Redox Behavior of Rhodium 9,10-Phenanthrenediimine Complexes

A rhodium(II)/rhodium(III) redox couple for C-H bond amination with alkylazides: a rhodium(III)-nitrenoid intermediate with a tetradentate [14]-macrocyclic ligand

The catalytic activity of a rhodium(II) dimer complex, [RhII(TMAA)]2 (TMAA = tetramethyltetraaza[14]annulene), in C-H amination reactions with organic azides is explored. Organic azides (N3-R) with an electron-withdrawing group such as a sulfonyl group (trisylazide; R = S(O)2iPr3C6H2 (Trs)) and a simple alkyl group (R = (CH2)4Ph, (CH2)2OCH2Ph, CH2Ph, or C6H4NO2) are employed in intra- and intermolecular C-H bond amination reactions. The spectroscopic analysis using ESI-mass and EPR spectroscopy techniques on the reaction intermediate generated from [RhII(TMAA)]2 and N3-R reveals that a rhodium(III)-nitrenoid species is an active oxidant in the C-H bond amination reaction. DFT calculations suggest that the species can feature a radical localised nitrogen atom. The DFT calculation studies also indicate that the amination reaction involves hydrogen atom abstraction from the organic substrate R'-H by the NR moiety of 2N˙R and successive rebound of the generated organic radical intermediate R'˙ to [RhIII(NH-R)(TMAA)], giving [RhII(TMAA)] and R'-NH-R (amination product).

Rhodium Diamidobenzene Complexes: A Tale of Different Substituents on the Diamidobenzene Ligand

Diamidobenzene ligands are a prominent class of redox-active ligands owing to their electron reservoir behaviour, as well as the possibility of tuning the steric and the electronic properties of such ligands through the substituents on the N-atoms of the ligands. In this contribution, we present Rh(iii) complexes with four differently substituted diamidobenzene ligands. By using a combination of crystallography, NMR spectroscopy, electrochemistry, UV-vis-NIR/EPR spectroelectrochemistry, and quantum chemical calculations we show that the substituents on the ligands have a profound influence on the bonding, donor, electrochemical and spectroscopic properties of the Rh complexes. We present, for the first time, design strategies for the isolation of mononuclear Rh(ii) metallates whose redox potentials span across more than 850 mV. These Rh(ii) metallates undergo typical metalloradical reactivity such as activation of O₂ and C–Cl bond activations. Additionally, we also show that the substituents on the ligands dictate the one versus two electron nature of the oxidation steps of the Rh complexes. Furthermore, the oxidative reactivity of the metal complexes with a [CH₃]⁺ source leads to the isolation of a unprecedented, homobimetallic, heterovalent complex featuring a novel π-bonded rhodio-o-diiminoquionone. Our results thus reveal several new potentials of the diamidobenzene ligand class in organometallic reactivity and small molecule activation with potential relevance for catalysis.

Diamidobenzene ligands are versatile platforms in organometallic Rh-chemistry. They allow the isolation of tunable mononuclear ate-complexes, and the formation of a unprecedented homobimetallic, heterovalent complex.

Bulky bis(aryl)triazenides: just aspiring amidinates? A structural and spectroscopic study

The triazenide ligand is compared to the isoelectronic formamidinate with regards donor capacity, coordination chemistry and capacity to stabilise reactive main group species.

Palladium(ii) and platinum(ii) complexes of glyoxalbis(N-aryl)osazone: molecular and electronic structures, anti-microbial activities and DNA-binding study

A new family of palladium(ii) and platinum(ii) complexes of redox non-innocent osazone ligands that exhibit moderate antileishmanial activity were isolated.

Coordination, reactivity, and structural properties of electron-rich ethoxy- and dimethylamino-substituted 1,3-diketiminate ligands and their complexes

Two new anionic ligands L and their metal complexes ZnL₂, (CH₃)₂AlL, and LRh(CO)₂ (shown) have been prepared and fully characterized.

Glyoxalbis(2-methylmercaptoanil) complexes of nickel and ruthenium: radical versus non-radical states

The molecular and electronic structures of nickel(ii) and ruthenium(ii) complexes of glyoxalbis(2-methylmercaptoanil) and their reduced and oxidized analogues are reported.

Rhodium(ii) dimers without metal–metal bonds

Oxidation of (^MeBDI)Rh(cyclooctene) (^MeBDI = [2,6-Me₂C₆H₃NCMe]₂CH) with Br₂ or I₂ produces paramagnetic halide-bridged Rh^II dimers [(^MeBDI)Rh]₂(μ-X)₂ without a direct Rh–Rh bond.

Ruthenium, osmium and rhodium complexes of 1,4-diaryl 1,4-diazabutadiene: radical versus non-radical states

Molecular and electronic structures of the ruthenium, osmium and rhodium complexes of 1,4-di(3-nitrophenyl)-1,4-diazabutadiene (L^DAB) and their redox series are reported.

Cobalt in a bis-β-diketiminate environment

The reaction of Co(2)(mesityl)(4) with acetonitrile leads to the formation of a planar, low spin, bis-β-diketiminate cobalt(II) complex, (1-mesitylbutane-1,3-diimine)(2)Co (1). EPR spectroscopy, magnetic studies, and DFT calculations reveal the Co(II) ion to reside in a tetragonal ligand field with a (2)B(2)(d(yz))(1) ground state electronic configuration. Oxidation of 1 with ferrocenium hexafluorophosphate furnishes (1-mesitylbutane-1,3-diimine)(2)Co(THF)(2)PF(6) (2). The absence of significant changes in the metal-ligand bond metrics of the X-ray crystal structures of 1 and 2 supports ligand participation in the oxidation event. Moreover, no significant changes in C-C or C-N bond lengths are observed by X-ray crystallography upon oxidation of a β-diketiminate ligand, in contrast to typical redox noninnocent ligand platforms.

Carbon-carbon reductive elimination from homoleptic uranium(IV) alkyls induced by redox-active ligands

The synthesis, characterization, and reactivity of the homoleptic uranium(IV) alkyls U(CH(2)C(6)H(5))(4) (1-Ph), U(CH(2)-p-CH(3)C(6)H(4))(4) (1-p-Me), and U(CH(2)-m-(CH(3))(2)C(6)H(3))(4) (1-m-Me(2)) are reported. The addition of 4 equiv of K(CH(2)Ar) (Ar = Ph, p-CH(3)C(6)H(4), m-(CH(3))(2)C(6)H(3)) to UCl(4) at -108 °C produces 1-Ph in good yields and 1-p-Me and 1-m-Me(2) in moderate yields. Further characterization of 1-Ph by X-ray crystallography confirmed η(4)-coordination of each benzyl ligand to the uranium center. Magnetic studies produced an effective magnetic moment of 2.60 μ(B) at 23 °C, which is consistent with a tetravalent uranium 5f(2) electronic configuration. Addition of 1 equiv of the redox-active α-diimine (Mes)DAB(Me) ((Mes)DAB(Me) = [ArN═C(Me)C(Me)═NAr]; Ar = 2,4,6-trimethylphenyl (Mes)) to 1-Ph results in reductive elimination of 1 equiv of bibenzyl (PhCH(2)CH(2)Ph), affording ((Mes)DAB(Me))U(CH(2)C(6)H(5))(2) (2-Ph). Treating an equimolar mixture of 1-Ph and 1-Ph-d(28) with (Mes)DAB(Me) forms the products from monomolecular reductive elimination, 2-Ph, 2-Ph-d(14), bibenzyl, and bibenzyl-d(14). This is confirmed by (1)H NMR spectroscopy and GC/MS analysis of both organometallic and organic products. Addition of 1 equiv of 1,2-bis(dimethylphosphino)ethane (dmpe) to 1-Ph results in formation of the previously synthesized (dmpe)U(CH(2)C(6)H(5))(4) (3-Ph), indicating the redox-innocent chelating phosphine stabilizes the uranium center in 3-Ph and prevents reductive elimination of bibenzyl. Full characterization for 3-Ph, including X-ray crystallography, is reported.

Theoretical investigations on electronic spectra and the redox-switchable second-order nonlinear optical responses of rhodium(I)-9,10-phenanthrenediimine complexes

The redox-switchable second-order nonlinear optical (NLO) properties of a series of Rh(I) complexes have been studied based on density functional theory (DFT) calculations. The analysis of the electronic structure shows that the Rh(I) ion acts as the oxidation center in a one-electron-oxidized process, while both the Rh(I) ion and the 9,10-phenanthrenediimine (phdi) ligand act as reduction centers in a one-electron-reduced process. Different redox centers lead to different charge-transfer (CT) features, which alter the static first hyperpolarizabilities of the neutral complexes. Our DFT calculations indicated that these complexes show large second-order NLO responses and that the redox process can significantly enhance these NLO responses. For complexes 2 and 3, the β(tot) values of the one-electron-reduced species 2(-) and the one-electron-oxidized species 3(+) are ~10.0 and ~8.5 times larger, respectively, than those of the corresponding neutral complexes. Therefore, complexes 2 and 3 are promising candidates for redox-switchable NLO molecular materials. The large NLO responses of the oxidized species are mainly related to ligand-to-ligand charge-transfer (LLCT) transitions when combined with intraligand charge-transfer (ILCT) transitions, while the results for the reduced species are strongly associated with metal-to-ligand charge-transfer (MLCT) transitions.

Manifestations of noninnocent ligand behavior

The potential of redox-active ligands to behave "noninnocently" in transition-metal coordination compounds is reflected with respect to various aspects and situations. These include the question of establishing "correct" oxidation states, the identification and characterization of differently charged radical ligands, the listing of structural and other consequences of ligand redox reactions, and the distinction between barrierless delocalized "resonance" cases M(n)/L(n) ↔ M(n+1)L(n-1) versus separated valence tautomer equilibrium situations M(n)/L(n) ⇌ M(n+1)L(n-1). Further ambivalence arises for dinuclear systems with radical bridge M(n)(μ-L(•))M(n) versus mixed-valent alternatives M(n+1)(μ-L(-))M(n), for noninnocent ligand-bridged coordination compounds of higher nuclearity such as (μ(3)-L)M(3), (μ(4)-L)M(4), (μ-L)(4)M(4), or coordination polymers. Conversely, the presence of more than one noninnocently behaving ligand at a single transition-metal site in situations such as L(n)-M-L(n-1) or L(•)-M-L(•) may give rise to corresponding ligand-to-ligand interaction phenomena (charge transfer, electron hopping, and spin-spin coupling) and to redox-induced electron transfer with counterintuitive oxidation-state changes. The relationships of noninnocent ligand behavior with excited-state descriptions and perspectives regarding material properties and single-electron or multielectron reactivity are also illustrated briefly.