posted on 2016-01-11, 00:00authored byRoss J. Beattie, Peter
S. White, Joseph L. Templeton
The Tp′W(CO)(HCCH)(NCHMe)
(Tp′ = hydridotris(3,5-dimethylpyrazolyl)borate)
molecule contains an electron-rich 1-azavinylidene ligand where the
preferred site for electrophile addition is the α-nitrogen.
As is observed in other cases for electrophile addition to a 1-azavinylidene
with a cis-alkyne ligand, this fragment will add
small electrophiles such as H+, Me+, and Et+ to the α-nitrogen lone pair, maintaining a WII metal center and forming the coordinated imine products [Tp′W(CO)(HCCH)(NHCHMe)][BF4], [Tp′W(CO)(HCCH)(N(Me)CHMe)][OTf], and [Tp′W(CO)(HCCH)(N(Et)CHMe)][OTf].
In contrast, when a one-electron oxidant is employed, net electrophile
addition at the β-carbon can be achieved. Changing the nature
of the electrophile to [CPh3][BF4] leads to
a rapid one-electron oxidation and selective radical combination to
form the imido product [Tp′W(CO)(HCCH)(NCH(CH3)CPh3)][BF4]. This net addition of an electrophile at
the 1-azavinylidene β-carbon concomitantly oxidizes the metal
center to WIV and raises the CO stretching frequency to
2109 cm–1 versus 1946 cm–1 for
the WII imine complex. This reactivity is also seen with
[CPh2(C6H4OMe)][BF4].
This variable reactivity demonstrates the flexibility of the 1-azavinylidene
ligand when it is paired with an adjacent alkyne ligand.