Porous Materials Based on 3-Dimensional Td-Directing

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Porous Materials Based on 3-Dimensional Td-Directing Functionalized Adamantane Scaffolds and Applied as Recyclable Catalysts Houssein Nasrallah, and Jean-Cyrille Hierso Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04508 • Publication Date (Web): 26 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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Chemistry of Materials

Porous Materials Based on 3-Dimensional Td-Directing Functionalized Adamantane Scaffolds and Applied as Recyclable Catalysts Houssein Nasrallah,† and Jean-Cyrille Hierso*,†,‡ †Institut

de Chimie Moléculaire de l’Université de Bourgogne (ICMUB - UMR CNRS 6302), Université de Bourgogne Franche-Comté (UBFC), 9 avenue Alain Savary, 21078 Dijon Cedex, France ‡ Institut

Universitaire de France (IUF), 103 Boulevard Saint Michel, 75005 Paris Cedex, France

ABSTRACT: Porous materials have been of high scientific and technological interest owing to their unique performances in many topical applications related to multiphasic functional systems: gas separation and storage, heterogeneous catalysis, energy conversion, etc. We review herein the synthetic strategies applied for using functionalized adamantane derivatives as polyhedral (mainly tetrahedral, Td-directing) building units of three-dimensional (3-D) porous supramolecular structures and nanomaterials, either purely organic or within metal hybrid frameworks. The resulting materials are currently used in varied heterogeneous (or supported) transition metal catalysis and organocatalysis, including recent high-value asymmetric synthesis. Characterization, synthetic applications and recycling properties of catalytic materials based on adamantane-scaffold are discussed, and this review highlights the structuring advantages of variously functionalized-adamantanes to reach high surface area and controlled porosity for exploiting confinement effects related to modified kinetics compared to homogeneous reactions, and pertinent chemo- and enantioselectivity issues.

1. INTRODUCTION Adamantane is a cycloaliphatic hydrocarbon with fused chair cyclohexane rings, which essentially corresponds to diamond lattice cage-structure. Synthetic access to adamantane scaffold and its derivatization into polyfunctionalized molecules has been investigated since the fifties and is well-documented (Figure 1).1 Adamantane gathers several features that make it unique in many research fields, from functional materials to biomedical applications. For instance, adamantanes are introduced in biochemicals in order to enhance the lipophilicity and global stability of the drugs. In terms of structure, adamantane is attractive because of its bulky spherical shape, its rigidity, and a high potential for symmetrical (and unsymmetrical) polyfunctionalization. This renders adamantane an ideal sp3-C building block for the reasoned construction and arrangement of highsymmetry directional ensembles. X

X X = Br, C CH , alkyl, etc.

X Adamantane X

v. Ragué Schleyer (1957) X

topics Adamantane synthesis, structure, properties Self-assembly of high-symmetry coordination cages (general) MOFs and self-assembled supramolecular coordination complexes (general) Adamantane functionalization Medicinal chemistry of adamantanes

X X

unequally polyfunctionalized adamantane derivatives applied at producing biologically active compounds,5 has been complemented by a comprehensive review from Schreiner group on the highly advanced medicinal chemistry of adamantane derivatives.6 Schreiner, Hierso, and others have extended the survey on the general functionalization and typical applications of functionalized adamantanes to diamantanes and triamantanes, and some other polymantanes (tetramantane, etc.).7, 8, 9 The biological activity of multivalent adamantane scaffolds and their synthetic modes have been surveyed, focusing in particular on peptide-based systems.10 More recently, Williams and co-workers reviewed the performances of homogeneous metal catalysts and organocatalysts in which phosphine and carbene derivatives bears adamantane alkyl groups.11

X = Br, I, OH, COOH, Ph, etc.

Diamondoids applications (general)

Figure 1. Representative polyfunctionalized adamantanes.

Diamondoids functionalization (general)

A number of relevant reviews are available that focused on particular aspects of adamantane and lower-diamondoids (diamantane, triamantane) chemistry and applications (Table 1). von R. Schleyer early on discussed the interrelation of the chemical and physical properties of adamantanes with the diamondoid structure, together with the development of adamantane scaffolds synthetic routes.2 This pioneering approach introduced adamantanes to the field of three-dimensional, coordination-driven self-assemblies based on directional-bonding methodologies.3,4 A survey of the synthesis of

Diamondoids for functional applications Adamantanes derivatization for peptide biomedical applications Homogeneous catalysts with adamantanes scaffolds

authors / year v. R. Schleyer et al. /1964 Stang et al. / 2002

ref 2

Stang et al. / 2013

4

Kovalev et al./ 2012 Schreiner et al./ 2013 Schreiner, Hierso et al./ 2014 Hierso, Schreiner et al./ 2015 Sun et al./ 2015

5

Bianco et al./2015

10

Williams 2016

11

et

al./

3

6 7 8 9

Table 1. Review papers dealing with adamantanes syntheses and/or applications.

ACS Paragon Plus Environment


From a perspective more oriented towards high performance functional materials, it thus appeared timely to us to provide a concise survey highlighting the synthesis and catalytic applications of adamantane derivatives operating as polyhedral building units of supramolecular structures and porous materials. Herein, we related this approach to the application of the resulting structures in heterogeneous (or supported) transition metal catalysis, and in organocatalysis –including high-value asymmetric versions– which are topics that have met with a growing interest in the last decade. Notably, the interest of adamantane-based materials widely extends to capture and sensing capacities which are not discussed here.12, 13 This review cut-off was first semester 2018. 2. POROUS MATERIALS USED FOR CATALYTIC APPLICATIONS At the interface of organic, organometallic and inorganic chemistry, materials chemists have made significant progress in the construction of porous materials by assembling different building units (Figure 2).14, 15, 16 The chemical nature of their building units, as wells as their pore size, are two essential parameters to categorize porous materials. Based on chemical nature, three main groups can be distinguished: (i) inorganic porous materials (i. e. zeolites, porous metals oxides and silica, etc.), plus two other broad categories of materials that incorporate organic functions, (ii) organic-inorganic hybrid materials consisting in the assembly of a metal precursor via coordination with polyfunctionalized organic building unit to form a coordination polymer (CP), and (iii) the organic porous materials constructed by the assembly of merely polyfunctionalized organic units, often called Porous Organic Polymers (POPs). The most widespread coordination polymers (CPs) are known as Metal-Organic Frameworks (MOFs). Nevertheless, a debate to unambiguously define a MOF regarding other coordination polymers is existing.17, 18, 19 Reedijk and co-workers surveyed various definitions of MOFs proposed by different research groups.17 They discussed some important definitions such as 2-D and 3-D networks, 3-D–only networks, as well as likely and/or proven porosity, networks with frames, etc. Their review highlighted also the utilization of the term Metal-Organic Polymers (MOPs) by various research groups. Relevant historical perspective of MOF development and personal views have been also reported relating to definition and nomenclature.18, 19 Clearly this variety of terminology, together with a significant range of different specific properties for the CPs, made more difficult their unanimous classification. The present review is not addressing this issue. Unlike CPs, the classification of POPs is arguably easier and main categories can be defined such as: Covalent Organic Frameworks (COFs), Hyper Cross-linked Polymers (HCPs), Porous Aromatic Frameworks (PAFs), and various microporous polymers (PIMs, CMPs), see Figure 2.15, 20, 21 Among this family of POP materials, the Covalent Organic Frameworks (COFs) are in majority under well-defined crystalline form. Finally, in the porous materials described above, while a large set is polymeric in nature, an array of organic porous materials are of oligomeric-type such as calixarenes, cryptophanes and other cavitands.22

Organic

Porous Materials

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Cavitands, cryptophanes, calixarenes Porous Organic Polymers (POPs)

Inorganic

Zeolites, SiO2 (MCM-41, 48, etc.), M(O)x

Covalent Organic Frameworks (COFs) Hyper Cross-linked Polymers (HCPs) -

Page 2 of 27

Figure 2. Porous materials general categorization according to the nature of building units (see ref. 14 for a variant). Depending on their pore size, porous materials are divided into microporous (50 nm).23 Polyfunctionalized organic molecules are widely available for tuning porous materials porosity and specific area in CPs.24, 25 Though, limitations exist concerning thermal and chemical stability of organics, which remain detrimental for their applications as scaffold in functional porous materials. This is especially true for demanding catalytic applications in which recovering and/or recycling are primary objectives.14, 26 The choice of organic building units is essential to reach high performance porous materials, including controlled porosity, high periodicity, thermal and chemical stability. Thus, intrinsic features of a relevant organic building block are: (i) its rigidity, since the formation of pores requires a rigid structure that promotes cavity creation and avoids oligomeric packing;27, 28 (ii) a high symmetry, this property combined with an appropriate rigidity favors the construction of periodic porous structures;29 (iii) a controlled bulkiness, since steric restriction by incorporation of bulky substituents on organic block prevents undesired intercalation during porous materials synthesis. Accordingly, a tetrahedral geometry may help generating highly ordered periodic 3-D architectures and provides high symmetry for promoting a general sphericity and high surface area of porous materials.30, 31 In addition, extended tetrahedral symmetry is advantageous during porosity formation in material synthesis by impeding close packed arrangement.32, 33, 34 Overall, adamantane building blocks that can be functionalized in various directions, constitute ideal candidates for the construction of porous materials, which meet all the previous requirements as scaffold, both within CPs and POPs catalytic materials. 3. POLYFUNCTIONALIZED ADAMANTANE SCAFFOLDS We describe in this section the methods reported for trifunctionalization and tetrafunctionalization of the adamantane scaffold, specifically in relation with further construction of threedimensional structures (3-D). Several reviews addressed the general synthesis of adamantanes and their mono- and difunctionalization (Table 1). Mostly, tri- and tetrabromoadamantane, as well as tri- and tetraphenyladamantane, are the precursors preferred for introducing functional groups in poly-substituted scaffolds and we focused at these reports. 3.1 Bromoadamantanes derivatization. Bromination of adamantane 1 is increasingly difficult from mono- to tetrafunctionalization (Figure 3). Harsh conditions are required to produce tri- and tetrabromoadamantane, 4 and 5, and the undesired formation of mixtures including mono- and dibromoadamantane (2 and 3) is often occurring. The synthesis of 1,3,5-tribromoadamatane 4 was described by Sletter using Lewis acidic conditions with AlBr3.35 By using iron powder Yurchenko achieved the synthesis of 1,3,5tribromoadamantane with high selectivity and better yield in milder conditions.36 For tetrabromination the Sletter method using metallic bomb,35 was improved by Scollot using AlCl3 in refluxing bromine.37 This latter method remains currently the most popular choice for 1,3,5,7-tetrabromoadamantane 5 preparation.38

Porous Aromatic Frameworks (PAFs) Polymers with Intrinsic Microporosity (PIMs) Conjugated Microporous Polymers (CMPs)

Br (a): Br2

2 (a): 85% Sletter et al.35 (b): 75% Schreiner et al.39

Br2, Fe

(b): NaOH aq. CBr4, PTC 1

Metal Organic Frameworks (MOFs)

OrganicInorganic Hybrid

Coordination polymers

Metal-Organic-Polymers (MOPs) Porous Coordination-Polymers (PCPs)

Br2, Fe rt, 2 h

Br

Br 3

64% Lightner et al.40


Br

Br Br 4

75 °C, 24 h

Br2, AlCl3

98% Yurchenko et al.36 80% Sletter et al.35

Br

75 °C, 24 h Br

Br Br 5

58% Scollot et al.37 75% Sletter et al.35

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Figure 3. Adamantane polybromination methods. Alternative monoand dibromination methods give good yields of products.39, 40 1,3,5-Tribromoadamantane 4 is used as starting material in the synthesis of many trifunctionalized adamantanes (Figure 4). Hydroxylation of 4 was achieved to produce adamantane triol 6, which was used as precursor to prepare adamantane 1,3,5-tricarboxylic acid 7 7. This latter is used to obtain the triamine 8 by Hoffmann degradation, tritopic 8 is employed in the synthesis of coordination polymers.41 Domasevitch (2012) Br

Br

Ag2SO4

HO

HCOOH

OH

H2SO4

Br 4

HO2C

CO2H

15% oleum

OH 6 65%

CO2H 7 72%

1) SOCl2

H 2N

2) 20% NH3 3) Br2, NaOH

NH2 NH2 8 56%

tetracyanoadamantane 22 (Figure 7).51 Hydrolysis of 22 produces the tetracarboxylic acid adamantane 23. This method is a very good alternative to prepare 23, reducing the number of synthetic steps and avoiding steel bomb use reported by Landa,52 or others.53 To prepare the 1,3,5,7-tetraaminoadamantane, 21, Ritter amination failed from 1,3,5,7-tetrabromoadamantane 5.54 To overcome this issue, Scollott applied photochemical Ritter amination37 to 1,3,5,7tetraiodoadamantane 18a,55 to obtain the 1,3,5,7-tetraacetamide adamantane 20. This latter was then hydrolyzed in an acidic medium to obtain the 1,3,5,7-tetraaminoadamantane hydrochloride 21•HCl.37 Domasevitch reported the alternative synthesis of 1,3,5,7tetraaminoadamantane 21 by Hoffman degradation of the amide 24 obtained via Gutierrez cyanation (Figure 7).41 Migulin et al. described the synthesis of 1,3,5,7-tetrahydroxyadamantane 19 by 1,3,5,7tetrabromoadamantane 5 hydroxylation in concentrated H2SO4 in the presence of Ag2SO4.56

Figure 4. Adamantane amino-trifunctionalization starting from 1,3,5tribromoadamantane 4 via triol and carboxylic acid. Starting from 4 Malik et al. have prepared 1,3,5-triethynyladamantane 9, which was used in the synthesis of thermally stable polymers (Figure 5).42 Maison et al., starting either from tribromide 4 or triethynyladamantane 9, have prepared a series of expanded trifunctional adamantanes including some terminal triol, tricarboxylic acid, triester, triamine, and tricyanide derivatives with ethylene and propylene spacer (see for instance 10-13a,b and 14-16, in Figures 5 and 6, respectively).43, 44

(a): CH3I, AlBr3, X = I, 18a, 91%

Br

X

(b): CCl4, AlCl3, X = Cl, 18b, 98%

Br

X

Br

X

Migulin (1999) (c): H2SO4, AgSO4 X = OH, 19 75%

Br 5 NaCN h

CH3CN H2O, h

Scollot (1980)

NHAc

HClaq.

AcHN

X

NHAc

NHAc 20 51 %

79 %

Gutierrez (2004)

CN

CO2H

KOHaq. autoclave

CN 22 73 %

NH2

Br2

CN NC

NH2

H 2N

HO2C

yield n.d.

Domasevitch (2012) SOCl2 NH3aq. 92%

CO2H 23

CO2H

21

NH2

(4 HCl*)

NaOHaq. 70% CONH2

H2NOC

CONH2 CONH2 24

*1,3,5,7-tetraaminoadamantane hydrochloride was isolated by Scollott's method Malik (1992)

Br

4

Maison (2008)

HO

AlCl3

n-BuLi

Figure 7. Adamantane tetrafunctionalization based on 1,3,5,7tetrabromoadamantane 5.

TsO TsCl, Py

HCHO

t-BuOK

OH

9

10

84%

94%

11 55%

OTs

TsO

HO Maison (2008) AIBN, Bu3SnH 4

FG

CN NC 2

CN

FG 2

2

2

NC

12 67%

FG

2 2

FG reagent yield

COOH, HCl, H2O 13a 95%

CH2NH2 DIBAL 13b 91%

Figure 5. Synthesis of triethynyladamantanes derivatives from 4. Maison et al. have also disclosed an efficient strategy to access unequally tetrafunctionalized adamantanes possessing three identical arms and a protected amino function in the 1-position (Figure 6, 17). Maison efficient synthetic access to A3B tetrafunctionalized adamantanes is widely applicable to prepare tetrahedral multivalent species for biomedical purposes.45, 46

3.2 (Poly)phenyladamantane derivatization. Phenylation reactions, conducted under Lewis acidic conditions, produced the other main class of tri- and tetrafunctionalized adamantane precursors for porous materials synthesis. Various phenylation of adamantane in positions 1-(25), 1,3-(26), 1,3,5-(27) and 1,3,5,7-(28) are achieved from reaction of bromoadamantane 2 with benzene in the presence of AlCl3 and t-BuBr (Figure 8, top). The number of phenyl groups introduced is carefully controlled by experimental conditions.50 Newman (1972)

Br 2

Maison (2004)

9

MeLi CO2

CO2H TMS-CH2N2 2 MeOH 14 2 97%

HO2C 2 HO2C

MeO2C

CO2Me LiAlH 4 2 HO

2

MeO2C

2

15 63%

OH 2 HO

2

2

2

t-BuBr benzene

AlCl3 benzene t-BuBr

2

AlCl3

t-BuBr benzene AlCl3

Ph

Ph

B

HO2C

2

2 17 56%

A

A

Ph 27 64%

Ph Ph

Ph + Ph 27

Ph Ph

28

RuCl3 H5IO6

HOOC COOH

COOH 7 96%

CO2Me Ph

Ph CO2H

2

26

Ph

NHAc

HO2C

Ph

Maison (2008) Ph

16 99%

Br2, Fe NOBF4, MeCN

Ph +

Ph + 25

Ph

28 44%

RuCl3 SOCl2 MeOH

MeO2C

CO2Me CO2Me 29 35%

Figure 8. Bromoadamantane polyphenylation, and subsequent tri- and tetracarboxylic acid adamantane synthesis via Sharpless oxidation.

A3B tetrafunctionalization

A

Figure 6. Expanded scope of equally trifunctionalized adamantanes, and A3B tetrafunctionalization from 9. 1,3,5,7-Tetrabromoadamantane 5 (Figure 3) is a key precursor for accessing symmetrical tetrafunctional adamantanes (Figure 7, 18-22). Notably, it serves to form 1,3,5,7-tetracarboxylic acid adamantane 23 (Figure 7) that is widely used as core in dendron and dendrimers promoted catalysis.47, 48, 49 Mostly, tetracarboxylic acid adamantane 23 is isolated from its methyl ester derivative after direct esterification to facilitate workup and chromatographic purification advantages.50 In 2004, the photochemical cyanation of 5 was reported by Gutierrez, producing in a good yield single step reaction the 1,3,5,7-

From these phenylated adamantanes (27 and 28), Maison et al. further developed a practical method for yielding adamantane tri- and tetracarboxylic acids, via Sharpless oxidation by using per-iodic acid as oxidant and RuCl3 as catalyst (Figure 8, bottom).57 The authors obtained excellent yield of adamantane 1,3,5-tricarboxylic acid 7, but because of solubility limitation of 1,3,5,7-tetraphenyladamantane 28, lower yield of 1,3,5,7-adamantane tetracarboxylic acid 23 (via methyl ester 29). Bianco’s group employed a Newman-Maison approach50 from 25 to prepare the A3B derivatives (Figure 6) 1,3,5-triphenyl-7bromoadamantane, 1,3,5-triphenyl-7-acetylaminoadamantane and 1,3,5-tricarboxy-7-acetylaminoadamantane, usable as building blocks


Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

for dendrimers in biomedical applications.58, 59, 60, 61 The postfunctionalization of 1,3,5,7-tetraphenyladamantane 28 was described for the use of resulting blocks (typically tetrasubstituted arylbromo, aryliodo, arylamino blocks, etc.) in the development of molecular electronic devices,62, 63 in biological conjugation (towards 3-D nucleosidic “sticky ends”),64 in flame retardant of polycarbonate polymers,65 in Atomic-Force Microscopy (AFM) as a part of molecular tips,66, 67 and essentially for the construction of materials used in gas sorption and catalytic applications. In this context, several functional groups on phenyl were introduced from tetraphenyl 28 via electrophilic substitution in para-position of the phenyl moieties (Table 2).64, 65, 68, 69, 70, 71, 72 This successfully allowed building further molecules and functional materials with a global conservation of the original tetrahedral symmetry from the rigid adamantane primary tecton. E

Ph E

E+

Ph

Ph Ph

28

E

electrophilic substitution

Page 4 of 27

4.1. Porous materials integrating 1,3,5,7-tetraphenyladamantane with –C≡C– ethynyl linkers. Ding et al. achieved the heterogenization of Shibasaki’s [La/Binol] catalyst for enantioselective epoxidation of ,-unsaturated ketones, using polytopic modified Binol derivatives (Binol = 2,2'-dihydroxy-1,1'-binaphthyl). They employed 1,3,5,7tetraphenyladamantane for synthesizing the tetrahedral chiral Binolbased branched derivative 32 (Figure 10).78 Adamantane-Binol tetratopic linker 32 was prepared by palladium-catalyzed Sonogashira reaction from iodinated 30a with a methoxymethyl (MOM)-protected Binol B1 substituted at one aromatic moiety by an ethynyl functional group (Figure 10). The 3-D MOP 33 was then synthesized by coordination of the Binol part of 32 to lanthanum from the precursor complex La(OiPr)3.78 A series of ditopic (aa-ag) and tritopic (ah) Binol-based linkers were used for providing analogous La-Based 1-D and 2-D MOPs, respectively (Figure 10). These materials were isolated by filtration and washed with THF. The resulting materials were characterized by Scanning electron microscopy (SEM) and powder X-ray diffraction studies (PXRD), showing non-crystalline structures with a surface composed of micrometer-sized particles (ranging from 1 to 12 µm). More in-depth characterization of the various materials (like porosity size, surface area, or La loading) was not reported.

30a-f E

entry 1

–E

main reagents

yield (%)

ref.

–I, 30a

(CH3CO2)2IPh, I2

80

68, 69

2

–Br, 30b

Br2

60

70

3

–NO2, 30c

HNO3/H2O, Ac2O, AcOH

35

64

4

–CHO, 30d

TiCl4/CH3OCHCl2

84

71

5

–COCH3, 30e

AlCl3/CHCOCl

68

72

6

–SO2Cl, 30f

HSO3Cl

57

65

Table 2. 1,3,5,7-tetraphenyladamantane post-functionalization. Direct functionalization of 28 by SEAr can be followed by synthetic modifications which give access to a wide range of functional groups (FGs) hold on the aromatic moieties of 1,3,5,7-tetraaryladamantanes (FGs = NH2, N3, COOH, SH, alkynyl, etc.).64, 73, 74, 75, 76 Importantly, the tetraiodinated compound 30a is a convenient precursor for such functionalizations, notably used for direct alkyne groups incorporation via the versatile palladium-catalyzed alkynylation cross-couplings (aka Dieck-Heck and Sonogashira-Tohda-Hagihara alkynylation reactions).77 Formed from 30a, terminal alkyne derivatives of type 31 (Figure 9),64 open access to further post-functionalization of adamantylphenyl scaffold using easily accessible reactive ethynyl groups.

30a

OMOM OMOM

+

1) Pd(PPh3)2Cl2 (1 mol%), CuI (0.5 mol%), Et3N 2) 12 M HCl

B1

32

Adamantane-Binol tetratopic linker

64%

La(OiPr)3 OPPh3 THF

OH OH

=

33 10-20 mg scale yield n.d.

= La

Binol-based di- and tritopic linkers

aa

ae

ad

ac

ab

af

ag

ah

Figure 10. 3-D lanthanum-based MOP 33 synthesized from tetrahedral adamantane-Binol linker 32 (top) and other polytopic linkers used for 1-D and 2-D MOPs synthesis (bottom). 30a

+

Si

1) [Pd(PPh3)2Cl2], CuI Et3N 2) KF, MeOH

31 74%

The catalytic activities of this series of MOPs were compared for enantioselective epoxidation of a α,β-unsaturated ketones with cumene hydroperoxide (CMHP, 88%) as the oxidant in the presence of Ph3P=O (15 mol%) and MS (4 Å) (Table 3), to assess the impact of the arrangement of chiral units on catalytic properties of MOPs.78

Figure 9. Ethynyl-terminated 1,3,5,7-tetraphenyladamantane 31. O

Then, tetrahedral ethynyl compound 31 can be used, in turn, in a Heck or Sonogashira cross-coupling with a halogenated organic partner for further molecular extension. Thus, tetrafunctionalized precursors 30a and 31 provide chiefly used building blocks for the integration of Tddirecting robust adamantane tectons in structuring 3-D porous materials. This is discussed in the following sections. 4. POROUS MATERIALS BASED ON TETRAPHENYLADAMANTANES

Ph  

Ph

chalcone

entry 1 2 3 4 5 6


CMHP 1.5 equiv 5 mol % [La/Binol] MOP 4 Å MS, THF, OPPh3 30 min, RT

linker 32 aa ab ac ad ae

yield (%) 99 99 99 99 99 99

HO

O Ph 



O

O

Ph

ee (%) (R,S) 95.0 83.7 82.9 95.5 97.6 93.3

CMHP

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7 8 9

af ag ah

99 99 99

95.1 84.2 91.5

Table 3. Enantioselective La-catalyzed epoxidation of α, βunsaturated ketone 1,3-diphenyl-2-propenone (chalcone) by MOPs from linkers 32 and aa-ah (Figure 10). Using 1-D and 2-D MOPs, the authors observed the benefits of longer linker lengths (ac and ad vs aa and ab) for higher enantiomeric excess (ee) above 95.5 (with ad 97.6 ee, Table 3, entry 5). A significant effect on the ee is apparently related to the angle of coordinating functions on a phenyl spacer (ad, af, ag, Tables 3 entries 5, 7 and 8), indicating a significant structural effect of the coordination polymers, with the para mutual-positioning leading to better enantioselectivity. From the adamantane-Binol linker 32 the 3-D La-based MOP 33 gave a good 95.0 ee, which was comparable to that attained in homogeneous reactions.79 Five recycling runs were realized for chalcone epoxidation with the MOP based on linker ad (yield>99% with 96.594.9% ee in the four first runs), and ICP analyses (inductively coupled plasma atomic emission spectroscopy) showed in solution 0.32, 0.39 and 0.2 ppm La content, after the three first runs, respectively.78 The authors extended their studies of chiral materials to the formation of polytopic phosphonite derivatives from Binol-linkers (Figure 11).80 Each linker was modified by reaction with tris(dimethylamino) phosphine [P(NMe2)3] to give the phosphonite linkers 34 (incorporating adamantane) and the series ba-bh. Rhodium complexation with these phosphonite linkers was achieved to generate insoluble 1-D, 2-D and 3-D MOPs, ca-ch and 35, respectively. The complexation was confirmed by cross-polarization magic-anglespinning (CP-MAS) 31P NMR analysis. These materials were found mostly amorphous from powder X-ray diffraction patterns (PXRD), but their specific surface area and porosity were not reported. Phosphonite-based linkers

Spacer

P(NMe2)3

OH OH

O O P

Spacer

Toluene, reflux

n

O P [Rh] O NMe2

P

95 (R) 96 (R) 93 (R) 95 (R) 97 (S)

95 (R) 97 (R) 95 (R) 96 (R) 98 (S)

96 (R) 97 (R) 96 (R) 96 (R) 95 (S)

Table 4. Rhodium-catalyzed asymmetric hydrogenation of dehydroamino acid methyl esters. Interestingly, while MOPs ca-ch (linkers aa-ah, Table 4) led to products of R configuration, with the tetrafunctionalized adamantanebased rhodium MOP 35 the S enantiomer was obtained generally with high ee (Table 4, entry 9).80 This inversion indicates that 3-D arrangement of 35 is strongly influencing the stereochemistry in the course of the hydrogenation. Notably, the kinetic profile of the reaction with each heterogeneous system has shown that the catalytic activity of rhodium catalysts incorporating tri- and tetratopic ligands bh and 34 was the highest, overpassing the performances of homogenous counterparts.80 The resultant homochiral MOPs were recycled without significant loss in enantioselectivity but a drop in reactivity, which was overcome by employment of a continuous flow reactor that allows for high efficiency for extended reaction periods. Garcia et al. reported the synthesis of 3-D COF 36 by palladiumcatalyzed Sonogashira coupling of tetraiodide 30a with 1,4'diethynylbiphenyl through –C≡C– linkers (Figure 12).84 The COF preparation led to some residual palladium deposition and ICP-OES analysis (inductively coupled plasma optical emission spectrometry) indicated that around 0.1 wt% palladium remained inside the pores. The resulting low solubility organic material 36 was isolated by filtration, and mixed with freshly prepared nanoparticles of palladium (final loading 0.5 wt%, Figure 12), or gold (loading 0.5 wt%), to serve as a support in the final hybrids PdNPs@COF 37 and AuNPs@COF 37b, respectively. A reducing post-treatment of the palladium NPs was applied with washing with distilled water and drying at 80 °C then flushing in hydrogen flow at 150 °C. The 0.5 wt% AuNPs@COF 37b material was prepared starting from an aqueous solution of chloroauric acid hydrate (HAuCl4·3H2O) and post-reduced. MeOH NH3 aq. RT, 30 min

NMe2 n

ab ac af ag ah 34 Binol Yield (%) 76 74 79 64 70 65 Scale (mg) 526 621 663 537 855 1277

30a +

(PPh3P)PdCl2

PdNPs

CuI, Et3N

MeOH 2h

PdNPs@COF 37 > 100 mg yield n. a.

Rh(COD)BF4 CH2Cl2

Spacer

Me2N

ae af ag ah 35

Pd(OAc)2

34, ba-bh

32, aa-ah n = 1,2,3 or 4

5 6 7 8 9

99% 500 mg

O O

36

n

35, ca-ch yield n. a. (not available) scale : 10-20 mg

Figure 11. Synthesis of rhodium-based MOPs 35, ca-ch. MOPs materials ca-ch and 35 catalyzed heterogeneous asymmetric hydrogenation of -dehydroamino acid methyl esters in quantitative yields (Table 4).80 General studies using [Rh/phosphonite] complexes under homogeneous conditions have shown that most active species contains two monodentate phosphorous ligands bonded to one RhI center (in a P–Rh–P motif like depicted in Figure 11).81, 82, 83 With the heterogeneized rhodium catalysts, good to high ee (ranging between 93 and 98%) were obtained in all cases. O R

OCH3 NHAc

entry 1 2 3 4

[Rh/phosphonite] MOPs (1 mol %) 40 atm H2 toluene, RT

linker aa ab ac ad

R=H 95 (R) 95 (R) 94 (R) 94 (R)

O R

OCH3 NHAc

ee (%) (config.) R = CH3 R = Ph 95 (R) 96 (R) 96 (R) 95 (R) 94 (R) 94 (R) 96 (R) 96 (R)

Figure 12. 3-D COF 36 via Sonogashira coupling and PdNPs@COF 37 from palladium nanoparticles deposition. Transmission electron microscopy (TEM) images of 36 and 37. Reprinted with permission from ref 84. Copyright 2016 Royal Society of Chemistry. Surface characterization for COF 36 was realized following N2 adsorption–desorption isotherms. Typical Langmuir isotherm of COF in the range of P/P0 = 0-10−3 indicated microporosity and adsorption– desorption isotherms in the range of P/P0 = 0.1–1.0 was characteristic of mesoporous materials. The surface area of COF 36 following Brunauer, Emmett and Teller (BET) method was found to be SBET = 557 m2.g-1, and the total pores volume Vpor = 0.35 cm3.g-1. These values decreased to 555 m2.g-1 and 0.22 cm3.g-1, respectively after palladium nanoparticles supporting, showing a significant filling of the intrinsic porosity (–37%) . The calculated pore-size of 36 gave



values of 0.4 and 4.2 nm, which correlated the coexistence of a microand mesoporosity in the COF. Transmission electron microscopy (TEM) of the PdNPs@COF 37 showed a dispersion of spherical palladium nanoparticles with average diameter c.a. 4 nm, which corresponds to the mesopores size (Figure 12). No long-range organization of the metal NPs was observed, but the significant total pore volume decrease after NPs deposition, and their general size roughly corresponding to mesoporosity suggested an encapsulation of NPs into the COF. The PXRD patterns showed two peaks at 2θ, 41° and 47° that are specific to the (111) and (200) crystallographic facets of palladium NPs, while no specific peaks were observed for 3-D COF 36. This indicated the amorphous nature of the organic matrix.84 Gold deposition led to a diffraction line at 2θ, 38° attributed to the (111) crystallographic facet of gold NPs (particle size of around 9.6 nm). The thermogravimetric analysis (TGA) revealed thermal stability up to 300 °C for COFs 36 and Pd and Au hybrids 37 suggesting that COF preserved its properties after metal NPs deposition. The authors used PdNPs@COF 37 for olefin hydrogenation in nitrostyrene (Table 5).84 This reaction is challenging regarding chemoselectivity,85, 86 and accordingly was also tested with a benchmark catalyst palladium on charcoal (Pd/C*) characterized by a dispersion of smaller non-encapsulated particles (c.a. 2 nm). Full selectivity to p-ethylnitrobenzene P1a was achieved using PdNPs@COF 37 in 1 h reaction (Table 5, entry 2). Conversely, from Pd/C* catalyst the amino derivative was obtained with unselective hydrogenation of both nitro and olefin functions. This difference in the activity and selectivity was tentatively related to the NPs size of palladium. Smaller NPs in Pd-NPs/C leads to fast hydrogenation without selectivity, while larger confined NPs would induce a chemical discrimination regarding the two functions. By using AuNPs@COF 37b the hydrogenation of both the C=C and nitro groups occurred, and it required higher reaction temperature (140 °C) without reaching complete conversion after 6 h. Finally, catalyst PdNPs@COF 37 could be recycled at least 10 runs. The authors mentioned that the analysis of the liquid phase after the reaction by ICP-OES indicated no leaching of either palladium or gold, irrespective of the reaction conditions. NO2 [Pd] 0.5 mol% 30 atm H2

+

n-heptane, 90 °C

S1

NH2

NO2

P1a

entry

catalyst (0.5 mol %)

t (h)

conv. (%)

1 2 3

37 37 Pd/C*

0.5 1 0.5

74 100 100

NH2

BET surface area at 474 m2.g-1 and pores volume around 0.15 cm3.g-1 (Table 6, entry 3). Porous materials based on planar trifunctionalized linkers di and dh gave CMPs ei and eh with BET surface area around 400 m2.g-1 and smaller 0.07 and 0.13 cm3.g-1 pore volumes, respectively (Table 6, entries 1-2).87 The 3-D architectures have a substantially higher porous volume. For these materials the pore sizes were mainly distributed around 0.6–2.0 nm in the microporous range. Polymers in the series were characterized at the molecular level by 13C CP-MAS NMR spectroscopy with peaks emerging at  90 ppm for alkynyl groups incorporated into the polymers. TGA results showed that the decomposition of these framework starts at 300 °C. polyyne

CMP

di dj dh 31

ei ej eh 38

L*

P1c

selectivity (%) P1a 100 100 -

P1b -

P1c 100

Table 5. Catalytic activity and selectivity in 4-nitrostyrene reduction with PdNPs@COF 37 and Pd/C*. Li et al. reported the heterogeneization of BINAP derivatives by reacting dibrominated phosphine oxide BINAP-O B2 with tri- and tetradentate polyyne linkers in palladium-catalyzed Sonogashira cross-coupling reactions, forming extended hexagonal frameworks as conjugated microporous polymer CMPs (Figure 13).87 The 1,3,5,7tetraphenyladamantane-based CMP 38 (Figure 13) was synthetized by using the tetrahedral linker 31 (see Figure 9). Following the same procedure, 2-D or 3-D CMPs ei, eh and ej were prepared from linkers di, dh and dj. The synthesis of CMPs networks bearing terminal alkyne –C≡C–H was evidenced by FT-IR with a sharp band observed around 3300 cm-1. Among these porous materials, adamantane-based CMPs 38 displayed the highest BET surface area and pore volume, with respective values of SBET = 509 m2.g-1and Vpor = 0.17 cm3.g-1 (Table 6, entry 4). Tetrafunctionalized linker dj gave CMP ej with a

yield (%) scale (mg) 53 70 88 99

200 280 310 228

L*

L*

(PPh3)4Pd CuI

+

Et3N B2 =

L*

L*

CMP

polyyne Br

L*

POPh2 POPh2

L*

Br

dj

di

31

dh

Figure 13. BINAP-O-based CMPs ei, ej, eh and 38. The BINAP-O moieties were reduced using HSiCl3 to recover BINAP. Accordingly, 31P MAS NMR signal attributed to phosphine oxide at 25.1 ppm shifted to 15.8 ppm after reduction. Then, ruthenium was incorporated in reduced CMPs (red-CMPs) using benzene Ru(II) chloride dimer as precursor for in-situ coordination with the chelating phosphine. entry

CMP

1 2 3 4

ei eh ej 38

+

P1b

Page 6 of 27

SBET (m2.g-1) 391 407 474 509

Vpor (cm3.g-1) 0.07 0.13 0.15 0.17

Smicro (m2.g-1) 146 261 298 334

Table 6. Characterization of CMPs ei, ej, eh and adamantane-based 38. Vpor at P/P0 = 0.99 and Smicro for micropores surface area. [Ru/red-CMP] catalytic materials were tested in asymmetric hydrogenation of methyl acetoacetate (Table 7, entries 1-4). In the first 2 h, the conversion obtained were moderate ranging from 24-30% with a highest 91% ee obtained using the adamantane based reduced CMP [Ru/red-38].87 Upon 24 h of reaction [Ru/red-38] achieved asymmetric hydrogenation of a range of β-keto esters in high conversion yield >99% with ee ranging from 90% to 99% (Table 7, entry 5). O

O

[Ru/red-CMP] O

entry 1 2 3 4

[Ru/red-CMP] [Ru/red-ei] [Ru/ red-eh] [Ru/ red-ej] [Ru/ red-38]


50 Bar H2, MeOH, 52 °C, 2 h

conv. (%) 26 27 24 30

OH

O O

ee (%) (Config.) 67 (R) 72 (R) 71 (R) 91 (R)

Page 7 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5

Conditions : [Ru/red-38]

1

OH O

50 Bar H2, MeOH, 52 °C, 24 h

R1

99%

O

R2

R = Me, Et, ClCH2, i-Pr R2 = Me, Et, PhCH2, i-Pr (90-99% ee)

Table 7. Asymmetric hydrogenation of methyl acetoacetate and performances of [Ru/red-38] on other -ketoesters in 24 h. The authors suggested that the larger surface area and porous volume obtained from adamantane-based scaffold in 38 leads to a slightly better diffusion of the substrate to the chiral active centers, with apparently also more controlled stereogenic interactions. Catalytic material 38 could be recovered by centrifugation/filtration. The colorless filtrate collected after asymmetric hydrogenation of methyl acetoacetate did not afford additional product, suggesting according the authors the heterogeneous nature of the reaction system (Ru leaching measurement was not reported). The solid catalyst was reused three cycles without significant deterioration in the activity but the enantioselectivity decreased from 94% to 90%. Li et al. also used this set of red-CMPs to coordinate iridium for asymmetric hydrogenation of quinaldine S2 into P2 (Table 8).88 [Ir/BINAP] complexes are widely employed in homogeneous hydrogenation reactions.89, 90 However, depending on the iridium loading and concentration, the formation of hydride-bridged dimeric iridium complexes, which were inactive in catalysis have been documented.91,92 Li et al. hypothesized that porous materials with dispersed anchoring sites might prevent the formation of inactive dimers.88 Quinaldine hydrogenation at low 0.05 mol% iridium catalyst loading using the various [Ir/red-CMP] catalytic materials were compared with the homogenous counterpart [Ir/BINAP] (Table 8, entries 1-5). Among [Ir/red-CMP] catalytic materials, [Ir/red-38] showed the best performances (Table 8, entry 5, 17% conversion for 54% ee). This trend was confirmed at 0.25 mol % with a total conversion with 72% ee obtained from [Ir/red-38] (Table 8, entry 7), while only 23 % of quinaldine was converted by using 0.5 mol % of homogeneous [Ir/BINAP] with the 71% ee (entry 6). Recycling by filtration allowed five catalyst reuse without significant decrease in the activity. The colorless filtrate from the asymmetric hydrogenation of quinaldine did not afford further product, but iridium leaching measurement was not reported.

N

entry

catalyst

1 2 3 4 5 6 7

[Ir/BINAP] [Ir/red-ei] [Ir/red-eh] [Ir/red-ej] [Ir/red-38] [Ir/BINAP] [Ir/red-38] (1st run) 2nd run 3rd run 4th run 5th run 6th run

8 9 10 11 12

[Au]

[Au]

[Pd(PPh3)2Cl2]

[Ir/red-CMP] or [Ir/BINAP] (0.25-0.05 mol%)

S2

gold-functionalized POP 39 was obtained.93 Tritopic alkynes di and dh were also used for structuring comparable [Au/NHC]-containing porous organic polymer fi and fh, respectively (Figure 14). The structuring alkyne C–C≡C–C bond formation in the polymers 39, fi and fh was monitored by the disappearance in FT-IR of C≡C–H and Ar–I stretching bands at 3300 and 3000 cm-1, respectively. A 18.45 wt% loading of gold was determined in 39 (and 18.51% for fh and 20.42% for fi). Consistent with above detailed studies (Table 6),87 the 3-D structuring effects of the tetrahedral adamantane-based tecton 31 was clearly observed on the BET surface area of 39. Accordingly a SBET = 506 m2.g-1 was obtained for 39, while notably smaller SBET values of 350 m2.g-1 and 258 m2.g-1 were obtained for fh and fi, respectively.93 Further synthetic studies by the group focused at developing some kinetic control in producing 39. They established a critical influence of the concentration conditions. POP syntheses were achieved with varying concentration of [Au/NHC] (0.24 mmol) and 31 (0.12 mmol), which were polycondensed using the same catalyst loading conditions but within different amounts of solvent (50, 80, 100, 120, and 150 mL of toluene). A transition from nonporous to microporous structures, or the coexistence of micro- and mesoporous region were found in the framework depending on these concentrations. The BET surface areas increased gradually from a very low value at high concentration (SBET =16 m2.g-1, 50 mL solvent, nonporous) to a highest value (798 m2.g-1, 100 mL, microporous) and then decreased to a medium value (S= 506 m2.g-1, 150 mL, micro and mesoporous domains).93 This kinetic– structure phenomena was analyzed and attributed to pore blocking by the branch−branch cross-effect at high concentrations and high reaction rate. With the gradually reduced concentration, the reaction rate decreased and the branch−branch cross effect weakened. Microporous POPs with the highest surface area were prepared when the pore blocking reached a lower degree. Finally, POPs having micro- and mesoporous structures in the framework with median SBET were synthesized at higher dilution. Thermogravimetric analysis showed that these materials have thermal stability up to ca 300 °C.

40 bar H2, I2 CH2Cl2, 25 °C, 2 h

N H

[Au]



Au/NHC I

P2

ratio (mol%) 0.05 0.05 0.05 0.05 0.05 0.5 0.25

conv. (%) 5 9 12 10 17 23 99

ee (%) 70 48 54 50 54 71 72

0.25 0.25 0.25 0.25 0.25

99 99 99 99 90

72 71 70 70 69

Table 8. Asymmetric hydrogenation of quinaldine S2 under heterogeneous [Ir/red-CMP] and homogeneous conditions ([Ir/BINAP]. Still based on the coupling of polyyne synthons (Figure 13), Li et al. described the direct incorporation of Au/NHC fragments into Porous Organic Polymer (POP) frameworks looking for gold-catalyzed alkyne hydration with efficient recycling of the hybrid catalytic materials. Ethynyl-adamantane linker 31 (Figure 9) was reacted with iodine-functionalized [Au/NHC] complexes in a palladium-catalyzed Sonogashira coupling. The 1,3,5,7-tetraphenyladamantane-based

[Au]

+

[Au]

CuI, Et3N di dh 31

[Au]

[Au]

iPr iPr

N

Au Cl N

iPr

iPr

fi 52%, 365 mg fh 47%, 381 mg 39 50%, 408 mg

I

Figure 14. [Au/NHC] incorporation into porous organic polymers. POP 39 was employed as an efficient and reusable heterogeneous catalyst in alkyne hydration towards carbonyl function formation. This reaction has been reported in homogeneous conditions using a related system [Au/(NHC)–AgSbF6].94 The hydration of phenylacetylene S3a (Figure 15) was achieved using POPs fi (75%), fh (83%) and 39 (86 %); the latter system allowed five runs recycling without significant activity loss. Analysis of the aqueous reaction solution after each cycle by ICP showed gold leaching did not reach the detection limit of 1 mg/L.



R

R'

O

39

+ H 2O

R R'

AgSbF6, (MeOH)/H2O S3a-f R = H, MeO, Me, 120 °C, 24 h F, t-Bu, Et R' = Me, Ph, p-C6H4-n-Bu S3g = C4H9

P3a-f 82-96%

with Yb(III) from 1.15 to 1.40 wt% at 77 K. These results could be of interest for further developing heterogeneous catalysis (hydrogenation, hydroelementation, reductions, etc.) based on 40 combined with transition metals (Ru, Ir, Rh, Pd, etc.), taking profit of porous confinement to activate relatively inert CO2 and H2 trapped molecules.

P3g 64%

C 4H 9

O

Figure 15. POP 39 used for Au-catalyzed alkyne hydration. Conditions: 39, 5 mg for 1 mmol of S3a-f or 10 mg for 1 mmol of S3g. POPs 39 efficiently promoted S3a-f hydration (Figure 15, 82-96% yield). A significant size effect was evidenced from more hindering S3g (2.11 nm long). A low conversion below 12% was obtained with nonporous and microporous POPs 39, while a yield of 64 % was isolated using the mesoporous resin 39 of median SBET. Porosity size effect for promoting reactivity was clearly illustrated here with substrate S3g. Chang et al. described the synthesis of adamantane-based porous polymer bearing p-dicarboxylic benzene groups by coupling the ethynyl linker 31 with 2,5-dibromoterephthalic acid. Ytterbium(III) was then chelated by carboxylate groups starting from YbCl3 (Figure 16).95 A high SBET of 970 m2.g-1 was achieved for POP 40, which showed a noticeable decrease after Yb(III) complexation (SBET = 885 m2.g-1). A microporous pore size about 0.65 nm was found for 40. After coordination, a larger micro/mesopores distribution was observed ranging from 1.4 to 19.0 nm. This size increase indicates a significant change in the pore structure by lanthanide coordination, as well as the flexibility of the network. The incorporation of Yb(III) ions was confirmed by X-ray photoelectron spectroscopy (XPS) measurements, the polymer emitted Yb 4d photoelectrons with the corresponding binding energy at 185 eV. The authors estimated surface relative atomic ratios of C (69.88 at%), O (27.79%), and Yb (2.33 at%) using XPS elemental analysis. The concentration of Yb was smaller than the value of 4.44 at% calculated from an ideal structure, showing inhomogeneity of the deposition. Nevertheless, XPS analyzes only a very small portion of emerging surface of the materials. Both 40 and 41 were found to be thermally stable up to 200 °C and degraded gradually as the temperature further increased.

COOH 31 + Br

Br

Page 8 of 27

Pd(PPh3)Cl2 HO

CuI

41, HC(OMe)3

Br

OMe Br

100 % OMe

MeOH, RT, 12 h

O

OMe

41, HC(OMe)3 O

MeOH, RT, 12 h

O

OMe

100 %

Figure 17. Catalytic acetalization of 4-bromobenzaldehyde and furfural. Conditions: 30 mg of 41 for 0.1 mmol of substrate. Template-driven synthesis is a relevant method for the shape control of materials, since resultant shapes originating from the sacrificed templates are predictable.97 Son et al. developed a template synthesis of hollow porous organic polymers (so-called H-POPs), based on tetraphenyladamantane, as a general solution to modulate and improve the accessibility of catalytic species into porous materials.98 By using inorganic silica spheres as template (average size distribution 310 ± 20 nm), organic networks formed at the surface of silica spheres via palladium-catalyzed Sonogashira cross-coupling of tetraethynyl 31 with the aryl dibromide spacer B3 (Figure 18). The hollow structure was obtained after selective etching of silica with HF (Figure 18). Through the screening of synthetic conditions such as solvent and amount of reagents, synthetic methods for thickness control of shells can be developed.97 TMS

31

+

2

Br

Br

Pd-catalyzed Sonogashira coupling

SiO2

Etching HF

H-POP

SiO2 B3 TMS

42

43

H-POP

O

HOOC 43

40 O

84% 150 mg

[Yb] O

41 92% 102 mg

OH

O

O

2 YbCl3

O [Yb]

Figure 16. Coordination of Yb(III) to micro/mesoporous materials 41. Since Lewis acidic Yb(III) had been used in homogeneous and heterogeneous acetalization and related reactions,96 the lanthanide polymer 41 was tested on the acetalization of pbromobenzaldehyde and furfural (Figure 17). Quantitative yields of acetals were obtained with these substrates, and five runs were achieved with a low gradual loss of activity from recycling (8590% yield in 5th run).95 Ytterbium plausible leaching was not reported. Notably, with these porous materials an enhanced CO2 adsorption capacity was reported from POP 40 to Yb(III)-loaded 41 (1.56 and 2.36 mmol.g-1 at 298 K, respectively) despite the reduction of SBET. The H2 uptake also increased after coordination

Figure 18. Hollow-POPs synthesis strategy. The dibromide spacer B3 difunctionalized with TMS-protected alkyne group, allowed post-modification of H-POP 43 (Figure 19). A chiral pyrrolidine moiety was incorporated at H-POP 43 following a Cu(I)catalyzed azide/alkyne cycloaddition with (S)-N-Boc-2azidomethylpyrrolidine.98 Acidic deprotection of resulting H-POP 44 was carried out to give enantiopure pyrrolidine-modified H-POP 45 (Figure 19). These synthetic steps were monitored by FT-IR. H-POP 43 displayed identifiable stretching bands for terminal alkyne at 3300 cm-1. The carbonyl band of Boc protecting groups in H-POP 44 appeared at 1691 cm-1 and naturally disappeared after deprotection while stretching band for N–H appeared in H-POP 45 at 1675 cm-1.


Page 9 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


malonates S'4b and S'4c (Table 9, entries 1-3). A positive confinement effect99 was attributed to reduced pore size based on the performance in enantioselectivity for 4-nitrocinnamaldehyde S4e conversion found to be superior to homogeneous catalyst (60% ee vs 92% ee for H-POP 45).100 A very good stability of H-POP 45 organocatalyst was shown with successive recycling processes in which activity and enantioselectivity were conserved after five runs.98

H-POP

43

1) click reaction CuI, DIPEA N3 N Boc BocN N

N

O

N

H

H-POP

44

+ R'O2C S4a-e

N

N

PhCO2H DMF/H2O, RT, 72 h

HN N

N

CO2R'

H R

entry S4 (R) S'4 (R') conv.(%) ee (%) 1 S4a (H) S'4a (Me) 93 66 2 S4a (H) S'4b (Et) 49 66 3 S4a (H) S'4c (PhCH2) traces / 4 S4b (OMe) S'4a (Me) 92 48 5 S4c (Cl) S'4a (Me) 87 81 6 S4d (Br) S'4a (Me) 96 83 7 S4e (NO2) S'4a (Me) 94 92 Table 9. Catalytic addition of malonates to cinnamaldehydes catalyzed by H-POP 45.

2) CF3COOH (deprotection)

N

H-POP

45

N

S'4a-c

R'O2C O

P4

N

NBoc

N

R

10 mol % H-POP 45 CO2R'

N

yield n.d.100-250 mg scale NH

Figure 19. Synthesis of pyrrolidine-modified H-POP 45 The hollow structure of H-POP 45 was analyzed by TEM, which evidenced materials of regular spherical shape with size around 360 ± 24 nm, consistent with the size of template silica spheres (Figure 20). According PXRD analysis, these H-POPs 43-45 were amorphous. For 43, the BET surface was found SBET = 573 m2.g-1 with a microporous volume Vpor = 0.22 cm3.g-1. The post-synthetic modification with (S)N-Boc-2-azidomethylpyrrolidine to 44 resulted in a decrease of both surface area and microporous volume to SBET = 327 m2.g-1 and Vpor = 0.10 cm3.g-1. After deprotection, for 45 the surface area and microporous volume increased back to SBET = 466 m2.g-1 and Vpor = 0.16 cm3.g-1.98 TGA indicated that H-POPs are thermally stable up to 190 °C and gradually decomposed upon temperature increasing.

Figure 20. TEM images of H-POP 45 (left) and a non-hollow polymer (right). Reprinted with permission from ref 98. Copyright 2017 Royal Society of Chemistry. The activity of H-POP 45 was studied in organocatalysis for the 1,4asymmetric addition of dimethylmalonate S'4a to cinnamaldehyde S4a. A very good conversion was obtained (93%) with a moderate ee of 66 %. With “non-hollow” version of this materials only 30 % of conversion was obtained. This enhancement in the catalytic performance of hollow POP 45 versus non-hollow POPs was attributed to shorter diffusion pathways of substrates and reagents into 45.98 Accordingly, the authors mentioned that as the shell thicknesses of H-POP increased catalytic activity generally gradually decreased. The scope of asymmetric addition of malonates onto 1,4-insaturated aldehydes was further investigated with 45 used as porous organocatalyst (Table 9). Steric effects emerged from the microporosity, as shown by the activity decrease with the larger

Overall, convenient cross-coupling reactions between terminal alkyne and halide functions (typically dibromides and diiodides) allowed the building of Td-directed adamantane-based porous materials following three different strategies. (i) First, the addition of C2-symmetric atropoisomeric chiral fragments bearing chelating donors (Binol for oxygen, BINAP for phosphine, etc.) to rigid adamantane scaffolds allowed the immobilization of discrete metal complexes into 3-D porous structures. Metals as diverse as La, Rh, Ir and Ru were then exploited for asymmetric versions of α,βunsaturated ketone epoxidation, and selective hydrogenation reactions of α,β-unsaturated aminoesters, -ketoesters and quinaldine, respectively. This general strategy also endorsed the anchoring of Au and Yb complexes within achiral environments by carbene or oxygen donors, respectively, for phenylacetylene hydration and acetal protection of p-bromobenzaldehyde and furfural. (ii) Second, the confinement into preformed porous frameworks of in-situ prepared palladium and gold nanoparticles was found to be beneficial for olefin selectivity in nitro-selective hydrogenation of pvinylnitrobenzene. (iii) Third, the synthesis of hollow 3-D porous materials was achieved from template SiO2 silica spheres, the resulting materials being post-functionalized by azide-alkyne cycloaddition to form triazole-linked chiral organocatalysts. These spherical-shape porous materials performed asymmetric addition of malonates onto 1,4insaturated aldehydes by pendant chiral pyrrolidines, with a positive confinement effect on the enantioselectivity that was attributed to small pore size. Conversely, detrimental size-effects on conversion were observed with bigger malonate reagents. Based on tetrafunctionalized adamantanes incorporating ethynyl linkers, the large scope of 3-D porous catalytic materials formed, and their many successful applications achieved from transition metal catalysis, lanthanides Lewis acid chemistry and organocatalysis, is very promising. These significant advances are attributable to the rigidity and stability of the tectons concerning the building of porous materials and their recovering, but also to the high reactivity of alkyne groups for suitable tecton functionalization based on Pd(0)-catalyzed Sonogashira and alkyne cycloaddition reactions. Though, other valuable linkers for adamantyl tectons are approachable, as detailed in the following sections. 4.2. Porous materials integrating 1,3,5,7-tetraphenyladamantane with –C=C– ethenyl linkers. The reactivity of alkyne function of tetraethynyl compound 31 was also successfully exploited to generate ethenyl –C=C– linked porous materials.



Chang et al. described a radical polymerization reaction between ethynyl 31 and 1,3,5-benzenetrithiol using AIBN initiator to produce a porous organic polymer (POP) incorporating –C=C– linkers formed by C–S coupling (46, Figure 21).101 The microporous organic polymer 46 contained both thiol groups and disulfide bridges that resulted from oxidative reaction between unreacted thiol groups. POP 46 was characterized in the solid state by 13C NMR evidencing the disappearance of acetylenic carbon at 90 ppm, while the peaks of the vinyl carbons overlapped with those of the aromatic carbons and the adamantane carbon peaks were observed at 38 and 44 ppm. In FT-IR spectrum, the stretching vibration band of acetylene at 2100 cm-1 accordingly disappeared, the weak peak at 2550 cm-1 was assigned to the stretching vibration of a thiol group. The microporous polymer was found to have amorphous spherical structures with diameters ranging from 20 to 30 nm by SEM measurements. The BET surface area of the microporous polymer determined by nitrogen adsorption– desorption isotherms were 575.8 m2.g-1 (consistent with other values for such adamantane-based 3-D organic polymers) and a total pore volume of 1.76 cm3.g-1. The thermogravimetric analysis (TGA) of the polymer showed thermal stability up to 300 °C under nitrogen.101 Encapsulated gold nanoparticles (NPs) were grown into POP 46 from an aqueous solution of HAuCl4 and NaBH4 with the polymer dispersed in ethanol. A narrow dispersion of spherical gold nanoparticles of 7.3 nm size was observed by TEM (Figure 21, bottom). The porosity incorporating a high density of sulfur atoms help controlling the growth of gold nanoparticles. Accordingly, SBET significantly decreases at 103.5 m2.g-1 after encapsulated gold nanoparticles synthesis. This decreasing may be related to the gold NPs occupation within the hybrid material AuNPs@POP 47. The Xray diffraction pattern of AuNPs@POP 47 displayed diffraction peaks corresponding to the (111) and (200) planes of a typical fcc Au crystal phase from Au NPs. XPS suggested Au(I) surface with a sulfur S2p peak of binding energy at 162 eV, related to sulfur chemisorbed on the Au surface.101 The average loading of gold determined by TGA was ca 7 wt%.

SH 31

+ HS

SH

AIBN DMF

S

S

S

S

S 46

S

Page 10 of 27

without significant loss in activity. Metal leaching measurement in solution from gold NPs was not reported.

HO

NO2

47 NaBH4, H2O

HO

NH2 quantitative

10 min, RT

Figure 22. 4-Nitrophenol reduction using gold supported NPs. Conditions: catalyst loading 10 mg of 47 for 50 mmol of substrate. Cui et al. reported the synthesis of 3-D POPs hj, hk and 49 by coupling the tetrafunctionalized pinacol-boron units gj, gk or adamantane-based 48 with a ditopic chiral norbornadiene triflate using palladium-catalyzed Suzuki coupling (Figure 23).102 Tecton 48 was obtained from the coupling of tetrabromide 30b64 with bis(pinacolato)diborane in the presence of [Pd(dppf)Cl2] in DMSO (synthetic details were not reported). 13C CP-MAS NMR confirmed diene bonding in these materials with peaks assigned for sp2-C around 115 and 125 ppm, and attributed to norbornadiene methylene (CH2) around 47 ppm. These POPs are insoluble in water, hydrochloric acid (6 M), sodium hydroxide (8 M) and in the common organic solvents that were tested, consistent with a dense cross-linked network. TGA showed that the decomposition of these framework starts at about 550 °C as additional proof of robustness. The PXRD patterns indicated that these POPs are amorphous. POPs hj, hk and 49 were used for supporting rhodium complex to respectively give ij, ik and 50, further applied in 1,4-asymmetric conjugation addition. Rhodium was incorporated in each material by coordination with diene from a postmetalation synthetic step using [Rh(C2H4)2Cl]2 in 1,4-dioxane.102 The BET surface area of adamantane-based 49 was found to be SBET = 252 m2.g-1, while higher values of 312 m2.g-1 and 471 m2.g-1 were respectively measured for silicon Si-centered materiel hk, and Ccentered materials hj. Total pores volumes of 0.295, 0.304 and 0.393 cm3.g-1 respectively, followed the same trend and the pore size distribution calculated using nonlocal density functional theory reveals microporous distributions at around 0.7 nm for these POPs. SBET trends contrast to related studies (see above) in which adamantane linkers resulted in higher SBET porous materials above 500 m2.g-1.84, 87, 93 The dense packing of more flexible polymers is presumably at the origin of this situation. After the incorporation of rhodium complex in 49 and hk the surface sorption by N2 absorption/desorption isotherms indicates the absence of porosity and a negligible surface area. A decrease in the BET surface area 186 m2.g1 was observed in the case on ij. ICP-OES analyses of the materials ij, ik and 50 indicated rhodium loadings of 20.5, 19.0 and 16.5 wt% respectively. 102

70% 165 mg

O O B

HAuCl4 aq. NaBH4

OTf

O B O Y

+

Pd(dppf)Cl2, K2CO3

TfO B O O

O B O

1,4-dioxane, H2O 110 °C, 72 h

gj : Y = C, gk : Y = Si, 48 : Y = adamant-1,3,5,7-yl (Ad)

Encapsulted AuNPs 47

[Rh]

[Rh]

[Rh(C2H4)2Cl]2

Y

Figure 21. Thiol-based porous material 46 and TEM image of encapsulated AuNPs@POP 47. Reprinted with permission from ref 101. Copyright 2015 Royal Society of Chemistry. The AuNPs@POP 47 catalyzed the reduction of 4-nitrophenol to 4aminophenol using NaBH4 in water (Figure 22). A total conversion was obtained after 10 min. Additionally, filtration of this catalytic materials allowed five successive runs in recycling experiments

Y

1,4-dioxane 25 °C [Rh] [Rh]

ij, ik, 50

yields n. a. c.a. 6 mg scale

hj, hk, 49 Y = C : 91%, 48.2 mg Y = Si : 81%, 44.0 mg Y = Ad : 75%, 49.0 mg

Figure 23. 3-D POPs hj, hk and 49 synthesized by Suzuki coupling, and [Rh/norbornadiene] supported porous materials ij, ik and 50.


Page 11 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The POPs incorporating [Rh/norbornadiene] moieties were tested in asymmetric conjugation addition of 3-methoxyphenylboronic acid to 2-cyclohexenone (Figure 24).102 A high yield with a good enantioselectivity (92%, 87% ee) was obtained with POP ij, while lower values were obtained with ik and 50 affording the product in 76-78 % yield with moderate 76% ee and 68% ee, respectively. These results were consistent with surface properties where a higher surface area and thus a structuring porosity leads to better catalytic performances especially concerning enantioselectivity. In these [Rh/norbornadiene] POPs, Rh(I) is immobilized mainly by coordination to the diene moieties of the organic matrix, and this interaction is weak. Thus, rhodium species got loss during the reaction and recovering procedure. Nevertheless, upon completion of the reaction, POP gj could be recovered by centrifugation and reused for a next cycle after the addition of 0.004 mol [Rh(C2H4)2Cl]2 without significant loss of activity and enantioselectivity. The % yields and % ee for five successive runs were determined as 91 (ee 86%), 90 (ee 85%), 90 (ee 84%), 88 (ee 84%) and 87 (ee 84%), respectively. O

B(OH)2

ij, ik or 50 (4 mol%) KOH 50 (mol%)

+

from ij : 92%, 87% ee ik : 78%, 76% ee 50 : 76%, 68% ee OMe

O (R)

1,4-dioxane/H2O 50 °C, 8 h

OMe

Figure 24. Rhodium-catalyzed asymmetric conjugation addition of 3methoxyphenyl boronic acid to 2-cyclohexenone in heterogeneous conditions. Some flexible polymers have the ability to swell in organic solvents. This swelling may facilitate and expand contacts between the catalytic sites and the reacting substrates.103, 104 Accordingly, Chen et al. looked for porous materials with enhanced flexibility, and reported the synthesis of 4-(N,N-dimethylamino)pyridine)-based (DMAP) polymers (Figure 25).105 The diolefin DMAP linker B4 was reacted in palladium-catalyzed Heck coupling either with triiodinated jh or tetraiodinated adamantane 30a to respectively form the precipitating polymeric matrix 2-D kh, and 3-D 51 (Figure 25). I

N

N

I

I

jh

N

Pd(OAc)2, Et3N DMF, 100 °C, 3 days

B4

N

kh 75%, 473 mg

N

30a (Table 2) N

B4

Pd(OAc)2, Et3N DMF, 100 °C, 3 days

N

N

51 82%, 530 mg

lh as rigid counternpart of kh

N

N

lh

Figure 25. Porous material kh and 51 with flexible vinyl linkers synthesis via Heck coupling, and the rigid counterpart lh. After polymerization process, the FT-IR analysis of POPs kh and 51 confirmed the disappearance of vibration bands characteristic of the reagents, i. e. substituted terminal olefins in B4 (913 and 998 cm-1) and Ar–I stretching (504 cm-1). SEM indicated that these materials

consist of fairly uniform granules with diameters ranging from 500 to 600 nm, while TEM imaging showed that the polymers adopted highly dense textures different from the typical morphologies of porous polymers. The XRD studies showed the amorphous nature of kh and 51. The polymeric matrix kh and 51 showed negligible N2 adsorption indicating a mostly nonporous nature. Accordingly, very low BET surfaces, SBET = 9 and 3 m2.g-1, were measured for kh and 51, respectively.105 Conversely, the use of a traditional rigid alkynyl linker in lh counterpart led to a surface area SBET = 508 m2.g-1.106 The content of DMAP moieties are 1.96 mmol.g−1 and 1.77 mmol.g−1. TGA showed that kh and 51 give initial weight losses of 2%, below 100 °C, which was ascribed to the removal of trapped water and organic solvent molecules, then desolvated materials were stable up to 250 °C. The authors evaluated the catalytic performances of kh and 51 in the acylation of 1-phenylethanol using acetic anhydride (Figure 26).Indeed, 4-DMAP is an effective acylation catalyst but, due to its intrinsic toxicity, strategies for recycling DMAP are desirable.105 OH

OAc + 2 Ac2O

kh, 51 or lh (5 mol%) 25 °C

from kh, TOF 11 51, TOF 5.6 lh, TOF 2

quantitative

Figure 26. Acylation of 1-phenylethanol organocatalyzed either by flexible POPs 2-D kh and 3-D 51, or by rigid 2-D POP lh. In the absence of catalyst, trace amount of product was observed over 12 h. In contrast, both materials exhibited excellent catalytic activity at room temperature. A total conversion was obtained in 2.5 h using 5 mol% kh (99% conv., TOF 11) while 3.5 h was needed with 51 (98% conv., TOF 6). These activities were found higher than the one reported for rigid materials lh, for which total conversion was achieved only after 10 h.105, 106 In this reaction conducted at room temperature directly in the organic reagents (so-called “solvent-free”) the activity seems to be related to the polymer intrinsic flexibility and its swelling ability, which arguably may allow a better diffusion of the substrates towards the active catalytic sites. The substrate scope of the acylation reaction with 51 was then extended to various alcohols with acetic and isobutyric anhydrides under base- and solvent-free conditions. Thus, from catalytic applications view, in addition to pyridine-based flexible polymers usable as recyclable organocatalysts for benzylic alcohols acylation, the synthesis of thiol-based rigid polymers allowed encapsulation of gold NPs that were active in nitro-selective reduction of p-nitrophenols. Weakly-bonded [Rh/norbornadiene] complex supported on a recoverable organic support were also used for asymmetric conjugation addition of phenyl boronic acid to 2cyclohexenone with moderate enantioselectivity maintained over several cycles. From porous materials design approach, the association of olefinic linkers with Td-directing adamantane tectons has been achieved by either radical polymerization between tridentate thiophenols and acetylenic fonctions, or by palladium-catalyzed Suzuki and Heck reactions with a bidentate triflate or a vinylpyridine, respectively. Interestingly, the resulting porous polymers, if not fully insoluble, may present some higher degree of flexibility which may induce in turn swelling properties in organic reagents (herein phenylethanol for instance). In general, the lack of rigidity has a tendency to reduce the surface area and porosity size of polymeric frameworks because of some polymer packing effects that are assumed to be detrimental to further catalytic applications. However, the situation is more intricate when a significant swelling of flexible polymer is at work, since in some cases, presented above, the activity can be related to the polymer intrinsic flexibility and swelling ability, to explain a better diffusion of the substrates towards the active catalytic sites. 4.3. Porous materials integrating 1,3,5,7-tetraphenyladamantane with aromatic benzene linkers. The wide scope of reactions available for aromatic benzene derivatives functionalization (SEAr,



organometallic cross-couplings, Diels-Alder reactions, etc.) make those motifs essential linkers for building porous materials. The existence of weak -stacking interactions between structurally planar aromatics is also a factor that may both modify (or influence) the synthetic course of porous materials, but also their final structure and interactions with molecular hosts (this is well-known in layered 2-D materials). Accordingly, benzene linkers have been used also in association with 3-D Td-directing adamantane scaffolds. Nguyen et al. described the construction of benzene linked POPs using cobalt-mediated [2+2+2] cyclisation of monomer B5 with tetrafunctionalized alkynyl adamantane-based 31, or C-centered tetraphenylyne dj (Figure 27).107

Page 12 of 27

organophosphate Paraoxon pesticide, by using 6 mol% loading of catalyst in MeOH (Figure 28). A better activity was observed with adamantane-based POP 53, for which the initial methanolysis rate was found to be c.a. 2.5 fold faster than that for nj. This performance was attributed to both the higher surface area of 53 and the hydrophobic nature of adamantane that would enhance the solvophobic encapsulation of the Paraoxon derivative. O 2N O

O P

53 (6 mol% La)

OMe OMe

MeOH, 60 °C

MeO

OH

O P

OMe + OMe

Initial rate 53 = 17 ×10-7 M.s-1 nj = 7×10-7 M.s-1

NO2

Paraoxon derivative

Figure 28. Paraoxon derivative PNDPM (p-nitrophenyl dimethyl phosphate) methanolysis catalyzed by POP 53. O

Y

+ 2

OEt O

1) Co(CO)8 1,4-dioxane 100 °C

Y HO

OH

2) HCl conc.

B5 HO

31: Y = Ad dj: Y = C

52: Y = Ad, 97%, 80 mg mj : Y = C, 98%, 64 mg

HO

(OMe) La O

Y

O

La(acac)3 MeOH

O

(MeO)La

53: Y = Ad, 97%, 80 mg nj: Y = C, 98%, 64 mg

O

Figure 27. Benzene linked POPs synthesis via cobalt-catalyzed [2+2+2] cyclization and La(III) complexes immobilization. The La loading measured by ICP-OES for 53 and nj were found to be 16 and 22 wt% respectively, values which reasonably matched the theoretical expected values (20 and 23 wt%, respectively). The porous materials which were based on Td-directing functionalized adamantanes 52 and 53 displayed significantly higher BET surface area than that of mj and nj based on tetrakis(4-ethynyl)methane: SBET = 1165 vs 1050 m2.g-1, and 650 vs 265 cm3.g-1, respectively (Table 10). The same trend was observed for the porous volume, with a notably higher value of 0.28 cm3.g-1 for 53.107

Recycling of catalytic porous material 53 after methanolysis was achieved, but a significant loss in activity was observed with each run, which was related to significant La leaching for each runs (run 1: 0.1%, run 2: 0.6%, run 3: 4. 3% of initial metal loading for 53, and 10.1% for nj in run 1).107 Catalysis initial rate decreased six times in the third run (from 1.7 × 10-6 M.s-1 to 2.9 × 10-7 M.s-1). This recycling issue was studied by changing the reaction and solvent, for achieving hydrolysis of PNDPM in 30 vol% EtOH. The use of EtOH helps to solubilize the substrate. Under these conditions 53 retains significantly more catalytic activity over three cycles of PNDPM hydrolysis (with leaching minimized at 0.1-0.2% for 53 and nj). While there is minimal leaching of La(III) ions from the pores of 53 the degradation of [La/(catecholate)] is more significant in methanol. The choice of reaction media is thus important in the decomposition of PNDPM. Apparently, methanol does not solvate this substrate and its products correctly, and sequestration of insolubles inside the pores clogged them, which significantly reduced catalytic activity upon recycling. Sánchez et al. described the synthesis of benzene linked porous organic polymers (called porous aromatic frameworks, PAFs) 54 and pj by a palladium-catalyzed Suzuki coupling of the phenyl-1,4diboronic acid conducted with either tetrafunctionalized iodoadamantane 30a, or the smaller C-centered analogue oj (Figure 29).108 Incorporation of amino groups to the resulting highly aromatic 3-D frameworks, was achieved via an electrophilic nitration (for intermediates 55 and qj) followed by reduction step to obtain aminofunctionalized PAFs 56 and rj (Figure 29).

I

Entry 1 2 3 4

POP 52 mj 53 nj

SBET (m2.g-1) 1165 1050 650 265

Vpor (cm3.g-1) 0.59 0.51 0.28 0.09

MeOH uptake (mmol.g-1) / / 5.8 1.9

I Y

I

B(OH)2 +

Pd(OAc)2 NaHCO3, PPh3

Y

DMF/H2O B(OH)2

I 30a: Y = Ad oj: Y = C

Table 10. Properties of porous materials 52, 53, mj and nj, and MeOH uptake for La/POPs. The Lewis-acid activity of La(III) ion in solvolysis of phosphate triesters could be exploited in the decomposition of toxic organophosphate-based nerve agents. The authors hypothesized that [La/monochatelocate] moieties stabilized into POP cavities would be able to bind phosphate triesters together with hydroxylated reagents inside a micropore, and therefore accelerate phosphate decomposition. In addition to the dianionic catecholate ligand, the third anionic ligand of the La(III) ion (i.e., MeO− or HO−) may serve as a pool of nucleophiles in the solvolysis/hydrolysis of phosphate esters. Thus, they investigated first MeOH uptake of these POPs, showing properties consistent with pore volumes (Table 10).107 The adsorbed MeOH amount with 53 was 5.8 mmol.g-1 against 1.9 mmol.g-1 for nj. These catalytic materials where then tested in the methanolysis of pnitrophenyl dimethyl phosphate (PNDPM), a derivative of the toxic

HNO3/TFA NH2

54: Y = Ad pj: Y = C amounts, yields n. a.

NO2 NH2

NO2 SnCl4, H2O

Y

Y

THF H 2N

O 2N H 2N 56: Y = Ad, 164 mg rj: Y = C amount, yield n. a.

O 2N 55: Y = Ad, 164 mg qj: Y = C amount, yield n. a.

Figure 29. Highly aromatic benzene linked porous materials (54, pj) and post-functionalized nitro- (55, qj) and amino-derivatives (56 and rj).


Page 13 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Peptide coupling of (NBoc)-L-Proline to amino groups into PAFs 56 and rj followed by deprotection in acidic medium allowed the introduction of a pendant Proline group into PAFs (Figure 30).108 The porous materials 57 and rj can then be treated with the dimeric Rh(I) precursor [Rh(COD)Cl]2 in the presence of AgSbF6 in THF for 3 h, generating metallized PAFs 58 and sj, with [Rh] content of 0.19 mmol.g-1 for 58 and 0.36 mmol.g-1 for sj. PAFs synthesis steps were monitored by FT-IR, and the vibration bands at 1345 and 1527 cm-1 confirmed the introduction of nitro groups into 55 and qj; these bands disappeared after reduction to 56 and rj. Primary amine bond stretching bands were observed at 3369 cm-1 and 3439 cm-1. The characteristic bond of secondary amine stretch at 3228 cm-1 confirmed the introduction of L-Proline in PAFs 57 and sj. The bands attributed to pendant L-Proline were conserved after rhodium incorporation, which led to the formation of 58 and tj, respectively. A kind of bimodal heterogeneous catalyst was thus formed, which potentially combined organocatalysis opportunities (from available basic LProline) and transition metal reactivity.108 HN O

O NH

H N

[Rh]

HN

O

O NH

HN

centered parent material the authors chose tj as catalyst to evaluate the scope of the reaction and recyclability opportunities. The authors demonstrated a high recyclability of the porous material in the tandem conversion of benzaldehyde into P6 (consistent yields > 90% for both reactions) over ten successive cycles, and the concentration of rhodium in the resulting solution was less than 0.01 ppm (ICP) indicating that no significant metal leaching occurred during this tandem catalysis.108 O H

+ NC

CN

Base

CN

CN

[Rh] H2

CN

CN

P5

P6

Figure 31. Tandem Knoevenagel condensation/hydrogenation reactions conducted with [Rh/Prolinamide]– PAFs. Sánchez and coworkers, expended the synthetic application of the amine-functionalized PAFs 56 to phosphazene incorporation (Figure 32).109

HN

N N3

HN

N P N N

N3

1) NBoc-L-proline 56 or rj

2) TFA

[Rh(COD)Cl2]

Y

NH

AgSbF6

O

Y

NH

56

HN

O HN

N H

O [Rh] 58 : Y = Ad, 108 mg tj : Y = C, amount, yield n. a.

Figure 30. Highly aromatic benzene-linked porous materials postfunctionalized with L-Proline pendant groups and partial metallization with Rh complexes. According to TGA, highly aromatic non-functionalized POP 54 was stable up to 535 °C. A SBET over 514 m2·g−1, for a pore volume (Barrett−Joyner−Halenda, BJH Method) of 0.59 cm3·g−1 was found for POP 54, against a smaller value of SBET = 393 m2·g−1 for analogous pj, while a high pore volume of 0.58 cm3·g−1 was measured (Table 11, entries 1 and 4). The introduction of amino groups in the porous materials decreased the surface area to ca 413 m2·g−1 in adamantanebased PAFs 56 and to 248 m2·g−1 in C-centered PAFs rj (Table 11, entries 2, 4). These values diminished down to 98 m2·g−1 and 130 m2·g−1, respectively when the bifunctional prolinamide ligands and rhodium metal were introduced (58 and tj, Table 11 entries 3 and 6). Fairly large pore volumes are preserved, above 0.25 cm3.g-1.108 polymer 54 56 58 pj rj tj

SBET (m2.g-1) 514 413 98 393 248 130

benzene

Y = Ad

N H

Vpor (cm3.g-1) 0.59 0.48 0.27 0.58 0.32 0.25

Table 11. Properties of PAFs 54, 56, 58, pj, rj and tj. The authors described Knoevenagel condensation of an aldehyde with malononitrile (or ethyl cyanoacetate), which allowed the formation of a carbon−carbon coupling product mediated by pyrrolidine groups, with a subsequent hydrogenation of this intermediate assisted by the heterogeneized rhodium complexes (Figure 31).108 The Knoevenagel condensation was first carried out by the prolinamide-functionalized PAFs 57 and sj, leading to the intermediate alkene P5, which was isolated in similar excellent yields (90 and 92%) using these two catalysts. The metallized PAFs 58 and tj were employed with success in tandem Knoevenagel-hydrogenation reaction to obtain the product P6 in quantitative yield. Because of their similar performances and due to the easier availability of Td-C-

N P(N(CH3)2)3

N3

HN

57: Y = Ad, 200 mg sj : Y = C, amount, yield n. a.

entry 1 2 3 4 5 6

NaNO3 NaN3

O

HN

Y

N3 59 536 mg, yield n. a.

N

Y N N P N N 60 221 mg, yield n. a.

N

P

N

N N N P N

Figure 32. Synthesis of phosphazene polyaromatic framework 60. The amino groups were first replaced by azide, which incorporation in 59 was monitored by FT-IR with a characteristic peak at 2110 cm-1 (asymmetric stretching frequency of the azido group). The PAF 59 was treated with tris(dimethylamino)phosphine via Staudinger reaction, to obtain the phosphazene-functionalized materials 60. Several versions of PAFs 60 were prepared that integrated different phosphazene amount. EA of nitrogen content allowed distinguishing three phosphazene-based porous materials with 0.29 mmol.g-1 (60a), 0.79 mmol.g-1 (60b) and 1.32 mmol.g-1 (60c) of nitrogen. The SBET surface area for 60a was 410 m2.g-1 and the estimated pore volume was 0.26 cm3·g−1 (for precursor 56 SBET = 413 cm3·g−1, Vpore = 0.48 cm3·g−1). The increase of phosphazene loading led to progressive diminishing of the surface area and porosity to 225 m2.g-1 and 0.19 cm3·g−1 for 60b, down to 100 m2.g-1 and 0.14 cm3.g-1 for 60c. The authors attributed this effect to the size of phosphazene groups, which would partially block the pores of the materials. TGA confirmed the thermal stability of the polymers up to 400 °C.109 The various versions of porous materials 60a-c were used as heterogeneous organocatalysts in polyesters synthesis by δvalerolactone ring opening polymerization (ROP) using 5 mol% of catalyst (Table 12). The conversion was c.a. 90% after 45 h using the catalyst 60a (Table 12, entry 1). Under similar conditions, the conversion decreased to 56% and 39% using catalytic materials with lower porous volume 60b and 60c, respectively (entries 2-3). The homogeneous system 2Ph-Phos needed 7 days reaction to obtain 90% of conversion (Table 12, entry 4). The authors suggested that this notable catalytic activity improvement from heterogeneous systems might result of pores acting as nanometer-sized reactors for catalysis. The catalytic material 60b was recycled by filtration and used in four successive runs. The conversion varied between 87 and 92 % in each run. First run needed more time (72 h) to achieve 89 % of conversion, while in the second and successive runs only 45 h were sufficient to achieve maximal conversions, showing probably rearrangement of porous matrix during the first reaction.



O O

+ MeOH

-valerolactone

60a-c or 2Ph-Phos (5 mol%)

O

Toluene, 110 °C

O O

OH n

PO(OH)2

N N N P N

1) PdCl2(PPh3)2,, HPO(OEt)2 Et3N, C6H6, 80 °C

O

30a

catalyst 60a 60b 60c 2Ph-Phos

time (h) 45 45 45 168 (7 days)

(HO)2OP

2) HCl

2Ph-Phos

entry 1 2 3 4

Page 14 of 27

PO(OH)2

conv. (%) 99 56 39 90

Table 12. ROP of cyclic ester δ-valerolactone 105 using heterogeneous organocatalysts 60a-c and homogeneous counterpart 2Ph-Phos. For introducing aromatic linkers into 3-D adamantane-based porous materials, cobalt cycloaddition of alkynes or convenient biphenyl synthesis from palladium-catalyzed Suzuki reaction have been used. The aromatic rings introduced open the way to a variety of useful postfunctionalization reactions which includes, to date: nitration, amination, amidation, azidation and subsequently, even the introduction of pendant iminophosphorane groups (phosphazenes). It is easy to anticipate that more functions can be valuably associated to such aromatic porous materials by convenient post-functionalization (like sulfonate groups for instance, which may be susceptible of increasing water-swelling of porous materials or of introducing new multiphasic catalytic processes). From catalytic applications outlook, the conceptually innovative combination of organocatalysis and transition metal catalysis has been introduced for tandem Knoevenagel condensation/Rh-hydrogenation, by using a sophisticated porous amide multisite catalyst. The independent use of La immobilized complexes and phosphazene polyaromatic frameworks for, respectively, methanolysis of hazardous organophosphates (known as acetylcholinesterase inhibitors) and ring opening polymerization of cyclic ester δ-valerolactone (metal-free) have been also pertinent regarding sustainable chemistry development. It is somewhat surprising that little discussion concerned the high aromatic ring content in these porous polymers. This, in relation with the conceivable influence of weak supramolecular interactions (such as, but not exclusively, – stacking) i) first, in the formation of porous structures and second ii) in the search for their exploitation in catalytic reactions.110 Additionally, such porous polymers have not been exploited to date for nanoparticles encapsulation (preformed or in-situ grown). The structure and reactivity of the resulting composites would be pertinent to investigate in link with the highly aromatic environment. Progress in those aspects is expectable in the near future. 4.4. Porous materials integrating 1,3,5,7-tetraphenyladamantane with -donors coordinating functional groups: phosphonate- and carbene-CPs. Neumann et al. reported the phosphonated 61 derivative of 1,3,5,7-tetraphenyladamantane, which was prepared by palladocatalyzed phosphonation of 30a (Figure 33). The monomer 61 was engaged in coordinative polymerization independently with titanium,111 and vanadium,112 to produce the coordination polymers (CPs) 62 and 63, respectively. FT-IR of monomer 61 showed characteristic bands for an alkyl arylphosphonic acid: C–H(Ar) 3008, C–H(Alk) 2921 and 2580, C=C 1604, P=O 1139, and P–O 997 and 928 cm-1. Upon formation of the vanadium phosphonate 63,112 there was significant changes in the energies associated to P–O with bands at 1028, 993, and 952 cm-1 indicating the formation of P–O–V bonds. Consistently, in the titanium phosphonate 62 the disappearance of the vibration band at 928 cm–1 was assigned to P–O–H disruption, and the raise of a new band at 1026 cm–1 can be assigned to P–O–Ti bond formation.111 At the same time the band P=O at 1137 cm–1 was almost not shifted, indicating a local environment unchanged that suggested no direct Ti coordination via these (P,O) atoms. TGA analysis indicated that thermal decomposition of the organic component of the hybrid materials (decomposition into CO2) starts above 360 °C.

61 PO(OH)2

O O O P

O

O P

O

M

O P O O

O P O O

O O O P O

O O P O

O

P O

O

P O

71%, 1700 mg

Ti[OCH(CH3)2]4 or V(O)CH(CH3)3 DMSO

O

62 : M = Ti, 49%, 910 mg 63 : M = V, 62%, 790 mg

Figure 33. Synthesis of [Ti/phosphonate]- and [V/phosphonate]MOPs 62 and 63 from tetrafunctionalized Td-directing adamantane 61. For CP 62 various preparative pathways yielded insoluble materials with a Ti/P ratio ranging between ~1.0 and 3.4. For 63 the elemental analysis by energy dispersive X-ray spectroscopy provided a mean V/P ratio 1:1. Porous titanium-based CP 62 had a SBET = 557 m2.g-1 and a Vpor = 0.42 cm3.g-1 with the coexistence of micropores and mesopores (average size of 1.35 nm and 3.8 nm, respectively). The X-ray powder diffraction pattern of the hybrid material 62 showed two broad peaks at 2θ values 5.88° (d spacing 15.2 Å) and 11.28° (d spacing 7.9 Å) evidencing a weak periodicity.111 The presence of three additional diffuse peaks at higher 2θ values indicated the presence of some amorphous material, which overall suggested a paracrystalline material. As can be seen from Figure 34, the calculated micropore sizes have an average diameter of ca. 15.6 Å, consistent with the calculated micropore size of 13.5±4 Å, derived from the adsorption branch of the isotherm applying Saito–Foley (SF) pore model. TEM and SEM analysis showed the formation of nanospheres (180-300 nm diameter) as nearly the only type of specimen morphology. Some of the nanospheres were broken and appeared in microscopy with hollow and/or sponge-like interior. The exploitation of this [Ti/phosphonate]CP remains to be developed, especially its inclusion potential. 111 The nitrogen sorption isotherm of vanadium phosphonate 63 is consistent with a mesoporous material of SBET = 118 m2.g-1.112 The mesopore size distribution curve derived from the desorption branch of the isotherm using the BJH method showed the main maximum corresponding to a pore diameter of 3.8 to 3.9 nm. Clearly, the mesoporosity is very similar to the one observed for titanium phosphonate 62 but the surface area is substantially lower, arguably due to the absence of microporosity in 63 compared to 62. The PXRD pattern showed a single strong and relatively broad peak at 2θ value of 6.3° (d spacing 14 Å) and four broad low-intensive peaks at 2θ value of 11.85, 19.1, 28.6, and 40.9°, evidencing a certain degree of periodicity in the material.112

Figure 34. Computer-generated model of a fragment of Ti-CP 62. Reproduced with permission from ref 111. Copyright 2006 WileyVCH Verlag GmbH & Co. KGaA, Weinheim.


Page 15 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Porous [V/phosphonate]-CP 63 was employed in aerobic oxidation of benzylic alcohols. Some very good conversions (92-99%) with high selectivity (> 95%, no benzoic acid from benzaldehyde overoxidation, or ethers from acid condensation of the benzylic alcohol formed) toward aldehydes P7a-e were obtained with the substrates S7a-e (Table 13, entries 1-5). In contrast, only a moderate conversion and selectivity was observed with 2,4,6-trimethylbenzyl alcohol (S7g, entry 6). The byproducts were 7% 2,4,6-trimethylbenzoic acid and 14% unidentified product (possibly 3,5-dimethylphthalide by GCMS). Since S7g is larger than other substrates, the authors proposed that the mesoporous catalyst is shape/size selective (Table 13), as observed in some other cases.93, 98 Recycling of these phosphonatebased materials was not reported.

N

CuI, KOH 30a

DMSO

CH3I

N

N

NH

N

N

R S7a-f

entry 1 2 3 4 5 6

alcohol S7a S7b S7c S7d S7e S7g

R H 4-MeO 4-Me 4-NO2 4-CF3 2,4,6-Me

conv. (%) >99 >99 98 92 >99 56

selectivity (%) 96 98 96 >98 >98 79

Table 13. Aerobic oxidation of benzylic alcohols S7a-f catalyzed by 63. Conditions: catalyst loading 20 mg of 63, for 1 mmol of substrate. Nie et al. reported the synthesis of the N-heterocyclic carbene (NHC) precursor 1,3,5,7-tetrakis(4-(imidazol-1yl)phenyl)adamantane 64 (Figure 35).113 Compound 64 was formed starting from tetraiodide 30a as precursor, engaged in copperpromoted Ullmann C–N cross-coupling with imidazole. Compound 64 was converted into methylimidazolium salt 65 by reaction with methyliodide. This later was treated with Pd(OAc)2 to generate the coordination polymer 66 (Figure 35). The presence of [Pd/NHC] fragments in polymer 66 was confirmed by IR-FT and 13C NMR. The IR-FT spectra of 65 presents the stretching band of quaternary C=N groups at 1551 cm-1, which disappeared after palladium introduction in CP 66.113 Additionally, the carbene characteristic peak was detected in 13C NMR spectra at 168 ppm. The BET surface area and total pore volume for 66 were significantly lower than classical values from Tddirecting adamantane-based scaffolds, with SBET = 54 m2.g-1 and Vpor = 0.06 cm3.g-1, respectively. The adsorption isotherms display a steep nitrogen gas uptake at low relative pressure (P/P0 < 0.001) reflecting abundant micropore structure, a slight hysteresis loop implying a spot of mesopore and a sharp rise at high pressure regions (P/P0 = 0.8–1.0) indicating the presence of macropores in this material. The pore-size distribution calculated by DFT confirmed the presence of a primary micropore and a spot of meso- and macropore, this global irregularity was reflected in SEM analysis. The XRD study revealed that CP 66 was mostly amorphous and TGA showed that this material is stable up to about 350 °C. The palladium content of the material was not reported.

N N

I

N N

N [Pd]

[Pd]

N

Pd(OAc)2

[Pd] N

110 °C 2 days

N

N Pd

DMF

I N N

N

[Pd] N

[Pd]

P7a-f

I

N

O R

N 65 94%, 1310 mg

N

N

O2 (2 bar) toluene, 100 °C 15 h

N

N

I

OH

N

N I

64

N

[V/phosphonate]-CP 63

N I

N N

N

N

[Pd]

66 91%, 1080 mg

N

Figure 35. Synthesis of adamantane-based MOP 66. The coordination polymer 66 was employed in palladium-catalyzed Suzuki cross-coupling of the electron-rich (deactivated) 4bromoanisole with phenylboronic acid and its catalytic activity was compared with homogeneous palladium precatalysts in the absence of additional ligands (Table 14). [Pd] catalyst MeO

Br

B(OH)2

+

(0.5 mol%)

MeO

EtOH/H2O (1:2) 50 °C, 4 h

entry 1 2 3 4

catalyst [Pd/NHC]-CP 66

conversion (%) >98 85 70 77

Pd(OAc)2 Pd(PPh3)4 [Pd(dppf)Cl2] Table 14. Bromoanisole arylation with [Pd/NHC]-CP 66.

A total conversion was observed using [Pd/NHC] coordination polymer 66, which confirmed the positive NHC ligand effect on the oxidative addition of bromoanisole. Accordingly, the palladium homogeneous catalysts were found to be less active (Table 14, entries 2-4). The catalytic material 66 was recycled three times with a slow gradual decrease in activity. Unclear results were reported when the reaction of 4-bromoanisole with phenylboronic acid was conducted in the presence of polyvinylpyrrolidone (PVP) – traditionally used for stabilizing colloidal palladium– the yield decreased to 53%, indicating according the authors soluble inactive palladium species formed in the process. Yet, it was mentioned that ICP analysis did not reveal palladium leaching into the solution during the reaction.113 In Suzuki coupling [Pd/NHC] CP 66 also catalyzed the arylation with phenylboronic acid of a more diverse range of substituted haloarenes: high yields ranging from 94 to 99 % (Table 15, entries 13) were achieved with para-substituted iodo- and bromoarenes. More demanding substrates were identified with sterically ortho-hindered bromoaryl and para-substituted chlorobenzenes (entries 4-8).113 entry 1 2 3

haloarene

yield (%) 96

O I O

94

I

O Br

4

99 66

Br

5


Br

63


6

Page 16 of 27

71 Br

O

7 8

58

Cl

52

Cl

Table 15. Scope of Suzuki coupling of phenylboronic acid with haloarenes catalyzed by [Pd/NHC]-CP 66. Thus, thermally stable and robust coordination polymers with early (Ti and V) and late (Pd) transition metals were developed using coordinating phosphonate and carbenes pendant groups, respectively. The 3-D Td-directing tetratopic adamantane tectons were achieved from palladium-catalyzed C–P and Cu-catalyzed C–N coupling of tetraiodinated adamantane 30a with esters of phosphorous acids and imidazole, respectively. [V/phosphonate] porous materials catalyzed aerobic oxidation of benzylic alcohols to aldehydes with some size effects attributed by the authors to mesoporosity limitation. Conversely, the stabilization of palladium immobilized with a NHC donor remained uncertain in the conditions reported, even if Suzuki cross-coupling application was achieved with good to moderate success. 5. POROUS MATERIALS TETRAAMINOADAMANTANE

BASED

ON

1,3,5,7-

3-D microporous materials with inherent basicity attracted recent interest for their potential catalytic performances, for instance in the Knoevenagel condensation reaction (used as a base-catalyzed reaction of industrial interest).114 Such base-functionalized porous materials could be synthesized by combining polytopic tetrahedral building units in Schiff base reactions. Accordingly, the synthesis of 3-D porous tetraimine-based covalent organic frameworks involved the use of a tetrahedral alkyl amine, 1,3,5,7- tetraaminoadamantane 21 (Figure 7). Accordingly, Yan et al. reported the synthesis of 3-D COF 67 by condensation of tetraaminoadamantane 21 with1,3,5triformylbenzene (Figure 36). N N

N O

NH2 H 2N

NH2 NH2

+

O

N

solvothermolysis AcOHaq. 120 °C, 5 days

O

21

N N

N

N

N N 67 82%, 28 mg

N N

Figure 36. Synthesis of 3-D COF 67. This synthesis was conducted under solvothermal conditions in 10:1 vol. mixture of mesitylene and 3M aqueous acetic acid, followed by heating at 120 °C for five days, to give 67 as a crystalline solid in 82% yield. Polymer 67 was found to be insoluble in water and common organic solvents (acetone, ethanol, hexanes, DMF, THF). The FT-IR spectra of 67 showed stretching band at 1635 cm-1 attributed to a C=N vibration, which confirmed the formation of imine links in this materials (Figure 36). SEM images of 67 showed that the organic framework crystallized with a uniform polyhedral morphology and a particle size of about 300 nm. TGA measurements showed a good thermal stability of the material up to 350 °C.114 Powder X-ray diffraction analysis confirmed the crystallinity of the COF 67 with an intense 2θ peak at 8.78°. An extended structure was modeled for COF 67 in the space group I43d (Figure 37). Excellent agreement between the experimental and simulated PXRD patterns was obtained. Accordingly, COF 67 has microporous cavities with a diameter of 7.8 Å and rectangular windows with a size of 7.8 x 11.3 Å2 (Figure 37).114

Figure 37. Modeling of extended structure for COF 67: a) atomic connectivity in 67 with atom surfaces represented, b) microporous cavity (as pink sphere) with a diameter of 7.8 Å in 67, c) rectangular window with a size of 7.8×11.3 Å2 in 67. Reprinted with permission from ref 114. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. For 67 nitrogen-gas adsorption at 77 K exhibited the signature feature of microporous materials with isotherms characterized by a sharp uptake under low relative pressures in the range of P/P0 = 10-5– 10-2. The lack of hysteresis indicates that the adsorption and desorption mechanisms are similar and that the adsorption is reversible. The BET surface area of the COF 67 was found to be SBET = 730 m2.g-1. The total pore volume was estimated at P/P0 = 0.90 to be of Vpor = 0.43 cm3.g–1. The pore-size distributions was calculated on the basis of nonlocal density functional theory and showed a narrow pore width around 8.3 Å, which was in excellent agreement with the pore size of 7.8 Å predicted from the crystal structure modeling.114 Taking advantage of the basic properties of its imino groups and well–defined pore size, the COF 67 was employed in Knoevenagel condensation reaction. Benzaldehydes with various sizes were tested for coupling with malononitrile; the size of reactants was calculated as the longest distance of these molecules in two perpendicular directions, Figure 38: benzaldehyde (6.1x8.7 Å2), 4methylbenzaldehyde (6.1x 10.1 Å2), 4-phenylbenzaldehyde (6.1x13.3 Å2), 4-(4-methylphenyl)benzaldehyde (6.1x14.7 Å2), and malononitrile (4.5x6.9 Å2). A high conversion was observed with benzaldehyde S8a leading to product P8a of size 7.6 x 10.4 A2, which is smaller than pore window. However, the conversion dramatically dropped while using substituted substrates with sizes larger than the pore window (S8b-c). This strongly suggested that the reaction occurred in the pores, and the very low conversions in substituted benzaldehydes S8b-c were realized on surface (or were finished once the pores were clogged). For comparison, catalytic reactions with various related homogeneous catalysts (1-adamantylamine, N,N',N''(1,3,5-benzenetriyltrimethylidyne)triadamantyl- amine, and 2,4,6tris((adamantylamino)methylene)cyclohexane-1,3,5-trione) were carried out under the same conditions. These catalysts showed all good to high conversion (77–97%) without any detectable size-selectivity effect, which supports the fact that COF 67 is a size-selective catalyst. This size-selectivity with COF 67 was further evidenced by reacting simultaneously in the same batch the benzaldehydes S8a and S8c with malononitrile. As expected, a high selectivity was observed with a 97 % conversion of benzaldehyde S8a to P8a, and only 2 % of product P8c obtained. The catalyst can be recovered by filtration and reused for Knoevenagel condensation reactions at least in three successive runs without activity loss.114


Ar CHO

+

S8a-d

CN CN

COF 67 (5 mol%) benzene 10 h, RT

4.5 Å

CN R P8a-d (%)

CN

7.6 Å

6.1 Å

simulation.115 Structural similarities with COF 67 were noticeable,114 clearly confirming the usefulness of Td-directing tetraamine and tetraimine adamantane scaffolds in the synthesis of highly periodic 3D porous crystalline frameworks under solvothermal conditions.116, 117 The application of the BET model in the low pressure region (0.005 < P/P0 < 0.07) showed some top high specific surface areas of 2259 m2.g−1 for 68 and 2071 m2.g−1 for 68-F. The calculated micropore-size distributions showed a narrow pore width of 1.36 nm for 68 and 1.28 nm for 68-F, which were in good agreement with the pore size predicted from their calculated crystal structures at 1.42 Å.115 The proximity of acidic and basic sites of 68 and 68-F allowed the acid−base catalyzed one-pot cascade reaction combining the hydrolysis of an acetal (catalyzed by the acidic sites of boroxine group) followed by Knoevenagel aldehyde condensation catalyzed by the basic sites of imine groups (Figure 40).115

NC

NC

O S8a 8.7 Å

P8a (96%) 10.4 Å NC

S8b

CN

7.6 Å

6.1 Å

O

P8b (4%) 12.2 Å

10.1 Å

NC S8c

14.7 Å

NC CN P8d (3%) 16.8 Å

Figure 38. Catalytic activity of 67 in the Knoevenagel condensation reaction as a function of the reactants and products size. Fang and Qiu et al. extended the synthetic use of tetraaminoadamantane 21 to the synthesis of 3-D crystalline polymers 68 and 68-F with dual linkages, which involves the formation of imine groups as 4-connected nodes, and triangular boroxine rings as a 3connected nodes (Figure 39).115 Their synthesis was carried out by solvothermal reaction of 21 and four equivalents of either 4formylphenylboronic acid (COF 68), or 2-fluoro-4formylphenylboronic acid (COF 68-F) in a 1:1 vol. mixture of dioxane and mesitylene, followed by heating at 120 °C for 3 days. The yield of white crystalline solid was 82% for 68 and 85% for 68-F. Similar to COF 67,114 these products were insoluble in common organic solvents, including polar DMSO, and N-methyl-2-pyrrolidone. N N O

NH2 NH2 +

N

NH2 21

HO

R B

OH

N

R

N

4

O 68 or 68-F acidic boroxine groups

S8a

R

R

68 or 68-F basic imine groups

R P9a : R = COOEt, 93% P9b : R = COCH3 93% P8 : R = CN 96-98%

Figure 40. One pot cascade deacetalization-Knoevenagel condensation catalyzed by COFs 68 and 68-F. Conditions: S9 and carbon acid derivatives, 1 mmol; catalyst 10 mol%; CDCl3, RT, 20 h. The reaction was performed at 1 mmol scale of reagents in CDCl3 (product distribution monitored by 1H NMR) with 10 mol% of COF 68 or 68-F. First benzaldehyde dimethyl acetal S9 was hydrolyzed to give benzaldehyde S8a, which further reacted with either malononitrile, ethyl cyanoacetate, or acetylacetone to respectively form benzylidene malononitrile, ethyl trans-α-cyanocinnamate, or 3benzylidene-2,4-pentanedione (P8-P9b). Quantitative conversion of S9 (100%) led to the formation of benzaldehyde intermediate in high yield to give, through consecutive Knoevenagel condensation, benzylidene malononitrile in 98% yield from COF 68 and 96% from 68-F. Replacing malononitrile reactant with ethyl cyanoacetate or acetylacetone gave similar conversions above 90% (Figure 40). PXRD and N2 adsorption analysis post-cascade confirmed the conservation of COF structures, indicating a high stability of these materials in the acid−base reaction. The catalyst can be isolated from the reaction mixture by filtration and reused three times with no loss of activity. 115

N N

B O

dioxane/ mesitylene

H 2N

OMe S9

15.4 Å 7.6 Å

S8d

OMe

P8c (4%)

13.3 Å O

R

CN

7.6 Å

6.1 Å

O

6.1 Å

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6.9 Å

Page 17 of 27

120 °C, 3 days

O B

B O

N R

R

R= H 68 82%, 26.7 mg R= F 68-F 85%, 30.9 mg

N N

N N

Figure 39. Synthesis of dual linked 3-D COFs 68 and 68-F. FT-IR monitoring of 68 and 68-F synthesis showed the disappearance of the OH stretching band at 3215 cm−1, the carbonyl stretching band around 1680 cm−1 and the N−H stretching band around 3300 cm−1 for 21, suggesting a full conversion of these groups.115 Accordingly, the characteristic stretching band of a C=N imine group emerged around 1640 cm−1, as well as B−O and B3O3 ring vibrations (around 715, 1300 and 1340 cm−1). 13C crosspolarization magic-angle-spinning NMR confirmed the presence of carbon from the imine group at 168 ppm for 68 and 166 ppm for 68F. Scanning electron microscopy (SEM) study reveals that both 3-D COFs exhibit rectangular morphology with random distribution of particles sizing about 200 nm. From TGA analysis these COFs, built up of dual labile covalent linkages, showed a remarkably high thermal stability up to 400 °C. The crystalline nature of both COFs was confirmed by PXRD analysis, and was further studied by

Usually, during COF synthesis, dynamic reversible reactions are useful for the formation of covalent organic frameworks with high crystallinity and periodicity.117 However, because of this reversibility final robustness of the material could be compromised. Herein 3-D Td-directing adamantane scaffolds prolonged with imine and boroxine linkages are advantageous to provide thermodynamic stability which sometimes lack to the final COFs and MOFs highly-ordered structures. Thus, tetraamine adamantane scaffolds are clearly recommendable as precursors in the synthesis of highly periodic 3-D porous crystalline frameworks by derivatization to generate imino group as linkers. Their use as recyclable bimodal acid/base catalysts in one-pot cascade deacetalization-Knoevenagel condensation is also a conceptual advance attributable to the exploitation of valuable rigid polyfunctionalized Td-directing adamantane derivatives. 6. SUMMARY AND OUTLOOK Polyfunctionalized adamantane derivatives are now clearly identified by the community as valuable building blocks for structuring porous materials. In the search for the development of rigid 3-D Td-directing tectons based on adamantane, the predictable synthetic limitations for introducing convenient functional groups have been progressively overcome. This is attributable to the optimization of 1,3,5,7tetrabromo (5) and tetraiodo-adamantane (18a) synthesis, and to the access to the ubiquitous 1,3,5,7-tetraphenyladamantane (28), which is versatile enough to allow further installation on its aromatic rings of halogen groups (30a and 30b), sp-C unsaturated functions (31), and



other suitable leaving groups (48). Complementary to these precursors, tetracarboxylic- (23) and tetraamino-adamantane (21) offer further synthetic opportunities already successfully exploited for materials synthesis. From this pool of starting adamantane derivatives, a second generation of valuable tectons is reachable, based on efficient crosscoupling reactions, which include Sonogashira, Suzuki, Heck, Ullmann couplings, and also cobalt and copper cycloadditions, as well as condensations such as peptide coupling, and a range of radicalbased polymerization reactions. By using this same set of reactions, the targeted porous materials are also accessible from functionalized adamantanes coupled with additional polytopic linkers (benzenetrithiol, bistriflate-norbornadiene, phenyl-diboronic acid, triformylbenzene, etc.), linkers which may be eventually also chemically active for further materials elaboration or metal coordination (B1-B5). Three broad families of porous materials, usable as catalytic recyclable or recoverable materials, have been developed from 3-D Td-directing adamantanes tectons: i) coordination polymers (CPs), which incorporate metal linkers; ii) porous organic polymers (POPs), which are able afterwards to immobilize metal complexes or encapsulate metal nanoparticles; and iii) highly crystalline covalent organic frameworks (COFs), with well-defined porous size and distribution. Because of adamantane scaffold, mostly robust porous materials are obtained, as usually evidenced by routine TGA analysis, and/or successful recycling runs in heterogeneous catalysis. In addition, 3-D adamantane-based material synthesis is mature enough now to allow identifying the best ways to obtain very rigid frameworks, which generally guarantee SBET around or above 500 m2.g-1 and micro/mesoporosity with porous volumes ranging between 0.15 to 0.50 cm3.g-1. These features are better obtained from Tddirecting blocks incorporating adamantyl–phenyl–alkynyl “all-rigidelements” chains. An appealing, yet challenging, synthetic development might be the extension of the structuring role played by adamantane to related polymantane building blocks, either more rigid such as the lower diamondoids diamantane and triamantane,7 or that would introduce an intriguing degree of flexibility with for example bisadamantane scaffolds. On the other hand, the replacement of terminal linear alkynyl groups for more flexible linkers, like for instance alkenyl groups, already introduced a certain degree of flexibility in the resulting polymers. This may decrease the surface area and porosity size (access) because of polymer packing. However, this effect has not been found necessarily detrimental to catalytic applications, when pertinent organic swelling features were given to the materials because of this new flexibility. Accordingly, swelling may facilitate access to reactive sites in catalysis. This approach probably deserves more attention in relation with solvent-free catalysis. Biphasic conditions, undescribed to date with 3-D adamantane-based materials, would be also concerned. The synthesis of highly crystalline materials achieving long-range periodicity and much higher surface area (SBET from 700 m2.g-1 to above 2000 m2.g-1) is accessible from dynamic reversible linking reactions conducted under solvothermal conditions in time-prolonged reactions (120°C, 3-5 days). In this case covalently-weaker nitrogencontaining or boron-containing linkages (imine bonds, etc.) is advantageous to form the crystalline COFs, all the more that thermally stable materials can be formed (up to 350-400 °C). The general challenges in materials characterization have been appropriately discussed elsewhere,21 and naturally applied to porous materials based on 3-dimensional Td-directing functionalized adamantane scaffolds. However, in relation with heterogeneous catalysis, the development of proper models for molecular interactions into a define pore is needed to understand and control, the many sizeselectivity and confinement effects already reported. Beside some necessary forthcoming in-situ experimental characterization tools, the development of computational structure–property prediction tools15 is highly desirable. This becomes all the more important that heterogeneized asymmetric catalytic reactions constantly developed –

Page 18 of 27

including the enzyme-reminding recoverable organocatalysts–. Such computational approach would also benefit to the understanding and exploitation of weak non-covalent  interactions for porous materials arrangement and catalyst design. Concerning catalytic reactivity, proof-of-concepts for tandem and cascade bimodal catalysis have been provided from sophisticated porous organic polymers. The combination of organocatalysis and transition metal catalysis is thus particularly appealing and further progress can be expected from the encapsulation of nanoparticles into the porosity of organocatalytic porous materials. Such confined space playing the role of a second coordination sphere for the catalyst surface, thus may induce new selectivity (including new enantioselectivity) and modified activity of the catalysts (faster, slower, size-selective, etc.). A better control of catalyst surface and encapsulation influence over the reactions is also desirable, and in this context a great challenge and opportunity is clearly a better understanding of kinetics related to these catalytic materials, which are mostly not available in current studies. Still, from the catalytic applications perspective, progress has to be done in general concerning the recycling methods and work-up (to avoid metal leaching, polymer matrix loss, porosity deterioration, intercalation, packing, etc.). In this context, the number of recycling experiments should generally reach more routinely greater cycle numbers (> 5-6),118 and more in depth-analysis of the consequences of the catalytic runs should be addressed more systematically (beyond catalyst morphology by microscopy, some systematic studies of porosity/surface area modification post-catalysis as well as analysis of leaching in solution). The robustness of the porous materials built from adamantane derivatives should facilitate these requirements. All these technical and engineering challenges will naturally complement the creative thinking of materials and synthetic chemists to further propose convenient strategies in tectons designing and their arrangement towards specific catalytic applications with innovative properties not covered by other porous materials and homogenous systems. AUTHOR INFORMATION Corresponding Author [email protected] ORCID Jean-Cyrille Hierso: 0000-0002-2048-647X Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the ANR program ICARE_1 (ANR-16CE07-0007-02), by the CNRS, Université de Bourgogne, Conseil Régional de Bourgogne through the plan d'actions régional pour l'innovation (PARI) and the fonds européen de développement regional (FEDER) programs. We sincerely thank D. Poinsot, R. Axet, I. Gerber, Y. Min and P. Serp for very fruitful discussions.


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Graphical Abstract



Page 20 of 27

REFERENCES

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Porous Materials Based on 3-Dimensional Td-Directing

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