Polymer networks cross-linked by reversible metal-ligand interactions possess versatile mechanical properties achieved simply by varying the metal species and quantity. Although prior experiments have revealed the dependence of the network’s...
In this work nine lithium N, N′-bis(trimethylsilyl)amidinates-TMEDA complexes and the dimeric adduct of 2-ethylbenzonitrile lithium bis(trimethylsilyl)amide were synthesized and characterized. The later compound was found to be a kinetically stabilized intermediate in the amidinate formation. The solid state structures of the amidinates revealed several types of crystalline supramolecular structures, generated by the degree and nature of involvement of the amidinate π system in the intermolecular interactions. The π system was also found to play a significant role in the intramolecular bonding with the lithium atom. The π bonding is obtained at the expense of the σ bond, weakening it, while keeping the metal-ligand bond length almost invariable. This facile π interaction along with the Li-N σ-bond activation can imply that similar involvements of the π systems may take place in the salt metathesis reactions with these compounds.
Bis(amidinate) titanium and zirconium bis-(dimethylamido) complexes were prepared, and the dynamic behavior of the titanium complex containing perfluorinated amidinate ligand (11) was studied in detail. The variabletemperature NMR revealed the presence of two species in solution, in line with the different connection modes of the ligand to the metal center. The resulting complexes were tested as catalysts in the polymerization of propylene, and the resulting polymers were consistent with elastomeric high-molecularweight atactic polypropylenes.
Anion-exchange
membrane fuel cells (AEMFCs) have attracted the
attention of the scientific community during the past years, mostly
because of the potential for eliminating the need for using costly
platinum catalysts in the cells. However, the broad commercialization
of AEMFCs is hampered by the low chemical stability of the cationic
functional groups in the anion-conducting membranes required for the
transportation of hydroxide ions in the cell. Improving the stability
of these groups is directly connected with the ability to recognize
the different mechanisms of the OH– attack. In this
work, we have synthesized eight different carbazolium cationic model
molecules and investigated their alkaline stability as a function
of their electronic substituent properties. Given that N,N-diaryl carbazolium salts decompose through a
single-electron-transfer mechanism, the change in carbazolium electron
density leads to a very significant impact on their chemical stability.
Substituents with very negative Hammett parameters demonstrate unparalleled
stability toward dry hydroxide. This study provides guidelines for
a different approach to develop stable quaternary ammonium salts for
AEMFCs, making use of the unique parameters of this decomposition
mechanism.
Lithium N,N'-bis(trimethylsilyl)heterocyclic amidinate complexes with 3- and 4-pyridyl and 3-furyl carbon substituents were prepared by addition of the corresponding nitriles to LiN(SiMe(3))(2) (LiNTMS(2)) solution. In the presence of N,N,N',N' tetramethylethylene diamine (TMEDA), both pyridyl amidinates crystallize as coordination polymers with an amidinate-Li-pyridyl backbone. The 4-pyridyl derivative (7) creates a linear polymer with amidinate-Li-TMEDA units as side chains, whereas the 3-pyridyl polymer (6) has a two-dimensional (2D) network structure in which TMEDA serves as a cross-linker. Solvation of the reaction mixture of 3-furonitrile and LiNTMS(2) with TMEDA affords the monomeric 3-furyl amidinate Li TMEDA complex (3). Crystals of the Li(2)O complex {[3-furyl-C-(NTMS)(2)Li](4).Li(2)O}.C(7)H(8) (4) are obtained from toluene by partial hydrolysis of the unsolvated 3-furyl amidinate (2). Degradation of the polymer (7) to monomeric units can be achieved by solvation in toluene or by reaction with TMS(2)NLi.TMEDA that affords crystals of the complex {NTMS(2)Li.[4-C(5)H(4)N-C(NTMS)(2)Li.TMEDA]}(2).(NTMS(2)Li.TMEDA) (8). The formation of these aggregates can be rationalized by directed substitution of TMEDA with pyridyl moieties and by the laddering principle.
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