Abstract:A new series of molybdenum and tungsten tricarbonyl pincer complexes, bearing pyridine-based PONOPtype pincer ligands, have been synthesized and fully characterized. Addition of HBF 4 •Et 2 O to these tricarbonyl complexes generated seven-coordinate molybdenum and tungsten hydride complexes, and these compounds have been isolated in good yields. These metal hydrides show fluxional behavior in solution.The hydride ligands on these metal complexes are acidic in nature and are readily deprotonated by bases. The m… Show more
“…The ν CO frequencies, in particular the symmetric CO stretch, which presents the highest stretching frequency, are indicative for the increasing electron donor strength/decreasing π-acceptor strength of the PNP ligands. When compared to previous results, they follow the order PNP-TADDOL < PNP-BIPOL < PNP-Ph < PNP Me -Ph < PNP Me -Et < PNP-Cy < PNP- i Pr ≈ PNP Me - i Pr < PNP-Et < PNP- t Bu as shown in Scheme 3 and Table 1 [13, 15–17, 27, 31]. …”
The new chiral PNP pincer ligand N2,N6-bis((3aR, 8aR)-2,2-dimethyl-4,4,8,8-tetraphenyltetrahydro[1,3]dioxolo[4,5-e][1,3,2]dioxaphosphepin-6-yl)pyridine-2,6-diamine (PNP-TADDOL) was synthesized in 80% isolated yield. Complexes of the type [M(PNP-TADDOL)(CO)3] (M = Cr, Mo, and W) were prepared via a solvothermal approach. This methodology constitutes a fast, simple, and practical synthetic method to obtain complexes of that type in high isolated yields. The X-ray structure of the molybdenum complex is presented.Graphical abstract
“…The ν CO frequencies, in particular the symmetric CO stretch, which presents the highest stretching frequency, are indicative for the increasing electron donor strength/decreasing π-acceptor strength of the PNP ligands. When compared to previous results, they follow the order PNP-TADDOL < PNP-BIPOL < PNP-Ph < PNP Me -Ph < PNP Me -Et < PNP-Cy < PNP- i Pr ≈ PNP Me - i Pr < PNP-Et < PNP- t Bu as shown in Scheme 3 and Table 1 [13, 15–17, 27, 31]. …”
The new chiral PNP pincer ligand N2,N6-bis((3aR, 8aR)-2,2-dimethyl-4,4,8,8-tetraphenyltetrahydro[1,3]dioxolo[4,5-e][1,3,2]dioxaphosphepin-6-yl)pyridine-2,6-diamine (PNP-TADDOL) was synthesized in 80% isolated yield. Complexes of the type [M(PNP-TADDOL)(CO)3] (M = Cr, Mo, and W) were prepared via a solvothermal approach. This methodology constitutes a fast, simple, and practical synthetic method to obtain complexes of that type in high isolated yields. The X-ray structure of the molybdenum complex is presented.Graphical abstract
“…Note that in some cases of linear 1‐alkenes, the E / Z selectivities (2.3:1–4:1) are close to the thermodynamic ratio . Similarly, a number of transition‐metal catalysts (for example, Fe, Ru, Co, Mo, and Ir) offer regioselectivities for 2‐alkenes; unfortunately the stereoselectivities demonstrated in these works are moderate or low. In 2012, Grotjahn disclosed an imidazolyl‐phosphine Ru complex for E ‐selective isomerizations of linear allyl substrates (for example, O ‐, N ‐allyl substrates and allylarenes).…”
A unique Ir complex ( NC P)Ir with the pyridine-phosphine pincer as the sole ligand, featuring a dual agostic interaction between the Ir and two σ C-H bonds from a tBu substituent, has been prepared. This complex exhibits exceptionally high activity and excellent regio- and stereoselectivity for monoisomerization of 1-alkenes to trans-2-alkenes with wide functional-group tolerance. Reactions can be performed in neat reactant on a more than 100 gram scale using 0.005 mol % catalyst loadings with turnover numbers up to 19000.
“…After the discovery and pioneering work by the groups of Shaw and van Koten , in the 1970s, pincer ligands and their associated complexes have become a mature organometallic research field. Many different types of pincer ligands have been designed that vary in the donor atoms, steric or electronic influence substituents on the donors, and the rigidity of the ligand in general (PCP, − POC sp2 OP, − POC sp3 OP, POCN, − PNP, − PONOP, − PIMCOP, and NNN, − among others). The highly versatile and often electron-donating nature of the pincer ligand framework allows for almost endless customization.…”
The electronic and steric properties of a new class of electron-rich and sterically hindered tertamethylated PNP pincer ligands (Me 4 PNP R = 2,6-bis[(dialkylphosphino)propyl]pyridine with R = i Pr, t Bu) are discussed. Introducing the methyl groups on the pincer arm prevents dearomatization of the pincer framework and increases the bulkiness and electron-donating capacity of the ligand. Highly reactive Ni I species are thus prevented from dimerizing and can be analyzed by a wide variety of spectroscopic methods. X-ray diffraction study shows that steric bulk has an important influence on the resulting geometric and spectroscopic properties of the Ni I complexes. Complexes 5 and 6, which contain i Pr groups on the phosphorus atoms, show a very rare seesaw geometry around the metal center, while t Bu complexes 7 and 8 show a distorted square-planar geometry. Computational analysis reveals that the SOMO for all complexes has a d x 2 −y 2 character with the spin density mostly residing on the nickel.
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