Structure and Reactivity of η²(3e)-Nitrile and η²(3e)-Alkyne Complexes of Niobium

Abstract

Nitrile adducts of the type Tp*Nb(CO)(PhC⋮CMe)(RC⋮Ν) (Tp* = hydridotris(3,5-dimethylpyrazolyl)borate; R = Me (2a), Et (2b), PhCH2 (2c), 4-MeOC6H5 (2d), Ph (2e), 4-CF3C6H5 (2f)) have been prepared in fair yields by CO displacement from Tp*Nb(CO)2(PhC⋮CMe) (1) and fully characterized. 13C NMR spectroscopy (δ 180−190 for the nitrile and alkyne contact carbons) and X-ray crystallography (for 2b) indicate an η2-coordination and an unprecedented formally three-electron-donor (3e) behavior for both the alkyne and the nitrile. Electrochemical studies suggest that an equilibrium between this η2(3e)-nitrile/η2(3e)-alkyne form and an otherwise undetected η1(2e)-nitrile/η2(4e)-alkyne form exists in solution. PhC⋮N is thermally displaced from 2e by PMe2Ph or PhC⋮CMe to give Tp*Nb(CO)(PhC⋮CMe)(PMe2Ph) or Tp*Nb(CO)(PhC⋮CMe)2 (3a), respectively. 3a does not react with PMe2Ph. Protonation of 2b with HBF4 induces nitrile/alkyne coupling to give the niobacycle Tp*NbF(CPhCMeCPhNH), via the isolated η2-iminoacyl alkyne cation [Tp*Nb(CO)(η2-CPhNH)(PhC⋮CMe)][BF4], characterized by NMR spectroscopy at 243 K. Protonation of 3a with HBF4 leads to the niobacycle Tp*NbF[C(Ph)C(Me)C(CMeCPhH)O] (X-ray structure), in which CO is included in the alkyne coupling reaction. The intermediate [Tp*Nb(CO)(CMeCPhH)(PhC⋮CMe)][BF4] has been characterized by NMR at 193 K. A deshielded Cα (δ 205), a shielded Cβ (δ 97), and a reduced 1 J CH (114 Hz) suggest that a β-CH agostic interaction stabilizes this vinyl alkyne cation. Protonation of the new phosphinoalkyne derivative Tp*Nb(CO)(PhC⋮CMe)(PPh2C⋮CPh) affords the stable phosphonium complex {Tp*Nb(CO)[Ph2P(H)C⋮CPh](PhC⋮CMe)}[BF4], in which the two alkyne ligands act as 3e donors.

CH Bond Activation of Unsaturated Hydrocarbons by a Niobium Methyl Cyclopropyl Precursor. Cyclopropyl Ring Opening and Alkyne Coupling Reaction

“…This bonding motif, initially highlighted by Curtis, has been observed in several group 5 complexes, , including in the Tp Me2 Nb family. We have observed these structures in reactions coupling a coordinated alkyne with acyl, iminoacyl, and rearranged ethyl or allyl groups. The case of the allyl group is particularly noteworthy, since the allyl and cyclopropyl groups are isomers.…”

Actinide‐Mediated Cyclization of 1,2,4,5‐Tetracyanobenzene: Synthesis and Characterization of Self‐Assembled Trinuclear Thorium and Uranium Macrocycles

“…12 The formation of an actinide ketimide complex ( 5 ) seems to be the crucial step in the synthesis of the unique 5,6‐dicyano‐1‐methyl‐3‐( N ‐methylamino)isoindolyl ligand. Numerous examples of metal‐mediated nitrile coupling reactions with compounds such as amines,13a imines,13b ketones,13c alkynes,13d amidines,13e oximes,13f and other nitriles13g are known. However, to the best of our knowledge, the coupling of a nitrile with a ketimide functional group has not been observed with transition metals, once again highlighting the differences between transition metals and the actinides.…”

A comparative study of the topology of the experimental electron density within 2 and 4e− donor alkyne complexes

“…Interestingly, these bonding descriptions can be related to chemical reactivity as the alkyne ligand in half-sandwich Cp(CO) 2 Mn-(η 2 -2e-alkyne) complexes is easily displaced, by acetonitrile for instance, 24 while L n Nb(η 2 -4-e-alkyne) complexes are inert to substitution. 6,25 This is correlated with lower bond ellipticity at the Nb⋯C bcp's which should indicate greater contribution to This journal is © The Royal Society of Chemistry 2012 the bonding perpendicularly to the NbCC plane. As a last element to estimate the covalency in the MCC region, it appears that the bond order 26 for the CuC bond is, as expected, significantly lower for the Nb complex (Table 1).…”