Transition Metal Complexes of Bulky, Electron-Rich <i>N</i>-Phosphanyl-Substituted N-Heterocyclic Carbenes (NHCP Ligands). Small Bite Angle Four-Membered (κ-<i>C</i>,κ-<i>P</i>) Chelate Structures

Abstract

Bulky, electron-rich N -phosphanyl-substituted N-heterocyclic carbenes (NHCP ligands: Et CNP 3, Mes CNP 5a, Dipp CNP 5b ) are potential ligands for strained, very small bite angle four-membered transition metal chelate complexes with (κ- C,κ- P ) metal coordination. Various compounds of this type have been synthesized and were fully characterized inter alia by X-ray crystallography. Reactions of isolated NHCP species 3, 5a, and 5b with appropriate metal precursors furnished a series of (κ- C,κ- P )-coordinated four-membered NHCP chelate complexes for the metals of the nickel triad and for ruthenium. The cis -dimethyl Pd(II) complex ( Mes CNP-κ C,κ- P )PdMe 2 ( 8 ) reacted at room temperature with acceptor-substituted alkenes such as fumaronitrile and maleic anhydride to yield the corresponding η 2 -olefin complexes 9 and 10 with reductive elimination of ethane. Palladacyclopentadiene complex 11 was formed by addition of two equivalents of dimethyl acetylene dicarboxylate to complex 8 via alkyne–alkyne coupling.