N-heterocyclic olefins as dative carbon donor ligands for diaminoplumbylenes: Syntheses and crystal structures of adducts with 1,3,4,5-tetramethyl-2-methyleneimidazoline

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.