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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. 

The formation of dimer [(μ-Cl)Rh-(κ3(P,Si,Si)PhP(o-C6H4CH2SiiPr2)(o-C6H4CH2SiiPrnPr))]2 (Rh-3) with an n-propyl group on one of the silicon atoms as a minor product was affected by the reaction of [RhCl(COD)]2 with proligand PhP(o-C6H4CH2SiHiPr2)2, L1. The major product of the reaction was monomeric 14-electron Rh(III) complex [ClRh(κ3(P,Si,Si)PhP(o-C6H4CH2SiiPr2)2)] (Rh-1). Computations revealed that the monomer–dimer equilibrium is shifted toward the monomer with four isopropyl substituents on the two Si atoms of the ligand as in Rh-1; conversely, the dimer is favored with only one n-propyl as in Rh-3, and with less bulky alkyl substituents such as in [ClRh(κ3(P,Si,Si)PhP(o-C6H4CH2SiMe2)2]2 (Rh-2). Computations on the mechanism of formation of Rh-3 directly from [RhCl(COD)]2 are in agreement with the experimental findings and it is found to be less energetic than if stemming from Rh-1. Additionally, a Si–O–Si complex, [μ-Cl-Rh{κ3(P,Si,C)PPh(o-C6H4CH2SiiPrO SiiPr2CH-o-C6H4)}]2, Rh-4, is generated from the reaction of Rh-1 with adventitious water as a result of intramolecular C–H activation. 

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. 

Robust earth-abundant transition metal-based photocatalysts are needed for photocatalytic CO2 reduction. A series of six Ni(II) complexes have been synthesized with a tridentate CNC pincer ligand composed of two imidazole or benzimidazole derived N-heterocyclic carbene (NHC) rings and a pyridyl ring with different R substituents (R = OMe, Me, H) para to N of the pyridine ring. These complexes have been characterized using spectroscopic, analytic, and crystallographic methods. The electrochemical properties of all complexes were studied by cyclic voltammetry under N2 and CO2 atmospheres. Photocatalytic reduction of CO2 to CO and HCO2– was analyzed using all the complexes in the presence and absence of an external photosensitizer (PS). All of these complexes are active as photocatalysts for CO2 reduction with and without the presence of an external PS with appreciable turnover numbers (TON) for formate (HCO2–) production and typically lower amounts of CO. Notably, all Ni(II) CNC-pincer complexes in this series are also active as self-sensitized photocatalysts. Complex 4Me with a benzimidazole derived CNC pincer ligand was found to be the most active self-sensitized photocatalyst. Ultrafast transient absorption spectroscopy (TAS) experiments and computational studies were performed to understand the mechanism of these catalysts. Whereas sensitized catalysis involves halide loss to produce more active complexes, self-sensitized catalysis requires some halide to remain coordinated to allow for a favorable electron transfer between the excited nickel complex and the sacrificial electron donor. This then allows the nickel complex to undergo CO2 reduction catalysis via NiI or Ni0 catalytic cycles. The two active species (NiI¬ and Ni0) demonstrate distinct reactivity and selectivity which influences the formation of CO vs. formate as the product. 

MOF NU-1000 was employed to host Ni tripodal complexes prepared from new organometallic precursors [HNi(κ4(E,P,P,P)-E(o-C 6 H 4 CH 2 PPh 2 ) 3 ], E = Si (Ni-1), Ge (Ni-2). The new heterogenous catalytic materials, Ni-1@NU-1000 and Ni-2@NU-1000 show the advantages of both homogenous and heterogeneous catalysts. They catalyze the hydroboration of aldehydes and ketones more efficiently than the homogenous Ni-1 and Ni-2, under aerobic conditions, and allowing recyclability of the catalyst.