Experimental and theoretical study of dimer-of-dimers-type tetrarhodium(ii) complexes bridged by 1,4-benzenedicarboxylate linkers

Abstract

Two dimer-of-dimers-type tetrarhodium complexes, [Rh₄(piv)₆(BDC)] ([1]; piv = pivalate) and [Rh₄(piv)₆(F₄BDC)] ([2]), in which two paddlewheel-type dirhodium units are linked by 1,4-benzenedicarboxylate (BDC) and 1,4-tetrafluorobenzenedicarboxylate (F₄BDC), respectively, have been synthesized and characterized via single-crystal X-ray diffraction analyses, ESI-MS, ¹H NMR, infrared spectroscopy, Raman spectroscopy, and elemental analyses. Crystal structure analyses of [1(THF)₄] and [2(THF)₄], which are crystallized from THF solutions of [1] and [2], respectively, revealed that dihedral angles (ϕ) between two –CO₂ units and phenyl rings of the BDC linker in [1(THF)₄] are almost co-planar (ϕ = 2.8°), whereas those of the F₄BDC linker in [2(THF)₄] are largely inclined (ϕ = 78.3°). Density functional theory calculations clarified that (i) their dihedral angles of optimized geometries of [1(THF)₄] and [2(THF)₄] are almost the same as their experimental geometries, and (ii) the rotation energy barriers of phenyl moieties in [1(THF)₄] and [2(THF)₄] estimated by potential energy surface analyses are 12.0 and 8.4 kcal mol⁻¹, respectively, indicating that hydrogen bondings are formed between two –CO₂ units and four hydrogen atoms of phenyl rings of the BDC linker in [1(THF)₄], whereas two –CO₂ units and four fluorine groups on the phenyl ring of the F₄BDC linker in [2(THF)₄] are electrostatically and sterically repulsed. Electrochemical properties and electronic structures of [1(THF)₄] and [2(THF)₄] are strongly influenced by the electronic states of dicarboxylate linkers, whereas absorption spectra are strongly influenced by the dihedral angles between two –CO₂ units and phenyl rings of dicarboxylate linkers.