Diversity of Isomerization Patterns and Protolytic Forms in

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

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Diversity of Isomerization Patterns and Protolytic Forms in Aminocarbene PdII and PtII Complexes Formed upon Addition of N,N′‑Diphenylguanidine to Metal-Activated Isocyanides Svetlana A. Katkova,† Mikhail A. Kinzhalov,† Peter M. Tolstoy,† Alexander S. Novikov,† Vadim P. Boyarskiy,*,† Anastasiia Yu. Ananyan,‡ Pavel V. Gushchin,† Matti Haukka,§ Andrey A. Zolotarev,† Alexander Yu. Ivanov,† Semen S. Zlotsky,∥ and Vadim Yu. Kukushkin*,†,‡ †

Saint Petersburg State University, 7/9 Universitetskaya Nab., 199034 Saint Petersburg, Russian Federation Institute of Macromolecular Compounds of Russian Academy of Sciences, Bolshoii Pr. 31, 199004 Saint Petersburg, Russian Federation § Department of Chemistry, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä, Finland ∥ Ufa State Petroleum Technological University, Kosmonavtov 1, 420062 Ufa, Bashkortostan, Russian Federation ‡

S Supporting Information *

ABSTRACT: Reaction of the palladium(II) and platinum(II) isocyanide complexes cis[MCl2(CNR)2] [M = Pd, R = C6H3(2,6-Me2) (Xyl), 2-Cl-6-MeC6H3, cyclohexyl (Cy), t-Bu, C(Me)2CH2(Me)3 (1,1,3,3-tetramethylbuth-1-yl abbreviated as tmbu); M = Pt, R = Xyl, 2-Cl-6MeC6H3, Cy, t-Bu, and tmbu] with N,N′-diphenylguanidine (DPG) leads to DPG-derived metal-bound deprotonated acyclic diaminocarbene (ADC) species. This reaction occurs via a two-step process, involving the initial coupling of the guanidine with one of the isocyanides and leading to deprotonated monocarbene monochelated species, while the next addition grants the deprotonated bis-carbene bis-chelated metal compounds. DPG behaves as nucleophile, deprotonating base, and chelator. The addition of DPG proceeded with different regioselectivity depending on the metal center and, in a larger extent, on the substituent R in RNCs. The X-ray diffraction studies for the deprotonated mono- and bis-carbene complexes confirmed the regioisomerism of these species and allowed the identification of ADC protolytic forms stabilized in the solid-state. 1D (1H and 13C{1H}) and 2D (1H,1H-NOESY; 1H,15N-HSQC; 1H,15N-HMBC) solution NMR of the obtained systems demonstrated their configuration isomerism accompanied by prototropic tautomerism. Together, the solid-state and solution data provide an insight into the flexible character of ADC species.

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INTRODUCTION Additions of various N-nucleophiles to metal-activated isocyanides are a useful synthetic route to metal-bound diaminocarbenes otherwise difficult to obtain. Moreover, the nucleophilic attack on coordinated isocyanide is a way not only to classic ADC complexes but also to chelated ADC species (if polynucleophiles are used; see reviews (refs 1−4) and references therein and refs 5−7) or to NHC (in the case of bifunctional reagents bearing as nucleophilic and electrophilic centers).8−19 Thus, formed metal−ADC species are recognized as promising catalysts for many organic transformations that involve consecutive oxidative addition−reductive elimination steps (for reviews, see refs 20 and 21; for our recent works, see refs 22−26). In particular, palladium−ADC derivatives demonstrate outstanding performance in numerous cross-coupling reactions9,17,23−29 including supported catalysis,30 while platinum−ADCs are efficient in hydrosilylation of alkynes.16,22 Very strong donor properties of ADC ligands facilitate oxidative addition, whereas the steric hindrance of ADC, easily tunable from low to high, promotes the reductive elimination step. One of the important advantages of metal−ADCs as catalysts compared to those of metal−NHCs (for works on net electron © XXXX American Chemical Society

donor and steric properties of ADCs, see refs 31−37) is the rotational flexibility of the ligand itself within the coordination sphere of the complex. The rotational flexibility of ADC ligands is thought to fulfill the catalyst steric contradictory requirements at various steps of catalytic cycles (for works on steric flexibility of ADCs in comparison to NHCs, see refs 38−41). In addition, the configuration/conformation of the carbene ligand affects its electronic and steric properties, which largely determine the chemical properties of the complex and ultimately its catalytic activity.42,43 This flexibility, as reflected in the configuration/conformation isomerism and/or acid−base equilibria, has received little attention, and only the relatively easy deprotonation of the NH group of metal-bound ADCs observed upon synthetic transformations7,44 or upon treatment with a base45−51 have been reported. Of note in this context that carbenes after their HN-deprotonation should be treated as formamidinyls insofar as, in accord with the IUPAC definition,52 carbenes are neutral species. The situation is not that obvious when HN-deprotonated carbene Received: July 26, 2017

A

DOI: 10.1021/acs.organomet.7b00569 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

R = C6H3(2,6-Me2) (Xyl) 1, 2-Cl-6-MeC6H3 2, cyclohexyl (Cy) 3, t-Bu 4, and C(Me)2CH2(Me)3 (1,1,3,3-tetramethylbuth-1-yl abbreviated as tmbu) 5; M = Pt, R = Xyl 6, 2-Cl-6-MeC6H3 7, Cy 8, t-Bu 9, and tmbu 10] were employed as the reactivity partners for the nucleophilic addition of DPG. It is well-documented that DPG exists in a tautomeric equilibrium in solution, which is an intramolecular process of proton transfer between three N centers.59−62 Owing to this tautomerism, the N sites of DPG are rather similar in their nucleophilicity and, in turn, in their reactivity toward metalactivated substrates. Notably, DPG as nucleophile has previously been employed for Pt-mediated coupling with nitrile RCN ligands,63,64 but it has never been applied for similar reactions involving isomeric isocyanides RNC. We started our studies from attempted reactions between an uncomplexed isocyanide (CNXyl was taken as a model) and DPG in CHCl3. In these blank experiments, no reaction was observed either at 20−25 °C for 1 day or even upon reflux of the reaction mixture for 5 h. These observations indicate that the additions of DPG along the CN triple bond of the ligated isocyanides described later in this work are metal-mediated. In our metal-involving experiments, we observed generations of regioisomers (abbreviated as R1 and R2; Scheme 2), which in turn participate in configuration (configuration isomers C1 and C2) and tautomeric (tautomers τ1 and τ2) equilibria. Our synthetic experiments are disclosed below in two consecutive sections. Reaction of cis-[MCl2(CNR)2] with 2 equiv of DPG. After addition of 2 equiv of DPG in CHCl3 to any one of starting isocyanide complexes 1−10 (Scheme 3) upon ultrasound treatment that additionally facilitate dissolution, the reaction mixture became homogeneous within approximately 1−2 min, and this observation collaterally indicates a fast transformation.

species are bound to a metal center because of charge redistribution within the complex. To avoid this ambiguity, we conditionally use the term “deprotonated carbenes” along this report for all species, irrespective of their acid−base forms, derived from the nucleophilic addition to metal-isocyanides. In summary, both isomerism and acid−base equilibria of metal-bound ADCs deserve separate studies that should employ as logical models ADCs bearing multiple N centers and branched structures. Our studies focus specifically on palladium(II)− and platinum(II)−ADC species derived from the unreported coupling between metal-activated RNC ligands and N,N′-diphenylguanidine (abbreviated in this work as DPG; Scheme 1). First, Scheme 1. Tautomeric Equilibrium of DPG

we studied the reactions between isocyanides, activated by PdII or PtII centers, and DPG. These reactions proceeded with distinct regioselectivity depending on the metal center and the substituent R in RNCs and led to DPG-derived metal−ADC species. Second, we inspected X-ray structures of the formed ADC complexes and established the regioisomerism of these species and verified ADC protolytic forms stabilized in the solid-state. Third, we studied the obtained systems in solutions by 1D (1H and 13C{1H}) and 2D (1H,1H-NOESY; 1H,15N-HSQC; 1 15 H, N-HMBC) NMR spectroscopies and observed their configuration isomerism accompanied by prototropic tautomerism. Together, our data provide an insight into the flexible character of ADC species; these results are further detailed in the sections below.

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Scheme 2. Observed Equilibria in 11−18

RESULTS AND DISCUSSION Reaction of Metal-Complexed Isocyanides with DPG. The most developed protocols to metal−ADCs are based upon the addition of monofunctional (e.g., amines and alcohols) or bifunctional (e.g., hydrazines, diamines, or amidines) nucleophiles to RNCs activated by coordination.3,4 If the additions of monofunctional N-nucleophiles are well-studied, then the additions of bifunctional nucleophiles are substantially less explored, and the known examples include (i) reactions of nucleophiles with strongly reduced nucleophilicity of one of the reaction centers (e.g., 4-nitrophenylhydrazine, N,N-diphenylhydrazine, carbo- and sulphohydrazides, and hydrazones), which afforded classic ADC species27,53−55 and (ii) reactions of nucleophiles with the same or similar nucleophilicity/basicity of the attacking nitrogen atoms (e.g., hydrazine and most of monosubstituted hydrazines, alkylene- or arene-1,2-diamines, amidines, and 2-aminoazaheterocycles), which lead to a range of cyclic chelated ADC structures in their protic and deprotonated forms.6,12,13,56,57 The addition of unsymmetrical polyfunctional nucleophiles (i.e., containing more than two different nucleophilic centers in one molecule) to coordinated isocyanides has never been reported in the past, but namely these nucleophiles should lead to the branched ADCs with multiple donor centers. To fill the gap in the generation of such ADC species, strongly basic N,N′-diphenylguanidine (pKa = 9.15 ± 0.03)58 was taken as the polyfunctional nucleophile and the known palladium(II) and platinum(II) isocyanide complexes cis-[MCl2(CNR)2] [M = Pd, B


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Organometallics Scheme 3. Reaction between cis-[MCl2(CNR)2] (M = Pd and Pt) and DPG

substituent R in isocyanide ligands. Involvement of aryl isocyanides leads to a larger proportion of regioisomers R1 and application of alkyl isocyanides favor regioisomer R2. A similar dependence of the structure of the resulting carbene complexes on the substituent at the isocyanide ligand was observed earlier by some of us for the nucleophilic addition involving phenylbenzamidine, PhC(NH)NHPh. In that reaction, XylNC ligands were attacked by the NHPh moiety, whereas t-BuNC reacted with the CNH center of the amidine.44 Concurrently, replacement of palladium to platinum center leads to similar products but with quantitative differences in the isomeric ratios (except the reduction of the PtII centers in 9 and 10). It is believed that the stabilization of one or the other regioisomeric form depends on various overlapping factors that include, in particular, steric hindrance and, as indicated later, noncovalent interactions and attribution of the stabilization to only one factor is perhaps too speculative. Reaction of cis-[MCl2(CNR)2] with 4 equiv of DPG. When complexes 1−8 were treated with 4 equiv of DPG in CHCl3 (Scheme 3, Route B) upon reflux for 8 h (1−3, 6, 7) or 2 days (4, 5, 8), we observed the generation of deprotonated bis-carbene complexes 19−26 in 85−98% 1H NMR yields (Table 2). Complexes 19−26 were isolated from the reaction mixtures as colorless solids in 43−76% yields. As for case of deprotonated monocarbene complexes 11−18 and here the strong decrease in isolated yields in comparison with the 1H NMR yields is due to difficulty in purification of the title products from DPG·HCl. The structures of 19−26 were studied in solution by 1H NMR and 1H,15N-HMBC NMR spectroscopies, and we found that 21−23 (M = Pd, R = Cy, t-Bu, tmbu) and 26 (M = Pt, R = Cy) feature two identical chelated rings with the protonated NAlk centers (Scheme 3). Complexes 19, 20, 24, and 25 (M = Pd, Pt, R = Xyl, 2-Cl-6-MeC6H3) exhibit structures with the asymmetrically protonated NAr groups where the N(H)R hydrogen atom of the first NCN fragment is involved in an intramolecular hydrogen bond with the NAr nitrogen atom of the second NCN fragment. The same yields of 19−26 (up to 100% 1H NMR yields 51− 84% isolated yields) were achieved upon reflux of 11−18 with 2 equiv of DPG in CHCl3 for 8 h (Route C). We note that the isolated yields for Route C are higher than those for Route B because amount of the formed byproduct is two times less for Route C. The reactions were 100% regioselective, and only one of two possible regioisomeric deprotonated bis-carbene complexes was formed in each case. The regioselectivity of Route C does not depend on the isomeric composition of the initial mixture of 11−18. This suggests a sufficiently fast regioisomerization of 11−18 species

In the case of palladium(II) complex 1 (R = Xyl), the progress of the reaction was monitored by IR and 1H NMR at −20, 0, 20, and 40 °C, and after 1−2 min, we observed the generation of regioisomers 11-R1 and 11-R2τ1 in ca. 4:1 ratio and in overall 98% 1H NMR yield. In the case of platinum(II) congener 6 (R = Xyl), we also detected a regioisomeric mixture of 16-R1 and 16-R2τ1 at a 3:1 ratio that is also formed in overall 98% 1H NMR yield. The mixtures of the solid isomeric species were isolated in 89% (11) and 73% (16) yields after separation of DPG·HCl. When palladium and platinum complexes 2 and 7 (R = 2-Cl6-MeC6H3), respectively, react with 2 equiv of DPG (CHCl3, RT, 5 min) mixtures of regioisomers 12-R1 and 12-R2τ1 (3:1 ratio; 96% 1H NMR yield) or 17-R1 and 17-R2τ1 (2:1 ratio; 94% 1H NMR yield) were obtained along with some yet unidentified products. Further removal of the solvent followed by repeated extraction of the formed solid residue with Et2O allowed the isolation of a mixture of solid regioisomers 12-R1 and 12-R2τ1 (ca. 3:1) and a mixture of 17-R1 and 17-R2τ1 (ca. 2:1), both in ca. 64% isolated yields. The complexes are partially decomposed upon the isolation, and this explains the moderate yields. The reaction between 3 or 8 (R = Cy) and DPG in a 1:2 molar ratio under the same conditions gives a mixture of regioisomers 13-R1 and 13-R2τ2 (a 1:1 ratio; 99% 1H NMR yield) in the case of PdII, and one complex, 18-R2τ1, (90% 1H NMR yield) for PtII. The structures of 13-R1 and 18-R2τ1 in the solid state were additionally confirmed by single-crystal X-ray diffraction (see below). When a mixture of 4 (R = t-Bu) and 2 equiv of DPG in CHCl3 was kept at RT for approximately 30 min, the solution changed its color from yellow to pale yellow. The 1H NMR monitoring of the reaction demonstrated the presence of two configuration isomers (14-R2τ1c1 and 14-R2τ1c2; ca. 2:1 ratio; 95% 1H NMR yield) of one tautomer and regioisomer 14-, whose structures were verified by the 1H,1H-NOESY NMR experiments (see below). In the case of 5 (R = tmbu), the reaction with 2 equiv of DPG also resulted in formation of a mixture of two configuration isomers (15-R2τ1c1 and 15-R2τ1c2 in ca. 2:1 ratio; 95% 1H NMR yield). Complexes 14 and 15 were isolated as solids in 76 and 70% yields, respectively. Note that the reaction of DPG with platinum(II) complexes 9 (R = t-Bu) and 10 (R = tmbu) either at RT or upon reflux leads to a broad mixture of yet unidentified products including platinum black. It is evidently from these experiments that the formation of deprotonated monocarbene species in the reaction between cis-[MCl2(CNR)2] and DPG mainly depends on the nature of C


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Organometallics

Figure 1. 1H,1H-NOESY NMR spectrum of a solution of 13 in CDCl3 showing a mixture of 13-R1 and 13-R2τ2.

proposed formulas. In the HRESI+-MS of 11−26, ions [M + H]+ and/or [M − Cl]+ with the characteristic isotopic distribution were detected. The FTIR spectra of 11−26 indicated the availability of the NCN moiety. Thus, intense absorption at 1590−1550 cm−1 was assigned to ν(NCN), and corresponding ν(N−H) bands emerge in the range of 3460−3240 cm−1. The FTIR spectra of 11−18 display one strong ν(CN) stretching vibration in the range between 2218 and 2179 cm−1 due to one unreacted isocyanide ligand, whereas starting complexes 1−8 display two overlapped ν(CN) bands in the interval 2270− 2200 cm−1.66,67 The FTIR spectra of 19−26 show no bands in the range between 2300 and 2190 cm−1 corresponding to ν(CN) collaterally confirming the transformation of both isocyanide ligands into the aminocarbenes.6 NMR Study of Isomeric Mixtures. All regioisomers/ tautomers/configuration isomers, which are possible for the studied complexes are represented in Scheme 3. The regioisomeric ratio was not changed when complexes 11−18 were left to stand in CDCl3 for 2 days at 40 °C. In order to find out which of these configuration isomers actually exist in CDCl3 solutions at room temperature, we have recorded and analyzed the 1H,1H-NOESY NMR spectra of 13, 14, and 18, as well as 1 15 H, N-HSQC and 1H,15N-HMBC NMR spectra of 14, parts of which are shown in Figures 1−4. In all cases, from the experimental NMR spectra it is hard to decide whether the molecules exist in c1 or in c2 configuration isomeric form in solution and therefore below we omit this part of the nomenclature. In the spectrum of 13 (Figure 1), we assign the signal at δ 4.35 to the NH2 protons of regioisomer R1, because this signal exhibits cross-peaks with the CH signals of the phenyl moieties and no cross-peaks with Cy’s. Besides, similar chemical shifts have been previously observed for the structurally analogous guanidinato moiety of the platinum(II) complex [PtCl2{H2NC(NPh)2}{NHC(Et)NC(NHPh)NPh}].64 The other two

under the reaction conditions that proceeds with the splitting of the CN bond of the aminocarbene fragment. In other words, this is experimental evidence favoring the reversibility of the formation of the deprotonated carbene ligands in 11−18 via palladium- or platinum-mediated reaction of isocyanides in 1−8 with DPG as N-nucleophile. Similar reversible formation of palladium diaminocarbene backbone in a solution was reported by Slaughter and co-workers for intramolecular chelate ring opening in a palladium(II) bis-carbene complex,65 and later by us for intermolecular regioisomerization of palladium binuclear aminocarbene complexes.7 An interesting feature of deprotonated bis-carbenes 19−26 is that in each case only one of the several possible tautomers is formed. Moreover, the structure of the tautomers is determined by R of RNC. In the case of aryl complexes 19, 20, 24, and 25, this is the asymmetric tautomer, whereas in the case of alkyl complexes 21−23 and 26, it is the symmetrically protonated form. We believe the reason for this dependence is first of all the different basicity of the ArN and AlkN nitrogen centers. Characterization of 11−26. Isomeric mixtures of 11−17 and complexes 18-R1τ1 and 19−26 were characterized by elemental analyses (C, H, N), HRESI+-MS, and FTIR. Complexes 13-R1, 14-R2τ1c1, 18-R2τ1, 19, 22, 24, and 25 were additionally studied by single-crystal X-ray diffraction. In solution, all equilibria associated with 11−18 were verified by 1D (1H and 13C{1H}) and 2D (1H,1H-NOESY; 1H,15N-HSQC; 1 15 H, N-HMBC) NMR spectroscopies (“NMR Study of Isomeric Mixtures”). In the NMR spectra of all studied species, apart from the discussed main peaks, some extra signals of low intensity were observed. These signals are presumably from some minor isomeric forms as HRESI-MS and elemental analyses are not affected by the presence of these minor species. The solid isomeric mixtures of 11−17, complexes 18-R2τ1 and 19−26 gave satisfactory microanalyses (C, H, N) for the D


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Figure 2. 1H,1H-NOESY NMR spectrum of a solution of 14 in CDCl3 showing two forms of 14-R2τ1 isomers (labeled as “major” and “minor”).

Figure 3. 1H,15N-HSQC (blue) and 1H,15N-HMBC (red) NMR spectra of a solution of 14 in CDCl3, confirming that both major and minor forms correspond to 14-R2τ1 isomers.

NH signals of equal intensity at δ 5.5 and 5.9 show cross-peaks with the aromatic CH signals and neither of them has crosspeaks with the CH signals of Cy’s, which is consistent with the R2τ2 structure. The intensity ratio of R1/R2τ2 is close to unity. Exchange peaks with the residual water are pronounced for the NH signals at δ 5.9 and 4.35, which suggests that these protons are structurally similar and allows the completion of

the assignment as shown in Figure 1 (Ha and Hb protons). An exchange between the Ha proton of R2τ2 and the NH2 protons of R1 is evidenced by the corresponding cross-peak. In the spectrum of 14 (Figure 2), two pairs of NH signals, viz., a pair at δ 5.4 and 6.2 (labeled as “major form” in Figure 2), and a 3-fold less intense pair of signals at δ 6.0 and 7.6 (labeled as “minor form”) were observed. The partial assignment of these E


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Organometallics

Figure 4. 1H,1H-NOESY NMR spectrum of a solution of 18 in CDCl3 showing a pattern consistent with 18-R2τ1 isomer.

Table 1. Coupling between the Isocyanide Ligands and 4 equiv of DPG (CHCl3, Reflux), Complexes, and Their Numbering regioisomer R2

staring complexes (nos.) 1 2 3 4 5 6 7 8

(M (M (M (M (M (M (M (M

= = = = = = = =

Pd, R = Xyl) Pd, R = 2Cl-6MeC6H3) Pd, R = Cy) Pd, R = t-Bu) Pd, R = tmbu) Pt, R = Xyl) Pt, R = 2Cl-6MeC6H3) Pt, R = Cy)

coupling product (nos.)

regioisomer R1

tautomer τ1

11 12 13 14 15 16 17 18

11-R1 12-R1 13-R1 not detected not detected 16-R1 17-R1 not detected

11-R2τ1 12-R2τ1 not detected 14-R2τ1c1 and 14-R2τ1c2 15-R2τ1c1 and 15-R2τ1c2 16-R2τ1 17-R2τ1 18-R2τ1

ratio between the detected tautomer τ2 forms in the solutions not detected not detected 13-R2τ2 not detected not detected not detected not detected not detected

4:1 3:1 1:1 2:1 2:1 3:1 2:1

1

overall H NMR yield

isolated yielda

98% 96% 99% 95% 95% 98% 94% 90%

89% 64% 74% 76% 70% 73% 64% 68%

a

The differences between 1H NMR and isolated yields are mostly because of the difficulties in separation of the target complexes and DPG·HCl exhibiting similar solubilities.

nitrogens and the other with the NH−t-Bu nitrogens. From the available data it is difficult to judge by which conformational change the major and minor forms differ. Tentatively, one can speculate that these forms correspond to c1 and c2 isomers. This could explain the large difference in the chemical shifts of Hb protons, as in c1 this proton should be deshielded by the neighboring CN triple bond. However, this assignment would require some additional evidence. Finally, in the spectrum of 18 (Figure 4) two NH signals of equal intensity at δ 5.6 and 6.3 were observed. The former resonance is split into a doublet and has cross-peaks with the CH/CH2 signals of the Cy moieties and no cross-peaks with the CH resonances of Ph’s. In contrast, the NH peak at δ 6.3 has cross-peaks with the aromatic signals and no cross-peaks with the CH/CH2 resonances of Cy’s. All these findings are consistent with the R2τ1 structure. Other NH signals, which could be attributed to regioisomer R1, were not observed.

signals can be done on the basis of presence/absence of crosspeaks with various CH proton signals. Within each pair of the NH signals, one of them, Hb, has a cross-peak with the t-Bu proton resonance, while the other one, Ha, has a cross-peak with the aromatic CHs, which suggests the 14-R2τ1 form. Interestingly, the chemical shifts of Hb protons are quite different, viz., δ 7.6 and 5.4. Moreover, the addition of trifluoroacetic acid to a CDCl3 solution of chelated complex 14 led to disappearance of the resonances of the imine protons of both conformers in the range δ 5.4−6.3 and appearance of two singlets at ca. δ 10, characteristic of diaminocarbene complexes. The assignment of both forms to 14-R2τ1 isomer is further supported by the 1H,15N-HSQC and 1H,15N-HMBC spectra shown in Figure 3 (in blue and in red, respectively). Indeed, the NH signals of the major and minor forms exhibit very similar patterns of cross-peaks: one cross-peak with the NH−Ph F


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Figure 5. View of two different regioisomers, viz., 13-R1 (left) and 18-R2τ1 (right), with the atomic numbering schemes. Thermal ellipsoids are drawn with the 50% probability. Hydrogen labels are omitted for simplicity. Selected bond lengths (Å) and angles (deg) for 13-R1: Pd1−Cl1 2.4010(9), Pd1−N4 2.044(3), Pd1−C1 1.995(4), Pd1−C21 1.957(4), N5−C21 1.145(6), N5−C22 1.453(5), N4−C8 1.305(5), N4−C15 1.435(5), C8−N3 1.354(5), C1−N1 1.271(5), C1−N2 1.454(5), N1−C2 1.447(5), N2−C8 1.370(5), N2−C9 1.435(5), C1−Pd1−N4 80.64(14), C21−N5− C22 174.3(4), C8−N4−Pd1 114.0(2), N1−C1−N2 114.4(3), C1−N1−C2 123.2(3), N5−C21−Pd1 168.9(4), C8−N2−C1 116.5(3), N4−C8−N2 116.8(3), C10−C9−N2 120.9(4); for 18-R2τ1: Pt1−Cl1 2.3805(12), Pt1−N4 2.050(5), Pt1−C1 1.987(6), Pt1−C21 1.904(6), C8−N4 1.346(7), C8−N2 1.340(7), C8−N3 1.367(7), N4−C15 1.406(7), N2−C1 1.332(7), N1−C1 1.331(7), N1−C2 1.467(7), N3−C9 1.418(8), C22−N5 1.452(7), N5−C21 1.156(9), C1−Pt1−N4 77.8(2), N4−C8−N3 119.4(5), N2−C8−N4 121.5(5), N2−C8−N3 119.1(5), C8−N4−Pt1 110.2(4), C1−N2−C8 111.7(5), C1−N1−C2 123.7(5), C8−N3−C9 130.0(5), N1−C1−N2 117.9(5), C21−N5−C22 170.1(6), N5−C21−Pt1 173.3(5).

The results of our analysis of the NMR spectra of 13, 14, and 18 (Figures 1−4) as well as the results of similar analysis of the NMR spectra of 15−17 (spectra not shown) are listed in Table 1. X-ray Diffraction Studies. Structure of the Deprotonated Monocarbene Complexes. The crystallographic data and processing parameters for 13-R1, 14-R2τ1, and 18-R2τ1 are listed in Table S1; the plots for the structures can be found in Figures 5 and 6. Bond lengths and angles are given in the figure captions. In the crystals structure of each of the deprotonated monocarbene species, the almost undistorted square-planar environment around the metal center is completed with one chloride, one deprotonated aminocarbene species, which acts as a bidentate ligand forming the five-membered C,N-chelated ring, and one unreacted isocyanide ligand. The unreacted isocyanide is located in the cis position to the carbon atom from the deprotonated aminocarbene ligand. In 14-R2τ1, and 18-R2τ1, the exocyclic and endocyclic C−N bonds of the metallacycles exhibit similar lengths and lie between single CN bond (e.g., 1.469(10) Å in amines)68 and double CN bond (e.g., 1.279(8) Å in imines)68 distances. In 13-R1, the C−N bonds are different (i.e., C1−N1 is a double CN bond, C1−N2, C9−N2, and C15−N4 is a single CN bonds, and C8−N2, C8−N3, C8−N4 exhibit intermediate values between single and double CN bond distances). In the isocyanide ligand, the CN has a normal value for the triple CN bond in various (RNC)MII (M = Pd and Pt).69,70 The X-ray powder diffraction data for 14 fully agree with the calculated powder diffraction data for the single-crystal structure of 14-R2τ1 thus suggesting homogeneity of the sample. Structure of the Deprotonated Bis-carbene Complexes. The crystallographic data and processing parameters for complexes 19, 22, 24, and 25 are listed in Table S1, the plots for the structures can be found in Figures 7, 8, and S1, and bond lengths and angles are given as in Figure 8’s caption (22) and in

Figure 6. View of 14-R2τ1c1 with the atomic numbering schemes. Thermal ellipsoids are drawn with the 50% probability. Hydrogen labels are omitted for simplicity. Selected bond lengths (Å) and angles (deg): Pd1−Cl1 2.3631(6), Pd1−N4 2.0225(19), Pd1−C1 1.979(2), Pd1−C19 1.940(2), N1−C1 1.328(3), N1−C2 1.492(3), N2−C1 1.332(3), N2−C6 1.353(3), N3−C6 1.361(3), N3−C7 1.409(3), N4−C6 1.323(3), N4−C13 1.429(3), N5−C19 1.147(3), N5−C20 1.460(3), C1−Pd1−N4 78.19(8), C1−N1−C2 129.1(2), C1−N2−C6 111.79(18), C6−N3−C7 131.7(2), C6−N4−Pd1 111.66(15), C19−N5−C20 177.1(2), N1−C1−N2 120.0(2), N5−C19−Pd1 174.7(2), N4−C6−N2 120.6(2).

Table 3 (19, 24). Note that in the solid phase as in the solution 22 features two identical chelated rings with the “true” carbene fragments displaying the equal CN bond distances whereas complexes 19, 24, and 25 form structures with the asymmetrically G


Article

Organometallics

Figure 7. Views of Pd complex 19 (left) and Pt complex 24 (right) with the atomic numbering schemes. Thermal ellipsoids are drawn with the 50% probability. Hydrogen labels are omitted for simplicity.

Figure 8. View of Pd complex 22 with the atomic numbering scheme. Thermal ellipsoids are drawn with the 50% probability. Hydrogen labels are omitted for simplicity. Selected bond lengths (Å) and angles (deg): Pd1−C14 1.9902(18), Pd1−N1 2.0967(16), N1−C7 1.322(2), N1−C1 1.421(2), N2−C7 1.366(3), N2−C8 1.409(3), N3−C14 1.328(3), N3−C7 1.365(2), N4−C14 1.344(2), N4−C15 1.489(2), C14−Pd1−N1 77.11(7), C7−N1−C1 121.64(16), C7−N1−Pd1 109.71(12), C7−N2−C8 129.40(17), C14−N3−C7 111.71(16), C14−N4−C15 129.04(17).

(N−H 2.74 Å, N−N 3.28 Å) or even Bondi’s72 vdW radii (N−H 2.75 Å, N−N 3.10 Å). At the same time, the solid-state structure of 22 displays two weaker intramolecular hydrogen bonds symmetrically located between two deprotonated diaminocarbene ligands (N4···N4 distance 3.117(3) Å, N4···H4 distance 2.60(2) Å). The corresponding angles in all cases are in range 124.2(17)−156(2)° and these contacts could be interpreted as hydrogen bond accordingly to the IUPAC criterion.73 In addition, in structures of all deprotonated biscarbene complexes 19, 22, 24, and 25, the distances between centroids of the phenyl rings by the nitrogens connected to the metal-center are less than the sum of two Rowland’s vdW radii of carbon which suggests the presence of π−π stacking between these phenyl rings.74 Parameters of the π-stacking are given in Table S2. Hirshfeld Surface Analysis for X-ray Structures of the Deprotonated Mono- and Bis-carbene Complexes. As shown above, many of the features of the studied complexes are due to their configuration isomerism accompanied by prototropic tautomerism, which in turn are due to the presence of intramolecular hydrogen bonds. The Hirshfeld surface analysis for the obtained X-ray structures of the deprotonated mono- and bis-carbene complexes (except disordered 20) reveal that in all studied cases the crystal packing is determined mainly by van der Waals forces and π−π stacking rather than intermolecular hydrogen bonding (or other localized specific intermolecular interactions). The contributions of intermolecular N−H hydrogen bonding to the molecular Hirshfeld surfaces in all studied cases are small (3−6%). For the deprotonated monocarbene complexes, the observed noticeable contributions of

Table 2. Coupling between the Isocyanide Ligands and 4 equiv of DPG (CHCl3, Reflux), Complexes, and Their Numbering staring complexes (nos.) 1 2 3 4 5 6 7 8

(M (M (M (M (M (M (M (M

= = = = = = = =

Pd, R = Xyl) Pd, R = 2Cl-6MeC6H3) Pd, R = Cy) Pd, R = t-Bu) Pd, R = tmbu) Pt, R = Xyl) Pt, R = 2Cl-6MeC6H3) Pt, R = Cy)

coupling product (nos.) 19 20 21 22 23 24 25 26

1

overall H NMR yield

isolated yielda

98 95 98 94 94 97 90 85

74% 57% 52% 68% 70% 76% 55% 43%

a

The differences between 1H NMR and isolated yields are mostly because of the difficulties in separation of the target complexes and DPG·HCl exhibiting similar solubilities

protonated nitrogen atoms. In 19, 24, and 25, C−N bonds of the first NCN fragments have similar lengths and their values lie between single and double CN bonds distance (see above). The second NCN fragments exhibit different CN bond distances, viz., as one single and one double. In the structures of 19, 24, and 25, the first NCN fragment is involved in strong intramolecular hydrogen bond N3−H3···N7 with the NR group of the other NCN moiety thus forming a six-membered cycle. The distances between the N7 and H3 atoms (19: 1.99(3) Å; 24: 1.96(3) Å; 25: 1.968(3) Å) and between N3 and N7 (19: 2.764(2) Å; 24: 2.769(3) Å; 25: 2.761(4) Å) atoms are substantially less than the sum of their Rowland’s71 H


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Organometallics

compounds. DPG behaves as nucleophile, deprotonating base, and chelator. The addition of DPG proceeded with different regioselectivity depending on the substituent R in RNCs. In the first stage, the reaction with the CNAr and CNAlk ligands led to different regioisomers of the monochelate complexes formed as major products. The bis-chelate species generated on the next step are different tautomers in case of CNAr and CNAlk. Concurrently, replacement of palladium to platinum center leads to similar products but with quantitative differences in the isomeric ratios (except the reduction of the PtII centers in 9 and 10). It is important to point out that examples for the metal-mediated regioselective addition to CN triple bond of nitriles RCN and isocyanides RNC are scarce, and the formation of two different regioisomers was observed only for the addition of DPG to metal-bound dialkylcyanamides, R2NCN64 and upon the metal-mediated coupling of RNC with N-phenylbenzamidine.44 Second, we analyzed X-ray structures of the deprotonated mono- and bis-carbene complexes and confirmed the regioisomerism of these species and verified ADC protolytic forms stabilized in the solid-state. We studied the obtained systems in solutions by 1D (1H and 13C{1H}) and 2D (1H,1H-NOESY; 1 15 H, N-HSQC; 1H,15N-HMBC) NMR spectroscopies and observed their configuration isomerism accompanied by prototropic tautomerism. Together, the solid-state and solution data provide an insight into the flexible character of ADC species. Our data collaterally support the idea that outstanding performance of ADC-based metal catalysts over their NHC analogs, reported in many instances, is due to rotational and acid−base flexibility of ADCs, and these properties together fulfill the catalyst steric contradictory requirements at various steps of catalytic cycles, although additional studies on a wider range of examples to attest to this are still required. We are also exploring the catalytic properties of the deprotonated carbenes derived from reactions of metal-activated RNCs and DPG in order to develop useful catalytic systems and to establish catalyst structure−activity relationships in various cross-coupling systems.

Table 3. Selected Bond Lengths (Å) and Angles (deg) for 19, 24, and 25 19 M1−C14 M1−C36 M1−N1 M1−N5 N1−C7 N1−C1 N2−C7 N2−C14 N3−C14 N3−C15 N4−C7 N4−C8 N5−C29 N5−C23 N6−C29 N6−C36 N7−C36 N8−C29 N8−C30 C14−M1−N1 C36−M1−N5 C7−N1−C1 C7−N1−M1 C7−N2−C14 C14−N3−C15 C7−N4−C8 C29−N5−C23 C29−N5−M1 C29−N6−C36 C36−N7−C37 C29−N8−C30 N1−C7−N2 N2−C14−M1 N5−C29−N6 N6−C36−M1

24

Bond Lengths (Å) 1.9795(18) 1.964(2) 1.9863(17) 1.979(2) 2.0964(14) 2.0925(18) 2.1200(14) 2.1048(19) 1.324(2) 1.326(3) 1.424(2) 1.433(3) 1.352(2) 1.342(3) 1.339(2) 1.346(3) 1.327(2) 1.334(3) 1.434(2) 1.432(3) 1.369(2) 1.370(3) 1.412(2) 1.414(3) 1.304(2) 1.308(3) 1.426(2) 1.436(3) 1.363(2) 1.364(3) 1.411(2) 1.408(3) 1.282(2) 1.288(3) 1.359(2) 1.351(3) 1.432(2) 1.434(3) Angles (deg) 77.75(6) 77.15(8) 80.70(6) 80.10(8) 119.08(14) 119.11(18) 109.27(11) 109.90(15) 111.56(15) 110.90(19) 125.49(15) 125.7(2) 130.31(16) 129.9(2) 119.40(15) 118.46(19) 111.49(12) 112.55(16) 119.50(15) 118.64(19) 120.70(15) 119.8(2) 124.63(16) 124.7(2) 122.57(15) 122.2(2) 118.54(12) 119.57(17) 117.50(16) 116.8(2) 110.80(12) 111.91(16)

25 1.973(3) 1.980(3) 2.093(2) 2.112(2) 1.336(4) 1.424(4) 1.370(4) 1.336(4) 1.336(4) 1.423(4) 1.370(4) 1.411(4) 1.308(4) 1.434(4) 1.357(4) 1.407(4) 1.292(4) 1.357(4) 1.436(4) 77.13(11) 79.92(11) 119.3(2) 109.92(19) 111.1(2) 125.2(3) 129.4(3) 119.0(2) 112.3(2) 118.9(3) 120.4(3) 124.0(3) 121.8(3) 119.9(2) 116.9(3) 111.9(2)

■

Cl−H intermolecular contacts to the molecular Hirshfeld surfaces (9−10%), whereas such contributions are absent for the deprotonated bis-carbene complexes. It can be concluded that the N−H hydrogen bonding, which must inevitably be present in these deprotonated mono- and bis-carbene species, are mainly intramolecular in all studied cases. For details of the Hirshfeld surface analysis for X-ray structures of the complexes see Table S3 and Figures S2−S8).

EXPERIMENTAL SECTION

Materials and Instrumentation. Solvents, PdCl2, K2[PtCl4], all isocyanides, and DPG were obtained from commercial sources and used as received, whereas CH2Cl2 and CHCl3 were purified by the conventional distillation over calcium chloride. Complexes 1−8 were prepared by the known procedures.22,54,67 C, H, and N elemental analyses were carried out on a Euro EA3028-HT. Mass spectra were obtained on a Bruker micrOTOF spectrometer equipped with electrospray ionization (ESI) source. MeOH, MeCN, or MeOH/ DMSO mixture were used as solvents. The instrument was operated both at positive and negative ionization modes using m/z range of 50−3000. The capillary voltage of the ion source was set at −4500 V (ESI+) or 3500 V (ESI−) and the capillary exit at ±(70−150) V. The nebulizer gas pressure was 0.4 bar and drying gas flow 4.0 L/min. The most intensive peak in the isotopic pattern is reported. Infrared spectra (4000−400 cm−1) were recorded on a Shimadzu FTIR 8400S instrument and a PerkinElmer Spectrum BX FT-IR spectrometer in KBr pellets. 1D (1H, 13C{1H}) NMR spectra and 2D (1H,1H-NOESY; 1 15 H, N-HSQC; 1H,15N-HMBC) NMR correlation experiments were recorded on Bruker Avance III 400 MHz spectrometer (400.13 MHz for 1H, 100.61 MHz for 13C). The spectra were measured at 298 ± 1 K (for 11−18 and 26) and low temperatures (218 K for 19, 21, and 24; 223 K for 25; 233 K for 20) using CDCl3 (Sigma-Aldrich, USA) as solvent and referenced to tetramethylsilane as external standard.

■

CONCLUSIONS The results of this work can be considered from the following perspectives. First, in the course of this study, we have observed a novel metal-mediated coupling between N,N′-diphenylguanidine (DPG), taken as the polyfunctional nucleophile, and the CN triple bond of an isocyanide in the palladium(II) and platinum(II) complexes. This coupling led to the guanidinederived metal-bound deprotonated acyclic diaminocarbene species. The integration of DPG and RNC occurs via the two-step process, involving the initial coupling of the guanidine with one of the isocyanides and leading to the deprotonated monocarbene monochelated species, while the next addition grants the deprotonated bis-carbene bis-chelated metal I


Article

Organometallics

solvent), 7.09−7.04 (m, 1.4Ha+b), 6.95−6.90 (m, 2.7Ha+b), 6.65−6.61 (m, 2.7Ha+b), 6.09 (t, 0.3H, J = 7.5, Hb), 6.04 (t, 1.3H, J = 7.5, Ha) (Ar’s); 5.65 (s, 0.2H, NHb), 4.52 (s, 2H, NH2a), 2.22 (s, 1.7H, 2Meb), 2.21 (s, 6.0H, 2Mea), 2.13 (s, 6.0H, 2Mea), 2.11 (s, 1.7H, 2Meb). 13 C{1H} NMR (CDCl3, δ): 160.63, 160.44, 159.17, 157.86, 150.68, 150.48, 143.27, 139.25, 138.10, 137.48, 134.84, 134.20, 130.26, 130.23, 130.19, 129.95, 129.80, 129.55, 129.45, 129.29, 129.20, 128.73, 128.63, 128.17, 127.90, 127.66, 127.55, 127.29, 127.27, 127.24, 126.48, 125.70, 125.63, 122.98, 122.89 (CN, CN, and C from Ar), 19.25 and 18.54 (Mea); 19.20 and 18.50 (Meb). Mixture of 12-R1 (a) and 12-R2τ1 (b).

Ultrasound irradiation was performed at ultrasonic bath Transsonic 460/H (with a frequency of 55 kHz and HF peak 170 W). X-ray Structure Determinations. The crystals of 13-R1, 14-R2τ1, 18-R2τ1, 19, 22, 24, and 25 were immersed in cryo-oil, mounted in a MiTeGen or Nylon loop, and measured at 100−170 K on a Bruker SMART APEX II, Bruker Kappa Apex II Duo, Rigaku Oxford Diffraction Supernova, or on a Bruker Kappa Apex II diffractometer using Mo Kα (λ = 0.71073) or Cu Kα (λ = 1.54184) radiation. The Apex2,75 CrysAlisPro,76 or Denzo-Scalepack77 program packages were used for cell refinements and data reductions. Multiscan, numerical, or analytical absorption corrections (SADABS78 or CrysAlisPro)76 were applied to the intensities before structure solution. The structures were solved by direct methods or charge flipping method using the SHELXS-9779 or SUPERFLIP80 software including using the OLEX2 program complex.81 Structural refinements were carried out using SHELXL79 software. In 25, the pairs of methyl and chlorine ring substituents were disordered over alternative sites with occupancy ratios of 0.43/0.57 and 0.54/0.46. A series of both geometric and displacement constraints and restraints were applied to these disordered moieties. The N−H hydrogen atoms in 14-R2τ1, 19, 22, 24, and 25 were located from the difference Fourier map and refined isotropically. All other H atoms were positioned geometrically and constrained to ride on their parent atoms, with C−H = 0.93−0.98 Å, N−H = 0.86 Å, and Uiso = 1.2−1.5Ueq (parent atom). The crystallographic details are summarized in Table S1. The structural information has been deposited in CCDC 1541820−1541823, 1542913, 1542914, and 1551262 and are available in the database of organic compound crystal structures at the site http://www.ccdc.cam.ac.uk/data_request/cif. Computational Details. The Hirshfeld molecular surface was generated by CrystalExplorer 3.1 program82,83 based on the results of the X-ray study for structure with the best (lowest) R-factor, viz., 18-R2τ1. The normalized contact distances, dnorm,84 based on Bondi’s van der Waals radii,72 were mapped into the Hirshfeld surface. On Figures S2−S8 in the color scale, negative values of dnorm are visualized by the red color indicating contacts shorter than the sum of van der Waals radii. The white color denotes intermolecular distances close to van der Waals contacts with dnorm equal to zero. In turn, contacts longer than the sum of van der Waals radii with positive dnorm values are colored with blue. Synthetic Work. General Procedure for the Reaction of 1−8 with DPG (2 equiv). Solid DPG (53 mg, 0.250 mmol) was added to a solution (3, 4, and 8) or a suspension (1, 2, and 5−7) of cis-[MCl2(CNR)2] (0.125 mmol) in chloroform (2 mL) in air at RT. The reaction mixture was ultrasound-treated for 1−2 min until formation of a homogeneous solution was observed. The solution was then evaporated to dryness in air at RT. The oily residue was placed under a layer of hexane (2 mL) and rubbed with a glass wand until crystallization. The target product was extracted with five 2 mL portion of Et2O and dried in air at RT. No other purifications were needed to obtain analytically pure material. Mixture of 11-R1 (a) and 11-R2τ1 (b).

Yield 52 mg, 64%. Calcd (%) for C29H24N5Cl3Pd: C 53.15, H 3.69, N 10.69; Found: C 54.10, H 3.78, N 11.05. HRESI+-MS, m/z: 618.0417 [M − Cl]+ [618.0438 calcd] IR (KBr, selected bands, cm−1): 3467 (w), 3395 (w), ν(N−H); 2927 (m-w) ν(C−H from Ar and Me); 2184 (s), ν(CN); 1646 (s), 1618 (vs), 1541 (s), 1486 (s), ν(CN and CC from Ar); 760 (m), 700 (m), δ(C−H from Ar). 1H NMR (CDCl3, δ): 7.57−7.15 (m, 20Ha+b, NHb and CHCl3 from the solvent), 7.05−7.02 (m, 2.6Ha+b), 6.90−6.82 (m, 2.9Ha+b), 6.76−6.66 (m, 2.5Ha+b), 6.10 (t, 0.5H, J = 7.8, Hb), 6.05 (t, 0.9H, J = 7.8, Ha) (Ar’s); 5.70 (s, 0.5H, NHb), 4.56 (s, 1.9H, NH2a), 4.06 (s, 0.5H, NHb), 2.34 (s, 1.2H, Meb), 2.33 (s, 3H, Mea), 2.18 (s, 3H, Mea), 2.16 (s, 1.2H, Meb). 13C{1H} NMR (CDCl3, δ): 163.02 (CN); 159.37, 158.10, 148.87, 148.63, 143.01, 137.79, 137.17, 137.00, 136.90, 134.64, 130.81, 130.73, 130.31, 130.17, 130.10, 130.06, 130.00, 129.65, 129.58, 129.56, 129.44, 129.34, 128.19, 128.17, 128.12, 128.05, 128.00, 127.24, 126.98, 126.87, 126.64, 126.60, 125.77, 125.56, 125.42, 125.30, 125.17, 123.16, 123.02 (C from Ar); 19.42 and 19.00 (Mea); 19.37 and 18.93 (Meb). Mixture of 13-R1 (a) and 13-R2τ2 (b).

Yield 52 mg, 74%. Calcd (%) for C27H34N5ClPd: C 56.85, H 6.01, N 12.28; found: C 57.38, H 6.01, N 13.38. HRESI+-MS, m/z: 570.1615 [M + H]+ [570.1610 calcd]; 534.1864 [M − Cl]+ [534.1844 calcd]. IR (KBr, selected bands, cm−1): 3146 (w), ν(N−H); 2932 (s), 2858 (m), ν(C−H from Ar and Cy); 2205 (s), ν(CN); 1647 (vs), 1615 (vs), 1541 (s), 1496 (s), ν(CN and CC from Ar); 1450 (m), δ(H−C−H from Cy); 731 (vs) δ(C−H from Ar). 1H NMR (CDCl3, δ): 7.49−7.20 (m, 20H, 20Ha+b), 7.15−7.13 (m, 2H, 2Ha+b) (Ar’s); 5.89 (s, br, 1H, NHb), 5.51 (s, br, 1H, NHb), 4.36 (s, br, 2H, NH2a), 3.90−3.86 (m, 2H, CH, Cya), 3.03−2.98 (m, 1H, CH, Cyb), 1.97−1.21 (m, 48H, CH2, Cya+b). 13C{1H} NMR (CDCl3, δ): 158.19 (2C, CN); 156.93, 154.87, 154.28, 143.71, 138.98, 138.46, 132.29, 131.13, 130.33, 130.11, 129.90, 129.65, 129.41, 129.21, 129.10, 128.51, 128.38, 128.32 (C from Ar); 63.84, 63.77, 54.81, 32.13, 32.06, 31.72, 31.64, 25.91, 24.83, 24.81, 24.66, 24.59, 24.03, 24.01, 22.83, 22.73, 22.44, 22.32 (Cy). Crystals of 13-R1 suitable for X-ray diffraction study were obtained by slow evaporation of a Et2O solution of mixture of 13-R1 and 13-R2τ2 at 20−25 °C.

Yield 68 mg, 89%. Calcd (%) for C31H30N5ClPd: C 60.59, H 4.92, N 11.40; Found: C 60.60, H 5.23, N 10.94. HRESI+-MS, m/z: 614.1285 [M + H]+ [614.1297 calcd]. IR (KBr, selected bands, cm−1): 3462 (vs), 3393 (m), ν(N−H); 3061 (m-w), 3037 (m-w), 2923 (m-w), 2917 (m-w), 2852 (w), ν(C−H from Ar and Me); 2185 (s), ν(CN); 1637 (s), 1616 (vs), 1589 (s), 1560 (s), 1489 (s), ν(CN and CC from Ar); 756 (m), 696 (m), δ(C−H from Ar). 1H NMR (CDCl3, δ): 7.55−7.19 (m, 15Ha+b, NHb and CHCl3 from the J


Article

Organometallics 14-R2τ1c1.

[M + H]+ [703.1910 calcd], 666.2080 [M − Cl]+ [666.2123 calcd]. IR (KBr, selected bands, cm−1): 3464 (m), 3393 (m), ν(N−H); 3061 (m-w), 3037 (m-w), 2934 (m-w), 2917 (m-w), 2849 (w), ν(C−H from Ar and Me); 2179 (vs), ν(CN); 1633 (s), 1610 (vs), 1590 (s), 1555 (vs), 1492 (s), ν(CN and CC from Ar); 760 (m), 696 (m), δ(C−H from Ar).1H NMR (CDCl3, δ): 7.56−7.21 (m, 18.1Ha+b, NHb and CHCl3 from the solvent), 7.06−7.02 (m, 1.7Ha+b), 6.92 (t, J = 9.0, 3.1Ha+b), 6.61−6.57 (m, 2.7Ha+b), 6.04−5.96 (m, 1.8H, Ha+b) (Ar’s); 4.56 (s, 2H, NH2a), 2.23 (s, 2.0H, 2Mea), 2.22 (s, 6.0H, 2Meb), 2.14 (s, 6H, 2Meb), 2.12 (s, 2.0H, 2Mea). 13C{1H} NMR (CDCl3, δ): 160.81, 159.92 (CN); 154.78, 150.90, 150.80, 150.20, 149.33, 142.09, 137.81, 137.14, 134.43, 134.29, 133.53, 130.36, 130.27, 130.23, 129.99, 129.60, 129.43, 129.30, 129.17, 128.33, 128.27, 128.21, 127.78, 127.30, 127.27, 127.22, 127.20, 127.12, 127.05, 125.92, 125.73, 122.50, 122.38 (C from Ar); 19.16 and 18.48 (Mea); 19.11 and 18.43 (Meb). Mixture of 17-R1 (a) and 17-R2τ1 (b).

Yield 49 mg, 76%. Calcd (%) for C23H30N5ClPd: C 53.29, H 5.83, N 13.51; Found: C 53.75, H 6.05, N 13.32. HRESI+-MS, m/z: 482.1518 [M − Cl]+ [482.1531 calcd]. IR (KBr, selected bands, cm−1): 3456 (m), 3402 (s), ν(N−H); 2966 (m), 2924 (m-w), ν(C−H from Ar and Me); 2218 (m), ν(CN); 1601 (s), 1562 (vs), 1497 (m-w), 1416 (vs), ν(CN and CC from Ar); 756 (m-w), 696 (m-w), δ(C−H from Ar). 1H NMR (mixture of 14-R2τ1c1 (a) and 14-R2τ1c2 (b)) (CDCl3, δ): 7.62 (s, br, 0.5H, NHb), 7.41 (t, J = 7.7, 2.6H, Ara+b), 7.32 (dd, J = 8.3, 10.3, 2H, Ara+b), 7.24 (dd, J = 6.0, 7.9, 7.1H, Ara+b), 7.07−6.99 (m, 1.4H, Ara+b), 6.23 (s, br, 0.8H, NHa), 6.04 (s, br, 0.4H, NHb), 5.44 (s, br, 0.8H, NHa), 1.59 (s, 9H, t-Bua), 1.44 (s, 11.4H, t-Bua+b), 1.10 (s, 3.8H, t-Bub). 13C{1H} NMR (mixture of 14-R2τ1c1 and 14-R2τ1c2) (CDCl3, δ): 184.82 (2C, Ccarbene); 168.95 (CN); 147.81, 143.81, 139.82, 139.46, 130.24, 129.69, 129.22, 128.36, 127.51, 127.19, 126.02, 125.88, 125.64, 123.75, 123.25, 121.75, 120.75 (C from Ar); 55.31, 54.05, 30.22, 29.67, 29.03, 28.76 (t-Bu). Crystals of 14-R2τ1c1 suitable for X-ray diffraction study were obtained by slow evaporation of a Et2O solution at 20−25 °C. Mixture of 15-R2τ1c1 (a) and 15-R2τ1c2 (b).

Yield 60 mg, 64%. Calcd (%) for C29H24N5Cl3Pt: C 46.82, H 3.25, N 14.29; Found: C 47.61, H 3.75, N 14.15. HRESI+-MS, m/z: 707.1025 [M − Cl]+ [707.1052 calcd], 743.0787 [M + H]+ [743.0818 calcd]. IR (KBr, selected bands, cm−1): 3467 (w), 3395 (w), ν(N−H); 2925 (s) ν(C−H from Ar and Me); 2183 (s), ν(CN); 1620 (vs), 1595 (s), 1562 (s), 1489 (s), ν(CN and CC from Ar); 756 (m), 695 (m), δ(C−H from Ar). 1H NMR (CDCl3, δ): 7.54−6.69 (m, 35Ha+b), 6.10−6.01 (m, 2.1H, Ha+b) (Ar’s), 5.68 (s, 0.6H, NHb), 4.54 (s, 1.9H, NH2a), 2.32 (s, 1.8H, Meb), 2.30 (s, 3H, Mea), 2.15 (s, 3H, Mea), 2.14 (s, 1.8H, Meb). 13C{1H} NMR (CDCl3, δ): 160.24 (CN); 148.96, 141.80, 130.41, 130.37, 130.13, 129.89, 129.58, 129.35, 129.14, 128.49, 128.36, 128.18, 127.98, 127.2, 126.86, 126.76, 126.58, 125.98, 125.85, 125.16, 125.07, 125.05, 124.32, 123.92, 122.95, 122.70, 122.54, 115.41(C from Ar); 19.31 and 18.85 (Meb); 19.37 and 18.91 (Mea). 18-R2τ1.

Yield 55 mg, 70%. Calcd (%) for C31H46N5ClPd: C 59.04, H 7.35, N 11.11; Found: C 58.60, H 6.92, N 11.50. HRESI+-MS, m/z: 594.2811 [M − Cl]+ [594.2783 calcd]. IR (KBr, selected bands, cm−1): 3456 (m), 3404 (m), ν(N−H); 2951 (m-w), ν(C−H from Ar, CH2 and Me); 2201 (m-w), ν(CN); 1599 (s), 1556 (s), 1497 (m-w), 1416 (vs), ν(CN and CC from Ar); 746 (w), 696 (w), δ(C−H from Ar). 1H NMR (CDCl3, δ): 7.67 (s, br, 0.4H, NHb), 7.46−7.37 (m, 4.9H, Ara+b), 7.34− 7.28 (m, 3.3H, Ara+b), 7.25−7.20 (m, 11.6H, Ara+b), 7.10−6.98 (m, 2.9H, Ara+b), 6.21 (s, br, 1.0H, NHa), 6.04 (s, br, 0.6H, NHb), 5.45 (s, br, 0.9H, NHa), 1.87−1.46 (m, 25.7H, tmbua+b), 1.13 (s, 9H, tmbua), 1.05−1.06 (d, 16.4H, tmbua+b), 0.93 (s, 6.5H, tmbub). 13C{1H} NMR (CDCl3, δ): 185.16 (Ccarbene); 168.91, 168.72 (C=N), 147.93, 143.88, 139.90, 139.54, 130.22, 129.69, 129.16, 128.35, 128.33, 127.55, 127.20, 126.69, 125.91, 125.80, 125.66, 123.79, 122.01, 121.71, 120.84 (C from Ar); 61.59, 60.64, 59.31, 58.08, 53.55, 53.10, 52.07, 51.58, 31.87, 31.75, 31.68, 31.62, 31.10, 30.81, 30.46, 30.39, 29.69, 29.63, 29.15, 28.86 (tmbu). Mixture of 16-R1 (a) and 16-R2τ1 (b).

Yield 56 mg, 68%. Calcd (%) for C27H34N5ClPt. C 49.20, H 5.20, N 10.63; Found: C 49.61, H 5.69, N 11.12. HRESI+-MS, m/z: 623.2454 [M − Cl]+ [623.2456 calcd]. IR (KBr, selected bands, cm−1): 3403 (s, br), 3056 (w), ν(N−H); 2927 (s), 2853 (m), ν(C−H from Ar and Cy); 2206 (s), ν(CN); 1599 (s), 1555 (vs), 1491 (s), 1418 (m), ν(CN and CC from Ar); 1450 (m), δ(H−C−H from Cy); 754 (w), 697 (m-w), δ(C−H from Ar). 1H NMR (CDCl3, δ): 7.49−7.32 (m, 5H), 7.28−7.20 (m, 4H), 7.03 (t, J = 7.3, 1H) (Ar’s); 6.31 (s, br, 1H, NH), 5.64 (d, br, J = 7.4, 1H, NH), 4.15−4.06 (m, 1H, CH, Cy), 3.99−3.96 (m, 1H, CH, Cy), 2.17−1.21 (m, 22H, CH2, Cy). 13C{1H} NMR (CDCl3, δ): 174.15, 172.32 (CN); 154.93, 142.61, 139.28, 133.36, 130.41, 129.24, 128.57, 128.45, 128.06, 126.52, 125.90, 123.41, 120.75(C from Ar); 55.07, 52.37, 32.86, 32.19, 25.62, 24.85, 24.79, 22.69 (Cy). Crystals of 18-R2τ1 suitable for X-ray diffraction study were obtained by slow evaporation of a Et2O solution at 20−25 °C. General Procedure for the Reaction of 1−8 with DPG (4 equiv). Solid DPG (106 mg, 0.500 mmol) was added to a solution

Yield 64 mg, 73%. Calcd (%) for C31H30N5ClPt: C 52.95, H 4.30, N 9.96; Found: C 53.40, H 4.83, N 10.42. HRESI+-MS, m/z: 703.1867 K


Article

Organometallics

m/z: 745.2983 [M + H]+ [745.2953 calcd]. IR (KBr, selected bands, cm−1): 3406 (s), 3233 (m-w), ν(N−H); 2926 (m), 2851 (m-w), ν(C−H from Ar and Cy); 1597 (vs), 1557 (vs), 1497 (m), 1400 (vs), ν(CN and CC from Ar); 1448 (m), δ(H−C−H from Cy); 756 (w), 694 (m-w), δ(C−H from Ar). 1H NMR (CDCl3, δ): 7.31 (d, J = 7.9, 4H, Ar), 7.17 (t, J = 7.9, 4H, Ar), 6.92 (t, J = 7.3, 2H, Ar), 7.84 (m, 6H, Ar), 6.65 (dd, J = 2.8, 6.3, 4H, Ar), 6.09 (s, br, 3H, NH), 4.17−4.13 (m, 2H, CH, Cy), 2.17 (d, J = 9.2, 4H, Cy), 1.79 (dd, J = 4.0, 9.3, 4H, Cy), 1.66 (dd, J = 3.9, 8.7, 2H, Cy), 1.46−1.40 (m, J = 12.0, 23.7, 4H, Cy), 1.34−1.27 (m, 6H, 2Cy).13C{1H} NMR (CDCl3, δ): 198.74 (Ccarbene), 166.23 (CN), 145.19, 140.21, 129.11, 128.24, 125.88, 123.97, 122.36, 120.49, 51.54, 33.13, 31.59, 25.76, 24.85, 22.66. 22.

(3, 4, and 8) or a suspension (1, 2, and 5−7) of cis-[MCl2(CNR)2] (0.125 mmol) in chloroform (3 mL) in air at RT. The reaction mixture was then refluxed for 8 h (1−3, 6, and 7) or 2 days (4, 5, and 8) under vigorous stirring; the color of the mixture turned from light yellow to dark brown. The solution was then evaporated to dryness in air at RT, the rest was washed with three 2 mL portions of MeOH and dried in air at RT. No other purifications were needed to obtain analytically pure material. 19.

Yield 73 mg, 74%. Calcd (%) for C44H42N8Pd. C 66.96, H 5.36, N 14.20; Found: C 67.01, H 5.25, N 14.05. HRESI+-MS, m/z: 789.2676 [M + H]+ [789.2640 calcd]. IR (KBr, selected bands, cm−1): 3398 (m), 3367 (m), ν(N−H); 3055 (m-w), 3028 (m-w), 2918 (m-w), 2850 (m-w), 2831 (m-w), ν(C−H from Ar and Me); 1614 (vs), 1605 (vs), 1573 (vs), 1510 (s), 1448 (s), ν(CN and CC from Ar); 765 (m), 753 (m), 694 (m), δ(C−H from Ar). 1H NMR (CDCl3, δ, 218 K): 12.48 (s, NH), 7.11−6.62 (m, 26H, Ar’s, NH), 6.03 (s, br, 1H, NH), 5.83 (s, br, 1H, NH), 2.36 (s, 6H, Me), 2.15 (s, 6H, Me). 13 C{1H} NMR (CDCl3, δ, 218 K): 187.69 (Ccarbene); 166.69, 165.93, 153.17, 146.01, 145.27, 142.25, 140.41, 140.09, 136.30, 134.76, 129.91, 129.59, 129.23, 128.67, 128.47, 128.36, 127.86, 127.50, 126.52, 126.32, 126.03, 125.26, 124.11, 123.07, 121.35, 118.66, 19.80, 19.04. Crystals of 19 suitable for X-ray diffraction study were obtained by slow evaporation of a methanol/chloroform (1:1, v/v) solution at 20−25 °C. 20.

Yield 59 mg, 68%. Calcd (%) for C36H42N8Pd·1/4CHCl3. C 60.22, H 5.89, N 15.50; Found: C 60.70, H 6.01, N 15.09. HRESI+-MS, m/z: 693.2640 [M + H]+ [693.2640 calcd]. (KBr, selected bands, cm−1): 3394 (m), ν(N−H); 2962 (w), 2952 (w), 2868 (w), ν(C−H from Ar and Me); 1596 (s), 1560 (vs), 1498 (m), 1452 (m), ν(CN and CC from Ar); 743 (m), 696 (m), δ(C−H from Ar). 1H NMR (CDCl3, δ): 7.25−7.17 (m, 9H, Ar), 6.97 (t, 2H, J = 7.1, Ar), 6.87− 6.84 (m, 6H, Ar), 6.69 (dd, 4H, J = 2.2, 7.1, Ar), 6.11 (s, br, 2H, NH), 6.04 (s, br, 2H, NH), 1.51 (s, 18H, But). 13C{1H} NMR (CDCl3, δ): 199.80 (Ccarbene); 166.21 (CN), 145.45, 140.13, 129.07, 128.10, 125.94, 123.88, 122.75, 121.59 (C from Ar); 54.27, 29.35 (But). Crystals of 22 suitable for X-ray diffraction study were obtained by slow evaporation of a methanol/chloroform (1:1, v/v) solution at 20−25 °C. 23.

Yield 59 mg, 57%. Calcd (%) for C42H36N8Cl2Pd. C 60.77, H 4.37, N 13.50; Found: C 61.31, H 5.03, N 13.90. HRESI+-MS, m/z: 829.1585 [M + H]+ [829.1548 calcd]. IR (KBr, selected bands, cm−1): 3395 (m), 3362 (m), ν(N−H); 1616 (s), 1598 (s), 1572 (s), 1509 (s), 1448 (m-w), ν(CN and CC from Ar); 767 (m), 752 (m), 688 (m), δ(C−H from Ar), 517 ν(C−Cl). 1H NMR (CDCl3, δ, 233 K): 12.42−12.41 (m, 1H, NH), 7.43 (s, br, 1H, NH), 7.34−6.59 (m, 26H, Ar), 6.02−5.88 (m, br, 2H, NH), 2.39−2.16 (m, 6H, 2Me). 13C{1H} NMR (CDCl3, δ, 233 K): 177.54, 177.38, 170.79, 156.88, 155.67, 145.99, 144.95, 141.84, 140.28, 139.68, 136.11, 134.87, 129.94, 129.72, 129.35, 128.80, 128.49, 127.82, 127.52, 126.82, 125.77, 124.78, 123.92, 123.13, 121.74, 119.01, 19.77, 19.09. 21.

Yield 70 mg, 70%. Calcd (%) for C44H58N8Pd·3/4CHCl3. C 60.06, H 6.62, N 12.52; Found: C 60.07, H 6.82, N 12.61. HRESI+-MS, m/z: 805.3889 [M + H]+ [805.3892 calcd]. IR (KBr, selected bands, cm−1): 3447 (s), 3393 (s), ν(N−H); 2953 (m-w), ν(C−H from Ar, CH2 and Me); 1636 (m), 1601 (s), 1562 (s), 1499 (m), 1400 (vs), ν(CN and CC from Ar); 743 (w), 696 (m-w), δ(C−H from Ar). 1H NMR (CDCl3, δ): 7.24−7.16 (m, 9H, Ar), 6.96 (t, 2H, J = 6.8, Ar), 6.86−6.84 (m, 6H, Ar), 6.69−6.67 (m, 4H, Ar), 6.07 (s, br, 2H, NH), 6.00 (s, br, 2H, NH), 2.03 (s, 4H, 2CH2, tmbu), 1.55 (s, 12H, 4CH3, tmbu), 1.05 (s, 18H, 6CH3, tmbu). 13C{1H} NMR (CDCl3, δ): 200.32 (Ccarbene); 166.46, 166.36 (CN); 145.95, 140.64, 140.54, 129.41, 128.53, 126.45, 124.20, 123.13, 123.09, 122.12, 121.98, 58.98, 58.85, 52.15, 32.16, 32.12, 31.93, 30.09, 29.98. 24.

Yield 49 mg, 52%. Calcd (%) for C40H46N8Pd·CHCl3. C 56.95, H 5.48, N 12.96; Found: C 57.15, H 5.57, N 12.63. HRESI+-MS, L


Article

Organometallics Yield 84 mg, 76%. Calcd (%) for C44H42N8Pt·1/2CHCl3. C 57.00, H 4.57, N 11.95; Found: C 56.56, H 4.60, N 11.81. HRESI+-MS, m/z: 878.3280 [M + H]+ [878.3253 calcd]. IR (KBr, selected bands, cm−1): 3399 (m), 3367 (m), ν(N−H); 3055 (m-w), 3028 (m-w), 2916 (m-w), 2850 (m-w), 2831 (m-w), ν(C−H from Ar and Me); 1609 (vs), 1601 (vs), 1568 (vs), 1515 (s), 1495 (s), ν(CN and CC from Ar); 765 (m), 752 (m), 695 (m), δ(C−H from Ar). 1H NMR (CDCl3, δ, 218 K): 12.68 (s, 1H, NH), 7.40 (s, br, 1H, NH), 7.16−6.66 (m, 26H, Ar’s), 6.02 (s, br, 1H, NH), 5.88 (s, br, 1H, NH), 2.38 (s, 6H, Me), 2.17 (s, 6H, Me). 13C{1H} NMR (CDCl3, δ, 218 K): 177.36, 170.78 (CN); 155.51, 154.73, 145.98, 144.82, 141.78, 141.67, 140.30, 139.65, 136.08, 134.85, 130.03, 129.75, 129.34, 128.82, 128.59, 127.90, 127.57, 126.82, 126.09, 125.95, 125.76, 124.82, 123.15, 122.93, 121.62, 118.74, 19.92, 19.21. Crystals of 24 suitable for X-ray diffraction study were obtained by slow evaporation of a methanol/ chloroform (1:1, v/v) solution at 20−25 °C. 25.

Crystal data, results of the Hirshfeld surface analysis for X-ray structures, 1H and 13C{1H} NMR and ESI-MS spectra (PDF) Accession Codes

CCDC 1541820−1541823, 1542913−1542914, and 1551262 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 data_request@ccdc. cam.ac.uk, 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 Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Mikhail A. Kinzhalov: 0000-0001-5055-1212 Alexander S. Novikov: 0000-0001-9913-5324 Vadim P. Boyarskiy: 0000-0002-6038-0872 Matti Haukka: 0000-0002-6744-7208 Vadim Yu. Kukushkin: 0000-0002-2253-085X Yield 64 mg, 55%. Calcd (%) for C42H36N8Cl2Pt·1/2CHCl3. C 52.17, H 3.76, N 11.45; Found: C 52.70, H 3.93, N 11.51. HRESI+-MS, m/z: 919.2164 [M + H]+ [919.2160 calcd]. IR (KBr, selected bands, cm−1): 3398 (vs), 3379 (vs), ν(N−H); 2922 (m-w), 2852 (m-w), ν(C−H from Ar and Me); 1614 (s), 1605 (s), 1571 (s), 1516 (m), 1448 (m-w), ν(CN and CC from Ar); 765 (m), 752 (m), 694 (m), δ(C−H from Ar), 528 ν(C−Cl). 1H NMR (CDCl3, δ, 223 K): 12.47−12.39 (m, 1H, NH), 7.45−7.38 (m, br, 1H, NH), 7.33−6.63 (m, 26H, Ar), 6.04−5.94 (m, br, 2H, NH), 2.43−2.11 (m, 6H, 2Me). 13C{1H} NMR (CDCl3, δ, 223 K): 188.71, 188.67 (Ccarbene); 168.22, 168.15, 166.68 (CN); 153.01, 144.93, 144.01, 141.96, 140.15, 138.41, 138.32, 137.98, 137.68, 136.26, 132.38, 131.91, 131.34, 131.24, 129.97, 129.66, 129.30, 128.91, 128.81, 128.40, 127.19, 127.03, 126.98, 126.85, 126.77, 126.53, 126.31, 126.13, 125.35, 125.14, 124.92, 124.30, 124.22, 123.08, 121.61, 119.23, 20.28, 20.21, 19.62, 19.56. 26.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The synthetic part of this work was supported by Russian Science Foundation (grant 14-43-00017-P), whereas the structural studies were supported by Saint Petersburg State University (project 12.37.214.2016) for the deprotonated monocarbene complexes and Russian Foundation for Basic Research (grant 15-03-01563) for the deprotonated bis-carbene complexes. Physicochemical studies were performed at the Center for Magnetic Resonance, Center for X-ray Diffraction Studies, and Center for Chemical Analysis and Materials Research (all belong to Saint Petersburg State University).

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Yield 42 mg, 43%. Calcd (%) for C40H46N8Pt·1/2CHCl3. C 54.43, H 5.25, N 12.54; Found: C 54.00, H 5.18, N 12.03. HRESI+-MS, m/z: 834.3534 [M + H]+ [834.3566 calcd]. IR (KBr, selected bands, cm−1): 3419 (s), 3241 (m-w), ν(N−H); 2929 (m), 2852 (m-w), ν(C−H from Ar and Cy); 1601 (vs), 1556 (vs), 1490 (m), 1424 (vs), ν(CN and CC from Ar); 1448 (m), δ(H−C−H from Cy); 757 (w), 697 (m-w), δ(C−H from Ar). 1H NMR (CDCl3, δ): 7.38−7.11 (m, 12H, Ar), 6.97−6.86 (m, 6H, Ar), 6.73−6.67 (m, 2H, Ar), 6.19−6.09 (m, br, 4H, NH), 4.25−4.16 (m, 2H, CH, Cy), 2.20−1.23 (m, 20H, 2Cy). 13C{1H} NMR (CDCl3, δ): 186.50 (Ccarbene), 170.57 (CN), 144.81, 140.03, 129.21, 128.21, 126.45, 124.58, 122.47, 120.42, 51.44, 33.08, 25.74, 24.82.

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REFERENCES

(1) Luzyanin, K. V.; Pombeiro, A. J. L. Carbene Complexes Derived from Metal-Bound Isocyanides: Recent Advances. In Isocyanide Chemistry: Applications in Synthesis and Materials Science; Nenajdenko, V., Ed.; Wiley-VCH: Weinheim, 2012; p 531−550. (2) Vignolle, J.; Catton, X.; Bourissou, D. Chem. Rev. 2009, 109, 3333−3384. (3) Michelin, R. A.; Pombeiro, A. J. L.; Guedes da Silva, M. F. C. Coord. Chem. Rev. 2001, 218, 75−112. (4) Boyarskiy, V. P.; Bokach, N. A.; Luzyanin, K. V.; Kukushkin, V. Y. Chem. Rev. 2015, 115, 2698−2779. (5) Kinzhalov, M. A.; Luzyanin, K. V.; Boyarskiy, V. P.; Haukka, M.; Kukushkin, V. Y. Russ. Chem. Bull. 2013, 62, 758−766. (6) Kinzhalov, M. A.; Timofeeva, S. A.; Luzyanin, K. V.; Boyarskiy, V. P.; Yakimanskiy, A. A.; Haukka, M.; Kukushkin, V. Y. Organometallics 2016, 35, 218−228. (7) Mikherdov, A. S.; Kinzhalov, M. A.; Novikov, A. S.; Boyarskiy, V. P.; Boyarskaya, I. A.; Dar’in, D. V.; Starova, G. L.; Kukushkin, V. Y. J. Am. Chem. Soc. 2016, 138, 14129−14137. (8) Bertani, R.; Mozzon, M.; Michelin, R. A. Inorg. Chem. 1988, 27, 2809−2815. (9) Hashmi, A. S. K.; Lothschutz, C.; Böhling, C.; Rominger, F. Organometallics 2011, 30, 2411−2417. (10) Hashmi, A. S. K.; Yu, Y.; Rominger, F. Organometallics 2012, 31, 895−904.

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