Sterically variable dizinc complexes bearing bis(iminopyridyl)phenolate ligands: synthesis, structures and reactivity studies

A series of chiral dizinc complexes of the type [(2,6-{ArN=C(Me)C5H3N}2C6H3O)Zn2(micro-Cl)Cl2] [Ar=2,6-i-Pr2C6H3 (), 2,6-Me2C6H3 (), 2,4,6-Me3-C6H2 (), 2,4-Me2C6H3 ()] can be conveniently prepared in good yield by the template reaction of 2,6-{O=C(Me)C5H3N}2C6H3OH with an excess of the corresponding aniline and two equivalents of zinc dichloride in n-BuOH at elevated temperature. Alternatively, the pro-ligands, 2,6-{(ArN=C(Me)C5H3N}2C6H3OH [Ar=2,6-i-Pr2C6H3 (L1-H), 2,6-Me2C6H3 (L2-H), 2,4,6-Me3C6H2 (L3-H), 2,4-Me2C6H3 (L4-H)], can be isolated and then treated with two equivalents of zinc dichloride to afford . Interaction of with two equivalents of NaOAc in the presence of TlBF4 gives the diacetate-bridged salt [(L1)Zn2(micro-OAc)2](BF4) () while with Nadbm (dbm=dibenzoylmethanato) the bis(dbm)-chelated salt [(L1)Zn2(dbm)2](BF4) () is obtained. Hydrolysis occurs on reaction of with TlOEt to furnish [(L1)Zn2(micro-OH)Cl2] () as the only isolable product. Conversely, reaction of with Tlhp (hp=2-pyridonate) affords the neutral bis(pyridonate)-bridged trimetallic complex [(L1)Zn3(micro-hp)2Cl3] () as the major product along with as the minor one. Complex and mixtures of / act as modest activators for the ring-opening polymerisation of epsilon-caprolactone. Single crystal X-ray diffraction studies have been performed on , , , , and reveal Zn...Zn separations in the range: 3.069(4)-4.649(6) A.     

Radical Polymerization of Vinyl Acetate with Bis(tetramethylheptadionato)cobalt(II): Coexistence of Three Different Mechanisms

Poly(vinyl acetate) by OMRP : Increasing the steric encumbrance of the β‐diketonate R substituents in vinyl acetate (VAc) polymerization mediator [Co{OC(R)CHC(R)O}₂] from Me to tBu sufficiently weakens the Co^III? PVAc bond of the polymer chain to allow it to operate by both associative (degenerative transfer) and dissociative (organometallic radical polymerization, OMRP) mechanisms (see scheme). The Co^III? PVAc species also acts as a transfer agent in the absence of Lewis bases, whereas the Co^II complex shows catalytic chain transfer (CCT) activity.

     

Electronic and Steric Ligand Effects in the Radical Polymerization of Vinyl Acetate Mediated by β‐Ketoiminate Complexes of Cobalt(II)

Complexes Co[OC(Ph)CHC(Me)NAr]₂ [Ar=Ph, 1 ; o,o′‐C₆H₃Me₂ (Xyl), 2 ; p‐C₆H₄CF₃, 3 ] are tested in the polymerization of vinyl acetate (VAc) initiated by V‐70 (0.8 equiv) at 30 °C. Polymerization occurs without any notable induction time yielding PVAc with relatively low polydispersity, but with higher than expected M_n values, which indicates inefficient trapping processes. The apparent polymerization rate constant varies in the order 2 > 1 > 3 . Controlled polymer growth is also observed when the polymerization is conducted in the presence of a much higher V‐70/ 1 ratio, demonstrating that this system can also function as a transfer agent in a degenerative transfer process. Competition between chain growth and catalyzed chain transfer (CCT) is also observed, the latter prevailing at higher temperatures. Comparison of these results with previous reports on bis(β‐diketonato) complexes allows a separate assessment of ligand electronic and steric effects in the ability to control polymerization.     

Ligand adducts of bis(acetylacetonato)iron(II): a 1H NMR study

We report here a thorough (1)H NMR study of Fe(acac)(2) solutions in a wide variety of noncoordinating and coordinating solvents, as well as the interaction of this complex with Et(3)N, pyridine, PMe(2)Ph, and R(2)PCH(2)CH(2)PR(2) [R = Ph (dppe), Et (depe)] in C(6)D(6). The study reveals that Fe(acac)(2) is readily transformed into Fe(acac)(3) in solution under aerobic conditions and that the commercial compound is usually contaminated by significant amounts of Fe(acac)(3). The (1)H NMR resonances of Fe(acac)(2) are rather solvent-dependent and quite different than those reported in the literature. The compound is unstable in CDCl(3) and stable in CD(2)Cl(2), C(6)D(6), CD(3)CN, acetone-d(6), DMSO-d(6), THF-d(8), and CD(3)OD. The addition of the above-mentioned ligands (L) reveals only one paramagnetically shifted band for each type of acac and L proton, the position of which varies with the L/Fe ratio, consistent with rapid ligand exchange equilibria on the NMR time scale. A fit of the NMR data at a high L/Fe ratio allows the calculation of the expected resonances for all protons in the Fe(acac)(2)L(2) molecules. The system with the bidentate depe ligand shows evidence for a slow ligand exchange at low depe/Fe ratios, proposed to involve a species with the cis-chelated mononuclear Fe(acac)(2)(depe) structure, whereas the fast exchange at a higher ratio is proposed to involved a trans-Fe(acac)(2)(κ(1)-depe)(2) complex. Complex Fe(acac)(2)(dppe) cannot be investigated in solution because of low solubility in a noncoordinating solvent and because of the poor dppe competition for binding in coordinating solvents. The compound was crystallized, and its X-ray structure reveals a 1-dimensional polymeric structure with dppe-bridged Fe centers having the trans-octahedral Fe(acac)(2)(κ(1)-dppe)(2) coordination environment.     

Use of Stille-type cross-coupling as a route to oligopyridylimines

The new tin reagents, 2-(n-Bu₃Sn)-6-{C(R)OCH₂CH₂O}-C₅H₃N, (R=H a, Me b), have been employed in Stille-type cross-coupling reactions with a range of oligopyridylbromides generating, following a facile deprotection step, a series of formyl- and acetyl-functionalised oligopyridines. Condensation reactions with 2,6-diisopropylaniline has allowed access to families of novel sterically bulky multidentate N,N,N,N (tetradentate), N,N,N,N,N (pentadentate), N,N,N,N,N,N (sexidentate) and N,N,N,N,N,N,N (heptadentate) nitrogen donor ligands. This work represents a straightforward and rapid synthetic route for the preparation of oligopyridylimines, which are expected to act as useful components for the self-assembly of polymetallic complexes.     

Prominence of the Instability of a Stabilizing Agent in the Changes in Physical State of a Hybrid Nanomaterial

Shaping ability of hybrid nanomaterials is a key point for their further use in devices. It is therefore crucial to control it. To this end, it is necessary that the macroscopic properties of the material remain constant over time. Here, we evidence by multinuclear Magic-Angle Spinning Nuclear Magnetic Resonance spectroscopic study including ¹⁷O isotope exchange that for a ZnO-alkylamine hybrid material, the partial carbonation of amine into ammonium carbamate molecules is behind the conversion from highly viscous liquid to a powdery solid when exposed to air. This carbonation induces modification and reorganization of the organic shell around the nanocrystals and affects significantly the macroscopic properties of the material such as it physical state, its solubility and colloidal stability. This study, straightforwardly extendable, highlights that the nature of the functional chemical group allowing connecting the stabilizing agent (SA) to the surface of the nanoparticles is of tremendous importance especially if the SA is reactive with molecules present in the environment.     

Copper–Carbene Intermediates in the Copper‐Catalyzed Functionalization of OH Bonds

Copper–carbene [Tp^xCu?C(Ph)(CO₂Et)] and copper–diazo adducts [Tp^xCu{η¹‐N₂C(Ph)(CO₂Et)}] have been detected and characterized in the context of the catalytic functionalization of O?H bonds through carbene insertion by using N₂?C(Ph)(CO₂Et) as the carbene source. These are the first examples of these type of complexes in which the copper center bears a tridentate ligand and displays a tetrahedral geometry. The relevance of these complexes in the catalytic cycle has been assessed by NMR spectroscopy, and kinetic studies have demonstrated that the N‐bound diazo adduct is a dormant species and is not en route to the formation of the copper–carbene intermediate.     

Mechanistic insights into the cobalt-mediated radical polymerization (CMRP) of vinyl acetate with cobalt(III) adducts as initiators

Over the past few years, cobalt-mediated radical polymerization (CMRP) has proved efficient in controlling the radical polymerization of very reactive monomers, such as vinyl acetate (VAc). However, the reason for this success and the intimate mechanism remained basically speculative. Herein, two mechanisms are shown to coexist: the reversible termination of the growing poly(vinyl acetate) chains by the Co(acac)2 complex (acac: acetylacetonato), and a degenerative chain-transfer process. The importance of one contribution over the other strongly depends on the polymerization conditions, including complexation of cobalt by ligands, such as water and pyridine. This significant progress in the CMRP mechanism relies on the isolation and characterization of the very first cobalt adducts formed in the polymerization medium and their use as CMRP initiators. The structure proposed for these adducts was supported by DFT calculations. Beyond the control of the VAc polymerization, which is the best ever achieved by CMRP, extension to other monomers and substantial progress in macromolecular engineering are now realistic forecasts.