Marcin Brynda

Amidinato– and Guanidinato–Cobalt(I) Complexes: Characterization of Exceptionally Short Co–Co Interactions

Low-coordinate, carbonyl-free first row transition metal(I) complexes are relatively rare but are finding increasing use in the activation of small molecules, as enzyme mimics, and so forth. These complexes are generally very reactive species that are stabilized by a variety of sterically bulky, mono-, di-, tri-, and higher dentate ligands. Perhaps the most versatile of these are the b-diketiminates (e.g., [{ArNC(Me)}2CH] (nacnac ; Ar= 2,6-diisopropylphenyl)), which have been utilized in the preparation of a range of Group 5–12 first row transition metal(I) complexes that have shown fascinating chemistry. In recent years, we have employed the bulky amidinate and guanidinate ligands ([(ArN)2CR] (R= tBu; piso ), N(C6H11)2 (giso ), or NiPr2 (priso )) for the stabilization of a variety of Group 2, 13, 14, and 15 metal(I) complexes, and planar four-coordinate lanthanide(II) complexes. These studies have highlighted close analogies (but also differences) between the stabilizing and ligating properties of the bulky amidinates and guanidinates, and the properties of b-diketiminates. With these characteristics in mind, we extended the coordination chemistry of the bulky ligand piso to the preparation of the first amidinato–iron(I) complex, [(k-N,N’-piso)Fe(h-toluene)] (cf. [(k-N,N’-nacnac)Fe(h-benzene)]), which was shown to weakly activate dinitrogen to give [{(k-N-,h-Ar-piso)Fe(m-N)}2] (cf. [{(k N,N’-nacnac)Fe(m-N)}2] ), with an accompanying change in the coordination mode of the piso ligand. Subsequent reports from another research group detailed unprecedented amidinato–chromium(I) complexes, which included the diamagnetic, amidinate bridged species, [{Cr[m-N(Ar’)C(R)N(Ar’)]}2] (R=H or Me, Ar’=Ar or 2,6-Et2C6H3), that contain very short Cr–Cr quintuple bonds (ca. 1.74 ). These results motivated us to extend the coordination chemistry of bulky amidinate and guanidinate ligands toward other first row transition metal(I) centers. We were particularly interested in preparing analogues of the bdiketiminate-stabilized cobalt(I) system [(k-N,N’-nacnac)Co(h-toluene)] (1), which, like other cobalt(I) complexes, has been shown to activate an assortment of small molecules. In addition, we believed that the previously demonstrated coordinative flexibility of our ligands relative to that of b-diketiminates could yield varying complex types, depending on the reaction conditions employed. Herein, we report the first amidinato– and guanidinato– cobalt(I) complexes, two dimeric examples of which exhibit the shortest Co–Co interactions reported to date. Preliminary further reactivity studies of these complexes are also reported. The paramagnetic cobalt(II) precursor complexes 2a–c (Scheme 1), were readily prepared in good yields by saltmethathesis reactions between CoX2 (X=Br or I) and the potassium salt of the appropriate ligand. The structural characterization of one complex, [{(priso)CoI}2], is the first to be reported for an amidinato– or guanidinato–cobalt(II) halide complex, and shows the complex to be dimeric with distorted tetrahedral cobalt centers. Upon reduction of 2a–c with potassium (or magnesium) in toluene, the amidinato– and guanidinato–cobalt(I) complexes 3a–c were obtained as crystalline solids in high yields. It is noteworthy that no nitrogen-coordinated complexes were obtained when the reductions were carried out under a dinitrogen atmosphere, as was the case with the reduction in toluene that gave 1. Reduction of 2a or 2b with potassium in cyclohexane under a dinitrogen atmosphere afforded the dimeric cobalt(I) complexes 4a and 4b as extremely air-sensitive solids in good yields. We have seen no evidence so far for the conversion of [*] Prof. C. Jones, Dr. C. Schulten, Dr. R. P. Rose, Dr. A. Stasch, W. D. Woodul, Prof. K. S. Murray, Dr. B. Moubaraki School of Chemistry, Monash University PO Box 23, VIC, 3800 (Australia) Fax: (+61)3-9905-4597 E-mail: cameron.jones@sci.monash.edu.au

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.

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).

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.

Multifrequency pulsed EPR studies of biologically relevant manganese(II) complexes

Electron paramagnetic resonance studies at multiple frequencies (MF EPR) can provide detailed electronic structure descriptions of unpaired electrons in organic radicals, inorganic complexes, and metalloenzymes. Analysis of these properties aids in the assignment of the chemical environment surrounding the paramagnet and provides mechanistic insight into the chemical reactions in which these systems take part. Herein, we present results from pulsed EPR studies performed at three different frequencies (9, 31, and 130 GHz) on [Mn(II)(H2O)6]2+, Mn(II) adducts with the nucleotides ATP and GMP, and the Mn(II)-bound form of the hammerhead ribozyme (MnHH). Through line shape analysis and interpretation of the zero-field splitting values derived from successful simulations of the corresponding continuous-wave and field-swept echo-detected spectra, these data are used to exemplify the ability of the MF EPR approach in distinguishing the nature of the first ligand sphere. A survey of recent results from pulsed EPR, as well as pulsed electron-nuclear double resonance and electron spin echo envelope modulation spectroscopic studies applied to Mn(II)-dependent systems, is also presented.

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).

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.

Density Functional Theory calculations on the magnetic properties of the model tyrosine radical-histidine complex mimicking tyrosyl radical YD · in photosystem II

Results of Density Functional Theory (DFT) theoretical investigations, which use a model tyrosyl (Tyr) radical and tyrosyl-histidine (Tyr-His) complex to mimick the YD· radical in Photosystem II (PSII) are presented and compared to experimental results from 15N Electron-Nuclear Double Resonance spectroscopy (ENDOR) studies of the τ nitrogen coupling from His-189 in the PSII Tyr-His complex. The DFT calculations are performed using an optimized geometry of the tyrosine radical and Tyr-His complex. The conformational space of the Tyr-His tandem is explored by varying the relative geometry of the two components; relevant parameters, such as the spin distribution on the phenoxy-ring carbons of the Tyr radical and the EPR hyperfine tensors, are calculated at each geometry and compared with the available experimental data. The isotropic 15N-ENDOR signal arising from spin delocalization on the His hydrogen-bonded to the PSII tyrosine radical is analyzed in terms of the DFT obtained parameters. The calculations of the g tensor using the Gauge Independent Atomic Orbital (GIAO) approach are presented and the influence of the geometry of the Tyr-His complex on the deviation of the g-tensor elements from the free electron values is discussed.

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.

The Manganese-Calcium Cluster of the Oxygen-Evolving System: Synthetic Models, EPR Studies, and Electronic Structure Calculations

Manganese plays a variety of roles in enzymes [1], such as for example arginase, which catalyzes the hydrolysis of arginine, forming urea and ornithine as products [2–4]. Given the biological accessibility of at least three oxidation states — MnII, MnIII, and MnIV — it is not surprising that manganese is also involved in important redox reactions [5]. Beside the well-known manganese superoxide dismutase and manganese catalase enzymes where Mn plays a role that can also be served by Fe or other metals, manganese exclusively acts as an oxidizer of water in the pentanuclear Mn4Ca cluster of Photosystem II. This cluster carries out the fourelectron oxidation of two H2O molecules to O2, providing nearly all of the free molecular oxygen in our atmosphere.