A new monoclinic polymorph of dichloridotetrakis(dimethyl sulfoxide)ruthenium(II)

The title compound, cis,fac-dichloridotetrakis(dimethyl sulfoxide)-κ3 S,κO-ruthenium(II), [RuCl2(C2H6OS)4], was obtained from newly synthesized ruthenium complexes of 3-amino-2-chloropyridine. The Ru atom has a distorted octahedral coordination with two cis-oriented chloride ligands and four dimethyl sulfoxide ligands. Three of the sulfoxide ligands are S-bonded in a fac configuration, while the fourth is O-bonded. The title compound represents a new, and fourth, polymorph of the complex. Two other monoclinic forms and an orthorhombic modification have been reported previously.

The title compound, cis,fac-dichloridotetrakis(dimethyl sulfoxide)-3 S,O-ruthenium(II), [RuCl 2 (C 2 H 6 OS) 4 ], was obtained from newly synthesized ruthenium complexes of 3-amino-2-chloropyridine. The Ru atom has a distorted octahedral coordination with two cis-oriented chloride ligands and four dimethyl sulfoxide ligands. Three of the sulfoxide ligands are S-bonded in a fac configuration, while the fourth is O-bonded. The title compound represents a new, and fourth, polymorph of the complex. Two other monoclinic forms and an orthorhombic modification have been reported previously.  A new monoclinic polymorph of dichloridotetrakis(dimethyl sulfoxide)ruthenium(II) G. Georgieva, G. Gencheva, B. L. Shivachev and R. P. Nikolova

Comment
In the field of medicinal chemistry ruthenium complexes have gained considerable attention as non-platinum anticancer agents (Galanski et al., 2003). In order to obtain new complexes with potential antitumor properties we studied the reactions of Ru(III) and 3-amino-2-chloropyridine (acp) under different conditions. We found that along with the complexation reaction a redox process also takes places leading to the formation of ruthenium complexes with the studied bidentate N-ligand in different oxidation states. The reported titled compound is a side product of this reaction.
The new polymorph modification of the title compound ( Fig. 1) was obtained studying the substitution reaction of the acp-ligands from the inner coordination sphere of trans-[Ru(IV)Cl 4 (acp) 2 ] in a hot DMSO solution (120-130 °C) (Fig.   3). The resulting orange crystals were determined by crystal structure analysis to be cis-RuCl 2 (DMSO) 4 ( Fig. 1). Two monoclinic (Mercer & Trotter, 1975;Alessio et al., 1988;Pigge et al., 2005) and one orthorhombic (Attia & Calligaris, 1987;Srivastava & Fronczek, 2003) modifications have been previously reported. The compound reported here is also monoclinic with similar type of ruthenium coordination but different three-dimensional arrangement of the structural units.
Of the four DMSO ligands, three are S-coordinated, with Ru-S distances of 2.240 (2), 2.264 (2) and 2.278 (2) Å. As already reported, the Ru-S distance trans to the O-bonded DMSO (2.240 (2) Å) is significantly shortened, being more than 0.02 Å shorter than the other two.
The 0.05 Å longer S-O distance in the O-bonded DMSO ligand, is indicative for the weakened double-bond character compared to the other three DMSO ligands. As it was mentioned above the only difference between the polymorphs is the three-dimensional arrangement of the structural units. The difference is easily recognized if the number and arrangement of short O···H contacts between the neighboring entities is considered.
We calculated the O···H contacts with distances shorter than the sum of Van der Waals radii (e.g. less than 2.5 Å) for all previously reported polymorphs and for the second title compound.
In the case of the orthorhombic structure there is only one such contact of 2.296 Å between O3(S-bonded DMSO) and H6(O-bonded DMSO) (Srivastava & Fronczek, 2003); for the same polymorph refined earlier by Attia & Calligaris, 1987, the obtained distance is 2.391 Å and occurs between O3 and H5 atoms). The structural units connected by that short contact are chain-like arranged along c axis. Similar arrangement of the structural units is observed in the monoclinic modifications with β angle close to 90° (Mercer & Trotter, 1975;Pigge et al., 2005). The obtained O···H distances are 2.375 and 2.443 Å for the structures reported in 1995 and 2005 respectively. In the second monoclinic polymorph with β angle of 116.8°t here are no O···H contacts below 2.5 Å, the shortest one is of 2.546 Å and occurs between O2 and H7 (both oxygen and hydrogen atoms belong to S-bonded DMSO molecules). Nevertheless, the arrangement of the units connected through the "shortest" contact is chain-like and is analogous to the one in the other polymorphs.
supplementary materials sup-2 In the structure of (I) there are two O···H contacts shorter than 2.5 Å, both between atoms belonging to only S-bonded DMSO molecules. As it is shown in Fig. 2 the structural units are connected by the described contacts in two directions in a layer-like arrangement differing from the chain-like one found in earlier reported polymorphs.

Refinement
The methyl H atoms were placed in idealized positions (C-H methyl = 0.96 Å). All H atoms were constrained to ride on their parent atoms, with U iso (H) = 1.5U eq (C methyl ). Fig. 1. View of the structure and the atom-numbering scheme of (I) showing 50% probability displacement ellipsoids. H atoms are shown as small spheres of arbitrary radii.