A coordinative partnership in tungsten chemistry: N-bound isothiocyanate and a tridentate pyrazolone-Schiff base in a hexacoordinate [WO2]2+ complex revealed by DFT and spectroscopy.

This work presents the synthesis and detailed structural elucidation of a new cis-dioxotungsten(VI) complex, featuring a novel hexacoordinate geometry. The complex was formed using a custom-synthesized Schiff base ligand, N-(4'-acetylidene-3'-methyl-1'-phenyl-2'-pyrazolin-5'-one)-4-amino-2,3-dimethyl-1-phenyl-3-pyrazolin-5-one (Hampph-aap), designed to provide specific coordination pockets for metal binding. The reaction of this ligand with the precursor complex, tetraisothiocyanatodioxotungstate(VI) [WO2(NCS)4]2-, proceeded in a 1:1 molar ratio, yielding a diamagnetic product with the empirical formula [WO2(L)(NCS)]. Comprehensive spectroscopic analysis, including FT-IR, UV-Vis, multinuclear NMR, and high-resolution mass spectrometry, confirmed the successful formation of the complex. The data collectively demonstrate that the Hampph-aap ligand behaves as a monobasic tridentate chelator, binding to the tungsten center after deprotonation. To move beyond experimental characterization and gain deeper insight into the molecular framework, Density Functional Theory (DFT) calculations at the WB97XD/SDD level with SMD solvation were employed. The computational study involved full geometry optimization and subsequent simulation of vibrational, electronic, and NMR spectra. The theoretically optimized structure is in excellent agreement with the experimental data, robustly confirming an octahedral geometry around the central tungsten atom. A key finding from both experimental and computational analyses is the unambiguous identification of the thiocyanate co-ligand binding through its nitrogen atom (N-bonded isothiocyanate) based on comparative theoretical analysis between N- and S-coordinated systems. Beyond structure, DFT analysis revealed significant nonlinear optical (NLO) potential and proposed catalytic relevance in epoxidation, supported by DOS and preliminary docking studies. This combined experimental and theoretical approach provides a conclusive and multi-faceted understanding of the complex's structural, electronic, and applied properties.