The photoionization and dissociative photoionization of Im(iPr)2, 1,3-diisopropylimidazolin-2-ylidene, was investigated by imaging photoelectron photoion coincidence (iPEPICO) with vacuum ultraviolet (VUV) synchrotron radiation. A lone-pair electron of the carbene carbon atom is removed upon ionization and the molecular geometry changes significantly. Only 0.5 eV above the adiabatic ionization energy, IEad =7.52±0.1 eV, the carbene cation fragments, yielding propene or a methyl radical in parallel dissociation reactions with appearance energies of 8.22 and 8.17 eV, respectively. Both reaction channels appear at almost the same photon energy, suggesting a shared transition state. This is confirmed by calculations, which reveal the rate-determining step as hydrogen-atom migration from the isopropyl group to the carbene carbon center forming a resonance-stabilized imidazolium ion. Above 10.5 eV, analogous sequential dissociation channels open up. The first propene-loss fragment ion dissociates further and another methyl or propene is abstracted. Again, a resonance-stabilized imidazolium ion acts as intermediate. The aromaticity of the system is enhanced even in vertical ionization. Indeed, the coincidence technique confirms that a real imidazolium ion is produced by hydrogen transfer over a small barrier. The simple analysis of the breakdown diagram yields all the clues to disentangle the complex dissociative photoionization mechanism of this intermediate-sized molecule. Photoelectron photoion coincidence is a promising tool to unveil the fragmentation mechanism of larger molecules in mass spectrometry.
β,β'-Bisporphyrins are intrinsically chiral porphyrin dimers with fascinating properties. The configurational stability at their axes can be directed by variation of the central metal atoms. Herein, we present a regioselective functionalization of the monomeric 2-amino-tetraphenyl-porphyrin as a versatile substrate for dimerization by oxidative coupling. By simple variation of the reaction conditions (solvent and oxidant), the oxidation selectively gave either the axially chiral C,C-coupled diaminobisporphyrin in high yields or, under Ullmann conditions, the twofold N,C-linked achiral dimer, also in good yields. A generalized mechanism for the coupling reaction is proposed based on DFT calculations. The axially chiral β,β'-coupled porphyrin dimers were isolated as racemic mixtures, but can be resolved by HPLC on a chiral phase. TDDFT and coupled-cluster calculations were used to explain the spectroscopic properties of the aminoporphyrins and their dimers and to elucidate the absolute configurations of the C,C-coupled bisporphyrins.
The synthesis of different ruthenium(II) complexes with an [Ru(NHC)4]2+ (NHC = N‐heterocyclic carbene) core is reported. The reaction of [(η6‐C6H4MeiPr)RuCl(μ‐Cl)]2 with 1 equiv. of the NHC iPr2Im in thf leads to the clean formation of [(η6‐C6H4MeiPr)Ru(iPr2Im)Cl2] (1), but use of an excess of the carbene at higher temperatures affords side products such as [Ru(iPr2Im)4H]Cl (9). Complexes of the type [Ru(R2Im)4Cl2] [R = Me (2), nPr (3), MeiPr (6)] were synthesized by the reaction of [Ru(PPh3)3Cl2] with Me2Im, nPr2Im, and MeiPrIm. Compound 6 was isolated as a mixture of isomers that differ in the relative orientation of the asymmetrically substituted NHC ligand with respect to the Ru–Cl vector. Replacement of the chlorido ligands in 2 and 3 with acetonitrile leads to the formation of [Ru(R2Im)4(CH3CN)]Cl2 [R = Me (4), nPr (5)]. At higher temperatures, complex 6 eliminates HCl under C–H activation of one of the NHC ligand iPr groups to give the cyclometallation product 7. The reaction of [Ru(PPh3)3Cl2] with iPr2Im afforded the cyclometallation product [Ru(iPr2Im)3{iPr(C3H6)Im}Cl] (8) or the hydrido complex [Ru(iPr2Im)4(H)]Cl (9) as the main product depending on the reaction conditions. The complexes 1, 3, 4, and 7 as well as two isomers of 6 were structurally characterized.
Synthesis and characterization of molybdenum bisimido complexes featuring the extremely bulky Pr* framework [Pr* = 2,6‐bis(diphenylmethyl)‐4‐methylphenyl] are reported. The octahedral halide complex [Mo(NPr*)2Cl2(THF)2] (1) is readily available from ammonium dimolybdate and the amine Pr*NH2, and features two THF molecules in the coordination sphere of molybdenum. The chloride ligands in 1 may be exchanged with methyl or benzyl groups using Grignard reagents. The resulting complexes [Mo(NPr*)2Me2] (2) and [Mo(NPr*)2Bz2] (3) are monomeric and, in contrast to 1, do not coordinate further neutral two‐electron donor ligands in neither the solid state nor in solution.
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