Bis(tert-butylimido)bis(N,O-chelate)tungsten(VI) Complexes: Probing Amidate and Pyridonate Hemilability

Abstract

Four new bis(tert-butylimido)bis(N,O-chelate)tungsten(VI) complexes (3-6), in which the N,O-chelate is an amidate or pyridonate ligand, have been synthesized and characterized. Computational analysis has been used to calculate the relative energies of different stereoisomers and shown how the steric demand of each ligand influences coordination and bonding modes. The electronically saturated complexes have been employed to evaluate 1,3-N,O-chelated metal-ligand interactions. Complexes 3-6 were treated with electrophilic reagents, which resulted in strikingly different reactivity patterns between the amidate and the pyridonate ligated complexes. The observed reactivity differences are accompanied by direct observation of different trends in the hemilability of these two different classes of 1,3-N,O-chelates.

Ta-Catalyzed Hydroaminoalkylation of Alkenes: Insights into Ligand-Modified Reactivity Using DFT

“…41,42 Furthermore, these strained 4-membered metallacycles are also known to readily adopt κ 1 binding modes. 43,44 To begin, computational work focused on simplified 5coordinate tantalum complexes Cl 2 TaR(NPhMe) 2 , with R as either an electron-withdrawing (chloro) (II) or electrondonating (methyl) (III) ligand to explore σ-donor effects upon catalysis (Figure 1). For ease of computational resources, Nmethyl aniline and propene were selected as model substrates.…”

“…A key experimental trend in Ta-catalyzed HAA is the observation that a more electrophilic metal center offers enhanced reactivity. The unsymmetric N , O -chelating ligand of complex I presents computational challenges in that multiple coordination geometries can be readily accessed in the κ 2 binding mode. , Furthermore, these strained 4-membered metallacycles are also known to readily adopt κ 1 binding modes. , To begin, computational work focused on simplified 5-coordinate tantalum complexes Cl 2 TaR­(NPhMe) 2 , with R as either an electron-withdrawing (chloro) ( II ) or electron-donating (methyl) ( III ) ligand to explore σ-donor effects upon catalysis (Figure ). For ease of computational resources, N -methyl aniline and propene were selected as model substrates.…”

“…Thus, alternative mechanisms were explored for TS­(C) in I . Amidate ligands are known to be hemilabile, to access O-bond monodentate binding modes with early transition metals. , TS­(C) was sought in which the amidate was bound κ 1 through oxygen; however, repeated attempts at geometry optimization reliably resulted in a κ 2 chelate as shown in Figure . Thus, no alternative path of lower energy could be found despite extensive effort.…”

Effects of Coordinating a Hemilabile Ligand to 14e Cp*M(NO) Scaffolds (M = Mo, W)

“…Given the facts outlined in the preceding paragraph, we then decided to focus our attention on 18e Cp*W­(NO)­(κ 2 -P-N) complexes in which κ 2 -P-N is a bidentate hemilabile ligand because we expected that the labile portion of the ligand (i.e., the N-terminus) would allow a controlled opening of a site in the tungsten’s coordination sphere in the presence of a hydrocarbon substrate. Such hemilability has been invoked to explain the unique properties, such as stereochemical flexibility and nonrigidity, , and enhanced catalytic activity , of complexes possessing these types of ligands. Specifically, the κ 2 → κ 1 shifts exhibited by hemilabile phosphine–pyridine ligands have been well-documented for these complexes. − The first objective of this investigation was therefore the synthesis of a typical Cp*W­(NO)­(κ 2 -P-N) compound with the initial P-N ligand chosen for a variety of steric and electronic reasons, being 2-[(diisopropylphosphino)­methyl]-3-methylpyridine ( i Pr 2 PN).…”

Accessing Pentagonal Bipyramidal Geometry with Pentadentate Pincer Amido-bis(amidate) Ligands in Group IV and V Early Transition Metal Complexes

“…In the last two decades, interest in amidates and the closely related sulfonamidate and phosphoramidate ligands has grown extensively thanks in great part to the seminal efforts of the Schafer lab. , Amidates bearing relatively bulky substituents are excellent supporting ligands for a wide range of elements across the periodic table including rare-earths, − actinides, , early transition metals, late transition metals, , and the p-block. − Additionally, these compounds have been shown to be versatile catalysts for various reactions such as hydroamination and hydroaminoalkylation of unsaturated substrates, − ring opening polymerization of cyclic esters, , or olefin polymerization . Titanium and zirconium peptide­(amidate) conjugates have been reported by Erker, − and more recently amidate uranium complexes have been employed by Arnold and co-workers as CVD precursors …”