Guo, Zhifang, Blair, Victoria L., Deacon, Glen B., and Junk, Peter C. (2022) Europium is different: solvent and ligand effects on oxidation state outcomes and C-F activation in reactions between europium metal and Pentafluorophenylsilver. Chemistry: a European journal, 28 (3). e202103865.

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Abstract

Unique outcomes have emerged from the redox transmetallation/ protolysis (RTP) reactions of europium metal with [Ag(C6F5)(py)] (py=pyridine) and pyrazoles (RR ' pzH). In pyridine, a solvent not normally used for RTP reactions, the products were mainly Eu-II complexes, [Eu(RR ' pz)(2)(py)(4)] (RR ' pz=3,5-diphenylpyrazolate (Ph(2)pz) 1; 3-(2-thienyl)-5-trifluoromethylpyrazolate (ttfpz) 2; 3-methyl-5-phenylpyrazolate (PhMepz) 3). However, use of 3,5-di-tert-butylpyrazole (tBu(2)pzH) gave trivalent [Eu(tBu(2)pz)(3)(py)(2)] 4, whereas the bulkier N,N '-bis(2,6-difluorophenyl)formamidine (DFFormH) gave divalent [Eu(DFForm)(2)(py)(3)] 5. In tetrahydrofuran (thf), the usual solvent for RTP reactions, C-F activation was observed for the first time with [Ag(C6F5)(py)] in such reactions. Thus trivalent [{Eu-2(Ph(2)pz)(4)(py)(4)(thf)(2)(mu-F)(2)}{Eu-2(Ph(2)pz)(4)(py)(2)(thf)(4)(mu-F)(2)}] (6), [Eu-2(ttfpz)(4)(py)(2)(dme)(2)(mu-F)(2)] (7), [Eu-2(tBu(2)pz)(4)(dme)(2)(mu-F)(2)] (8) were obtained from the appropriate pyrazoles, the last two after crystallization from 1,2-dimethoxyethane (dme). Surprisingly 3,5-dimethylpyrazole (Me(2)pzH) gave the divalent cage [Eu-6(Me(2)pz)(10)(thf)(6)(mu-F)(2)] (9). This has a compact ovoid core held together by bridging fluoride, thf, and pyrazolate ligands, the last including the rare mu(4)-1 eta(5)(N2C3): 2 eta(2)(N,N '): 3 kappa(N): 4 kappa(N ') pyrazolate binding mode. With the bulky N,N '-bis(2,6-diisopropylphenyl)formamidine (DippFormH), which often favours C-F activation in RTP reactions, neither oxidation to Eu-III nor C-F activation was observed and [Eu(DippForm)(2)(thf)(2)] (10) was isolated. By contrast, Eu reacted with Bi(C6F5)(3) and Ph(2)pzH or tBu(2)pzH in thf without C-F activation, to give [Eu(Ph(2)pz)(2)(thf)(4)] (11) and [Eu(tBu(2)pz)(3)(thf)(2)] (12) respectively, the oxidation state outcomes corresponding to that for use of [Ag(C6F5)(py)] in pyridine.

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