Mixed N/O ligands

The known chemistry of Mn1" with nitrogen ligands, other than the macrocycles (Section 41.4.8), is still quite limited but see also the mixed N, O ligands, Section 41.4.7. Perhaps the tendency for Mn1" to disproportionate, in many ligand systems, is responsible for the failure to obtain [Mn N(SiMe3)2 3], when these species have been obtained84 for all other metals Scm to Fem. 

The crystallographic structure of the D. vulgaris protein has been reported by Lindley and collaborators (132). The structure was solved to a resolution of 1.7 A. The major findings are consistent with most of the conclusions derived from the Mossbauer work done in the D. desulfuricans protein. The protein was found to contain two distinct clusters of the same nuclearity. Also, one cluster has a mixed N, O, S ligand environment, while the other has a regular iron-sulfur core... 

The bidentate, mixed N,P ligand (o-amino-phenyl)diphenylphosphine-N,P (Hzapp, (188)) reacts with (34) or (227) to yield the six-coordinate cationic complex [TcNCl(H2app)2]+ (262), in which the amines are not deprotonated. The X-ray crystal stnirtiirp of this romr.lpx was elucidated, showing coordination of the two bidentate ligands with mutual cis P coordination. 

Mixed N—O donor atom sexa- and octadentote ligands 69... 

Mixed N—O donor atom quinque- and sexadentate ligands 96... 

Schiff bases commonly employ polyamines as reagents, the donor nature of the ligands depending both on the type of aldehyde or ketone used and the nature of primary amine or diamine. Scheme 1 illustrates some ways of obtaining Schiff bases of different denticities via 1 1 reactions of aldehydes with ketones to yield mixed N, O, S, and Se donor sets. Condensation of aldehydes and ketones with a-amino acids is also known. Where two moles of a carbonyl compounds and one mole of diamine are reacted, more elaborate Schiff bases with higher denticity result, also exemplified in Scheme 1. 

A key step towards hydrogenation catalysts with broader substrate scope was the development of mixed N,P ligands. Such ligands represent an even more effective way of desymmetrization than diphosphines, based on the different electronic properties of the two coordinating heteroatoms, a soft P atom with o-donor and 7c-acceptor properties and a hard N atom with mainly o-donor character (Figure 3). 

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