Crystalline Germanium(I) and Tin(I) Centered Radical Anions

Abstract An isostructural series of heavy Group 14 E(I) radical anions (Ge, Sn, Pb), stabilized by a bulky xanthene‐based diamido ligand are reported. The radical anions were synthesised by the one‐electron reduction of their corresponding E(II) precursor complexes with sodium naphthalenide in THF, yielding the radical anions as charge‐separated sodium salts. The series of main group radicals have been comprehensively characterized by EPR spectroscopy, X‐ray crystallography and DFT analysis, which reveal that in all cases, the spin density of the unpaired electron almost exclusively resides in a p‐orbital of π symmetry located on the Group 14 center.


S1)
Materials and methods

S4)
Computational details and additional data

S5)
Tabulated EPR and DFT data. Comparison to literature

S6)
Additional pulse EPR data

S4
Preparation of 2-Pb: A solution of sodium naphthalenide was freshly prepared by transferring a solution of naphthalene (29.2 mg, 0.228 mmol) in THF (3 mL) into a Schlenk flask containing sodium metal (20 mg, 0.87 mmol) at room temperature. The reaction mixture was stirred for 12 h at room temperature, producing a deep green solution. This solution was added dropwise to a solution of 1-Pb (200 mg, 0.228 mmol) in THF (2 mL) at -95 °C, forming a dark orange solution. This solution was used directly for the EPR characterization. This solution was found to be extremely, air, moisture and temperature sensitive -decomposing slowly even at -78 °C. Unfortunately, all attempts to crystallize this radical anion failed, likely due to the compound's extremely unstable nature.
Due to the high thermal sensitivity of 2-Pb both in solution and the solid state, the compound could not be characterized by common techniques such as elemental analysis and mass spectrometry. That said, the compound has been rigorously characterized by EPR spectroscopy and DFT analysis.

S2) X-ray crystallographic studies
Single-crystal X-ray diffraction data were collected using a Rigaku Supernova dual-source diffractometer.
Crystals of both 2-Ge and 2-Sn were both found to be highly temperature sensitive, instantly decomposing in room temperature oil. As such, crystals of both were selected at low temperature (-40 °C) under a 2:1 mixture of n-hexane and Paratone-N oil, using a mCHILL cold mounting device. S3 Crystals were mounted on Micromount loops, which were quickly plunged into liquid nitrogen before being transferred onto to goniometer of the diffractometer, which was cooled by an Oxford Cryosystems open flow N2 cooling device. S4 Data were collected at 150 K using mirror monochromated Cu Kα (λ = 1.5418 Å) radiation. Data collected were processed using the CrysAlisPro package, including unit cell parameter refinement and inter-frame scaling (which was carried out using SCALE3 ABSPACK within CrysAlisPro). S5

S3.1 EPR sample preparation.
The quartz EPR tube was first placed under vacuum, flushed three times with argon and cooled to −78 °C before a solutions of 2-Ge or 2-Sn was transferred into the tube. We note that best results were achieved by pre-cooling the transfer cannula by passing through just-above-freezing THF immediately before transfer. The transfer of 2-Pb into an EPR tube proved challenging, with the procedure described above leading to the formation of a significant amount of metallic lead, which prevented the acquisition of any meaningful spectra.
Improved results were achieved by cooling the transfer cannula with liquid dinitrogen immediately before it was used.

S3.2 EPR Measurements
Continuous Wave (CW) EPR measurements were performed on a Bruker E580 spectrometer equipped with an ER4122 SHQ resonator, an Oxford instruments E900 cryostat and ITC 4 temperature controller. The spectrum of the 2-Ge complex was recorded at 80 K with a field modulation amplitude of 0.05 mT and a power of 0.047 mW. The spectrum of the 2-Sn complex was recorded at 120 K with a field modulation amplitude of 0.05 mT and a power of 0.63 mW. The spectrum of the 2-Pb complex was recorded at 10 K with a field modulation amplitude of 1.0 mT and a power of 0.047 mW.
Pulse EPR measurements were performed between 10-30 K using a Bruker dielectric resonator (MD5), Oxford instruments CF935 liquid helium cryostat and ITC-503 temperature controller. Electron spin echo-detected (ESE) field-swept spectra were measured using the pulse sequence: tp-τ-2tp-τ-echo. The length of p/2 microwave pulse was generally set to tp = 8 ns. The interpulse distance was varied in the range τ = 120-500 ns.

S3.3 Spin Hamiltonian Simulations
EPR spectra were simultaneously fit assuming a spin S = ½ ground state. The electron Zeeman, nuclear Zeeman and hyperfine terms were treated exactly. Spectral simulations were performed numerically using the EasySpin package 38,39 in MATLAB. The g tensor, metal hyperfine tensor were assumed to be collinear. An anisotropic linewidth was used for all simulations. Linewidth parameters (Sys.lw, Sys.HStrain) used are tabulated below.
All other parameters can be found in Tables S5.1 and S5.3. For the simulation of three pulse ESEEM and HYSCORE data the two equivalent 14 N hyperfine tensors collinear with the g-tensor of the system were S9 sufficient to reproduce all spectral features. Inclusion of orientation selection did not significantly improve the fitting.

S4)
Additional computational data

S4.1 Computational details
Geometry optimizations and calculations of spectroscopic parameters were performed using ORCA. S9 The crystallographic coordinates were used as input for unconstrained geometry optimizations with the TPSS functional, while the hybrid TPSSh functional was used for calculation of spectroscopic properties owing to its well-documented performance for EPR parameters of open-shell systems. Optimizations were carried out both with and without the latest generation of dispersion corrections by Grimme and coworkers (D4). S10 Firstprinciples calculations of core properties such as the hyperfine coupling interaction necessitate the use of flexible all-electron basis sets to describe the distribution of charge and spin density close to the nucleus, while the nature of the target properties and the atomic weight of the elements involved require an adequate treatment of relativistic effects. S11-S12 For these reasons we adopted the zero-order regular approximation (ZORA) S13-S15 as a scalar relativistic Hamiltonian, in combination with all-electron ZORA-compatible basis sets.
These were the purpose-made segmented all-electron relativistically contracted (SARC-ZORA) basis sets of valence TZVP quality for Sn and Pd, S16-S17 and the ZORA-recontracted S18 def2-TZVP basis sets S19 for lighter atoms, including Ge, with the exception of C and H for which the ZORA-def2-ZVP basis set was used. Universal auxiliary basis sets (SARC/J) by Pantazis and coworkers were used for Coulomb fitting in the resolution of the identity approximation. Optimizations were performed with high integration accuracy ("DefGrid2"), which was further increased for the calculation of EPR parameters ("DefGrid3", combined with locally dense grids on the centers of interest). The hyperfine coupling calculations considered all contributions to the hyperfine coupling tensor, i.e. the Fermi contact term, the spin dipolar, and the spin-orbit coupling components. Picture-change effects were included in the calculation of EPR parameters. The mean-field approximation to the Breit-Pauli operator S20-S22 was used as the effective spin-orbit coupling operator.

S5) Tabulated EPR and DFT data. Comparison to literature
Fitted spin Hamiltonian parameters for 2-Ge, 2-Sn and 2-Pb are listed in Table S5.1 below. In all fittings, the g and metal hyperfine tensors were assumed to be collinear. For 2-Ge, the metal hyperfine structure is poorly resolved. As such two fitting were attempted, one where the hyperfine tensor was allowed to vary all three principal components independently (rhombic symmetry) and one where two of the principals were constrained to be the same (axial symmetry). For 2-Sn and 2-Pb all spin Hamiltonian parameters are well defined, although the sign of the hyperfine components is ambiguous. In Table S5 to the precise conformation of the ligand (hinge angle, flanking N-coordinated substituents etc.) -or at least the degree of bending seen is insufficient to cause much effect. This is perhaps unsurprising. As described in the main text, the isotropic component of the hyperfine tensor is mainly derived from the Fermi contact term (AFC spin density at the nucleus) and, as such, reports on the s orbital character of the SOMO. The AFC for all DFT models was estimated to be very small, as compared to the maximum value (i.e. 100% s orbital character), S23 consistent with the unpaired spin being localized in an un-hybridised metal p-orbital, which is orientated perpendicular to the plane of the ligand i.e. spin density is located above and below the plane of the ligand. This also likely explains the good agreement seen for between 2-Ge and the earlier reported, planar Ge(I) radical from the group of Jones. S24 While the metal hyperfine tensor cannot discriminate between the three DFT model sets, the g tensor arguably can, although the effect is subtle. Here model set I and set III best reproduce the experimental values, with model set II systemically predicting a more rhombic g tensor for all complexes. As corrections to the g tensor for these complexes primarily stem from spin-orbit coupling (SOC) induced mixing within the p-orbital S13 manifold, this parameter is more sensitive to changes in the bonding of the complex that result from bending of the ligand backbone, and to a lesser extent loss of the flanking substituents.
Looking at each complex individually, we see that the DFT calculated parameters for 2-Ge show the best agreement with experiment, matching the g tensor and anisotropic hyperfine term. All slight over-estimate the Aiso (30 MHz), which is approximately zero experimentally. We again emphasize though that this difference is negligible when considered as a percentage of the maximum possible hyperfine coupling value i.e. Amax(Ge) ≈ 2300 MHz, with ΔA = (Aexp-Amax) = 1.3%. Which is simply to say that any small change in the s-orbital character of the SOMO can have a dramatic effect on the calculated metal hyperfine couplings. The easiest feature to identify in these spectra is a doublet associated with the two double quantum transitions of the spin manifold (να, νβ). These appear at higher frequencies and are typically sharp and are described by the following analytical expressions: Where νI is the nuclear Larmor frequency [ν( 14 N) = 1.077 MHz, 350 mT], A is the hyperfine coupling, K is the quadrupole coupling divided by 4 (e 2 qQ/4h) and η the quadrupole asymmetry parameter. S25 (Table S5.3). S27 Interestingly, high frequency correlations marked with an asterisk in Figure S6. show good agreement with those generated from DFT models described above, although the isotropic hyperfine coupling is systematically underestimated by ≈ 2 MHz A full analysis of 14 N ESEEM and HYSCORE data of 2-Pb was not achieved in this study. Preliminary analysis ( Figure S6.7) though indicates that the 14 N hyperfine coupling for this complex is approximately two-fold larger than that seen for both 2-Ge and 2-Sn, but still below 10 MHz. As before, the quadrupole coupling is of the order of 3.5 MHz.  a) best fit assuming Axx≠Ayy≠Azz i.e. hyperfine tensor is rhombic; b) best fit assuming Axx=Ayy≠Azz i.e. hyperfine tensor is axial; c) Ill defined -within the experimental linewidth; d) DFT model including dispersion corrections i.e. correctly reproduces the hinged structure; e) DFT model excluding dispersion corrections i.e. the ligand is planar; f) Truncated DFT model including dispersion corrections i.e. symmetric about the Cs mirror plane; g) giso = (gxx+gyy+gzz)/3; h) ganiso = gc/2; where [ga gb gc] = ([g1 g2 g3] -giso). The principal tensor values (gxx, gyy, gzz) are reordered such that |g1| ≤ |g2| ≤ |g3|; i) g(η) = (ga-gb)/gc.      Table S5.3. Figure S6.3. X-band HYSCORE spectrum (τ = 196 ns) of 2-Ge superimposed with a spin Hamiltonian simulation using the same parameters as in Figure S6.2. The top simulation includes only one 14 N nucleus whereas the bottom simulation includes two equivalent 14 N nuclei. Note that while both simulations reproduce the intense cross peaks in the center of the spectrum, only the bottom simulation reproduces weaker high frequency correlations marked with an asterisk e.g. the frequency correlations at (-13, -3.9) and (-3.9, -13) MHz etc. that appear at twice the frequency of the prominent double quantum 14 N peaks are indicative of two equivalent nitrogen environments. The simulation is described in S3.3 and all fitted parameters listed in Table S5.3. Figure S6.4. X-band HYSCORE spectra of 2-Sn recoded on the low field edge (351.6 mT) and in the center (365 mT) of its EPR spectrum. All EPR parameters are listed in S3.2. As above, the ridge centered at ≈15 MHz in the (+,+) quadrant is consistent with weakly (dipolar) coupled 1 H nuclei; a coupling of <3 MHz is equivalent to a metal-1 H inter-spin distance of <3 Å. This is approximately the distance between the metal center and the hydrogens of the pendant Dipp groups. The two intense cross-peaks in the (-, +) quadrant cenetred at (-3.4, -6.5) and (-6.5, -3.4) MHz are consistent with the double quantum transitions of a 14 N nucleus with a hyperfine coupling A ≈ 3 MHz. Figure S6.5. X-band three pulse ESEEM spectra of 2-Sn recoded in the center of its EPR spectrum (365 mT). All EPR parameters are listed in S3.2. The black line shows the data, the red line a simulation using the spin Hamiltonian formalism. The simulation is described in S3.3 and all fitted parameters listed in Table S5.3. Figure S6.6. X-band HYSCORE spectrum (τ = 196 ns) of 2-Sn superimposed with a spin Hamiltonian simulation using the same parameters as in Figure S6.5. The top simulation includes only one 14 N nucleus whereas the bottom simulation includes two equivalent 14 N nuclei. Owing to poorer data quality, higher frequency correlations are not clearly resolved and as such both fitting equally reproduce the data. We note however that we see no evidence for two nitrogen environments. The simulation is described in S3.3 and all fitted parameters listed in Table S5.3. Figure S6.7. X-band HYSCORE spectrum of 2-Pb recoded on the low field edge (298 mT) of its EPR spectrum. All EPR parameters are listed in S3.2. As above, the cross-peak centered at ≈15 MHz in the (+,+) quadrant is consistent with weakly (dipolar) coupled 1 H nuclei. Cross-peaks in the (-, +) quadrant cenetred at (-9.8, -5.2) and (-5.2, -9.8) MHz are consistent with the double quantum transitions of a 14 N nucleus with a hyperfine coupling of A ≈ 8 MHz.