Synthesis, Crystal Structure, and Adsorption Properties of Werner-type Cu(II) Complex [Cu(CF3SO3)2(4-methylpyridine)4]

Abstract

A Werner-type Cu(II) complex, ¡-[Cu(CF 3 SO 3 ) 2 (4-mepy) 4 ] (¡-PAC-1-CF 3 SO 3 , PAC: porous assembly of coordination complex and 4-mepy: 4-methylpyridine), was synthesized, crystallographically characterized, and its adsorption properties were compared with those of the derivative ¡-[Cu(PF 6 ) 2 (4-mepy) 4 ] (¡-PAC-1-PF 6 ).Porous assemblies of coordination complexes (PACs), which are formed by the assembly of discrete coordination complexes without internal spaces via intermolecular interactions (van der Waals, electrostatic, and hydrogen-bonding interactions) that are weaker than coordination bonds, 1 3 have the potential to provide the following unique host guest features: (i) diversity in assembled structures with guest molecules, resulting from weak and flexible intermolecular interactions; 1h,1i (ii) formation of host frameworks consisting of two or more different discrete coordination complexes; 1j (iii) facile fabrication of films because of high solubility; 1k,1l and (iv) polymorphism without guest molecules, which is controlled by a kind of preadsorbed guests. 1c,1i These characteristics of PACs could supply not only fundamental information about their pores but also unprecedented, useful porous functions.The Werner-type complex [Ni(NCS) 2 (4-mepy) 4 ] (4-mepy: 4-methylpyridine) and its derivatives are well known as examples of PACs; their preparation and guest-inclusion abilities were first reported in 1957. 1a Among them, [Ni(NCS) 2 (4-mepy) 4 ] has been thoroughly studied because of its interesting guest-inclusion properties derived from pseudopolymorphism, such as a dense ¡-form, and guest-including ¢-and £-forms. The ¡-form has no space for including guest molecules because of its dense packing structure, whereas the ¢-and £-forms are channel-type and layer-type clathrates, respectively. 1i In particular, this complex easily affords an empty ¢-form that adsorbs several gases with type I isotherms.We have recently found that a Cu(II)-based PAC, [Cu(PF 6 ) 2 (4-mepy) 4 ] (PAC-1-PF 6 ), which is a quasi-Wernertype metal complex, has interesting guest-recognition abilities. 4 The dense ¡-form, ¡-PAC-1-PF 6 , has inorganic PF 6 ¹ anions covered with only fluorine atoms in the framework, and it adsorbs CO 2 gas with structural transformations; this is the first example of gas-adsorption properties in the dense ¡-form. On the other hand, when ¡-PAC-1-PF 6 is recrystallized from acetone/n-hexane and 2-butanone/n-hexane, some of the weakly coordinated PF 6 ¹ anions are easily released from the axial sites and Lewis-base guests (acetone and 2-butanone) attach to these sites instead, forming the guest-including £-form, £-{[Cu(PF 6 ) 2 (4-mepy) 4 ][Cu(PF 6 )(4-mepy) 4 (acetone)]¢ PF 6 ¢4acetone} (£-PAC-1-PF 6¸2 .5acetone), £-{[Cu(PF 6 ) 2 (4-mepy) 4 ][Cu(PF 6 )(4-mepy) 4 (2-butanone)]¢PF 6 ¢3.5(2-butanone)} (£-PAC-1-PF 6¸2 .25(2-butanone)). The prefix "quasi-" derives from such labile axial bonds. These unprecedented phenomena have never been observed in any other Werner-type metal complexes. These result...