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Bulky N‑Phosphinomethyl-Functionalized N‑Heterocyclic Carbene Chelate Ligands: Synthesis, Molecular Geometry, Electronic Structure, and Their Ruthenium Alkylidene Complexes
journal contribution
posted on 2013-01-14, 00:00 authored by Hiyam Salem, Martin Schmitt, Ulrike Herrlich (née
Blumbach), Erik Kühnel, Marcel Brill, Philipp Nägele, André Luiz Bogado, Frank Rominger, Peter HofmannA new, extremely bulky, and electron-rich N-phosphinomethyl-functionalized
N-heterocyclic carbene ligand, 5a (tBuNHCPtBu), and a somewhat less
bulky congener, 5b (MesNHCPtBu), forming five-membered chelate rings with metal centers,
have been synthesized in four steps starting from the easily accessible
di-tert-butyl(hydroxymethyl)phosphine oxide (1). 5a was isolated and fully characterized by
spectroscopic methods including UV-photoelectron spectroscopy and
X-ray diffraction. The reaction of 5a with [Ru(COD)Cl2]n under hydrogen pressure or
with [Ru(p-cymene)Cl2]2 led
to the formation of the unsaturated dinuclear complex [Ru(tBuNHCPtBu)(μ-Cl)(Cl)]2 (6), which serves as a precursor for a series
of ruthenium carbene complexes (7a–f) using substituted phenyldiazomethanes (p-X-C6H4(CH)N2; X = H (a), Br
(b), CF3 (c), NO2,
(d), CH3 (e)) and trimethylsilyldiazomethane
(f). Treatment of 6 with phosphine or pyridine
ligands led to the formation of the mononuclear adducts, [Ru(tBuNHCPtBu)(Cl2)(PR3)] (R = Me (8), Ph (9), Cy (10)) and [Ru(tBuNHCPtBu)(Cl2)(py)n] (n = 1 (11), 2 (12); if (py)2 = bipy (13)), which were synthesized
in order to find alternative precursor complexes because the dimer 6 showed very low solubility in most organic solvents. Complex 7a was obtained analytically pure on a different route via
transmetalation from a silver complex bearing tBuNHCPtBu (15) to
the first-generation Grubbs catalyst as the ruthenium precursor. Complexes 7a–c and 7e were characterized
by X-ray diffraction analysis, revealing a geometry that can be viewed
as both a distorted square pyramid and a distorted trigonal bipyramid
with the two chloro ligands in a cis configuration.
The steric bulk, especially of 5a with its N-tBu moiety, stabilizes 16 VE Ru complexes. In contrast
to ligand 5a, the somewhat less bulky MesNHCPtBu ligand 5b has allowed its
direct metalation with two ruthenium alkylidene precursors, affording
the two new carbene complexes 17 and 18.
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cis configurationcarbene complexes 17ruthenium alkylidene precursorsRuthenium Alkylidene ComplexesAComplexes 7square pyramidhydrogen pressureMesNHCPtBu ligand 5 bdimer 6ligand 5alternative precursor complexesMolecular Geometrychloro ligands7 eComplex 7ruthenium precursorCHpyridine ligandsCF5 bElectronic Structurespectroscopic methods6Hmetal centers16 VE Ru complexessteric bulk
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