Engineering femtosecond organometallic chemistry: Photochemistry and dynamics of ultrafast chelation of cyclopentadienylmanganese tricarbonyl derivatives with pendant benzenecarbonyl and pyridinecarbonyl groups

Article abstract of DOI:10.1021/om2003656

The chelation following photodissociation of CO for cyclopentadienyl manganese tricarbonyl derivatives with a bifunctional side chain has been investigated. Previous studies show that steady-state irradiation of 1 (Mn{μ⁵-C₅H₄CH₂COR}-(CO) ₃, R = 2-pyridyl) leads to CO dissociation and formation of O-chelate 2 with smaller amounts of N-chelate 3. Subsequently, 2 rearranges thermally to 3. A new preparation for 1 is reported, while analogues 4 (R = phenyl) and 5 (R = 4-pyridyl) are prepared for the first time. Steady-state UV-vis, FTIR, and NMR studies of 4 and 5 in heptane demonstrate that O-chelates 6 and 7, respectively, are formed with the side-chain oxygen but decay on the minute time scale. The linkage isomerization of O-chelate 2 to 3 is faster than the decay observed for the O-chelate 6 (R = 2-pyridyl versus Ph), even in the presence of 0.1 M pyridine for the latter. Following irradiation of 4 during time-resolved infrared studies in heptane, ultrafast O-chelation is observed but not ultrafast solvent coordination. Ultrafast O-chelation is also observed for 5 along with an unidentified transient. Following irradiation of 1, ultrafast O-and N-chelation are observed, to the exclusion of ultrafast solvent coordination. This result suggests that chelate formation is a subpicosecond process and that both chelates are formed independently. A split in the otherwise degenerate stretching band s for 4 and 5 in FTIR spectra suggests that there is significant electronic communication between the side chain and the metal carbonyl groups. The results suggest that ultrafast chelation is favored by side-chain conformations that position a functional group near the metal center.

Full text of DOI:10.1021/om2003656

ARTICLE
pubs.acs.org/Organometallics
Engineering Femtosecond Organometallic Chemistry: Photochemistry
and Dynamics of Ultrafast Chelation of Cyclopentadienylmanganese
Tricarbonyl Derivatives with Pendant Benzenecarbonyl and
Pyridinecarbonyl Groups
Edwin J. Heilweil,‡ Jermaine O. Johnson,† Karen L. Mosley,† Philippe P. Lubet,† Charles Edwin Webster,*,†
and Theodore J. Burkey*,†
†Department of Chemistry, The University of Memphis, Memphis, Tennessee 38152-3550, United States
‡Optical Technology Division, Physics Laboratory, National Institute of Standards and Technology, Gaithersburg,
Maryland 20899-8443, United States
S Supporting Information
b
ABSTRACT: The chelation following photodissociation of CO
for cyclopentadienyl manganese tricarbonyl derivatives with a
bifunctional side chain has been investigated. Previous studies
show that steady-state irradiation of 1 (Mn{η5-C5H4CH2COR}-
(CO)3, R = 2-pyridyl) leads to CO dissociation and formation
of O-chelate 2 with smaller amounts of N-chelate 3. Subsequently, 2 rearranges thermally to 3. A new preparation for 1 is reported,
while analogues 4 (R = phenyl) and 5 (R = 4-pyridyl) are prepared for the first time. Steady-state UVꢀvis, FTIR, and NMR studies
of 4 and 5 in heptane demonstrate that O-chelates 6 and 7, respectively, are formed with the side-chain oxygen but decay on the
minute time scale. The linkage isomerization of O-chelate 2 to 3 is faster than the decay observed for the O-chelate 6 (R = 2-pyridyl
versus Ph), even in the presence of 0.1 M pyridine for the latter. Following irradiation of 4 during time-resolved infrared studies in
heptane, ultrafast O-chelation is observed but not ultrafast solvent coordination. Ultrafast O-chelation is also observed for 5 along
with an unidentified transient. Following irradiation of 1, ultrafast O- and N-chelation are observed, to the exclusion of ultrafast
solvent coordination. This result suggests that chelate formation is a subpicosecond process and that both chelates are formed
independently. A split in the otherwise degenerate stretching bands for 4 and 5 in FTIR spectra suggests that there is significant
electronic communication between the side chain and the metal carbonyl groups. The results suggest that ultrafast chelation is
favored by side-chain conformations that position a functional group near the metal center.
’ INTRODUCTION
In many cases photochelation competes with ultrafast ligand
recombination and solvent coordination, which occur in a pico-
second or less.6 Our studies have shown that chelation generally
occurs on two time scales.4,5 The first time scale is subpicose-
cond, where chelation competes with ligand recombination and
solvent coordination. For some compounds the chelation occurs to
the exclusion of both ligand recombination and solvent coordina-
tion even when a solvent such as acetonitrile makes strong and
kinetically stable bonds with the metal center.5a The second time
scale is about 100 ns for species in alkane solvents where a co-
ordinated solvent molecule must be displaced for chelation to occur.
The time scale is often longer in more reactive solvents where a
strongly coordinated solvent must be displaced. Since ultrafast
solvent coordination and ligand recombination can lead to unpro-
ductive processes and delays for light-driven molecular devices, it is
important to understand how these processes can be eliminated.
UV irradiation of cyclopentadienyl manganesetricarbonylderi-
vatives with pendant functional groups leads to chelate formation
When a coordinated functional group is part of a chelate ring,
strain and conformational restrictions can be expected to affect
bond energies, vibrational and electronic transitions, and the
rates of addition or dissociation of the functional group. We are
interested in designing efficient photochromic organometallics
based on the mechanism of a photoreversible linkage isomeriza-
tion where chelation following the photodissociation of a ligand
is a key process. Understanding how structural features influence
chelate stability or its rate of formation is invaluable for this
project.1 An early study of photochelation reported that a series
of CpMn(CO)(η1-PꢀP), where PꢀP were diphosphines, all
formed four- to six-membered rings within 100 ns following
irradiation.2 The quantum yield (0.6) for chelate formation was
independent of ring size, indicating that the product-determining
step preceded chelation. In contrast, the quantum yield for
chelation of (η5-C5H4R)Mn(CO)3 (R = COCH2OCH3, CO-
CH2SCH3, CO(CH2)2SCH3, CH2CO2CH3, (CH2)2CO2CH3)
varied with the structure of R (0.6ꢀ1.0),3 and the rates of
Received: May 2, 2011
Published: October 24, 2011
chelation varied from 2 ꢁ 106 to greater than 5 ꢁ 109 sꢀ1 4,5
.
r
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Scheme 1. The Photochrome 2/3 Produced Following Irra-
diation of 1
of nitrogen-purged 1.2 M HCl was cannulated into the flask. Sodium
bicarbonate (5%, 10 mL) was added, and the solution was extracted with
three 50 mL aliquots of diethyl ether. The combined ether layers were
back-extracted with two 50 mL aliquots of water and dried overnight with
magnesium sulfate. The ether was reduced by 30% before recrystalliza-
tion at 4 °C (4.92 g, 52% yield). 1H NMR (270 MHz, CDCl3): δ (ppm)
4.20(s, 2 H, CH2), 4.67(m, 2 H, 3,4-Cp), 4.85 (m, 2 H, 2,5-Cp), 7.49 (m,
1 H, 5-py), 7.84 (m, 1 H, 4-py), 8.05 (m, 1 H, 3-py), 8.70 (m, 1 H, 6-py).
Synthesis of 4, Tricarbonyl[η5-(2-oxo-2-phenyl)ethyl)cy-
clopentadienyl]manganese. The procedure was the same as for 1
except where noted: potassium tert-butoxide (4.5 g, 40 mmol), methyl
cymantrene (6.3 mL, 40 mmol), and ethyl benzoate (7.1 mL, 50 mmol)
instead of ethyl picolinate. The mixture was stirred for 24 h. The ether
was evaporated with a nitrogen stream to provide a solid product, which
was dissolved in 150 mL of anhydrous diethyl ether outside the glove-
box. The solution was washed with water (3 ꢁ 25 mL), dried with
MgSO4, filtered, and gradually cooled to ꢀ16 °C. A yellow product
formed overnight (1.56 g, 32%). Overall yield: 23%. Elemental analysis
found for C16H11MnO4, MW = 322.19: analysis (calculated) C 59.45
(59.64) H 3.80 (3.44). 1H NMR (270 MHz, CDCl3): δ (ppm) 3.94 (s,
1 H), 4.72 (m, 2 H), 4.78 (m, 2 H), 7.49 (t, J = 7.4 Hz, 2 H), 7.60 (t, J =
7.4 Hz, 1 H), and 7.98 (d, J = 7.4 Hz, 2 H). 13C NMR (67.5 MHz,
CDCl3): δ (ppm) 37.79, 82.25, 84.43, 97.35, 128.43, 128.97, 133.78,
136.09, 195.92, and 225.07. FTIR (NaCl, heptane): 2024 (4 cmꢀ1
fwhm), 1946 and 1938 cmꢀ1 (apparent 13 cmꢀ1 fwhm for overlapping
peaks).
Scheme 2
Synthesis of 5. Tricarbonyl[η5-(2-oxo-2-(4-pyridyl)ethyl)-
cyclopentadienyl]manganese. The procedure was the same as for
1 except where noted: potassium t-butoxide (2.3 g, 20.5 mmol), methyl
cymantrene (2.5 mL, 14.7 mmol), and ethyl isonicotinate (1.8 mL, 13.4
mmol) instead of ethyl picolinate. After workup, the solid residue was
crystallized in 30 mL of argon-purged toluene (2.12 g, 49% yield). Ele-
mental analysis found for C15H10MnNO4, MW = 323.18 (calculated): C
55.78 (55.75) H 3.02 (3.12) N 4.29 (4.33). 1H NMR (270 MHz,
CDCl3): δ (ppm) 3.97 (s, 2 H, CH2), 4.76 (br s, 2 H, 3,4-Cp), 4.81 (br s,
2 H, 2,5-Cp), 7.77 (d, 5.0 Hz, 2 H, 3,5 pyr), 8.87 (d, 5.0 Hz, 2 H, 2,6 pyr).
13C NMR (67.5 MHz, CDCl3): δ (ppm) 38.02, 82.45, 84.49, 95.79,
121.23, 141.95, 151.37, 195.48, and 225.00. FTIR (NaCl, heptane):
2026 (4 cmꢀ1 fwhm), 1949 and 1940 cmꢀ1 (apparent 17 cmꢀ1 fwhm for
overlapping peaks).
Ligand Substitution of O-Chelate from 4 with Pyridine
Monitored via UVꢀVis Spectroscopy. In a typical experiment, a
solution of 4 (3.2 mg) in 10 mL of heptane (1 mM) was stirred overnight
in a glovebox. Two milliliters were transferred to a quartz screw cap
cuvette (l = 1 cm), and a UVꢀvis spectrum was recorded. The cuvette
was positioned 5 cm from an RPR-3000 lamp and irradiated (60 s) until
A635 (absorbance at 365 nm) reached 1.0 OD. The pyridine concentra-
tion was brought to 10 mM by injecting 10 μL of 2 M pyridine into the
irradiated solution, and spectra were obtained at 30 s intervals. The in-
tercepts for Figures 7, 8, 10, and 11 are within experimental error of zero.
FTIR Spectra of Irradiated Samples. In a typical experiment, 4
(29 mg) in 10 mL of heptane (9 mM) was irradiated in a 0.1 mm path
length CaF2 cell at room temperature. The cell was irradiated for 30 s
twice and then 60 s with a Pen Ray lamp. Readings were taken im-
mediately (<5 s) after irradiation and 4 min after irradiation. The cell was
positioned 10 cm away from the lamp.
for esters,3 sulfides,3 pyridines,7 ketones,7 nitriles,8 alkenes,8 al-
kynes,8 and carbamates.9 In the case of 1 (Scheme 1), a photo-
chrome10 is produced based on blue (2) and purple (3) linkage
isomers.7 2 is unstable at room temperature and rearranges to 3
within 5 min but can be reversed by irradiation with visible light.
At the lowest temperatures and shortest irradiation times, the
ratio of 2 to 3 increases to 3:1 and suggests that irradiation
initially favors the formation of 2. An important issue is the extent
that both isomers are formed from an excited state directly, or by
photoisomerization via secondary photolysis, or by thermal
isomerization following irradiation. In this study, steady-state
irradiation studies of analogues that are not capable of forming an
N-chelate (4 and 5, Scheme 2) and ultrafast, time-resolved
infrared studies (TRIR) of 1, 4, and 5 are used to identify
transient species following irradiation, to understand the selec-
tivity for isomer formation, and to measure isomerization rates.
’ EXPERIMENTAL SECTION
General Conditions. Unless otherwise noted, all reactions were
carried out under nitrogen or argon in a glovebox or Schlenk flask, and all
steady-state irradiations were conducted with a single RPR-3000 UV
lamp (Southern New England Ultraviolet Company) or Pen Ray lamp
(UVP). Infrared spectra were obtained with a Thermo Nicolet 380.
UVꢀvis spectra were obtained with an Agilent 8453 spectrophotometer
(A0 = absorbance at time = 0, At = absorbance at time t).11
TRIR Spectroscopy. The transient absorption apparatus at NIST
has been described in detail previously.5 Spectra were averaged for
1000ꢀ3000 laser shots. For 1, nanosecond spectra were obtained fol-
lowing 266 nm irradiation, and picosecond spectra were obtained
following 289 nm irradiation. For 4 and 5, all picosecond and nanose-
cond spectra were obtained following 355 nm irradiation. In typical
experiments, 1 (23 mg) was dissolved in 80 mL of heptane (0.85 mM), 4
Synthesis of 1, Tricarbonyl[η5-(2-oxo-2-pyridyl)ethyl)-
cyclopentadienyl]manganese. Potassium tert-butoxide (6.1 g,
55 mmol) was dissolved in anhydrous DMF (20 mL) in a 100 mL Schlenk
flask, followed by methyl cymantrene (5 mL, 32 mmol) and argon-
purged ethyl picolinate (5 mL, 35 mmol). After stirring for 12 h, 50 mL
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Scheme 3. Irradiation of 4
Figure 2. NMR spectra for low-temperature irradiation of 4 in toluene-
d8: (lower) at room temperature before irradiation; (upper) 5 min total
irradiation at ꢀ20 °C. The probe temperature was 25 °C during
acquisition.
Figure 1. UVꢀvis spectra of 3 mM 4 in heptane at room temperature
following UV irradiation.
Figure 3. FTIR spectral changes after 30 s irradiation of 9 mM 4 in
heptane at room temperature. Difference spectra were recorded 0, 3, and
60 min after irradiation.
(39 mg) in 75 mL of heptane (1.6 mM), and 5 (21 mg) in 50 mL of
heptane (1.3 mM).
method.7 The structure of 1 was confirmed by comparing spectra
with published results. Lower yields (<30%) were obtained in
DMSO, and a similar procedure using BuLi in THF at ꢀ78 °C
produced no detectable product. 4 and 5 were likewise prepared
in DMF, although a lower isolated yield was obtained for 4.
Steady-State and Flash Photolysis of 4. The formation of
intermediates and products following irradiation of 4 (Scheme 3
and TOC figure) were identified using UVꢀvis, NMR, and IR
spectroscopies. A 3 mM heptane solution of 4 is faint yellow with
little visible absorbance (Figure 1). UV irradiation changes the
solution to blue with a peak forming at 635 nm and decaying with
a half-life of 24 h at 23 °C. UV irradiation of 4 in toluene-d8
likewise changes the solution to blue as new NMR peaks appear;
some peaks are obscured by those for solvent and 4, while peaks
at 6.8, 5.1, 3.2, and 2.8 ppm are well resolved (Figure 2, area ratios
of the latter three peaks are 1:1:1). UV irradiation of 4 in heptane
led to bleaching of starting material peaks at 2024 and 1946/
1938 cmꢀ1 (overlapping) in difference FTIR spectra and new
absorption peaks at 1948 and 1887 cmꢀ1 (Figure 3). After
60 min in the dark at room temperature, the peaks at 1948 and
1887 cmꢀ1 decreased by 50%, and the bleached peaks similarly
recovered. Difference TRIR spectra for 4 reveal the peak at
1887 cmꢀ1 appears in less than 100 ps with no change to 1 μs
(Figure 4). In the presence of 1 mM pyridine, the bleach recovery
is almost completely inhibited, but the absorption peak at
1887 cmꢀ1 decays by 50% in 70 min, while two additional peaks
Computational Methods. Theoretical calculations have been
carriedoutusing the Gaussian0912 implementation of PBEPBE(Perdewꢀ
BurkeꢀErnzerhof exchange correlation functional13) density functional
theory14 using the default pruned fine grids for energies (75, 302),
default pruned coarse grids for gradients and Hessians (35, 110) (neither
grid is pruned for manganese), and nondefault SCF convergence for
geometry optimizations (10ꢀ6). The basis set for manganese was the
Hay and Wadt basis set and effective core potential combination
(LANL2DZ)15 as modified by Couty and Hall, where the two outermost
p functions have been replaced by a split of the optimized 4p functions,16
supplemented with a single set of f polarization functions.17 The 6-
31G(d0)18,19 basis sets were used for all other atoms. The density fitting
approximation20 for the fitting of the Coulomb potential was used for all
calculations; auxiliary density-fitting basis functions were generated auto-
matically (by the procedure implemented in Gaussian 09.A02) for the
specified AO basis set. Spherical harmonic d and f functions were used
throughout; that is, there are five angular basis functions per d function and
seven angular basis functions per f function. All structures were fully
optimized, and analytical frequency calculations were performed on all
structures to ensure stationary points are a zeroth-order saddle point (a local
minimum), or first-order saddle point (a transition state) was achieved.
’ RESULTS
Synthesis. A new synthesis of 1 in DMF based on the acy-
lation of the methyl cymantrene carbanion is a considerable im-
provement in yield (50%) and purification over the previous
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Figure 4. TRIR difference spectra acquired after 355 nm pulsed laser
irradiation of 1.6 mM 4 in heptane at 25 °C. The spectra are offset
vertically by 0.05 OD units. Bleaches and absorptions at higher
wavenumbers were not recorded. The FTIR difference spectrum was
obtained after steady-state UV irradiation.
Figure 7. Plot of ln(A0/At) at 635 nm after irradiation of 1 mM 4 and
addition of 0.1 M pyridine in heptane. kobs = 3.95(0.03) ꢁ 10ꢀ3 sꢀ1
R = 0.9995, baseline correction = 0.005.
,
Figure 5. FTIR spectral changes after 1 min irradiation of 1 mM 4 with
1 mM pyridine in heptane at 25 °C. Difference spectra were recorded
0, 10, 30, 50, and 70 min after irradiation.
Figure 8. Plot of kobs versus [pyridine] monitored at 635 nm for
irradiation of 4 in heptane, k1 = 3.8(0.2) ꢁ 10ꢀ2 Mꢀ1 sꢀ1, R = 0.997.
first- or second-order kinetics, and the rate of decay actually
increased with time (Figures 1S and 2S, Supporting Information).21
In the presence of 0.1 M pyridine, the decay of A635 displayed first-
order kinetics (Figure 7), and an isosbestic point is observed at
476 nm (Figures 3S and 4S, Supporting Information). Plots of
ln(A0/At) versus time at 635 or 680 nm were linear for pyridine
concentrations of 1ꢀ100 mM (Figures 5Sꢀ17S, Supporting
Information). The observed rate constants obtained from the slopes
of these plots were plotted versus pyridine concentration (Figure 8)
to yield a slope of k1 = 3.8(0.2) ꢁ 10ꢀ2 Mꢀ1 sꢀ1
.
Steady-State and Flash Photolysis of 5. Results for 5 were
similar to 4 (Scheme 4). The steady-state UV irradiation of a pale
yellow solution containing 5 changed to blue as a peak at 704 nm
increased with irradiation time (Figure 18S, Supporting In-
formation) and then decreased by >50% in the dark in 30 min
(Figure 9). A plot of ln(A0/At) versus time for the decay at
700 nm after irradiation of 5 yielded kobs = 5.1(0.1) ꢁ 10ꢀ4 sꢀ1 in
the absence of pyridine (Figure 10) and 4.9(0.1) ꢁ 10ꢀ4 sꢀ1 in the
presence of 1 mM pyridine (Figure 11). Difference FTIR spectra of
5 after UV irradiation at ꢀ10 °C reveal bleaches at 2027, 1948/
1940 cmꢀ1 (overlapping) and new peaks at 1952 and 1894 cmꢀ1
(Figure 12). As the new peaks decay (50% in 100 min), the
bleached peaks only partially recover and two new broad absorp-
tion features grow from 1907 to 1930 cmꢀ1 and 1870 to 1840 cmꢀ1
Figure 6. FTIR spectral changes after 1 min irradiation of 1 mM 4 with
10 mM pyridine in heptane at 25 °C (CaF2, 0.95 mm). Difference
spectra were recorded 0, 5, 10, 15, and 20 min after irradiation.
appear at 1932 and 1865 cmꢀ1 (Figure 5). These latter peaks
grow more rapidly in the presence of 10 mM pyridine, and the
50% decay of the 1887 cmꢀ1 peak is also faster (20 min, Figure 6).
Rate constants for the decay of the absorption at 635 nm
observed after irradiation of 4 were obtained at different concentra-
tions of pyridine. In the absence of pyridine, the decay did not follow
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Scheme 4. Irradiation of 5
Figure 11. Plot of ln(A0/At) at 700 nm following irradiation of 1 mM
5 with 1 mM pyridine in heptane at 25 °C, kobs = 4.9(0.1) ꢁ 10ꢀ4 sꢀ1
R = 0.999, first point and baseline correction.
,
Figure 9. UVꢀvis spectra following irradiation of 1 mM 5 with no
added pyridine in heptane at 25 °C, delay = 0, 10, 20, 30, 40, 50, and
60 min.
Figure 12. FTIR spectral changes following irradiation of 1 mM 5 in
heptane at ꢀ10 °C. Spectra are recorded at time 0, 10, 20, 40, 60, 80, and
100 min.
Flash Photolysis of 1. Steady-state photolysis studies of 1
were reported previously.7 TRIR spectra for 1 in heptane reveal
a bleach feature near 1940 cmꢀ1 and new absorption peaks at
1947, 1887, and 1878 cmꢀ1 (Figure 14). The peaks are fully
developed within 133 ps, where the intensity ratio of 1887 to
1878 cmꢀ1 is about 3:1. This ratio does not change up to 500 ns
and is the same as that initially observed at low temperature and
short irradiation times in FTIR spectra.7
Computations of Metal-Bound CO Infrared Stretching
Frequencies. Several low-energy conformations of 1, 4, and 5
were determined computationally, and the metal-bound CO
frequencies are reported in Table 1 along with experimental
results for 1ꢀ8 (see spectra in Supporting Information, Figures
19Sꢀ24S). As expected, the computed harmonic frequencies are
blue-shifted compared to experimental frequencies, but the
relative frequencies are consistent. The computed asymmetric
stretching vibrations are not degenerate, and compared to 1 the
difference is much larger for 4 and 5. Stationary points and
computed frequencies are also reported for 8, related chelate
isomers, and σ-ethane complexes in the Supporting Information,
Scheme S1 and Tables S3 and S4. All the chelate isomers have
higher computed frequencies than the computed frequencies for
8. Only the σ-ethane complexes and coordinatively unsaturated
isomers (no chelated side chains) have lower frequencies that 8.
Figure 10. Plot of ln(A0/At) at 700 nm following irradiation of 1 mM
5 without pyridine in heptane at 25 °C, kobs = 5.1(0.1) ꢁ 10ꢀ4 sꢀ1
R = 0.998, first point correction.
,
with an apparent 20 min half-life. Similarly, TRIR spectra also
reveal absorption peaks at 1952 and 1894 cmꢀ1 and bleaches at
1948 and 1940 cmꢀ1 (Figure 13). The peak at 1894 cmꢀ1 appears
completely formed within 67 ps and does not change in amplitude
or shape for delays to 2 μs. The peak is asymmetric with a “shoulder”
on the low-frequency side and has an apparent 12 cmꢀ1 fwhm.
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’ DISCUSSION
Because 4 has virtually no visible absorption (Figure 1, 0 min), the
peak at 635 nm can be attributed entirely to product. The shape and
peak position in the visible spectrum, including a shoulder at
700 nm, are almost identical to those reported for 2.22 The areas
and shifts of the new NMR peaks at 5.1, 3.2, and 2.8 ppm after
irradiation in toluene-d8 (Figure 2) are assigned to the α-Cp, β-Cp,
and methylene peaks of 6, respectively, and are consistent with those
observed for 2 (5.4, 4.1, and 3.3 ppm in methylcyclohexane-d14)
considering solvent differences and potential steric and shielding
effects resulting from the presence of the phenyl group and
restricted conformations of the pyridine ring. The Mn-bound CO
IR frequencies of 6 (1948 and 1887 cmꢀ1, Figure 3) are nearly
identical to those for 2 (1947 and 1887 cmꢀ1 versus 1941 and
1878 cmꢀ1 for 3).7 These assignments are consistent with the
computational results inTable 1. Insummation, the results suggest
that 6 and 2 are O-chelates because 6 cannot be an N-chelate and
the spectral properties for 6 are similar to 2 but distinctly different
from those for 3, where N-chelation is confirmed by its X-ray
crystal structure.7
Formation and Decay of the O-Chelate from 4. The spec-
troscopic results suggest that an O-chelate is formed following
irradiation of 4. The electronic transition at 635 nm for a blue
product along with NMR and IR peaks similar to those reported for
2 (Figures 1ꢀ6, Scheme 3, Table 1)7 are assigned to O-chelate 6.
In the absence of pyridine, the O-chelate 6 decays as the bleach
of 4 recovers, indicating recombination with carbon monoxide.
The recovery of 4 from O-chelate 6 can be inhibited by pyridine:
minor recovery of 4 is observed as two new peaks (1932 and
1865 cmꢀ1) assigned to the pyridine adduct 7 grow during the
decay of O-chelate 6 (Figure 5). The assignment of the peaks for
the pyridine adduct 7 is consistent with the peak intensity
dependence on pyridine concentration (Figures 5 and 6) and
the computed CO frequencies relative to those of 4 and 6
(Table 1 and Figure 24S, Supporting Information). Even in the
presence of 100 mM pyridine, the observed rate constant for the
decay of O-chelate 6 (3.8 ꢁ 10ꢀ3 sꢀ1, a bimolecular substitution)
is slower than the observed rate constant for the decay of
O-chelate 2 (7 ꢁ 10ꢀ3 sꢀ1, a unimolecular rearrangement).
The results indicate that an O-chelate is the first and only
intermediate formed after irradiation of 4 and is therefore the
precursor to all subsequent species. For example, immediately
after steady-state irradiation of 4 in the presence of pyridine,
O-chelate 6 is observed before pyridine adduct 7 formation,
suggesting that pyridine adduct 7 is formed only from 6 and
not directly from 4 (Scheme 3). This result is consistent with
picosecond to microsecond TRIR studies, where only O-chelate
6 is observed (Figure 4). Therefore, as expected, formation of
chelate will be much faster than dispersed bimolecular addi-
tion. Previous studies show that solvent-coordinated species are
converted to a chelate within 200 ns;1,23 thus, the long-lived
Figure 13. TRIR difference spectra acquired after 355 nm pulsed laser
irradiation of 1.3 mM 5 in heptane. The spectra are offset vertically by
0.05 OD units. The FTIR difference spectrum was obtained after steady-
state UV irradiation of 0.5 mM 5 in heptane.
Figure 14. TRIR difference spectra acquired after pulse laser irradiation
of 1 (picosecond, 266 nm, 0.9 mM; nanosecond, 289 nm, 1.4 mM). The
FTIR spectrum was obtained after steady-state UV irradiation of 0.6 mM
1 in heptane.
Table 1. CO Stretching Bands of Cymantrene Derivatives in Heptane
symmetric
asymmetric
experimental, cmꢀ1 (fwhm)b
experimental, cmꢀ1 (fwhm)b
computational,a cmꢀ1
computational,a cmꢀ1
1c
2
2024(4)
1947
2035
1982
1970
2036
2036
1980
1962
1987
1943(9)
1975, 1973
1943
1887
3
1941
1878
1931
4d
5e
6
2024(4)
2026(4)
1948(3)
1932
1946, 1938(13)
1948, 1940(17)
1887(4)
1978, 1970
1978, 1971
1941
7
1865(12)
1921
8
1952(3)
1894(5)
1948
a Average value for several conformers for 1, 4, and 5 (see Discussion for further details). b Apparent full width at half-maximum. c 3 mM 1 in heptane.
d 1 mM 4 with 10 mM pyridine in heptane. e 0.5 mM 5 in heptane.
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Scheme 5. Mechanism of the Decay of 6
allowing the carbon monoxide to escape from solution impedes
the decay of O-chelate 6, and the blue chromophore of O-chelate
6 is observed even after 24 h (Figure 1). We conclude that the
relative stability of an O-chelate can increase if a low-energy path
to a stable linkage isomer is eliminated.
Formation and Decay of the O-Chelate from 5. Steady-state
and TRIR results for 5 are similar to those for 4. Following
irradiation of 5 in the absence of added Lewis base, a blue
chromophore assigned to the O-chelate 8 (A704) has a half-life
(t1/2 = 30 min, Figure 9) that is substantially longer than the half-
life for the O-chelate 2 (t1/2 = 3 min at 25 °C),7 but shorter than
the half-life observed for the O-chelate 6 (t1/2 = 60 min,
Figure 3). Two factors can contribute to the faster decay of the
O-chelate 8 versus that of 6. First, the MnꢀO bond in 8 is
weakened by the reduced electron density at the side-chain
carbonyl (caused by the pyridyl group). Second, even though
the O-chelate 8 cannot isomerize to an N-chelate (a unimolecular
process), the pyridyl groups of 5 or O-chelate 8 can add to the
metal center of another O-chelate 8 (both bimolecular pro-
cesses). Thus, the very broad features at 1920 and 1850 cmꢀ1 in
Figure 12 are attributed to formation of oligomers where the
pyridyl group of one complex adds to the metal center of another
and contributes to the decay of O-chelate 8. The lack of an
isosbestic point in Figure 9 indicates O-chelate 8 does not simply
revert back to 5. This result is supported by FTIR spectra, where
the recovery of 5 after irradiation only partially accounts for the
peaks assigned to O-chelate 6 are unlikely to be due to a heptane-
coordinated intermediate. There is also no evidence of ultrafast
solvent coordination similar to that observed for other metal
centers that bind to alkane solvents,1,6b,23ꢀ25 suggesting that the
chelation excludes ultrafast solvent coordination and that the side
chain is in a conformation that favors the ultrafast chelation. It is
possible the heptane-coordinated intermediate could be unde-
tected, but only if the heptane-coordinated intermediate and the
chelate have identical peaks and the heptane-coordinated inter-
mediate converts only to the chelate.
decay of peaks assigned to O-chelate 8 (1952 and 1894 cmꢀ1
,
A mechanism proposed for the decay of 6 is shown in
Scheme 5. At low pyridine concentration, the rate of decay of
6 is equal to the rate of formation of 4 (Figure 3). Scheme 5 is
consistent with this observation for the steady-state assumptions
for recovery of 4 (kꢀ1. k2[CO] . k3[pyridine]) or formation
Figure 12). The higher stretching frequencies for O-chelate 8
compared to O-chelate 6 are attributed to the electron-with-
drawing nature of the pyridine ring, which decreases electron
donation to the metal and consequently the back-bonding to the
metal-CO groups.
of 7 (k . k3[pyridine] . k2[CO]; see Supporting Infor-
On ultrafast time scales, a new absorption peak observed at
1894 cmꢀ1 is likewise attributed to the O-chelate 8 (Figure 13).
The peak has a shoulder near 1885 cmꢀ1, where a peak is often
detected for heptane-coordinated complexes.4,5 This shoulder
persists up to 2 μs with no significant decay, but it is gone before
standard FTIR spectra can be recorded (several seconds). In
previous studies for CpMn and AreneCr complexes, chelation
occurs with sulfur- and nitrogen-containing side chains that dis-
place coordinated heptane, and in no case did a heptane-
coordinated complex last longer than 200 ns.1,4,5 We cannot
rule out the possibility of a heptane-coordinated complex be-
cause the side-chain oxygen may not displace the heptane as
effectively as a side chain with sulfur or nitrogen. Nevertheless, no
matter how slow the O-chelate 8 is formed from a heptane-
coordinated complex, the latter is unlikely to persist longer than
100 μs in these experiments because the bimolecular rate con-
stant for solvent displacement is on the order of 107 Mꢀ1 sꢀ1 and
the concentration of pyridine groups from 5 and all photopro-
ducts is 1 mM.4 From these results, one might conclude that a
heptane-coordinated complex is not formed from 5. Likely as-
signments for the shoulder near 1885 cmꢀ1 include transient
pyridine σ or π complexes with the pyridine group like those
proposed for the irradiation of 17 or pendant pyridine groups on
arene chromium carbonyl complexes.1 We turned to the com-
putational results to explore the plausible structures. None of the
computed frequencies for the computed transient unimolecular
species match this experimental result. However, the results for
the computed red shift of CO frequencies for the heptane-
coordinated complex closely match those for the experimentally
observed transient: the computed red shift for 8 of 10 to 11 cmꢀ1
ꢀ1
mation). At high pyridine concentrations, the rate is first-order
in [pyridine], consistent with a steady-state assumption where
k
ꢀ1 . k2[CO] + k3[pyridine] and k3[pyridine] . k2[CO]. The
kinetic results suggest the rate-determining step precedes attack
of CO or pyridine and is likely to be the dissociation of the
MnꢀO bond. If this mechanism is correct, then it also indicates
that the formation of O-chelate 6 from the solvent-coordinated
10 in Scheme 5 is faster than bimolecular displacement of solvent
by a dispersed nucleophile. Nonetheless, a mechanism that ex-
cludes 10 where 6 is attacked directly by CO or pyridine is also
consistent with the results.
A comparison of 6 with the O-chelate 2 is revealing: 2 is
thermally unstable (t1/2 ≈ 3 min, 25 °C), as it undergoes
a linkage isomerization to the N-chelate 3 (Scheme 1).7 DFT
calculations and experimental results suggested that the isomer-
ization occurs via a metal walk along the π system between the
two heteroatoms instead of a dissociative mechanism that has a
higher barrier (dissociation of the MnꢀO bond, followed by
rotation of the side chain, and formation of the MnꢀN bond).7
Thus, it appears that the bridge between the two functional
groups provides a conduit for rearrangement, and removing the
nitrogen in 2 should increase the lifetime of the O-chelate since
migration along the side chain would not lead to a more stable
isomer. Indeed the half-life of O-chelate 6 in the absence of added
Lewis base (t1/2 = 60 min, 1948 and 1887 cmꢀ1, Figure 3) is
longer than that observed for 2. Since the O-chelate 6 cannot
undergo a linkage isomerization to a more stable isomer, the
primary path of decay is recombination with carbon monoxide,
which results in the recovery of the bleach of 4. In contrast,
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in the CO frequency for the model solvent-coordinated complex
is very similar to the experimentally observed red shift of 9 cmꢀ1
(see Table S3, Supporting Information).
the metal carbonyl groups are responsible for the loss of near
degeneracy of the two asymmetric stretching frequencies. A
much larger interaction for 4 and 5 compared to 1 is supported
by the larger frequency differences determined computationally
(Table 1).32 This interpretation is consistent with the observa-
tion that these complexes undergo ultrafast chelation, which
requires that the side-chain functional group must be near the
metal center at the instant of CO dissociation.
Irradiation of 1. The TRIR spectra for the irradiation of
1 (Figure 14) are distinctly different from those obtained for 4
and 5. The assignment of two, well-resolved peaks at 1887 and
1878 cmꢀ1 to O-chelate 2 and N-chelate 3, respectively, is
consistent with previous assignments based on steady-state FTIR
and NMR experiments and a crystal structure of isolated 3.7 As
noted above, the assignment of the 1887 cmꢀ1 to O-chelate 2 is
also consistent with the spectra observed for the O-chelate 6.
In the previous study, lower temperatures and shorter irradia-
tion times were used to minimize the rearrangement of 2 to 3, but
a minor amount of 3 was always observed.7 It was unclear
whether both 2 and 3 were directly formed from the excited
state of 1, if 3 was formed by secondary photolysis of 2, or
subsequent thermal rearrangement. This distinction is important
in determining the mechanism of chelation and the design of
selective photochromes. To minimize the possibility of thermal
or photoisomerization of 2 to 3 that might occur during steady-
state irradiation, TRIR spectroscopy was used with ultrashort
laser pulses and time delays. The appearance of O-chelate 2 and
N-chelate 3 in the first 100 ps is limited by vibrational relaxation
effects.22 Assessment of the laser pulse duration, sample absor-
bance, the pulse intensity, and quantum yields demonstrates that
it is impossible for a secondary photolysis to account for
rearrangement of 2 to 3 (calculation details are in the Supporting
Information). The results suggest that both isomers are formed
directly from the excited state of 1 and not by rearrangement of
the O-chelate 2 to the N-chelate 3 because the thermal rearran-
gement is slow (ΔG‡ = 20.3 kcal/mol at 25 °C).7
’ SUMMARY
Three cyclopentadienyl manganese tricarbonyl derivatives, 1,
4, and 5, with different side chains (Mn{η5-C5H4CH2COR}-
(CO)3, R = 2-pyridinyl 1, phenyl 4, and 4-pyridinyl 5) have been
prepared and studied. The species formed following UV irradia-
tion of 1, 4, and 5 have been characterized and identified with
both experimental (FTIR, TRIR, UVꢀvis, and NMR) and com-
putational (DFT) techniques. Photodissociation of CO from
model compounds 4 and 5 produces O-chelates 6 and 8,
respectively, within 100 ps. In the case of 5, there is evidence
of ultrafast formation of another species, and the infrared peaks
are consistent with a solvent-coordinated species, but kinetic data
indicate this assignment is unlikely. The O-chelates 2, 6, and 8
each decay on minute time scales in the presence of added or
pendant pyridine groups, but in the absence of any pyridine
group the O-chelate 6 persists for 24 h. Photodissociation of CO
from 1 produces simultaneously O-chelate 2 and N-chelate 3
within 100 ps. Because the observation of metal carbonyl species
is limited by vibrational relaxation (ca. 100 ps), this time scale is
the upper limit for the formation of the chelates 2 and 3.
Importantly, ultrafast solvent coordination can occur within a
few picoseconds and provides a benchmark for the rate of ultra-
fast chelation: if ultrafast solvent coordination is limited by
competing ultrafast chelation, then the chelation is almost
certainly occurring in less than a picosecond. These are the time
scales that will ensure optimum performance of optically driven
molecular devices.
Within experimental error, the relative intensities of 2 and 3 do
not change from 10ꢀ10 to 10 s.26 Like the product distribution
observed for cymantrene derivatives with sulfide side chains,4,5
the initial distribution of O-chelate 2 and N-chelate 3 likely
depends on thermodynamically favored conformations at the
instant a photon is absorbed. The initial chelate distribution is a
result of the fact that chelation of proximal functional groups is
much faster than solvent coordination and side-chain conforma-
tional changes required to form a ring at room temperature.27
If solvent coordination were to occur, a transient absorption
peak would be expected near 1888 cmꢀ1 like that observed for an
isooctane-coordinated complex formed following irradiation of
methyl cyclopentadienyl manganese tricarbonyl.28 This peak would
overlap the O-chelate 6 peak. As noted above, solvent displacement
via N-chelation should occur within 200 ns,1 and the latter should
result in a change in the relative intensities of the 1887 and
1878 cmꢀ1 peaks. Within experimental error, there is no change
in these peaks up to several seconds. Therefore, we propose there is
no appreciable ultrafast solvent coordination for this system.
Metal Carbonyl Spectra. Metal tricarbonyls normally exhibit
two infrared metalꢀcarbonyl stretching peaks where the lower
energy peak corresponds to nearly degenerate asymmetric stretch-
’ ASSOCIATED CONTENT
S
Supporting Information. Time-resolved UV spectra,
b
plots of ln(A0/At) from UV spectra, IR spectra of 1, 4, and 5,
kinetic derivations for Scheme 5, details for computational results
for CO frequencies, and computed energies of stationary points
related to 8. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: cewebstr@memphis.edu; tburkey@memphis.edu.
’ ACKNOWLEDGMENT
ing vibrations. For example, both (η5-C5H5)Mn(CO)3 (in
29
This work was partly supported by the National Science
Foundation under Grant No. CHE-0911528 (T.J.B. and C.E.W.)
and the National Institute of Standards and Technology under
Award No. 70NANB8H129 (T.J.B.).
cyclohexane) and (η5-CH3C5H4)Mn(CO)3 (in n-hexane)
30
have CO vibrational stretches at 1944 cmꢀ1, which are substan-
tially broader and larger than their respective stretches at 2027
and 2025 cmꢀ1 31
In contrast, three CO stretching peaks are
.
observed for 4 and 5 before irradiation (see Figures 19Sꢀ22S in
the Supporting Information) and after irradiation as bleaches
(Figures 3, 5, 6, and 12). We propose that for the lowest energy
conformers the interactions of the side chains of 4 and 5 with
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