cyclo-Tetrakis(μ-2,4,6-trimethylphenyl-κC 1:κC 1)bis(trimethylphosphane)-1κP,3κP-tetracopper(I)

The molecular structure of the title complex consists of an eight-membered Cu4C4 ring with an alternating arrangement of copper(I) atoms and μ-mesityl groups. Two of the copper(I) atoms are additionally linked to PMe3 ligands, giving a distorted trigonal coordination.

The title compound, [Cu 4 (C 9 H 11 ) 4 (C 3 H 9 P) 2 ] or [Cu 4 (Mes) 4 (PMe 3 ) 2 ] (Mes = 2,4,6-trimethylphenyl), was synthesized from copper(I) mesityl and trimethylphosphane in THF as solvent. The molecular structure of the complex has C 2 symmetry and consists of four copper(I) atoms bridged by four -mesityl groups, giving an eight-membered puckered {Cu 4 C 4 } ring. Additionally, two copper(I) atoms at opposite corners of the Cu 4 rhomb are each linked to a terminal PMe 3 ligand. The PMe 3 -bearing copper(I) atoms exhibit a distorted trigonal-planar coordination mode whereas the remaining Cu atoms linked to two mesityl groups are nearly linearly coordinated.
In order to get some insight into the reactivity of mesitylcopper towards sterically less demanding phosphanes, trimethylphosphane was chosen as a ligand. Treatment of a data reports solution of mesitylcopper in THF with PMe 3 at room temperature led to the formation of the tetranuclear complex [Cu 4 (Mes) 4 (PMe 3 ) 2 ] (1).
The molecular structure of (1) comprises four copper(I) atoms that are linked by four -mesityl groups to give an eight-membered {Cu 4 C 4 } ring (Fig. 1). Additionally, two copper atoms at diametrically opposite positions of the ring are each linked to a terminal PMe 3 group. The tetranuclear copper complex exhibits crystallographic C 2 symmetry with the diad axis passing through the center of the C10-C15 bond. The rhombic arrangement of the copper atoms is nearly planar, with marginal deviations of 0.0087 Å from the mean plane through the four copper atoms. The relatively small CuÁ Á ÁCu distances at the edges of the rhomb [2.4603 (5)-2.4625 (5) Å ] suggest cuprophilic interactions. The CuÁ Á ÁCu separations between the copper atoms at opposite corners of the rhomb are 4.2013 (5) Å for Cu2Á Á ÁCu2 i [symmetry code: (i) -x + 1, y, -z + 1 2 ] and 2.5657 (7) Å for Cu1Á Á ÁCu1 i . Similar shaped arrangements of four Cu atoms were observed in the derivatives [Cu 4 (Mes) 4 (THT) 2 ], [Cu 4 (o-Tol) 4 (SMe 2 ) 2 ] (Lend-ers et al., 1991) and [Cu 4 Ph 4 (SMe 2 ) 2 ] (Olmstead & Power, 1990). Complex (1) exhibits two types of differently coordinated Cu atoms (Table 1). Cu1 is surrounded by two mesityl groups with Cu-C distances of 2.005 (3) and 2.006 (3) Å . In comparison with Cu 4 Mes 4 , the Cu-C distances are slightly enlarged by around 0.014 Å . However, the bending of the C1-Cu1-C10 i unit [138.3 (1) ] is clearly more pronounced than in [Cu 4 Mes 4 ] (164.05-165.70 ). Apart from two mesityl groups, Cu2 bears a PMe 3 unit as a third ligand. The increased coordination number leads to a further enlargement of the Cu-C distances with values of 2.093 (3) and 2.095 (3) Å . The coordination around Cu2 is planar with a C-Cu-C angle of 163.0 (1) and C-Cu-P angles of 97.9 (1) and 99.0 (1) (sum of the angles around Cu2: 359.9 ). Comparison of the bond lengths of compound (1) and related [Cu 4 Mes 4 L 2 ] complexes reveals that the ligand PMe 3 leads to a larger increase of the Cu-C distances for the tricoordinate copper atoms than other ligands investigated so far. In [Cu 4 Mes 4 L 2 ] complexes with L = piperidine, allyl methyl sulfide, 2,5-dithiahexane, tetrahydrothiophene and bis{2-[1-(dimethylamino)ethyl]phenylthiolato}magnesium, the mean Cu-C distances for the tricoordinated copper atoms are in the range 2.054-2.064 Å . In the case of the dicoordinated Cu there is no particular effect. Furthermore, there is a slight influence on the C-Cu-C angles for the dicoordinated [138.3 (1) ] and the tricoordinate copper atoms [163.0 (1) ], which are smaller than in the [Cu 4 Mes 4 L 2 ] complexes mentioned above (140.3-142.8 and 165.0-170.2 , respectively).

Figure 2
Partial packing diagram for 1 in a view down the crystallographic b axis. The intermolecular C-HÁ Á Á interactions are shown as gray dashed lines.

Synthesis and crystallization
A solution of 0.46 g (2.5 mmol) mesitylcopper (Meyer et al., 1989) in 10 ml of THF was treated with 0.13 ml (1.25 mmol) of trimethyl phosphane. The reaction mixture was stirred for one h at 293 K. The reaction product [Cu 4 (Mes) 4 (PMe 3 ) 2 ] (1) was precipitated by the addition of 30 ml of n-hexane. After filtration, the colorless product was washed with diethyl ether (2 Â 5 ml) and dried under vacuum. Single crystals suitable for X-ray analysis were obtained by slow diffusion of n-hexane into a THF solution of the product. Yield: 0.33 g (60%). C 42 H 62 Cu 4 P 2 (883.01 g mol À1 ). Analysis: Cu 29.0% (calc.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.

Funding information
We acknowledge the financial support within the funding programme Open Access Publishing by the German Research Foundation (DFG).  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.