Bipyridine and phenanthroline IR-spectral bands as indicators of metal spin state in hexacoordinated complexes of Fe(ii), Ni(ii) and Co(ii)

Abstract

Possible stable structures of various 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen) complexes, [Ni(bpy)(3)](2+), [Co(bpy)(3)](2+), [Fe(bpy)(3)](2+) and Fe(phen)(2)(NCS)(2), were optimized for different spin states of the metals, and the spectra computed for every form were compared with the experimental IR spectra of the compounds. It is demonstrated that the changes in spin states of the metals influence both geometry and vibrational spectra of the complexes. Spectral changes are predicted not only in the low frequency range, corresponding to metal-ligand vibrations, but also in the mid-IR range, where ligand vibrations are active. Detailed computational analysis in combination with the corresponding spectroscopic experiment shows that the spectral changes are of a similar character for complexes with the same ligands independent on the central metal and can be used as spectroscopic markers of the electronic state of the latter. Found spectral markers have been validated at a number of complexes of Fe(II), Ni(II), Co(II), Zn(II) and Cu(II) with bpy and phen ligands.

Correlations between metal spin states and vibrational spectra of a trinuclear Fe(II) complex exhibiting spin crossover

“…The actual increase is observed for the corresponding experimental band at 1554 cm À1 at high temperatures (Fig. 3), similar to the case of the abovementioned tris(pyrazol-1-yl)methane, phenanthroline and bipyridine complexes [11,12]. According to the computations (Fig.…”

“…A similar enhancement of IR bands associated with in-plane ring vibrations involving N atoms coordinated to the metal centers was caused by the LS / HS transition of SCO complexes of Fe(II) with tris(pyrazol-1-yl)methane [11] and phenanthroline [12] ligands as well as of tris-bipyridine complexes of Fe(II), Co(II) and Ni(II).12.…”

“…Metalto-ligand bond lengths are predicted with a largest deviation of about 4% from the experiment. Similarly to the mononuclear Fe(II) SCO complexes with tris(pyrazol-1-yl)methane [11] and phenanthroline and bipyridine ligands [12] considered elsewhere, the strongest dependence on the metal spin state is found for the Fe(II)-ligand bonds: 202e222 transition causes elongation of the Fe 2 eN distance by~0.2 Å, whereas other structural parameters including Fe 1,3 eligand bonds remain essentially unchanged. These results are in agreement with the X-ray data [27] for a similar complex with C 2 H 4 OH substituent at N 4 .…”

“…We have shown for trisbipyridine and phenanthroline complexes [12] that the main factor influencing IR spectra is the population of the 3d-orbitals, which are split into two groups for metal atoms in an octahedral environment (Scheme S1, SI). The differences between the IR spectra with populated and empty highest 3d-orbitals of the metal center appear to be essentially independent on the identity of the metal.…”

“…Density functional theory (DFT) has been shown to effectively reveal the bands most sensitive to the spin state [10e12]. Moreover, in case of tris-bipyridine and phenanthroline complexes of Fe(II), Co(II), Ni(II), Cu(II), and Zn(II) the differences between the IR spectra with (singly or doubly) populated and non-populated highest 3d-orbitals of the metal center appear to be essentially independent on the identity of the metal [12]. The mid-IR spectroscopic markers of transition-metal electronic/spin state are therefore transferable.…”