Electronic states and vibrational structures of sym‐ and unsym‐Co3(dipyridylamine)4Cl2 using temperature‐controlled Raman and surface‐enhanced Raman spectroscopy

We employed temperature‐controlled Raman spectroscopy to obtain the vibrational structures of the linear tricobalt metal‐string complex Co3(dpa)4Cl2 (dpa = di(2‐pyridyl) amide) and to identify its low‐lying electronic states. The density functional theory (DFT) method B3LYP*‐D3 was used to obtain the molecular structures, vibrational frequencies, and electronic levels of varied spin states. Co3(dpa)4Cl2 has sym‐ and unsym‐forms of Co–Co metal bonding. The Raman intensities of pyridyl breathing bands appeared to be sensitive to the coordinated Co atom and hence could be used to identify both forms. In s‐Co3 crystal, the split pyridyl Raman breathing bands indicated both symmetric and unsymmetric forms. This is explained that the molecule populated both the 2A2 (the ground state) and possibly 2B (reduced symmetry of 2E) states even at 77 K. The 2B state is coupled to the 4B state, which has an unsymmetric Co–Co bond. As the temperature increased to 423 K, the intensity of the low‐wavenumber pyridyl breathing band further increased and was attributed to the 4B state. At 77–323 K, the Raman spectra of u‐Co3 crystal populated mostly in the 4B state. From the SERS measurements, the samples were prepared in solution phase, and u‐Co3 displayed spectral behavior similar to that of s‐Co3, implying that the global minimal geometry is a symmetric form. Raman curves of s‐ and u‐Co3(dpa)4Cl2 crystal at 77 K in the low wavenumber region are presented. The Co–Co stretch band is at 314 cm−1 in the unsymmetric form. No conversion between these forms is observed in the temperature range of 77–623 K.