Synthesis and structural characterization of Ti(III) and Mo(III) complexes supported by PNP pincer ligands European Journal of Chemistry 14 (3) (2023) 311-315 European Journal of Chemistry ISSN 2153-2249 (Print) / ISSN 2153-2257 (Online) – Copyright © 2023 The Authors – Atlanta Publishing House LLC – Printed in the USA. This work is published and licensed by Atlanta Publishing House LLC – CC BY NC – Some Rights Reserved. https://dx.doi.org/10.5155/eurjchem.14.3.311-315.2463 European Journal of Chemistry View Journal Online View Article Online Synthesis and structural characterization of Ti(III) and Mo(III) complexes supported by PNP pincer ligands Rita Ruivo 1, Luis Alves 2,* and Ana Martins 1,* 1 Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal 2 Centro de Química Estrutural, Associação do Instituto Superior Técnico para a Investigação e Desenvolvimento, Av. António José de Almeida no 12, 1000-043 Lisboa, Portugal * Corresponding author at: Centro de Química Estrutural, Associação do Instituto Superior Técnico para a Investigação e Desenvolvimento, Av. António José de Almeida no 12, 1000-043 Lisboa, Portugal; Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal. e-mail: luis.g.alves@tecnico.ulisboa.pt (L. Alves), ana.martins@tecnico.ulisboa.pt (A. Martins). 10.5155/eurjchem.14.3.311-315.2463 Received: 21 June 2023 Received in revised form: 04 July 2023 Accepted: 11 July 2023 Published online: 30 September 2023 Printed: 30 September 2023 New Ti(III) and Mo(III) complexes of formulae [(PNP-Ph)TiCl3], 1, and [(PNP-iPr)MoCl3], 2, where PNP-Ph = N,N’-bis(diphenylphosphino)-2,6-diaminopyridine and PNP-iPr = N,N’- bis(diisopropylphosphino)-2,6-diaminopyridine were synthesised, in moderate yields, by reaction of MCl3·(THF)3 (M = Ti and Mo) with the suitable ligand precursor. The solid-state molecular structures of complexes 1 and 2 were obtained by single-crystal X-ray diffraction. Crystal data for C37H41Cl3N3O2P2Ti (1·(C4H8O)2): triclinic, space group P-1 (no. 2), a = 10.0945(4) Å, b = 10.3002(4) Å, c = 18.6233(7) Å, α = 92.412(2)°, β = 91.108(2)°, γ = 101.705(3)°, V = 1893.65(13) Å3, Z = 2, µ(MoKα) = 0.559 mm-1, Dcalc = 1.361 g.cm-3, 20760 reflections measured (2.021 ≤ Θ ≤ 27.130), 8327 unique (Rint = 0.0399, Rsigma = 0.0414) which were used in all calculations. The final R1 was 0.0316 (I > σ(I)) and wR2 was 0.0850 (all data). Crystal data for C17H33Cl3MoN3P2 (2): tetragonal, space group I41/a (no. 88), a = b = 19.468(4) Å, c = 31.711(6) Å, α = β = γ = 90°, V = 12019(5) Å3, Z = 16, µ(MoKα) = 0.816 mm-1, Dcalc = 1.202 g.cm-3, 42367 reflections measured (2.569 ≤ Θ ≤ 25.347), 5498 unique (Rint = 0.1408, Rsigma = 0.1293) which were used in all calculations. The final R1 was 0.1005 (I > σ(I)) and wR2 was 0.3194 (all data). The coordination geometry around the titanium and molybdenum centers is best described as octahedral, with three donor atoms of the PNP ligand and one chlorine atom occupying the equatorial plane. The axial positions of the octahedron are occupied by the other two chlorido ligands in both complexes. The NH spacer groups in the PNP ligands have an important role in the establishment of hydrogen bonds between the complexes and molecules of the solvent or neighbouring species. Titanium Molybdenum Pincer ligands Metal complexes Hydrogen bonding Single-crystal X-ray diffraction Cite this: Eur. J. Chem. 2023, 14(3), 311-315 Journal website: www.eurjchem.com 1. Introduction Pincer ligands are tridentate chelating species that bind tightly to three adjacent coplanar coordination positions of a metal center, typically in a meridional configuration [1]. The stereochemical parameters of this type of ligands can be easily controlled by modification of the substituents at the donor sites and/or spacers [2,3]. Due to those characteristics, interest in pincer ligands has increased over the last few years as their metal complexes display interesting thermodynamic and kinetic properties [4]. Anionic pincer ligands based on 1,3- disubsituted benzenes were the first to be described and are still the most used pincer structures [5]. More recently, pincer ligands based on 2,6-diaminopyridines have attracted consi- derable attention because their mid to late transition metal complexes have shown important catalytic applications [6-10]. On the contrary, early transition metal complexes supported by this type of ligand are still much less explored, limited to vanadium species [11,12]. These complexes were found to catalyse the homocoupling of phenylmagnesium bromide in the presence of iodomethane as an oxidising agent, at room temperature, to give phenylbenzene [12]. Although a variety of molybdenum complexes in several oxidation states can be found in the literature [13-25], Mo(III) complexes are scarcely reported [26-28]. In the presence of CoCp*2 (Cp* = η5-C5H5) as a reducing agent and 2,4,6-colidinium trifluoromethanesulfonate as a proton source, complexes of the formula [(R-PNPR’)MoI3] (R = H, Ph, Me, MeO, ferrocenyl, and ruthenocenyl; R’ = tBu, iPr, Ph, and adamantyl) were effective catalysts to produce ammo- nia from molecular dinitrogen under ambient conditions [28]. Here, we describe the synthesis and structural characterisation of new Ti(III) and Mo(III) complexes supported by PNP pincer ligands based on 2,6-diaminopyridine. 2. Experimental 2.1. General considerations The compounds N,N’-bis(diphenylphosphino)-2,6-diami- nopyridine (PNP-Ph) [29] and N,N’-bis(diisopropylphosphino)- 2,6-diaminopyridine (PNP-iPr) [21] were prepared according to the literature. ABSTRACT RESEARCH ARTICLE KEYWORDS https://dx.doi.org/10.5155/eurjchem.14.3.311-315.2463 https://www.eurjchem.com/ https://dx.doi.org/10.5155/eurjchem.14.3.311-315.2463 mailto:luis.g.alves@tecnico.ulisboa.pt mailto:ana.martins@tecnico.ulisboa.pt http://www.eurjchem.com/ https://crossmark.crossref.org/dialog/?doi=10.5155/eurjchem.14.3.311-315.2463&domain=pdf&date_stamp=2023-09-30 312 Ruivo et al. / European Journal of Chemistry 14 (3) (2023) 311-315 2023 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.14.3.311-315.2463 Scheme 1. Synthetic route for the preparation of [(PNP)MCl3] (M = Ti, 1, and M = Mo, 2) complexes. All other reagents were commercial grade and were used without further purification. All manipulations were performed under an inert atmosphere of dry oxygen-free nitrogen by means of standard Schlenk and glovebox techniques. Solvents were pre-dried using 4 Å molecular sieves and refluxed over sodium benzophenone in an atmosphere of N2 and collected by distillation. The 1H and 31P{1H} NMR spectra were recorded on a Bruker AVANCE-250 spectrometer at 296 K and referenced to tetramethylsilane and H3PO4 (85%), respectively. Elemental analyses (C, H, N) were performed in a Fisons CHNS/O analyser Carlo Erba Instruments EA-1108 equipment at the Laboratório de Análises do Instituto Superior Técnico. 2.2. Synthesis and characterization 2.2.1. N,N’-Bis(diphenylphosphino)-2,6-diaminopyridine, PNP-Ph A solution of Ph2PCl (7.20 mL, 38.96 mmol) in toluene (60 mL) was added dropwise to a solution of triethylamine (5.11 mL, 36.66 mmol) and 2,6-diaminopyridine (2.00 g, 18.33 mmol) in the same solvent (100 mL), at 0 °C. The reaction mixture was heated to 50 °C, and the precipitate formed was filtered off. The solvent was removed under vacuum and the remaining oil was recrystallized from toluene/n-hexane (1:1). Color: White. Yield: 91%. 1H NMR (250.0 MHz, CDCl3, δ, ppm): 7.51-7.38 (m, 21H, Py), 6.55 (dd, J = 7.9 Hz, J = 1.2, 2H, Py), 5.36 (d, J = 8.2 Hz, 2H, NH). 31P{1H} NMR (101.2 MHz, CDCl3, δ, ppm): 27.9. 2.2.2. N,N’-Bis(diisopropylphosphino)-2,6-diaminopyridine, PNP-iPr Triethylamine (4.71 mL, 33.78 mmol) was added to a suspension of 2,6-diaminopyridine (1.82 g, 16.68 mmol) in toluene (100 mL). The mixture was cooled to 0 °C, and iPr2PCl (5.35 mL, 33.65 mmol) was added dropwise. Upon further cooling to -70 °C, a 2.5 M solution of n-BuLi in hexane (13.51 mL, 33.78 mmol) was slowly added. The solution was allowed to reach room temperature and then stirred overnight at 80 °C. The reaction mixture was filtered, and the solvent was removed under vacuum. The remaining oil was recrystallized from toluene/n-hexane (1:1). Color: White. Yield: 97%. 1H NMR (250.0 MHz, CDCl3, δ, ppm): 7.20 (t, J = 8.1 Hz, 1H, Py), 6.38 (dd, J = 8.0 Hz, J = 2.1, 2H, Py), 4.38 (d, J = 10.7 Hz, 2H, NH), 1.69 (m, J = 7.0, J = 1.9 Hz, 4H, CH(CH3)2), 1.05-0.93 (m, 24H, CH(CH3)2). 31P{1H} NMR (101.2 MHz, CDCl3, δ, ppm): 49.2. 2.2.3. [(PNP-Ph)TiCl3], 1 A solution of PNP-Ph (250 mg, 0.52 mmol) in THF was added to a suspension of TiCl3·(THF)3 (175 mg, 0.47 mmol) in the same solvent, and the reaction mixture was refluxed overnight. The solvent was evaporated, and the brown solid obtained was washed with small portions of toluene and dried in vacuum. Crystals suitable for single crystal X-ray diffraction were obtained by slow concentration of a THF solution at room temperature using a H-shape glass tube containing the solution of the compound and a small volume of toluene in each side of the tube. Color: Brown. Yield: 55%. Anal. calcd for C29H25Cl3N3P2Ti·(C4H8O)2·(H2O)2: C, 54.73; H, 5.59; N, 5.18. Found: C, 54.04; H, 4.75; N, 4.90%. 2.2.4. [(PNP-iPr)MoCl3], 2 A solution of PNP-iPr (341 mg, 1.00 mmol) in toluene was added to a suspension of MoCl3·(THF)3 (374 mg, 0.90 mmol) in the same solvent. The reaction mixture was refluxed overnight, providing a green precipitate. The solid was filtered off, washed with small portions of Et2O, and dried under vacuum. Crystals suitable for single-crystal X-ray diffraction were obtained by slow concentration of a CH2Cl2 solution at room temperature using a H-shape glass tube containing the solution of the compound and a small volume of toluene in each side of the tube. Color: Green. Yield: 64%. Anal. calcd for C17H33Cl3 MoN3P2·(CH2Cl2)0.3: C, 36.51; H, 5.95; N, 7.38. Found: C, 36.38; H, 6.28; N, 7.36%. 2.3. General procedure for X-ray crystallography Suitable crystals of compounds 1 and 2 were coated and selected in Fomblin® oil in an inert atmosphere of nitrogen. The crystals were then mounted on a loop external to the glovebox environment and data was collected using graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) on a Bruker AXS-KAPPA APEX II diffractometer. Cell parameters were retrieved using Bruker SMART software and refined using Bruker SAINT on all observed reflections [30]. Absorption corrections were applied using SADABS [31]. The structures were solved by direct methods using SIR97 [32]. Structure refinement was done using SHELXL-2018/3 [33]. These programs are part of the WinGX software package version 2021.3 [34]. Hydrogen atoms bonded to nitrogen were located on the difference map and refined freely. Hydrogen atoms bonded to carbons were inserted in the calculated positions and allowed to refine riding in the parent atom. Compound 2 crystalized with diffuse and disordered solvent molecules that could not be modeled. Therefore, they were removed using the Squeeze/PLATON sequence [35]. A total void of 3774 Å3 containing 1516 electrons per unit cell was found and fits well for two molecules of dichloromethane (42 electrons) in the asymmetric unit. The poor diffracting power and crystal quality of 2 (as attested by the Rint value obtained) precluded the final refinement to lower the corresponding R values. Crystallo- graphic and experimental details of data collection and crystal structure determinations for compounds 1 and 2 are available in Table 1. Illustrations of molecular structures were made with ORTEP-3 for Windows [36]. 3. Results and discussion Treatment of MCl3·(THF)3 (M = Ti and Mo) with the suitable PNP ligand afforded complexes [(PNP-Ph)TiCl3], 1, and [(PNP- iPr)MoCl3], 2, where PNP-Ph = N,N’-bis(diphenylphosphino)- 2,6-diaminopyridine and PNP-iPr = N,N’-bis(diisopropylphos- phino)-2,6-diaminopyridine, in moderated yields as shown in Scheme 1. Ruivo et al. / European Journal of Chemistry 14 (3) (2023) 311-315 313 2023 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.14.3.311-315.2463 Table 1. Crystal data and details of structure refinement for compounds 1 and 2. Parameter 1·(C4H8O)2 2 Empirical formula C37H41Cl3N3O2P2Ti C17H33Cl3MoN3P2 Formula weight 775.89 543.69 Temperature (K) 296(2) 296(2) Crystal system, space group Triclinic, P-1 Tetragonal, I41/a a, (AÅ ) 10.0945(4) 19.468(4) b, (AÅ ) 10.3002(4) 19.468(4) c, (AÅ ) 18.6233(7) 31.711(6) α, (°) 92.412(2) 90 β, (°) 91.108(2) 90 γ, (°) 101.705(3) 90 Volume (Å3) 1893.65(13) 12019(5) Z 2 16 ρcalc (g/cm3) 1.361 1.202 μ (mm-1) 0.559 0.816 F(000) 806 4464 Crystal size (mm3) 0.10 × 0.20 × 0.35 0.20 × 0.20 × 0.20 Θ range for data collection (°) 2.021 to 27.130 2.569 to 25.347 Index ranges -12 ≤ h ≤ 12, -9 ≤ k ≤ 13, -23 ≤ l ≤ 20 -23 ≤ h ≤ 23, -23 ≤ k ≤ 23, -33 ≤ l ≤ 38 Reflections collected 20760 42367 Independent reflections 8327 [Rint = 0.0399, Rsigma = 0.0414] 5498 [Rint = 0.1408, Rsigma = 0.1293] Data/restraints/parameters 8327/0/441 5498/25/235 Goodness-of-fit on F2 1.058 1.016 Final R indexes [I≥2σ(I)] R1 = 0.0316, wR2 = 0.0818 R1 = 0.1005, wR2 = 0.2728 Final R indexes [all data] R1 = 0.0380, wR2 = 0.0850 R1 = 0.2049, wR2 = 0.3194 Largest diff. peak/hole (e Å-3) 0.340/-0.374 0.797/-0.428 Table 2. Relevant intermolecular hydrogen bond distances (Å) and angles (°) of compounds 1 and 2. Compound D-H···A d(D-H) d(H···A) d(D···A) (DĤA) Symmetry operation 1 N(2)-H(2N)···O(2_$1) 0.84 2.02 2.8574(1) 174 x, 1+y, z N(3)-H(3N)···O(1) 0.82 2.05 2.8562(1) 171 1-x, 1-y, 1-z 2 N(2)-H(2N)···Cl(3_$1) 0.86 2.51 3.3523(7) 166 -¼+y, ¼-x, ¼-z N(3)-H(3N)···Cl(2_$2) 0.86 2.44 3.2843(7) 168 ¾-y, ¼+x, ¼+z Figure 1. ORTEP diagram of 1 showing thermal ellipsoids at 40% probability level. Selected hydrogen atoms are omitted for clarity. Dashed lines represent hydrogen bonds. Selected bond lengths (Å) and angles (°): Ti(1)-N(1) 2.235(1), Ti(1)-P(1) 2.5487(4), Ti(1)-P(2) 2.5839(5), Ti(1)-Cl(1) 2.3146(4), Ti(1)-Cl(2) 2.3650(4), Ti(1)-Cl(3) 2.3310(4); P(1)-Ti(1)-N(1) 77.63(3), P(2)-Ti(1)-N(1) 76.90(3), P(1)-Ti(1)-Cl(2) 102.62(2), P(2)-Ti(1)-Cl(2) 102.89(2), P(1)-Ti(1)-P(2) 154.37(2), N(1)-Ti(1)-Cl(2) 179.13(3), Cl(1)-Ti(1)-Cl(3) 168.85(2). Crystals of 1 and 2 suitable for single-crystal X-ray diffraction were obtained by slow concentration of THF and dichloromethane solutions, respectively, at room temperature, using an H-shaped glass tube containing the solution of the compound and toluene in each side of the tube. In these conditions, the evaporation of the solvent is determined by dissolution and diffusion into toluene. Complex 1 crystallized in the triclinic P-1 space group, while complex 2 crystallized in the tetragonal I41/a space group. ORTEP diagrams of complexes 1 and 2 and the selected bond lengths and angles are shown in Figures 1 and 2, respectively. The solid-state molecular structures of complexes 1 and 2 determined by single crystal X-ray diffraction show distorted octahedral trivalent titanium and molybdenum centers. The three donor atoms of the PNP ligand and one chloride atom occupy the equatorial plane of both octahedra, which is usually defined as the one containing the donor atoms of the pincer ligand. The P-M-Cl angles are wider than 90°, while the N-M-P and the P-M-P angles are narrower than 90° and 180°, respectively, as a consequence of the chelating nature of the pincer ligand. The axial positions are occupied by two chlorine atoms that define Cl-M-Cl angles that deviate from linearity (168.85(2)° in 1 and 176.7(2)° in 2) and are contracted toward the pyridine ring. The same pattern is observed for the P-M-P angles (154.37(2)° in 1 and 156.1(1)° in 2). This feature is also observed in other M(III) complexes (M = V [11,12], Cr [12,37,38], Mo [27], Ru [39], and In [40]) supported by PNP ligands based on 2,6-diaminopyridine. Complex 1 crystallized with two molecules of THF that interact with the NH spacers of the PNP ligand by N-H···O hydrogen bonds with bridging distances of 2.02 and 2.05 Å. Complex 1 is isostructural with vanadium(III) trichlorido [(PNP-Ph)VCl3]·(C4H8O)2 already reported in the literature [11]. Complex 2 shows two hydrogen bonds of the type N-H···Cl established between the amino spacers and the chlorine atoms of neighboring molecules, which are responsible for the molecular assembly. The N(3)- H(3N)···Cl(2) interactions between molecules of 2, at the supramolecular level, define helicoidal chains that grow along the a direction. These chains interact with each other by N(2)- H(2N)···Cl(3) hydrogen bonds with a distance of 2.51 Å, creating a three-dimensional network as shown in Figure 3. Detailed hydrogen bond lengths and angles found in complexes 1 and 2 are shown in Table 2. 314 Ruivo et al. / European Journal of Chemistry 14 (3) (2023) 311-315 2023 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.14.3.311-315.2463 Figure 2. ORTEP diagram of 2 showing thermal ellipsoids at 30% probability level. Selected hydrogen atoms were omitted for clarity. Dashed lines represent hydrogen bonds. Selected bond lengths (Å) and angles (°): Mo(1)-N(1) 2.173(8), Mo(1)-P(1) 2.556(4), Mo(1)-P(2) 2.489(4), Mo(1)-Cl(1) 2.232(5), Mo(1)-Cl(2) 2.421(3), Mo(1)-Cl(3) 2.474(3); P(1)-Mo(1)-N(1) 77.8(3), P(2)-Mo(1)-N(1) 78.7(3), P(1)-Mo(1)-Cl(2) 103.1(1), P(2)-Mo(1)-Cl(2) 100.1(1), P(1)-Mo(1)-P(2) 156.1(1), N(1)-Mo(1)-Cl(2) 176.3(2), Cl(1)-Mo(1)-Cl(3) 176.7(2). Figure 3. Polyhedral view of the crystal structure of 2 along the b-axis showing chains growing in the a direction. Dashed blue lines represent hydrogen bonds. 4. Conclusions [(PNP-Ph)TiCl3] and [(PNP-iPr)MoCl3] (PNP-Ph = N,N’- bis(diphenylphosphino)-2,6-diaminopyridine and PNP-iPr = N,N’-bis(diisopropylphosphino)-2,6-diaminopyridine) were synthesized and characterized. The solid-state molecular structures of both complexes were determined by single-crystal X-ray diffraction and reveal distorted octahedral geometries. The equatorial planes of the octahedra are occupied by the three donor atoms of the PNP ligands and one chlorine atom. The axial positions are occupied by the other two chlorido ligands. The presence of NH amino spacer groups in PNP ligands plays an important role in the establishment of hydrogen bonds between the complexes and the molecules of the solvent or neighboring species. Such interactions are also responsible for the supramolecular assembly of [(PNP-iPr)MoCl3] that reveals a three-dimensional network defined by helicoidal chains growing in the a direction. To our knowledge, [(PNP-Ph)TiCl3] is the first titanium complex supported by a PNP pincer ligand based on 2,6-diaminopyridine to be described. Acknowledgements The authors are grateful to Fundação para a Ciência e a Tecnologia, Portugal, for funding (UID/QUI/00100/2019, UIDB/00100/2020, UIDP/00100/2020, LA/P/0056/2020 and CATSUS PD/BD/135533/2018. Supporting information CCDC-2224820 and 2224821 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/, or by e-mailing data_request@ ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44(0)1223-336033. Disclosure statement Conflict of interest: The authors declare that they have no conflict of interest. Ethical approval: All ethical guidelines have been adhered to. Sample availability: Samples of the compounds are available from the author. CRediT authorship contribution statement Conceptualization: Ana Martins; Investigation: Rita Ruivo, Luis Alves; Writing - Original Draft: Luis Alves; Writing - Review and Editing: Luis Alves, Ana Martins; Supervision: Ana Martins; Project Administration: Ana Martins. Funding Fundação para a Ciência e a Tecnologia, Portugal https://dx.doi.org/10.13039/501100001871 ORCID and Email Rita Ruivo rita.alvarez.ruivo@tecnico.ulisboa.pt https://orcid.org/0000-0002-4223-0651 Luis Alves luis.g.alves@tecnico.ulisboa.pt https://orcid.org/0000-0002-7938-9850 Ana Martins ana.martins@tecnico.ulisboa.pt https://orcid.org/0000-0002-3922-5501 https://www.ccdc.cam.ac.uk/structures/ mailto:data_request@ccdc.cam.ac.uk mailto:data_request@ccdc.cam.ac.uk https://dx.doi.org/10.13039/501100001871 mailto:rita.alvarez.ruivo@tecnico.ulisboa.pt https://orcid.org/0000-0002-4223-0651 mailto:luis.g.alves@tecnico.ulisboa.pt https://orcid.org/0000-0002-7938-9850 mailto:ana.martins@tecnico.ulisboa.pt https://orcid.org/0000-0002-3922-5501 Ruivo et al. / European Journal of Chemistry 14 (3) (2023) 311-315 315 2023 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.14.3.311-315.2463 References [1]. Peris, E.; Crabtree, R. H. Key factors in pincer ligand design. Chem. Soc. Rev. 2018, 47, 1959–1968. [2]. Lawrence, M. A. W.; Green, K.-A.; Nelson, P. N.; Lorraine, S. C. Review: Pincer ligands—Tunable, versatile and applicable. Polyhedron 2018, 143, 11–27. [3]. van Koten, G.; Milstein, D. Organometallic pincer Chemistry; Springer: Berlin, Germany, 2012. [4]. van Koten, G.; Gossage, R. A. The privileged pincer-metal platform: Coordination chemistry & applications; Springer: Berlin, Germany, 2015. [5]. Murugesan, S.; Kirchner, K. Non-precious metal complexes with an anionic PCP pincer architecture. Dalton Trans. 2016, 45, 416–439. [6]. Bauer, G.; Hu, X. Recent developments of iron pincer complexes for catalytic applications. Inorg. Chem. Front. 2016, 3, 741–765. [7]. Szabó, K. J.; Wendt, O. F. Pincer and pincer-type complexes: Applications in organic synthesis and catalysis; John Wiley & Sons, 2014. [8]. Gorgas, N.; Alves, L. G.; Stöger, B.; Martins, A. M.; Veiros, L. F.; Kirchner, K. Stable, yet highly reactive nonclassical iron(II) polyhydride pincer complexes: Z-selective dimerization and hydroboration of terminal alkynes. J. Am. Chem. Soc. 2017, 139, 8130–8133. [9]. Bertini, F.; Glatz, M.; Stöger, B.; Peruzzini, M.; Veiros, L. F.; Kirchner, K.; Gonsalvi, L. Carbon dioxide reduction to methanol catalyzed by Mn(I) PNP pincer complexes under mild reaction conditions. ACS Catal. 2019, 9, 632–639. [10]. Mastalir, M.; Pittenauer, E.; Stöger, B.; Allmaier, G.; Kirchner, K. Three different reactions, one catalyst: A Cu(I) PNP pincer complex as catalyst for C-C and C-N cross-couplings. Org. Lett. 2017, 19, 2178– 2181. [11]. Ruivo, R.; Alves, L. G.; Kirchner, K.; Martins, A. M. Supramolecular structures of V(III) complexes supported by PNP pincer ligands. J. Mol. Struct. 2017, 1149, 229–234. [12]. Mastalir, M.; Glatz, M.; Stöger, B.; Weil, M.; Pittenauer, E.; Allmaier, G.; Kirchner, K. Synthesis, characterization and reactivity of vanadium, chromium, and manganese PNP pincer complexes. Inorganica Chim. Acta 2017, 455, 707–714. [13]. Mastalir, M.; de Aguiar, S. R. M. M.; Glatz, M.; Stöger, B.; Kirchner, K. A convenient solvothermal synthesis of group 6 PNP pincer tricarbonyl complexes. Organometallics 2016, 35, 229–232. [14]. Arashiba, K.; Sasaki, K.; Kuriyama, S.; Miyake, Y.; Nakanishi, H.; Nishibayashi, Y. Synthesis and protonation of molybdenum– and tungsten–dinitrogen complexes bearing PNP-type pincer ligands. Organometallics 2012, 31, 2035–2041. [15]. Arashiba, K.; Eizawa, A.; Tanaka, H.; Nakajima, K.; Yoshizawa, K.; Nishibayashi, Y. Catalytic nitrogen fixation via direct cleavage of nitrogen–nitrogen triple bond of molecular dinitrogen under ambient reaction conditions. Bull. Chem. Soc. Jpn. 2017, 90, 1111–1118. [16]. de Aguiar, S. R. M. M.; Stöger, B.; Pittenauer, E.; Puchberger, M.; Allmaier, G.; Veiros, L. F.; Kirchner, K. A complete series of halocarbonyl molybdenum PNP pincer complexes - Unexpected differences between NH and NMe spacers. J. Organomet. Chem. 2014, 760, 74–83. [17]. Kinoshita, E.; Arashiba, K.; Kuriyama, S.; Eizawa, A.; Nakajima, K.; Nishibayashi, Y. Synthesis and catalytic activity of molybdenum– nitride complexes bearing pincer ligands: Molybdenum-nitride complexes bearing pincer ligands. Eur. J. Inorg. Chem. 2015, 2015, 1789–1794. [18]. de Aguiar, S. R. M. M.; Schröder-Holzhacker, C.; Pecak, J.; Stöger, B.; Kirchner, K. Synthesis and characterization of TADDOL-based chiral group six PNP pincer tricarbonyl complexes. Monatsh. Chem. 2019, 150, 103–109. [19]. Tanaka, H.; Arashiba, K.; Kuriyama, S.; Sasada, A.; Nakajima, K.; Yoshizawa, K.; Nishibayashi, Y. Unique behaviour of dinitrogen- bridged dimolybdenum complexes bearing pincer ligand towards catalytic formation of ammonia. Nat. Commun. 2014, 5, 3737. [20]. Álvarez, M.; Galindo, A.; Pérez, P. J.; Carmona, E. Molybdenum and tungsten complexes with carbon dioxide and ethylene ligands. Chem. Sci. 2019, 10, 8541–8546. [21]. Benito-Garagorri, D.; Becker, E.; Wiedermann, J.; Lackner, W.; Pollak, M.; Mereiter, K.; Kisala, J.; Kirchner, K. Achiral and chiral transition metal complexes with modularly designed tridentate PNP pincer-type ligands based on N-heterocyclic diamines. Organometallics 2006, 25, 1900–1913. [22]. Oztopcu, O.; Holzhacker, C.; Puchberger, M.; Weil, M.; Mereiter, K.; Veiros, L. F.; Kirchner, K. Synthesis and characterization of hydrido carbonyl molybdenum and tungsten PNP pincer complexes. Organometallics 2013, 32, 3042–3052. [23]. de Aguiar, S. R. M. M.; Öztopcu, Ö.; Troiani, A.; de Petris, G.; Weil, M.; Stöger, B.; Pittenauer, E.; Allmaier, G.; Veiros, L. F.; Kirchner, K. Formation of mono oxo molybdenum(IV) PNP pincer complexes: Interplay between water and molecular oxygen: Formation of mono oxo molybdenum(IV) PNP pincer complexes: Interplay between water and molecular oxygen. Eur. J. Inorg. Chem. 2018, 2018, 876–884. [24]. Aguiar, S. R. M. M. de; Stöger, B.; Pittenauer, E.; Allmaier, G.; Veiros, L. F.; Kirchner, K. Structural diversity of halocarbonyl molybdenum and tungsten PNP pincer complexes through ligand modifications. Dalton Trans. 2016, 45, 13834–13845. [25]. de Aguiar, S. R. M. M.; Öztopcu, Ö.; Stöger, B.; Mereiter, K.; Veiros, L. F.; Pittenauer, E.; Allmaier, G.; Kirchner, K. Synthesis and reactivity of coordinatively unsaturated halocarbonyl molybdenum PNP pincer complexes. Dalton Trans. 2014, 43, 14669–14679. [26]. Kinoshita, E.; Arashiba, K.; Kuriyama, S.; Miyake, Y.; Shimazaki, R.; Nakanishi, H.; Nishibayashi, Y. Synthesis and catalytic activity of molybdenum–dinitrogen complexes bearing unsymmetric PNP-type pincer ligands. Organometallics 2012, 31, 8437–8443. [27]. Stucke, N.; Krahmer, J.; Näther, C.; Tuczek, F. Molybdenum complexes supported by PN3P pincer ligands: Synthesis, characterization, and application to synthetic nitrogen fixation: Molybdenum complexes supported by PN3P pincer ligands: Synthesis, characterization, and application to synthetic nitrogen fixation. Eur. J. Inorg. Chem. 2018, 2018, 5108–5116. [28]. Itabashi, T.; Mori, I.; Arashiba, K.; Eizawa, A.; Nakajima, K.; Nishibayashi, Y. Effect of substituents on molybdenum triiodide complexes bearing PNP-type pincer ligands toward catalytic nitrogen fixation. Dalton Trans. 2019, 48, 3182–3186. [29]. Schirmer, W.; Flörke, U.; Haupt, H.-J. Darstellung, Eigenschaften und Molekülstrukturen von Komplexen des versteiften dreizähnigen Chelatliganden N,N’-Bis(diphenylphosphino)-2,6-diaminopyridin mit MII - und M0-Ubergangsmetallen [MII = Ni, Pd, Pt; M0 = Cr, Mo, W]. Z. Anorg. Allg. Chem. 1987, 545, 83–97. [30]. Bruker (1997-2003). SAINT. Bruker AXS Inc., Madison, Wisconsin, USA. [31]. Sheldrick, G. M. SADABS, Program for Empirical Absorption Corrections, University of Göttingen, 1996. [32]. Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. SIR97: a new tool for crystal structure determination and refinement. J. Appl. Crystallogr. 1999, 32, 115–119. [33]. Sheldrick, G. M. Crystal structure refinement with SHELXL. Acta Crystallogr. C Struct. Chem. 2015, 71, 3–8. [34]. Farrugia, L. J. WinGX and ORTEP for Windows: an update. J. Appl. Crystallogr. 2012, 45, 849–854. [35]. Spek, A. L. Structure validation in chemical crystallography. Acta Crystallogr. D Biol. Crystallogr. 2009, 65, 148–155. [36]. Farrugia, L. J. ORTEP-3 for Windows - a version of ORTEP-III with a Graphical User Interface (GUI). J. Appl. Crystallogr. 1997, 30, 565–565. [37]. Gong, D.; Liu, W.; Chen, T.; Chen, Z.-R.; Huang, K.-W. Ethylene polymerization by PN3-type pincer chromium(III) complexes. J. Mol. Catal. A Chem. 2014, 395, 100–107. [38]. Alzamly, A.; Gambarotta, S.; Korobkov, I. Synthesis, structures, and ethylene oligomerization activity of bis(phosphanylamine)pyridine chromium/aluminate complexes. Organometallics 2013, 32, 7107– 7115. [39]. Matayoshi, K.; Seragaki, M.; Mukai, K.; Asato, E.; Takara, S. Synthesis and crystal structure of [RuCl3{N,N′-bis(diphenylphosphino)-2,6- diaminopyridine}]·3CH3OH. X-ray Struct. Anal. Online 2012, 28, 57– 58. [40]. Haupt, H.-J.; Flörke, U. Crystal structure of N,N’-bis(diphenyl phosphino)-2,6-diaminopyridine-trichloroindium(III) bis-(tetra hydrofurane), (NHP(C6H5)2)2(C5H3N)Cl3In(C4H8O)2. Z. Kristallogr. Cryst. Mater. 1991, 196, 299–301. Copyright © 2023 by Authors. This work is published and licensed by Atlanta Publishing House LLC, Atlanta, GA, USA. The full terms of this license are available at http://www.eurjchem.com/index.php/eurjchem/pages/view/terms and incorporate the Creative Commons Attribution-Non Commercial (CC BY NC) (International, v4.0) License (http://creativecommons.org/licenses/by-nc/4.0). By accessing the work, you hereby accept the Terms. This is an open access article distributed under the terms and conditions of the CC BY NC License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited without any further permission from Atlanta Publishing House LLC (European Journal of Chemistry). No use, distribution, or reproduction is permitted which does not comply with these terms. Permissions for commercial use of this work beyond the scope of the License (http://www.eurjchem.com/index.php/eurjchem/pages/view/terms) are administered by Atlanta Publishing House LLC (European Journal of Chemistry). http://www.eurjchem.com/index.php/eurjchem/pages/view/terms http://creativecommons.org/licenses/by-nc/4.0 http://www.eurjchem.com/index.php/eurjchem/pages/view/terms 1. Introduction 2. Experimental 2.1. General considerations 2.2. Synthesis and characterization 2.2.1. N,N’-Bis(diphenylphosphino)-2,6-diaminopyridine, PNP-Ph 2.2.2. N,N’-Bis(diisopropylphosphino)-2,6-diaminopyridine, PNP-iPr 2.2.3. [(PNP-Ph)TiCl3], 1 2.2.4. [(PNP-iPr)MoCl3], 2 2.3. General procedure for X-ray crystallography 3. Results and discussion 4. Conclusions Acknowledgements Supporting information Disclosure statement CRediT authorship contribution statement Funding ORCID and Email References PrintField10: PrintField11: PrintField12: PrintField13: PrintField14: PrintField20: PrintField21: PrintField22: PrintField23: PrintField24: