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

Eleven new mononuclear manganese(111) complexes prepared from two hexadentate ligands, Li and L2, with different degrees of steric bulk in the substituents are reported. Li 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-Sa1(2)323 to indicate the 323 alkyl connectivity in the starting 5 tetraamine and the substitution (R) on the phenolate ring. Li 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 Li, namely, [MnL1]NO3, (1e). The BPh, and PF6- complexes [MnL1]3Ph(4), (1a), [MnL2]3Ph(4), (2a), [MnL1]PF6, (lb'), and [MnL2]-PF6, (2b), with both ligands Li and L2, remain high-spin (HS) over the measured temperature range. However, the onohydrate of (1b) [MnL1]PF6 center dot H2O, (1b), shows gradual spin-crossover (SCO), as do the ClO4, BF4-, and NO3 complexes [MnL1]ClO4 center dot H2O, (1c), [MnL2]ClO4, (2c), [MnL1](3)F4 center dot H2O, (Id), [MnL1]3F(4).0.4H(2)O, (2d), [MnL1]NO3, (1e), and [MnL2]NO3.EtOH, (2e). The three complexes formed with ethoxy-substituted ligand L2 all show a higher Tin than the analogous complexes with methoxy-substituted ligand Li. Analysis of distortion parameters shows that complexes formed with the bulkier ligand L2 exhibit more deformation from perfect octahedral geometry, leading to a higher T-1/2 in the SCO examples, where Tin 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 (lb) 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).