Binding properties of solvatochromic indicators [Cu(X)(acac)(tmen)] (X = PF₆^- and BF₄^-, acac^- = acetylacetonate, tmen = N,N,N′,N′-tetramethylethylenediamine) in solution and the solid state

The solvatochromic indicator [Cu(acac)(tmen)(H₂O)] ·PF₆ (1·H₂O) has been synthesized and crystallographically characterized. 1·H₂O binds an H ₂O molecule at the Cu(II) axial site, while the PF₆ ^- anion is coordination free. The binding properties of [Cu(PF ₆)(acac)(tmen)] (1) and [Cu(BF₄)(acac)(tmen)] (2) have been investigated in solution and the solid state. The donor number of the PF₆^- anion (DN_PF6) was determined from the UV-vis spectra of 1 in 1,2-dichloroethane. The value of DN_PF6 of the PF₆^- anion is slightly larger than that of the tetraphenylborate anion (BPh₄^-), which is known as a noncoordinating anion. In the solid state, 1 and 2 reversibty bind and release H₂O molecules at the Cu(II) axial sites. The coordinated H ₂O molecules in 2 are more easily removed than those in 1 because of the strong Lewis basicity of the BF₄^- anion compared to the PF₆^- ion. The lower melting point of 1 versus 2 is attributed to the loose binding of the PF₆^- anions to the Cu(II) centers, which induces the dynamic nature of the crystal.