Comment

Complexes of divalent transition metals with mixed N-donor and acetate-related ligands have been studied widely due to their close structural analogy with reduced non-heme iron(II) proteins (Hagen et al., 1993). During the course of investigations into possible complexes formed by cobalt(II), pyridine and acetate derivatives, the title compound, (I) (Fig. 1), was isolated. The red crystals consist of neutral dinuclear cobalt(II) molecules in which each cobalt ion is coordinated (Table 1) by two pyridine N atoms and one monodentate trifluoroacetate ion. Two further trifluoroacetate ions bridge the metal nuclei in a ₂-manner, and the coordination shell is completed by a single ₂-bridging water molecule. The water molecule H atoms make intramolecular hydrogen bonds (Table 2) to the uncoordinated O atoms of the non-bridging trifluoroacetate ions.

The overall molecular architecture of (I) is similar to that of related compounds (Corkery & Hockless, 1997; Turpeinen et al., 1987; Hagen et al., 1993). The Co1-OW and Co2-OW distances of 2.190 (3) and 2.196 (3) Å, respectively, are the same within experimental uncertainty. The Co-N distances for the pyridine molecules (weak acceptors) trans to the water O atom ( neutral) are significantly shorter than those trans to the fluoroacetate O atoms (weak donor) as a result of the well known trans influence [for example, Co1-N4 = 2.139 (4) Å versus Co1-N6 = 2.128 (4) Å]. The Co1-OW-Co2 angle is 116.86 (15)°, which is well within the expected range of values (e.g. Corkery & Hockless, 1997; Turpeinen et al., 1997; Hagen et al., 1993). There is no evidence of intermolecular hydrogen bonding or any other directional forces between the individual molecules. In terms of crystal packing, the molecules are arranged in layers in the ab plane (Fig. 2).

Figure 1 The molecular structure of (I), showing 50% displacement ellipsoids for the non-H atoms. The minor disorder components are indicated by dashed C-F bonds.

Figure 2 Packing diagrams for (I), showing the layered arrangement of molecules (left) and the plan of the layers (right).

Experimental

CoCl₂·4H₂O (0.502 g) was mixed with Na₂CO₃ (0.154 g) and distilled water (approximately 10 ml) was added with stirring. Following this, trifluoroacetic acid (1 ml) was added dropwise. Pyridine (approximately 1 ml) was added and the mixture was reduced to dryness at 343 K on a rotary evaporator. Further pyridine (5 ml) was added and a pink precipitate was formed by addition of hexane (30 ml). The precipitate was dissolved in chloroform and mixed crystals (blue and red) were grown by vapour transport of diethyl ether. The blue crystals were shown to be pyridinium trichloropyridinecobalt(II) (Hahn et al., 1997) by X-ray single-crystal analysis and the red crystals the title compound, (I).

Crystal data

[Co2(C2F3O2)4(C5H5N)4(H2O)] Mr = 903.86 Triclinic, a = 9.4211 (19) Å b = 10.741 (2) Å c = 19.185 (4) Å = 78.17 (3)° = 79.00 (3)° = 76.00 (3)° V = 1823.2 (6) Å3 Z = 2 Dx = 1.647 Mg m-3 Mo K radiation Cell parameters from 13594 reflections = 1-27.5° = 1.02 mm-1 T = 180 (2) K Block, red 0.28 × 0.10 × 0.10 mm

Data collection

Nonius KappaCCD diffractometer Thin-slice and scans Absorption correction: multi-scan(SORTAV; Blessing 1995)Tmin = 0.775, Tmax = 0.890 19699 measured reflections 8305 independent reflections 6603 reflections with I > 2(I) Rint = 0.044 max = 27.5° h = -12 12 k = -13 13 l = -20 24

Refinement

Refinement on F2 R[F2 > 2(F2)] = 0.079 wR(F2) = 0.240 S = 1.05 8305 reflections 483 parameters H atoms treated by a mixture of independent and constrained refinement w = 1/[2(Fo2) + (0.128P)2 + 5.9158P] where P = (Fo2 + 2Fc2)/3 (/)max = 0.007 max = 1.69 e Å-3 min = -1.14 e Å-3

Table 1 Selected geometric parameters (Å, °) Co1-OW 2.190 (3) Co1-O2A 2.070 (4) Co1-O8A 2.092 (4) Co1-O3B 2.098 (4) Co1-N6 2.128 (4) Co1-N4 2.139 (4) Co2-OW 2.196 (3) Co2-O1A 2.095 (4) Co2-O3A 2.074 (4) Co2-O2B 2.095 (4) Co2-N5 2.124 (4) Co2-N7 2.138 (4) Co1-OW-Co2 116.86 (15)

Table 2 Hydrogen-bond geometry (Å, °) D-HA D-H HA DA D-HA OW-H1O8B 0.96 (5) 1.67 (5) 2.616 (5) 165 (5) OW-H2O1B 0.96 (5) 1.68 (5) 2.610 (5) 161 (4)

Difference maps indicated that the F atoms attached to C2B and C8B were disordered over two sets of positions. Refined occupancies (sum constrained to unity) of 0.667 (6):0.333 (6) and 0.601 (6):0.399 (6) resulted for the major and minor components of C2B and C8B, respectively. The disordered F atoms were modelled with isotropic displacement parameters. The F atoms around C1B and C3B may also be slightly disordered but this was not resolved in the present data. The water molecule H atoms were located in a difference map and refined with distance restraints (O-H = 0.96 Å). Pyridine H atoms were placed in idealized locations (C-H = 0.93 Å) and refined as riding with the constraint U_iso(H) = 1.2U_eq(carrier) applied. The higest peak and depest hole in are located 1.51 Å from atom F1C and 0.49 Å from F2A, respectively.

Data collection: COLLECT (Nonius, 1998); cell refinement: SCALEPACK (Otwinowski & Minor, 1997); data reduction: DENZO (Otwinowski & Minor, 1997) and SCALEPACK; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 2003); software used to prepare material for publication: SHELXL97.