Activation of C–X (X=Cl or Br) bonds in 2-halobenzaldehydes as their 2-pyridylhydrazone derivatives: oxidative addition to tungsten(0) to give aryl–tungsten(II) complexes

Condensation of 2-pyridylhydrazine, (NC5H4)NHNH2, with the 2-halobenzaldehydes 2,6-dichlorobenzaldehyde, 2-chlorobenzaldehyde or 2-bromobenzaldehyde gave the 2-pyridylhydrazones, (C6H3Cl2-2,6)CHNNH(C5H4N) (1a), (C6H4Cl-2)CHNNH(C5H4N) (1b) or (C6H4Br–2)CHNNH(C5H4N) (1c), respectively. Treatment of 1a with [W(CO)3(NCEt)3] in warm tetrahydrofuran caused oxidative addition of the CCl bond to give the aryl–tungsten(II) complex [W(CO)3Cl{(C6H3Cl-6)CHNNH(C5H4N)C,N,N}] (2a). Similarly, the hydrazones 1b and 1c, when treated with [W(CO)3(NCEt)3], gave the aryl–tungsten(II) complexes [W(CO)3Cl{(C6H4)CHNNH(C5H4N)C,N,N}] (2b) and [W(CO)3Br{(C6H4)CHNNH(C5H4N)C,N,N}] (2c). 1H and 13C–{1H} NMR, IR and mass spectral data are given.