The Stabilization of Three-Coordinate Formal Mn(0) Complex with NHC and Alkene Ligation

Article abstract of DOI:10.1016/j.chempr.2018.09.002

Low-coordinate zero-valent metal species are implicated as key intermediates in various transition-metal-catalyzed and -mediated reactions. However, knowledge on this type of metal species has been mainly restricted to the metals in groups 8–10, and that

Full text of DOI:10.1016/j.chempr.2018.09.002

Article
The Stabilization of Three-Coordinate Formal
Mn(0) Complex with NHC and Alkene Ligation
Jun Cheng, Qi Chen, Xuebing
Leng, Zhongwen Ouyang,
Zhenxing Wang, Shengfa Ye,
Liang Deng
zxwang@hust.edu.cn (Z.W.)
shengfa.ye@kofo.mpg.de (S.Y.)
deng@sioc.ac.cn (L.D.)
HIGHLIGHTS
Three-coordinate formal Mn(0)
complexes have been isolated
S = 3/2 ground-spin state and
strong metal-to-alkene p
backdonation
A rare example of Mn-mediated
reductive coupling of unsaturated
hydrocarbons
2
H activation by low-coordinate
formal Mn(0) alkene complex
We report three-coordinate formal Mn(0) complexes [(NHC)Mn(dvtms)] that have
an S = 3/2 ground-spin state and feature highly covalent metal-ligand bonding.
These complexes can perform reductive coupling with unsaturated hydrocarbons
2 2 2
and show diversified reactivity toward H O, H , CO, and I . The results highlight
the synthetic utility of low-coordinate low-valent manganese complexes,
warranting further exploration.
Cheng et al., Chem 4, 1–17
December 13, 2018 ª 2018 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2018.09.002

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Article
The Stabilization of Three-Coordinate
Formal Mn(0) Complex
with NHC and Alkene Ligation
1
1
1
3
3,
2,
Jun Cheng, Qi Chen, Xuebing Leng, Zhongwen Ouyang, Zhenxing Wang, * Shengfa Ye, *
and Liang Deng1,4,
*
SUMMARY
The Bigger Picture
Low-coordinate zero-valent metal species are implicated as key intermediates
in various transition-metal-catalyzed and -mediated reactions. However, knowl-
edge on this type of metal species has been mainly restricted to the metals in
groups 8–10, and that on the earlier transition-metal analogs is very limited.
Low-coordinate zero-valent
transition-metal species are a
class of organometallic
compounds that can mediate
chemical bond activation and also
be used as catalysts and starting
materials for the synthesis of new
organic and coordination
2
2
Herein, we report three-coordinate formal Mn(0) complexes [(NHC)Mn(h :h -
dvtms)] (NHC = N-heterocyclic carbene, dvtms = divinyltetramethyldisiloxane).
Spectroscopic and computational studies established that these formal Mn(0)
complexes have an S = 3/2 ground-spin state and feature strong metal-to-
alkene p backdonation. As a result of the high covalency of the metal-ligand
bonding, these formal Mn(0) complexes are best described as resonance among
compounds. Among versatile
transition metals in the periodical
table, low-coordinate zero-valent
manganese complexes have long
been sought after. However,
because of their high chemical
activity, these species are not
easily accessed, and fundamental
understanding of their properties
is limited. Herein, we report the
stabilization of three-coordinate
formal zero-valent manganese
complexes through the use of
N-heterocyclic carbene and
alkene ligands. The study also
reveals the intriguing reactivity of
this type of reactive metal species
in mediating alkene
2
–
a series of limiting canonical structures of Mn(0)-dvtms, Mn(II)-[dvtm] , and
4
–
Mn(IV)-[dvtms]
coupling with unsaturated hydrocarbons and show diversified reactivity toward
O, H , CO, and I , which hint at the synthetic utility of the three-coordinate
. These formal Mn(0) complexes can perform reductive
H
2
2
2
formal Mn(0) species.
INTRODUCTION
Low-valent transition-metal species that simultaneously have low coordination
numbers are important reactive intermediates proposed in many transition-metal-
1
–3
catalyzed reactions.
in the activation of small molecules, e.g., H
ordinate low-valent 3d metal complexes were found to exhibit peculiar magnetic
This type of metal complex also shows intriguing reactivity
4
–11
2
, N
2
, CO, and CO
2
.
Recently, low-co-
1
2–19
properties that imply their potential usage as new magnetic materials.
Associated with these fascinating chemical and physical properties are their unusual
electronic and geometric features, which in turn pose a great challenge to synthetic
2
,3,20–23
chemists.
The inherent difficulty in preparing low-coordinate low-valent tran-
transformation and dihydrogen
activation.
sition-metal complexes likely lies in the proneness of low-coordinate metal species
to bind ligands to achieve coordination saturation and the readiness of low-valent
metal species to undergo oxidative addition reactions because of their reduction
potency. In this regard, one could reason that, among the metals in the same row
of the periodic table, low-coordinate low-valent early transition-metal complexes
should be highly reactive and could be more challenging to access than their late
transition-metal analogs.
This perception seems to gain support from the status quo of the accessibility of low-
coordinate zero-valent 3d metal complexes. Plenty of low-coordinate Ni(0), Co(0),
and Fe(0) complexes featuring phosphine, N-heterocyclic carbene (NHC), and
Chem 4, 1–17, December 13, 2018 ª 2018 Elsevier Inc.
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Figure 1. Examples of Highly Reactive Formal Mn(0) Complexes
2
4–45
isocyanide ligands have been reported.
In contrast, isolable low-coordinate
zero-valent transition-metal complexes of the earlier groups (groups 4–7), even for
the ones with a formal oxidation state of zero, are rare. In 2013, Roesky et al. pub-
lished a two-coordinate manganese complex supported by a cyclic aminoalkyl car-
4
6
2 2
bene (cAAC) ligand [Mn(Me -cAAC) ] (Figure 1). Its isolation apparently benefits
from the strong p-accepting capability of cAAC. Spectroscopic and theoretical
studies indicated that the pronounced Mn-to-cAAC backdonation endowed the
Mn(I) or even Mn(II) nature of this formal Mn(0) complex. Consequently, the cAAC
ligand possesses somewhat radical anion character, and the interaction of
[
Mn(Me
2 2 2
-cAAC) ] with H causes the conversion of the cAAC ligands into alkyl
ligands. In 2014, Jones reported another two-coordinate formal Mn(0) complex
Mes
i
i
[
(
nacnac)MgMn(N(C
6
H
2
-2,6-(CHPh
2
)
2
-4-Pr )(SiPr
-2,6-(CHPh
nacnac = [(MesNCMe)
3
))] (Figure 1) prepared from the
i
i
reaction of the Mn(II) complex [MnBr(THF)(N(C
6
H
2
2
)
2
-4-Pr )(SiPr
3
))]
2
with
Mes
Mes
1ꢀ 47
the Mg(I) dimer [Mg( nacnac)]
2
(
2
CH)] ). This manga-
nese complex features a unique Mn–Mg bond, and can be viewed as either a Mn(0)-
Mg(II), Mn(I)-Mg(I), or Mn(II)-Mg(0) species. It reacts with Mn(II) and Cr(II) halides to
form dinuclear Mn-Mn and Mn-Cr complexes, respectively, and its interactions
i
i
with N
2
O and Pr NCNPr resulted in two-electron reduction of the substrates.
ꢀ
Isolable five-coordinate 17e Mn(0) species are equally hard to access. Early studies
ꢀ
found that 17e species Mn(CO)
5
and Mn(CO)
3
(PR
3
)
2
could be generated
8 2
(CO) L ]
from photochemical substitution of dinuclear manganese complexes [Mn
L = CO, phosphine) but are hard to isolate because of their high activity.
2
4
8–51
(
A
recent study by Figueroa and co-workers showed that the use of sterically encum-
ꢀ
bering isocyanide ligands allows preparation and isolation of 17e species
0
0
52,53
[
Mn(CO)
3
(CNAr*)
2
] (Ar* = 2,6-(2 ,6 -diisopropylphenyl)phenyl, Figure 1).
The
metalloradical species displays intriguing reactivity of one-electron redox
1State Key Laboratory of Organometallic
Chemistry, Center for Excellence in Molecular
Synthesis, Shanghai Institute of Organic
Chemistry, University of Chinese Academy of
Sciences, Chinese Academy of Sciences, 345
Lingling Road, Shanghai 200032, P.R. China
reactions to convert to Mn(I) and Mn(ꢀI) complexes. Relevant to this, Chirik and
iPr
iPr
2
co-workers reported a five-coordinate complex, [( PDI)Mn(THF) ] ( PDI = 2,6-
i
54
(
2 6 3 2 5 3
2,6- Pr -C H N=CMe) C H N). Spectroscopic studies indicated that the elec-
tronic structure of its S = 3/2 ground-spin state was best formulated as a high-spin
2Max-Planck Institut f u¨ r Kohlenforschung,
Kaiser-Wilhelm-Platz 1, 45470 M u¨ lheim an der
Ruhr, Germany
Mn(II) center antiferromagnetically coupled to a bis(imino)pyridine triplet diradical.
The limited knowledge on low-coordinate zero-valent 3d metal complexes of the
metals in groups 4–7 urges further exploration. Toward this end, we wish to report
the synthesis, characterization, and reactivity of the first three-coordinate formal
3
Wuhan National High Magnetic Field Center &
School of Physics, Huazhong University of Science
and Technology, Wuhan 430074, P.R. China
4Lead Contact
2
2
zero-valent group 7 metal complexes [(NHC)Mn(h :h -dvtms)] (NHC = IPr, 1; IMes,
; Et -cAAC, 3; dvtms = divinyltetramethyldisiloxane). Complexes 1–3 were pre-
pared from one-pot reactions of MnCl with NHC, dvtms, and KC . Spectroscopic
studies and theoretical calculations reveal an S = 3/2 ground-spin state for these
*
Correspondence: zxwang@hust.edu.cn (Z.W.),
2
2
shengfa.ye@kofo.mpg.de (S.Y.),
deng@sioc.ac.cn (L.D.)
2
8
https://doi.org/10.1016/j.chempr.2018.09.002
2
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Scheme 1. Preparation Route for the Formal Mn(0) Complexes 1–3
formal Mn(0) complexes that feature highly covalent metal-ligand bonding arising
from pronounced metal-to-alkene p backdonation. Consequently, the electronic
structure of these formal Mn(0) complexes is best described as intermediate among
several limiting canonical forms ranging from Mn(0) to Mn(IV). The formal Mn(0) com-
plexes can facilitate reductive coupling of their dvtms ligand with alkene, allene, and
alkyne to form seven-membered mangana(II) cycles, which represents a rare
example of such transformations promoted by well-defined manganese complexes.
The dvtms ligand in 1 can also be protonated by H
. In addition, release of dvtms was observed when 1 was treated with I
2
2
O and partially hydrogenated by
H
2
or CO.
These findings demonstrate the capability of the combined ligand set of NHC
with vinylsilanes to stabilize low-coordinate formal zero-valent transition-metal com-
plexes besides late transition metals and highlights the synthetic utility of low-coor-
dinate NHC-Mn-alkene complexes.
RESULTS AND DISCUSSION
Synthesis and Characterization of the Three-Coordinate Formal Mn(0)
Complexes
Low-valent manganese complexes are mostly known for five- and six-coordinate
Mn(ꢀI), Mn(0), and Mn(I) complexes with CO, isocyanide, cyclopentadienyl, and
5
0–53,55–59
phosphine ligands,
coordination numbers of 2–4 are rare.
of a combined ligand set of NHC with the bis(alkene) ligand dvtms in stabilizing
and low-valent manganese complexes with the lower
1
6,17,46,47,60–63
Noting this and also the success
6
4
65
31,32
35–38
39,41
three-coordinate Pt(0), Pd(0), Ni(0),
Co(0),
and Fe(0)
complexes re-
ported by Mark o´ , Beller, Hazari, Johnson and co-workers, and us, we were curious
whether this ligand set could allow stabilization of three-coordinate formal Mn(0)
species or not. To our delight, our exploration proved its effectiveness.
Treatment of MnCl
2
with 1 equiv of the free NHC ligand (IPr, IMes, or Et
2
-cAAC) in tetra-
ꢁ
hydrofuran (THF), followed by addition of 1 equiv of dvtms and 2 equiv of KC
8
at ꢀ40 C,
gave deep brown mixtures. After workup and recrystallization, low-coordinate manga-
2
2
2
nese complexes [(NHC)Mn(h :h -dvtms)] (NHC = IPr, 1; IMes, 2; Et -cAAC, 3) were
isolated in good yields (62%, 41%, and 65%, respectively) as crystalline solids from
the mixtures (Scheme 1). The synthetic protocol is identical to that used for the prepa-
35–39,41
ration of the analogous formal Fe(0) and Co(0) complexes,
but the reduction step
ꢁ
has to be run at a low temperature (ꢀ40 C) as the trials to synthesize 1 at room temper-
ature gave oily material that resisted crystallization. The failure suggests the high activity
of the low-coordinate manganese species. With the aim of accessing similar low-coor-
dinate manganese complexes bearing other alkene ligands, the reduction reactions of
2 8
MnCl with IPr and KC in the presence of trimethylsilylethylene, isoprene, and diallyl
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Figure 2. Molecular Structures of 1 and 3
Molecular structures of 1(anti-isomer, left) and 3 (right) show 30% probability ellipsoids and the partial atom numbering scheme. Hydrogen atoms are
˚
omitted for clarity. Selected distances (A) and angles (deg) for 1 (anti-isomer): Mn1-C1 2.086(4), Mn1-C2 2.087(6), Mn1-C3 2.092(5), Mn1-C4 2.086(4),
0
0
0
Mn1-C5 2.106(7), C2-C3 1.382(8), C4-C5 1.432(10), C1-Mn1-C2 97.0(2), C1-Mn1-C4 93.32(17), C2-Mn1-C3 38.6(2), C4-Mn1-C5 40.0(3); for 3: Mn1-C1
.0274(11), Mn1-C2 2.1320(11), Mn1-C3 2.1165(11), Mn1-C4 2.1010(11), Mn1-C5 2.1155(11), C2-C3 1.4322(16), C4-C5 1.4319(16), C1-Mn1-C2 91.29(4),
2
C1-Mn1-C4 100.00(4), C2-Mn1-C3 39.40(4), C4-Mn1-C5 39.70(4).
ether were examined. Unfortunately, these reactions produced brown oily mixtures
from which no desired manganese complex could be isolated.
As isolated, 1 and 2 present as green solids, and 3 is red. They are soluble in toluene,
diethyl ether, and THF, and slightly soluble in n-hexane. Complexes 1–3 are air and
moisture sensitive; however, they show good stability in both solid state and solution
ꢁ
when stored under a nitrogen atmosphere and at ꢀ30 C. Complexes 1–3 have been
1
characterized by H NMR (Figures S11–S13), superconducting quantum interference
device (SQUID) measurement (Figure 3; see also Figure S42), electron paramagnetic
resonance (EPR) spectroscopy (Figures 4 and 5; see also Figure S43), absorption
spectroscopy (Figure S6), elemental analysis, and single-crystal X-ray diffraction
study (Figure 2; see also Figure S1 and Data S1 and S2). Consistent with their different
color, the absorption spectra of 1 and 2 in THF feature two absorption bands at ca.
4
00 and 660 nm, whereas two bands at the maxima of 480 and 705 nm with large ab-
sorption coefficients were observed in the spectrum of 3 (Figure S6). The difference in
the absorption spectra may arise from the different electronic nature of their carbene
6
6,67
1
ligands.
spectra measured in benzene-d
shifted signals in the range +20 to ꢀ30 ppm (Figures S11–S13). Solution magnetic
susceptibility measurements (Evans method in benzene-d at room temperature)
indicated magnetic moments of 4.0, 3.9, and 3.7 m for 1–3, respectively, which are
for 3d ions with an S = 3/2 state.
Complexes 1–3 are paramagnetic, as suggested by their H NMR
6
exhibiting heavily broadened, paramagnetically
6
B
close to the spin-only value of 3.87 m
B
Molecular Structures
The solid-state structures of 1 and 3 were established by single-crystal X-ray diffrac-
tion studies (Figure 2; see also Figure S1, Table S1, and Data S1 and S2). The man-
ganese centers in both complexes are coordinated by one NHC ligand and one
2
2
h :h -dvtms ligand, forming a trigonal planar geometry. The Mn–C(carbene) bond
˚
distances in 1 and 3 are 2.086(4) and 2.027(2) A, respectively, which are shorter
4
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Figure 3. Magnetization Measurements of 3
Temperature dependence of the magnetic moment, meff, of a solid sample of 3 recorded in a 1.0 T
magnetic field and variable field and variable temperature dependence of the magnetization of 3
(
inset). The solid lines represent best fits using the parameters given in the text.
than those of high-spin Mn(II) complexes [Mn
2
Cl
2
(m-Cl)
2
(IPr)
2
2
] (2.183(5) and
6
8
69
70
˚
˚
˚
2
.196(5) A), [(IPr)Mn(Mes)
2
] (2.195(2) A), [(IPr)Mn(CH
2
SiMe
3
)
] (2.236(2) A),
7
0
˚
[
(IPr)Mn(CH(SiMe
3
)
2
)
2
] (2.277(4) A), and also the Mn(II) dialkyl complexes 4–7 and
˚
9
(2.177(8)–2.189(4) A, see below).The contracted Mn–C(carbene) separations in 1
and 3 might be related to the high covalency of the Mn–C(carbene) bond in low-val-
ent manganese complexes. Similar phenomena were also observed in low-coordi-
3
5,39
2
2
nate iron- and cobalt-NHC complexes.
The h :h -dvtms ligand in 1 is disordered
and was modeled as a mixture of syn- and anti-isomers based on the relative align-
ment of the vinyl moieties. The anti-isomer has Mn–C(alkene) distances in a narrow
range of 2.086(4)–2.106(7), and C(alkene)–C(alkene) distances of 1.382(8) and
1
.432(10). These C(alkene)–C(alkene) bonds are distinctly longer than the C=C
7
1
˚
bonds in vinylsilanes (1.32 A), indicating strong p backdonation from the formal
Mn(0) center to the dvtms ligand. But they are comparable with those found for
7
2
2
2
73
˚
˚
[(dppe)Fe(h :h -dvtms)] (1.409(5) A),
[
[
CpCo(CH
2
CHSiMe
˚
2 2 2 3
CH )] (1.438(5) A), and [Ti(CH CH )(OAr*) (PMe )] (1.425(5) A).
3
)
2
]
(1.408(2) A),
7
4
75
˚
(Cp*) Ti(CH
2
2
2
The titanium complexes are thought to be intermediate between the
limiting electronic structures of Ti(II) ethylene and Ti(IV) metallacyclopropane.
However, when compared with the corresponding C–C bonds in the
7
6
˚
metalla- and thia-cyclopropanes, [(PPh
3
)
2
Pt(trans-NCCHCH
˚
2
CN)] (1.53(4) A),
and thioethylene CH CH
1.484(3) A), the C(alkene)–C(alkene) distances in 1 are apparently shorter. Also,
t
77
[
(
Cp*Ta(CH
2
CH
2
)(CHBu )(PMe
3
)] (1.477(4) A),
2
2
S
7
8
˚
these C(alkene)–C(alkene) distances are longer than those in manganese complexes
5
2
79
˚
6 5 2 2
H )(h -NHCH CH=CH )] (1.37(1) A) but close to that in
[
(h -C
Li [MnH(CH
dvtms ligand presents in a syn form. The Mn–C(alkene) distances also fall into a nar-
5
H
4
(CH
3
))Mn(CO)C(C
8
0
2
2
˚
2 2
] (1.41(1) A). In the crystal structure of 3, the h :h -
4
2
CH
2
)(dmpe-H)
˚
row range of 2.101(1)–2.132(1) A, which are comparable with those in 1 and slightly
˚
shorter than the Mn(II)–C bonds in [(THF) Mn(1,3-(SiMe ) C H ) ] (2.174(2) A) and
2 3 2 3 3 2
0
˚
81
[
(
(tmeda)Mn(1,1 ,3-(SiMe
3
)
3
C
3
H
2
)
2
] (2.189(3) A). The C(alkene)–C(alkene) distance
˚
1.432(2) A) of 3 is comparable with those in 1. Thus, among these structure data,
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Figure 4. Resonance Field versus Microwave Frequency (Quantum Energy) of EPR Transitions
for 3
The Hamiltonian parameters used are: S = 3/2, g
x
= 1.97(2), g
y
= 1.97(2), g
z
= 1.97(2), jDj = 3.22(3)
ꢀ
1
ꢀ1
cm , E = 0.14(3) cm . Green, blue, and red curves are the simulations using the best-fitted spin
Hamiltonian parameters with the magnetic field H parallel to the x, y, and z axes of the ZFS tensor,
respectively. The vertical dashed line represents the frequency (375 GHz) used in Figure 5 at which
the spectra were recorded or simulated.
the C(alkene)–C(alkene) distances in 1 and 3 indicate strong backdonation from their
formal Mn(0) center to the dvtms ligand, but the backdonation is not strong enough
to allow the assignment of these complexes as manganacyclopropanes.
The accessibility of 1–3 proved the effectiveness of the combined ligand set of NHC
with dvtms in stabilizing zero-valent metal complexes besides late transition metals.
Comparison of the structures of a series of formal zero-valent metal complexes
2
2
involving different metal centers, [(NHC)M(h :h -dvtms)] (M = Ni, Co, Fe, Mn;
3
2,35,39
NHC = IMes or IPr)
showed a stepwise increase in the M–C(carbene) distances
˚
from 1.91 to 1.95 to 2.03 and to 2.09 A upon going from Ni to Co to Fe and to Mn. In
parallel, the M–C(alkene) bonds also gradually lengthen (Table S4). Both observa-
tions thus reveal the overwhelming effect of the differential atomic radii of the ele-
ments from Ni to Co to Fe and to Mn that largely dictates the variation of the
metal–ligand bond distances. Despite the subtle structure change, the detailed
electronic interaction between the dvtms ligand and the metal center in the series
of complexes (NHC)M(dvtms) could be different. Interestingly, irrespective of the
2
2
metal center in [(NHC)M(h :h -dvtms)], the C(alkene)–C(alkene) bond lengths are
nearly identical. This reflects the strong reduction potency of Mn(0) over the zero-
valent late 3d transition metals, because 1 has the longest M–C(alkene) bond
distances.
Electronic Structure of the Three-Coordinate Formal Mn(0) Complexes
Noting the rarity of low-coordinate formal Mn(0) complexes, spectroscopic charac-
terization and theoretical studies on 1 and 3 have been performed to probe their
electronic structures. Because analogous results were found for both complexes,
we focus our discussion on 3 (Figures 3, 4, 5, and 6; see also Figures S43 and S44)
and summarize the data for 1 (Figures S42, S45, and S46) in the Supplemental
Information.
6
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Figure 5. High-Frequency EPR Spectrum of 3 with Its Simulations at 375 GHz and 9 K
The blue trace represents the spectrum simulated using the positive D value, for which the magnitude
is determined in Figure 4. The red trace is the spectrum simulated using the corresponding negative
D value. The other Hamiltonian parameters used are the same as those in Figure 4.
From variable temperature (2–295 K) magnetic susceptibility and variable tempera-
ture and variable-field magnetization measurements (Figure 3), a quartet ground
state (S = 3/2) has been established for 3. The solid line in Figure 3 represents a
simulation using the usual S = 3/2 spin Hamiltonian (Equation 1) with giso
=
ꢀ
1
1
.99(1), D = ꢀ3.8(1) cm , E/D = 0.04(2), and temperature-independent paramagne-
ꢀ6
tism = 10 3 10 electromagnetic units (emu). Here, D and E are the axial and
rhombic zero-field splitting (ZFS) parameters, and g is the intrinsic g matrix.
ꢀ
ꢃ
ꢁ
ꢂ
2
1
E
2
2
! c!
b
b
b
b
H = D S ꢀ SðS + 1Þ + D S ꢀ S
+ b B g S
(Equation 1)
z
x
y
3
The high-frequency (HF)-EPR experiments on the polycrystalline powder samples of
3
were carriedoutat 9 K andvarious frequenciesto accuratelydeterminethespinHamil-
tonian parameters. The results are plotted in Figure 4 as the field dependence of the
resonances on the frequencies. Obviously, one of the zero-field transitions is observed
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ꢀ
1
0
0
2
2
at about 6.4 cm . This corresponds to the effective spitting 2jD jðD = D + 3E Þ
between the two Kramers doublets of a quartet state (S = 3/2). A least-square fit to a
complete two-dimensional array of the resonances yields the following parameters:
= 1.97(2), jDj = 3.22(3) cmꢀ , and E = 0.14(3) cm , which
1
ꢀ1
x y z
g = 1.97(2), g = 1.97(2), g
are in reasonable agreement with those determined by the magnetization and X-band
ꢀ1
EPR studies (jDj = 3.5–3.8 cm ) supplied in the Supplemental Information. To confirm
thesignofD, asingle-frequencyspectrum(375GHz)wassimulatedasshownin Figure5,
which confirms the negative sign of D (D = ꢀ3.22(3) cmꢀ1).
To gain further insights into the nature of 3, we have undertaken detailed computa-
tional investigations using a wavefunction-based multireference completely active
space self-consistent field (CASSCF) approach. To this end, an active space consist-
ing of 7 electrons distributed into 12 orbitals was chosen. In addition to the five Mn
3
d and two dvtms C=C p* orbitals, the Mn 4d shell was also included in the active
8
2,83
space to account for double-shell effects.
To lower the computational cost, a
0
truncated model (3 ), in which the Dipp and Et substituents in the carbene ligand
were replaced by Ph and Me, was used. Figure 6 depicts the resulting natural orbitals
Chem 4, 1–17, December 13, 2018
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0
Figure 6. Natural Orbitals of 3 Derived from the Ground-State CASSCF(7,12) Calculations
The occupation number along with the dominant atomic contribution is indicated near each orbital.
derived from the ground-state CASSCF(7,12) calculations. One can identify three
singly occupied, essentially nonbonding orbitals of predominant Mn 3d parentage
(dz2, dxz, and dyz) and strong p backdonation from the Mn dxy and dx2-y2 orbitals
to the dvtms C=C p* orbitals, as demonstrated by two pairs of the bonding and an-
tibonding combinations labeled as Mn dxy/x2-y2 + px/y* and Mn dxy/x2-y2 – px/y*,
0
respectively. In a simplified picture, the p backdonation in 3 can be interpreted
as electron transfer from a formal Mn(0) center to the dvtms ligand via two channels,
each involving two electrons. This process would eventually lead to formation of a
4
ꢀ
formal Mn(IV) center and a tetra-anionic ligand ([dvtms] ), once it was completely
0
finished. The CASSCF(7,12) wavefunction of 3 has a principal electron configuration
2
2
1
1
1
0
0
x y x y
of (dxy + p *) (dx2-y2+p *) (dz2) (dxz) (dyz) (dxy – p *) (dx2-y2–p *) with a weight of
7
7
3%, and none of the remaining electron configurations has a weight exceeding
%. Notably, the non-negligible negative spin population (ꢂ0.25) found for each
p bond of dvtms is substantially lower than 1 as expected for a C=C p* radical
8
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Figure 7. Canonical Forms of the Low-Coordinate Manganese Species at an S = 3/2 State
The half arrows on Mn and alkene moieties denote the 3d and unpaired 2p electrons, respectively.
(
Figure 6). Thus, the negative spin density in dvtms mainly arises from spin polariza-
tion, for which the a- and b-electron transfers from the doubly occupied dxy/x2-y2 or-
8
4,85
bitals of a formal Mn(0) center cannot take place simultaneously.
The nearly
identical contributions of the Mn dxy and dx2-y2 atomic orbitals and the dvtms C=C
p* orbitals in Mn dxy/x2-y2 + px/y* and Mn dxy/x2-y2 – px/y* indicate almost the highest
degree of metal-ligand covalency that renders unambiguous assignment of the
8
6
physical oxidation state of the metal center difficult. It has been reported that un-
der such circumstances, attempts to assign the physical oxidation state using X-ray
8
7,88
absorption spectroscopy fail as well.
Therefore, the bonding of 3 has to be inter-
preted as resonance among a series of limiting canonical electronic structures of a
high-spin Mn(0) center (SMn = 3/2) ligated by a dvtms ligand (A in Figure 7), an inter-
2
-
mediate-spin Mn(II) ion (SMn = 3/2) bound to a dianionic ligand ([dvtms] ) (B in Fig-
ure 7), a high-spin Mn(II) (SMn = 5/2) antiferromagnetically coupled to a triplet dia-
$
$2–
nionic ligand ([dvtms] ) (Sdvtms = 1) yielding an overall quartet state (Stotal = 3/2)
0
(
B in Figure 7), and a high-spin Mn(IV) ion (SMn = 3/2) chelated by a tetra-anionic
4
-
ligand ([dvtms] ) (C in Figure 7). Such highly covalent metal-ligand interactions
were encountered in our earlier study, and the electronic structures were also
8
9
interpreted as resonance among several limiting bonding situations. The B3LYP
calculations of complex 3 also deliver a qualitatively identical bonding picture
(
Figure S44). As expected, 1 was also computed to feature the same electronic struc-
ture despite involving a different NHC ligand (Figures S45–S46). Thus, the calcula-
tions indicate that the high stability of these formal Mn(0) complexes (NHC)
Mn(dvtms) apparently results from the good p-accepting nature of dvtms in addition
to the sterically demanding and strong s-donating nature of NHC.
Reactivity of the Three-Coordinate Formal Mn(0) Complexes
Complexes 1–3 are rare examples of low-coordinate formal Mn(0) alkene complexes.
Given the limited knowledge on low-coordinate manganese alkene complexes, the
90–93
growing interest in manganese-catalyzed organic transformations in recent years
prompted us to perform a careful reactivity study on these manganese complexes.
These low-coordinate manganese complexes are found to be reactive toward
alkenes, alkynes, and allenes. The reactions of 4-(trifluoromethyl)styrene, 2,5-nor-
bornadiene, 1-phenyl-1-propyne, and phenyl allene with 1 or 3 occurred at room
temperature, from which Mn(II) complexes bearing carbodianionic chelates 4–7
were isolated in high yields (Scheme 2). Single-crystal X-ray diffraction studies unam-
biguously established the structures of 4–7 as Mn(II) complexes (Figure 8; see also
Figures S2 and S3, Tables S1 and S2, and Data S3, S4, S5, and S6) resulting from
manganese-mediated reductive coupling of the two alkene moieties of the dvtms
Chem 4, 1–17, December 13, 2018
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Scheme 2. Reactions of the Formal Mn(0) Complexes with Alkene, Alkyne, and Allene
ligand in 1 or 3 with the added unsaturated hydrocarbons. As a representative, Fig-
ure 8 depicts the structures of 4 and 6. The unique manganabicyclo[4.3.1]decane
frameworks in 4–7 contain metallacycloheptane rings. The ones in 4 and 7 adapt a
twisted chair conformation, and those in 5 and 6 are in a boat conformation. The sub-
3
stituents, silyl, aryl, and alkyl groups, on the C(sp ) atoms of the seven-membered
manganacycles are all cisoid to each other. The observed stereospecificity might
be traced back to the steric demanding nature of the NHC ligands, which renders
the reductive coupling reactions stereoselective. Metallacycloheptanes are key in-
termediates in a plethora of transition-metal catalyzed coupling reactions of alkenes,
and their isolation proved challenging due to their readiness to undergo b-hydride
9
4–96
elimination.
Considering this, the accessibility of 4–7 most likely reflects the
unique steric feature of the poly-ring system that might prevent the manganese cen-
ter from approaching the b-H atoms. Complexes 4–7 were further characterized by
1
H NMR (Figures S14–S18), solution magnetic susceptibility measurements, absorp-
tion spectroscopy (Figures S7 and S8), and elemental analysis. Their long Mn–C dis-
tances and large magnetic moments indicate their high-spin (S = 5/2) Mn(II) nature.
The reactions of the formal Mn(0) complexes with unsaturated hydrocarbons repre-
sent the first examples of reductive couplings of alkenes and alkynes mediated
by well-defined manganese complexes. Such reactions are useful methods for
C–C bond formation and are often promoted by low-valent transition-metal spe-
9
7–101
cies of Ti, Zr, Cr, Fe, Ni, and Co.
Probably because of the difficulty in access-
ing low-coordinate low-valent manganese species, manganese-mediated reductive
coupling reactions have been rarely reported. To our knowledge, the coupling of
norbornadiene with 1,3-butadiene catalyzed by the Ziegler-Natta catalyst Mn(acac)
2
i
2,5
3
with Pr MgCl or Et Al, which produced 3-vinyltricyclo[4.2.1.0 ]non-7-ene and
5
-(buta-1,3-dien-1-yl)bicyclo[2.2.1]hept-2-ene in low yields, is the only relevant
1
02
example.
To get more insight into this reductive coupling reaction, the influence
of the electronic properties of the mono-alkene was examined by comparing the
relative rates of the reactions of 1 with 4-trifluoromethylstyrene, styrene, and 4-me-
thyoxystyrene. The controlled reactions on NMR scale revealed that, while all
the alkenes could react with 1 to afford similar reductive coupling products as
1
suggested by their similar resultant H NMR spectra (Figure S19), the reaction times
required for complete conversion of 1 (6, 48, and 192 hr, respectively) are different.
The enhanced rates of the reaction with electron-deficient alkenes are consistent with
those of the reductive coupling reactions mediated by other transition metals and hints
10
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Figure 8. Molecular Structures of 4 and 6
Molecular structures of 4 (left) and 6 (right) show 30% probability ellipsoids and the partial atom numbering scheme. Hydrogen atoms are omitted for clarity.
˚
Selected distances (A) and angles (deg) for 4: Mn1-C1 2.189(4), Mn1-C2 2.153(5), Mn1-C3 2.140(5), C2-C30 1.535(7), C30-C31 1.552(8), C31-C32 1.535(8), C32-C33
1
1
.544(7), C3-C33 1.547(7), C1-Mn1-C2 130.76(17), C1-Mn1-C3 124.68(16), C2-Mn1-C3 104.36(18); for 6: Mn1-C1 2.177(8), Mn1-C2 2.142(7), Mn1-C3 2.158(7), C2-C34
.574(9), C34-C35 1.552(9), C3-C37 1.564(9), C36-C37 1.518(10), C35-C36 1.345(9), C1-Mn1-C2 134.3(2), C1-Mn1-C3 120.7(3), C2-Mn1-C3 102.6(3).
9
7–101
at the electron richness of the manganese species.
Accordingly, the formation
of 4–7 may be rationalized by a classic reductive coupling mechanism mediated by
9
7–101,103
low-valent transition metals.
As an example, Scheme 3 shows a feasible
pathway for the reactions of 1 with alkenes. The reaction might start from the ligand-
replacement reaction of one alkene moiety of the dvtms ligand in 1 by the added alkene
molecule, 4-(trifluoromethyl)styrene or 2,5-norbornadiene, which gives new bis(alkene)
formal Mn(0) species D. Species D undergoes a reductive coupling reaction, presum-
ably via manganacyclopropane intermediate E, to form manganacyclopentane F.
Further insertion of the appended alkene moiety of F into its metallacyclopentane
ring could eventually produce the manganacycloheptanes. In the C–C bond formation
104
steps, the capability of the a-silyl group to stabilize carboanion,
steric repulsions
imparted by the bulky NHC ligands, and ring constraints are among the key determi-
nants that govern the regio- and stereoselectivity of the coupling reaction. Considering
the different ground-spin state between 4–7 (S = 5/2) and 1 and 3 (S = 3/2), these reduc-
105,106
tive coupling reactions must involve a spin crossover.
Whether the C–C bond
formation steps occurring on the energy surface of S = 3/2 or S = 5/2 remains an
open question. Thus, elucidating the reaction mechanism and addressing the above
question needs a detailed experimental and theoretical mechanistic investigation.
The low-coordinate formal Mn(0) complexes also showed diversified reactivity
toward H
2
O, H
ꢁ
2
, I
2
, and CO. Hydrolysis of 1 with a large excess amount of
SiOSiMe CH=CH (8) in 92% NMR
O, the corresponding di-deuterium compound,
(8-d ), was obtained in a high yield (Scheme 4).
were characterized by NMR (Figures S20–S28) and mass
spectrometry. The deviation of the quantitative deuteration ratio of 8-d could be
due to the presence of a small amount of H O in deuterated water. In addition, the
capability of low-coordinate low-valent transition-metal complexes in activating C–H
H
2
O at ꢀ78 C furnished mono-alkene EtMe
2
2
2
yield. Upon replacing H
CH DCHDSiMe OSiMe CH=CH
Compounds 8 and 8-d
2 2
O by D
2
2
2
2
2
2
2
2
1
07,108
bonds could also partly make a contribution.
The formation of these mono-
alkenes hints at the contribution of the mangana(II)-cyclopropane form (B in Figure 7)
in the canonical structures of the three-coordinate formal Mn(0) alkene complexes.
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Scheme 3. Proposed Mechanism for the Manganese-Mediated Reductive Coupling Reactions of 1 with Alkenes
Complex 1 can also react with H
complex (9) in 71% yield (Scheme 4). The reaction presents rare examples of H
vation by low-valent manganese species. Precedents are only known for the reaction
2
(1 atm) at room temperature to give a Mn(II) alkyl
2
acti-
1
09
2
of photo-excited manganese atoms with H in a cryogenic matrix and the reaction
4
6
1
of Roesky’s [Mn(Me
2
-cAAC)
2
] with H
2
.
Complex 9 has been characterized by H
NMR (Figure S29), UV-visible spectroscopy (Figures S7 and S8), solution magnetic
susceptibility measurement, X-ray diffraction studies (Figure 9; see also Table S3
and Data S7), as well as elemental analysis. Comparing its structure with that of 1 re-
vealed that the hydrogenation reaction resulted in addition of two hydrogen atoms
to the two terminal carbon atoms of the alkene moieties of the dvtms ligand. Indeed,
exposure of 1 to D
mation of (DCH CH
could be explained by a route starting from the s-bond metathesis of 1 in form B
2
(1 atm.) in toluene followed by hydrolysis with H
2
O led to the for-
2
2
(CH Si) O (10-d ) in a high yield (Scheme 4). The generation of
3
)
2
2
2
9
2
with H to yield manganese(II) alkyl hydride species, followed by migratory insertion
(
Scheme S1). Alternatively, the classic route involves oxidative addition of 1, in form
A, with H to afford Mn(II) species (IPr)MnH (dvtms) that undergoes further migratory
insertion reactions to produce 9 (Scheme S1). Notably, as the combined H, H, C,
2
2
1
2
13
1
3
and C DEPT-135 NMR analysis (Figures S30–S38) indicate a high D incorporation in
the methyl position of 10-d2 (112%), the elementary steps in the proposed paths
(
2
Scheme S1) could be reversible. Complex 9 is unreactive toward H (1 atm.) at
room temperature, which might account for the unsuccessful attempt to perform
catalytic hydrogenation of dvtms using 1 as the catalyst.
Different from the aforementioned reactions, the interactions of 1 with I
not lead to functionalization of dvtms, rather to its release. Treatment of 1 with 1
equiv of I afforded the Mn(II) complex [(IPr)MnI (THF)] (11) and dvtms in high yields
Scheme 4). On the other hand, the exposure of 1 with an excess amount of CO
1 atm.) produced [(IPr)Mn(CO) (12) and dvtms (Scheme 4). Complexes 11 and
2 have been characterized by single-crystal X-ray diffraction analyses (Figures S4
2
and CO did
2
2
(
(
4 2
]
1
and S5; see also Table S3 and Data S8 and S9), NMR (Figures S39–S41) and absorp-
tion spectroscopy (Figures S9 and S10). These reaction outcomes are distinct from
the well-established reactivity of transition-metal alkyl complexes with I
2
and CO,
1
10,111
wherein iodination and CO-insertion reactions usually occur.
These reactions
seem to be more congruent with the Mn(0)-alkene canonical form (A in Figure 7) of
the formal Mn(0) alkene complexes.
Conclusion
In this study, we showed that the stabilization and isolation of low-coordinate formal
Mn(0) species can be achieved through the use of a combined ligand set of mono-
dentate NHC and dvtms and that the resultant manganese complexes [(NHC)
Mn(dvtms)] have rich reactivity. The low-coordinate manganese complexes [(NHC)
2
Mn(dvtms)] were readily prepared from the reactions of NHC with MnCl , dvtms,
12
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2 2 2 2 2
Scheme 4. Reactions of 1 with H O, D O, H , D , I , and CO
8
and KC . They represent rare examples of isolable low-coordinate formal zero-valent
transition-metal complexes besides known late transition-metal complexes. Spec-
troscopic characterization in combination with theoretical studies revealed their
S = 3/2 ground-spin state and strong p backdonation from the metal centers to
the alkene moieties. As a result of the high covalency of the metal-ligand interaction,
these formal Mn(0) complexes are better interpreted as intermediate between a se-
ries of limiting canonical structures of Mn(0), Mn(II), and Mn(IV). In accord with the
electronic nature, [(NHC)Mn(dvtms)] exhibits intriguing reactivity of reductive
coupling with alkenes and alkynes to form Mn(II) dialkyl complexes, which is unprec-
edented for manganese complexes. In addition, [(NHC)Mn(dvtms)] can react with
H
2
O to give mono-alkene EtMe
2
SiOSiMe
2
CH=CH
2
, can be hydrogenated by H
to give Mn(II) diiodide, and
2
to form Mn(II) dialkyl compound, can be oxidized by I
2
can also undergo a ligand-substitution reaction with CO to form a Mn(0) carbonyl
complex. In the latter two reactions, the dvtms ligand was released without further
conversion. These results highlight the synthetic utility of low-coordinate low-valent
manganese complexes, which warrants further exploration.
EXPERIMENTAL PROCEDURES
Full experimental procedures are provided in the Supplemental Information.
DATA AND SOFTWARE AVAILABILITY
Crystallographic data of complexes 1, 3, 4, 5, 6, 7, 9, 11, and 12 have been deposited
in the Cambridge Crystallographic Data Center under accession numbers
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Figure 9. Molecular Structure of 9
Molecular structure of 9 shows 30% probability ellipsoids and the partial atom numbering scheme.
˚
Hydrogen atoms are omitted for clarity. Selected distances (A) and angles (deg): Mn1-C1 2.181(5),
Mn1-C2 2.129(7), Mn1-C3 2.148(9), C2-C4 1.601(16), C3-C5 1.572(14), C1-Mn1-C2 127.8(2), C1-Mn1-
C3 115.2(3), C2-Mn1-C3 115.2(3).
CCDC: 1845539, 1845540, 1845541, 1845542, 1845543, 1845544, 1845545,
1
845546, and 1845547, respectively.
SUPPLEMENTAL INFORMATION
Supplemental Information includes Supplemental Experimental Procedures, 46 fig-
ures, 1 scheme, 4 tables, and 9 data files and can be found with this article online at
https://doi.org/10.1016/j.chempr.2018.09.002.
ACKNOWLEDGMENTS
The work was supported by the National Key Research and Development Pro-
gram (2016YFA0202900), the National Natural Science Foundation of China
(
21725104, 21690062, and 21432001), and the Strategic Priority Research Pro-
gram of the Chinese Academy of Sciences (XDB20000000). S.Y. gratefully ac-
knowledges the financial support from the Max Planck Society. We are deeply
indebted to Dr. Eckhard Bill for fruitful discussions and to Mr. Andreas G o¨ bel
for SQUID measurements.
AUTHOR CONTRIBUTIONS
Investigation, J.C., S.Y., and L.D.; Synthetic Studies, J.C. and Q.C.; Crystallographic
Studies, X.L.; EPR Experiments, S.Y., Z.O., and Z.W.; SQUID Experiments, S.Y.;
Computational Experiments, S.Y.; Writing – Original Draft, J.C., Z.W., S.Y., and
L.D.; Writing – Review & Editing, Z.W., S.Y., and L.D.; Supervision, L.D.
14
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DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: May 28, 2018
Revised: July 30, 2018
Accepted: September 10, 2018
Published: October 4, 2018
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