Synthesis and Characterization of a Tetraaza Macrocyclic Nickel(II) Complex Bearing Two Amidine Pendant Arms: Unprecedented Strong Metal-Pendant Arm Interaction

Abstract

There has been continuing interest in the preparation of new polyaza macrocyclic compounds bearing various types of functional pendant arms, since chemical properties of such compounds are influenced by the nature of the functional groups. Some coordinated organic nitriles (R-CN) in transition metal complexes react with H2O, R'OH, or R''-NH2 under relatively mild conditions to produce amides (RCONH2), imidate esters (R-C(=NH)-OR'), or amidines (R-C(=NH)NHR''). Recently, such reactions have been successfully applied to the preparation of functionalized polyaza macrocyclic compounds. For example, the trans-octahedral complexes [NiL] and [NiL] bearing two coordinated amide or imidate ester groups have been prepared by the reaction of [NiL] with water or methanol. It has been revealed that, in the case of [NiL], the axial Ni-N (pendant imidate ester group) distance (2.100(3) A) is considerably shorter than the in-plane Ni-N (tertiary amino group) distance (2.118(3) A) and is similar to the Ni-N (secondary amino group) distance (2.097(3) A). This trend is different from the observation that the axial Ni-X (X = N or O atom of a functional pendant arm) distances of most di-N-functionalized macrocyclic complexes, such as [NiL], [NiL], and [NiL], are much longer than all of the in-plane Ni-N distances. It has been also reported that the imidate ester groups of [NiL] are quite resistant to hydrolysis, unlike most coordinated imidate esters. This has been attributed to the strong Ni-N (pendant imidate ester group) interactions. However, the effects of strong axial Ni-X (functional pendant arm) interactions on chemical properties of such macrocyclic complexes are not thoroughly investigated. Therefore, we have been interested in the design and synthesis of various types of macrocyclic complexes with relatively short axial Ni-X (functional pendant arm) bonds. Herein, we report a new nickel(II) complex [NiL](ClO4)2· 2CH3CN (1) bearing two pendant amidine groups (-CH2C(=NH)-NH-(CH2)2CH3). The complex was prepared by the reaction of [NiL](ClO4)2 with n-propylamine. Interestingly, the Ni-N (pendant amidine group) distance of 1 is considerably shorter than the Ni-N (pendant imidate ester group) distance of [NiL]. Furthermore, the oxidation potential of 1 is extraordinarily lower than those of [NiL] and other related trans-octahedral nickel(II) complexes. The effects of the pendant amidine groups upon the structure and chemical properties of 1 are described. The cobalt(III) complexes [CoL] and [CoL], in which the amidine group (-C(=NH)-NH-) is incorporated into the macrocyclic framework, have been prepared. However, as far as we know, 14-membered tetraaza macrocyclic complexes bearing amidine pendant arms are not reported to date.