Cyclotrimerisation of isocyanates catalysed by low-coordinate Mn(ii) and Fe(ii) m-terphenyl complexes

Article abstract of DOI:10.1039/C6CC07243G

Two- and three-coordinate m-terphenyl complexes of manganese and iron are efficient catalysts for the selective cyclotrimerisation of primary aliphatic isocyanates affording isocyanurates in short reaction times and under mild conditions.

Full text of DOI:10.1039/C6CC07243G

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DOI: 10.1039/C6CC07243G
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Cyclotrimerisation of isocyanates catalysed by low-coordinate
Mn(II) and Fe(II) m-terphenyl complexes
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
a
a
Helen R. Sharpe, Ana M. Geer, Huw E. L. Williams, Toby J. Blundell, William Lewis, Alexander J.
a
a
Blake and Deborah L. Kays *
DOI: 10.1039/x0xx00000x
www.rsc.org/
Two- and three-coordinate m-terphenyl complexes of manganese
and iron are efficient catalysts for the selective cyclotrimerisation
of primary aliphatic isocyanates affording isocyanurates in short
reaction times and under mild conditions.
The cyclotrimerisation of isocyanates is a direct and an atom-
economical approach for the synthesis of isocyanurates. These
compounds are commonly used as additives in polyurethane
foams, coating materials and composites in order to enhance
1
Scheme 1. Cyclotrimerisation of isocyanates catalysed by m-terphenyl Mn(II) (1, 3) and
Fe(II) (2, 4) complexes, where Tmp = 2,4,5-Me , Mes = 2,4,6-Me and R = Et
their physical properties. They are also used as key
3
C
H
6 2
3 6 2
C H
2
components in applications such as microporous materials,
n
n
(
5a), Pr (5b), Hex (5c) or Bn (5d).
3
4
selective ion-bonding, medicines and drug delivery. Lewis
bases (e.g. amines, phosphines, N-heterocyclic carbenes ),
5
6
7
Herein, we demonstrate that two-15 and three-coordinate
manganese(II) and iron(II) diaryl complexes stabilised by
m-terphenyl ligands are efficient precatalysts for the
cyclotrimerisation of isocyanates (Scheme 1). These complexes
offer high activity, selectivity towards alkyl isocyanates, and
high selectivity to the formation of isocyanurate products.
8
9
rare-earth and main group complexes have been used as
precatalysts for the cyclooligomerisation of isocyanates. Less
attention has been focused on the use of transition metal
species for this purpose; an early example reported the use of
copper(II) and nickel(II) halides, and palladium(0) complexes
have been traditionally more common for this chemistry.11
Many existing cyclotrimerisation methodologies face issues
such as low catalyst activity, by-product formation, long
reaction times, high reaction temperatures or difficulty with
product separation.
10
The reaction between [2,6-Tmp
) and one equivalent of the corresponding metal
dihalide [MnBr or FeCl (THF)1.5] in a mixture of toluene and
THF at room temperature yields the three-coordinate diaryl
complexes (2,6-Tmp M(THF) [M = Mn ( ), Fe ( )], with
concomitant formation of the lithium halide. Complexes and
were isolated as pale pink ( ) or yellow ( ) highly air- and
2 6 3 2
C H Li] (8; Tmp = 2,4,5-
3 6 2
Me C H
2
2
2
C
H
6 3
)
2
1
2
There is interest in the use of low-coordinate transition
metal compounds as stoichiometric and catalyst precursors for
diverse transformations, leading to unusual reactivity and
1
2
1
2
moisture-sensitive crystals. They have been fully characterised
and their solid state structures have been determined via X-ray
1
2
13
14
unique mechanisms, and two- and three-coordinate
compounds have been investigated for their efficacy as
catalyst precursors. As part of our research into low-
coordinate transition metal m-terphenyl complexes, we are
investigating the efficacy of these species as precatalysts.
crystallography. The molecular structures of
and ESI) display a distorted trigonal planar geometry [Σ(angles
at M): 360.0 ) and 359.9 )] around the metal centre with
1 and 2 (Figure 1

(1
 (2
the coordination sites occupied by two m-terphenyl ligands,
both in a syn conformation, and a THF molecule. This is
contrary to that observed for the related two-coordinate
a.School of Chemistry, University of Nottingham, University Park, Nottingham, NG7
2
RD, UK.
compounds (2,6-Ar
2
C
6
H
3
)
2
M [Ar = 2,4,6-Me
3
C
6
H
2
(Mes);
b.
Centre for Biomolecular Sciences, University of Nottingham, University Park,
Nottingham NG7ꢀ2RD, United Kingdom
Email: Deborah.Kays@nottingham.ac.uk
Electronic Supplementary Information (ESI) available: full experimental details for
i
15,16
2
,6- Pr C
2 6
H
3
(Dipp); M = Mn, Fe],
which feature the more
sterically demanding Mes and Dipp substituents. Three-
coordinate geometries have been observed previously when
the m-terphenyl ligand features asymmetric flanking
1
1
,
2
,
5a
-d and 6-8, kinetic experiments, crystallographic data and CIF files. CCDC-
500842-1500845 and -1515783. See DOI: 10.1039/x0xx00000
1
7,18
substituents.
This journal is © The Royal Society of Chemistry 20xx
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successful for primary aliphatic isocyanates (Table 1). No
View Article Online
DOI: 10.1039/C6CC07243G
cyclotrimerisation was observed via NMR spectroscopy when
aromatic isocyanates [phenyl-, p-tolyl- (p-Tol) and 3,5-
dimethoxyphenyl isocyanate] were used as substrates, even
when the reaction was heated to 60
Table S1). Secondary and tertiary aliphatic isocyanates (entries
7-32, Table S1) were not effective in this reaction possibly
C for 48 h (entries 21-26,
2
due to the incremented steric bulk of the substrates which
could prevent access to the metal centre. This steric effect is
illustrated in the notably longer reaction times observed for
longer aliphatic chains (entries 1 and 7). The temperature also
has an important effect on the catalytic transformation: for
n
PrNCO the catalytic activity improves significantly when
raising the temperature from room temperature to 60
C
(entries 7 and 8). This exclusive reactivity towards the
cyclotrimerisation of alkyl isocyanates is in contrast to that
exhibited by other transition metal catalysts.
Figure 1. Molecular structure of
2 with anisotropic displacement ellipsoids set at
5
0% probability. Hydrogen atoms have been omitted for clarity. Selected bond
To investigate the reactivity towards aryl isocyanates, a
lengths (Å) and angles (
.1303(13), C(1) Mn(1)
dihedral angle 78.77(6).
Fe(1) O(1) 2.0583(14), C(1)
C(25) Fe(1) O(1) 110.83(7), Ar

) for
C(1’) 138.13(6), C(1)
: Fe(1) C(1) 2.073(2),
Fe(1) C(25) 133.89(8), C(1)
Ar dihedral angle 83.91(8).
1
and
2
;
1
: Mn(1)
Mn(1)
Fe(1)
Fe(1)

C(1) 2.1395(11), Mn(1)
O(1) 110.93(3), Ar
C(25) 2.0734(19),
O(1)
2





Ar stoichiometric reaction between complex
4 and one
2







O(1) 115.22(7), equivalent of 3,5-dimethoxyphenyl isocyanate was performed
in toluene. Due to the paramagnetic nature of the compound
precluding useful NMR studies the reactions were monitored


The M
C distances for 1 and 2 (Figure 1) are comparable to
by IR spectroscopy. No reaction was observed after 12 h at
room temperature, but raising the temperature to 60 C for 16
those for other three-coordinate M(II) complexes with aryl

19
ligands. The C
and , respectively] are considerably larger than the value of
20 expected for a trigonal planar complex. This observation
MC angles [138.13(6) and 133.89(8) for 1
h resulted in a colour change from yellow to a dark red
solution, which is accompanied by the appearance of new
peaks in the IR spectra. Whilst no peaks were observed in the
aromatic isocyanurate region of the spectrum, the appearance
2

1
is consistent with the steric repulsion imposed by the two
bulky m-terphenyl ligands. This angle is also similar to the
-
1
of a strong peak at 1676 cm is consistent with formation of
an Fe-amido species resulting from the insertion of the
1
7
related compound (2,6-Naph
The torsion angle between the two central aryl rings for both
and is slightly more acute than the corresponding interplanar
angles in related two-coordinate m-terphenyl compounds.
An initial assessment of the catalytic activity of and
the cyclooligomerisation of isocyanates was carried out using
ethyl isocyanate as a model substrate in C at room Entry
2 6 3 2 2 7
C H ) CoOEt (Naph = 1-C10H ).
1
3
,5-dimethoxyphenyl isocyanate into the Fe

C bond.20
2
15,16
1
2
for Table 1. Catalytic cyclotrimerisation of isocyanates using 1-4.[a]
Cat
T
Time
(h)
Conv.
(%)
6
D
6
Isocyanate
Product
[
b]
(
mol%)
1 (5)
1 (2)
(C)
rt[c]
rt
rt
rt
rt
rt
rt
60
60
60
60
60
60
60
60
60
60
60
60
temperature with 5 mol% catalyst loading (Scheme 1, Table 1,
entries 1 and 4). Remarkably, when using the manganese(II)
1
2
3
4
5
6
7
8
9
EtN=C=O
EtN=C=O
EtN=C=O
EtN=C=O
EtN=C=O
EtN=C=O
5a
5a
5a
5a
5a
5a
5b
5b
5b
5b
5b
5c
5c
5c
5c
5d
5d
5d
5d
1
2.8
5
97
98
98
97
98
99
96
98
88
94
>99
97
98
99
99
99
65
>99
>99
species
just an hour, but a significant decrease in the rate of the
reaction was observed with the iron(II) complex . Lowering
the catalyst loading for to 2 mol% and 0.5 mol% also affords
high conversions in short times (entries 2 and 3).
The catalytic activity of and was compared with the
related two-coordinate species (2,6-Mes M [M = Mn ( ),
(entries 5 and 6). Whilst the manganese(II)
compounds and reached completion in comparable times,
a notable difference was found between the two iron
complexes. The two-coordinate iron(II) complex reaches high
conversion in shorter time for the cyclotrimerisation reaction
compared to (entries 6 and 4, respectively). Notably, no
1 as the precatalyst the reaction reaches completion in
1 (0.5)
2 (5)
3 (5)
4 (5)
1 (5)
1 (5)
2 (5)
3 (5)
4 (5)
1 (5)
2 (5)
3 (5)
4 (5)
1 (5)
2 (5)
3 (5)
4 (5)
24
2.5
6.5
19
14
14
14
14
14
14
14
14
14
14
14
14
2
1
n
PrN=C=O
PrN=C=O
PrN=C=O
PrN=C=O
n
1
2
n
C
2 6
H
3
)
2
3
n
1
5
10
Fe (4)]
n
11
12
13
14
15
16
17
18
PrN=C=O
1
3
n
n
n
n
HexN=C=O
HexN=C=O
HexN=C=O
HexN=C=O
BnN=C=O
BnN=C=O
BnN=C=O
BnN=C=O
4
2
dimerisation of the isocyanates to form uretediones, a
common by-product of cyclotrimerisation reactions,7,8b,9c was
observed for any of the catalytic reactions. The scope of the
reaction was evaluated for a variety of aliphatic and aromatic
isocyanates using complexes
ESI), whereupon it was determined that the catalysis was only
19
1
[a] Reaction conditions: 0.6 mL of C
6 6
D . [b] Determined by H NMR spectroscopy.
3 and 4 (Table 1 and Table S1,
[c] rt = room temperature (20C ± 2).
2
| J. Name., 2012, 00, 1-3
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Although this species could not be isolated, it was hydrolysed
with water affording anilide derivative (Scheme 2) which was
View Article Online
DOI: 10.1039/C6CC07243G
6
characterised via spectroscopy and single-crystal X-ray
diffraction studies (see ESI). That there is no reaction towards
isocyanates indicates that this putative Fe-amido species is
deactivated towards further insertion.
Experimental
observations
indicate
that
the
a
cyclotrimerisation most likely proceeds through
homogeneous mechanism; no induction period is observed
1
when monitoring the reaction by H NMR. In addition, the rate
of the catalysis is unaffected by the presence of a drop of
21
mercury. Moreover, the reaction does not appear to be
radical-mediated as performing the reaction in the presence of
a radical trap leads to no decrease in the reaction rate.22
Kinetic experiments were performed at varying concentrations
of precatalyst (3) and isocyanate (EtNCO). Analysis of the rate
order reveals first-order behaviour with respect to both the
precatalyst and the isocyanate (Figures S1-S4, ESI).
Scheme 3. Proposed catalytic cycle for the cyclotrimerisation of isocyanates
catalysed by m-terphenyl Mn(II) (1, 3) and Fe(II) (2, 4) complexes.
A catalytic cycle for the cyclotrimerisation reaction is
proposed in Scheme 3. Taking into account the inherent
coordinative unsaturated nature and the +II oxidation states of
possible for the unreactive isocyanates (featuring aryl and
secondary alkyl substituents, for example) to react after the
coordination of the first or second isocyanate (intermediates
A
the metals in complexes 1-4, it is reasonable to assume that
and ) producing mixed cyclotrimers. In fact, when preliminary
B
23
the catalysts act as Lewis acids, in a similar manner to that
postulated for Yb complexes.8c Initially, the isocyanate
cross coupling reactions were carried out with isocyanate
mixtures (Table S2, ESI) we observed the formation of mixed
trimers by 1H NMR and GC-MS. Especially interesting is the
reaction between EtNCO and p-TolNCO (1:2 ratio) where the
possible isocyanurate isomers are obtained with a degree of
coordinates to the metal centre to give
A. Previous
calculations on a Pd(0) cyclotrimerisation catalyst have found
that the C=N coordination mode is slightly more stable than
11b
C=O, but both states lie close in energy. This coordination
renders the carbon atom more electrophilic. Following this, an
outer-sphere attack of the nucleophilic nitrogen of the
incoming isocyanate at the electropositive carbon yields
selectivity [ratios are 21:50:29 for (EtNCO)
3 2
:(EtNCO) (p-
TolNCO):(EtNCO) (p-TolNCO)]. Furthermore, we do not
2
observe a promoting effect within our monomer mixture; the
presence of the more reactive monomer (in our case EtNCO)
11a,24
intermediate
isocyanate on the coordinated linear dimer results in the
formation of intermediate . Finally, ring closure to give the
B
.
Subsequently, nucleophilic attack of a free
i
does not promote the formation of (p-TolNCO)
3
or ( PrNCO)
3
.
The synthesis of asymmetrically-substituted isocyanurates is of
interest as they are an important class of molecules with
C
cyclotrimer, which is displaced by a new isocyanate moiety,
regenerates the catalyst. An alternative reaction pathway has
been proposed by Paul et al. where the coordination of the
isocyanate to the metal centre results in an increase of the
nucleophilic character of the coordinated isocyanate.11b
Therefore, the stepwise formation of the cyclotrimer would
take place by nucleophilic attack of the coordinated isocyanate
onto free isocyanate.
4a,b
pharmaceutical and agricultural utility; this reactivity offers
the possibility of obtaining these compounds through an atom
economical one-pot reaction using only isocyanate precursors.
In summary, we have shown that an m-terphenyl ligand

(2,6-Tmp
C
2 6
H
3
) can be used to access three-coordinate
manganese(II) and iron(II) complexes. Low-coordinate m-
terphenyl species were found to be efficient precatalysts
1-4
for the cyclotrimerisation of primary aliphatic isocyanates
under mild conditions. These precatalysts show a selectivity
towards aliphatic monomers not generally exhibited by other
catalysts, with preliminary results offering the possibility of the
selective synthesis of cross-coupled isocyanurates. These
results highlight the potential of two- and three-coordinate
m-terphenyl complexes as precursors for catalytic
transformations.
The proposed mechanism is consistent with the substantial
difference in the catalytic activity of the manganese and iron
complexes which is most probably due to the more Lewis
acidic manganese(II) centre compared to the iron(II) centre.
The proposed mechanism suggests that the steric demands
decrease as the linear chain grows further away from the
sterically encumbered metal centre. Therefore it should be
We gratefully acknowledge the support of the University of
Nottingham, the EPSRC and Leverhulme Trust. We also thank
Dr Adrienne Davis, University of Nottingham for helpful NMR
discussions, the EPSRC UK National Mass Spectrometry Facility
at Swansea University and Dr Mick Cooper at the University of
Nottingham for mass spectrometry and Mr Stephen Boyer
Scheme 2. Insertion of one equivalent of R-NCO (R = 3,5-(OMe)
Fe-C bond of complex followed by hydrolysis.
2 6 3
C H ) into an
4
(London Metropolitan University) for elemental analyses.
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