Linkage Isomerization as a Mechanism for Photochromic Materials: Cyclopentadienylmanganese Tricarbonyl Derivatives with Chelatable Functional Groups

Abstract

Two, bifunctional side-chain cyclopentadienylmanganese tricarbonyl complexes, 7 (pyridine and ketone side chain) and 14 (thioamide and pyridine side chain), were prepared and converted to chelates following CO dissociation by UV irradiation. Both pyridine (8) and carbonyl (9) chelates are observed following irradiation of 7. In contrast, only thioamide chelate ( 16) is observed following irradiation of 14 even though a pyridine group was available. Visible irradiation isomerizes the pyridine chelate 8 to the carbonyl chelate 9, which thermally isomerizes back to 8 at 25 °C in a few minutes, demonstrating a photochromic response from a cyclopentadienyl-manganese complex based on a linkage isomerization of a tethered functional group. DFT calculations predicted that the activation enthalpy of thermal linkage isomerization would be 20.8 kcal/mol and that the mechanism is likely an associative process that does not involve a simple dissociation of the manganese bond to the side-chain ketone. The DFT calculations were supported by subsequent dynamic NMR experiments that yielded an activation enthalpy and entropy of 21.4 ( 0.8 kcal/mol and 3.5 ( 0.1 eu, respectively. The studies indicate that a compound with a tethered, coordinated functional group, which is otherwise not labile, can isomerize by a low-energy pathway if there is an appropriate "conduit" to another functional group with a stronger metal-ligand bond. Thus, the preparation of photochromic organometallic complexes based on linkage isomerization will require a bridge that inhibits an associative walk between functional groups if they are to be bistable.

Photochemistry of Chromium Arene Tricarbonyl Complexes with Tethered Pyridinyl and Propenyl Groups: Investigations of the Effect of Ring Size on Chelate Formation, Structure, and Linkage Isomerization

“…), 4.36 (br d, J = 6.6 Hz, 2 H, 2,6-Ph), 4.25 (br t, J = 6.2 Hz, 1 H, 4-Ph), 2.56 (t, J = 7.5 Hz, 2 H, CH 2 ), 1.99 (m, 2 H, CH 2 ), 1.73 (m, 2 H, CH 2 ) 13. C NMR (C 6 D 6 ): δ 233.8, 161.5, 149.8, 135.9, 122.7, 121.2, 113.6, 93.8, 92.6, 90.1, 37.4, 34.5, 30.9.…”

Spectroscopic Studies of Photochemical Transformations of Cymantrenylquinazolinone Derivatives

“…It is also well known that one of the basic properties of cyclopentadienyltricarbonyl manganese (cymantrene) complexes is the possibility of ligand exchange at the manganese atom under UV irradiation leading to a significant change in the spectral properties of the molecules . Earlier we found that the photolysis of cymantrenylalkylamides of carboxylic acids results in the formation of thermodynamically stable dicarbonyl chelates due to the coordination of the carbonyl fragment –NC=O to the Mn atom.…”

How does binding of imidazole-based inhibitors to heme oxygenase-1 influence their conformation? Insights combining crystal structures and molecular modelling

“…The CSD search yielded 14 individual crystal structures, 13 of which are organometallic compounds binding several metallic centres in a metallocene-like coordination mode. The five structures selected [i.e BIXSUY (Pappas & Chirik, 2014), DIVXUC (To et al, 2008), FOLNEA (Kowalski et al, 2005), PUFLAD (Ahmed et al, 1997) and VAZLAK (Wang et al, 2005)] are those for which coordination does not constrain significantly the overall conformation of the molecule. None of these five selected structures contain an imidazole ring.…”