Search for: All records

The synthesis of the bidentate Mo complex tetrachlorido[6-(3’-butylimidazolium-1’-yl)-2-(3’’-butylimidazol-1’’-yl-2’’-idene-κC2)phenyl-κC1]molybdenum(IV) 3 was carried out using the metalation followed by transmetalation methodology. The transmetallation process led to a bidentate complex after reaction of DCM with HNMe2 formed an acidic ammonium ion that protonated the bidentate complex. Optimization of the synthetic methodology provided the tetrachlorido[6-(3’-butylimidazolium-1’-yl)-2-(3’’-butylimidazol-1’’-yl-2’’-idene-κC2)phenyl-κC1]molybdenum(IV) complex in high yield. The crystal structure of the bidentate complex, 3, is reported herein. Attempts to avoid the acidic reaction conditions with different solvents or starting materials produced the bis-ligated Mo complex bis[2,6-bis(3’-butylimidazol-1’-yl-2’-idene-κC2)phenyl-κC1]molybdenum(IV) dichloride based on MS analysis. Electronic and coordinative unsaturation in the resulting bidentate complex, 3, open new possibilities for coordination of incoming substrates while also allowing access to the pincer-like motif via oxidative addition. Access to the interconversion of a NHC to/from imidazolium opens new avenues of non-innocent ligand pathways for proton shuttling in reactions such as nitrogen reduction to ammonia. 

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. 

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. 

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. 

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.