Crystal structure of [(1,2,3,4,11,12-η)-anthracene]tris(trimethylstannyl)cobalt(III)

The first reported structure of a cobalt complex containing an η6-anthracene ligand is presented. The anthracene ligand is nearly flat and coordinates the metal asymmetrically, such that the ring junction carbon atoms are slightly further from the cobalt center than are the other four.

The asymmetric unit of the title structure, [Co( 6 -C 14 H 10 ){Sn(CH 3 ) 3 } 3 ], contains two independent molecules. Each anthracene ligand is 6 -coordinating to a Co III cation and is nearly planar [fold angles of 5.4 (3) and 9.7 (3) ], as would be expected for its behaving almost entirely as a donor to a high-oxidation-state metal center. The slight fold in each anthracene ligand gives rise to slightly longer Co-C bond lengths to the ring junction carbon atoms than to the other four. Each Co III cation is further coordinated by three Sn(CH 3 ) 3 ligands, giving each molecule a three-legged piano-stool geometry. In each of the two independent molecules, the trio of SnMe 3 ligands are modeled as disordered over two positions, rotated by approximately 30%, such that the C atoms nearly overlap. In one molecule, the disorder ratio refined to 0.9365 (8):0.0635 (8), while that for the other refined to 0.9686 (8):0.0314 (8). The molecules are well separated, and thus no significant intermolecular interactions are observed. The compound is of interest as the first structure report of an 6 -anthracene cobalt(III) complex.

Chemical context
Oxidation derivatives of unstable low-valent species often provide indirect support for their formulations. For example, thermally unstable alkyl isocyanide complexes of formally M(ÀII) that were proposed to be 'K 2 [M(CNtBu) 4 ],' M = Fe (Brennessel et al., 2007), Ru (Corella et al., 1992), were reacted at low temperature in situ with SnPh 3 Cl to afford isolable and readily characterizable derivatives, trans-M(SnPh 3 ) 2 (CNtBu) 4 . Similarly, it was planned to derivatize the formally Co(ÀI) anion [Co(C 10 H 8 ) 2 ] À , C 10 H 8 = naphthalene, which is the analog of the well-characterized and isolable anthracene cobaltate [Co(C 14 H 10 ) 2 ] À (C 14 H 10 = anthracene; Brennessel et al., 2002). To date, the only established instance of [Co(C 10 H 8 ) 2 ] À is as part of the highly specific triple salt [K(18-crown-6)] 3 [Co(C 10 H 8 )(C 2 H 4 ) 2 ] 2 [Co(C 10 H 8 ) 2 ] (Brennessel et al., 2006). But before applying this procedure to the naphthalene system, we chose to first apply it to the wellbehaved anthracene system to test the feasibility of the derivatization. Thus, one equivalent of SnMe 3 Cl was added in situ to a THF solution of [K(THF) x ][Co(C 14 H 10 ) 2 ] (Brennessel et al., 2002), which produced an intense violet, pentane-soluble species. Rather than being the expected '[Co(C 14 H 10 ) 2 (SnMe 3 )]' formally Co(I) species, however, after further investigation it was determined to be the title compound, [Co( 6 -C 14 H 10 )(SnMe 3 ) 3 ] (I), based on singlecrystal X-ray diffraction. ISSN 1600-5368 Similar reactions using SnPh 3 and Sn(cyclohexyl) 3 produced only intractable mixtures. Filtration of the reaction mixture left a very reactive dark-gray filter cake, which appeared to be from the deposition of Co metal. A tentative balanced equation has been proposed based on the initial evidence (see equation below). No yield was obtained, but if the equation holds, a quantitative yield would only be 33.3% based on cobalt. Single crystals were grown from a saturated pentane solution in a 243 K freezer and NMR experiments (see Synthesis and crystallization) were performed on the single crystals, which corroborated the structure analysis from diffraction data.

Structural Commentary
The structure contains two independent molecules of (I) (Fig. 1) that are metrically very similar. Each molecule contains one anthracene and three SnMe 3 ligands in a threelegged-piano-stool geometry. In each of the two independent molecules, the trio of tin ligands are disordered with a 30 rotation of the set, although the minor component of the disorder is very small (<10% by mass in both cases). The anthracene ligands in both molecules are coordinated in an 6 mode and are nearly planar, with only the slightest bends at the imaginary lines joining atoms C1 and C4 [5.4 (3) ] and C24 and C27 [9.7 (3) ]. The Co-C distances to the ring junction carbon atoms are slightly longer by 0.17 Å than those to the metal-coordinating non-ring junction atoms (Table 1). This has been referred to as a 'flat-slipped' coordination mode, and is likely due to an antibonding component of the anthracene HOMO at the ring-junction carbon atoms (Zhu et al., 2006). Thus the anthracene ligand is displaced slightly from the symmetric coordination mode found in 6 -benzene metal complexes, in order to maximize the bonding overlaps with the four non-ring-junction carbon atoms. Because the metal is formally d 6 Co III , the -donation from the anthracene ligand is likely the most important contribution to its bonding.

Database Survey
Structures of 6 -coordinated anthracene transition metal complexes are few [Cambridge Structural Database, Version 5.35, update No. 3, May 2014;Groom & Allen, 2014], but range from Ti (Seaburg et al., 1998) to Co (this work). Although one ligand in the titanium complex, [Ti(dmpe)-( 4 -C 14 H 10 )( 6 -C 14 H 10 )] [dmpe = 1,2-bis(dimethylphosphino), is considered 6 -coordinating based on Ti-C bond lengths, the fold angle between the plane consisting of non-ring-junction metal-coordinating carbon atoms and the rest of the ligand is nearly 20 , very likely placing it on the cusp of an 4 coordination mode. However, both [Cr(C 14 H 10 )(CO) 3 ] (Hanic & Mills, 1968) and [Mo(C 14 H 10 )(PMe 3 ) 3 ] (Zhu et al., 2006) have nearly planar anthracene ligands (6.6 and 5.5-5.8 , respectively). The small fold angles and the M-C(ring junction) bond lengths that are slightly longer than the M-C(nonring junction) ones exemplify the 'flat-slipped' coordination mode (Table 1). For these cases of early transition metals, the -donation of anthracene is supplemented by -backbonding to the anthracene LUMO; however, the C-C bond lengths are not all that different from those seen in normal-valent late transition metal complexes, and all are elongated relative to those in free anthracene (Table 1).

Figure 1
The two independent molecules of (I), showing the atom numbering. The minor components of the disorder are shown with dashed lines and boundary ellipsoids. The two orientations of the SnMe 3 ligand set fit in essentially the same volume because the methyl groups are overlapped. Displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms have been omitted.
In the structures of later transition metal compounds, the 6 'flat-slipped' coordination mode is found in normal-or slightly sub-valent metal complexes, and the fold angle appears to be sensitive to oxidation state. In structures with Ru II coordination centers (Garcia et al., 2010;Konovalov et al., 2011) the fold angles are 3.1 and 4.4 , respectively. As the oxidation state decreases, as in the cases of Fe I (Schnö ckelborg et al., 2012;Hatanaka et al., 2012) and Rh I (Woolf et al., 2011), the fold angles increase slightly to 15.8, 9.1, 9.2, and 13.8 , respectively. Although fold angles may be subject to a variety of additional effects, including packing and sterics, in general the trend is that these angles increase with greater electron-acceptor behavior. This has been examined for the series Cp*Fe(C 14 H 10 )(À/0/+) and Cp*Fe(C 10 H 8 )(À/0/+), Cp* = C 5 Me 5 , by a combination of X-ray crystallography and DFT methods (Schnö ckelborg et al., 2012). In low oxidation states, the fold angles are significant and the ring-junction carbon atoms are bent away from the metal, thus making the coordination 4 . Whereas the folds become almost non-existent (<10 ) for normal valent oxidation states and the coordination is 6 , consistent with what is observed in (I), a formally Co III , d 6 metal atom.
To date, the analogous reaction using naphthalene instead of anthracene has not been performed.

Synthesis and crystallization
A clear blue solution of CoBr 2 (0.500 g, 2.29 mmol) in THF (60 ml, 195 K) was added to a deep-blue solution of K[C 14 H 10 ] (6.86 mmol) in THF (60 ml, 195 K). To the resulting deep pinkish-red solution was added SnMe 3 Cl (0.455 g, 2.29 mmol) in THF (20 ml, 195 K), which dulled the color. After slow warming to room temperature, the solution was filtered to remove KBr and KCl. The solvent was removed under vacuum, and the product was extracted into pentane (25 ml) and filtered to give an intense violet solution. After the filtrate was cooled to and kept at 273 K for one h, the violet supernatant was carefully transferred to another vessel and placed in a freezer (243 K) for two days, during which time big purple-black crystals of the title complex formed. No attempts to establish the yield or obtain bulk elemental analyses were carried out. However, the product was characterized using the single crystals in solution by NMR and in the solid state by single-crystal X-ray diffraction. 1 Table 1 Comparison of (I) with free anthracene and selected 'flat-slipped' structures (Å , ).

Figure 2
Anthracene numbering scheme for comparisons in Table 1

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. In each of the two independent molecules, the trio of SnMe 3 ligands are modeled as disordered over two positions, such that the carbon atoms nearly overlap. In the molecule containing Co1 the disorder ratio refined to 0.9366 (8):0.0634 (8). That for the other molecule refined to 0.9685 (8):0.0315 (8). Despite the small fraction of the minor components, when the disorders are not modeled, the R1 residual increases from 0.0375 to 0.0538. For each disorder model, analogous bond lengths and angles were heavily restrained to be similar. Anisotropic displacement parameters for pairs of near-isopositional carbon atoms were constrained to be equivalent. The rather large residual peak in the difference map (1.93 electrons per Å 3 , located 1.74 Å from atom C4) has no chemical meaning. It (and other similar smaller peaks) is likely due to a very minor twin component whose twin law is [1 0 0 / 0 1 0 / À0.623 À0.754 1], a 180 degree rotation about [001] (Parsons et al., 2003).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. The largest residual peak of 1.93 electrons per Å 3 , located 1.74 Å from atom C4, has no chemical meaning. It (and other smaller peaks that likewise having no chemical meaning) is likely due to a very minor twin component whose twin law is [-1 0 0 / 0 -1 0 / -0.623 -0.754 1], a 180 degree rotation about [001] (Parsons, 2003). In each of the two independent molecules, the trio of SnMe 3 ligands are modeled as disordered over two positions, such that the carbon atoms nearly overlap. In the molecule containing Co1 the disorder ratio refined to 0.9366 (8):0.0634 (8). That for the other molecule refined to 0.9686 (8):0.0314 (8). Despite the small mass of the minor components, when the disorders are not modeled, the R1 residual increases from 0.0375 to 0.0538. For each disorder model, analogous bond lengths and angles were heavily restrained to be similar. Anisotropic displacement parameters for pairs of near-isopositional carbon atoms were constrained to be equivalent. H atom positions of cobalt-coordinated carbon atoms were refined freely, but with relative thermal parameters as described below. All other H atoms were placed geometrically and treated as riding atoms: sp 2 , C-H = 0.95 Å, with U iso (H) = 1.2U eq (C), and methyl, C-H = 0.98 Å with U iso (H) = 1.5U eq (C).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ.    (5)