Geometric and Electronic Structure of aC₁-Symmetric Ruthenium-Aryloxide Metathesis Catalyst: An Experimental and Computational Study

Abstract

Spectroscopic and computational methods were employed to establish the geometric and electronic structure of the Ru-pseudohalide metathesis catalyst RuCl(OC6Br5)(IMes)(py)(CHPh) 5 (IMes = 1,3-dimesitylimidazol-2-ylidene, py = pyridine). NOESY-NMR and electronic absorption spectroscopic analysis of 5, carried out in parallel with analysis of the known trans- and cis-anionic structures RuCl2(IMes)(py)2(CHPh) (3a) and Ru(o-cat)(IMes)(py)(CHPh) 4a (o-cat = ortho-catecholate), confirms the trans-anionic geometry of the bromoaryloxide complex. An additional Ru···Br dative interaction is indicated by DFT analysis. The lower energy barrier to Ru−CNHC rotation vs N−CMes rotation is demonstrated for 4a and 5 and is suggested for the corresponding dichloride catalysts. The location and intensity of the principal visible absorption band is strongly conserved for 3a and 5a and is proposed as a useful signature to facilitate identification of the trans-anionic geometry in related RuX2LL′(CHPh) species.

Mechanistic Studies of Hoveyda–Grubbs Metathesis Catalysts Bearing S‐, Br‐, I‐, and N‐coordinating Naphthalene Ligands

“…In particular, examination of 3.7-4.3 ppm region, with the multiplet characteristic for NCH 2 CH 2 N fragment of the cis-Cl 2 isomer, revealed presence of additional sharp singlet, expected for symmetric structure of the trans-Cl 2 form, in which the four protons become equivalent due to symmetry and exchange processes. [30,45] The peak integration agreed with intensity of the minor form observed in benzylidene region, and content of the both species increased with temperature in parallel manner. Considering that the initiation mechanism consists of the isomerization process, higher contents of the active trans-Cl 2 forms should translate into more active catalysts.…”

Decomposition of Vanadium(V) Alkylidenes Relevant to Olefin Metathesis

“…In the proposed vanadium­(III) intermediate 8 , bearing the chlorinate aryloxide ligand, DFT revealed that coordination of an orthochlorine to vanadium may occur with little energetic cost (1.1 kcal/mol) (Chart S4). This is reminiscent of the ruthenium alkylidene complex bearing a brominated aryloxide ligand (OC 6 Br 5 )­(IMes)­(py)­Ru­(Cl)­[CH­(C 6 H 5 )] (IMes = 1,3-dimesitylimidazol-2-ylidene, py = pyridine), in which DFT analysis revealed a ruthenium–bromine interaction . In the case of 8 , the chlorine coordination allows for facile oxidative addition to generate the vanadium­(V) chloride species 12 (Figure ).…”

“…This is reminiscent of the ruthenium alkylidene complex bearing a brominated aryloxide ligand (OC 6 Br 5 )(IMes)(py)Ru(Cl)[CH(C 6 H 5 )] (IMes = 1,3dimesitylimidazol-2-ylidene, py = pyridine), in which DFT analysis revealed a ruthenium−bromine interaction. 68 In the case of 8, the chlorine coordination allows for facile oxidative addition to generate the vanadium(V) chloride species 12 (Figure 2). Compound 12 may then act as the source of the chlorine atom observed in the product, 10, via a comproportionation reaction with another equivalent of the vanadium(III) species 8 (Figure 2).…”

A New Family of Halogen‐Chelated Hoveyda–Grubbs‐Type Metathesis Catalysts

“…As weak donors, we considered covalently bonded halogen atoms, which are a rare class of ligands in coordination chemistry. This type of interaction has been postulated in only a few benzylidene–ruthenium complexes described in the literature, and only for structures with the donor atom occupying the sixth coordination site and, usually, weakly bonded to the metal center 19–21. To the best of our knowledge, stable pentacoordinated ruthenium complexes in which one of the ligands is a covalently bonded halogen atom remain unknown.…”

“…This type of interaction has been postulated in only a few benzylidene-ruthenium complexes described in the literature, and only for structures with the donor atom occupying the sixth coordination site and, usually, weakly bonded to the metal center. [19][20][21] To the best of our knowledge, stable pentacoordinated ruthenium complexes in which one of the ligands is a covalently bonded halogen atom remain unknown. Starting from this point, we investigated the reaction between the commercially available complex 4 and 2-halostyrene derivatives under ligand-exchange conditions.…”