Jelte

A highly trans-bent dialumene has marginal Al=Al double-bond character and, in solution, can dissociate into monomeric aluminyl fragments. Reactivity studies reveal that both the dialuminene and aluminyl monomer can be trapped by varying the reaction partner. DFT calculations elaborate the origins of the extreme trans-bending, weak Al=Al bond, and define substituent effects in dialumenes.

Abstract

Dialumenes are neutral Al^I compounds with Al=Al multiple bonds. We report the isolation of an amidophosphine-supported dialumene. Our X-ray crystallographic, spectroscopic, and computational DFT analyses reveal a long and extreme trans-bent Al=Al bond with a low dissociation energy and bond order. In solution, the dialumene can dissociate into monomeric Al^I species. Reactivity studies reveal two modes of reaction: as dialumene or as aluminyl monomers.

Nature Chemistry, Published online: 03 September 2021; doi:10.1038/s41557-021-00780-5

The periodic table of elements should be celebrated not only for the order it brings, but also for the fascinating stories underlying this icon of science, suggests Juris Meija.

Host guest-chemistry with a variety of siloxane ligands has been established over the years. This review tackles the recent developments in the field, recapitulates the historical aspects of siloxane coordination chemistry and describes the specific Si−O bond character with respect to different siloxane linkages. Implications on Si−O bond activation are included, and the limits of siloxane coordination are redefined.

Abstract

Siloxanes have evolved into a multi-million dollar business due to their manifold of commercial and industrial applications. As siloxanes have high hydrophobicity, low basicity, high flexibility and also high chemical inertness in common, their chemistry differs significantly from that of organic ethers. The discovery of organic crown ethers, for instance, is commonly accepted as the birth of synthetic host-guest chemistry. Regarding the chemical properties of siloxanes, cyclic siloxanes which formally resemble silicon analogues of crown ethers, have received considerably less interest in terms of their host-guest chemistry. Hence, only little is known about siloxane coordination chemistry in the chemical community and the number of published works in this field has been very low till lately. In the last few years, the field has significantly advanced and elegant methods were established to enable the Si−O−Si unit for coordination. This review therefore summarizes the recent developments in the field, recapitulates the historical aspects of siloxane coordination chemistry and describes the specific Si−O bond character with regard to different siloxane linkages. Implications on Si−O bond activation are included and the limits of siloxane coordination are redefined.

This Minireview showcases key advances on the use of polar s-block organometallic reagents in bioinspired solvents [water, glycerol and Deep Eutectic Solvents (DESs)], under air, at room temperature and in the presence of moisture, a trio of conditions that for decades has been considered incompatible with the manipulation of these reagents.

Abstract

Challenging conventional wisdom that s-block organometallic reagents such as Grignard or organolithiums need to be used under protecting inert atmosphere (N₂ or Ar), employing dry organic solvents with a strict temperature control, this Minireview focusses on recent advances on the use of these commodity reagents while operating under air, at room temperature and in the presence of moisture. Key for the success of these approaches has been the use of the following sustainable solvents: i) water; ii) Deep Eutectic Solvents (DESs); or iii) biomass-derived polyols (like glycerol) or ethereal solvents [i. e., 2-MeTHF or cyclopentyl methyl ether (CPME)]. The versatility of these air and moisture compatible synthetic protocols has been demonstrated for a myriad of key organic transformations, including nucleophilic additions of RLi/RMgX reagents to unsaturated organic molecules (i. e., ketones, imines, esters, amides or nitriles) as well as ortho- and lateral lithiation of aromatic substrates, Pd catalysed cross-couplings and anionic polymerisation of styrenes. Extension of these studies to lithium amides (LiNR₂) or phosphides (LiPPh₂) has enabled the development of more sustainable and efficient methods for C−N and C−P bond forming processes. These unconventional s-block metal mediated transformations have also been successfully incorporated in one-pot tandem processes in combination with transition-metal and organo-catalysis. Remarkably, in some cases the conversions and chemoselectivities observed are superior to those detected in common toxic organic solvents, while working under inert atmosphere conditions with strict temperature control. The key role played by the choice of solvent in these transformations and how it can affect the constitution of the s-block organometallic species present in solution is also discussed.

LALALALALALA: Six Lewis acid (LA) functions attached to a trisilacyclohexane core unit form a hexa-Lewis acid with a relatively rigid backbone and different types of Lewis-acidic cavities.

Abstract

We report the preparation of hexadentate poly-Lewis acids (PLA) based on 1,3,5-trisilacyclohexane backbones bearing two alkynyl groups attached to each of the silicon atoms. A rigid hexadentate PLA bearing six Lewis-acidic catecholatoboryl-substituents was prepared by a tin-boron exchange reaction. Its structure, determined by X-ray diffraction, is the first of a Lewis-acid-functionalised donor-free trisilacyclohexane. Flexible hexadentate PLA were prepared by hydroboration or hydrosilylation of hexavinyltrisilacyclohexane, resulting in PLA with six 9-BBN, SiCl₃, SiCl₂Me or SiClMe₂ groups. The Lewis-acidity of the last one was increased by conversion with silver triflate, resulting in a PLA with six highly acidic silyl triflate groups attached to the 1,3,5-trisilacyclohexane unit as TfOSiMe₂-C₂H₄- groups. Host-guest experiments of the above PLA demonstrated the suitability of the flexible representatives for complexation of neutral Lewis-based guest molecules under formation of 1 : 6 adducts (host: guest).

Activating C−H bonds with Si: Intermolecular allylic/benzylic and homoallylic/homobenzylic C−H activations by a divalent silicon species were accomplished by the treatment of a cyclic (alkyl)(amino)silylene with cyclohexene or 1,2,3,4‐tetrahydronaphthalene in neat conditions. The reaction of the silylene and cyclohexene is likely initiated by allylic dehydrogenation to generate a silyl radical and cyclohexyl radical, which are responsible for the homoallylic C−H activation.

Abstract

Direct activation of inert C(sp³)−H bonds by main group element species is yet a formidable challenge. Herein, the dehydrogenation of cyclohexene and 1,2,3,4‐tetrahydronaphthalene through the allylic/benzylic and homoallylic/homobenzylic C−H bond activation by cyclic (alkyl)(amino)silylene 1 in neat conditions is reported to yield the corresponding aromatic compounds. As for the reaction of cyclohexene, allylsilane 3 and 7‐silanorbornene 4 were also observed, which could be interpreted as a direct dehydrogenative silylation reaction of monoalkenes at the allylic positions. Experimental and computational studies suggest that the dehydrogenation of cyclohexene at the homoallylic position was accomplished by a combination of silylene 1 and radical intermediates such as hydrosilyl radical INT1 or cyclohexenyl radical H, which are generated in the initial step of the reaction.

The first chalcogen‐expanded Si₇E siliconoids are readily accessible from the silylene‐substituted Si₆ siliconoid and elemental chalcogen (E=S, Se, Te). The cluster core incorporates two additional vertices in one step: a pentacoordinate silicon and the chalcogen atom (⋅=silicon).

Abstract

Reactions of silylenes with heavier chalcogens (E) typically result in Si=E double bonds or their π‐addition products. In contrast, the oxidation of a silylene‐functionalized unsaturated silicon cluster (siliconoid) with Group 16 elements selectively yields cluster expanded siliconoids Si₇E (E=S, Se, Te) fully preserving the unsaturated nature of the cluster scaffold as evident from the NMR signatures of the products. Mechanistic considerations by DFT calculations suggest the intermediacy of a Si₆ siliconoid with exohedral Si=E functionality. The reaction thus may serve as model system for the oxidation of surface‐bonded silylenes at Si(100) by chalcogens and their diffusion into the silicon bulk.

The reaction of pentaphosphaferrocene [Cp*Fe(η⁵‐P₅)] with cationic electrophiles results in the formation of the complexes [Cp*Fe(η⁵‐P₅R)][B(C₆F₅)₄] (R=SiEt₃, H, Me). These compounds represent the first structurally characterised coordination complexes bearing elusive pentaphospholes (cyclo‐P₅R) as ligands, including the parent cyclo‐P₅H.

Abstract

Electrophilic functionalisation of [Cp*Fe(η⁵‐P₅)] (1) yields the first transition‐metal complexes of pentaphospholes (cyclo‐P₅R). Silylation of 1 with [(Et₃Si)₂(μ‐H)][B(C₆F₅)₄] leads to the ionic species [Cp*Fe(η⁵‐P₅SiEt₃)][B(C₆F₅)₄] (2), whose subsequent reaction with H₂O yields the parent compound [Cp*Fe(η⁵‐P₅H)][B(C₆F₅)₄] (3). The synthesis of a carbon‐substituted derivative [Cp*Fe(η⁵‐P₅Me)][X] ([X]⁻=[FB(C₆F₅)₃]⁻ (4 a), [B(C₆F₅)₄]⁻ (4 b)) is achieved by methylation of 1 employing [Me₃O][BF₄] and B(C₆F₅)₃ or a combination of MeOTf and [Li(OEt₂)₂][B(C₆F₅)₄]. The structural characterisation of these compounds reveals a slight envelope structure for the cyclo‐P₅R ligand. Detailed NMR‐spectroscopic studies suggest a highly dynamic behaviour and thus a distinct lability for 2 and 3 in solution. DFT calculations shed light on the electronic structure and bonding situation of this unprecedented class of compounds.

To gain insight into the nonlinear optical properties and reactivity of organotetrel chalcogenide clusters, we studied the silicon homologues, [(RSi)₄S₆] (R=Ph, Np, Sty), and their products upon reaction with [AuCl(PPh₃)], [{RSi(μ‐S)}₂{AuPPh₃(μ‐S)}₂]. Quantum chemical studies of cluster dimers elucidated the differences in the habit of silicon and tin compounds, and DFT calculations of crystalline models confirmed the applicability of these methods.

Abstract

We report the extension of the class of organotetrel sulfide clusters with further examples of the still rare silicon‐based species, synthesized from RSiCl₃ with R=phenyl (Ph, I), naphthyl (Np, II), and styryl (Sty, III) with Na₂S. Besides known [(PhSi)₄S₆] (IV), new compounds [(NpSi)₄S₆] (1) and [(StySi)₄S₆] (2) were obtained, the first two of which underwent reactions with [AuCl(PPh₃)] to form ternary complexes. DFT studies of cluster dimers helped us understand the differences between the habit of {Si₄S₆}‐ and {Sn₄S₆}‐based compounds. Crystalline 1 showed a pronounced nonlinear optical response, while for intrinsically amorphous 2, the chemical damage threshold seems to inhibit a corresponding observation. Calculations within the independent particle approximation served to rationalize and compare electronic and optical excitations of [(RSi)₄S₆] clusters (R=Ph, Np). The calculations reproduced the measured data and allowed for the interpretation of the main spectroscopic features.

The first parent arsanylalanes and ‐gallanes stabilized only by a Lewis base were synthesized. These compounds are accessible via a salt metathesis reaction of LB⋅E′H₂Cl and KAsH₂ and a H₂ elimination reaction between LB⋅E′H₃ and AsH₃, respectively. In addition, the unprecedented branched compounds IDipp⋅E′H(EH₂)₂ (E′=Al, Ga; E=As, P could be obtained.

Abstract

The synthesis and characterization of the unprecedented compounds IDipp⋅E′H₂AsH₂ (E′=Al, Ga; IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) are reported, the first monomeric, parent representatives of an arsanylalane and arsanylgallane, respectively, stabilized only by a LB (LB=Lewis Base). They are prepared by a salt metathesis reaction of KAsH₂ with IDipp⋅E′H₂Cl (E′=Al, Ga). The H₂‐elimination pathway through the reaction of AsH₃ with IDipp⋅E′H₃ (E′=Al, Ga) was found to be a possible synthetic route with some disadvantages compared to the salt metathesis reaction. The corresponding organo‐substituted compounds IDipp⋅GaH₂AsPh₂ (1) and IDipp⋅AlH₂AsPh₂ (2) were obtained by the reaction of KAsPh₂ with IDipp⋅E′H₂Cl (E′=Al, Ga). The novel branched parent compounds IDipp⋅E′H(EH₂)₂ (E′=Al, Ga; E=P, As) were synthesized by salt metathesis reactions starting from IDipp⋅E′HCl₂ (E′=Al, Ga). Supporting DFT computations give insight into the different synthetic pathways and the stability of the products.

A unique photoinduced reaction that couples a triarylphosphine, an alkene, and water to produce 2‐(cyclohexa‐2,5‐dienyl)ethylphosphine oxide is reported. The three components are all readily available, and their intermolecular coupling significantly increases molecular complexity. The resulting products are applicable to the Wittig olefination.

Abstract

A unique photoinduced reaction that couples a triarylphosphine, an alkene, and water to produce 2‐(cyclohexa‐2,5‐dienyl)ethylphosphine oxide is reported herein. The alkene inserts into a C(aryl)−P bond of the arylphosphine, the aryl ring is dearomatized into the cyclohexadienyl ring, and the phosphorus is oxidized. The three components are all readily available, and their intermolecular coupling significantly increases molecular complexity. The products formed are applicable to the Wittig olefination.

Upon reaction of silylated [Ge₉] clusters with 1,3,2‐diazaborolidines boranyl‐functionalization is achieved. The presented experiments aim for the generation of frustrated Lewis acid–base pairs as predicted from quantum‐chemical calculations.

Abstract

The unique three‐dimensional structure of spherical, homoatomic nine‐atom germanium clusters opens various possibilities for the spatial arrangement of functional groups. Ligands comprising lone pairs have recently been introduced in the cluster sphere, and we now report the addition of a boranyl group to the cluster featuring a Ge−B exo‐cluster bond. The reaction of the twofold‐silylated cluster [Ge₉{Si(TMS)₃}₂]²⁻ (TMS=trimethylsilyl) with 2‐chloro‐1,3,2‐diazaborolidines DAB^R‐Cl leads to the first boranyl‐functionalized [Ge₉] clusters [Ge₉{Si(TMS)₃}₂DAB^R]⁻ (R=methyl (1 a), iso‐propyl (2 a), ortho‐tolyl (3 a)). The anions 2 a and 3 a were structurally characterized as [NHC^DippCu]⁺ complexes (NHC^Dipp=1,3‐di(2,6‐diisopropylphenyl)imidazolylidine) through single crystal X‐ray structure determination. Quantum‐chemical calculations manifest the frustrated Lewis pair (FLP) character of the boranyl‐functionalized cluster [Ge₉{Si(TMS)₃}₂BCy₂]⁻ (4 a).

A neutral 2π electronic three‐membered aromatic disilaborirane is reported where aromatic stabilization arises from the overlap of a p orbital on boron and two σ* MOs of silicon. Also, the intriguing reactivity of the disilaborirane is studied and a four‐membered heterocycle is obtained.

Abstract

We report the design, synthesis, structure, bonding, and reaction of a neutral 2π aromatic three‐membered disilaborirane. The disilaborirane is synthesized by a facile one‐pot reductive dehalogenation of amidinato‐silylene chloride and dibromoarylborane with potassium graphite. Despite the tetravalent arrangement of atoms around silicon, the three‐membered silicon‐boron‐silicon ring is aromatic, as evidenced by NMR spectroscopy, nucleus independent chemical shift calculations, first‐principles electronic structure studies using density functional theory (DFT) and natural bond orbital (NBO) based bonding analysis. Trimethylsilylnitrene, generated in situ, inserts in the Si−Si bond of disilaborirane to obtain a four‐membered heterocycle 1‐aza‐2,3‐disila‐4‐boretidine derivative. Both the heterocycles are fully characterized by X‐ray crystallography.

With naphthalene and (Me₃Si)₃CSiCl₃ as the starting materials, silacyclopropanyl complexes of potassium and divalent ytterbium are synthesized, which can readily convert into silaspiro‐benzocycloheptenyl and cyclobutenosilaindan derivatives, showing the first example of insertion of a metal‐substituted silylene fragment into naphthalene's aromatic ring and rearrangements into the structurally intriguing silacycles.

Abstract

Synthesis of silacycle compounds are of fundamental and application importance. Herein we report the first example of insertion of metal‐substituted silylene fragment into naphthalene's aromatic ring. More significantly, this insertion is followed by interesting rearrangements to yield silaspiro‐benzocycloheptenyl and cyclobutenosilaindan derivatives. The formation of cyclobutenosilaindan derivative includes the C−C bond cleavage and 4π electrocyclization steps; the formation of silaspiro‐benzocycloheptenyl derivative is more complicated, including the C−C bond cleavage, reversible 4π electrocyclization, C−H bond activation and C−Si bond cleavage. DFT investigations were carried out to shed light on the mechanistic aspects of these two rearrangements. The formed cyclobutenosilaindan potassium can readily react with PhOH, MeOTf, EtOTf, PhCH₂Cl or PhCOCl at room temperature to afford the hydrogen, alkyl, benzyl or benzoyl substituted cyclobutenosilaindans in high yields.

INEPT and DEPT sequences are routinely used to enhance the NMR signals for low γ nuclides such as ¹⁵N or ¹³C.  The enhancement relies on polarization transfer between the protons J-coupled and the low γ nuclide.  The pulse sequences incorporate delays based on the reciprocal of the J-coupling constant between the protons and the low γ nuclide.  In the case of ¹⁵N INEPT, the enhancement for each scan can be as much as γ_H/γ_N (~ 10) compared to that from a conventional one-pulse sequence with inverse gated decoupling.  Furthermore, the recycle delay for the INEPT sequence depends on the the ¹H T₁ relaxation time rather than that of ¹⁵N.  ¹H T₁'s are typically an order of magnitude (or more) less than those of ¹⁵N so the recycle delays required for ¹⁵N INEPT spectra are at least ten (and possibly 100 times) shorter than those required for one-pulse data collection.  These two factors mean that the true time saving for a ¹⁵N INEPT measurement compared to a one-pulse ¹⁵N measurement can be on the order of 100 - 1000 times.  There are, however cases where ¹⁵N INEPT signals are attenuated or entirely nonexistent.  Attenuated ¹⁵N INEPT signals are observed when the protons (with short T₁) coupled to ¹⁵N exchange with those of water (longer T₁) on a time scale of seconds.*  The problem arises because of saturation transfer during the inverse gated decoupling used during the acquisition time. The partially saturated protons are unable to transfer as much polarization to the ¹⁵N as they would were they fully polarized. The problem can be reduced if a recycle delay much greater than the T₁ relaxation time of the water protons is employed.  If the protons bound to ¹⁵N undergo exchange with other labile protons at a rate fast with respect to the ¹H-¹⁵N J coupling interaction, polarization transfer from ¹H to ¹⁵N is not possible and a ¹⁵N INEPT signal cannot be observed.  This is demonstrated in the figure below.
Concentrated solutions of the methyl ester of anthranilic acid and anthranilic acid were prepared in DMSO-d₆.  The ¹⁵N NMR data were collected on a 600 MHz instrument with a cryoprobe.  The left-hand panel of the figure compares the ¹⁵N one-pulse spectrum with inverse gated decoupling (bottom) to the INEPT spectrum (top) for the methyl ester.  The spectra were collected with the same number of scans. For the methyl ester, the ¹⁵N bound protons do not exchange with any other labile protons.  The enhancement in the ¹⁵N INEPT spectrum is clear.  Similar spectra for anthranilic acid are shown on the right-hand side of the figure.  In anthranilic acid, the ¹⁵N bound -NH₂ protons undergo intramolecular exchange with the acid proton at a rate fast with respect to the one-bond ¹⁵N-¹H coupling constant (~90 Hz).  As a result, polarization transfer is not possible and no INEPT signal is observed.  The same is true for the meta- and para- isomers (data not shown). 

Thank you to Jin Hong for sharing her experience with collecting ¹⁵N INEPT data for anthranilic acid and Mojmir Suchy for kindly providing the samples.

* G.D. Henry and B.D. Sykes, J. Magn. Reson. B, 102, 193 (1993).

Germaborenes with the first authentic Ge=B double bond were synthesized. Upon photoactivation, germaborenes undergo a reversible [2+2] cycloaddition reaction with an arene moiety.

Abstract

Phosphine‐stabilized germaborenes featuring an unprecedented Ge=B double bond with short B⋅⋅⋅Ge contacts of 1.886(2) (4) and 1.895(3) Å (5) were synthesized starting from an intramolecular germylene–phosphine Lewis pair (1). After oxidative addition of boron trihalides BX₃ (X=Cl, Br), the addition products were reduced with magnesium and catalytic amounts of anthracene to give the borylene derivatives in yields of 78 % (4) and 57 % (5). These halide‐substituted germaborenes were characterized by single‐crystal structure analysis, and the electronic structures were studied by quantum‐chemical calculations. According to an NBO NRT analysis, the dominating Lewis structure contains a Ge=B double bond. The germaborenes undergo a reversible, photochemically initiated [2+2] cycloaddition with the phenyl moiety of a terphenyl substituent at room temperature, forming a complex heterocyclic structure with Ge^IV in a strongly distorted coordination environment.

2D ¹H J-RESolved spectroscopy (JRES) is able to separate the ¹H chemical shift and J coupling interactions in the F2 and F1 domains of the 2D data, respectively. The F2 projection represents the pure-shift ¹H decoupled ¹H NMR spectrum while the individual F1 slices at each chemical shift reveal the ¹H - ¹H J coupling for each resonance.  When this technique is applied to a spin system with both homonuclear ¹H-¹H coupling and heteronuclear coupling, it has the ability to provide both the homonuclear and heteronuclear coupling constants.  This is demonstrated in the figure below for 2,3-difluoro pyridine which has both ¹H-¹H and ¹H-¹⁹F coupling.
The top trace in the figure is the ¹H NMR spectrum showing the complex resonances due to both the homonuclear and heteronuclear coupling.  The 2D JRES spectrum is highlighted in grey.  The ¹H-¹H coupling is shown in the F1 slices which were summed to produce the blue, red and green vertical traces in the figure for ¹H resonances A, C and B, respectively.  These traces are identical to the resonances in the separately collected ¹H spectrum with ¹⁹F decoupling shown in the bottom trace of the figure.  The F2 projection of the JRES spectrum is shown in the trace directly on top of the 2D spectrum, colour coded in yellow.  The F2 projection represents the ¹H decoupled ¹H spectrum showing only the ¹H- ¹⁹F coupling.  It can be compared to the separately collected PSYCHE pure-shift ¹H spectrum, colour coded in orange which is very nearly identical.  Clearly this very simple, often overlooked, technique can provide a great deal of both homonuclear and heteronuclear coupling information. 

Give it to me straight: Addition of metabolically relevant ¹³C‐acetate and parahydrogen to solutions containing activated catalyst gives rise to enhancement of ¹³C NMR signals by up to ≈100‐fold at 9.4 T. The hyper‐polarization is shown to proceed directly, unlike previous efforts that have required either synthesis of catalyst‐binding composite structures, and/or the indirect relay of spin order through other species.

Abstract

Herein, we demonstrate “direct” ¹³C hyperpolarization of ¹³C‐acetate via signal amplification by reversible exchange (SABRE). The standard SABRE homogeneous catalyst [Ir‐IMes; [IrCl(COD)(IMes)], (IMes=1,3‐bis(2,4,6‐trimethylphenyl), imidazole‐2‐ylidene; COD=cyclooctadiene)] was first activated in the presence of an auxiliary substrate (pyridine) in alcohol. Following addition of sodium 1‐¹³C‐acetate, parahydrogen bubbling within a microtesla magnetic field (i.e. under conditions of SABRE in shield enables alignment transfer to heteronuclei, SABRE‐SHEATH) resulted in positive enhancements of up to ≈100‐fold in the ¹³C NMR signal compared to thermal equilibrium at 9.4 T. The present results are consistent with a mechanism of “direct” transfer of spin order from parahydrogen to ¹³C spins of acetate weakly bound to the catalyst, under conditions of fast exchange with respect to the ¹³C acetate resonance, but we find that relaxation dynamics at microtesla fields alter the optimal matching from the traditional SABRE‐SHEATH picture. Further development of this approach could lead to new ways to rapidly, cheaply, and simply hyperpolarize a broad range of substrates (e.g. metabolites with carboxyl groups) for various applications, including biomedical NMR and MRI of cellular and in vivo metabolism.

Lineup of suspects: Interest in kinetically stabilized pnictogenium ions prompted the re‐investigation of the bis(ferrocenyl)phosphenium ion, the only claimed donor‐free divalent phosphenium ion. Iron was found guilty of charge compensation.

Abstract

The bis(ferrocenyl)phosphenium ion, [Fc₂P]⁺, reported by Cowley et al. (J. Am. Chem. Soc. 1981, 103, 714–715), was the only claimed donor‐free divalent phosphenium ion. Our examination of the molecular and electronic structure reveals that [Fc₂P]⁺ possesses significant intramolecular Fe⋅⋅⋅P contacts, which are predominantly electrostatic and moderate the Lewis acidity. Nonetheless, [Fc₂P]⁺ undergoes complex formation with the Lewis bases PPh₃ and IPr to give the donor–acceptor complexes [Fc₂P(PPh₃)]⁺ and [Fc₂P(IPr)]⁺ (IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazole‐2‐ylidene).