Tetrazol-5-ylidene Gold(III) Complexes from Sequen-tial [2 + 3] Cycloaddition of Azide to Metal-bound Iso-cyanides and N4-AlkylationMikhail A. Kinzhalov,*,a Alina S. Legkodukh,a Tatyana B. Anisimova,a Alexander S. Novikov,a

Vitalii V. Suslonov,a Konstantin V. Luzyanin*a,b and Vadim Yu. Kukushkina

aSaint Petersburg State University, 7/9 Universitetskaya Nab., Saint Petersburg 199034, Russian Federation, e-mail: m.kinzhalov@spbu.rubDepartment of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, United King-dom, e-mail: konstantin.luzyanin@liverpool.ac.uk Supporting Information is available

ABSTRACT: Reaction between equimolar amounts of the isocyanide complexes [AuCl3(CNR)] [R = 2,6-Me2C6H3 (Xyl) 1a, 2,4,6-Me3C6H2 (Mes) 1b, Cy 1c, t-Bu 1d] and tetrabutylammonium azide (2) proceeds in CH2Cl2 at room temperature for ca. 10 min furnishing the gold(III)-tetrazolates [n-Bu4N][AuCl3(CN4R)] (3a–d) that were obtained in 89−95% yields after purification. Subsequent reaction between equimolar amounts of 3a–d and methyl trifluoromethanesulfonate (MeOTf) proceeds in CH2Cl2 at −70 °С for ca. 30 min to give the corresponding gold(III) complexes [AuCl3(CaN(Me)N2NbR)]a–b (5a–d) bearing 1,4-disubsti-tuted tetrazol-5-ylidene ligands (69−75%). Complexes 3a–d were obtained as pale-yellow solids and characterized by elemental analyses (C, H, N), HRESI−-MS, FTIR, 1H and 13C{1H} NMR spectroscopies. Complexes 5a–d were obtained as colorless solids and characterized by elemental analyses (C, H, N), HRESI+-MS, 1D (1H and 13C{1H}) and 2D (1H,13C-HMBC) NMR spectroscopies. In addition, the structures of 3a, 3b, 3c, and 5a were established by single-crystal X-ray diffraction. Analysis of the Wiberg bond in-dices (WI) for gas phase optimized model structures of 3a–c and 5a computed using the natural bond orbital (NBO) partitioning scheme disclosed a higher degree of electron density delocalization in the CN4 moiety of carbene 5a when compared to tetrazolate 3a–c. Results of DFT calculations for a model sys-tem reveal that the mechanism for the cycloaddition of an azide to the isocyanide ligand in [AuCl3(CNMe)] is stepwise and involves nucleophilic attack of N3– on the N atom of CNMe followed by ring-closure. The addition is both kinetically and thermodynamically favorable and occurs via the forma-tion of an acyclic NNNCN-intermediate, whereas the cyclization is the rate-determining step.

INTRODUCTIONIn the last decade, N-heterocyclic carbenes (NHCs) have emerged as valuable ancillary lig-ands for a wide range of applications, embracing contemporary transition-metal catalysis,1 organic synthesis,2 materials research,3 and medicine.3a,4

Many structural types of NHCs are known, includ-ing imidazolylidene, imidazolinylidene, thiazo- and oxazolylidene, and triazolylidene architec-tures.5 Among NHCs containing multiple nitrogen atoms within the cycle, triazolylylidene species

(Figure 1A) are well studied, while those based upon tetrazolylidene (1B), and their metal com-plexes (1C) are still scarce.6

Thus, deprotonation of the corresponding 1,3-disubstituted tetrazolium salts with a base at a low temperature led to generation of free 1,3-dis-ubstituted tetrazol-5-ylidene carbenes (Scheme 1, Route A).6a,b Alternatively, 5-azido-1,3-diphenyl-tetrazolium salt reacted with sodium azide to give 1,3-diphenyltetrazol-5-ylidene (Route B).6c An iso-meric 2,3-diphenyltetrazol-5-ylidene was pre-

1

pared via the same strategy using LiN(SiMe3)2 as a base.6d Uncomplexed 1,3-disubstituted and 2,3-disubstituted tetrazol-5-ylidenes were found to be thermally unstable even at room temperature (RT), and undergo a ring-opening to give 3-cyano-triazene or cyanoazimine upon thermal decompo-sition at RT, correspondingly.6b-d Until now, prepa-ration of uncomplexed 1,4-disubstituted tetrazol-5-ylidenes have not been reported, while the se-quential lithiation of the CH-acidic 1-substituted tetrazole followed by a transmetallation with a gold(I) substrate, and an in situ N4-alkylation brought about the formation of the single re-ported

Figure 1. 1,2,4-triazol-2-ylidene (A),1a mesoionic 1,3-disubstituted and 2,3-disubstituted tetrazol-5-yli-dene carbenes (B),6 and gold(III)-complex with 1,4-disubstituted tetrazol-5-ylidene carbenes (C).

Scheme 1. Reported preparation of free 1,3-disubstituted and gold(I)-complex with 1,4-dis-ubstituted tetrazol-5-ylidenes.

Scheme 2. [2 + 3] Cycloaddition of azide to AuIII-bound isocyanides (Route E) and subsequent N4-alkylation (Route F) leading to gold(III)-complexes with 1,4-disubstituted tetrazol-5-ylidenes.

example of a metal complex featuring 1,4-disub-stituted tetrazol-5-ylidene (Route C). Alterna-tively, this compound was prepared via a dipolar cycloaddition of azide to xylyl isocyanide at gold(I) center followed by a N4-alkylation (Route D).6e No other examples of metal complexes with 1,4-disubstituted tetrazol-5-ylidene ligands have been reported.

In pursuit of our studies on reactivity on metal-bound isocyanides,7 we envisaged that the gold(III) complexes with 1,4-disubstituted tetra-zol-5-ylidene can be accessed directly from gold(III)-isocyanides. Owing the rapidly expanding field of applications of gold-NHC species,8 we re-

port herein on preparation and structural charac-terization of gold(III) species bearing rare 1,4-dis-ubstituted tetrazol-5-ylidene ligands.

RESULTS AND DISCUSSION[2 + 3] Cycloaddition of azide to AuIII-bound isocyanides. Reaction between equimolar amounts of the isocyanide complexes [AuCl3(CNR)] [R = 2,6-Me2C6H3 (Xyl) 1a, 2,4,6-Me3C6H2 (Mes) 1b, Cy 1c, t-Bu 1d] and tet-rabutylammonium azide (2) proceeds in CH2Cl2 at RT for ca. 10 min to give the tetrazolate derivat-ives [n-Bu4N][AuCl3(CN4R)] in nearly quantitative 1H NMR yield (Scheme 2, Route E).

2

After evaporation of the solvent, all complexes were further purified by recrystallization from a CH2Cl2/Et2O mixture to afford [n-Bu4N][AuCl3(CN4R)] (3a–d) in good to excellent (89−95%) isolated yield. Other organic azides, i.e., 4-methylbenzenesulfonyl azide and methyl (E)-2-azido-3-(4-chlorophenyl)acrylate do not re-act with complex 1a even upon prolonged heat-ing, e.g., for 24 h in refluxing CH2Cl2. Furthermore, no reaction between uncomplexed azide and an isocyanide was detected even after 3d reflux in CH2Cl2 implying that the observed [2 + 3] cy-cloaddition is gold(III)-mediated.

N4-alkylation of the tetrazolate precur-sors. Despite a great significance of the tetrazole moiety in synthetic chemistry, medicine, and ma-terials science,9 preparation of alkylated tetra-zoles are limited in scope.9a A general alkylation route includes SN2 reaction between an alkyl halide and a free tetrazole. Broad application of this procedure is hampered by the formation of isomeric dialkylated mixtures, moderate yields, and requirements for the expensive alkyl halides.9a,b In organometallic chemistry, alkylation of N-coordinated tetrazolate ligands is well known

and it was shown that the regioselectivity of alky-lation is often controlled by the nature of a metal center.10 Alkylation of C-coordinated tetrazoles is limited to a few examples of reactions that pro-ceed at the gold(I) center (Scheme 1, Route D). By this route complexes with 1,4-disubstituted tetrazol-5-ylidene ligands were prepared.6e

In this study, the reaction between equimolar amounts of complexes 3a–d and methyl trifluo-romethanesulfonate (MeOTf) proceeds in CH2Cl2 at −70 °С for ca. 30 min to give the correspond-ing gold(III) complexes [AuCl3(CaN(Me)N2NbR)]a–b

(5a–d) bearing 1,4-disubstituted tetrazol-5-yli-dene ligands (Scheme 2, Route F). These tetra-zol-5-ylidene carbene species were formed in 87−96% NMR yield; subsequent isolation and pu-rification ensured 69−75% isolated yields for 5a–d (see Experimental section). For 5a, as a repre-sentative species, we undertook the sequence of cycloaddition/alkylation in one-pot manner with-out the separation of intermediate compounds 3a–d. In this one-pot system, NMR yield of 5a was comparable (71%) to that determined for the sequential procedure (73%).

Figure 2. 1H,13C-HMBC NMR spectrum of 5b. Characterization of the gold(III) com-

plexes. Complexes 3a–d were obtained as pale-yellow solids and characterized by elemental analyses (C, H, N), HRESI−-MS, FTIR, 1H and 13C{1H} NMR spectroscopies. Complexes 5a–d were obtained as colorless solids and character-ized by elemental analyses (C, H, N), HRESI+-MS, 1D (1H and 13C{1H}) and 2D (1H,13C-HMBC) NMR spectroscopies. In addition, the structures of 3a, 3b, 3c, and 5a were established by single-crystal X-ray diffraction.

Tetrazolate complexes 3a–d and tetrazol-5-yli-dene carbene species 5a–d gave satisfactory C, H, and N elemental analyses, which are consis-tent with the proposed formulas. The HRESI– mass spectra of 3a–d demonstrated the corresponding [AuCl3(CN4R)]− ion along with the characteristic isotopic distribution. In the HRESI+-MS of 5a–d, the ions [M – 2Cl + Na]+ with the characteristic isotopic distribution were detected. The IR spec-

tra of 3a–d display no bands in the range be-tween 2300 and 2000 cm–1 due to ν(C≡N), thus collaterally supporting the transformation of iso-cyanide ligands into tetrazolates.

The cycloaddition of 2 to 1a–d is accompanied by a pronounced downfield δ 13C shift of the iso-cyanide quaternary C atom to the range common for metal-bonded tetrazolate ring [M]–CN4R (δC 150–165 ppm).7a,11 In 3a–d, the CN4R 13C signals were found to resonate at δC 138–144 ppm; that is ca. 30 ppm downfield shifted vs. signals in the starting 1a–d (e.g. δC 112  for C≡N in 1c and 1d7e). These δ values for the CN4R 13C signal are upfield relatively to those (δC 173–187) reported for a series of the gold(I) tetrazolate complexes [Au(PPh3)(CN4R)].6e

The methylation of the tetrazolate ring in 3a–d is accompanied by a downfield δ 13C shift tetrazo-late quaternary C atom to the range that is spe-cific for M-NHC (δC 150–224).12 The Ccarbene 13C sig-

3

nals in 5a–d were found to resonate in the range of δC 150–155, that is ca. 10 ppm downfield shifted vs. CN4R 13C signals in 3a–d. These δ val-ues for the Ccarbene 13C signal are upfield relatively to those (δC 186.9) reported for the known gold(I) complex bearing the tetrazole-derived NHC ligand [Au(PPh3)(CN(Xyl)N2N(Me)](CF3SO3).6e Such upfield 13C shift was previously reported for other gold(I)-NHC complexes [δC 176 for AuCl(NHC) and δC 151 for AuCl3(NHC)13].

Chemical shift of the methyl group installed in 5a–d was assigned upon inspection of the gradi-ent-enhanced 1H,13C-HMBC NMR spectrum of 5b (Figure 2). Herein, the signal at δH 4.57 (hydro-gens of the methyl group) exhibits relevant cross-peaks with the quaternary signal of the carbene moiety at δC 154.1 due to 3JCH between those.

X-ray diffraction studies. Crystal data collec-tion and refinement details for complexes 3a, 3b, 3c, and 5a are summarized in Table S1, while plots are provided on Figures 3, 4, S1, and S2. Selected bond lengths and angles are summa-rized in Table S2 and legends to Figures 3 and 4.

Figure 3. View of complex 3a with the atomic num-bering scheme. Thermal ellipsoids are drawn with the 50% probability. Tetrabutylammonium ion and hydrogen labels are omitted for simplicity. Selected bond lengths (Å) and angles (°): Au1‒Cl1 2.3234(11), Au1‒Cl2 2.2763(10), Au1‒Cl3 2.2762(10), Au1‒C1 1.999(3), C1‒N1 1.338(4), C1‒N4 1.315(4), N1‒N2 1.364(4), N2‒N3 1.288(5), N3‒N4 1.359(5), Cl1‒Au1‒Cl2 91.20(4), C1‒Au1‒Cl2 89.60(11), N1‒C1‒N4 108.4(3).

Figure 4. View of complex 5a with the atomic num-bering scheme. Thermal ellipsoids are drawn with the 50% probability. Hydrogen labels are omitted for simplicity. Selected bond lengths (Å) and angles (°): Au1‒Cl1 2.3033(8), Au1‒Cl2 2.2876(9), Au1‒Cl3 2.2978(9), Au1‒C1 2.004(3), C1‒N1 1.335(4), C1‒N4 1.337(4), N1‒N2 1.369(4), N2‒N3 1.271(4), N3‒N4 1.357(4), N4‒C10 1.469(5), Cl1‒Au1‒Cl2 91.93(3), C1‒Au1‒Cl2 89.24(10), N1‒C1‒N4 104.0(3), C1‒N4‒C10 130.5(3).

The coordination polyhedra of all complexes are built up by one tetrazolato (3a–c) or carbene lig-and (5a) and three chlorides furnishing a typical slightly distorted square-planar coordination ge-ometry. All bond angles around the AuIII cores are close to 90° varying from 87.17(14)° to 92.22(4)° (Table S2). The Au‒Cl1 (2.3098(11) and 2.3056(12) Å) distances opposite to the carbon atom are slightly longer than the other two Au‒Cl bond lengths (2.2793(12) and 2.2826(12) or 2.2846(12) and 2.2842(12) Å) reflecting substan-tial ground-state trans-influence of the both tetra-zolato and carbene ligands. In 5a, the C–N bonds of the carbene fragment exhibit similar lengths and their values lie between single CN bonds (e.g., 1.469(10) Å in amines14) and the double CN bond distance (e.g., 1.279(8) Å in imines14). In the tetrazolate species 3a–c, CN4 fragment exhibit distinct single and double CN bond distances re-flecting the lower degree of the electron delocal-ization when compared to that one in the carbene species 5a.

We undertook the analysis of the Wiberg bond indices (WI) for gas phase optimized model struc-tures of 3a–c and 5a computed using the natural bond orbital (NBO) partitioning scheme). Higher

degree of electron density delocalization in the CN4 moiety of carbene 5a when compared to tetrazolate 3a–c was observed (see Table 1). Herein, in carbene 5a WI values for C1–N1 and N2–N3 contacts are higher and for N3–N4 and N4–C1 contacts are lower than those in tetrazolate 3a–c. Obtained theoretical calculation data agree well with the experimental X-ray diffraction obser-vations.

Table 1. Calculated Wiberg bond indices for gas phase optimized structures of 3a–c and 5a.

BondBond lengths (Å)3a 3b 3c 5a

C1–N1 1.20 1.20 1.22 1.30N1–N2 1.16 1.16 1.19 1.16N2–N3 1.61 1.61 1.59 1.68N3–N4 1.29 1.29 1.30 1.18N4–C1 1.53 1.53 1.53 1.30

Theoretical considerations of the gold(III)-mediated cycloaddtion of azide and iso-

4

cyanide. In order to shed light on the mechanism of the AuIII-mediated [2+3] cycloaddition, a quan-tum chemical DFT study of this reaction in the model system [AuCl3(CNMe)] + N3– was under-taken (CPCM-B3LYP/6-311+G(d,p)//gas-B3LYP/6-31G(d) level of theory, MWB60 pseudopotential and appropriate contracted basis set for the gold atoms). Three types of mechanisms for this for-mal CA have been proposed. One of them is con-certed (via one cyclic transition state TSc) and the other two are stepwise (involve the formation of acyclic zwitterionic intermediates INTa and INTb followed by the cyclization) (Scheme 3).

We were unable to locate on the potential en-ergy surface any transition states for the con-certed pathway or acyclic NNNNC-intermediate INTb. Despite numerous attempts, in all cases, optimization led to the formation of initial species, which are separated from each other, or to product P. Thus, pathways B and C have been excluded from consideration. At the same time, all minima for structures corresponding to path-way A, viz., acyclic NNNCN-intermediate INTa and both transition states TSs1a and TSs2a, were successfully found. Scheme 3. Proposed stepwise (A and C) and concerted (B) mechanisms for the formal CA of N3

– to [AuCl3(CNMe)].

The process is initiated by the exergonic forma-tion of the orientation complex [AuCl3(CNMe)]•N3–. The first transition state TSs1a corresponds to the formation of the C–N bond giving the acyclic NNNCN-intermediate INTa, and the second transition state TSs2a is associated with the ring closure forming the C-tetrazolate complex [AuCl3(CN4Me)]– (P). The sec-ond step is the rate determining (Figure 5, Ta-bles S3 and S4 in Supporting Information).

Gs, kcal/mol

20

10

0

R

TSs1a

INTa

1.4

18.1

6.4

30

40 40.2

P

PdII

AuIII

1.0

27.6

13.0

45.1

TSs2a10

Figure 5. Energy profiles of R → P transformation for PdII- (Ref. 7a) and AuIII-mediated CA.

Based upon quantum chemical calculations, we stated that (i) the formation of the C-tetrazolate gold complexes from AuIII-coordinated isocyanides and free azides is favorable from both kinetic and thermodynamic viewpoints, and (ii) the AuIII cen-ter promotes CA better than the PdII center as previously reported.7a The same trend was estab-lished for the nucleophilic addition to isocyanide bound to more electron poor (e.g. AuIII) centers when compared to more electron rich (e.g. PdII).15 We checked the tendency toward decomposition of C-tetrazolate gold complexes via the path [AuCl3(CN4Me)]– → N2 + [AuCl3(N=C=NMe)]– (by TSdec), and concluded that the gold(III) center is excellent stabilizer for such anionic heterocycle (Gs = 40.2 kcal/mol, Gs = –32.1 kcal/mol for [AuCl3(CN4Me)]– vs. Gs = 36.5 kcal/mol, Gs = –35.2 kcal/mol for trans-[PdCl(PH3)2(CN4Me)] and Gs = 10.8 kcal/mol, Gs = –60.2 kcal/mol for free CN4Me– 7a (Tables S3 and S4).

FINAL REMARKSIn pursuit of these studies, we established that gold(III) complexes with rare NHC species, i.e. 1,4-disubstituted tetrazol-5-ylidenes, can be ac-cessed directly from gold(III)-isocyanides. Herein, the [2 + 3] cycloaddition between equimolar amounts of the isocyanide complexes [AuCl3(CNR)] and tetrabutylammonium azide fur-nished the corresponding tetrazolate derivatives [n-Bu4N][AuCl3(CN4R)] in good to excellent yield. This reaction is gold(III)-mediated insofar as no reaction between free azide and uncomplexed isocyanide was observed. Results of DFT calcula-tions for a model system reveal that the mecha-nism for the cycloaddition of an azide to the iso-cyanide ligand in [AuCl3(CNMe)] is stepwise and involves nucleophilic attack of N3– on the N atom of CNMe followed by ring-closure. The addition is both kinetically and thermodynamically favorable and occurs via the formation of an acyclic NNNCN-intermediate, whereas the cyclization is the rate-determining step.

5

In the next step, selective N4-methylation of [n-Bu4N][AuCl3(CN4R)] with equimolar amount of MeOTf leads to the corresponding gold(III) com-plexes with 1,4-disubstituted tetrazol-5-ylidene carbene ligands, that were obtained in good yield after purification. With a representative example we demonstrated that the sequence of cycloaddi-tion/alkylation can be undertaken in one-pot man-ner without the separation of intermediate com-pounds [n-Bu4N][AuCl3(CN4R)] giving target gold(III)-NHCs in yields comparable to the sequen-tial procedure. A sequence of gold(III)-mediated [2 + 3] cycloaddition between isocyanide and azide followed by the N4-methylation of the inter-mediate tetrazolate formed represents a unique approach to gold(III) complexes with 1,4-disubsti-tuted tetrazol-5-ylidene carbene ligands that can hardly be accessed by other routes. Further stud-ies aiming at an expansion of a family of gold(III)-NHCs and their application in catalysis are under-way in our group.

EXPERIMENTAL SECTIONMaterials and Instrumentation. Solvents and reagents were obtained from commercial sources and used as received, apart from CH2Cl2, which was purified by the conventional distillation over CaH2. Gold com-plexes 1a–c were synthesized according to the previ-ously reported methods.7e,f C, H, and N elemental analy-ses were carried out on a Euro EA3028-НТ. Mass spec-tra were obtained on a Bruker micrOTOF spectrometer equipped with electrospray ionization (ESI) source. MeCN was used as solvent. The instrument was oper-ated both at positive and negative ionization modes us-ing 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 bars and drying gas flow 4.0 L/min. The most intensive peak in the isotopic pattern is re-ported. Infrared spectra (4000−400 cm–1) were recorded on a Perkin Elmer Spectrum BX FT-IR spec-trometer in KBr pellets. 1D (1H, 13C{1H}) NMR spectra and 2D (1H,13C-HMBC) NMR correlation experiments were recorded on Bruker Avance III 400 MHz spectrome-ter (400.13 MHz for 1H, 100.61 MHz for 13C). The spectra were measured at 298±1 K using CDCl3 (Sigma-Aldrich, USA) as solvent and referenced to tetramethylsilane as external standard.

X-ray Structure Determinations. Crystals of 3a–c and 5a were obtained by slow evaporation of CH2Cl2:Et2O solutions. Single-crystal X-ray diffraction ex-periment carried out using Agilent Xcalibur diffractome-ters with monochromated MoKα radiation. Crystal was fixed on a micro mount, placed on diffractometer and measured at 100(2) K, except 3a, which measured at 200(2) K due to fast decomposition at a lower tempera-ture. The unit cell parameters were refined by least square techniques in the 2θ range of 6.0–55.0 for MoKα. The structure has been solved by the Superflip16 structure solution program using Charge Flipping and refined by means of the ShelXL17 program, incorporated in the OLEX2 program package.18 Empirical absorption correction was applied in CrysAlisPro (Agilent Technolo-gies, 2013) program complex using spherical harmonics (for all samples except 4f), implemented in SCALE3 ABSPACK scaling algorithm. Supplementary crystallo-graphic data for this paper have been deposited at

Cambridge Crystallographic Data Centre (1557394–1557397) and can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif.

Computational details. The DFT calculations have been carried out at the CPCM-B3LYP/6-311+G(d,p)//gas-B3LYP/6-31G(d) level of theory using the MWB60 pseu-dopotential and appropriate contracted basis set for the gold atoms19 with the help of the Gaussian-09 program package.20 No symmetry operations have been applied. The Hessian matrix was calculated analytically for the optimized structures in order to prove the location of correct minima or saddle points, and to estimate the thermodynamic parameters, the latter being calculated at 25 °C. The nature of all transition states was investi-gated by the analysis of vectors associated with the imaginary frequency and by the calculations of the in-trinsic reaction coordinates (IRC) using the Gonzalez–Schlegel method.21 The total energies corrected for sol-vent effects were estimated at the single-point calcula-tions on the basis of gas-phase equilibrium geometries using the polarizable continuum model in the CPCM version21d,e with CH2Cl2 as solvent. The UAKS model was applied for the molecular cavity, and dispersion, cavita-tion, and repulsion terms were also considered. The en-tropic term in CH2Cl2 solution (Ss) was calculated ac-cording to the procedure described by Wertz22 and Cooper and Ziegler23 (see eqs. 1–4), as well as en-thalpies and Gibbs free energies in solution (Hs and Gs) using the expressions 5 and 6 (all equations are pro-vided in the Supporting Information). As it was shown previously,24 this approach is sufficiently accurate for the description of CAs to the CN triple bond providing results close to those obtained by such methods as MP2, MP4, CCSD(T), CBS-Q, and G3B3. Note that this approach was also used in our previous studies on Pd II-mediated integration of isocyanides and azide ion.7a

The Cartesian atomic coordinates of the calculated equilibrium structures are presented in Supporting In-formation, Table S5. The Cartesian atomic coordinates of the calculated equilibrium structures are provided as a separate xyz file.

Cycloaddition of azide to gold(III)-isocyanides. A solution of [n-Bu4N]N3 (2) (28 mg, 0.100 mmol) in dry CH2Cl2 (1.5 mL) was added to a solution of [AuCl3(CNR)] (1a–d; 0.100 mmol) in dry CH2Cl2 (1.5 mL) at RT. An im-mediate change of a solution color to a pale-yellow oc-curred, and the reaction mixture was left to stand for ca. 10 min at RT. The reaction mixture was then evapo-rated to dryness, and the pale-yellow solid was re-dis-solved and re-crystallized from a CH2Cl2:Et2O (1:1, v/v) mixture at RT.

3a (65 mg, 91%). Pale-yellow solid. Anal. calculated for C25H45N5Cl3Au: C, 41.76; H, 6.31; N, 9.74%. Found: C, 42.23; H, 6.12; N, 9.41%. HRESI−-MS, m/z: calcd. for C9H9N4Cl3Au− 474.9558, found 474.9553 [M–Bu4N]−. IR (KBr, selected bands, cm–1): 2964, 2933, and 2875 ν(C−H); 1469, 1388, and 1370 ν(C=N) and ν(N=N). 1H NMR (400.13 MHz, CDCl3, δ): 1.03 (t, 3JH,H = 7.1 Hz, 12H, Me from n-Bu), 1.44−1.51 (m, 8H, γ-CH2 from n-Bu), 1.71 (с, 8H, β-CH2 from n-Bu), 2.11 (с, 6H, Me from Xyl), 3.30 (с, 8H, NCH2), 7.16 (d, 3JH,H = 7.6 Hz, 2H, m-CArH), 7.32 (t, 3JH,H = 7.6 Hz, 1H, p-CArH). 13C{1H} NMR (100.61 MHz, CDCl3, δ): 13.7 (Me from n-Bu), 18.4 (Me from Xyl), 19.8 (γ-CH2 from n-Bu), 24.1 (β-CH2 from n-Bu), 59.1 (α-CH2 from n-Bu), 128.5 (m-CAr), 130.2 (p-CAr), 133.8 (ipso-CAr), 136.7 (o-CAr), 143.5 (CN4).

3b (70 mg, 95%). Pale-yellow solid. Anal. calculated for C26H47N5Cl3Au: C, 42.60; H, 6.46; N, 9.55%. Found: C, 42.67; H, 6.50; N, 10.43%. HRESI−-MS, m/z: calcd. for C10H11N4Cl3Au– 488.9715, found 488.9709 [M–Bu4N]−. IR

6

(KBr, selected bands, cm–1): 2963, 2934, and 2875 ν(C−H); 1472, 1396, and 1381 ν(C=N) and ν(N=N). 1H NMR (400.13 MHz, CDCl3, δ): 1.05 (t, 3JH,H = 7.3 Hz, 12H, Me from n-Bu), 1.45−1.54 (m, 8H, γ-CH2 from n-Bu), 1.69−1.77 (m, 8H, β-CH2 from n-Bu), 2.07 (с, 6H, o-CarMe), 2.36 (с, 3H, p-CarMe), 3.32−3.36 (m, 8H, NCH2), 6.97 (с, 2H, m-CArH). 13C{1H} NMR (100.61 MHz, CDCl3, δ): 13.7 (Me from n-Bu), 18.3 (o-Me from Mes), 19.8 (γ-CH2 from n-Bu), 21.1 (p-Me from Mes), 24.1 (β-CH2 from n-Bu), 59.1 (α-CH2 from n-Bu), 129.2 (m-CAr), 131.2 (ipso-CAr), 136.2 (o-CAr), 140.1 (p-CAr), 143.8 (CN4).

3c (63 mg, 90%). Pale-yellow solid. Anal. calculated for C23H47N5Cl3Au: C, 39.69; H, 6.73; N, 9.94%. Found: C, 39.63; H, 6.80; N, 10.05%. HRESI−-MS, m/z: calcd. for C7H11N4Cl3Au– 452.9715, found 452.9705 [M–Bu4N]−. IR (KBr, selected bands, cm–1): 2958, 2933, and 2873 ν(C−H); 1485, 1456, and 1382 ν(C=N) and ν(N=N). 1H NMR (400.13 MHz, CDCl3, δ): 1.04 (t, 3JH,H = 7.3 Hz, 12H, Me from n-Bu), 1.28–1.37 (m, 1Н, СH from Су) 1.43–1.54 (m, 10H, СH2 from Су + γ-CH2 from n-Bu), 1.68–1.76 (m, 9H, СH from Су + β-CH2 from n-Bu), 1.91–2.24 (m, 6H, α- and β-CH2 from Cy), 3.30–3.34 (m, 8H, N-CH2 from n-Bu), 4.69–4.77 (m, 1H, CH from Cy). 13C{1H} NMR (100.61 MHz, CDCl3, δ): 13.7 (Me from n-Bu), 19.8 (γ-CH2 from n-Bu), 24.1 (β-CH2 from n-Bu), 25.1 (γ-CH2 from Cy), 25.3 (β-CH2 from Cy), 33.0 (α-CH2 from Cy), 59.0 (α-CH2 from n-Bu), 59.2 (CH from Cy), 141.3 (CN4).

3d (60 mg, 89%). Pale-yellow solid. Anal. calculated for C21H45N5Cl3Au: C, 37.59; H, 6.76; N, 10.44%. Found: C, 37.91; H, 6.76; N, 10.25%. HRESI−-MS, m/z: calcd. for C5H9N4Cl3Au– 426.9558, found 426.9552 [M–Bu4N]−. IR (KBr, selected bands, cm–1): 2962, 2937 and 2875 ν(C−H); 1473, 1391 1370 ν(C=N) and ν(N=N). 1H NMR (400.13 MHz, CDCl3, δ): 1.03 (t, 3JH,H = 7.3 Hz, 12H, Me from n-Bu), 1.45−1.54 (m, 8H, γ-CH2), 1.68−1.76 (m, 8H, β-CH2), 1.91 (с, 9H, Me from t-Bu), 3.31−3.35 (m, 8H, NCH2). 13C{1H} NMR (100.61 MHz, CDCl3, δ): 13.7 (Me from n-Bu), 19.8 (γ-CH2 from n-Bu), 24.1 (β-CH2 from n-Bu), 31.1 (Me from t-Bu), 59.1 (α-CH2 from n-Bu), 60.6 (C from t-Bu), 138.8 (CN4).

N4-alkylation of complexes 3a–d. Solution of MeOTf (4, 7 mg, 0.042 mmol) in CH2Cl2 (1.5 mL) was added to solution any one of 3a–d (0.040 mmol) in CH2Cl2 (1.5 mL) at –70 °C to form a pale-yellow solution. The reaction mixture was kept at –70 °C for ca. 30 min and then warmed to RT and left to stand for addi-tional 30 min, where upon was it evaporated to dryness at RT, and the residue was washed with CH2Cl2/Et2O mixture (1:10, v/v) and purified by recrystallization from a CH2Cl2/Et2O mixture at −18 °С.

5a (15 mg, 75%). Colorless solid. Anal. calculated for C10H12N4Cl3Au: C, 24.43; H, 2.46; N, 11.40%. Found: C, 24.87; H, 2.55; N, 11.30%. HRESI−-MS, m/z: calcd. for C10H12N4ClAuNa+ 443.0309, found 443.0309 [M−2Cl+Na]+. 1H NMR (400.13 MHz, CDCl3, δ): 2.19 (с, 6H, Me from Xyl), 4.59 (с, 3H, Me-N), 7.31 (d, 3JH,H = 7.6 Hz, 2H, m-CArH), 7.52 (t, 3JH,H = 7.6 Hz, 1H, p-CArH). 13C{1H} NMR (100.61 MHz, CDCl3, δ): 18.3 (Me from Xyl), 38.8 (Me–N), 129.5 (m-CAr), 131.6 (ipso-CAr), 132.6 (p-CAr), 136.0 (o-CAr), 154.3 (CN4).

5b (15 mg, 73%). Colorless solid. Anal. calculated for C11H14N4Cl3Au: C, 26.13; H, 2.79; N, 11.08%. Found: C, 26.55; H, 2.89; N, 10.99%. HRESI−-MS, m/z: calcd. for C11H14N4ClAuNa+ 457.0470, found 457.0473 [M−2Cl+Na]+. 1H NMR (400.13 MHz, CDCl3, δ): 2.13 (с, 6H, o-Me from Mes), 2.43 (с, 3H, p-Me from Mes), 4.57 (с, 3H, Me-N), 7.09 (c, 2H, m-CArH). 13C{1H} NMR (100.61 MHz, CDCl3, δ): 18.2 (m-Me from Mes), 21.3 (p-

Me from Mes), 38.8 (Me–N), 129.1 (ipso-CAr), 130.2 (m-CAr), 135.6 (o-CAr), 143.1 (p-CAr), 154.2 (CN4).

5c (13 mg, 70%). Colorless solid. Anal. calculated for C8H14N4Cl3Au: C, 20.46; H, 3.01; N, 11.93%. Found: C, 20.98; H, 3.11; N, 11.78%. HRESI+-MS, m/z: calcd. for C8H14N4ClAuNa+ 421.0465, found 421.0454 [M−2Cl+Na]+. 1H NMR (400.13 MHz, CDCl3, δ): 1.31–1.67 (m, 3H, γ-СН and β-CH2), 1.84–1.88 (m, 1Н, γ-СН), 2.01–2.34 (m, 6H, 2 α- and 1 β-CH2 from Cy), 4.41 (с, 3H, Me-N), 4.91–4.99 (m, 1H, CH from Cy). 13C{1H} NMR (100.61 MHz, CDCl3, δ): 24.5 (γ-CH2 from Cy), 24.9 (β-CH2 from Cy), 32.6 (α-CH2 from Cy), 38.3 (Me–N), 63.8 (N–CH from Cy), 141.3 (CN4).

5d (12 mg, 69%). Colorless solid. Anal. calculated for C6H12N4Cl3Au: C, 16.25; H, 2.73; N, 12.63%. Found: C, 16.86; H, 2.82; N, 12.47%. HRESI+-MS, m/z: calcd. for C6H12N4ClAuNa+ 395.0309, found 395.0309 [M−2Cl+Na]+. 1H NMR (400.13 MHz, CDCl3, δ): 2.03 (с, 9H, Me from t-Bu), 4.44 (с, 3H, Me–N). 13C{1H} NMR (100.61 MHz, CDCl3, δ): 30.6 (Me from t-Bu), 38.7 (Me–N), 67.2 (C from t-Bu), 150.6 (CN4).

ASSOCIATED CONTENT Supporting InformationThe Supporting Information is available free of charge on the ACS Publications website. Crystallographic data and structure refinement de-tails, spectroscopic data for prepared compounds, theoretical calculation data and Cartesian atomic co-ordinates (PDF)X-ray diffraction data (CIF) Cartesian atomic coordinates of the calculated equi-librium structures (XYZ)

AUTHOR INFORMATIONCorresponding Authors* Mikhail A. Kinzhalov, e-mail: m.kinzhalov@spbu.ru * Konstantin V. Luzyanin, e-mail: konstantin.luzyanin@liverpool.ac.uk

The authors declare no competing financial in-terests.

ACKNOWLEDGMENTS This work was supported by the Russian Science Foundation (grant 14-43-00017-P). 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).

REFERENCES(1) (a) Hopkinson, M. N.; Richter, C.; Schedler, M.;

Glorius, F. Nature 2014, 510, 485–496 and references cited therein. (b) Díez-González, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612−3676. (c) Glorius, F. N-Heterocyclic carbenes in transition metal catalysis; Springer, 2007. (d) N-Heterocyclic Carbenes: From Lab-oratory Curiosities to Efficient Synthetic Tools; Diez-Gonzalez, S., Ed.; RSC Publishing, 2011.

(2) (a) Nolan, S. P. N-Heterocyclic carbenes in syn-thesis Wiley, 2006. (b) Marion, N.; Díez-González, S.; Nolan, S. P. Angew. Chem. Int Ed. 2007, 46, 2988–3000. (c) Enders, D.; Balensiefer, T. Acc. Chem. Res. 2004, 37, 534–541.

7

(3) (a) Mercs, L.; Albrecht, M. Chem. Soc. Rev. 2010, 39, 1903–1912. (b) Oisaki, K.; Li, Q.; Furukawa, H.; Czaja, A. U.; Yaghi, O. M. J. Am. Chem. Soc. 2010, 132, 9262–9264. (c) Visbal, R.; Concepcion Gimeno, M. Chem. Soc. Rev. 2014, 43, 3551–3574.

(4) (a) Hindi, K. M.; Panzner, M. J.; Tessier, C. A.; Cannon, C. L.; Youngs, W. J. Chem. Rev. 2009, 109, 3859–3884. (b) Hickey, J. L.; Ruhayel, R. A.; Barnard, P. J.; Baker, M. V.; Berners-Price, S. J.; A., F. J. Am. Chem. Soc. 2008, 130, 12570–12571.

(5) (a) F. E. Hahn; Jahnke, M. C. Angew. Chem. Int. Ed. 2008, 47, 3122–3172. (b) Jahnke, M. C.; Hahn, F. E. Coord. Chem. Rev. 2015, 293–294, 95–115.

(6) (a) Norris, W. P.; Henry, R. A. Tetrahedron Lett. 1965, 1213–1215. (b) Araki, S.; Wanibe, Y.; Uno, F.; Morikawa, A.; Yamamoto, K.; Chiba, K.; Butsugan, Y. Chem. Ber. 1993, 126, 1149–1155. (c) Araki, S.; Ya-mamoto, K.; Yagi, M.; Inoue, T.; Fukagawa, H.; Hattori, H.; Yamamura, H.; Kawai, M.; Butsugan, Y. Eur. J. Org. Chem. 1998, 121–127. (d) Lowack, R. H.; Weiss, R. J. Am. Chem. Soc. 1990, 112, 333–338. (e) Gabrielli, W. F.; Nogai, S. D.; McKenzie, J. M.; Cronje, S.; Rauben-heimer, H. G. New J. Chem. 2009, 33, 2208–2218.

(7) (a) Kinzhalov, M. A.; Novikov, A. S.; Luzyanin, K. V.; Haukka, M.; Pombeiro, A. J. L.; Kukushkin, V. Y. New J. Chem. 2016, 40, 521–527. (b) Luzyanin, K. V.; Pombeiro, A. J. L.; Haukka, M.; Kukushkin, V. Y. Organometallics 2008, 27, 5379–5389. (c) Luzyanin, K. V.; Tskhovrebov, A. G.; Guedes da Silva, M. F. C.; Haukka, M.; Pombeiro, A. J. L.; Kukushkin, V. Y. Chem.-Eur. J. 2009, 15, 5969–5978. (d) Tskhovrebov, A. G.; Luzyanin, K. V.; Dolgushin, F. M.; Guedes da Silva, M. F. C.; Pombeiro, A. J. L.; Kukushkin, V. Y. Organometallics 2011, 30, 3362–3370. (e) Anisimova, T. B.; Kinzhalov, M. A.; Guedes da Silva, M. F. C.; Novikov, A. S.; Kukushkin, V. Y.; Pombeiro, A. J. L.; Luzyanin, K. V. New J. Chem. 2017, 41, 3246–3250. (f) Anisimova, T. B.; Kinzhalov, M. A.; Kuznetsov, M. L.; Guedes da Silva, M. F. C.; Zolotarev, A. A.; Kukushkin, V. Y.; Pombeiro, A. J. L.; Luzyanin, K. V. Organometallics 2016, 35, 3569–3576.

(8) (a) Nösel, P.; Müller, V.; Mader, S.; Moghimi, S.; Rudolph, M.; Braun, I.; Rominger, F.; Hashmi, A. S. K. Adv. Synth. Catal. 2015, 357, 500–506. (b) Kumar, R.; Nevado, C. Angew. Chem. Int Ed. 2017, 56, 1994–2015.

(9) (a) Ostrovskii, V. A.; Koren, A. O. Heterocycles 2000, 53, 1421–1448. (b) L. Wang; K. Zhu; Q. Chen; He, M. J. Org. Chem. 2014, 79, 11780–11786. (c) Fernandez-Bachiller, M. I.; Perez, C.; Monjas, L.; Rademann, J.; Rodríguez-Franco, M. I. J. Med. Chem. 2012, 55, 1303–1317. (d) Aromí, G.; Barrios, L. A.; Roubeau, O.; Gamez, P. Coord. Chem. Rev. 2011, 255, 485–616. (e) Tao, G. H.; Guo, Y.; Joo, Y. H.; Twamley, B.; Shreeve, J. M. J. Mater. Chem. 2008, 18, 5524–5530.

(10) (a) N. E. Takach; Nelson, J. H. Inorg. Chem. 1981, 20, 1258–1263. (b) Beck, W.; Burger, K.; Fehlhammer, W. P. Chem. Ber. 1971, 104, 1816–1825.

(11) (a) Kim, Y.-J.; Kim, D.-H.; Lee, J.-Y.; Lee, S.-W. J. Organomet. Chem. 1997, 538, 189–197. (b) Kim, Y.-J.; Kwak, Y.-S.; Lee, S.-W. J. Organomet. Chem. 2000, 603, 152–160. (c) Wehlan, M.; Thiel, R.; Fuchs, J.; Beck, W.; Felhammer, W. P. J. Organomet. Chem. 2000, 613, 159–169. (d) Kim, Y.-J.; Joo, Y.-S.; Han, J.-T.; Han, W. S.; Lee, S.-W. Dalton Trans. 2002, 3611–3618. (e) Kim, Y.-J.; Kwak, Y.-S.; Joo, Y.-S.; Lee, S.-W. Dalton Trans. 2002, 144–151. (f) Kim, Y.-J.; Han, J.-T.; Kang, S.; Han, W. S.; Lee, S. W. Dalton Trans. 2003, 3357–3364. (g) Chang, X.; Lee, K.-E.; Kim, Y.-J.; Lee, S. W. Inorg. Chim. Acta 2006, 359, 4436–4440. (h) Kim, Y.-J.; Lee, K.-E.; Jeon,

H.-T.; Huh, H. S.; Lee, S. W. Inorg. Chim. Acta 2008, 361, 2159–2165.

(12) (a) Fantasia, S.; Petersen, J. L.; Jacobsen, H.; Cavallo, L.; Nolan, S. P. Organometallics 2007, 26, 5880–5889. (b) Huynh, H. V.; Han, Y.; Jothibasu, R.; Yang, J. A. Organometallics 2009, 28, 5395–5404.

(13) Huynh, H. V.; Guo, S.; Wu, W. Organometallics 2013, 32, 4591–4600.

(14) Allen, H. F.; Kennard, O.; Watson, D. G.; Bram-mer, L.; Guy Orpen, A.; Taylor, T. J. Chem. Soc. Perkin Trans 2 1987, 1987, S1–S19.

(15) (a) Boyarskiy, V. P.; Bokach, N. A.; Luzyanin, K. V.; Kukushkin, V. Y. Chem. Rev. 2015, 115, 2698–2779. (b) Hahn, F. E.; Langenhahn, V.; Pape, T. Chem. Com-mun. 2005, 5390–5392. (c) Hahn, F. E.; Langenhahn, V.; Meier, N.; Lügger, T.; Fehlhammer, W. P. Chem.-Eur. J. 2003, 9, 704–712. (d) Dumke, A. C.; Pape, T.; Kösters, J.; Feldmann, K.-O.; Schulte to Brinke, C.; Hahn, F. E. Organometallics 2013, 32, 289–299.

(16) (a) Palatinus, L.; van der Lee, A. J. Appl. Cryst. 2008, 975–984. (b) Palatinus, L.; Prathapa, S. J.; van Smaalen, S. J. Appl. Cryst. 2012, 575–580.

(17) Sheldrick, G. M. Acta Cryst. C. 2015, 71, 3–8.(18) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.;

Howard, J. A. K.; Puschmann, H. J. Appl. Cryst. 2009, 42, 339–341.

(19) Andrae, D.; Häußermann, U.; Dolg, M.; Stoll, H.; Preuß, H. Theor. Chim. Acta 1990, 77, 123–141.

(20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scal-mani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Iz-maylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; A., M. J.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; J., C.; Fox, D. J., Gaussian 09, Revision C.01, Wallingford, CT, Gaussian, Inc., 2010.

(21) (a) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154–2161. (b) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523–5527. (c) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1991, 95, 5853–5860. (d) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995–2001. (e) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. J. Comput. Chem. 2003, 24, 669–681.

(22) Wertz, D. H. J. Am. Chem. Soc. 1980, 102, 5316–5322.

(23) Cooper, J.; Ziegler, T. Inorg. Chem. 2002, 41, 6614–6622.

(24) (a) Novikov, A. S.; Kuznetsov, M. L.; Pombeiro, A. J. L. Chem.-Eur. J. 2013, 19, 2874–2888. (b) Kuznetsov, M. L.; Kukushkin, V. Y.; Pombeiro, A. J. L. J. Org. Chem. 2010, 75, 1474–1490. (c) Kuznetsov, M. L.; Kukushkin, V. Y. J. Org. Chem. 2006, 71, 582–592. (d) Kuznetsov, M. L.; Nazarov, A. A.; Kozlova, L. V.; Kukushkin, V. Y. J. Org. Chem. 2007, 72, 4475–4477.

8

Table of Contents artwork

9