Structural Lessons from Main-Group Metallocenes

In situ Grignard Metalation Method, Part II: Scope of the One-Pot Synthesis of Organocalcium Compounds

The in situ Grignard Metalation Method (iGMM) is a straightforward one-pot strategy to synthesize alkaline-earth metal amides in multi-gram scale with high yields via addition of bromoethane to an ethereal suspension of a primary or secondary amine and magnesium (Part I) or calcium (Part II). This method is highly advantageous because no activation of calcium is required prior to the reaction. Contrary to the magnesium-based iGMM, there are some limitations, the most conspicuous one is the large influence of steric factors. The preparation of Ca(hmds)₂ succeeds smoothly within a few hours with excellent yields opening the opportunity to prepare large amounts of this reagent. Side reactions and transfer of the iGMM to substituted anilines and N-heterocycles as well as other H-acidic substrates such as cyclopentadienes are studied. Bulky amidines cannot be converted directly to calcium amidinates via the iGMM but stoichiometric calciation with Ca(hmds)₂ enables their preparation.

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.

In Situ Grignard Metalation Method for the Synthesis of Hauser Bases

The in situ Grignard Metalation Method (iGMM) is a straightforward one‐pot procedure to quickly produce multigram amounts of Hauser bases R₂N‐MgBr which are valuable and vastly used metalation reagents and novel electrolytes for magnesium batteries. During addition of bromoethane to a suspension of Mg metal and secondary amine at room temperature in an ethereal solvent, a smooth reaction yields R₂N‐MgBr under evolution of ethane within a few hours. A Schlenk equilibrium is operative, interconverting the Hauser bases into their solvated homoleptic congeners Mg(NR₂)₂ and MgBr₂ depending on the solvent. Scope and preconditions are studied, and side reactions limiting the yield have been investigated. DOSY NMR experiments and X‐ray crystal structures of characteristic examples clarify aggregation in solution and the solid state.

Liquid-like properties of cyclopentadienyl complexes of barium: molecular dynamics simulations of nanoscale droplets

Cyclopentadienyl complexes of barium have great utility in materials science and engineering, in particular, as precursors in the atomic layer deposition processes, which are required to be fluidic as well as thermally stable and volatile. Here, we investigated the liquid-like properties of cyclopentadienyl barium complexes including (Me₅C₅)₂Ba, (^tBu₃C₅H₂)₂Ba, (ⁱPr₄C₅H)₂Ba, (ⁱPr₅C₅)₂Ba, and [(SiMe₃)₃C₅H₂]₂Ba, using molecular dynamics simulations of nanoscale droplets. The compounds were modeled using a recently developed generic force field, GFN-FF. Nanoscale droplets with about 5.0 nm diameters were formed by aggregating 96 molecules of each compound. Simulation results reveal that substituting methyl groups of (Me₅C₅)₂Ba with other alkyl and silyl moieties has a non-negligible effect on the intra- and intermolecular structure and dynamics. In particular, in contrast to more flexible (Me₅C₅)₂Ba, the substitution with five iso-propyl groups to form (ⁱPr₅C₅)₂Ba adds rigidity to the complex with restricted orientational fluctuations for two cyclopentadienyl ligands and arranges molecules parallel to each other with greater probability. In addition, comparison between (^tBu₃C₅H₂)₂Ba, with three tert-butyl groups, and its silyl analogue, [(SiMe₃)₃C₅H₂]₂Ba, reveals that intermolecular interactions between the molecules with silyl groups are softer than those with tert-butyl groups and result in broader radial distribution functions, whereas the dynamic properties are similar for both compounds. This work suggests that molecular dynamics simulations contribute to molecular-level understanding of the effect of chemical substitution in organometallic compounds on the intra- and intermolecular properties of molecular liquids.

Cyclopentadienyl Complexes of the Alkaline Earths in Light of the Periodic Trends

The electron-rich cyclopentadienyl and the analogous indenyl and fluorenyl ligands (collectively denoted here as Cp') have been impactful in stabilizing electron-deficient metal centers including the highly electropositive alkaline earths. Being in the s-block, the group 2 metals follow a major periodic variation in their atomic and ionic properties which is reflected in those Cp' compounds. This article presents an overview of this class of compounds for all the five metals from beryllium to barium (radium is excluded for its radioactivity), highlighting their systematic variation.

One-Step Synthesis and Schlenk-Type Equilibrium of Cyclopentadienylmagnesium Bromides

In the in situ Grignard metalation method (iGMM), the addition of bromoethane to a suspension of magnesium turnings and cyclopentadienes [C5 H6 (HCp), C5 H5 -Si(iPr)3 (HCpTIPS )] in diethyl ether smoothly yields heteroleptic [(Et2 O)Mg(CpR )(μ-Br)]2 (CpR =Cp (1), CpTIPS (2)). The Schlenk equilibrium of 2 in toluene leads to ligand exchange and formation of homoleptic [Mg(CpR )2 ] (3) and [(Et2 O)MgBr(μ-Br)]2 (4). Interfering solvation and aggregation as well as ligand redistribution equilibria hamper a quantitative elucidation of thermodynamic data for the Schlenk equilibrium of 2 in toluene. In ethereal solvents, mononuclear species [(Et2 O)2 Mg(CpTIPS )Br] (2'), [(Et2 O)n Mg(CpTIPS )2 ] (3'), and [(Et2 O)2 MgBr2 ] (4') coexist. Larger coordination numbers can be realized with cyclic ethers like tetrahydropyran allowing crystallization of [(thp)4 MgBr2 ] (5). The interpretation of the temperature-dependency of the Schlenk equilibrium constant in diethyl ether gives a reaction enthalpy ΔH and reaction entropy ΔS of -11.5 kJ mol-1 and 60 J mol-1 , respectively.

Optical and nonlinear optical properties of Ln(Tp)2, where Ln = La,…,Lu and Tp = tris(pyrazolyl)borate: a DFT+TD-DFT study

DFT calculations of electronic, structural, thermodynamic properties, magnetic moment, static and dynamic polarizability and hyperpolarizability of Ln(Tp)₂ (Ln = rare earths, Tp = ring-unsubstituted tris(pyrazolyl)borate) complexes.

Isolation and structural and electronic characterization of salts of the decamethylferrocene dication

Ferrocene and its decamethyl derivative [Cp*2Fe] are the most common standards for nonaqueous electrochemical investigations because of their well-defined and only mildly solvent-dependent reversible Fe(II)/Fe(III) redox couple. Higher oxidation states have only rarely been studied. We report the isolation and crystallographic and spectroscopic characterization of surprisingly stable Fe(IV) salts of the [Cp*2Fe](2+) dication, produced by oxidation of [Cp*2Fe] with AsF5, SbF5, or ReF6 in neat sulfur dioxide as well as [XeF](Sb2F11) in neat hydrogen fluoride. The Sb2F11(-) salt exhibits a metallocene with the expected mutually parallel arrangements of the Cp* rings, whereas the As2F11(-), AsF6(-), SbF6(-), and ReF6(-) salts manifest tilt angles ranging from 4° to 17°. Both (57)Fe Mössbauer spectroscopy and superconducting quantum interference device magnetization studies reveal identical d-orbital splitting with an S = 1, (3)E ground state based on the 3d electronic configuration e2g (3)a1g (1) of all [Cp*2Fe](2+) salts.

Heavy Grignard Reagents: Challenges and Possibilities of Aryl Alkaline Earth Metal Compounds

Compounds of the type aryl--M--X, with M=Ca, Sr, Ba and X as any kind of ligand (such as halide, phosphanide, amide, aryl), are presented. The low reactivity of the heavy alkaline earth metals calcium, strontium, and barium enforces an activation prior to use for the direct synthesis. The insertion of these metals into C--I bonds of aryl iodides (direct synthesis) yields aryl metal iodides and has to be performed at low temperatures and in THF. Aryl alkaline-earth-metal compounds show some characteristics: 1) the ease of ether cleavage enforces low reaction temperatures, 2) for Sr and Ba the Schlenk equilibrium is shifted towards homoleptic MI2 and MPh2, 3) high solubility of diaryl alkaline-earth-metal derivatives in THF even at low temperatures initiated quantum chemical investigations on the aggregation behavior, and 4) a strong low field shift of the 13C resonances of the ipso carbon atoms in NMR spectra was observed. First results from quantum chemical calculations on diaryl dicalcium(I) suggest a long Ca--Ca bond with a considerable Ca--Ca bond dissociation energy. Initial results on a selection of applications such as metallation, metathesis, and addition reactions of aryl calcium compounds are presented as well.

Aryl Calcium Compounds: Syntheses, Structures, Physical Properties, and Chemical Behavior

Organocalcium chemistry is still in its infancy. The direct synthesis of activated calcium and (substituted) iodobenzenes allows for the large-scale and high-yield synthesis of aryl calcium iodides. The influence of the substitution patterns of the phenyl group, halogen atom, and solvent is discussed. Aryl calcium iodides show a Schlenk equilibrium that enables the isolation of diaryl calcium derivatives. Owing to the high reactivity of aryl calcium halides, low temperatures have to be maintained throughout the preparative procedures in order to avoid side reactions. A decrease of reactivity and, hence, an enhanced stability at higher temperatures can be achieved by shielding of the calcium atom by increasing the coordination number of the metal center or by substitution of the iodide anion by bulky groups.

Recent developments in the field of organic heterobimetallic compounds of the alkaline-earth metals

Heterobimetallic compounds of the alkaline-earth metals show a wide structural variety with strongly differing reactivity patterns. The combination of magnesium and alkali metal amides yields cyclic molecules with an extreme high reactivity which often are considered as "inverse crowns" with the metal atoms as coordination sites for Lewis bases. In other metallates of the alkaline-earth metals an activation of alkyl groups succeeds. In alkaline-earth metal zincates an inverse coordination of the type M(2)[(mu-R)(2)ZnR](2) is observed and the alkyl groups are in bridging positions between zinc and the s-block metals thus forming a very reactive M-C-Zn three-center-two-electron bond. Furthermore, the metals of the carbon group form alkaline-earth metal-silicon, -germanium and -tin bonds or, in the presence of very strong Lewis bases, even solvent-separated ion pairs. For electronegative substituents at tin an inverse coordination mode such as M[(mu-R)(2)SnR](2) is observed.

100 years after grignard: where does the organometallic chemistry of the heavy alkaline Earth metals stand today?

Even a century after the discovery of the Grignard reagents, similar compounds of the heavy alkaline earth metals are still lacking. Owing to the industrial need for such compounds (in particular as anionic polymerization catalysts), the last few years have been marked by a rapid, extensive development which has culminated in the first structurally characterized alkylbarium derivatives such as the crown ether complex of bis(triphenylmethyl)barium (1). hmpt=hexamethylphosphoric acid triamide.

Syntheses and solid state structures of tris(pyrazolyl)methane complexes of sodium, potassium, calcium, and strontium: comparison of structures with analogous complexes of lead(II)

The reaction of NaI with 2 equiv of HC(pz)(3) or HC(3,5-Me(2)pz)(3) (pz = pyrazolyl ring) leads to the formation of [[HC(pz)(3)](2)Na](I) (1) and [[HC(3,5-Me(2)pz)(3)](2)Na](I) (2), respectively. Both compounds have trigonally distorted octahedral arrangements about the sodium. A similar reaction of KPF(6) with HC(3,5-Me(2)pz)(3) results in the formation of [[HC(3,5-Me(2)pz)(3)](2)K](PF(6)) (3), a complex also shown crystallographically to have a trigonally distorted octahedral arrangement about the potassium, which is an unusually low coordination number for this large metal ion. The complex [[HC(pz)(3)](2)Sr](BF(4))(2) (4) forms in the reaction of Sr(acac)(2) (acac = acetylacetonate) with HBF(4).Et(2)O followed by 2 equiv of HC(pz)(3). The structure is highly distorted, showing kappa(3) bonding of both tris(pyrazolyl)methane ligands and, in addition, interactions with the metal from three fluorine atoms from the BF(4)(-) counterions. The symmetrical structure of 1 and the nine-coordinate structure of 4 are both very different from the distorted, six-coordinate structure [[HC(pz)(3)](2)Pb](BF(4))(2), indicating that for this compound the lone pair on lead(II) is influencing the structure. The reaction of M(acac)(2) (M = Sr, Ca) with H[B[3,5-(CF(3))(2)C(6)H(3)](4)] followed by 2 equiv of HC(pz)(3) produces [[HC(pz)(3)](2)(Hacac)Sr][B[3,5-(CF(3))(2)C(6)H(3)](4)](2) (5) (when the reaction is done in CH(2)Cl(2)), [[HC(pz)(3)](2)(Me(2)CO)(2)Sr][B[3,5-(CF(3))(2)C(6)H(3)](4)](2) (6) (when the reaction is done in acetone), and [[HC(pz)(3)](2)(Hacac)Ca][B[3,5-(CF(3))(2)C(6)H(3)](4)](2)(7), respectively. The structures of all three complexes show a distorted eight-coordinate arrangement of the ligands about the metal. Crystal data: 1 is orthorhombic, Pnma, a = 16.931(1), b = 22.368(3), c = 7.937(2) A, alpha = 90, beta = 90, gamma = 90 degrees, Z = 4; 2 is trigonal, R3, a = 10.7483(8), b = 10.7483(8), c = 35.395(4) A, alpha = 90, beta = 90, gamma = 120 degrees, Z = 3; 3 is monoclinic, P2(1)/c, a = 9.144(4), b = 13.377(6), c = 15.988(7) A, alpha = 90, beta = 92.291(10), gamma = 90 degrees, Z = 2; 4 is hexagonal, P6(5), a = 9.42530(10), b = 9.42530(10), c = 55.3713(5) A, alpha = 90, beta = 90, gamma = 120 degrees, Z = 6; 5 is monoclinic, P2/n, a = 14.1601(3), b = 13.1756(3), c = 27.1826(6) A, alpha = 90, beta = 90.1744(7), gamma = 90 degrees, Z = 2; 6 is monoclinic, P2/n, a = 14.2709(7), b = 13.2646(7), c = 27.4189(13) A, alpha = 90, beta = 90.3850(10), gamma = 90 degrees, Z = 2; 7 is monoclinic, P2/n, a = 14.2388(2), b = 13.1919(2), c = 26.7879(3) A, alpha = 90, beta = 90.0650(8), gamma = 90 degrees, Z = 2.