Electronic and Molecular Structures of trans-Dioxotechnetium(V) Polypyridyl Complexes in the Solid State

The structures of novel Tc(V) complexes trans-[TcO2(py)4]Cl·2H2O (1a), trans-[TcO2(pic)4]Cl·2H2O (2a), and trans-[TcO2(pic)4]BPh4 (2b) were determined by X-ray crystallography, and their spectroscopic characteristics were investigated by emission spectroscopy and atomic scale calculations. The cations adopt a tetragonally distorted octahedral geometry, with a trans orientation of the apical oxo groups. trans-[TcO2(pic)4]BPh4 has an inversion center located on technetium; however, for trans-[TcO2(py)4]Cl·2H2O and trans-[TcO2(pic)4]Cl·2H2O, a strong H bond formed by only one of the oxo substituents introduces an asymmetry in the structure, resulting in inequivalent trans Tc–N and TcO distances. Upon 415 nm excitation at room temperature, the complexes exhibited broad, structureless luminescences with emission maxima at approximately 710 nm (1a) and 750 nm (2a, 2b). Like the Re(V) analogs, the Tc(V) complexes luminesce from a 3Eg excited state. Upon cooling the samples from 278 to 8 K, distinct vibronic features appear in the spectra of the complexes along with increases in emission intensities. The low temperature emission spectra display the characteristic progressions of the symmetric OTcO and the Tc–L stretching modes. Lowest-energy, triplet excited-state distortions calculated using a time-dependent theoretical approach are in good agreement with the experimental spectra. The discovery of luminescence from the trans-dioxotechnetium(V) complexes provides the first opportunity to directly compare fundamental luminescence properties of second- and third-row d2 metal-oxo congeners.