Tracking the Progress and Mechanism Study of a Solvothermal in Situ Domino N-Alkylation Reaction of Triethylamine and Ammonia Assisted by Ferrous Sulfate

Article abstract of DOI:10.1021/acs.inorgchem.7b01190

Atom economic in situ domino N-alkylation reactions of triethylamine/ammonia with 2-(hydroxymethyl)quinolin-8-ol (HL-OH) assisted by FeSO₄·7H₂O were realized under mild solvothermal conditions at 120 °C in acetonitrile. The resulting tripodal 2,2′,2″-[nitrilotris(methylene)]tris(quinolin-8-ol) (H₃L₃-N) forms a linear trimer [Fe₃(L₃-N)₂] (1). Electrospray ionization mass spectrometry of the reaction solution provides evidence for the intermediates of three steps, while crystallography and X-ray photoelectron spectroscopy characterize the trimer. Shortening the time of the reaction allowed for the organic intermediates to be isolated, which led to a proposed mechanism. The method provides a facile way to produce symmetric tertiary amine from widely used NEt₃ and NH₃. The results represent an example of the in situ Fe²⁺-catalyzed domino reaction in which Fe²⁺ is coordinated by the generated ligands and is involved in each step until the final cluster 1.

Full text of DOI:10.1021/acs.inorgchem.7b01190

Communication
pubs.acs.org/IC
Tracking the Progress and Mechanism Study of a Solvothermal in
Situ Domino N‑Alkylation Reaction of Triethylamine and Ammonia
Assisted by Ferrous Sulfate
Jin-Ping Zhong,†,∥ Bin Liu,‡,∥ Tao Yang,†,‡ Yue-Jin Liu,‡ Zhong-Hong Zhu,† Bing-Feng Shi,§
Mohamedally Kurmoo,⊥ and Ming-Hua Zeng*,†,‡
†Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources, Department of Chemistry and Pharmaceutical
Sciences, Guangxi Normal University, Guilin 541001, P. R. China
‡Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry of Education Key Laboratory for the
Synthesis and Application of Organic Functional Molecules, and College of Chemistry and Chemical Engineering, Hubei University,
Wuhan 430062, P. R. China
§Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China
⊥Institut de Chimie de Strasbourg, CNRS, UMR 7177, Universite
́
de Strasbourg, 4 rue Blaise Pascal, 67070 Strasbourg, France
S
* Supporting Information
novel structures and physical properties.3,4 The investigation of
the mechanism of such in situ metal/ligand reactions is helpful in
understanding the processes that can lead to the rational design
of target molecules.4
Rarely have domino reactions, where simple starting materials
undergo a series of step-by-step reactions in one pot, been
reported using the solvothermal method.6 Domino reactions are
desirable for the construction of complicated compounds
because it is atom, step, and time economic (Scheme 1a).6,7
Fe-catalyzed domino reactions are appealing for their cheapness
and hypotoxicity (Table S1). However, in situ metal/ligand
ABSTRACT: Atom economic in situ domino N-alkyla-
tion reactions of triethylamine/ammonia with 2-
(hydroxymethyl)quinolin-8-ol (HL-OH) assisted by
FeSO4·7H2O were realized under mild solvothermal
conditions at 120 °C in acetonitrile. The resulting tripodal
2,2′,2″-[nitrilotris(methylene)]tris(quinolin-8-ol) (H3L3-
N) forms a linear trimer [Fe3(L3-N)2] (1). Electrospray
ionization mass spectrometry of the reaction solution
provides evidence for the intermediates of three steps,
while crystallography and X-ray photoelectron spectrosco-
py characterize the trimer. Shortening the time of the
reaction allowed for the organic intermediates to be
isolated, which led to a proposed mechanism. The method
provides a facile way to produce symmetric tertiary amine
from widely used NEt3 and NH3. The results represent an
example of the in situ Fe2+-catalyzed domino reaction in
which Fe2+ is coordinated by the generated ligands and is
involved in each step until the final cluster 1.
Scheme 1. (a) General Scheme of Domino Reactions and (b)
in Situ Multistep N-Alkylation of NEt3 or Ammonia Assisted
by FeSO4·7H2O To Produce the Cluster 1
ne of the primary goals in chemistry is to understand how
Oreactions progress while identifying the intermediates so
that schemes and mechanisms can be drawn.1−3 Observing and
tracking these reactions using the appropriate methods are
prerequisites to attaining these goals.2,3 Choosing a reaction
procedure is an important basis for direct synthesis and for
further reactions.3,4 The solvothermal and hydrothermal
techniques are becoming important synthetic tools and have
generated otherwise unattainable organic molecules, coordina-
tion polymers (or metal−organic frameworks), and clusters.3−5
Often in situ metal/ligand reactions take place when a
solvothermal technique is employed in which the starting ligands
undergo reactions assisted by the metals to produce oligomeric
ligands that are difficult to synthesize or even inaccessible using
traditional organic synthetic approaches.4 In addition, the in situ
produced ligands could further participate in the assembly of
transition metals to give functional coordination complexes with
Received: May 13, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01190
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
domino reactions are rare, and mechanistic study is still in its
infancy.
2-(Hydroxymethyl)quinolin-8-ol (HL-OH) is known to
undergo an in situ C−C coupling reaction in the presence of
ethanol, Co(H2O)6(NO3)2, and triethylamine under solvother-
mal conditions to give 1,2-bis(8-hydroxyquinolin-2-yl)ethane-
1,2-diol, which resulted in the formation of red crystals of the
single-molecule magnet [CoII7(bhqe)3(OH)2(H2O)6]·
2C2H5OH·H2O.8 In contrast, 1,2-bis(benzo[d]imidazol-2-yl)-
1,2-ethanediol undergoes in situ dehydration under solvothermal
conditions in the presence of water, methanol, Zn-
(H2O)6(NO3)2, and triethylamine to form 1,2-bis(benzo[d]-
imidazol-2-yl)-1,2-ethenol (H3Ln) and crystallizes in the highly
luminescent [Zn5(H2Ln)6](NO3)4·8H2O·2CH3OH, which is a
nontoxic bioimaging marker for tumor cells.3b The importance of
in situ oligomerization of ligands in a solvothermal reaction thus
prompted us to explore a low-redox-potential metal such as Fe2+
in the reaction of HL-OH.
An unprecedented Fe2+-catalyzed domino reaction of the
widely used organic base NEt3 with HL-OH, producing a linear
trinuclear iron cluster [Fe3(L3-N)2] (1; Scheme 1b and Figure
S1), has been observed. Crystal data for 1 and selected bond
lengths and angles are summarized in Table S2. 2,2′,2″-
[Nitrilotris(methylene)]tris(quinolin-8-ol) (H3L3-N) is poten-
tially expected to lose three protons of its alcohols, giving six
negative charges for a pair to obtain the neutral trimeric complex
(Figure S2); therefore, each Fe atom has to be divalent. This was
confirmed by X-ray photoelectron spectroscopy (XPS; Figure
S3c). The two peripheral Fe atoms are seven-coordinated by four
N and three O atoms of each L3-N, and the central Fe atom
bridges the two through coordination to the six O atoms (Figure
S4a). The coordination sphere around the seven-coordinated
FeII is a capped flattened octahedron, which is suggested by
calculation of the continuous-shape measurement value (Table
S3). The molecular ion peak ([Fe3(L3-N)2]+; m/z 1138.12) of
cluster 1 was recorded by electrospray ionization mass
spectrometry (ESI-MS; Figure S5 and Table S4).
It is clear from the crystal structure of compound 1 that the in
situ metal/ligand reaction of HL-OH and NEt3 in the presence of
FeSO4·7H2O generated the coordinating tripodal amine ligand.
Quite surprising was the involvement of the commonly used
organic base NEt3 as the nitrogen source. When the reaction was
performed without NEt3, neither crystals of cluster 1 nor the ESI-
MS signal related to the Fe3(L3-N)2 fragment was observed in the
reaction solution and precipitate. The mild conditions and the
use of the most abundant metal are very encouraging as potential
commercially economic terms for industrial application.9
Consequently, we set out to unravel the reaction pathway in
order to provide a mechanism.
Figure 1. (a) Selected peaks of the time-dependent ESI-MS spectra of
the reaction of NEt3 with HL-OH in the presence of FeSO4·7H2O. (b)
Proposed mechanism (the intermediates in squares have the core
structures of the detected fragments in part a).
and a new species has appeared at m/z 1138.13, which is assigned
to the molecular ion of cluster [Fe2 FeIII(L3-N)2]+ (F5,
II
1138.13). After 48 h, the intensities of F2−F4 have become
very weak and that of F5 is the most prominent peak. The change
of the intensities of the peaks of F1−F4 indicates their
intermediate nature and that NEt3 undergoes stepwise N-
alkylation. The time-dependent ESI-MS spectra of the reaction
precipitates (Figure S7b) are similar to those of the reaction
solutions, confirming that domino N-alkylation produces cluster
1. To our knowledge, this is the first time that the N-alkylation
reaction of amine with alcohol was monitored by ESI-MS and the
key intermediates were successfully detected.
The amount of reaction precipitate increases with time. The
XRD pattern of the reaction precipitate resembles more and
more that simulated for the crystal (Figure S6), also suggesting
that the amount of cluster 1 increases. This is also confirmed by
the increase of the peak intensity of 1 in the time-dependent ESI-
MS of the reaction precipitate (Figure S7 and Table S5).
Through variation of reaction time, the N-alkylated organic
intermediates HL-NEt2 and H2L2-NEt and the product H3L3-N
could be obtained postsynthetically through acidification of the
reaction solution and precipitate, followed by extraction and
thin-layer chromatography (Figures S7d, S8, and Table S6).
From the results of the above analyses, a plausible mechanism
is proposed (Figure 1b and Table S7). First, coordination of HL-
OH with Fe2+ in the presence of NEt3 produces pincer-type
intermediate I ([FeIII(L-O)(H2O)4(CH3CN)]+, m/z 342.03). It
is known that the manganese pincer complexes (Mn-PNP) are
efficient catalysts for N-alkylation of primary and secondary
amines with alcohol via a hydrogen-borrowing (BH) pathway.10
Although it can also undergo a nucleophilic substitution pathway
(Table S8), the alkaline environment is not suitable for it. Then
the coordinated CH2O group transforms to the CHO group, as
proposed via the BH mechanism.11 It is known that the tertiary
ESI-MS of the reaction solutions as a function of time (1, 5, 24,
and 48 h) of the solvothermal treatment was used to search for
information on the reaction process (Figures 1a and S7). The
spectrum after 1 h shows four main sets of peaks at m/z 342.03,
432.00, 518.06, and 543.11, which were assigned to the
respective species [FeIII(L-O)(H2O)4(CH3CN)]+ (F1, m/z
342.05), [FeII(L-NEt2)K2(OH)3(H2O)]+ (F2, m/z 432.02),
II
[Fe2 (L2-NEt)(CH3OH)(OH)]+ (F3, m/z 518.05), and
[FeII(L3-N) + 2H]+ (F4, m/z 543.11). These fragments contain
the deprotonated starting ligand (L-O) and the monosubstituted
(L-NEt2), disubstituted (L2-NEt), and trisubstituted (L3-N)
amine intermediates, suggesting a possible step-by-step reaction.
After 5 h, the main F1−F4 peaks are still present and their
intensities have changed marginally. By 24 h, F1 has disappeared
B
DOI: 10.1021/acs.inorgchem.7b01190
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Communication
amine could undergo N−C bond cleavage via an iminium ion
intermediate catalyzed by an iron complex.12 Furthermore, the
reductive amination of tertiary amine and aldehyde has been
reported via the iminium cation intermediate.13 Then,
coordination of NEt3 with the central Fe atom leads to α-CH
activation to give the coordinated iminium ion intermediate A.
The reaction between the iminium ion and aldehyde results in
intermediate II ([FeIII(L-NEt2)(OH)3]+) and acetaldehyde,
which has so far been elusive possibly because of its low boiling
point and high reactivity. II is also a tertiary amine reacting with
another molecule of HL-OH via N-alkylation to give
intermediate III ([FeII(L2-NEt)(OH)]+; m/z 518.06). In III,
the L2-NEt ligand will also coordinate with Fe2+. This could
preserve the Fe2+ cation from dissociation and then facilitate the
third step N-alkylation to give intermediate IV ([FeII(L3-N)]+).
In IV, Fe2+ is trapped by ligand L3-N and hinders coordination of
the other ligand to Fe2+. Finally, two IV intermediates are easily
linked by one Fe2+ atom through the O atoms of L3-N to produce
the final product 1. The key intermediates (I−IV) are the cores
of the fragments F1−F4 observed by ESI-MS. The Fe ion is
indispensable for this transformation because without FeSO4·
7H2O or when using other sulfate, no related fragments were
detected by MS (Figure S12). Furthermore, the ligand HL-OH
could provide suitable electronic and steric environments for
Fe2+, which is helpful for coordination of NEt3 followed by a
further reaction with the in situ generated aldehyde group. HL-
OH is also helpful in stabilizing the N-alkylation intermediates,
assisting further reaction of the intermediates with another two
HL-OH ligands, from which triple N-alkylation is achieved.
However, other alcohols [(1-methyl-1H-benzo[d]imidazol-2-
yl)methanol and (1H-benzo[d]imidazol-2-yl)methanol] do not
undergo this transformation, as judged by the absence of any
crystals or crystalline products as well as the intermediates and
molecular fragments corresponding to 1 in the MS of the
solutions. Fe2+ is preserved in the final [Fe3(L3-N)2]+ cluster.
This could explain why the amount of FeSO4·7H2O is slightly
more than HL-OH, different from the commonly used catalyst
loading. In contrast to N-alkylation reactions of low-weight
amines in the literature that always end after two steps,14,15 the
present procedure goes to the third step leading to the
symmetrical tertiary amine.
Direct N-alkylation of amine with alcohol constitutes an
attractive environmentally friendly process because water is
generated as the main byproduct.10,14−17 N-alkylation of tertiary
amine with alcohol has rarely been reported (Table S9).18 The
products are usually formed from single or double N-alkylation
of tertiary amine catalyzed by RuCl3·3H2O18a or under harsh
conditions.18b,c To our knowledge, this is the first example of a
triple domino N-alkylation of NEt3 assisted by iron catalyst under
solvothermal conditions. Furthermore, these results show that
the time-dependent ESI-MS is a valuable tool for monitoring the
domino reaction, from which the reaction could be controlled to
gain the desired intermediates.
As a cheap and readily available nitrogen source, ammonia has
been applied in N-alkylation with alcohol.19 The N-alkylation of
NH3 was realized at the same reaction condition, producing the
same cluster 1 with improved yield (Figures S9, S10, and Table
S10). A possible mechanism involving triple BH pathways is
proposed (Figure S10). In effect, NH3 is a good enough nitrogen
source to give complex 1.
ature resulting in a trimeric cluster. For the first time, the stepwise
intermediates are identified by ESI-MS. Estimation of their
quantities at different reaction times followed by their isolation
reveals three consecutive N-alkylation steps that allowed for a
mechanism to be established. The selective synthesis of organic
ligands HL-NEt2, H2L2-NEt, and H3L3-N is realized by
controlling the reaction time. It is also less hazardous and
more economic than those in the literature. Application of the
procedure for the reaction of ammonia was also successfully
performed with improved yield. Because Fe2+ is present in all of
the intermediates and products, it is reasonable to conclude that
the chelating ligands HL-OH, L-NEt2/L-NH2, and L2-NEt/L2-
NH could provide suitable electronic and steric environments for
Fe2+, which could promote multiple N-alkylations and the
stepwise assembly. This could give some insight of the
coordination catalysis and self-catalysis. The results represent a
vivid example of the metal-catalyzed in situ domino reaction. The
synthesis of other multidentate ligands via this strategy is
ongoing.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01190.
Experimental details, crystal data, mass spectrometry,
XRD, XPS, and NMR (PDF)
Accession Codes
CCDC 1540483−1540484 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: zmh@mailbox.gxnu.edu.cn.
■
ORCID
Bin Liu: 0000-0002-2268-3653
Tao Yang: 0000-0003-1864-8545
Bing-Feng Shi: 0000-0003-0375-955X
Mohamedally Kurmoo: 0000-0002-5205-8410
Ming-Hua Zeng: 0000-0002-7227-7688
Author Contributions
∥These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation of
China for Distinguished Young Scholars (Grant 21525101),
NSFC (Grants 91422302 and 21371037), NSFGX (Grants
2014GXNSFFA118003 and CMEMR2014-A02), BAGUI schol-
ar program (Grant 2014A001). M.K. was funded by the CNRS,
France.
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