Manganese Tricarbonyl Complexes with Asymmetric 2-Iminopyridine Ligands: Toward Decoupling Steric and Electronic Factors in Electrocatalytic CO2 Reduction

Article abstract of DOI:10.1021/acs.inorgchem.6b01477

Manganese tricarbonyl bromide complexes incorporating IP (2-(phenylimino)pyridine) derivatives, [MnBr(CO)₃(IP)], are demonstrated as a new group of catalysts for CO₂ reduction, which represent the first example of utilization of (phenylimino)pyridine ligands on manganese centers for this purpose. The key feature is the asymmetric structure of the redox-noninnocent ligand that permits independent tuning of its steric and electronic properties. The α-diimine ligands and five new Mn(I) compounds have been synthesized, isolated in high yields, and fully characterized, including X-ray crystallography. Their electrochemical and electrocatalytic behavior was investigated using cyclic voltammetry and UV-vis-IR spectroelectrochemistry within an OTTLE cell. Mechanistic investigations under an inert atmosphere have revealed differences in the nature of the reduction products as a function of steric bulk of the ligand. The direct ECE (electrochemical-chemical-electrochemical) formation of a five-coordinate anion [Mn(CO)₃(IP)]^a, a product of two-electron reduction of the parent complex, is observed in the case of the bulky DIPIMP (2-[((2,6-diisopropylphenyl)imino)methyl]pyridine), TBIMP (2-[((2-tert-butylphenyl)imino)methyl]pyridine), and TBIEP (2-[((2-tert-butylphenyl)imino)ethyl]pyridine) derivatives. This process is replaced for the least sterically demanding IP ligand in [MnBr(CO)₃(IMP)] (2-[(phenylimino)methyl]pyridine) by the stepwise formation of such a monoanion via an ECEC(E) mechanism involving also the intermediate Mn-Mn dimer [Mn(CO)₃(IMP)]₂. The complex [MnBr(CO)₃(IPIMP)] (2-[((2-diisopropylphenyl)imino)methyl]pyridine), which carries a moderately electron donating, moderately bulky IP ligand, shows an intermediate behavior where both the five-coordinate anion and its dimeric precursor are jointly detected on the time scale of the spectroelectrochemical experiments. Under an atmosphere of CO₂ the studied complexes, except for the DIPIMP derivative, rapidly coordinate CO₂, forming stable bicarbonate intermediates, with no dimer being observed. Such behavior indicates that the CO₂ binding is outcompeting another pathway: viz., the dimerization reaction between the five-coordinate anion and the neutral parent complex. The bicarbonate intermediate species undergo reduction at more negative potentials (ca. a2.2 V vs Fc/Fc⁺), recovering [Mn(CO)₃(IP)]^- and triggering the catalytic production of CO.

Full text of DOI:10.1021/acs.inorgchem.6b01477

Article
pubs.acs.org/IC
Manganese Tricarbonyl Complexes with Asymmetric
2‑Iminopyridine Ligands: Toward Decoupling Steric and Electronic
Factors in Electrocatalytic CO2 Reduction
Steven J. P. Spall,† Theo Keane,† Joanne Tory,‡ Dean C. Cocker,† Harry Adams,† Hannah Fowler,†
Anthony J. H. M. Meijer,† Frantisek Hartl,*,‡ and Julia A. Weinstein*,†
̌
†Department of Chemistry, University of Sheffield, Sheffield S3 7HF, U.K.
‡Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, U.K.
S
* Supporting Information
ABSTRACT: Manganese tricarbonyl bromide complexes incorporating IP
(2-(phenylimino)pyridine) derivatives, [MnBr(CO)3(IP)], are demonstra-
ted as a new group of catalysts for CO2 reduction, which represent the first
example of utilization of (phenylimino)pyridine ligands on manganese
centers for this purpose. The key feature is the asymmetric structure of the
redox-noninnocent ligand that permits independent tuning of its steric and
electronic properties. The α-diimine ligands and five new Mn(I)
compounds have been synthesized, isolated in high yields, and fully
characterized, including X-ray crystallography. Their electrochemical and
electrocatalytic behavior was investigated using cyclic voltammetry and
UV−vis−IR spectroelectrochemistry within an OTTLE cell. Mechanistic
investigations under an inert atmosphere have revealed differences in the
nature of the reduction products as a function of steric bulk of the ligand.
The direct ECE (electrochemical−chemical−electrochemical) formation of
a five-coordinate anion [Mn(CO)3(IP)]−, a product of two-electron reduction of the parent complex, is observed in the case of
the bulky DIPIMP (2-[((2,6-diisopropylphenyl)imino)methyl]pyridine), TBIMP (2-[((2-tert-butylphenyl)imino)methyl]-
pyridine), and TBIEP (2-[((2-tert-butylphenyl)imino)ethyl]pyridine) derivatives. This process is replaced for the least sterically
demanding IP ligand in [MnBr(CO)3(IMP)] (2-[(phenylimino)methyl]pyridine) by the stepwise formation of such a
monoanion via an ECEC(E) mechanism involving also the intermediate Mn−Mn dimer [Mn(CO)3(IMP)]2. The complex
[MnBr(CO)3(IPIMP)] (2-[((2-diisopropylphenyl)imino)methyl]pyridine), which carries a moderately electron donating,
moderately bulky IP ligand, shows an intermediate behavior where both the five-coordinate anion and its dimeric precursor are
jointly detected on the time scale of the spectroelectrochemical experiments. Under an atmosphere of CO2 the studied
complexes, except for the DIPIMP derivative, rapidly coordinate CO2, forming stable bicarbonate intermediates, with no dimer
being observed. Such behavior indicates that the CO2 binding is outcompeting another pathway: viz., the dimerization reaction
between the five-coordinate anion and the neutral parent complex. The bicarbonate intermediate species undergo reduction at
more negative potentials (ca. −2.2 V vs Fc/Fc+), recovering [Mn(CO)3(IP)]− and triggering the catalytic production of CO.
precursor results in the formation of the Mn−Mn dimer
[Mn(CO)3(R-bpy)]2.8,9 Notably, neither the primary reduction
product [MnBr(CO)3(R-bpy•−)] nor the five-coordinate
radical intermediates [Mn(CO)3(R-bpy)]• have been detected
by either UV−vis or IR spectroscopy.2,7 Nanosecond time-
resolved infrared (TRIR) studies reveal that no detectable
solvent adduct is formed before the dimerization of Mn species
on this time scale; instead, the five-coordinate species is
observed, which rapidly dimerizes.10 For some of the Re
analogues, a one-electron-reduced complex, [ReCl(CO)3(R-
bpy•−)], was observed by IR spectroscopy and identified by the
he interest in solar fuels in terms of both photocatalytic
and electrocatalytic CO2 reduction, in the latter case
1
T
utilizing sustainable electricity, has been increasing markedly in
the new millennium. The recent demonstration of the
electrocatalytic activity of manganese2 analogues of the
archetypal Re(I) catalysts3−6 for CO2 reduction has given a
new impetus to research into noble-metal-free catalytic systems.
[MnBr(CO)3(α-diimine)] complexes have been shown to
outperform rhenium-based analogues with regard to CO2
reduction under certain conditions.7 Most notably, the presence
of a Brønsted acid7−10 appears to be a prerequisite for catalysis
with a range of tricarbonyl Mn α-diimine complexes.
ca. 15−20 cm−1 decrease in the ν
̃
(CO) energy,11−13 as was the
Mechanistic studies5,10 of the active 2,2′-bipyridine-based (R-
bpy) manganese catalysts have shown that one-electron
reduction of the parent complex [MnBr(CO)3(R-bpy)]
Received: June 22, 2016
© XXXX American Chemical Society
A
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Article
five-coordinate radical [Re(CO)3(tBu-bpy)]• by an additional
15−20 cm−1 shift.
coordinate anions, with the steric bulk of the ligand preventing
dimerization, but does not act as a catalyst for CO2 reduction
due to insufficiently negative reduction potential.8
Two mechanisms have been proposed10,14−18 for the
ultimate reduction of [Mn(CO)3(α-diimine)]2 in the presence
of CO2, which can be referred to as the anionic and the
oxidative addition19 pathways. The anionic pathway involves
reduction of the dimer [Mn(CO)3(α-diimine)]2 at a potential
more negative than that of the parent complex, generating the
five-coordinate anion [Mn(CO)3(α-diimine)]−, to which CO2
coordinates and is catalytically reduced in the presence of a
Brønsted acid (the source of H+). The anionic pathway is
broadly similar to the two-electron pathway observed for Re
complexes.20,21 In contrast, the uncommon second pathway
identified using pulsed EPR studies19 involves coordination of
CO2 to the dimer [Mn(CO)3(2,2′-bpy)]2 in the presence of a
Brønsted acid in a concerted oxidative addition step. This
process is shown to generate a low-spin MnII−COOH complex,
from which CO is subsequently released.
Introduction of the pyridine moiety allows one to reach the
required reduction potentials, while the Ph group attached to
the CN fragment can be decorated with sterically demanding
substituents, ensuring steric bulk while only slightly affecting
the electronic properties. As the phenyl moiety lies out of plane
with the conjugated α-diimine (because of steric effects), the π
electrons of the phenyl substituent are decoupled from the
metallacycle formed by the metal center and the α-diimine.
Therefore, functionalization of the phenyl ring in the R1 and R2
positions with large sterically hindering groups (that also have a
+I effect) will have only very minimal effects on the electronics
of the active site of the molecule (vide infra). These ligands
offer an opportunity to separate steric and electronic effects in a
chelating α-diimine ligand to a certain extent. Thus, the
possibility arises of a systematic variation of the steric hindrance
by changing R1 and R2 groups, while the R group strongly
influences the electronics (but could also hinder the CN
bond).
Since the catalytic CO2 reduction with the use of
[MnX(CO)3(α-diimine)] (α-diimine = R-bpy; X = halide or
pseudohalide) has been shown to proceed in many cases via a
dimerization step, immobilization of the catalyst22 or
introduction of sterically hindering groups at bpy may have a
profound effect on the catalytic activity.23 Indeed, it has
recently been shown that the use of bipyridines incorporating
bulky groups in the 6,6′-positions24,25 (or another bulkier
heterocyclic ligand26) largely inhibits dimerization in the
catalytic cycle. The result is the formation of the stable five-
coordinate anion via the two-electron transfer (ECE) at the first
cathodic wave. However, coordination of CO2 to the five-
coordinate anion produces a stable species which must be
reduced at considerably more negative potentials27 in order for
catalysis to be observed. It has recently been shown that in the
presence of a Lewis acid, Mg2+,7,28 the catalytic overpotential29
is decreased by approximately 400 mV.
These ligands are readily accessible via simple synthetic
routes, which are suitable for the purpose of comparatively
independent alteration of steric and electronic effects (Chart 1).
Chart 1. General Structure of the Complexes with the
Asymmetric α-Diimine Ligands (2-R1-6-R2-phenyl)(R-
a
imino)pyridine
A similar behavior was observed for [MnBr(CO)3(R-DAB)]
complexes featuring nonaromatic 1,4-diazabuta-1,3-diene (R-
DAB)8,9,30 ligands. The reduction potentials of the dimers
[Mn(CO)3(R-DAB)]2 are almost identical with those of the
parent complexes, implying that the five-coordinate anion is
produced directly upon reduction and reacts readily with CO2
in solution to form a stable bicarbonate complex8,30 and, as
with sterically hindered 2,2′-bipyridine ligands,25 a much more
negative potential (below 2 V vs Fc/Fc+) must be applied to
trigger catalytic CO2 reduction. Functionalization of the α-
a
Numbers given in parentheses correspond to the Mn complexes.
When R = H, the ligands will be derivatives of [(phenylimino)-
methyl]pyridine: IMP (R1,2 = H), IPIMP (R1 = Pr, R2 = H), and
DIPIMP (R1,2 = Pr), TBIMP (R1 = Bu, R2 = H). R = CH3 gives
TBIEP, [((tert-butylphenyl)imino)ethyl]pyridine.
i
i
t
The potential of such ligands34−38 has been convincingly
illustrated by the recent work on a Re tricarbonyl complex with
2-[((2-cyclohexyl-1-methyl)methyl)imino]pyridine36 (both the
one-electron-reduced parent complex and the neutral five-
coordinate Re(0) species were detected), and Mo pyridine-
monoimides.34
Herein we report a new series of manganese-based catalysts
for CO2 reduction. We will show that a change in the structure
of the ligands within the same series affects the efficiency of the
process and the relative distribution of the intermediate species,
demonstrating the versatile and tunable nature of these types of
catalysts.
t
diimine with a sterically bulky group such as Bu should also
modify the electronic properties of the ligand. In particular, this
change should affect the energy of the LUMO, the reduction
potential, and catalytic activity.23−33
Introducing steric bulk23,25 to prevent unwanted reactions of
the catalytic species, including dimerization as either Mn−Mn9
or C(imino)−C(imino) bound species,21 while at the same
time reducing the risk of increased overpotential is a
challenging task. Molecular designs that allow for steric and
electronic effects to be decoupled are required.
In this paper we have investigated a family of tricarbonyl
manganese complexes featuring asymmetric α-diimine ligands,
iminopyridines (IP).21,23,34,35 They combine an accessible
−CN− imino bond of the diazabuta-1,3-diene derivatives
DAB32 with the aromatic pyridine part, thereby being a
“hybrid” of 2,2′-bipyridine ligands and nonaromatic R-DAB
ligands. Each of the parts is important: for instance, a Mn(I)
complex with Ph-DAB demonstrates formation of five-
EXPERIMENTAL SECTION
■
All solvents were supplied by VWR and used as received. The
compounds were purchased from either Sigma-Aldrich or Strem
Chemicals and, unless stated, used as received. Tetrabutylammonium
hexafluorophosphate, [Bu4N][PF6], was recrystallized from hot
ethanol and dried overnight in a vacuum oven before use in the
electrochemical studies. TBIEP (2-[((2-tert-butylphenyl)imino)ethyl]-
B
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pyridine) and TBIMP were synthesized as previously described;35 the
analytical data matched those reported previously. Unless otherwise
stated, UV−vis spectra were recorded on a Carry 50 Bio
spectrophotometer and IR spectra on a PerkinElmer Spectrum 1
FT-IR spectrometer.
[MnBr(CO)3(TBIMP)] (4). [MnBr(CO)5] (0.84 mmol, 0.23 g) was
combined with DIPIMP (0.84 mmol, 0.2 g) in diethyl ether (20 mL)
and refluxed under aerobic conditions for 4 h. The product was
formed in quantitative yield. 1H NMR (400 MHz, CDCl3): δ 9.27 (d, J
= 4.4 Hz, 1H), 8.50 (s, 1H), 8.12 (d, J = 6.9 Hz, 1H), 8.03 (t, J = 7.2
Hz, 1H), 7.91 (d, J = 7.1 Hz, 1H), 7.61 (t, J = 6.2 Hz, 1H), 7.57 (d, J =
7.4 Hz, 1H), 7.30 (t, 1H), 1.43 (s, 1H). HRMS (TOF-ESI, +ve): m/z
(M + Na+) calcd for C19H18N2O3NaBrMn 478.9774, found 478.9789.
[MnBr(CO)3(TBIEP)] (5). [MnBr(CO)5] (0.8 mmol, 0.22 g) was
combined with TBIEP (0.8 mmol, 0.2 g) and refluxed under aerobic
conditions in diethyl ether (20 mL) for 4 h. The product was formed
Syntheses. IMP (2-[(Phenylimino)methyl]pyridine). Aniline (11.3
mmol, 1.05 g, 1.02 mL) was added to 2-pyridinecarboxaldehyde (11.3
mmol, 1.2 g, 1.1 mL) in flame-dried glassware and stirred for 1 h.
Hexane (10 mL) was added and the solution dried over sodium
sulfate. The solution was filtered, concentrated under vacuum, and
placed in a freezer overnight. The large yellow needlelike crystals that
1
1
in quantitative yield. H NMR (400 MHz, CDCl3): δ 9.28 (d, J = 5.0
formed were filtered and washed with hexane. Yield: 73%. H NMR
Hz, 1H), 8.04 (td, J = 7.8, 1.3 Hz, 1H), 7.95 (d, J = 7.7 Hz, 1H), 7.88
(dd, J = 6.1, 3.4 Hz, 1H), 7.65−7.54 (m, 2H), 7.34−7.27 (m, 2H),
2.39 (s, 3H), 1.39 (s, 8H). HRMS (TOF-ES, +ve): m/z (M + Na+)
calcd for C20H20N2O3NaBrMn 492.9935, found 492.9934.
(400 MHz, CDCl3): δ 8.72 (d, J = 4.7 Hz, 1H), 8.61 (s, 1H), 8.21 (d, J
= 7.9 Hz, 1H), 7.83 (td, J = 7.7, 1.6 Hz, 1H), 7.47−7.35 (m, 3H), 7.29
(d, J = 7.7 Hz, 3H), 1.58 (s, 4H).
IPIMP (2-[2-((Isopropylphenyl)imino)methyl]pyridine). 2-Isopro-
pylaniline (12.4 mmol, 1.7 g, 1.8 mL) was mixed with 2-
pyridinecarboxaldehyde (12.4 mmol, 1.3 g, 1.2 mL) in flame-dried
glassware and stirred for 1 h. Hexane (20 mL) was added and the
solution dried over sodium sulfate. The solution was filtered and
solvent removed under vacuum, yielding the product as a brown oil.
Previous reports indicated that this compound could not be
crystallized; therefore, the oil was used in the next reaction step
[MnBr(CO)3(bpy)] (6). This compound was prepared following the
literature procedure;2 analytical data are in agreement with the
literature data. [MnBr(CO)5] (1.28 mmol, 0.35 g) was combined with
2,2′-bipyridine (1.28 mmol, 0.2 g) in diethyl ether (20 mL) and
refluxed under aerobic conditions for 4 h. The product was formed in
1
80% yield. H NMR (500 MHz, CDCl3): δ 9.27 (d, J = 4.3 Hz, 1H),
8.12 (d, J = 6.5 Hz, 1H), 7.99 (t, 1H), 7.53 (t, 1H). HRMS (TOF-ESI,
+ve): m/z (M + Na+) calcd for C13H8N2O3NaBrMn 396.8991, found
369.8988.
1
without further purification (purity by NMR >97%). H NMR (400
MHz, CDCl3): δ 8.72 (ddd, J = 4.8, 1.6, 0.9 Hz, 1H), 8.54 (s, 1H),
8.26 (dd, J = 7.9, 0.9 Hz, 1H), 7.82 (ddd, J = 7.9, 1.7, 0.8 Hz, 1H),
7.45−7.32 (m, 2H), 7.31−7.22 (m, 2H), 7.08−6.96 (m, 1H), 3.56 (dp,
J = 13.8, 6.8 Hz, 1H), 1.26 (dd, J = 6.8, 4.1 Hz, 7H).
Cyclic Voltammetry. Cyclic voltammetry was performed using a
Princeton Applied Research VersaSTAT3 potentiostat on 2 mM 1−5
in Grubbs dried HPLC-grade acetonitrile containing 2 × 10−1
M
[Bu4N][PF6] as supporting electrolyte. A glassy-carbon working
electrode (surface area 0.07 cm2, polished on alumina and paper)
and a Pt-wire counter electrode were used with a 0.1 M KCl Ag/AgCl
reference electrode.
The solutions were deoxygenated by bubbling thoroughly with
bottled N2 (BOC), and the N2 atmosphere was maintained over the
samples during the experiment. To test for catalytic current in the
presence of CO2, the samples were bubbled thoroughly with bottled
CO2 (BOC) and cyclic voltammograms (CVs) were recorded under
an atmosphere of CO2 (some residual water might be present in the
CO2 used to saturate the samples). Water was then added (0.3−6 mL
of the solution of each sample) to test the effects of Brønsted acid.
Ferrocene was added as the internal standard at the end of all
experiments.
Spectroelectrochemistry. Infrared spectroelectrochemistry was
performed using an EmStat3 or EmStat3+ potentiostat (PalmSens,
Houten, The Netherlands). The solution of 4 mM complex in the
presence of 3 × 10−1 M [Bu4N][PF6] in dry acetonitrile was analyzed
using an optically transparent thin-layer electrochemical (OTTLE) cell
equipped with Pt minigrid working and auxiliary electrodes, an Ag-
microwire pseudoreference electrode, and CaF2 windows. Samples
were prepared under an argon atmosphere; for electrocatalytic
measurements, the solutions were bubbled with CO2 on a frit (a
few minutes) to saturation under normal pressure. Parallel IR and
UV−vis spectral monitoring during the spectroelectrochemical
experiment was performed on a Bruker Vertex 70v FT-IR
spectrometer or PerkinElmer Spectrum 1 and a Scinco S-3100
spectrophotometer, respectively. Thin-layer CVs were recorded in the
course of the experiment.
DIPIMP (2-[((2,6-Diisopropylphenyl)imino)methyl]pyridine). 2,6-
Diisopropylaniline (11.3 mmol, 2 g, 2.1 mL) was mixed with 2-
pyridinecarboxaldehyde (11.3 mmol, 1.2 g, 1.1 mL) in flame-dried
glassware and stirred for 2 h. Hexane (10 mL) was added and the
solution dried over sodium sulfate. The solution was filtered and
concentrated before being placed in a freezer overnight. Light brown
to yellow crystals were formed, which were filtered off and washed
with hexane. Yield: 67%. 1H NMR (400 MHz, CDCl3): δ 8.73 (ddd, J
= 4.8, 1.7, 0.9 Hz, 1H), 8.31 (s, 1H), 8.27 (dt, J = 7.9, 1.0 Hz, 1H),
7.90−7.82 (m, 1H), 7.42 (ddd, J = 7.5, 4.9, 1.2 Hz, 1H), 7.22−7.07
(m, 3H), 2.97 (hept, J = 6.9 Hz, 2H), 1.62 (s, 1H), 1.18 (d, J = 6.9 Hz,
13H).
Complexes 1−6 were prepared from [MnBr(CO)5] and the
corresponding ligand, using diethyl ether as a solvent. The products
were collected by centrifugation and washed with diethyl ether to
1
afford analytically pure 1−6. H NMR spectra for 1−5 are given in
Figure SI22−SI26 in the Supporting Information.
[MnBr(CO)3(IMP)] (1). [MnBr(CO)5] (1.1 mmol, 0.3 g) was
combined with IMP (1.1 mmol, 0.2 g) in diethyl ether (20 mL) and
refluxed under aerobic conditions for 4 h.39 The product was formed
1
in quantitative yield. H NMR (500 MHz, CDCl3): δ 9.26 (d, J = 5.0
Hz, 1H), 8.45 (s, J = 27.9 Hz, 1H), 8.14−7.79 (m, 3H), 7.68−7.36 (m,
5H). HRMS (TOF-ES, + ve): m/z (M + Na+) calcd for
C15H10N2O3NaBrMn 422.9153, found 422.9149.
[MnBr(CO)3(IPIMP)] (2). [MnBr(CO)5] (0.89 mmol, 0.24 g) was
combined with IPIMP (0.89 mmol, 0.2 g) in diethyl ether (20 mL)
and refluxed under aerobic conditions for 4 h. The product was
1
formed in quantitative yield. H NMR (500 MHz, CDCl3): δ 9.27 (s,
1H), 8.39 (s, 1H), 8.04 (s, 1H), 7.94 (d, J = 4.1 Hz, 1H), 7.78 (d, J =
7.0 Hz, 1H), 7.61 (s, 1H), 7.48 (d, J = 15.3 Hz, 1H), 7.43 (d, J = 6.7
Hz, 1H), 7.37 (t, 1H), 7.30 (d, J = 6.9 Hz, 1H), 3.58 (s, 1H), 3.03 (d, J
= 35.6 Hz, 1H), 1.47−1.11 (m, 1H). HRMS (TOF-ES, +ve): m/z (M
+ Na+) calcd for C18H16N2O3NaBrMn 464.9622, found 464.9644.
[MnBr(CO)3(DIPIMP)] (3). [MnBr(CO)5] (0.75 mmol, 0.2 g) was
combined with DIPIMP (0.75 mmol, 0.2 g) in diethyl ether (20 mL)
and refluxed under aerobic conditions for 4 h. The product was
Gas Chromatography Linked to Electrolysis. Bulk electrolysis
was performed on a 0.17 mM solution of each of the complexes in a 60
mL solution of acetonitrile/water (9/1 v/v). The cell setup consisted
of a Pt-mesh working electrode, a Pt-rod counter electrode in a
semiporous compartment, and an Ag-wire pseudoreference electrode
in a 0.1 M KCl solution. The potential of the Fc/Fc+ recorded in this
setup using a glassy-carbon 3 mm diameter electrode was +0.350 V vs
Ag wire pseudoreference. Hence, in order to reach the potential
necessary for the CO2 reduction as estimated from the CV data, the
potential was held at −1.9 V vs Ag wire for all samples: i.e., −2.25 V vs
Fc/Fc+. Prior to electrolysis, a CV was recorded in the bulk electrolysis
cell using a glassy-carbon working electrode. Gas samples (100 μL)
were withdrawn from the head space at regular intervals and analyzed
1
formed in 97% yield. H NMR (500 MHz, CDCl3): δ 9.30 (s, 1H),
8.41 (s, 1H), 7.99 (d, J = 50.7 Hz, 2H), 7.63 (s, 1H), 7.34 (s, 2H), 4.04
(s, 1H), 2.91 (s, 1H), 1.34 (d, J = 3.1 Hz, 6H), 1.05 (dd, J = 80.6, 35.1
Hz, 6H). HRMS (TOF-ES, +ve): m/z (M + Na+) calcd for
C21H22N2O3NaBrMn 507.0092, found 507.0107.
C
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by a gas chromatograph fitted with a thermal conductivity detector
(Perkin ElmerArnel autosystem XL). H2 was used as the carrier gas in
CO-quantification experiments. N2 was used as a carrier gas in the
control experiment.
DFT, the pyridine and phenyl rings lie approximately
orthogonally to one another (dihedral angles between the
corresponding planes are MnTBIEP 84.55°, MnIPIMP 83.64°,
MnDIPIMP 78°), resulting in little orbital overlap between
these two moieties, with the exception of MnIMP, where the
two ring systems were significantly less orthogonal (56.54°).
The crystal structures have revealed significant steric
X-ray Crystallography. Crystals were grown using the antisolvent
crystallization method, with the solvent dichloromethane and hexane
as the antisolvent. Single-crystal X-ray diffraction data were collected
on a Bruker SMART APEX-II CCD diffractometer operating a Mo Kα
sealed-tube X-ray source or a Bruker D8 Venture diffractometer
equipped with a PHOTON 100 dual-CMOS chip detector and
operating a Cu Kα IμS microfocus X-ray source. The data were
processed using Bruker APEX3 software and corrected for absorption
using empirical methods (SADABS) based upon symmetry-equivalent
reflections combined with measurements at different azimuthal
angles.40 The crystal structures were solved and refined using the
Bruker SHELXTL software package.
t
hindrance between the substituents R = Me and R1 = Bu in
MnTBIEP, which inhibits rotation of the Ph ring and confers
conformational rigidity. Rotation of the Ph ring in MnDIPIMP
i
is also inhibited by the two Pr groups and hence also has
conformational rigidity. In contrast, MnIMP and MnIPIMP
exhibit much smaller steric hindrance, facilitating the rotation of
the phenyl ring.
Computational Investigations of Molecular Structures
and Frontier Orbitals by DFT. The optimized geometries of
the studied complexes with frontier orbitals overlaid as
calculated by DFT are displayed in Table 1. As anticipated,
the phenyl group lies out of the plane of the chelating diimine.
The HOMO is localized predominantly over the axial Br−Mn−
C(O) bonds with almost no contribution from the phenyl
moiety. The LUMO resides largely on the imine, pyridyl, and
metal center, with minimal contribution from the C1−C2 and
C1−C6 σ bonds of the phenyl groups. In the case of MnIMP,
due to the lack of substitution at R1 and R2, the phenyl moiety
is less sterically hindered and thus is positioned closer to the
plane of the imino-pyridine fragment, resulting in a small
degree of involvement of the phenyl π system in the low-energy
unoccupied orbitals. This trend continues in the other low-
energy unoccupied orbitals (see Figures SI1−SI10 in the
Supporting Information).
The energies of the HOMO in all complexes are within 0.02
eV of each other, and all compounds in the IMP subset of
complexes have a LUMO that lies within 0.03 eV of those of
the other complexes. In contrast, MnTBIEP shows a difference
in LUMO energy of +0.19 eV in comparison with IPIMP. This
larger difference in LUMO energy comes as result of
methylation at the R position. In contrast, adding two isopropyl
groups at the R1 and R2 positions resulted in an energy
difference of just 0.02 eV between MnIMP and MnDIPIMP.
The results of the calculations on the trends in the energies of
HOMO/LUMO are in full agreement with the experimentally
determined redox potentials (see below). These results imply
that an almost complete separation between the steric and the
electronic effects in the context of few-electron reductions can
indeed be achieved in this series of complexes. Changing the R
group will strongly affect the energy of the LUMO while also
having some impact on the steric properties at the carbon of
the imino CN bond, while changing the R1 or R2 groups
should have considerable effects on the steric hindrance of the
molecule (protecting the Mn and imino-N centers) but hardly
affect its electronic properties.
Computational Methods. Density functional theory (DFT)
calculations were performed using the Gaussian 09 program package.41
All calculations utilized the global hybrid exchange correlation
functional B3LYP,42,43 a “mixed” basis set consisting of the SDD
basis set as defined in Gaussian for Mn and the 6-311G(d,p) basis set
for all other atoms.44−47 The solvent acetonitrile was included using
the polarizable continuum model (PCM) as implemented in
Gaussian.48,49 All species were modeled at the lowest multiplicity
appropriate for the electron count, and the restricted formalism was
used for closed-shell cases. An “ultrafine” integral grid, as defined by
Gaussian, was used and all geometries were confirmed as minima by
the absence of imaginary frequencies in their vibrational spectra as
calculated within the harmonic approximation. The values of
vibrational frequencies have been scaled by 0.966 to match
experimental ν(CO) values of the parent neutral complexes.
RESULTS AND DISCUSSION
■
X-ray Crystallography. The crystal structures of the
complexes [MnBr(CO)3(α-diimine)] (α-diimine = TBIEP,
IMP, IPIMP, DIPIMP) are shown in Figure 1, and selected
bond distances and angles are given in Table SI1 in the
Supporting Information. Similar bond lengths are observed for
the four complexes, and these are in good agreement with
related [MnBr(CO)3(α-diimine)] species reported in the
literature. The X-ray data are in good agreement with the
results obtained through DFT calculations. As predicted by
The experimental and calculated carbonyl vibrational
frequencies of the studied complexes are shown in Table 2.
The calculated frequencies are in good agreement with the
experimental values. Some systematic discrepancies are
apparent: the high-energy A′(1) mode tends to be under-
estimated by ∼10 cm−1, the A″ mode tends to have a lower
deviation of only ∼2 cm−1, and the low-energy A′(2) mode
tends to be overestimated by ∼10 cm−1. It is clear that
attachment of the methyl group at R increases the electron
density on the metal center and thus also the Mn to CO π
back-bonding, as evidenced by the smaller values of ν(CO) for
Figure 1. X-ray crystal structures of the studied complexes shown with
thermal ellipsoids at the 50% probability level. CCDC 1457930
(MnTBIEP), 1457931 (MnDIPIMP), 1457932 (MnIPIMP), 1457933
(MnIMP). Full crystallographic details are given in the Supporting
Information.
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Table 1. Frontier Orbitals of the Complexes 1−5 and the Corresponding Five-Coordinate Anions Calculated at the B3LYP/
a
SDD+6-311G(d,p)/IEFPCM Level
a
Isovalue of 0.04 (e bohr−3)1/2
.
MnTBIEP in comparison to the IMP subseries (complexes 1−
4). However, substitution at R1 and R2 has only a slight effect
on the frequencies. It should be noted that the magnitude of
these effects is small (<10 cm−1) and that it is beyond the scope
of this computational work to unravel the various factors
effecting changes in CO stretching frequencies.50
the corresponding [Mn(CO)3(H2O)(α-diimine)]+ (cationic
aquo complexes) match the experimental data well. We
therefore use the calculated ν(CO) wavenumbers for the
hydrolyzed aquo and reduced (dimer and anion) species to aid
the analysis of the IR spectra and product assignment in the
course of the corresponding cathodic IR-SEC experiments
(vide infra).
The results of DFT calculations (Table 2) of IR spectra for
the parent Br complexes [MnBr(CO)3(αdiimine)] (1−5) and
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Table 2. Experimentally Obtained and Calculated Harmonic Vibrational Frequencies, Scaled by 0.966, of Carbonyl Stretching
Vibrations, ν(CO), of the Mn Complexes in Their Neutral Form (1−5) and Transient One-Electron-Reduced Form, as well as
a
Five-Coordinate Anion, a Cationic Aquo Complex, and an Mn−Mn Bound Dimer
ν(CO)/cm−1
species
calcd
2020, 1943, 1931
exptl
[MnBr(CO)3(IMP)] (1)
[MnBr(CO)3(IMP)]−
2029, 1941, 1926
not obsd
1992, 1906, 1897
b
[Mn(CO)3(H2O)(IMP)]+
[Mn(CO)3(IMP)]−
2046, 1966, 1957
2051, 1964, 1958
1930, 1826
1906, 1830, 1813
[Mn(CO)3(IMP)]2
1964, 1918, 1891, 1882, 1872, 1868
2020, 1945, 1929
1994, 1949, 1902, 1875
2029, 1943, 1923
not obsd
[MnBr(CO)3(IPIMP)] (2)
[MnBr(CO)3(IPIMP)]−
[Mn(CO)3(H2O)(IPIMP)]+
[Mn(CO)3(IPIMP)]−
1988, 1905, 1891
2044, 1963, 1956
2049, 1959 (br)
1929, 1824
1905, 1826, 1808
[Mn(CO)3(IPIMP)]2
1964, 1917, 1890, 1881, 1866, 1860
2019, 1945, 1929
1981, 1949, 1901, 1882, 1862
2028, 1944, 1922
not obsd
[MnBr(CO)3(DIPIMP)] (3)
[MnBr(CO)3(DIPIMP)]−
[Mn(CO)3(H2O)(DIPIMP)]+
[Mn(CO)3(DIPIMP)]−
[Mn(CO)3(DIPIMP)]2
[MnBr(CO)3(TBIMP)] (4)
[MnBr(CO)3(TBIMP)]−
[Mn(CO)3(H2O)(TBIMP)]+
[Mn(CO)3(TBIMP)]−
[Mn(CO)3(TBIMP)]2
1985, 1906, 1890
b
2045, 1964, 1957
2050, 1960 (br)
1903, 1824, 1806
1929, 1829/1822
not obsd
1965, 1918, 1890, 1880, 1860, 1850
2020, 1947, 1925
2029, 1945, 1923
not obsd
1988, 1907, 1890
2045, 1965, 1956
not obsd
1906, 1827, 1807
1928, 1823
1964, 1916, 1889, 1879, 1862, 1854
2018, 1944, 1921
not obsd
[MnBr(CO)3(TBIEP)] (5)
[MnBr(CO)3(TBIEP)]−
[Mn(CO)3(H2O)(TBIEP)]+
[Mn(CO)3(TBIEP)]−
2028, 1943, 1917
not obsd
1980. 1904, 1883
2042, 1962, 1950
2048, 1960, 1954
1922, 1814 (br)
not obsd
1897, 1819, 1798
[Mn(CO)3(TBIEP)]2
1958, 1909, 1880, 1870, 1850, 1841
a
b
In acetonitrile at 293 K. Positions are approximate, as the parent CO stretching vibrations obscure those of the cationic aquo complex.
t
Adding electron-donating groups (iPr, Bu) to the phenyl
ring of the IMP subseries does not have a large effect on the
ν(CO) frequency, the band positions being virtually unchanged
among complexes 1−4. The two higher-frequency bands are at
2028−2029 and 1943−1944 cm−1 for all five complexes, while
the lowest ν(CO) band is seen at 1923−1922 cm−1 for the IMP
series but is shifted to lower energy, 1917 cm−1 in
[MnBr(CO)3(TBIEP)], where the increased π back-donation
is caused by R = Me. This invariability of ν(CO) frequencies,
while the dihedral angles between the planes of the pyridine
and phenyl moieties of the IP ligands are clearly changing
drastically, from ∼56 to ∼84°, confirms the opportunity of the
somewhat independent tuning of electronic and steric factors.
Calculations performed on the five-coordinate anions of the
studied compounds, [Mn(CO)3(diimine)]−, show trends very
similar to what has been observed in the parent complexes.
Both the HOMO and LUMO are predominantly delocalized
over the tricarbonyl-Mn and α-diimine metallacycle, with little
participation from the phenyl ring, with the exception of
MnIMP, the LUMO of which has a significant contribution
from the phenyl moiety.
Figure 2. Electronic absorption spectra of the manganese complexes
1−5 studied in this work, in DCM at 293 K.
Cyclic Voltammetry. Electrochemical studies showed
significant differences between the cathodic path of MnTBIEP
(5) and those of the IMP subseries (1−4).
Under an N2 atmosphere, [MnBr(CO)3(TBIEP)] shows a
single reduction wave at Ep,c −1.53 V and an intense anodic
wave at Ep,a = −1.3 V observed on the reverse anodic scan. This
behavior is similar to that of [MnBr(CO)3(iPr-DAB)] (iPr-
DAB = 1,4-diisopropyl-1,4-diazabuta-1,3-diene),8 which is
reduced by an ECE mechanism. The initial one-electron
reduction results in dissociation of the bromide to form a five-
coordinate radical, [Mn(CO)3(TBIEP)]•, which is concom-
itantly reduced to the five-coordinate anion [Mn-
The UV−vis absorption spectra (Figure 2) are consistent
with the nature of the frontier orbitals obtained from the
calculated data. The lowest energy absorption band for the
complexes of the IMP subseries 1−4 occurs at approximately
the same ca. 500 nm position. In contrast MnTBIEP (5)
exhibits an absorption band with a maximum at a shorter
wavelength, 460 nm, due to electron donation from the Me
group which destabilizes the LUMO.
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(CO)3(TBIEP)]− (reoxidized at −1.3 V) at the potential
required for the reduction of [MnBr(CO)3(TBIEP)]. A small
anodic wave at −0.6 V is characteristic of oxidation of
[Mn(CO)3(TBIEP)]2 formed in the course of the anodic
path of the five-coordinate anion, and indicates that
dimerization can still occur with R = CH3. The dimer could
also be produced in a reaction of [Mn(CO)3(TBIEP)]− with
neutral [MnBr(CO)3(TBIEP)] on the cathodic scan, but the
absence of a cathodic wave for reduction of [Mn-
(CO)3(TBIEP)]2 indicates that its reduction potential is too
close to that of [MnBr(CO)3(TBIEP)] for a separate reduction
wave to be observed.
The CV traces of MnIMP obtained under a N2 atmosphere
show three cathodic reduction peaks at Ep,c = −1.28, −1.41, and
−1.54 V and a strong anodic peak at Ep,a = −1.24 V. The first
reduction at −1.28 V can be assigned to the cation
[Mn(CO)3(H2O)(IMP)]+; it is likely that the peak at −1.41
V is due to remaining nonhydrolyzed [MnBr(CO)3(IMP)] or a
solvent adduct,11 while the −1.54 V wave corresponds to the
reduction of the dimeric species (see also the IR spectroelec-
trochemical section below). The five-coordinate anion [Mn-
(CO)3(IMP)]− is probably the reduction product at all three
different cathodic waves (the parent complex [MnBr-
(CO)3(IMP)], aquo complex, and the IMPMn−MnIMP
dimer), as evidenced by its anodic wave at Ep,a = −1.24 V on
the reverse anodic scan (accompanied by the dimer oxidation
above −0.5 V).
Over time a smaller cathodic wave emerges at Ep,c = −1.35 V,
due to the aquo-coordinated cationic complex forming via
hydrolysis of the parent Br complex (see Figures SI11 and SI12
in the Supporting Information). Under an atmosphere of CO2
the anodic wave of [Mn(CO)3(TBIEP)]− at −1.3 V disappears,
and the profile of the CV also changes (Figure 3), with a broad
cathodic wave of [MnBr(CO)3(TBIEP)] shifted slightly
negatively, indicating an interaction with CO2. However,
similar to the case for Mn-bpy complexes,2 catalytic reduction
of CO2 in the absence of a Brønsted acid was not observed (the
small peak beginning around −2.18 V is due to a small amount
of water entering the CV cell when it is being saturated with
CO2). Addition of 0.3 mL of water leads to significant current
enhancement at −2.18 V, in line with what has been observed
with [MnBr(CO)3(iPr-DAB)].8
Figure 3. Cyclic voltammograms of 1 mM MnTBIEP (top panel) and
MnTBIMP (bottom panel) in acetonitrile with 0.2 M [Bu4N][PF6] as
supporting electrolyte, under a N2 atmosphere (black), CO2
atmosphere (red), and CO2 with 4.7% added water (blue) at a scan
rate of 0.1 V s−1.
Under a CO2 atmosphere, the increased cathodic current is
seen at ∼−2.2 V for all complexes. We believe this is due to
some amount of the bicarbonate complex being formed, likely
due to traces of water in the CO2 used.33 When 10% water is
added to the CO2-saturated solution, a strong current
enhancement is observed at −2.21 V. Importantly, CVs
recorded under a N2 atmosphere in acetonitrile in the presence
of water do not show catalytic current enhancement (see
Figures SI14−SI18 in the Supporting Information); thus, both
CO2 and water are required for the current enhancement to be
observed.
Under a CO2 atmosphere, no anodic wave corresponding to
reoxidation of the five-coordinate anion is observed for the least
sterically hindered [Mn(CO)3(IMP)]− (Figure 4) or for the
monosubstituted complexes 2 and 4, a behavior indicative of a
rapid reaction of the anion with CO2. A diminished but clear
anodic wave of [Mn(CO)3(TBIEP)]− can be observed under a
CO2 vs a N2 atmosphere (Figure 3), suggesting that
[Mn(CO)3(TBIEP)]− associates with CO2 less efficiently.
While, similarly to MnIPIMP, no anodic wave corresponding
to [Mn(CO)3(TBIMP)]− reoxidation under a CO2 atmosphere
Figure 4. Cyclic voltammograms of 1 mM MnIMP in acetonitrile with
0.2 M [Bu4N][PF6] at a scan rate of 0.1 V s−1, under an atmosphere of
N2 (black), CO2 (red), and CO2 with 4.7% H2O (blue).
could be observed, indicating that CO2 association is rapid, the
overall current enhancement for this complex is comparatively
low, indicating lower efficiency at reducing CO2, perhaps due to
the bicarbonate intermediate somewhat preventing the recovery
of the five-coordinate catalytic species.
Under N2, reduction of [MnBr(CO)3(IPIMP)] is seen at Ep,c
−1.49 V, accompanied by a wave at Ep,c −1.29 V, assigned to
the cationic aquo complex [Mn(CO)3(H2O)(IPIMP)]+
(Figure 5). As discussed above, upon addition of CO2 the
oxidation wave of the anion [Mn(CO)3(IPIMP)]− is not
observed, indicating a rapid reaction between the five-
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bicarbonate complex reduction) or current enhancement below
−2 V is observed and the anodic peak due to oxidation of
[Mn(CO)3(DIPIMP)]− does not fully disappear. This suggests
that the reduced complex is less prone to interact with CO2, as
would be expected due to the increased steric hindrance and
i
structural rigidity of the complex arising from the two Pr
substituents at the N-phenyl rings.
IR and UV−Vis Spectroelectrochemistry under an
Inert Atmosphere. IR spectroscopy20 is an ideal tool to
monitor the cathodic processes in the studied complexes, due
to presence of the carbonyl ligands as strong IR reporters.
Table 2 gives the key experimental and calculated vibrational
frequencies for the starting complexes and several relevant
intermediate and dimer species. IR spectroelectrochemistry
(IR-SEC) was used to probe the intermediates produced upon
reduction and to monitor their presence during CO2 reduction.
IR spectra of MnTBIEP (Figure 7) show, upon the first
reduction, depletion of the parent ν(CO) bands, with new
bands growing in at 1922 and 1898 cm−1 and a broad feature at
1814 cm−1. The bands at 1922 and 1814 cm−1 can be assigned
to the five-coordinate anion [Mn(CO)3(TBIEP)]−, an assign-
ment supported by DFT calculations. The band at 1898 cm−1,
Figure 5. Cyclic voltammograms of 1 mM MnIPIMP in acetonitrile
with 0.2 M [Bu4N][PF6] at a scan rate of 0.1 V s−1, under an
atmosphere of N2 (black), CO2 (red), and CO2 with 4.7% H2O (blue).
coordinate anion and CO2. Some current enhancement at
−2.26 V is observed upon saturation with CO2, which is
enhanced greatly upon the addition of 0.3 mL of water (the
current enhancement corresponds to the cathodic wave of the
bicarbonate complex, identified in the IR spectra (vide infra):
some catalysis occurs due to hydrolysis caused for example by
residual water in the electrolyte or in the CO2).
CV of MnTBIMP (Figure 3, bottom panel) is similar to that
of MnIPIMP and MnTBIEP with a strong cathodic wave at
−1.45 V. At ca. −2.28 V current enhancements ascribed to CO2
reduction can be observed under CO2 and CO2 with added
H2O, though the icat/ip values (Table SI1 in the Supporting
Information) are somewhat lower in comparison to the other
complexes studied here. Importantly, the anodic wave of the
five-coordinate anion reoxidation is not detected for MnIPIMP
and MnTBIMP but is clearly seen for slower reacting
MnTBIEP and MnDIPIMP anions.
MnDIPIMP shows significant differences in the CV traces in
comparison to the other complexes of the IMP subseries
(Figure 6). Similarly to the IMP and IPIMP complexes, a
formation of a cationic aquo complex ([Mn(CO)3(H2O)-
(DIPIMP)]+) is observed in solution. However, upon
saturation with CO2 no additional processes (intermediate
Figure 7. IR spectral changes accompanying in situ reduction of
complexes in Ar-saturated acetonitrile/0.2 M [Bu4N][PF6] within an
OTTLE cell. (top) For MnTBIEP, a direct reduction of the parent
complex (black line) to the five-coordinate anion (green line) is
observed. :(P) [MnBr(CO)3(TBIEP)]; (A) [Mn(CO)3(TBIEP)]−;
(M) an unassigned side product. (bottom) For MnTBIMP, a direct
reduction of the parent complex (black line) to the five-coordinate
anion (green line) is observed: (P) [MnBr(CO)3(TBIMP)]; (A)
[Mn(CO)3(TBIMP)]−.
Figure 6. Cyclic voltammograms of 1 mM MnDIPIMP in acetonitrile
with 0.2 M [Bu4N][PF6] at a scan rate of 0.1 V s−1, under an
atmosphere of N2 (black), CO2 (red), and CO2 with 4.7% H2O (blue).
H
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which grows in after the five-coordinate anion begins to form,
could tentatively be attributed to a decomposition product.
UV−vis spectroelectrochemistry (Figure SI13 in the Support-
ing Information) supports this notion, as only a band at ca. 570
nm has been detected, which corresponds to the five-
coordinate anion. Differently from the MnIMP and MnIPIMP
complexes (see below), there is no indication of dimer
([Mn(CO)3(TBIEP)]2) formation during the reduction of
MnTBIEP on the time scale of the experiments performed.
MnTBIMP mirrors the behavior of MnTBIEP with the bands
at 2029, 1945, and 1923 cm−1 corresponding to the parent
complex being replaced concertedly with bands at 1928 and
1823 cm−1 corresponding to the five-coordinate anion, with no
intermediate species being observed. This would suggest that
the direct formation of the five-coordinate anion is due to the
cm−1 upon reduction. This is assigned to the aquo complex
[Mn(CO)3(H2O)(IMP)]+.
Differently from MnIMP, MnIPIMP showed concurrent
formation of the dimer and the five-coordinate anion upon
reduction of the parent complex (Figure 9, top). The
t
steric demands of the Bu group, since the mono-iPr derivative
2 does exhibit dimer formation (Table 2).
The results of the IR-SEC study of MnIMP are shown in
Figure 8. The first reduction of MnIMP in CH3CN under an Ar
Figure 8. IR spectral changes accompanying in situ reduction of
MnIMP in Ar-saturated acetonitrile/0.2 M [Bu4N][PF6] within an
OTTLE cell. The parent complex [MnBr(CO)3(IMP)] (P, black line),
and aquo cation [Mn(CO)3(H2O) (IMP)]+ (H, additional features in
the red spectrum) are reduced to the dimer [MnBr(CO)3(IMP)]2 (D,
green line) followed by reduction of the dimer to the five-coordinate
anion [Mn(CO)3(IMP)]− (A, blue line). The intermediate spectrum
(red line) recorded between those of the parent complex and the
dimer also shows the features of the aquo complex.
Figure 9. IR spectral changes accompanying in situ reduction of the
complexes in Ar-saturated acetonitrile/0.2 M [Bu4N][PF6] within an
OTTLE cell. (top) MnIPIMP, with concurrent formation of a dimer
and a five-coordinate anion on reduction of the parent complex being
observed: (P) [MnBr(CO)3(IPIMP)]; (D) [Mn(CO)3(IPIMP)]2;
(A) [Mn(CO)3(IPIMP)]−; (H) [Mn(CO)3(H2O)(IPIMP)]+. (bot-
tom) MnDIPIMP, with reduction of the parent complex to a five-
coordinate anion being observed: (P) [MnBr(CO)3(DIPIMP)]; (A)
[Mn(CO)3(DIPIMP)]−; (H) [Mn(CO)3(H2O)(DIPIMP)]+; (M)
[Mn(CO)3(MeCN)(DIPIMP)]•.
atmosphere is accompanied by depletion of the parent IR bands
at 2029, 1941, and 1926 cm−1. Simultaneously, the growth of
new bands at 1994, 1949, 1902, and 1875 cm−1 is seen, which
are characteristic of the Mn−Mn dimer8,9,13,30 [Mn-
(CO)3(IMP)]2. Additionally, a peak at 2051 cm−1 grows in
initially, which is assigned to the intermediate aquo cation
[Mn(CO)3(H2O)(IMP)]+ observed also by cyclic voltamme-
try. Further reduction of the dimer leads to formation of broad
absorption bands at 1826 and 1930 cm−1, once the formation of
the dimer species is complete. These features are characteristic
of the formation of the five-coordinate anion8,9,13 [Mn-
(CO)3(IMP)]−. UV−vis spectroelectrochemistry performed
in parallel with the IR-SEC experiment confirms the presence
of both of these species (Figure SI14 in the Supporting
Information) via the broad absorption band at ca. 800 nm
(assigned to the dimer) and the intense absorption at ca. 675
nm (assigned to the five-coordinate anion).8 All complexes in
the IMP subseries exhibited a small transient peak at ca. 2050
introduction of the isopropyl substituent at the phenyl ring
leads to the observation of a small amount of the five-
coordinate anion [Mn(CO)3(IPIMP)]− (absorbing at 1929 and
1824 cm−1), which grows in alongside peaks indicative of dimer
formation (1981, 1949, 1901, 1882, and 1862 cm−1).
Importantly, the IR absorption bands, corresponding to both
the dimer and the five-coordinate anion, grew in simulta-
neously. UV−vis spectroelectrochemistry confirmed the
presence of both dimer and five-coordinate species in this
case, as is evident from Figure SI13 in the Supporting
Information).
In contrast, only the five-coordinate anion is detected already
from the onset of the reduction of MnDIPIMP under the
experimental conditions used. In this case, there is no evidence
for the dimer formation during the reduction of the parent
complex. As shown in Figure 9 (bottom), an intense peak at
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1823 cm−1, assigned to the five-coordinate anion, grew in,
followed closely by smaller peaks at 2007 and 1899 cm−1. The
second peak assigned to the five-coordinate anion at 1929 cm−1
was masked by the absorption of the parent complex at the
beginning of the reduction process. We tentatively assign the
peaks at 2007/1899 cm−1 to the solvent-coordinated radical
species [Mn(CO)3(MeCN)(DIPIMP)]•, in analogy with
[Re(CO)3(PrCN)(iPr-PyCa)]17 (iPr-PyCa = (isopropylimino)-
pyridine; PrCN = butyronitrile) which shows ν(CO) bands at
2005 and 1885 (br) cm−1. Further, since the anodic wave of the
dimer oxidation is not observed in the CV of MnDIPIMP, but a
1e reduced radical species is observed in IR-SEC, it is evident
that the DIPIMP ligand prevents dimerization.
complex results in the formation of the two-electron-reduced
five-coordinate anion that reacts efficiently with CO2; no dimer
is observed during the reduction of MnIMP (Figure 10) or
MnIPIMP (Figure 11).
MnTBIMP shows behavior intermediate to that of
t
MnTBIEP and MnIPIMP: similarly to MnTBIEP, the Bu
substituent prevent dimer formation upon reduction. However,
differently from MnTBIEP, and similar to MnIPIMP, a rapid
reaction with CO2 takes place, which in the case of MnTBIEP is
considerably slowed by the R = Me group. It is important to
note that if Mn−Mn dimer is reduced at the same or even less
negative potentials than that of the parent complex, it will not
be detected in the studies.9 Thus, the comments above
regarding the absence of dimer formation only relate to the
Br complexes studied here. Substituting Br− with a different
group, which would lead to the parent complex being reduced
at less negative potentials, may permit detection of these
species. Five-coordinate complex formation appears to correlate
with a less negative first reduction potential (see Table 3). A
Figure 10. IR spectral changes accompanying in situ reduction of
MnIMP ([MnBr(CO)3(IMP)]) in CO2-saturated acetonitrile/0.2 M
[Bu4N][PF6] within an OTTLE cell: (P) [MnBr(CO)3(IMP)]; (B)
[Mn(CO)3(IMP)(η1-OCO2H)]; (C) [Mn(CO)5]−; (H) [Mn-
(CO)3(H2O)(IMP)]+; (F and S) free bicarbonate (OCO2H−) and
subordinate formate (OCHO−) accompanying the catalytic reduction
of CO2 to CO.
Table 3. Cathodic Potentials (V, vs Fc/Fc+) of the Parent
Complexes [MnBr(CO)3(IP)] (1 mM, Acetonitrile, 0.2 M
[Bu4N][PF6]) and Corresponding Cationic Mn Aquo
Derivatives Formed in Situ by Partial Hydrolysis
b
complex
Ep,c
catalytic potential
a
[MnBr(CO)3(IMP)] (1)
−1.41, −1.54
−1.28
−1.49
−1.29
−1.44
−1.23
−1.45
−1.53
−1.35
−2.21
[Mn(CO)3(H2O)(IMP)]+
[MnBr(CO)3(IPIMP)] (2)
[Mn(CO)3(H2O)(IPIMP)]+
[MnBr(CO)3(DIPIMP)] (3)
[Mn(CO)3(H2O)(DIPIMP)]+
[MnBr(CO)3(TBIMP)] (4)
[MnBr(CO)3(TBIEP)] (5)
[Mn(CO)3(H2O) (TBIEP)]+
−2.26
−2.16
∼−2.28
−2.18
a
This process probably corresponds to a reduction of the dimer.
Largely coinciding with the reduction of a bicarbonate complex (see
b
Figure 11. IR spectral changes accompanying in situ reduction of
MnIPIMP ([MnBr(CO)3(IPIMP)]) in CO2-saturated acetonitrile/0.2
M [Bu4N][PF6] in an OTTLE cell: (P) [MnBr(CO)3(IPIMP)]; (A)
[Mn(CO)3(IPIMP)]−; (B) [Mn(CO)3(IPIMP)(η1-OCO2H)]; (C)
[Mn(CO)5]−; (H) [Mn(CO)3(H2O)(IPIMP)]+; (F and S) free
bicarbonate (OCO2H−) and subordinate formate (OCHO−) accom-
panying the catalytic reduction of CO2 to CO.
the spectroelectrochemical section).
comparable correlation was found for Mn-R-DAB complexes
and sterically hindered 2,2′-bipyridines already reported in the
literature. These complexes also exhibit less negative first
reduction potentials in comparison to their less sterically
hindered counterparts and form five-coordinate anions directly
upon reduction.8,25
IR and UV−Vis Spectroelectrochemistry under a CO2
Atmosphere. Electrochemical behavior under a CO2 atmos-
phere is vastly different from that under a N2 or Ar atmosphere.
The electrocatalytic reduction of CO2 with the four Mn
complexes can be described in terms of three different types of
behavior, largely controlled by the steric hindrance of the active
imino CN bond. MnIMP and MnIPIMP are relatively
unhindered, and the catalytic behaviors are almost identical.
The initial reduction of parent and/or the cationic aquo
The catalytic process at the initial cathodic wave is, however,
inhibited by the rapid formation of a stable bicarbonate
complex, absorbing at 2036, 1940, 1924, and 1671 cm−1 for the
IPIMP species, in line with the reports for sterically hindered
Mn-mesityl-bipyridine25 complexes and Mn-R-DAB com-
plexes.8 A further negative potential shift of ca. 0.7 V is needed
to reduce the bicarbonate complex, resulting in the recovery of
the five-coordinate anion that triggers the catalytic conversion
of CO2.
J
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For the nonhindered IMP and IPIMP ligands the five-
coordinate anion reacts rapidly and is not observed in the IR
spectra on this time scale (for IMP) and only at a low
concentration (for IPIMP). The production of CO in the thin
solution layer results in the displacement of the α-diimine
ligand in the five-coordinate anion, forming the pentacarbonyl
species [Mn(CO)5]− clearly seen in the IR spectra via the
growth of bands at 1897 and 1865 cm−1 (species C in Figures
10−13). Remarkably, in these two cases only a comparatively
Figure 12. IR spectral changes accompanying in situ reduction of
MnDIPIMP in CO2-saturated acetonitrile/0.2 M [Bu4N][PF6] within
an OTTLE cell: (P) [MnBr(CO)3(DIPIMP)]; (A) [Mn-
(CO)3(DIPIMP)]−; (B) [Mn(CO)3(DIPIMP)(η1-OCO2H)]; (H)
aquo complex [Mn(CO)3(H2O)(DIPIMP)]+; (C) [Mn(CO)5]−;
(M) [Mn(CO)3(MeCN)(DIPIMP)]•; (F and S) free bicarbonate
(OCO2H−) and subordinate formate (OCHO−).
Figure 13. IR spectral changes accompanying in situ reduction of
complexes in CO2-saturated acetonitrile/0.2 M [Bu4N][PF6] within an
OTTLE cell. (top) For MnTBIEP: (P) [MnBr(CO)3(TBIEP)]; (A)
[Mn(CO)3(TBIEP)]−; (B) [Mn(CO)3(TBIEP)(η1-OCO2H)]; (C)
[Mn(CO)5]−; (F and S) free bicarbonate (OCO2H−) and formate
(OCHO−) accompanying the catalytic reduction of CO2 to CO.
(bottom) For MnTBIMP: (P) [MnBr(CO)3(TBIMP)]; (B) [Mn-
(CO)3(TBIMP)(η1-OCO2H)]; (C) [Mn(CO)5]−; (F and S) free
bicarbonate (OCO2H−) and formate (OCHO−) accompanying the
catalytic reduction of CO2 to CO.
small amount of free bicarbonate or free formate (1685, 1638,
and 1604 cm−1 for the IPIMP species) relative to [Mn(CO)5]−
is observed, marking the high catalytic efficiency toward CO
production.
Upon reduction of the more CN hindered DIPIMP
complex, the five-coordinate anion formed does not react with
CO2 efficiently and a metastable population of the anionic five-
coordinate MnDIPIMP species [Mn(CO)3(DIPIMP)]− is
detected even under a high excess of CO2. Interestingly, and
differently from the other complexes in the Mn-IP series, the
formation of a bicarbonate complex is only detected at the
potential corresponding to the reduction of CO2-associated
species at around −2 V vs Fc/Fc+, while on prior coordination
of CO2 to the five-coordinate anion at the parent MnDIPIMP
cathodic wave no bicarbonate ligand signature is detected. At
the catalytic potential where the bicarbonate complex is
reduced, the conversion of CO2 to CO is also inefficient. A
high concentration of the five-coordinate anion is still seen,
converting slowly to [Mn(CO)5]− when the concentration of
CO increases; at the same time the production of free
bicarbonate (and free formate) is much higher in comparison
to the MnIMP and MnIPIMP cases, marking the low catalytic
efficiency toward CO production. Notably, the lower CO-
stretching band of [Mn(CO)3(DIPIMP)]− becomes shifted
from its standard position (1829/1822 cm−1) to lower energy
(ca. 1810 cm−1) at the advanced stage of the catalytic
conversion. This shift may indicate the presence of an
observable adduct of the five-coordinate anion, most likely
with CO2 or formate (over the Mn−NC bond). In this
context it is interesting to note that the related Re-IP complex37
forms the carbonate complex in two 1e-reduction steps, via a
direct coordination to the Re center, without CN being
directly involved.
In the case of MnTBIEP the imino CN bond is hindered
both at the carbon atom via the methyl group and by the tert-
butyl group on the phenyl moiety. There are similarities with
but also differences from the hindered DIPIMP complex, which
does not have a hindering group at the C atom of the imino
CN moiety. Upon reduction of the parent complex in CO2-
saturated acetonitrile the five-coordinate anion [Mn-
(CO)3(TBIEP)]− coordinates CO2, forming the bicarbonate
complex readily (similar to IMP and IPIMP) with the
characteristic IR absorption band at 1673 cm−1.8,30
A small amount of the five-coordinate anion [Mn-
(CO)3(TBIEP)]− is observed in the initial step. Lowering the
potential to around −1.5 V vs Fc/Fc+ results in catalytic
conversion of the bicarbonate complex; however, similar to
MnDIPIMP this conversion is not efficient in comparison with
MnIMP and MnIPIMP. This is shown via the slower growth of
[Mn(CO)5]− in comparison to IPIMP and the greater
quantities of free bicarbonate produced. As with MnDIPIMP
the five-coordinate anion “adduct” form is observed with the
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Article
lower energy CO-stretching band shifted to a lower wave-
number (from 1814 to 1803 cm−1). Thus, hindering the imine
C atom does not affect adduct formation between CO2 and
[Mn(CO)3(TBIEP)]−.
optimizing steric vs electronic effects in CO2 reduction,
whereby the thermodynamic factors, the rate of CO2
coordination, and the rate of decomposition of catalyst
precursor species need to be balanced.
However, at the negative potentials where the bicarbonate
complex is reduced (recovering the catalytic five-coordinate
anion) the hindrance provided by the methyl and tert-butyl
groups also negatively affects the catalytic formation of CO2 to
CO (as evidenced by large amounts of free bicarbonate and
slow formation of [Mn(CO)5]− at lower CO concentration). It
is not very clear whether this greater hindrance is due directly
to the presence of the methyl group on the C position or
whether this is due to the tert-butyl group inhibiting rotation of
the phenyl moiety and preventing the five-coordinate anion
from adopting a more suitable (pyramidal) geometry for CO2
association.
The main transformation pathways of 1−5 upon reduction
under an inert atmosphere, and under an atmosphere of CO2,
are summarized schematically in Figure 14.
Again, MnTBIMP behaves in a fashion similar to that of
MnTBIEP. Upon reduction the parent complex rapidly
associates CO2, forming the bicarbonate complex; as the
reduction potential is lowered further, the bicarbonate complex
is reduced, forming CO which is able to displace the TBIMP
and forming [Mn(CO)5]−. One important difference is that
significantly less (if any) five-coordinate anion is observed in
the presence of CO2 than was the case with both MnTBIEP
Figure 14. Main transformation pathways of 1−5 upon reduction
under (a) an inert atmosphere and (b) an atmosphere of CO2. A is
detected for 3 only due to the comparatively slower reaction of [3]−
with CO2.
t
and MnDIPIMP. This suggests that Bu is not as sterically
i
demanding as two Pr groups in these systems, as CO2 is still
able to coordinate.
Estimation of Electrocatalytic Activity toward CO
Production using Gas Chromatography. The CO
concentration as a function of time in the course of controlled
potential electrolysis estimated by GC analysis of the headspace
of the electrolysis cell shows a gradual buildup of CO in the
course of the electrolysis (Figure SI19 in the Supporting
Information). A comparison with the performance of [MnBr-
(CO)3(bpy)] catalyst investigated under identical conditions
(see Figure S19) shows that the efficiency of CO production
for the new catalysts 1−5 is comparable to that of
[MnBr(CO)3(bpy)], with the least sterically hindered
MnIMP complex being somewhat more efficient. Due to the
large volumes used in the experiment, considerable secondary
processes occur during bulk electrolysis, manifested in the loss
of the initial intense yellow-red color of the solution as the
reaction progressed, which was concomitant with an increase in
current toward the end of the electrolysis. These deviations
from an ideal behavior suggest that, as CO2 is depleted in
solution, competing catalyst degradation pathways begin to
occur, precluding reliable estimates of efficiencies.
Estimation of efficiency from the CV data was done by the
relative icat/ip values (Table S1 in the Supporting Information)
following the method described in refs 4 and 7. Comparing the
current values detected in the CV at −2.24 V (vs Fc/Fc+)
recorded under a CO2 and N2 atmosphere in acetonitrile/water
also shows that the performances of 1−5 are comparable to one
another and are comparable to that of [Mn(CO)3(bpy)Br], at
30−60% efficiency. It is important that the most sterically
protected complexes, MnDIPIMP and MnTBIEP, seem to be
performing better as far as icat/ip values are concerned but that
the least sterically hindered complex, MnIMP, is the most
efficient in the series. These observations are different from the
observation of MnTBIMP producing more CO than [MnBr-
(CO)3(bpy)] in the bulk electrolysis/GC experiments. While
these data can only be considered in relative terms, they do
show the potential of these complexes to act as a test bed for
CONCLUSIONS
■
A series of Mn(I) tricarbonyl electrocatalysts for CO2 reduction
which employ, for the first time, asymmetric α-diimine ligands,
imino-pyridines, has been developed, and their catalytic activity
has been confirmed and evaluated in detail.
We have demonstrated through conventional and thin-layer
cyclic voltammetry, UV−vis and IR spectroscop, and DFT
computational analysis the π decoupling of the phenyl from the
Mn(pyridine-CCN) metallacycle. The practical effect of this
feature is the ability to disentangle steric and electronic effects
of the α-diimine ligand on the catalytic properties. Until now,
introduction of sterically bulky groups, which are also typically
electron donating, was coming at the price of an increased
overpotential required for CO2 reduction. The use of an
asymmetric α-diimine has allowed us to probe the effect of
adding ever greater sterically demanding groups without much
change in the catalytic potential. We have demonstrated that a
systematic increase in the steric hindrance of the R1 and R2
groups in the IMP subseries results in the switch of the nature
of the first reduction product detected on the time scale of the
experiment under an inert gas atmosphere, from a dimer to a
five-coordinate anion, at a very similar reduction potential. In
the absence of sterically hindering groups on the phenyl ring,
MnIMP, a dimer is formed, while increasing the steric
i
hindrance by adding Pr groups to the R1 and R2 positions
(MnDIPIMP) resulted in direct formation of the five-
coordinate anion, in line with prior observations for similar
sterically hindered ligands.23,26 MnIPIMP (in which case the
dimer may be reduced at the parent cathodic wave due to
slightly negatively shifted reduction potential vs that for
MnIMP) exhibited behavior intermediate to that of MnIMP
and MnDIPIMP with both the dimer and the five-coordinate
anion observed to be formed concurrently. MnTBIMP and
L
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Article
ORCID
MnTBIEP both formed the five-coordinate anion directly upon
reduction of the parent complex.
Anthony J. H. M. Meijer: 0000-0003-4803-3488
Julia A. Weinstein: 0000-0001-6883-072X
Under a CO2 atmosphere, all of the complexes reduce CO2
to CO. The buildup of CO in the thin-layer spectroelec-
trochemical cell resulted in the displacement of the α-diimine
ligand, forming [Mn(CO)5]−. The complex containing the
most sterically demanding ligand, DIPIMP, is as anticipated
least susceptible to α-diimine displacement with CO, forming
exclusively the five-coordinate anion upon the first reduction; it
also has the least propensity to coordinate CO2, resulting in a
considerable buildup of the concentration of the five-coordinate
anion. An intermediate formation of the bicarbonate is also
likely, as a band at 1686 cm−1 is present at intermediate times.
Of particular interest is that the least sterically hindered
complex, MnIMP, seemed to form a CO2-associated complex
directly upon the first reduction, with no significant formation
of the dimer being observed on the time scale of the
experiment. This behavior is similar to that reported for the
symmetric nonaromatic Mn-R-DAB (R = alkyl) compounds.8,30
The formation of a stable bicarbonate complex, either through
the coordination to the metal center or via the imino CN
bond,23,37 leads to the need for increased overpotential. From
that point of view, the steric hindering (protection) of the
metal center/the imino CN bond in the Mn(IP) complexes
is advantageous, as it disfavors the Mn−Mn dimerization (when
MnIMP is compared with MnDIPIMP). However, such steric
crowding also slows the catalytic conversion of CO2 to CO at
the negative overpotentials, as can be seen in the GC data and
from the icat/ip values. A difference in the reactivity of
MnTBIMP and MnTBIEP, where no dimer formation has
been detected for either of the complexes in the IR-SEC
experiments but where MnTBIEP exhibits slower CO2
conversion due to R = CH3, alters the HOMO−LUMO gap
in comparison to the IMP series as well as introduces additional
steric bulk, further supporting the notion that it is possible to
separate steric and electronic factors to a large extent. Balancing
these factors by careful ligand design may lead to the optimal
solution.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful to E. J. Carrington, T. M. Roseveare,
and C. M. Kiker for assistance in interpreting the X-ray
diffraction data, Drs. A. Haynes, E. S. Davies, and S. Parker for
discussions, and G. Chandrakumar for experimental assistance.
Support of the University of Sheffield and its SURE scheme,
Shine DTC, the University of Reading (Project D14-015), the
EPSRC, and the RSC Undergraduate Bursary (T.K. and H.F.)
is gratefully acknowledged.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b01477.
Calculated frontier orbitals from HOMO-3 to LUMO+3
for all studied complexes, complete CV measurements,
control experiments, crystallographic data, and H NMR
spectra of the new complexes (PDF)
Crystallographic data (CIF)
Crystallographic data (CIF)
1
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B. A.; Preses, J. M.; Wishart, J. F. Mechanism of the Formation of a
Mn-Based CO2 Reduction Catalyst Revealed by Pulse Radiolysis with
Time-Resolved Infrared Detection. J. Am. Chem. Soc. 2014, 136,
5563−5566.
AUTHOR INFORMATION
■
(11) Johnson, F. P. A.; George, M. W.; Hartl, F.; Turner, J. J.
Electrocatalytic Reduction of CO2 Using the Complexes [Re(bpy)
(CO)3L]n (n = + 1, L = P(OEt)3, CH3CN; n = 0, L = Cl−, Otf−; bpy =
Corresponding Authors
*E-mail for F.H.: F.Hartl@reading.ac.uk.
*E-mail for J.A.W.: Julia.Weinstein@sheffield.ac.uk.
−
2,2′-Bipyridine; Otf− = CF3SO3 ) as Catalyst Precursors: Infrared
M
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