Light-induced metastable states in oxalatenitrosylruthenium(II) and terpyridinenitrosylruthenium(II) complexes

Two extremely long lived metastable states (SI and SII) can be accessed by irradiation with light in the blue-green spectral range at temperatures below 200 K in Cs(2)[Ru(ox)(NO)Cl(3)], [Ni(cyclam)][Ru(ox)(NO)Cl(3)].3H(2)O, and [Ru(terpy)(NO)(OH)Cl][PF(6)]. The crystal structures of the ground states of the oxalate-containing compounds are presented, and the influence of the atomic distances of the cations/anions is discussed with respect to the decay temperatures. The radiationless thermal decay of the metastable states is detected by differential scanning calorimetry (DSC) for the three compounds. Both metastable states decay exponentially in time under isothermal conditions. The excited states are energetically separated from the ground state by potential barriers given by the activation energy of the Arrhenius law. In [Ni(cyclam)][Ru(ox)(NO)Cl(3)].3H(2)O the enthalpy maximum of the thermal decay of SII appears at 182 K, which is a relatively high decay temperature for SII. The reason for this strong temperature shift compared to those of the other compounds could be due to the polarization effect of Ni(2+) on the electron density at the Ru site via the Cl atom.     

Lifetime of Parahydrogen in Aqueous Solutions and Human Blood

Molecular hydrogen has unique nuclear spin properties. Its nuclear spin isomer, parahydrogen (pH₂), was instrumental in the early days of quantum mechanics and allows to boost the NMR signal by several orders of magnitude. pH_2-induced polarization (PHIP) is based on the survival of pH₂ spin order in solution, yet its lifetime has not been investigated in aqueous or biological media required for in vivo applications. Herein, we report longitudinal relaxation times (T₁) and lifetimes of pH₂ ( ) in methanol and water, with or without O₂, NaCl, rhodium-catalyst or human blood. Furthermore, we present a relaxation model that uses T₁ and for more precise theoretical predictions of the H₂ spin state in PHIP experiments. All measured T₁ values were in the range of 1.4–2 s and values were of the order of 10–300 minutes. These relatively long lifetimes hold great promise for emerging in vivo implementations and applications of PHIP.     

Metal–ligand cooperativity in the soluble hydrogenase-1 from Pyrococcus furiosus

Metal–ligand cooperativity is an essential feature of bioinorganic catalysis. The design principles of such cooperativity in metalloenzymes are underexplored, but are critical to understand for developing efficient catalysts designed with earth abundant metals for small molecule activation. The simple substrate requirements of reversible proton reduction by the [NiFe]-hydrogenases make them a model bioinorganic system. A highly conserved arginine residue (R355) directly above the exogenous ligand binding position of the [NiFe]-catalytic core is known to be essential for optimal function because mutation to a lysine results in lower catalytic rates. To expand on our studies of soluble hydrogenase-1 from Pyrococcus furiosus (Pf SH1), we investigated the role of R355 by site-directed-mutagenesis to a lysine (R355K) using infrared and electron paramagnetic resonance spectroscopic probes sensitive to active site redox and protonation events. It was found the mutation resulted in an altered ligand binding environment at the [NiFe] centre. A key observation was destabilization of the Ni_a³⁺–C state, which contains a bridging hydride. Instead, the tautomeric Ni_a⁺–L states were observed. Overall, the results provided insight into complex metal–ligand cooperativity between the active site and protein scaffold that modulates the bridging hydride stability and the proton inventory, which should prove valuable to design principles for efficient bioinspired catalysts.

Metal–ligand cooperativity is an essential feature of bioinorganic catalysis.     

Dihydropentalene und 6-Vinylfulvene aus cyclopentadienylcyclopropenen. Notiz zur Umsetzung von 1,2,3-Tris[(tert-butyl)thio]cyclopropenylium-tetrafluoroborat mit cyclopentadienid

Dihydropentalenes and 6-Vinylfulvenes from Cyclopentadienyl-cyctopropenes. Reaction of 1,2,3-Tris[(tert-butyl)thio]cyclopropenylium Tetrafluoroborate with Cyclopentadienide . Reaction of 1,2,3-tris[(tert-butyl)thio]cyclopropenylium tetrafluoroborate ( 6a ) with sodium Cyclopentadienide gives, besides of 7,8-bis{(tert-buty)thio calicene ( 7a ), rearranged products 7,8,8-tris[(tert-butyl)thio]-6-vinylpentafulvene ( 9a ) and 4,5.6-tris[(tert-butyl}thio]-1,2-dihydropentalene ( 10a )) in varying amounts depending on reaction conditions. Vinyl-carbenes 12 and 13 are supposed to be possible intermediates.     

Modeling highly branched structures: Description of the solution structures of dendrimers,polyglycerol, and glycogen

We show that Random Branching Theory (RBT) accurately describes the structures of various synthetic and natural highly branched polymers in solution. We test the theory against data taken from the literature, including radii of gyration of glycogen, hyperbranched polyglycerols, and polyamidoamine dendrimers and the small‐angle X‐ray scattering profiles of these same dendrimers. In particular, all these polymers can be described adequately by sequentially branching units, packed together in a random close packing arrangement. Combined with previous tests against experiments and computer simulations, the evidence presented here shows that RBT is a simple, but surprisingly useful, theory of highly branched polymers' solution structure. We suggest that it is sufficiently powerful to be useful in the design of new polymers. Our most surprising conclusion is that random attachment of component parts produces a good model of regularly branched polymers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1525–1538, 2011     

Graphene layer growth: Collision of migrating five-member rings

A reaction pathway is explored in which two cyclopenta groups combine on the zigzag edge of a graphene layer. The process is initiated by H addition to a five-membered ring, followed by opening of that ring and the formation of a six-membered ring adjacent to another five-membered ring. The elementary steps of the migration pathway are analyzed using density functional theory to examine the region of the potential energy surface associated with the pathway. The calculations are performed on a substrate modeled by the zigzag edge of tetracene. Based on the obtained energetics, the dynamics of the system are analyzed by solving the energy transfer master equations. The results indicate energetic and reaction-rate similarity between the cyclopenta combination and migration reactions. Also examined in the present study are desorption rates of migrating cyclopenta rings which are found to be comparable to cyclopenta ring migration.