Optical investigation of spin-crossover in cobalt(II) bis-terpy complexes

Abstract

The spin transition of the [Co(terpy) 2 ] 2+ complex (terpy = 2,2 0 :6 0 ,2 00 -terpyridine) is analysed based on experimental data from optical spectroscopy and magnetic susceptibility measurements. The single crystal absorption spectrum of [Co(terpy) 2 ](ClO 4 ) 2 shows an asymmetric absorption band at 14 400 cm À1 with an intensity typical for a spin-allowed d-d transition and a temperature behaviour typical for a thermal spin transition. The single crystal absorption spectra of suggest that in this compound, the complex is essentially in the highspin state at all temperatures. However, the increase in intensity observed in the region of the low-spin MLCT transition with increasing temperature implies an unusual partial thermal population of the low-spin state of up to about 10% at room temperature. Finally, highspin ! low-spin relaxation curves following pulsed laser excitation for [Co(terpy) 2 ](ClO 4 ) 2 dispersed in KBr discs, and as a comparison for the closely related [Co(4-terpyridone) 2 ](ClO 4 ) 2 spin-crossover compound are given.

Heteroleptic Complexes of the Tridentate Pyridine‐2,6‐di‐tetrazolate Ligand

“…5H 2 O . Strong orbital contributions for Co‐complexes are often observed,,, which agrees with g = 2.25 of the Co‐complex presented here.…”

“…When dissolving the two Co complexes in pyridine two almost identical spectra result with long‐wavelength maxima at 950 and 500 nm and a third band at around 400 nm partially obscured by the intense π→π* band (Table ). For octahedral high spin d 7 Co(II) complexes three d‐d transitions ( 4 T 1g (F)→ 4 T 2g (F), 4 T 1g (F)→ 4 T 1g (P) und 4 T 1g (F)→ 4 A 2g (F)), can be expected and we assume the same underlying species [Co(pydtz)(Py) 3 ] in both cases. This is in line with assumed higher ligand strength of Py compared with H 2 O and a facile ligand exchange reaction transferring both starting materials to the same complex species in solution, and confirms our assumptions on the ease of coligand exchange based on the DSC‐TG experiments.…”

Rational Design of Polymers for Selective CO2 Reduction Catalysis

“…The interplay between Co loading and coordination was investigated using UV/Vis spectroscopy for p1 and a series of coordinated P1 x polymers (Figure a). Upon the addition of Co II to p1 in 0.6 or 1 equivalents to every two tpy units ( P1 0.6 or P1 1 , respectively), three absorption maxima are observed at 440, 505, and 550 nm, attributed to Co II bis(tpy) metal‐to‐ligand charge transfer transitions . At concentrations of Co corresponding to mono(tpy) stoichiometry ( P1 2 ) and a further excess of Co ( P1 5 ), the spectra show concomitant loss of the Co II bis(tpy) transitions and emergence of a broad absorption in the UV region around 380 nm, characteristic of the Co II mono(tpy) complex (Figure a, inset) .…”

Assembly of Supramolecular Networks with the Inclusion of T4(2)6(2) Water Tapes and R6 Water Clusters

“…The Co-N or Co-O bond lengths in the title complexes are normal in comparison with those reported in the literature. The distorted octahedra of cobalt ions in complex 2 can be explained by Jahn-Teller effect for d 7 configuration [18]. Five crystallographically independent water molecules are present, which generate together with the anions (DBrS 2-) a complex 2D H-bond network.…”

“…Finally, each anions (DBrS 2-) is linked to such tape through the H-bond. This contains different kind of rings that may be described using the graph-set approach suggested by Etter, Bernstein et al [18]. The above mentioned water clusters are represented as R 6 6 (12) (A) and R 4 4 (8) (B) whereas other rings involve the one carboxylate oxygen atoms in the dibromosuccinoate anions: we can find R 6 6 (12) (C) rings with the carboxyl group linking one tetrameric cluster and one anion, R 4 4 (9) (D) rings that involve one carboxylate oxygens atom from DBrS 2-and three water molecules (Fig.…”