Complexes with Hybrid Phosphorus-NHC Ligands: Pincer-Type Ir Hydrides, Dinuclear Ag and Ir and Tetranuclear Cu and Ag Complexes

Three types of hybrid phosphorus-imidazolium
salts, 1-methyl-3-(3-((diphenylphosphino)­methyl)­benzyl)-1H-imidazol-3-ium hexafluorophosphate (2·PF6), 1-methyl-3-(3-(di-tert-butylphosphinooxy)­phenyl)­imidazolium
iodide (8a), and 3-(3-((diphenylphosphoryl)­methyl)­phenyl)-1-methyl-1H-imidazol-3-ium iodide (11) have been prepared
and used as precursors to phosphine-NHC, phosphinite-NHC, and phosphoryl-NHC
metal complexes, respectively. The structure of 11 has
been determined by X-ray diffraction. The Ag­(I) and Ir­(I) complexes
of the phosphine-NHC ligand, [Ag­(μ-P-NHC,κC,κP)]2(PF6)2 (3) and [Ir­(cod)­(μ-P-NHC,κC,κP)]2(PF6)2 (4), were obtained and characterized by NMR, ESI-MS,
elemental analysis, and X-ray diffraction. Both complexes are dinuclear
and dicationic, with two P-NHC ligands bridging the two metal centers.
The presence of the P donor led for 3 to an unprecedented
structure compared to that of related Ag­(I) complexes with trans spanning
bis-NHC ligands. Complex 4 is the first example of a
dinuclear iridium complex with a hybrid P-NHC ligand. The new hydrido,
Ir­(III) pincer-type complex [IrH­(CNHCCCNHC)­(MeCN)]­PF6 (7) is suggested to
have a square-pyramidal structure. The tetranuclear Ag­(I) complex
with the phosphinite-NHC ligand, [Ag2(μ3-I)­(μ-PO-NHC,κP,κCNHC)]2 (9a) has a cubane-type
structure, with alternating silver and iodine apexes and two PO-NHC
ligands bridging opposite edges of the Ag4 tetrahedron.
The Ir­(III) pincer complexes [IrH­(I)­(PO-NHC,κP,κC,κCNHC)Me] (10a) and [IrH­(I)­(PO-NHC,κP,κC,κCNHC)n‑Bu] (10b), with Me or n-Bu substituents on the nitrogen
atom, respectively, have been prepared and characterized. Ag­(I) and
Cu­(I) complexes with the phosphoryl-NHC ligand are reported and the
centrosymmetric structure of the latter, [Cu­(OP-NHC,κCNHC)2(μ-I)­{Cu­(μ-I)}]2 (13), was established by X-ray diffraction and
consists of a central Cu2(μ-I)2 rhombus
connected by single iodide bridges to two Cu­(OP-NHC,κCNHC)2 moieties. The Ir­(III) hydride
pincer complexes 10a,b were tested as catalyst precursors
for the C–H bond activation of alkanes. Although their efficiency
was significantly lower for transfer dehydrogenation from cyclooctane
(coa) to t-butylethylene (tbe) than that of known
PCP-Ir systems, these results represent the first attempts to study
the catalytic properties of hybrid P-NHC iridium pincer complexes.