{μ-5-[1,3-Bis(2,4,6-trimethylphenyl)-3H-imidazolium-2-yl]-2-(2-oxoethenyl-1κC 1)furan-3-yl-2κC 3}-μ-hydrido-bis(tetracarbonylrhenium) tetrahydrofuran 0.67-solvate

The title complex, [Re2(C27H25N2O2)H(CO)8]·0.67C4H8O, was formed as a product in the reaction of a rhenium(I)–Fischer carbene complex with a free NHC carbene. The coordination environment about the two Re atoms is slightly distorted octahedral, including a bridging H atom. The imidazolium and furan groups are almost coplanar, whereas the mesityl substituents show an almost perpendicular arrangement with respect to both heterocyclic units. Molecules of the complex pack in such a way as to form channels parallel with the bc unit-cell face diagonal running through the unit face diagonal. These channels are partially occupied by tetrahydrofuran solvent molecules.

The title complex, [Re 2 (C 27 H 25 N 2 O 2 )H(CO) 8 ]Á0.67C 4 H 8 O, was formed as a product in the reaction of a rhenium(I)-Fischer carbene complex with a free NHC carbene. The coordination environment about the two Re atoms is slightly distorted octahedral, including a bridging H atom. The imidazolium and furan groups are almost coplanar, whereas the mesityl substituents show an almost perpendicular arrangement with respect to both heterocyclic units. Molecules of the complex pack in such a way as to form channels parallel with the bc unit-cell face diagonal running through the unit face diagonal. These channels are partially occupied by tetrahydrofuran solvent molecules.

Comment
The title complex (1) was formed as a product in the reaction of a rhenium(I)-Fischer carbene complex with a free NHC carbene, prepared in situ by the deprotonation of N,N′-dimesitylimidazolium chloride (HIMesCl) with n-butyllithium.
Formation of this complex is quite unique and involves four reaction sites. Rhenium complexes can mimic transition metal Lewis acids to catalyse Friedel-Crafts C-C bond formation. The catalytic activity of the rhenium complexes is initiated by decarbonylation under heating to form the active species [ReBr(CO) 4 ]. In complex 1 the position most susceptible to nucleophilic attack is the 2-position on the furan ring. This is attributed to the electron withdrawing effect of the carbene ligand on the ring. Subsequent C-H activation leads to hydride migration and Re-Re bond breaking.
The resulting complex now contains both a Re-C and Re-H bond. The Re atom from the original Fischer carbene moiety is left with a vacant coordination site enabling the newly formed hydride to form a bridge between the two Re atoms. Ketenyl complexes of tungsten have been previously reported by Kreissl et al. (1976Kreissl et al. ( , 1977 and later these types of complexes were also described for rhenium(VII) complexes (Li et al., 2006). The insertion of a bridging CO can lead to the ketene formation observed in complex 1.
The geometry of the dimesitylimidazol-2-yl moiety is similar to those observed for previously published dimesitylimidazol-2-yl structures, in particular for some recent examples bonded to a C atom (Naeem et al., 2010;Chia et al., 2011). As usual, the planes of the rings of the mesityl substituents are close to perpendicular to that of the imidazole ring (Table 1). However, the furan ring is close to being coplanar with the imidazole ring (Table 1).
The overall shape of the molecule of the complex does not lend itself to efficient close-packing in the crystal. The molecules of the complex pack in layers parallel with the A face of the unit cell with the two C-H moieties of each imidazol ring in close contact with the Re(CO) 4 moieties of molecules in the next layer. Such an arrangement leaves voids in the crystal structure which form channels running parallel with the b,-c face diagonal of the unit cell (Fig. 2). The thf solvent molecules occupy these channels (Fig. 3). The channels allow egress of thf molecules out of the crystal without significant degradation of the structure. Thus loss of solvent has occurred leading to a partial occupancy of the site by the thf solvate of 0.666 (13) of a molecule per asymmetric unit. Uncertainty in the precise positions of the thf molecules within the channels leads to the molecule being ill-defined in the structure solution and refinement, thus the thf molecule needed to be treated as a rigid body.

Refinement
All H atoms were included in calculated positions and allowed to ride on the atom to which each is bonded (except for H1 which bridges the two Re atoms the coordinates of which were not varied). The isotropic adp's for each H were set to 1.2 × the equivalent isotropic adp of the atom to which each is bonded (1.5 × for H1). A poorly defined thf solvent molecule was treated as a rigid body and a common isotropic adp was refined for all its non-H atoms. A common site occupation factor for all the atoms of the thf molecule refined to a value of 0.666 (13).     where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 2.10 e Å −3 Δρ min = −0.83 e Å −3

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.