Preparation of 1,3‐diphosphabuta‐1,3‐dienes from sterically encumbered phosphaalkyne and phosphaethenyllithium

Kinetically stabilized 2‐lithio‐1‐(2,4,6‐tri‐t‐butylphenyl)‐1‐phosphapropene was allowed to react with a bulky phosphaalkyne Mes*CP (Mes* = 2,4,6‐t‐Bu₃C₆H₂) followed by quenching with iodomethane or benzyl bromide to give the corresponding 1,3‐diphosphabuta‐1,3‐dienes. The presence of the bulky Mes* group on the 1‐phosphorus atom prevents intramolecular [2+2] cyclization and gave the PC PC skeleton, whereas Mes*CP reacted with half an equivalent of nucleophile to afford the PCPC four‐membered ring compounds. X‐ray crystallography of 4‐benzyl‐1,3‐diphosphabuta‐1,3‐diene confirmed the molecular structure showing conjugation on the 1,3‐diphosphabuta‐1,3‐diene moiety. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:357–360, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20104     

Alternative Synthesis and Structures of C‐monoacetylenic Phosphaalkenes

An alternative synthesis of C‐monoacetylenic phosphaalkenes trans‐Mes*P=C(Me)(C≡CR) (Mes* = 2, 4, 6‐^tBu₃Ph, R = Ph, SiMe₃) from C‐bromophosphaalkenes cis‐Mes*P=C(Me)Br using standard Sonogashira coupling conditions is described. Crystallographic studies confirm cis‐trans isomerization of the P=C double bond during Pd‐catalyzed cross coupling, leading exclusively to trans‐acetylenic phosphaalkenes. Crystallographic studies of all synthesized compounds reveal the extend of π‐conjugation over the acetylene and P=C π‐systems.     

Two Sterically Encumbered 1,3,2‐Dioxaphospholanes – Reactions,Comparison of Crystal Structures and Computational Explanations

3,4,5,6‐Tetrachlorobenzo‐3‐(2,4,6‐tri‐tert‐butylphenyl)‐1,3,2‐dioxaphospholane ( 2 ) and benzo‐3‐(2,4,6‐tri‐tert‐butylphenyl)‐1,3,2‐dioxaphospholane ( 4 ), in which the reactive P^III‐center lies close to the sterically demanding Mes* group (Mes* = 2,4,6‐tri‐tert‐butylphenyl), were prepared from Mes*–Br and the corresponding P‐chloro‐phospholane. Compounds 2 and 4 reacted with various oxidants, azides, MeSO₃CF₃ or [(tht)AuCl] (tht = tetrahydrothiophene) to give the expected products. All crystal structures of the products display a strongly distorted Mes* system with a boat conformation of the phenyl ring and appreciable out‐of‐plane deviations of phosphorus and the ortho‐tert‐butyl groups to opposite sides of the ring. Quantum chemical calculations at the DFT (density functional theory) level of theory were used in order to discriminate between intra‐ and intermolecular forces, which are responsible for these distortions.     

Phosphorus Centers of Different Hybridization in Phosphaalkene‐Substituted Phospholes

Phosphole‐substituted phosphaalkenes (PPAs) of the general formula Mes*P?C(CH₃)?(C₄H₂P(Ph))?R 5 a – c (Mes*=2,4,6‐tBu₃Ph; R=2‐pyridyl ( a ), 2‐thienyl ( b ), phenyl ( c )) have been prepared from octa‐1,7‐diyne‐substituted phosphaalkenes by utilizing the Fagan–Nugent route. The presence of two differently hybridized phosphorus centers (σ²,λ³ and σ³,λ³) in 5 offers the possibility to selectively tune the HOMO–LUMO gap of the compounds by utilizing the different reactivity of the two phosphorus heteroatoms. Oxidation of 5 a – c by sulfur proceeds exclusively at the σ³,λ³‐phosphorus atom, thus giving rise to the corresponding thioxophospholes 6 a – c . Similarly, 5 a is selectively coordinated by AuCl at the σ³,λ³‐phosphorus atom. Subsequent second AuCl coordination at the σ²,λ³‐phosphorus heteroatom results in a dimetallic species that is characterized by a gold–gold interaction that provokes a change in π conjugation. Spectroscopic, electrochemical, and theoretical investigations show that the phosphaalkene and the phosphole both have a sizable impact on the electronic properties of the compounds. The presence of the phosphaalkene unit induces a decrease of the HOMO–LUMO gap relative to reference phosphole‐containing π systems that lack a P?C substituent.     

SiP Double Bonds: Experimental and Theoretical Study of an NHC‐Stabilized Phosphasilenylidene

An experimental and theoretical study of the first compound featuring a Si?P bond to a two‐coordinate silicon atom is reported. The NHC‐stabilized phosphasilenylidene (IDipp)Si?PMes* (IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene, Mes*=2,4,6‐tBu₃C₆H₂) was prepared by SiMe₃Cl elimination from SiCl₂(IDipp) and LiP(Mes*)SiMe₃ and characterized by X‐ray crystallography, NMR spectroscopy, cyclic voltammetry, and UV/Vis spectroscopy. It has a planar trans‐bent geometry with a short Si? P distance of 2.1188(7) Å and acute bonding angles at Si (96.90(6)°) and P (95.38(6)°). The bonding parameters indicate the presence of a Si?P bond with a lone electron pair of high s‐character at Si and P, in agreement with natural bond orbital (NBO) analysis. Comparative cyclic voltammetric and UV/Vis spectroscopic experiments of this compound, the disilicon(0) compound (IDipp)Si?Si(IDipp), and the diphosphene Mes*P?PMes* reveal, in combination with quantum chemical calculations, the isolobal relationship of the three double‐bond systems.     

Synthesis of a Molecule with Four Different Adjacent Pnictogens

The synthesis of a molecule containing four adjacent different pnictogens was attempted by conversion of a Group 15 allyl analogue anion [Mes*NAsPMes*]^? (Mes*=2,4,6‐tri‐tert‐butylphenyl) with antimony(III) chloride. A suitable precursor is Mes*N(H)AsPMes* ( 1 ) for which several syntheses were investigated. The anions afforded by deprotonation of Mes*N(H)AsPMes* were found to be labile and, therefore, salts could not be isolated. However, the in situ generated anions could be quenched with SbCl₃, yielding Mes*N(SbCl₂)AsPMes* ( 4 ).     

Imino‐Bridged Bisphosphaalkenes (2,4‐Diphospha‐3‐azapentadienes)

Deprotonation of aminophosphaalkenes (RMe₂Si)₂C?PN(H)(R′) (R=Me, iPr; R′=tBu, 1‐adamantyl (1‐Ada), 2,4,6‐tBu₃C₆H₂ (Mes*)) followed by reactions of the corresponding Li salts Li[(RMe₂Si)₂C?P(M)(R′)] with one equivalent of the corresponding P‐chlorophosphaalkenes (RMe₂Si)₂C?PCl provides bisphosphaalkenes (2,4‐diphospha‐3‐azapentadienes) [(RMe₂Si)₂C?P]₂NR′. The thermally unstable tert‐butyliminobisphosphaalkene [(Me₃Si)₂C?P]₂NtBu ( 4 a ) undergoes isomerisation reactions by Me₃Si‐group migration that lead to mixtures of four‐membered heterocyles, but in the presence of an excess amount of (Me₃Si)₂C?PCl, 4 a furnishes an azatriphosphabicyclohexene C₃(SiMe₃)₅P₃NtBu ( 5 ) that gave red single crystals. Compound 5 contains a diphosphirane ring condensed with an azatriphospholene system that exhibits an endocylic P?C double bond and an exocyclic ylidic P⁽⁺⁾? C^(?)(SiMe₃)₂ unit. Using the bulkier iPrMe₂Si substituents at three‐coordinated carbon leads to slightly enhanced thermal stability of 2,4‐diphospha‐3‐azapentadienes [(iPrMe₂Si)₂C?P]₂NR′ (R′=tBu: 4 b ; R′=1‐Ada: 8 ). According to a low‐temperature crystal‐structure determination, 8 adopts a non‐planar structure with two distinctly differently oriented P?C sites, but ³¹P NMR spectra in solution exhibit singlet signals. ³¹P NMR spectra also reveal that bulky Mes* groups (Mes*=2,4,6‐tBu₃C₆H₂) at the central imino function lead to mixtures of symmetric and unsymmetric rotamers, thus implying hindered rotation around the P? N bonds in persistent compounds [(RMe₂Si)₂C?P]₂NMes* ( 11 a , 11 b ). DFT calculations for the parent molecule [(H₃Si)₂C?P]₂NCH₃ suggest that the non‐planar distortion of compound 8 will have steric grounds.     

Base‐Stabilized Phosphinidene Boranes by Silylium‐Ion Abstraction

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.     

Coordination Behavior of Sterically Protected Phosphaalkenes on the AuCl Moiety Leading to Catalytic 1,6‐Enyne Cycloisomerization

Mes*‐substituted 2,3‐dimethyl‐1,4‐diphosphabuta‐1,3‐diene, 1,2‐diphenyl‐3,4‐diphosphinidenecyclobutene, 2,2‐bis(methylsulfanyl)‐1‐phosphaethene, and 3,3‐diphenyl‐1,3‐diphosphapropenes (Mes*=2,4,6‐tri‐tert‐butylphenyl) were employed as P ligands of gold(I) complexes. The (E,E)‐2,3‐dimethyl‐1,4‐diphosphabuta‐1,3‐diene functioned as a P2 ligand for digold(I) complex formation with or without intramolecular Au–Au contact, which depends on the conformation of the 1,3‐diphosphabuta‐1,3‐diene. The 1,2‐diphenyl‐3,4‐diphosphinidenecyclobutene, which has a rigid s‐cis P?C? C?P skeleton, afforded the corresponding digold(I) complexes with a slight distortion of the planar diphosphinidenecyclobutene framework and intramolecular Au–Au contact. In the case of the 2,2‐bis(methylsulfanyl)‐1‐phosphaethene, only the phosphorus atom coordinated to gold, and the sulfur atom showed almost no intra‐ or intermolecular coordination to gold. On the other hand, the 1,3‐diphosphapropenes behaved as nonequivalent P2 ligands to afford the corresponding mono‐ and digold(I) complexes. Some phosphaalkene–gold(I) complexes showed catalytic activity for 1,6‐enyne cycloisomerization without cocatalysts such as silver hexafluoroantimonate.     

An electrochemical study of PCl3 and POCl3 in the room temperature ionic liquid [C4mpyrr][N(Tf)2

Voltammetric studies of PCl3 and POCl3 have not been reported in the literature to date, probably due to the instability of these molecules in conventional aprotic solvents giving unstable and irreproducible results. From a previous study [Amigues et al. Chem. Commun. 2005, 1-4], it was found that ionic liquids have the ability to offer a uniquely stable solution phase environment for the study of these phosphorus compounds. Consequently, the electrochemistry of PCl3 and POCl3 has been studied by cyclic voltammetry on a gold microelectrode in the ionic liquid [C4mpyrr][N(Tf)2] (1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide). For both compounds, reduction and oxidation waves were observed and a tentative assignment of the waves is given. For PCl3, the reduction was thought to proceed via the following mechanism: PCl3 + e- <=> PCl3-, PCl3- <=> Cl- + P*Cl2, and Cl- + PCl3 <=> PCl4-. For POCl3, the suggested reduction mechanism was analogous to that of PCl3: POCl3 + e- <=> POCl3-, POCl3- <=> Cl- + P*OCl2, and Cl- + POCl3 <=> POCl4-. In both cases P*Cl2 and P*OCl2 are likely to engage in further reactions. Potential step microdisk chronoamperometry was carried out on the reductive waves of PCl3 and POCl3 to measure diffusion coefficients and number of electrons transferred. It was found that the diffusion of PCl3 was unusually slow (3.1 x 10(-12) m2 s(-1)): approximately 1 order of magnitude less than that for POCl3 (2.2 x 10(-11) m2 s(-1)). For both PCl3 and POCl3, a "split wave" was observed, with an overall electron count of 1. This observation is shown to be consistent with and to "fingerprint" the mechanisms proposed above.     

A Tricyclic Hexaphosphane

The reaction of the functionalized cyclo‐tetraphosphane [ClP(μ‐PMes*)]₂ (Mes*=2,4,6‐tri‐tert‐butylphenyl) with different Lewis bases led to the formation of an unprecedented tricyclic hexaphosphane, Mes*P₆Mes*. The formation of this compound was investigated by spectroscopic and theoretical methods, revealing an unusual ring expansion reaction. The title compound was fully characterized by experimental and computational methods.     

Towards Spontaneous Heterolysis of the Homonuclear P?P Bond in Diphosphines: The Case of Diazaphospholeniumtriphospholides

Computational studies on a series of polyphospholyl‐substituted N‐heterocyclic phosphines (CH)₂(NR)₂ P? P_n(CH)_5?n (R=Me, n=1–5) disclosed that increasing formal replacement of CH units in the phosphole ring by phosphorus atoms is associated with an increase in P? P distances and charge separation, and a decrease in covalent bond orders. Altogether, these trends imply that the CH versus P substitution enhances ionic P? P bond polarization in these compounds. Experimental verification of this hypothesis was obtained for the triphospholyl diazaphospholenes (CR)₂(NR′)₂P? P₃(CtBu)₂ ( 8 a : R=H, R′=tBu; 8 b : R=Me, R′=Mesityl [Mes]), which were prepared through metathesis reactions from suitable precursors and identified by solution and solid‐state NMR data and a single‐crystal X‐ray diffraction study of 8 a . Analysis of J_PP coupling patterns suggested that both species are characterized by the absence of a strong covalent P? P bond connecting both rings. This interpretation was confirmed by the finding of a unique P? P distance of 2.79 Å for crystalline 8 a , and further supported by computational studies, which led to the conclusion that both species are better described as diazaphospholenium‐triphospholide contact ion pairs rather than covalent molecules. Variable‐temperature (VT) NMR spectra of 8 b showed a collapse of J_PP couplings between atoms in different rings, which indicates scrambling of the diazaphospholenium and triphospholide units between different molecules in solution, and further substantiates the proposed view on the molecular structure.     

The Synthetic Potential of C-Halophosphaalkenes

Abstract.

Methodologies for the functionalization of phosphaalkenes Mes*P=CHal₂ were developed. Lithiation with n-butyllithium yielded carbenoids Mes*P=CLiHal which were reacted with various electrophiles such as acid chlorides, carbonyl compounds, and metal halides. The dihalophosphaalkenes were also converted to monohalophosphaalkenes; the latter proved to be suitable for Stille-type cross coupling reaction with Grignard reagents. New phosphaalkenes of the type (E)-Mes*P=C(H)Ar with a variety of functionalities were obtained in high yield and isomeric purity.     

Aluminium Diphosphamethanides: Hidden Frustrated Lewis Pairs

The synthesis and characterisation of two aluminium diphosphamethanide complexes, [Al(tBu)₂{κ²P,P′‐Mes*PCHPMes*}] ( 3 ) and [Al(C₆F₅)₂{κ²P,P′‐Mes*PCHPMes*}] ( 4 ), and the silylated analogue, Mes*PCHP(SiMe₃)Mes* ( 5 ), are reported. The aluminium complexes feature four‐membered PCPAl core structures consisting of diphosphaallyl ligands. The silylated phosphine 5 was found to be a valuable precursor for the synthesis of 4 as it cleanly reacts with the diaryl aluminium chloride [(C₆F₅)₂AlCl]₂. The aluminium complex 3 reacts with molecular dihydrogen at room temperature under formation of the acyclic σ²λ³,σ³λ³‐diphosphine Mes*PCHP(H)Mes* and the corresponding dialkyl aluminium hydride [tBu₂AlH]₃. Thus, 3 belongs to the family of so‐called hidden frustrated Lewis pairs.     

Syntheses and reactivity of novel unsaturated cyclic compounds containing phosphorus atoms

Kinetic stabilization of compounds containing heavy main-group elements through the use of bulky substituents is of current interest. The widely used 2,4,6-tri-t-butylphenyl group (Mes*) is recognized as a powerful bulky protecting group and has enabled us to successfully prepare various types of phosphorus compounds with unusual structures. When a phosphaalkyne carrying Mes* was treated with tBuLi and then quenched with MeOH, a 1,3-diphosphacyclobutene was obtained, whereas, when MeI was used as a quencher, a 1,3-diphosphacyclobutane-2,4-diyl was formed almost quantitatively as a stable biradical compound. The reaction mechanism for the formation of both compounds can be explained by a phosphide intermediate, which can be formed via dimerization of the phosphaalkyne promoted by tBuLi. Some other diphosphacyclobutane-2,4-diyls with various substituents were prepared in a similar fashion and showed interesting reactivities including ring expansion, oxidation, isomerization and so on.     

Über Phosphorstickstoffverbindungen,XXXII. Über Vier- und fünfgliedrige Heterocyclen mit Phosphor,Stickstoff und Kohlenstoff im Ring

On reacting of oxamide with PCl₅ the syntheses of the new N-, C- and pentavalent phosphorus containing heterocycles I and II (see “Inhaltsübersicht”), built up from interconnected four- and fivemembered ring systems, have been achieved. Reaction of N, N′- dimethyloxamide with PCl₃ yields the compound III which may be chlorinated to IV. An intermolecular reaction between the PCl₃- and carbonyl groups of IV gives V. The fivemembered ring systems III and V may each be linked together via N? CH₃ bridges, i. e. via P? N(CH₃)? P and P(O)? N(CH₃)? P(O) units, respectively. N, N′- dimethyloxamide reacts with PCl₅ to form a mixture of fivemembered heterocycles containing trivalent phosphorus (as a PCl group) and chlorinated carbon.     

Three‐Coordinate Nickel(I) Complexes Stabilised by Six‐, Seven‐ and Eight‐Membered Ring N‐Heterocyclic Carbenes: Synthesis,EPR/DFT Studies and Catalytic Activity

Comproportionation of [Ni(cod)₂] (cod=cyclooctadiene) and [Ni(PPh₃)₂X₂] (X=Br, Cl) in the presence of six‐, seven‐ and eight‐membered ring N‐aryl‐substituted heterocyclic carbenes (NHCs) provides a route to a series of isostructural three‐coordinate Ni^I complexes [Ni(NHC)(PPh₃)X] (X=Br, Cl; NHC=6‐Mes 1 , 6‐Anis 2 , 6‐AnisMes 3 , 7‐o‐Tol 4 , 8‐Mes 5 , 8‐o‐Tol 6 , O‐8‐o‐Tol 7 ). Continuous wave (CW) and pulsed EPR measurements on 1 , 4 , 5 , 6 and 7 reveal that the spin Hamiltonian parameters are particularly sensitive to changes in NHC ring size, N substituents and halide. In combination with DFT calculations, a mixed SOMO of ∣3d〉 and ∣3d〉 character, which was found to be dependent on the complex geometry, was observed and this was compared to the experimental g values obtained from the EPR spectra. A pronounced ³¹P superhyperfine coupling to the PPh₃ group was also identified, consistent with the large spin density on the phosphorus, along with partially resolved bromine couplings. The use of 1 , 4 , 5 and 6 as pre‐catalysts for the Kumada coupling of aryl chlorides and fluorides with ArMgY (Ar=Ph, Mes) showed the highest activity for the smaller ring systems and/or smaller substituents (i.e., 1 > 4 ≈ 6 ? 5 ).     

The Complexed 1,3‐Diphospha‐2‐Arsaallyl Radical and Its Cationic and Anionic Derivatives

Photolysis of [Cp*As{W(CO)₅}₂] ( 1 a ) in the presence of Mes*P?PMes* (Mes*=2,4,6‐tri‐tert‐butylphenyl) leads to the novel 1,3‐diphospha‐2‐arsaallyl radical [(CO)₅W(μ,η²:η¹‐P₂AsMes*₂)W(CO)₄] ( 2 a ). The frontier orbitals of the radical 2 a are indicative of a stable π‐allylic system that is only marginally influenced by the d orbitals of the two tungsten atoms. The SOMO and the corresponding spin density distribution of the radical 2 a show that the unpaired electron is preferentially located at the two equivalent terminal phosphorus atoms, which has been confirmed by EPR spectroscopy. The protonated derivative of 2 a , the complex [(CO)₅W(μ,η²:η¹‐P₂As(H)Mes*₂)W(CO)₄] ( 6 a ) is formed during chromatographic workup, whereas the additional products [Mes*P?PMes*{W(CO)₅}] as the Z‐isomer ( 3 ) and the E‐isomer ( 4 ), and [As₂{W(CO)₅}₃] ( 5 ) are produced as a result of a decomposition reaction of radical 2 a . Reduction of radical 2 a yields the stable anion [(CO)₅W(μ,η²:η¹‐P₂AsMes*₂)W(CO)₄]^? in 7 a , whereas upon oxidation the corresponding cationic complex [(CO)₅W(μ,η²:η¹‐P₂AsMes*₂)W(CO)₄][SbF₆] ( 8 a ) is formed, which is only stable at low temperatures in solution. Compounds 2 a , 7 a , and 8 a represent the hitherto elusive complexed redox congeners of the diphospha‐arsa‐allyl system. The analogous oxidation of the triphosphaallyl radical [(CO)₅W(μ,η²:η¹‐ P₃Mes*₂)W(CO)₄] ( 2 b ) also leads to an allyl cation, which decomposes under CH activation to the phosphine derivative [(CO)₅W{μ,η²:η¹‐P₃(Mes*)(C₅H₂tBu₂C(CH₃)₂CH₂)}W(CO)₄] ( 9 ), in which a CH bond of a methyl group of the Mes* substituent has been activated. All new products have been characterized by NMR spectrometry and IR spectroscopy, and compounds 2 a , 3 , 6 a , 7 a , and 9 by X‐ray diffraction analysis.     

Highly Electron‐Deficient and Air‐Stable Conjugated Thienylboranes

Introduced herein is a series of conjugated thienylboranes, which are inert to air and moisture, and even resist acids and strong bases. X‐ray analyses reveal a coplanar arrangement of the thiophene rings, an arrangement which facilitates p–π conjugation through the boron atoms despite the presence of highly bulky 2,4,6‐tri‐tert‐butylphenyl (Mes*) or 2,4,6‐tris(trifluoromethyl)phenyl (^FMes) groups. Short B???F contacts, which lead to a pseudotrigonal bipyramidal geometry in the ^FMes species, have been further studied by DFT and AIM analysis. In contrast to the Mes* groups, the highly electron‐withdrawing ^FMes groups do not diminish the Lewis acidity of boron toward F^? anions. These compounds can be lithiated or iodinated under electrophilic conditions without decomposition, thus offering a promising route to larger conjugated structures with electron‐acceptor character.     

Chiral Organoborane Lewis Pairs Derived from Pyridylferrocene

In an effort to develop a new class of redox‐active chiral Lewis pairs, pyridine and borane moieties with different steric and electronic properties were introduced onto a planar chiral 1,2‐disubstituted ferrocene framework. Metathesis of lithiated, stannylated, or mercuriated pyridylferrocenes with boron halides afforded (pR)‐2‐[bis(pentafluorophenyl)boryl]‐1‐(3,5‐dimethylpyrid‐2‐yl)ferrocene ( 4‐Pf ), (pR)‐2‐[dimesitylboryl]‐1‐(3,5‐dimethylpyrid‐2‐yl)ferrocene ( 4‐Mes ), (pS)‐2‐(bis(pentafluorophenyl)boryl)‐1‐(2‐trimethylsilylpyrid‐6‐yl)ferrocene ( 5‐Pf ), or (pS)‐2‐[dimesitylboryl]‐1‐(2‐trimethylsilylpyrid‐6‐yl)ferrocene ( 5‐Mes ). The borylated products were analyzed by multinuclear NMR spectroscopy, HRMS, and single‐crystal X‐ray diffraction. Chiral HPLC and optical‐rotation measurements were employed to assess the stereoselectivity of the borylation process and to establish the correct stereochemical assignments. The strength of the B–N interactions were investigated in solution and in the solid state. Compounds 4‐Pf and 4‐Mes formed robust ‘closed’ B?N heterocyclic systems that proved to be perfectly stable to air and moisture, whereas 5‐Pf established a dynamic equilibrium, in which the B?N heterocycle was observed exclusively at room temperature, but opened up at high temperature according to ¹⁹F NMR exchange spectroscopy data. As a consequence, 5‐Pf reacted readily with a molecule of water to generate a ring‐opened pyridinium borate. The combination of bulky borane and bulky pyridyl groups in 5‐Mes led to a completely ‘open’ frustrated Lewis pair system with uncomplexed pyridine and borane groups, even at room temperature. Electrochemical studies were performed and the effect of preparative ferrocene oxidation on the structural features was also explored.