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Dichloro(4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane)chromium(III) chloride, Dichloro(4,10-dibenzyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane) chromium(III) chloride, and Dichloro(4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2] hexadecane)chromium)(III) chloride have been prepared by the reaction of anhydrous chromium(III) chloride with the appropriate cross-bridged tetraazamacrocycle. Aquation of these complexes proved difficult, but Chlorohydroxo(4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane)chromium)(III) chloride was synthesized directly from chromium(II) chloride complexation followed by exposure or the reaction to air in the presence of water. The four complexes were characterized by X-ray crystal structure determination. All contain the chromium(III) ion in a distorted octahedral geometry and the macrocycle in the cis-V configuration, as dictated by the ethylene cross-bridge. Further characterization of the hydroxo complex reveals a magnetic moment of μeff = 3.95 B.M. and electronic absorbtions in acetonitrile at λmax = 583nm (ε = 65.8 L/cm·mol), 431nm (ε = 34.8 L/cm·mol) and 369nm (ε = 17 L/cm·mol).
Chromium(III) complexes have played an important historical role in the development of transition metal photochemistry, primarily due to their relative kinetic inertness.1 Enhancing that stability in order to study aquation rates of the ion has been achieved by using macrocyclic ligands.2 The photochemical behavior of Cr3+cyclam complexes have been thoroughly studied due to their low photoreactivity and long excited-state lifetimes.3–6 More recently, stable chromium(III) cyclam complexes have been used to generate NO by photolysis of bound NO2− in oxygenated aqueous solutions.7–10 Even more complex stability can be gained by bridging the macrocycle between donor atoms.11 Recently, cyclams bridged between adjacent nitrogen atoms have been complexed to Cr3+ in order to study the effect of ligand constraint on the photochemistry.12–14
Our own research program has centered on transition metal complexes of the ethylene cross-bridged tetraazamacrocycles first synthesized by Weisman.15,16 We have previously demonstrated a large advantage in kinetic stability verses unbridged macrocycles for these cross-bridged ligands.17–19 In addition, the cis-V configuration of the bridged macrocycle dictated by the short ethylene cross-bridge, results in a distorted octahedral geometry around most first-row transition metal ions.17–22 The extent of the distortion depends on the size of the parent macrocycle ring and the radius of the metal ion.18–22 These facts have led us to conclude that the chromium(III) complexes of the cross-bridged tetraazamacrocycles might have interesting photochemical properties. In addition, chromium is a catalytically active metal ion. Cross-bridged tetraazamacrocyles have demonstrated an ability to stabilize first-row transition metal ions to make them catalytically useful.18,23–27 For these various reasons, we have begun to synthesize and characterize chromium complexes of the cross-bridged tetraazamacrocycles. This report focuses on the synthesis and structural characterization of several of these chromium(III) complexes.
The ethylene cross-bridged ligands 4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2] tetradecane (1); 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (2); and 4,10-dibenzyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (3) were prepared according to literature procedures (Figure 1).15,16 Electronic spectra were recorded using a Shimadzu UV-240 UV-Vis Spectrometer. Magnetic moments were obtained on finely ground solid samples at ambient temperatures using a Johnson Matthey MSB Auto magnetic susceptibility balance. Metal complexation was carried out as follows:
Anhydrous CrCl3 (0.316 g, 2.00 mmol) and ligand 2 (0.508 g, 2.00 mmol) were added to 20 ml of anhydrous DMF in an inert atmosphere glovebox. The reaction was stirred at 50–60 °C for 18 hours during which it became a dark green solution. After cooling, the solution was removed from the glovebox and excess diethyl ether was added, causing a green precipitate to form. The precipitate was filtered off, washed with ether and dried under vacuum, giving 0.317 g (77%) of the green solid product. Anal. Calcd. (found) for CrC14H30N4Cl3·1DMF·5 H2O : C 35.45 (35.56); 8.23 (8.38); N 12.16 (11.82). X-ray quality, dark purple crystals were grown from ether diffusion into a DMF solution.
Anhydrous CrCl3 (0.316 g, 2.00 mmol) and ligand 1 (0.453 g, 2.00 mmol) were added to 20 ml of anhydrous DMF in an inert atmosphere glovebox. The reaction was stirred at 50–60 °C for 18 hours during which it became a dark purple solution. After cooling, the solution was removed from the glovebox and excess diethyl ether was added, causing a purple precipitate to form. The precipitate was filtered off, washed with ether and dried under vacuum, giving 0.731 g (95%) of the purple solid product. Anal. Calcd. (found) for CrC12H26N4Cl3 2·H2O : C 34.26 (34.03); 7.16 (6.87); N 13.32 (13.04). X-ray quality, dark purple crystals were grown from ether diffusion into a DMF solution.
Anhydrous CrCl3 (0.316 g, 2.00 mol) and ligand 3 (0.757 g, 2.00 mmol) were added to 20 ml of anhydrous DMF in an inert atmosphere glovebox. The reaction was stirred at 50–60 °C for 18 hours during which it became a dark purple solution with dark purple precipitate. After cooling, the solution was removed from the glovebox. The dark purple precipitate product was filtered off, washed with ether and dried under vacuum, giving 0.731 g (93%) of the purple solid product. Anal. Calcd. (found) for CrC24H34N4Cl3·H2O : C 51.94 (51.66); 6.53 (6.34); N 10.10 (10.09). X-ray quality, dark purple crystals were grown from ether diffusion into a DMF solution.
Ligand 2 (0.254 g, 1.00 mol) was dissolved in 20 ml of DMF. Anhydrous CrCl2 (0.123 g, 1.00 mol) was added and the solution was stirred at 50–60 °C for 48 hours in the air. The reaction originally had a green color, but became dark blue over the reaction time. The solution was cooled and filtered. Ether diffusion into the filtrate resulted in the formation of blue, X-ray quality crystals of the product (0.0958 g yield, 23%). Anal. Calcd. (found) for [CrC14H30N4Cl(OH)]Cl·1.67 H2O : C 39.63 (39.76); H 8.15 (8.39); N 13.20 (12.87).
The diffraction data sets were collected on a Stöe IPDS-II imaging plate diffractometer using Moα radiation (λ = 0.71073 Å). The crystals were kept at 150K during data collection using the Oxford Cryosystems Cryostream Cooler. The structures were solved using direct methods (SHELXS-97) and refined against F2 (SHELXL-97). H atoms were placed in idealised positions or located on the difference map, and refined using a riding model with C-H = 0.97 Å, N-H = 0.91 Å, O-H = 0.85 Å and Uiso(H) = 1.2 or 1.5 times Ueq of the carrier atom. All non-H atoms were refined anisotropically. The WinGX package was used for refinement and production of data tables, and ORTEP-3 was used for structure visualization.28 All ORTEP representations show ellipsoids at the 50% probability level. CCDC 699882 – 699885 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif.
The syntheses of the dichloride complexes were fairly straightforward, involving the complexation of CrCl3 with the respective ligand in DMF. The reactions were generally done in an inert atmosphere glovebox with anhydrous solvents in order to protect the proton sponge ligands16,19,21 from protonation, which can defeat complexation. The product obtained in all cases from this reaction was [Cr(L)Cl2]Cl. Interestingly, the complexes with cyclen-based ligands 1 and 3 were purple powders, while the complex of cyclam-based ligand 2 was isolated as a green powder. However, upon crystallization, this complex yielded purple crystals. Perhaps the color change was due to interaction with DMF, which was present according to the elemental analysis of the green powder, but was not present in the purple crystals.
Aquation of [Cr(2)Cl2]Cl was attempted in water, but no satisfactory product was obtained. For this reason, we attempted to synthesize the aqua complex from CrCl2 and the ligand by reacting them in DMF open to the air. The blue mono-hydroxo complex was isolated in low yield from this reaction. The protonation state of the bound oxygen is assumed from the total number of ions and the apparent 3+ oxidation state of the chromium in the complex. The magnetic moment of this complex was determined on the solid as μeff = 3.95 B.M., which corresponds to n = 3, the chromium(III) ion.29 UV-Vis Spectroscopy was performed on this complex in acetonitrile. The complex was a blue color in an acetonitrile solution. Three absorption bands were present with the greatest band occurring at λmax= 583 nm (ε = 66 L/cm·mol), a secondary peak at 431 nm (ε = 35 L/cm·mol) and a very small peak at approximately 369 nm (ε = 17 L/cm·mol). These bands are consistent with the distorted octahedral geometry of other Cr3+ cyclam derivatives.14,30,31
Crystals suitable for X-ray structure determination were obtained for all four complexes (Figures 2 and and3).3). The cis-V conformation expected to be dictated by the ligand cross-bridge is evident for all of the complexes structurally characterized here. Apparently, neither the identity of the metal ion, nor that of the alkyl substituents effects this conformation. To our knowledge, this same ligand conformation has been seen in all published metal complexes with ethylene cross-bridged cyclams and cyclens.
There are two independent Cr(1)Cl2+ cations in the unit cell of this structure, although the bond lengths and angles are similar in both. The thermal ellipsoids are quite a bit smaller for the cation labeled Cr(1), so the following discussion will refer to that cation. Comparison between the two cyclen-based structures (Figure 2) reveals little difference in the metal coordination geometry due to the differing alkyl groups. The bulkier benzyl substituents are directed away from the chloride binding site of Cr3+. In Mn2+ and Fe2+ complexes with ligand 3, π–π stacking between benzyl groups of adjacent molecules is apparent.32 In the Cr3+ structure, the chloride counter ions (not present in the other cases) appear to disrupt this interaction. The Nax-Cr-Nax and Neq-Cr-Neq bond angles for both cyclen complexes differ little: 160.83(19)° and 83.50(18)° for Cr(1)Cl2+ versus 160.35(19)° and 83.6(2)° for Cr(3)Cl2+, respectively. Likewise, the Cr-Nax and Cr-Neq bond lengths are very similar: averaging 2.103 Å and 2.050 Å for Cr(1)Cl2+ versus 2.117 Å and 2.064 Å for Cr(3)Cl2+, respectively. These parameters indicate that the larger benzyl groups do little to disrupt the binding of Cr3+ to the bridged cyclen macrocycle. Aromatic pendants can function as antennae to absorb more light for photochemical processes,8 motivating the use of this ligand.
The two cyclam based structures (Figure 3) differ not in macrocycle substitution, but in monodentate ligands at the cis sites not occupied by the cross-bridged macrocycle. The Nax-Cr-Nax and Neq-Cr-Neq bond angles for both cyclam complexes differ little: 172.46(11)° and 84.63(11)° for Cr(2)Cl2+ versus 172.23(14)° and 84.07(14)° for Cr(2)Cl(OH)+, respectively. Likewise, the Cr-Nax and Cr-Neq bond lengths are very similar: averaging 2.156 Å and 2.102 Å for Cr(2)Cl2+ versus 2.149 Å and 2.112 Å for Cr(2)Cl(OH)+, respectively. The Cr-Cl bond in Cr(2)Cl(OH)+ is 2.3341 Å, which is similar to the Cr-Cl bond distances in Cr(2)Cl2+, which average 2.331 Å. The Cr-Cl bond distances in the cyclen complexes average 2.318 Å for Cr(1)Cl2+ and 2.319 Å for Cr(3)Cl2+. These distances are similar to what has been reported in the literature for Cr3+-Cl, with an average for several other complexes being 2.335 Å.33 The Cr-OH bond in Cr(2)Cl(OH)+ is much shorter, at 1.964 Å. The same literature source33 lists an average for this bond at 1.929 Å, although it does not specify the oxidation state of the chromium. Our value is significantly longer than this, but it is definitely shorter than the Cr-OH2 value specified for six-coordinate Cr3+, given as 1.997 Å.33 The magnetic moment, UV-Vis spectrum, and number of other anions present in the crystal structure (vide supra) all point to a Cr3+-OH species as being most likely for his complex.
In a final comparison, the ring size of the parent macrocycle makes a difference on how fully the metal ion is engulfed by the bridged macrocycle. The easiest to use parameter to discuss this trend is the Nax-Cr-Nax bond angle, where Nax is an axially coordinated nitrogen. This bond angle averages 160.59° in the smaller cyclen cases, while it averages 172.35° for the cyclam complexes. The larger the bond angle, the closer to linearity and thus the better fit, or complementarity, between the ligand and the preferred octahedral geometry of the chromium(III) ion. A more subtle difference in the Neq-Cr-Neq angles shows the same trend: this angle averages 83.55° for the cyclen complexes and 84.35° for the cyclam complexes. Finally, the Cr—N bond distances are somewhat affected by the ligand size as well. The four Cr—N bond distances average 2.083 Å in the cyclen complexes, while this average is 2.130 Å in the cyclam complexes. The average value for a number of Cr—NR3 bonds in the literature is 2.093 Å.33
The synthesis, X-ray crystal structures, and some other characterization of four different Cr3+ complexes of ethylene cross-bridged tetraazamacrocycles have been presented. Because of the utility of these ligands in the coordination of other metal ions, and the catalytic and photochemical utility of Cr3+ coordination compounds, these complexes will likely provide much interesting chemistry for future study.
TJH thanks the Research Corporation (CC6505) and the Oklahoma State Regents for Higher Education for monetary support. This work was supported by NIH Grant P20 RR016478 (to TJH) from the INBRE Program of the National Center for Research Resources.
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