[(1,2,5,6-η)-Cycloocta-1,5-diene](1-ethyl-3-isopropyl-1,3-imidazol-2-ylidene)(triphenylphosphane)rhodium(I) tetrafluoridoborate

The cationic complex of the title compound contains an RhI atom with a pseudo-square planar coordination environment. It is ligated by an N-heterocyclic carbene, a triphenylphosphane, and a bidentate cylcooctadiene ligand. Charge balance is achieved from an out-sphere tetrafluoridoborate anion.


Structure description
N-heterocyclic carbenes (NHCs) have emerged as excellent spectator ligands in homogeneous catalysis, especially in transfer hydrogenation reactions. Transfer hydrogenation of unsaturated bonds is a reaction of great interest and it exemplifies some of the key aspects of green chemistry (Ruff et al., 2016;Zuo et al., 2014). The N-heterocyclic carbene (NHC) ligands can be tuned sterically and electronically by having different alkyl groups on the nitrogen atoms (Gusev, 2009). Many imidazole-and triazole-based NHC-rhodium and -iridium complexes have been synthesized and structurally characterized (Herrmann et al., 2006;Wang & Lin 1998;Chianese et al., 2004;Nichol et al., 2009Nichol et al., , 2010Nichol et al., , 2011Nichol et al., , 2012Idrees et al., 2017a,b;Huttenstine et al., 2011). Their catalytic activities in the transfer data reports hydrogenation of ketones and imines has also been studied and reported (Hillier et al., 2001;Gnanamgari et al., 2007;Albrecht et al., 2002).
The molecular structure of the title salt, [RhC 34 H 41 N 2 P] + (BF 4 ) À , (4), is illustrated in Fig. 1. No solvent molecules were found in the structure of (4). The coordination environment around the rhodium(I) ion, formed by the coordination to the metal of the two olefinic bonds of the cyclooctadiene (COD) ligand, the carbene carbon atom of the NHC ligand, and the phosphorus atom from triphenylphosphane, is slightly distorted square-planar. The Rh-C(NHC) bond length is found to be 2.035 (3) Å in (4). The C(NHC)-metal-P(PPh 3 ) bond angle is 88.37 (8) . The N-C(carbene)-N bond angle in the imidazole-based carbene is 104.7 (2) .
Several non-covalent interactions exist between atoms that are closer than the sum of the van der Waals radii and are reported in Table 1. Fig. 2 shows the crystal packing diagram for compound (4) with these interactions shown as dashed orange lines. The majority of these interactions exist as weak, unconventional C-HÁ Á ÁF hydrogen bonds between the ligands and the fluorine atoms of the tetrafluoridoborate anion. From the NHC ligand, the hydrogen atom on the fivemembered ring, H21, interacts with F4. H24 from the isopropyl wingtip group and H22A from the ethyl wingtip group interact with F2 and F3, respectively. H28B and H32B from the double bonds of the COD ligand interact with F4 and F2, respectively. H12, a hydrogen atom in the ortho position of a phenyl ring on the triphenylphosphane ligand interacts with F1.

Synthesis and crystallization
1-Ethyl imidazole (compound 1) was purchased from Strem and used without further purification, and ligand syntheses were performed in air using reagent-grade solvents, which were used without further purification. NMR spectra were recorded at room temperature in CDCl 3 on a 400 MHz (operating at 162 MHz for 31 P) Varian spectrometer and Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) x; y À 1; z; (ii) x À 1 2 ; y À 1 2 ; z.

Figure 2
Crystal packing diagram of compound (4) with non-covalent interactions shown with dotted orange lines.

Figure 3
Reaction scheme summarizing the synthesis of the N-heterocylic carbene ligand through the formation of the title salt (4).

Figure 1
A view of the molecular entities in compound (4), showing the atom labeling. Displacement ellipsoids are drawn at the 50% probability level.   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.