Bis[μ-N,N′-bis(2,6-diisopropylphenyl)ethene-1,2-diamido]-1,4(η2);1:2κ4 N:N;3:4κ4 N:N-bis(diethyl ether)-1κO,4κO-di-μ-hydrido-2:3κ4 H:H-2,3-dichromium(II)-1,4-dilithium(I) pentane hemisolvate

The title compound, [Cr2Li2(C26H36N2)2(μ-H)2(C4H10O)2]·0.5C5H12, is a binuclear chromium complex bridged by two hydrogen atoms. Each chromium atom is coordinated in a distorted square-planar geometry by one chelating bis(2,6-diisopropylphenyl)ethene-1,2-diamido ligand via its two N atoms. Additionally, two diametrically opposed lithium ether adducts coordinate in an η4 mode on the backbone of the ligands. There is a crystallographic inversion center in the middle of the Cr2H2 ring. One of the isopropyl groups is disordered over two positions in a 0.567 (7):0.433 (7) ratio. Disorder is also observed in the pentane hemisolvate molecule.

The title compound, [Cr 2 Li 2 (C 26 H 36 N 2 ) 2 (-H) 2 (C 4 H 10 O) 2 ]Á-0.5C 5 H 12 , is a binuclear chromium complex bridged by two hydrogen atoms. Each chromium atom is coordinated in a distorted square-planar geometry by one chelating bis(2,6diisopropylphenyl)ethene-1,2-diamido ligand via its two N atoms. Additionally, two diametrically opposed lithium ether adducts coordinate in an 4 mode on the backbone of the ligands. There is a crystallographic inversion center in the middle of the Cr 2 H 2 ring. One of the isopropyl groups is disordered over two positions in a 0.567 (7):0.433 (7) ratio. Disorder is also observed in the pentane hemisolvate molecule.  Structurally characterized binuclear chromium complexes that are bridged by two hydrogen atoms were reported only four times before (Fryzuk et al., 1994;MacAdams et al., 2003;Albahily et al., 2008;Rozenel et al., 2009). This is the first time such kind of complex is reported with ethene-1,2-diamido ligands. A similar coordination of alkali metals on binuclear metal-1,2-diiminoethane complexes was observed before (Baker et al., 2005).

Related literature
We became interested in chromium hydride derivatives because they are postulated as intermediates formed in selective oligomerization of ethylene following β-hydride elimination of the hepta-or nonametallacycle, prior to elimination of 1hexene or 1-octene. Cr dihydride species are suggested to be formed in side-chain reactions during the ethylene tetramerization process (Overett et al., 2005). In order to explore the chemistry of these kinds of complexes, we reacted a binuclear chromium diimine complex (Peitz et al., 2009) with 1,4-dilithiobutane to build up chromacyclopentanes which decompose at room temperature and form a binuclear dihydrido-bridged chromium complex.
The molecular structure of the title compound shows that two chromium(II) centers are bridged by two hydrogen atoms to form a binuclear complex. Each metal center is coordinated by one chelating diimine igand, (i-Pr) 2 C 6 H 3 -NC(H)-C(H)N-C 6 H 3 (i-Pr) 2 , via both N atoms of each ligand. Due to its redox properties this ligand acts as electron acceptor which leads to the shortened C-C and elongated C-N bond lengths in the ligand backbone in comparison to the free diimine ligand, thus forming an ethene-1,2-diamido unit. Additionally, diametrically opposed to each other, two lithium ether adducts coordinate in a η 4 mode on the backbone of the ligands which are twisted in an angle of 62.1 (1) ° against each other. The coordination geometry on each chromium center can be best described as distorted square planar (mean deviation from the best plane defined by Cr1-N1-C1-C2-N2 0.046 Å). The Cr1-Cr1′ distance of 2.5779 (5) Å is around 0.14 Å shorter than those found in all the other structurally characterized dihydride-bridged chromium dimers and solution. Crystallization at -30 °C yielded 0.082 g (22%) of red-brown single crystals suitable for X-ray analysis.

S3. Refinement
H1 was located via the difference Fourier map and refined isotropically. All other H atoms were placed in idealized positions with d(C-H) = 0.99 (CH 2 ), 0.98 (CH 3 ) and 0.95-1.00 Å (CH) and refined using a riding model with U iso (H) fixed at 1.5 U eq (C) for CH 3 and 1.2 U eq (C) for CH 2 and CH.

Figure 1
The molecular structure of the title compound showing the atom-labelling scheme. Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms, solvent and the disorder of the coordinated diethyl ether are not shown for clarity.

ether)-1κO,4κO-di-µ-hydrido-2:3κ 4 H:H-2,3-dichromium(II)-1,4-dilithium(I) pentane hemisolvate
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. 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.