Sodium and Potassium Ion Directed Self-Assembled Multinuclear Assembly of Divalent Nickel or Copper and l-Leucine Derived Ligand

l-Leucine derived ligand (H2Ll-leu), KOH, and Ni(II) salt in 2:2:1 ratio self-assembled into a rather large (∼13 Å) supramolecular assembly with the formula [K{Ni(HLl-leu)2}3]+ (1). Structural characterization showed three [Ni(HLl-leu)2] units encapsulated K+ similar to organic crown ethers/cryptand. Substituting Ni(II) with Cu(II) and K+ with Na+ in the above reaction resulted in a set of structurally identical assemblies with the general formula [M′{M(HLl-leu)2}3]+, where M′ is either K+ or Na+ and M is either Cu(II) or Ni(II); [Na{Ni(HLl-leu)2}3]ClO4 (2), Na{Ni(HLl-leu)2}3]OTf (3), [K{Cu(HLl-leu)2}3]ClO4 (4), [Na{Cu(HLl-leu)2}3]ClO4 (5), [K{Cu(HLl-leu)2}3]NO3 (6). Electrospray Ionization (ESI)-mass spectra of the assemblies in MeOH showed the retention of assemblies in solution. Visible spectroscopic studies showed retention of assembly 1 in N,N-dimethylformamide (DMF) which is stable even after the addition of 5 equiv of [18]-crown-6. The assemblies in 2−6 show various degrees of dissociation to [M(HLl-leu)2] and M′, in stronger H-bonding methanol. The dissociation can be reversed upon addition of excess KNO3/NaNO3 salt. Structural characterization of [Cu(HLl-leu)2(MeCN)] (7) along with its transformation to [K{Cu(HLl-leu)2}3]+ in the presence of K+ salt demonstrated that the assembly formation proceeds through an alkali metal ion induced ligand reorientation within the [Cu(HLl-leu)2] units which is further stabilized by six strong H-bonds holding the assembly. Interestingly, visible spectra of 1 and 2 shows that minor structural changes caused by replacing K+ with Na+ is sufficient to shift the d-d transition of Ni(II) by ∼70 nm, thereby providing an indirect way of distinguishing K+ and Na+, none of which have spectroscopic signature in the visible range.