The Staudinger ligation of azides and phosphines has found widespread use in the field of chemical biology, but the mechanism of the transformation has not been characterized in detail. In this work, we undertook a mechanistic study of the Staudinger ligation with a focus on factors that affect reaction kinetics and on the identification of intermediates. The Staudinger ligation with alkyl azides was second-order overall and proceeded more rapidly in polar, protic solvents. Hammett analyses demonstrated that electron-donating substituents on the phosphine accelerate the overall reaction. The electronic and steric properties of the ester had no significant impact on the overall rate but did affect product ratios. Finally, the structure of an intermediate that accumulates under anhydrous conditions was identified. These findings establish a platform for optimizing the Staudinger ligation for expanded use in biological applications.

A family of cationic, neutral, and anionic bis(imino)pyridine iron alkyl complexes has been prepared, and their electronic and molecular structures have been established by a combination of X-ray diffraction, Mössbauer spectroscopy, magnetochemistry, and open-shell density functional theory. For the cationic complexes, [((iPr)PDI)Fe-R][BPh(4)] ((iPr)PDI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)N═CMe)(2)C(5)H(3)N; R = CH(2)SiMe(3), CH(2)CMe(3), or CH(3)), which are known single-component ethylene polymerization catalysts, the data establish high spin ferrous compounds (S(Fe) = 2) with neutral, redox-innocent bis(imino)pyridine chelates. One-electron reduction to the corresponding neutral alkyls, ((iPr)PDI)Fe(CH(2)SiMe(3)) or ((iPr)PDI)Fe(CH(2)CMe(3)), is chelate-based, resulting in a bis(imino)pyridine radical anion (S(PDI) = 1/2) antiferromagnetically coupled to a high spin ferrous ion (S(Fe) = 2). The neutral neopentyl derivative was reduced by an additional electron and furnished the corresponding anion, [Li(Et(2)O)(3)][((iPr)PDI)Fe(CH(2)CMe(3))N(2)], with concomitant coordination of dinitrogen. The experimental and computational data establish that this S = 0 compound is best described as a low spin ferrous compound (S(Fe) = 0) with a closed-shell singlet bis(imino)pyridine dianion (S(PDI) = 0), demonstrating that the reduction is ligand-based. The change in field strength of the bis(imino)pyridine coupled with the placement of the alkyl ligand into the apical position of the molecule induced a spin state change at the iron center from high to low spin. The relevance of the compounds and their electronic structures to olefin polymerization catalysis is also presented.

Monomeric imidozirconocene complexes of the type Cp2(L)Zr=NCMe3 (Cp = cyclopentadienyl, L = Lewis base) have been shown to activate the carbon-hydrogen bonds of benzene, but not the C-H bonds of saturated hydrocarbons. To our knowledge, this singularly important class of C-H activation reactions has heretofore not been observed in imidometallocene systems. The M=NR bond formed on heating the racemic ethylenebis(tetrahydro)indenyl methyl tert-butyl amide complex, however, cleanly and quantitatively activates a wide range of n-alkane, alkene, and arene C-H bonds. Mechanistic experiments support the proposal of intramolecular elimination of methane followed by a concerted addition of the hydrocarbon C-H bond. Products formed by activation of sp2 C-H bonds are generally more thermodynamically stable than those formed by activation of sp3 C-H bonds, and those resulting from reaction at primary C-H bonds are preferred over secondary sp3 C-H activation products. There is also evidence that thermodynamic selectivity among C-H bonds is sterically rather than electronically controlled.

Iron dialkyl complexes, [N 3 ]Fe(CH 2 SiMe 3 ) 2 , with three different classes of tridentate, nitrogen-based "[N 3 ]" ligands, aryl-substituted bis(imino)pyridines, terpyridine, and pyridine bis(oxazoline), have been synthesized and evaluated in the catalytic hydrosilylation of olefins with tertiary silanes. The 2,2′:6′,2″-terpyridine (terpy) complex, (terpy)Fe-(CH 2 SiMe 3 ) 2 , was prepared either via alkylation of (terpy)-FeCl 2 with LiCH 2 SiMe 3 or by pyridine displacement from (pyridine) 2 Fe(CH 2 SiMe 3 ) 2 by free terpyridine. The arylsubstituted bis(imino)pyridine compounds, ( R PDI)Fe(CH 2 SiMe 3 ) 2 ( R PDI = 2,6-(2,6-R 2 -C 6 H 3 NCMe) 2 C 5 H 3 N), with smaller 2,6-dialkyl substituents (R = Et, Me) or a 2-i Pr substituent ( 2-iPr PDI)Fe(CH 2 SiMe 3 ) 2 ( 2-iPr PDI = 2,6-(2-i Pr-C 6 H 4 N CMe) 2 C 5 H 3 N, are effective precursors (0.5 mol %) for the anti-Markovnikov hydrosilylation of 1-octene with (Me 3 SiO) 2 MeSiH and (EtO) 3 SiH over the course of 1 h at 60 °C. No hydrosilylation activity was observed with Et 3 SiH. The most hindered member of the series, ( iPr PDI)Fe(CH 2 SiMe 3 ) 2 , and the pyridine bis(oxazoline) iron compound, (R,R)-( iPr Pybox)Fe(CH 2 SiMe 3 ) 2( iPr Pybox = 2,6-bis[isopropyl-2-oxazolin-2-yl]pyridine), were inactive for the hydrosilylation of 1-octene with all tertiary silanes studied. By contrast, the terpyridine precursor, (terpy)Fe(CH 2 SiMe 3 ) 2 , reached >95% conversion at 60 °C with Et 3 SiH and (Me 3 SiO) 2 MeSiH. In addition, the hydrosilylation of vinylcyclohexene oxide was accomplished in the presence of 1.0 mol % (terpy)Fe(CH 2 SiMe 3 ) 2 , demonstrating functional group compatibility unique to this compound that is absent from bis(imino)pyridine iron compounds. The electronic structures of all three classes of iron dialkyl compounds have been evaluated by a combination of X-ray diffraction, magnetochemistry, Mossbauer spectroscopy, and density functional theory calculations. All of the compounds are best described as high-spin iron(III) compounds with antiferromagnetic coupling to chelate radical anions.

The [Cp*CpZrNCMe3(thf)] system provides the first direct measurement of kinetic selectivity in sp, sp2, and sp3 CH bond activation with Group 4 imido complexes. This feature allows the design of selectivity and mechanistic experiments to probe the 1,2‐RH‐addition event. Substrates reacting with the highest relative rates generally form the most thermodynamically stable products. Cp*=η5‐C5Me5, Cp=η5‐C5H5.

Anhydrous iron dibromide complexes bearing bidentate α-diimine ligands Ar N=C(Me)-(Me)C=N Ar and Ar BIAN (BIAN = bis(imino)acenaphthene; Ar = dpp and Mes; dpp = 2,6diisopropylphenyl; Mes = 2,4,6-trimethylphenyl) have been prepared and characterized by 1 H NMR spectroscopy. The aryl-substituted BIAN complexes were structurally characterized by single-crystal X-ray diffraction, and their metrical parameters are consistent with a redoxinnocent chelating ligand. A high-spin iron(II) electronic structure description for the Ar BIAN iron complexes is supported by Mössbauer spectroscopy, solution state magnetic measurements, and quantum-chemical calculations. Upon reduction, the iron complexes promote catalytic hydrosilylation of 1-hexene with phenylsilane at 22 o C.

Enrichment of ecosystems with excess nutrients is occurring at an alarming rate and has fundamentally altered ecosystems worldwide. Salt marshes, which lie at the land-sea interface, are highly effective at removing anthropogenic nutrients through the action of macrophytes and through microbial processes in coastal sediments. The response of salt marsh bacteria to excess nitrogen has been documented; however, the role of fungi and their response to excess nitrogen in salt marsh sediments is not fully understood. Here, we document the response of salt marsh fungal communities to long-term excess nitrate in four distinct marsh habitats within a northern temperate marsh complex. We show that salt marsh fungal communities varied as a function of salt marsh habitat, with both fungal abundance and diversity increasing with carbon quantity. Nutrient enrichment altered fungal communities in all habitats through an increase in fungal abundance and the proliferation of putative fungal denitrifiers. Nutrient enrichment also altered marsh carbon quality in low marsh surface sediments where fungal response to nutrient enrichment was most dramatic, suggesting nutrient enrichment can alter organic matter quality in coastal sediments. Our results indicate that fungi, in addition to bacteria, likely play an important role in anaerobic decomposition of salt marsh sediment organic matter.

Mössbauer studies of three two-coordinate linear high-spin Fe(2+) compounds, namely, Fe{N(SiMe3)(Dipp)}2 (1) (Dipp = C6H3-2,6-(i)Pr2), Fe(OAr')2 (2) [Ar' = C6H3-2,6-(C6H3-2,6-(i)Pr2)2], and Fe{C(SiMe3)3}2 (3), are presented. The complexes were characterized by zero- and applied-field Mössbauer spectroscopy (1-3), as well as zero- and applied-field heat-capacity measurements (3). As 1-3 are rigorously linear, the distortion(s) that might normally be expected in view of the Jahn-Teller theorem need not necessarily apply. We find that the resulting very large unquenched orbital angular momentum leads to what we believe to be the largest observed internal magnetic field to date in a high-spin iron(II) compound, specifically +162 T in 1. The latter field is strongly polarized along the directions of the external field for both longitudinal and transverse field applications. For the longitudinal case, the applied field increases the overall hyperfine splitting consistent with a dominant orbital contribution to the effective internal field. By contrast, 2 has an internal field that is not as strongly polarized along a longitudinally applied field and is smaller in magnitude at ca. 116 T. Complex 3 behaves similarly to complex 1. They are sufficiently self-dilute (e.g., Fe···Fe distances of ca. 9-10 Å) to exhibit varying degrees of slow paramagnetic relaxation in zero field for the neat solid form. In the absence of EPR signals for 1-3, we show that heat-capacity measurements for one of the complexes (3) establish a geff value near 12, in agreement with the principal component of the ligand electric field gradient being coincident with the z axis.