High nuclearity Pt carbonyls in alkali-metal X zeolites

Abstract

Diffuse reflectance time-resolved UV–VIS spectroscopy has been used together with FTIR spectroscopy and ¹³CO–¹²CO isotopic exchange for the investigation of anionic Pt carbonyl complexes [Pt₃(CO)₃(µ-CO)₃]_n^2– formed in alkali-metal X zeolites (alkali metal = Li⁺, Na⁺, K⁺ and Cs⁺) from [Pt(NH₃)₄]²⁺. It is shown that, compared with solutions, the zeolite matrix does not alter electronic transitions, while the vibrational frequencies of the CO ligands are appreciably changed. This latter effect is explained by the interaction of linear CO ligands with oxygen atoms of the zeolite lattice [an upward frequency shift of CO stretching vibration ν(CO)₁] and the location of the bridging COs in the vicinity of alkali-metal cations [a downward frequency shift of CO stretching vibration ν(CO)_b]. The effect of increasing the nuclearity of Chini complexes on the increase of ν(CO)₁(at 2000–2100 cm^–1) is much higher than the influence of increasing the electropositivity of the alkali-metal cations in the X matrix. Inside all the alkali-metal X zeolites and under all carbonylation conditions used, Pt₆ species (n= 2) are formed. The decreasing size and electropositivity of alkali-metal cations in the sequence Cs⁺ > K⁺ > Na⁺ > Li⁺ assist in stacking of more triangular units and the appearance of the Pt₉ and Pt₁₅(n= 3 and 5, respectively) carbonyl complexes.