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

Bulky, electron-rich N-phosphanyl-substituted N-heterocyclic carbenes (NHCP ligands: ^EtCNP 3, ^MesCNP 5a, ^DippCNP 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 (^MesCNP-κC,κ-P)­PdMe₂ (8) reacted at room temperature with acceptor-substituted alkenes such as fumaronitrile and maleic anhydride to yield the corresponding η²-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.