English

• 0.   Introduction

  Molybdenum is the only element of the second transition series, being essential for life^[1-2] and is present in all forms of life, ranging from ancient bacteria to humans and with the exception of nitrogenases, mostly, catalyze oxido transfer reactions^[3-4]. It is the only biometal important for microorganisms, plants, and animals with a large variety of stable and accessible oxidation states of all the second series of transition metals. Further, the coordination chemistry of molybdenum(Ⅵ) assumes special importance due to its relatively harmless nature to the environment^[5] and is of significant interest and attention due to its biological activity, viz. anticancer, antibacterial, and antitumor properties^[6-8]. The coordination chemistry of molybdenum gained considerable attention because of its versatile applications in the field of catalysis and biology^[9-10], physiology, and industry^[11-13]. The coordination chemistry of molybdenum attracts more and more attention, due to the chemistry of its variety of oxidation states, coordination number, impact on structure, reactivity, and because of the potential applications of molybdenum compounds^[14-17]. The dioxomolybdenum complexes have been widely studied, as models for the active sites of oxotransfermolybdoenzymes like xanthine oxidase, xanthine dehydrogenase, nitrate reductase and sulfite and aldehyde oxidase^[18-20]. In the catalytic activity of molybdoenzymes the oxidation state of molybdenum varies between Ⅵ and Ⅳ states and Mo(Ⅴ) coexists along with Mo(Ⅵ) and Mo(Ⅳ). The study of molybdenum complexes with dianionictridenate ligands is particularly significant because the coordination of cis-MoO₂²⁺ with di-anionic tridentate ligand systems provides an open active site on molybdenum^[21]. Many reports are available for the studies of dioxomolybdenum complexes with O, N, and S-containing ligands^[22-24].

  In view of the significant role played by molybdenum in biological systems, catalysis, and industry, and only much less work on oxido metal ion complexes of malonoyldihydrazones. Therefore, we aimed to synthesize a monometallic complex [MoO₂(H₂L)(H₂O)] of dioxomolybdenum(Ⅵ) from bis(2-hydroxy-1-naphthaldehyde) malonoyldihydrazone (H₄L, Scheme 1) and characterized it by using different spectroscopic techniques, such as ¹H NMR, ¹³C NMR, IR, and UV-Vis. The structure of the complex has also been confirmed by single-crystal X-ray crystallography.

  Scheme 1

  

  Scheme 1.  Structural formula of H₄L

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  1.   Experimental

  1.1   Materials and instrumentation

  Solvents were reagent grade and used as received. Other chemicals were E-Merck, Himedia, or equivalent grades, and all reagents were used as received. All operations were performed under aerobic conditions. Infrared spectra in a range of 4 000-200 cm^-1 were recorded as KBr discs by using a BX-Ⅲ/FTIR Perkin Elmer Spectrophotometer. The ¹H NMR and ¹³C NMR spectra were recorded on Bruker Avance Ⅱ 400 and 100 MHz in DMSO-d₆ solution using TMS as an internal standard. Electronic spectra were recorded on a Perkin-Elmer Lambda-25 spectrophotometer.

  1.2   Preparation of H₄L

  To an aqueous methanol solution of malonoyldihydrazine (0.66 g, 1 mmol), 2-hydroxy-1-naphthaldehyde (1.72 g, 1 mmol) was added and the reaction mixture was stirred for about half an hour at 40 ℃. The yellow precipitate thus obtained was filtered and washed with hot methanol and dried over anhydrous CaCl₂. Yield: 95%. IR data (cm^-1, KBr): 3 431, 3 197, 3 042 (vs br, ν_OH+NH), 1 700(s), 1 666 (vs, ν_C=O), 1 624, 1 621, 1 540, 1 320, 1 187, 743, 529, 510. ¹H NMR (400 MHz, DMSO-d₆): δ 12.47-11.55 (s, 1H, OH), 9.23, 9.17 (s, 1H, NH), 7.92-7.16 (m, Ar-H), 8.88, 8.23 (s, 2H, C(H)=N), 2.2 (s, 2H, —CH₂).

  1.3   Synthetic procedure for the complex

  H₄L (1.00 g, 2.27 mmol) and MoO₂(acac)₂ (0.81 g, 2.5 mmol) was taken in 50 mL methanol and stirred well at 60 ℃ to give a homogeneous solution. The yellow precipitate was filtered and washed three times with hot methanol (10 mL each time), and dried under a vacuum. The resulting solution was cooled and filtered. The filtrate was kept for crystallization which yielded blocks of orange crystals within four days. Yield: 92%.

  1.4   Single crystal X-ray diffraction

  Single crystal X-ray data were collected at 298 K, using Xcalibur Eos Gemini diffractometer equipped with a monochromated Mo Kα radiation (λ=0.071 073 nm). The CrysAlis Pro (Agilent, 2013) software package was used for data collection and reduction. The crystal structure was solved by SHELXT and refined by SHELXL-2014^[25-26]. All non-hydrogen atoms were refined anisotropically, whereas the hydrogen atoms were placed at a calculated position and refined in the final refinement.

  2.   Results and discussion

  The synthesis of the complex involved the reaction of the stoichiometric amounts of MoO₂(acac)₂ with the ligand in a 1∶1 molar ratio in methanol under stirring conditions (Scheme 2). The complex was insoluble in benzene, hexane, toluene, CCl₄, CHCl₃, ether, etc. The complex was sparingly soluble in acetonitrile, water, methanol, and dichloromethane, and completely soluble in solvents such as dimethyl sulfoxide (DMSO) and dimethyl formamide. The molar conductance value for the complex is 1.8 Ω^-1·cm²·mol^-1 at 1 mmol·L^-1 dilution in DMSO solution. This value revealed the non-electrolytic nature of the complex in the solution^[27].

  Scheme 2

  

  Scheme 2.  Schematic diagram showing the preparation of the complex

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  2.1   Mass spectra

  The complex has been studied by ESI mass spectroscopy. Molybdenum had six isotopes of roughly equal abundance, and the peaks were at m/z values of 694.12, 606.30, 588.29, and 567.14, respectively (Fig.S1, Supporting information). The third prominent peak at m/z value of 588.29 is assigned to [MoO₂(H₅L)(H₂O)]⁺ (587.41), which results from either loss of coordinated methanol molecule followed by coordination of one water molecule or loss of one coordinated water molecule from [MoO₂(H₅L)(H₂O)₂]. The fourth prominent peak at the m/z value of 567.14 corresponds to the loss of coordinated methanol molecules from the complex. Recently, we have shown that in the monometallic molybdenum complex in which both the naphthoxido oxygen atoms are bonded to the molybdenum ion center, and the methylene group is further activated leading to its condensation with sulfoxido oxygen atoms^[28].

  2.2   Electronic spectra

  The electronic spectrum of the complex in the DMSO solution is shown in Fig.S2. The H₄L ligand (Fig.S3) showed absorption bands at 316 nm (ε=8 735 dm³·mol^-1·cm^-1), 365 nm (ε=7 763 dm³·mol^-1·cm^-1), and 414 nm (ε=2 628 dm³·mol^-1·cm^-1) in the DMSO solution, respectively. The band in the region of 294-316 nm is attributed to the intraligand π→π* transition centered on the phenyl/naphthyl rings. Other absorption bands which were observed in the region of 324-365 nm, owe their origin due to n→π* transitions centered on > C=O and > C=N groups, respectively^[29-30]. Apart from the intra-ligand band, an additional band was observed at 433 nm (ε=1 930 dm³·mol^-1·cm^-1), which was not visible in those of the free dihydrazone. Because of the 4d⁰ configuration, Mo(Ⅵ) complex is not expected to show any absorption in the visible region. Therefore, this band may be attributed to the ligand-to-metal charge transfer transition. These results are consistent with the results reported for dioxomolybdenum(Ⅵ) complexes^[31].

  2.3   IR spectra

  The IR spectra of H₄L and the complex are shown in Fig.S4 and S5. The complex showed the bands at 3 396, 3 323, 3 202, 3 176, and 3 080 cm^-1, which are attributed to joint contribution from stretching vibrations of secondary —NH groups and naphtholic/ phenolic —OH groups, respectively. The presence of multiple bands in the region of 3 396-3 080 cm^-1 in the complex indicates the presence of both —NH and —OH groups revealing that all the —CONH— and —OH groups are not bonded to the molybdenum atom. The carbonyl groups exhibited the bands at 1 683 and 1 673 cm^-1. This is attributed to the carbonyl stretching frequency. The disappearance of the narrow band at 1 677 and 1 700 cm^-1 in H₄L on complexation and the appearance of either a strong triplet or strong doublet in the region of 1 651-1 683 cm^-1 indicates the enolization of the half part of the dihydrazone and the uncoordinated nature of one > C=O group. The complex exhibited two peaks at 454 and 594 cm^-1, respectively. These bands were assigned to Mo—O bands (carbonyl and phenolate/naphtholate), respectively^[32]. A medium-intensity band was observed at 1 562 cm^-1 which is attributed to the mixed contribution of amide Ⅱ + ν_C—O (naphtholic). The infrared spectra of the complex exhibited two new strong bands at 938 and 916 cm^-1 which are assigned to the symmetric and asymmetric Mo=O stretching vibrations, respectively of the cis-[MoO₂]²⁺ unit^[33].

  2.4   ¹H NMR

  The ¹H NMR and ¹³C NMR spectra of H₄L and the complex are shown in Fig.S6 and S7. The ¹H NMR spectra of the complex showed peaks at δ 12.55, 12.10, and δ 10.03, 9.70, respectively. While the former signals appearing downfield are attributed to —OH proton, the latter peaks displaying upfield are attributed to —NH proton. The integration of the signals in the region of δ 9.62-12.55 corresponds to two protons, one due to the phenolic/naphtholic —OH group, while the other one due to the secondary —NH group which remains uncoordinated. The azomethine proton signals appear at δ 9.70. The signal shifted downfield by 0.30 as compared to those in the uncoordinated ligands^[34]. The phenyl/naphthyl proton signals appeared in the region of δ 6.73-8.89 in uncoordinated dihydrazones while in the region of δ 6.92-8.89 in the complex.

  2.5   Molecular structure

  ORTEP plot of the crystal of the complex, with a numbering scheme, is shown in Fig. 1. The complex crystallizes with the solvent (acetonitrile) of crystallization. Crystal data and structure refinement are given in Table 1. Selected bond lengths and bond angles are given in Table 2. The complex crystallizes in the monoclinic space group P2₁/c. Although dihydrazone is a tetrabasic octa-dentate ligand, it functions as a dibasic tridentate ligand in the complex binding. The metal center has distorted octahedral coordination environments, connected to one azomethine nitrogen atom of H₂L^2-, two terminal oxido groups, two oxygen atoms of H₂L^2-, and one oxygen atom of the coordinated water molecule. The Mo=O distances are 0.169 0(6) and 0.169 3(5) nm, respectively, and are within the range of 0.162-0.172 nm observed for dioxidomolybdenum(Ⅵ) complexes^[35-36]. These Mo=O distances are markedly shorter than the Mo—O4 distance of the coordinated water molecule which is found to be 0.231 4(5) nm. Additionally, the length of the trans-Mo—N1 and Mo—O4 (H₂O) bonds (0.223 0(5) and 0.231 4(5) nm) are greater than those of the terminal Mo=O bonds. This is, probably, a direct consequence of the well-known trans effect of Mo=O bonds. The cis- and trans-internal octahedral (N, O)—Mo—(N, O) bond angles are found in the 72.18(17)°-104.4(2)° and 148.8(2)°-171.2(2)° ranges, respectively. A five-membered and another six-membered chelate rings are formed with the angle of 72.18(17)° and 80.16(19)° subtended at molybdenum atom, respectively. This type of hydrazone coordination has a widespread occurrence among complexes with several transition metals^[37-38].

  图 1

  

  图 1.  ORTEP plot of the complex with a 50% probability

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  表 1

  

  表 1  Crystal data and details of structure refinement of the complex

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  Parameter	[MoO2(H2L)(H2O)]
  Empirical formula	C25H22MoN6O8
  Formula weight	630.42
  Crystal system	Monoclinic
  Space group	P21/c
  a / nm	1.385 12(7)
  b / nm	1.378 27(8)
  c / nm	1.402 54(8)
  β / (°)	90.771(4)
  Z	4
  Data, restraint, number of parameters	5 202, 0, 364
  Goodness-of-fit on F2	1.013
  Final R indices [I≥2σ(I)]	R1=0.082 7, wR2=0.186 7
  Final R indices (all data)	R1=0.131 3, wR2=0.228 0
  Largest diff. peak and hole / (e·nm-3)	1 340, -880

  表 2

  

  表 2  Bond lengths and bond angles of the complex

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  Mo1—O4	0.231 2(5)	Mo1—O3	0.169 6(5)	Mo1—O5	0.199 1(4)
  Mo1—N1	0.222 8(5)	Mo1—O1	0.192 7(5)	Mo1—O2	0.168 9(6)
  O3—Mo1—O4	83.5(2)	O3—Mo1—O5	97.4(2)	O3—Mo1—N1	160.4(2)
  O5—Mo1—O4	80.04(18)	O5—Mo1—N1	72.15(17)	N1—Mo1—O4	78.45(17)
  O1—Mo1—O4	80.54(18)	O1—Mo1—O5	148.8(2)	O1—Mo1—N1	80.14(19)
  O2—Mo1—O4	171.29(19)	O2—Mo1—O3	105.1(3)	O2—Mo1—O5	97.3(2)
  O2—Mo1—N1	92.8(2)	O2—Mo1—O1	98.2(2)

  2.6   Packing diagram

  The packing diagram of the crystal of the complex is mediated by weak hydrogen bonds, C—H…O interactions, and N…O interaction of the lattice-held acetonitrile molecule (Table 3). In the crystal structure of the complex (Fig. 2), the two complex molecules are linked to each other through H-atoms of the coordinated water molecule by one intermolecular O—H…O hydrogen bonds with intermolecular H-bonded distance of 0.266 6 nm. There is a highly directional intramolecular hydrogen bonding in the complex in which the free carbonyl oxygen atom is hydrogen bonded to the free naphtholic —OH oxygen atom of the same ligand molecule with the O—H…O distance being about 0.376 5 nm. The oxygen atom of the free naphtholic —OH group of the first coordinated ligand molecule is H-bonded to the molybdenyl oxygen atom of the third complex molecule with the O—H…O distance being 0.381 6 nm. C—H…O interaction is also partially responsible for crystal packing with the C…O distances falling in the region of 0.313 3-0.339 9 nm. The oxygen atom of the coordinated water molecule belonging to the first complex molecule is bonded to the free carbonyl oxygen atom of the second complex molecule with an O—H…O distance being 0.266 6 nm. The acetonitrile nitrogen atom is extensively H-bonded to different H-atoms of coordinated dihydrazone molecule with C—H…N distance being 0.271 0 nm as well as to the oxygen and hydrogen atoms both of coordinated water molecule present in the second complex molecule with N…O and N…H distance being 0.287 1 and 0.214 1 nm, respectively. These are engaged in cooperative water-to-acetonitrile interaction. The discrete type of solvent clusters, i.e., water, in particular, have been reported by different workers. In the literature, there are also some polymeric water clusters which may be chains (1D clusters), layers (2D clusters), and a handful of frameworks (3D clusters)^[39].

  表 3

  

  表 3  Hydrogen bond parameters of the complex

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  D—H…A	d(D—H) / nm	d(H…A) / nm	d(D…A) / nm	∠D—H…A / (°)
  O4—H4A…N5	0.086	0.214	0.287 1(11)	143
  O4—H4B…O6#1	0.086	0.182	0.266 6(6)	168
  O7—H7A…N4	0.082	0.185	0.254 9(7)	142
  C15—H15…O2#2	0.093	0.247	0.330 2(9)	150
  C22—H22…O5#3	0.093	0.247	0.339 5(9)	174
  Symmetry codes: #1: x, 1/2-y, -1/2+z; #2: 1-x, 1-y, -z; #3: 1+x, y, z.

  图 2

  

  图 2.  Packing diagram of the complex showing hydrogen bonding along b-axis

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  3.   Conclusions

  In the present study, we have synthesized a monometallic molybdedum(Ⅵ) complex obtained from the reaction of MoO₂(acac)₂ with bis(2-hydroxy-1-naphthaldehyde)malonoyldihydrazone. The complex is monomeric in nature. The dihydrazone ligand is present in keto-enol forms and functions as a monobasic tridentate ligand coordinating with the metal center through phenolate/naphtholate oxygen atoms and azomethine nitrogen atoms.

  Supporting information is available at http://www.wjhxxb.cn

  
  Acknowledgments: The author would like to thank Head SAIF, North-Eastern Hill University, Shillong-793022, India, for providing UV-Vis, IR, and NMR spectra. Conflicts of interest: The authors declare no conflict of interest.
  
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  Scheme 1  Structural formula of H₄L

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  Scheme 2  Schematic diagram showing the preparation of the complex

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  图 1  ORTEP plot of the complex with a 50% probability

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  图 2  Packing diagram of the complex showing hydrogen bonding along b-axis

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  表 1  Crystal data and details of structure refinement of the complex

  Parameter	[MoO2(H2L)(H2O)]
  Empirical formula	C25H22MoN6O8
  Formula weight	630.42
  Crystal system	Monoclinic
  Space group	P21/c
  a / nm	1.385 12(7)
  b / nm	1.378 27(8)
  c / nm	1.402 54(8)
  β / (°)	90.771(4)
  Z	4
  Data, restraint, number of parameters	5 202, 0, 364
  Goodness-of-fit on F2	1.013
  Final R indices [I≥2σ(I)]	R1=0.082 7, wR2=0.186 7
  Final R indices (all data)	R1=0.131 3, wR2=0.228 0
  Largest diff. peak and hole / (e·nm-3)	1 340, -880

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  表 2  Bond lengths and bond angles of the complex

  Mo1—O4	0.231 2(5)	Mo1—O3	0.169 6(5)	Mo1—O5	0.199 1(4)
  Mo1—N1	0.222 8(5)	Mo1—O1	0.192 7(5)	Mo1—O2	0.168 9(6)
  O3—Mo1—O4	83.5(2)	O3—Mo1—O5	97.4(2)	O3—Mo1—N1	160.4(2)
  O5—Mo1—O4	80.04(18)	O5—Mo1—N1	72.15(17)	N1—Mo1—O4	78.45(17)
  O1—Mo1—O4	80.54(18)	O1—Mo1—O5	148.8(2)	O1—Mo1—N1	80.14(19)
  O2—Mo1—O4	171.29(19)	O2—Mo1—O3	105.1(3)	O2—Mo1—O5	97.3(2)
  O2—Mo1—N1	92.8(2)	O2—Mo1—O1	98.2(2)

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  表 3  Hydrogen bond parameters of the complex

  D—H…A	d(D—H) / nm	d(H…A) / nm	d(D…A) / nm	∠D—H…A / (°)
  O4—H4A…N5	0.086	0.214	0.287 1(11)	143
  O4—H4B…O6#1	0.086	0.182	0.266 6(6)	168
  O7—H7A…N4	0.082	0.185	0.254 9(7)	142
  C15—H15…O2#2	0.093	0.247	0.330 2(9)	150
  C22—H22…O5#3	0.093	0.247	0.339 5(9)	174
  Symmetry codes: #1: x, 1/2-y, -1/2+z; #2: 1-x, 1-y, -z; #3: 1+x, y, z.

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