Impact of steric bulk on photoinduced ligand exchange reactions in Mn(I) photoCORMs

Article abstract of DOI:10.1016/j.ica.2020.119845

The investigation of ligand exchange reactions in Mn(I)-based photoactivated CO-releasing molecules, or “photoCORMs,” has largely focused on the electronic effects of the ligand set. In this work, we report the effects of sterically bulky bidentate (NN) ligands on the efficiency for CO release and the formation of photochemical intermediates in fac-[Mn(NN)(CO)₃(L)]ⁿ⁺ photoCORMs. The vibrational and electronic absorption spectroscopy and photochemistry of two new Mn(I) photoCORMs with a sterically bulky 6,6′-dimethyl-2,2′-bipyridine ligand, fac-[Mn(6,6′-Me₂bpy)(CO)₃Br] (6,6′-Me₂bpy-Br) and fac-[Mn(6,6′-Me₂bpy)(CO)₃(py)]⁺ (6,6′-Me₂bpy-py), are reported in comparison to two previously reported analogues, fac-[Mn(4,4′-Me₂bpy)(CO)₃Br] (4,4′-Me₂bpy-Br) and fac-[Mn(4,4′-Me₂bpy)(CO)₃(py)]⁺ (4,4′-Me₂bpy-py) with the 4,4′-dimethyl-2,2′-bipyridine ligand. The steric demands of the methyl substituents in the 6,6′ positions on bpy significantly distort the structure, as the crystal structure shows contraction between the equatorial CO ligands and tilting of the bidentate ligand relative to the 4,4′-Me₂bpy complexes. The movement of the methyl substituents from the 4,4′ to the 6,6′ positions on bpy has little impact on the electronic properties of the complexes, as observed by FTIR and electronic absorption spectroscopy, while the steric bulk of 6,6′-Me₂bpy increases the quantum yield of CO release (Φ_CO) and increases the lability of the Br^? and py ligands compared to the 4,4′-Me₂bpy complexes with less steric bulk.

Full text of DOI:10.1016/j.ica.2020.119845

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Impact of steric bulk on photoinduced ligand exchange reactions in Mn(I)
photoCORMs
Shabnam Pordel, Briana R Schrage, Christopher J. Ziegler, Jessica K. White
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Impact of steric bulk on photoinduced ligand exchange reactions in Mn(I) photoCORMs
Shabnam Pordela, Briana R Schrageb, Christopher J. Zieglerb, Jessica K. White*a
a
Department of Chemistry and Biochemistry, Clippinger Laboratories, Ohio University, Athens, OH
45701, United States (whitej7@ohio.edu)
b Department of Chemistry, Knight Chemical Laboratory, University of Akron, Akron, OH 44325, United
States
Abstract
The investigation of ligand exchange reactions in Mn(I)-based photoactivated CO-releasing molecules,
or “photoCORMs,” has largely focused on the electronic effects of the ligand set. In this work, we report
the effects of sterically bulky bidentate (NN) ligands on the efficiency for CO release and the formation of
photochemical intermediates in fac-[Mn(NN)(CO)3(L)]n+ photoCORMs. The vibrational and electronic
absorption spectroscopy and photochemistry of two new Mn(I) photoCORMs with a sterically bulky 6,6′-
dimethyl-2,2′-bipyridine ligand, fac-[Mn(6,6′-Me2bpy)(CO)3Br] (6,6′-Me2bpy-Br) and fac-[Mn(6,6′-
Me2bpy)(CO)3(py)]+ (6,6′-Me2bpy-py), are reported in comparison to two previously reported analogues,
fac-[Mn(4,4′-Me2bpy)(CO)3Br] (4,4′-Me2bpy-Br) and fac-[Mn(4,4′-Me2bpy)(CO)3(py)]+ (4,4′-Me2bpy-py)
with the 4,4′-dimethyl-2,2′-bipyridine ligand. The steric demands of the methyl substituents in the 6,6′
positions on bpy significantly distort the structure, as the crystal structure shows contraction between the
equatorial CO ligands and tilting of the bidentate ligand relative to the 4,4′-Me2bpy complexes. The
movement of the methyl substituents from the 4,4′ to the 6,6′ positions on bpy has little impact on the
electronic properties of the complexes, as observed by FTIR and electronic absorption spectroscopy, while
the steric bulk of 6,6′-Me2bpy increases the quantum yield of CO release (ΦCO) and increases the lability of
the Br− and py ligands compared to the 4,4′-Me2bpy complexes with less steric bulk.
Keywords
Manganese; carbon monoxide; photoCORM; ligand exchange; photochemistry
1

1. Introduction
Carbon monoxide is a gaseous signaling molecule that participates in several cell signaling pathways.
As a result, endogenously produced CO plays important physiological roles such as anti-inflammation,
cytoprotection, vasodilation, and blood vessel formation.[1–6] Recent research has also indicated
therapeutic potential for CO, such as anti-bacterial and anti-cancer activity.[3,5,7–10] At high
concentrations, CO is anti-proliferative and pro-apoptotic in cancer cells, while it is anti-apoptotic and anti-
inflammatory in healthy tissue.[3] Due to the toxicity and storage safety issues arising from inhalation of
gaseous CO,[11] delivery of the small molecule with high spatiotemporal control is an important goal. The
tendency of CO to act as a Lewis base, forming coordinate covalent (or dative) bonds with transition metal
ions, has enabled the development of CO-releasing molecules (termed “CORMs”), in which the CO is
deactivated through formation of metal-CO bond.[12–14] Upon bond cleavage, the CO is released and its
pharmacological activity is restored.
CORMs that release CO through photochemical bond dissociation are known as “photoCORMs,” or
photo-activated CO-releasing molecules.[15–21] PhotoCORMs allow for CO to remain coordinated to a
metal complex pro-drug until irradiation with UV or visible light at a desired location, such as a tumor or
infection, releases one or more equivalents of CO. The wavelength required for bond cleavage and the
number of CO molecules released per metal complex depend on the molecular architecture. While many
photoCORMs have been reported featuring Fe, Ru, Cr, W, Mo, and Re,[2,15,22–29] those featuring Mn(I)
have been increasingly studied due to their ability to release multiple equivalents of CO with visible
light,[7,8,30–52] a desirable feature to decrease photoinduced damage to healthy cells and tissue.[53]
A frequently studied architecture for Mn(I) photoCORMs is fac-[Mn(NN)(CO)3(L)]n+, in which NN is
typically a diimine bidentate ligand and L is a monodentate ancillary ligand such as Br−, CH3CN,
phosphines, or N-containing heterocycles. The identities of NN and L influence the photophysical
properties and photochemical ligand dissociation. Excitation with visible light results in population of a
lowest-lying metal-to-ligand charge transfer (MLCT) state, which may also have a significant amount of
2

halide-to-ligand charge transfer (XLCT) character if L is a halide.[15,19] The reduced electron density on
the metal weakens the π-backbonding between Mn and CO ligands, allowing ligand dissociation. It is
generally reported that the quantum yield for CO release (ΦCO) increases as π-acidity of the bidentate ligand
increases.[15] For example, in the set of fac-[Mn(NN)(CO)3Br] compounds in which NN = (2-
phenyliminomethyl)quinoline (pimq), 2-quinoline-N-(2′-methylthiophenyl)methyleneimine (qmtpm), and
2-pyridyl-N-(2′-methylthiophenyl)methyleneimine (pmtpm), the value for ΦCO with 509 nm irradiation
increases from NN = pimq to pmtpm to qmtpm as conjugation on the NN ligand increases.[54]
The efficiency and rate of CO release are also influenced by the nature of the monodentate ancillary
ligand L, as π-donor ligands tend to provide greater CO release efficiency compared to π-acceptor ligands.
In the case of the fac-[Mn(qmtpm)(CO)3L]n+ and fac-[Mn(pmtpm)(CO)3L]n+ complexes, the CO release
rate is greater when L = Br− compared to L = CH3CN.[54] The π-donor Br− destabilizes the Mn(dπ) orbitals
relative to the case when L is the π-acceptor CH3CN, resulting in a lower energy MLCT state and enhanced
absorptivity at longer wavelengths. Similarly, in photoCORMs with the fac-[Mn(azpy)(CO)3(L)]n+ (azpy
= 2-phenylazopyridine) architecture, the rate of CO release from the compound when L = Br− is greater
than the rate when L = PPh3.[33,34] Monodentate ligands that coordinate through imidazolyl groups also
increase the quantum yield for CO dissociation compared to those with piperazinyl groups, as observed in
the fac-[Mn(phen)(CO)3(L)]+ complexes, where phen = 1,10-phenanthroline and L = imidazoledanysl or 1-
dansylpiperazine.[39,41]
Our previous work with fac-[Mn(NN)(CO)3Br] and fac-[Mn(NN)(CO)3(py)]+ photoCORMs, in which
NN = 4,4′-dimethyl-2,2′-bipyridine (4,4′-Me2bpy), 2,2′-bipyridine (bpy), and 4,4′-dimethylester-2,2′-
bipyridine (4,4′-dmebpy), highlighted the strong influence of the monodentate L ligand in the
photochemical ligand dissociation reactions.[47] We discovered that a π-donating Br− directs
photodissociation of an equatorial CO in the first step, while a π-accepting py directs photodissociation of
the axial CO in the first step. We observed this trend regardless of the electron-donating (methyl) or
electron-withdrawing (methyl ester) substituents at the 4,4′ positions of 2,2′-bipyridine. Additionally, in
3

agreement with reports of related Mn(I) photoCORMs, the ΦCO increases with increasing NN π-acidity
(4,4′-Me2bpy < bpy < 4,4′-dmebpy) in each series, and all compounds with L = Br− displayed larger ΦCO
values than their L = py analogues.
The purpose of this work is to broaden our investigation by probing the impact of a sterically bulky NN
bidentate ligand on the photochemical ligand dissociation efficiency and mechanism in fac-
[Mn(NN)(CO)3L]n+ photoCORMs. The impact of steric distortion on fac-[Mn(NN)(CO)3L]n+ photoCORMs
has not been clearly investigated. In this study, we utilized 6,6′-dimethyl-2,2′-bipyridine (6,6′-Me2bpy) to
prepare two new photoCORMs, fac-[Mn(6,6′-Me2bpy)(CO)3Br] (6,6′-Me2bpy-Br) and fac-[Mn(6,6′-
Me2bpy)(CO)3(py)]+ (6,6′-Me2bpy-py) for direct comparison to our previously reported fac-[Mn(4,4′-
Me2bpy)(CO)3Br]
(4,4′-Me2bpy-Br)
and
fac-[Mn(4,4′-Me2bpy)(CO)3(py)]+
(4,4′-Me2bpy-py)
analogues.[47] The photoCORMs discussed in this work are shown in Figure 1. The methyl groups at the
4,4′- and 6,6′-positions are expected to impart similar electronic effects but significantly different steric
effects as a result of the placement of the substituents relative to the Mn-CO bonds. Herein we present an
investigation into the impact of steric bulk on the structure and photophysical and photochemical properties
of fac-[Mn(NN)(CO)3L]n+ photoCORMs.
Figure 1. Structural representations and naming scheme for the Mn(I) photoCORMs.
4

2. Experimental section
2.1. Materials
Bromopentacarbonylmanganese(I)
was
purchased
from
Strem
Chemicals.
Silver
trifluoromethanesulfonate (AgOTf) was purchased from Acros Organics. Acetonitrile, pyridine,
dichloromethane, diethyl ether, methanol, hexane, and chloroform were purchased from Fisher Scientific.
Potassium tris(oxalato)ferrate(III) was purchased from Alfa Aesar. Acetonitrile-d3 were purchased from
Cambridge Isotope Laboratories. [Mn(4,4′-Me2bpy)3]ClO4,[55] fac-[Mn(4,4’-Me2bpy)(CO)3Br] (4,4′-
Me2bpy-Br),[56] and fac-[Mn(4,4’-Me2bpy)(CO)3py]+ (4,4′-Me2bpy-py),[47] were synthesized according
to reported procedures.
2.2. Methods
All reactions and experiments were performed in the dark (unless otherwise noted) to prevent
unintentional photodecomposition.
2.2.1. Synthesis of fac-[Mn(6,6′-Me2bpy)(CO)3Br] (6,6′-Me2bpy-Br)
The synthetic procedure was adapted from a previously reported procedure for related Mn(I)
compounds.[56] Mn(CO)5Br (270 mg, 0.98 mmol) and 6,6′-dimethyl-2,2′-dipyridyl (200 mg, 1.0 mmol)
were added to 30 mL of diethyl ether and heated at reflux for 4 h under Ar. After cooling to room
temperature, the yellow precipitate was collected by vacuum filtration and washed with diethyl ether. Yield:
320 mg (0.79 mmol, 81%). 1H NMR (500 MHz, CD3CN): δ 8.01 (2H, d), 7.93 (2H, t), 7.48 (2H, m), 3.1
(6H, s). Elemental analysis calculated for C15H12O3N2MnBr: 44.69% C, 3.00% H, 6.94% N. Found: 44.6%
C, 2.98% H, 7.00% N. HR-ESI(+)-MS (CH3CN): [M−Br+CH3CN]+, m/z = 364.048 (calcd m/z = 364.049).
2.2.2. Synthesis of fac-[Mn(6,6′-Me2bpy)(CO)3(py)](OTf) (6,6′-Me2bpy-py)
The synthetic procedure was adapted from a previously reported procedure for related Mn(I)
compounds.[47] A mixture of fac-[Mn(6,6′-Me2bpy)(CO)3Br] (100 mg, 0.24 mmol) and AgOTf (110 mg,
5

0.42 mmol) in 15 mL of CH2Cl2 was stirred at room temperature for 16 h. The precipitated AgBr was
filtered through Celite, and the solvent was evaporated under reduced pressure. The obtained yellow solid
was dissolved in 20 mL of methanol followed by addition of 1 mL of pyridine. The solution was heated at
reflux under argon for 5 h. After cooling to room temperature, the solvent was removed under reduced
pressure. The yellow residue was dissolved in 5 mL of CH2Cl2 and added dropwise to 100 mL of hexane
to induce precipitation. The yellow solid was collected by vacuum filtration and washed with diethyl ether.
Yield: 90 mg (0.16 mmol, 67 %). 1H NMR (500 MHz, CD3CN): δ 7.97 (2H, t), 7.90 (2H, d), 7.76 (1H, m),
7.66 (2H, d), 7.62 (2H, d), 7.16 (2H, t), 3.15 (6H, s). Elemental analysis calculated for
C21H17O6N3MnF3S•0.5H2O: 45.01% C, 3.24 % H, 7.50% N, 5.72% S. Found: 45.22% C, 3.00% H, 7.53%
N, 5.76% S. HR-ESI(+)-MS (CH3CN): [M−OTf]+, m/z = 402.064 (calcd m/z = 402.065).
2.2.3. Elemental analysis
Elemental analysis (C, H, N, S) was performed by Atlantic Microlabs, Inc. (Norcross, GA). Prior to
combustion, samples were protected from light to prevent photodecomposition.
2.2.4. High resolution electrospray ionization mass spectrometry (HR-ESI MS)
HR-ESI(+)-MS spectra were recorded using a Thermo Fisher Scientific Q Exactive Plus hybrid
quadrupole–Orbitrap mass spectrometer with solvent flow rate of 20 μL/min in the positive mode. A
potential of 4.0 kV was applied to ionize the 5 μM solution of each compound in CH3CN.
2.2.5. Single crystal X-ray crystallography
Crystals suitable for X-ray diffraction were obtained by slow vapor diffusion of diethyl ether into a
CH2Cl2 solution of each complex. X-ray intensity data were measured on a Bruker PHOTON II CPAD-
based diffractometer with dual Cu/Mo ImuS microfocus optics (Cu Kα radiation, λ = 1.54178 Å, Mo Kα
radiation, λ =0.71073 Å). Crystals were mounted on a cryoloop using Paratone oil and placed under a steam
of nitrogen at 100 K (Oxford Cryosystems). The detector was placed at a distance of 5.00 cm from the
crystal. The data were corrected for absorption with the SADABS program. The structures were refined
6

using the Bruker SHELXTL Software Package (Version 6.1)[57] and were solved using direct methods
until the final anisotropic full-matrix, least squares refinement of F2 converged. Crystallographic data for
4,4′-Me2bpy-Br, 6,6′-Me2bpy-Br, 4,4′-Me2bpy-py, and 6,6′-Me2bpy-py are provided in Tables S1 and S2.
CCDC numbers 1989788-1989791 contain the supplementary crystallographic data for this paper and can
be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/structures.
2.2.6. Computational methods
The geometries for 6,6′-Me2bpy-Br, 6,6′-Me2bpy-py, and proposed photochemical intermediates were
fully optimized using the M06 level[58,59] of density functional theory (DFT) with the SMD continuum
solvation model for CH3CN solvent[60] in the Gaussian 16 program.[61] The 6-31G* basis set was utilized
for C, H, N, and O,[62] while the SDD energy consistent pseudopotentials were utilized for Mn and Br.[63]
To confirm the presence of local minima on the potential energy surfaces, frequency analysis was
performed after geometric optimization. The Avogadro program (Version 1.2.0)[64] was used to obtain the
calculate C−O stretching frequencies which were multiplied by an frequency scaling factor of 0.96 for
comparison to experimental data.
2.2.7. 1H NMR
1H NMR spectra were obtained on a Bruker Ascend 500 MHz spectrometer at 298 K. The chemical
shifts were referenced to the CD3CN solvent peak (δ = 1.94 ppm).
2.2.8. Fourier transform infrared (FTIR) spectroscopy
FTIR spectra were collected using a Shimadzu IRAffinity-1S Fourier transform infrared
spectrophotometer and a CaF2 liquid IR cell with CH3CN solvent.
7

2.2.9. Electronic absorption spectroscopy
Electronic absorption spectra were recorded with an Agilent Cary 8454 diode array UV-visible
spectrophotometer with 1 nm resolution and 0.5 s integration time in a 1 × 1 cm quartz cuvette at room
temperature. Extinction coefficients in CH3CN were measured in triplicate.
2.2.10. Photolysis experiments
The photolysis studies were carried out with CH3CN solutions of the compounds at room temperature
and 405 nm LEDs (Luxeon Star LED, Quadica Developments, Inc., Lethbridge, Alberta, Canada). Each
sample was irradiated for varied time intervals and analyzed by the appropriate spectroscopic method
(FTIR, electronic absorption and 1H NMR analysis). Analogous dark control experiments were performed
in the absence of irradiation. Ferrioxalate chemical actinometry was performed to determine the photon
flux of the 405 nm light source (2.7 × 10−8 mol photons/s).[65] The quantum yield of CO dissociation (ΦCO)
was measured using electronic absorption spectroscopy by monitoring the decrease in the MLCT
absorbance band at very early times (<10% change in absorbance). All samples were absorbance matched
at the irradiation wavelength (405 nm). The rate of moles of reactant lost was obtained from the slope of
the moles of reactant vs irradiation time plot. Values were corrected for the fraction of light absorbed by
the sample at the 405 nm. The ΦCO values were calculated by dividing the rate of moles of reactant lost (in
moles reactant/s) by the photon flux (moles of photons/s). Representative data for 6,6′-Me2bpy-Br and 6,6′-
Me2bpy-py are provided in Figure S1.
3. Results and discussion
3.1. Synthesis and characterization
The new compounds 6,6′-Me2bpy-Br and 6,6′-Me2bpy-py were prepared by adapting the previously
reported methods for the 4,4′-Me2bpy analogues.[47,56]. It is critical to keep all Mn(I)-containing solids
8

and solutions in the dark, as rapid photodecomposition occurs under ambient light conditions. Elemental
analysis (C, H, N, S) and ESI-MS were consistent with the proposed compositions, and the 1H NMR spectra
in CD3CN (Figures S2 and S3) were consistent with the proposed structures.
3.2. Structural analysis
A comparison of bond lengths and bond angles obtained by single crystal X-ray diffraction provides
important information about the impact of steric bulk on the fac-[Mn(NN)(CO)3(L)]n+ complexes. The
crystal structures for 4,4′-Me2bpy-Br, 6,6′-Me2bpy-Br, 4,4′-Me2bpy-py and 6,6′-Me2bpy-py are provided in
Figure 2, and tabulated bond distances and angles are provided in Tables S3 and S4. For 6,6′-Me2bpy-Br
and 4,4′-Me2bpy-py, there are three and two multiple equivalents in the asymmetric unit, respectively. In
general, the bond lengths are not impacted significantly between the 4,4′-Me2bpy complexes and their 6,6′-
Me2bpy analogues. The most notable exception is the Mn(1)−N(1) and Mn(1)-N(2) bonds (i.e. the bonds
between Mn and the N atoms on Me2bpy), which increase from 2.023(3) and 2.040(3) Å to 2.079(2) and
2.080(2) Å for 4,4′-Me2bpy-Br and 6,6′-Me2bpy-Br, respectively, as a result of this interference between
the methyl substituents and equatorial CO ligands. A comparison of bond angles suggests the structural
impacts of steric bulk. In 4,4′-Me2bpy-Br, the C(1)−Mn(1)−C(3) bond angle (between the two equatorial
CO ligands) is 88.8(2)°, whereas this same bond angle in 6,6′-Me2bpy-Br is contracted to 84.1(1)°, as a
result of the interference between the 6,6′-methyl groups and the equatorial CO ligands (Figure 3). It should
be noted that we cannot rule out contribution of crystal packing effects toward the differences in these bond
angles, as a small yet significant difference in Mn-C bond lengths are observed as a function of methyl
group placement (Table S3). Finally, the methyl substituents at the 6,6′-positions cause the bidentate ligand
to tilt out of the “normal” plane as a result of interferences with the equatorial CO ligands (Figure 3). By
defining the “normal” Me2bpy ligand plane with Mn(1), N(1), and N(2), and the Me2bpy plane with N(1),
N(2), C(8), and C(11) (as shown in Figure S4), the 6,6′-Me2bpy ligand is tilted toward L by ~21° and ~26°
in the structure of 6,6′-Me2bpy-Br and 6,6′-Me2bpy-py, respectively, while the 4,4′-Me2bpy ligand only tilts
by ~9° and ~10° in 4,4′-Me2bpy-Br and 4,4′-Me2bpy-py, respectively. Tilting of the ligand is consistent
9

with similar tilt angles reported in Ru(II) complexes with bulky bidentate ligands such as 6,6′-Me2bpy, 2,2′-
biquinoline, 2,9-dimethyl-1,10-phenathroline, and 3,6-dimethyldipyridylphenazine.[66–69]
Figure 2. Crystal structures of 4,4′-Me2bpy-Br, 6,6′-Me2bpy-Br, 4,4′-Me2bpy-py and 6,6′-Me2bpy-py with
thermal ellipsoids set at 50% probability level. Triflate counter ion for 4,4′-Me2bpy-py and 6,6′-Me2bpy-py
is omitted for clarity.
10

Figure 3. Crystal structures of 4,4′-Me2bpy-Br (A), 6,6′-Me2bpy-Br (B), 4,4′-Me2bpy-py (C) and 6,6′-
Me2bpy-py (D) viewed from the side (left) and down the Br-Mn or py-Mn bond (right). Thermal ellipsoids
set at 50% probability level.
11

3.4. Vibrational spectroscopy
FTIR spectral analysis provides insight into the electron density on the Mn center by virtue of the C−O
stretching frequencies. The FTIR spectra for all four compounds are shown in Figure 4, and the
experimental vibrational frequencies compared to the calculated values predicted from DFT calculations
are presented in Table 1. The FTIR spectra for all fac-[Mn(NN)(CO)3Br] and fac-[Mn(NN)(CO)3(py)]+
compounds exhibit three vibrational bands in the C−O stretching region (1800-2200 cm−1), in agreement
with the compounds’ Cs symmetry. The νCO values for the compounds are determined by the ligand set’s
impact on the Mn center’s electron density, as decreasing the electron density on Mn causes weaker Mn-
CO π-backbonding and higher C-O stretching frequency. Similar to our previously reported 4,4′-Me2bpy
compounds,[47] replacing the π-donor Br− with π-accepting py in the 6,6′-Me2bpy compounds shifts νCO
from 2023, 1926, and 1919 cm−1 to 2042, 1954, and 1939 cm−1, respectively. The placement of the methyl
groups at either the 4,4′- or 6,6′-positions of bpy have very little impact on the vibrational frequencies,
indicating that the electron donation is not significantly different at the ortho vs para positions of the
pyridine rings.
12

Figure 4. FTIR spectra for 4,4′-Me2bpy-Br (black solid), 6,6′-Me2bpy-Br (blue solid), 4,4′-Me2bpy-py
(black dashed) and 6,6′-Me2bpy-py (blue dashed) in rt CH3CN.
Table 1. Experimental and calculated C−O vibrational frequencies for fac-[Mn(NN)(CO)3Br] and fac-
[Mn(NN)(CO)3(py)]+ (NN = 4,4′-Me2bpy and 6,6′-Me2bpy) in rt CH3CN.
Compound
Experimental ν(CO) (cm−1) a
Calculated ν(CO) (cm−1) b
4,4′-Me2bpy-Br c
6,6′-Me2bpy-Br
4,4′-Me2bpy-py c
6,6′-Me2bpy-py
2026, 1933, 1919
2019, 1938, 1929
2023, 1926, 1919
2015, 1938, 1928
2042, 1954, 1939
2033, 1953, 1947
2039, 1950, 1936
2031, 1951, 1946
a Data collected in rt CH3CN in a CaF2 cell.
b Obtained from the energy optimized structures using the M06 level of DFT and the 6-31G* basis set for
H, C, N, and O and the SDD basis set for Mn and Br.
c From reference 47.
3.5. Electronic absorption spectroscopy
The electronic absorption spectra of the four photoCORMs in CH3CN are presented in Figure 5, and the
absorption maxima and extinction coefficients are provided in Table 2. For fac-[Mn(NN)(CO)3Br]
13

compounds, the lowest energy absorbance band is known to have both Mn(dπ)→NN(π*) singlet metal-to-
ligand charge transfer (1MLCT) and Br(p)→NN(π*) singlet halide-to-ligand charge transfer (1XLCT)
character, while the fac-[Mn(NN)(CO)3(py)]+ analogues’ lowest energy band is expected to have primarily
1MLCT character. As expected based on the previously reported 4,4′-Me2bpy-Br and 4,4′-Me2bpy-py
complexes,[47] a blue shift in the lowest energy band is observed by replacing the Br− in 6,6′-Me2bpy-Br
(λmax = 397 nm; ε = 2800 M−1cm−1) with py to afford 6,6′-Me2bpy-py (λmax = 374 nm; ε = 3300 M−1cm−1)
due to the stabilization of the Mn(dπ) orbitals by the π-accepting py. In comparing the effects of the methyl
substituent placement on Me2bpy, a small blue shift in the MLCT band is observed for the 6,6′-Me2bpy
complexes compared to their 4,4′-Me2bpy analogues. Moving the methyl substituents from the 4,4′
positions to the 6,6′ positions causes shifting from 411 to 397 nm (4,4′-Me2bpy-Br and 6,6′-Me2bpy-Br,
respectively) and from 378 to 374 nm for 4,4′-Me2bpy-py and 6,6′-Me2bpy-py, respectively. While the
cause of this small blue shift is not clear, and it may be a combination of both steric and electronic factors,
a similar trend has been observed in related compounds featuring lowest energy 1MLCT absorbance bands.
For the analogous fac-[Re(Me2bpy)(CO)3(py)]+ compounds in CH3CN, the 1MLCT band was found at 369
and 364 nm for 4,4′-Me2bpy and 6,6′-Me2bpy, respectively.[70] Similarly, moving the methyl substituents
1
from 4,4′ to 6,6′ caused a shift from 441 to 418 nm for the MLCT band for [Mo(Me2bpy)(CO)4] in
acetone.[71] This shift was also observed for [Ru(bpy)2(Me2bpy)]2+, in which the lowest energy band,
comprising both Ru(dπ)→bpy(π*) and Ru(dπ)→Me2bpy(π*) MLCT transitions, has a maximum at 455 and
453 nm for 4,4′-Me2bpy and 6,6′-Me2bpy, respectively.[72]
14

Figure 5. Electronic absorption spectra for 4,4′-Me2bpy-Br (black solid), 6,6′-Me2bpy-Br (blue solid), 4,4′-
Me2bpy-py (black dashed) and 6,6′-Me2bpy-py (blue dashed) in rt CH3CN. Inset: zoom-in region
highlighting the lowest energy absorbance bands.
Table 2. Electronic absorption spectroscopy data for fac-[Mn(NN)(CO)3Br] and fac-[Mn(NN)(CO)3(py)]+
(NN = 4,4′-Me2bpy and 6,6′-Me2bpy) in rt CH3CN.
MLCT
λ, nm (ε × 10−4, M−1cm−1)
411 (2.9)
π→π*
λ, nm (ε × 10−4, M−1cm−1)
290 (18)
Compound
4,4′-Me2bpy-Br a
6,6′-Me2bpy-Br
4,4′-Me2bpy-py a
6,6′-Me2bpy-py
a From reference 47.
397 (2.8)
328 (9.8)
378 (3.4)
287 (18)
374 (3.3)
330 (13)
3.6. Photochemical ligand dissociation
1
In our previous work using both FTIR and H NMR to interrogate the photointermediates produced
during irradiation with visible light,[47] we reported that irradiation of 4,4′-Me2bpy-Br and 4,4′-Me2bpy-
py in CH3CN results in formation of two dicarbonyl intermediates. In the first step, one CO is exchanged
with a solvent molecule, and in the second step, the monodentate ligand L is replaced with a second solvent
15

molecule. The carbonyl ligands provide a convenient probe into the ligand exchange processes that occur
during photolysis of the fac-[Mn(NN)(CO)3(L)]n+ complexes with visible light, as the number of νCO signals
and relative frequencies can be correlated to the molecule’s point group and ligand set. Figure 6 highlights
the comparison between the FTIR spectral changes during photolysis of the NN = 4,4′-Me2bpy compounds
(A and B) with the NN = 6,6′-Me2bpy compounds (C and D). No spectral shifts are observed by FTIR, 1H
NMR, or electronic absorption spectroscopy when samples are stored in solution in the dark for at least 1 h
1
(Figures S5-S8). H NMR analysis (Figures S9-S10) complements the information obtained by FTIR
because, although it gives no clear information about the number of CO ligands coordinated, it provides
information about whether a mirror plane bisecting the NN ligand is present.
In the case of 4,4′-Me2bpy-Br and 6,6′-Me2bpy-Br, the most notable difference is that while two sets of
intermediate νCO bands are observed for 4,4′-Me2bpy-Br (Fig. 6A), only one set of intermediate bands is
observed for 6,6′-Me2bpy-Br (Fig. 6C). The loss of the original signals at 2023, 1926, 1919 cm−1 is
accompanied by the appearance of new bands at 1960 and 1876 cm−1. Considering the similar frequencies
between this set and the second intermediate bands for the 4,4′-Me2bpy-Br analogue, this intermediate is
likely a dicarbonyl intermediate in which one CO and Br− are both replaced by CH3CN solvent. The
observed νCO bands are found at frequencies similar to those predicted by DFT calculations (Table S5) for
either cis,cis-[Mn(6,6′-Me2bpy)(CO)2(CH3CN)2]+ (1967 and 1908 cm−1) or cis,trans-[Mn(6,6′-
Me2bpy)(CO)2(CH3CN)2]+ (1966 and 1908 cm−1). As no evidence of a Br−-containing intermediate is
observed on the time scale of this experiment, we propose that the Mn-Br bond is considerably more labile
following the first CO dissociation with the addition of steric bulk at the 6,6′ positions than in the 4,4′-
Me2bpy analogue. 1H NMR analysis for 4,4′-Me2bpy-Br indicated that loss of Br− from the cis,cis-[Mn(4,4′-
Me2bpy)(CO)2(CH3CN)Br] intermediate forms cis,cis-[Mn(4,4′-Me2bpy)(CO)2(CH3CN)2]+ followed by
conversion to the cis,trans-[Mn(4,4′-Me2bpy)(CO)2(CH3CN)2]+ isomer (in which both CO ligands are in
equatorial positions and both CH3CN ligands are in axial positions). A similar analysis of 6,6′-Me2bpy-Br
1
by H NMR (Figure S9) shows only the formation of a symmetric intermediate, cis,trans-[Mn(6,6′-
16

Me2bpy)(CO)2(CH3CN)2]+. DFT calculations predict that the νCO are very similar between the cis,cis and
1
cis,trans isomer, so only H NMR can differentiate between these two species. We propose that the
structural distortion imparted by the 6,6′-Me2bpy ligand increases the photolability, causing the formation
and
consumption
of
cis,cis-[Mn(6,6′-Me2bpy)(CO)2(CH3CN)Br]
and
cis,cis-[Mn(6,6′-
Me2bpy)(CO)2(CH3CN)2]+ to occur on a timescale too fast to observe by FTIR and 1H NMR spectroscopy
(Scheme 1).
Figure 6. FTIR spectra for 4,4′-Me2bpy-Br (A), 4,4′-Me2bpy-py (B), 6,6′-Me2bpy-Br (C), and 6,6′-Me2bpy-
py (D), in rt CH3CN (1 mM) following irradiation with λ = 405 nm.
17

Scheme 1. Proposed photochemical ligand exchange mechanism for fac-[Mn(6,6′-Me2bpy)(CO)3Br] in
CH3CN.
The FTIR spectra collected during photolysis of 4,4′-Me2bpy-py (Figure 6B) and 6,6′-Me2bpy-py
(Figure 6D) show very similar changes. In our previous work with 4,4′-Me2bpy-py,[47] we found that the
first photochemical intermediate is cis,trans-[Mn(4,4′-Me2bpy)(CO)2(CH3CN)(py)]+ which forms through
exchange of the axial CO with CH3CN. Absorption of a photon is then required to exchange py with CH3CN
to form the second intermediate, cis,trans-[Mn(4,4′-Me2bpy)(CO)2(CH3CN)2]+. Unlike the L = Br−
analogue, this process does not occur in the absence of light. In the FTIR spectrum (Figure 6B), the
cis,trans-[Mn(4,4′-Me2bpy)(CO)2(CH3CN)(py)]+ bands at 1948 and 1867 cm−1 are followed by a slightly
higher energy set (1962 and 1879 cm−1) that appear as shoulders on the first intermediate bands,
corresponding to cis,trans-[Mn(4,4′-Me2bpy)(CO)2(CH3CN)2]+.[47] The same trend is observed for 6,6′-
Me2bpy-py, in which the original bands at 2039, 1950, and 1936 cm−1 decrease in intensity as the new bands
at 1948 and 1863 cm−1 appear (Figure 6D). Further irradiation causes the decrease in the first intermediate
bands concomitant with the appearance of the second intermediate bands at 1958 and 1875 cm−1. These two
intermediates are assigned as cis,trans-[Mn(6,6′-Me2bpy)(CO)2(CH3CN)(py)]+ and cis,trans-[Mn(6,6′-
1
Me2bpy)(CO)2(CH3CN)2]+, respectively, based on the 4,4′-Me2bpy analogues and the H NMR of the
photointermediates (Figure S10). The theoretical νCO bands calculated by DFT for cis,trans-[Mn(6,6′-
18

Me2bpy)(CO)2(CH3CN)(py)]+ (1893 and 1953 cm−1) and cis,trans-[Mn(6,6′-Me2bpy)(CO)2(CH3CN)2]+
(1908 and 1966 cm−1), which highlight a small increase in stretching frequency upon replacement of py
with CH3CN, are in sound agreement with experimental results (Table S5).
The most notable difference arising from the placement of the methyl substituents is the lability of the
py ligand from the cis,trans-[Mn(NN)(CO)2(CH3CN)(py)]+ intermediate. A sample of 6,6′-Me2bpy-py was
irradiated in CH3CN for 25 s (red line, Figure 7). At this time, both the first and second intermediates,
cis,trans-[Mn(NN)(CO)2(CH3CN)(py)]+ and cis,trans-[Mn(NN)(CO)2(CH3CN)2]+, respectively, were
observed. The FTIR spectrum was then measured after the sample was stored in the dark for 5 min (gray
line, Figure 7). At this point, the first intermediate completely converted into the second intermediate,
indicating thermal dissociation of py from this dicarbonyl intermediate, as shown in Scheme 2. This is in
stark contrast to the analogous experiment with 4,4′-Me2bpy-py, in which this py exchange required
irradiation.[47] The structural distortion imparted by the 6,6′-Me2bpy apparently weakens the Mn-py bond
in this intermediate, facilitating facile exchange with solvent. This thermal exchange of L with solvent in
the dark was observed for 4,4′-Me2bpy-Br,[47] and the Br− ligand is even more labile in the 6,6′-Me2bpy
case, as no cis,cis-[Mn(6,6′-Me2bpy)(CO)2(CH3CN)Br] intermediate can be observed on the timescales of
these experiments. This steric bulk also leads to a shorter irradiation time required to fully dissociate all CO
ligands from 1 mM CH3CN solutions of 6,6′-Me2bpy-py vs 4,4′-Me2bpy-py, as shown in Figures 6B and
6D.
19

Figure 7. FTIR spectra for 6,6′-Me2bpy-Br in CH3CN after irradiation for 25 s with 405 nm light to form
the first intermediate (red line), and after storing the sample (containing the original species and first
intermediate) in the dark in solution for 5 minutes (gray line). The first and second intermediates are
indicated with circles and asterisks, respectively.
Scheme 2. Proposed photochemical ligand exchange mechanism for fac-[Mn(6,6′-Me2bpy)(CO)3(py)]+ in
CH3CN.
The quantum yield for exchange of the first CO ligand (ΦCO) from Mn(I) photoCORMs has been
reported to generally increase with increasing π-acidity of the NN ligand. We investigated the impact of
steric distortion on ΦCO by irradiating solutions of 6,6′-Me2bpy-Br and 6,6′-Me2bpy-py in CH3CN with 405
nm light (flux = 2.7 × 10−8 mol photons/s). The electronic absorption spectra for the photolysis of 6,6′-
Me2bpy-Br in CH3CN is presented in Figure S1. For 6,6′-Me2bpy-Br and 6,6′-Me2bpy-py, the values for
ΦCO were calculated to be 0.38 ± 0.01 and 0.27 ± 0.01, respectively. The greater ΦCO for 6,6′-Me2bpy-Br
compared to 6,6′-Me2bpy-py is consistent with the trend for 4,4′-Me2bpy-Br and 4,4′-Me2bpy-py (ΦCO
=
20

0.20 ± 0.01 and 0.15 ± 0.01, respectively).[47] The increased ΦCO for each NN = 6,6′-Me2bpy complex
compared to its 4,4′-Me2bpy analogue is presumably due to the steric distortion caused by the location of
the methyl substituents and resulting bond angle distortions, as discussed above, which destabilize the
metal-ligand bonds and makes them more susceptible to exchange with solvent. For 6,6′-Me2bpy-Br and
6,6′-Me2bpy-py, it should be noted that the measured ΦCO is an “apparent” ΦCO, as exchange of the first CO
and Br−/py occur on similar timescales. The inability to resolve these two processes from each other
complicates the measurement of this value; however, the presence of an isosbestic point during early
photolysis times (Figure S1) allows for determination of a quantum yield for the photochemical process.
As irradiation of 6,6′-Me2bpy-Br and 6,6′-Me2bpy-py with 405 nm leads to the eventual loss of all three
coordinated CO ligands, as observed by FTIR analysis, the question remains about the fate of the rest of
the metal complex. Analyses of related Mn(I) photoCORMs described the photoproducts containing
oxidized Mn species, including [Mn(NN)3]2+, [Mn(NN)2(solvent)2]2+, MnO2, and solvated Mn2+
ion.[40,41,44,54] During irradiation of the fac-[Mn(NN)(CO)3L]n+ complexes, the 1H NMR signals become
significantly broadened, suggesting the formation of a paramagnetic species. An analysis of the electronic
absorption spectra of solutions following exhaustive photolysis (Figure S11), in comparison to the free 4,4′-
Me2bpy and 6,6′-Me2bpy ligands, suggests that a significant amount of free 6,6′-Me2bpy is present in
solution following irradiation of both 6,6′-Me2bpy-Br and 6,6′-Me2bpy-py. In contrast, the electronic
absorption spectra for the 4,4′-Me2bpy-Br and 4,4′-Me2bpy-py photoproduct solutions are much more
similar to the spectrum of [Mn(4,4′-Me2bpy)3]2+ (Figure S11). Additionally, free 6,6′-Me2bpy is detected
1
by H NMR in solution after photolysis (Figure S12), while 4,4′-Me2bpy is not. The 6,6′-position of the
methyl groups is expected to inhibit the formation of [Mn(6,6′-Me2bpy)3]2+, resulting in a greater amount
of free ligand in solution.
21

4. Conclusions
Two new fac-[Mn(NN)(CO)3(L)]n+ photoCORMs, 6,6′-Me2bpy-Br and 6,6′-Me2bpy-py, featuring a
sterically bulky bidentate ligand, were prepared, and their photophysical and photochemical properties were
investigated compared to the previously reported 4,4′-Me2bpy analogues. While the placement of the
methyl substituents has little impact on the electronic properties of the complexes, the steric distortion is
apparent in the crystal structures of the four compounds. Notably, the steric bulk of the 6,6′-Me2bpy ligand
causes a contraction of the angle between the two equatorial CO ligands and a significantly canted bidentate
ligand relative to the 4,4′-Me2bpy complexes. The steric bulk enhances the ligand exchange efficiencies
and increases the labilities of both CO and ancillary Br− or py ligands during irradiation with visible light
while maintaining stability in solution in the dark. Work is underway to further study the impact of ligand
sets on photochemical ligand dissociation in photoCORMs.
Acknowledgments
We thank the Department of Chemistry and Biochemistry and the College of Arts and Sciences at Ohio
University for start-up funds. We also acknowledge the Ohio Supercomputer Center for an allocation of
computing time.
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests:
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