No abstract

Reported N2 complexes of cobalt do not have substantial weakening of the N−N bond. Using diketiminate ligands to enforce three-coordinate geometries, we have synthesized several novel CoNNCo complexes. In formally univalent complexes, cobalt is poorer than iron at weakening the N−N bond, but in formally zerovalent complexes, cobalt and iron give similar N−N weakening. The weakening is due to cobalt-to-N2 π-backbonding, and potassium cations pull more electron density into N2. These results show that the low coordination number of a trigonal-planar geometry is impetus enough to make even the electronegative cobalt weaken the N−N bond of N2.

Monomeric copper(I) alkyl complexes that possess the N-heterocyclic carbene (NHC) ligands IPr, SIPr, and IMes [IPr ) 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, SIPr ) 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, IMes ) 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] react with amines or alcohols to release alkane and form the corresponding monomeric copper(I) amido, alkoxide, or aryloxide complexes. Thermal decomposition reactions of (NHC)Cu I methyl complexes at temperatures between 100 and 130°C produce methane, ethane, and ethylene. The reactions of (NHC)Cu(NHPh) complexes with bromoethane reveal increasing nucleophilic reactivity at the anilido ligand in the order (SIPr)Cu(NHPh) < (IPr)Cu(NHPh) < (IMes)Cu(NHPh) < (dtbpe)Cu(NHPh) [dtbpe ) 1,2-bis(di-tert-butylphosphino)ethane]. DFT calculations suggest that the HOMO for the series of Cu anilido complexes is localized primarily on the amido nitrogen with some pπ anilido −dπ Cu π*-character. [(IPr)Cu(µ-H)] 2 and (IPr)Cu(Ph) react with aniline to quantitatively produce (IPr)Cu(NHPh)/dihydrogen and (IPr)Cu(NHPh)/benzene, respectively. Analysis of the DFT calculations reveals that the conversion of [(IPr)Cu(µ-H)] 2 and aniline to (IPr)Cu(NHPh) and dihydrogen is favorable with ∆H ≈ −7 kcal/mol and ∆G ≈ −9 kcal/mol.

Attempted syntheses of (smif)2Ti (smif = 1,3-di-(2-pyridyl)-2-azaallyl) based on metatheses of TiClnLm (n = 2–4) with M(smif) (M = Li, Na), in the presence of a reducing agent (Na/Hg) when necessary, failed, but several apparent Ti(II) species were identified by X-ray crystallography and multidimensional NMR spectroscopy: (smif){Li(smif-smif)}Ti (1, X-ray), [(smif)Ti]2(μ-κ3,κ3-N,N(py)2-smif,smif) (2), (smif)Ti(κ3-N,N(py)2-smif,(smif)H) (3), and (smif)Ti(dpma) (4). NMR spectroscopy and K-edge XAS showed that each compound possesses ligands that are redox non-innnocent, such that d1 Ti(III) centers AF-couple to ligand radicals: (smif){Li(smif-smif)2−}TiIII (1), [(smif2−)TiIII]2(μ-κ3,κ3-N,N(py)2-smif,smif) (2), [(smif2−)TiIII](κ3-N,N(py)2-smif,(smif)H) (3), and (smif2−)TiIII(dpma) (4). The instability of the (smif)2Ti relative to its C-C coupled dimer, 2, is rationalized via the complementary nature of the amide and smif radical anion ligands, which are also common to 3 and 4. Calculations support this contention.

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Methane functionalization via L(n)M=E active species (L(n) = beta-diketiminate, dihydrophosphinoethane; M = Fe-Ni, E = NCF(3), NCH(3), O) through a hydrogen atom abstraction (HAA)/radical rebound (RR) mechanism is calculated to be thermodynamically and kinetically feasible. The enthalpies of each reaction decrease in the order Fe > Co > Ni and with the proximity of CF(3) supporting ligand substituents ("fluorination") to the metal center. For HAA, lower abstraction enthalpies were calculated for L(n) = beta-diketiminate and E = NCF(3) rather than dhpe and NCH(3), respectively, whereas the opposite trend was found for RR enthalpies. The overall functionalization thermodynamics were optimal for L(n) = beta-diketiminate and E = NCH(3), with similar enthalpies for E = O when M = Ni. The HAA kinetics further implicate fluorinated (beta-diket)Ni=O as the most promising methane functionalization complexes, with calculated activation barriers as low as 8.1 kcal mol(-1).

Recently, Nishibayashi et al. reported a dimolybdenum-dinitrogen complex that is catalytic for complete reduction of dinitrogen to ammonia. This catalyst is different from the Schrock molybdenum catalyst in two fundamental aspects: it contains two metal centers, and the oxidation state is Mo(0) instead of Mo(III). We show that a remarkable feature of the bimetallic complex is the bond-mediated delocalized electronic states, resulting from the two metal centers bridged by a dinitrogen ligand. Using first-principles calculations, we found that this property makes the bimetallic complex the effective catalyst, as opposed to the originally postulated monometallic fragment. A favorable reaction pathway is identified, and the nature of the intermediates is examined. Furthermore, studies of the intermediate states led us to propose possible deactivation processes of the catalyst. The finding that the central bimetallic unit (Mo-N2-Mo) is relevant for catalytic activity may provide a guideline for the development of more efficient dinitrogen-reducing catalysts.

Synthesis and characterization of the monomeric complex ( t bpy)Pt(Me)(NHPh) ( t bpy = 4,4′-di-tert-butyl-2,2′-dipyridyl) has been accomplished. Mechanistic studies reveal that 1,2-addition of dihydrogen across the Pt−anilido bond to initially produce ( t bpy)Pt(Me)(H) and free aniline is catalyzed by elemental Pt rather than through a pathway that involves direct activation of H2 by Pt and 1,2-addition across the Pt−NHPh bond.