Abstract

Abstract

Abstract

The oxygen-atom-transfer (OAT) reactivity of [L^iPrMoO₂(OPh)] (1, L^iPr=hydrotris(3-isopropylpyrazol-1-yl)borate) with the tertiary phosphines PEt₃ and PPh₂Me in acetonitrile was investigated. The first step, [L^iPrMoO₂(OPh)]+PR₃[L^iPrMoO(OPh)(OPR₃)], follows a second-order rate law with an associative transition state (PEt₃, ΔH ^≠=48.4 (±1.9) kJ mol⁻¹, ΔS ^≠=−149.2 (±6.4) J mol⁻¹ K⁻¹, ΔG ^≠=92.9 kJ mol⁻¹; PPh₂Me, ΔH ^≠=73.4 (±3.7) kJ mol⁻¹, ΔS ^≠=−71.9 (±2.3) J mol⁻¹ K⁻¹, ΔG ^≠=94.8 kJ mol⁻¹). With PMe₃ as a model substrate, the geometry and the free energy of the transition state (TS) for the formation of the phosphine oxide-coordinated intermediate were calculated. The latter, 95 kJ mol⁻¹, is in good agreement with the experimental values. An unexpectedly large O-P-C angle calculated for the TS suggests that there is significant O-nucleophilic attack on the PC σ* in addition to the expected nucleophilic attack of the P on the MoO π*. The second step of the reaction, that is, the exchange of the coordinated phosphine oxide with acetonitrile, [L^iPrMoO(OPh)(OPR₃)] + MeCN [L^iPrMoO(OPh)(MeCN)] + OPR₃, follows a first-order rate law in MeCN. A dissociative interchange (I_d) mechanism, with activation parameters of ΔH ^≠=93.5 (±0.9) kJ mol⁻¹, ΔS ^≠=18.2 (±3.3) J mol⁻¹ K⁻¹, ΔG ^≠=88.1 kJ mol⁻¹ and ΔH ^≠=97.9 (±3.4) kJ mol⁻¹, ΔS ^≠=47.3 (±11.8) J mol⁻¹ K⁻¹, ΔG ^≠=83.8 kJ mol⁻¹, for [L^iPrMoO(OPh)(OPEt₃)] (2 a) and [L^iPrMoO(OPh)(OPPh₂Me)] (2 b), respectively, is consistent with the experimental data. Although gas-phase calculations indicate that the MoOPMe₃ bond is stronger than the MoNCMe bond, solvation provides the driving force for the release of the phosphine oxide and formation of [L^iPrMoO(OPh)(MeCN)] (3).

Intermediates in the oxygen atom transfer from Mo^VI to P^III, [Tp^iPrMoOX(OPR₃)] (Tp^iPr=hydrotris(3-isopropylpyrazol-1-yl)borate; X=Cl⁻, phenolates, thiolates), have been isolated from the reactions of [Tp^iPrMoO₂X] with phosphines (PEt₃, PMePh₂, PPh₃). The green, diamagnetic oxomolybdenum(IV) complexes possess local C₁ symmetry (by NMR spectroscopy) and exhibit IR bands assigned to ν(MoO) (approximately 950 cm⁻¹) and ν(PO) (1140–1083 cm⁻¹) vibrations. The X-ray crystal structures of [Tp^iPrMoOX(OPEt₃)] (X=OC₆H₄-2-sBu, SnBu), [Tp^iPrMoO(OPh)(OPMePh₂)], and [Tp^iPrMoOCl(OPPh₃)] have been determined. The monomeric complexes exhibit distorted octahedral geometries, with coordination spheres composed of tridentate fac-Tp^iPr and mutually cis monodentate terminal oxo, phosphoryl (phosphine oxide), and monoanionic X ligands. The electronic structures and stabilities of the complexes have been probed by computational methods, with the three-dimensional energy surfaces confirming the existence of a low-energy steric pocket that restricts the conformational freedom of the phosphoryl ligand and inhibits complete oxygen atom transfer. The reactivity of the complexes is also briefly described.

The biological transformation of nitrogen oxyanions is widespread in nature and gives rise to a robust biogeochemical cycle. The first step in nitrate reduction is carried out by the enzyme nitrate reductase (NR). Although NR always catalyzes the same chemical reaction (conversion of nitrate into nitrite), its location in the cell, structure, and function are organism-dependent. We use protein sequence data to determine phylogenetic relationships and to examine similarities in structure and function. Three distinct clades of NR are apparent: the eukaryotic assimilatory NR (Euk-NR) clade, the membrane-associated prokaryotic NR (Nar) clade, and a clade that includes both the periplasmic NR (Nap) and prokaryotic assimilatory NR (Nas). The high degree of sequence similarity and a phylogenetic distribution that follows taxonomic classification suggest a monophyletic origin for the Euk-NR early on in the evolution of eukaryotic cells. In contrast, sequence conservation, phylogenetic analysis, and physiology suggest that both Nar and Nap were acquired by horizontal gene transfer. Nap and Nas share a lesser degree of similarity, with Nap a subclade of Nas. Nap from strict anaerobic bacteria such as Desulfovibrio desulfuricans is ancestral to facultative species and may provide an evolutionary link between Nap and Nas. We observed conserved binding sites for molybdenum and pterin cofactors in all four proteins. In pathways involving Euk-NR, Nas, and Nar, for which ammonia is the end product, nitrite is reduced to ammonia by a siroheme nitrite reductase. Nap, however, is coupled to a pentaheme nitrite reductase. In denitrification, whether Nar or Nap is involved, nitrite is reduced to nitric oxide by either a cytochrome cd₁or a copper-containing nitrite reductase. This complexity underscores the importance of nitrate reduction as a key biological process.