Cyclooctatetraenide-based single-ion magnets featuring bulky cyclopentadienyl ligand

We report a family of organometallic rare-earth complexes with the general formula (COT)M(Cpttt) (where (COT)2− = cyclooctatetraenide, (Cpttt)− = 1,2,4-tri(tert-butyl)cyclopentadienide, M = Y(iii), Nd(iii), Dy(iii) and Er(iii)). Similarly to the prototypical Er(iii) analog featuring pentamethylcyclopentadienyl ligand (Cp*)−, (COT)Er(Cpttt) behaves as a single-ion magnet. However, the introduction of the sterically demanding (Cpttt)− imposes geometric constraints that lead to a simplified magnetic relaxation behavior compared to the (Cp*)− containing complexes. Consequently, (COT)Er(Cpttt) can be viewed as a model representative of this organometallic single-ion magnet architecture. In addition, we demonstrate that the increased steric profile associated with the (Cpttt)− ligand permits preparation, structural characterization and interrogation of magnetic properties of the early-lanthanide complex, (COT)Nd(Cpttt). Such a mononuclear derivative could not be obtained when a (Cp*)− ligand was employed, a testament to larger ionic radius of this early lanthanide ion.

Toluene, diethyl ether, and pentane were dried using the commercial MBraun SPS-800 solvent purification system, degassed via three freeze-pump-thaw cycles, and stored over 4Å molecular sieves (MS) under argon atmosphere. Tetrahydrofuran and benzene-d6 were distilled from sodium in presence of benzophenone and stored over 4Å MS under argon atmosphere. 1,3,5,7-Cyclooctatetraene was dried over 4Å MS, vacuum transferred, degassed via three freeze-pump-thaw cycles, and stored under argon atmosphere. All of the liquid state nuclear magnetic resonance (NMR) spectra were recorded on a Bruker 500 MHz (11.7 T) NMR spectrometer. The 1 H and 13 C chemical shifts were calibrated to the residual signals from the deutered solvent. 6 In acquisition of 1 H spectra 1 s relaxation delay was used. 89 Y spectra were recorded using a Bruker anti-ringdown (aring) pulse sequence with 5 s interscan relaxation. The liquid-state 89 Y NMR spectra were referenced to an aqueous solution of YCl 3 ·6 H 2 O at 0 ppm. Attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) spectra were collected using a Bruker ALPHA spectrometer equipped with MIR source, KBr beamsplitter, RT-DLaTGS detector and Platinum ATR accessory housed in argonfilled glovebox. The data were collected between 4000 and 400 cm -1 with 4 cm -1 averaging S-2 over 24 scans. The transmission FTIR experiments aimed at probing the f-f optical transitions were performed at the IR beamline of the Swiss Light Source at the Paul Scherrer Institut, Switzerland, which is equipped with a Bruker IFS125 HR spectrometer with an optical path difference of up to 12.6 m. A CaF 2 beam splitter and a liquid nitrogen-cooled InSb detector were used to access the energy range of ≈ 6000 -7000 cm -1 . The spectral resolution was set to 0.05 cm -1 after verifying that the observed linewidths are not instrument limited by performing test scans with a resolution of 0.005 cm -1 . Polycrystalline solid samples were finely ground and pressed into pellets of 0.3 mm thickness with a previously outgassed commercial KBr powder (Sigma Aldrich). The samples were prepared in inert He gas environment and transferred into a continuous-flow He cryostat equipped with NIR grade quartz windows. The heatsink base was kept at low temperature (3 K).
Single crystal X-Ray diffraction (SCXRD) data were collected using either a Bruker D8 Advance diffractometer equipped with APEX II CCD detector or an Oxford Diffraction Excalibur S diffractometer equipped with Sapphire 3 CCD detector, both utilizing a Mo K α source (λ = 0.71073 Å), and performing φ-and ω-scans. The air-sensitive single crystals were isolated in the glovebox and covered with oil to prevent the decomposition.
The crystals were promptly mounted on the goniometer and placed in a stream of cold dinitrogen to cool the sample down (100 K) and to prevent decomposition. The structures were solved using either the direct methods implemented in SHELXS 7 or the intrinsic phasing method implemented in SHELXT 8 and refined by least-squares approach using SHELXL 2018/3,, 9 all interfaced via Olex 2 software. 10 All non-hydrogen atoms were refined anisotropically. For all M ttt and Nd ttt ·THF hydrogen atoms were located in the difference Fourier map and refined isotropically. For each aromatic ring a FLAT instruction was applied and C-C (and in some cases aromatic C-H) distances were restrained with SADI instructions. Rigid body restraints (DELU instruction) were applied to carbon atoms constituting each aromatic ring. For K 2 [(COT)Nd(Cp*)] 4 (OTf) 2 the hydrogen atoms that could not be located in the difference maps were placed in calculated positions using AFIX 137 and 43 instructions for methyl group and aromatic protons, respectively, and refined using the riding model. The C-C distances in the non-disordered (COT) 2rings were fixed using the DFIX instruction to 1.41 Å, a value based on the other structures featuring this ligand. A combination of DELU, RIGU, ISOR and EADP instructions was applied to refine the disorder present within the crystal structure.
The static susceptibility measurements were performed on solid polycrystalline samples with a Quantum Design MPMS-XL SQUID magnetometer. The following values of magnetic field were used 0.2 kOe, 2 kOe and 10 kOe for the temperature ranges of 2-20 K, 20-80 K and 80-300 K, respectively, in order to prevent any saturation effect in the case of Er ttt and Dy ttt ; in the case of Nd ttt 2 kOe and 10 kOe were used for the temperature S-3 ranges of 2-20 K and 20-300 K, respectively. The AC magnetic susceptibility measurements were performed on Quantum Design MPMS-XL SQUID magnetometer (0.01-1000 Hz frequency range). Owing to the extreme air sensitivity of the materials, for all of the measurements the samples were sealed in quartz EPR tubes. All materials were mixed with eicosane. Each mixture was placed at the bottom of an EPR tube and sealed under high vacuum. The bottom of the tube was warmed at 40°C to melt eicosane, which upon cooling formed a solid matrix adhering to the walls of the tube. The measurements were all corrected for the intrinsic diamagnetic contribution as calculated with Pascal's constants 11 and also for the diamagnetic contribution of the eicosane.

Computational methods
The geometry optimizations for molecular models were performed in ORCA 5.0.2 computational package 12-14 utilizing the PBE0 functional. 15 Zeroth-Order Regular Approximation (ZORA) [16][17][18] was used to account for the relativistic effects. ZORA-def2-TZVPP and SARC-ZORA-TZVPP basis sets were used for main group and rare-earth elements, respectively, while an additional auxiliary basis set SARC-J was applied. 19-23 A higher precision of the DFT grid was achieved by increasing the radial grid on the rare-earth metal to 6. To speed up the calculations, the 'resolution of the identity' approximation in terms of a Split-RI-J variant in combination with the 'chain-of-spheres exchange' approximation (RIJCOSX) 24 was used to generate the Coulomb and the Hartree-Fock S-5 exchange integrals. Additionally, a faster convergence in the SCF steps was achieved by the applying KDIIS 25 and SOSCF 26 keywords. In all calculations the DFT-D3 diffusion correction with Becke-Johnson damping function (D3BJ) was applied. 27,28 Calculation of the electronic structure and magnetic properties were performed with State-Averaged Complete Active Space Self-Consistent Field approach with Restricted Active Space State Interaction method (SA-CASSCF/RASSI-SO) included in the Open-Molcas 19.11 version. 29 The performed multi-configurational approach used the Douglas-Kroll Hamiltonian to treat the relativistic effects in two steps. In the basis-set generation scalar terms are included and are further used in the determination of the CASSCF wavefunctions and energies. 30 The calculated CASSCF wavefunctions are mixed within the RASSI-SO method to account for spin-orbit coupling. 31,32 The active space was chosen to span the seven 4f orbitals populated with three, nine and eleven 4f electrons for Nd(III), Dy(III) and Er(III), respectively. In described active space, state-averaged on the CASSCF level were considered for the CASPT2 step. In the final step, spin-orbit coupling was introduced with the RASSI module and the SINGLE_ANISO routine was used to compute the magnetic properties. To give more insights into the orientation of the magnetic axis, the molecular electrostatic potential is calculated from the ab initio S-6 LOPROP charge analysis 41 (eq. 1)

Calculations of percent buried volume in the investigated complexes
To gain an insight into the steric differences between the newly synthesized (Cp ttt )and the published (Cp*)derivatives we performed calculations of the 'percent buried volume' using the SambVca 2.1 software package. 44 This parameter quantifies the volume fraction of the imaginary sphere of a pre-defined radius that is centered around the metal ion that is occupied by the ligand. Consequently, it can be used as a gauge for the steric properties of the ligand environment. 45 We used the experimentally determined SCXRD structures as a starting point for the buried volume calculations. The sphere radius was selected based on the distance to the furthest methyl carbon belonging to the tert-butyl moieties located on the same side of the cyclopentadienyl ring as the metal center. This was kept constant across derivatives containing the same metal ions (e.g. Er ttt and Er*) to ensure comparability of the results. As anticipated, the presence of the (Cp ttt )ligand leads to an increased buried volume compared with the respective (Cp*)containing species. These results substantiate our conclusions regarding the impact of the ligand sterics on the solid-state structure of the compounds discussed in this report.              Oe external DC field.        S-51   S-53