A Diruthenium Complex of a "Nindigo" Ligand

The compound ((mu-Nindigo)[Ru(acac)(2)](2)} = 1, H-2(Nindigo) = indigo-N,N'-diphenylimine and acac(-) = 2,4-pentanedionate, has been structurally characterized in the rac form, which exhibits two edge-sharing six-membered chelate rings involving ruthenium, and the former beta-diketiminato functions with a twist angle of 33.9 degrees around the central C-C bond. The metric parameters suggest a neutral pi acceptor bridge containing coupled s-trans configurated alpha-diimines, which are coordinated by two ruthenium(II) centers. DFT calculations confirm the experimental structure and oxidation state assignment of the rac form; both diastereoisomers are present in solution according to H-1 NMR spectroscopy. A very intense long-wavelength MLCT absorption at 630 nm (epsilon = 66 800 M-1 cm(-1)) and a weaker near-IR band at 1120 nm (epsilon = 3000 M-1 cm(-1)) are observed for the CH3CN solution. Reversible one-electron reduction and oxidation steps were studied by cyclic voltammetry, differential pulse voltammetry, EPR, and UV-vis-NIR spectroelectrochemistry to exhibit metal-centered oxidation and mixed metal/ligand-centered reduction. These results are supported by TD-DFT calculations of the species rac- or meso-1(n), n = 3+, 2+, +, 0, -, 2-

Varying electronic structural forms of ruthenium complexes of non-innocent 9,10-phenanthrenequinonoid ligands

Bis(acetylacetonato) ruthenium complexes [Ru(acac) 2(Q(1-3))], 1-3, incorporating redox non-innocent 9,10-phenanthrenequinonoid ligands (Q(1) = 9,10-phenanthrenequinone, 1; Q(2) = 9,10-phenanthrenequinonediimine, 2; Q(3) = 9,10-phenanthrenequinonemonoimine, 3) have been characterised electrochemically, spectroscopically and structurally. The four independent molecules in the unit cell of 2 are involved in intermolecular hydrogen bonding and p-p interactions, leading to a 2D network. The oxidation state-sensitive bond distances of the coordinated ligands Q(n) at 1.296(5)/1.289(5) A (C-O), 1.315(3)/1.322 (4) A (C-N), and 1.285(3)/1.328(3) A (C-O/C-N) in 1, 2 and 3, respectively, and the well resolved H-1 NMR resonances within the standard chemical shift range suggest DFT supported variable contributions from valence formulations [RuIII(acac)(2)(Q.(-))] (spin-coupled) and [RuII(acac) (2)(Q(0))], respectively. Complexes 1-3 exhibit one oxidation and two reduction steps with comproportionation constants Kc similar to 10(7)-10(22) for the intermediates. The electrochemically generated persistent redox states 1n (n = 0, 1-, 2-) and 2(n)/3(n) (n = 1+, 0, 1-, 2-) have been analysed by UV-vis-NIR spectroelectrochemistry and by EPR for the paramagnetic intermediates in combination with DFT and TD-DFT calculations, revealing significant differences in the oxidation state distribution at the {Ru-Q} interface for 1(n)-3(n). In particular, the diminished propensity of the NH-containing systems for reduction results in the preference for RuII(Q(0)) relative to RuIII(Q(.-)) (neutral compounds) and for RuII(Q(.-)) over the RuIII(Q(2-)) alternative in the case of the monoanionic complexes

Stabilizing the Elusive ortho-Quinone/Copper(I) Oxidation State Combination through pi/pi Interaction in an Isolated Complex

The heterodinuclear compound [(PhenQ)Cu(dppf)](BF(4)), PhenQ= 9,10-phenanthrenequinone and dppf = 1,1'-bis(diphenylphosphino)ferrocene, was identified structurally and spectroscopically (NMR, IR, UV-vis) as a copper(l) complex of a completely unreduced ortho-quinone. Crystallographic and DFT calculation results suggest that this stabilization of a hitherto elusive arrangement is partially owed to intramolecular pi/pi interactions phenyt/PhenQ. Intermolecular PhenQ/PhenQ pi stacking is also observed in the crystal. According to DFT calculations, the pi interactions are responsible for the considerably distorted coordination geometry at Cu(1) with one short and one longer Cu-O and Cu-P bond, respectively, and with bond angles at copper ranging from 99 degrees to 133 degrees. Electrochemical reduction proceeds reversibly at Low temperatures to yield an EPR spectroscopically characterized semiquinone-copper(I) species

Tuning Spin–Spin Coupling in Quinonoid-Bridged Dicopper(II) Complexes through Rational Bridge Variation

Bridged metal complexes [{Cu­(tmpa)}₂­(μ-L¹_–2H)]­(ClO₄)₂ (<b>1</b>), [{Cu­(tmpa)}₂­(μ-L²_–2H)]­(ClO₄)₂ (<b>2</b>), [{Cu­(tmpa)}₂­(μ-L³_–2H)]­(BPh₄)₂ (<b>3</b>), and [{Cu­(tmpa)}₂­(μ-L⁴_–2H)]­(ClO₄)₂ (<b>4</b>) (tmpa = tris­(2-pyridyl­methyl)­amine, L¹ = chloranilic acid, L² = 2,5-dihydroxy-1,4-benzoquinone, L³ = (2,5-di-[2-(methoxy)-anilino]-1,4-benzoquinone, L⁴ = azophenine) were synthesized from copper­(II) salts, tmpa, and the bridging quinonoid ligands in the presence of a base. X-ray structural characterization of the complexes showed a distorted octahedral environment around the copper­(II) centers for the complexes <b>1</b>–<b>3</b>, the donors being the nitrogen atoms of tmpa, and the nitrogen or oxygen donors of the bridging quinones. In contrast, the copper­(II) centers in <b>4</b> display a distorted square-pyramidal coordination, where one of the pyridine arms of each tmpa remains uncoordinated. Bond-length analyses within the bridging ligand exhibit localization of the double bonds inside the bridge for <b>1</b>–<b>3</b>. In contrast, complete delocalization of double bonds within the bridging ligand is observed for <b>4</b>. Temperature-dependent magnetic susceptibility measurements on the complexes reveal an antiferromagnetic coupling between the copper­(II) ions. The strength of antiferromagnetic coupling was observed to depend on the energy of the HOMO of the bridging quinone ligands, with exchange coupling constants <i>J</i> in the range between −23.2 and −0.6 cm^–1 and the strength of antiferromagnetic coupling of <b>4</b> > <b>3</b> > <b>2</b> > <b>1</b>. Broken-symmetry density functional theory calculations (DFT) revealed that the orientation of magnetic orbitals in <b>1</b> and <b>2</b> is different than that in <b>3</b> and <b>4</b>, and this results in two different exchange pathways. These results demonstrate how bridge-mediated spin–spin coupling in quinone-bridged metal complexes can be strongly tuned by a rational design of the bridging ligand employing the [O] for [NR] isoelectronic analogy

Tuning Spin–Spin Coupling in Quinonoid-Bridged Dicopper(II) Complexes through Rational Bridge Variation

Bridged metal complexes [{Cu­(tmpa)}₂­(μ-L¹_–2H)]­(ClO₄)₂ (<b>1</b>), [{Cu­(tmpa)}₂­(μ-L²_–2H)]­(ClO₄)₂ (<b>2</b>), [{Cu­(tmpa)}₂­(μ-L³_–2H)]­(BPh₄)₂ (<b>3</b>), and [{Cu­(tmpa)}₂­(μ-L⁴_–2H)]­(ClO₄)₂ (<b>4</b>) (tmpa = tris­(2-pyridyl­methyl)­amine, L¹ = chloranilic acid, L² = 2,5-dihydroxy-1,4-benzoquinone, L³ = (2,5-di-[2-(methoxy)-anilino]-1,4-benzoquinone, L⁴ = azophenine) were synthesized from copper­(II) salts, tmpa, and the bridging quinonoid ligands in the presence of a base. X-ray structural characterization of the complexes showed a distorted octahedral environment around the copper­(II) centers for the complexes <b>1</b>–<b>3</b>, the donors being the nitrogen atoms of tmpa, and the nitrogen or oxygen donors of the bridging quinones. In contrast, the copper­(II) centers in <b>4</b> display a distorted square-pyramidal coordination, where one of the pyridine arms of each tmpa remains uncoordinated. Bond-length analyses within the bridging ligand exhibit localization of the double bonds inside the bridge for <b>1</b>–<b>3</b>. In contrast, complete delocalization of double bonds within the bridging ligand is observed for <b>4</b>. Temperature-dependent magnetic susceptibility measurements on the complexes reveal an antiferromagnetic coupling between the copper­(II) ions. The strength of antiferromagnetic coupling was observed to depend on the energy of the HOMO of the bridging quinone ligands, with exchange coupling constants <i>J</i> in the range between −23.2 and −0.6 cm^–1 and the strength of antiferromagnetic coupling of <b>4</b> > <b>3</b> > <b>2</b> > <b>1</b>. Broken-symmetry density functional theory calculations (DFT) revealed that the orientation of magnetic orbitals in <b>1</b> and <b>2</b> is different than that in <b>3</b> and <b>4</b>, and this results in two different exchange pathways. These results demonstrate how bridge-mediated spin–spin coupling in quinone-bridged metal complexes can be strongly tuned by a rational design of the bridging ligand employing the [O] for [NR] isoelectronic analogy

A Diruthenium Complex of a “Nindigo” Ligand

The compound {(μ-Nindigo)­[Ru­(acac)₂]₂} = <b>1</b>, H₂(Nindigo) = indigo-<i>N</i>,<i>N</i>′-diphenylimine and acac^– = 2,4-pentanedionate, has been structurally characterized in the <i>rac</i> form, which exhibits two edge-sharing six-membered chelate rings involving ruthenium, and the former β-diketiminato functions with a twist angle of 33.9° around the central C–C bond. The metric parameters suggest a neutral π acceptor bridge containing coupled <i>s</i>-<i>trans</i> configurated α-diimines, which are coordinated by two ruthenium­(II) centers. DFT calculations confirm the experimental structure and oxidation state assignment of the <i>rac</i> form; both diastereoisomers are present in solution according to ¹H NMR spectroscopy. A very intense long-wavelength MLCT absorption at 630 nm (ε = 66 800 M^–1 cm^–1) and a weaker near-IR band at 1120 nm (ε = 3000 M^–1 cm^–1) are observed for the CH₃CN solution. Reversible one-electron reduction and oxidation steps were studied by cyclic voltammetry, differential pulse voltammetry, EPR, and UV–vis–NIR spectroelectrochemistry to exhibit metal-centered oxidation and mixed metal/ligand-centered reduction. These results are supported by TD-DFT calculations of the species <i>rac</i>- or <i>meso</i>-<b>1</b>^<i>n</i>, <i>n</i> = 3+, 2+, +, 0, −, 2–