Mono- and Dinuclear Manganese Carbonyls Supported by 1,8-Disubstituted (L = Py, SMe, SH) Anthracene Ligand Scaffolds

Article abstract of DOI:10.1021/acs.inorgchem.5b02737

Presented herein is a synthetic scheme to generate symmetric and asymmetric ligands based on a 1,8-disubstituted anthracene scaffold. The metal-binding scaffolds were prepared by aryl chloride activation of 1,8-dichloroanthracene using Suzuki-type couplings facilitated by [Pd(dba)₂] as a Pd source; the choice of cocatalyst (XPhos or SPhos) yielded symmetrically or asymmetrically substituted scaffolds (respectively): namely, Anth-S_Me2 (3), Anth-N2 (4), and Anth-NS_Me (6). The ligands exhibit a nonplanar geometry in the solid state (X-ray), owing to steric hindrance between the anthracene scaffold and the coupled aryl units. To determine the flexibility and binding characteristics of the anthracene-based ligands, the symmetric scaffolds were complexed with [Mn(CO)₅Br] to afford the mononuclear species [(Anth-S_Me2)Mn(CO)₃Br] (8) and [(Anth-N2)Mn(CO)₃Br] (9), in which the donor moieties chelate the Mn center in a cis fashion. The asymmetric ligand Anth-NS_Me (6) binds preferentially through the py moieties, affording the bis-ligated complex [(Anth-NS_Me)₂Mn(CO)₃Br] (10), wherein the thioether-S donors remain unbound. Alternatively, deprotection of the thioether in 6 affords the free thiol ligand Anth-NS_H (7), which more readily binds the Mn center. Complexation of 7 ultimately affords the mixed-valence Mn^I/Mn^II dimer of formula [(Anth-NS)₃Mn₂(CO)₃] (11), which exhibits a fac-{Mn(CO)₃} unit supported by a triad of bridging thiolates, which are in turn ligated to a supporting Mn(II) center (EPR: |D| = 0.053 cm^-1, E/|D| = 0.3, A_iso = -150 MHz). All of the metal complexes have been characterized by single-crystal X-ray diffraction, IR spectroscopy and NMR/EPR measurements - all of which demonstrate that the meta-linked, anthracene-based ligand scaffold is a viable approach for the coordination of metal carbonyls.

Full text of DOI:10.1021/acs.inorgchem.5b02737

Article
pubs.acs.org/IC
Mono- and Dinuclear Manganese Carbonyls Supported by 1,8-
Disubstituted (L = Py, S , S ) Anthracene Ligand Scaffolds
Me
H
Taylor A. Manes and Michael J. Rose*
Department of Chemistry, The University of Texas at Austin, 1 University Station, A5300, Austin, Texas 78712, United States
S Supporting Information
*
ABSTRACT: Presented herein is a synthetic scheme to generate symmetric and asymmetric ligands based on a 1,8-disubstituted
anthracene scaffold. The metal-binding scaffolds were prepared by aryl chloride activation of 1,8-dichloroanthracene using
Suzuki-type couplings facilitated by [Pd(dba) ] as a Pd source; the choice of cocatalyst (XPhos or SPhos) yielded symmetrically
2
or asymmetrically substituted scaffolds (respectively): namely, Anth-S 2 (3), Anth-N2 (4), and Anth-NS (6). The ligands
Me
Me
exhibit a nonplanar geometry in the solid state (X-ray), owing to steric hindrance between the anthracene scaffold and the
coupled aryl units. To determine the flexibility and binding characteristics of the anthracene-based ligands, the symmetric
scaffolds were complexed with [Mn(CO) Br] to afford the mononuclear species [(Anth-S 2)Mn(CO) Br] (8) and [(Anth-
5
Me
3
N2)Mn(CO) Br] (9), in which the donor moieties chelate the Mn center in a cis fashion. The asymmetric ligand Anth-NS (6)
3
Me
binds preferentially through the py moieties, affording the bis-ligated complex [(Anth-NS ) Mn(CO) Br] (10), wherein the
Me 2
3
thioether-S donors remain unbound. Alternatively, deprotection of the thioether in 6 affords the free thiol ligand Anth-NS (7),
H
I
II
which more readily binds the Mn center. Complexation of 7 ultimately affords the mixed-valence Mn /Mn dimer of formula
(Anth-NS) Mn (CO) ] (11), which exhibits a fac-{Mn(CO) } unit supported by a triad of bridging thiolates, which are in turn
[
3
2
3
3
−
1
ligated to a supporting Mn(II) center (EPR: |D| = 0.053 cm , E/|D| = 0.3, A = −150 MHz). All of the metal complexes have
iso
been characterized by single-crystal X-ray diffraction, IR spectroscopy and NMR/EPR measurementsall of which demonstrate
that the meta-linked, anthracene-based ligand scaffold is a viable approach for the coordination of metal carbonyls.
INTRODUCTION
Rigid scaffolds have been used to study wide-bite and trans-
■
10,13
chelating phosphines.
(DBFphos) was used to generate trans square-planar and
For example, a dibenzofuran system
Molecular scaffolds can be a valuable tool, as they provide
different binding and reactivity motifs to metal centers.
1
−4
4,14
tetrahedral systems. A further study by Lu et al.
installed a
Small changes to the framework can modulate the overall
binding geometry or affinity, which can then enhance or
diminish the functionality of the complex. The purpose of a
scaffold is to place donors or substituents in advantageous
geometries to perform a designed function, and in many cases
chelating ligands provide such a setting. Scaffold chelates can be
phenyl arm to distance the backbone from the donating
phosphine atoms. This elongation prevents the furan oxygen
from binding the metal center and accommodates a broader
range of bite angles to allow for isomerization of intermediates,
particularly in penta- and hexacoordinated catalytic sys-
14−16
tems.
Substituted anthracene scaffolds also have wide
5
1
7
classified into three groups: cis-chelating ligands, wide-bite-
versatility in inorganic chemistry. Studies have used
anthracene as a trans-chelating scaffold by placing phosphines
on a phenyl linker at the para position, as seen in Figure 1. The
anthracene system is advantageous to trans-spanning ligands
due to its aromatic frame and strategic placement of donor
atoms. By using para-substituted phenyl linkers, the spacing of
the donor atoms is restricted enough to allow only trans
6
7,8
angle ligands, and trans-spanning ligands.
Scaffolding
approaches can be used to enforce one or more of these
ligation motifs to encourage a specific function of the metal
complex. In many catalytic processes the ligand field and its
arrangement around the metal center play an important role in
9
−11
catalytic efficiency.
Some scaffolds allow flexibility such that
1
,12
a complex can undergo isomerization during catalysis,
but
others (such as the one we present in the current work) are
more rigid and enforce a single binding geometry.
Received: November 24, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b02737
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
solution was dried over Na SO and concentrated in vacuo to afford a
2
4
brown-yellow oil. The oil was dissolved in a minimal amount of CHCl3
and loaded onto a silica column packed with a 7/1 mixture of hexanes
and ethyl acetate. Pure product was separated using 7/1 hexanes/ethyl
acetate as eluent. The symmetric thioether ligand 3 was thus obtained
as a pale yellow powder. Yield: 86% (750 mg). X-ray-quality yellow
needles were grown using vapor diffusion of pentane into a solution of
1
the complex in THF. H NMR (400 MHz, CDCl ): δ 2.42 (s, 6H),
3
Figure 1. iprDPDBFphos (left) and para-substituted anthracene
center) scaffolds showing trans-chelated coordination environments
incorporating a phenyl linker, and a model cis-chelation scaffold
right).
4
7,8
7.29 (m, 4H), 7.39 (m, 6H), 7.53 (t, 2H), 8.03 (d, 2H), 8.55 (s, 1H),
8
1
1
3
.67 (s, 1H). 13C NMR (400 MHz CDCl3): δ 15.72, 123.52, 125.24,
(
25.56, 126.21, 126.71, 126.90, 127.76, 127.83, 128.49, 129.93, 131.77,
38.45, 139.97, 141.01. Selected IR frequencies (solid state, cm ):
044 (w), 2917 (w), 1583 (m), 1434 (m), 872 (s). MS (CI+): m/z
−1
(
calcd. for C H S (M), 422.1163; observed, 422.1169.
28
22 2
1
,8-Bis(pyridin-3-yl)anthracene, Anth-N2 (4). In a Schlenk flask,
,8-dichloroanthracene (500 mg, 2.02 mmol), pyridin-3-yl boronic
acid (496 mg, 4.04 mmol), and anhydrous sodium carbonate (428 mg,
.04 mmol) were added, and the flask was taken to the drybox. Next,
Pd(dba) ] (34.8 mg, 0.0606 mmol) and XPhos (28.9 mg, 0.0606
mmol) were added to the reaction mixture. Dry THF (70 mL) was
added to the reaction to produce a violet solution. The reaction was
removed from the drybox and placed on a Schlenk line under N2
chelation. Trans-spanning ligands not using the scaffolding
approach have been susceptible to conformational instability
18
1
due to their flexible nature; however, the anthracene system
avoids these issues due to its rigid aromatic frame.
4
[
2
The anthracene model can be altered to act as a cis-chelating
agent by shifting the donor linkage point to the meta position.
In the present case (Figure 1, right) the donors are positioned
at the meta positionin contrast to the para position used in
the trans-spanning works. This conformation still retains its
rigidity and stability factors; however with the donors angled
inward, a cis-ligation motif should be highly favored. Another
novel facet of the present system is the asymmetric nature of
the donor “arms”. We report a selective coupling method to
incorporate both nitrogen and sulfur donors to a single ligand
frame. The intent of the present work involving these cis-
chelating ligands is to study their metal-binding affinity and
structural aspects within the scope of a manganese(I) carbonyl
environment. Mn carbonyl complexes have proven useful in
catalysis, CO delivery, and detection and are common in the
where degassed H O (10 mL) was added. The reaction was heated to
2
85 °C and refluxed overnight to obtain a dark purple/red solution. The
reaction was cooled to ambient temperature, quenched with saturated
NH Cl (5 mL), extracted with DCM (70 mL), and washed with
4
saturated brine (2 × 100 mL). The resulting brown/yellow organic
solution was dried over Na SO and concentrated in vacuo to afford a
2
4
brown/yellow oil. The brown/yellow oil was dissolved in a minimal
amount of CHCl and loaded on a silica column packed with a 7:1
3
ratio of hexanes and ethyl acetate. Pure product was separated using
MeOH as eluent. The symmetric pyridine ligand 4 was thus obtained
as a dark yellow powder. Yield: 80% (536 mg). X-ray quality yellow
needles were grown using vapor diffusion of pentane into a solution of
1
the complex in THF. H NMR (400 MHz, CDCl ): δ 7.35 (dd, 2H),
3
19−23
study of redox-active ligands.
In this work we look to
7.41 (d, 2H), 7.57 (dd, 2H), 7.79 (dt 2H), 8.10 (d, 2H), 8.33 (s, 1H),
evaluate and to understand the fundamental coordination
chemistry of meta-linked anthracene scaffolds in the Mn
environment to determine its suitability to support mono-
nuclear metal carbonyl complexes.
8.59 (s, 1H), 8.61 (dd, 1H), 8.71 (s, 1H). 13C NMR (400 MHz,
CDCl
31.81, 135.96, 136.57, 137.11, 148.70, 150.36. Selected IR frequencies
): δ 122.40, 122.90, 125.38, 126.98, 127.39, 128.57, 130.09,
3
1
−1
(solid state, cm ): 3030 (w), 1410 (s), 1181 (s), 1023 (s), 879 (s).
MS (CI+): m/z calcd for C H N (M), 332.1313; observed,
24
16
2
3
32.1314.
-(8-Chloroanthracen-1-yl)pyridine, Anth-NCl (5). In a Schlenk
flask were placed 1,8-dichloroanthracene (500 mg, 2.02 mmol),
pyridin-3-yl)boronic acid (248 mg, 2.02 mmol), and anhydrous
EXPERIMENTAL SECTION
General Considerations. All synthetic procedures were carried
■
3
out under inert atmosphere (N ) via Schlenk line or drybox techniques
2
(
unless otherwise stated. Dry solvents were purified using a two-column
alumina purification system (Pure Process Technology). Deuterated
solvents were purchased from Cambridge Isotope Laboratories and
used without further purification; the standard freeze−pump−thaw
technique was used to degas deuterated solvents as necessary. The
starting material 1,8-dichloroanthraquinone was purchased from MP
sodium carbonate (214 mg, 2.02 mmol), and the flask was taken to the
drybox. Next, [Pd(dba) ] (11.6 mg, 0.0202 mmol) and SPhos (8.3 mg,
2
0
.0202 mmol) were added to the reaction mixture. Dry THF (70 mL)
was added to generate a purple solution. The reaction mixture was
removed from the drybox and placed on a Schlenk line under N , at
which point H O (10 mL) was added to the reaction mixture. The
reaction mixture was heated to 85 °C and refluxed overnight to give an
orange solution. The reaction mixture was cooled to ambient
conditions, quenched with saturated NH Cl (5 mL), extracted with
DCM (70 mL), and washed with saturated brine (2 × 100 mL). The
resulting brown-yellow organic solution was dried over Na SO , and
concentrated in vacuo to afford a brown-yellow oil. The oil was then
dissolved in a minimal amount of CHCl and loaded onto a silica
column packed with a mixture of 7/1 hexanes and ethyl acetate. Pure
product was separated using 4/1 hexanes/ethyl acetate as eluent. The
asymmetric precursor 5, a yellow powder, was obtained in 37% yield
2
2
Biomedicals; [Pd(dba) ] (Strem), XPhos, SPhos, and 3-pyridinylbor-
2
onic acid (Oakwood), (3-(methylthio)phenyl)boronic acid (Syntho-
nix), and tert-nonyl mercaptan (Sigma) were purchased and used
without further purification.
4
Synthesis of Ligands and Synthons. 1,8-Bis(3-(methylthio)-
phenyl)anthracene, Anth-S 2 (3). In a Schlenk flask were placed 1,8-
2
4
Me
dichloroanthracene (500 mg, 2.02 mmol), (3-(methylthio)phenyl)-
boronic acid (678 mg, 4.04 mmol), and anhydrous sodium carbonate
428 mg, 4.04 mmol), and the flask was taken to the drybox. Next,
3
(
[
Pd(dba) ] (34.8 mg, 0.0606 mmol) and XPhos (28.9 mg, 0.0606
2
mmol) were added to the reaction mixture. Dry THF (70 mL) was
added to the reaction mixture to generate a violet solution. The
reaction mixture was removed from the drybox and placed on a
Schlenk line under N , at which point degassed H O (10 mL) was
1
(215 mg). H NMR (400 MHz, CDCl
): δ 7.39 (t, 1H), 7.46 (d, 1H),
3
7.56 (m, 3H), 7.95 (m, 2H), 8.09 (d, 1H), 8.54 (s, 1H), 8.77 (d, 1H),
8.81 (s, 1H), 8.88 (s, 1H). 13C NMR (400 MHz, CDCl
): δ 104.99,
2
2
3
added. The reaction mixture was heated to 85 °C and refluxed
overnight to give a dark purple-red solution. The reaction mixture was
then cooled to ambient conditions and quenched with saturated
121.81, 123.25, 125.35, 125.64, 127.28, 127.49, 127.53, 128.58, 129.21,
130.44, 132.23, 136.16, 136.87, 137.37, 148.96, 150.55. Selected IR
frequencies (solid state, cm ): 3048 (w), 2968 (w), 2860 (w), 1615
−1
NH Cl (5 mL), extracted with DCM (70 mL), and washed with
(m), 1407 (m), 877 (s), 737 (s). MS (CI+): m/z calcd for C H ClN
4
19 12
saturated brine (2 × 100 mL). The resulting brown-yellow organic
(M), 289.0658; observed, 289.0659.
B
DOI: 10.1021/acs.inorgchem.5b02737
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
3
-(8-(3-(Methylthio)phenyl)anthracen-1-yl)pyridine, Anth-NSMe
6). In a Schlenk flask were placed 5 (500 mg, 1.73 mmol), (3-
methylthio)phenyl)boronic acid (290 mg, 1.73 mmol), and
746 (s). Anal. Calcd for C H O S BrMn: C, 58.04; H, 3.46; Found:
C, 56.08; H, 3.35.
31
22
3 2
(
(
[(Anth-N2)Mn(CO) Br] (9). Under drybox conditions, 4 (20 mg,
3
anhydrous sodium carbonate (183 mg, 1.73 mmol), and the flask
0.060 mmol) was dissolved in 10 mL of THF. Separately,
was taken to the drybox. Next, [Pd(dba) ] (29.8 mg, 0.051 mmol) and
[Mn(CO) Br] (16 mg, 0.060 mmol) was dissolved in THF and
2
5
XPhos (24.3 mg, 0.051 mmol) were added to the reaction mixture.
Dry THF (70 mL) was added to generate a purple solution. The
reaction mixture was taken from the drybox and placed on a Schlenk
line under N , where H O (10 mL) was added. The reaction mixture
was heated to 85 °C and refluxed overnight to afford a purple-red
solution. The reaction mixture was cooled to ambient temperature,
added to the ligand solution to generate a dark yellow solution. The
reaction mixture was stirred at room temperature overnight, generating
a cloudy yellow solution. The solvent was removed in vacuo to afford a
yellow powder, which was redissolved into DCM and filtered over a
bed of Celite to remove a flocculent suspension of solids. The yellow
filtrate was concentrated in vacuo to afford an analytically pure yellow
powder of 9. Yield: 65% (22 mg). X-ray-quality yellow needles were
2
2
quenched with saturated NH Cl (5 mL), extracted with DCM (70
4
mL), and washed with saturated brine (2 × 100 mL). The resulting
grown using vapor effusion of a solution of the complex in DCM into
1
brown-yellow organic solution was dried over Na SO , and
n-hexane. H NMR (400 MHz, CDCl
3
): δ 7.59 (b 3H), 8.03 (d, 3H),
2
4
concentrated in vacuo to afford a brown-yellow oil. The oil was
8.28 (s, 1H), 8.59 (d, 3H), 9.10 (s, 2H), 9.51 (s, 1H), 9.80 (s, 2H),
−1
dissolved in a minimal amount of CHCl and loaded on a silica
9.93 (s, 1H). Selected IR frequencies (solid state, cm ): 3068 (w),
3
2
015 (vs, νCO), 1927 (vs, νCO), 1888 (vs, νCO), 1413 (m), 1188 (m),
column packed with a mixture of 7/1 hexanes and ethyl acetate. Pure
product was separated using 1/1 hexanes/ethyl acetate as eluent. The
7
37 (s). Anal. Calcd for C H O N BrMn: C, 58.83; H, 2.93; N, 5.08;
27 16
3
2
Found: C, 58.07; H, 2.91; N, 5.00.
[(Anth-NS ) Mn(CO) Br] (10). Under drybox conditions, 6 (50 mg,
asymmetric ligand 6 was thus obtained as a pale yellow powder. Yield:
1
9
1
1
7% (632 mg). H NMR (400 MHz, CDCl ): δ 2.46 (s, 6H), 7.24 (m,
Me 2
3
3
0
.132 mmol) was dissolved in 10 mL of THF. Separately,
H), 7.39 (m, 6H), 7.55 (m, 2H), 7.82 (dt 1H), 8.07 (d, 1H), 8.08 (d,
H), 8.49 (s, 1H), 8.57 (s, 1H), 8.62 (dd, 1H), 8.72 (s, 1H). 13C NMR
[Mn(CO) Br] (36 mg, 0.132 mmol) was dissolved in THF (10
5
mL) and added to the existing ligand solution, generating a dark
yellow solution. The reaction mixture was stirred overnight with no
color change. The solvent was removed in vacuo, affording a dark
yellow powder. The yellow powder was redissolved in minimal THF,
and loaded onto a short (2 mL pipet) column of neutral alumina. The
desired product was eluted with THF to afford yellow-orange flakes.
For further purification, the flakes were dissolved in fluorobenzene (10
mL) and allowed to stand overnight to produce an orange precipitate
of analytically pure 10. Yield: 32% (41 mg). X-ray-quality yellow
(
400 MHz, CDCl ): δ 15.75, 122.78, 122.98, 125.16, 125.44, 125.51,
3
1
1
1
2
26.39, 126.65, 126.74, 127.13, 127.81, 127.85, 128.53, 128.63, 129.84,
30.15, 131.68, 131.89, 136.06, 136.60, 137.17, 138.45, 139.91, 140.87,
−1
48.60, 150.32. Selected IR frequencies (solid state, cm ): 3057 (w),
955 (w), 1584 (s), 1410 (s), 1325 (s), 1021 (s), 880 (s). MS (CI+):
m/z calcd for C H NS (M), 377.1238; observed, 377.1241.
26
19
3
-(8-(Pyridin-3-yl)anthracen-1-yl)benzenethiol, Anth-NSH (7).
Under drybox conditions NaH (64 mg, 2.65 mmol) was suspended
in 10 mL of DMF. Separately tert-nonyl mercaptan (424 mg, 2.65
mmol) was dissolved in 10 mL of DMF and then mixed with the NaH
solution to produce the sodium thiolate salt and H2 gas.
Approximately 5 min after the mercaptan and NaH had reacted, 6
was suspended in ∼10 mL of DMF and added, generating a pink-
purple solution. The Schlenk flask was then carefully taken from the
needles were grown by layering a solution of the complex in CHCl
3
1
with pentane. H NMR (400 MHz, CDCl ): δ 2.43 (s, 3H), 7.34 (d,
3
2
H), 7.44 (d, 2H), 7.56 (t, 2H), 7.88 (d, 1H), 8.01 (d, 2H), 8.29 (s,
1
H), 8.47 (s, 1H), 8.76 (d, 1H), 8.81 (s, 1H). Selected IR frequencies
−1
(solid state, cm ): 3060 (w), 2948 (w), 2017 (vs, νCO), 1931 (vs,
νCO), 1900 (vs, νCO), 1414 (m), 872 (s), 632 (s). Anal. Calcd for
drybox to the hood and placed under an N atmosphere on a Schlenk
2
C H O N S BrMn: C, 67.83; H, 3.93; N, 2.88; Found: C, 67.86; H,
55
38
3
2 2
line. The reaction mixture was set to reflux at 160 °C overnight. The
resulting red-purple solution was cooled to room temperature and
quenched with 4 equiv of concentrated HCl in ∼50 mL of water. The
reaction mixture was manually stirred for several minutes until a
flocculent yellow precipitant was observed. The precipitate was filtered
3
.89; N, 2.87.
(Anth-NS) Mn (CO) ] (11). Under drybox conditions, 7 (100 mg,
[
3
2
3
0
.275 mmol) was dissolved in 5 mL of THF, generating a pale yellow
solution. Next, KOAc was added as a solid (27 mg, 0.275 mmol) and
stirred with the ligand in solution for approximately 5 min. Finally,
over a frit and air-dried. The resulting yellow powder 7 was thus
isolated in 72% yield (175 mg). H NMR (400 MHz, CDCl ): δ 3.69
[
Mn(CO) Br] (75 mg, 0.275 mmol) was dissolved in 5 mL of THF.
1
5
3
The Mn was then added, producing a darker yellow solution. As the
reaction progressed, the KOAc slowly dissolved, thus facilitating
deprotonation of the thiol. The reaction mixture was stirred at room
temperature for 9 h, generating an orange-yellow solution not like the
starting yellow solution; a copious white precipitate was also observed.
The orange solution was filtered over a bed of Celite, and the solvent
was removed in vacuo, producing an orange powder. The powder was
washed with DCM, eliminating a yellow solution (residual [Mn-
(
8
b 1H), 7.29 (m, 3H), 7.42 (m, 4H), 7.55 (m, 3H), 8.07 (dd, 2H),
.45 (s, 1H), 8.58 (s, 1H), 8.65 (dd, 1H), 8.76 (s, 1H). 13C NMR (400
MHz, CDCl ): δ 123.29, 123.30, 125.56, 125.79, 126.80, 127.13,
3
1
1
1
(
(
3
27.50, 127.73, 128.33, 128.89, 128.95, 129.35, 130.27, 130.38, 131.00,
31.40, 132.07, 132.27, 136.46, 137.01, 137.48, 139.86, 141.55, 148.98,
−1
50.81. Selected IR frequencies (solid state, cm ): 2962 (m), 2917
m), 2871 (m), 1588 (s), 1557 (s), 1310 (s), 1032 (s), 875 (vs), 784
+
vs), 747 (vs). MS (+ESI): m/z calcd for C H NS (M + H) ,
25
17
(CO) Br] and other impurities). The remaining solid was dissolved in
5
64.1154; observed, 364.1159.
Synthesis of Metal Complexes. [(Anth-S 2)Mn(CO) Br] (8).
THF and stirred with 2 equiv of NEt Br, generating another white
4
Me
3
precipitate (KBr). The solution was again filtered over Celite, affording
an orange solution. The solvent was then evacuated, producing an
orange-yellow powder. The yellow-orange material was dissolved in
Under drybox conditions, 3 (50 mg, 0.115 mmol) was dissolved in 10
mL of THF. Separately, [Mn(CO) Br] (32 mg, 0.115 mmol) was
5
dissolved in 10 mL of THF and added to the ligand solution. The dark
yellow solution was stirred at room temperature overnight with no
apparent color change. The solvent was removed under vacuum,
resulting in dark yellow flakes. The flakes were then redissolved in a
minimal amount of THF and loaded onto a short (2 mL pipet)
column of neutral alumina. The complex was eluted with THF to
afford 8 as a yellow powder after evacuation of solvent. Yield: 41% (30
mg). X-ray-quality yellow needles were grown using vapor effusion of a
either THF or MeCN and precipitated by vapor diffusion of Et O (>3
days), thus producing a pale yellow powder (27 mg, yield 20%). X-ray-
2
quality yellow needles were grown using a slow vapor diffusion of Et O
2
into a solution of the complex in MeCN. EPR (MeCN/tol (3/1) glass,
X-band, 85 K): g = 2.16, A = 40 G; g = 2.00, A = 80 G. Selected IR
−1
frequencies (solid state, cm ): 3044 (m), 1985 (vs, νCO), 1903 (vs,
νCO), 1881 (vs, νCO), 1550 (m), 1407 (m), 875 (m), 739 (s). Solid-
state magnetic susceptibility: μeff = 5.96 μ . FD−MS: calcd m/z,
B
1
DCM solution of the complex into n-hexane. H NMR (400 MHz,
1280.1619; observed m/z, 1281.1697.
Physical Measurements. H and 13C NMR spectra were collected
on a Varian DirecDrive 400 MHz spectrometer, and chemical shifts (δ,
ppm) were referenced to the respective solvents. Infrared spectra were
measured under ambient conditions on a Bruker Alpha Fourier
1
CDCl ): δ 3.01 (s, 6H), 7.35 (d, 2H), 7.48 (d, 2H), 7.53 (t, 2H), 7.60
3
(
1
2
t, 2H), 7.75 (d, 2H), 7.81 (s, 2H), 8.10 (d, 2H), 8.45 (s, 1H), 8.62 (s,
H). Selected IR frequencies (solid state, cm ): 3044 (w), 2917 (w),
023 (vs, νCO), 1933 (vs, νCO), 1918 (vs, νCO), 1580 (m) 879 (m),
−1
C
DOI: 10.1021/acs.inorgchem.5b02737
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Scheme 1. Synthetic Scheme for the Preparation of Ligands and Precursors 3−7
a
Reaction conditions: (i) Pd(dba) /Xphos, Na CO , THF/H O (7/1), (3-(methylthio)phenyl)boronic acid (isolated yield 86%); (ii) Pd(dba) /
2
2
3
2
2
Xphos, Na CO , THF/H O (7/1), (pyridin-3-yl)boronic acid (isolated yield 80%); (iii) Pd(dba) /Sphos, Na CO , THF/H O (7/1), (pyridin-3-
2
3
2
2
2
3
2
yl)boronic acid (isolated yield 37%); (iv) Pd(dba) /Xphos, Na CO , THF/H O (7/1), (3-(methylthio)phenyl)boronic acid (isolated yield 97%);
2
2
3
2
(
v) tert-nonyl mercaptan, NaH, DMF (isolated yield 72%).
Scheme 2. Preparation of Manganese Carbonyl Complexes
a
Reaction conditions: (i) THF, [Mn(CO) Br], room temperature, overnight; (ii) THF, [Mn(CO) Br], KOAc, room temperature, 9 h.
5
5
transform infrared (FTIR) spectrometer equipped with a diamond
ATR crystal. Mass spectrometry (MS) data were measured on an
Agilent Technologies 6530 Accurate Mass QTofLC/MS instrument.
EPR spectra were obtained with a Bruker Biospin EMXplus 114 X-
band spectrometer equipped with a liquid nitrogen cryostat. The solid-
state magnetic moments were determined using a Johnson-Matthey
MSB Mk1 magnetic susceptibility balance. Elemental analysis was
performed by Midwest Microlab LLC. Details regarding the X-ray
D
DOI: 10.1021/acs.inorgchem.5b02737
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
diffraction data, refinement, and instrumentation can be found in the
Supporting Information. DFT calculations (Firefly software package)
centers when in electron-deficient environments (minus a CO
ligand) generates more attractive coordination conditions for
the thioether moiety. Complexation of 3 afforded [(Anth-
2
4
for the dimer 11 were performed using the coordinates of the crystal
structure as a starting point, with the 6-31G** basis set and the pure
2
5,26
SMe2)Mn(CO) Br] (8) in good yield. Alternatively, the more
3
functional PW91.
Molecular orbitals were visualized using
2
7
strongly donating bis(pyridine) scaffold 4 resulted in complete
MacMolPlt,
and spin density plots were generated using
28,29
formation of [(Anth-N2)Mn(CO) Br] (9) in both non-
gOpenMol.
3
coordinating and coordinating solvents (DCM and THF,
respectively).
RESULTS AND DISCUSSION
■
The higher affinity of the pyridine donor for the Mn(I)
center is also apparent in the ligation motif of [(Anth-
NS ) Mn(CO) Br)] (10), derived from the asymmetric
Synthetic Rationale. Ligand Construction. The synthesis
of the starting material 1,8-dichloroanthracene (2) was
Me 2
3
performed on the basis of a hybrid of procedures previously
ligand Anth-NSMe (6). Reaction of 6 with manganese carbonyl
bromide was pursued in THF to accommodate the binding
needs of both the pyridine and thioether donor sets. While
3
0,31
published,
and the spectroscopic details can be found in the
Supporting Information. The synthesis of the anthracene type
ligands is summarized in Scheme 1 and was executed through
the Suzuki cross-coupling cycle. The Suzuki cycle is catalyzed
by palladium, and the functionality of the catalyst is dependent
on the selected phosphine ligand. In attempts to generate
1
initial results (IR, H NMR) indicated the ligation of both N
and S donors, further purification by column chromatography,
differential solubility, and crystallization afforded an unexpected
product. Figure 4 shows the structure of 10, wherein the
thioether moieties remained unbound. The unbound thioether
ligands 3 (Anth-S 2) and 4 (Anth-N2), we first utilized
Me
tetrakis(triphenylphosphine)palladium(0), but we observed no
reactivity of the substrates in the presence of this catalyst. On
the basis of literature precedents, we also attempted reactions
19,35
motif has been observed previously for Mn(I) carbonyls.
Further attempts at promoting thioether binding were
performed with trimethylamine N-oxide (TMAO), a common
decarbonylation agent. However, following addition of TMAO,
complex 10 remained as the only isolable product after
purification and crystallization.
using alkylphosphonium salts in the presence of [Pd(dba) ],
2
32
yet still observed no reactivity. The use of XPhos and
Pd(dba) ] had been reported to activate aryl−chloride bonds
[
2
to generate new C−C bonds between aryl halides and aryl or
Free Thiol Ligand and Mn Complexation. To promote
sulfur binding to the Mn(I) center, the Anth-NSMe ligand (6)
33
heterocyclic boronic acids. The XPhos/[Pd(dba) ] system
2
provided positive results with 1,8-dichloroanthracene, and as
such, ligands 3 and 4 were prepared using this system. Efforts to
prepare an asymmetric ligand with the XPhos/[Pd(dba)2]
system demonstrated that symmetric dicoupling was preferred
regardless of catalyst loading or amounts of either boronic acid.
was modified to generate the free thiol ligand Anth-NS (7).
H
Deprotection of the thioether to generate the free thiol was
achieved by reaction with tert-nonyl mercaptan and NaH in
DMF overnight at 160 °C. The subsequent metalation of thiol
7
in THF in the presence of potassium acetate (KOAc) and the
This is presumably due to the well-documented steric
i
Mn(I) carbonyl for 9 h afforded an orange-yellow solution that
contained primarily one product, as suggested by IR spectros-
copy. Reactions for longer timesor in the presence of
hindrance of the pendant Ar( Pr) groups near the metal site,
3
which have been shown to accelerate the final reductive
elimination step. The steric bulk present when this phosphine
NEt OAc in place of KOAcafforded intractable mixtures of
4
ligates promotes a PdL system that is much more active than
1
34
species; reactions for shorter times with KOAc afforded
incomplete complexation of the Mn carbonyl starting salt, as
determined by IR spectroscopy. Thus, following 9 h with
KOAc, the solution was filtered and evaporated, yielding a
yellow powder. The insolubility of this product in DCM or PhF
suggested the formation of a charged complex. As a result, the
yellow powder was washed with DCM to remove residual
other PdL (n = 2, 4) species. Thus, an alternative Pd/
n
phosphine system was required to synthesize a suitable
precursor to an asymmetric ligand.
Further screening proved that SPhos, a less sterically
hindering phosphine, promoted the selective formation of the
monosubstituted intermediate when reacted with 1 equiv of the
(3-pyridinyl)boronic acid substituent. Anth-NCl (5) was thus
[
Mn(CO) Br] and uncomplexed ligand, affording the crude
isolated, in which a single pyridine moiety had coupled, leaving
one chlorine site available for further reactivity. Upon reliable
synthesis of 5, we returned to the established XPhos system in
order to couple the thioether substituent at the remaining
chlorine site. Excess thioether (1.1−1.5 equiv) was used to
ensure complete reaction of the chloro substituent, generating
Anth-NSMe (6) in high yield. Finally, the synthesis of the free
5
−1
solid (ν 2044, 1985, and 1890 cm ). Cation exchange from
K to NEt4 using NEt Br in THF (performed in an N -filled
CO
+
+
4
2
drybox) resulted in a complex that was more soluble in organic
solvent (DCM, PhF, etc.); the complex exhibited a very similar
−1
IR spectrum (νCO 2028, 1985, and 1883 cm ) in the solid
state, consistent with a simple counterion exchange. Sub-
sequent crystallization in either THF or MeCN (Et O vapor
thiol ligand Anth-NS (7) was achieved by deprotection of 6
2
H
diffusion) eliminated the highest energy ν feature (2028
with tert-nonyl mercaptan in DMF (160 °C).
CO
−1
cm ) and generated a new lower (i.e., lowest) energy feature at
1879 cm . Surprisingly, X-ray analysis of the final product
revealed the formula of the resulting neutral complex to be
Complexation with Mn Carbonyl Bromide. The propen-
sities for coordination of the ligands were tested via
complexation with the manganese(I) carbonyl source [Mn-
−1
(
CO) Br]. Reaction conditions can be found in Scheme 2. In
[(Anth-NS)
3
Mn
Mn center (tricarbonyl fragment) and one oxidized Mn
center (with an N donor set). On the basis of the IR data,
it is evident that while the K →NEt
2 3
(CO) ] (11; vide infra), bearing one formally
5
I
II
the case of the bis(thioether) ligand 3, complexation in
noncoordinating solvents such as DCM proved unsuccessful.
However, reactions in THF resulted in ligation of the Anth-S2
scaffold to the metal center, presumably by facilitating the loss
of CO ligands promoted by the formation of intermediate THF
S
3 3
+
+
exchange did not alter
4
the composition of the initial product, extended incubation in
MeCN or THF during crystallization did result in an autoredox
1
9
I
adducts. The weakly coordinating nature of THF to metal
process wherein one of the Mn centers reduced solvent (or
E
DOI: 10.1021/acs.inorgchem.5b02737
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
I
II
other available moiety) to generate the mixed-valent Mn /Mn
final product.
Structural Characteristics. Ligand X-ray Structures. The
crystal structures of Anth-S 2 (3) and Anth-N2 (4) (see
Me
Figures S2 and S3 in the Supporting Information, respectively)
reflect their interesting 3D nature. Due to steric bulk the
pyridine and aryl thioether moieties are not coplanar with the
anthracene backbone. The torsion angles of the phenyl
thioether moieties with respect to the anthracene scaffold are
1
14 and 123°, while torsion angles of the pyridine moieties are
6
2 and 64°. Additionally, the meta linkage of the aryl units
points the N2 and S 2 donors away from the scaffold,
Me
maximizing the available space for binding a metal carbonyl
fragment. In both cases, the donor atoms are positioned on the
same side (i.e., “up−up”, not “up−down”) of the scaffold, thus
facilitating the binding of the ligand to a single metal center.
Indeed, the N atoms of the pyridine moieties of 4 appear ideally
arranged for metal binding (tilted inward), whereas the S atoms
of the thioether moieties in 3 (tilted outward) must overcome
some steric hindrance to approach the metal site in a cis
fashion. Despite these structural outcomes, it is true that we
cannot be certain of the orientation of the donors in the bulk
Figure 3. ORTEP diagram (30% ellipsoids) of [(Anth-N2)Mn-
(
CO) Br] (9) with the labeling scheme. H atoms are omitted for
3
clarity.
cis-binding motif, each Mn center binds the three CO ligands in
a facial orientation, with the orthogonal face composed of the
two ligand donors and halide. Interestingly, in the case of 8, the
metal core buckles underneath the ligand, somewhat occupying
the vacancy between the two aryl linkers. This is attributable to
(solid) material or of the dynamic conformational flexibility in
solution during complexation. X-ray-quality crystals for the
unbound asymmetric ligands were not obtained despite many
attempts. However, the asymmetric ligands Anth-NSMe (6) and
Anth-NSH (7) can be seen in the crystal structures of the
complexes [(Anth-NS ) Mn(CO) Br] (10) and [(Anth-
the steric influence effect of the thioether CH groupsboth
3
directed “upward” in 8. The Mn−S bond distances of
2
.3903(13) and 2.3850(13) Å are slightly longer than those
Me
2
3
reported for previous Mn(I) thioether carbonyls, such as
2
NS) Mn (CO) ] (11) (Figures 4 and 5), the details of which
3
2
3
19
36
.3001(8) and 2.362(2) Å. A decrease in the torsion angle
are discussed below.
Structures of Metal Complexes. The structures of
associated with the Mn complex is also observed, as the free
rotation is restricted by the Mn center. Angles decrease from
complexes [(Anth-S 2)Mn(CO) Br] (8) and [(Anth-N2)-
Me
3
∼
115° in the free ligand to 57° in the metal complex. The
Mn(CO) Br] (9) (Figures 2 and 3, respectively) both exhibit
3
remaining distances of interest may be found in Table 1.
Table 1. Selected Bond Distances (Å), Bond Angles (deg),
and Torsion Angles (deg) for [(Anth-S 2)Mn(CO) Br]
Me
3
(
8), [(Anth-N2)Mn(CO) Br] (9), and [(Anth-
3
a
NS ) Mn(CO) Br] (10)
Me 2
3
8
9
10
Mn−N1
Mn−N2
Mn−S1
Mn−S2
Mn−C(O)
2.135(3)
2.139(3)
2.102(4)
2.093(4)
2.3903(13)
2.3850(13)
1.800(5)
1.818(4)
1.837(4)
1.800(4)
90.38(10)
1.917(7)
1.813(5)
1.816(5)
84.86(15)
1
1
.817(5)
.800(5)
N −Mn−N2
1
S1−Mn−S2
Mn−Br
torsion
83.63(5)
2.5069(8)
57.6(6)
2.5217(6)
45.8(4)
2.5329(9)
74.4(6) (py)
Figure 2. ORTEP diagram (30% ellipsoids) of [(Anth-SMe2)Mn-
1
14.5(5) (SMe
)
(
CO) Br] (8) with the labeling scheme. H atoms are omitted for
3
a
clarity.
Torsion angles were measured using the ortho and peri carbons of
the anthracene and respective substituents.
ligation of the bidentate ligand in a cis fashion to a
{
Mn(CO) Br} fragment. The bite angle between the
Similar to the case for 8, complex 9 exhibits a cis chelation
3
thioether-S donors in 8 is 83.63°, contributing to the pseudo-
octahedral coordination environment. Analogously, the bite
angle between the pyridine-N donors in 9 is 90.38°,
contributing to a nearly ideal octahedral shape. Both cases
demonstrate that the combination of ligand flexibility and
donor spacing promotes facile metal binding. As a result of the
motif of the bis(pyridine) ligand 4 to the {Mn(CO) Br}
3
fragment. As a result, the facial arrangement of CO ligands is
analogous to that in 8, and the Mn−C and Mn−Br bond
distances are quite similar (Table 1). The pyridine arms in 8 are
twisted from planarity with respect to the anthracene plane
(torsion angle 45.8(4)°). Upon inspection of the torsion angles
F
DOI: 10.1021/acs.inorgchem.5b02737
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
in the unbound ligand 4 (torsion angle 62.0(3)°), it is apparent
that the presence of the Mn center in 8 is responsible for the
further twisting of the pyridine moieties toward the anthracene
plane. As a result, in the presence of the Mn ion, the proximal
ortho C−H units at the “top” of the pyridine are in close
py
contact in complex 8 (H···H = 2.19 Å), versus the much longer
H···H contact of 3.54 Å in unbound ligand 4. It is notable that
both values are still greater than the sum of the van der Waals
radii of the H atoms (2.18 Å), suggesting at least a minimal
extent of steric clash in the case of the Mn complex.
While we were unable to grow crystals of the asymmetric
ligand 6 (Anth-NSMe), we did obtain crystals of its
corresponding metal complex, [(Anth-NS ) Mn(CO) Br]
Me 2
3
(10), depicted in Figure 4. The most notable feature of the
Figure 4. ORTEP diagram (30% ellipsoids) of [(Anth-NSMe)2Mn-
CO) Br] (10) with the labeling scheme. H atoms are omitted for
(
3
clarity.
complex is that two ligands bind a single manganese center via
their nitrogen donors. The thioether moiety on each ligand
remained unbound, at an approximate distance of ∼8.43 Å
from the metal center. The higher affinity of pyridine-N versus
the thioether-S for the Mn(I) center is thus suggested by this
structure, as described in Synthetic Rationale. Interestingly, the
Figure 5. Three views of the ORTEP diagram (30% ellipsoids) of
[(Anth-NS) Mn (CO) ] (11) with the labeling scheme. The “axial”
3 2 3
view (middle view) shows the “asymmetric” orientation of the
anthracene moieties about the dimer “core”. The “core” view (bottom
view) eliminates the anthracene scaffolds and is depicted for visual
clarity. In the top view, H atoms are omitted for clarity, while the
bottom view excludes all ligand-based C and H atoms. Note: in the
bottom view N /S , N /S , and N /S are the chelate pairings.
Mn−N bonds in 10 (2.102(4), 2.093(4) Å) are slightly
py
shorter than the Mn−N bonds in 9 (2.135(3), 2.139(3) Å),
py
suggesting that the steric effects of the bis(pyridine) chelate 4
prevent complete encroachment of the Npy donors in 9 to the
metal center. Additionally, despite the fact that there are two
separate ligands (no chelate effect), and considering the steric
implications, the pyridine moieties are still bound close to each
other in a cis fashion. This is likely attributable to the
propensity of the manganese tricarbonyl moiety to retain its
1
1
2
2
3
3
center that retains its CO ligands; the Mn(I) and Mn(II)
centers are bridged by the three μ -S thiolate bridges. Further
2
I
II
evidence for the Mn −Mn dimer is provided in Spectroscopy,
and selected bond distances and angles may be found in Table
2. Overall, the ligation motif exhibited by complex 11
demonstrates a successful attempt to bis-ligate a metal center
with an asymmetric anthracene-based scaffold in a cis
orientation. However, further work will be required to extend
the series of mononuclear complexes by sterically preventing
dimerization across the thiolato-S donor(s) and adjacent
metals.
̀
“piano stool” configuration, vis a vis the preference of strongly
back-bonding CO ligands to bind trans to stronger σ donors.
The lack of binding between the thioether-S and Mn(I)
center prompted us to explore the ligation properties of the
mixed pyridine/thiolate ligand Anth-NSH (7). The resulting
structure of [(Anth-NS) Mn (CO) ] (11) is shown in Figure 5,
3
2
3
which reveals two manganese centers ligated by three pyridine/
thiolate chelates. During the synthesis, isolation, and crystal-
lization of the complex, the presence of the more nucleophilic
thiolate had two primary effects: (i) the intended effect of
promoting both N and S binding and (ii) the unintended effect
of promoting a redox event at one of the manganese centers
Spectroscopy. Infrared Spectra. Figure 6 depicts the
infrared spectrum obtained for each Mn complex. Considering
only the donor atoms ligated directly to the metal center, the
complexes [(Anth-S 2)Mn(CO) Br] (8), [(Anth-N2)Mn-
Me
3
(
despite isolation/crystallization under an inert atmosphere).
(CO) Br] (9), and [(Anth-NS ) Mn(CO) Br] (10) have
3 Me 2 3
The result is that one metal center is oxidized to a Mn(II) that
binds all three Npy donors, while the other remains a Mn(I)
approximate C symmetry. Evaluation of the appropriate
s
character tables predicts each complex should exhibit three
G
DOI: 10.1021/acs.inorgchem.5b02737
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 2. Selected Bond Distances (Å) and Bond Angles
associated with a CO trans to the halide. The other two
stretches seen in each complex are observed as two sharp
(
deg) for [(Anth-NS) Mn (CO) ] (11) and Calculated Bond
3 2 3
5
−1
Distances and Bond Angles from the S = / DFT-Optimized
features in close proximity in the vicinity of 1910 ± 20 cm .
2
Structure
On the other hand, [(Anth-NS) Mn (CO) ] (11) has
3
2
3
approximate C3v symmetry (again considering only the
symmetry of the metal centers plus donor atoms). In this
case, the group theory evaluation suggests that only two
stretches should be visible in the IR spectra. However, the
spectrum of a crystalline sample of 11 reveals three carbonyl
1
1
calcd
1
1
1
1
Mn −N1
2.269(10)
2.310(11)
2.282(8)
2.600(4)
2.651(3)
2.610(3)
2.426(3)
2.411(4)
2.403(3)
1.768(12)
2.321
2.285
2.329
2.553
2.601
2.556
2.469
2.450
2.448
1.767
1.765
1.761
98.89
103.17
103.21
Mn −N2
Mn −N3
Mn −S1
−1
stretches overall occurring at 1985, 1905, and 1881 cm . First,
Mn −S2
−1
1
it is notable that the highest ν(CO) value in 11 (1985 cm ) is
Mn −S3
1
lower than the highest ν(CO) values in 8−10. This is
attributable to the stronger σ-donor strength of the three
Mn −S1
2
Mn −S2
2
thiolato-S donors bound in a position trans to the fac-
Mn −S3
+
2
{
Mn(CO)} fragment. However, as described in Structural
Mn −CO
2
Characteristics, the three thiolato-S donors are not crystallo-
1
1
.777(15)
.771(12)
I
II
graphically equivalent. As noted above, the mixed Mn −Mn
oxidation state system incurs a disparity in Mn −S bond
1
x
N −Mn −S
N −Mn −S
N −Mn −S
103.2(3)
98.9(2)
1
1
1
2
3
distances into two sets: two short bonds and one long bond.
The IR data suggest that the inequivalent Mn−S bonding
motifs lower the predicted symmetry of the molecule from C
2
1
103.2(2)
3
1
3v
to C , thus affording similar absorption spectra to complexes 8−
s
10. Additionally, it is notable that the crystalline solid, powder,
and solution IR spectra of 11 (Figure S21 in the Supporting
Information) all exhibit the higher energy feature (1990, 1985,
−
1
1
985 cm , respectively) and the lower energy feature(s) (1903
−1
−1
−1
and 1878 cm , sharp; 1892 cm , broad; 1899 cm , broad;
respectively), strongly suggesting that the structure observed in
the crystal structure is retained in solution. Indeed, 11 can be
dissolved in noncoordinating solvent (CH Cl , PhF) or
2
2
coordinating solvent (THF) and reisolated as the pure complex
by either solvent evaporation or recrystallization; no loss of CO
or other change in composition is observed. Furthermore, the
FD-MS (gentle ionization method) of 11 reveals a peak (M +
H) at m/z 1281.1697 (Figure S22 in the Supporting
Information), consistent with the calculated mass of m/z
1
1
280.1619. There is no evidence to suggest that the structure of
1 in solution is different from that in the crystal.
Electron Paramagnetic Resonance (EPR) and Magnetism.
Regarding the thiolate-based complex [(Anth-NS) Mn (CO) ]
3
2
3
1
(
11), the charge-balance, lack of observable features in the H
NMR spectrum, and differential bond metrics led us to
hypothesize the presence of a paramagnetic Mn(II) center in
the complex. As the structure clearly indicates discrete
{
Mn(CO) } and {Mn(N3)(S3)} moieties, it was reasonable
3
to hypothesize a mixed-valence dimer containing the well-
known Mn(I) tricarbonyl “piano stool” moiety and a distinct
(
and localized) Mn(II) center bound by the remaining
heteroatoms. As such, we pursued EPR studies of the
complexboth in the solid state and in solution.
The solid-state EPR spectrum of polycrystalline 11 over a
range of temperatures (288−88 K) is shown in Figure 7. Unlike
the case for the solution phase, the primary features can be
observed at both room temperature and at 88 K, although the
Figure 6. Infrared spectra of 8 (bottom green trace), 9 (middle blue
trace), 10 (middle red trace), and 11 (top black trace) showing
carbonyl stretches associated with each complex.
55
Mn hyperfine features are not resolved at any temperature.
The spectrum exhibits a primary feature at g = 2.02, which itself
could be consistent with either an S = 1/2 or an S = 5/2
37,38
carbonyl stretches. As seen in Figure 8, indeed all three exhibit
system.
However, the four additional features at both lower
such properties. Complex 8 exhibits CO stretches at 2023,
fields (g = 5.56, 2.86) and higher fields (g = 1.50, 1.22) are
inconsistent with an S = 1/2 system but consistent with an S =
5/2 Kramers manifold. To provide further evidence for the S =
5/2 configuration for the Mn(II) ion in 11, we pursued
−1
1
1
2
933, and 1918 cm , complex 9 exhibits CO stretches at 2015,
−1
927, and 1888 cm , and complex 10 exhibits CO stretches at
−1
017, 1931, and 1900 cm . Each of these complexes has a CO
−
1
39
stretch above 2000 cm , which could be most closely
simulations of the data (EasySpin; CW solid state module).
H
DOI: 10.1021/acs.inorgchem.5b02737
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
−1
(
0.1−1 cm ) are only associated with lower coordination
numbers and/or the presence of halides. The best-fit E/|D| ratio
of 0.30 in the simulation was indicative of a predominantly
rhombic system. Indeed, despite the seemingly symmetric N3S3
donor set (apparently axial coordination environment), closer
inspection of the bond distances (Table 2) coincide with the
rhombic interpretation. In the structure of 11 (Figure 5), the
display of supporting anthracene moieties is not in the
arrangement of a C “propeller”: while two anthracene units
3
are aligned in counterclockwise fashion, the third orients in a
clockwise orientation. This arrangement of anthracene moieties
translates into the first coordination sphere, wherein there are
two similar Mn−N bonds (2.269(10), 2.282(8) Å) and one
distinct Mn−N bond (2.310(11) Å); the same trend is
observed for the Mn−S bonds (2.600(4) and 2.610(3) Å
versus 2.651(3) Å). Thus, the local asymmetry of the N S
coordination sphere appears sufficient to induce rhombic
character in the system.
Solutions of 11 (3/1 MeCN/toluene) at room temperature
afforded no observable features; however, frozen glasses at 85 K
afforded the spectrum shown in Figure 9. The primary feature
Figure 7. Solid-state powder EPR spectra of [(Anth-NS) Mn (CO) ]
3
2
3
(
11) acquired in the range of 88−288 K, with prominent features
3
3
labeled with the associated g values. Instrument parameters: ν = 9.43
GHz, modulation frequency 100 kHz, modulation amplitude 8 G,
microwave power 2 mW.
As shown in Figure 8, an excellent agreement between
simulation (top) and experiment (bottom, 88 K) was obtained
Figure 9. EPR spectrum of [(Anth-NS) Mn (CO) ] (11) obtained at
3
2
3
Figure 8. Simulated (top) and experimental (bottom, 88 K) solid-state
EPR spectra for [(Anth-NS) Mn (CO) ] (11). Prominent features are
8
5 K in a frozen glass of 3/1 MeCN/toluene. Inset: expanded view of
3
2
3
the primary feature in which the hyperfine coupling is observed.
Instrument parameters: T = 85 K, ν = 9.43 GHz, modulation
frequency 100 kHz, modulation amplitude 8 G, microwave power 2
mW.
labeled with the associated g-values. Simulation parameters: S = 5/2;
−
1
55
E/|D| = 0.3; |D| = 0.053 cm ; isotropic Mn hyperfine Aiso = −150
MHz; line broadening (Lorentzian/Gaussian) 32/16.
using a g tensor of 2.0. High-spin Mn(II) (or any high-spin d5)
complexes exhibit spherical electron density about the metal
center and therefore generally have small Zeeman interactions;
of the broad spectrum is centered near g ≈ 2.00.
37,38
In this
solution spectrum, the metal-centered nature of the unpaired
electron is apparent in the two sets of hyperfine features due to
the I = 5/2 Mn nucleus. More specifically, there is one set of
40
55
this leads to g values near 2.0. Indeed, DFT spin density
calculations (Figure 10) reveal the expected spherical spin
density around the Mn center.
hyperfine features near g = 2.1 (40 G), as well as a distinct set
of broader hyperfine features near g = 2.0 (80 G). Such nuclear
hyperfine is not observed in the solid-state sample due to the
close proximity of Mn centers in the solid state (faster
relaxation times) in comparison to the dilute solution (1 mM).
It is also notable that the minor features observed in the solid-
state spectrum at higher field (g = 5.56, 2.86) are somewhat
masked by the broadness of the solution spectrum. Importantly,
the simulated EPR spectrum (Figure S23 in the Supporting
2
+
Regarding the zero field splitting (ZFS), the simulated EPR
−1
spectrum utilized |D| = 0.053 cm and E/|D| = 0.30. As
40
discussed in a review by Neese and co-workers, octahedral
Mn(II) complexes without halides exhibit very small ZFS
−1
parameters (|D| ≈ 0.01−0.1 cm ). Although thiolate donors
were not considered in that review, the bridging nature of the
thiolates in 11, coupled with their covalent character, renders
them more akin to oxygen donors than halides. Indeed, the
majority of six-coordinate Mn(II) complexes with mixed N/O
−4
−1
Information) using Aiso = −150 MHz (−50 × 10 cm ) along
with drastically diminished line broadening exhibited inde-
pendent sets of hyperfine features near g = 2.1 (45 G) and g =
−1
ligation exhibit |D| ≈ 0.01−0.1 cm . Much higher |D| values
I
DOI: 10.1021/acs.inorgchem.5b02737
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 10. DFT calculated energies of the S = 5/2, 3/2, and 1/2 configurations using the X-ray coordinates of 11 (6-31G**/PW91). The highest
energy SOMO in each case is shown above its respective configuration.
4
−1
2
.0 (60 G). Such an Aiso value (−50 × 10− cm ) is consistent
To confirm that the spin density was indeed localized on the
with those compiled by Neese and co-workers (|A | ≈ (68−
formally manganese(II) {Mn(N )(S )} unit, a spin density plot
iso
3
3
4
−1 40
1
00) × 10− cm ). Overall, these datacoupled with
was generated from the DFT calculations (Figure 11). The
observations that the IR spectrum in solution closely matches
that of the solid-state IR (vide supra)suggests that the
connectivity of 11 determined by X-ray diffraction is largely
retained in solution.
The solid-state magnetic properties of 11 were also
investigated. The magnetic susceptibility of 11 at 298 K was
determined to be μeff = 5.96 μ , quite close to the predicted
B
spin-only value for an S = 5/2 system (μSO = 5.92). Thus, the
consistent EPR behavior in both solution and the solid state, as
well as the solid state μ value, suggests that Mn(II) ion in the
eff
dimer exists in all cases in the S = 5/2 configuration. This
conclusion was further supported by DFT calculations (next
section).
Density Functional Theory (DFT). The dimeric structure
of [(Anth-NS) Mn (CO) ] (11) was examined by DFT
3
2
3
calculations to determine the lowest energy spin multiplicity
of the complex (Figure 10). To this end, spin multiplicity
calculations were performed on the unoptimized X-ray
coordinates of 11, under the presumption of S = 1/2, 3/2,
and 5/2 configurations. Although the S = 3/2 calculation did
not converge as readily S = 1/2 and S = 5/2 calculations, the
high-spin S = 5/2 configuration was found to be roughly 25.1
kcal/mol more stable than the lower spin states. This result is
consistent with the experimentally determined magnetic
Figure 11. DFT calculated spin density plot for the geometry
optimized structure of 11 (6-31G**/PW91).
strong localization of the spin on the Mn(II) center is isotropic,
as a result of the evenly distributed spin across the five d
orbitals. We also analyzed the DFT calculated nuclear charges,
which were found to be quite distinct for the two Mn centers.
Both the calculated Mulliken charges (0.825 versus 0.274) and
Lowdin charges (0.164 versus −0.527) clearly indicate discrete
oxidation states for the {Mn(N )(S )} ion and the {Mn(CO) }
susceptibility value of 5.96 μ . After this unequivocal result
B
was obtained, optimization of the coordinates in the S = 5/2
case afforded very good to excellent agreement with the bond
distances and angles derived from X-ray analysis (see Table 2).
Indeed, closer inspection of the bond distances surrounding the
3
3
3
ion, respectively. On the basis of all of the DFT results, we can
conclude the dimer 11 is a mixed-valence system best described
as a class I system, where there is no delocalization of spin or
charge, and also no observable IVCT band.
{
Mn(N )(S )} unit reveal bond metricsespecially the Mn−N
3
3
bondsmore closely associated with high-spin versus low-spin
systems. The Mn−N bond lengths of 2.269(10), 2.310(11),
and 2.282(8) Å found in the dimer 11 are similar to those in
other reported high-spin systems (e.g., Mn−N = 2.2499(16)
Å) and are much longer than those in reported low-spin
systems (e.g., Mn−N = 2.003(2) Å).
CONCLUSION
■
We have demonstrated the design and synthesis of anthracene
ligand scaffolds bearing meta-linked aryl moieties for metal
binding. Previous work by others described trans ligation using
anthracene scaffolds with para-linked aryl donors, but by
41
42
J
DOI: 10.1021/acs.inorgchem.5b02737
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
shifting the donors to the meta positon, the cis-ligation motif
was favored. We also report for the first time a synthetic
method for the synthesis of asymmetric anthracene scaffolds.
The cis ligation by both symmetric and asymmetric ligands was
observed in both monometallic and bimetallic complexes, the
latter of which was supported by a mixed pyridine/thiolate
ligand. The mixed pyridine/thiolate ligand afforded the mixed-
valence dimer 11, which includes a localized high-spin Mn(II)
ion as determined by solution EPR, solid EPR, magnetic
moments, and DFT calculations. In future endeavors, similar
anthracene chelators will be implemented with iron(II)
carbonyl species to pursue facially coordinating, small-molecule
mimics of mono-[Fe] hydrogenasean ongoing topic of
research in our laboratory. Additional studies will be performed
to discern if the anthracene scaffold approach can achieve both
structural and functional modeling of this natural organo-
metallic complex.
(10) Kranenburg, M.; van der Burgt, Y.; Kamer, P.; van Leeuwen, P.
W. Organometallics 1995, 14, 3081−3089.
(11) Kamer, P. C. J.; Van Leeuwen, P. W. N. M.; Reek, J. N. H. Acc.
Chem. Res. 2001, 34, 895−904.
12) Hamann, B. C.; Hartwig, J. F. J. Am. Chem. Soc. 1998, 120,
694−3703.
13) Haenel, M. W.; Jakubik, D.; Rothenberger, E.; Schroth, G.
Chem. Ber. 1991, 124, 1705−1710.
14) Ding, K.; Miller, D. L.; Young, V. G.; Lu, C. C. Inorg. Chem.
011, 50, 2545−2552.
15) Jimenez-Rodríguez, C.; Roca, F. X.; Bo, C.; Benet-Buchholz, J.;
Escudero-Adan, E. C.; Freixa, Z.; van Leeuwen, P. W. N. M. Dalton
(
3
(
(
2
(
́
́
Trans. 2006, 268−278.
(16) Fairlamb, I. J. S.; Grant, S.; Tommasi, S.; Lynam, J. M.; Bandini,
M.; Hao, D.; Zhenyang, L.; Whitwood, A. C. Adv. Synth. Catal. 2006,
348, 2515−2530.
(17) Lamm, J. H.; Vishnevskiy, Y. V.; Ziemann, E.; Kinder, T. a.;
Neumann, B.; Stammler, H. G.; Mitzel, N. W. Eur. J. Inorg. Chem.
014, 2014, 941−947.
18) Smith, R. C.; Protasiewicz, J. D. Organometallics 2004, 23,
215−4222.
(19) Lumsden, S. E.; Durgaprasad, G.; Thomas Muthiah, K.; Rose,
2
(
4
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.5b02737.
M. J. Dalton Trans. 2014, 43, 10725−10738.
(20) Rimmer, R. D.; Pierri, A. E.; Ford, P. C. Coord. Chem. Rev. 2012,
2
56, 1509−1519.
21) Hartl, F.; Rosa, P.; Ricard, L.; Le Floch, P.; Zal
Chem. Rev. 2007, 251, 557−576.
22) Deeming, A. J.; Karim, M.; Bates, P. A.; Hursthouse, M. B.
Polyhedron 1988, 7, 1401−1403.
23) Begum, N.; Hyder, M. I.; Kabir, S. E.; Hossain, G. M. G.;
Nordlander, E.; Rokhsana, D.; Rosenberg, E. Inorg. Chem. 2005, 44,
(
́ ̌
is, S. Coord.
Crystallographic data (CIF)
Complete X-ray data tables, H and 13C NMR spectra,
and mass spectral characterizations of ligands and
complexes (PDF)
1
(
(
9
887−9894.
AUTHOR INFORMATION
■
(24) Granovsky, A.. http://classic.chem.msu.su/gran/gamess/index.
Corresponding Author
html.
(25) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.;
*E-mail for M.J.R.: mrose@cm.utexas.edu.
Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B: Condens. Matter
Mater. Phys. 1993, 48, 4978.
Notes
The authors declare no competing financial interest.
(26) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.;
Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B: Condens. Matter
Mater. Phys. 1992, 46, 6671−6687.
ACKNOWLEDGMENTS
■
(27) Bode, B. M.; Gordon, M. S. J. Mol. Graphics Modell. 1998, 16,
This research was supported by the Robert A. Welch
Foundation (F-1822), and the ACS Petroleum Research
Fund (53542-DN13). The authors thank Dr. Vince Lynch for
assistance with X-ray data collection and the refinement of
structures, and we thank Dr. Kory Mueller for aiding in the
collection of EPR spectra. We also acknowledge Zhu-Lin “Sam”
Xie for assistance in performing DFT calculations.
1
(
(
33−138.
28) Laaksonen, L. J. Mol. Graphics 1992, 10, 33−34.
29) Bergman, D. L.; Laaksonen, L.; Laaksonen, A. J. Mol. Graphics
Modell. 1997, 15, 301−306.
30) Goichi, M.; Segawa, K.; Suzuki, S.; Toyota, S. Synthesis 2005,
116−2118.
31) Perez-Trujillo, M.; Maestre, I.; Jaime, C.; Alvarez-Larena, A.;
(
2
(
́
Piniella, J. F.; Virgili, A. Tetrahedron: Asymmetry 2005, 16, 3084−3093.
REFERENCES
(32) Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 2719−2724.
■
(33) Martin, R.; Buchwald, S. L. Acc. Chem. Res. 2008, 41, 1461−
(
1) Birkholz, M.-N.; Freixa, Z.; van Leeuwen, P. W. N. M. Chem. Soc.
Rev. 2009, 38, 1099−1118.
1
473.
34) Christmann, U.; Vilar, R. Angew. Chem., Int. Ed. 2005, 44, 366−
74.
35) Gonzalez, M. A.; Carrington, S. J.; Fry, N. L.; Martinez, J. L.;
Mascharak, P. K. Inorg. Chem. 2012, 51, 11930−11940.
36) Connolly, J.; Goodban, G. W.; Reid, G.; Slawin, A. M. Z. J.
Chem. Soc., Dalton Trans. 1998, 2225−2232.
37) Pike, R. D.; Rieger, A. L.; Rieger, P. H. J. Chem. Soc., Faraday
Trans. 1 1989, 85, 3913−3925.
38) Tao, L.; Stich, T. A.; Butterfield, C. N.; Romano, C. A.; Spiro, T.
(
3
(
(
2
(
1
2) Freixa, Z.; van Leeuwen, P. W. N. M. Coord. Chem. Rev. 2008,
52, 1755−1786.
3) Freixa, Z.; van Leeuwen, P. W. N. M. Dalt. Trans. 2003, 1890−
901.
4) Marlier, E. E.; Tereniak, S. J.; Ding, K.; Mulliken, J. E.; Lu, C. C.
Inorg. Chem. 2011, 50, 9290−9299.
5) Elsevier, C. J.; Reedijk, J.; Walton, P. H.; Ward, M. D. Dalton
Trans. 2003, 1869−1880.
6) van Leeuwen, P. W. N. M.; Kamer, P. C. J.; Reek, J. N. H. Pure
Appl. Chem. 1999, 71, 1443−1452.
7) Kaganovsky, L.; Cho, K.; Gelman, D. Organometallics 2008, 27,
139−5145.
8) Bessel, C. A.; Aggarwal, P.; Marschilok, A. C.; Takeuchi, K. J.
Chem. Rev. 2001, 101, 1031−1066.
9) Kaganovsky, L.; Gelman, D.; Rueck-Braun, K. J. Organomet.
Chem. 2010, 695, 260−266.
(
(
(
(
(
(
G.; Tebo, B. M.; Casey, W. H.; Britt, R. D. J. Am. Chem. Soc. 2015,
137, 10563−10575.
(
5
(
(39) Stoll, S.; Schweiger, A. J. Magn. Reson. 2006, 178, 42−55.
(40) Duboc, C.; Collomb, M. N.; Neese, F. Appl. Magn. Reson. 2010,
37, 229−245.
(
(41) Walesa-Chorab, M.; Kubicki, M.; Korabik, M.; Patroniak, V.
Dalt. Trans 2013, 42, 9746−9754.
K
DOI: 10.1021/acs.inorgchem.5b02737
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
42) Saha, A.; Majumdar, P.; Goswami, S. J. Chem. Soc. Dalt. Trans.
2
000, 1703−1708.
L
DOI: 10.1021/acs.inorgchem.5b02737
Inorg. Chem. XXXX, XXX, XXX−XXX