Two
molecules of 4,5-dichloroimidazole react with dibromomethane
to give the diimidazole 1. Reaction of 1 with iodomethane or 2-fluorobenzyl bromide yields the monoalkylated
imidazolium/imidazole salts [2]I and [3]Br,
respectively. Salt [2]I reacts with Ag2O followed
by transmetalation to [Rh(Cp*)(Cl)2]2 to give
the RhIII-NHC complex [4], bearing an NHC
ligand with a pendant imidazole group. The pendant imidazole can be
deprotonated using NaOAc to yield complex [5], bearing
a doubly C-metalated C∧C chelate ligand. Reaction
of [2]I or [3]Br with NaOAc and [Rh(Cp*)(Cl)2]2 yields the C∧C chelate complexes
[7] and [9], respectively, in a one-pot
reaction. The imine ring nitrogen atom in complexes [7] and [9] can be protonated using HBF4·Et2O to give complexes [8]BF4 and [11]BF4, each bearing a C(NHC)∧C(pNHC) chelate ligand (pNHC =
protic NH,NR-NHC ligand). Alkylation of the imine ring nitrogen atom
in [9] yields complex [10]BF4, bearing a unique unsymmetrical (NHC)∧C(NHC′)
chelate ligand.
The IrIII complexes 4 and 5 bearing bis‐NHC ligands (NHC = N‐heterocyclic carbene) composed of one classical NR,NR NHC and one N,NR NHC donor were prepared by the reaction of the azolium/azole compounds 2I and 3Br, respectively, with [{Cp*IrCl(µ‐Cl)}2] (Cp*=η5‐C5Me5) in the presence of NaOAc as base. Most likely, the salts 2I and 3Br were first selectively deprotonated at the C2 position of the disubstituted (NR,NR) diazaheterocycle to generate an NHC donor, which then coordinated to the IrIII center. Subsequently, NaOAc promoted C–H bond activation at the pendant imidazole moiety of the intermediate IrIII mono‐NHC complexes led to the formation of the six‐membered iridacycles 4 and 5, which bear a chelating, doubly C‐metalated C(NHC)^C(NHC′) bis‐NHC ligand. The IrIII complexes 4 and 5 were tested as precatalysts for the reduction of imines with molecular hydrogen. Moderate to good activity was observed at a catalyst loading of 5 mol‐% and an H2 pressure of 3 bar in MeOH.
The treatment of PdCl 2 (NCPh) 2 with Ph 3 PdNPh (1) gives the expected complex trans-PdCl 2 [N(Ph)-PPh 3 ] 2 (3). However, the reaction of PdCl 2 (COD) (COD ) 1,5-cyclooctadiene) with 1 or Ph 3 PdN-1-Np (2) (Np ) naphthyl) occurs through nucleophilic attack of 1 or 2 on one olefinic bond of the COD ligand followed by proton abstraction on the adjacent methylene group, giving the η 1 -allyl complexes [Ph 3 P(R)NH‚‚‚Cl 2 Pd(C 8 H 11 )] (R ) Ph, 4; Np, 5). The X-ray structure of 4 has been determined and shows two interesting facts: (i) the η 1 -η 2 -bonded cyclooctadienyl ligand, containing a η 1 -allyl fragment, and (ii) the presence of a strong H bond between one of the Cl ligands and the proton of the NH group. This H bond persists in solution, as shown by NMR and molar conductance measurements. The abstraction of a chloride on 4 by reaction with AgClO 4 cleaves the H bond and gives a mixture of the salt [Ph 3 PN-(H)Ph](ClO 4 ) ( 6) and the neutral η 1 -η 2 -cyclooctadienyl complex [Pd(µ-Cl)(C 8 H 11 )] 2 (7). Complex 7 is an adequate precursor for the synthesis of other stable η 1 -allyl complexes, and no η 1 -η 3 allyl interconversion has been observed.
Intricately interwoven topologies are continually being synthesized and are ultimately equally versatile and significant at the nanoscale level; however, reports concerning ravel structures, which are highly entwined new topological species, are extremely rare and fraught with tremendous synthesis challenges. To solve the synthesis problem, a tetrapodontic pyridine ligand L1 with two types of olefinic bond units and two Cp*M‐based building blocks (E1, M=Rh; E2, M=Ir) featuring large conjugated planes was prepared to perform the self‐assembly. Two unprecedented [5+10] icosanuclear molecular 4‐ravels containing four crossings were obtained by parallel‐displaced π⋅⋅⋅π interactions in a single‐step strategy. Remarkably, reversible structural transformations between the 4‐ravel and the corresponding metallocage could be realized by concentration changes and solvent‐ and guest‐induced effects. X‐ray crystallographic data and NMR spectroscopy provide full confirmation of these phenomena.
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