Oxygenextrusion from amidate ligands to generate terminal TaO units under reducing conditions. How to successfully use amidate ligands in dinitrogen coordination chemistry

Abstract

A series of mixed Cp* amidate tantalum complexes Cp*Ta(RNC(O)R')X(3) (where R = Me(2)C(6)H(3), (i)Pr, R' = (t)Bu, Ph, X = Cl, Me) have been prepared via salt metathesis and their fundamental reactivities under reducing conditions have been explored. Reaction of the tantalum chloro precursors with potassium graphite under N(2) or Ar leads to the stereoselective formation of the terminal tantalum oxo species, Cp*Ta=O(η(2)-RN=CR')Cl. This represents the formal extrusion of oxygen from the amidate ligand to the reduced tantalum center and is accompanied by the formation of the iminoacyl fragment bound to Ta(v). Amidate dinitrogen complexes, [Cp*TaCl(RNC(O)(t)Bu)](2)(μ-N(2)) (where R = Me(2)C(6)H(3), (i)Pr) were synthesized via salt metathesis from the known [Cp*TaCl(2)](2)(μ-N(2)) precursor, establishing that amidate ligands can support dinitrogen complexes, but not the reduction process often necessary for their synthesis.

Four‐Membered Heterometallacyclic d0 and d1 Complexes of Group 4 Metallocenes with Amidato Ligands

“…However, CÀO bond cleavage is known for amidate complexes of other early transition metals and under reducing conditions, a terminal oxo complex was formed (Scheme 3, C). [14] Interestingly, it is possible to stabilize rare Zr III species with a highly strained [PNP] ligand, as has been shown recently (Scheme 3, A). [15] These results encouraged us to investigate the use of amides as reagents for the synthesis of various Group 4 metallocene complexes with the view of expanding the scope of d 1 complexes with amidate systems on these early transition metals.…”

“…Four-membered metallacycles stabilizing early-transition metals (A), [15] amidate complexes of Ti and Zr (B), [13] and cleavage product of an amidate complex under reducing conditions (C). [14] Scheme 4. Synthesis of the chlorido complexes 1 a, b, and 2 (Dipp=2,6-diisopropylphenyl).…”

“…Amidate complexes of Group 4 metals are stable for d 0 metals as Ti IV or Zr IV (Scheme , B ). However, CO bond cleavage is known for amidate complexes of other early transition metals and under reducing conditions, a terminal oxo complex was formed (Scheme , C ) 14…”

“… Four‐membered metallacycles stabilizing early‐transition metals ( A ),15 amidate complexes of Ti and Zr ( B ),13 and cleavage product of an amidate complex under reducing conditions ( C ) 14…”

Reactivity and Structure of Complexes of Small Molecules: Dinitrogen

“…When an excess of 130 The chloride ligands could be substituted to synthesize the amidate complexes, with elongated NN bond (1.292(4)Å). 131 Alternatively, reduction of the Cp*TaCl 4 with an excess of KC 8 (5 equiv.) under N 2 in the presence of ArS-SAr provided the Ta V ,Ta V complex [Cp*Ta(S-4-(MeC 6 H 4 ) 2 )] 2 (μ-η 1 :η 1 -N 2 ) (d(NN) 1.281(7)Å) (scheme 35, bottom).…”