Thermal and photochemical reactivity of manganese tricarbonyl and tetracarbonyl complexes with a bulky diazabutadiene ligand

Article abstract of DOI:10.1021/ic500025k

The manganese tricarbonyl complex fac-Mn(Br)(CO)₃( ⁱPr₂Ph-DAB) (1) [ⁱPr₂Ph-DAB = (N,N′-bis(2,6-di-isopropylphenyl)-1,4-diaza-1,3-butadiene)] was synthesized from the reaction of Mn(CO)₅Br with the sterically encumbered DAB ligand. Compound 1 exhibits rapid CO release under low power visible light irradiation (560 nm) suggesting its possible use as a photoCORM. The reaction of compound 1 with TlPF₆ in the dark afforded the manganese(I) tetracarbonyl complex, [Mn(CO)₄(ⁱPr ₂Ph-DAB)][PF₆] (2). While 2 is comparatively more stable than 1 in light, it demonstrates high thermal reactivity such that dissolution in CH₃CN or THF at room temperature results in rapid CO loss and formation of the respective solvate complexes. This unusual reactivity is due to the large steric profile of the DAB ligand which results in a weak Mn-CO binding interaction.

Full text of DOI:10.1021/ic500025k

Article
pubs.acs.org/IC
Thermal and Photochemical Reactivity of Manganese Tricarbonyl
and Tetracarbonyl Complexes with a Bulky Diazabutadiene Ligand
Veeranna Yempally,† Samuel J. Kyran,‡ Rajesh K. Raju,† Wai Yip Fan,§ Edward N. Brothers,†
Donald J. Darensbourg,*,‡ and Ashfaq A. Bengali*,†
†Department of Chemistry, Texas A&M University at Qatar, Doha, Qatar
§Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 119077
‡Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States
S
* Supporting Information
ABSTRACT: The manganese tricarbonyl complex fac-Mn-
(Br)(CO)3(iPr2Ph-DAB) (1) [iPr2Ph-DAB = (N,N′-bis(2,6-di-
isopropylphenyl)-1,4-diaza-1,3-butadiene)] was synthesized
from the reaction of Mn(CO)5Br with the sterically
encumbered DAB ligand. Compound 1 exhibits rapid CO
release under low power visible light irradiation (560 nm)
suggesting its possible use as a photoCORM. The reaction of
compound 1 with TlPF6 in the dark afforded the manganese-
(I) tetracarbonyl complex, [Mn(CO)4(iPr2Ph-DAB)][PF6]
(2). While 2 is comparatively more stable than 1 in light, it
demonstrates high thermal reactivity such that dissolution in
CH3CN or THF at room temperature results in rapid CO loss and formation of the respective solvate complexes. This unusual
reactivity is due to the large steric profile of the DAB ligand which results in a weak Mn−CO binding interaction.
inhalation.11a An important consideration in the design of
these molecules is the facility with which they release CO either
INTRODUCTION
■
Transition metal complexes with 1,4-diaza-1,3-butadiene
thermally or photochemically under visible light irradiation
(photoCORMs). Several strategies have been adopted to
improve the performance of metal carbonyl complexes in this
respect. The use of sterically encumbered bulky chelating
ligands for the activation of small molecules and C−H bonds
has been successfully employed thus far,1b,c,12 but extending the
same strategy for CORM synthesis is largely unexplored and
needs attention.
To ascertain the impact of bulky DAB ligands upon the
properties of the corresponding manganese carbonyl com-
plexes, the thermal and photochemical reactivity of the
sterically encumbered fac-Mn(Br)(CO)3(iPr2Ph-DAB) (1)
[iPr2Ph-DAB = (N,N′-bis(2,6-di-isopropylphenyl)-1,4-diaza-
1,3-butadiene)] and related cationic complexes is reported in
this study (Figure 1). These complexes exhibit interesting
reactivity including efficient thermal and photochemical CO
loss. As such, the present findings suggest a role for steric
manipulation of ligand characteristics in the design of CORMs.
The experimental results were found to be consistent with
theoretical calculations using DFT.
(DAB) ligands bearing bulky aryl or alkyl groups on the
nitrogen atoms have garnered much attention owing to their
unusual oxidation state at the metal center and easily tunable
stereoelectronic properties.1 Low lying and long-lived MLCT
excited states2 and excellent coordination properties3 are some
of the key advantages of DAB ligands over conventional
phosphine ligands.4 Unusual electron donor and acceptor
properties coupled with tunable absorption in the visible region
allow metal-DAB complexes to be used in the preparation of
photoactive materials5 and in olefin polymerization chemistry.6
Over the past few decades, several studies aimed at exploring
the potential application of [MX(CO)n(DAB)] (M = Ru or Re
and X = halide or metal) compounds in the electrocatalytic
reduction of CO2 and in the synthesis of photoluminescent
materials have been conducted.7,8
While a considerable number of reports have been published
on the photoreactivity of [MnX(CO)3(α-diimine)] (X = Br,
Cl) complexes,9 less is known for DAB based manganese
tricarbonyl complexes with bulky aryl substituents on the
nitrogen atoms.10 Manganese tricarbonyl complexes have
recently received much attention in the field of CO releasing
molecules (CORMs).11 CO gas is known to dilate blood vessels
in a manner similar to nitric oxide and has been shown to
possess anti-inflammatory and anti-apoptotic properties.11b
Metal carbonyl complexes can be used as CO carriers, thus
circumventing the inherent toxicity associated with CO
Received: January 6, 2014
Published: April 1, 2014
© 2014 American Chemical Society
4081
dx.doi.org/10.1021/ic500025k | Inorg. Chem. 2014, 53, 4081−4088

Inorganic Chemistry
Article
fac-[Mn(CO)3(iPr2Ph-DAB)(CH3CN)]PF6 (3a). A round-bottom
flask was charged with 0.100 g of 1 (0.17 mmol), 0.087 g TlPF6 (0.24
mmol), and 20 mL of anhydrous acetonitrile under nitrogen
atmosphere. The solution was left overnight with constant stirring at
room temperature. The reaction mixture was filtered through a Celite
pad to remove thallium bromide and evaporated to yield a red residue
which was washed several times with diethyl ether and dried in vacuo
to afford 0.085 g (73% yield) of red solid, 3a. IR data in
dichloromethane (νCO in cm‑1): 2054 (s), 1979 (s), 1963 (m).
NMR data in CDCl3: 1H δ 8.50 (s, 2 H), 7.40 (m, 6 H), 3.10 (s, 2 H),
2.77 (s, 2 H), 2.54 (s, CH3, acetonitrile), 1.28 (s, 12 H), 1.19 (s, 12
H); 19F δ −73.3 (d, J = 712 Hz); 31P δ −144.5 (pentet, J = 712 Hz).
UV−vis data in dichloromethane (λmax in nm): 365, 487. HRMS (EI+)
[M - PF6]+ (calc.), 556.232; [M - PF6]+ (exp.), 556.237.
Figure 1. Molecular structures of the synthesized complexes.
fac-[Mn(CO)3(iPr2Ph-DAB)(CNBut)]PF6 (3b). A round-bottom
flask was charged with 0.100 g of 1 (0.17 mmol), 0.070 g TlPF6
(0.20 mmol), 0.022 g CNtBu (0.26 mmol), and 20 mL of anhydrous
dichloromethane under a nitrogen atmosphere. The solution was left
overnight with constant stirring at room temperature. The reaction
mixture was filtered through a Celite pad to remove thallium bromide,
and the filtrate was evaporated to yield a red residue which was washed
several times with hexane and dried in vacuo to afford 0.120 g (96%
yield) of red solid, 3b. IR data in dichloromethane (νCO in cm−1):
2051 (s), 1983 (m, br), νCN: 2187 cm‑1. NMR data in CDCl3: 1H δ
8.51 (s, 2 H), 7.40 (m, 6 H), 3.26 (s, 2 H), 2.58 (s, 2 H), 1.75 (s, But,
9 H), 1.42 (s, 12 H), 1.21 (s, 12 H); 19F δ −72.3 (d, J = 734 Hz); 31P δ
−144.5 (pentet, J = 712 Hz). UV−vis data in dichloromethane (λmax in
nm): 356, 473. HRMS (EI+) [M - PF6]+ (calc.), 598.257; [M - PF6]+
(exp.), 598.284.
EXPERIMENTAL SECTION
■
General. Unless otherwise stated, all experiments were performed
in the absence of light. Anhydrous solvents and Schlenk techniques
were used for the synthesis of all complexes. Manganese pentacarbonyl
bromide (Mn(CO)5Br) and thallium hexafluorophosphate (TlPF6)
i
were purchased from Strem Chemicals, and the ligands Pr2Ph-DAB
and tert-butyl isocyanide (CNBut) from Sigma-Aldrich, and used
without further purification. NMR spectra were recorded on either a
Varian INOVA 500 or a Bruker Advance II 400 spectrometer. 1H, 19F,
and 13C NMR spectra were referenced to residual solvent resonances.
Infrared spectra were obtained on a Bruker Tensor 27 or Vertex 80
FTIR spectrometer. UV−vis spectra were obtained using a Perkin-
Elmer Lambda 950 spectrometer. Mass spectral analysis for all
compounds was performed by the laboratory for Biological Mass
Spectroscopy at Texas A&M University, College Station, TX.
fac-ReBr(CO)3(iPr2Ph-DAB) (4). A round-bottom flask was
i
fac-Mn(Br)(CO)3(iPr2Ph-DAB) (1). A round-bottom flask was
charged with 0.318 g of Mn(CO)5Br (1.16 mmol), 0.436 g of Pr2Ph-
charged with 0.220 g of Re(CO)5Br (0.54 mmol), 0.205 g Pr2Ph-
i
DAB (0.54 mmol), and 20 mL of anhydrous toluene under a nitrogen
atmosphere. The solution was refluxed with constant stirring for 3 h.
After cooling to room temperature and evaporation of solvent, a solid
purple colored residue was recovered which was redissolved in
dichloromethane and upon slow evaporation of solvent at 0 °C
afforded 0.340 g (86% yield) of dark violet crystals of 4. IR data in
dichloromethane (νCO in cm−1): 2035 (s), 1971 (s), 1930 (m). NMR
data in CDCl3: 1H δ 8.68 (s, 2 H), 7.40 (m, 6 H), 4.00 (septet, 2 H),
2.77 (septet, 2 H), 1.36 (quartet, 12 H), 1.14 (quartet, 12 H); 13C{1H}
δ 166.5 (s), 148.1 (s), 141.4 (d), 128.5 (s, 1), 124.9 (t), 28.5 (d), 26.9
(d), 23.0 (d). UV−vis data in acetonitrile (λmax, nm): 325, 375, 499.
HRMS (EI+) ; [M - Br]+ (calc.), 647.223; [M - Br]+ (exp.), 647.228.
CW Photolysis Experiments (CO release studies). The light
source for these experiments was a 100 W Xe arc lamp coupled to a
monochromator to isolate the appropriate photolysis wavelength. The
irradiation power at the sample was measured to be 0.3 mW with an
optical power meter (ThorLabs). The samples were prepared in dim
light and placed in an IR cell with CaF2 windows positioned 2 cm away
from the exit slit of the monochromator. The changes in the IR spectra
during irradiation were monitored with a Bruker Vertex 80 FTIR.
Time Resolved IR Studies. Samples were irradiated in a 0.75 mm
path length temperature controlled cell (Harrick Scientific) with CaF2
windows using 355 nm light from a Nd:YAG laser (Quantel, Brilliant
B). For low temperature experiments, a 0.5 mm path length variable
temperature IR cell (Specac) was used. Changes in the IR spectra
following a single shot of the UV laser were monitored using rapid
scan FTIR (Bruker Vertex 80).
X-ray Crystallography. Diffraction data for 1, 2, 3a, 3b, and 4
were obtained by placing the crystals under streaming nitrogen (150
K) in a SMART Apex CCD diffractometer. The space groups were
determined on the basis of systematic absences and intensity statistics.
The structures were solved by direct methods and refined by full-
matrix least-squares on F2. Anisotropic displacement parameters were
determined for all non-hydrogen atoms. Hydrogen atoms were placed
at idealized positions and refined with fixed isotropic displacement
parameters. The following is a list of programs used: data collection
and cell refinement, APEX2;14 data reductions, SAINTPLUS version
6.63;15 absorption correction, SADABS;16 structural solutions,
SHELXS-97;17 structural refinement, SHELXL-97;18 graphics and
DAB, and 20 mL of anhydrous diethyl ether under a nitrogen
atmosphere. The solution was left overnight with constant stirring at
room temperature. The solvent was evaporated to yield a solid violet
colored residue which was redissolved in dichloromethane and upon
evaporation of the solvent at 0 °C afforded 0.600 g (87% yield) of dark
violet crystals of 1. IR data in dichloromethane (νCO in cm−1): 2035
(s), 1971 (s), 1930 (m). NMR data in CDCl3: 1H δ 8.16 (s, 2 H), 7.17
(m, 6 H), 3.85 (s, 2 H), 2.69 (s, 2 H), 1.17 (s, 12 H), 1.92 (s, 12 H);
13C{1H} δ 164.5(s), 149.1(s), 140.4 (d), 128.5 (s, 1), 124.9 (t),
28.5(d), 26.9 (d), 23.3 (d). UV−vis data in dichloromethane (λmax,
nm): 366, 582. HRMS (EI+) ; [M - Br]+ (calc.), 515.210; [M - Br]+
(exp.), 515.210.
fac-Mn(Br)(CO)3(bpy) (bpy = 2,2′-bipyridine). This complex
was synthesized according to literature procedure13 and characterized
using IR spectroscopy in dichloromethane (νCO in cm−1): 2022 (s),
1934 (s), 1912 (m).
[Mn(CO)4(iPr2Ph-DAB)]PF6 (2). Method 1: A round-bottom flask
was charged with 0.100 g of 1 (0.17 mmol), 0.075 g of TlPF6 (0.21
mmol), and 20 mL of anhydrous dichloromethane under a nitrogen
atmosphere. The solution was left overnight with constant stirring at
room temperature. The reaction mixture was filtered through a Celite
pad to remove thallium bromide and the filtrate was evaporated to
yield a yellow oily residue which was washed several times with diethyl
ether and dried in vacuo to afford 0.055 g (47% yield) of a yellow-
orange solid, 2. IR data in dichloromethane (νCO in cm−1): 2109 (w),
2042 (sh), 2027 (s), 2006 (m). NMR data in CDCl3: 1H δ 8.67 (s, 2
H), 7.47 (m, 6 H), 3.76 (s, 2 H), 2.83 (s, 2 H), 1.31 (s, 12 H); 19F δ
−72.1 (d, J = 712 Hz); 31P δ −144.8 (septet, J = 712 Hz). UV−vis
data in dichloromethane (λmax in nm): 350, 406. HRMS (EI+) [M -
PF6]+ (calc.), 543.202; [M -PF6]+(exp.), 543.205. Method 2: A round-
bottom flask was charged with 0.100 g of 1 (0.17 mmol), 0.065 g
TlPF6 (0.18 mmol), and 20 mL of anhydrous 1,2-dichlorethane under
a nitrogen atmosphere. After stirring for 1 h at room temperature, CO
gas was bubbled through the solution for 2 h. After additional stirring
for 16 h, the solution was filtered through a Celite pad to remove
thallium bromide and evaporated to yield a brown solid residue which
was washed several times with diethyl ether and dried in vacuo to
afford 0.088 g (75% yield) of yellow-orange solid, 2.
4082
dx.doi.org/10.1021/ic500025k | Inorg. Chem. 2014, 53, 4081−4088

Inorganic Chemistry
Article
publication materials, Mercury version 3.0.19 The data is available in
the Supporting Information.
DFT Calculations. All calculations were performed in the Gaussian
09 suite of programs using density functional theory (DFT).20 All
geometries were optimized with the ωB97XD functional21 using the
Def2-SVP basis set.22 Frequency calculations were also performed on
the optimized geometries to confirm energy minima by the absence of
imaginary frequencies.
impact upon the Mn−CO interaction in this complex, the Mn−
CO bond dissociation enthalpy (BDE) in 1 and in the less
encumbered fac-Mn(Br)(CO)3(Ph-DAB) species was calcu-
lated using DFT. The Mn−CO BDEs calculated at 37.1 kcal/
mol (axial) and 26.9 kcal/mol (equatorial) were found to be
significantly lower in 1 compared to fac-Mn(Br)(CO)3(Ph-
DAB) at 49.6 kcal/mol (axial) and 34.1 kcal/mol (equatorial).
The weakening of the Mn−CO bond as a result of increased
steric crowding around the metal center suggests that the
iPr2Ph-DAB ligand may be of utility in the design of efficient
CORMs or photoCORMs.
RESULTS AND DISCUSSION
■
MnBr(CO)3(iPr2Ph-DAB) (1). Reaction of Mn(CO)5Br with
i
the Pr2Ph-DAB ligand at room temperature affords 1 in good
Visible Light Photolysis. When a heptane solution of 1 is
exposed to room light at ambient temperature, the color
changes from dark violet to yellow within 30 min. This reaction
is reversible and, in the absence of light, the yellow solution
converts back to dark violet. IR analysis of the yellow solution
indicates the presence of Mn(CO)5Br and evaporation resulted
in the isolation of yellow crystals which were confirmed to be
yield. The crystal structure (Figure 2) shows a facial
i
the free Pr2Ph-DAB ligand by X-ray analysis. The reversibility
in the dark can then be explained as a thermal reaction between
the free diimine ligand and the pentacarbonyl to reform 1.
Complex 1 is stable in the dark for several days without
decomposition. Due to biological considerations, an important
design parameter for photoCORMs is the requirement that
they release CO upon visible light irradiation. The wavelength
range for such photodynamic therapies is generally 630−850
nm, and viable targets should undergo CO release under these
irradiation conditions.24 While most photoCORMs readily
demonstrate CO release under UV irradiation,11d,e,h,25 very few
examples exist for similar photo activity with visible light (λ >
400 nm). Complex 1 has an absorption maxima at 582 nm
(CH2Cl2), and to determine the rate of CO photorelease, its
decomposition was monitored by IR spectroscopy following
irradiation at 560 nm. As shown in Figure 3, the CO bands of 1
steadily decrease in intensity without the formation of any
intermediates that could be detected under these conditions. A
small broad peak at 2140 cm−1 grows in and is assigned to free
CO in solution providing clear evidence for CO release upon
visible light photolysis. The half-life for the CO release process
was 12 min and, given the low power of the light source used
(0.3 mW), this rate compares very favorably to those found for
some other Mn complexes. For example, photolysis of
[Mn(CO)3(tpa)]ClO4 (tpa = tris(2-pyridyl)amine) with
∼100 mW visible light (λ > 350 nm) yields an apparent CO
release rate constant of 6 × 10−3 s−1 (t1/2 = 2 min).11g
Figure 2. Thermal ellipsoid (probability level 50%) plot of 1 with
select atom labeling. Hydrogen atoms omitted for clarity.
arrangement of the CO ligands and the relative intensities of
the three CO stretching bands observed in the IR spectrum
(Table 1) are consistent with this geometry. An interesting
structural feature in 1 is the significant deviation from linearity
in the axial ∠Br−Mn−CO = 172.51°, which is most likely due
to steric congestion around the Mn center because of the bulky
iPr2Ph-DAB ligand. This bond angle is in good agreement with
a DFT calculated value of 170.71° and indicates the level of
theory applied is suitable for describing the current systems.
The extent of this angular deviation is unusual by comparison
with other Mn(Br)(CO)3(diimine) complexes in which the
axial ∠CO−Mn−Br ranges from 177.54° to 178.89°.11e,23 In
the electronically similar yet sterically different Mn(Br)-
(CO)3(Ph-DAB) complex, DFT calculations predict an almost
linear (179.9°) ∠CO−Mn−Br.
The Re analog (4) was also synthesized and compared to 1;
the crystal structure (Figure S1) indicates a larger value for the
axial ∠CO−Re−Br = 176.05°. The steric influence of the
iPrPh2-DAB ligand upon this bond angle is diminished in this
case as the larger Re center provides more space to
accommodate the coordinated ligands. To isolate the steric
Recently, rapid CO photorelease from two manganese
complexes at λ > 520 nm was reported.26 Thus, complex 1,
which exhibits CO dissociation at 560 nm, is a rare example of a
visible light induced photoCORM.11f,l While more rigorous
experiments, to include myoglobin assays and quantification of
a
Table 1. CO Stretching Wavenumbers for Several Complexes at 293 K
c
complex
vCO (cm‑1) (exp)
vCO (cm‑1) (calc.)
Mn(Br)(CO)3(R-DAB) (1)
2035, 1971, 1930
2189, 2142, 2093
Mn(Br)(CO)3(bpy)
2022, 1934, 1912
[Mn(CO)4(R-DAB)][PF6] (2)
[Mn(CH3CN)(CO)3(R-DAB)][PF6] (3a)
[Mn(THF)(CO)3(R-DAB)][PF6]
[Mn(DHF)(CO)3(R-DAB)][PF6]
[Mn(CNtBu)(CO)3(R-DAB)][PF6] (3b)
2109, 2042, 2027, 2006
2054, 1979, 1963
2253, 2197, 2187, 2181
2215, 2162, 2145
2055, 1979, 1965
2206, 2142, 2135
2053, 1979, 1964
b
2187 (CN), 2051, 1983
2211, 2161, 2159
a
b
c
i
[R = Pr2Ph, bpy = bipyridyl]. Unless otherwise noted, solvent = CH2Cl2. Broad. Unscaled.
4083
dx.doi.org/10.1021/ic500025k | Inorg. Chem. 2014, 53, 4081−4088

Inorganic Chemistry
Article
Figure 4. Spectral changes upon 355 nm photolysis of a THF solution
of 1 at 228 K. The initially formed Mn(Br)(THF)(CO)2(iPr2Ph-DAB)
species (1952 cm−1, 1895 cm−1) converts to the mer-Mn(Br)-
(CO)3(iPr2Ph-DAB) isomer (2043 cm−1, 1978 cm−1, 1936 cm−1)
within 33 s of photolysis.
Figure 3. IR spectra obtained upon exposure of 1 to 560 nm light.
Inset shows the decay of the 2035 cm−1 peak as a function of time.
CO release, are required to assess the feasibility of such
complexes for their use as photoCORMs, our intent was simply
to demonstrate the unusual dissociation of the Mn−CO bond
in 1 under visible light irradiation. By analogy with similar
diimine complexes, visible light induced cleavage of the Mn−
CO bond is likely due to a MLCT transition from a
combination Mn(1)−CO π and Br p centered orbital to the
π* orbitals of the diimine ligand resulting in reduced Mn →
CO π backbonding and subsequent CO loss.11f,26 To provide
support for this explanation, Tamm-Dancoff time dependent
density functional calculations were performed on complex 1. A
calculated transition at 563 nm has a major contribution from
the promotion HOMO-1 → LUMO which were found to have
orbital characteristics (Figures S3, S4) similar to the other
diimine systems mentioned above.
Scheme 1
and show that the Mn-THF bond in Mn(Br)(THF)-
(CO)2(iPr2Ph-DAB) (14.7 kcal/mol) is almost 10 kcal/mol
weaker than in Mn(Br)(CO)3(bpy) (24.5 kcal/mol). However,
the transition state for the final step in the isomerization is not
likely to involve significant Mn−THF bond disruption, since a
10 kcal/mol difference in BDE would result in a larger
difference in rate than that observed here.
UV Photolysis. Previous studies have demonstrated that
Mn(Br)(CO)3(R-DAB) complexes (R = iPr2, t-Bu) undergo fac
→ mer isomerization upon UV photolysis.9 Ultrafast studies
show that the isomerization proceeds by way of equatorial CO
dissociation (<400 fs) followed by Br movement (11 ps) from
the axial to equatorial plane and reattachment of CO to yield
the meridonal species.27 To study the effect of the iPr2Ph-DAB
ligand upon the rate of this isomerization, a THF solution of 1
was photolyzed with 355 nm light from a Nd:YAG laser. At 228
K, the spectral changes shown in Figure 4 confirm the initial
formation of a dicarbonyl complex absorbing at 1952 cm−1 and
1895 cm−1 which converts to a species with three CO bands. By
analogy with previous studies,9h the dicarbonyl species is
assigned to the solvated Mn(Br)(THF)(CO)2(iPr2Ph-DAB)
complex, and based on the relative intensities of the three CO
bands, the final product is identified as the meridonal isomer
(Scheme 1). A similar experiment was conducted with the less
sterically encumbered Mn(Br)(CO)3(bpy) complex. At 228 K,
the rate of conversion from the THF solvate to the meridonal
isomer was 20 times faster for complex 1 compared to
Mn(Br)(CO)3(bpy). The difference in rates may be attributed
i
The results presented above suggest that the Pr2Ph-DAB
ligand imparts unusual structural and photoreactivity properties
to complex 1, primarily as a result of the steric bulk of this
molecule. To assess the influence of this ligand upon the
thermal reactivity of the Mn−CO bond in a molecule with a
higher CO to metal ratio, an important design consideration in
CORMs, the ionic complex, [Mn(CO)4(iPr2Ph-DAB)][PF6]
(2), was synthesized.
[Mn(CO)4(iPr2Ph-DAB)][PF6](2). Addition of TlPF6 to a
dichloromethane solution of 1 in the absence of added CO
followed by crystallization provided 2 in low yield. As shown in
Scheme 2, initial generation of the Lewis acid−base adduct
[Mn(CO)3(iPr2Ph-DAB)(PF6)] followed by its partial decom-
position during the crystallization process to generate CO
which can coordinate to the residual tricarbonyl likely results in
the formation of 2.28 When the reaction was conducted in the
presence of added CO a much improved yield of 2 was realized.
The crystal structure of 2 is shown in Figure 5. As expected of a
i
to differences in the steric profile of the bpy and Pr2Ph-DAB
ligands and the resulting impact upon the Mn-THF bond
strength. DFT calculations are consistent with this conclusion
4084
dx.doi.org/10.1021/ic500025k | Inorg. Chem. 2014, 53, 4081−4088

Inorganic Chemistry
Article
Scheme 2
Figure 6. Thermal ellipsoid (probability level 50%) plot of 3a with
select atom labeling. Hydrogen atoms and counteranion omitted for
clarity.
Figure 5. Thermal ellipsoid (probability level 50%) plot of 2 with
select atom labeling. Hydrogen atoms and counteranion omitted for
clarity.
cationic complex with four strong π accepting ligands, the CO
stretching bands in 2 are shifted to significantly higher
wavenumbers relative to 1 (Table 1). Similar to 1, the steric
i
influence of the P2Ph-DAB ligand upon the geometry of 2 is
evident in the axial ∠CO−Mn−CO, which at 170.75° is
significantly less than that found in analogous complexes where
this angle ranges from 174.14° to 178.9°.29
Figure 7. Spectral changes observed upon reaction of a dichloro-
methane solution of 2 with 0.5 M CH3CN at 288 K to yield 3a.
Thermal Reactivity. The expected weakening of the Mn−
CO bonds due to the crowded metal center in 2 is strikingly
apparent in the thermal reactivity of this complex. For example,
dissolution of the yellow-orange solid in acetonitrile at 288 K
results in an instantaneous color change to red. The IR
spectrum of this red solution shows three peaks with relative
intensities suggestive of a facial tricarbonyl complex. This
species was isolated and the crystal structure shown in Figure 6
confirms its identity as the acetonitrile complex [Mn(CH3CN)-
(CO)3(iPr2Ph-DAB)][PF6] (3a).
To monitor the formation of the acetonitrile adduct, a
dichloromethane solution of 2 was reacted with 0.5 M CH3CN
at 288 K. The IR spectral changes shown in Figure 7 confirm a
clean conversion of 2 into 3a under these conditions.
Unfortunately, the mechanism of this substitution reaction
and the possibility of estimating the Mn−CO bond enthalpy
could not be investigated, because at higher temperatures and
concentrations of CH3CN, additional unassigned peaks were
observed and the rate of decay of 2 did not match the growth of
the product. An associative mechanism for CO substitution
appears unlikely given the steric bulk of the iPr2Ph-DAB ligand.
However, when the concentration of CO was increased by
bubbling it through the reaction solution for several minutes,
no significant effect (less than a factor of 2) was observed on
the decay rate of 2. This observation is not consistent with a
dissociative substitution mechanism either and would suggest
an interchange (Ia or Id) pathway for the displacement of CO
from 2.
Spectral changes in the infrared observed upon dissolution of
2 in other coordinating solvents such as THF and 2,3-
dihydrofuran (DHF) suggest the displacement of an axial CO
ligand at room temperature to form the [Mn(THF)-
(CO)3(iPr2Ph-DAB)][PF6] and [Mn(DHF)(CO)3(iPr2Ph-
DAB)][PF6] complexes, respectively. The rate of CO
substitution is slower in these solvents compared to CH3CN.
More basic ligands such as pyridine and other amines reacted
with 2 to afford a monocarbonyl complex (single peak in the IR
spectrum) which was unstable and could not be isolated.
Despite the different donor characteristics of the solvent
ligands, the substituted tricarbonyl complexes have similar CO
stretching wavenumbers (Table 1). This observation suggests
that the electronic environment of the Mn center is not
influenced by the ligand donor/acceptor abilities. To ascertain
the extent of this insensitivity, the isonitrile ligand CNt-Bu, with
electronic characteristics significantly different than those of
4085
dx.doi.org/10.1021/ic500025k | Inorg. Chem. 2014, 53, 4081−4088

Inorganic Chemistry
Article
THF and CH3CN, was used to synthesize the [Mn(CNt-
Bu)(CO)3(iPr2Ph-DAB)][PF6] (3b) complex. The crystal
structure of 3b is shown in Figure 8.
In agreement with experimental findings, the DFT
calculations predict that displacement of CO in 2 by CH3CN
and THF is thermodynamically favored with ΔG°rxn (298 K) =
−5.7 kcal/mol and −1.5 kcal/mol, respectively. In all cases,
Mn−L BDEs are calculated to be significantly lower in
complexes with the larger (iPr2Ph-DAB) ligand compared to
those of the electronically similar yet smaller (Ph-DAB) ligand.
Of all the ionic complexes studied, calculations indicate that the
strongest interaction is between the Mn center and the
isonitrile ligand. This observation is consistent with the greater
σ donating ability of isonitrile compared to the other ligands
used and, as expected for a cationic complex, suggests a reduced
role for π backbonding in stabilizing the Mn-L interaction.
Photoactivity. Compared to 1, complex 2 shows enhanced
thermal reactivity with respect to CO loss. However, unlike 1, it
is stable in room light. For example, a dichloromethane solution
of 2 showed no evidence of decomposition after several hours
under room light. This relative photostability is also manifested
in the rate of CO release from 2 compared to 1. While
irradiation of 2 at its absorption maxima of 420 nm results in
CO release, the half-life of 180 min is significantly longer than
that observed for 1 (35 min) under similar conditions.
Figure 8. Thermal ellipsoid (probability level 50%) plot of 4b with
select atom labeling. Hydrogen atoms and counteranion omitted for
clarity. The unit cell contains two identical molecules.
CONCLUSIONS
Similar to the structures observed for 1, 2, and 3a, the axial
∠CO−Mn−CNt−Bu = 172.38° shows a large deviation from
linearity. Surprisingly, the CO stretching wavenumbers in this
species are comparable to those of the other solvated
complexes (Table 1). Since DAB ligands are considered to be
electron reservoirs,30 it is possible that they impart a “leveling”
effect upon the metal electron density resulting in similar CO
stretching wavenumbers for several of the observed complexes.
The DFT calculated CO band positions for these complexes
(Table 1) are also within 20 cm−1 (unscaled) of each other.
The extreme lability of the CO ligand in 2 is in dramatic
contrast to similar complexes such as [Mn(CO)4(phen)]+ and
[Mn(CO)4(dppe)]+ which demonstrate CO substitution only
at high temperatures.31 For example, [Mn(CO)4(dppe)]+ is
stable for several months in acetonitrile at room temperature.
As shown in Table 2, the Mn−CO bond in 2 is remarkably
weak (BDE = 25.3 kcal/mol) especially compared to that in
■
i
The coordination of a bulky diimine ligand, Pr2Ph-DAB, to a
Mn(1) center yields complexes with unusual thermal and
photochemical properties that may be of utility in the design of
CORMs or photoCORMs. For example, while UV photolysis
of metal carbonyl complexes results in CO loss, fac-Mn(Br)-
(CO)3(iPr2Ph-DAB) demonstrates facile photochemical CO
release under visible light (560 nm) irradiation. In contrast to
some cationic manganese tetracarbonyl complexes which are
stable toward thermal displacement of CO, the cationic
complex [Mn(CO)4(iPr2Ph-DAB)][PF6] undergoes rapid
substitution of CO, even by solvent molecules such as
CH3CN and THF. Thus, dissolution of the tetracarbonyl
complex in acetonitrile at 288 K results in instantaneous
formation of fac-[Mn(CH3CN)(CO)3(iPr2Ph-DAB)][PF6]
species. The crystal structures of the synthesized complexes
are consistent with considerable crowding around the metal
center and theoretical calculations using DFT confirm the
weakening of the metal−CO interaction is primarily due to the
steric bulk of the diimine ligand. These results indicate the
Table 2. Calculated Mn−L BDEs for Several Complexes
Demonstrating the Steric Influence of the Diimine Ligand
upon the Strength of This Interaction
i
possible use of the Pr2Ph-DAB ligand in the synthesis of
[Mn(L)(CO)3(iPr2Ph-DAB)]+
(kcal/mol)
[Mn(L)(CO)3(Ph-DAB)]+
(kcal/mol)
reactive metal carbonyl complexes with applications as CORMs
or other photoactive materials.
L
CO
25.3
30.1
26.1
43.5
35.4
40.3
41.6
52.8
CH3CN
THF
ASSOCIATED CONTENT
■
S
* Supporting Information
CNtBu
X-ray crystallographic data for 1, 2, 3a, 3b, and 4. UV−vis
spectra and calculated structures for the modeled complexes.
This material is available free of charge via the Internet at
http://pubs.acs.org.
[Mn(CO)4(Ph-DAB)]+ (35.4 kcal/mol) and [Mn-
(CO)4(bpy)]+ (31.4 kcal/mol). Given the similar Mn−CO
BDEs in these latter two complexes, the weaker interaction in 2
is clearly a result of steric rather than electronic differences
between the diimine ligands. In a related Mn complex, (η3-
allyl)Mn(CO)4, increasing the steric bulk of the allyl ligand also
resulted in an increase in the rate of CO loss.32 Since facile loss
of coordinated CO ligands is an important characteristic of
CORMs, these findings suggest that incorporation of the bulky
iPr2Ph-DAB ligand into metal carbonyl complexes may be
useful in the synthesis of viable targets.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: djdarens@mail.chem.tamu.edu (D.J.D.).
*E-mail: ashfaq.bengali@qatar.tamu.edu (A.A.B).
Notes
The authors declare no competing financial interest.
4086
dx.doi.org/10.1021/ic500025k | Inorg. Chem. 2014, 53, 4081−4088

Inorganic Chemistry
Article
Oskam, A. Organometallics 1994, 13, 2641−2650. (h) Kleverlaan, C. J.;
Hartl, F.; Stufkens, D. J. J. Photochem. Photobiol. A 1997, 103, 231.
(10) Chen, H.; Bartlett, R. A.; Dias, H. V. R.; Olmstead, M. M.;
Power, P. P. Inorg. Chem. 1991, 30, 2487−2494. (b) García-Escudero,
L. A.; Miguel, D.; Turiel, J. A. J. Organomet. Chem. 2006, 691, 3434−
3444. (c) Ghosh, M.; Weyhermuller, T.; Wieghardt, K. Dalton Trans.
2008, 37, 5149−5151. (d) Graham, A. J.; Akrigg, D.; Sheldrick, B.
Cryst. Struct. Commun. 1977, 6, 571.
ACKNOWLEDGMENTS
■
This publication was made possible by funding from the Qatar
National Research Fund (member of Qatar Foundation). The
experimental work was supported by NPRP grant 4-1517-1-245
and the theoretical studies by NPRP grant 09-143-1-022. The
statements made herein are solely the responsibility of the
authors.
(11) (a) Schatzschneider, U. Eur. J. Chem. 2010, 10, 1451−1467.
(b) Motterlini, R.; Otterbein, L. E. Nature 2010, 9, 728−743.
(c) Crook, S. H.; Mann, B. E.; Meijer, A. J. H. M.; Adams, H.; Sawle,
P.; Scapens, D. A.; Motterlini, R. A. Dalton Trans. 2011, 40, 4230−
4235. (d) Dordelmann, G.; Meinhardt, T.; Sowik, T.; Krueger, A.;
Schatzschneider, U. Chem. Commun. 2012, 48, 11528−11530.
REFERENCES
■
(1) (a) Baker, R. J.; Jones, C.; Mills, D. P.; Pierce, G. A.; Waugh, M.
Inorg. Chim. Acta 2008, 361, 427−435. (b) Dong, Q.; Su, J.-H.; Gong,
S.; Li, Q.-S.; Zhao, Y.; Wu, B.; Yang, X.-J. Organometallics 2013, 32,
2866−2869. (c) Dong, Q.; Zhao, Y.; Su, Y.; Su, J.-H.; Wu, B.; Yang, X.-
J. Inorg. Chem. 2012, 51, 13162−13170. (d) Gao, J.; Li, S.; Zhao, Y.;
Wu, B.; Yang, X.-J. Organometallics 2012, 31, 2978−2985. (e) Greulich,
(e) Dordelmann, G.; Pfeiffer, H.; Birkner, A.; Schatzschneider, U.
̈
Inorg. Chem. 2011, 50, 4362−4367. (f) Gonzalez, M. A.; Carrington, S.
J.; Fry, N. L.; Martinez, J. L.; Mascharak, P. K. Inorg. Chem. 2012, 51,
11930−11940. (g) Gonzalez, M. A.; Yim, M. A.; Cheng, S.; Moyes, A.;
Hobbs, A. J.; Mascharak, P. K. Inorg. Chem. 2011, 51, 601−608.
(h) Govender, P.; Pai, S.; Schatzschneider, U.; Smith, G. S. Inorg.
Chem. 2013, 52, 5470−5478. (i) Hallett, A. J.; Angharad Baber, R.;
Guy Orpen, A.; Ward, B. D. Dalton Trans. 2011, 40, 9276−9283.
(j) Pfeiffer, H.; Rojas, A.; Niesel, J.; Schatzschneider, U. Dalton Trans.
2009, 38, 4292−4298. (k) Rudolf, P.; Kanal, F.; Knorr, J.; Nagel, C.;
Niesel, J.; Brixner, T.; Schatzschneider, U.; Nuernberger, P. J. Phys.
Chem. Lett. 2013, 4, 596−602. (l) Ward, J. S.; Lynam, J. M.; Moir, J.
W. B.; Sanin, D. E.; Mountford, A. P.; Fairlamb, I. J. S. Dalton Trans.
2012, 41, 10514−10517. (m) Mohr, F.; Niesel, J.; Schatzschneider, U.;
Lehmann, C. W. Z. Anorg. Allg. Chem. 2012, 638, 543−546.
́ ̌
S.; Kaim, W.; Stange, A. F.; Stoll, H.; Fiedler, J.; Zalis, S. Inorg. Chem.
1996, 35, 3998−4002. (f) Koten, G. V.; Vrieze, K. Adv. Organomet.
Chem. 1982, 21, 151−239. (g) Ray, K.; Petrenko, T.; Wieghardt, K.;
Neese, F. Dalton Trans. 2007, 35, 1552−1566.
(2) (a) Kumar, A.; Sun, S.-S.; Lees, A. Top. Organomet Chem. 2010,
29, 37−71. (b) Nieuwenhuis, H. A.; Stufkens, D. J.; Oskam, A. Inorg.
Chem. 1994, 33, 3212−3217. (c) Vlce
Organometallics 2010, 29, 115−158.
̌
k, A., Jr. Photophysics of
(3) (a) Bock, H.; tom Dieck, H. Angew. Chem., Int. Ed. 1966, 5, 520−
522. (b) Vrieze, K. J. Organomet. Chem. 1986, 300, 307−326.
(4) (a) Bigioni, M.; Ganis, P.; Panunzi, A.; Ruffo, F.; Salvatore, C.;
Vito, A. Eur. J. Inorg. Chem. 2000, 2000, 1717−1721. (b) Ferrara, M.
L.; Giordano, F.; Orabona, I.; Panunzi, A.; Ruffo, F. Eur. J. Inorg. Chem.
1999, 1999, 1939−1947. (c) Togni, A.; Venanzi, L. M. Angew. Chem.,
Int. Ed. 1994, 33, 497−526. (d) Vrieze, K. J. Organomet. Chem. 1986,
300, 307−326.
(12) (a) Eiβler, A.; Klaring, P.; Emmerling, F.; Braun, T. Eur. J. Inorg.
̈
Chem. 2013, 2013, 4775−4788. (b) Li, J.; Zhang, K.; Huang, H.; YU,
A.; Hu, H.; Cui, H.; Cui, C. Organometallics 2013, 32, 1630−1635.
(13) Staal, L. H.; Oskam, A.; Vrieze, K. J. Organomet. Chem. 1979,
170, 235−245.
(5) (a) Burgess, J.; Chambers, J.; Haines, R. Trans. Met. Chem. 1981,
6, 145−151. (b) Guillon, T.; Boggio-Pasqua, M.; Alary, F.; Heully, J.-
L.; Lebon, E.; Sutra, P.; Igau, A. Inorg. Chem. 2010, 49, 8862−8872.
(c) Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159−244.
(d) Piau, R. E.; Guillon, T.; Lebon, E.; Perrot, N.; Alary, F.; Boggio-
Pasqua, M.; Heully, J.-L.; Juris, A.; Sutra, P.; Igau, A. New. J. Chem.
2012, 36, 2484−2492. (e) Zal
Chem. 2004, 43, 7978−7985.
(6) (a) Guan, Z.; Popeney, C. Top. Cat. 2009, 26, 179−220. (b) Ittel,
S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100, 1169−
1204. (c) Yuan, J.; Wang, X.; Mei, T.; Liu, Y.; Miao, C.; Xie, X. Trans.
Met. Chem. 2011, 36, 433−439.
(7) (a) Hartl, F.; Aarnts, M. P; Peelen, K. Collect. Czech. Chem.
Commun. 1996, 61, 1342−1352. (b) Johnson, F. P. A.; George, M. W.;
Hartl, F.; Turner, J. J. Organometallics 1996, 15, 3374−3387.
(c) Rossenaar, B. D.; Hartl, F.; Stufkens, D. J. Inorg. Chem. 1996,
35, 6194−6203. (d) Kumar, B.; Llorente, M.; Froehlich, J.; Dang, T.;
Sathrum, A.; Kubiak, C. P. Annu. Rev. Phys. Chem. 2012, 63, 541−569.
(e) Rossenar, B. D.; Hartl, F.; Stufkens, D. J. Inorg. Chem. 1996, 35,
6194−6203.
(8) (a) Chartrand, D.; Castro Ruiz, C. A.; Hanan, G. S. Inorg. Chem.
2012, 51, 12738−12747. (b) Sacksteder, L.; Lee, M.; Demas, J. N.;
DeGraff, B. A. J. Am. Chem. Soc. 1993, 115, 8230−8238. (c) Sacksteder,
L.; Zipp, A. P.; Brown, E. A.; Streich, J.; Demas, J. N.; DeGraff, B. A.
Inorg. Chem. 1990, 29, 4335−4340. (d) Wrighton, M.; Morse, D. L. J.
Am. Chem. Soc. 1974, 96, 998−1003. (e) Li, Y.; Liu, Y.; Guo, J.; Wu,
F.; Tian, W.; Li, B.; Wang, Y. Synth. Met. 2001, 118, 175.
(14) APEX2, v 2009.7−0; Bruker AXS, Inc., Madison, WI, 2007.
(15) SAINTPLUS: Program for Reduction of Area Detector Data, 1034,
v 6.63; Bruker AXS Inc., Madison, WI, 2007.
(16) Sheldrick, G. M., SADABS: Program for Absorption Correction of
1036 Area Detector Frames; Bruker AXS Inc., Madison, WI, 2001.
(17) Sheldrick, G. M., SHELXS-97: Program for Crystal Structure 1038
́ ̌
is, S.; Ben Amor, N.; Daniel, C. Inorg.
Solution; Universitat Gottingen, Gottingen, Germany, 1997.
̈
̈
̈
(18) Sheldrick, G. M., SHELXL-97: Program for Crystal Structure
1040 Refinement; Universitat Gottingen, Gottingen, Germany, 1997.
̈
̈
̈
(19) Macrae, C. F.; Edgington, P. R.; McCabe, P.; Pidcock, E.;
Shields, G. P.; Taylor, R.; Towler, M.; van de Streek, J. J. Appl.
Crystallogr. 2006, 39, 453−457.
(20) Frisch, M. J.; et al. Gaussian 09, revision B.01; Gaussian Inc.,
Wallingford, CT, 2009.
(21) Chai, J. D.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2008, 10,
6615−6620.
(22) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7,
3297−3305.
(23) (a) García-Escudero, L. A.; Miguel, D.; Turiel, J. A. J. Organomet.
Chem. 2006, 691, 3434−3444. (b) Valín, M. L.; Moreiras, D.; Solans,
X.; Font-Altaba.; García-Alonso, F. J. Acta Crystallogr., Sect. C 1986, 42,
417−418. (c) Graham, A. J.; Akrigg, D.; Sheldrick, B. Cryst. Struct.
Commun. 1977, 6, 571. (d) Alvarez, C. M.; Garcia-Rodriguez, R.;
Miquel, D. Dalton Trans. 2007, 36, 3546−3554.
(24) (a) Szacilowski, K.; Macyk, W.; Drzewiecka-Matuszek, A.;
Brindell, M.; Stochel, G. Chem. Rev. 2005, 105, 2647−2694. (b) Farrer,
N. J.; Salassa, L.; Sadler, P. J. Dalton Trans. 2009, 38, 10690−10701.
(25) (a) Niesel, J.; Pinto, A.; N’DOngo, H. W. P.; Merz, K.; Ott, I.;
Gust, R.; Schatszsschneider, U. Chem. Commun. 2008, 44, 1798−1800.
(b) Kunz, P. C.; Huber, W.; Rojas, A.; Schatszsschneider, U.; Spingler,
B. Eur. J. Inorg. Chem. 2009, 2009, 5358−5366.
(9) (a) Hartl, F.; Mahabiersing, T.; Le Floch, P.; Mathey, F.; Ricard,
L.; Rosa, P.; Zal
Rosa, P.; Ricard, L.; Le Floch, P.; Zal
́
is,
̌
S. Inorg. Chem. 2003, 42, 4442−4455. (b) Hartl, F.;
́
is, S. Coord. Chem. Rev. 2007, 251,
̌
557−576. (c) Kleverlaan, C. J.; Hartl, F.; Stufkens, D. J. J. Organomet.
Chem. 1998, 561, 57−65. (d) Kokkes, M. W.; Stufkens, D. J.; Oskam,
A. Inorg. Chem. 1985, 24, 2934−2942. (e) Rosa, A.; Ricciardi, G.;
Baerends, E. J.; Stufkens, D. J. J. Phy. Chem. 1996, 100, 15346−15357.
(f) Rosa, A.; Ricciardi, G.; Baerends, E. J.; Stufkens, D. J. Inorg. Chem.
1998, 37, 6244−6254. (g) Stor, G. J.; Morrison, S. L.; Stufkens, D. J.;
(26) Carrington, S. J.; Chakraborty, I.; MAscharak, P. K. Chem
Commun. 2013, 49, 11254−11256.
̌
(27) (a) Vlcek, A., Jr.; Farrell, I. R.; Liard, D. J.; Matousek, P.;
Towrie, M.; Parker, A. W.; Grills, D. C.; George, M. W. J. Chem. Soc.,
4087
dx.doi.org/10.1021/ic500025k | Inorg. Chem. 2014, 53, 4081−4088

Inorganic Chemistry
Article
Dalton Trans. 2002, 701. (b) Farrell, I. R.; Vlce
̌
k, A., Jr. Coord. Chem.
Rev. 2000, 208, 87−101.
(28) Several examples of sixteen electron cationic metal complexes
with weakly coordinating anions are present in the literature. See, for
example, Beck, W.; Sunkel, K. Chem. Rev. 1988, 88, 1405−1421.
̈
(29) (a) Scheiring, T.; Kaim, W.; Fielder, J. J. Organomet. Chem.
2000, 598, 136−141. (b) Carriedo, G. A.; Per
Miguel, D.; Riera, V.; García-Granda, S.; Per
Organomet. Chem. 1996, 511, 77−84.
́
ez-Martínez, J. A.;
́
ez-Carreno, E. J.
̃
(30) (a) Blackmore, K. J.; Lal, N.; Ziller, J. W.; Heyduk, A. F. Eur. J.
Inorg. Chem. 2009, 2009, 735−743. (b) Lionetti, D.; Medvecz, A. J.;
Ugrinova, V.; Quiroz-Guzman, M.; Noll, B. C.; Brown, S. N. Inorg.
Chem. 2010, 49, 4687−4697. (c) Lippert, C. A.; Soper, J. D. Inorg.
Chem. 2010, 49, 3682−3684. (d) Heneline, M. R.; Clapp, L. A.;
Hardcastle, K. I.; Soper, J. D. Polyhedron 2010, 29, 164−169.
(31) (a) Darensbourg, D. J.; Freolich, J. A. J. Am. Chem. Soc. 1977,
99, 5940−5946. (b) Behrens, H.; Lampe, R. J.; Merbach, P.; Moll, M.
J. Organomet. Chem. 1978, 159, 201−217. (c) Uson, R.; Riera, V.;
Gimeno, J.; Laguna, M. Transition Met. Chem. 1977, 2, 123−130.
(32) Palmer, G. T.; Basolo, F. J. Am. Chem. Soc. 1985, 107, 3122−
3129.
4088
dx.doi.org/10.1021/ic500025k | Inorg. Chem. 2014, 53, 4081−4088