1,4-diazadienes ligands

Complexes of the type (COT )Ln(DAD(THF) (Ln = Sm, Yb COT = CgHfj(SiMe3)2-l,4 DAD = 1,4-diaza-dienes) have been prepared in a one-pot reaction by treatment of elemental Sm or Yb with equimolar amounts of l,4-bis(trimethylsilyl)cyclooctatriene and 1,4-diazadiene ligands (Scheme 228). H and 171Yb NMR data show that the Sm derivatives contain Sm3+ ions and coordinated DAD radical anions, while in the case of Yb the neutral DAD ligand is coordinated to divalent ytterbium.813... 

With the formation of 2, two interdependent stereocentres are formed, the bridgehead iron and carbon atoms. The imine carbon atoms in 1 are prochiral, and the alkyne can approach either of them from either the re- or the si-face. With chiral substituents at the nitrogen atoms, the approach of one face may be favoured over the other. This would lead to diastereoselectivity which has been investigated with different types of diazadiene ligands and a series of chiral N-substituents. Depending on both the type of diazadiene (C2 and non-C2 symmetric) and the chiral N-substituents, diastereoselectivities from 0 to > 99% have been observed. 

Kreisel et al. [41] reported on a dinuclear compound that exhibits a very short chromium-chromium distance. The geometry around each chromium atom is trigonal planar with each metal being coordinated by two N atoms from two different diazadiene ligands as well as by the neighboring Cr atom (Figure 9.2). The extremely short Cr-Cr distance of 1.8028(9) A, made it the shortest metal-metal distance reported when the paper was published. 

The complexes with diazadiene ligands (Mo-5) were also examined for OMRP and ATRP. The sttucture of the ligands and the polymerization conditions were cmcial for the dual activity for example, the diaryl diazadiene complex... 

Employing chiral diazadiene ligands, the dimerization of butadiene or the codimerization of isoprene and 1,3-pentadiene can be performed in an enantioselective manner. The latter provides (-)-l,7-dimethyl-l,5-cyclooctadiene in 89% yield and up to 61% ee using the C2-symmetric (l/ )-menthyl-diazadiene ligand. 

Stoffelbach F, Richard P, Poli R, Jenny T, Savary C. Half-sandwich Mo(III) complexes with asymmetric diazadiene ligands. Inorg Chim Acta. 2006 359 4447-4453. lizuka Y, Li ZM, Satoh K, et al. Chiral (-)-DIOP ruthenium complexes for asymmetric radical addition and living radical polymerization reactions. Eur J Org Chem. 

These include pyrazolylborates such as Tp RuH(H2)2,31 and diazadiene compounds of RN=CR —CR =NR ligands.32 Nitrido and imido compounds are discussed in Section 18-F-ll there are a few amido complexes known.33... 

Cationic alkyl complexes of nickel and palladium of type (22-XXXII) stabilized by rigid chelating diazadiene (DAD) ligands polymerize 1-alkenes the nickel complex is particularly active.122 The activities are significantly higher than those of neutral nickel complexes. 

Diazadiene or diimine ligands of the general type R-N=CH-CH=N-R (Rdab) are known to stabilize complexes in low oxidation states. Cloke and coworkers have... 

Another type of nitrogen containing ligands is diazadienes or di-imines of the general form R-N=CH-CH=N-R (DAD). Although many transition metal complexes with these ligands have been synthesized, the preparation of titanium analogs is not easily achieved. These titanium compounds are extremely air and moisture sensitive... 

Metal vapour s)mthesis in rare-earth chemistry has been used as early as 1977 to prepare rare-earth butadiene and alk)me complexes by interaction of butadienes and alk)mes (respectively) with vaporised metals (Evans, 1987 Evans et al., 1977), and we have seen in Section 3 that low-valent scandium complexes can also be made by this technique. Additionally, condensation of rare-earth vapours with a 7i-acceptor such as 1,4-di-(t-Butyl)diazadiene (DAD) has produced compounds of general formula [R(DAD)3], except with scandium where the composition is [Sc(DAD)2]. Extensive metal-ligand electron transfer is effective in these molecules so the real oxidation state of the rare earth (at least at room temperature) is in fact + 3. Note that the first structurally characterised organosamarium(II)... 

---