Pyridine-2,6-diimine systems

Uppard, S.J. and Berg, J.M. (1994) Principles of Bioinorganic Chemistry, University Science Books, Mill Valley, CA. 

and Schwederski, B. (1994) Bioinorganic Chemistry, John Wiley Sons Ltd, Chichester. 

Nazeeruddin, M.K., Pechy, P., Renouard, T., Zakeeruddin, S.M., Humphry-Baker, R., Comte, P., liska, P., Cevey, L., Costa, E., Shklover, V., Spiccia, L, Deacon, 

Schubert, U.S., Hofmeier, H., and Newkome, G.R. (2006) Modem Ter-pyridine Chemistry, Wiley-VCH Verlag GmbH, Weinheim. 

Berreau, L.M. (2006) Activation of SmaU Molecules, Wiley-VCH Verlag GmbH Co. KGaA, Weinheim, p. 287. 

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The formation of diimine systems by Schiff -base-type condensation of suitable aldehydes and primary amines has been widely applied. Those reported are mostly strong field systems and their relevance to the spin crossover field is generally in systems of the kind [Fe(diimine)2(NCS)2]. The effect of the incorporation of substituents likely to hinder coordination has been studied. Robinson and Busch noted a fundamental difference at room temperature in the electronic properties of the [Fe N6]2+ derivatives of 2-pyridi-nalmethylhydrazone and 2-pyridinal-dimethylhydrazone, those of the former being low spin and those of the latter high spin [49]. The temperature-dependence of the magnetism of the latter complex was not reported but may well be of interest. However, spin crossover [Fe(diimine)3]2+ systems have been characterised for systems where the incorporation of appropriate substituents has reduced the ligand field. 

This contrasts with the purely high spin nature of the complex of 2,6-bis(quinolin-2-yl)pyridine [62] 46 and is consistent with the reduced steric barrier to coordination from substitution adjacent to the donor atom within five-membered rings, evident in the diimine systems. 

The initial reports of synthetic iron(II) spin crossover involved [Fe(diimine)2(NCS)2] complexes where the diimine was either 1,10-phenanthroline or 2,2 -bipyridine. The spin crossover situation persists in such complexes for a wide range of diimine species and for certain unidentate pyridine derivatives/ bridging diimine systems and bis(unidentate) systems. In addition, mixed aromatic-aliphatic N4 quadridentate donors have been incorporated into this general class. The anionic groups can be varied and spin crossover is also observed in [Fe(diimine)2X2] systems when X =NCSe, [NCBH3] TCNQ [N(CN)2] and 2X =[WS4f... 

The [Fe(diimine)2X2] system has been modified by replacing the diimines by unidentate nitrogen donors. [Fe(diimine)(py)2(NCS)2] is a crossover system when the diimine is 2,2 -bipyrimidine or phen [99] but [Fe(py)4(NCS)2] is purely high spin [100]. However, [Fe(py)4(NCS)2] systems containing substituted pyridine derivatives have been shown to exhibit thermal SCO [101], while 4,4 -bipyridine derivatives are able to bridge Fe(II) centres and form polynuclear structures containing SCO [Fe(py)4(NCS)2] centres [102]. SCO is maintained in certain instances when the diimines are replaced by an N4 quadridentate [103,104]. 

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). 

The effects of introducing halogens in the 2 and 6 position of phenyl imine catalysts was also studied in diimine pyridine iron dichloride/MAO systems [13]. These catalysts afford linear products with a low olefin content, generally less than one (olefin) functionality per chain. The latter is due to a fast transfer of iron bound alkyl groups to the aluminum compounds that are present in excess. After hydrolysis, alkanes are obtained. When a high ratio of aluminum alkyl to iron catalyst is used, polyethene waxes are obtained due to the statistically favored alkyl group exchange between the metal species. 

Baugh et al. [22] synthesized and characterized a series of nickel(II) and iron(II) complexes of the general formula [LMX2] containing bidentate (for M = Ni) and tridentate (for M = Fe) heterocycle-imine ligands. Activation of these pre-catalysts with methyl aluminoxane yields active catalyst systems for the oligomerization/polymerization of ethylene. Compared to a-diimine nickel and bis (immo)pyridine iron catalysts, both metal systems provide only half of the steric protection and consequently the catalytic activities are significantly lower. 

Many of the metal ion complexes which catalyze these reactions have the metal coordinated to a planar N-donor ligand, such as a substituted porphyrin or salen, (N,N -bis(salicylidene)-l,2-ethylene-diimine, and no alkoxide is initially present. These systems also require a co-catalyst which often is an imidazole or pyridine derivative. Hie rate of epoxide opening is found to be either first-order or second-order in the concentration of the catalyst for different systems. For a first-order system, the ring opening can be pictured as shown in the following diagram ... 

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