Nickel, Ruthenium, and Rhodium NCN-Pincer ... - ACS Publications

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Nickel, Ruthenium, and Rhodium NCN-Pincer Complexes Featuring a Six-Membered N‑Heterocyclic Carbene Central Moiety and Pyridyl Pendant Arms Yanmin Jiang, Chris Gendy, and Roland Roesler* Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4 Canada S Supporting Information *

ABSTRACT: NCN pincer ligand precursor 1·HX (X = Cl, PF6), having a six-membered N-heterocyclic carbene central moiety and pyridyl pendant arms, was prepared via N−C crosscoupling of 2-bromo-6-tert-butylpyridine with propane-1,3diamine, followed by ring closure with triethylorthoformate. The free ligand 1 was not accessible via deprotonation, yet its copper(I) complex 2 was prepared by reacting 1·HCl with CuI and potassium hexamethyldisilazide. It was used further as a transmetalation agent to prepare nickel(II) and ruthenium(II) dihalide NCN-pincer complexes 3a, 4a, and 4b, featuring a distorted square-pyramidal geometry at the metal, with the carbene ligand in the axial position. The rhodium(III) NCNpincer complex 6b was obtained from 2 and [Rh(cod)Cl]2 via transmetalation followed by oxidation with Cu(I) in THF. The rhodium(III) NCC-pincer complex 9b was prepared under similar conditions in acetonitrile, where a spontaneous rollover cyclometalation occurred. Complex 9b has a distorted octahedral geometry, with the acetonitrile ligand trans to the carbene. Very short Ru−CNHC and Rh−CNHC bonds were measured in complexes 4a, 4b, and 9b. Halogen exchange was observed by 1H and 13C NMR spectroscopy in both ruthenium and rhodium systems during the transmetalation stage, and the purification was accomplished by pushing the respective equilibrium with excess halide.

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INTRODUCTION The robust and easily tunable 2,2′:6′,2″-terpyridine NNNpincer ligands (terpy or tpy) have found numerous applications in coordination and materials chemistry, as well as catalysis.1 Notable are recent advances in the activation of dinitrogen,2 labilization of the NH bonds in ammonia,3 carbon dioxide reduction,4 and especially water splitting.5 The replacement of pyridine moieties with N-heterocyclic carbenes (NHCs) in the terpyridine scaffold is expected to increase the electron density at the coordinated metal,6 although the precise impact of this exchange on the reactivity at the metal center is difficult to predict. Six-membered NHCs were serendipitously incorporated in the central position of potential NCN-pincer architectures A (M = Pd, Rh)7 well before they could be isolated as free ligands (Chart 1), although the NCN coordination mode has not been documented in these derivatives.8 Complexes B on the other hand were the product of targeted, modular synthesis (M = Cr, Fe).9a,b Iron derivatives in particular were extensively investigated, and the strongly electron-donating ligand proved to be capable to stabilize complexes having the metal in formal oxidation states 0, I, II, and III.9d,e Some of these iron complexes were shown to be proficient at promoting lactide polymerization.9c The photophysical properties of derivatives C (M = Ru, Pd, Pt) have been discussed,10 while palladium © XXXX American Chemical Society

derivatives D have shown catalytic activity toward Heck coupling.11 Pincer complexes E, (M = Ni, Rh, Ir) incorporating a polycyclic, chiral backbone derived from (1R)-camphor have been investigated as catalysts for hydrogenation and transfer hydrogenation reactions.12 Only two types of PCP pincer ligands incorporating six-membered NHCs in the central positions have been reported to date. Complexes F (M = Ru, Os, Rh, Ir) were obtained from the corresponding N2CH2 precursors either via a double C−H bond activation or via a single C−H bond activation followed by hydride abstraction.13 The methodology was expanded to the synthesis of dinuclear rhodium complexes G with perylene-based PCP ligands.14 C2v-symmetric NCN-ligands featuring pyridyl donors bonded directly to the more common 5-membered NHCs in the central position have been shown to coordinate exclusively in a bidentate fashion as in H, due to the excessively large NN bite imposed by the narrower carbene angle (Chart 2).15 Longer pendant arms allow for pincer coordination and a number of complexes I (M = Ru, Pd, Pt) have been described.16 Their benzimidazole analogues J are also well-known (M = Ru, Ni, Pd, Pt).17 The nonpincer coordination chemistry of Group 11 and 12 metals with the ligands shown in complexes I has been Received: January 12, 2018

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DOI: 10.1021/acs.organomet.8b00022 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Chart 1. Pincer Complexes with Six-Membered NHCs in the Central Position

Scheme 1. Synthesis of Ligand Precursors 1·HX and Copper(I) Complex 2

Chart 2. Complexes Featuring C2v-Symmetric NCN-Pincer Ligands with Five-Membered NHCs in the Central Position and Pyridyl Pendant Arms

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RESULTS AND DISCUSSION Ligand Synthesis. The desired N,N′-dipyridylpropane-1,3diamine was obtained by reaction of two equivalents 2-bromo6-tert-butylpyridine20 with propane-1,3-diamine (Scheme 1) in toluene under Buchwald−Hartwig amination conditions: 0.5 mol % tris(dibenzylideneacetone)dipalladium (Pd2(dba)3), 1 mol % rac-2,2′-bis(diphenylphosphino)-1,1′-binaphthalene (rac-BINAP), and tBuONa. The product was obtained as a thick red oil that was used in the subsequent synthetic step without further purification. Depending on the presence of minor impurities that affect its rate of exchange in CDCl3, the 1 H NMR spectrum of this precursor may or may not feature a resonance corresponding to the NH proton. The rate of exchange also has an impact on the coupling pattern of the methylene bridge adjacent to this nitrogen (triplet, 3JHH = 6.7 Hz, or triplet of doublets, 3JHH = 6.7 Hz, 3JHH = 6.1 Hz). The method could not be expanded to the synthesis of the mesityland 2,4,6-triisopropylphenyl-substituted pyridyl analogues: the C−N cross-coupling to the propane-1,3-diamine backbone was unsuccessful for these substrates. The ring-closing reaction to give ligand precursor 1·HCl or 1·HPF6 (Scheme 1) was completed in a classical fashion using excess triethylorthoformate and the respective dry ammonium salt. The N2CH proton in 1·HCl resonates at 10.72 ppm, with the corresponding carbon resonance at 148.7 ppm. The N2CH bridge turned out to be extremely sensitive toward hydrolytic cleavage, with all synthesis and handling requiring scrupulous exclusion of moisture. A silver complex to be used in transmetalation could not be synthesized because the water byproduct of the reaction between 1·HCl and Ag2O induced hydrolytic ring opening of the ligand. The hydrolysis of N,N′-disubstituted1,4,5,6-tetrahydropyrimidinium ions under basic conditions to give acyclic aminoformamides is well-known.21 Interestingly, closely related derivatives C were prepared in protic solvents, including water and methanol, and hydrolytic opening of the

extensively investigated, and these moieties have also been incorporated in tetradentate cyclophane ligands. Of note is that all NCN-pincer ligands featuring central NHC moieties and pyridyl pendant arms that have been reported to date lack any substitution of the pyridyl ring, precluding steric control of the chemistry at the metal center. We recently reported the reversible, ligand-assisted activation of ammonia on a nickel complex of type B (MLn = Ni·KCl, R = Dipp, R′ = Ph).18 The carbene assumed a highly uncommon, noninnocent role in this reaction, with the highly nucleophilic carbene carbon binding the NH proton while the nickel center connected to the amide functionality, through a mechanism that was not yet elucidated. Aiming to improve on the stability of the ligands in B and further expand the chemistry of NHCbased pincer systems,19 we explored the synthesis and properties of ligands 1, formally obtained by adorning the ligands in complexes of type C with tert-butyl groups (Scheme 1). The replacement of the imine pendant arms with pyridyl functionalities was expected to substantially improve the robustness of the metal systems. B


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Organometallics central ring was not reported as being a problem.10 As observed with analogues B, the free carbene 1 was not stable and could not be accessed via deprotonation even at low temperatures. Copper(I) Iodide Complex 2. Copper carbene complexes have been found to be excellent transmetallating reagents, employable when more common avenues to carbene derivatives are not successful.9a,22 1·HPF6 showed no reactivity toward Stryker’s reagent, ([(PPh3)CuH]6),23 but this reagent led to deprotonation and subsequent copper-carbene bond formation with 1·HCl. However, the resulting copper complex did not undergo transmetalation with [Rh(cod)Cl]2 (see Supporting Information). Fortunately, the more suitable transmetallating agent 2 could be prepared by adding a THF suspension of the ligand precursor 1·HCl to a THF suspension of CuI containing potassium hexamethyldisilazide (KHMDS) at low temperature. The resulting copper complex was isolated in 37% yield as a pale orange solid (Scheme 1) that could be recrystallized from toluene. The broad 1H NMR signals suggested that at room temperature complex equilibria could be present in CD2Cl2, as previously described for copper(I) complexes with nitrogenbased pincer ligands.24 A crystallographic determination revealed an ionic structure, [Cu(1)2]2[ICu(μ-I)2CuI], in the solid state. The planar, doubly charged [ICu(μ-I)2CuI] anion featuring bridging and terminal iodide ligands has been observed before.25 In the [Cu(1)2] cation, the copper center is tetracoordinated by two chelating ligands in a coordination geometry that could be described as strongly distorted (flattened) tetrahedral, or distorted disphenoidal (Figure 1).26

very long, yet not unprecedented for tetracoordinated pyridyl complexes of copper(I). Nickel(II) Complex 3a. Reaction of 2 with NiBr2(dme) in THF at room temperature gave nickel(II) dibromide complex 3a as a green, paramagnetic solid in 66% yield (Scheme 2). Scheme 2. Synthesis of Nickel(II) and Ruthenium(II) Complexes 3 and 4

Single crystals were grown by slow evaporation of a saturated dichloromethane solution. The complex adopts a distorted trigonal bipyramidal geometry in the solid state (Figure 2), with

Figure 2. Solid-state structure of 3a with thermal ellipsoids drawn at 50% probability level. All hydrogen atoms and the dichloromethane crystallization solvent were omitted for clarity. Selected bond lengths (Å) and angles (deg): Ni1−C1 = 1.887(5), Ni1-N = 2.218(4), 2.229(4), Ni1−Br = 2.4752(9), 2.4836(9); N3−Ni1−N4 = 157.83(16), Br1−Ni1−Br2 = 164.81(3), N1−C1−N2 = 120.5(5).

the two nitrogen atoms in the axial position forming a fairly acute N3−Ni1−N4 angle of 157.83(16)°. The Br1−Ni1−Br2 angle is in turn remarkably wide, at 164.81(3)°, so that the description of the structure as distorted square pyramidal with the carbene carbon in the axial position is equally accurate. Such geometry is fairly unusual in pincer complexes of nickel, and when observed, it is associated with steric blocking of the sixth coordination site at the metal. Only one (terpy)NiBr2 compound is available for comparison, and its metric parameters are similar, except for a much narrower Br−Ni− Br angle of 129.51(6)°.27 A comparison of the pincer ligand binding reveals that the C−Ni bond in 3a is 0.1 Å shorter than the central N−Ni bond in its terpy analogue, while the N−Ni bonds in 3a are 0.1 Å longer than their counterparts in the terpy complex. The effective magnetic moment of 3a, determined using Evans method,28 had a value of 3.51. The elemental analysis, crystal structure, and mass spectrum of 3a indicate that no significant bromide−iodide exchange leading to 3c and 3b has taken place during the synthesis.

Figure 1. Solid-state structure of the cation in 2 with thermal ellipsoids drawn at 50% probability level. All hydrogen atoms and the Cu2I42− counterion were omitted for clarity. Selected bond lengths (Å) and angles (deg): Cu1−C1 = 1.930(4), Cu1−C23 = 1.930(4), Cu1−N1 = 2.374(3), Cu1−N5 = 2.396(3); C1−Cu1−C23 = 151.87(16), N1− Cu1−N5 = 120.85(12).

Both noncoordinating pyridine groups are rotated by ca. 180°, with the nitrogen atoms oriented away from the metal. The carbene ligands form a C1−Cu1−C23 angle of 151.87(16)°, which could be seen as the axial arrangement of the disphenoidal geometry, while the pyridyl ligands form a more acute N1−Cu1−N5 angle of 120.85(12)° in the equatorial plane. The two copper−carbene bonds Cu1−C1 and Cu1− C23 are equivalent (1.930(4) Å) and fall in the typical range for NHC complexes. The two copper-pyridyl bonds Cu1−N1 and Cu1−N5 are similar in length (2.374(3) and 2.396(3) Å) and C


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Organometallics Ruthenium(II) Complexes 4a and 4b. Reaction of 2 with [RuCl2(p-cymene)]2 in THF at room temperature (Scheme 2) gave a product that was isolated as a burgundy colored, diamagnetic powder. Multinuclear NMR characterization in CD2Cl2 revealed the presence of a major constituent, as well as closely related impurities (Figure 3). The major product,

Figure 4. Solid-state structure of 4a with thermal ellipsoids drawn at 50% probability level. All hydrogen atoms and the THF crystallization solvent were omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru1−C1 = 1.820(6), Ru1-N = 2.117(5), 2.132(6), Ru1−Cl = 2.446(2), 2.441(2); N3−Ru1−N4 = 160.5(2), Cl1−Ru1−Cl2 = 168.87(6).

Figure 3. Progress of the halogen exchange reaction of crude 4a with LiI in CD2Cl2, monitored by 1H NMR.

identified as [Ru(1)Cl2], 4a, exhibited a resonance corresponding to the tert-butyl signal at 2.05 ppm in the 1H NMR spectrum and one corresponding to the carbene carbon at 240.9 ppm in the 13C NMR spectrum. This represents a substantial downfield shift from 148.7 ppm in the ligand precursor 1·HCl, and compares well with the carbene 13C chemical shift in ruthenium analogues C (235−237 ppm).10 The signals at 2.02 ppm (1H NMR) and 238.0 ppm (13C NMR) were assigned to the [Ru(1)I2] impurity, 4b, while the remaining resonance at 2.04 ppm in the 1H NMR spectrum was attributed to [Ru(1)ClI], 4c. The halogen exchange equilibrium could be pushed both ways with AgCl or LiI (Figure 3), allowing for the isolation of pure 4a and 4b. Single crystals of 4a were grown by slow evaporation of a saturated THF solution. The complex adopts a square pyramidal geometry with the carbene carbon in the axial position in the solid state, very similar to that of 3a (Figure 4). Only few pincer complexes of pentacoordinated ruthenium(II) dihalide (LRuX2) are known,29 and neither of them features a terpy ligand. Most frequently, the sixth coordination site is occupied by carbon monoxide or a phosphine, and in some cases where the sixth coordination site is available, agostic and anagostic interactions involving the ligand substituents have been identified at this position.29b,d,f The two Ru−Cl bonds are almost colinear, with the Cl1−Ru−Cl2 angle measuring 168.87(6)°. The N3−Ru1−N4 angle is, at 160.5(2)°, fairly typical of pincer geometry. The Ru−C bond in 4a is 0.15 Å shorter than the corresponding distance in the octahedral, homoleptic compound C,10 reflecting the lack of a trans substituent in the former. Single crystals of 4b were grown from CH2Cl2 and a crystallographic determination revealed a structure similar to that observed for 4a (Figure 5). Aside from the length of the ruthenium-halogen bonds, the metric parameters of the structure are effectively identical to those measured in 4a, with one notable exception. The I−Ru−I bond angle in 4b is,

Figure 5. Solid-state structure of 4b with thermal ellipsoids drawn at 50% probability level. All hydrogen were omitted for clarity. Selected bond length (Å) and angles (deg): Ru1−C1 = 1.815(2), Ru1-N = 2.0956(18), 2.1227(18), Ru1−I = 2.7213(2), 2.7043(2); N1−Ru1− N4 = 161.15(7), I1−Ru1−I2 = 177.978(8).

with 177.978(8)°, even wider than its Cl−Ru−Cl counterpart in 4a. This is a result of the steric interaction of the larger halogen with the tert-butyl groups that block the coordination site in trans to the carbene carbon, and brings the geometry at the metal very close to an idealized square pyramid. At 1.820(6) and 1.815(2) Å, the Ru−C bonds in complexes 4a and 4b are the shortest Ru−CNHC bonds reported to date. They are substantially shorter than the closest reported values of 1.885(7) and 1.892(3) Å, measured in pincer and pentadentate complexes featuring pyridyl pendant arms.30 The average Ru− CNHC bond measures 2.05(5) Å over 1279 structures (vs 2.07(5) Å over 792 structures for Ru−Caryl).31 To this remarkably short bond contribute, among other influences, the lack of a trans substituent, the σ-donor ability of sixmembered carbenes, which is slightly superior to that of their five-membered counterparts, and the rigid geometry of the pincer ligand, with the pendant arms holding the metal close to the central donor. In turn, the Ru−N bonds in 4a and 4b are, with 2.096(2)−2.132(6) Å, somewhat longer than the average reported Ru−Npyridyl bond in 2,2′:6′,2″-terpyridine complexes (2.07(2) Å).31 The steric strain between the tert-butyl D


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Organometallics

of Rh(III) rollover cyclometalation product 9b, which was isolated as an orange solid (Scheme 3). In THF, the reaction did not proceed past 6b, and after isolation, pure 6b did not convert to 9b in acetonitrile, in the presence or absence of CuI. Rollover cyclometalation has been observed on a few occasions in complexes with N-pyridyl- and N-(1,8-naphthyridyl)-NHC ligands, exclusively on iridium and rhodium(I), and in all cases a sterically demanding substituent was present in the 6 position of the pyridyl ring.32 It appears therefore that the steric hindrance of the tert-butyl groups on the pendant arms has a non-negligible contribution to the driving force behind the formation of 9b. Mechanistically, precursor 5b could undergo rollover cyclometalation to generate Cs-symmetric Rh(III)-pincer complex 7b. Reductive elimination of HI in 7b, potentially under the influence of the intramolecular free pyridine site generated in the previous step, could lead to 8b. The oxidation of the latter to 9b could be accomplished with the copper iodide byproduct present in the reaction mixture, as postulated above for the transformation of 5 to 6. It is highly unlikely that Rh(III) complex 6b is an intermediate in the formation of 9b. It is generally accepted that rollover cyclometalation requires a Rh(I) precursor for C−H oxidative addition,33 and there are no obvious paths for the conversion of 6 to a Rh(I) intermediate such as 5. Reductive elimination of I2 is thermodynamically unfavorable and the reduction of Rh(III) to Rh(I) in the presence of CuI is incompatible with the postulated role of CuI as an oxidant in the formation of 6. It is rather likely that the higher polarity and coordinating ability of acetonitrile lowers the activation barrier for the formation of 7b from the common intermediate 5b. The critical role of donor solvents in promoting efficient rollover cyclometalation is wellknown.33,34 No spectroscopic evidence for the existence of intermediates 5, 7, and 8 could be obtained. Single crystals of 9b were grown by slow evaporation of a saturated acetonitrile solution and the solid-state structure confirmed the proposed geometry of the compound (Figure 7).

substituents is alleviated by a twisting of the planes of pyridyl rings, which form dihedral angles of 18.6° (3a), 18.1° (4a), and 14.1° (4b) with each other. Rhodium(III) Complexes 6b and 9b. Reaction of 2 with [Rh(cod)Cl]2 in THF at room temperature gave a diamagnetic compound initially believed to be Rh(I) complex 5 (Scheme 3). Scheme 3. Synthesis of Rhodium Complexes 6a−c and 9b

It was isolated as a brown solid and shown to have timeaveraged C2v symmetry by 1H NMR, and limited solubility in organic solvents. Halogen exchange with AgCl in CH3CN-d3, however, was monitored by 1H and 13C NMR spectroscopy (Figure 6) and indicated that the isolated product was the ionic

Figure 6. Progress of the halogen exchange reaction of crude 6b with AgCl in CH3CN-d3, monitored by 1H NMR. * corresponds to CH3CN-d2.

Rh(III) iodide 6b, structurally very similar to Ru(II) complex 4b described above. In the presence of excess AgCl, diiodide complex 6b led to the formation of dichloride complex 6a, via the chloroiodide intermediate 6c. The existence of the latter is incompatible with a monohalide complex such as 5. The needle-shaped crystals of 6b were too thin for a diffraction experiment and its identity could not be confirmed crystallographically, but the ESI-HRMS spectrum featured the expected [6b-I]+ ion. The transmetalation of 2 with [Rh(cod)Cl]2 is expected to yield in the first step C2v-symmetric Rh(I)-pincer complex 5, which could be oxidized to 6 by the copper iodide byproduct. This proposal is supported by the observation of a copper mirror on the walls of the reaction vessel. The reaction of 2 with [Rh(cod)Cl]2 in acetonitrile took an entirely different path than in THF, leading to clean formation

Figure 7. Solid-state structure of 9b with thermal ellipsoids drawn at 50% probability level. All hydrogen atoms and the acetonitrile crystallization solvent were omitted for clarity. Selected bond length (Å) and angles (deg): Rh1−C10 = 1.903(3), Rh1−C8 = 1.994(3), Rh1−N4 = 2.346(3), Rh1−N5 = 2.130(3), Rh1−I = 2.6535(3), 2.6687(3); C8−Rh1−N4 = 158.78(11), C10−Rh1−N5 = 172.85(12), I1−Rh1−I2 = 174.332(11).

Rollover cyclometalation led to the formation of a monoanionic NCC-pincer ligand on a pseudooctahedral Rh(III) center. The pyridyl-NHC-aryl pincer ligand architecture has not been described before; however, its analogous 6-phenyl-2,2′bipyridyl (pyridyl−pyridyl-aryl) is ubiquitous in coordination chemistry. A handful of examples have been obtained via rollover cyclometalation in 2,2′:6′,2″-terpyridine on platinum E


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Organometallics and ruthenium.35 With 1.903(3) Å, the Rh−C10 bond in 9b is extremely short, on par with the shortest reported Rh−CNHC bonds, which were measured in SCS- and PCP-pincer systems (1.892(12) - 1.905(6) Å).36 For comparison, the average Rh− CNHC bond measures 2.03(4) Å over 829 structures (vs 2.01(5) Å over 722 structures for Rh−Caryl).31 The substantial steric influence of the tert-butyl substituents onto the coordination site in trans to the carbene ligand is reflected by the bond angles involving the acetonitrile ligand: N4−Rh1−N5 = 109.74(10)° and C8−Rh1−N5 = 91.48(11)°. The dihedral angle formed by the best planes of the two pyridyl rings, which was discussed above as an indicator of the steric strain between the tert-butyl substituents, is only 3.7° in 9b, reflecting the alleviating effect rollover cyclometalation had on the steric strain in this molecule. In spite of the marked differences in the σ-donor and πacceptor properties of NHCs vs the phenyl anion,37 a comparison of the M−CNHC bonds in 4a,b and 9b with all M−Caryl bonds reported in the literature may be warranted. It reveals that the shortest Ru−Caryl bonds measure 1.901(9)− 1.939(3) Å and are found in NCN-pincer complexes (vs Ru− CNHC = 1.820(6) and 1.815(2) Å in 4a and 4b, respectively).38 The shortest reported Rh−Caryl bonds are also found in NCNpincer complexes and measure 1.860(12)−1.878(5) Å and (vs Rh−CNHC = 1.903(3) Å in 9b).39 Although the Ni−CNHC bond length in 3a does not stand out (1.887(5) Å vs the shortest Ni−CNHC bonds reported in the literature: 1.742(2)−1.795(7) Å and the average Ni−CNHC bond: 1.90(5) Å over 896 structures),18,40 it can be noted that the shortest reported Ni− Caryl bonds measure 1.788(10)−1.819(4) Å and are found, with one exception, in NCN-pincer complexes (vs the average Ni− Caryl bond of 1.90(3) Å over 880 structures).41 From this comparison, it can be concluded that, for the three examined metals at least, the rigid structure and ligand metric parameters in NCN-pincer complexes are conducive to the formation of very short metal−carbon bonds involving the central carbene or aryl moiety. The observation that the M−Caryl bonds compiled above are somewhat shorter than the M−CNHC bonds for M = Rh, somewhat longer for M = Ni, and noticeably longer for M = Ru emphasizes the complexity of the factors that influence this bonding parameter.

CNHC bonds were measured in 4a, 4b, and 9b, the shortest on record for the respective metal-carbene complexes, attesting to the superior binding ability of the NCN and NCC-pincer ligands developed herein. The coordination of molecules small enough to fit into the coordination pocket in trans to the carbene moiety of square pyramidal complexes 1MX2, as well as the chemistry of NHC-based pincer ligands obtained via rollover cyclometalation, are currently being investigated.

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EXPERIMENTAL SECTION

General Considerations. Unless otherwise specified, all operations were carried out with careful exclusion of air and moisture using standard Schlenk and glovebox techniques. Solvents used in the preparation of air- and/or moisture-sensitive compounds were dried by using an MBraun Solvent Purification System fitted with alumina columns (dichloromethane, toluene, and pentane), or dried by refluxing over potassium and subsequent distillation (THF, benzene and hexanes) under a positive pressure of argon. Starting materials were purchased from commercial suppliers, or prepared according to literature procedures. Deuterated solvents were vacuum-distilled over potassium (THF-d8, benzene-d6 and toluene-d8) or CaH2 (CD2Cl2 and CH3CN-d3) and then degassed using three freeze−pump−thaw cycles. All NMR spectra were acquired on Bruker Avance III 400 or Avance I 400 spectrometers and chemical shifts are reported in δ units (ppm) using the solvent as an internal reference: CH2Cl2-d1 (5.32 ppm, 1H) and CH2Cl2-d2 (54.00 ppm, 13C); CHCl3 (7.24 ppm, 1H) and CDCl3 (77.23 ppm, 13C); acetone-d5 (2.05 ppm, 1H) and acetoned6 (29.92 ppm, 13C); CH3CN-d2 (1.94 ppm, 1H) and CH3CN-d3 (1.39 ppm, 13C); benzene-d5 (7.16 ppm, 1H) and benzene-d6 (128.39 ppm, 13 C). 2-Bromo-6-tert-butylpyridine.20 In a two-neck flask, 2,6dibromopyridine (12.30 g, 51.92 mmol) and copper iodide (2.48 g, 13.0 mmol, 25 mol %) were stirred in THF (50 mL) at −78 °C. A solution of tert-butylmagnesium chloride (78.2 mL, 1.0 M in THF, 78.2 mmol) was added dropwise, and the reaction mixture was stirred for 24 h while it warmed up from −78 °C to room temperature. The reaction was quenched by addition of saturated aqueous ammonium chloride solution (30 was extracted by dichloromethane (3 × 20 mL). Once the organic phase was dried with magnesium sulfate and the solvent was removed at reduced pressure, the residue was distilled (bp ∼80 °C/3.3 Torr), affording 8.06 g (72%) of product as a clear liquid. 1 H NMR (400 MHz, 298 K, acetone-d6): δ = 1.33 (s, 9H, C(CH3)3), 7.38 (dd, 1H, 3JHH = 7.8 Hz, 4JHH = 0.7 Hz, CH), 7.44 (dd, 1H, 3JHH = 7.7 Hz, 4JHH = 0.7 Hz, CH), 7.65 (t, 1H, 3JHH = 7.8 Hz, CH). N,N′-Bis-[6-tert-butyl-2]pyridylpropane-1,3-diamine. 1,3-Diaminopropane (1.921 g, 25.92 mmol), 2-bromo-6-tert-butylpyridine (12.220 g, 57.076 mmol), sodium tert-butoxide (6.231 g, 64.83 mmol), Pd2(dba)3 (0.122 g, 0.133 mmol, 0.5 mol %), and rac-BINAP (0.161 g, 0.258 mmol, 1 mol %) were loaded into a one-neck flask. Toluene (50 mL) was condensed, and the resulting suspension was refluxed for 12 h. After the suspension was allowed to cool to room temperature, the solids were removed by filtration, and all volatiles from the filtrate were removed in vacuo. The crude product was left behind in a quantitative yield as a deep red gel of satisfactory purity and was used for subsequent steps without further purification. 1H NMR (400 MHz, 298 K, CDCl3): δ = 1.29 (s, 18H, C(CH3)3), 1.92 (pent, 2H, 3JHH = 6.7 Hz, (CH2)2CH2), 3.41 (t, 4H, 3JHH = 6.7 Hz, CH2NH), 6.15 (dd, 2H, 3JHH = 8.2 Hz, 4JHH = 0.4 Hz, C3H), 6.57 (dd, 2H, 3JHH = 7.5 Hz, 4 JHH = 0.5 Hz, C5H), 7.31 (t, 2H, 3JHH = 7.7 Hz, C4H). 13C{1H} NMR (DEPT-Q, 100 MHz, 298 K, CDCl3): δ = 30.0 ((CH2)2CH2), 30.3 (C(CH3)3), 37.3 (C(CH3)3), 39.9 (NHCH2), 103.8 (C3H), 107.8 (C5H), 137.6 (C4H), 158.0 (N2C2), 168.2 (tBuC6). Ligand Precursor 1·HCl. Dry ammonium chloride (2.005 g, 37.48 mmol) and N,N′-bis(6-tert-butyl-2-pyridyl)propane-1,3-diamine (8.820 g, 25.90 mmol) were stirred in triethylorthoformate (25 mL) for 12 h at 120 °C. After the mixture was allowed to cool to room temperature, all volatiles were removed in vacuo, and the residue was washed once with diethyl ether (30 mL) and then pentane (30 mL),

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CONCLUSIONS The work presented herein proves that NCN-pincer ligands with a central NHC moiety and pyridyl pendant arms can be conveniently adorned with sterically demanding substituents in the 6,6’ position of the pyridyl rings, allowing for an effective protection of the coordination site in trans to the carbene moiety. Specifically, ligand 1 allowed for the isolation of pincer complexes 1MX2 (MX2 = NiBr2, 3a; RuCl2, 4a; RuI2, 4b; RhI2+, 6b) displaying a square pyramidal geometry that is uncharacteristic for these metals and reflects the steric impact of the pendant arms onto the sixth coordination site of the metal. The formal replacement of the imine pendant arms in complexes B by pyridyl groups in 1 leads to a stabilization of the carbene structure, which was exclusively observed in all metal complexes, to the detriment of the diaminoalkyl moiety observed in B. This is easily explained through the reduced competitive delocalization of the NHC-nitrogen lone pairs of electrons onto the aromatic pyridyl ring, vs the imine groups. Rollover cyclometalation in Rh(I) precursor 5 eventually led to the isolation of Rh(III) complex 9b, featuring a novel monoanionic pincer ligand. Very short Ru−CNHC and Rh− F


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Organometallics

(N2Ccarbene). HRMS (ESI+), m/z (%): [12C22H3035ClN4102Ru]+, calcd 487.1202, found 487.1198. Ruthenium(II) Diiodide Complex 4b. THF (30 mL) was added at room temperature to a mixture of copper complex 2 (394 mg, 0.364 mmol) and (p-cymene)ruthenium dichloride dimer (223 mg, 0.364 mmol). The mixture was stirred for 12 h, resulting in the formation of a burgundy solution and a brown precipitate. The solids were removed by filtration, and excess LiI (127 mg, 0.949 mmol) was added to the burgundy filtrate. After the mixture was sonicated for 12 h, the liquid phase was filtered and concentrated down to a volume of 5 mL. An equal volume of toluene was condensed onto the solution, leading to the precipitation of red-brown solids. The liquid part was decanted off, and the solids were washed with hexanes (10 mL), yielding 280 mg (54%) brown powder. 1H NMR (400 MHz, 298 K, CD2Cl2): δ = 2.02 (s, 18H, C(CH3)3), 2.67 (pent, 2H, 3JHH = 6.0 Hz, (CH2)2CH2), 4.15 (t, 4H, 3JHH = 6.0 Hz, CH2N), 6.94 (dd, 2H, 3JHH = 8.2 Hz, 4JHH = 0.9 Hz, C3H), 7.28 (dd, 2H, 3JHH = 8.1 Hz, 4JHH = 1.0 Hz C5H), 7.62 (t, 2H, 3JHH = 8.1 Hz, C4H). 13C{1H} NMR (DEPT-Q, 100 MHz, 298 K, CD2Cl2): δ = 22.1 ((CH2)2CH2), 31.3 (C(CH3)3), 39.6 (C(CH3)3), 43.6 (NCH2), 106.6 (C3H), 118.9 (C5H), 137.1 (C4H), 160.7 (N2C2), 174.5 (tBuC6), 238.0 (N2Ccarbene). HRMS (ESI+), m/z (%): [12C22H30127IN4102Ru]+, calcd 579.0559, found 579.0554. Anal. Calcd for C22H30I2N4Ru: C 37.46; H 4.29; N 7.94. Found: C 37.09; H 4.36; N 7.74. Rhodium(III) Iodide Complex 6b. THF (50 mL) was condensed onto a mixture of copper complex 2 (309 mg, 0.285 mmol) and [Rh(cod)Cl]2 (140 mg, 0.284 mmol). The mixture was stirred at room temperature for 12 h, forming an orange solution and a brown precipitate. The precipitate was filtered off, and all volatiles from the filtrate were removed in vacuo. The solid residue was washed with toluene (2 × 10 mL), yielding the product as a brown powder (150 mg, 31%). 1H NMR (400 MHz, 298 K, CD3CN): δ = 1.80 (s, 18H, C(CH3)3), 2.63 (pent, 2H, 3JHH = 6.0 Hz, (CH2)2CH2), 4.36 (t, 4H, 3 JHH = 6.0 Hz, CH2N), 7.47 (dd, 2H, 3JHH = 8.3 Hz, 4JHH = 1.0 Hz, C3H), 7.68 (dd, 2H, 3JHH = 8.2 Hz, 4JHH = 1.1 Hz, C5H), 8.12 (t, 2H, 3 JHH = 8.3 Hz, C4H). 13C{1H} NMR (DEPT-Q, 100 MHz, 298 K, CD3CN): δ = 20.4 ((CH2)2CH2), 30.5 (C(CH3)3), 39.9 (C(CH3)3), 46.3 (NCH2), 112.9 (C3H), 123.6 (C5H), 143.2 (C4H), 158.6 (N2C2), 172.8 (tBuC6). HRMS (ESI+), m/z (%): [12C22H30127I2N4103Rh]+, calcd 706.9609, found 706.9596. Rhodium(III) Iodide Complex 9b. Acetonitrile (30 mL) was added at room temperature to a mixture of copper complex 2 (475 mg, 0.438 mmol) and [Rh(cod)Cl]2 (216 mg, 0.438 mmol). The mixture was stirred for 12 h, resulting in the formation of an orange solution and a red precipitate. The solids were removed by filtration, and all volatiles from the filtrate were removed in vacuo. The residue was washed with toluene (2 × 10 mL), yielding 320 mg (49%) product as an orange powder. 1H NMR (400 MHz, 298 K, CD3CN): δ = 1.36 (s, 9H, C(CH3)3), 1.69 (s, 9H, C(CH3)3), 2.37 (pent, 2H, 3 JHH = 5.9 Hz, (CH2)2CH2), 4.12 (t, 4H, 3JHH = 5.9 Hz, CH2N), 6.92 (d, 1H, 3JHH = 7.8 Hz, CH), 7.28 (dd, 1H, 3JHH = 8.5 Hz, 4JHH = 0.9 Hz, CH), 7.44 (dd, 1H, 3JHH = 8.0 Hz, 4JHH = 0.9 Hz, CH), 7.68 (dd, 1H, 3JHH = 7.8 Hz, 3JHRh = 1.1 Hz, CH), 7.89 (t, 1H, 3JHH = 8.2 Hz, CH). HRMS (ESI+), m/z (%): [12C24H32127IN5103Rh]+, calcd 620.0757, found 620.0748.

leaving behind 6.904 g (69%) product as a beige solid. 1H NMR (400 MHz, 298 K, CD2Cl2): δ = 1.40 (s, 18H, C(CH3)3), 2.60 (pent, 2H, 3 JHH = 5.8 Hz, (CH2)2CH2), 4.51 (t, 4H, 3JHH = 5.8 Hz, CH2N), 7.43 (d, 2H, 3JHH = 7.8 Hz, CH), 7.47 (d, 2H, 3JHH = 8.0 Hz, CH), 7.91 (t, 2H, 3JHH = 8.0 Hz, C4H), 10.72 (s, 1H, N2CH). 13C{1H} NMR (DEPT-Q, 100 MHz, 298 K, CD2Cl2): δ = 19.6 ((CH2)2CH2), 30.3 (C(CH3)3), 38.2 (C(CH3)3), 44.3 (NCH2), 110.2 (C3H), 119.6 (C5H), 140.9 (C4H), 148.7 (N2CH), 150.0 (N2C2), 169.9 (tBuC6). HRMS (ESI+), m/z (%): [12C22H31N4]+, calcd 351.2549, found 351.2552. Anal. Calcd for C22H31ClN4: C 68.29; H 8.08; N 14.48. Found: C 67.36; H 8.07; N 14.42. Ligand Precursor 1·HPF6. Dry ammonium hexafluorophosphate (577 mg, 3.54 mmol) and N,N′-bis(6-tert-butyl-2-pyridyl)propane-1,3diamine (804 mg, 2.36 mmol) were stirred in triethylorthoformate (5 mL) for 12 h at 120 °C. After the mixture was allowed to cool to room temperature, all volatiles were removed in vacuo, and the residue was extracted with THF (15 mL). THF was removed in vacuo, and the residue was subsequently washed with diethyl ether (5 mL) and then pentane (10 mL), leaving behind 860 mg (73%) product as a yellow powder. 1H NMR (400 MHz, 298 K, CD2Cl2): δ = 1.42 (s, 18H, C(CH3)3), 2.58 (pent, 2H, 3JHH = 5.9 Hz, (CH2)2CH2), 4.20 (t, 4H, 3 JHH = 5.9 Hz, CH2N), 7.25 (dd, 2H, 3JHH = 8.2 Hz, 4JHH = 0.4 Hz, C3H), 7.48 (dd, 2H, 3JHH = 7.7 Hz, 4JHH = 0.4 Hz, C5H), 7.93 (t, 2H, 3 JHH = 8.0 Hz, C4H), 10.77 (s, 1H, N2CH). Copper(I) Iodide Complex 2. Potassium hexamethyldisilazide (0.670 g, 3.36 mmol) and copper iodide (0.640 g, 3.36 mmol) were mixed in THF (30 mL) at −40 °C. After stirring for 1 h, the resulting suspension was added via cannula transfer to a suspension of 1·HCl (1.302 g, 3.365 mmol) in THF (20 mL) at −78 °C. The reaction mixture was stirred and allowed to warm up to room temperature over 24 h. All volatiles were removed in vacuo, and the residue was extracted with toluene (60 mL). The solvent was removed in vacuo, and the remaining product was washed twice with hexanes (2 × 30 mL), yielding the product as a pale orange solid (0.680 g, 37%). 1H NMR (400 MHz, 298 K, CD2Cl2): δ = 1.38 (br, 18H, C(CH3)3), 2.31 (br, 2H, (CH2)2CH2), 3.97 (br, 4H, CH2N), 7.13−7.90 (br, 6H, CH). Anal. Calcd for C44H60I2N8Cu2: C 48.85; H 5.59; N 10.36. Found: C 49.55; H 6.23; N 10.72. Nickel(II) Bromide Complex 3a. THF (10 mL) was condensed onto a mixture of copper complex 2 (200 mg, 0.185 mmol) and NiBr2(dme) (114 mg, 0.369 mmol). The mixture was stirred at room temperature for 12 h, resulting in the formation of a dark brown solution and a brown-green precipitate. The liquid phase was decanted off, and the brown-green solids were extracted with dichloromethane (20 mL). The solids were removed by filtration, and the solvent was removed in vacuo. The solid residue was washed with pentane (20 mL), affording 138 mg (66%) of product as deep green crystals. HRMS (ESI+), m/z (%): [12C22H3079BrN458Ni]+, calcd 487.1007, found 487.1005. Anal. Calcd for C22H30Br2N4Ni·CH2Cl2: C 42.24; H 4.93; N 8.57. Found: C 42.79; H 5.21; N 8.74. Ruthenium(II) Dichloride Complex 4a. THF (30 mL) was added at room temperature to a mixture of copper complex 2 (318 mg, 0.294 mmol) and (p-cymene)ruthenium dichloride dimer (180 mg, 0.294 mmol). The mixture was stirred for 12 h, resulting in the formation of a burgundy solution and a brown precipitate. The solids were removed by filtration, and excess AgCl (86 mg, 0.600 mmol) was added to the burgundy filtrate. After sonicating the mixture for 12 h, the liquid phase was filtered and concentrated down to a volume of 5 mL. An equal volume of toluene was condensed onto the solution, leading to the precipitation of burgundy solids. The liquid part was decanted off, and the solids were washed with hexanes (10 mL), yielding 190 mg (62%) burgundy microcrystalline product. 1H NMR (400 MHz, 298 K, CD2Cl2): δ = 2.04 (s, 18H, C(CH3)3), 2.69 (pent, 2H, 3JHH = 6.0 Hz, (CH2)2CH2), 4.18 (t, 4H, 3JHH = 6.0 Hz, CH2N), 6.95 (dd, 2H, 3JHH = 8.2 Hz, 4JHH = 0.9 Hz, C3H), 7.29 (dd, 2H, 3JHH = 8.0 Hz, 4JHH = 1.0 Hz C5H), 7.64 (t, 2H, 3JHH = 8.1 Hz, C4H). 13 C{1H} NMR (DEPT-Q, 100 MHz, 298 K, CD2Cl2): δ = 22.4 ((CH2)2CH2), 32.1 (C(CH3)3), 39.8 (C(CH3)3), 43.5 (NCH2), 106.5 (C3H), 118.2 (C5H), 137.5 (C4H), 161.2 (N2C2), 174.3 (tBuC6), 240.9

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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.8b00022. Complete NMR spectra for all new diamagnetic compounds, as well as the relevant crystallography tables (PDF) Accession Codes

CCDC 1569177−1569181 contain the supplementary crystallographic data for this paper. These data can be obtained free of G


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Organometallics

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Roland Roesler: 0000-0002-1235-3976 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this work was provided by the Natural Sciences and Engineering Research Council of Canada in the form of a Discovery Grant #262037 to R.R.

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REFERENCES

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