Metal−Metal Bonding and Structures of Metal String Complexes Cr₃(dpa)₄Cl₂, Cr₃(dpa)₄(NCS)₂, and [Cr₃(dpa)₄Cl₂](PF₆) from IR, Raman, and Surface-Enhanced Raman Spectra

Abstract

We recorded infrared, Raman, and surface-enhanced Raman scattering (SERS) spectra of metal−string complexes Cr3(dpa)4X2 (dpa = di(2-pyridyl)amido, X = Cl, NCS) and [Cr3(dpa)4Cl2](PF6) and dipyridylamine (Hdpa) to determine their vibrational frequencies and to study their structures. For the SERS measurements these complexes were adsorbed on silver nanoparticles in aqueous solution to eliminate the constraints of a crystal lattice. From the results of analysis of the vibrational normal modes we assign the infrared band at 346 cm−1 to the Cr3 asymmetric stretching vibration of the symmetric form and the Raman line at 570 cm−1 to the Cr−Cr stretching mode for the unsymmetric form of Cr3(dpa)4Cl2. Complex Cr3(dpa)4Cl2 exhibits both symmetric (s-) and unsymmetric (u-) forms in solution but Cr3(dpa)4(NCS)2 only the s-form. The structures for both complexes in their ground states have the s-form. The oxidized complex [Cr3(dpa)4Cl2](PF6) has only a u-form for which the Cr−Cr stretching mode is assigned to the band at 570 cm−1. From the variation with temperature from 23 to 60 °C of the intensity of this line, we obtained the proportion of the u-form Cr3(dpa)4Cl2; the enthalpy change is thus obtained to be ΔH = 46.2 ± 3.3 kJ mol−1 and the entropy change is ΔS = 138 ± 10.3 J K−1 mol−1 for the reaction u-Cr3(dpa)4Cl2 ↔ s-Cr3(dpa)4Cl2. From the spectral intensities and band frequencies in SERS spectra, Hdpa is expected to adsorb on a silver nanoparticle with the amido nitrogen and pyridyl rings tilted from the silver surface, whereas the trichromium complex with the chromium ion line is orthogonal to the silver surface normal in aqueous silver solution.