Highly efficient MoIV3 ?sbIII cluster frustrated Lewis pair hydrogenation

Article abstract of DOI:10.1039/c9dt04138a

Frustrated Lewis pairs (FLPs) featuring Lewis acid-base synergistic action have been recognized as one of the most powerful bifunctional catalytic systems. Each Lewis component, however, is usually composed of a single atom and hence lacks a cooperative e

Full text of DOI:10.1039/c9dt04138a

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Highly efficient Mo3IV⋯SbIII cluster frustrated Lewis
pair hydrogenation†
Cite this: Dalton Trans., 2019, 48,
17445
Fang Yua,b and Li Xu
*
a
Received 24th October 2019,
Accepted 7th November 2019
DOI: 10.1039/c9dt04138a
rsc.li/dalton
Frustrated Lewis pairs (FLPs) featuring Lewis acid–base synergistic most important types of clusters comprising from several to
action have been recognized as one of the most powerful bifunc- hundreds of oxygen-bridged metal centres.6–9 POMs contain
tional catalytic systems. Each Lewis component, however, is numerous Lewis basic bridging/terminal oxygen atoms, but
usually composed of a single atom and hence lacks a cooperative they are not efficient FLP systems because of the absence of
effect. In this work, cluster FLPs (CFLPs) comprised of a triangularly active Lewis acid sites. As a consequence, they are widely
Mo–Mo bonded MoI3V Lewis acidic cluster component and a Lewis exploited for catalyzing oxidation reactions but their appli-
basic SbIII partner in H[SbI3IIMoI3VMoVI O55(OH)2py3] (1) have been cations in hydrogenation catalysis (at high temperature10 or
15
developed. The more extensive cooperative actions both within via electrochemical reduction11) are extremely rare. Recently,
MoI3V and between Mo3IV and SbIII account for the excellent catalytic triangularly Mo–Mo bonded MoI3V Lewis acidic cluster com-
performance of 1, providing a new way of thinking for the designed ponents have been successfully incorporated into POMs.12–15
synthesis of highly efficient FLP catalysts.
In contrast to the reported FLP systems bearing strong Lewis
basic pnictogenium(III) (PnIII) components,1–4 however, they
have comparatively weak Lewis basic oxygen partners, which
limits their catalytic efficiency in terms of the reaction rate,
temperature, catalyst dosage and recyclability. We thus have an
idea to incorporate strongly Lewis basic PnIII components
into {MoI3V}–POM hybrid clusters to enhance both the FLP
activity and stability. Herein, we wish to report the first
example of CFLPs comprised of MoI3V⋯PnIII Lewis pairs,
Introduction
Main group FLP hydrogenation initiated by the discovery of
the metal-free heterolytic cleavage of dihydrogen is one of the
most exciting recent developments in bifunctional catalysis.1,2
Later, it has been rapidly broadened to transition metal3,4 and
rare earth FLPs,5 respectively, as they can also serve as Lewis
acidic components. These reported FLP systems usually com-
prise a single Lewis acid–base pair in which each component
is composed of a single main group, transition metal or rare
earth atom and hence lack a cooperative effect. With this con-
sideration in mind, we propose a new concept: a cluster FLP
(CFLP) that comprises Lewis acidic (or basic) cluster com-
ponents instead of a single atom centre. It features interatomic
bonding among neighboring Lewis acidic (or basic) com-
ponents and hence allows for more extensive synergistic
effects not only between the FLP partners but also within the
FLP components. Polyoxometalates (POMs) are one of the
[SbI3IIMoI3VMoV15I O55(OH)2
py3]·NH2(CH3)2·4NH(CH3)2·2H2O
VI
({SbI3IIMoI3VMo }-1), which exhibits remarkably enhanced cata-
15
lytic performance as a consequence of the much more power-
ful MoI3V⋯SbIII CFLP.
Resuls and discussion
VI
{SbI3IIMoI3VMo }-1 was obtained by the organic acid-assisted
15
solvothermal oxidative aggregation of [Mo3O2(O2CCH3)6
(H2O)3]2+ and heterometallic oxide (Sb2O3) (Fig. S1†), present-
ing a new synthetic strategy to incorporate heteroatoms into
CFLP hybrid clusters (Fig. 1). They were initially obtained by
the incomplete oxidation of [Mo3O4(H2O)9]4+ in a low yield.
Later, the more robust bioxo-capped hexaacetate-bridged
[MoI3VO2(O2CCH3)6(H2O)3]2+ was employed to alleviate the Mo3IV
oxidative aggregation by the MoI3V(μ3-O)2 → MoI3V(μ3-O)(μ2-O)3
skeletal conversion,16 providing a universal preparative route
resulting in a much higher and repeatable yield.13 As shown in
aState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter Chinese Academy of Sciences, Fuzhou, Fujian 350002,
P. R. China. E-mail: xli@fjirsm.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing100049, China
†Electronic supplementary information (ESI) available: Full experimental and
computational details. CCDC 1949159 (1). For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/C9DT04138A
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Fig. 2 Chain-like MoI3V⋯SbIII Lewis cluster pairs in the CFLP hybrid
III
VI
cluster {SbI3IIMo3IVMo }-1 (MoI3V⋯Sb , 4.5–4.7 Å).
15
Table 1 Average bond lengths (Å) in CFLP hybrid clusters 1–7 and the
discrete 8 and MoIV-electron transfer (ΔMoIV) based on 8
Fig. 1 Solvothermal syntheses of CFLP hybrid clusters 2–7 by the
partial oxidative aggregation of the bioxo-capped hexacarboxylate-
bridged trimolybdenum(IV) cluster precursor [MoI3VO2(O2CMe)6(H2O)3]
ZnCl4 under different reaction conditions by tuning the components
and ratios of mixed solvents, temperature, basic/acidic agents and het-
d(Mo–Mo)
d(MoIV–μ2-O–)
–MoIV –M
Clusters
MoI3V
MoV2
ΔMOIV
erometal oxides (polyhedron color: Mo4+, purple; Mo5+, green; Mo6+
,
{SbI3IIMo3IVO6Mo1V5I }-1
{γ-MoI6VO8Mo7VI}-2
2.527
2.511
2.525
2.487
2.545
1.927
1.926
1.939
1.930
1.936
2.055 0.245
2.087 0.150
2.090 0.151
0.255
2.088 0.095
2.011 0.161
2.062 0.190
0
black green; P5+, yellow; Na+, blue; Zn2+, agua; Ge4+, dark yellow).
Reaction conditions: (i) py/DMF/H2O = 9 : 1 : 1, 398 K, 2 days; (ii) py/
glycol/H2O = 7 : 1 : 3, GeO2, 421 K, 2 days; (iii) py/DMF/H2O = 1 : 9 : 1,
348 K, 2 days; (iv) py/EtOH/H2O = 8 : 1 : 1, 348 K, 2 days; (v) py/MeOH/
H2O = 7 : 2 : 1, Na2SiO3, 421 K, 2 days; (vi) py/H2O = 7 : 3, H3PO4, 409 K,
3 days.
{β-MoI6VO8Mo7VI}-3
{Mo6IVO2Mo1V0I Ge3}-4
{Mo3IVO7Mo1V0I P4Zn}-5
{Mo1IV2O5Mo4VMoV3INa}-6
{Mo6IVO6Mo6VMoV9I}-7
2.512 2.674 1.932
2.506 2.718 1.934
[MoI3VO4(C2O4)3Mepy3]2−-8 2.499
1.918
VI
Fig. S1,† the formation of {SbI3IIMoI3VMo }-1 can be understood
15
by relating it to non-tetrahedral Dawson variants negative charge of the bridging oxygen atoms (Ob) induced by
[HnPnIIIMo1V8I O60]9−n (Pn = pnictide, n = 2, Pn = As,17 n = 3, Pn = the 6e-reduction. The endohedral SbO3 has considerably
Bi18) with an endohedral PnO3 group. It is monoanionic and shorter Sb–O distances (1.958(8) and 1.988(5) Å) than SbO4
charge balance by one protonated dimethyl amine counterca- (1.993(8)–2.140(5) Å) and [P2Mo12SbI2IIO40]2− (2.076(5)–2.094(5)
+
tion NH2(CH3)2 most often occurs by the high temperature Å).22 There are two protons within the cage that are attached to
decomposition of DMF. It is not soluble in water and common the two capping oxygen atoms (O25 and its equivalent)
organic solvents. As shown in Fig. S2,† it exhibits high thermal revealed by a low BVS value (1.169, Table S1†).
stability and loses dimethyl amine and water molecules below
The crystal structure of 1 is given in Fig. S4.† It is crucial
VI
300 °C, pyridine dative ligands below 400 °C and molybdenum that the crystal packing of {SbI3IIMo3IVMo }-1 occurs in such a
15
oxide below 800 °C. The polyhedral representation and atomic- capping manner that the Lewis basic Sb3 and Lewis acidic
III
VI
ally labeled structure of {SbI3IIMo3IVMo }-1 are given in Fig. 2 Mo3IV form chain-like MoI3V⋯Sb CFLPs as shown in Fig. 2,
15
and S3,† respectively. It has crystallographic mirror symmetry wherein the MoI3V triangles are capped by Sb3 at distances (4.6,
(defined by Mo1, Mo10 and three Sb atoms) and features six 5.0 Å) similar to the sum of their Allinger’s van der Waals radii
[Mo3O4] incomplete cubane-type cluster units one of which (4.91 Å). Such capping distances are perfect for cooperatively
marked in purple has three much shorter Mo–Mo bonds promoting the heterolytic cleavage of hydrazine substrates
(Table 1, av. 2.527(1) Å) than the other five (MoVI⋯MoVI, av. between them to produce active hydride and proton intermedi-
3.333 Å) and is explicitly assigned as the triangular MoIV–MoIV ates for transfer hydrogenation as detailed below.
bonded [MoI3VO4] Lewis acidic cluster unit.12–15,16,19–21 Each
DFT theoretical calculations of 1 have been conducted
MoIV is loosely datively bonded to pyridine with weak MoIV–py using B3LYP-6-31G*(C,H,O,N)/LanL2DZ (Mo, Sb) on the opti-
bonding (Mo–N 2.24(1), 2.232(7) Å) accounting for the MoIV mized geometry of the anionic [Sb3IIIMo3IVMo1V5I O55(OH)2py3]−
Lewis acidic activity.12–15 The occurrence of the important elec- nanocage. The MoIV atoms have an averaged Mulliken charge
trophilic addition of two Lewis basic SbIII in 6e-reduced of 1.462 (Mo1, 1.521; Mo2, 1.433), which results in an appro-
III
17,18
{SbI3IIMoI3VMo }-1 as compared to the parent {Pn Mo
}
is priate electron transfer of 0.245 from each MoIV Lewis acid in
VI
15
VI
18
driven by both the charge balance requirement and the more contrast to 8 (MoIV, 1.217). Such MoIV-electron transfer is
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IV
3n
Table 2 Relationship between the selectivity and the yield of CFLPs 1–7 and Mo –μ2-Ox Lewis pairs, heterometal M and HOMO, related catalysts
8 and 9 and catalyst-free conditions are also included for comparison
Mo –Ox
Catalysisa (%)
Selectivity
IV
3n
HOMO
Mo (%)
CFLP catalysts 1–7 and related catalysts 8 and 9
n
x
{MoI3V}nlinkage
M
Yield
{SbI3IIMo3IVO6Mo1V5I }-1
{γ-MoI6VO8Mo7VI}-2
{β-MoI6VO8Mo7VI}-3
{Mo6IVO2Mo1V0I Ge3}-4
{Mo3IVO7Mo1V0I P4Zn}-5
{Mo1IV2O5Mo4VMoV3INa}-6
{Mo6IVO6Mo6VMoV9I}-7
[Mo3IVO4(C2O4)3Mepy3]2−-8
Sb2O3-9
1
2
2
2
1
4
2
6
8
8
2
7
5
6
—
γ
SbIII
—
—
65.4 MoIV
67.4 MoIV
65.6 MoIV
59.3 MoIV
62.1 MoIV
58.6 MoIV
56.2 MoV
68.3 MoIV
100
100
99.8
78.0
46.9
47.9
40.8
16.8
3.9
99.8
99.8
99.8
97.4
99.8
97.7
14.2
β
γ
GeIV
ZnII, PV
Na+
—
—
α, β
—
<5
<0.5
No
a GC yield, nitrobenzene (10 μL, 0.097 mmol), N2H4·H2O (85%, 20 μL, 0.3 mmol), EtOH (2 mL), 1, 1 mol%, 60 °C, 0.5 h; 2–9, 3 mol%, 80 °C, 2 h.
helpful to enhance both the Lewis acidity of MoIV and the yield at 80 °C. 0.1 and 0.33 mol% 1 result in 90.9% and 92.9%
Lewis basicity of the surrounding oxygen and heterometal yields accompanied by unreacted PhNO2 and the byproduct
atoms. Also notable are HOMO compositions in which the azoxybenzene, respectively (Fig. S8†), which are much greater
MoIV contributions are essential to catalytic activity.15 As
shown in Table 2 and Fig. S5,† the HOMO of 1 is mainly com-
posed of MoIV (65.4%) as in the cases of 2–6, differing from
the HOMO of 7 that is dominated by MoV (56.2%) and is
believed to responsible for the unusually low catalytic activity.
A comparison of the catalytic performance of the CFLP
hybrid clusters 1–7 and related compounds 8 and 9 in the
hydrazine-mediated transfer hydrogenation of nitrobenzene to
aniline, one of the most important industrial hydrogenation
reactions,23 is given in Table 2. Their catalytic efficiency is
largely dependent on the number and arrangement of Lewis
acidic {MoI3V}n active units because of the weak Lewis basic
oxygen partners when additional Lewis basic partners are
absent for 2–7 (Fig. S6†).15 The head-to-head γ-linkage
Fig. 3 Catalytic yields of 1 and 2 at different temperatures. Reaction
conditions: catalysts, 3 mol%, PhNO2, 10 μL, 0.097 mmol, N2H4·H2O,
20 μL, 0.3 mmol, EtOH, 2 mL, 2 h.
between two neighboring Mo3IV triangles (Fig. S7†) allows for
the most effective three pairs of MoIV–MoIV synergistic actions,
accounting for the best catalytic performance of {γ-MoI6VMo7VI}-
V
2 (yield > 99.8%, Table 2).14,15 {Mo3IVMo P 4ZnII}-5 containing
VI
10
a single MoI3V Lewis acidic unit results in only moderate yield
(47.9%), although it also exhibits excellent selectivity (>97%).
V
II
Like {MoI3VMo P 4Zn }-5, {SbI3IIMoI3VMo1V5I }-1 has only one MoI3V
VI
10
active Lewis acidic unit. However, the additional stronger SbIII
Lewis base offers us a unique opportunity to examine the con-
tribution of the Lewis basic partners to the CFLP system. As
indicated in Table 2, {Sb3IIIMoI3VMo }-1 exhibits perfect selecti-
VI
15
vity (100%) and conversion (100%) although it has only a
single MoI3V Lewis acidic unit. Moreover, it also exhibits excel-
lent catalytic efficiency (Fig. 3, yield, 99.8%) at room tempera-
ture, which is much higher than that of {γ-MoI6VMo7VI}-2 (yield,
5.8%). The SbIII-enhanced catalytic activity is also revealed by a
comparison of catalyst (mol%)-yield plots as shown in Fig. 4.
0.5–3 mol% 1 retains the perfect catalytic performance (yields
Fig. 4 Catalyst (0.1–3 mol%)-dependent yields for the hydrogenation
of nitrobenzene to aniline, reaction conditions: PhNO2, 10 μL,
0.097 mmol, N2H4·H2O, 20 μL, 0.3 mmol, EtOH, 2 mL, 2 h, 1, 60 °C, 2,
> 99%) even at 60 °C, whereas 0.5 mol% 2 results in only 36% 80 °C.
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Table 3 Hydrazine-mediated hydrogenation of nitroarenes to aryla-
a
VI
mines catalysed by {SbI3IIMo3IVMo }-1
15
Entry
1
Reaction
Yield (%)
99.8
2
3
4
5
99.8
99.8
99.8
99.8
Fig. 5 Time-dependent catalysis yields for the hydrogenation of nitro-
benzene to aniline. Reaction conditions: catalysts, 3 mol%, PhNO2,
10 μL, 0.097 mmol, N2H4·H2O, 20 μL, 0.3 mmol, EtOH, 2 mL, 80 °C.
than the 8.0% yield obtained using 0.1 mol% 2. A comparison
of time–yield plots (Fig. 5 and Fig. S9†) further indicates the
6
7
99.8
81.2
VI
highly improved catalytic performance of {SbI3IIMo3IVMo }-1
15
that achieves complete conversion within 30 minutes in con-
trast to 120 minutes required for {γ-MoI6VMo7VI}-2. Notable is
the initial (<5 min) formation of the PhNHOH intermediate
for the former (Fig. S9†), which reveals the direct multiple
route (Scheme S1†) as in the case of the latter. The absence of
the PhNHOH intermediate for the latter may be related to the
two MoI3V Lewis acidic active units that stabilize up to six
hydrides allowing for the complete conversion of PhNO2 to
PhNH2 and 2H2O in each catalysis cycle.14 The remarkably
8
9
61.2
39.2
a Determined by GC-2014, reaction conditions: nitrobenzene (10 μL,
0.097 mmol), N2H4·H2O (20 μL, 0.3 mmol), 3 mol% catalysts, EtOH
(2 mL), 2 h.
VI
enhanced performance of {SbI3IIMoI3VMo }-1 is believed to be
15
related to the stronger Lewis basicity and higher stability of
the SbIII partners promoting and presumably dominating the
bifunctional synergistic actions of the MoI3V⋯SbIII CFLPs cata-
lyzing Sb: → H+ → N → H− → MoIV heterolytic cleavage of
hydrazine. Such MoI3V⋯SbIII CFLP synergistic actions for gener-
ating and stabilizing the proton/hydride intermediates
obtained by hydrazine heterolysis play an essential role in the
excellent catalytic performance since both Sb2O3 and the dis-
crete MoI3VO4-type cluster 8 have very low activity (yield < 5%,
Table 2). Table 3 and Fig. S10† describe the influence of
the nitroarene substituents on the catalysis yields of
{Sb3IIIMoI3VMoV15I }-1. The Lewis basic substituents trans to NO2
remarkably reduce the catalytic activity as revealed by the
lowest yield (39.2%) with the strongest basicity (trans-NH2).
This might indicate the essential role of the Lewis acidic Mo3IV
partners that can be deactivated by the non-frustrated –HN2
Lewis base. The yield of the cis-OH derivative (99.8%) is much
higher than that of the trans-OH derivative (61.2%) presumably
as a consequence of the inner OH⋯O2N hydrogen bond and
the resulting steric hindrance of the former which reduce the
Lewis basicity and the MoIV-bonding ability of the cis-OH
Fig. 6 Recyclability of 1 and 2 in the heterogeneous catalysis hydro-
genation of nitrobenzene to aniline. Catalysis conditions:
PhNO2, 10 μL, 0.097 mmol, N2H4·H2O, 20 μL, 0.3 mmol, EtOH, 2 mL,
80 °C, for 1: fresh and cycle 1, 0.5 h, cycle 2, 1 h, cycles 3–10, 2 h. For 2:
fresh and cycles 1–4, 2h.
3 mol%,
VI
group. The recyclability of the insoluble {SbI3IIMo3IVMo }-1
15
heterogeneous catalyst has been examined as shown in Fig. 6, under similar catalysis conditions. The markedly improved
VI
S11 and S12.† It exhibits both perfect selectivity (100%) and recyclability of {SbI3IIMo3IVMo }-1 can be assigned to the more
15
excellent conversion (>95%) for cycles 1–10, and hence shows stable SbIII Lewis basic sites that may largely contribute to the
much better recyclable performance than {γ-MoI6VMo7VI}-2 Mo3IV⋯SbIII CFLP activity.
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excellent catalytic performance of the CFLP hybrid cluster H
[SbI3IIMo3IVMoV15I O55(OH)2py3] as a consequence of the extensive
synergistic effects both within the Lewis acidic MoI3V cluster
component and between MoI3V and SbIII Lewis basic partners.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work is supported by the National Natural Science
Foundation of China (21873102) and the Strategic Priority
Research Program of Chinese Academy of Sciences, Grant No.
XDB 20000000.
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