Synthesis of tetra-pincer nickel(ii) and palladium(ii) complexes of resorcin[4]arene-octophosphinite [Res(OPR2)8] and rhodium-catalyzed regioselective hydroformylation reaction

Article abstract of DOI:10.1039/c9dt02499a

The condensation reaction of resorcinol with pentanal yielded resorcin[4]arene 1 which on bromination using N-bromosuccinimide at room temperature produced tetra-bromide derivative 2. The reactions of 2 with chlorodiphenylphosphine and o-phenylenephosphoro-chloridite yielded octaphosphinite 3 (hereafter referred to as octaphos) and octaphosphite 4, respectively. The reactions of 3 with Ni(COD)₂ or Pd₂(dba)₃·CHCl₃ in appropriate molar ratios yielded tetra-pincer complexes 5 and 6, respectively. The structures of both the complexes were established by single crystal X-ray diffraction studies. The resorcin[4]arene backbone adopts a boat structure in these complexes. Typically, the Rh-catalyzed hydroformylation of styrene prevalently delivers a branched (b) chiral aldehyde. A unique resorcin[4]arene skeleton based octaphos 3 was employed in the Rh-catalyzed hydroformylation of styrene. The hydroformylation of styrene with a metal to ligand ratio of 1:1 (M:L) was found to be regioselective, producing a linear (l) aldehyde as a major product with 100% conversion in 3 h. The l:b ratio surprisingly increased when the ortho positions of styrene were populated by methyl and chloro substituents. The hydroformylation of p-nitro styrene triggered a remarkably high linear:branched aldehyde ratio of 2.4 (71% linear aldehyde) despite its electron withdrawing nature. The highest linear selectivity of 97% (l:b ratio 27.8) was achieved in the case of 2,4,6-trimethylstyrene.

Full text of DOI:10.1039/c9dt02499a

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Synthesis and tetra-pincer nickel(II) and palladium(II) complexes of
DOI: 10.1039/C9DT02499A
resorcin[4]arene-octophosphinite [Res(OPR2)8] and rhodium-catalyzed
regioselective hydroformylation reaction†
Guddekoppa S. Ananthnaga§⁂, Dipanjan Mondala⁂, Joel T. Magueb and
Maravanji S. Balakrishnaa
Abstract. The condensation reaction of resorcinol with pentanal yielded resorcin[4]arene 1
which on bromination using N-bromosuccinimide at room temperature produced tetra-bromide
derivative 2. The reactions of 2 with chlorodiphenylphosphine and o-phenylenephosphoro-
chloridite yielded octaphosphinite 3 (here after referred as octaphos) and octaphosphite 4,
respectively. The reactions of 3 with Ni(COD)2 or Pd2(dba)3·CHCl3 in appropriate molar ratios
yielded tetra-pincer complexes 5 and 6, respectively. The structures of both the complexes were
established by single crystal X-ray diffraction studies. The resorcin[4]arene backbone adopts a
boat structure in these complexes. Typically, the Rh-catalyzed hydroformylation of styrene
prevalently delivers branched (b) chiral aldehyde. A unique resorcin[4]arene skeleton based
octaphos 3 was employed in the Rh-catalyzed hydroformylation of styrene. Hydroformylation
of styrene with metal to ligand ratio of 1:1 (M:L) was found to be regioselective producing
linear (l) aldehyde as major product with 100% conversion in 3h. The l:b ratio surprisingly
increased when the ortho positions of styrene were populated by methyl and chloro substituents.
In the hydroformylation of p-nitro styrene, despite electron withdrawing nature, triggered a
remarkably high linear:branced aldehyde ratio of 2.4 (71% linear aldehyde). Highest linear
selectivity of 97% (l:b ratio 27.8) was achieved in the case of 2,4,6-trimethylstyrene.
aPhosphorus Laboratory, Department of Chemistry, Indian Institute of Technology Bombay, Powai,
Mumbai 400076, India.
bDepartment of Chemistry, Tulane University, New Orleans, Louisiana 70118, USA.
§Present address: Chemistry Department, B. M. S. College of Engineering, Bull Temple Road,
Basavanagudi, Bengaluru 560019, Karnataka, India
*Author to whom correspondence should be addressed. E-mail: krishna@chem.iitb.ac.in,
msb_krishna@iitb.ac.in (M. S. Balakrishna); Fax: +91-22-5172-3480/2576-7152.
⁂Contributed equally to this work
† Electronic supplementary information (ESI): Tables of selected structural data and selected
spectroscopic data. CCDC 1917466 and 1917467. For ESI and crystallographic data in CIF or other
electronic format see DOI:
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View Article Online
DOI: 10.1039/C9DT02499A
Introduction
Resorcin[4]arenes are a class of macrocyclic compounds used as a platform for the
synthesis of functionalized receptors in the field of supramolecular chemistry.1-3 The
resorcin[4]arenes are structurally similar to calix[4]arenes and can easily be prepared by the
condensation of resorcinol with aldehydes.1 These bowl shaped compounds are used in
molecular recognition where they can serve as hosts for small guest molecules and cations.4
The interactions responsible for the molecular recognition are H-bonding, π-stacking, and
dipole-dipole interactions.5-8 These macrocycles are used as HPLC stationary phases,11,12 in the
selective extraction of lanthanides and actinides9,10 and also as NMR chiral shift agents.6,13 The
macrocyclic ring in resorcin[4]arenes can adopt four symmetrical arrangements, viz, crown,
chair, boat, and saddle (Chart 1). However, boat and chair are the two preferred conformations
depending on the substituents on the cavitands.14,15
Chart 1 Possible stereoisomers of resorcin[4]arene.
The adjacent phenolic units of the resorcin[4]arene backbone can be bridged using
methylene, dialkylsilicate, phosphoryl, and heterophenylene linkers to make the skeleton more
rigid (Chart 2).1,5,7,9,14,16-18 The length and nature of the bridge are known to affect the cavity
size of these cavitands.5,19-21 The resorcin[4]arenes have two zones that could be accessed for
functionalization namely the upper and the lower rims of the molecule. Functionalization of
the lower rim usually begins from an already functionalized aldehyde. The upper rim of the
molecule possesses two possible sites for chemical modifications, the phenolic groups and the
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ortho positions of the resorcinol moieties.16,22,23 The derivatization of phenolic –ODHOIg: 1r0o.1u03p9s/Cb9yDT02499A
phosphine moieties results in the formation of pincer-capable polyphosphorus ligands (Chart
2).24-28 Such ligands can be used to make either conventional polynuclear complexes29-35 or
aromatic ortho-C—H bond activated multi-pincer complexes and also in host-guest
chemistry17,18,36,37 to trap small molecules. Puddephatt and co-workers have synthesized PdII,
CuI and AgI complexes of resorcin[4]arene-based octa- and tetraphosphinites and also mixed
tetracarboxylate-tetraphosphonite derivatives (Chart 2).24-26,28 However, reports on pincer
complexes based on these cavitands or their applications in catalytic reactions are scant in the
literature.38-43
View Article Online
Chart 2 Substituted resorcin[4]arenes.
Hydroformylation of olefins is one of the most significant reactions in homogeneous
catalysis because of their important industrial significance,44 for example, in pharmaceuticals,
agrochemicals, and other fine chemicals.45-47 Numerous hydroformylation catalysts have been
developed over the years48-52 to improve the catalyst stability, activity and regioselectivity.
Most efficient hydroformylation processes utilize rhodium catalysts with monophosphines or
bisphosphines to achieve regioselectivity as well as stereoselectivity.53 Since styrenes are
excellent substrates to produce branched aldehydes, most of the investigations have been
centered around the advancement of chiral ligands or procedures to make ideal branched
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enantioisomer in high optical purity.54 Nevertheless, ongoing research on Rh-catalVyiezweAdrticle Online
DOI: 10.1039/C9DT02499A
hydroformylation of styrene has uncovered that, opposite selectivity for linear aldehyde may
likewise be conceivable through the choice of solvent,55 biphasic system56 or fine-tuned
ligand57 that assist in achieving highest linear regioselectivity.
A couple of models regarding hydroformylation of styrene concerning the linear product
can be found in the literature. Reek and co-workers utilized a library of SUPRAPhos
phosphine-phosphoroamidite ligands and achieved 72% (l:b ratio 2.6) and 70%(l:b ratio 2.4)
of the linear aldehyde from styrene and 1-octene, respectively.51 Zhang et al. reported highest
selectivity towards the formation of linear aldehyde up to 96% (l:b ratio 22) in the case of 3-
phenylpropanal involving a substituted tetraphosphorus/Rh system.58 Sémeril et al. have
reported the synthesis and application of a class of new diphosphites with a conical calixarene
skeleton, which demonstrated high regioselectivity for the linear products in the
hydroformylation of styrene (up to 75% linear aldehyde, l:b ratio 3.0) and allyl benzyl ether
(l:b ratio up to 20).59 These results encouraged us to explore octaphos 3 in the hydroformylation
of styrene and its derivatives.
Herein, we describe the synthesis, tetranickel(II), and tetrapalladium(II) pincer complexes
and catalytic investigation of octaphos/Rh system in hydroformylation reactions.
Results and discussion
The resorcin[4]arene derivative (1) was synthesized by the acid catalyzed condensation of
resorcinol and pentanal in 89% yield.60 The resorcin[4]arenes derived from aromatic aldehydes
are soluble only in polar organic solvents. In order to enhance the solubility of the
resorcin[4]arene, aliphatic aldehyde, pentanal was used.61 The aromatic bromination of 1 using
N-bromosuccinimide in 2-butanone resulted in tetrabromide 2 in good yield. The mass
spectrum of 2 showed the base peak at 1094.9937 which corresponds to [M+3Na–2H]+.
4

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DOI: 10.1039/C9DT02499A
Scheme 1 Synthesis of octaphos ligands 2 and 3 and NiII and PdII complexes of octophos 3.
The reaction of tetrabromoresorcin[4]arene (2) with chlorodiphenylphosphine in the
presence of triethylamine yielded octaphos 3 in good yield. The 31P{1H} NMR spectrum of 3
showed two single resonances of equal intensities at 122.6 ppm and 116.4 ppm indicating the
presence of two different types of phosphorus environments or possibility of having two
different compounds. Other spectroscopic and analytic data confirmed the presence of only one
product probably with boat conformation (Chart 1) having two phosphorus environments. For
further confirmation, similar reaction of o-phenylenephosphorochloridite with 2 was
31
performed to obtain octaphosphite 4. The P{1H} NMR spectrum of 4 also displayed two
singlets of equal intensities at 137.9 ppm and 137.4 ppm. The resorcin[4]arene backbone in
both the ligands 3 and 4 found to prefer boat conformation that leads to two 31P{1H} resonances
one each for the outward, and the upward directed –OPR2 groups.28
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The reaction of octaphos 3 with four equivalents of Ni(COD)2 or two eqDuOivI: a10l.e10n3t9s/Co9DfT02499A
Pd2(dba)3·CHCl3 in THF under refluxing conditions afforded pincer complexes 5 and 6,
respectively, as outlined in Scheme 1. The 31P{1H} NMR spectrum of nickel complex 5 showed
a single resonance at 141.6 ppm, whereas that of palladium complex 6 consists of two sharp
singlets at 142.6 and 141.8 ppm. This may be due to the rapid interconversion of boat
conformations in the case of nickel complex. As expected, the low temperature (–20 °C) 31P{1H}
NMR spectrum of nickel complex 5 also displayed two single resonances at 139.7 and 147.1
ppm. The two signals were observed because of the boat conformation of the resorcin[4]arene
backbone in both the pincer complexes. At room temperature boat interconversion appears to
be faster in nickel complexes compared to palladium analogue. The structures of both nickel
and palladium complexes were established by single crystal X-ray diffraction studies.62
Molecular structures of pincers 5 and 6
The perspective views of the molecular structures of complexes 5 and 6 along with atom
labeling scheme are shown in Figs. 1 and 2. The X-ray quality crystals of 5 and 6 were obtained
by slow evaporation of dichloromethane solution at room temperature. The X-ray crystal
structures of 5 and 6 confirm the oxidative addition of all the four C–Br bonds to the metal
centers.
6

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View Article Online
Fig. 1 The molecular structure of 5. All hydrogen atoms and lattice solvents areDoOmI: 10it.1t0e3d9/Cfo9DrT02499A
clarity. Selected bond length (Å) and bond angles (°): Ni1–Br1 2.3146(8), Ni2–Br2 2.294(3),
Ni1–P1 2.1471(15), Ni1–P2 2.1553(14), Ni2–P3 2.1495(18), Ni2–P4 2.1444(18), Ni1–C18
1.892(4), Ni2–C38 1.881(5), P1–O1 1.643(4), P2–O2 1.632(3), P3–O3 1.637(4), P4–O4
1.635(4), Br1–Ni1–P1 99.66(4), Br1–Ni1–P2 96.97(4), Br2–Ni2–P3 101.74(10), Br2–Ni2–P4
94.09(9), Br1–Ni1–C18 178.13(14), Br2–Ni2–C38 166.9(2), P1–Ni1–C18 81.89(15), P2–Ni1–
C18 81.58(14), P3–Ni2–C38 82.79(16), P4–Ni2–C38 81.70(16), P1–Ni1–P2 162.63(6), P3–
Ni2–P4 164.17(6).
The resorcin[4]arene skeleton adopts the boat conformation in both the complexes 5 and 6,
in which the two opposite aryl rings along with phosphinite substituents are flattened keeping
the other two aryl rings upright and nearly parallel (31P NMR data supported this conformation).
The adjacent aryl rings with phosphinite substituents are nearly perpendicular to each other.
The metallocycles and the corresponding benzene rings are coplanar. The Ni1–C18 and Ni2–
C38 bond distances in 5 are 1.892(4) Å and 1.881(5) Å, respectively. The average M–C bond
distance in nickel complex 5 [1.884 Å] is slightly shorter than that in palladium complex 6
[2.011 Å], which is expected due to the smaller atomic radius of nickel compared to palladium.
The average M–P bond distances in complexes 5 [2.149 Å] and 6 [2.274 Å] are comparable
with those of the reported pincer compounds, [NiCl{C6H3-2,6-(OPPh2)2}] [2.157 Å],
[NiI{C6H3-2,6-(OPPh2)2}] [2.158 Å]39, 63-65 and [PdI{2,6-C6H3(OPR2)2}] [Pd–P, 2.261 to 2.294
Å].66, 67 The C18–Ni1–Br1 bond angle [178.13(14)°] is almost linear, whereas the C38–Ni2–
Br2 bond angle [166.9(2)°] deviates appreciably from linearity. In the case of palladium
complex 6, all the C–Pd–Br bond angles are nearly linear.
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View Article Online
DOI: 10.1039/C9DT02499A
View 1
View 2
Fig. 2 The molecular structure of 6. All the hydrogen atoms are omitted for clarity. View 1:
Ball and Stick Model View 2: Partial Space Filling Model. Selected bond length (Å) and bond
angles (°): Pd1–Br1 2.463(2), Pd2–Br2 2.4845(12), Pd1–P1 2.277(4), Pd1–P2 2.288(4), Pd2–
P3 2.266(3), Pd2–P4 2.266(3), Pd1–C13 2.009(11), Pd2–C36 2.015(8), P1–O1 1.640(8), P2–
O2 1.648(8), P3–O3 1.642(6), P4–O4 1.634(6), Br1–Pd1–P1 98.54(12), Br1–Pd1–P2
101.40(13), Br2–Pd2–P3 103.54(7), Br2–Pd2–P4 97.13(7), Br1–Pd1–C13 176.2(3), Br2–Pd2–
C36 177.0(3), P1–Pd1–C13 79.9(3), P2–Pd1–C13 80.4(3), P3–Pd2–C36 79.5(3), P4–Pd2–C36
79.9(3), P1–Pd1–P2 159.86(13), P3–Pd2–P4 158.55(9).
8

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DOI: 10.1039/C9DT02499A
Hydroformylation reactions of styrene derivatives
The literature report on hydroformylation of styrenes using rhodium-phosphorus donor
systems suggest that the strong π-acceptor ligands are more effective and produce linear and/or
branched derivatives.51, 58, 59 Further, in hydroformylation reaction, active catalyst generated in
situ from a mixture of ligand (2PR3 or a bisphosphine (PP)) and [Rh(acac)(CO)2] yielded better
results with higher conversion rates compared to isolated complexes of the type [Rh(acac)(κ,κ-
PP)]68 or [Rh(PR3)(CO)(acac)]69. In the present study, octaphos 3 was reacted with one and
Scheme 2 Synthesis of Rh complex 7, 8 and resorcinol based bisphosphine ligand
[C6H3Br(OPPh2)2](11)
four equivalents of [Rh(acac)(CO)2] in CH2Cl2 to form mono- and tetrarhodium complexes 7
and 8, respectively (Scheme 2). The 31P{1H} NMR spectrum of 1:1 product, complex 7,
showed two singlets for uncoordinated phosphorus atoms of two types due to boat
conformation and a set of multiplets due to coordinated PPh2 moieties showing 1JRhP coupling
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(ESI, Fig. S5). 31P{1H} NMR spectrum of 1:4 product, complex 8, showed tDhOreI:e10.1s0e3t9s/Co9DfT02499A
resonances probably due to boat conformation with rhodium binding to PPh2 moieties from
adjacent aromatic groups to minimize steric congestion arising from bromine atom for
chelating mode of coordination (ESI, Fig. S7). To compare the catalytic performance of
octaphos, its monomeric analogue, bisphosphinite, [C6H3Br(OPPh2)2] (11) was prepared and
spectroscopically characterized by NMR and HRMS data and its in situ generated complex
with [Rh(acac)(CO)2] was directly used in catalysis.
View Article Online
Scheme 3 Hydroformylation of styrene
The octaphos 3, bisphosphinite 11 and two isolated complexes 7 and 8 were employed in
the Rh-catalyzed hydroformylation of styrene derivatives. The catalytic activity and
regioselectivity towards the linear product were investigated initially with PPh3 and
bisphosphinite [C6H3Br(OPPh2)2] (11). Later, the reaction conditions were optimised (Table 1)
for octaphos 3/Rh system with respect to parameters such as: CO/H2 pressure, temperature and
ligand-to-rhodium (L/Rh) ratio. After initial screening at different temperatures and syngas
pressure, the difference in effectiveness was rather pronounced. Change in the CO/H2 pressure
induced changes in the regioselectivity, but the catalytic activity was slightly affected. For
example, on increasing the pressure from 10 to 20 bar, the selectivity for linear aldehyde went
up from 50% to 62% (Table 1, entries 1 and 2). The decrease in reaction temperature resulted
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in less conversion and hence higher pressure was applied with increase in the reaction VtieiwmAerticle Online
DOI: 10.1039/C9DT02499A
(entries 5 and 6). A slight decrease of l:b ratio and slight increase in isomerization were noticed
when the L/Rh ratio was changed from 1:1 to 2:1 (entries 3 and 4). Surprisingly, changing the
L/Rh ratio from 4:1 to 6:1 resulted in least conversion with least linear selectivity (Table 1,
entries 9 and 10). Based on this data, most appropriate L/Rh ratio of 1:1 was employed. Increase
in the temperature from 80 to 110 °C resulted in improved activity and regioselectivity (Table
1, entries 1, 3, and 4). Optimised reaction condition: solvent as toluene, 110 °C, 20 atm H2/CO,
substrate/L/Rh = 1000/1/1, was employed, which showed highest linear selectivity (entry 8).
The catalytic ability of PPh3, under these optimised conditions showed very low selectivity
(entry 7). Low catalyst loading (substrate/L/Rh = 2000/1/1) also resulted in lower selectivity
as well as low conversion (entries 13 and 14). The hydroformylation reactions with two isolated
complexes 7 and 8 showed good linear selectivity but with poor conversions (entries 15 and
16). Similar catalytic reaction using [C6H3Br(OPPh2)2](11)/Rh resulted in more branched
isomer with 56% conversion (entry 17). The in situ generated 1:1 precatalyst with octaphos 3
was found to be superior in its regio selectivity and also conversion rates. Therefore, in situ
generated complex was utilized for further investigation towards the hydroformylation of
different styrene subsidiaries.
Using the optimised reaction conditions, a series of styrene derivatives were
hydroformylated using the Rh/octaphos 3 catalyst. The substrate to catalyst ratio was 1000, and
the catalyst concentration was 1.0 mM. The reaction was terminated after 3 h (Table 2). It was
found that styrene substituted with electron-withdrawing group showed lower linear selectivity
than that of styrene with electron-donating groups (Table 2). The steric hindrance of the
substrates on the regioselectivity of the hydroformylation is likewise noteworthy. On
introducing methyl group at ortho position of the styrene, the reactivity and the regioselectivity
were improved significantly (entry 8). On adding methyl group at the other ortho position, the
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View Article Online
reactivity was enhanced to form linear product in almost quantitative yield (entry 2).
DOI: 10.1039/C9DT02499A
Unexpectedly, in the hydroformylation of p-nitro styrene, despite the presence of electron
withdrawing substituent, highlighted the higher linear aldehyde selectivity (entry 5). Earlier
report correlated the lower conversion of 2,4,6-trimethylstyrene, due to the bulkiness of
substrate, that prevents the substrate from coordinating to the metal centre during the
transformation.58 Nonetheless, octaphos 3 showed remarkably high conversion along with
almost quantitative linear selectivity of 97% (l:b ratio up to 27.8) with the same styrene
derivative (entry 2)
Table 1 Hydroformylation of styrene using ligand 3 under different reaction conditionsa
Entry L/[Rh]
CO/H2 Temp. Time
Conversione
l:b ratiof
l/b (%)g
TON
(Bar)
10
20
20
30
20
40
20
20
20
30
20
20
20
20
20
20
20
(° C)
80
(h)
2
2
3
3
2
4
4
3
3
4
3
3
3
3
3
3
3
(%)
43
1
2
1:1
1:1
2:1
2:1
1:1
1:1
PPh3
1:1
6:1
4:1
1:4
1:4
1:1
1:1
7
1.0
1.6
1.4
1.5
1.4
1.2
0.9
1.6
0.9
0.8
1.4
1.3
1.7
1.4
1.5
1.4
0.7
50/50
62/38
58/42
60/40
58/42
55/45
47/53
62/38
47/53
44/56
58/42
57/43
63/37
58/42
60/40
58/42
45/55
430
680
690
840
420
900
280
1000
20
90
68
3
90
69
4
110
80
84
5
42
6
90
90
7
110
110
110
110
110
85
28
8
100
traces (2)
74
9
10
11
12
13b
14b
15c
16c
17d
740
840
790
460
1380
600
520
560
84
79
85
23
110
110
110
110
69
60
52
8
1:1
56
a S/C=1,000, Rh(acac)(CO)2 = [Rh] = 1.0 mM, toluene as solvent, b S/C=2,000. c using metal
d
e
complexes 7 and 8, L= [C6H3Br(OPPh2)2](11), Conversion of the styrenes determined on
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f
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the basis of GC. l:b = linear:branched ratio, determined on the basis of GC analysis.
DOI: 10.1039/C9DT02499A
Percentage of linear aldehyde and branched aldehyde in all aldehydes.
Table 2 Hydroformylation of styrene derivativesa
Entry Substrate
Conversionb
l:b ratioc
l/b (%)d
TON
(%)
99
98
98
98
99
99
53
98
29
98
1
2
3
4
5
6
7
8
9
10
4-Bromostyrene
1.5
27.8
1.6
1.6
2.4
5
60/40
97/3
940
980
980
980
990
990
530
980
290
980
2,4,6-Trimethylstyrene
3-Methylstyrene
4-Chlorostyrene
3-Nitrostyrene
62/38
62/38
71/29
83/17
41/59
81/19
97/3
2-Chlorostyrene
2-Fluorostyrene
2-Methylstyrene
α-Methylstyrene
4-Vinylanisole
0.7
4.4
34.9
1.5
60/40
a S/C = 1000, Temp. = 110°C, Pressure = 20 bar (1:1=CO:H2), time = 3 h, solvent = toluene. b
Conversion of the substituted styrenes determined on the basis of GC. c l/b = linear:branched
d
ratio determined on the basis of GC analysis. Percentage of linear aldehyde and branched
aldehyde in all aldehydes.
Scheme 4 Generally accepted reason behind the formation of linear and branch aldehyde.
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It is interesting to analyse the data presented in Table 2. Although, hydroformylation
DOI: 10.1039/C9DT02499A
reactions have been studied extensively, a very few studies concerning the mechanism of the
hydroformylation of styrene to yield linear product are available in the literature.70-72 The high
regioselectivity for the hydroformylation of styrene towards linear aldehyde can be symbolized
through steric cooperation between the ligands and the substrate. One of the most reasonable
justification of this regioselectivity is the ready formation of a η3-Rh complex as shown in
Scheme 3. Previously, for the hydroformylation of styrene catalyzed by Rh complex, it was
opined that the significant difference in the steric environment between the ligand and the
substrate provide remarkable regioselectivity to form linear aldehyde.58, 59, 68 In the present
study, the dependency of the selectivity on the steric nature of the ligand is comparable as it
incorporates bulky resorcin[4]arene skeleton. The steric hindrance furnished by octaphos 3
prevents the formation of η3-Rh-complex over η1-Rh complex (Scheme 4) and hence the
preferential formation linear aldehyde with higher regioselectivity takes place.
Conclusions
In summary, resorcin[4]arene-based pincer-capable octaphos ligands and their tetra-pincer
complexes were synthesized. The resorcin[4]arene backbone in both the ligands and in the
pincer complexes adopts the boat conformation.
The Rh-catalyzed hydroformylation of styrene affords selectively the branched aldehyde,
however, one can reverse the selectivity towards linear product by choosing an appropriate
ligand framework as demonstrated by octaphos 3. Octaphos effected moderate regeoselectivity
for styrene, whereas in the case of substituted styrenes very high regeoselectivity towards linear
aldehyde (97% linear aldehyde) was achieved. These resulted have demonstrated an excellent
steric cooperation between the ligands and the substrate.
These pincer complexes with potential catalytic applications can also act as hosts for small
molecules because of their large surface area for intermolecular interactions. This may also
14

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help in tuning the catalytic activity of these cavitands for various other organic transformatVioiewnAsr.ticle Online
DOI: 10.1039/C9DT02499A
Work in this direction is in progress in our laboratory.
Experimental section
General procedures. All manipulations were performed under rigorously anaerobic
conditions using Schlenk techniques. All the solvents were purified by conventional methods73
and distilled prior to use. The precursor, Pd2(dba)3·CHCl3 was prepared according to the
published procedure.74 Ni(COD)2 was purchased from Aldrich chemicals and used as such
without further purification. Other chemicals were obtained from commercial sources and
purified prior to use.
1
31
Instrumentation. The H and P{1H} NMR (δ in ppm) spectra were recorded using either
Bruker AV 400 or AV 500 spectrometers operating at appropriate frequencies. The 31P NMR
spectra were acquired using broad band proton decoupling. The spectra were recorded in
CDCl3 (DMSO-d6) solutions with CDCl3 (DMSO-d6) as internal lock. TMS and H3PO4 (85%
1
in D2O) were used as internal and external standards for H and 31P{1H} NMR, respectively.
The signals are quoted as s (singlet), t (triplet), m (multiplet) and br (broad). The microanalyses
were performed using Carlo Erba Model 1112 elemental analyzer. The mass spectra were
recorded using Waters Q-Tof micro (YA-105). The melting points were observed in capillary
tubes.
Synthesis of resorcin[4]arene (1). Resorcinol (20.0 g, 0.1816 mol) was dissolved in a
mixture of water (20 mL), ethanol (95%, 60 mL) and aqueous HCl (37%, 18 mL). To this clear
stirred solution was added slowly pentanal (15.7 g). The reaction was exothermic and
temperature was maintained below 50 °C using a cold water bath. After the completion of
addition, the reaction mixture was heated to 80 °C for 16 h. The yellow needles that separated
were collected and washed with 30 mL of cold 1:1 ethanol-water mixture. A light yellow
material thus obtained was dried at 80 °C for 12 h and used for the next step without further
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View Article Online
purification. Yield: 89% (29 g). 1H NMR (400 MHz, DMSO-d6):  8.85 (s, 8H, OH), 7.12 (s,
DOI: 10.1039/C9DT02499A
4H, ArH), 4.18 (t, 3JHH = 7.8 Hz, 4H, CH), 2.01 (m, 8H, CH2), 1.30–1.12 (m, 16H, CH2), 0.81
(t, 3JHH = 7.4 Hz, 12H, CH3).
Synthesis of tetrabromoresorcin[4]arene (2). To a well-stirred solution of 1 (20 g, 0.028
mmol) in 2-butanone (350 mL) was added N-bromosuccinimide (30 g, 0.168 mol) in portions.
After 10 min, the product started to precipitate and the mixture was stirred for another 4 h. The
precipitate was collected by filtration, washed with cold 2-butanone and dried under vacuum,
to obtain 2 as an off-white solid. Yield: 71% (20.6 g). Mp >270 °C. Anal. Calcd. for
1
C44H52Br4O8: C, 51.38; H, 5.10%. Found: C, 51.65; H, 5.14%. H NMR (400 MHz, DMSO-
d6):  9.11 (s, 8H, OH), 7.35 (s, 4H, ArH), 4.33 (t, 3JHH = 7.7 Hz, 4H, CH), 2.16 (m, 8H, CH2),
3
1.32 (m, 8H, CH2), 1.17 (m, 8H, CH2), 0.84 (t, JHH = 7.3 Hz, 12H, CH3). MS (EI): m/z =
1094.99 [M+3Na–2H]+
Synthesis of (tetrabromo)resorcin[4]arene octaphosphinite (3). To an ice-cold solution
of tetrabromoresorcin[4]arene (2) (2 g, 1.945 mmol) and triethylamine (2.19 g, 3.0 mL, 21.64
mmol) in diethyl ether (50 mL) was added dropwise a solution of chlorodiphenylphosphine
(3.69 g, 3.0 mL, 16.72 mmol) in the same solvent (30 mL). After the completion of addition,
the reaction mixture was stirred for 12 h at room temperature and then filtered through Celite
to remove the amine hydrochloride salt. The solvent was removed under vacuum to obtain the
product as a white solid. The crude product was recrystallized from toluene to obtain
analytically pure 3. Yield: 70% (3.4 g). Mp: >268 °C. Anal. Calcd. for C140H124Br4O8P8: C,
1
67.21; H, 5.00%. Found: C, 67.23; H, 5.13%. H NMR (400 MHz, CDCl3): δ 7.82-6.52 (m,
84H, ArH), 4.32 (m, 4H, CH,), 2.16 (m, 8H, CH2), 1.37 (m, 8H, CH2), 1.21 (m, 8H, CH2), 0.81
(m, 12H, CH3). 31P{1H} NMR (162 MHz, CDCl3): δ 122.6 (s), 116.4 (s).
Synthesis of (tetrabromo)resorcin[4]arene octaphosphite (4). To an ice-cold solution of
tetrabromoresorcin[4]arene (2) (1.0 g, 0.9725 mmol and triethylamine (1.09 g, 1.5 mL, 10.76
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mmol) in diethyl ether (50 mL) was added dropwise
a
solutiDoOnI: 10.o10f3V9ie/Cw9ADrtiTc0le2O4n9l9inAe
o-
phenylenephosphorochloridite (1.4 g, 8.02 mmol) in the same solvent (30 mL). After the
completion of addition, the reaction mixture was stirred for 12 h at room temperature and then
filtered through Celite to remove the amine hydrochloride salt. The solvent was removed under
vacuum to obtain the product as a white solid. The crude solid was recrystallized from toluene
to obtain analytically pure 4. Yield: 60% (1.4 g). Mp: >270 °C. Anal. Calcd. for C92H76Br4O24P8:
C, 51.80; H, 3.59%. Found: C, 51.63; H, 3.27%. 1H NMR (400 MHz, CDCl3): δ 7.82-6.52 (m,
84H, ArH), 4.32 (m, 4H, CH), 2.16 (m, 8H, CH2), 1.37 (m, 8H, CH2), 1.21 (m, 8H, CH2), 0.81
(m, 12H, CH3). 31P{1H} NMR (162 MHz, CDCl3): δ 137.9 (s), 137.4 (s).
Synthesis of (tetrakisnickel)resorcin[4]arene (5). A mixture of octaphosphinite (3)
(0.036 g, 0.0144 mmol) and Ni(COD)2(0.016 g, 0.0578 mmol) was refluxed in THF (15 mL)
for 6 h. Then the reaction mixture was allowed to cool to room temperature, filtered through
Celite and all the volatilities were removed under vacuum. The yellow mass thus obtained was
washed with hot petroleum ether to afford pale yellow powder, which was further recrystalized
from a 1:1 mixture of dichloromethane/petroleum ether to obtain analytically pure 5. Yield:
86% (0.034 g). Mp: >270 °C. Anal. Calcd. for C140H124Br4Ni4O8P8: C, 61.44; H, 4.57%. Found:
C, 61.47; H, 4.22%. 1H NMR (400 MHz, CDCl3): δ 8.12-6.61 (m, 84H, ArH), 4.73 (t, 3JHH
=
7.3 Hz, 2H, CH), 3.75 (t, 3JHH = 6.4 Hz, 2H, CH),1.92 (m, 8H, CH2), 1.31 (m, 8H, CH2), 0.91
31
(m, 8H, CH2), 0.81 (m, 12H, CH3). P{1H} NMR (162 MHz, CDCl3, 25 °C): δ 141.6 (s).
31P{1H} NMR (162 MHz, CDCl3, –20 °C): δ 139.7 (s), 147.1 (s).
Synthesis of (tetrakispalladium)resorcin[4]arene (6). A mixture of octaphosphinite (3)
(0.09 g, 0.036 mmol) and Pd2(dba)3·CHCl3 (0.075 g, 0.072 mmol) was refluxed in THF (15
mL) for 6 h. Then the reaction mixture was allowed cooled to room temperature, filtered
through Celite and all the volatilities were removed under reduced pressure. The yellow mass
thus obtained was washed several time with hot petroleum ether to afford a pale yellow
17

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crystalline product. Yield: 79% (0.084 g). Mp: >270 °C. Anal. Calcd. ViewfoArrticle Online
DOI: 10.1039/C9DT02499A
C140H124Br4O8P8Pd4·(C4H8O)2: C, 57.87; H, 4.59%. Found: C, 57.84; H, 4.42%. 1H NMR (400
MHz, CDCl3): δ 8.22-6.48 (m, 84H, ArH), 4.79 (t, 3JHH = 7.3 Hz, 2H, CH), 3.74 (m, 2H, CH),
1.94 (m, 8H, CH2), 1.57 (m, 8H, CH2), 1.32 (m, 8H, CH2), 0.78 (m, 12H, CH3). 31P{1H} NMR
(162 MHz, CDCl3): δ142.6 (s), 141.8 (s).
X-Ray crystallography. A crystal of each of the compounds 5 and 6 suitable for X-ray crystal
analysis was mounted on a Cryoloop with a drop of Paratone oil and placed in the cold nitrogen
stream of the Kryoflex attachment of the Bruker APEX CCD diffractometer. A full sphere of
data was collected using 606 scans in ω (0.3° per scan) at φ = 0, 120 and 240° using the
SMART75 software package, or the APEX276 program suite. The raw data were reduced to F2
values using the SAINT+ software76. Multiple measurements of equivalent reflections
provided the basis for an empirical absorption correction as well as a correction for any crystal
deterioration during the data collection (SADABS76). Both the structures were solved by direct
methods and refined by full-matrix least-squares procedures using the SHELXTL program
package76. The poor quality of the refinement in case of nickel(II) complex can be attributed
to the extensive disorder. Hydrogen atoms attached to carbon were placed in calculated
positions and included as riding contributions with isotropic displacement parameters tied to
those of the attached non-hydrogen atoms. Since the crystals were not diffracting well, X-ray
crystallography data showed high R1 values for both the compounds 5 and 6. There were some
significant disorder in the phenyl and nBu groups in both the compounds which were modelled
to their correct positions. However, in compound 5 significant peaks appeared around the
n
methyl region of the disordered Bu groups were left as such. In compounds 5 and 6 small
regions of electron density peaks well-separated from the main molecule were present after the
final refinement, indicating partially occupied and/or disordered solvent sites. Since the
disorder could not be resolved and the nature and number of solvent molecules could not be
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View Article Online
determined, all-electron density associated with the solvent molecule(s) were removed using
DOI: 10.1039/C9DT02499A
the SQUEEZE procedure in PLATON.77 The results indicated 59 electrons, and a volume of
905 Å3 for 5, whereas 374 electrons, and the volume of 1619 Å3 for 6. The given chemical
formula and other crystal data do not take into account the unknown solvent molecule(s).
Crystallographic data for the structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 1917466
(compound 5), 1917467 (compound 6).
General Procedure for the Hydroformylation of styrene
In a glovebox filled with nitrogen, octaphos 3 (0.1 mol %) and [Rh(acac)(CO)2] (0.1 mol%) in
1 mL toluene were added to a 2 mL vial and after stirring for 5 min, styrene (0.48 mmol) and
additional solvent were added in the above solution to make up the total volume of the reaction
mixture to 10 mL. The solution was then transferred into an autoclave fitted with a mechanical
stirrer. Once closed, the autoclave was flushed twice with syngas (CO/H2 1:1 v/v), pressurised
with a CO/H2 mixture and heated. The reaction was terminated after 3 h. Octaphos 3 was first
applied to optimize the hydroformylation conditions of styrene with respect to parameter
changes of CO/H2 pressure, temperature and ligand-to-rhodium (L/Rh) ratio. The effect of
CO/H2 pressure was verified with CO/H2 pressure ranging from 10 to 30 psi atm. The reactions
were performed with various L/Rh ratio and reactor was heated to desired temperature (a range
of 80 to 110 °C) with a stirring speed of 340 rpm. After completion of the reaction, the reactor
was cooled to room temperature and the remaining synthesis gas was carefully released in a
well-ventilated fume hood. The reaction mixture was quantitatively analysed by gas
chromatography. Catalytic conversions were determined by gas chromatography (GC) on a
Rtx®-1 capilary column (inner diameter 0.25 mm, film thickness 0.25 µm, length 30.0 m).
Retention times were compared with authentic samples.
Conflicts of interest
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DOI: 10.1039/C9DT02499A
There are no conflicts to declare.
Acknowledgments
We are grateful to the Science & Engineering Research Board, New Delhi, India, for
financial support of this work through grant No.SB/S1/IC-08/2014. GSA thanks CSIR New
Delhi and IIT Bombay for fellowship. DM thanks Department of Science and Technology,
New Delhi, for Inspire Fellowship. We also thank the Department of Chemistry
Instrumentation Facilities, IIT Bombay, for spectral and analytical data. JTM thanks the
Louisiana Board of Regents for the purchase of the CCD diffractometer and the Chemistry
Department of Tulane University for the support of the X-ray laboratory.
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Å, b = 21.8228(12) Å, c = 25.4931(14) Å, α = 90°, β = 114.048(10)°, γ = 90°, V =
14259.7(13) Å3, Z = 4, Dc = 1.353 g cm-3, µ(Mo-Kα) = 1.858 mm-1, F(000) = 5928, T =
100 K, GoF = 1.091, final R1 = 0.0800 and wR2 = 0.2393 for I>2σ (I), R1 = 0.0992 and
wR2 = 0.2558 for all data. CCDC 1917466. Crystal data for 6: C140H124Br4O8P8Pd4, M =
2927.38, monoclinic (C2/c), a = 27.9334(7) Å, b = 21.8518(4) Å, c = 26.0254(5) Å, α =
90°, β = 114.212(10) °, γ = 90°, V = 14488.4(5) Å3, Z = 4, Dc = 1.342 g cm-3, µ(Mo-Kα) =
6.464 mm-1, F(000) = 5888, T =150 K, GoF = 1.048, final R1 = 0.0889 and wR2 = 0.2276
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Synthesis and tetra-pincer nickel(II) and palladium(II) comDpOlI:e1x0.1e0s39/Co9DfT02499A
resorcin[4]arene-octophosphinite [Res(OPR2)8] and rhodium-catalyzed
regioselective hydroformylation reaction
Guddekoppa S. Ananthnaga#, Dipanjan Mondala, Joel T. Magueb and Maravanji S.
Balakrishna,a
This paper describes the synthesis of resorcin[4]arene based octaphosphinite ligands, tetra-
pincer complexes of NiII and PdII and rhodium-octaphosphinite catalyzed hydroformylation of
styrene and its derivatives.