Rhodium Amidinate Dimers as Structural and Functional Hubs for Multimetallic Assemblies Daniel Chartrand and Garry S. Hanan* Department of Chemistry, Universite ́ de Montre ́ al, Montre ́ al, Quebec, H3T 1J4 Canada * S Supporting Information ABSTRACT: The synthesis and characterization of multichromophore assemblies based on a dirhodium tetra-N,N′-diphenylisonicotinamidinate dimer are reported. The pyridyl moieties were used to coordinate up to four positively charged rhenium(I) chromophores of the form fac- [Re(bpy)(CO) 3 L]PF 6 (bpy = 2,2′-bipyridine, L = a pyridyl group on the Rh 2 dimer). The mono-, bis-, tris-, and tetrarhenium assemblies were isolated by size-exclusion chromatography, and their spectroscopic and electrochemical properties were studied and compared with DFT and time-dependent (TD) DFT models of the original rhodium dimer and the mono- and tetrarhenium assembly. The rhenium chromophores modify the properties of the rhodium dimer: for example, the first oxidation of the Rh 2 dimer (Rh-Rh δ* orbital) increased from the original 210 mV versus SCE in acetonitrile, by 45 mV per rhenium complex added, finishing at 390 mV for the tetrarhenium complex. The rhodium dimers display solvatochromism with acetonitrile (MeCN) due to the formation of an axial adduct and has an association constant that increased by a factor of 3.8 when the dimer has four rhenium chromophores. The absorption data clearly exhibited the cumulative effect of the addition of rhenium chromophores in the 230 to 400 nm range. The main visible band, a metal-dimer-to-ligand charge transfer ( 1 M 2 LCT) transition determined by TD-DFT, red-shifts from 541 nm to 603 nm, while the main near-IR band, a 1 Rh 2 (π*→σ*) transition, has a small blue-shift (∼26 cm -1 /Re), varying from 837 to 831 nm upon addition of the four Re(I) chromophores. This was observed in TD-DFT also with a total shift of 105 cm -1 for the tetrarhenium assembly. In terms of emission, the rhenium excited state was completely quenched upon coordination to the dimer, suggesting fast electron transfer of the rhodium dimer. All other aspects of the rhenium chromophore are similar to the parent complex where L = pyridine, showing similar redox couples and additive spectral characteristics. ■ INTRODUCTION The rhodium dimer with its paddle-wheel motif is greatly affected by the nature of its four ligands. 1 Its tetra-acetate and tetracarboxamidinate form are effective catalysts, 2 its bis-acetate form has anticancer properties, 3 and in general it serves as a building block for supramolecular assemblies. 4 Amidinate-based rhodium dimers have a chemistry of their own and have been studied for the last 30 years. 5 They are in general more inert then their acetate analogues, but still possess rich electro- chemistry and photochemistry, making them an interesting choice from which to build polynuclear complexes. 6 Rhenium triscarbonyl diimine chromophores also possess a very rich history due to their high stability, high-energy excited state, and capacity for reductive and oxidative quenching of their excited state. 7 They can also be functionalized to form supramolecular assemblies. 6a,b As such, they are good candidates for functional assemblies, since they can act as photosensitizers that supply electrons for catalysts (e.g., hydrogen evolution), 8 as photocatalysts that reduce carbon dioxide to carbon monoxide, 9 or as photosensitizers for other processes, such as photoisomerization. 10 The rhenium arche- type used in this study is [Re(bpy)(CO) 3 L] + , where bpy = 2,2′- bipyridine and L is a neutral ligand. In natural photosynthetic systems, light energy is gathered by light-harvesting antennae and is subsequently transferred to a reaction center. 11 Our approach to gather light energy involves attaching chromophores to the paddle-wheel motif of the rhodium dimer, thus grafting up to four chromophores in a very close space with a strict 90° angle between each metal center, similar to tetrapyridylporphyrin assemblies. 12 Both tetra- amidinate and tetra-acetate rhodium dimers have been designed to incorporate functional groups able to bond to metal ions. 6a,b,13 The tetra-amidinate dimers were found to be more robust and allowed subsequent reactions to be performed on the metal dimer. 6a We have already demonstrated the use of isonicotinic amidinate rhodium dimers to assemble four rhenium chromophores, but since the dimer themselves are not known as active catalysts, unsaturated assemblies (one to three chromophores) are more promising, as they intrinsically possess further coordination sites to make larger assemblies of chromophores or even complete photocatalytic systems. The properties of these four units, 1 to 4 as seen in Chart 1 (full in Chart S1), are presented herein, with a focus on the effect on the dimer core upon addition of each rhenium chromophore and the coordination of axial acetonitrile. To this end, theoretical calculations were also performed to establish and Received: November 4, 2013 Published: December 17, 2013 Article pubs.acs.org/IC © 2013 American Chemical Society 624 dx.doi.org/10.1021/ic4024585 | Inorg. Chem. 2014, 53, 624-636