Ligand multitopic

The tetradentate ligands (340) and (341) form 1 1 metakligand complexes with [IrCl(cod)]2.548 The complexes were tested in the asymmetric hydrogenation of prochiral olefins, providing enantioselectivities up to 36%. The multitopic ligands L, (342) and (343), bind to Ir1 to form [IrL] species which have been characterized by elemental analysis, mass spectrometry, IR and NMR spectroscopy.549 The complexes show enantioselectivities of up to 30% for the hydrogenation of prochiral olefins under mild reaction conditions. 

Exodentate multitopic ligands are used to link transition metal ions into building blocks. Some examples of such ligands are 2,4,6-tris(4-pyridyl)-l,3,5-triazine, oligopyridines, and 3- and 4-pyridyl-substituted porphyrins. 

Romero-Salguero, F.J., and Lehn, J.-M., Synthesis of multitopic bidentate ligands based on alternating pyridine and pyridazine rings. Tetrahedron Lett., 40, 859, 1999. 

Figure 1.2 A selection of MOFs based on tetranuclear Zn40(C02)6, dinuclear Cu2(C02)4 and ID Zn202(C02)2 secondary building units (left) and a range of multitopic carboxylate ligands (top). Reprinted with permission from D. Britt, D. Tranchemontagne and O.M. Yaghi, Proc. Natl. Acad. Set. U.S.A, 105, 11623.

Principal attention in the incorporation of radical multitopic ligands into MOF phases has focused on the well known Tt-acceptors TCNE (tetracya-noethylene) and TCNQ (7,7,8,8-tetracyanoquinodimethane), which in their mononegative forms have an unpaired spin that can couple with... 

Use of exodentate multitopic ligands that also act as nodes in the architecture of the resulting polymer. This strategy is less predictable but nevertheless has already generated a number of high-symmetry mineralomimetic architectures. 

The unlimited possibilities of designing multidentate and multitopic ligands that take advantage of the coordination geometry of specific metal ions to assemble a variety of supramolecular multinuclear architectures are far beyond the scope of this chapter, and the reader will find inspiring and fascinating examples in several reviews, as well as in other chapters of this work (see Self-Assembled Links Catenanes, Templated Synthesis of... 

The self-supported heterogeneous BINOL-Ti catalysts developed by Ding [280], using in situ assembly of bridged multitopic BINOL ligands and Ti(O Pr)4 in the presence of water, showed excellent enantioselectivities in the oxidation of sulfides and can be recycled eight times over a month without loss of activity or enantioselectivity [136b] (Scheme 14.118). 

Another direction has been to develop heterogeneous applications of diruthenium paddlewheels [97]. By using multitopic carboxylate ligands, for example, 1,4-benzenedicarboxyate, RUj tetracar-boxylates are assembled into microporous networks. Interesting applications such as the catalytic hydrogenation of alkenes, the oxidation of primary aliphatic alcohols, and photocatalytic hydrogen production from water have been reported [98-101]. 

As a different immobilization strategy for the heterogeneous asymmetric catalyst. Ding et cd. developed a self-supported polymer catalyst for the flow asymmetric reduction [140]. Self-assembly of chiral multitopic ligands and reactive metal ions afforded homochiral metal-organic coordination polymers, which exhibited... 

Coordinative bonding has also been used to assemble polynuclear metallo-helicates where multitopic ligands wrap around cations of appropriate coordination... 

Figure 2.42. Molecular structures of new multitopic bipyridine-bridged diethynylthiophene ligands and schematic representation of energy hopping and end-trapping.

The preparation of ferrocenyl substituted pyridines, such as 4-ferrocenylpyiidine and l,r-di(2-pyiidyl)feiTocene (L), allowed the preparation of complexes with formula [Rh(cod)L2]CI04. The multitopic ligands, A,A -bis[(A)-prolyl)phenylenediamine, A,A -bis [(A)-pyrrolidin-2-yl]methyl)phenylenediamine, A,Af -bis[(6 )-A-benzylprolyl]phe-nylenediamine, and A,A -bis [(A)-Af-benzyl-pyrrolidin-2-yl]methyl)phenylenediamine, were synthesized and their coordination properties with Rh(i) studied. The ligand bis(bipyridine)-calix[4]arene (BBPC) reacted with 2 equiv. of [Rh(nbd)2]BF4 or [Rh(nbd)(CH3CN)2]BF4 yielding the bimetallic compound [Rh(nbd)(BBPC)]2[BF4]2 396. ... 

Fig. 13 Supramolecular, redox-active polymers formed by coordination bonding, yielding redox-active gels (utilizing multitopic cyclam bis-terpyridine ligands, CHTT) (reprinted with permission from [210], Copyright 2009 American Chemical Society)...

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