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

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.

Mixed valency in quadruple hydrogen-bonded dimers of bis(biimidazolate)dirhodium complexes

Dirhodium complexes with biimidazole (H2bim) ligands [Rh2(O2CR)2(H2bim)2Cl2] (R = Bu ([1Cl2]), Pr ([2Cl2])), [Rh2(O2CBu)2(H2bim)2](PF6)2 ([1](PF6)2), [Rh2(O2CBu)2(H2bim)2(PPh3)2](PF6)2 ([1(PPh3)2](PF6)2), and [Rh2(O2CPr)2(H2bim)2(PPh3)2]Cl2 ([2(PPh3)2]Cl2) have been synthesized. Deprotonation of the biimidazole complexes afforded the quadruply hydrogen-bonded dimers of the biimidazolate complexes [Rh2(O2CR)2(Hbim)2(PPh3)2]2 (R = Bu ([1'(PPh3)2]2) and Pr ([2'(PPh3)2]2)). Complementary hydrogen bonds between the Hbim(-) ligands are not coplanar because the Hbim(-) ions in one dirhodium complex are not parallel (the dihedral angle between them is ca. 15°). A cyclic voltammogram of [1'(PPh3)2]2 shows two sets of two consecutive oxidation waves in CH2Cl2. The one-electron-oxidized species of [1'(PPh3)2]2 showed no intervalence charge-transfer band in the electronic spectrum and an axially symmetrical ESR spectrum with hyperfine structure because of two phosphorus atoms. These observations show that the odd electron is localized in a σ(Rh-Rh) orbital on one dirhodium unit. Theoretical calculations indicate that an oxidized complex [Rh2(O2CMe)2(bim)2(PMe3)2](-) hydrogen bonded with a biimidazole complex [Rh2(O2CMe)2(H2bim)2(PMe3)2](2+) was a stable mixed-valence complex.

Assembled structures of tetrakis(biimidazole)dirhodium complexes hydrogen-bonded with common inorganic anions

Eight new structures of dirhodium complexes, each with four biimidazole (H2bim) ligands, were obtained: [Rh2(H2bim)4(H2O)2](NO3)4·4H2O (I), [Rh2(H2bim)4(H2O)2](ClO4)4·5H2O (II), [Rh2(H2bim)4(MeOH)2](ClO4)4 (III), [Rh2(H2bim)4(DMF)2](BF4)4 (IV), [Rh2(H2bim)4(Mepy)2](SiF6)2·8H2O (V), [{Rh2(H2bim)4(pz)}2(μ-pz)](SiF6)(ClO4)6·12.7H2O (VI), [{Rh2(H2bim)4(pz)}2(μ-pz)](ClO4)8·11.4H2O (VII) and [Rh2(H2bim)4(μ-pz)](SiF6)2·6H2O (VIII). The unbridged Rh-Rh bond distances range between 2.6313 (8) and 2.7052 (5) Å. The dirhodium units adopt a staggered conformation with torsion angles N-Rh-Rh-N of 37.6 (4)-48.98 (8)°. Various assembled structures were constructed by hydrogen bonding between the complexes and the anions: a discrete structure in (IV), one-dimensional structure in (II), two-dimensional structures in (I), (III), (VI), (VII) and (VIII) and a three-dimensional structure in (V).

Two- and three-dimensional assembled structures constructed from amidate-bridged paddlewheel complexes with group 6 oxometallate ions

Monocationic acetamidate-bridged dinuclear units are two-dimensionally connected by axially coordinating group 6 oxometallate ions in the structures [{M2(acam)4}2(M'O4)] (M = Rh, Ru; M' = Cr, Mo, W; Hacam = acetamide). In these sheet structures, dinuclear units are connected to each other by NH...O hydrogen bonds. The sheets are also connected with NH...O hydrogen bonds. Amidate-bridged dirhodium units that have bulkier alkyl groups reacted with CrO4(2-) or MoO4(2-) give three-dimensional diamondoid structures [{Rh2(pram)4}2(CrO4)] · 1.08H2O, [{Rh2(pram)4}2(MoO4)] · 2.44H2O and [{Rh2(buam)4}2(MoO4)] (Hpram = propionamide; Hbuam = butyramide). In these structures the NH group of the amidate ligands hydrogen bonds with O atoms in metallate ions.