Tetradentate amine

The inability of amines to deprotonate upon coordination, and thus to compensate the charge of the MOs+ core, can be overcome by combination with other types of donors-including N-donor groups-in the ligand, as has already been discussed above for oxocyclam. A prominent example is provided by technetium complexes of tetradentate amine oxime ligands. 

The branched tetradentate ligand tris(2-aminoethyl)amine (tren) forms rather stable metal complexes with most transition metal ions. It is a very hard and basic ligand and consequently its iron(II) complexes are all high-spin. Later we will discuss hexadentate derivatives of this ligand which form crossover complexes (see Sect. 3.2). 

The combination of pyridine and aliphatic nitrogen donors creates an intermediate ligand field which has yielded a long list of spin crossover systems [2]. Recently, Kahn et al. obtained two new tetradentate ligands in which two 2-pyridylmethyl groups replace two hydrogen atoms on the same amine function in 1,2-diaminoethane and 1,3-diaminopropane (4, 5) [5-6]. 

About twenty years ago we reported on the di-isothiocyanato iron(II) complex of the tetradentate ligand tpa (tris(2-pyridylmethyl)amine) [7] (6). It was shown that this complex exhibits the spin crossover phenomenon with a critical temperature Tm of about 170 K. Several different solvated phases of the same system have since been characterized by Chansou et al. [8]. The unsolvated phase which can be isolated from an aqueous solution has been investigated by nuclear forward scattering (NFS), nuclear inelastic scattering (NIS) [9], extended x-ray absorption fine structure (EXAFS) spectroscopy, conventional Mossbauer spectroscopy, and by measurements of the magnetic susceptibility (SQUID) [10-13]. The various measurements consistently show that the transition is complete and abrupt and it exhibits a hysteresis loop between 102 and 110 K. 

Itoh et al. (151) employed a tetradentate amine and synthesized the complex [Znn(l)(MeCN)]PF6, where 1 represents the monoanion of 2-methylthio-4-ferf-6-[[bis[2-(2-pyridyl)ethyl]amino]methyl]phenol (see Fig. 31 for 1). This complex was chemically [with (NH4)2[Ce(N03)6]) or electrochemically oxidized yielding the (phenoxyl)zinc species Zn1 (1 )(MeCN )]PF6. It displays phenoxyl K-K tran-... 

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