A Redox-Switchable Germylene and its Ligating Properties in Selected Transition Metal Complexes

Recent advances in the chemistry of isolable carbene analogues with group 13-15 elements

Carbenes (R2C:), compounds with a divalent carbon atom containing only six valence shell electrons, have evolved into a broader class with the replacement of the carbene carbon or the RC moiety with main group elements, leading to the creation of main group carbene analogues. These analogues, mirroring the electronic structure of carbenes (a lone pair of electrons and an empty orbital), demonstrate unique reactivity. Over the last three decades, this area has seen substantial advancements, paralleling the innovations in carbene chemistry. Recent studies have revealed a spectrum of unique carbene analogues, such as monocoordinate aluminylenes, nitrenes, and bismuthinidenes, notable for their extraordinary properties and diverse reactivity, offering promising applications in small molecule activation. This review delves into the isolable main group carbene analogues that are in the forefront from 2010 and beyond, spanning elements from group 13 (B, Al, Ga, In, and Tl), group 14 (Si, Ge, Sn, and Pb) and group 15 (N, P, As, Sb, and Bi). Specifically, this review focuses on the potential amphiphilic species that possess both lone pairs of electrons and vacant orbitals. We detail their comprehensive synthesis and stabilization strategies, outlining the reactivity arising from their distinct structural characteristics.

Bi- and tridentate coordination behaviour of a novel bis(phosphinimino)methanide ligand

Herein, we report the synthesis of a novel ferrocenyl-functionalized bis(phosphinimino)methane ligand (CH2 (PPh2 NFc)2 ). Deprotonation of CH2 (PPh2 NFc)2 with KN(SiMe3 )2 gave the dimeric species [K{CH(PPh2 NFc)2 }]2 , which was further reacted with ECl2 (E=Ge, Sn) to yield the tetrylene compounds [{CH(PPh2 NFc)2 }ECl]. The ligand and the resulting tetrylenes were examined for their electrochemical properties with the aid of cyclic voltammetry. Furthermore, the reaction of the tetrylenes [{CH(PPh2 NFc)2 }ECl] with [AuC6 F5 (tht)] resulted in the bimetallic complexes [{(AuC6 F5 )CH(PPh2 NFc)2 }ECl] with an unusual Au coordination on the ligand backbone.

A complete series of N-heterocyclic tetrylenes (Si-Pb) with a 1,1'-ferrocenediyl backbone enabled by 1,3,2-diazaborolyl N-substituents

The use of bulky 1,3,2-diazaborolyl N-substituents has allowed the synthesis of the complete series of ferrocene-based N-heterocyclic tetrylenes fc[(N{B})2E] (fc = 1,1'-ferrocenediyl, {B} = (HCNC6H3-2,6-iPr2)2B, E = Si-Pb). The silylene fc[(N{B})2Si] is inert towards NH3, CO2 or N2O under ambient conditions and thus significantly less reactive than the N-aryl homologue fc[(NC6H3-2,6-iPr2)2Si]. In accord with its higher reactivity, computational results indicate a more pronounced ambiphilicity of fc[(NC6H3-2,6-iPr2)2Si]. Our computational investigation of the model compound fc[(NBMe2)2Si] suggests that silylenes of this type may be superior to fc[(NC6H3-2,6-iPr2)2Si] in terms of ambiphilicity.

Reactions of Mesityl Azide with Ferrocene‐Based N‐Heterocyclic Germylenes, Stannylenes and Plumbylenes, Including PPh 2 ‐Functionalised Congeners

The reactivity of ferrocene‐based N‐heterocyclic tetrylenes [{Fe(η⁵−C₅H₄−NSitBuMe₂)₂}E] (E=Ge, Sn, Pb) towards mesityl azide (MesN₃) is compared with that of PPh₂‐functionalised congeners exhibiting two possible reaction sites, namely the E^II and P^III atom. For E=Ge and Sn the reaction occurs at the E^II atom, leading to the formation of N₂ and an E^IV=NMes unit. The germanimines are sufficiently stable for isolation. The stannanimines furnish follow‐up products, either by [2+3] cycloaddition with MesN₃ or, in the PPh₂‐substituted case, by NMes transfer from the Sn^IV to the P^III atom. Whereas [{Fe(η⁵−C₅H₄−NSitBuMe₂)₂}Pb] and other diaminoplumbylenes studied are inert even under forcing conditions, the PPh₂‐substituted congener forms an addition product with MesN₃, thus showing a behaviour similar to that of frustrated Lewis pairs. The germylenes of this study afford copper(I) complexes with CuCl, including the first structurally characterised linear dicoordinate halogenido complex [CuX(L)] with a heavier tetrylene ligand L.

Planar-chiral 1,1'-diaminoferrocenes

Planar-chiral homologues of 1,1'-diaminoferrocene, which bear a single additional substituent adjacent to one of the amino groups, are prepared as racemic mixtures in a few steps and in good yields from ferrocene. Various substituents relevant to steric shielding, coordination and further functionalisation are used, giving access to ferrocene-based planar-chiral diimines and diamines as well as stable N-heterocyclic carbenes and tetrylenes by transformations analogous to procedures established for 1,1'-diaminoferrocene.

Ferrocenyl-substituted low-coordinated heavier group 14 elements

Several examples of stable low-coordinated species of heavier group 14 elements (Si, Ge, Sb, Pb) such as divalent species and multiple-bond compounds have been reported. With the goal in mind to create unprecedented low-coordinated species of heavier group 14 elements that exhibit considerably increased redox stability, ferrocenyl (Fc)-substituted low-coordinated species of heavier group 14 elements were designed. In this short account article, recent progress on the synthesis of Fc-based low-coordinated species of heavier group 14 elements is summarized.

Isolation of a Diylide-Stabilized Stannylene and Germylene: Enhanced Donor Strength through Coplanar Lone Pair Alignment

The preparation of the first stable diylide‐substituted stannylene and germylene (Y₂E, with E=Ge, Sn and Y=[PPh₃‐C‐SO₂Tol]⁻) is reported. The synthesis is easily accomplished in one step from the sulfonyl‐substituted metalated ylide YNa and the corresponding ECl₂ precursors. Y₂Ge and Y₂Sn exhibit unusual structures in the solid state and in solution, in which the three adjacent lone pairs in the C‐E‐C linkage are arranged coplanar to each other. As shown by DFT studies, this bonding situation is preferred over the typical π‐donation from the ligands into the empty p‐orbital at the metal due to the strong anion‐stabilizing ability of the sulfonyl groups in the ylide backbone and their additional coordination to the metal. The alignment of the three lone pairs leads to a remarkable boost of the HOMO energy and thus of the donor strengths of the tetrylenes. Hence, Y₂Ge and Y₂Sn become stronger donors than their diamino or diaryl congeners and comparable to cyclic alkyl(amino)carbenes. First reactivity studies confirm the high reactivity of Y₂Ge and Y₂Sn, which for example undergo an intramolecular C−H activation reaction via metal–ligand cooperation.

Reactive Dimerization of an N-Heterocyclic Plumbylene: C-H Activation with PbII

The N-heterocyclic plumbylene [Fe{(η⁵ -C₅ H₄ )NSiMe₃ }₂ Pb:] is in equilibrium with an unprecedented dimer in solution, whose formation involves the cleavage of a strong C-H bond and concomitant formation of a Pb-C and an N-H bond. According to a mechanistic DFT assessment, dimer formation does not involve direct Pb^II insertion into a cyclopentadienyl C-H bond, but is best described as an electrophilic substitution. The bulkier plumbylene [Fe{(η⁵ -C₅ H₄ )NSitBuMe₂ }₂ Pb:] shows no dimerization, but compensates its electrophilicity by the formation of an intramolecular Fe-Pb bond.

Germylene stabilized group 12 metal complexes and their reactivity with chalcogens

This manuscript reports the first examples of germylene stabilized cadmium complexes 3, 6, and 7, and novel germylene zinc complexes 2 and 5.

Redox-responsive phosphonite gold complexes in hydroamination catalysis

Very high activities were observed in the redox-induced hydroamination of alkynes by employing a redox-active gold(i) complex featuring an electron-deficient, terphenyl-substituted phosphonite-based ligand. The hydroamination proceeds roughly two-fold faster with the in situ oxidized catalysts than with their reduced form.

N-Functionalized Ferrocenes: Subvalent Group XIV Element Chlorides and tert-Butyllithium-Induced C-C Bond Cleavage under Mild Conditions

The ferrocene derivative (η⁵ -Cp)Fe{η⁵ -C₅ H₃ -1-(ArNCH)-2-(CH₂ NMe₂ )} (1; Ar=2,6-iPr₂ C₆ H₃ )) reacts diastereoselectively with LiR by carbolithiation and subsequent hydrolysis to give (η⁵ -Cp)Fe{η⁵ -C₅ H₃ -1-(ArHNCHR)-2-(CH₂ NMe₂ )} (3: R=tBu; 4: R=Ph; 5: R=Me) in high yields. For R=tBu, the organolithium derivative (η⁵ -Cp)Fe{η⁵ -C₅ H₃ -1-(ArLiNCHR)-2-(CH₂ NMe₂ )} (2) was isolated. Compound 2 reacts with GeCl₂ ⋅dioxane and SnCl₂ to give the metallylene amide chlorides (η⁵ -Cp)Fe{η⁵ -C₅ H₃ -1-(ArMNCHtBu)-2-(CH₂ NMe₂ )} 6 (M=GeCl) and 7 (M=SnCl), respectively, which each contain three stereogenic centers. The potential of 7 as a ligand in transition-metal chemistry is demonstrated by formation of its complex (η⁵ -Cp)Fe{η⁵ -C₅ H₃ -1-(ArMNCHtBu)-2-(CH₂ NMe₂ )} [9, M= Sn(Cl)W(CO)₅ ]. Treatment of 3 with tert-butyllithium at room temperature causes an unprecedented carbon-carbon bond cleavage whereas under kinetic control, lithiation at the Cp-3 position takes place, which leads to the isolation of (η⁵ -Cp)Fe{η⁵ -C₅ H₃ -1-(ArHNCHtBu)-2-(CH₂ NMe₂ )-3-SiMe₃ } (10).

New reactivity at the silicon bridge in sila[1]ferrocenophanes

We describe two new types of reactivity involving silicon-bridged [1]ferrocenophanes. In an attempt to form a [1]ferrocenophane with a bridging silyl cation, the reaction of sila[1]ferrocenophane [Fe(η-C₅H₄)₂Si(H)TMP] (12) (TMP = 2,2,6,6-tetramethylpiperidyl) towards the hydride-abstraction reagent trityl tetrakis(pentafluorophenyl)borate ([CPh₃][B(C₆F₅)₄]) was explored. This yielded the unusual dinuclear species [Fe(η-C₅H₄)₂Si(TMP·H)(η-C₅H₃)Fe(η-C₅H₄)Si(H)TMP][B(C₆F₅)₄] [13][B(C6F5)4] in low yield. The formation of [13]+ is proposed to involve abstraction of hydride from the silicon bridge in 12 with subsequent C-H bond cleavage of a cyclopentadienyl group by the resulting electrophilic transient silyl cation intermediate. We also explored the reaction of dimethylsila[1]ferrocenophane [Fe(η-C₅H₄)₂SiMe₂] (1) with the Au(i) species AuCl(PMe₃). This was found to result in addition of the Au-Cl bond across the Cp_ipso-Si bond to yield the ring-opened species [1'-(chlorodimethylsilyl)-ferrocenyl](trimethylphosphine)gold(i), [Fe(C₅H₄SiMe₂Cl){C₅H₄Au(PMe₃)}] (14). This represents the first example of ring-opening addition of a metallocenophane with a reagent possessing a transition metal-halogen bond.

A Redox-Active Bis(ferrocenyl)germylene and Its Reactivity

Bis(ferrocenyl)germylene Fc*₂ Ge: [2; Fc*=2,5-bis(3,5-di-tert-butylphenyl)-1-ferrocenyl] was isolated in the form of red crystals from the reaction of the sterically demanding ferrocenyl lithium dimer (Fc*Li)₂ and GeI₂ . Bis(ferrocenyl)germylene 2 exhibits extraordinary thermal stability in hydrocarbon solution and the solid state, as well as stable redox behavior. Moreover, it undergoes a ligand-redistribution reaction with GeCl₂ ⋅(dioxane) to afford the corresponding chlorogermylene, which was isolated upon coordination with PBu₃ .

An acyclic zincagermylene: rapid activation of dihydrogen at sub-ambient temperature

The first example of a stable zincagermylene, :Ge(TBoN)(ZnL*) (TBoN = N(SiMe₃){B(DipNCH)₂}, Dip = C₆H₃Prⁱ₂-2,6; L* = -N{C₆H₂[C(H)Ph₂]₂Me-2,6,4}(SiPrⁱ₃)) is prepared and shown to have unprecedented reactivity for a germylene, with respect to the activation of dihydrogen. Computational analyses point towards this being partially derived from the electron releasing nature of the amido-zinc fragment, which leads to a narrowing of the HOMO-LUMO gap in the compound.

N-Heterocyclic germylenes and stannylenes of the type [Fe{(η 5-C5H4)NR}2E] with bulky alkyl substituents

The 1,1′-diaminoferrocene derivatives [Fe(η 5-C5H4–NHAd)2] (Ad=2-adamantyl) and [Fe(η 5-C5H4–NHtBu)2] were investigated in terms of their suitability for the synthesis of N-heterocyclic tetrylenes of the type [{Fe(η 5-C5H4–NR)2}E] (E=Ge, Sn). The synthesis of these target compounds was easily achieved with R=tBu, but failed with R=Ad. In the latter case, the stannylene was not sufficiently stable for isolation and decomposed to the aminoiminoferrocene derivative [Fe(η 5-C5H4–NHAd)(η 5-C5H4–N=Ad′)] (Ad′=adamant-2-ylidene). Attempts to synthesise [{Fe(η 5-C5H4–NAd)2}Ge] afforded intractable material, from which the unusual compound [μ 2-{Fe(η 5-C5H4–NAd)2}Ge2(μ 3-O)(GeCl2)] was obtained by serendipity. It contains GeO, stabilised by adduct formation with GeCl2 and the target germylene. [Fe(η 5-C5H4–NHtBu)2], [Fe(η 5-C5H4–NHAd)2], [Fe(η 5-C5H4–NHAd)(η 5-C5H4–N=Ad′)], [μ 2-{Fe(η 5-C5H4–NAd)2}Ge2(μ 3-O)(GeCl2)] and [{Fe(η 5-C5H4–NtBu)2}E] (E=Ge, Sn) were structurally characterised by single-crystal X-ray diffraction.

Reactivity of a Base-Stabilized Germanium(I) Dimer toward Group 9 Metal(I) Chloride and Dimanganese Decacarbonyl

The reactivity of the 2-imino-5,6-methylenedioxylphenylgermanium(I) dimer toward group 9 metal(I) chloride and dimanganese decacarbonyl is described. [LGe:]₂ (1, L = 2-imino-5,6-methylenedioxylphenyl) underwent a disproportionation reaction with 1.5 equiv of group 9 metal(I) chloride [MCl(cod)]₂ (M = Rh, Ir) in toluene to afford a mixture of the group 9 metallogermylene-chlorometal(I) complexes [LGeμ-{M(cod)}₂Cl] (M = Rh (2), Ir (4)) and chlorogermylene-chlorometal(I) complexes [L(Cl)GeM(cod)Cl] (M = Rh (3), Ir (5)), respectively. The disproportionation property of 1 is further evidenced by its reaction with 0.5 equiv of Mn₂(CO)₁₀ in refluxing toluene to form a mixture of the manganogermylene dimer [(LGe)μ-{Mn(CO)₄}]₂ (7) and free ligand [LH] (8). Compounds 2-5, 7, and 8 were elucidated by NMR spectroscopy, X-ray crystallography, and DFT calculations, respectively.

Redox-Active N-Heterocyclic Germylenes and Stannylenes with a Ferrocene-1,1'-diyl Backbone

We describe ferrocene-based N-heterocyclic germylenes and stannylenes of the type [Fe{(η⁵ -C₅ H₄ )NR}₂ E:] (1 RE; E=Ge, Sn; R=neopentyl (Np), mesityl (Mes), trimethylsilyl (TMS)), which constitute the first examples of redox-functionalised N-heterocyclic tetrylenes (NHTs). These compounds are thermally stable and were structurally characterised by means of X-ray diffraction studies, except for the neopentyl-substituted stannylene 1 NpSn, the decomposition of which afforded the aminoiminoferrocene [fc(NHCH₂ tBu)(N=CHtBu)] (2) and the spiro tin(IV) compound (1 Np)₂ Sn (3). DFT calculations show that the HOMO of the NHTs of our study is localised on the ferrocenylene backbone. A one-electron oxidation process affords ions of the type 1 RE^+. . In contrast to the NHC system 1 RC, the localised ferrocenium-type nature of the oxidised form does not compromise the fundamental tetrylene character of 1 RE^+. .

Palladium complexes of ferrocene-based phosphine ligands as redox-switchable catalysts in Buchwald–Hartwig cross-coupling reactions

Redox-switchable catalysis: Palladium(ii) complexes of two differently substituted [1]phosphaferrocenophanes FcPR (R = Mes, biaryl) and of diphenylferrocenyl phosphine Ph₂PFc were applied in redox-switchable Buchwald–Hartwig cross-coupling reactions.