Synthesis of (Arylmido)niobium(V) Complexes Containing Ketimide, Phenoxide Ligands, and Some Reactions with Phenols and Alcohols

Abstract

Reactions of Nb(NAr)(N=C t Bu 2 ) 3 ( 3a , Ar = 2,6-Me 2 C 6 H 3 ) with 1.0, 2.0, or 3.0 equiv of Ar′OH (Ar′ = 2,6- i Pr 2 C 6 H 3 ) afforded Nb(NAr)(N=C t Bu 2 ) 2 (OAr′), Nb(NAr)(N=C t Bu 2 )(OAr′) 2 , or Nb(NAr)(OAr′) 3 , respectively (at 25 °C), whereas the reaction with 2.0 equiv of 2,6- t Bu 2 C 6 H 3 OH afforded Nb(NAr)(N=C t Bu 2 ) 2 ( O -2,6- t Bu 2 C 6 H 3 ) upon heating (70 °C) without the formation of bis(phenoxide) and the reaction of 3a with 2.0 equiv of 2,4,6-Me 3 C 6 H 2 OH afforded Nb(NAr)(N=C t Bu 2 )( O -2,4,6-Me 3 C 6 H 2 ) 2 (HN=C t Bu 2 ). Similar reactions of 3a with 1.0 equiv of (CF 3 ) 3 COH or 2.0 equiv of (CF 3 ) 2 CHOH afforded Nb(NAr)(N=C t Bu 2 ) 2 [OC(CF 3 ) 3 ](HN=C t Bu 2 ) or Nb(NAr)(N=C t Bu 2 )[OCH(CF 3 ) 2 ] 2 (HN=C t Bu 2 ), respectively. On the basis of their structural analyses and the reaction chemistry, it was suggested that these reactions proceeded via coordination of phenol (alcohol) to Nb and the subsequent proton (hydrogen) transfer to the ketimide (N=C t Bu 2 ) ligand. The reaction of Nb(NAr)(N=C t Bu 2 ) 2 (OAr′) with 1.0 equiv of 2,4,6-Me 3 C 6 H 2 OH gave the disproportionation products Nb(NAr)(N=C t Bu 2 )(OAr′) 2 and Nb(NAr)(N=C t Bu 2 )( O -2,4,6-Me 3 ...

Synthesis and Structural Analysis of Four Coordinate (Arylimido)niobium(V) Dimethyl Complexes Containing Phenoxide Ligand: MAO-Free Ethylene Polymerization by the Cationic Nb(V)–Methyl Complex

“…These complexes were prepared from Nb­(NAr)­(NMe 2 ) 3 , not from Nb­(NAr)­Cl 3 (dme) because of difficulty for removal of coordinated solvent (dme). In fact, as reported previously, treatment of Nb­(NAr)­Cl 3 (dme) (Ar = 2,6-Me 2 C 6 H 3 , 2,6- i Pr 2 C 6 H 3 ), with Li­(O-2,6- i Pr 2 C 6 H 3 ) in Et 2 O gave the dichloride, Nb­(NAr)­Cl 2 (O-2,6- i Pr 2 C 6 H 3 )­(dme), and the coordinated dme could not be removed even upon heating (100 °C) or by treating with 3 equiv of NiBr 2 or ZnCl 2 in toluene at 50 °C . In contrast, as described in this paper (Scheme ), reactions of Nb­(NAr)­(NMe 2 ) 3 with 2,6- t Bu 2 C 6 H 3 OH (1.0 equiv) in n -hexane gave Nb­(NAr)­(NMe 2 ) 2 (O-2,6- t Bu 2 C 6 H 3 ).…”

“…The complexes 2a – c and 3a were identified by NMR spectra and elemental analysis, and their structures were determined by X-ray crystallographic analysis (shown below, Figure ). Attempted treatment of 3a with 3 equiv of NiBr 2 or ZnCl 2 in toluene, to remove the coordinated HNMe 2 , recovered 3a , confirmed by X-ray crystallography (and elemental analysis), , although certain changes in resonances were observed in the 1 H NMR spectrum . Attempted reaction of 3a with 2.5 equiv of MeMgBr lead to failed isolation of the dimethyl complex .…”

“…(Arylimido)­niobium­(V) complexes that contain three dimethylamide ligands, Nb­(NAr)­(NMe 2 ) 3 [Ar = 2,6- i Pr 2 C 6 H 3 ( 1a ), 2,6-Me 2 C 6 H 3 ( 1b ), 2-MeC 6 H 4 ( 1c )], have been chosen, because, as described in the Introduction (Scheme ), the complexes were used for synthesis of Nb­(NAr)­Me 2 [2-(2,6-Me 2 C 6 H 3 )­NCH 2 (C 5 H 4 N)] through the bis­(alkoxide) analogues, Nb­(NAr)­[OCH­(CF 3 ) 2 ] 2 [2-(2,6-Me 2 C 6 H 3 )­NCH 2 (C 5 H 4 N)], by treatment with MeMgBr and attempted removal of coordinated solvent (dme) from the dichloride complex, Nb­(N-2,6- i Pr 2 C 6 H 3 )­Cl 2 (O-2,6- i Pr 2 C 6 H 3 )­(dme), was unsuccessful …”

“…Imido (M = NR) ligand, employed in this study, has been known as an effective ligand for stabilization of the high oxidation state in early transition metals even in their reactions with Al alkyls, recently exemplified by (arylimido)­vanadium complexes by solution V K-Edge EXAFS (extended X-ray absorption fine structure) analysis . As summarized in Chart , there are thus many reports for synthesis and reaction chemistry of the niobium complexes containing both tert -butyl-imido and β-diketiminate ligands (also shown as D in Chart ), , and some reports for the (arylimido)­niobium­(V) complexes ,,,− especially containing monoanionic chelate guanidinato ligands were known. However, synthesis of the low coordinative (4 coordinate) unsaturated organo-niobium­(V) complexes containing an anionic donor (monodentate) ligands still have been limited, , although the complexes should exhibit promising catalysis capabilities on the basis of results in the phenoxide-, imidazolin-2-iminato-, or imidazolidin-2-iminato-modified (arylimido)­vanadium­(V) dichloride complexes for ethylene (co)­polymerization and the dialkyl complexes as precursors for the alkylidene complexes which act as catalytically active for olefin metathesis polymerization. , …”

“…In this paper, a series of 4 coordinate (arylimido)­niobium­(V) complexes have been prepared and we previously reported synthesis of the complexes with the other geometries. , Although it has been generally postulated that the pre-edge position, the intensities, and the edge absorption in the K edge XANES (X-ray absorption near edge structure) spectra are dependent upon the coordination geometry as well as the oxidation states, ,− however, almost no reports for the detailed spectral analysis for a series of four (and six) coordinate niobium­(V) complexes have been known. We thus explored the effect of ligands and basic geometry including assignment of peak(s) with major d-p hybrid orbitals on the XANES spectra through time-dependent density functional theory (TD-DFT) , analysis in addition to the relationship with the crystallographic analysis data and chemical shifts in the 93 Nb NMR spectra.…”