Synthesis and study of olefin metathesis catalysts supported by redox-switchable diaminocarbene[3]ferrocenophanes?

Dalton Transactions Pub Date: 2013-07-25 DOI: 10.1039/C3DT51278A

Abstract

A redox-switchable ligand, N,N′-dimethyldiaminocarbene[3]ferrocenophane (5), was synthesized and incorporated into a series of Ir- and Ru-based complexes. Electrochemical and spectroscopic analyses of (5)Ir(CO)2Cl (15) revealed that 5 displayed a Tolman electronic parameter value of 2050 cm?1 in the neutral state and 2061 cm?1 upon oxidation. Moreover, inspection of X-ray crystallography data recorded for (5)Ir(cis,cis-1,5-cyclooctadiene)Cl (13) revealed that 5 was sterically less bulky (%VBur = 28.4) than other known diaminocarbene[3]ferrocenophanes, which facilitated the synthesis of (5)(PPh3)Cl2Ru(3-phenylindenylid-1-ene) (18). Complex 18 exhibited quasi-reversible electrochemical processes at 0.79 and 0.98 V relative to SCE, which were assigned to the Fe and Ru centers in the complex, respectively, based on UV-vis and electron pair resonance spectroscopic measurements. Adding 2,3-dichloro-5,6-dicyanoquinone over the course of a ring-opening metathesis polymerization of cis,cis-1,5-cyclooctadiene catalyzed by 18 ([monomer]0/[18]0 = 2500) reduced the corresponding rate constant of the reaction by over an order of magnitude (pre-oxidation: kobs = 0.045 s?1; post-oxidation: kobs = 0.0012 s?1). Subsequent reduction of the oxidized species using decamethylferrocene restored catalytic activity (post-reduction: kobs = up to 0.016 s?1, depending on when the reductant was added). The difference in the polymerization rates was attributed to the relative donating ability of the redox-active ligand (i.e., strongly donating 5versus weakly donating 5++++++) which ultimately governed the activity displayed by the corresponding catalyst.

Graphical abstract: Synthesis and study of olefin metathesis catalysts supported by redox-switchable diaminocarbene[3]ferrocenophanes
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