Trispyrazolylborates

Metalated container molecules can be viewed as a class of compounds that have one or more active metal coordination sites anchored within or next to a molecular cavity (Fig. 2). A range of host systems is capable of forming such structures. The majority of these compounds represent macrocyclic molecules and steri-cally demanding tripod ligands, as for instance calixarenes (42), cyclodextrins (43,44), and trispyrazolylborates (45-48), respectively. In the following, selected types of metalated container molecules and their properties are briefly discussed and where appropriate the foundation papers from relevant earlier work are included. Porphyrin-based hosts and coordination cages with encapsulated metal complexes have been reviewed previously (49-53) and, therefore, only the most recent examples will be described. Thereafter, our work in this field is reported. 

The preparation and characterization of several octahedral Ru(II) complexes containing s-trans coordinated dienes have been reported. The Zn mediated reduction of Ru(acac)3 in the presence of a 1,3-diene affords (diene)Ru(acac)2 complexes as a mixture of diastereo-mers (eg. 129)13a b. Reaction of [(trispyrazolylborate)RuCl]jt or [(NH3)4Ru(acetone)2]2+ [CIO4 ]2 with acyclic dienes yields complex 130 or cation 131 respectively130,14. Coordination of the ligand as an s-trans diene was indicated either by crystal structure or by determining C2v symmetry on the basis of NMR spectroscopy. 

Former studies of Moro-oka et al. with model complexes of the trispyrazolylborate ligands and have already shown,... 

Trispyrazolylborates are models for tris-histidine active sites in zinc enzymes, e.g., the matrix metalloproteinases involved in breakdown of extracellular matrices. Inhibition of these metalloproteinases may prove valuable in the treatment of, inter alios, cancer and arthritis, so efforts are being made to find appropriate ligands to block the zinc active site. The search has recently moved on from hydroxamates to hydroxypyridinones - l-hydroxy-2-pyridinone is a cyclic analogue of hydroxamic acid. As reported in Section II.B.2 earlier, hydroxypyridinones form stable five-coordinate complexes on reaction with hydrotris(3,5-phenylmethylpyrazolyl)borate zinc hydroxide. Modeling studies suggest that hydroxypyridinonate ligands should be able to access the active site in the enzyme with ease (110). 

Complexes of molybdenum form the largest set of any of the transition metals. Many of these compounds display spectroscopic properties, isomerism and structural features that illustrate the coordination chemistry of polypyrazolylborate complexes. In addition, this system illustrates that trispyrazolylborate ligands are more bulky than their formal analogue C5H5. 

Bis(trispyrazolylborate)titanium(II) complexes, preparation and reactivity, 4, 266 Bis-vinylic tellurides, preparation, 9, 605 Bis( -xylene)lead(II)bis(tetrachloroaluminate), preparation,... 

TDMPP = P[(2,6-OMe)2C6H3]3, IMes= 1,3-dimesitylimidazoli-nium, PP3 = P(CH2CH2PPh2)3, Tp=trispyrazolylborate,... 

Molecular complexes are also formed, examples being trispyrazolylborate complexes of Yb47 and /3-dike complexes of europium.48... 

An overview of trispyrazolylborate (15) complexes of iridium has been included in a wider review of Rh and Ir complexes including these and derivative ligands." " ... 

Only a handful of pseudotetrahedral or octahedral oxosulfido-Mo(VI) and -Mo(V) complexes are known, and their chemistry is described in Section 11. Mononuclear sulfido-Mo(V) complexes are unstable due to their susceptibility to redox, polynucleation, and hydrolysis reactions. Trispyrazolylborate derivatives such as sfructuraUy characterized Tp Mo SX2 (X = Cl, OPh derivative X2 = bdt, catecholate) are most commonly encountered. The Tp complexes exhibit short Mo=S bonds (ca. 2.13 A), intense S(ls)- - n (Mo=S) XAS transitions characteristic of terminal thio ligands, and EPR iso see g-Factor) values substantially lower than their 0x0 analogues. ... 

A number of pentavalent trispyrazolylborate derivatives (see Tris(pyrazolyl)borates) have been reported, especially for niobium. Most of them are monomeric Tp MCl2X (X = O, NR) derivatives, jj and jj -pyrazolato (16) or 1,2,4-triazolato derivatives have been prepared as potential sources of nitrides for CVD. The reactions between dilithium l,4-diaza-l,3-diene and pentachlorides have been investigated. ... 

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