A solid-state 55Mn NMR spectroscopy and DFT investigation of manganese pentacarbonyl compounds

Abstract

Central transition ⁵⁵Mn NMR spectra of several solid manganese pentacarbonyls acquired at magnetic field strengths of 11.75, 17.63, and 21.1 T are presented. The variety of distinct powder sample lineshapes obtained demonstrates the sensitivity of solid-state ⁵⁵Mn NMR to the local bonding environment, including the presence of crystallographically unique Mn sites, and facilitates the extraction of the Mn chemical shift anisotropies, CSAs, and the nuclear quadrupolar parameters. The compounds investigated include molecules with approximate C_4v symmetry, LMn(CO)₅ (L = Cl, Br, I, HgMn(CO)₅, CH₃) and several molecules of lower symmetry (L = PhCH₂, Ph_3−nCl_nSn (n = 1, 2, 3)). For these compounds, the Mn CSA values range from <100 ppm for Cl₃SnMn(CO)₅ to 1260 ppm for ClMn(CO)₅. At 21.1 T the ⁵⁵Mn NMR lineshapes are appreciably influenced by the Mn CSA despite the presence of significant ⁵⁵Mn quadrupolar coupling constants that range from 8.0 MHz for Cl₃SnMn(CO)₅ to 35.0 MHz for CH₃Mn(CO)₅. The breadth of the solid-state ⁵⁵Mn NMR spectra of the pentacarbonyl halides is dominated by the CSA at all three applied magnetic fields. DFT calculations of the Mn magnetic shielding tensors reproduce the experimental trends and the magnitude of the CSA is qualitatively rationalized using a molecular orbital, MO, interpretation based on Ramsey’s theory of magnetic shielding. In addition to the energy differences between symmetry-appropriate occupied and virtual MOs, the d-character of the Mn MOs is important for determining the paramagnetic shielding contribution to the principal components of the magnetic shielding tensor.