Two chromium(II) acetate complexes with N-heterocyclic carbene (NHC) coligands

The crystal structures of two chromium(II) acetate complexes with N-heterocyclic carbene coligands were determined.


Chemical context
Since its discovery in 1844 by Peligot (Peligot et al., 1844), chromium(II) acetate has frequently been used as the starting material for a large variety of chromium(II) compounds (Cotton et al., 2005).Treatment of chromium(II) acetate with donor ligands L gives dinuclear complexes [Cr 2 (OAc) 4 L 2 ] that adopt paddle-wheel structures with the ligands L at axial positions.This structure pattern was first observed for the dihydrate [Cr 2 (OAc) 4 (H 2 O) 2 ] (van Niekerk et al., 1953;Cotton et al., 1971) and later on for a large number of ligands comprising oxygen and, particularly, nitrogen donor atoms (Cotton et al., 2005).
Recently, we reported on chromium(II) silylamide complexes that were generated from chromium(II) acetate as starting material.In the course of these investigations, the application of NHC coligands proved very successful.Typically, a suspension of chromium(II) acetate in THF was first treated with the NHC ligand to give deeply violet-coloured solutions.Treatment of the in situ generated chromium(II) acetate NHC complex with Li 2 Me 2 Si(NPh) 2 led to [Cr{Me 2 Si(NPh) 2 (NHC) 2 }] (Heiser & Merzweiler, 2022).We were now interested in the isolation and structural characterization of chromium(II) acetate NHC complexes.
[Cr 2 (OAc) 4 (IDipp) 2 ]•2THF (1) and [Cr 2 (OAc) 4 (IMes) 2 ] (2) crystallize in the triclinic system, space group P1 with Z = 1.The crystal structure of 1 consists of discrete [Cr 2 (OAc) 4 (IDipp) 2 ] units and two molecules of tetrahydrofuran per formula unit.Compound 2 crystallizes without solvate molecules.In both complexes, the Cr 2 (OAc) 4 units exhibit classical paddle-wheel structures with crystallographically imposed 1 symmetry.The coordination sphere of the chromium atoms consists of four acetate oxygen atoms at the base of a square pyramid and the NHC carbon atom at the apex.
The Cr-O distances in 1 range from 2.012 (2) to 2.025 (1) A ˚and in 2 from 2.024 (2) to 2.027 (2) A ˚(Tables 1  and 2).Similar distances have been reported for 15 chromium(II) acetate derivatives that are currently deposited in the CSD database (Groom et al., 2016).The shortest

Table 1
Selected geometric parameters (A ˚, � ) for 1.Overall, the geometric parameters of both compounds are very similar.However, it is worth mentioning that complexes 1 and 2 differ in the mutual orientation of the NHC ligands and the paddle-wheel core.In the case of compound 1, the imidazolidine ring adopts an eclipsed orientation with respect to the O1-Cr-O2 unit as indicated by the torsion angles N2-C5-Cr-O1

Supramolecular features
Compound 1 displays a weak C-H� � �O hydrogen bridge [D� � �A: 3.411 (6) A ˚; Table 3] between the C6-H6 group of the imidazolidine ring and the tetrahydrofuran oxygen atom O5.In the case of compound 2, there is a weak C-H� � �O hydrogen bridge [D� � �A: 3.527 (4) A ˚; Table 4, Fig. 3] between the acetate carbon atom C2 and the acetate oxygen atom O2 ii of a neighbouring complex unit.Furthermore, there is a complementary hydrogen bridge between C2 ii and O2.As a result, the chromium acetate complexes are catenated by R 2 2 (8) hydrogen-bond motifs along the direction of the crystallographic a axis.Moreover, the supramolecular structure is supported by weak C-H� � �� hydrogen bonds (Fig. 4), which are formed between neighbouring mesityl groups.The distance between the methyl carbon atom C15 and the centroid of the aromatic ring C17 iii -C22 iii is 3.340 (4) A ˚.

Synthesis of [Cr 2 (OAc) 4 (IDipp) 2 ] (1)
To a suspension of chromium(II) acetate (470 mg, 1.39 mmol) in toluene (12 ml) was added a solution of IDipp (1180 mg, 2.78 mmol) in toluene (8 ml).The solution was stirred at 300 K overnight.The chromium(II) acetate dissolved and a change of colour from dark red to violet was observed.Insoluble material was filtered off and the solution was concentrated slightly in vacuo.Upon standing at 248 K for two days, the product crystallized in the form of violet crystals, which were filtered off and dried in vacuo.Single crystals of the product were obtained upon cooling down a THF solution of 1 to 248 K. Yield: 660 mg (40%).
(2 � 5 ml).After reducing the volume to half the amount, the solution was heated to dissolve the precipitated product.Upon standing at 267 K for two days, the product crystallized in a form of violet single crystals, which were filtered off and dried in vacuo.Yield: 650 mg (40%

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5.All hydrogen atoms were positioned geometrically and refined using a riding model with U iso (H) = 1.2(CH and CH 2 ) or 1.5(CH 3 ) times U eq (C).

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.

Figure 1
Figure 1Molecular structure of 1 in the crystal.Displacement ellipsoids are at the 50% probability level.There is a disorder over two orientations concerning the THF molecule and three of the i Pr groups.In each case, only the major orientation is displayed.H atoms not involved in C-H� � �O hydrogen bonds are omitted for clarity.Intermolecular C-H� � �O hydrogen bonds shown as dashed lines.[Symmetry code: (i) À x + 1; À y + 1; À z + 1.

Figure 3
Figure 3 Crystal structure of 2, intermolecular C-H� � �O hydrogen bonds are shown as dashed lines.

Table 5
Experimental details.