Platinum complexes with 2,2 :6 ,2"terpyridine

Apart from the platinum(ii) diimine system, analogous platinum(ii) systems with planar tridentate ligands are also hot topics in current chemical research. Their useful physical and biological properties, such as interaction with DNA and proteins, were first reported by Lippard and coworkers [150, 151]. Subsequent reports by Che and coworkers described the luminescence properties of a series of platinum(ii) complexes with terpyridine ligands, [Pt(trpy)X] (X = Cl, Br, I, N3,... 

For example, in 1963 the photochemistry of magnesium phthalocyanine with coordinated uranium cations was studied in pyridine and ethanol and indicated the occurrence of PET to the uranium complex . A rapid photoinduced electron transfer (2-20 ps) followed by an ultrafast charge recombination was shown for various zinc and magnesium porphyrins linked to a platinum terpyridine acetylide complex . The results indicated the electronic interactions between the porphyrin subunit and the platinum complex, and underscored the potential of the linking para-phenylene bisacetylene bridge to mediate a rapid electron transfer over a long donor-acceptor distance. 

From the studies reviewed above it has become evident that competion studies between thiols/thioethers and intact double-helical DNA are required, to find out whether or not the formation of the Pt-GG chelate is a driving force that can overcome the Pt-S interactions. Even then, one should realize that we are only dealing with a model, as in the cell other metals might also play a role in the disruption of Pt-S bonds. In this respect it should be mentioned that it was recently reported that addition of transition metals such as Zn11 or Cu11 can cleave even the Pt-S bond in thiolated terpyridine-platinum complexes at neutral pH [87],... 

Many recents studies have focused on applications of metallointerca-lation, which is also an important noncovalent interaction of metal complexes with nucleic acids. Intercalation is a common mode of association of small molecules with DNA, where a flat aromatic heterocyclic moiety inserts and stacks in between the DNA base pairs (13). Lippard and coworkers (14) determined in 1974 that platinum(II) complexes containing an aromatic heterocyclic ligand such as terpyridine could intercalate in DNA. Figure 2c shows such a stacking interaction of such a complex in a dinucleotide (15). Recently, we have found that intercalation is not restricted to completely flat, square planar complexes, but partial intercalation of ligands coordinated to octahedral metal centers is feasible as well... 

Platinum(ii)-containing complexes with inherently chiral binaphthol derivatives display a versatile scaffold between random coils and singleturn helical strands, in which the conformational transition is controlled by the Pt... Pt and n-n interactions of allq7nylplatinum(ii) terpyridine moiety upon solvent and temperature modulation. ... 

The intercalation of platinum complexes containing bidentate or tridentate aromatic amines was first proposed for binding to tRNA and then to DNA [53]. The structures of the relevant complexes of palladium and platinum are shown in Figure 1.5 and a requisite is the presence of planar ligands such as bipyridine, terpyridine, or o-phenanthroline, which are coplanar with the coordination plane formed by the four ligand donor atoms and the metal ion. The similar pyridine derivative, [Pt(en)py2], where the pyridine Ugands are forced by nonbonded steric constraints to be out of the coordination plane, does not intercalate. 

The X-ray crystal structure of a [Pt(terpy)Cl]—AMP complex has also been briefly reported [59]. In this case, the complex consists of a hydrogen bonded AMP base pair, that mimics the rare Hoogsteen model [60] with NH2—Ni and NH2—Ny hydrogen bonds (see Appendix 1) sandwiched between two platinum—terpyridine molecules. The nucleotide is not locked into a helix in this structure and so the sequence can be described as I—I—BP—I—I—BP, emphasizing the propensity of platinum complexes for stacking interactions [61]. 

A wide-ranging study of several cyclometallated analogs of platinum(ii) terpyridine/chloro complexes has been undertaken.Crystal structures were determined, and substitution kinetics experiments in methanol were conducted. The nucleophiles Br and Gl were employed, and in two cases, [Pt(N-N-C)Cl] and [Pt(N-G-N)Gl], substitution kinetics with TU, DMTU, and TMTU were measured. N-N-CH is 6-phenyl-2,2 -bipyridine, N-CH-N is l,3-di(2-pyridyl)benzene, TU is thiourea, and DMTU and TMTU are the dimethyl and tetramethyl analogs of TU. The complexes are schematically represented below 5. 

The strong cr-donating ability of the alkynyl unit that is capable of raising the energy of the LF excited states, together with the interest in metal alkynyl systems, has led to the synthesis of the first series of luminescent alkynyl complexes of platinum]ii) terpyridine, [Pt]trpy)C=C-R], which was reported by Yam and coworkers [156]. The complexes were prepared in reasonable yields by reacting the platinum]ii) terpyridyl complex, [Pt]trpy)]NCMe)]]OTf)2, with various aryl-alkynes in the presence of sodium hydroxide or triethylamine in methanol [Scheme 10.20). 

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