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

Paddlewheel-type and half-paddlewheel-type diruthenium(II,II) complexes with 1,8-naphthyridine-2-carboxylate

Paddlewheel-type diruthenium(II,II) complexes are paramagnetic with two unpaired electrons (S = 1) and can be utilized as versatile building blocks for higher-order structures, such as supramolecular complexes, coordination polymers, and metal-organic frameworks, although they are generally highly air-sensitive. In this study, we developed an air-stable paddlewheel-type diruthenium(II,II) complex with two electron-withdrawing 1,8-naphthyridine-2-carboxylate (npc) ligands, [Ru2(μ-npc)2(O2CMe)2] (1). The two acetate ligands in 1 can be replaced by other carboxylate ligands; the solvothermal reactions of 1 with benzoic acid (HO2CPh) yields the heteroleptic [Ru2(μ-npc)2(O2CPh)2] (2), whereas its reaction with 1,8-naphthyridine-2-carboxylic acid (Hnpc) produces the homoleptic [Ru2(μ-npc)2(η2-npc)2] (3). The molecular structures of 1-3 were characterized using paramagnetic 1H NMR, ESI-TOF-MS, elemental analyses, and single-crystal X-ray diffraction, which revealed that 1 and 2 form conventional paddlewheel-type structures, where two npc and two carboxylate ligands coordinate to the Ru2 core in a cis-2 : 2 arrangement, whereas 3 forms a half-paddlewheel-type structure, with the Ru2 core coordinated by two bridging μ-npc and two chelating η2-npc ligands. Temperature-dependent magnetic susceptibility measurements of 1-3 showed large zero-field splittings (D = 227, 238, and 240 cm-1, respectively) due to the Ru24+ center, and their effective magnetic moments at 300 K, ranging from 2.78 to 2.90μB, are consistent with the spin-only value of 2.83μB for an S = 1 system. Electrochemical analyses revealed that 1-3 are redox-active and undergo reversible redox processes; their cyclic voltammetry (CV) diagrams showed an oxidation wave associated with the Ru25+/Ru24+ process and two sequential reduction waves corresponding to the reduction of two npc ligands. Notably, 1-3 show intense broad absorption bands at approximately 500-800 nm, theoretically assigned to the metal-ligand charge transfers (MLCTs) from the d(Ru2) to π*(npc) orbitals.

Unique vapochromism of a paddlewheel-type dirhodium complex accompanied by dynamic structural and phase transitions

The one-dimensional coordination polymer [Rh2(HA)4]n (1G; HA = hexanoate) exhibits a drastic vapochromic color change from green to red upon exposure to pyridine (py) vapor. Heating the red discrete complex [Rh2(HA)4(py)2] (1R) at 338 K affords the purple discrete tetrarhodium complex [Rh2(HA)4(py)]2 (1P), which is an intermediate species in the vapochromic transformation of 1G to 1R. The obtained complexes 1G, 1R, and 1P differ not only in their color in the solid state, but also in their temperature-dependent phase transition properties.

Coordination-Induced Self-Assembly of a Heteroleptic Paddlewheel-Type Dirhodium Complex

A novel heteroleptic paddlewheel-type dirhodium (Rh2) complex [Rh2(O2CCH3)3(PABC)] (1; PABC = para-aminobenzenecarboxylate), which has an amino group as a potential donor site for coordination with the metal ion, was synthesized and characterized by 1H NMR, ESI-TOF-MS, infrared spectra, and elemental analysis. The slow evaporation of N,N-dimethylformamide (DMF)-dissolved 1 produces the purple-colored crystalline polymeric species [Rh2(O2CCH3)3 (PABC)(DMF)]n (1P). Single-crystal and powder X-ray diffraction analyses, as well as thermo-gravimetric analysis, clarified that 1P formed a one-dimensional polymeric structure, in which the two axial sites of the Rh2 ion in 1P are coordinated by a DMF molecule and an amino group of the PABC ligand of the neighboring molecule 1, by coordination-induced self-assembly (polymerization) with an Rh-amino bond. The reversible structural change (self-assembly and disassembly transformations) between the discrete species [Rh2(O2CCH3)3(PABC)(DMF)2] (1D; green solution) and the polymeric species 1P (purple solid) was accompanied by a color change, which easily occurred by the dissolution and evaporation procedures with DMF.

Synthesis, Characterization, Absorption Properties, and Electronic Structures of Paddlewheel-Type Dirhodium(II) Tetra-μ-(n-naphthoate) Complexes: An Experimental and Theoretical Study

The reactions of [Rh₂(O₂CCH₃)₄(OH₂)₂] with n-naphthalenecarboxylic acids (n = 1: 1-HNC, n = 2: 2-HNC) afford the dirhodium tetra-μ-(n-naphthoate) complexes [Rh₂(1-NC)₄] (1) and [Rh₂(2-NC)₄] (2), respectively. Single crystal X-ray diffraction analyses of [1(OCMe₂)₂] and [2(OCMe₂)₂], which were obtained by recrystallization from acetone (OCMe₂) solutions of 1 and 2, reveal that the dirhodium cores are coordinated by four equatorially bridging naphthoate ligands and two axial OCMe₂ ligands. Density functional theory (DFT) calculation confirmed that (i) the single Rh⁻Rh bond is formed between the two Rh ions and (ii) the electronic structures between two Rh ions in [1(OCMe₂)₂] and [2(OCMe₂)₂] are best described as π⁴δ²σ²δ*²π*⁴ and δ²π⁴σ²δ*²π*⁴, respectively. Time-dependent DFT (TDDFT) calculations clarify the absorption band characters of [1(OCMe₂)₂] and [2(OCMe₂)₂]; the former shows the bands due to d⁻d and metal⁻to⁻metal-ligand charge transfer (MMLCT) excitations in the visible light region, whereas the latter shows the bands due to only d⁻d excitations in the same region. The electrochemical properties and thermal stabilities of [1(OCMe₂)₂] and [2(OCMe₂)₂] were also investigated in this study.