Supramolecular allosteric cofacial porphyrin complexes

Nature routinely uses cooperative interactions to regulate cellular activity. For years, chemists have designed synthetic systems that aim toward harnessing the reactivity common to natural biological systems. By learning how to control these interactions in situ, one begins to allow for the preparation of man-made biomimetic systems that can efficiently mimic the interactions found in Nature. To this end, we have designed a synthetic protocol for the preparation of flexible metal-directed supramolecular cofacial porphyrin complexes which are readily obtained in greater than 90% yield through the use of new hemilabile porphyrin ligands with bifunctional ether-phosphine or thioether-phosphine substituents at the 5 and 15 positions on the porphyrin ring. The resulting architectures contain two hemilabile ligand-metal domains (RhI or CuI sites) and two cofacially aligned porphyrins (ZnII sites), offering orthogonal functionalities and allowing these multimetallic complexes to exist in two states, "condensed" or "open". Combining the ether-phosphine ligand with the appropriate RhI or CuI transition-metal precursors results in "open" macrocyclic products. In contrast, reacting the thioether-phosphine ligand with RhI or CuI precursors yields condensed structures that can be converted into their "open" macrocyclic forms via introduction of additional ancillary ligands. The change in cavity size that occurs allows these structures to function as allosteric catalysts for the acyl transfer reaction between X-pyridylcarbinol (where X = 2, 3, or 4) and 1-acetylimidazole. For 3- and 4-pyridylcarbinol, the "open" macrocycle accelerates the acyl transfer reaction more than the condensed analogue and significantly more than the porphyrin monomer. In contrast, an allosteric effect was not observed for 2-pyridylcarbinol, which is expected to be a weaker binder and is unfavorably constrained inside the macrocyclic cavity.     

Charge transfer excitations in cofacial fullerene-porphyrin complexes

Porphyrin and fullerene donor-acceptor complexes have been extensively studied for their photo-induced charge transfer characteristics. We present the electronic structure of ground states and a few charge transfer excited states of four cofacial porphyrin-fullerene molecular constructs studied using density functional theory at the all-electron level using large polarized basis sets. The donors are base and Zn-tetraphenyl porphyrins and the acceptor molecules are C(60) and C(70). The complexes reported here are non-bonded with a face-to-face distance between the porphyrin and the fullerene of 2.7 to 3.0 A?. The energies of the low lying excited states including charge transfer states calculated using our recent excited state method are in good agreement with available experimental values. We find that replacing C(60) by C(70) in a given dyad may increase the lowest charge transfer excitation energy by about 0.27 eV. Variation of donor in these complexes has marginal effect on the lowest charge transfer excitation energy. The interfacial dipole moments and lowest charge transfer states are studied as a function of face-to-face distance.     

A molecular-clip-based approach to cofacial zinc-porphyrin complexes

We have synthesized molecular-clip-based cofacial zinc-porphyrin complexes. The complexes are shown to be new efficient receptors for the complexation of 1,4-diazabicyclo[2,2,2]octane, acridinium ions, 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA) and 1,2,4,5-benzenetetracarboxylic dianhydride (BTDA). Large binding constants (K_asso) in the range of 2.0 × 10⁴-1.1 × 10⁸ M⁻¹ were obtained for the 1:1 complexes of molecular tweezers (3) and rectangle (8) by inserting the hosts between the two porphyrin rings. The values of K and the spectroscopic results suggest that 3 and 8 serve as effective building blocks to capture the hosts. Semi-empirical calculations show that the guests position themselves within the cleft of the bis-porphyrins.     

Cationic cryptand complexes of tin(II)

A series of cationic cryptand complexes of tin(II), [Cryptand[2.2.2]SnX][SnX(3)] (10, X = Cl; 11, X = Br; 12, X = I) and [Cryptand[2.2.2]Sn][OTf](2) (13), were synthesized by the addition of cryptand[2.2.2] to a solution of either tin(II) chloride, iodide, or trifluoromethanesulfonate. The complexes could also be synthesized by the addition of the appropriate trimethylsilyl halide (or pseudohalide) reagent to a solution of tin(II) chloride and cryptand[2.2.2]. The complexes were characterized using a variety of techniques including NMR, Raman, and temperature-dependent M?ssbauer spectroscopy, mass spectrometry, and X-ray diffraction.     

Crown ether-based cryptand/tropylium cation inclusion complexes

Host–guest complexation between crown ether-based cryptand hosts and a carbonium ion, tropylium hexafluorophosphate was studied. ¹H NMR, NOESY NMR, and electrospray ionization mass spectrometry were employed to characterize these inclusion complexes. The contrast tests of ¹H NMR and association constants indicated that cryptands are much better hosts for tropylium hexafluorophosphate than the corresponding simple crown ethers. C–H?O hydrogen bonding, face-to-face π-stacking interactions, and charge-transfer interactions are thought to be the main driving forces for the formation of these host–guest complexes. These multiple non-covalent interactions may jointly contribute to the complex formation and considerably reinforce the complex stability. Moreover, the complexation between dibenzo-24-crown-8-based cryptand 4 and tropylium hexafluorophosphate 7 can be reversibly controlled by adding KPF₆ and then DB18C6 in 1:1 acetonitrile/chloroform, providing a new cation-responsive host–guest recognition motif for supramolecular chemistry.     

Theoretical predictions of cofacial bis(actinyl) complexes of a stretched Schiff-base calixpyrrole ligand

Actinyl and actinyl-transition metal complexation by a polypyrrolic macrocycle with anthracenyl linkers between the N(4)-donor compartments was evaluated using relativistic density functional theory which predicts that a highly unusual cofacial bis-actinyl structure complex is stable.     

A convergent coordination chemistry-based approach to dissymmetric macrocyclic cofacial porphyrin complexes

We report a highly convergent and modular approach for the synthesis of dissymmetric cofacial porphyrin complexes, which is based upon the weak-link approach to supramolecular coordination chemistry. Specifically, we have utilized a halide-induced ligand rearrangement reaction, which is capable of providing heteroligated mixed-metal porphyrin complexes in quantitative yield. Significantly, the adoption of a coordination chemistry based approach for the synthesis of these complexes allows for facile in situ regulation of the porphyrin-porphyrin interactions through the addition of external chemical stimuli.     

Preparation and structural studies of a new series of bimetallic complexes

A new series of bimetallic Lewis acid, Cu(NCO)₂(NCS)₂Hg, and its complexes, L₂Cu(NCO)₂(NCS)₂Hg (L =thf,dmso, pyridine, nicotinamide and Ph₃P are prepared and studied by physical and spectroscopic methods.     

Transition metal ion directed bimetallic macrocyclic complexes

Template condensation of 1,4-phenylenediamine, formaldehyde, 1,3-diaminopropane and benzil in a 1:4:4:2, molar ratio results in the formation of a new series of binuclear decaazamacrocyclic complexes: dichloro/nitrato [1-phenyl bis(8,9-diphenyl-1,3,7,10,14 pentaazacyclo-pentadeca-7,9-diene) metal (II)], [M₂LX₄](M = MnII, Co^II, Ni^II, CuII and Zn^II, X = Cl or NO₃), which were characterised by i.r., ¹H-n.m.r., e.p.r., u.v–vis. spectral studies, conductivity and magnetic susceptibility measurements. All the complexes show octahedral geometry.     

A facile route to bimetallic ruthenium dipyridophenazine complexes

Using achiral coordinatively unsaturated metal complex building-blocks, the two step synthesis of a bimetallic complex containing independent [Ru(II)dppz] units tethered together by a linking 4,4'dipyridyl-1,5-pentane ligand is reported. Photophysical studies on this prototype system indicate that the characteristic luminescence of the [Ru(II)dppz] moieties is perturbed by self-quenching processes. Preliminary binding studies on the complex with natural and synthetic duplex DNA is reported. Luminescence and calorimetric titrations reveal that the complex does not show enhanced binding affinity with respect to analogous monometallic complexes. This result is interpreted by a consideration of the length and rigidity of the linker employed in the complex.     

Single-site catalysis by bimetallic zinc calixarene inclusion complexes

[reaction: see text] Bimetallic alkylzinc calixarene inclusion complexes were prepared and used in the ring-opening polymerization of L-lactide. Polymers with high molecular weight and a low degree of polydispersity were obtained. 1H NMR and diffusion NMR experiments suggest that a single-site mechanism is operative.     

Redox-switchable conjugated bimetallic ruthenium(II) complexes

The conjugated bimetallic ruthenium(II) complex composed of 1,4-phenylenediamine as a bridging ligand was synthesized by photo-irradiation to show redox-switching of the emission properties of the terminal Ru(II) units depending on the redox state of the π-conjugated bridging spacer.     

Roof-shaped halide-bridged bimetallic complexes via ring expansion reaction

Binucleating behavior of rigid triptycene-based ligands has been studied. It has been demonstrated that trans-spanned transition-metal mononuclear complexes bearing 1,8-bis(diisopropylphosphino)triptycene (L1) and 1-diisopropylphosphino-8-diphenylphosphinotriptycene (L2) react with an appropriate transition-metal precursor via a ring-expansion pathway to form unusual bimetallic quasi-closed structures. New palladium and rhodium complexes featuring strongly bent (ca. 115 degrees ) M2(mu-Cl2) cores with very closely spanned metal centers (less than 3 A) have been prepared using the described ring-expansion reaction and have been fully characterized. Despite a constrained arrangement of the binuclear system, halogen bridges in all new compounds were stable in both the solid state and solution showing no tendency for dissociation even in the presence of added Lewis bases. Spontaneous resolution of the dissymmetric Pd2(mu-Cl)2Cl2(1-diisopropylphosphino-8-diphenylphosphinotriptycene) (2) into enantiopure antipodes is discussed as well.     

Combinatorial synthesis of bimetallic complexes with three halogeno bridges

Methods for the synthesis of bimetallic complexes in which two different metal fragments are connected by three chloro or bromo bridges are reported. The reactions are general, fast, and give rise to structurally defined products in quantitative yields. Therefore, they are ideally suited for generating a library of homo- and heterobimetallic complexes in a combinatorial fashion. This is of special interest for applications in homogeneous catalysis. Selected members of this library were synthesized and comprehensively characterized; single-crystal X-ray analyses were performed for 15 new bimetallic compounds.     

Flexible cofacial binuclear metal complexes derived from alpha,alpha-bis(salicylimino)-m-xylene

The tetradentate Schiff-base ligand SIXH2 (alpha,alpha-bis(salicylimino)-m-xylene), prepared from salicylaldehyde and m-xylylenediamine, forms cofacial binuclear complexes with Pd and Cu. Of the two isomers possible (trans-syn and trans-anti) for M2(SIX)2, these complexes crystallize exclusively as the trans-anti isomer. In ansolvous Pd2(SIX)2, the metal-containing planes are approximately parallel, with PdPd 4.416(1) A. Pd2(SIX)2 also forms a crystalline solvate, in which the molecules adopt a more open conformation with longer metal-metal distances (5.109(1) and 5.112(1) A). The M...M distance is significantly longer in Cu2(SIX)2 (6.653(1) A), because of conformational changes in the m-xylylene moieties and substantial tetrahedral distortion about Cu.     

Inclusion [2]complexes based on the cryptand/diquat recognition motif

Seven diquat-based inclusion [2]complexes were studied by proton NMR spectroscopy, electrospray ionization mass spectrometry, and X-ray analysis. The hosts used in these inclusion [2]complexes are bis(5-hydroxymethyl-1,3-phenylene)-32-crown-10, a bis(m-phenylene)-26-crown-8-based cryptand, and five bis(m-phenylene)-32-crown-10-based cryptands. Bis(m-phenylene)-32-crown-10-based cryptands have been proved to be able to complex diquat much more strongly than bis(m-phenylene)-32-crown-10 itself and one containing a pyridyl moiety has one of the highest K_a values reported to date. These hosts form 1:1 complexes with diquat in solution and in the solid state. It was found that the improved binding from bis(m-phenylene)-32-crown-10 to bis(5-hydroxymethyl-1,3-phenylene)-32-crown-10 was due to a supramolecular cryptand structure formed by chelation of the two terminal OH moieties of bis(5-hydroxymethyl-1,3-phenylene)-32-crown-10 with a water molecule as a hydrogen-bonding bridge.     

Synthesis of pendant-type anthraquinone-bridged cofacial dinuclear platinum(II) complexes and their emission properties

Anthraquinone-bridged mononuclear and dinuclear complexes, [PtCl(AQ-amide-tpy)](PF6) (1), [Pt2Cl2(AQ-amide-tpy2)](PF6)2 (2), and [Pt2Cl2(AQ-eth-tpy2)](PF6)2 (3), were synthesized and their photochemical properties were investigated. Amide-bound mononuclear complex 1 exhibited only metal-to-ligand charge transfer (MLCT) absorption and emission, whereas dinuclear complex 2 exhibited a low-energy emission around 700 nm at room temperature. Emission lifetime analysis indicated that this emission was originated from the metal-metal-to-ligand charge transfer (MMLCT) excited state, implying the existence of an intramolecular Pt-Pt interaction at the photoexcited state. 3 with rigid ethynylene linkers showed a low-energy absorption around 520 nm (epsilon = approximately 1100 M(-1) cm(-1)) in addition to an 1MLCT absorption, which was ascribed to a 3MLCT absorption from the consideration of the Pt-Pt distance on a geometry-optimized structure. The emission of 3 appeared at 600 nm, which is higher in energy compared with the emission of 2. It is postulated that the restriction of the Pt-Pt distance flexibility in the rigid structure of 3 prevents the significant increase of the Pt-Pt interaction at the excited state.     

Rapid phosphorus triester hydrolysis catalyzed by bimetallic tetrabenzimidazole complexes

Bimetallic complexes based on the binucleating ligand N,N,N',N'-tetrakis[(2-benzimidazolyl)methyl]-2-hydroxy-1,3-diaminopropane (1L) and its new toluoyl ester derivative (2L) catalyze the hydrolysis of phosphorus triesters at ambient temperature with activities rivalling the fastest known systems.     

New high-selectivity hydrogenation catalysts prepared from bimetallic acetate complexes

The catalytic properties of new Pd-Zn/Al₂O₃ catalysts in selective acetylene hydrogenation in an acetylene-ethylene mixture at 30–120°C and atmospheric pressure are reported. The catalysts prepared from the bimetallic complex Pd-Zn(OOCMe)₄(OH₂) are much more selective than the catalysts prepared by simultaneously supporting the homonuclear complexes Pd₃(OOCMe)₆ and Zn(OOCMe)₂ · 2H₂O. It is demonstrated by diffuse reflectance IR spectroscopy of adsorbed CO that the heat treatment of the supported bimetallic complex at 250°C in flowing H₂ yields a Pd-Zn alloy on the surface. It is this alloy that ensures the high selectivity of the Pd-Zn/Al₂O₃ catalysts.