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

Abstract

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 pCO. In the early stage of the interaction, strong and irreversibly bound monocarbonyl species characterized by ν̃COads > ν̃COgas 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 (−ΔadsH ∼ 120 and ∼100 kJ/mol, respectively) is much larger than the −ΔadsH 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 −ΔadsH 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 Δν̃CO and −ΔHads quantities for a large set of non-d/d0/d10 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 Δν̃CO versus −ΔadsH 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/d0 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.