Structural and spectroscopic properties of trans-dichlorobis(2,2-dimethyl-1,3-diaminopropane)chromium(III) chloride

The structure of trans-[CrCl2(Me2tn)2]Cl (Me2tn = 2,2-dimethylpropane-1,3-diamine) has been determined by a single-crystal X-ray diffraction study at 150 K. The analysis reveals that there are two independent Cr(III) complex cations in the structure, one with crystallographic inversion symmetry and the other with two-fold rotation symmetry, which are conformational isomers of each other. In both conformations, the chromium atom adopts a distorted octahedral structure with the four nitrogen atoms of two Me2tn ligands occuping the equatorial plane and two chlorine atoms occuping trans-axial positions. The six-membered chelate rings are in stable chair conformations with N–Cr–N angles of 87.03(8)° and 88.99(8)°. The two chelate rings in the centrosymmetric complex cation 1 are anti, while those in the rotation-symmetric complex cation 2 are in syn conformations. The mean Cr–N and Cr–Cl bond lengths are 2.0922 and 2.3253 Å, respectively. The infrared and UV–visible absorption spectra of trans-[CrCl2(Me2tn)2]Cl have also been measured. The resolved band maxima of the electronic d–d spectrum are fitted with a secular determinant for a quartet energy state of the d3 configuration in a tetragonal field including configurational but neglecting spin-orbit coupling. It is confirmed that the nitrogen atoms of the Me2tn ligand have a strong σ-donor character, but the chloro ligand has weak σ- and π-donor properties toward the chromium(III) ion.