Tetrahedral Tetraphosphonic Acids. New Building Blocks in Supramolecular Chemistry Alexandra Schü trumpf, † Erdoğ an Kirpi, ‡ Aysun Bulut, §,□ Flavien L. Morel, ∥ Marco Ranocchiari, ⊥ Enno Lork, † Yunus Zorlu, # Simon Grabowsky, † Gü ndoğ Yü cesan,* ,§ and Jens Beckmann* ,† † Institut fü r Anorganische Chemie und Kristallographie, Universitä t Bremen, Leobener Straße, 28359 Bremen, Germany ‡ Department of Chemistry, Yildiz Technical University, Davutpasa Campus, Esenler, 34010 Istanbul, Turkey § The Faculty of Chemical and Metallurgical Engineering, The Department of Bioengineering, Yildiz Technical University, Davutpas ̧ a Cad. No.127, 34210 Esenler, I ̇ stanbul, Turkey ∥ Department of Chemistry and Applied Biosciences, ETH Zü rich, Vladimir-Prelog-Weg 1, 8093 Zü rich, Switzerland ⊥ Laboratory for Catalysis and Sustainable Chemistry, Paul Scherrer Institut, 5232 Villigen, Switzerland # Gebze Technical University, Department of Chemistry, P.O. Box 141 Gebze, 41400 Kocaeli Turkey □ The Department of Chemistry, Boğaziç i University, Bebek, 34342 Istanbul, Turkey * S Supporting Information ABSTRACT: The structural characterization of tetraphenylmethane tetrakis- 4-phosphonic acid (4a) and its closely related Si-analogue tetraphenylsilane tetrakis-4-phosphonic acid (4b) is reported. Crystals of 4a, obtained by crystallization at 165 °C from water under hydrothermal conditions, comprise a dense threefold interpenetrated hydrogen-bonded network. Lower calculated density cocrystals 4b·4 EtOH, attained by recrystallization from ethanol, give rise to a porous honeycomb network that shows no interpenetration. Isolated from the mother liquor, the encapsulated ethanol is rapidly lost under ambient conditions leaving amorphous 4b. While the amorphous 4b is reasonably soluble in methanol, crystalline 4a is virtually insoluble in common organic solvents. ■ INTRODUCTION The construction of porous materials such as MOFs and COFs requires the judicious choice of rigid building blocks that contain functional groups suitable for metal coordination to form the desired organic−inorganic hybrid framework. 1−4 In particular, trigonal planar aromatic carboxylates provide structural rigidity and controllable design elements to produce isoreticular expansions with well-established secondary building units. 5−8 Most of the aromatic carboxylates with suitable sizes for MOF synthesis have been exploited to reach the optimum surface areas and gas adsorption properties. Although all these efforts to produce carboxylate-based MOFs have provided valuable insight into coordination chemistry, alternative bridging ligands with wider selection of adhesive functional groups such as phosphonates 9−12 and arsenates 13 have been vastly neglected. The expansion of MOF research into different functional groups is required not only to produce novel MOFs with improved properties, but also to address stability issues. Metal-organophosphonates form a well-known class of hybrid frameworks possessing wide range of applications including sorption, separation, optics, and catalysis; 14−16 but their synthesis has been limited mostly to aliphatic diphoshonic acids and a comparatively few aromatic phosphonic acids. 17−47 Prominent examples include the trigonal building blocks 1,3,5- benzene tri(phosphonic acid), 34−40 1,3,5-benzene tri(p-phenyl- phosphonic acid), 41−45 1,3,5-triazine tri(phosphonic acid), 46 and 1,3,5-triazine tri(p-phenylphosphonic acid). 47 Phospho- nates have tetrahedral RPO 3 2− structure with three oxygen atoms available for metal binding. The rich coordination modes of tetrahedral phosphonates on metal centers are expected to produce many new secondary building units with different transition elements. 48 On the other hand, tetraphosphonates have multiple modes of tunable negative charges between 0 and −8, which is dependent on the reaction pH conditions. Therefore, they could initiate the formation of interesting metal oxidation states and novel metal clusters to balance the negative charges especially with transition metals possessing multiple oxidation states such as vanadium, manganese, and molybde- num. All these charge balance efforts could result in pH controlled tunable coordination chemistry producing interest- ing magnetic and catalytic properties. 49−52 MOFs constructed using the aromatic tetrahedral ligands are among the ones Received: June 11, 2015 Revised: August 17, 2015 Published: August 21, 2015 Article pubs.acs.org/crystal © 2015 American Chemical Society 4925 DOI: 10.1021/acs.cgd.5b00811 Cryst. Growth Des. 2015, 15, 4925−4931