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The mechanism of C(sp2)-H borylation of fluorinated arenes with B2Pin2 (Pin = pinacolato) catalyzed by bis(phosphino)pyridine (iPrPNP) cobalt complexes was studied to understand the origins of the uniquely high ortho-to-fluorine regioselectivity observed in these reactions. Variable time normalization analysis (VTNA) of reaction time courses and deuterium kinetic isotope effect measurements established a kinetic regime wherein C(sp2)-H oxidative addition is fast and reversible. Monitoring the reaction by in situ NMR spectroscopy revealed the intermediacy of a cobalt(I)-aryl complex that was generated with the same high ortho-to-fluorine regioselectivity associated with the overall catalytic transformation. Deuterium labeling experiments and stoichiometric studies established C(sp2)-H oxidative addition of the fluorinated arene as the selectivity-determining step of the reaction. This step favors the formation of ortho-fluoroaryl cobalt intermediates due to the ortho fluorine effect, a phenomenon whereby ortho fluorine substituents stabilize transition metal-carbon bonds. Computational studies provided evidence that the cobalt-carbon bonds of the relevant intermediates in (iPrPNP)Co-catalyzed borylation are strengthened with increasing ortho fluorine substitution. The atypical kinetic regime involving fast and reversible C(sp2)-H oxidative addition in combination with the thermodynamic preference for forming cobalt-aryl bonds adjacent to fluorinated sites are the origin of the high regioselectivity in the catalytic borylation reaction.

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BACKGROUND: Stannous fluoride (SnF2) is a compound present in many commercially available dentifrices; however, oxidative decomposition negatively impacts its efficacy. Stannous oxidation is often mitigated through the addition of complexing agents or sources of sacrificial stannous compounds. The authors have found that the addition of zinc phosphate significantly improved stannous stability more effectively than other stabilization methods. The authors evaluated the chemical speciation of stannous compounds within a variety of formulations using x-ray absorption near edge spectroscopy (XANES), a technique never used before in this manner. These data were compared and correlated with several antimicrobial experiments. METHODS: XANES data of various commercially available compounds and Colgate TotalSF were performed and analyzed against a library of reference compounds to determine the tin chemical speciation. The antibacterial assays used were salivary adenosine triphosphate, short-interval kill test, plaque glycolysis, and anaerobic biofilm models. RESULTS: XANES spectra showed a diverse distribution of tin species and varying degrees of SnF2 oxidation. In vitro antimicrobial assessment indicated significant differences in performance, which may be correlated to the differences in tin speciation and oxidation state. CONCLUSIONS: Driven by the excipient ingredients, SnF2 dentifrices contain a distribution of tin species in either the SnF2 or Sn(IV) oxidation state. The addition of zinc phosphate provided significant robustness against oxidation, which directly translated to greater efficacy against bacteria. PRACTICAL IMPLICATIONS: The choice of inactive ingredients in a dentifrice with active SnF2 can dramatically impact product stability.

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The hydrogenolysis of titanium-nitrogen bonds in a series of bis(cyclopentadienyl) titanium amides, hydrazides and imides by proton coupled electron transfer (PCET) is described. Twelve different N-H bond dissociation free energies (BDFEs) among the various nitrogen-containing ligands were measured or calculated, and effects of metal oxidation state and N-ligand substituent were determined. Two metal hydride complexes, (η5-C5Me5)(py-Ph)Rh-H (py-Ph = 2-pyridylphenyl, [Rh]-H) and (η5-C5R5)(CO)3Cr-H ([Cr]R-H, R= H, Me) were evaluated for formal H atom transfer reactivity and were selected due to their relatively weak M-H bond strengths yet ability to activate and cleave molecular hydrogen. Despite comparable M-H BDFEs, disparate reactivity between the two compounds was observed and was traced to the vastly different acidities of the M-H bonds and overall redox potentials of the molecules. With [Rh]-H, catalytic syntheses of ammonia, silylamine and N,N-dimethylhydrazine have been accomplished from the corresponding titanium(IV) complex using H2 as the stoichiometric H atom source. The data presented in this study provides the thermochemical foundation for the synthesis of NH3 by proton coupled electron transfer at a well-defined transition metal center.

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A comprehensive study into the mechanism of bis(phosphino)pyridine (PNP) cobalt-catalyzed C-H borylation of 2,6-lutidine using B2Pin2 (Pin = pinacolate) has been conducted. The experimentally observed rate law, deuterium kinetic isotope effects, and identification of the catalyst resting state support turnover limiting C-H activation from a fully characterized cobalt(I) boryl intermediate. Monitoring the catalytic reaction as a function of time revealed that borylation of the 4-position of the pincer in the cobalt catalyst was faster than arene borylation. Cyclic voltammetry established the electron withdrawing influence of 4-BPin, which slows the rate of C-H oxidative addition and hence overall catalytic turnover. This mechanistic insight inspired the next generation of 4-substituted PNP cobalt catalysts with electron donating and sterically blocking methyl and pyrrolidinyl substituents that exhibited increased activity for the C-H borylation of unactivated arenes. The rationally designed catalysts promote effective turnover with stoichiometric quantities of arene substrate and B2Pin2. Kinetic studies on the improved catalyst, 4-(H)2BPin, established a change in turnover limiting step from C-H oxidative addition to C-B reductive elimination. The iridium congener of the optimized cobalt catalyst, 6-(H)2BPin, was prepared and crystallographically characterized and proved inactive for C-H borylation, a result of the high kinetic barrier for reductive elimination from octahedral Ir(III) complexes.

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Synthesis of coordinatively unsaturated Cp*Co(IPr) (2), is accomplished by addition of free N-heterocyclic carbene IPr to [(Cp*Co)2-µ-(η(4):η(4)-toluene)] (1). Stoichiometric reactivity is consistent with a 16 electron species, as 2 undergoes ligand addition/NHC displacement and reversible reaction with dihydrogen. Cp*Co(IPr) represents an elusive example of a stable Cp*CoL fragment.

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Cobalt dialkyl and bis(carboxylate) complexes bearing α-diimine ligands have been synthesized and demonstrated as active for the C(sp(3))-H borylation of a range of substituted alkyl arenes using B2Pin2 (Pin = pinacolate) as the boron source. At longer reaction times, rare examples of polyborylation were observed, and in the case of toluene, all three benzylic C-H positions were functionalized. Coupling benzylic C-H activation with alkyl isomerization enabled a base-metal-catalyzed method for the borylation of remote, unactivated C(sp(3))-H bonds.

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The interactions between commercial antiperspirant (AP) salts [aluminum chlorohydrate (ACH), activated ACH, aluminum sesquichlorohydrate (ASCH), zirconium aluminum glycine (ZAG), activated ZAG), pure aluminum polyoxocations (Al13-mer, Al30-mer), and the zirconium(IV)-glycine complex Zr6 (O)4 (OH)4 (H2O)8 (Gly)8]12+(-) (CP-2 or ZG) with Bovine serum albumin (BSA) were studied using zeta potential and turbidity measurements. The maximal turbidity, which revealed the optimal interactions between protein and metal salts, for all protein-metal salt samples was observed at the isoelectric point (IEP), where the zeta potential of the solution was zero. Efficacy of AP salts was determined via three parameters: the amount of salt required to flocculate BSA to reach IEP, the turbidity of solution at the IEP, and the pH range over which the turbidity of the solution remains sufficiently high. By comparing active salt performance from this work to traditional prescreening methods, this methodology was able to provide a consistent efficacy assessment for metal actives in APs or in water treatment.

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A bis(imino)pyridine cobalt-catalyzed hydroboration of terminal alkynes with HBPin (Pin = pinacolate) with high yield and (Z)-selectivity for synthetically valuable vinylboronate esters is described. Deuterium labeling studies, stoichiometric experiments, and isolation of catalytically relevant intermediates support a mechanism involving selective insertion of an alkynylboronate ester into a Co-H bond, a pathway distinct from known precious metal catalysts where metal vinylidene intermediates have been proposed to account for the observed (Z) selectivity. The identity of the imine substituents dictates the relative rates of activation of the cobalt precatalyst with HBPin or the terminal alkyne and, as a consequence, is responsible for the stereochemical outcome of the catalytic reaction.

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The catalytic hydrogenolysis of the titanium-amide bond in (η(5)-C5Me4SiMe3)2Ti(Cl)NH2 to yield free ammonia is described. The rhodium hydride, (η(5)-C5Me5)(py-Ph)RhH (py-Ph = 2-phenylpyridine), serves as the catalyst and promotes N-H bond formation via hydrogen atom transfer. The N-H bond dissociation free energies of ammonia ligands have also been determined for titanocene and zirconocene complexes and reveal a stark dependence on metal identity and oxidation state. In all cases, the N-H BDFEs of coordinated NH3 decreases by >40 kcal/mol from the value in the free gas phase molecule.

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The addition of carbon dioxide to ((tBu)PNP)CoH [(tBu)PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine] resulted in rapid insertion into the Co-H bond to form the corresponding κ(1)-formate complex, which has been structurally characterized. Treatment of ((tBu)PNP)CoH with PhSiH3 resulted in oxidative addition to form trans-((tBu)PNP)CoH2(SiH2Ph), which undergoes rapid exchange with excess free silane. With 0.5 mol % ((tBu)PNP)CoH, the catalytic hydrosilylation of CO2 with PhSiH3 to a mixture of oligomers containing silyl formate, bis(silyl)acetyl, and silyl ether subunits has been observed.

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Addition of terminal or internal alkynes to a base-free titanocene oxide results in synthesis of the corresponding oxometallocyclobutene. With appropriate cyclopentadienyl substitution, these compounds undergo reversible C-C reductive elimination offering a unique approach to cyclopentadienyl modification.

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Partial hydrolysis of AlCl3 with Ca(OH)2 and the amino acid glycine enables the selective transformation of the Al13 Keggin structures, outlining the ε → δ → γ isomerization process. Through this, a new γ-Al13 Keggin structure was able to be isolated and characterized through (27)Al NMR and single-crystal XRD.

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A discrete, hexanuclear zirconium metallocycle has been synthesized, isolated, and characterized by single-crystal X-ray diffraction. It is sustained in a chair-like conformation by glycolic acids. Formation and growth of the complex has been observed using in situ time-resolved dynamic light scattering measurements.

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