The silver complex of the tripodal N-heterocyclic carbene ligand TIME Me , [(TIME Me ) 2 Ag 3 ]-(PF 6 ) 3 (3), reacts with copper(I) bromide and (dimethyl sulfide)gold(I) chloride to yield the corresponding D 3 -symmetrical copper(I) and gold(I) complexes [(TIME Me ) 2 Cu 3 ](PF 6 ) 3 (4) and [(TIME Me ) 2 Au 3 ](PF 6 ) 3 (5). Single-crystal X-ray diffraction, spectroscopic, and computational studies of this series of metal NHC complexes are described. The group 11 metal complexes of the TIME Me ligand exhibit isostructural geometries, with three metal ions bridging two of the TIME Me ligands. Each metal ion is linearly coordinated to two carbene centers, with each of the carbenoid carbons stemming from a different ligand. Overall, the molecules possess D 3 symmetry. The electronic structure of these newly synthesized compounds was elucidated with the aid of DFT calculations. In contrast to the common assumption that NHCs are pure σ-donor ligands, our calculations reveal the existence of both σ-and π-type interactions between the metal ions and the carbenoid carbons. A study of the closely related D 2d -symmetrical species Pd(CN 2 Bu t 2 C 2 H 2 ) 2 (6) and the simplified D 2h -symmetrical model complexes M(IM Me C:) 2 (8-10; M ) Ag, Cu, Au) allowed for quantitative comparison of the two different types of bonding interactions. It was found that π-back-bonding interactions in these diaminocarbene model species contribute to approximately 15-30% of the complexes' overall orbital interaction energies.

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.

The synthesis and spectroscopic characterization of the mononuclear uranium complex [((ArO)(3)tacn)U(III)(NCCH(3))] is reported. The uranium(III) complex reacts with organic azides to yield uranium(IV) azido as well as uranium(V) imido complexes, [((ArO)(3)tacn)U(IV)(N(3))] and [((ArO)(3)tacn)U(V)(NSi(CH(3))(3))]. Single-crystal X-ray diffraction, spectroscopic, and computational studies of this analogous series of uranium tris-aryloxide complexes supported by triazacyclononane are described. The hexadentate, tris-anionic ligand coordinates to the large uranium ion in unprecedented fashion, engendering coordinatively unsaturated and highly reactive uranium centers. The macrocyclic triazacyclononane tris-aryloxide derivative occupies six coordination sites, with the three aryloxide pendant arms forming a trigonal plane at the metal center. DFT quantum mechanic methods were applied to rationalize the reactivity and to elucidate the electronic structure of the newly synthesized compounds. It is shown that the deeply colored uranium(III) and uranium(V) species are stabilized via pi-bonding interaction, involving uranium f-orbitals and the axial acetonitrile and imido ligand, respectively. In contrast, the bonding in the colorless uranium(IV) azido complex is purely ionic in nature. The magnetism of the series of complexes with an [N3O3-N(ax)] core structure and oxidation states +III, +IV, and +V is discussed in context of the electronic structures.

Migratory insertion of benzonitrile into both An−C bonds of the bis(alkyl) and bis(aryl) complexes (C5Me5)2AnR2 yields the actinide ketimido complexes (C5Me5)2An[−NC(Ph)(R)]2 (where An = Th, R = Ph, CH2Ph, CH3; An = U, R = CH2Ph, CH3) and provides a versatile method for the construction of electronically and sterically diverse ketimide ligands. The Th(IV) compounds represent the first examples of thorium ketimide complexes. The uranium complexes are surprisingly unreactive, and both the uranium and thorium bis(ketimido) complexes display unusual electronic structure properties. The combined chemical and physical properties of these complexes suggest a higher An−N bond order due to significant ligand-to-metal π-bonding in the actinide ketimido interactions and indicate that the f-electrons in mid-valent organouranium complexes might be far more involved in chemical bonding and reactivity than previously thought. We also report herein the structures of the known thorium and uranium complexes (C5Me5)2Th(CH2Ph)2, (C5Me5)2ThMe2, (C5Me5)2U(CH2Ph)2, and (C5Me5)2UMe2.

Electron-rich uranium coordination complexes display a pronounced reactivity toward small molecules. In this Feature article, the exciting chemistry of trivalent uranium ions coordinated to classic Werner-type ligand environments is reviewed. Three fundamentally important reactions of the [(((R)ArO)3tacn)U]-system are presented that result in alkane coordination, CO/CO2 activation, and nitrogen atom-transfer chemistry.

The highly reactive, six-coordinate tris-aryloxide U(III) species, [((t-BuArO)3tacn)U] (1) reacts with CO2 in a 2e- reduction to produce CO and a dinuclear U(IV/IV) mu-oxygen bridged complex [{((t-BuArO)3tacn)U}2(mu-O)] (2). This reaction proceeds via a dinuclear CO2-bridged intermediate 3. Also, mononuclear 1 was treated with 1 atm of CO to yield dinuclear [{((t-BuArO)3tacn)U}2(mu-CO)] (4) with a CO ligand bridging two uranium ions in an unprecedented mu:eta1,eta1 fashion. The mixed-valent azido-bridged U(III/IV) complex 5 was synthesized from trivalent 1 and tetravalent [((t-BuArO)3tacn)U(N3)] and serves as an isostructural analogue of triatomic-bridged intermediate 3 as well as an electronic model for mixed-valent 4.

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...

The synthesis, X-ray structure, stability, and photophysical properties of several trivalent lanthanide complexes formed from two differing bis-bidentate ligands incorporating either alkyl or alkyl ether linkages and featuring the 1-hydroxy-2-pyridinone (1,2-HOPO) chelate group in complex with Eu(III), Sm(III) and Gd(III) are reported. The Eu(III) complexes are among some of the best examples, pairing highly efficient emission ( Eu tot Φ ~ 21.5 %,) with high stability (pEu ~ 18.6) in aqueous solution, and are excellent candidates for use in biological assays. A comparison of the observed behavior of the complexes with differing backbone linkages shows remarkable similarities, both in stability and photophysical properties. Low temperature photophysical measurements for a Gd(III) complex were also used to gain insight into the electronic structure, and were found to agree with corresponding TD-DFT calculations for a model complex. A comparison of the high resolution Eu(III) emission spectra in solution and from single crystals also revealed a more symmetric coordination geometry about the metal ion in solution due to dynamic rotation of the observed solid state structure.