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The 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 CH2Cl2at room temperature for ∼10 min to furnish the gold(III) tetrazolates [n-Bu4N][AuCl3(CN4R)] (3a–d), which were obtained in 89–95% yields after purification. Subsequent reaction between equimolar amounts of 3a–dand methyl trifluoromethanesulfonate (MeOTf) proceeds in CH2Cl2at −70 °C for ∼30 min to give the corresponding gold(III) complexes [AuCl3(CaN(Me)N2NbR)]a–b(5a–d) bearing 1,4-disubstituted tetrazol-5-ylidene ligands (69–75%). Complexes 3a–dwere obtained as pale-yellow solids and characterized by elemental analyses (C, H, N), HRESI–-MS, FTIR, and 1H and 13C{1H} NMR spectroscopies. Complexes 5a–dwere obtained as colorless solids and characterized by elemental analyses (C, H, N), HRESI+-MS, and 1D (1H and 13C{1H}) and 2D (1H,13C-HMBC) NMR spectroscopies. In addition, the structures of 3a, 3b, 3c, and 5awere established by single-crystal X-ray diffraction. Analysis of the Wiberg bond indices (WI) for gas phase-optimized model structures of 3a–cand 5acomputed using the natural bond orbital (NBO) partitioning scheme disclosed a higher degree of electron density delocalization in the CN4moiety of carbene 5awhen compared to tetrazolate 3a–c. Results of DFT calculations for a model system 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 formation of an acyclic NNNCN intermediate, whereas the cyclization is the rate-determining step.

The reaction between the equimolar amounts of cis-[PdCl2(CNR1)2] (R1= cyclohexyl (Cy) (1), tBu (2)) and the carbohydrazides R2CONHNH2(R2= Ph (5), 4-ClC6H4(6), 3-NO2C6H4(7), 4-NO2C6H4(8), 4-CH3C6H4(9), 3,4-(MeO)2C6H3(10), naphth-1-yl (11), fur-2-yl (12), 4-NO2C6H4CH2(13), Cy (14), 1-(4-fluorophenyl)-5-oxopyrrolidin-3-yl (15), (pyrrolidin-1-yl)C(O) (16)) proceeds in refluxing CHCl3for ca. 4 h. A subsequent workup provided the aminocarbene species cis-[PdCl2{C(NHNHC(O)R2)N(H)R1}(CNR1)] (18–33) in good to excellent (80–95%) isolated yields. The coupling of equimolar amounts of cis-[PdCl2(CNR1)2] (R1= Cy (1), tBu (2), 2,6-Me2C6H3(Xyl) (3), 2-Cl-6-MeC6H3(4)) and PhSO2NHNH2(17) occurs similarly and affords the aminocarbenes cis-[PdCl2{C(NHNHS(O)2Ph)N(H)R1}(CNR1)] (34–37; 60–90%). Complexes 18–37were characterized by elemental analyses (C, H, N), ESI+-MS, IR, and 1D (1H, 13C{1H}) and 2D (1H,1H-COSY, 1H,13C-HMQC/1H,13C-HSQC, 1H,13C-HMBC) NMR spectroscopy, as well as by X-ray diffraction for three species (34, 37, and 38).

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-6-MeC6H3, 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.