Synthesis, Characterization and Crystal Structure of trans-Aquahydroxobis(2,2-dimethyl-1,3-propanediamine)chromium(III) Diperchlorate

Experimental

Caution! Although we experienced no difficulty with the perchlorate salt described in this study, the compound is potentially explosive, and should be handled with very great care.

Materials and Synthesis. The free ligand 2,2-dimethyl-1,3-propanediamine was obtained from Aldrich Chemical Co. and used as supplied. All chemicals were reagent grade materials and used without further purification. The starting material trans-[Cr(Me2tn)2Cl2]Cl was prepared as described previously.10 A suspension of trans-[Cr(Me2tn)2Cl2]Cl (0.5 g, 1.38 mmol) in water (5 mL) containing pyridine (3 mL) was heated at 100 ℃ for 15 minutes. Then sodium perchlorate (1.0 g, 8.17 mmol) was added, and the solution was allowed to stand at room temperature for one day to give the pink crystals suitable for X-ray structural analysis. Yield 62%. Anal. Found: C, 24.40; H, 6.31; N, 11.35%. Calc. for Cr(C10H32N4O2)(ClO4)2: C, 24.45; H, 6.57; N, 11.40%. Visible spectral data for an aqueous solution, λmax in nm (ε in M−1 cm−1): 403 (68.5), 502 (41.2). IR spectrum (KBr, cm−1): 3423 m and 3371 s (ν OH), 3224 vs and 33170 s (ν NH), 2966 vs and 2878 s (ν CH), 1730 m, 1604 vs and 1563 vs (δ NH2), 1479 vs (δ CH2), 1374 m, 1223 m, 1152 vs, 1118 s (ν CN), 1081 vs (νas ClO), 1038 s, 988 s and 975s, 889 s and 841 m (ρ NH2), 810 m and 775 s (ρ CH2), 682 s, 636 s and 627 vs (δ OClO), 541 m, 460 m and 438 m (ν Cr-N).

Physical Measurements. The room-temperature visible absorption spectrum was recorded on a HP 8453 diode array spectrophotometer. The mid-infrared spectrum was obtained using a JASCO 460 plus series FT-IR spectrometer in a KBr pellet. Analyses for C, H and N were performed on a Carlo Erba 1108 Elemental Vario EL analyzer.

Crystal Structure Analysis. A plate pink crystal of title complex having approximate dimensions 0.21 × 0.21 × 0.05 mm3 was coated with paratone-N oil because the crystal loses its crystallinity on exposure to air. The diffraction data was measured with synchrotron radiation (λ = 0.62998 Å) on a 2D SMC ADSC Quantum-210 detector with a silicon (111) double-crystal monochromator at the Pohang Accelerator Laboratory, Korea and a nitrogen cold stream (100 K). The ADSC Quantum-210 ADX program11 was used for collection of data, and HKL3000sm (Ver. 703r)12 was used for cell refinement, reduction, and absorption correction. The structure was solved by direct methods with SHELXTL-XS (Ver. 2013/4) program and refined by full-matrix leastsquares calculations with the SHELXTL-XL (Ver. 2013/4) program.13 Molecular graphics were produced using DIAMOND-3.14 All non-hydrogen atoms were refined with anisotropic displacement coefficients. Hydrogen atoms were assigned isotropic displacement coefficients, U(H) = 1.2U(C, N) or 1.5U (C-methyl), and their coordinates were allowed to ride on their respective carbons except coordinated one of the water molecule hydrogen. The least-squares refinement of the structural model was performed under geometry restraints and displacement parameter restraints such as, DFIX, EXYZ and EADP for coordinated hydroxyl group and water molecule. The crystallographic experimental data and refinement parameters are summarized in Table 1.

Table 1.Crystallographic data for trans-[Cr(Me2tn)2(OH)(H2O)](ClO4)2

 

Results and Discussion

Crystallography. To determine the molecular structure unambiguously, single-crystal X-ray structure determination was carried out which revealed that the title complex salt consisted of a [Cr(Me2tn)2(OH)(H2O)]2+ cation and two perchlorate anions. The structure analysis showed the monoclinic space group of P21/n with Z = 2. Principal bond lengths and angles are listed in Table 2. An ellipsoid plot of the complex molecule together with atomic numbering is illustrated in Figure 2. Hydrogen atoms are shown as circles of arbitrary radii.

Table 2.Symmetry code: (i) −x, −y+1, −z.

Figure 2.Molecular structure of trans-[Cr(Me2tn)2(OH)(H2O)](ClO4)2.

In molecule the chromium(III) center is roughly coplanar with the four N atoms and adopts an octahedral geometry, where the four N atoms of two bidentate Me2tn ligands occupy the equatorial sites. The two O atoms of hydroxy group and water molecule coordinate to the Cr metal center in the trans arrangement. The two chelate rings of Me2tn ligands in complex are only in stable anti chair-chair conformation. The conformational arrangement is very similar to anti conformation which was found in trans-anti-[Cr(Me2tn)2(NCS)2]NCS·1/2H2O, trans-anti-[Cr(Me2tn)2Cl2]ClO4 and trans-anti-[Cr(Me2tn)2Cl2]2ZnCl4.6-9 The anti conformational arrangement is quite different from syn conformer observed in trans-anti/syn-[Cr(Me2tn)2Cl2]Cl, and trans-anti/syn-[Cr(Me2tn)2Br2]2Br2·HClO4·H2O containing the two conformers.5,6

The difference observed in the conformations of the two chelate rings seems to be responsible for the differences in the crystallographic disorder, packing forces, and hydrogenbonding networks among the solvent molecules, complex cations and anions in the complexes. The Cr–N bond distances for nitrogen atoms of the Me2tn are in the range of 2.084 (2) to 2.088 (2) Å and these lengths are in agreement with those observed in trans-anti-[Cr(Me2tn)2(NCS)2]NCS·1/2H2O, trans-anti-[Cr(Me2tn)2Cl2]ClO4, trans-anti-[Cr(Me2tn)2Cl2]2ZnCl4, trans-anti-[Cr(tn)2Cl2]ClO4, trans-anti-[Cr(tn)2Br2]ClO4, trans-[Cr(en)2Br2]ClO4, trans-[Cr(15aneN4)F2]ClO4, trans-[Cr(2,2,3-tet)F2]ClO4, cis-β-[Cr(2,2,3-tet)Cl2]ClO4 and cis-β-[Cr(2,2,3-tet)(N3)2]Br.6-9,15-21 The oxygen atoms in trans position coordinated chromium(III) both carry one well defined H atom, the displacement parameter of which refined to an acceptably value. Each oxygen atom is also associated with a weaker electron density peak at the correct position to define a second hydrogen atom. The displacement parameters associated with these minor peaks refine to acceptable values if fixed site occupancy of 0.5 is assumed. This situation can be modelled in terms of disorder across the two oxygen sites of the water and hydroxyl ligands. One hydroxyl group plus two perchlorate anions thus balance the +3 charge on chromium, as required. The postulation of disorder at the axial sites accounts for the nearly equal Cr-O bond lengths. Because of X-ray diffraction is such a weak analytical technique with regard to hydrogen placement, the ultimate interpretation of the structure remains unclear. A similar interpretation of a hydroxo/aquo ambiguity is found in trans-[Cr(5,12-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) (OH)(H2O)](ClO4)2·H2O.22 The mean Cr1-O1A bond is 1.971 (2) Å in length and the O1A-Cr1-O1Ai bond angle is constrained to 180°. The Cr-O bond length is slightly shorter than the distance for the Cr-OH2 and longer than that for Cr-OH as expected. The bond length is also comparable to the values of Cr-O bond lengths found in trans-[Cr(cyclam)(OH)(H2O)]Br2·H2O, trans-[Cr(cyclam)(OH)(H2O)](SF3SO3)2, trans-[Cr(cyclam)(OH)2]ClO4, trans-[Cr(cyclam)(nic-O)2]ClO4, cis-[Cr(cyclam)(ONO)2]NO2, [Cr(cyclam)ox]ClO4, [Cr(acac)(edda)] and [Cr(cyclam)(acac)](ClO4)2·1/2H2O.23-29

The six-membered ring is in stable chair conformation with the N1-Cr1-N2 angle of 90.34 (4)°. The average Cr–N–C, N–C–C and C–C–C bond angles in the six-membered chair rings are 119.05 (8), 114.08 (2) and 109.43 (2)°, respectively. The presence of two substituted methyl groups on the carbon of the Me2tn molecule does not disturb the essential features of the chelate ring. The mean C-N and C-C distances in Me2tn ligand are typical, and comparable to those observed in trans-[Cr(Me2tn)X2]+ (X = Cl, Br, NCS, N3) complexes.5-9 The uncoordinated ClO4− anions remain outside the coordination sphere. The perchlorate anion shows a distorted tetrahedral arrangement of oxygen atoms around the central chloride atom with Cl-O distances in the range 1.427 (2)–1.447 (2) Å and the O-Cl-O angles ranging from 107.61 (9) to 110.96 (8)°.

In the title complex, the crystal lattice is stabilized by hydrogen bonding interactions between the NH groups of the Me2tn ligand, hydrogens of OH or H2O ligands, and the oxygens of the ClO4− anion. Thus, the [Cr(Me2tn)2(OH)(H2O)]2+ cations and ClO4− anions are linked together by a network of hydrogen bonds (Fig. 3). The differences found in the conformations of the two six-membered chelate rings may be attributed to the differences in the hydrogen bonding networks, compositional disorder, and crystal packing forces between the chromium(III) complex cation and counter anion present in the lattice.

Figure 3.Hydrogen-bonded thick sheet structure, viewed along the b axis. Hydrogen bonding is denoted by blue dashed lines (color figure online).

 

Conclusions

The X-ray crystallographic analysis of newly prepared complex indicates that the chromium atom is in a distorted octahedral environment, coordinated by two bidentate, 2,2-dimethyl-1,3-propanediamine ligands and two O atoms of hydroxy group and water molecule in trans position. The average Cr-N(Me2tn) and Cr-O bonds are 2.086 (2) and 1.907 (9) Å, respectively. The carbon atoms of the two Me2tn chelate rings in the complex adopt a stable anti conformation on the coordination plane not including syn conformer. The crystal-packing and hydrogen-bonding forces as well as the counter anion dimensions are probably responsible for the observed different conformation.