Spiroconjugation over a boron atom: facile synthesis, structures and vibrational spectra of crystalline 1,3-disubstituted (propen-1,3-diolato)(oxalato)boron molecules

Abstract

Preparation of single crystals of spiroconjugated bis-chelated boron compounds containing ligands such as propen-1,3-diolates and oxalate, for application as lasing materials in dye laser technology, is burdened by their insolubility in organic solvents and the high melting temperatures. In this work, on the example of (diphenylpropen- 1,3-diolato)(oxalato)boron (1) it is demonstrated that the solvothermal method can be successfully applied as facile, convenient and fairly inexpensive method to overcome the difficulties with preparation of such materials in single crystalline form directly from boric acid and the ligands. Solid-state IR spectra (recorded at room temperature and low temperature) and Raman spectra (recorded at room temperature) and equilibrium molecular geometries of 1 and three other disubstituted β-ketoboron molecules [RC(O)CHC(O)R'](O2CCO2)B (R = R' = t-butyl, 2; R = methyl, R' = phenyl, 3; R = R' = 2-pyridyl, 4) in the ground electronic states were analyzed in detail, based on HF SCF (HF/3-21G, HF/6-31G), MP2 (MP2/6-31G) and DFT (B3LYP/6-31G) calculations. Minima with spiro-chelated tetrahedral (sp3) BO4 or trigonal-planar (sp2) BO3 coordination were located on the potential energy hypersurfaces for all systems. The tetrahedral structures are more stable relative to the trigonal structures (for example, ~37 kcal mol−1 in the case of 2), accounting for the observation that they represent the actual conformers in the solid state. Vibrational criteria for spectroscopic distinction between trigonal and tetrahedral boron−oxygen coordination geometry are presented.