Extreme g-Tensor Anisotropy and Its Insensitivity to Structural Distortions in a Family of Linear Two-Coordinate Ni(I) Bis-N-heterocyclic Carbene Complexes

Abstract

We report a new series of homoleptic Ni(I) bis-Nheterocyclic carbene complexes with a range of torsion angles between the two ligands from 68°to 90°. Electron paramagnetic resonance measurements revealed a strongly anisotropic g-tensor in all complexes with a small variation in g ∥ ∼ 5.7−5.9 and g ⊥ ∼ 0.6. The energy of the first excited state identified by variable-field far-infrared magnetic spectroscopy and SOC-CASSCF/NEVPT2 calculations is in the range 270−650 cm −1 . Magnetic relaxation measured by alternating current susceptibility up to 10 K is dominated by Raman and direct processes. Ab initio ligand-field analysis reveals that a torsion angle of <90°causes the splitting between doubly occupied d xz and d yz orbitals, which has little effect on the magnetic properties, while the temperature dependence of the magnetic relaxation appears to have no correlation with the torsion angle.

Cationic Nickel d9‐Metalloradicals [Ni(NHC)2]+

“…In general, the complexes bearing NHCs with saturated backbones ( 2 + : g 2 =1.02, g 3 =0.84 and 4 + : g 2 =1.37, g 3 =1.07) revealed higher g 2 and g 3 values than their counterparts with unsaturated NHC‐backbones ( 1 + : g 2 ∼0.46, g 3 =outside the range of the magnetic field and 3 + : g 2 =0.68, g 3 =0.58) and the N ‐Dipp substituted carbenes led to higher g 2 / g 3 values compared to the N ‐Mes substituted NHCs. Thus, we found the most extreme g ‐tensor anisotropy for compound 1 + ( g 1 =5.77, g 2 ∼0.46, g 3 =outside the range of the magnetic field), which is in the same region as reported for the complexes C1 − C4 ( g 1 =5.66–5.89, g 2 =0.56–0.62, g 3 =0.55–0.58) [18] . For the cAAC Me stabilized complex 5 + g ‐tensors which are much less anisotropic ( g 1 =3.73, g 2 =2.50, g 3 =1.67) were observed.…”

“…Measurements of the magnetic moments μ eff in solution of compounds 1 + – 5 + (Evans method) in THF‐d 8 or CD 2 Cl 2 ( 5 + ) revealed values between 2.26–3.15 μ B . All values are significantly larger than the spin‐only value of 1.73 μ B , but also differ certainly from the values of 3.0–3.3 μ B observed for linear complexes [Ni I (NHC) 2 ] + C1 − C4 stabilized by six‐ and seven‐membered NHC ligands [18] . To get further insight into the magnetic properties of 1 + – 5 + EPR experiments were performed on frozen solutions of each of the complexes ( 1 + – 4 + in THF and 5 + in DCM) as well as on polycrystalline powder samples (compare Table 2 and Figure 6).…”

“…To get further insight into the magnetic properties of 1 + – 5 + EPR experiments were performed on frozen solutions of each of the complexes ( 1 + – 4 + in THF and 5 + in DCM) as well as on polycrystalline powder samples (compare Table 2 and Figure 6). The powder spectra of 1 + – 4 + revealed highly anisotropic g ‐tensors, with g 1 values between 5.09–5.77, as it was observed previously for comparable compounds ( C1 − C4 ) [18] . In contrast to Whittlesey's complexes C1 − C4 , the five‐ring NHC complex cations produced very different g 2 and g 3 values, depending on the carbene.…”

“…It was demonstrated that these complexes reveal an orbitally degenerate ground state 2 Δ, which results from a unique crystal‐field splitting ( vide infra ), and therefore leads to very large magnetic anisotropy. The noticeable differences in the low‐temperature magnetic relaxation of these compounds were attributed to different vibrational modes and to spin‐phonon coupling, while the different torsion angles of the ligands seem to have no influence on the relaxation times, respectively [18] …”

“…They also described some structural and magnetic similarities to complex C1 , but did not identify SIM behavior for this complex. Just recently, Whittlesey and co‐workers expanded their work on linear Ni(I) complexes ligated with six‐ or seven‐membered NHC ligands and presented three related complexes [Ni I (7‐Mes) 2 ] + C2 , [Ni I (6‐Xyl) 2 ] + C3 , [Ni I (7‐Xyl) 2 ] + C4 including a detailed discussion about their magnetic properties, focusing on the extreme g ‐tensor anisotropy and its dependence on structural distortion [18] . It was demonstrated that these complexes reveal an orbitally degenerate ground state 2 Δ, which results from a unique crystal‐field splitting ( vide infra ), and therefore leads to very large magnetic anisotropy.…”