Martin Stanford

The ambiphilic nature of geometrically constrained Group 15 complexes bearing the N,N-bis(3,5-di-tert-butyl-2-phenolate)amide pincer ligand (ONO³⁻) is explored. Despite their differing reactivity towards nucleophilic substrates with polarised element–hydrogen bonds (e.g., NH₃), both the phosphorus(III), P(ONO) (1 a), and arsenic(III), As(ONO) (1 b), compounds exhibit similar reactivity towards charged nucleophiles and electrophiles. Reactions of 1 a and 1 b with KOtBu or KNPh₂ afford anionic complexes in which the nucleophilic anion associates with the pnictogen centre ([(tBuO)Pn(ONO)]⁻ (Pn=P (2 a), As (2 b)) and [(Ph₂N)Pn(ONO)]⁻ (Pn=P (3 a), As (3 b)). Compound 2 a can subsequently be reacted with a proton source or benzylbromide to afford the phosphorus(V) compounds (tBuO)HP(ONO) (4 a) and (tBuO)BzP(ONO) (5 a), respectively, whereas analogous arsenic(V) compounds are inaccessible. Electrophilic substrates, such as HOTf and MeOTf, preferentially associate with the nitrogen atom of the ligand backbone of both 1 a and 1 b, giving rise to cationic species that can be rationalised as either ammonium salts or as amine-stabilised phosphenium or arsenium complexes ([Pn{ON(H)O}]⁺ (Pn=P (6 a), As (6 b)) and [Pn{ON(Me)O}]⁺ (Pn=P (7 a), As (7 b)). Reaction of 1 a with an acid bearing a nucleophilic counteranion (such as HCl) gives rise to a phosphorus(V) compound HPCl(ONO) (8 a), whereas the analogous reaction with 1 b results in the addition of HCl across one of the As−O bonds to afford ClAs{(H)ONO} (8 b). Functionalisation at both the pnictogen centre and the ligand backbone is also possible by reaction of 7 a/7 b with KOtBu, which affords the neutral species (tBuO)Pn{ON(Me)O} (Pn=P (9 a), As (9 b)). The ambiphilic reactivity of these geometrically constrained complexes allows some insight into the mechanism of reactivity of 1 a towards small molecules, such as ammonia and water.

Complementary chemistry: The reactivity of geometrically constrained phosphorus(III) and arsenic(III) complexes towards ionic nucleophiles and electrophiles has been explored. These studies show that anionic nucleophiles readily associate with the heavier pnictogen(III) centres (see scheme), suggesting that such an association may play an important role in the mechanism for the bond activation of NH₃ and H₂O.

The ansa-aminohydroborane 1-NMe₂-2-(BH₂)C₆H₄ crystallizes in an unprecedented type of dimer containing a B−H bond activated by one FLP moiety. Upon mild heating and without the use of any catalyst, this molecule liberates one equivalent of hydrogen to generate a diborane molecule. The synthesis and structural characterization of these new compounds, as well as the kinetic monitoring of the reaction and the DFT investigation of its mechanism, are reported.

Your best bet for B−B: The ansa-aminohydroborane 1-NMe₂-2-(BH₂)C₆H₄ crystallizes in an unprecedented type of dimer containing a hydride bridge between two boron centers. Upon mild heating and without the use of a catalyst, this molecule liberates one equivalent of hydrogen to generate a boron–boron bond (see scheme).

The first terminal manganese phosphinidene complex was quantitatively synthesized from a terminal alkylborylene complex. Its structure and bonding, as well as the reaction mechanism, were investigated through a combination of experimental and computational studies.

Lucky 7: The first stable, neutral Group 7 terminal phosphinidene was synthesized through a one-step conversion from a terminal borylene complex, thereby revealing an unexpected reactivity for transition-metal borylene complexes.

Disproportionation reactions of Si₂Cl₆ in the presence of [nBu₄N]Cl in halogenated solvents yield the compound [nBu₄N][C(SiCl₃)₃] (1). X-ray structure analysis of 1 proves the existence of a planar carbanion, which is stabilized by three trichlorosilyl groups. Quantum chemical analysis shows the presence of highly polar bonds in the anion. Planarization of the anion can be explained by the interaction of the occupied lone pair at the carbon atom with the antibonding σ* orbitals of the Si–Cl bonds (negative hyperconjugation).

Disproportionation of Si₂Cl₆ in the presence of [nBu₄N]Cl in halogenated solvents yields the compound [nBu₄N][C(SiCl₃)₃]. X-ray structure analysis proves the existence of a highly symmetric carbanion, which is stabilized by three trichlorosilyl groups.

A diverse range of Lewis acidic alkyl, vinyl and aryl boranes and borenium compounds that are capable of new carbon–carbon bond formation through selective migratory group transfer have been synthesised. Utilising a series of heteroleptic boranes [PhB(C₆F₅)₂ (1), PhCH₂CH₂B(C₆F₅)₂ (2), and E-B(C₆F₅)₂(C₆F₅)C=C(I)R (R=Ph 3 a, nBu 3 b)] and borenium cations [phenylquinolatoborenium cation ([QOBPh][AlCl₄], 4)], it has been shown that these boron-based compounds are capable of producing novel allyl- boron and boronium compounds through complex rearrangement reactions with various propargyl esters and carbamates. These reactions yield highly functionalised, synthetically useful boron substituted organic compounds with substantial molecular complexity in a one-pot reaction.

A diverse range of Lewis acidic alkyl, vinyl and aryl boranes and borenium compounds that are capable of new carbon–carbon bond formation through selective migratory group transfer have been synthesised. Utilising a series of heteroleptic boranes as well as borenium cations, synthetic pathways to highly functionalised, synthetically useful boron-substituted organic compounds with substantial molecular complexity have been achieved in a one-pot reaction.

A potassium diboryllithate (B₂LiK) was synthesized and structurally characterized. DFT calculations, including NPA and AIM analyses of B₂LiK, revealed ionic interactions between the two bridging boryl anions and Li⁺ and K⁺. Upon standing in benzene, B₂LiK deprotonated the solvent to form a hydroborane and a phenylborane. On the basis of DFT calculations, a detailed reaction mechanism, involving deprotonation and hydride/phenyl exchange processes, is proposed. An NBO analysis of the transition state for the deprotonation of benzene suggests that the deprotonation should be induced by the coordination of benzene to the K⁺.

A ’LiK ’ of work: A potassium diboryllithate, B₂LiK, was synthesized and structurally characterized. The bonding situation in this compound was examined by NMR, XRD, NPA, and AIM analyses. B₂LiK is able to deprotonate benzene with concomitant formation of phenylborane as the major product. A detailed reaction mechanism based on DFT calculations suggests that the deprotonation of benzene should be initiated by a transition state involving the coordination of benzene to K⁺.

The oxidative addition reaction of X−H σ-bonds to Group 13 (E=Al, Ga, In) containing compounds has been computationally explored within the density functional theory framework. These reactions, which proceed concertedly involving the E^IE^III oxidation, are exothermic and associated with relatively low activation barriers. In addition, the following trends in reactivity are found: (i) the activation barriers are lower for the X−H bonds involving the heavier element in the same group (ΔE^≠: C>Si; N>P; O>S), (ii) the process becomes kinetically more favorable in going from left to right in the same period (ΔE^≠: C>N>O; Si≈P>S), and (iii) the activation barrier systematically increases when heavier Group 13 elements are involved in the transformation (ΔE^≠: Al<Ga<In). These reactivity trends are analyzed and quantitatively rationalized in detail by means of the activation-strain model of reactivity in combination with the energy decomposition analysis method.

Understanding barriers: Reactivity trends in the oxidative addition of X−H σ-bonds to Group 13 (E=Al, Ga, In) element-containing compounds are analyzed in detail by means of computational methods. The corresponding activation barriers are lower for the X−H bonds involving the heavier element in the same group and in going from left to right in the same period (see figure). In addition, the activation barrier systematically increases when heavier Group 13 elements are involved in the transformation (ΔE^≠: Al<Ga<In).

A variety of (2-thioxoimidazol-4-yl)phosphanes have been synthesized with the prospect of further functionalization. Following a standard reaction protocol, P-amino (2a,b), P-chloro (3a,b), and P-hydrogeno (8a) phosphane derivatives were synthesized. Treatment of chlorophosphanes 3a,b with Lewis acids in dichloromethane/diethyl ether did not afford the expected phosphenium cation, instead ethoxyphosphane derivatives 5a,b were isolated. Treatment of secondary phosphane 8a with potassium hexamethyldisilazide afforded 2-thioxoimidazol-4-yl-substituted phosphanide derivative 10a, which was used to access dinuclear phosphanido borane 11a and tungsten complexes 12a. Furthermore, the first strong NMR evidence for P-anionic bis(imidazole-2-ylidene) 13a is provided. The first attempts to synthesize an (2-thioxoimidazol-4-yl)-substituted phosphanyl radical or its dimer, the tetrakis(2-thioxoimidazol-4-yl)-substituted diphosphane, led exclusively to tris(2-thioxoimidazole-4-yl)phosphane 14a. These compounds were fully characterized spectroscopically and the structures of 2a, 3a, 7b, 8a, 10a, 11a, and 14a were determined by single-crystal X-ray crystallography. The challenges associated with the synthesis of some of the titled compounds were studied computationally.

2-Thioxoimidazole substituents have been investigated for the first time in the synthesis of low-coordinate phosphorus compounds of types I–III and revealed some surprising new features. The synthesis of some of the compounds have been studied computationally to provide further insights into some of the difficulties encountered in the syntheses.

Reduction of silylated 1,3-dichloro-1,3-diphospha-2,4-diazane, [ClP(µ-NHyp)]₂ [Hyp = Si(SiMe₃)₃], with activated magnesium in dimethoxyethane allowed the isolation and full characterization of the first stable silylated biradicaloid, [P(µ-NHyp)]₂. The oxidation of [P(µ-NHyp)]₂ with selenium and sulfur was studied. Activation of molecules bearing double bonds was achieved in the reaction with acetone and carbon disulfide, yielding heteroatom cage compounds.

Silylated 1,3-dichloro-1,3-diphospha-2,4-diazane, [ClP(µ-NHyp)]₂ [Hyp = Si(SiMe₃)₃], was reduced with activated magnesium to afford the silylated biradicaloid [P(µ-NHyp)]₂, which was isolation and fully characterized. The activation of molecules bearing single and double bonds with [P(µ-NHyp)]₂ was studied, yielding heteroatom cage compounds.

Coupling of carbon monoxide with nitrogen monoxide was achieved at a frustrated Lewis pair template. This unique reaction uses hydride as an auxiliary, which reductively activates carbon monoxide at the frustrated Lewis pair. The CO/NO coupling reaction then takes place through a pathway involving a radical reaction in which the hydrogen atom auxiliary is eventually removed again.

Couples and pairs: Coupling of carbon monoxide with nitrogen monoxide was achieved at a frustrated Lewis pair template. This unique reaction uses hydride as an auxiliary, which reductively activates carbon monoxide at the frustrated Lewis pair. The CO/NO coupling reaction then takes place through a pathway involving a radical reaction in which the hydrogen atom auxiliary is eventually removed again.

The reactivity of diazadiphosphapentalene 1 towards various substrates was investigated. Reaction of 1 with ammonia–borane resulted in transfer hydrogenolysis concomitantly with the cleavage of a P−N bond. By treatment of 1 with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), oxidation took place at one of the phosphorus atoms of 1, and a P^V/P^III mixed-valence derivative was isolated. At the same time, it was demonstrated that only one of the phosphorus atoms in 1 behaves as an electron donor for electrophiles and Lewis acids. The former afforded an intramolecularly coordinated phosphine-phosphenium species, whereas the latter demonstrates the ligand property of 1. UV irradiation induced rearrangement of 1 into another example of another diazadiphosphapentalene.

Reactivity series: Reactivity of diazadiphosphapentalene 1 towards various substrates was investigated. Activation of ammonia–borane by 1 proceeded concomitantly with the cleavage of the P−N bond in 1, while electrophiles and Lewis acids readily coordinated to the P atom between two carbon atoms in 1 (see scheme). Under irradiation with a Hg(Xe) lamp, photoisomerization of 1 occurred to afford a new diazadiphosphapentalene derivative.

A series of benzyl(diphenylphosphino) and o-phenyl(diphenlyphosphino) substituted germylenes and plumbylenes were synthesized by nucleophilic substitution between the respective lithium reagent and tetrylene halide. The Lewis pairs were characterized by X-ray crystallography and NMR spectroscopy. The reactivity of the tetrylenes was investigated with respect to azide addition. In the germylene case, the germaniumimide was formed as the kinetically controlled product, which rearranges upon heating to give the phosphinimide. The stannylene and plumbylene derivatives react with adamantylazide to give the azide adducts. 1-Pentene reacts diastereoselectively with the phosphagermirane to give a cyclic addition product. Trimethysilylacetylene shows an addition with the benzylphosphino-substituted germylene and plumbylene to give the cycloheteropentene molecules. The addition product between phenylacetylene and the four membered Ge-P adduct shows after addition at room temperature a 1,4-phenylmigration to give a cyclic phosphine. Alkylnitrene insertion into a Ge−C bond of the alkyne addition product of the phosphagermirane was found in reaction with adamantylazide.

Nucleophillic substitution: Intramolecular germylene and plumbylene adducts were synthesized, structurally characterized, and their reactivity with respect to azide and alkyne addition was studied. Pentene reacts diastereoselectively with the phosphagermirane to give a cyclic addition product. Trimethysilylacetylene shows an addition with the benzylphosphino-substituted germylene and plumbylene to give the cycloheteropentene molecules (see scheme).

The reaction of a metastable SiCl₂ solution with the sterically less-demanding carbene N,N-diisopropylimidazo-2-ylidene (IPr) yields the salt [(IPr₃Si₃Cl₅)⁺]Cl⁻ (1-Cl), containing a silyl cation with a Si₃ backbone. Salt 1 is highly reactive, but it can be used as a reagent in deuterated dichloromethane, whereby dehalogenation with Me₃SiOTf (OTf=O₃SCF₃) gives the dicationic silyl halide [(IPr₃Si₃Cl₄)]²⁺ 2. Quantum chemical calculations show that the HOMO is localized at the negatively charged central silicon atom of 1 and 2, and thus although both compounds are cations they are better described as silanides, which was also corroborated by NMR investigations.

More like silanides: The salt [(IPr₃Si₃Cl₅)]⁺Cl⁻ (IPr=N,N-diisopropylimidazo-2-ylidene), containing a silyl cation with a Si₃ backbone, was synthesized. Dehalogenation with Me₃SiO₃SCF₃ gives the di-cationic silyl halide [(IPr₃Si₃Cl₄)]²⁺. Quantum chemical calculations show that although both compounds are cations they are better described as silanides, as also corroborated by NMR investigations.