A Neutral Borylene and its Conversion to a Radical by Selective Hydrogen Transfer.

A successful selective reduction of X2B-Tip (Tip = 1,3,5-iPr3-C6H2, X = I, Br) with KC8 and Mg metal, respectively, in the presence of a hybrid ligand (C6H4(PPh2)LSi) leads to a stable low-valent five-membered ring as a boryl radical [C6H4(PPh2)LSiBTip][Br] (1) and neutral borylene [C6H4(PPh2)LSiBTip] (2). Compound 2 reacts with 1,4-cyclohexadiene, resulting in hydrogen abstraction to afford the radical [C6H4(PPh2)LSiB(H)Tip] (3). Quantum chemical studies reveal that compound 1 is a B-centered radical, and compound 2 is a phosphane and silylene stabilized neutral borylene in a trigonal planar environment, whereas compound 3 is an amidinate-centered radical. Although compounds 1 and 2 are stabilized by hyperconjugation and π-conjugation, they display high H-abstraction energy and basicity, respectively.

Cyanate Formation via Photolytic Splitting of Dinitrogen.

Light-driven N2 cleavage into molecular nitrides is an attractive strategy for synthetic nitrogen fixation. However, suitable platforms are rare. Furthermore, the development of catalytic protocols via this elementary step suffers from poor understanding of N-N photosplitting within dinitrogen complexes, as well as of the thermochemical and kinetic framework for coupled follow-up chemistry. We here present a tungsten pincer platform, which undergoes fully reversible, thermal N2 splitting and reverse nitride coupling, allowing for experimental derivation of thermodynamic and kinetic parameters of the N-N cleavage step. Selective N-N splitting was also obtained photolytically. DFT computations allocate the productive excitations within the {WNNW} core. Transient absorption spectroscopy shows ultrafast repopulation of the electronic ground state. Comparison with ground-state kinetics and resonance Raman data support a pathway for N-N photosplitting via a nonstatistically vibrationally excited ground state that benefits from vibronically coupled structural distortion of the core. Nitride carbonylation and release are demonstrated within a full synthetic cycle for trimethylsilylcyanate formation directly from N2 and CO.

Nitrogen Fixation via Splitting into Nitrido Complexes.

The large carbon footprint of the Haber-Bosch process, which provides ammonia for fertilizers but also the feedstock for all nitrogenous commercial products, has fueled the quest for alternative synthetic strategies to nitrogen fixation. Owing to the extraordinarily strong N≡N triple bond, the key step of the Haber-Bosch reaction, i.e., the dissociative adsorption of N2, requires high temperatures. Since the first report in 1995, a wide variety of molecular transition metal and f-block compounds have been reported that can fully cleave N2 at ambient conditions and form well-defined nitrido complexes. We here provide a comprehensive survey of the current state of N2 splitting reactions in solution and follow-up nitrogen transfer reactivity. Particular emphasis is put on electronic structure requirements for the formation of suitable molecular precursors and their N-N scission reactivity. The prospects of N2 splitting for the synthesis of nitrogen containing products will be discussed, ranging from ammonia and heterocumulenes to organic amines, amides or nitriles via proton coupled electron transfer, carbonylation, or electrophilic functionalization of N2 derived nitrido complexes. Accomplishments and challenges for nitrogen fixation via N2 splitting are presented to offer guidelines for the development of catalytic platforms.

Selectivity of tungsten mediated dinitrogen splitting vs. proton reduction.

Mo complexes are currently the most active catalysts for nitrogen fixation under ambient conditions. In comparison, tungsten platforms are scarcely examined. For active catalysts, the control of N2 vs. proton reduction selectivities remains a difficult task. We here present N2 splitting using a tungsten pincer platform, which has been proposed as the key reaction for catalytic nitrogen fixation. Starting from [WCl3(PNP)] (PNP = N(CH2CH2PtBu2)2), the activation of N2 enabled the isolation of the dinitrogen bridged redox series [(N2){WCl(PNP)}2]0/+/2+. Protonation of the neutral complex results either in the formation of a nitride [W(N)Cl(HPNP)]+ or H2 evolution and oxidation of the W2N2 core, respectively, depending on the acid and reaction conditions. Examination of the nitrogen splitting vs. proton reduction selectivity emphasizes the role of hydrogen bonding of the conjugate base with the protonated intermediates and provides guidelines for nitrogen fixation.

Dinitrogen Splitting Coupled to Protonation.

The coupling of electron- and proton-transfer steps provides a general concept to control the driving force of redox reactions. N2 splitting of a molybdenum dinitrogen complex into nitrides coupled to a reaction with Brønsted acid is reported. Remarkably, our spectroscopic, kinetic, and computational mechanistic analysis attributes N-N bond cleavage to protonation in the periphery of an amide pincer ligands rather than the {Mo-N2 -Mo} core. The strong effect on electronic structure and ultimately the thermochemistry and kinetic barrier of N-N bond cleavage is an unusual case of a proton-coupled metal-to-ligand charge transfer process, highlighting the use of proton-responsive ligands for nitrogen fixation.

Square-Planar Cobalt(III) Pincer Complex.

A series of square-planar cobalt(II) complexes with pincer ligands {N(CH2CH2PtBu2)2}(-) ({L1(tBu)}(-)), {N(CH2CH2PtBu2)(CHCHPtBu2)}(-) ({L2(tBu)}(-)), and {N(CHCHPtBu2)2}(-) ({L3(tBu)}(-)) was synthesized. Ligand dehydrogenation was accomplished with a new, high-yield protocol that employs the 2,4,6-tri-tert-butylphenoxy radical as hydrogen acceptor. [CoCl{Ln(tBu)}] (n = 1-3) were examined with respect to reduction, protonation, and oxidation, respectively. One-electron oxidations of [CoCl(L1(tBu))] and [CoCl(L2(tBu))] lead to ligand-centered radical reactivity, like amide disproportionation into cobalt(II) amine and imine complexes. In contrast, oxidation of [CoCl{L3(tBu)}] with Ag(+) enabled the isolation of thermally stable, square-planar cobalt(III) complex [CoCl{L3(tBu)}](+), which adopts an intermediate-spin (S = 1) ground state with large magnetic anisotropy. Hence, pincer dehydrogenation gives access to a new platform for high-valent cobalt in square-planar geometry.