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 = d9-1 = 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.
