Phosphate coordination, monodentate

Figure 4-49. Hydrolysis of a monodentate phosphate resulting from intramolecular attack by a coordinate hydroxide nucleophile.

Fig. 18. Average environment of calcium ions in amorphous calcium phosphates as determined by X-ray absorption spectroscopy near the K absorption edge of calcium. The diagram shows two bidentate and two monodentate phosphate tetrahedra and two water molecules contributing to the eight oxygens of the first coordination sphere of the calcium ions. A third shell, possibly comprising the two phosphorus atoms of the monodentate phosphate ions, can be seen in some of the preparations. The positions of the protons are not established (Holt et al., 1988, 1989b Holt and Hukins, 1991).

Monodentate phosphate compounds. The monodentate phosphate compounds are stronger Lewis bases and have a higher coordinating ability than ethers. Their basicity is in the order phosphate <... 

As seen from the data in Table II, the enzyme enhances the eflFect of bound Mn on the C-3 protons of acetol phosphate, indicating the formation of an enzyme-Mn-substrate bridge complex. However, the enzyme-bound Mn has a smaller effect on the C-1 protons, indicating that the presence of the enzyme alters the structure of the Mn coordination complexes. Thus, in the absence of enzyme, from distance calculations and from stability constants, inorganic Mn forms a monodentate phosphate complex with acetol phosphate. 

Figure 14 Catalytic mechanisms proposed for the purple acid phosphatases (1) attack of a terminal hydroxide on the Fe on a monodentate phosphate ester substrate coordinated to the divalent metal site (2) attack of the bridging hydroxide on a bridging phosphate ester (3) attack of a hydroxide ion generated in the second coordination sphere of the Fe on a monodentate phosphate ester.

A combination of kinetic and labeling studies established a mechanism involving attack by copper-bound hydroxide, followed by PO bond cleavage. Further details of the mechanisms of these reactions has come from a detailed study of the hydrolysis of ci5-[Ir(en)2(0H) 0P(=0)(0R)2 ] (36) complexes (R = ethyl or 4-nitrophenyl). The reaction involves intramolecular attack by coordinated hydroxide, and rate enhancements of 10 are found. The products of the reactions are not the chelated phosphate esters (37) expected from a knowledge of cobalt(III) chemistry, but monodentate phosphate monoesters (38). This is assigned to relative differences in the sizes of the metal ions and the basicity of the coordinated... 

Figure 8. Bidentate (type A, left) and monodentate (type B, right) phosphate coordination to tantalum ions, with the possibility for the formation of intermolecular hydrogen bonding.

Perhaps the most extensively studied catalytic reaction in acpreous solutions is the metal-ion catalysed hydrolysis of carboxylate esters, phosphate esters , phosphate diesters, amides and nittiles". Inspired by hydrolytic metalloenzymes, a multitude of different metal-ion complexes have been prepared and analysed with respect to their hydrolytic activity. Unfortunately, the exact mechanism by which these complexes operate is not completely clarified. The most important role of the catalyst is coordination of a hydroxide ion that is acting as a nucleophile. The extent of activation of tire substrate througji coordination to the Lewis-acidic metal centre is still unclear and probably varies from one substrate to another. For monodentate substrates this interaction is not very efficient. Only a few quantitative studies have been published. Chan et al. reported an equilibrium constant for coordination of the amide carbonyl group of... 

In addition to the remarkable effects on reactivity, there is significant cooperativity between the two metal centers in the dinuclear Co(m) complex (24) for coordinating phosphate diesters. The equilibrium constant for monodentate coordination of dimethyl phosphate to a mononuclear Co(m) complex (Figure 6.5) is only about 2.8 m 1 [43], but is >330 m 1 for bridging dimethyl phosphate to the dinuclear Co(m) center in 24 [75],... 

Purple acid phosphatases (PAPs) catalyze the hydrolysis of phosphate monoesters with mildly acidic pH optima (5-7) utilizing a binuclear metal center containing a ferric ion and a divalent metal ion. PAPs are also characterized by their purple color, the result of a tyrosine (Tyr) to Fe3+ charge transfer transition at about 560nm.113 All known mammalian PAPs are monomeric and have a binuclear Fe3+-Fe2+ center, whereas the kidney bean and soybean enzymes are dimeric and have an Fe3 + -Zn2+ center in each subunit. The X-ray structures for kidney bean PAP114 and the PAP115 from rat bone reveal that despite a sequence similarity of only 18%, they share very similar catalytic sites. The structure of the kidney bean PAP shows the two metal ions at a distance of 3.1 A, with a monodentate bridging Asp-164. These and other residues involved in metal coordination can be seen in Fig. 21. 

There is a great variety of coordination patterns of the derived anionic ligands (dichalcogeno-imidodiphosphinates and -phosphates), which can be described as monodentate, bidentate chelating (symmetrical or unsymmetrical) and bridging, or with the aid of metal connectivity terminology.120,121 Several types can be distinguished (Table 7) ... 

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