Martin Stanford

“… I suggest two possible changes that might make better practices in scientific publishing. A mechanism is needed to recognize the valuable task that reviewers perform and I would also like to have a brief explanation of the individual author contributions to each manuscript …” Read more in the Editorial by Richard N. Zare.

A strong σ-donor abnormal N-heterocyclic carbene (aNHC), 1,3-bis(2,6-diisopropylphenyl)-2,4-diphenylimidazol-5-ylidene (L1), reacted with B(C₆F₅)₃ in the presence of CO₂ and N₂O to form the adducts [aNHC·CO₂·B(C₆F₅)₃] (1) and [aNHC·N₂O·B(C₆F₅)₃] (2), respectively, in high yields at room temperature within a very short period of time. Furthermore, this aNHC/B(C₆F₅)₃ reagent system was found to be very efficient for the ring-opening of tetrahydrofuran (thf) and tetrahydrothiophene (tht), affording 3 and 4, respectively, within 5–10 min at room temperature in good yields. Interestingly, this aNHC forms the air-stable Lewis acid/base adduct [aNHC·B(C₆F₅)₃] (5) with B(C₆F₅)₃ in the absence of any suitable substrate upon keeping this mixture in solution. DFT calculations showed that the Lewis acid/base adduct 5 is energetically less stable than compounds 1–4, and this may be the driving force for the formation of compounds 1–4 in the presence of small molecules such as CO₂, N₂O, thf and tht. All the new compounds were characterized spectroscopically (¹H, ¹³C, ¹⁹F and ¹¹B NMR, IR spectroscopy) in solution and the solid-state structures were unambiguously established by single-crystal X-ray diffraction analysis.

The abnormal N-heterocyclic carbene, 1,3-bis(2,6-diisopropylphenyl)-2,4-diphenylimidazole-5-ylidene, reacts with B(C₆F₅)₃ in the presence of CO₂ or N₂O to form [aNHC·CO₂·B(C₆F₅)₃] or [aNHC·N₂O·B(C₆F₅)₃], respectively, in high yields at room temperature within a very short period of time.

Mononuclear Bi^I and Bi^II compounds remained elusive for a long time, but recently, the isolation and characterization of such species were reported. They can be handled in solution at ambient temperature and show exceptional properties, which are expected to reveal new reactivity patterns, enabling the development of new catalytic processes. TEMPO=2,2,6,6-tetramethyl-1-piperidinyl N-oxide.

Lewis-base-free diphosphinoalumane and 1-hydro-2-chlorophosphinoalumane derivatives bearing a bulky aryl substituent were synthesized by the reaction of the corresponding lithium phosphide and dichloroalumane. Structures of these phosphinoalumane derivatives were determined by spectroscopic and X-ray crystallographic analyses. Because of the efficient steric protection by the bulky aryl substituent, the aluminum centers in these phosphinoalumane derivatives have tricoordinate geometry. Reactions of the phosphinoalumane derivatives with organolithium reagents and bases were investigated.

Lewis-base-free diphosphinoalumane and 1-hydro-2-chlorophosphinioalumane derivatives bearing a bulky aryl substituent were synthesized from the corresponding lithium phosphide and dichloroalumane. Their structures were determined spectroscopically and by X-ray crystallography. Because of the efficient steric protection by the bulky aryl substituent, their aluminum centers are tricoordinate.

The storage of energy in a safe and environmentally benign way is one of the main challenges of today’s society. Ammonia–borane (AB=NH₃BH₃) has been proposed as a possible candidate for the chemical storage of hydrogen. However, the efficient release of hydrogen is still an active field of research. Herein, we present a metal-free bis(borane) Lewis acid catalyst that promotes the evolution of up to 2.5 equivalents of H₂ per AB molecule. The catalyst can be reused multiple times without loss of activity. The moderate temperature of 60 °C allows for controlling the supply of H₂ on demand simply by heating and cooling. Mechanistic studies give preliminary insights into the kinetics and mechanism of the catalytic reaction.

Two boron atoms collaborate: A highly efficient bis(borane) Lewis acid catalyst, which can be reused multiple times without loss of activity, catalyzes the release of 2.46 equivalents of H₂ per H₃N–BH₃ molecule. The dehydrogenation can be initiated and stopped on demand simply be heating to 60 °C or cooling to room temperature. Mechanistic studies provide insight into the mode of action of the catalyst.

Hückel π aromaticity is typically a domain of carbon-rich compounds. Only very few analogues with non-carbon frameworks are currently known, all involving the heavier elements. The isolation of the triboracyclopropenyl dianion is presented, a boron-based analogue of the cyclopropenyl cation, which belongs to the prototypical class of Hückel π aromatics. Reduction of Cl₂BNCy₂ by sodium metal produced [B₃(NCy₂)₃]²⁻, which was isolated as its dimeric Na⁺ salt (Na₄[B₃(NCy₂)₃]₂⋅2 DME; 1) in 45 % yield and characterized by single-crystal X-ray diffraction. Cyclic voltammetry measurements established an extremely high oxidation potential for 1 (E_pc=−2.42 V), which was further confirmed by reactivity studies. The Hückel-type π aromatic character of the [B₃(NCy₂)₃]²⁻ dianion was verified by various theoretical methods, which clearly indicated π aromaticity for the B₃ core of a similar magnitude to that in [C₃H₃]⁺ and benzene.

Chemical reduction of Cl₂BNCy₂ afforded Na₄[B₃(NCy₂)₃]₂⋅2 DME, which contains the triboracyclopropenyl dianion [B₃(NCy₂)₃]²⁻, a boron-based analogue of the prototypical Hückel 2π aromatic [C₃H₃]⁺. Both X-ray diffraction and density functional theory are indicative of BB multiple bonding and the presence of a cyclically delocalized 2π electron system.

Contrary to the classical silylene dimerization leading to a disilene structure, phosphine stabilized hydro- and chloro-silylenes (2 a,b) undergo an unique dimerization via silylene insertion into SiX σ-bonds (X=H, Cl), which is reversible at room temperature. DFT calculations indicate that the insertion reaction proceeds in one step in a concerted manner.

Contrary to the case of classical silylene dimerization to form a disilene, the phosphine-stabilized hydro- and chloro-silylenes undergo an unique dimerization via silylene insertion into SiX σ-bond (X=H, Cl), which is reversible at room temperature. DFT calculations indicate that the insertion reaction proceeds in one step in a concerted manner.

The reaction of the arylchlorosilylene–NHC adduct ArSi(NHC)Cl [Ar=2,6-Trip₂-C₆H₃; NHC=(MeC)₂(NMe)₂C] 1 with one molar equiv of LiPH₂^.dme (dme=1,2-dimethoxyethane) affords the first 1,2-dihydrophosphasilene adduct 2 (ArSi(NHC)(H)PH). The latter is labile in solution and can undergo head-to-tail dimerization to give [ArSi(H)PH]₂ 3 and “free” NHC. Further stabilization of 2 by complexation with {W(CO)₅} affords the isolable 1,2-dihydrophosphasilene–tungsten complex 4 [ArSi(NHC)(H)P(H)W(CO)₅]. Additionally, the new 1-silyl-2-hydrophosphasilene ArSi(NHC)(H)PSiMe₃ 5 could be synthesized and structurally characterized. DFT studies confirmed that the SiP bond in 2 and 4 is mostly zwitterionic with drastically decreased double-bond character.

Closer to the roots: 1,2-Dihydrophosphasilene derivative 1 could be synthesized from the corresponding arylchlorosilylene–N-heterocyclic carbene (NHC) adduct and LiPH₂ and is stabilized by NHCSi donor–acceptor complexation. Remarkably, the Si–P double-bond character in 1 is more like the betain-like resonance form 1′.

The reduction of N,C,N-chelated bismuth chlorides [C₆H₃-2,6-(CHNR)₂]BiCl₂ [where R=tBu (1), 2′,6′-Me₂C₆H₃ (2), or 4′-Me₂NC₆H₄ (3)] or N,C-chelated analogues [C₆H₂-2-(CHN-2′,6′-iPr₂C₆H₃)-4,6-(tBu)₂]BiCl₂ (4) and [C₆H₂-2-(CH₂NEt₂)-4,6-(tBu)₂]BiCl₂ (5) is reported. Reduction of compounds 1–3 gave monomeric N,C,N-chelated bismuthinidenes [C₆H₃-2,6-(CHNR)₂]Bi [where R=tBu (6), 2′,6′-Me₂C₆H₃ (7) or 4′-Me₂NC₆H₄ (8)]. Similarly, the reduction of 4 led to the isolation of the compound [C₆H₂-2-(CHN-2′,6′-iPr₂C₆H₃)-4,6-(tBu)₂]Bi (9) as an unprecedented two-coordinated bismuthinidene that has been structurally characterized. In contrast, the dibismuthene {[C₆H₂-2-(CH₂NEt₂)-4,6-(tBu)₂]Bi}₂ (10) was obtained by the reduction of 5. Compounds 6–10 were characterized by using ¹H and ¹³C NMR spectroscopy and their structures, except for 7, were determined with the help of single-crystal X-ray diffraction analysis. It is clear that the structure of the reduced products (bismuthinidene versus dibismuthene) is ligand-dependent and particularly influenced by the strength of the NBi intramolecular interaction(s). Therefore, a theoretical survey describing the bonding situation in the studied compounds and related bismuth(I) systems is included. Importantly, we found that the C₃NBi chelating ring in the two-coordinated bismuthinidene 9 exhibits significant aromatic character by delocalization of the bismuth lone pair.

Tune in to bismuth: Fine tuning of the structure of N,C,N- and N,C-chelating ligands allowed isolation of a unique set of organobismuth(I) compounds ranging from dibismuthene to di-coordinated bismuthinidene. The organobismuth compounds contain a BiC₃N ring, which has significant aromatic character as a result of the delocalization of the bismuth lone pair.

Activation of CO₂ by the bis(amidinato)silylene 1 and the analogous bis(guanidinato)silylene 2 leads to the structurally analogous six-coordinate silicon(IV) complexes 4 (previous work) and 8, respectively, the first silicon compounds with a chelating carbonato ligand. Likewise, CS₂ activation by silylene 1 affords the analogous six-coordinate silicon(IV) complex 10, the first silicon compound with a chelating trithiocarbonato ligand. CS₂ activation by silylene 2, however, yields the five-coordinate silicon(IV) complex 13 with a carbon-bound CS₂²⁻ ligand, which also represents an unprecedented coordination mode in silicon coordination chemistry. Treatment of the dinuclear silicon(IV) complexes 5 and 6 with CO₂ also affords the six-coordinate carbonatosilicon(IV) complexes 4 and 8, respectively.

CO₂ and CS₂ activation by the bis(amidinato)silylene 1 and the analogous bis(guanidinato)silylene 2 leads to the respective six- or five-coordinate silicon(IV) complexes 3–5. Compounds 3–5 were characterized by NMR spectroscopic studies in the solid state and in solution and by crystal structure analyses.

The synthesis of a phosphorus(III) compound bearing a N,N-bis(3,5-di-tert-butyl-2-phenoxy)amide ligand is reported. This species has been found to react with ammonia and water, activating the EH bonds in both substrates by formal oxidative addition to afford the corresponding phosphorus(V) compounds. In the case of water, both OH bonds can be activated, splitting the molecule into its constituent elements. To our knowledge, this is the first example of a compound based on main group elements that sequentially activates water in this manner.

Divide and conquer: A phosphorus(III) compound bearing a tridentate N,N-bis(3,5-di-tert-butyl-2-phenoxy)amide ligand is reported. This species has been found to react with ammonia and water activating the EH bonds (E=N and O, respectively) in both substrates to afford the corresponding phosphorus(V) compounds. In the case of water, both OH bonds can be activated.