Crystal structure of [μ-1κ2 C 1,C 4:2(1,2,3,4-η)-1,2,3,4-tetraphenylbuta-1,3-diene-1,4-diyl]bis(tricarbonylosmium)(Os—Os)

A crystal structure of the osmole complex (μ-η4-C4Ph4)Os2(CO)6 revealed an eclipsed sawhorse molecular geometry with no bridging or semi-bridging carbonyl ligands.


Chemical context
Metallacyclopentadiene complexes, known as metalloles, with the formula (-4 -C 4 R 4 )M 2 (CO) 6 are typically produced by C-C bond-coupling reactions of alkynes with group 8 metal carbonyls (Mathur et al., 2014). These metalloles have been shown to adopt one of two possible geometries in the solid state, i.e. one in which the carbonyl ligands of the M 2 (CO) 6 units are eclipsed in a so-called sawhorse conformation, or one in which the carbonyls are staggered with one CO semibridging the metal-metal bond. Ferroles (M = Fe) almost always adopt the staggered non-sawhorse conformation (Kumar et al., 2014;Iyoda et al., 1997;Jeannin et al., 1994;Heim et al., 1992;Daran & Jeannin, 1984), while ruthenoles (M = Ru) display an equal propensity to adopt either the sawhorse or the nonsawhorse conformation (Yang, 2014;Mathur et al., 2008Mathur et al., , 2014Tunik et al., 1997). Only two osmole (M = Os) complexes have been examined by X-ray crystallographic analysis, and both of them exhibit the sawhorse conformation. One of these is (-4 -2,3-dimethylbutadiene)Os 2 (CO) 6 (I) (see Scheme 1), which was prepared by reacting Os 3 (CO) 12 with 2,3-dimethylbutadiene, and in which the osmacyclopentadiene ring contains H atoms in the 2,5-positions and methyl groups in the 3,4-positions (Dodge et al., 1963). The other one is (-4 -FcC 2 -C CFc) 2 Os 2 (CO) 6 (II, Fc is ferrocenyl), which was a product of the reaction of Os 3 (CO) 10 (NCMe) 2 with 1,4-bis(ferrocenyl)butadiyne, and in which the osmacyclopentadiene ring is substituted by ferrocenyl-C C-groups in the 2,5-positions and by ferrocenyl groups in the 3,4-positions as a result of head-to-head coupling of the butadiyne starting material (Adams et al., 2002). ISSN 2056-9890 Our goal was to obtain the crystal structure of the title osmole (-4 -C 4 Ph 4 )Os 2 (CO) 6 (III) (see Scheme 2) containing a tetraphenylbutadiene moiety, which was first reported over 46 years ago but which has never been structurally characterized (Gambino et al., 1971). Gambino et al. prepared III by a three-step process: Os 3 (CO) 12 was heated with diphenylacetylene (tolan) to produce Os 3 (CO) 8 (C 4 Ph 4 ) (IV), which was treated with CO to yield Os 3 (CO) 9 (C 4 Ph 4 ) (V). This was then treated with excess CO to produce III. The overall yield for III based on Os 3 (CO) 12 was not mentioned, but it was clearly less than 4% since the yields for the first two steps were reported to be about 10 and 40%, respectively. In order to obtain a significant quantity of III for crystal growing attempts, we sought a higher yield method of preparing this osmole complex. We turned to microwave heating since it had been shown to offer improved efficiency for the preparation of certain other osmium carbonyl complexes (Johnson & Powell, 2008;Leadbeater & Shoemaker, 2008;Jung et al., 2012;Pyper et al., 2013).

Structural commentary
The molecular structure of compound III is illustrated in Fig. 1. All four phenyl rings are disordered over two slightly different orientations (Fig. 2), and the refined occupancies of the major components are 0.50 (3), 0.510 (12), 0.519 (18), and 0.568 (12). Two of the carbonyl ligands are also disordered over two slightly different positions and the occupancies of the major components are 0.568 (16) and 0.625 (13). Each C or O atom in the minor components is displaced less than 1 Å from its counterpart in the major components. The geometrical features of the central portion of III are quite similar to those of the two (-4 -C 4 R 4 )Os 2 (CO) 6 osmoles that have been previously characterized by X-ray crystallography, with planar osmacyclopentadiene rings and eclipsed sawhorse conformations of the carbonyls. Thus, there are no bridging or semibridging CO ligands. The R groups (phenyl rings) in III are intermediate in size compared to those of the other two osmoles, one of which (i.e. I) had small butadiene substituents of H and Me, while the other (i.e. II) had large substituents of Fc-C C-and Fc. The Os-Os bond lengths of 2.74 Å for I, 2.7494 (2) Å for III, and 2.7556 (7) Å for II might reflect an inverse correlation between the strength of the metal-metal bond and the steric bulk of the butadiene substituents, The molecular structure of the title compound, showing the positions of the major phenyl-ring and carbonyl-ligand components, as well as the atom-labelling scheme. Displacement ellipsoids are shown at the 50% probability level and H atoms have been omitted for clarity.

Figure 2
A ball-and-stick view of the asymmetric unit of III, with partial atom labeling. All components of the disordered carbonyl ligands and phenyl rings are shown (the minor ones in pale blue).
although the rudimentary nature of the crystal structure report for I precludes a definitive conclusion concerning this trend (the only bond length included in the description of the structure of I was the Os-Os distance and no s.u. value was given). The average bond lengths between the Os atoms that lie within the metallacyclpentadiene rings and the 2,5-C atoms of the rings are 2.09 (1) Å for II and 2.10 (1) Å for III, while the other Os atoms in II and III have an average distance of 2.31 (4) and 2.32 (4) Å , respectively, from the four C atoms in the metallacyclpentadiene rings. The central C-C distances in the C 4 R 4 groups are 1.48 (1) Å for II and 1.461 (5) Å for III, and these are both longer than the other two C-C distances on either side of them [average of 1.42 (1) Å for II and 1.420 (5) Å for III], supporting the designation of these groups as dienes. There are five unique torsion angles within each metallacyclpentadiene ring, and the average values of these are 8 for II and 0.7 for III. Thus, the planarity of the metallacyclpentadiene ring in III is less distorted than it is in II, which is most likely a consequence of the smaller steric bulk of the R groups in III.

Database survey
A search of the Cambridge Structural Database (Version 5.39, last update February 2018; Groom et al., 2016) for metallole complexes of the type (-4 -C 4 R 4 )M 2 (CO) 6 , where M is any transition metal, gave 14 hits. The only hit containing Os atoms was complex II with a sawhorse conformation and no bridging carbonyl ligands. Eight of the hits were for ruthenoles, four with non-sawhorse conformations and semibridging CO ligands and four with sawhorse conformations without bridging carbonyls. The five remaining hits were for ferroles, all of which have semibridging CO ligands.

Supramolecular features
There are only two intermolecular nonbonding distances in the structure of III that are shorter than the sum of the van der Waals radii. A weak C19A-H19AÁ Á ÁO2 i hydrogen bond (Table 1) and a close O2Á Á ÁO5 ii contact of 2.941 (9) Å [symmetry code: (ii) À 1 2 + x, 1 À y, z]. These combine to stack molecules of III along the direction of the b axis of the unit cell ( Fig. 3).

Synthesis and crystallization
Dodecacarbonyltriosmium(0) (100 mg, 0.110 mmol) and MeCN (8 ml) were placed in a 35 ml glass reaction vessel, then sealed with a PTFE cap and placed in a CEM Discover-SP microwave reactor. The mixture was stirred and heated at 403 K for 8 min to yield a green solution in which the major component was known to be Os 3 (CO) 11 (NCMe), as noted in a previous report (Jung et al., 2009). The reaction vessel was removed from the microwave reactor and allowed to cool to room temperature. Diphenylacetylene (118 mg, 0.662 mmol) was added to the vessel and it was returned to the microwave reactor. This solution was stirred and heated at 433 K for 6 min. The solvent was removed by rotary evaporation, then the residue was dissolved in CH 2 Cl 2 and subjected to thinlayer chromatography (TLC) using an eluent of 1:1 (v/v) hexanes/CH 2 Cl 2 . Three yellow bands were collected. The top band consisted of 34.1 mg (22.8% yield) of complex III. IR ( CO , hexane): 2081 (s), 2051 (vs), 2018 (m), 1998 (s), and 1968 (m) cm À1 . The second band consisted of a mixture of complex III and an unidentified product. The third band consisted of 4.1 mg (3.2% yield) of complex IV. Crystals of III were grown by slow evaporation of an n-hexane solution at room temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The C atoms in the four phenyl rings were disordered over two slightly different orientations. Each phenyl ring was split into two components (A and B), which were refined as rigid hexagons. H atoms were included in idealized positions and allowed to ride on their parent atoms: C-H = 0.95 Å with U iso (H) = 1.2U eq (C). The refined occupancy ratios were C11-C16 0.519 (18) Table 1 Hydrogen-bond geometry (Å , ). (12)  139 Symmetry code: (i) Àx þ 1; y þ 1 2 ; Àz þ 1 2 .

Figure 3
The overall packing of III, viewed along the b-axis direction.

-diyl]bis(tricarbonylosmium)(Os-Os)
Crystal data Extinction correction: SHELXL2018 (Sheldrick, 2015b), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.000017 (2) 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.