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In the present work, the interaction of diatomic molecules with charged transition metal clusters and complexes was investigated. Temperature controlled isothermal kinetic studies served to elucidate the adsorption behavior of transition metal clusters. Infrared multiple photon dissociation (IR-MPD) experiments in conjunction with density functional theory (DFT) computations enabled the analysis of adsorbate induced changes on the structure and spin multiplicity of transition metal cores. A tandem cryo trap setup was used for the kinetic and spectroscopic investigations of the given compounds as isolated species in the gas phase. The presented investigations enabled insight into the metal-adsorbate bonding and provided cluster size and adsorbate coverage dependent information on cluster surface morphologies.
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.
We report the N2 cryo adsorption kinetics of selected gas phase mixed rhodium–iron clusters [RhiFej]+ in the range of i = 3–8 and j = 3–8 in 26 K He buffer gas by the use of a cryo tandem RF-hexapole trap–Fourier transform ion cyclotron resonance mass spectrometer. From kinetic data and fits, we extract relative rate constants for each N2 adsorption step and possible desorption steps. We find significant trends in adsorption behavior, which reveal adsorption limits, intermittent adsorption limits, and equilibrium reactions. For those steps, which are in equilibrium, we determine the Gibbs free energies. We conclude on likely ligand shell reorganization and some weakly bound N2 ligands for clusters where multiple N2 adsorbates are in equilibrium. The relative rate constants are transferred to absolute rate constants, which are slightly smaller than the collision rate constants calculated by the average dipole orientation (Langevin) theory. The calculated sticking probabilities increase, in general, with the size of the clusters and decrease with the level of N2 adsorption, in particular, when reaching an adsorption/desorption equilibrium. We receive further evidence on cluster size dependent properties, such as cluster geometries and metal atom distributions within the clusters through the accompanying spectroscopic and computational study on the equiatomic i = j clusters [Klein et al., J. Chem. Phys. 156, 014302 (2022)].