Dibromido(2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene)cobalt(III) bromide

In the title compound, [CoBr2(C14H24N4)]·Br, the CoIII ion is located on an inversion centre and possesses a distorted octahedral coordination geometry in which four nitrogen donors of the ligand molecule are in the equatorial plane and two Br− ions occupy both the axial sites to give a trans isomer. The Br- counter- anion is also located on an inversion centre.

In the title compound, [CoBr 2 (C 14 H 24 N 4 )]ÁBr, the Co III ion is located on an inversion centre and possesses a distorted octahedral coordination geometry in which four nitrogen donors of the ligand molecule are in the equatorial plane and two Br À ions occupy both the axial sites to give a trans isomer. The Brcounter-anion is also located on an inversion centre.

Comment
Most of the known synthetic macrocyclic ligands and their metal complexes have been prepared and characterized during the last few decades. Most commonly they are quadridentates containing nitrogen donor atoms, although compounds containing oxygen and sulfur donors are also known (Douglas, 1978). Metal template synthesis of multidentate and macromonocyclic ligands have been established over the last three decades as offering high yield and selective routes to new ligands and their complexes (Comba et al., 1986). Transition metal macrocyclic complexes have received much attention as active part of metalloenzymes (Chaudhary et al., 2002) as biomimic model compounds (Jones et al., 1979) due to their resemblance with natural proteins like hemerythrin and enzymes. They also played an important role as catalysts in oxidation and epoxidation processes (Chandra et al., 2008). There are some recent reports about some macrocyclic Co II and Co III complexes which showed high activity towards H 2 evolution electrochemically (Hu et al., 2007) or photochemically (Fihri, Artero, Pereira & Fontecave, 2008;Fihri, Artero, Razavet et al., 2008;Du et al., 2008). The title compound has been observed to evolve H 2 electrocatalytically in acetonitrile (Hu et al., 2007). Unfortunately, it is found that this compound does not show any catalytic activity towards H 2 evolution in a well known photosystem consisting of tris (2,2 ' -bipyridine)ruthenium(II) as a photosensitizer, methylviologen (N,N ' -dimethyl-4,4 ' -bipyridinium) as an electron mediator, and ethylenediaminetetraacetic acid disodium salt as a sacrificial electron donor. Because of our on-going studies on the H 2 -evolving activity of Pt II based molecular catalysts (Yamauchi et al., 2009), attempts have been made to obtain the Pt II complex of the present macrocyclic ligand. However the metal exchange from Co III to Pt II has been unsuccessful so far, presunably due to the extremely high stability of the Co III complex, during the course of these studies we have succeeded in the x-ray crystal structure determination of the present compound.
The Co III ion and the Brion involved as a counter anion are respectively located at crystallographic inversion centers.
Because of these requirements four nitrogen donors, two of them are independent, comprise a crystalloaphically planar geometry and the Co III ion is also located exactly on the same plane. The vector defined by the Co-Br bond is slightly declined from the vector which is peependicular to the basal plane consisting of the four nitrogen donor atoms which can be recognized from the N-Co-Br angles; [N2-Co1-Br1=91.78 (4)° and N1-Co1-Br1=89.14 (4)°]. It is also observed that the Co-N, N═C and N-C bond distances of 1.9208 (13), 1.288 (2) and 1.472 (2) Å, respectively, are in accordance with the reported values for similar Co III imine type macrocyclic complexes [2,9-dimethyl-3,10-diphenyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene)cobalt(III); Co-N, N═C and N-C distances are 1.923 (13), 1.278 (3) and 1.478 (3) Å, respectively] (Baird et al., 1993). The N2, C4, C5 i , C6 i and N1 i atoms form an envelope geometry in which the triangle defined by atoms C4, C5 i and C6 i is canted by 60.77 (11)° with respect to the least square plane defined by N1 i , C6 i , C4 and N2 atoms Recrystallization of the crude product by a method reported in the same paper resulted in the formation of dark green crystals suitable for X-ray diffraction analysis.

Refinement
All H atoms were placed in idealized positions (methyl C-H = 0.96 Å, methylene C-H = 0.97 Å), and included in the refinement in a riding-model approximation, with U iso (H) =1.5U eq (methyl C) and U iso (H) =1.2U eq (methylene C). In the final difference Fourier map, the highest peak was located 0.97 Å from atom Br1. The deepest hole was located 1.92 Å from atom Br1. Fig. 1. The molecular structure of (I) showing the atom-labeling scheme. Displacement ellipsoids are drawn at the 50% probability level.

Special details
Experimental. The first 50 frames were rescanned at the end of data collection to evaluate any possible decay phenomenon. Since it was judged to be negligible, no decay correction was applied to the data.