Metallacyclic Zinc Complexes of Alkylidene-Linked Bitopic Bis(pyrazolyl)methane Ligands: Unusual Exocyclic Bridging Fluoride Ligands

The alkylidene-linked, bitopic bis(pyrazolyl)methane ligands 1,1,3,3-tetrakis(1-pyrazolyl)propane ([CH(pz)2]2CH2;
L1; pz = 1-pyrazolyl), 1,1,4,4-tetrakis(1-pyrazolyl)butane ([CH(pz)2]2(CH2)2; L2), and 1,1,5,5-tetrakis(1-pyrazolyl)pentane ([CH(pz)2]2(CH2)3; L3) react with Zn(BF4)2·5H2O in tetrahydrofuran (THF) to afford complexes with the empirical formula [ZnL](BF4)2
(L = L1 (1), L2 (2), and L3 (3)). 1H NMR studies indicate that all three complexes are fluxional in solution. Recrystallization of
the complexes from acetonitrile yields the crystallographically characterized compounds [Zn(μ-[CH(pz)2]2CH2)(H2O)(CH3CN)]3(BF4)6·2.5CH3CN (1a), {[Zn2(μ2-[CH(pz)2]2[CH2]2)2(μ2-F)](BF4)3·CH3CN·0.5H2O}∞ (2a), and [Zn3(μ-[CH(pz)2]2(CH2)3)3(H2O)5](BF4)6·4CH3CN (3a). Compounds 1a and 3a are trimeric metallacycles in which acetonitrile and water molecules complete the
coordination spheres of the octahedral or trigonal bipyramidal Zn2+ ions. Compound 2a consists of one-dimensional chains of
dimeric metallacycles connected through the trigonal bipyramidal zinc cations by exocyclic bridging fluoride ligands, the origin of
which must be fluoride abstraction from the BF4- counterions. The polymeric chains of 2a are also supported by π−π stacking
between adjacent pyrazolyl groups, and the compound contains highly ordered solvent molecules that alternate in layers with the
intricate gridlike structure of the cationic polymers.