Stereoelectronic Profiling of Expanded-Ring N-Heterocyclic Carbenes

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Stereoelectronic Profiling of Expanded-Ring N‑Heterocyclic Carbenes Anuj Kumar,† Dan Yuan,‡ and Han Vinh Huynh*,† †

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Dushu Lake Campus, Soochow University, Suzhou 215123, People’s Republic of China

‡

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S Supporting Information *

ABSTRACT: Heterobis(carbene) complexes of palladium(II) and gold(I) containing expanded-ring N-heterocyclic carbenes (erNHCs) have been prepared in order to study their electronic properties. erNHCs with mesityl substituents were found to exhibit anisotropic interferences, which hampered ranking of their donicities by 13C NMR spectroscopy. The anisotropy effects were found to be stronger in the linear gold complexes, where a smaller coordination number allows the wingtips to spread out more. erNHCs with flexible N-benzyl groups are more suitable, and their donor strengths were found to gradually increase from five- to seven-membered heterocycles. The same trend can also be obtained by comparing the 1J(C−H) coupling constants of the respective salts, although significant differences between seven- and eightmembered erNHCs could not be detected. The %Vbur values of erNHCs obtained from structures of their palladium and gold complexes revealed that the anisotropic interferences increase with overall steric bulk.

N

Chart 1. Expanded-Ring NHCs and the Averaged Carbonyl Stretches of Their trans-[RhCl(CO)2(erNHC)] Complexes

onclassical N-heterocyclic carbenes (NHCs) derived from six- and seven-membered heterocycles have recently attracted more attention as they often display superior activities compared to their classical five-membered counterparts in transition-metal catalysis.1−6 This has been ascribed to an enlarged N−C−N angle, which should have significant implications on the stereoelectronic profiles of such expandedring NHCs (erNHCs).7−13 The widened angle should lead to an increased p-character of the carbene lone pair orbital resulting in a destabilization of the highest occupied molecular orbital.14 Concurrently, the steric impact of the wingtip groups on the metal center is enhanced.15−20 The stepwise addition of methylene groups in simple expanded-ring NHCs should also increase the positive inductive effect of the backbone. Overall, one can suppose that both the donating ability and the steric bulk increase with increasing ring size of the NHC. Previously, some attempts were made to evaluate the electronic properties of erNHCs using IR spectroscopy of rhodium- or iridium carbonyl complexes (Chart 1).21,22,15 However, these can only provide information about the net donor−acceptor effect, and deconvolution into individual contributions is not possible. Moreover, intuitive and conclusive results could not be obtained due to the limited resolution of the methodology and varied choices of ring sizes and substituents. For example, essentially identical averaged wavenumbers were obtained for six- and seven-membered NHCs, while their differences to fiveand eight-membered NHCs were somewhat greater.22 Clearly, the elucidation of their donating abilities is not an easy task, since the increased steric bulk could negatively © XXXX American Chemical Society

interfere with the determination of pure electronic effects. In recent years, we have introduced a unified electronic parameter that can be used to compare the (primarily) σ-donicities of Werner-type and organometallic ligands on the same scale, that is, the Huynh electronic parameter (HEP).23,24 The methodology measures the impact of a trans-ligand L on the 13Ccarbene NMR signal of the 1,3-diisopropylbenzimidazolin-2-ylidene (iPr2-bimy) reporter ligand in complexes of the type trans[PdBr2(iPr2-bimy)L]. This signal is abbreviated as the HEP value. Due to the aforementioned challenge, expanded-ring NHCs constitute an interesting and important ligand class to Received: March 20, 2019

A

DOI: 10.1021/acs.inorgchem.9b00786 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry further test the scope and limitations of the HEP. Herein, we report the systematic, stereoelectronic profiling of six-, seven-, and even eight-membered NHCs using heterobis(NHC) complexes of palladium(II) and gold(I) using more recent methods. In particular, we reveal the impact of aromatic substituents in bulky ligands that provides a more detailed understanding of the HEP. Moreover, the use of 1J(C−H) coupling constants for the same purpose was investigated in detail. Finally, %Vbur values were determined using solid-state structures of the palladium(II) and gold(I) complexes.

The upfield shifts observed here point to an enhanced anisotropy effect of the aromatic mesityl groups of the expanded-ring NHCs on the trans-standing iPr2-bimy ligand.26 Two different resonances belonging to two diverse carbene centers were observed in the 13C NMR spectrum of each complex. The more downfield carbene signals at 201.3 and 212.3 ppm can be easily assigned to the 6-Mes and 7-Mes ligands, respectively,27 while their HEP signals were found at 176.6 ppm (6-Mes) and 176.9 ppm (7-Mes). Comparison of these values with those of IMes (177.2 ppm) and SIMes (177.6 ppm)25 would suggest stronger donicities of fivemembered NHCs over expanded-ring NHCs, which is, however, not intuitive (Figure 1, vide supra).

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RESULTS AND DISCUSSION Heterobis(carbene) Complexes of Palladium(II). Complex probes of the type trans-[PdBr2(iPr2-bimy)(erNHC)] are required to evaluate the donicity of NHCs with larger ring sizes by HEP. Initially, commonly used N-mesityl-substituted six- and seven-membered saturated erNHCs were targeted, since their salt precursors 6-Mes·HBr (A) and 7-Mes·HBr (B) are most easily prepared. For this purpose, the amidinium bromides were treated with Ag2O and the dimeric [PdBr2(iPr2bimy)]2 complex in a one-pot reaction in dichloromethane at ambient temperature for 24 h (Scheme 1, route I). The Scheme 1. Synthetic Strategy for trans-Hetero-bis(carbene) PdII Complexes

Figure 1. Comparison of N-mesityl substituted NHCs on the HEP scale.

It appears that the aforementioned increased anisotropy affecting the isopropyl C−H protons also interferes with the HEP determination of complexes 1 and 2. This is understandable, since the larger N−C−N angles of erNHCs would place the mesityl substituents closer to the palladium center and the iPr2-bimy reporter ligand compared to five-membered analogues. The increased anisotropic exposure leads to a smaller HEP value. To find further support for our hypothesis, single crystals of complexes 1·CHCl3 and 2·CHCl3 suitable for X-ray diffraction were grown by slow evaporation of their solutions in chloroform. The molecular structures depicted in Figure 2 reveal square-planar geometries, in which each palladium(II) center is coordinated by one iPr2-bimy, an expanded-ring NHC, and two bromido ligands in a trans arrangement. In both cases, the bond length between Pd1−C1 is shorter than that between Pd1−C14 pointing to a stronger coordination of the iPr2-bimy ligand. Previously, we concluded that such bond distances do not strictly correlate with the ligand’s donor strengths, since bond parameters are generally affected by a complicated interplay of various factors including crystal packing, solvation, and counterion effects.25 Notably, the N− C−N angles were found to be 117.6(3)° and 119.4(4)° for 1 and 2, respectively, which are significantly larger than that for the SIMes analogue [cf. 107.3(2)°].25 Due to the larger N− C−N angles, the distances between the centroids of the mesityl rings and iPr2-bimy carbene carbon are expected to become shorter. Indeed, these distances of 4.971(9) and 4.963(9) Å for 1 and 4.931(5) and 4.999(5) Å for 2, respectively, are significantly smaller than those of the SIMes complex [cf. 5.140(4) and 5.646(4) Å].25 We believe that this leads to a greater exposure of the iPr2-bimy reporter in complexes 1 and 2 to the anisotropy of the mesityl rings. Consequently, an unusual upfield shift of 13Ccarbene NMR resonances is observed, which interferes with the actual HEP determination of the erNHCs. The electronic influence of the expanded-ring

expected heterobis(carbene) complexes trans-[PdBr2(iPr2bimy)(6-Mes)] (1) and trans-[PdBr2(iPr2-bimy)(7-Mes)] (2) were obtained in low yields of 30 and 32%, respectively. Alternatively, KHMDS can be used as an external base for the in situ generation of the free erNHCs for the cleavage of dimeric [PdBr2(iPr2-bimy)]2 in THF (Scheme 1, route II), which also yielded the expected palladium complexes (1, 30%) and (2, 34%). The formation of the complexes was supported by positive mode ESI mass spectrometry, which shows dominant signals for the respective [M − Br]+ fragments at m/z = 709 (1) and 723 (2), respectively. In their 1H NMR spectra recorded in CDCl3, new methylene signals were observed in the range of 2.26−3.91 ppm for the newly introduced expanded-ring NHCs. Moreover, the isopropyl C−H protons of the iPr2bimy reporter ligand resonate as septets centered at 4.78 ppm (1) and 4.75 ppm (2), respectively. Notably, these resonances are significantly upfield compared to that of the five-membered SIMes analogue trans-[PdBr2(iPr2-bimy)(SIMes)] at 5.25 ppm. Generally, such signals appear in the more downfield range of 6.36−5.20 ppm for HEP probes of five-membered NHCs.25 B

DOI: 10.1021/acs.inorgchem.9b00786 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Molecular structures of one independent molecule each for complexes 1·CHCl3 and 2·CHCl3 showing 50% probability ellipsoids; hydrogen atoms and solvent molecules are omitted for clarity. Selected bond lengths (Å) and angles (deg) for 1·CHCl3: Pd1−C1 1.995(3), Pd1− C14 2.069(3), Pd1−Br1 2.448(0), Pd1−Br2 2.467(0); C1−Pd1−C14 177.6(2), Br1−Pd1−Br2 168.5(4), N3−C14−N4 117.6(3). 2·CHCl3: Pd1−C1 2.012(5), Pd1−C14 2.061(5), Pd1−Br1 2.4321(7), Pd1−Br2 2.4398(7); C1−Pd1−C14 176.8(2), Br1−Pd1−Br2 172.1(3), N3−C14− N4 119.4(4).

backbone is thus diluted by the anisotropy of the aromatic wingtip groups. In order to circumvent such anisotropic interferences, erNHCs with more flexible substituents were targeted. Specifically, we opted for the benzyl group, since the respective erNHCs could be directly compared to a series of previously studied dibenzyl-substituted classical five-membered NHCs.25 In addition, N,N′-dibenzylic systems are also synthetically more viable and easier to handle than the simple dialkyl analogues, which are known to undergo ring-opening reactions with ease. With this intent, the six- and seven-membered N,N′dibenzyl-substituted ligand precursors 6-Bn·HBr (C) and 7Bn·HBr (D) were synthesized following modified literature procedures.28 However, attempts to prepare the respective heterobis(NHC) complexes via route I using C/D and Ag2O failed (Scheme 1). Instead, hydrolyzed products of the expanded-ring heterocycles were observed due to the unavoidable formation of water as the byproduct in these reactions. Nevertheless, the targeted heterobis(carbene) complexes trans-[PdBr2(iPr2-bimy)(6-Bn)] (3) and trans[PdBr2(iPr2-bimy)(7-Bn)](4) could be isolated in low yields (17 and 20%) following route II using the precursors C/D and KHMDS as an external base (Scheme 1). Dominant signals for the respective [M − Br]+ fragments observed in their ESI mass spectra support the formation of the complexes. Their 1H NMR spectra reveal signals for both types of NHC ligands. More importantly, the resonances for the isopropyl C−H protons at 5.93 ppm (3) and 5.98 ppm (4) are now in the typical range of five-membered NHC complexes.25 This signifies a markedly reduced anisotropic interference in the complexes 3 and 4 compared to 1 and 2. Their HEP signals were detected at 180.6 ppm (3) and 182.0 ppm (4), indicating that both erNHCs are more electron donating compared to the direct five-membered analogue SIBn (cf. 180.1 ppm).25 Based on these values, the NHCs can be ranked as 5-Bn < 6Bn < 7-Bn in order of increasing electron-donating ability on the HEP scale (Figure 3), which is in agreement with the notion that additional methylene groups increase the +I effect of the NHC backbone.

Figure 3. Comparison of N-benzyl-substituted NHCs on a HEP scale.

Inspired by the positive outcome, and to push the limits of HEP further, we next focused on eight-membered NHCs. The salt 8-Bn·HBr (E) and the respective complex trans[PdBr2(iPr2-bimy)(8-Bn)] (5) were prepared in analogy to the smaller counterparts (Scheme 1). Complex 5 shows spectroscopic characteristics similar to those of 3 and 4. However, a notable feature of 5 is a slightly more upfield isopropyl C−H proton resonance at 5.88 ppm, which may already point to some small contributions of anisotropy even from the more flexible benzyl substituents. Indeed, its HEP value was detected also more upfield at 180.7 ppm positioning 8-Bn in between 7-Bn and 6-Bn in terms of donicity (Figure 3). This observation could be explained by an even larger N− C−N angle in 8-Bn, which would place the N-benzyl substituents even closer to the iPr2-bimy sensor for anisotropy to interfere. Single crystals for all three complexes 3−5 were grown for X-ray analyses. The molecular structures depicted in Figure 4 confirm the expected square-planar geometry with the erNHCs trans to the iPr2-bimy ligand. As observed for complexes 1 and 2, the palladium distances to the erNHCs are significantly longer than to the iPr2-bimy reporter. As anticipated, all erNHCs exhibit significantly larger the N−C−N angles, that is, 118.6(5)° for 3, 118.3(3)° for 4 and 122.8(3)° for 5, than the five-membered SIBn analogue [109.2(6)°].25 It is also notable that the angle for 8-Bn is by far the largest, while those for 6-Bn and 7-Bn are essentially identical. For the more flexible benzyl groups, it is not viable to consider the distances between the C

DOI: 10.1021/acs.inorgchem.9b00786 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Molecular structures of complexes 3, 4, and 5 showing 50% probability ellipsoids. For 5, only one independent molecule is shown; hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg) for 3: Pd1−C1 2.025(6), Pd1−C14 2.053(6), Pd1−Br1 2.446(8), Pd1−Br2 2.450(8), C18···C1 4.654(5), C25···C1 4.849(3); C1−Pd1−C14 177.1(3), Br1−Pd1−Br2 177.5(4), N3−C4−N4 118.6(5). 4: Pd1−C20 2.019(3), Pd1−C1 2.052(3), Pd1−Br1 2.451(4), Pd1−Br2 2.448(4), C13···C20 4.628(1), C6···C20 4.906(2); C20−Pd1−C1 175.5(3), Br1−Pd1−Br2 176.6(5), N1−C1−N2 118.3(3). 5: Pd1−C1 2.029(3), Pd1−C14 2.053(3), Pd1−Br1 2.456(4), Pd1−Br2 2.448(4), C20···C1 4.670(7), C27···C1 4.651(8); C1−Pd1−C14 177.5(3), Br1−Pd1−Br2 178.6(6), N3−C14−N4 122.8(3).

aryl centroid to the iPr2-bimy carbene atom. Instead, the separation between the benzylic carbon atoms and the reporter carbon could be used to gauge anisotropic exposure. Complex 5 exhibits an averaged distance of 4.661 Å, which is slightly shorter than that found for complexes 3 and 4 of 4.752 and 4.767 Å, respectively. It is therefore intuitive to suppose that the increased steric bulk in 8-Bn induced by a larger heterocycle also increases its anisotropy influence markedly. Heterobis(carbene) Complexes of Gold(I). In addition to heterobis(carbene) complexes of palladium(II), those of gold(I) can also be used to gauge the donating abilities of NHCs. Previously, we have observed an excellent correlation with a linear regression coefficient of R2 = 0.98 for iPr2-bimy 13 Ccarbene NMR signals in [Au(iPr2-bimy)(NHC)]BF4/PF6 and trans-[PdBr2(iPr2-bimy)(NHC)] complexes.29 As such, we were also interested in evaluating the anisotropy effects of

erNHCs on the gold system, which exhibits a different coordination geometry than the HEP complexes. The absence of two additional bromido ligands in gold(I) complexes may result in a different outcome. This study calls for the preparation of [Au(iPr2-bimy)(erNHC)]BF4 complexes, which in principle could be obtained by chlorido substitution of [AuCl(iPr2-bimy)] with in situ generated erNHCs. However, this approach turned out to be nonviable in the case of erNHCs. Therefore, an “inverse” approach was targeted, in which [AuX(erNHC)] complexes had to be prepared first, followed by halido substitution with the iPr2-bimy probe. With this intent, a series of [AuBr(erNHC)] complexes were synthesized by reaction of the erNHC ligand precursors A− E with [AuCl(tht)] in the presence of KHMDS starting at −78 °C, but with gradual warming to ambient temperature (Scheme 2). The complexes [AuBr(6-Mes)] (6) and [AuBrD

DOI: 10.1021/acs.inorgchem.9b00786 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 2. Synthetic Routes to Hetero-bis(carbene) AuI Complexes

Figure 5. Molecular structures of complexes 12 and 14 showing 50% probability ellipsoids; hydrogen atoms and BF4 anion are omitted for clarity. Selected bond lengths (Å) and angles (deg) for 12: Au1−C1 2.022(4), Au1−C14 2.035(4); C1−Au1−C14 175.6(6), N3−C14−N4 120.6(4). 14: Au1−C1 2.025(3), Au1−C14 2.044(3), C26···C1 4.678(3), C19···C1 4.775(3); C1−Au1−C14 175.6(3), N3−C14−N4 119.3(3).

peaks for the respective [Au(iPr2-bimy)(erNHC)]+ cations in all cases. Again and due to an increased anisotropy, the isopropyl methine 1H NMR signals of the mesityl-substituted erNHC complexes (3.93 ppm for 11; 3.82 ppm for 12) are much more upfield than those for the benzyl-containing ones (5.00 ppm for 13, 4.80 ppm for 14 and 4.84 ppm for 15). Compared to the equivalent signals in the palladium(II) complexes 1−5, these resonances are also more upfield. Two distinct carbene signals are found in 13C NMR spectrum of each complex. The carbene atoms of the erNHCs resonate from 201.8−212.4 ppm, which are more downfield compared to their precursor complexes 6−10. Comparison of the iPr2-bimy carbene signals does not show any conclusive trend. For example, the 6-Mes complex 11 exhibits a more downfield shift compared to the 7-Mes complex 12 (187.8 vs 187.4 ppm). In the benzyl series, 7-Bn complex 14 gives rise to a more downfield shift than its 6-Bn analogue 13 (187.9 vs 187.7 ppm), which is more intuitive. However, the 8-Bn complex 15 shows the most upfield shift in this series at 187.1 ppm. Overall, these results show that the gold(I) probes are even more prone to anisotropic interferences exhibited by substituents than the palladium complexes. This can be reasoned by the reduced number of ligands in the linear gold(I) complexes, which allows the wingtips of the erNHCs to spread out more efficiently. In turn, the iPr2-bimy reporter is more exposed to the anisotropic region of the aryl groups influencing its chemical shifts. Single crystals of the seven-membered erNHC complexes 12 and 14 suitable for X-ray diffraction were obtained by slow evaporation of their concentrated solutions in acetonitrile. Both compounds adopt a linear arrangement, where the gold(I) center is coordinated by the erNHC and the iPr2-bimy

(7-Mes)] (7) with N-mesityl wingtips were obtained in good yields (69 and 75%), while the N-benzyl derivatives [AuBr(6Bn)] (8), [AuBr(7-Bn)] (9), and [AuBr(8-Bn)] (10) were isolated in lower yields (21−31%) after column chromatography. The formation of the AuI complexes 6−10 was evident by 1H NMR spectroscopy, which shows the erNHCs’ resonances with exception of the downfield NCHN signals characteristic for their ligand precursors. In the 13C NMR spectra, the gold-bound carbene signals were observed in the range of 192.3−204.7 ppm. Single crystal X-ray diffraction studies of all five complexes also confirm their identity as linear complexes, in which the AuI center is ligated by one erNHC and a bromido ligand (see Supporting Information). The Au− C distances and N−C−N angles are comparable to those reported for similar gold(I) erNHC complexes.30−32 The seven-membered heterocycle in complexes 7 and 9 adopts an envelope-like conformation. However, conformational flexibility of 7-NHCs has been described recently.32 AuBr(erNHC)] complexes 6−10 provide access to the targeted heterobis(NHC) complexes [Au( i Pr 2 -bimy)(erNHC)]BF4 11−15 in good yields (69−95%) when treated with iPr2-bimy·HBF4 and Ag2O at ambient temperature (Scheme 2). All complexes are stable in solid state and in solution.29 For some related heterobis(carbene) gold(I) complexes, we have previously observed ligand redistribution processes in solution. Notably, complexes 11−15 resist such decompositions, which was attributed to the stronger electrondonating power of the erNHCs. The increased electron density on the AuI centers may result in stronger Au−carbene bonds due to enhanced π back-donation, which are non-negligible in electron-rich d10 complexes. Support for their formation was first obtained by positive mode ESI MS, which shows base E

DOI: 10.1021/acs.inorgchem.9b00786 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

In contribution to a better understanding of erNHCs, it is also of our interest to test if five- to eight-membered NHCs with N-mesityl and N-benzyl substituents can be discerned by their 1J(C−H) coupling constants. Moreover, the chemical shift of the acidic proton may provide some useful insights into the acidity of the salts and inversely on the basicity of the parent NHC as its conjugate base. In addition, the shielding capability of the N-substituents could be tested as well. To avoid any other influences,36 only equimolar solutions (0.05 mmol/0.6 mL) of bromide salts were analyzed in DMSO-d6. Distinct 13C satellites for the acidic C2−H proton could be observed in 1H NMR spectra of the salts within 50 scans, and the respective 1J(C−H) coupling constants are given in Chart 2 together with the chemical shifts of the acidic protons. The signals for the mesityl series show a gradual upfield shift with increasing ring size from 5 to 7, which can be explained by both increasing +I effect of the backbone as well as increased impact of the mesityl groups as the N−C−N angle increases. The eight-membered heterocycle behaves differently and shows a more downfield shift compared to the sevenmembered ring. Maybe, the increased flexibility of the backbone could lead to a more relaxed N−C−N angle slightly decreasing the shielding effect of the mesityl rings. However, this claim is tentative due to the lack of structural data for the 8-Mes system. The chemical shifts for the benzyl series do not show any conclusive trend. Here, the eight-membered cycle even exhibits the most downfield resonance. The increased rotational freedom of the benzyl group could randomly expose the C2−H proton to shielding and deshielding areas, leading to complicated interplay of various effects. The 1J(C−H) coupling constants, on the other hand, provide a clearer picture. In both series, the coupling constants decrease with increasing ring size, although the differences between seven- and eight-membered rings are marginal. This would imply increasing the donating ability of the NHCs in the order 5-NHC < 6-NHC < 7-NHC ≈ 8-NHC, which is mostly consistent with HEP. With values of