Coordination binding modes

Tetranuclear Ni11 complexes (889), (890a), and (890b) with unusual /i3-l,l,3 and /r4-1,1,3,3 azide binding modes have been obtained from discrete pyrazolate-based dinuclear building blocks.2141 (890a) and (890b) represent the only known examples of such EE/EO /i4 coordination of azide in molecular compounds. 

The results obtained have suggested a sandwich conformation of the host-guest complex in which DABCO was protected from the solvent and other molecules. The cross-peaks between coordinated DABCO and complex protons in the 2D ROESY spectra have indicated the accommodation of the DABCO molecule in the host cavity and confirmed the sandwich binding mode. 

The nature of the target to be attacked by any drug obviously depends on the specific application. Many cytotoxic metal complexes target DNA because of its importance in replication and cell viability. Coordination compounds offer many binding modes to polynucleotides, including outer-sphere noncovalent binding, metal coordination to nucleobase and phosphate backbone... 

Coordination Chemistry and Binding Modes of Amino Acids... 

The versatile binding modes of the Cu2+ ion with coordination number from four to six due to Jahn-Teller distortion is one of the important reasons for the diverse structures of the Cu-Ln amino acid complexes. In contrast, other transition metal ions prefer the octahedral mode. For the divalent ions Co2+, Ni2+, and Zn2+, only two distinct structures were observed one is a heptanuclear octahedral [LnM6] cluster compound, and the other is also heptanuclear but with a trigonal-prismatic structure. 

Beside four-coordinate Be2+ structures, a couple of five-coordinate structures are known, e.g., Dehnicke s [BeCl(12-crown-4)]+ (179). The combination of beryllium cations and different crown ethers is of particular interest, not only because the Nobel Prize was awarded in part for the development of crown ethers (15), but also because crown ethers are common building blocks in all kinds of chemistry (180,181), with a wide range of applications. Since Be2+ is the smallest metal ion, the binding modes of Be2+ and crown ethers can lead to unexpected structural motifs. 

The availability of different metal ion binding sites in 9-substituted purine and pyrimidine nucleobases and their model compounds has been recently reviewed by Lippert [7]. The distribution of metal ions between various donor atoms depends on the basicity of the donor atom, steric factors, interligand interactions, and on the nature of the metal. Under appropriate reaction conditions most of the heteroatoms in purine and pyrimidine moieties are capable of binding Pt(II) or Pt(IV) [7]. In addition, platinum binding also to the carbon atoms (e.g. to C5 in 1,3-dimethyluracil) has been established [22]. However, the strong preference of platinum coordination to the N7 and N1 sites in purine bases and to the N3 site in pyrimidine bases cannot completely be explained by the negative molecular electrostatic potential associated with these sites [23], Other factors, such as kinetics of various binding modes and steric factors, appear to play an important role in the complexation reactions of platinum compounds. 

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