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Synthesis 2015; 47(24): 3849-3858
DOI: 10.1055/s-0035-1560324
paper
© Georg Thieme Verlag Stuttgart · New York

Simple Solubilization of the Traditional 2,2′:6′,2′′-Terpyridine Ligand­ in Organic Solvents by Substitution with 4,4′′-Di-tert-butyl Groups

Baptiste Laramée-Milette
a   Département de Chimie, Pavillon J.-A. Bombardier, 5155 Chemin de la Rampe, Université de Montréal, Montréal, Québec, H3T 2B1, Canada   Email: garry.hanan@umontreal.ca
,
Thomas Auvray
a   Département de Chimie, Pavillon J.-A. Bombardier, 5155 Chemin de la Rampe, Université de Montréal, Montréal, Québec, H3T 2B1, Canada   Email: garry.hanan@umontreal.ca
,
Sacha Nguyen
a   Département de Chimie, Pavillon J.-A. Bombardier, 5155 Chemin de la Rampe, Université de Montréal, Montréal, Québec, H3T 2B1, Canada   Email: garry.hanan@umontreal.ca
b   Département de Chimie, 1 Rue de la Technologie, UT Lyon 1, 69622 Villeurbanne cedex, France
,
Samantha Tremblay
a   Département de Chimie, Pavillon J.-A. Bombardier, 5155 Chemin de la Rampe, Université de Montréal, Montréal, Québec, H3T 2B1, Canada   Email: garry.hanan@umontreal.ca
,
Christophe Lachance-Brais
a   Département de Chimie, Pavillon J.-A. Bombardier, 5155 Chemin de la Rampe, Université de Montréal, Montréal, Québec, H3T 2B1, Canada   Email: garry.hanan@umontreal.ca
,
Mélanie Donguy
a   Département de Chimie, Pavillon J.-A. Bombardier, 5155 Chemin de la Rampe, Université de Montréal, Montréal, Québec, H3T 2B1, Canada   Email: garry.hanan@umontreal.ca
b   Département de Chimie, 1 Rue de la Technologie, UT Lyon 1, 69622 Villeurbanne cedex, France
,
Valeska Taylor
a   Département de Chimie, Pavillon J.-A. Bombardier, 5155 Chemin de la Rampe, Université de Montréal, Montréal, Québec, H3T 2B1, Canada   Email: garry.hanan@umontreal.ca
,
Denis Deschênes
a   Département de Chimie, Pavillon J.-A. Bombardier, 5155 Chemin de la Rampe, Université de Montréal, Montréal, Québec, H3T 2B1, Canada   Email: garry.hanan@umontreal.ca
,
Garry S. Hanan*
a   Département de Chimie, Pavillon J.-A. Bombardier, 5155 Chemin de la Rampe, Université de Montréal, Montréal, Québec, H3T 2B1, Canada   Email: garry.hanan@umontreal.ca
› Author Affiliations
Further Information

Publication History

Received: 17 July 2015

Accepted after revision: 02 September 2015

Publication Date:
22 September 2015 (online)

 
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Abstract

A simple one-pot procedure is described for the synthesis and purification of 4′-substituted 4,4′′-di-tert-butyl-2,2′:6′,2′′-terpyridine ligands. The new series of ligand display improved solubility in most organic solvents in comparison to traditional 2,2′:6′,2′′-terpyridine ligands.


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Key words

terpyridine - tert-butyl - tridentate ligand - one-pot synthesis - coordination chemistry

For the past few decades, the 2,2′:6′,2′′-terpyridine (tpy) motif has been the ligand of choice of coordination chemists due to its ability to bond to a vast variety of metal ions.[1] The metal complexes generated via the coordination of tridentate ligands are more appealing as compared to their bidentate analogues since there is no possibility of diastereomer formation in their octahedral complexes.[2] They have found applications in several areas of research, such as anion-sensing,[3] biomedical applications,[4] catalysis,[5] and dye-sensitized solar cells.[6]

A number of strategies have been developed over the years for the synthesis of both symmetrical and nonsymmetrical terpyridine ligands. Among the most utilized techniques, the Tschitschibabin and Kröhnke-type syntheses allow the easy preparation of 4′-aryl- and 4′-heteroaryl-2,2′:6′,2′′-terpyridines by a ring-assembly methodology.[7] As shown by several authors, the substitution in the 4′-terpyridine position by an aryl group can have a major influence on the properties of the metal complexes (Figure [1, a]).[8] Also, the careful design and introduction of the substituents into the 4′-position and further coordination to metal ions can give rise to one-, two-, and three-dimensional polymers and assemblies.[9] One major drawback of this approach is that as the size of the supramolecular species become larger, its solubility usually decreases.

Zoom Image
Figure 1 Substitutions patterns of a) regular 4′-aryl and 4′-heteroaryl terpyridine ligands, b) Le Bozec’s 4,4′,4′′-tri-tert-butyl terpyridine ligand, and c) Tzschucke’s nonsymmetric alkylated terpyridine ligands.

To overcome such problems, several groups have used solubilizing groups such as tert-butyl units in order to maintain a modicum of solubility in most organic solvents.[10] As an example, there are numerous references relating to 4,4′,4′′-tri-tert-butyl-2,2′:6′,2′′-terpyridine ligand since it was first synthesized by Le Bozec in the early 1990s (Figure [1, b]).[10] It is somewhat surprising that aside from Le Bozec’s ligand, there are only a few reports on the use of tert-butyl-terpyridine ligands in supramolecular coordination chemistry.[11] A few years ago, Tzschucke et al. reported the preparation of symmetrically and nonsymmetrically alkylated terpyridines (R = Me, t-Bu), however, this method required multi-step reaction conditions involving inert atmosphere conditions and expensive Pd(II) catalysts (Figure [1, c]).[12] Only a handful of other examples of 4,4′′-di-tert-butyl-terpyridine ligand have been synthesized in which the 4′-position of the central pyridine is fully accessible for functionalization.[12] [13] A strategy that allows easy access to solubilized 4′-aryl and 4′-heteroaryl terpyridine ligands would lead to the synthesis of supramolecular materials and assemblies not otherwise accessible with regular 2,2′:6′,2′′-terpyridine units.[14] Herein, we report a one-pot synthesis of several 4′-aryl- and 4′-heteroaryl-4,4′′-di-tert-butyl-2,2′:6′,2′′-terpyridine ligands (Scheme [1,]Table [1]) as well as three metal complexes [Fe(II), Ni(II), Cu(II)] in order to demonstrate their metal ion coordination ability.

Zoom Image
Scheme 1 One-pot synthesis of 4′-aryl- and 4′-heteroaryl-4,4′′-di-tert-butyl-2,2′:6′,2′′-terpyridine

Table 1 Various Ligands Prepared in Refluxing EtOHa

Ligand

R

Yield (%)

 1

27

 2

25

 3

38

 4

40

 5

27

 6

71

 7

47

 8

50 (15)b

 9

90

10

24

a For reaction sequence, see Scheme [1].

b Yield of the previously published mechanically ground two-step reaction.[14]

The synthetic procedure is similar to the previously mentioned one-pot synthesis;[15] however, the isolation and purification of the ligand had to be adapted due to the greater solubility of the ligands in most organic solvents (Table S1, Supporting Information).

The 2-acetyl-4-tert-butylpyridine precursor was obtained via the acetylation of 4-tert-butylpyridine,[13] however, the maximum yield that we obtained for this reaction was 25%. The enolate of 2-acetyl-4-tert-butylpyridine was obtained by the addition of KOH at ambient temperature in ethanolic solution. The aldol condensation and Michael addition proceed at room temperature in most cases. The cyclization of the central pyridine was accomplished in ethanol at reflux, so the diketone intermediate would still be solubilized. After a few hours at reflux, a small amount of precipitate appeared on the wall of the flask; however, most of the product remained in solution. The solvent was reduced to a minimum volume (~25 mL) under vacuum and saturation of the remaining solution with water led to the precipitation of a brown-beige compound. Filtration of the aqueous suspension over a bed of Celite was followed by an aqueous wash to remove excess KOH and NH4OH as well as water-soluble impurities. The precipitate was then dissolved in a minimum amount of acetone and the ligand was precipitated by the addition of hexanes. Recrystallization of the ligand from a minimum amount of hot methanol afforded pure material (Table [1]). In all of the cases, the 1H NMR, 13C NMR as well as the COSY NMR supported the formation of the desired ligands and their purity. In terms of the solubility in organic solvent, it appears that the di-tert-butylterpyridine ligands have an increased solubility in a wide range of organic solvents in which their homologues without the tert-butyl group are only sparingly soluble or insoluble. In fact, almost any organic solvent except hexanes (and presumably other aliphatic solvents) can be used to dissolve the new ligands with the tert-butyl units.

Zoom Image
Figure 2 ORTEP diagram of ligand 1 with anisotropic displacement ellipsoids at 50% probability

Table 2 Synthesis of First-Row Transition-Metal Complexes

Complex

Reaction conditions

Yield (%)

4Fe

CHCl3–acetone (3:1), reflux, 3 h

46

4Ni

63

4Cu

52

X-ray quality crystals of 1 were grown from a solution of 1 in a mixture of water and ethanol by slow evaporation. As depicted in Figure [2], diffraction analyses reveal that the three pyridyl rings are almost coplanar (θplane ˂10°), which is explained by the presence of four weak C–H···N hydrogen bonds (C–HH···N distance <2.55 Å). Intramolecular H repulsion force the plane of the 4′-phenyl ring to adopt an angle of approximately 50° compared to the central pyridyl ring.

Among various combinations of metals and ligands 1–10, the coordination of three first-row transition metal ions, Fe(II), Ni(II), and Cu(II) to ligand 4 was investigated (Table [2]). It is worth noting that other terpyridine metal complexes using these metals ions are known to catalyze several types of organic reactions.[16] The presence of the bromine atom on the backbone of the molecules is also of interest for further post-coordination modification (e.g., Suzuki–Miyaura, Negishi, Heck, and other coupling reactions), thus potentially giving access to symmetrical, nonsymmetrical, and also polyterpyridines ligands for the synthesis of supramolecular materials.[17]

The complexation was achieved by mixing the ligand (2 equiv) and the metal precursor (1 equiv) in a mixture of chloroform–acetone (3:1) and refluxing for 3 hours (Table [2]). The metal–ligand coordination was confirmed by a strong color change during the reaction [Fe(II) = deep purple; Ni(II) = pale yellow; Cu(II) = cyan]. The reaction process was followed by thin layer chromatography (silica gel, hexane–ethyl acetate, 7:3). The solvent was removed under reduced pressure and the resulting material was dissolved in a minimum amount of acetonitrile. The resulting complex was then subjected to metathesis by pouring it into an aqueous solution with an excess of KPF6 (10 equiv). The precipitate was then collected by filtration.

Zoom Image
Figure 3 ORTEP diagram of 4Fe complex with anisotropic displacement ellipsoids at 50% probability; hydrogen atoms, toluene solvent molecules, and PF6 counter-anion were omitted for clarity

The 1H NMR spectra were obtained in CD3CN for all of the compounds and they supported the formation of the complexes. While 4Fe is diamagnetic, complexes 4Ni and 4Cu are paramagnetic and signals observed in 1H NMR spectra are shifted and broadened when compared to characteristic diamagnetic metal complexes. Due to this behavior, proton count and assignments of the observed peaks were impossible in both cases. However, X-ray quality crystals of all of the complexes were grown from suitable solvents.

Crystals of 4Fe were obtained by the slow evaporation of an acetone–toluene solution over few days while 4Ni crystallized from an acetone–hexane solution by slow evaporation. As displayed in Figure [3] and Figure [4], the iron and nickel compounds are homoleptic complexes where the two ligands adopt a meridional arrangement and are almost perpendicular one to the other (88° and 82° for the Fe(II) and Ni(II) complexes, respectively). In both cases, the metal ions adopt a distorted octahedral geometry. The Fe–N as well as the Ni–N bond distances and the average of bite angles (N–MII–N) are analogous to those found in similar metal complexes.[18] [19]

Zoom Image
Figure 4 ORTEP diagram of 4Ni complex with anisotropic displacement ellipsoids at 50% probability; hydrogen atoms, acetone solvent molecules, and PF6 counter-anion were omitted for clarity

The copper compound crystallized from a chloroform solution at ambient temperature by slow evaporation. To our surprise, it appeared that complexation of the Cu(II) metal ion did not give the expected homoleptic complex.[18] Instead, as depicted in Figure [5, a] rare binuclear centrosymmetric Cu(II) dimer was obtained, where the copper ions adopt a slightly distorted square-pyramidal geometry.[20]

The solid-state structure and refinement data for ligand 1 and complexes 4Fe, 4Ni, and 4Cu are presented in Table [3].

The length of the Cu–Cl bond with the apical chlorine of the pyramid (2.78 Å) is significantly longer than the Cu–Cl bond with the chlorine atom pseudo-planar to the terpyridine ligand (2.23 Å). It is worth mentioning that under high-resolution mass spectrometry conditions, only half of the dimer was observed, even when the mildest possible conditions were used for the ionization (applied potential = 30–50 V). It is difficult to speculate on the fate of the dimeric complex once in solution since it is unclear if the dimer is unstable only under the ionization conditions used for the HRMS experiment or if the Cu–Cl bond with the chlorine in the apical position of the square-based pyramid is broken once the complex is solubilized.

Zoom Image
Figure 5 ORTEP diagram of 4Cu complex with anisotropic displacement ellipsoids at 50% probability; hydrogen atoms, chloroform solvent molecule, and PF6 counter-anion were omitted for clarity.

In conclusion, we have demonstrated a simple one-pot reaction for the synthesis of 4,4′′-di-tert-butyl-2,2′:6′,2′′-terpyridine units with several 4′-aryl and 4′-heteroaryl substituents. Further complexation with different 1st row transition metal ions led to the synthesis of a dimeric Cu(II) species as well as two homoleptic Fe(II) and Ni(II) metal complexes. Both ligands and complexes show much greater solubility in most common organic solvents. Based on this research, new metallo-supramolecular one-dimensional polymer species as well as two- and three-dimensional assembly are now under investigation.

Table 3 Solid-State Structure and Refinement Data for Ligand 1 and Complexes 4Fe, 4Ni, and 4Cu

1

4Fe

4Ni

4Cu

Formula

C29H31N3

[C54H60Br2FeN6] [(PF6)2]·(C7H8)2

[C54H60Br2N6Ni] [(PF6)2]·(C3H6O)

[C54H60Cl2Br2Cu2N6] [(PF6)2]·(CHCl3)

color/form

colorless block

purple needle

yellow needle

cyan needle

Mw [g·mol–1]

438.70

1346.73

1392.75

833.05

temp [K]

100

150

150

100

wavelength [Å]

0.71073

1.54178

1.54178

0.71073

crystal system

monoclinic

monoclinic

tetragonal

triclinic

unit cell dimension

a [Å]

14.7283(5)

14.5883(6)

32.2042(13)

9.1227(3)

b [Å]

6.2151(2)

25.7565(10)

32.2042(13)

13.7546(4)

c [Å]

26.2221(9)

19.6856(8)

24.5290(11)

15.4580(6)

α [°]

90

90

90

99.306(2)

β [°]

91.6740(1)

107.436(2)

90

107.1230(1)

γ [°]

90

90

90

104.4900(1)

V [Å3]

2399.29(14)

7056.9(5)

25439(2)

1735.68(10)

space group

P21/n

P21/c

I 41 c d

P-1

Z

4

4

16

2

d calcd [gcm–3]

1.214

1.441

1.455

1.653

μ [mml–1]

0.070

4.132

3.074

2.193

F(000)

972

3136

11345

866

reflection collected

45580

13410

223250

31419

independent reflections

4794

13410

11995

7509

goodness-of-fit (GOF) on F2

1.038

1.038

1.037

1.111

R1 (F) [I > 2σ(I)]

0.0392

0.0977

0.0468

0.0515

wR(F 2) [I > 2σ(I)]

0.1013

0.1148

0.1198

0.1386

R1 (F) (all data)

0.0414

0.2539

0.0537

0.0531

wR(F2 ) (all data)

0.1036

0.2666

0.1259

0.1399

largest difference peak/hole [e Å–3]

0.2180

0.802

0.776

1.126

NMR spectra were recorded in CDCl3 and CD3CN at 25 °C on a Bruker AV-400 spectrometer at 400 MHz for 1H NMR and 100 MHz for 13C NMR. Chemical shifts (δ) are reported in part per million (ppm) relative to TMS, and are referenced to the residual solvent signal (δ = 1.94 ppm for CD3CN and 7.26 ppm for CDCl3). Absorption spectra were measured in deaerated spectroscopic grade solvent at r.t. on a Cary 300 UV/Vis-NIR spectrophotometer from Agilent Technologies. The high-resolution mass spectrometry (HRMS) experiments were performed on a Bruker Daltonics microTOF spectrometer using electrospray ionization. IR spectra were recorded on a Bruker alpha-p FT-IR spectrometer equipped with a single reflection diamond ATR module.


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X-ray Crystal Structure Determination

X-ray diffraction data collection for ligand 2 and the metal complex 4Cu were carried out on a Bruker APEX II area detector diffractometer equipped with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). Data collection for the metal complexes 4Fe and 4Ni were carried out on a Bruker APEX II DUO Kappa-CCD diffractometer equipped with an Oxford Cryosystem liquid N2 device, using Cu-Kα radiation (λ = 1.54178 Å). The crystal–detector distance was 38 mm. The cell parameters were determined (APEX2 software) from reflections taken from three sets of 100 frames. The structures were solved by direct methods using the program SHELXS-97. The refinement and all further calculations were carried out using SHELXL-97. The H-­atoms were included in calculated positions and treated as riding atoms using SHELXL default parameters. The non-H atoms were refined anisotropically, using weighted full-matrix least-squares on F2.


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4′-Phenyl-4,4′′-di-tert-butyl-2,2′:6′,2′′-terpyridine (1); Typical Procedure

2-Acetyl-4-tert-butylpyridine (2.5 g, 14 mmol) was added to a solution of benzaldehyde (0.75 g, 7 mmol) in EtOH (100 mL). KOH pellets (1.1 g, 20 mmol) and aq NH3 (25 mL, 29.3%, 30 mmol) were then added to the solution. The solution was stirred at r.t. for 4 h. The solvent was reduced to 20 mL and saturated with H2O (200 mL). The off-white solid was filtered over Celite, washed with H2O (150 mL), and dissolved in a minimum amount of acetone. The solvent was reduced to 5–10 mL and was saturated with hexanes. The precipitate was collected by filtration. Recrystallization from hot MeOH afforded 4′-phenyl-4,4′′-di-tert-butyl-2,2′:6′,2′′-terpyridine (1). Yield: 800 mg (27%); white crystalline solid; mp 157–159 °C.

IR (ATR, neat): 2955, 2923, 2867, 1585, 1546, 1370, 891, 840, 695 cm–1.

1H NMR (400 MHz, CDCl3): δ = 8.81 (d, J = 2 Hz, 2 H), 8.75 (s, 2 H), 8.65 (d, J = 5 Hz, 2 H), 7.93 (d, J = 7 Hz, 2 H), 7.51 (t, J = 7 Hz, 2 H), 7.44 (t, J = 7 Hz, 2 H), 7.37 (dd, J = 5 Hz, 2 H), 1.45 (s, 18 H).

13C NMR (100 MHz, CDCl3): δ = 161.2, 160.9, 156.0, 155.8, 150.4, 148.9, 138.5, 135.7, 128.9, 127.4, 121.1, 118.8, 118.3, 35.0, 30.5.

HRMS (ESI): m/z [M + H]+ calcd for C29H31N3: 422.25907; found: 422.25969; difference: 1.46 ppm.


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4′-(4-Methylphenyl)-4,4′′-di-tert-butyl-2,2′:6′,2′′-terpyridine (2)

Following the typical procedure, 2-acetyl-4-tert-butylpyridine (1.0 g, 5.6 mmol) and 4-methylbenzaldehyde (340 mg, 2.8 mmol) afforded 2. Yield: 310 mg (25%); white solid; mp 135–137 °C.

IR (ATR, neat): 2959, 2866, 1585, 1542, 1370, 818 cm–1.

1H NMR (400 MHz, CDCl3): δ = 8.80 (d, J = 2 Hz, 2 H), 8.72 (s, 2 H), 8.64 (d, J = 6 Hz, 2 H), 7.83 (d, J = 8 Hz, 2 H), 7.36 (dd, J = 5 Hz, 2 H), 7.31 (d, J = 8 Hz, 2 H), 2.43 (s, 3 H), 1.45 (s, 18 H).

13C NMR (100 MHz, CDCl3): δ = 160.7, 156.2, 155.9, 150.2, 149.0, 139.0, 135.6, 129.6, 127.2, 121.0, 118.4, 118.2, 35.0, 30.5, 21.3.

HRMS (ESI): m/z [M + H]+ calcd for C30H33N3: 436.27472; found: 436.27393; difference: 1.81 ppm.


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4′-(4-Methoxyphenyl)-4,4′′-di-tert-butyl-2,2′:6′,2′′-terpyridine (3)

Following the typical procedure, 2-acetyl-4-tert-butylpyridine (2.5 g, 14 mmol) and 4-methoxybenzaldehyde (960 mg, 7 mmol) afforded 3. Yield: 1.21 g (38%); white solid; mp 145–147 °C.

IR (ATR, neat): 2961, 2901, 2867, 1583, 1542, 1370, 824 cm–1.

1H NMR (400 MHz, CDCl3): δ = 8.80 (d, J = 2 Hz, 2 H), 8.70 (s, 2 H), 8.64 (d, J = 5 Hz, 2 H), 7.89 (d, J = 9 Hz, 2 H), 7.36 (dd, J = 5 Hz, 2 H), 7.03 (d, J = 9 Hz, 2 H), 3.87 (s, 3 H), 1.45 (s, 18 H).

13C NMR (100 MHz, CDCl3): δ = 160.8, 160.4, 156.2, 155.8, 149.8, 149.0, 130.8, 128.5, 121.0, 118.2, 118.0, 114.2, 55.3, 34.9, 30.5.

HRMS (ESI): m/z [M + Na]+ calcd for C30H33N3ONa: 474.25158; found: 474.24922; difference: 4.98 ppm.


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4′-(4-Bromophenyl)-4,4′′-di-tert-butyl-2,2′:6′,2′′-terpyridine (4)

Following the typical procedure, 2-acetyl-4-tert-butylpyridine (2.5 g, 14 mmol) and 4-bromobenzaldehyde (1.3 g, 7 mmol) afforded 4. Yield: 1.40 g (40%); white solid; mp 174–177 °C.

IR (ATR, neat): 2961, 2902, 2866, 1583, 1542, 1370, 893, 824 cm–1.

1H NMR (400 MHz, CDCl3): δ = 8.80 (d, J = 2 Hz, 2 H), 8.70 (s, 2 H), 8.64 (d, J = 5 Hz, 2 H), 7.79 (d, J = 9 Hz, 2 H), 7.63 (t, J = 9 Hz, 2 H), 7.38 (dd, J = 5 Hz, 2 H), 1.45 (s, 18 H).

13C NMR (100 MHz, CDCl3): δ = 161.0, 156.0, 155.8, 149.1, 149.0, 137.5, 132.0, 128.9, 123.4, 121.2, 118.4, 118.3, 35.0, 30.5.

HRMS (ESI): m/z [M + H]+ calcd for C29H30BrN3: 502.16770; found: 502.16668; difference: 2.03 ppm.


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4′-(4-Dimethylaminophenyl)-4,4′′-di-tert-butyl-2,2′:6′,2′′-terpyridine (5)

Following the typical procedure, 2-acetyl-4-tert-butylpyridine (1.0 g, 5.6 mmol) and 4-(dimethylamino)benzaldehyde (420 mg, 2.8 mmol) afforded 5. Yield: 355 mg (27%); yellow solid; mp 143–145 °C.

IR (ATR, neat): 2955, 2867, 1583, 1528, 1359, 813 cm–1.

1H NMR (400 MHz, CDCl3): δ = 8.79 (d, J = 2 Hz, 2 H), 8.70 (s, 2 H), 8.65 (d, J = 5 Hz, 2 H), 7.90 (d, J = 9 Hz, 2 H), 7.35 (dd, J = 5 Hz, 2 H), 6.82 (d, J = 9 Hz, 2 H), 3.04 (s, 6 H), 1.45 (s, 18 H).

13C NMR (100 MHz, CDCl3): δ = 160.6, 156.5, 155.7, 151.1, 150.0, 149.0, 128.1, 125.6, 120.9, 118.2, 117.3, 112.2, 40.3, 34.9, 30.5.

HRMS (ESI): m/z [M + H]+ calcd for C31H36N4: 465.30127; found: 465.29973; difference: 3.11 ppm.


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4′-(4-Nitrophenyl)-4,4′′-di-tert-butyl-2,2′:6′,2′′-terpyridine (6)

Following the typical procedure, 2-acetyl-4-tert-butylpyridine (2.5 g, 14 mmol) and 4-nitrobenzaldehyde (1.1 g, 7.2 mmol) afforded 6. Yield: 2.33 g (71%); beige solid; mp 210–214 °C.

IR (ATR, neat): 2961, 2901, 2867, 1583, 1542, 1370, 824 cm–1.

1H NMR (400 MHz, CDCl3): δ = 8.79 (br s, 2 H), 8.71 (br s, 2 H), 8.63 (br s, 2 H), 8.33 (br d, J = 7 Hz, 2 H), 8.02 (br d, J = 7 Hz, 2 H), 7.38 (br s, 2 H), 3.87, 1.45 (s, 18 H).

13C NMR (100 MHz, CDCl3): δ = 161.4, 156.8, 155.9, 149.5, 148.5, 148.2, 145.4, 128.7, 124.5, 121.9, 119.1, 118.7, 35.4, 30.9.

HRMS (ESI): m/z [M + Na]+ calcd for C29H30N4O2Na: 489.22610; found: 489.22777; difference: 3.41 ppm.


#

4′-(4-Cyanophenyl)-4,4′′-di-tert-butyl-2,2′:6′,2′′-terpyridine (7)

Following the typical procedure, 2-acetyl-4-tert-butylpyridine (2.5 g, 14 mmol) and 4-formylbenzonitrile (925 mg, 7 mmol) afforded 7. Yield: 1.71 g (47%); beige solid; mp 205–208 °C.

IR (ATR, neat): 2962, 2903, 2868, 1586, 1542, 1371, 894, 840 cm–1.

1H NMR (400 MHz, CDCl3): δ = 8.79 (d, J = 1 Hz, 2 H), 8.74 (s, 2 H), 8.64 (d, J = 5 Hz, 2 H), 8.00–7.94 (m, 4 H), 7.38 (d, J = 4 Hz, 2 H), 1.45 (s, 18 H).

13C NMR (100 MHz, CDCl3): δ = 168.8, 161.0, 156.1, 155.7, 149.1, 149.0, 142.1, 133.5, 128.0, 127.6, 121.3, 118.7, 118.3, 35.0, 30.5.

HRMS (ESI): m/z [M + H]+ calcd for C30H30N4: 447.25432; found: 447.25335; difference: 2.17 ppm.


#

4′-(3-Pyridinyl)-4,4′′-di-tert-butyl-2,2′:6′,2′′-terpyridine (8)

Following the typical procedure, 2-acetyl-4-tert-butylpyridine (2.5 g, 14 mmol) and nicotinaldehyde (755 mg, 7 mmol) afforded 8. Yield: 1.49 g (50%); white solid; mp 197–200 °C.

IR (ATR, neat): 2963, 2868, 1587, 1543, 1371, 1024 cm–1.

1H NMR (400 MHz, CDCl3): δ = 9.05 (d, J = 2 Hz, 1 H), 8.71 (d, J = 2 Hz, 1 H), 8.60 (s, 2 H), 8.59 (dd, J = 5 Hz, 1 H), 8.56 (d, J = 5 Hz, 2 H), 8.22 (dt, J = 8 Hz, 1 H), 7.46 (dd, J = 8 Hz, 1 H), 7.36 (dd, J = 5 Hz, 2 H), 1.39 (s, 18 H).

13C NMR (100 MHz, CDCl3): δ = 161.4, 156.0, 155.4, 149.4, 148.8, 147.7, 146.9, 135.2, 134.4, 124.0, 121.4, 118.6, 118.5, 34.9, 30.3.

HRMS (ESI): m/z [M + H]+ calcd for C28H30N4: 423.25432; found: 423.25314; difference: 2.79 ppm.


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4′-(4-Pyridinyl)-4,4′′-di-tert-butyl-2,2′:6′,2′′-terpyridine (9)

Following the typical procedure, 2-acetyl-4-tert-butylpyridine (2.5 g, 14 mmol) and isonicotinaldehyde (755 mg, 7 mmol) afforded 9. Yield: 2.66 g (90%); white solid; mp 161–165 °C.

IR (ATR, neat): 2962, 2868, 1585, 1540, 1371, 821 cm–1.

1H NMR (400 MHz, CDCl3): δ = 8.81 (d, J = 2 Hz, 2 H), 8.81–8.77 (m, 4 H), 8.66 (d, J = 5 Hz, 2 H), 7.82 (d, J = 5 Hz, 2 H), 7.40 (dd, J = 5 Hz, 2 H), 1.47 (s, 18 H).

13C NMR (100 MHz, CDCl3): δ = 161.4, 156.9, 156.0, 150.9, 149.5, 147.9, 146.5, 122.1, 121.8, 118.8, 118.7, 35.4, 30.9.

HRMS (ESI): m/z [M + H]+ calcd for C28H30N4: 423.25432; found: 423.25272; difference: 3.78 ppm.


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4′-(2-Furanyl)-4,4′′-di-tert-butyl-2,2′:6′,2′′-terpyridine (10)

Following the typical procedure, 2-acetyl-4-tert-butylpyridine (2.5 g, 14 mmol) and 2-furancarbaldehyde (677 mg, 7 mmol) afforded 10. Yield: 280 mg (24%); beige solid; mp 158–161 °C.

IR (ATR, neat): 2961, 2902, 2866, 1582, 1544, 1370, 893, 839 cm-1.

1H NMR (400 MHz, CDCl3): δ = 8.77 (s, 2 H), 8.71 (s, 2 H), 8.65 (d, J = 5 Hz, 2 H), 7.58 (s, 1 H), 7.37 (dd, J = 5 Hz, 2 H), 7.12 (d, J = 3 Hz, 1 H), 6.56 (dd, J = 3 Hz, 1 H), 1.44 (s, 18 H).

13C NMR (100 MHz, CDCl3): δ = 160.8, 155.9, 149.0, 143.6, 139.6, 121.1, 118.2, 114.9, 112.1, 109.1, 35.0, 30.5.

HRMS (ESI): m/z [M + H]+ calcd for C27H29N3O: 412.23834; found: 412.23756; difference: 1.89 ppm.


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Metal Complexes; General Procedure

In a 100 mL round-bottomed flask charged with 4′-(4-bromophenyl)-4,4′′-di-tert-butyl-2,2′:6′,2′′-terpyridine (4; 100 mg, 0.20 mmol) was added CHCl3 (25 mL). To the clear solution was added an acetone solution (25 mL) of the metal salt (0.10 mmol). An instant change in coloration was observed. The mixture was left under reflux for 3 h. After this time, the solvent was removed under vacuum and the residue was dissolve in a minimum amount of acetone. The metathesis of the counter-anion was then achieved by the addition of an aq solution of KPF6 (10 equiv), leading to the instant precipitation of a colored precipitate in 45–65% yield.


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4Fe (11)

Following the general procedure, Fe(SO4)(H2O)7 (27.8 mg) was used to prepare 4Fe, which precipitated as a purple solid; yield: 60 mg (46%); >245 °C (dec.).

IR (ATR, neat): 2965, 2909, 2872, 1615, 832, 556 cm–1.

1H NMR (400 MHz, CD3CN): δ = 9.23 (s, 2 H), 8.59 (s, 2 H), 8.29 (d, J = 9 Hz, 2 H), 8.01 (d, J = 9 Hz, 2 H), 7.08 (dd, J = 6 Hz, 2 H), 7.02 (d, J = 6 Hz, 2 H), 1.27 (s, 18 H).

13C NMR (100 MHz, CDCl3): δ = 164.7, 161.5, 158.5, 153.1, 149.8, 136.9, 133.7, 130.8, 125.4, 122.5, 122.1, 36.1, 30.3.

HRMS (ESI): m/z [M – PF6]+ calcd for C29H30BrF6FeN3P: 1201.22216; found: 1201.21883; difference: 2.77 ppm; [M – 2 PF6]2+ calcd for C29H30BrFeN3: 528.12872; found: 528.12949; difference: 1.46 ppm.


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4Ni (12)

Following the general procedure, Ni(OAc)2(H2O)4 (28.5 mg) was used to prepare 4Ni, which precipitated as a beige solid; yield: 85 mg (63%); >225 °C (dec.).

IR (ATR, neat): 2966, 2908, 2872, 1611, 1288, 1137, 823, 739, 482 cm–1.

1H NMR (400 MHz, CD3CN): δ = 76.05 (br s), 71.56 (br s), 41.80 (br m), 11.03 (br s), 10.72 (br s), 7.40 (br s) 1.03 (br s).

13C NMR (100 MHz, CDCl3): No peak was observed due to the paramagnetic nature of the compound.

HRMS (ESI): m/z [M – PF6]+ calcd for C29H30BrF6N3NiP: 1203.22167; found: 1203.21858; difference: 2.57 ppm; [M – 2 PF6]2+ calcd for C29H30BrN3Ni: 529.12857; found: 529.12845; difference: 0.23 ppm.


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4Cu (13)

Following the general procedure, Cu(OAc)2 (18.2 mg) was used to prepare 4Cu, which precipitated as a cyan solid; yield: 75 mg (52%); >285 °C (dec.).

IR (ATR, neat): 2962, 2906, 2872, 1612, 1409, 1394, 1255, 1009, 821 cm–1.

1H NMR (400 MHz, CD3CN): δ = 9.05 (br s), 7.15 (br s), 1.06 (br s).

13C NMR (100 MHz, CDCl3): No peak was observed due to the paramagnetic nature of the compound.

HRMS (ESI): m/z [M]+ calcd for C29H30BrClCuN3: 599.05821; found: 599.05988; difference: 2.79 ppm.


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Acknowledgment

The authors thank the Natural Sciences and Engineering Research Council (NSERC) of Canada and the Université de Montréal’s Direction des Relations Internationales for financial aid. MD and SN thank the Ministère de l’Enseignement Supérieur et de la Recherche for funding. We are grateful to UdeM NMR and XRD services for their help.

Supporting Information

    Supporting information for this article is available online at http://dx.doi.org.accesdistant.sorbonne-universite.fr/10.1055/s-0035-1560324.
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Figure 1 Substitutions patterns of a) regular 4′-aryl and 4′-heteroaryl terpyridine ligands, b) Le Bozec’s 4,4′,4′′-tri-tert-butyl terpyridine ligand, and c) Tzschucke’s nonsymmetric alkylated terpyridine ligands.
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Scheme 1 One-pot synthesis of 4′-aryl- and 4′-heteroaryl-4,4′′-di-tert-butyl-2,2′:6′,2′′-terpyridine
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Figure 2 ORTEP diagram of ligand 1 with anisotropic displacement ellipsoids at 50% probability
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Figure 3 ORTEP diagram of 4Fe complex with anisotropic displacement ellipsoids at 50% probability; hydrogen atoms, toluene solvent molecules, and PF6 counter-anion were omitted for clarity
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Figure 4 ORTEP diagram of 4Ni complex with anisotropic displacement ellipsoids at 50% probability; hydrogen atoms, acetone solvent molecules, and PF6 counter-anion were omitted for clarity
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Figure 5 ORTEP diagram of 4Cu complex with anisotropic displacement ellipsoids at 50% probability; hydrogen atoms, chloroform solvent molecule, and PF6 counter-anion were omitted for clarity.