Synthesis and Spectroscopic Characterization of Copper(II)−Nitrito Complexes with Hydrotris(pyrazolyl)borate and Related Coligands

This study focuses on the geometric (molecular) structures, spectroscopic properties, and electronic structures of copper(II)−nitrito complexes as a function of second coordination sphere effects using a set of closely related coligands. With anionic hydrotris(pyrazolyl)borate ligands, one nitrite is bound to copper(II). Depending on the steric demand of the coligand, the coordination mode is either symmetric or asymmetric bidentate, which leads to different ground states of the resulting complexes as evident from EPR spectroscopy. The vibrational spectra of these compounds are assigned using isotope substitution and DFT calculations. The results demonstrate that νsym(N−O) occurs at higher energy than νasym(N−O), which is different from the literature assignments for related compounds. UV−vis absorption and MCD spectra are presented and analyzed with the help of TD-DFT calculations. The principal binding modes of nitrite to Cu(II) and Cu(I) are also investigated applying DFT. Using a neutral tris(pyrazolyl)methane ligand, two nitrite ligands are bound to copper. In this case, a very unusual binding mode is observed where one nitrite is η1-O and the other one is η1-N bound. This allows to study the properties of coordinated nitrite as a function of binding mode in one complex. The N-coordination mode is easily identified from vibrational spectroscopy, where N-bound nitrite shows a large shift of νasym(N−O) to >1400 cm-1, which is a unique spectroscopic feature. The optical spectra of this compound exhibit an intense band around 300 nm, which might be attributable to a nitrite to Cu(II) CT transition. Finally, using a bidentate neutral bis(pyrazolyl)methane ligand, two η1-O coordinated nitrite ligands are observed. The vibrational and optical (UV−vis and MCD) spectra of this compound are presented and analyzed.