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Pyridine and quinoline undergo selective C–H activation in the 2-position with Rh and Ir complexes of a boryl/bis(phosphine) PBP pincer ligand, resulting in a 2-pyridyl bridging the transition metal and the boron center. Examination of this reactivity with Rh and Ir complexes carrying different non-pincer ligands on the transition metal led to the realization of the possible isomerism derived from the 2-pyridyl fragment connecting either via B–N/C–M bonds or via B–C/N–M bonds. This M–C/M–N isomerism was systematically examined for four structural types. Each of these types has a defined set of ligands on Rh/Ir besides 2-pyridyl and PBP. A pair of M–C/M–N isomers for each type was computationally examined for Rh and for Ir, totaling 16 compounds. Several of these compounds were isolated or observed in solution by experimental methods, in addition to a few 2-quinolyl variants. The DFT predictions concerning the thermodynamic preference within each M–C/M–N isomeric match the experimental findings very well. In two cases where DFT predicts, The thermodynamic preference between two isomeric products of C–H activation of pyridine, with 2-pyridyl bridging boron and iridium or rhodium, primarily depends on the M–C bond strength.

This report examines reactions of a series of Ir complexes supported by the diarylboryl/bis(phosphine) PBP pincer ligand with ethylene: (PBP)IrH4 (1), (PBP)IrH2(CO) (2), and (PBP)Ir(CO)2 (3). The outcomes of these reactions differ from those typical for Ir complexes supported by other pincer ligands and do not give rise to simple ethylene adducts or products of insertion of Ir into the C–H bond of ethylene. Instead, the elements of ethylene are incorporated into the molecules to result in B–C bonds. In the case of 2 and 3, ethylene addition results in the formation of B/Ir bridging ethylidene complexes 5 and 6. For 6, the addition of ethylene (and the analogous addition of 1-hexene) is shown to be partially reversible. Addition of ethylene to 2 and 3 is remarkable because they are saturated at Ir and yet the net outcome is such that ethylene binds without replacing any ligands already present. A mechanistic inquiry suggests that dissociation of CO from 3 or 6 is necessary in order for the addition or loss of ethylene to proceed., (PBP)Ir pincer complexes containing a boryl-iridium linkage reversibly bind ethylene as an ethylidene bridging B and Ir.

(PBP)Ir(CO)2 possesses a Lewis acidic boryl center and a basic monovalent Ir center. Heterolytic cleavage of O–H, N–H, and F–H bonds by this molecule was examined. (PBP)Ir(CO)2 reacted with water, methanol, ethanol, cyclohexanol, phenols, benzoic acid, and triethylamine trihydrofluoride to add the O–H and F–H units across the B–Ir bond with preservation of both carbonyl ligands. For the benzoate adduct, one of the CO ligands is lost upon thermolysis with the benzoate bridging B and Ir. Thermolysis of (PBP)Ir(CO)2 with nBuNH2 ultimately resulted in a monocarbonyl product of the N–H cleavage with the resultant amido group bridging B and Ir. Dicarbonyl intermediates were observed in this reaction. Structures of the X–H cleavage products and the possible mechanistic pathways are discussed. Mechanistic experiments indicate that the reaction with ROH substrates proceeds by initial protonation of Ir, consistent with slower reactions with less acidic ROH substrates (and no reaction with tBuOH). For amines, the re...

Syntheses of Re and Mn carbonyl complexes of the diarylamido/bis(phosphine) PNP pincer ligand are reported. The saturated (PNP)Re(CO)3 and (PNP)Mn(CO)3 react cleanly with HBr in acetic acid to undergo decarbonylation with concomitant addition of HBr across the N-M bond. The resultant (PNHP)MBr(CO)2 (M = Mn or Re) complexes can be dehydrobrominated with triethylamine to yield the unsaturated (PNP)M(CO)2. (PNP)Re(CO)2 has high affinity for CO and abstracts CO from (PNP)Mn(CO)3.

Correction for ‘Cooperative C–H activation of pyridine by PBP complexes of Rh and Ir can lead to bridging 2-pyridyls with different connectivity to the B–M unit’ by Yihan Cao et al., Chem. Sci., 2021, 12, 14167–14173, https://doi.org/10.1039/D1SC01850G.

A series of Rh complexes derived from a PBP-type pincer ligand have been synthesized and characterized. It was previously reported that reaction of [(COD)RhCl]2 with 2,2′-bis(diisopropylphino)triphenylborane (1) resulted in a mixture of complexes containing a Z-type borane interaction (2-Cl), a boryl pincer (3a-Cl), and a η2 binding of the B–Ph bond to Rh (4-Cl). In this work, we demonstrate that analogous complexes are accessible by replacement of chloride with potentially bidentate acetylacetonate, carboxylate, and trifluoromethanesulfonate ligands. In addition, a new type of isomer was observed in complexes with acetate and pivalate, where the carboxylate bridges between Rh and B (3b-OAc, 3b-OPiv). All of these types of complexes are isomeric, and the preference for particular isomers for different anionic ligands varies. These isomers differ and are related by a change in the coordination mode of the oxygenous ligands and the migration of the Ph group between B and Rh.

This work reports on the synthesis of several new complexes of Ir supported by a diarylboryl/bis(phosphine) PBP pincer ligand. The previously reported complexes (PBP)Ir(Ph)(Cl) (1) and (PBP)Ir(H)(Cl) (2) were converted to the new complexes (PBP)IrH4 (3) and (PBP)Ir(Ph)(H) (4). Complexes 3 and 4 serve similarly as precatalysts for transfer dehydrogenation of cyclooctane. The turnover numbers achieved were relatively modest but were increased (to 220 at 200 °C) when 1-hexene was used as a sacrificial hydrogen acceptor vs tert-butylethylene. The dicarbonyl complex (PBP)Ir(CO)2 (6) was also synthesized, by the reaction of CO with either 3 or 4. Intermediates (PBPhP)Ir(H)(CO)2 (5) and (PBP)IrH2(CO) (7) were observed in these reactions. Complex 7 could be obtained in pure form by comproportionation of 3 and 6. Solid-state structures of 3 and 6 were determined by X-ray crystallography.

Silver halide complexes of a borane/bis(phosphine) ligand have been prepared and characterized. With AgF, the borane abstracts fluoride, resulting in a zwitterionic complex. With AgCl, AgBr, and AgI, the halide stays coordinated to Ag, with little to no Ag-B interaction.

Complexes (PCPR)Re(O)(OAc)2 [R = iPr (4a) and tBu (4b); PCP = κ3-P,C,P-2,6-(R2PCH2)2C6H3] undergo unexpected irreversible hydrolysis to yield (PCPR)Re(O)(OAc)(OH) (3a/3b) and free AcOH. 3a and 3b are highly fluxional in solution, possibly via AcOH loss and the intermediacy of (PCPR)Re(O)2, which was isolated for R = tBu (5b).

The unexpectedly facile insertion of Rh or Ir into a B-Ph bond (reversible for Rh) converts a borane/bis(phosphine) precursor into a boryl/bis(phosphine) PBP pincer ligand. Interconversions between the boryl/borane/borate central functionality are demonstrated in reactions with dihydrogen.

A novel PNN-type pincer ligand has been accessed via imine reduction with LiAlH4 to provide the phosphine diamino proligand, PNHNH (1). The ligand, 1, as well as PNHP can be metallated directly, via N–H cleavage, with L2ReOX2(OEt) precursors to access six-coordinate (PNP)ReOCl2 (2) and (PNNH)ReOX2 (3-Cl, X = Cl; 3-Br, X = Br) in good yield. 3-Cl and 3-Br undergo dehydrohalogenation upon treatment with NEt3, furnishing the five-coordinate phosphine/diamido PNN-type compounds (PNN)ReOX (4-Cl, X = Cl; 4-Br, X = Br) in excellent yield, presenting as a mixture of rotameric diasteromers. The reversibility of this deprotonation, and the coordinative unsaturation of 4-Cl is shown in reactions with HCl(aq) and PMe3 providing 3-Cl and 4-PMe3, respectively. Treatment of 4-Cl with AgOAc, AgOTf, or NaHBEt3 lead to formation of (PNN)ReO(OAc) (4-OAc), (PNN)ReO(OTf) (4-OTf), and (PNN)ReO(H) (4-H), all isolated in excellent yields in varying diasteromeric ratios. The nature of the isomerism was analyzed based on solid-state structural studies and solution NMR data.

A two-step reaction to convert terminal alkynes into triborylalkenes is reported. In the first step, the terminal alkyne and pinacolborane (HBpin) are converted into an alkynylboronate, which is catalyzed by an iridium complex supported by a SiNN pincer ligand. In the second step, treatment of the reaction mixture with CO generates a new catalyst which mediates dehydrogenative diboration of alkynylboronate with pinacolborane. The mechanism of the diboration remains unclear but it does not proceed via intermediacy of hydroboration products or via B2 pin2.

A one-pot synthesis of arene-based PCP/PNP ligands has been developed. The reaction of 1,3-bis(bromomethyl)benzene or 2,6-bis(bromomethyl)pyridine with various chlorophosphines in acetonitrile afforded bis-phosphonium salts. These salts can then be reduced by magnesium powder to yield PCP or PNP ligands. In comparison to traditional synthetic methods for making PCP/PNP ligands involving the use of secondary phosphines, this new alternative method allows for the use of chlorophosphines, which are cheaper, safer to handle, and have a broader range of commercially available derivatives. This is especially true for the chlorophosphines with less bulky alkyl groups. Moreover, the one-pot procedure can be extended to allow for the direct synthesis of PCP/PNP nickel complexes. By using nickel powder as the reductant, the resulting nickel halide was found to directly undergo metalation with the PCP or PNP ligand to generate nickel complexes in high yields.

The stereochemistry, kinetics, and mechanism of olefin polymerization catalyzed by a set of zirconium-based metallocenes was studied by NMR using dissolution dynamic nuclear polarization (DNP). Hyperpolarized 1-hexene was polymerized in situ with a C2 symmetric catalyst, [(EBI)ZrMe][B(C6F5)4] (EBI = rac-(C2H4(1-indenyl)2)), and a C2v symmetric catalyst, [(Cp)2ZrMe][B(C6F5)4] (Cp = cyclopentadienyl). Hyperpolarized 13C NMR spectra were used to characterize product tacticity following initiation of the reaction. At the same time, a signal gain of 3 orders of magnitude from 13C hyperpolarization enabled the real time observation of catalyst-polymeryl species and deactivation products, such as vinylidene and a Zr-allyl complex. The compounds appearing in the reaction provide evidence for the existence of β-hydride elimination and formation of a dormant site via a methane-generating mechanism. The presence of a deactivating mechanism was incorporated in a model used to determine kinetic parameters of the react...

Transition-metal mediated C-H functionalization has emerged as a powerful method in the chemistry relevant to the synthesis of pharmaceuticals, agrochemicals, and advanced materials. Because organic molecules typically contain multiple types of C-H bonds, selective C-H functionalization is a major ongoing challenge. C-H activation of heteroatom-containing organics has often been approached via the use of the directing effect, whereby the coordination to the basic heteroatom directs the reactive metal center to a specific C-H bond. We now report a different approach where the nitrogen donor in pyridine derivatives coordinates to an ancillary Lewis acidic boryl ligand directly attached to the metal (iridium) center, as opposed to the metal itself. This topology directs the iridium center to activate a different C-H bond than in the cases of directing donor coordination to the metal. Using this strategy, we demonstrate ortho-regiospecific C-H activation of pyridines and an example of the subsequent functionalization via C-C bond formation.

While investigating rhodium-catalyzed Negishi coupling, it was observed that the (PNP)Rh fragment readily inserted into zinc–carbon bonds to form isolable molecules with covalent rhodium–zinc bonds.

The first Ni-promoted prototype reaction based on the tandem C-H activation of heteroarenes with alkene isomerization is demonstrated, leading to the branched hydroheteroarylation products. Simultaneously, the reaction selectivity can be chemically switched to linear adducts through Ni-Al tandem catalysis.

The two-step approach for the synthesis of triborylalkenes starts with the conversion of terminal alkynes and pinacolborane (HBpin) into alkynylboronate catalyzed by a SiNN pincer ligand supported iridium complex, followed by treatment of the reaction mixture with CO for the generation of a new catalyst.

The stereochemistry, kinetics, and mechanism of olefin polymerization catalyzed by a set of zirconium-based metallocenes was studied by NMR using dissolution dynamic nuclear polarization (DNP). Hyperpolarized 1-hexene was polymerized in situ with a C2 symmetric catalyst, [(EBI)ZrMe][B(C6F5)4] (EBI = rac-(C2H4(1-indenyl)2)), and a C2v symmetric catalyst, [(Cp)2ZrMe][B(C6F5)4] (Cp = cyclopentadienyl). Hyperpolarized (13)C NMR spectra were used to characterize product tacticity following initiation of the reaction. At the same time, a signal gain of 3 orders of magnitude from (13)C hyperpolarization enabled the real time observation of catalyst-polymeryl species and deactivation products, such as vinylidene and a Zr-allyl complex. The compounds appearing in the reaction provide evidence for the existence of β-hydride elimination and formation of a dormant site via a methane-generating mechanism. The presence of a deactivating mechanism was incorporated in a model used to determine kinetic parameters of the reaction. On this basis, rate constants were measured between 0.8 and 6.7 mol % of catalyst. The concentration dependence of the rate constants obtained indicates a second-order process for polymerization concomitant with a first-order process for deactivation. The simultaneous observation of both processes in the time evolution of (13)C NMR signals over the course of several seconds underlines the utility of hyperpolarized NMR for quantifying early events in polymerization reactions.

The first Ni-promoted prototype reaction based on the tandem C–H activation of heteroarenes with alkene isomerization is demonstrated, leading to the branched hydroheteroarylation products. Simultaneously, the reaction selectivity can be chemically switched to linear adducts through Ni–Al tandem catalysis.

We previously reported a highly efficient protocol for bimetallic Ni-Al-catalyzed hydroheteroarylation of styrene with benzimidazole based on C-H bond activation. We have now delineated the mechanism of this process, providing a rationale for an observed switch in regioselectivity in the presence of the Lewis acid, AlMe3. The present mechanistic study gives insights for the rational development of catalysts that exhibit required linear/branched selectivity.

An amino-linked nitrogen heterocyclic carbene mediates the carbon—carbon coupling reaction through direct C—H functionalization of benzene (II) and pyridine (IV) with aryl iodides (I) in the absence of a metal catalyst.

We have disclosed a new mode of a chemically regioselective switch for C–H bond functionalization of benzimidazole derivatives via a cooperative effect invoked by Ni–Al bimetallic catalysis to create a steric requirement for obtaining the linear product of styrene insertion. Yet, excluding the AlMe3 cocatalyst switches the reaction toward branch selectivity.

In this paper, a novel broadband miniaturized antenna is presented. The proposed antenna can provide a wide operating bandwidth for DTV signal reception in the UHF band in mobile handheld devices. This antenna was utilizing eight quarter-wavelength slots with a compact size of 65×8.5 mm2. In proposed antenna, the L-shape 50-ohm microstrip line coupling fed was used to achieve broadband impedance matching. The antenna is able to realize an onmidirectional radiation characteristic across the entire operating band.

An amino-linked nitrogen heterocyclic carbene (amino-NHC), 1-tBu, has been shown to mediate carbon–carbon coupling through the direct C–H functionalization of benzene and pyridine in the absence of a metal catalyst. Using EPR, the first spectroscopic evidence corroborating the single electron transfer mechanism for the metal-free carbon–carbon coupling manifold, as reported by others, is introduced.

A convenient method for the synthesis of highly substituted isoquinolines and isoquinolinium salts by the nickel-catalyzed cyclization of ortho-haloketoximes and -ketimines, respectively, with alkynes is described. The reaction of ortho-haloketoximes and various alkynes in the presence of [Ni(PPh(3))(2)Br(2)] and zinc powder in a mixture of acetonitrile and tetrahydrofuran at 80 °C for 15 hours gave 1,3,4-trisubstituted isoquinoline products in moderate to excellent yields and high regioselectivity. The corresponding isoquinoline N-oxide was found to be the intermediate in the cyclization reaction pathway. In contrast, the reaction of ortho-haloketimines and alkynes under similar catalytic conditions in tetrahydrofuran at 70 °C for two hours gave 1,2,3,4-tetrasubstituted isoquinolinium salts in good to excellent yields.

A convenient method for the synthesis of highly substituted isoquinolines and isoquinolinium salts by the nickel-catalyzed cyclization of ortho-haloketoximes and -ketimines, respectively, with alkynes is described. The reaction of ortho-haloketoximes and various alkynes in the presence of [Ni(PPh(3))(2)Br(2)] and zinc powder in a mixture of acetonitrile and tetrahydrofuran at 80 °C for 15 hours gave 1,3,4-trisubstituted isoquinoline products in moderate to excellent yields and high regioselectivity. The corresponding isoquinoline N-oxide was found to be the intermediate in the cyclization reaction pathway. In contrast, the reaction of ortho-haloketimines and alkynes under similar catalytic conditions in tetrahydrofuran at 70 °C for two hours gave 1,2,3,4-tetrasubstituted isoquinolinium salts in good to excellent yields.