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

Abstract

The reaction between K 2[Biphen] ([Biphen] 2−=3,3′-Di- t-butyl-5,5′,6,6′-tetramethyl-1,1′-Biphenyl-2,2′-diolate) and Mo(NAr Cl)(CH- t-Bu)(OTf) 2(dme) (Ar Cl=2,6-Cl 2C 6H 3) in the presence of ten equivalents of triethylamine gave Mo(NHAr Cl)(C- t-Bu)[Biphen] ( 4a) in 40–50% yield. Addition of K 2[ S-Biphen] to Mo(NAr Cl)(CHCMe 2Ph)(OTf) 2(THF) in THF led to the isolation of Mo(NHAr Cl)(CCMe 2Ph)[ 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 H 2[Biphen] to Mo(CCH 2SiMe 3)[N( i-Pr)Ar″)] 3 (Ar″=3,5-dimethylphenyl) produced Mo(CCH 2SiMe 3)[Biphen][N( i-Pr)Ar″)] in situ, which when treated with one equivalent of 1-adamantanol gave a mixture of Mo(CCH 2SiMe 3)[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(C 3R 3)[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 C 3Me 3 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.