A cationic rhodium(I) N-heterocyclic carbene complex isolated as an aqua adduct

The title complex, aqua[1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene](η4-cycloocta-1,5-diene)rhodium(I) tetrafluoridoborate, [Rh(C8H12)(C27H36N2)(H2O)]BF4, exihibits a square-planar geometry around the Rh(I) atom, formed by a bidentate cycloocta-1,5-diene (cod) ligand, an N-heterocylcic carbene and an aqua ligand. The complex is cationic and a BF4 − anion balances the charge. The structure exists as a hydrogen-bonded dimer in the solid state, formed via interactions between the aqua ligand H atoms and the BF4 − F atoms.


Experimental
Crystal data [Rh(C 8 H 12   of the complex (Herrmann, et al., 2006). Here, we report the crystal structure of the title compound, 1, as a stable aqua adduct that also exhibits H-F bonding interactions in the solid state.
The asymmetric unit of 1 contains a full molecule with the Rh(I) ion in a square planar geometry ( Figure 1). The coordination sphere of the Rh(I) is completed through bonds to cycooctadiene, the carbene, and an aqua ligand, creating a complex cation. Charge balance is achieved with a non-coordinating tetrafluoroborate anion.The existence of nearly idealized square planar geometry can be supported using a recently reported metric, τ 4 , for determining molecular shape in four coordinate complexes (Yang, et al., 2007). Here, a τ 4 value near zero is determined for square planar complexes; however, as the value approaches one, tetrahedral geometry is observed. By evaluation of the bond angles around the central rhodium atom, the τ 4 parameter was determined to be 0.028 for the title compound. The carbene atom, C1, deviates from an idealized sp 2 hybridization in that the N1-C1-N2 bond angle is 103.77 (19)°.
Some related cationic rhodium -imidazol-2-ylidene carbene complexes with tetrafluroborate counteranions have been reported (Nichol, et al., 2009(Nichol, et al., , 2010Bappert, et al., 2004) albeit, with neutral donors other than H 2 O. In the case of 1, presumably due to the steric of the cod and carbene ligands, H 2 O was found to be the only neutral ligand of appropriate size to occupy the fourth coordination site. In summary, we have reported the crystal structure of a cationic rhodium carbene complex containing an aqua ligand.
The structure exists as a hydrogen-bonded dimer in the solid state. Future work aims to investigate the reactivity of this and other similar complexes for various organic transformations.

Experimental
All chemicals were purchased commercially, except for the neutral rhodium (I) carbene complex, [(cod)Rh(NHC)Cl], which was prepared according to the procedure of Yu, et al. (2006). The following manipulations were carried out under an inert supplementary materials sup-2 nitrogen atmosphere. The cationic compound, [(cod)Rh(NHC)H 2 O]BF 4 , was synthesized by mixing the neutral rhodium compound (0.170 mmol) with water (10 drops) and silver tetrafluoroborate (0.170 mmol) in dichloromethane (20 ml). A yellow solution was obtained after stirring at room temperature for four days along with the formation of a white precipitate.
The solution was gravity filtered over Celite and the filtrate was dried in vacuo to give a dark orange product (63.33%).
X-ray quality crystals were obtained by dissolving the product in a small amount of dichloromethane and layering with pentane. 1

Refinement
Most hydrogen atoms were placed at calculated geometries and allowed to ride on the position of the parent atom. Hydrogen thermal parameters were set to 1.2 times the equivalent isotropic U value of the parent atom. C-H distances were constrained as follows: C aromatic -H 0.95 Å, CH 3 0.98 Å, CH 1.00 Å, and CH 2 0.99 Å. The hydrogen atoms of the aqua ligand were found from the difference Fourier map and allowed to freely refine.
Crystal data [Rh(C 8  as those based on F, and R-factors based on ALL data will be even larger.
The hydrogen atoms on the water ligand were located from the difference map and their positions were allowed to refine freely.