METAL-MEDIATED SUPRAMOLECULAR ASSEMBLIES OF PORPHYRINS

2000/2001XIV Ciclo1971Versione digitalizzata della tesi di dottorato cartacea. Nell'originale cartaceo manca la pag. 6

New Insight into a Deceptively Simple Reaction: The Coordination of bpy to RuII-Carbonyl Precursors - The Central Role of thefac-[Ru(bpy)Cl(CO)3]+Intermediate and theChloride Rebound Mechanism

This work demonstrates how a careful reexamination of well-trodden fields can fill conceptual gaps that previously escaped full understanding. The coordination of 2,2'-bipyridine (bpy) to the known Ru(II)-chlorido-carbonyl precursors \u2013 the dinuclear [RuCl2(CO)3]2 (P1) and the polymeric [RuCl2(CO)2]n (P2) \u2013 has been investigated by several groups in the past, and a remarkably large number of ruthenium mono(bpy) carbonyls were identified and fully characterized. Many were investigated as catalysts or key intermediates for the photochemical, electrochemical, and photo-electrochemical reduction of CO2, and for the water\u2013gas shift reaction. Nevertheless, even though most \u2013 if not all \u2013 the reaction products are known already, a careful exam of the literature led us to believe that a convincing general scheme interconnecting them all was still missing and important questions remained unanswered. For this reason, we investigated the reactivity of two mononuclear Ru(II)-carbonyl-dmso precursors, trans,cis,cis-[RuCl2(CO)2(dmso-O)2] (P3) and fac-[RuCl2(CO)3(dmso-O)] (P4) \u2013 that can be considered as \u2018activated forms\u2019 of P2 and P1, respectively \u2013 towards the coordination of bpy. Compounds P3 and P4, allowed us to gain new mechanistic insight and a deeper level of understanding. In particular, we found that coordination of bpy to P4 (or P1) generates first the tricarbonyl cation fac-[Ru(bpy)Cl(CO)3]+.This key intermediate undergoes the facile and selective nucleophilic attack on the CO trans to Cl (by RO\u2013 in alcoholic solvents or OH\u2013 from adventitious water in other solvents), leading to all other species. We also demonstrated that Cl\u2013 \u2013 even when in large excess \u2013 is unable to replace a carbonyl on fac-[Ru(bpy)Cl(CO)3]+. However, the chloride set free from the precursor, competes efficiently with bpy for the coordination to Ru(II) (chloride rebound mechanism)

Synthesis and characterization of trans-di-(4-pyridyl)porphyrin dimers

Preparation and characterization of a small library of symmetric trans-di(4-pyridyl) porphyrin dimers, obtained by either Glaser\u2013Hay or Sonogashira coupling reactions from appropriately prepared trans-di-4-pyridylporphyrin precursors, is presented. The porphyrin dimers are differentiated by a phenyl-alkynyl bridge of increasing length at one meso-position, while for all the derivatives the two remaining opposite meso-positions are tailored with a phenyl moiety bearing a short polyether chain. Coordination of the four pyridyl groups with appropriate metal fragments may be exploited to construct tubular hollow structures, with varied internal sizes, depending on the choice of the porphyrin dimer component

Neutral 1,3,5-Triaza-7-phosphaadamantane-Ruthenium(II) Complexes as Precursors for the Preparation of Highly Water-Soluble Derivatives

The monodentate phosphane ligand 1,3,5-triaza-7-phosphaadamantane (PTA) imparts excellent water solubility to its complexes. We aimed to prepare precursors with one or more PTA coligands for solubility and one or more labile ligands for facile replacement by a linker. For this purpose, we investigated the reactivity of the neutral isomers trans- and cis-RuCl2(PTA)4 (1 and 2) towards 2,2\u2032-bipyridine (bpy), as a model chelating diimine linker. The new derivatives mer-[Ru(bpy)Cl(PTA)3]Cl (9) and fac-[Ru(bpy)Cl(PTA)3]Cl (10) were prepared and characterized. We also found that PTA reacts rapidly with cis,fac-RuCl2(dmso-O)(dmso-S)3 (11) and trans-RuCl2(dmso-S)4 (13) under mild conditions through the replacement of pairs of mutually trans dmso ligands with high selectivity, even when in stoichiometric defect. Thus, 11 affords cis,cis,trans-RuCl2(dmso-S)2(PTA)2 (12), whereas 13 gives 1. The two dmso ligands of 12 can be replaced selectively by chelating diimines such as bpy to afford the less symmetrical all-cis product cis,cis-Ru(bpy)Cl2(PTA)2 (16)

Modulating the Shape of Short Metal-Mediated Heteroleptic Tapes of Porphyrins

In view of developing artificial light-responsive complex systems, the preparation of discrete and robust heteroleptic assemblies of different chromophores in precisely defined positions is of great value since they would allow to investigate directional processes unavailable in symmetrical architectures. Here we describe the preparation, through a modular stepwise approach, and characterization of four novel and robust metal-mediated heteroleptic 4+3 porphyrin tapes, labeled D-4-T-4-D-4, D-3-T-4-D-3, D-4-T-3-D-4, and D-3-T-3-D-3, where a central meso-tetrapyridylporphyrin (either 3 '-TPyP=T-3 or 4 '-TPyP=T-4) is connected to two equal cis-dipyridylporphyrins (either 3 ' cisDPyMP=D-3 or 4 ' cisDPyMP=D-4) through four {t,c,c-RuCl2(CO)(2)} fragments. Whereas D-4-T-4-D-4 is flat, the tapes containing at least one 3 ' PyP, i. e. D-3-T-4-D-3, D-4-T-3-D-4, and D-3-T-3-D-3, have unprecedented - and well defined - 3D geometries, and each exists in solution as a pair of stereoisomers in slow conformational equilibrium. The X-ray molecular structures of two such conformers, the C-shaped (D-3-T-4-D-3)(C) and the z-shaped (D-4-T-3-D-4)(z), were determined and are fully consistent with the solution NMR findings

Self-Assembled Ruthenium(II)Porphyrin-Aluminium(III)Porphyrin-Fullerene Triad for Long-Lived Photoinduced Charge Separation

A very efficient metal-mediated strategy led, in a single step, to a quantitative construction of a new three-component multichromophoric system containing one fullerene monoadduct, one aluminium(III) monopyridylporphyrin, and one ruthenium(II) tetraphenylporphyrin. The Al(III) monopyridylporphyrin component plays the pivotal role in directing the correct self-assembly process and behaves as the antenna unit for the photoinduced processes of interest. A detailed study of the photophysical behavior of the triad was carried out in different solvents (CH2Cl2, THF, and toluene) by stationary and timeresolved emission and absorption spectroscopy in the pico- and nanosecond time domains. Following excitation of the Alporphyrin, the strong fluorescence typical of this unit was strongly quenched. The time-resolved absorption experiments provided evidence for the occurrence of stepwise photoinduced electron and hole transfer processes, leading to a chargeseparated state with reduced fullerene acceptor and oxidized ruthenium porphyrin donor. The time constant values measured in CH2Cl2 for the formation of charge-separated state Ru-Al+-C60 - (10 ps), the charge shift process (Ru-Al+-C60 - \u2192 Ru+-Al-C60 -), where a hole is transferred from Al-based to Ru-based unit (75 ps), and the charge recombination process to ground state (>5 ns), can be rationalized within the Marcus theory. Although the charge-separating performance of this triad is not outstanding, this study demonstrates that, using the self-assembling strategy, improvements can be obtained by appropriate chemical modifications of the individual molecular components

(15)N NMR spectroscopy unambiguously establishes the coordination mode of the diimine linker 2-(2'-pyridyl)pyrimidine-4-carboxylic acid (cppH) in Ru(II) complexes

We investigated the reactivity of three Ru(II) precursors - trans,cis,cis-[RuCl2(CO)2(dmso-O)2], cis,fac-[RuCl2(dmso-O)(dmso-S)3], and trans-[RuCl2(dmso-S)4] - towards the diimine linker 2-(2'-pyridyl)pyrimidine-4-carboxylic acid (cppH) or its parent compound 4-methyl-2-(2'-pyridyl)pyrimidine ligand (mpp), in which a methyl group replaces the carboxylic group on the pyrimidine ring. In principle, both cppH and mpp can originate linkage isomers, depending on how the pyrimidine ring binds to ruthenium through the nitrogen atom ortho (N(o)) or para (N(p)) to the group in position 4. The principal aim of this work was to establish a spectroscopic fingerprint for distinguishing the coordination mode of cppH/mpp also in the absence of an X-ray structural characterization. By virtue of the new complexes described here, together with the others previously reported by us, we successfully recorded (1)H,(15)N-HMBC NMR spectra at natural abundance of the (15)N isotope on a consistent number of fully characterized Ru(ii)-cppH/mpp compounds, most of them being stereoisomers and/or linkage isomers. Thus, we found that (15)N NMR chemical shifts unambiguously establish the binding mode of cppH and mpp - either through N(o) or N(p) - and can be conveniently applied also in the absence of the X-ray structure. In fact, coordination of cppH to Ru(ii) induces a marked upfield shift for the resonance of the N atoms directly bound to the metal, with coordination induced shifts (CIS) ranging from ca. -45 to -75 ppm, depending on the complex, whereas the unbound N atom resonates at a frequency similar to that of the free ligand. Similar results were found for the complexes of mpp. This work confirmed our previous finding that cppH has no binding preference, whereas mpp binds exclusively through N(p). Interestingly, the two cppH linkage isomers trans,cis-[RuCl2(CO)2(cppH-\u3baN(p))] () and trans,cis-[RuCl2(CO)2(cppH-\u3baN(o))] () were easily obtained in pure form by exploiting their different solubility properties

Synthesis and characterization of a hydrophilic conjugated 4+4 Re(I)-porphyrin metallacycle

The preparation and the full characterization, including the X-ray structure determination, of a polar trans-dipyridylporphyrin functionalized with two short polyoxyethylene chains is reported. Reaction of the porphyrin with a Re(I) complex yielded a 4+4 metallacycle showing an improved solubility and a lower tendency to aggregate with respect to analogous porphyrin cyclic derivatives. These properties allowed a full NMR characterization of the metallacycle including VT 1H DOSY-NMR experiments and, for the first time, the recording of a 13C-NMR spectrum giving further insight into the structural definition of these type of metallacycles. Spray deposition of the metallacycle on a heated mica substrate shows the formation of regular ring-like nano-structures which are not formed by the parent porphyrin

Giant Shape-Persistent Tetrahedral Porphyrin System: Light-Induced Charge Separation

Tetraphenylmethane appended with four pyridylpyridinium units works as a scaffold to self-assemble four ruthenium porphyrins in a tetrahedral shape-persistent giant architecture. The resulting supramolecular structure has been characterised in the solid state by X-ray single crystal analysis and in solution by various techniques. Multinuclear NMR spectroscopy confirms the 1 : 4 stoichiometry with the formation of a highly symmetric structure. The self-assembly process can be monitored by changes of the redox potentials, as well as by modifications in the visible absorption spectrum of the ruthenium porphyrin and by a complete quenching of both the bright fluorescence of the tetracationic scaffold and the weak phosphorescence of the ruthenium porphyrin. An ultrafast photoinduced electron transfer is responsible for this quenching process. The lifetime of the resulting charge separated state (800 ps) is about four times longer in the giant supramolecular structure compared to the model 1 : 1 complex formed by the ruthenium porphyrin and a single pyridylpyridinium unit. Electron delocalization over the tetrameric pyridinium structure is likely to be responsible for this effect