Helge Mueller-Bunz

Iridium Complexes Containing Mesoionic C Donors: Selective C(sp3)H versus C(sp2)H Bond Activation, Reactivity Towards Acids and Bases, and Catalytic Oxidation of Silanes and Water

Metalation of a C2-methylated pyridylimidazolium salt with [IrCp*Cl2]2 affords either an ylidic complex, resulting from C(sp(3))-H bond activation of the C2-bound CH3 group if the metalation is performed in the presence of a base, such as AgO2 or Na2CO3, or a mesoionic complex via cyclometalation and thermally induced heterocyclic C(sp(2))-H bond activation, if the reaction is performed in the absence of a base. Similar cyclometalation and complex formation via C(sp(2))-H bond activation is observed when the heterocyclic ligand precursor consists of the analogous pyridyltriazolium salt, that is, when the metal bonding at the C2 position is blocked by a nitrogen rather than a methyl substituent. Despite the strongly mesoionic character of both the imidazolylidene and the triazolylidene, the former reacts rapidly with D(+) and undergoes isotope exchange at the heterocyclic C5 position, whereas the triazolylidene ligand is stable and only undergoes H/D exchange under basic conditions, where the imidazolylidene is essentially unreactive. The high stability of the Ir-C bond in aqueous solution over a broad pH range was exploited in catalytic water oxidation and silane oxidation. The catalytic hydrosilylation of ketones proceeds with turnover frequencies as high as 6,000 h(-1) with both the imidazolylidene and the triazolylidene system, whereas water oxidation is enhanced by the stronger donor properties of the imidazol-4-ylidene ligands and is more than three times faster than with the triazolylidene analogue.

Solvent-Dependent Switch of Ligand Donor Ability and Catalytic Activity of Ruthenium(II) Complexes Containing Pyridinylidene Amide (PYA) N-Heterocyclic Carbene Hybrid Ligands

Chelating ligands incorporating both N-[1-alkylpyridin-4(1H)-ylidene]amide (PYA) and N-heterocyclic carbene (NHC) donor sites were prepared and used for the synthesis of ruthenium(II) complexes. Cyclic voltammetry, NMR, and UV-vis spectroscopy of the complexes indicate a solvent-dependent contribution of the limiting resonance structures associated with the ligand in solution. The neutral pyridylidene imine structure is more pronounced in apolar solvents (CH2Cl2), while the mesoionic pyridinium amide form is predominant in polar solvents (MeOH, DMSO). The distinct electronic properties of these hybrid PYA-NHC ligands in different solvents have a direct influence on the catalytic activity of the ruthenium center, e.g., in the dehydrogenation of benzyl alcohol to benzaldehyde. The activity in different solvents qualitatively correlates with the solvent permittivity.

Dinuclear ruthenium complexes containing a new ditopic phthalazin-bis(triazole) ligand that promotes metal–metal interactions

Much attention has been paid to heterocyclic N-containing ligands due to their applicability as bridging ligands in the synthesis of redox active dinuclear metal complexes. With this aim, we report the synthesis and full characterization of a novel phthalazine-triazole ligand (1,4-bis(1-methyl-1H-1,2,3-triazol-4-yl)phthalazine). Moreover, we show that the phthalazine nitrogen atoms of this N-heterocyclic ligand are more reactive towards alkylating agents than the triazole groups. New ruthenium(II) complexes containing this ligand have been obtained and characterized both structurally and electrochemically. The geometry imposed by the ligand allows the placement of two ruthenium centers in very close proximity so that efficient through-space interactions take place, a concept of crucial importance for electron transfer processes.

Synthesis of a sterically modulated pyridine–NHC palladium complex and its reactivity towards ethylene

A new cationic pyridine-carbene palladium complex has been prepared that features a C,N-bidentate coordinating ligand with a shielded pyridine and a sterically less protected carbene moiety; evaluation of this complex in ethylene polymerization revealed competitive reductive elimination processes and provides guidelines for further catalyst design.

(2S,4S)-3-Benzoyl-4-benzyl-2-tert-but-yl-1,3-oxazolidin-5-one.

In the title compound, C21H23NO3, the central oxazolidinone ring is approximately planar, the maximum deviation from the plane through the central ring being 0.043 (1) Å. The tert-butyl and benzyl substituents are cis to each other and trans to the N-benzoyl group. The interplanar angle between the aromatic rings of the C-benzyl and N-benzoyl groups is 81.10 (4)°.

tert-Butyl-dimethyl-silanol hemihydrate.

The crystal structure of the title compound, C6H16OSi·0.5H2O, reveals an asymmetric unit containing two molecules of the silanol and a single water molecule. There is evidence of hydrogen bonding between the three molecules in the asymmetric unit. The H atoms of the silanol OH groups and the water H atoms are each disordered equally over two positions.