Ab Initio Chemical Synthesis of Designer Metal Phosphate Frameworks at Ambient Conditions Alok Ch. Kalita, † Nayanmoni Gogoi, ‡ Ritambhara Jangir, † Subramaniam Kuppuswamy, † Mrinalini G. Walawalkar, † and Ramaswamy Murugavel* ,† † Department of Chemistry, Indian Institute of Technology-Bombay, Powai, Mumbai 400076, India ‡ Department of Chemical Sciences, Tezpur University, Napaam 784028, India * S Supporting Information ABSTRACT: Stepwise hierarchical and rational synthesis of porous zinc phosphate frameworks by predictable and directed assembly of easily isolable tetrameric zinc phosphate [Zn(dipp)(solv)] 4 (dippH 2 = diisopropylphenyldihydrogen phosphate; solv = CH 3 OH or dimethyl sulfoxide) with D4R (double-4-ring) topology has been achieved. The preformed and highly robust D4R secondary building unit can be coordinatively interconnected through a varied choice of bipyridine-based ditopic spacers L1−L7 to isolate eight functional zinc phosphate frameworks, [Zn 4 (dipp) 4 (L1) 1.5 (DMSO)]· 4H 2 O (2), [Zn 4 (dipp) 4 (L2) 1.5 (CH 3 OH)] (3), [Zn 4 (dipp) 4 (L1) 2 ] ( 4), [Zn 4 (dipp) 4 (L3) 2 ] ( 5 ), [Zn 4 (dipp) 4 (L4) 2 ] ( 6 ), [Zn 4 (dipp) 4 (L5) 2 ](7), [Zn 4 (dipp) 4 (L6) 2 ](8), and [Zn 4 (dipp) 4 (L7) 2 ](9), in good yield. The preparative procedures are simple and do not require high pressure or temperature. Surface area measurements of these framework solids show that the guest accessibility of the frameworks can be tuned by suitable modification of bipyridine spacers. ■ INTRODUCTION The discovery of microporous aluminophosphates (AlPO) x by Flanigen and co-workers in 1982 resulted in an outburst of activity in the area of porous solids due to their potential utility in a wide range of applications. 1 Consequently, a host of open framework microporous metal phosphates have been reported, and many of these materials have found applications in sorption, catalysis, magnetism, etc. 2−6 In this context, a rational or retrosynthetic approach for assembling designer phosphate or silicate porous materials from preformed molecular building blocks is of paramount importance. Control over the volume and architecture of the pores or the ability to modify their chemical/physical environments has remained a major unsolved challenge for several decades. Several research groups have focused their investigations toward this objective, since such zeolite modifications will eventually allow the modulation of framework−guest interactions and thus lead to tailored properties and functions. However, the process of assembling designer silicate- or phosphate-based materials has turned out to be a formidable challenge, due to the nonavailability of soluble precursors bearing SiO 4 or PO 4 tetrahedra. It was believed in 1990s that the isolation of compounds such as the kinetically stabilized organosilanetriols, (R′RN)Si(OH) 3 (R = aryl; R′ = SiMe 3 ), would eventually afford an easy access to porous solids via Si 4 O 12 M 4 molecular cages that resemble the secondary building units (SBUs) found in zeolites. 7−9 Starting from organic-soluble phosphonic acids and phosphate esters, a similar but not identical approach was also envisaged to assemble porous solids. 10,11 No doubt both of these approaches, involving silanols and phosphonic acids, yielded a plethora of structurally diverse secondary building blocks, 7−9 however, it has not been possible to convert any of these M- siloxane or -phosphonate SBUs into porous solids by appropriate chemical modifications or reactions. A few years ago, we had shown that the reaction of Zn(OAc) 2 ·2H 2 O with diisopropylphenyl dihydrogen phosphate (dippH 2 ) in methanol produces a zinc phosphate, [Zn(dipp)- (CH 3 OH)] n . 11a−c The inability to obtain good-quality single crystals for this species, owing to the rapid loss of methanol, prevented the exact determination of the form of association (cluster versus layered solid). This led us to replace methanol by a stronger Lewis base; for example, substituted pyridines establish that the association number in these system in fact is four. 11 The core of these tetrameric clusters adopts a cubane- like shape that resembles the double-4-ring (D4R) SBUs of zeolitic materials (Figure 1). The fact that there are four zinc centers occupying the alternate corners of the cubane (as if at the four vertices of a tetrahedron) and that they are highly Lewis acidic led us to anticipate the possibility of interconnect- ing the cubanes by employing bridging ligands with stronger Lewis basic character. However, all attempts to link the D4R cages by bridging Zn centers through bidentate Lewis bases such as 4,4′-bipyridine or its extended analogues persistently yielded an insoluble noncrystalline precipitate. This precluded precise structural analysis of the resulting products by Received: April 6, 2014 Published: August 14, 2014 Article pubs.acs.org/IC © 2014 American Chemical Society 8959 dx.doi.org/10.1021/ic500810d | Inorg. Chem. 2014, 53, 8959−8969