Oxidative Dehydrogenation of Hexaaza Macrocyclic Nickel(II) Complexes Bearing Two N-Alkyl Pendant Arms: Preparation of New Complexes Containing Imidoylamidine Linkages

Abstract

Metal-promoted reactions of coordinated organic compounds, such as oxidative dehydrogenation reactions, have attracted particular attention because of their potential applications in organic chemistry, catalysis, and biochemistry. The saturated tetraaza macrocyclic nickel(II) complex [NiL] is known to react with HNO3 to yield [NiL] containing four isolated imino groups. The reaction of [NiL] with HNO3 produces [NiL] and [NiL] containing only one or two imino groups in the six-membered chelate ring(s). On the other hand, [NiL] reacts with H2O2 to produce the dinuclear complex [Ni2L], in which two unsaturated macrocyclic units are linked together by a C=C bond. Literature up to date clearly shows that the number and position of the imino groups in the product is strongly influenced by the structural characteristics of the macrocycle. In this work, we attempted the oxidative dehydrogenation of the hexaaza macrocyclic complexes [NiL] and [NiL] bearing two N-alkyl groups and prepared [NiL] (L = 1,8dimethyl-1,3,6,8,10,13-hexaazacyclotetradeca-2,6,9,13-tetraene) and [NiL] (L = 1,8-dibenzyl-1,3,6,8,10,13-hexaazacyclotetradeca-2,6,9,13-tetraene) containing two imidoylamidine (–N=CH-N(R)-CH=N-) linkages. Interestingly, [NiL](ClO4)2 consists of two stable conformational isomers and forms a 2D supramolecular structure. Synthesis and chemical properties of [NiL] and [NiL] are reported. Imidoylamidines are widely used as precursors for the synthesis of various organic compounds and polymers, exhibiting useful properties. Recently, some nickel(II) complexes of imidoylamidines (HN=CR-NH-CR=NH; R = alkyl group) have been also prepared and characterized. However, [NiL] and [NiL] are rare examples of 14membered hexaaza macrocyclic complexes that contain two imidoylamidine linkages.