Kyle Pearce

While electrophilic aromatic substitution is well-established for carbon and silicon-based electrophiles, it is extremely underdeveloped for germanium. Herein we report the first effective intermolecular Germa–Friedel–Crafts reaction. The process can be modified to enable selective synthesis of the mono- or the di-aryl germane derivatives, ArGeCl₃ and Ar₂GeCl₂.

ABSTRACT

Electrophilic aromatic substitution is a textbook transformation for the lighter Group 14 elements. In contrast to acylation, alkylation, and silylation, electrophilic germylation is extremely underdeveloped. Herein we report the first effective intermolecular Germa–Friedel–Crafts reaction. Specifically, combining the key industrial precursor GeCl₄, with Al₂Cl₆ and an inexpensive hindered pyridine base enables electrophilic C-H germylation of a range of arenes. The process can be applied for selective synthesis of either the mono- or the di-aryl germanes, ArGeCl₃ and Ar₂GeCl₂, respectively. ArGeCl₃ are versatile intermediates that can be transformed in situ into the synthetically desirable ArGe(alkyl)₃ derivatives. Mechanistic and computational analysis support an S_EAr-type process where Al₂Cl₆ is the key halophilic activator that generates a germanium electrophile able to effect C–H germylation in combination with the base. Overall, this work demonstrates that a high-yielding intermolecular Germa–Friedel–Crafts reaction is possible provided an appropriate Brønsted base is used.

Breaking the crown: Reduction of P₄ generally results in larger polyphosphide Zintl anions: P_m ⁿˉ. However, using a hydrocarbon-soluble Mg⁰ complex, P₄ can be fully reduced to mononuclear P³ˉ. The metal crown opens to a two-armed pincer-like ligand that efficiently stabilizes the phosphido anion. Latter P³ˉ anion reacts as a base, a three-fold nucleophile or a reducing agent.

Abstract

Traditional bulk syntheses of phosphorus compounds start with P₄ to PCl₃ oxidation but more sustainable methods cleave P─P bonds reductively. This generally results in larger polyphosphide Zintl anions: P_m ⁿˉ. We report a relatively selective full reduction of P₄ at room temperature to give a unique hydrocarbon-soluble s-block metal complex of the P³ˉ anion. Key to this chemistry is a recently reported redox-active metal crown complex: (BDI*)MgNa₃N″₂ (VI); N″ = N(SiMe₃)₂ and BDI* = HC[(tBu)C═N-DIPeP]₂, DIPeP = 2,6-CHEt₂-phenyl. The reduction of P₄ according to 2 VI + 0.25 P₄ → (BDI*)MgNa₅N″₃P (1) + 0.5 [(BDI*)Mg]₂ + 0.33 (NaN″)₃ is calculated to be exothermic (ΔH = −40.5 kcal mol⁻¹). The crystal structure of 1 shows a strongly bound (BDI*)MgP²ˉ anion with two chelating [Na-N″-Na⁺] and [Na-N″-Na-N″-Na⁺] arms of unequal length. Although these arms are highly fluxional and rapidly exchange ions, they effectively stabilize the P³ˉ anion. DFT calculations confirm the highly ionic nature of the complex and describe P³ˉ as full valence-shell anion with four lone-pairs of electrons. Reactivity studies show that the P³ˉ anion can react as a triple Brønsted base, a three-fold nucleophile or as a reducing agent.

The heavier, the better! We report that the caesium congener is the most active and versatile catalyst amongst a full group set of well-defined crown ether supported alkali metal diphenyl phosphides for catalytic hydrophosphination of various substrates and phosphines including aryl and alkyl phosphines. Furthermore, methods to generate P-chiral products, unsymmetrical ethylene-bridged bisphosphines, and alkene isomerization are presented.

Abstract

We present a study of the ability of our recently reported well-defined crown ether-coordinated alkali metal phosphides 1^AM to catalyze hydrophosphination (HP) of alkynes and alkenes. In a comparative study including reaction monitoring by ¹H NMR spectroscopy, we show that the activity of caesium compound 1^Cs greatly exceeds that of its lighter congeners, enabling us to solve some reported challenges of catalytic hydrophosphination. Through the rarely used application of dialkyl phosphines, we were able to produce trialkyl phosphines from HP of styrene derivatives and activated as well as non-activated alkynes by catalytic HP with ⁿ Bu₂PH and ^t Bu₂PH. Using ^t BuPhPH, P-chiral products were obtained, still in racemic mixtures showcasing this system's potential. We also proved the method is suited for preparing unsymmetrical ethylene-bridged bisphosphines by HP of Ph₂P(vinyl) isolable in high yields. These advances hint that well-defined organocaesium compounds could make a long-term impact in chemistry.

Displaying strong controlled metalating power, hydrocarbon-soluble sodium neopentyl can deprotonate challenging non-activated arenes and alkenes, with sodiated intermediates successfully undergoing onward functionalisation reactions. By characterising key reaction intermediates, and running computational studies on models, insight has been gained on the structure, bonding, and special reactivity of this promising new reagent.

Abstract

While recent studies have shown that organosodium reagents can offer excellent promise in organic synthesis, their widespread applications can be hindered by their limited solubility in organic solvents, lack of stability, and uncontrolled reactivity. Improving this immature area, here we report the structure of unsolvated neopentyl sodium, which exhibits a unique discrete tetrameric motif and good solubility in hexane. Combining spectroscopic and crystallographic studies with computational bonding analysis, key insights into its constitution and stability have been gained. Expanding its synthetic potential, supporting neopentyl sodium with Lewis donor PMDETA (N,N,N′,N″,N″-pentamethyldiethylenetriamine) allows for the direct metalation of challenging non-activated arenes and alkenes, including the regioselective meta-functionalisation of selected 1,3-disubstituted alkyl arenes. Shedding light on sodium-mediated metalation protocols, key organometallic intermediates have been isolated and structurally defined, which can subsequently be intercepted with CO₂ or a boron electrophile to be directly used in Pd-catalysed Suzuki cross coupling reactions.

Salt metathesis between Al^I and Sn^II complexes gives an electron-rich complex with an AlSn bond which can be described as a weak polar bond between Al^II and Sn^I open shell fragments. Electronic structure and reactivity are discussed.

The recently discovered class of potassium aluminyl complexes (R₂Al⁻K⁺) enables facile access to heterobimetallic Al-metal bonds by salt metathesis. Reaction of Al^I complex (^DIPPBDI-H)Al⁻K⁺ with the low-valent Sn^II complex (^DIPPBDI)SnCl gave (^DIPPBDI-H)AlSn(^DIPPBDI) (1); ^DIPPBDI = HC[C(Me)N(DIPP)]₂, DIPP = 2,6-diisopropylphenyl and ^DIPPBDI-H = H₂CC(N-DIPP)-C(H)C(Me)-N-DIPP. The crystal structure shows an asymmetric complex with a planar but strongly distorted trigonal coordination geometry at Al and trigonal pyramidal coordination at Sn. This geometry supports a donor–acceptor bonding model in which (^DIPPBDI-H)Al⁻ donates an sp ² electron pair in an empty p-orbital of (^DIPPBDI)Sn⁺. Extensive computational analyses support polar covalent AlSn bonding which based on energy decomposition analysis is best described as the interaction of two neutral open shell Al^II/Sn^I fragments: (^DIPPBDI-H)Al^•/(^DIPPBDI)Sn^•. Effective-oxidation state analysis assigns the oxidation states Al^I/Sn^II supporting the description of the bond as a (^DIPPBDI-H)Al⁻ → (^DIPPBDI)Sn⁺ donor–acceptor interaction. Reacting the electron-rich, polarized Al^δ+Sn^δ− bond with alkynes gave cis-insertion, indicating a concerted reaction with a four-membered ring transition state. Reaction with CO₂ selectively gave AlO₂CSn insertion, supporting the nucleophilic character of the electron-rich Sn center. The dianionic (^DIPPBDI-H)^{2 −} ligand is in these reactions not inert and reacts as C-centered nucleophile with alkyne or CO₂ resulting in dianionic tridentate ligands.

The synthesis, structures and dynamic magnetic properties of erbium-silole and erbium-stannole complexes are reported and compared to those of the analogous erbium-germole complex. Remarkable similarities in the single-molecule magnet properties of the three complexes are unearthed, highlighting the minimal role of the heavier group 14 elements in slow magnetic relaxation behaviour.

Abstract

The synthesis, structures and magnetic properties of an η⁵-silole complex and an η⁵-stannole complex of erbium are reported. The sandwich complex anions [(η⁵-Cp^Si)Er(η⁸-COT)]^– and [(η⁵-Cp^Sn)Er(η⁸-COT)]^–, where Cp^Si is [SiC₄-2,5-(SiMe₃)₂-3,4-Ph₂]^2– (1_Si ), Cp^Sn is [SnC₄-2,5-(SiMe₃)₂-3,4-Me₂]^2– (1_Sn ) and COT=cyclo-octatetraenyl, were obtained as their [K(2.2.2-cryptand)]⁺ salts and found to be isostructural, with remarkably similar bond lengths and angles, differing only in the lengths of the Er–E interactions (E=Si, Sn). The parallels in the molecular structures of 1_Si and 1_Sn are reflected in their dynamic magnetic properties, which show single-molecule magnet behaviour in zero applied field, with effective energy barriers of 115±7 and 125±3 cm^–1, respectively, along with comparable magnetic relaxation times. Analysis of the two complexes using ab initio calculations reveals differences at a quantitative level, but overall similar electronic structures, with the thermally activated relaxation likely to proceed via the first-excited Kramers doublet. Comparing 1_Si and 1_Sn with the previously reported germanium analogue 1_Ge reveals that swapping one heavier group 14 element for another in complexes of the type [(η⁵-Cp^E)Er(η⁸-COT)]^– has a minimal impact on the SMM behaviour.

Reactions of [(2,6-Dipp₂C₆H₃)Sn(μ-H)]₂ (Dipp=2,6-i-Pr₂C₆H₃) with [(BDI)AeH]₂ (BDI=HC{(Me)CNDipp}₂; Ae=Mg, Ca) provide heterobimetallic bis-μ₂-hydrido species, [[(BDI)Ae-μ-(H)₂-Sn(C₆H₃-2,6-Dipp₂)]. Although both compounds react with hex-1-ene to provide alkaline earth derivatives of tetravalent organostannides, the magnesium reagent promotes twofold addition of the alkene whereas the calcium-based reaction is arrested after a single hydrostannylation event.

Abstract

Reactions of a m-terphenylhydridostannylene with β-diketiminato magnesium and calcium hydrides provide bis-μ₂-hydrido species, the heterobimetallic constitutions of which are maintained after the addition of THF donor solvent. In both cases, reactions with hex-1-ene result in the formation of tetravalent organostannyl alkaline earth derivatives. Whereas the magnesium reagent undergoes facile twofold addition, the calcium-centered process is arrested after a single alkene reduction event. This contrasting behavior is assessed to result from the heavier alkaline earth element's ability to form a persistent polyhapto interaction with an arene substituent of the m-terphenyl ligand.

Reactions of the dimeric alkaline earth hydrides, [(BDI)AeH]₂ (BDI=HC{(Me)CNDipp}₂; Dipp=2,6-i-Pr₂C₆H_3; Ae=Mg, Ca), with Ph₃GeH in the presence of THF provide H₂ and [(BDI)AeGePh₃(THF)]. Although the synthesis of the magnesium derivative is complicated by THF ring opening, the calcium reagent is a viable source of the triphenylgermanide nucleophile.

Abstract

The dimeric calcium and magnesium hydrides, [(BDI)AeH]₂ [BDI=HC{(Me)CNDipp}₂, Dipp=2,6-i-Pr₂C₆H₃; Ae=Mg or Ca] do not react with Ph₃GeH in non-coordinating solvent. Addition of THF, however, induces deprotonation and access to monomeric Ae-germanide complexes, [(BDI)Ae{GePh₃}(THF)], both of which have been structurally characterized. Although this process is facile when Ae=Ca, the analogous magnesium-based reaction requires heating to temperatures >100 °C, under which conditions germanide formation is complicated by THF ring opening and the generation of an alkaline earth germyl-C-terminated n-butoxide, [(BDI)Mg{μ₂-O(CH₂)₄GePh₃}]. Reactions of [(BDI)Ca{GePh₃}(THF)] with N,N′-di-isopropylcarbodiimide and benzophenone provide the respective germylamidinate and germylalkoxide derivatives, [(BDI)Ca{(i-PrN)₂CGePh₃}(THF)] and [(BDI)Ca{OC(GePh₃)Ph₂}(THF)], demonstrating its potential as a well-defined and soluble source of the [Ph₃Ge]⁻ anion in nucleophilic addition reactions.