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

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.

Iminophosphonamido-Supported Plumbylenes and Plumbyliumylidenes: Synthesis and Properties

A series of low-valent lead(II) species, i.e., plumbylenes and plumbyliumylidenes supported by an N,N'-di-tert-butyliminophosphonamide ligand, have been synthesized. Chloroplumbylene [Ph₂P(N^tBu)₂PbCl] (1) was readily prepared by the reaction of the corresponding lithium iminophosphonamide and PbCl₂ in THF. The substitution reaction of 1 with KN(SiMe₃)₂ in THF afforded the corresponding aminoplumbylene [Ph₂P(N^tBu)₂PbN(SiMe₃)₂] (2) in the form of colorless crystals. The structures of these plumbylenes in the solid state and in solution were exhaustively characterized using multinuclear NMR spectroscopy and X-ray diffraction analysis. In the crystalline state, 1 adopts a nearly linear polymeric structure in the direction of the c axis via Pb-Cl bonds, with alternating four-membered PbN₂P rings. The chloride-abstraction reaction of 1 using Na[B(C₆F₅)₄] in fluorobenzene proceeded efficiently at ambient temperature to furnish plumbyliumylidene [Ph₂P(N^tBu)₂Pb:]⁺ (3⁺), which was isolated as the air-sensitive yellow borate salt 3[B(C₆F₅)₄]. Plumbyliumylidene 3[B(C₆F₅)₄] acts as a Lewis acid catalyst for the hydroboration of benzophenone and benzaldehyde at catalyst loadings of as low as 0.1 mol % under ambient conditions. Furthermore, the details of the reaction mechanism are discussed on the basis of the results of DFT calculations.

N-heterocyclic carbene and cyclic (alkyl)(amino)carbene adducts of plumbanes and plumbylenes

Lewis-acid/base adducts of N-heterocyclic carbenes (NHCs) and the cyclic (alkyl)(amino)carbene cAAC^Me (1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene) with selected lead(II) and lead(IV) compounds are presented. The reaction of the NHCs Me₂Im^Me (1,3,4,5-tetramethyl-imidazolin-2-ylidene), iPr₂Im^Me (1,3-di-isopropyl-4,5-dimethyl-imidazolin-2-ylidene), Dipp₂Im (1,3-bis-(2,6-di-isopropylphenyl)-imidazolin-2-ylidene) and cAAC^Me (1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene) with PbI₂ yielded the NHC-containing plumbylenes NHC·PbI₂ (NHC = Me₂Im^Me (1), iPr₂Im^Me (2), Dipp₂Im (3) and cAAC^Me·PbI₂ (4)). Using the Pb(IV) compound PbCl₂Ph₂, the plumbane adducts NHC·PbCl₂Ph₂ (NHC = Me₂Im^Me (5), iPr₂Im^Me (6), Dipp₂Im (7)) and cAAC^Me·PbCl₂Ph₂ (8)) were isolated in high yields. Reduction of the lead(IV) adducts 5 and 6 with excess KC₈ afforded the diaryl substituted plumbylenes Me₂Im^Me·PbPh₂ (9) and iPr₂Im^Me·PbPh₂ (10), which are stable in the solid state but decompose in solution.

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.

Forever young: the first seventy years of ferrocene

The discovery of ferrocene, [Fe(η5-C5H5)2], seventy years ago has significantly influenced chemical research and provided a key impetus for establishing and rapidly expanding organometallic chemistry, which has continued at a...

CH Activation of Cationic Bismuth Amides: Heteroaromaticity, Derivatization, and Lewis Acidity

Cationization of Bi(NPh₂)₃ has recently been reported to allow access to single- and double-CH activation reactions, followed by selective transformation of Bi-C into C-X functional groups (X = electrophile). Here we show that this approach can successfully be transferred to a range of bismuth amides with two aryl groups at the nitrogen, Bi(NR^aryl₂)₃. Exchange of one nitrogen-bound aryl group for an alkyl substituent gave the first example of a homoleptic bismuth amide with a mixed aryl/alkyl substitution pattern at the nitrogen, Bi(NPhiPr)₃. This compound is susceptible to selective N-N radical coupling in its neutral form and also undergoes selective CH activation when transformed into a cationic species. The second CH activation is blocked due to the absence of a second aryl moiety at nitrogen. The Lewis acidity of neutral bismuth amides is compared with that of cationic species "[Bi(aryl)(amide)(L)_n]⁺" and "[Bi(aryl)₂(L)_n]⁺" based on the (modified) Gutmann-Beckett method (L = tetrahydrofuran or pyridine). The heteroaromatic character of [Bi(C₆H₃R)₂NH(triflate)] compounds, which are iso-valence-electronic with anthracene, is investigated by theoretical methods. Analytical methods used in this work include nuclear magnetic resonance spectroscopy, single-crystal X-ray diffraction, mass spectrometry, and density functional theory calculations.

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.

Application of ferrocene-bridged N-heterocyclic carbene stabilised bis-phosphinidenes in Sn(II) complexation

Two new bidentate ferrocene-bridged bis(N-heterocyclic carbene-phospinidenes) (bisNHCPs) were successfully isolated by treating 1,1'-bis-(dichlorophosphine)ferrocene with N-heterocyclic carbenes, followed by dechlorination using sodium naphthalenide. The bisNHCPs were used in complexation of various Sn(II) halides and Sn(II) bistriflate (SnX₂ with X = Cl, Br, I, OTf). Transmetalation to a CuCl complex and Sn(II) transfer to a bisimine was performed to investigate the stannyliumylidenes' reactivity.

A Stable N-Heterocyclic Silylene with a 1,1'-Ferrocenediyl Backbone

The N-heterocyclic silylene [{Fe(η5 -C5 H4 -NDipp)2 }Si] (1DippSi, Dipp=2,6-diisopropylphenyl) shows an excellent combination of pronounced thermal stability and high reactivity towards small molecules. It reacts readily with CO2 and N2 O, respectively affording (1DippSiO2 )2 C and (1DippSiO)2 as follow-up products of the silanone 1DippSiO. Its reactions with H2 O, NH3 , and FcPH2 (Fc=ferrocenyl) furnish the respective oxidative addition products 1DippSi(H)X (X=OH, NH2 , PHFc). Its reaction with H3 BNH3 unexpectedly results in B-H, instead of N-H, bond activation, affording 1DippSi(H)(BH2 NH3 ). DFT results suggest that dramatically different mechanisms are operative for these H-X insertions.

d-d Dative Bonding Between Iron and the Alkaline-Earth Metals Calcium, Strontium, and Barium

Double deprotonation of the diamine 1,1'-(tBuCH2 NH)-ferrocene (1-H2 ) by alkaline-earth (Ae) or EuII metal reagents gave the complexes 1-Ae (Ae=Mg, Ca, Sr, Ba) and 1-Eu. 1-Mg crystallized as a monomer while the heavier complexes crystallized as dimers. The Fe⋅⋅⋅Mg distance in 1-Mg is too long for a bonding interaction, but short Fe⋅⋅⋅Ae distances in 1-Ca, 1-Sr, and 1-Ba clearly support intramolecular Fe⋅⋅⋅Ae bonding. Further evidence for interactions is provided by a tilting of the Cp rings and the related 1 H NMR chemical-shift difference between the Cp α and β protons. While electrochemical studies are complicated by complex decomposition, UV/Vis spectral features of the complexes support Fe→Ae dative bonding. A comprehensive bonding analysis of all 1-Ae complexes shows that the heavier species 1-Ca, 1-Sr, and 1-Ba possess genuine Fe→Ae bonds which involve vacant d-orbitals of the alkaline-earth atoms and partially filled d-orbitals on Fe. In 1-Mg, a weak Fe→Mg donation into vacant p-orbitals of the Mg atom is observed.

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.