Multicomponent Self-Assembly of Diaminobenzoquinonato-Bridged Manganese(I) Metallosupramolecular Rectangles: Host-Guest Interactions, Anticancer Activity, and Visible-Light-Induced CO Releasing Studies.

The one-pot self-assembly of Mn2(CO)10, a bis-chelated diaminobenzoquinonato (ON∩ON) bridge (L), and a linear ditopic linker (N∩N) (L') has resulted into the formation of M4L2L2'-type manganese(I)-based tetranuclear metallorectangles of the general formula [{(CO)3Mn(µ-η4-L)Mn(CO)3}2(µ-(N-N)2)] (1-8), wherein L is 2,5-bis(n-butylamine)-1,4-benzoquinone (bbbq) and/or 2,5-bis(phenethylamino)-1,4-benzoquinone (bpbq) and N-N is 4,4'-bipyridine (bpy), trans-1,2-bis(4-pyridyl) ethylene (bpe), phenyl-1,4-bis(isonicotinate) (pbin), and N,N'-bis(4-pyridylformamide)-1,4-benzene (bpfb). Metallorectangles 1-8 were characterized by infrared, ultraviolet-visible (UV-vis) absorption, and 1H nuclear magnetic resonance spectroscopies, elemental analysis, and electrospray ionization-mass spectrometry. The molecular and crystal structures of 1•n(CHCl3) and coronene⊂3•coronene were determined by the single-crystal X-ray diffraction method. Host-guest binding abilities of 1, 3, and 7 with coronene, pyrene, and 4,4'-dihydroxybiphenyl were investigated using UV-vis absorption and emission spectroscopic techniques. Formation of host-guest complexes was further confirmed by the single-crystal X-ray structural analysis. Absorption of spectra of these metallorectangles showed high-intensity metal-ligand charge transfer as a broad band in the visible region. In vitro cytotoxicity assays were performed on 1, 3, 5, and 7 against lung, colon, and cervical cancer cells as well as normal cells. Compounds 5 and 7 were identified as visible-light-induced CO-releasing molecules from the myoglobin assay.

Self-Assembled Manganese(I)-Based Selenolato-Bridged Tetranuclear Metallorectangles: Host-Guest Interaction, Anticancer, and CO-Releasing Studies.

Supramolecular one-step self-assembly of dimanganese decacarbonyl, diaryl diselenide, and linear dipyridyl ligands (L = pyrazine (pz), 4,4'-bipyridine (bpy), and trans-1,2-bis(4-pyridyl)ethylene (bpe)) has resulted in the formation of selenolato-bridged manganese(I)-based metallorectangles. The synthesis of tetranuclear Mn(I)-based metallorectangles [{(CO)3Mn(µ-SeR)2Mn(CO)3}2(µ-L)2] (1-6) was facilitated by the oxidative addition of diaryl diselenide to dimanganese decacarbonyl with the simultaneous coordination of linear bidentate pyridyl linker in an orthogonal fashion. Formation of metallorectangles 1-6 was ascertained using IR, UV-vis, NMR spectroscopic techniques, and elemental analyses. The molecular mass of compounds 2, 4, and 6 were determined by ESI-mass spectrometry. Solid-state structural elucidation of 2, 3, and 6 by single-crystal X-ray diffraction methods revealed a rectangular framework wherein selenolato-bridges and pyridyl ligands define the shorter and longer edges, respectively. Also, the guest binding capability of metallorectangles 3 and 5 with different aromatic guests was studied using UV-vis absorption and emission spectrophotometric titration methods that affirmed strong host-guest binding interactions. The formation of the host-guest complex between metallorectangle 3 and pyrene has been explicitly corroborated by the single-crystal X-ray structure of 3•pyrene. Moreover, select metallorectangles 1-4 and 6 were studied to explore their anticancer activity, while CO-releasing ability of metallorectangle 2 was further appraised using equine heart myoglobin assay.

Molecular interaction of manganese based carbon monoxide releasing molecule (MnCORM) with human serum albumin (HSA).

In the present study, the interaction between the HSA and MnCORM in vitro under physiological conditions, was investigated through ultraviolet-visible (UV-vis) absorption, fluorescence, time-resolved fluorescence, circular dichroism (CD), Fourier transform infrared (FT-IR) spectroscopic techniques and in silico molecular docking methods. Binding parameters such as the binding constant, number of binding sites and binding force were obtained from the fluorescence data. Thermodynamic interaction revealed that the reaction was spontaneous (ΔG < 0) and hydrogen bond and van der Waals interaction were primarily involved in the binding. The changes induced in the secondary structure conformation due to the MnCORM interaction were monitored using CD and FT-IR spectroscopic techniques. The results showed reduction in α-helix conformation and corresponding increase in ß-sheet and unordered structures due to slight unfolding. The time-resolved fluorescence decay confirmed the static quenching mechanism of the MnCORM. The molecular docking studies revealed that the MnCORM interacted at Sudlow's site II of domain IIIA through hydrogen bond and van der Waals interactions. In order to understand the drug distribution and elimination, studies on the drug molecule interaction with HSA are vital. Therefore, it is evident that MnCORM interacts with HSA through ground state complex formation and thus suitable for in vivo delivery.

Single-Pot Self-Assembly of Heteroleptic Mn(I)-Based Aminoquinonato-Bridged Ester/Amide-Functionalized Dinuclear Metallastirrups: Potential Anticancer and Visible-Light-Triggered CORMs.

Multicomponent self-assembly of Mn2(CO)10, a bis-chelating aminoquinonato (ON∩ON) bridge (L), and an ester/amide-functionalized flexible neutral ditopic linker (L') has resulted into the formation of M2LL'-type manganese(I)-based dinuclear metallastirrups of general formula [{(CO)3Mn(µ-η4-L)Mn(CO)3}(µ-L')] (1-10). Compounds 1-10 were accomplished via orthogonal bonding of the aminoquinone ligand (2,5-bis(n-butylamino)-1,4-benzoquinone/2,5-bis(phenethylamino)-1,4-benzoquinone) and ditopic pyridyl ligand to manganese carbonyl. The resultant metallastirrups were characterized using elemental analyses and IR, UV-vis, 1H NMR, and electrospray ionization-mass spectroscopic techniques. The molecular structure of 6 was confirmed by single-crystal X-ray diffraction methods. Furthermore, molecular recognition capabilities of 1, 5, 7, and 9 were evaluated with aromatic compounds containing hydroxy/amine functionalities. Anticancer activities of compounds 1-3, 5-7, 9, and 10 were investigated against three cancer cell lines, that is, lung (A549), colon (HCT-15), and cervical (HeLa) as well as on normal cells (HEK 293). Compound 9 showed a broad-spectrum inhibition toward these cancer cells upon exposure to visible light. The myoglobin assay was performed using UV-vis absorption spectroscopy to investigate the visible-light-triggered CO release from 5 and 9 that could be related to their ability to effectively inhibit cancer cells. In addition, morphological studies confirmed the induction of autophagy due to the treatment of cancer cells using compound 9.

Amide-Functionalized Chalcogen-Bridged Flexible Tetranuclear Rhenacycles: Synthesis, Characterization, Solvent Effect on the Structure, and Guest Binding.

The synthesis of flexible rhenium(I)-based amide-functionalized chalcogen-bridged tetranuclear metallacycles of general formula [{(CO)3Re(µ-ER)2Re(CO)3}2(µ-L)2] (1-8) was achieved by treating rhenium carbonyl with dialkyl/diaryl chalcogenide (RE-ER; E = S and Se) in the presence of ditopic flexible or semiflexible pyridyl ligand with amide functionality (L = N,N'-bis(4-pyridylcarboxamide)-1,2-ethane (bpce) and N,N'-bis(4-(4-pyridylcarboxamide)phenyl)methane (bpcpm)). Compounds 1-8 were formed by multicomponent self-assembly under one-pot reaction conditions via oxidative addition of dialkyl/diaryl chalcogenide to rhenium carbonyl with pyridyl ligands. The resultant metallacyclophanes were characterized using elemental analyses, infrared, ultraviolet-visible, and NMR spectroscopic techniques. Metallacyclophanes 1-3 and 7 were structurally characterized by single-crystal X-ray diffraction methods. The solvent-induced structural change of flexible tetranuclear metallacyclophane 2 was demonstrated by crystallizing 2 in dichloroethane and dimethylformamide. Molecular recognition capabilities of 2 and 7 were studied with few aromatic compounds containing ethereal linkages.

Synthesis, Structural Characterization, and Host-Guest Studies of Aminoquinonato-Bridged Re(I) Supramolecular Rectangles.

Aminoquinonato bridged Re(I)-based metallarectangles have been constructed via an orthogonal bonding approach. Self-assembly of Re2(CO)10 and aminoquinone ligands in the presence of ditopic linear pyridyl ligands has resulted in the formation of metallarectangles of the general formula [{(CO)3Re(µ-η(4)-L)Re(CO)3}2(µ-N-L'-N)2] (1-4), wherein 1, L = 2,5-bis(n-butylamino)-1,4-benzoquinonato (bbbq) and N-L'-N = 4,4'-bipyridine (bpy); 2, L = 2,5-bis(phenethylamino)-1,4-benzoquinonato (bpbq) and N-L'-N = 4,4'-bipyridine; 3, L = 2,5-bis(n-butylamino)-1,4-benzoquinonato (bbbq) and N-L'-N = trans-1,2-bis(4-pyridyl)ethylene (bpe) and 4, L = 2,5-bis(phenethylamino)-1,4-benzoquinonato (bpbq) and N-L'-N = trans-1,2-bis(4-pyridyl)ethylene (bpe). Metallarectangles 1-4 have been characterized by elemental analysis, IR, NMR, and UV-vis absorption spectroscopic techniques. The molecular structures of 1 and 4 were determined by single-crystal X-ray diffraction methods. The molecular recognition capability of 1 and 3 with pyrene and triphenylene has been investigated using UV-vis absorption and emission spectroscopic techniques. The formation of host-guest complex has been further corroborated by the single-crystal X-ray structural evidence of carceplex system (3⊃pyrene)·(DMF).

Self-assembly of oxamidato bridged ester functionalised dirhenium metallastirrups: synthesis, characterisation and cytotoxicity studies.

A new set of ester functionalised Re(i)-based oxamidato bridged neutral dinuclear metallacycles were synthesised by self-assembly of four components from three building blocks in a facile one-pot reaction via an orthogonal bonding approach. Oxidative addition of oxamide ligands (H2L = N,N'-diphenyloxamide, and N,N'-dibenzyloxamide) to rhenium carbonyl (Re2(CO)10) in the presence of semi-rigid and flexible ditopic pyridyl ligands (L' = o-phenylene diisonicotinate (pdi), ethane diyl di-4-pyridine carboxylate (etdp) and 1,4-butane diyl di-4-pyridine carboxylate (budp)) having ester functionality afforded neutral dirhenium metallacycles of the general formula [(CO)3Re(µ-L)(µ-L')Re(CO)3] (1-5) under solvothermal reaction conditions. The metallacyclic compounds were characterised using elemental analyses, IR, UV-vis and NMR spectroscopic techniques. Structural analyses of 2-5 by single crystal X-ray diffraction methods revealed a stirrup like molecular framework in which two fac-Re(CO)3 units are bridged together by dissymmetrical NO∩ON bis-chelation of oxamide ligands (as a pedestal of stirrups) and further connected by a flexible ditopic tecton (as an arched anchor of stirrups) in an orthogonal fashion. The cytotoxicity activities of dirhenium metallacycles 1-5 were studied in vitro against three different cancer cell lines and normal cells.

Self-assembly of a fac-Mn(CO)3-core containing dinuclear metallacycles using flexible ditopic linkers.

Syntheses of manganese(i)-based dinuclear metallacycles have been accomplished under facile one-pot reaction conditions at room temperature. Self-assembly of four components has resulted in the formation of M2L2-type metallacycles [Mn(CO)3Br(µ-NLN)]2 (1-5) using pentacarbonylbromomanganese as the metal precursor and flexible ligands such as 1,2-bis(4-pyridyl)ethane (bpa), 1,2-bis(4-pyridyl)propane (bpp), 1,2-ethanediyl di-4-pyridine carboxylate (edp), 1,4-butanediyl di-4-pyridine carboxylate (budp), and 1,6-hexanediyl di-4-pyridine carboxylate (hedp) as linkers. The metallacycles have been characterized on the basis of IR, NMR, UV-vis, and ESI-mass spectroscopic techniques and single-crystal X-ray diffraction methods. The host capability of the metallacycles has been demonstrated using single-crystal X-ray crystallography.

Self-assembly of Thiolato-Bridged Manganese(I)-Based Metallarectangles: One-pot Synthesis and Structural Characterization.

A new series of thiolato-bridged manganese(I)-based supramolecular rectangles have been achieved by three-precursor self-assembly of Mn2(CO)10, diaryl disulfides (RSSR), and linear ditopic azine ligands (L) [L = pyrazine (pz), 4,4'-bipyridine (bpy), and trans-1,2-bis(4-pyridyl)ethylene (bpe)] using a one-pot synthetic strategy. Oxidative addition of RSSR (diphenyl disulfide and p-tolyl disulfide) to manganese decacarbonyl in the presence of rigid bidentate ligands (L) afforded metallarectangles of the general formula [{(CO)3Mn(µ-SR)2Mn(CO)3}2(µ-L)2] (1-6). Compounds 1-6 were characterized using elemental analyses and NMR, IR, and UV-vis absorption spectroscopic techniques. The molecular structures of metallarectangles 1, 3, and 5 were elucidated by single-crystal X-ray diffraction methods. The guest binding ability of 3 and 5 has been investigated with two aromatic guests using electronic absorption and fluorescence emission spectroscopy, and the results revealed a strong binding interaction between host-guest species.

One-pot synthesis of oxamidato-bridged hexarhenium trigonal prisms adorned with ester functionality.

Oxamidato-bridged Re(I)-based hexanuclear trigonal prisms with ester functionality have been synthesised via a multicomponent self-assembly process under solvothermal conditions. The self-assembly of Re2(CO)10, oxamide ligands (H2L1 = N,N'-dibutyloxamide, H2L2 = N,N'-dioctyloxamide, H2L3 = N,N'-didodecyloxamide and H2L4 = N,N'-dibenzyloxamide) and phenyl-1,3,5-tris(isonicotinate) (ptin) resulted in the formation of metallaprisms with the general formula [{(CO)3Re(µ­Î·(4)-L)Re(CO)3}3(µ3-ptin)2] (1­4). The metallaprisms 1­4 have been characterised using spectroscopic techniques, and the molecular structure of 4 has been elucidated by single-crystal X-ray diffraction methods. Investigations on the guest binding ability of 2 with a few aromatic alcohols and L-tryptophan using UV­vis and fluorescence spectroscopic titration experiments revealed strong host­guest interactions. The luminescence enhancement studies of 2 and 3 have been carried out using organic-aqueous solvent mixtures.

Diaqua-dichloridobis(pyridine-κN)manganese(II).

The molecular title compound, [MnCl(2)(C(5)H(5)N)(2)(H(2)O)(2)], lies about an inversion centre. The Mn(II) atom is in an all-trans octa-hedral environment defined by two water mol-ecules, two chloride anions and two pyridine ligands. An inter-molecular hydrogen-bonding inter-action between a water mol-ecule and a chloride anion bonded to an adjacent Mn(II) atom generates an eight-membered ring. The crystal packing exhibits two inter-molecular π-π stacking inter-actions between the aromatic rings, with centroid-centroid distances of 3.485 (12) and 3.532 (12) Å.

2,2'-[Naphthalene-1,5-diylbis(nitrilo-methanylyl-idene)]diphenol.

The title compound, C(24)H(18)N(2)O(2), lies about an inversion centre and the asymmetric unit contains one half-mol-ecule. An intra-molecular O-H⋯N hydrogen bond generates a six-membered ring, producing an S(6) ring motif. The crystal packing exhibits inter-molecular π-π stacking inter-actions between the aromatic rings with a centroid-centroid distance of 3.851 (2) Å.

Butane-1,4-diyl bis-(pyridine-4-carboxyl-ate).

The mol-ecule of the title compound, C(16)H(16)N(2)O(4), lies about an inversion centre; the butane chain adopts an extended zigzag conformation. The dihedral angle between the pyridine ring and the adjacent COO group is 3.52 (s14)°.

Heptacarbonyl-1κC,2κC-(4-phenyl-pyridine-1κN)di-µ-phenyltellurido-1:2κTe:Te-dirhenium(I).

In the title complex, [Re(2)(C(6)H(5)Te)(2)(C(11)H(9)N)(CO)(7)], two Re atoms are coordinated in slightly distorted octa-hedral coordination environments and are bridged by two Te atoms, which are coordinated in trigonal-pyramidal environments. The torsion angle for the Te-Re-Te-Re sequence of atoms is 17.06 (3)°. The crystal structure is stabilized by weak C-H⋯O and C-H⋯π inter-actions. In addition, there are Te⋯Te distances [4.0392 (12) Å] and O⋯O distances [2.902 (19) Å] which are shorter than the sum of the van der Waals radii for these atoms. A short inter-molecular lone pair⋯π distance [C O⋯Cg = 3.31 (2) Å] is also observed.

Bis(µ-phenyl-tellurido-κTe:Te)bis-[tetra-carbonyl-rhenium(I)].

The title compound, [Re(2)(C(6)H(5)Te)(2)(CO)(8)], crystallizes with two mol-ecules in the asymmetric unit, in which two Re atoms are coordinated in a slightly distorted octa-hedral environment and are bridged by two Te atoms, which show a distorted trigonal-pyramidal geometry. The torsion angles for the Te-Re-Te-Re sequence of atoms are 19.29 (18) and 16.54 (16)° in the two mol-ecules. Thus, the Re-Te four-membered rings in the two mol-ecules deviate significantly from planarity. Two intra-molecular C-H⋯O inter-actions occur in one of the mol-ecules. Te-Te [4.0551 (10) Å] inter-actions between the two mol-ecules and weak inter-molecular C-H⋯O inter-actions stabilize the crystal packing.

Aggregate of alkoxy-bridged Re(I)-rectangles as a probe for photoluminescence quenching.

Alkoxy-bridged rhenium(I) rectangles [{(CO)(3)Re(mu-OR)(2)Re(CO)(3)}(2)(mu-bpy)(2)] (1, R = C(4)H(9); 2, R = C(8)H(17); 3, R = C(12)H(25); bpy = 4,4'-bipyridine) comprising long alkyl chains form optically transparent aggregates and exhibit luminescence enhancement in the presence of water. The aggregation of Re(I)-rectangle was followed using a light-scattering technique. Presumably, the enhanced luminescence efficiency resulted from restriction of torsional molecular motion in the aggregates. In addition, the rate of bimolecular quenching of Re(I)-aggregates in the triplet excited state by various electron donors (amines) and acceptors (quinones) was efficient. These results indicate that the excited state of aggregated Re(I) surfactants with an electron acceptor and donor facilitate the electron-transfer quenching process after they became preassociated inside the Re(I)-aggregated species. These synthesized compounds may be useful fluorescent materials in optoelectronic applications.

CH...pi interaction for rhenium-based rectangles: an interaction that is rarely designed into a host-guest pair.

Alkoxy- and thiolato-bridged Re(I) molecular rectangles [{(CO)3Re(mu-ER)2Re(CO)3}2(mu-bpy)2] (ER = SC4H9, 1a; SC8H17, 1b; OC4H9, 2a; OC12H25, 2b; bpy = 4,4'-bipyridine) exhibit strong interactions with several planar aromatic molecules. The nature of their binding was studied by spectral techniques and verified by X-ray diffraction analysis. Standard absorption and fluorescence titrations showed that a relatively strong 1:1 interaction occurs between aromatic guests such as pyrene and these rectangles. The results of a single-crystal X-ray diffraction analysis show that the recognition of 1 with a pyrene molecule is mainly due to CH...pi interactions and the face of the guest pyrene is located over the edges of the bpy linkers of 1. This is a fairly novel example of an interaction that is rarely designed into a host-guest pair. Furthermore, the interaction of 1 with Ag+ results in the self-organization of supramolecular arrays, as revealed by solid-state data.

Photoluminescence electron-transfer quenching of rhenium(I) rectangles with amines.

Electron-transfer (ET) reactions from aromatic amines to excited states of rhenium(I)-based molecular rectangles [{Re(CO)3(mu-bpy)Br}{Re(CO)3(mu-L)Br}]2 (bpy = 4,4'-bipyridine, L = 4,4'-dipyridylacetylene (dpa), I; L = 4,4'-dipyridylbutadiyne (dpb), II; and L = 1,4-bis(4'-pyridylethynyl)benzene (bpeb), III) were investigated in a dichloromethane solution using luminescence quenching techniques. Direct evidence for the ET reaction was obtained from the detection of the amine cation radical in this system using time-resolved transient absorption spectroscopy. The values of the luminescence quenching rate constants, kq, of the 3MLCT excited state of Re(I) rectangles with amines were found to be higher than those for the monomeric Re(I) complexes and other Re(I)-based metallacyclophanes. The observed kq values were correlated well with the driving force (Delta G degrees) for the ET reactions. In addition, a semiclassical theory of ET was successfully applied to the photoluminescence quenching of Re(I) rectangles with amines.

Synthesis and photophysical properties of neutral luminescent rhenium-based molecular rectangles.

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.

Self-assembly of octarhenium-based neutral luminescent rectangular prisms.

Self-assembly of rhenium-based nanoscale rectangular prismatic boxes has been achieved in quantitative conversion. The fac-rhenium corner provided three mutually perpendicular coordination sites and served as a good candidate for the construction of 3-D boxes. These are the first Re-based, neutral, luminescent prisms of M(8)L(2)L'(8) type that has been characterized crystallographically. Their luminescent properties and molecular recognition capabilities make these molecular prisms interesting supramolecules.