Isomeric separation in donor–acceptor systems of Pd(II) and Pt(II) and a combined structural, electrochemical and spectroelectrochemical study

Compounds of the form [(pap)M(Q2−)] (pap = phenylazopyridine; Q = 3,5-di-tert- butyl-benzoquinone, M = Pd, 1a and 1b, M = Pt, 2a and 2b; Q = 4-tert-butyl- benzoquinone, M = Pd, 3a and 3b; M = Pt, 4a and 4b) were synthesized in a one- pot reaction. The geometrical isomers, which are possible because of the built in asymmetry of these ligands, have been separated by using different temperatures and variable solubility. Structural characterization of 1b shows that the metal centers are in a square planar environment, the pap ligand is in the unreduced neutral state and the quinones are in the doubly reduced, Q2−catecholate form. Cyclic voltammetric measurements on the complexes display two one-electron oxidations and two one-electron reductions. EPR and vis-NIR spectra of the one-electron oxidized forms of the complexes indicate that the first oxidation takes place on the Q2− ligands to produce a metal bound semiquinone (Q˙−) radical. Reduction takes place on the pap ligand, generating metal bound pap˙− as seen from the 14N (I = 1) coupling in their EPR spectrum. All the complexes in their [(pap)M(Q2−)] neutral forms show strong absorptions in the NIR region which are largely LLCT (ligand to ligand charge transfer) in origin. These NIR bands can be tuned over a wide energy range by varying the metal center as well as the Q ligand. In addition, the intensity of NIR bands can be switched on and off by a simple electron transfer at relatively low potentials. DFT studies were used to corroborate these findings

a combined structural, electrochemical and spectroscopic study

Reactions of [(az-H)Pd(μ-Cl)2Pd(az-H)] (az = azobenzene) with the zwitterionic, p-benzoquinonemonoimine-type ligands 4-(n-butylamino)-6(n-butylimino)-3-oxocyclohexa-1,4-dien-1-olate (Q1) or 4-(isopropylamino)-6(isopropylimino)-3-oxocyclohexa-1,4-dien-1-olate) (Q2) in the presence of a base leads to the formation of the mononuclear complexes [(az-H)Pd(Q1-H)] (1) and [(az-H)Pd(Q2-H)] (2) respectively. Structural characterization of 2 shows an almost square planar coordination geometry around the Pd(II) centre, a short Pd–C bond, a slight elongation of the N[double bond, length as m-dash]N double bond of the az-H ligand and localization of the double bonds within the Q2-H ligand. Additionally, intermolecular N–H–O interactions exist between the uncoordinated N–H and O groups of two different molecules. Cyclic voltammetry of the complexes reveals an irreversible oxidation and two reversible reduction processes. A combination of electrochemical and UV-vis-NIR and EPR spectroelectrochemical studies are used to show that both coordinated ligands participate successively in the redox processes, thus revealing their non-innocent character

Straightforward approach to efficient oxidative DNA cleaving agents based on Cu(II) complexes of heterosubstituted cyclens

The Cu(II) complexes of cyclen and two of its heterosubstituted analogues were shown to be efficient oxidative DNA cleavers. The reactivity strongly depends on the heteroatom inserted into the macrocycle (O > S > N)

The redox series [Ru(bpy)2(L)]n, n = +3, +2, +1, 0, with L = bipyridine, “click” derived pyridyl-triazole or bis-triazole: a combined structural, electrochemical, spectroelectrochemical and DFT investigation

The compounds [Ru(bpy)2(L1)](ClO4)2 (1(ClO4)2), [Ru(bpy)2(L2)](ClO4)2 (2(ClO4)2), [Ru(bpy)2(L3)](ClO4)2 (3(ClO4)2), [Ru(bpy)2(L4)](ClO4)2 (4(ClO4)2), [Ru(bpy)2(L5)](ClO4)2 (5(ClO4)2), and [Ru(bpy)2(L6)](ClO4)26(ClO4)2 (bpy = 2,2′-bipyridine, L1 = 1-(4-isopropyl- phenyl)-4-(2-pyridyl)-1,2,3-triazole, L2 = 1-(4-butoxy- phenyl)-4-(2-pyridyl)-1,2,3-triazole, L3 = 1-(2-trifluoromethyl- phenyl)-4-(2-pyridyl)-1,2,3-triazole, L4 = 4,4′-bis-{1-(2,6-diisopropyl- phenyl)}-1,2,3-triazole, L5 = 4,4′-bis-{(1-phenyl)}-1,2,3-triazole, L6 = 4,4′-bis-{1-(2-trifluoromethyl-phenyl)}-1,2,3-triazole) were synthesized from [Ru(bpy)2(EtOH)2](ClO4)2 and the corresponding “click”-derived pyridyl- triazole or bis-triazole ligands, and characterized by 1H-NMR spectroscopy, elemental analysis, mass spectrometry and X-ray crystallography. Structural analysis showed a distorted octahedral coordination environment about the Ru(II) centers, and shorter Ru–N(triazole) bond distances compared to Ru–N(pyridine) distances in complexes of mixed-donor ligands. All the complexes were subjected to cyclic voltammetric studies, and the results were compared to the well-known [Ru(bpy)3]2+ compound. The oxidation and reduction potentials were found to be largely uninfluenced by ligand changes, with all the investigated complexes showing their oxidation and reduction steps at rather similar potentials. A combined UV-vis-NIR and EPR spectroelectrochemical investigation, together with DFT calculations, was used to determine the site of electron transfer in these complexes. These results provided insights into their electronic structures in the various investigated redox states, showed subtle differences in the spectroscopic signatures of these complexes despite their similar electrochemical properties, and provided clues to the unperturbed redox potentials in these complexes with respect to ligand substitutions. The reduced forms of the complexes display structured absorption bands in the NIR region. Additionally, we also present new synthetic routes for the ligands presented here using Cu-abnormal carbene catalysts

Reactivity of TCNE and TCNQ derivatives of quinonoid zwitterions with Cu(I)

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Heterobimetallic Cu–dppf (dppf = 1,1′-Bis(diphenylphosphino)ferrocene) Complexes with “Click” Derived Ligands: A Combined Structural, Electrochemical, Spectroelectrochemical, and Theoretical Study

Heterodinuclear complexes of the form [(dppf)­Cu­(L)]­(BF₄) (dppf = 1,1′-bis­(diphenylphosphino)­ferrocene), where L are the chelating, substituted 4,4′-bis­(1,2,3-triazole) or 4-pyridyl­(1,2,3-triazole) ligands, were synthesized by reacting [Cu­(dppf)­(CH₃CN)₂]­(BF₄) with the corresponding “click” derived ligands. Structural characterization of representative complexes revealed a distorted-tetrahedral coordination geometry around the Cu­(I) centers, with the donor atoms being the P donors of dppf and the N donors of the substituted triazole ligands. The “local-pseudo” symmetry around the iron center in all the investigated complexes of dppf is between that of the idealized <i>D</i>_5<i>h</i> and <i>D</i>_5<i>d</i>. Furthermore, for the complex with the mixed pyridine and triazole donors, the Cu–N bond distances were found to be shorter for the triazole N donors in comparison to those for the pyridine N donors. Electrochemical studies on the complexes revealed the presence of one oxidation and one reduction step for each. These studies were combined with UV–vis–near-IR and EPR spectroelectrochemical studies to deduce the locus of the oxidation process (Cu vs Fe) and to see the influence of changing the chelating “click” derived ligand on both the oxidation and reduction processes and their spectroscopic signatures. Structure-based DFT studies were performed to get insights into the experimental spectroscopic results. The results obtained here are compared with those of the complex [(dppf)­Cu­(bpy)]­(BF₄) (bpy = 2,2′-bipyridine). A comparison is made among bpy, pyridyl-triazole, and bis-triazole ligands, and the effect of systematically replacing these ligands on the electrochemical and spectroscopic properties of the corresponding heterodinuclear complexes is investigated

Heterobimetallic Cu–dppf (dppf = 1,1′-Bis(diphenylphosphino)ferrocene) Complexes with “Click” Derived Ligands: A Combined Structural, Electrochemical, Spectroelectrochemical, and Theoretical Study

Heterodinuclear complexes of the form [(dppf)­Cu­(L)]­(BF₄) (dppf = 1,1′-bis­(diphenylphosphino)­ferrocene), where L are the chelating, substituted 4,4′-bis­(1,2,3-triazole) or 4-pyridyl­(1,2,3-triazole) ligands, were synthesized by reacting [Cu­(dppf)­(CH₃CN)₂]­(BF₄) with the corresponding “click” derived ligands. Structural characterization of representative complexes revealed a distorted-tetrahedral coordination geometry around the Cu­(I) centers, with the donor atoms being the P donors of dppf and the N donors of the substituted triazole ligands. The “local-pseudo” symmetry around the iron center in all the investigated complexes of dppf is between that of the idealized <i>D</i>_5<i>h</i> and <i>D</i>_5<i>d</i>. Furthermore, for the complex with the mixed pyridine and triazole donors, the Cu–N bond distances were found to be shorter for the triazole N donors in comparison to those for the pyridine N donors. Electrochemical studies on the complexes revealed the presence of one oxidation and one reduction step for each. These studies were combined with UV–vis–near-IR and EPR spectroelectrochemical studies to deduce the locus of the oxidation process (Cu vs Fe) and to see the influence of changing the chelating “click” derived ligand on both the oxidation and reduction processes and their spectroscopic signatures. Structure-based DFT studies were performed to get insights into the experimental spectroscopic results. The results obtained here are compared with those of the complex [(dppf)­Cu­(bpy)]­(BF₄) (bpy = 2,2′-bipyridine). A comparison is made among bpy, pyridyl-triazole, and bis-triazole ligands, and the effect of systematically replacing these ligands on the electrochemical and spectroscopic properties of the corresponding heterodinuclear complexes is investigated

Electrochromic Platinum(II) Complexes Derived from Azobenzene and Zwitterionic Quinonoid Ligands: Electronic and Geometric Structures

The ligands azobenzene (az) and the zwitterionic 4-(isopropylamino)-6-(isopropyliminio)-3-oxocyclohexa-1,4-dien-1-olate (Q) were used to synthesize the mononuclear complex [(Q_‑H)­Pt­(az_‑H)] (<b>1</b>), and the dinuclear complex [(Q_‑H)­Pt­(μ-az_‑2H)­Pt­(Q_‑H)] (<b>2</b>). Structural characterization of the complexes shows a distorted-square-planar environment around the Pt­(II) centers and localization of the double bonds within the Q_‑H ligand on metal coordination. Furthermore, the NN azo bond is elongated in the metal complexes in comparison to free az, owing to π back-bonding from Pt­(II) to az. Complexes <b>1</b> and <b>2</b> display multiple reversible reduction steps in their cyclic voltammograms. The complexes also exhibit strong absorptions in the visible region, the position and intensity of which can be influenced by the chromophore [(Q_‑H)­Pt]. UV–vis–near-IR spectroelectrochemical studies show that the absorption of these complexes in the visible as well as the near-IR region can be controlled by electron transfer steps. Depending on the charge state of the complexes, they are found to be either transparent in the near-IR region but strongly absorbing in the visible or vice versa, thus displaying strong electrochromic behavior. EPR spectroelectrochemical studies together with DFT calculations and comparison with the complex [(Q_‑H)­Pd­(az_‑H)] (<b>3</b>) are used to locate the site of electron transfer in these complexes and to elucidate their electronic properties in the various redox states. Complex <b>2</b> is a rare example where doubly deprotonated azobenzene acts as a bridging ligand

Dinuclear Quinonoid-Bridged d⁸ Metal Complexes with Redox-Active Azobenzene Stoppers: Electrochemical Properties and Electrochromic Behavior

The ligands 2,5-bis­[2,6-(diisopropyl)­anilino]-1,4-benzoquinone (L¹) and azophenine (L²) were reacted with [(az_‑H)­M­(μ-Cl)₂M­(az_–H)] (M = Pd, Pt, az = azobenzene) to generate the complexes [(az_‑H)­Pd­(μ-L¹_‑2H)­Pd­(az_‑H)] (<b>1</b>), [(az_‑H)­Pt­(μ-L¹_‑2H)­Pt­(az_‑H)] (<b>2</b>), and [(az_‑H)­Pt­(μ-L²_‑2H)­Pt­(az_‑H)] (<b>3</b>). Structural characterization of <b>1</b> and <b>2</b> revealed a distorted-square-planar environment around the metal centers, localization of double bonds within the L¹_‑2H ligand, and binding of L¹_‑2H to the metal centers through anionic O^– and neutral imine type donors. Furthermore, the NN double bond within az_‑H displayed a slight elongation in comparison to that in free az owing to back-bonding from the dπ metal orbitals to the π* orbitals of az_‑H. All complexes show an irreversible oxidation step and three stepwise, reversible one-electron-reduction steps in their cyclic voltammograms. The redox potentials of the complexes are seen to be strongly dependent on the nature of the bridging ligand. UV–vis–near-IR spectroelectrochemical measurements show that these complexes are strongly absorbing in the visible or the near-IR region, depending on the charged state of the metal complexes. The position and intensity of the absorption bands can be tuned by varying the bridging ligand and the metal center. Additionally, the absorption bands can be tuned by simple one-electron-transfer steps. EPR spectroelectrochemistry and DFT calculations have been used to shed light on the electronic structures of these metal complexes in their various redox states and to interpret the results obtained from the UV–vis–near-IR spectroelectrochemistry measurements. In this work, a comparison is being made among d⁸ metal complexes containing bridging quinones with a [O,O,O,O], [O,N,O,N], or [N,N,N,N] donor set, and the advantages of using the isoelectronic [NR] for [O] substitution on the quinonoid ligands for generating electrochromic metal complexes are discussed. In doing so, we also present complex <b>3</b>, which is a rare example of a dinuclear metal complex containing the azophenine bridge

Electrochemistry, Chemical Reactivity, and Time-Resolved Infrared Spectroscopy of Donor–Acceptor Systems [(Q^<i>x</i>)Pt(pap^<i>y</i>)] (Q = Substituted <i>o</i>‑Quinone or <i>o</i>‑Iminoquinone; pap = Phenylazopyridine)

The donor–acceptor complex [(^O,NQ^2–)­Pt­(pap⁰)] (<b>1</b>; pap = phenylazopyridine, ^O,NQ⁰ = 4,6-di-<i>tert</i>-butyl-<i>N</i>-phenyl-<i>o</i>-iminobenzoquinone), which displays strong π-bonding interactions and shows strong absorption in the near-IR region, has been investigated with respect to its redox-induced reactivity and electrochemical and excited-state properties. The one-electron-oxidized product [(^O,NQ^•–)­Pt­(pap⁰)]­(BF₄) ([<b>1</b>]­BF₄) was chemically isolated. Single-crystal X-ray diffraction studies establish the iminosemiquinone form of ^O,NQ in [<b>1</b>]⁺. Simulation of the cyclic voltammograms of <b>1</b> recorded in the presence of PPh₃ elucidates the mechanism and delivers relevant thermodynamic and kinetic parameters for the redox-induced reaction with PPh₃. The thermodynamically stable product of this reaction, complex [(^O,NQ^•–) Pt­(PPh₃)₂]­(PF₆) ([<b>2</b>]­PF₆), was isolated and characterized by X-ray crystallography, electrochemistry, and electron paramagnetic resonance spectroscopy. Picosecond time-resolved infrared spectroscopic studies on complex <b>1b</b> (one of the positional isomers of <b>1</b>) and its analogue [(^O,OQ^2–)­Pt­(pap⁰)] (<b>3</b>; ^O,OQ = 3,5-di-<i>tert</i>-butyl-<i>o</i>-benzoquinone) provided insight into the excited-state dynamics and revealed that the nature of the lowest excited state in the amidophenolate complex <b>1b</b> is primarily diimine-ligand-based, while it is predominantly an interligand charge-transfer state in the case of <b>3</b>. Density functional theory calculations on [<b>1</b>]^<i>n</i>+ provided further insight into the nature of the frontier orbitals of various redox forms and vibrational mode assignments. We discuss the mechanistic details of the newly established redox-induced reactivity of <b>1</b> with electron donors and propose a mechanism for this process