Synthesis of M[1-H] complexes
Treatment of H2L with 1 equiv of Pb(HMDS)2 (HMDS = N(SiMe3)2) afforded an insoluble orange-red solid in 95% yield in weakly-coordinating solvents, such as benzene and toluene, which is likely polymeric or oligomeric {LPb}n. Upon “dissolution” in tetrahydrofuran (THF), this compound reacted with the solvent to form LPb(THF)2 (1-THF2; “1” respresents the “LPb” fragment). After recrystallization, 1-THF2 was obtained in 85% yield as an orange-red crystalline solid, and its identity was verified using elemental analysis, multinuclear nuclear-magnetic-resonance (NMR) spectroscopy, and X-ray crystallography. The 207Pb NMR spectroscopy of five-coordinate 1-THF2 revealed a single resonance at 422.8 ppm, which was shifted upfield compared with the resonance of three-coordinate, β-diketiminate-supported chloro-, amido-, and alkyl-plumbylenes (approximately 3000–1400 ppm)16-18. The coordination of THF molecules on the axial positions of “LPb” reveals its Lewis acidic nature.
An initial attempt to synthesize the hydride species was conducted by adding KBsBu3H (1 equiv.) to the THF solution of the compound {LPb}n at −35 °C. A rapid color change from orange-red to bright yellow was observed immediately upon the addition of KBsBu3H. The 1H NMR spectrum of the reaction mixture, which was obtained within 5 min, revealed formation of a new species (K[1-H]); a broad resonance was found at δ = 39.64 ppm (full width at half maximum H = 60 Ηz, see Supplementary Fig. S4). This species however decayed very quickly at room temperature (within 30 min) to produce a black insoluble powder, which was presumably lead metal, together with potassium salt K2L (potassium salt of dianionic L2–) and H2 as evidenced by 1H NMR spectroscopy. The extremely downfield-shifted 1H resonance is likely attributed to the hydride ligand at the Pb center. The 1H NMR spectra of the terphenyl-stabilized lead(II) hydrides reported by Wesemann12 and Power13 featured hydride signals located at approximately 33 to 35 ppm. The unusually high frequency of the 1H NMR shifts of the lead(II) hydrides was predicted theoretically by Vícha and Straka19, who described the spectroscopic abnormality of a lead(II) hydride species originating from the spin-orbit heavy atom on the light atom effect (i.e., SO-HALA effect)20-23. The 1H resonance of K[1-H] is close to theoretical predictions with respect to monomeric lead hydrides supported by tridentate pincer-type (33.8 ppm) and β-diketiminato (52.5 ppm) ligands19.
The formation of K2L as the decomposition product of K[1-H] implies that a potassium ion may have been initially substituted for the “PbH” moiety of the [LPbH]– ([1-H]–) anion of K[1-H], leading to a sequence of decomposition reactions. The interference of the potassium ion may be excluded by the use of crown ethers, which have high binding affinity toward alkali metal ions. The addition of KBsBu3H to {LPb}n in the presence of an excess amount of 18-crown-6 (3–4 equiv.) in THF rapidly produced a bright yellow solution, and no decomposition products was observed visually or by 1H NMR spectroscopy after 3 hours of reaction (Fig. 3). The reaction mixture was subjected to a workup, resulting in the production of [K18c6][1-H], a yellow powder, in 81% yield. Although [K18c6][1-H] was also soluble in benzene, it decomposed within 3 h. The 1H NMR spectrum of [K18c6][1-H] in d8-THF exhibited sharp resonance at δ = 41.43 ppm that was flanked by satellites resulting from 207Pb–1H coupling (1JPbH = 1312 Hz; Fig. 4a). To the best of our knowledge, this unusual 1H resonance is at the lowest field observed thus far for a diamagnetic compound. The coupling constant of [K18c6][1-H] was considerably higher than those reported for [ArPb(μ-H)]2 (700–750 Hz)12,13, and it was much lower than that reported for Me3PbH (2379 Hz)24. The magnitude of the spin–spin coupling constant (J) is directly related to the Fermi contact mechanism24-26, and the 1JPbH of the Pb–H bond is proportional to the percentage of 6s orbitals of Pb involved in this type of bonding. Thus, the coupling constants of low-valent lead(II) hydrides found in [K18c6][1-H] and [AriPrPb(μ-H)]2 were expected to be lower than that of tetravalent Me3PbH due to the pronounced inert-pair effect in Pb(II) species4. The high stability of [K18c6][1-H] allowed us to probe the 207Pb NMR resonance of this species. The 207Pb{1H} NMR spectrum of [K18c6][1-H] contains a single sharp resonance at 1450.1 ppm (Fig. 4b). To prove that this resonance is associated with the Pb center in [K18c6][1-H], a proton-coupled 207Pb NMR spectrum was obtained, and it exhibits a doublet signal centered at 1450.1 ppm with a 1JHPb of 1320 Hz; this finding was consistent with the coupling constant observed in the 1H NMR spectrum (Fig. 4c). On the basis of 1H-207Pb heteronuclear-multiple-quantum-coherence (HMQC) NMR experiments, we were able to observe the correlation between the hydride signal and Pb resonance (Fig. 4d). Furthermore, the 2H NMR spectrum of the deuterium-labeled species ([K18c6][1-D], vide infra) exhibited resonance at 41 ppm (Supplementary Fig. S19), and the species’ Pb signal in the 207Pb{1H} NMR spectrum was broad (H = 690 Ηz; Fig. 4e) due to deuteride coupling (nuclear spin I = 1). 1JPbD (~ 200 Hz) is expected to be approximately 1/6.5 as large as 1JPbH because the coupling constants of varying isotopes are proportional to their gyromagnetic ratios (γD/γH = 1/6.488). The 207Pb NMR resonance of [K18c6][1-H] is shifted upfield compared with that of [AriPrPb(μ-H)]2 (3736 ppm)12, and it is closer to that of the base-stabilized [AriPrPbH(NHC)] (NHC = 1,3,4,5-tetramethylimidazol-2-ylidene; δ = 834 ppm), which possesses higher coordination at the Pb center12.
Furthermore, the presence of the [1-H]– moiety of [K18c6][1-H] in the THF solution was probed by performing electrospray ionization–mass spectrometry in negative ion-mode, which revealed the presence of [1-H]– from [K18c6][1-H] (m/z = 720.33, Supplementary Fig. S23) and [1-D]– from [K18c6][1-D] (m/z = 721.42, Supplementary Fig. S24). Finally, infrared spectroscopy was performed to detect Pb–H and Pb–D vibrations. Although no clear Pb–H stretching frequency was identified, likely due to the overlap with other signals, a Pb–D vibration was identified at 1053 cm–1, which was close to the theoretically predicted value 1065 cm–1 (Supplementary Fig. S21). Collectively, these spectroscopic findings enabled the unambiguous assignment of the hydride ligand bound to the lead center in [K18c6][1-H].
The half-life (t1/2) of [K18c6][1-H] is approximately 2 days in THF at ambient temperature, as indicated by the disappearing hydride signal in the 1H NMR spectrum (Supplementary Fig. S12, vide infra). It is also noteworthy when only the equimolar of 18-crown-6 was employed in this reaction, the resulting lead hydride species decomposed within 2 h. This observation indicates that the necessary sequester of K+ was the key to stabilizing the [1-H]– moiety. When 1 equiv. of [2.2.2]-cryptand was employed instead of 18-crown-6 as the chelator for the potassium ion, a lead hydride species [([2.2.2]-cryptand)K][LPbH] ([Kcrypt222][1-H]) was isolated as a yellow powder in 90% yield (Fig. 3). Notably, the compound [Kcrypt222][1-H] is much more stable in solution and solid-state form than the compounds K[1-H] and [K18c6][1-H]. The solid form of [Kcrypt222][1-H] can be stored at room temperature and exhibited no observable decomposition for 30 days. Single crystals of [Kcrypt222][1-H] were obtained by performing pentane diffusion in the saturated THF solution at −35 °C, and X-ray crystallography clearly indicated that [Kcrypt222][1-H] as an anionic and monomeric plumbylene complex with [([2.2.2]-cryptand)K)] as the counter ion (Fig. 5a). The lead center is 0.215 Å out of the ligand plane (defined by three nitrogen atoms), and this deviation is larger than that observed in 1・THF2 (0.024 Å). Whereas complex 1・THF2 possesses two THF molecules bound to the axial positions, only one ligand (i.e., H– ligand) is likely bound to the Pb center and responsible for the displacement of Pb. The crystal packing diagram of [Kcrypt222][1-H] exhibits no close contact between the lead centers on separate molecules, indicating that [Kcrypt222][1-H] is a distinct, monomeric molecule in the solid-state structure (Supplementary Fig. S26b). The NMR features of [Kcrypt222][1-H] and [K18c6][1-H] are essentially identical (for the [1-H]– moiety, Supplementary Fig. S15), and the discussion of the spectroscopic characterization of the [1-H]– moiety focuses mainly on [K18c6][1-H] due to the limited solubility of [Kcrypt222][1-H] in THF. The employment of crown ethers to sequester K+ appeared to be a valid strategy for enhancing the stability of the [1-H]– anion. This inference was confirmed by adding KPF6 as a free K+ source to THF solutions containing [K18c6][1-H] and [Kcrypt222][1-H], which resulted in the immediate decomposition of the lead hydride species in both cases.
Several factors likely contributed to the uncommon stability of the [1-H]– moiety, which was observed despite a lack of commonly used sterically demanding substituents (such as the 2,6-diisopropylphenyl and 2,4,6-trimethylphenyl groups in the ligand system) that provide sufficient steric protection. First, dihydrogen bonding (DHB) interactions between the ortho hydrogens on the five-phenyl rings of the pyrrolyl donors (HPh) and hydride ligand (HPb) may have stabilized the Pb–H moiety (Fig. 6a). The 1H one-dimensional nuclear-Overhauser-effect spectroscopy (i.e., 1-D NOSEY) of [K18c6][1-H] in d8-THF at −20 °C revealed a correlation between HPb and HPh, suggesting they were in close proximity (Fig. 6b). In the density-functional-theory-optimized structure, two HPh were pointing toward HPb at a short distance of 2.317 Å, which was less than the sum of the van der Waals radii of H (2rH = 2.4 Å; Fig. 4a)27. Our natural bond orbital analysis results further supported the presence of DHB in this system. In particular, natural population analysis revealed that the charges in the hydride (−0.333) and ortho hydrogens (0.233) were consistent with our hypothesis for HPbδ–×××HPhδ+ interactions. The results of second-order-perturbation-energy analysis supported our assumption, indicating a donor–acceptor interaction of 1.76 kcal/mol from the Pb–H σ bonding pair to the σ* of ortho C–H bonds. Collectively, the experimental observations and computational results indicated the presence of DHB, which provided secondary coordination sphere interactions in the [1-H]– moiety and, consequently, stabilized Pb–H bonding. Second, strong donation from the dianionic pincer ligand L to the Pb center may have prevented homolytic cleavage of the Pb–H bond, which could have, in turn, led to an anionic [LPb×]– fragment and H× generating dianionic diplumbynes and H2. Furthermore, the kinetic stabilization provided by the phenyl groups on the pyrrolyl donor should not be neglected in the [1-H]– moiety.
Low-valent Ge and Sn hydride species readily undergo hydroelementation reactions with carbonyl compounds28-32. We discovered that the hydroplumbylation reaction of [1-H]– with benzaldehyde cleanly produced a Pb(II) benzyloxy compound [1-OBz]– in 92% yield (for M = [Kcrypt222]). Alternatively, this species could be obtained as orange-yellow crystals in 85% yield by adding Lewis basic potassium benzyloxide to {LPb}n in the presence of [2.2.2]-cryptand (Fig. 3). After hydride transfer to benzaldehyde, the characteristic downfield lead hydride signal in the 1H NMR spectrum was replaced by the CH2 resonance of the benzyloxy group at 5.37 ppm. The 207Pb NMR resonance of [Kcrypt222][1-OBz] was identified at δ = 847.3 ppm, indicating an upfield shift relative to [Kcrypt222][1-H] (1450.1 ppm). The molecular structure of [Kcrypt222][1-OBz], which was clarified using X-ray crystallography, revealed that the benzyloxy group was bound to the lead center with a Pb–O bond length of 2.153(4) Å (average of two independent molecules in the unit cell) and Npy–Pb–O angle (avg.) of 90.16(13)° (Fig. 5b). Interestingly, ortho hydrogens on the five-phenyl rings of the pyrrolyl donors (HPh) were pointing toward the oxygen atom with averaged HPhδ+×××Oδ– distances of 2.70 Å. These short distances between partially positive HPh and partially negative O were reminiscent of the aforementioned DHB interactions. Notably, [Kcrypt222][1-OBz] reacted cleanly with HBpin to regenerate the lead hydride species [Kcrypt222][1-H] and the borated product BzOBPin; this was verified through 1H NMR spectroscopy and mass spectrometry. The deuterium-labeled [1-D]– species was obtained similarly by using DBpin and was obtained in 94% yield. Remarkably, the successful generation of [1-H]– suggests the use of lead(II) hydride to perform the catalytic hydroboration of carbonyl compounds3.