Tunable magnetic exchange interactions in manganese-doped inverted core-shell ZnSe-CdSe nanocrystals.

Magnetic doping of semiconductor nanostructures is actively pursued for applications in magnetic memory and spin-based electronics. Central to these efforts is a drive to control the interaction strength between carriers (electrons and holes) and the embedded magnetic atoms. In this respect, colloidal nanocrystal heterostructures provide great flexibility through growth-controlled 'engineering' of electron and hole wavefunctions in individual nanocrystals. Here, we demonstrate a widely tunable magnetic sp-d exchange interaction between electron-hole excitations (excitons) and paramagnetic manganese ions using 'inverted' core-shell nanocrystals composed of Mn(2+)-doped ZnSe cores overcoated with undoped shells of narrower-gap CdSe. Magnetic circular dichroism studies reveal giant Zeeman spin splittings of the band-edge exciton that, surprisingly, are tunable in both magnitude and sign. Effective exciton g-factors are controllably tuned from -200 to +30 solely by increasing the CdSe shell thickness, demonstrating that strong quantum confinement and wavefunction engineering in heterostructured nanocrystal materials can be used to manipulate carrier-Mn(2+) wavefunction overlap and the sp-d exchange parameters themselves.

Direct spectroscopic observation of large quenching of first-order orbital angular momentum with bending in monomeric, two-coordinate Fe(II) primary amido complexes and the profound magnetic effects of the absence of Jahn- and Renner-Teller distortions in rigorously linear coordination.

The monomeric iron(II) amido derivatives Fe{N(H)Ar*}(2) (1), Ar* = C(6)H(3)-2,6-(C(6)H(2)-2,4,6-Pr(i)(3))(2), and Fe{N(H)Ar(#)}(2) (2), Ar(#) = C(6)H(3)-2,6-(C(6)H(2)-2,4,6-Me(3))(2), were synthesized and studied in order to determine the effects of geometric changes on their unusual magnetic properties. The compounds, which are the first stable homoleptic primary amides of iron(II), were obtained by the transamination of Fe{N(SiMe(3))(2)}(2), with HN(SiMe(3))(2) elimination, by the primary amines H(2)NAr* or H(2)NAr(#). X-ray crystallography showed that they have either strictly linear (1) or bent (2, N-Fe-N = 140.9(2) degrees ) iron coordination. Variable temperature magnetization and applied magnetic field Mossbauer spectroscopy studies revealed a very large dependence of the magnetic properties on the metal coordination geometry. At ambient temperature, the linear 1 displayed an effective magnetic moment in the range 7.0-7.50 mu(B), consistent with essentially free ion magnetism. There is a very high internal orbital field component, H(L) approximately 170 T which is only exceeded by a H(L) approximately 203 T of Fe{C(SiMe(3))(3)}(2). In contrast, the strongly bent 2 displayed a significantly lower mu(eff) value in the range 5.25-5.80 mu(B) at ambient temperature and a much lower orbital field H(L) value of 116 T. The data for the two amido complexes demonstrate a very large quenching of the orbital magnetic moment upon bending the linear geometry. In addition, a strong correlation of H(L) with overall formal symmetry is confirmed. ESR spectroscopy supports the existence of large orbital magnetic moments in 1 and 2, and DFT calculations provide good agreement with the physical data.

Structure of the biliverdin radical intermediate in phycocyanobilin:ferredoxin oxidoreductase identified by high-field EPR and DFT.

The cyanobacterial enzyme phycocyanobilin:ferredoxin oxidoreductase (PcyA) catalyzes the two-step four-electron reduction of biliverdin IXalpha to phycocyanobilin, the precursor of biliprotein chromophores found in phycobilisomes. It is known that catalysis proceeds via paramagnetic radical intermediates, but the structure of these intermediates and the transfer pathways for the four protons involved are not known. In this study, high-field electron paramagnetic resonance (EPR) spectroscopy of frozen solutions and single crystals of the one-electron reduced protein-substrate complex of two PcyA mutants D105N from the cyanobacteria Synechocystis sp. PCC6803 and Nostoc sp. PCC7120 are examined. Detailed analysis of Synechocystis D105N mutant spectra at 130 and 406 GHz reveals a biliverdin radical with a very narrow g tensor with principal values 2.00359(5), 2.00341(5), and 2.00218(5). Using density-functional theory (DFT) computations to explore the possible protonation states of the biliverdin radical, it is shown that this g tensor is consistent with a biliverdin radical where the carbonyl oxygen atoms on both the A and the D pyrrole rings are protonated. This experimentally confirms the reaction mechanism recently proposed (Tu, et al. Biochemistry 2007, 46, 1484).

Probing the coupling between proton and electron transfer in photosystem II core complexes containing a 3-fluorotyrosine.

The catalytic cycle of numerous enzymes involves the coupling between proton transfer and electron transfer. Yet, the understanding of this coordinated transfer in biological systems remains limited, likely because its characterization relies on the controlled but experimentally challenging modifications of the free energy changes associated with either the electron or proton transfer. We have performed such a study here in Photosystem II. The driving force for electron transfer from Tyr(Z) to P(680)(*+) has been decreased by approximately 80 meV by mutating the axial ligand of P(680), and that for proton transfer upon oxidation of Tyr(Z) by substituting a 3-fluorotyrosine (3F-Tyr(Z)) for Tyr(Z). In Mn-depleted Photosystem II, the dependence upon pH of the oxidation rates of Tyr(Z) and 3F-Tyr(Z) were found to be similar. However, in the pH range where the phenolic hydroxyl of Tyr(Z) is involved in a H-bond with a proton acceptor, the activation energy of the oxidation of 3F-Tyr(Z) is decreased by 110 meV, a value which correlates with the in vitro finding of a 90 meV stabilization energy to the phenolate form of 3F-Tyr when compared to Tyr (Seyedsayamdost et al. J. Am. Chem. Soc. 2006, 128,1569-1579). Thus, when the phenol of Y(Z) acts as a H-bond donor, its oxidation by P(680)(*+) is controlled by its prior deprotonation. This contrasts with the situation prevailing at lower pH, where the proton acceptor is protonated and therefore unavailable, in which the oxidation-induced proton transfer from the phenolic hydroxyl of Tyr(Z) has been proposed to occur concertedly with the electron transfer to P(680)(*+). This suggests a switch between a concerted proton/electron transfer at pHs < 7.5 to a sequential one at pHs > 7.5 and illustrates the roles of the H-bond and of the likely salt-bridge existing between the phenolate and the nearby proton acceptor in determining the coupling between proton and electron transfer.

Large differences in secondary metal-arene interactions in the transition-metal dimers ArMMAr (Ar = terphenyl; M = Cr, Fe, or Co): implications for Cr-Cr quintuple bonding.

Quantum mechanical calculations, using both CASPT2 and DFT methods, for the model systems (MeMMMe, PhMMPh, (MeMMMe)(C6H6)2, Ar(S)MMAr(S), Ar (#)MMAr(#); M = Cr, Fe, Co; Ar(S) = C6H4-2(C6H5), Ar(#) = C6H3-2,6(C6H3-2,6-Me2)2) are described. These studies were undertaken to provide a multireference description of the metal-metal bond in the simple dimers MeMMMe and PhMMPh (M = Cr, Fe, Co) and to determine the extent of secondary metal-arene interaction involving the flanking aryl rings of the terphenyl ligands in quintuply bonded Ar'CrCrAr' (Ar' = C6H3-2,6(C6H3-2,6-Pr(i)2)2). We show that in the Cr-Cr species the Cr-arene interaction is a feeble one that causes only a small weakening of the quintuple bond. In sharp contrast, in the analogous Fe and Co species strong eta(6)-arene interactions that preclude significant metal-metal bonding are predicted.

Reaction of a sterically encumbered iron(I) aryl/arene with organoazides: formation of an iron(V) bis(imide).

Reaction of 3,5-Pr(i)2Ar*Fe(eta6-C6H6)(3,5-Pr(i)2Ar* = C6H1-2,6-(C6H2)-2,4,6-Pr(i)3)(2)-3,5-Pr(i)2) with (N3C6H3)-2,6-Mes2 (Mes = (C6H2)-2,4,6-Me3) afforded the dimeric iron(II) amido/aryl complex {CH2C6H(2)-2(C6H3)-2-N(H)FeAr*-3,5-Pr(i)2)-3,5-Me2}2 (1) which arises via methyl hydrogen abstraction by nitrogen and dimerization of the radical via C-C bond formation; in contrast, reaction of 3,5-Pr(i)2Ar*Fe(eta6)-C6H6) with N3(1-Ad) (1-Ad = 1-adamantanyl) gave the iron(V) bis(imido) complex 3,5-Pr(i)2Ar*Fe{N(1-Ad)}2 (2).

Addition of H2 to distannynes under ambient conditions.

Dihydrogen reacts directly with a range of distannynes at ca. 25 degrees C under one atmosphere pressure to afford symmetric hydrogen bridged or unsymmetric stannylstannane products in high yield.

Effects of the alkali metal counter ions on the germanium-germanium double bond length in a heavier group 14 element ethenide salt.

The first, well-characterized 1,2-dilithium salt of a group 14 element ethenide species, [[(dioxane)(0.5)(Et2O)LiGeC6H3-2,6-Mes2]2]infinity, shows that the positions of the cations have a large effect on the length of the Ge-Ge double bond.

Azacrown ether-copper(ii)-hexanoate complexes. From monomer to 1-D metal organic polymer.

The synthesis and characterisation of two new Cu(ii)-hexanoate-azacrown ether species are described. The first monomeric complex is a "classical" macrocyclic Cu(ii) complex with two monodentate axially coordinated carboxylates serving as counterions. The second is an adduct of the former complex with the copper(ii) hexanoate paddlewheel dimer, Cu(2)(hexanoate)(4), forming an infinite 1-D chain of alternating monomer-dimer units. The electronic and magnetic properties of both species are discussed based on UV-vis, IR, X-ray and EPR studies combined with the DFT calculations. The UV-vis titration results prove that complex and the polymeric species are in equilibrium in toluene solution.

Asymmetric dinuclear copper(I) complexes of bis-(2-(2-pyridyl)ethyl)-2-(N-toluenesulfonylamino)ethylamine with short copper-copper distances.

Addition of two equivalents of CuCl to deprotonated bis-(2-(2-pyridyl)ethyl)-2-(N-toluenesulfonylamino)ethylamine (PETAEA) and its derivatives yielded new types of dinuclear Cu(I) complexes, Cu(mu-PETAEA)CuCl, Cu(mu-PEMAEA)CuCl, and Cu(mu-PENAEA)CuCl (PEMAEA is the 4-methoxyphenyl derivative of PETAEA and PENAEA is the 4-nitrophenyl derivative), exhibiting a four coordinate N(4)Cu center, a two coordinate NCuCl center, and a metal-metal distance within the range of 2.6572(8) to 2.6903(3) A. Analysis of the covalent radii for four coordinate and two coordinate copper(I), the acute copper-nitrogen-copper angles, and density functional theory (DFT) calculations suggest a weak attraction between the two copper atoms. The complexes apparently formed in a two-step process with the formation of the tetracoordinate mononuclear complex preceding the coordination of a second equivalent of CuCl to the lone pair of the sulfonamidate ligand.

Synthesis and characterization of the homologous M-M bonded series Ar'MMAr' (M = Zn, Cd, or Hg; Ar' = C6H3-2,6-(C6H3-2,6-Pr(i)2)2) and related arylmetal halides and hydride species.

The synthesis and structural characterization of the first homologous, molecular M-M bonded series for the group 12 metals are reported. The compounds Ar'MMAr' (M = Zn, Cd, or Hg; Ar' = C(6)H(3)-2,6-(C(6)H(3)-2,6-Pr(i)(2))(2)) were synthesized by reduction of the corresponding arylmetal halides by alkali metal/graphite (Zn or Hg) or sodium hydride (Cd). These compounds possess almost linear C-M-M-C core structures with two-coordinate metals. The observed M-M bonds distances were 2.3591(9), 2.6257(5), and 2.5738(3) A for the zinc, cadmium, and mercury species, respectively. The shorter Hg-Hg bond in comparison to that of Cd-Cd is consistent with DFT calculations which show that the strength of the Hg-Hg bond is greater. The arylmetal halides precursors (Ar'MI)(1 or 2), and the highly reactive hydrides (Ar'MH)(1 or 2), were also synthesized and fully characterized by X-ray crystallography (Zn and Cd) and multinuclear NMR spectroscopy. The arylzinc and arylcadmium iodides have iodide-bridged dimeric structures, whereas the arylmercury iodide, Ar'HgI, is monomeric. The arylzinc and arylcadmium hydrides have symmetric (Zn) or unsymmetric (Cd) mu-H-bridged structures. The Ar'HgH species was synthesized and characterized by spectroscopy, but a satisfactory refinement of the structure was precluded by the contamination of monomeric Ar'HgH by Ar'H. It was also shown that the decomposition of Ar'Cd(mu-H)(2)CdAr' at room temperature leads to the M-M bonded Ar'CdCdAr', thereby supporting the view that the reduction of the iodide proceeds via the hydride intermediate.