Directed assembly and reactivity of olefins within a one-dimensional ladder-like coordination polymer based on a dinuclear Zn( II) platform{ Giannis S. Papaefstathiou, Ivan G. Georgiev, Tomislav Fris ˇc ˇic ´ and Leonard R. MacGillivray* Received (in Columbia, MO, USA) 31st March 2005, Accepted 8th June 2005 First published as an Advance Article on the web 8th July 2005 DOI: 10.1039/b504477d A ladder-like coordination polymer involving a dinuclear Zn Schiff-base complex possesses organic linkers that undergo [2 + 2] photodimerisation in the solid state. Methods that provide reliable control of chemical reactivity in the organic solid state can enable chemists to conduct stereoselective and solvent-free syntheses 1 by design 2 and construct functional solid-state materials (e.g. switches, nano-scale actuators). 3 Unfortunately, reliable control of organic solid-state reactivity, particularly in the case of bimolecular reactions, has largely remained elusive owing to vexatious structure effects of molecular close packing. 4 In this context, we have described a modular method to direct the [2 + 2] photodimerisation in the solid state. 2,5 We have shown that rigid, bifunctional molecules (e.g. resorcinol) can be used as templates to juxtapose olefins (e.g. trans-1,2-bis(4-pyridyl)ethylene or 4,49-bpe) via hydrogen bonds within finite molecular assemblies [e.g. 2(resorcinol)?2(4,49-bpe)] for the photoreaction. That the olefins are assembled within finite assemblies means that the solid- state reactivity is largely ‘insulated’ from structural effects of close packing. 4 To enable properties of transition-metal ions to be integrated within such reactive solids, we have recently extended the method to the dinuclear, cationic Zn complex [Zn 2 L(OH)] 2+ (1) (where LH 5 2,6-bis[N-(2-pyridylethyl)formimidoyl]-4-methylphe- nol) which assembled 4,49-bpe, via coordination bonds, within the finite tetranuclear complex [2(1)(4,49-bpe) 2 ] 4+ (Scheme 1). 6 The positioning of the olefins was dictated by the separation distance of the Zn ions within 1. The olefins of the tetranuclear assembly reacted via a single-crystal-to-single-crystal transformation to give rctt-tetrakis(4-pyridyl)cyclobutane (4,49-tpcb) stereospecifically in 100% yield. During experiments to generalize 1 as a template to direct the [2 + 2] photodimerisation in the solid state, 2 we have discovered the ability of 1 to assemble with 4,49-bpe to form an infinite, one-dimensional (1D) ladder-like coordination polymer in [(1)(4,49-bpe) 2 ](ClO 4 ) 2 ?4H 2 O (Scheme 1). 7 A single-crystal X-ray structure analysis{ of the solid reveals that the olefins are organized within the 1D polymer in a position suitable for a [2 + 2] photoreaction. Photoreactivity experiments demonstrate that the crystals are photoactive, reacting to give 4,49-tpcb stereospecifically in near 100% yield. Whereas coordination polymers, or metal–organic frameworks (MOFs), have gained considerable attention owing to the ability of such extended structures to confer unique bulk physical properties (e.g. host– guest, magnetic) to solids, 8 [(1)(4,49-bpe) 2 ](ClO 4 ) 2 ?4H 2 O represents a rare case of a MOF with organic units that undergo [2 + 2] photodimerisation in the solid state. 9,10 Pale yellow single crystals of [(1)(4,49-bpe) 2 ](ClO 4 ) 2 ?4H 2 O were obtained by slow evaporation of an ethanolic solution (25 mL) of [Zn 2 L(OH)](ClO 4 ) 2 (0.32 g, 0.5 mmol) and 4,49-bpe (0.91 g, 0.5 mmol) (molar ratio: 1:1) over a period of 2 days (yield: 72%). The composition of [(1)(4,49-bpe) 2 ](ClO 4 ) 2 ?4H 2 O was confirmed via single-crystal and powder X-ray diffraction data, as well as thermal gravimetric analysis. A single-crystal X-ray structure analysis of [(1)(4,49-bpe) 2 ]- (ClO 4 ) 2 ?4H 2 O (Fig. 1) demonstrates that, similar to [2(1)(4,49-bpe) 2 ](ClO 4 ) 4 ?4H 2 O, the metal and organic components have assembled such that the bipyridines are organized, via Zn–N bonds, in a face-to-face stacked arrangement (Fig. 1). The two Zn atoms of 1 are separated by 3.19 A ˚ (cf. [2(1)(4,49-bpe) 2 ](ClO 4 ) 4 ?4H 2 O]: 3.14 A ˚ ) while the carbon–carbon double (CLC) bonds of the stacked olefins lie parallel and separated by 3.71 A ˚ (cf. [2(1)(4,49-bpe) 2 ]?4H 2 O: parallel, 3.64 A ˚ ). The geometry of the stacked olefins conforms to criteria of Schmidt for [2 + 2] photoreaction in a solid. 11 In contrast to [2(1)(4,49-bpe) 2 ](ClO 4 ) 4 ?4H 2 O, however, each Zn ion of [(1)(4,49-bpe) 2 ](ClO 4 ) 2 ?4H 2 O lies in an octahedral, rather than a Department of Chemistry, University of Iowa, Iowa City, IA 52242-1294, USA. E-mail: len-macgillivray@uiowa.edu; Fax: 319-335-1270; Tel: 319-335-3504 { Electronic supplementary information (ESI) available: Figs. S1–S5: TGA and XRD spectra. See http://dx.doi.org/10.1039/b504477d Scheme 1 COMMUNICATION www.rsc.org/chemcomm | ChemComm 3974 | Chem. Commun., 2005, 3974–3976 This journal is ß The Royal Society of Chemistry 2005