The crystal and molecular structure of ammonium-bis(malonato) oxovanadium(IV) dihydrate

The title compound, (NH4)2[VO(C3H2O4)2]·2H2O, crystallizes in the monoclinic space group P21/n, with a=7.1889(7), b=19.254(2), c=9.879(2) Å, β=108.19(1)o, and Z=4. The VO2+ cation is five-fold coordinated with two malonate anions acting as bidentate ligands and a water molecule. Infrared and Raman spectra are also reported to attain a wider insight into the compound characteristics.


Introduction
Increasing interest about the coordination and structural chemistry of oxovanadium(IV) complexes has arisen during the last years, probably due to the increasing evidences of the biological relevance of this cation.~-5As part of our model studies related to vanadium metabolism, we have investigated different simple VO 2 § complexes which may be useful as model systems for vanadium biochemistry.As this cation shows a great preference for oxygen donors, a-6 model complexes with such type of ligands are of special interest.
Some years ago we studied the spectroscopic and thermal behavior of a great number of salts of the complex anion bis(malonato)oxovanadium(IV) 7'8 and more recently we have extended these studies to a number of salts derived from a similar complex containing benzylmalonato as a ligand of the vanadyl(IV) cation.9 Both systems generate complexes with the biologically relevant O = V(O)4_ environment.
Despite the interest in these model systems, no structural information is so far available for any salt

475
of these complex anions.Therefore, we have examined the crystal and molecular structure of the hydrated ammonium salt of the bis(malonato)oxovanadium(IV) complex anion.The structural information is complemented with spectroscopic data, obtained from infrared and Raman measurements.

Sample preparation
The investigated salt, (NH4)2[VO(C3H204)2] 9 2H20, was obtained by reaction between NH4VO3 with malonic acid in the presence of ammonium carbonate.7'~~ The crude, very soluble, product was recrystallized twice from small portions of water.Finally, the material was dissolved in a greater water volume and left to evaporate slowly at room temperature.In this way, a large number of well formed crystals, with dimensions and characteristics suitable for the crystallographic study, could be obtained.

Instrumentation
The IR spectra were recorded between 4000 and 200 cm-~ with a Perkin Elmer 580B spectrophotometer, using the KBr pellet technique.Raman spectra of finely powdered samples were obtained with the FRA 106 Raman-attachment of a Bruker FTIR IFS66 spectrometer.The 1064 nm-line of a Nd:YAG solid state laser was used for excitation.

Crystallographic study
Crystal data, data collection procedures, structure determination methods, and refinement results are summarized in Table 1.The unit cell parameters were obtained by least-squares refinement of [sin 0/X] 2 val-  The structure was solved by Patterson and Fourier methods with neutral atomic scattering factors.The water hydrogen atoms were located in a difference Fourier map and incorporated in the refinement at fixed positions with a common isotropic parameter which, in the final run, converged to U = 0.07(1) ,~2.The hydrogen atoms of the malonato ligands were also located in the difference Fourier map.However, they were positioned stereochemically and during the refinement kept riding on the carbon atoms to which they are attached with a common isotropic thermal parameter which converged to U --0.065(9) ,~2.Most of the ammonium hydrogen atoms could not be determined reliably in the final difference Fourier map.This is probably due to rotational disorder of the NH~ ions, a fact which could be confirmed through the spectroscopic measurements.4.

Structure of the compound
The observed short N(2)..-O( 14) contact distance of 2.714(7) A, suggests a strong N-H .... O bond.In fact, the peak of 0.7 electron/~, 3 found in the difference Fourier map along the N(2)...O( 14) contact can be assigned to one of the corresponding ammonium hydrogens.A discrete rotational disorder of this NH~ion mainly around this bond probably accounts for the other peaks found surrounding N(2) atom.N(1)'"O contact distances longer than 2.84 ,~ are observed for the other ammonium ion.This and the constellation of peaks found around N(I) seem to imply that this group is also rotationally disordered.

hlfrared and Raman spectra
The IR-spectrum shows great similarities with those previously obtained for some other salts of the same complex anion, v It is additionally complicated by the presence of the typical NHJ-vibrations.The Raman spectrum gave some additional information about the vibrational behavior.
The analysis of the NH~--vibrations appears specially interesting to attain complementary information about the behavior of these ions in the crystal lattice.According to Waddington, 16 if the free rotation of the NH~" ion is hindered in a crystal lattice, then a number of spectroscopic peculiarities may be observed.If the N + H4 ion is involved in hydrogen bonding, one may expect a shift of the IR-active antisymmetric stretching mode, v3, to lower frequencies in comparison with the value usually found for the "free" ion.In this case, a combination mode (v2 + v4) as well as the first overtone of the anti-symmetric bending vibration of this ion (2v4) attains sufficient intensity, probably by Fermi resonance with one of the energy-lowered v3-components.On the other hand, an additional weak band, originated in the interaction of v4 with one torsional N + oscillation of the H4 ion (v4 + v6), can also often be observed.
In the present case such peculiarities could be clearly observed, therefore confirming the hindered rotation of at least one of the ammonium ions as suggested by the structural analysis.The combination mode is seen as a weak shoulder at 301 1 cm-~ on the low energy side of a strong IR band centered at 3160 cm-~; the 21) 4 overtone also appears as a shoulder on this strong band at 2865 cm -~, whereas (v4 + v6) is observed as a weak band at 1949 cm -~.The expected splittings of v3 and v4 ~6 are not evident in our IR spectra due to the fact that these modes become overlapped with other vibrational modes.The v3 region is strongly broadened by the presence of the O-H stretchings of the water molecules; the v4 mode, which is located at 1407 cm -I, is overlapped by the v~(COO ) and ~(CH2) modes of the malonate groups.The v6-torsional mode appears as a very weak IR band at ca. 505 cm -I.
In the Raman spectrum the Va~ and v~ modes of the carboxylate groups are found at 1622 and 1414 cm -t, respectively, both as medium-intensity bands.
The characteristic v(V-O) stretching is found at 980cm i in the IR spectrum and at 978 cm-~ in the Raman spectrum as a very strong and well defined band in both cases.On the other hand, an IR band multiplet of medium intensity, with components at 470, 462(sh) and 444 cm-1 (468 and 436 cm -i in the Raman spectrum) can be assigned to the V-O-stretching vibrations (cf.also Ref. 9).We could also identify a weak IR band located at 818 cm t (817 cm i in the Raman spectrum) assignable to a rocking mode of the coordinated water.
Finally, it is worth mentioning that our IR-spectrum resembles that corresponding to a previously reported tetrahydrate of the same salt.17 In this case, the v(V-O) stretching was found at 990 cm-~ and the rocking mode of the coordinated water at 850 cmwhereas no information, neither on the v(V-O) mode nor on the peculiarities of the NH4 ~ vibrations were given.

Figure 1 Fig. 1 .
Figure 1 shows an ORTEW s plot of the compound showing the labelling of the non-hydrogen atoms and their vibrational ellipsoids.Atomic fractional coordi-