Kristy M. DeWitt, Tung T. To, and Edwin J. Heilweil


Laser Applications Group, Optical Technology Division, Physics Laboratory

National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8443


            Previous work has shown that the cyclopentadienylmanganese complex (h5-C3H4CH3)Mn(CO)2(3-cyanomethylpyridine) can act as a bistable photochromic organometallic complex via linkage isomerization.[1]  Steady-state room temperature experiments in solution show that the red-orange pyridine nitrogen bound complex, Mn-Py, isomerizes to the yellow cyano bound isomer, Mn-NC, upon visible irradiation.  Further irradiation with UV light reverses ligand coordination back to the pyridine ring, restoring the red-orange solution color. 

A time-resolved transient infrared study was performed to investigate the mechanism and time-scale of this linkage isomerization.  To date, only the visible pathway has been studied in isooctane.  Strong bleach features are observed at 1932 and 1868 cm-1, corresponding to loss of the pyridine-coordinated complex.  An absorption feature at 1985 cm-1 appears immediately and decays completely within 400 ps, while absorption features at 1952 and 1888 cm-1 grow in at a similar rate to the loss of the 1985 cm-1 peak.  Previous FTIR experiments have shown that absorbances corresponding to the final cyano-coordinated complex appear at 2025, 1943, and 1885 cm-1.1

Based on these results we propose the following mechanism.  Following absorption of a 400 nm photon, the cyanomethylpyridine ligand dissociates from the Mn center.  The peak at 1985 cm-1 corresponds to a “bare radical”, triplet state, or some other form of ring-slipped intermediate that relaxes to form an isooctane-solvated complex.  The unstable alkane solvate then converts to the chelate product on the >nanosecond timescale.  Additional confirmation of this mechanism was obtained by comparing to transient infrared studies of methylcyclopentadienyl manganese tricarbonyl (MMT) in isooctane where both MMT and (h5-C3H4CH3)Mn(CO)2(3-cyanomethylpyridine) form an identical transient Mn-alkane solvate (and CO-stretch absorption features) upon UV and visible irradiation, respectively.   

In summary, our results indicate that the linkage isomerization pathway for (h5-C3H4CH3)Mn(CO)2(3-cyanomentylpyridine) from the pyridine-coordinated to the nitrile-coordinated isomer proceeds through a solvated intermediate.  Future work will include studying the reverse isomerization, and exploring the photochemistry of tethered bifunctional ligands to potentially eliminate the solvated intermediate.


Authors:           Kristy M. DeWitt and Tung T. To

Mentor:            Edwin J. Heilweil

Room #:           D101

Address:           100 Bureau Drive Stop 8443

                        Gaithersburg, Maryland 20899-8443

Phone:              301-975-5327

Email:               kdewitt@nist.gov, tung.to@nist.gov, edwin.heilweil@nist.gov

Sigma Xi:          Not members of Sigma Xi

Category:         Chemistry

[1] To, T. T.; Barnes, C. E.; Burkey, T. J.  Organometallics 2004, 23, 2708.