Alternative (κ1-N:η6-arene vs. κ2-N,N) coordination of a sterically demanding amidinate ligand: are size and electronic structure of the Ln ion decisive factors?

Abstract

The amine elimination reaction of equimolar amounts of ansa-bis(amidine) C₆H₄-1,2-{NC(tBu)NH(2,6-iPr₂C₆H₃)}₂ (L¹H) and [(Me₃Si)₂N]₂Yb(THF)₂ affords a bis(amidinate) Yb^II complex [C₆H₄-1,2-{NC(tBu)N(2,6-iPr₂C₆H₃)}₂]Yb(THF) (1) in 68% yield. Complex 1 features a rather rare η¹-amido:η⁶-arene coordination of both amidinate fragments to the Yb^II ion, resulting in the formation of a bent bis(arene) structure. Oxidation of 1 by I₂ regardless of the molar ratio of reagents (2 : 1 or 1 : 1) leads to the formation of the Yb^III species [{(2,6-iPr₂C₆H₃)NC(tBu)NH}-C₆H₄-1,2-{NC(tBu)N(2,6-iPr₂C₆H₃)}]YbI₂(THF)₂ (2) in which only one amidinate fragment is coordinated to the ytterbium ion in κ²-N,N′-chelating coordination mode, while the second NCN fragment is protonated in the course of the reaction and is not bound to the metal ion. The outcome of the salt metathesis reaction of LaCl₃ with lithium amidinates [C₆H₄-1,2-{NC(tBu)N(2,6-R₂C₆H₃)}₂Li₂] (R = Me, iPr) is proven to be strongly affected by the substituent 2,6-R₂C₆H₃ on the amidinate nitrogens. When R = iPr, the salt metathesis reaction occurs smoothly and results in the formation of an ate-chloro-amidinate complex [C₆H₄-1,2-{NC(tBu)N(2,6-iPr₂C₆H₃)}₂]La(μ²-Cl)Li(THF)(μ²-Cl)₂Li(THF)₂ (3) in which the La^III ion is coordinated by both amidinate fragments in a “classic” κ²-N,N′-chelating fashion. In the case of R = Me, the reaction requires prolonged heating for completion. Moreover, the salt metathesis reaction is accompanied by the fragmentation of the ligand and affords a trinuclear chloro-amidinate complex [C₆H₄-1,2-{NC(tBu)N(2,6-Me₂C₆H₃)}₂]La{[(tBu)C(N-2,6-Me₂C₆H₃)₂]La(THF)}₂(μ²-Cl)₄(μ³-Cl)₂ (4) containing one dianionic ansa-bis(amidinate) and two monoanionic [(tBu)C(N-2,6-Me₂C₆H₃)₂] amidinate fragments. DFT calculations are conducted to determine the factor that governs this change in coordination mode and, in particular, the effect of the metal oxidation state.