(Hetero)bimetallic and Tetranuclear Complexes of Pincer-Bridged N

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(Hetero)bimetallic and Tetranuclear Complexes of Pincer-Bridged N‑Heterocyclic Carbene Ligands Qiaoqiao Teng† and Han Vinh Huynh* Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Republic of Singapore

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S Supporting Information *

ABSTRACT: Site-selective mono-palladation of potentially pentadentate dipropyl-pyridine-2,6-dicarboxamide bridged diNHC proligands 1Bn/Me can be achieved by using different bases, affording complexes of the type trans-[PdCl2(diNHC)] (3Bn/Me) and [PdCl(N′,N,N′)]BF4 (4Bn/Me), respectively, as metallo-proligands capable of binding additional metals. The 2nd palladation of both metalloligands using Pd(OAc)2, however, gave the same tetrapalladium complex 5, in which the ligand coordinates in a μ,κ4CN3,κC mode due to the entropically favorable tetradentate encapsulation of one PdII ion, leaving a monodentate NHC coordination to the 2nd palladium center. Single deprotonation of the diazolium complex 4Bn affords the benzimidazolium complex 7 as the key intermediate in the formation of the tetrapalladium complex 5. Metallo-NHC precursor 7 also provides easy access to the hetero-bimetallic palladium/gold complex 8.

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INTRODUCTION N-heterocyclic carbenes (NHCs) are prevalent in organometallic chemistry not only because of their generally strong electron donation1 and the enhanced M−NHC bonds of their complexes but also due to the ease of modifications of their Nsubstituents that can give rise to almost limitless variations.2 In addition to structural changes, stereoelectronic tuning is also possible. For example, stereoelectronic flexibility of NHCs can be achieved by choice of N-alkyl or N-aryl substituents with stark influences on performance of the respective complexes in catalysis.3 Decorating the N-wing tips with an appropriate choice of functional groups has shown to give resulting complexes with unique properties such as hemilability,4 water solubility,5 recoverability,6 and biocompatibility.7 Incorporation of additional hetero-donor atoms, on the other hand, will result in di- or polydentate ligands, leading to a large topological diversity, which even resulted in the isolation of NHC-containing metallo-supramolecules, which are gaining increasing attention in recent years.8 In this respect, we have reported a dipropyl-pyridine-2,6dicarboxamide bridged diNHC ligand, which combines soft organometallic NHC donors with a hard Werner-typical N′,N,N′-pincer unit. By making use of the hard-soft-acidbase (HSAB) concept, metal discrimination between soft gold(I) and hard cobalt(III) centers could be achieved, which led to a controlled and stepwise access to the first heterotrinuclear, double-stranded NHC-helicate (Scheme 1).9 Palladium(II), on the other hand, is borderline in terms of the HSAB concept and known to favorably coordinate to both of these donor sets. This should lead to a rather different reactivity and possibly to the formation of new polynuclear architectures. Along these lines, we herein disclose our © XXXX American Chemical Society

Scheme 1. Coordination Chemistry of a Dipropyl-Pyridine2,6-Dicarboxamide Bridged diNHC Ligand

exploration on the coordination chemistry of dipropylpyridine-2,6-dicarboxamide bridged diNHC ligands with palladium(II).

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RESULTS AND DISCUSSION Selective Carbene Palladation. In an attempt to explore the coordination chemistry of the aforementioned potentially pentadentate diNHC ligands with a group 10 metal, the silver complexes 2Bn/Me, generated by mixing the dibenzimidazolium Received: September 8, 2018

A

DOI: 10.1021/acs.organomet.8b00664 Organometallics XXXX, XXX, XXX−XXX

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Organometallics salts 1Bn/Me with Ag2O, were reacted with [PdCl2(CH3CN)2] in the presence of tetramethylammonium chloride (Scheme 2).

Table 1. Selected NMR Data of Salts 1Bn/Me, Their Ag(I)Complexes 2Bn/Me, and Pd(II)-Complexes 3−8

Scheme 2. Synthesis of trans-[PdCl2(diNHC)] Complexes 3Bn/Me

complex NC(H)N 1Bna 1Mea 2Bna 2Meb 3Bnc 3Mec 4Bnc 4Mea 5a 6a 7c 8a

9.35 9.14

9.98 9.50

9.67

CON(H) 8.75 8.54 8.57 9.02 9.20 9.12

NC(H)N 142.7 142.9 -d -d 180.3 179.4 143.1 143.4 180.8, 180.9, 179.4, 180.8,

161.5 165.5 142.5 179.0

CON(H) 164.9 164.8 164.4 162.3 164.3 164.1 171.7 171.7 173.0, 173.0, 173.3, 172.9,

171.7 171.6 171.7 171.8

C2, C6 149.7 149.5 149.5 148.0 149.8 149.3 152.9 153.1 152.8, 152.7, 152.3, 152.6,

152.4 152.4 151.9 152.5

a

Measured in CD3CN. bMeasured in d6-DMSO. cMeasured in CDCl3. dThe signals were not resolved despite prolonged acquisition time.

Single crystals of complexes 3Bn/Me suitable for X-ray diffraction studies were obtained by either slow diffusion of diethyl ether into a solution in acetonitrile (3Bn) or slow evaporation of a saturated solution in chloroform (3Me). Their molecular structures are depicted in Figures 1 and 2 with selected bond parameters. Both 3Bn and 3Me crystallize as mononuclear complexes, in which the palladium centers are coordinated by two chlorido ligands and the trans-spanning diNHC in square-planar geometries, giving rise to 18membered palladacycles. In line with solution NMR data, the two NHC moieties in complex 3Bn are syn to each other, placing the pincer bridge above and the two N-benzyl groups below the coordination plane. The two NHC planes are close to coplanarity with a small dihedral angle of ∼7°, and the two Pd−CNHC distances of 2.037(3) and 2.044(3) Å are essentially equal. Additionally, two discrete molecules of complex 3Bn are paired up by intermolecular hydrogen bonding between the CONH hydrogen atoms of one molecule with a chlorido ligand of the second molecule (Figure 1). The solid-state molecular structure of 3Me is vastly different (Figure 2). Unusually, its 18-membered palladacycle is almost helically twisted around the C−Pd−C vector, leading to an anti-arrangement of the two NHC moieties in the solid state. Moreover, the pincer bridge is held closely to the complex center via short intramolecular contacts between one chlorido ligand and the amide protons, giving a rather compact overall structure apparently to increase packing efficiency (cf. Table S1). In comparison to the structure in 3Bn, this arrangement increases the interplanar angle between the two NHCs to ∼15°. In contrast to 3Bn, the two Pd−CNHC bonds of 2.059(5) and 2.010(5) Å are also distinctively different outside 3σ. Nevertheless, all of these bonds still fall in the range of those typically observed for trans-[PdCl2(NHC)2] complexes.10 Clearly, complex 3Bn cannot adopt such a rare structure due to the steric congestions induced by the benzyl substituents. Upon dissolution, complex 3Me unfolds itself to a structure akin to 3Bn as evidenced by the rather simple NMR spectra found in solution (vide supra). Selective Pincer-Complex Formation. The direct metalation of the dibenzimidazolium salts showed a very different selectivity. When the carbene precursors 1Bn/Me were treated with triethylamine as an external base, followed by addition of [PdCl2(CH3CN)2], site-elective metalation occurred, furnish-

Notably, a clean NHC transfer from silver to palladium was observed without interference of the pincer unit, which led to the formation of the mononuclear complexes of the general formula trans-[PdCl2(diNHC)] (diNHC = dipropyl-pyridine2,6-dicarboxamide bridged dibenzimidazolin-2-ylidene) with benzyl (3Bn) or methyl (3Me) N-substituents. The complexes 3Bn/Me were isolated as air-stable and pale-yellow solids. Complex 3Bn exhibited better solubilities in polar organic media such as chlorinated solvents, CH3CN, and DMSO, while both proofed insoluble in less polar solvents including hexane, diethyl ether, and ethyl acetate. Their formation is supported by positive mode ESI mass spectrometry, which shows base peaks at m/z 803 and 650 for the [M − Cl]+ fragments of the complexes 3Bn/Me, respectively. In the NMR spectra, only one set of signals were observed, indicating a symmetrical bonding of the ligand in both cases. However, the CH2 protons become diastereotopic owing to the restricted rotational freedom arising from palladation. For complex 3Bn, eight signals are observed for the four chemically distinct CH2 groups. The benzylic protons appear as two doublets at 6.44 and 5.19 ppm with geminal coupling constants of 2J(H,H) = 15.1 Hz, while the propylene protons are resonating as six multiplets centered at 5.77, 4.57 (NCH2), 3.73, 3.60, 2.82, 2.49 ppm (NHCH2 and CH2CH2CH2). Its carbene and carbonyl carbon atoms are observed at 180.3 and 164.3 ppm, respectively (Figure S1). Complex 3Me has a similar splitting pattern for the propylene protons, while the NCH3 protons resonate as one singlet at 3.97 ppm. The chemical shifts of the carbene and carbonyl carbon atoms are found at 179.4 and 164.1 ppm, respectively. The signal for the CONH hydrogen atoms of both complexes remains as one triplet at 9.20 (3Bn) and 9.12 ppm (3Me), respectively (Table 1). B

DOI: 10.1021/acs.organomet.8b00664 Organometallics XXXX, XXX, XXX−XXX

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Figure 2. Molecular structure of 3Me showing 50% probability ellipsoids; most hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and bond angles [deg]: Pd1−C1 2.010(5), Pd1−Cl1 2.3069(13), Pd1−C22 2.059(5), Pd1−Cl2 2.3255(12), C1−Pd1− Cl1 89.09(14), Cl1−Pd1−C22 88.13(14), C22−Pd1−Cl2 96.73(14), Cl2−Pd1−C1 86.27(14), C1−Pd1−C22 175.0(2), Cl1−Pd1−Cl2 174.38(5); PdC2Cl2/NHC (left, right) dihedral angle: 65.6, 77.1°.

Scheme 3. Synthesis of Pincer-Type [PdCl(N′,N,N′)]BF4 Complexes 4Bn/Me

of the salt precursors (Figures S3 and S4). Moreover, significant downfield shifts of the 13C carbonyl carbon signals (1Bn/Me: 164.9/164.8, 4Bn/Me: 171.7 ppm for both) were noted. These observations indicate the successful coordination of the N′,N,N′ pincer moieties to the palladium centers.11 The signals for the benzimidazolium moieties and methylene protons of the tether remain largely unchanged, which indicates a similar degree of flexibility as that of the parent salts (Table 1). The identity of complex 4Me was additionally corroborated by X-ray diffraction analysis of a single crystal obtained by diffusion of diethyl ether to its solution in acetonitrile. The molecular structure depicted in Figure 3 confirms the coordination of the N′,N,N′ pincer to the palladium(II) center with two dangling azolium moieties. One chlorido ligand completes the coordination sphere of the square-planar complex. The two N3−Pd1−N4 and N5−Pd1−N4 bite angles within the Pd-pincer unit measure ∼80°. The two Pd−Namido bonds of 2.042(2) and 2.036(2) Å are significantly longer than the central Pd−Npy bond of 1.924(2) Å. Second Metalation Steps. Having successfully established site-selective palladations of the potentially pentadentate ligands via either carbene (3Bn/Me) or N′,N,N′-pincer coordination (4Bn/Me), we focused on attempts to introduce a second palladium center into the system. For this purpose, the benzyl-substituted complexes 3Bn and 4Bn were chosen, since they were generally isolated in better yields. These metalloligand precursors require two base equivalents for

Figure 1. Molecular structures and pairing of 3Bn showing 50% probability ellipsoids; most hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and bond angles [deg]: Pd1−C1 2.037(3), Pd1−Cl1 2.3009(9), Pd1−C28 2.044(3), Pd1−Cl2 2.3295(9), C1− Pd1−Cl1 90.02(9), Cl1−Pd1−C28 91.04(9), C28−Pd1−Cl2 89.73(9), Cl2−Pd1−C1 89.31(9), C1−Pd1−C28 173.38(13), Cl1− Pd1−Cl2 178.84(3); PdC2Cl2/NHC (left, right) dihedral angle: 69.9, 63.2°.

ing the palladium-chlorido pincer complexes [PdCl(N′,N,N′)]BF4 (4Bn/Me) in good yields (Scheme 3). Apparently, only the CONH groups were deprotonated by the amine base, while the benzimidazolium moieties remain unaffected. The two cationic complexes were isolated as yellow solids insoluble in hexane, diethyl ether, and ethyl acetate, but readily soluble in the polar organic solvents including CHCl3, CH2Cl2, CH3CN, and DMSO. The methyl-substituted complex 4Me is also soluble in MeOH and H2O. In the positive mode ESI mass spectra, base peaks due to the [M − BF4]+ fragments are observed at m/z 802 and 652, respectively, confirming the formation of these diazolium-tagged, cationic complexes. NMR spectroscopic studies reveal the disappearance of the CONH protons, which were observed in the downfield region C

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complex 5 as a common product in yields of 67% and 72%, respectively (Scheme 4). The initially anticipated dinuclear complex was not observed at all. Complex 5 has two types of palladium(II) centers. One of them is encapsulated in a tetradentate κ4CN3 fashion by the N′,N,N′ pincer unit and one NHC. The additional NHC coordination results in the formation of a new sevenmembered metallocycle as the driving force. The second palladium(II) center, on the other hand, is coordinated by the remaining NHC together with one terminal and two bridging chlorido ligands, giving rise to a tetranuclear species. Formation of the tetranuclear complex 5 was first proven by ESI mass spectrometry, which shows an isotopic envelope centered at m/z 1888 assignable to [M + H]+. The 1H NMR spectrum shows the absence of all acidic CONH and NCHN protons characteristic for the salt precursor 1Bn (Figure 4). Two inequivalent carbonyl carbon signals are found at 173.0 and 171.7 ppm, and two carbene atoms resonate wellseparated at 180.8 (C7) and 161.5 ppm (C8) in the 13C NMR spectrum. This observation suggests the coordination of both the N′,N,N′ pincer and NHC donors in an asymmetric mode, which is further corroborated by signal doubling of all the other hydrogen and carbon atoms of the ligand (Table 1). The very different 13Ccarbene NMR signals indicate two electronically distinct palladium(II) centers. The downfield resonance at 180.8 ppm can be rationally assigned to the coordinatively saturated and electron-rich PdII-pincer units. The more upfield chemical shift at 161.5 ppm is assigned to the carbene donors of the chlorido-bridged dipalladium unit,

Figure 3. Molecular structure of 4Me showing 50% probability ellipsoids; hydrogen atoms, BF4 anion, and solvent molecules are omitted for clarity. Selected bond lengths [Å] and bond angles [deg]: Pd1−Cl1 2.3211(7), Pd1−N3 2.042(2), Pd1−N4 1.924(2), Pd1−N5 2.036(2), N3−Pd1−Cl1 100.68(7), Cl1−Pd1−N5 98.45(7), N5− Pd1−N4 80.56(9), N4−Pd1−N3 80.23(9), Cl1−Pd1−N4 177.92(7), N3−Pd1−N5 160.69(9).

subsequent activation, and Pd(OAc)2 was chosen as a suitable reagent. Surprisingly, mixing either 3Bn or 4Bn with Pd(OAc)2 in the presence of excess tetramethylammonium chloride as a source for additional chlorido ligands gave the tetrapalladium Scheme 4. Further Metallations of 3Bn and 4Bn

D

DOI: 10.1021/acs.organomet.8b00664 Organometallics XXXX, XXX, XXX−XXX

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Figure 4. 1H and 13C NMR spectra of tetranuclear complex 5 (CD3CN). Two CONCH2 and three CH2CH2CH2 proton signals are not resolved or coincide with solvent signals.

z 802 to 766 in the positive mode ESI mass spectrum was observed due to the loss of the chlorido ligand upon cyclization. The unsymmetrical bonding of the ligand leads to signal doubling in the NMR spectra. As expected, the 1H NMR signals of the dangling benzimidazolium moiety resemble those of the precursor salt 1Bn (Table 1). In contrast, the cyclized pincer unit gives rise to diastereotopic hydrogen signals, which is also observed for the tetranuclear complex 5. Complex 7 was then reacted with Pd(OAc) 2 and tetramethylammonium chloride, which indeed gave the tetranuclear complex 5 in an improved yield of 86%. Hence, formation of complex 5 from complex 4Bn is a two-step process involving (i) carbene generation and coordination to the PdIIpincer unit, followed by (ii) palladation of the dangling benzimidazolium moiety and subsequent dimerization. The formation of complex 5 from bis(NHC) complex 3Bn is less expected, but must involve at least one Pd−NHC bond cleavage. The study of the exact mechanism is not within the scope of this study. At the moment, however, one can speculate that PdII-pincer formation occurs first, possibly giving the targeted charged dipalladium species. The latter is unstable, and an entropically favorable rearrangement occurs (chelate effect), followed by dimerization, to give the final neutral product 5.

which can also be confirmed by comparison with analogues dimeric [PdX2(NHC)]2 complexes.12 Such dimeric complexes are known to be readily cleaved by pyridines.13 Indeed, the dipalladium pyridine adduct 6 was obtained by reacting complex 5 with pyridine, which additionally corroborated its “dimeric” and chlorido-bridged nature. The dipalladium complex 6 shows slightly better solubilities in common organic solvents. Characterizations with ESI mass spectrometry and elemental analysis confirm its formation. More important, NMR spectroscopy shows additional signals due to the presence of the pyridine ligand. Upon bridge-cleavage of the complex 5 with pyridine, a downfield shift of the second carbene carbon from 161.5 to 165.5 ppm was observed, while the signal for the NHC attached to the PdII-pincer unit remains almost unaffected (Table 1). To study a possible pathway for the formation of the tetranuclear complex 5, complex precursor [PdCl(N′,N,N′)]BF4 (4Bn) was first treated with 0.6 equiv of Ag2O. The reaction proceeded smoothly at ambient temperature, yielding complex 7 in a good yield of 84%. In this reaction, one can propose the formation of an intermediate silver-NHC species, which immediately transfers the carbene to the PdII-pincer unit with concurrent precipitation of chlorido ligand as silver chloride. The other azolium moiety remained intact and dangling (Scheme 4). A remarkable shift of base peak from m/ E

DOI: 10.1021/acs.organomet.8b00664 Organometallics XXXX, XXX, XXX−XXX

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Figure 5. Section of the 2D 1H,13C-HMBC NMR spectrum of complex 8.

NHC unit, which resonate as a singlet due to rotational freedom (Figure 5). Single crystals suitable for X-ray diffraction analysis were obtained for complex 8 via diffusion of diethyl ether into a concentrated solution in CH3CN. The determined structure confirms the expected connectivity (Figure 6). The palladium(II) center is coordinated in tetradentate fashion by the N′,N,N′ pincer and one NHC to give a square-planar geometry. The dihedral angle between the [PdCN 3 ] coordination plane and the NHC plane is ∼55°. The gold(I) center, on the other hand, is coordinated by the second NHC and one chlorido ligand in a linear fashion.

The remaining benzimidazolium salt moiety in complex 7 could also offer access to hetero-bimetallic complexes. It was hence reacted with 0.6 equiv of Ag2O in the presence of 1 equiv of tetramethylammonium chloride to afford the silver(I)NHC intermediate, which was directly used for the transmetalation to gold(I) by addition of [AuCl(THT)] (Scheme 4). The yellow hetero-bimetallic complex 8 was yielded with good solubilities in CHCl3, CH2Cl2, CH3CN, and DMSO. ESI mass spectrometry shows a base peak at m/z 998 for the [M + H]+ fragment. The 1H NMR spectrum of complex 8 shows the absence of the NCHN proton signal characteristic for the benzimidazolium moiety in its precursor 7. Moreover, the second set of the propylene protons become diastereotopic upon coordination to the gold(I) center. Notably, the 13C NMR spectrum of complex 8 in CD3CN shows two downfield signals at 180.7 and 179.0 due to the two distinct carbene carbon atoms. The signal at 180.7 ppm can be readily assigned to the carbene donor of the PdII-pincer unit as it is similar to that observed in the complexes 5 (180.8 ppm) and 6 (180.9 ppm). The new 13Ccarbene NMR signal at 179.0 ppm is in the range typically reported for [AuCl(NHC)] (NHC = benzimidazolin-2-ylidene) complexes and hence was assigned to the carbene donor bound to gold(I).14 This tentative assignment was further corroborated by 2D 1H,13CHMBC spectroscopy, which shows correlation peaks of the 180.7 ppm signal to the two diastereotopic benzylic protons of PdII-pincer complex fragment. The carbene signal at 179.0 ppm shows cross-peaks to the benzylic protons of the Au−

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CONCLUSION We have presented vastly different coordination modes of dipropyl-pyridine-2,6-dicarboxamide bridged diNHC ligands toward palladium(II) in comparison to gold(I) and cobalt(III) reported in our previous work.7 The first palladations of the potentially pentadentate ligands were controllable in a siteselective manner, giving rise to the formation of either bis(NHC) or N′N,N′-pincer complexes. Attempts at second palladations consistently led to a reorganization of the ligand to a tetra- and monodentate binding mode, which gave rise to chlorido-bridged tetrapalladium complexes with very different palladium(II) centers. The favorable κ4CN3 tetradentate coordination mode can be exploited for the preparation of Pd-NHC metalloligands, which provide access to well-defined (hetero)bimetallic dipalladium or palladium/gold complexes with a potential for cooperative catalysis. Research along this F

DOI: 10.1021/acs.organomet.8b00664 Organometallics XXXX, XXX, XXX−XXX

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Found: C, 58.70; H, 5.01; N, 11.86. LRMS (ESI) calcd for C41H39ClN7O2Pd: 802 ([M − Cl]+), found: 802. Palladium(II) Dicarbene Complex 3Me. This complex was prepared in analogy to 3Bn from PdCl2 (18 mg, 0.10 mmol), tetramethylammonium chloride (15 mg, 0.14 mmol), and complex 2Me (70 mg, 0.10 mmol). It was purified by column chromatography (SiO2, CH2Cl2/MeOH: 40/1). Yield: 36 mg, 0.05 mmol, 52%. 1H NMR (500 MHz, CDCl3): δ 9.12 (t, 3J(H,H) = 5.8 Hz, 2 H, CONH), 7.95 (d, 2 H, C3/5−H), 7.82 (t, 1 H, C4−H), 7.49 (d, 2 H, Ar−H), 7.30−7.27 (m, 2 H, Ar−H), 7.23 (t, 2 H, Ar−H), 7.06 (d, 2 H, Ar−H), 5.81−5.75 (m, 2 H, NCH2), 4.61−4.56 (m, 2 H, NCH2), 3.97 (s, 6 H, NCH3), 3.77−3.70 (m, 2 H, NHCH2), 3.51−3.48 (m, 2 H, NHCH2), 2.84−2.80 (m, 2 H, CH2CH2CH2), 2.40−2.37 (m, 2 H, CH2CH2CH2). 13C{1H} NMR (126 MHz, CDCl3): δ 179.4 (Ccarbene), 164.1 (CO), 149.3 (C2/6), 138.9 (C4), 135.6, 134.1, (Ar−C), 124.6 (C3/5), 123.9, 112.1, 110.6 (Ar−C, two are coincident), 47.3 (NCH2), 38.1 (NHCH2), 34.9 (NCH3), 26.0 (CH2CH2CH2). Anal. Calcd for C29H31Cl2N7O2Pd: C, 50.71; H, 4.55; N, 14.27. Found: C, 50.34; H, 4.17; N, 13.95. LRMS (ESI) calcd for C29H31ClN7O2Pd: 650 ([M − Cl]+), found: 650. Palladium(II) (N′,N,N′)-Pincer Complex 4Bn. Salt precusor 1Bn (84 mg, 0.10 mmol) was mixed with NEt3 (30 μL, 0.22 mmol) in CH3CN (10 mL) and stirred for 15 min at 70 °C. [PdCl2(CH3CN)2] (PdCl2: 18 mg, 0.10 mmol) was added, and the mixture was kept stirring for 2 h at 70 °C. The resulting mixture was filtered and dried in vacuo. The residue was dissolved in CH2Cl2 (20 mL) and extracted with H2O (5 × 20 mL). The organic layer was collected and dried over Na2SO4 to give the product as a yellow solid (72 mg, 0.08 mmol, 81%). 1H NMR (500 MHz, CDCl3): δ 9.98 (s, 2 H, NCHN), 7.88 (t, 1 H, C4−H), 7.73 (d, 2 H, C3/5−H), 7.55 (d, 2 H, Ar−H), 7.47 (d, 2 H, Ar−H), 7.43−7.38 (m, 8 H, Ar−H), 7.31−7.27 (m, 6 H, Ar−H), 5.64 (s, 4 H, NCH2Ph), 4.60 (br−s, 4 H, NCH2), 3.43 (br−s, 4 H, CONCH2), 2.42 (br−s, 4 H, CH2CH2CH2). 13C{1H} NMR (126 MHz, CDCl3): δ 171.7 (CO), 152.9 (C2/6), 143.1 (NCHN), 140.2 (C4), 133.3, 132.3, 131.9, 130.0, 129.8, 128.9, 127.6 (Ar−C, two are coincident), 124.7 (C3/5), 114.3, 114.2 (Ar−C), 52.1 (NCH2Ph), 46.5 (NCH2), 42.9 (CONCH2), 29.7 (CH2CH2CH2). 19F NMR (282.38 MHz, CDCl3): δ −74.44, −74.50 (BF4). Anal. Calcd for C41H39BClF4N7O2Pd: C, 55.30; H, 4.41; N, 11.01. Found: C, 55.32; H, 4.56; N, 11.06. LRMS (ESI) calcd for C41H39ClN7O2Pd: 802 ([M − BF4]+), found: 802. Palladium(II) (N′,N,N′)-Pincer Complex 4Me. This complex was prepared in analogy to 4Bn from 1Me (68 mg, 0.10 mmol), NEt3 (30 μL, 0.22 mmol), and [PdCl2(CH3CN)2] (PdCl2: 18 mg, 0.10 mmol). However, 4Me is soluble in H2O, which hampers the removal of the HNEt3Cl salt. Corresponding signals were observed in its 1H and 13C NMR spectra. The crude product yield is 45 mg, 0.06 mmol, 61%. The product yield is about 39% based on the 1H NMR spectrum. 1H NMR (500 MHz, CD3CN): δ 9.50 (s, 2 H, NCHN), 7.99 (t, 1 H, C4−H), 7.88 (d, 2 H, C3/5−H), 7.69 (d, 2 H, Ar−H), 7.57 (m, 4 H, Ar−H), 7.48 (d, 2 H, Ar−H), 4.47 (t, 3J(H,H) = 6.0 Hz, 4 H, NCH2), 3.99 (s, 6 H, NCH3), 4.00 (br−s, 4 H, CONCH2), 2.29 (m, 3J(H,H) = 6.0 Hz, 4 H, CH2CH2CH2). 13C{1H} NMR (126 MHz, CD3CN): δ 171.7 (CO), 153.1 (C2/6), 143.4 (NCHN), 141.0 (C4), 133.2, 132.4, 127.6, 127.5 (Ar−C), 124.6 (C3/5), 114.6, 113.8 (Ar−C), 46.6 (NCH2), 43.0 (CONCH2), 34.1 (NCH3), 29.8 (CH2CH2CH2). 19 F NMR (282.38 MHz, CD3CN): δ −74.86, −74.95 (BF4). 1H and 13 C NMR spectra are shown in the Supporting Information. LRMS (ESI) calcd for C29H31ClN7O2Pd: 650 ([M − BF4]+), found: 650. Tetrapalladium(II) Tetracarbene Complex 5. This complex could be obtained via reacting (A): 3Bn (84 mg, 0.10 mmol), (B) 4Bn (89 mg, 0.10 mmol), or (C) 7 (85 mg, 0.10 mmol) with Pd(OAc)2 (86 mg, 0.10 mmol) in the presence of tetramethylammonium chloride (33 mg, 0.30 mmol) in DMSO (10 mL) at 90 °C overnight. A common workup procedure was used to purify the product. After the solvent was removed by vacuum distillation, the residue was dissolved in CH2Cl2 (30 mL) and then extracted with H2O (5 × 20 mL). The organic layer was combined and dried over NaSO4. After being concentrated to 5 mL, the filtrate was subjected to colum chromatography (SiO2, CH2Cl2/MeOH: 20/1) to yield the product

Figure 6. Molecular structure of 8 showing 50% probability ellipsoids; solvent molecules, hydrogen atoms, and the benzyl substituents on N1 and N7 are omitted for clarity Selected bond lengths [Å] and bond angles [deg]: Pd1−C1 1.984(3), Pd1−N3 2.015(3), Pd1−N4 1.957(3), Pd1−N5 2.045(3), Au1−C28 1.975(3), Au1−Cl1 2.2679(3), C1−Pd1−N3 93.97(12), N3−Pd1−N4 80.80(11), N4− Pd1−N5 79.70(11), N5−Pd1−C1 105.55(12), N4−Pd1−C1 174.52(12), N3−Pd1−N5 160.46(11), C28−Au1−Cl1 179.05(10); PdCN3/NHC dihedral angle: 55.3°.

line is currently underway in our laboratory with focus on the incorporation of various transition metals distinct in their reactivities.

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EXPERIMENTAL SECTION

General Considerations. All operations were performed without taking precautions to exclude air and moisture, and all solvents and chemicals were used as received. The purity of all new compounds was confirmed by 1H and 13C{1H} NMR spectroscopy and elemental analysis. 1H and 13C{1H} NMR spectra were recorded at 298 K on a Bruker ACF 300 or AMX 500 spectrophotometer, and the chemical shifts (δ) were internally referenced to the residual solvent signals relative to tetramethylsilane (1H, 13C). ESI mass spectra were measured using a Finnigan MAT LCQ spectrometer. Elemental analyses were performed on an ElementarVario Micro Cube elemental analyzer at the Department of Chemistry, National University of Singapore. As most of the products are hydroscopic solid or viscous oil, satisfactory results were only achieved with limited samples and reported here. The salt precursors 1 and silver-dicarbene complexes 2 were prepared as previous.9 Palladium(II) Dicarbene Complex 3Bn. A solution of [PdCl2(CH3CN)2] (PdCl2: 18 mg, 0.10 mmol) in CH3CN (20 mL) and tetramethylammonium chloride (15 mg, 0.14 mmol) in MeOH (2 mL) was slowly added to a solution of complex 2Bn (86 mg, 0.10 mmol) in CH2Cl2 (10 mL). The mixture was kept stirring overnight before the precipitation was filtered off through Celite. The filtrate was concentrated to 3 mL, which was subjected to column chromatography (SiO2, hexane/ethyl acetate: 1/5) to obtain the product as a pale yellow solid (53 mg, 0.06 mmol, 63%). 1H NMR (500 MHz, CDCl3): δ 9.20 (t, 3J(H,H) = 5.3 Hz, 2 H, CONH), 8.06 (d, 2 H, C3/5−H), 7.89 (t, 1 H, C4−H), 7.48 (d, 2 H, Ar−H), 7.37 (t, 4 H, Ar−H), 7.27−7.21 (m, 10 H, Ar−H), 7.02 (t, 2 H, Ar−H), 6.67 (d, 2 H, Ar−H), 6.44 (d, 2J(H, H) = 15.1 Hz, 2 H, NCHHPh), 5.77 (m, 2 H, NCH2), 5.19 (d, 2J(H, H) = 15.1 Hz, 2 H, NCHHPh), 4.57 (m, 2 H, NCH2), 3.73 (m, 2 H, NHCH2), 3.60 (m, 2 H, NHCH2), 2.82 (m, 2 H, CH2CH2CH2), 2.49 (m, 2 H, CH2CH2CH2). 13 C{1H} NMR (126 MHz, CDCl3): δ 180.3 (Ccarbene), 164.3 (CO), 149.8 (C2/6), 139.0 (C4), 135.7, 134.9, 134.8, 129.3, 128.71, 128.68 (Ar−C), 125.0 (C3/5), 124.8, 123.8, 112.5, 112.2 (Ar−C), 53.6 (NCH2Ph), 47.8 (NCH2), 38.5 (NHCH2), 26.3 (CH2CH2CH2). Anal. Calcd for C41H39Cl2N7O2Pd: C, 58.69; H, 4.68; N, 11.68. G

DOI: 10.1021/acs.organomet.8b00664 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

NMR (282.38 MHz, CDCl3): δ −74.58, −74.59 (s, BF4). Anal. Calcd for C41H38BF4N7O2Pd: C, 57.66; H, 4.48; N, 11.48. Found: C, 57.38; H, 4.34; N, 11.09. LRMS (ESI) calcd for C41H38N7O2Pd: 766 ([M − BF4]+), found: 766. Palladium(II) and Gold(I) Hetero-bimetallic Dicarbene Complex 8. Complex 7 (85 mg, 0.10 mmol) was reacted with Ag2O (14 mg, 0.06 mmol) in the presence of tetramethylammonium chloride (16 mg, 0.15 mmol) in CH2Cl2 (20 mL) and MeOH (2 mL) for 5 h. The resulting suspension was filtered into a solution of [AuCl(THT)] (32 mg, 0.10 mmol) in CH2Cl2 (5 mL) through Celite. The combined mixture was stirred for 5 h before the solvent was removed in vacuo. Washing the residue with H2O (3 × 10 mL) gave the crude product, which was then subjected to column chromatography (SiO2, CH2Cl2/MeOH: 30/1) to afford the product as a yellow solid (63 mg, 0.06 mmol, 63%). 1H NMR (500 MHz, CD3CN): δ 8.07 (t, 1 H, C4−H), 7.68−7.65 (t, 2 H, C3− and C5− H), 7.54 (d, 1 H, Ar−H), 7.48 (d, 1 H, Ar−H), 7.35−7.33 (m, 3 H, Ar−H), 7.27−7.19 (m, 13 H, Ar−H), 5.77 (d, 2J(H, H) = 15.2 Hz, 1 H, NCHHPh), 5.58−5.52 (m, 1 H, NCH2), 5.31 (s, 2 H, NCH2Ph), 5.21 (d, 2J(H, H) = 15.2 Hz, 1 H, NCHHPh), 4.55−4.49 (m, 2 H, NCH2), 4.39−4.29 (m, 2 H, NCH2 and CONCH2), 4.25−4.17 (m, 1 H, CONCH2), 2.78−2.72 (m, 1 H, CH2CH2CH2), 2.10−2.02 (m, 1 H, CH2CH2CH2) two CONCH2 and two CH2CH2CH2 protons were not resolved or coincide with solvent signals. 13C{1H} NMR (126 MHz, CD3CN): δ 180.8 (Pd−Ccarbene), 179.0 (Au−Ccarbene), 172.9, 171.8 (CO), 152.6, 152.5 (C2 and C6), 142.2 (C4), 136.8, 136.6, 135.1, 134.7, 134.0, 133.5, 129.8, 129.18, 129.15, 128.4, 128.2, 125.4, 125.3, 125.0, 124.9, 124.8, 124.5, 113.3, 112.9, 112.8, 112.1 (Ar−C, C3 and C5, two are coincident), 52.7, 52.3 (NCH2Ph), 47.8, 45.7 (NCH2), 44.8, 42.1 (CONCH2), 32.6, 28.9 (CH2CH2CH2). Anal. Calcd for C41H37AuClN7O2Pd: C, 49.31; H, 3.73; N, 9.82. Found: C, 49.69; H, 4.02; N, 9.88. LRMS (ESI) calcd for C41H38AuClN7O2Pd: 998 ([M + H]+), found: 998.

as yellow solids (Yields obtained from method A/B/C: 63/68/81 mg, 0.03/0.04/0.04 mmol, 67/72/86%). 1H NMR (500 MHz, CD3CN): δ 8.11 (t, 2 H, C4−H), 7.71 (d, 2 H, C3−H), 7.67 (d, 2 H, C5−H), 7.53 (d, 2 H, Ar−H), 7.45−7.39 (m, 6 H, Ar−H), 7.36−7.20 (m, 20 H, Ar−H), 7.18−7.13 (m, 4 H, Ar−H), 7.07 (t, 2 H, Ar−H), 6.97 (d, 2 H, Ar−H), 5.91 (d, 2J(H, H) = 15.8 Hz, 2 H, NCHHPh), 5.72 (d, 2 J(H, H) = 15.8 Hz, 2 H, NCHHPh), 5.60−5.54 (m, 2 H, NCH2), 5.43 (d, 2J(H, H) = 16.4 Hz, 2 H, NCHHPh), 5.06−4.97 (m, 4 H, NCHHPh and NCH2), 4.75−4.70 (m, 2 H, NCH2), 4.57−4.53 (m, 2 H, CONCH2), 4.35−4.30 (m, 4 H, NCH2 and CONCH2), 2.74−2.69 (m, 2 H, CH2CH2CH2), two CONCH2 and three CH2CH2CH2 were not resolved or coincide with solvent signals. 13C{1H} NMR (126 MHz, CD3CN): δ 180.8 (C7carbene), 173.0, 171.7 (CO), 161.5 (C8carbene), 152.8, 152.4 (C2 and C6), 142.2 (C4), 136.9, 136.2, 135.2, 134.9, 134.5, 134.1, 129.8, 129.6, 129.1, 129.0, 128.8, 128.3, 124.8, 124.7, 124.4, 124.3, 124.2, 113.2, 112.4, 112.1, 111.9 (Ar−C and C3, C5, two are coincident), 52.9, 52.3 (NCH2Ph), 47.7, 47.1 (NCH2), 45.1, 42.2 (CONCH2), 31.6, 29.0 (CH2CH2CH2). Anal. Calcd for C82H74Cl4N14O4Pd4: C, 52.19; H, 3.95; N, 10.39. Found: C, 52.38; H, 4.30; N, 10.01. LRMS (ESI) calcd for C42H42Cl2N7O3Pd2: 976 ([M/2 + H + MeOH]+), found: 976; calcd for C82H75Cl4N14O4Pd4: 1887 ([M + H]+), found: 1887. Dipalladium Dicarbene Pyridine Complex 6. This complex was obtained by reacting the tetranuclear complex 5 (44 mg, 0.02 mmol) with pyridine (18 μL, 0.22 mmol) in CH2Cl2 (10 mL) for 2 h. The reaction was dried and the residue washed with diethyl ether to afford the product as a yellow solid (46 mg, 0.04 mmol, 96%). 1H NMR (500 MHz, CD3CN): δ 8.70 (d, 2 H, Py−H), 8.07 (t, 1 H, C4−H), 7.85 (t, 1 H, Py−H), 7.69 (d, 1 H, C3−H), 7.64 (d, 1 H, C5−H), 7.53−7.50 (m, 3 H, Ar−H), 7.44 (d, 2 H, Py−H), 7.37−7.17 (m, 13 H, Ar−H), 7.09 (t, 1 H, Ar−H), 7.03 (d, 1 H, Ar−H), 6.03 (d, 2 J(H, H) = 15.8 Hz, 1 H, NCHHPh), 5.80 (d, 2J(H, H) = 15.8 Hz, 1 H, NCHHPh), 5.66 (d, 2J(H, H) = 15.8 Hz, 1 H, NCHHPh), 5.53− 5.46 (m, 1 H, NCHH), 5.18 (d, 2J(H, H) = 15.8 Hz, 1 H, NCHHPh), 5.14−5.08 (m, 1 H, NCHH), 4.85−4.79 (m, 1 H, NCHH), 4.53− 4.49 (m, 1 H, CONCH2), 4.41−4.35 (m, 1 H, NCHH), 4.32−4.28 (m, 1 H, CONCH2), 2.87−2.81 (m, 1 H, CH2CH2CH2), 2.35−2.29 (m, 1 H, CH2CH2CH2), 1.77−1.72 (m, 1 H, CH2CH2CH2), two CONCH2 and one CH2CH2CH2 were not resolved or coincide with solvent signals. 13C{1H} NMR (126 MHz, CD3CN): δ 180.9 (C7carbene‑cyclized), 173.0, 171.6 (CO), 165.5 (C8carbene), 152.7, 152.4 (C2 and C6), 151.7 (Py−C), 142.2 (C4), 139.7 (Py−C), 137.0, 136.7, 135.2, 135.1, 134.6, 134.5, 129.8, 129.6, 129.09, 129.06, 129.00, 128.3, 125.8, 124.82, 124.76, 124.73, 124.23, 124.20, 124.1, 113.2, 112.3, 112.1, 111.8 (Ar−C, Py−C, C3 and C5), 52.9, 52.4 (NCH2Ph), 47.8, 47.0 (NCH2), 45.1, 42.1 (CONCH2), 31.8, 29.0 (CH2CH2CH2). Anal. Calcd for C46H42Cl2N8O2Pd2: C, 54.03; H, 4.14; N, 10.96. Found: C, 53.93; H, 4.38; N, 11.05. LRMS (ESI) calcd for C42H42Cl2N7O3Pd2: 976 ([M − Py + MeOH + H]+), found: 976. Palladium Monocarbene Complex 7. This complex was obtained by reacting complex 4Bn (89 mg, 0.10 mmol) with Ag2O (14 mg, 0.06 mmol) in CH2Cl2 (30 mL) overnight. The resulting suspension was filtered to afford the product as a yellow solid (72 mg, 0.08 mmol, 84%). 1H NMR (500 MHz, CDCl3): δ 9.67 (s, 1 H, NCHN), 8.03 (t, 1 H, C4−H), 7.81 (d, 1 H, C3−H), 7.76 (d, 2 H, C5−H and Ar−H), 7.52−7.45 (m, 5 H, Ar−H), 7.42 (d, 2 H, Ar−H), 7.38 (t, 4 H, Ar−H), 7.33−7.28 (m, 6 H, Ar−H), 7.24−7.20 (m, 2 H, Ar−H), 5.94 (d, 2J(H, H) = 15.1 Hz, 1 H, NCHHPh), 5.62 (d, 2J(H, H) = 15.1 Hz, 1 H, NCHHPh), 5.59−5.53 (m, 1 H, NCH2), 5.51 (s, 2 H, NCH2Ph), 4.63−4.59 (m, 1 H, CONCH2), 4.51−4.44 (m, 3 H, NCH2 and CONCH2), 4.02−3.96 (m, 1 H, NCH2), 2.99−2.94 (m, 1 H, CH2CH2CH2), 2.02−1.95 (m, 2 H, CH2CH2CH2), two CONCH2 and one CH2CH2CH2 were not resolved or coincide with solvent signals. 13C{1H} NMR (126 MHz, CDCl3): δ 179.4 (Ccarbene), 173.3, 171.7 (CO), 152.3, 151.9 (C2 and C6), 142.5 (NCHN), 141.5 (C4), 135.9, 134.8, 134.6, 133.1, 132.2, 131.8, 130.05, 129.95, 129.90, 129.7, 129.0, 128.8, 128.1, 127.8, 125.1, 124.83, 124.79, 124.4, 114.3, 114.2, 113.4, 111.1 (Ar−C, C3 and C5), 52.5, 52.0 (NCH2Ph), 46.3, 45.8 (NCH2), 44.4, 42.2 (CONCH2), 30.4, 28.9 (CH2CH2CH2). 19F

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00664. NMR spectral data, crystallographic data of complexes 3Bn, 3Me, 4Me, 8 (PDF) Accession Codes

CCDC 1864192−1864195 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Qiaoqiao Teng: 0000-0002-7343-8092 Han Vinh Huynh: 0000-0003-4460-6066 Present Address †

School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Singapore Ministry of Education and the National University of Singapore for financial support H

DOI: 10.1021/acs.organomet.8b00664 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

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(WBS R-143-000-669-112). Technical support from staff at the CMMAC of our department is appreciated.

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REFERENCES

(1) Huynh, H. V. Electronic Properties of N-Heterocyclic Carbenes and Their Experimental Determination. Chem. Rev. 2018, 118, 9457− 9492. (2) Huynh, H. V. The Organometallic Chemistry of N-Heterocyclic Carbenes; Wiley: Chichester, U.K., 2017. (3) Selected examples: (a) Díez-González, S.; Nolan, S. P. Stereoelectronic Parameters Associated with N-Heterocyclic Carbene (NHC) Ligands: A Quest for Understanding. Coord. Chem. Rev. 2007, 251, 874−883. (b) Di Giuseppe, A.; Castarlenas, R.; Pérez-Torrente, J. J.; Lahoz, F. J.; Oro, L. A. Hydride-Rhodium(III)-N-Heterocyclic Carbene Catalysts for Vinyl-Selective H/D Exchange: A Structure− Activity Study. Chem. - Eur. J. 2014, 20, 8391−8403. (c) Wu, W.; Teng, Q.; Chua, Y.-Y.; Huynh, H. V.; Duong, H. A. Iron-Catalyzed Cross-Coupling Reactions of Arylmagnesium Reagents with Aryl Chlorides and Tosylates: Influence of Ligand Structural Parameters and Identification of a General N-Heterocyclic Carbene Ligand. Organometallics 2017, 36, 2293−2297. (d) Teng, Q.; Wu, W.; Duong, H. A.; Huynh, H. V. Ring-Expanded N-Heterocyclic Carbenes as Ligands in Iron-Catalysed Cross-Coupling Reactions of Arylmagnesium Reagents and Aryl Chlorides. Chem. Commun. 2018, 54, 6044− 6047. (4) Selected examples: (a) Bernhammer, J. C.; Huynh, H. V. Benzimidazolin-2-ylidene Complexes of Palladium(II) Featuring a Thioether Moiety: Synthesis, Characterization, Molecular Dynamics, and Catalytic Activities. Organometallics 2014, 33, 1266−1275. (b) Nair, A. G.; McBurney, R. T.; Walker, D. B.; Page, M. J.; Gatus, M. R. D.; Bhadbhade, M.; Messerle, B. A. Ruthenium(II) Complexes of Hemilabile Pincer Ligands: Synthesis and Catalysing the Transfer Hydrogenation of Ketones. Dalton Trans 2016, 45, 14335−14342. (c) Liu, Y.; Kean, Z. S.; d’Aquino, A. I.; Manraj, Y. D.; Mendez-Arroyo, J.; Mirkin, C. A. Palladium(II) Weak-Link Approach Complexes Bearing Hemilabile N-Heterocyclic Carbene−Thioether Ligands. Inorg. Chem. 2017, 56, 5902−5910. (d) d’Aquino, A. I.; Cheng, H. F.; Barroso-Flores, J.; Kean, Z. S.; Mendez-Arroyo, J.; McGuirk, C. M.; Mirkin, C. A. An Allosterically Regulated, Four-State Macrocycle. Inorg. Chem. 2018, 57, 3568−3578. (5) (a) Schaper, L.-A.; Hock, S. J.; Herrmann, W. A.; Kühn, F. E. Synthesis and Application of Water-Soluble NHC Transition-Metal Complexes. Angew. Chem., Int. Ed. 2013, 52, 270−289. (b) Baquero, E. A.; Silbestri, G. F.; Flores, J. C.; de Jesús, E. Solvent-Reversible Addition of Alkyne C−H Bonds to Water-Soluble NHC Platinum(0) Complexes. Organometallics 2017, 36, 2271−2274. (c) Nguyen, V. H.; El Ali, B. M.; Huynh, H. V. Stereoelectronic Flexibility of Ammonium-Functionalized Triazole-Derived Carbenes: Palladation and Catalytic Activities in Water. Organometallics 2018, 37, 2358− 2367. (6) (a) Zhou, H.; Wang, Y.-M.; Zhang, W.-Z.; Qu, J.-P.; Lu, X.-B. NHeterocyclic Carbene Functionalized MCM-41 as An Efficient Catalyst for Chemical Fixation of Carbon Dioxide. Green Chem. 2011, 13, 644−650. (b) Martínez-Olid, F.; Andrés, R.; de Jesús, E.; Flores, J. C.; Gómez-Sal, P.; Heuzé, K.; Vellutini, L. Magnetically Recoverable Catalysts Based on Mono- or Bis-(NHC) Complexes of Palladium for the Suzuki−Miyaura Reaction in Aqueous Media: Two NHC−Pd Linkages Are Better Than One. Dalton Trans 2016, 45, 11633−11638. (c) Kandathil, V.; Fahlman, B. D.; Sasidhar, B. S.; Patil, S. A.; Patil, S. A. A Convenient, Efficient and Reusable NHeterocyclic Carbene-Palladium(II) Based Catalyst Supported on Magnetite for Suzuki−Miyaura and Mizoroki−Heck Cross-Coupling Reactions. New J. Chem. 2017, 41, 9531−9545. (7) Zhao, W.; Ferro, V.; Baker, M. V. Carbohydrate-N-Heterocyclic Carbene Metal Complexes: Synthesis, Catalysis and Biological Studies. Coord. Chem. Rev. 2017, 339, 1−16. (8) (a) Yuan, D.; Huynh, H. V. A Comparative Study on Dinuclear and Multinuclear Ni(II), Pd(II), and Pt(II) Complexes of a ThiolatoFunctionalized, Benzannulated N-Heterocyclic Carbene Ligand. Inorg. I

DOI: 10.1021/acs.organomet.8b00664 Organometallics XXXX, XXX, XXX−XXX