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Comparing Main Group and Transition-Metal Square-Planar Complexes of the Diselenoimidodiphosphinate Anion: A Solid-State NMR Investigation of M[N(iPr2PSe)2]2 (M = Se, Te; Pd, Pt)

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journal contribution
posted on 07.04.2008 by Bryan A. Demko, Roderick E. Wasylishen
A comparison of the square-planar complexes of group 10 (PdII, PtII) and 16 (SeII, TeII) centers with the tetraisopropyldiselenoimidodiphosphinate anion, [N(iPr2PSe)2], is made on the basis of the results of a solid-state 31P, 77Se, 125Te, and 195Pt NMR investigation. Density functional theory calculations of the respective chemical shift and 14N electric field gradient tensors in these compounds complement the experimental results. The NMR spectra were analyzed to determine the respective phosphorus, selenium, tellurium, and platinum chemical shift tensors along with numerous indirect spin–spin coupling constants. Special attention was given to observed differences in the NMR parameters for the transition metal and main-group square-planar complexes. Residual dipolar coupling between 14N and 31P, not observed in the solid-state 31P NMR spectra of the Pd(II) and Pt(II) complexes, was observed at 4.7 and 7.0 T for M[N(iPr2PSe)2]2 (M = Se, Te) yielding average values of R(31P,14N)eff = 890 Hz, CQ(14N) = 2.5 MHz, 1J(31P,14N)iso= 15 Hz, α = 90°, β = 17°. The span, Ω, and calculated orientation of the selenium chemical shift tensor for the diselenoimidodiphosphinate anion is found to depend on whether the selenium is located within a pseudoboat or distorted-chair MSe2P2N six-membered ring. The largest reported values of 1J(77Se,77Se)iso, 405 and 435 Hz, and 1J(125Te,77Se)iso, 1120 and 1270 Hz, were obtained for the selenium and tellurium complexes, respectively; however, in contrast a correspondingly large value of 1J(195Pt,77Se)iso was not found. The chemical shift tensors for the central atoms, Se(II) and Te(II), possess positive skews, while for Pt(II) its chemical shift tensor has a negative κ. This observed difference for the shielding of the central atoms has been explained using a qualitative molecular orbital approach.