Selective preparation of pyridine derivatives from two different alkynes and a nitrile was achieved by a novel procedure in which an alkyne and a nitrile couple first to give an azazirconacyclopentadiene followed by reaction with the second alkyne in the presence of 1 equiv of NiCl(2)(PPh(3))(2). This procedure gives only single products of pyridine derivatives from two different symmetrical alkynes and a nitrile. Our novel procedure can be used even with two similar alkyl-substituted alkynes such as 3-hexyne and 4-octyne. Two possible pyridine isomers from 3-hexyne, 4-octyne, and acetonitrile could be completely and independently prepared as single products by this method. The origin of the selectivity comes from the addition order of two different alkynes. This method was applied for the formation of pyridones and iminopyridines using isocyanate and carbodiimide derivatives instead of nitriles, respectively. Reaction of an alkyne with Cp(2)ZrEt(2) and an isocyanate or a carbodiimide gives an azazirconacycle. Treatment of the azazirconacycle with the second alkyne in the presence of 1 equiv of NiCl(2)(PPh(3))(2) gave a pyridone or an iminopyridine derivative. The use of two different unsymmetrical alkynes afforded the pyridine with five different substituents when the first alkyne has a trialkylsilyl group and the second alkyne has a phenyl group as functional groups. On the other hand, azazirconacyclopentadienes reacted with propargyl bromide in the presence of CuCl with excellent regioselectivity to give tetrasubstituted pyridine derivatives as single products. With the assistance of the trialkylsilyl groups, pyridines with all different substituents including H were also prepared.

The Cr(III) porphyrin complexes [Cr(tpp)(Cl)(H2O)] (1) and [(Cr(tpp)(Cl)(py)] (2) (tpp represents the dianion of 5,10,15,20-tetraphenylporphine) crystallized from a chloroform–toluene mixture in the tetragonal space group I4, Z = 2, a = 13.559(5), b = 13.559(5), c = 9.770(3) Å, V = 1796(3) Å3, and from a dichloroethane–toluene mixture containing a small amount of pyridine (py) in the monoclinic space group P21/n, Z = 4, a = 14.655(5), b = 23.498(4), c = 13.152(2) Å, β = 101.54(1)°, V = 4437(2) Å3, respectively. The axial Cr–O bond length for 1 and the axial Cr–N bond length for 2 are 2.239(3) and 2.140(5) Å, respectively. Laser irradiation of the toluene solution of [Cr(tpp)(Cl)(L)] causes the photodissociation of the axial ligand L, where L represents H2O or 3-cyanopyridine, to give the five-coordinate intermediate [Cr(tpp)(Cl)]. The rate constant of the axial ligand recombination reaction falls into a narrow range around 1 × 109 mol−1 kg s−1 at 25.0 °C for both reactions. The activation parameters indicate the very high reactivity of the five-coordinate intermediate. The diffusion-controlled process is regarded as a rate-determining step for the recombination.

A series of mono- and dinuclear Ru(bpy)2 complexes (bpy = 2,2‘-bipyridine) containing 2,2‘-bis(benzimidazol-2-yl)-4,4‘-bipyridine (bbbpyH2) were prepared. The mononuclear complex [Ru(bpy)2(bbbpyH2)](ClO4)2·CH3OH·4H2O was characterized by an X-ray structure determination. Crystal data are as follows: triclinic, space group P1̄, a = 14.443(4) Å, b = 15.392(4) Å, c = 11.675(2)Å, α = 101.44(2)°, β = 107.85(2)°, γ = 96.36(2)°, V = 2380(1) Å3, Z = 2. The coordination geometry of the ruthenium(II) ion is approximately octahedral. The dihedral angle between the two pyridyl rings in bbbpyH2 is 9.4(3)°, which is close to coplanar, in the complex. Mono- and dinuclear complexes exhibit broad charge-transfer absorption bands at 420−520 nm and emission at 660−720 nm in CH3CN solution with lifetimes of 200−800 ns at room temperature. Transient difference absorption spectra and resonance Raman (rR) spectra were used to assign the charge-transfer bands in the 420−520 nm region and to identify the lowest excited states. Both absorption and emission spectra are sensitive to solvent and solution pH. Deprotonation of the dinuclear complex raises the energies of the π* orbitals of the bbbpyH2 ligand, so that they become closer in energy to the π* orbitals of bpy. The intervalence band of [(bpy)2Ru(bbbpyH2)Ru(bpy)2]5+ is observed at 1200 nm ( ε = 170 M-1 cm-1) in CH3CN. The value of the electronic coupling matrix element, H AB, was determined as 120 cm-1. Upon deprotonation, the IT band was not observed. It is therefore concluded that a superexchange pathway occurs predominantly via the Ru(II) dπ−bbbpyH2 π* interaction, since deprotonation decreases the interaction. The role of the intervening fragments in the bridging ligand is discussed from the viewpoint of orbital energies and their orbital mixing with Ru dπ orbitals.