Synthesis and Characterization of a Benzene−Dimolybdenum Complex with a New Bridging Mode

Cyclo-E5-bridged trinuclear triple-decker complexes (E = P, As) containing a triply-bonded Mo2 unit and their isomerisation

The reaction of the low-coordinate quadruply-bonded dimolybdenum complex Mo2[μ,κ2-PhB(N-2,6-iPr2C6H3)2]2 (1) with Cp*Fe(η5-E5) (E = P, As) gives two trinuclear species Cp*Fe(μ3,η5:2:2-E5)Mo2[μ,κ2-PhB(N-2,6-iPr2C6H3)2]2 (E = P (4) and As (5)). 4 undergoes facile isomerisation upon heating to give Cp*FeMo2[κ2-PhB(N-2,6-iPr2C6H3)2](μ3,κ:κ:η2-P2)[μ3,κ:κ:η3κ-P3PhB(N-2,6-iPr2C6H3)2] (6), where the FeMo2P5 core motif displays a cubane-like structure.

Ammonia Activation, H2 Evolution and Nitride Formation from a Molybdenum Complex with a Chemically and Redox Noninnocent Ligand

Treatment of the bis(imino)pyridine molybdenum η⁶-benzene complex (^iPrPDI)Mo(η⁶-C₆H₆) (^iPrPDI, 2,6-(2,6-iPr₂C₆H₃N═CMe)₂C₅H₃N) with NH₃ resulted in coordination induced haptotropic rearrangement of the arene to form (^iPrPDI)Mo(NH₃)₂(η²-C₆H₆). Analogous η²-ethylene and η²-cyclohexene complexes were also synthesized, and the latter was crystallographically characterized. All three compounds undergo loss of the η²-coordinated ligand followed by N-H bond activation, bis(imino)pyridine modification, and H₂ loss. A dual ammonia activation approach has been discovered whereby reversible M-L cooperativity and coordination induced bond weakening likely contribute to dihydrogen formation. Significantly, the weakened N-H bonds in (^iPrPDI)Mo(NH₃)₂(η²-C₂H₄) enabled hydrogen atom abstraction and synthesis of a terminal nitride from coordinated ammonia, a key step in NH₃ oxidation.

Distortions of π-coordinated arenes with anionic character

A qualitative analysis of the distortions that operate on the π system of bridging arenes with anionic character is presented and substantiated by computational studies at the density functional B3LYP and CASSCF levels. The observed effects of bonding to two metal atoms and of the negative charge are an expansion of the arene ring due to the partial occupation of π* orbitals, an elongation or compression distortion accompanied by a loss of the equivalence of carbon-carbon bonds due to a Jahn-Teller distortion of the arene dianions, and a ring puckering due to a second-order Jahn-Teller distortion that may appear independently of the existence of the first-order effect. The workings of the orbital mixing produced by these distortions have been revealed in a straightforward way by a pseudosymmetry analysis of the HOMOs of the distorted conformations. The systems studied include Li(I) and Y(III) adducts of benzene, as well as trimethylsilyl-substituted derivatives in the former case. An analysis of the structural data of a variety of purported di- and tetraanionic arene ligands coordinated to transition metals in several bridging modes has reproduced the main geometrical trends found in the computational study for the benzene and trimethylsilyl-substituted benzene dianions, allowing a classification of the variety of structural motifs found in the literature.