No abstract

Supramolecular structures containing transition metal ions with potential inclusion and host-guest applications have been extensively explored in recent years. [1][2][3][4] Stang and co-workers contributed to the formation of a combinatorial library of cyclic molecular polygons via the systematic combination of building blocks with predetermined angles. 5 The initial focus was directed toward molecular squares, i.e., macrocycles fabricated by ciscoordinated transition metal corners and rigid or semirigid bifunctional ligand edges. 6-8 Subsequent efforts were devoted to the synthesis of molecular rectangles to improve selectivity and sensitivity in molecular recognition and separation. 9-12 Hupp et al. were the first to report the preparation of rhenium thiolatebased rectangles that exhibited interesting electrochemistry. 9,10 Subsequent work by the same group led to the synthesis, characterization, and preliminary binding properties of a new class of tetracationic rectangular molecules with triflate as counterion. 10 To tune the cavity size, Sullivan et al. recently also reported the preparation of a series of molecular rectangles based on fac-Re-(CO) 3 corners containing 4,4′-bipyridine as one side and two η 2 -alkoxy or hydroxy bridges as the other. 12 We report herein the synthesis and characterization of a new class of molecular rectangles 3 (Scheme 1) that exhibit luminescence in solution at room temperature. Unlike earlier "rectangles", compounds 3 are neutral, possess larger cavities, and have no counterions inside the channels.Activation of Re(CO) 5 Br (1) with Me 3 NO at 0°C followed by addition of 0.5 equiv of 4,4′-bipyridine in toluene gave a paleyellow, shiny product {Re(CO) 4 Br} 2 (µ-bpy) (2a) in 58% yield. Further treatment of 2a at 5°C in CH 2 Cl 2 with 2 equiv of Me 3 -NO and a required amount of acetonitrile yielded intermediate-{(Re(CO) 3 (NCMe)Br} 2 (µ-bpy), which produced the molecular rectangle 3a (14% yield) after subsequent titration with pyrazine. Following a similar strategy, the bimetallic edges 2b and 2c were synthesized from trans-1,2-bis(4-pyridyl)ethylene and pyrazine, respectively. Compounds 2a and 2b were then converted into 3b and 3c by reacting with trans-1,2-bis-(4-pyridyl)ethylene and pyrazine, respectively.Rectangles 3 and their corresponding edges 2 were characterized spectroscopically. The FAB-MS analyses of 3a and 2a exhibit signals corresponding to the molecular ions at m/z ) 1880 and m/z ) 916, respectively, with the experimental isotope pattern matching the calculated values. The 1 H NMR spectrum of 3a shows the presence of two predominant isomeric forms in solution at room temperature, most likely due to the orientation of the CO/Br trans-ligand pairs with respect to the plane containing the Re atoms.Yellow single crystals of 3a were grown from acetone, and X-ray diffraction studies were carried out. 13 An ORTEP diagram of 3a is shown in Figure 1. The structure consists of a molecular rectangle in which two Br(CO) 4 Re-bpy-Re(CO) 4 Br edges are Crystal data for 3a...

Self-assembly of rhenium(I)-based molecular rectangles containing long alkyl chains has been achieved in one-pot synthesis by solvothermal methods. An enormous enhancement in the emission intensity, quantum yield, and lifetime of the rectangles has been observed when the solvent medium is changed from organic to aqueous. Addition of water favors the aggregation of Re(I) molecular rectangle resulting in the luminescence enhancement, and this phenomenon has been traced out using light scattering techniques.

Ruthenium(II)-polypyridyl complexes of similar size but with variable reduction potential undergo efficient photoinduced electron-transfer reactions with phenolate ions in aqueous medium. All these reactions are exergonic and are in accordance with the Marcus theory of electron transfer. At high negative ∆G°Marcus inverted region is observed in this bimolecular photoinduced charge separation reaction.

A series of neutral luminescent molecular rectangles [[Re(CO)(3)(mu-bpy)Br][Re(CO)(3)(mu-L)Br]](2) (1-4) having fac-Re(CO)(3)Br as corners and 4,4'-bipyridine (bpy) as the bridging ligand on one side and other bipyridyl ligands of varying length (L) on the other side have been synthesized and characterized. The crystal structure of 1 shows a rectangular cavity with the dimensions of 11.44 x 7.21 A. When the cavity size is tuned from 1 to 4, a dimension of 11.4 x 20.8 A could be achieved, as revealed by the molecular modeling. These rectangles exhibit luminescence in solution at room temperature. In particular, compound 4 containing 1,4-bis(4'-pyridylethynyl)benzene (bpeb) as bridging ligand shows the excited-state lifetime of 495 ns. Fine-tuning of the cavity size of the rectangles improves their excited-state properties. These properties facilitate the study of excited-state electron-transfer reactions with electron acceptors and donors and host-guest binding. Crystallographic information: 1.6CH(3)COCH(3) is monoclinic, P2(1)/c, with a = 12.0890(2), b = 24.2982(2), and c = 12.8721(2) A, beta = 107.923(1) degrees, and Z = 2.

The rate constants for the photoinduced electron transfer reactions of Ru(II)-polypyridyl complexes (k q ) with 2,6-disubstituted phenolate ions in aqueous acetonitrile are highly sensitive to change in the (Ru(NN) 3 2`) bulkiness of the ligand in as well as the phenolate ion. The decrease in value with the increase in Ru(NN) 3 2`k q the size of the ligand in and the phenolate ion is ascribed to the decrease in the electronic coupling Ru(NN) 3 2m atrix element, between the donor and acceptor with the increase in the electron transfer distance. The o H DA o, hydrophobic interaction or possible pÈp stacking between the pyridine rings of and the aryl Ru(NN) 3 2m oiety of ArO~leads to less steric e †ect.

A novel one‐pot synthesis of rhenium‐based triangular prismatic cages has been accomplished in excellent yields by a self‐assembly strategy. Reaction of Re2(CO)10 with 2,4,6‐tri‐4‐pyridyl‐1,3,5‐triazine (tpt) in the presence of an aliphatic alcohol (1‐butanol, 1‐octanol or 1‐dodecanol) afforded molecular prisms, [{(CO)3Re(μ2‐OR)2Re(CO)3}3(μ3‐tpt)2] (1, R = C4H9; 2, R = C8H17; 3, R = C12H25) by solvothermal methods. Fine tuning of the solubility is achieved by tailoring the length of the alkyl chain of the alcohol. Compounds 1−3 reported here are the first Re‐containing molecular prisms of the M6L2L′6 type. In addition, they are the first neutral, prismatic cages without counterions that are highly air and moisture stable. Compounds 2 and 3 are soluble in most common organic solvents and they exhibit a large solvatochromism: the value of λmaxab could be tuned over 125 nm simply by changing the polarity of the solvent.

The one-pot synthesis of soluble, neutral, luminescent rectangular supramolecules in high yields by solvothermal methods and fine tuning of the molecular architecture in one dimension from ~7 Å to a nanometer scale, ~21 Å, have been achieved.