Synthesis & characterization of a metal hydride complex

Objectives:

1. To synthesize a cobalt hydride complex

2. To deduce its chemical structure based on the spectral data

 

Introduction:

Metal hydride complexes are very crucial as the intermediates in many catalytic processes such as alkene oligomerization and hydrogenation. Covalently bonded metal hydride complexes are known for all the transition metals. The complexes often contain the metal in a low oxidation state with the ligands of phosphines, carbon monoxide or cyclopentadiene.

 

Isomerization of alkene is always a possibility in any homogenous catalytic reaction that involved alkene. Migratory insertion of alkene into the metal hydrogen (M-H) bond can occur in a Makovnikov addition or anti-Makovnikov manner. Alkene isomerization is a process that involves Makovnikov addition followed by a β-elimination which is shown in the diagram 1 below:

Figure 1: Isomerization of butene by hydride mechanism

In the Figure 1, it shows that the but-2-ene is synthesized from but-1-ene through the hydride mechanism in which metal hydride acts as the intermediate.

One of the well known processes of homogeneous catalytic reaction is hydrogenation of alkene. The metal hydride complex plays a very important intermediate in the hydrogenation of alkene, for example, Wilkinson’s catalyst in the hydrogenation of alkene. The following figure 2 shows how the metal hydride acts as an intermediate in the particular process.

 

Firstly, Wilkinson’s complex will dissociates one phosohine ligand to form a 14 electron complex. This followed by the oxidative addition of hydrogen to form a metal hydride intermediate. In the following step, alkene is added into the metal complex via ligand addition. Migratory insertion of alkene into the M-H bond leads to the formation of alkyl ligand that bonded to rhodium metal. Alkane is formed at the end of the process via reductive elimination between a hydride and an alkyl group. The catalyst is being reused in the hydrogenation process and the process is repeating again.

 

In this experiment, metal hydride complex is being synthesized and characterized by using proton nuclear magnetic resonance (¹H NMR) spectrophotometer in order to determine the number of proton present. IR spectrophotometer is also used in characterizing the metal complex.

 

Materials:

Sodium borohydride, ethanol, cobalt(II) nitrate hydrate, triphenylphoshate, methanol, dichloromethane

 

Instruments:

FT-IR spectrophotometer, NMR spectrophotometer

 

Apparatus:

Melting point apparatus, hotplate stirrer, magnetic stirrer bar, Buchner funnel, beaker, Erlenmeyer flask, Hirsch funnel

 

Procedure:

Part A: Preparation of Metal hydride

Part B: Characterization of Metal hydride

 

Results and calculation:

Table 1: Weight of hydridotetrakis(triphenylphosphito)cobalt(I)

Weight of empty sample vial	12.5925g
Weight of ( sample vial + metal hydride)	19.3553 g
Weight of metal hydride	6.7628 g

Calculating percentage yield of metal complex

One mole of Co²⁺ reacts with one mole of P(C₅H₅)₃.

Mole number of Co²⁺ = 1.5240g / 200.93g mol^-1

= 0.0076 mol

Mole number of P(C₆H₅)₃ = 6.8037g/ 261.97g mol^-1

= 0.026mol

Thus, Co²⁺ is the limiting reagent since P(C₆H₅)₃ is in excess.

Theoretical mass of hydridotetrakis(triphenylphosphito)cobalt(I)

= 0.0076mol x 1107.81g mol^-1

= 8.4194g

Percentage yield = 6.7628 g / 8.4194g x 100%

= 80.32%

 

Table 1: Integration value and number of proton present for each peak in NMR spectrum

Value of integration	= 60	= 1
Ratio	60	1
Number of protons present	60	1

Note: One triphenylphosphate contains 15 H

Since one triphenylphosphate contains 15H, so the presence of 60H indicates that there is four triphenylphosphate ligands binded to the metal hydride complex.

Table 2: Significant peaks of cobalt(II) nitrate hydrate in IR spectrum (Appendix I)

Significant signals	Wavenumber (cm-1)
Expected (from table)	Observed (from spectrum)
O-H stretch	3200-3550	3403
Asymmetric NO2 stretch	1450-1600	1629
Symmetric NO2 stretch	1260-1375	1384

Table 3: Significant peaks of triphenylphosphite in IR spectrum (Appendix II)

Significant signals	Wavenumber (cm-1)
Expected (from table)	Observed (from spectrum)
Aromatic C=C stretch	1400-1600	1481, 1590
=C-H stretch	3010-3100	3062, 3038
C-P stretch	700	absent

Table 4: Significant signals of hydridotetrakis(triphenylphosphito)cobalt(I) in IR spectrum (Appendix III)

Significant signals	Wavenumber (cm-1)
Expected (from table)	Observed (from table)
=C-H stretch	3010-3100	3067
Aromatic C=C stretch	1400-1600	1490, 1591
Co-H stretch	1745-1933	absent
C-P stretch	700	691

Note: The C-P stretch value is obtained from journal. (Kurita et al, 2003)

Electron counting of hydridotetrakis(triphenylphosphito)cobalt(I)

Hydride: donates 2 electrons, each triphenylphosphito donates 2 electrons

Hydride: -1, triphenylphosphosphito: neutral

Oxidation state of cobalt = Co(I), with d⁶

Total electron count = 2 + 4(2) + 6

= 16 electrons

 

Discussion:

The percentage yield of hydridotetrakis(triphenylphosphito)cobalt(I) is 80.32% in which the mass of product obtained experimentally is 6.7628 g.

In this experiment, sodium borohydride (white) was used to provide hydride to metal complex to form a cobalt hydride complex. Ethanol acts as a medium to allow the cobalt complex (greenish brown) can form in the solid state since the cobalt hydride complex is not soluble in ethanol. The product was washed with ethanol, water and methanol is to remove any other unreacted starting materials. This method can reduce the presence of impurities.

From the IR spectrum of triphenylphophate, the significant signals include aromatic C=C (1481cm^-1 and 1590cm^-1) and =C-H (3062cm^-1 and 3038cm^-1). However, the expected C-P stretch at 700cm^-1 did not present. According to the IR spectrum of product, the significant signals that present are =C-H- stretch (3067cm^-1), aromatic C=C (1490cm^-1 and 1591cm^-1) and C-P stretch (691cm^-1). However, there is another expected significant signal did not present in the IR spectrum which is the Co-H stretch (1745-1933cm^-1).

Based on the ¹H NMR spectrum of product, the integration of the first signal (at positive ppm value) shows 66mm while the second signal (at negative ppm value) shows 1.1mm. From the spectrum, number of protons present in the first and second signals is 60 H and 1 H respectively. Hence, the molecular structure of hydridotetrakis(triphenylphosphito)cobalt(I) is deduced as monocapped tetrahedral with the hydride as the face-capping ligand. The figure 3 below shows the structure of hydridotetrakis(triphenylphosphito)cobalt(I).

Figure 3: Molecular structure of hydridotetrakis(triphenylphosphito)cobalt(I)

The fomal oxidation state of cobalt in the metal complex is Co(I). This is because there is only one hydride carrying one negative charge bonded to cobalt while the other four triphenylphosphine ligands are neutral. Based on the electron counting in the calculation and result part, the total electron is 16. The metal complex is stable in 16 electrons because the four sterically bulky triphenylphosphine ligands blocked the vacant site and hence other ligand is not allowed to bind to cobalt. As a result, hydridotetrakis(triphenylphosphito)cobalt(I) has a total number of 16 electrons.

Precaution steps:

1. Make sure the sodium borohydride will never contact with acid or water since it will liberates hydrogen.

2. Dichloromethane is highly volatile and must be used in the fume hood only.

 

 

Flickr Tags: synthesis,characterization,metal hydride,IR,Makovnikov addition,β-elimination,hydridotetrakis(triphenylphosphito)cobalt(I)

Metal complexes of Saccharin Synthesis & Characterization

Objectives:

1. To synthesize tetraaqua-bis(o-sulfobenzoimido)copper(II) and tetraaqua-bis(o-sulfobenzoimido)cobalt(II)

2. To characterize tetraaqua-bis(o-sulfobenzoimido)copper(II) and tetraaqua-bis(o-sulfobenzoimido)cobalt(II)

 

Introduction:

Saccharin (C₇H₅NO₃S), also known as 1,2-benzisothiazol-3(2H)-one is discovered by Remsen and Fahlberg in 1879 (as cited in Jovanovski, G., 2000). Saccharin and aspartame are artificial sweeteners which were developed over the years in order to eliminate the caloric intake in the diet associated with carbohydrate sugars. Saccharin is about 500 times sweeter than sugar. Its water soluble sodium salt is widely used as synthetic sweetener for diabetics as well as addictive in dietetic products.

The saccharin was intensively investigated due to its suspected carcinogenic nature. In the studies of Price and his co-workers, it has been shown that it causes urinary bladder carcinomas in mice when implanted in the bladders of mice (as cited in Jovanovski, G., 2000). Since that time, extensive work was carried out to investigate the effect of saccharin on human metabolism. As a result, saccharin has been categorized into the list of potential human carcinogens.

Saccharin contains three functional groups (carbonyl, imino and sulfonyl) connected to each other in a five-membered ring which is condensed to a relatively stiff benzene ring as shown in Figure 1. Saccharin has a high degree of hapticity. It acts as monodentate ligand by using nitrogen atom, carbonyl oxygen atom or sulfonyl oxygen atom and as bidentate ligand via either two atoms. It also acts as a neutral ligand donor. The following figure shows the structures of saccharin and saccharinate anion.

Figure 1 Structure of saccharin and saccharinate anion

From the Günzler, H. & Gremlich, H. ‘s studies of saccharin (as cited in Teleb, S.M, 2004), the spectrum of free saccharin (Hsac) exhibits a weak band at 3215 cm^-1 due to the v(N-H) vibration. Based on spectral investigations for a series of metal saccarinates (Teleb, S.M., 2004), the wavenumber of v(CO) mode can be used to make certain predictions on the type of metal-saccharinate bonding (bonding between metal and nitrogen). Teleb also found that the lowering in v(CO) wavenumber is more pronounced in ionic bond between metal and saccharinate anion. The v(CO) stretch of free saccharin can be observed at the wavenumber of 1725cm^-1 (Jovanovski, G., Soptrajanov, B., & Kamenar, B., 1990, as cited in Teleb, S.M., 2004) and at 1675cm^-1 in the spectrum of sodium saccharinate (Jovanovski, G. et al., 1988, as cited in Teleb, S.M., 2004). Besides, the wavenumbers of sulphonyl stretching, v(SO₂) in free saccharin is observed at 1360cm^-1 and 1180cm^-1 (Jovanovski, G., Tanceva S., & Soptrajanov, B., 1995) for asymmetric and symmetric modes respectively.

In this experiment, cobalt(II) and copper(II) complexes of saccharin are synthesized in order to identify their spectral characterisations. Other metal complexes, such as iron(II), nickel(II), zinc(II) complexes of saccharin may be prepared by using the same method.

 

Materials:

Sodium saccharinate dihydrate, copper(II) sulfate pentahydrate, cobalt(II) chloride hexahydrate

 

Apparatus:

Magnetic stirring hotplate, magnetic stirring bar, beakers, Hirsch funnel, sand bath

 

Instruments:

FT-IR spectrophotometer, UV-Vis Spectrophotometer

 

Procedures:

Part A: Preparation of tetraaqua-bis(o-sulfobenzoimido)copper(II)

1. Copper(II) sulfate pentahydrate was placed in a beaker.

2. Sodium saccharin dihydrate and water were added into the beaker.

3. The mixture was stirred until dissolution occurs with warming on hotplate.

4. Light blue solution was warmed on a sand bath with stirring in order to concentrate the solution.

5. Allowed the contents to cool down slowly to room temperature.

6. The beaker was further cooled down in ice bath for 30 minutes and the crystal was collected by suction filtration.

7. The crystal was washed with minimum ice cold water and dried over silica gel in a dessicator.

 

Part B: Preparation of tetraaqua-bis(o-sulfobenzoimido)cobalt(II)

1. This compound was prepared by using the same procedures in part A. Cobalt(II) chloride hexahydrate, sodium saccharin dihydrate were dissolved in 6ml of water.

 

Part C: Characterization of metal complexes

1. The yield and colour of metal complexes were recorded.

2. IR spectra of the complexes were obtained.

3. UV-Vis spectra of complexes were obtained by using DMF as solvent. λ_max was determined.

 

Results and calculations:

 

Table 1.1: Significant peaks of starting materials in IR spectrum

Starting materials	Sodium saccharinate dihydrate, Na(sac).2H2O	Copper(II) sulphate pentahydrate, CuSO4.5H2O
O-H stretch	3364 cm-1	3339 cm-1
C=O stretch	1648 cm-1	-
SO stretch	1335 cm-1 (asym.), 1143 cm-1 (sym.)	-

 

Table 1.2: Significant peaks of tetraaqua-bis(o-sulfobenzoimido)copper(II) in IR spectrum

Vibrational mode	Wavenumber (cm-1)
O-H stretch	3567, 3503, 3414
C=O stretch	1618
SO stretch	1353(asym.), 1164(sym.)

 

Table 1.3: Significant peaks of tetraaqua-bis(o-sulfobenzoimido)copper(II) in IR spectrum

Vibrational mode	Wavenumber (cm-1)
O-H stretch	3567, 3503, 3414
C=O stretch	1618
SO stretch	1353(asym.), 1164(sym.)

 

References:

1. International Associates. (1985). Inorganic Syntheses. Canada: John Wiley & Sons, Inc.

2. Jovanovski G. (2000). Metal Sccharinates and Their Complexes with N-donor Ligands. Metodij University, Macedonia 73(3), 843-868.

3. Teleb, S.M. (2004). Spectral and Thermal Studies of Saccharinato Complexes. Journal of the Argentine Chemical Society, 92(4), 31-40.