Two scandium-biuret complexes: [Sc(C₂H₅N₃O₂)(H₂O)₅]Cl₃·H₂O and [Sc(C₂H₅N₃O₂)₄](NO₃)₃

The scandium(III) cations in the structures of penta­aqua­(biuret-[kappa]²O,O')scandium(III) trichloride monohydrate, [Sc(C₂H₅N₃O₂)(H₂O)₅]Cl₃·H₂O, (I), and tetra­kis(biuret-[kappa]²O,O')scandium(III) trinitrate, [Sc(C₂H₅N₃O₂)₄](NO₃)₃, (II), are found to adopt very different coordinations with the same biuret ligand. The roles of hydrogen bonding and the counter-ion in the establishment of the structures are described. In (I), the Sc³⁺ cation adopts a fairly regular penta­gonal bipyramidal coordination geometry arising from one O,O'-bidentate biuret mol­ecule and five water mol­ecules. A dense network of N-H...Cl, O-H...O and O-H...Cl hydrogen bonds help to establish the packing, resulting in dimeric associations of two cations and two water mol­ecules. In (II), the Sc³⁺ cation (site symmetry 2) adopts a slightly squashed square-anti­prismatic geometry arising from four O,O'-bidentate biuret mol­ecules. A network of N-H...O hydrogen bonds help to establish the packing, which features [010] chains of cations. One of the nitrate ions is disordered about an inversion centre. Both structures form three-dimensional hydrogen-bond networks.

S1. Comment

Compared with other first-row transition elements, the coordination chemistry of Sc³⁺ has not been extensively investigated, perhaps in part owing to the historical high cost of scandium compounds. Even so, it is clearly defined as a relatively weak class-a acceptor and shows a preference for O-atom donor ligands (Greenwood & Earnshaw, 1997), although coordination to N atoms and halide ions has been observed (e.g. Ripert et al., 1999). Scandium(III) is notably flexible in its coordination preference; for example, it can adopt six- [e.g. Sc(H₂O)₆.Sc(CH₃SO₃)₆; Lindquist-Reis et al., 2006], seven- [e.g. Sc(H₂O)₇.3Cl; Lim et al., 2000], eight- [Sc(H₂O)₈.3(CF₃SO₃); Abbasi et al., 2005] and nine-coordination [Sc(H₂O)₉.3(CF₃SO₃); Castellani et al., 1995] in its mononuclear aqua complexes.

Biuret (biur), H₂N—CO—NH—CO—NH₂ (or C₂H₅N₃O₂), has long been recognized as a ligand in coordination chemistry (Wiedemann, 1848). In low-pH or neutral conditions, biuret shows O,O'-bidentate coordination to metal cations [e.g. with Zn (Nardelli et al., 1963), Cu (Freeman & Smith, 1966) or Ni (Lawson & Harrison, 2005)]. When biuret is deprotonated in basic conditions, N,N'-bidentate coordination can arise [e.g. with Cu (Pajunen & Pajunen, 1982)]. In this paper, we describe the syntheses and structures of two new scandium–biuret complexes, Sc(biur)(H₂O)₅.3Cl.H₂O, (I), and Sc(biur)₄.3NO₃, (II). In the former, the Sc³⁺ ion is seven-coordinate, and in the latter, eight coordinate.

Compound (I) is a hydrated molecular salt containing a new [Sc(biur)(H₂O)₅]³⁺ complex ion, three charge-balancing chloride ions and one uncoordinated water molecule. The Sc³⁺ ion in (I) is coordinated to seven O atoms (Fig. 1 and Table 1) in a slightly distorted pentagonal–bipyramidal arrangement. The O,O-bidenate biuret molecule occupies two of the equatorial positions, and a six-membered chelate ring results. The mean equatorial O—Sc—O bond angle is 73.0° and the axial O6—Sc1—O7 bond angle is 177.98 (4)°. The axial Sc—O bond lengths are slightly shorter than their equatorial equivalents. In [Sc(H₂O)₇]³⁺.3X^- (X = Cl and Br; Lim et al., 2000) a very similar scandium geometry arises although the complete complex cation is generated by twofold symmetry in these cases.

It has been noted previously (Carugo et al., 1992) that the biuret molecule can be regarded as two planar amide fragments linked by an NH bridge. Here, the dihedral angle between the N1/C1/O1/N2 and N2/C2/O2/N3 units is 13.21 (9)°. The Sc³⁺ cation deviates from the N1/C1/O1/N2 and N2/C2/O2/N3 mean planes by -0.501 (3) and 0.157 (3) Å, respectively.

On progressing along the chain formed by atoms N1, C1, N2, C2 and N3 (Fig. 1), the C—N bond lengths in (I) show a short–long–long–short (slls) pattern, although all of them are intermediate between typical C—N single (1.47 Å) and C═N double (1.30 Å) bonds, consistent with a significant degree of electronic delocalization over the entire biuret molecule (Carugo et al., 1992). This slls bond-length alternation pattern is the most common one seen in biurets, although others are also possible (Harrison, 2007).

The component species in (I) are linked by a dense network of N—H···Cl, O—H···Cl and O—H···O hydrogen bonds (Table 2). Perhaps the most interesting of these are the O—H···O links, which result in [Sc(biur)(H₂O)₅.H₂O]₂ inversion dimers (Fig. 2) in which the graph-set motif (Bernstein et al., 1995) is R²₄(12). Of the other hydrogen bonds, atoms Cl1, Cl2 and Cl3 accept four, six and five, respectively.

The asymmetric unit of (II) contains one Sc³⁺ ion (site symmetry 2), two biuret molecules and two nitrate ions, one of which is disordered about an inversion centre. Crystal symmetry generates an [Sc(biur)₄]³⁺ complex ion (Fig. 3), in which the Sc^III ion adopts a squashed square–antiprismatic geometry (Table 3 and Fig. 4). The r.m.s. deviation from the mean plane of the four O atoms forming one square face [O1, O2, O3ⁱ and O4ⁱ; (i) 1 - x, y, 1/2 - z] is 0.004 Å and Sc1 is displaced from this plane by 1.190 (1) Å. Owing to crystal symmetry, the same values apply to the other four O atoms (O1ⁱ, O2ⁱ, O3 and O4). The dihedral angle between the two sets of O atoms is 1.20 (9)°.

Both biuret molecules in (II) exhibit the slls C—N bond-length pattern. The dihedral angle between the N1/C1/O1/N2 and N2/C2/O2/N3 fragments is 20.35 (9)°, indicating an unusually high degree of twisting, and that between N4/C3/O3/N5 and N5/C4/O4/N6 is 11.76 (18)°. The deviations of the Sc³⁺ ion from the mean planes of the biuret fragments are -0.139 (4) Å for N1/C1/O1/N2, -1.323 (4) Å for N2/C2/O2/N3, 0.848 (5) Å for N4/C3/O3/N5 and -0.171 (5) Å for N5/C4/O4/N6.

The structure of (II) is completed by two nitrate ions, one of which is disordered about an inversion centre. A network of N—H···O hydrogen bonds (Table 4) link the component species into a three-dimensional array. The most notable are the N3—H3A···O4ⁱⁱⁱ [symmetry code (iii) -x + 1, y + 1, -z + 1/2] and N6—H6A···O2^vi [symmetry code: (vi) -x + 1, y - 1, -z + 1/2] links, which lead to [010] chains of cations containing R²₂(8) loops (Fig. 5).

The complex ion containing Sm³⁺ equivalent to that seen in (II) has been described by Haddad (1987) in Sm(biur)₃.(NO₃)₃, and indeed the overall structures of (II) and Sm(biur)₃.(NO₃)₃ are isostructural but not isomorphous.

A short nitro-O6···C1 contact of 2.904 (3) Å occurs in the crystal structure of (II), which could be another example of a short through-space O···C electrostatic interaction (O'Leary & Wallis, 2007).

Related literature

For related literature, see: Abbasi et al. (2005); Bernstein et al. (1995); Carugo et al. (1992); Castellani et al. (1995); Greenwood & Earnshaw (1997); Haddad (1987); Harrison (2007); Lawson & Harrison (2005); Lim et al. (2000); Nardelli et al. (1963); O'Leary & Wallis (2007); Pajunen & Pajunen (1982); Ripert et al. (1999).

S2. Experimental

To prepare (I), 0.1 M aqueous solutions of ScCl₃ (10 ml) and biuret (10 ml) were mixed and a small quantity of dilute hydrochloric acid was added, resulting in a colourles soluton. Colourless blocks of (I) grew over several days as the water slowly evaporated. Compound (II) was prepared in the same way, with 0.1 M Sc(NO₃)₃ replacing the scandium chloride solution. Large colourless slabs of (II) grew as the water evaporated.

S3. Refinement

N-bound H atoms were positioned geometrically (N—H = 0.88 Å) and refined as riding [U_iso(H) = 1.2U_eq(N)]. Water H atoms were located in difference maps and refined as riding in their as-found relative positions [U_iso(H) = 1.2U_eq(O)].

Computing details

For both compounds, data collection: COLLECT (Nonius, 1998); cell refinement: HKL SCALEPACK (Otwinowski & Minor, 1997); data reduction: HKL DENZO and SCALEPACK (Otwinowski & Minor, 1997), and SORTAV (Blessing, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. A view of the molecular structure of (I), showing 50% probablity displacement ellipsoids (arbitrary spheres for the H atoms). Hydrogen bonds are indicated by double-dashed lines.
	Fig. 2. A view of the centrosymmetric [Sc(biur)(H2O)5.H2O]2 dimer in (I), with the hydrogen bonds indicated by double-dashed lines. [Symmetry code: (vi) -x + 1, -y + 1, -z + 2.]
	Fig. 3. A view of the [Sc(biur)4]^3+ complex ion in (II), showing 50% probability displacement ellipsoids (arbitrary spheres for the H atoms). [Symmetry code: (i) -x + 1, y, -z + 1/2.]
	Fig. 4. A detail of (II), showing the squashed square–antiprismatic geomtery of the scandium(III) ion. [Symmetry code: (i) -x + 1, y, -z + 1/2.]
	Fig. 5. Part of a hydrogen bonded [010] chain of [Sc(biur)]^3+ cations in (II), with the N—H···O hydrogen bonds indicated by double-dashed lines. [Symmetry codes: (iii) -x + 1, y + 1, -z = 1/2, (vi) -x + 1, y - 1, -z + 1/2.] Table 1 (i)-->(vii).

(vii).