Heteroleptic lead(II)-halide complexes supported by a bulky iminoanilide ligand

Introduction

Lead had been extensively used in industry, as an anti-knock additive in petrol and in metal piping, until its high toxicity was judged prohibitive. The coordination chemistry of lead has, on the other hand, attracted little attention. It is only in the past two decades that bulky ancillary ligands have yielded low-coordinate lead(II) complexes and that their reactivity has been explored. Until then, lead(II) complexes had been plagued by light sensitivity and by a propensity to decompose and form insoluble lead(0) during uncontrolled redox processes.

Major advances in lead(II) coordination chemistry were achieved with monodentate terphenyl and bidentate β-diketiminato ligands, and a number of remarkable two- and three-coordinate complexes were structurally characterised. Terphenyls have, for instance, yielded the encumbered two-coordinate [Pb{Ar′}₂] (Simons et al., 1997) and [{Ar′}PbSi(SiMe₃)₃] (Klett et al., 1999), where Ar′=C₆H₃Mes₂-2,6; the halo-bridged dimers [{Ar″}Pb(μ-Br)]₂ (Pu et al., 2000) and [{Ar″}Pb(μ-I)]₂ (Filippou et al., 2004), where Ar″=C₆H₃Trip₂-2,6; and the two-coordinate organolead(II) compound [{Ar″}PbR], where R=Me, ^tBu, or Ph (Pu et al., 2000). More recently, the groups of Fulton and Roesky have reported on monometallic lead(II) compounds stabilised by ubiquitous β-diketiminates. The halides [{BDI^iPr}PbX], where X=Cl, Br, or I, were disclosed in 2007 (Chen et al., 2007); the compounds were obtained by salt metathesis between PbX₂ and [{BDI^iPr}Li]. The missing halide in this family (Figure 1), [{BDI^iPr}PbF], was later reported by Roesky and co-workers, upon fluorination of [{BDI^iPr}PbNMe₂] with pentafluoropyridine (Jana et al., 2011). A variety of terminal aryloxides (Fulton et al., 2007), alkoxides (Tam et al., 2009), phosphides (Yao et al., 2007; Tam et al., 2012), amides and anilides (Harris et al., 2014), and alkyl (Jana et al., 2009; Taylor et al., 2015) mononuclear compounds [{BDI^iPr}PbX] were also synthesised by salt metathesis reactions, most of them from the chloro complex [{BDI^iPr}PbCl]. Fulton and co-workers also revealed a rare case of lead(II) cation, [{BDI^iPr}Pb]⁺.[B(C₆F₅)₄]⁻ (Taylor et al., 2011). Their preparation of [{BDI^Me}PbCl] and [{BDI^H}PbCl] showed that bulky isopropyl groups in ortho positions of the β-diketiminate, as in {BDI^iPr}⁻, were not mandatory to obtain stable, monometallic lead(II) compounds (Tam et al., 2015). The role of mononuclear chloro complexes, in particular as precursors for salt metathesis reactions, was prominent in these studies. Finally, a case of Cl-bridged aminoanilido lead(II) dimer was reported (Vaňkátová et al., 2011), where the distances between each metal centre and the two bridging chlorides exhibited a large difference, 2.620(2) and 3.167(2) Å.

Figure 1:

Three-coordinate mononuclear lead(II) halides and their proligands.

We have used the iminoanilide {N^N^iPr}⁻ to synthesise a variety of alkaline-earth complexes (Figure 1). Our interest in this ligand framework stems in particular from its structural similarities with the versatile {BDI^iPr}⁻, although the more rigid nature of the backbone in {N^N^iPr}⁻ imparts original properties to this ligand (Liu et al., 2012, 2013). We have also developed aminophenolato (Wang et al., 2014) and organochalchogenolato (Pop et al., 2014) lead(II) complexes, as well as simple alkoxides devoid of bulky ancillary ligands (Wang et al., 2015), and have shown that they were competent molecular catalysts. As part of our ongoing program in main group chemistry, we report here on the synthesis and molecular solid-state structure of the chloro complex [{N^N^iPr}PbCl] and on its derivatisation to [{N^N^iPr}PbⁱPr].

Results and discussion

The room temperature treatment in THF of anhydrous PbCl₂ with an equimolar amount of {N^N^iPr}K, freshly prepared by deprotonation of the proligand with potassium hydride, afforded the chloro complex [{N^N^iPr}PbCl] (1) (Scheme 1). The product was isolated in 62% yield as analytically pure orange crystals upon recrystallisation from a pentane solution. The identity of 1 was established on the basis of nuclear magnetic resonance (NMR) spectroscopy and X-ray diffraction crystallography, and its purity was corroborated by combustion analysis. Pure, microcrystalline [{N^N^iPr}PbI] (2) was obtained in a similar fashion in 31% non-optimised yield upon reaction of PbI₂ and {N^N^iPr}K. Both compounds are readily soluble in common non-protic organic solvents, including aliphatic hydrocarbons.

Scheme 1:

Syntheses of [{N^N^iPr}PbCl] (1) and [{N^N^iPr}PbI] (2).

The NMR spectroscopic data for the two complexes were recorded in benzene-d₆. The ¹H NMR spectrum of complex 1 displays a diagnostic singlet at 8.21 ppm for the imine Ar-CH=N hydrogen, with a distinctive ³J_HPb coupling constant of 56 Hz consistent with the binding N_imine to the metal. The corresponding resonance appears at 8.11 ppm in the ¹H spectrum of complex 2, which exhibits broad resonances at 298 K, testifying to fluxional processes in solution. In the ¹³C{¹H} NMR spectra, the resonances for Ar-CH=N carbon atoms are detected at 167.17 and 168.22 ppm for 1 and 2, respectively. The ²⁰⁷Pb NMR spectra were recorded for the two complexes. No resonance could be detected for 2, perhaps because the complex starts decomposing to unidentifiable species in solution within 3–4 h, or perhaps owing to strong coupling to iodide (I=5/2 for ¹²⁷I), broadening the resonance that could not be distinguished from the baseline. We note that, similarly, the ²⁰⁷Pb resonance for [{BDI^iPr}PbI] has, to our knowledge, been reported neither in the seminal paper (Chen et al., 2007) nor later. On the other hand, the ²⁰⁷Pb resonance for 1 was detected at +1317 ppm, that is, in the region diagnostic of three-coordinate β-diketiminato lead(II) chlorides, compared, for instance, with the resonances for [{BDI^iPr}PbCl] (+1413 ppm; Jana et al., 2009) as well as [{BDI^Me}PbCl] and [{BDI^H}PbCl] (+1227 and +1388 ppm, respectively; Tam et al., 2015). The resonance for the Cl-bridged aminoanilide [{(2-Me₂NCH₂)C₆H₄N(SiMe₃)}Pb(μ-Cl)]₂ was somewhat deshielded (+1816 ppm), although this may result from the different nature of the ancillary ligand rather than from the four-coordinate environment of the metal centres in this dinuclear complex (Vaňkátová et al., 2011).

The molecular solid-state structure of the chloro complex 1 was established by single-crystal X-ray diffraction analysis. The asymmetric unit contains two similar, but not identical, molecules of [{N^N^iPr}PbCl]: 1a and 1b. The distance of the lead atom to the mean planes defined by N_anilide-C-C-C=N_imine, that is, 0.53 (for 1a) and 0.32 Å (for 1b), is the main difference between the two independent components. The structure of 1a is depicted in Figure 2, and only the geometric parameters for this component are discussed in detail. A summary of the interatomic distances and angles data for 1a and 1b, showing their close similarities, is collated in Table 1.

Figure 2:

Molecular solid-state structure of one of the two independent components (1a) in [{N^N^iPr}PbCl] (1).

H atoms are omitted for clarity. Selected interatomic distances (Å) and angles (degrees): Pb1-N1=2.246(6), Pb1-N9=2.336(6), Pb1-Cl1=2.5912(19), N1-C2=1.357(9), N1-C10=1.449(9), C2-C7=1.434(10), C7-C8=1.421(9), C8-N9=1.309(9), N9-C22=1.434(9), N1-Pb1-N9=80.60(2), N1-Pb1-Cl1=94.60(16), N9-Pb1-Cl1=84.98(15), C2-N1-C10=119.50(6), C2-N1-Pb1=131.80(5), C10-N1-Pb1=108.60(4), C8-N9-C22=118.60(6), C8-N9-Pb1=125.20(5), C22-N9-Pb1=115.70(4). Pb1 sits 0.53 Å above the N1-C2-C7-C8-N9 mean plane.

The chloride Cl1 sits in endo conformation, that is, roughly perpendicular to the N_anilide-C-C-C=N_imine plane (aka N1-C2-C7-C8-N9) as seen for other β-diketiminato Pb(II) halides. The Pb1-Cl1 bond length (2.5912(19) Å) in 1a compares well with those measured in Fulton’s β-diketiminates [{BDI^iPr}PbCl] (2.5653(7) Å) and [{BDI^Me}PbCl] (2.5757(11) Å), and it is much shorter than in the polymeric [{BDI^H}PbCl] (2.8081(11) and 2.9928(11) Å) (Tam et al., 2015). The Pb1-N1 distance is expectedly shorter for the negatively charged N_amide atom (2.246(6) Å) than the Pb1-N9 distance (2.336(6) Å) for the neutral N_imine atom. By comparison, the two pertaining Pb-N bonds are commensurate in β-diketiminato complexes, for example, in [{BDI^iPr}PbCl] (2.290(2) and 2.280(2) Å), in [{BDI^H}PbCl] (2.279(3) and 2.285(4) Å), or in [{BDI^Me}PbCl] (2.288(3) and 2.306(3) Å) (Tam et al., 2015). On the other hand, the pertaining Pb-N bonds were much longer (2.459(3) and 2.426(3) Å) in the dimeric complex [{L}Pb]₂ incorporating the dianionic N-{2-([4-N-ethylthiose-micarbazone]methyl)phenyl}-p-toluene-sulfonamide ligand {L}⁻ and featuring four-coordinate lead atoms (Pedrido et al., 2008). The geometry of N1 and N9 is trigonal planar, with the sum of the angles reaching, respectively, 359.9 and 359.5°. The geometry of the lead(II) atom is pyramidal, with the sum of the angles amounting to 260.2°. Note, however, that the angles show a certain deviation from the ideal value of 90° expected for three-coordinate lead(II) centres with purely 6s² unpaired electrons, thus showing some contribution of the empty p_z orbital to the electronic lone pair in 1a. The degree of pyramidalisation (DoP) of the metal in 1a, defined by DoP=360−∑i=13θ(i)/0.9 where θ(i) are the angles around lead(II) (Maksić and Kovačević, 1999), comes to 111%, exceeding by about 10%–15% those found in most chloro lead(II) β-diketiminates. However, both values are very similar to those measured in [{BDI^H}PbCl] (sum of angles=261°, DoP=110%) (Tam et al., 2015).

The existence of two distinct molecules, 1a and 1b, in the asymmetric unit of the molecular structure of 1 can be tentatively linked to the presence of the close molecules 1a′ and 1b′ in the crystal lattice (but in adjacent asymmetric units). Hence, the lead atoms, Pb1 in 1a and Pb2 in 1b, are located at short range of the chlorides Cl1′ and Cl2′ from the respective neighbouring symmetry-related molecules 1a′ and 1b′. Because of packing forces, the different distances Pb2···Cl2′ (3.353 Å; see Figure 3) and Pb1···Cl1′ (3.428 Å) may, hence, impact the respective positions of the lead atoms Pb1 and Pb2 with respect to the mean NCCCN plane. Note that these two distances are too long for a Pb-Cl bond. For instance, the Pb-Cl bond lengths in the polymeric [{BDI^H}PbCl], 2.8081(11) and 2.9928(11) Å (Tam et al., 2015), were much shorter than the Pb1···Cl1′ and Pb2···Cl2′ distances in 1a and 1b. On the other hand, in [{N(SiMe₃)C(Ph)C(SiMe₃)(C₅H₄N-2)}PbCl] bearing a pyridyl-1-azaallyl monoanionic ligand, the Pb-Cl bond lengths were in the range 2.599(2)–2.641(2) Å, whereas there was no interaction between the metal atoms and chlorides situated at a greater distance, 3.086(2)–3.173(2) Å (Leung et al., 2005).

Figure 3:

Representation of the molecular solid-state structure of one of the two independent components (1b) in [{N^N^iPr}PbCl] (1), showing the short Pb2···Cl2′ and Pb2′···Cl2 distances with the symmetry-related 1b′.

Selected interatomic distances (Å) and angles (degrees): Pb2-Cl2=2.574(2), Pb2-Cl2′=3.353(2), Cl2-Pb2-Cl2′=78.09(6). See Table 1 for other key parameters.

In an attempt to prepare a lead(II) alkyl, 1 was reacted with an equimolar amount of ⁱPrMgBr. The orange microcrystalline [{N^N^iPr}PbCH(CH₃)₂(THF)] (3) was isolated from pentane, but X-ray quality crystals eluded us. The identity of 3 was authenticated by its NMR data recorded in benzene-d₆. The ¹H NMR spectrum given in Figure 4 shows the formation of the targeted product. The high-field heptet at 0.01 ppm was assigned to [Pb]-CH(CH₃)₂. In the ¹³C{¹H} NMR spectrum, the metal-bound isopropyl generates resonances at 8.77 ppm for PbCH(CH₃)₂ and 25.34 ppm for PbCH(CH₃)₂. The presence of one molecule of THF that could not be removed in vacuo was detected in the ¹H and ¹³C{¹H} NMR spectra, with resonances at δ_1H=3.49 and 1.14 ppm and at δ_13C=69.66 and 24.82 ppm. These chemical shifts, different from those of free THF in benzene-d₆, suggest binding of THF to the metal. This would result in a 20-electron complex if the imine remained coordinated. The CH=N imine remains intact in 3, as corroborated by a singlet at 7.95 ppm in the ¹H NMR spectrum and a downfield resonance at 171.48 ppm in the ¹³C{¹H} spectrum.

Figure 4:

¹H NMR spectrum (benzene-d₆, 400 MHz, 298 K) of [{N^N^iPr}PbⁱPr] (3).

^†Silicone grease; ^‡residual pentane.

Conclusion

The iminoanilido complex [{N^N^iPr}PbCl] (1), a useful addition to the limited number of soluble three-coordinate lead(II) chlorides, has been synthesised and structurally characterised. This complex a priori opens access upon derivatisation to a larger panel of other lead(II) complexes, as exemplified by the preparation of a lead(II)-alkyl complex [{N^N^iPr}PbCH(CH₃)₂] (3); further examples can be anticipated from the extensive work in this area by the group of Fulton. The coordination sphere of 1 shows noticeable differences with that of the analogous β-diketiminato complex [{BDI^iPr}PbCl]. A summary of the coordination spheres in the iminoanilido [{N^N^iPr}PbCl] (1) and Fulton’s congeneric β-diketiminato [{BDI^iPr}PbCl] (Chen et al., 2007) is provided in Figure 5; only the most pertinent bond lengths and angles are given. In conjunction with the data obtained earlier with large alkaline earths (Liu et al., 2012, 2013), the results disclosed here illustrate that subtle differences of coordination patterns are to be expected when one or the other of the two structurally related ligand platforms {N^N^iPr}⁻ and {BDI^iPr}⁻ are used. This may bear substantial implications for applicative purposes, for instance in molecular catalysis where control of the first coordination sphere around the central metal takes a prominent role.

Figure 5:

Comparison of interatomic distances (Å) and angles between [{N^N^iPr}PbCl] (1) and Fulton’s directly related [{BDI^iPr}PbCl] (Chen et al., 2007).

Average values for the two independent components 1a and 1b are given for complex 1.

Experimental section