Planar-Chiral 1,1′-Diiminoferrocenes [Fe{η5-C5H3-1-(N=CHPh)-2-(C≡CAr)}{η5-C5H4(N=CHPh)}] Containing a Bulky Ethynyl Substituent Adjacent to One of the Imino Groups

The planar-chiral stannylated diaminoferrocene [Fe{η⁵-C₅H₃-1-(SnnBu₃)-2-(NH₂)}{η⁵-C₅H₄(NH₂)}] (A) and its diimine derivative [Fe{η⁵-C₅H₃-1-(SnnBu₃)-2-(N=CHPh)}{η⁵-C₅H₄(N=CHPh)}] (B) gave unsatisfactory results in attempts to achieve Stille cross-coupling reactions with PhBr in the presence of [PdCl₂(PPh₃)₂] or [Pd(PPh₃)₄]/CuO as catalysts. Analogous attempts aiming at the Stille cross-coupling of the iodo derivative [Fe{η⁵-C₅H₃-1-I-2-(N=CHPh)}{η⁵-C₅H₄(N=CHPh)}] (C) with PhSnnBu₃ are equally fruitless, whereas Sonogashira cross-coupling reactions of C are successfully performed with ethynylbenzene, ethynyl-2,4,6-trimethylbenzene, 4-ethynyl-1,1′-biphenyl, 9-ethynylphenanthrene, and 1-ethynylpyrene, affording [Fe{η⁵-C₅H₃-1-(N=CHPh)-2-(C≡CAr)}{η⁵-C₅H₄(N=CHPh)}] (Ar = phenyl, mesityl, 1,1′-biphenyl-4-yl, 9-phenanthryl, and 1-pyrenyl for 1, 2, 3, 4, and 5, respectively) in ≈60% yield under optimized conditions (C/ArC≡CH/[Pd(PtBu₃)₂]/CuI/NiPr₂Et = 1/4/0.5/3/40, toluene, 70 °C, 16 h). 4 (Ar = 9-phenanthryl) is structurally characterized by single-crystal X-ray diffraction (XRD). The suitability of the diimines of this study for metal chelation is demonstrated by reactions with [PdCl₂(MeCN)₂]. [PdCl₂(C)] and [PdCl₂(5)] are structurally characterized by XRD. The 1-pyrenyl unit present in [PdCl₂(5)] effectively blocks one side of the N,N-chelated metal center by wall-like steric shielding.