Michael Nippe

Exchange coupling and magnetic blocking in bipyrimidyl radical-bridged dilanthanide complexes.

The synthesis and magnetic properties of three new bipyrimidyl radical-bridged dilanthanide complexes, [(Cp*(2)Ln)(2)(μ-bpym(•))](+) (Ln = Gd, Tb, Dy), are reported. Strong Ln(III)-bpym(•-) exchange coupling is observed for all species, as indicated by the increases in χ(M)T at low temperatures. For the Gd(III)-containing complex, a fit to the data reveals antiferromagnetic coupling with J = -10 cm(-1) to give an S = (13)/(2) ground state. The Tb(III) and Dy(III) congeners show single-molecule magnet behavior with relaxation barriers of U(eff) = 44(2) and 87.8(3) cm(-1), respectively, a consequence of the large magnetic anisotropies imparted by these ions. Significantly, the latter complex exhibits a divergence of the field-cooled and zero-field-cooled dc susceptibility data at 6.5 K and magnetic hysteresis below this temperature.

Towards a comprehensive understanding of visible-light photogeneration of hydrogen from water using cobalt(II) polypyridyl catalysts

Homogeneous aqueous solutions of photocatalytic ensembles, consisting of [Ru(bpy)3]2+ as a photosensitizer, ascorbic acid/ascorbate as the electron source, and 10 distinct Co2+-based molecular catalysts, were evaluated for visible-light induced hydrogen evolution using high-throughput screening. The combined results demonstrate that Co2+ complexes bearing tetradentate ligands yield more active photocatalytic compositions than their congeners with pentadentate ligands while operating with high catalyst stability. Additionally, molecular Co2+ catalysts with cis open coordination sites appear to be significantly more active for hydrogen evolution than those with trans open sites. As evidenced by mass spectrometric analysis of the reactor headspace and associated deuteration experiments, the H2 gas generated in all instances was derived from aqueous protons. One of the most promising cis-disposed Co2+ species, [Co(bpyPY2Me)(CH3CN)(CF3SO3)](CF3SO3) (1), engages in highly efficient hydrogen evolving photocatalysis, achieving a turnover number of 4200 (H2/Co) and a turnover frequency of 3200 (H2/Co per h) at pH 4 under simulated sunlight (AM 1.5G, 100 mW cm−2) at room temperature. At equimolar concentrations of photosensitizer and 1, the total hydrogen produced appears to be exclusively limited by the photostability of [Ru(bpy)3]2+, which was observed to decompose into an Ru(bpy)2–ascorbate adduct, as evidenced by HPLC and ESI-MS experiments. Lowering the operating temperature from 27 to 5 °C significantly attenuates bpy dissociation from the sensitizer, resulting in a net ∼two-fold increase in hydrogen production from this composition. The primary electron transfer steps of this photocatalytic ensemble were investigated by nanosecond transient absorption spectroscopy. Photoexcited [Ru(bpy)3]2+ undergoes reductive quenching by ascorbic acid/ascorbate (kq = 2.6 × 107 M−1 s−1), releasing [Ru(bpy)3]+ from the encounter solvent cage with an efficiency of 55 ± 5%. In the presence of catalyst 1, [Ru(bpy)3]+ generated in the initial flash-quench experiment transfers an electron (ket = 2 × 109 M−1 s−1) at an efficiency of 85 ± 10% to the catalyst, which is believed to enter the hydrogen evolution cycle subsequently. Using a combinatorial approach, all ten Co2+ catalysts were evaluated for their potential to operate under neutral pH 7.0 conditions. Catalyst 7, [Co(PY4MeH2)(CH3CN)(CF3SO3)](CF3SO3), was revealed to be most promising, as its performance metrics were only marginally affected by pH and turnover numbers greater than 1000 were easily obtained in photocatalytic hydrogen generation. These comprehensive findings provide guidelines for the development of molecular compositions capable of evolving hydrogen from purely aqueous media.

Catalytic proton reduction with transition metal complexes of the redox-active ligand bpy2PYMe

A new pentadentate, redox-active ligand bpy2PYMe has been synthesized and its corresponding transition metal complexes of Fe2+ (1), Co2+ (2), Ni2+ (3), Cu2+ (4), and Zn2+ (5) have been investigated for electro- and photo-catalytic proton reduction in acetonitrile and water, respectively. Under weak acid conditions, the Co complex displays catalytic onset at potentials similar to those of the ligand centered reductions in the absence of acid. Related Co complexes devoid of ligand redox activity catalyze H2 evolution under similar conditions at significantly higher overpotentials, showcasing the beneficial effect of combining ligand-centered redox activity with a redox-active Co center. Furthermore, turnover numbers as high as 1630 could be obtained under aqueous photocatalytic conditions using [Ru(bpy)3]2+ as a photosensitizer. Under those conditions catalytic hydrogen production was solely limited by photosensitizer stability. Introduction of an electron withdrawing CF3 group into the pyridine moiety of the ligand as in bpy2PYMe-CF3 renders its corresponding Co complex 6 less active for proton reduction in electro- and photocatalytic experiments. This surprising effect of ligand substitution was investigated by means of density functional theory calculations which suggest the importance of electronic communication between Co1+ and the redox-active ligand. Taken together, the results provide a path forward in the design of robust molecular catalysts in aqueous media with minimized overpotential by exploiting the synergy between redox-active metal and ligand components.

Stable dye-sensitized solar cell electrolytes based on cobalt(ii)/(iii) complexes of a hexadentate pyridyl ligand

Dye-sensitized solar cells (DSCs) can be fabricated from lowcost components with simple high-throughput printing techniques thereby providing a viable alternative to conventional photovoltaics. A recent step-change in DSC research has been the successful application of transition-metal complexes (e.g., [Co(bpy)3] , bpy = 2,2’-bipyridine), organometallics (e.g., ferrocene), and organic compounds as redox mediators, replacing formerly used corrosive iodine/iodide electrolytes while maintaining impressive energy conversion efficiencies. An attractive feature of cobalt complexes, in particular, is that the structure, electronic properties, and redox chemistry can be tuned by varying the ligand environment. In this respect, the type and number of donor atoms (denticity) that the ligands used to form the cobalt coordination sphere is of paramount importance. First, for related families of ligands, such as polypyridyls, a higher denticity results in a higher overall stability constant (b) of the complex through the chelate effect. The stability of complexes is important when contemplating their application in many fields, including renewable energy research focusing on DSCs and solar-driven hydrogen generation from water. Dissociation of the bpy ligands from [Co(bpy)3] 2+ was recently identified as a significant issue for its applicability as a catalyst in photo-electrochemical water splitting devices, which was overcome by replacing the bpy ligands with a pentadentate ligand. Secondly, the denticity of the ligands will affect the reorganization energies associated with electron transfer processes involving the Co redox states, and will influence the rates of important charge-transfer steps, such as charge recombination and dye regeneration. The latter will affect the driving force required for efficient dye regeneration and therefore ultimately the maximum obtainable DSC efficiency. 9] To date only a small number of cobalt complexes have been tested in DSCs, with the majority based on bidentate ligands such as bpy and phen (1,10-phenanthroline) and their derivatives. In a recent article about applying Co-tpy complexes (tpy = 2,2’;6’,2’’-terpyridine) as DSC redox mediators, Gr tzel and co-workers highlight the importance of ligand substitution to achieve higher open-circuit photovoltage (VOC). [3g] We recently reported a study of DSCs employing cobalt complexes based on a combination of a pentadentate ligand (2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine, PY5Me2) and a weakly bound monodentate ligand. We were able to finetune the redox potential of the complex by choosing monodentate ligands with varying Lewis basicity, thereby creating the opportunity to adjust the driving force for dye regeneration. Here we report for the first time the application of the Co complex of a hexapyridyl ligand (6,6’-bis(1,1-di(pyridin-2-yl)ethyl)-2,2’-bipyridine, bpyPY4) as a redox mediator in DSCs, and compare its photovoltaic performance and stability to the reference mediator [Co(bpy)3] 2+/3+ (Co-bpy). A major motivation for using a hexadentate ligand was to develop a redox shuttle based on complexes with very high thermodynamic stability.

Exchange coupling and magnetic blocking in dilanthanide complexes bridged by the multi-electron redox-active ligand 2,3,5,6-tetra(2-pyridyl)pyrazine

The syntheses and magnetic properties of six new compounds featuring the radical-bridged dilanthanide complexes [(Cp*2Ln)2(μ-tppz˙)]+ (Ln = Gd, 1; Tb, 2; Dy, 3; tppz = 2,3,5,6-tetra(2-pyridyl)pyrazine) and [(Cp*2Ln)2(μ-tppz˙)]− (Ln = Gd, 4; Tb, 5, Dy, 6) are reported. Cyclic voltammograms for compounds 1–3 reveal that the tppz ligand can reversibly undergo multiple redox changes. Hence, in the two sets of compounds isolated, 1–3 and 4–6, the redox-active ligand tppz exists in the monoanionic (tppz˙−) and trianionic (tppz˙3−) forms, respectively. Substantial LnIII–tppz˙− exchange coupling is found for the cationic tppz˙− radical-bridged species of 1–3, as suggested by a rise in χMT at low temperatures. For the Gd compound 1, fits to the data yielded a coupling constant of J = −6.91(4) cm−1, revealing antiferromagnetic coupling to give an S = 13/2 ground state. Both of the TbIII and DyIII-containing compounds 2 and 3 exhibit single-molecule magnet behavior under zero applied dc field. Importantly, the Dy congener shows a divergence of the field-cooled and zero-field-cooled dc susceptibility data at 2.8 K and magnetic hysteresis below 3.25 K. Interestingly, the coupling constant of J = −6.29(3) cm−1 determined for the trianionic tppz˙3− radical-bridged Gd compound 4 is of similar magnitude to that of the tppz˙−-bridged analogue 1. However, the anionic tppz˙3−-bridged species containing TbIII and DyIII centers, compounds 5 and 6, do not exhibit slow magnetization dynamics under zero and applied dc fields. Computational results indicate a doublet ground state for the bridging tppz˙3− unit, with a different distribution for the spin density orientation towards the LnIII centers. These results have important implications for the future design of molecule-based magnets incorporating exchange-coupled lanthanide-radical species.

Remarkable regioselectivity in the preparation of the first heterotrimetallic Mo[quadruple bond]W...Cr chain.

Addition of CrCl(2) to the dinuclear synthon MoW(dpa)(4) yields a regioselectively formed heterotrimetallic Mo[quadruple bond]W...Cr chain; computational studies suggest that the polarization of the Mo[quadruple bond]W quadruple bond partially accounts for this unexpected selectivity.

A well-defined terminal vanadium(III) oxo complex.

The ubiquity of vanadium oxo complexes in the V+ and IV+ oxidation states has contributed to a comprehensive understanding of their electronic structure and reactivity. However, despite being predicted to be stable by ligand-field theory, the isolation and characterization of a well-defined terminal mononuclear vanadium(III) oxo complex has remained elusive. We present the synthesis and characterization of a unique terminal mononuclear vanadium(III) oxo species supported by the pentadentate polypyridyl ligand 2,6-bis[1,1-bis(2-pyridyl)ethyl]pyridine (PY5Me2). Exposure of [V(II)(NCCH3)(PY5Me2)](2+) (1) to either dioxygen or selected O-atom-transfer reagents yields [V(IV)(O)(PY5Me2)](2+) (2). The metal-centered one-electron reduction of this vanadium(IV) oxo complex furnishes a stable, diamagnetic [V(III)(O)(PY5Me2)](+) (3) species. The vanadium(III) oxo species is unreactive toward H- and O-atom transfer but readily reacts with protons to form a putative vanadium hydroxo complex. Computational results predict that further one-electron reduction of the vanadium(III) oxo species will result in ligand-based reduction, even though pyridine is generally considered to be a poor π-accepting ligand. These results have implications for future efforts toward low-valent vanadyl chemistry, particularly with regard to the isolation and study of formal vanadium(II) oxo species.

Slow Magnetic Relaxation in a Lanthanide-[1]Metallocenophane Complex

The first example of a lanthanide metallocenophane complex has been isolated as [Li(THF)4][DyFc3Li2(THF)2] (1). The molecular structure of complex 1 differs dramatically from those of main group and transition metal ferrocenophane complexes and features a distorted trigonal prismatic geometry around the Dy(III) ion and close intramolecular Dy···Fe distances. Furthermore, complex 1 exhibits all characteristics of a soft single-molecule magnet.

Electrocatalytic CO2 Reduction by Imidazolium-Functionalized Molecular Catalysts

We present the first examples of CO2 electro-reduction catalysts that feature charged imidazolium groups in the secondary coordination sphere. The functionalized Lehn-type catalysts display significant differences in their redox properties and improved catalytic activities as compared to the conventional reference catalyst. Our results suggest that the incorporated imidazolium moieties do not solely function as a charged tag but also alter mechanistic aspects of catalysis.

Dinuclear Cobalt Complexes with a Decadentate Ligand Scaffold: Hydrogen Evolution and Oxygen Reduction Catalysis

A new decadentate dinucleating ligand containing a pyridazine bridging group and pyridylic arms has been synthesized and characterized by analytical and spectroscopic techniques. Four new dinuclear cobalt complexes featuring this ligand have been prepared and thoroughly characterized both in the solid state (X-ray diffraction) and in solution (1D and 2D NMR spectroscopy, ESI-MS, and electrochemical techniques). The flexible but stable coordination environment provided by the ligand scaffold when coordinating Co in different oxidation states is shown to play a crucial role in the performance of the set of complexes when tested as catalysts for the photochemical hydrogen evolution reaction (HER) and chemical oxygen reduction reaction (ORR).