Report No xxxx - Instytut Fizyki JÄ drowej PAN

PHASE TRANSITIONS AND REORIENTATIONAL MOTIONS OF THE
COMPLEX CATIONS AND NH 3 LIGANDS IN POLYCRYSTALLINES
[Co(NH 3 ) 6 ](ClO 4 ) 3 AND [Zn(NH 3 ) 4 ](BF 4 ) 2
Natalia Górska a , Łukasz Hetmańczyk a , Edward Mikuli a , Anna Migdał-Mikuli a , Krystyna
Hołderna-Natkaniec b , Weronika Kasperkowiak b
a Jagiellonian University, Faculty of Chemistry, Department of Chemical Physics,
30-060 Kraków, ul. Ingardena 3, Poland; b A. Mickiewicz University, Institute of Physics,
61-606 Poznań, ul. Umultowska 85, Poland
Hexaaminecobal(III) chlorate(VII) and tetraaminezinck(II) tetrafluoroborate
compounds (chemical formula: [Co(NH 3 ) 6 ](ClO 4 ) 3 and ([Zn(NH 3 ) 4 ](BF 4 ) 2 ), respectively) are
particularly interesting molecular materials because of different reorientational motions of the
complex cation and NH 3 ligands. In [Co(NH 3 ) 6 ] 2+ cation the Co 3+ ion occupies the center of
an octahedron, while in [Zn(NH 3 ) 4 ] 2+ the Zn 2+ ion occupies the center of a tetrahedron of the
NH 3 ligands. The ClO - 4 and BF - 4 anions have a tetrahedral structure and also perform fast
reorientational motions. Both substances undergo a several solid-solid phase transitions which
are attributed to the changes in the molecular groups arrangements as well as in their
reorientational dynamics. The DSC measurements [1,2] performed in the temperature range
90-300 K detected the phase transitions which thermodynamic parameters are presented in
Table. 1.
Table. 1. Thermodynamics parameters of the phase transitions on heating
Compound T C [K] ∆H [J·mol -1 ]
-1
∆S [J·mol
-1·K
[Co(NH 3 ) 6 ](ClO 4 ) 3 103.2 137 1.4
[Zn(NH 3 ) 4 ](BF 4 ) 2 179.4
120.6
106.5
1370
930
80
7.6
7.7
0.7
1
The proton magnetic resonance H NMR measurements were performed on a labmade
spectrometer operating in the double modulation system at 25 MHz [3]. Fig. 1a and 1b
show the temperature dependencies of the slope line width (δH) and the second moment (M 2)
of NMR line in the studied compounds. In order to propose a model of internal dynamics, the
second moment value was calculated from the van Vleck formula [4]. The onset of the
following reorientations was taken into account during the heating of these compounds: the
reorientation of NH 3 ligands about the M-N axis and the reorientation of octahedral or
tetr ahedral cation, respectively. It was assumed that the N-H bond distances were of 1.00 Å
and all the N-H bonds were tetrahedraly directed with respect to the Zn-N bonds, or
octahedrally directed with respect to the Co-N bonds, and the distance Zn-N and Co-N takes
the value of 2.01 and 2.28 Å, respectively. The M Rigid 2 obtained the value of 47.3 and 43.8 G 2 ,
for [Zn(NH 3 ) 4 ](BF 4 ) 2 and [Co(NH 3 ) 6 ](ClO 4 ) 3 , respectively. The observed value of the second
moment of 1 H NMR line for [Zn(NH 3 ) 4 ](BF 4 ) 2 close to 15 G 2 at 90 K can be interpreted as
resulting from the NH 3 ligands reorientation. On heating at T C1 also the reorientations of
tetrahedral [Zn(NH 3 ) 4 ] 2+ cation set on. The observed value of the second moment of 1 H NMR
line for [Co(NH 3 ) 6 ](ClO 4 ) 3 close to 17 G 2 at 90 K can be interpreted also as resulting from the
NH 3 ligands reorientation. On heating the reorientations of octahedral [Co(NH 3 ) 6 ] 3+ cation set
on, too.
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