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In contrast to the reported CCC-NHC pincer ligands that contain normal N-heterocyclic carbenes (NHC), herein we report an imidazole-based abnormal NHC (aNHC) pincer ligand, CCC-aNHC. The CCC-aNHC pincer Pt complex with two aNHC donors was synthesized via the in situ metalation and transmetalation methodology. The 1,3-phenylene(bis-2-phenyl-3-butyl imidazolium) diiodide salt was reacted with Zr(NMe2)4 to generate a CCC-aNHC pincer zirconium complex in situ. It was transmetalated to Pt using [Pt(COD)Cl2]. Electrospray ionization of the Pt pincer complex [(BuCa‑iCa‑iCBu)-PtI] in acetonitrile generated an intense peak at m/z = 696.2375, which was assigned to the dinitrogen adduct [M−I+N2]+ of the cationic CCC-aNHC pincer Pt(II) complex [(BuCa‑iCa‑iCBu)Pt− N2]+, representing a rare example of the platinum dinitrogen organometallic complex. The super electron-donating ability of the pincer ligands with abnormal NHC enabled the cationic CCC-aNHC pincer Pt(II) complex to selectively bind N2 over MeCN in a first-order analysis. A collision-induced dissociation (CID) study was conducted on the N2 and MeCN adducts, suggesting that more energy was required to dissociate N2 than MeCN. A computational study suggested that the N2 adduct was kinetically stable in the gas phase whereas the MeCN adduct was thermodynamically preferred. The computational results reconciled the mass spectral data experiment with an attempt to isolate the N2 adduct. DFT computation suggested that N2 dissociation is more challenging due to higher energy transition states, and there is a competitive pathway of N2 tumbling within the coordination sphere of the Pt. This tumbling path is not available from the MeCN ligand due to ligand structural differences. 

Monoligated and bis-ligated CCC-NHC pincer Fe complexes with n-butyl substituents have been synthesized by the Zr metalation/transmetalation route. Both the direct metalation/transmetalation and transmetalation from the isolated (BuCiCiCBu)ZrNMe2Cl2, 3, yielded the octahedrally coordinated Fe(III) bis-ligated complex [(BuCiCiCBu)2Fe]Cl, 2a. Transmetalation from in situ and isolated (BuCiCiCBu)ZrCl3, 5, in the presence of excess TMSCl and 1 equiv of the Fe source yielded the monoligated (BuCiCiCBu)FeCl2, 4. Conditions that convert [(BuCiCiCBu)2Fe]+, 2, to (BuCiCiCBu)FeCl2, 4, complex have been found. Characterization included 1H NMR, UV−visible, femtosecond transient absorption spectroscopies, TDDFT computations, and mass spectroscopy along with X-ray crystallographic structure determinations. 

The first catalytic application of a CCC-NHC pincer Ir (III) dimer complex for the C – H functionalization of N-methylindoles with a-aryl-a-diazoacetates at the C-3 position is reported herein. The Ir pincer dimer complex was used as a catalyst at 3 mol% loading. The best reaction conditions involved a combination of catalyst and substrates in a specific order. It resulted in the activation of the C–H bond with the formation of a new C–C bond to generate a-aryl-a-indolyl acetates with more than 99% conversion at room temperature without requiring any additives. Isolated yields of 84–97% were obtained for a range of substrates. Under these catalytic conditions, the unprotected N–H indoles did not react with a-aryl-a-diazoacetate. These catalytic conditions provided an efficient synthetic route to the synthesis of a-aryl-a-indolyl acetates. 

The metalation/transmetalation strategy using [Zr(NMe2)4] as initial metalating reagent offers an efficient approach to the synthesis of CCC–NHC pincer complexes. Many CCC–NHC pincer complexes have been prepared via this methodology. As efficient as this methodology is, many questions remained as to the mechanism for the process, particularly the requirement of two equivalents of Rh per proligand for good yields. Previously, no intermediates have been reported to shed light on the mechanism. In the process of investigating an intermediate and the mechanism of the metalation/transmetalation methodology, a new mixed valent bimetallic CCC–NHC pincer Rh complex with two chloro ligands bridged between a [(CCC–NHC)Rh(III)] and a [Rh(I)(COD)] fragment was isolated and fully characterized. The investigation of the Rh(III)/Rh(I) bimetallic intermediate in the CCC–NHC pincer metalation/transmetalation methodology led to an improved stoichiometric synthesis of CCC–NHC pincer Rh complexes. It was found that switching the proligand from iodide to chloride counterion obviated the need for an extra equivalent of Rh. The iodide bridged Rh(III)/Rh(I) intermediate was much more stable and prevented further reaction in comparison to the chloride congener. When it was switched to only chloride present the reaction quickly gave efficient, complete transmetalation with only a 1:1 ratio of proligand:Rh. 

To date, no CCC‐NHC pincer complexes of Re have been reported in the literature. The first CCC‐NHC pincer complex of Re is reported. It was fully characterized by¹H and¹³C nuclear magnetic resonance (NMR) spectroscopy, mass spectroscopy, elemental analysis, and X‐ray crystallographic methods to determine the molecular structure. It was synthesized via transmetallation from an isolated Zr precursor and was found to be air stable. The catalytic activity of the CCC‐NHC Re(I) pincer complex was demonstrated for the borrowing hydrogen coupling reaction between benzylic ketones and primary alcohols to generate a new C–C bond in an environmentally friendly catalysis requiring no activating groups for the alcohol functionality. This borrowing hydrogen coupling reaction produced a stoichiometric amount of water as the only by‐product and did not require the conversion of the primary alcohol to a leaving group. A broad range of substrates was examined, and isolated yields from 53% to 92% were obtained. A catalytic cycle for the CCC‐NHC Re(I) pincer complex catalyzed borrowing hydrogen coupling reaction is proposed.