Transition-metal-containing thermotropic liquid crystals (LCs) have been a topic of great interest in both the LC and materials chemistry communities. [1,2] The d-metal centers in these mesogens can impart the resulting LC mesophases with unique structural, electronic, magnetic, and photonic properties, [2] and even catalytic reactivity. [3] One recent avenue of research within the field of metallomesogens has been the design of polymerizable derivatives. The ability to covalently link these metal-containing units together via chain-addition [4± 8] or step-growth [4,9] polymerization methods affords anisotropic linear polymers or crosslinked networks with superior chemical, mechanical, and thermal stability, characteristics which are important for potential applications.[4] Radical polymerization of acrylate-containing metallomesogens is one of the most effective methods of making anisotropic metallopolymers and networks.[4±8] However, this polymerization technique suffers from two inherent problems, especially when transition-metal LC monomers are employed. First, there is usually some LC phase destabilization when acrylate tails are used in place of normal n-alkyl or n-alkoxy tails. This effect has been attributed to the greater steric bulk and polarity of the terminal acrylate moiety perturbing the packing of the LC tails.[6] Second, metallomesogens containing certain transitionmetal ions with unpaired electrons or high redox activities have been found to quench radical polymerization processes, with accompanying degradation of the metal center.[6] Radical polymerization of metallomesogens have been most successful with monomers containing stable, closed-shell metal centers. [6] Recently, 1,3-dienoxy tails have been employed as an alternative reactive-tail system in the design of polymerizable thermotropic and lyotropic LCs.[10±12] This tail system was originally developed as a means of making polymerizable analogs COMMUNICATIONS 602
Colorless single crystals of Cd2[μ8‐MTB]·3H2O·DMF (1) were prepared in DMF/H2O solution [1: space group C2/c (no. 15) with a = 1821.30(6), b = 2175.08(6), c = 1269.87(4) pm, β = 129.684(1)°]. The connection between the methane‐p‐benzoate tetraanions (MTB4–) and the Cd2+ cations leads to a three‐dimensional framework with channels extending along [110] and [110] with openings of 670 pm × 360 pm. The channel‐like voids accommodate water molecules and N,N‐dimethylformamide (DMF) molecules not bound to Cd2+. Colorless single crystals of [Cd4(2,2′‐bipy)4(μ7‐MTB)2]·7DMF (2) were prepared in DMF in the presence of 2,2′‐bipyridine [2: space group P1 (no. 2) with a = 1224.84(4), b = 1418.85(5), c = 2033.49(4) pm, α = 85.831(2)°, β = 88.351(2)°, γ = 68.261(1)°]. The coordination of MTB4– to Cd2+ results in infinite layers parallel to (001). The layers, not connected by any hydrogen bonds, contain small openings of about 320 pm × 340 pm.
The cover picture shows coordination polymers with the anion of methanetetrabenzoic acid (MTB) featuring tetrahedrally directed functional groups, which will favour frameworks related to the structure of diamond often being called diamondoid. If interpenetrating structures are avoided, these frameworks will provide voids able to accomodate guest molecules. A natural analogue is known with the mineral melanophlogite. Cd2[µ8‐MTB]·3H2O·DMF (1) has channel‐like voids of 670 pm x 360 pm diameter, which accomodate water molecules and DMF. In [Cd4(2,2'‐bipy)4(µ7‐MTB)2]·7DMF (2) a coligand is used, preventing formation of a three‐dimensionally polymeric framework. However, the layers of 2 stacked in …AA… sequence have small openings of 340 pm x 320 pm width. More details are discussed in the article by S. Harms, R. Köferstein, H. Görls, and C. Robl on page 912.
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