Metal-mediated reactions between dialkylcyanamides and acetamidoxime generate unusual (nitrosoguanidinate)nickel(II) complexes

Abstract

The nitrosoguanidinate complexes [Ni{NH=C(NR₂)NN(O)}₂] (R₂ = Me₂1, (CH₂)₄O 2, (CH₂)₄3, (CH₂)₅4, (Me)Ph 5, Ph₂6, (p-MeC₆H₄)₂7) were obtained in low-to-moderate (12–26%) yields but reproducible yields in an unexpected metal-mediated reaction in MeOH between the nickel salt NiCl₂·2H₂O, N,N-disubstituted cyanamides NCNR₂, and the amidoxime MeC(=NOH)NH₂. These complexes were formed along with a spectrum of cyanamide–oxime coupling products. The IR and X-ray data indicate the delocalization within the NNO and NCN systems of the nitrosoguanidinate ligand. This delocalization was additionally confirmed by inspection of Wiberg bond indices for the selected bonds. In the X-ray structure of 5, the rare metallophilic contacts Ni⋯Ni between stacks of the square-planar complexes were detected and these non-covalent interactions were studied by non-relativistic and relativistic DFT calculations and topological analysis of the electron density distribution within the framework of Bader's theory (QTAIM method). The estimated strength of the Ni⋯Ni interactions is 1.3–1.9 kcal mol⁻¹ and they are mostly determined by crystal packing effects and weak attractive interactions between adjacent metal centers due to the overlapping of their d_z² and p_z orbitals.

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Introduction

The reactions of metal-activated conventional nitriles NCR (R = alkyl and/or aryl) and such unconventional nitrile species as cyanamides NCNR₂ have been intensively studied during the last two decades and impressive examples of differences in reactivity between these two types of nitriles are available in the literature (see, for instance, our review¹). Thus, the guanidine {PhN(H)}₂C=NH reacts with dialkylcyanamide species bearing the cis-(NCNR₂)₂Pt^II functionality to form eight-membered metallacycles stabilized due to an extended chain of conjugation (Scheme 1), whereas conventional nitrile complexes cis-(NCR)₂Pt^II form 1,3,5-triazapentadienate derivatives {N̲H=C(R)NC(R)=N̲H}Pt^II (Scheme 1, a).² Dialkylcyanamide ligands in cis-(NCNR₂)₂Pt^IV-type complexes undergo hydration coupling to form six-membered metallacycles,³ while conventional nitrile ligands form metal-bound carboxamides in their iminol form (b).⁴ Amidrazone derivatives were isolated in the reaction of amidoximes with trans-(NCNR₂)₂Pt^IV (c).⁵ However, when complexes with the trans-(NCR)₂Pt^IV moiety were treated with amidoximes, the reaction resulted in a bis-coupling product.⁶ These qualitative differences in reactivity between dialkylcyanamide and conventional nitrile ligands were documented mainly for platinum(II and IV) complexes and only a few relevant examples were reported for nickel(II) species.⁷

Scheme 1 Comparison of the metal-mediated conversions of cyanamide and nitrile ligands, emphasizing different reaction routes for these two types of nitrile species.

The reactions of nickel(II)-activated conventional nitriles with oximes were studied previously and a cascade reaction leading to the 1,3,5-triazapentadienate systems {N̲H=C(R)NC(R)=N̲H}Ni^II has been reported by Kopylovich and Pombeiro et al. (d).^7,8 Recently, we reported on the reaction between Ni^II-bound dialkylcyanamides and oximes that leads to the generation of coordinated and uncomplexed coupling products featuring the {NH=C(NR₂)ONCR¹R²}Ni^II and [NH=C(NR₂)ONCR¹R²]⁺ moieties. The latter reaction differs from the reaction with conventional nitriles giving 1,3,5-triazapentadienate systems (d).^8a,e

In continuation of our studies of the reactivity of disubstituted cyanamides and conventional nitriles with different nucleophiles in the presence of nickel(II),^8a,9 we turned our attention to amidoximes and observed an unusual transformation in the system Ni^II/NCNR₂/MeC(=NOH)NH₂ leading to the nitrosoguanidinate complexes [Ni{N̲H=C(NR₂)NN̲(O)}₂]. This transformation is realized only for cyanamides and not for conventional nitriles and for acetamidoxime and not for other oximes. We report herein on the reaction of acetamidoxime with cyanamides in the presence of nickel salts that leads to a novel family of nitrosoguanidinate complexes, whose chemistry is still almost unexplored because of the harmful and explosive properties of uncomplexed nitrosoguanidines.

Results and discussion

For the study of the Ni^II-involving reactions between cyanamides and amidoximes we addressed the nickel salts NiX₂·nH₂O (X = OTf, n = 0; Cl, n = 0, 2 or 6; Br, n = 3; I, n = 6; NO₃, n = 6), N,N-disubstituted cyanamides NCNR₂ (R₂ = Me₂, (CH₂)₄O, (CH₂)₄, (CH₂)₅, (Me)Ph, Ph₂, (p-MeC₆H₄)₂), and acetamidoxime MeC(=NOH)NH₂. These reactions were studied in methanol in which the nickel salts are rather well soluble. In the system NiX₂·nH₂O/NCNR₂/MeC(=NOH)NH₂, unexpected (nitrosoguanidinate)Ni^II complexes 1–7 (Scheme 2) were reproducibly obtained in low-to-moderate (12–26%) yields. The previously reported addition products featuring the {N̲H=C(NR₂)ON̲=C(NH₂)Me}Ni^II moiety, which were also formed in the reaction and released as oily residues, were identified by HRESI⁺-MS and IR spectroscopy.

Scheme 2 Ni^II-Mediated generation of nitrosoguanidinate species 1–7 (R₂ = Me₂1, (CH₂)₄O 2, (CH₂)₅3, (CH₂)₄4, Me/Ph 5, Ph₂6, (p-MeC₆H₄)₂7).

In the synthetic experiments, treatment of NCNR₂ with the amidoxime MeC(=NOH)NH₂ in the presence of NiX₂·nH₂O in methanol at 50 °C, followed by evaporation of the reaction mixture in air at RT (20–25 °C), results in the generation of 1–7 along with oily residues of other Ni-containing products. Among the tested nickel salts, the highest yields of the nitrosoguanidinate complexes were achieved for NiCl₂·2H₂O. The usage of the highly hydrated salt NiCl₂·6H₂O or the addition of one drop of H₂O to the reaction mixtures leads to similar yields to those obtained in the reaction with NiCl₂·2H₂O, whereas the usage of non-hydrated NiCl₂ leads to slightly lower yields. We optimized the reaction conditions by varying the ratio between the components of the system NiCl₂·2H₂O/NCNMe₂/MeC(=NOH)NH₂ (molar ratios 1/2.5/2.5, 1/2.5/5, 1/5/2.5, and 1/10/2.5 were used) and also temperature (RT, 50 °C, and 70 °C). The optimal condition is a system with a 1/2.5/2.5 ratio among the reactants and a temperature of 50 °C that gave 1 in 15% yield. The solvent choice is important; thus, 1–7 were not formed in acetone, while the application of undried ethanol in the system NiCl₂·2H₂O/NCNMe₂/MeC(=NOH)NH₂ also led to 1 and this compound was isolated in 12% yield.

The positive-mode HRESI-MS spectra of the oily residues obtained from NiCl₂·2H₂O/NCNR₂/MeC(=NOH)NH₂ systems exhibit several groups of peaks with characteristic isotopic patterns corresponding to the [H₂N=C(NR₂)ON̲=C(Me)NH₂]⁺, [Ni{NH=C(NR₂)ONC(Me)NH₂}₂]²⁺, [NiCl{NH=C(NR₂)ONC(Me)NH₂}]⁺, and [NiCl{NH=C(NR₂)ONC(Me)NH₂}₂]⁺ (R = Me₂, (CH₂)₄O, (CH₂)₄, (CH₂)₅, Ph₂, (Me)Ph) ions, which reflect the formation of the coupling product. It is noteworthy that these signals appeared in the HRESI⁺-MS spectra already after 2 h of reaction. The IR spectra of the reaction mixture NiCl₂·2H₂O/NCNMe₂/MeC(=NOH)NH₂ were measured at different time intervals (0 h, 2 h, and 4 h). In the first and the second spectra, an absorption band from ν(C≡N) at 2216–2264 cm⁻¹ was observed. In the IR spectra recorded after 4 h, this band was not found, whereas several new intensive bands from ν(C=N) in the range of 1635–1675 cm⁻¹ were observed instead that indicates total conversion of NCNR₂ into coordinated NH=C(NR₂)ONC(NH₂)Me. These observations give evidence of Ni^II-mediated amidoxime–cyanamide coupling leading to nickel(II) complexes featuring the {N̲H=C(NR₂)ON̲C(NH₂)Me}Ni^II functionality. These observations are in agreement with the previously reported Ni^II-mediated oxime–cyanamide coupling leading to nickel(II) complexes {N̲H=C(NR₂)ON̲C(R′)Me}Ni^II (R′ = Me, Ph)^8a and also Zn^II-¹⁰ and Pt^II-mediated¹¹ amidoxime–cyanamide coupling leading to complexes with N̲H=C(NR₂)ON̲C(NH₂)R′ ligands. The coupling products were not isolated in pure forms, probably due to high kinetic lability of nickel(II) complexes and the existence of nickel(II) products with coordinated carbimidoylamidoxime in several forms.^8a

We also conducted a metal-free blank experiment and found that keeping a mixture of NCNMe₂ and MeC(=NOH)NH₂ in a 1 : 1 molar ratio in the absence of any nickel(II) salt under the conditions of the nickel(II)-mediated reaction does not give the coupling products or uncomplexed nitrosoguanidine species even after 5 d. Therefore, one can conclude that the formation of nitrosoguanidine species is metal-mediated.

Other compounds that behave as simultaneous nucleophiles and “NO” sources, such as HONH₂ ¹² and HON=CMe₂,¹³ were tested in the Ni^II-involving reaction with NCNMe₂. However, no nitrosoguanidinate species were obtained and/or identified in the reaction mixture. The known 1,3,5-triazapentadienate complex [Ni{HN̲=C(OMe)NC(OMe)=N̲H}₂]¹⁴ was isolated from the system NiCl₂·2H₂O/NCNMe₂/HON=CMe₂ in 5% yield. The application of the other amidoximes R′C(=NOH)NH₂ (R′ = PhCH₂, Ph, p-BrC₆H₄) in the reaction does not lead to 1–7. We assume that this difference in reactivity could be explained by the greater nucleophilicity of HON=C(NH₂)Me than that of the other tested amidoximes. The application of conventional nitriles NCR (R = Me, Ph) in the reaction with NiCl₂·2H₂O/HON=C(NH₂)Me in MeOH does not lead to similar nitrosoamidine complexes. The difference between cyanamides and conventional nitriles in reactivity is mostly due to the presence, in cyanamides, of the NR₂ group exhibiting strong +M effect.

Based on our observations and the literature data, we assume that the formation of 1–7 is a result of a few consecutive Ni^II-mediated reactions. Initially, the acetamidoxime reacts with Ni^II-activated cyanamide to form coupling products featuring the {N̲H=C(NR₂)ON̲=C(NH₂)Me}Ni^II chelated ring (Scheme 3). The generation of stable isostructural coupling products was previously observed for the relevant system Ni^II/NCNR₂/ketoxime.^8a Probably, the other amidoximes are not active in the generation of (nitrosoguanidinate)Ni^II species due to their lower nucleophilicity.¹⁵

Scheme 3 A plausible mechanism for the generation of (nitrosoguanidinate)Ni^II complexes.

These coupling products could further react with wet MeOH to give O=C(NR₂)NH₂, HNR₂, NH₃, CO₂, and the parent amidoxime MeC(=NOH)NH₂ (c). Similar transformations were postulated for the Ni^II-mediated reaction of dialkylcyanamides with pyrazoles in undried MeOH.⁹ Ammonia, which was accumulated in the reaction mixture, reacted with either the (NCNR₂)Ni^II (b) or {HN̲=C(NR₂)ON̲=C(NH₂)Me}Ni^II (c) species, producing complexes featuring the {HN=C(NR₂)NH₂}Ni^II functionality via nucleophilic addition (b) or Pinner-like nucleophilic substitution (c) routes. Addition of external ammonia to the NiCl₂·2H₂O/NCNMe₂/MeC(=NOH)NH₂ system in methanol does not change the yield of the reaction.

Ammonia is likely generated in situ from NCNR₂ (or its coupling products) via nickel(II)-mediated hydrolysis, whereupon NH₃ adds to another nitrile group to give a guanidine moiety (c). Previously, one of us reported the relevant (Co^II/oxime)-mediated conversion of nitriles NCR (R = alkyl, aryl) leading to amidines RC(NH₂)=NH.¹⁶ In the latter transformation, again, the addition of external ammonia does not affect the yield of the formed amidines. Furthermore, the generation of coupling products with in situ generated ammonia was observed upon metal-mediated syntheses of 1,3,5-triazapentadienes (for a review on triazapentadiene complexes see ref. 8b), phthalocyanines,^8c,17 diacylamides,¹⁸N-acylamidines,¹⁹ imidic anhydrides,³ and amidines.^16,20 Our current experiments and the previous data^8a,17–20 suggest that the mechanism of the reaction shown in Scheme 3 is not simple and perhaps involves internal generation of ammonia at a nickel(II) center as one of the fast reaction steps.

Independently, MeC(=NOH)NH₂ could be subjected to oxidative hydrolysis to give an “NO” source. The relevant Ni^II-mediated nitrosation with hydroxylamine formed in situ upon ketoxime hydrolysis is known.²¹ Once NO is formed, it coordinates to the Ni^II center (d) and then couples with the guanidine ligand producing the {N̲H=C(NR₂)NN̲(O)}Ni^II group (e). Step e of the nitrosyl–guanidine coupling is analogous to that observed in the reaction between the amidinate complex [Fe{RNC(Me)NR}₃] (R = p-MeC₆H₄) and NO.²² This reaction leads to [Fe{N(O)N(R)C(Me)NR}{RNC(Me)NR}₂] and the complex is formed through intramolecular nitrosyl–amidinate coupling at the iron center. It is noteworthy that formally reverse reaction—i.e. when the NHC–N₂O adducts (NCH = 1,3-dimesitylimidazol-2-ylidene and 1,3-diisopropylimidazol-2-ylidene) featuring the same N₂CNN=O linkage react with the Ni(COD)₂ complex—includes the N–N bond cleavage leading to the nitrosyl nickel complex and guanidinate ligands.²³

We added gaseous NO to the reaction mixture NiCl₂·2H₂O/NCNMe₂/MeC(=NOH)NH₂ (MeOH, 50 °C, 1 d) and kept it for several days for slow evaporation. We observed the formation of 1, but its yield was exactly the same (15%) as in the experiment without the addition of NO. These observations also support the idea that NO is formed in situ as one of the fast reaction steps.

In the context of the generation of nitrosoguanidine nickel(II) species, it is noteworthy that uncomplexed nitrosoguanidines are known and they are typically synthesized by reduction of the corresponding nitroguanidines with Zn dust or RANEY® nickel/H₂;²⁴ other methods for the preparation of the nitrosoguanidines are still undeveloped. Nitrosoguanidine itself is an explosive crystalline material,²⁵N-alkylated nitrosoguanidines were studied as intermediates of the biosynthetic conversion of L-arginine to nitric oxide,²⁶ whereas another nitrosoguanidine derivative, viz. N-methyl-N′-nitro-N-nitrosoguanidine, is widely applied for directed mutagenesis in the selection of plants and microorganisms²⁷ and it is also used in industry as an exciting explosive with a high thermal stability.²⁸

Characterization of 1–7

Complexes 1–7 were characterized by C, H, and N elemental analyses, HRESI⁺-MS, FTIR, ¹H and ¹³C{¹H} NMR, TG/DTA, and single-crystal X-ray diffraction (for 1 and 3–5). Nitrosoguanidinate complexes 1–7 are stable until 194–240 °C (Fig. S7, S13, S19, S25, S31, S37 and S43; ESI†) and then gradually decompose at higher temperatures. All complexes gave satisfactory C, H, and N elemental analyses for the proposed formulas. The HRESI⁺-MS spectra exhibit several groups of peaks with characteristic isotopic patterns corresponding to the [M + H]⁺, [M + Na]⁺, [2M + Na]⁺, [3M + Na]⁺ ([M] = [Ni{NH=C(NR₂)NN(O)}₂]) ions.

In the IR spectra of 1–7, absorption bands from ν(N–H) (3352–3476 cm⁻¹), ν(C=N) (1566–1608 cm⁻¹), and ν(N=O) (1386–1414 cm⁻¹) appeared at the expected ranges (Fig. S3, S9, S15, S21, S27, S33 and S39; ESI†).²⁹ It is noteworthy that ν(N=O) of the nitroso group of the complexed nitrosoguanidinates exhibit substantially smaller values than those for the uncomplexed neutral H₂NC(=NH)NHNO species²⁵ (1520 cm⁻¹), thus demonstrating reduction of the multiplicity of the N=O bond upon deprotonation and complexation. The ν(N=O) absorption bands of 1–7 exhibit intermediate values between the values of double (1500–1480 cm⁻¹ in nitrosamines R₂N–N=O²⁹) and single (1310–1250 cm⁻¹ in azoxy species R–N=N → O²⁹) NO stretches, thus indicating intermediate bond order within the nitroso fragment. These data are consistent with X-ray structure parameters of 1 and 3–5 whose inspection suggests the delocalization. This delocalization was also supported by analysis of the calculated Wiberg bond indices for selected bonds (see later).

In the ¹H NMR spectra (Fig. S5, S11, S17, S23, S29, S35 and S41; ESI†), the signals of the =NH protons appeared in the 3.56–3.75 ppm range for 1 and 4 and 4.33–4.65 ppm for 2, 3, and 5–7. In the ¹³C{¹H} NMR, resonances of the methyl groups are at 21 ppm for 7 and in the range 37–41 ppm for 1 and 5; NCH₂ appear at 46–49 ppm for 2, 3, and 4; CH_Ar emerge at 126–131 ppm, and C_ipso at 126–142 ppm for 5, 6, and 7. One signal in the interval 164.91–167.25 ppm refers to the quaternary carbon atom of the nitrosoguanidinate moiety N(H)=C (Fig. S6, S12, S18, S24, S30, S36 and S42; ESI†).

Complexes 1–7 are red or purple colored. The absorption spectra of all complexes 1–7 display similar spectral patterns with several intensive absorption bands in the near-UV region. The most intense absorption bands are located in the 251–254 (lg ε 3.27–3.35) and 312–329 (lg ε 3.02–3.47) nm ranges along with four bands of low intensity in the intervals 395–407 (lg ε 2.26–2.63), 459–466 (lg ε 1.95–2.09), 526–538 (lg ε 2.01–2.20), and 558–570 (lg ε 1.93–2.14) nm (in CH₂Cl₂; Fig. S4, S10, S16, S22, S28, S34 and S40; ESI†). For theoretical discussions on the nature of the singlet excited electronic states of 1 and the assignment of the most significant transitions see the ESI.†

Solid-state structures of the nitrosoguanidinate complexes

X-ray structure determination. Single-crystal X-ray diffraction studies were performed for five compounds, viz. 1 and 3–5 (Fig. 1 and Fig. S45–S48, Table S2; ESI†). The geometric characteristics of 1 and 3–5 agree with each other and also with the reported X-ray structure of [Ni{N̲H=C(NH₂)NN̲(O)}].³⁰ In all structures, the nickel centers exhibit a planar environment (geometry index τ₄ = 0 (ref. 31)).

Fig. 1 Molecular structure of 5 with the atomic numbering scheme. ADP ellipsoids are drawn at the 50% probability level.

The N(1)–N(3) and N(1)–O(1) bond lengths are 1.3006(16)–1.320(4) Å and 1.240(3)–1.256(4) Å, respectively. These distances are coherent with those in the previously reported complex [Ni{N̲H=C(NH₂)NN̲(O)}₂] (N(1)–N(3) 1.285(9) Å; N(1)–O(1) 1.260(9) Å (ref. 30)). The distances N(1)–N(3) (1.3006(16)–1.320(4) Å) exhibit values similar to the N–N distance in pyridazine (1.304(19) Å)³² favoring intermediate N(1)N(3) bond order. The distances N(1)–O(1) have intermediate values between single N_sp²–O (1.394(18) Å in oximes³²) and double N=O (1.218(13) Å in the nitro group³²) bonds. Thus, the N(1)–N(3) and N(1)–O(1) bonds have intermediate bond order, which is in agreement with the IR data for complexes 1–7 (see above). The C(1)–N(3) bond length (1.391(2)–1.411(5) Å) is only slightly longer than the typical single C_sp²–N(2) bond (e.g. 1.376(11) Å in imidazole³²), thus reflecting its single bond character. These statements are consistent with the theoretically calculated bond orders (Tables 1 and 2).

Table 1 Calculated Wiberg bond indices for selected bonds in gas phase optimized structures of 1, 3, 4, and 5 (M06/6-311+G* level of theory)

Bond	1	3	4	5
Ni1–N2	0.56	0.55	0.56	0.56
N2–C1	1.44	1.45	1.44	1.45
C1–N4	1.20	1.19	1.20	1.18
C1–N3	1.15	1.15	1.15	1.16
N3–N1	1.40	1.40	1.40	1.39
N1–Ni1	0.55	0.55	0.55	0.55

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Table 2 Geometric characteristics of the uncomplexed nitrosoguanidine H₂N(=NH)NHNO,²⁵ [Ni{N̲H=C(NH₂)NN̲(O)}₂],³⁰1, and 3–5

Distance, Å	H2N(=NH)NHNO^25	R = NH2 ^30	R = NMe2 (1)	R = N(CH2)5 (3)	R = N(CH2)4 (4)	R = N(CH3)Ph (5)	R = NA(CAH3A)PhA (5)
Ni(1)–N(1)		1.879(7)	1.873(3)	1.8684(11)	1.877(2)	1.869(3)	1.878(3)
Ni(1)–N(2)	1.260(2)	1.851(6)	1.841(3)	1.8473(11)	1.846(2)	1.844(2)	1.841(3)
N(1)–O(1)	1.302(2)	1.260(9)	1.256(4)	1.2496(15)	1.250(3)	1.242(3)	1.240(3)
N(1)–N(3)	1.391(2)	1.285(9)	1.311(4)	1.3006(16)	1.305(3)	1.320(4)	1.307(4)
N(3)–C(1)	1.315(2)	1.404(11)	1.411(5)	1.4087(17)	1.408(3)	1.407(4)	1.404(4)
C(1)–N(2)	1.309(2)	1.297(11)	1.318(4)	1.3112(17)	1.314(3)	1.303(4)	1.319(4)
C(1)–N(4)		1.321(11)	1.332(4)	1.3377(17)	1.331(3)	1.347(4)	1.334(4)
N(4)–C(2)			1.457(5)	1.4684(17)	1.478(3)	1.457(4)	1.462(4)
N(4)–C(3)			1.471(4)			1.441(4)	1.440(4)

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Angle, °	(NH2)2CN2O	R = NH2	R = NMe2	R = N(CH2)5	R = N(CH2)4	R = N(CH3)Ph	R = NA(CAH3A)PhA
Ni(1)–N(1)–N(3)		119.2(5)	119.0(2)	119.84(9)	119.33(15)	118.91(19)	118.73(19)
Ni(1)–N(1)–O(1)	113.5(1)	124.0(5)	123.3	123.46(9)	122.89(16)	124.4(2)	123.6(2)
N(1)–Ni(1)–N(2)	110.8(1)	80.9(3)	81.46(12)	81.42(5)	81.39(9)	81.42(11)	81.45(11)
Ni(1)–N(2)–C(1)	115.0(2)	113.4(6)	114.1(2)	113.61(9)	113.29(16)	113.9(2)	113.8(2)
O(1)–N(1)–N(3)	124.8(2)	116.8(7)	117.7(3)	117.67(11)	117.78(19)	116.6(3)	117.7(2)
N(1)–N(3)–C(1)	120.2(2)	107.8(7)	107.8(3)	108.01(11)	107.16(18)	107.1(2)	108.0(2)
N(3)–C(1)–N(2)		118.6(7)	117.6(3)	117.75(12)	118.8(2)	118.6(3)	117.9(3)
N(3)–C(1)–N(4)		114.7(7)	115.2(3)	114.81(11)	115.1(2)	116.0(3)	117.3(3)
N(2)–C(1)–N(4)		126.7(8)	127.1(3)	127.41(12)	126.1(2)	125.4(3)	124.8(3)

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All other bonds are of normal values (for a detailed discussion see the ESI†). Inspection of the crystallographic data did not reveal the presence of any metallophilic contacts in crystal structures of 1, 3, and 4, but in 5 this type of non-covalent interaction was detected and confirmed theoretically (see the Computational studies section later).

Analysis of the Wiberg bond indices. This analysis for some selected bonds in the gas phase optimized structures of 1, 3, 4, and 5, computed using the natural bond orbital (NBO) partitioning scheme at the M06/6-311+G* level of theory, reveals a noticeable degree of electron density delocalization in the chelated fragments of these species, viz., Wiberg bond indices are close to 1.5 for N2–C1 and N3–N1 contacts and noticeably deviate from 1.0 for C1–N4 and C1–N3 contacts (Table 1). These theoretical calculation data agree well with the experimental (IR and X-ray diffraction) observations and the results of our previous DFT study on electron density delocalization in [Ni{NH=C(NMe₂)ON=CMe₂}₂(H₂O)₂]²⁺ (ref. 8a) and [H₂N]C(R)ON]C(R′)NH₂]⁺ (ref. 33) species.

Computational studies. Inspection of the crystallographic data reveals the presence of metallophilic interactions Ni⋯Ni in the crystal structure of 5 obtained at 100 K (structure 5-100) and at room temperature (structure 5-RT). Indeed, in both cases the distance between appropriate metal centers is lower than the sum of their van der Waals (vdW) radii. One should take into consideration that such radii cannot be unambiguously defined and to date several values of vdW radii for nickel based on different approaches have been proposed, varying from 1.63 (ref. 34) to 2.40 Å.³⁵ Taking into consideration that metallophilic interactions often impart important properties to functional materials (viz., magnetism, conductivity, or luminescence³⁶) and, in addition, that only one theoretical study was performed by us on the metallophilic nature of Ni^II⋯Ni^II short contacts and their energies were reported,⁹ in the current work a detailed computational study of this phenomenon has been performed.

In order to confirm or disprove the hypothesis on the existence of these non-covalent interactions and to quantify their energies from a theoretical viewpoint, we carried out non-relativistic and relativistic DFT calculations and topological analysis of the electron density distribution within the framework of Bader's theory (QTAIM method)³⁷ for the oligomeric clusters 5-100 and 5-RT as model systems (Table 3 and Table S3†). This approach has already been successfully used by us in studies on the non-covalent interactions and properties of coordination bonds in various transition metal complexes.^9,38 The contour line diagram of the Laplacian distribution ∇²ρ(r), bond paths, and selected zero-flux surfaces for Ni⋯Ni metallophilic interactions in 5-100 is shown in Fig. 2. To visualize the studied non-covalent interactions, reduced density gradient (RDG) analysis³⁹ was carried out, and the RDG isosurface for 5-100 was plotted (Fig. 2).

Fig. 2 Contour line diagram of the Laplacian distribution ∇²ρ(r), bond paths and selected zero-flux surfaces (left) and the RDG isosurface referring to the non-covalent interactions (right) for metallophilic interactions Ni⋯Ni in 5-100 (M06/6-311+G* level of theory). Bond critical points (3, −1) are shown in blue, nuclear critical points (3, −3) – in pale brown, ring critical points (3, +1) – in orange, and cage critical points (3, +3) – in light green. Length is given in units of Å; RDG isosurface values are given in hartree.

Table 3 Values of the density of all electrons – ρ(r), Laplacian of electron density – ∇²ρ(r), energy density – H_b, potential energy density – V(r), and Lagrangian kinetic energy – G(r) (hartree) at the bond critical points (3, −1), corresponding to metallophilic interactions Ni⋯Ni in 5-100 and 5-RT, as well as energies of these interactions E_int (kcal mol⁻¹), defined by two approaches

Method/basis	ρ(r)	∇^2ρ(r)	H b	V(r)	G(r)	E int ^a	E int ^b
a E int = −V(r)/2.^40 b E int = 0.429G(r).^41
5 -100
M06/6-311+G*	0.010	0.023	0.000	−0.005	0.005	1.6	1.4
M06/DZP-DKH	0.010	0.023	0.000	−0.005	0.005	1.6	1.4
M06/MDF10(Ni) and 6-311+G* (other atoms)	0.010	0.022	0.000	−0.006	0.006	1.9	1.6
5 -RT
M06/6-311+G*	0.009	0.021	0.001	−0.004	0.005	1.3	1.4
M06/DZP-DKH	0.009	0.021	0.001	−0.004	0.005	1.3	1.4
M06/MDF10(Ni) and 6-311+G* (other atoms)	0.009	0.021	0.000	−0.005	0.005	1.6	1.4

---

The QTAIM analysis demonstrates the presence of appropriate bond critical points (BCPs) for metallophilic interactions Ni⋯Ni both in 5-100 and in 5-RT. The lengths of the corresponding contacts (3.30 Å for 5-100 and 3.36 Å for 5-RT) are slightly larger than the sum of Bondi's (the shortest)³⁴ vdW radii for Ni (3.26 Å), but noticeably lower than Batsanov's⁴² and Alvarez's³⁵ vdW radii for Ni (4.00 and 4.80 Å, respectively). The low magnitude of the electron density (0.009–0.010 hartree), positive values of the Laplacian (0.021–0.023 hartree), and zero or close to zero (0.001 hartree) positive energy density in these BCPs are typical for non-covalent interactions.

We have defined energies for these contacts according to the procedures proposed by Espinosa et al.⁴⁰ and Vener et al.⁴¹ (Table 3). The estimated strength of these non-covalent interactions is 1.3–1.9 kcal mol⁻¹. It is noteworthy that previously⁹ we studied relevant Ni⋯Ni intramolecular metallophilic interactions in the dinuclear [Ni₂(μ₂-Ph₂Pz)₂(1,3,5-triazapentadienate)₂] species that were stronger (the estimated energy is 3.5–4.4 kcal mol⁻¹). The balance between the Lagrangian kinetic energy G(r) and potential energy density V(r) at the BCPs reveals the nature of these interactions in 5: if the ratio −G(r)/V(r) > 1 is satisfied, then the nature of appropriate interactions is purely non-covalent; in case the ratio −G(r)/V(r) < 1, some covalent component is observed.⁴³ Based on this criterion one can state that the covalent contribution is absent for metallophilic interactions Ni⋯Ni both in 5-100 and in 5-RT. The estimate of the properties of electron density in BCPs and energies for the metallophilic interactions Ni⋯Ni in 5-100 and 5-RT is almost independent of the basis set used (non-relativistic or relativistic approaches).

We believe that in 5 the emergence of nickel(II)⋯nickel(II) contacts is determined by crystal packing effects and weak attractive interactions between the adjacent metal centers due to the overlapping of their d_z² and p_z orbitals (Fig. 3).

Fig. 3 Weak attractive interactions between the adjacent metal centers due to the overlapping of their d_z² and p_z orbitals.

The results of our combined DFT and QTAIM studies reveal that theoretically determined energies for the ligand-supported⁹ and ligand-unsupported (this work) metallophilic interactions nickel(II)⋯nickel(II) are certainly lower than those found for the Pd^II⋯Pd^II (4.3–6.0 kcal mol⁻¹ (ref. 44)) and Pt^II⋯Pt^II (3.9–11.7 kcal mol⁻¹ (ref. 45)) systems, and the strength of metallophilic interactions for compounds of group 10 elements logically increases on going down the group.

Concluding remarks

The results of this work can be considered from a few perspectives. Firstly, we observed an unusual transformation in the system NiCl₂·2H₂O/NCNR₂/MeC(=NOH)NH₂ in methanol leading to the nitrosoguanidinate complexes [Ni{N̲H=C(NR₂)NN̲(O)}₂]. Although the achieved yields are rather low and this reaction at present cannot be recommended for synthetic purposes, the observed transformation explicitly demonstrates that nitrosoguanidinate metal species can be obtained from stable and broadly commercially available precursors, thus avoiding the utilization of toxic (for high toxicity of N-nitroso species, see ref. 46) and explosive nitrosoguanidines for sequestering metal centers.

Secondly, to our knowledge, the coordination chemistry of nitrosoguanidine/ate species is at a very early stage of growth, and metal nitrosoguanidinate derivatives are currently represented exclusively by the [M{N̲H=C(NH₂)NN̲(O)}₂] (M = Ni, Pd), [Co{N̲H=C(NH₂)NN̲(O)}₂(NH₂CON₂O)], and [Cu{N̲H=C(NH₂)NN̲(O)}₂]·Me₂SO complexes that were obtained by the reaction of pre-prepared nitrosoguanidine (H₂NC(=NH)NHNO²⁵) with appropriate metal salts.³⁰ The years that had passed since the preparation of these complexes did not bring any further nitrosoguanidinate metal compounds. In this work, we present a novel route to metal nitrosoguanidinates and we hope that, after further development, it might represent a method for the generation of nitrosoguanidinate metal species from rather safe precursors and for the further elaboration of nitrosoguanidine coordination chemistry.

Thirdly, we observed a novel difference in reactivity between conventional nitriles and cyanamides that typifies the family of so-called push–pull nitriles.

Finally, the retrosynthetic approach to the obtained nitrosoguanidinate species suggests that these complexes could be generated via metal-mediated coupling between nitrosyl ligands and guanidines. This reaction should be relevant to the reported coupling between the amidinate ligands in [Fe{RNC(Me)NR}₃] and NO²² that leads to the [Fe{N(=O)N(R)C(Me)NR}{RNC(Me)NR}₂] complex (52% yield) formed through the nitrosyl–amidinate integration at the iron center. As a further development of the project, we intend to study nitrosyl–guanidinate coupling involving, on the one hand, complexes featuring linear and/or bent nitrosyl ligands and, on the other hand, various N-substituted guanidines. Further studies directed toward widening the family of metal nitrosoguanidinates and establishment of their relevance to our nitroso-⁴⁷ and dinitrosoalkane⁴⁸ ligand systems are in progress in our group.

Experimental section

Materials and instrumentation

All dialkylcyanamides, nickel(II) salts and solvents were purchased from commercial sources and used as received. Nickel(II) chloride dihydrate was prepared according to the reported procedure.⁴⁹ The amidoximes R′C(=NOH)NH₂ (R′ = Me, PhCH₂, Ph, p-BrC₆H₄) were synthesized according to literature methods.⁵⁰ Microanalyses (C, H, N) were carried out on a Euro EA3028-HT analyzer. Electrospray ionization mass spectra were obtained on a Bruker micrOTOF spectrometer equipped with an electrospray ionization (ESI) source. The instrument was operated in positive ion mode using an m/z range of 50–3000. The capillary voltage of the ion source was set at −4500 V (HRESI⁺-MS) and the capillary exit at +100 V. For HRESI⁺-MS the complexes were dissolved in MeOH and MeCN was used as an ionization agent. The absorption spectra were recorded on a Shimadzu UV 1800 spectrophotometer in CH₂Cl₂. FTIR spectra were recorded on Shimadzu FTIR-8400S (4000–400 cm⁻¹) and Shimadzu IRAffinity-1S (4000–300 cm⁻¹) spectrometers using KBr pellets. ¹H and ¹³C{¹H} NMR (400.13 and 100.613 MHz, respectively) spectra were recorded on a Bruker Avance 400 spectrometer in Me₂SO-d₆ and CDCl₃ at ambient temperature; residual solvent signals were used as the internal standard (Fig. S19–S20, S25–S26, S31–S32, S37–S38, S43–S44, S49–S50 and S55–S56; ESI†). TG/DTA measurements (Fig. S21, S27, S33, S39, S45, S51 and S57; ESI†) were performed with a NETZSCH TG 209 F1 Libra thermoanalyzer and MnO₂ powder was used as a standard. The initial weights of the samples were in the range 0.6–1.6 mg. The experiments were run in an open alumina crucible in a stream of argon at a heating rate of 10 K min⁻¹. The final temperature of the experiments was 600 °C. Analysis of thermal data was performed with Proteus analysis software.

X-ray diffraction

Single-crystal XRD experiment was carried out on Agilent Technologies “Xcalibur” and “SuperNova” diffractometers with monochromated MoKα and CuKα radiation sources, respectively. The crystal was kept at 100(2) K during experiment. The structures were solved by means of the Superflip⁵¹ structure solution program using Charge Flipping and refined by means of the ShelXL⁵² program incorporated into the OLEX2 program package.⁵³ Empirical absorption corrections were applied using the CrysAlisPro (Agilent Technologies, 2012) program package using spherical harmonics implemented in SCALE3 ABSPACK scaling algorithm. CCDC numbers 1530060 and 1530062–1530064† contain the supplementary crystallographic data for the structures.

Computational details

Single point calculations based on the experimental X-ray geometries of 5-100 and 5-RT (quasi-solid-state approach) were carried out at the DFT level of theory using the M06 functional⁵⁴ (this functional was specifically developed to describe weak dispersion forces and non-covalent interactions) with the help of the Gaussian-09 ⁵⁵ program package. Three approaches were used, viz. (i) the Douglas–Kroll–Hess 2^nd order scalar relativistic calculations⁵⁶ that requested a relativistic core Hamiltonian were carried out using DZP-DKH basis sets⁵⁷ for all atoms; (ii) the effective core potential calculations were carried out using the multielectron fit fully relativistic energy-consistent pseudopotential MDF10 of the Stuttgart/Cologne group that described 10 core electrons and the appropriate contracted basis set for the nickel atom⁵⁸ and the standard 6-311+G* basis sets for other atoms; (iii) the non-relativistic calculations were carried out using the standard 6-311+G* basis sets for all atoms. The topological analysis of the electron density distribution with the help of the atoms in molecules (QTAIM) method developed by Bader³⁷ was performed using the Multiwfn program (version 3.3.8).⁵⁹ The full geometry optimization procedure was carried out with the help of the Gaussian-09 ⁵⁵ program package at the CAM-B3LYP/6-311++G** (for 1 in dichloromethane solution) and M06/6-311+G* (for 1, 3, 4, and 5 in gas phase) levels of theory. No symmetry restrictions have been applied during the geometry optimization and experimental X-ray data were used as starting points. The solvent effects were taken into account using the SMD continuum solvation model of Truhlar et al.⁶⁰ The Hessian matrix was calculated analytically for all optimized structures in order to prove the location of correct minima (no imaginary frequencies). The Cartesian atomic coordinates of model structures are presented in Table S3 (ESI†).

Synthetic work

Ni^II-Mediated reaction of dimethylcyanamide with amidoximes: generation of 1–7. MeC(=NOH)NH₂ (7.4 mg, 0.1 mmol) powder was added to a stirred suspension of NiCl₂·2H₂O (40 mg, 0.24 mmol) in MeOH (15 mL) placed in a 20 mL round bottomed flask. Then any one of the corresponding cyanamides NCNR₂ (R₂ = Me₂, (CH₂)₄O, (CH₂)₄, (CH₂)₅, (Me)Ph, Ph₂, (p-MeC₆H₄)₂; 0.6 mmol) was added to the mixture and it was left to stand for 24 h at 50 °C in the closed flask without stirring. The color of the homogeneous solution turned from light green to dark brown or dark violet (R₂ = Ph₂). The resulting solutions were evaporated in open air at RT, furnishing, after 2–3 d, crystalline precipitates of 1–7 along with the released oily residues of other Ni-containing products. The precipitates formed were separated from the oily residues by washing with three 1.5 mL portions of MeOH and dried in a desiccator over CaCl₂ at RT.
[Ni{N̲H=C(NMe₂)NN̲(O)}₂] (1). 10.7 mg, 15%. Red needles. Anal. calcd for C₆H₁₄N₈NiO₂: C, 24.94; H, 4.88; N, 38.78. Found: C, 25.01; H, 4.88, N, 38.77%. HRESI⁺, m/z: 289.0674 ([M + H]⁺ requires 289.0671), 311.0496 ([M + Na]⁺ requires 311.0491), 599.1078 ([2M + Na]⁺ requires 599.1084), 889.1634 ([3M + Na]⁺ requires 889.1632). ν_max(KBr)/cm⁻¹: 3374 m, ν(N–H); 2936 w, ν(C–H); 2886 w, ν(C–H); 1608 s ν(C=N), 1386 s ν(N=O). ¹H NMR (CDCl₃, δ): 3.05 (s, 6H, CH₃), 3.19 (s, 6H, CH₃), 3.72 (s, 1H, NH), 3.75 (s, 1H, NH). ¹³C{¹H} NMR (CDCl₃, δ): 37.20 and 40.47 (CH₃), 167.20 (N(H)=C). On heating in a capillary (2° min⁻¹) this complex turns brown and decomposes at 270–272 °C. TG/DTA: 210–244 °C (weight loss 34.7%; NH₂CONMe₂, requires 34.9%), 244–609 °C (weight loss 43.2%; 75.5% NiO, requires 74.7%). Crystals suitable for XRD were obtained from the reaction mixture by its slow evaporation.
[Ni{N̲H=C(NC₄H₈O)NN̲(O)}₂] (2). 10.4 mg, 12%. Red powder. Anal. calcd for C₁₀H₁₈N₈NiO₄: C, 32.20; H, 4.86; N, 30.04. Found: C, 32.29; H, 4.82, N, 30.00%. HRESI⁺, m/z: 113.0713 ([NCNC₄H₈OH]⁺ requires 113.0715), 373.0886 ([M + H]⁺ requires 373.0883), 395.0702 ([M + Na]⁺ requires 395.0702), 767.1516 ([2M + Na]⁺ requires 767.1507). ν_max(KBr)/cm⁻¹: 3403 w, ν(N–H); 2983 w, ν(C–H); 2921 w, ν(C–H); 2867 w, ν(C–H); 1599 s, ν(C=N); 1414 s, ν(N=O). ¹H NMR (DMSO-d₆, δ): 3.52–3.55 (m, 4H, CH₂^α), 3.61–3.64 (m, 4H, CH₂^β), 4.65 (s, 2H, NH). ¹³C{¹H} NMR (DMSO-d₆, δ): 48.46 (OCH₂), 65.50 (NCH₂), the N(H)=C signal was not detected. On heating in a capillary (2° min⁻¹) this complex turns brown and decomposes at 284–285 °C. TG/DTA: 220–299 °C (weight loss 53.9%), 299–610 °C (weight loss 19.9%).
[Ni{N̲H=C(NC₅H₁₀)NN̲(O)}₂] (3). 10.3 mg, 12%. Red needles. Anal. calcd for C₁₂H₂₂N₈NiO₂: C, 39.05; H, 6.01; N, 30.36. Found: C, 39.03; H, 6.15, N, 30.22%. HRESI⁺, m/z: 369.1315 ([Ni{NHC(NC₅H₁₀)NN(O)}₂H]⁺ requires 369.1297), 391.1126 ([M + Na]⁺ requires 391.1117), 759.2321 ([2M + Na]⁺ requires 759.2336), 1129.3452 ([3M + Na]⁺ requires 1129.3510). ν_max(KBr)/cm⁻¹: 3362 m, ν(N–H); 2936 w, ν(C–H); 2850 w, ν(C–H); 1596 s, ν(C=N); 1404 s, ν(N=O). ¹H NMR (DMSO-d₆, δ): 1.51–1.57 (m, 12H, CH₂), 3.50 (s, 8H, NCH₂), 4.44, 5.20 (two s, 2H, NH). ¹³C{¹H} NMR (CDCl₃, δ): 23.92, 25.79 (CH₂), 46.25, 48.95 (br NCH₂), 165.59 (N(H)=C). On heating in a capillary (2° min⁻¹) this complex turns brown and decomposes at 255–256 °C. TG/DTA: 204–295 °C (weight loss 63.9%), 295–610 °C (weight loss 16.8%). Crystals suitable for XRD were obtained from the reaction mixture by its slow evaporation.
[Ni{N̲H=C(NC₄H₈)NN̲(O)}₂] (4). 10.5 mg, 13%. Red needles. Anal. calcd for C₁₀H₁₈N₈NiO₂: C, 35.22; H, 5.32; N, 32.86. Found: C, 35.02; H, 5.25; N, 32.60%. HRESI⁺, m/z: 341.1003 ([M + H]⁺ requires 341.0984), 363.0836 ([M + Na]⁺ requires 363.0804), 703.1764 ([2M + Na]⁺ requires 703.1710), 1045.2612 ([3M + Na]⁺ requires 1045.2571). ν_max(KBr)/cm⁻¹: 3352 m, ν(N–H); 2954 w, ν(C–H); 2876 w, ν(C–H); 1600 s, ν(C=N); 1396 m, ν(N=O). ¹H NMR (CDCl₃, δ): 1.94 (m, 4H, CH₂), 2.05 (m, 4H, CH₂), 3.38 (m, 4H, NCH₂), 3.55 (s, 2H, NH), 3.61 (m, 4H, NCH₂). ¹³C{¹H} NMR (CDCl₃, δ): 25.13, 25.74 (CH₂), 46.15, 49.58 (NCH₂), 164.91 (N(H)=C). TG/DTA: 212–322 °C (weight loss 57.1%), 322–484 °C (weight loss 10.5%). Crystals suitable for XRD were obtained from the reaction mixture by its slow evaporation.
[Ni{N̲H=C(N(CH₃)Ph)NN̲(O)}₂] (5). 15.3 mg, 15%. Red needles. Anal. calcd for C₁₆H₁₈N₈NiO₂: C, 46.52; H, 4.39; N, 27.13. Found: C, 46.57; H, 4.50, N, 27.22%. HRESI⁺, m/z: 413.0977 ([M + H]⁺ requires 413.0984), 435.0800 ([M + Na]⁺ requires 435.0804), 847.1708 ([2M + Na]⁺ requires 847.1710). ν_max(KBr)/cm⁻¹: 3394 m, ν(N–H); 3050 w, ν(C–H); 2928 w, ν(C–H); 1608 s, ν(C=N); 1578 s, ν(CN) and/or δ(CH_Ar), 1496 m, ν(Ph), 1392 s, ν(N=O). ¹H NMR (DMSO-d₆, δ): 3.36 (s, 6H, CH₃), 4.11, 5.38 (two s, 2H, NH), 7.13 (m, 6H, Ph), 7.47 (m, 4H, Ph). ¹³C{¹H} NMR (CDCl₃, δ): 41.34 (CH₃), 126.47, 128.70, 130.56 (CH_Ar); 141.73 (C_ipso), 167.02 (N(H)=C). On heating in a capillary (2° min⁻¹) this complex turns brown and decomposes at 259–260 °C. TG/DTA: 215–452 °C (weight loss 59.7%), 452–609 °C (weight loss 5.7%). Crystals suitable for XRD were obtained from the reaction mixture by its slow evaporation.
[Ni{N̲H=C(NPh₂)NN̲(O)}₂] (6). 33.1 mg, 26%. Purple thin needles. Anal. calcd for C₂₆H₂₂N₈NiO₂: C, 58.13; H, 4.13; N, 20.86. Found: C, 58.03; H, 4.50, N, 20.22%. HRESI⁺, m/z: 537.1295 ([M + H] requires 537.1297), 559.1116 ([M + Na]⁺ requires 559.1117), 1095.2397 ([2M + Na]⁺ requires 1095.2336). ν_max(KBr)/cm⁻¹: 3476 w, ν(N–H); 3394 w, ν(N–H), 3060 w, ν(C–H); 1566 s, ν(C=N); 1492 m δ(CH_Ar); 1392 s, ν(N=O). ¹H NMR (CDCl₃, δ): 4.43 (s, 2H, NH), 7.30 (m, 20H, Ph). ¹³C{¹H} NMR (CDCl₃, δ): 127.30, 127.99, 129.93 (CH_Ar), 142.08 (C_ipso), 167.2 (N(H)=C). On heating in a capillary (2° min⁻¹) this complex turns brown and decomposes at 284–285 °C. TG/DTA: 240–262 °C (weight loss 8.6%), 262–276 °C (weight loss 27.5%), 276–324 °C (weight loss 28.3%), 324–610 °C (weight loss 10.1%).
[Ni{N̲H=C(N(p-Tol)₂)NN̲(O)}₂] (7). 33.8 mg, 24%. Purple thin needles. Anal. calcd for C₃₀H₃₀N₈NiO₂: C, 60.73; H, 5.10; N, 18.89. Found: C, 59.95; H, 5.50, N, 19.22%. HRESI⁺, m/z: 593.1933 ([M + H]⁺ requires 593.1923), 1185.3823 ([2M + H]⁺ requires 1185.3769). ν_max(KBr)/cm⁻¹: 3394 m, ν(N–H); 3034 w, ν(C–H), 3062 w, ν(C–H); 2920 w, ν(C–H); 1570 s, ν(C=N); δ(CH_Ar); 1390 s, ν(N=O). ¹H NMR (CDCl₃, δ): 2.32 (s, 12H, CH₃), 4.33 (s, 2H, NH), 7.08–7.13 (d, 16H, CH_Ar). ¹³C{¹H} NMR (CDCl₃, δ): 21.19 (CH₃), 126.31 (C_ipso), 126.92, 130.48 (CH_Ar), 137.07 (C_ipso), 167.25 (N(H)=C). On heating in a capillary (2° min⁻¹) this complex turns purple and decomposes at 289–290 °C. TG/DTA: 214–349 °C (weight loss 62.6%), 349–569 °C (weight loss 9.1%).

Acknowledgements

The authors are grateful to the Russian Science Foundation (grant 14-13-0060). Physicochemical studies were performed at the Center for Magnetic Resonance, Center for X-ray Diffraction Studies, Center for Chemical Analysis and Materials Research, and the Center for Thermogravimetric and Calorimetric Research (all belonging to Saint Petersburg State University). The authors thank the reviewers for valuable suggestions.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1530060 and 1530062–1530064. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt01960b

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