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

William W Brennessel; John E Ellis

doi:10.1107/S1600536812038287

. 2012 Sep 12;68(Pt 10):m1257–m1258. doi: 10.1107/S1600536812038287

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

William W Brennessel ^a,^*, John E Ellis ^a

PMCID: PMC3470144 PMID: 23125588

Abstract

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.

Related literature

For reports on the only other homoleptic ethylenemetalate, the ethyleneferrate, see: Jonas (1979 ▶, 1981 ▶); Jonas et al. (1979 ▶); Jonas & Krüger (1980 ▶). For reports on the anion of the title complex, but with different cations, see: Jonas (1979 ▶, 1981 ▶, 1984 ▶, 1985 ▶); Jonas et al. (1979 ▶); Jonas & Krüger (1980 ▶). For the initial report of this anion synthesized from cobalt(II) bromide, see: Brennessel et al. (2006 ▶). For neutral and cationic structurally characterized homoleptic ethylene transition metal complexes, see for [Pt⁰]: Howard et al. (1983 ▶); for [Cu⁺]: Santiso-Quiñones et al. (2007 ▶); for [Ag⁺]: Reisinger et al. (2009 ▶); for [Au⁺]: Dias et al. (2008 ▶). For details of the preparation and purification of reagents and solvents, and for descriptions of the equipment and techniques, see: Brennessel (2009 ▶). For a description of the Cambridge Structural Database, see: Allen (2002 ▶). For the bond-length of ethylene gas, see: Lide (2003 ▶).

Experimental

Crystal data

[K(C₁₈H₃₆N₂O₆)][Co(C₂H₄)₄]
M _r = 586.73
Orthorhombic,
a = 25.836 (3) Å
b = 10.4820 (12) Å
c = 22.544 (3) Å
V = 6105.4 (12) Å³
Z = 8
Mo Kα radiation
μ = 0.74 mm⁻¹
T = 173 K
0.50 × 0.24 × 0.16 mm

Data collection

Siemens SMART CCD Platform diffractometer
Absorption correction: multi-scan (SADABS; Sheldrick, 1996 ▶) T _min = 0.709, T _max = 0.891
45000 measured reflections
7010 independent reflections
4825 reflections with I > 2σ(I)
R _int = 0.056

Refinement

R[F ² > 2σ(F ²)] = 0.035
wR(F ²) = 0.073
S = 1.02
7010 reflections
414 parameters
16 restraints
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 0.31 e Å⁻³
Δρ_min = −0.31 e Å⁻³

Data collection: SMART (Bruker, 2003 ▶); cell refinement: SAINT (Bruker, 2003 ▶); data reduction: SAINT; program(s) used to solve structure: SIR97 (Altomare et al., 1999 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: SHELXTL (Sheldrick, 2008 ▶); software used to prepare material for publication: SHELXTL.

Supplementary Material

Crystal structure: contains datablock(s) I, global. DOI: 10.1107/S1600536812038287/su2495sup1.cif

e-68-m1257-sup1.cif^{(47KB, cif)}

Structure factors: contains datablock(s) I. DOI: 10.1107/S1600536812038287/su2495Isup2.hkl

e-68-m1257-Isup2.hkl^{(343.1KB, hkl)}

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Table 1. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C24—H24A⋯O5ⁱ	0.99	2.59	3.326 (6)	131

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Symmetry code: (i) Inline graphic .

Acknowledgments

This research was supported by the US National Science Foundation and the donors of the Petroleum Research Fund, administered by the American Chemical Society.

supplementary crystallographic information

Comment

The reductive synthesis of the anion from cobaltocene (CoCp₂) with alkali metals has been reported previously in a patent (Jonas, 1979) and in review articles (Jonas & Krüger, 1980; Jonas, Schieferstein et al., 1979; Jonas, 1981, 1984, 1985). 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 CoCp₂ (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. The recently reported anion [Co(η²-C₂H₄)₂(η⁴-C₁₀H₈)]^-, bis(ethylene)naphthalenecobaltate(–I), which occurs twice independently as part of a triple salt (Brennessel et al., 2006), has bond lengths that are statistically identical (1.410 (8) Å, avg) to those of the title complex (1.401 (6) Å, avg). The isoelectronic iron structure, [Li(tmeda)]₂[Fe(η²-C₂H₄)₄], has been determined with two unique C–C distances of 1.410 and 1.433 Å (Jonas, Schieferstein et al., 1979); relative to that of free ethylene = 1.333 Å (Lide, 2003). No standard uncertainties were reported, but they are likely to be somewhat large (R1 = 7.8%). Also, the ethylene ligands have an asymmetry due to different contact distances with the lithium cations. If we take the average C–C bond length of the iron structure at face value, 1.42 Å, then it appears to be slightly longer that of the title complex, 1.401 (6) Å, which would be consistent with a more reduced metal center. All other structurally characterized homoleptic ethylene transition metal compounds are either neutral: Pt (Howard et al., 1983); or cationic: Cu⁺ (Santiso-Quiñones et al., 2007), Ag⁺ (Reisinger et al., 2009), and Au⁺ (Dias et al., 2008).

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[C₁₀H₈], (13.7 mmol) in THF (50 ml, 195 K) and from a second flask containing bright blue anhydrous CoBr₂ (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 CoBr₂ 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). Et₂O (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 re-coordination of naphthalene). The slurry was filtered, and the product was washed with Et₂O (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, °): a = 15.498 (5), b = 14.768 (5), c = 10.744 (4), B = 94.253 (4); V = 2452.2 (14) Å³; Z = 4; T = 173 (2) K; 2161 reflections (1813 for [I > 2σ(I)]).