Alebel Nibret

Title : 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 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 ^oC 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. Activation parameters determined for the reaction of 5a with DMAP as 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.

Keywords: niobium; chloranilic acid; substitution; mechanism; kinetics; crystal structures

Audience Take Away Notes: 

• Identical twin metals (Nb(V) and Ta(V)
• Physical and chemical properties of materials
• It describes the study directed towards the synthesis, characterization, and substitution kinetics of novel niobium(V) and tantalum(V) complexes containing selected O, O’-bidentate ligands that could potentially be used for the separation of niobium from tantalum
• There is also a search for complexes that will be attractive and useful as advanced materials for high-technology applications and in the nuclear power environment
• This study included a systematic assessment of the physical and chemical behavior of the free ligands, co-crystals, and complexes as well as the factors that govern their stability, reactivity, coordination geometry, bond lengths and angles, π-π stacking interaction, and inter- and intramolecular networks of hydrogen bonds. It became evident that the crystal stabilization was influenced by intermolecular π-π interactions and inter- and intramolecular hydrogen and halogen bond networks, producing a stabilized crystal lattice

Biography:

Dr. Alebel Nibret Belay was born in 1988. He holds of B.Sc in applied Chemistry from Mada Walabu University and his M.Sc. in Physical Chemistry from Haramaya University, Ethiopia in 2010 and 2012, respectively. In 2012, Belay joined as a lecturer and researcher in the College of Science Department of Chemistry at Bahir Dar University, Ethiopia. He received his PhD from the University of the Free State, South Africa (2018) followed by 5 years as an assistant professor at Bahir Dar University, Ethiopia. He joined the College of Chemistry and Chemical Engineering, Hunan University, Changsha, China in 2023 and his research backbone lies on bio-based nanomaterials, chemical synthesis, and coordination chemistry of inorganic/organic materials using crystallography, encompassing both fundamental scientific exploration and industrialization aspects. He has published  10 research articles in SCI(E) journals.