Magnetic Properties and Electronic Structure of the S = 2 Complex [Mn^III{(OPPh₂)₂N}₃] Showing Field-Induced Slow Magnetization Relaxation

Abstract

The high-spin S = 2 Mn(III) complex [Mn-{(OPPh 2 ) 2 N} 3 ] (1 Mn ) exhibits field-induced slow relaxation of magnetization (Inorg. Chem. 2013, 52, 12869). Magnetic susceptibility and dual-mode X-band electron paramagnetic resonance (EPR) studies revealed a negative value of the zerofield-splitting (zfs) parameter D. In order to explore the magnetic and electronic properties of 1 Mn in detail, a combination of experimental and computational studies is presented herein. Alternating-current magnetometry on magnetically diluted samples (1 Mn /1 Ga ) of 1 Mn in the diamagnetic gallium analogue, [Ga-{(OPPh 2 ) 2 N} 3 ], indicates that the slow relaxation behavior of 1 Mn is due to the intrinsic properties of the individual molecules of 1 Mn . Investigation of the single-crystal magnetization of both 1 Mn and 1 Mn /1 Ga by a micro-SQUID device reveals hysteresis loops below 1 K. Closed hysteresis loops at a zero direct-current magnetic field are observed and attributed to fast quantum tunneling of magnetization. High-frequency and -field EPR (HFEPR) spectroscopic studies reveal that, apart from the second-order zfs terms (D and E), fourth-order terms (B 4 m ) are required in order to appropriately describe the magnetic properties of 1 Mn . These studies provide accurate spin-Hamiltonian (sH) parameters of 1 Mn , i.e., zfs parameters |D| = 3.917(5) cm −1 , |E| = 0.018(4) cm −1 , B 0 4 = B 4 2 = 0, and B 4 4 = (3.6 ± 1.7) × 10 −3 cm −1 and g = [1.994(5), 1.996(4), 1.985(4)], and confirm the negative sign of D. Parallel-mode Xband EPR studies on 1 Mn /1 Ga and CH 2 Cl 2 solutions of 1 Mn probe the electronic−nuclear hyperfine interactions in the solid state and solution. The electronic structure of 1 Mn is investigated by quantum-chemical calculations by employing recently developed computational protocols that are grounded on ab initio wave function theory. From computational analysis, the contributions of spin−spin and spin−orbit coupling to the magnitude of D are obtained. The calculations provide also computed values of the fourthorder zfs terms B 4 m , as well as those of the g and hyperfine interaction tensor components. In all cases, a very good agreement between the computed and experimentally determined sH parameters is observed. The magnetization relaxation properties of 1 Mn are rationalized on the basis of the composition of the ground-state wave functions in the absence or presence of an external magnetic field.

Record High Magnetic Anisotropy in Three-Coordinate MnIII and CrII Complexes: A Theoretical Perspective

“…In the early years of SMMs, the focus has been on TM cluster, particularly that of Mn­(III) ions, as this offers an easy source of a negative D parameter for the chemists and unearthed numerous SMMs, albeit with smaller U eff / T B values. , The Mn­(III) ions are very robust and can be easily incorporated in cluster aggregation and are relatively redox stable but exhibit only small D values of the order of ∼|5| cm –1 . While several low-coordinate TM ions were pursued recently for potential SMMs, Mn­(III) has not been studied in detail, perhaps due to the perception that the expected D values are rather small . Apart from U eff values, the blocking temperature is an important criterion, which is often very small for TM SMMs.…”