Early transition metal complexes bearing a C-capped tris(phenolate) ligand incorporating a pendant imine arm: synthesis, structure, and ethylene polymerization behavior

The ligand 3-[2,2′-methylenebis(4,6-di-tert-butylphenol)-5-tert-butylsalicylidene-(2,6-diisopropyl)phenylimine] (L1H3) was reacted with MCl4 (M = Ti, Zr) or MCl5 (M = Nb, Ta) to give complexes of the type [MCl2(L1H2)2] (M = Ti (1); Zr (2)), [NbCl3(L1H)] (3), or [TaCl4(L1H2)] (4), respectively. Single crystal X-ray diffraction of 1−4 revealed common “iminium” species resulting in zwitterionic complexes. Reaction of [V(Np-tol)(On-Pr)3] with L1H3 afforded [{(VNp-tol)(L1H)}2(µ-On-Pr)2] (5), and a second complex [(VO)2(µ-O)(L3H)2] (6) (L3H being derived from 3-[2,2′-methylenebis(4,6-di-tert-butylphenol)-5-tert-butylsalicylidene-p-tolylimine]). The condensation reaction between 3-[2,2′-methylenebis(4,6-di-tert-butylphenol)-5-tert-butyl-2-hydroxybenzaldehyde] (L0H3) and o-phenylenediamine (1,2-diaminobenzene) afforded two products: a pseudo-16-membered hydrogen bonded macrocyclic structure {1,2-bis-3-[2,2′-methylenebis(4,6-di-tert-butylphenol)-5-tert-butylsalicylidene-benzyldiimine]} (L5H6), or the benzimidazolyl bearing ligand (L6H3). The reaction of L5H6 or L6H3 with [VO(On-Pr)3] under varying conditions produced the complexes [(VO)(L5H4)] (7), [(VO)2(L5H)] (8), or [VO(L6H2)2] (9). L0H3 was reacted with a number of anilines to give the proligands {3-[2,2′-methylenebis(4,6-di-tert-butylphenol)-5-tert-butylsalicylidene-R-imine]}, where R = NC6H5 (L2H3), NC6H4-Me (L3H3), and NC6H2-Me3 (L4H3). Reactions of these ligands with [VO(On-Pr)3] formed bischelating complexes of the form [(VO)(L2−4H2)2] (10, 11, and 12, respectively). The reaction of L1H3 with trimethylaluminum led to a bis-aluminum complex {(AlMe2)[AlMe(NCMe)]L1} (13). The ability of complexes 1−12 to polymerize ethylene in the presence of an organoaluminum cocatalyst was investigated. Procatalysts 1 and 2 were found to produce negligible activities in the presence of dimethylaluminum chloride (DMAC) and the reactivator ethyltrichloroacetate (ETA), whereas 3 and 4 were found to be completely inactive for polymerization using a variety of different organoaluminum cocatalysts. Using the combination of DMAC and ETA, complexes 5−12 were found to be highly active catalysts; in all cases, the polymer formed was of high molecular weight linear polyethylene

Electronic Control of the Protonation Rates of Fe–Fe Bonds

Protonation at metal–metal bonds is of fundamental interest in the context of the function of the active sites of hydrogenases and nitrogenases. In diiron dithiolate complexes bearing carbonyl and electron-donating ligands, the metal–metal bond is the highest occupied molecular orbital (HOMO) with a “bent” geometry. Here we show that the experimentally measured rates of protonation (<i>k</i>_H) of this bond and the energy of the HOMO as measured by the oxidation potential of the complexes (<i>E</i>_1/2^ox) correlate in a linear free energy relationship: ln <i>k</i>_H = ((<i>F</i>(<i>c</i> – β<i>E</i>_1/2^ox))/(<i>RT</i>)), where <i>c</i> is a constant and β is the dimensionless Brønsted coefficient. The value of β of 0.68 is indicative of a strong dependence upon energy of the HOMO: measured rates of protonation vary over 6 orders of magnitude for a change in <i>E</i>_1/2^ox of ca. 0.55 V (ca. 11 orders of magnitude/V). This relationship allows prediction of protonation rates of systems that are either too fast to measure experimentally or that possess additional protonation sites. It is further suggested that the nature of the bridgehead in the dithiolate ligand can exert a stereoelectronic influence: bulky substituents destabilize the HOMO, thereby increasing the rate of protonation

Investigation of the Ultrafast Dynamics Occurring during Unsensitized Photocatalytic H₂ Evolution by an [FeFe]-Hydrogenase Subsite Analogue

Biomimetic compounds based upon the active subsite of the [FeFe]-hydrogenase enzyme system have been the focus of much attention as catalysts for hydrogen production: a clean energy vector. Until recently, use of hydrogenase subsite systems for <i>light-driven</i> hydrogen production has typically required the involvement of a photosensitizer, but the molecule [(μ-pdt)­(μ-H)­Fe₂(CO)₄(dppv)]⁺, (<b>1</b>; dppv = <i>cis</i>-1,2-C₂H₂(PPh₂)₂; pdt = 1,3-propanedithiolate) has been reported to catalyze the evolution of hydrogen gas under sensitizer-free conditions. Establishing the molecular mechanism that leads to photohydrogen production by <b>1</b> is thus an important step that may enable further development of this family of molecules as solar fuel platforms. Here, we report ultrafast UV_pump–IR_probe spectroscopy of <b>1</b> at three different excitation wavelengths and in a range of solvents, including under the conditions required for H₂ production. Combining spectroscopic measurements of the photochemistry and vibrational relaxation dynamics of <b>1</b> with ground-state density functional theory (DFT) calculations shows that, irrespective of experimental conditions, near-instantaneous carbonyl ligand loss is the main photochemical channel. No evidence for a long-lived excited electronic state was found. These results provide the first time-resolved data for the photochemistry of <b>1</b> and offer an alternative interpretation of the underlying mechanism of light-driven hydrogen generation

Tri- and tetra-dentate imine vanadyl complexes: synthesis, structure and ethylene polymerization/ring opening polymerization capability

A number of vanadyl complexes bearing tri- or tetradentate phenoxyimine ligands have been structurally characterized and shown to exhibit high catalytic activity for ethylene polymerization in the presence of diethylaluminium chloride and ethyltrichloroacetate; at high temperatures, such complexes were also capable of Q4 the ROP of ε-caprolactone

Vanadyl complexes bearing bi-dentate phenoxyimine ligands: synthesis, structural studies and ethylene polymerization capability

Vanadyl complexes bearing bi-dentate phenoxyimine ligands: synthesis, structural studies and ethylene polymerization capabilit