Countercation-Controlled Nuclearity of Zr/Hf Peroxo Oxalates

Understanding fundamental differences between zirconium and hafnium chemistry contributes to our fundamental understanding of the periodic table and leads to devising necessary separations for high-precision nuclear and microelectronics applications, developing water-based nanolithographic processes, and creating new robust metal-organic frameworks for catalysis and separations. Here we crystallize a rich matrix of polynuclear Zr and Hf species differentiating in complexation with peroxide and oxalate in mild acid, where the countercations influence polymerization. Hf only complexes oxalate, yielding polymeric {(N(CH3)(4))(4)[Hf-2(OH)(2)(C2O4)(5)]}(n), dimeric Na-6[Hf-2(OH)(2)(C2O4)(6)], and Li2K4[Hf-2(OH)(2)(C2O4)(6)] and mononuclear K4Hf(C2O4)(4), Rb4Hf(C2O4)(4), and Cs4Hf(C2O4)(4). Zr complexes both peroxide and oxalate to yield the ring structures (N( CH3)(4))(6) [Zr-6(O-2)(6)(OH)(6)(C2O4)(6)], Li (12)[Zr-8(O-2)(12)(OH)(4)(C2O4)(8)], K-18[Zr-12(O-2)(18)(OH)(6)(C2O4)(12)], and Rb-24[Zr-16(O-2)(24)(OH)(8)(C2O4)(16)]. The Zr ring nuclearity increases with countercation size, while Hf polymerization decreases with increasing countercation size. The Zr rings feature nine-coordinate face-sharing polyhedra in both solution and the solid state, unprecedented in Zr coordination complexes. These studies describe differentiating the coordination chemistry of Zr/Hf, exploiting simple aqueous reagents that could be further developed for aqueous synthesis of materials as well as challenging chemical separations.