Electronic properties of 4,4',5,5'-tetramethyl-2,2'-biphosphinine (tmbp) in the redox series fac-[Mn(Br)(CO)3(tmbp)], [Mn(CO)3(tmbp)]2, and [Mn(CO)3(tmbp)](-): crystallographic,spectroelectrochemical, and DFT computational study

Stepwise electrochemical reduction of the complex fac-[Mn(Br)(CO)(3)(tmbp)] (tmbp = 4,4',5,5'-tetramethyl-2,2'-biphosphinine) produces the dimer [Mn(CO)(3)(tmbp)](2) and the five-coordinate anion [Mn(CO)(3)(tmbp)](-). All three members of the redox series have been characterized by single-crystal X-ray diffraction. The crystallographic data provide valuable insight into the localization of the added electrons on the (carbonyl)manganese and tmbp centers. In particular, the formulation of the two-electron-reduced anion as [Mn(0)(CO)(3)(tmbp(-))](-) also agrees with the analysis of its IR nu(CO) wavenumbers and with the results of density functional theoretical (DFT) MO calculations on this compound. The strongly delocalized pi-bonding in the anion stabilizes its five-coordinate geometry and results in the appearance of several mixed Mn-to-tmbp charge-transfer/IL(tmbp) transitions in the near-UV-vis spectral region. A thorough voltammetric and UV-vis/IR spectroelectrochemical study of the reduction path provided evidence for a direct formation of [Mn(CO)(3)(tmbp)](-) via a two-electron ECE mechanism involving the [Mn(CO)(3)(tmbp)](*) radical transient. At ambient temperature [Mn(CO)(3)(tmbp)](-) reacts rapidly with nonreduced fac-[Mn(Br)(CO)(3)(tmbp)] to produce [Mn(CO)(3)(tmbp)](2). Comparison with the analogous 2,2'-bipyridine complexes has revealed striking similarity in the bonding properties and reactivity, despite the stronger pi-acceptor character of the tmbp ligand.     

Electron transfer between two silyl-substituted phenylene rings: EPR/ENDOR spectra,DFT calculations,and crystal structure of the one-electron reduction compound of a di(m-silylphenylenedisiloxane)

Reduction of a solution of octamethylcyclo-di(m-silylphenylenedisiloxane) 4 in THF on a potassium mirror leads to EPR/ENDOR spectra characterized by a large coupling (approximately 20 MHz) with two protons, similar to the spectra obtained after reduction of the m-disilylbenzene derivative 5, consistent with a localization of the extra electron on a single ring of 4. The spectra recorded after reduction of 4 at low temperature in the presence of an equimolar amount of 18-crown-6 exhibit couplings of approximately 10 MHz with four protons and indicate that embedding the counterion in crown-ether provokes the delocalization of the unpaired electron on the two phenyl rings of 4. The measured hyperfine interactions agree with those calculated by DFT for the optimized structure of 4(.-). Direct information on the structure of this anion is obtained from the X-ray diffraction of crystals grown at -18 degrees C in reduced solutions containing 4, potassium, and crown ether in a THF/hexane mixture. Both DFT and crystal structures clearly indicate the geometry changes caused by the addition of an electron to 4: the interphenyl distance drastically decreases, leading to a partial overlap of the two rings. The structure of 4(.-) is a model for an electron transfer (ET) transition state between the two aromatic rings. The principal reason for the adoption of this structure lies in the bonding interaction between the LUMO (pi orbitals) of these two fragments; moreover, the constraints of the macrocycle probably contribute to the stabilization of this structure.     

Synthesis of a 2-benzocymantrenylpyridine and further mechanistic insights

The synthesis of a new benzocymantrene derivative containing an appended pyridyl moiety was attempted by reacting a lithioindenylpyridine with (CO)₅MnBr. The reaction led to the formation of small amounts of the targeted benzocymantrene, which was subsequently characterized by structural X-ray diffraction analysis. Alternative treatment of the same ligand with (η¹-benzyl)pentacarbonylmanganese led to the formation of a five-membered manganacycle resulting from the C-H bond activation at the five-membered ring of the indenyl group. To rationalize the origin of the poor yield in the benzocymantrene derivative a complete study of the reaction profile was carried out by means of a state-of-the-art DFT-D2 method. The theory indicated that the main difficulty in producing the targeted benzocymantrene stems from the relatively high activation energy barrier necessary to convert a (η¹-indenyl)Mn(CO)₅ intermediate into a (η³-indenyl)Mn(CO)₄ transient species.     

Phosphinines and Diphosphaferrocenes: Recent Advances

Recent developments in the chemistry of phosphinines and phosphaferrocenes are reviewed. Phosphinine-based macrocycles such as silacalix-[4]-phosphinines efficiently stabilize gold(0). 2,2'Biphosphinines which behave as powerful chelates show a greater ability than their nitrogen counterparts for the stabilization of highly reduced and electron-excessive transition metal centers. On the other hand, diphosphaferrocenes coordinate various metal centers via a side-on bonding mode. Palladium(0) complexes of octa-alkyl diphosphaferrocenes behave as efficient catalysts both in the Suzuki cross-coupling reaction and in the synthesis of arylboronic esters.     

Stereoselective Aminoxylation of Biradical Titanium Enolates with TEMPO

A highly efficient and straightforward aminoxylation of titanium(IV) enolates from (S)‐N‐acyl‐4‐benzyl‐5,5‐dimethyl‐1,3‐oxazolidin‐2‐ones with TEMPO has been developed. A wide array of functional groups on the acyl moiety, including alkyl and aryl substituents, olefins, esters, or α‐cyclopropyl, as well as α‐trifluoromethyl groups, are well tolerated. This transformation can therefore produce the α‐aminoxylated adducts in excellent yields with high diastereomeric ratios (d.r.). In turn, parallel additions to the α,β‐unsaturated N‐acyl counterparts give the corresponding γ‐adducts with complete regioselectivity in moderate to good yields. Removal of the piperidinyl moiety or the chiral auxiliary converts the resultant adducts into enantiomerically pure α‐hydroxy carboxyl derivatives, alcohols, or esters in high yields under mild conditions. Finally, a new mechanistic model based on the biradical character of the titanium(IV) enolates has been proposed.