Bonds and lone pairs in the flexible coordination sphere of lead(II) Annegret K. Hall,a Jack M. Harrowfield,*a Ali Morsali,b Ali A. Soudib and Alexander Yanovskyc a Special Research Centre for Advanced Mineral and Materials Processing, University of Western Australia, Nedlands WA 6907, Australia b Department of Chemistry, University of Zanjan, P.O. Box 45195-313, Zanjan, Iran c X-ray Structural Centre, Academy of Sciences of Russia, Moscow B-334, 117813, Russia Received 13th March 2000, Accepted 18th April 2000, Published 25th April 2000 The single crystal XRD characterisation of a ’mixed anion’ complex of Pb(II) with 1,10-phenanthroline, [Pb(phen)(O2CCH3)(O2ClO2)], shows the complex to be polymeric as a result of acetate ligand bridging, the Pb atom being in an unsymmetrical eight-coordinate, N2O6, environment and the crystal array reflecting a strong influence of p–p stacking of the heteroaromatic chelate ligands.The structure is used to raise issues concerning the coordination sphere of lead(II) which are of relevance to the rational design of solid state arrays. An issue frequently discussed in considering the coordination and stereochemistry of heavy metals is that of the 'stereochemical activity' of valence shell lone electron pairs.1–7 Extensive recent structural studies of lead(II) compounds8,9 in particular have provided a basis for a rather detailed analysis of the evidence for coordination sphere distortions which may be a consequence of the presence of such pairs. It appears that in complexes of lead(II) [and probably in those of related species such as Tl(I)10 and Bi(III)11], the nature and form of the coordination sphere is generally determined by a number of factors, possibly including lone pair–bond pair repulsions, of comparable influence, so that seemingly minor differences in ligands or in the crystal array can have quite marked effects upon the coordination stereochemistry.Since the presence of a lone pair is not directly detected but is inferred on the basis of the spatial distribution of atoms assumed to be donors to the central metal, the identification of these donor atoms is fundamental to the analysis of any particular system. Interestingly, this alone is not a straightforward process. In a recent report12 of the crystal structure of lead(II) 2- carboxyethylphosphonate, for example, three inequivalent Pb atoms, each considered to have a coordination site occupied by a lone pair, were identified.The assigned Pb– O distances in these three species varied over a relatively large range (2.267–2.738 Å), a not uncommon observation for particular Pb(II)–donor atom interactions,8,9 and the given coordination number assignments of 3, 4 and 5 were (apparently) based upon assumption of an upper limit of approximately 2.75 Å for Pb–O. Under this assumption, each lead atom clearly has a 'hemidirected'9 coordination sphere. In lead(II) hydrogenphenylphosphonate,13 however, a 'holodirected'9 coordination sphere has been identified based on Pb–O separations ranging from 2.605–2.843 Å, and if other oxyanions such as nitrate and perchlorate, assumed to be relatively weak donors, are considered as ligands, Pb–O separations as great as 3.41 Å have been associated with bonding (and 'holodirected' coordination, though not necessarily coordination with a DOI: 10.1039/b001972k stereochemically inactive lone pair).If, then, an increased Pb–O interaction cutoff distance of 2.9 Å is applied to lead(II) 2-carboxyethylphosphonate, the 3-, 4- and 5- coordinate Pb atoms become 5-, 6- and 7-coordinate, respectively, and all may be considered 'holodirected' in their coordination. It is still possible to discern 'holes' in the coordination spheres which may be indicative of the presence of a lone pair but their identification is much more equivocal and the framework defined by the bonding array is now much less readily identified.These problems become more acute in complexes of the mixed ligand type we describe in the present communication. Reaction between 1,10-phenanthroline (phen) and an equimolar mixture of lead(II) acetate and lead(II) perchlorate by diffusion along a thermal gradient in methanol solution (the 'branched tube' method14) provided crystalline material analysing as [Pb(phen)(O2CCH3)(O2ClO2)].15 Determination of the structure of this material by X-ray crystallography (Table 1) showed the complex in the solid state (Fig. 1) to be a polymeric species with various similarities to both polymeric [Pb(phen)(NO3)2(OH2)] and dimeric [Pb(phen)(O2CCH3)2].8 The polymer backbone in the present system is formed by bridging coordination of both acetate oxygen atoms involved in chelation of one lead atom to different adjacent lead atoms, so that (Pb–O–C–O) strings are cross-linked to give a double-stranded polymer unlike the single-stranded polymer of [Pb(phen)(NO3)2(OH2)], though both share the characteristics of alternating dispositions of the 1,10- phenanthroline ligands to opposite sides of the chain.The Pb2O2 rhombs which share edges with PbOCO quadrilaterals to make the polymer core are similar to the central Pb2O2 rhomb of dimeric [Pb(phen)(O2CCH3)2], though the longer edge of the rhomb in [Pb(phen)(O2CCH3)(O2ClO2)]n (2.736 Å) is considerably shorter than that of the simple acetate (3.366 Å).Consistent with the assumption that it should be a weaker ligand than acetate, the perchlorate anion of [Pb(phen)(O2CCH3)(O2ClO2)]n is symmetrically bidentate CrystEngComm, 2000, 13nTable 1 Crystal data for the Pb(II) complexa Data Properties C14H11ClN2O6Pb 545.9 Triclinic Formula MCrystal system Space group 1 P a/Å 7.084(3) b/Å 10.627(4) c/Å 11.038(5) a/° 71.59(3) b/ ° 87.83(4) g/ ° 85.62(4) V/Å3 786.1(6) Dc /g cm–3 2.306 T/K 293(2) Total independent 5099 reflections Observed 3313 reflectionsb R1 0.053 wR2 0.136 aCrystal dimensions 0.60 0.30 0.20 mm. A unique diffractometer data set was collected using Siemens R3m/V diffractometer; monochromatic MoKa radiation, l= 0.71073 Å.b Used in a full matrix with I > 2 s(I). Click here for full crystallographic data (CCDC no. 1350/19). Table 2 Pb···O geometry (Å)a Pb1–O1 Pb1–O2 Pb1–O2' Pb1–O1'' Pb1–O3'' Pb1–O4'' a Symmetry codes: ' : –1 + x, –y, –z; '' : –x, –y, –z. towards the lead atom but does not appear to be involved in further, bridging interactions. Thus, each lead atom can be considered to be 8-coordinate with an N2O6 donor atom array, the range of Pb–O distances (Table 2), in particular, being similar to those of the systems described above. Although 'holodirected', this donor atom array is like those of [Pb(phen)(NO3)2(OH2)]n and [Pb(phen)(O2CCH3)2]2 in that there is an obvious 'hole' in the coordination sphere which might well be taken as the site of a valence shell lone pair.A striking feature of most crystalline arrays containing heteroaromatic bases8,16 is the presence of 'stacks' (chargetransfer arrays)17,18 of these planar species in which the mean molecular planes are close to parallel and separated by a distance of ~3.5 Å, close to that of the planes in graphite. Just as is the case when hexagonal BN is compared with graphite,19 the projection of one plane of 2.495(6) 2.507(7) 2.736(9) 2.719(9) 2.935(17) 2.945(17) atoms onto another can differ considerably and the projections of one 1,10-phenanthroline molecule onto its nearest stacking neighbour in the three complexes [Pb(phen)(O2CCH3)(O2ClO2)]n, [Pb(phen)(O2CCH3)2]2 and [Pb(phen)(NO3)2(OH2)]n are shown in Fig.2. The differences are obvious and presumably are at least partly a consequence of the nature of the other components of the arrays, though the structural evidence alone does not allow any facile assignment of priority to the different factors which may be recognised. If the inherent energy difference between 'holo-' and 'hemidirected' coordination arraysabout Pb(II) is indeed 30–50 kJ mol–1,9 it may be that the stacking interaction makes a contribution of such magnitude to the total lattice energy. Because of the relatively close approach of large ligand molecules involved in stacking, some mutual distortion of the metal coordination spheres might be anticipated. Significantly, ’holes’ in the coordination spheres of Pb in the above three complexes are all close to the region where the phenanthroline entity of a neighbouring complex must enter to stack.Thus, evidence here, at least, for the stereochemical activity of a valence shell lone pair may be simply an artefact of the manner in which a continuous solid structure is divided by convention into entities which do not necessarily define the strongest interactions giving rise to the total structure. A question arises which, put simply, is whether the array determines the stacking or the stacking determines the array? In relation to the influence of aromatic units generally upon Pb(II) coordination chemistry, it is of interest to contrast the recent assignment of a hexahapto interaction with a phenyl group of the tetraphenylborate anion to bonding involving the formal lone pair,20 and the failure to detect evidence of lone pair activity in a plumbocene derivative.21 (a) (b) Fig.2 Projections of nearest neighbour pairs in the p–p stacks of heteroaromatic bases in Pb(II) complexes. (c) Fig. 1 Ellipsoid surfaces of the anisotropic atomic displacement parameters to a probability of 0.5, shown using an Xtal-generated POV-Ray plot.16 (a) A view of the Pb2O2 rhomb forming the basic link of the polymeric structure; (b) a view perpendicular to a polymeric strand of the solid showing the chelating and bridging functions of the acetate ligand and the full donor atom environment of the Pb atoms, for a 3D presentation click the image or here; (c) a projection of the structure down a showing the neighbouring polymeric chains and p–p interactions between the phenanthroline ligands.References 1 R. D. Hancock, in Perspectives in Coordination Chemistry, ed. A. F. Williams, C. Floriani and A. E. Merbach, VCHA:VCH, Basel, 1992, p. 129. 2 R. D. Hancock, M. S. Shaikjee, S. M. Dobson and J. C. A. Boeyens, Inorg. Chim. Acta, 1988, 154, 229. 3 P. Pyykkö, Chem. Rev., 1988, 88, 563. 4 A. Bashall, M. McPartlin, B. P. Murphy, D. E. Fenton, S. J. Kitchen and P. A. Tasker, J. Chem. 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Chem., 1976, 54, 3430. 15 1,10-Phenanthroline (0.4 g) was placed in one arm of the branched tube and a mixture of lead(II) perchlorate trihydrate (0.46 g) and lead(II) acetate (0.36 g) in the other. Methanol was carefully added to fill both arms, then the tube was sealed and the ligand-containing arm immersed in a bath at 60 °C while the other was at ambient temperature. After 15 d, white crystals, mp 273 °C, had deposited in the cooler arm. Analysis: found C, 31.0; H, 2.1; N, 5.2; calculated for C14H11ClN2O6Pb C, 30.80; H, 2.03; N, 5.13%. 16 S. R. Hall, H. D. Flack and J. M. Stewart, The XTAL 3.2 Reference Manual, Universities of Western Australia, Geneva and Maryland, 1992; S. R. Hall, D. J. du Boulay and R. Olthof-Hazekamp, POV-Ray. Xtal3.7 User’s Manual, University of Western Australia. 17 G. Ferguson, C. Glidewell and E. S. Lavender, Acta Crystallogr., Sect. B, 1999, B55, 591. 18 Molecular Complexes, ed. R. Foster, Paul Elek (Scientific Books) Ltd, London, 1973. 19 N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, Pergamon Press, Oxford, 1984, pp. 235–236. 20 M. Di Vaira, F. Mazi and P. Stoppioni, Eur. J. Inorg. Chem., 1999, 833. 21 D. J. Burkey, T. P. Hanusa and J. C. Huffman, Inorg. Chem., 2000, 39, 153. CrystEngComm © The Royal Society of Chemistry 2000