Three-way cooperativity in d8 metal complexes with ligands displaying chemical and redox non-innocence

Electron transfer catalysis mediated by 3d complexes of redox non-innocent ligands possessing an azo function: a perspective

It was first reported almost two decades ago that ligands with azo functions are capable of accepting electron(s) upon coordination to produce azo-anion radical complexes, thereby exhibiting redox non-innocence. Over the past two decades, there have been numerous reports of such complexes along with their structures and diverse characteristics. The ability of a coordinated azo function to accept one or more electron(s), thereby acting as an electron reservoir, is currently employed to carry out electron transfer catalysis since they can undergo redox transformation at mild potentials due to the presence of energetically accessible energy levels. The cooperative involvement of redox non-innocent ligand(s) containing an azo group and the coordinated metal centre can adjust and modulate the Lewis acidity of the latter through selective ligand-centred redox events, thereby manipulating the capacity of the metal centre to bind to the substrate. We have summarized the list of first row transition metal complexes of iron, cobalt, nickel, copper and zinc with redox non-innocent ligands incorporating an azo function that have been exploited as electron transfer catalysts to effectuate sustainable synthesis of a wide variety of useful chemicals. These include ketazines, pyrimidines, benzothiazole, benzoxazoles, N-acyl hydrazones, quinazoline-4(3)H-ones, C-3 alkylated indoles, N-alkylated anilines and N-alkylated heteroamines. The reaction pathways, as demonstrated by catalytic loops, reveal that the azo function of a coordinated ligand can act as an electron sink in the initial steps to bring about alcohol oxidation and thereafter, they serve as an electron pool to produce the final products either via HAT or PCET processes.

Influence of Multisite Metal-Ligand Cooperativity on the Redox Activity of Noninnocent N2S2 Ligands

Metal-ligand cooperativity (MLC) relies on chemically reactive ligands to assist metals with small-molecule binding and activation, and it has facilitated unprecedented examples of catalysis with metal complexes. Despite growing interest in combining ligand-centered chemical and redox reactions for chemical transformations, there are few studies demonstrating how chemically engaging redox active ligands in MLC affects their electrochemical properties when bound to metals. Here we report stepwise changes in the redox activity of model Ru complexes as zero, one, and two BH₃ molecules undergo MLC binding with a triaryl noninnocent N₂S₂ ligand derived from o-phenylenediamine (L1). A similar series of Ru complexes with a diaryl N₂S₂ ligand with ethylene substituted in place of phenylene (L2) is also described to evaluate the influence of the o-phenylenediamine subunit on redox activity and MLC. Cyclic voltammetry (CV) studies and density functional theory (DFT) calculations show that MLC attenuates ligand-centered redox activity in both series of complexes, but electron transfer is still achieved when only one of the two redox-active sites on the ligands is chemically engaged. The results demonstrate how incorporating more than one multifunctional reactive site could be an effective strategy for maintaining redox noninnocence in ligands that are also chemically reactive and competent for MLC.

Dinuclear RuII complexes with quinonoid bridges: tuning the electrochemical and spectroscopic properties of redox-switchable NIR dyes through judicious bridge design

Bridging quinonoid ligands are important platforms for generating metal-based switchable optoelectronic and magnetic materials. A possible sound way of influencing the properties of the aforementioned materials is to modify the direct metal-ligand interface. We present herein a series of dinuclear RuII complexes where the set of donor atoms at the bridging quinonoid ligands range from [O,O,O,O], [O,O,O,N], [O,N,O,N] and [O,N,O,N']. Additionally, the substituents on the N-donors were varied as well (a total of eight different quinonoid bridges are compared). We also present a mononuclear RuII complex for comparison purposes. The dinuclear complexes act as switchable NIR dyes, absorbing in the NIR region in their mixed-valent RuII/RuIII form but not in the neighboring RuII/RuII and RuIII/RuIII states. The switching potentials (the potentials at which NIR absorptions appear) and the λmax of the NIR band can be fine-tuned by varying the donor atoms as well as the electron-donating ability of the substituents on the nitrogen atoms (tuning E by ca. 0.4 V and λmax by ca. 450 nm). Introducing more electron-rich substituents at the nitrogen atoms of the bridge results in higher band energies and more cathodic redox potentials. Unsymmetrical bridging ligands increase the thermodynamic stability of the mixed-valent state. Whereas almost all of the mixed-valent species presented here belong to the delocalised type III of the Robin-Day classification, the most unsymmetrical complex 2O,N(Mes) shows characteristic signs of a borderline Class-II-III compounds. This comprehensive study thus establishes the lesser used unsymmetrically substituted quinones as excellent bridges for generating and tuning a series of properties in their corresponding metal complexes.

Insensitivity of Magnetic Coupling to Ligand Substitution in a Series of Tetraoxolene Radical-Bridged Fe2 Complexes

The elucidation of magnetostructural correlations between bridging ligand substitution and strength of magnetic coupling is essential to the development of high-temperature molecule-based magnetic materials. Toward this end, we report the series of tetraoxolene-bridged Fe^II₂ complexes [(Me₃TPyA)₂Fe₂(^RL)]ⁿ⁺ (Me₃TPyA = tris(6-methyl-2-pyridylmethyl)amine; n = 2: ^OMeLH₂ = 3,6-dimethoxy-2,5-dihydroxo-1,4-benzoquinone, ^ClLH₂ = 3,6-dichloro-2,5-dihydroxo-1,4-benzoquinone, Na₂[^NO₂L] = sodium 3,6-dinitro-2,5-dihydroxo-1,4-benzoquinone; n = 4: ^SMe₂L = 3,6-bis(dimethylsulfonium)-2,5-dihydroxo-1,4-benzoquinone diylide) and their one-electron-reduced analogues. Variable-temperature dc magnetic susceptibility data reveal the presence of weak ferromagnetic superexchange between Fe^II centers in the oxidized species, with exchange constants of J = +1.2(2) (R = OMe, Cl) and +0.3(1) (R = NO₂, SMe₂) cm^-1. In contrast, X-ray diffraction, cyclic voltammetry, and Mössbauer spectroscopy establish a ligand-centered radical in the reduced complexes. Magnetic measurements for the radical-bridged species reveal the presence of strong antiferromagnetic metal-radical coupling, with J = -57(10), -60(7), -58(6), and -65(8) cm^-1 for R = OMe, Cl, NO₂, and SMe₂, respectively. The minimal effects of substituents in the 3- and 6-positions of ^RL^x-• on the magnetic coupling strength is understood through electronic structure calculations, which show negligible spin density on the substituents and associated C atoms of the ring. Finally, the radical-bridged complexes are single-molecule magnets, with relaxation barriers of U_eff = 50(1), 41(1), 38(1), and 33(1) cm^-1 for R = OMe, Cl, NO₂, and SMe₂, respectively. Taken together, these results provide the first examination of how bridging ligand substitution influences magnetic coupling in semiquinoid-bridged compounds, and they establish design criteria for the synthesis of semiquinoid-based molecules and materials.

When Do Strongly Coupled Diradicals Show Strongly Coupled Reactivity? Thermodynamics and Kinetics of Hydrogen Atom Transfer Reactions of Palladium and Platinum Bis(iminosemiquinone) Complexes

The 2,2'-biphenylene-bridged bis(iminosemiquinone) complexes ( ^tBuClip)M [ ^tBuClipH₄ = 4,4'-di- tert-butyl- N, N'-bis(3,5-di- tert-butyl-2-hydroxyphenyl)-2,2'-diaminobiphenyl; M = Pd, Pt] can be reduced to the bis(aminophenoxide) complexes ( ^tBuClipH₂)M by reaction with hydrazobenzene (M = Pd) or by catalytic hydrogenation (M = Pt). The palladium complex with one aminophenoxide ligand and one iminosemiquinone ligand, ( ^tBuClipH)Pd, is generated by comproportionation of ( ^tBuClip)Pd with ( ^tBuClipH₂)Pd in a process that is both slow (0.06 M^-1 s^-1 in toluene at 23 °C) and only modestly favorable ( K_com = 1.9 in CDCl₃), indicating that both N-H bonds have essentially the same bond strength. The mono(iminoquinone) complex ( ^tBuClipH)Pt has not been observed, indicating that the platinum analogue shows no tendency to comproportionate ( K_com < 0.1). The average bond dissociation free energies (BDFE) of the complexes have been established by equilibration with suitably substituted hydrazobenzenes, and the palladium bis(iminosemiquinone) is markedly more oxidizing than the platinum compound, with hydrogen transfer from ( ^tBuClipH₂)Pt to ( ^tBuClip)Pd occurring with Δ G° = -8.9 kcal mol^-1. The palladium complex ( ^tBuClipH₂)Pd reacts with nitroxyl radicals in two observable steps, with the first hydrogen transfer taking place slightly faster than the second. In the platinum analogue, the first hydrogen transfer is much slower than the second, presumably because the N-H bond in the monoradical complex ( ^tBuClipH)Pt is unusually weak. Using driving force-rate correlations, it is estimated that this bond has a BDFE of 55.1 kcal mol^-1, which is 7.1 kcal mol^-1 weaker than that of the first N-H bond in ( ^tBuClipH₂)Pt. The two radical centers in the platinum, but not the palladium, complex thus act in concert with each other and display a strong thermodynamic bias toward two-electron reactivity. The greater thermodynamic and kinetic coupling in the platinum complex is attributed to the stronger metal-ligand π interactions in this compound.

Di- vs. tetra-substituted quinonediimines: a drastic effect on coordination chemistry

N,N′-Di-substituted benzoquinonediimine ligands behave differently in coordination chemistry compared to the well-known N,N′,N′′,N′′′-tetra-substituted analogues.

Benzoquinonoid-bridged dinuclear actinide complexes

We report the coordination chemistry of benzoquinonoid-bridged dinluclear thorium(iv) and uranium(iv) complexes with the tripodal ligand tris[2-amido(2-pyridyl)ethyl]amine ligand,L.

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

The new dinucleating redox-active ligand (L^H4 ), bearing two redox-active NNO-binding pockets linked by a 1,2,3-triazole unit, is synthetically readily accessible. Coordination to two equivalents of Pd^II resulted in the formation of paramagnetic (S=1/2 ) dinuclear Pd complexes with a κ² -N,N'-bridging triazole and a single bridging chlorido or azido ligand. A combined spectroscopic, spectroelectrochemical, and computational study confirmed Robin-Day Class II mixed-valence within the redox-active ligand, with little influence of the secondary bridging anionic ligand. Intervalence charge transfer was observed between the two ligand binding pockets. Selective one-electron oxidation allowed for isolation of the corresponding cationic ligand-based diradical species. SQUID (super-conducting quantum interference device) measurements of these compounds revealed weak anti-ferromagnetic spin coupling between the two ligand-centered radicals and an overall singlet ground state in the solid state, which is supported by DFT calculations. The rigid and conjugated dinucleating redox-active ligand framework thus allows for efficient electronic communication between the two binding pockets.

Group 5 chemistry supported by β-diketiminate ligands

β-Diketiminate (BDI) ligands are widely used supporting ligands in modern organometallic chemistry and are capable of stabilizing various metal complexes in multiple oxidation states and coordination environments.

Colour tuning by the stepwise synthesis of mononuclear and homo- and hetero-dinuclear platinum(ii) complexes using a zwitterionic quinonoid ligand

The colour of a mononuclear Pt(ii) complex bearing a [O,N,N,O] quinonoid ligand is judiciously tuned by stepwise deprotonation and successive homo- and hetero-metalation.

Probing bistability in FeIIand CoIIcomplexes with an unsymmetrically substituted quinonoid ligand

A quinone ligand with a [O,O,O,N] donor set is presented. Its Fe^IIcomplex displays bistability as a function of temperature, pressure and photoexcitation and its Co^IIcomplex displays a redox-induced spin-state change.

Correction: New avenues for ligand-mediated processes--expanding metal reactivity by the use of redox-active catechol, o-aminophenol and o-phenylenediamine ligands

Correction for 'New avenues for ligand-mediated processes - expanding metal reactivity by the use of redox-active catechol, o-aminophenol and o-phenylenediamine ligands' by Daniël L. J. Broere et al., Chem. Soc. Rev., 2015, DOI: 10.1039/c5cs00161g.

Dinuclear palladium complexes with two ligand-centered radicals and a single bridging ligand: subtle tuning of magnetic properties

The facile and tunable preparation of unique dinuclear [(L(⋅))Pd-X-Pd(L(⋅))] complexes (X = Cl or N3), bearing a ligand radical on each Pd, is disclosed, as well as their magnetochemistry in solution and solid state is reported. Chloride abstraction from [PdCl(NNO(ISQ))] (NNO(ISQ) = iminosemiquinonato) with TlPF6 results in an unusual monochlorido-bridged dinuclear open-shell diradical species, [{Pd(NNO(ISQ))}2(μ-Cl)](+), with an unusually small Pd-Cl-Pd angle (ca. 93°, determined by X-ray). This suggests an intramolecular d(8)-d(8) interaction, which is supported by DFT calculations. SQUID measurements indicate moderate antiferromagnetic spin exchange between the two ligand radicals and an overall singlet ground state in the solid state. VT EPR spectroscopy shows a transient signal corresponding to a triplet state between 20 and 60 K. Complex 2 reacts with PPh3 to generate [Pd(NNO(ISQ))(PPh3)](+) and one equivalent of [PdCl(NNO(ISQ))]. Reacting an 1:1 mixture of [PdCl(NNO(ISQ))] and [Pd(N3)(NNO(ISQ))] furnishes the 1,1-azido-bridged dinuclear diradical [{Pd(NNO(ISQ))}2(κ(1)-N;μ-N3](+), with a Pd-N-Pd angle close to 127° (X-ray). Magnetic and EPR measurements indicate two independent S = 1/2 spin carriers and no magnetic interaction in the solid state. The two diradical species both show no spin exchange in solution, likely because of unhindered rotation around the Pd-X-Pd core. This work demonstrates that a single bridging atom can induce subtle and tunable changes in structural and magnetic properties of novel dinuclear Pd complexes featuring two ligand-based radicals.