Crystal structures of two mononuclear complexes of terbium(III) nitrate with the tripodal alcohol 1,1,1-tris(hydroxymethyl)propane

Two new mononuclear complexes of ten- and nine-coordinate terbium(III) were synthesized from Tb(NO3)3·5H2O and the tripodal alcohol 1,1,1-tris(hydroxymethyl)propane (H3 L Et) to be employed as potential building blocks for heterometallic 3d–4f block metal aggregates.

Two new mononuclear cationic complexes in which the Tb III ion is bis-chelated by the tripodal alcohol 1,1,1-tris(hydroxymethyl)propane (H 3 L Et , C 6 H 14 O 3 ) were prepared from Tb(NO 3 ) 3 Á5H 2 O and had their crystal and molecular structures solved by single-crystal X-ray diffraction analysis after data collection at 100 K. Both products were isolated in reasonable yields from the same reaction mixture by using different crystallization conditions. The highersymmetry complex dinitratobis[1,1,1-tris(hydroxymethyl)propane]terbium(III) nitrate dimethoxyethane hemisolvate, [Tb(NO 3 ) 2 (H 3 L Et ) 2 ]NO 3 Á0.5C 4 H 10 O 2 , 1, in which the lanthanide ion is 10-coordinate and adopts an s-bicapped squareantiprismatic coordination geometry, contains two bidentate nitrate ions bound to the metal atom; another nitrate ion functions as a counter-ion and a halfmolecule of dimethoxyethane (completed by a crystallographic twofold rotation axis) is also present. In product aquanitratobis[1,1,1-tris(hydroxymethyl)propane]terbium(III) dinitrate, [Tb(NO 3 )(H 3 L Et ) 2 (H 2 O)](NO 3 ) 2 , 2, one bidentate nitrate ion and one water molecule are bound to the nine-coordinate terbium(III) centre, while two free nitrate ions contribute to charge balance outside the tricapped trigonal-prismatic coordination polyhedron. No free water molecule was found in either of the crystal structures and, only in the case of 1, dimethoxyethane acts as a crystallizing solvent. In both molecular structures, the two tripodal ligands are bent to one side of the coordination sphere, leaving room for the anionic and water ligands. In complex 2, the methyl group of one of the H 3 L Et ligands is disordered over two alternative orientations. Strong hydrogen bonds, both intra-and intermolecular, are found in the crystal structures due to the number of different donor and acceptor groups present.

Chemical context
Our interest in developing synthetic routes for the synthesis of mono-or polynuclear complexes containing lanthanide (III) ions is based on the possibility that these compounds behave as single-ion (SIM) or single-molecule (SMM) magnets (Benelli & Gatteschi, 2015;Gatteschi et al., 2006;Frost et al., 2016;Meng et al., 2016). In such chemical species, it is usually possible to exploit the strong spin-orbit coupling, the relatively high-spin angular momentum and the large magnetic anisotropy presented by lanthanides to maximize the energy barrier for the reversal of the magnetization (Luzon & Sessoli, 2012;Vieru et al., 2016;Sessoli & Powell, 2009) and therefore increase the technological applicability of these materials.
With this objective in mind, our first steps were the synthesis and characterization of complexes containing Ln III ions that ISSN 2056-9890 could be used as building blocks for polynuclear 3d-4f block metal aggregates. The first report of a heterometallic complex of this type that showed SMM behaviour described the tetranuclear molecule [{Cu II LTb III (Hfac) 2 } 2 ] [H 3 L = 1-(2hydroxybenzamido)-2-(2-hydroxy-3-methoxy-benzylidene amino)ethane and Hfac = hexafluoroacetylacetone], obtained by self-assembly (Osa et al., 2004). Magnetic studies of the product revealed ferromagnetic exchange and slow relaxation of the magnetization at low temperatures, with a potential energy barrier Á/kB of 21 K (14.7 cm À1 ).
After this report, many other heterometallic complexes containing 3d and 4f ions with different structures and nuclearities were characterized as single-molecule magnets . In 2014, a trinuclear complex of dysprosium(III) and iron(II) presented the largest potential energy barrier reported to date for this type of system. The molecule, formulated as [Fe II 2 Dy III L 2 (H 2 O)]ClO 4 Á2H 2 O, L = 2,2 0 ,2 00 -{[nitrilotris(ethane-2,1-diyl)]tris(azanediyl)methylene}tris(4chlorophenol), and also synthesized in a self-assembly reaction, presents two iron(II) ions in different coordination environments (octahedral and distorted trigonal prismatic) bound to a dysprosium(III) ion in quasi-D 5h symmetry, which is pointed out by the authors as fundamental for the observed SMM behaviour and for the impressive potential energy barrier of 459 K (319 cm À1 ) (Liu et al., 2014). This value, although lower than the record figures reported for lanthanide-containing SIM compounds (Liu et al., 2016), still reveals the potential of mixed 3d-4f metal complexes to behave as quantum magnets.
Despite these good results, most of the experimental procedures employed for the preparation of these polynuclear compounds involve self-assembly reactions, which often compete with the rational design of the desired molecules. Many efforts have been directed recently to the development of synthetic routes that allow for greater predictability of the formed products, both structural and with respect to their magnetic properties, employing simple and elegant experimental procedures that include modular synthesis approaches (Kahn, 1997;Stumpf et al., 1993).

Structural commentary
The crystals of product 1 contain the mononuclear complex [Tb(H 3 L Et ) 2 (NO 3 ) 2 ](NO 3 )Á0.5glyme (Fig. 1), in which the terbium(III) ion is 10-coordinate, being connected to six hydroxyl groups of the tripodal alcohol molecules and to two bidentate nitrate ions. There is also one nitrate ion (acting as a counter-ion); a solvating dimethoxyethane (glyme) molecule is shared between two units of the cationic complex. The complete gylme molecule is completed by a crystallographic twofold rotation axis.
The geometric arrangement of the oxygen donor atoms about the metal atom in 1 is closer to a distorted s-bicapped square antiprism, Fig. 2, than to an s-bicapped dodecahedron (Rohrbaugh & Jacobson, 1974). The choice of the bicapped square-antiprismatic coordination sphere is mainly based on the angles between the coordinating oxygen atoms presented in Table 1, which are closer to the expected 90 values of the square planes in the former (Fig. 2) than to the alternating ca 77 and 100 angles in the latter (Rohrbaugh & Jacobson, 1974).
The mean square planes represented in Fig. 2 form a dihedral angle of 5.58 in the complex cation of 1. The capping atoms, O1 and O5, both belong to the bidentate NO 3 À ligands and form the two longest Tb-O bonds in the structure of 1, 2.5697 (13) and 2.5874 (14) Å , respectively. Because of the typically small bite angles of the chelating nitrate ions, 49.50 (4) for O2-Tb1-O1 and 50.12 (4) for O4-Tb1-O5, the Tb-O1 and Tb-O5 bonds are significantly bent towards O2 and O4, respectively, creating additional structural distortion.
The average Tb-O bond involving the bidentate nitrate ligands in 1 [2.549 Å , and Table 2] is shorter than that described by Delangle and co-workers for the lanthanum(III) cation [La(H 3 L 1 ) 2 (NO 3 ) 2 ] + , H 3 L 1 = cis,cis-1,3,5-trihydroxycyclohexane; average = 2.681 Å ; Delangle et al., 2001]. This agrees with the smaller effective ionic radius of the Tb III ion as compared to that of La III (for example 1.095 versus 1.216 Å for nine-coordination respectively; Shannon, 1976). The effective ionic radius for 10-coordinate terbium(III) is not available in the literature. The mean Tb-O bond to the tripodal H 3 L Et ligands is 2.404 Å , again significantly shorter than in the lanthanum(III)-cyclic triol analogue mentioned above (average = 2.542 Å ). The lack of other reported lanthanide complexes with a bis(tripodal alcohol)-bis(bidentate nitrate) coordination environment similar to that found in 1 restricts further comparisons.
The slow mixing of a hexane layer into the same reaction mixture that gave product 1 afforded another set of colourless crystals, product 2, in high yield (see Synthesis and crystallization). As for 1, crystals of 2 were practically insoluble at room temperature in hexane, toluene, thf, glyme and acetonitrile, but soluble in the last three solvents after heating at ca 323 K.
Single-crystal X-ray diffraction analysis of 2 revealed again a mononuclear complex, this time of formula [Tb(H 3 L Et ) 2 -(NO 3 )(H 2 O)](NO 3 ) 2 (Fig. 3), in which the coordination number of the metal atom is nine. In this case, the terbium(III) atom is coordinated by six hydroxyl groups of the tripodal alcohols, a bidentate nitrate ion and one water molecule probably coming from the Tb(NO 3 ) 3 Á5H 2 O starting material. Two distinct non-coordinating nitrate anions complete the charge balance in the product.
The geometry adopted by the metal atom in 2 is close to a tri-capped trigonal prism, as reported for complexes [Ln(H 3 L 1 ) 2 (NO 3 )(H 2 O)](NO 3 ) 2 (Ln = Ho III , Eu III and Yb III ; H 3 L 1 = cis,cis-1,3,5-trihydroxycyclohexane; Husson et al., Plot of the coordination sphere (left) and schematic representation of the coordination environment about the terbium(III) atom in product 1. The two mutually rotated square faces O2-O12-O14-O23 and O13-O22-O24-O4 are capped by atoms O1 and O5, respectively. Table 1 Selected non-bonding angles ( ) in the molecular structure of product 1.  , 1997), appears less suitable to characterize 2 because of a much less regular placement of the coordinating oxygen atoms in the two square planes, O10-O12-O13-O23 and O1-O2-O22-O24, that are typical of this polyhedral arrangement. The coordination of the Tb III atom by the two tripodal ligands in both 1 and 2 is very similar. In the [M 3 M 0 (L Et ) 2 (dpm) 3 ] complexes (M and M 0 = d-block metals), as above (Accorsi et al., 2006;Totaro et al., 2013;Westrup et al., 2014;Gregoli et al., 2009), the central metal is six-coordinate and the two tripodal ligands are inverted about that atom in an approximately octahedral arrangement; here, the C B Á Á ÁM Á Á ÁC B 0 angle is close to 180 (where C B and C B 0 are the bridgehead carbon atoms in the tripodal ligand). In our complexes 1 and 2, with 10-and 9-coordinate atoms, the tripodal ligands are tilted apart, with C11-Tb1-C21 angles of 129.7 and 135.5 , respectively; this arrangement allows more space for the extra ligands in the coordination sphere. In both 1 and 2, all the extra ligands, nitrate ions and water molecules, lie on the plane that bisects the tripodal ligands; the number of extra coordinating atoms determines the distribution in the bisecting plane and overall geometrical patterns, as described above.
According to Table 2, the metal-oxygen distances involving the H 3 L Et ligands in 1 and 2 vary from 2.3583 (13) to 2.4749 (14) (complex 1) and from 2.3545 (9) to 2.4344 (9) Å (complex 2), these ranges being slightly larger than those reported for the Ln 3+ complexes of the trihydroxycyclohexane ligands (Delangle et al., 2001). This probably arises from the different flexibilities of H 3 L Et and the cyclic alcohols used in the syntheses, which allow for distortions of the lanthanide coordination environments. Also, the more crowded environment of the 10-coordinated metal ion in 1 as compared to 2 probably causes the larger observed variation.
The Tb-O bond lengths involving the nitrate ions in 2 are intermediate when compared to the analogous complexes of Eu III , Ho III and Yb III (Delangle et al., 2001;Husson et al., 1999) (Table 3). This is in agreement with the gradual decrease of the effective ionic radii of these ions (1.120, 1.095, 1.072 and 1.042 Å for Eu III , Tb III , Ho III and Yb III , respectively, in 9coordinate environments; Shannon, 1976). The same pattern is observed for the average metal-oxygen bond of the water molecule (Table 3).
It has been demonstrated (Delangle et al., 2001) that the formation of Ln(H 3 L) 2 complexes (Ln = La III , Pr III , Nd III , Eu III and Yb III ; L = cis,cis-1,3,5-or cis,cis-1,2,3-trihydroxycyclohexane) in solution is strongly dependent on the metal:ligand ratio and on the chemical nature of the metal ion, its ionic radius, the polarity of the solvent and the nature of the counter-ion, either nitrate or triflate.
In the present work, the reaction between hydrated terbium(III) nitrate and H 3 L Et led to the isolation of two distinct products, 1 and 2, from the same reaction mixture, with modification only of the crystallization conditions. Product 2, [Tb(H 3 L Et ) 2 (NO 3 )(H 2 O)](NO 3 ) 2 , was obtained in higher yield and after a shorter time interval (24 h) than the more symmetrical 1,  Table 2 Metal-oxygen distances (Å ) in the two complexes, 1 and 2.
preparation of 2 is also easier to reproduce than that of 1; the former appears to be favoured by addition of a less polar solvent (hexane) to the reaction mixture. The isolation of 1, on the other hand, seems to be subjected to a very subtle control of the crystallization conditions, and this is probably the reason why there are fewer reports of similar, anhydrous Ln(H 3 L) 2 products in the literature. The presence of solvating glyme in the crystals of 1 suggests that the use of other solvents with different stereo requirements could be a strategy to help the crystallization of this water-free complex.

Supramolecular features
The three hydroxyl groups in both complexes are all donor groups to hydrogen bonds. The acceptor atoms are oxygen atoms of nitrate ions and, in complex 1, an oxygen atom of the glyme molecule (Fig. 4). In complex 2, the water ligand forms two hydrogen bonds to two non-coordinating nitrate ions (Fig. 5). Thus, in both compounds, all the ions and the glyme molecule are linked in an extensive three-dimensional hydrogen-bonded network.
In both complexes, there are also some intermolecular C-HÁ Á ÁO interactions, which may be described as 'weak hydrogen bonds'. These are included in Tables 4 and 5 with the stronger O-HÁ Á ÁO bonds.

Database survey
Delangle and co-workers (Delangle et al., 2001;Husson et al., 1999) reported the preparation of a variety of mononuclear complexes of various lanthanide(III) ions, specifically La III , Pr III , Nd III , Ho III , Eu III and Yb III , with the trialcohols cis,cis-1,3,5-trihydroxycyclohexane (H 3 L 1 ) and cis,cis-1,2,3-trihydroxycyclohexane(H 3 L 2 ) as models for the coordination of monosaccharides. In those compounds, as in 1 and 2, the metal atoms are coordinated to two trialcohol molecules and bidentate/monodentate O-donor anions (nitrate or triflate), or to these anions and water molecules.
Monosaccharide-derived polyols have also been used as chelating ligands for lanthanide(III) ions. LnCl 3 and Ln(NO 3 ) 3 (Ln = La III , Tb III and Sm III ) were shown to form chain-like complexes with d-galactitol in which the alditol provides three hydroxyl groups to coordinate one metal ion and three other hydroxyl groups to coordinate another; in all     (7) cases, there are two alditol molecules bound to each lanthanide (Su et al., 2002;Yu et al., 2011). Other authors have employed erythritol, whose molecule functions as two bidentate ligands or as a three-hydroxyl donor to a variety of lanthanide(III) chlorides (Ce, Pr, Nd, Eu, Gd and Tb; Yang et al., 2012;Yang, Xie et al., 2005;. These studies describe several possible binding modes of these polyols to lanthanide ions. As far as tripodal alcohol ligands are concerned, mononuclear yttrium(III) complexes of 1,1,1-tris(hydroxymethyl)propane (H 3 L Et ) and 1,1,1-tris(hydroxymethyl)ethane (H 3 L Me ), as well as of the aminopolyalcohol (HOCH 2 ) 3 CN(CH 2 CH 2 OH) 2 , H 5 L N(EtOH)2 , were described by Chen and co-workers while investigating chelate complexes for radiotherapeutic applications (Chen et al., 1997). In two of the reported products, those prepared from H 3 L Me and H 5 L N(EtOH)2 , the coordination sphere of the eight-coordinate yttrium atom contains chloride instead of nitrate ligands. A more recent study (Xu et al., 2015), in its turn, describes a dysprosium(III) complex with H 3 L Et that is isostructural to product 2 (present work) and has been employed to investigate possible biomedical applications of the binding of rare earth metal ions to the apoferritin protein.

Synthesis and crystallization
All experimental operations were performed under N 2(g) (99.999%, Praxair) or under vacuum of 10 À3 Torr, using Schlenk and glove-box techniques. Solvents (dimethoxy-ethane and hexane) were purified according to procedures described in the literature (Perrin & Armarego, 1997). Terbium(III) nitrate pentahydrate and 1,1,1-tris(hydroxymethyl)propane (H 3 L Et ) were purchased from Aldrich; the latter was dissolved in thf/toluene (1:1), crystallized at 153 K, isolated by filtration and stored under N 2 at room temperature prior to use. Elemental analysis (C, H and N) were performed under argon by MEDAC Laboratories Ltd. (Chobham, Surrey, UK), using a Thermal Scientific Flash EA 1112 Series Elemental Analyzer. Infrared spectra (FTIR, Nujol mulls) were obtained on a BIORAD FTS 3500GX instrument in the range of 400-4000 cm À1 .

Synthesis of [Tb
A solution containing 1.91 g (4.39 mmol) of Tb(NO 3 ) 3 Á-5H 2 O in 50 ml of dimethoxyethane (glyme) received the addition of 1.11 g (8.27 mmol) of solid 1,1,1-tris(hydroxymethyl)propane to form a colourless solution that was refluxed for 15 min. After this period of time, the heating was turned off and a 32 ml aliquot of the reaction mixture was withdrawn for the isolation of product 2 (described below). The remaining 18 ml were cooled down to 153 K for four days, without forming any solid. The solution was then dried under vacuum and the resulting solid was almost completely redissolved in 7.5 ml of glyme. A fine suspension was obtained which, after seven days at 153 K, gave colourless crystals that were isolated and dried under vacuum (complex 1). Yield: 360 mg, 0.547 mmol (12.5% based on the total amount of terbium employed in the reaction). If the yield was extrapolated to the total volume of the reaction mixture (50 ml) instead of the 18 ml effectively employed for crystallization, it could reach 34.7%. Elemental analysis: calculated for [Tb(H 3 L Et ) 2 (NO 3 ) 2 ](NO 3 )Á0.5glyme (C 14 H 33 N 3 O 16 Tb) C 25.54, H 5.05, N 6.38%. Found C 25.34, H 5.08, N 6.60%. FTIR (Nujol mull, cm À1 , s = strong, m = medium, w = weak, sh = shoulder): 3359m, 3220m (O-H); 1050sh, 1020s, 942s, mainly (C-O); 1271s a (NO 2 ), 1041s s (NO 2 ).

Refinement
Crystal data, data collection and structure refinement details for both complexes 1 and 2 are summarized in Table 6. Disorder was noted in both structures: in compound 1, the methylene groups in the three CH 2 OH groups in one tripodal ligand were each found to be disordered over two sets of sites, with an occupancy ratio of 0.911 (7) : 0.089 (7), whereas in 2, the disorder is in a terminal methyl group, which is disordered over two orientations, with an occupancy ratio of 0.827 (4) : 0.173 (4).
All the hydroxyl and water hydrogen atoms were located clearly in difference maps and were refined freely and satisfactorily. All the remaining hydrogen atoms were set in idealized positions and refined as riding on the parent carbon atoms.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.