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

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.