Donor Strengths Determination of Pnictogen and Chalcogen Ligands by the Huynh Electronic Parameter and Its Correlation to Sigma Hammett Constants

Abstract

The suitability and accuracy of the Huynh electronic parameter (HEP) was further tested to reveal remote substituent effects in pyridines, which are located five or six bonds away from the reporter probe. These values show an excellent correlation to Hammett σ‐constants of the respective substituents with coefficients of R ²=0.9856 (σ _m) and R ²=0.9857 (σ _p). Based on this observation, a methodology for the re‐evaluation of certain Hammett constants with larger uncertainties has been proposed and demonstrated. Moreover, the scope of HEP was extended to various neutral pnictogen and chalcogen donors during which “transphobia effects” were revealed for mixed NHC complexes containing phosphites, arsine and stibine for the first time.

Remote substituent effects can be revealed by Huynh electronic parameters and an excellent correlation to Hammett constants was observed. In addition, the electronic properties of selected pnictogen and chalcogen ligands were determined on a unified scale.

Introduction

The properties of metal complexes are determined by the nature of the metal center and the stereoelectronic signatures imposed by the ligands. Often the steric bulk of a particular ligand can be easily estimated by its Lewis structure drawing, while its electronic properties are more difficult to judge. This is especially so, when the ligand has multiple substituents of different inductive and mesomeric effects, which could enhance or oppose each other. In order to compare the donating abilities of ligands experimentally, a few electronic parameters have been developed (Figure 1).

Figure 1.

Selected experimental electronic parameters.

The ligand electrochemical parameter E _L (LEP) has been introduced by Lever based on redox potentials of for example, Ru^II/III metal complexes.1 The E _L values of a large number of Werner‐type ligands have been tabulated, which reflect their relative capacity to stabilize a metal in a certain oxidation state. Ligands with smaller E _L values can therefore stabilize the Ru^III state in the Ru^II/III couple better than those with larger E _L values. Although E _L values are not a direct measure for the donating ability of a ligand, they are nevertheless related to its donating power.2 The drawback is that only complexes with reversible redox chemistry can be considered for the determination of E _L values. The requirement for less common electrochemical setups and the exclusion of non‐innocent ligands is a further limitation.

In organometallic chemistry, the most commonly used parameter is the so‐called Tolman electronic parameter (TEP) developed in 1970, which compares the A₁ carbonyl IR stretching frequencies of [Ni(CO)₃L] complexes, in which L denotes the ligand of interest.3 This methodology evaluates the amount of π‐backdonation from the nickel(0) center to the carbonyl ligands, which is in turn influenced by the donor/acceptor power of the ligand L. For phosphines, for which σ‐donor and π‐acceptor strengths are approximately inversely proportional, TEP has become a valuable tool of assessment. For ligands that do not show such behavior, interpretation of TEPs is less straightforward, since deconvolution into donor and acceptor contributions is not possible. Another disadvantage of TEP and its Rh/Ir variants4 is the requirement for highly toxic materials, that is, [Ni(CO)₄] or carbon monoxide. The largest drawback, however, is that the majority of Werner‐type ligands cannot be systematically probed using carbonyl‐based methods. For example, amines and pyridines do not react with [Ni(CO)₄].

We have introduced a new electronic parameter, which addresses these shortcomings and allows direct comparison of Werner‐type and organometallic ligands on a unified scale without the use of highly toxic materials.5 Our parameter, that is, Huynh electronic parameter (HEP), evaluates the influence of a particular trans‐ligand L on the ¹³C_carbene NMR shift of the ⁱPr₂‐bimy reporter ligand (i.e. HEP signal) in square‐planar complexes of the type trans‐[PdBr₂(ⁱPr₂‐bimy)L]. It was found that a weaker donor would lead to a relative upfield shift, whereas a stronger donor would lead to a downfield shift of the HEP signal. Different types of NHCs,6 phosphines, isocyanides, and N‐donors have been evaluated.7, 8, 9, 10 Furthermore, much smaller electronic differences among similar ligands can be resolved, providing useful information for the fine‐tuning of complexes. To test its suitability further, we herein report a systematic evaluation of remote substituent effects by using meta‐ and para‐substituted pyridines and correlated the HEP results to the respective σ _p and σ _m Hammett constants.11 Subsequently, the HEP scale was extended to various neutral pnictogen and chalcogen ligands such as phosphites, arsines, stilbenes, etc. to provide insights into their electronic differences.

Results and Discussion

HEPs of substituted pyridines and correlation to σ‐Hammett constants

One advantage of the HEP is the ease of complex probe preparation. Thus, simple mixing of the dimeric complex [PdBr₂(ⁱPr₂‐bimy)]₂ (I)12 with two equivalents of the substituted pyridines in dichloromethane at ambient temperature already results in rapid reactions. In all cases, the initial orange suspensions clear into bright‐yellow solutions (Table 1). To ensure completion, the mixtures were allowed to stir for 2 h. Overall, 26 new trans‐[PdBr₂(ⁱPr₂‐bimy)(Py‐R)] complexes were prepared with 13 meta‐ (2–14) and 13 para‐substituted pyridines (15–27) and compared to the previously reported parent complex trans‐[PdBr₂(ⁱPr₂‐bimy)(Py)] (1).13, 14

Table 1.

Synthesis of trans‐[PdBr₂(ⁱPr₂‐bimy)(Py‐R)] complexes 2–27, in which Py‐R=meta or para‐substituted pyridines, and their characteristic ¹H NMR signals [ppm], HEP values [ppm],^[a] and Hammett σ‐constants in CDCl₃.

R‐group	δNCH	δPyH2,6	HEP	σ m/p
3‐CN (2)	6.25	9.53, 9.43	157.33	0.56
3‐NO2 (3)	6.28	10.10, 9.55	157.38	0.71
3‐CHO (4)	6.31	9.67, 9.43	158.54	0.35
3‐Br (5)	6.29	9.26, 9.11	158.63	0.39
3‐I (6)	6.29	9.36, 9.12	158.68	0.35
3‐Cl (7)	6.30	9.18, 9.07	158.72	0.37
3‐F (8)	6.30	9.12, 9.02	158.73	0.34
3‐CO2H (9)	6.33	9.81, 9.37	159.04	0.37
3‐OH (10)	6.32	8.63, 8.54	159.60	0.12
3‐Ph (11)	6.37	9.39, 9.12	159.80	0.06
3‐Et (12)	6.35	8.94, 8.92	160.33	−0.07
3‐Me (13)	6.34	8.91, 8.89	160.41	−0.07
3‐NH2 (14)	6.33	8.51, 8.44	160.47	−0.16
4‐CN (15)	6.25	9.42	157.79	0.66
4‐CF3 (16)	6.29	9.40	158.38	0.54
4‐CHO (17)	6.30	9.43	158.76	0.42
4‐Cl (18)	6.29	9.08	158.98	0.23
4‐CO2H (19)	6.31	9.36	159.00	0.45
4‐Br (20)	6.29	8.94	159.03	0.23
4‐I (21)	6.28	8.79	159.21	0.18
4‐Ph (22)	6.38	9.16	160.16	−0.01
4‐OMe (23)	6.33	8.92	160.39	−0.27
4‐Me (24)	6.33	8.93	160.43	−0.17
4‐Et (25)	6.34	8.96	160.58	−0.15
4‐NH2 (26)	6.34	8.61	161.35	−0.66
4‐NMe2 (27)	6.36	8.61	161.97	−0.83

[a] HEP values are given with the second decimal in subscript for comparison. Detailed discussion on standard deviations of HEP can be found in References [5], [7] and [8].

All complexes are air‐ and moisture‐stable and were characterized by ¹H and ¹³C NMR spectroscopies, ESI mass spectrometry and elemental analysis. Single crystals for six complexes were also obtained as representatives by slow evaporation of their solutions, and X‐ray diffraction analyses confirm their desired compositions (vide infra). The ¹H NMR spectra of all compounds show the presence of the ⁱPr₂‐bimy and the pyridine derivative in an expected 1:1 ratio. Upon coordination, the ortho‐proton signals of the pyridine derivatives shift downfield by 0.4–0.6 ppm (i.e. δPyH_2,6 in Table 1). Another striking feature is the downfield shift of 1.04–1.17 ppm observed for the isopropyl C−H protons (i.e. δNCH in Table 1) of the complexes in comparison to those in the 1,3‐diisopropyl‐benzimidazolium bromide salt at 5.21 ppm. These suggest the presence of anagostic C−H⋅⋅⋅Pd interactions, which are quite common for d⁸ metal complexes with ⁱPr₂‐bimy ligands.15 Similar spectral observations were also made for the related compounds with the parent pyridine and bridging bipyridine ligands.13

More important for our purpose are the ¹³C NMR chemical shifts of the ⁱPr₂‐bimy reporter ligand, that is, HEP values. The good solubility of the complexes allows an easy detection of these signals, and they are summarized in Table 1. In general, electron releasing substituents lead to small downfield shifts, while electron‐withdrawing ones induced an upfield trend with respect to the parent unsubstituted pyridine complex (R=H), which has an HEP value of 160.0₁ ppm.8 Notably, remote changes five (meta) or six (para) bonds away can be differentiated by the ⁱPr₂‐bimy reporter signal in line with chemical intuition. In this respect, it is worth mentioning that a standard deviation of SD=0.01 ppm can be estimated by the full‐width‐at‐half‐maximum (fwhm≈0.02 ppm) for typical ¹³C NMR signals.5, 7, 8

In organic chemistry, electronic substituent effects have been quantified by the Hammett σ‐constants, which have been defined using meta and para substituted benzoic acids, while no constants were considered for ortho derivatives due to steric interferences.16 They are typically based on conductance measurements of ionization constants of the appropriately substituted benzoic acids [Eq. (1)], in which K _H is the ionization constant for benzoic acid in water at 25 °C, and K _X is the ionization constant for a meta‐ or para‐substituted benzoic acid.11, 16

$${\displaystyle \sigma \left(\mathrm{X}\right)=\log K{}_{\mathrm{X}}-\log K{}_{\mathrm{H}}}$$ (1)

Electron‐withdrawing substituents would increase the acidity giving rise to positive values of Hammett σ‐constants and vice versa. In addition, these constants have also been shown to reflect the electron densities of substituted pyridines.17 Hence, the HEP values obtained for complexes 2–27 were correlated to the well‐accepted and widely used Hammett constants in order to further evaluate our methodology in terms of accuracy (Table 2).

Table 2.

Selected NMR spectroscopic data of complexes 32–34 and trans‐36–39 measured in CDCl₃.

Complex	Ligand	1HMe [ppm]	HEP [ppm]	2 J(C,P) [Hz]
32	TBD	1.74	165.74	–
33	DBU	1.77	166.32	–
34	NEt3	1.74	157.96	–
trans‐36	P(OiPr)3	1.77	175.25	287.1
trans‐37	P(OPh)3	1.51	171.65	289.8
trans‐38	P(O‐2,4‐tBu‐Ph)3	1.59	171.48	289.6
trans‐39	AsPh3	1.79	169.17	–

Notably, an excellent correlation was already observed using all data with linear regression coefficients of R ²=0.9773 and R ²=0.9816 for the meta and para series, respectively. In an early paper, McDaniel and Brown highlighted that there were considerable variations in the σ‐values for the 3‐ and 4‐CO₂H substituents (0.37 and 0.45) with large estimated limits of uncertainty of around 0.1.18 Based on this statement, the correlation study was repeated with the exclusion of the carboxylic acid group, which led to further improvements of the coefficients to R ²=0.9856 (meta) and R ²=0.9857 (para). The linear regression graphs and equations are depicted in Figure 2.

Figure 2.

Graphs and equations for the regressions of (i) HEP vs. σ _m Hammett constants (upper) and (ii) HEP vs. σ _p Hammett constants (lower).

The resulting equations allow interconversion of HEP into Hammett σ constants or vice versa [e.g., Eqs. (2), (3)]. For example, using the reported HEP value for 3‐ethynylpyridine (3‐Py‐C₂H) of 159.2 ppm8 and Equation 2, we can predict a σ _m value of 0.20 for the ethynyl substituent, which is very close to the reported value of 0.21.11

$${\displaystyle \sigma {}_{\mathrm{m}}=(\mathrm{HEP}-160.0433)/-4.1235}$$ (2)

$${\displaystyle \sigma {}_{\mathrm{p}}=(\mathrm{HEP}-159.7855)/-2.7171}$$ (3)

Since Hammett σ‐values are determined from acidity constants of benzoic acids, it is actually understandable why the uncertainties for the carboxylic acid substituent are rather large. The increased acidities of isophthalic (pK _a1=3.70) and terephthalic acid (pK _a1=3.54) compared to that of the parent benzoic acid (pK _a=4.19) cannot be solely attributed to the electron‐withdrawing nature of the CO₂H group. Due to the presence of two identical acid functions in these molecules, the proton dissociation is statistically enhanced. Thus, the electronic influence of the CO₂H group is likely to be overestimated resulting in larger σ‐values. On the other hand, the HEP values of the pyridine carboxylic acid complexes 9 and 19 reflect the intrinsic electronic property of the molecules without such interferences (Figure 3). Application of Equations 2 and 3 with these HEP values gives lower calculated constants of σ _m*=0.25 and σ _p*=0.29.

Figure 3.

Determination of the Hammett constant and HEP for the carboxylic acid substituent.

McDaniel and Brown also noted that ionic substituents have Hammett constants with larger uncertainties.18 An example is the carboxylate group (CO₂ ⁻)_, which has been assigned a zero value for σ _p without any consideration of cation effects.18 However, we suppose that the electronic influence of this group cannot be cation‐independent. To test this notion, we subjected the complex trans‐[PdBr₂(ⁱPr₂‐bimy)(Py‐4‐CO₂H)] (19) to common alkali metal hydroxides to obtain HEP probes of pyridyl‐4‐carboxylates with different alkali metal ions (Scheme 1).

Scheme 1.

Synthesis of HEP probes with different alkali pyridinecarboxylate ligands.

Upon deprotonation, an increased donating ability is expected for the resulting pyridinecarboxylate ligand, which is indeed confirmed by the larger HEP values. As anticipated, different cations induce a different response, and the downfield shifts are smallest for Li⁺ and largest for Na⁺ and K⁺. This observation implies that the negative charge is best stabilized by the lithium cation, which is in line with its largest charge/size ratio leading to the best hard/hard interactions with the carboxylate group. The HEP value peaks with sodium and gradually decrease with increasing size of the cation. The difference between potassium and rubidium is larger as the latter is known to form compounds with higher coordination numbers. Scheme 1 also depicts the calculated σ _p* constants for the different alkali metal carboxylates using Equations 2 and 3.

With the availability of HEP data summarized in Table 1, it is lastly also of interest to study if the HEP value of a meta‐substituted pyridine could be predicted by that of its para‐analogue and vice versa. In order to test such a possibility, HEPs of meta‐ and para‐substituted pyridines bearing identical substituents were plotted against each other where available. With inclusion of all ten substituents, a very good linear regression coefficient of R ²=0.9701 was obtained. Exclusion of the NH₂ group as an outlier led to a further improvement to R ²=0.9846 (Figure 4).

Figure 4.

Regression graphs and equation of HEP (meta) vs. HEP (para).

Based on this encouraging result, it appears that substituents in meta‐ and para‐position exhibit a similar (almost linear) effect on the HEP, potentially allowing for predictions from each other. This could be particularly powerful for groups with primarily inductive effects, while those with dominant mesomeric contributions have to be treated with more caution. The latter is understandable as mesomeric effects have a drastically different impact in meta‐ compared to para‐position.

Determining the HEPs of selected pnictogen donors

Having established an excellent correlation of HEP with Hammett constants, we focused on the extension of our method to previously unaccounted ligands. In particular, triethylamine (NEt₃), 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (TBD) and 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) are widely used organic bases, but they can also stabilize active metal species by coordination. It is therefore surprising that their donating abilities have not been explicitly compared yet. Hence, the respective HEP probes were routinely prepared by exposure of [PdBr₂(ⁱPr₂‐bimy)]₂ (I)12 to the free amine bases. The yellow products trans‐[PdBr₂(ⁱPr₂‐bimy)(TBD)] (32) and trans‐[PdBr₂(ⁱPr₂‐bimy)(DBU)] (33) formed rapidly, while prolonged stirring with an excess of triethylamine was required to fully convert complex I into trans‐[PdBr₂(ⁱPr₂‐bimy)(NEt₃)] (34). TBD is a bicyclic guanidine containing an imine and a secondary amine function that are available for coordination. However, the ligand shows selective coordination of the imine nitrogen to metal centers such as Pd, Cu, Ag, Fe and Li,19, 20 which is also confirmed by single crystal X‐ray diffraction of TBD complex 32 (vide infra). Coordination via the imine function is also expected for the DBU ligand. The three base adducts 32–34 have good solubilities in most organic solvents, and no cis‐isomers were detected. In the ¹³C NMR spectra, the three carbene carbon atoms resonate in the order of 34 (NEt₃, 157.9₆ ppm)<32 (TBD, 165.7₄ ppm)<33 (DBU, 166.3₂ ppm, Table 2).21 This trend indicates that the aliphatic amine NEt₃ is a significantly weaker donor compared to the unsaturated imines, which is in agreement with a greater availability of the lone pair of sp ²‐hybridized imine donors. The weaker donating effect of NEt₃ comparable to the weakest pyridines (Table 1) also explains its sluggish reaction with dimer I. In addition, DBU is a stronger ligand than TBD, which is reasonable by considering the additional −I effect from the third nitrogen atom in the TBD ligand.

In addition to nitrogen donors, HEP has also been used to evaluate different types of phosphines.8, 22, 23 It is thus of interest to extend the scope to other pnictogen donors such as phosphites, arsines and stibines. This study was initiated with P(OMe)₃, P(OⁱPr)₃, P(OPh)₃ and P(O‐2,4‐^tBu‐Ph)₃, which are diverse in terms of stereoelectronic properties. Instant color changes from orange to pale yellow were observed for most reaction mixtures indicative of a generally strong donating ability of these ligands and the fast formation of mixed NHC/phosphite complexes, which are surprisingly rare.24, 25 Similar to their phosphine counterparts, electronically driven trans–cis isomerizations were observed. The thermodynamically preferred cis‐arrangement in these complexes is due to the so‐called “transphobia effect” of phosphorus donors.12, 26, 27 This term was introduced to describe the general difficulty of placing a phosphorus trans to a carbon donor.28 The isomerization is fastest for the P(OMe)₃ and P(OⁱPr)₃ complexes containing the smallest phosphites. Hence, only the cis‐[PdBr₂(ⁱPr₂‐bimy){P(OMe)₃}] (cis‐35) and cis‐[PdBr₂(ⁱPr₂‐bimy){P(OⁱPr)₃}] (cis‐36) complexes were isolated in the standard procedures. The process is slower for P(OPh)₃ complex, and signals for both trans/cis‐[PdBr₂(ⁱPr₂‐bimy){P(OPh)₃}] (trans‐/cis‐37) complexes were captured in the NMR spectra. The complex trans‐[PdBr₂(ⁱPr₂‐bimy){P(O‐2,4‐^tBu‐Ph)₃}] (trans‐38) resists isomerization due to the enhanced steric bulk, and a HEP value of 171.4₈ ppm was detected for the P(O‐2,4‐^tBu‐Ph)₃ ligand without problems (Table 2).

Since trans‐configured complex probes are required for the HEP method, direct NMR scale reactions were carried out to capture the signals of the initially formed trans‐isomers for the less bulky phosphites before complete isomerization. Upon mixing the reactants in NMR tubes, the samples were immediately measured after addition of CDCl₃. Indeed, only one set of signals due to the trans‐[PdBr₂(ⁱPr₂‐bimy){P(OⁱPr)₃}] (trans‐36) or trans‐[PdBr₂(ⁱPr₂‐bimy){P(OPh)₃}] (trans‐37) complexes was observed in the respective ¹H NMR spectra. ³¹P and ¹³C NMR data were measured immediately thereafter. This approach was successful for complex trans‐37, and a HEP of 171.6₅ ppm was obtained for the P(OPh)₃ ligand. However, signals due to both trans‐ and cis‐isomers were detected in the ¹³C NMR spectrum of 36. To resolve the carbene signal of trans‐36 in a shorter time, the NMR reaction was repeated with ¹³C2‐labeled complex I,5 which allowed detection of the HEP signal for P(OⁱPr)₃ at 175.2₅ ppm with a single scan. Unfortunately, the signals of trans‐35 complex could not be captured even within such a short time due to its very rapid trans–cis isomerization. Overall, the donating ability of the phosphites decreases in the order P(OⁱPr)₃≫P(OPh)₃>P(O‐2,4‐^tBu‐Ph)₃ (Table 2). Generally, they are also weaker donors compared to phosphines with the same substituents. No HEP value was obtained for P(OMe)₃ due to its strong transphobia. However, it is intuitive to place it in between P(OⁱPr)₃ and P(OPh)₃, which is reflected in the trends of the available cis complexes (Table 3).

Table 3.

Selected NMR spectroscopic data of complexes cis‐35–37, cis‐39 and cis‐40 measured in CDCl₃.

Complex	Ligand	1HMe [ppm]	HEP [ppm]	2 J(C,P) [Hz]
cis‐35	P(OMe)3	1.74, 1.66	170.79	21.4
cis‐36	P(OiPr)3	1.71, 1.69	171.98	22.9
cis‐37	P(OPh)3	1.57, 1.14	169.27	22.9
cis‐39	AsPh3	1.66, 0.89	169.34	–
cis‐40	SbPh3	1.66, 1.04	167.46	–

Similar trans–cis isomerizations were also observed for mixed NHC/AsPh₃ and NHC/SbPh₃ complexes. The process is particularly rapid for the antimony compound, and only cis‐[PdBr₂(ⁱPr₂‐bimy)(SbPh₃)] (cis‐40) could be detected by all means. Complex trans‐[PdBr₂(ⁱPr₂‐bimy)(AsPh₃)] (39), on the other hand, isomerizes slower and could be fully characterized by direct NMR reaction, giving a HEP value of 169.1₇ ppm. This arsine is therefore a weaker donor compared to phosphines and phosphites. Comparison of the cis‐39 and cis‐40 could indicate that the stibine is a weaker donor than the arsine (Table 3), which is in agreement with TEP studies.29

After having obtained the carbene signals of the trans‐configured pnictogen complexes 36–39, the isomerization processes were purposely monitored by ¹H NMR spectroscopy in CDCl₃. Differentiation of trans‐ versus cis‐configured products can be best achieved by analyzing the ¹H NMR signals of methyl groups in the ⁱPr₂‐bimy ligand. For the symmetrical trans‐isomers, only one doublet is observed, while non‐equivalence in the cis‐isomers leads to two doublets of equal integration. In addition, the phosphite complexes can be distinguished by the ³¹P coupled carbene signals. The trans‐isomers exhibit ² J(C,P) coupling constants of >280 Hz (Table 2), while <23 Hz are observed for cis‐isomers (Table 3).

The P(OⁱPr)₃ complex trans‐36 fully converted to cis‐36 within two hours. For the P(OPh)₃ complex 37, only 17 % was converted to the cis‐isomer after eight days. However, the ¹H NMR spectrum of the product mixture obtained from the initial lab‐scale reaction (30 min stirring in CH₂Cl₂) shows a trans–cis conversion of 50 %. Besides better mixing, the different solvent would be the key factor in influencing the isomerization rate. The increased polarity of CH₂Cl₂ compared to CDCl₃ should favor the formation of the cis‐isomer with a larger dipole moment. Hence, by repeating the reaction in the even more polar organic solvent MeOH, cis‐[PdBr₂(ⁱPr₂‐bimy){P(OPh)₃}] (cis‐37) was exclusively obtained after 10 hours of stirring at ambient temperature. The P(O‐2,4‐^tBu‐Ph)₃ complex trans‐38 did not change at all after ten days. Heating the NMR sample in an oil bath (65 °C) for several days led to decomposition to palladium black, but no cis‐isomer was detected. The isomerization of the AsPh₃ complex 39 seems to reach equilibrium after six days giving rise to a trans/cis mixture with a ratio of 1:3.5. Due to transphobia, only single crystals of the cis‐adducts could be obtained (vide infra).

Determining the HEPs of selected neutral chalcogen donors

Neutral chalcogen donors form weaker bonds with metal centers compared to their pnictogen counterparts. Their weak donor capability makes them suitable candidates for the design of hemilabile hybrid ligands for catalysis. For example, complexes of thioether‐functionalized NHC ligands have shown promising catalytic activities and an interesting structural dynamics.30, 31, 32, 33, 34 However, less attention has been paid to the measurement of their electron donating abilities compared to those of the other ligands. Herein, the electron donating ability of two common thioethers, that is, dimethylsulfide (DMS) and tetrahydrothiophene (THT) are assessed. We were also interested in studying the coordination chemistry and electron donating properties of analogous neutral oxygen donors. Unfortunately, dialkyl ethers are even weaker donors and were not able to cleave the dimeric complex I. Pyridine N‐oxide (PNO) as a formally neutral oxygen donor was included instead. The increased dipole moment due to charge separation leads to a stronger nucleophilic property of the oxygen atom, which may allow the cleavage reaction to proceed. The two thioethers smoothly cleaved complex I, affording complexes trans‐[PdBr₂(ⁱPr₂‐bimy)(DMS)] (41) and trans‐[PdBr₂(ⁱPr₂‐bimy)(THT)] (42). However, an excess of PNO was required to afford the desired complex trans‐[PdBr₂(ⁱPr₂‐bimy)(PNO)] (43). The two thioethers are expected to have very similar electron donating properties. However, ¹³C NMR spectroscopy could still resolve the small difference as indicated by the HEP values of complexes 41 and 42 at 163.4₆ and 163.6₃ ppm, respectively. The cyclic THT ligand was found to be slightly stronger donating than the DMS ligand. The PNO complex 43 shows the most upfield HEP of 155.7₄ ppm indicating a weak donor property of the PNO ligand, which explains the difficulty met in the preparation of this complex.

Overall, the ligands investigated in this work cover a HEP range of 20 ppm (Figure 5). The phosphites are among the strongest donors, but they are still weaker than their direct phosphine counterparts,8 which in turn are inferior to common NHCs in terms of electron donation.7 The HEP value of triphenylarsine is smaller than all P‐donors, but significantly larger than all N‐donors. The latter can be differentiated into sp ²‐ and sp ³‐hybridized N‐donor atoms and aromaticity of the heterocycle. Generally, sp ²‐hybridized N‐donors are stronger due to the less hindered lone pair, and aliphatic imines such as DBU and the guanidine TBD are stronger than aromatic pyridines. We also note that the donating ability of pyridines can be fine‐tuned over a range of 5 ppm. Thioethers are found in between aliphatic imines and pyridines, and pyridine‐N‐oxide is the weakest donor on the HEP scale thus far.

Figure 5.

The donor strengths of selected organometallic and Werner‐type ligands on the HEP scale.

Solid‐state molecular structures

Single crystals of complexes 4, 9, 15⋅CH₂Cl₂, 19⋅2 CH₂Cl₂, 26⋅0.5 CHCl₃, 27⋅0.5 CHCl₃⋅0.5 CH₂Cl₂, 32, cis‐35⋅CH₂Cl₂, cis‐36⋅CHCl₃, cis‐37⋅2 CHCl₃, cis‐39, cis‐40, 42 and 43⋅CHCl₃ were obtained by slow evaporation of their solutions (see Supporting Information), and X‐ray diffraction reveals square‐planar geometries expected for Pd^II complexes. In all cases, the ⁱPr₂‐bimy ligand is perpendicular to the [PdCBr₂L] coordination plane.

The structures of the pyridine adducts 4, 9, 15, 19, 26 and 27 closely resemble those of previously published analogues13 and do not require additional comments (Figure S1). The remaining complexes, on the other hand, are rare examples and their molecular structures are depicted in Figure 6.

Figure 6.

Molecular structures of 32, cis‐35⋅CH₂Cl₂, cis‐36⋅CHCl₃, cis‐37⋅2CHCl₃, cis‐39, cis‐40, 42 and 43⋅CHCl₃ showing 50 % probability ellipsoids; hydrogen atoms and solvent molecules are omitted for clarity. Selected bond length [Å] and bond angles [°]: 32, Pd1−C1 1.967(4), Pd1−Br1 2.4380(7), Pd1−Br2 2.4341(7), Pd1−N3 2.075(4); C1‐Pd1‐Br1 86.3(1), C1‐Pd1‐Br2 89.2(1), Br1‐Pd1‐N3 93.4(1), Br2‐Pd1‐N3 91.2(1); PdCBr₂N/NHC dihedral angle θ 88.1°. cis‐35, Pd1−C1 1.980(7), Pd1−Br1 2.4914(9), Pd1−Br2 2.472(1), Pd1−P1 2.204(2); C1‐Pd1‐Br1 84.8(2), C1‐Pd1‐P1 89.7(2), Br1‐Pd1‐Br2 94.21(3), Br2‐Pd1‐P1 91.30(6); θ 89.6°. cis‐36, Pd1−C1 1.988(3), Pd1−Br1 2.4853(4), Pd1−Br2 2.4779(4), Pd1−P1 2.2133(8); C1‐Pd1‐Br2 85.90(8), C1‐Pd1‐P1 90.41(8), Br1‐Pd1‐Br2 92.22(2), Br1‐Pd1‐P1 91.33(2); θ 83.8°. cis‐37, Pd1−C1 1.992(4), Pd1−Br1 2.4751(5), Pd1−Br2 2.4767(5), Pd1−P1 2.204(1); C1‐Pd1‐Br1 88.1(1), C1‐Pd1‐P1 90.4(1), Br1‐Pd1‐Br2 94.64(2), Br2‐Pd1‐P1 86.88(3); θ 82.8°. cis‐39, Pd1−C1 1.970(2), Pd1−Br1 2.4818(3), Pd1−Br2 2.4624(3), Pd1−As1 2.3568(3); C1‐Pd1‐Br2 85.44(6), C1‐Pd1‐As1 90.40(6), Br1‐Pd1‐Br2 93.92(1), Br1‐Pd1‐As1 90.09(1); θ 88.4°. cis‐40, Pd1−C1 1.971(3), Pd1−Br1 2.4699(4), Pd1−Br2 2.4771(4), Pd1−Sb1 2.4967(3); C1‐Pd1‐Br1 86.44(9), C1‐Pd1‐Sb1 95.66(9), Br1‐Pd1‐Br2 95.55(1), Br2‐Pd1‐Sb1 82.49(1); θ 84.2°. 42, Pd1−C1 1.969(3), Pd1−Br1 2.4286(4), Pd1−Br2 2.4342(4), Pd1−S1 2.3784(7); C1‐Pd1‐Br1 86.85(8), C1‐Pd1‐Br2 87.63(8), Br1‐Pd1‐S1 87.13(2), Br2‐Pd1‐S1 98.76(2); θ 87.1°. 43, Pd1−C1 1.935(2), Pd1−Br1 2.4213(3), Pd1−Br2 2.4201(3), Pd1−O1 2.113(2); C1‐Pd1‐Br1 88.42(6), C1‐Pd1‐Br2 86.46(6), Br1‐Pd1‐O1 88.86(4), Br2‐Pd1‐O1 96.39(4); θ 88.2°.

Compound 32 is the first mixed NHC/guanidine complex. The N‐heterocycle is also oriented perpendicular to the coordination plane and coordinates via the imine instead of the two amine moieties, confirming once again the superior donating ability of sp ²‐N‐donors over sp ³‐hybridized counterparts. As common for N‐ligands, a trans arrangement is observed making them ideal candidates for HEP analyses.

In contrast, only cis‐isomers of the phosphite adducts 35–37 crystallized as a consequence of rapid trans–cis isomerization due to transphobia. Previously reported mixed NHC/phosphite complexes all have a trans arrangement, since they contain the very bulky NHCs IPr and SIPr.25 Compared to these, the averaged Pd–C separation of 1.987 Å in our cis complexes is significantly shorter indicating stronger bonds. They are also identical within error margin. The cis‐configuration also leads to shorter and stronger Pd−P bonds with an average length of 2.207 Å.

Similar observations were made for the arsine and stibine complexes cis‐39 and cis‐40, which are the first structurally characterized cis‐isomers. The Pd−C bonds of 1.970(2) and 1.971(3) are identical and equal to that observed for the cis‐[PdBr₂(ⁱPr₂‐bimy)(PPh₃)]12 complex within 3σ. On the other hand, bonds to palladium steadily elongate going from phosphorus {2.2624(8) Å}12 to arsenic {2.3568(3) Å} to antimony {2.4967(3) Å}, which is due to the increasing atom size going down the group. All these bonds are significantly shorter and stronger than those observed in related trans complexes.35

Complexes 42 and 43 are also the first of their kind. The thioether and N‐oxide donors adopt preferably a trans orientation with respect to the NHC. The Pd–C distance for the thioether complex of 1.969(3) Å is substantially longer than that in the pyridine‐N‐oxide complex Pd1‐C1 1.935(2) Å, which is reflective of their different donor strengths. The latter is equal to that observed for complex formed with acetonitrile,12 which is also a very weak donor.

Conclusions

The study of 26 complex probes of the type trans‐[PdBr₂(ⁱPr₂‐bimy)(Py‐R)] using the Huynh electronic parameter (HEP) have revealed remote substituent effects in meta‐ and para‐substituted pyridines five and six bonds away from the NMR probe. Moreover, the influence of these substituents on the ¹³C_carbene NMR signal of the ⁱPr₂‐bimy reporter ligand shows an excellent correlation to their respective sigma Hammett constants (σ _m and σ _p), providing further support for the suitability of HEP as an efficient and accurate electronic parameter. The resulting regression equations could also be used for the re‐evaluation of certain Hammett constants that carry larger uncertainties. This has been demonstrated for the CO₂H and CO₂M (M=alkali metal) substituents. In addition HEPs of meta‐ and para‐substituted pyridines bearing the same substituent also showed an excellent linear correlation. In principle, this allows for HEP predictions of para‐substituted pyridines from that of the respective meta‐isomer and vice versa. Finally, attempts were made to extend the library of HEP values to various ligands that hitherto have not been covered yet. In particular, these include phosphites, arsines, stibines, thioethers and N‐oxides. The mixed NHC/ligand complexes of the pnictogens show an interesting and electronically‐driven trans–cis isomerization. The so‐called “transphobia” effect previously described for phosphorous ligands have thus been extended to include arsenic and antimony compounds. In addition to reporting new types of complexes, this work also expands the toolbox for chemists and provides valuable electronic data for the proper comparison of different ligand classes on a unified scale.

Supporting Information

Supporting Information for this article is given via a link at the end of the document. The Supporting Information contains a detailed description of the synthetic work, the characterization of all analytes as well as selected crystallographic data.

Conflict of interest

The authors declare no conflict of interest.

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Supplementary

Q. Teng, P. S. Ng, J. N. Leung, H. V. Huynh, Chem. Eur. J. 2019, 25, 13956.