Fac/Mer Isomerism in Fe(II) Complexes

1. I7 – Fac/Mer Isomerism in Low Spin Fe (II) Complexes
Rafia Aslam
1327166
05/12/13

2. Abstract
The investigation involved reacting benzylamine with 2-pyridine carboxaldehyde with addition of iron (II)
tetrafluoroborate producing a low spin octahedral complex displaying fac/mer isomerism. Through NMR the ratio
of fac/mer isomerism was analysed whilst UV-vis spectroscopy determined the extinction coefficients of the metal-
ligand-charge-transfer transitions. An IR spectrum was also recorded. An overall yield of 79.7% was obtained.

Introduction
The 5 d-orbitals, 3dxy, 3dxz, 3dyz, 3dx
2
-y
2
and 3dz
2
, are degenerate on an isolated metal ion. The orientation of these
orbitals in space explains why splitting occurs upon ligand attachment. The 3dxy, 3dxz and 3dyz orbitals have lobes
pointing between their respective axes; the latter’s along their axes.
The ligand attaches to the central metal ion via co-ordinate bonding. Lone pairs are donated across the x, y and z
axes, incurring repulsion. Hence the 3dx
2
-y
2
and 3dz
2
orbitals become higher in energy than 3dxy, 3dxz and 3dyz . This
observation forms the basis of the crystal field theory, founded by German physicists Hans Bethe and John
Hasbrouck in the 1930s. It considers the “spin pairing energy (P)” and “ligand field splitting parameter (∆₀)”.
Pairing of electrons generates repulsion, requiring energy to overcome. The crystal field theory describes how P <
∆₀ in low spin complexes where ligands are strong field therefore an additional electron enters either 3dxy, 3dxz or
3dyz orbitals (refer to Fig 1 below). However, should P > ∆₀, this electron would enter dz² or dx²-y² orbitals. This
occurs in high spin complexes where ligands are weak field, accounting for the large number of unpaired electrons.
(i) (ii)




Fig 1i – high spin complex; Fig 1ii – low spin complex
1
ΔO represents the energy absorbed by the complex when an electron is excited to a higher orbital, leading to the
complex transmitting a complementary colour; for example, [Fe(H2O)6]
2+
appears as a pale green hexa-aqua ion
because it absorbs red and blue light.
Furthermore, most iron complexes are paramagnetic as they contain unpaired electrons, whereas some are low
spin complexes and therefore diamagnetic as all the electrons are paired.
The extents of the pairing energy and ΔO are affected by ligand.
2
Low spin d
6
complexes are formed with ligands
occurring at the top end of the spectrochemical series, as they create larger splitting fields. The experiment
involves a π-acceptor ligand as electrons from the d-orbitals on iron can be transferred into the anti-bonding π *
orbitals on the ligand, increasing the splitting parameter, leading to low spin complex formation.

Experimental Method
0.32g of 2-pyridine carboxaldehyde was dissolved in 10cm
3
of methanol and added to 0.32g of benzylamine. The
pale yellow solution was stirred for 10 minutes before addition of 0.337g of [Fe(OH2)6][BF4]2 (Mr 337.55).
3

Energy Δo Energy Δo
dz² and dx²-y²
3dxy, 3dxz and 3dyz
dz² and dx²-y²
3dxy, 3dxz and 3dyz

3. The resulting dark purple solution was heated to reflux for 1 hour at 75°C before being cooled to a lukewarm
temperature. Stirring was continued as 7cm
3
of diethyl ether were added for recrystallisation, indicated by a
glittery appearance. A dark purple powder was isolated via Buchner filtration, washed with 20cm
3
of diethyl ether
and dried under suction. IR and H
1
NMR were recorded using Perkin Elmer Spectrum 100 FT-IR and 250 MHz
Bruker ACF NMR respectively. A 100cm
3
volumetric solution was prepared from weighing an accurately known
mass (close to 0.25g) of product and 100cm
3
HPLC grade acetonitrile. 1cm
3
of solution was transferred into
another 100cm
3
volumetric flask using a 1ml volumetric pipette, made to volume with HPLC grade acetonitrile.
Using quartz cuvettes, with pure acetonitrile as the blank, wavelength and absorbance were recorded via Varian
Cary 50 UV-VIS Spectrophotometer.
Results and discussion

where L =
(E)-N-Benzyl-1-(2-pyridinyl) methanimine
[4]
0.651g of [FeL3][BF4]2 was obtained, giving a yield of 79.7% (See Appendix C-1 for calculation). This could have been
higher if product had not been lost in apparatus during transfer onto weighing boats. The peak at 1.95ppm in the
H
1
NMR appears due to acetonitrile-D3 which was the solvent used, suggesting that the real yield would indeed
have been lower than 79.7% had a small amount of solvent not dissolved.
The IR spectrum shows the main peaks confirming the presence of imine ligands.
Wavenumber (cm
-1
) Corresponding bond Functional Group
1448.92 C-H Methylene bridging (-CH2-)
1592.97 C=C Aromatic
1615.38 C=N Imine
3036.55 C-H Alkane
Table 1; IR Data for Main Peaks in [FeL3][BF4]2 (See Appendix S-1 for Spectrum)
NMR
Table 2; H
1
NMR Data for [FeL3][BF4]2 (See Appendix S-2 for Spectrum)
The peak at 8.5ppm represents the imine hydrogen in the facial isomer. However, the corresponding peaks for
meridional isomers are too small and therefore not integrated by the NMR machine. Hence, a quantitative
identification of the ratio is not possible. The possible reasons for this could be that the NMR was carried out
within a few minutes of the product being dried under suction and isolate, hence the more favourable facial
isomers were present and interchange had not yet occurred, which explains the high fac: mer ratio. Nevertheless
Chemical shift (ppm) Corresponding to H- environment Splitting pattern
2.15 -N=CH- -C-C4H4N Singlet
3.25 -N=CH- -C-C4H4N Singlet
5.0 Ar-CH2- -N=CH- Doublet
5.45 Ar-CH2- -N=CH- Doublet
6.8 -N=CH- -C-C4H4N Singlet
7.1 -CH=N- -CH2-Ar Triplet
7.2 -CH=N- -CH2-Ar Triplet
7.35 -CH=N- -CH2-Ar Triplet
7.55 Ar-CH2- -N=CH- Doublet
7.9 -CH=N- -CH2-Ar Triplet
8.5 -N=CH- -C-C4H4N Singlet

4. it can be noted that 3 singlet peaks for the imine hydrogen atoms in mer would occur within the region of 8.2 to
9ppm as each is in a different plane, and hence, different environment, so the integration value for one of these
peaks is multiplied by 3. This can be used to calculate a fac/mer ratio.
Facial isomerism is generally more favourable than meridional in octahedral complexes as the 3 ligands lie on the
same plane at 90° to each other and with equal metal ion-ligand bond lengths. In meridional isomerism, one of the
ligands occupies a different plane at an angle of 180°. With time, the fac isomers interchange to mer so the ratio
becomes 1:1. In this experiment, the ligand is bidentate and the product consists of a racemic mixture of fac and
mer isomers with a relatively slow interchange.
5






Fig 2 – Fac/Mer isomerism in [FeL3][BF4]2
UV-vis Spectroscopy
Wavelength (nm) Absorbance (a.u)
516.5 0.1364
561.3 0.1836
Table 3; UV-vis Data for [FeL3][BF4]2 (See Appendix S-3 for Spectrum)
Electron transfer occurs from Fe
2+
into low lying π* antibonding orbitals in the aromatic ligands. This metal-ligand
charge transfer reduces build up of negative charge on the metal ion, leading to the final product displaying
intense colour. UV-vis spectrometry measures how much light a compound absorbs at specific wavelengths. Beer’s
law enables the calculation of molar extinction coefficients:-
A = εcl (A= absorbance, ε= molar extinction coefficient, c= concentration, I= path length)
“ε” accounts for the amount of light absorbed at a given wavelength. This value is different for different
compounds. Compounds possessing high ε values, such as [FeL3][BF4]2, absorb enough light to enable detection at
low concentrations.
A concentration of (3.019 ± 0.018) x 10
-5
mol dm
-3
was calculated (See Appendix C-2 for calculation).

Molar extinction coefficient
A = εcl
- A = 0.1836, c = 3.0188x10
-5
mol dm
-3
, I = 1 cm (width of quartz cuvette)
- ε = 0.1836/(3.0188x10
-5
x 1) = 6081.9 L mol
-1
cm
-1


- A = 0.1364, c = 3.0188x10
-5
mol dm
-3
, I = 1 cm
- ε = 0.1364/(3.0188x10
-5
x 1) = 4518.4 L mol
-1
cm
-1


MER- Ligands in different planes to each
other so 3 singlet peaks on H
1
NMR
FAC- All imine hydrogen atoms equivalent
so on same plane; one singlet on H
1
NMR

5. Energy of light absorption
E = hc/λ (E= energy, h = Planck’s constant, c = speed of light, λ = wavelength)
- λ = 561.3nm
- E = {(6.626x10
-34
) x (3x10
8
)} / (5.613x10
-7
) = 3.54x10
-19
J

- λ = 516.5nm
- E = {(6.626x10
-34
) x (3x10
8
)} / (5.165x10
-7
) = 3.85x10
-19
J

Conclusion
Analysis from UV-vis spectroscopy shows that lower wavelengths of light give higher frequencies which positively
correlate with the relative amount of energy needed to excite electrons to higher energy levels. It was also
understood that absorbance is proportional to the concentration of solution used. The experiment involved a
diluted volumetric solution resulting in lower absorbance as there were less molecules present to interact with
light. If concentration remained constant, a lower absorbance resulted in a lower ε value.
The complex displayed fac/mer isomerism. A high fac: mer ratio was observed due to lack of meridional isomers
present in the NMR spectrum.
Experimental limitations involved the assumption that Beer’s Law is valid for a variety of concentration ranges, yet
the law does not consider factors such as polychromatic radiation containing several wavelengths of light, or
electrostatic attractions that exist between molecules at high concentrations. These factors affect the linear
relationship between absorbance and concentration. Therefore it is more accurate to plot a calibration curve,
where the absorbances of several standard solutions of known concentrations are recorded and compared with
the absorbance of the solution whose concentration is unknown.

References
1. Jim Clark, More About 3d Orbitals, Chemguide, 2011, accessed on 26
th
Dec 2013,
< http://www.chemguide.co.uk/inorganic/complexions/colour2.html> .
2. P. W. Atkins, T. Overton, J. Rourke, M. Weller and F. Armstrong, Shriver and Atkin’s Inorganic Chemistry, Oxford
University Press, Oxford, 5
th
edn, 2010, ch. 21, pp. 474-477.
3. Sigma Aldrich, Iron (II) tetrafluoroborate hexahydrate, United Kingdom, accessed on 28
th
December 2013,
< http://www.sigmaaldrich.com/catalog/product/aldrich/401668?lang=en&region=GB > .
4. Chemspider, (E)-N-Benzyl-1-(2-pyridinyl)methanimine, Royal Society of Chemistry, United Kingdom, accessed on
8
th
January 2014, < http://www.chemspider.com/Chemical-Structure.9404407.html?rid=e8b057ca-d580-4275-
b59d-535e61dbc4c7 > .
5. S. E. Howson, L. E. N. Allan, N. P. Chmel, G. J. Clarkson, R. V. Gorkum and P. Scott, Chem. Commun., 2009,
13, 1727-1729.

6. Appendices
Appendix C-1; Calculation for % yield
% yield = (mass obtained/theoretical mass) x 100
- Mass obtained = 0.651g

- For theoretical mass:

Mass of [Fe(OH2)6][BF4]2 / Mr
= 0.337 / (55.85 + [6x15.9994] + [12x1.00794] + [2x10.811] + [8x18.998403]) = 9.9838x10
-4
moles

- 1:1 ratio hence moles of [FeL3][BF4]2 = 9.9838x10
-4


9.9838x10
-4
x (55.845 + [2x10.811] + [8x18.998403] + 3[Mr of Ligand]) =
9.9838x10
-4
x {55.845 + [2x10.811] + [8x18.998403] + 3([13x12.0107] + [12x1.00794] + [2x14.0067])}
= 0.817g

- % yield = 0.651 / 0.817 x 100 = 79.7% (1.d.p)

Appendix C-2; Calculation for concentration of product in 100cm
3
volumetric solution with HPLC acetonitrile
0.25g required for UV-vis spectroscopy
Mass of weighing boat used = 0.911 (± 0.0005) g
1.158g (product + weighing boat) – 0.911g = 0.247g product used.
Error propagation: √(d
2
x + d
2
y) = √(0.0005
2
+0.0005
2
) = 7.07x10
-4
g  (0.247 ± 0.0007) g

Mass / Mr = 0.247 / 818.197564 = 3.019x10
-4
moles
Error propagation: √(0.000707/0.247)
2
= 2.862x10
-3

Ans x (3.019x10
-4
) = 8.640x10
-7
 (3.019 ± 0.009 ) x 10
-4
moles

Volumetric flask = 100 (± 0.08) ml
c = n/v  3.019x10
-4
/ (100/1000) = 3.019x10
-3
mol dm
-3
in 100cm
3
acetonitrile
Error propagation: √{ (0.009x10
-4
/3.019x10
-4
)
2
+ (0.08/100)
2
} = 3.087x10
-3

Ans x (3.019x10
-3
) = 9.318x10
-6
 (3.087 ± 0.009) x 10
-3
mol dm
-3

Graduated pipette = 1(± 0.005) ml


3.019x10
-4
/ 100 = 3.019x10
-6
mol dm
-3
in 1cm
3
solution
Error propagation: √{ (0.009x10
-4
/3.019x10
-4
)
2
+ (0.005/1)
2
} = 5.821x10
-3

Ans x (3.019x10
-6
) = 1.757x10
-8
 (3.02 ± 0.02) x 10
-6
moles

c = n/v  3.019x10
-6
/ (100/1000) = 3.019x10
-5
mol dm
-3

Error propagation: √{ (0.01757x10
-6
/3.019x10
-6
)
2
+ (0.08/100)
2
} = 5.875x10
-3

Ans x (3.019x10
-5
) = 1.774x10
-7
(3.019 ± 0.018) x 10
-5
mol dm
-3

7. Appendix S-1; IR Spectrum of [FeL3][BF4]2

Appendix S-2; H
1
NMR Spectrum of [FeL3][BF4]2

8. Appendix S-3; UV-vis Absorption Spectrum of [FeL3][BF4]2