The electron-rich, six-coordinate tris-aryloxide uranium(III) complex [((AdArO)3tacn)U(III)] [where (AdArOH)3tacn = 1,4,7-tris(3-adamantyl-5-tert-butyl-2-hydroxybenzyl)1,4,7-triazacyclononane] reacts rapidly with CO2 to yield [((AdArO)3tacn)U(IV)(CO2)], a complex in which the CO(2) ligand is linearly coordinated to the metal through its oxygen atom (eta1-OCO). The latter complex has been crystallographically and spectroscopically characterized. The inequivalent O-C-O bond lengths [1.122 angstroms (A) for the O-C bond adjacent to uranium and 1.277 A for the other], considered together with magnetization data and electronic and vibrational spectra, support the following bonding model: U(IV)=O=C*-O- <--> U(IV)-OC-O-. In these charge-separated resonance structures, the uranium center is oxidized to uranium(IV) and the CO2 ligand reduced by one electron.

Chemists have long sought to mimic enzymatic hydrogen activation with structurally simpler compounds. Here, we report a functional [NiFe]-based model of [NiFe]hydrogenase enzymes. This complex heterolytically activates hydrogen to form a hydride complex that is capable of reducing substrates by either hydride ion or electron transfer. Structural investigations were performed by a range of techniques, including x-ray diffraction and neutron scattering, resulting in crystal structures and the finding that the hydrido ligand is predominantly associated with the Fe center. The ligand's hydridic character is manifested in its reactivity with strong acid to liberate H(2).

Metal-alkane complexes are believed to be key intermediates in C-H activation processes. The C-H σ bond of saturated hydrocarbons is strong and notoriously unreactive, and thus, selective intermolecular carbon-hydrogen bond activation has been identified as a fundamental and practical challenge to synthetic chemists. 1 Although theoretical chemists have made significant progress to elucidate the fundamental nature of metal-alkane interactions, detailed structural information for metal-alkane adducts is exceedingly rare. Most known examples of transition metal-alkane complexes to date have been detected in gas phases, matrices, and solutions in situ. 2 In virtually all reported cases, the metal-alkane adducts were identified spectroscopically as fleeting intermediates at cryogenic temperatures.Noteworthy exceptions were recently reported by George et al. 3 and Geftakis and Ball. 4 The latter group generated a cyclopentane adduct, [(Cp)Re(CO) 2 (C 5 H 10 )], via photolysis of [(Cp)Re(CO) 3 ] that was detected NMR-spectroscopically as an intermediate in neat cyclopentane solution at -93°C. On the basis of a comparison of the experimentally determined 13 C and 1 H coupling constants and chemical shifts with those of structurally closely related, -agostic bonded C-H moieties, an η 2 -H,C metal-alkane interaction was proposed (see below).In 1997, Reed et al. reported the only example of an X-ray diffraction analysis of a simple alkane in the coordination sphere of a metal complex. 5 In this iron porphyrin complex, (dap)Fe‚(nheptane), the hydrophobic pocket of a double A-framed porphyrin supported the heptane-iron adduct through a host/guest effect.We report here the X-ray diffraction analysis of a series of alkane adducts of the low-valent, coordinatively unsaturated, tris-aryl oxide uranium(III) complex [((ArO) 3 tacn)U] (1, Scheme 1). 6,7 These species exhibit evidence for bonding interactions between the uranium ion as well as the macrocyclic ligand and the axial alkane and, thus, raise the question whether the axial alkane is held in place through metal-alkane coordination, a host-guest effect, or a combination of both.Recrystallization of highly reactive 1 from neat n-pentane, n-hexane, benzene, and/or toluene, or mixtures thereof, did not yield single crystals suitable for X-ray diffraction analysis. We found, however, that cube-shaped, red-brown crystals could be obtained from an n-pentane solution if trace amounts of cyclohexane were present in the glovebox atmosphere. If a solution of 1 in n-pentane is treated with 50 equiv of cyclohexane or cyclopentane, cubeshaped crystals of [((ArO) 3 tacn)U(cy-C6)]‚(cy-C6) (1a) and [((ArO) 3 tacn)U(cy-C5)]‚(cy-C5) (1b) can be isolated reproducibly.The X-ray diffraction analysis of both complexes clearly revealed atom positions and connectivities of one molecule of cycloalkane in the coordination sphere of the uranium(III) center and a second molecule of cycloalkane cocrystallized in the lattice. The quality of the X-ray data, however, did not allow for discussion of metri...

Reaction of [(((Ad)ArO)(3)tacn)U(III)] (1) or [((Me(3)Si)(2)N)(3)U(III)] (3) with tetramethylimidazol-2-ylidene (Me(4)IMC:) yields novel N-heterocyclic carbene complexes [(((Ad)ArO)(3)tacn)U(III)(Me(4)IMC:)] (2) and [((Me(3)Si)(2)N)(3)U(III)(Me(4)IMC:)] (4). Uranium complexes 2 and 4 represent the first examples of compounds with an N-heterocyclic carbene ligand coordinated to a low-valent uranium center. The paramagnetic complexes 1, 2, and 4 were characterized by (1)H NMR, UV-vis-NIR, and EPR spectroscopy as well as SQUID magnetization measurements and X-ray diffraction analyses. DFT studies indicate a significant degree of pi-bonding in the U(III)-carbene entity.

In recent years, sterically encumbering ligands have been used increasingly to synthesize coordinatively unsaturated, highly reactive metal complexes.[1] The steric constraints imposed by spectator ligands and chelators often translate into molecular and electronic structural changes as they directly impact the coordination mode and metal-ligand orbital interactions; the latter being particularly important in atom-transfer chemistry. Relative to complexes with less customized ligands, unsaturated metal ions with molecularly engineered ligand environments often show altered and increased reactivity as a result of the increased steric pressure. Sterically tailored, highly reactive transition-metal complexes achieve a wide variety of small-molecule activation and atom-transfer chemistry very successfully.[2-6] The strategy of applying steric pressure, however, is not always sufficient to achieve certain desired reactivities. The most recent class of transition-metal-based nitrogen-transfer reagents (aziridination catalysts), for example, takes advantage of high-valent transition-metal nitrido complexes that readily transfer the imidoacyl entity to olefins upon activation with trifluoroacetic acid anhydride (TFAA). [7][8][9] The MN activation with TFAA is indispensable due to the highly covalent dp-pp interaction, thus resulting in very strong metal-nitrido triple bonds. [10][11][12] In uranium chemistry, however, the valence f orbitals do not participate in bonding to the same extent as metal d orbitals. As a result, the UN formal triple bond in the uranium imido and the elusive nitrido species is considerably more ionic in nature and can be described as U(d. Accordingly, we expected sterically pressured uranium imido and/or nitrido complexes with custom-designed chelators to exhibit increased nucleophilicity toward organic substrates. We therefore aimed to synthesize high-valent uranium imido and nitrido complexes of varying steric demand to explore possible applications for nitrogen-and group-transfer chemistry.We recently reported the reaction of the trivalent precursor complex [(( tBu ArO) 3 tacn)U] (1, ( tBu ArOH) 3 tacn = 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane) [13]