Synthesis and electronic structure of reduced bis(imino)pyridine manganese compounds

Article abstract of DOI:10.1002/ejic.201100569

The synthesis and electronic structure of reduced aryl-substituted bis(imino)pyridine manganese compounds have been explored. Stirring a THF slurry of [(^iPrPDI)MnCl₂] {^iPrPDI = 2,6-(2,6-iPr ₂-C₆H₃N=CMe)₂C₅H ₃N} with excess Na and catalytic (0.5 mol-%) naphthalene furnished the bis(THF) compound [(^iPrPDI)Mn(THF)₂]. Performing the reduction with excess Na(Hg) in toluene furnished the bis(chelate) manganese compound [(^iPrPDI)₂Mn]. For both compounds, a combination of EPR spectroscopy, magnetic measurements and metrical parameters determined from X-ray diffraction established high-spin Mn^II compounds with reduced, redox-active bis(imino)pyridine ligands. Substitution of the THF ligands with carbon monoxide yielded [(^iPrPDI)Mn(CO)₂], a low-spin Mn^I, d⁶ compound with an experimentally observed bis(imino)pyridine-centred radical. Oxidation and reduction of this compound furnished [(^iPrPDI)Mn(CO)₃]⁺ and [( ^iPrPDI)Mn(CO)₂]^-, respectively, and provided a series of three manganese carbonyl compounds over three oxidation states. Elucidation of the electronic structure of these compounds established that oxidation events within the series are ligand-rather than manganese-based, most likely a result of the stable low-spin Mn^I, d⁶ electron configuration imparted by the strong-field carbonyl ligands. Copyright

Full text of DOI:10.1002/ejic.201100569

FULL PAPER
DOI: 10.1002/ejic.201100569
Synthesis and Electronic Structure of Reduced Bis(imino)pyridine Manganese
Compounds
Sarah K. Russell,[a] Amanda C. Bowman,[b] Emil Lobkovsky,[c] Karl Wieghardt,[b] and
Paul J. Chirik*[a]
Keywords: Manganese / Catalysts / Chelates / Redox chemistry / Non-innocent ligands
The synthesis and electronic structure of reduced aryl-substi-
tuted bis(imino)pyridine manganese compounds have been
explored. Stirring a THF slurry of [(iPrPDI)MnCl2] {iPrPDI =
2,6-(2,6-iPr2–C6H3N=CMe)2C5H3N} with excess Na and cata-
carbon monoxide yielded [(iPrPDI)Mn(CO)2], a low-spin MnI,
d6 compound with an experimentally observed bis(imino)-
pyridine-centred radical. Oxidation and reduction of this
compound furnished [(iPrPDI)Mn(CO)3]+ and [(iPrPDI)Mn-
lytic (0.5 mol-%) naphthalene furnished the bis(THF) com- (CO)2]–, respectively, and provided a series of three manga-
pound [(iPrPDI)Mn(THF)2]. Performing the reduction with ex-
cess Na(Hg) in toluene furnished the bis(chelate) manganese
compound [(iPrPDI)2Mn]. For both compounds, a combination
of EPR spectroscopy, magnetic measurements and metrical
parameters determined from X-ray diffraction established
high-spin MnII compounds with reduced, redox-active bis-
(imino)pyridine ligands. Substitution of the THF ligands with
nese carbonyl compounds over three oxidation states. Eluci-
dation of the electronic structure of these compounds estab-
lished that oxidation events within the series are ligand-
rather than manganese-based, most likely a result of the
stable low-spin MnI, d6 electron configuration imparted by
the strong-field carbonyl ligands.
Early investigations into the electronic structure of bis-
(imino)pyridine manganese compounds were pioneered by
Wieghardt and coworkers.[8] A one-electron reduction of
the high-spin, MnII bis(chelate) complex [(4–OMePDI)2Mn]-
Introduction
Aryl-substituted bis(imino)pyridine ligands, ArPDI [(Ar–
N=CMe)2C5H3N; Ar = aryl group], have received renewed
attention in first-row transition-metal chemistry because of
their relative ease of synthesis,[1] structural modularity[2]
and ability to support a range of catalytically active com-
pounds.[3,4] One notable feature of this ligand class is its
so-called “redox-activity” or “non-innocence” whereby the
conjugated π system of the ligand engages in reversible elec-
tron-transfer events with the metal.[5–8] Accordingly, many
reduced bis(imino)pyridine metal compounds have formal
oxidation states that are deceiving. Prominent among these
compounds are first-row metal dinitrogen derivatives and
examples are now known for vanadium,[9] chromium,[10]
iron,[11] cobalt,[12] nickel[13] and a second-row example, ru-
thenium.[14] Reduced bis(imino)pyridine manganese exam-
ples are a conspicuous absence in this family of compounds
and the electronic structure and possible catalytic activity
of such species are of interest.
(PF6)2
{
4–OMePDI = 2,6-(4-OMe-C6H4N=CMe)2C5H3N}
to the corresponding monocation resulted in two single-
electron reductions of the bis(imino)pyridine chelates and
oxidation of the metal to yield the low-spin MnIII com-
pound [(4–OMePDI1–)2MnIII](PF6). Crystallographic charac-
terization of both compounds has provided reference values
for the bond length distortions of a neutral and monored-
uced chelate in bis(imino)pyridine manganese chemistry.
The high activity of [(iPrPDI)FeCl2] and [(iPrPDI)CoCl2]
{
iPrPDI = 2,6-(2,6-iPr2–C6H3N=CMe)2C5H3N} when acti-
vated with methylaluminoxane (MAO) in olefin polymeri-
zation[15,16] prompted a similar evaluation of the corre-
sponding manganese compound. Gambarotta and cowork-
ers synthesized [(iPrPDI)MnCl2] and established its struc-
ture as a five-coordinate, pseudo-square-pyramidal com-
pound with a neutral bis(imino)pyridine chelate, similar to
the iron and cobalt analogues.[17,18,19] No activity for ethyl-
ene polymerization was observed upon activation with
MAO, behaviour attributed to the high-spin MnII (S = 5/2)
ground state. Alkyl complexes were also explored and the
four-coordinate, distorted-planar methyl complex [(iPrPDI)-
MnCH3] was synthesized and crystallographically charac-
terized. Performing the alkylation of [(iPrPDI)MnCl2] with
LiCH2SiMe3 yielded an anionic manganese alkyl, [(iPrPDI)-
[a] Department of Chemistry, Princeton University,
Princeton, New Jersey, 08544 USA
Fax: +1-609-258-6746
E-mail: pchirik@princeton.edu
[b] Max-Planck Institute for Bioinorganic Chemistry,
Stiftstrasse 34-46, 45470 Mülheim an der Ruhr, Germany
[c] Department of Chemistry and Chemical Biology, Cornell
University,
Ithaca, New York, 14853 USA
Eur. J. Inorg. Chem. 2012, 535–545
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
535

P. J. Chirik et al.
FULL PAPER
Mn(CH2SiMe3)]–, along with a bimetallic alkyl complex re- lated iron[11a,11c] and cobalt compounds.[12] Stirring a THF
sulting from C–C coupling of deprotonated imine methyl solution of [(iPrPDI)MnCl2][17] with 2 equiv. of sodium
groups. This reactivity highlights the chemical non-inno- metal and a catalytic quantity of naphthalene (0.05 equiv.)
cence of the bis(imino)pyridine ligand in addition to its re- in the presence of 1 atm of dinitrogen for 16 hours fur-
dox activity.
nished a dark brown solid identified as the bis(imino)pyr-
Cámpora and coworkers have also explored bis(imino)- idine manganese bis(THF) compound, [(iPrPDI)Mn-
pyridine manganese alkyl chemistry with the goal of under- (THF)2], in 61% yield [Equation (1)]. Analysis of the solid
standing the lack of polymerization activity. Addition of by infrared spectroscopy did not reveal any strong bands
the free bis(imino)pyridine ligand, iPrPDI, to various MnII diagnostic of N2 coordination.
dialkyl precursors resulted in migration of one of the alkyl
groups to the para-pyridine position of the chelate. This
route, following oxidative hydrolysis, provides a convenient
synthetic route to 4-alkyl-substituted bis(imino)pyridines.[20]
Use of β-silyl-substituted alkyls such as [CH2SiMe3] en-
abled the isolation of a rare bis(imino)pyridine manganese
dialkyl complex, [(iPrPDI)Mn(CH2SiMe3)2], which over
time in solution underwent migration of one of the alkyl
ligands to the para-pyridine position of the bis(imino)-
The 1H NMR spectrum of [(iPrPDI)Mn(THF)2] recorded
in [D6]benzene at 23 °C proved uninformative; only broad
and featureless peaks were observed that were not readily
assigned. Routine characterization was accomplished by
aqueous degradation and integration of 1 equiv. of free bi-
s(imino)pyridine ligand to a number of equivalents of liber-
ated THF. A consistent ratio of 1:2 bis(imino)pyridine/THF
was observed from batch to batch. Repeating the aqueous
pyridine, supporting the intermediacy of the manganese di-
alkyl compounds in the rearrangement process.[21] Synthesis
of the pyridine-stabilized bis(imino)pyridine manganese alkyl
cation [(iPrPDI)Mn(CH2SiMe3)(py)]+ was accomplished by
protonation of the corresponding neutral dialkyl. Unlike the
analogous bis(imino)pyridine iron[22] and cobalt[23] alkyl
cations, the manganese compounds are inactive to ethylene
polymerization, again attributed to the S = 5/2 ground
state.
2
degradation with D2O produced no observable H NMR
signals, confirming that the bis(imino)pyridine remains in-
tact during the reduction procedure.
Bis(imino)pyridine manganese bis(triflato) compounds
[(iPrPDI)Mn(OTf)2] and [(MesPDI)Mn(OTf)2] {MesPDI =
2,6-(2,4,6-Me3–C6H2N=CMe)2C5H3N} have been reported
by Britovsek and coworkers and explored in oxidation ca-
talysis.[24] Once again, no catalytic turnover was observed,
which was attributed to the high-spin d5 configuration of
the MnII centre.
Single crystals of [(iPrPDI)Mn(THF)2] suitable for X-ray
diffraction were obtained from a concentrated diethyl ether
solution stored at –35 °C. A representation of the solid-state
structure is presented in Figure 1 and selected bond lengths
and angles are reported in Table 1. The overall molecular
geometry of the compound is best described as pseudo-
square pyramidal with THF molecules occupying both the
basal and apical positions. The bond lengths of the bis-
(imino)pyridine chelate are diagnostic of a two-electron re-
duction[5,6] with elongated Nimine–Cimine bond lengths of
1.367(3) and 1.340(3) Å. Accordingly, the Cipso–Cimine bond
As part of our continuing efforts in bis(imino)pyridine
chemistry with reduced first-row metal ions, the reduction
of [(iPrPDI)MnCl2] was of interest. On the basis of the
rich catalytic chemistry observed with [(iPrPDI)Fe-
(N2)2],[11a,11c,25] we sought to prepare a manganese ana-
logue either with dinitrogen or other suitable neutral, labile
ligands.[24,26] Given the lack of catalytic activity observed
previously with high-spin bis(imino)pyridine manganese(II)
complexes, two-electron reduced manganese compounds
were targeted. However, we were curious as to whether the
redox-active bis(imino)pyridine would prevent formation of
low-oxidation state manganese compounds. Here we de-
scribe several synthetic strategies for the two-electron re-
duction of [(iPrPDI)MnCl2] and elucidation of the electronic
structure of the isolated products. Metal- versus bis(imino)-
pyridine-centred reductions are evaluated as a function of
the additional ligands in the coordination sphere of the
manganese ion.
Results and Discussion
Figure 1. Representation of the solid-state structure of [(iPrPDI)-
Mn(THF)2] at 30% probability ellipsoids. Hydrogen atoms are
Our studies commenced with the reduction of [(iPrPDI)-
MnCl2] using methods that have proven successful with re- omitted for clarity.
536
www.eurjic.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 535–545

Reduced Bis(imino)pyridine Manganese Compounds
lengths are shortened to 1.441(3) and 1.421(3) Å. The Mn– magnetometry (Figure 2). A fit of the data for the major
Nimine distances of 2.1213(15) and 2.1946(15) Å along with component yielded E/D = 0.209 and a small zero-field split-
the Mn–Npy bond length of 2.0514(17) Å are consistent ting parameter (D) of 0.604 cm–1, in agreement with the
with a high-spin MnII compound.
values obtained from SQUID magnetometry. For the major
component, the large g-anisotropy is consistent with man-
ganese-centred spins and principally metal-based SOMOs.
Table 1. Selected bond lengths [Å] and angles [°] for [(iPrPDI)Mn-
(THF)2].
Mn(1)–N(1)
Mn(1)–N(2)
Mn(1)–N(3)
Mn(1)–O(1)
Mn(1)–O(2)
2.1213(15) N(1)–C(2)
2.0514(17) N(3)–C(8)
2.1946(15) C(2)–C(3)
2.1909(15) C(7)–C(8)
2.1998(16) N(2)–C(3)
N(2)–C(7)
1.367(3)
1.340(3)
1.441(3)
1.421(3)
1.363(2)
1.374(2)
N(2)–Mn(1)–N(1) 75.92(6)
N(2)–Mn(1)–N(3) 74.68(6)
N(1)–Mn(1)–N(3) 146.13(6) N(2)–Mn(1)–O(2)
O(1)–Mn(1)–O(2)
N(2)–Mn(1)–O(1)
93.85(6)
169.19(7)
96.84(7)
The solid-state magnetic moment of [(iPrPDI)Mn-
(THF)2] was determined by a magnetic susceptibility bal-
ance and produced a value of 3.7 μB at 23 °C, consistent
with an S = 3/2 ground state. The magnetic behaviour of
[(iPrPDI)Mn(THF)2] was also studied using SQUID magne-
tometry. A plot of μeff vs. temperature is presented in Fig-
ure 2. A fit of the data yielded an average g value of 2.10
and E/D = 0.21 and D = 0.61 cm–1.
Figure 3. Experimental (bottom) and simulated (top) EPR spectra
for [(iPrPDI)Mn(THF)2]. Data collected at 10 K in 1:1 toluene/
pentane glass (microwave frequency 9.44 GHz, microwave power
0.32 mW, modulation 25 G). gx = 2.829, gy = 2.471, gz = 2.302; Axx
= 176ϫ10–4 cm–1, Ayy = 352ϫ10–4 cm–1, Azz = 150ϫ10–4 cm–1; D
= 0.61 cm–1, E/D = 0.21.
The catalytic performance of [(iPrPDI)Mn(THF)2] was
evaluated in olefin hydrogenation and diene cycloisomeriza-
tion reactions and compared to the corresponding iron and
cobalt dinitrogen compounds.[11a,12b] Attempts to hydroge-
nate 1-hexene or cyclohexene in the presence of 10 mol-%
[(iPrPDI)Mn(THF)2] at both 23 and 65 °C with 4 atm of H2
produced no turnover. Likewise, α, ω-dienes such as diallyl
tert-butylamine and diethyl diallyl malonate produced no
[2+2] cyclization in the presence of 10 mol-% [(iPrPDI)-
Mn(THF)2] at 65 °C. The lack of reactivity is probably due
to the high-spin MnII centre in [(iPrPDI)Mn(THF)2].
In an attempt to obviate formation of the bis(THF) man-
ganese complex, the reduction of [(iPrPDI)MnCl2] was car-
ried out in the absence of THF. Stirring a pentane slurry of
[(iPrPDI)MnCl2] with an excess of 0.5% Na(Hg) for two
days under a dinitrogen atmosphere followed by filtration
and recrystallization of the product from diethyl ether at
–35 °C resulted in isolation of a red solid identified as
[(iPrPDI)2Mn] in 46% yield [Equation (2)].
Figure 2. Variable temperature SQUID magnetization data for
[(iPrPDI)Mn(THF)2]. Experimental data are represented by the
open circles, the simulation is the top curve and the bottom line
the paramagnetic impurity (see text). The data are corrected for
underlying diamagnetism.
These data, in combination with the metrical parameters
determined from X-ray diffraction, suggest that the elec-
tronic structure of [(iPrPDI)Mn(THF)2] is best described as
a high-spin MnII, d5 compound (SMn = 5/2), antiferromag-
netically coupled to a bis(imino)pyridine triplet diradical
(SPDI = 1). The toluene/pentane glass (1:1) EPR spectrum
of the compound, recorded at 10 K and presented in Fig-
ure 3, also supports this electronic structure description. A
rhombic signal corresponding to an S = 3/2 compound is
observed for the major species, while a second signal is pres-
ent in small amounts (≈1%) for a second unidentified minor
component. Variable amounts of the second minor compo-
nent were observed in every sample of [(iPrPDI)Mn(THF)2]
and was also observed in approximately 2–3% by SQUID
Eur. J. Inorg. Chem. 2012, 535–545
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
537

P. J. Chirik et al.
FULL PAPER
1
As with [(iPrPDI)Mn(THF)2], the [D6]benzene H NMR centre, the Nimine–Cimine bond lengths are slightly elongated
spectrum of [(iPrPDI)2Mn] is uninformative exhibiting only to 1.324(3) and 1.338(3) Å while the Cimine–Cipso bond
broad and featureless resonances. Single crystals suitable for lengths are shortened to 1.450(3) and 1.430(3) Å. The free
X-ray diffraction were obtained from a concentrated diethyl imine arms have Nimine–Cimine bond lengths of 1.298(3) and
ether solution at –35 °C and a representation of the solid- 1.281(3) Å, consistent with a neutral ligand.
state structure is presented in Figure 4. Selected bond
A magnetic moment of 3.9 μB was measured in the solid
lengths and angles are reported in Table 2. The overall mo- state (23 °C) for [(iPrPDI)2Mn], consistent with three un-
lecular geometry is best described as a cis-divacant octahe- paired electrons and an overall S = 3/2 ground state.
dron with pseudo trans-imine arms of each bis(imino)pyr- SQUID magnetization data were also collected on a poly-
idine outside the primary coordination sphere of the man- crystalline sample between 2 and 300 K (Figure 5). At tem-
ganese centre. The presence of two κ2 rather than tradi- peratures below 20 K a rise in the effective magnetic mo-
tional κ3 chelates is likely steric in origin and a consequence ment is observed due to intermolecular ferromagnetic cou-
of the large 2,6-diisopropyl aryl rings that prevent forma- pling in the solid state. Overall, these interactions are weak
tion of a six-coordinate idealized octahedral compound. A as evidenced by the Weiss constant of 0.96 K. At tempera-
similar coordination geometry was observed in the bis(chel- tures above 25 K, the value of μeff plateaus at 3.9 μB, consis-
ate) iron compound [(EtPDI)2Fe] {EtPDI = 2,6-(2,6-Et2– tent with both the value measured on the magnetic suscep-
C6H3N=CMe)2C5H3N}, while the compound with the tibility balance and the spin-only value for three unpaired
smaller aryl substituents, (4–OMePDI)2Fe, is hexacoordinate electrons. Simulation of the data also yielded an average g
with two κ3 chelates.[27]
value of 1.990, a D value of –1.759 cm–1 and an E/D value
of 0.30.
Figure 5. Variable temperature SQUID magnetization data for
[(iPrPDI)2Mn]. Experimental data are represented by the open
circles; data are corrected for underlying diamagnetism. The simu-
lation is the curve.
Figure 4. Representation of the solid-state structure of [(iPrPDI)2-
Mn] at 30% probability ellipsoids. Hydrogen atoms and aryl groups
are omitted for clarity.
The EPR spectrum of the compound, recorded in 1:1
toluene/pentane glass (Figure 6), exhibits a rhombic signal.
The observation of large (and relatively isotropic) Mn hy-
perfine values is consistent with a MnII centre with princi-
pally metal-based SOMOs. On the basis of these data along
with the metrical parameters from X-ray diffraction studies,
Table 2. Selected bond lengths [Å] and angles [°] for [(iPrPDI)2Mn].
Mn(1)–N(1)
Mn(1)–N(2)
Mn(1)–N(4)
Mn(1)–N(5)
N(1)–C(2)
2.2637(18) C(2)–C(3)
2.0320(16) C(7)–C(8)
2.1406(19) N(3)–C(8)
2.1311(16) N(6)–C(41)
1.450(3)
1.468(3)
1.298(3)
1.281(3)
1.430(3)
1.489(3)
[(iPrPDI)2Mn] is best described as a high-spin MnII (SMn
=
1.324(3)
1.338(4)
C(35)–C(36)
C(40)–C(41)
N(4)–C(35)
5/2) compound antiferromagnetically coupled to two bis-
(imino)pyridine radical anions.
N(2)–Mn(1)–N(5)
N(2)–Mn(1)–N(4)
N(4)–Mn(1)–N(5)
171.05(7)
95.26(7)
79.40(7)
N(1)–Mn(1)–N(2)
N(1)–Mn(1)–N(5)
N(1)–Mn(1)–N(4)
74.33(7)
114.13(7)
109.11
During the synthesis of [(iPrPDI)Mn(THF)2] and
[(iPrPDI)2Mn] by a two-electron reduction of [(iPrPDI)-
MnCl2], an immediate colour change to dark orange-red
The metrical parameters of the bis(imino)pyridine che- was observed in both cases, suggesting a common interme-
late are consistent with a one-electron reduction (Table 2). diate. On the basis of a precedent from both bis(imino)pyr-
Care must be exercised when interpreting these data and idine iron[11] and cobalt[12] chemistry, [(iPrPDI)MnCl]
comparing them to compounds with traditional κ3-bis- seemed a likely one-electron reduced intermediate. To test
(imino)pyridines. A more reasonable metrical comparison this possibility, an independent synthetic route to the com-
is to bidentate, redox-active α-iminopyridines.[28] For the pound was sought. Stirring a THF slurry of [(iPrPDI)-
portions of the chelates coordinated to the manganese MnCl2] with an equimolar quantity of [(iPrPDI)Mn-
538
www.eurjic.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 535–545

Reduced Bis(imino)pyridine Manganese Compounds
mirror plane. The values reported in the table reflect the
two molecules and the symmetry element. Also included
are the corresponding metrical parameters for the related
bis(imino)pyridine iron bromide compound [(iPrPDI)-
FeBr(THF)].[29] The overall molecular geometry is best de-
scribed as idealized square pyramidal with the THF ligand
occupying the apical position.
Figure 6. Experimental (bottom) and simulated (top) EPR spectra
for [(iPrPDI)2Mn]. Data collected at 10 K in 1:1 toluene/pentane
glass. Axx
= = =
–85ϫ10–4 cm–1, Ayy –78ϫ10–4 cm–1, Azz
–92ϫ10–4 cm–1; D = –0.5 cm–1, E/D = 0.33; σ(E/D) = 0.07.
(THF)2] furnished a dark-red solid identified as [(iPrPDI)-
MnCl(THF)] in 91% yield [Equation (3)]. This compound
was also isolated from reduction of [(iPrPDI)MnCl2] with
sodium metal in the presence of 5 mol-% naphthalene.
When using this synthetic route, the reaction was initiated
in THF solution to generate NaC10H8 and the solvent was
then replaced with diethyl ether to avoid a two-electron re-
duction to [(iPrPDI)Mn(THF)2]. Even with this precaution,
some decomposition and two-electron reduction was ob-
served and thus the comproportionation method is the pre-
ferred synthetic route.
Figure 7. Representation of the solid-state structure of [(iPrPDI)-
MnCl(THF)] at 30% probability ellipsoids. Hydrogen atoms are
omitted for clarity.
Table 3. Selected bond lengths [Å] and angles [°] for [(iPrPDI)-
MnCl(THF)] and [(iPrPDI)FeBr(THF)].
[(iPrPDI)MnCl(THF)]
[(iPrPDI)FeBr(THF)]
2.008(3)
M–Npy
2.065(2)
2.058(2)
2.283(2)
2.314(2)
2.293(1)
2.298(1)
1.293(2)
1.296(3)
1.454(3)
1.441(3)
95.27(8)
102.39(9)
163.12(7)
160.63(7)
M–Nimine
2.172(3)
2.150(3)
2.416(1)
M–X (X = Cl, Br)
Nimine–Cimine
1.299(4)
1.306(4)
1.446(5)
1.433(5)
92.64(15)
Cipso–Cimine
Npy–M–O
Npy–M–X
167.89(9)
The [D6]benzene 1H NMR spectrum of [(iPrPDI)-
MnCl(THF)] at 20 °C exhibits five paramagnetically broad-
ened and shifted resonances between –10 and 50 ppm,
The Nimine–Cimine bond lengths of 1.293(2) and
which are useful for routine characterization of the com- 1.296(3) Å are slightly elongated from the corresponding
pound. Aqueous degradation liberated 1 equiv. of THF per values of 1.273(2) and 1.279(8) Å reported for [(iPrPDI)-
equiv. of free bis(imino)pyridine, confirming THF coordi- MnCl2][17] and fall between the accepted values for a neu-
nation in bulk samples. Attempts to isolate a THF-free ver- tral and monoreduced chelate,[5,6] rendering the ligand oxi-
sion of the compound by recrystallization from diethyl dation state assignment ambiguous. The Cimine–Cipso dis-
ether or pentane have been unsuccessful, resulting in the tances of 1.454(3) and 1.441(3) Å are a more pronounced
decomposition and isolation of an unidentified, insoluble distortion and are clearly more consistent with a one-elec-
white powder.
tron bis(imino)pyridine reduction.[5,6] Similar bond pertur-
Single crystals of [(iPrPDI)MnCl(THF)] suitable for X- bations are present in the related iron compound, [(iPrPDI)-
ray diffraction analysis were obtained from layering pent- FeBr(THF)],[29] where a high-spin ferrous ion was assigned
ane on to a THF solution of the compound at –35 °C. A with a monoreduced bis(imino)pyridine chelate. We there-
representation of the solid-state structure is presented in fore conclude that [(iPrPDI)MnCl(THF)] also contains a
Figure 7 and selected bond lengths and angles are reported bis(imino)pyridine radical anion that is antiferromag-
in Table 3. There are two independent molecules in the netically coupled to a high-spin MnII centre producing an
asymmetric unit each with a crystallographically imposed overall S = 2 ground state. The high-spin, d5 configuration
Eur. J. Inorg. Chem. 2012, 535–545
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
539

P. J. Chirik et al.
FULL PAPER
is corroborated by the relatively long Mn–Npy and Mn– Mn], suggesting a low-spin state. The Nimine–Cimine bond
Nimine bond lengths (Table 3). Accordingly, a solid-state lengths of 1.325(2) and 1.321(2) Å are elongated from the
magnetic moment of 4.6 μB was measured at 23 °C, consis- neutral chelate and are diagnostic of a monoreduced bis-
tent with an S = 2 ground state. However, it should be (imino)pyridine radical anion.[5,6] Accordingly, the Cimine
–
noted that a traditional high-spin MnI description with a Cipso distances of 1.443(2) and 1.445(2) Å also support this
neutral chelate would also produce a magnetic moment ligand oxidation state assignment.
consistent with four unpaired electrons.
Bis(imino)pyridine Manganese Carbonyl Compounds
Reduced bis(imino)pyridine manganese compounds with
strong-field organometallic ligands were also explored with
the goal of accessing lower metal oxidation and spin
states.[30] Addition of 4 atm of carbon monoxide to a tolu-
ene solution of [(iPrPDI)Mn(THF)2] followed by solvent re-
moval and subsequent recrystallization from diethyl ether
at –35 °C furnished a dark-pink solid identified as [(iPrPDI)-
Mn(CO)2] in 87% yield [Equation (4)]. The infrared spec-
trum exhibits two strong bands centred at 1915 and
1860 cm–1 in pentane solution. These bands are observed at
1896 and 1836 cm–1 in the solid state (KBr pellet). Compar-
ing the pentane values to the corresponding bands for
[(iPrPDI)Fe(CO)2] (νCO = 1974 and 1914 cm–1) establishes a
more reducing metal centre in the manganese complex. A
summary of all carbonyl stretching frequencies determined
by infrared spectroscopy for the compounds in this work is
reported in Table 4.
Figure 8. Representation of the solid-state structure of [(iPrPDI)-
Mn(CO)2] at 30% probability ellipsoids. Hydrogen atoms are omit-
ted for clarity.
Table 5. Selected bond lengths [Å] for the series of bis(imino)pyr-
idine manganese carbonyl complexes prepared in this study.
[(iPrPDI)Mn(CO)2]–
(
iPrPDI)Mn(CO)2 [(iPrPDI)Mn(CO)3]+
Mn(1)–N(1)
Mn(1)–N(2)
Mn(1)–N(3)
Mn(1)–C(34)
Mn(1)–C(35)
N(1)–C(2)
N(3)–C(8)
C(2)–C(3)
1.983(2)
1.886(2)
1.979(2)
1.752(3)
1.771(3)
1.343(3)
1.351(3)
1.411(4)
1.399(4)
1.388(3)
1.389(3)
1.988(1)
1.916(1)
1.996(1)
1.782(1)
1.762(1)
1.325(2)
1.321(2)
1.443(2)
1.445(2)
1.363(2)
1.360(1)
2.054(3)
1.961(3)
2.093(3)
1.858(4)
1.784(4)
1.298(4)
1.294(4)
1.476(5)
1.477(5)
1.350(4)
1.352(4)
C(7)–C(8)
N(2)–C(3)
N(2)–C(7)
Table 4. Solid-state infrared stretching frequencies [ν(CO)] for bis-
(imino)pyridine manganese compounds prepared in this work.
Data for [(iPrPDI)Fe(CO)2] taken from ref.[11a]
To further explore the ground-state electronic structure
of [(iPrPDI)Mn(CO)2], a solid-state magnetic moment was
recorded at 23 °C. An effective magnetic moment of 1.8 μB
was measured, consistent with an S = 1/2 ground state and
the short metal–ligand bond lengths observed in the X-ray
structure. The doublet ground state was also confirmed by
EPR spectroscopy. The spectrum was collected at 10 K in
1:1 toluene/pentane glass and is presented in Figure 9. A
ν(CO) [cm–1]
[Na(OEt2)3][(iPrPDI)Mn(CO)2]
[(iPrPDI)Mn(CO)2]
1806, 1719
1896, 1836, 1915, 1860[a]
2052, 1967, 1943
1974, 1914[a]
[(iPrPDI)Mn(CO)3][BPh4]
[(iPrPDI)Fe(CO)2]
[a] Recorded in pentane.
The solid-state structure of [(iPrPDI)Mn(CO)2] was deter- rhombic signal was observed and simulation of the data
mined by X-ray diffraction and is presented in Figure 8 and produced manganese hyperfine coupling constants (I55Mn
=
selected bond lengths and angles are reported in Table 5. 5/2) of Axx = 23ϫ10–4 cm–1, Ayy = 8.0ϫ10–4 cm–1 and Azz
The overall molecular geometry is best described as square = 32.5ϫ10–4 cm–1. These values are relatively small and
pyramidal with carbonyl ligands in the apical and one of suggest a principally ligand, rather than metal-based
the basal positions, similar to the geometry of the iron ana- SOMO. By comparison, low-spin MnII compounds with
logue [(iPrPDI)Fe(CO)2]. The Mn–Nimine bond lengths of manganese-based SOMOs typically exhibit A values larger
1.988(1) and 1.996(1) Å and the Mn–Npy distance of than 100 cm–1.[31] The relatively small g-anisotropy (g =
1.916(1) Å are significantly shorter than the metal–nitrogen 2.007, 2.202, 1.998) is also consistent with this electronic
bonds in[(iPrPDI)MnCl2], [(iPrPDI)Mn(THF)2]or [(iPrPDI)2- structure description.
540
www.eurjic.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 535–545

Reduced Bis(imino)pyridine Manganese Compounds
exhibits two strong CO bands at 1806 and 1719 cm–1, in-
dicative of a high degree of reduction from the electron-rich
manganese centre.
Single crystals suitable for X-ray diffraction were ob-
tained from a concentrated diethyl ether solution at –35 °C.
A representation of the solid-state structure is presented in
Figure 10 and selected bond lengths and angles are reported
in Table 5. The overall molecular geometry is best described
as distorted-trigonal pyramidal, with the two carbonyl li-
gands occupying the equatorial positions, likely to maxim-
ize π-backbonding interactions. The sodium cation is lig-
ated by three disordered diethyl ether molecules and is also
coordinated to one of the oxygen atoms of the carbonyl
ligands.
Figure 9. Experimental (bottom) and simulated (top) EPR spectra
for [(iPrPDI)Mn(CO)2]. Data collected at 10 K in 1:1 toluene/pent-
ane glass. gx = 2.070, gy = 2.202, gz = 1.998; Axx = 23ϫ10–4 cm–1,
Ayy = 8.0ϫ10–4 cm–1, Azz = 32.5ϫ10–4 cm–1.
On the basis of the crystallographic, magnetic and EPR
spectroscopic data, the electronic structure of [(iPrPDI)-
Mn(CO)2] is best described as a low-spin MnI, d6 com-
pound with a bis(imino)pyridine radical anion as the
SOMO. The d6 electronic configuration at the manganese
centre is well suited for π backbonding with the carbonyl
ligands and accounts for the low infrared stretching fre-
quencies as compared to the iron congener. It is also inter-
esting to consider that a “simple” ligand-substitution reac-
tion of the σ-donor THF for the π-acceptor CO induces a
change in the metal oxidation state where [(iPrPDI2–)MnII-
(THF)2] transforms to [(iPrPDI–)MnI(CO)2].
Figure 10. Truncated representation of the solid-state structure of
[Na(OEt2)3][(iPrPDI)Mn(CO)2] at 30% probability ellipsoids. The
Attempts to prepare [(iPrPDI)Mn(CO)2] by a sodium am- diethyl ether carbon and hydrogen atoms are omitted for clarity.
algam reduction of [(iPrPDI)MnCl2] under an atmosphere
of carbon monoxide, which was also used to prepare
[(iPrPDI)Fe(CO)2],[11a] did not yield the manganese dicar-
bonyl but rather the corresponding anion [Na(OEt2)3]-
The bond lengths of the bis(imino)pyridine chelate are
diagnostic of a two-electron reduction. The Nimine–Cimine
[(iPrPDI)Mn(CO)2] as
a
yellow solid [Equation (5)].
bonds elongate to 1.343(3) and 1.351(3) Å, while the Cipso
–
Cimine distances shorten to 1.411(4) and 1.399(4) Å. These
are the largest distortions observed in the family of bis-
(imino)pyridine manganese compounds prepared to date
and likely indicative of the closed shell dianion [iPrPDI]2–
(S = 0).
Decreasing the amount of sodium amalgam from 5 to
2 equiv. also furnished the manganese dicarbonyl anion
along with remaining [(iPrPDI)MnCl2] starting material.
The one-electron oxidation of [Na(OEt2)3][(iPrPDI)-
Mn(CO)2] was explored as an alternative synthetic route to
[(iPrPDI)Mn(CO)2]. Addition of 1 equiv. of AgBPh4 to a
benzene solution of [Na(OEt2)3][(iPrPDI)Mn(CO)2] cleanly
furnished the neutral manganese dicarbonyl [(iPrPDI)-
Mn(CO)2].
To complete a series of bis(imino)pyridine manganese
Diamagnetic [Na(OEt2)3][(iPrPDI)Mn(CO)2] was readily carbonyl compounds over three different oxidation states,
1
characterized by H and 13C NMR spectroscopy. Because the oxidation of the neutral compound was also explored.
of a limited solubility in hydrocarbon solvents, spectra were Addition of 1 equiv. of [Cp2Fe][BPh4] to a benzene solution
recorded in [D8]THF. The imine methyl groups appear at δ of [(iPrPDI)Mn(CO)2] followed by addition of pentane re-
= 2.62 ppm and the meta-pyridine at δ = 8.12 ppm, as ex- sulted in precipitation of a tan solid identified as a new
pected for a diamagnetic compound with no contributions bis(imino)pyridine manganese compound albeit in low
from low-lying paramagnetic excited states. The solid-state yield. Analysis of the isolated solid by infrared spectroscopy
(KBr) infrared spectrum of [Na(OEt2)3][(iPrPDI)Mn(CO)2] revealed three strong carbonyl stretches centred at 2052,
Eur. J. Inorg. Chem. 2012, 535–545
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
541

P. J. Chirik et al.
FULL PAPER
1967 and 1943 cm–1, suggesting that the oxidation product subsequent one-electron reduction of the neutral compound
was in fact the bis(imino)pyridine manganese tricarbonyl [(iPrPDI1–)MnI(CO)2,] is also ligand-based and furnishes
cation [(iPrPDI)Mn(CO)3][BPh4]. With this information in [(iPrPDI2–)MnI(CO)2]–, where the bis(imino)pyridine is in its
hand, the procedure was repeated in the presence of 1 atm dianionic form. Similar ligand-based redox events have
of carbon monoxide and furnished the desired [(iPrPDI)- been reported for pincer manganese carbonyl com-
Mn(CO)3][BPh4] compound in near quantitative yield pounds.[30]
[Equation (6)]. The 18-electron manganese compound is
diamagnetic and accordingly exhibits sharp 1H and 13C
NMR spectra in [D8]THF (see Experimental Section).
Figure 12. Electronic structure summary of a series of bis(imino)-
pyridine manganese carbonyl complexes that vary by three oxi-
dation states. The oxidation and reduction processes are ligand-
rather than metal-based and all compounds are low-spin MnI.
Single crystals suitable for X-ray diffraction were ob-
tained from a fluorobenzene solution layered with pentane
and stored at –35 °C. A representation of the solid-state
structure is presented in Figure 11 and selected bond
lengths and angles are reported in Table 5. The overall mo-
lecular geometry is best described as an idealized octahe-
dron with a meridional arrangement of the three carbonyl
ligands. The metrical parameters of the bis(imino)pyridine
are consistent with a neutral chelate (Table 5). These values,
coupled with the relatively short Mn–N bond lengths are
consistent with a low-spin MnI compound, suggesting that
the oxidation of [(iPrPDI1–)MnI(CO)2] is principally ligand
based.[30]
A similar series of bis(imino)pyridine iron carbonyl com-
plexes that differ by three oxidation states has also been
prepared and the electronic structures determined by a
combination of X-ray diffraction, spectroscopic experi-
ments and DFT calculations.[32] The neutral compound
[(iPrPDI)Fe(CO)2] is highly covalent and the best electronic
structure description involves resonance structures between
the [(iPrPDI0)Fe0(CO)2] and [(iPrPDI2–)FeII(CO)2] canonical
forms, where in the latter case the bis(imino)pyridine che-
late is in its closed shell dianionic form. Reducing the neu-
tral carbonyl compound by one electron produced the an-
ionic complex [(iPrPDI2–)FeI(CO)2]–, where the reduction is
iron-based and furnished a low-spin FeI compound with a
dianionic chelate. Oxidation of [(iPrPDI)Fe(CO)2] yielded
the corresponding cation [(iPrPDI0)FeI(CO)2]+, which is
also a low-spin FeI compound with a neutral bis(imino)pyr-
idine chelate. For iron, redox chemistry within the carbonyl
series of compounds involves cooperative metal and ligand
oxidation and reduction events whereas in the correspond-
ing manganese derivatives, the redox chemistry is princi-
pally ligand-based. For the latter, it is likely that the low-
spin MnI, d6 configuration is preferred for the strong-ligand
field imparted by the carbonyl ligands (and also favours π
backbonding), which in turn relegates redox events to the
bis(imino)pyridine chelate.
Conclusions
Figure 11. Truncated representation of the solid-state structure of
[(iPrPDI)Mn(CO)3][BPh4] at 30% probability ellipsoids. The
[BPh4]– anion and the hydrogen atoms are omitted for clarity.
A family of reduced bis(imino)pyridine manganese com-
plexes have been synthesized and the role of the redox-
active ligand on the overall electronic structure of the com-
Having prepared examples of bis(imino)pyridine manga- pounds established. Attempts to prepare a manganese dini-
nese carbonyl complexes that vary by three oxidation states, trogen derivative have been unsuccessful, likely due to the
comparison of the electronic structures is useful to deter- preferred electronic structure of the metal ion in a weak
mine the site (metal vs. ligand) of reduction. A summary of ligand field. For a principally σ-donating ligand such as
the electronic structures is presented in Figure 12. A one- THF, a high-spin MnII compound results with a doubly
electron reduction of the cation [(iPrPDI0)MnI(CO)3]+ to reduced bis(imino)pyridine diradical. Substitution of the
[(iPrPDI1–)MnI(CO)2] occurs at the bis(imino)pyridine che- THF ligands by carbon monoxide results in a reduction of
late and with loss of a carbon monoxide ligand. Likewise, the metal to low-spin MnI, d6 with a concomitant oxidation
542
www.eurjic.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 535–545

Reduced Bis(imino)pyridine Manganese Compounds
(ER4102ST) and a helium flow cryostat (Oxford Instruments ESR
910). Microwave frequencies were calibrated with a Hewlett–Pack-
ard frequency counter (HP5352B), and the field control was cal-
ibrated with a Bruker NMR field probe (ER035M). The spectra
were simulated with the program GFIT (by E. Bill) for the calcula-
tion of powder spectra with effective g values and anisotropic line
widths (Lorentzian line shapes were used). In each case a 2 mm
solution of the complex was prepared in toluene. Data were re-
corded at 296 K with a frequency of 9.43 GHz, a modulation am-
plitude of 10 G and a microwave power of 1.26 mW. Simulations
were conducted using the W95EPR software package.
of the ligand to its radical anion form. The bis(imino)pyr-
idine radical is the SOMO of [(iPrPDI)Mn(CO)2] and was
observed by EPR spectroscopy providing direct experimen-
tal evidence for the redox activity of the chelate. Oxidation
and reduction of the neutral manganese dicarbonyl com-
pound was also carried out and the MnI, d6 electronic con-
figuration was maintained throughout a series of com-
pounds that vary by three oxidation states.
[(iPrPDI)Mn(THF)2]: A 100 mL round-bottomed flask was charged
with [(iPrPDI)MnCl2] (1.00 g, 1.65 mmol), sodium metal (0.078 g,
3.37 mmol, 2.05 equiv.) and naphthalene (0.010 g, 0.082 mmol,
0.05 equiv.). THF (approximately 20 mL) was added to the flask
and the resulting reaction mixture was stirred for 16 h. During this
time a colour change from orange to brown was observed. After
reduction was complete (as indicated by the formation of the brown
solution), the THF was removed in vacuo. The resulting solid was
dissolved in diethyl ether and filtered through Celite. The filtrate
was collected, concentrated, stored at –35 °C and yielded 0.687 g
(61%) of a dark brown solid identified as [(iPrPDI)Mn(THF)2].
Crystals suitable for X-ray analysis were obtained from a diethyl
ether solution stored at –35 °C. C41H59MnN3O2 (680.87): calcd. C
75.89, H 9.16, N 6.48; found C 76.14, H 9.20, N 6.36. Magnetic
susceptibility: μeff = 3.7 μB (magnetic susceptibility balance, 23 °C).
Experimental Section
General Considerations: All air- and moisture-sensitive manipula-
tions were carried out using a standard vacuum line, Schlenk and
cannula techniques, or in an MBraun inert atmosphere dry box
containing an atmosphere of purified nitrogen. Solvents for air-
and moisture-sensitive manipulations were initially dried and
deoxygenated using literature procedures. [D6]Benzene was pur-
chased from Cambridge Isotope Laboratories and dried with 4 Å
molecular sieves. The complexes [(iPrPDI)MnCl2] and [Cp2Fe]-
[BPh4] were prepared according to literature procedures.[17,18]
1H NMR spectra were recorded with Varian Mercury 300, Inova
400, 500 and 600 spectrometers operating at 299.76, 399.78, 500.62
and 599.78 MHz, respectively. 13C NMR spectra were recorded
1
with an Inova 500 spectrometer operating at 125.893 MHz. All H
and 13C NMR chemical shifts are reported relative to SiMe4 using
the 1H (residual) and 13C chemical shifts of the solvent as a second-
ary standard. For diamagnetic complexes, many assignments were
made based on COSY and HSQC NMR experiments. Solution
magnetic moments were determined by Evans method using a fer-
rocene standard and are the average value of at least two indepen-
dent measurements. Gouy balance measurements were performed
with a Johnson Matthey instrument that was calibrated with
HgCo(SCN)4. Peak widths at half heights are reported for para-
magnetically broadened and shifted resonances. Infrared spectra
were collected with a Thermo Nicolet spectrometer. Elemental
analyses were performed at Robertson Microlit Laboratories, Inc.,
in Madison, NJ.
[(iPrPDI)2Mn]: A 100 mL round-bottomed flask was charged with
mercury (45.41 g, 226.38 mmol) and pentane (approximately
30 mL). Sodium metal (0.227 g, 9.87 mmol) was cut into pieces and
added to the flask. The amalgam was stirred for 10 min, then solid
[(iPrPDI)MnCl2] (1.00 g, 1.65 mmol) was added. The reduction was
stirred for 48 h during which time a colour change in the solution
from orange to brown was observed. After 48 h the solution was
decanted from the amalgam and filtered through Celite. The re-
maining product was extracted into diethyl ether and filtered
through Celite. Recrystallization from diethyl ether at –35 °C
yielded 0.386 g (46%) of a red solid identified as [(iPrPDI)2Mn].
Crystals suitable for X-ray analysis were obtained from a diethyl
ether solution stored at –35 °C. C66H86MnN6 (1018.38): calcd. C
77.84, H 8.51, N 8.25; found C 77.94, H 8.52, N 8.39. Magnetic
susceptibility: μeff = 3.9 μB (magnetic susceptibility balance, 23 °C).
Single crystals suitable for X-ray diffraction were coated with poly-
isobutylene oil in a drybox, transferred to a nylon loop and then
quickly transferred to the goniometer head of a Bruker X8 APEX2
diffractometer equipped with a molybdenum X-ray tube (λ =
0.71073 Å). Preliminary data revealed the crystal system. A hemi-
sphere routine was used for data collection and determination of
lattice constants. The space group was identified and the data were
processed using the Bruker SAINT+ program and corrected for
absorption using SADABS. The structures were solved by direct
methods (SHELXS) completed by subsequent Fourier synthesis
and refined by full-matrix least-squares procedures.
[(iPrPDI)MnCl(THF)]:
A 100 mL round-bottomed flask was
charged with [(iPrPDI)Mn(THF)2] (0.500 g, 0.734 mmol) and
[(iPrPDI)MnCl2] (0.450 g, 0.741 mmol). THF (approximately
20 mL) was added to the flask and the resulting reaction mixture
was stirred for 16 h. During this time a colour change from brown
to dark red was observed. Following removal of the THF in vacuo,
the product was extracted into diethyl ether and filtered through
Celite. The solution was evaporated to dryness to yield 0.673 g
(91%) of a dark-red solid identified as [(iPrPDI)MnCl(THF)].
C37H51ClMnN3O (644.22): calcd. C 69.28, H 7.58, N 7.34; found
C 69.01, H 8.22, N 7.32. Magnetic susceptibility: μeff = 4.6 μB (mag-
netic susceptibility balance, 23 °C). 1H NMR ([D6]benzene, 20 °C):
δ = –7.91 (J = 530 Hz), –1.80 (J = 1013 Hz), 4.92 (J = 656 Hz),
10.17 (J = 954 Hz), 43.23 (J = 1701 Hz) ppm.
SQUID magnetization data of crystalline powdered samples were
recorded with a SQUID magnetometer (Quantum Design) at
10 kOe between 5 and 300 K for all samples. Values of the magnetic
susceptibility were corrected for the underlying diamagnetic in-
crement by using tabulated Pascal constants[33] and the effect of
the blank sample holders (gelatin capsule/straw). Samples used for
magnetization measurements were recrystallized multiple times and
[(iPrPDI)Mn(CO)2]: A thick-walled glass vessel was charged with
of [(iPrPDI)Mn(THF)2] (0.500 g, 0.734 mmol) and toluene (approx-
imately 20 mL). The vessel was removed from the dry box and sub-
merged in liquid nitrogen. On the high vacuum line, the vessel was
evacuated and 4 atm of CO were added. The solution was warmed
to room temperature and stirred for 1 h. During this time, a pink
solution formed. The excess CO was removed on the high vacuum
1
checked for chemical composition by H NMR spectroscopy. The
program julX written by E. Bill was used for (elements of) the simu-
lation and analysis of magnetic susceptibility data.
X-band EPR data were collected with a Bruker ELEXSYS E500
spectrometer equipped with the Bruker standard cavity
Eur. J. Inorg. Chem. 2012, 535–545
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
543

P. J. Chirik et al.
FULL PAPER
line and the vessel placed back in the box. The solution was filtered
through Celite and all solvent removed in vacuo to yield 0.378 g
(87%) of a dark-pink solid identified as [(iPrPDI)Mn(CO)2]. Crys-
tals suitable for X-ray diffraction were grown from a diethyl ether
solution stored at –35 °C. C35H43MnN3O2 (592.68): calcd. C 70.93,
H 7.31, N 7.09; found C 70.70, H 7.37, N 6.91. Magnetic suscep-
Acknowledgments
We thank the U. S. National Science Foundation and the Deutsche
Forschungsgemeinschaft (DFG) for a Cooperative Activities in
Chemistry between U. S. and German Investigators grant. We also
thank Eckard Bill for assistance with the simulations of EPR data.
1
tibility: μeff = 1.9 μB ([D6]benzene, 23 °C). H NMR ([D6]benzene,
20 °C): δ = –2.41 (J = 633 Hz), 3.52 (J = 87 Hz), 5.98 (J = 108 Hz),
[1] E. C. Alyea, P. H. Merrel, Inorg. Met. Org. Chem. 1974, 4, 535.
[2] V. C. Gibson, C. Redshaw, G. A. Solan, Chem. Rev. 2007, 107,
1745.
7.24 (J = 40 Hz), 13.34 (J = 157 Hz) ppm. IR: ν(CO) 1836,
˜
1896 cm–1 (KBr); 1860, 1915 cm–1 (pentane).
[3] a) P. J. Chirik, in: Catalysis Without Precious Metals (Ed.:
R. M. Bullock), Wiley-VCH, Weinheim, Germany, 2010, pp.
83–106; b) P. J. Chirik, K. Wieghardt, Science 2010, 327, 5967.
[4] a) C. Bianchini, G. Giambastiani, I. G. Rios, G. Mantovani,
A. Meli, A. M. Segarra, Coord. Chem. Rev. 2006, 250, 1391; b)
T. Matsugi, T. Fujita, Chem. Soc. Rev. 2008, 37, 1264.
[5] Q. Knijnenburg, S. Gambarotta, P. H. M. Budzelaar, Dalton
Trans. 2006, 5442.
[6] S. C. Bart, K. Chlopek, E. Bill, M. W. Bouwkamp, E. Lobkov-
sky, F. Neese, K. Wieghardt, P. J. Chirik, J. Am. Chem. Soc.
2006, 128, 13901.
[7] K. P. Butin, E. K. Beloglazkina, N. V. Zyk, Russ. Chem. Rev.
2005, 74, 531.
[8] B. de Bruin, E. Bothe, T. Weyhermüller, K. Wieghardt, Inorg.
Chem. 2000, 39, 2936.
[9] Vanadium: I. Vidyaratne, S. Gambarotta, I. Korobkov,
P. H. M. Budzelaar, Inorg. Chem. 2005, 44, 1187.
[10] Chromium: I. Vidyaratne, J. Scott, S. Gambarotta, P. H. M.
Budzelaar, Inorg. Chem. 2007, 46, 7040.
[(iPrPDI)Mn(CO)2][Na(OEt2)3]: A thick-walled glass vessel was
charged with mercury (45.41 g, 226.38 mmol) and toluene (approxi-
mately 50 mL). Sodium metal (0.227 g, 9.87 mmol) was cut into
pieces and added to the vessel. The amalgam was stirred for 10 min
and then solid [(iPrPDI)MnCl2] (1.00 g, 1.65 mmol) was added. The
vessel was submerged in liquid nitrogen and 4 atm of carbon mon-
oxide were added on the high vacuum line. The reaction mixture
was warmed to room temperature and stirred for 48 h during which
time a colour change from orange to dark yellow was observed.
After 48 h the vessel was degassed and placed in the glove box.
The solution was decanted from the amalgam and filtered through
Celite. The remaining product was extracted into diethyl ether and
filtered through Celite. Recrystallization from diethyl ether at
–35 °C yielded 0.772 g (92%) of dark-yellow crystals identified as
[(iPrPDI)Mn(CO)2][Na(OEt2)3]. Crystals suitable for X-ray diffrac-
tion were obtained from a concentrated diethyl ether solution at
1
–35 °C. H NMR ([D6]benzene, 20 °C): δ = 0.99 [d, J = 7.6 Hz, 12
[11] Iron: a) S. C. Bart, E. Lobkovsky, P. J. Chirik, J. Am. Chem.
Soc. 2004, 126, 13794; b) J. Scott, I. Vidyaratne, I. Korobkov,
S. Gambarotta, P. H. M. Budzelaar, Inorg. Chem. 2008, 47, 896;
c) S. K. Russell, J. M. Darmon, E. Lobkovsky, P. J. Chirik, In-
org. Chem. 2010, 49, 2782.
[12] Cobalt: a) V. C. Gibson, M. J. Humphries, K. P. Tellmann,
D. F. Wass, A. J. P. White, D. J. Williams, Chem. Commun ; b)
A. C. Bowman, C. Milsmann, C. C. Atienza, E. Lobkovsky, K.
Wieghardt, P. J. Chirik, J. Am. Chem. Soc. 2010, 132, 1676.
[13] Nickel: D. Zhu, I. Thapa, I. Korobkov, S. Gambarotta,
P. H. M. Budzelaar, Inorg. Chem. (ASAP) 2011.
H, CH(CH3)], 1.29 [d, J = 7.6 Hz, 12 H, CH(CH3)], 1.38 (br. s, 8
H, THF), 2.62 [s, 6 H, C(CH3)], 2.85 [sept, J = 7.6 Hz, 4 H,
CH(CH3)], 3.49 (br. s, 8 H, THF), 7.09 (br. s, 6 H, m-Ar and p-
Ar), 7.34 (t, J = 7.6 Hz, 1 H, p-py), 8.12 (d, J = 7.6 Hz, 2 H, m-
py) ppm. 13C NMR ([D6]benzene, 20 °C): δ = 17.48 [C(CH3)], 24.54
[CH(CH3)2], 25.54 [CH(CH3)2], 27.87 [CH(CH3)2], 117.47 (m-py),
123.02 (m-Ar), 124.85 (p-Ar), 126.41 (p-py), 140.09, 142.86, 146.81,
154.47 (quaternary carbons), 232.93 (CO) ppm. IR (KBr): ν =
˜
(CO) 1719, 1806 cm–1.
[(iPrPDI)Mn(CO)3][BPh4]: A thick-walled glass vessel was charged
with [(iPrPDI)Mn(CO)2] (0.200 g, 0.337 mmol) and [Cp2Fe][BPh4]
(0.170 g, 0.337 mmol). The vessel was then evacuated on the high
vacuum line and benzene (approximately 10 mL) was added at li-
quid-nitrogen temperature. While the reaction remained frozen,
4 atm of carbon monoxide were added. The solution was warmed
to room temperature and stirred for 30 min during which time a
colour change from purple to tan was observed. The vessel was
degassed and placed in the drybox where the solution was transfer-
red to a vial and pentane (approximately 10 mL) was added. Col-
lection of the precipitate yielded 0.233 g (76%) of a tan solid iden-
tified as [(iPrPDI)Mn(CO)2][BPh4]. C60H63BMnN3O3 (939.92):
[14] Ruthenium: M. Gallagher, N. L. Wieder, V. K. Dioumaev, P. J.
Carroll, D. H. Berry, Organometallics 2010, 29, 591.
[15] a) B. L. Small, M. Brookhart, J. Am. Chem. Soc. 1998, 120,
7143; b) B. L. Small, M. Brookhart, A. M. A. Bennett, J. Am.
Chem. Soc. 1998, 120, 4049; c) A. M. A. Bennett (Dupont) WO
98/27124, 1998; Chem. Abstr. 1998, 129, 122973x.
[16] a) G. J. P. Britovsek, V. C. Gibson, B. S. Kimberely, P. J. Mad-
dox, S. J. McTavish, G. A. Solan, A. J. P. White, D. J. Williams,
Chem. Commun. 1998, 849; b) G. J. P. Britovsek, M. Bruce,
V. C. Gibson, B. S. Kimberely, P. J. Maddox, S. Mastroianni,
S. J. McTavish, C. Redhsaw, G. A. Solan, S. S. Stromberg,
A. J. P. White, D. J. Williams, J. Am. Chem. Soc. 1999, 121,
8728.
[17] D. Reardon, G. Aharnian, S. Gambarotta, G. P. A. Yap, Orga-
nometallics 2002, 21, 786.
[18] H. Sugiyama, D. Reardon, G. Aharnian, S. Gambarotta,
G. P. A. Yap, P. H. M. Budzelaar, J. Am. Chem. Soc. 2002, 124,
12268.
1
calcd. C 76.67, H 6.67, N 4.47; found C 76.39, H 6.91, N 4.38. H
NMR ([D8]THF): δ = 1.08 [d, J = 5.2 Hz, 12 H, CH(CH3)2], 1.36
[d, J = 5.2 Hz, 12 H, CH(CH3)2], 2.17 [s, 6 H, C(CH3)], 2.57 [sept,
J = 5.2 Hz, 4 H, CH(CH3)2], 6.73 (br. s, 8 H, BPh4), 6.87 (br. s, 8
H, BPh4), 7.38 (br. s, 4 H, BPh4), 7.07 (m, 6 H, Ar), 7.74 (m, 3 H,
[19] For
a related bis(aldimino)pyridine-manganese dibromide
m-py and p-py) ppm. IR (KBr): ν = (CO) 1943, 1967, 2052 cm–1.
compound, see: D. A. Edwards, M. F. Mahon, W. R. Martin,
K. C. Molloy, J. Chem. Soc., Dalton Trans. 1990, 3161.
˜
[20] a) J. Cámpora, C. M. Pérez, A. Rodríguez-Delgado, A. M.
Naz, P. Palma, E. Álvarez, Organometallics 2007, 26, 1104; b)
J. Cámpora, P. Palma, C. M. Pérez, A. Rodríguez-Delgado, E.
Álvarez, E. Gutiérrez-Puebla, Organometallics 2010, 29, 2960.
[21] C. M. Pérez, A. Rodríguez-Delgado, P. Palma, E. Álvarez, E.
Gutiérrez-Puebla, J. Cámpora, Chem. Eur. J. 2010, 16, 13834.
[22] a) M. W. Bouwkamp, E. Lobkovsky, P. J. Chirik, J. Am. Chem.
Soc. 2005, 127, 9660; b) A. M. Tondreau, C. Milsmann, A. D.
CCDC-827876 {for [(iPrPDI)Mn(THF)2]}, -827877 {for [(iPrPDI)2-
Mn)]}, -827878 {for [(iPrPDI)Mn(CO)3][BPh4]}, -827879 {for
[(iPrPDI)Mn(CO)2][Na(OEt2)3]}, -827880 {for [(iPrPDI)Mn-
(CO)2]}, -844416 {for [(iPrPDI)MnCl(THF)]} contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
544
www.eurjic.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 535–545

Reduced Bis(imino)pyridine Manganese Compounds
Patrick, H. M. Hoyt, E. Lobkovsky, K. Wieghardt, P. J. Chirik, [28]
J. Am. Chem. Soc. 2010, 132, 15046.
C. C. Lu, E. Bill, T. Weyhermüller, E. Bothe, K. Wieghardt, J.
Am. Chem. Soc. 2008, 130, 3181.
R. J. Trovitch, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc.
2008, 130, 11631.
A. T. Radosevich, J. G. Melnick, S. A. Stoian, D. Bacciu, C.-
H. Chen, B. M. Foxman, O. V. Ozerov, D. G. Nocera, Inorg.
Chem. 2009, 48, 9214.
[23] C. C. H. Atienza, C. Milsmann, E. Lobkovsky, P. J. Chirik, An-
gew. Chem. Int. Ed. 2011, 50, 8143.
[24] G. J. P. Britovsek, J. England, S. K. Spitzmesser, A. J. P. White,
D. J. Williams, Dalton Trans. 2005, 945.
[29]
[30]
[25] a) M. W. Bouwkamp, A. C. Bowman, E. Lobkovsky, P. J. Chi-
rik, J. Am. Chem. Soc. 2006, 128, 13360; b) K. T. Sylvester, P. J.
Chirik, J. Am. Chem. Soc. 2009, 131, 8772; c) S. K. Russell,
P. J. Chirik, J. Am. Chem. Soc. 2011, 133, 8858.
[26] S. C. Bart, E. Lobkovsky, E. Bill, K. Wieghardt, P. J. Chirik,
Inorg. Chem. 2007, 46, 7055.
[31] P. H. Rieger, Coord. Chem. Rev. 1994, 135–136, 203.
[32] A. M. Tondreau, C. Milsmann, E. Lobkovsky, P. J. Chirik, In-
org. Chem., in press.
[33] G. A. Bain, J. F. Berry, J. Chem. Educ. 2008, 85, 532.
Received: June 4, 2011
Published Online: October 5, 2011
[27] B. M. Wile, R. J. Trovitch, S. C. Bart, A. M. Tondreau, E. Lob-
kovsky, C. Milsmann, E. Bill, K. Wieghardt, P. J. Chirik, Inorg.
Chem. 2009, 48, 4190.
Eur. J. Inorg. Chem. 2012, 535–545
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
545