Crystal Structure of 4′-Phenyl-1′,4′-Dihydro-2,2′:6′,2″-Terpyridine: An Intermediate from the Synthesis of Phenylterpyridine

Abstract

The intermediate compound 4′-phenyl-1′,4′-dihydro-2,2′:6′,2″-terpyridine (pdhtpy) was isolated for the first time during the synthesis of 4′-phenyl-2,2′:6′,2″-terpyridine (ptpy) and characterised by single-crystal X-ray diffraction. Pdhtpy crystallises in the triclinic crystal system with space group P$\bar{1}$ with the following unit cell parameters at 100 K: a = 6.1325(4) Å; b = 8.2667(5) Å; c = 16.052(2) Å; α = 86.829(2)°; β = 82.507(2)°; γ = 84.603(2)°; V = 802.49(9) ų. The absence of stabilising electron-withdrawing groups renders pdhtpy prone to oxidative conditions. Pdhtpy was obtained as a mixture with ptpy, confirmed by Rietveld refinement of the powder X-ray diffraction pattern. Notably, pdhtpy is the first solid-state 1,4-dihydropyridine lacking electron-withdrawing groups at both positions 3 and 5, distinguishing it from Hantzsch esters and related compounds.

1. Introduction

The synthesis of functionalised six-membered rings with a single nitrogen atom is achieved through ring-forming reactions, like Hantzsch synthesis of 1,4-dihydropyridines [1,2] and Chichibabin or Kröhnke pyridine syntheses [2,3,4]. The source of nitrogen atoms in these reactions could be aqueous ammonia, ammonium salts, urea, or functionalised amines [5,6,7,8]. These syntheses are widely used to obtain 2,2′:6′,2″-terpyridine and its analogues [7,8,9]. In the ring-formation reaction of the central pyridine ring of terpyridines, the last intermediate contains 1,4-dihydropyridine ring, which undergoes aromatisation through oxidation (Scheme 1) [10]. However, to the best of our knowledge, these intermediates in terpyridine synthesis have not yet been isolated due to their instability.

In contrast to unfunctionalized 1,4-dihydropyridines, Hantzsch esters could be isolated due to carboxylate groups in positions 3 and 5. This electron-withdrawing functionalisation contributes to the conjugated π-system, thereby preventing the aromatisation of the core six-membered ring. Some other isolated examples of stabilised 1,4-dihydropyridines had substituents such as ketones [11], often introduced in a cycle connecting positions 2 and 3 of the central ring [12,13,14,15,16]; cyano groups [11,12,13]; or a nitro group [13]. Only a few examples are known where one substituent is present in either position 3 or 5 [14,15]. Alternatively, the stability of the central ring against oxidation can be chemically secured once the hydrogen in position 1 is substituted (by an organyl group), and such compounds remain stable even if no other substituents are present in positions 2, 3, 5, and 6 [16]. The stability against oxidation could also be achieved by replacing both hydrogens in position 4, such as in 4-oxo-1,4-dihydropyridines [17,18]. Although in these last examples such compounds are called 1,4-dihydropyridines by nomenclature, they are not strictly so in a chemical sense, as one of the hydrogen atoms is not present in the stated positions.

1,4-Dihydropyridines are of high importance in medicine as calcium channel blockers [19,20,21]. Namely, nifedipine and its later analogues have been used as antihypertensive drugs for decades [20]. This class of compounds also shows other beneficial biological activity, such as anti-diabetic, neuro-protective, anticancer, and many more [21]. It should be noted that only the Hantzsch esters or their analogues could be investigated for biological activity due to their stability [20].

Previously, 4′-phenyl-2,2′:6′,2″-terpyridine (ptpy) was used by us as a ligand in coordination chemistry of rare earth elements [22,23]. Due to the good sensitisation luminescence of Eu³⁺ and Tb³⁺ by ptpy, it was used for the semi-quantitative detection of these two lanthanides in water, with the process having the potential to be used in urban mining [24]. In addition, complexes of Eu³⁺ and Tb³⁺ with ptpy are proposed as solid-state photoluminescence standards [25].

In this work, we obtained 4′-phenyl-1′,4′-dihydro-2,2′:6′,2″-terpyridine (pdhtpy) as an intermediate in the synthesis of ptpy. The structure of pdhtpy was investigated using single-crystal X-ray diffraction analysis. We also show that despite its instability, it is principally possible to obtain pdhtpy as a bulk, although in a mixture with ptpy.

2. Materials and Methods

2-Acetylpyridine (>99%, Sigma-Aldrich), benzaldehyde (>98%, Acros), KOH (85%, Grüssing), 25% aqueous ammonia (Merck), and solvents were used as received unless otherwise stated.

Crystals of pdhtpy were obtained via a modified literature synthesis of ptpy [7]. Benzaldehyde (2.08 mL, 20 mmol) was dissolved in ethanol (100 mL) in a 250 mL beaker. To this solution, 2-acetylpyridine (4.52 mL, 40 mmol) and KOH pellets (3.08 g, 47 mmol) were added, and the beaker was covered with a Petri dish. The reaction mixture was stirred at room temperature for 4 h. Afterwards, the mixture was filtered, and the dark brown filtrate was transferred to a 250 mL round-bottom flask. Aqueous ammonia (3.35 mL, 45 mmol) was added to it. The flask was closed with a stopper, and the mixture was stirred at room temperature overnight. The resulting solid was filtered off and washed with ethanol (3 × 10 mL). The product was recrystallised from a CHCl₃-MeOH 1:1 mixture in a beaker covered with a watch glass. Crystals of both ptpy and pdhtpy were obtained.

A bulk synthesis of pdhtpy was attempted via a modified procedure from the literature for ptpy [7]. The ethanol was degassed by ultrasonication. A 250 mL Schlenk flask was evacuated and refilled with argon three times. Benzaldehyde (2.08 mL, 20 mmol), ethanol (100 mL), and 2-acetylpyridine (4.52 mL, 40 mmol) were added to the flask under a counterflow of argon. The flask was briefly evacuated and refilled with argon. Subsequently, KOH pellets (3.30 g, 50 mmol) and aqueous ammonia (50 mL, 670 mmol) were added under a counterflow of argon, followed by brief evacuation and backfilling. The reaction mixture was stirred at room temperature for 4 h. The resulting yellow solid was filtered under air and washed with ethanol (3 × 10 mL), and then analysed as obtained. Yield: 1.788 g (29%; calculated for the mixture pdhtpy/ptpy as 60:40).

The single-crystal X-ray diffraction data was obtained with a BRUKER AXS D8 Venture diffractometer equipped with an IμS microfocus X-ray source (Mo-K_α) and a PHOTON100 detector at 100 K. The single crystal was mounted on a goniometer head using a perfluorinated ether (viscosity 1800 cSt). The BRUKER AXS Apex software (v2021.4-0) was used for data collection. Data processing was accomplished with XPREP. Structure solutions were carried out with direct methods using SHELXT [26], and the obtained crystal structures were refined with least square techniques using SHELXL [27] on the graphical platform shelXle [28]. The crystallographic information file (CIF) and the CheckCIF report file for pdhtpy are available as Supplementary Materials. The powder X-ray diffraction data was obtained with a Malvern PANalytical X’pert PRO diffractometer equipped with the X’celerator linear detector (length 2.122° in 2θ). Measurement was performed in reflection geometry with Cu-K_α X-ray radiation in a 5–60° 2θ range with a step of 0.017° and an exposure time of 120 s per step. The Rietveld refinement was performed using TOPAS Academic 7 software [29]. The crystal structure of ptpy was retrieved from CCDC 618883 [8]. The instrumental resolution function was obtained by fitting the powder X-ray diffraction data of LaB₆ (NIST SRM^® 660) using the TCHZ pseudo-Voigt peak shape function. The simple axial model was used to determine the asymmetry due to axial divergence. A background polynomial of 8th order was used. Preferred orientation was corrected for both structures using a spherical harmonic function of 2nd order. The refined lattice parameters were a = 12.192(2) Å, b = 11.780(2) Å, and c = 10.984(2) Å, V = 1577.6(3) ų for ptpy (SG Pbcn); a = 6.204(1) Å, b = 8.379(1) Å, c = 16.168(2) Å, α = 85.57(2)°, β = 82.98(1)°, γ = 85.73(2)°, and V = 830.8(2) ų for pdhtpy (SG P$\bar{1}$). Final agreement factors were R_wp 9.3%, R’_wp 14.0%, and GOF 7.8. The Rietveld refinement yielded weight fractions of 40.185(0.298) wt% for ptpy and 59.815(0.298) wt% for pdhtpy; rounded to the nearest 5 wt%, these values correspond to 40 wt% and 60 wt%, respectively. The refinement also yielded crystallographic densities of 1.303 g·cm⁻³ for ptpy (from the literature: 1.302 g·cm⁻³ at 295 K [30]) and 1.245 g·cm⁻³ for pdhtpy (for comparison, for the single-crystal data reported in this work: 1.289 g·cm⁻³ at 100 K).

The interatomic distances reported were determined using Diamond software (v4.6.8). The Hirshfeld surface analysis [31] was performed using CrystalExplorer software (v21.5) [32]. The Hirshfeld surfaces were colour-mapped with the normalised contact distance (d_norm), from red for intermolecular short interactions (distances shorter than the sum of the van der Waals radii) through white to blue (distances longer than the sum of the van der Waals radii). Please note that CrystalExplorer Software automatically sets the X-H interatomic distances for Hirschfeld surface calculation, namely the C-H to 1.083 Å and N-H to 1.000 Å. This makes the intermolecular bond H1-H3′ 2.136 Å instead of 2.26(2) Å, when determined directly from the crystal structure model.

3. Results and Discussion

4′-Phenyl-1′,4′-dihydro-2,2′:6′,2″-terpyridine (pdhtpy) crystallises in a triclinic crystal system, space group P$\bar{1}$, with lattice parameters (at 100 K) a = 6.1325(4) Å; b = 8.2667(5) Å; c = 16.052(2) Å; α = 86.829(2)°; β = 82.507(2)°; and γ = 84.603(2)° (CCDC 2432533). An excerpt of the crystal structure showing the molecular unit is presented in Figure 1. The interatomic distances in the central 1,4-dihydropyridine ring correspond to the expected values. Interatomic distances N1-C1 and N1-C5 are 1.384(2) and 1.383(2) Å, respectively (lit. 1.370–1.417 Å [33,34]). Interatomic distances C1-C2 and C4-C5 are 1.338(3) and 1.341(2) Å, respectively (lit. 1.339–1.401 Å [35,36]). Interatomic distances C2-C3 and C3-C4 are 1.527(2) and 1.512(2) Å, respectively (lit. 1.487–1.548 Å [34,37]). The central ring is planar, with all atoms (N1, C1, C2, C3, C4, and C5) having a distance of less than 0.04 Å from the plane. The hydrogen atom bonded to the nitrogen atom is also in the plane of the central ring, being a distance of 0.04(2) Å from the plane. The angles between the bonds of C3 atoms are close to the tetrahedral (109.5°), namely, the angles of H3-C3-C2, H3-C3-C4, and H3-C3-C16 are 107.9(2)°, C16-C3-C2 111.8(2)°, C16-C3-C4 111.0(2)°, and C2-C3-C4 110.2(2)°. Thus, from the interatomic distances and angles, it can be concluded that the atoms N1, C1, C2, C4, and C5 of the central ring form a conjugated aromatic π-system, whereas atom C3 exhibits sp³ hybridisation. The side pyridine rings are coplanar with the central ring, with a torsion angle below 2°. All three nitrogen-containing rings are convergent in terms of the pointing of their nitrogen atoms and, therefore, lie in a cisoid arrangement. The phenyl ring is almost perpendicular to the central ring, with a torsion angle of 80.1(2)°.

The conformation of pdhtpy differs markedly from that of the 4′-phenyl-2,2′:6′,2″-terpyridine (ptpy). In ptpy, the three pyridine rings adopt a transoid arrangement: the nitrogen atoms of the outer rings are oriented opposite to those of the central ring, and the inter-ring dihedral angle is about 8° [30]. In pdhtpy, however, the relative orientation of the three nitrogen-bearing rings more closely resembles the geometry observed for ptpy when it is bound to metal centres, as in crystal structures of complexes with transition-metal cations [30,38,39,40] and rare-earth element cations [22,23,41,42].

Crystal packing of pdhtpy is presented in Figure 2. Hirshfeld surface analysis of pdhtpy (Figure 3) reveals two short intermolecular contacts present in the crystal packing. Along the crystallographic direction [100], translationally related molecules display a H···H distance of 2.26(2) Å between the H atoms in positions 1 and 4 of the 1,4-dihydropyridine rings of neighbouring molecules (Contacts 1 and 1′ in Figure 3), generating a chain-like motif. This feature is not typical for Hantzsch esters, as they typically show an intermolecular hydrogen bond between the N-H of 1,4-dihydropyridine and C=O groups [43,44,45,46].

A second interaction in the crystal packing of pdhtpy occurs along crystallographic direction [001]: a C-H···C point-to-face (T-shape) contact between a pyridine-ring hydrogen and an adjacent phenyl ring (Contacts 2 and 2′ in Figure 3). Together with the previous interaction, these two σ-π attraction contacts produce a quasi-double-chain packing arrangement. By contrast, the crystal packing of ptpy shows neither comparable short contacts nor π-π stacking; instead, it adopts a fully offset packing mode [30,47].

The 1,4-dihydropyridine pdhtpy is an intermediate in the synthesis of ptpy (Scheme 2). Previously, intermediates 3-phenyl-1-(2-pyridinyl)-2-propen-1-one and 3-phenyl-1,5-bis(pyridin-2-yl)pentane-1,5-dione (Scheme 2) were investigated crystallographically [48,49]. Their synthesis could be easily performed by stopping the reaction at the corresponding point by using only one equivalent of 2-acetylpyridine or by omitting the source of nitrogen for the central ring, respectively. For the synthesis of ptpy, the intermediate pdhtpy should undergo oxidation [10]. As the single crystal was obtained in a semi-closed synthetic system, it suggests that access to atmospheric oxygen should be sufficient for this oxidation. In contrast, for the aromatisation of Hantzsch esters, which have a 1,4-dihydropyridine ring, more harsh conditions are required by using oxidising agents like CrO₃, KMnO₄, HNO₃, or I₂ [50].

As the pdhtpy does not have substituent groups in positions 3′ and 5′ of the central ring, it is easily prone to oxidation and, therefore, aromatisation of the central six-membered ring to form ptpy. In the synthesis attempt under inert conditions, the crude solid product mixture consisted of pdhtpy and ptpy, in the approximate ratio of 60:40, determined by quantitative Rietveld refinement of the powder X-ray diffraction pattern (Figure 4). Due to the close affinity of both compounds and the overall instability of pdhtpy, their separation proved unfeasible. For example, pdhtpy can be sublimed at lower temperatures than ptpy in a dynamic vacuum; however, once in the gas phase, it transforms to ptpy, which forms outside the heating zone. This shows that pdhtpy can undergo spontaneous oxidative dehydrogenation and does not always require an additional oxidising agent (Scheme 2).

The lower sublimation temperature of pdhtpy can be rationalised by its lower crystallographic density: 1.245 g·cm⁻³ for pdhtpy versus 1.302 g·cm⁻³ for ptpy (both at room temperature). As sublimation enthalpy increases with crystallographic density [51], the less densely packed pdhtpy, therefore, requires less energy to sublime.

4. Conclusions

The reaction between 2-acetylpyridine and benzaldehyde in the presence of ammonia, under an oxidiser-deficient atmosphere (oxygen from air), yielded 4′-phenyl-1′,4′-dihydro-2,2′:6′,2″-terpyridine (pdhtpy) in crystalline form. Pdhtpy is an intermediate in the synthesis of 4′-phenylterpyridine (ptpy). While ptpy exhibits molecular packing with offset σ···π contacts, pdhtpy shows a chain-like supramolecular structure as hydrogen atoms of 1,4-dihydropyridine (in positions 1 and 4) of the following pdhtpy molecules have a short intermolecular contact of 2.26(2) Å. This is the first isolation of a solid-state 1,4-dihydropyridine that does not contain electron-withdrawing groups at the 3- and/or 5-positions, distinguishing it from Hantzsch esters and similar compounds. Given the pharmaceutical interest in 1,4-dihydropyridines, particularly as calcium channel blockers, pdhtpy represents a novel subclass of these compounds worthy of further investigation. One of the viable strategies to stabilise such 1,4-dihydropyridines could be in situ complexation during their formation.