Localized Mixed-Valence and Redox Activity within a Triazole-Bridged Dinucleating Ligand upon Coordination to Palladium

Selective Manipulation of Well-Defined Trinuclear Pd(II)-Complexes

Several strategies are available to design well-defined multimetallic molecular entities bearing functional ligands. Substoichiometric exchange reactions in the coordination sphere of pre-existing multinuclear precursors are relatively underexploited in this context. Palladium(II) acetate is not a mononuclear compound in the solid state but rather exists as a trimer, i. e. [Pd3(OAc)6]. Although this material is ubiquitously used to synthesize mononuclear Pd species, it may principally also lend itself to selective exchange of some of the edge-sharing acetate units in its triangular motif, whilst keeping the overall multinuclear architecture intact. Strikingly, little is known about the controlled manipulation and substoichiometric substitution chemistry of this well-defined conglomerate. We herein conclusively demonstrate that, for the first time, the targeted exchange of two or four acetate units from the Pd3(acetate)6 platform is possible, thereby installing either one or two new tridentate ligands onto this trinuclear architecture. Follow-up exchange and substitution chemistry is available without disrupting the multimetallic nature of the core structure. New complexes 2-7 are all conclusively characterized using multinuclear NMR spectroscopy, UV-vis and IR spectroscopy as well as X-ray diffraction analysis.

Ni(II) complexes of a new tetradentate NN'N''O picolinoyl-1,2-phenylenediamide-phenolate redox-active ligand at different redox levels

Three square planar nickel(II) complexes of a new asymmetric tetradentate redox-active ligand H₃L² in its deprotonated form, at three redox levels, open-shell semiquinonate(1-) π radical, quinone(0) and closed-shell dianion of its 2-aminophenolate part, have been synthesized. The coordinated ligand provides N (pyridine) and N' and N'' (carboxamide and 1,2-phenylenediamide, respectively) and O (phenolate) donor sites. Cyclic voltammetry on the parent complex [Ni(L²)] 1 in CH₂Cl₂ established a three-membered electron-transfer series (oxidative response at E_1/2 = 0.57 V and reductive response at -0.32 V vs. SCE) consisting of neutral, monocationic and monoanionic [Ni(L²)]^z (z = 0, 1+ and 1-). Oxidation of 1 with AgSbF₆ affords [Ni(L²)](SbF₆) (2) and reduction of 1 with cobaltocene yields [Co(η⁵-C₅H₅)₂][Ni(L²)] (3). The molecular structures of 1·CH₃CN, 2·0.5CH₂Cl₂ and 3·C₆H₆ have been determined by X-ray crystallography at 100 K. Characterization by ¹H NMR, X-band EPR (g_av = 2.006 (solid); 2.008 (CH₂Cl₂-C₆H₅CH₃ glass); 80 K) and UV-VIS-NIR spectral properties established that 1, 2 and 3 have [Ni^II{(L²)˙^2-}], [Ni^II{(L²)^-}]⁺/1⁺ and [Ni^II{(L²)^3-}]^-/1^- electronic states, respectively. Thus, the redox processes are ligand-centred. While 1 possesses paramagnetic S_t (total spin) = 1/2, 2 and 3 possess diamagnetic ground-state S_t = 0. Interestingly, the variable-temperature (2-300 K) magnetic measurement reveals that 1 with the S_t = 1/2 ground state attains the antiferromagnetic S_t = 0 state at a very low temperature, due to weak noncovalent interactions via π-π stacking. Density functional theory (DFT) electronic structural calculations at the B3LYP level of theory rationalized the experimental results. In the UV-VIS-NIR spectra, broad absorptions are recorded for 1 and 2 in the range of 800-1600 nm; however, such an absorption is absent for 3. Time-dependent (TD)-DFT calculations provide a very good fit with the experimental spectra and allow us to identify the observed electronic transitions.

Ligand-Centered Triplet Diradical Supported by a Binuclear Palladium(II) Dipyrrindione

Oligopyrroles form a versatile class of redox-active ligands and electron reservoirs. Although the stabilization of radicals within oligopyrrolic π systems is more common for macrocyclic ligands, bidentate dipyrrindiones are emerging as compact platforms for one-electron redox chemistry in transition-metal complexes. We report the synthesis of a bis(aqua) palladium(II) dipyrrindione complex and its deprotonation-driven dimerization to form a hydroxo-bridged binuclear complex in the presence of water or triethylamine. Electrochemical, spectroelectrochemical, and computational analyses of the binuclear complex indicate the accessibility of two quasi-reversible ligand-centered reduction processes. The product of a two-electron chemical reduction by cobaltocene was isolated and characterized. In the solid state, this cobaltocenium salt features a folded dianionic complex that maintains the hydroxo bridges between the divalent palladium centers. X-band and Q-band EPR spectroscopic experiments and DFT computational analysis allow assignment of the dianionic species as a diradical with spin density almost entirely located on the two dipyrrindione ligands. As established from the EPR temperature dependence, the associated exchange coupling is weak and antiferromagnetic (J ≈ −2.5 K), which results in a predominantly triplet state at the temperatures at which the measurements have been performed.

The coordination and redox chemistry of the dipyrrindione scaffold, which is found in several heme metabolites, is investigated in heteroleptic palladium(II) complexes. The bis(aqua) complex undergoes a deprotonation-driven dimerization to form a hydroxo-bridged binuclear species. Crystallographic, electrochemical, and spectroscopic data, as well as computational analysis, demonstrate that a two-electron reduction of the binuclear complex leads to a diradical dianion with spin density delocalized over the two dipyrrindione ligands.

Redox-Active Supramolecular Heteroleptic M4 L2 L'2 Assemblies with Tunable Interior Binding Site

Three Pt4 L2 L'2 heteroleptic rectangles (1-3), containing ditopic redox-active bis-pyridine functionalized perylene bisimide (PBI) ligands PBI-pyr2 (L) are reported. Co-ligand L' is a dicarboxylate spacer of varying length, leading to modified overall size of the assemblies. 1 H NMR spectroscopy reveals a trend in the splitting and upfield chemical shift of the PBI-hydrogens in the rectangles with respect to free PBI, most pronounced with the largest strut length (3) and least with the smallest strut length (1). This is attributed to increased rotational freedom of the PBI-pyr2 ligand over its longitudinal axis (Npy -Npy ), due to increased distance between the PBI-surfaces, which is corroborated by VT-NMR measurements and DFT calculations. The intramolecular motion entails desymmetrization of the two PBI-ligands, in line with cyclic voltammetry (CV) data. The first (overall two-electron) reduction event and re-oxidation for 1 display a subtle peak-to-peak splitting of 60 mV, whilst increased splitting of this event is observed for 2 and 3. The binding of pyrene in 1 is probed to establish proof of concept of host-guest chemistry enabled by the two PBI-motifs. Fitting the binding curve obtained by 1 H NMR titration with a 1:1 complex formation model led to a binding constant of 964±55 m-1 . Pyrene binding is shown to directly influence the redox-chemistry of 1, resulting in a cathodic and anodic shift of approximately 46 mV on the first and second reduction event, respectively.

Ligand mixed-valence and electrical conductivity in coordination complexes containing a redox-active phenalenol-substituted ligand

A new redox-active hydrazone ligand bearing a phenalenol group is described (phpl), which produces neutral six-coordinate Fe and Co complexes (1 & 2) with the ligands identified in different oxidation states; an open-shell anion radical and closed-shell dianion. An intense and very low-energy intervalence charge transfer (IVCT) band is identified in solid-state and in solution in the complexes. Single crystals of 1 are semiconducting (at 300 K, σ = 3.05 × 10^-4 S cm^-1 with E_a = 245 meV).

Impact of Group 10 Metals on the Solvent-Induced Disproportionation of o-Semiquinonato Complexes

The oxidation of [M^II (3,5-DTBCat)(DTBbpy)] (M=Ni ([Ni]), Pd ([Pd]), and Pt ([Pt]); 3,5-DTBCat=3,5-di-tert-butylcatecholato; DTBbpy=4,4'-di-tert-butyl-2,2'-bipyridine) afforded the dimeric {[Ni^II (3,5-DTBSQ)(DTBbpy)](PF₆ )}₂ ({[Ni](PF₆ )}₂ ; 3,5-DTBSQ=3,5-di-tert-butylsemiquinonato) and monomeric semiquinonato (SQ) complexes [M^II (3,5-DTBSQ)(DTBbpy)](PF₆ ) (M=Pd ([Pd](PF₆ )) and Pt ([Pt](PF₆ ))). The negative solvatochromic properties of the SQ complexes allowed us to estimate the relative order of their dipole moments: [Pd](PF₆ )>[Pt](PF₆ )>{[Ni](PF₆ )}₂ . The complexes [Pd](PF₆ ) and [Pt](PF₆ ) adopt monomeric structures and are stable in CH₂ Cl₂ and toluene, whereas they gradually disproportionate at room temperature to [M] and 3,5-di-tert-butylbenzoquinone (3,5-DTBBQ) in polar solvents such as THF, MeOH, EtOH, DMF, or DMSO. The results of spectroscopic studies suggested that the oxidized nickel complex adopts a monomeric structure ([Ni](PF₆ )) in CH₂ Cl₂ , but a dimeric structure ({[Ni](PF₆ )}₂ ) in the other investigated solvents. In polar solvents, {[Ni](PF₆ )}₂ may disproportionate to [Ni] and 3,5-DTBBQ at 323 K, thereby demonstrating a significant solvent- and metal-dependence in temperature. The relative activities of {[Ni](PF₆ )}₂ and [M](PF₆ ) toward disproportionation are related to the electrochemically estimated K_dis values in CH₂ Cl₂ and DMF. The present work demonstrates that solvent polarity and the dipole moments of the SQ complexes promote disproportionation, which can be controlled by a judicious choice of the metal ion, solvent, and temperature.

Radical-Type Reactivity and Catalysis by Single-Electron Transfer to or from Redox-Active Ligands

Controlled ligand-based redox-activity and chemical non-innocence are rapidly gaining importance for selective (catalytic) processes. This Concept aims to provide an overview of the progress regarding ligand-to-substrate single-electron transfer as a relatively new mode of operation to exploit ligand-centered reactivity and catalysis based thereon.

Hydrosilylation of carbonyl and carboxyl groups catalysed by Mn(i) complexes bearing triazole ligands

Manganese(i) complexes bearing readily accessible triazole ligands are effective catalysts for the hydrosilylation of carbonyl and carboxyl compounds.

Stabilization of different redox levels of a tridentate benzoxazole amidophenoxide ligand when bound to Co(iii) or V(v)

The electronic structure of Co and V complexes of a tridentate benzoxazole-containing aminophenol ligand NNOH2 were characterized by both experimental and theoretical methods.

Six-coordinate [CoIII(L)2]z (z = 1-, 0, 1+) complexes of an azo-appended o-aminophenolate in amidate(2-) and iminosemiquinonate π-radical (1-) redox-levels: the existence of valence-tautomerism

Aerobic reaction of the ligand H2L1, 2-(2-phenylazo)-anilino-4,6-di-tert-butylphenol, CoCl2·6H2O and Et3N in MeOH under refluxing conditions produces, after work-up and recrystallization, black crystals of [Co(L1)2] (1). When examined by cyclic voltammetry, 1 displays in CH2Cl2 three one-electron redox responses: two oxidative, E11/2 = 0.30 V (peak-to-peak separation, ΔEp = 100 mV) and E21/2 = 1.04 V (ΔEp = 120 mV), and one reductive E1/2 = -0.27 V (ΔEp = 120 mV) vs. SCE. Consequently, 1 is chemically oxidized by 1 equiv. of [FeIII(η5-C5H5)2][PF6], affording the isolation of deep purple crystals of [Co(L1)2][PF6]·2CH2Cl2 (2), and one-electron reduction with [CoII(η5-C5H5)2] yielded bluish-black crystals of [CoIII(η5-C5H5)2][Co(L1)2]·MeCN (3). A solid sample of 1 exhibits temperature-independent (50-300 K) magnetism, revealing the presence of a free radical (S = 1/2), which exhibits an isotropic EPR signal (g = 2.003) at 298 K and at 77 K an eight-line feature characteristic of hyperfine-interaction of the radical with the Co (I = 7/2) nucleus. Based on X-ray structural parameters of 1-3 at 100 K, magnetic and EPR spectral behaviour of 1, and variable-temperature (233-313 K) 1H NMR spectral features of 1-3 and 13C NMR spectra at 298 K of 2 and 3 in CDCl3 point to the electronic structure of the complexes as either [CoIII{(LAP)2-}{(LISQ)}˙-] or [CoIII{(L1)2}˙3-] (delocalized nature favours the latter description) (1), [CoIII{(LISQ)˙-}2][PF6]·2CH2Cl2 (2) and [CoIII(η5-C5H5)2][CoIII{(LAP)2-}2]·MeCN (3) [(LAP)2- and (LISQ)˙- represent the redox-level of coordinated ligands o-amidophenolate(2-) ion and o-iminobenzosemiquinonate(1-) π-radical ion, respectively]. Notably, all the observed redox processes are ligand-centred. To the best of our knowledge, this is the first time that six-coordinate complexes of a common tridentate o-aminophenolate-based ligand have been structurally characterized for the parent 1, its monocation 2 and the monoanion 3 counterparts. Temperature-dependent 1H NMR spectra reveal the existence of valence-tautomeric equilibria in 1-3. Density Functional Theory (DFT) calculations at the B3LYP-level of theory corroborate the electronic structural assignment of 1-3 from experimental data. The origins of the observed UV-VIS-NIR absorptions for 1-3 have been assigned, based on time-dependent (TD)-DFT calculations.

Aminotriazole Mn(I) Complexes as Effective Catalysts for Transfer Hydrogenation of Ketones

A catalytic system based on complexes comprising abundant and cheap manganese together with readily available aminotriazole ligands is reported. The new Mn(I) complexes are catalytically competent in transfer hydrogenation of ketones with 2-propanol as hydrogen source. The reaction proceeds under mild conditions at 80 °C for 20 h with 3 % of catalyst loading using either KO t Bu or NaOH as base. Good to excellent yields were obtained for a wide substrate scope with broad functional group tolerance. The obtained results by varying the substitution pattern of the ligand are consistent with an out-sphere mechanism for the H-transfer.

Alkali Cation Effects on Redox-Active Formazanate Ligands in Iron Chemistry

Noncovalent interactions of organic moieties with Lewis acidic alkali cations can greatly affect structure and reactivity. Herein, we describe the effects of interactions with alkali-metal cations within a series of reduced iron complexes bearing a redox-active formazanate ligand, in terms of structures, magnetism, spectroscopy, and reaction rates. In the absence of a crown ether to sequester the alkali cation, dimeric complexes are isolated wherein the formazanate has rearranged to form a five-membered metallacycle. The dissociation of these dimers is dependent on the binding mode and size of the alkali cation. In the dimers, the formazanate ligands are radical dianions, as shown by X-ray absorption spectroscopy, Mössbauer spectroscopy, and analysis of metrical parameters. These experimental measures are complemented by density functional theory calculations that show the spin density on the bridging ligands.

Reversible Ligand-Centered Reduction in Low-Coordinate Iron Formazanate Complexes

Coordination of redox-active ligands to metals is a compelling strategy for making reduced complexes more accessible. In this work, we explore the use of redox-active formazanate ligands in low-coordinate iron chemistry. Reduction of an iron(II) precursor occurs at milder potentials than analogous non-redox-active β-diketiminate complexes, and the reduced three-coordinate formazanate-iron compound is characterized in detail. Structural, spectroscopic, and computational analysis show that the formazanate ligand undergoes reversible ligand-centered reduction to form a formazanate radical dianion in the reduced species. The less negative reduction potential of the reduced low-coordinate iron formazanate complex leads to distinctive reactivity with formation of a new N-I bond that is not seen with the β-diketiminate analogue. Thus, the storage of an electron on the supporting ligand changes the redox potential and enhances certain reactivity.

Nickel-Alkyl Complexes with a Reactive PNC-Pincer Ligand

Based on previous work related to the design and application of rigid tridentate phosphine-pyridine-phenyl coordination offered by a PNC-pincer ligand upon cyclometalation to nickel, the synthesis, spectroscopic and solid state characterization and redox-reactivity of two NiII(PNC) complexes featuring either a methyl (2CH3 ) or CF3 co-ligand (2CF3 ) are described. One-electron oxidation is proposed to furnish C-C reductive elimination, as deduced from a combined chemical, electrochemical, spectroscopic and computational study. One-electron reduction results in a ligand-centered radical anion, as supported by electrochemistry, UV spectroelectrochemistry, EPR spectroscopy, and DFT calculations. This further attenuates the breadth of chemical reactivity offered by such PNC-pincer ligands.

Ambient-Stable Bis-Azoaromatic-Centered Diradical [(L•)M(L•)] Complexes of Rh(III): Synthesis, Structure, Redox, and Spin-Spin Interaction

Bis-azoaromatic electron traps, viz. 2-(2-pyridylazo)azoarene 1, have been synthesized by colligating electron-deficient pyridine and azoarene moieties, and they act as apposite proradical templates for the formation of stable open-shell diradical complexes [(1^•-)Rh^III(1^•-)]⁺ ([2]⁺), starting from the low-valent electron reservoir [Rh^I]. The less stable monoradical [Rh^III(1^•-)Cl₂(PPh₃)₃] (3) has also been isolated as a minor product. These π-radical complexes are multiredox systems, and the electron transfer processes occur exclusively within the pincer-type NNN ligand backbone 1. Molecular and electronic structures of the diradicals and monoradicals have been ascertained with the aid of X-ray diffraction, electrochemical, spectroelectrochemical, and spectral (electronic, IR, NMR, and EPR) studies. In the diradicals [2]⁺, the orthogonal disposition of two ligand π orbitals linked via a closed-shell metal center (t₂⁶) impedes significant coupling between the radicals. Indeed, the observed magnetic moment of [2a]⁺ lies near ∼2.3 μ_B over the temperature range 50-300 K. A very weak antiferromagnetic (AF) intramolecular spin-spin interaction between two ligand π arrays in [(1^•-)Rh^III(1^•-)]⁺ have been found experimentally (J ≈ -5 cm^-1), and this is further substantiated by density functional theory (DFT) calculations at the (U)B3LYP/6-31G(d,p) level.

Catalytic Synthesis of N-Heterocycles via Direct C(sp3)-H Amination Using an Air-Stable Iron(III) Species with a Redox-Active Ligand

Coordination of FeCl3 to the redox-active pyridine-aminophenol ligand NNOH2 in the presence of base and under aerobic conditions generates FeCl2(NNOISQ) (1), featuring high-spin FeIII and an NNOISQ radical ligand. The complex has an overall S = 2 spin state, as deduced from experimental and computational data. The ligand-centered radical couples antiferromagnetically with the Fe center. Readily available, well-defined, and air-stable 1 catalyzes the challenging intramolecular direct C(sp3)-H amination of unactivated organic azides to generate a range of saturated N-heterocycles with the highest turnover number (TON) (1 mol% of 1, 12 h, TON = 62; 0.1 mol% of 1, 7 days, TON = 620) reported to date. The catalyst is easily recycled without noticeable loss of catalytic activity. A detailed kinetic study for C(sp3)-H amination of 1-azido-4-phenylbutane (S1) revealed zero order in the azide substrate and first order in both the catalyst and Boc2O. A cationic iron complex, generated from the neutral precatalyst upon reaction with Boc2O, is proposed as the catalytically active species.