Jenalyn D. Osuna

Oxomolybdenum Chemistry Results and Discussion Molybdenum is a transition element that is involved in many bioinorganic materials. It is readily convertible to several oxidation states. It has the ability to form stable bonds with nitrogen, oxygen and sulfur. It has a high susceptibility to both nucleophilic and electrophilic attack. (Housecroft, C. E. and Sharpe, A. G., 2005) These type of reactions are responsible for the chemistry of Mo complexes and are important in catalytic oxygen atom transfer reactions such as those mediated by industrial oxidation catalysts and molybdo-enzymes such as sulfide oxidase, xanthine oxidase and nitrate reductase. (Meissler G.L. and Tarr D.A.) Nucleophilic reaction involves the change in the oxidation state of the metal and involves the interaction between an electron-rich nucleophile to a partially positive, electron-deficient atom. (Housecroft, C. E. and Sharpe, A. G., 2005) It involves redox reaction and transfer of oxygen atom. It is represented by the following reaction: Electrohilic reaction, on the other hand, involves the coming together of an electron rich species and a chemical species that is deficit with electrons or electronically unsaturated. It involves a simple protonation reaction. (Housecroft, C. E. and Sharpe, A. G., 2005) The oxidation states of the species are not changed. It is represented by the following reaction: Metal oxides of molybdenum play biological functions involving several kinds of catalytic reactions. Its ability to do so is attributed to the oxo-metal active sites present in its structure. The oxoligand has the ability to stabilize high-valent metal centers through both π and σ base interactions of the d-orbitals and the ligand p-orbitals. For these reasons, molybdenum complexes can undergo polyoxometallate formation. Polyoxometallate formation refers to the formation of metal compounds or complexes of multiple oxygen bonds, which is most pronounced at the oxidation states of elements such as V(V), Mo(VI), and W(VI). (Meissler G.L. and Tarr D.A.) Oxygen transfer or oxo-transfer reaction is defined by the reaction: Where M is the metal center, O is oxygen and L is the ligand. XO or X can either be an oxygen atom donor or acceptor but is not an oxide. Atom transfer results to the oxidation or reduction of the metal center having oxidation state z or z+2. On the other hand, the transferable or transferred oxygen is directly bound in a terminal or bridging mode to the metal center. Metal oxidation and substrate reduction are enhanced by a strongly reducing metal center, a strong M-O bond, and an oxygen atom donor with a relatively weak X-O bond. (Cotton, F. A. and Wilkinson, G., 1972) The exercise was divided into two parts. The first part involves the preparation of oxomolybdenum complexes such as the cis-MoO2(S2CNEt2) 2, red compound, purple compound and yellow compound while the second part involves the oxygen transfer chemistry of the red compound. Synthesis of cis-MoO2(S2CNEt2)2 In the exercise, cis-MoO2(S2CNEt2) 2 was the first compound to be synthesized. The first step involves the synthesis of N,N-diethyldithiocarbamate ligand using diethylamine solution, carbon disulfide and sodium hydroxide. The synthesis proper of cis-MoO2(S2CNEt2) 2 happened after the addition of sodium molybdate(VI) dehydrate in acidic medium. The observations on the synthesis of cisMoO2(S2CNEt2) 2 is shown on Table 4.1. Table 4.1. Observations on the synthesis of cis-MoO2(S2CNEt2) 2 . Action Taken Observations NaOH + H2O colorless solution Addition of diethylamine colorless solution with bubbling Addition of CS2 yellowish solution; observable odor After swirling for 10 minutes yellowish solution Addition of Na2MoO4.2H2O yellowish solution Addition of 12 M HCl formation of red precipitate; solution turned blue-green Filtration Filtrate blue-green solution Residue yellow-brown precipitate After washing with dH2O yellow-brown precipitate After washing with ethanol yellow-brown precipitate After washing with ether yellow-brown precipitate After drying yellow-brown precipitate Initially, N,N-diethyldithiocarbamate was synthesized by mixing 0.40 mL diethylamine, 0.17 g NaOH and 8.3 mL water in an Erlenmeyer flask. The solution was swirled for 5 minutes and then treated with 0.25 mL carbon disulfide. The reaction involved is shown on the following equation: N,N-diethyldithiocarbamate was synthesized in situ since it easily decompose releasing sulfur, nitrogen and sodium oxides with heat. Also, preparing it in situ will also prevent the formation of impurities. The flask was covered with watch glass and was swirled again for another 10 minutes. About 0.58 g of sodium molybdate(VI) dihydrate, the source of Mo(VI) was then added to the mixture, followed by dropwise addition of a solution containing 1.30 mL concentrated HCl in 16.70 mL water for about 15 minutes. The mixture was swirled vigorously during the dropwise addition. Doing so allows the complete interaction of the reacting species, homogenization of the pH since too acidic condition may reduce Mo(VI) to Mo(IV) and prevention of the reaction of molybdenum complexes with HCl that may produce undesired products. The reaction involved is as follows: At this point, yellow-brown precipitate started to form and it was isolated by filtration using sintered funnel. This was washed with water and ethanol. The over-all reaction for the synthesis of cisMoO2(S2CNEt2) 2 is: The theoretical yield computed is about 1.0169 g for cis-MoO2(S2CNEt2)2 and the reaction involved for its formation is electrophilic in nature. The structure of the cis and trans isomers of MoO2(S2 CNEt2)2 is shown on Figure 4.1. It shows that there are two possible configuration for the synthesized product. However, only the cis isomer is favored. This is because the O-Mo and Mo-O in cis- isomer preserve the integrity of the σ and π bond as described on Figure 4.2. This does not happen in trans- isomer. In the trans- isomer, presence of O-Mo bonds weakens the other Mo-o bond. N N S S S O O S S O O S S N S Mo Mo N Fig. 4.1 cis- and trans-isomers of MoO2(S2CNEt2) 2, respectively. M-O sigma bond M-O pi bond Fig. 4.2 M-O sigma and pi bond exist in cis- isomer of MoO2 (S2CNEt2)2. Based on the NMR data provided on Table 4.2, the compound shows that it has a chemical shift of 1.32 and 3.80 which corresponds to a triplet and quartet and designated by –CH3 and –CH2 respectively. This means that the compound consisted of 20 H atom, which supports that the synthesized product is the cis isomer. Table 4.2. H1 NMR data of the oxomolybdenum complexes. Compound Chemical Shift, δ (multiplicity) 1.32 (triplet), 3.80 (quartet) P none R 1.38 (triplet), 3.87 (multiplet), 3.93 (multiplet) Y 1.42 (triplet), 3.90 (multiplet) From cis-MoO2(S2CNEt2)2, several colorful oxomolybdenum complexes were synthesized by assuming different ratio with the oxo and N,N-diethyldithiocarbamate ligand and the molybdate ion. The colorful complexes are purple, red and yellow compounds. The molecular formulas as well as the structures of these compounds were deduced from the H NMR, IR and MS data provided which were summarized on Tables 4.2, 4.3 and 4.4. Table 4.3. IR data of the oxomolybdenum complexes. v/cm-1 Compound- P, R, Y 1440, 1380, 1360, 1280,-, 1150, P, R, Y 1100, 1090,- R 950 Y 940, 920 (shoulder) P 755 (750) P 295, 220 Y Band Assignment C-N stretch C-H stretch Mo=O stretch Mo=O stretch Mo=O stretch O-Mo-O bridge Mo-Cl stretch Table 4.4. MS data of the oxomolybdenum complexes. P, m/Z = 832 R, m/Z = 410 Element % # of atoms % # of atoms C- H- N- S- Cl 0 0 0 0 Mo- O- Molecular Mo2O3(S2CNEt2 ) 4 MoO(S2CNEt2) 2 Formula Y, m/Z = 480 % # of atoms- MoOCl 2(S2CNEt2) 2 The purple compound, P The purple compound was synthesized by adding 0.05 g triphenylphospine (PPh3) in 3.5 mL methanol to the dissolved 0.17 g cis-MoO2(S2CNEt2) 2 in 2 ml dichloromethane. The solution was swirled in order to ensure the complete interaction of the components and allowed to stand for 10 minutes to induce formation of precipitate. The purple solid was isolated through suction filtration and the resulting product was washed with methanol. The observations during the synthesis of purple compound are summarized on Table 4.5. Table 4.5. Observations on the synthesis of purple compound. Action Taken Observations 0.17 g cis-MoO2(S2CNEt2) 2 + 2ml dichloromethane yellow-brown mixture PPh3 + methanol clear solution Mixture + PPh3 solution formation of red-violet solution After standing for 15 minutes red-violet solution Filtration Filtrate deep purple to black solids Residue light pink liquid The molecular formula and structure of the purple compound was deduced from the given IR, NMR and MS data. Figure 4.3 shows the synthesized purple compound. N N S S S Mo O Mo S S O S O O S S N N Fig. 4.3 Mo2VO3(S2CNEt2) 4 or the purple compound, P. The chemical reactions involved are as follows: And the over-all chemical reaction involved is: Since there is a change in the oxidation state of the molybdate ion from VI to V, then it can be concluded that the reaction involved is a nucleophilic reaction. In the reaction, PPh3 was oxidized and acted as an acceptor of oxygen atom. Methanol and dicholoromethane were used as solvents. The purple compound initially appeared as very dark purple to black crystals after drying. However, scratching it using a spatula showed that the compound was indeed, purple. This could be attributed to the principle of tribochromism. Tribochromism is the change in the color of the compound due to mechanical friction. (Housecroft, C. E. and Sharpe, A. G., 2005) The red compound, R The red compound was synthesized by dissolving 0.33 g of cis-MoO2 (S2CNEt2) 2 and 0.33 g PPh3 in 1.7 mL 1,2-dicholoethane. The mixture was then place in a micro-refux set-up under the hood since the compound is moderately air-sensitive. Then, excess PPh 3 was added. Refluxing prevents the airoxidation of the resulting compound. Oxygen can oxidize Mo(IV) to Mo(VI) and this will take over in the reaction described below: This was the also the reason why the mouth of the condenser was covered with tissue paper. Refluxing also drive the reaction forward and allow the complete interactions of the reactants. The mixture was refluxed for 15 minutes. Upon completion of the reflux, the reaction mixture was poured with swirling into 50 mL cold ethanol to wash. The crystals formed were again filtered and washed with minimal ethanol and dried. Excess PPh3 was added in order to ensure that one oxo-ligand was removed from every cis-MoO2 (S2CNEt2) 2molecule. Based from MS data, the formula for R compound was deduced. The observations on the synthesis of the red compound are summarized on Table 4.6. Table 4.6. Observations on the synthesis of red compound. Action Taken Observations cis-MoO2 (S2CNEt2) 2 + PPh3 + 1,2-dichloromethane red-purple solution Addition of 1,2-dichloromethane red-purple solution After reflux red-purple solution Addition of cold ethanol formation of red-purple crystals Filtration Filtrate purple liquid Residue red-purple crystals Washing with ethanol red-purple crystals Washing with ether red-purple crystals Since there is again a change in the oxidation number of the metal ion, then the reaction is nucleophilic. Based from the IR, NMR and MS data, the structure of R compound was deduced to be: O S S Mo N S S N Fig. 4.4. Mo IV O(S2 CNEt2) 2 or the red compound, R. The yellow compound, Y The yellow compound was synthesized dissolving 0.17 g of cis-MoO2(S2CNEt2) 2 in 11.7 mL acetone and filtering it using fluted filter paper. The filtrate was then treated with 0.8 ml 12 M HCl and swirled for 10 minutes. The mixture was allowed to stand in the ice bath for a couple of hours. Table 4.7 shows the summarized observations during the synthesis of the yellow compound. Table 4.7. Observations on the synthesis of the yellow compound, Y. Actions taken Observations 0.17 g cis- MoO2(S2CNEt2) + 11.7 mL acetone Red-orange mixture Filtration Filtrate Red-orange liquid Residue Yellow-brown solids Addition of 0.8 mL HCl to filtrate Turbid yellowish mixture Swirling for 10 minutes Formation of yellow precipitate in light green liquid After standing No change Filtration Filtrate clear light green liquid Residue pale yellow powdery solid HCl acted as the donor of the Cl - ion, the electrophile. There is no change in the oxidation number of the metal ion. Thus the reaction is electrophilic in nature. The reaction involved based on the predicted molecular formula in mass spectroscopy data is described below. The product obtained can exist in three isomeric forms. However, the top most form as can be seen on Figure 4.5 is the most favored orientation of the ligands because of the less steric hindrance between the O and Cl atom. Oxygen and chlorine atoms, being in a mer position or existing at one plane and the dithiocarbamato at the axial position gives the least steric hindrance for the yellow ccompund, Y. O S N Mo S S Cl Cl O Cl S N Mo S N S Cl S S N N O Cl S Mo S Cl S S N Fig. 4.5. Three isomers of MoVIOCl 2(S2CNEt2) 2 or the yellow compound, Y. Oxygen-transfer Chemistry In order to investigate the related reactions involved in the synthesis, several test tube reactions were performed. First, cis-MoO2 (S2CNEt2 ) 2 and R compound, both dissolved in dichloromethane were mixed and resulted to a bloody-red solution. Theoretically, the resulted solution must be purple in color, specifically the purple compound, P. The reaction involved is: yellow-brown red purple This reaction is a nucleophilic reaction since there is a change in the oxidation number of the metal ion. Second experiment was done by allowing compound R dissolved in dichloromethane to be exposed to air. As it has been said previously, R compound is air sensitive and may cause the oxidation of Mo(IV) to Mo(VI). This, when R compound is oxidized, the red compound turned yellow brown. The reaction involved is: Red yellow-brown This reaction is also a nucleophilic reaction since there is a change in the oxidation number of the metal ion. The last experiment done was the addition of hydrogen peroxide solution to a solution of R in dichloromethane. This was swirled for one minute and then excess PPh3 was added. Yellow compound reacted with PPh3 produces purple compound, P. The reaction involved is: Yellow purple The reaction is again nucleophilic since a change in oxidation state occurred. It can be concluded then that for an oxo ligand to transfer, the oxygen atom donor should contain a metal center with high oxidation number. The reaction is enhanced by a strongly oxidizing metal center, a weak M=O bond, and an oxygen atom acceptor which forms relatively strong bond with oxygen. Conclusion The exercise involves the synthesis of cis-MoO2(S2CNEt2) 2 by preparing first the N,Ndithiocarbamate ligand in situ and eventually adding the sodium molybdate(VI) dihydrate. This was followed by dropwise addition of HCl with vigorous agitation. Vigorous agitation allows the regulation of pH in the solution avoiding the formation of side products. The yellow -brown cis-MoO2(S2CNEt2) 2 was then isolated and washed. Electrophilic reaction happened since there is no change in the oxidation number of the metal ion. The cis- isomer predominates in the reaction since at this conformation, σ and π bond are preserved. The synthesized cis-MoO2(S2CNEt2) 2 was used for the synthesis of other colored oxomolybdenum complexes such as purple, red and yellow compound. The molecular formulas and structures of the synthesized compounds were deduced using the IR, H 1 NMR and MS data that was provided. Purple compound was synthesized by the addition of 0.05 g PPh3 on the product. This allows the formation of a very deep-purple to black crystals which have the molecular formula of Mo2 VO3(S2CNEt2) 4. Since the oxidation number of Molybdate ion is changed from +6 to +5, it is considered as nucleophilic reaction. The synthesis of red compound is also a nucleophilic reacti on. This was done by adding 0.33 g PPh3 to cis-MoO2(S2CNEt2) 2 yielding the red-purple compound MoIV O(S2CNEt2) 2. PPh3, for both purple and red compound, acted as oxo-acceptor giving OPPh3 as part of the product. Yellow compound, on the other hand involves electrophilic addition of Cl - ion from HCl to cisMoO2(S2CNEt2). The product of this reaction is Mo VIOCl 2(S2CNEt2) 2 which exist in three isomers but the isomer with least steric hindrance is favored. The oxygen transfer chemistry of the synthesized compounds was further studied by mixing cisMoO2(S2CNEt2) 2 and R compound yielding purple compound. Exposing R compound to air yielded cisMoO2(S2CNEt2) 2 and addition of hydrogen peroxide and excess PPh 3 to yellow compound yielded the purple compound. It was then concluded that for an oxo-ligand transfer to be effective, the oxygen atom donor should contain a metal center with high oxidation number. Also, the reaction is enhanced by a strongly oxidizing metal center, a weak M=O bond, and an oxygen atom acceptor which forms relatively strong bond with oxygen. References 1. Housecroft, C. E. and Sharpe, A. G. (2005). Inorganic Chemistry. 2nd ed. UK: Pearson Education International. pp-. Meissler G.L. and Tarr D.A. Inorganic Chemistry. 3rd. ed. USA: Pearson Education International. pp-. Cotton, F. A. and Wilkinson, G. (1972). Advanced Inorganic Chemistry: A comprehensive text. 3rd ed. USA: John Wiley and Sons, Inc. pp. 923-926 SAMPLE CALCULATIONS 1. Molecular Formula of compound P Assume that the given ration (m/Z) is the mass of each element for every 100 g sample. a) # of moles of C, H, N, S nx=  nc =  nH =  nN =  nS = b) number of atoms of C, N, H, S # of atoms = moles of atom (  for C = =- ≈ 20 atoms  for H = =- = 40 atoms  for N = =- ≈ 4 atoms  for S = =- ≈ 8 atoms c) mass of Mo and O 1 mole Mo = 2 moles S2CN(C2H5) 2 From the data S8C20N4H40 or S8 C4N4 (C8H20)2; S2(x)CxNx(C2(x)H5(x)) 2 = S2(4)C4N4(C2(4)H5(4))2 x =4 ) Thus, 4 moles of S2CN(C2H5) 2 = 2 moles Mo Mass Mo = = 23.0625 g Mo Mass O = 100 g – (mC + mH+ mN + ms + mMo) = 100 g –(28.75 g + 4.85 g + 6.65 g + 30.85 g + 23.06 g) = 5.84 g d) number of atoms of Mo and O  nMo = =- moles for Mo =- moles =- ≈ 2 atoms  nO = for O = = 3.0368 ≈ 3 atoms e) molecular formula of compound P C20H10N4 S8Mo2O3 ; or Mo2O3(S2CNEt2)4 2. Theoretical Yield a) cis-MoO2(S2CNEt2) 2 0.58 g Na2MoO4 · 2 H2O x ( )x )x( =- g cis-MoO2(S2CNEt2) 2 b) derivative (Purple compound) 0.17 g cis-MoO2 (S2CNEt2) 2 x ( =- g x( )x( )