Mononuclear ligand-stabilized complexes

In 1996 Stack and co-workers reported an unusual 3 1 (copper 02 stoichiometry) reaction between a mononuclear copper(I) complex of a A-permethylated (lR,2R)-cyclohexanediamine ligand with dioxygen. The end product of this reaction, stable at only low temperatures (X-ray structure at —40 °C) is a discrete, mixed-valence trinuclear copper cluster (1), with two Cu11 and a Cu111 center (Cu-Cu 2.641 and 2.704 A).27 Its spectroscopic and magnetic behavior were also investigated in detail. The relevance of this synthetic complex to the reduction of 02 at the trinuclear active sites of multicopper oxidases4-8 was discussed. Once formed, it exhibits moderate thermal stability, decomposed by a non-first-order process in about 3h at —10 °C. In the presence of trace water, the major isolated product was the bis(/i-hydroxo)dicopper(II) dimer (2). 

The L-N3 ligand has also been used to stabilize mononuclear [Mo O] complexes possessing pendant phosphate ester groups (42) and Mo "P distances of 4-8 A (80, 81). The NMR linewidths are strongly dependent upon the Mo—P distance, and reasonable Mo—P distances were derived from longitudinal relaxation times using the Solomon equation 154). Molecular modeling studies of Mo-co indicate that phosphate coordination to the Mo atom is stereochemically feasible, but the P ENDOR studies of the Mo(V) states of xanthine oxidase (67) estimate the Mo—P distance as 7-12 A and seem to rule out such a structure. 

Heteroleptic organolanthanide complexes containing reactive Ln-X bonds and stabilizing ancillary ligands are key precursor compounds in catalytic transformations. Mononuclearity is usually a prerequisite for both good solubility and reactivity. Utilization of bulky ligands, ate complexation, and donor functionalization are applicable procedures for generating mononuclear complexes. 

Finally, insertion of alkynes into dihydrogen mononuclear cationic rhenium complexes, stabilized by the triphos (= l,l,l-tris(diphenylphosphinomethyl)ethane) ligand, leads to vinylidene derivatives which react with water or ethanol eliminating silanols or methane, Fig. 18. Solvents were acetone or THF no chromatography was used. 

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