Máté J. Bezdek

Coordination-induced weakening of ammonia, water, and hydrazine X-H bonds in a molybdenum complex.

Coordinated scission of N–H or O–H bonds Ammonia and water both have well-explored acid-base chemistry at room temperature, revolving around proton exchange. In contrast, radical chemistry involving H-atom exchange is comparatively rare in these molecules in the absence of a high-energy stimulus. Bezdek et al. now show that coordination of ammonia or water to a molybdenum complex substantially weakens the N–H or O–H bonds, so much so that heating to 60°C liberates hydrogen (see the Perspective by Hoover). Theoretical and electrochemical analyses reveal the underpinnings of the bond-weakening phenomenon. Science, this issue p. 730; see also p. 707 Breaking N–H and O–H bonds in ammonia and water is easier when the N and O atoms bind to a molybdenum center. 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.

Cobalt-Catalyzed 1,1-Diboration of Terminal Alkynes: Scope, Mechanism, and Synthetic Applications

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.

C(sp2)–H Borylation of Fluorinated Arenes Using an Air-Stable Cobalt Precatalyst: Electronically Enhanced Site Selectivity Enables Synthetic Opportunities

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(O2CtBu)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.

Expanding Boundaries: N2 Cleavage and Functionalization beyond Early Transition Metals

Early transition metals are well known to catalyze the cleavage and functionalization of N2 . In this Highlight, recent work showing that a rhenium catalyst is also capable of carrying out this difficult task is summarized, and a synthetic cycle for the stoichiometric incorporation of atmospheric N2 into acetonitrile is presented.

Insight into Transmetalation Enables Cobalt-Catalyzed Suzuki–Miyaura Cross Coupling

Among the fundamental transformations that comprise a catalytic cycle for cross coupling, transmetalation from the nucleophile to the metal catalyst is perhaps the least understood. Optimizing this elementary step has enabled the first example of a cobalt-catalyzed Suzuki–Miyaura cross coupling between aryl triflate electrophiles and heteroaryl boron nucleophiles. Key to this discovery was the preparation and characterization of a new class of tetrahedral, high-spin bis(phosphino)pyridine cobalt(I) alkoxide and aryloxide complexes, (iPrPNP)CoOR, and optimizing their reactivity with 2-benzofuranylBPin (Pin = pinacolate). Cobalt compounds with small alkoxide substituents such as R = methyl and ethyl underwent swift transmetalation at 23 °C but also proved kinetically unstable toward β–H elimination. Secondary alkoxides such as R = iPr or CH(Ph)Me balanced stability and reactivity. Isolation and structural characterization of the product following transmetalation, (iPrPNP)Co(2-benzofuranyl), established a planar, diamagnetic cobalt(I) complex, demonstrating the high- and low-spin states of cobalt(I) rapidly interconvert during this reaction. The insights from the studies in this elementary step guided selection of appropriate reaction conditions to enable the first examples of cobalt-catalyzed C–C bond formation between neutral boron nucleophiles and aryl triflate electrophiles, and a model for the successful transmetalation reactivity is proposed.

Ammonia Activation, H2 Evolution and Nitride Formation from a Molybdenum Complex with a Chemically and Redox Noninnocent Ligand

Treatment of the bis(imino)pyridine molybdenum η6-benzene complex (iPrPDI)Mo(η6-C6H6) (iPrPDI, 2,6-(2,6-iPr2C6H3N═CMe)2C5H3N) with NH3 resulted in coordination induced haptotropic rearrangement of the arene to form (iPrPDI)Mo(NH3)2(η2-C6H6). Analogous η2-ethylene and η2-cyclohexene complexes were also synthesized, and the latter was crystallographically characterized. All three compounds undergo loss of the η2-coordinated ligand followed by N-H bond activation, bis(imino)pyridine modification, and H2 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 (iPrPDI)Mo(NH3)2(η2-C2H4) enabled hydrogen atom abstraction and synthesis of a terminal nitride from coordinated ammonia, a key step in NH3 oxidation.

Thermodynamics of N–H bond formation in bis(phosphine) molybdenum(II) diazenides and the influence of the trans ligand

A series of bis(phosphine) molybdenum(ii) diazenides [(dppe)2Mo(NNCy)(I)], [(dppe)2(CH3CN)Mo(NNCy)][BArF24] and [(dppe)2)(3,5-(CF3)2C6H3CN)Mo(NNCy)][BArF24] (dppe = 1,2-bis(diphenylphosphino)ethane; Cy = cyclohexyl; ArF24 = (3,5-(CF3)2C6H3)4) were synthesized and structurally characterized. Treatment of the diazenido complexes with a stoichiometric amount of [H(OEt2)2][BArF24] afforded the corresponding molybdenum(iv) hydrazido species [(dppe)2Mo(NNHCy)(I)][BArF24], [(dppe)2(CH3CN)Mo(NNHCy)][BArF24]2 and [(dppe)2(3,5-(CF3)2C6H3CN)Mo(NNHCy)][BArF24]2, enabling the study of N-H bond dissociation free energies (BDFEs) in the classical Chatt-type bis(phosphine) diazenide platform as a function of ligand (L) trans to the nitrogenous fragment. Deprotonation and electrochemical experiments established that the trans nitrile 3,5-(CF3)2C6H3CN afforded the least reducing molybdenum(iv) hydrazido complex in the series ( = -1.32 V vs. Fc/Fc+) with the most acidic N-H bond (pKa < 2.6, THF), whereas the ligands CH3CN ( = -1.60 V, pKa < 5.5) and I- ( = -2.03 V, pKa = 9.3) gave more reducing complexes with less acidic N-H bonds. Computational (DFT) studies confirm weak N-H bond strengths of 32.8 (L = I-), 35.4 (L = CH3CN) and 36.2 kcal mol-1 (L = 3,5-(CF3)2C6H3CN) in the hydrazido series.

Determining and Understanding N-H Bond Strengths in Synthetic Nitrogen Fixation Cycles

The fixation of atmospheric dinitrogen to ammonia using molecular catalysts has been a long-standing challenge in homogeneous catalysis and synthetic chemistry. New approaches to this problem may offer more energy-efficient and carbon-neutral routes to this important industrial compound. Despite the ubiquity of ammine, amide, imide and diazenide ligands in coordination chemistry, little thermodynamic data is available for understanding N-H bond strengths in molecules bearing these nitrogenous fragments. This article presents an overview of both computational and experimental approaches for the determination of N-H bond dissociation free energies in a variety of compounds relevant to nitrogen fixation to ammonia. The influence of metal oxidation state, ancillary ligand and identity of the nitrogen donor are highlighted. Implications for future design of molecular systems for the reduction of dinitrogen are discussed.

Ultrafast Photophysics of a Dinitrogen-Bridged Molybdenum Complex

Among the many metal-dinitrogen complexes synthesized, the end-on bridging (μ2, η1, η1-N2) coordination mode is notoriously unreactive for nitrogen fixation. This is principally due to the large activation energy for ground-state nitrogen-element bond formation and motivates exploration of the photoexcited reactivity of this coordination mode. To provide the foundation for this concept, the photophysics of a dinitrogen-bridged molybdenum complex was explored by ultrafast electronic spectroscopies. The complex absorbs light from the UV to near-IR, and the transitions are predominantly of metal-to-ligand charge transfer (MLCT) character. Five excitation wavelengths (440, 520, 610, 730, and 1150 nm) were employed to access MLCT bands, and the dynamics were probed between 430 and 1600 nm. Despite the large energy space occupied by electronic states (ca. 1.2 eV), the dynamics were independent of the excitation wavelength. In the proposed kinetic model, photoexcitation from a Mo-N═N-Mo centered ground state populates the π*-state delocalized over two terpyridine ligands. Due to a large terpyridine-terpyridine spatial separation, electronic localization occurs within 100 fs, augmented by symmetry breaking. The subsequent interplay of internal conversion and intersystem crossing (ISC) populates the lowest 3MLCT state in 2-3 ps. Decay to the ground state occurs either directly or via a thermally activated metal-centered (3MC) trap state having two time constants (10-15 ps, 23-26 ps [298 K]; 103 ps, 612 ps [77 K]). ISC between 1MLCT and 3MLCT involves migration of energized electron density from the terpyridine π* orbitals to the Mo-N═N-Mo core. Implication of the observed dynamics for the potential N-H bond forming reactivity are discussed.