cathodic shift in the Cu^I/Cu^II redox potential was observed, culminating with 1^PMe3 at −780 mV. This E_1/2 value is remarkably reducing, and to the best of our knowledge represents the most cathodic Cu^I/Cu^II redox couple for this geometry reported to date. The trend in redox potentials suggested that an increase in the number of alkyl groups on phosphorus caused an increase in the σ-donation of the phosphinimine nitrogen to Cu, thereby creating increasingly electron-rich metal centers across the series. However, 1^PMe2(NEt2), which would be expected to have the strongest σ-donation due to the increased anionic character on the nitrogen donor atom, was found to have a Cu^I/Cu^II couple of −740 mV, a value that is 40 mV more anodic than the Cu^I/Cu^II redox couple of 1^PMe3 and similar to the redox couple observed for 1^PMe2Ph. Combined with the structural data, these results indicate that the secondary coordination sphere plays a role in the electronic structures of the metal centers across this series. We thus turned to density functional theory (DFT) to generate an impression of the valence manifold that could be carried into further experimental studies with this system.

Fig. 4 Cyclic voltammograms of 1^PMe3 (red), 1^PMe2Ph (blue), 1^PMe2(NEt2) (black), 1^PMePh2 (green), and 1^PPh3 (purple), showing their Cu^I/Cu^II redox couples in isobutyronitrile (IBN).

DFT calculations on 1^PR3 and comparison to literature analogs

DFT calculations on 1^PR3 were compared to a series of literature examples selected for the similarities they offer in terms of structure and donor type (Fig. 5a). The most facile point of comparison is tren-Cu^I, which shares the architecture and N₄ donor set of P₃tren. We have similarly included the TMG₃tren-Cu^I complex due to parallels between the basic guanidyl groups of this ligand and the phosphinimine groups on 1^PR3. Other recent ligand design efforts have generated Cu^I centers in similar geometries but with widely varying redox potentials. Karlin and coworkers reported a series of ^XTMPA-Cu^I complexes (TMPA = tris(2-pyridylmethyl)amine) wherein a systematic change in the Cu^I/Cu^II redox couple was attained through substitution at the para-positions of the TMPA pyridyl rings.⁴⁸ Introducing electron-donating groups at these positions resulted in a shift of the Cu^I/Cu^II couple to more negative potentials, ultimately attaining a potential of −0.70 V (vs. Fc/Fc⁺) for the dimethylamino-substituted ^XTMPA derivative (^NMe2TMPA). This value was ∼0.3 V more cathodic than the Cu^I complex bound by the parent ligand (^HTMPA).

Fig. 5 (a) Cu^I complexes from the literature for computational comparison to 1^PR3, and (b) DFT-calculated MO diagram for Cu^I complexes (energies referenced arbitrarily to 1e of 1^PMe3).

DFT geometry optimizations were performed on the cations of 1^PR3, tren-Cu^I, TMG₃tren-Cu^I, ^HTMPA-Cu^I, ^tBuTMPA-Cu^I, ^OMeTMPA-Cu^I, and ^NMe2TMPA-Cu^I. In accord with available experimental data, it was found that MeCN coordination to ^HTMPA-Cu^I was energetically favorable, forming a 20 e⁻ complex. The other Cu^I complexes preferentially retained an open coordination site trans to the tertiary nitrogen. The geometries for all four-coordinate complexes were best described as trigonal pyramidal, and the five-coordinate MeCN adduct of ^HTMPA-Cu^I adopted a C_3v geometry that is best described as trigonal bipyramidal. In all cases, the calculated bond distances about copper provided excellent agreement with experimental data (see ESI†). One observation that will be important to the ensuing discussion is that the trend in Cu–N_ax distances observed experimentally (1^PMe2Ph < 1^PMe3 < 1^PPh3) was reproduced using these computational methods.

The calculations revealed an interesting trend in the ligand field splitting pattern. The pseudo-C_3v-symmetric geometries allow for valence manifolds containing nearly degenerate e-sets of d_xz/d_yz character (1e) and d_x²−y²/d_xy character (2e), along with an a₁-symmetric d_z² orbital. For ^HTMPA-Cu^I-MeCN, ^tBuTMPA-Cu^I, ^OMeTMPA-Cu^I, ^NMe2TMPA-Cu^I, TMG₃tren-Cu^I, and tren-Cu^I, 1e is lowest in energy, followed by 2e, and then a₁ as the HOMO, creating 2 : 2 : 1 d-manifold splitting pattern. As expected, ^HTMPA-Cu^I–MeCN exhibits the highest energy a₁ due to the ligand field effect of the additional axial donor. Removal of MeCN leads to the trigonal pyramidal ^HTMPA-Cu^I model compound suitable for comparison to the other four-coordinate complexes in this series (Fig. 5b); this four-coordinate species retains the 2 : 2 : 1 splitting pattern of the other ^XTMPA congeners.

For 1^PR3, the d-manifolds switch to 2 : 1 : 2 configurations, with the 2e set containing the HOMO and HOMO−1 (Fig. 5b for 1^PMe3). This inversion in the valence manifold splitting pattern may be thought to result from two primary contributors. First, the stronger ligand field strength of the partially anionic phosphinimine nitrogen σ-donors creates a larger split between 1e and 2e, effectively raising the energy of 2e in 1^PR3 relative to 2e in the reference compounds. Second, a significant variation in the relative energy of a₁ (d_z²) was observed. The energy of a₁ decreases from TMPA-Cu^I to ^tBuTMPA-Cu^I, ^OMeTMPA-Cu^I, ^NMe2TMPA-Cu^I, TMG₃tren-Cu^I, and tren-Cu^I. As expected, this trend was found to correlate closely with the Cu–N_ax bond distance, as the ligand field strength in the axial direction will exert the most influence on the destabilization of d_z² (R² = 0.98; Fig. 6, yellow); i.e. the longer the Cu–N_ax distance, the lower the a₁ energy. The 1^PR3 complexes, however, provide notable deviations from this trend. Using 1^PMe3 as an example, the predicted Cu–N_ax distance of 2.244 Å would be expected to result in an a₁ energy of ca. 23.3 kcal mol⁻¹, but the energy of a₁ is instead calculated to lie nearly 6 kcal mol⁻¹ lower in energy (17.5 kcal mol⁻¹, Fig. 6, red). This unexpected and significant decrease in the energy of a₁ appears to work in concert with the aforementioned destabilization of 2e to create the 2 : 1 : 2 ligand field splitting pattern. Similar effects for 1^PMe2Ph, 1^PMe2(NEt2), and 1^PMe2Ph are observed, to varying degrees (Fig. 6, blue, black, and green).

Fig. 6 Calculated energies of a₁ as a function of the calculated Cu–N_ax bond distances (R² = 0.98).

On considering the ylidic character of the phosphinimines and the geometric constraints imposed by the tren backbone of the ^R3P₃tren ligands, the possibility arose that the deviations in the energy of a₁ may reflect stabilization from cationic electrostatic fields emanating from the phosphonium residues. This view is supported by evaluation of the electrostatic potential along the canonical z-axis radiating away from the copper centers in the complexes evaluated above. In the absence of electrostatic fields in the secondary coordination sphere, one would expect the potential experienced by a test charge placed along this vector to diminish with a ca. 1/r dependence, according to the coulombic interaction of that test charge with the Cu ion. Indeed, this behavior is observed for ^HTMPA-Cu^I, ^tBuTMPA-Cu^I, ^OMeTMPA-Cu^I, ^NMe2TMPA-Cu^I, and tren-Cu^I (Fig. 7). However, deviations from 1/r behavior of the electrostatic potential are apparent in the cases of 1^PR3 (Fig. 7). In these cases, clear maxima along the z-axis are observed at ca. 2.75 Å, a position that lies in-plane with the three nearest phosphonium carbon atoms, which are ca. 2.5 Å from the z-axes. Evaluation of the electrostatic potential gradients along the z-axes for 1^PR3 predicts the presence of local electrostatic fields, located at ca. 2.2 Å from the copper centers, ranging in magnitude from 3–12 V nm⁻¹ (Fig. S1–S3†). These field strengths are on par with what has been measured both in enzymes using vibrational Stark spectroscopy^2–4 and in organized matter via neutron and synchrotron/X-ray diffraction studies,⁵ and it would therefore be large enough in magnitude to interact with the electron pair in a₁. The TMG₃tren-Cu^I complex also shows a modest deviation from the expected 1/r dependence in electrostatic potential, rising at ca. 2 Å to a plateau in electrostatic potential that persists to ca. 4 Å from the copper center (Fig. 7). This may be thought to result from the contribution of ionic resonance pictures available to the guanidinyl units in which a cationic charge is located on one of the amino nitrogen atoms. The modest deviation in electrostatic potential in this case is consistent with the crystallographic data, which indicate that the TMG groups show minor contributions from these ionic resonance structures.

Fig. 7 One-dimensional electrostatic potential map along the canonical z-axes of the Cu^I complexes.

Deviations in the a₁ orbital energies (Fig. 6) increase in order from 1^PMePh2 < 1^PMe3 < 1^PMe2Ph ≈ 1^PMe2(NEt2). This ordering was found to directly correlate with the calculated strength of the electrostatic field present along the canonical z-axes of the molecules (Fig. 8, green triangles), thereby implicating the cationic phosphonium residues as having a through-space electrostatic stabilizing effect on a₁ (Fig. 9a). The strengths of the electrostatic fields in 1^PR3 were also found to inversely correlate with the calculated Cu–N_ax distances (Fig. 8, purple diamonds). This observation suggests that electrostatic attractions between the occupied a₁ orbitals and the phosphonium residues work in concert with the rigidity of the ligand backbones to pull the N_ax atoms closer to the copper centers (Fig. 9b). This stereoelectronic effect is significant in that it represents a physical consequence of electrostatic stabilization of a₁. Together, the observations discussed within this section support a direct relationship between the physical properties observed in this system and the magnitude of the electrostatic fields emanating from the cationic phosphonium residues in the secondary coordination spheres in 1^PR3.

Fig. 8 Correlations between the ∼2.75 Å local maxima in electrostatic potential and (i) a₁ orbital energies (green triangles, left y-axis, R² = 0.96) and (ii) calculated Cu–N_ax bond lengths (purple diamonds, right y-axis, R² = 0.94).

Fig. 9 (a) qualitative MO diagram showing the impact of the Cu–N_ax distance on the a₁ orbital without stabilizing ESFs (blue), and with stabilizing ESFs (red), and (b) the stereoelectronic impact of an enhanced attraction between a₁ and the phosphonium residues as a function of the strength of the internal ESFs.

Chemical oxidation of 1^PMe3 and DFT analysis of the Cu^II products

To validate the computationally predicted valence manifolds for 1^PR3, we performed chemical oxidations of one of the P₃tren-Cu^I complexes. The 2 : 1 : 2 d-orbital splitting patterns calculated for 1^PR3 suggest that 1 e⁻ oxidations would generate holes with significant 2e character. The resulting Cu^II complexes would contain doubly degenerate electronic ground states, which would then be susceptible to primary Jahn–Teller distortions. By contrast, the 2 : 2 : 1 d-orbital splitting pattern predicted for the other compounds in this class would yield singly degenerate S = 1/2 states upon oxidation, precluding Jahn–Teller distortions from occurring. Thus, the presence or absence of a Jahn–Teller distortion in the Cu^II analog of 1^PMe3 may be used to resolve the structure of the Cu^I valence manifold.

Upon treatment of 1^PMe3 with AgBAr^F₄·4MeCN in THF, the colorless solution immediately turned red, concomitant with precipitation of a dark solid (Ag⁰). The Cu^II product proved resistant to crystallization, but independent synthesis of the dication through ligation of cupric triflate with ^Me3P₃tren in MeCN afforded [(^Me3P₃tren)Cu][OTf]₂ (2^PMe3, Fig. 10a). This red material shows the same UV-vis spectral features as the oxidation product described above and the same cyclic voltammetry trace as 1^PMe3 (Fig. S15†). Single crystals of 2^PMe3 were grown by diffusion of pentane into a 1,2-difluorobenzene (DFB) solution of the complex; however, radiative decay of the crystalline sample during X-ray diffraction experiments (Cu-Kα radiation source, λ = 1.5406 Å) resulted in incomplete data set collection and subsequently poor X-ray model refinement (R = 27%). Although the exact bond lengths and angles derived from the preliminary solid-state structure of 2^PMe3 are not statistically reliable, the data were adequate to reveal a four-coordinate Cu^II complex clearly distorted to a pseudo-C_s symmetry via a Jahn–Teller distortion (Fig. 10b).

Fig. 10 (a) Synthetic schemes for 2^PMe3 and 3^PMe3, (b) solid state structures of 2^PMe3 and 3^PMe3 with select bond distances and angles tabulated below (structural data for 2^PMe3 are preliminary due to incomplete data set collection stemming from radiative sample degradation during the XRD experiment), and (c) DFT-derived molecular orbital diagrams showing the β (spin down) d-manifold for 2^PMe3 and 3^PMe3.

This result may be contrasted with the 1-electron oxidation of 1^PMe3 using triphenylmethyl chloride, which resulted in the formation of the yellow/green five-coordinate complex [(^Me3P₃tren)CuCl][OTf] (3^PMe3, Fig. 10a), which exists in the solid-state as a pseudo-C_3v-symmetric complex with an axial chloride ligand. Significant differences in the solid-state geometries between 2^PMe3 and 3^PMe3 are highlighted in the selected bond distances and angles shown in Fig. 10b. Complex 2^PMe3 exhibits shorter Cu–N_eq distances, a longer Cu–N_ax distance, and a marked distortion of the equatorial donors from a nearly trigonal geometry in 3^PMe3 (<N_eq–Cu–N_eq = 113°–126°) to a T-shaped geometry in 2^PMe3 (<N_eq–Cu–N_eq = 106°–147°).

DFT geometry optimization calculations on the cationic portion of 2^PMe3 replicated the Jahn–Teller distortion observed experimentally, regardless of the starting geometry (Fig. 10c, left, β (spin down) orbitals). As expected, the distortion lifted the degeneracy of 2e, increasing the energy of the singly occupied d_x²−y² and decreasing the energy of d_xy. In an effort to examine the uniqueness of the Jahn–Teller distortion observed in 2^PMe3, the analogous, four-coordinate Cu^II complexes bound by ^HTMPA, ^tBuTMPA-Cu^I, ^OMeTMPA-Cu^I, ^NMe2TMPA, TMG₃tren, and tren were evaluated by the same methods as those used for 2^PMe3. The calculated Cu^II ions were found to exhibit C_3v symmetry, closely matching the experimental geometries, and consistent with the presence of an a₁-symmetric vacancy. Comparable DFT analysis of the five-coordinate complex 3^PMe3 supports the conclusion that its 3-fold symmetry results from Cl⁻ destabilization of d_z², thereby reverting the ligand field splitting to a 2 : 2 : 1 pattern (Fig. 10c, right, β orbitals) that is inactive toward a Jahn–Teller distortion for a d⁹ metal ion.

Time-dependent density functional theory (TD-DFT) predictions added support that the valence manifold structures of the red Cu^II species 2^PMe3 and the green Cu^II species 3^PMe3 both persist in the solution phase. The experimentally measured UV-vis spectrum of 2^PMe3 (Fig. 11a and b, solid orange trace) contains three prominent features at ca. 400 nm (25 000 cm⁻¹), 526 nm (19 000 cm⁻¹), and 700 nm (14 200 cm⁻¹) that were reproduced computationally (Fig. 11a and b, dashed orange trace). The feature at 400 nm is assigned as the d_yz → d_x²−y² transition, and the feature at 526 nm is assigned as the d_xz → d_x²−y² transition. The feature calculated at 700 nm is best described as a ligand to metal charge transfer (LMCT) from the phosphinimine moieties to the empty d_x²−y² β orbital, though the experimental and calculated intensities are both predicted to be low. A single broad feature is observed in the near infrared spectrum (NIR) (Fig. 11b, solid orange trace) at ca. 1190 nm (8403 cm⁻¹) that was reproduced computationally (Fig. 11b, dashed orange trace) as a combination of the d_xy → d_x²−y² and d_z² → d_x²−y² transitions.

Fig. 11 (a) Experimental and TD-DFT generated UV-vis spectra of 2^PMe3 and 3^PMe3, (b) near-IR region of the electronic spectra for 2^PMe3 and 3^PMe3, and (c) tabulated d–d transitions for 2^PMe3 and 3^PMe3. Calculated d–d transitions are indicated on the spectra with arrows.

The experimental UV-vis spectrum of 3^PMe3 (Fig. 11a, solid blue trace) contains prominent features at ca. 360 nm (27 800 cm⁻¹) and 750 nm (13 300 cm⁻¹), which were also reproduced computationally (27 800 cm⁻¹ and 14 388 cm⁻¹) (Fig. 11a, dashed blue trace). Although the d_xz/d_yz → d_z² transitions are predicted to be within this energy range (23 444, 23 489 cm⁻¹), they are likely not observed due to low intensities (ε_calc = 13–14 M⁻¹ cm⁻¹). A single broad feature is observed in the NIR spectrum of 3^PMe3 (Fig. 11b, solid blue trace) that was reproduced computationally (Fig. 11b, dashed blue trace) as the d_xy/d_x²−y² → d_z² transitions. The calculated energies of the various d–d transitions for 2^PMe3 and 3^PMe3 are tabulated in Fig. 11c and are marked on the spectra in Fig. 11a and b with arrows (orange for 2^PMe3, blue for 3^PMe3).

DFT evaluation of the Cu^I/Cu^II redox potentials

The calculated redox potentials for this series of complexes were evaluated as a way to gauge the extent to which the Jahn–Teller distortions, the phosphinimine donor strengths, and the through space electrostatic effects individually contribute to the highly cathodic Cu^I/Cu^II couples. As mentioned above, Karlin and co-workers have shown that variations in the donor strength of TMPA-based ligands significantly impacts Cu^I/Cu^II redox potentials, and the Tolman group has used distal charged residues to influence Cu^II/Cu^III redox potentials as a function of the sign of the charged group. Jahn–Teller effects have also been shown to impact the thermodynamics of electron transfer. For example, a ∼0.6 V shift in redox potentials for a series of Cu^I bis(1,10-phenanthroline) complexes has been ascribed to the varying extent of Jahn–Teller distortion available to the complexes as a function of the steric bulk in their secondary coordination spheres.^49–51

In the present case, the trends in the experimental redox potentials were reproduced computationally for 1^PMe3 and the ^XTMPA-Cu^I complexes⁴⁸ discussed above (Table 1). The mean unsigned error of 120 mV is on par with the current state-of-the-art.⁵² In order to evaluate the impact of the Jahn–Teller distortion in 2^PMe3 on the Cu^I/Cu^II redox potential, a C_3v-symmetric analog of 2^PMe3 was evaluated by constraining the N_eq–Cu–N_eq angles to 120°. The structure optimized with this constraint was then used to calculate a hypothetical Cu^I/Cu^II redox potential in which the Cu^I symmetry is maintained upon oxidation of 1^PMe3 (denoted as C_3v-Cu^I/C_3v-Cu^II). This redox couple (−630 mV) is on par with the calculated redox potential for ^NMe2TMPA-Cu^I (−617 mV), indicating that the phosphinimine donors act as strong σ-donors to create electron-rich Cu^I centers. Importantly, however, the C_3v-Cu^I/C_3v-Cu^II potential is calculated to be substantially more anodic than the Cu^I/Cu^II redox potential that takes into account the Jahn–Teller distortion on oxidation (C_3v-Cu^I/C_s-Cu^II, −890 mV; Table 1). This 260 mV shift in potential indicates that the geometric change accompanying oxidation provides a substantial driving force (ca. 6 kcal mol⁻¹) for electron transfer from 1^PMe3.

Table 1 Calculated and experimental Cu^I/Cu^II redox couples for complexes with available experimental data.⁴² The error represents the signed error between experiment and calculation. Average unsigned error = 120 mV

Complex	Calc. pot. (mV)	Exp. pot. (mV)	Error (mV)
1^PMe3 (C3v-Cu^I/Cs-Cu^II)	−890	−780	−110
1^PMe3 (C3v-Cu^I/C3v-Cu^II)	−630		+150
1^PMe2Ph	−579	−750	+171
1^PMe2(NEt2)	−822	−740	−82
1^PMePh2	−640	−650	+10
^HTMPA-Cu^I	−232	−400	+168
^tBuTMPA-Cu^I	−303	−460	+157
^OMeTMPA-Cu^I	−311	−490	+179
^NMe2TMPA-Cu^I	−617	−700	+83

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It remained to consider the origin of the >350 mV range in redox potentials for 1^PR3. The potentials do not clearly track with the donor strengths of the phosphinimines, and the characterization of the oxidation as being 2e (d_x²−y²/d_xy)-based suggests that the extent of electrostatic stabilization of a₁ is not responsible. This is supported by the observation that the magnitude of the electrostatic potential along the canonical z-axis does not scale with the Cu^I/Cu^II redox couple. We thus hypothesized that the steric bulk of the phosphinimine substitutions was responsible for modulating the extent to which the Jahn–Teller distortion stabilized the oxidized Cu^II ions.

This steric effect was evaluated through calculation of a modified Tolman cone angle (θ*) for the phosphinimine substituents in the ^R3P₃tren ligands (see ESI† for details on the calculation of θ*). Upon plotting the experimental Cu^I/Cu^II redox potentials versus θ*, a direct relationship between steric bulk in the secondary coordination sphere and the Cu^I/Cu^II redox potential is realized (Fig. 12). Indeed, for cone angles >140°, a linear relationship (R² = 0.99) is observed. This observation can be explained by considering that the maximum amount of stabilization of the Cu^II species (and thus the most cathodic Cu^I/Cu^II redox potential) will occur for ligands with small values of θ*, for which the limited steric profile of the ligand allows for the most distortion by the tethered arms of the ligand backbone. As the steric profile of the ligand increases, the Cu^II species become increasingly prohibited from accessing Jahn–Teller-distorted geometries (Fig. 13), resulting in a systematic anodic shift in the Cu^I/Cu^II redox potentials. The non-linearity of the data shown in Fig. 12 on inclusion of 1^PMe3 suggests that a lower bound for the cone angle at which the Jahn–Teller distortion is inhibited lies between 123° and 141°. Overall, these data indicate that while electrostatic effects appear to be critical for engendering a Jahn–Teller distortion in 2^PR3, the steric profile of the secondary coordination sphere is responsible for fine-tuning the Cu^I/Cu^II redox potentials.

Fig. 12 Plot of redox potential versus θ*, showing a linear relationship between potential and the steric bulk of the ligand for values of θ* ≥ 141° (black trendline, R² = 0.99). The blue trendline (R² = 0.77) includes all five data points.

Fig. 13 Depiction of the inhibition of an “ideal” Jahn–Teller distortion with increased steric bulk in the secondary coordination sphere.

Conclusions

This study demonstrates the ability of strategically-placed charged residues to directly impact the valence manifolds of homogeneous inorganic complexes. This technique is independent of the specific donor bound to the metal center, allowing for orthogonal control of valence orbital energies by way of through-bond and through-space effects. In the present example, electrostatic fields (ESFs) emanating from cationic phosphonium residues in the secondary coordination spheres of 1^PR3 stabilize the valence MO a₁ (d_z²) orbitals. This stabilization would typically result in an anodic shift in redox potential, but the strong σ-donation from the phosphinimine donors provides an orthogonal destabilization of the Cu^I 2e set, producing counter-intuitive shifts in Cu^I/Cu^II redox potentials. The resulting inversion of the valence manifold brought by these complementary effects generates a system capable of undergoing a Jahn–Teller distortion upon oxidation, providing an additional thermodynamic driving force for electron transfer. Studies to assess the impact of ESFs on small molecule binding and reactivity are underway.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was financially supported by the National Institute of General Medical Sciences of the National Institutes of Health (award number R35GM128794), the Charles E. Kaufman Foundation of the Pittsburgh Foundation (award number KA2016-85227), and the University of Pennsylvania. We also thank the Vagelos Institute for Energy Science and Technology (VIEST) for a Graduate Research Fellowship, awarded to ABW.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2045438–2045442. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0sc06364a

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