Tetranuclear carboxylates

Only four tetranuclear carboxylates have been reported so far, and they only involve small RE(III) (RE = Y, Dy, Tm, and Lu). Their structures are all centrosymmetric and fall into two types. The sttucture of linear tetramer, [YL3(H20)2]4 (L = p-hydroxybenzoate), is shown in Figure 6(a). The four Y ions are linked together by simple... 

The tetranuclear structure observed for basic zinc/acetate/[Zn40(02CCH3)6] is also observed for basic zinc pivalate and benzoate. Solution studies showed an equilibrium between hydrated basic structures and the hydrated form of 3,1 bridging structures in the presence of a trace amount of water. Zinc crotonate shows a less common basic carboxylate polymeric structure, [Zn5(OH)2 (02CCHCHCH3)8]n.369... 

A direct catalytic conversion of esters, lactones, and carboxylic acids to oxazolines was efficiently achieved by treatment with amino alcohols in the presence of the tetranuclear zinc cluster Zn4(0C0CF3)60 as catalyst, essential for condensation and cyclodehydration reactions. For example, the use of (5)-valinol allowed the easy synthesis of oxazolines 125 and 126 in satisfactory yields <06CC2711>. A one-pot direct preparation of various 2-substituted oxazolines (as well as benzoxazoles and oxadiazoles) was also performed from carboxylic acids and amino alcohols (or aminophenols or benzhydrazide) using Deoxo-Fluor reagent <06TL6497>. 

Tetranuclear manganese(III) carboxylate complexes have three different configurations for their metal centers fused open cubane, planar, or butterfly (see Figure 22). There is only one example of the first type namely [Mn4Q2(OAc)2(BSP)2] (105). The Mn Mn separations in (105) vary between 2.875(1)A and 3.122(1)A. Variable temperature magnetic measurements for (105) indicated weak antiferromagnetic interactions (/=—10.0cm and / = —3.7cm ) between the manganese(III) centers. Cyclic voltammetry of (105) in methanol shows one quasireversible oxidation wave at 0.01 V and two quasireversible reduction waves at about —0.4 V and —0.7 V vs. SCE. 

Butterfly tetranuclear manganese(III) carboxylate complexes are monoanionic with the exception of the bpy complex. An example of a butterfly core complex is (Bu 4N)[Mn402(02CPh)7-(imac)2]-5CH3CN (108). 

Use of the tripodal ligand Saltren creates a tetranuclear complex [Mn402(Sal-tren)2][MnCl4] 2CH3CN (172) having a fused open cubane structure without a carboxylate group. At 300 K the value of the effective magnetic moment for (172) is 5.1 /Ub, which decreases to 2.0 /iB at 4.2 K. The redox properties of (172) in propylene carbonate have been studied by cyclic voltammetry. Two oxidation responses are observed at = —0.32V and 0.43 V vs. SCE. 

The earliest example of a coordination tetrahedron was reported by Saalfrank and coworkers [144], Their study showed the formation of the tetranuclear magnesium(II) complex 30 bridged by six tetra-carboxylate ligands 29, representing the first reported coordination tetrahedron exhibiting M4L6 stoichiometry (Fig. 21). This serendipitous discovery was followed by the formation of other, pre-designed tetrahedral architectures [19,122,145]. 

There is no evidence for any toxic effects of chromium(III), which is an essential trace element in mammals (required daily intake 50-200 /tg) and participates in glucose and lipid metabolism. In the low-molecular-weight Cr binding substance (LMWCr), an oligopeptide, a tetranuclear Crm carboxylate complex may be present.44... 

Tetranuclear Co(III) carboxylate complexes having the formula [Co404(02CR)2(bpy)4](C104)2 have been prepared and characterized. These are built around a central cubane-type [ 0404]" + core (Figure 14) and, most interestingly, exhibit a reversible one-electron oxidation to give what are formally Co(III)3Co(rV) species. ... 

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