A wide array of forms of palladium has been utilized as precatalysts for Heck and Suzuki coupling reactions over the last 15 years. Historically, nearly every form of palladium used has been described as the active catalytic species. However, recent research has begun to shed light on the in situ transformations that many palladium precatalysts undergo during and before the catalytic reaction, and there are now many suggestions in the literature that narrow the scope of types of palladium that may be considered true “catalysts” in these coupling reactions. In this work, for each type of precatalyst, the recent literature is summarized and the type(s) of palladium that are proposed to be truly active are enumerated. All forms of palladium, including discrete soluble palladium complexes, solid-supported metal ligand complexes, supported palladium nano- and macroparticles, soluble palladium nanoparticles, soluble ligand-free palladium, and palladium-exchanged oxides are considered and reviewed here. A considerable focus is placed on solid precatalysts and on evidence for and against catalysis by solid surfaces vs. soluble species when starting with various precatalysts. The review closes with a critical overview of various control experiments or tests that have been used by authors to assess the homogeneity or heterogeneity of catalyst systems.

On the Nature of the Active Species in Palladium Catalyzed Mizoroki–Heck and Suzuki–Miyaura Couplings – Homogeneous or Heterogeneous Catalysis, A Critical Review

π-Bonding and the Lone Pair Effect in Multiple Bonds between Heavier Main Group Elements

The chemistry of expanded porphyrins, which are higher homologues of porphyrins, has been intensively explored for the last three decades. Expanded porphyrins exhibit structures, electronic properties, coordination chemistry, and reactivities that are entirely different from those of porphyrins. Through these studies, it has become increasingly apparent that expanded porphyrins are attractive in views of aromaticity and multimetal coordination, or as functional dyes, nonlinear optical materials, ion receptors, or stable organic radicals. As such, we have continuously witnessed the emergence of expanded porphyrins that exhibit unprecedented structures and properties, as is highlighted by the facile realization of Mobius aromatic and even antiaromatic systems with twisted molecular structures. In this Review, the recent progress of the chemistry of expanded porphyrins after the seminal Review by Sessler and Seidel in 2003 is presented.

Expanded Porphyrins: Intriguing Structures, Electronic Properties, and Reactivities

Photoinduced metal-free atom transfer radical polymerization (ATRP) of methyl methacrylate was investigated using several phenothiazine derivatives and other related compounds as photoredox catalysts. The experiments show that all selected catalysts can be involved in the activation step, but not all of them participated efficiently in the deactivation step. The redox properties and the stability of radical cations derived from the catalysts were evaluated by cyclic voltammetry. Laser flash photolysis (LFP) was used to determine the lifetime and activity of photoexcited catalysts. Kinetic analysis of the activation reaction according to dissociative electron-transfer (DET) theory suggests that the activation occurs only with an excited state of catalyst. Density functional theory (DFT) calculations revealed the structures and stabilities of the radical cation intermediates as well as the reaction energy profiles of deactivation pathways with different photoredox catalysts. Both experiments and calculation...

Mechanism of Photoinduced Metal-Free Atom Transfer Radical Polymerization: Experimental and Computational Studies.

Multifunctional ligands with podand topology provide intrinsically well-defined coordination geometries. Several subgroups of multidentate ligand systems comprising dual functionality have been developed so far. Recent advances made in this research area reflect its topicality. Work on metal complexes of ambiphilic ligands consisting of Lewis-acidic Group 13 element bridgehead atoms and additional donor functionalities is in the current focus of interest. The intrinsic topology of tetradentate ligands may introduce a fascinating reactivity and interesting electronic properties to the metal complexes. Janus-head type ligands are very promising candidates for the preparation of multimetallic complexes.

Multidentate ligand systems featuring dual functionality

Isolation of some sterically protected unsymmetrical diphosphenes: nature of the phosphorus-phosphorus double bond

Remarkable structure deformation in phenothiazine trimer radical cation.

A complete series of copper(II) halide complexes [CuX(tptm)]
				(X = F (1), Cl (2), Br (3), I (4); tptm = tris(2-pyridylthio)methyl) with a novel CuII–C(sp3) bond has been prepared by the reactions of [Cu(tptm)(CH3CN)]PF6
				(5·PF6) with corresponding halide sources of KF or n-Bu4NX (X = Cl, Br, I), and the trigonal bipyramidal structures have been confirmed by X-ray crystallography and/or EPR spectroscopy. The iodide complex 4 easily liberates the iodide anion in acetonitrile forming the acetonitrile complex 5 as a result. The EPR spectra of the complexes showed several superhyperfine structures that strongly indicated the presence of spin density on the halide ligands through the Cu–X bond. The results of DFT calculations essentially matched with the X-ray crystallographic and the EPR spectroscopic results. Cyclic voltammetry revealed a quasi-reversible reduction wave for CuII/CuI indicating a trigonal pyramidal coordination for CuI states. A coincidence of the redox potential for all [CuX(tptm)]0/+ processes indicates that the main oxidation site in each complex is the tptm ligand.

A complete series of copper(II) halide complexes (X = F, Cl, Br, I) with a novel Cu(II)-C(sp3) bond.

An ab initio calculation on proton transfer in the benzoic acid dimer

The contribution of 3d orbitals of sulfur atom to the electronic structures of the sulfur compounds, H2CS, CH3SH, C6H5SH, and C6H5SCH3 have been studied by means of ab initio MO method. The results have been compared with those of the corresponding oxygen compounds in order to clarify the peculiar properties of the sulfur compounds. The global molecular properties are scarcely affected by the 3d orbitals of sulfur atom since they are energetically highly located. However, the local characters such as charge distributions and LUMO are largely varied by the participation of 3d orbitals.

Toshio Matsushita

Papers

Isolation of some sterically protected unsymmetrical diphosphenes: nature of the phosphorus-phosphorus double bond

Remarkable structure deformation in phenothiazine trimer radical cation.

A complete series of copper(II) halide complexes (X = F, Cl, Br, I) with a novel Cu(II)-C(sp3) bond.

An ab initio calculation on proton transfer in the benzoic acid dimer

An MO study of organic sulfur compounds. Comparison with the corresponding oxygen compounds.