(2.2.2-Cryptand)potassium tetrakis(η2-ethylene)cobaltate(−I)

The title salt, [K(C18H36N2O6)][Co(C2H4)4], is one of only two known homoleptic ethylenemetalates. The cation and anion are well separated, which gives an unperturbed tetrahedral anion as is expected for a formally Co−I d 10 metal center. The considerable elongation of the C=C bonds of the ethylene ligands [average 1.401 (6) Å], relative to that of free ethylene (1.333 Å), is consistent with metal→π* back-bonding models. One arm of the 2.2.2-cryptand (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) complexant is disordered and was modeled over two positions with a refined occupancy ratio of 0.559 (2):0.441 (2). In the crystal, the cationic K(2.2.2-cryptand) units are linked via C—H⋯O hydrogen bonds, forming inversion dimers. There are no other significant intermolecular interactions in the crystal structure.

The title salt, [K(C 18 H 36 N 2 O 6 )][Co(C 2 H 4 ) 4 ], is one of only two known homoleptic ethylenemetalates. The cation and anion are well separated, which gives an unperturbed tetrahedral anion as is expected for a formally Co ÀI d 10 metal center. The considerable elongation of the C C bonds of the ethylene ligands [average 1.401 (6) Å ], relative to that of free ethylene (1.333 Å ), is consistent with metal!* back-bonding models. One arm of the 2.2. 2-cryptand (4,7,13,16,21,24-hexaoxa-1,10diazabicyclo[8.8.8]hexacosane) complexant is disordered and was modeled over two positions with a refined occupancy ratio of 0.559 (2):0.441 (2). In the crystal, the cationic K(2.2.2cryptand) units are linked via C-HÁ Á ÁO hydrogen bonds, forming inversion dimers. There are no other significant intermolecular interactions in the crystal structure.
Herein we report on the first structure of the title anion and the reductive synthesis from cobalt(II) bromide using potassium naphthalene as the reducing agent. Because the advantages of having cyclopentadienide (Cp -) as a support ligand in the reduction from CoCp 2 (Jonas & Krüger, 1980; see discussion on page 533) were not available in our synthesis, we had to be certain that ethylene gas was present in excess, to assist naphthalene in supporting the metal center in its various oxidation states from +2 to -1. This was achieved with low temperatures, specifically 195 K, at which point ethylene appeared to be "infinitely" soluble in THF. Even at the very cold, but slightly warmer, temperature of 213 K, ethylene appeared to have finite solubility. The other interesting point in the synthesis was that naphthalene would (re)coordinate to cobalt when THF was removed under reduced pressure. Therefore the isolation of the final product required that additional ethylene gas be reintroduced to the diethyl ether slurry to displace any (re)coordinated naphthalene. It was easy to determine when the naphthalene was fully displaced because the slurry lost all trace of red and became pale yellow to off-white. Details on the isolated red naphthalenecobaltates(-I) can be found elsewhere (Brennessel et al., 2006, Brennessel, 2009. A search of the Cambridge Structural Database (CSD, Version 5.33, update No. 4, August 2012;Allen, 2002), indicated the presence of 629 structures containing an ethylene ligand, but only 29 are with first row transition metals containing at least two ethylene ligands.
The molecular structure of the title anion is illustrated in Fig. 1, and the title K + 2.2.2-cryptand cationic unit in Fig. 2.
In the crystal, the cationic K + 2.2.2-cryptand units are linked via a pair of C-H···O hydrogen bonds to form inversion dimers (Table 1). There are no other significant intermolecular interactions in the crystal structure.

Experimental
Details on the preparation and purification of reagents and solvents, and descriptions of the equipment and techniques can be found elsewhere (Brennessel, 2009). Note that the following synthetic procedure results in a salt for which the potassium cation is complexed by 18-crown-6. Unfortunately single crystals that were grown of this complex resulted in very poor quality data (see below), and thus a different potassium complexant was incorporated for this study. To obtain the title complex the 2.2.2-cryptand salt, an aliquot of the yellow filtrate prior to the addition of 18-crown-6, was transferred to a flask containing excess 2.2.2-cryptand. Light yellow needles of the title complex were then grown from a pentane-layered THF solution at 273 K.
Argon was removed in vacuo from a flask containing deep green potassium naphthalene, K[C 10 H 8 ], (13.7 mmol) in THF (50 ml, 195 K) and from a second flask containing bright blue anhydrous CoBr 2 (1.000 g, 4.57 mmol) also in THF (50 ml, 195 K), and replaced with ethylene. At this low temperature ethylene is extremely ("infinitely") soluble and the flask system would develop a slight vacuum whenever the valve to the ethylene tank was closed. After ca. 15 psi of gas were drawn from the tank, both the tank and the flasks were closed off and argon was reintroduced to the line. Using argon pressure (a Hg bubbler was attached to the flask system to keep the pressure near 1 atm), the CoBr 2 solution was transferred to the reducing agent via cannula, producing a pale yellow solution, which was then warmed slowly to room temperature (with the system open to the Hg bubbler!). The solution was filtered to remove KBr. 18-crown-6 (1.208 g, 4.57 mmol) in THF (20 ml) was added to the yellow filtrate. The solvent was removed in vacuo, which caused the solution to turn reddish as some naphthalene (re)coordinated to some of the product (see below). Et 2 O (75 ml) was added and argon was once again replaced with ethylene, at which point the slurry lost its red color and became nearly colorless.
The lines to the flasks were freed of ethylene and replaced with argon (the flask was not evacuated to avoid possible recoordination of naphthalene). The slurry was filtered, and the product was washed with Et 2 O (20 ml) and dried in vacuo, yielding an off-white solid (1.796 g, 83%). Although the product contained paramagnetic impurities which caused severe broadening of NMR spectral peaks, the material was sufficiently pure by this synthetic method for use in subsequent reactions. Pale yellow blocks of the18-crown-6 salt, which were grown from a pentane-layered THF solution at 273 K, were not suitable for a single-crystal X-ray experiment. The anion was badly disordered over a crystallographic twofold axis and no satisfactory model was obtained, thus the reason for the aliquot that was extracted for use with 2.2.2-cryptand to produce crystals of the title salt (see above). Crystal data for the 18-crown-6 salt: Monoclinic, C2/c; Cell constants (Å,

Refinement
One arm of the 2.2.2-cryptand complexant (atoms O5,O6,C21-C26 & O5′,O6′,C21′-C26′) was modeled as disordered over two positions with a refined occupancy ratio of 0.559 (2):0.441 (2). Corresponding bond lengths and angles in the two orientations of the cryptand arm disorder were restrained to be similar. Anisotropic displacement parameters for spatially close atoms from the two orientations were constrained to be equivalent. H atoms on the ethylene ligands were located in a difference Fourier map and were freely refined. All other H atoms were placed geometrically and treated as riding atoms: C-H = 0.99 Å with U iso( H) = 1.2U eq (C).   The molecular structure of the title cationic K + 2.

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. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.