Peculiarities of the reaction of (SPY-5-34)-dichloro-(κ2(C,O)-2-formylbenzylidene)(1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene)ruthenium with potassium hydridotris(pyrazolyl)borate

The reaction of (SPY-5-34)-dichloro-(κ2(C,O)-2-formylbenzylidene)(H2IMes)ruthenium (H2IMes=1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene) with potassium hydridotris(pyrazolyl)borate (KTp) in dichloromethane yielded an unusual ruthenium complex chloro(κ3(N,N,N)-chlorotris(pyrazolyl)borate)(κ2(C,C)-1-(2,4,6-trimethylphenyl)-3-(4,6-dimethylphenyl-2-methylidene)-4,5-dihydroimidazol-2-ylidene)ruthenium (2). In 2, a chlorotris(pyrazolyl)borate ligand, which had been created during this reaction, binds in κ3(N,N,N)-mode to the central ruthenium atom. Additionally, a double C–H activation of a methyl group of the H2IMes resulted in the formation of a chelating N-heterocyclic biscarbene ligand and liberation of the former 2-formylbenzylidene as 2-methylbenzaldehyde. Formally, a double hydrogen transfer from a methyl group of the H2IMes to the initial carbene carbon occurred. 2 was characterized by NMR spectroscopy, elemental analysis and single crystal X-ray structure determination. The reaction of KTp with (SPY-5-34)-dichloro(κ2(C,O)-2-ethoxycarbonylbenzylidene)(H2IMes)ruthenium, on the other hand, gave the expected product chloro(κ3(N,N,N)-hydridotris(pyrazolyl)borate)(H2IMes)(2-ethoxycarbonylbenzylidene)ruthenium (6). Compound 6 was characterized by NMR spectroscopy, elemental analysis and single crystal X-ray structure determination. Investigations of the relative activities of these complexes in model ring opening metathesis polymerizations showed a pronounced thermal latency. Polymerizations proceeded at temperatures above 100 °C in case of 6 and 130 °C in case of 2.

This report describes two unprecedented reaction-pathways which occurred during the reaction of (SPY-5-34)-dichloro-(κ2(C,O)-2-formylbenzylidene)(H2IMes)ruthenium with KTp. The first is the formation of chlorotris(pyrazolyl)borate from hydridotris(pyrazolyl)borate. The second is a formal double hydrogen transfer from a methyl group of the H2IMes ligand to a carbene carbon, transforming the H2IMes into a chelating biscarbene ligand.Figure optionsDownload as PowerPoint slide