Amines tetranuclear

Involvement of two nucleophilic nitrogen atoms is thus typical for the amino heterocycles. The mutual disposition of the pyridine and amine nitrogen atoms allows the formation of chelate structures for the cobalt complexes of purine, 221 and 222. Structures with the N, iV -five-membered metal cycles were proven for the tri- and tetranuclear complexes of silver ) with 8-aminoquinoline (223) (92IC4370), and polymeric copper- and rhodium-acetate clusters (224). Another coordination mode can be found in the complexes of 4-amino-3,5-bis(pyridin-2-yl)-l,2,4-triazole, (225 or... 

The three different tetranuclear structures which have been observed in the crystalline state are the two compact structures 6 and 8 and the chain structure 7a. Structure 6 is found in [Co4(NH3),2(OH)6]C16-8H20 and its amine analogs (52 59). The analogous ammonia and ethylenediamine chromium(III) complexes Cr4(NH3)12(OH)66+ and Cr4(en)6(0H)66+ have been characterized quite recently (40, 41, 42, 60). Structure 7a has so far been observed (42) only in a chromium(III) amine complex, Cr4(en)6(OH)66+, but, as discussed in Section IV, both structures 7b and 7c are possible structures for the tetranuclear aqua chromium(III) species. Structure 8 is known from the so-called rhodoso complex, Cr4N12(OH)66+ [N12 = (NH3)12 or (en6] (61, 62). 

The circular dichroism (CD) spectra of optically active di-, tri-, and tetranuclear complexes of chromium(III) and cobalt(III) have been reported and used to establish the complexes absolute configurations (55 59, 111, 115, 116, 152-157). The changes in circular dichroism resulting from ion pairing have been studied for the tetranuclear hexol Co j(OH)2Co(NH,)4J, h+ and have been shown to be attributable to the vicinal effect of the chiral oxygen centers produced stereospecifically by the ion-pair formation (56). For a series of trinuclear cobalt (III) amine complexes, cis-Co(CN)2[(OH)2Co(N4)2 J3 +, it was shown that the main CD contributions due to the two chiral Co(OH)4(CN)2 and Co(N4)(OH)2 centers are additive (155). In the case of the related tetranuclear complex Co((OH)2Co(en)2J,< + this postulate of additivity of CD spectra proved unsatisfactory (57). 

My co-worker J. Sedlmeier then held the view that the amine-containing iron carbonyl complexes were also ionic compounds (VII, 14, 21). Hence the compound Fe2(CO)4(en)3 (en= 1,2-ethylenediamine) was formulated as [Fe(en)3]2+[Fe(CO)4]2. Systematic investigations revealed that reactions of the iron carbonyls with other nitrogen and oxygen donors likewise involved valency disproportionation of the metal with concomitant formation of mono- and polynuclear carbonylferrates, viz., [Fe(CO)4]2, [Fe2(CO)8]2-, [Fe3(CO)n]2-. R. Werner (VII, 15, 17, 19, 20) even discovered and characterized compounds containing the tetranuclear anion [Fe4(CO)13]2, the first being that from pyridine and iron carbonyl, viz.,... 

In 3.22, one of the Zr d orbitals is unavailable for bonding due to the ancillary tridentate ligand, which positions the amine donor and the chloride to overlap effectively with that d orbital. This explanation is supported by MO calculations. X-Ray structures of related binuclear dinitrogen-Ti end-on complexes, e.g., 3.23 (Beydoun et al., 1992) support the conclusions of Fryzuk and coworkers. Compound 3.23 has (Z)-disposed neutral amine donors instead of ( )-disposed phosphine ligands in 3.22. Three other edge-on bi- and tetranuclear dinitrogen complexes are known (see Fryzuk et al., 1993, structures III-V). 

An even more interesting vista is opened up by a recent report of a tetranuclear [Mni Os] compound with the terdentate ligand (241). The reaction of aqueous solutions of the amine and MnCl2 in water with O2 produces a black solution, and the addition of NaBr, and adjustment to pH 8, gives needle-like crystals of [Mn406L4]Br3 50HQ5 6H20. 

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