Investigation of Bimetallic Am(III) and Nd(III) Tetrazolate Complexes Under Ambient and High-Pressure Conditions

The bimetallic complexes [(M(pmtz)₂(H₂O)₃)₂(μ-pmtz)](pmtz)·7H₂O (M³⁺ = Nd³⁺ and Am³⁺, Nd1 and Am1, pmtz^- = 5-(pyrimidyl)tetrazolate) were synthesized and characterized by single-crystal X-ray diffraction and UV-vis-NIR absorption spectroscopy, completing the [(An(pmtz)₂(H₂O)₃)₂(μ-pmtz)](pmtz)·nH₂O, (An³⁺ = Pu³⁺-Cf³⁺, Pu1-Cf1) series. All M-N_tetrazolate bonds (M³⁺ = Pu³⁺-Cf³⁺, Ce³⁺, Nd³⁺, Sm³⁺, and Dy³⁺) are significantly shorter than the corresponding M-N_pyrimidyl bonds, with an average difference of approximately 0.15-0.2 Å. Computational results further support this trend, indicating that the Am/Nd-N_tetrazolate bonds possess a greater covalent character than the corresponding Am/Nd-N_pyrimidyl bonds. Also, the f-orbital contribution to the Am-N bonds in Am1 is greater than those of Nd-N bonds in Nd1. Variable-pressure solid-state absorption spectra of Am1 and isomorphous Nd1 demonstrate that 5f → 5f transitions exhibit significantly greater red shifts, peak broadening, and merging compared to those of 4f → 4f transitions. Notably, the group H transition (⁷F₀ → ⁵L₆) in Am1 shifts at a rate of 41.13 ± 1.47 cm^-1/GPa, while the ⁴I_9/2 → ²K_13/2 transition in Nd1 shifts at a rate of 8.39 ± 0.86 cm^-1/GPa. These findings highlight the greater pressure sensitivity and covalency of 5f orbitals relative to those of 4f orbitals.