Crystal structure of (18-crown-6)potassium(I) [(1,2,3,4,5-η)-cycloheptadienyl][(1,2,3-η)-cycloheptatrienyl]cobalt(I)

The reaction of bis(anthracene)cobaltate(−I) with excess cycloheptatriene, C7H8, resulted in a new 18-electron cobaltate containing two different seven-membered ring ligands, based on single-crystal X-ray diffraction. This compound is of interest as the first to possess cycloheptatrienyl and cycloheptadienyl ligands in an anionic metal complex.

The reaction of bis(anthracene)cobaltate(ÀI) with excess cycloheptatriene, C 7 H 8 , resulted in a new 18-electron cobaltate containing two different sevenmembered ring ligands, based on single-crystal X-ray diffraction. The asymmetric unit of this structure contains two independent cation-anion pairs of the title complex, [K(18-crown-6)][Co( 3 -C 7 H 7 )( 5 -C 7 H 9 )], where 18-crown-6 stands for 1,4,7,10,13,16-hexaoxacyclooctadecane (C 12 H 24 O 6 ), in general positions and well separated. Each (18-crown-6)potassium cation is in contact with the 3 -coordinating ligand of one cobaltate complex. Each 3 -coordinating ligand behaves as an allylic anion whose exo-diene moiety is bent away from the allylic plane, and thus is not involved directly in the bonding. The metalcoordinating portions of the anionic 5 ligands are planar and one of these ligands is modeled as disordered over two positions, with occupancy ratio 0.699 (5):0.301 (5), such that one orientation is rotated by one carbon atom with respect to the other. The diffraction intensities were integrated according to non-merohedral twin law [1 0 0/0 1 0/0.064 0 1], a 180 rotation about reciprocal lattice axis [001], and the masses of the twin domains refined to equal amounts. As both ligands are formally coordinated as anions, the cobalt atom is best considered to be Co I . This compound is of interest as the first to possess cycloheptatrienyl and cycloheptadienyl ligands in an anionic metal complex.

Chemical context
To date there is only one crystal structure reported of a homoleptic cycloheptatriene (CHT) transition metal complex, Zr( 6 -C 7 H 8 ) 2 (Green & Walker, 1989), presumably because such molecules tend to isomerize. In the case of this zirconium species, room-temperature syntheses produced a mixture of it and its hydrogen-migrated isomer Zr( 7 -C 7 H 7 )( 5 -C 7 H 9 ). For the titanium analog, although the homoleptic CHT complex was initially observed by NMR, no crystals were obtained, and it readily isomerized. Metal vapor co-condensation reactions of titanium and iron with CHT also led to the isomerized forms (Timms & Turney, 1976;Blackborow et al., 1976). Cocondensation of molybdenum atoms with CHT resulted in Mo( 6 -C 7 H 8 ) 2 , which could be isolated at room temperature, but was observed to isomerize to Mo( 7 -C 7 H 7 )( 5 -C 7 H 9 ) with a half-life of ca 200 h .
Given the tendency for homoleptic CHT complexes to isomerize, we decided to investigate whether this would occur in the late transition metal low-valent cobalt system. The 18electron anionic precursor bis(anthracene)cobaltate(ÀI) was chosen because it had been demonstrated that the anthracene ligands are quite labile (Brennessel et al., 2002;Brennessel & Ellis, 2012). Under an argon atmosphere, excess CHT was introduced dropwise to a cold tetrahydrofuran solution of bis(anthracene)cobaltate(ÀI). Red-brown single crystals of the isolated product suitable for an X-ray diffraction experiment revealed a new 18-electron cobalt complex anion containing two different cyclic ligands, [Co( 3 -C 7 H 7 )( 5 -C 7 H 9 )] À , which confirmed that isomerization had occurred and that both anthracene ligands had been displaced. As no spectroscopy had been performed, it is unknown if an anionic intermediate like '[Co(-C 7 H 8 ) 2 ] À ' was initially formed, and if formed, whether it had any lifetime in cold and/or roomtemperature solutions.

Structural commentary
There are two independent contact ion pairs of [K(18-crown-6)][Co( 3 -C 7 H 7 )( 5 -C 7 H 9 )], (I), in the asymmetric unit (Figs. 1 and 2). The potassium cations are complexed by 18-crown-6 cyclic ethers and are in contact with carbon atoms of the 3coordinating ligands of the cobalt anions, with KÁ Á ÁC distances ranging from 3.207 (3) to 3.538 (4) Å . The longest KÁ Á ÁC distance is well within the sum of the van der Waals radii for potassium and carbon of 4.45 Å (Bondi, 1964). The C 7 H 7 ligands are bonded 3 to the cobalt atoms, and their Co-C and C . . . C bond lengths are consistent with their formulations as anionic allylic ligands with exo-diene moieties, i.e., 3cycloheptatrienyl ligands (see Table 1). Especially noteworthy are the lengths of the double bonds in the exo-diene portions, which are normal for C C bonds and show that the exo-diene moieties are independent of the allylic coordination to the metal centers. The Co-C bond lengths have the typical longshort-long pattern seen in other 3 -cycloheptatrienyl transition metal species (Table 2), and the exo-diene portions of these ligands are essentially planar and are bent away from the plane of the allylic regions by 28.0 (4) and 27.2 (4) , for anions containing Co1 and Co2, respectively. Interestingly, the tropylium cation (CHT + ) also has the formula C 7 H 7 ; however, tropylium as a ligand is aromatic, and thus planar and with similar C . . . C bond lengths. The 5 -coordinating ligands are essentially planar in their cobalt-bonded regions with r.m.s. deviations from planarity of 0.050 and 0.051 Å for planes C8-C12 and C22-C26, respectively (see Figs. 1 and 2).
With cobalt bound to three ('allyl') and five ('pentadienyl') carbon atoms of the seven-membered rings as described above, it is thought best to consider the cobalt atom as formally Co I with two anionic ligands. Extended Hü ckel MO Structure of the first independent molecule of (I), with displacement ellipsoids shown at the 50% probability level. H atoms have been omitted. Thin lines indicate the primarily electrostatic interactions between the K + cation and the crown ether and 3 ring.

Figure 2
Structure of the second independent molecule of (I), with displacement ellipsoids shown at the 50% probability level. H atoms and the minor component of the disordered ring have been omitted. Thin lines indicate the primarily electrostatic interactions between the K + cation and the crown ether and 3 ring.
calculations on [Fe( 3 -C 7 H 7 )(CO) 3 ] À (Hofmann, 1978), whose structure has been reported (Sepp et al., 1978), not only demonstrated that there is a preference for the metal to bind through the 3 -allylic region of the ligand rather than through the diene segment, but showed that there is more charge localization on the ring for the former conformation over the latter.
The exact mechanism of isomerization has not been determined for (I), including whether the hydrogen transfer is intraor intermolecular. In one DFT study on selected early transition metal complexes, the mechanism for hydrogen migration was determined to be intramolecular, and a metal hydride intermediate was predicted to be favored over a direct ligandto-ligand transfer (Herbert et al., 2004). The same conclusion was reached in kinetic studies on similar molybdenum complexes . If these studies can be extended to the cobalt system, then it could be proposed that the hydrogen migration occurs via a '[CoH(-C 7 H 7 )(-C 7 H 8 )] À ' intermediate.

Database survey
As mentioned above, there is exactly one homoleptic CHT structure in the Cambridge Structural Database to date (CSD, Version 5.36, update No. 1, November 2014;Groom & Allen, 2014), namely Zr( 6 -C 7 H 8 ) 2 (Green & Walker, 1989). All others have been structurally characterized after isomerization, including (I). There are 23 structures containing an 5cycloheptadienyl ligand, but only 12 structures containing an 3 -cycloheptatrienyl ligand bonded to a single metal atom. Of the latter, just three are anionic; they are of the form [AsPh 4 ][M(CO) 3 ( 3 -C 7 H 7 )], M = Fe (Sepp et al., 1978) and M = Ru, Os (Astley et al., 1990). (I) is the first example of an anionic transition metal complex containing both cycloheptadienyl and cycloheptatrienyl ligands to be reported.

Synthesis and crystallization
All operations were performed under an atmosphere of 99.5% argon further purified by passage through columns of activated BASF catalyst and molecular sieves. Standard Schlenk techniques were employed for all reactions with a double manifold vacuum (0.01 Torr) line. Solutions were transferred via stainless steel double-ended needles (cannulas) and glasscovered magnetic stir bars were employed. Cycloheptatriene was distilled from Na/K alloy. Excess cycloheptatriene was added dropwise to a deep pinkish-red solution of [K(18-crown-6)(THF) 2 ][Co( 4 -C 14 H 10 ) 2 ] (0.500 g, 0.579 mmol; Brennessel et al., 2002;Brennessel & Ellis, 2012) in THF (50 ml,195 K). The solution was slowly warmed to room temperature, at which point it was deep yellowish brown. After the solvent was removed in vacuo and heptane (70 ml) was added, the slurry was filtered. The product was washed with pentane (20 ml) and dried in vacuo, yielding a blackish-gray solid [0.292 g, 92%, based on cobalt  Comparison of bond lengths (Å ) and fold angles ( ) for selected later transition metal complexes containing 3 -cycloheptatrienyl ligands, with numbering according to Fig. 3. Fold angles are defined as the angles between the C1-C2-C3 (allylic) and C1-C3-C4-C5-C6-C7 (exo-diene) mean planes..

Figure 3
Numbering scheme used for the 3 -cycloheptatrienyl ligands in Table 2. and using the formulation of (I)]. This product was only characterized by single-crystal X-ray diffraction. Red-brown blocks were grown from a pentane-layered THF solution at 273 K.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The refinement stalled at R 1 = 0.19, at which point the structure was examined for twinning (Parsons et al., 2003). Non-merohedral twinning was identified and the data were re-integrated accordingly. Application of twin law [1 0 0 / 0 1 0 / 0.064 0 1], a 180 rotation about reciprocal lattice direction [001], reduced the R 1 residual to its final value of 0.043 (Table 3). The mass ratio of the twin components refined to 0.5040 (7):0.4960 (7). The 5 -coordinating ligand C8-C14 is modeled as disordered over two positions with site occupancy ratio 0.699 (5):0.301 (5), such that the ethyl linkage is shifted by one carbon atom (see Fig. 4). Analogous bond lengths and angles between the two positions of the disordered ring were heavily restrained to be similar. Anisotropic displacement parameters for pairs of proximal atoms from the two components of the disorder were constrained to be equivalent (Sheldrick, 2015).
H-atom positions of ring-ligand carbon atoms, except those in the minor component of the disorder, were located in a difference map and refined freely. All other H atoms were placed geometrically and treated as riding atoms: methine and sp 2 , C-H = 1.00 Å , and methylene, C-H = 0.99 Å , with U iso (H) = 1.2U eq (C). View of the ring ligand disorder. Displacement ellipsoids are shown at the 50% probability level and H atoms have been omitted. The numbering scheme of the minor component of the disorder was chosen to show the mirror-like symmetry that allows both orientations to fit within essentially the same volume.  Data collection: SMART (Bruker, 2003); cell refinement: SAINT (Bruker, 2003); data reduction: SAINT (Bruker, 2003); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: SHELXTL (Sheldrick, 2015); software used to prepare material for publication: SHELXTL (Sheldrick, 2015). Special details Refinement. The structure was integrated and refined as a non-merohedral twin (Parsons et al., 2003). Application of twin law [-1 0 0 / 0 -1 0 / 0.064 0 1], a 180° rotation about reciprocal lattice [001], reduced the R residual from 19.0% to its final value of 4.3%. The mass ratio of the twin components refined to 0.5039 (7):0.4961 (7). The η 5 -coordinating ligand C8-C14 is modeled as disordered over two positions, 0.697 (5):0.303 (5), such that the ethyl linkage is shifted by one carbon atom. Analogous bond lengths and angles between the two positions of the disordered ring were heavily restrained to be similar. Anisotropic displacement parameters for pairs of proximal atoms from the two components of the disorder were constrained to be equivalent. H atom positions of ring-ligand carbon atoms, except those in the minor component of the disorder, were refined freely. All other H atoms were placed geometrically and treated as riding atoms: methine and sp 2 , C-H = 1.00 Å, and methylene, C-H = 0.99 Å, with U iso (H) = 1.2U eq (C).

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