Synthesis reveals distinct binding differences and the ansa-effect in alkaline earth metallocenes, suggesting implications for complex stability.
The synthesis and structural characterisation of a series of alkaline earth ansa-octaphenylmetallocenes (Mg, Ca, Sr, Ba) bearing an ethylene bridge are described. The complexes [AE(C5Ph4CH2)2(thf)n] (AE = Mg (1), Ca (2) n = 1; AE = Sr (3), Ba (4), n = 2) were obtained through reductive dimerisation of 1,2,3,4-tetraphenylfulvene, facilitated by zero-valent metals and fully characterised by NMR spectroscopy. Single-crystal XRD studies reveal distinct binding differences of the Cp ligands to Mg in complex 1 compared to the heavier analogues (η3vs. η5). Complex 3 is the first structurally characterised ansa-metallocene complex of Sr. An ansa-effect was observed for the Ba complex 4 which showed good stability at room temperature in contrast to the previously described non-bridged analogue. Efficient transmetallation from the Ca ansa complex 2 to FeCl2 provided the new ansa-ferrocene complex [Fe(C5Ph4CH2)2] (5). Structural, spectroscopic and electrochemical properties of this bent ferrocenophane complex were compared to those of the known unbridged octaphenylferrocene.
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Shephard et al. (2025) studied this question.
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