Reversible Insertion of 1,3‐Di‐tert‐butylimidazol‐2‐ylidene into a RuH Bond and Bimetallic Activation of a NC Bond of Imidazoline

Abstract

N-Heterocyclic carbenes (NHCs) are strong 2-electron donors and hardly exhibit p-accepting character. [1] NHCs stabilize unsaturated metal centres and have been recently applied as stable spectator ligands in various catalytic transformations of organic molecules in combination with mononuclear transition-metal complexes. [2] However, NHCs can undergo degradation pathways, [3] because they are multifunctional compounds, namely, a lone pair of electrons at C2, which acts as a donor; p-electron lone pairs at N1 and N3; and a C=C double bond between C4 and C5. Because of this multifunctionality, NHCs sometimes undergo a chemical transformation that is possibly related to a deactivation process for catalytically active species.So far, there have been fewer reports on reactions of NHCs with polynuclear complexes than with mononuclear ones, [4] though a variety of patterns are expected to be obtained in the reaction involving a multimetallic system owing to the multifunctional character of NHC molecules. We previously reported the reactions of dinuclear homo-and heterometallic tetrahydrido complexes, [Cp*Ru(m-H) 4 RuCp*] and [Cp*Ru(m-H) 4 OsCp*], with tert-phosphorus ligand that do not contain aryl substituents. [5,6] We examined the reaction of [Cp*Ru(m-H) 4 RuCp*] (1) with 1,3-di-tert-butylimidazol-2-ylidene (ItBu), which is a multifunctional, strong 2-electron donor. We report herein an unprecedented reversible insertion of 1,3-di-tert-butylimidazol-2-ylidene into the ruthenium-hydride bond of 1 and the dynamic process of the resulting dinuclear imidazoline complex. Furthermore, a bimetallic-system-assisted ringopening reaction, that is, the cleavage of the NÀC bond of the imidazoline ligand to generate pseudo-diazaruthenacycle is also reported.The treatment of 1 with 2.3 equivalents of ItBu in tetrahydrofurane at room temperature resulted in the formation of a dinuclear ruthenium m-h 1 :h 2 -4-imidazoline complex 2 as a result of migration of the hydrides from the ruthenium atom to the carbenic carbon (C2) [Eq.(1)]. The yield of the conversion from 1 to 2 reached 56 % under these conditions. The yield inevitably reached a plateau because of the presence of the reverse process, that is, the de-insertion of ItBu. Therefore, a large excess amount of ItBu must be added to carry out the conversion. Time-conversion curves of the reaction of 1 with ItBu are given in the Supporting Information. Complex 2 was efficiently isolated by crystallization from cold pentane (À30 8C).In complex 2, the 4-imidazoline molecule is coordinated as a bridging ligand through a C=C bond and one of the nitrogen atoms. Single crystals of 2 suitable for X-ray diffraction studies were obtained from cold (À30 8C) pentane. The X-ray diffraction data clearly showed the dinuclear structure bridged by 4-imidazoline that was formed by hydride migration to the C2 of the ItBu molecule. The molecular structure of 2 is illustrated in Figure 1 with the relevant bond lengths and angles. [7] The imidazoline molecule is coordinated as 4-electron donor, and...

C−N Bond Activation and Ring Opening of a Saturated N‐Heterocyclic Carbene by Lateral Alkali‐Metal‐Mediated Metalation

“…Although NHCs are generally accepted as robust spectator ligands,anumber of studies have reported that when coordinated to transition metals they can undergo both CÀH and C À Na ctivation. [15] To date,t his reactivity has been missing in s-block chemistry,w ith the exception of Hills [16] work in the formal insertion of BeH 2 into aC À Nbond of the unsaturated NHC IPr and the aforementioned reduction of SIPr with Li metal. [14] Thef ormation of 1 and 2 shows that under the conditions studied both alkali metal MCH 2 SiMe 3 reagents are able to promote the cleavage of the SIMes C À N bond, that is significantly different from the reactivity observed when potassium bis(trimethyl)silyl amide is treated with unsaturated NHCs,w here only donor-acceptor coordination of the NHC to the Kcenter is observed.…”

Coupling of an N‐Heterocyclic Carbene on Iron with Alkynes to Form η5‐Cyclopentadienyl‐Diimine Ligands

“…3 The reported examples involve the migratory insertion of an NHC ligand into metal–carbon bonds,2b–e and the migration of alkyl or aryl groups to the carbene carbon of NHC ligands followed by ring opening 3. The ring opening of NHC ligands has been also demonstrated with a dinuclear ruthenium hydride complex4 and with main group hydrides 5. The removal of an N‐bound substituent of NHC ligands has been demonstrated with Ni, Ru, and Fe complexes 6.…”

C−N Bond Activation and Ring Opening of a Saturated N‐Heterocyclic Carbene by Lateral Alkali‐Metal‐Mediated Metalation

“…[15] To date,t his reactivity has been missing in s-block chemistry,w ith the exception of Hills [16] work in the formal insertion of BeH 2 into aC À Nbond of the unsaturated NHC IPr and the aforementioned reduction of SIPr with Li metal. [15] To date,t his reactivity has been missing in s-block chemistry,w ith the exception of Hills [16] work in the formal insertion of BeH 2 into aC À Nbond of the unsaturated NHC IPr and the aforementioned reduction of SIPr with Li metal.…”