Molybdenum Dithiolene Complexes as Structural Models for the Active Sites of Molybdenum(IV) Sulfide Hydrodesulfurization Catalysts

Abstract

The removal of sulfur (as H2S) from organosulfur species in petroleum feedstocks (hydrodesulfurization, HDS) is carried out on an enormous scale by using heterogeneous catalysts based on MoS2 (usually doped with Co). Partially hydrogenated thiophenes are postulated intermediates in the MoS2‐catalyzed hydrodesulfurization of thiophene. The present contribution describes new molecular models for the proposed active sites in HDS catalysis. The models are derived from a mixed‐ligand (push–pull) molybdenum trisdithiolene {[Mo(tfd)2(bdt)]; tfd = S2C2(CF3)2, bdt = S2C6H4}: selective intraligand alkyne binding converts the bdt group to a labile Mo‐chelating benzodithiin, which can be substituted with a variety of weak donor ligands. The complexes [Mo(tfd)2(dht)2] and [Mo(tfd)2(tht)2] (dht = 2,5‐dihydrothiophene; tht = tetrahydrothiophene) were synthesized and crystallographically characterized. The Mo(tfd)2 substructures closely resemble the presumed active site in MoS2 HDS catalysts. The coordination geometries at molybdenum are approximately trigonal prismatic, and the metal bears two strongly bound dithiolene (tfd) groups and two comparatively weakly bound thioether ligands (dht or tht). Competitive binding experiments establish that tht binds more strongly to the Mo center than dht (Keq = 6.5 ± 0.5). Preliminary reactivity studies reveal that [Mo(tfd)2(dht)2] decomposes to [Mo(tfd)3], thiophene, and unidentified species upon heating. Further, [Mo(tfd)2(tht)2] induces the isomerization of 1,4‐cyclohexadiene to 1,3‐cyclohexadiene at elevated temperatures.

Coordination compounds containing bis-dithiolene-chelated molybdenum(IV) and oxalate: comparison of terminal with bridging oxalate

“…] are a clear indication of fully reduced (dianionic) ene-dithiolate (tfd 2À ), such that the oxidation state of the metal is +IV. The Mo-S bond lengths, ranging from 2.3265 (14) to 2.3390 (15) Å , are as expected for tfd complexes of Mo IV (Nguyen et al, 2010). Regarding the bonded oxalate, the average Mo-O bond length is 2.12 Å [Mo1-O1 = 2.104 (3) Å and Mo1-O2 = 2.135 (3) Å ].…”

“…Compounds where oxalate coordinates to molybdenum are rare, although some examples have been synthesized, mostly in the context of nitrogenase models, where oxalate was deemed a model for homocitrate (Demadis & Coucouvanis, 1995). Stimulated by our previous results on the molybdenum(IV) dithiolene fragment Mo(tfd) 2 [tfd 2À = S 2 C 2 (CF 3 ) 2 2À ] with a labile 'cap' (Harrison et al, 2007;Nguyen et al, 2010), we added oxalate to the Mo(tfd) 2 fragment, as described in the 'Synthesis and crystallization' section (x5). The [Mo(tfd) 2 (ox)] 2À (1 2À ) and [(tfd) 2 Mo(-ox)Mo-(tfd) 2 ] 2À (2 2À ) anions were indeed obtained, offering an opportunity for a structural comparison.…”

“…Also, both metal centres much more closely resemble a trigonal prismatic structure than an octahedral structure, as is expected for d 2 tris-chelates involving dithiolenes. Using the X-M-X trans criterion (Beswick et al, 2004;Nguyen et al, 2010), the geometry around molybdenum in 1 2À is 88% trigonal-prismatic. Using the same method, the geometry around molybdenum in 2 2À analyzes as 99% trigonal-prismatic.…”

“…Organic and inorganic starting materials were obtained from Sigma-Aldrich. Mo(tfd) 2 (tht) 2 (tht = tetrahydrothiophene) was synthesized from Mo(tfd) 2 (bdt) (bdt = S 2 C 6 H 4 ) as in Nguyen et al (2010). Mo(tfd) 2 (bdt) was synthesized as in Harrison et al (2007).…”

Diverse reactivities of aromaticity–unsaturation coexisted metalladithiolene rings

“…The structural data of 4 indicated some electron deficiency of the bdt ligand. The meta-coordinated 2,3-dihydro-1,4-dithiin moiety (bdte(CH 2 ) 2 moiety) in 5 is easy to replace by oxo ligand, mnt ligand (S 2 C 2 (CN) 2 ), and two sulfide ligands, giving 6 (Chart 7(b)) [37]. [Mo(bdt)(tfd) 2 ] catalizes the formations of 2,3-dihydro-1,4-dithiin derivatives (7) from alkene and bis-o-phenylene tetrasulfide (alkene ¼ ethylene, 1-hexene, cyclohexene, allyl alcohol, cis-2-pentene, trans-2-pentene), as shown in Chart 7(c) [38].…”