Valence Tautomerism of p‐Block Element Compounds – An Eligible Phenomenon for Main Group Catalysis?

P(V)-bis(amidophenolate) ligand cooperation: stoichiometric CO-bond cleavage in aldehydes and ketones

The cooperation between a geometrically constrained, highly electrophilic phosphorus(V) center, and an electronically rich tetradentate bis(amidophenolate) ligand enables the cleavage of the CO bond from typical aldehydes and ketones delivering iminio phosphoramidate species. The amphiphilic nature of these products, which is demonstrated through their reaction with typical Lewis acids and bases, enables their use as a mild source of silylium cations from silanes, allowing the selective reductive coupling of aldehydes to ethers under catalytic conditions.

Syntheses and exploration of the catalytic activities of organotin(IV) compounds

Six organotin(IV) compounds (1-6) have been synthesized by reaction of the polydentate pro-ligands H3L and H2L, respectively, with the corresponding diorganotin chlorides. All of the compounds were characterized by FT-IR spectroscopy, 1H, 13C{1H}, and 119Sn (1H) NMR spectroscopy, HRMS spectrometry, and single-crystal X-ray diffraction. The solid-state structures show that all of the compounds are monomeric (except compound 3) and contain a penta-coordinated tin atom. Compound 3 is a dimer with two hexa-coordinated tin atoms. Compounds 1-3 contain a non-coordinated hydroxymethyl group. All of the compounds have been screened for their catalytic efficacy in the synthesis of 1,2 disubstituted benzimidazoles using o-phenylenediamine and aldehyde derivatives. It has been observed that both the Lewis acidic Sn(IV) centre and the hydroxymethyl group (hydrogen bond donor) catalyse the reactions with a product yield of up to 92%.

Gradual solid-state redox-isomerism in the lanthanide series

Oxidation of [(ArBIG-bian)2-Yb2+(dme)] (1) (ArBIG-bian = 1,2-bis[(2,6-dibenzhydryl-4-methylphenyl)imino]acenaphthene; dme = 1,2-dimethoxyethane) by 0.5 equivalent of Me2NC(S)S-S(S)CNMe2 in dme at ambient temperature affords a mixture of two products, [(ArBIG-bian)2-Yb3+{SC(S)NMe2}1-(dme)] and [(ArBIG-bian)1-Yb2+{SC(S)NMe2}1-(dme)], which represent two redox-isomers (2a and 2b, respectively). Their ratio in solution depends on the solvent as well as on the temperature. In the solid state, a decrease of temperature (350 → 100 K) caused an electron transfer from the Yb2+ ion to the ArBIG-bian radical-anion in isomer 2b to afford isomer 2a. Accordingly, the ratio of isomers 2a and 2b changes from 1 : 1 (350 K) to 3 : 1 (100 K). In contrast, in the dimer [(dme)(dpp-bian)1-Yb2+(μ-Cl)2Yb3+(dpp-bian)2-(dme)] (dpp-bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene), which is the sole example of a lanthanide complex that reveals solid-state redox-isomerism (valence tautomerism) reported so far, the electron transfer from the Yb2+ ion to the dpp-bian radical-anion takes place at around 150 K and is completed within a temperature interval of ca. 7 K.

Visible-Light Activation of Diorganyl Bis(pyridylimino) Isoindolide Aluminum(III) Complexes and Their Organometallic Radical Reactivity

We report on the synthesis and characterization of a series of (mostly) air-stable diorganyl bis(pyridylimino) isoindolide (BPI) aluminum complexes and their chemistry upon visible-light excitation. The redox non-innocent BPI pincer ligand allows for efficient charge transfer homolytic processes of the title compounds. This makes them a universal platform for the generation of carbon-centered radicals. The photo-induced homolytic cleavage of the Al-C bonds was investigated by means of stationary and transient UV/Vis spectroscopy, spin trapping experiments, as well as EPR and NMR spectroscopy. The experimental findings were supported by quantum chemical calculations. Reactivity studies enabled the utilization of the aluminum complexes as reactants in tin-free Giese-type reactions and carbonyl alkylations under ambient conditions, which both indicated radical-polar crossover behavior. A deeper understanding of the physical fundamentals and photochemical process was provided, furnishing in turn a new strategy to control the reactivity of bench-stable aluminum organometallics.

Double 1,2-Carbon Migration at Mixed Heavier Sn=Ge Vinylidenes

1,2-migration is one recurring isomerization reaction in organic chemistry. In contrast, double 1,2-migration remains rare and limited to transition-metal complexes. Herein, we describe the synthesis, characterization and reactivity of mixed heavier Sn=Ge vinylidenes. Double 1,2-carbon migration enables the isomerization of the stannagermenylidene (3) to the germastannenylidene (4). X-ray diffraction analysis and DFT calculations revealed that 3 and 4 feature a Sn=Ge double bond. The reaction of 3 with IMe4 (1,3,4,5-tetramethylimidazoline-2-ylidene) results in the electron redistribution in the Sn=Ge core to give the germylone-stannylene adduct (5). Moreover, treatment of 3 with 0.25 equiv. of (AlCp*)4 produces the heteronuclear aluminyl stannagermyne (6).

Valence Tautomerism Induced Proton Coupled Electron Transfer:X-H Bond Oxidation with a Dinuclear Au(II) Hydroxide Complex

We report the preparation and characterization of the dinuclear AuII hydroxide complex AuII 2(L)2(OH)2 (L=N,N'-bis (2,6-dimethyl) phenylformamidinate) and study its reactivity towards weak X-H bonds. Through the interplay of kinetic analysis and computational studies, we demonstrate that the oxidation of cyclohexadiene follows a concerted proton-coupled electron transfer (cPCET) mechanism, a rare type of reactivity for Au complexes. We find that the Au-Au σ-bond undergoes polarization in the PCET event leading to an adjustment of oxidation levels for both Au centers prior to C(sp3)-H bond cleavage. We thus describe the oxidation event as a valence tautomerism-induced PCET where the basicity of one reduced Au-OH unit provides a proton acceptor and the second more oxidized Au center serves as an electron acceptor. The coordination of these events allows for unprecedented radical-type reactivity by a closed shell AuII complex.

Mononuclear organogermanium(IV) catalysts for a [3 + 2] cycloaddition reaction

Three mononuclear Ge(IV) compounds, [(C6H5)2Ge(C13H8N2O4)] (1), [(C6H5)2Ge(C14H10N2O5)] (2), and [(C6H5)2Ge(C14H11NO3)] (3), have been synthesized by the reaction of pro-ligands H2L1 (C13H10N2O4), H2L2 (C14H12N2O5), and H2L3 (C14H13NO3) with (C6H5)2GeCl2 in the presence of triethylamine. All compounds were characterized by FT-IR spectroscopy and NMR spectroscopy. Single crystal X-ray diffraction analysis shows that the germanium(IV) atom exhibits a five-coordinated geometry in compounds 1 and 2. All compounds were screened as Lewis acid catalysts in the [3 + 2] cycloaddition reaction between sodium azide and various nitriles. The reactions resulted in the formation of 5-substituted 1H-tetrazoles with yields of up to 96%. Based on the experimental findings and DFT calculations, a plausible mechanism is proposed for the [3 + 2] cycloaddition reaction.

Heteroatom-Based Diradical(oid)s

Heteroatom-centered diradical(oid)s have been in the focus of molecular main group chemistry for nearly 30 years. During this time, the diradical concept has evolved and the focus has shifted to the rational design of diradical(oid)s for specific applications. This review article begins with some important theoretical considerations of the diradical and tetraradical concept. Based on these theoretical considerations, the design of diradical(oid)s in terms of ligand choice, steric, symmetry, electronic situation, element choice, and reactivity is highlighted with examples. In particular, heteroatom-centered diradical reactions are discussed and compared with closed-shell reactions such as pericyclic additions. The comparison between closed-shell reactivity, which proceeds in a concerted manner, and open-shell reactivity, which proceeds in a stepwise fashion, along with considerations of diradical(oid) design, provides a rational understanding of this interesting and unusual class of compounds. The application of diradical(oid)s, for example in small molecule activation or as molecular switches, is also highlighted. The final part of this review begins with application-related details of the spectroscopy of diradical(oid)s, followed by an update of the heteroatom-centered diradical(oid)s and tetraradical(oid)s published in the last 10 years since 2013.

A crystalline T-shaped planar group 14 anion

Isolable T-shaped planar pnictogen compounds R₃Pn were reported more than three decades ago and have been attracting burgeoning interest in recent years; T-shaped planar group 14 anions, isoelectronic to R₃Pn, however, are still unknown. Herein, we report the synthesis, full characterization, and reactivity of the first crystalline T-shaped planar group 14 anion 4 bearing a trinitrogen pincer ligand. DFT calculations indicate that the tricoordinate germanium center features both an unoccupied 4p orbital and two lone pairs of electrons. Its electron-rich nature allows for the nucleophilic attack on the methyl iodine giving methyl-substituted complex 5 and facile oxidation of the germanium center by elemental sulfur and selenium to furnish unpresented organic anions bearing terminal Ge[double bond, length as m-dash]Ch (Ch = S or Se) double bonds.

On the Dynamic Behavior of Pacman Phosphanes─A Case of Cooperativity and Redox Isomerism

In solution, the Pacman chlorophosphane (2Cl) shows fast exchange of the endo/exo-orientation of the two P-Cl bonds in the molecule featuring cooperativity. Experimental and quantum mechanical investigations of the inversion on the phosphorus(III) centers reveal a crucial role of chloride ions in the dynamic process. To confirm the results, the homologous Pacman halogen-phosphanes 2X were prepared by halogen exchange reactions (X = F, Br, and I). Besides accelerated dynamic behavior for the heavier analogues, significant differences in the molecular structure are caused by the halogen exchange reactions, including the formation of an endo-endo substituted Pacman fluorophosphane as well as dicationic species by phosphorus halogen bond dissociation. The latter process can be regarded as redox isomerism since two P^III atoms in 2X become P^V centers in the dications.

Stimuli Responsive Silylene: Electromerism Induced Reversible Switching Between Mono- and Bis-Silylene

Electromerism is a very well-known phenomenon in transition metal chemistry. In main group chemistry, this concept has only started getting attention recently. We report stimuli responsive low-valent silicon compounds exhibiting electromerism. A mixed-valent silaiminyl-silylene 1, [LSi-Si(NDipp)L] (L=PhC(N^t Bu)₂ ), was synthesized in a single step from amidinate-chlorosilylene. Compound 1 has two interconnected Si atoms in formally +I and +III oxidation states. Upon treatment with Lewis acidic Cu^I X (X=mesityl, Cl, Br, I), electron redistribution occurs resulting in the formation of [{LSi(NDipp)Si(L)}-CuX], in which both silicon atoms are in the +II formal oxidation state. Removal of the copper center from [{LSi(NDipp)Si(L)}-CuX] by using a Lewis basic carbene led to reformation of the precursor [LSi-Si(NDipp)L]. Thus, the process is fully reversible. This showcases the first example of Lewis acid/base-induced reversible electromerism in silicon chemistry.

Bis(catecholato)germane: An Effective Catalyst for Friedel-Crafts Alkylation Reaction

Bis(catecholato)germane, [Ge(C6H4O2)2(H2O)2] (1) was synthesized by the reaction of catechol and germanium oxide in water according to a reported method. Complex 1 was characterized by FT-IR spectroscopy, NMR spectroscopy and...