Immobilization of copper complexes with (1,10-phenanthrolinyl)phosphonates on titania supports for sustainable catalysis

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S. Brandès, F. herbst, S. Rigolet, A. Lemeune and I. P. Beletskaya, J. Mater. Chem. A, 2017, DOI:

10.1039/C7TA01195D.

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Immobilization of copper complexes with (1,10-phenanthrolinyl)phosphonates on titania

supports for sustainable catalysis

Alexander Mitrofanov,^a,bStéphane Brandès,^aFrédéric Herbst,^cSéverinne Rigolet,^d

Alla BessmertnykhꢀLemeune,^a,*Irina Beletskaya^b,*

^aInstitut de Chimie Moléculaire de l’Université de Bourgogne (ICMUB), UMR CNRS 6302, 9

Av. Alain Savary, 21078 Dijon, France. Eꢀmail: Alla.Lemeune@uꢀbourgogne.fr

^bDepartment of Chemistry, Lomonosov Moscow State University, Leninskie Gory, GSPꢀ1,

Moscow 119991, Russia. Eꢀmail: beletska@org.chem.msu.ru

^cLaboratoire Interdisciplinaire Carnot de Bourgogne, UMR CNRS 6303, 9 Av. Alain Savary,

21078 Dijon, France

^dInstitut de Science des Matériaux de Mulhouse, Université de HauteꢀAlsace, UMR CNRS

7361, 15 rue Jean Starcky, Mulhouse, 68057, France

Different strategies for the immobilization of copper complexes with 1,10ꢀphenanthroline

(phen) using the phosphonate anchoring group were investigated to prepare robust and porous

heterogeneous catalysts. Homoleptic and heteroleptic copper(I) complexes with phen bearing

the bis(trimethylsiloxy)phosphoryl anchoring group (Pphen-Si) at different positions of phen

backbone were prepared and covalently incorporated into titania (TiO₂) xerogels by using the

solꢀgel process or grafted onto a surface of mesoporous TiO₂(S_BET= 650 m²g^ꢀ1). Copper(I)

bis(Pphen-Si) complexes were the only complexes that were successfully anchored onto TiO₂

surface because the heterogenization was often accompanied by undesirable dissociation of

copper complexes. Hybrid materials based on copper(I) chelates with one phen ligand were

obtained following a twoꢀstep procedure involving the immobilization of Pphen-Si chelators

and their successive complexation with copper(I) ions. Most porous material Cu/6b/SM/A

displaying BET surface area of 243 m²g^ꢀ1and bearing 0.57 mmol g^ꢀ1of the complex was

synthesized according to the surface modification of TiO₂. Excellent catalytic performance of

the reusable Cu/6b/SM/A material in the Sonogashiraꢀtype coupling and the Huisgen 1,3ꢀ

dipolar cycloaddition was also demonstrated. This solid represents the first example of

mesoporous TiO₂ꢀsupported transition metal catalysts.

Introduction

Copper catalysts could replace precious metal complexes in many industrial processes

involving in pharmaceutical, dye and polymer productions.^1ꢀ3This is of major importance in

view of societal concerns relating to environmental and sustainable chemistry. Last two

decades, significant improvements in homogeneous Cuꢀcatalysed C–C^4ꢀ7and C–Het (Het = O,

S, Se, N, P)^6,8crossꢀcoupling reactions, additions to unsaturated C–C bonds,⁹the Huisgen

1,3ꢀdipolar cycloaddition,¹⁰and other organic transformations^2,9,11ꢀ15were reported.

Homogeneous catalysts are well suited for increasing the reaction scope and fine tuning the

reaction rate and its selectivity. However, the separation of organic products from toxic

copper compounds are particularly troublesome^16,17due to exceptional coordination properties

of this metal, which avidly bind a huge range of organic molecules containing oxygen and

nitrogen donor sites. To solve this problem and recover the catalyst, several strategies were

explored. It was reported that copper nanoparticles and copperꢀmodified dendrimers can

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occasionally replace homogeneous catalysts.^18,19However, their use in industrial processes is

also hampered by the difficult purification of products from catalysts. Consequently, copper

nanoparticles, salts and oxides were immobilized onto insoluble supports, ether organic

(polystyrene,^20,21polyaniline²²and cellulose²³) or inorganic (zeolites,²⁴hydrotalcite,^25,26

hydroapatites,²⁷fluorapatites,²⁸silica,^20,29,30alumina^31ꢀ34and other^35ꢀ38). In these materials,

copper ions or nanoparticles are weakly bonded to the solid supports bearing hydroxy,

sulfonate or amine coordinating sites. These catalysts benefit from a spatial separation of

catalytic species and can be easily recovered, but their stability is quite low. As a result,

copper leaching is observed that leads to metal contamination of products and the decrease in

the catalytic activity of the recovered solids.

Another promising approach to robust catalytic materials is a direct immobilization of

relevant copper complexes onto solid supports such as silicas,^21,39ꢀ41zeolites,⁴²metalꢀorganic

frameworks (MOFs)⁴³or graphene.⁴⁴For instance, heterogenized copper N,Nꢀ and N,Oꢀ

chelates (Cu/L) were applied as catalysts in the Huisgen cycloaddition,^21,41,45the Mannich

threeꢀcomponent coupling reaction,⁴⁶the Sonogashiraꢀtype coupling,⁴⁷the Ullmannꢀtype

arylation of amines,⁴⁸the oxidative carbonylation of methanol³⁹and other organic

transformations. Surprisingly, catalyst leaching is difficult to control even for these materials.

As a result, it is still a common industrial practice to avoid, whenever possible, the use of

copper catalysts, in particular during the later steps of the synthesis of complex molecules in

pharmaceutical and agrochemical industries. There is thus a need to develop more efficient

immobilization strategies for the heterogenization of copper chelates.

The choice of the solid support is a key point for the efficient catalyst immobilization.

Inorganic supports offer various inherent advantages over organic and biopolymers. They are

insoluble in common organic solvents and water, do not swell and display a high structural,

thermal, mechanical and chemical stability. Rigid structure of these supports allows for a

spatial separation of catalytic centres. Recent investigations of hybrid organicꢀinorganic

materials based on phosphonates revealed that metal oxides or polymeric phosphonate

networks can be used as a solid support for heterogenization of transition metal complexes.^49ꢀ

52

Among them, TiO₂are particularly interesting because resulting molecular materials are

costꢀeffective and display exceptional thermal and chemical stability stemming from the

robustness of Ti–O(P) and P–C bonds. Several strategies are currently available for

immobilization of phosphonates onto TiO₂supports including solꢀgel (SG) processes and

surface modification (SM) reactions.^53ꢀ55Examples of TiO₂ꢀsupported transition metal

catalysts are still limited to a few reports. Immobilization of palladium complexes with

phosphine ligands bearing phosphonate anchoring groups on TiO₂matrices was carefully

studied but all obtained materials were inefficient in the Sonogashira coupling reaction.⁵⁶

Ru(II) and Ir(I) complexes with bipyridine were incorporated into TiO₂matrices in order to

prepare heterogeneous catalysts for reduction of aromatic and unsaturated ketones.^53,57Under

appropriate conditions, these solids catalysed heterogeneous hydrogenation with practically

useful chemoselectivity. The titaniaꢀsupported Co(I) complex prepared by using SG method

was an efficient catalyst for the hydroformylation of alkenes in contrast to a relevant

homogeneous complex.⁵⁸It has to be noted that all reported hybrid materials based on titania

were nonꢀporous since they displayed low specific surface areas (< 100 m²g). This is a

serious drawback for their catalytic performance.

In the present work, we report the heterogenization of copper complexes with phen

ligands bearing the phosphonate anchoring group using titania matrices. This approach to the

immobilization of copper catalysts combines several potential advantages including

thermodynamic stability of copper chelates with phen ligands, their catalytic efficiency and

versatility, the strong covalent linkage of phosphonates to TiO₂networks and excellent

mechanical, thermal and chemical properties of titania, which is widely used in industry as a

2

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support for inorganic catalysts. In particular, we were interested to control the porosity of the

materials and prepare costꢀeffective solid catalysts with surface characteristics that are

relatively close to those of mesoporous materials commonly used as heterogenized catalysts

such as functionalized ordered silicas. In this regard, the immobilization of phen ligands

functionalized by the phosphonate group (Pphen) and their copper(I) complexes (Cu/Pphen)

was investigated in details. First, Pphen/SG and Cu/Pphen/SG materials were prepared

reacting Pphen ligands or their complexes with titanium isopropoxide (Ti(OiPr)₄) according

to the SG process (Scheme 1, routes A and C, Figure 1). Alternatively, these ligands and

complexes were grafted onto the surface of mesoporous titanium oxide (S_BET= 650 m²g^ꢀ1)

yielding Pphen/SM and Cu/Pphen/SM materials (Scheme 1, routes B and D, Figure 1).

Moreover, the complexation of copper ions with Pphen/SM solids was explored to prepare

heterogenized chelates. The structural characteristics of hybrid materials and the integrity of

the immobilized molecules were investigated by different physicochemical methods,

including elemental analysis, infrared spectroscopy, nitrogen sorption isotherms, EDX

spectrometry, scanning electron microscopy (SEM), transmission electron microscopy (TEM)

and solidꢀstate NMR. Finally, the catalytic performance of most porous material Cu/6b/SM/A

was examined

Scheme 1. Schematic representation of the immobilization of copper complexes with phen ligands according to

the SG process (routes A and C) and the SM reaction (routes B and D).

in the Sonogashiraꢀtype and Huisgen cycloaddition reactions. These studies demonstrated that

Cu/6b/SM/A is stable in the presence of strong bases like triethylamine and cesium carbonate

and efficient as a catalyst for both reactions. This solid catalyst can be readily recovered and

reused up to 10 times without loss of activity. To the best of our knowledge, this is the first

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example of mesoporous titaniaꢀsupported transition metal catalysts.

Figure 1. Structures of Pphen ligands and Cu/Pphen complexes.

Experimental

Unless otherwise noted, all chemicals and starting materials were obtained

commercially from Acros® or Aldrich® and used without further purification. Complex

Cu(PPh₃)₃Br,⁵⁹3,8ꢀdibromoꢀ1,10ꢀphenanthroline,⁶⁰2ꢀchloroꢀ9ꢀ(4ꢀmethoxyphenyl)ꢀ1,10ꢀ

phenanthroline,⁶¹4ꢀ(diethoxyphosphoryl)phenylboronic acid pinacol ester⁶²and 4ꢀ

(diethoxyphosphorylphenyl)ꢀ4’ꢀphenylboronic acid pinacol ester⁶²were prepared according

to literature methods. Mesoporous TiO₂was synthesized from Ti(OiPr)₄according to the solꢀ

gel process in the presence of limited amount of water (20 equiv) in THF (see the Supporting

Information).⁶³The empirical formula of hydrated TiO₂((TiO_x(OH)_4ꢀ2x)·m(H₂O)·n(iPrOH), x

= 1.6ꢀ1.8; m = 0.1ꢀ0.4; y = 0.1ꢀ0.2) was calculated based on the elemental analysis of xerogel

dried at 80 °C under reduced pressure for 12 h.

Phenanthrolinyl phosphonates 6aꢀ8a,⁶⁴

heteroleptic copper(I) complexes

Cu(Pphen)(PPh₃)Br (1aꢀ3a)⁶⁵and homoleptic complex [Cu(Pphen)₂]PF₆(4a)⁶⁵were obtained

according to our previous reports.

Analytical thinꢀlayer chromatography (TLC) was carried out using Merck silica gel 60

plates (precoated sheets, 0.2 mm thick, with fluorescence indicator F254). Column

chromatography purification was carried out on silica gel (Silica 60, 63ꢀ200 ꢁm, Aldrich) and

neutral alumina (Aluminium oxide 90, 63ꢀ200 ꢁm, Merck). Centrifugation was performed at

6000 rpm for 5 min. Catalytic reactions were carried out using Carousel 12 Plus equipment

for parallel synthesis (Radleys).

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¹H, ³¹P and ¹³C NMR spectra were acquired either on a Bruker Avance III 500 MHz or

a Bruker Avance III Nanobay 300 MHz spectrometer. Chemical shifts are expressed in parts

per million (ppm) and referenced to residual nonꢀdeuterated solvent signals.^66,67The coupling

constants are expressed in unit of frequency (Hz). The unambiguous assignment of signals in

¹H and ¹³C NMR spectra was performed using gradientꢀenhanced COSY, HMQS and

NOESY correlation experiments. MALDIꢀTOF massꢀspectra were obtained on a Bruker

Ultraflex II LRF 2000 massꢀspectrometer in positive ion mode with dithranol matrix.

Accurate mass measurements (HRMS) were made on a THERMO LTQ Orbitrap XL.

Microanalyses (CHN) were performed on a Thermo Finnigan Flash 1112 analyser. Cu, P and

Ti elemental analyses were performed with inductively coupled plasma optical emission

spectrometers ICPꢀOES (DUO) ICAP 7400. FTIR spectra were registered on FTꢀIR Nexus

(Nicolet) and Bruker Vector 22 spectrophotometers. MicroꢀATR accessory (Pike) was used in

order to obtain FTIR spectra of polycrystalline solid complexes. Thermogravimetric (TGA)

measurements were performed on a Netzsch STA 409 PC Luxx analyser. Samples were

purged in an N₂(30 mL min^ꢀ1) / O₂(10 mL min^ꢀ1) stream during analysis and heated to 1000

°C in alumina crucibles with a heating rate of 10 K min^ꢀ1. Powder Xꢀray diffraction

experiments were performed on an Empyrean diffractometer from the PANalytical company

in the range 3° < 2θ < 50°. The uncrushed samples (few milligrams) were placed between two

Mylar sheets and the analysis was performed in transmission mode using a focusing Xꢀray

mirror equipped with fixed divergent and antiꢀscattering slits (aperture 0.5°) and 0.02 rad

Soller slits. The data collection was run with a copper anticathode Xꢀray tube (Cu Kα₁=

1.54060 Å / Cu Kα₂= 1.54443 Å) and with a X'Celerator detector equipped with an antiꢀ

scattering slit of 5 mm. Accurate mass measurements (HRMS) were recorded on a

Thermo LTQ Orbitrap XL apparatus equipped with an electrospray ionisation (ESI)

source. Nitrogen adsorptionꢀdesorption isotherms were measured with Micromeritics ASAP

2010 or 2020 analyser at 77 K with samples outgassed at 393 K in reduced pressure (10^ꢀ5torr)

for at least 6 h. Specific surface areas were calculated by the BET method.⁶⁸Mesopore

characterizations were performed by the BarrettꢀJoynerꢀHalenda method.⁶⁹Diffuse

reflectance spectra were recorded in the solid state at room temperature on a Agilent Carry

5000 UVꢀVisꢀNIR spectrometer. Continuous wave (CW) EPR spectra of solid samples were

recorded on a Bruker ELEXSYS 500. The instrument was equipped with a 4122 SHQE/0405

Xꢀband resonant cavity operating at 9.43 GHz, a Xꢀband high power dual gunꢀoscillator

bridge, and a quartz cryostat cooled at 100 K with a stream of nitrogen. The temperature was

regulated with an ER 4131VT accessory. All apparatus as well as the data acquisition were

controlled using Xepr software. The magnetic field was swept from 250 to 360 mT through

2048 points. Spectra were recorded at 6 mW power, 100 kHz frequency modulation, 0.5 mT

modulation amplitude, 10 ms time constant and 40 ms conversion time. The ³¹P solidꢀstate

NMR experiment was performed at room temperature on Bruker Avance II 400 spectrometer

operating at B₀= 9.4T equipped with a Bruker double channel 4mm probe at a Larmor

frequency of 161.99MHz. The spectrum was recorded with a π/2 pulse duration of 3.5 ꢀs and

31

a recycling delay of 60 s at a spinning frequency of 14 kHz. P spin lattice relaxation times

(T₁) was measured with the saturationꢀrecovery pulse sequence. ³¹P spectrum was referenced

to H₃PO₄(85% in water). Deconvolutions of the spectrum was performed using Dmfit

software (http://nmr.cemhti.cnrsꢀorleans.fr/dmfit/) with Gaussian/Lorentzian functions. Fieldꢀ

emission scanning electron microscopy (FESEM) was realized using a JEOL JSM 7600F

instrument located in the ARCEN analysis centre of the University of Bourgogne (Dijon).

Images were made using GentleBeamꢀHigh SEM mode.

Transmission electron microscopy (TEM) analyses were conducted using a JEOL JEMꢀ2100F

microscope operating at 200 kV and located in the ARCEN analysis centre of the University

of Bourgogne (Dijon). EDX spectrometry in STEM and TEM mode was used for chemical

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mapping and qualitative elemental analysis using a Bruker XFlash Detector 5030

spectrometer fitted on the JEMꢀ2100F microscope.

All measurements except SEM and TEM imaging and solid state NMR were performed at the

"Pôle Chimie Moléculaire", the technological platform for chemical analysis and molecular

synthesis (http://www.wpcm.fr) which relies on the Institute of the Molecular Chemistry of

University of Burgundy and Welience^TM, a Burgundy University private subsidiary.

Synthesis of 1,10-phenanthrolinyl phosphonates and complex 5a.

Diethyl (4-(9-(4-methoxyphenyl)-1,10-phenanthrolin-2-yl)phenyl)phosphonate (8a). A

mixture of 2ꢀchloroꢀ9ꢀ(4ꢀmethoxyphenyl)ꢀ1,10ꢀphenanthroline (160 mg, 0.5 mmol), 4ꢀ

(diethoxyphosphoryl)phenylboronic acid pinacol ester (187 mg, 0.55 mmol), Pd(dppf)Cl₂

(20.4 mg, 0.025 mmol) and Cs₂CO₃(815 mg, 2.5 mmol) in dioxane was stirred for 3 h at

reflux under Ar. The mixture was filtered and concentrated under reduced pressure. The

residue was purified by column chromatography on silica gel (eluent СH₂Cl₂/MeOH (0–1%

MeOH in СH₂Cl₂)). Beige solid. Yield 70% (174 mg). ν

_max/ cm^ꢀ12982 (СН), 2937 (СН),

2905 (СН), 1602, 1587, 1578, 1544, 1488, 1441, 1421, 1392, 1362, 1300, 1246 (Р = О),

1174, 1133, 1113, 1095, 1016 (РОС), 958 (РОС), 891, 837, 792, 780, 746. δ_H(300 MHz,

3

CDCl₃, 300 K) 1.38 (t, J_H,H7.1, 6Н, Me), 3.95 (s, 3Н, ОMe), 4.19 (m, 4Н, СН₂), 7.14 (d,

³J_H,H8.8, 2Н, mꢀН), 7.79 (ABꢀsystem, J_AB8.7, 2Н, 5,6ꢀН), 8.06 (dd, ³J _H,P13.0, ³J_H,H8.3, 2Н,

3

mꢀH), 8.13 (d, J_H,H8.7, 1Н, 8ꢀH), 8.18 (d, J_H,H8.7, 1Н, 3ꢀH), 8.30 (d, J_H,H₄8.7, 1Н, 7ꢀH),

8.37 (d, J_H,H8.7, 1Н, 4ꢀH), 8.45 (d, ³J_H,H8.8, 2Н, оꢀН), 8.57 (dd, J_H,H8.3, J_H,P3.9, 2Н, оꢀ

Н). δ_C(125 MHz, CDCl₃, 300 K) 16.4 (d, J_C,P6, 2C, Me), 55.4 (1C, OMe), 62.2 (d, J_C,P5,

2C, OCH₂), 114.3 (2C, mꢀC), 119.6 (1C, 8ꢀC), 120.0 (1C, 3ꢀC), 125.4 (1C, 6ꢀC), 126.7 (1C,

3

2

3

5ꢀC), 127.5 (1C, 4a or 6aꢀC), 127.7 (d, J_C,P15, 2C, oꢀC), 128.3 (1C, 4a or 6aꢀC),128.8 (d,

¹J_C,P187, 1C, ipsoꢀC), 129.0 (2C, oꢀC), 132.0 (1C, pꢀC), 132.4 (d, J_C,P10, 2C, mꢀC), 136.8

2

4

(1C, 7ꢀC), 137.1 (1C, 4ꢀC), 143.4 (d, J_C,P3, 1C, pꢀC),146.0 (1C, 10a or 10bꢀC), 146.1 (1C,

10a or 10bꢀC), 155.3 (1C, 2 or 9ꢀC), 156.4 (1C, 2 or 9ꢀC), 161.0 (1C, pꢀC). δ_P(121 MHz,

CDCl₃, 300 K) 18.82. HRMS (ESI): m/z: found: 521.1586; calc. for C₂₉H₂₇N₂NaO₄P

([M+Na]⁺): 521.1601.

Tetraethyl ((1,10-phenanthrolin-3,8-diyl)bis(4,1-phenylene))bis(phosphonate) (10a). A

mixture

of

3,8ꢀdibromoꢀ1,10ꢀphenanthroline

(169

mg,

0.5

mmol),

4ꢀ

(diethoxyphosphoryl)phenylboronic acid pinacol ester (425 mg, 1.25 mmol), Pd(OAc)₂(11.2

mg, 0.05 mmol), PPh₃(40 mg, 0.15 mmol) and Cs₂CO₃(815 mg, 2.5 mmol) in dioxane (4 ml)

was stirred at reflux for 40 h under Ar. The reaction mixture was cooled to room temperature

and filtered. The filtrate was concentrated under reduced pressure and the residue was purified

by column chromatography on silica gel (eluent – CH₂Cl₂). White solid. Yield 52% (155 mg).

ν

_max/ cm^ꢀ12983 (СН), 1603, 1556, 1475, 1432, 1392, 1365, 1244 (Р = О), 1163, 1132, 1099,

1042, 1014 (РОС), 959 (РОС), 937, 838, 815, 793, 762, 731. δ_H(300 MHz, CDCl₃, 300 K)

3

4

1.34 (t, J_H,H7.1, 12H, Me), 4.15 (m, 8Н, СН₂), 7.86 (dd, J_H,H8.3, J_H,P3.8, 2Н, оꢀН), 7.90

(s, 2Н, 5,6ꢀН), 7.98 (dd, ³J_H,P13.0, ³J_H,H8.3, 2Н, mꢀH), 8.42 (d, ⁴J_H,H2.3, 2Н, 4,7ꢀН), 9.42 (d,

⁴J_H,H2.3, 2Н, 2,9ꢀН). δ_C(125 MHz, CDCl₃, 300 K) 16.4 (d, ³J_C,P6 Hz, 4C, Me), 62.3 (d, ²J_C,P

5, 4C, OCH₂), 127.35 (2C, 5,6ꢀC), 127.6 (d, J_C,P13, 4C, oꢀC), 129.2 (d, J_C,P187, 2C, ipsoꢀ

C), 129.3 (2C, 4a,6aꢀC), 132.7 (d, J_C,P10, 4C, mꢀC), 133.9 (2C, 4,7ꢀC), 134.9 (2C, 3,8ꢀC),

3

1

2

4

141.4 (d, J_C,P3, 2C, pꢀC),145.5 (2C, 10a,10bꢀC) 149.4 (2C, 2,9ꢀC). δ_P(121 MHz, CDCl₃,

300 K) 18.14. HRMS (ESI): m/z found: 605.1944; calc. for C₃₂H₃₄N₂NaO₆P₂([M+Н]⁺):

605.1965.

Tetraethyl

((1,10-phenanthrolin-3,8-diyl)bis(([1,1'-biphenyl]-4',4-diyl)))bis-

(phosphonate) (11a). A mixture of 3,8ꢀdibromoꢀ1,10ꢀphenanthroline (169 mg, 0.5 mmol), 4ꢀ

((diethoxyphosphoryl)phenyl)ꢀ4’ꢀphenylboronic acid pinacol ester (520 mg, 1.25 mmol),

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Pd(OAc)₂(11.2 mg, 0.05 mmol), PPh₃(40 mg, 0.15 mmol) and Cs₂CO₃(815 mg, 2.5 mmol)

in dioxane (4 ml) was stirred at reflux for 24 h under Ar. The reaction mixture was cooled and

filtered. The filtrate was concentrated under reduced pressure. The residue was purified by

column chromatography on silica gel (eluent – CH₂Cl₂). White solid. Yield 76% (226 mg).

ν

_max/ cm^ꢀ12985 (СН), 1601, 1493, 1476, 1434, 1388, 1367, 1236 (Р = О), 1162, 1132, 1050,

1017 (РОС), 939, 917, 848, 822, 780, 733, 695. δ_H(300 MHz, CDCl₃, 300 K) 1.34 (t, J_H,H

3

7.1, 12H, Me), 4.15 (m, 8Н, СН₂), 7.72–7.94 (m, 18Н), 8.42 (d, ⁴J_H,H2.3, 2Н, 4,7ꢀН), 9.46 (d,

⁴J_H,H2.3, 2Н, 2,9ꢀН). δ_C(125 MHz, CDCl₃, 300 K) 16.4 (d, J_C,P6, 4C, Me), 62.2 (d, J_C,P

3

2

5.3, 4C, OCH₂), 126.9 (2C, 5,6ꢀC), 127.19 (d, ³J_C,P9, 4C, oꢀC), 127.75 (d, ¹J_C,P187, 2C, ipsoꢀ

3

2

C), 128.1 (d, J_C,P9, 8C, m,oꢀC), 129.33 (2C, 4a,6aꢀC), 132.5 (d, J_C,P9, 4C, mꢀC), 133.3

(2C, 4,7ꢀC), 135.1 (2C, 3,8ꢀC), 137.3 (2C, ArꢀC), 140.0 (2C, Ar ꢀC), 144.2 (d, ⁴J_C,P3, 2C, pꢀ

C),145.3 (2C, 10a,10bꢀC), 149.4 (2C, 2,9ꢀC). δ_P(121 MHz, CDCl₃, 300 K) 18.70. HRMS

(ESI) m/z: found: 779.2388; calc. for C₄₄H₄₂N₂NaO₆P₂([M+Н]⁺): 779.2410.

[Cu(8a)₂]PF₆(5a). Phenanthroline phosphonate 8a (125 mg, 0.25 mmol) was dissolved in

dichloromethane (CH₂Cl₂) (5 mL) under Ar. A solution of [Cu(CH₃CN)₄]PF₆(46.6 mg, 0.125

mmol) in CH₂Cl₂(10 mL) was prepared under Ar and then added with a syringe to the stirred

solution of the ligand. Then the reaction mixture was stirred at room temperature for 10 min,

cooled to room temperature and concentrated under reduced pressure. The solid residue was

dissolved in CH₂Cl₂(2 mL) and the solution was layered with diethyl ether (10 mL). A

precipitate was collected by filtration and then dried under vacuum. Brown crystals. Yield

80% (110 mg). ν

_max/ cm^ꢀ12982 (CH), 2906 (СН), 1605, 1552, 1545, 1490, 1422, 1391, 1358,

1322, 1303, 1247 (P = O), 1175, 1134, 1110, 1042, 1014 (POC), 959 (POC), 904, 867, 834,

782, 750, 722. δ_H(500 MHz, CD₂Cl₂, 300 K) 1.26 (t, ³J_H,H7.1, 12H, Me), 3.45 (s, 6Н, ОMe),

3.95 (m, 8Н, СН₂), 5.99 (d, ³J_H,H8.4, 4H, mꢀН), 7.04 (dd, ³J_H,P12.6, ³J_H,H8.0, 4H, mꢀH), 7.31

(d, ³J_H,H8.4, 4Н, оꢀН), 7.62 (dd, ³J_H,H8.0, ⁴J_H,P2.4, 4Н, оꢀН), 7.85 (d, ³J_H,H8.3, 2Н), 7.93 (d,

3

³J_H,H8.3, 2Н), 8.06 (s, 4Н), 8.48 (d, J_H,H8.3, 2Н), 8.59 (d, J_H,H8.3, 2Н). δ_P(121 MHz,

CD₂Cl₂, 300 K) 18.81. HRMS (ESI): m/z found 1059.2703; calc. for C₅₈H₅₄CuN₄O₈P₂: ([Mꢀ

PF₆]⁺) 1059.2707.

General procedure for the preparation of silyl phosphonate esters 6b-11b. To a 0.1 M

solution of diethyl phosphonate esters 6aꢀ11a in dry dichloromethane TMSBr (6 equiv. for

each diethoxyphosphoryl group) was added under Ar and the resulting mixture was stirred at

1

room temperature until a complete conversion of diethyl esters according H NMR

spectroscopy (24–48 h). Evaporation of volatiles under reduced pressure afforded silyl

phosphonate esters 6bꢀ11b in quantitative yield. The compounds were introduced in the next

step without additional purification.

Bis(trimethylsilyl) 1,10-phenanthrolin-3-ylphosphonate (6b). δ_H(300 MHz, CDCl₃, 300

K) 0.25 (s, 18H, Me), 7.65 (dd, ³J_H,H8.1, ³J_H,H4.3, 1H, 8ꢀH), 7.82 (d, ³J_H,H9.2 Hz, 1H, 6ꢀH),

3

7.85 (d, ³J_H,H9.2, 1H, 5ꢀH), 8.26 (dd, J_H,H8.1, ⁴J_H,H1.7, 1H, 7ꢀH), 8.68 (dd, J_H,P15.2, ⁴J_H,H

2.0, 1H, 4ꢀH), 9.17 (dd, J_H,H4.3, J_H,H1.7, 1H, 9ꢀH), 9.33 (dd, J_H,P5.4, ⁴J_H,H2.0, 1H, 9ꢀH).

3

4

3

δ_P(121 MHz, CDCl₃, 300 K) ꢀ3.61.

(1,10-Phenanthrolin-3-yl)phosphonic acid (6c). To a solution of 6a (79.1 mg, 0.25 mmol)

in dry CH₂Cl₂(2.5 mL) TMSBr (198 µL, 1.5 mmol) was added under Ar and the resulting

mixture was stirred at room temperature until a complete conversion of the diethyl ester

1

according H NMR spectroscopy (24 h). Then MeOH (100 µL) was added and the reaction

mixture was filtered. The precipitate was washed with MeOH (1 mL x 2), CH₂Cl₂(1 mL) and

dried under reduced pressure. White solid. Yield 98% (63.7 mg). Found C 55.27, H 3.35, N

10.98. Calc. for C₁₂H₉N₂O₃P: C 55.39; H 3.49; N 10.77 %. δ_H(300 MHz, CD₃OD, 300 K)

3

8.35–8.45 (m, 3H, 5,6,8ꢀH), 9.09 (d, J_HP14.5, 1H, 4ꢀH), 9.31–9.37 (br. m, 2H, 7,9ꢀH), 9.50

(d, ³J_HP5.5, 1H, 2ꢀH). δ_P(300 MHz, CD₃OD, 300 K) 12.16.

Immobilization of copper complexes Cu/Pphen.

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General procedure for the preparation of Cu/Pphen/SG xerogels. Silyl phosphonate ester 6b

or 9b was dissolved in THF and treated with solid Сu(PPh₃)₃Br at room temperature for 30

min to obtain complexes 1b or 3b, respectively. Then a 0.8 M solution of Ti(OiPr)₄in THF

was added to yield a transparent coloured solution. After stirring of the resulting mixture for

40 min, water diluted by THF (5 M solution) was added dropwise to this solution. A rapid

formation of gels or precipitates was observed. The reaction mixture was kept without stirring

for 48 h at room temperature. The precipitate was collected by filtration, washed with THF,

methanol and diethyl ether and dried under reduced pressure (2 mmHg) at 80 ^°C for 24 h. The

amount of reagents and solvents employed for the preparation of complexes 1b and 3b and in

the solꢀgel process, yields of Cu/Ppen/SG xerogels are summarized in Table S1. Elemental

analyses of xerogels Cu/Ppen/SG are reported in Tables S2 and S3.

1

31

Complex Cu(6b)(PPh₃)Br (1b) is sensitive to moisture and was characterized by H and P

spectroscopy in a crude mixture obtained after evaporation of volatiles. This mixture contains

the complex 1b and PPh₃.

Cu(6b)(PPh₃)Br (1b). δ_H(300 MHz, CDCl₃, 300 K) 0.21 (s, 18H, Me), 7.19 (m, 9H, m,pꢀH),

7.29 (m, 6H, oꢀH), 7.82 (br s, 1H, 8ꢀH), 8.05 (br. d, ³J_H,H8.8, 1H, 6ꢀH), 8.18 (br. d, ³J_H,H8.8,

1H, 5ꢀH), 8.65 (br s, 1H, 7ꢀH), 8.75 (br s, 1H, 4ꢀH), 8.80 (br s, 1H, 9ꢀH), 9.15 (br s, 1H, 2ꢀH).

δ_P(121 MHz, CDCl₃, 300 K) ꢀ4.67 (br s).

General procedure for the surface modification of mesoporous titanium oxide. Silyl

phosphonate esters 6b–8b were dissolved in CH₂Cl₂and treated with solid Сu(PPh₃)₃Br or

[Cu(CH₃CN)₄]PF₆at room temperature for 30 min under Ar to prepare complexes 1b, 2b, 4b

or 5b. After the reaction was completed, a transparent coloured solution was cannulated into a

flask containing TiO₂under Ar. The suspension was stirred for 2 days at room temperature.

The solid was collected by filtration, washed with THF, methanol and ether and dried for 24 h

at 80 ^oC under reduced pressure (2 mmHg). The amount of reagents and solvents used for the

preparation of complexes and the solꢀgel process are reported in Table S4. Elemental analyses

of solids Cu/Pphen/SM are summarized in Tables S5 and S6.

Immobilization of ligands PPhen-Si.

General procedure for the preparation of Pphen/SG xerogels. Silyl phosphonate ester 6b or

9b–11b was dissolved in THF. A 0.8 M solution of Ti(OiPr)₄in THF was added to this

solution and the reaction mixture was stirred for 40 min. Then water diluted by THF (5 M

solution) was added dropwise to this solution. A rapid formation of gels or precipitates was

observed. Then the reaction mixture was kept without stirring for 48 h at room temperature.

The precipitate was collected by filtration, washed with THF, methanol and diethyl ether and

°

dried under reduced pressure (2 mmHg) at 80 C for 24 h. The amount of reagents and

solvents used for the solꢀgel process and yields of xerogels Pphen/SG are reported in Table

S7. Elemental analyses of solids Pphen/SG are presented in Table S8.

Surface modification of mesoporous TiO₂by chelate 6b. Silyl phosphonate diester 6b

prepared from phosphonate 6a (316 mg, 1 mmol) was dissolved in CH₂Cl₂(20 mL) under Ar.

A transparent uncoloured solution was cannulated into a flask containing hydrated titanium

oxide (10 mmol) under Ar. The suspension was stirred for 48 h at room temperature. The

solid (959 mg) was collected by filtration, washed with THF, MeOH and ether and dried for

o

24 h at 80 C under reduced pressure (2 mmHg). The elemental analysis of solid 6b/SM is

reported in Tables S7 and S8.

Complex formation with grafted ligand 6b.

A solution of Сu(PPh₃)₃Br or [Cu(CH₃CN)₄]PF₆in CH₂Cl₂was added to material 6b/SM

placed into a Schlenk tube under Ar. The suspension was stirred for 24 h at room temperature.

The brown solids were collected by centrifugation, washed with CH₂Cl₂, MeOH and diethyl

ether and dried under reduced pressure (2 mmHg) for 24 h at 80 ^oC.

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The amount of reagents and yields of materials Cu/6b/SM are reported in Table S9.

Elemental analyses of Cu/6b/SM are summarized in Tables S10 and S11.

Catalytic reactions.

General procedure for Sonogashiraꢀtype coupling. A 8 ml glass vial was charged with alkyne

(0.75 mmol), aryl iodide (0.5 mmol), Cs₂CO₃(325.8 mg, 1 mmol), Cu/6b/SM/A (43 mg, 5

mol% calculated onto the grafted complex), PPh₃(13.1 mg, 10 mol%) and toluene (2 mL)

under Ar. A vial was closed with Teflon cap and the reaction mixture was stirred at reflux for

16 h. After cooling to room temperature, the organic phase was separated by centrifugation,

and the solid catalyst was washed with toluene (2 mL) and MeOH (5 mL x 2). The combined

toluene phases were evaporated to dryness under reduced pressure. The crude products were

analysed by ¹H NMR spectroscopy. Mesitylene was used as an internal standard. The spectral

data of coupling products were in good agreement with the literature data.^70ꢀ75The results are

summarizes in Table 5 (entries 1, 7ꢀ16) and Scheme 6.

Recycling of Cu/6b/SM/A in the reaction of phenylacetylene with pꢀiodoanisole was

carried out. After washing with toluene and MeOH, the catalyst was dried under reduced

pressure for 3 h and used in the next reaction cycle as reported above for the freshly prepared

Cu/6b/SM/A. The results are summarized in Table 5 (entries 3ꢀ6).

A hot filtration test was performed for the reaction of phenylacetylene with pꢀ

iodoanisole (Table 5, entry 2). After 1 h of heating, a half of the reaction mixture was taken

by syringe equipped with a Acrodisc® syringe filter with Supor® membrane (pore size 10

ꢁm) and transferred into a 8 ml glass vial under Ar. The vial was charged with Cs₂CO₃(163

mg, 0.5 mmol), PPh₃(6.6 mg, 10 mol%). Then both reaction mixtures were stirred at reflux

for additional 12 h and monitored by ¹H NMR spectroscopy. The results are shown in Figure

S36.

Reaction of phenylacetylene with pꢀiodoanisole (Table 5, entry 2) was also performed

using materials 3b/SG/20 and 3b/SG/10 as catalysts. The material loading was calculated

using copper content to obtain 5 mol% of grafted complex. In both cases, complete

conversion of aryl halide was not achieved even after 72 h of reflux.

The compare the catalytic activity of materials 1b/SM and 2b/SM, the reaction of

phenylacetylene with pꢀiodoanisole (Table 5, entry 2) was performed in the presence of the

catalysts containing 5 mol% of grafted ligands. The conversion of aryl halide was 99 and 60

%, respectively, after 72 h of heating. Catalytic properties of materials 4b/SM and 5b/SM

with respect to this reaction were also investigated using 5 mol% of grafted catalysts. For

both materials, no coupling products were obtained after 16 h of reflux.

General procedure for Huisgen cycloaddition. A 8 ml glass vial was charged with alkyne

(0.25 mmol), azide (0.25 mmol), triethylamine (34.7 µL, 0.25 mmol), Cu/6b/SM/A (4.3 mg,

1 mol%, calculated onto the grafted complex) and fresh distilled THF (1 mL) under Ar. A vial

o

was closed with Teflon cap and the reaction mixture was stirred at 60 C for 3 h. After

cooling to room temperature, the organic phase was separated by centrifugation, and the solid

catalyst was washed with THF (1 mL), methanol (1 mL x 2) and separated by centrifugation.

The combined THF phases were evaporated to dryness under reduced pressure. The crude

products were analysed by using ¹H NMR spectroscopy. Mesitylene was added as an internal

standard. The spectral data of obtained products were in good agreement with the literature

data.^76ꢀ80The product yields are presented in Table 6.

A hot filtration test was performed for the reaction of phenylacetylene with pꢀ

nitrobenzyl azide (Table 6, entry 1). After 1h of heating, a half of the reaction mixture was

taken by syringe equipped with a Acrodisc® syringe filter with Supor® membrane (pore size

10 ꢁm) and introduced into a 8 ml glass vial under Ar. Then both reaction mixtures were

1

stirred at reflux for additional 2 h and monitored by H NMR spectroscopy. The results are

shown in Figure S37.

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Recycling of Cu/6b/SM/A in the reaction of phenylacetylene with pꢀnitrobenzyl azide

(Table 6, entry 1) was carried out. After washing, the catalyst was separated by centrifugation

and dried under reduced pressure for 3 h and used in the next reaction cycle as reported above

for the freshly prepared Cu/6b/SM/A. The product yields in 10 consecutive catalytic

reactions are shown in Figure 5.

Results and discussion

Preparation of heterogenized catalysts

Synthesis of copper(I) complexes bearing phosphonate anchoring groups. Recently we

reported the synthesis of air and moisture stable copper(I) complexes with

(diethoxyphosporyl)phenanthroline and triphenylphosphine ligands (6a, 7a and 9a) which

catalyzed C–C and C–Het (Het = N, P) crossꢀcoupling reactions.^64,65,81These complexes

could also be useful for the preparation of heterogeneous catalysts if the diethoxyphosphoryl

substituent could be transformed into more reactive phosphonic acid or

bis(trimethylsiloxy)phosphoryl groups (Scheme 1).^53,82,83

Generally, the reaction of alkyl phosphonic acid diesters with TMSBr affords

trimethylsilyl (TMS) diesters in high yields under mild conditions. However, when a solution

of complex 1a was reacted with TMSBr in CH₂Cl₂at room temperature a complicated

31

mixture of products was obtained according to P NMR analysis. Therefore, we decided to

introduced the phosphorous anchoring group before the insertion of copper ions into chelators

6ꢀ11 (Scheme 2).

Scheme 2. Synthesis of molecular building blocks for the preparation of hybrid materials: silyl esters 6bꢀ11b,

phosphonic acid 6c and copper complexes 1bꢀ5b.

First, phosphonic acid 6c was chosen as a model ligand. This compound was prepared

in high yield (98%) by the treatment of diester 6a with TMSBr followed by addition of

MeOH. The acid 6c was stable in air but hardly soluble in all organic solvents and aqueous

media. All our attempts to prepare copper complexes reacting this ditopic chelator with

tris(triphenylphosphine)copper(I) bromide (Cu(PPh₃)₃Br) and tetrakis(acetonitrile)copper(I)

hexafluorophosphate ([Cu(CH₃CN)₄]PF₆) were unsuccessful, probably due to a low

selectivity of the complexation reaction under studied experimental conditions.

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Next, TMS (1,10ꢀphenanthrolinyl)phosphonic acid diesters 6bꢀ9b were prepared by

reacting compounds 6aꢀ9a with TMSBr in dichloromethane at room temperature when

1

monitored by H NMR spectroscopy. These moisture sensitive compounds were obtained in

quantitative yields and introduced in the next step without additional purification. Thus, after

the reaction of 6a,7a and 9a with TMSBr run to completion, the volatiles were evaporated to

dryness and the residue was reacted with Cu(PPh₃)₃Br in CH₂Cl₂to prepare heteroleptic

complexes 1bꢀ3b. Alternatively, ligands 8b and 9b were treated with [Cu(CH₃CN)₄]PF₆to

yield homoleptic bis(phenanthroline) copper(I) complexes 5b and 4b, respectively. The

progress of complexation was monitored by ³¹P NMR spectroscopy using the difference in the

chemical shift of phosphorous nuclei of ligands 6b–9b and corresponding complexes 1b–5b

(ꢁ

δ 1–2 ppm). After a full consumption of chelators, the volatiles were evaporated and the

residues were introduced in the SG process (route A, Scheme 1) or anchored onto a

mesoporous titania support prepared by us recently⁶³(route B, Scheme 1).

Preparation of xerogels from copper(I) complexes 1b and 3b (route A, Scheme 1). The

first series of materials has been prepared from heteroleptic complexes 1b and 3b and

Ti(OiPr)₄according to the twoꢀstep SG process^54,84as shown in Scheme 3. This oneꢀpot coꢀ

condensation route provides a homogeneous distribution of functional groups within the

hybrid solid. Copper complexes 1b and 3b were reacted with an excess of Ti(OiPr)₄in THF

to form titanium phosphonates (Step I). Subsequently, the hydrolysis, condensation and coꢀ

condensation reactions (Step II) were initiated by a dropwise addition of water diluted by

THF to form a polymeric titania network containing covalently linked copper complexes. A

gel which was formed immediately was aged at room temperature for two additional days.

Then the solid was filtered, washed and dry at 80 °C under reduced pressure for 24 h.

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Scheme 3. Representative reactions involved in the preparation of hybrid xerogels Cu/Pphen/SG.

Five different materials Cu/1b/SG/n (n = 3, 10) and Cu/3b/SG/n (n = 3, 10, 20) were

prepared varying the Ti:Cu molar ratio from 3 to 20 as indicated in Table 1 (see also the

Supporting Information, Table S1). The xerogels were first characterized by elemental

analysis, inductively coupled plasma optical emission spectroscopy (ICPꢀOES),

thermogravimetric (TG) analysis, nitrogen adsorption–desorption at 77 K and FTIR

spectroscopy. The molar ratio of components used for the material preparation, the chemical

composition of xerogels and the

Table 1. Chemical composition and surface properties of xerogels Cu/Pphen/SG.

Enꢀ Xerogel

try

Chemical composition of the xerogel

Pphen : Cu :

S_BET

Cu/Pphen

: Ti(OiPr)₄

molar ratio

in the

TiO₂

[m²g^ꢀ1]

Cu/Pphen/SG

Pphen

molar

ratio in

synthesis

the xeroꢀ the

gel^[a]

xeroꢀ

gel^[b]

1

2

3

4

5

1:10

1:3

1:20

1:10

1:3

(Cu(PPh₃)Br)_0.34(C₁₂H₇N₂O₂P)(TiO₂)₁₀(H₂O)_9.2(C₃H₇OH)_3.51:10

(Cu(PPh₃)Br) _0.65(C₁₂H₇N₂O₂P)(TiO₂)_3.6(H₂O)₈(C₃H₇OH)_3.51:3.6

0.34:1 10

0.65:1

0.38:1 270

0.40:1 145

1b/SG/10

1b/SG/3

3b/SG/20

3b/SG/10

3b/SG/3

0

(Cu(PPh₃)Br) _0.38(C₁₂H₆N₂O₄P₂)(TiO₂)₁₈(H₂O)₄₀(C₃H₇OH)₅

1:18

(Cu(PPh₃)Br)_0.40(C₁₂H₆N₂O₄P₂)(TiO₂)₁₀(H₂O)₂₀(C₃H₇OH)_3.41:10

(Cu(PPh₃)Br)_0.40(C₁₂H₆N₂O₄P₂)(TiO₂)_2.6(H₂O)_10.7(C₃H₇OH)_1.71:2.6

0.40:1

0

[a] The molar ratio was calculated from the P, N, Ti content determined by elemental analysis and ICPꢀOES. [b]

The molar ratio was calculated from the Cu, P, N content determined by elemental analysis and ICPꢀOES.

Expected Cu:Pphen ratio is 1:1.

calculated Brunauer–Emmett–Teller (BET) surface area are summarized in Table 1.

Additional data on the material composition and their surface properties are presented in

Tables S2, S3 and S12. The formulas of materials were derived from the content of six

elements (C, H, N, P, Ti, Cu) assuming the presence of adsorbed water and isopropyl alcohol

molecules that is obvious for xerogel samples. As seen in Table 1, the Pphen:TiO₂molar

ratio in materials was close to that of the starting compounds used in the reactions. In contrast,

the content of copper atoms in all five Cu/Pphen/SG solids was lower than expected values.

These data indicated that complexes 1b and 3b partially dissociated during the SG process.

The successful incorporation of PPhen ligands in Cu/Pphen/SG xerogels was further

confirmed by the analysis of filtrates obtained after the washing step. Combined liquid phases

were evaporated to dryness and analysed by ³¹P and ¹H NMR spectroscopies. The absence of

signal sets corresponding to phen derivatives indicated that the incorporation of Pphen into

the xerogels was quantitative. Thus, resulting hybrid materials contain the Pphen and

Cu(Pphen)(PPh₃)Br moieties linked to titania networks.

The presence of adsorbed water and isopropanol molecules in xerogels 1b/SG/n and

3b/SG/n was confirmed by TG analysis. In general, thermal patterns were similar for all

studied samples. As an example, the TG curve of xerogel 3b/SG/20 is shown in Figure S22.

A progressive weight loss comprised between 70 and 530 °C was observed. Accordingly, the

separation of the initial loss of adsorbed solvents and the subsequent calcination of organic

components was unclear. This thermal behaviour is a typical feature of titania xerogels in

which solvent molecules are strongly chemisorbed on the titania surface.^85ꢀ87Nevertheless,

semiquantitative consideration of thermal data indicated that the loss of the initial weight for

all samples was about 10–15% when the temperature was increased up to 200–250 °C, that

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was about a half of maximal percentages of adsorbed solvents calculated from the chemical

composition of the solids (Table 1). This difference can be explained by the presence of

residual nonꢀhydrolysed isopropoxy groups (iPrO–Ti) and nonꢀcondensed hydroxy groups

(HO–Ti). These moieties are involved in the thermal reactions (condensation and

decomposition) only when the temperature rises above 250 °C.

To prove the structure of organic moieties embedded into xerogels 1b/SG/n and

3b/SG/n and their covalent link to titania support, FTIR spectra of the solids were recorded

(Figure 2, S23 and S24). Roughly, the spectra of all hybrid solids were remarkably similar in

appearance. In Figure 2, FTIR spectrum of 3b/SG/3 is compared to those of the relevant

ligand 9a and the heteroleptic copper complex 3a.

Bands having the greatest intensities are located between 900 and 1260 cm^ꢀ1and

associated with vibrations of the heteroaromatic moiety and its phosphonate substituent.^88ꢀ90

Notably, the shape of the 3b/SG/3 spectrum in this region is significantly different from those

of ligand 9a and complex 3a that points to the covalent bonding of the phosphonate group to

the titania matrix. In particular, a strong broad band in 950–1150 cm^ꢀ1region observed for

material 3b/SG/3 is commonly associated with MetalꢀOꢀP stretching vibrations.^91,92

Moreover, characteristic vibrations of phen heterocycle are also observed in two frequency

regions, namely 700–900 and 1350–1600 cm^ꢀ1in all three spectra. Despite the overlapping of

Pphen and PPh₃vibration bands, the latter can be identified owing to the presence of a

relatively strong band at 1430 cm^ꢀ1typical of the vibration of the phenyl ring directly attached

to a phosphorous atom and a medium band at 690 cm^ꢀ1.⁹³The O–H stretching bands at 3500–

3700 cm^ꢀ1, associated with Ti–OH groups and adsorbed water, and weak bands in 2900–3100

cm^ꢀ1region, characteristic of the aromatic and aliphatic C–H bonds are also present in the

spectrum of 3b/SG/3.

A

B

C

Figure 2. FTIR spectra of 9a(A), 3a(B) and 3b/SG/3(C).

It has to be noted, that stretching vibrations of the phosphonate group in 950–1250 cm^ꢀ

¹region were commonly used for disclosing of the binding mode of phosphonate molecules to

a titania network.^83,91,92However, recently Blockhuas et al. have demonstrated that FTIR

spectroscopy is not well suited for the investigation of the bonding mode in hybrid materials

because P–O and P=O stretches are also depend on hydrogen bonding.⁹⁴For solids 1b/SG/n

and 3b/SG/n, this analysis is even more complicated due to the overlapping these stretches

with vibrations of the heterocyclic moiety. For example, a weak band at approximately 1250

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cm^ꢀ1can be assigned to vibration of the uncoordinated P=O group or the heteroaromatic

fragment.

Powder Xꢀray diffraction measurements indicate that all materials are nonꢀcrystalline.

The porosity of xerogels 1b/SG/n and 3b/SG/n was examined by nitrogen adsorption–

desorption at 77 K. Interestingly, BET surface areas were dependent on the number of

phosphonate groups at phen ligands and the amount of Ti(OiPr)₄employed for the

incorporation of complexes. Solids 1b/SG/3 and 1b/SG/10 containing monophosphonate

residues were nonꢀporous. Material 3b/SG/20 prepared from the copper complex with

(phen)diphosphonate 3b and 20 equiv of Ti(OiPr)₄exhibited a microꢀ and mesoporosity and a

remarkable specific surface area (BET surface area of 270 m²g^ꢀ1). However, in a series of

xerogels 3b/SG/20–3b/SG/3, this value dropped drastically (Table 1 and S12) with the

decrease of titania percentage that is a serious drawback for their applications in catalysis.

To increase porosity of hybrid materials and the content of copper ions, another

strategy of immobilization was investigated. The complexes were grafted onto a surface of

preꢀformed TiO₂.

Surface modification of TiO₂by copper(I) complexes (route B, Scheme 1). Recently we

have reported the preparation of nonꢀordered mesoporous titania exhibited a remarkable

specific surface area of 580–650 m²g^ꢀ1and pore volumes of 0.5–1.2 cm³g^ꢀ1.⁶³This costꢀ

effective hydrated TiO₂is readily available according to a templateꢀfree SG process and

promising for the preparation of heterogeneous catalysts. A surface modification of TiO₂by

phosphonates is shown in Scheme 4. Silyl phosphonate diesters are reacted with a preꢀformed

titania support bearing reactive hydroxy groups. Accordingly, water is no more needed for the

immobilization and the SM reaction can be performed in all organic solvents in which the

targeted complexes can be dissolved. Copper complexes 1b and 2b are readily soluble in

chlorinated solvents and we chose weakly coordinating CH₂Cl₂as a reaction medium to

prevent the dissociation of copper complexes.

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Scheme 4. Schematic representation of grafting of copper(I) complexes with Pphen-Si ligands onto a titania

support.

Complexes 1b and 2b smoothly reacted with TiO₂powders in CH₂Cl₂at room

temperature. Hybrid solids Cu/Pphen/SM were filtrated after 48 h of stirring, dried at 80 °C

under reduced pressure and examined by elemental and ICPꢀOES analyses (Tables 2, S4ꢀS6).

Table 2. Chemical composition and surface properties of xerogels Cu/Pphen/SM.

Enꢀ Material

try

Chemical composition of the xerogel

Pphen:

TiO₂molar

Cu :

Pphen

S_BET

Cu/Pphen

: TiO₂

[a]

[m²g^ꢀ1]

molar

ratio in the

synthesis

ratio in the molar

solid^[b]

ratio in

the solid^[c]

1

2

3

1:10

(Cu(PPh₃)Br) _0.18(C₁₂H₇N₂O₂P)(TiO₂)_9.7(H₂O)₁₁(C₃H₇OH)_0.81:9.7

(Cu(PPh₃)Br) _0.14(C₁₂H₇N₂O₂P) (TiO₂)_9.5(H₂O)₁₁(C₃H₇OH) 1:9.5

0.18:1^[d]317

0.14:1^[d]nd^[e]

1b/SM

2b/SM

4b/SM

Cu(C₁₂H₇N₂O₂P)₂PF₆(TiO₂)₁₁(H₂O)₁₈(C₃H₇OH)_0.6

1:11

1:2^[f]

290

4

1:20

Cu(C₂₅H₁₇N₂O₂P)₂PF₆(TiO₂)₁₉(H₂O)₂₂

1:19

270

5b/SM

[a] A TiO₂powder with BET surface areas of 650 m²g^ꢀ1was prepared according to the solꢀgel process. [b] The

molar ratio was calculated from the P, N, Ti content determined by elemental analysis and ICPꢀOES. [c] The

molar ratio was calculated from the Cu, P, N content determined by elemental analysis and ICPꢀOES. [d]

Expected Cu:Pphen ratio is 1:1. [e] Not determined. [f] Expected Cu:Pphen ratio is 1:2.

The Ti:N and Ti:P molar ratios indicated quantitative grafting of Pphen ligands.

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Surprisingly, Cu:Pphen molar ratio was low in both materials as it was previously observed

for Cu/Ppen/SG xerogels. Thus, once again resulting hybrid materials contain Pphen and

Cu(Pphen)(PPh₃)Br moieties linked to the titania support. These results can be explained by

1

kinetic lability of heteroleptic copper complexes (Scheme 5). In fact, according to H and ³¹P

NMR studies of chelates 1 and 2 only one major species exists in CDCl₃solution for each of

these complexes (Figures S1ꢀS4, S16 and S17). In these mononuclear species, phen moieties

are coordinated to copper centres by two nitrogen atoms and a rapid exchange of Pphen

1

ligands is highly probable.⁶⁵For example, line broadening in the room temperature H NMR

spectrum of complex 1b could be caused by the ligand exchange (Figure S16). The

immobilization of the Pphen ligand which could be present in solution in a low concentration

should proceed more rapidly than the incorporation of bulky heteroleptic complex Cu/Pphen.

Accordingly, the complexation equilibrium could be moved to the nonꢀcoordinated ligand that

should provide for the formation of material modified by free chelator moieties.

Scheme 5. Schematic representation of Cu/Pphen complexes grafting on titania support.

To check this hypothesis, kinetically more stable⁶⁵homoleptic complexes 4b and 5b

were grafted onto TiO₂surface (Table 2). As evidenced by Cu:Ti, Cu:P, Cu:N molar ratios

obtained for these materials (Tables S6), the immobilization was successful in both cases.

FTIR spectra of solids 4b/SM and 5b/SM shown in Figures S25ꢀS26 corroborated the

proposed molecular structure of materials.

Porosity of Cu/Pphen/SM materials was studied by N₂sorption measurements (Table

2 and S12). The shapes of all isotherms were similar and resembled to that of the nonꢀ

modified titania support. In contrast, BET surface areas of hybrid materials were smaller

compared to that of the asꢀsynthesized TiO₂. This indicates the successful grafting of

complexes onto a surface of mesopores.

Being unable to overcome the dissociation of heteroleptic copper complexes, we have

turned to a stepwise strategy for the preparation of heterogeneous catalysts. This method

involves the incorporation of free Pphen ligands into a titania support followed by their

complexation with copper ions.

Immobilization of Pphen-Si ligands followed by the insertion of copper ions (routes C

and D, Scheme 1). The first series of materials have been prepared according to the SG

process as described above for copper complexes, with the only difference that Pphen-Si

chelators were used as molecular precursors (Scheme 1 and 3). Materials 6b/SG and 9b/SG

was characterized by elemental analysis, ICPꢀOES, nitrogen adsorption–desorption at 77 K

(Table 3, S7 and S8, entries 1 and 2). As seen from these data, Ti:N and Ti:P molar ratios are

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very close to theoretical values. However, both solids display a pretty low BET surface area

of 215 and 160 m²g^ꢀ1, respectively. Our attempt to increase a xerogel porosity by using

bulkier ligands in which the phen moiety and phosphonate substituents were separated by

phenyl spacers (compounds 10b and 11b, Figure 1) have met with only a partial success

(Table 3, entries 3 and 4). The BET surface areas of xerogels 10b/SG and 11b/SG were 270

and 300 m²g^ꢀ1, respectively, even when Ti(OiPr)₄was used in a large excess (20 equiv).

Table 3. Chemical composition and surface properties of xerogels Pphen/SG.

Enꢀ Material

Pphen:

Chemical composition of the material

Pphen:TiO₂

S_BET

try

Ti(OiPr)₄

molar ratio in

the synthesis

1:10

molar ratio in [m²g^ꢀ1]

the solid^[a]

Pphen/SG

1

(С₁₂H₇N₂O₂P)(TiO₂)_9.2(H₂O)₁₂(C₃H₇OH)_1.91:9.2

215

6b/SG

2

3

4

1:10

1:20

(С₁₂H₆N₂O₄P₂)(TiO₂)₁₀(H₂O)_19.5(C₃H₇OH)_1.21:10

(С₂₄H₁₄N₂O₄P₂)(TiO₂)_17.9(H₂O)₂₇(C₃H₇OH)_3.51:17.9

(С₃₆H₂₂N₂O₄P₂)(TiO₂)_18.8(H₂O)₃₇(C₃H₇OH)_0.91:18.8

160

270

300

9b/SG

10b/SG

11b/SG

[a] The molar ratio was calculated from the P, N, Ti content determined by elemental analysis and ICPꢀOES.

Next, silyl phosphonate 1b was grafted onto a surface of TiO₂in CH₂Cl₂at room temperature

as described above for copper complexes. As expected, the reaction proceeds quantitatively

yielding porous hybrid material 6b/SM which was firstly characterized by elemental analysis

(Table S7 and S8), EDX spectrometry, FTIR (Figure S31) and ³¹P MAS NMR spectroscopies.

In particular, the ³¹P MAS NMR spectrum shows a broad signal typical of nonꢀordered solids

based on titania at about 5 ppm (Figure 3). The ³¹P chemical shifts in amorphous titania

phosphates were reported between ꢀ21 and ꢀ4 ppm.⁹⁵Thus, the cleavage of C–P bond was not

observed under these experimental conditions and phosphorous atoms in the material are

bonded to the phen scaffold and the three oxygen atoms. Phosphorous signals of layered

titanium phosphonates are obviously observed at ꢀ4 ppm as sharp signals⁹⁵that doesn't match

with our spectrum. These data indicate that phen moieties are separated one from others onto

the titania surface. The experimental spectrum of 6b/SM was deconvoluted as shown in

Figure 3. According to this analysis, the experimentally observed signal is a superposition of

three sharp resonances originated from nonꢀequivalent phosphorous sites. Based on relative

positions of three signals in ³¹P MAS NMR spectra of titaniaꢀsupported phenylphosphonic

acid,^83,94,96the major signal (

exhibited a tridentate binding mode and the two other signals were assigned to monoꢀ (

ppm) and bidentate ( 7.2 ppm) phosphonate groups.

δ

4.8 ppm, 80%) was attributed to the phosphonate group

ꢀ2.7

δ

Interestingly, qualitative EDXꢀSTEM analysis of this material indicated the presence

of Br among with the other expected elements (Figures S34 and S35). We have assumed that

the phen ligand was partially protonated by traces of hydrobromic acid when it was reacted

with TMSBr.

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spectre

AMI-303

³¹P MAS

4mm 14kHz

fit

Bandes de rotation

*

4,8ppm

80%

7,2pm

6%

*

-2,7ppm

14%

*

120

100

80

60

40

20

0

-20

-40

-60

-80

-100

(ppm)

Figure 3. Experimantal (blue) and simulated (red) ³¹P MAS NMR spectrum of material 6b/SM. The stars

indicate

spinning sidebands.

Nitrogen adsorption–desorption isotherms of solid 6b/SM and bare TiO₂support are

presented in Figure 4A and pore size distributions for these materials are shown in Figure 4B.

The features of two isotherms are very similar. They demonstrate a gradual increase in the

adsorbed nitrogen volume in function of the relative pressure making a hysteresis loop. The

morphology of the curves can be considered as a combination of type II and type IV

isotherms that are typical for nonꢀordered porous solids containing both mesopores and

macropores with a large distribution of the pore size. The BET surface area was lowered from

650 to 366 m²g^ꢀ1after grafting the ligand 6b. This surface modification has also caused a

significant decrease in the amount of small mesopores in TiO₂(Figure 4B).

A

B

1000

900

800

700

600

500

400

300

200

100

0

0.2

0.4

0.6

0.8

1

P/P₀

Figure 4. Nitrogen adsorption–desorption studies: isotherms for asꢀsynthesized hydrated TiO₂(black), solid

6b/SM (red) and Cu/6b/SM/A (blue) (A); pore size distribution (BJH calculations) for as synthesized TiO₂

(black), 6b/SM (red) and Cu/6b/SM/A (blue) (B).

As can be seen from Tables 3 and S12, porosity of material 6b/SM was superior to

those of hybrid materials Pphen/SG prepared by the SG method. In addition, in this material,

chelator moieties are located on the titania surface and more accessible for reactant molecules

than in Pphen/SG materials in which the ligands are buried inside the solids. Thus, 6b/SM

material was chosen for the preparation of the heterogeneous catalyst. The insertion of copper

ions was performed by stirring this solid with various copper(I) complexes in CH₂Cl₂at room

temperature (Table 4). The chelation was sluggish due to a strong steric hindrances induced

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by the solid support and/or the partial protonation of the titaniaꢀsupported phen ligand. First,

solid 6b/SM was reacted with 1.1 equiv of Cu(PPh₃)₃Br which we previously used to prepare

related Pphen complexes 1ꢀ3 under homogeneous conditions. According to the elemental

analysis and ICPꢀOES data for resulting materials, Cu:Pphen molar ratio was only 0.16 (entry

1). When Cu(PPh₃)₃Br amount was increased up to 2.6 equiv, the complexation of 29% of the

grafted ligand was achieved (entry 2). Fortunately, [Cu(MeCN)₄]PF₆complex bearing less

bulky ligands was more reactive and gave the targeted material Cu/6b/SM/A (entry 3).

Empirical formula of this material was derived from its elemental analysis (Table S10 and

S11). In addition, the presence of fluor in the powder was confirmed by EDXꢀTEM analysis

(Figure S36). It has to be also noted, that brom was not found in the studied sample. These

data are in agreement with our hypothesis that the titaniaꢀsupported phen ligand is partially

protonated in the material 6b/SM. Accordingly, bromine anion is no more present in the solid

after the complexation reaction.

Table 4. Insertion of copper(I) ions into material 6b/SM.

Entry

Solid

Copper complex (equiv of grafted 6b)

Cu:6b

molar ratio^[a]

0.16:1

S_BET

[m²g^ꢀ1]

1

2

3

Cu(PPh₃)₃Br (1.1)

Cu(PPh₃)₃Br (2.6)

Cu(MeCN)₄PF₆(2.6)

nd^[b]

Cu/6b/SM/P1

Cu/6b/SM/P2

Cu/6b/SM/A

0.29:1

1:1

nd^[b]

243

[a] The molar ratio was calculated from the Cu, P, N content determined by elemental analysis and ICPꢀOES.

[b]Not determined.

Nitrogen adsorption–desorption isotherm of Cu/6b/SM/A at 77 K is presented in

Figure 4A and compared to those of bare titania support and starting material 6b/SM. Upon

the stepwise derivatization of TiO₂, no change in the shape of isotherms was observed,

whereas a marked decrease in the BET surface area (from 650 to 246 m²g^ꢀ1, Table S12) and

pore volume (from 1.36 to 0.55 cm³g^ꢀ1, Table S12) was noted, which is consistent with the

presence of a significant amount of grafted complex on the surface.

The morphology of solids 6b/SM and Cu/6b/SM/A was studied by SEM and TEM

microphotographies and compared to that of the bare TiO₂support. SEM images are shown in

Figure 5. Asꢀsynthesized hydrated TiO₂is composed of the strongly aggregated nanoparticles

displaying a similar shapes and quite narrow distribution of the grain size. As seen in Figure

5, grafting of the Pphen ligand (material 6b/SM) and the following insertion of copper(I) ions

(material Cu/6b/SM/A) do not influence on the morphology of solids. TEM images of 6b/SM

and Cu/6b/SM/A are shown in Figure S33. The mesoporous nanospheroids with a diameter

ranging from 5 to 20 nm are irregularly distributed in the space and separated by large widths

of hundreds of nanometres. Calculated external specific surface area for nonꢀaggregated

anatase nanoparticles of this size is in the range of 75–300 m²g^ꢀ1that evidences the presence

of interior mesopores in at least bare TiO₂(S_BET= 650 m²g^ꢀ1). This morphology perfectly fits

the catalytic application providing the accessibility of catalytic sites and a mass transfer of

reagents and products through large canals separating the nanoparticles. Powder Xꢀray

measurements demonstrate that these samples are nonꢀcrystalline (Figure S37).

A

B

C

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100 nm

10 nm

100 nm

Figure 5. SEM microphotographs of bare hydrated TiO₂(A), solid 6b/SM (B) and material Cu/6b/SM/A (C).

Upper microphotographs of each series were obtained from powders deposited on a silicon wafer as ethanol

suspensions. Bottom images were observed using samples prepared by the dispersion the materials on a

conductive carbon tape and carbon coating (8 nm).

All Cu/Pphen/SG and Cu/Pphen/SM materials except 5b/SM change their brown

colour to greenꢀblue being exposure to air during a few weeks. Oxidation of copper(I) ions

was assumed and proved by EPR spectroscopy and the diffuse reflectance spectroscopy. As a

representative example, EPR spectrum of Cu/6b/SM/A stored in the air is presented in Figure

S38. The spectrum recorded in Xꢀband frequencies at 100 K shows a broad line in the region

of 2500–3600 G due to an intermolecular spin exchange caused by spin coupling between

paramagnetic copper(II) centres located in close proximity. As a result, the anisotropic g_i

values cannot be determined by simulation experiments due to the broad linewidth that

precluded any studies of a copper ion environment in the solid state. We also observed a very

large and weak band in the region of 700–950 nm in the diffuse reflectance spectrum of

Cu/6b/SM/A due to dꢀd electron transitions in copper(II) complexes.^97,98However, a weak

intensity of this band and its broadness also excluded any conclusions on the copper(II) ion

environment (Figure S39).

Catalytic Reactions

Sonogashira coupling. Catalytic crossꢀcoupling reaction of aryl halide with terminal

acetylenes is a wellꢀknown and useful method in organic synthesis which is frequently

employed for the synthesis of biologically active molecules, heterocycles, natural products, or

polymers.^99,100Commonly, this reaction is performed in anhydrous solvents in the presence of

tertiary amines, palladium catalysts and with copper salts as coꢀcatalysts. Palladiumꢀfree

conditions were also widely explored^99,101ꢀ103but heterogenized copper complexes are still

limited by a few examples.^47,104ꢀ107

In the preliminary experiment, a model reaction of pꢀiodoanisole with phenylacetylene

in toluene at 110 °C was performed in the presence of cesium carbonate and 5 mol% of

Cu/6b/SM/A (Table 5, entry 1). The reaction proceeded slowly yielding the product of C–C

coupling in 27 and 77 % yield after 16 and 72 h, respectively. When 10 mol% of PPh₃was

added, the reaction afforded the target product 12 in 98% yield after 16 h of reflux. The

reaction performed in the dark proceeds as well as in the daylight. This indicates that any

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photocatalytic processes were not observed under studied conditions. To determine whether a

nonꢀsupported copper catalyst could catalyse the reaction, a hot filtration test was performed

(Figure S40). The filtered solution didn't show any catalytic activity that testified a

heterogeneous catalytic process. The positive role of PPh₃in this heterogeneous reaction can

stem from its influence on the structure of intermediate complexes involving in the catalytic

cycle or its ability to reduce inactive Cu(II) complexes to copper(I) chelates.¹⁰⁸

To prove that grafted phenanthroline complexes are true catalytic centres and clarify

the role of phen ligand, titaniaꢀsupported Cu(MeCN)₄PF₆was prepared by the impregnation

of TiO₂powder in CH₂Cl₂solution of this complex at room temperature. This material and

nonꢀmodified titatium support didn't promote the studied reaction indicating that titaniaꢀ

supported Cu/Pphen complexes are involved in the catalytic cycle.

Next, the reaction scope was explored. Coupling products 12 were obtained in nearꢀ

quantitative yield starting from aryl halides with electronꢀdonating or electronꢀwithdrawing

Table 5. Reaction of alkynes with aryl iodides catalysed by Cu/6b/SM/A. ^[a]

Entry

1^[c]

Alkyne

Aryl halide

Product

Yield^[b][%]

27^[d]

2(1st cycle)

3(2nd cycle)

4(3d cycle)

5(4th cycle)

6(5th cycle)

7

99

98

99

8

99

9

10

11

12

13

14

15

16

99

[a] Reaction conditions: 0.75 mmol of the alkyne, 0.5 mmol of the aryl halide, 1 mmol of Cs₂CO₃, 5 mol%

Cu/6b/SM/A (calculated onto the grafted complex) and 10 mol% of PPh₃were refluxed in toluene (2 mL) under

1

Ar. [b] The yield was determined by H NMR spectroscopy of the crude product. [c] Reaction was performed

without PPh₃. [d] 77% conversion of the aryl halide was observed after 72 h.

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groups including sterically bulky oꢀisomers. Phenylacetylene can also be replaced by different

aliphatic or aromatic alkynes without any loss of selectivity and product yield (entries 11ꢀ16).

After the reaction runs to completion, the catalyst can be recovered by centrifugation,

washing with MeOH and subsequent drying at 80 °C under reduced pressure. The catalytic

performance of the recovered solid was explored in the reaction of pꢀiodoanisole with

phenylacetylene. The catalyst was reused in five consecutive cycles retaining completely the

selectivity and the product yield (entries 2ꢀ6). The recycled solids were also introduced in the

reactions with different aryl iodides (entries 7ꢀ10) and shown the same catalytic efficiency as

catalyst Cu/6b/SM/A. When oꢀiodophenol (13) or oꢀiodo(trifluoroacetyl)aniline (14) were

reacted with phenylacetylene, cascade catalytic reactions involving the Sonogashira coupling

and the addition of N–H or O–H bonds to the triple bond are observed to yield benzofuran 15

and indole 16 (Scheme 6). The cyclization products were obtained in quantitative yield.

Scheme 6. Cascade reactions of iodides 13 and 14 with phenylacetylene.

It's worthy to note that materials 3b/SG/10, 3b/SG/20, 1b/SM and 2b/SM based on

copper complexes with one phen ligand also catalyse the reaction of phenylacetylene with pꢀ

iodoanisole. However, they are less convenient in practice due to a smaller number of

catalytic centres as compared to Cu/6b/SM/A. In contrast, 4b/SM and 5b/SM prepared by

grafting bis(Pphen)copper complexes 4b and 5b were inactive like their counterpart 4a and

5a, respectively, which were studied under homogeneous conditions. These results are in

accordance with previous reports⁸¹and can be explained by a slow dissociation kinetics of

copper complexes bearing two phen ligands.

Huisgen 1,3-dipolar cycloaddition. Copperꢀcatalysed alkyneꢀazide cycloaddition is a novel

synthetic paradigm having a remarkable practical and ecological impact and widely

employing in the synthesis of many functional molecules including pharmaceuticals, dyes,

sensors, and bioconjugates.^109ꢀ116This reaction allows to deliver sophisticated molecules or

libraries of substituted 1,2,3ꢀtriazoles due to its selectivity, excellent functional compatibility,

reliability and simplicity of experimental conditions. However, the separation of targeted

molecules from

resting copper byꢀproducts is still challenging and has a primordial importance for further

developing of this synthetic method. ^{33,35,41,117ꢀ120}In this regard, we have explored the alkyneꢀ

azide cycloaddition in the presence of material Cu/6b/SM/A (Table 6). First, the reaction

conditions were optimized using the reaction of phenylacetylene with pꢀnitrobenzyl azide as a

model reaction. The solvent was found to be a key parameter influencing the reaction rate.

The cycloaddition product was not detected when the reaction was run in toluene or

dichloromethane. Polar coordinating solvents like THF or DMF were required to obtain a

complete conversion to 1,2,3ꢀtriazole and in THF, the reaction was rapid. When it was

performed at 60 °C in the presence of triethylamine and 5 mol% of Cu/6b/SM/A, the product

was isolated in nearꢀquantitative yield after 1 h of stirring (Table 8, entry 1). Complete

conversion was also achieved employing 1 mol% of the catalyst although the reaction time

should be increased up to 3 h (entry 2). Noteworthy, the reaction was not observed under

studied conditions without the catalyst and in presence of the nonꢀmodified TiO₂powder. A

hot filtration test revealed that the cycloaddition proceeds as a heterogeneous process even

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being performed in coordinating THF solvent and in the presence of triethylamine that is a

strong ligand for copper ions (Figure S41).

Table 6. Huisgen cycloaddition catalysed by Cu/6b/SM/A.^[a]

Entry

1^[c]

Alkyne, R₁

H

Azide

Product

Yield^[b][%]

99

2

3

4

5

6

7

8

H

99

95

99

96

92

H

Me

tBu

CO₂Me

CN

[a] Reaction conditions: 0.25 mmol of the alkyne, 0.25 mmol of the azide, 0.25 mmol of NEt₃, 1 mol% of

Cu/6b/SM/A (calculated onto the grafted complex) in THF (1 mL) at 60 °C under Ar. [b] Isolated yields. [c]

The reaction was performed with 5 mol% of Cu/6b/SM/A (calculated onto the grafted complex) during 1 h.

Figure 6. Recycling of Cu/6b/SM/A in the Huisgen cycloaddition of phenylacetylene with pꢀnitrobenzyl azide

(Table 8, entry 1).

The reaction was successful and run without complications with azides bearing benzyl

and even alkyl residues (entries 3ꢀ5). The aromatic alkynes gave high yields of click products

independently of whether electronꢀdonor or electronꢀwithdrawing substituents were present in

the aromatic ring (entries 5ꢀ8).

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The catalyst recovering and refining was straightforward. After reaction completion,

the catalyst was isolated by centrifugation, washed with THF and introduced in the next cycle.

Ten consecutive reactions of phenylacetylene with pꢀnitrobenzyl azide were carried out giving

the targeted 1,2,3ꢀtriazole in comparable yields (90ꢀ98 %) (Figure 6). The content of resting

copper derivatives in the final products was determined by using ISPꢀOES technique. The

average value of copper content was found to be of 48 ppm and 15 ppm for the Sonogashira

coupling (Table 5, entry 2) and the Huisgen cycloaddition (Table 8, entry 1) reactions,

respectively, that is among of the lowest values reported for heterogeneous copper

catalysts.^23,121ꢀ127

The morphology of recovered catalysts was also investigated by using SEM

microphotography of the solid recovered after the Sonogashiraꢀtype coupling performed in

toluene in the presence of PPh₃and strong Cs₂CO₃base. As seen in Figure 7, a morphology of

Cu/6b/SM/A remains intact after the reaction completion and the powder is composed of

uniform agglomerates of primary nanoparticles.

A

B

100 nm

Figure 7. SEM microphotographs of solid Cu/6b/SM/A (A), and the recycled catalyst after Sonogashira

coupling (B). The images were obtained using samples prepared by the dispersion the materials on a conductive

carbon tape and carbon coating (8 nm).

Conclusions

In the present work, different strategies for the covalent immobilization of copper

complexes with phen ligands functionalized by phosphonate groups (Pphen) were

systematically explored. Reported SG methods for the preparation of titaniaꢀsupported

transition metal complexes were compared to the SM of hydrated mesoporous titania that we

have synthesized recently. The resulting hybrid materials were characterized as both a bulk

solids and at the molecular level by using different physicochemical methods including

elemental analysis, infrared spectroscopy, nitrogen sorption isotherms, SEM

microphotography.

Immobilization of labile heteroleptic copper(I) complexes with Pphen ligands is

accompanied by a partial removal of metal ions whatever the experimental procedure was

used for the covalent link of these complexes to TiO₂supports. This drawback can be

overcome by using the stepwise method involving the immobilization of Pphen-Si ligands

and their consecutive complexation with copper(I) ions. Most porous material Cu/6b/SM/A

in which Pphen ligand is linked to the surface by a single phosphonate group located at 3ꢀ

position of the phenanthroline backbone and coordinated to copper(I) ions was prepared by

the postꢀsynthetic modification of hydrated mesoporous TiO₂(S_BET= 650 m²g^ꢀ1). This

material exhibits BET surface area of 246 m²g^ꢀ1contains 0.57 mmol of the grafted complex

,

24

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DOI: 10.1039/C7TA01195D

per gram of the solid and benefits from the high thermal and chemical robustness, the

nanosized morphology, the mesoporosity and the spatial separation of catalytic sites.

Excellent catalytic performance of Cu/6b/SM/A in the Sonogashiraꢀtype coupling and

the Huisgen 1,3ꢀdipolar cycloaddition was also demonstrated. Fairly low catalyst loading, a

simple recovery and reuse of the solid were achieved in both studied reactions proceeding

through principally different reaction pathways. This catalytic versatility of copper catalyst is

highly desirable for sustainable chemistry but still rarely reported owing to the challenge in

the preparation of robust catalysts applicable for a wide range of experimental conditions. Our

ongoing work focuses on the screening of catalytic reactions promoted by Cu/6b/SM/A. This

work also paves the way towards costꢀeffective heterogenized transition metal catalysts

prepared by grafting of metal complexes decorated by phosphonate anchoring groups onto the

surface of mesoporous TiO₂that we have prepared recently according the nonꢀtemplating solꢀ

gel process.

Acknowledgements

Myriam Heydel, Fanny Picquet and Marcel Soustelle are warmly acknowledged for their

technical support. The authors are very grateful to Remi Chassagnon for the cooperation in

TEM studies and helpful discussions. A.Yu. Mitrofanov thanks the Russian Foundation for

Basic Research (grant No. 16ꢀ33ꢀ60207) for the financial support and the French government

for PhD fellowship. Financial support from the CNRS and the Burgundy Region (PARI IME

SMT8 and PARI II CDEA programs) is also acknowledged. This work was carried out in the

frame of the International Associated French–Russian (LIA) Laboratory of Macrocycle

Systems and Related Materials (LAMREM) of CNRS and RAS.

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29

Article Doi

Immobilization of copper complexes with (1,10-phenanthrolinyl)phosphonates on titania supports for sustainable catalysis

DOI: 10.1039/c7ta01195d

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