Nickel(ii)-mediated cyanamide-pyrazole coupling highlights distinct reactivity of NCNR2 and NCR nitrile ligands

The reaction between pyrazole (PzH), the N,N-disubstituted cyanamides NCNR2 (R2 = Me2, Et2, C4H8, C5H10, C5H10O), and the nickel salts NiX2·nH2O (X = NO3, OTf, ClO4, Cl, OTs, n = 6; Br, n = 3) affords complexes featuring ligands derived from the NiII-mediated cyanamide–pyrazole coupling. The composition of the products and their yields are strongly affected by the choice of NiX2·nH2O and a molar ratio between the reactants. The [Ni(PzH)2{NHC(NR2)Pz}2](X)2 ([1–5](X)2; R2 = Me21, Et22, C4H83, C5H104, C5H10O 5; X = NO3, OTf – in all combinations, and R2 = Me21 for X = Cl, Br) complexes (named here as bis-chelate complexes), featuring two chelated ligands derived from the cyanamide–pyrazole coupling, were obtained selectively from NiX2·nH2O when the molar ratio between the reactants NiX2 : PzH : NCNR2 was 1 : 4 : (2.3). By using NiX2·6H2O (X = ClO4, OTs) and with the molar ratio NiX2 : PzH : NCNR2 1 : 3 : (3.5), the pure tris-chelate species [Ni{NHC(NR2)Pz}3]X2 ([6–7](ClO4)2, R2 = C4H86, C5H107; [8](OTs)2, R2 = Me2) were isolated. Structurally similar complexes are not formed under the coupling conditions when conventional nitriles NCR (R = Alk; for instance, NCMe) were applied as the nitrile component of the reaction. Quantum-chemical calculations demonstrate that generation of the tris-chelate [Ni{NHC(NR2)Pz}3]2+ species is thermodynamically driven with the energetic difference 4.6 kcal mol−1 between appropriate bis- and tris-chelate products, while for conventional nitrile derivatives the bis-chelate products are thermodynamically more preferable. The structures of eight complexes, viz. [1](Cl)2, [1–2](NO3)2, [4](NO3)2, [1–2](OTf)2, [3](OTf)2·Me2CO, and [9](OTf)2 were established by single crystal X-ray diffraction.