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Mechanical vs Electronic Strain: Calculated Configurations of Alkynyl-Pt(II)-Phosphine Macrocycles

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posted on 06.04.2020, 20:43 by Eric A. Buchanan, Josef Michl
Optimized geometries of macrocycles composed of four Pt­(PR3)2 linkers and four rods terminated with ethynyl or pyridyl moieties have been calculated at the PBE0/Def2-TZVPP//PBE0/Def2-SVP level of density functional theory for all combinations of cis and trans configurations at the Pt centers. For uncharged complexes with four p,p′-bis­(ethynyl)­biphenyl rods and neutral Pt centers, the energy of the planar oval-shaped all-trans isomer lies 17 kcal/mol below that of the puckered square-shaped all-cis isomer. In this case, the electronic strain associated with the cis arrangement at the Pt atoms overrules the mechanical strain associated with the bending of triple bonds. For cationic complexes containing two bipyridyl and two p,p′-bis­(ethynyl)­biphenyl rods, with a +1 charge on each Pt center, a puckered rectangular structure with all-cis Pt centers is found to be 38 kcal/mol lower in energy than the isomer with all four Pt centers trans. The results have been analyzed in terms of three additive factors, referred to as electronic, mechanical, and electrostatic strains.