1,3-Bis(trimethylsilyl)-2-phenyl-1-aza-3-phosphapropenide Anions as Bidentate Ligands for the Alkaline Earth Metals Magnesium, Calcium, Strontium, and Barium

Benchmark study of popular density functionals for calculating binding energies of three-center two-electron bonds

Density functional theory (DFT) can be used to study the three-center two-electron (3c2e) bonding mode, which is universal in catalysts containing alkaline-earth (Ae) and boron-group (Bg) elements. However, because of the delocalization pattern of the 3c2e bond, the wavefunction cannot be accurately described by DFT methods. The calculated energies of Ae and Bg catalysts therefore fluctuate greatly when different functionals are used, largely because of inconsistent DFT-calculated binding energies of 3c2e bonds. Nevertheless, with the development of supercomputers and theoretical calculation software, the DFT method is becoming increasingly popular for studying Ae and Bg catalysts. In this study, we compared the performances of 21 functionals with the high-level composite G3B3 method in calculations for the binding energies of 3c2e bonds. Several frequently used post-Hartree-Fock methods were also tested. The calculation results indicate that the M06-2X, MN12-L, and MN15 functionals give consistent and reliable binding energies for common 3c2e bonds. © 2018 Wiley Periodicals, Inc.

Intensely Photoluminescent Diamidophosphines of the Alkaline-Earth Metals, Aluminum, and Zinc

The positively charged and weakly polarizable s-block metals commonly do not usually have phosphine ligands in molecular complexes. Herein, we report mono- and dinuclear small diamidophosphine complexes of the alkaline-earth metals Mg, Ca, and Sr, which were prepared from simple precursors and a phosphine-functionalized diamine ligand N,N-bis(2-(diphenyl-phosphino)phenyl)ethane-1,2-diamine (PNHNHP). The alkaline-earth metal based complexes [(PNNP)Mg]₂ and [(PNNP)M(thf)₃ ] (M=Ca, Sr), exhibit unusual coordination spheres and show bright fluorescence, both in the solid state and in solution. For comparison, the even stronger luminescent Al and Zn complexes [(PNNP)Zn]₂ and [(PNNP)AlCl] were prepared. Emission lifetimes in the nanosecond range and high photoluminescence quantum yields up to 93 % are observed at room temperature.

Alkaline earths as main group reagents in molecular catalysis

The past decade has witnessed some remarkable advances in our appreciation of the structural and reaction chemistry of the heavier alkaline earth (Ae = Mg, Ca, Sr, Ba) elements. Derived from complexes of these metals in their immutable +2 oxidation state, a broad and widely applicable catalytic chemistry has also emerged, driven by considerations of cost and inherent low toxicity. The considerable adjustments incurred to ionic radius and resultant cation charge density also provide reactivity with significant mechanistic and kinetic variability as group 2 is descended. In an attempt to place these advances in the broader context of contemporary main group element chemistry, this review focusses on the developing state of the art in both multiple bond heterofunctionalisation and cross coupling catalysis. We review specific advances in alkene and alkyne hydroamination and hydrophosphination catalysis and related extensions of this reactivity that allow the synthesis of a wide variety of acyclic and heterocyclic small molecules. The use of heavier alkaline earth hydride derivatives as pre-catalysts and intermediates in multiple bond hydrogenation, hydrosilylation and hydroboration is also described along with the emergence of these and related reagents in a variety of dehydrocoupling processes that allow that facile catalytic construction of Si-C, Si-N and B-N bonds.

Metal carbonyl complexes of phosphaamidines. Coordinative integrity detected in C-amino(λ3,σ2)-phosphaalkene isomers coordinated through n(P) HOMO−1 donor orbitals

L(Cr,Mo,W)(CO)₅coordination ofN,P-disubstituted phosphaamidines stabilizes molecules prone to isomerization. NMR spectra are consistent with the average mirror symmetry.

Stable cyclic carbenes and related species beyond diaminocarbenes

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ-donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.

Heterofunctionalization catalysis with organometallic complexes of calcium, strontium and barium

Despite the routine employment of Grignard reagents and Hauser bases as stoichiometric carbanion reagents in organic and inorganic synthesis, a defined reaction chemistry encompassing the heavier elements of Group II (M = Ca, Sr and Ba) has, until recently, remained unreported. This article provides details of the recent progress in heavier Group II catalysed small molecule transformations mediated by well-defined heteroleptic and homoleptic complexes of the form LMX or MX 2 ; where L is a mono-anionic ligand and X is a reactive σ-bonded substituent. The intra- and intermolecular heterofunctionalization (hydroamination, hydrophosphination, hydrosilylation and hydrogenation) of alkenes, alkynes, dienes, carbodiimides, isocyanates and ketones is discussed.

Synthesis of phosphaformamidines and phosphaformamidinates

A large-scale synthetic route to a variety of phosphaformamidines and phosphaformamidinates, a type of derivative that was not accessible by the methods previously known for preparing phosphaamidines and phosphaamidinates, is reported. Thermally stable ethyl N-arylformimidates 1 (ArN=CH(OEt), Ar = 2,4,6-(Me)3Ph or 2,6-(iPr)2Ph) readily reacted with lithium dialkyl- and diarylphosphanides to afford the corresponding N-aryl phosphaformamidines in 80 and 60% yield, respectively, whereas with lithium (aryl)(silyl)phosphanide, the N-aryl-N-silylphosphaformamidine (60% yield) was obtained. Addition of primary lithium arylphosphanides to 1 followed by addition of a stoichiometric amount of nBuLi gave rise to the respective phosphaformamidinates (70-88% yield). Methanolysis of the products afforded the N-aryl-N-hydrogenophosphaformamidines (90-95% yield). The solid-state structure of one of the phosphaformamidinates is also presented.

Syntheses and Structures of Strontium, Barium, and Europium Bis(diphosphanylamido) Complexes

Bis(diphosphanylamido) complexes of strontium and europium, [{(Ph(2)P)(2)N}(2)M(THF)(3)] (M = Sr (1), Eu (2)), have been prepared by reaction of [K(THF)(n)()][N(PPh(2))(2)] (n = 1.25, 1.5) and MI(2). The single-crystal X-ray structures of compounds 1 and 2 always show a eta(2) coordination of the ligand via the nitrogen and one phosphorus atom. In solution, a dynamic behavior of the ligand is observed, which is caused by the rapid exchange of the two different phosphorus atoms. As a result of the radius of the larger ion, treatment of [K(THF)(n)()][N(PPh(2))(2)] with BaI(2) gives the coordination polymer [{(Ph(2)P)(2)N}(2)Ba(THF){(Ph(2)P)(2)N}K](n)() (3). Two of the three {(Ph(2)P)(2)N}(-) ligands of compound 3 bind to the metal in eta(2) (N, P) while the third shows a heteroallylic (P, P) coordination mode. In the solid state, the infinite chain is formally held together by pi coordination of the phenyl rings to the potassium atoms.

Complexes of the Heavier Alkaline Earth Metals Ca, Sr, and Ba with O-Functionalized Phosphanide Ligands

Metathesis reactions between either SrI(2) or BaI(2) and 2 equiv of the potassium phosphanide [[(Me(3)Si)(2)CH]-(C(6)H(4)-2-OMe)P]K yield, after recrystallization, the complexes [[([Me(3)Si](2)CH)(C(6)H(4)-2-OMe)P](2)M(THF)(n)] [M = Sr, n = 2 (5); Ba, n = 3 (6)]. Similar metathesis reactions between MI(2) and 2 equiv of the more sterically demanding potassium phosphanide [[(Me(3)Si)(2)CH](C(6)H(3)-2-OMe-3-Me)P]K yield the chemically isostructural complexes [[([Me(3)Si](2)CH)(C(6)H(3)-2-OMe-3-Me)P](2)M(THF)(2)] [M = Ca (9), Sr (7), Ba (8)]. Compounds 5-9 have been characterized by multi-element NMR spectroscopy and X-ray crystallography. Complex 9 is thermally unstable and decomposes at room temperature to give the tertiary phosphane [(Me(3)Si)(2)CH](C(6)H(3)-2-OMe-3-Me)P(Me) and an unidentified Ca-containing product. Compounds 5 and 6 also decompose at elevated temperatures to give the corresponding tertiary phosphane [(Me(3)Si)(2)CH](C(6)H(4)-2-OMe)P(Me) and intractable metal-containing products. The decomposition of 5, 6, and 9 suggests that these compounds undergo an intramolecular methyl migration from the O atom in one phosphanide ligand to the P atom of an adjacent phosphanide ligand to give species containing dianionic alkoxo-phosphanide ligands.

An N,P-disubstituted-2-aminophosphaalkene and lithium and potassium complexes of the deprotonated “phosphaamidinate” anion

The reaction of DippPH2(Dipp = 2,6-iPr2C6H3) with DippN=C(p-CH3C6H4)Cl in refluxing xylenes affords DippP=C(p-CH3C6H4)N(H)Dipp; deprotonation with alkali metal reagents produces unique lithium and potassium complexes with the ligand in a different geometry to that of the free phosphaamidine.