Conjugation between 3D and 2D aromaticity: does it really exist? The case of carborane-fused heterocycles.

Although several synthesized icosahedral carborane fused 2D π-ring systems are known, and even considerable conjugation has been noted between them in some cases, the phenomenon itself is not fully understood. Based on the results of our computational study, it can be concluded that the 2D aromatic character of the fused (exo) five-membered ring is low, even in cases where significant conjugation was proposed in previous studies. Moreover, the carborane moiety constricts the bonding properties of the exo ring, thus prohibiting or promoting different Lewis resonance structures. These results will shed further light on the design and electronic modulation of new carborane-based materials.

A new access to diazaphospholes via cycloaddition-cycloreversion reactions on triazaphospholes.

A novel bis-CF3-substituted diazaphosphole was synthesized selectively from hexafluoro-2-butyne and a 3H-1,2,3,4-triazaphosphole derivative. The [4+2] cycloaddition and subsequent cycloreversion reaction under elimination of pivaloyl nitrile affords the product in high yield. The heterocycle coordinates via the phosphorus atom to a W(CO)5-fragment and shows stronger π-accepting properties than the triazaphosphole.

Screening of transition metal doped copper clusters for CO2 activation.

Activation of CO2 is the first step towards its reduction to more useful chemicals. Here we systematically investigate the CO2 activation mechanism on Cu3X (X is a first-row transition metal atom) using density functional theory computations. The CO2 adsorption energies and the activation mechanisms depend strongly on the selected dopant. The dopant electronegativity, the HOMO-LUMO gap and the overlap of the frontier molecular orbitals control the CO2 dissociation efficiency. Our calculations reveal that early transition metal-doped (Sc, Ti, V) clusters exhibit a high CO2 adsorption energy, a low activation barrier for its dissociation, and a facile regeneration of the clusters. Thus, early transition metal-doped copper clusters, particularly Cu3Sc, may be efficient catalysts for the carbon capture and utilization process.

Observation of the Reaction Intermediates of Methanol Dehydrogenation by Cationic Vanadium Clusters.

A mass spectrometric study of the reactions of vanadium cationic clusters with methanol in a low-pressure collision cell is reported. For comparison, the reaction of methanol with cobalt cationic clusters was studied. For vanadium, the main reaction products are fully dehydrogenated species, and partial dehydrogenation and non-dehydrogenation species are observed as minors, for which the relative intensities increase with cluster size and also at low cluster source temperature cooled by liquid nitrogen; no dehydrogenation products were observed for cobalt clusters. Quantum chemical calculations explored the reaction pathways and revealed that the fully dehydrogenation products of the reaction between Vn + and methanol are Vn (C)(O)+ , in which C and O are separated owing to the high oxophilicity of vanadium. The partial dehydrogenation and non-dehydrogenation species were verified to be reaction intermediates along the reaction pathway, and their most probable structures were proposed.

Bulky 1,1'-bisphosphanoferrocenes and their coordination behaviour towards Cu(i).

Two bulky mesityl substituted dppf-analogues Fe(C5H4PMes2)2 (Mes = 2,4,6-Me3C6H2, 1) and Fe(C5H4PMes2)(C5H4PPh2) (Mes = 2,4,6-Me3C6H2, Ph = C6H5, 3) have been prepared and their properties as donor ligands have been explored using heteronuclear NMR spectroscopy and in particular via1JP-Se coupling, cyclic voltammetry and DFT calculations. Based on the results obtained, a series of mono- and dinuclear Cu(i) complexes have been prepared with these new diphosphane ligands using Br-, I-, and BF4- as counter anions. For the very bulky ligand 1 rare and unprecedented double bridging complexation modes have been observed containing two non-planar Cu2Br2 units, while for the other dinuclear complexes planar Cu2Br2 units have been found. The Cu(i) complexes of 1 and 3 were then used as catalysts for CO2-fixation reaction with terminal alkynes, and complexes with ligand 3 were found to be more efficient than those with 1. DFT calculations performed on compounds 1, 3 and their Cu(i) complexes were able to verify the trend of these catalytic reactions.

Stretching the P-C Bond. Variations on Carbenes and Phosphanes.

The stability and the structure of adducts formed between four substituted phosphanes (PX3, X:H, F, Cl, and NMe2) and 11 different carbenes have been investigated by DFT calculations. In most cases, the structure of the adducts depends strongly on the stability of the carbene itself, exhibiting a linear correlation with the increasing dissociation energy of the adduct. Carbenes of low stability form phosphorus ylides (F), which can be described as phosphane → carbene adducts supported with some back-bonding. The most stable carbenes, which have high energy lone pair, do not form stable F-type structures but carbene → phosphane adducts (E-type structure), utilizing the low-lying lowest unoccupied molecular orbital (LUMO) of the phosphane (with electronegative substituents), benefiting also from the carbene-pnictogen interaction. Especially noteworthy is the case of PCl3, which has an extremely low energy LUMO in its T-shaped form. Although this PCl3 structure is a transition state of rather high energy, the large stabilization energy of the complex makes this carbene-phosphane adduct stable. Most interestingly, in case of carbenes with medium stability both F- and E-type structures could be optimized, giving rise to bond-stretch isomerism. Likewise, for phosphorus ylides (F), the stability of the adducts G formed from carbenes with hypovalent phosphorus (PX-phosphinidene) is in a linear relationship with the stabilization of the carbene. Adducts of carbenes with hypervalent phosphorus (PX5) are the most stable when X is electronegative, and the carbene is highly nucleophilic.

Four consecutive reactions in one pot: cascade formation of an unprecedented triphosphatricyclo[3.2.1.02,7]oct-3-ene.

A hitherto unknown triphosphatricyclo[3.2.1.02,7]oct-3-ene was obtained in high yield by a total of four consecutive pericyclic reactions from di-2-pyrone and excess TMS-C[triple bond, length as m-dash]P. DFT calculations gave insight into the reaction mechanism. The chiral phosphorus cage readily forms coordination compounds with Au(i) and Cu(i).

Controllable access to P-functional [3]ferrocenophane and [4]ferrocenophane frameworks.

Condensation of secondary 1,1'-diaminoferrocenes with phosphorus trihalides (PCl3 or PBr3) yielded either P-halo-1,3-diaza-2-phospha[3]ferrocenophanes or 1,1'-diaminoferrocenediyl-bis(dichlorophosphines), respectively. The latter provide controlled access to both [3]- and [4]ferrocene frameworks. Thus, reductive coupling with magnesium gave diaza-diphospha[4]ferrocenophane-annelated tetraphosphetanes, while a reaction with LiNMe2 produced a P-chloro-diazaphospha[3]ferrocenophane. Condensation of diaminoferrocenes and aminodichlorophosphines were mostly unselective, but afforded in one case a 3-amino[3]ferrocenophane. All reaction products were characterised by spectroscopic and single-crystal XRD studies. DFT studies indicate that the product selectivity in the reactions studied depends on a combination of kinetic and thermodynamic effects, which correlate subtly with the steric bulk of the N-substituents. Cyclic voltammetry measurements revealed that the ferrocenophanes can undergo multiple oxidation events, the first of which may according to DFT studies be located in both the ferrocene and aminophosphine units. The quantum chemical studies provided also insight into stereochemical aspects like ring strain in the ferrocenophane units.