Substituent Effects on Spin State in a Series of Mononuclear Manganese(III) Complexes with Hexadentate Schiff-Base Ligands

Abstract

Eleven new mononuclear manganese(III) complexes prepared from two hexadentate ligands, L1 and L2, with different degrees of steric bulk in the substituents are reported. L1 and L2 are Schiff bases resulting from condensation of N,N'-bis(3-aminopropyl)ethylenediamine with 3-methoxy-2-hydroxybenzaldehyde and 3-ethoxy-2-hydroxybenzaldehyde respectively, and are members of a ligand series we have abbreviated as R-Sal2323 to indicate the 323 alkyl connectivity in the starting tetraamine and the substitution (R) on the phenolate ring. L1 hosts a methoxy substituent on both phenolate rings, while L2 bears a larger ethoxy group in the same position. Structural and magnetic properties are reported in comparison with those of a previously reported analogue with L1, namely, [MnL1]NO3, (1e). The BPh4(-) and PF6(-) complexes [MnL1]BPh4, (1a), [MnL2]BPh4, (2a), [MnL1]PF6, (1b'), and [MnL2]PF6, (2b), with both ligands L1 and L2, remain high-spin (HS) over the measured temperature range. However, the monohydrate of (1b') [MnL1]PF6·H2O, (1b), shows gradual spin-crossover (SCO), as do the ClO4(-), BF4(-), and NO3(-) complexes [MnL1]ClO4·H2O, (1c), [MnL2]ClO4, (2c), [MnL1]BF4·H2O, (1d), [MnL2]BF4·0.4H2O, (2d), [MnL1]NO3, (1e), and [MnL2]NO3·EtOH, (2e). The three complexes formed with ethoxy-substituted ligand L2 all show a higher T1/2 than the analogous complexes with methoxy-substituted ligand L1. Analysis of distortion parameters shows that complexes formed with the bulkier ligand L2 exhibit more deformation from perfect octahedral geometry, leading to a higher T1/2 in the SCO examples, where T1/2 is the temperature where the spin state is 50% high spin and 50% low spin. Spin state assignment in the solid state is shown to be solvate-dependent for complexes (1b) and (2e), and room temperature UV-visible and NMR spectra indicate a solution-state spin assignment intermediate between fully HS and fully low spin in 10 complexes, (1a)-(1e) and (2a)-(2e).

Spin Crossover in cis Manganese(III) Quinolylsalicylaldiminates

“…It follows that the spin crossover is incomplete at both ends of the spin transition over this temperature range. Similar incomplete and gradual spin crossover is also observed in [Mn(3‐OMe‐sal 2 bapen)]BF 4 · H 2 O and [Mn(3,5‐Br‐sal 2 bapen)]OTf · EtOH 21,22. In contrast, in [Mn(qsal‐Br) 2 ] + χ M T rises more steadily from 2.00 to 3.05 cm 3 mol –1 K as the sample is warmed to room temperature.…”

“…Similar small changes in the positions of the LMCT bands are also observed in [Mn II (H5‐Me‐php)(5‐Me‐php)]ClO 4 and [Mn III (3,5‐ t Bu 2 ‐php) 2 ]ClO 4 {5‐Me‐php = 5‐methyl(pyridin‐2‐ylhydrazonomethyl)phenolate; 3,5‐ t Bu 2 ‐php = 3,5‐di‐ tert ‐butyl(pyridin‐2‐ylhydrazonomethyl)phenolate} 33. Moreover, the position of the band at between 430–460 nm is consistent with the Mn III complexes being high spin 21…”

Schiff Base Ligands Derived from l‐Histidine Methyl Ester: Characterization, Racemization, and Dimerization of Their Transition‐Metal Complexes

“…The Mn1–O1 and Mn1–O4 bond lengths are 1.885(7) and 1.913(7) Å, respectively, which are consistent with those of Mn III –O(phenolato) bonds in a series of Mn II , Mn III , and Mn IV complexes with a similar tripodal Schiff base ligand . However, the Mn1–N1 bond at 2.267(10) Å and the Mn1–N4 bond at 2.268(10) Å are significantly longer than typical Mn III –N(imine) bonds (and those of the corresponding Co III complex described below), owing to the Jahn–Teller effect of a high‐spin Mn III ion.…”

Spin-state switches in molecular materials chemistry

“…The spincrossover phenomenon is most heavily studied in iron(II) chemistry, 12 and is also well-known in iron(III) 13 and cobalt(II) 14 compounds. Complexes of other 3 d metal ions, notably recently manganese(III), 15 can also exhibit the effect however.…”