Martin Stanford

Persistent radicals undergo hydrogen atom abstraction reactions with a great variety of substrates, but not with dihydrogen. It has now been found that the TEMPO radical splits dihydrogen under mild conditions in the presence of the strong bulky B(C₆F₅)₃ boron Lewis acid. The reaction is thought to proceed by a typical frustrated Lewis pair mechanism with the TEMPO radical acting as the active Lewis base. The reaction was analyzed by DFT, which indicates that no significant spin density on the hydrogen atoms is accumulated along the H₂ splitting reaction path.

Persistent radicals undergo hydrogen atom abstraction reactions with a great variety of substrates, but not with dihydrogen. It has now been found that the TEMPO radical splits dihydrogen under mild conditions in the presence of the strong bulky B(C₆F₅)₃ boron Lewis acid. The reaction is thought to proceed by a typical frustrated Lewis pair mechanism with the TEMPO radical acting as the active Lewis base.

The regioselective syntheses of 1,2-azaborinines is achieved using an unsymmetrical iminoborane through both catalytic and stepwise modular routes. The 1,2-azaborinine ring can be selectively functionalized in the 4- and/or 6-position through control of the stepwise reaction sequence, allowing access to vinyl-functionalized and redox-active, luminescent, donor-functionalized 1,2-azaborinines. The electrochemistry and photochemistry of a tetraarylamine-substituted 1,2-azaborinine are studied. Cyclic voltammetry of this compound, relative to a non-B,N-substituted reference molecule, showed an additional oxidation wave assigned to the oxidation of the azaborinine ring, while emission spectroscopy indicated that the azaborinine was significantly more fluorescent than the reference.

The regioselective synthesis of 1,2-azaborinines is achieved using an unsymmetrical iminoborane through both catalytic and stepwise modular routes. The 1,2-azaborinine ring can be selectively functionalized in the 2- and/or 4-position through control of the stepwise reaction sequence, allowing access to vinyl-functionalized and redox-active, luminescent, donor-functionalized 1,2-azaborinines.

Hydrogen can be selectively removed from organotin trihydrides to generate the corresponding organohydrostannylene intermediates. Depending on the size of the substituent and the mode of generation, the intermediates undergo further reactions. Herein, we report on the formation of a variety of organotin hydrides with tin in the oxidation states Sn^II, Sn^I–Sn^III and Sn^III–Sn^III, all accessed by the controlled removal of hydrogen from the tetravalent Ar′Sn^IV trihydride (Ar′=2,6-dimesitylphenyl, mesityl=2,4,6-trimethylphenyl).

Tin-tin time! Formation of a variety of organotin hydrides with tin in the oxidation states Sn^II, Sn^I–Sn^III, and Sn^III–Sn^III, all accessed by the controlled removal of hydrogen from the tetravalent Ar′Sn^IV trihydride (Ar′=2,6-dimesitylphenyl, mesityl=2,4,6-trimethylphenyl; see scheme), is reported.

The reaction of N-heterocyclic carbene (NHC)-stabilized hydrosilylene 1, tBu₃SiSi(H)NHC, with one and two equivalents of benzophenone gave rise to bicyclic compounds 2 and 3, tBu₃Si(R)[Si-CH₂-N(CMeCMeNMe)-CPh₂-O] (R = H, OCHPh₂). The NHC plays a crucial role in this reactivity, as it is directly involved in C–C bond formation and supplies the C–H-activated methyl group. In the formation of 3, the terminal Si–H bond of the hydrosilylene is involved in transition-metal-free hydrosilylation. All steps in these mechanisms are based on NHC-stabilized zwitterionic transition states and intermediates and were investigated by utilizing DFT calculations. In addition, the CO₂ activation of 1 to yield cis/trans-cyclotrisiloxane 4, [(tBu₃Si)(H)Si-O]₃, stereoselectively and its mechanistic study are reported.

Reaction of an N-heterocyclic carbene stabilized hydrosilylene with benzophenone yields a bicyclic compound through C–C bond formation and transition-metal-free hydrosilylation. In contrast, the reaction of the same silylene with carbon dioxide affords a cis/trans-cyclotrisiloxane by oxygen transfer. Both reaction mechanisms are studied in detail by DFT calculations.

The ionization of 1,1-dihydridocyclopentasilane 7 has been found to yield the cyclic polysilanylsilyl cation 8 instead of the expected hydrogen-substituted silylium ion 6. The silyl cation 8 is stabilized by the formation of an intramolecular Si−H−Si bridge, which also provides the thermodynamic driving force for its formation. In general, the preference for the formation of Si−H−Si bridges can be used to scavenge and identify transient intermediates in the Lewis acid induced rearrangement of polysilanes. The validity of this concept has been demonstrated for one central step in this chemistry, the ring-contraction reaction of cyclohexasilanes to form silylcyclopentasilanes.

The hydrogen trick: A new synthetic approach has been established for trapping highly reactive and short-lived cationic intermediates of sila-Wagner–Meerwein rearrangements of polysilanes (see figure). The preference for 2e–3c Si−H−Si bridges can be used to identify transient intermediates by low-temperature ²⁹Si NMR spectroscopy. This method allows for a general understanding of rearrangement processes in Lewis acid catalyzed polysilane transformations.

A simple method to access borylphosphonium iodides [RH₂P–BH₂·NMe₃]I (1a: R = Me; 1b: R = Et; 1c: R = nPr) by the addition of iodoalkanes to PH₂–BH₂·NMe₃ was developed. Complexes 1a–c were characterized by multinuclear NMR spectroscopy, and 1a and 1b additionally by single-crystal X-ray diffraction. It was possible to synthesize the Lewis-base-stabilized organosubstituted phosphanylborane MePH–BH₂·NMe₃ (2) from [MePH₂–BH₂·NMe₃]I (1a). Thermolysis of 2 generated a soluble, low-molecular-mass poly(alkylphosphinoborane) consisting of at least 40 repeat units, as identified by ESI-MS. These results are promising for the future preparation of a wide range of Lewis-base-stabilized phosphanylboranes, which are of interest as precursors to poly[(alkylphosphino)boranes] and are otherwise difficult to access by conventional metal-catalyzed methods.

A route for the synthesis of monoalkyl-substituted Lewis-base-stabilized phosphinoboranes via borylphosphonium precursors is presented. Mild thermolysis of the Lewis-base-stabilized methylphosphinoborane generates the corresponding polymer consisting of at least 40 repeat units, which exemplifies the applicability of this method.

The reduction of [Cp′′′Ni(η³-P₃)] (1; Cp′′′=η⁵-1,2,4-tBu₃C₅H₂) with potassium produces the complex anion [(Cp′′′Ni)₂(μ,η^2:2-P₈)]²⁻ (2), which contains a realgar-like P₈ unit. The anionic triple-decker sandwich complex [(Cp′′′Ni)₂(μ,η^3:3-P₃)]⁻ (3) with a cyclo-P₃ middle deck is obtained when 1 is treated with NaNH₂ as a nucleophile. Na[3] can subsequently be oxidized with AgOTf to the neutral triple-decker complex [(Cp′′′Ni)₂(μ,η^3:3-P₃)] (4). In contrast, 1 reacts with LiPPh₂ to give the anionic compound [(Cp′′′Ni)₂(μ,η^2:2-P₆PPh₂)]⁻ (5), a complex containing a bicyclic P₇ fragment capped by two Cp′′′Ni units. Protonation of Li[5] with HBF₄ leads to the neutral complex [(Cp′′′Ni)₂(μ,η^2:2-(HP₆PPh₂)] (6). Adding LiNMe₂ to 1 results in [Cp′′′Ni(η²-P₃NMe₂)]⁻ (7) becoming accessible, a complex which forms as a result of nucleophilic attack at the cyclo-P₃ ring of 1. The complexes K₂[2], Na[3], 4, 6, and Li[7] were fully characterized and their structures determined by single-crystal X-ray diffraction.

Stacking the deck: The reaction of [(η⁵-1,2,4-tBu₃C₅H₂)Ni(η³-P₃)] with potassium leads to the formation of a realgar-like P₈ ligand. Triple-decker sandwich complexes with a P₃ middle deck or unprecedented bicyclic P₇ ligands are formed with nucleophiles.

Reaction of the functionalized tetrel cluster [Ge₉R₃]^– {R = Si(SiMe₃)₃} with coinage metal N-heterocyclic carbene (NHC) complexes affords the first Zintl cluster transition metal complexes coordinated by NHC molecules. In [(η³-Ge₉R₃)M(NHC^Dipp)], the D_3d-symmetric cluster polyhedron coordinates with a triangular Ge₃ face to the coinage metals M = Cu, Ag, Au, which themselves also bind to the NHC. The role of fine tuning of electronic properties by using an NHC ligand is discussed for the transformation reaction of [(η³-Ge₉R₃)Ag(NHC^Dipp)] to yield [Ag(η³-Ge₉R₃)₂][Ag(NHC^Dipp)₂]. All complexes are fully characterized including single-crystal X-ray structure analyses.

Whereas intermetalloid cluster formation from ligand-free Zintl ions affords highly polar solvents, functionalized Zintl cluster anions allow for other solvents. Reactions with coinage metal N-heterocyclic carbene (NHC) complexes give a deep insight into the reactivity of Zintl clusters, as shown by the series [(η³-Ge₉R₃)M(NHC^Dipp)], M = Cu, Ag, Au and the subsequent transformation to [M(η³-Ge₉R₃)₂][M(NHC^Dipp)₂] for M = Ag.

The first carbodicarbene stabilized by flanking cyclopropenylidenes is reported. Tetraphenylcarbodicyclopropenylidene (2) is accessed by deprotonation of the corresponding triafulvene cyclopropenium salt, and has been spectroscopically characterized in [D₈]THF solution at −60 °C. Main-group and transition-metal complexes of 2 have been accessed, and have revealed the high sigma donating ability, and exclusive η¹ binding of this neutral all carbon ligand. Variable temperature NMR spectroscopy studies reveal varying degrees of free rotation in the flanking cyclopropenylidene groups of 2 in its coordination compounds.

Carbo loading: The first carbodicarbene stabilized by flanking cyclopropenylidenes is accessed by deprotonation of the corresponding triafulvene cyclopropenium salt. Main-group and transition-metal complexes of it have been accessed, and have revealed the high σ-donating ability, and exclusive η¹ binding of this neutral all carbon ligand.

The phosphorus/boron-substituted hexatriene systems 6 undergo thermally induced electrocyclic ring closure to yield the cyclohexadiene-derived P/B frustrated Lewis pairs (FLPs) 7. Subsequent TEMPO oxidation gives the phenylene-bridged FLPs 8. Both systems activate dihydrogen and the thermally robust FLPs undergo carbon–carbon coupling reactions at a mesityl group upon treatment with dimethyl acetylenedicarboxylate at elevated temperatures.

The phosphorus/boron-substituted hexatriene gave the phenylene-bridged P/B frustrated Lewis pairs (FLP) by thermally induced electrocyclic ring closure followed by oxidation with the 2,2,6,6-tetramethylpiperidine-N-oxyl radical. The thermally robust FLP reacted with dimethyl acetylenedicarboxylate by C−C coupling with the mesityl group (see picture).

The reaction of stibinidene and bismuthinidene ArM [where Ar=C₆H₃-2,6-(CH=NtBu)₂; M=Sb (1), Bi (2)] with transition metal (TM) carbonyls Co₂(CO)₈ and Mn₂(CO)₁₀ produced unprecedented ionic complexes [(ArM)₂Co(CO)₃]⁺[Co(CO)₄]⁻ and [(ArM)₂Mn(CO)₄]⁺[Mn(CO)₅]⁻ [where M=Sb (3, 5), Bi (4, 6)]. The pnictinidenes 1 and 2 behaved as two-electron donors in this set of compounds. Besides the MTM bonds, the topological analysis also revealed a number of secondary interactions contributing to the stabilization of cationic parts of titled complexes.

Generous donation: The synthesis, structure and bonding analysis of stibinidene and bismuthinidene ionic complexes of cobalt and manganese is presented. The pnictinidenes are coordinated as terminal ligands and act as two-electron donors in this set of compounds, an unprecedented feature in the chemistry of heavy Group 15 elements.

The construction of single-atom-thick sheets of boron on a silver substrate, which was published in late 2015, represents a significant advance towards the realization of useful two-dimensional materials based solely on boron. This Highlight provides background information on the topic of boron allotropes and an outlook for further work in this area.

A new stable heterocyclic germylene, in which the divalent germanium atom lies between a nitrogen atom and a phosphanylidene phosphorane group, was synthesized. Experimental and theoretical studies revealed the peculiar effect of phosphanylidene phosphorane substituent, which is a stronger π-donor towards germanium than an amino group is. Because of the weak phosphorus–germanium π-bond, this new germylene compound shows an enhanced reactivity compared to classical N-heterocyclic germylenes.

‘Germs’ of a different type: A stable heterocyclic germylene, in which the divalent germanium atom lies between a nitrogen atom and a phosphanylidene phosphorane group, was synthesized. Experimental and theoretical studies revealed the peculiar effect of the phosphanylidene phosphorane substituent, which presents π-donor abilities stronger than that of amino groups.

The red-colored tetraborane(4) [B₄(hpp)₄]^3+. (3; hpp=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate) with a rhomboid B₄ skeleton stabilized by four N donors, was synthesized by the reaction of the strong hydride abstraction reagent [(acridine)BCl₂][AlCl₄] with the electron-rich diborane(4) [HB(hpp)]₂ (1). The salt 3[AlCl₄]₃ was structurally characterized and the presence of unpaired electrons proven by EPR measurements. The unprecedented radical tricationic 3 is distinguished by a high positive charge and boron atoms in a low oxidation state (less than two).

Share and share alike: Reaction of the doubly base-stabilized diborane(4) [HB(hpp)]₂ with the strong hydride abstraction reagent [(acridine)BCl₂][AlCl₄] leads to the unprecedented red-colored tetraborane(4) [B₄(hpp)₄]^3+. with a rhomboid B₄ skeleton and four-center, five-electron bonding.

We report the preparation of N-heterocyclic carbene (NHC)-stabilized compounds containing P=B double bonds. The reaction of the highly functionalized phosphinoborane Mes*(SiMe₃)P−B(Cl)Cp* with Lewis bases allows access to base-stabilized phosphinidene boranes Mes*P=B(L)Cp* (L=4-dimethylaminopyridine (DMAP), NHC) by Me₃SiCl elimination. The formation of these species is shown to proceed through transient borylphosphide anions generated by Me₃Si abstraction.

Intermediate intercepted! Syntheses of a range of base-stabilized P=B doubly-bonded compounds proceeds by SiMe₃ abstraction (see scheme; Mes*=2,4,6-tritertbutylphenyl; Cp*=1,2,3,4,5-pentamethylcyclopentadienyl). The interception of an anionic borylphosphide intermediate offers insight into thermal Me₃SiCl elimination, for example in chemical vapor deposition processes.

The geminal frustrated Lewis pair tBu₂PCH₂B(Fxyl)₂ (1; Fxyl=3,5-(CF₃)₂C₆H₃) is accessible in 65 % yield from tBu₂PCH₂Li and (Fxyl)₂BF. According to NMR spectroscopy and X-ray crystallography, 1 is monomeric both in solution and in the solid state. The intramolecular P⋅⋅⋅B distance of 2.900(5) Å and the full planarity of the borane site exclude any significant P/B interaction. Compound 1 readily activates a broad variety of substrates including H₂, EtMe₂SiH, CO₂/CS₂, Ph₂CO, and H₃CCN. Terminal alkynes react with heterolysis of the C−H bond. Haloboranes give cyclic adducts with strong P−BX₃ and weak R₃B−X bonds. Unprecedented transformations leading to zwitterionic XP/BCX₃ adducts occur on treatment of 1 with CCl₄ or CBr₄ in Et₂O. In less polar solvents (C₆H₆, n-pentane), XP/BCX₃ adduct formation is accompanied by the generation of significant amounts of XP/BX adducts. FLP 1 catalyzes the hydrogenation of PhCH=NtBu and the hydrosilylation of Ph₂CO with EtMe₂SiH.

Complementary twins: The geminal frustrated Lewis pair tBu₂PCH₂B[3,5-(CF₃)₂C₆H₃]₂ (1) is accessible from tBu₂PCH₂Li and [3,5-(CF₃)₂C₆H₃]₂BF. Compound 1 readily activates element–element single, double, and triple bonds. Remarkably, CX₄ undergoes C−X bond heterolysis with formation of unprecedented XP/BCX₃ adducts (X=Cl, Br; see scheme).