Revisiting the Coordination Chemistry of Molybdenum(V): Novel Complexes with Pyrazinoate and Picolinate Ligands

Abstract

Reactions of (pyH)₅[MoOCl₄(H₂O)]₃Cl₂ with picolinic and pyrazinoic acids yielded three new dinuclear molybdenum(V) complexes: (pyH)₂[Mo₂O₄Cl₂(pic)₂]·CH₃CN (1), (pyH)₂[Mo₂O₄Cl₂(pic)₂]·CH₃CH₂CN (2) and (pyH)₂[Mo₂O₄Cl₂(pyraz)₂]·CH₃CN (3) (pic⁻ = picolinate, pyraz⁻ = pyrazinoate and pyH⁺ = protonated pyridine). The compounds were characterized by single-crystal X-ray diffraction, infrared and ¹H NMR spectroscopy, elemental analysis, and TG/DSC measurements. All display a robust {Mo^V₂O₄}²⁺ core with the heteroaromatic ligands bound in a N,O-bidentate chelating manner.

1. Introduction

In continuation of our studies on the {Mo^V₂O₄}²⁺ coordination chemistry with quinaldinate (anionic form of quinoline-2-carboxylic acid, abbreviated as quin⁻) [1], our investigation was extended to two related ligands: picolinate (pic⁻) and pyrazinoate (pyraz⁻). The structural formulae of the ligands are shown in Scheme 1, while Scheme 2 depicts the structure of the {Mo^V₂O₄}²⁺ unit. The dinuclear unit dominates the coordination chemistry of molybdenum(V) [2,3,4,5,6,7,8,9,10]. Compounds containing this structural fragment often form unexpectedly across diverse ligand systems, owing to the remarkable stability of the {Mo^V₂O₄}²⁺ core. Regardless of the ligand environment, its structural characteristics remain unchanged [2]. With the bridging unit being non-planar, the metal ions are brought into close proximity, with a typical separation of 2.5–2.6 Å. Such a distance is consistent with a direct bonding interaction between two d¹ ions, known as a single metal–metal bond [11]. Consequently, {Mo^V₂O₄}²⁺ compounds are diamagnetic. Another characteristic of the {Mo^V₂O₄}²⁺ structural fragment is the presence of two Mo=O bonds, which labilize bonds with ligands in their trans position. Overall, the dinuclear unit has six coordination sites that can accommodate a variety of N-, O- or S-donor ligands. In their absence, the coordination demands of molybdenum centers are met through aggregation into clusters, enabled by alkoxide and oxide ligands. In particular, the latter can adopt doubly, triply or even quadruply bridging roles [2,12]. Our previous reactions with quinaldinate yielded a discrete dinuclear anionic complex, [Mo₂O₄(quin)₃]⁻ [1]. In this complex, quinaldinate ligands coordinated to the dinuclear unit in two distinctly different ways: two quinaldinates bound in the usual manner, i.e., as a N,O-bidentate chelating ligand to a single metal ion [13], whereas one adopted an O,O′-bridging fashion where carboxylate oxygens bound to separate metal ions. In the latter case, the carboxylate functioned as a third bridge between the metal ions thus forming a coordination motif frequently observed for carboxylate ligands in {Mo^V₂O₄}²⁺ chemistry [14,15,16].

The present study focuses on the products of the (pyH)₅[MoOCl₄(H₂O)]₃Cl₂ reactions with picolinic and pyrazinoic acid. Since picolinate shares the same donor set as quinaldinate, it was expected to behave analogously, although differences in solubility could influence reaction outcomes. Pyrazinoate, by contrast, offers greater structural versatility due to the presence of an additional nitrogen donor atom. Engagement of this site in coordination may lead to diverse bridging modes. Three novel crystalline compounds were obtained: (pyH)₂[Mo₂O₄Cl₂(pic)₂]·CH₃CN (1), (pyH)₂[Mo₂O₄Cl₂(pic)₂]·CH₃CH₂CN (2) and (pyH)₂[Mo₂O₄Cl₂(pyraz)₂]·CH₃CN (3) (pyH⁺ stands for protonated pyridine). The molecular structures of 1 and 3 were determined by means of single-crystal X-ray diffraction analysis, whereas the identity of 2 was deduced from its infrared and ¹H NMR spectra, elemental analysis and TG/DSC analysis.

2. Discussion

2.1. Synthetic Considerations

Reaction of the mononuclear (pyH)₅[MoOCl₄(H₂O)]₃Cl₂ in the mixture of acetonitrile and nitromethane with picolinic acid afforded a single product, crystalline (pyH)₂[Mo₂O₄Cl₂(pic)₂]·CH₃CN (1) of an orange-red color in a relatively good yield. The base, used for deprotonation of picolinic acid, was pyridine. In its absence, the reaction mixture retained its initial deep dark red, almost violet color and no solid could be isolated. An excess of the acid and base was used. The mononuclear [MoOCl₄(H₂O)]⁻ ion underwent both substitution of the labile aqua and chloride ligands and dimerization, ultimately forming a stable {Mo^V₂O₄}²⁺ structural core. The chloride substitution was not complete: one-fourth of chlorides remained coordinated. The synthetic procedure for compound 1 was modified by replacing acetonitrile with propionitrile. The substitution of nitrile solvent did not alter the reaction course as the analogous crystalline product, (pyH)₂[Mo₂O₄Cl₂(pic)₂]·CH₃CH₂CN (2), formed. In this compound, propionitrile molecules of crystallization fulfill the same role as acetonitrile in compound 1. The formation of compounds 1 and 2 is summarized in the balanced equation shown below.

2 (pyH)₅[MoOCl₄(H₂O)]₃Cl₂ + 6 picH + 6 py → 3 (pyH)₂[Mo₂O₄Cl₂(pic)₂] + 12 HCl + 10 pyHCl

For the formation of the analogous pyrazinoate compound, (pyH)₂[Mo₂O₄Cl₂(pyraz)₂]·CH₃CN (3), a different solvent system, namely a mixture of acetonitrile and methanol, can be used. In addition, pyridine was replaced with triethylamine as the base. The formation of compound 3 is summarized in the equation shown below.

2 (pyH)₅[MoOCl₄(H₂O)]₃Cl₂ + 6 pyrazH + 6 Et₃N → 3 (pyH)₂[Mo₂O₄Cl₂(pyraz)₂] + 12 HCl + 4 pyHCl + 6 Et₃NHCl

Our previous studies have shown that the use of methanol as a solvent typically results in the depletion of chloride ligands from the molybdenum(V) coordination sphere. Notably, this was not the case with the synthesis of (pyH)₂[Mo₂O₄Cl₂(pyraz)₂]·CH₃CN (3).

2.2. Crystal Structures

The crystal structures of (pyH)₂[Mo₂O₄Cl₂(pic)₂]·CH₃CN (1) and (pyH)₂[Mo₂O₄Cl₂(pyraz)₂]·CH₃CN (3) show pronounced similarities. Both compounds crystallize in triclinic space group P 1. Their solid-state structures consist of dinuclear complex anions, [Mo₂O₄Cl₂(pic)₂]²⁻ in 1 and [Mo₂O₄Cl₂(pyraz)₂]²⁻ in 3, pyridinium ions as counter-cations and acetonitrile solvent molecules of crystallization. The ORTEP drawing of the dinuclear complex anions of 1 and 3 are shown in Figure 1 and Figure 2. In the complex anions, the ligands, picolinate and pyrazinoate coordinate in a bidentate chelating manner through nitrogen and one carboxylate oxygen atom. The coordination environment of each molybdenum ion thus consists of three oxides and one chloride apart from the N- and O-donors of the picolinate/pyrazinoate. The ClNO₄ donor set occupies vertices of a highly distorted octahedron. The relative position of ligands in the complex anions is such that their symmetry closely approaches that of the C₂ point group with a two-fold rotation axis bisecting the Mo–Mo vector. The similarity between the two complex anions extends also to the bonding parameters. The relevant bonds and angles are given in

Table 1. Relevant geometric parameters [Å,°] in complex anions of 1 and 3.

	[Mo2O4Cl2(pic)2]2− in 1	[Mo2O4Cl2(pyraz)2]2− in 3
Mo–Mo	2.5704(6)	2.5592(6)
“fold” angle *	151.1(2)	151.4(3)
Mo–Cl	2.4653(19), 2.4750(18)	2.4678(19), 2.469(2)
Mo–N	2.238(6), 2.254(6)	2.256(7), 2.271(7)
Mo–O(COO–)	2.155(5), 2.180(5)	2.154(6), 2.202(6)

* Defined as a dihedral angle between two Mo(μ₂-O)₂ planes.

. The Mo–Mo bonds amount 2.5704(6) Å in 1 and 2.5592(6) Å in 3 and as such fall within the typical range observed for {Mo^V₂O₄}²⁺ compounds [2]. For instance, a very similar Mo–Mo bond length, namely 2.5682(3) Å, is observed in a quinaldinate complex [Mo₂O₄(quin)₃]⁻ [1] whose metal ions are bridged in addition by two oxides also with a carboxylate moiety. On the other hand, the picolinate and pyrazinoate complexes display more bent Mo(μ₂-O)₂Mo bridging units than [Mo₂O₄(quin)₃]⁻, as shown by the so-called “fold” angles, 151.1(2)° (in compound 1) and 151.4(3)° (in 3) vs. 163.36(16)° (in a quinaldinate complex). The picolinate/pyrazinoate ligands in [Mo₂O₄Cl₂(pic)₂]²⁻/[Mo₂O₄Cl₂(pyraz)₂]²⁻ have coordinated with the nitrogen donor occupying the site cis to the Mo=O moiety, whereas the carboxylate oxygen occupies the trans site. By contrast, the N,O-donors of the chelating quinaldinate ligands in [Mo₂O₄(quin)₃]⁻ occupy positions that are both cis with respect to the Mo=O moiety. The comparison of the ligands’ bond lengths is thus limited only to the nitrogen-to-molybdenum bonds and shows them to be very similar. Significant differences, as a result of the operating trans influence of the Mo=O moiety, may be observed in the carboxylate-to-molybdenum bonds: the Mo–O(carboxylate) bonds in 1 [2.155(5), 2.180(5) Å] and 3 [2.154(6), 2.202(6) Å] are longer than in [Mo₂O₄(quin)₃]⁻ [2.0826(16), 2.0859(19) Å].

In the structures of 1 and 3, the pyridinum cations are hydrogen-bonded to the complex anions. In both, one pyridinium cation is attached to one of the two bridging oxides, whereas the other pyridinium cation binds to the non-coordinated carboxylate oxygen of one of the two picolinates/pyrazinoates. Additionally, π···π interactions occur between pyridinium cations in the solid-state structure of 1.

2.3. TG Analysis

In an oxidizing atmosphere up to 800 °C, all three compounds display very similar thermal behavior, consistent with their similar crystal structures. TG and DSC curves for (pyH)₂[Mo₂O₄Cl₂(pic)₂]·CH₃CN (1) and (pyH)₂[Mo₂O₄Cl₂(pyraz)₂]·CH₃CN (3) are shown in Figure 3 and Figure 4, whereas those of (pyH)₂[Mo₂O₄Cl₂(pic)₂]·CH₃CH₂CN (2) form part of the Supplementary Materials. A detailed description is given only for 3. A complex multi-step decomposition process was revealed. The onset of the first step is at ca. 140 °C. It is completed by 230 °C and accounts for 16.33% of mass loss. This initial decomposition cannot be attributed solely to the release of nitrile solvent molecules, as the observed mass loss is significantly greater than the calculated acetonitrile content, 5.30%. Upon further heating, additional mass losses occur in two steps. The latter stage is highly exothermic (peak at ca. 430 °C) and corresponds to the formation of molybdenum(VI) oxide, calcd./found as follows: 37.18%/36.53%. Its mass remains constant within the 450–610 °C interval. At higher temperatures, molybdenum(VI) oxide begins to sublime, leading to a continuous decrease in the mass of the solid residue.

3. Experimental

3.1. General Remarks

Chemicals were purchased from Aldrich (St. Louis, MO, USA) and used as received. (pyH)₅[MoOCl₄(H₂O)]₃Cl₂ was prepared following the published procedure [17]. Microanalyses (C, H, N) were performed by the in-house facility on a Perkin–Elmer 2400 II instrument (Shelton, CT, USA). The infrared spectra were measured directly on solid samples using ATR with a Bruker Alpha II FTIR spectrometer (Rheinstetten, Germany). Spectra were obtained between 4000 and 400 cm⁻¹ with 2 cm⁻¹ resolution and 25 scans and are shown as acquired without corrections or other manipulations. Spectra revealed characteristic absorption bands for vibrations of carboxylate [18] and terminal Mo=O groups [19]. TG-DSC measurements were performed on a Mettler Toledo TGA/DSC1 instrument (Schwerzenbach, Switzerland) in a temperature range from 25 to 800 °C with a heating rate of 5 K/min. During the measurement, the furnace was purged with synthetic air with a flow rate of 50 mL/min. 150 µL alumina crucible was used and the initial masses of the samples were around 10 mg. The blank curve was subtracted. Molybdenum content in the samples was determined as MoO₃, solid residue in the 450–630 °C temperature interval. ¹H NMR spectra of DMSO-d₆ solutions were obtained on a Bruker Avance NEO (Rheinstetten, Germany) at 600 MHz. The spectra were referenced to the central peak of the residual resonance for DMSO-d₆ at 2.50 ppm [20]. Chemical shifts (δ) are given in ppm, multiplicities are reported as follows: s = singlet, d = doublet, t = triplet, q = quartet and m = multiplet. Data were processed with MestReNova program (version 14.2.2) [21]. With the exception of the nitrile resonances, the ¹H NMR spectra of compounds 1 and 2 are essentially identical. Moreover, the number and the shape of the resonances associated with the picolinate (in compounds 1 and 2) or pyrazinoate ligands (in 3) strongly indicate the presence of exchange processes within the dinuclear complex anions. Nevertheless, in all three spectra, the sum of integrals is consistent with the compounds’ composition, i.e., the molar ratio between the pyH⁺ cations and the heteroaromatic ligands is 1:1.

3.2. X-Ray Crystallography

The crystals were mounted on the tip of a glass fiber with a small amount of silicon grease and transferred to a goniometer head. Data were collected on an Agilent SuperNova diffractometer (Oxfordshire, UK) with molybdenum (Mo-K_α, λ = 0.71073 Å) microfocus sealed X-ray source at 150 K. CrysAlis PRO [22] was used for data processing. Structures were solved with Olex² software (version 1.5) [23] using ShelXT (version 2018/3) [24] and refined with least square methods in ShelXL [25]. Anisotropic displacement parameters were determined for all non-hydrogen atoms. Hydrogen atoms were placed at calculated positions and were allowed to ride on their parent atoms. Figures depicting the structures were prepared using Mercury [26]. The crystallographic data were deposited to the CCDC and assigned the deposition numbers 2492841 (1) and 2492842 (3).

Crystal data for 1, C₂₄H₂₃Cl₂Mo₂N₅O₈ (772.25 g mol⁻¹): triclinic P 1, a = 7.8948(2) Å, b = 8.7632(3) Å, c = 10.9092(4) Å, α = 96.103(3)°, β = 103.765(3)°, γ = 102.504(3)°, V = 705.69(4) ų, Z = 1, ρ_calcd = 1.817 g cm⁻³, μ = 1.134 mm⁻¹; 11,929 reflections collected (5512 independent, R_int = 0.0341). The final residues were R = 0.0327, and wR₂ = 0.0766 [5167 reflections, I > 2σ(I)], and R = 0.0370, and wR₂ = 0.0798 [all data].

Crystal data for 3, C₂₂H₂₁Cl₂Mo₂N₇O₈ (774.24 g mol⁻¹): triclinic P 1, a = 6.9296(3) Å, b = 10.0862(4) Å, c = 10.8704(3) Å, α = 105.379(3)°, β = 94.753(3)°, γ = 107.344(4)°, V = 688.35(5) ų, Z = 1, ρ_calcd = 1.868 g cm⁻³, μ = 1.165 mm⁻¹; 11,260 reflections collected (5417 independent, R_int = 0.0269). The final residues were R = 0.0376, and wR₂ = 0.0876 [5099 reflections, I > 2σ(I)], and R = 0.0415, and wR₂ = 0.0919 [all data].

3.3. Preparation of (pyH)₂[Mo₂O₄Cl₂(pic)₂]·CH₃CN (1)

Picolinic acid (185 mg, 1.50 mmol) and pyridine (160 mg, 2.0 mmol) were added to the mixture of acetonitrile (15 mL) and nitromethane (10 mL). Afterwards, (pyH)₅[MoOCl₄(H₂O)]₃Cl₂ (214 mg, 0.50 mmol of Mo(V)) was added. The resulting mixture of a dark red color was stored in a closed Erlenmeyer flask under ambient conditions. Orange crystals of 1 were filtered off after 4 days. Yield: 145 mg; 75%. Found C, 36.98; H, 2.62; N, 9.08%. C₂₄H₂₃Cl₂Mo₂N₅O₈ (M = 772.25 g/mol) requires C, 37.33; H, 3.00%; N, 9.07%. TG determination of the Mo content, 23.7%. Required Mo, 24.9%. IR (ATR, cm⁻¹): 3083m [(C−H) in pic⁻ and pyH⁺], 2250w [ν(C≡N) in CH₃CN], 1661vs, 1652vs, 1634vs, 1591vs [ν_as(COO⁻) in pic⁻], 1377vs, 1346vvs [ν_s(COO⁻) in pic⁻], 946vvs, 929vs [ν(Mo=O)]. ¹H NMR ((CD₃)₂SO with 0.03% v/v TMS, 600 MHz): δ several resonances in the 9.36–7.19 range (m, 8H, pic⁻), 8.88 (d, J = 5.0 Hz, 4H, pyH⁺), 8.49 (t, J = 7.8 Hz, 2H, pyH⁺), 7.99–7.96 (m, 4H, pyH⁺), 2.07 (s, 3H, CH₃CN) ppm.

3.4. Preparation of (pyH)₂[Mo₂O₄Cl₂(pic)₂]·CH₃CH₂CN (2)

Picolinic acid (185 mg, 1.50 mmol) and pyridine (160 mg, 2.0 mmol) were added to the mixture of propionitrile (15 mL) and nitromethane (10 mL). Afterwards, (pyH)₅[MoOCl₄(H₂O)]₃Cl₂ (214 mg, 0.50 mmol of Mo(V)) was added. The resulting dark-red solution was stored in a closed Erlenmeyer flask under ambient conditions. Orange crystalline solid of 2 was filtered off after 4 days. Yield: 135 mg; 69%. Found C, 37.92; H, 3.05; N, 9.12%. C₂₅H₂₅Cl₂Mo₂N₅O₈ (M = 786.28 g/mol) requires C, 38.19; H, 3.20%; N, 8.91%. TG determination of the Mo content, 23.5%. Required Mo, 24.4%. IR (ATR, cm⁻¹): 3083m [(C−H) in pic⁻ and pyH⁺], 2246w [ν(C≡N) in CH₃CH₂CN], 1661vs, 1633vs, 1590vs [ν_as(COO⁻) in pic⁻], 1378vs, 1345vvs [ν_s(COO⁻) in pic⁻], 946vvs, 928vs [ν(Mo=O)]. ¹H NMR ((CD₃)₂SO with 0.03% v/v TMS, 600 MHz): δ several resonances in the 9.36–7.22 range (m, 8H, pic⁻), 8.89 (d, J = 5.2 Hz, 4H, pyH⁺), 8.51 (t, J = 7.7 Hz, 2H, pyH⁺), 8.01–7.98 (m, 4H, pyH⁺), 2.46 (q, J = 7.6 Hz, 2H, CH₃CH₂CN), 1.14 (t, J = 7.6 Hz, 3H, CH₃CH₂CN) ppm.

3.5. Preparation of (pyH)₂[Mo₂O₄Cl₂(pyraz)₂]·CH₃CN (3)

(pyH)₅[MoOCl₄(H₂O)]₃Cl₂ (214 mg, 0.50 mmol of Mo(V)) was dissolved in the mixture of acetonitrile (10 mL) and methanol (5 mL). To this solution, pyrazinoic acid (140 mg, 1.13 mmol) and triethylamine (0.33 mL, 2.37 mmol) were added. The resulting mixture of a brown-red color was stored in a closed Erlenmeyer flask under ambient conditions. Orange-red crystalline solid 3 was filtered off on the following day. Yield: 142 mg; 73%. Single crystals of 3 (used for X-ray structure analysis) were obtained from a slightly modified reaction mixture. Pyrazinoic acid (143 mg, 1.15 mmol) and pyridine (120 mg, 1.50 mmol) were added to the mixture of acetonitrile (10 mL), methanol (5 mL) and nitromethane (10 mL). Afterwards, (pyH)₅[MoOCl₄(H₂O)]₃Cl₂ (161 mg, 0.39 mmol of Mo(V)) was added. The resulting mixture was stored in a closed Erlenmeyer flask under ambient conditions. Within a couple hours, all the solids were consumed and the color of the solution changed to orange-red. On the following day, large orange crystals of 3 were obtained.

The crystalline solids obtained from both preparations exhibit identical infrared spectra. Found C, 34.12; H, 2.59; N, 12.52%. C₂₂H₂₁Cl₂Mo₂N₇O₈ (M = 774.24 g/mol) requires C, 34.13; H, 2.73%; N, 12.66%. TG determination of the Mo content, 24.4%. Required Mo, 24.8%. IR (ATR, cm⁻¹): 3071m, 3057m [ν(C−H) in pyraz⁻ and pyH⁺], 2247w [ν(C≡N) in CH₃CN], 1666vvs, 1634vvs [ν_as(COO⁻) in pyraz⁻], 1375vvs, 1333vvs [ν_s(COO⁻) in pyraz⁻], 949vvs, 928vvs [ν(Mo=O)]. ¹H NMR ((CD₃)₂SO with 0.03% v/v TMS, 600 MHz): δ several resonances in the 9.38–8.80 range (m, 6H, pyraz⁻), 8.83 (d, J = 4.9 Hz, 4H, pyH⁺), 8.37 (t, J = 7.8 Hz, 2H, pyH⁺), 7.89–7.86 (m, 4H, pyH⁺), 2.07 (s, 3H, CH₃CN) ppm.

4. Conclusions

Reactions of the mononuclear (pyH)₅[MoOCl₄(H₂O)]₃Cl₂ with either picolinic or pyrazinoic acid resulted in the analogous dinuclear [Mo₂O₄Cl₂(pic)₂]²⁻ and [Mo₂O₄Cl₂(pyraz)₂]²⁻ complexes which retained some chloride ligands and had their heteroaromatic ligands bound in a N,O-bidentate chelating manner. The coordination behavior of picolinate and pyrazinoate contrasts with that of the similar quinaldinate which was bound also via its carboxylate moiety in a O,O′-bidentate bridging manner. Notably, pyrazinoate did not fully exploit its coordination potential as its second nitrogen atom did not participate in bonding interactions with molybdenum.