Two Structures Toward Understanding Evolution from Surfactant-Polyoxometalate Lamellae to Surfactant-Encapsulated Polyoxometalates

Surfactant-POM (polyoxometalate) phases are fascinating in both their self-assembly behavior and their utility as catalysts, probes, and photochromic, electrochromic, and magnetic devices. Well-ordered lamellar phases are formed when the surfactant:POM ratio is 4:1 or 2:1, and these have been described in great detail from single-crystal X-ray diffraction studies. However, the surfactant-encapsulated clusters (SECs) with much larger surfactant:POM ratios do not form single-crystals readily. Thus less is known about their structural detail, and the evolution from the well-ordered lamellar phases to the SECs with increasing surfactant:POM ratio has not been detailed. We present here two structures that have resulted from an investigation of understanding the evolution of the surfactant-POM lamellar phase as the surfactant:POM ratio increases. [H 2 SiMo 12 O 40 ][CH 3 CN] 2 [C 16 H 33 N(CH 3 ) 3 ] 4 (monoclinic #4, P 2 1 a = 12.636(2) Å, b = 20.577(4) Å, c = 22.364(4) Å, β = 93.394(4)°) holds true to the preference of 4:1 surfactant:POM ratio in well-ordered crystalline phases, whereas [H x SiMo 12 O 40 ][C 16 H 33 N(CH 3 ) 3 ] 5 [CH 3 CN] 4 (triclinic No. 2, P 1̅, a = 12.513(7) Å, b = 23.37(1) Å, c = 24.44(1) Å, α = 93.418(8)°, β = 92.046(8)°, γ = 99.113(7)°) provides the first example of a surfactant-POM phase with a surfactant:POM ratio >4. This structure provides a glimpse of the structural evolution from ordered lamellar POM-surfactant phases to more disordered phases such as the SECs.