Uncovering Redox Non-innocent Hydrogen-Bonding in Cu(I)-Diazene Complexes

Abstract

The life-sustaining reduction of N2 to NH3 is thermoneutral yet kinetically challenged by high-energy intermediates such as N2H2. Exploring intramolecular H-bonding as a potential strategy to stabilize diazene intermediates, we employ a series of [ xHetTpCu]2(μ-N2H2) complexes that exhibit H-bonding between pendant aromatic N-heterocycles (xHet) such as pyridine and a bridging trans-N2H2 ligand at copper­(I) centers. X-ray crystallography and IR spectroscopy clearly reveal H-bonding in [pyMeTpCu]2(μ-N2H2) while low-temperature 1H NMR studies coupled with DFT analysis reveals a dynamic equilibrium between two closely related, symmetric H-bonded structural motifs. Importantly, the xHet pendant negligibly influences the electronic structure of xHetTpCuI centers in xHetTpCu­(CNAr2,6‑Me2 ) complexes that lack H-bonding as judged by nearly indistinguishable ν­(CN) frequencies (2113–2117 cm–1). Nonetheless, H-bonding in the corresponding [ xHetTpCu]2(μ-N2H2) complexes results in marked changes in ν­(NN) (1398–1419 cm–1) revealed through resonance Raman studies. Due to the closely matched N–H BDEs of N2H2 and the pyH0 cation radical, the aromatic N-heterocyclic pendants may encourage partial H-atom transfer (HAT) from N2H2 to xHet through redox-non-innocent H-bonding in [ xHetTpCu]2(μ-N2H2). DFT studies reveal modest thermodynamic barriers for concerted transfer of both H-atoms of coordinated N2H2 to the xHet pendants to generate tautomeric [ xHetHTpCu]2(μ-N2) complexes, identifying metal-assisted concerted dual HAT as a thermodynamically favorable pathway for N2/N2H2 interconversion.

A guide to secondary coordination sphere editing

“…With interest in dinitrogen (N 2 ) reduction and intermediates relevant thereto, in 2021 Shafaat and Warren studied the N 2 coordination chemistry of a pyridine-substituted trispyrazolylborate (Tp) copper(I) compound (Scheme 26). 66 As a product of two-electron, two-proton reduction, diazene (HNQNH) is a reactive small-molecule, that at room temperature, undergoes disproportionation to N 2 and hydrazine (N 2 H 4 ). In this study, the authors showed that {[ xHet TpCu] 2 (m-N 2 H 2 )} exhibited pendant hydrogen bonding interactions with a pyridine positioned in the SCS.…”

Evaluating Diazene to N2 Interconversion at Iron‐Sulfur Complexes

“…As highlighted in the introduction, stable dicopper diazene complexes with appended H atom acceptors have been used to study the energetics of H atom transfer at N 2 H 2 without an initial exogenous electron transfer [15a] . In order to compare to the Sellmann system, we chose to calculate the energy of the N 2 species that would be formed by PCET from N 2 H 2 in stable diazene complex 1‐L 1 .…”

“… Limiting resonance structures of the dicopper‐N 2 H 2 complexes (a) and (b), and the N 2 tautomer (c) [15a] . The hydrogen bond is considered to be transferrable in the resonance structures, similar to the “resonance‐assisted hydrogen bonding” concept [21] …”

Boosting the interlayer-confined nitrate reduction reaction by in situ electrochemical potassium ion intercalation

“…5b, the intensities of the ν 1 and ν 3 modes are obviously stronger in the K + -containing electrolyte solution than that in the Na + -containing electrolyte solution, verifying the vital role of K + in the insertion of NO 3 − . As the reaction proceeds, a vibration mode located at 548 cm −1 appears, which can be assigned to the Cu-N bond [53]. The intensity increases gradually with the reaction time, suggesting that NO 3 − or the intermediate species are bonded to the catalyst surface by forming a Cu-N bond.…”