Synthesis of Molybdenum Imido 2-Adamantylidene Complexes through α Hydrogen Abstraction

Abstract

Addition of 2-adamantylMgBr in diethyl ether to Mo(NAr) 2 (Cl) 2 (DME) (Ar = 2,6-i-Pr 2 C 6 H 3 ) gave Mo(NAr) 2 (2-Ad) 2 (2-Ad = 2-adamantyl, DME = 1,2-dimethoxyethane), from which Mo(NAr)(Adene)(OTf) 2 (DME) (1, Adene = 2-adamantylidene) was prepared upon addition of triflic acid. Methods analogous to those that have been successful for synthesizing neopentylidene and neophylidene complexes were used to form Mo(NAr)-(Adene)(OMes) 2 (2; Mes = 2,4,6-trimethylphenyl), [Mo(NAr)(Adene)(OC 6 F 5 ) 2 ] 2 (3), Mo(NAr)(Adene)(OC 6 F 5 ) 2 (MeCN) (4), Mo(NAr)(Adene)(Cl) 2 (bipy) (5; bipy = 2,2′bipyridine), Mo(NAr)(Adene)(Cl)(OHMT) (6; OHMT = O-2,6-Mesityl 2 C 6 H 3 ), Mo(NAr)-(Adene)(Cl)(OHMT)(t-BuCN) (6(t-BuCN)), and Mo(NAr)(Adene)(OHMT)(Pyr) (7; Pyr = pyrrolide). Analogous methods beginning with Mo, and Mo(N-t-Bu)(Adene)(OHMT)(Pyr)(THF) (8d). X-ray structural studies of 1-5, 6(t-BuCN), 7, 8a, 8c, and 8d suggest that one of the Adene β protons is engaged in an agostic interaction with the metal when the electron count is less than 18. Preliminary olefin metathesis studies confirm that Adene complexes react with cyclooctene and 1-decene in the manner expected for high-oxidation-state Mo alkylidene complexes.

Synthesis of Molybdenum Perfluorophenylimido 2-Adamantylidene Complexes

“…The MoC bonds are longer in 3a and 3b compared to other compounds prepared here that contain less than 18e ( 4 – 6 ). The difference between the two MoC α C β angles in the alkylidene is small in 3a and 3b , as was also found in analogous NAr and N- t -Bu complexes …”

“…We conclude that high oxidation state Mo pentafluorophenylimido 2-adamantylidene complexes are readily prepared through α hydrogen abstraction reactions analogous to those reported for Mo 2,6-diisopropylphenylimido and tert -butylimido Adene complexes . More advanced NMR and theoretical studies in the last several years that have resulted in a reinterpretation of agostic CH interactions, in general, and the nature of the MC bond in alkylidenes, in particular, will be required in order to understand the bonding details within the Mo­(Adene) system.…”

“…We found what we interpreted as strong CH β agostic interactions in the methyl group in the first structurally characterized MoCMePh complexes . Therefore, we turned to synthesizing and studying Mo­(Adene) (Adene = 2-adamantylidene) complexes where the two CH β bonds are identical. Few 2-adamantylidene complexes of any metal had been published and no d 0 2-adamantylidene complexes were known prior to our report last year of Mo imido Adene complexes in which the imido substituent is 2,6-diisopropylphenyl (Ar) or tert -butyl.…”

“…Therefore, we turned to synthesizing and studying Mo­(Adene) (Adene = 2-adamantylidene) complexes where the two CH β bonds are identical. Few 2-adamantylidene complexes of any metal had been published and no d 0 2-adamantylidene complexes were known prior to our report last year of Mo imido Adene complexes in which the imido substituent is 2,6-diisopropylphenyl (Ar) or tert -butyl. We found that the difference in the two MoC α C β bond angles in the Adene ligand is as large as 26°.…”

Increasing Olefin Metathesis Activity of Silica‐Supported Molybdenum Imido Adamantylidene Complexes through E Ligand σ‐Donation

“…[24][25][26] In this work, we explore the grafting and the metathesis activity of well-defined bis-pyrrolide Mo imido adamantylidene precursors of the form Mo-(Pyr) 2 (= NR)(= Ad) (Pyr = 2,5-dimethylpyrrolyl; R = 2,6diisopropylphenyl (dipp; 1), C 6 F 5 (2), t Bu (3); Ad = adamantyl; Scheme 1). [17,27,28] We show in particular that, while the compound with the most electron withdrawing imido substituent (Ar F5 ) is the only one active in molecular form, there is a great increase in activity upon grafting these compounds onto silica, leading to significant activity for all investigated compounds. Furthermore, we show that the activities of the different materials correlate with the reduction of the calculated energy difference between the tbp and sp metallacyclobutanes.…”