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Two trinuclear complexes of rhodium and iridium have been obtained by reaction of the tris-N-heterocyclic carbene ligand TIMEN^iPr and [(COD)MCl]2 or [(COD)2M](BF4) (M = Rh and Ir). The new complexes have been fully characterized by means of NMR spectroscopy and single-crystal X-ray diffraction studies. The trinuclear rhodium complex shows efficient activity in cyclization of acetylenic carboxylic acids, ranking among the highest known for this type of reaction.

Detailed cyclic voltammetric and UV−visible−near-infrared electronic absorption spectral data have been obtained for a series of pentamethylcyclopentadienyl complexes of uranium(IV) and thorium(IV) of the general formula (C5Me5)2An(L1)(L2), where L1, L2 = Cl, SO3CF3, CH3, CH2Ph, imido (&dbd;N-2,4,6-^tBu3C6H2), hydrazonato (η²(N,N‘)-RNN&dbd;CPh2; R = CH3, CH2Ph, Ph), ketimido (−N&dbd;C(Ph)(R); R = CH3, CH2Ph, Ph) ligands, and for the hexavalent uranium bis(imido) complex (C5Me5)2U(&dbd;NPh)2. The electrochemical and spectroscopic behavior of the tetravalent uranium complexes falls cleanly into distinct categories based on the nature of L1 and L2. If both ligands are simple σ-donors (Cl, SO3CF3, CH3, CH2Ph), a reversible U(IV)/U(III) voltammetric wave is the only metal-based process observed, and it occurs between ~−1.8 and −2.6 V vs [(C5H5)2Fe]^+/0. If either L1 or L2 is a nitrogen-donor ligand (imido, hydrazonato, ketimido), then both a U(IV)/U(III) reduction wave and a U(V)/U(IV) oxidation wave are observed. The reduction step occurs in the same potential region as for the σ-donor complexes, and the oxidation wave occurs in the range from ~+0.2 to −0.7 V vs [(C5H5)2Fe]^+/0. This oxidation wave is reversible, indicating that the unusual pentavalent uranium oxidation state is kinetically stable on a voltammetric time scale, and the potential of the oxidation step indicates that the pentavalent state is thermodynamically stabilized from interaction with the nitrogen-donor ligand(s). The separation between reduction and oxidation processes in these nitrogen-donor complexes remains nearly constant over the series of eight complexes, with an average value of 2.09 V. Additional ligand-based redox processes are also observed and assigned on the basis of the existence of corresponding voltammetric waves in the Th(IV) complexes and other cyclopentadiene complexes. The electronic absorption spectra for all U(IV) complexes are comprised of two distinct regions:  a lower energy region (E < 15 000 cm^-1) containing the narrow f−f transitions arising from within the 5f orbital manifold and a higher energy region (E > 15 000 cm^-1) containing broad, unstructured, or poorly structured bands derived from both metal-localized 5f−6d transitions and molecular-based transitions including ligand-localized and metal-to-ligand charge-transfer transitions. A definite trend in intensities of these transitions is observed, depending on the nature of L1 and L2. If L1 and L2 are both simple σ-donor ligands, the f−f transition intensities are relatively weak (molar absorptivity ε ≈ 10−80 M^-1 cm^-1), consistent with observations for most classical coordination complexes of 5f² electronic configuration, and the broad, higher energy bands have ε values in the 3000−5000 M^-1 cm^-1 range. If L1 and/or L2 is a hydrazonato ligand, the f−f transition intensities increase to ~30−120 M^-1 cm^-1 and the broad, higher energy bands develop significantly greater intensities (ε ≈ 15 000−20 000 M^-1 cm^-1). Finally, for the imido and ketimido complexes of U(IV), the f−f transition intensities increase to ~50−400 M^-1 cm^-1, and the broad, higher energy bands continue to carry substantial intensity (ε ≈ 10 000−15 000 M^-1 cm^-1) while also extending to lower energy. This interesting trend in f−f transition intensity is interpreted in the context of an intensity-stealing mechanism from the charge-transfer excited states that reflects an enhanced degree of covalent interaction between the actinide metal center and L1/L2 for the nitrogen-donor ligand systems.

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&dbd;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.

The silver complex of the tripodal N-heterocyclic carbene ligand TIME^Me, [(TIME^Me)2Ag3](PF6)3 (3), reacts with copper(I) bromide and (dimethyl sulfide)gold(I) chloride to yield the corresponding D3-symmetrical copper(I) and gold(I) complexes [(TIME^Me)2Cu3](PF6)3 (4) and [(TIME^Me)2Au3](PF6)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 D3 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 D2d-symmetrical species Pd(CN2Bu^t2C2H2)2 (6) and the simplified D2h-symmetrical model complexes M(IM^MeC:)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 new tris-carbene ligand [TIME^t-Bu] has been synthesized and fully characterized. Reaction of the free carbene with a copper(I) salt provided an unprecedented dinuclear Cu(I)−Cu(I) complex, in which the cuprous ion is coordinated in a trigonal planar ligand environment of three different carbon ligators. Interestingly, one of the three chelating carbon atoms can be formed only via C−H activation of the unsaturated imidazole backbone of the carbene chelator. Accordingly, the title complex is described as a bis-carbenealkenyl copper(I) complex.