The synthesis, reactivity, and structural study of multiple imido complexes of group 6.

Abstract

The first reported tris(imido) complexes of group 6, (Li(THF)₄) (M(NAr)₃Cl) (M = W (4) or Mo (9)), are formed upon treatment of M(NAr)₂Cl₂(THF)₂ complexes with two equiv of LiNHAr. Tris(imido) complexes constitute the kinetic products of reaction between M(NAr)₂Cl₂(THF)₂ and 2 equiv of LiNHAr. [(Li(THF)₄) (M(NAr)₃Cl)] can back react with the H₂NAr formed in the reaction to give the thermodynamic products M(NAr)₂(NHAr)₂ (M = W (29), Mo (11)) irreversibly. Mechanistic evidence for the formation of the bis(imide) complex W(NAr)₂Cl₂(THF)₂ (2) from W(NAr)Cl₄(THF) (1) show the reaction proceeds by an α-H abstraction of nascent W(NAr)(NHAr)₂Cl₂(THF)ₓ. Thus treatment of W(NAr)(NEt₂)Cl₂ (8) with LiNHAr yields 2 and Et₂NH. Mechanistic evidence shows that the formation of tris(imide) complexes 4 and 9 proceeds through an intermolecular α-H deprotonation. In support of this, no 4 is formed upon treatment of W(NAr)₂(NEt₂)Cl (3) with LiNHAr. It is possible to displace the chloride with a variety of nucleophiles to form analogs to complexes 4 and 9. For example, treatment of complex 4 with PMe₃ yields W(NAr)₃(PMe₃) (5), whereas reaction of complex 9 with pyridine yields Mo(NAr)₃(pyr) (16). Alkyl analogs [Li(THF)₄][W(NAr)₃Me] (19) and [Li(THF)₄][(Mo(NAr)₃Me) (17) are formed upon reaction of 4 and 9 with MeLi, respectively. Construction of an orbital interaction diagram for tris(imido) complexes (M(NAr)₃L), as well as related CpM(NAr)₂Cl complexes, reveals that the HOMO is a three-fold symmetric ligand-based non-bonding orbital. This makes these complexes highly reactive with a variety of electrophiles. Thus protonation of 17 with [HNMe₃][BPh₄] yields Mo(NAr)₂(NHAr)Me (23), methylation of 4 with methyl iodide yields W(NAr)₂[N(Me)Ar]I (33), and reaction of CO₂ with 5 yields W(NAr)₂ [N(C(O)O)Ar](PMe₃) (38). M(NAr)₃L and CpM(NAr)₂Cl complexes catalyze the metathesis of carbodiimides and olefins. The rate of ring opening metathesis polymerization (ROMP) of cyclic olefins is greatly enhanced in the presence of Lewis acid activators.