Immobilized Boron-Centered Heteroscorpionates: Heterocycle Metathesis and Coordination Chemistry

Abstract

The preparation of a resin-supported boron-scorpionate ligand and its nickel(II) coordination complexes are reported. The supported ligand is prepared as its potassium salt, making it a general reagent suitable for chelation of any transition metal ion. Resin-immobilized benzotriazole (Bead-btz) reacted cleanly with KTp* (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) by heterocycle metathesis in warm dimethylformamide (DMF) to yield bead-Tp′K, {resin-btz(H)B(pz*)2}K. Significantly, bead-Tp′K readily bound nickel(II) from simple salts with minimal leaching of the nickel ion. Bead-Tp′NiNO3 reacts further with cysteine thiolate (ethyl ester), imparting the deep green color to the beads characteristic of a TpRNiCysEt coordination sphere. Bead-Tp′NiCysEt exhibited an oxygen sensitivity similar to Tp*NiCysEt in solution (Inorg. Chem. 1999, p 5690) and also independently verified for a selenocystamine analogue, Tp*NiSeCysAm. Addition of fresh cysteine thiolate ethyl ester to oxidized bead-Tp′NiCysEt reproduced the original green color. Heterocycle metathesis was also used to prepare KTp′ as a white solid. Reaction with nickel(II) gave (Tp′)2Ni, separable into two different isomers. The air-sensitive molybdenum(0) complex, [PPh4][Tp′Mo(CO)3], was also prepared and the Cs complex symmetry demonstrated by infrared and 13C NMR spectroscopies. Immobilized TpmMo(CO)3 was prepared from the previously reported resin-supported tris(pyrazolyl)methane. In contrast to its weak coordination of nickel(II) (Inorg. Chem. 2009, p 3535), bead-Tpm proved a strong chelate toward this second row metal. The supported scorpionates described here should find use in studies of selective metal-protein binding, metalloprotein modeling, and heterogeneous catalysis, and render such scorpionate applications amenable to combinatorial methods.