Interaction of Iron–Sulfur Clusters with N₂: Biomimetic Systems in the Gas Phase

Abstract

The active centers of the nitrogen-fixing enzymes nitrogenases comprise iron−sulfur clusters. The binding of the substrate N 2 to these clusters plays a fundamental role for the subsequent ammonia synthesis. However, due to the complexity of the natural system and the lack of suitable synthetic models, the interaction of N 2 with the iron−sulfur clusters remains largely elusive. In this contribution, we demonstrate the gas-phase preparation and investigation of the cationic Fe 2 S 2 + , Fe 3 S 3 + , and Fe 4 S 4 + clusters. These clusters represent the first cluster model systems containing more than one iron and sulfur atom, respectively, which are found to bind N 2 . Temperature-dependent kinetic measurements allow, for the first time, for the determination of experimental binding energies of N 2 to iron−sulfur clusters. In addition, concurrent firstprinciples simulations reveal the ground state and isomeric structures of the iron−sulfur clusters with and without adsorbed N 2 and provide a conceptual understanding of the interaction between N 2 and iron−sulfur clusters. In particular, we present molecular level details of N 2 bonding such as the identification of the adsorption sites, the bond geometry, the bond strength, and the nature of the cluster-N 2 binding interaction, which is found to be promoted by occupation of the empty antibonding orbitals of the free N 2 molecule through interaction with the frontier orbitals of the iron−sulfur complex involving d-orbitals of iron hybridized with sulfur porbitals.

Dual Iron Sites in Activation of N2 by Iron–Sulfur Cluster Anions Fe5S2– and Fe5S3–

“…The end-on coordination mode is the most prevalent manner observed for N 2 adsorption on metal clusters . Previous investigations have elucidated that π back-donation of d- electrons from a single metal atom (the adsorption site) to π* orbitals of N 2 prevails in this binding mode (as shown in Scheme a). ,− In this work, an unprecedented interaction between the d – d bonding orbitals of dual iron sites and the π* orbitals of N 2 (Scheme b) in the end-on binding mode was disclosed to explain the superior reactivity of polynuclear iron–sulfur cluster anions Fe 5 S 2,3 – toward N 2 among the systematically investigated Fe x S y – ( x = 1–8, y = 0– x ). This is the first example to reveal that the dual iron site rather than a single Fe atom plays a crucial role to capture N 2 in the common end-on binding mode.…”

“…Dinitrogen is an abundant and easily accessible resource, but it is highly inert under ambient conditions. , The activation and functionalization of N 2 have therefore been a long-standing important research subject . In nature, nitrogen fixation is mediated by nitrogenase enzymes, in which N 2 reduction takes place at the active iron-based cofactors (FeMo/FeV/FeFe-cofactor) consisting of multinuclear iron–sulfur clusters in the weak-field sulfur-dominated environment. − Spectroscopic studies have indicated that N 2 binds to the iron site of the FeMoco in an end-on configuration. − Encouraged by the outstanding reactivity and unique structures of the nitrogenases, there has been intense research activity focusing on the iron–sulfur clusters with the ability to bind and reduce N 2 . ,− Despite remarkable achievements in the synthesis of complex iron–sulfur clusters in condensed-phase systems, examples involving N 2 binding to iron–sulfur clusters remain scarce. − Only very recently, several synthesized mononuclear and multinuclear iron–sulfur complexes were reported to bind N 2 and induce the further functionalization of N 2 . ,, …”

“…However, a systematic investigation on the reactivity of iron–sulfur clusters toward N 2 is still scarce. To date, only Landman et al reported that Fe n S n + ( n = 2–4) can adsorb multiple N 2 molecules in an end-on coordination mode via temperature-dependent kinetic experiments in an ion trap reactor and disclosed that the contacted Fe atom plays an important role in the interaction between clusters and N 2 molecule …”

“…To date, only Landman et al reported that Fe n S n + (n = 2−4) can adsorb multiple N 2 molecules in an end-on coordination mode via temperaturedependent kinetic experiments in an ion trap reactor and disclosed that the contacted Fe atom plays an important role in the interaction between clusters and N 2 molecule. 14 The end-on coordination mode is the most prevalent manner observed for N 2 adsorption on metal clusters. 41 Previous investigations have elucidated that π back-donation of d-electrons from a single metal atom (the adsorption site) to π* orbitals of N 2 prevails in this binding mode (as shown in Scheme 1a).…”

Nitrogen Activation and Transformation on Monometallic Niobium Boron Oxide Cluster Anions at Room Temperature: A Dual-Site Mechanism

“…The combined use of experimental and theoretical techniques is a powerful methodology for the investigation of model reactions between N 2 and gas-phase species and has been used to reveal N 2 activation mechanisms in an unperturbed environment at a strictly molecular level. − Specifically, the gas-phase species ,,,− that possess the ability to cleave the NN triple bond under thermal collision conditions commonly involve the cooperation of more than one TM atom, for adsorbing and activating N 2 , with the exception of the previously reported Os/N 2 system . It is noteworthy that, although there are increasing reports on the N 2 activation mediated by gas-phase ions, the transformation of N 2 has rarely been reported. , Considering that the empty p orbital in boron is similar to the d orbital of transition metals and can, therefore, participate in efficient “donor–acceptor” processes upon interaction with N 2 , several B-based catalysts have been reported to be able to activate N 2 . − Thus, boron-doped clusters probably are promising systems exhibiting a high reactivity toward N 2 activation.…”