An NMR and relativistic DFT investigation of one-bond nuclear spin–spin coupling in solid triphenyl group-14 chlorides

Chlorine-35 Solid-State Nuclear Magnetic Resonance Spectroscopy as an Indirect Probe of the Oxidation Number of Tin in Tin Chlorides

Ultrawideline ³⁵Cl solid-state nuclear magnetic resonance (SSNMR) spectra of a series of 12 tin chlorides were recorded. The magnitude of the ³⁵Cl quadrupolar coupling constant (C_Q) was shown to consistently indicate the chemical state (oxidation number) of the bound Sn center. The chemical state of the Sn center was independently verified by tin Mössbauer spectroscopy. C_Q(³⁵Cl) values of >30 MHz correspond to Sn(IV), while C_Q(³⁵Cl) readings of <30 MHz indicate that Sn(II) is present. Tin-119 SSNMR experiments would seem to be the most direct and effective route to interrogating tin in these systems, yet we show that ambiguous results can emerge from this method, which may lead to an incorrect interpretation of the Sn oxidation number. The accumulated ³⁵Cl NMR data are used as a guide to assign the Sn oxidation number in the mixed-valent metal complex Ph₃PPd^ImSnCl₂. The synthesis and crystal structure of the related Ph₃PPt^ImSnCl₂ are reported, and ¹⁹⁵Pt and ³⁵Cl SSNMR experiments were also used to investigate its Pt-Sn bonding. Plane-wave DFT calculations of ³⁵Cl, ¹¹⁹Sn, and ¹⁹⁵Pt NMR parameters are used to model and interpret experimental data, supported by computed ¹¹⁹Sn and ¹⁹⁵Pt chemical shift tensor orientations. Given the ubiquity of directly bound Cl centers in organometallic and inorganic systems, there is tremendous potential for widespread usage of ³⁵Cl SSNMR parameters to provide a reliable indication of the chemical state in metal chlorides.

Synthesis and Structure of Sn14Cl6(CH2SiMe3)12: Toward Nanoclusters of 4-Coordinate α-Sn

Orange crystals of a Sn₁₄ cluster have been isolated in up to 22% yield from a reaction between Me₃SiCH₂SnCl₃, SnCl₄, and LiAlH₄. The structure determined by single crystal X-ray diffraction shows three unique Sn atoms in a 6:6:2 ratio, with all Sn atoms 4-coordinate, similar to the tetrahedral bonding in elemental gray Sn. The solid state ¹¹⁷Sn MAS NMR spectrum shows the three types of distinct Sn atoms in the expected 3:3:1 intensity ratio with respective chemical shifts of 87.9, -66.6, and -607.1 ppm relative to Me₄Sn. The chemical shift of the two Sn atoms without ligands (bonded only to Sn), at -607.1 ppm, is the most upfield, and is the closest to the chemical shift, reported here, of bulk gray tin (-910 ppm). First-principles density functional theory calculations of the chemical shielding tensors corroborate this assignment. While the core coordination is distorted from the ideal tetrahedral arrangement in the diamond structure of gray tin, this Sn₁₄ cluster, as the largest reported cluster with all 4-coordinate Sn, represents a major incremental step toward being able to prepare atomically precise nanoparticles of gray tin.

Lead(II) complex formation with L-cysteine in aqueous solution

The lead(II) complexes formed with the multidentate chelator L-cysteine (H2Cys) in an alkaline aqueous solution were studied using (207)Pb, (13)C, and (1)H NMR, Pb LIII-edge X-ray absorption, and UV-vis spectroscopic techniques, complemented by electrospray ion mass spectrometry (ESI-MS). The H2Cys/Pb(II) mole ratios were varied from 2.1 to 10.0 for two sets of solutions with CPb(II) = 0.01 and 0.1 M, respectively, prepared at pH values (9.1-10.4) for which precipitates of lead(II) cysteine dissolved. At low H2Cys/Pb(II) mole ratios (2.1-3.0), a mixture of the dithiolate [Pb(S,N-Cys)2](2-) and [Pb(S,N,O-Cys)(S-HCys)](-) complexes with average Pb-(N/O) and Pb-S distances of 2.42 ± 0.04 and 2.64 ± 0.04 Å, respectively, was found to dominate. At high concentration of free cysteinate (>0.7 M), a significant amount converts to the trithiolate [Pb(S,N-Cys)(S-HCys)2](2-), including a minor amount of a PbS3-coordinated [Pb(S-HCys)3](-) complex. The coordination mode was evaluated by fitting linear combinations of EXAFS oscillations to the experimental spectra and by examining the (207)Pb NMR signals in the chemical shift range δPb = 2006-2507 ppm, which became increasingly deshielded with increasing free cysteinate concentration. One-pulse magic-angle-spinning (MAS) (207)Pb NMR spectra of crystalline Pb(aet)2 (Haet = 2-aminoethanethiol or cysteamine) with PbS2N2 coordination were measured for comparison (δiso = 2105 ppm). The UV-vis spectra displayed absorption maxima at 298-300 nm (S(-) → Pb(II) charge transfer) for the dithiolate PbS2N(N/O) species; with increasing ligand excess, a shoulder appeared at ∼330 nm for the trithiolate PbS3N and PbS3 (minor) complexes. The results provide spectroscopic fingerprints for structural models for lead(II) coordination modes to proteins and enzymes.

Lead(II) binding to the chelating agent D-penicillamine in aqueous solution

A spectroscopic investigation of the complexes formed between the Pb(II) ion and D-penicillamine (H2Pen), a chelating agent used in the treatment of lead poisoning, was carried out on two sets of alkaline aqueous solutions with CPb(II) ≈ 10 and 100 mM, varying the H2Pen/Pb(II) molar ratio (2.0, 3.0, 4.0, 10.0). Ultraviolet-visible (UV-vis) spectra of the 10 mM Pb(II) solutions consistently showed an absorption peak at 298 nm for S(-) → Pb(II) ligand-to-metal charge-transfer. The downfield (13)C NMR chemical shift for the penicillamine COO(-) group confirmed Pb(II) coordination. The (207)Pb NMR chemical shifts were confined to a narrow range between 1806 ppm and 1873 ppm for all Pb(II)-penicillamine solutions, indicating only small variations in the speciation, even in large penicillamine excess. Those chemical shifts are considerably deshielded, relative to the solid-state (207)Pb NMR isotropic chemical shift of 909 ppm obtained for crystalline penicillaminatolead(II) with Pb(S,N,O-Pen) coordination. The Pb LIII-edge extended X-ray absorption fine structure (EXAFS) spectra obtained for these solutions were well-modeled with two Pb-S and two Pb-(N/O) bonds with mean distances 2.64 ± 0.04 Å and 2.45 ± 0.04 Å, respectively. The combined spectroscopic results, reporting δ((207)Pb) ≈ 1870 ppm and λmax ≈ 298 nm for a Pb(II)S2NO site, are consistent with a dominating 1:2 lead(II):penicillamine complex with [Pb(S,N,O-Pen)(S-HnPen)](2-n) (n = 0-1) coordination in alkaline solutions, and provide useful structural information on how penicillamine can function as an antidote against lead toxicity in vivo.

Shielding and indirect spin-spin coupling tensors in the presence of a heavy atom: an experimental and theoretical study of bis(phenylethynyl)mercury

Magnetic shielding and indirect spin-spin coupling phenomena are tensorial properties and both their isotropic and anisotropic parts do affect NMR spectra. The involved interaction tensors, σ and J, can nowadays be theoretically calculated, although the reliability of such methods in the case of anisotropic parameters, Δσ and ΔJ, in systems involving heavy nuclei, yet demands testing. In this communication the results of the experimental and theoretical investigations of bis(phenylethynyl)mercury (I) labeled with (13)C isotope at positions neighboring Hg are reported. The theoretical calculations of molecular geometry and values of NMR parameters for I have been performed by the ZORA/DFT method, including the relativistic scalar and spin-orbit coupling contributions, using the PBE0 functional and TZP (or jcpl) basis set. These values have been confronted with the experimentally measured ones. The isotropic parameters have been measured by the standard (13)C and (199)Hg NMR spectra. The shielding anisotropies for the atoms in the central part of molecule I have been determined in a liquid sample using magnetic relaxation measurements. The relaxation data have been interpreted within the rotational diffusion theory, assuming the symmetrical top reorientation model. The anisotropies of one-bond (13)C-(199)Hg and two-bond (13)C-Hg-(13)C spin-spin couplings have been determined exploiting the temperature-dependent (13)C NMR spectra of I in the ZLI1167 liquid-crystal phase. We have found that our theoretical calculations reproduce experimental values of both isotropic and anisotropic NMR parameters very well.

Reconnaissance of diverse structural and electronic environments in germanium halides by solid-state 73Ge NMR and quantum chemical calculations

Solid-state 73Ge nuclear magnetic resonance (NMR) is an attractive technique for the characterization of solid germanium-containing materials, but experiments can be exceedingly difficult in practice due to the unfavourable NMR properties of the 73Ge nucleus. Presented herein is a series of solid-state 73Ge NMR experiments on germanium halides (GeX4 and GeX2, where X = I, Br, and Cl) conducted at moderate (9.4 and 11.7 T) and ultrahigh (21.1 T) magnetic fields, intended to characterize the 73Ge NMR response in highly symmetric and asymmetric coordination environments. Quadrupole coupling constants range from 0.16 to 35 MHz. Isotropic chemical shifts for the GeX4 series trend with halide electronegativity, as found for the analogous silicon and tin halides. The indirect spin-spin coupling constant 1J(73Ge, 127I) is estimated from 73Ge MAS NMR to be 35 ± 10 Hz in GeI2, with the reduced coupling constant agreeing with those of other group 14 halides. Quantum chemical calculations using GIPAW DFT are in reasonable accord with experimental quadrupole couplings, but fail for chemical shielding. A preliminary NMR crystallographic study of GeI2 and GeCl2 incorporating 127I and 35Cl NMR spectra has led to plausible conclusions reflecting the structural homology of these compounds, although definitive characterization remains elusive.

Measurement of delta(1)J((199)Hg, (31)P) in [HgPCy3(OAc)2]2 and relativistic ZORA DFT investigations of mercury-phosphorus coupling tensors

Using 31P solid-state NMR spectroscopy, anisotropy in the indirect 199Hg-31P spin-spin coupling tensor (DeltaJ) for powdered [HgPCy3(OAc)2]2 (1) has been measured as 4700 +/- 300 Hz. Zeroth-order regular approximation (ZORA) density functional theory (DFT) calculations, including scalar and spin-orbit relativistic effects, performed on 1 and a series of other related compounds show that DeltaJ(199Hg, (31)P) arises entirely from the ZORA Fermi-contact-spin-dipolar cross term. The calculations validate assumptions made in the spectral analysis of 1 and in previous determinations of DeltaJ in powder samples, namely that J is axially symmetric and shares its principal axis system with the direct dipolar coupling tensor (D). Agreement between experiment and theory for various 199Hg, 31P spin-spin coupling anisotropies is reasonable; however, experimental values of 1J(199Hg, 31P)(iso) are significantly underestimated by the calculations. The most important improvements in the agreement were obtained as a result of including more of the crystal lattice in the model used for the calculations, e.g., a change of 43% was noted for 1J(199Hg, 31P)(iso) in [HgPPh3(NO3)2]2 depending on whether the two or three nearest nitrate ions are included in the model. Finally, we have written a computer program to simulate the effects of non-axial symmetry in J and of non-coincidence of the J and D on powder NMR spectra. Simulations clearly show that both of these effects have a pronounced impact on the 31P NMR spectrum of 199Hg-31P spin pairs, suggesting that the effects should be observable experimentally if a suitable compound can be identified.

Probing lead(II) bonding environments in 4-substituted pyridine adducts of (2,6-Me2C6H3S)2Pb: an X-ray structural and solid-state 207Pb NMR study

The effect of subtle changes in the sigma-electron donor ability of 4-substituted pyridine ligands on the lead(II) coordination environment of (2,6-Me(2)C(6)H(3)S)(2)Pb (1) adducts has been examined. The reaction of 1 with a series of 4-substituted pyridines in toluene or dichloromethane results in the formation of 1:1 complexes [(2,6-Me(2)C(6)H(3)S)(2)Pb(pyCOH)](2) (3), [(2,6-Me(2)C(6)H(3)S)(2)Pb(pyOMe)](2) (4), and (2,6-Me(2)C(6)H(3)S)(2)Pb(pyNMe(2)) (5) (pyCOH = 4-pyridinecarboxaldehyde; pyOMe = 4-methoxypyridine; pyNMe2 = 4-dimethylaminopyridine), all of which have been structurally characterized by X-ray crystallography. The structures of 3 and 4 are dimeric and have psi-trigonal bipyramidal S(3)N bonding environments, with the 4-substituted pyridine nitrogen and bridging sulfur atoms in axial positions and two thiolate sulfur atoms in equatorial sites. Conversely, compound 5 is monomeric and exhibits a psi-trigonal pyramidal S(2)N bonding environment at lead(II). The observed structures may be rationalized in terms of a simple valence bond model and the sigma-electron donor ability of the 4-pyridine ligands as derived from the analysis of proton affinity values. Solid-state (207)Pb NMR experiments are applied in combination with density functional theory (DFT) calculations to provide further insight into the nature of bonding in 4, 5, and (2,6-Me(2)C(6)H(3)S)(2)Pb(py)(2) (2). The lead chemical shielding (CS) tensor parameters of 2, 4, and 5 reveal some of the largest chemical shielding anisotropies (CSA) observed in lead coordination complexes to date. DFT calculations using the Amsterdam Density Functional (ADF) program, which take into account relativistic effects using the zeroth-order regular approximation (ZORA), yield lead CS tensor components and orientations. Paramagnetic contributions to the lead CS tensor from individual pairs of occupied and virtual molecular orbitals (MOs) are examined to gain insight into the origin of the large CSA. The CS tensor is primarily influenced by mixing of the occupied MOs localized on the sulfur and lead atoms with virtual MOs largely comprised of lead 6p orbitals.