Pyridine-diimine ligands

Gibson described the synthesis of four-coordinate cationic aluminium alkyls 1 which were reported to be well-defined aluminium polymerization catalysts [12]. However, the polydispersities of the products obtained were high (2.9-6.3), showing that there is not a single well-defined active species. The experiments were carried out in metal autoclaves, and Fe and Co complexes of pyridine-diimine ligands are extremely active in ethene polymerization [34], so a transition-metal impurity does not seem an unreasonable explanation. 

Complexes with pyridine-2,6-diimine ligands, five-coordinate [NiX2(174)] (X = C1, Br) or six-coordinate [Ni(174)2]X2, were usually assumed to have innocent neutral ligands. The X-ray structure and spectroscopic characteristics of [Ni(174)2](PF6) confirm that the complex contains the neutral ligand ([174] ) and a divalent nickel ion.579 The cyclic voltamogram of this complex in CH2C12 shows three reversible one-electron-transfer processes at = 1.19 V, —1.30 V, and — 1.82V (vs. Fc+/Fc), of which the first is a one-electron oxidation, while the other two correspond to two successive one-electron reductions. The spectroscopic data allow one to assign these processes as follows ([174]1 is a one-electron reduced radical form of [174] ) [Nini(174)°2]3+ [NiII(174)02]21 - " [NiI(174)°2]+ = " [NiI(174)°(174)1 ]°. 

Scheme 13 Hemilabile axial pyridine donor a-diimine ligand and complexes...

Let us start with the pyridine-2,6-diimine ligand (L) shown in Scheme 14.110... 

Low-valence transition metal complexes of a-diimine ligands are highly colored because of the presence of low-energy metal to a-diimine charge transfer (MLCT) transitions. For a series of d6-M(CO)., (a-diimine) (M=Cr,Mo,W) and d8- M (CO)3 (a-diimine) (M =Fe, Ru) complexes, we have studied the spectroscopic and photochemical properties (1-10). The a-diimine ligands used are 1,4-diaza-1,3-butadiene (R-DAB), pyridine-2-car-baldehyde-imine (PyCa), 2,2 -bipyridine (bipy) or 1,10-phenanthroline (phen) molecules. A close relationship was deduced between the photochemical behavior of these complexes and their resonance Raman (rR) spectra, obtained by excitation into the low-energy MLCT band. 

In Table 1, the diimine ligands were either 2,2/-bipyridine or 1,10-phenanthroline, or one of their derivatives, although 2,2,-bipyrazinc and derivatives of 2,2 -bipyrimidine are also listed. Some monodentate chelating ligands contained one or more pyridine functionality and could bind an... 

Other diimine ligands are shown in (104-106) with systems containing one pyridine and one other heterocyclic mono-aza ligand, and the C atoms of the py ring can be replaced by one or more N atoms to give, for example, (107). Other coupled sets are pyrazole plus pyrazole (108), pyrazole plus imidazole (109), and imidazole plus imidazole (110) together with coupled pyrazines (111), pyrimidines (112), (113), and pyridazines (114). 

Condensation of an aldehyde or ketone with a primary amine yields a Schiff base, a class of compounds that have been widely studied as chelating ligands. iV,iV -Diimine ligands, resulting from condensation with pyridine-2-aldehyde or 2-pyridyl ketones, are discussed in Section 8.2.2, and N,0-ligands are discussed here. The condensation products of... 

Figure 5. Illustrative example of an EPR spectrum of a ligand-centred radical species in isotropic solution at room temperature. The spectrum reveals hyperfine couplings with all nitrogen and hydrogen atoms of the le-reduced pyridine-2,6-diimine ligand.

The debate on hydroxide attack at coordinated diimine and pyridine ligands coordinated to transition metal ions such as Fe " continues. Catalysis of hydrolysis of the diimine ligand tptz, (34), by Cu " has been suggested to proceed by attack by hydroxide or by water at one of the pyridine rings of tptz when it is coordinated to the copper. ... 

Fig. 3.23 Schematic structures of the Os3(CO)io(diimine) clusters, of their zwitterions and biradicals, and of the diimine ligands used R-PyCa = pyridine-2-carbaldehyde iV-alkyllmine R-AcPy = 2-acetylpyridine JV-alkylimine R-DAB = N,N -dialkyl-l,4-diaza-1,3-butadiene. Copyright American Chemical Society 1998 [121]...

The diimine ligands 2-(phenylazo)pyridine and its p-methyl derivative [(33) R = H or Me] form bis complexes [Ru(LL)2XY]" with ruthenium(II). The complexes with X = Y = Cl, Br, or I (n = 0) react with tertiary phosphines by a second-order process with /c2 values apparently controlled by the bulk of the entering ligand. The complexes with X = py, Y = OH2, and X = Y = OH2 n = 2) react quite quickly with donor solvents such as acetronitrile or dimethyl sulfoxide the bis-aquo complex also reacts rapidly with pyridine. Complexes d5-[Ru(LL)2X2], when LL = an aryl-azooxime [(34) with R = aryl], readily lose HX on addition of a base such... 

When a bulky P N ligand was used, the electronically favored 2,1-insertion and the sterically favored 1,2-insertion compete with each other, providing a regioirregular styrene/CO copolymer. Regiocontrolled but atactic copolymers were obtained using pyridine-imidazole or diimine ligands. 

On the basis of these analyses, we hypothesized the use of an earth-abundant transition-metal complex containing a non-redox-active metal center and redox-active (non-innocent) ligand as a catalyst. We demonstrated our prediction on [(PDl)Ca(THF)3], where PDI is a non-innocent pyridine-2,6-diimine ligand, and the catalyzed benzyhc (of the MeCH2Ph substrate) C-H bond alkylation by unsubstituted and diphenyl (termed the iio or-(io or)-substituted diazocarbene precursors, N2CH2 and N2CPh2. 

In recent years, this section of this report has been dominated by the photochemical and photophysical properties of Re(CO)3(diimine)X complexes. Although the main deluge of research is over, there are still examples to be found. The photochemical/physical properties of monomer and polymer species with ReI(CO)3(phen) chromophores has been investigated by Wolcan and Ferraudi, and proton-controlled photoisomerisation of rhenium(I) tricarbonyl bipyridine linked to amine or azacrown ether groups by a styryl pyridine bridging ligand has been discovered by Perutz et alP... 

---