The tetrahedral geometry of organolead(IV) compounds can be readily transformed by using an organic ligand containing a dangling-arm oxygen functionality. The acidity of the Pb center results in so-called secondary bonding between O and Pb thereby pushing the geometry at Pb toward a trigonal bipyramidal (tbp) structure. Replacing a phenyl group by a chlorine atom dramatically enhances this phenomenon. Thus for (o-methoxybenzyl) triphenyllead (4), and (o-methoxybenzyl)diphenyllead chloride (5), the Pb–O internuclear distances are 3.362(4) and 2.845(3) Å, respectively; 83% (4) and 70% (5) of the sum of the van der Waals Pb and O radii. Within the group 14 element congeners the structural analysis of the (o-methoxybenzyl)triphenylE compounds, E = Si, Ge, Sn, and now Pb, demonstrates the relative acidities of E are Si < Ge < Sn < Pb.
Since the structural analysis of the pyridine adduct of trimethyltinchloride, PyMe3SnCl (Hulme 1963), there has been a consistent interest in the so-called hypercoordinated compounds of the heavy group 14 elements, where 5 and 6 coordinate materials exhibit interesting structural and chemical variations compared to the normal tetrahedral arrangement of these elements in their tetravalent state (Khan & Foucher 2016). In the case of silicon, the interest of early researchers (Chuit et al. 1993; Bassindale et al. 2003; 2010) was focused upon the study of proposed intermediates in the mechanism of displacement reactions at the silicon center. In the more studied case of tin, the search for entertaining structural motifs (Colton & Dakternieks 1988), potential biological activity (Li et al. 1996; Lébl et al. 2003; Raychaudhury et al. 2005; Vargas et al. 2017), and interesting new organotin polymers and oligomers (Khan et al. 2015), are all stimulants for this research activity.
Related studies on germanium and lead are less prevalent and indeed the study of organolead chemistry in general has become a quiet backwater within the general area of organometallic chemistry. Although a check on “organolead” in the current literature will result in many very interesting recent studies, they are focused upon lead perovskite salts, where the organo-ammonium cation reveals the reason for “organo”; however, no lead-carbon bond interactions are present (Cai et al. 2013). Reports on intramolecularly hyper-coordinated Pb compounds are very limited. An early such interaction was reported for (Me2NCH2CH2CH2)Ph2PbI, with a Pb–N internuclear distance of 2.677 Å (Figure 1A) (Zickgraf et al. 1998). A later report described the synthesis and structure of the related (o-Me2NCH2-C6H4)Ph2Pb(Cl) with a Pb–N internuclear distance of 2.64(18) Å (Figure 1B) (Cristea et al. 2009). In both examples the intramolecular Pb–N interaction is trans axial to the halogen atom in a structure transitioning to trigonal bipyramidal (tbp).
We have a longstanding interest in intramolecular hypercoordination within the group 14 elements and have reported on the variations of intramolecular interactions for the (omethylthiobenzyl)triphenyl-element system, (o-MeSC6H4CH2)EPh3, E = Si, Ge, Sn, Pb, Figure 2A, involving potential, and discovered, E–S interactions (Munguia et al. 2003). From this study it was clear that for the tetraorganometallics (E(C)4) the Lewis acidity of the central element was restricted to Sn and Pb, as might be expected due to the significantly more metallic character of these heavy elements. More recently a synthetic, structural, and DFT study of the related oxygen triphenylbenzyl derivatives (o-MeOC6H4CH2)EPh3, E = Si (1), Ge (2), Sn (3), Pb (4) (Figure 2B) detailed the extent of any intramolecular E–O interactions (López-Cardoso et al. 2018).
For the E = Si and Ge compounds, 1 and 2, both the experimental structural analysis and computational calculations resulted in no significant intramolecular interaction between E and the dangling arm O atom. The internuclear E–O distance was well above the sum of the van der Waals radii for E and O (Chauvin 1992), and the geometries at the central element E, were effectively tetrahedral, 98% (1); 96.5% (2). We had previously published the structure of the tin compound 3 and observed a distinct deviation of the geometry at Sn toward a trigonal bipyramidal structure and noted a Sn–O intermolecular distance of 3.074 Å, which represented 79% of the sum of the Sn and O VdW radii (Munguia et al 2007). Clearly the tin atom was, as predicted by the DFT calculations, considerably more acidic than both Si and Ge in the series 1, 2, 3, and 4.
Unfortunately, at the time of that article the lead compound, (o-MeOC6H4CH2)PbPh3 (4), was not available in crystalline form; however, the DFT calculation predicted a significant Pb–O interaction of 3.45 Å and a distortion from a tetrahedral structure at the Pb atom due to that intramolecular interaction.
We now report the structure of 4, and that of the related compound (o-MeOC6H4CH2)PbPh2Cl, 5, where the replacement of a phenyl group by chlorine atom will be expected to dramatically enhance any Lewis acidity of the lead atom and thereby augment any intramolecular Pb–O interaction.
MATERIALS AND METHODS
Experimental: Synthesis of o-methoxybenzylchlorodiphenyllead
A 10 mL screw capped vial equipped with a small magnetic stir bar was charged with 0.14 g (0.25 mmol) of o-methoxybenzyltriphenyllead, 4 (López-Cardoso et al. 2018), in 2 mL of CDCl3. To the vial, maintained at −25 oC, was added dropwise via a micro syringe 0.25 mL of (1.0 M) HCl solution in ether. The reaction was maintained at low temperature for 30 min. 207Pb NMR spectroscopic monitoring showed that the initial resonance of 4 at −152.1 ppm disappeared and was replaced by a new resonance at 83.8 ppm due to the formation of (o-MeOC6H4-CH2)PbPh2Cl. The white crystalline solid 0.085 g (66 % yield) was recrystallized from hexane/CDCl3 mixture. 1HNMR (CDCl3): 3.61(s, 3H, O-Me), 3.74 (s, 2H, CH2), 6.70–7.51 (complex multiplets, 14H, Ph); 13CNMR: 40.2 (CH2), 55.1 (O-Me), 109.5, 121.3,127.0, 127.8, 129.4. 129.9, 130.2, 125.8, 155.5, 159.2 (Ph); 207Pb: 83.8 ppm.
The X-ray intensity data of compound 4 was measured on a Photon 200 Bruker D8 VENTURE Duo system equipped with a microfocus (MoKα, λ = 0.71073 Å) and a HELIOS multilayer optics monochromator. The X-ray intensity data of compound 5 were measured on a Bruker SMART APEX CCD system equipped with a fine-focus tube (MoKα, λ = 0.71073 Å) and a graphite monochromator. In both cases, the frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. In addition, it was corrected for absorption effects using the multi-scan method (SADABS). The structure was solved with the intrinsic phasing method and refined using the Bruker SHELXTL Software Package.
RESULTS AND DISCUSSION
The synthesis and characterization of 4 was as previously described (López-Cardoso et al. 2018) involving the in-situ reaction between o-methoxybenzyl chloride and Ph3PbCl in the presence of an excess of magnesium turnings, Equation 1. The 1H and 13C NMR characterization of the material was as reported, with the exception that our 207Pb NMR exhibited a single resonance at −152.1 ppm, rather than that reported at -181 ppm. Compound 4 crystallized in space group P21/c and the ORTEP structural representation is illustrated in Figure 3 and selected crystallographic data are reported in Table 1.
Compound 4 has a single conformer in the asymmetric unit cell with a Pb–O distance of 3.362(4) Å. As noted above, the DFT calculations (using the basis set LACVP) on 4 predicted an intramolecular Pb–O interaction of 3.45 Å. This longer than experimental internuclear distance was typical of the other calculated vs experimental values for all the group 14 series of compounds, and probably reflects the current inadequacy of the basis sets used for the heavy element materials and possible crystal packing forces. However, the result is that compound 4 does exhibit a significant intramolecular hypercoodination as predicted. Furthermore, the geometry at the Pb atom exhibits an experimental value of 74% Td structure, vide infraTable 3 and further discussion, illustrating the strength of the intramolecular Pb–O interaction even in this tetraorganolead material.
The transformation of 4 to the monochloro derivative (o-MeOC6H4CH2)PbPh2Cl (5) was readily accomplished in good yield by treatment of 4 with one equivalent of an HCl/Et2O solution, Equation 2.
The 1H, 13C and 207Pb NMR data for 5 are in accord with the formulation. In particular the 207Pb NMR chemical shift of 83.8 ppm exhibits the change expected for the transformation of the tetraorganolead 4 (with limited hypercoordination) to a triorganolead chloride with significant hypercoordination (Margolis et al. 2003). These NMR trends are similar to those well-established by 119Sn NMR studies on related materials (Holaćek et al. 1983; Colton & Dakternieks 1988; Munguia et al. 2007).
The ORTEP representation of the structure of 5, also with a single conformer in the unit cell, is illustrated in Figure 4 and exhibits a much closer Pb–O internuclear distance of 2.845 Å, compared to 4 at 3.362 Å, and a significantly lower Td structure (31%, Table 2), i.e. it is much closer to a tbp geometry.
In Table 2 we summarize the pertinent structural data for 4 and 5, along with the data for 1, 2 and 3 for ready comparison. The estimates of Td geometries involve comparing the sum of the equatorial angles for tetrahedral and trigonal bipyramidal, 328.5 and 360° respectively, to the observed values. The resulting data are in accord with the increasing Lewis acidity of the central element and intramolecular E–O interactions; the replacement of Ph group in 4 by Cl in 5 is dramatic.
We performed DFT calculations using the LanL2DZ basis set from the Gaussian 09 system (Frisch et al. 2010), which has been suggested for heavy elements such as Pb (Wadt & Hay 1985; Lyczko 2017). These computations, Table 3, predicted an intramolecular Pb–O distance in 4 of 3.54 Å, and 2.69 Å for 5, somewhat less accurate than the LACVP basis set used by López-Cardoso et al. The data do however predict the significantly reduced Pb–O internuclear distance upon substituting a Ph group of 4 by a chlorine in 5. The other geometrical aspects of the two molecules 4 and 5 are reasonably in agreement with the experimental data in terms of bond lengths and angles, and also illustrate the reduced percentage of Td structure from 4 (91%) to 5 (36%) using the Eq method.
Overall, these results confirm the gradually increasing acidity of the elements in the order Si < Ge < Sn <Pb with respect to both O (this work) and S (Munguia et al. 2003) dangling arm ligands in homologous systems. Both the decreasing Pb–O internuclear distances and the increasing tbp geometry at Pb arrive at the same conclusion. The synthesis, and structural/spectroscopic analysis, of the new organolead compound 5 further confirms the tendency for increasingly electronegative groups at the central Pb center to augment the Lewis acidity of that group 14 metal.
We are thankful for support by the Welch Foundation (Grant AH-0546), the Kresge Foundation for upkeep of our NMR facility, and the NSF MRI program (CHE-1827875 for purchase the VENTURE system.