Inter- and intramolecular phosphonium salt cocatalysis in cyclic carbonate synthesis catalysed by a bimetallic aluminium(salen) complex

Article abstract of DOI:10.1016/j.tetlet.2012.03.090

Quaternary phosphonium salts can be used as cocatalysts for the conversion of epoxides and carbon dioxide into cyclic carbonates catalysed by bimetallic aluminium(salen) complexes at ambient temperature and one atmosphere pressure. The phosphonium groups

Full text of DOI:10.1016/j.tetlet.2012.03.090

Tetrahedron Letters 53 (2012) 2736–2740
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Inter- and intramolecular phosphonium salt cocatalysis in cyclic carbonate
synthesis catalysed by a bimetallic aluminium(salen) complex
⇑
Michael North , Pedro Villuendas, Carl Young
School of Chemistry and University Research Centre in Catalysis and Intensified Processing, Bedson Building, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Quaternary phosphonium salts can be used as cocatalysts for the conversion of epoxides and carbon diox-
ide into cyclic carbonates catalysed by bimetallic aluminium(salen) complexes at ambient temperature
and one atmosphere pressure. The phosphonium groups could also be attached onto the salen ligands
of the bimetallic aluminium(salen) complex to form one-component catalysts, and in this case the cata-
lytic activity was heavily influenced by the solubility of the catalyst in the epoxide substrate. The results
are consistent with a previously proposed catalytic cycle for cyclic carbonate synthesis catalysed by
bimetallic aluminium(salen) complexes.
Received 19 December 2011
Revised 25 February 2012
Accepted 22 March 2012
Available online 29 March 2012
Keywords:
Cyclic carbonate
Salen
Ó 2012 Elsevier Ltd. All rights reserved.
Phosphonium salt
Aluminium
The synthesis of cyclic carbonates from epoxides and carbon
dioxide (Scheme 1) is one of the few reactions of carbon dioxide
which is of commercial importance.1 Cyclic carbonates have
numerous applications including: electrolytes for lithium-ion bat-
teries, degreasers, paint strippers, lacquers, plasticisers, cure-
agents, carbon dioxide removal from natural gas and enhanced
oil recovery from depleted wells.2 They also make excellent polar
aprotic solvents.2,3 for a wide range of chemical transformations
and are intermediates in the synthesis of monoethylene glycol4
and dimethyl carbonate.4,5 The formation of cyclic carbonates is
the combination of a bimetallic l-oxo-bridged aluminium complex
and a quaternary ammonium bromide forms an exceptionally ac-
tive catalyst system for the synthesis of cyclic carbonates.9,10
But
BnEt2N
Br
NEt2Bn
Br
But
But
But But
O
N
N
O
O
N
Al
N
O
O
O
O
N
Al
O
highly exothermic (
D
Hf = À140 kJ molÀ1 for ethylene carbonate6)
Al
however, the reaction occurs only in the presence of a catalyst.
Commercially, quaternary ammonium1,5 or phosphonium4 halides
are used as catalysts, though they necessitate the use of high reac-
tion temperatures and pressures along with highly purified carbon
dioxide. The reaction can be further catalysed by the use of a Lewis
acid to activate the epoxide and a large number of Lewis acid
catalysts have been developed for this reaction.7
N
But But
But
1
Br
Br
Et2N
2
NEt2Bn
R
Building on our previous work on the use of bimetallic
2: R = Bn
N
N
O
O
Schiff-base complexes in asymmetric catalysis,8 we showed that
3: R = (CH2)3silica
Al
O
O
O
2
O
O
+ CO2
4
R
Initially, we reported that the combination of complex 1 and
tetrabutylammonium bromide (TBAB) catalysed cyclic carbonate
synthesis at room temperature and atmospheric pressure.
Subsequently, we attached ammonium groups covalently to the
salen ligand, giving homogeneous 2 or heterogeneous 3 one-
component catalysts which had similar catalytic activity to
R
Scheme 1. Synthesis of cyclic carbonates.
⇑
Corresponding author. Tel.: +44 191 222 7128; fax: +44 191 222 6929.
E-mail addresses: michael.north@ncl.ac.uk, michael.north@newcastle.ac.uk
(M. North).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.03.090

M. North et al. / Tetrahedron Letters 53 (2012) 2736–2740
2737
Entries 1–3 are the control experiments showing that complex 1,
TBAB or tributylamine all have very low catalytic activity when
used as the only catalyst. Entry 4 highlights the very strong syner-
gistic effect observed when using complex 1 and TBAB together
since TBAB can provide both bromide to ring-open the epoxide
and tributylamine to activate carbon dioxide. Entry 5 shows that
complex 1 and tributylamine also display a synergistic effect, albeit
a much weaker one than observed for complex 1 and TBAB. In this
case, the amine may play the role of nucleophile and leaving group
normally undertaken by bromide, as well as activating the carbon
dioxide.
1 (2.5 mol%) / additive
or
O
O
8 or 9 (2.5 mol%)
O
O
+ CO2
0-27 °C; 1-8 bar
R
5a-k
R
6a-k
a: R = Ph; b: R = CH2OPh; c: R = CH2OH; d: R = CH2Cl;
e: R = 4-ClC6H4; f: R = 4-MeC6H4; g: R = C8H17; h: R = Bu;
i: R = Et; j: R = Me; k: R = H
Scheme 2. Synthesis of cyclic carbonates 6a–k.
complex 1.11,12 Immobilised catalyst 3 could also be used to syn-
thesise cyclic carbonates in a gas-phase flow reactor at atmo-
spheric pressure and 100 °C.12,13 Catalysts 1–3 were tolerant of
impurities present in flue gas,12,14 allowing the development
of an integrated system for energy production with utilisation of
the waste carbon dioxide in cyclic carbonate synthesis.15 Recently,
we showed that the salen ligand could be simplified to an acen
ligand, thus giving complex 4 which, in the presence of TBAB, also
catalysed the synthesis of cyclic carbonates at room temperature
and atmospheric pressure.16
Kinetic studies showed that reactions catalysed by 1/TBAB were
second order in TBAB concentration and first order in the concen-
tration of all other reaction components.10,17 Tributylamine formed
in situ from TBAB was also shown to be catalytically active. Hence,
a catalytic cycle was proposed in which complex 1 acts as a Lewis
acid, activating the epoxide towards ring-opening by bromide sup-
plied by one TBAB molecule. A second TBAB generates tributyl-
Table 1, entry 6 reveals that triphenylphosphine is as effective a
cocatalyst as tributylamine, whilst entry 7 shows that in contrast,
triphenylphosphine oxide has no catalytic activity as the results
are comparable to entry 1. Entries 8–12 illustrate that the combina-
tion of complex 1 and a quaternary phosphonium halide can form an
active catalyst system. Entries 8 and 9 compare the results obtained
using tetraphenylphosphonium chloride and triphenyl-(benzyl)
phosphonium chloride. The former should not be capable of gener-
ating triphenylphosphine in situ, whilst the latter contains a P–Csp3
bond at which a substitution reaction could occur to form triphen-
ylphosphine. However, the results obtained using both of these
additives were essentially identical. Entry 10 shows that increasing
the concentration of triphenyl(benzyl)phosphonium chloride by a
factor of four did not result in any increase in the catalytic activity.
However, as entry 11 shows, changing the halide from chloride to
bromide did result in a large increase in reaction rate and conver-
sion, consistent with bromide being a better nucleophile than chlo-
ride, resulting in an increased rate of epoxide ring-opening. Finally,
as entry 12 shows, adding tributylamine as a third catalyst compo-
nent (to activate the carbon dioxide) was not effective as there was
no difference in catalytic activity between entries 11 and 12.
The results shown in Table 1 indicate that the combination of
complex 1 and either triphenylphosphine or a quaternary phos-
phonium halide can exhibit synergistic catalysis. The most effec-
tive system is complex 1 and triphenyl(benzyl)phosphonium
bromide. However, even this is about a factor of eight less active
than the combination of complex 1 and TBAB (compare entries 4
and 11). This can be explained on the basis that whilst both
triphenyl(benzyl)phosphonium bromide and TBAB can provide
bromide to ring-open the epoxide, only the latter can form an
amine to activate the carbon dioxide. Adding tributylamine to
the triphenyl(benzyl)phosphonium bromide system (entry 12)
was therefore expected to show a further enhancement of the cat-
alytic activity, but its failure to do so may be due to ylide formation
from triphenyl(benzyl)phosphonium bromide which would gener-
ate hydrogen bromide as a by-product and hence protonate the
amine, inhibiting its interaction with carbon dioxide.
amine which activates the carbon dioxide as
a zwitterionic
carbamate, stabilised by the second aluminium ion present in the
bimetallic complex. This allows the carbonate bond to form intra-
molecularly, and thus accounts for the high catalytic activity of
complexes 1–4.
CoII and CoIII(salen) complexes have also been used as catalysts
for cyclic carbonate synthesis,7 and in these cases the attachment
of quaternary phosphonium groups onto the salen ligand provided
one-component catalysts with enhanced catalytic activity.18 In
view of this, and the common use of quaternary phosphonium salts
as cocatalysts for Lewis acid catalysed cyclic carbonate synthesis,7
we decided to investigate the use of complex 1 with quaternary
phosphonium salt cocatalysts as well as the replacement of the
quaternary ammonium units in complex 2 with quaternary phos-
phonium groups. The results of this study are reported in this
Letter.
Initially, two-component catalyst systems comprising complex
1 and various additives were used to catalyse the conversion of
styrene oxide 5a into styrene carbonate 6a at 27 °C and one atmo-
sphere pressure (Scheme 2), the results being presented in Table 1.
Table 1
Effect of additives on the activity of complex 1 for the synthesis of styrene carbonate 6a
Entry
1 (mol %)
Additive (mol %)
Conversion after 3 ha (%)
Conversion after 6 ha (%)
2
Conversion after 24 ha (%)
1
2
3
4
5
6
7
8
2.5
0
0
None
TBAB (2.5)
Bu3N (2.5)
TBAB (2.5)
Bu3N (2.5)
Ph3P (2.5)
Ph3PO (2.5)
Ph4PCl (2.5)
BnPh3PCl (2.5)
BnPh3PCl (10)
BnPh3PBr (2.5)
BnPh3PBr + Bu3N (2.5 each)
2
15
12
1
3
b
b
1
b
b
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
62
4
5
2
1
3
2
12
13
7
10
33
34
13
33
29
21
73
69
4
b
9
10
9
27
28
10
11
12
a
Conversions determined by GCMS analysis.
Not determined.
b

2738
M. North et al. / Tetrahedron Letters 53 (2012) 2736–2740
Initially complexes 8/9a were used to catalyse the synthesis of
NH3 Cl
NH3 Cl
OH
styrene carbonate 6a at 27 °C and one atmosphere carbon dioxide
pressure (Scheme 2) and the results are presented in Tables 2 and
3. Both complexes were found to be effective one-component cat-
alysts, giving conversions of 85–96% after a reaction time of 24 h.
Therefore, the study was extended to epoxides 5b–k and the re-
sults obtained using catalyst 8a with chloride counterions were
rather mixed. Epoxides 5b–f were converted into cyclic carbonates
6b–f with 66–99% conversions after 24 h (Table 2, entries 2–6), and
glycidol 5c was a particularly reactive substrate, giving 99% con-
version after just 3 h (Table 2, entry 3). In contrast, aliphatic epox-
ides 5g–i gave only 2–5% conversion into cyclic carbonates 6g–i
after reaction times of 24 h (Table 2, entries 7–9). Propylene oxide
5j did however give a good yield of propylene carbonate 6j after
24 h at 0 °C (Table 2, entry 10), and ethylene oxide 5k was con-
verted into ethylene carbonate 6k in 53% yield in a reaction carried
out in a sealed reactor at room temperature and eight atmospheres
carbon dioxide initial pressure for 24 h (Table 2, entry 11).
The mixed results obtained with catalyst 8a appeared to be due
to the solubility of the catalyst in epoxide which is both the sub-
strate and the solvent during the early stages of the reaction. Thus,
catalyst 8a was seen to be completely soluble in epoxides 5a–f, j
under the reaction conditions, but did not dissolve in epoxides
5g–i. Support for this hypothesis came from a reaction carried
out with epoxide 5h with propylene carbonate added as a solvent
for the reaction.10,17 In this case, catalyst 8a dissolved in the reac-
tion mixture and the conversion of 5h into 6h increased to 35%. In
contrast, catalyst 9a with bromide counterions was found to be a
more consistent and highly effective catalyst, giving high conver-
sions (89–99%) and isolated chemical yields (79–98%) after reac-
tion times of 24 h for all of epoxides 5a–k except for the highly
non-polar 1,2-decene oxide 5g (Table 3). For reactions catalysed
by complex 9a, glycidol 5c and 1,2-butene oxide 5i were both par-
ticularly reactive substrates (Table 3, entries 3 and 9). Once again,
the catalytic activity of complex 9a was related to its solubility as
the catalyst was seen to dissolve in all of the reaction mixtures, ex-
cept that involving epoxide 5g.
+
Cl Ph3P
CHO
8
NaOMe/EtOH, , 16 h 72% 7-9a: R1 = R2 = H
Δ
b: R1 = H; R2 = But
R1
R1
c: R1 = (R,R)-(CH2)4; R2 = But
d: R1 = (R,R)-(CH2)4;R2 = H
N
N
OH HO
Cl Ph3P
Ph3P Cl
R2
R2
7a-d
44-74%
Al / I2 / MePh, EtOH, RT, 16 h
PPh3 Cl
R2
PPh3 Br
R2
R1
R1
N
N
O
O
R1
R1
N
N
O
O
Bu4NBr
74-90%
Al
O
Al
O
R2
R2
PPh3 Cl
PPh3 Br
2
2
8a-d
9a-d
Scheme 3. Synthesis of one-component catalysts.
Since the two-component catalyst systems derived from com-
plex 1 and triphenyl(benzyl)phosphonium halides were catalyti-
cally active, it was decided to prepare a range of one-component
analogues of complex 2, but with the ammonium groups replaced
with phosphonium salts. The synthesis of these one-component
catalysts started from known19 ligands 7a–c and ligand 7d
which was prepared from the known20 phosphonium salt 8 and
(R,R)-1,2-diaminocyclohexane (Scheme 3). Ligands 7a–d were then
converted into bimetallic aluminium (salen) tetrachlorides 8a–d,
using the methodology we have previously reported10 for the syn-
thesis of complex 1, and then subjected to ion-exchange with TBAB
to give tetrabromides 9a–d.21
Since the results with complexes 8/9a had indicated that the
solubility of the catalyst in epoxide was important for its catalytic
activity and that complexes with bromide counterions were more
active than those with chloride counterions, the synthesis of com-
plexes 9b–d was undertaken (Scheme 3). It was expected that the
introduction of tert-butyl and/or cyclohexyl groups into these cat-
alysts would make them more non-polar and hence more soluble
in epoxides, especially unfunctionalised epoxides such as 5g–k.
The catalytic activity of complexes 9a–d was determined at 27 °C
Table 2
Synthesis of cyclic carbonates 6a–k catalysed by complex 8a (2.5 mol %)
Entry
Epoxide
Conversion after 3 ha (%)
Conversion after 6 ha (%)
Conversion after 24 ha (%)
Yieldc
1
2
3
4
5
6
7
8
5a (R = Ph)
11
30
99
41
3
4
0
1
25
49
99
78
20
10
0
85
79
99
99
76
66
2
77
76
91
96
63
5b (R = CH2OPh)
5c (R = CH2OH)
5d (R = CH2Cl)
5e (R = 4-ClC6H4)
5f (R = 4-MeC6H4)
59
b
5g (R = C8H17
5h (R = Bu)
5i (R = Et)
)
b
b
2
5
9
10
11
0
2
3
5jd (R = Me)
5ke (R = H)
82
53
b
b
b
b
b
b
a
b
c
Conversions determined by 1H NMR analysis.
Not determined.
Isolated chemical yield of pure cyclic carbonate after 24 h.
Reaction at 0 °C.
d
e
Initial pressure of 8 atmospheres.

M. North et al. / Tetrahedron Letters 53 (2012) 2736–2740
2739
Table 3
Synthesis of cyclic carbonates 6a–k catalysed by complex 9a (2.5 mol %)
Entry
Epoxide
Conversion after 3 ha (%)
Conversion after 6 ha (%)
Conversion after 24 ha (%)
Yieldc
1
2
3
4
5
6
7
8
5a (R = Ph)
64
57
99
67
46
18
1
74
72
99
95
70
59
1
96
89
99
97
99
99
15
99
91
79
95
89
97
5b (R = CH2OPh)
5c (R = CH2OH)
5d (R = CH2Cl)
5e (R = 4-ClC6H4)
5f (R = 4-MeC6H4)
97
b
5g (R = C8H17
5h (R = Bu)
5i (R = Et)
)
40
72
97
96
98
88
9
10
11
99
99
99
5jd (R = Me)
5ke (R = H)
b
b
b
b
b
b
a
b
c
Conversions determined by 1H NMR analysis.
Not determined.
Isolated chemical yield of pure cyclic carbonate after 24 h.
Reaction at 0 °C.
d
e
Initial pressure of 8 atmospheres.
Table 4
Synthesis of cyclic carbonates 6a,g catalysed by complexes 9a–d (2.5 mol %)
Entry
Catalyst
Epoxide
Conversion after 3 ha (%)
Conversion after 6 ha (%)
Conversion after 24 ha (%)
1
2
3
4
5
6
7
8
9a
9a
9b
9b
9c
9c
9d
9d
5a (R = Ph)
5g (R = C8H17
5a (R = Ph)
5g (R = C8H17
5a (R = Ph)
5g (R = C8H17
5a (R = Ph)
64
1
87
4
85
7
61
1
74
1
96
15
99
80
99
84
99
31
)
)
)
)
99
11
99
10
81
3
5g (R = C8H17
a
Conversions determined by 1H NMR analysis.
and one atmosphere carbon dioxide pressure using both styrene
oxide 5a and 1,2-decene oxide 5g as substrates (Scheme 2), and
the results are presented in Table 4. It is apparent from the data
in Table 4, that all four complexes 9a–d were capable of catalysing
the synthesis of styrene carbonate with >96% conversion after 24 h,
but that complexes 9b,c which both contain four hydrophobic tert-
butyl groups were the most active catalysts, giving 99% conversion
after a reaction time of just 6 h (Table 4, entries 1,3,5 and 7). In
contrast, the introduction of two cyclohexyl units into the catalyst
structure resulted in only a small increase in catalyst efficiency
(Table 4, entries 1 and 7). The same trend is seen even more dra-
matically in the reaction involving the use of highly non-polar
epoxide 5g. In this case, the introduction of two cyclohexyl units
into the catalyst structure 9d doubles the conversion of epoxide
5g into cyclic carbonate 6g after 24 h (Table 4, entries 2 and 8),
but the conversion is still only 31%. In contrast, the introduction
of four tert-butyl groups into the catalyst (9b) dramatically in-
creases the conversion of epoxide 5g into cyclic carbonate 6g from
15% to 80% after 24 h (Table 4, entries 2 and 4). The introduction of
cyclohexyl and tert-butyl groups into the catalyst 9c resulted in
only a small additional increase in the conversion of epoxide 5g,
to 84%. Complexes 9b,c were observed to be totally soluble in reac-
tions involving epoxides 5a,g.
Quaternary phosphonium halides have been shown to be effec-
tive cocatalysts for use with bimetallic aluminium(salen) com-
plexes for the synthesis of cyclic carbonates at atmospheric
pressure and ambient temperature or lower. Phosphonium
bromides are more effective cocatalysts than the corresponding
phosphonium chlorides, which is consistent with the catalytic
role of the phosphonium halide being to supply the nucleophile
used to ring-open the epoxide substrate. When used as a two-
component catalyst system, the combination of complex 1 and a
quaternary phosphonium halide is significantly less active than
the combination of complex 1 and TBAB, but still much more active
than other catalyst systems for cyclic carbonate synthesis.7 This is
consistent with the important role played by a tertiary amine
generated in situ from a tetraalkylammonium halide cocatalyst
in activating the carbon dioxide.
One-component catalysts could also be prepared by covalently
attaching quaternary phosphonium groups onto the salen ligand
and again, complexes with bromide counterions were more active
than those with chloride counterions. The catalytic activity of these
one-component catalysts is heavily influenced by their solubility in
the epoxide substrate. Thus, by introducing the hydrophobic
tert-butyl groups onto the salen ligands, even highly non-polar
epoxides could be converted into cyclic carbonates with high
conversions at ambient pressure and temperature.
References and notes
1. Peppel, W. J. Ind. Eng. Chem. 1958, 50, 767–770.
2. Schäffner, B.; Schäffner, F.; Verevkin, S. P.; Börner, A. Chem. Rev. 2010, 110,
4554–4581.
3. (a) North, M.; Pizzato, F.; Villuendas, P. ChemSusChem 2009, 2, 862–865; (b)
North, M.; Omedes-Pujol, M. Tetrahedron Lett. 2009, 50, 4452–4454; (c) Clegg,
W.; Harrington, R. W.; North, M.; Pizzato, F.; Villuendas, P. Tetrahedron:
Asymmetry 2010, 21, 1262–1271; (d) North, M.; Villuendas, P. Org. Lett. 2010,
12, 2378–2381; (e) North, M.; Omedes-Pujol, M. Belstein. J. Org. Chem. 2010, 6,
1043–1055; (f) Beattie, C.; North, M.; Villuendas, P. Molecules 2011, 16, 3420–
3432; (g) Morcillo, M.; North, M.; Villuendas, P. Synthesis 2011, 1918–1925; (h)
Lenden, P.; Ylioja, P. M.; González-Rodríguez, C.; Entwistle, D. A.; Willis, M. C.
Green Chem. 2011, 13, 1980–1982.
4. Knifton, J. F.; Duranleau, R. G. J. Mol. Catal. 1991, 67, 389–399.
5. Fukuoka, S.; Kawamura, M.; Komiya, K.; Tojo, M.; Hachiya, H.; Hasegawa, K.;
Aminaka, M.; Okamoto, H.; Fukawa, I.; Konno, S. Green Chem. 2003, 5, 497–507.
6. (a)Matheson Gas Data Book; Yawes, C. L., Braker, W., Eds., 7th Ed.; McGraw-Hill,
2001; p 374; (b) Aresta, M.; Dibenedetto, A. Dalton Trans. 2007, 36, 2975–2992.
7. (a) North, M.; Pasquale, R.; Young, C. Green Chem. 2010, 12, 1514–1539; (b)
Decortes, A.; Castilla, A. M.; Kleij, A. W. Angew. Chem., Int. Ed. 2010, 49, 9822–
9837.

2740
M. North et al. / Tetrahedron Letters 53 (2012) 2736–2740
8. For leading references see: (a) Belokon’, Y. N.; Caveda-Cepas, S.; Green, B.;
Ikonnikov, N. S.; Khrustalev, V. N.; Larichev, V. S.; Moscalenko, M. A.; North, M.;
Orizu, C.; Tararov, V. I.; Tasinazzo, M.; Timofeeva, G. I.; Yashkina, L. V. J. Am. Chem.
Soc. 1999, 121, 3968–3973; (b) Belokon’, Y. N.; Gutnov, A. V.; Moskalenko, M. A.;
Yashkina, L. V.; Lesovoy, D. E.; Ikonnikov, N. S.; Larichev, V. S.; North, M. Chem.
Commun. 2002, 244–245; (c) Belokon’, Y. N.; North, M.; Maleev, V. I.; Voskoboev,
N. V.; Moskalenko, M. A.; Peregudov, A. S.; Dmitriev, A. V.; Ikonnikov, N. S.;
Kagan, H. B. Angew. Chem., Int. Ed. 2004, 43, 4085–4089; (d) Blacker, J.;
Clutterbuck, L. A.; Crampton, M. R.; Grosjean, C.; North, M. Tetrahedron:
Asymmetry 2006, 17, 1449–1456; (e) Belokon’, Y. N.; Clegg, W.; Harrington, R.
W.; Ishibashi, E.; Nomura, H.; North, M. Tetrahedron 2007, 63, 9724–9740; (f)
Belokon’, Y. N.; Clegg, W.; Harrington, R. W.; North, M.; Young, C. Inorg. Chem.
2008, 47, 3801–3814; (g) Belokon’, Y. N.; Hunt, J.; North, M. Tetrahedron:
Asymmetry 2008, 19, 2804–2815; (h) Belokon’, Y. N.; Clegg, W.; Harrington, R.
W.; Maleev, V. I.; North, M.; Omedes Pujol, M.; Usanov, D. L.; Young, C. Chem. Eur.
J. 2009, 15, 2148–2165; (i) Chechik, V.; Conte, M.; Dransfield, T.; North, M.;
Omedes-Pujol, M. Chem. Commun. 2010, 46, 3372–3374.
11. Meléndez, J.; North, M.; Villuendas, P. Chem. Commun. 2009, 2577–2579.
12. Meléndez, J.; North, M.; Villuendas, P.; Young, C. Dalton Trans. 2011, 40, 3885–
3902.
13. North, M.; Villuendas, P.; Young, C. Chem. Eur. J. 2009, 15, 11454–11457.
14. North, M.; Wang, B.; Young, C. Energy Environ. Sci. 2011, 4, 4163–4170.
15. Metcalfe, I. S.; North, M.; Pasquale, R.; Thursfield, A. Energy Environ. Sci. 2010, 3,
212–215.
16. North, M.; Young, C. Catal. Sci. Technol. 2011, 1, 93–99.
17. North, M.; Pasquale, R. Angew. Chem., Int. Ed. 2009, 48, 2946–2948.
18. (a) Miao, C.-X.; Wang, J.-Q.; Wu, Y.; Du, Y.; He, L.-N. ChemSusChem 2008, 1,
236–241; (b) Chang, T.; Jin, L.; Jing, H. ChemCatChem 2009, 1, 379–383.
19. (a) Haikarainen, A.; Sipilä, J.; Pietikäinen, P.; Pajunen, A.; Mutikainen, I. J. Chem.
Soc., Dalton Trans. 2001, 991–995; (b) Bahramian, B.; Mirkhani, V.;
Tangestaninejad, S.; Moghadam, M. J. Mol. Catal. A 2006, 244, 139–145; (c)
Teixeira, J.; Silva, A. R.; Branco, L. C.; Afonso, C. A. M.; Freire, C. Inorg. Chim. Acta
2010, 363, 3321–3329.
20. Thakur, S. K.; Hosangadi, B. D. Tetrahedron 1996, 52, 8755–8762.
21. All new compounds had spectroscopic data consistent with their structures.
9. Meléndez, J.; North, M.; Pasquale, R. Eur. J. Inorg. Chem. 2007, 3323–3326.
10. Clegg, W.; Harrington, R. W.; North, M.; Pasquale, R. Chem. Eur. J. 2010, 16,
6828–6843.