N‑Heterocyclic-Carbene Complexes Readily Prepared from Di-μ-hydroxopalladacycles Catalyze the Suzuki Arylation of 9‑Bromophenanthrene

New cyclometalated palladium complexes
of general formula [Pd­(Bmim)­(X)­(C∧N)] have been
synthesized by a novel reaction route
involving di-μ-hydroxo-palladacycles [{Pd­(μ-OH)­(C∧N)}2] (C∧N = 2-benzoylpyridine
(Bzpy), I, previously unreported, or C∧N = 2-phenylpyridine (Phpy), II)] and 1,3-butylmethyl­imidazolium
salts [HBmim]­X (X: Cl, Br, I, or saccharinate (Sacc); a, b, c, or d complexes, respectively).
This simple acid–base reaction could not be achieved under
identical conditions when corresponding di-μ-acetate complexes
were used as starting materials. An alternative pathway to NHC/imidate
complexes has also been explored by reacting IIb with
[Ag­(Phthal)­(SMe2)]2 (Phthal = phthalimidate, e) to obtain [Pd­(Bmim)­(Phthal)­(Phpy)], IIe. Structural
characterization by X-ray diffraction of complexes Id, IIb, IId, and IIe has confirmed
the proposed formulas. The mononuclear complexes have shown to catalyze
the scalable Suzuki–Miyaura cross-coupling of 9-bromophenanthrene
with a wide scope of aryl boronic acids, irrespective of their electronic
properties and at a very low catalyst concentration of 0.01%.