Hexafluorosilicate and tetrafluoroborate coordination to lead(II) di- and tri-imine complexes â€“ Unusual fluoroanion coordination modes

Lead(II) tetrafluoroborate and hexafluorosilicate complexes with 2,20-bipyridyl, 1,10-phenathroline and 2,20:60 ,200-terpyridyl have been prepared from the ligand and lead salt in aqueous/MeCN. Crystal structures are reported for [Pb(bipy)2(SiF6)], [Pb(phen)2(SiF6)] and [Pb(bipy)2(BF4)2] which are dinuclear with each lead coordinated ‘‘cis’’ to the two diimines and with the bridging fluoroanions completing eight or nine-coordination. [Pb(phen)2(BF4)2] is eight-coordinate and mononuclear with ‘‘cis’’ diimines and two j-BF4 groups. [Pb(phen)2(H2O)2(SiF6)] is also mononuclear with a j-SiF6 group and two coordinated water molecules. Reaction of Pb(BF4)2 with 2,20:60,200-terpyridyl gave only [Pb(terpy)3][BF4]2, but Pb(SiF6) produced [Pb(terpy)(H2O)(SiF6)], which is a chain polymer with bridging SiF6 groups and significant pstacking of the imine rings. The work has identified a number of coordination modes of the SiF6 anion, which has been little used in coordination chemistry but proves to be versatile and also stable (to decomposition/hydrolysis). 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/).


Introduction
'Non-coordinating' anions have played a key role in many areas of coordination and organometallic chemistry and catalysis, despite the recognition that in appropriate cases anion coordination does occur, and sometimes this can be surprisingly strong [1][2][3][4]. More recently, efforts have focussed on designing anions with a weaker coordinating ability than the archetypal examples ClO 4 À or CF 3 SO 3 À , such as fluorinated-arylborates or -aluminates [4]. With the majority of metal ions fluoroanions such as BF 4 À , PF 6 À or SbF 6 À have a very limited tendency to coordinate. They are also usually chemically inert and are much more readily available than fluorinated-arylborates or -aluminates. In a recent investigation of lead(II) complexes with crown ethers, oxathia-and oxaselena-macrocycles, we used lead(II) tetrafluoroborate or hexafluorophosphate as sources of lead(II), and found that not only did the fluoroanions readily enter the first coordination sphere of the lead to exhibit a range of coordination modes (j 1 , j 2 , or l 2 ), but also that they were readily decomposed in some systems, with formation of coordinated or free fluoride [5]. Decomposition of the anions also occurred in some tin(II) crown ether systems [6]. In order to explore both the coordinating abilities of fluoroanions and this unexpected reactivity, we investigated lead(II) tetrafluoroborate and hexafluorosilicate complexes of 2,2 0 -bipyridyl (bipy), 1,10-phenanthroline (phen) and 2,2 0 :6 0 ,2 00 -terpyridyl (terpy), and report the results here. A large number of lead(II) complexes with these imine ligands are known, but most are with oxoanions (ClO 4 À , NO 3 À or O 2 CR À , etc.) [7]. The [Pb(bipy) x (PF 6 ) 2 ] (x = 2 or 4) have recently been described [8], hence we did not include the hexafluorophosphate complexes in this work. Lead, with a covalent radius of 1.46 Å A 0 , often forms compounds with high coordination numbers and with irregular geometries, which reflect both the steric demands of the ligands and inter-ligand replusions. The presence of a formal lone pair, which may or may not be stereochemically active, may also influence the geometries observed [7].

Results and discussion
The lead(II) salts used for the syntheses were an aqueous solution of Pb(BF 4 ) 2 , and solid Pb(SiF 6 )Á2H 2 O, which was dissolved in the minimum volume of water. The structure of the latter contains nine-coordinate lead in two environments, one provided by six fluorides and three water molecules, the other with five fluorines and four water molecules [9]. The Pb(BF 4 ) 2 solution, as supplied, contains some excess acid to prevent hydrolysis. The acid caused no problems with bipy or phen, but crystals obtained from the terpy reaction were reproducibly found by X-ray crystal structure determination to be [ 2 or Pb(SiF 6 ) with either bipy or phen (or terpy, in the case of Pb(SiF 6 )) in a mixture of H 2 O and MeCN, afforded white, beige or pale pink solids. Crystals were obtained by allowing aliquots of the reaction solutions to evaporate slowly in air. Although water is not evident in the IR spectra of the bulk powders, (after washing with diethyl ether and drying in vacuo), water is present in several of the crystal structures.

X-ray structures
As a result of the labile nature of Pb(II) in solution and the unpredictable geometries present, the information provided by spectroscopic techniques is quite limited. X-ray crystallographic analysis is the key characterisation technique for complexes of this type and therefore the structural features of the new complexes are described first, followed by a discussion of relevant spectroscopic data. Microanalytical data confirm the stoichiometries of the bulk samples.
The structure of [Pb(bipy) 2 (SiF 6 )] ( Fig. 1) shows it to be a centrosymmetric dimer. Each lead ion is coordinated to two chelating bipy ligands, disposed ''cis'' with d(Pb-N) = 2.531(5)-2.760(5) Å. These values can be compared with those reported for [Pb(bipy) 2 (Y) 2 ] (Y = ClO 4 , NO 3 ), which are also dimers with bridging oxo-anions [10], with d(Pb-N) in the range 2.512 (7) The bulk product isolated from the reaction of Pb(SiF 6 ) and phen was the analogous [Pb(phen) 2 (SiF 6 (Fig. 4). The structure is surprisingly complicated; the PbN 4 geometry is similar to that found in the other diimine complexes [10,12], but instead of there being four l 2 -BF 4 groups as one might have expected, the coordination sphere of each lead centre is completed by four fluorines from three BF 4 À anions, one of which is j 1 , one is j 2 , one is l 2 , and the fourth one chelates (j 2 ) to one lead and forms a single bridge to the second lead. This gives eight-(or nine-when the water is present) coordinate lead centres. The Pb-F distances span the range 2.725(8)-3.028(9) Å, and probably reflect the packing within the crystal lattice rather than any strong directional preference.
In contrast, [Pb(phen) 2 (BF 4 ) 2 ] is monomeric, with eight-coordinate lead, composed of two chelating diimines and two j 2 -BF 4 À groups ( Fig. 5), and thus is closely related to that of [Pb(phen) 2 (H 2 O) 2 (SiF 6 )], with the two water molecules replaced by a chelating tetrafluoroborate anion. The Pb-F bonds are surprisingly disparate (by $0.15 Å) between the two anions.  The literature structure [8] of the dimeric [Pb(bipy) 2 (PF 6 ) 2 ] also shows Pb-F contacts significantly within the van der Waals radii sum, and reveals the complexity of fluoroanion coordination in these systems. Although not discussed by the authors, the structure (Fig. 6) shows each lead is j 2 -coordinated to a PF 6 À group, whilst a third PF 6 À group is coordinated in an asymmetric j 3 mode, and the fourth PF 6 À j 2 -coordinated to one lead and bridging to the second one via a long Pb-F interaction.
In summary, the core geometry of the Pb(diimine) 2 units has the diimines ''cis'' disposed on one hemisphere of the lead, and is relatively little affected by the anions present, which tend to fill the remaining space around the lead centre. In contrast to oxoanions, which coordinate strongly to the lead [7,8,10,12], the coordination adopted with the weakly bound fluoroanions is likely to be influenced by crystal packing effects, which results in the irregular and varying geometries found.
Different structures are found in the complexes of terpy. Species with a 1:1 Pb:terpy ratio were reported with lead(II) halides or oxo-salts [13], which have 1-D chain structures and bridging anions. The fluorosilicate complex prepared in this work has the formula [Pb(terpy)(H 2 O)(SiF 6 )] and shows a planar N 3 -coordinated terpy. The coordinated water is disordered above and below the PbN 3 plane. The Pb-O distances (2.87(2) and 3.05(1) Å) are very long (although still well within the sum of the van der Waals radii, 3.5 Å), suggesting weak interactions. Both the microanalysis and the IR spectrum indicate the bulk sample is anhydrous; the coordinated water present in the crystal structure presumably fills a void in the lattice, and is easily removed in vacuo. The SiF 6 2groups linking the lead-terpy units into chains are disordered (Fig. 7), but seem to adopt two distinct and reasonably well-defined orientations. The SiÀF distances are not significantly different to those found in the free anion ($1.70 Å) [14], but the PbÀF bond distances span a wide range and, in view of the disorder, are not discussed here (although they are included in the CIF). The coordinated F1A/F1B site positions refine very close together and strongly suggest this atom position is identical for both orientations. Although one must interpret the dimensions with care because of this disorder, it appears that in one orientation, the SiF 6 2À units bridge the between neighboring chains (Fig. 7), with the distance between the planes = 3.566 Å. As mentioned above, reaction of terpy with Pb(BF 4 ) 2 gave [Pb(terpy) 3 ][BF 4 ] 2 . The cation has been reported before in the ClO 4 À salt [13], in which the cation is disordered. The structure of [Pb(terpy) 3 ][BF 4 ] 2 is described in ESI, and although this is also disordered, the disorder has been successfully modelled in the alternative space-group PÀ3c1, which avoids the unreasonably close approach of some atoms in the reported structure [15].   The coordination modes of the BF 4 À and PF 6 À groups to Pb(II) described above are similar to those observed in other systems, including the crown ether, thia-and selena-crown systems [5], and demonstrate that FÁ Á ÁPb coordination is favoured. The unusual dimer structure of [Pb(bipy) 2 (BF 4 ) 2 ], which shows four different coordination modes of the BF 4 À groups, probably indicates there is little to choose between them on bond energy grounds, and the structure adopted reflects crystal packing requirements, maximising the lattice energy.
These unit, the lead is either 8-or 10-coordinate (the latter is shown in Fig. 7); the p-stacking between neighbouring chains also plays a role here.

Spectroscopic data
The detailed information that can be obtained from spectroscopic data on this series of Pb(II) complexes is limited. The IR spectra (Nujol) do confirm the presence of the imines and the fluoroanions (Experimental), but do not reliably distinguish the anion coordination modes. The stretching modes of the BF 4 À and SiF 6 2À groups are significantly broadened, although resolved splittings which might be expected in view of the lower symmetries arising from the Pb(II) coordination are not seen, presumably because these interactions are weak. The 1 H NMR spectra (CD 3 CN or D 2 O   . Notably, and in contrast to the results in the Pb(II) crown ether systems [5], there was no evidence for decomposition of the fluoroanions in any of the new complexes reported here.

Experimental
The ligands and lead tetrafluoroborate (50% solution in water) were obtained from Aldrich, and lead hexafluorosilicate dihydrate from Alfa Aesar, and were used as received. IR spectra were recorded as Nujol mulls between CsI plates using a Perkin Elmer Spectrum 100 spectrometer over the range 4000-200 cm À1 . 1 H and 19 F{ 1 H} NMR spectra were recorded using a Bruker DPX-400 spectrometer and referenced to the residual solvent resonance and external CFCl 3 respectively. Microanalytical measurements were performed by London Metropolitan University.

[Pb(bipy) 2 (BF 4 ) 2 ]
Pb(BF 4 ) 2 as a 50% aqueous solution (0.206 g, 0.27 mmol) was added to a solution of 2,2 0 -bipyridyl (0.081 g, 0.52 mmol) in CH 3 CN (5 mL), leading to the precipitation of a small amount of fine white powder. The reaction mixture was stirred for 24 h, then it was filtered and a portion was removed for crystal growth. From the remaining solution, the solvent was removed in vacuo to yield a pale pink powder, which was washed with Et 2 O (5 mL) and dried in vacuo. Yield: 0.113 g, 60%. Anal. Calc. for C 20

[Pb(bipy) 2 (SiF 6 )]
A solution of 2,2 0 -bipyridyl (0.081 g, 0.52 mmol) in CH 3 CN (5 mL) was added to Pb(SiF 6 )Á2H 2 O (0.100 g, 0.26 mmol) dissolved in deionised water (15 mL), leading to the precipitation of a small amount of fine white powder. The reaction mixture was stirred for 1 h, then it was filtered and a small aliquot removed for crystal growth. The solvent was removed in vacuo from the rest, to yield a pale pink powder, which was washed with Et 2 O (5 mL) and dried in vacuo. Yield: 0.062 g, 36%. Anal. Calc. for C 20

[Pb(phen) 2 (BF 4 ) 2 ]
Pb(BF 4 ) 2 as a 50% aqueous solution (0.197 g, 0.26 mmol) was added to a solution of 1,10-phenanthroline (0.095 g, 0.52 mmol) in CH 3 CN (5 mL), leading to the precipitation of a small amount of fine white powder. The reaction mixture was stirred for 2 h, then it was filtered and the solvent removed in vacuo to yield a white powder, which was washed with Et 2 O (5 mL) and dried in vacuo.

[Pb(phen) 2 (SiF 6 )]
A solution of 1,10-phenanthroline (0.100 g, 0.55 mmol) in CH 3 CN (5 mL) was added to Pb(SiF 6 )Á2H 2 O (0.102 g, 0.26 mmol) dissolved in deionised water (15 mL), giving a colourless solution. The reaction mixture was stirred for 2 h, then the solvent volume was reduced in vacuo until a white powder precipitated, which was isolated by filtration, washed with CH 3 CN and dried in vacuo.

X-ray crystallography
Details of the crystallographic data collection and refinement parameters are given in Table 1. Crystals suitable for single crystal X-ray analysis were obtained as described above. Data collections used a Rigaku AFC12 goniometer equipped with an enhanced sensitivity (HG) Saturn724+ detector mounted at the window of an FR-E+ SuperBright molybdenum (k = 0.71073) rotating anode generator with VHF Varimax optics (70 micron focus) with the crystal held at 100 K (N 2 cryostream). Structure solution and refinements were performed with either SHELX(S/L)97 or SHELX(S/L)2013 [20] and were straightforward, except where detailed below. H atoms bonded to C were placed in calculated positions using the default C-H distance and refined using a riding model. [{Pb(phen) 2 (SiF 6 )} 2 ]Á[Pb(phen) 2 (H 2 O) 2 (SiF 6 )]Á11H 2 O contains 13 water molecules, but it was not possible to refine hydrogens positions that formed a consistent hydrogen bonding network. Hydrogen atoms for all water molecules were therefore omitted form the refinement, but included in the formula. [Pb(terpy) 3 ] [BF 4 ] 2 has the cation disordered around the 3-fold axis with the coordinated terpy forming both a right and left handed spiral, each with 50% occupancy. ADPs of paired atoms were constrained to be the same. One of the BF 4 anions is disordered about the 3-fold axis, ADPs of paired atoms were constrained to be the same and distance restraints for some B-F distances were applied.
[Pb(terpy)(H 2 O)(SiF 6 )] contains an SiF 6 anion that is disordered rotationally about the coordinated F-Si-F axis. This was refined in two parts and the occupancies converged on ca. 50/50. Geometrical restraints (FLAT) and equal ADP constraints for paired atoms were applied.

Conclusions
Several new diimine complexes of the lead(II) salts, Pb(BF 4 ) 2 and Pb(SiF 6 ), have been structurally and spectroscopically characterised. All show significant fluoroanion coordination, leading to highly irregular geometries at Pb(II). The work has demonstrated that the dianionic SiF 6 2À can coordinate to Pb(II) to form monomer, dimer and chain polymer complexes, involving a variety of SiF 6 2À coordination modes. There was no evidence for fragmentation or hydrolysis of the fluoroanions in these systems, contrasting with Pb(II) macrocyclic systems [5]. These results suggest that the stable and readily available SiF 6 2À merits further study as a weakly coordinating dianion in other metal systems. The dianionic charge, which results in only half as many anions needed to balance the charge, is also capable of generating different structural motifs to those found with the monoanionic fluoroanions (BF 4 À , PF 6 À , etc.).