Synthesis & Characterization of a Metal Hydride Complex

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Title: Synthesis & Characterization of a Metal Hydride Complex

Objective

1.     To synthesis a cobalt hydride complex and deduce its chemical structure based on the spectral data.

Introduction

            Hydrogen atom, H can coordinate to the transition metal center as σ-donor ligand or as σ-ligand. When it acts as a σ-ligand, it coordinates to the metal center through the single bond between the two hydrogen atoms, H-H and results in a dihydrogen complex. However, it can also coordinate to the transition metal center through hydride form, H^- and produce dihydride complex, and are commonly known as covalent hydrides. The following figure shows the difference between the dihydrogen and dihydride complex.

                                                              (no image available)

(I)                                            (II)

Figure 1 Structure (I) and (II) represents a general structure for dihydrogen and dihydride metal complex respectively

            Metal hydride complexes are important as intermediates in many catalytic processes such as alkene oligomerization and hydrogenation. Recently, there also have been a lot of researches on metal hydride complex as a potential candidate for fuel storage for energy consumption applications and as prospective materials for neutron radiation shielding (Stepien, 2005). However, it requires techniques of compressing gaseous hydrogen to pressure of a few gigapascals to synthesize hydrides of most transition metals, such as cobalt hydrides. In order to synthesize metal hydride complexes, a number of preparation methods can be used included (i) protonation (requires an electron rich basic metal center), (ii) from hydride donors (main group metal hydrides), (iii) from H₂ (via oxidative addition – requires a coordinatively unsaturated metal center),  and (iv) from a ligand (β-elimination).

A metal hydride may have acidic or basic character depending on the electronic nature of the metal involved and its ligand set. Early transition metal hydrides tend to carry significant negative charge on the H atom whereas later more electronegative transition metals favour a more positive charge on the H atom, thus the term hydride should not be taken literally. In this experiment, we are going to synthesize organophosphine derivative of cobalt hydride complexes from a hydride donor, sodium borohydride (NaBH₄) in the presence of excess ligands. Sodium borohydride is an ionic hydride which liberates hydrogen gas immediately after dissolve in water. It is also a good reducing agent that finds wide application in laboratory and on a technical scale, especially in bleaching the wood pulp. It is used in this experiment to reduce the oxidation state of the cobalt metal center and to provide source of hydride ions as ligand. On the other hand, cobalt hydride complex with the triphenylphosphite is the first examples of metal hydrides stabilized by phosphite ligands. Triphenylphosphite is a bulky ligand when coordinate to the metal center through the lone pair electrons on the P-atom. This bulky ligand will exert steric effect on the metal complexes and thus blocks the larger size ligand from coordinate to the cobalt metal center. Obviously, the smaller size of hydride has no problem to coordinate to the cobalt metal center. However, coordinated hydride ligand often cause distorted geometry in this cobalt complex.

Procedures

Part A: Preparation of Metal Hydride

1.     A solution of 0.5 g of sodium borohydride in 10 mL is added dropwise to a stirred solution of cobalt(II) nitrate hydrate (1.5 g) and triphenylphosphite (8.0 g) in 30 mL ethanol at 25 °C.

2.     After 15 minutes, the solid is filtered, washed with ethanol, water and finally methanol and dried at the pump.

3.     The product was recrystallized by dissolving in 30 mL of dichloromethane and filtered to obtain a clear dichloromethane solution.

Part B: Characterization of Product

1.     The yield of the product was recorded.

2.     IR and ¹H NMR spectrum of the complex was obtained.

Results & Calculations

Table 1 Weight of materials used and products formed.

Materials	Weight (g)
NaBH4	0.5781 g
Co(NO3)2 • 6 H2O	1.5873g
P(OPh)3	8.0773 g
Beaker	105.7246g
Beaker + Product	107.4530g
Product	1.4284g

Co²⁺ + 4 P(OPh)₃ + H^- + e^-                 HCo[P(OPh)₃]₄

Moles of Co²⁺ = Moles of Co(NO₃)₂ • 6 H₂O used

                        = 1.5873 g / 291.0352 g mol^-1

                        = 5.45 x 10^-3 mol

Moles of H^- = Moles of NaBH₄ used × 4

                    = (0.5781 g / 37.83 g mol^-1) × 4

                    = 6.11 x 10^-2 mol

Moles of P(OPh)₃ used = 8.0773 g / 310.28 g mol^-1

                                      = 2.60 x 10^-2 mol

If P(OPh)₃ is the limiting reagent, then:

Moles of HCo[P(OPh)₃]₄ produced = Moles of P(OPh)₃ / 4

         = 2.60 x 10^-2 mol / 4

         = 6.51 x 10^-3  mol

.

From the calculation above, it shows that Co(NO₃)₂ • 6 H₂O is the limiting reagent.

Therefore, moles of HCo[P(OPh)₃]₄ produced = Moles of Co²⁺

                                                                           = 5.45 x 10^-3 mol

Theoretical weight of HCo[P(OPh)₃]₄ produced = 5.45 x 10^-3 mol × 1301.05 g mol^-1

                                                                              = 7.091g

Percentage yield of HCo[P(OPh)₃]₄ = (1.4284 g / 7.091 g) × 100 %

                                                          = 20.14 %

Table 2 IR frequencies of starting material and products formed.

Compound 1: Cobalt (II) nitrate hydrate

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

Compound 2: Triphenylphosphite

Significant signals
Expected (from table)	Wavenumber (cm-1)	Observed (from spectrum)
Aromatic C=C stretch	1400-1600	1481, 1590
=C-H stretch	3010-3100	3062, 3038
C-P stretch	700	746

Compound 3: Hydirotetrakis(triphenylphosphito)cobalt (II)

Significant signals
Expected (from table)	Wavenumber (cm-1)	Observed (from spectrum)
=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

Table 3 ¹H NMR spectrum of complex

Chemical shift (ppm)	~ − 11.5	~ 7.5
Division	11mm / 10 = 1.1 mm	66 mm
Ratio	1.1 / 1.1 = 1	66 / 1.1 = 60
Integration	1	60
Types of Proton	−H	12 × −C6H5

Discussion

From the experiment above, the percentage yield of product is calculated to be 20.14 % . Sodium borohydride (NaBH₄) was used for it is a good reducing agent and provides the hydride ions, H^-  and electrons to the complex, which reduces Co²⁺ to Co⁺. From the IR spectrum of Co(NO₃)₂ • 6 H₂O, the peaks found were namely;

i)                O-H stretching frequency at 3403 cm^-1

ii)              bending frequency of O-H at 1629 cm^-1

iii)             Asymmetric stretching frequency of NO₂ at 1384 cm^-1.

As for hexahydrate nitrate ions, the IR frequencies included δ (O-H) at around 1575-1675 cm^-1, _as(NO₂) between 1260-1375 cm^-1 and 1450-1600 cm^-1. On the other hand, IR spectrum of P(OPh)₃ consist of sp² C-H stretch frequency at 3062 cm^-1 and 3038 cm^-1, aromatic C=C stretch (1590 cm^-1, 1481 cm^-1), and P-C stretch (746 cm^-1).

When comparing the IR spectrum, there were correlation between that starting material and the final product. It consisted of sp² C-H stretch (3067 cm^-1), aromatic C=C stretch (1591 cm^-1, 1490 cm^-1) and P-C stretch (757 cm^-1). Absence of asymmetric stretching frequency of NO₂ in the IR spectrum indicates that the complex formed does not contain any of the nitrate ions. Besides, there is also no O-H stretch frequency in the IR spectrum of the complex, indicating the complex if free from water molecules. Interestingly, no ν(M-H) can be detected in the infrared spectra of the cobalt complex that we have synthesized, but the presence of hydride ligands is confirmed by the appearance of a quintet pattern in the high-field NMR spectra (Levison & Robinson, 1972).

From the ¹H NMR spectrum, there are only two types of proton present, which are –H at around −11.5 ppm and −C₆H₅ at around 7.5 ppm. Since there is only one –H, this could be attributed to the only one hydride ligand present in the complex synthesized. On the other hand, there is twelve −C₆H₅ functional group present in the complex, resulting in P(OPh)₃ groups in the complex. Since there is no other ligands attached to the Co metal center after comparing both IR and ¹H NMR spectrum, the chemical structure of the complex synthesized can be deduced as HCo[P(OPh)₃]₄. In this complex, the formal oxidation state for Co metal center is Co(I), which was reduced from Co(II). As there is only one anionic ligand attached to the Co metal center, with H^- as a single negative charge and P(OPh)₃ is a neutral ligand. Thus, the oxidation state of Co metal center in this neutral complex should be Co(I). Excess electrons that were used to reduce the oxidation state of Co(II) was obtained from the NaBH₄. Below structure depicts the arrangement of the synthesized complex.

The P-atoms of the four triphenylphosphite ligands are disposed in a distorted tetrahedral geometry around the Co(I) ion. Hydride ligand is located at a location trans to one of the P-atoms, showing a monocapped tetrahedral complex with the hydride as the face-capping ligand. From the journal, (Crane & Young, 2004) has shown that the hydride ligand trans to one of the P-atoms was strongly indicated by the long Co-P bond distance of 2.1191 (7) Å caused by the trans influence of the hydride, and the pattern of bond angles subtended at the cobalt center. The location was confirmed by the high residual electron density observed at this position in the difference Fourier map and the subsequent successful free refinement of the positional parameters for the hydride ligand, with a Co-H distance of 1.36 (2) Å.

On the other hand, since cobalt is in Group 9, Co(I) has dⁿ = d^9-1 = d⁸, contributing 8 electrons towards electron counting. The anionic hydride ligand will contribute 2 electrons, and the four neutral P(OPh₃) ligands will contribute 8 electrons, each contributes 2 electrons. Hence, the total electrons for the complex HCo[P(OPh)₃]₄ would be 8 + 2 + 8 =18 electrons.

Conclusion

The percentage yield of this complex is 20.14%. After comparing IR and ¹H NMR spectrum, the cobalt hydride complex that have been synthesized is having the chemical formula of HCo[P(OPh)₃]₄. The geometry of this complex is monocapped tetrahedral and has an 18 electrons complex. The formal oxidation state of Co is Co (I).

References

1.     Daniel J. Goebbert, Etienne Garand, Torsten Wende, Risshu Bergmann, Gerard Meijer, Knut R. Asmis & Daniel M. Neumark (2009). Infrared Spectroscopy of the Microhydrated Nitrate Ions NO₃^- (H₂O)_1-6. J. Phys. Chem. A, 113, pp. 7584 – 7592.

2.     J. J. Levison & S. D. Robinson (1972). Inorganic Syntheses, Volume XIII. United States, U.S.: McGraw-Hill, Inc. Chapter 4, pp. 105 – 111.

3.     Jonathan D. Crane & Nigel Young (2004). Hydridotetrakis(triphenylphosphito)cobalt(I). Acta Crystallographica, E(60), m487 – m488.

4.     Zdzislaw M. Stepien (2005). Formation of Cobalt Hydrides in Low Temperature Field Evaporation. Optica Applicata, XXXV(3), pp. 363 – 368.