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Steric Aspects of the Binding of Monofunctional Platinum(II) Complexes to Sites on Nucleobases:  Metal Complex “Flatness” as a Structural Element of Speciation

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journal contribution
posted on 13.11.1998, 00:00 by Elizabeth Yuriev, John D. Orbell
Through the use of an established repulsive energy methodology, the endocyclic nitrogens of the common nucleobases have been probed by monofunctional PtII(A)3 moieties, where A ≡ NH3 and (A)3 ≡ diethylenetriamine (dien) and N1,N1,N4,N7,N7-pentamethyldiene (pmdien). These three complexes have been selected to represent, respectively, a gradual buildup of bulk on either side of the coordination plane. The values of the resulting steric parameters, ER and IS, are compared between the three complexes and benchmarked to previously reported values for the spherically symmetrical Cr(CO)5 probe. This study indicates that steric effects for coordination to nucleobases of “flatter” metal species are more subtle than those for more encompassing species and are determined predominantly by the distribution of their neighboring exocyclic substituents rather than their individual nature (e.g., amino, oxo, or methyl). A means of quantifying metal complex flatness with respect to the coordination of metal species to endocyclic nitrogens of nucleobases is suggested. For the Pt(NH3)3(nucleobase) and Pt(dien)(nucleobase) systems, a quantitative relationship has been established between steric effects and conformational flexibility, in terms of rotation about the Pt−N bond. These results suggest that, in the absence of external constraints such as crystal packing forces, the base/coordination plane dihedral angle may be inferred from steric considerations. Over a wide range of Pt/N4 dihedral angle, rotation about the Pt−N bond is, surprisingly, less sterically demanding when the flanking substituent is an amino rather than an oxo group.