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.

We review recent studies of processes relevant to photoinduced linkage isomerization of organometallic systems with the goal of preparing organometallics with an efficient and ultrafast photochromic response. The organometallic system thus corresponds to two linkage isomers with different electronic environments that are responsible for different optical properties. Much of this work has focused on examining processes following irradiation of cyclopentadienyl manganese tricarbonyl derivatives (compounds 3-21) including solvent coordination, thermal relaxation, solvent displacement by tethered functional groups (chelation), dissociation of tethered functional groups, and linkage isomerization. A new platform is investigated for obtaining a photochromic response in new experiments with arene chromium dicarbonyl complexes. A photochromic response is observed for arene chromium dicarbonyl complexes with tethered pyridine and olefin functional groups based on light-driven linkage isomerization on the nanosecond time scale. Irradiation at 532 nm of 23 ([Cr{eta(6)-C(6)H(5)CH(2-Py-kappaN)CH(2)CH=CH(2)}(CO)(2)]) (Py = pyridine) results in the isomerization to 22 ([Cr{eta(6)-C(6)H(5)CH(2-Py)CH(2)-eta(2)-CH=CH(2)}(CO)(2)]), and 355 nm irradiation isomerizes 22 to 23. The ultrafast linkage isomerization has been investigated at room temperature in n-heptane solution on the picosecond to microsecond time scale with UV- or visible-pump and IR-probe transient absorption spectroscopy by comparing the dynamics with model compounds containing only a tethered pyridine. Irradiation of 24 ([Cr{eta(6)-C(6)H(5)(CH(2))(3)(2-Py)}(CO)(3)]) and 25 ([Cr{eta(6)-C(6)H(5)(CH(2))(2)(2-Py)}(CO)(3)]) at 289 nm induces CO loss to immediately yield a Cr-heptane solvent coordinated intermediate of the unsaturated Cr fragment, which then converts to the kappaN(1)-pyridine chelate within 200 and 100 ns, respectively. Irradiation of 26 ([Cr{eta(6)-C(6)H(5)CH(2)(2-Py)}(CO)(3)]) also induces CO loss to immediately yield three species: the Cr-heptane solvent coordinated intermediate, a kappaN(1)-Py nitrogen chelate, and an agostic eta(2)-chelate in which the pyridine is coordinated to the metal center via a C-H agostic bond as opposed to the nitrogen lone pair. Both the transient Cr-heptane coordinated intermediate and the agostic pyridine chelate convert to the stable kappaN(1)-pyridine chelate within 50 ns. Similar reaction dynamics and transient species are observed for the chelate 33 ([Cr{eta(6)-C(6)H(5)CH(2)(2-Py)-kappaN}(CO)(2)]) where a Cr-Py bond, not a Cr-CO bond, initially cleaves.

Cyclopentadienylmanganese complexes of the general formula (η5-C5H4CH3)Mn(CO)2L, where L is a nonchelatable, bifunctional ligand, were found to be photochromic. Irradiation of (η5-C5H4CH3)Mn(CO)2L(3-cyanomethylpyridine) with alternating visible and UV light produced alternating yellow and red solutions, and fatigue of this response was inhibited when free 3-(cyanomethyl)pyridine was present during irradiation. Similar results were observed when L is pyridine in the presence of dispersed acetonitrile. Irradiation of (η5-C5H4CH3)Mn(CO)3 and a pyridine derivative RC5H4N (R = 3-CH2CN, 2-CH2CN, 4-CHCHPh, 4-CHCH2) generated (η5-C5H4CH3)Mn(CO)2L in situ, which likewise showed a photochromic response. The results demonstrate that the linkage isomerization occurs by unimolecular and bimolecular processes and that linkage isomerization is an effective photochromic mechanism for organometallics.