1.2- Diimines chelate rings

Coordinated nitrogen donor atoms can be involved in chelate-forming template reactions by virtue of nucleophilic addition to carbonyl compounds. An early and rather specific example does not allow the possibility of elimination following the addition step (equation 46).171 More recent work on ruthenium(III) and osmium(III) results in the formation of a-diimine chelate rings... 

Complexes with other Bidentate 1,4-Diimines. The thermodynamic stability of the five-membered iron(II) 1,4-diimine chelate ring in low-spin complexes is illustrated by the facile autoxidation of saturated amine complexes to the corresponding colored 1,4-diimine complexes. The first example of this oxidative dehydrogenation is shown in equation (14), and the more recently studied " reaction (15) is known to involve iron(III) species and radical intermediates. Interestingly, the complex [Fe(CN)4(HN=CHCH=NH)] can be obtained by oxidizing [Fe(CN)4(en)] , whereas the tris-ligand complex of this smallest diimine ligand has yet to be prepared. 

D2J,] Ilia a-diimine chelate rings allylic type of delocalization in six-membered chelate rings C2/1 nib localized double bonds... 

Hydrazine reacts with ds-[Fe(bipy)2(CNMe)2f by a second-order reaction, involving attack by nitrogen at the cyano-carbon as the ratedetermining step. The second hydrazine-nitrogen then attacks the other cyano-carbon rapidly to give an inverted diimine chelate ring (21). Rate constants and Arrhenius parameters for this reaction were reported. The ligand oxidation reaction sequence which converts ethane-1,2-diamine coordinated to iron(II) in the [Fe(CN)4(en)] anion into the parent diimine... 

Reaction of [Ni(TMTAA)] with acid in the presence of A1C13 gives complex (606) that features two isolated diimine units in the six-membered chelate rings (Equation (21)),1535 while hydrogenation of [Ni(TMTAA)] leads to fully saturated cyclam macrocycles (607) (Equation (22)) 1536... 

A // = 40kJmol-1 AS = —204JK 1mol 1). The activation parameters indicate associative activation for both steps (212). Associative chelate ring closure here is consistent with the mechanistic pattern established in organometallic systems (213), where this process is associative for [M(CO)5(diimine)] where M = Mo or W (though dissociative for M = first-row Cr (214)) and the diimine does not carry bulky substituents. 

In 1996, Brookhart and co-workers developed a remarkable class of Pd complexes with sterically encumbered diimine ligands (Scheme 4, S4-1, S4-2, S4-4, and S4-5). These examples are capable of mediating the co-polymerization of ethylene with methyl acrylate (MA) to furnish highly branched PE with ester groups on the polymer chain ends by a chain-walking mechanism (Scheme 10). " This represents the first example of transition metal-catalyzed ethylene/MA co-polymerization via an insertion mechanism. The mechanism for co-polymerization is by 2,1-insertion of MA and subsequent chelate-ring expansion, followed by the insertion of ethylene units. The discovery of these diimine Pd catalysts has stimulated a resurgence of activity in the area of late transition metal-based molecular catalysis. Recently, the random incorporation of MA into linear PE by Pd-catalyzed insertion polymeriza-... 

The hexahydropyrimidine (58), formed from l-phenylpropane-l,2-dione and propane-1,3-diamine, is an excellent precursor for the a-diimine macrocyclic complexes (60), presumably via the amino ketone (59) (Scheme 36).126 In this case, intramolecular cyclization of (59) to (58) is reversible, so that the metal ion can exert a thermodynamic template effect in formation of the complex (60). This represents a further example of a long-known phenomenon in which a metal ion can stabilize an a-diimine structure by virtue of the formation of stable five-membered chelate rings. Many 2-hydroxy- or 2-mercapto-amines undergo reaction with a-dicarbonyl compounds to yield heterocyclic compounds rather than a-diimines. However, in the presence of suitable metal... 

Schiff bases having two nitrogen atoms as donors may be derived either from condensation of dialdehydes and diketones with two molecules of an amine, or from reaction of diamines with aldehydes or ketones. In Section 20.1.2.1, it has been pointed out that coordination through the N atom may occur only under particular circumstances. However, in the case of diimines the formation of chelate rings stabilizes the metal-nitrogen bond. Thus, they can form both mono-41 and bis-chelate42 complexes. 

Simple a-diimines are hydrolytically unstable, but can be stabilized as metal complexes by virtue of the formation of stable five-membered chelate rings.68 69 a-Diketones and glyoxal undergo metal template reactions with amines to yield complexes of multidentate ligands such as (34),70 (35)71 and (36).72>73 In the last case, the metal exerts its stabilizing influence on the a-diimine partner in an equilibrium process (Scheme 5). The same phenomenon occurs with amino alcohols74 75 in addition to amino thiols. The thiolate complexes (37) can be converted to macrocyclic complexes by alkylation in a kinetic template reaction (Scheme 5).76 77... 

Substitution of 1,3-diimine or Iminato Six-membered Chelate Rings 459... 

There are two metal centres, six chelate rings and three bridges, a total of 11 stereogenic centres, which leads to many possible isomers. Even if one limits the degrees of freedom of the chelate rings by using planar a,a -diimine ligands, there are still 16 possible isomers. In practice only two enantiomers are usually observed. 

The five- and six-membered transition metal chelate rings contain examples of compounds which have been most extensively studied, theoretically as well as experimentally. The tris-(gIyoxalbis-N-me thy limine )iron (II) chelate 23 synthesized by Krumholz 68> is the structurally simplest example of a vast number of intensely coloured chelates containing different 1,2-diimine ligand systems which have been recently reviewed 69>. (pd)n -Bonding was thought to be so important for this type of chelates that it could explain their properties by an aromatic 70> or quasiaromatic 71-72) behaviour. These types of chelates will be considered specifically in Chapt. V. 

Type B, containing two chelate rings in planar quadratic arrangement, may be observed in 1,2-diimine chelates 124 l2s> 43. But an X-ray structure determination of nickel glyoxime, 26) showed that it does not possess ideal D2h symmetry in the crystal because of contacts to atoms of neighbouring molecules. 

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