Distinct Palladium(II) Carbene Complexes Supported by Six

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Article Cite This: ACS Omega 2018, 3, 6587−6594

Distinct Palladium(II) Carbene Complexes Supported by SixMembered 1,3-Disubstituted Permidin-2-ylidene, Six-Membered N‑Heterocyclic Carbenes Sojung Lee, Bulat Gabidullin, and Darrin Richeson* Department of Chemistry and Biomolecular Sciences and Centre for Catalysis Research and Innovation, University of Ottawa, Ottawa, Ontario K1N6N5, Canada

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S Supporting Information *

ABSTRACT: The first synthesis, isolation, and characterization of permidin-2-ylidene complexes of Pd(II) is reported with entry resulting from either a direct reaction with isolable six-membered N-heterocyclic carbene or from the enetetramine, arising from dimerization of the carbene. Furthermore, a simplified method to prepare N,N′disubstituted perimidinium bromide salts, precursors to 1,3-disubstituted perimidin-2-ylidene, was achieved using ammonium bromide as a source of weak acid. Through synthesis and nuclear magnetic resonance spectroscopic analysis of a carbene-phosphinidine adduct, an interrogation of the fundamental π-bonding ability of 1,3diisopropylperimidin-2-ylidene revealed this interaction to be weak and of a similar order to unsaturated imidazol-2-ylidenes.

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INTRODUCTION N-Heterocyclic carbenes (NHCs) are well-established supporting ligands for metal complexes that span the periodic table. The strong σ donation and reduced tendency to dissociate along with the tunable steric protection provided by these ligands have resulted in their development as important tools in organometallic chemistry and particularly as ancillary ligands in catalyst design.1−7 In this respect, the design of NHC frameworks is a significant endeavor because, through changes to donor properties, they affect the electronic environment of the transition metal center and, by modification of their structural features, they can alter the steric environment of a metal complex. The most common NHCs in the literature have five-membered ring architectures represented by the unsaturated 1,3-disubstituted imidazol-2-ylidenes and the saturated analogues, 1,3-disubstituted imidazolin-2-ylidenes. Variation and investigation of steric and electronic features of these species have been extensively explored.8−12 Recent efforts have focused on understanding and modulating the relative σ-and π-bondings in the M−NHC interaction.13−19 By contrast, stable versions of the six-membered heterocyclic perimidine-based carbenes (perimidin-2-ylidene) have been isolated for more than a decade, yet this alternative framework is almost unexplored and only a handful of metal complexes of Ru,20,21 Os,20 Rh,7,22−26 and Ir27,28 have been isolated and characterized. Although no Pd complexes of these carbenes have been isolated, there are some reports of catalytic activity from such complexes.29,30 Both experimental7,22,23,27,29,30 and computationally derived data31,32 have been used to examine the interaction of these particular carbene scaffolds with metal centers. The limited number of isolated perimidin-2-ylidene metal complexes in combination with the promise in applications in © 2018 American Chemical Society

catalysis encouraged our effort to isolate and characterize new Pd(II) complexes stabilized by six-membered perimidine-based carbenes. As an entry point to this chemistry, we report a simple method to prepare N,N′-disubstituted perimidinium bromide salts, which serve as either precursors to a persistent 1,3-disubstituted perimidin-2-ylidene or the related enetetramine, the formal C−C bonded dimer of a carbene. Either of these species could be employed for direct access to Pd(II) complexes of the targeted perimidin-2-ylidene ligands and the first isolation and spectroscopic and structural analyses of such species. Furthermore, we report on computational analysis indicating thermodynamic favorability of dimerization of these permidin-2-ylidenes and experimental confirmation of the weak π-acceptor ability using spectroscopic analysis of a carbene-phosphinidine adduct.

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RESULTS AND DISCUSSION

The original reports for the isolation of 1,3-disubstituted perimidin-2-ylidenes employed the deprotonation of the cationic perimidinium species such as 1 and 2.22,23 A key complication of this approach was that many of the perimidinium cations were obtained with formate or tosylate counterions, and clean deprotonation first required an anion exchange for chloride. In an effort to simplify the preparation of perimidinium halide, we explored the replacement of organic acids in the cyclization step with ammonium bromide, as shown in Scheme 1. This improved approach successfully yielded N,N′-diisopropylperimidinium bromide, Received: March 7, 2018 Accepted: June 6, 2018 Published: June 19, 2018 6587

DOI: 10.1021/acsomega.8b00437 ACS Omega 2018, 3, 6587−6594

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amine, {C10H6(iPrN)(C6H4CH2N)C}2 4. Specifically, in the H NMR spectra, the original single resonance for the benzyl CH2 group was replaced with eight doublets, consistent with the formation of cis and trans isomers of the enetetramine with diastereotopic protons for the CH2 moieties. Fortunately, crystals of one of the isomers of compound 4 were obtained, and single-crystal X-ray analysis established the proposed dimerization. A structural diagram of the transisomer for {C10H6(iPrN)(BnN)C}2 4 is presented in Figure 1 with

Scheme 1. Preparation of Benzyl-Substituted Perimidinium Bromide, C10H6(iPrN)(BnN)CH+Br− 2 Achieved by the Reaction of N-Isopropyl Perimidine with Benzyl Bromide

1

C10H6(iPrN)2CH+Br−, in 77% yield. Preparation of the previously reported benzyl-substituted perimidinium bromide, C10H6(iPrN)(BnN)CH+Br− 2, was achieved by the reaction of N-isopropyl perimidine with benzyl bromide (Scheme 1).23 Synthetic details and characterization of 1 and 2 including spectroscopic analyses and single-crystal X-ray diffraction analyses are provided in the Supporting Information. The central positively charged HCN2 moieties in 1 and 2 gave characteristic downfield 1H nuclear magnetic resonance (NMR) signals at δ 9.7 or 10.2 with associated 13C NMR signals at δ 149.9 or 150.9, respectively. The remaining signals for the cations appear at appropriate chemical shifts and have integration values consistent with their formulations. In our hands, the deprotonation of bromide salts 1 and 2 was best achieved using KN(SiMe3)2 as a strong nonnucleophilic base (Scheme 2). As anticipated, the deprotona-

Figure 1. Molecular structure of the enetetramine trans{C10H6(iPrN)(BnN)C}2 4. The asymmetric unit consisted of two half molecules of 4. One of these with the symmetry equivalent atoms is shown. Selected atoms and their symmetry equivalents have been labelled, and hydrogen atoms have been omitted for clarity.

Table 1. Selected Bond Lengths [Å] and Angles [deg] for One Molecule in the Asymmetric Unit of {C10H6(BnN)(iPrN)C}2 4

Scheme 2. Deprotonation of Bromide Salts 1 and 2 Achieved Using KN(SiMe3)2 as a Strong Non-Nucleophilic Base

bond lengths (Å) C(11)−C(11)′ C(11)−N(1) C(11)−N(2) C(1)−N(1) C(12)−N(1) C(9)−N(2) C(19)−N(2)

1.333(3) 1.419(2) 1.432(2) 1.399(2) 1.462(2) 1.435(2) 1.496(2)

bond angle (deg) N(1)−C(11)−N(2) N(2)−C(11)−C(11)′ N(1)−C(11)−C(11)′ C(12)−N(1)−C(1)1 C(11)−N(1)−C(1) C(1)−N(1)−C(12) C(19)−N(2)−C(11) C(11)−N(2)−C(9) C(9)−N(2)−C(19)

114.31(13) 121.0(2) 124.4(2) 122.35(13) 114.84(13) 120.48(14) 112.95(13) 109.23(13) 114.90(13)

selected crystallographic data provided in Table 1. These results revealed a molecule with a central CC bond located on an inversion center, with a C11−C11′ linkage that displayed a bond length of 1.333(3) Å, a value similar to the reported analogue trans-{C 1 0 H 6 (cyclo-C 7 H 1 3 N)(pMeC6H4CH2N)C}2 with a CC distance of 1.348(4) Å.23 The two halves of the molecule are slightly twisted along this CC axis with an N1−C11−C11′−N2′ angle of 6.47°. Furthermore, the six-membered heterocycles are not planar. The N1 and N2 centers displayed different geometries with N1 being planar (∑ angles 358°), whereas N2 is pyramidal (∑ angles 337°). As a result the iPr substituents on N2 centers project out of the central C2N4 plane. The C11−N1 and C11− N2 bond lengths [1.419(2) and 1.432(2) Å] are increased compared to those of the perimidinium starting material 2 [1.318(3) and 1.316(3) Å], indicating that the formation of the C11−C11′ π bond has led to a reduced degree of C−N π-

tion of 1 proceeded smoothly to yield the reported carbene 3.22,23 The key NMR observations for this deprotonation are the disappearance of the C−H signal of the central HCN2 unit in 1H NMR with concomitant loss of the associated 13C signal at δ 149.9 and appearance of a new downfield Ccarbene signal at δ 241.7. Somewhat surprisingly, the deprotonation of 2 was not as straightforward as anticipated. This compound has been reported to yield 1-isopropyl-3-benzyl-perimidin-2-ylidene, C10H6(iPrN) (C6H4CH2N)C, based on initial spectroscopic signatures. 23 However, like the related {C 10 H 6 (cycloC7H13N)(p-MeC6H4CH2N)C}2, which was formed by dimerization of the initially formed perimidin-2-ylidenes,23 the NMR spectra of the products from deprotonation of 2 suggested dimerization to form the corresponding enetetr6588


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ACS Omega bonding compared to the starting material C10H6(iPrN)(BnN)CH+Br 2. The dimerization of N-stabilized carbenes has been reviewed and rationalized in terms of thermodynamic values for the C C bond formation and steric protection provided by the Nsubstituents. A simple comparison of the computed singlettriplet energy gap (ES/T) with an estimated value for the CC bond energy provides a thermodynamic model that can be used to rationalize the dimerization of the carbene.33−38 Essentially, if the value of ES/T for two carbenes is less than the energy released from the formation of the CC double bond, estimated at 172 kcal/mol, the dimerization product is favored. In the case of perimidin-2-ylidene C10H6(iPrN)(BnN)C, a density functional optimization optimization of both the singlet and triplet model species was computed using the B3LYP functional and the TZVP basis set, and this calculation yielded an ES/T of 57.7 kcal/mol. Thus, the energy released on CC bond formation minus the ES/T for two carbene units yields approximately 56.6 kcal/mol as the energy that could be gained by forming the dimer, {C10H6(iPrN)(BnN)C}2 4. The more subtle role of steric protection in this dimerization is shown by the comparison of the structures of free carbene 3 and enetetramine 4. In spite of a smaller computed ES/T for monomeric 3 (B3LYP, TZVP) of 54.8 kcal/mol, simply replacing a benzyl substituent with an isopropyl group favors isolation of the free monomeric carbene. These observations are closely parallel to the reported result for related carbene/ enetetramine species.23 In addition to knowledge of ES/T, the σ-donor/π-acceptor ability of NHCs is a fundamental feature that is key for their application in organometallic chemistry and catalysis. A common method for accessing the donor ability of a ligand, and a carbene specifically, is through the Tolman electronic parameter (TEP),39 and the donor abilities of perimidin-2ylidenes have been analyzed using the TEP.7,23,26 Two recent reports have proposed a means of assessing the π-acceptor ability of NHCs through NMR-based analysis of either C−P or C−Se bonding.40,41 For example, 31P NMR chemical shifts of easily synthesized carbene-phenylphosphinidene adducts provided a correlation of relative π-acceptor properties of NHCs. The π-acceptor ability of perimidine-2-ylidenes has not been investigated by this method, and therefore, we carried out the synthesis of phenylphosphinidene species 5, as depicted in Scheme 3. Compound 5 gave a 31P NMR shift of −53.5 ppm,

Given the limited application of perimidine-based carbenes as ligands, the broad application of Pd(II) complexes in catalysis, and the preliminary reports of in situ generated permidine-2-ylidene/Pd(II) catalysts,29,30 we explored the organometallic chemistry of Pd(OAc)2 complexes bearing carbene ligands derived from 1 and 2. With ready access to carbene 3 via simple deprotonation, the direct reaction of 2 equivalents of 3 with Pd(OAc)2 (Scheme 4) was used to Scheme 4. Direct Reactions of either Carbene 3 or Enetetramine 4 with Pd(OAc)2 to Successfully Obtain the Bis(carbene) Pd(II) Species, [C10H6(iPrN)2C]2Pd(OAc)2 6

successfully obtain the bis(carbene) Pd(II) species [C10H6(iPrN)2C]2Pd(OAc)2 6. Both 1H and 13C NMR spectroscopies as well as high-resolution mass spectroscopy (HRMS) provided convincing evidence for our formulation of 6. The NMR spectra indicated a symmetrical structure with a 1:1 ratio of carbene and acetate ligands. Additionally, the 13C NMR spectra gave a resonance assigned to the carbene carbon center at δ 197.3, which was shifted downfield from the free carbene 3, as evidence of successful ligand coordination. An additional key indication for coordinated carbene came from the observation in the 1H NMR spectrum of a large change in the chemical shift for the C−H septet of the iPr groups from 4.08 ppm in free 3 to 8.34 ppm in 6. Similar changes in the chemical shift have been noted for perimidine-based carbenes coordinated to Rh(I) complexes.22,23 The observation of a downfield 1H NMR shift for CH protons experiencing axial M···H−C contacts with square-planar transition-metal d8 complexes was first reported in 1967 for a Ni(II) complex and has remained an active area of investigation.42 The observation that the iPr methyne protons display a deshielding of 4.26 ppm is consistent with the exceptional downfield shifts for analogous interactions appearing in recent reports and strongly suggest that these protons are positioned above and below the PdL4 plane.43−47 Recently, an elegant combination of experimental and computational analysis has been used to reveal the more intimate details of such interactions.48,49 Single crystals of 6 were obtained from toluene and provided, through X-ray analysis, definite confirmation for the connectivity of 6. The results of this analysis are presented in Figure 2 with a summary of selected bonding parameters in Table 2. These results showed that 6 displayed a pseudosquare planar, divalent palladium center, Pd1, with transcoordinated acetate and carbene ligands. As expected for a planar metal coordination geometry, the interligand angles sum to 360°. The Pd−Ccarbene distance of 2.064(2) Å is similar to literature values found for square planar Pd(II)−NHC complexes.50,51 Within the coordinated carbene ligand, the

Scheme 3. Synthesis of Phenylphosphinidene Species 5

and this high field shift was typical of reported phosphinidenes formed from unsaturated imidazol-2-ylidenes, indicative of an electron-rich phosphorus center and a low degree of P−C πbonding. Furthermore, the room-temperature 1H and 13C NMR spectra of 5 displayed sharp symmetrical features that suggested that the P−Ph group was freely rotating relative to the CN2 plane of the carbene fragment and providing additional evidence of rather weak P−C π-bonding. 6589


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tunately, the analogous synthetic procedure employing direct reaction of 4 and Pd(OAc)2 provided the successful isolation of [C10H6(iPrN)(BnN)C]2Pd(OAc)2 7. The NMR data for 7 was analogous to 6, which provided a clear indication for the formation of the trans-diacetatobis(carbene)Pd(II) complex, [C10H6(iPrN)(BnN)C]2Pd(OAc)2. Perhaps the clearest indication of success in isolation of this species was the observation that the septet resonance for the isopropyl CH proton was deshielded by 4.32 ppm compared to the enetetramine starting material 2 and appeared at δ 7.82. Additionally, the benzyl CH2 resonance of 7 also exhibited a similar deshielding effect and was observed at δ 7.45. Single crystals of 7 were obtained, and their X-ray analysis confirmed the identity and connectivity of 7, as shown in Figure 3. Structural features including the metal coordination

Figure 2. Molecular structure of [C10H6(iPrN)2C]2Pd(OAc)2 6. The structure showed substitutional disorder of acetate and bromide anions with Br as a minor component that is not depicted. For clarity, only selected atoms are shown as thermal ellipsoids and labelled. The remaining carbon and oxygen atoms are rendered in wire frame. Hydrogen atoms have been omitted.

average N−Ccarbene bond length became slightly shorter compared to the free carbene species.22 This feature is consistent with the concept that the carbene does not participate in significant Pd−Ccarbene π-bonding. The two coordinated carbene ligands are coplanar, with these planes approximately perpendicular to the Pd pseudo-square planar coordination (dihedral angle of PdO2C2 and N1−C11−N2 = 85.7°). This carbene ligand arrangement positions the isopropyl CH moieties directly above and below the planar Pd(II) center, which is entirely consistent with the observed strong deshielding of these protons in the 1H NMR spectra of 6. Because our attempts to generate a free carbene ligand via deprotonation of 2 led only to isolation of enetetramine 4, we chose to investigate the viability of using 4 as the precursor for a Pd-coordinated 1-isopropyl-3-benzyl-perimidin-2-ylidene, C10H6(iPrN)(BnN)C ligand. The use of enetetramines in the synthesis of metal−carbene complexes has been reviewed,52,53 and we have previously employed this approach to prepare a Rh(I) complex of perimidin-2-ylidene C[N(3,5-Me)C6H3]2C10H6 with 3,5-dimethyl phenyl substituents.54 For-

Figure 3. X-ray structure of [C10H6(iPrN)(BnN)C]2Pd(OAc)2 7. For clarity, only selected atoms are shown as thermal ellipsoids and labelled. The remaining carbon and oxygen atoms are rendered in wire frame. Hydrogen atoms have been omitted.

geometry and the relative orientation of the coordinated ligands were analogous to 6 (Table 2). For example, the Pd− Ccarbene (C11) bond distance was 2.061(2) Å, the carbene

Table 2. Selected Bond Lengths [Å] and Angles [deg] for [C10H6(iPrN)2C]2Pd(OAc)2 6 and [C10H6(iPrN)(BnN)C]2Pd(OAc)2 7 compound 6

compound 7 Bond Lengths (Å)

C(11)−Pd(1) O(1)−Pd(1) N(1)−C(11) N(2)−C(11)

2.064(2) 2.016(7) 1.350(3) 1.350(3)

C(11)−Pd(1)−C(11)′ C(11)−Pd(1)−O(1) C(11)−Pd(1)−O(1)′ C(11)i−Pd(1)−O(1) C(11)i−Pd(1)−O(1)′ N(1)−C(11)−N(2) N(2)−C(11)−Pd(1) N(1)−C(11)−Pd(1) C(11)−N(1)−C(1) C(11)−N(1)−C(12) C(1)−N(1)−C(12) C(11)−N(2)−C(9) C(11)−N(2)−C(15) C(9)−N(2)−C(15)

180.0 98.47(16) 81.53(16) 81.53(16) 98.47(16) 119.2(2) 120.1(2) 120.0(2) 123.1(2) 116.1(2) 120.8(2) 122.9(2) 116.0(2) 121.0(2)

C(11)−Pd(1) O(1)−Pd(1) N(1)−C(11) N(2)−C(11)

2.061(2) 2.034(2) 1.343(3) 1.345(3)

C(11)−Pd(1)−C(11)′ C(11)−Pd(1)−O(1) C(11)−Pd(1)−O(1)′ C(11)′−Pd(1)−O(1) C(11)′−Pd(1)−O(1)′ N(1)−C(11)−N(2) N(1)−C(11)−Pd(1) N(2)−C(11)−Pd(1) C(11)−N(1)−C(2) C(11)−N(1)−(12) C(2)−N(1)−C(12) C(11)−N(2)−C(10) C(11)−N(2)−C(19) C(10)−N(2)−C(19)

180.0 82.63(8) 97.37(8) 97.37(8) 82.63(8) 118.6(2) 118.8(2) 121.6(2) 124.1(2) 118.6(2) 117.3(2) 123.0(2) 116.5(2) 120.4(2)

Bond Angle (deg)

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were refined anisotropically. All hydrogen atoms were placed in idealized positions. The crystal of 1 was twinned and the refinement was done over two domains using HKLF5 data. The ratio of the domains is 0.68:0.32. The sum of the occupancies for Br anions was constrained to two and after refinement gave 0.895(3), 0.877(4), 0.149(4), and 0.080(4) for Br(1), Br(2), Br(3), and Br(4), respectively. Enhanced rigid-bond restraints (RIGU) were applied to all atoms. Other restraints were not used. No additional restraints or constraints were used for 2 and 7. In 4, all molecules are lying on inversion centers with the asymmetric unit consisting of two half molecules. One of the isopropyl groups C(41)−C(40)−C(42) is disordered over two positions with 0.60(1):0.40(1) occupancy ratio. It was refined using bond length and angle restraints (SADI and SAME) and restraints on the atomic displacement parameters (SIMU and RIGU). Structure 6 shows substitutional disorder of acetate and bromide anions with 0.917(3):0.083(3) occupancy ratio. The compound crystallized with two tetrahydrofuran (THF) solvate molecules per one complex. The THF molecule is disordered over two positions with 0.725(7):0.275(7) occupancy ratio. The resulting formula corresponded to (C17H20N2)2(CH3COO)1.84Br0.16Pd (C4H8O)2. Preparation of Enetetramine 4, {C10H6(iPrN)(BnN)C}2. Compound 2 (0.50 g, 1.3 mmol) was dissolved in 15 mL of 1:1 chlorobenzene/THF in a glovebox. To this solution was added K[N(SiMe3)2] (0.26 g, 1.3 mmol) dissolved in 5 mL of 1:1 chlorobenzene/THF. The reaction mixture was stirred for 1 h, and the solvent was removed under vacuum. The resulting pale brown viscous liquid was extracted with toluene, followed by solvent removal under vacuum to yield 4 (0.35 g, 0.58 mmol, 45%). 1 H NMR (C6D6, 600 MHz): δ 7.18−7.56 (m, 16H, CH), 7.06−7.15 (m, 10H, CH), 6.74−7.01 (m, 10H, CH), 6.20− 6.61 (m, 6H, CH), 5.39−5.53 (m, 2H, CH), 5.52 (d, 1H, CH2, J = 13.4 Hz), 5.39 (d, 1H, CH2, J = 16.0 Hz), 5.21 (d, 1H, CH2, J = 16.9 Hz), 5.12 (d, 1H, CH2, J = 16.9 Hz), 5.0 (m, 1H, CH2), 4.9 (d, 1H, CH2, J = 16.1 Hz), 4.44 (d, 1H, CH2, J = 16.1 Hz), 4.23 (d, 1H, CH2, J = 13.4 Hz), 3.87 (sept, 1H, CHMe2, J = 6.7 Hz), 3.37 (sept, 1H, CHMe2, J = 6.1 Hz), 3.30 (sept, 1H, CHMe2, J = 6.7 Hz), 3.27 (sept, 1H, CHMe2, J = 7.0 Hz), 1.49 (d, 6H, CH3, J = 7.0 Hz), 1.01−1.20 (m, 6H, CH3), 0.85 (d, 6H, CH3, J = 6.7 Hz), 0.74 (m, 6H, CH3). 13 C{1H} NMR (C6D6, 150 MHz): δ 162.9 (CC), 144.9, 144.0, 143.7, 143.2, 142.8, 141.3, 139.3, 138.6, 138.2, 137.9, 137.8, 137.2, 136.4, 136.3, 136.2, 136.1, 135.8, 135.4, 132.3, 131.52, 130.36, 129.0, 128.9, 128.7, 128.6, 128.5, 128.3, 127.4, 127.1, 127.0, 126.8, 122.6, 121.1, 119.8, 119.4, 119.3, 118.5, 117.6, 117.5, 117.4, 117.1, 116.4, 116.1, 114.8, 106.4, 105.3, 105.0, 104.79, 104.4 (Carom), 59.4 (CH2), 56.5 (CHMe2), 55.8 (CH2), 54.4 (CH2), 53.4 (CHMe2), 51.6 (CHMe2), 47.3 (CHMe2), 47.1 (CH2), 23.3, 22.4, 22.3, 22.2, 21.4, 21.2, 18.9, 18.6 (CH3). Preparation of Phenylphosphinidene C10H6(iPrN)2C PC6H5 (5). Dichlorophenylphosphine (0.179 g, 1.00 mmol) was added to a solution of carbene 3 (0.252 g, 1.00 mmol) in pentane (10 mL) at room temperature in a nitrogen-filled glovebox. A yellow solid precipitated immediately. The mixture was stirred overnight and filtered in the glovebox. The solids were washed with diethyl ether and dried under reduced pressure. In a glovebox, the solids were combined in a reaction

ligands were coplanar, and the sum of the four interligand angles around the Pd center was 360°, consistent with a square planar coordination geometry. This first successful synthesis, isolation, and characterization of permidin-2-ylidene complexes of Pd(II) complexes is a significant addition to the coordination chemistry of these ligands and encourages further exploration and application of these carbenes, which are captured in a unique molecular architecture. We documented that both direct reaction with the isolated carbene and cleavage of enetetramine, the dimer of permidin-2-ylidene, presented routes to the Pd complex synthesis. The accessibility and use of this family of carbenes is further strengthened by the report of using ammonium ion as a weak acid in the generation of the carbene precursor because this avoids issues that previously arose from the use of organic acids. Additional fundamental characteristics of permidin-2-ylidines such as the critical role of steric protection, provided by the N-R substituents, in prohibiting the thermodynamically favored carbene dimerization and the weak π-acceptor nature of these ligands supported through structural and spectroscopic analyses have also been provided. With all of these features in mind and the extensive literature on the use of Pd−NHCs in catalytic reactions, our future studies will explore the catalytic potential of these complexes.

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EXPERIMENTAL SECTION Unless otherwise noted, all manipulations are carried out in either a nitrogen-filled glovebox or under nitrogen using standard Schlenk techniques. Reaction solvents were sparged with nitrogen and then dried by passage through a column of activated alumina using an apparatus purchased from Anhydrous Engineering. Deuterated benzene and chloroform and toluene were purchased from Aldrich Chemical Company. Acetone, benzyl bromide, 1,8-diaminonaphthalene, formic acid, LiAlH4, MeLi, KN(SiMe3)2, triethylorthoformate, and Pd(OAc)2 were purchased from Aldrich Chemical Company and used without further purification. 1-Isopropylperimidine, C10H6N(iPrN)CH, 1-isopropyl-3-benzylperimidinium bromide, 2, and 1,3-isopropylperimidine-2-ylidene, 3, were prepared by literature methods.22,54 1 H spectra were run on either a Bruker 300, 400, or 600 MHz spectrometer, and 13C{1H} NMR spectra were run on either a Bruker 75, 100, or 150 MHz spectrometer using the residual protons of the deuterated solvent for reference. Elemental analyses were performed by Midwest Micro lab in Indianapolis, IN, USA. High-resolution lock mass accuracy mass spectrometry was conducted on an electrospray global quadrupole time-of-flight spectrometer at the John L Holmes Mass Spectrometry Facility at the University of Ottawa. Crystallographic Analysis. Crystals were mounted on thin glass fibers using Parabar 10312 cryoprotectant. Prior to data collection, the crystals were cooled to 200(2) K. The data were collected on a Bruker AXS single-crystal diffractometer equipped with a sealed Mo tube (wavelength 0.71073 Å) and an APEX II CCD detector. The raw data collection and reduction were done with the Bruker APEX II software package.55 Semiempirical absorption corrections based on equivalent reflections were applied using TWINABS56 for 1 and SADABS57 for other datasets. Systematic absences in the diffraction dataset and unit-cell parameter were consistent with the assigned space groups. The structures were solved by direct methods and refined with full-matrix least-squares procedures based on F2, using SHELXL58 and WinGX.59 All non-H atoms 6591


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flask with magnesium (0.048 g, 2.00 mmol) and 10 mL of THF. The mixture was then stirred at room temperature overnight, and the volatiles were removed. The product was extracted with benzene, filtered, and the volatiles were removed under reduced pressure. The final compound was obtained as an orange solid; yield 69% (0.248 g, 0.69 mmol). 1 H NMR (C6D6, 600 MHz): δ 7.28−7.35 (m, 6H, CH), 7.10−7.12 (m, 1H, CH), 7.03−7.07 (m, 2H, CH), 6.47 (d, 2H, CH, J = 7.7 Hz), 3.91 (sept, 2H, CHMe2, J = 7.0 Hz), 0.98 (d, 12H, CH3, J = 7.0 Hz). 13C{1H} NMR (C6D6, 150 MHz): δ 144.4, 141.4 (d, JPC = 76.4 Hz), 141.0 (d, Ccarbene, JPC = 122.9 Hz) 136.6, 136.2 (d, JPC = 15.1 Hz), 135.8, 129.3, 128.3, 126.9, 117.51, 103.9 (Carom), 65.7 (d, JPC = 5.6 Hz, CHMe2), 18.6 (CH3). 31P NMR (C6D6, 121 MHz): δ −51.5. Preparation of {C10H6(iPrN)2C}2Pd(OAc)2 (6). In a nitrogen-filled glovebox, a vial was charged with Pd(OAc)2 (0.090 g, 0.4 mmol), 10 mL of THF, and a stir bar. To this solution, carbene 3 (0.099 g, 0.4 mmol) predissolved in 10 mL of toluene was added dropwise. During the addition, the reaction solution turned into brown color. The reaction was stirred overnight at room temperature; then, the volume of the solvent was removed under vacuum and cooled to −25 °C to give colorless crystals of 6 (0.140 g, 48% yield). 1 H NMR (C6D6, 300 MHz): δ 8.34 (sept, 4H, CHMe2, J = 7.2 Hz), 7.27 (m, 4H, CH), 6.50 (m, 4H, CH), 6.4 (d, 4H, CH, J = 8.1 Hz), 1.96 (s, 6H, CH3), 1.91 (d, 24H, CH3, J = 7.2 Hz).13C{1H} NMR (C6D6, 75 MHz): δ 197.0 (C−Pd), 178 (CO), 132, 134.7, 131.5, 130.7, 121.4, 128.5, 128.4, 127.2, 121.0, 106.9 (Carom), 65 (CHMe2), 23.2 (CH3CO), 19.0 (CH3) Preparation of {C10H6(iPrN)(BnN)C}2Pd(OAc)2 (7). In a nitrogen-filled glovebox, a vial was charged with Pd(OAc)2 (0.056 g, 0.25 mmol), 10 mL of THF, and a stir bar. To this solution, enetetramine 4 (0.076 g, 0.25 mmol) predissolved in 10 mL of toluene was added dropwise. During the addition, the solution turned into dark brown color. The reaction mixture was stirred overnight at room temperature; then, the solvent was removed under vacuum and cooled to −25 °C to give colorless crystals of 7 (0.129 g, 68% yield). 1 H NMR (C6D6, 300 MHz): δ 7.82 (sept, 2H, CHMe2, J = 7.0 Hz), 7.45 (d, 4H, CH2, J = 7.3 Hz), 6.73−7.07 (m, 18H, CH), 6.60 (m, 2H, CH), 6.28 (d, 2H, CH, J = 8.0 Hz), 1.89 (s, 6H, CH3), 1.64 (d, 12H, CH3, J = 7.0 Hz). 13C{1H} NMR (C6D6, 75 MHz): δ 199.0 (C−Pd), 174.9 (CO), 136.2, 131, 133.5, 132.2, 129.5, 128.5, 128.4, 127.2, 126.9, 126.6, 126.5, 126.1, 122.1, 120.8, 128.0, 106.9 (Carom), 61.3 (CH2), 59.5 (CHMe2), 22.1 (CH3CO), 18.4 (CH3).

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.R.). ORCID

Darrin Richeson: 0000-0003-1503-2323 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We thank the Natural Sciences and Engineering Research Council (NSERC) of Canada for funding as well as the Canada Foundation for Innovation for computing resources.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00437. Additional experimental details including the synthesis of compounds 1−3, analytical data, figures of the X-ray structure of the cation of 1 and the cation of 2, and table of selected bond lengths (Å) and angles (deg) for 1 and 2 (PDF) CCDC 1824116−1824120 contain the supplementary crystallographic data for this paper (ZIP) 6592


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Distinct Palladium(II) Carbene Complexes Supported by Six

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