Redox Activity and Bond Activation in Iridium–Diamidobenzene Complexes: A Combined Structural, (Spectro)electrochemical, and DFT Investigation

Noninnocent ligands are special because of their ability to act as electron reservoirs and tune reactivity at a metal center “on-demand”. In the following we present two iridium­(III) complexes with a diamidobenzene ligand: one that is coordinatively unsaturated and a second one that is a coordinatively saturated, regular 18 valence electron complex. We show the electrochemical interconversion between the two complexes and propose a mechanism for the same. Both the complexes have been isolated in pure forms and characterized by spectroscopic, (spectro)­electrochemical, and crystallographic techniques. Additionally, results from DFT calculations are presented to decipher the bonding situation within the two complexes and to investigate the bond activation pathway leading to the interconversion of one form into another. In this work we make use of the increasingly popular concept of using redox steps at noninnocent ligands to tune bond activation and chemical reactivity at the metal center

Redox Activity and Bond Activation in Iridium–Diamidobenzene Complexes: A Combined Structural, (Spectro)electrochemical, and DFT Investigation

Noninnocent ligands are special because of their ability to act as electron reservoirs and tune reactivity at a metal center “on-demand”. In the following we present two iridium­(III) complexes with a diamidobenzene ligand: one that is coordinatively unsaturated and a second one that is a coordinatively saturated, regular 18 valence electron complex. We show the electrochemical interconversion between the two complexes and propose a mechanism for the same. Both the complexes have been isolated in pure forms and characterized by spectroscopic, (spectro)­electrochemical, and crystallographic techniques. Additionally, results from DFT calculations are presented to decipher the bonding situation within the two complexes and to investigate the bond activation pathway leading to the interconversion of one form into another. In this work we make use of the increasingly popular concept of using redox steps at noninnocent ligands to tune bond activation and chemical reactivity at the metal center

Uncommon <i>cis</i> Configuration of a Metal–Metal Bridging Noninnocent Nindigo Ligand

In contrast to several reported coordination compounds of <i>trans</i>-Nindigo ligands [Nindigo = indigo-bis­(<i>N-</i>arylimine) = LH₂] with one or two six-membered chelate rings involving one indole N and one extracyclic N for metal binding, the new diruthenium complex ion [(acac)₂Ru­(μ,η²:η²-L)­Ru­(bpy)₂]²⁺ = <b>2</b>²⁺ exhibits edge-sharing five- and seven-membered chelate rings in the first documented case of asymmetric bridging by a Nindigo ligand in the <i>cis</i> configuration [L^2– = indigo-bis­(<i>N</i>-phenylimine)­dianion]. The dication in compound [<b>2</b>]­(ClO₄)₂ displays one Ru­(α-diimine)₃ site and one ruthenium center with three negatively charged chelate ligands. Compound [<b>2</b>]­(ClO₄)₂ is obtained from the [Ru­(bpy)₂]²⁺-containing <i>cis</i> precursor [(LH)­Ru­(bpy)₂]­ClO₄ = [<b>1</b>]­ClO₄, which exhibits intramolecular H-bonding in the cation. Four accessible oxidation states each were characterized for the <b>1</b>^<i>n</i> and <b>2</b>^<i>n</i> redox series with respect to metal- or ligand-centered electron transfer, based on X-ray structures, electron paramagnetic resonance, and ultraviolet–visible–near-infrared spectroelectrochemistry in conjunction with density functional theory calculation results. The structural asymmetry in the Ru^III/Ru^II system <b>2</b>²⁺ is reflected by the electronic asymmetry (class I mixed-valence situation), leaving the noninnocent Nindigo bridge as the main redox-active site

Ancillary Ligand Control of Electronic Structure in o-Benzoquinonediimine-Ruthenium Complex Redox Series: Structures, Electron Paramagnetic Resonance (EPR), and Ultraviolet−Visible−Near-Infrared (UV-vis-NIR) Spectroelectrochemistry

The compounds Ru­(acac)₂(Q) (<b>1</b>), [Ru­(bpy)₂(Q)]­(ClO₄)₂ ([<b>2</b>]­(ClO₄)₂), and [Ru­(pap)₂(Q)]­PF₆ ([<b>3</b>]­PF₆), containing Q = <i>N,N</i>′-diphenyl-<i>o</i>-benzoquinonediimine and donating 2,4-pentanedionate ligands (acac^–), π-accepting 2,2^/-bipyridine (bpy), or strongly <i>π-</i>accepting 2-phenylazopyridine (pap) were prepared and structurally identified. The electronic structures of the complexes and several accessible oxidized and reduced forms were studied experimentally (electrochemistry, magnetic resonance, ultraviolet-visible-near-infrared (UV-vis-NIR) spectroelectrochemistry) and computationally (DFT/TD-DFT) to reveal significantly variable electron transfer behavior and charge distribution. While the redox system <b>1</b>⁺–<b>1</b>^– prefers trivalent ruthenium with corresponding oxidation states Q⁰–Q^2– of the noninnocent ligand, the series <b>2</b>²⁺–<b>2</b>⁰ and <b>3</b>²⁺–<b>3</b>^– retain Ru^II. The bpy and pap co-ligands are not only spectators but can also be reduced prior to a second reduction of Q. The present study with new experimental and computational evidence on the influence of co-ligands on the metal is complementary to a report on the substituent effects in <i>o</i>-quinonediimine ligands [Kalinina et al., <i>Inorg. Chem</i>. <b>2008</b>, <i>47</i>, 10110] and to the discussion of the most appropriate oxidation state formulation Ru^II(Q⁰) or Ru^III(Q^• –)

Ancillary Ligand Control of Electronic Structure in o-Benzoquinonediimine-Ruthenium Complex Redox Series: Structures, Electron Paramagnetic Resonance (EPR), and Ultraviolet−Visible−Near-Infrared (UV-vis-NIR) Spectroelectrochemistry

The compounds Ru­(acac)₂(Q) (<b>1</b>), [Ru­(bpy)₂(Q)]­(ClO₄)₂ ([<b>2</b>]­(ClO₄)₂), and [Ru­(pap)₂(Q)]­PF₆ ([<b>3</b>]­PF₆), containing Q = <i>N,N</i>′-diphenyl-<i>o</i>-benzoquinonediimine and donating 2,4-pentanedionate ligands (acac^–), π-accepting 2,2^/-bipyridine (bpy), or strongly <i>π-</i>accepting 2-phenylazopyridine (pap) were prepared and structurally identified. The electronic structures of the complexes and several accessible oxidized and reduced forms were studied experimentally (electrochemistry, magnetic resonance, ultraviolet-visible-near-infrared (UV-vis-NIR) spectroelectrochemistry) and computationally (DFT/TD-DFT) to reveal significantly variable electron transfer behavior and charge distribution. While the redox system <b>1</b>⁺–<b>1</b>^– prefers trivalent ruthenium with corresponding oxidation states Q⁰–Q^2– of the noninnocent ligand, the series <b>2</b>²⁺–<b>2</b>⁰ and <b>3</b>²⁺–<b>3</b>^– retain Ru^II. The bpy and pap co-ligands are not only spectators but can also be reduced prior to a second reduction of Q. The present study with new experimental and computational evidence on the influence of co-ligands on the metal is complementary to a report on the substituent effects in <i>o</i>-quinonediimine ligands [Kalinina et al., <i>Inorg. Chem</i>. <b>2008</b>, <i>47</i>, 10110] and to the discussion of the most appropriate oxidation state formulation Ru^II(Q⁰) or Ru^III(Q^• –)

Sensitivity of a Strained C–C Single Bond to Charge Transfer: Redox Activity in Mononuclear and Dinuclear Ruthenium Complexes of Bis(arylimino)acenaphthene (BIAN) Ligands

The new compounds [Ru­(acac)₂(BIAN)], BIAN = bis­(arylimino)­acenaphthene (aryl = Ph (<b>1a</b>), 4-MeC₆H₄ (<b>2a</b>), 4-OMeC₆H₄ (<b>3a</b>), 4-ClC₆H₄ (<b>4a</b>), 4-NO₂C₆H₄ (<b>5a</b>)), were synthesized and structurally, electrochemically, spectroscopically, and computationally characterized. The α-diimine sections of the compounds exhibit intrachelate ring bond lengths 1.304 Å < d­(CN) < 1.334 and 1.425 Å < d­(CC) < 1.449 Å, which indicate considerable metal-to-ligand charge transfer in the ground state, approaching a Ru^III(BIAN^•–) oxidation state formulation. The particular structural sensitivity of the strained peri-connecting C–C bond in the BIAN ligands toward metal-to-ligand charge transfer is discussed. Oxidation of [Ru­(acac)₂(BIAN)] produces electron paramagnetic resonance (EPR) and UV–vis–NIR (NIR = near infrared) spectroelectrochemically detectable Ru^III species, while the reduction yields predominantly BIAN-based spin, in agreement with density functional theory (DFT) spin-density calculations. Variation of the substituents from CH₃ to NO₂ has little effect on the spin distribution but affects the absorption spectra. The dinuclear compounds {(μ-tppz)­[Ru­(Cl)­(BIAN)]₂}­(ClO₄)₂, tppz = 2,3,5,6-tetrakis­(2-pyridyl)­pyrazine; aryl (BIAN) = Ph ([<b>1b</b>]­(ClO₄)₂), 4-MeC₆H₄ ([<b>2b</b>]­(ClO₄)₂), 4-OMeC₆H₄ ([<b>3b</b>]­(ClO₄)₂), 4-ClC₆H₄ ([<b>4b</b>]­(ClO₄)₂), were also obtained and investigated. The structure determination of [<b>2b</b>]­(ClO₄)₂ and [<b>3b</b>]­(ClO₄)₂ reveals <i>trans</i> configuration of the chloride ligands and unreduced BIAN ligands. The DFT and spectroelectrochemical results (UV–vis–NIR, EPR) indicate oxidation to a weakly coupled Ru^IIIRu^II mixed-valent species but reduction to a tppz-centered radical state. The effect of the π electron-accepting BIAN ancillary ligands is to diminish the metal–metal interaction due to competition with the acceptor bridge tppz

Sensitivity of a Strained C–C Single Bond to Charge Transfer: Redox Activity in Mononuclear and Dinuclear Ruthenium Complexes of Bis(arylimino)acenaphthene (BIAN) Ligands

The new compounds [Ru­(acac)₂(BIAN)], BIAN = bis­(arylimino)­acenaphthene (aryl = Ph (<b>1a</b>), 4-MeC₆H₄ (<b>2a</b>), 4-OMeC₆H₄ (<b>3a</b>), 4-ClC₆H₄ (<b>4a</b>), 4-NO₂C₆H₄ (<b>5a</b>)), were synthesized and structurally, electrochemically, spectroscopically, and computationally characterized. The α-diimine sections of the compounds exhibit intrachelate ring bond lengths 1.304 Å < d­(CN) < 1.334 and 1.425 Å < d­(CC) < 1.449 Å, which indicate considerable metal-to-ligand charge transfer in the ground state, approaching a Ru^III(BIAN^•–) oxidation state formulation. The particular structural sensitivity of the strained peri-connecting C–C bond in the BIAN ligands toward metal-to-ligand charge transfer is discussed. Oxidation of [Ru­(acac)₂(BIAN)] produces electron paramagnetic resonance (EPR) and UV–vis–NIR (NIR = near infrared) spectroelectrochemically detectable Ru^III species, while the reduction yields predominantly BIAN-based spin, in agreement with density functional theory (DFT) spin-density calculations. Variation of the substituents from CH₃ to NO₂ has little effect on the spin distribution but affects the absorption spectra. The dinuclear compounds {(μ-tppz)­[Ru­(Cl)­(BIAN)]₂}­(ClO₄)₂, tppz = 2,3,5,6-tetrakis­(2-pyridyl)­pyrazine; aryl (BIAN) = Ph ([<b>1b</b>]­(ClO₄)₂), 4-MeC₆H₄ ([<b>2b</b>]­(ClO₄)₂), 4-OMeC₆H₄ ([<b>3b</b>]­(ClO₄)₂), 4-ClC₆H₄ ([<b>4b</b>]­(ClO₄)₂), were also obtained and investigated. The structure determination of [<b>2b</b>]­(ClO₄)₂ and [<b>3b</b>]­(ClO₄)₂ reveals <i>trans</i> configuration of the chloride ligands and unreduced BIAN ligands. The DFT and spectroelectrochemical results (UV–vis–NIR, EPR) indicate oxidation to a weakly coupled Ru^IIIRu^II mixed-valent species but reduction to a tppz-centered radical state. The effect of the π electron-accepting BIAN ancillary ligands is to diminish the metal–metal interaction due to competition with the acceptor bridge tppz

Tuning Ligand Effects and Probing the Inner-Workings of Bond Activation Steps: Generation of Ruthenium Complexes with Tailor-Made Properties

Activating chemical bonds through external triggers and understanding the underlying mechanism are at the heart of developing molecules with catalytic and switchable functions. Thermal, photochemical, and electrochemical bond activation pathways are useful for many chemical reactions. In this Article, a series of Ru^II complexes containing a bidentate and a tripodal ligand were synthesized. Starting from all-pyridine complex <b>1</b>²⁺, the pyridines were stepwise substituted with “click” triazoles (<b>2</b>²⁺–<b>7</b>²⁺). Whereas the thermo- and photoreactivity of <b>1</b>²⁺ are due to steric repulsion within the equatorial plane of the complex, <b>3</b>²⁺–<b>6</b>²⁺ are reactive because of triazoles in axial positions, and <b>4</b>²⁺ shows unprecedented photoreactivity. Complexes that feature neither steric interactions nor axial triazoles (<b>2</b>²⁺ and <b>7</b>²⁺) do not show any reactivity. Furthermore, a redox-triggered conversion mechanism was discovered in <b>1</b>²⁺, <b>3</b>²⁺, and <b>4</b>²⁺. We show here ligand design principles required to convert a completely inert molecule to a reactive one and vice versa, and provide mechanistic insights into their functioning. The results presented here will likely have consequences for developing a future generation of catalysts, sensors, and molecular switches

Tuning Ligand Effects and Probing the Inner-Workings of Bond Activation Steps: Generation of Ruthenium Complexes with Tailor-Made Properties

Activating chemical bonds through external triggers and understanding the underlying mechanism are at the heart of developing molecules with catalytic and switchable functions. Thermal, photochemical, and electrochemical bond activation pathways are useful for many chemical reactions. In this Article, a series of Ru^II complexes containing a bidentate and a tripodal ligand were synthesized. Starting from all-pyridine complex <b>1</b>²⁺, the pyridines were stepwise substituted with “click” triazoles (<b>2</b>²⁺–<b>7</b>²⁺). Whereas the thermo- and photoreactivity of <b>1</b>²⁺ are due to steric repulsion within the equatorial plane of the complex, <b>3</b>²⁺–<b>6</b>²⁺ are reactive because of triazoles in axial positions, and <b>4</b>²⁺ shows unprecedented photoreactivity. Complexes that feature neither steric interactions nor axial triazoles (<b>2</b>²⁺ and <b>7</b>²⁺) do not show any reactivity. Furthermore, a redox-triggered conversion mechanism was discovered in <b>1</b>²⁺, <b>3</b>²⁺, and <b>4</b>²⁺. We show here ligand design principles required to convert a completely inert molecule to a reactive one and vice versa, and provide mechanistic insights into their functioning. The results presented here will likely have consequences for developing a future generation of catalysts, sensors, and molecular switches

Tuning Ligand Effects and Probing the Inner-Workings of Bond Activation Steps: Generation of Ruthenium Complexes with Tailor-Made Properties

Activating chemical bonds through external triggers and understanding the underlying mechanism are at the heart of developing molecules with catalytic and switchable functions. Thermal, photochemical, and electrochemical bond activation pathways are useful for many chemical reactions. In this Article, a series of Ru^II complexes containing a bidentate and a tripodal ligand were synthesized. Starting from all-pyridine complex <b>1</b>²⁺, the pyridines were stepwise substituted with “click” triazoles (<b>2</b>²⁺–<b>7</b>²⁺). Whereas the thermo- and photoreactivity of <b>1</b>²⁺ are due to steric repulsion within the equatorial plane of the complex, <b>3</b>²⁺–<b>6</b>²⁺ are reactive because of triazoles in axial positions, and <b>4</b>²⁺ shows unprecedented photoreactivity. Complexes that feature neither steric interactions nor axial triazoles (<b>2</b>²⁺ and <b>7</b>²⁺) do not show any reactivity. Furthermore, a redox-triggered conversion mechanism was discovered in <b>1</b>²⁺, <b>3</b>²⁺, and <b>4</b>²⁺. We show here ligand design principles required to convert a completely inert molecule to a reactive one and vice versa, and provide mechanistic insights into their functioning. The results presented here will likely have consequences for developing a future generation of catalysts, sensors, and molecular switches