Saturday, June 29, 2013

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:

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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.

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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

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Part B: Characterization of Metal hydride

image

 

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

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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).

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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.

 

 

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