Abstract
The new linker molecule 4-phosphono-biphenyl-4′-carboxylic acid (H2O3P-(C6H4)2-COOH, H3BPPA) has been structurally elucidated in hydrogen-bonded networks with the ammonium cation NH4(H2BPPA)(H3BPPA) (1) and the hexaamminecobalt(III) cation [Co(NH3)6](BPPA)·4H2O (2). The protic O-H and N-H hydrogen atoms were found and refined in the low-temperature single-crystal X-ray structures. The hydrogen bonds in both structures are so-called charge-assisted; that is, the H-bond donor and/or acceptor carry positive and/or negative ionic charges, respectively. The H-bonded network in 1 consists of one formally mono-deprotonated 4-phosphonato-biphenyl-4′-carboxylic acid group; that is, a H2BPPA− anion and a neutral H3BPPA molecule, which together form a 3D hydrogen-bonded network. However, an almost symmetric resonance-assisted hydrogen bond (RAHB) bond [O···H = 1.17 (3) and 1.26 (3) Å, O···H···O = 180 (3)°] signals charge delocalization between the formal H2BPPA− anion and the formally neutral H3BPPA molecule. Hence, the anion in 1 is better formulated as [H2BPPA···H···H2BPPA]−. In the H-bonded network of 2 the 4-phosphonato-biphenyl-4′-carboxylic acid is triply deprotonated, BPPA3−. The [Co(NH3)6]3+ cation is embedded between H-bond acceptor groups, –COO− and –PO3− and H2O molecules. The incorporation of sixteen H2O molecules per unit cell makes 2 an analogue of the well-studied guanidinium sulfonate frameworks.
Highlights
IntroductionThe organophosphonic acid function, which has a pKa1 of 2.0 for the first and a pKa2 of
The organophosphonic acid function, which has a pKa1 of 2.0 for the first and a pKa2 of6.59 for the second proton, is capable of forming strong metal-to-ligand coordinative bonds in thermodynamically stable complexes with high stability constants [1]
The ammonium monohydrogenphosphonato-biphenyl-carboxylic acid crystallized with one molecule of the free
Summary
The organophosphonic acid function, which has a pKa1 of 2.0 for the first and a pKa2 of. 6.59 for the second proton, is capable of forming strong metal-to-ligand coordinative bonds in thermodynamically stable complexes with high stability constants [1]. Organophosphonates can contain additional functional groups such as carboxylate, hydroxyl or amino in the organo-moiety which presents a tunable functionality with a wide variety of structural motifs and properties [1,3,17]. Carboxy-phosphonates can form porous or 3D metal-ligand networks [18,19,20]. The extended biphenyl-based variant 4-phosphono-biphenyl-4’-carboxylic acid (H3 BPPA) was unknown so far (Scheme 1). We present the new linker 4-phosphono-biphenyl-4’-carboxylic acid, H3 BPPA, and its deprotonated structure in hydrogen-bonded networks with NH4 + and [Co(NH3 )6 ]3+ cations
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