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).

Sandwich, Triple-Decker and Other Sandwich-like Complexes of Cyclopentadienyl Anions with Lithium or Sodium Cations

Density functional theory, DFT, calculations were carried out on complexes containing cyclopentadienyl anions and lithium or sodium cations; half-sandwich, sandwich and sandwich-like complexes (among them triple-decker ones) are analyzed. Searches performed through the Cambridge Structural Database revealed that crystal structures containing these motifs exist, mostly structures with sodium cations. The DFT calculations performed here include geometry optimization and frequency calculations of the complexes at the ωB97XD/aug-cc-pVTZ level, followed by the partitioning of the energy of interaction via the Energy Decomposition Analysis scheme, EDA, at the BP86-D3/TZ2P level. Additional calculations and analyses were performed using both the Quantum Theory of Atoms in Molecules, QTAIM, and the Natural Bond Orbital analyses, NBO. The results of this work show that the electrostatic interaction energy is the most important attractive contribution to the total interaction energy of each of the complex systems analyzed here, and that complexation itself leads to minor electron charge shifts.

Expanding the series of alkali metal plumbolyl complexes to Na and K

Herein we detail the straightforward and scalable synthesis of sodium and potassium complexes of the 2,5-bis(tert-butyldimethylsilyl)-3,4-diphenylplumbolyl dianion (Pbl^TBS,Ph). Their solid-state structures were found to comprise either monomeric solvates or coordination polymers depending on the alkali metal ion and crystallization medium. These complexes were readily prepared with high yields and purity compared to known routes to the dilithium congener of Pbl^TBS,Ph and are well-positioned to serve as convenient precursors for salt metathesis-type reactions leading to metal complexes of the understudied Pbl^TBS,Ph ligand.

Introduction of plumbole to f-element chemistry

Herein, we present the synthesis and characterization of heteroleptic lanthanide complexes bearing a dianionic η5-plumbole ligand in their coordination sphere. The reaction proceeds via a salt elimination reaction between the dilithioplumbole ([Li(thf)]2[1,4-bis-tert-butyl-dimethylsilyl-2,3-bis-phenyl-plumbolyl] = [Li2(thf)2(η5-LPb)]) and specifically designed [Ln(η8-COTTIPS)BH4] precursors (Ln = lanthanide, La, Ce, Sm, Er; COTTIPS = 1,4-bis-triisopropylsilyl-cyclooctatetraenyl), that are capable of stabilizing a planar plumbole moiety in the coordination sphere of different trivalent lanthanide ions. In-depth ab initio calculations show that the aromaticity of the dianionic plumbole is retained upon coordination. Electron delocalization occurs from the plumbole HOMO to an orbital of mainly d-character at the lanthanide ion. The magnetic properties of the erbium congener were investigated in detail, leading to the observation of magnetic hysteresis up to 5 K (200 Oe s-1), an unequivocal proof for single molecule magnet behavior in this system. The magnetic behavior of the erbium species can be modulated by manipulating the position of the lithium cation in the complex, which directly influences the bonding metrics in the central [(η5-LPb)Er(η8-COTTIPS)]- fragment. This allowed us to assess a fundamental magneto-structural correlation in an otherwise identical inner coordination sphere.

Triple-decker complexes incorporating three distinct deck architectures

The reactivity of the dilithioplumbole ([Li₂(thf)₂(μ,η⁵-L^Pb)], L^Pb = 1,4-bis-tert-butyl-dimethylsilyl-2,3-bis-phenyl-plumbolyl) towards the reactive pnictogen precursors P₄, pentaphosphaferrocene, and pentaarsaferrocene ([Cp*Fe(η⁵-E₅)] (Cp* = η⁵-C₅Me₅, E = P, As)) is reported. The reaction with P₄ afforded a phospholyl lithium complex, via lead-phosphorus exchange, while the reactions with [Cp*Fe(η⁵-E₅)] yielded the first examples of Pb-Fe-Li heterotrimetallic triple-decker polypnictogenides with three different deck motifs.

Heavier element-containing aromatics of [4n+2]-electron systems

While the implication of the aromaticity concept has been dramatically expanded to date since its emergence in 1865, the classical [4n+2]/4n-electron counting protocol still plays an essential role in evaluating the aromatic nature of compounds. Over the last few decades, a variety of heavier heterocycles featuring the formal [4n+2] π-electron arrangements have been developed, which allows for assessing their aromatic nature. In this review, we present recent developments of the [4n+2]-electron systems of heavier heterocycles involving group 13-15 elements. The synthesis, spectroscopic data, structural parameters, computational data, and reactivity are introduced.

Metallaaromatic Chemistry: History and Development

Since the prediction of the existence of metallabenzenes in 1979, metallaaromatic chemistry has developed rapidly, due to its importance in both experimental and theoretical fields. Now six major types of metallaromatic compounds, metallabenzenes, metallabenzynes, heterometallaaromatics, dianion metalloles, metallapentalenes and metallapentalynes (also termed carbolongs), and spiro metalloles, have been reported and extensively studied. Their parent organic analogues may be aromatic, non-aromatic, or even anti-aromatic. These unique systems not only enrich the large family of aromatics, but they also broaden our understanding and extend the concept of aromaticity. This review provides a comprehensive overview of metallaaromatic chemistry. We have focused on not only the six major classes of metallaaromatics, including the main-group-metal-based metallaaromatics, but also other types, such as metallacyclobutadienes and metallacyclopropenes. The structures, synthetic methods, and reactivities are described, their applications are covered, and the challenges and future prospects of the area are discussed. The criteria commonly used to judge the aromaticity of metallaaromatics are presented.

Metallacyclopentadienes: synthesis, structure and reactivity

Metallacyclopentadienes, which possess two M-C(sp²) bonds and feature the structure of M(C[upper bond 1 start]R¹[double bond, length as m-dash]CR²-CR³[double bond, length as m-dash]C[upper bond 1 end]R⁴), are an important class of five-membered metallacycles. They are considered as both reactive intermediates in the stoichiometric and catalytic transformations of organic molecules and useful precursors to main group element compounds, and have received considerable attention in organometallic chemistry, coordination chemistry and synthetic organic chemistry over the past six decades because of their unique metallacyclic structure. This review comprehensively presents the synthesis, structure and reactivity of the s-, p-, d- and f-block metallacyclopentadienes distributed in the whole periodic table. In addition, their application in synthetic organic chemistry and polymer chemistry is summarized. This review aims to be beneficial for the design and synthesis of novel metallacyclopentadienes, and for promoting the rapid development of metallacyclic chemistry.

Transition-Metal Complexes Featuring Dianionic Heavy Group 14 Element Aromatic Ligands

The synthesis of dilithio-stannoles and -plumboles, dianionic aromatic compounds containing tin and lead atoms in their π-skeletons, opened a new field of transition-metal chemistry. Since the discovery of ferrocene (Cp₂Fe), which is composed of anionic aromatic ligands (Cp: cyclopentadienyl) and Fe(II), ferrocene-type sandwich complexes have long played important roles in many fields of chemistry. During the last few decades, the electronic and structural properties of the Cp ligand have been modified by introducing electron-donating, electron-withdrawing, and sterically encumbered substituents on the skeletal carbon atoms to obtain desirable properties of the resulting sandwich complexes. In terms of modifying the Cp ligand, we focused our attention on introducing a heavy group 14 atom into the π-skeleton. This idea was originally inspired by a question of whether or not aromaticity was retained after the replacement of a skeletal carbon atom by a heavy group 14 atom. After we succeeded in the synthesis of aromatic dilithio-stannoles and -plumboles, revealing that the concept of conventional aromaticity was expanded to lead-containing π-systems, we undertook the present project on applying these dianionic aromatic heavy Cp analogues as ligands for transition-metal complexes. The combination of a stannole and Cp*Ru units accomplished the creation of a neutral triple-decker complex and an anionic ruthenocene, which was not be accessible using Cp and its related ligands that are composed of only carbon atoms. The anionic ruthenocene reacted with electrophiles to afford ruthenocene-type sandwich complexes, and the structures of the stannole skeletons were highly dependent on the substituents on the tin atoms, in sharp contrast to the planar Cp ligand. The dianionic plumbole ligand was also found to function as an η⁵-coordinating ligand in an anionic ruthenocene, which is noteworthy in terms of incorporating the heaviest group 14 atom into a π-ligand to produce a ferrocene-type sandwich complex. The anionic ruthenocene bearing the plumbole ligand reacted with electrophiles to afford ruthenocene-type plumbole complexes, which have oxidation potentials lower than those of the corresponding tin analogues, demonstrating the effect of introduction of a lead atom heavier than a tin atom. In the reactions of dilithiostannoles with group 4 metals, the resulting complexes were found to have exotic electronic structures that cannot be constructed by the Cp ligand. The transition-metal complexes derived from dilithio-stannoles and -plumbole therefore exhibit remarkable differences as well as similarities to the traditional Cp-based transition-metal complexes. These results spotlight the introduction of heavy group 14 atoms into carbon-based π-skeletons, which can perturb the electronic properties of conventional transition-metal complexes and open a new chemistry of transition-metal complexes.

The aromatic dianion metalloles

Metalloaromatic species are unique and important both experimentally and theoretically. Significant progress has been made during the past few decades. New aromatic systems have challenged and extended the concept of aromaticity remarkably. In this perspective, recent results on the study of the dianion aromatic metalloles and their corresponding analogues are reviewed. These include the dilithio group 14 metalloles, group 13 metalloles and transition metal metalloles. X-ray crystallography has made a key contribution to the understanding of the structures. Various theoretical tools, such as NICS and AdNDP, make it possible to measure the aromaticity beyond Hückel's rule. The dianion butadiene skeletons play a key role in these metalloles and can be regarded as non-innocent ligands, which accept the electrons from the metal center and thus form the aromatic rings. By simply changing the central metals to different metals, the metallole analogues such as dicupra[10]annulenes and spiroaromatic palladoles can also be generated, which opens a door to synthesize other metalla-macrocyclic aromatics. Key challenges and envisioned opportunities for the future, such as applying these dianion metalloles as novel ligands of transition metals and generating new types of organometallic aromatic system, are also discussed.

Synthesis and reactivity of a ruthenocene-type complex bearing an aromatic π-ligand with the heaviest group 14 element

An anionic ruthenocene with an η⁵-coordinating plumbole ligand was prepared and reacted with electrophiles to afford plumbole complexes.

An anionic ruthenocene derived from a dilithioplumbole complex was prepared. In the complex, the plumbole ligand coordinates a ruthenium atom in an η⁵-fashion, similar to the cyclopentadienyl ligand in ferrocene. The ruthenocene that has the aromatic π-ligand with the heaviest group 14 element reacted with electrophiles to afford the plumbole complexes wherein the plumbole ligands show deviation from planarity, in contrast to the planar plumbole ring in the anionic ruthenocene. The bent angles of the plumbole ligands are dependent on the substituents on the lead atoms. Cyclic voltammetry measurements revealed that the plumbole complexes are oxidized more easily than the corresponding stannole complexes.

Reactivity of heavy carbene analogues towards oxidants: a redox active ligand-enabled isolation of a paramagnetic stannylene

We report the first isolated paramagnetic stannylene enabled by a redox active ligand.

Diverse coordination modes in tin analogues of a cyclopentadienyl anion depending on the substituents on the tin atom

The coordination mode of stannole monoanion ligands is highly dependent on the types of substituents on the tin atom.