Variation of the Sterical Properties of the N-Heterocyclic Carbene Coligand in Thermally Triggerable Ruthenium-Based Olefin Metathesis Precatalysts/Initiators

A series of ruthenium complexes based on the kappa(2)( C,N) - (2- (benzo [h] quinolin-10-yl)methylidene ruthenium dichloride fragment featuring different neutral coligands L (L = 1,3-bis (2,6-diis opropylp henyl) -4, S- dihydroimidazol-2-ylidene (SIPr), 1,3-bis(2,4,6-trimethylpheny1)-4,5-dihydroimidazol-2-ylidene (SIMcs), 1,3-bis(2,4-dimethylpheny1)-4,5-dihydroimidazol-2-ylidene (81Xyl), and 1,3-bis(2-methylpheny1)4,5-dihydroimidazol-2-ylidene (SITO1)) was prepared, characterized, and tested in the thermally induced ring-opening metathesis polymerization of dicydopentadiene. In addition, the corresponding tricydohe#1phosphine derivative was investigated for comparison. All compounds were isolated as their trans-clichloro isomers. NMR spectroscopic features as well as structural features are, particularly within the NHC-bearing complexes, very similar, but their polymerization activity at elevated temperatures is distinctly different. While the SIMes derivatiie shows the desired properties, i.e., latency at room temperature and pronouneed polymerization activity at elevated temperature, 'all other preinitiators do not. The preinitiator featuring the SIPr coligarid is the most latent one, needing temperatures > 140 degrees C to show moderate: activity in the polymerization of dicydopentadiene. Compounds bearing the smaller N-heterocyclic carbene congeners are stable and latent at room temperature, but decompose upon heating, diminishing the polymerization activity at elevated temperatures. Density functional calculations show that the SIMes -derivative is the easiest to activate and yields the most stable 14-electron intermediate. Finally calculations reveal a distinct influence of the nature Of the N-heterocyclic carbene ligand on the position of the equilibrium of cis- and trans-dichloro isbrhers of the Complexes. -While the SLPr and the SIMes derivatives prefer the cisconfiguration, all other derivatives favor, at least in solvents with low dielectric constants, the trans-configuration. These computational findings were supported by the isolation and full characterization of the cis-dichloro isomer of the SIMes-bearing preinitiator obtained upon heating of its trans-isomer at 140 degrees C.