Parallel Metallocenes of Germanium, Tin, and Lead

Benchmark Study on the Calculation of 207Pb NMR Chemical Shifts

A benchmark set for the computation of 207Pb nuclear magnetic resonance (NMR) chemical shifts is presented. The PbS50 set includes conformer ensembles of 50 lead-containing molecular compounds and their experimentally measured 207Pb NMR chemical shifts. Various bonding motifs at the Pb center with up to seven bonding partners are included. Six different solvents were used in the measurements. The respective shifts lie in the range between +10745 and -5030 ppm. Several calculation settings are assessed by evaluating computed 207Pb NMR shifts for the use with different density functional approximations (DFAs), relativistic approaches, treatment of the conformational space, and levels for geometry optimization. Relativistic effects were included explicitly with the zeroth order regular approximation (ZORA), for which only the spin-orbit variant was able to yield reliable results. In total, seven GGAs and three hybrid DFAs were tested. Hybrid DFAs significantly outperform GGAs. The most accurate DFAs are mPW1PW with a mean absolute deviation (MAD) of 429 ppm and PBE0 with an MAD of 446 ppm. Conformational influences are small as most compounds are rigid, but more flexible structures still benefit from Boltzmann averaging. Including explicit relativistic treatments such as SO-ZORA in the geometry optimization does not show any significant improvement over the use of effective core potentials (ECPs).

Pyramidanes: newcomers to the anti-van't Hoff-Le Bel family

In this feature article, an overview of the chemistry of pyramidanes, as a novel class of main group element clusters, is given. A general introduction sets the scene, briefly presenting the non-classical pyramidal geometry of tetracoordinate carbon, as opposed to the classical tetrahedral configuration. Pyramidanes, as the simplest organic compounds possessing a pyramidal carbon atom, are then discussed from both computational and experimental viewpoints, to show the theoretical predictions on the stability and thus the feasibility of pyramidanes has finally culminated in the isolation of the first stable representatives of the pyramidane family featuring heavy main group elements at the apex of the square pyramid. Synthetic strategies towards pyramidanes, as well as their peculiar structural features, non-classical bonding situations, and specific reactivity, are presented and discussed in this review.

Heavier N-heterocyclic half-sandwich tetrylenes

ansa-Half-sandwich complexes of the group 14 elements germanium, tin and lead are reported, which represent a new class of Lewis amphiphilic tetrylenes and bridge the gap between classical N-heterocyclic systems and group 14 metallocenes. These compounds can form complexes both with carbenes and transition metal fragments.

Benchmark Study on the Calculation of 119Sn NMR Chemical Shifts

A new benchmark set termed SnS51 for assessing quantum chemical methods for the computation of ¹¹⁹Sn NMR chemical shifts is presented. It covers 51 unique ¹¹⁹Sn NMR chemical shifts for a selection of 50 tin compounds with diverse bonding motifs and ligands. The experimental reference data are in the spectral range of ±2500 ppm measured in seven different solvents. Fifteen common density functional approximations, two scalar- and one spin-orbit relativistic approach are assessed based on conformer ensembles generated using the CREST/CENSO scheme and state-of-the-art semiempirical (GFN2-xTB), force field (GFN-FF), and composite DFT methods (r²SCAN-3c). Based on the results of this study, the spin-orbit relativistic method combinations of SO-ZORA with PBE0 or revPBE functionals are generally recommended. Both yield mean absolute deviations from experimental data below 100 ppm and excellent linear regression determination coefficients of ≤0.99. If spin-orbit calculations are not affordable, the use of SR-ZORA with B3LYP or X2C with ωB97X or M06 may be considered to obtain qualitative predictions if no severe spin-orbit effects, for example, due to heavy nuclei containing ligands, are expected. An empirical linear scaling correction is demonstrated to be applicable for further improvement, and respective empirical parameters are given. Conformational effects on chemical shifts are studied in detail but are mostly found to be small. However, in specific cases when the ligand sphere differs substantially between conformers, chemical shifts can change by up to several hundred ppm.

Removal of lead ions from water using thiophene-functionalized metal-organic frameworks

Heavy metal ion contamination in water is a hazard to all life. The removal of such ions from aqueous media by porous metal-organic frameworks (MOFs) is a growing field. Herein we report the use of thiophene containing MOFs, notably DUT-67, for the removal of Pb2+ ions from water under both batch and flow conditions with a max loading of 98.5 mg Pb2+ per g MOF. Solution-phase 207Pb NMR spectroscopy and an observed decrease in adsorption with non-thiophene containing systems support a mechanism of adsorption via a putative 6π-η5-thiophene-Pb2+ coordination.

Synthesis, structure and a nucleophilic coordination reaction of Germanetellurones

A germylene germanetellurone adduct L(Cp)GeTe(GeCl₂), a pentafluorophenyl gold(i) germanethione and germaneselone compounds L(Me)GeE(AuC₆F₅) (E = S and Se) are herein reported.

Lead-poisoned zinc fingers: quantum mechanical exploration of structure, coordination, and electronic excitations

Density functional theory (DFT) structure calculations and time-dependent DFT electronic excitation calculations on simple mononuclear lead structures confirm recent reports on the stabilization of tricoordinated structural domains in poisoned proteins. However, the possibility of the formation of tetracoordinated lead complexes should not be disregarded in studies on mechanisms of lead toxicity because structures with both coordination modes are plausible and might contribute to observed UV spectra. Reported calculations along with detailed molecular orbital analysis confirm that the intense UV signal at around 260 nm is an indicator of the ligand-to-metal charge transfer (LMCT) band where the electrons are transferred from the sulfur 3p orbital to the lead 6p orbital. The composition of the LMCT band reveals significant excitations not only from the Pb-S bonding orbitals but also from sulfur lone-pair orbitals to the Pb-S antibonding orbitals for which the electron density is largely localized on the Pb "6p-like" molecular orbitals. There is a solid indication that the stereochemically active pair orbital of lead is not strongly hybridized and remains largely of the 6s character in tricoordinated lead structures and is minimally hybridized in tetracoordinated lead structures. Computed UV spectra of lead model complexes are compared to experimental UV spectra of model lead peptides. The comparison shows a good agreement with the major spectral trends and changes observed in these experiments.

Control of the stereochemical impact of the lone pair in lead(II) tris(pyrazolyl)methane complexes. Improved preparation of Na[B[3,5-(CF3)2C6H3]4]

The reaction of Pb(acac)2 with 2 equiv of [H(Et2O)2][B[3,5-(CF3)2C6H3]4] (HBAr(f)) in CH2Cl2 followed by addition of 2 equiv of either HC(pz)3 or HC(3,5-Me2pz)3 (pz = pyrazolyl ring) leads to the formation of [Pb[HC(pz)3]2][B[3,5-(CF3)2C6H3]4]2 (1) and [Pb[HC(3,5-Me2pz)3]2][B[3,5-(CF3)2C6H3]4]2 (2), respectively. The cation in 1 has a distorted octahedral structure with a stereochemically active lone pair on lead(II). In contrast, the cation in 2 is trigonally distorted octahedral with the lone pair on the lead(II) clearly stereochemically inactive. The driving force for this cation to have a stereochemically inactive lone pair is that in this geometric arrangement the interligand distances between adjacent 3-position methyl groups are close to 4.0 A, the sum of the van der Waals radii of two methyl groups. To facilitate this chemistry, the synthesis of Na[B[3,5-(CF3)2C6H3]4], needed to prepare HBAr(f), has been dramatically improved. The main change is to add NaBF4 to the reaction mixture before forming the Grignard from the reaction of magnesium and 3,5-(CF3)2C6H3Br. The Grignard reacts with the NaBF4 as it forms, reducing the danger of explosion and leading to a higher isolated yield of the product. Crystallographic information: 1 is triclinic, P1, a = 13.0133(6) A, b = 17.2210(7) A, c = 24.7634(11) A, alpha = 71.7300(10) degrees, beta = 82.3630(10) degrees, gamma = 70.5120(10) degrees, Z = 2; 2 is triclinic, P1, a = 12.756(4) A, b = 13.469(4) A, c = 17.160(5) A, alpha = 82.454(7) degrees, beta = 89.904(8) degrees, gamma = 72.995(7) degrees, Z = 1.