2,2'-Azobis(pyridine) (abpy) as a multiply reducible tetradentate ligand. EPR evidence for the configurational dependence of intramolecular electron transfer in the stereoisomeric tris-chelate ruthenium complexes [Ru(abpy)n(bpy)3-n]m (n = 2, 3; m = 2+ to 3-)

Implementation of 2,2'-azobispyridine radical mono- and dianions in dinuclear rare earth metal complexes

The seminal isolation of a dinuclear rare earth metal complex comprising a bridging 2,2'-azobispyridyl radical anion, [(Cptet2Y)2(μ-abpy˙)](BPh4), is presented, which was obtained from a one-electron chemical oxidation of [(Cptet2Y)2(μ-abpy)]. The unprecedented compounds were characterized by crystallography, spectroscopy and DFT computations. The radical character was proven by EPR spectroscopy.

Radical versus Nonradical States of Azobis(benzothiazole) as a Function of Ancillary Ligands on Selective Ruthenium Platforms

The paper deals with the electronic impact of ancillary ligands on the varying redox features of azobis(benzothiazole) (abbt) in the newly introduced mononuclear ruthenium complexes [Ru(pap)₂(abbt)]ⁿ (1ⁿ) and [Ru(bpy)₂(abbt)]ⁿ (2ⁿ), where pap = 2-phenylazopyridine and bpy = 2,2'-bipyridine. In this regard, the complexes [Ru^II(pap)₂(abbt^•-)]ClO₄ ([1]ClO₄), [Ru^II(pap)₂(abbt⁰)](ClO₄)₂ ([1](ClO₄)₂), [Ru^II(bpy)₂(abbt⁰)](ClO₄)₂ ([2](ClO₄)₂), and [Ru^II(bpy)₂(abbt^•-)]ClO₄ ([2]ClO₄) were structurally and spectroscopically characterized. Unambiguous assignments of the aforestated radical and nonradical forms of abbt in 1⁺/2⁺ and 1²⁺/2²⁺, respectively, were made primarily based on their redox-sensitive azo (N═N) bond distances as well as by their characteristic electron paramagnetic resonance (EPR)/NMR signatures. Although the radical form of abbt^•- was isolated as an exclusive product in the case of strongly π-acidic pap-derived 1⁺, the corresponding moderately π-acidic bpy ancillary ligand primarily delivered an oxidized form of abbt⁰ in 2²⁺, along with the radical form in 2⁺ as a minor (<10%) component. The oxidized abbt⁰-derived [1](ClO₄)₂ was, however, obtained via the chemical oxidation of [1]ClO₄. Both 1⁺ and 2²⁺ displayed multiple closed by reversible redox processes (one oxidation O1 and four successive reductions R1-R4) within the potential window of ±2.0 V versus saturated calomel electrode. The involvement of metal-, ligand-, or metal/ligand-based frontier molecular orbitals along the redox chain was assigned based on the combined experimental (structure, EPR, and spectroelectrochemisry) and theoretical [density functional theory (DFT): molecular orbitals, Mulliken spin densities/time-dependent DFT] investigations. It revealed primarily ligand (abbt/pap or bpy)-based redox activities, keeping the metal ion as a simple spectator. Moreover, frontier molecular orbital analysis corroborated the initial isolation of the radical and nonradical species for the pap-derived 1⁺ and bpy-derived 2²⁺ as well as facile reduction of pap and abbt in 1⁺ and 2⁺, respectively.

A rare earth metallocene containing a 2,2'-azopyridyl radical anion

Introducing spin onto organic ligands that are coordinated to rare earth metal ions allows direct exchange with metal spin centres. This is particularly relevant for the deeply buried 4f-orbitals of the lanthanide ions that can give rise to unparalleled magnetic properties. For efficacy of exchange coupling, the donor atoms of the radical ligand require high-spin density. Such molecules are extremely rare owing to their reactive nature that renders isolation and purification difficult. Here, we demonstrate that a 2,2′-azopyridyl (abpy) radical (S = 1/2) bound to the rare earth metal yttrium can be realized. This molecule represents the first rare earth metal complex containing an abpy radical and is unambigously characterized by X-ray crystallography, NMR, UV-Vis-NIR, and IR spectroscopy. In addition, the most stable isotope ⁸⁹Y with a natural abundance of 100% and a nuclear spin of ½ allows an in-depth analysis of the yttrium–radical complex via EPR and HYSCORE spectroscopy. Further insight into the electronic ground state of the radical azobispyridine-coordinated metal complex was realized through unrestricted DFT calculations, which suggests that the unpaired spin density of the SOMO is heavily localized on the azo and pyridyl nitrogen atoms. The experimental results are supported by NBO calculations and give a comprehensive picture of the spin density of the azopyridyl ancillary ligand. This unexplored azopyridyl radical anion in heavy element chemistry bears crucial implications for the design of molecule-based magnets particularly comprising anisotropic lanthanide ions.

Unambiguous characterization of the first 2,2′-azobispyridine radical-containing rare earth metal complex through X-ray crystallography, DFT computations, EPR and HYSCORE spectroscopy.

Inter-ligand electronic coupling mediated through a dimetal bridge: dependence on metal ions and ancillary ligands

The metal–metal bond orbitals and the ancillary ligands influence inter-ligand charge transfer through the dimetal bridge.

mer and fac isomerism in tris chelate diimine metal complexes

In this perspective, we highlight the issue of meridional (mer) and facial (fac) orientation of asymmetrical diimines in tris-chelate transition metal complexes. Diimine ligands have long been the workhorse of coordination chemistry, and whilst there are now good strategies to isolate materials where the inherent metal centered chirality is under almost complete control, and systematic methodologies to isolate heteroleptic complexes, the conceptually simple geometrical isomerism has not been widely investigated. In systems where the two donor atoms are significantly different in terms of the σ-donor and π-accepting ability, the fac isomer is likely to be the thermodynamic product. For the diimine complexes with two trigonal planar nitrogen atoms there is much more subtlety to the system, and external factors such as the solvent, lattice packing and the various steric considerations play a delicate role in determining the observed and isolable product. In this article we discuss the possibilities to control the isomeric ratio in labile systems, consider the opportunities to separate inert complexes and discuss the observed differences in their spectroscopic properties. Finally we report on the ligand orientation in supramolecular systems where facial coordination leads to simple regular structures such as helicates and tetrahedra, but the ability of the ligand system to adopt a mer orientation enables self-assembled structures of considerable beauty and complexity.

Tris(2,2'-azobispyridine) complexes of copper(II): X-ray structures, reactivities, and the radical nonradical bis(ligand) analogues

Tris(abpy) complexes of types mer-[Cu(II)(abpy)3][PF6]2 (mer-1(2+)[PF6(–)]2) and ctc-[Cu(II)(abpy)2(bpy)][PF6]2 (ctc-2(2+)[PF6(–)]2) were successfully isolated and characterized by spectra and single-crystal X-ray structure determinations (abpy = 2,2′-azobispyridine; bpy = 2,2′-bipyridine). Reactions of mer-1(2+) and ctc-2(2+) ions with catechol, o-aminophenol, p-phenylenediamine, and diphenylamine (Ph–NH–Ph) in 2:1 molar ratio afford [CuI(abpy)2](+) (3(+)) and corresponding quinone derivatives. The similar reactions of [Cu(II)(bpy)3](2+) and [Cu(II)(phen)3](2+) with these substrates yielding [Cu(I)(bpy)2](+) and [Cu(I)(phen)2](+) imply that these complexes undergo reduction-induced ligand dissociation reactions (phen = 1,10-phenanthroline). The average −N═N– lengths in mer-1(2+)[PF6(–)]2 and ctc-2(2+)[PF6(–)]2 are 1.248(4), while that in 3(+)[PF6(–)]·2CH2Cl2 is relatively longer, 1.275(2) Å, due to dCu → πazo* back bonding. In cyclic voltammetry, mer-1(2+) exhibits one quasi-reversible wave at −0.42 V due to Cu(II)/Cu(I) and abpy/abpy(•–) couples and two reversible waves at −0.90 and −1.28 V due to abpy/abpy(•–) couple, while those of ctc-2(2+) ion appear at −0.44, −0.86, and −1.10 V versus Fc(+)/Fc couple. The anodic 3(2+)/3(+) and the cathodic 3(+)/3 redox waves at +0.33 and −0.40 V are reversible. The electron paramagnetic resonance spectra and density functional theory (DFT) calculations authenticated the existence of abpy anion radical (abpy(•–)) in 3, which is defined as a hybrid state of [Cu(I)(abpy(0.5•–))(abpy(0.5•–))] and [Cu(II)(abpy(•–))(abpy(•–))] states. 3(2+) ion is a neutral abpy complex of copper(II) of type [Cu(II)(abpy)2](2+). 3 exhibits a near-IR absorption band at 2400–3000 nm because of the intervalence ligand-to-ligand charge transfer, elucidated by time-dependent DFT calculations in CH2Cl2.

The Redox Series [M(bpy)2(Q)]n+, M = Ru or Os, Q = 3,5-Di-tert-butyl-N-phenyl-1,2-benzoquinonemonoimine. Isolation and a Complete X and W Band EPR Study of the Semiquinone States (n = 1)

The complexes [M(bpy)(2)(Q)](PF(6)) (bpy = 2,2'-bipyridyl; M = Ru, Os; Q = 3,5-di-tert-butyl-N-phenyl-1,2-benzoquinonemonoimine) were isolated and studied by X and W band EPR in a dichloromethane solution at ambient temperatures and at 4 K. For M = Ru, the (14)N hyperfine splitting confirms the Ru(II)/semiquinone formulation, although at a > 1 mT, the (99,101)Ru satellite coupling is unusually high. W band EPR allowed us to determine the relatively small g anisotropy Delta g = g(1) - g(3) = 0.0665 for the ruthenium complex. The osmium analogue exhibits a much higher difference Delta g = 0.370, which is attributed not only to the larger spin-orbit coupling constant of Os versus that of Ru but also to a higher extent of metal contribution to the singly occupied molecular orbital. The difference Delta E between the oxidation and reduction potentials of the radical complexes is larger for the ruthenium compound (Delta E = 0.87 V) than for the osmium analogue (Delta E = 0.72), confirming the difference in metal/ligand interaction. The electrochemically generated states [M(bpy)(2)(Q)](n+), n = 0, 1, 2, and 3, were also characterized using UV-vis-near-infrared spectroelectrochemistry.

Molecule-Bridged Mixed-Valent Intermediates Involving the RuIOxidation State

The diruthenium(II) complexes {(mu-L)[RuCl(Cym)]2}(PF6)n, Cym = p-cymene = 4-isopropyltoluene, L = 2,2'-azobispyridine = abpy and n = 1, or L = 2,5-bis(1-phenyliminoethyl)pyrazine = bpip and n = 2, were synthesized and characterized by NMR (n = 2) or EPR spectroscopy (n = 1). Whereas the (n = 1) species are ligand radical-bridged RuIIRuII complexes, the three-electron reduction under loss of both chloride ions produces the ions {(mu-L)[Ru(Cym)]2}+, which could be identified as RuI(4d7)-containing mixed-valent species (Ru0RuI or RuIRuII) through UV-vis-NIR spectroelectrochemistry (intervalence charge-transfer bands around 1500 nm) and EPR (rhombic g tensor anisotropy). The weak metal-metal interaction of the dsigma electrons from the eg set is responsible for the small electrochemical coupling with comproportionation constants Kc approximately 102.

Complex Reduction Chemistry of (abpy)PtCl2, abpy = 2,2‘-Azobispyridine: Formation of Cyclic [(μ,η2:η1-abpy)PtCl]22+with a New Coordination Mode for abpy and a Near-Infrared Ligand-to-Ligand Intervalence Charge Transfer Absorption of the One-Electron Reduced State

The structurally characterized (abpy)PtCl2, abpy = 2,2'-azobispyridine, reveals a strong metal/ligand pi interaction as supported by DFT calculations. Unexpectedly, its chemical or electrochemical reduction occurs irreversibly to yield EPR-detectable [(micro,eta2:eta2-abpy)[PtCl2]2]*- and, as the main product of chloride dissociation, the structurally identified and DFT-calculated dinuclear [(micro,eta2:eta1-abpy)PtCl]2(2+) with a novel coordination mode for abpy and isolated as tetrachlorozincate. Stepwise reversible one-electron reduction of that dimer, separated by 0.24 V, exhibits an intense near-infrared band for the monocationic intermediate [(abpy(-I))(abpy)Pt2Cl2]*+ at 1220 nm (epsilon = 3370 M(-1) cm(-1)) which is attributed to a ligand-to-ligand intervalence charge-transfer transition.

Metal-promoted aromatic ring amination and deamination reactions at a diazo ligand coordinated to rhodium and ruthenium

Reactions of MCl(3).3H(2)O (M = Rh and Ru) with the ligand 2-[(2-N-arylamino)phenylazo]pyridine [HL(1); NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(H) (HL(1a)), NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(CH(3)) (HL(1b)), and NH(4)C(5)N=NC(6)H(4)N(H)C(5)H(4)N (HL(1c))] in the presence of dilute NEt(3) afforded multiple products. In the case of rhodium, two green compounds, viz. [Rh(L(1))(2)](+) ([2](+)) and [RhCl(pap)(L(1))](+) ([3](+)), where L(1) and pap stand for the conjugate base of [HL(1)] and 2-(phenylazo)pyridine, respectively, were separated on a preparative thin layer chromatographic plate. The reaction of RuCl(3).3H(2)O, on the other hand, produced two brown compounds, viz. [RuCl(HL(1))(L(1))] (4) and [RuCl(pap)(L(1))] (5), respectively, as the major products. The X-ray structures of the representative complexes are reported. Except for complex 2, and 4, the products are formed due to the cleavage of an otherwise unreactive C(phenyl)-N(amino) bond. In complex 4, one of the tridentate ligands (HL(1)) does not use its maximum denticity and coordinates as a neutral bidentate donor. Plausible reasons for the differences in their modes of coordination of the ligands as in 2 and 4 have been discussed. The ligand pap in the cationic mixed ligand complex [3](+) reacts instantaneously with ArNH(2) to produce an ink-blue compound, [RhCl(HL(2))(L(1))](+) ([6](+)) in a high yield. The ligand HL(2) is formed due to regioselective fusion of ArNH(2) residue at the para carbon of the phenyl ring (with respect to the azo fragment) of pap in [3](+). The above complexes are generally intensely colored and show strong absorptions in the visible region, which are assigned to intraligand charge transfer transitions. These complexes undergo multiple and successive one-electron-transfer processes at the cathodic potentials. Electrogenerated cationic complexes of ruthenium(III), [4](+) and [5](+), showed rhombic EPR spectra at 77 K.

A Bis(azo-imine)palladium(II) System with 10 Ligand pi Electrons. Synthesis, Structure, Serial Redox, and Relationship to Bis(azooximates) and Other Species

The first azo-imine chelate system, Pd(N(H)C(R)NNPh)(2) (Pd(RA)(2)), has been isolated in the form of diamagnetic solids by the 6e(-)-6H(+) reduction of bis(phenylazooximato)palladium(II), Pd(N(O)C(R)NNPh)(2) (abbreviated Pd(RB)(2)), with ascorbic acid in a mixed solvent (R = Ph, alpha-naphthyl). Selected spectral features are described. The X-ray structures of Pd(PhA)(2) and Pd(PhB)(2) have revealed trans-planar geometry consistent with metal oxidation state of +2. Bond length trends within the chelate rings are rationalized in terms of steric and electronic factors. In Pd(PhA)(2) a total of 10 ligand pi electrons are present, each formally monoanionic ligand contributing five. Model EHMO studies have revealed that the filled HOMO (a(u)) in Pd(RA)(2) is a bonding combination of two ligand pi orbitals with large azo contributions. The LUMO (b(g)) is roughly the corresponding antibonding combination. The outer pi-electron configuration of Pd(RA)(2) is (a(u))(2)(b(g))(0). Four successive voltammetric responses, two oxidative and two reductive, are observed. The E(1/2) range is -1.3 to +0.8 V vs SCE for Pd(PhA)(2) in a 1:9 MeCN-CH(2)Cl(2) mixture (Pt electrode). EPR and electronic spectra of the electrogenerated one-electron-oxidized complex Pd(PhA)(2)(+) are described. The azo-imine system is compared with imine-imine and azo-azo systems. Crystal data for the complexes are as follows. Pd(PhA)(2): crystal system monoclinic; space group C2/c; a = 18.167(5) Å, b = 7.420(3) Å, c = 16.527(6) Å; beta = 92.70(3) degrees; V = 2225(1) Å(3); Z = 4; R = 2.61%, R(w) = 3.58%. Pd(PhB)(2): crystal system monoclinic; space group P2(1)/n; a = 5.735(5) Å, b = 10.797(6) Å, c = 18.022(11) Å; beta = 97.73(6) Å; V = 1105(1) Å(3); Z = 2; R = 3.37%; R(w) = 3.40%.