Bis[l-4-(ethylammoniomethyl)-3,5- dimethylpyrazolato-jN:N]bis[(g- 1,5-cyclooctadiene)rhodium(I)] dichloride dichloromethane methanol solvate

In the title compound, [Rh2(C8H15N3)2(C8H12)2]Cl2 CH2Cl2 CH3OH, the dinuclear Rh complex has C2 symmetry and the two pyrazolato ligands act as -bridges. The coordination of each Rh cation is completed by one cyclooctadiene (COD) ligand. It is shown that the average RhÐC(COD) distance is linearly dependent on the RhÐ N(pyrazole) distance in this type of compound, and this is ascribed to the steric hindrance produced by the packing.

In the title compound, [Rh 2 (C 8 H 15 N 3 ) 2 (C 8 H 12 ) 2 ]Cl 2 ÁCH 2 -Cl 2 ÁCH 3 OH, the dinuclear Rh I complex has C 2 symmetry and the two pyrazolato ligands act as "-bridges. The coordination of each Rh I cation is completed by one cyclooctadiene (COD) ligand. It is shown that the average RhÐC(COD) distance is linearly dependent on the RhÐ N(pyrazole) distance in this type of compound, and this is ascribed to the steric hindrance produced by the packing.

Comment
Research into the coordination chemistry of pyrazole-derived ligands has progressed rapidly over the last two decades. Mukherjee (2000) published an extensive review, completing those presented by La Monica & Ardizzoia (1997) and Tro®menko (1972Tro®menko ( , 1986Tro®menko ( , 1993. Only four structures of dinuclear rhodium(I) complexes with pyrazole bridges and cyclooctadiene ligands (cod) (Louie et al., 1984;Cano et al., 1997;Esquius et al., 2000) are present in the Cambridge Structural Database (CSD, release of November 2001; Allen & Kennard, 1993). A feature of these compounds is the variation of the RhÐC and RhÐN bond distances without a clear reason. In order to increase understanding of this distance variation, the title compound, (I), was prepared, which is similar to those previously published by Esquius et al. (2000).
The molecular structure of (I) is shown in Fig. 1 and selected geometric details are given in Table 1. The structure of (I) consists of discrete molecules separated by van der Waals interactions and weak hydrogen bonds ( Table 2).
The methanol molecules were located as disordered, and atom O1 seems to form a hydrogen bond with a Cl À anion [O1Á Á ÁCl1 i 3.118 (4) A Ê ; symmetry code (i) 1 2 À x, 1 2 À y, 1 À z]. Each Rh atom is linked to four C atoms of a cyclooctadiene ligand and two N atoms of two different pyrazole units. The pyrazole acts as a "-N,N H -bridge between two Rh atoms. The A view of the molecular structure of (I) showing 50% probability displacement ellipsoids and the atom-numbering scheme. The H atoms, the Cl À anions and the dichloromethane and methanol solvent molecules have been omitted for clarity.

Figure 2
A graph of average RhÐC versus average RhÐN bond lengths in "-pyrazole-[Rh(COD)] 2 units. RhÐN1ÐN2ÐRh i torsion angle is 2. 43 (19) . The planarity of this moiety is similar to that observed when the pyrazole lacks a bulky substituent in position 4 (Louie et al., 1984;Esquius et al., 2000). The dihedral angle between the Rh/N1/ N2/Rh i and pyrazole planes is 20.17 (10) . The ethylammoniomethyl moiety is planar and twisted by 87.0 (2) with respect to the pyrazole plane.
If the average RhÐC(COD) and RhÐN(pyrazole) lengths are compared, it is observed that <RhÐC> increases when <RhÐN> increases (Fig. 2), while the NÐN and CÐC lengths remain practically constant [average values in the ®ve structures are 1.360 (7) and 1.375 (12) A Ê , respectively]. This suggests that the bond lengths involving the Rh atom are more affected by the steric hindrance of the packing than by electronic effects. This is corroborated by the two electronically more similar pyrazole ligands, 3,5-dimethyl-4-[N-(isopropyl)aminomethyl]pyrazolyl and 3,5-dimethyl-4-(ethylammonium)methylpyrazolate, presenting the upper and lower limiting values.

Experimental
To prepare (I), [RhCl(COD)] 2 (0.08 g, 0.16 mmol) dissolved in CH 2 Cl 2 (5 ml) was added to a solution of 3,5-dimethyl-4-(ethylamino)methylpyrazole (0.08 g, 0.32 mmol) in CH 2 Cl 2 (5 ml) and the mixture stirred for 15 h. The solvent was evaporated to dryness in vacuo and the residue was washed with Et 2 O and dissolved in a minimum amount of CH 2 Cl 2 . The title complex was precipitated by adding hexane to the solution. A yellow±orange solid was ®ltered off and dried in vacuo. Crystals of (I) were obtained by evaporation of a methanol solution.

Data collection
Enraf±Nonius CAD-4 diffractometer 3/2 scans 6120 measured re¯ections 5860 independent re¯ections 4234 re¯ections with I > 2'(I) R int = 0.061 max = 30 h = À17 3 16 k = 0 3 36 l = 0 3 18 3 standard re¯ections frequency: 120 min intensity decay: none Methanol atoms O1 and C19 were located from a difference Fourier synthesis. Their occupancy factor of 0.5 was assigned according to the peak heights. The molar ratio with respect to the remaining formula was con®rmed by elemental analysis. The H atoms on N15 were re®ned freely. The positions of 27 H atoms were geometrically computed (CÐH = 0.93±0.97 A Ê ) and re®ned using a riding model, with U iso (H) = 1.2U eq (C). Dichloromethane H atoms were located from a difference Fourier synthesis, while methanol H atoms were not located.