Synthesis and Reactivity of NHC-Based Rhodium Macrocycles

Using a general synthetic procedure
employing readily accessed terminal alkene-functionalized pro-ligands
and macrocyclization by ring-closing olefin metathesis, rhodium carbonyl
complexes have been prepared that contain lutidine (1a; n = 1) and pyridine (1b; n = 0) derived tridentate CNC macrocycles with dodecamethylene
spacers. In solution, 1a shows temperature-invariant
time-averaged C2 symmetry by 1H NMR spectroscopy (CD2Cl2, 500 MHz), whereas
in the solid-state, two polymorphs can be obtained showing different
conformations of the alkyl spacer about the metal–carbonyl
bond (asymmetric and symmetric). In contrast, time-averaged motion
of alkyl spacer in 1b can be halted by cooling below
225 K (CD2Cl2, 500 MHz), and the complex crystallizes
as a dimer with an interesting unsupported Rh···Rh
bonding interaction (3.2758(6) Å). Oxidative addition reactions
of 1a and 1b, using MeI and PhICl2, have been studied in situ by 1H NMR spectroscopy, although
pure Rh­(III) adducts can be ultimately isolated only with the pyridine-based
macrocyclic ligand. The lutidine backbone of 1a can be
deprotonated by addition of K­[N­(SiMe3)2], and
the resulting neutral dearomatized complex (5) has been
fully characterized in solution, by variable-temperature 1H NMR spectroscopy, and in the solid state, by X-ray diffraction.