Bipyridine Adducts of Molybdenum Imido Alkylidene and Imido Alkylidyne Complexes

Abstract

Seven bipyridine adducts of molybdenum imido alkylidene bispyrrolide complexes of the type Mo(NR)(CHCMe2R′)(Pyr)2(bipy) (1a–1g; R = 2,6-i-Pr2C6H3 (Ar), adamantyl (Ad), 2,6-Me2C6H3 (Ar′), 2-i-PrC6H4 (AriPr), 2-ClC6H4 (ArCl), 2-t-BuC6H4 (ArtBu), and 2-MesitylC6H4 (ArM), respectively; R′ = Me, Ph) have been prepared using three different methods. Up to three isomers of the adducts are observed that are proposed to be the trans and two possible cis pyrrolide isomers of syn alkylidenes. Sonication of a mixture containing 1a–1g, HMTOH (2,6-dimesitylphenol), and ZnCl2(dioxane) led to the formation of MAP species of the type Mo(NR)(CHCMe2R′)(Pyr)(OHMT) (3a–3g). DCMNBD (2,3-dicarbomethoxynorbornadiene) is polymerized employing 3a–3g as initiators to yield >98% cis,syndiotactic poly(DCMNBD). Attempts to prepare bipy adducts of bisdimethylpyrrolide complexes led to formation of imido alkylidyne complexes of the type Mo(NR)(CCMe2R′)(Me2Pyr)(bipy) (Me2Pyr = 2,5-dimethylpyrrolide; 4a – 4g) through a ligand-induced migration of an alkylidene α proton to a dimethylpyrrolide ligand. X-ray structures of Mo(NAr)(CHCMe2Ph)(Pyr)2(bipy) (1a), Mo(NAriPr)(CHCMe2Ph)(Pyr)(OHMT) (3d), Mo(NAr)(CCMe2Ph)(Me2Pyr)(bipy) (4a), and Mo(NArT)(CCMe3)(Me2Pyr)(bipy) (ArT = 2-(2,4,6-i-Pr3C6H2)C6H4; 4g) showed structures with the normal bond lengths and angles.

Understanding Synthetic Peculiarities of Cationic Molybdenum(VI) Imido Alkylidene N‐Heterocyclic Carbene Complexes

“…Products bearing only one NHC have not been observed, indicating the necessity of two stabilizing neutral ligands. Complex 38 is the first isolated Mo imido alkylidyne species, which is not stabilized by a bipyridine ligand . In addition, bis NHC complexes of this type have not been reported to the best of our knowledge.…”

“…We propose that elimination of hexafluoro‐ tert ‐butanol from complexes of type 9 and 10 , and subsequent protonation at the NHC of 9 or 10 by HOCMe(CF 3 ) 2 yields stable anionic molybdenum amido alkylidyne species accompanied by an imidazolium cation and, in this case, probably unstable [Mo(imido)(CCMe 2 Ph)(NHC)(OR)] complexes. Such molybdenum imido alkylidyne species were reported to form readily upon addition of bipyridine to several bis‐pyrrolide complexes and can be stabilized by bipyridine or two NHC ligands (vide infra). A few rare examples of neutral and anionic amido alkylidyne species have been reported earlier, and seem to form readily under a variety of circumstances from imido alkylidenes .…”

High‐Oxidation State Molybdenum and Tungsten Complexes Relevant to Olefin Metathesis

“…Interestingly, attempts to prepare bipy adducts of bisdimethylpyrrolide complexes led to the formation of imido alkylidyne complexes of the type Mo(NR)(CCMe 2 R ′ )(Me 2 Pyr)(bipy) through the ligand-induced migration of an alkylidene α proton to a dimethylpyrrolide ligand (Eq. (1.4)) [55].…”

“…Mo(NAr)(CHR ′ )(Pyr) 2 R ′ = t-Bu or CMe 2 Ph [11] Mo(N-2,6-Br 2 -4-MeC 6 H 2 )(CH-t-Bu)(Pyr) 2 [11] Mo(NAd)(CHCMe 2 Ph)(Pyr) 2 [11] Mo(NAd)(CHCMe 2 Ph)(Pyr) 2 (PMe 3 )* [12] Mo(NR)(CHCMe 2 Ph)(Pyr) 2 (bipy) R = Ar*, Ad, Ar Me2 , Ar Cl , Ar iPr , Ar tBu , Ar Mes [55] Mo(NAr)(CHCMe 2 Ph)X 2 X = Me 4 Pyr*, i-Pr 2 Pyr*, Ph 2 Pyr*, Indolide* [12] Mo(NAr)(CH-t-Bu)(Ph 2 Pyr) 2 [12] Mo(NAr)(CHCMe 2 Ph)(indolide) 2 (THF) [12] Mo(NAr)(CHCMe 2 Ph)(R 2 Pz) 2 R 2 Pz = 3,5-diphenylpyrazolide* or 3,5-di-t-butylpyrazolide [32] (continued overleaf ) [38] W(NC 6 F 5 )(CH-t-Bu)(Pyr) 2 (DME) [18] 1.2 New Imido Ligands and Synthetic Approaches 5 [34] (continued overleaf ) *An X-ray structure was obtained for this compound bipy, 2,2-bipyridyl; DME, 1,2-dimethoxyethane; Mes, 2,4,6-trimethylphenyl; NAd, N-1-Admantyl; NAr, N-2,6-i-Pr 2 C 6 H 3 ; NAr R2 , N-2,6-R 2 C 6 H 3 ; NAr R , N-2-RC 6 H 4 ; NAr 3,5Me2 , N-3,5-Me 2 C 6 H 3 ; Biphen 2− , 3,3 ′ -di-t-Bu-5,5 ′ ,6,6 ′ -tetramethyl-1,1 ′ -biphenyl-2,2 ′ -diolate; OBr 2 Bitet, 3,3 ′ -dibromo-2 ′ -(tert-butyldimethylsilyloxy)-5,5 ′ ,6,6 ′ ,7,7 ′ ,8,8 ′ -octahydro-1,1 ′ -binaphthyl-2-olate; OHIPT, hexaisopropylterphenoxide = O-2,6-(2,4,6-i-Pr 3 C 6 H 2 ) 2 C 6 H 2 ; OHMT, hexamethylterphenoxide = O-2,6-Mes 2 C 6 H 2 ; ODFT, decafluoroterphenoxide = O-2,6-(C 6 F 5 ) 2 C 6 H 3 ; ODPP, 2,6-diphenylphenoxide; ODIPP, O-2,6-(i-Pr) 2 C 6 H 3 ; OR F6 , OCMe(CF 3 ) 2 ; OR F3 , OCMe 2 (CF 3 ); OBiphenTMS, O-3,3 ′ -di-tert-butyl-5,5 ′ ,6,6 ′ -tetramethyl-2 ′ -(trimethylsilyloxy)biphenyl-2-olate; OBINAP-TBS, O-2 ′ -(tert-butyldimethylsilyloxy)-1,1 ′ -binaphthyl-2-olate; ODPP R , 2,6-(2,5-R 2 pyrrolyl) 2 Phenoxide (R = i-Pr or Ph); Pyr, pyrrolide; MesPyr, 2-mesitylpyrrolide; CNPyr, 2-cyanopyrrolide; Me 4 Pyr, 2,3,4,5-tetramethylpyrrolide; R 2 Pyr, 2,5-R 2 pyrrolide; TBS, dimethyl-t-butylsilyl; Trip, 2,4,6-triisopropylphenyl.…”

“…An X-ray structure showed that the NAr species was the unsymmetric dimer, {Mo(NAr)(syn-CHCMe 2 Ph)(η 5 -NC 4 H 4 )(η 1 -NC 4 H 4 )}{Mo(NAr)(syn-CHCMe 2 Ph)(η 1 -NC 4 H 4 ) 2 }, in which the nitrogen in the Bipyridine adducts of imido alkylidene bispyrrolide complexes have proven useful as intermediates in the often-unsuccessful synthesis and isolation of complexes that contain the parent pyrrolide. For example, Mo(NR)(CHCMe 2 R ′ ) (Pyr) 2 (bipy) (NR = NAr, NAd, NAr Me2 , NAr iPr , NAr Cl , NAr tBu , and NAr Mes ; R ′ = Me, Ph) can be prepared and readily isolated [55]. The sonication of a mixture containing the bispyrrolide bipy adduct, HMTOH, and ZnCl 2 (dioxane) (to remove the bipy) led to formation of MAP species of the type Mo(NR) [39].…”