Structural, electronic and spectroscopic features of the dihydride Pd4(dppm)4(H)22+ cluster catalyst

Abstract

1H NMR, vibrational, and UV–vis spectroscopic measurements for Pd2(dppm)2Cl2 (dppm = Ph2PCH2PPh2), [Pd4(dppm)4(X)2](BF4)2 (X = Cl, H), and [Pd4(dmpm)4(H)2](Br)2 (dmpm = Me2PCH2PMe2) were performed to address the structure of the recently identified title cluster. Its dmpm analogue was prepared from the reaction between Pd2(dmpm)2Br2 and NaBH4 in methanol under inert atmosphere, and exhibits the expected nonet (rel. int. 1:8:28:56:70:56:28:8:1) at –5.21 ppm (in (CD3)2CO), contrasting with that of Pd4(dppm)4(H)22+ (δ + 5.15 ppm, (CD3)2CO). This significant difference is explained by the presence of the PdH residues in the deshielding region of the dppm-phenyl groups. The vibrational spectra in the low-frequency region are consistent with a centrosymmetric structure, and a scattering at 144 cm–1 in the FT-Raman spectra is observed; a peak that is assigned to ν(Pd2) on the basis of a comparison with the well established M2-bonded Pd2(dppm)2Cl2 data ((Pd2) = 149 cm–1). On the basis of the qualitative temperature behavior of the band maxima and width, the two lowest energy absorption bands in the UV–vis spectra are assigned to d σ [Formula: see text] d σ * type transition. EHMO computations predict four well-isolated frontier MO levels, defined as d σ *(Pd2)/d σ *(PdH) (LUMO + 1), d σ *(Pd2) (LUMO), d σ *(Pd2)/d σ *(PdH) (HOMO), and d σ (Pd2) (HOMO –1), and simple selection rules (u « g) indicate that only two low-energy electronic transitions are orbitally allowed, consistent with the UV–vis findings. Computer modelings show the presence of large cavities above and under the Pd4 plane, described by inner-cavity nonbonded H···H distances of ~5–8 Å.Key words: cluster, palladium, hydride, structure, spectroscopy.