Crystallization and Structure Determination of Fac-triammin-aquo-oxalato-cobalt (iii)-nitrate Monohydrate

The title compound, fac-triammin-aquo-oxalato-cobalt(III)-nitrate monohydrate, fac-[Co(NH3)3(C2O4)(H2O)]NO3· H2O (2), was prepared according to an original synthetic protocol published exactly 100 years ago by Alfred Werner by dissolving the indigo-blue non-electrolyte complex mer-triammin-chloro-oxalato-cobalt(III), mer-[Co(NH3)3(C2O4)Cl] (1), in boiling half-concentrated nitric acid. Contrary to the literature, it did not crystallize directly from the reaction mixture, but crystallization could be induced by saturating the solution with NaClO4. The structure of 2 has monoclinic (P21/n) symmetry. The crystal structure displays an extensive array of N–H· · · O and O–H· · · O hydrogen bonding.


Results and Discussion
The title compound fac-triammin-chloro-oxalato-cobalt (III)-nitrate monohydrate, fac-[Co(NH3)3(C2O4)(H2O)]NO3• H2O (2), was prepared according to the original procedure reported by Werner [16].Nitric acid was added to a blue suspension of 1 in water and the mixture was heated to boiling for a few minutes until a clear, purple solution had formed (Scheme 1).

Scheme 1. Preparation of the title compound fac-[Co(NH3)3(C2O4)(H2O)]NO3• H2O (2).
According to Werner, beautiful, well-formed, red-purple crystals of 2 ("prä chtig karmoisinrote Nadeln") should form upon slight cooling of the reaction mixture.In our case, however, crystallization did not occur even on prolonged standing of the solution at 0 °C.It is well established from studies by Siebert et al. [19,20] that perchlorates are often particularly suitable for the isolation of various series of Werner-type cobalt(III) ammine complexes.Quite often, the perchlorates are less soluble than other salts (e.g., chlorides, bromides, nitrates) and exhibit a highly crystallinity [19,20].Thus, we attempted to prepare the new perchlorate derivative [Co(NH3)3(C2O4)(H2O)]ClO4 by saturating the solution of 2 with solid NaClO4.Undisturbed standing of the reaction mixture at 0 °C for 2 days afforded well-formed, purple, needle-like crystals.However, an X-ray diffraction study of the product surprisingly revealed the presence of the original nitrate fac-[Co(NH3)3(C2O4)(H2O)]NO3• H2O (2).Obviously, the large excess of NaClO4 had induced the crystallization of 2, whereas the perchlorate salt could not be isolated.

Experimental Section
Cautionary note: Perchlorates are potentially explosive and should always be handled with great care!The title compound, fac-triammin-chloro-oxalato-cobalt (III)-nitrate monohydrate, fac-[Co(NH3)3(C2O4)(H2O)]NO3• H2O (2), was prepared according to the original procedure given by Werner [16].A 5 g-sample of indigo-blue mer-[Co(NH3)3(C2O4)Cl] (1) [12] was suspended in a mixture of water (30 mL) and concentrated nitric acid (20 mL).The mixture was heated to boiling until after a few minutes a clear purple solution had formed.In contrast to the original preparation, crystallization did not occur simply upon cooling of the reaction mixture to room temperature.However, crystallization could be induced by saturating the solution with solid sodium perchlorate, NaClO4.Under these conditions, purple crystals of fac-[Co(NH3)3(C2O4)(H2O)]NO3• H2O (2) were obtained after undisturbed standing at 0 °C for 2 days.
The X-ray diffraction data for compound 2 were measured at 210(2) K on a Stoe IPDS diffractometer.Structure solution and refinement were carried out using the programs SHELXS-97 [21] and SHELXL-97 [22].The hydrogen atoms were freely refined using restraints.CCDC reference number: 1027030.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.

Conclusions
Exactly 100 years after being first reported by Alfred Werner, the crystal structure of the title compound, fac-triammin-chloro-oxalato-cobalt(III)-nitrate monohydrate, fac-[Co(NH3)3(C2O4)(H2O)]NO3• H2O (2) has been determined by X-ray diffraction.The crystal structure analysis confirmed both the composition and the facial coordination geometry of this complex which Werner had correctly proposed without the help of modern analytical techniques.

Figure 2
Figure 2 shows the crystal packing of compound 2 in the solid state.The crystal structure displays an extensive network of N-H• • • O and O-H• • • O hydrogen bonding interactions (Table 3).The coordinated water molecule is connected through O-H• • • O hydrogen bonds with the water of crystallization and the uncoordinated nitrate anion.The latter also forms an N-H• • • O hydrogen bond to the NH3 ligand in trans-position to the coordinated water in a neighboring cation.The other two, cis-oriented, NH3 ligands in the cation form a pair of N-H• • • O hydrogen bonds with the uncoordinated oxalate oxygen atoms of