X-ray structure analysis of symmetrically substituted 1,1′-diformylruthenocene

The crystal and molecular structure of 1,1′-formylruthenocene has been determined. The molecules self-assemble in a two-dimensional structure by C—H⋯O and C—H⋯π interactions with cisoid relative orientations of the two formyl groups.

1,1 0 -Diformylruthenocene, [Ru(C 6 H 5 O) 2 ], crystallizes in the orthorhombic system in the P2 1 2 1 2 1 space group at room temperature. There are two crystallographically independent molecules in the asymmetric unit. The cyclopentadienyl rings have eclipsed configuration. The molecules self-assemble in a two-dimensional structure by C-HÁ Á ÁO and C-HÁ Á Á interactions with cisoid relative orientations of the two formyl groups. The crystal studied was refined as an inversion twin.

Chemical context
Ferrocene and its derivatives are among the most important metallocenes. The general chemistry of ruthenocenes and osmocenes has not been researched much, as they are much less reactive and much more expensive. The ruthenocene skeleton is less 'superaromatic' than the ferrocene skeleton (Nesmeyanov et al., 1972). It has long been reported that ruthenocene is more reactive towards lithiation than its ferrocene analogue (Rausch et al., 1960). The presence of less negative charge on the cyclopentadienyl rings of ruthenocene than on those of ferrocene explains this higher reactivity as well as the higher acidity of the ruthenocene system (Sanders & Mueller-Westerhoff, 1996).
Numerous applications of 1,1 0 -disubstituted derivatives of ferrocene and ruthenocene in asymmetric catalysis (Dai & Hou, 2010), biochemistry and material sciences (Š tě pnička, 2008), have been reported. Different types of substituents on the Cp ring often result in significant changes in the reactivity and properties of ruthenocene as a result of the electronic and steric factors that influence the molecular entity. In general, ferrocene (Fc) and its heavier analogue ruthenocene (Rc) have similar structures (Muratov et al., 2014). The molecular structures of formyl ferrocene, 1,1 0 -diformyl ferrocene and formyl ruthenocene are known in the literature (Braga et al., 1999;Muratov et al., 2014). The structures of 1,1 0 -disubstituted ferrocenes containing carboxylic or carbonyl groups have the potential to form a large number of intermolecular interactions, building blocks in two or three dimensions, and to mould the intermolecular hydrogen bonds and CO networks to achieve highly organized superstructures (Braga & Grepioni, 1997). The structure of the ferrocene analogue of the title compound has been published (Braga et al., 1999;MacGillivray et al., 1999). We report here the crystal and molecular structure of 1,1 0 -diformylruthenocene, which has not previously been reported. ISSN 2056-9890

Structural commentary
The title compound contains two crystallographic independent molecules (A and B, Fig. 1a) in the asymmetric unit, which possess the same rotameric conformations. In both molecules, the carbon atoms of the cyclopentadienyl rings form pentagonal prisms, which bind to the ruthenium atom (sandwich array). These Cp rings are in partially eclipsed positions. The two -CHO groups of the cyclopentadienyl rings are in cisoid relative conformations. Bond lengths in the two independent molecules are given in Table 1. The C1-C11-C12-C6 and C31-C21-C26-C32 torsion angles are 2.5 (9) and 6.0 (9) , respectively, which suggests that molecule A is more eclipsed than molecule B. In the reported crystal structure of the Fc(CHO) 2 analogue, there are also two independent molecular units in the asymmetric unit, but with different rotameric conformations. Similarly, the torsion angles C11-O1-O2-C12 and C31-O21-O22-C32 are 2 (1) and 7 (1) , respectively. The torsion angle in diformyl ferrocene, which has a staggered configuration in one of the molecules in the asymmetric unit is 42.4 (Balavoine et al., 1991;Mueller-Westerhoff et al., 1993). The crystal structure of the diacetylruthenocene molecule reported earlier also shows a cis configuration for the acetyl group, but one acetyl group is rotated by 180 with respect to the other (Trotter, 1963).
The Cp(centroid)Á Á ÁCp(centroid) distances in molecules A and B are 3.621 and 3.616 Å , respectively. The difference could be due to the electronic effects of the two symmetrically substituted formyl groups. It was also observed that the C and O atoms of both formyl groups are nearly coplanar to the plane of their respective Cp ring. A comparison of the two complex molecules in the asymmetric unit was performed by calculation of the molecular overlay (Mercury; Macrae et al., 2008) (Fig. 1b), resulting in the values D r.m.s. = 0.0622 and D max = 0.1208.

Supramolecular features
The molecules self-assemble in a two-dimensional structure assisted by C-HÁ Á ÁO and C-HÁ Á Á interactions (Desiraju, 1996), as shown in Fig. 2. Numerical details are given in Table 2. All secondary interactions that are shorter than the sum of the van der Waals radii of the atoms involved minus research communications Table 1 Lengths of the Ru-C and C-C bonds in the Cp rings of the A and B molecules..  Figure 1 (a) ORTEP representation of the two crystallographically independent ruthenocene complex molecules in the asymmetric unit at 50% probability level with atomic labelling. (b) The superimposed molecules from the asymmetric unit.
0.12 Å are included. The molecules form columns that are arranged in two-dimensional sheets parallel to the ab plane. The short contacts of each molecule result in a special neighbouring array in three-dimensions, forming V-type assemblies as shown in Fig. 3. In particular, the space group P2 1 2 1 2 1 permits close packing of molecules (Braga et al., 1999).

Quantum-chemical calculations
DFT quantum-chemical calculations were performed using !B97X-D based on 6-31 G* with SPARTAN16 (Wavefunction, 2017). The DFT structure optimization of 1,1 0 -diformylruthenocene was performed starting from the X-ray data. The energy of molecule A, where the molecule is eclipsed and the formyl groups are in a cisoid geometry, is 0.73 Kcal more stable than that of the molecule with a transoid geometry for the two formyl groups. When the energy of the two molecules calculated together was compared with the sum of the energies obtained independently for each molecule, it was observed that the asymmetric unit A-B is more stable by 14.14 Kcal. This observation may be partly due to the presence of the two C-HÁ Á ÁO hydrogen-bonding interactions between the two independent molecules as shown in Fig. 3.

Synthesis and crystallization
All reactants were purchased from Aldrich Chemical Co. and 1,1 0 -diformyl ruthenocene was synthesized as reported earlier (Trotter, 1963). Yellow needle-like crystals of ruthenocene dialdehyde were obtained by slow evaporation of a saturated dichloromethane/hexane solution (v:v = 2:8) at ambient temperature.

Figure 3
Packing arrangement along the c axis with dashed lines indicating the intermolecular contacts.

1,1′-Diformylruthenocene
Crystal data Absolute structure: Refined as an inversion twin Absolute structure parameter: 0.43 (12) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Ru1-C6-C12-O2 93.6 (15) Ru2-C26-C32-O22 −95.5 (13)