Crystal structure of μ-carbonyl-1:2κ2 C:C-carbonyl-1κC-(1η5-cyclopentadienyl)iodido-2κI-[μ-2-(pyridin-2-yl)ethene-1,1-diyl-1κC 1:2κ2 N,C 1]ironpalladium(Fe—Pd) benzene monosolvate

The title binuclear μ-pyridylvinylidene FePd complex (FePd1) was obtained from Cp(CO)2FeI and 2-ethynylpyridine in diisorpopylamine in the presence of PdCl2


Chemical context
Transition metal -pyridylethynyl complexes attract considerable research interest since they can act as precursors for pyridylvinylidene complexes (Chou et al., 2008) and as buildings blocks for supramolecular assemblies in molecular electronics (Le Stang et al., 1999), as well as materials for nonlinear optics (Wu et al., 1997).
Thus, while the alkynylation of FpI with terminal arylacetylens HC C-Ar proceeds along the typical Sonogashira pathway to afford FpC C-Ar in reasonable yields (Nakaya et al., 2009), the same reaction of o-pyridylacetylene did not result in the Sonogashira alkynylation product, but afforded the binuclear complex FePd1 where the metal atoms are bridged through the carbonyl and pyridylvinylidene ligands, the pyridyl nitrogen atom being bound to the palladium atom. Although additional experimental and probably theoretical studies are needed to reveal the true reaction pathway, one can assume the formation of FePd1 to be caused by the following successive steps in the palladium coordination sphere: (i) the oxidation addition of FpI at the Fe-I bond, (ii) the acetylene-vinylidene rearrangement of the -pyridylacetylene ligand followed by (iii) insertion of the Cp(CO) 2 Fefragment into the Pd C bond and accompanied by (iv) formation of the bridging carbonyl group and the Pd-N bond ( Fig. 1, pathway A). Presumably, it is the Pd-N bond that efficiently stabilizes FePd1, thereby favoring pathway A. This stabilization cannot occur in the case of reactions of arylacetylenes, and the typical Sonogashira reaction proceeds via the formation of a pyridylethynyl complex followed by the Fe-C-reductive elimination (Sonogashira, 1998) (Fig. 1,  pathway B).

Structural commentary
The molecular structure of the title compound is shown in The reaction pathway.
C2-Pd1 angles are 141.7 (2) and 137.0 (2) , respectively, and the Fe1-C2 and Pd1-C2 distances are 1.942 (3) Å and 2.012 (3) Å , respectively]. In addition, the iron and palladium atoms are linked through the bridging pyridylvinylidene fragment coordinated by the C3 atom. The four-membered ring Fe1-C2-Pd1-C3 thereby formed is folded slightly by 11.61 (14) along the Fe1Á Á ÁPd1 line with a short metal-metal distance of 2.5779 (4) Å [for comparison the values of the covalent radii for these metals are r(Fe) = 1.32, r(Pd) = 1.39 Å ; Cordero et al., 2008]. The Fe1-C3 distance of 1.836 (3) Å is noticeably longer compared to the analogous distances in mononuclear iron vinylidene complexes: for example, 1.744 (4) Å in ( 5 -C 5 H 5 )Fe(SnPh 3 )(CO)( C CHPh) (Adams et al., 1999) and 1.744 (9) Å in ( 5 -C 5 Me 5 )Fe(CO)-(TMS)( C C(TMS)Ph) (Kalman et al., 2014), and the Fe1-C3-C4 angle of 156.9 (2) is noticeably deviated from linearity. At the same time, the Pd1-C3-C4 angle is 118.58 (19) , which suggests an unsymmetrical coordination of the C3 atom to the iron and palladium atoms. This asymmetry can be explained by the 2 -coordination of the Fe C double bond to the palladium atom. It is noteworthy that in Fe-Mtype binuclear 2 -vinylidene complexes, the coordination to the metal atoms is characterized by approximately equal values for the Fe-C-C and M-C-C angles [131.8-145.3 according to a CCDC (Groom et al., 2016) search]. The C3-C4 distance of 1.328 (4) Å in the vinylidene fragment corresponds with typical C C double-bond lengths in olefins. Besides coordination to C3, the palladium atom binds to the pyridylvinylidene fragment via the nitrogen atom of the pyridine ring to a five-membered chelating ring (the ring is almost planar and the maximum deviation from the mean plane is 0.02 Å for atoms C3 and C4). The iodine atom completes the coordination sphere of the 16-electron palladium atom, which corresponds to a slightly distorted squareplanar geometry [the dihedral angle between the N1/Pd1/C3 and I1/Pd/C2 planes is 3.2 (1) ].

Supramolecular features
In the crystal, the complexes form centrosymmetrical dimers (Fig. 3) due to -stacking interactions between the pyridylvinylidene fragments with an interplanar distance of 3.36 Å and a shortest interatomic C5Á Á ÁC9(1 À x, Ày, Àz) distance of 3.339 (4) Å . The outer plane of the pyridylvinylidene fragment in the dimer is additionally shielded by the solvating benzene molecule, which is oriented by one of its C-H groups to the centroid a of the five-membered chelating palladacycle [the C6S-H6SAÁ Á ÁCg1 distance is 2.67 Å ; Cg1 is the centroid of the five-membered ring, the angle between the Cg1Á Á ÁH6SA vector and the ring normal is 9.7 , and the C6S-H6SAÁ Á ÁCg1 angle is 160 ].

Synthesis and crystallization
A mixture of Cp(CO) 2 FeI (127.3 mg, 0.419 mmol) and PdCl 2 (76 mg, 0.429 mmol) in diisopropyl amine (4 ml) was heated to 315 K and H-C C(2-C 5 H 4 N) (0.3 ml) was added. The mixture was stirred for 16 h at 333 K and the diisopropyl amine was removed under reduced pressure. The crude mixture was extracted with dichloromethane, the extract was filtered through celite, and the solvent was evaporated to dryness. The residue was dissolved in a dichloromethanehexane (1:1) mixture and chromatographed on a silica column (9.5 Â 1 cm). A dark-yellow band was eluted with dichloromethane and the eluate was evaporated to yield Cp(CO) 2 Fe(-C=CH(2-C 5 H 4 N)PdI (FePd1) (29 mg, 12%) as a brown solid. Red-brown crystals of the complex suitable for X-ray diffraction analysis were obtained after recrystallization Centrosymmetric stacked dimer in the crystal packing. Atoms labelled with the suffix A are generated by the symmetry operation (1 À x, Ày, Àz).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. Atom H4 of the vinyl group was located in a difference Fourier map and refined freely. All other H atoms were fixed geometrically and refined using a riding model with U iso (H) = 1.2U eq (C).

Computing details
Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008). 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.