Investigation of the Zero‐Field Splitting in Six‐ and Seven‐Coordinate Mononuclear Mn^II Complexes with N/O‐Based Ligands by Combining EPR Spectroscopy and Quantum Chemistry

Abstract

Abstract The electronic properties of a series of seven mononuclear Mn II complexes with an N/O and N/Cl coordination sphere have been investigated by X‐ and Q‐band EPR spectroscopy as well as by density functional theory (DFT). The [Mn(terpy)(CF 3 CO 2 ) 2 (H 2 O)] ( 1 ), [Mn(pb) 2 (CF 3 SO 3 ) 2 ] ( 2 ), [Mn(pb) 2 (NO 3 )(H 2 O)](NO 3 ) ( 3 ), [Mn(bpy) 2 (CF 3 CO 2 ) 2 ] ( 4 ) and [Mn(pb) 2 Cl 2 ] ( 5 ) complexes are six‐coordinate, whereas [Mn(terpy)(NO 3 ) 2 (H 2 O)] ( 6 ) and [Mn(bpea)(NO 3 ) 2 ] ( 7 ) are seven‐coordinate [terpy = 2,2′:6,2″‐terpyridine, pb = (–)‐pinene[5,6]bipyridine, bpy = 2,2′‐bipyridine, bpea = N , N ‐bis(2‐pyridylmethyl)ethylamine]. The X‐ray structure of new complex 1 has also been resolved, whereas the structures of 2 – 7 have been already reported. The D values of the N/O‐coordinated complexes 1 – 4 , 6 and 7 all fall into the narrow range of 0.035–0.086 cm –1 . Complex 5 exhibits a slightly larger D value of 0.146 cm –1 , in accordance with its N/Cl coordination sphere. The sign of D has been determined from the low‐temperature Q‐band EPR spectra for all complexes with the exception of 1 because of its large E / D ratio. The zero‐field splitting (ZFS) parameters of 1 – 7 have been predicted by DFT calculations using two types of functionals, BP86 and B3LYP. The calculations were initiated from the crystal structures as well as from optimized structures in which only the positions of the hydrogen atoms were relaxed. The calculated D values show that B3LYP is not suited to predict the ZFS for Mn II complexes with only N/O‐based ligands, whereas with BP86 the quality of the prediction is satisfactory. Moreover, with BP86, the sign of D is correctly reproduced within the series when experimentally determined.