Cryo spectroscopy of N2 on cationic iron clusters

Abstract

Infrared photodissociation (IR-PD) spectra of iron cluster dinitrogen adsorbate complexes [Fen(N2)m]+ for n = 8–20 reveal slightly redshifted IR active bands in the region of 2200–2340 cm−1. These bands mostly relate to stretching vibrations of end-on coordinated N2 chromophores, a μ1,end end-on binding motif. Density Functional Theory (DFT) modeling and detailed analysis of n = 13 complexes are consistent with an icosahedral Fe13+ core structure. The first adsorbate shell closure at (n,m) = (13,12)—as recognized by the accompanying paper on the kinetics of N2 uptake by cationic iron clusters—comes with extensive IR-PD band broadening resulting from enhanced couplings among adjacent N2 adsorbates. DFT modeling predicts spin quenching by N2 adsorption as evidenced by the shift of the computed spin minima among possible spin states (spin valleys). The IR-PD spectrum of (17,1) surprisingly reveals an absence of any structure but efficient non-resonant fragmentation, which might indicate some weakly bound (roaming) N2 adsorbate. The multiple and broad bands of (17,m) for all other cases than (17,1) and (17,7) indicate a high degree of variation in N2 binding motifs and couplings. In contrast, the (17,7) spectrum of six sharp bands suggests pairwise equivalent N2 adsorbates. The IR-PD spectra of (18,m) reveal additional features in the 2120–2200 cm−1 region, which we associate with a μ1,side side-on motif. Some additional features in the (18,m) spectra at high N2 loads indicate a μ1,tilt tilted end-on adsorption motif.

Probing the binding and activation of small molecules by gas-phase transition metal clustersviaIR spectroscopy

“…70 In case of binding of multiple ligands to a cluster, the M–L binding energy often strongly drops after ‘saturation’ of the cluster – caused by steric or electronic effects – such that additional (molecular) ligands may also act like messengers. 71 As a side note, embedding neutral and charged molecules or clusters in superfluid He droplets (at about 0.4 K) is by now a well-established technique for their spectroscopy – including in the IR – in a nearly non-disturbing environment at low temperature. 72 The fast dissipation of energy from the molecule into the superfluid He surrounding minimizes hot-bands and band shifts due to (cross-) anharmonicities.…”

Catalysis of dinitrogen activation and reduction by a single Fe13 cluster and its doped systems

“…38,39 Enormous efforts have been made to explore the application of cluster catalysts in various reactions. 3,[40][41][42][43][44][45][46][47][48] Recently, a study on the Mo 3 S 4 Fe cluster has revealed the indispensable role of Fe sites in N 2 reduction. 49 In addition to the dependence on active sites and electronic structures, 50,51 metal clusters often exhibit prominent stability because of their symmetric structures, such as Mackay icosahedrons, 52 in which to the number of shells, that is, the total atom number N corresponds to 13, 55, and 147 (i.e., n = 1, 2, and 3), respectively.…”

“…38,39 Enormous efforts have been made to explore the application of cluster catalysts in various reactions. 3,40–48 Recently, a study on the Mo 3 S 4 Fe cluster has revealed the indispensable role of Fe sites in N 2 reduction. 49…”

Dinitrogen Activation in the Gas Phase: Spectroscopic Characterization of C−N Coupling in the V3C++N2Reaction

“…Vibrational action spectroscopy has proven particularly useful to characterize the N 2 – metal cluster interaction [10] . Monitoring the red shift of the N 2 stretching frequency (unbound: 2330 cm −1 ) allows to differentiate between weaker‐bound N 2 in an end‐on coordination (2330–2100 cm −1 ) [10a–f] and a more strongly interacting side‐on coordinated μ 2 ‐N 2 (1500–1400 cm −1 ) [10g] . Cleavage of the N≡N triple bond leads to the formation of metal nitride species, whose presence has been inferred indirectly by spectroscopic identification of an activated N−N intermediate along a computed reaction path that elucidated a stepwise across edge above surface (AEAS) mechanism [10g] .…”