A-diimine ligands

The general structure of bidentate a-diimine ligands used in this study was shown in Scheme... 

In a series of studies of the spectroscopy and photochemistry of nickel(O) -a-diimine complexes, the structural differences among the complexes NiL2 and Ni(CO)2L (L Q-diimine) have been examined by means of molecular orbital calculations and electronic absorption Raman resonance studies.2471, 472 Summing up earlier work, the noninnocence of a-diimine ligands with a flat — N=C—C=N— skeleton in low-valent Ni chemistry and the course of substitution reactions of Ni° complexes with 1,4-diaza-1,3-dienes or a,a -bipyridine have been reviewed.2473... 

Catalysts Derived from Cyclic a-Diimine Ligands. 206... 

Numerous modifications to the bis(aryl)-a-diimine ligand motif have been reported, especially in the patent literature. The review by Ittel includes an index of diimine and related ligands in proprietary publications up to the year 2000 [26]. Instead, this discussion will focus on academic reports of Ni(II) and Pd(II) polymerization catalysts bearing a-diimine or other closely related chelating neutral nitrogen ligands. 

Fig. 8 Teraryl-substituted a-diimine ligand-based Ni(II) polymerization catalysts...

A specific goal we have in mind is to design cyclophane-based a-diimine ligands for overcoming the low thermal stability of the regular a-diimine catalyst systems. 

Fig. 11 Late transition metal polymerization catalysts from cyclic a-diimine ligands...

Scheme 12 Concept of hemilabile axial donor a-diimine ligand...

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

Since their discovery over a decade ago, late transition metal a-diimine polymerization catalysts have offered new opportunities in the development of novel materials. The Ni(II) catalysts are highly active and attractive for industrial polyolefin production, while the Pd(II) catalysts exhibit unparalleled functional group tolerance and a propensity to form unusually branched polymers from simple monomers. Much of the success of these catalysts derives from the properties of the a-diimine ligands, whose steric bulk is necessary to accelerate the insertion process and inhibit chain transfer. 

Bpym is a bis(a-diimine) ligand, and its similarity to the well-known 2,2 -bipyridine (bpy) and 1,10-phenanthroline (phen) ligands has attracted the... 

The influence of steric effects on the rates of oxidative addition to Rh(I) and migratory CO insertion on Rh(III) was probed in a study of the reactivity of a series of [Rh(CO)(a-diimine)I] complexes with Mel (Scheme 9) [46]. For a-diimine ligands of low steric bulk (e.g. bpy, L1, L4, L5) fast oxidative addition of Mel was observed (103-104 times faster than [Rh(CO)2l2] ) and stable Rh(III) methyl complexes resulted. For more bulky a-diimine ligands (e.g. L2, L3, L6) containing ortho-alkyl groups on the N-aryl substituents, oxidative addition is inhibited but methyl migration is promoted, leading to Rh(III) acetyl products. The results obtained from this model system demonstrate that steric effects can be used to tune the relative rates of two key steps in the carbonylation cycle. 

A. P. Zipp, L. Sacksteder, J. Streich, A. Cook, J. N. Demas, and B. A. DeGraff, Luminescence of Phenium(I) complexes with highly sterically hindered a-diimine ligands, Inorg. Chem. 32, 5629-5632 (1993). 

A variation on the a-diimine ligand is the phenoxy-imine ligand, which has been studied mostly with the group 4 metals Ti and Zr [Ittel et al., 2000 Makio et al., 2002 Milano et al., 2002 Mitani et al., 2003 Saito et al., 2001, 2002 Tian and Coates, 2000]. A typical phenoxy-imine initiator is shown below, where R1 is usually a phenyl or substituted phenyl group. 

Diaza-1,3-butadiene (a-Diimine) Ligands Their Coordination Modes and the Reactivity of Their Metal Complexes, 21, 152 Diene-Iron Carbonyl Complexes, 1, 1... 

Figure 5-78. The nickel(n) complex of 2-aminoethanethiol reacts smoothly with biacetyl at nitrogen to give a diimine ligand.

---