Amidrazone complexes from a cascade platinum(II)-mediated reaction between amidoximes and dialkylcyanamides

Article abstract of DOI:10.1021/ic4000878

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(NCNAlk ₂)₂] (Alk₂ = Me₂, 1a; C ₅H₁₀, 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; NO ₂, 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(NR ₂)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′)NNH ₂}] (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 ¹H NMR techniques, while 5b, 5d, 5g, 7b, and 7e were also studied using single-crystal X-ray diffraction.