Ring-Opening of σ-Thienyl and σ-Furyl Ligands at Ditungsten (M⋮M) Centers

A series of compounds of formula 1,2-M 2 (σ-Th) 2 (NMe 2 ) 4, 1, has been prepared, where M = Mo and/or W and Th = 2-thienyl[2-Th], 3-thienyl[3-Th], 5-methyl-2-thienyl[2,5-MeTh], and 2-benzothienyl[2-BTh]. In hydrocarbon solvents these compounds exist as a mixture of anti and gauche rotamers having a central CN 2 M⋮MN 2 C core. Addition of t BuOH or CF 3 Me 2 COH to hydrocarbon solutions of 1, where M = W, lead to ring-opened products, 2, when the thienyl ligand is attached via the 2-carbon position. No ring-opening occurs for 3-thienyl derivatives. W 2 (OR) 5 (μ-CCH 2 CHCHS)(σ-2-Th), 2, where one of the 2-thienyl rings has been opened, has been fully characterized and shown to be derived from a ring-opened μ-vinylidene intermediate W 2 (O t Bu) 4 (μ-CCHCHCHS)(σ-2-Th). Rather interestingly, the compound W 2 (σ-2-Th)(NMe 2 ) 5 reacts with t BuOH to give only W 2 (2-Th)(O t Bu) 5 and HNMe 2 . Reactions between 1 and the less sterically demanding alcohols i PrOH and t BuCH 2 OH lead to W 2 (OR) 6 compounds with liberation of HNMe 2 and the sulfur containing hydrocarbon. Compound 1 (M = W, 2-Th) reacts with CO 2 to give W 2 (NMe 2 ) 2 (O 2 CNMe 2 ) 2 (σ-2-Th) 2, 3, while with pivalic anhydride W 2 (O 2 C t Bu) 4 is formed via reductive elimination of the C−C coupled product 2,2‘-bithiophene and t BuCONMe 2 (4 equiv). 1 (M = W, 2-Th) and CO yield a product formulated as W 2 (NMe 2 ) 4 (μ-CCHCHCHS)(σ-2-Th)(CO) 3 wherein one ring has opened to form a μ-vinylidene thiolate ligand. Attempts to prepare 1,2-W 2 L 2 (NMe 2 ) 4 compounds are described where L = an η 1 -nitrogen containing heterocycle but only for L = 2-methylindolyl was a compound successfully characterized. Reactions of these compounds with t BuOH gave W 2 (O t Bu) 6 and LH (2 equiv). The compound W 2 (σ-2-Fu) 2 (NMe 2 ) 4 was prepared and characterized (2-Fu = 2-furyl) and shown to undergo ring-opening in its reaction with t BuOH to give W 2 (O t Bu) 5 (μ-CCH 2 CHCHO)(σ-2-Fu), 4, in an analogous manner to the 2-Th complex. The complexes 1 (M = W, 2-Th), 2, 3, and 4 have been characterized by single crystal X-ray studies. The results described herein are compared to other ring-opening reactions of S, N, and O organic heterocyclic compounds as models for the activation of S, N, and O containing fossil fuels in hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and hydrodeoxygenation (HDO) processes. The cleavage of the C−X bonds at the (M⋮M) 6+ centers (M = Mo, W and X = S, N and O) is also compared with earlier cleavage reactions involving C⋮O, RC⋮N, R 2 C O, and Ar 2 C S compounds.