A Trigonal Prismatic Cobalt(II) Complex as a Single Molecule Magnet with a Reduced Contribution from Quantum Tunneling

Abstract

Herein, we report a new trigonal prismatic cobalt(II) complex that behaves as a single molecule magnet. The obtained zero‐field splitting, which is also directly accessed by THz‐EPR spectroscopy (−102.5 cm−1), results in a large magnetization reversal barrier U of 205 cm−1. Its effective value, however, is much lower (101 cm−1), even though there is practically no contribution from quantum tunneling to magnetization relaxation.

Coordination [CoII2] and [CoIIZnII] Helicates Showing Slow Magnetic Relaxation

“…These dependencies can be reproduced invoking quantum tunneling (B 1 /(1 + B 2 H 2 )), direct (AH n T), Raman (CT m ) and Orbach (τ 0 exp(-Δ/k B T)) relaxation processes (Table S5). The obtained values for the Orbach relaxation barrier (Δ) were significantly smaller than expected from dcmagnetometry and NMR data, which is consistent with the data obtained previously for other Co(II)-based SMMs, where the magnetic relaxation was described using a combination of Raman and direct relaxation pathways with no Orbach contribution, 25,[44][45] or where the Orbach barrier was smaller than predicted by other methods. [46][47][48] In conclusion, two new complexes are presented with Co (II) ions in a trigonal geometry intermediate between TP-TAP that reduces the large magnetic anisotropy observed in complexes with either of both geometries, verifying previously published predictions.…”

Observability of Paramagnetic NMR Signals at over 10 000 ppm Chemical Shifts

“…Observation and qualitative analysis Following the experimental observation [60] of 1 H and 13 C NMR signals in [ tBu (PNP)Fe-H], our DFT estimates, augmented by contact and pseudocontact terms, predicted a 31 P chemical shift of about À11 000 ppm at room temperature [B3LYP/6-311G(d,p); def2-TZVP (Fe only)], close to where the signal was found experimentally (Figure 2) using 200, 400, and 600 MHz NMR spectrometers. In a 14.1 Tesla magnet, the 31 P NMR resonance was broad and barely visible.…”

“…Both factors may be the reason for the small disagreement between calculated and experimental H 12 shift (Figure 5). Calculated 13 C and other 1 H hyperfine tensors were largely unaffected by the spin-orbit coupling effects, which were neglected in the subsequent analysis.…”

“…[ 3 , 4 ] At the same time, there is a growing interest in the detection and analysis of paramagnetic intermediates in catalytic reaction cycles, particularly for the first‐row transition metals, where the reactive molecular fragments tend to be either directly bonded, or otherwise in close proximity to the metal. [ 5 , 6 , 7 , 8 , 9 , 10 , 11 ] Other areas of current interest are single‐molecule magnets (SMMs),[ 12 , 13 , 14 , 15 ] contrast agents in magnetic resonance imaging (MRI),[ 16 , 17 , 18 ] and pseudocontact shift (PCS) probes in structural biology. [ 19 , 20 , 21 , 22 ] Several recent studies used magneto‐structural correlations derived by multireference ab initio methods to refine chemical structures using paramagnetic NMR shifts.…”

Observability of Paramagnetic NMR Signals at over 10 000 ppm Chemical Shifts

“…[3,4] At the same time, there is a growing interest in the detection and analysis of paramagnetic intermediates in catalytic reaction cycles, particularly for the first-row transition metals, where the reactive molecular fragments tend to be either directly bonded, or otherwise in close proximity to the metal. [5][6][7][8][9][10][11] Other areas of current interest are singlemolecule magnets (SMMs), [12][13][14][15] contrast agents in magnetic resonance imaging (MRI), [16][17][18] and pseudocontact shift (PCS) probes in structural biology. [19][20][21][22] Several recent studies used magneto-structural correlations derived by multireference ab initio methods to refine chemical structures using paramagnetic NMR shifts.…”