Synthesis, Single Crystal X-ray Structure, Spectroscopy and Substitution Behavior of Niobium(V) Complexes Activated by Chloranilate as Bidentate Ligand

Abstract

Chloranilic acid (2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone, caH2) as a bidentate ligand for Nb(V) as a metal center is presented. The different coordination behavior of caH2 is well illustrated by a monomeric (Et4N)cis-[NbO(ca)2(H2O)OPPh3]·3H2O.THF (5) and a novel tetranuclear compound (Et4N)4[Nb4O4(ca)2(μ2-O)2Cl8]·2CH3CN (6) via self-assembly, respectively. These were obtained in > 80% yields and characterized by IR, UV/Vis and NMR (1H, 13C{1H}, 31P{1H}) spectroscopy and single crystal X-ray diffraction, and they included a systematic assessment of the solid-state behavior. The anionic metal complexes showed different coordination modes at the Nb(V): [Nb4O4(ca)2(μ2-O)2Cl8]4− (6a; distorted octahedral) and cis-[NbO(ca)2(H2O)(OPPh3)]− (5a; D5h distorted pentagonal bipyramidal), respectively. The tetranuclear complex 6a is substitution inert, while cis-[NbO(ca)2(H2O)OPPh3]− (5a) allowed a systematic ligation kinetic evaluation. The substitution of the coordinated triphenylphosphine oxide by a range of pyridine-type entering nucleophiles, 4-N,N-dimethyl-aminopyridine (DMAP), pyridine (py), 4-methylpyridine (4Mepy), 3-chloropyridine (3Clpy) and 3-bromopyridine (3Brpy) in acetonitrile at 31.2 °C was carefully evaluated. The subtle interplay between the main group ligand systems and the hard, early transition metal Nb(V) complex (5a) was well illustrated. The entering monodentate ligands showed a 15-fold reactivity range increase in the order 3Brpy < 3Clpy < 4Mepy < py < DMAP in broad agreement with the Brønsted-donating ability of the nucleophiles. The activation parameters determined for the reaction of 5a with DMAP as the entering ligand yielded ΔH≠kf = 52 ± 1 kJ mol−1 and ΔS≠kf = −108 ± 3 J K−1 mol−1 for the enthalpy and entropy of activation, respectively, indicating an associative substitution mechanism. The study presents an important contribution to the structure/reactivity relationships in Nb(V) complexes stabilized by chloranilic acid as a bidentate ligand.