Imposing high-symmetry and tuneable geometry on lanthanide centres with chelating Pt and Pd metalloligands

Abstract

Exploitation of HSAB preferences allows for high-yield, one-pot syntheses of lanthanide complexes chelated by two Pd or Pt metalloligands, [M^II(SAc)₄]²⁻ (SAc⁻ = thioacetate, M = Pd, Pt). The resulting complexes with 8 oxygen donors surrounding the lanthanides can be isolated in crystallographically tetragonal environments as either [NEt₄]⁺ (space group: P4/mcc) or [PPh₄]⁺ (space group: P4/n) salts. In the case of M = Pt, the complete series of lanthanide complexes has been structurally characterized as the [NEt₄]⁺ salts (except for Ln = Pm), while the [PPh₄]⁺ salts have been structurally characterized for Ln = Gd–Er, Y. For M = Pd, selected lanthanide complexes have been structurally characterized as both salts. The only significant structural difference between salts of the two counter ions is the resulting twist angle connecting tetragonal prismatic and tetragonal anti-prismatic configurations, with the [PPh₄]⁺ salts approaching ideal D_4d symmetry very closely (φ = 44.52–44.61°) while the [NEt₄]⁺ salts exhibit intermediate twist angles in the interval φ = 17.28–27.41°, the twist increasing as the complete 4f series is traversed. Static magnetic properties for the latter half of the lanthanide series are found to agree well in the high temperature limit with the expected Curie behavior. Perpendicular and parallel mode EPR spectroscopy on randomly oriented powder samples and single crystals of the Gd complexes with respectively Pd- and Pt-based metalloligands demonstrate the nature of the platinum metal to strongly affect the spectra. Consistent parametrization of all of the EPR spectra reveals the main difference to stem from a large difference in the magnitude of the leading axial term, B⁰₂, this being almost four times larger for the Pt-based complexes as compared to the Pd analogues, indicating a direct Pt(5d_z²)–Ln interaction and an arguable coordination number of 10 rather than 8. The parametrization of the EPR spectra also confirms that off-diagonal operators are associated with non-zero parameters for the [NEt₄]⁺ salts, while only contributing minimally for the [PPh₄]⁺ salts in which lanthanide coordination approximates D_4d point group symmetry closely.