Energy‐Level Tailoring in a Series of Redox‐Rich Quinonoid‐Bridged Diruthenium Complexes Containing Tris(2‐pyridylmethyl)amine as a Co‐Ligand

Abstract

Reactions of [{Ru(tmpa)}2(μ-Cl)2][ClO4]2, (2[ClO4]2, tmpa=tris(2-pyridylmethyl)amine) with 2,5-dihydroxy-1,4-benzoquinone (L1), 2,5-di-[2,6-(dimethyl)-anilino]-1,4-benzoquinone (L2), or 2,5-di-[2,4,6-(trimethyl)-anilino)]-1,4-benzoquinone (L3) in the presence of a base led to the formation of the dinuclear complexes [{Ru(tmpa)}2(μ-L1-2H)][ClO4]2 (3[ClO4]2), [{Ru(tmpa)}2(μ-L2-2H)][ClO4]2 (4[ClO4]2), and [{Ru(tmpa)}2(μ-L3-2H)][ClO4]2 (5[ClO4]2). Structural characterization of 5[ClO4]2 showed the localization of the double bonds within the quinonoid ring and a twisting of the mesityl substituents with respect to the quinonoid plane. Cyclic voltammetry of the complexes show two reversible oxidation and quinonoid-based reduction processes. Results obtained from UV/Vis/NIR and EPR spectroelectrochemistry are invoked to discuss ruthenium- versus quinonoid-ligand-centered redox activity. The complex 3[ClO4]2 is compared to the reported complex [{Ru(bpy)}2(μ-L1-2 H)]2+ (12+, bpy=2,2′-bipyridine). The effects of substituting the bidentate and better π-accepting bpy co-ligands with tetradentate tmpa ligands [pure σ-donating (amine) as well as σ-donating and π-accepting (pyridines)] on the redox and electronic properties of the complexes are discussed. Comparisons are also made between complexes containing the dianionic forms of the all-oxygen-donating L1 ligand with the L2 and L3 ligands containing an [O,N,O,N] donor set. The one-electron oxidized forms of the complexes show absorption in the NIR region. The position as well as the intensity of this band can be tuned by the substituents on the quinonoid bridge. In addition, this band can be switched on and off by using tunable redox potentials, making such systems attractive candidates for NIR electrochromism.

Radical ligand-containing single-molecule magnets

“…Efforts to generate benzosemiquinonoid-containing transition metal complexes with a nuclearity of greater than one have resulted in numerous dinuclear compounds, including those containing metal centers bridged by derivatives of 2,5-dihydroxy-1,4-benzoquinonoids [172][173][174][175][176][177][178][179][180][181][182][183][184][185][186][187][188][189], bis(pyrazolyl)benzoquinonoids [190][191][192][193], bis(phosphine)benzoquinonoids [194,195], bis(amino) benzoquinonoids [196][197][198][199][200][201][202], and bis(amino)diiminobenzoquinonoids [86,[203][204][205][206]. Indeed, the radical form of these ligands has shown enhanced magnetic coupling compared to their non-radical redox isomers [174,180,182].…”

Functional metal complexes based on bridging “imino”-quinonoid ligands

“…Complexes 5 2+ and 6 2+ with tmpa as co-ligand display two one-electron oxidation steps as stated for the corresponding bpy complexes above [19]. However, 5 2+ and 6 2+ show only one reduction step confirming that the additional reduction steps observed for 1 2+ -4 2+ The difference between the potentials of the two oxidation steps have often been used in diruthenium complexes to determine the comproportion constant (K c = 10 ΔE/59 , with E in mV)) which is related to the thermodynamic stability of the mixed-valent forms [20].…”

“…Intriguingly, the bonding situation described above was also found in the sole Scheme 5. Bond localization within the bridge in dinuclear complexes of (L 5 ) 2- [19].…”

“…For the investigation of redox properties and metal-metal interaction, two different kinds of diruthenium complexes have been designed. In one of these, the metal centers in the starting compounds are low spin (LS) Ru(II) centers that contain strongly or moderately π-accepting co-ligands such as 2,2´-bipyridine (bpy) or tris(2-methylene-pyridine)amine (tmpa) (Chart 4), making these complexes diamagnetic [17][18][19]. In the second case, the metal centers are LS Ru(III), that contain donating ligands like acetylacetonato (acac) as co-ligands, making such complexes paramagnetic with the presence of two spin centers within the same molecule (Chart 4) [13a,18].…”

Ruthenium Hydride Complexes with Zwitterionic Quinonoid Ligands – Isomer Separation, Structural Properties, Electrochemistry, and Catalysis

“…[2][3][4] The redox noninnocence of such molecules imparts many interesting properties to them. [5][6][7][8][9][10][11][12] Thus, metal complexes of quinonoid ligands have been extensively investigated due to their valence ambiguity and captivating electronic structures, [13][14][15][16][17][18] their engrossing magnetic properties, [19][20][21][22] their use as bridges for molecular and supramolecular systems, [19,[23][24][25][26][27][28][29][30][31][32][33][34][35][36] and in homogeneous catalysis. [37][38][39][40] In recent years, we have developed the chemistry of the potentially antiaromatic zwitterionic quinonoid ligands Q [41][42][43][44] (Scheme 1) and their metal complexes.…”