Received 11 December 2009
Two polymorphs of a lead(II) complex with 8-hydroxy-2-methylquinoline and thiocyanate
Two distinct polymorphs of bis(2-methylquinolin-8-olato)-3N,O:O;3O:N,O-bis[(isothiocyanato-N)lead(II)], [Pb2(C10H8NO)2(NCS)2], (I), forming dinuclear complexes from a methanolic solution containing lead(II) nitrate, 2-methylquinolin-8-ol (M-Hq) and KSCN, crystallized concomitantly as colourless prisms [form (Ia)] and long thin colourless needles [form (Ib)]. In both cases, the complexes lie across a centre of inversion. The polymorphs differ substantially in their conformation and in their interactions, viz. PbS and - for form (Ia) and PbS, Pb and C-H for form (Ib).
The synthesis of novel organic-inorganic hybrid materials in the field of supramolecular and crystal engineering has been a subject of rapid growth in recent years (Moulton & Zaworotko, 2001). Bidentate ligands containing soft and hard atoms have potential applications in catalytic and stoichiometric reactions (Kaim & Schwederski, 1995), as well as in advanced materials (Soldatov et al., 2004). Recent reports of quinolin-8-ol (Hq) and its derivatives with lead(II) salts include [Pb(q)(NCS)]n (Shahverdizadeh et al., 2008), [Pb2(M-q)2(NO3)2(CH3OH)2] (M-q is 2-methylquinolin-8-olate; Mohammadnezhad et al., 2009a), [Pb2(M-q)2(C2H3O2)2] (Mohammadnezhad et al., 2009b), [Pb2(Cl-q)2(C2H3O2)2] (Cl-q is 5-chloroquinolin-8-olate; Mohammadnezhad et al., 2009c) and [Pb4(q)6(NO3)2] (Zhang et al., 2008).
As part of our interest in exploring the effect of steric hindrance in lead(II) complexes with mixed ligands, we have examined isothiocyanate due to its various coordination modes, i.e. single or bridging coordination through S, N or both, in the presence of 2-methylquinolin-8-ol (M-Hq). In this paper, we report the crystal structures of two polymorphs of [Pb(M-q)(NCS)]2, (Ia) and (Ib), which show that the steric hindrance of a methyl group leads to the coordination of isothiocyanate only via the N atom, not via the S atom, in contrast with the bidentate coordination observed in [Pb(q)(NCS)]n (Shahverdizadeh et al., 2008).
Polymorph (Ia) crystallizes in the triclinic space group P, while polymorph (Ib) crystallizes concomitantly in the monoclinic space group P21/c. Perspective drawings of these compounds are shown in Figs. 1 and 2, respectively; in both cases, the Pb2O2 core lies across the crystallographic inversion centre. In both structures, the PbII ion is four-coordinated, with the M-q ligand acting as a bidentate chelate, along with a bridging phenoxy O atom and the N atom from the isothiocyanate. As expected, these four coordinating atoms around the PbII centre in both polymorphs show a hemidirected geometry with a stereochemically active lone pair. Selected bond distances and bond angles are presented in Table 1. It is notable that the major structural difference between these two polymorphs is the NCS coordination angle. In (Ia), the Pb1-N2-C1 angle is 149.7 (4)°, in contrast with a value of 136.8 (6)° in (Ib). All other angles except O1-Pb1-N2, N2-Pb1-O1i and O1i-Pb1-N1 are similar [symmetry code: (i) -x, -y + 1, -z + 1 for (Ia); (i) -x + 1, -y + 2, -z + 1 for (Ib)]. The different conformations of the polymorphs are strongly reflected in the torsion angles containing thiocyanate atoms N2 and C1, but also in the torsion angle Pb1-O1-C8-C7 (see Table 1). The PbPb distances are 3.9397 (3) and 4.0212 (14) Å for (Ia) and (Ib), respectively.
Interestingly, in contrast with the previously reported coordination polymer [Pb(q)(SCN)]n (Shahverdizadeh et al., 2008), the steric hindrance imposed by the methyl group at the C2 position prevents the coordination of SCN as a bidentate ligand. Indeed, the larger S atom is unable to coordinate to the PbII ion in either case, and hence the polymeric nature is disrupted and the coordination number decreases to four. Nevertheless, PbS interactions are formed for Pb1S1ii and PbivS1iii [3.6009 (14) Å] and for Pb1S1iii and PbivS1ii [3.6649 (14) Å] in compound (Ia) (Fig. 3) [symmetry codes: (ii) 1 + x, y, z; (iii) -1 - x, 1 - y, 2 - z; (iv) -x, 1 - y, 2 - z]. Different types of interaction are seen in polymorph (Ib). In addition to PbS interactions with distances of 3.506 (3) Å, an interaction is observed between the PbII ion and the centroid (Cg) of the C5-C10 benzene ring in the molecule at the symmetry position (x, y + 1, z), with PbCg = 3.171 Å (Fig. 4).
Structure (Ia) contains sheets of molecules in the ac plane held together by PbS interactions (Fig. 5). The sheets have the aromatic wings of the ligand protruding from each side and interdigitate to provide - stacking, with perpendicular distances of 3.288 and 3.377 Å (Fig. 6), reflecting the different positions of the N atoms in the overlapping heterocyclic rings.
In contrast, structure (Ib) contains chains of molecules along b which are held together by PbS and Pbring interactions. The aromatic surfaces protruding from these chains make a herringbone formation in the bc plane (Fig. 7), with C-H interactions for enhancement [between C6-H6 and the centroid (Cg) of the C5-C10 ring at symmetry position (-x + 1, y - , -z + ), with C6Cg = 3.458 (7) Å, H6Cg = 2.70 Å and C6-H6Cg = 137°].
| || Figure 1 |
The structure of polymorph (Ia), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. [Symmetry code: (i) -x, -y + 1, -z + 1.]
| || Figure 2 |
The structure of polymorph (Ib), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. [Symmetry code: (i) -x + 1, -y + 2, -z + 1.]
| || Figure 3 |
The PbS interactions (dashed lines) in (Ia). [Symmetry codes: (ii) 1 + x, y, z; (iii) -1 - x, 1 - y, 2 - z; (iv) -x, 1 - y, 2 - z.]
| || Figure 4 |
The PbS interactions (dashed lines) in (Ib), with distances of 3.506 (3) Å. The distance from atom Pb1 to the centroid of the C5-C10 ring of an adjacent molecule (also dashed lines) is 3.171 Å. [Symmetry codes: (i) -x + 1, -y + 2, -z + 1; (ii) x, y + 1, z.]
| || Figure 5 |
The contents of the unit cell of (Ia), in a projection parallel to the quinoline rings.
| || Figure 6 |
The - interactions in (Ia). The overlap of the quinoline rings with a perpendicular distance of 3.288 Å is shown in the upper part, and that with a perpendicular distance of 3.377 Å is shown in the lower part.
| || Figure 7 |
The herringbone formation of the aromatic surfaces in (Ib). H atoms have been omitted for clarity.
Lead nitrate (0.33 g, 1 mmol), 2-methylquinolin-8-ol (0.16 g, 1 mmol) and KSCN (0.19 g, 2 mmol) were loaded into a convection tube. The tube was filled carefully with methanol and kept at 333 K. Crystals were collected from the side arm after several days; they were a mixture of large crystals of KNO3, colourless prisms of (Ia) and long thin colourless needles of (Ib).
Aromatic H atoms were included in the model with Uiso(H) = 1.2Ueq(C), and their positions were constrained to ideal geometry using an appropriate riding model, with C-H = 0.95 Å. For methyl groups, N-C-H angles (109.5°) were kept fixed, while the torsion angle was allowed to refine, with the starting positions based on the circular Fourier synthesis averaged using the local threefold axis; C-H = 0.98 Å and Uiso(H) = 1.5Ueq(C).
For both polymorphs, data collection: SMART (Bruker, 2003); cell refinement: SAINT (Bruker, 2003); data reduction: SAINT and SADABS (Sheldrick, 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2009); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).
Supplementary data for this paper are available from the IUCr electronic archives (Reference: SF3124 ). Services for accessing these data are described at the back of the journal.
The authors thank Shahid Beheshti University for supporting this study.
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