Abstract
The crystal structure of di-chlorido-(4,10-dimethyl-1,4,7,10-tetra-aza-bicyclo-[5.5.2]tetra-deca-ne)chromium(III) hexa-fluorido-phosphate, [CrCl2(C12H26N4)]PF6, (I), has monoclinic symmetry (space group P21/n) at 150 K. The structure of the related di-chlorido-(4,11-dimethyl-1,4,8,11-tetra-aza-bicyclo-[6.6.2]hexa-deca-ne)chromium(III) hexa-fluorido-phosphate, [CrCl2(C14H30N4)]PF6, (II), also displays monoclinic symmetry (space group P21/c) at 150 K. In each case, the Cr(III) ion is hexa-coordinate with two cis chloride ions and two non-adjacent N atoms bound cis equatorially and the other two non-adjacent N atoms bound trans axially in a cis-V conformation of the macrocycle. The extent of the distortion from the preferred octa-hedral coordination geometry of the Cr(III) ion is determined by the parent macrocycle ring size, with the larger cross-bridged cyclam ring in (II) better able to accommodate this preference and the smaller cross-bridged cyclen ring in (I) requiring more distortion away from octa-hedral geometry.
Highlights
The crystal structure of dichlorido(4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane)chromium(III) hexafluoridophosphate, [CrCl2(C12H26N4)]PF6, (I), has monoclinic symmetry at 150 K
The extent of the distortion from the preferred octahedral coordination geometry of the CrIII ion is determined by the parent macrocycle ring size, with the larger cross-bridged cyclam ring in (II) better able to accommodate this preference and the smaller cross-bridged cyclen ring in (I) requiring more distortion away from octahedral geometry
Ethylene cross-bridged tetraazamacrocycles were introduced to coordination chemists in 1990 by Weisman and Wong (Weisman et al, 1990). Their transition metal complexes have become important to the fields of oxidation catalysis (Yin et al, 2007; Dong et al, 2013), medical/biological imaging (Boswell et al, 2004; Sprague et al, 2007; Silversides et al, 2011) and chemokine receptor antagonism (Lewis et al, 2005; Valks et al, 2006; Smith et al, 2012) due to the combination of restricted macrocycle configuration and kinetic inertness inherent to these ligands.Chromium(III) complexes have played an important role in characterizing new ligands due to their relative kinetic inertness (Cotton & Wilkinson, 1988)
Summary
Ethylene cross-bridged tetraazamacrocycles were introduced to coordination chemists in 1990 by Weisman and Wong (Weisman et al, 1990). The N atoms of each macrocycle occupy four coordination sites, while two chloride ions in a cis arrangement complete the coordination of CrIII This socalled cis-V conformation, expected to be dictated by the ligand cross-bridge, is apparent for both of the complexes structurally characterized here. Neither the identity of the metal ion, nor that of the alkyl substituents affects this conformation This same conformation has been seen in all known metal complexes with ethylene cross-bridged cyclam and cyclen ligands. The ring size of the parent macrocycle alters the degree to which the metal ion is engulfed by the bridged macrocycle This is most clearly evident in the N2—Cr1—N4 bond angle between two axially bound nitrogen atoms. Symmetry codes: (i) x; Ày þ 12; z þ 12; (ii) Àx; y þ 12; Àz þ 12; (iii) Àx þ 1; y À 12; Àz þ 12; (iv) x; Ày þ 32; z þ 12
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