Synthetic Strategies for Trapping the Elusive trans-Dirhodium(II,II) Formamidinate Isomer: Effects of Cis versus Trans Geometry on the Photophysical Properties

Abstract

The cis- and trans-dirhodium­(II,II) complexes cis-[Rh2(μ-DTolF)2(μ-np)­(MeCN)4]­[BF4]2 (1; DTolF = N,N′-di-p-tolylformamidinate and np = 1,8-naphthyridine), cis- and trans-[Rh2(μ-DTolF)2(μ-qxnp)­(MeCN)3]­[BF4]2 [2 and 3, respectively, where qxnp = 2-(1,8-naphthyridin-2-yl)­quinoxaline], and trans-[Rh2(μ-DTolF)2(μ-qxnp)2]­[BF4]2 (4) were synthesized and characterized. A new synthetic methodology was developed that consists of the sequential addition of π-accepting axially blocking ligands to favor formation of the first example of a bis-substituted formamidinate-bearing trans product. Isolation of the intermediates 2 and 3 provides insight into the mechanistic requirements for obtaining 4 and the cis analogue, cis-[Rh2(μ-DTolF)2(μ-qxnp)2]­[BF4]2 (5). Density functional theory calculations provide support for the synthetic mechanism and proposed intermediates. The metal/ligand-to-ligand charge-transfer (ML-LCT) absorption maximum of the trans complex 4 at 832 nm is red-shifted by 1173 cm–1 and exhibits shorter lifetimes of the 1ML-LCT and 3ML-LCT excited states, 3 ps and 0.40 ns, respectively, compared to those of the cis analogue 5. The shorter excited-state lifetimes of 4 are attributed to the longer Rh–Rh bond of 2.4942(8) Å relative to that in 5, 2.4498(2) Å. A longer metal–metal bond reflects a decreased overlap of the Rh atoms, which leads to more accessible metal-centered excited states for radiationless deactivation. The 3ML-LCT excited states of 4 and 5 undergo reversible bimolecular charge transfer with the electron donor p-phenylenediamine when irradiated with low-energy light. These results indicate that trans isomers are a source of unexplored tunability for potential p-type semiconductor applications and, given their distinct geometric arrangement, constitute useful building blocks for supramolecular architectures with potentially interesting photophysical properties.