The Synthesis and Structure of a Scandium Nitrate Hydroxy-Bridged Dimeric Complex Supported by Bipyridyl Ligands

The current discussion on whether scandium, yttrium and lanthanum should represent Group 3 in the Periodic Table or whether lutetium should replace lanthanum in the group has prompted us to further explore the structural chemistry of the Group 3 elements and compare the coordination numbers and coordination geometries adopted. The steric and electronic properties of the coordinated ligands have a major influence on the structures adopted. We report the synthesis and crystal structure determination of an unusual dinuclear scandium complex [(bipy)(NO3)2Sc(µ-OH)2Sc(NO3)2(bipy)] obtained by the reaction of hydrated scandium nitrate with 2,2′-bipyridyl (bipy) in either ethanol or nitromethane. The crystal structure of the complex shows that the scandium centers are eight coordinate, and the structure obtained contrasts with related complexes found in the lanthanide series [Ln(bipy)2(NO3)3] and [Ln(phen)2(NO3)3] (phen = phenanthroline) and in [M(terpy)(NO3)3] (M = Sc, Er–Lu), where these complexes are all mononuclear.


Introduction
The form and arrangement of the Periodic Table is of considerable current interest [1]. Some of this interest comes from the discussion of whether lanthanum should be classed as a Group 3 element along with scandium and yttrium [2] or whether lutetium is better suited to be a member of this group than lanthanum [3,4]. From the viewpoint of structural chemistry, the coordination number and geometry adopted by the +3 metal ion in a complex is dependent on the size of the ion, with the steric requirements of the ligands playing a secondary role; ligand field effects do not contribute to complexes of the Ln(III) ions or for Sc(III) and Y(III). For example, the smaller Sc(III) ion forms a seven coordinate aqua ion in contrast to the nine coordinate [La(H 2 O) 9 ] 3+ ion that is typical of the larger early lanthanides and the eight coordinate [M(H 2 O) 8 ] 3+ ions formed by Y(III) and the heavier actinides, which have intermediate ionic radii [5]. A recent systematic structural study of analogous complexes of Sc(III), Y(III), La(III) and Lu(III) showed that there were 29 sets of compounds where at least three of the elements form compounds with the same ligands have been identified and their crystal structures determined [6]. In 14 of the sets, the scandium and lutetium complexes have the same coordination number; but in the remaining 15 they do not. Since the ionic radii of Sc(III) and Lu(III) are quite similar (0.75 Å (6 coordinate) and 0.98 Å (8 coordinate), respectively [7]), the observation that approximately half of the cases where Sc(III) and Lu(III) form equivalent compounds indicates that the nature of the ligands in these complexes is important in determining the resultant coordination number and geometry. This has prompted us to investigate the coordination chemistry of Sc(III) complexes further looking for differences in coordination geometry and number with similar ligand sets.

Results and Discussion
Using synthesis conditions similar to those used by ourselves and others for [Ln(bipy) 2 (NO 3 ) 3 ] (Ln = Y, La-Lu) [10][11][12], we obtained a somewhat powdery product, but by reaction of hot dilute ethanolic solutions of hydrated scandium nitrate and bipy, in a 1:2 ratio, followed by very slow cooling (immersed in a Dewar of very hot water) yielded small colorless crystals suitable for study by synchrotron single crystal X-ray diffraction methods.
The infrared spectrum of the product contains absorptions due to both the nitrate group and the 2,2 -bipyridyl ligand, and it is not always possible to distinguish them. Thus, absorptions at 1020 and 1033 cm −1 are probably due, respectively, to a ring breathing vibration and to ν1(A 1 ) of the nitrate group. Strong bands at 1315 and 1331 cm −1 are assigned to a ligand vibration and to a vibration of the coordinated nitrate group, as are three absorptions at 1441, 1476 and 1537 cm −1 . A broad, weak vibration centered upoñ 3090 cm −1 is probably due to ν(O-H) stretching vibrations (See Supplementary Materials Figure S1a). A MALDI mass spectrum of the compound (See Figure S1b) displayed a molecular ion with a m/z peak at 682.94 [M -H] + , suggesting that a dimeric complex had formed. The X-ray single-crystal structure determination of the product identified it to be an unexpected dimeric complex, [(bipy)(NO 3 ) 2 Sc(µ-OH) 2 Sc(NO 3 ) 2 (bipy)]. We investigated the use of nitromethane as an alternative solvent for the reaction, following the example of Junk et al. [15], who isolated [In(bipy) 2 (NO 3 ) 3 ] from this medium, but the product of this reaction was identical to that obtained from ethanol.
Junk et al. [15] obtained a similar compound, [(bipy)(NO3)2In(μ-OH)2In(NO3)2(bipy)], on one occasion from the reaction of hydrated indium nitrate with 2,2′-bipyridyl in nitromethane. The poorly formed crystals gave a structure of rather low precision, but average In-N distances are rather shorter, at 2.25 Å, and In-OH and In-O (NO3) distances longer at 2.14 and 2.52 Å, respectively, than in the scandium compound. On the basis of ionic radii [7], they would be expected to be some 0.04 Å longer. The isolation of this compound indicates that hydrolysis of [M(H2O)6] 3+ ions occurs for larger metals than scandium.
Junk et al. [15] obtained a similar compound, [(bipy)(NO 3 ) 2 In(µ-OH) 2 In(NO 3 ) 2 (bipy)], on one occasion from the reaction of hydrated indium nitrate with 2,2 -bipyridyl in nitromethane. The poorly formed crystals gave a structure of rather low precision, but average In-N distances are rather shorter, at 2.25 Å, and In-OH and In-O (NO 3 ) distances longer at 2.14 and 2.52 Å, respectively, than in the scandium compound. On the basis of ionic radii [7], they would be expected to be some 0.04 Å longer. The isolation of this compound indicates that hydrolysis of [M(H 2 O) 6 ] 3+ ions occurs for larger metals than scandium.
The corresponding reaction between scandium nitrate and phenanthroline in methanol results in a rather similar complex, eight coordinate [(phen)(NO 3 ) 2 Sc(µ-OMe) 2 Sc(NO 3 ) 2 (phen)], this time with two methoxy bridges [23] [16]; at pH 3, it is known to hydrolyze [18] [24]. Evidently the smaller radius of the Sc 3+ ion and greater polarizing power makes it a stronger Lewis acid than the lanthanide ions, promoting hydrolysis of the aqua ion and concomitant dimerization; this is reflected in the pKa value for Sc 3+ (aq) of 4.3, compared with the respective values of 8.5 and 7.6 for La 3+ (aq) and Lu 3+ (aq) [25], a cause of the significant differences between the chemistry of scandium and the lanthanides [6].

Materials and Methods
Hydrated scandium nitrate, 2,2 -bipyridine, and solvents were obtained as commercial products (Aldrich) and were used without purification.
A hot solution of hydrated scandium nitrate (0.10 g, 0.31 mmol) in ethanol (5 mL) was mixed with 2,2 -bipyridine (0.10 g, 0.64 mmol) in hot ethanol (25 mL) and allowed to stand with very slow cooling overnight, Colorless crystals were obtained on standing for 2 days. The reaction was repeated using nitromethane instead of ethanol as the solvent and the same reaction product was obtained.
The IR spectrum of the solid was recorded on a Nicolet Avatar 360 FTIR spectrometer. IR: ν /cm The crystal data, data collection parameters, and structure solution and refinement details for the crystal structure of [(bipy)(NO 3 ) 2 Sc(µ-OH) 2 Sc(NO 3 ) 2 (bipy)] are summarized in Table S1 (Supplementary Materials). The crystal data was collected on a Bruker AXS SMART diffractometer (Madison, WI, USA), equipped with an Oxford Cryostream cooling apparatus, at Station 9.8 of the CCLRC Daresbury Laboratory, UK, using monochromatic X-ray radiation of wavelength 0.6911 Å. Structure solution was achieved by direct methods and refined by full-matrix least-squares on F 2 using SHELXL-2014 [26] within the OLEX-2 suite [27], with all ordered non-hydrogen atoms assigned anisotropic displacement parameters. Hydrogen atoms attached to carbon atoms were placed in idealized positions and allowed to ride on the relevant carbon atom. The unique hydroxy hydrogen atom was located in the electron density difference map and refined freely. In the final cycles of refinement, a weighting scheme that gave a relatively flat analysis of variance was introduced and refinement continued until convergence was reached.

Conclusions
Reaction of hot dilute ethanolic solutions of hydrated scandium nitrate and bipy led to the formation of a dimeric, di-hydroxy bridge scandium complex, [(bipy)(NO 3 ) 2 Sc(µ-OH) 2 Sc(NO 3 ) 2 (bipy)]. An X-ray crystal structure determination of the product shows that the scandium centers are eight coordinate, and this contrasts to related complexes of  Table S1: Crystal data and structure refinement for bath333n; Table S2: Fractional Atomic Coordinates (×10 4 ) and Equivalent Isotropic Displacement Parameters (Å 2 × 10 3 ) for bath333n. U eq is defined as 1/3 of the trace of the orthogonalized U IJ tensor; Table S3: Anisotropic Displacement Parameters (Å 2 × 10 3 ) for bath333n. The Anisotropic displacement factor exponent takes the form: −2π 2 [h 2 a × 2U 11 + 2hka × b × U 12 + . . . ]; Table S4: Bond Lengths for bath333n; Table S5: Bond Angles for bath333n; Table S6: Hydrogen Bonds for bath333n; Table S7: Torsion Angles for bath333n; Table S8: Hydrogen Atom Coordinates (Å × 10 4 ) and Isotropic Displacement Parameters (Å 2 × 10 3 ) for bath333n; Figure S1: MALDI mass spectrum and solid state IR of the complex; Figure S2: Surface plot of D-norm surface-0_3117_1_0733; Figure  Author Contributions: S.A.C. and P.R.R. wrote the manuscript. S.A.C. carried out the synthetic work and S.S. determined the crystal structure with support from S.J.T. and J.E.W., S.A.C., P.R.R. and S.S. evaluated the data, and S.A.C. supervised the project. All the authors discussed and analyzed the data. All authors have read and agreed to the published version of the manuscript.

Funding:
We are grateful to the Engineering and Physical Sciences Research Council (UK) for a studentship to SS (EP/D072859/1) and for a Senior Fellowship to PRR (EP/D072859/1). We thank Louise Slope for obtaining the infrared spectrum.