Zerovalent carbene complexes of Pd containing the 1,3,4,5-tetramethylimidazol-2-ylidene
ligand (tmiy) have been synthesized. Pd(COD)(alkene) (COD = cyclooctadiene) reacts with
the nucleophilic carbene tmiy to produce the complexes Pd(tmiy)2(alkene) (alkene = maleic
anhydride (MAH) (2), tetracyanoethylene (TCNE) (3)). Spectroscopic studies on the complexes
provide strong evidence of the almost purely donor nature of the carbene ligand. Oxidative
addition of hydrocarbyl halide and dihalide substrates to 2 and 3 yield the PdII derivatives
Pd(tmiy)2(Ph)I (4), Pd(tmiy)2I2 (5), Pd(tmiy)2(4-nitrophenyl)I (6), and Pd(tmiy)2Br2 (7). The
zerovalent Ni complex Ni(tmiy)2 was produced in situ from the reaction of Ni(COD)2 with
tmiy, and the oxidative addition of organic halides yields Ni(tmiy)2(o-tolyl)Br (8), Ni(tmiy)2(Me)I (9), and Ni(tmiy)2I2 (10). X-ray crystal structures of complexes 8 and 10 are reported
which reveal square-planar coordination with the carbene ligands inclined at significant
angles to the coordination planes of the complexes. Metal to ligand bond distances are all
indicative of the electron density induced on the metal center by the donor carbene ligands.
Halide abstraction from 6 in the presence of n-butyl acrylate leads to migratory insertion of
the olefin and elimination of the Heck coupling product. Under stoichiometric reaction
conditions hydrocarbyl−imidazolium ions are also produced as byproducts. Selected
complexes were tested as catalysts for the Heck reaction (Pd) and Suzuki coupling (Pd, Ni)
and were found to be highly active. The observed reaction behavior is interpreted, and a
mechanism for the Heck coupling is provided.
A general synthetic methodology is reported for tris(bidentate)ruthenium(II) complexes containing three different polypyridyl ligands, based on the sequential addition of the ligands to the oligomer [RuíCObCX],,. The tris-(heteroleptic) complexes were characterized by FAB mass spectrometry and NMR spectroscopy. An X-ray crystal structure determination was made for the complex [Ru(Me2bpy)(phen)(bpa)](PF6)2'C6Hi4 [C40H43F12N7P2RU, M = 1062.8; Me2bpy = 4J4'-dimethyl-2,2'-bipyridine, phen = 1,10-phenanthroline, bpa = bis(2-pyridyl)amine]: triclinic, space group PI, a = 14.57(3) k,b= 13.50(3) k, c = 12.73(3) Á, a = 68.6(2)°, ß = 63.5(1)°, = 79.8(2)°, V = 2082 Á3, Z = 2. Aspects of the electrochemistry, spectroscopy, and photophysics of the tris-(heteroleptic) species are discussed.
The complexes {Cp*(PP)Ru}2(μ-C⋮CC⋮C) (PP = dppm 5a, dppe 5b) have been synthesized
from RuCl(PP)Cp* (1a/b) via the corresponding vinylidenes [Ru(CCH2)(PP)Cp*]+ (2a/b), deprotonation (KOBut) to the ethynyls Ru(C⋮CH)(PP)Cp* (3a/b), oxidative coupling
([FeCp2][PF6]) to the bis(vinylidenes) [{Ru(PP)Cp*}2{μ-(CCHCHC)}]2+ (4a/b), and
deprotonation [dbu (4a), KOBut (4b)]. Electrochemistry of 5a/b revealed the expected
sequence of four 1e redox steps, which occurred at significantly lower E° values than found
for the Ru(PPh3)2Cp analogue. Single-crystal X-ray structure determinations are reported
for 1a/b, 2a/b, 3a/b, 4a/b, and 5a/b, together with the oxidized products [5b][PF6]
n
(n = 1,
2). In the monocation [5b][PF6] the Ru−C(1) [1.931(2) Å] and C−C distances [1.248−1.338(3) Å] are intermediate between those found in 5b and the dication [5b]2+. The short Ru−C
[1.857(5) Å] and experimentally equal C−C distances [1.269−1.280(6) Å] in [5b][PF6]2
confirm
the anticipated dicarbene-cumulene structure for the RuCCCCRu bridge.
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