Molybdenum alkylidyne complexes that contain a 3,3′-di-t-butyl-5,5′,6,6′-tetramethyl-1,1′-biphenyl-2,2′-diolate ([Biphen]2−) ligand

The reaction between K2[Biphen] ([Biphen]2−=3,3′-Di-t-butyl-5,5′,6,6′-tetramethyl-1,1′-Biphenyl-2,2′-diolate) and Mo(NArCl)(CH-t-Bu)(OTf)2(dme) (ArCl=2,6-Cl2C6H3) in the presence of ten equivalents of triethylamine gave Mo(NHArCl)(C-t-Bu)[Biphen] (4a) in 40–50% yield. Addition of K2[S-Biphen] to Mo(NArCl)(CHCMe2Ph)(OTf)2(THF) in THF led to the isolation of Mo(NHArCl)(CCMe2Ph)[S-Biphen] (4b) in ∼40% yield. An X-ray crystal study of 4b confirmed the proposed structure and also revealed that one ortho chloride approaches within 2.93 Å of the metal approximately trans to the alkylidyne ligand. Addition of one equivalent of H2[Biphen] to Mo(CCH2SiMe3)[N(i-Pr)Ar″)]3 (Ar″=3,5-dimethylphenyl) produced Mo(CCH2SiMe3)[Biphen][N(i-Pr)Ar″)] in situ, which when treated with one equivalent of 1-adamantanol gave a mixture of Mo(CCH2SiMe3)[Biphen](OAd) (9) and three equivalents of HN(i-Pr)Ar″, from which 9 could be isolated as a beige powder in 46% yield. An X-ray study of 9 confirmed that it is a pseudotetrahedral species in which the MoC bond length is 1.707(15) Å and the MoCC angle is 168.3(11)°. Addition of ten equivalents of 2-butyne or 3-hexyne to a pale yellow solution of 9 produced the molybdacyclobutadiene complexes Mo(C3R3)[Biphen](OAd) (R=Me or Et; 10a and 10b, respectively) in high yield. Both 10a and 10b decompose slowly in solution, even in the presence of added alkyne. An X-ray structure of the decomposition product of 10a revealed it to have the stoichiometry of 10a plus one additional equivalent of 2-butyne. The most unusual feature of the structure of this alkyne complex is a fusion of the C3Me3 portion of the metallacyclobutadiene ring to carbons in position 5 and 6 in the [Biphen]2− backbone to create a σ allyl linkage. These results suggest that Mo biphenolate alkylidyne complexes are not likely to be stable under conditions where alkynes are metathesized.