Lewis Acid Behavior of MoF₅ and MoOF₄: Syntheses and Characterization of MoF₅(NCCH₃), MoF₅(NC₅H₅)_n, and MoOF₄(NC₅H₅)_n (n = 1, 2)

Abstract

The Lewis acid−base adducts MoF 5 (NC 5 H 5 ) n and MoOF 4 (NC 5 H 5 ) n (n = 1, 2) were synthesized from the reactions of MoF 5 and MoOF 4 with C 5 H 5 N and structurally characterized by X-ray crystallography. Whereas the crystal structures of MoF 5 (NC 5 H 5 ) 2 and MoOF 4 (NC 5 H 5 ) 2 are isomorphous containing pentagonal-bipyramidal molecules, the fluorido-bridged, heptacoordinate [MoF 5 (NC 5 H 5 )] 2 dimer differs starkly from monomeric, hexacoordinate MoOF 4 (NC 5 -H 5 ). For the weaker Lewis base CH 3 CN, only the 1:1 adduct, MoF 5 (NCCH 3 ), could be isolated. All adducts were characterized by Raman spectroscopy in conjunction with vibrational frequency calculations. Multinuclear NMR spectroscopy revealed an unprecedented isomerism of MoOF 4 (NC 5 H 5 ) 2 in solution, with the pyridyl ligands occupying adjacent or nonadjacent positions in the equatorial plane of the pentagonal bipyramid. Paramagnetic MoF 5 (NC 5 H 5 ) 2 was characterized by electron paramagnetic resonance (EPR) spectroscopy as a dispersion in solid adamantane as well as in a diamagnetic host lattice of MoOF 4 (NC 5 H 5 ) 2 ; EPR parameters were computed using ZORA with the BPW91 functional using relativistic all-electron wave functions for Mo and simulated using EasySpin. Density functional theory calculations (B3LYP) and natural bond orbital analyses were conducted to elucidate the distinctive bonding and structural properties of all adducts reported herein and explore fundamental differences observed in the Lewis acid behavior of MoF 5 and MoOF 4 .