Resurgence of Organomanganese(I) Chemistry. Bidentate Manganese(I) Phosphine-Phenol(ate) Complexes

Article abstract of DOI:10.1021/acs.inorgchem.9b00941

As part of the United Nations 2019 celebration of the periodic table of elements, we are privileged to present our studies with the element manganese in this Forum Article series. Catalysis with organomanganese(I) complexes has recently emerged as an important area with the discovery that pincer manganese(I) complexes that can activate substrates through metal-ligand cooperative mechanisms are active (de)hydrogenation catalysts. However, this rapidly growing field faces several challenges, and we identify these in this Forum Article. Some of our efforts in addressing these challenges include using alternative precursors to Mn(CO)₅Br to prepare manganese(I) dicarbonyl complexes, the latter of which is usually a component of active catalysts. Specifically, the synthesis of a new bidentate phosphine-phenol ligand along with its corresponding coordination chemistry of five new manganese(I) complexes is described. The complexes having two phenol-phenolate moieties interact with the secondary coordination sphere to enable facile loss of the bromido ligand and even one of the CO ligands to afford manganese(I) dicarbonyl centers.

Full text of DOI:10.1021/acs.inorgchem.9b00941

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Resurgence of Organomanganese(I) Chemistry. Bidentate
Manganese(I) Phosphine−Phenol(ate) Complexes
Karthika J. Kadassery,† Samantha N. MacMillan,‡ and David C. Lacy*,†
†Department of Chemistry, University at Buffalo, State University of New York, Buffalo, New York 14260, United States
‡Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States
S
* Supporting Information
ABSTRACT: As part of the United Nations 2019 celebration
of the periodic table of elements, we are privileged to present
our studies with the element manganese in this Forum Article
series. Catalysis with organomanganese(I) complexes has
recently emerged as an important area with the discovery
that pincer manganese(I) complexes that can activate substrates
through metal−ligand cooperative mechanisms are active
(de)hydrogenation catalysts. However, this rapidly growing
field faces several challenges, and we identify these in this
Forum Article. Some of our efforts in addressing these
challenges include using alternative precursors to Mn(CO)5Br
to prepare manganese(I) dicarbonyl complexes, the latter of
which is usually a component of active catalysts. Specifically, the synthesis of a new bidentate phosphine−phenol ligand along
with its corresponding coordination chemistry of five new manganese(I) complexes is described. The complexes having two
phenol−phenolate moieties interact with the secondary coordination sphere to enable facile loss of the bromido ligand and even
one of the CO ligands to afford manganese(I) dicarbonyl centers.
electrons are in a high-spin configuration, resulting in a
nullification of ligand-field stabilization. In other words, the
frontier orbitals of high-spin manganese(2+) complexes are not
very different from complex to complex, and thereby the
chemistry is not as versatile and is often viewed as less
interesting than nearby first-row metals.6 This chemical nature
has even earned manganese the nickname the “black-sheep” of
divalent transition organometals,7 sharing more similarities
with organomagnesium and organozinc than nearby organo-
transition metals.
The similarity to redox inert divalent metals enables it to
replace various 2+ cations like zinc(II), calcium(II), and
magnesium(II) as a structural component to proteins.8
However, unlike magnesium and zinc, manganese has rich
redox chemistry with accessible d counts from d8 to d0 without
evoking exotic oxidation states. This has significant implica-
tions to life on Earth because a critical component of the
molecular machinery in photosynthesis is a tetranuclear
Mn4CaO center that oxidizes water to O2.9 Additionally, it is
becoming widely recognized that MnII is a crucial metal ion for
microbial growth and function.10
INTRODUCTION
■
Celebrating Manganese in the Year of the Periodic
Table. The United Nations has proclaimed 2019 as the Year of
the Periodic Table to celebrate the 150th anniversary of the
periodic table. We are privileged with the opportunity to
showcase manganese in this Inorganic Chemistry Forum Article
that commemorates humankind’s achievement of codifying the
chemical elements. Manganese is a first-row transition metal
that has been used since prehistoric times in the form of black
mineral pigments found in a variety of locations around
Europe.1 Manganese was one of the first elements identified,
having been systematically discovered in 1774 by Scheele and
Gahn.2 In modern times, it finds practical application in
metallurgy and a variety of areas in society. Manganese is an
element that touches almost every field of chemistry, but as
inorganic chemists, we are primarily interested in its chemical
properties. Hence, the following discussion on manganese
contains our limited perspective with an emphasis on
homogeneous organometallic manganese(I) chemistry.
In keeping with the theme of The Year of the Periodic Table,
we first turn to the periodic table to intuit the types of
properties manganese is expected to have. Manganese is
unique among the first-row transition metals because it has no
naturally occurring second-row congener, and rhenium (its
third-row congener) is one of the least abundant transition
metals in the Earth’s crust.3 In its common 2+ formal oxidation
state, manganese is additionally unique, owing to its half-filled
3d subshell. Except in the most unusual of cases,4,5 the five d
Special Issue: Celebrating the Year of the Periodic Table: Emerging
Investigators in Inorganic Chemistry
Received: March 31, 2019
© XXXX American Chemical Society
A
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Examples of mononuclear molecular manganese complexes
with unambiguous Mnn+ oxidation states with d counts from d8
to d0 include the following: [Mn−1(CO)3{CNArDipp}2]−
Manganese(I) accommodates an extra L-type ligand, and
therefore bidentate ligands are sufficient in stabilizing catalysts
of the metal−ligand cooperative (MLC) sort.32 For the 2+
ions, the supporting ligand needs to contain at least two
strong-field L-type ligands to keep the complexes low-spin (for
iron). This aspect of manganese has many added benefits
including flexibility in ligand design, and phosphine-free
bidentate NN ligands can be employed.33,34 Collectively, this
new and growing body of data surrounding manganese(I)
(de)hydrogenation catalysts led us to explore uncharted areas
with the goal of uncovering new applications of manganese in
areas such as energy and catalysis.
Besides the fact that manganese(I) chemistry has lagged
behind the organometallic chemistry of other first-row
transition metals (e.g., iron, cobalt, nickel, etc.), some of the
major challenges facing catalysis with organomanganese(I) are
as follows: (i) The only relevant manganese(I) precursor for
catalysts is Mn(CO)5Br, which is rather expensive and is
derived from Mn2(CO)10. (ii) All of the bifunctional ligands
used in MLC manganese(I) systems rely exclusively on anionic
N-atom donors for the bifunctional activation of substrates.
(iii) Synthetic routes to prepare the catalytically active 16-
electron manganese(I) dicarbonyl complex are lacking. These
generally require “luck of the draw” syntheses, where
coordination of a tridentate ligand produces the required
precatalyst formulation. Recently, Kirchner and co-workers
demonstrated that “bifunctional ligands” are not even
necessary to enact hydrogenation chemistry with manganese-
(I) precatalysts,35 and thus the search for novel ligands and less
expensive precursor materials should constitute major foci in
the resurgence of organomanganese(I) chemistry.
In this report, we detail some of the new efforts ongoing in
our laboratory to address these three challenges. First, two
synthons derived from Mn2(CO)10 are used to prepare
manganese(I) catalysts; the first of these is the in situ prepared
butterfly tetranuclear complex described below, and the other
is MeMn(CO)5. Second, we have been exploring phenolic-
based phosphine ligands to augment the nitrogen-only
heteroatoms found in bifunctional systems. Finally, the
phenolic platform interacts with the secondary coordination
sphere and appears to modulate the liberation of bromido
ligands and even carbonyl ligands. This work is an extension of
an earlier report where we prepared a tridentate bis-
(phosphine)−phenol (POP) pincer ligand.36 The resulting
POP-Mn complexes were constrained in their geometry, which
limited access to dicarbonyl catalysts. To relax the constraints
imposed by the POP system, we synthesized a new bidentate
monophosphine ligand that affords access to a variety of
coordination modes including manganese(I) dicarbonyl
complexes.
( A r D i p p
=
2 , 6 - [ 2 , 6 - ( i P r ) 2 C 6 H 3 ] 2 C 6 H 3 ) , 1 1
[Mn0(CO)3{CNArDipp}2],12 CpMnI(CO)3 (Cp = cyclopenta-
dienyl),13 MnIICp2,14 MnIII(acac)3 (acac = acetylacetonate),15
[MnIVMe6]2−,16 [MnV(O)H3buea]− (H3buea = tris[(N′-tert-
butylureaylato)-N-ethylene]amine),17 [MnVI(NtBu)4]2−,18 and
[MnVII(NtBu)3Cl].18 Much of the fundamental research in
organometallic chemistry relied on the element manganese.
For instance, the substitution chemistry of six-coordinate d6
Mn(CO)5X was instrumental in the discovery of the cis
effect.19 Migratory insertions was first observed on the
MeMn(CO)5 complex, and the first M−M bond in a molecular
species was discovered with Mn2(CO)10.20,21
Molecular manganese complexes have an important role in
industrial chemistry on a massive scale as catalysts in radical-
chain aerobic oxidations22 and as an antiknock fuel additive.23
In the latter case, methylcyclopentadienylmanganese(I)
tricarbonyl was produced on an industrial scale to replace
tetraethyllead, demonstrating that organomanganese(I) com-
plexes can be economically produced in large quantities. To
this end, manganese has the potential to compete with
expensive metals in nearly every corner of homogeneous
catalysis, whether it be coupling reactions,24 C−H bond
activation,25 epoxidations,26 small-molecule activation,27 and
more. This potential is limited only by available knowledge, of
which is lacking because, in our opinion, the chemistry of
manganese has lagged behind the other transition metals.28
Thus, there is ample reason to explore the fundamental
chemistry of manganese, and only time will tell what kind of
new reactions manganese will catalyze to open possible
avenues toward replacing toxic and expensive metals in
synthesis.
Resurgence of Organomanganese(I) Chemistry. An
excellent example of how this can unfold is the discovery that
MnI ions are useful in a variety of (de)hydrogenation reactions
previously performed by ruthenium, iron, and cobalt pincer
catalysts.29 This finding, which came decades after the first
analogous ruthenium examples,30 stimulated a resurgence of
catalysis with organomanganese(I). Langer and Zell recently
discussed some of the unique properties of manganese(I)
compared to analogous d6 ruthenium(II) and iron(II) systems
that will enable this new chemistry.31 One of the most
important differences is that 16-electron manganese(I)
complexes can support four neutral L-type ligands instead of
three. The general formulation for a 16-electron manganese(I)
catalyst is MnI(XL2)(CO)2, where XL2 is the supporting
bifunctional ligand(s) (Chart 1). In contrast to M2+ (Fe, Ru)
16-electron catalysts, the formulation is Y−MII(XL2)(CO),
where Y is a supporting X-type ligand (e.g., hydride).
RESULTS AND DISCUSSION
■
Alternative Precursors to Mn(CO)5Br To Access
Manganese(I) Catalysts. Our roots in exploring the
fundamental chemistry of organomanganese(I) centers origi-
nated from our interest in the finding that cymantrene
[CpMn(CO)3] can mediate a stoichiometric water-splitting
reaction by the action of, presumably, a single photon.37 Using
a variety of organomanganese(I) complexes with and without
water-derived ligands (e.g., Chart 2), we showed that only
certain heteronuclear MnI-OH complexes produce H2 under
photochemical conditions. In particular, the tetramer [Mn-
(CO)3(μ3-OH)]4 (1) exhibited unique behavior in that it was
Chart 1
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Chart 2. Manganese(I) Complexes with and without Water-
Derived Ligands Tested for Photochemical H2
Production39,40
Scheme 2. Previously Reported POP-MnI Complexes36
the only complex that produced H2 gas under rigorously dry
conditions through a CO-photolysis-induced H-atom-transfer
reaction.38,38 This prompted our investigation into the
formation mechanism of the tetramer so as to develop an
atom-precise methodology to install labeled D and 18O atoms
in 1.
led to the generation of an analogous tricarbonyl compound,
[H-PO][PO]Mn(CO)3 (5; Scheme 3), the synthetic details of
which are described in the next section.
During this initial study, we discovered a novel tetranuclear
butterfly complex (2) with bridging carbonato ligands derived
exclusively from Mn2(CO)10 and Me3NO (Scheme 1). The
Scheme 3. Single-Step Preparation of the H-PO Complex 5
Scheme 1. Butterfly Complex En Route to 141
Although complex 2 is an interesting alternative synthon to
Mn(CO)5Br, the workup for such reactions can be problematic
owing to low yields and/or residual Me3NO. On our quest for
higher yielding routes, in an earlier report, we explored the
applicability of MeMn(CO)5 as a manganese(I) synthon and
found that the MeMn(CO)5 complex is able to rapidly install
MnI(CO)n groups in Brønsted acidic ligands.43 MeMn(CO)5
can be derived directly from Mn2(CO)10 in good yield using
NaK and MeI in a glovebox, although MeMn(CO)5 itself is air-
stable. Besides its high price, the use of Mn(CO)5Br can be
disadvantageous because it is typically required to prepare H-
LMn(Br)(CO)2 (H-L = tridentate bifunctional pincer ligand)
and then must be activated with a strong base (e.g., KOtBu) to
generate the active 16-electron manganese(I) dicarbonyl
complex (Chart 1). Alternatively, MeMn(CO)5 circumvents
the need to prepare the precatalyst because CH4 is readily
liberated. For instance, refluxing MeMn(CO)5 with 1 equiv of
H-POP or 2 equiv of H-PO yielded the corresponding
tricarbonyl complexes 5′ and 5, respectively, via the release of
CH4 (Schemes 3 and 4). This method’s usefulness extends into
a variety of applications that include the activation of N−H,
O−H, and even C−H bonds on preligands; a summary from
our previous report with MeMn(CO)5 as an alternative
synthon is shown in Scheme 4.
formation of carbonate from Mn2(CO)10 and Me3NO was
unexpected because it implied the formation of CO3 from
2−
CO2 and Me3NO, which is not possible without a reductant.
Therefore, it was presumed that the Mn0 centers were the
source of electrons and, indeed, 2 contained four MnI(CO)3
fragments charge-balanced by two carbonato groups. In
addition to two capping NMe3 ligands, the structure also
contained a rare μ2-ONMe3 ligand. The treatment of 2 with
exactly 4 equiv of water resulted in complete conversion to the
tetramer 1 and Me3NO·2H2O, providing a synthetic route to
the fully deuterated and partially 18O-labeled tetramer.41
The reaction of 2 with water to yield 1 suggested an acid−
base chemistry, where the basic carbonato ligand moieties in 2
deprotonated the incoming water molecules, thus installing the
OH group in the MnI(CO)3 fragments. We envisioned the
applicability of this route for direct activation of the Brønsted
acidic bonds on various ligand platforms for generation of the
corresponding manganese(I) tricarbonyl complexes. Such
manganese(I) tricarbonyl complexes are often precursors for
generation of the manganese(I) dicarbonyl active catalyst in
MLC chemistry.42 In fact, treating 2 with our phenolic pincer
ligand, tridentate bis(phosphine) phenol−2,6-bis-
[(diphenylphosphino)methyl]-4-methylphenol (H-POP), in-
stead of water, led to deprotonation of the phenol group as
expected and yielded [POP]Mn(CO)3 (5′; Scheme 2).36
Following the same procedure as that for our new bidentate
p h e n o li c l ig a n d , mo n o p h o s p h i n e p h e n ol − 2-
[(diphenylphosphino)methyl]-4,6-dimethylphenol (H-PO),
Coordination Chemistry of [H-PO]MnI Complexes. As
stated, we synthesized the tridentate H-POP as a supporting
ligand for manganese(I) complexes. One of the key findings
from that study was that the Mn−O−C bond angle of the
mononuclear complex 5′ was much smaller (103.4°) than
those of other POP-MnI complexes from the same report
(≈120−124°). The result was that the monomeric complex 5′
readily formed the inert dimer {5′}2 or was otherwise unstable
C
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43
PO]Mn(CO)3Br and induce coordination of the phenolic OH
were unsuccessful (e.g., through heating, vacuum, Me3NO,
etc.).44 Instead, 3 converts into [H-PO]2Mn(CO)3Br (4;
31P{1H} NMR = 55 ppm) and other unidentified products.
The same complex can be formed through the treatment of
Mn(CO)5Br with 2 equiv of H-PO at 70 °C under static
vacuum for 12 h (Scheme 6). Notably, in our previous report
with the H-POP system, we prepared a complex with
spectroscopic features nearly identical with those of 4
(31P{1H} NMR = 55 ppm; the ATR-FTIR νCO splitting
pattern matches mer-CO coordination) and was overall
consistent with a {[H-POP]Mn(CO)3Br}2 (4′) formulation
for which we proposed a dimeric structure with meridional CO
coordination (Scheme 2).
Scheme 4. Expedient Route to Catalysts with MeMn(CO)5
The XRD structure for 4 (Figure 1) supports our
hypothetical structure for 4′ (Scheme 2). Both of the OH
toward our attempts to generate highly active dicarbonyl
analogues. For this reason, we designed H-PO to be a less
constrained platform. The procedure uses 2,4-dimethylphenol
in a Mannich reaction with dimethylamine and formaldehyde
and produces the preligand 2-[(dimethylamino)methyl]-4,6-
dimethylphenol (H-NO). H-NO was converted into H-PO
through treatment with diphenylphosphine in toluene at 150
°C in 93% yield as an air-sensitive white crystalline solid
(Scheme 5).
Scheme 5. Synthesis of H-PO
A simple ligand substitution between Mn(CO)5Br and H-
PO occurs at room temperature in tetrahydrofuran (THF) in
under 1 h. The result is liberation of CO and clean formation
of a new complex assigned as the monomeric [H-PO]Mn-
(CO)4Br (3) in quantitative yield [31P{1H} NMR = 40 ppm;
the attenuated-total-reflectance Fourier transform infrared
(ATR-FTIR) νCO splitting pattern matches that of tetracar-
bonyl coordination; Figures S4−S6]. This molecule was not
characterized by X-ray diffaction (XRD), but because of its
NMR and FTIR spectroscopic similarities to the previously
reported complex 3′ (31P{1H} NMR = 40 ppm),36 a proposed
structure is shown in Scheme 6.
Figure 1. X-ray crystallographically determined molecular structure of
4. Selected bond lengths (Å) and angles (deg): Mn−P1 2.3079(4);
Mn−P2 2.314(4); Mn−Br 2.5473(5); Mn−C1 1.784(2); Mn−C2
1.858(1); Mn−C3 1.866(1); O5···Br 3.386(1); O4···Br 3.220(1);
P1−Mn−P2 179.09(2); C1−Mn−C3 88.73(7); C1−Mn−C2
94.75(7); C2−Mn−C3 175.92(7). Ellipsoids are shown at 50%
probability and nonphenol H atoms removed for clarity.
moieties in the XRD structure are directed toward the bromido
ligand, with one having a short O···Br distance [O···Br =
3.386(1) Å] indicating hydrogen-bonding interaction. The
remaining coordination sites on manganese are occupied by
three CO ligands in a meridional fashion, consistent with the
ATR-FTIR spectrum, which additionally contains two different
νOH at 3350 and 3460 cm−1 that are tentatively assigned as the
hydrogen-bonded OH and the free OH moiety, respectively
(Figure S9). The singular peak in the 31P NMR spectrum for 4
indicates that the hydrogen-bonding group is not static in
solution (Figure S8).
Attempts to remove another molecule of CO from the
tetracarbonyl compound 3 and form the tricarbonyl [H-
Scheme 6. Coordination of H-PO with Mn(CO)5Br
Heating a solution of 4 in THF at 100 °C for 12 h under
static vacuum causes the formation of a new compound, 5, via
the release of one molecule of HBr. The 31P{1H} NMR
spectrum of 5 showed two peaks at 77 and 59 ppm
corresponding to the P atom on the phenolate PO and
phenolic H-PO ligands, respectively (Figure S11). The
molecular structure determined from XRD (Figure 2) is
consistent with that from ATR-FTIR for meridional
tricarbonyl coordination. The broad OH stretch around 2500
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Scheme 7. Regulated Secondary Coordination Spheres of 4
and 5 and the Synthesis of Dicarbonyl K[6]
Figure 2. X-ray crystallographically determined molecular structure of
5. Selected bond lengths (Å) and angles (deg): Mn−P1 2.2798(6);
Mn−P2 2.3035(6); Mn−O4 2.063(1); Mn−C1 1.854(2); Mn−C2
1.865(2); Mn−C3 1.777(1); O4···O5 2.580(1); P1−Mn−P2
172.55(3); Mn−O4−C4 122.39(8); C1−Mn−C2 173.85(6); C1−
Mn−C3 90.36(6); C2−Mn−C3 95.55(6). Ellipsoids are shown at
50% probability and nonphenol H atoms removed for clarity.
cm−1 in the ATR-FTIR (Figure S12) spectrum possibly
suggests that the free OH group is hydrogen-bonded to the O
atom on the adjacent ligand, again consistent with a short O···
O distance [2.580(1) Å] in the crystal structure.
5 closely resembles 5′ from our previous work except that
the phenolic C−O−Mn bond angle (103.4° in 5′) is less
constrained in 5 (122.4°).36 In the previous report, we showed
that 5′ reversibly reacts with HX (X = Cl, Br, I) to form {[H-
POP]Mn(CO)3X}2. Similarly, 5 reacts with HBr to reform 4.
The reversible addition of HBr to 5 and 5′ is presumably
enabled by the incorporation of interactions in the secondary
coordination sphere. For instance, Mn(CO)5Br does not
readily release its bromido ligand, which typically requires the
use of Ag+ salts. Additionally, molecules like H-PNP-Mn-
(CO)2Br (H-PNP = bis[(2-diisopropylphosphino)ethyl]-
amine; Scheme 4) require the addition of KOtBu to liberate
a Br ion and HOtBu. Hence, the interconversion of 4 and 5
may indicate that secondary coordination sphere interactions
modulate the energies associated with the liberation of
bromide.
Synthesis of Manganese(I) Dicarbonyl Complexes.
Treating 4 with 2 equiv of KH or 5 with 1 equiv of KH led to
complete deprotonation of the H-PO ligand(s) to generate a
dicarbonyl compound, K[(PO)2Mn(CO)2] (K[6]; (31P{1H}
NMR = 56 ppm; Scheme 7 and Figures S13−S15), and the salt
readily recrystallizes from THF at −35 °C. Crystallographic
data revealed a cis-dicarbonyl compound with two PO ligands
coordinated to Mn and K ions (Figure 3). The K ion is
additionally coordinated by three THF molecules, the latter of
which can be removed with dynamic vacuum at 40 °C.
The formation of manganese(I) dicarbonyl complexes is
challenging. Recently, manganese(I) dicarbonyl complexes
have been shown to be excellent catalysts for a variety of
hydrogenative and dehydrogenative reactions. However, their
synthesis is not readily accessible through typical methods used
to liberate CO (e.g., from a tricarbonyl) through photo-
chemical or chemical means.42 Rather, coordination reactions
Figure 3. X-ray crystallographically determined molecular structure of
the anion {[PO]2Mn(CO)2}− ([6]−). Selected bond lengths (Å) and
angles (deg): Mn−P1 2.2761(8); Mn−P2 2.2750(6); Mn−O3
2.075(1); Mn−O4 2.068(2); Mn−C1 1.748(2); Mn−C2 1.761(2);
P1−Mn−P2 168.43(3); Mn−O3−C21 133.7(1); Mn−O4−C42
131.8(1); C1−Mn−C2 86.7(1); O3−Mn−O4 81.40(6). Ellipsoids
are shown at 25% probability. THF molecules and K and H atoms are
not shown for clarity.
with tridentate ligands using Mn(CO)5Br is one of the few
methods that allows access to such dicarbonyl compounds. In
our previous report with the tridentate H-POP preligand, we
were unsuccessful in preparing POP-Mn(CO)2 from the
tricarbonyl complex despite having a density functional theory
(DFT)-computed Mn−CO bond dissociation free energy
(BDFE) of only 11 kcal/mol (Table 1). Rather, attempts at
liberating CO from 5′ using light or Me3NO resulted in
formation of the dimer {5′}2.36 Thus, the preparation of the
dicarbonyl [6]− using H-PO was somewhat unexpected.
The regulation of the secondary sphere and incorporation of
two phenolate−phosphine ligands appear to enable the
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a
additional support for its molecular identity in the absence of a
1H NMR spectrum or a high-quality XRD structure.
Table 1. Free Energies for Mn−CO Dissociation
b
c
complex
DFT
experiment
Mn−CO → Mn + CO
CONCLUSIONS
■
5 to 7 + CO
5′ to 7′+ CO
18
11
Being the third most abundant transition metal, next to iron
and titanium, manganese is in a position to replace expensive
and toxic metals in many fields including catalysis. To achieve
this goal, new knowledge regarding the fundamental
coordination and reaction chemistry of organomanganese
complexes must be pursued. Three challenges that we
identified are (i) the dearth of manganese precursors in
forming active manganese(I) pincer catalysts, (ii) the similarity
in bifunctional ligands all relying on N atoms, and (iii) the lack
of methods to prepare manganese(I) dicarbonyl complexes.
In addressing these challenges, we first demonstrated that
MeMn(CO)5 is a versatile synthon that can be used to directly
prepare a variety of coordinated ligand−metal complexes. We
also showed that by using a mixture of Mn2(CO)10 with excess
Me3NO under anhydrous conditions, a basic manganese(I)
carbonato intermediate is generated that also serves as an
excellent synthon for manganese(I) complexes. It was even
possible to generate a manganese(I) dicarbonyl complex
directly with H-PO and Mn2(CO)10 by the presumed oxidative
addition of a phenolic O−H bond by the Mn−Mn bond. Our
ongoing studies with phenolic-based phosphine complexes
show promise because the bidentate [PO]− and H-PO ligands
afforded several different coordination modes, including simple
access to the rare manganese(I) dicarbonyl motif. We
proposed that one facet of the phosphine−phenolate platform
is that modulation of the secondary coordination sphere
facilitates the liberation of ligands to the MnI center. New
synthetic routes and knowledge about the fundamental
chemistry of dicarbonyl complexes are important in the
resurgence of organomanganese(I) chemistry, where they
have shown to be active catalysts for a variety of trans-
formations and are ongoing in our laboratory.
d
[5-H]− to [6]− + CO
−8
e
f
[Mn(CO)6]+ to [Mn(CO)5]+ + CO
30
32
a
Energies in kcal/mol, standard conditions; complex 7 and 7′ are the
b
products of CO dissociation from 5 and 5′, respectively. K ions are
c
excluded from the calculations. B3LYP 6-31G* with the SMD
d
solvation model for toluene. Free energy for the intramolecular
e
ligand substitution reaction. Gas-phase free energy = 31 kcal/mol.
f
45
Gas-phase BDFEM−CO
.
formation of dicarbonyl complexes. To gain additional insight
into these factors, we used DFT computations to compare the
CO dissociation energies. As a benchmark, we computed a CO
dissociation energy of 30 kcal/mol for [Mn(CO)6]+, which is
close to the experimental Mn−CO BDFE of 32 kcal/mol
(Table 1).45 The loss of CO from the POP complex 5′ is only
11 kcal/mol. This low energy might be due to constraints on
the complex imposed by the short C−O−Mn angle discussed
above. This assumption is based on the analogous PO complex
5, which does not have the constrained C−O−Mn angle and
has a larger CO dissociation energy of 18 kcal/mol. However,
the conversion of [5-H]− (in silico removal of a proton from
5) to [6]− is exergonic (−8 kcal/mol). Note that this free
energy for conversion of the deprotonated form of 5 ([5-H]−)
into [6]− is an intramolecular ligand substitution reaction, not
a Mn−CO BDFE.
Within the context of preparing dicarbonyl complexes with
“alternative synthons”, the treatment of 2 equiv of H-PO with
Mn2(CO)10 in refluxing isopropyl alcohol affords a para-
magnetic bis(dicarbonyl) complex MnII[6]2 (Scheme 8),
Scheme 8. Preparation of Paramagnetic Bis(dicarbonyl)
EXPERIMENTAL SECTION
■
General Methods. All reagents were procured from commercial
sources and used without further purification unless otherwise noted.
Solvents were purified and collected from a PPT solvent system and
stored over 3 Å molecular sieves. Molecular sieves and basic alumina
were activated at 200 °C under vacuum (<100 mTorr) for 48 h before
use. Unless noted, all of the samples were prepared under nitrogen in
a VAC Genesis glovebox or argon using standard Schlenk-line
techniques. Deuterated solvents were degassed by a freeze−pump−
thaw method and stored in the glovebox in Strauss flasks. NMR
experiments were performed on Varian Mercury 300 MHz and Inova
400 MHz spectrometers. Transmission electron microscopy and
ATR-FTIR spectra were collected inside of a VAC Atmospheres
Omni glovebox using a Bruker Alpha IR spectrometer with an
ALPHA-P Platinum ATR module (diamond crystal). Cyclic
voltammetry was obtained using a SP-200 Bio-Logic potentiostat.
Electron paramagnetic resonance (EPR) spectra were collected on an
EMX Bruker X-band spectrometer on frozen solutions at 77 K.
Computational Methods. All calculations were performed using
QChem 4.4 and visualized using IQmol 2.11.1,46 and the B3LYP
functional, 6-31G* basis set, and solvent model based on density
(SMD) solvation model were used for toluene. Geometry
optimizations for manganese complexes were performed using the
crystal coordinates (the K ion was excluded in the calculation of [6]−
and the deprotonated form of 5) or constructed directly in the IQmol
visualizer (homoleptic Mn−CO complexes) as the starting points.
Frequency calculations were performed on all structures to ensure no
imaginary frequencies. Single-point energy calculations were used to
where one MnII ion sits between two [6]− molecular anions
(connectivity determined from XRD; Figure S17). The
intermediates involved were not identified, but heating (110
°C) 1:1 H-PO/Mn2(CO)10 in toluene-d8 produced a solution
1
with a H NMR signal at −7.9 ppm (Figure S19), which is
consistent with the formation of Mn(CO)5H. We propose that
an oxidative addition of the phenolic O−H bond across a
Mn0−Mn0 bond furnishes a hydride, and unidentified species
lead to the MnII[6]2 product. The same paramagnetic
bis(dicarbonyl) complex MnII[6]2 can be prepared by the
treatment of K[6] with 0.5 equiv of MnBr2, serving as an
F
DOI: 10.1021/acs.inorgchem.9b00941
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Forum Article
obtain the final electronic energy (enthalpy) and corrected by adding
the total enthalpy obtained from the frequency calculation (298 K).
Free energies (298 K) additionally required the total entropy from the
frequency calculation and energy of solvation in toluene.
growing yellow crystals at room temperature (30 mg, 62% crystalline
yield). Slight variations in the temperature led to the formation of
unidentified byproducts. A cleaner synthesis pathway is achieved
using Et3N. To a solution of Mn(CO)5Br (20 mg, 0.073 mmol, 1
equiv) in THF was added H-PO (51 mg, 0.16 mmol, 2.2 equiv). The
reaction mixture was subjected to three freeze−pump−thaw cycles
and then heated at 70 °C overnight. To this in situ prepared complex
4 was added excess Et3N (35 μL), leading to the immediate
precipitation of [Et3NH]Br. The reaction was stirred for 5 min and
the solution filtered using a fine frit. The solvent was removed under
vacuum and the residue washed with methanol. Drying the residue
under vacuum yielded pure 5 (52 mg, 92% yield) which shows poor
solubility in solvents like THF, toluene, dichloromethane (CH2Cl2),
and acetonitrile. Crystals suitable for XRD were obtained by layering a
solution of 5 in CH2Cl2 under diethyl ether at room temperature. 1H
NMR (toluene-d8, 300 MHz, 298 K, ppm): δ 1.87 (s, 3H, CH3), 2.13
(s, 3H, CH3), 2.40 (s, 3H, CH3), 2.41 (s, 3H, CH3), 3.56 (m, 4H,
CH2), 5.36 (s, 1H, CH), 6.54 (s, 1H, CH), 6.75 (s, 2H, CH), 6.89−
7.59 (20H, (C6H5)2P), 10.56 (s, 1H, OH). 31P{1H} NMR (toluene-
d8, 121 MHz, 298 K, ppm): δ 59.50, 77.33. Anal. Calcd (found) for 5·
1.5toluene (C55.5H53MnO5P2): C, 72.70 (72.47); H, 5.83 (5.89); N,
0.00 (0.44, trace Et3N from the reaction). ATR-FTIR (cm−1): 2540,
2045, 1962, 1892.
Synthesis of THF3·K[(PO)2Mn(CO)2] (THF3·K[6]). To a solution of
Mn(CO)5Br (90 mg, 0.33 mmol, 1 equiv) in THF (2 mL) was added
H-PO (220 mg, 0.69 mmol, 2.1 equiv). After evolution of the gases
ceased, the solution was subjected to one cycle of freeze−pump−thaw
and heated at 70 °C for 12 h. 31P{1H} NMR of the reaction mixture
showed complete conversion to 4. Excess KH (40 mg, 1.00 mmol, 3
equiv) was added to this in situ prepared 4, and the reaction mixture
was stirred until evolution of the bubbles stopped, during the course
of which a yellow precipitate formed. The reaction was then heated at
50 °C for 2 h. The yellow precipitate eventually dissolved, and the
solution turned bright orange. The reaction mixture was filtered using
a frit. Concentrating the filtrate (1 mL) and layering it under
petroleum ether afforded crystals of THF3·K[6] (269 mg, 90%
crystalline yield). Crystals suitable for XRD were obtained by storing
the concentrated filtrate at −35 °C. 1H NMR (CH2Cl2-d2, 300 MHz,
298 K, ppm): δ 1.65 (s, 3H, CH3), 2.23 (s, 3H, CH3), 3.22 (d, 1H,
CH2), 3.80 (d, 1H, CH2), 6.66 (s, 1H, CH), 6.90 (s, 1H, CH), 7.27−
7.78 (10H, (C6H5)2P). 31P{1H} NMR (THF, 121 MHz, 298 K,
Synthetic Procedures. Synthesis of 2-[(Dimethylamino)-
methyl]-4,6-dimethylphenol (H-NO). In a 50 mL round-bottomed
flask, open to air, 2,4-dimethylphenol (1.8 g, 15 mmol) and
dimethylamine (40 wt % aqueous solution, 3.8 mL, 30 mmol) were
mixed together at 0 °C. Formaldehyde (37 wt % aqueous solution, 2
mL, 25.6 mmol) was added, and the mixture was allowed to stir at
room temperature for 30 min, followed by reflux at 100 °C for 2 h.
After cooling to room temperature, NaCl was added to separate the
organic and aqueous layers. The top organic layer was collected, dried
over anhydrous Na2SO4, and later dried under vacuum to yield pure
1
H-NO as an oil (2.1 g, 78% yield). H NMR (CHCl3-d1, 400 MHz,
298 K, ppm): δ 2.21 (s, 3H, CH3), 2.22 (s, 3H, CH3), 2.31 (s, 6H,
CH3), 3.58 (s, 2H, CH2), 6.62 (s, 1H, CH), 6.86 (s, 1H, CH).
Synthesis of Monophosphine Phenol−2-[(Diphenylphosphino)-
methyl]-4,6-dimethylphenol (H-PO). In a 25 mL Strauss tube, H-NO
(500 mg, 2.8 mmol, 1 equiv) and diphenylphosphine (624 mg, 3.35
mmol, 1.2 equiv) were dissolved in toluene (1 mL). The solution was
freeze−pump−thawed once and then heated at 150 °C for 16 h. After
cooling to room temperature, the solvent was evaporated under
vacuum to obtain a viscous oil to which petroleum ether (10 mL) was
added, and the solution was stored at −35 °C. The white crystals
formed were collected using a frit and washed with cold petroleum
ether. Concentrating the washes under vacuum and storing again at
−35 °C yielded more white crystals (835 mg, 93% yield in total). The
washes were combined, and the solvent was pumped off to recover
residual diphenylphosphine, which was used for later H-PO syntheses.
1H NMR (CH2Cl2d2, 400 MHz, 298 K, ppm): δ 2.08 (s, 3H, CH3),
2.17 (s, 3H, CH3), 3.39 (s, 2H, CH2), 5.11 (s, 1H, OH), 6.50 (s, 1H,
CH), 6.76 (s, 1H, CH), 7.35−7.42 (10H, (C6H5)2P). 13C{1H} NMR
(toluene-d8, 101 MHz, 298 K, ppm): δ 16.08 (s), 20.50 (s), 31.56 (d,
J = 18.18 Hz), 122.99 (d, J = 7.07 Hz), 124.94 (d, J = 1.01 Hz),
129.60 (d, J = 9.09 Hz), 130.11 (d, J = 3.03 Hz), 133.14 (d, J = 24.24
Hz), 138.31 (s), 138.48 (s), 150.82 (s). 31P{1H} NMR (CH2Cl2-d2,
121 MHz, 298 K, ppm): δ −16.9.
Synthesis of [H-PO]Mn(CO)4Br (3). To a solution of Mn(CO)5Br
(47 mg, 0.17 mmol, 1 equiv) in THF was added H-PO (50 mg, 0.16
mmol, 1 equiv), and the reaction was stirred for 1 h at room
temperature. Evolution of the bubbles ceased within this 1 h. Layering
the solution under hexane and storing the vial at −35 °C yielded
orange crystals of 3 (76 mg, 78% crystalline yield). 1H NMR
(CH2Cl2-d2, 300 MHz, 298 K, ppm): δ 1.86 (s, 3H, CH3), 2.04 (s,
3H, CH3), 4.03 (d, 2H, CH2), 5.62 (s, 1H, CH), 5.71 (s, 1H, CH),
6.71 (s, 1H, OH), 7.48−7.60 (10H, (C6H5)2P). 31P{1H} NMR
(CH2Cl2-d2, 121 MHz, 298 K, ppm): δ 40.34. ATR-FTIR (cm−1):
3370, 2089, 2006, 1956, 1913.
Synthesis of [H-PO]2Mn(CO)3Br (4). To a solution of Mn(CO)5Br
(20 mg, 0.073 mmol, 1 equiv) in THF was added H-PO (51 mg, 0.16
mmol, 2.2 equiv). The reaction mixture was subjected to three
freeze−pump−thaw cycles and then heated at 70 °C overnight. THF
was removed under vacuum, and the yellow-orange residue was
washed with petroleum ether. Drying the solid yielded pure 4 (60 mg,
96% yield). Crystals suitable for XRD were obtained by layering a
THF solution of the compound under hexane at room temperature.
1H NMR (CH2Cl2-d2, 300 MHz, 298 K, ppm): δ 1.89 (s, 3H, CH3),
2.03 (s, 3H, CH3), 4.07 (d, 2H, CH2), 5.67 (s, 1H, CH), 5.76 (s, 1H,
CH), 6.68 (s, 1H, OH), 7.3−7.6 (10H, (C6H5)2P). 31P{1H} NMR
(THF, 121 MHz, 298 K, ppm): δ 55.29. Anal. Calcd (found) for 4,
C45H42BrMnO5P2: C, 62.88 (62.89); H, 4.92 (5.22); N, 0.00 (none
found). ATR-FTIR (cm−1): 3460, 3350, 2045, 1962, 1920.
ppm):
δ 56.07. Anal. Calcd (found) for THF3·K[6]
(C56H64KMnO7P2): C, 66.92 (66.25); H, 6.42 (6.14); N, 0.00
(none found). ATR-FTIR (cm−1): 1894, 1802.
Synthesis of MnII[(PO)2MnI(CO)2]2 (Mn[6]2). To a solution of H-
PO (66 mg, 0.2 mmol, 2 equiv) in 2 mL of isopropyl alcohol was
added Mn2(CO)10 (40 mg, 0.1 mmol, 1 equiv), and the reaction was
refluxed at 95 °C for 3 days. The solution was filtered using a frit to
collect the orange precipitate of Mn[6]2 (47 mg, 45% yield). The
precipitate was dissolved in a benzene/toluene mixture, layered under
hexane, and stored at −35 °C to yield orange needlelike crystals. Anal.
Calcd (found) for Mn[6]2 (C88H80Mn3O8P4): C, 68.00 (68.29); H,
5.19 (5.34); N, 0.00 (none found). ATR-FTIR (cm−1): 1928, 1846.
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.9b00941.
NMR, ATR-FTIR, and EPR spectra, connectivity
structure of MnII[6]2, DFT-optimized structures, and
X-ray crystallography (PDF)
Synthesis of [H-PO][PO]Mn(CO)3 (5). Mn(CO)5Br (17 mg, 0.06
mmol, 1 equiv) and H-PO (40 mg, 0.12 mmol, 2 equiv) were
dissolved in THF (1 mL). After stirring at room temperature for 1 h,
the reaction mixture was subjected to three freeze−pump−thaw
cycles and then heated at 100 °C overnight. The 31P{1H} NMR
spectrum of the reaction mixture showed complete conversion to 5.
Compound 5 was isolated by layering the solution under hexane and
Crystallographic structure (MOL)
Crystallographic structure (MOL)
Crystallographic structure (MOL)
Crystallographic structure (MOL)
G
DOI: 10.1021/acs.inorgchem.9b00941
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Forum Article
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Accession Codes
CCDC 1906794−1906796 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: DCLacy@Buffalo.edu.
■
ORCID
Karthika J. Kadassery: 0000-0003-2065-1356
Samantha N. MacMillan: 0000-0001-6516-1823
David C. Lacy: 0000-0001-5546-5081
Notes
The authors declare no competing financial interest.
(17) Taguchi, T.; Gupta, R.; Lassalle-Kaiser, B.; Boyce, D. W.;
Yachandra, V. K.; Tolman, W. B.; Yano, J.; Hendrich, M. P.; Borovik,
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M. B. Non-oxo chemistry of manganese in high oxidation states. Part
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ACKNOWLEDGMENTS
■
Financial support was provided by the University at Buffalo
and an ACS Petroleum Research Fund (ACS-PRF-57861-
DN13). This work was completed using the resources of the
Chemistry Instrument Center, University at Buffalo; contact
che-ic@buffalo.edu for sample inquiries. A. F. Cannella is
acknowledged for assistance with collecting EPR spectra.
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DOI: 10.1021/acs.inorgchem.9b00941
Inorg. Chem. XXXX, XXX, XXX−XXX