Electrostatic vs. inductive effects in phosphine ligand donor properties and reactivity

Enhanced rates and selectivity in enzymes are enabled in part by precisely tuned electric fields within active sites. Analogously, the use of charged groups to leverage electrostatics in molecular systems is a promising strategy to tune reactivity. However, separation of the through space and through bond effects of charged functional groups is a long standing challenge that limits the rational application of electric fields in molecular systems. To address this challenge we developed a method using the phosphorus selenium coupling value (JP–Se) of anionic phosphine selenides to quantify the electrostatic contribution of the borate moiety to donor strength. In this analysis we report the synthesis of a novel anionic phosphine, PPh2CH2BF3K, the corresponding tetraphenyl phosphonium and tetraethyl ammonium selenides [PPh4][SePPh2CH2BF3] and [TEA][SePPh2CH2BF3], and the Rh carbonyl complex [PPh4][Rh(acac)(CO)(PPh2(CH2BF3))]. Solvent-dependent changes in JP–Se were fit using Coulomb's law and support up to an 80% electrostatic contribution to the increase in donor strength of [PPh4][SePPh2CH2BF3] relative to SePPh2Et, while controls with [TEA][SePPh2CH2BF3] exclude convoluting ion pairing effects. Calculations using explicit solvation or point charges effectively replicate the experimental data. This JP–Se method was extended to [PPh4][SePPh2(2-BF3Ph)] and likewise estimates up to a 70% electrostatic contribution to the increase in donor strength relative to SePPh3. The use of PPh2CH2BF3K also accelerates C–F oxidative addition reactivity with Ni(COD)2 by an order of magnitude in comparison to the comparatively donating neutral phosphines PEt3 and PCy3. This enhanced reactivity prompted the investigation of catalytic fluoroarene C–F borylation, with improved yields observed for less fluorinated arenes. These results demonstrate that covalently bound charged functionalities can exert a significant electrostatic influence under common solution phase reaction conditions and experimentally validate theoretical predictions regarding electrostatic effects in reactivity.


Introduction
Spectroscopic and computational studies have cited oriented electric elds in active sites as key contributors to enzymatic reactivity. [1][2][3][4][5][6][7] Enzymes maintain and orient electric elds using polar and charged functional groups in the protein scaffold around the active site, a strategy that is appealing for synthetic molecular systems but difficult to mimic. [8][9][10][11][12] Multiple approaches have been proposed for incorporating similar electric elds into systems that lack an enzymatic superstructure. Electric elds can be directly applied to synthetic compounds through attachment to electrode surfaces or STM tips and these approaches have been shown to increase catalytic rates. [13][14][15][16][17][18][19][20][21][22][23] Another approach is to rationally append charged functional groups onto molecular scaffolds to offer control over the orientation and magnitude of electrostatic effects. Indeed, there has been enormous interest in modeling or leveraging electrostatic interactions in organic and inorganic molecules.  Computations predict that electrostatic effects can have a large impact on reactivity and catalysis. [47][48][49][50][51][52][53][54][55][56][57][58][59][60][61] A detailed understanding of the relative magnitudes of inductive and electrostatic charged substituent effects would be valuable in rationally tuning molecular reactivity. In this context, inductive represents through bond polarization of the molecule, similar to effects observed with electron donating or withdrawing groups, while electrostatic indicates through space polarization of a bond consistent with Coulomb's law. Experimentally parsing out the relative inductive and electrostatic contributions to reactivity and electronic structure from charged functional groups is challenging even in simple systems. A prime example of this is in classic Hammett literature where inductive through-bond and electrostatic throughspace inuences from substituents were predominantly treated as one lump effect, sometimes referred to as s i . 62 However, in a subset of this literature, there has been considerable debate on whether electrostatics or through-bond electron density factors are dominant in s i . Hammett originally considered the inuence of substituents to be entirely electrostatic in nature, a view supported by Ri, Eyring, and Westheimer. 63,64 Conversely, Jaffe considered substituent effects primarily through electron density, foreshadowing modern computational analyses. 65,66 The efficacy of both methods in rationalizing reactivity trends supports that both electrostatic and inductive interactions are active, and methods to distinguish between them are still being pursued to this day. [67][68][69] A more thorough understanding of the inductive and electrostatic factors inuencing the electronic structure and reactivity of distally charged ligands and complexes would be instructive, particularly as leveraging through-space interactions can serve as a strategy to break free-energy relationships. [70][71][72][73][74][75][76] Phosphines are ideal scaffolds for quantifying the inuence of electrostatics as these ligands feature prominently in catalysis and have well dened parameters for rationalizing reactivity trends, such as the Tolman Electronic Parameter (TEP) and cone angle. 77 Indeed, cationic and anionic moieties have previously been incorporated into phosphines, frequently leading to distinct properties or reactivity in comparison to neutral analogues. [78][79][80][81][82][83] Phosphine borate ligands specically have been prepared through the incorporation of triaryl-and triuoroborate and carborane functional groups, and have shown enhanced reactivity in polymerization, [84][85][86][87][88][89][90][91][92][93] cross coupling, [94][95][96] and hydrofunctionalization 97,98 reactions. These anionic phosphines are uniformly considered to be stronger donors than their neutral isostructural analogues. However, the origin of this increase (electrostatic or inductive) has remained elusive, and has largely been considered inductive by default. 68,69,[94][95][96][98][99][100][101] The ambiguity regarding through space effects of covalently bound distal anions limits the rational design of ligand scaffolds that leverage electric elds to enhance reactivity. For instance, a recent computational study predicts accelerated oxidative addition (OA) reactivity at a PMe 3 supported Pd complex in the presence of either an externally applied electric eld or a correctly positioned chloride ion. 53 This study suggests that similar effects may be replicated using distally charged anionic phosphines if a suitably thorough understanding of electrostatic effects in covalently bound anions can be obtained.
Herein we report a method to assign the electrostatic and inductive contributions of anionic functional groups to phosphine donor strength using the solvent dependence of phosphorus selenium coupling values (J P-Se ). This method is initially developed using the novel phosphine PPh 2 CH 2 BF 3 À (1) (Scheme 1). The R-BF 3 À functional group is ideal for this analysis as it provides a more compact charge in comparison to commonly used aryl borates. 96 Additionally, triuoroborate groups are reatively inert in comparison to aryl borates, which can engage in aryl-group transfer decomposition pathways 87 and intramolecular C-H oxidative addition reactivity. 99 The methylene linker to the phosphine precludes convoluting resonance inuences that may be present in aryl linkers. We report the synthesis of the potassium salt of this phosphine, K1, and assess the electrostatic impact of the R-BF 3 À moiety via both the Finally, an order of magnitude acceleration in the OA of aryl uorides mediated by K1 and Ni(COD) 2 is observed relative to neutral phosphines of comparable donor strengths, PEt 3 and PCy 3 . This nding suggests a unique impact of electrostatic effects beyond that expected from increased donor strength. This accelerated reactivity is applied to the catalytic deuoroborylation of uoroarenes, with improved reactivity observed with comparatively unactivated substrates. In summary, this work illustrates how electrostatic interactions from charged functional groups are a substantial contributor to phosphine donor strength in common organic solvents and that these electrostatic effects can be leveraged for increased reactivity and catalysis.

Results and discussion
Synthesis and characterization of PPh 2 CH 2 BF 3 K (K1) Synthesis of the phosphine proceeds readily via deprotonation of Ph 2 PH with KHMDS (KHMDS ¼ potassium hexamethyldisilazide), followed by dropwise addition to a stirring THF solution of potassium iodomethyltriuoroborate, and yields K1 as a white powder following workup (Scheme 1). We note that a related zwitterionic triphenyl phosphonium methyl tri-uoroborate has been previously synthesized. [102][103][104] The 1 H NMR of K1 shows the expected aromatic signals for the phenyl groups, and a doublet of quartets at 0.8 ppm from coupling of the CH 2 linker to phosphorus, boron, and uorine (Fig. S1 †). The 31 P{ 1 H} NMR spectrum shows a quartet at À15 ppm due to uorine coupling with an identical chemical shi to that observed for PPh 2 Et (Fig. S2 †). This observation is consistent with previous reports that charged phosphines have similar shis as their neutral analogues. 90,91,101 Analysis by 19 F{ 1 H} and 11 B{ 1 H} NMR indicates the expected shis and coupling for a R-BF 3 À group, supporting the presence of this anionic unit ( Fig. S3 and S5 †). 105 Compound K1 was structurally characterized via single-crystal X-ray diffraction (SXRD, Fig. 1). The SXRD structure shows the expected connectivity with an average B/P distance of 2.858(3)Å and a close association between K and B ($3Å, Fig. S89 †). While it is unclear if this association is preserved in solution, larger cations were chosen to limit ion pairing in further analyses (see below).

Tolman electronic parameter and J P-Se determination
To assay the donor strength of phosphine 1, its Tolman Electronic Parameter (TEP) was determined using a Rh carbonyl complex of the form Rh(acac)(CO)L (L ¼ phosphine). While the limitations of TEP in reecting M-L bond strengths has been noted previously, 106 it remains a standard in the literature for the comparison of phosphine donor strength. 107 The TEP is a measure of the donation of electron density from a phosphine to a metal complex through the combined effects of s donation (P / M) and p back-bonding (M / P) interactions. 83,107 TEP is traditionally determined from the A 1 -symmetrical n CO stretching frequency in Ni(CO) 3 L complexes, where a higher n CO indicates a less electron rich metal center resulting from weaker phosphine donors. However, the toxicity of the Ni(CO) 4 starting material has motivated the development of other model complexes to determine TEP. One such complex which displays a robust linear correlation between n CO and TEP is Rh(acac)(CO) L (see Fig. S67 † for equation), and accordingly the complex with L ¼ K1 was synthesized. 108 Addition of K1 to Rh(acac)(CO) 2 with PPh 4 Br affords [PPh 4 ] [Rh(acac)(CO)(PPh 2 (CH 2 BF 3 ))] (2) as a yellow solid. The SXRD structure of 2 shows a square planar geometry at Rh (Fig. 1) 103 Rh (J P-Rh ¼ 166 Hz) and 19 F (J P-F ¼ 10 Hz), consistent with the solid-state structure (Fig. S7 †). The solution IR spectrum of 2 in CH 2 Cl 2 shows a n CO of 1965 cm À1 , which correlates to a TEP of 2061.7 cm À1 (Fig. S67 †). This TEP is identical to that of PEt 3 (2061.7 cm À1 ), and is signicantly more donating than the related alkyldiaryl phosphine PPh 2 Et (2066.7 cm À1 ) (Fig. 2). 77 This result is consistent with the enhanced donation previously observed for phosphines with anionic borates. 68,83,86,[90][91][92]95,96,100,101,[109][110][111][112] In addition to metal carbonyl adducts, phosphine selenide compounds have also been used to quantify the donor strength and basicity of phosphines via their P-Se coupling constants (J P-Se ). 113,114 The use of NMR coupling constants is advantageous  due to greater instrumental resolution and sensitivity in the coupling value as compared to vibrational spectroscopy. 115 Changes in J P-Se report on changes in the s character of the P-Se bond. 116,117 The electron donating/withdrawing character of the substituents on P inuences this s character through hybridization changes as predicted by Bent's rule. 118,119 Electron withdrawing R groups on PR 3 increase the s character in the P-Se bond and thereby J P-Se , while electron donating R groups effect the opposite. 113 The  121 We then sought to compare our two experimental assays of phosphine donor strength. While the use of J P-Se to measure phosphine donor strength is well established, specic correlations between J P-Se and TEP have not been clearly dened. 118,122 Fitting of the reported J P-Se and TEP values for a series of 18 alkyl and aryl phosphines resulted in a reasonable linear correlation (R 2 ¼ 0.84, Fig. 2, S88 and Table S11 †). 77,108,121 Using this analysis to extrapolate a value of J P-Se from the experimentally determined TEP of 2 provides J P-Se ¼ 698 Hz, which is signicantly larger than the experimentally determined value of 687 Hz for [PPh 4 ][1 Se ] in DMSO-d 6 (Table S10 †). Surprisingly, measuring the J P-Se of [PPh 4 ][1 Se ] in CDCl 3 results in a signicant decrease in J P-Se to 657 Hz (D30 Hz), suggesting that phosphine 1 is a stronger donor in CDCl 3 than DMSO. Overall, the TEP and J P-Se values clearly indicate that the anionic charge promotes a large increase in the donor strength of phosphine 1. However, we wanted to further understand the origin of the large solvent dependence of this donor strength.

Analysis of donor strength solvent dependence
The presence of the charged borate in phosphine 1 and the discrepancy between TEP values determined via different methods prompted us to investigate how electrostatic effects contribute to these measurements. "Through-space" interactions have been suggested previously to explain anamolous J P-Se behavior in phosphines with 2-furyl and o-methoxyphenyl substituents, but a thorough analysis of this effect has not been undertaken. 123 A dependence on solvent ionic strength was proposed as a means of separating electrostatic and inductive contributions of a R-NMe 3 + substituent on the rate of acetate binding in iron porphyrins. 34 We reasoned that a similar solvent variation approach would be useful for separating inductive and electrostatic contributions to the donor strength of 1. Specically, the through-space electrostatic inuence of the charged group should be modied by the solvent dielectric (3), a measure of a medium's ability to shield a charge, 124 while the through-bond interactions should remain constant. IR spectra of 2 and the parent Rh(CO) 2 (acac) were initially acquired in MeCN, THF, and DCM. However, no variation of n CO outside of instrumental error is observed (Fig. S72 and S73 †). This is perhaps not surprising as the expected change in stretching frequency of $10 cm À1 is not large compared to the instrumental resolution (4 cm À1 ). Resolving dielectric induced shis is further limited by spectral convolution or broadening, likely from Rh-P rotational isomers of 2 with different stretching frequencies as has been observed in other carbonyl systems. 125 This manifests as substantially broader spectra for 2 than for Rh(CO) 2 (acac). These competing factors complicate the interpretation of donor strength in 2 and suggest that the higher sesitivity and resolution of J P-Se may make it a more conducive method for examining electrostatic effects.
While the inductive donor effects in [PPh 4 ][1 Se ] should be insensitive to 3, the electrostatic stabilization of the formally cationic phosphonium in the dominant resonance structure Se À -P + R 3 by the adjacent BF 3 À anion should increase as 3 decreases. 101,126 Lower 3 solvents will less effectively screen the anion, resulting in greater stabilization of the positive formal charge and a lower J P-Se . 136 Although it is difficult to predict solution structures, SXRD and DFT analysis (see below) of all phosphine selenides considered in this report show shorter distances between P and B than Se and B, supporting the feasibility of the anion stabilizing a formal positive charge on P (Table S15 †). We note that previous literature studies demonstrate some solvent dependence to J P-Se , with one report suggesting variation between 2-3%. 127,128 In our analysis, comparison of J P-Se in the anionic phosphine to a neutral analogue and restricting solvent choice to aprotic solvents serve as controls for any incidental trends. As mentioned, an overall decrease of 30 Hz in the J P-Se of [PPh 4 ][1 Se ] is observed upon moving from CDCl 3 (3 ¼ 4.8) to DMSO (3 ¼ 46.7). Expanding the solvent selection to include CD 3 CN, acetone-d 6 , CD 2 Cl 2 , and mixtures thereof shows a consistent decrease in J P-Se as 3 decreases (Fig. 3, S27-S30 and S12 †). In comparison, only a slight change of 7 Hz is observed across the same 3 range for the neutral congener SePPh 2 Et (Fig. 3, S37 and S12 †). Coulomb's law suggests that a linear dependence on 1/4p3 should be expected for a primarily electrostatic effect. Indeed, the observed solvent dependence of J P-Se for [PPh 4 ][1 Se ] follows this trend. The variable solvent coupling data for [PPh 4 ][1 Se ] was t to the linear relationship J P-Se ¼ 693(1) À 2.12(12) Â 10 3 Â (1/4p3) (R 2 ¼ 0.98). The neutral congener was also t and shows a shallower slope of À5(6) Â 10 2 and a worse R 2 ¼ 0.06 value (Table S13 †).
While this solvent trend for [PPh 4 ][1 Se ] is well modeled by an electrostatic effect, we also wanted to account for any ion pairing interactions. Ion pairing can range from separated free ions to close contact ion pairs, with varying degrees of ion solvation and association in between. These solvent separated ion pairs may also exist in an equilibrium, and generally it is difficult to precisely characterize the speciation of an ion pair in solution. 129 (Fig. 2, S34, S35 and S12 †). The magnitude of this change and the slope of the linear t (À7(2) Â 10 2 , R 2 ¼ 0.77) resembles those of the neutral analogues (Table S13 † (Table S14 †).
Overall, these controls with [TEA][1 Se ] support the assignment of electrostatic effects instead of ion pairing in rationalizing the observed solvent dependence.

Electrostatic contributions to donor strength
Assigning the solvent dependence of J P-Se as electrostatic in origin enables the separation of electrostatic and inductive contributions to donor strength (  (Table 1). While this analysis uses the extrapolated limits, the experimental data from the accessible range of 3 suggest that the electrostatic contribution is $50% (Table S12 †). These analyses show that electrostatic factors have a major, and even dominant, impact on the donor properties in these systems.
To further investigate the relative contribution of throughspace and through-bond effects, Density Functional Theory (DFT) calculations were performed to estimate J P-Se and compare with experimentally determined values (Table 1). For simplicity, two local geometries of the phosphine 1 Se were considered to capture limiting rotamers that may be present in solution: one transoid rotamer with a Se-P-C-B dihedral of 158 and one cis rotamer with a Se-P-C-B dihedral of 74 ( Fig. S80 and S81 †). A Boltzmann weighted average of these two extremes predicts a nearly 100% population of the transoid isomer at room temperature, with a calculated gas phase J P-Se of 792 Hz. We then performed optimizations of the transoid rotamer with explicit solvation to see if the observed experimental trends were reproduced computationally. Optimizations in CHCl 3 , DCM, and MeCN predict jJ P-Se j values of 648, 675, and 684 Hz respectively (Fig. S84-S86 and S12 †). These values are in remarkably good agreement with both the experimentally determined J P-Se as well as the observed trend with solvent dielectric.
To computationally deconvolute electrostatic contributions to J P-Se , we analyzed the effect of including point charges in the gas phase. Coulomb's law was used to estimate the electric eld at P generated by a negative point charge located at B in the transoid rotamer of 1 Se . This analysis gives an electric eld parallel to the P-Se bond (dened as the z-axis) of $ À1.07 VÅ À1 (Table S4 †). The electrostatic contribution to J P-Se was then determined by calculating the J P-Se for SePPh 2 Et with a negative point charge at $3.7Å from P in the z direction, resulting in an applied eld of comparable magnitude. The inclusion of this point charge lowers J P-Se to 809 Hz from 872 Hz in the neutral analogue, suggesting that the purely electrostatic contribution to J P-Se is 63 Hz. This electrostatic contribution is 79% of the total computed difference in J P-Se between 1 Se and SePPh 2 Et (80 Hz, Table 1), which is identical within error to that determined experimentally. Thus, DFT calculations support the experimental data demonstrating that electrostatic effects play a signicant role in the donor properties of 1.

Extension and comparison to other phosphines
The selenide of an anionic phosphine previously investigated for Pd catalyzed olen polymerization/oligomerization, [   CDCl 3 to DMSO-d 6 (Fig. 3, S38 and S12 † ] from their respective neutral congeners to other anionic phosphine systems is instructive, even in the absence of comparable solvent dependence studies. In one example a triptycene borate phosphine with a P/B distance of 3.03Å was compared to a silicon based neutral analogue. 96,111 The shi in J P-Se observed in CDCl 3 upon switching from the neutral to anionic version approached 90 Hz.  3 . The greater magnitude of the shi can be rationalized by the orientation of the anionic functional group, which is constrained to align with the P-Se bond in the triptycene case. The signicant change in J P-Se coupling observed in the triptycene case contrasts with another example featuring an anionic BPh 3 À group, SePPh 2 (p-BPh 3 Ph) À . 101 The difference in coupling between this compound and the neutral congener SePPh 3 is only 30 Hz in CDCl 3 , likely due to the larger distance between the charged group and the phosphine (6.49Å from DFT) and delocalization of the anionic charge into the aryl rings on boron. 96,99,101 These examples illustrate that the distance, orientation, and anion structure inuence the magnitude of the impact on phosphine donor properties.

C-F oxidative addition reactivity
The comparatively strong donor properties of phosphine 1 prompted up to consider its application in challenging oxidative addition (OA) reactions. Indeed, anionic phosphines have previously shown enhanced coupling reactivity with aryl chlorides in comparison to neutral isostructural congeners. [94][95][96] Uniquely, the J P-Se analysis carried out above with [PPh 4 ][1 Se ] allows for comparison between the reactivity of K1 and phosphines of quantitatively similar donor strengths. Comparison with PEt 3 is instructive as the J P-Se (684 Hz) closely matches that of [PPh 4 ][1 Se ] in DMSO (687 Hz) and the cone angles are similar (132 and 140 for PEt 3 and PPh 2 Et). 77 The reactivity of PCy 3 was also investigated as the J P-Se (675 Hz) closely matches that predicted for [PPh 4 ][1 Se ] in THF using the 1/4p3 linear t (3 ¼ 7.6, 671 Hz). While these comparisons do not perfectly account for enhanced ion pairing from K + or imperfect matching of cone angles, they are nonetheless useful to reveal trends in reactivity as a function of electrostatic contributions to donor strength.
The OA of aryl uoride bonds was chosen for this comparison due to a recent computational report suggesting that this reaction is accelerated in the presence of an electric eld. 53 Additionally, the OA of C 6 F 6 by Ni(COD) 2 (COD ¼ 1,5-cyclooctadiene) with PEt 3 has been previously reported to proceed very slowly, taking $4 weeks in hexane for completion. 130 Therefore, the rates of C 6 F 6 OA by Ni(COD) 2 with K1, PEt 3 and PCy 3 in THF were determined along with the overall conversion.
The combination of Ni(COD) 2 and 2 equivalents of K1 in THF generates a red solution with an absorbance in the UV-vis spectrum at 464 nm (Fig. S79 †). The 31 P{ 1 H} NMR spectrum of the reaction shows the appearance of three new resonances with some unreacted K1, indicating a mixture of differentially ligated species (Fig. S43 †). Addition of 10 equivalents of C 6 F 6 , as well as CF 3 Ph and OPPh 3 as internal standards, results in the disappearance of these resonances and the formation of a new doublet at 9.6 ppm consistent with coupling to a Ni-F (Fig. S48 †). Similarly, the 19 F NMR spectrum shows the appearance of resonances consistent with Ni-C 6 F 5 (À117.5, À166.4, and À167.1 ppm) and Ni-F (À383.6 ppm) moieties ( Fig. S44 and S45 †). The product resonances grow in with an average observed rate of 1.6(2) Â 10 À4 s À1 , and level off aer $7 hours with an average yield of 25(3)% (Fig. S47, S49 and Table S1 †). The reaction between Ni(COD) 2 , 2 equivalents of K1 and 21 equivalents of C 6 F 6 was also examined by monitoring the decay of the absorbance at 464 nm using UV-vis spectroscopy (Fig. S75 †). The observed rate of decay is on the same order of magnitude as the rate of formation determined by NMR methods (5.6(4) Â 10 À4 s À1 ). Reducing the amount of added K1 to 1 equivalent decreases the observed rate (2.8 Â 10 À4 s À1 ), which is inconsistent with a mechanism involving the dissociation of a ligand prior to OA (Fig. S74 †). The rate is also reduced with the addition of 3, 4, or 8 equivalents of K1, with the appearance of a new absorbance at 375 nm suggesting additional coordination of K1 to Ni may be possible (Fig. S76-S78 †). As a control, the absorbance at 464 nm was monitored in the absence of substrate and indicated minimal decay over the same time frame (Fig. S79 †).
Carrying out the same reaction with PEt 3 and PCy 3 results in the growth of similar NMR signals as those observed with K1, but with signicantly slower rates of product formation (3.7(7) Â 10 À6 s À1 and 2.0(1) Â 10 À5 s À1 , respectively) ( Fig. S50-S58 and Table S1 †). In contrast to K1, the OA product growth continues throughout the time the reaction was monitored, reaching 24(2)% at 114 h for PEt 3 and 66(15)% at 53 h for PCy 3 . The enhanced rate observed with K1 not only supports previous observations that anionic groups enhance rates of OA reactions, [94][95][96] but also demonstrates that the rate enhancement is greater than would be predicted on the basis of donor strength. This is perhaps most clearly illustrated by the one order of magnitude rate acceleration with K1 over PCy 3 , despite the nearly identical donor strengths predicted by our analysis. A distinct mechanism for electrostatic rate enhancement beyond an increase in donor strength has been suggested previously with an anionic carborane phosphine, wherein accelerated OA reactivity was attributed to ligand dissociation and transition state (TS) stabilization. 95 The exact nature of the rate acceleration with K1 is not yet clear. It is possible that the TS is lowered by the presence of an electric eld generated by the BF 3 À , as is predicted computationally, 53 but other factors, such as ion pairing with K + , may also be determinative. Regardless, these results emphasize that anionic charges enhance reactivity through mechanisms beyond simply increasing donor strength.

Catalytic deuoroborylation reactivity
The signicant rate acceleration observed in stoichiometric reactivity led us to investigate catalytic C-F borylation with K1 and Ni(COD) 2 . Deuoroborylation of uorinated arenes with Ni has been reported previously with N-heterocyclic carbene (NHC) and PCy 3 ligands. [131][132][133] The use of strongly donating ligands is consistent with the difficulty of the OA step, as C-F bonds have the highest BDE among carbon-halogen bonds. 131 Likewise, high catalyst loadings (10%), reaction temperatures (110 C), and long reaction times ($12 h) highlight the difficulty of these transformations.
This method was extended to other uoroarenes to examine the scope of reactivity (Fig. 4). The highest yield observed is for 1,3 diuorobenzene (50%), followed by uorobenzene (42%), with lower yields for more highly uorinated substrates. The trend of decreasing yields with higher levels of uorination contrasts with the trend observed with an NHC, where more highly uorinated substrates are more easily deuoroborylated. 133 Additionally, homocoupling is competitive in substrates with adjacent uorine atoms in our system. The uorobenzene borylation with K1 is higher than that with an NHC ligand (20%), 133 but in general other systems perform deuoroborylation of more diverse uoroarenes with higher yields than the current system. 131,132 Based on the observed formation of a C-F OA product with C 6 F 6 , it is likely that deuoroborylation proceeds through a traditional OA, transmetalation, and reductive elimination mechanism. The differing reactivity trend with arene uorination implies that the enhanced OA rate in the present system makes transmetalation the turnover limiting step. This hypothesis is consistent with increased yields with an excess of B 2 pin 2 as well as the need for more nucleophilic hydroxide or alkoxide additives over uoride sources. We speculate that in situ generated methoxide may facilitate transmetalation of B 2 pin 2 by exchanging with Ni-F intermediates, as a similar sbond metathesis of B 2 pin 2 with Ni alkoxide complexes has been reported. 135 However, the specic role played by Cs + and MeOH remains unclear. Faster rates and unique reaction conditions for this system (i.e. lower temperature, base, water and alcohol) suggest that it may offer complementary reactivity to the established neutral phosphine systems and motivates further investigation. Regardless of specic methodological applications, the fact that the inclusion of a BF 3 À group enables a diarylalkyl phosphine to perform comparably with an NHC in C-F borylation highlights how electrostatic effects can both dramatically enhance stroichiometric OA reactivity as well as catalytic processes.

Conclusions
In conclusion, a new anionic phosphine ligand has been synthesized and demonstrates that the inclusion of an anionic triuoroborate group dramatically increases the donor properties relative to neutral analogues. Furthermore, a series of experiments and calculations have demonstrated that a signicant portion of the increase in donor strength arises from electrostatic as opposed to inductive effects. The electrostatic interactions in this ligand accelerate OA rates by an order of magnitude compared to ligands of similar donor strength, consistent with theoretical predictions. This enhanced OA reactivity can be leveraged for the catalytic deuoroborylation of C-F bonds with reactivity trends that differ from previously reported examples. While there are several reports on the inuence of appended anionic groups on mono-or polydentate phosphine ligands, this work is the rst case where the relative contributions from inductive versus electrostatic donation have been disentangled. The ability of charged groups to stabilize specic resonance structures, such as the zwitterionic Se À -P + R 3 structure, offers tremendous potential in tuning catalytic systems as we demonstrate here. The fact that a major portion of the increase in donor strength arises from electrostatic effects in common organic solvents with charge-diffuse counterions has important implications for ligand design as the directionality of electric elds provides a unique variable for inuencing reactivity and breaking classic free-energy relationships.

Data availability
Detailed experimental and computational data can be found in the ESI. †

Author contributions
The project was planned out and the manuscript was written

Conflicts of interest
There are no conicts to declare.