Abstract
Interaction of robust chromium(III) fluoride complexes with sodium or lithium cations in solution lead to hypsochromic spectral shifts of increasing magnitude along the series: trans-[CrF 2(py) 4] +, mer-[CrF 3(terpy)], and fac-[CrF 3(Me 3tacn)]. Crystalline products isolated from solution exhibit μ 2-bridging by the fluoride ligands in a linear fashion between Na +-ions and chromium centres in catena-[Na(H 2O) 4(μ-F)- trans-{CrF(py) 4}](HCO 3) 2 and in the dimers [Li(H 2O) n (μ-F)- trans-{CrF(py) 4}] 2+ ( n = 3, 4). The uncharged chromium complexes fac-[CrF 3(Me 3tacn)] and mer-[CrF 3(terpy)] have been synthesized from mer-[CrF 3(py) 3] and shown to precipitate sodium salts from solution, of which 3[CrF 3(Me 3tacn)]·2Na(Bph 4)·solv and 6[CrF 3(terpy)]·4Na(Bph 4)·solv have been crystallographically characterized. In these clusters, the neutral fluoride complexes bring the Na + cation separation down to 3.610 Å and 3.369 Å, respectively, which is much closer than the inter-cation distance in NaCl and comparable to that of NaF. DFT calculations support the notion of a strong interaction between Na + ions and neutral chromium(III) fluoride complexes. The calculations reproduce the magnitude and the counter-intuitive sign of the spectral shifts induced by second sphere complexation in solution, which originates in a breakdown of the assumption of parameter transferability in ligand-field descriptions.
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