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Ru K-edge XAS data indicate that second generation ruthenium-based olefin metathesis precatalysts (L = N-heterocyclic carbene) possess a more electron-deficient metal center than in the corresponding first generation species (L = tricyclohexylphosphine). This surprising effect is also observed from DFT calculations and provides a simple rationale for the slow phosphine dissociation kinetics previously noted for second-generation metathesis precatalysts.

The reaction of MesP=CPh2 with the least sterically demanding N-heterocyclic carbene (NHC = IMe) results in formation of the ‘abnormal’ (C4-substituted) 4-phosphino-NHC (1). In contrast, reaction with Me2IMe gives the unprecedented ‘normal’ C2 adduct, Me2IMe → P(Mes)=CPh2 (2). Particularly striking is the asymmetric and weak bonding of the NHC to the P=C moiety in 2. DFT calculations indicate that the P=C natural bond order in 2 (1.54) still reflects significant π-character to the bond (cf. MesP=CPh2: NBO = 1.98). Further computational analysis suggests that π-delocalization into the remote C-phenyl substituents is key to stabilizing the NHC adduct. [ABSTRACT FROM AUTHOR]

Rh-containing metallacycles, [(TPA)RhIII(κ2-( C, N)-CH2CH2(NR)2-]Cl; TPA= N, N, N,N-tris(2-pyridylmethyl)amine have been accessed through treatment of the RhI ethylene complex, [(TPA)Rh(η2 -CH2CH2)]Cl ([ 1]Cl) with substituted diazenes. We show this methodology to be tolerant of electron-deficient azo compounds including azo diesters (RCO2NNCO2R; R=Et [ 3]Cl, R= iPr [ 4]Cl, R= tBu [ 5]Cl, and R=Bn [ 6]Cl) and a cyclic azo diamide: 4-phenyl-1,2,4-triazole-3,5-dione (PTAD), [ 7]Cl. The latter complex features two ortho-fused ring systems and constitutes the first 3-rhoda-1,2-diazabicyclo[3.3.0]octane. Preliminary evidence suggests that these complexes result from N-N coordination followed by insertion of ethylene into a [Rh]N bond. In terms of reactivity, [ 3]Cl and [ 4]Cl successfully undergo ring-opening using p-toluenesulfonic acid, affording the Rh chlorides, [(TPA)RhIII(Cl)(κ1-( C)-CH2CH2(NCO2R)(NHCO2R)]OTs; [ 13]OTs and [ 14]OTs. Deprotection of [ 5]Cl using trifluoroacetic acid was also found to give an ethyl substituted, end-on coordinated diazene [(TPA)RhIII(κ2-( C,N)-CH2CH2(NH)2-]+ [ 16]Cl, a hitherto unreported motif. Treatment of [ 16]Cl with acetyl chloride resulted in the bisacetylated adduct [(TPA)RhIII(κ2-( C,N)-CH2CH2(NAc)2-]+, [ 17]Cl. Treatment of [ 1]Cl with AcNNAc did not give the RhN insertion product, but instead the N,O-chelated complex [(TPA)RhI (κ2-( O,N)-CH3(CO)(NH)(NC(CH3)(OCHCH2))]Cl [ 23]Cl, presumably through insertion of ethylene into a [Rh]O bond. [ABSTRACT FROM AUTHOR]

Herein, we report experimental, spectroscopic, and computational data that indicate that a rhodium ethylene complex, formally described as rhodium(I) and which forms a 2-rhoda(III) oxetane following reaction with H2O2, is more accurately described as a rhodium(III) metallacyclopropane. X-ray absorption spectroscopy clearly demonstrates a change in the oxidation state at rhodium following ligand coordination with tris(2-pyridylmethyl)amine. Both NMR and density functional theory studies suggest a high energy barrier to rotation of the coordinated ethylene, which is attributed to large geometric and electronic reorganization resulting from the loss of p-back-bonding. These results imply that the role of H2O2 in the formation of 2-rhoda(III) oxetanes is to oxidize the C2H4 fragment rather than the metal center, as has been previously suggested. [ABSTRACT FROM AUTHOR]