Indirect two-dimensional heteronuclear NMR spectroscopy. Phosphorus-31 and iron-57 spectra of organoiron complexes

Abstract

The indirect heteronuclear two-dimensional (2D) triple-resonance (S,I)-(/sup 1/H) NMR spectroscopy is introduced for measuring the chemical shift and scalar spin-spin coupling constants of an insensitive nucleus I via its scalar coupling J(S,I) by detection of the nucleus S of higher sensitivity. The versatility of this approach is demonstrated by extracting delta(/sup 57/Fe) and J(Fe,X) from (/sup 31/P,/sup 57/Fe)-(/sup 1/H) spectra of various dissolved ((/eta//sup 5/-Cp)(L/sub 2/(R)))Fe, ((/eta//sup 3/-allyl)(/eta//sup 5/-Cp)(L))Fe, and ((/eta//sup 1/,/eta//sup 2/-alkenyl)(/eta//sup 5/-Cp)(L))Fe complexes (R = alkyl, hydride; L = PR/sub 3/). In practice the sensitivity of 2D (/sup 31/P,/sup 57/Fe) spectra was found to be higher than that of the direct observation scheme by at least a factor (..gamma../sub P//..gamma../sub Fe/)/sup 5/2/. Due to the intrinsically higher resolving power of a two-dimensional experiment, small scalar couplings like /sup 2J/(Fe,F) and /sup 1/J(Fe,H) were readily obtained from indirect two-dimensional spectra. Combinations of (/sup 1/H,/sup 57/Fe) and (/sup 31/P,/sup 57/Fe) spectra yielded the relative signs of the J(Fe,X) couplings: /sup 1/J(Fe,P) is positive and increases with increasing ..pi..-acceptor power of the phosphorus ligand L from 55 (L = PMe/sub 3/, R = H) to 149 Hz (L = PF/sub 3/). /sup 1/J(Fe,H) is around +9 Hz (R = H), whereasmore » /sup 2/J(P,H) in these complexes was found to be negative. In all of the allyl complexes investigated, /sup 2J/(Fe,F) (L = PF/sub 3/) is positive and around 3 Hz. In the quasi-tetragonal and -trigonal iron complexes, delta(/sup 57/Fe) varies by about 4000 ppM. This can be rationalized qualitatively by the electronegativity of the atoms directly bonded to iron and the higher oxidation potential in the presence of more basic ligands L via the paramagnetic shielding term. 52 references, 5 figures, 5 tables.« less