A concerted evolution of supramolecular interactions in a {cation; metal complex; π-acid; solvent} anion-π system

Abstract

Combined Single-Crystal XRD, solution ¹³C and ¹⁹⁵Pt NMR, solid-state ¹⁹⁵Pt NMR and UV-vis measurements, together with comprehensive (TD)DFT calculations, are used to describe the supramolecular interactions and sequential concerted structural transformation occurring in a {PPh₄⁺;[Pt(CN)₄]²⁻;TCP;CH₂Cl₂} supramolecular system (TCP – tetracyanopyrazine). In the solid-state, a mixture of needle-like co-crystals first appears, (PPh₄)₂{[TCP][Pt(CN)₄]} (1) and (PPh₄)₂{[TCP][Pt(CN)₄]}·2CH₂Cl₂ (2), and subsequent recrystallization leads to block-shaped co-crystals of (PPh₄)₂{[TCP]₂[Pt(CN)₄]}·2CH₂Cl₂ (3), wherein double {[Pt(CN)₄]²⁻;TCP} and triple {[Pt(CN)₄]²⁻;TCP;CH₂Cl₂} and {PPh₄⁺;[Pt(CN)₄]²⁻;TCP} synthons present either in 1 or 2 are reproduced in 3. The structural pathway (1,2→3) is accompanied by a 1D-to-2D modular evolution of anion-π interactions between [Pt(CN)₄]²⁻ and π-acidic TCP that seems to be driven, as indicated by calculations, not by maximizing their strength but rather by optimizing other crystal interactions, e.g. between PPh₄⁺ cations. Formation of the corresponding {[Pt(CN)₄]²⁻;π-acid} anion-π contacts in solution is evidenced by ¹³C NMR shifts and by new low-energy electronic absorption in the visible region (UV-vis). Finally, a combined solution/solid-state ¹⁹⁵Pt NMR approach is used for the first time to shed light on the geometry and interactions involving such aggregates; it shows that the degree of trapping of [Pt(CN)₄]²⁻ near a π-acidic surface can be monitored by ¹⁹⁵Pt NMR chemical shifts. The results give fresh insight into block arrangement tactics and characterization of hybrid organic-inorganic co-crystal salts, and into aggregation controlled properties.