TETRADENTATE MACROCYCLIC COMPLEXES OF PLATINUM - EVALUATION OF THE STEREOCHEMICAL ALTERATIONS OF REDOX BEHAVIOR AND THE X-RAY CRYSTAL-STRUCTURE OF (1,4,7,10-TETRATHIACYCLODECANE)PLATINUM(II) CHLORIDE

The X-ray crystal structure of Pt([12]aneS4)2+ ([12]aneS4 = 1,4,7,10-tetrathiacycloduodecane) shows that the Pt(II) ion is displaced 0.33 A from the S4 plane but that the Pt-S bond length is 2.29 angstrom, the same as found earlier (Waknine et al. Inorg. Chem. 1991, 30,3691) for Pt([14]aneS4)2+ ([14]aneS4 = 1,4,8,11-tetrathiacyclotetradecane). This bond length is about 0.04 A shorter than expected on the basis of comparisons of covalent radii and on the basis of Cu-S bond lengths found for Cu(II) complexes with the same ligands. The shortened Pt-S bond suggests a relatively large pi-back-bonding contribution from the electron-rich Pt(II) to the sulfur ligand. The increasingly positive potential for the two-electron electrochemical oxidation of the Pt([n]aneS4)2+ complexes in chloride media as n decreases from 16 to 14 (Waknine et al.) has been shown by molecular mechanics calculations to originate mostly from the stereochemical repulsions between axial chloride in the Pt(IV) product and the equatorial [n]aneS4 ligand. A component of the observed electrochemical shifts appears to arise from the varying angular disposition of the pi-acceptor orbitals of the bonding S atoms as the ring size is changed. These varying angular, or ''electronic'', components of the Pt-S bond also result in substantial changes in the energies and intensities of the lowest energy metal-centered transitions. [Pt([12]aneS4)]Cl2.H2O crystallizes in the tetragonal space group P4/n with Z = 2, a = b = 10.8760 (9) angstrom, and c = 7.830(2) angstrom. The structure refined to R = 0.023 and R(w) = 0.028 for 793 unique, observed reflections.