Amidrazone Complexes from a Cascade Platinum(II)-Mediated Reaction between Amidoximes and Dialkylcyanamides

The aryl amidoximes R'C6H4C(NH2)═NOH (R' = Me, 2a; H, 2b; CN, 2c; NO2, 2d) react with the dialkylcyanamide platinum(II) complexes trans-[PtCl2(NCNAlk2)2] (Alk2 = Me2, 1a; C5H10, 1b) in a 1:1 molar ratio in CHCl3 to form chelated mono-addition products [3a-h]Cl, viz. [PtCl(NCNAlk2){NH═C(NR2)ON═C(C6H4R')NH2}]Cl (Alk2 = Me2; R' = Me, a; H, b; CN, c; NO2, d; Alk2 = C5H10; R' = Me, e; H, f; CN, g; NO2, h). In the solution, these species spontaneously transform to the amidrazone complexes [PtCl2{NH═C(NR2)NC(C6H4R')NNH2}] (7a-h; 36-47%); this conversion proceeds more selectively (49-60% after column chromatography) in the presence of the base (PhCH2)3N. The observed reactivity pattern is specific for NCNAlk2 ligands, and it is not realized for conventional alkyl- and arylnitrile ligands. The mechanism of the cascade reaction was studied by trapping the isocyanate intermediates [PtCl(NCO){NH═C(NR2)NC(C6H4R')NNH2}] (5a-h) and also by ESI-MS identification of the ammonia complexes [PtCl(NH3){NH═C(NR2)NC(C6H4R')NNH2}](+) ([6a-h](+)) in solution. The complexes [3a]Cl, [3c-h]Cl, 5a-h and 7a-h were characterized by elemental analyses, high resolution ESI-MS, IR, and (1)H NMR techniques, while 5b, 5d, 5g, 7b, and 7e were also studied using single-crystal X-ray diffraction.