Keywords agostic interactions; carbene ligands; low-coordinate complexes; palladium; rhodiumIn the last decade the use of diamino N-heterocyclic carbenes (NHCs) A as ancillary ligands for transition-metal catalysts and as organic catalysts on their own has proven very fruitful.[1] The efficiency of NHCs is attributed to their strong σ-donor properties and sterically demanding structure.[1,2] Not only have NHCs yielded improved transition metal catalysts, but they have also lead to the isolation of unusual low-coordinate metal complexes,[2] which often play a key role in catalytic processes. Recently, we reported the synthesis of stable cyclic (alkyl)(amino)carbenes (CAACs) B[3] and demonstrated that CAACs can be even stronger σ donors than NHCs, and lead to highly efficient palladium catalysts for the α-arylation of ketones and aldehydes. Here we report that the peculiar steric and electronic properties of rigid CAAC ligands allow the preparation of low-coordinate metal complexes, hitherto not isolable with any other ligands.To further evaluate the electronic properties of rigid CAAC B1, we attempted to prepare the [RhCl(CO) 2 (B1)] complex, since the CO stretching frequencies of this type of rhodium complexes are recognized as an excellent measure of the σ-donor ability of the ligand L, and a large amount of data is available for comparison.[4] Reaction of carbene B1 with half an equivalent of [{RhCl(cod) 2 } 2 ] (cod = 1,5-cyclooctadiene) afforded complex 1, which after purification by column chromatography on silica gel was obtained as orange crystals in 79% yield (Scheme 1). The structure of 1 was unambiguously determined by single-crystal X-ray diffraction.[5] Following the classical procedure, a solution of 1 in chloroform was treated with an excess of CO. Surprisingly, the 13 C NMR spectrum of the product (95% yield) showed only one CO signal, with a large coupling constant (J Rh,C = 134 Hz), and not two signals with smaller J values (J Rh,C = 33-82 Hz), as observed for [RhCl(CO) 2 (L)] complexes. [4] It is known that when bulky ligands L are present [RhCl(CO) 2 L] complexes can readily lose one CO ligand ** We are grateful to the NIH (R01 GM 68825) and RHODIA for financial support of this work.

The most comprehensive approach to analyze and characterize energetic materials is suggested and applied to enable rational, rigorous design of novel materials and targeted improvements of existing materials to achieve desired properties. We report synthesis, characterization of the structure and sensitivity, and modeling of thermal and electronic stability of the energetic, heterocyclic compound, 3,4-bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole-2-oxide (BNFF). The proposed novel, relatively simple synthesis of BNFF in excellent yields allows for an efficient scale up. Performing careful characterization indicates that these materials offer an unusual combination of properties and exhibit a relatively high energy density, high and controllable stability against decomposition, low melting temperature, and low sensitivity to initiation of detonation. First-principles calculations of activation barriers and reaction rate constants reveal the decomposition scenarios that govern the thermal stability and chemical behavior of BNFF, which appreciably differ from conventional nitro compounds. Details of the electronic structure and calculated electronic properties suggest that BNFF is an excellent candidate energetic material on its own and an attractive ingredient of modern energetic formulations to improve their stability and enable highly controllable chemical decomposition.

Small ring allenes are usually highly strained and highly reactive species, and for a long time considered only as transient intermediates. The recent isolation of a five membered heterocyclic allene 1f has raised questions and debate regarding the factors responsible for its stability. Since 1f has been derived by deprotonation of a pyrazolium ion 2f, it has been suggested that the stability of 1f comes from its aromatic character. Here we report computational evidence, including HOMA and NICS aromaticity indices, that allenes derived from 3,5-bis(pi-donor) substituted pyrazolium salts are weakly aromatic to nonaromatic, and that even their pyrazolium ion precursors have dramatically reduced aromaticity. Exocyclic delocalization, involving the pi-donor substituents, occurs at the expense of aromaticity and increases with the strength of the donor. Experimental support for these conclusions is found in the crystallographically determined structure of 3,5-bis(dimethlamino)pyrazolium ion 2g, which exhibits highly pyramidalized endocyclic nitrogen centers but planarized exocyclic ones, and from the facile C4 protonation to give a stable pyrazole-1,2-diium salt 3g, which has also been crystallographically characterized.