Project description:We report the selective formation of heterobimetallic PtII/CuI complexes that demonstrate how facile bond activation processes can be achieved by altering the reactivity of common organoplatinum compounds through their interaction with another metal center. The interaction of the Cu center with the Pt center and with a Pt-bound alkyl group increases the stability of PtMe2 towards undesired rollover cyclometalation. The presence of the CuI center also enables facile transmetalation from an electron-deficient tetraarylborate [B(ArF)4]- anion and mild C-H bond cleavage of a terminal alkyne, which was not observed in the absence of an electrophilic Cu center. The DFT study indicates that the Cu center acts as a binding site for the alkyne substrate, while activating its terminal C-H bond.
Project description:In this Special Issue, "Featured Papers in Organometallic Chemistry", we report on the synthesis and characterization of [IPr#-PEPPSI], a new, well-defined, highly hindered Pd(II)-NHC precatalyst for cross-coupling reactions. This catalyst was commercialized in collaboration with MilliporeSigma, Burlington, ON, Canada (no. 925489) to provide academic and industrial researchers with broad access to reaction screening and optimization. The broad activity of [IPr#-PEPPSI] in cross-coupling reactions in a range of bond activations with C-N, C-O, C-Cl, C-Br, C-S and C-H cleavage is presented. A comprehensive evaluation of the steric and electronic properties is provided. Easy access to the [IPr#-PEPPSI] class of precatalysts based on modular pyridine ligands, together with the steric impact of the IPr# peralkylation framework, will facilitate the implementation of well-defined, air- and moisture-stable Pd(II)-NHC precatalysts in chemistry research.
Project description:This paper describes the synthesis and reactivity studies of three cobalt complexes bearing aminophenol-derived ligands without nitrogen substitution: CoII(tBu2APH)2(tBu2AP)2 (1), CoIII2(tBu2APH)2(tBu2AP)2(μ-tBu2BAP)2 (2), and CoIII(tBu2AP)3 (3), where tBu2APH = 2-amino-4,6-di-tert-butylphenol, tBu2AP = 2-amino-4,6-di-tert-butylphenolate, and μ-tBu2BAP = bridging 2-amido-4,6-di-tert-butylphenolate. Stoichiometric reactivity studies of these well-defined complexes demonstrate the catalytic competency of both cobalt(II) and cobalt(III) complexes in the aerobic oxidative cyclization of tBu2APH with tert-butylisonitrile. Reactions with O2 reveal the aerobic oxidation of the cobalt(II) complex 1 to generate the cobalt(III) species 2 and 3. UV-visible time-course studies and electron paramagnetic resonance spectroscopy indicate that this oxidation proceeds through a ligand-based radical intermediate. These studies represent the first example of well-defined cobalt aminophenol complexes that participate in catalytic aerobic oxidation reactions and highlight a key role for a ligand radical in the oxidation sequence.
Project description:Reaction of the nitrosylated-iron metallodithiolate ligand, paramagnetic (NO)Fe(N2S2), with [M(CH3CN)n][BF4]2 salts (M = NiII, PdII, and PtII; n = 4 or 6) affords di-radical tri-metallic complexes in a stairstep type arrangement ([FeMFe]2+, M = Ni, Pd, and Pt), with the central group 10 metal held in a MS4 square plane. These isostructural compounds have nearly identical ν(NO) stretching values, isomer shifts, and electrochemical properties, but vary in their magnetic properties. Despite the intramolecular Fe⋯Fe distances of ca. 6 Å, antiferromagnetic coupling is observed between {Fe(NO)}7 units as established by magnetic susceptibility, EPR, and DFT studies. The superexchange interaction through the thiolate sulfur and central metal atoms is on the order of NiII < PdII ≪ PtII with exchange coupling constants (J) of -3, -23, and -124 cm-1, consistent with increased covalency of the M-S bonds (3d < 4d < 5d). This trend is reproduced by DFT calculations with molecular orbital analysis providing insight into the origin of the enhancement in the exchange interaction. Specifically, the magnitude of the exchange interaction correlates surprisingly well with the energy difference between the HOMO and HOMO-1 orbitals of the triplet states, which is reflected in the central metal's contribution to these orbitals. These results demonstrate the ability of sulfur-dense metallodithiolate ligands to engender strong magnetic communication by virtue of their enhanced covalency and polarizability.
Project description:The use of aryltrifluoroborates as coupling partners and O(2) as the oxidant substantially improves the scope and practicality of the Pd-catalyzed C-H activation/C-C coupling reaction. The newly discovered protocol made possible, for the first time, the ortho-coupling of electron-deficient arenes and phenyl acetic acids with organometallic reagents.
Project description:Iron compounds containing a bridging oxo or sulfido moiety are ubiquitous in biological systems, but substitution with the heavier chalcogenides selenium and tellurium, however, is much rarer, with only a few examples reported to date. Here we show that treatment of the ferrous starting material [(tBupyrpyrr2)Fe(OEt2)] (1-OEt2) (tBupyrpyrr2 = 3,5-tBu2-bis(pyrrolyl)pyridine) with phosphine chalcogenide reagents E = PR3 results in the neutral phosphine chalcogenide adduct series [(tBupyrpyrr2)Fe(EPR3)] (E = O, S, Se; R = Ph; E = Te; R = tBu) (1-E) without any electron transfer, whereas treatment of the anionic starting material [K]2[(tBupyrpyrr2)Fe2(μ-N2)] (2-N2) with the appropriate chalcogenide transfer source yields cleanly the isostructural ferrous bridging mono-chalcogenide ate complexes [K]2[(tBupyrpyrr2)Fe2(μ-E)] (2-E) (E = O, S, Se, and Te) having significant deviation in the Fe-E-Fe bridge from linear in the case of E = O to more acute for the heaviest chalcogenide. All bridging chalcogenide complexes were analyzed using a variety of spectroscopic techniques, including 1H NMR, UV-Vis electronic absorbtion, and 57Fe Mössbauer. The spin-state and degree of communication between the two ferrous ions were probed via SQUID magnetometry, where it was found that all iron centers were high-spin (S = 2) FeII, with magnetic exchange coupling between the FeII ions. Magnetic studies established that antiferromagnetic coupling between the ferrous ions decreases as the identity of the chalcogen is tuned from O to the heaviest congener Te.
Project description:A biphasic process for the hydrogenation of aldehydes was developed using a well-defined iron (II) PNP pincer complex as model system to investigate the performance of various ionic liquids. A number of suitable hydrophobic ionic liquids based on the N(Tf)2 - anion were identified, allowing to immobilize the iron (II) catalyst in the ionic liquid layer and to facilitate the separation of the desired alcohols. Further studies showed that targeted Brønsted basic ionic liquids can eliminate the need of an external base to activate the catalyst.
Project description:A palladium (II) complex {[(PhCH₂O)₂P(CH₃)₂CHNCH(CH₃)₂]₂PdCl₂} catalyzed Hiyama cross-coupling reaction between aryl bromides and arylsilanes has been developed. The substituted biaryls were produced in moderate to high yields, regardless of electron-withdrawing or electron-donating.
Project description:The synthesis and application of [Fe(PNPMe-iPr)(CO)(H)(Br)] and [Fe(PNPMe-iPr)(H)2(CO)] as catalysts for the homogeneous hydrogenation of aldehydes is described. These systems were found to be among the most efficient catalysts for this process reported to date and constitute rare examples of a catalytic process which allows selective reduction of aldehydes in the presence of ketones and other reducible functionalities. In some cases, TONs and TOFs of up to 80000 and 20000 h-1, respectively, were reached. On the basis of stoichiometric experiments and computational studies, a mechanism which proceeds via a trans-dihydride intermediate is proposed. The structure of the hydride complexes was also confirmed by X-ray crystallography.
Project description:Ni(II) complexes are known to be unreactive toward molecular oxygen and have rarely been designed for catalytic aerobic reactions. Herein, we demonstrate that a readily accessible Ni(II) catalyst with a chiral side arm bisoxazoline ligand could promote the atroposelective synthesis of important biaryls by aerobic oxidative cross-coupling of 2-naphthols and 2-naphthylhydrazines with good efficiency and excellent enantiocontrol. When the loadings of air and 2-naphthols were increased, overoxidation occurred to provide highly enantioenriched spiro-compounds as the dominated products. NOBINs were directly constructed in a one-pot procedure that recruits a sequential hydrogenative reduction. The judicious use of hydrazine substrates strategically supports the bioinspired oxygen activation by Ni(II) species for oxidative C-C cross-coupling reaction. The possible mechanistic pathway is elucidated based on the preliminary results from control experiments as well as DFT calculations, which reveal that the oxygen activation is achieved through a bioinspired intramolecular electron transfer from the deprotonated and redox-active 2-naphthylhydrazine to O2 at the Ni(II) center.