Lead ( II ) : Lewis acid and occasional base , as illustrated by its complex with 1 , 5-naphthalenedisulfonate and 5-methyl-1 , 10-phenanthroline

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Lead(II): Lewis acid and occasional base, as illustrated by its complex with 1,5-naphthalenedisulfonate and 5-methyl-1,10-phenanthroline † Jack Harrowfield* ,a and Pierre Thuéry* ,b A crystal structure determination of the Pb(II) coordination polymer [Pb(Mephen)(1,5-nds)(H2O)]n provides not only evidence of the common action of Pb(II) as a Lewis acid but also clear proof of its ability, in the solid state at least, to act as a Lewis base.This action as a base is attributed to the presence of a valence shell lone pair and its identification here is further evidence for the occasional but not universally detectable influence of the lone pair on the metal ion stereochemistry.
The suggestion by Moore and Pauling in 1941, in a powderdiffraction structural study of litharge (tetragonal PbO), 1 that a valence shell lone pair might determine the stereochemistry of the Pb(II) coordination environment proved to have a remarkable and enduring influence on subsequent diverse studies of the coordination chemistry of Pb(II). 2,3The gas phase electron configuration of Pb 2+ has the highest energy electron pair placed in the 6s orbital and thus it is often said that Pb(II) may have a stereochemically active 6s 2 lone pair, although clearly the electron pair cannot remain in an s orbital if it is to produce coordination sphere asymmetry and the supposedly inert nature of the 6s 2 electrons has been described as a "myth" of Pb(II) coordination chemistry. 2Indeed, DFT calculations on litharge 4 have shown that the lead 6s orbital interacts strongly with oxygen 2p orbitals to give a non-spherically-symmetrical electron density.Theoretical calculations have also indicated that Pb⋅⋅⋅Pb bonding may be significant 4,5 and consideration of the Hirshfeld surface 6 for tetragonal PbO (Fig. 1), derived for its crystal structure 7 using CrystalExplorer, 8 provides clear evidence for this, so that the actual influence of any lone pair in PbO is unclear.Given the delocalised electronic structure of solid PbO, it is possible that the characteristics of this material are not shared by the numerous coordination complexes of Pb(II) where multidentate ligands limit the proximity of the Pb(II) centres. 2,3,9- 113][14] or in some diorganolead "diplumbenes" 15 is there clear evidence for the retention of Pb⋅⋅⋅Pb interactions or "plumbophilicity", in some cases associated with Pb⋅⋅⋅Pb separations shorter than those in metallic Pb.3][4][5] Sulfur donor atoms, for example, are expected to have minimal orbital interaction with lead 6s and thus not to favour detectable lone pair effects on stereochemistry. 4,5It is worth noting here that transition metal complexes provide numerous examples where non-bonding d electrons appear to have no stereochemical influence (e.g. the three pairs of t2g electrons in [Co(NH3)6] 3+ ).
A difficulty in establishing that lone pair effects exist in Pb(II) complexes is that the lone pair itself is not directly observable.Various criteria of lone pair activity have therefore been explored and the VSEPRT (Valence Shell Electron Pair Repulsion Theory) approach, for example, has been used to rationalise the coordination sphere geometry of numerous aminocarboxylate complexes in terms of active lone pairs, 16 reflecting probably the specific nature of the donor atoms. 17here are, however, instances where the unsymmetrical coordination sphere geometry of a Pb(II) complex is mimicked in an analogous complex of a metal ion lacking a potential valence shell lone pair, 18 so that in general more sophisticated analyses based on whether the coordination sphere may be described as holo-or hemi-directed 10 (not always obviously 19 ) and/or whether there is an obvious coordination sphere vacancy, particularly one associated with unusually long bonds and large bond angles involving nearby donor atoms, 3 and on high-level computation, 2,3,20 have been applied.
One criterion of the presence of a lone pair that has been relatively little explored, however, is that of whether weak interactions of the lone pair as a donor can be discerned.In a survey of the structures of Pb(II) complexes of ligands containing aromatic units, 21 it has been deduced that approximately 3% of these structures provide evidence of interactions between the metal and these units, described by the authors as involving donation of the Pb(II) lone pair into antibonding orbitals of the aromatic systems.There is some ambiguity, however, as to whether these contacts do arise thus or are simply due to polyhapto π-donation from the aromatic ligand to the Pb(II) centre, 22 and in a more recent investigation concerning Pb(II) and several other metal ions and the effects of the interactions on luminescence properties, 23 it has been concluded (supported by DFT calculations) that the interaction is one of ligand donation to the metal ion.Where there should be no ambiguity is when the atom approaching Pb is incapable of donation, as expected for hydrogen bonding.While a careful investigation 24 of this possibility for [Pb(DOTAM)](ClO4)2⋅4.5H2O (DOTAM = 1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetra-azacyclododecane) led to the conclusion that any Pb-lone pair⋅⋅⋅H interaction was too minor to justify description as an hydrogen bond, this was qualified by the uncertainty in the exact orientation of the water molecule hydrogen atom involved.Two other possibilities drawn from the extant literature considered in reference 24, one involving an apparent Pb⋅⋅⋅H separation as short as 2.7 Å, also involved this difficulty.Nonetheless, our calculation of the Hirshfeld surface for the [Pb(DOTAM)](ClO4)2⋅4.5H2O structure as described provides evidence that there is a Pb-lone pair⋅⋅⋅H interaction beyond dispersion.There is less uncertainty in the orientation of an hydrogen atom substituent on an aromatic ring and in the structure of [Pb(Mephen)(1,5-nds)(H2O)] ( 1), where Mephen is 5-methyl-1,10-phenanthroline and 1,5-nds is 1,5naphthalenedisulfonate, a complex obtained during recent attempts to produce mixed-metal complexes with naphthalenedisulfonate ligands, 25 there is more conclusive evidence for hydrogen bonding by a Pb(II) lone pair.Complex 1 is the third to be obtained with Pb(II) and the 1,5-nds 2-ligand, after [Pb2(CH3CO2)2(1,5-nds)] 26 and [Pb(phen)(1,5nds)(H2O)2]⋅H2O, 27 which crstallize as three-and onedimensional polymers, respectively.
The evidence presented herein of Pb(II) lone pair hydrogen bonding reinforces the very extensive computational and indirect experimental evidence of the role of a lone pair of electrons in determining the metal ion's stereochemistry.While there must be a pair of electrons present within the valence shell and thus some consequences of this, the present results, placed in their literature context, also show that the bonding capacity of the lone pair is very limited.

Fig. 1 A
Fig. 1 A view of the Hirshfeld surface for the asymmetric unit of litharge, showing adjacent Pb (blue) and O (red) atoms (left).A view of a single Pb atom (green) of PbO and its four nearest neighbour O (red) atoms and the ten Pb atoms (blue) within 4 Å (right).
ligands are bound to either two or four lead atoms, with the sulfonate coordination modes κ 1 O and µ2κ 2 O,O':κ 2 O',O'' (µ2-κ 2 O,O':κ 1 O'' if the longest bond is disregarded) different from those in the previously reported Pb(II)

Fig. 3
Fig. 3 Views of the two-dimensional assembly (top) and the packing (bottom).Hydrogen atoms are omitted.

Fig. 4
Fig. 4 View of the lead(II) environment showing the hydrogen bond as a dashed line (top).Part of the Hirshfeld surface mapped with dnorm showing the three red dots corresponding to the interactions between Pb(II) and the three atoms O4 i , O6 i and H9 m (bottom).