Investigation of Pressure Effects in the Bimetallic Transplutonium Tetrazolate Complexes [(An(pmtz)2(H2O)3)2(μ-pmtz)]2(pmtz)2· n H2O (An3+ = Cm3+, Bk3+, and Cf3+)

Abstract

Understanding the effects of pressure on actinide compounds is an integral part of safe nuclear waste storage in deep geologic repositories and provides a means of systematically altering the structure and properties. However, detailing how the effects of pressure evolve across the actinide series in the later elements is not typically undertaken because of the challenges of conducting research on these unstable isotopes. Here, a family of bimetallic actinide complexes, [(An(pmtz)₂(H₂O)₃)₂(μ-pmtz)]₂(pmtz)₂·nH₂O (An³⁺ = Cm³⁺, Bk³⁺, and Cf³⁺, pmtz^- = 5-(pyrimidyl)tetrazolate; Cm1, Bk1, and Cf1), are reported and represent the first structurally characterized bimetallic berkelium and californium compounds. The pressure response as determined from UV-vis-NIR transitions varies for Cm1, Bk1, and Cf1. The 5f → 5f transitions in Cm1 are notably more sensitive to pressure compared to those in Bk1 and Cf1 and show substantial bathochromic shifting of several 5f → 5f transitions. In the case of Bk1, an ingrowth of a metal-to-ligand charge-transfer transition occurs at elevated pressures because of the accessible Bk³⁺/Bk⁴⁺ couple. For Cf1, no substantial transition shifting or emergence of MLCT transitions is observed at elevated pressures because of the prohibitive energetics of the Cf³⁺/Cf⁴⁺ couple and reduced sensitivity of the 5f → 5f transitions to the local coordination environment because of the more contracted 5f shell versus Cm³⁺ and Bk³⁺.