Improvement of the Thermal and Optical Performances of Protective Polydimethylsiloxane Space Coatings with Cellulose Nanocrystal Additives.

This work investigates the possibility of using cellulose nanocrystals (CNCs) as biobased nanoadditives in protective polydimethylsiloxane (PDMS) space coatings, to improve the thermal and optical performances of the material. CNCs produced from wood pulp were functionalized in different conditions with the objective to improve their dispersibility in the PDMS matrix, increase their thermal stability and provide photoactive functions. Polysiloxane, cinnamate, chloroacetate and trifluoroacetate moieties were accordingly anchored at the CNCs surface by silylation, using two different approaches, or acylation with different functional vinyl esters. The modified CNCs were thoroughly characterized by FT-IR spectroscopy, solid-state NMR spectroscopy and thermogravimetric analysis, before being incorporated into a PDMS space coating formulation in low concentration (0.5 to 4 wt %). The cross-linked PDMS films were subsequently investigated with regards to their mechanical behavior, thermal stability and optical properties after photoaging. Results revealed that the CNC additives could significantly improve the thermal stability of the PDMS coating, up to 140 °C, depending on the treatment and CNC concentration, without affecting the mechanical properties and transparency of the material. In addition, the PDMS films loaded with as low as 1 wt % halogenated nanoparticles, exhibited an improved UV-stability after irradiation in geostationary conditions.

Structural and electrochemical investigations of the high fluoride affinity of sterically hindered 1,8-bis(boryl)naphthalenes.

The reaction of 10-bromo-9-oxa-10-boraanthracene with the tetrakis(tetrahydrofuran)lithium salt of dimesityl-1,8-naphthalenediylborate in diethyl ether affords 1-(dimesitylboryl)-8-(10'-bora-9'-oxaanthryl)naphthalene (2). This diborane reacts with [Me3SiF2][S(NMe2)3)] to afford the anionic complex [2-mu2-F]-, which has been isolated as a [S(NMe2)3]+ salt. The cyclic voltammograms of diborane 2 as well as 1-(dimesitylboryl)-8-(10'-bora-9'-thiaanthryl)naphthalene (1) exhibit two reversible reductions at E(1/2) = -2.200 and -2.566 V (vs FcH/FcH+) for 1 and E(1/2) = -2.248 and -2.620 V (vs FcH/FcH+) for 2 corresponding to the sequential reduction of the two boron centers. These two waves simultaneously disappear upon fluoride addition, thus indicating the formation of fluoride chelate complexes [1-mu2-F]- and [2-mu2-F]-. To identify the origin of the high fluoride affinity displayed by these diboranes, the structures of 2 and [2-mu2-F]- have been studied experimentally and computationally. The crystallographic studies show that the structure of 2 is distorted, thus indicating the presence of important steric repulsions between the neighboring boryl moieties. By contrast, the structure of the anionic complex [2-mu2-F]- is much more sterically relaxed than that of 2, as indicated by a reduction of the B-B distance from 3.279(4) A in 2 to 2.922(7) A in [2-mu2-F]-. The structural results suggest that the high fluoride affinity displayed by 2 results, at least in part, from the relief of steric repulsions induced by fluoride binding. Finally, the nature of the bonding as well as the strength of the interactions involved in the B-F-B bridge of [2-mu2-F]- has been studied using density functional theory calculations and Atoms-In-Molecules analyses. These calculations indicate that the enthalpic gain associated with the formation of two B-F bonds in [2-mu2-F]- only amounts to a fraction of the energy of a terminal B-F bond. These calculations also suggest that the relief of steric repulsions induced by fluoride binding in 2 may contribute to the high fluoride affinity of these types of molecules.

Reactivity of the dimesityl-1,8-naphthalenediylborate anion: isolation of the borataalkene isomer and synthesis of 1,8-diborylnaphthalenes.

The anionic boron peri-bridged naphthalene derivative, namely dimesityl-1,8-naphthalenediylborate (1), undergoes a hydrolysis reaction to afford dimesityl-1-naphthylborane (2) whose structure has been determined. Upon standing at room temperature in toluene for an extended period of time, 1 undergoes a ring expansion reaction to afford 8,10,11a-trimethyl-7-mesityl-11aH-7-boratabenzo[de]anthracene (3). As shown by its crystal structure, compound 3 constitutes a rare example of a borataalkene and features a carbon-boron double bond of 1.475(6) Angstroms incorporated in a conjugated hexa-1-boratatriene system. The reaction of 1 with 9-chloro-9-borafluorene and 5-bromo-10,11-dihydrodibenzo[b,f]borepin results in the formation of diboranes 4 and 5 which bear two different boryl moieties at the peri-positions of naphthalene. These diboranes have been characterized by multinuclear NMR spectroscopy and X-ray single crystal analysis. The boron center of the borafluorenyl moiety is pi-coordinated to the ipso-carbon of a mesityl group with which it forms a contact of 2.730(3) Angstroms. The cyclic voltammogram of 2 in THF shows a quasi-reversible reduction wave at E(1/2)-2.41 V (vs. Fc/Fc+) corresponding to the formation of the radical anion. In the case of diboranes 4, 5 and 1-(dimesitylboryl)-8-(diphenylboryl)naphthalene (6), two distinct waves are observed at E(1/2)-2.14 and -2.56 V for 4, E(1/2)-2.26 and -2.78 V for 5, and E(1/2)-2.41 and -2.84 V for 6. The first reduction wave most likely indicates the formation of a radical anion in which the unpaired electron is sigma-delocalized over the two boron centers.

Transient azomethine-ylides from a stable amino-carbene and an aldiminium salt.

Catalytic amounts of a protic reagent such as tert-butyl alcohol promote the isomerization of a stable amino-aryl-carbene into a transient azomethine ylide. Deprotonation of an alkyl-aldiminium salt also leads to a transient azomethine ylide, but labeling experiments rule out the transient formation of the corresponding amino-alkyl-carbene. The potential hypersurface between model amino-carbene, aziridine, and azomethine ylide is investigated.

(Phosphino)(aryl)carbenes: effect of aryl substituents on their stabilization mode.

A broad range of (phosphino)(aryl)carbenes, 1b-d, 10a,b, and 14a,b, were prepared by photolysis of their diazo precursors. The influence of the steric and electronic properties of the aryl ring on the structure and stability of these carbenes was studied both experimentally and theoretically. Among the different stabilization modes investigated, those featuring an acceptor as well as a spectator aryl substituent result in stable or at least persistent carbenes that could be completely characterized by classical spectroscopic methods. In marked contrast, the new substitution pattern featuring a donor aryl ring results in a very fleeting carbene.