Spin Crossover in cis Manganese(III) Quinolylsalicylaldiminates

Abstract

Two new MnII/III redox pairs, [Mn(qsal‐Cl)2]0/+ 10/+ and [Mn(qsal‐Br)2]0/+ 20/+ have been synthesized employing the tridentate Schiff base ligands, 5‐X–N‐(8‐quinolyl)salicylaldimine (Hqsal‐X, X = Cl and Br). The neutral MnII complexes 1 and 2 were prepared from MnCl2 and Hqsal‐Cl and Hqsal‐Br while oxidation of 1 or 2 with AgOTf yields the cationic MnIII complexes, [Mn(qsal‐Cl)2]OTf, 1+, and [Mn(qsal‐Br)2]OTf, 2+. 1 and 2 have been characterized by single‐crystal X‐ray diffraction as CH2Cl2 solvates. The Mn centres adopt a strongly distorted octahedral geometry with cis O donors due to the pair of meridionally bound qsal‐X ligands. Electrochemical studies indicate two reversible one‐electron redox processes, Mn2+/3+ and Mn3+/4+. Spectroscopic studies show the LMCT bands move to lower wavelengths by ca. 30 nm while the νC=N stretches are little changed in the cationic complexes. Variable temperature magnetic susceptibility measurements indicate that 1+ and 2+ undergo gradual half spin crossover, despite having cis O donors. DFT calculations reveal a small HS–LS gap in the MnIII systems consistent with spin crossover and provide insight into the ligand design necessary for spin crossover in cis‐N4O2 MnIII compounds.

Spin Crossover in 3D Metal Centers Binding Halide-Containing Ligands: Magnetism, Structure and Computational Studies

“…However, solvated complexes tend to be more stable in the LS state. Harding and co-workers reported [72] the OTf − salts of two Mn(III) [Mn(L3) 2 ] + complexes containing the 5-X-N-(8-quinolyl)salicylaldimine ligands (Hqsal-X, L3, X = Cl and Br). Crystals of these complexes could be isolated from the oxidation of Mn(II) but were very sensitive to degradation.…”

Challenges in assignment of orbital populations in a high spin manganese(iii) complex

“…In reality only S=1 and S=2 spin states have been observed in small molecule complexes and a small subset of these show thermal switching between the high spin (HS) S=2 and the so-called low spin (LS) S=1 states. [30][31][32][33][34][35][36][37][38][39][40][41] As both the S = 1 and S = 2 states are Jahn-Teller active it is of interest to examine how the distortion is manifest in the HS form of spin crossover (SCO) complexes + cations, where R-Sal 2 323 is a hexadentate dianionic Schiff base ligand. 28,[30][31][32][33]38 To date an axial compression has been suggested for the HS forms of such complexes which has prompted some computational studies 40, 41 with the starting assumption that the magnetic orbital is d x 2 y 2 .…”

Homochiral Mn3+ Spin-Crossover Complexes: A Structural and Spectroscopic Study

“…Identification of the sign of D is particularly relevant to the current work because, although the majority of known (HS) Mn 3+ complexes display a pronounced axial elongation due to the Jahn–Teller effect, most Mn 3+ SCO complexes appear to have a marked compression in the spin quintet form. 19 , 31 , 33 , 37 , 69 , 88 , 105 − 112 An axially compressed 89 − 100 form may assist the transition to a spin triplet arrangement as the energetic order of orbitals should match that expected in the S = 1 form of the ion ( Figure 1 ). Diffraction alone, however, is not sufficient to discern if this is a genuine compression, but it can be resolved by measuring the sign of D by EPR at high fields.…”