Cryo infrared spectroscopy of N2 adsorption onto bimetallic rhodium-iron clusters in isolation

Abstract

We investigated the N₂ adsorption behavior of bimetallic rhodium-iron cluster cations [Rh_iFe_j(N₂)_m]⁺ by means of InfraRed MultiplePhotoDissociation (IR-MPD) spectroscopy in comparison with density functional theory (DFT) modeling. This approach allows us to refine our kinetic results [Ehrhard et al., J. Chem. Phys. (in press)] to enhance our conclusions. We focus on a selection of cluster adsorbate complexes within the ranges of i = j = 3-8 and m = 1-10. For i = j = 3, 4, DFT suggests alloy structures in the case of i = j = 4 of high (D_2d) symmetry: Rh-Fe bonds are preferred instead of Fe-Fe bonds or Rh-Rh bonds. N₂ adsorption and IR-MPD studies reveal strong evidence for preferential adsorption to Rh sites and mere secondary adsorption to Fe. In some cases, we observe adsorption isomers. With the help of modeling the cluster adsorbate complex [Rh₃Fe₃(N₂)₇]⁺, we find clear evidence that the position of IR bands allows for an element specific assignment of an adsorption site. We transfer these findings to the [Rh₄Fe₄(N₂)_m]⁺ cluster adsorbate complex where the first four N₂ molecules are exclusively adsorbed to the Rh atoms. The spectra of the larger adsorbates reveal N₂ adsorption onto the Fe atoms. Thus, the spectroscopic findings are well interpreted for the smaller clusters in terms of computed structures, and both compare well to those of our accompanying kinetic study [Ehrhard et al., J. Chem. Phys. (in press)]. In contrast to our previous studies of bare rhodium clusters, the present investigations do not provide any indication for a spin quench in [Rh_iFe_j(N₂)_m]⁺ upon stepwise N₂ adsorption.