Assembly of triangular trimetallic complexes by triamidophosphine ligands:: Spin-frustrated Mn2+ plaquettes and diamagnetic Mg2+ analogues with a combined through-space, through-bond pathway for 31P-31P spin-spin coupling

The reactions of 2 equiv of the ligand precursor P( CH2NHPh)(3) or P[ CH2NH-3,5-( CF3) 2C6H3](3) with 3 equiv of Mn[ N( SiMe3)(2)](2) provide high-yielding routes to the triangular trinuclear Mn( II) complexes [ P( CH2NPh)(3)] Mn-2(3)( THF)(3)center dot 1.5THF and [ P( CH2N-3,5-( CF3)(2)C6H3)(3)](2)Mn-3( THF)(3). The solid-state structures of these paramagnetic complexes have approximate C 3 symmetry. The magnetic moments from 300 to 1.8 K could be fit as a magnetic Jahn-Teller distorted isosceles triangle. These complexes exhibit spin frustration and possess an S = 1/2 ground state, as revealed by a plot of magnetization versus field at 1.8 K; at fields above 3.8 T, the occupation of an excited state with S = 3/2 becomes significant. The diamagnetic magnesium analogues were prepared by the reaction of the ligand precursor P( CH2NHPh)(3), P[ CH2NH-3,5-( CF3)(2)C6H3](3), or P( CH2NH-3,5- Me2C6H3)(3) with (Bu2Mg)-Bu-n. The solid-state structures of [ P( CH2NPh)(3)] Mg-2(3)( THF)(3), 1.5THF and [ P( CH2N-3,5-( CF3)(2)C6H3)(3)] Mg-2(3)( THF)(3) were determined. Solution H-1 NMR spectroscopy was used to demonstrate that the solid-state structures are maintained in solution. The aryl group of the terminal amido donor exhibits slow rotation on the NMR time scale, and this was found to be an electronic effect. Solution P-31 {H-1} NMR spectroscopy revealed an unexpected 15 Hz coupling between phosphorus nuclei in these complexes. Calculations on a model complex using density functional theory demonstrates that this coupling occurs via a combined through-space, through-bond pathway.