The reduction of N2 to NH3 is a requisite transformation for life.1 While it is widely appreciated that the iron-rich cofactors of nitrogenase enzymes facilitate this transformation,2-5 how they do so remains poorly understood. A central element of debate has been the exact site(s) of nitrogen coordination and reduction.6,7 The synthetic inorganic community placed an early emphasis on Mo8, because Mo was thought to be an essential element of nitrogenases3 and because pioneering work by Chatt and coworkers established that well-defined Mo model complexes could mediate the stoichiometric conversion of N2 to NH3.9 This chemical transformation can be performed in a catalytic fashion by two well-defined molecular systems that feature Mo centres.10,11 However, it is now thought that Fe is the only transition metal essential to all nitrogenases,3 and recent biochemical and spectroscopic data has implicated Fe instead of Mo as the site of N2 binding in the FeMo-cofactor.12 In this work, we describe a tris(phosphine)borane-supported Fe complex that catalyzes the reduction of N2 to NH3 under mild conditions, wherein >40% of the H+/e- equivalents are delivered to N2. Our results indicate that a single Fe site may be capable of stabilizing the various NxHy intermediates generated en route to catalytic NH3 formation. Geometric tunability at Fe imparted by a flexible Fe-B interaction in our model system appears to be important for efficient catalysis.13-15 We propose that the interstitial light C-atom recently assigned in the nitrogenase cofactor may play a similar role,16,17 perhaps by enabling a single Fe site to mediate the enzymatic catalysis via a flexible Fe-C interaction.18

One major pathway of mRNA decay in yeast occurs by deadenylation-dependent decapping, which exposes the transcript to 5Ј to 3Ј exonucleolytic degradation. We show that a second general pathway of mRNA decay in yeast occurs by 3Ј to 5Ј degradation of the transcript. We also show that the SKI2, SKI3, SKI6/ RRP41, SKI8 and RRP4 gene products are required for 3Ј to 5Ј decay of mRNA. The Ski6p/Rrp41p protein has homology to the Escherichia coli 3Ј to 5Ј exoribonuclease RNase PH, and both the Ski6p/Rrp41p and Rrp4p proteins are components of a multiprotein complex, termed the exosome, that contains at least three polypeptides with 3Ј to 5Ј exoribonuclease activities. These observations suggest that the exosome may be the nucleolytic activity that degrades the body of the mRNA in a 3Ј to 5Ј direction, and the exosome's activity on mRNAs may be modulated by Ski2p, Ski3p and Ski8p. Blocking both 3Ј to 5Ј and 5Ј to 3Ј decay leads to inviability, and conditional double mutants show extremely long mRNA half-lives. These observations argue that efficient mRNA turnover is required for viability and that we have identified the two major pathways of mRNA decay in yeast.

The ability of certain transition metals to mediate the reduction of N2 to NH3 has attracted broad interest in the biological and inorganic chemistry communities. Early transition metals such as Mo and W readily bind N2 and mediate its protonation at one or more N atoms to furnish M(NxHy) species that can be characterized and, in turn, extrude NH3. By contrast, the direct protonation of Fe-N2 species to Fe(NxHy) products that can be characterized has been elusive. Herein we show that addition of acid at low temperature to [(TPB)Fe(N2)][Na(12-crown-4)] results in a new S = 1/2 Fe species. EPR, ENDOR, Mössbauer, and EXAFS analysis, coupled with a DFT study, unequivocally assign this new species as [(TPB)Fe≡N-NH2]+, a doubly protonated hydrazido(2-) complex featuring an Fe-to-N triple bond. This unstable species offers strong evidence that the first steps in Fe-mediated nitrogen reduction by [(TPB)Fe(N2)][Na(12-crown-4)] can proceed along a distal or `Chatt-type' pathway. A brief discussion of whether subsequent catalytic steps may involve early or late stage cleavage of the N-N bond, as would be found in limiting distal or alternating mechanisms, respectively, is also provided.

C–H activation by transition metal oxo complexes is a fundamental reaction in oxidative chemistry carried out by both biological and synthetic systems. This centrality has motivated efforts to understand the patterns and mechanisms of such reactivity. We have therefore thoroughly examined the C–H activation reactivity of the recently synthesized and characterized late transition metal oxo complex PhB ( t BuIm)3CoIIIO. Precise values for the pK a and BDFEO–H of the conjugates of this complex have been experimentally determined and provide insight into the observed reactivity. The activation parameters for the reaction between this complex and 9,10-dihydroanthracene have also been measured and compared to previous literature examples. Evaluation of the rates of reaction of PhB­( t BuIm)3CoIIIO with a variety of hydrogen atom donors demonstrates that the reactivity of this complex is dependent on the pK a of the substrate of interest rather than the BDEC–H. This observation runs counter to the commonly cited reactivity paradigm for many other transition metal oxo complexes. Experimental and computational analysis of C–H activation reactions by PhB­( t BuIm)3CoIIIO reveals that the transition state for these processes contains significant proton transfer character. Nevertheless, additional experiments strongly suggest that the reaction does not occur via a stepwise process, leading to the conclusion that C–H activation by this CoIII–oxo complex proceeds by a pK a-driven “asynchronous” concerted mechanism. This result supports a new pattern of reactivity that may be applicable to other systems and could result in alternative selectivity for C–H activation reactions mediated by transition metal oxo complexes.

Tris(phosphine)borane ligated Fe(I) centers featuring N2H4, NH3, NH2, and OH ligands are described. Conversion of Fe-NH2 to Fe-NH3+ by addition of acid, and subsequent reductive release of NH3 to generate Fe-N2, is demonstrated. This sequence models the final steps of proposed Fe-mediated nitrogen fixation pathways. The five-coordinate trigonal bipyramidal complexes described are unusual in that they adopt S = 3/2 ground states and are prepared from a four-coordinate, S = 3/2 trigonal pyramidal precursor.

A new family of low-coordinate nickel imides supported by 1,2-bis(di-tert-butylphosphino)ethane was synthesized. The oxidation of nickel(II) complexes led to the formation of both aryl- and alkyl-substituted nickel(III) imides, and examples of both types have been isolated and fully characterized. The aryl substituent that proved most useful in stabilizing the Ni(III)-imide moiety was the bulky 2,6-dimesitylphenyl. The two nickel(III)-imide compounds showed different variable-temperature magnetic properties, but analogous EPR spectra at low temperatures. In order to account for this discrepancy, a low-spin/high-spin equilibrium was proposed to take place for the alkyl-substituted imide nickel(III) complex. This proposal was supported by DFT calculations. DFT calculations also indicated that the unpaired electron is mostly localized on the imide nitrogen for the nickel(III) complexes. The results of reactions carried out in the presence of hydrogen donors supported the findings from DFT calculations that the adamantyl substituent was a significantly more reactive hydrogen-atom abstractor. Interestingly, the steric properties of the 2,6-dimesitylphenyl substituent are important not only in protecting the Ni=N core but also in favoring one rotamer of the resulting nickel(III) imide, by locking in the phenyl ring in a perpendicular orientation with respect to the NiPP plane.

BIOCHEMISTRY: ANDERSON ET AL. 881 13 5-HydroxymethyldUMP could be a hydrolytic side product of the hypothetical intermediate having a methylene bridge between the 5 position of dUMP and the no. 5 atom of tetrahydrofolate.7 In all solvents examined by R. E. Cline, R. M. Fink, and K. Fink [J. Am. Chem. Soc., 81, 2521 (1959)], 5-hydroxymethyldeoxyuridine migrated to the same position as 5-hydroxymethyluracil or was slightly slower, and thymidine and thymine always migrated far ahead of both. We could not detect radioactivity on our chromatograms in the region between the starting line and the position of thymidine and thymine (RF, 0.64). 5-Hydroxymethyluracil showed an RF of 0.49. From the background level radioactivity on the chromatogram, 5-hydroxy-methyldUMP could not account for more than 5% of the synthesis. 5-HydroxymethyldUMP does not appear to be an intermediate in the E. coli dTMP synthetase reaction.The uridine nucleotides, uridine diphospho-acetylmuramyl I-ala-D-glu I.lys. D-ala D-ala (UDP-M~urNAc-pentapeptide) and uridine diphospho-acetylglucosamine (UDP-GlcNAc), are substrates for a reaction catalyzed by a particulate enzyme prepared from Staphylococcus aureus in which a linear glycopeptide com-882 BIOCHEMISTRY: ANDERSON ET AL. PROC. N. A. S.

Enabling the rational synthesis of molecular candidates for quantum information processing requires design principles that minimize electron spin decoherence. Here we report a systematic investigation of decoherence via the synthesis of two series of paramagnetic coordination complexes. These complexes, [M(C2O4)3](3-) (M = Ru, Cr, Fe) and [M(CN)6](3-) (M = Fe, Ru, Os), were prepared and interrogated by pulsed electron paramagnetic resonance (EPR) spectroscopy to assess quantitatively the influence of the magnitude of spin (S = (1)/2, (3)/2, (5)/2) and spin-orbit coupling (ζ = 464, 880, 3100 cm(-1)) on quantum decoherence. Coherence times (T2) were collected via Hahn echo experiments and revealed a small dependence on the two variables studied, demonstrating that the magnitudes of spin and spin-orbit coupling are not the primary drivers of electron spin decoherence. On the basis of these conclusions, a proof-of-concept molecule, [Ru(C2O4)3](3-), was selected for further study. The two parameters establishing the viability of a qubit are a long coherence time, T2, and the presence of Rabi oscillations. The complex [Ru(C2O4)3](3-) exhibits both a coherence time of T2 = 3.4 μs and the rarely observed Rabi oscillations. These two features establish [Ru(C2O4)3](3-) as a molecular qubit candidate and mark the viability of coordination complexes as qubit platforms. Our results illustrate that the design of qubit candidates can be achieved with a wide range of paramagnetic ions and spin states while preserving a long-lived coherence.