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Bimetallic cleavage of dinitrogen has emerged as a highly promising approach to synthesis using N2, particularly its conversion to NH3. It is generally considered that thermal bimetallic cleavage proceeds only through MNNM units with a delocalized 𝜋10 electronic configuration. We report herein a N2-bridged complex with a 𝜋8 configuration, [(PArNP)MoI]2(μ-1:1-N2) (2; PArNP = Ozerov’s anionic PNP pincer ligand). As expected, 2 displays a high barrier to thermal N2 cleavage which occurs only slowly at 110 °C (k = 1.65 x 10-4 s-1). However, at room temperature 2 catalyzes the conversion of N2 to NH3 by Cp*2Co and collidinium triflate. Experiments in the absence of reductant reveal that cleavage is catalyzed by Brønsted acids. DFT analysis indicates that this proceeds via protonation of the μ-N2 ligand, to give a diazenido bridge; N-N cleavage of this bridge is spin- and symmetry-allowed with a low calculated barrier (G‡ = 20 kcal/mol). The mononuclear product of cleavage of 2, (PArNP)Mo(N)I (1-(N)I), was characterized crystallographically and by EPR spectroscopy. 1-(N)I has a half-filled non-bonding d orbital; as a result, hydrogen-atom transfer or proton-coupled electron transfer to yield the corresponding imide is calculated to be much more thermodynamically favorable than analogous additions to the closed-shell nitrides derived from 𝜋10 complexes. This finding is calculated to be general for 𝜋8 versus 𝜋10 cleavage products, with implications for the design of molecular catalysts for N2 conversion to NH3. 

We report that the cationic iridium complex (iPrPCP)IrH+ undergoes addition of alkane C-H bonds, which is manifested by catalytic alkane transfer-dehydrogenation to give alkenes and by hydrogen isotope (H/D) exchange (HIE). Contrary to established selectivity trends found for C-H activation by transition metal complexes, strained cycloalkanes, including cyclopentane, cycloheptane, and cyclooctane, undergo C-H addition much more readily than n-alkanes which in turn are much more reactive than cyclohexane. Aromatic C-H bonds also undergo H/D exchange much less rapidly than those of the strained cycloalkanes, but much more favorably than cyclohexane. The order of reactivity toward dehydrogenation correlates qualitatively with the reaction thermodynamics, but the magnitude is much greater than can be explained by thermodynamics. Accordingly, the cycloalkenes corresponding to the strained cycloalkanes undergo hydrogenation much more readily than cyclohexene, despite the less favorable thermodynamics of such hydrogenations. Computational (DFT) studies allow rationalization of the origin of reactivity and the unusual selectivity. Specifically, the initial C-H addition is strongly assisted by 𝛽-agostic interactions, which are particularly favorable for the strained cycloalkanes. Subsequent to 𝛼-C-H addition, the H atom of the 𝛽-agostic C-H bond is transferred to the hydride ligand of (iPrPCP)IrH+, to give a dihydrogen ligand. The overall processes, C-H addition and 𝛽-H-transfer to hydride, generally show intermediates on the IRC surface but they are extremely shallow, such that the 1,2-dehydrogenations are presumed to be effectively concerted although asynchronous. 

The synthesis and characterization of a half-sandwich cobalt(II) complex supported by a bidentate, pendent-amine phosphine ligand (PPh2NBn2 = 1,5-diaza-3,7-diphosphacyclooctane) are reported. Oxidation of a cobalt(I)-phosphine precursor with silver(I) salts yielded the paramagnetic complex [CpCo(PPh2NBn2)]+. This species rapidly reacts with oxygen upon air exposure under ambient conditions, resulting in the insertion of oxygen into the cobalt-phosphine bonds. The aerobic oxidation of the phosphine induces the exchange and rearrangement of the ligands at cobalt. 

Two new compounds, Zn2FeSbO6 and Zn2MnSbO6, have been synthesized under high-pressure and high-temperature conditions. The synthesis, single-crystal and powder X-ray diffraction, X-ray absorption near-edge spectroscopy (XANES), optical second harmonic generation (SHG), and magnetic and heat capacity measurements were carried out for both compounds and are described. The lattice parameters are a = 5.17754(6) Å and c = 13.80045(16) Å for Zn2FeSbO6 and a = 5.1889(10) Å and c = 14.0418(3) Å for Zn2MnSbO6. Single-crystal X-ray diffraction analyses indicate that Zn2FeSbO6 consists of a cocrystal of superimposed Ni3TeO6 (NTO) and ordered ilmenite (OIL) components with a ratio of approximately 2:1 and Zn2MnSbO6 contains two nearly identical, but noncrystallographically related, OIL components in a ratio of approximately 6:1. 

A cobalt complex with a phenylenediamide redox-active ligand reacts with an electrophilic trifluoromethyl source to generate a robust Co–CF₃bond, while in contrast, treatment with alkyl electrophiles occurs at the ligand scaffold. 

The thioether–diphosphine pincer-ligated molybdenum complex (PSP)MoCl3 (1-Cl3, PSP = 4,5-bis(diisopropylphosphino)-2,7-di-tert-butyl-9,9-dimethyl-9H-thioxanthene) has been synthesized as a catalyst-precursor for N2 reduction catalysis with a focus on an integrated experimental/computational mechanistic investigation. The (PSP)Mo unit is isoelectronic with the (PNP)Mo (PNP = 2,6-bis(di-t-butylphosphinomethyl)pyridine) fragment found in the family of catalysts for the reduction of N2 to NH3 first reported by Nishibayashi and co-workers. Electrochemical studies reveal that 1-Cl3 is significantly more easily reduced than (PNP)MoCl3 (with a potential ca. 0.4 eV less negative). The reaction of 1-Cl3 with two reducing equivalents, under N2 atmosphere and in the presence of iodide, affords the nitride complex (PSP)Mo(N)(I). This observation suggests that the N2-bridged complex [(PSP)Mo(I)]2(N2) is formed and undergoes rapid cleavage. DFT calculations predict the splitting barrier of this complex to be low, in accord with calculations of (PNP)Mo and a related (PPP)Mo complex reported by Merakeb et al. Conversion of the nitride ligand to NH3 has been investigated in depth experimentally and computationally. Considering sequential addition of H atoms to the nitride through proton coupled electron-transfer or H-atom transfer, formation of the first N–H bond is thermodynamically relatively unfavorable. Experiment and theory, however, reveal that an N–H bond is readily formed by protonation of (PSP)Mo(N)(I) with lutidinium chloride, which is strongly promoted by coordination of Cl− to Mo. Other anions, e.g. triflate, can also act in this capacity although less effectively. These protonations, coupled with anion coordination, yield MoIV imide complexes, thereby circumventing the difficult formation of the first N–H bond corresponding to a low BDFE and formation of the respective MoIII imide complexes. The remaining two N–H bonds required to produce ammonia are formed thermodynamically much more favorably than the first. Computations suggest that formation of the MoIV imide is followed by a second protonation, then a rapid and favorable one-electron reduction, followed by a third protonation to afford coordinated ammonia. This comprehensive analysis of the elementary steps of ammonia synthesis provides guidance for future catalyst design. 

Pincer-ligated iridium complexes have been widely developed, and (pincer)Ir(III) complexes, particularly five-coordinate, are central to their chemistry. Such complexes typically bear two formally anionic ligands in addition to the pincer ligand itself. Yet despite the prevalence of halides as anionic ligands in transition metal chemistry there are relatively few examples in which both of these ancillary anionic ligands are halides or even other monodentate low-field anions. We report a study of the fragment (iPrPCP)IrCl2 (iPrPCP = 3 2,6 C6H3(CH2PiPr2)), and adducts thereof. These species are found to be thermodynamically disfavored relative to the corresponding hydridohalides. For example, DFT calculations and experiment indicate that one Ir-Cl bond of (iPrPCP)IrCl2 complexes will undergo reaction with H2 to give (iPrPCP)IrHCl or an adduct thereof. In the presence of aqueous HCl, (iPrPCP)IrCl2 adds a chloride ion to give an unusual example of an anionic transition metal complex ((iPrPCP)IrCl3–) with a Zundel cation (H5O2+). (iPrPCP)IrCl2 is not stable as a monomer at room temperature but exists in solution as a mixture of clusters which can add various small molecules. DFT calculations indicate that dimerization and trimerization of (iPrPCP)IrCl2 is more favorable than the analogous reactions of (iPrPCP)IrHCl, in accord with cluster formation being observed only for the dichloride complex. 

A set of six carboxylate-stabilized rhenium(I) tricarbonyl complexes supported by a 2,2’-bipyridine (bpy) ligand, Re(O2CR)(CO)3(bpy) (R = H, CH3, CHF2, R- or S CHBrCH(CH3)2, and C5H11), were prepared by acidolysis of the complex Re(OCO2C5H11)(CO)3(bpy) with the appropriate carboxylic acid and characterized by 1H and 13C-{1H} NMR and IR spectroscopy. The crystal structure of the complex, Re[R-O2CCHBrCH(CH3)2](CO)3(bpy), was determined by X-ray crystallography. Cytotoxicity results correlate positively with the Kb value of the carboxylate ligand. Apparently, the more substitutionally inert the carboxylate-stabilized complex is in a chloride-rich environment (similar to extracellular fluid) the greater the amount of cytotoxic [Re(CO)3(bpy)(H2O)]+ that forms in the cytosol. 

Iridium dibromide complexes of the phenyldiimine ligand 2,6-bis(1-((2,6-dimethylphenyl)imino)ethyl)phenyl, trans-(XyPhDI)IrBr2L, have been synthesized, and relative Ir-L BDFEs have been experimentally determined for a wide range of corresponding adducts of ligands L. An estimate of the absolute enthalpy of Ir-L binding has been obtained from dynamic NMR measurements. The results of DFT calculations are in very good agreement with the relative and absolute experimental values. Computational studies were extended to the formation of adducts of (XyPhDI)IrH2 and (XyPhDI)IrI, as well as other (pincer)IrI fragments, (Phebox)IrI and (PCP)IrI, to enable a comparison of electronic and steric effects with these archetypal pincer ligands. Attempts to reduce (XyPhDI)IrBr2(MeCN) to a hydride or an IrI complex yielded a dinuclear CN-bridged complex with a methyl ligand on the cyanide-C-bound Ir center (characterized by scXRD), indicating that C-CN bond cleavage took place at that Ir center. DFT calculations indicate that the C-CN bond cleavage occurs at one Ir center with strong assistance by coordination of the CN nitrogen to the other Ir center. 

Pincer-ligated iridium complexes have been widely developed, and (pincer)Ir(III) complexes, particularly five-coordinate, are central to their chemistry. Such complexes typically bear two formally anionic ligands in addition to the pincer ligand itself. Yet despite the prevalence of halides as anionic ligands in transition metal chemistry there are relatively few examples in which both of these ancillary anionic ligands are halides or even other monodentate low-field anions. We report a study of the fragment (iPrPCP)IrCl2 (iPrPCP = 3-2,6-C6H3(CH2PiPr2)), and adducts thereof. These species are found to be thermodynamically disfavored relative to the corresponding hydridohalides. For example, DFT calculations and experiment indicate that one Ir-Cl bond of (iPrPCP)IrCl2 complexes will undergo reaction with H2 to give the (iPrPCP)IrHCl or an adduct thereof. In the presence of aqueous HCl, (iPrPCP)IrCl2 adds a chloride ion to give an unusual example of an anionic transition metal complex ((iPrPCP)IrCl3–) with a Zundel cation (H5O2+). (iPrPCP)IrCl2 is not stable as a monomer at room temperature but exists in solution as a mixture of clusters which can add various small molecules. DFT calculations indicate that dimerization of (iPrPCP)IrCl2 is more favorable than dimerization of (iPrPCP)IrHCl, in accord with its observed tendency to form clusters.