Old Concepts, New Application – Additive-Free Hydrogenation of Nitriles Catalyzed by an Air Stable Alkyl Mn(I) Complex

Article abstract of DOI:10.1002/adsc.201901040

An efficient additive-free manganese-catalyzed hydrogenation of nitriles to primary amines with molecular hydrogen is described. The pre-catalyst, a well-defined bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dpre)(CO)₃(CH₃)] (dpre=1,2-bis(di-n-propylphosphino)ethane), undergoes CO migratory insertion into the manganese-alkyl bond to form acyl complexes which upon hydrogenolysis yields the active coordinatively unsaturated Mn(I) hydride catalyst [Mn(dpre)(CO)₂(H)]. A range of aromatic and aliphatic nitriles were efficiently and selectively converted into primary amines in good to excellent yields. The hydrogenation of nitriles proceeds at 100 °C with a catalyst loading of 2 mol % and a hydrogen pressure of 50 bar. Mechanistic insights are provided by means of DFT calculations. (Figure presented.).

Full text of DOI:10.1002/adsc.201901040

Title: Old Concepts, New Application - Additive-free Hydrogenation of
Nitriles Catalyzed by an Air Stable Alkyl Mn(I) Complex
Authors: Stefan Weber, Luis Veiros, and Karl Kirchner
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10.1002/adsc.201901040
Advanced Synthesis & Catalysis
Old Concepts, New Application - Additive-free Hydrogenation of
Nitriles Catalyzed by an Air Stable Alkyl Mn(I) Complex
Stefan Weber,[a] Luis F. Veiros,[b] and Karl Kirchner*,[a]
[a] Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt
9/163-AC, A-1060 Wien, Austria. E-mail: karl.kirchner@tuwien.ac.at
[b]
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av.
Rovisco Pais No. 1, 1049-001 Lisboa, Portugal.
Abstract: An efficient additive-free manganese-catalyzed hydrogenation of nitriles to
primary amines with molecular hydrogen is described. The pre-catalyst, a well-defined bench-
stable alkyl bisphosphine Mn(I) complex fac-[Mn(dpre)(CO)3(CH3)] (dpre = 1,2-bis(di-n-
propylphosphino)ethane), undergoes CO migratory insertion into the manganese-alkyl bond
to form acyl complexes which upon hydrogenolysis yields the active coordinatively
unsaturated Mn(I) hydride catalyst [Mn(dpre)(CO)2(H)]. A range of aromatic and aliphatic
nitriles were efficiently and selectively converted into primary amines in good to excellent
o
yields. The hydrogenation of nitriles proceeds at 100 C with a catalyst loading of 2 mol %
and a hydrogen pressure of 50 bar. Mechanistic insights are provided by means of DFT
calculations.
Keywords: manganese • alkyl complexes • hydrogenation • nitriles • migratory insertion
Introduction
Amines constitute an important class of organic compounds which are used as versatile
building blocks for the bulk and fine chemical industries.[1] A very attractive, atom-economic,
and sustainable route to obtain selectively primary amines is the hydrogenation of nitriles with
molecular hydrogen utilizing well-defined catalysts based on Earth abundant non-precious
metals.[2-4] In particular base metals such as iron and manganese are promising candidates as
these belong to the most abundant metals in the Earth crust, are inexpensive, and exhibit a low
environmental impact. While iron, cobalt, and nickel catalysts for the hydrogenation of polar
multiple bonds such as carbonyl compounds, imines, and nitriles had been subject of intense
investigation over the past decade,[5-11] low valent Mn(I) catalysts just appeared in 2016 as
new but very powerful players in this fast growing area.[6-11] As hydrogenation of nitriles is
concerned, several well-defined catalysts based on iron,[12] cobalt[13] and lately also
manganese (Scheme 1)[14] have been described.[15] It is interesting to note that most of these
reactions require the addition of additives (strong base) in order to activate the pre-catalysts.
1
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10.1002/adsc.201901040
Advanced Synthesis & Catalysis
Scheme 1. Manganese Catalysts for the Hydrogenation of Nitriles.
In this article, we report on the synthesis and application of a bench-stable and well-defined
alkyl bisphoshine Mn(I) complex as efficient pre-catalyst for the selective hydrogenation of
nitriles to give primary amines. It has to be noted that this is the first example of a manganese-
catalyzed hydrogenation of nitriles without the need of an additive (base) utilizing a well-
defined Mn(I) complex. The activation of the pre-catalyst takes advantage of the fact that (i)
CO ligands of Mn(I) alkyl carbonyl complexes are able to undergo migratory insertions to
form acyl complexes in the presence of strong field ligands such as CO or tertiary phosphines
- a well-known textbook reaction.[16] and that (ii) heterolytic cleavage of molecular hydrogen
by acyl complexes affords aldehydes and coordinatively unsaturated metal hydride
intermediates - a key step in both stoichiometric and catalytic hydroformylations of
alkenes.[17] Thus, the catalytic hydrogenation of nitriles is initiated by migratory insertion of a
CO ligand into the Mn-alkyl bond to yield acyl intermediates which undergo rapid
hydrogenolysis to form the active 16e- Mn(I) hydride catalysts as shown in Scheme 2.
Scheme 2. Formation of an Unsaturated Mn(I) Hydride Species via Alkyl Migration and
Hydrogenolysis of an Acyl Intermediate
Results and Discussion
In addition to the known Mn(I) complexes [Mn(CO)5(CH3)] (1),[18,19] fac-
[Mn(bipy)(CO)3(CH3)] (2),[20] the new Mn(I) complexes fac-[Mn(dpre)(CO)3(H)] (dpre = 1,2-
bis(di-n-propyl-phosphino)ethane) (4) and fac-[Mn(dpre)(CO)3(CH3)] (5)[21] were
synthesized, characterized, and applied. The hydride complex fac-[Mn(dpre)(CO)3(H)] (4)
was obtained by reacting a suspension of [Mn2(CO)10] and 1,2-bis(dipropylphosphino)ethane
in anhydrous n-pentanol at 140oC for 5 h in low yield (14%). The neutral methyl Mn(I)
complex 5 was obtained by reacting fac-[Mn(dpre)(CO)3(Br)] (3) with NaK and subsequently
2
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10.1002/adsc.201901040
Advanced Synthesis & Catalysis
with CH3I in 64 % isolated yield (Scheme 3). The resulting off-white complex was fully
1
characterized by H, 13C{1H} and 31P{1H} NMR and IR spectroscopy, and high-resolution
mass spectrometry. The IR spectrum contains strong CO stretching vibrations at 1974, 1888
and 1835 cm−1 which clearly indicate the coordination of three CO ligands to the metal center
1
in a facial arrangement. In the H and 13C{1H} NMR spectra the methyl group gives rise to
triplet resonances at -0.70 (J = 9.0 Hz) and -16.8 ppm (J = 18.2 Hz), respectively.
Scheme 3 Synthesis of fac-[Mn(dpre)(CO)3(CH3)] (5)
Scheme 4. Mn(I) Complexes Tested as Catalysts
The catalytic performance of complexes 1, 2, 4, and 5 (Scheme 4) was then investigated for
the hydrogenation of 4-fluorobenzonitrile as model substrate. Selected optimization
o
experiments are depicted in Table 1. With a catalyst loading of 3 mol% at 100 C and a
hydrogen pressure of 50 bar H2 in toluene for 18 h no reaction took place with complexes 1, 2
and 4 as pre-catalysts. This clearly emphasizes that strongly electron donating co-ligands such
as alkyl bisphosphines are required to achieve high catalytic activity. Moreover, the
coordinatively saturated and inert hydride complex 4 is inactive due to the lack of a vacant
coordination site. Accordingly, complex 4 is not an intermediate in the catalytic process
which is obviously a metal-centered, i.e., an inner-sphere, reaction. Under the same reaction
Table 1. Optimization of the Reaction Conditions for the Hydrogenation of 4-
Fluorobenzonitrile.a
3
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10.1002/adsc.201901040
Advanced Synthesis & Catalysis
entry catalyst
X (mol%) solvent T (oC) conversion (%)b yield (%)c
1
2
3
4
5
6d
7
8
[Mn(CO)5(CH3)] (1)
3
3
3
2
2
2
1
2
toluene 100
toluene 100
toluene 100
toluene 100
iPrOH 100
iPrOH 100
toluene 100
toluene 80
-
-
fac-[Mn(bipy)(CO)3(CH3)] (2)
fac-[Mn(dpre)(CO)3(H)] (4)
fac-[Mn(dpre)(CO)3(CH3)] (5)
fac-[Mn(dpre)(CO)3(CH3)] (5)
fac-[Mn(dpre)(CO)3(CH3)] (5)
fac-[Mn(dpre)(CO)3(CH3)] (5)
fac-[Mn(dpre)(CO)3(CH3)] (5)
-
-
-
-
>99
>99
-
95
93
-
37
-
11
-
aReaction conditions: 4-Fluorobenzonitrile (0.6 mmol), 50 bar H2, 5 mL solvent, 18 h.
c
bConversion determined by 19F{1H} NMR analysis Yield determined by 19F{1H} NMR
analysis using fluorobenzene as standard. dIn the absence of H2.
Table 2. Hydrogenation of Various (Hetero)aromatic Nitriles Catalyzed by 5.a
4
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Advanced Synthesis & Catalysis
aReaction conditions: Substrate (0.6 mmol), catalyst 5 (2 mol%), 5 mL anhydrous toluene,
100 °C 50 bar H2, 18 h. bIsolated yields as ammonium hydrochloride. cGC yield. d3 mol%. e72
h. f48 h
conditions with complex 5 but a catalyst loading of 2 mol%, 4-fluorobenzylamine was
obtained in 95 % yield (Table 1, entry 4). Similar results were obtained with 5 in iPrOH
(Table 1, entry 5), whereas no reaction took place in this solvent in the absence of H2 thus
5
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10.1002/adsc.201901040
Advanced Synthesis & Catalysis
clearly excluding a transfer hydrogenation process (Table 1, entry 6). At lower temperature
o
(80 C) or a lower catalyst loading (1 mol%), the catalytic activity dropped significantly and
no or lower conversions of 4-fluorobenzonitrile were achieved (Table 1, entries 7 and 8).
Table 3. Hydrogenation of Various Aliphatic Nitriles and Dinitriles Catalyzed by 5.a
aReaction conditions: Substrate (0.6 mmol), catalyst 5 (2 mol%), 5 mL anhydrous toluene,
100 °C 50 bar H2, 18 h. bIsolated yields as ammonium hydrochloride. cGC yield. d3 mol%. e72
h. f48 h
Having established the best reaction conditions, the applicability of catalyst 5 is
demonstrated in the selective hydrogenation of various nitriles, including substituted
6
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10.1002/adsc.201901040
Advanced Synthesis & Catalysis
aromatic, benzylic, aliphatic nitriles and a dinitriles. The results for aromatic substrates are
shown in Tables 2 and 3. Substrates with electron-withdrawing substituents such as halides
containing substrate are very efficiently converted to amines in excellent yields (Table 2,
entries 1-5). Good yields are achieved for substrates with electron-donating substituents such
as methyl, methoxy and amino groups (Table 2, entries 7-9). Heterocycles, for instance,
pyridine and thiophene derivatives are well tolerated (Table 2, entries 10-12).
No reaction was observed in the case of 2-(4-nitrophenyl)acetonitrile (Table 3, entry 1)
indicating catalyst deactivation. Dinitriles were reduced to the corresponding diamines in
good to excellent yields (Table 3, entries 4-6). Interestingly, aliphatic dinitriles, such as
adiponitrile (Table 3, entry 5) gave higher yields in comparison to the aromatic analogues
(Table 3, entry 4). Finally, linear aliphatic nitriles were reduced to the corresponding amines
in excellent yields (Table 3, entries 7-11).
A reasonable mechanism for the hydrogenation of nitriles was established by means of
DFT calculations in toluene with benzonitrile as model substrate.[22] The initiation step and
the key intermediates along the catalytic cycle are presented in Figures 1 and 2 (see
supporting information for details). The active species is formed by methyl migration to a CO
ligand in complex 5 (A in the calculations, Figure 1) producing acyl complex B which is
stabilized by a C-H agostic interaction. This step is endergonic (ΔG = 10.8 kcal/mol) with an
accessible barrier of 12.0 kcal/mol. Upon addition of H2 the dihydrogen complex D is formed.
This step has a barrier of 9.0 kcal/mol and is slightly endergonic (ΔG = 3.3 kcal/mol).
Protonation of the acyl ligand through H-transfer from the dihydrogen ligand takes place
yielding finally intermediate E, featuring a hydride ligand and an O-coordinated acetaldehyde.
The last step, from D to E, goes over a 5.4 kcal/mol barrier and has a favorable free energy
balance of -2.1 kcal/mol. The highest barrier along the initiation process is 18.1 kcal/mol
corresponding to the energy of transition state TSCD.
Figure 1. Free Energy Profile Calculated for the Formation and Hydrogenolysis an Acyl
Intermediate. Free Energies (kcal/mol) are Referred to fac-[Mn(dpre)(CO)3(CH3)] (5) (A in
the calculations).
A simplified catalytic cycle is depicted in Figure 2. The reaction begins with exchange
of the aldehyde in intermediate E by one molecule of nitrile to form F, the initial species in
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10.1002/adsc.201901040
Advanced Synthesis & Catalysis
the cycle with an N-coordinated benzonitrile. From here the reaction consists of two
consecutive H2 additions to the substrate going from a nitrile to an amine group. The catalytic
cycle starts with hydride transfer to the C-atom of the nitrile group producing an intermediate
with a side-bonded C=N double bond that is further stabilized by a C-H agostic interaction
(G). This is an endergonic step (ΔG = 7.0 kcal/mol) with a moderate barrier of 8.2 kcal/mol.
A ligand rearrangement from side to end on coordination leads to intermediate H. Here, the
PhCHC=N ligand is coordinated by the N-atom becoming a 4-electron donor and establishing
a Mn=N double bond as indicated by a short Mn-N distance (1.76 Å) and a Mn-N-C angle of
Figure 2. Simplified Catalytic Cycle for the Hydrogenation of Benzonitrile (Free energies in
kcal/mol are referred to fac-[Mn(dpre)(CO)3(CH3)] (5), barriers in italic and transition state
energies in parenthesis).
173° approaching linearity. The free energy balance for that rearrangement is very
favorable: ΔG = -19.0 kcal/mol. The path proceeds with H2 coordination reaching
intermediate K, a dihydrogen species. Overall, the coordination of dihydrogen overcomes a
8
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10.1002/adsc.201901040
Advanced Synthesis & Catalysis
significant barrier of 21.6 kcal/mol, being also unfavorable, from the thermodynamic point of
view, K being 19.9 kcal/mol less stable than H. The reaction continues with N-protonation via
H-transfer from the dihydrogen ligand. This leads to intermediate M presenting an N-
coordinated imine ligand, with a barrier of 6.9 kcal/mol and a favorable free energy balance of
ΔG = -20.1 kcal/mol. This concludes the first part of the mechanism, where one equivalent of
H2 was added to the initial benzonitrile forming the corresponding imine PhCH=NH.
The second part of the catalytic cycle starts with a rearrangement of the coordination
mode of the imine ligand in M, followed by hydride transfer to the imine C-atom resulting in
intermediate P containing a N-coordinated amido ligand. The overall process has a barrier of
14.1 kcal/mol and is favorable by -6.9 kcal/mol.[23] The mechanism proceeds with
coordination of the second H2 molecule resulting in the dihydrogen complex R which features
an amido co-ligand. Coordination of the second dihydrogen molecule is equivalent to what
was found for the first one with similar barrier and free energy balance (21.2 kcal/mol).
The final step along the path corresponds to protonation of the N-atom in R, in a facile
step with a negligible barrier of 2.0 kcal/mol and a very favorable free energy balance (ΔG =
-27.5 kcal/mol). In the final intermediate, S, the reaction product (benzylamine) is already
formed and remains N-coordinated to the metal. From S, ligand exchange with loss of the
product and addition of a new nitrile molecule closes the cycle and regenerates the starting
intermediate F with a neutral free energy balance of ΔG = 0 kcal/mol. The overall reaction
barrier is 26.8 kcal/mol, measured from H to the transition state of N-protonation (TSKL, see
supporting information). This is a high barrier that may be slightly overestimated but reflects
the reaction conditions (18 h at 100 °C).
Conclusion
In sum, we report an efficient hydrogenation of nitriles with molecular hydrogen catalyzed by
a well-defined, bench-stable bisphosphine Mn(I) complex. Our results indicate that an inner-
sphere (metal-centered) process without ligand participation is taking place. To the best of our
knowledge, this is the first example of an additive-free hydrogenation of nitriles catalyzed by
a well-defined Mn(I) complex. These reactions are atom economic implementing an
inexpensive, earth abundant non-precious metal catalyst. The catalytic process is initiated by
migratory insertion of a CO ligand into the Mn-alkyl bond to yield an acyl intermediate which
undergoes rapid hydrogenolysis to form the active 16e- Mn(I) hydride catalyst
[Mn(dpre)(CO)2(H)] - a conceptually new approach in Mn(I) hydrogenation chemistry. A
range of (hetero)aromatic and aliphatic nitriles were efficiently converted into primary
amines, respectively, in good to excellent isolated yields. The hydrogenation of nitriles
o
proceeds at 100 C with a catalyst loading of 2 mol %. A hydrogen pressure of 50 bar was
applied and the reaction time was 18 h. The mechanism obtained by means of DFT
calculations consists of two successive and equivalent H2 additions to the initial nitrile
substrate. The crucial features in the path are hydride transfer to the substrate C-atom,
followed by protonation of the corresponding N-atom, deriving from key hydride and
dihydrogen intermediates, respectively.
Experimental Section
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Advanced Synthesis & Catalysis
General Information. All reactions were performed under an inert atmosphere of argon by
using Schlenk techniques or in a MBraun inert-gas glovebox. The solvents were purified
according to standard procedures. The deuterated solvents were purchased from Aldrich and
dried over
3
Å
molecular sieves. Complexes [Mn(CO)5(CH3)] (1),[24] fac-
[Mn(bipy)(CO)3(CH3)] (2),[25] fac-[Mn(dpre)(CO)3(Br)] (dpre = 1,2-bis(di-n-propyl-
phosphino)ethane) (3),[14b] were synthesized according to literature.1H and 13C{1H}, and
31P{1H} NMR spectra were recorded on Bruker AVANCE-250, AVANCE-400, and
1
AVANCE-600 spectrometers. H and 13C{1H} NMR spectra were referenced internally to
residual protio-solvent, and solvent resonances, respectively, and are reported relative to
tetramethylsilane ( = 0 ppm). 31P{1H} NMR spectra were referenced externally to H3PO4
(85%) ( = 0 ppm). Hydrogenation reactions were carried out in a Roth steel autoclave using a
Tecsis manometer.
High resolution-accurate mass data mass spectra were recorded on a hybrid Maxis Qq-
aoTOF mass spectrometer (Bruker Daltonics, Bremen, Germany) fitted with an ESI-source.
Measured accurate mass data of the [M]+ ions for confirming calculated elemental
compositions were typically within 5 ppm accuracy. The mass calibration was done with a
commercial mixture of perfluorinated trialkyl-triazines (ES Tuning Mix, Agilent
Technologies, Santa Clara, CA, USA).
GC–MS analysis was conducted on a ISQ LT Single quadrupole MS (Thermo Fisher) directly
interfaced to a TRACE 1300 Gas Chromatographic systems (Thermo Fisher), using a Rxi-5Sil
MS (30 m, 0.25mm ID) cross-bonded dimethyl polysiloxane capillary column.
Synthesis. fac-[Mn(dpre)(CO)3(H)] (4). A microwave vial as charged with [Mn2(CO)10]
(300 mg, 0.77 mmol), 1,2-bis(dipropylphosphino)ethane (404 mg, 1.54 mmol) and anhydrous
n-pentanol (350 µL). The mixture was heated to 140 °C for 5 h. All volatiles were removed
under vacuum affording a red oil. Anhydrous MeOH (3 mL) was added and the yellow
solution was placed in the freezer giving a yellow solution and a colorless precipitate. The
suspension was centrifuged and the mother liquor was decanted. The solid was washed three
times with MeOH (1 mL) and the solid was dried under vacuum. Yield: 80 mg (14 %)
colorless solid. 1H NMR (δ, 400 MHz, C6D6, 20 oC): 1.56 – 0.85 (m, 25H), 0.76 (t, J = 6.9 Hz,
13
6H), 0.70 (t, J = 6.0 Hz, 6H), -9.25 (t, J = 48.6 Hz, 1H). C{1H} NMR (δ ,101 MHz, C6D6,
o
20 C): 224.6, 221.2, 32.6 (vm), 25.1 (vt, J = 13.1 Hz), 16.8 (vd, J = 6.1 Hz), 15.0 (vm).
o
31P{1H} NMR (δ ,162 MHz, C6D6, 20 C): 86.8 (s). ATR-IR (solid, cm-1): 1976 (CO), 1873
(CO), 1737 (MnH). HRMS (TOF ESI+): m/z calculated for C17H31MnO3P2 [M-H]+: 401.1202,
found 401.1196.
fac-[Mn(dpre)(CO)3(CH3)] (5). To a solution of fac-[Mn(dpre)(CO)3(Br)] (3) (500mg, 1.11
mmol) in anhydrous THF (30 mL) liquid Na/K alloy (2:3,110 mg, 3.33 mmol) was added and
stirred for 24 h. The solution was filtrated via a syringe filter and methyl iodide (1 mL) was
added and the mixture was stirred for 10 min. The solvent was removed under vacuum and
the residue was extracted with CH2Cl2 (15 mL) giving an orange solution. Upon removal of
the solvent a yellow oily residue was obtained which was dissolved in boiling anhydrous
MeOH (10 mL) and the flask was stored in the freezer resulting in the crystallization of an
10
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10.1002/adsc.201901040
Advanced Synthesis & Catalysis
1
o
off-white solid. Yield: 293 mg (64%) off-white solid. H NMR (, 400 MHz, C6D6, 20 C):
1.72 – 1.33 (m, 14H), 1.33-1.19 (m, 3H), 1.19 – 0.93 (m, 7H), 0.87 (t, J = 6.9 Hz, 6H), 0.80 (t,
J = 6.2 Hz, 7H), -0.70 (t, J = 9.0 Hz, 3H). 13C{1H} NMR (, 101 MHz, C6D6, 20 oC): 31.4 (vt,
J = 12.1 Hz) , 24.6 (vd, J = 10.1 Hz), 23.9 (vt, J = 20.2 Hz), 17.3 (vd, J = 64.6 Hz), 15.7 (vd, J
= 12.1 Hz), -16.8 (vt, J = 18.2 Hz), (CO not observed). 31P{1H} NMR (, 162 MHz, C6D6, 20
oC): 78.7 (s). ATR-IR (solid, cm-1): 1974 (CO), 1888 (CO), 1853 (CO). HRMS (TOF ESI+):
m/z calculated for C17H31MnO3P2 [M-CH3]+: 401.1202, found 401.1199.
General procedure for the hydrogenation of nitriles. Inside an Ar-flushed glovebox, the
nitrile (0.6 mmol, 1 equiv) and fac-[Mn(dpre)(CO)3(CH3)] (5) (0.006-0.022 mmol, 0.01-0.04
equiv.) were mixed with 5 mL of the anhydrous solvent and transferred into a steel autoclave,
which was three times evacuated and flushed with Ar prior use. The autoclave was flushed
three times with hydrogen, 50 bar H2 pressure were applied and the autoclave was placed in
an oil bath (80-100°C) and the reaction was run for 18 h. The autoclave was then cooled in an
ice bath for 10 min. The reaction mixture was filtrated through a PTFE membrane and
19
analyzed by F{1H} NMR for 4-fluorobenzonitrile as test substrate using fluorobenzene as
external standard.
Isolation of product as ammonium salt. The reaction mixture was filtrated through a short
plug of Celite and diluted with Et2O to 25 mL and 1 M HCl in Et2O was added drop wise until
precipitation was complete. The solution was decanted and the ammonium salt was several
times washed with Et2O and dried under vacuum.
Computational details. The computational results presented have been achieved in part using
the Vienna Scientific Cluster (VSC). All calculations were performed using the GAUSSIAN 09
software [26] without symmetry constraints. The optimized geometries were obtained with the
the PBE0 functional. That functional uses a hybrid generalized gradient approximation
(GGA), including 25 % mixture of Hartree-Fock[27] exchange with DFT[28] exchange-
correlation, given by Perdew, Burke and Ernzerhof functional (PBE).[29] The basis set used for
the geometry optimizations (basis b1) consisted of the Stuttgart/Dresden ECP (SDD) basis
set[30] to describe the electrons of manganese, and a standard 6-31G(d,p) basis set[31] for all
other atoms. Transition state optimizations were performed with the Synchronous Transit-
Guided Quasi-Newton Method (STQN) developed by Schlegel et al,[32] following extensive
searches of the Potential Energy Surface. Frequency calculations were performed to confirm
the nature of the stationary points, yielding one imaginary frequency for the transition states
and none for the minima. Each transition state was further confirmed by following its
vibrational mode downhill on both sides and obtaining the minima presented on the energy
profiles. The electronic energies (Eb1) obtained at the PBE0/b1 level of theory were converted
to free energy at 298.15 K and 1 atm (Gb1) by using zero point energy and thermal energy
corrections based on structural and vibration frequency data calculated at the same level. The
free energy values presented were corrected for dispersion by means of Grimme DFT-D3
method[33] with Becke and Johnson short distance damping.[34] Solvent effects (toluene) were
considered in all calculations using the Polarizable Continuum Model (PCM) initially devised
11
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10.1002/adsc.201901040
Advanced Synthesis & Catalysis
by Tomasi and coworkers[35] with radii and non-electrostatic terms of the SMD solvation
model, developed by Truhlar et al.[36]
Acknowledgements
KK gratefully acknowledges the Financial support by the Austrian Science Fund (FWF)
(Project No. P29584-N28). LFV thanks Fundação para a Ciência e Tecnologia,
UID/QUI/00100/2013.
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Advanced Synthesis & Catalysis
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Graphic Abstract:
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