Crystal structures of two cross-bridged chromium(III) tetraazamacrocycles

Abstract

The crystal structure of dichlorido (4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]-tetradecane)chromium(III) hexafluoridophosphate, [CrCl 2 (C 12 H 26 N 4 )]PF 6 , (I), has monoclinic symmetry (space group P2 1 /n) at 150 K. The structure of the related dichlorido (4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane)-chromium(III) hexafluoridophosphate, [CrCl 2 (C 14 H 30 N 4 )]PF 6 , (II), also displays monoclinic symmetry (space group P2 1 /c) at 150 K. In each case, the Cr III ion is hexacoordinate 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 octahedral 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 octahedral geometry. Chemical contextEthylene cross-bridged tetraazamacrocycles were introduced to coordination chemists in 1990 by Weisman and Wong (Weisman et al., 1990). Since then, 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). Yet, to date, only one report of the chromium coordination chemistry of these macrobicyclic ligands has appeared in the literature (Maples et al., 2009). In order to expand the range of metal ions that can be coordinated by these remarkable ligands (Hubin, 2003), we are exploring further the structural chemistry of chromium cross-bridged tetraazamacrocyclic complexes and report synthesis and crystal structures of dichlorido (4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane)chromium (III) hexafluoridophosphate, (I), and dichlorido (4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane)chromium(III)hexafluoridophosphate, (II). Structural commentaryEach of the title compounds crystallizes with a single positively-charged metal complex and one PF 6 À anion in the asymmetric unit. The metal ion in each complex adopts a distorted octahedral geometry. The N atoms of each macrocycle occupy four coordination sites, while two chloride ions in a cis arrangement complete the coordination of Cr III . This socalled cis-V conformation, expected to be dictated by the ligand cross-bridge, is apparent for both of the complexes structurally characterized here. Figs. 1 and 2 illustrate the local geometry about Cr III in (I) (dimethyl bridged-cyclen complex) and (...

Chromium(III)-pyrazole complexes. X-Ray crystal structures, 1H NMR investigation of ligand fluxional behavior and EPR studies

“…Complexes contain 6-coordinate pseudooctahedral Cr-centres with mer-CrCl 3 L 3 (L = pyrazole ligand) coordination sphere for 1 -6 and trans-CrCl 2 L 4 with trans pyrazoles in an anti configuration for 7. Bond lengths and angles are within the expected range for Cr III complexes (Table 2) [29,30]. For all the mer-Cr III complexes, the Cr-N bonds trans to a halogen atom are longer than those trans to a pyrazole, similarly to the analogous mer-[FeCl 3 (pz*H) 3 ] [8].…”

Catalytic oxidation properties of an acid-resistant cross-bridged cyclen Fe(ii) complex. Influence of the rigid donor backbone and protonation on the reactivity

“…Cross-bridged tetraazamacrocycles are known for their ability to yield very stable 50 and inert 30,51,52 transition metal complexes 22,[29][30][31]51,[53][54][55][56][57][58][59] due to their extreme rigidity. Consequently, they have proven to be able to stabilize high oxidation states.…”

Mapping of the Chromium and Iron Pyrazolate Landscape

“…Crystallographic data collection and refinement parameters for 1-6 and 7-8 are summarized in Table 14 and Bond lengths and angles are within the expected range for Cr III complexes (Table 3, Table 4, Table 5). 66,67 For all the mer-Cr III complexes, the Cr-N bonds trans to a halogen atom are longer than those trans to a pyrazole, similar to the analogous mer- 72.3013 72.2939 6Cr-N 2.066 2 Table 6. Intermolecular hydrogen bonding interactions are observed for 1-6 that play major role on the crystal packing of the complexes.…”