Sequential, kinetically controlled synthesis of multicomponent stereoisomeric assemblies.

The reversibility of noncovalent and metal–ligand interactions has widely been exploited to synthesize a plethora of supramolecular and coordination-based assemblies under thermodynamic control. Occasionally, entrapment in local energy minima leads to the formation of metastable products, which are often converted into lower-energy products upon prolonged reaction times. In contrast, self-assembled products in nature almost always arise according to the most expedient reaction pathway, that is, the kinetically selected. Herein we demonstrate a kinetically controlled approach to self-assembly, in which the sequence of addition of molecular structural units leads to the stereoselective formation of metallosupramolecular isomers. The use of platinum(II) (and other third-row transition metals) is particularly well-suited to a kinetic approach to self-assembly, not just because Pt–ligand bonds can be kinetically inert, but also because the metal ion can be conveniently tuned to produce a vast range of different ligand-exchange labilities. For instance, assemblies that utilize bis(phosphine) ligands as corner protecting groups often readily assemble at room temperature, while those that exploit neutral N-donor bidentate ligands, such as ethylene diamine, typically require several hours at elevated temperature to reach equilibrium. Furthermore, the mechanism of labilization, that is, the trans effect, is such that it is possible for a single metal center to possess cis exchangeable sites with dramatically different kinetic properties. We have recently prepared a metallosupramolecular trigonal prism that possesses an unsymmetrical cyclometalated C _ N corner protecting group, which was assembled in two steps by treating [LPt(dmso)] (where H2L = 2,6-diphenylpyridine) sequentially with 4,4’-bipy and tpt.3CSA (Scheme 1, steps a and b). The isolation of a single isomeric product from a possible fourteen products, as indicated by the H NMR spectrum of the hexa-PF6 salt (see the Supporting Information, Figure S1a), led us to ask, was this selectivity a result of each Pt center possessing one labile and one inert exchangeable site, or was the selectivity thermodynamic in origin? To answer this question, the sequence in which 4,4’-bipy and tpt were added to [LPt(dmso)] was reversed. [LPt(dmso)] was first treated with a third of an equivalent of tpt at room temperature in CH2Cl2 to give [(L Pt)3(tpt)] (Scheme 1, step c), which was then treated with 4,4’-bipy·2CSA, and after metathesis with NH4PF6, the hexa-PF6 salt was isolated in 99% yield (Scheme 1, step d). The H NMR spectrum of this product (see the Supporting Information, Figure S1b) also indicated the formation of a single species, yet there were clear differences between the spectra of the two isomers, in particular, for resonances HA-E. The product from route 1 was assigned as cis-[(HLPt)6(4,4’-bipy)3(tpt)2](PF6)6, in which the tpt ligand is coordinated cis to the nitrogen of the 2,6diphenylpyridine ligand, and the product from route 2 was assigned as trans-[(HLPt)6(4,4’-bipy)3(tpt)2](PF6)6, in which the tpt ligand is coordinated trans to the nitrogen of the 2,6diphenylpyridine ligand. This absolute assignment was made on the basis that a) the resonance of the ortho proton of tpt (HC) is more deshielded in the trans-to-nitrogen coordination site in comparison to the ortho proton 4,4’-bipy (HB), and b) the large and small relative separations between the resonances of HA and HB, and between the resonances of HC and HD. Scheme 1. Sequence-specific control over the formation of metallosupramolecular stereochemical isomers. a) 4,4’-bipy, CH2Cl2, RT, 18 h, 77%; b) (i) tpt·3CSA, CH2Cl2, RT, 1 h; (ii) NH4PF6, 97%; c) tpt, CH2Cl2, 18 h, 85%; d) (i) 4,4’-bipy·2CSA, CH2Cl2, RT, 3 h; (ii) NH4PF6, 99%. bipy= bipyridine, CSA=camphorsulfonic acid, dmso=dimethylsulfoxide, tpt= tris(4-pyridyl)triazine.