No abstract

Reaction of the bis-tridentate ligand bis[1-ethyl-2-[6'-(N,N-diethylcarbamoyl)pyridin-2'-yl]benzimidazol-5-yl]methane (L2) with Ln(CF(3)SO(3))(3).xH(2)O in acetonitrile (Ln = La-Lu) demonstrates the successive formation of three stable complexes [Ln(L2)(3)](3+), [Ln(2)(L2)(3)](6+), and [Ln(2)(L2)(2)](6+). Crystal-field independent NMR methods establish that the crystal structure of [Tb(2)(L2)(3)](6+) is a satisfying model for the helical structure observed in solution. This allows the qualitative and quantitative beta23 (bi,Ln1,Ln2)characterization of the heterobimetallic helicates [(Ln(1))(Ln(2))(L2)(3)](6+). A simple free energy thermodynamic model based on (i) an absolute affinity for each nine-coordinate lanthanide occupying a terminal N(6)O(3) site and (ii) a single intermetallic interaction between two adjacent metal ions in the complexes (DeltaE) successfully models the experimental macroscopic constants and allows the rational molecular programming of the extended trimetallic homologues [Ln(3)(L5)(3)](9+).

The reactions of the tris(pyridyl)tin(II) derivative [Li(thf)Sn(2-py(R))(3)] (py(R) = C(5)H(3)N-5-Me) (1) with the heavier group 13 alkyl compounds MEt(3), M = Ga or In, have been carried out. These led to formation of [{Li(thf)Sn(2-py(R))(3)}MEt(3)] adducts, which exhibit long Sn-M bonds and can be used for further lanthanoid metal coordination via the salt metathesis reaction with [Eu(Cp*)(2)(OEt(2))] (Cp* = η(5)-C(5)Me(5)) to give complexes [Eu{Sn(2-py(R))(3)MEt(3)}(2)]. In contrast, addition of the lighter group 13 analogue, (AlMe(3))(2), to 1 resulted in a pyridyl transfer reaction, yielding dimeric [AlMe(2)(2-py(R))](2). For comparison, the reaction of (AlMe(3))(2) with [Yb{Sn(2-py(R))(3)}(2)] (py(R) = C(5)H(3)N-3-Me) was explored, affording complex [Yb{Sn(2-py(R))(3)AlMe(3)}(2)]. All complexes have been characterised by NMR spectroscopy and X-ray crystallographic studies.

Reactions of a lithium tris(2-pyridyl)stannate with lanthanoid metal organic compounds resulted in the formation of novel donor-acceptor complexes, containing a low valent tin-lanthanoid bond; DFT calculations reveal that this bond exhibits some covalent character.

Complex twists: Following theoretical thermodynamic predictions, it is seen that the tetrametallic triple‐stranded lanthanide helicate [Ln4L3]12+ (see structure) dominates the speciation in solution at millimolar concentrations, despite its high positive charge. Isolation of the europium complex in the solid state unambiguously establishes its nanometric triple‐helical structure.