Heterogeneous Nonclassical Carbonyls Stabilized in Cu(I)− and Ag(I)−ZSM-5 Zeolites: Thermodynamic and Spectroscopic Features

A systematic analysis of the thermodynamic and spectroscopic features of a vast set of data on carbonyl-like complexes present in the literature, combined with some new experimental data here reported, allowed us to attempt to single out the electrostatic σ- and π-contributions in the formation of carbonyl bonding for the so-called “nonclassical carbonyls” hosted inside zeolitic nanocavities. In particular, the room-temperature adsorption of CO on well-defined Cu(I) and Ag(I)−ZSM-5 systems (as testified by X-ray absorption experiments) was studied by means of the joint use of IR spectroscopy and adsorption microcalorimetry. The formation in the zeolite pores of heterogeneous [Cu(CO) 2 ] + and [Ag(CO)] + complexes, the stoichiometry of which is in good agreement with the homogeneous nonclassical carbonyls, was monitored as a function of increasing p CO . In the early stage of the interaction, strong and irreversibly bound monocarbonyl species characterized by ν̃ COads > ν̃ COgas are formed on Cu(I) and Ag(I) sites. Conversely, labile adducts (electrostatic in nature) are formed on Na + and K + sites hosted in the same zeolite pores. The zero-coverage enthalpy of CO adsorbed on Cu(I) and Ag(I) sites (−Δ ads H ∼ 120 and ∼100 kJ/mol, respectively) is much larger than the −Δ ads H values measured for the two alkaline-metal adducts (∼35 and ∼28 kJ/mol for Na + and K +, respectively), despite the closeness of the charge/radius ratios of the two sets of metal cations [Na + and Cu(I); K + and Ag(I)]. The high −Δ ads H values and the irreversible nature of (a fraction) of the d- block metal carbonyls suggest the onset of a π-back-donation reinforcing the carbonyl bond with respect to a plain σ-coordination. A clear deviation from an empirical rule, which linearly correlate Δν̃ CO and −Δ H ads quantities for a large set of non -d/d 0 /d 10 metal carbonyls, was observed in the case of copper-and silver−carbonyls, confirming the interplay of σ- and π-back-donation contributions for such species, otherwise defined nonclassical carbonyls. The Δν̃ CO versus −Δ ads H empirical rule was found of general validity, in that it allows to infer the enthalpy values from the blue-shift of the C−O stretching frequency (and vice-versa) in the case of non -d/d 0 metal carbonyls, whereas it allows to roughly estimate by the deviation from the line the extent of the π-back-donation in the case of d- block metal carbonyls. Further, the spectroscopic and thermodynamic features of carbonyl species formed on (partially) reduced copper sites have shown that in the absence of strong electrostatic plus σ-coordinative components the carbonyl bond is surprisingly weak, despite the presence of π-back-donation.