Induction of Enzyme-like Peroxidase Activity in an Iron Porphyrin Complex Using Second Sphere Interactions.

Emulating enzymatic reactivity using small molecules has been a long-time challenging pursuit of the scientific community. Peroxidases, ubiquitous heme enzymes that are involved in hormone synthesis and the immune system, have been a prime target of such efforts due to their tremendous potential in the chemical industry as well as in wastewater treatment. Here it is demonstrated that inclusion of a second sphere guanidine moiety in an iron porphyrin not only makes this small molecule a veritable peroxidase catalyst but also offers an auxiliary binding site for organic substrates, facilitating their rapid oxidation with a green oxidant like H2O2. This small molecule analogue exhibits a "ping-pong" mechanism and Michaelis-Menten type kinetics, which is generally typical of metallo-enzymes and follows a mechanism of the natural enzyme in its entirety, including the formation of compound I as the primary oxidant.

Rational Design of Mononuclear Iron Porphyrins for Facile and Selective 4e-/4H+ O2 Reduction: Activation of O-O Bond by 2nd Sphere Hydrogen Bonding.

Facile and selective 4e-/4H+ electrochemical reduction of O2 to H2O in aqueous medium has been a sought-after goal for several decades. Elegant but synthetically demanding cytochrome c oxidase mimics have demonstrated selective 4e-/4H+ electrochemical O2 reduction to H2O is possible with rate constants as fast as 105 M-1 s-1 under heterogeneous conditions in aqueous media. Over the past few years, in situ mechanistic investigations on iron porphyrin complexes adsorbed on electrodes have revealed that the rate and selectivity of this multielectron and multiproton process is governed by the reactivity of a ferric hydroperoxide intermediate. The barrier of O-O bond cleavage determines the overall rate of O2 reduction and the site of protonation determines the selectivity. In this report, a series of mononuclear iron porphyrin complexes are rationally designed to achieve efficient O-O bond activation and site-selective proton transfer to effect facile and selective electrochemical reduction of O2 to water. Indeed, these crystallographically characterized complexes accomplish facile and selective reduction of O2 with rate constants >107 M-1 s-1 while retaining >95% selectivity when adsorbed on electrode surfaces (EPG) in water. These oxygen reduction reaction rate constants are 2 orders of magnitude faster than all known heme/Cu complexes and these complexes retain >90% selectivity even under rate determining electron transfer conditions that generally can only be achieved by installing additional redox active groups in the catalyst.

Catalytic Intramolecular Cycloaddition Reactions by Using a Discrete Molecular Architecture.

A discrete tetragonal tube-shaped complex (MT-1) has been synthesised by coordination-driven self-assembly of a carbazole-based tetraimidazole donor L and a Pd(II) 90° acceptor, that is, [cis-(dppf)Pd(OTf)2 ] (dppf=diphenylphosphinoferrocene, OTf=CF3 SO3- ). Complex MT-1 was characterised by multinuclear NMR, ESI-MS and single-crystal X-ray diffraction analysis (SCXRD), which showed its symmetrical tetrafacial tube-shaped architecture possessing a large cavity described by four aromatic walls. This coordination cage was successfully utilised as a molecular vessel to perform intramolecular cycloaddition reactions of O-allylated benzylidinebarbituric acid derivatives inside its confined nanospace. The presence of a catalytic amount of MT-1 promoted [4+2] cycloaddition reactions in a regio- and stereoselective manner, yielding the corresponding penta/tetracyclouracil derivatives in good yields under mild reaction conditions. This protocol is interesting compared with the literature reports for the synthesis of similar chromenopyran pyrimidinedione derivatives under high-temperature reflux conditions or solid-state melt reactions (SSMRs).

Versatility of Two Diimidazole Building Blocks in Coordination-Driven Self-Assembly.

A series of discrete molecular architectures was synthesized via self-assembly of two "linear" diimidazole donors [L1 = 1,4-di(1H-imidazol-1-yl)benzene and L2 = 1,1'-(2,5-dimethyl-1,4-phenylene)bis(1H-imidazole)] independently with cis-[(tmeda)Pd(NO3)2] [tmeda = N,N,N',N'-tetramethylethane-1,2-diamine], cis-[(tmeda)Pt(NO3)2], a diplatinum(II) acceptor (bisPt), and Pd(NO3)2. The donors L1 and L2 are isostructural with four hydrogens in the benzene ring of L1, whereas in L2 two of such hydrogens are substituted by methyl groups. The assemblies were characterized by NMR spectroscopy and electrospray ionization mass spectrometry analyses along with single-crystal structure determination of three of them. While the self-assembly of L1 with cis-[(tmeda)Pd(NO3)2] solely formed a [3 + 3] self-assembled molecular triangle (1), L2 with the same acceptor predominantly resulted in the formation of a [4 + 4] molecular square (3). Such a dramatic change in the final outcome in the coordination-driven self-assembly by simple alkyl substitution of isostructural donors is remarkable. Interestingly, self-assembly of L1 and L2 with analogous Pt(II) acceptor cis-[(tmeda)Pt(NO3)2] yielded mixtures of [3 + 3] triangle and [4 + 4] square, where the molecular triangles (4 and 6) were the predominant products in both the cases. The same donors in combination with a 0° acceptor bisPt independently formed the expected [2 + 2] metallo-macrocycles (8 and 9). Surprisingly, the macrocycle (9) involving L2 is found to exist in more than one conformation at room temperature. Moreover, the diimidazole donors formed unprecedented Pd6L12 molecular spheres (10 and 11) when they were separately treated with Pd(NO3)2. The imidazole moieties in the ligands are found to appear in versatile orientations in the synthesized molecules due to their rotational flexibility to produce required bite angles for the particular architecture.

Self-assembly of a redox active water soluble Pd6L8 'molecular dice'.

A water soluble 'molecular dice' was synthesised via coordination driven self-assembly of a Pd(II) ion with a flexible cationic tritopic donor and was fully characterised using NMR, ESI-MS and single crystal X-ray diffraction analysis. The donor-inherited redox active nature of the 'molecular dice' was studied using cyclic voltammetry.

Self-assembly of Metallamacrocycles Employing a New Benzil-based Organometallic Bisplatinum(II) Acceptor.

A benzil-based semi-rigid dinuclear-organometallic acceptor 4,4'-bis[trans-Pt(PEt(3))(2)(NO(3))(ethynyl)]benzil (bisPt-NO(3)) containing a Pt-ethynyl functionality was synthesized in good yield and characterized by multinuclear NMR ((1)H, (31)P, and (13)C), electrospray ionization mass spectrometry (ESI-MS), and single-crystal X-ray diffraction analysis of the iodide analogue bisPt-I. The stoichiometric (1:1) combination of the acceptor bisPt-NO(3) separately with four different ditopic donors (L(1)-L(4); L(1) = 9-ethyl-3,6-di(1H-imidazol-1-yl)-9H-carbazole, L(2) = 1,4-bis((1H-imidazol-1-yl)methyl)benzene, L(3) = 1,3-bis((1H-imidazol-1-yl)methyl)benzene and L(4) = 9,10-bis((1H-imidazol-1-yl) methyl)anthracene) yielded four [2 + 2] self-assembled metallacycles M(1)-M(4) in quantitative yields, respectively. All these newly synthesized assemblies were characterized by various spectroscopic techniques (NMR, IR, ESI-MS) and their sizes/shapes were predicted through geometry optimization employing the PM6 semi-empirical method. The benzil moiety was introduced in the backbone of the acceptor bisPt-NO(3) due to the interesting structural feature of long carbonyl C-C bond (∼1.54 Å), which enabled us to probe the role of conformational flexibility on size and shapes of the resulting coordination ensembles.

A Pd8 Tetrafacial Molecular Barrel as Carrier for Water Insoluble Fluorophore.

A new carbazole-based tetraimidazole ligand 1,3,6,8-tetra(1H-imidazol-1-yl)-9-methyl-9H-carbazole (L) has been synthesized. The unsymmetrical nature of L as well as the rotational freedom of imidazole donor moieties around C-N bond make it a special building unit, which upon treatment with cis-(tmeda)Pd(NO3)2 produced an unprecedented single linkage-isomeric Pd8 tetrafacial molecular nanobarrel (PSMBR-1) [tmeda = N,N,N',N'-tetramethylethane-1,2-diamine]. Unlike closed architectures, open barrel architecture of water-soluble PSMBR-1 makes it an ideal host for some water insoluble polyaromatic hydrocarbons in aqueous medium; one such inclusion complex coronene⊂PSMBR-1 was characterized by X-ray diffraction study. Moreover, the potential application of PSMBR-1 as carrier in aqueous medium for the transportation of water insoluble fluorophore (perylene) for live cell imaging is explored.

Fluorescent tris-imidazolium sensors for picric acid explosive.

Two new anthracene-functionalized fluorescent tris-imidazolium salts have been synthesized, characterized, and proven to be selective sensors for picric acid, which is a common constituent of many powerful explosives. Theoretical studies revealed an unusual ground-state electron transfer from picrate anion to the sensor molecules.