Unexpected transformation of mono- to bis-macrobicyclic dimethylglyoximate framework in a chloroform solution: Photochemical, MALDI-TOF MS and X-ray diffraction studies

Abstract

Abstract Crystallization of the 2-thiopheneboron-capped tris-dimethylglyoximate cobalt(II) clathrochelate from a crude chloroform unexpectedly gave single crystals of Co III Co II Co III bis-clathrochelate: in this complex, two macrobicyclic frameworks are linked by bridging Co 2 + ion. The N–O distances in the donor oxime groups of these X-rayed mono- and bis-clathrochelates are governed by the nature of their capping groups (1.38 and 1.31 A for the boron and cobalt “caps”). The average Co–N distances of the CoN 6 -coordination polyhedra of an encapsulated cobalt ion strongly depend on its oxidation states (1.97 and 1.91 A for the cobalt(II) and cobalt(III) ions). Co 2 + ion is shifted from the center of this polyhedron by 0.2 A due to the Jahn–Teller distortion, whereas Co 3 + ions occupy the centers of the ligands' cavities. The geometry of the Co II N 6 and Co III N 6 polyhedra is substantially different: due to the rigidity of the macrobicyclic tris-dioximate ligand and high Shannon radius of Co 2 + ion, the corresponding clathrochelate has a TP geometry, whereas the smaller Co 3 + ion causes the rotational–translation shortening of the CoN 6 polyhedron with a decrease in its height h from 2.45 to 2.24 A. The photochemical experiments with irradiation of a chloroform solution of the initial clathrochelate showed that the most probable pathway to the bis-clathrochelate includes photoinitiated radical oxidation of chloroform with air oxygen. The reactive species formed cause opening, rearrangement and re-capping of the macrobicyclic framework affording a new Co III Co II Co III bis-clathrochelate.