Unquenched orbital angular momentum in quasi-linear two-coordinate transition metal complexes featuring sterically bulky carbazole ligands

Abstract

A series of two-coordinate TM compounds, M(^(tbu)₂carb^(Ph₂CH)₂) (M = Mn, Fe, Co; ^(tbu)₂carb^(Ph₂CH)₂ = 1,8-bis(diphenylmethyl)-3,6-ditertbutylcarbazolide). Each of the complexes displays a bent geometry with a N–M–N bond angle ranging from 140.21(10)° to 144.37(15)°. Despite the non-linear geometry, M(^(tbu)₂carb^(Ph₂CH)₂)₂ (M = Fe, Co) have substantial spin–orbit coupling, and Co(^(tbu)₂carb^(Ph₂CH)₂)₂ displays slow magnetic relaxation with an U_eff value of 30.2 K and a large negative D of −169 cm⁻¹.

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Since the synthesis and crystallization of [Mn(C(SiMe₃)₂)₂], two-coordinate transition metal complexes have challenging targets for synthetic chemists.¹ Their limited coordination sphere confers on these complexes unique reactivity, spectroscopic, and magnetic properties that are not able to be achieved with traditional octahedral coordination geometries for transition metal complexes.^2,3 In particular, linear Fe(II) and Co(II) complexes have been targets for their unique magnetic properties.^4–9 In such systems, the linear coordination geometry results in easily accessible magnetic excited states, leading to unquenched orbital angular momentum and very large magnetic anisotropy for the complex. Two-coordinate compounds such as these have even exhibited single molecule magnetic behavior, including a thermal energy barrier to magnetic relaxation (U_eff) of up to 260 K.¹⁰

The main strategy for designing two-coordinate transition metal complexes typically requires the synthesis of sterically encumbering ligands designed to limit the ability of exogenous ligands and solvents to coordinate to the metal. Secondary silylamide ligands are commonly chosen for this purpose because their synthesis is highly modular.^6–11 These ligands are readily synthesized by reacting a primary amine with an array of silyl chlorides (ClSiR₃; R = Me, Ph, iPr). By changing the identity of the R group on the ClSiR₃, the steric profile of these ligands is easily tuned. However, the Si–N bond is highly susceptible to hydrolysis from trace amounts of water, adding to the synthetic challenge of these complexes. Secondary amines featuring C–N bonds are an attractive alternative to silylamides because the C–N bonds are not susceptible to hydrolysis. Bulky secondary amide ligands have been utilized to make two-coordinate transition metal compounds, such as Fe[N(tBu)₂]₂, which exhibit high degrees of spin–orbit coupling and excellent magnetic properties.⁴ However, the scope of these ligands is more limited than silylamides, making systematic studies of these complexes difficult.

Carbazole is a secondary amine that is easily modifiable at various points around its aromatic rings. Functionalization of the 1 and 8 positions on the carbazole backbone with a sterically bulky group maximizes the degree to which they will block coordination of the metal by other ligands.^12–17 This strategy has been used to synthesize two-coordinate transition metal complexes; however, close contacts between the metal and the phenyl groups of the ligand deleteriously impact their magnetic properties.¹⁵ Thus, designing a bulky carbazole ligand that limits these metal–ligand peripheral interactions will enable the synthesis of two-coordinate transition metal complexes exhibiting large magnetic anisotropy and spin–orbit coupling.

Herein we present the synthesis and magnetic characterization of a new super-bulky carbazole ligand, 1,8-bis(diphenylmethyl)-3,6-ditertbutylcarbazole (^(tbu)₂carbH^(Ph₂CH)₂) (1), featuring diphenyl methyl groups in the 1 and 8 positions of the carbazole backbone. This ligand was metalated to give a series of two-coordinate transition metal compounds, Mn(^(tbu)₂carb^(Ph₂CH)₂)₂ (2-Mn), Fe(^(tbu)₂carb^(Ph₂CH)₂)₂ (2-Fe) and Co(^(tbu)₂carb^(Ph₂CH)₂)₂ (2-Co). These compounds have been structurally characterized using X-ray crystallography, and their magnetic properties were probed by SQUID magnetometry. These compounds are the first two-coordinate compounds using carbazole ligands that feature substantial magnetic anisotropy.

Compound 1 was synthesized by an electrophilic aromatic substitution reaction between two equivalents of diphenylmethanol and one equivalent of 3,6-di-tertbutylcarbazole in yields of up to 60% (Scheme 1a). The ligand molecule crystallized in the monoclinic space group C2/c with a crystallographic C₂ axis passing through the nitrogen atom, bisecting the five-membered ring. The X-ray crystal structure of the ligand shows that the flanking phenyl rings of the carbazole are positioned perpendicular to its plane, creating a pocket in which the metal atom can sit while attached to the N atom (Fig. 1). Deprotonation of 1 with benzyl potassium immediately generated the potassium carbazolide, which was then reacted with a suitable transition metal halide in a 2 : 1 ligand to metal ratio to afford bis-ligated transition metal compounds in yields of up to 30%, (2-Mn, 2-Fe, 2-Co) (Scheme 1b). Powder X-ray diffraction patterns taken on the crystalline material were consistent with powder patterns simulated from the single crystal structures (Fig. S11–S12†), indicating the phase purity of the isolated compounds.

Scheme 1 a) Synthesis of 1. (b) Synthesis of 2-Mn, 2-Fe and 2-Co.

Fig. 1 Crystal structures of compounds 1 with numbering scheme (left) and 2-Co (right) (H = white, C = grey, N = blue, Co = brown). All non-H atoms are drawn as 50% thermal probability ellipsoids.

Compounds 2-Mn, 2-Fe and 2-Co each crystallized in the space group P1̄ and are isomorphic to each other. The asymmetric unit of these structures consists of a single molecule of 2-TM (Fig. 1). The crystal structure shows that each compound is a two-coordinate quasi-linear compound consisting of two ^(tbu)₂carb^(Ph₂CH)₂ ligands bonded to a transition metal ion. The N–TM bond distances range from 2.023(2) Å and 2.029(2) Å for 2-Mn to 1.945(3) Å for 2-Co. This decrease in bond length is consistent with the trend in ionic radii of these elements.¹⁸ The N₁–M and N₂–M bond distances in compounds 2-Mn, 2-Fe and 2-Co are in line with those of other high-spin TM compounds stabilized by carbazole-based ligands,^14,15 but are significantly longer than the typical N–M bond distances reported for two-coordinate M(amide)₂ compounds.^{2,4,5,9,19–25}

Each of these compounds exhibits a significant deviation from the ideal linear geometry of a two-coordinate complex, with N–M–N bond angles ranging from 140.21(10)° to 144.37(15)° for 2-Mn, 2-Fe, and 2-Co. In each of these structures, there is also a close contact between the ipso carbon of one of the flanking phenyl rings and the metal ranging from 2.662(2) Å in 2-Mn to 2.532(2) Å in 2-Co. This close contact provides an explanation for why the N–M–N bond angle deviates from linearity. By adopting a non-linear coordination geometry, the ligand brings the phenyl group into a position where it can interact with the partially filled 3d orbitals of the metal.

The static direct current (dc) magnetic susceptibility (χT) properties of compounds 2-Mn, 2-Fe and 2-Co were measured at temperatures ranging from 300 to 1.8 K under an applied magnetic field of 0.1 T (Fig. 2). The room temperature χT value of 3.96 cm³ K mol⁻¹ for 2-Mn is slightly lower than the theoretically calculated spin-only value for a high-spin Mn^II species (S = 5/2, χT = 4.38 cm³ K mol⁻¹), which is attributed to the difficulty in obtaining an accurate mass of the sample and eicosane due to the presence of static charge in the glove box. The magnetic susceptibility of 2-Mn gradually decreases with decreasing temperature, reaching a minimum value of 3.43 cm³ K mol⁻¹ at 1.8 K. This deviation from Curie behavior is indicative of presence of a small zero-field splitting in the system. Using the program PHI,²⁶ a fit of χT was obtained between 1.8 to 300 K, using a temperature independent paramagnetic (TIP) parameter of 0.001 cm³ mol⁻¹ K, g_iso = 1.80 and D = −0.44 cm⁻¹ where g_iso is the isotropic electronic g-factor and D is the axial zero-field splitting parameter. Further dc measurements at 1 T and 5 T were carried out for 2-Mn and a field-dependent χT behavior was observed at high temperatures (Fig. S14†). This behavior is likely due to the presence of a small superparamagnetic impurity.^27,28

Fig. 2 The temperature dependence of χT obtained for compounds 2-Mn (dotted green), 2-Fe (dotted orange) and 2-Co (dotted blue) between 1.8 and 300 K under an applied field of 0.1 T. The solid lines correspond to fits obtained from PHI.

The magnetic susceptibility of 2-Fe at 300 K is 3.69 cm³ K mol⁻¹, which is significantly higher than the expected spin-only values for high-spin Fe^II (S = 2, χT = 3.0 cm³ K mol⁻¹) systems. As the temperature decreases, χT increases, reaching a maximum of 4.74 cm³ K mol⁻¹ at 105 K. χT decreases precipitously as the temperature is lowered from 105 K, reaching a minimum value at 1.8 K. This behavior is indicative of strong spin–orbit coupling and qualitatively matches the behavior of a series of two-coordinate, high-spin Fe(amide)₂ compounds.^4,5,19 In those compounds, strong spin–orbit coupling was observed only when the N–Fe–N bond angle was close to 180° while substantial deviations from linearity resulted in significant quenching of the orbital angular momentum. Of particular note in that study was Fe[N(H)Ar^#]₂ (Ar^# = C₆H₃-2,6-(C₆H₂-2,4,6-Me₃)₂) which had a N–Fe–N bond angle of 140.9(2)° and displayed almost entirely quenched orbital angular momentum.^5,10 What is notable about 2-Fe is that it shows largely unquenched orbital angular momentum despite the significant deviation from linearity. One possible explanation is that the metal–ligand bond distances of 2-Fe are significantly longer than those of these other two-coordinate Fe(amide)₂ compounds (vide supra). As the Fe–N bonds are elongated, the ligand field for the molecule weakens, resulting in overall lower splitting between the Fe d-orbitals. This in turn lowers the energy separation between the ground state and low-lying magnetic excited states, resulting in the higher degree of spin–orbit coupling.

The dc magnetic susceptibility of 2-Co showed a room-temperature magnetic susceptibility of 2.56 cm³ K mol⁻¹ at 300 K, which is substantially larger than the expected spin-only value for a high spin Co^II ion (S = 3/2, χT = 1.88 cm³ K mol⁻¹). Upon cooling, a gradual decrease in χT is observed. Such behavior of χT may be attributed to spin–orbit coupling in 2-Co.^23,29 The best fit of the χT data of 2-Co yields g_iso = 2.38, D = −169 cm⁻¹ and zJ = −0.13 cm⁻¹. The large negative D value is consistent with significant axial anisotropy in 2-Co.

Temperature and frequency dependent alternating current (ac) magnetic susceptibility measurements were carried out for 2-Mn, 2-Fe, and 2-Co. Under zero applied dc field within the frequency range of 1–1000 Hz, all three compounds exhibit fast magnetic relaxation, with 2-Co showing the beginnings of out-of-phase signals near 1000 Hz (Fig. S15, S17–S21†). Upon application of a 1500 Oe dc magnetic field, 2-Co displayed temperature dependent out-of-phase magnetic susceptibility signals (χ′′) from 1.8 to 6.8 K (Fig. 3). Characteristic relaxation times for each temperature of 2-Co were determined by fitting the Cole–Cole plot of χ′ and χ′′ to a generalized Debye model using CC-FIT2.^30,31 Using eqn (1), the best fit of the relaxation times was obtained with a combination of Orbach and Raman relaxations, generating fit parameters of C = 9.33 s⁻¹ K⁻ⁿ, n = 1.82, τ₀ = 3.79 × 10^–6 s and U_eff = 30.2 K (Fig. 3).

τ⁻¹ = τ₀⁻¹ exp(U_eff/T) + CTⁿ (1)

Fig. 3 Out-of-phase ac magnetic susceptibility data for compound 2-Co from 1.8 to 6.8 K under an applied magnetic field of 1500 Oe. Solid lines correspond to the fit obtained from CC-FIT2 (top). Temperature dependence of relaxation times (τ) in the presence of an applied field of 1500 Oe (bottom).

While the U_eff of 2-Co is lower than the U_eff of other two-coordinate cobalt(II) SMMs, each of these complexes possesses a strictly linear coordination geometry.^29,32,33 The relaxation dynamics of 2-Co compares favorably to a previously reported bent two-coordinate Co(II) compound with N–Co–N bond angle of 148.66(9)° that displayed slow magnetic relaxation.³⁴ However, in that case, the magnetic relaxation was attributed exclusively to Raman relaxation.

In order to explain the magnetic properties seen in 2-Co, complete active space self-consistent field (CASSCF) along with the N-electron valence state perturbation theory (NEVPT2) were employed to calculate the g-factor and zero-field splitting parameters as well as to elucidate the effects of dynamic electron correlation (Fig. 4).35 The final active space for the CASSCF/NEVPT2 calculation of 2-Co consisted of the five 3d-orbitals on the cobalt center, two orbitals on the nitrogen atoms of the ligands and five second d-subshell orbitals. The calculated g-factor (2.50) and large negative D value (−161 cm−1) are in excellent agreement with those derived from the PHI fit (vide supra). The calculated E/D ratio (0.02) highlights the axial anisotropy of the metal center. The d-orbital splitting pattern reveals that orbitals dx2−y2 and dxy are almost degenerate and the lowest in energy. The dxz orbital is slightly higher in energy than dyz, while the dz2 orbital is the highest in energy. Based on the NEVPT2 results, the ground state electronic configuration of 2-Co is calculated to be (dxy, dx2−y2)3, dyz2, dxz1, dz21, indicating a non-Aufbau ground state. Such non-Aufbau ground state electronic configurations have been reported in select linear and quasi-linear two-coordinate cobalt(II) complexes32,34 and in a ferrocenium complex.36 This unconventional electronic configuration may arise to minimize the interelectronic repulsion between the dx2−y2 and dxy orbitals. The uneven filling of these nearly degenerate orbitals results in a first excited state that is only 464.6 cm−1 higher in energy than the ground state, consistent with the strong spin–orbit coupling that was experimentally observed in 2-Co.

Fig. 4 Energy diagram depicting the orbital contributions and electron occupations of seven active space orbitals (3d metal + two ligand-based orbitals) of 2-Co in ground state from CASSCF/NEVPT2 calculation. H-atoms are omitted for clarity (isosurface value 0.03).

In summary, we have synthesized and characterized a series of novel two-coordinate first-row transition metal complexes employing a sterically bulky carbazole-based ligand. Despite substantial deviation from linear geometry, unquenched orbital angular momentum is observed in 2-Fe and 2-Co. CASSCF/NEVPT2 calculations reveal a non-Aufbau electronic ground state for 2-Co. Under an applied dc field, 2-Co exhibits slow relaxation of the magnetization with U_eff = 30.2 K. Future work will examine methods for tuning the interaction between the metal center and the peripheral groups of the ligands to improve their magnetic properties.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this work is available in the main article and the ESI.† Crystallographic data for 1, 2-Mn, 2-Fe, and 2-Co has been deposited at the CCDC (2432502–2432505).†

Acknowledgements

The authors wish to thank John Berry (UW – Madison) for the use of the SQUID magnetometer. BSD thanks West Virginia University for their generous startup funding. The authors also acknowledge funding from the National Science Foundation (CHE – 1336071, X-ray Crystallography); (MRI – 1726534, High Performance Computational Cluster). The Quantum Design MPMS3 SQUID magnetometer with an EverCool System was supported by the UW-Madison Department of Chemistry.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, X-ray crystallographic structures. See DOI: https://doi.org/10.1039/d5dt00782h

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