The mechanism of a trinuclear cooperative dehydrogenative C−N bond‐forming reaction is investigated in this work, which avoids the use of chelate‐assisting directing groups. Two new highly efficient Ru/Cu co‐catalyzed systems were identified, allowing orders of magnitude greater TOFs than the previous state of the art. In‐depth kinetic studies were performed in combination with advanced DFT calculations, which reveal a decisive rate‐determining trinuclear Ru–Cu cooperative reductive elimination step (CRE).
Nucleophilic substitution of [(η 5 -cyclopentadienyl)(η 6 -chlorobenzene)iron(II)] hexafluorophosphate with sodium imidazolate resulted in the formation of [(η 5 -cyclopentadienyl)(η 6 -phenyl)iron(II)]imidazole hexafluorophosphate. The corresponding dicationic imidazolium salt, which was obtained by treating this imidazole precursor with methyl iodide, underwent cyclometallation with bis[dichlorido(η 5 -1,2,3,4,5-pentamethylcyclopentadienyl]iridium(III) in the presence of triethyl amine. The resulting bimetallic iridium(III) complex is the first example of an NHC complex bearing a cationic and cyclometallated [(η 5 -cyclopentadienyl)(η 6 -phenyl) iron(II)] + substituent. As its iron(II) precursors, the bimetallic iridium(III) complex was fully characterized by means of spectroscopy, elemental analysis and single crystal X-ray diffraction. In addition, it was investigated in a catalytic study, wherein it showed high activity in transfer hydrogenation compared to its neutral analogue having a simple phenyl instead of a cationic [(η 5 -cyclopentadienyl)(η 6 -phenyl)iron(II)] + unit at the NHC ligand.
Starting from [(η5‐cyclopentadienyl)(η6‐phenyl)iron(II)]imidazole, dicationic imidazolium salts were prepared by N‐alkylation. Reaction of these compounds with basic metal precursors such as mesityl copper(I) or palladium(II) acetate led to mono respectively dicationic transition metal NHC complexes (NHC=N‐heterocyclic carbene). Transmetalation using the copper(I) complexes opened up the access to NHC gold(I) compounds. PEPPSI‐type NHC complexes of palladium(II) and platinum(II) were prepared by offering a neutral pyridine ligand to the transition metal center. A rhodium(I) NHC complex was accessible by deprotonation of the dicationic imidazolium precursor and subsequent treatment with [(COD)Rh(μ2‐Cl)]2 (COD=1,5‐cyclooctadiene). The new NHC complexes were investigated by means of NMR spectroscopy, mass spectrometry as well as single crystal X‐ray structure analysis. Both, the palladium(II) containing PEPPSI‐type and the gold(I) complex, were investigated for their catalytic properties in typical model reactions such as cyclization reactions, Suzuki coupling and cyanation. In addition, a selenium adduct was synthesized in order to study the electronic properties of the underlying ligand backbone. Based on the chemical shift in the 77Se NMR spectrum, it is evident that these NHC ligands possess rather poor π‐acidity.
The synthesis and characterization
of a series of novel cationic
multimetallic transition-metal complexes based on the cationic phosphine
ligand (η6-diphenylphosphinobenzene)(η5-cyclopentadienyl)iron(II) hexafluorophosphate (1) are reported. Complexes of ligand 1 with the late
transition metals ruthenium, osmium, rhodium, and iridium as well
as palladium and platinum were isolated in generally good yields,
and the solid-state structures of most of them were determined. On
the basis of the 195Pt–31P NMR coupling
constant measured for trans-(1)2PtCl2 and the carbonyl absorption band in the IR
spectrum of trans-(1)2Rh(CO)Cl,
the electronic influence of the ligand on the metal center was evaluated.
These measurements are supported by density functional theory (DFT)
calculations, performed on the corresponding tricarbonylnickel(0)
complex in order to determine the Tolman electronic parameter (TEP)
of ligand 1.
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