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 C-2 symmetry by H-1 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 center dot center dot center dot Rh bonding interaction (3.2758(6) angstrom). Oxidative addition reactions of 1a and 1b, using MeI and PhICl2, have been studied in situ by H-1 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 H-1 NMR spectroscopy, and in the solid state, by X-ray diffraction.