Albert Gutiérrez

Antisymbiotic Self-Assembly and Dynamic Behavior of Metallamacrocycles with Allylic Corners

The design and synthesis of discrete supramolecular architectures by metal–ligand self-assembly is an area of current interest. The application of the directional bonding approach has proved to be very versatile, allowing the synthesis of many molecular triangles, squares, rectangles, pentagons, hexagons, and cages. Among them, homometallic molecular squares, formed by the reaction of cis-protected square planar complexes with linear symmetric ditopic ligands, have been the most widely studied. The assembly of metallamacrocycles containing different metal corners and/or nonsymmetric ligands using this method presents additional difficulties due to the possible generation of a mixture of isomers that are difficult to separate. To overcome this, they are generally obtained through modular self-assembly methodology. This strategy requires the initial synthesis of mononuclear complexes with strong covalently bound ligands that have additional donor sites available for coordination to further acceptor building blocks. Nevertheless, several molecular triangles and squares have been recently assembled following the directional bonding approach by using nonsymmetric linkers and identical metal corners. Interestingly, in some cases, unexpected self-selection of a single linkage isomer was observed. To analyze the large quantity of metallamacrocycles reported to date, the following classification is proposed, according to the nature of the assembled units: 1) species containing identical metal corners and symmetric linkers; 2) species containing identical metal corners and nonsymmetric linkers; and 3) species containing different metal corners and nonsymmetric linkers. Herein, we report the use for the first time of the directional bonding approach for the synthesis of metallamacrocycles displaying two different metal corners connected by a nonsymmetric ditopic linker. By the correct choice of the different units, that is, metal corners and ambidentate ligand, we have been able to promote its self-assembly producing molecular square architectures and, more interestingly, achieving the self-selection of a single linkage isomer. Evidently, the correct choice of a heteroditopic ligand in which the different (stereo)electronic characteristics of the donor groups could control the reactivity at the metal site is decisive. Thus, we focused our attention on the 4-PPh2py ligand (py=pyridine), given its hard-soft character. Furthermore, the electronic nature of the ancillary ligands in the metal corners on the proposed assembly is also crucial for the self-selection process. As a result, complex [Pd(h-2Me-C3H4) ACHTUNGTRENNUNG(cod)] ACHTUNGTRENNUNG(CF3SO3) was chosen as the source of {Pd(h-2-Me-C3H4)} + , a potential metal corner that could provide a varied reactivity to the system, as expected for the allyl palladium species. On the other hand, the complex [PdACHTUNGTRENNUNG(H2O)2 ACHTUNGTRENNUNG(dppp)] ACHTUNGTRENNUNG(CF3SO3)2 was chosen as the second metal corner supplier, {Pd ACHTUNGTRENNUNG(dppp)}2+ (dppp=bis(diphenylphosphino)propane). The selected metal fragments were expected to coordinate selectively to each of the donor atoms of the heteroditopic 4-PPh2py ligand. Following Pearson s antisymbiotic effect, the greatest stability is expected when the softest ligands are bonded trans to the hardest ones. With these three components in our hands, that is, [Pd(h2-Me-C3H4) ACHTUNGTRENNUNG(cod)] ACHTUNGTRENNUNG(CF3SO3), [Pd ACHTUNGTRENNUNG(H2O)2ACHTUNGTRENNUNG(dppp)] ACHTUNGTRENNUNG(CF3SO3)2, and 4-PPh2py, the self-assembly was explored in solution in dichloromethane at room temperature (Scheme 1). After several minutes, the color of the solution faded away and the P NMR spectrum (Supporting Information, Figure S1) showed two signals associated with the P atoms of the dppp ligand and those bonded to the allylpalladium fragment, respectively, indicating the presence of only one species in solution. This species, the targeted macrocycle 1, was obtained [a] Dr. I. Angurell, Dr. M. Ferrer, Dr. A. Guti rrez, Prof. M. Mart nez, Dr. L. Rodr guez, Prof. O. Rossell Departament de Quimica Inorg nica, Universitat de Barcelona Mart i Franqu s 1-11, 08028 Barcelona (Spain) Fax: (+34)934907725 E-mail : montse.ferrer@qi.ub.es [b] Dr. M. Engeser Kekul -Institut f r Organische Chemie und Biochemie der Universit t, Gerhard-Domagk-Str.1, 53121 Bonn (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201002605.

Kinetico-mechanistic insights on the assembling dynamics of allyl-cornered metallacycles: the Pt-Npy bond is the keystone.

The square-like homo- and heterometallamacrocycles [{Pd(η(3) -2-Me-C3 H4 )(L(n) )2 }2 {M(dppp)}2 ](CF3 SO3 )6 (dppp=1,3-bis(diphenylphosphino)propane) and [{Pd(η(3) -2-Me-C3 H4 )(L(1) )2 }2 {M(PPh3 )2 }2 ](CF3 SO3 )6 [py=pyridine, M=Pd, Pt, L(n=) 4-PPh2 py (L(1) ), 4-C6 F4 PPh2 py (L(2) )] containing allyl corners were synthesised by antisymbiotic self-assembly of the different palladium and platinum metallic corners and the ambidentate N,P ligands. All the synthesised assemblies displayed a complex dynamic behaviour in solution, the rate of which is found to be dependent on the electronic and/or steric nature of the different building blocks. A kinetico-mechanistic study by NMR line shape analysis of the dynamics of some of these assemblies was undertaken in order to determine the corresponding thermal activation parameters. Both an enhanced thermodynamic stability and slower dynamics were observed for platinum-pyridine-containing species when compared with their palladium analogues. Time-dependent NMR spectroscopy in combination with ESI mass spectrometry was used to study the exchange between the assemblies and their building blocks, as well as that occurring between different metallamacrocycles. Preliminary studies were carried out on the activity of some of the metallamacrocyclic compounds as catalytic precursors in the allylic substitution reaction, and the results compared with that of the monometallic allylic corner [Pd(η(3) -2-Me-C3 H4 )(L(1) )2 ](+) .

Bond Dissociation Energies of Metallo-supramolecular Building Blocks: Insight from Fragmentation of Selectively Self-Assembled Heterometallic Metallo-supramolecular Aggregates

A series of selectively self-assembled metallo-supramolecular square-like macrocycles with unsymmetric ditopic linkers and two different types of metal corners, i.e., {Pd(η3-2-Me-C3H4)} and {M(dppp)} with dppp = 1,3-bis(diphenylphosphino)propane and M = Pd2+ or Pt2+, have been studied in the gas phase using collision-induced dissociation. The aggregates show distinct fragmentation patterns determined by ligand length, i.e, aggregate size, and type of metal corner. Information on relative binding strength can be deduced. This is of particular interest for (methylallyl)Pd as a relatively new building block in metallo-supramolecular chemistry. The phosphane end of the unsymmetric ligand connected to (η3-2-Me-C3H4)Pd is bound significantly stronger than its pyridine end to (dppp)Pt and (dppp)Pd. These results are corroborated by DFT calculations.