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Rhodium Amidinate Dimers as Structural and Functional Hubs for Multimetallic Assemblies

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posted on 06.01.2014, 00:00 by Daniel Chartrand, Garry S. Hanan
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)3L]­PF6 (bpy = 2,2′-bipyridine, L = a pyridyl group on the Rh2 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 Rh2 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 (1M2LCT) transition determined by TD-DFT, red-shifts from 541 nm to 603 nm, while the main near-IR band, a 1Rh2(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.