Solution Conformational Features of the Unsymmetrical Metalla Crown Ether cis-Mo(CO)4{2-(2,2‘-O2C12H8)PO(CH2CH2O)2- 2-C20H12-2‘-OP(2,2‘-O2C12H8)} from 2D NMR Spectroscopy and Molecular Modeling

Abstract

A new unsymmetrical metalla crown ether, cis-Mo(CO)4{2-(2,2‘-O2C12H8)PO(CH2CH2O)2-2-C20H12-2‘-OP(2,2‘-O2C12H8)}, has been designed for solution conformation studies. The complex was prepared by the reaction of the α,ω-bis(phosphorus donor) polyether ligand 2-(2,2‘-O2C12H8)PO(CH2CH2O)2-2-C20H12-2‘-OP(2,2‘-O2C12H8) with Mo(CO)4(nbd). Separate 1H NMR resonances were observed for six of the eight methylene protons of the metalla crown ether, indicating that the motion of the metalla crown ether ring was restricted. The 1H, 13C, and 31P NMR chemical shifts of the metalla crown ether were completely assigned using a combination of 13C DEPT, 1H−1H COSY, 13C{1H} HETCOR, 31P{1H} HETCOR, 1H{13C} HMBC, and 1H−1H NOESY NMR experiments. The solution conformation was analyzed using 1H−1H vicinal coupling constants and 1H−1H NOESY NMR experiments. The vicinal 1H−1H coupling constants for the ethylene protons indicated that both ethylenes were gauche. The 1H−1H NOESY NMR experiments showed a number of close contacts between aromatic protons. Good qualitative agreement was observed between the H−H distances obtained from molecular modeling studies and the intensities of the direct 1H−1H NOEs of the aromatic protons. The structure of the metalla crown ether from the molecular modeling studies also explained the unusual shieldings and deshieldings of certain aromatic protons in the molecule.