Designing Molecular Assemblies in the Solid-State: Induced-Stacking by a Rebek Cleft
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Designing Molecular Assemblies in the Solid-State: Induced-Stacking by a Rebek Cleft Leonard R. MacGillivray* and Michelle M. Siebke Department of Chemistry University of Iowa Iowa City, IA USA 52242-1294 ABSTRACT Co-crystallization of 2,7-di-tert-butyl-9,9-dimethyl-4,5-xanthenedicarboxylic acid (1), a Rebek cleft, with 1,2-trans-bis(4-pyridyl)ethylene 2 yields a four-component molecular assembly, 2(1)·2(2), that is held together by four O-H···N hydrogen bonds. The cleft organizes the bipyridine such that two molecules of 2 lie in a stacked arrangement. INTRODUCTION A major goal of nanochemistry is the development of ‘bottom-up’ and ‘top-down’ methods that provide control of matter at the atomic and molecular scale, enabling chemists to gain access to new molecules and materials not obtainable using more traditional approaches to synthesis.1,2 In this context, a traditional area of contemporary chemistry that nanochemistry is expected to impact is chemical reactivity. Whereas contemporary organic synthesis is at a stage where many efforts focus upon reexamining procedures to optimize yield and atom economy,3 methods that control formation and breakage of covalent bonds at the nanoscale level possess an unprecedented potential to provide access to new and unusual chemical species with properties that, for example, may be used to design future nanotechnologies.1 To control reactivity at the nanoscale level, systems that juxtapose individual molecules in a position for reaction will have to be identified to direct formation of covalent bonds into predetermined shapes and patterns.2 To achieve this goal, it will be necessary to control structural consequences of noncovalent bonds (e.g. hydrogen bonds), since such forces have demonstrated efficacy in dictating the formation of nanoscale biosystems (e.g. viruses) and have been used to organize atoms and molecules in gases and liquids, as well as the solid state.4 We have introduced a template-based method for controlling reactivity in solids that employs bifunctional molecules based on resorcinol (e.g. 5-methoxyresorcinol) to organize, by way of hydrogen bonds, olefins linked to bipyridines [e.g. trans-1,2-bis(4-pyridyl)ethylene (4,4’bpe)] for [2+2] photoreaction (Scheme 1).5 In this approach, resorcinol functions as a linear hydrogen bond donor template6 that juxtaposes two hydrogen bond acceptor reactant molecules O H
N
N
H O
O H
N
O H
N
N
H O
N
H O
hv O H
template
N
N
reactants
crystal solid
H O
template
Scheme 1 Y2.8.1
product
within a nanometer-scale molecular assembly for reaction. The success of this ‘bottom-up’ approach lies in the ability of resorcinol to induce stacking of reactants within a discrete assembly, which eliminates many vexatious problems of intermolecular forces that have made such topochemical designs unreliable. The inspiration of this method stems from biology, where similar recognition processes involving linear templates and hydrogen bonds direct formation of covalent bonds in solution (e.g. DNA).6 We are currently identi
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