Although scores of transition metal complexes incorporating ammonia or water ligands have been characterized over the past century, little is known about how coordination influences the strength of the nitrogen-hydrogen and oxygen-hydrogen bonds. Here we report the synthesis of a molybdenum ammonia complex supported by terpyridine and phosphine ligands that lowers the nitrogen-hydrogen bond dissociation free energy from 99.5 (gas phase) to an experimentally measured value of 45.8 kilocalories per mole (agreeing closely with a value of 45.1 kilocalories per mole calculated by density functional theory). This bond weakening enables spontaneous dihydrogen evolution upon gentle heating, as well as the hydrogenation of styrene. Analogous molybdenum complexes promote dihydrogen evolution from coordinated water and hydrazine. Electrochemical and theoretical studies elucidate the contributions of metal redox potential and ammonia acidity to this effect.A mmonia and water are ubiquitous small molecules with strong bonds between hydrogen and the central atom (1). For over a century, transition metal-ammine (NH 3 ) and -aquo (H 2 O) compounds have defined bonding paradigms in chemistry (2), found application in cancer therapy (3), and promoted important fundamental chemical reactions such as electron transfer that rely on the inertness of the N-H or O-H bonds in the supporting ligands ( Fig. 1) (4).Common strategies for activation of ammonia and water include oxidative addition to a transition metal center (5-7), deprotonation by transition metal hydrides (8), reaction with bimetallic compounds (9), cooperative chemistry between a transition metal and a supporting ligand (10-12), and element-hydrogen (X-H) bond cleavage through reaction with main group compounds (13-16). Using most of these strategies, activation of the strong X-H bond is not typically coupled to H-H bond formation. One exception is the observation of H 2 elimination following oxidative addition of ammonia to a tantalum(III) complex (17).An alternative and less commonly explored strategy is homolytic cleavage of the X-H bond. Because of the high gas-phase bond dissociation free energies (BDFEs; 99.5 and 111.0 kcal/mol for NH 3 and H 2 O, respectively) (1), interaction with a transition metal or other appropriate reagent is necessary to induce bond weakening. As shown in Fig. 1, most classical transition metal compounds with ammine (aquo) ligands have N-H (O-H) bond strengths that are only slightly perturbed from the gas-phase value. Because experimental data are lacking, we used density functional theory (DFT) to compute N-H BDFEs.Coordination-induced bond weakening, whereby interaction of a ligand results in a considerable lowering of the X-H BDFE, has recently been identified or implicated in rare instances (18-23) and has been applied by Knowles's group (24) and others (25-27) in reactions of organic molecules involving N-H and O-H bonds, respectively. Cuerva's group (26, 27) and ours (28) However, this strategy has not yet been shown to be capable of ...
A cobalt-catalyzed method for the 1,1-diboration of terminal alkynes with bis(pinacolato)diboron (B2Pin2) is described. The reaction proceeds efficiently at 23 °C with excellent 1,1-selectivity and broad functional group tolerance. With the unsymmetrical diboron reagent PinB–BDan (Dan = naphthalene-1,8-diaminato), stereoselective 1,1-diboration provided products with two boron substituents that exhibit differential reactivity. One example prepared by diboration of 1-octyne was crystallized, and its stereochemistry established by X-ray crystallography. The utility and versatility of the 1,1-diborylalkene products was demonstrated in a number of synthetic applications, including a concise synthesis of the epilepsy medication tiagabine. In addition, a synthesis of 1,1,1-triborylalkanes was accomplished through cobalt-catalyzed hydroboration of 1,1-diborylalkenes with HBPin. Deuterium-labeling and stoichiometric experiments support a mechanism involving selective insertion of an alkynylboronate to a Co–B bond of a cobalt boryl complex to form a vinylcobalt intermediate. The latter was isolated and characterized by NMR spectroscopy and X-ray crystallography. A competition experiment established that the reaction involves formation of free alkynylboronate and the two boryl substituents are not necessarily derived from the same diboron source.
Cobalt catalysts with electronically enhanced site selectivity have been developed, as evidenced by the high ortho-to-fluorine selectivity observed in the C(sp2)–H borylation of fluorinated arenes. Both the air-sensitive cobalt(III) dihydride boryl 4-Me-(iPrPNP)Co(H)2BPin (1) and the air-stable cobalt(II) bis(pivalate) 4-Me-(iPrPNP)Co(O2 CtBu)2 (2) compounds were effective and exhibited broad functional group tolerance across a wide range of fluoroarenes containing electronically diverse functional groups, regardless of the substitution pattern on the arene. The electronically enhanced ortho-to-fluorine selectivity observed with the cobalt catalysts was maintained in the presence of a benzylic dimethylamine and hydrosilanes, overriding the established directing-group effects observed with precious-metal catalysts. The synthetically useful selectivity observed with cobalt was applied to an efficient synthesis of the anti-inflammatory drug flurbiprofen.
A bench-stable, 4-aryl-substituted terpyridine supported, high-spin cobalt(II) bis(acetate) complex, ( Ar Tpy)Co-(OAc) 2 ( Ar Tpy = 4′-(4-N,N′-dimethylaminophenyl)-2,2′:6′,2″terpyridine), is active for the C(sp 2 )−H borylation of arenes and heteroarenes with B 2 Pin 2 (Pin = pinacolato). Optimization of the catalytic borylation reaction revealed improved performance in the presence of LiOMe and turnover numbers of up to 100 have been observed using all air-stable components. EPR specstroscopy identified formation of inactive cobalt species, promoted by excess HBPin. A high-spin cobalt(II) bis[(diacetoxy)pinacolatoborate−κ 3 O,O,O] compound has been isolated and characterized by X-ray diffraction and is the result of catalyst deactivation.
Treatment of the bis(imino)pyridine molybdenum η-benzene complex (PDI)Mo(η-CH) (PDI, 2,6-(2,6-iPrCHN═CMe)CHN) with NH resulted in coordination induced haptotropic rearrangement of the arene to form (PDI)Mo(NH)(η-CH). Analogous η-ethylene and η-cyclohexene complexes were also synthesized, and the latter was crystallographically characterized. All three compounds undergo loss of the η-coordinated ligand followed by N-H bond activation, bis(imino)pyridine modification, and H loss. A dual ammonia activation approach has been discovered whereby reversible M-L cooperativity and coordination induced bond weakening likely contribute to dihydrogen formation. Significantly, the weakened N-H bonds in (PDI)Mo(NH)(η-CH) enabled hydrogen atom abstraction and synthesis of a terminal nitride from coordinated ammonia, a key step in NH oxidation.
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