Martin Stanford

The geminal chelate bis-borylalkanes 4 and 5 featuring strongly electrophilic B(C₆F₅)₂ and B(C₆F₅) groups, respectively, serve as efficient catalysts for the borylation of arenes and heteroarenes. The borylation reactions proceed under mild conditions with liberation of dihydrogen.

Geminal pentafluorophenyl-containing bis-borylalkanes efficiently catalyze the borylation reaction of a variety of arenes and heteroarenes at RT with liberation of dihydrogen.

The reaction of the bulky diphosphenes (Rind)P=P(Rind) (1; Rind=1,1,3,3,5,5,7,7-octa-R-substituted s-hydrindacen-4-yl) with two molecules of N-heterocyclic carbene (NHC; 1,3,4,5-tetramethylimidazol-2-ylidene) resulted in the quantitative formation of the NHC-bound phosphinidenes NHCP(Rind) (2), along with the cleavage of the P=P double bond. The reaction times are dependent on the steric size of the Rind groups (11 days for 2 a (R=Et) and 2 h for 2 b (R=Et, Me) at room temperature). The mechanism for the double bond-breaking is proposed to proceed via the formation of the NHC-coordinated, highly polarized diphospehenes 3 as an intermediate. Approach of a second NHC to 3 induces P−P bond cleavage and P−C bond formation, which proceeds through a transition state with a large negative Gibbs energy change to afford the two molecules of 2, thus being the rate-determining step of the overall reaction with the activation barriers of 80.4 for 2 a and 29.1 kJ mol⁻¹ for 2 b.

P=P bond-breaking: Bulky R₈-s-hydrindacen-4-yl (Rind)-substituted diphosphenes (R=Me, Et) react with two equivalents of N-heterocyclic carbene (NHC) to produce the NHC-coordinated phosphinidene adducts. The mechanism for P=P cleavage through formation and breaking of NHC-coordinated, highly polarized diphosphene intermediates is proposed.

There is growing interest in compounds containing functionalized E=E multiple bonds (E=Si, Ge, Sn, Pb) because of their potential to exhibit novel physical and chemical properties. However, compounds containing multiple functionalizations are rare, with scarcity increasing with increasing degree of substitution. The first ditetrelene R₂E=ER₂ in which the E=E bond is substituted by four heteroatoms (other than Si) is described. The tetraphosphadisilene {(Mes)₂P}₂Si=Si{P(Mes)₂}₂ (7) is readily isolated from the reaction between SiBr₄ and [(Mes)₂P]Li, the latter of which acts as a sacrificial reducing agent. The structure of 7 is presented, while the bonding in, and stability of 7 were probed using DFT calculations.

Tetraphosphadisilene compound {(Mes)₂P}₂Si=Si{P(Mes)₂}₂ is a readily accessible complex that is stable in the solid state, and is unlike tetraaminodisilenes (R₂N)₂Si=Si(NR₂)₂, which are typically unstable with respect to their silylene monomers (R₂N)₂Si. Tetraphosphadisilene is the first structurally characterized example of a ditetrelene substituted by more than two heteroatoms other than silicon.

Borinium ions, that is, two-coordinate boron cations, are the most electron-deficient isolable boron compounds. As borinium ions have only four formal valence electrons on boron, they should show a strong tendency to accept electron pairs on the boron atom to fill its valence shell. Thus chemical reactions of borinium ions are expected to give products in which the coordination number of boron is increased from two to three or four. However, contrary to this expectation, we found that the dimesitylborinium ion (Mes₂B⁺) undergoes twofold 1,2-carboboration reactions with two equivalents of diphenylacetylene to yield an unprecedented borinium ion (1⁺) with two substituted vinyl groups on the boron center. NMR spectroscopy and X-ray diffraction analysis of 1⁺, together with electronic-structure calculations, revealed that the positive charge is delocalized over the entire π-conjugated system. The fact that the chemical transformation of a borinium ion gives rise to a different borinium ion without a change in the coordination number is remarkable and should provide new insight into the chemistry of the Group 13 elements.

Still hungry: A two-coordinate diarylborinium ion undergoes twofold 1,2-carboboration with two equivalents of diphenylacetylene to form an unprecedented divinylborinium ion. NMR spectroscopy and X-ray diffraction analysis, together with theoretical calculations, revealed that the positive charge of the divinylborinium ion is delocalized over the entire π-conjugated system.

The first zwitterionic borata-bis(NHC)-stabilized phosphaketenyl germyliumylidene [(L₂(O=C=P)Ge:] 2 (L₂=(p-tolyl)₂B[1-(1-adamantyl)-3-yl-2-ylidene]₂) has been synthesized by salt-metathesis reaction of [L₂(Cl)Ge:] 1 with sodium phosphaethynolate [(dioxane)_nNaOCP]. Unexpectedly, its exposure to UV light affords, after reductive elimination of the entire PCO group, the unprecedented [L₂Ge-GeL₂] complex 3 in 54 % yields bearing the Ge₂²⁺ ion with Ge in the oxidation state +1. In addition, the 1,3-digermylium-2,4-diphosphacyclobutadiene [L₂Ge(μ-P)₂GeL₂] 4 and bis(germyliumylidenyl)-substituted diphosphene [(L₂Ge-P=P-GeL₂)] 5 could also be obtained in moderate yields. The formation of 3–5 and their electronic structures have been elucidated with DFT calculations.

Three birds with one stone: UV irradiation of the PCO-functionalized germyliumylidene 1 supported by a bis-NHC borate (L₂⁻) affords the first digermyliumylidene dication complex 2 as the main product through reductive elimination of the PCO group in 1 and subsequent Ge^I–Ge^I bond formation. In addition, the unusual 1,3-digermylium-2,4-diphosphacyclobutadiene diborate 3 and the unprecedented bis(germyliumylidenyl)diphosphene diborate 4 are also isolated in low yields.

The interaction among E₃ (E=Si, Ge, Sn) clusters and different ligands (L) encompassing five carbon-based donors (cyclic (alkyl)(amino)carbene (cAAC), N-heterocyclic carbene (NHC), saturated NHC (SNHC), mesoionic carbenes (MIC1, and MIC2)), two nitrogen-based donors (trimethylamine and pyridine), and two phosphorous-based donors (phosphinine and trimethylphosphine) in E₃(L)₃ complexes is explored through DFT computations. Although all carbenes form very strong bonds with E₃ clusters, cAAC makes the strongest bond with Si₃ and Ge₃ clusters, and MIC1 with the Sn₃ cluster. Nevertheless, other ligand-bound complexes are also viable at room temperature. This finding indicates that experimentalists may make use of them to synthesize the desired clusters based on precursor availability. The nature of the interaction in E−L bonds is analyzed through natural bond orbital analysis; energy decomposition analysis, in combination with the natural orbital for chemical valence; and adaptive natural density partitioning analysis. The LE σ-donation and LE π-back-donation play important roles in making contacts between L and E₃ clusters favorable; where the former is significantly more dominant over the latter.

Coaxing clusters together: The possibility of stabilizing E₃ (E=Si, Ge, Sn) clusters by various carbenes, trimethylamine, pyridine, phosphinine, and trimethylphosphine ligands (L) is explored through DFT computations (see figure). The LE σ donation and LE π-back-donation play important roles in making contacts between L and E₃ clusters favorable; the former is significantly more dominant over the latter.

C−H Borylation of arenes has been a subject of great interest recently because of its atom-economy and the wide applicability of borylated products in value-added synthesis. A new bis(silylene)cobalt(II) complex bearing a bis(N-heterocyclic silylene)-pyridine pincer ligand (SiNSi) has been synthesized and structurally characterized. It enabled the regioselective catalytic C−H borylation of pyridines, furans, and fluorinated arenes. Notably, it exhibited complementary regioselectivity for the borylation of fluorinated arenes compared to previously known catalytic systems, demonstrating that N-heterocyclic silylene donors have enormous potential in metal-catalyzed catalytic applications.

Strong-armed borylation: A new SiNSi cobalt pincer complex with two N-heterocyclic silylene (NHSi) arms enabled the direct and facile C−H borylation of heteroarenes and fluorinated arenes with complementary regioselectivity compared to previously reported transition-metal-based catalysts.

This Minireview focuses on the Lewis acidity/basicity “umpolung” of boron-based nucleophiles and phosphorus-based electrophiles.

Basically acidic: In the Lewis acidity/basicity “umpolung”, elements that traditionally exhibit acidity or basicity are modified to behave as Lewis bases or acids, respectively. This inversion of reactivity provides significant synthetic challenges, but also provides uniquely reactive species as demonstrated by recent attention on boron-based nucleophiles and phosphorus-based electrophiles.

Functional phosphanes having a P–N–B motif were obtained by treatment of chloro(organo)phosphanes with boryl amides. Compounds 4a–e are stable up to 100 °C and show no association in solution, even at low temperature (–70 °C). The functional phosphanes were characterized by NMR and IR spectroscopy, MS, and microanalysis, and phosphanes 4a, 4c, and 4e were additionally scrutinized by single-crystal X-ray crystallography. Preliminary studies on chlorine/lithium exchange of 4a led to anionic azaphosphaboriridine 5a′.

The synthesis and structure of several functional phosphanes having a P–N–B motif are presented. The title compounds exhibit remarkable thermal stability, unlike earlier reported examples, and enable chlorine/lithium exchange leading to the first anionic azaphosphaboriridine.