| Abstract |
The reaction between pyrazole (PzH), the N,N-disubstituted cyanamides NCNR2 (R-2 = Me-2, Et-2, C4H8, C5H10, C5H10O), and the nickel salts NiX2 center dot nH(2)O (X = NO3, OTf, ClO4, Cl, OTs, n = 6; Br, n = 3) affords complexes featuring ligands derived from the Ni-II-mediated cyanamide-pyrazole coupling. The composition of the products and their yields are strongly affected by the choice of NiX2 center dot nH(2)O and a molar ratio between the reactants. The [Ni(PzH)(2)\NH=C(NR2)Pz\(2)](X)(2) ([1-5](X)(2); R-2 = Me(2)1, Et(2)2, C(4)H(8)3, C(5)H(10)4, C5H10O 5; X = NO3, OTf - in all combinations, and R-2 = Me(2)1 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 center dot nH(2)O when the molar ratio between the reactants NiX2 : PzH : NCNR2 was 1 : 4 : (2.3). By using NiX2 center dot 6H(2)O (X = ClO4, OTs) and with the molar ratio NiX2 : PzH : NCNR2 1 : 3 : (3.5), the pure tris-chelate species [Ni\NH=C(NR2)Pz\(3)]X-2 ([6-7](ClO4)(2), R-2 = C(4)H(8)6, C(5)H(10)7; [8](OTs)(2), R-2 = Me-2) 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\NH-C(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)center dot Me2CO, and [9](OTf)(2) were established by single crystal X-ray diffraction. |
