DOI: 10.1002/chem.201202301 Control of Light-Promoted [2+2] Cycloaddition Reactions by a Remote Ancillary Regulatory Group That Is Covalently Attached to Rhenium Rectangles Zong-Zhan Lu, [a, b] Chung-Chou Lee, [a] Murugesan Velayudham, [a] Li-Wei Lee, [a] Jing-Yun Wu, [a] Ting-Shen Kuo, [c] and Kuang-Lieh Lu* [a] Introduction The control of molecular function by external stimuli, such as light or chemical, electrochemical, or thermal inputs, has been a subject of significant interest, owing to its potential applications on the molecular level. [1–4] In particular, the for- mation of CC bonds by photoinduced [2+2] cycloaddition reactions has attracted considerable interest. [5–9] The forma- tion of hydrocarbon functionalities through the cycloaddi- tion reactions of olefins has synthetic importance in the for- mation of cyclophanes [10] and biological significance in the generation of ladderane. [11] In general, weak forces, such as hydrogen bonding, p  p stacking, and coordinate bonding, are utilized to fix the reactive functional groups in a suitable orientation such that, upon the utilization of external stimu- li, [2+2] cycloaddition occurs. [4c, 6, 9c] Although light-activated [2+2] cycloaddition reactions occur very smoothly in most cases, [8, 9, 12, 13] controlling the reaction, in a preferred manner, remains a subject of tremendous importance. The [2+2] cycloaddition reaction has been studied by the groups of MacGillivray, Vittal, Biradha, and Lang, among others. [7, 9a–c, 13b, 14, 15] Strategies that have been reported to reg- ulate the reactivity of the photoinduced [2+2] cycloaddition reaction include: 1) manipulation by using a template (Fig- ure 1 a) ; [16a–c] 2) varying the counterions in the complexes; [15] Abstract: The high-yielding self-assem- bly of three neutral rhenium(I) rectan- gles, [Re 2 (CO) 6 (L)ACHTUNGTRENNUNG(bpe)] 2 (1a,L = 2,2’- biimidazolate (biim); 1b, L = 2,2’-bis- benzimidazolate (bbim); 1c, L = 2,2’- bis(4,5-dimethylimidazolate) (bdmim); bpe = trans-1,2-bis(4-pyridyl)ethylene), under hydrothermal conditions is de- scribed. The rectangles were structural- ly characterized by spectroscopic tech- niques and further confirmed by single- crystal X-ray diffraction. Upon irradia- tion with a Hg lamp at 365 nm, the bpe ligands of rectangles 1a and 1b under- went [2+2] photocycloaddition reac- tions to produce [{(Re(CO) 3 ) 2 L} 2 (4,4’- tpcb) 2 ] (2a, L = biim; 2b, L = bbim; 4,4’-tpcb = 1,2,3,4-tetrakis(4-pyridyl)cy- clobutane) through a single-crystal-to- single-crystal (SCSC) transformation. However, rectangle 1c, which con- tained methyl groups on the 2,2’-biimi- dazolate ligand, failed to undergo cy- cloaddition, even after prolonged irra- diation. This result indicates that the light-induced cycloaddition reaction can be preferentially controlled by the remote regulatory substituents, which are attached onto the same backbone of the rectangle complex. This transfor- mation is the first reported utilization of a remote ancillary regulatory ligand that is covalently attached onto a coor- dination compound to control the [2+2] cycloaddition reaction. Keywords: cycloaddition · photo- chemistry · rectangles · rhenium · self-assembly [a] Z.-Z. Lu, Dr. C.-C. Lee, Dr. M. Velayudham, L.-W. Lee, Dr. J.-Y. Wu, Prof.Dr. K.-L. Lu Institute of Chemistry, Academia Sinica, Taipei 115 (Taiwan) Fax: (+ 886) 2-27831237 E-mail: kllu@gate.sinica.edu.tw [b] Z.-Z. Lu Department of Chemistry, National Central University Jhongli 320 (Taiwan) [c] T.-S. Kuo Department of Chemistry, National Taiwan Normal University Taipei 106 (Taiwan) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201202301. Figure 1. Reported examples of controlled photoinduced [2+2] cyclo- ACHTUNGTRENNUNGaddition reactions: a) template assisted [2+2] cycloaddition reaction; b) argentophilic force controlled [2+2] cycloaddition reaction. # 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2012, 18, 15714 – 15721 15714