Modeling the signatures of hydrides in metalloenzymes: ENDOR analysis of a Di-iron Fe(µ-NH)(µ-H)Fe core.

The application of 35 GHz pulsed EPR and ENDOR spectroscopies has established that the biomimetic model complex L(3)Fe(µ-NH)(µ-H)FeL(3) (L(3) = [PhB(CH(2)PPh(2))(3)](-)) complex, 3, is a novel S = (1)/(2) type-III mixed-valence di-iron II/III species, in which the unpaired electron is shared equally between the two iron centers. (1,2)H and (14,15)N ENDOR measurements of the bridging imide are consistent with an allyl radical molecular orbital model for the two bridging ligands. Both the (µ-H) and the proton of the (µ-NH) of the crystallographically characterized 3 show the proposed signature of a 'bridging' hydride that is essentially equidistant between two 'anchor' metal ions: a rhombic dipolar interaction tensor, T ≈ [T, -T, 0]. The point-dipole model for describing the anisotropic interaction of a bridging H as the sum of the point-dipole couplings to the 'anchor' metal ions reproduces this signature with high accuracy, as well as the axial tensor of a terminal hydride, T ≈ [-T, -T, 2T], thus validating both the model and the signatures. This validation in turn lends strong support to the assignment, based on such a point-dipole analysis, that the molybdenum-iron cofactor of nitrogenase contains two [Fe-H(-)-Fe] bridging-hydride fragments in the catalytic intermediate that has accumulated four reducing equivalents (E(4)). Analysis further reveals a complementary similarity between the isotropic hyperfine couplings for the bridging hydrides in 3 and E(4). This study provides a foundation for spectroscopic study of hydrides in a variety of reducing metalloenzymes in addition to nitrogenase.

A nonclassical dihydrogen adduct of S = ½ Fe(I).

We have exploited the capacity of the "(SiP(iPr)(3))Fe(I)" scaffold to accommodate additional axial ligands and characterized the mononuclear S = ½ H(2) adduct complex (SiP(iPr)(3))Fe(I)(H(2)). EPR and ENDOR data, in the context of X-ray structural results, revealed that this complex provides a highly unusual example of an open-shell metal complex that binds dihydrogen as a ligand. The H(2) ligand at 2 K dynamically reorients within the ligand-binding pocket, tunneling among the energy minima created by strong interactions with the three Fe-P bonds.

Protonation of the dinitrogen-reduction catalyst [HIPTN3N]Mo(III) investigated by ENDOR spectroscopy.

Dinitrogen is reduced to ammonia by the molybdenum complex of L = [HIPTN3N](3-) [Mo; HIPT = 3,5-(2,4,6-iPr3C6H2)2C6H3]. The mechanism by which this occurs involves the stepwise addition of proton/electron pairs, but how the first pair converts MoN2 to MoN ═ NH remains uncertain. The first proton of reduction might bind either at Nß of N2 or at one of the three amido nitrogen (N(am)) ligands. Treatment of MoCO with [2,4,6-Me3C5H3N]BAr'4 [Ar' = 2,3-(CF3)2C6H3] in the absence of reductant generates HMoCO(+), whose electron paramagnetic resonance spectrum has greatly reduced g anisotropy relative to MoCO. (2)H Mims pulsed electron nuclear double-resonance spectroscopy of (2)HMoCO(+) shows a signal that simulations show to have a hyperfine tensor with an isotropic coupling, aiso((2)H) = -0.22 MHz, and a roughly dipolar anisotropic interaction, T((2)H) = [-0.48, -0.93, 1.42] MHz. The simulations show that the deuteron is bound to N(am), near the Mo equatorial plane, not along the normal, and at a distance of 2.6 Å from Mo, which is nearly identical with the (Nam)(2)H(+)-Mo distance predicted by density functional theory computations.

The structure of formaldehyde-inhibited xanthine oxidase determined by 35 GHz 2H ENDOR spectroscopy.

The formaldehyde-inhibited Mo(V) state of xanthine oxidase (I) has been studied for four decades, yet it has not proven possible to distinguish unequivocally among the several structures proposed for this form. The uniquely large isotropic hyperfine coupling for (13)C from CH(2)O led to the intriguing suggestion of a direct Mo-C bond for the active site of I. This suggestion was supported by the recent crystal structures of glycol- and glycerol-inhibited forms of aldehyde oxidoreductase, a member of the xanthine oxidase family. (1)H and (2)H ENDOR spectra of I(C(1,2)H(2)O) in H(2)O/D(2)O buffer now have unambiguously revealed that the active-site structure of I contains a CH(2)O adduct of Mo(V) in the form of a four-membered ring with S and O linking the C to Mo and have ruled out a direct Mo-C bond. Density functional theory computations are consistent with this conclusion. We interpret the large (13)C coupling as resulting from a "transannular hyperfine interaction".

Formation of {[HIPTN(3)N]Mo(III)H}(-) by heterolytic cleavage of H(2) as established by EPR and ENDOR spectroscopy.

MoN(2) (Mo = [(HIPTNCH(2)CH(2))(3)N]Mo, where HIPT = 3,5-(2,4,6-i-Pr(3)C(6)H(2))(2)C(6)H(3)) is the first stage in the reduction of N(2) to NH(3) by Mo. Its reaction with dihydrogen in fluid solution yields "MoH(2)", a molybdenum-dihydrogen compound. In this report, we describe a comprehensive electron paramagnetic resonance (EPR) and (1/2)H/(14)N electron nuclear double resonance (ENDOR) study of the product of the reaction between MoN(2) and H(2) that is trapped in frozen solution, 1. EPR spectra of 1 show that it has a near-axial g tensor, g = [2.086, 1.961, 1.947], with dramatically reduced g anisotropy relative to MoN(2). Analysis of the g values reveal that this anion has the Mo(III), [d(xz), d(yz)](3) orbital configuration, as proposed for the parent MoN(2) complex, and that it undergoes a strong pseudo-Jahn-Teller (PJT) distortion. Simulations of the 2D 35 GHz (1)H ENDOR pattern comprised of spectra taken at multiple fields across the EPR envelope (2 K) show that 1 is the [MoH](-) anion. The 35 GHz Mims pulsed (2)H ENDOR spectra of 1 prepared with (2)H(2) show the corresponding (2)H(-) signal, with a substantial deuterium isotope effect in a(iso). Radiolytic reduction of a structural analogue, Mo(IV)H, at 77 K, confirms the assignment of 1. Analysis of the 2D (14)N ENDOR pattern for the ligand amine nitrogen further reveals the presence of a linear N(ax)-Mo-H(-) molecular axis that is parallel to the unique magnetic direction (g(1)). The ENDOR pattern of the three equatorial nitrogens is well-reproduced by a model in which the Mo-N(eq) plane has undergone a static, not dynamic, PJT distortion, leading to a range of hyperfine couplings for the three N(eq)'s. The finding of a nearly axial hyperfine coupling tensor for the terminal hydride bound Mo supports the earlier proposal that the two exchangeable hydrogenic species bound to the FeMo cofactor of the nitrogense turnover intermediate, which has accumulated four electrons/protons (E(4)), are hydrides that bridge two metal ions, not terminal hydrides.

Tunable electrical conductivity in metal-organic framework thin-film devices.

We report a strategy for realizing tunable electrical conductivity in metal-organic frameworks (MOFs) in which the nanopores are infiltrated with redox-active, conjugated guest molecules. This approach is demonstrated using thin-film devices of the MOF Cu3(BTC)2 (also known as HKUST-1; BTC, benzene-1,3,5-tricarboxylic acid) infiltrated with the molecule 7,7,8,8-tetracyanoquinododimethane (TCNQ). Tunable, air-stable electrical conductivity over six orders of magnitude is achieved, with values as high as 7 siemens per meter. Spectroscopic data and first-principles modeling suggest that the conductivity arises from TCNQ guest molecules bridging the binuclear copper paddlewheels in the framework, leading to strong electronic coupling between the dimeric Cu subunits. These ohmically conducting porous MOFs could have applications in conformal electronic devices, reconfigurable electronics, and sensors.

Cu(I) beta-diketiminates for alkene aziridination: reversible Cu-arene binding and catalytic nitrene transfer from PhI=NTs.

beta-Diketiminato Cu(I)-lutidine complexes [RMeNN]Cu(2,4-lutidine) (R = Me (4a), (i)Pr (4b)) were prepared in high yield from Tl[RMeNN] and [CuBr(2,4-lutidine)(2)](2). Both 4a and 4b reversibly dissociate lutidine base in toluene to give [RMeNN]Cu(toluene) solvento complexes. A related base-free dicopper species [[Me(2)NN]Cu](2) (6) bridged via eta(2)-binding of opposing N-aryl rings could be isolated by the addition of Tl[Me(2)NN] to CuBr. The lutidine precursors serve as precatalysts for the aziridination of alkenes with PhI=NTs. Styrene, beta-methylstyrene, and cyclooctene gave the highest yields (59-96%) with a low olefin to PhI=NTs ratio (3:1) and 5 mol % catalyst loading.