Alexa Paretzki

Noninnocence of Indigo: Dehydroindigo Anions as Bridging Electron-Donor Ligands in Diruthenium Complexes

Complexes of singly or doubly deprotonated indigo (H2Ind) with one or two [Ru(pap)2](2+) fragments (pap = 2-phenylazopyridine) have been characterized experimentally [molecular structure, voltammetry, electron paramagnetic resonance (EPR), and UV-vis-near-IR spectroelectrochemistry] and by time-dependent density functional theory calculations. The compound [Ru(pap)2(HInd(-))]ClO4 ([1]ClO4) was found to contain an intramolecular NH---O hydrogen bond, whereas [{Ru(pap)2}2(μ-Ind(2-))](ClO4)2 ([2](ClO4)2), isolated as the meso diastereoisomer with near-IR absorptions at 1162 and 991 nm, contains two metals bridged at 6.354 Å distance by the bischelating indigo dianion. The spectroelectrochemical study of multiple reversible reduction and oxidation processes of 2(n) (n = 4+, 3+, 2+, 1+, 0, 1-, 2-, 3-, 4-) reveals the stepwise addition of electrons to the terminal π-accepting pap ligands, whereas the oxidations occur predominantly at the anionic indigo ligand, producing an EPR-identified indigo radical intermediate and revealing the suitability of deprotonated indigo as a σ- and π-donating bischelating bridge.

Identifying Intermediates of Sequential Electron and Hydrogen Loss from a Dicarbonylcobalt Hydride Complex

Coordination compounds of cobalt have recently received special attention in the context of hydrogen production (“splitting water with cobalt”) and conversion. Within these studies the mechanistic sequence of electron and proton or hydrogen transfer has frequently been discussed. Although cobalt carbonyl hydrides, especially [Co(CO)4H] have been the subject of many studies since their development by Hieber and co-workers in the 1930s and since their use in hydroformylation processes, the hydrogen-producing reactions were mainly reported with non-organometallic cobalt complexes, involving ligands such as oligodentate imines, glyoximes, phosphanes, and macrocycles. Herein we describe the synthesis and characterization of the first dicarbonylhydridocobalt complex [Co(CO)2(dippf)H] = [(1)H] with a 1,1’-diphosphinoferrocene ligand (dippf = 1,1’-bis(diisopropylphosphino)ferrocene) and its chemical and (spectro)electrochemical oxidation via [(1)H] to the structurally characterized product [Co(CO)2(dippf)] + = [(1)], which has formally lost a hydrogen atom and can be reduced to a Co species [(1)] (Scheme 1). In contrast to the above-mentioned complexes, in our series the presence of CO ligands makes it possible to monitor several intermediates by IR spectroelectrochemistry. The family of 1,1’-bis(diorganophosphino)ferrocene ligands has been widely used in catalysis and for functional molecular materials; these ligands can also be considered as redox noninnocent due to the reversible oxidation of the ferrocene framework. For instance, it was shown that the assignment of the oxidation site in ambivalent [FeRu] heterobimetallic complexes involving 1,1’-bis(diorganophosphino)ferrocene ligands is not trivial. Reaction of [Co(CO)4H] with dippf leads to compound [(1)H]. In addition to the H and P NMR spectroscopy the crystal structure analysis (Figure 1A, Table 1) confirms the configuration with one metal–hydride bond (at a disordered position) pointing toward the inside—probably a result of the steric crowding from the PiPr2 groups at the outside. Both hydride positions (at half occupancy) were thus found crystallographically as capping the approximately tetrahedral CuP2C2 coordination core. DFT calculations confirm the experimental structure as a stable arrangement (Table 1, see also Figure S1 in the Supporting Information). Cyclic voltammetry at ambient temperature (Figure 2A) and at 50 8C in CH2Cl2 (Figure S2) showed an oxidation of

Electronic, charge and magnetic interactions in three-centre systems

Three-centre systems [QCuQ]n containing electron transfer-active and donor-substituted o-iminoquinone ligands were synthesised and studied as models for electronic, charge and magnetic coupling within materials involving coordinative bonding. The neutral precursors (n = 0) involve spin-bearing semiquinone radical ions, Q˙−, bridged by paramagnetic copper(II). Crystal structures and electron paramagnetic resonance (EPR) measurements reveal a delicate sensitivity of structures and of spin–spin coupling in these three-centre configurations, depending on weak secondary interactions involving substituents at the coordinatively ambivalent (hemilabile) ligands Q×. Stepwise electron loss or uptake by [QCuQ]n is accompanied by considerable redox potential splitting and by spectroelectrochemically monitored change of UV-Vis-NIR absorption. Whereas the monoanions exhibit long-wavelength ligand-to-ligand intervalence charge transfer bands around 2000 nm for π(Q2−) → π*(Q˙−) transitions, the monocations are formed in a reorganisation step, achieving coordinative saturation at the metal. Weak secondary interactions are thus shown to be sufficient to cause significant structural changes as well as qualitative differences in spin–spin interaction and in excited state structures.

Analysis of Redox Series of Unsymmetrical 1,4-Diamido-9,10-anthraquinone-Bridged Diruthenium Compounds

The unsymmetrical diruthenium complexes [(bpy)2Ru(II)(μ-H2L(2-))Ru(III)(acac)2]ClO4 ([3]ClO4), [(pap)2RuII(μ-H2L(2-))Ru(III)(acac)2]ClO4 ([4]ClO4), and [(bpy)2Ru(II)(μ-H2L(2-))Ru(II)(pap)2](ClO4)2 ([5](ClO4)2) have been obtained by way of the mononuclear precursors [(bpy)2Ru(II)(H3L(-))]ClO4 ([1]ClO4) and [(pap)2Ru(II)(H3L(-))]ClO4 ([2]ClO4) (where bpy = 2,2′-bipyridine, pap = 2-phenylazopyridine, acac(-) = 2,4-pentanedionate, and H4L = 1,4-diamino-9,10-anthraquinone). Structural characterization by single-crystal X-ray diffraction and magnetic resonance (nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR)) were used to establish the oxidation state situation in each of the isolated materials. Cyclic voltammetry, EPR, and ultraviolet-visible-near-infrared (UV-vis-NIR) spectroelectrochemistry were used to analyze the multielectron transfer series of the potentially class I mixed-valent dinuclear compounds, considering the redox activities of differently coordinated metals, of the noninnocent bridge and of the terminal ligands. Comparison with symmetrical analogues [L2′Ru(μ-H2L)RuL2′](n) (where L′ = bpy, pap, or acac(-)) shows that the redox processes in the unsymmetrical dinuclear compounds are not averaged, with respect to the corresponding symmetrical systems, because of intramolecular charge rearrangements involving the metals, the noninnocent bridge, and the ancillary ligands.

Organosilicon Radicals with Si–H and Si–Me Bonds from Commodity Precursors

The cyclic alkyl(amino) carbene (cAAC) stabilized biradicals of composition (cAAC)2SiH2 (1), (cAAC)SiMe2-SiMe2(cAAC) (2), and (cAAC)SiMeCl-SiMeCl(cAAC) (3) have been isolated as molecular species. All the compounds are stable at room temperature for more than 6 months under inert conditions in the solid state. All radical species were fully characterized by single-crystal X-ray structure analysis and EPR spectroscopy. Furthermore, the structure and bonding of compounds 1-3 have been investigated by theoretical methods. Compound 1 contains the SiH2 moiety and this is the first instance, where we have isolated 1 without an acceptor molecule.