Thermochemistry of adducts of some bivalent transition metal bromides with triphenylphosphine

Article abstract of DOI:10.1016/j.tca.2005.06.016

The compounds [MBr_m(L)_n] (where M is Mn(II), Fe(II), Co(II), Ni(II), Cu(I) or Zn(II); L = triphenyphosphine (tpp); m = 1 or 2; n = 1.5, 2 or 3) were synthesized and characterized by melting points, elemental analysis, thermal analysis and electronic and IR spectroscopy. The enthalpies of dissolution of the adducts, metal(II) bromides and triphenylphosphine in methanol or in a solution of 10% (v/v) triethanolamine, 40% (v/v) acetonitrile and 50% (v/v) methanol were measured and by using thermochemical cycles, the following thermochemical parameters for the adducts, have been determined: the standard enthalpies for the Lewis acid/base reaction (Δ_rH ^θ), the standard enthalpies of formation (Δ _fH^θ), the standard enthalpies of decomposition (Δ_DH^θ), the lattice standard enthalpies (Δ_MH^θ) and the standard enthalpies of the Lewis acid/base reaction in the gaseous phase (Δ_rH ^θ(g)). The mean bond dissociation enthalpies of the metal(II)-phosphorus bonds (D?(M-P)) have been estimated.

Full text of DOI:10.1016/j.tca.2005.06.016

Thermochimica Acta 437 (2005) 100–105
Thermochemistry of adducts of some bivalent transition
metal bromides with triphenylphosphine
Pedro Oliver Dunstan∗
Instituto de Qu´ımica, Universidade Estadual de Campinas, C.P. 6154, CEP 13084-971, Campinas, Sa˜o Paulo, Brazil
Received 19 November 2004; received in revised form 1 June 2005; accepted 2 June 2005
Abstract
The compounds [MBrm(L)n] (where M is Mn(II), Fe(II), Co(II), Ni(II), Cu(I) or Zn(II); L = triphenyphosphine (tpp); m = 1 or 2; n = 1.5,
2 or 3) were synthesized and characterized by melting points, elemental analysis, thermal analysis and electronic and IR spectroscopy. The
enthalpies of dissolution of the adducts, metal(II) bromides and triphenylphosphine in methanol or in a solution of 10% (v/v) triethanolamine,
40% (v/v) acetonitrile and 50% (v/v) methanol were measured and by using thermochemical cycles, the following thermochemical parameters
␪
for the adducts, have been determined: the standard enthalpies for the Lewis acid/base reaction (ꢀrH ), the standard enthalpies of formation
␪
␪
␪
(ꢀfH ), the standard enthalpies of decomposition (ꢀDH ), the lattice standard enthalpies (ꢀMH ) and the standard enthalpies of the Lewis
␪
¯
acid/base reaction in the gaseous phase (ꢀrH (g)). The mean bond dissociation enthalpies of the metal(II)–phosphorus bonds (D(M−P)) have
been estimated.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Metal bromides; Transition metals; Thermochemistry; Metal(II)–phosphorus bonds; Dissolution enthalpies
1. Introduction
thermochemical parameters, to establish correlations among
the obtained parameters and to determine the energy involved
in the transition metal–phosphorus coordinated bond.
Thermochemical parameters related to transition
metal–phosphorus coordinated bonds do not exist in the
literature. Although coordination compounds between
metal(II) halides and triphenylphosphine are known, since
the last half of the 20th century, they have not been the
subject of any thermochemical study. Mostly they have been
synthesized, characterized and some electronic studies have
been made [1–8].
The thermodynamic properties of this kind of com-
pound are important to help understand metal–phosphorus
coordinated bonds. Also, the standard enthalpies of for-
mation are important to characterize and understand their
properties.
2. Experimental
2.1. Chemicals
Triphenylphosphine (BDH Chemicals Ltd.) was used as
obtained. The anhydrous metal(II) bromides used in the
preparation of the complexes were of reagent grade. Solvents
used in the synthesis of the adducts were purified by distilla-
˚
tion and stored over Linde 4 A molecular sieves.
2.2. Analytical
This paper reports the synthesis of complexes of some
bromides of divalent 3d-elements with triphenylphosphine.
They were synthesized with the purpose of obtaining several
Carbon and hydrogen contents were determined by micro-
analytical procedures. The metal contents were determined
by complexometric titration with 0.01 M EDTA solution [9]
of the aqueous solution of adduct samples. Bromide analysis
∗
Tel.: +55 19 37883088; fax: +55 19 37883023.
E-mail address: dunstan@iqm.unicamp.br.
0040-6031/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2005.06.016

P.O. Dunstan / Thermochimica Acta 437 (2005) 100–105
101
was by gravimetry, using 0.1 M AgNO3 solution, after the
2.6. Calorimetric measurements
adducts had been dissolved in water [10]. The capillary melt-
ing points of the adducts were determined with a UNIMELT
equipment from Thomas Hover.
Solution calorimetric determinations were done in an
LKB 8700-1 precision calorimeter as described elsewhere
[11]. The solution calorimetric measurements were per-
formed by dissolving samples of 3.3–95.5 mg of the adducts
or metal(II) bromides in 100 mL of methanol (or a mix-
ture of triethanolamine–acetonitrile–methanol for the Co(II)
compounds) and the triphenylphosphine in this latter solu-
tion, maintaining a molar relation equal to the stoichiom-
etry of the adduct. The accuracy of the calorimeter was
carried out by determining the heat of dissolution of
tris[(hydroxymethyl)amino]methane in 0.1 mol dm−3 HCl.
The result (−29.78 0.03 kJ mol−1) is in agreement with the
2.3. Adduct synthesis
The adducts were prepared by reaction of MBr2 dissolved
in hot n-butanol (FeBr2, CoBr2 or NiBr2), hot acetone (CuBr2
or ZnBr2) or hot tetrahydrofuran (MnBr2) with a hot solution
of triphenylphosphine in the same solvent used for dissolv-
ing the metal bromide. In all preparations a molar ratio
salt/ligand of 1/2 was used. A typical procedure is given
below.
To a solution of 2.20 g of NiBr2 (10 mmol) dissolved
in 40 mL of hot n-butanol, 5.30 g of triphenylphosphine
(20 mmol) dissolved in 40 mL of hot n-butanol was added
slowly and drop-wise with stirring. The mixture was refluxed
for several hours. After cooling, the dark green crystals were
filtered, washed with petroleum ether and dried for several
hours in vacuum. The product was stored in a desiccator over
calcium chloride.
value recommended by IUPAC (−29.763 0.003 kJ mol−1
)
[12].
3. Results and discussion
All the adducts obtained were solids. The yields, melting
points, colors, appearanceandanalyticaldataaresummarized
in Table 1. The adduct [CuBr(tpp)3] was obtained from the
reaction of CuBr2 and triphenylphosphine in methanol. This
was also observed by Tayin et al. [8]. The adduct of Fe(II) was
obtained with a stoichiometric relation salt/ligand of 1/1.5
and not with the stoichiometry of 1/2 found in the literature
[4].
2.4. Infrared spectra
Spectra were obtained with samples in KBr matrix for
the adducts and ligand. A Perkin-Elmer 1600 series FTIR
spectrophotometer in the 4000–400 cm−1 region was used.
3.1. Infrared spectra
2.5. Thermal studies
The infrared spectra of the adducts show dislocation of
some bands and the appearance of new ones when compared
with the spectra of the free ligand. For the complexes, the
band at 1089 cm−1 (x-sensitive q) observed in the free lig-
and is moved to higher frequencies and greatly increased in
its intensity. A new band in the region 726–705 cm−1 (x-
sensitive r) appeared after complexation. The strong band
at 514 cm−1 in the free ligand, assigned to an out-of-plane
ring deformation, moves 3–27 cm−1 to higher frequencies
after complexation. These facts are taken as indicative of
coordination of the triphenylphosphine to the metallic ions
TG/DTG and DSC measurements were obtained in an
argon atmosphere in a Du Pont 951 TG analyzer with samples
varying in mass from 2.41 to 5.38 mg (TG/DTG) and from
2.05 to 6.43 mg (DSC) and a heating rate of 10 K min−1in
the 293–673 K (DSC) and 298–1253 K (TG/DTG) tempera-
ture ranges. TG calibration for temperature was made with
metallic aluminum as a standard (mp = 660.37 ◦C) and the
equipment carried out the calibration for mass automatically.
The DSC calibration was made with metallic indium as a
l
␪
standard (mp = 165.73 ◦C, ꢀs H = 28.4 J g−1).
Table 1
Yields in percentage on preparation, melting points and analytical data of the compounds
Compound
Yield (%)
mpa (K)
Appearanceb
C (%)
H (%)
M (%)
Br (%)
Calculated
Found
Calculated
Found
Calculated
Found
Calculated
Found
[MnBr2(tpp)2]
82
64
91
41
34
62
534
365
423
388
438
492
li. ye. pw.
or. pw.
gre. bl. cr.
d. gre. cr.
wh. cr.
58.48
53.24
58.17
58.19
69.32
57.67
58.80
52.99
58.40
58.03
69.84
57.67
4.09
3.72
4.07
4.07
4.88
4.03
4.14
3.75
4.09
3.98
4.90
3.98
7.43
9.17
7.93
7.90
6.83
8.72
7.23
9.25
7.89
7.92
6.53
8.73
21.62
26.24
21.50
21.51
8.59
21.57
26.34
21.48
21.54
8.47
[FeBr2(tpp)1.5
[CoBr2(tpp)2]
[NiBr2(tpp)2]
[CuBr(tpp)3]
[ZnBr2(tpp)2]
a
]
wh. cr.
21.32
21.29
Melting point with decomposition.
b
li., light; ye., yellow; or., orange; bl., blue; gre., green; wh., white; pw., powder; cr., crystals; d., dark.

102
P.O. Dunstan / Thermochimica Acta 437 (2005) 100–105
Table 2
a
Infrared data for tpp and its complexes (cm−1
)
Compound
x-Sensitive q
x-Sensitive r
Out-of-plane ring
x-Sensitive y
deformation
tpp
1089m
1122m
1121s
1096s
1095s
1090s
1096s
514s
541s
541s
520s
519s
517s
522s
[MBr2(tpp)2]
[FeBr2(tpp)1.5
[CoBr2(tpp)2]
[NiBr2(tpp)2]
[CuBr(tpp)3]
[ZnBr2(tpp)2]
a
726m
723s
707m
709m
705m
707m
]
450w
439w
425w
Notation used is from Whiffen [15]. Intensity of bands: s, strong; m, medium; w, weak.
through the phosphorus atom [13–15]. Table 2 presents the
more important IR bands of the adducts.
lose some or all metal in the third step with the exceptions of
Fe and Ni adducts. In most cases a residue is observed, which
is all or part of the respective metal [16].
The DSC curves of the adducts are consistent with the
TG/DTG data and show endothermic peaks due to melting
or elimination of part of the ligand. Table 3 presents the ther-
moanalytical data of the adducts.
3.2. Thermal studies
Thermogravimetry and derivative thermogravimetry of
the adducts showed that the thermal dissociation processes
were of different types, with the lost of mass in 1 (Fe adduct),
2 (Ni adduct) or 3 (Mn, Co, Cu and Zn adducts) steps. Some
of these steps consist of two successive decomposition pro-
cesses (first step of Co, Ni or Cu adducts). They lose part of
the ligand (or all in the case of Fe adduct) in the first step,
following by loss of the rest of the ligand in the second step.
Part or all the bromine is lost in the third step with the excep-
tion of the adduct of Fe that loses part of the bromine together
with all the ligand in a unique step, the Mn adduct that loses
bromine in the second and third steps and the Ni adduct that
loses part of the bromine in the second step. The adducts also
3.3. Electronic spectra
Table 4 contains the band maxima assignments and calcu-
lated ligand field parameters for the adducts. For the adduct
of Mn(II), since only forbidden bands can be observed in the
electronic spectra of high spin (II) compounds, it is impossi-
ble to determine with accuracy the ligand field parameters. It
is, however, possible to deduce the local symmetry, which is
pseudo-tetrahedral [17,18], with two phosphorus atoms from
two ligand molecules and two bromide ions surrounding the
Table 3
Thermal analysis data of the compounds [MBr2(tpp)n]
␪
Compound
Mass lost (%)
Calculated
TG temperature range (K)
Species lost
DSC peak temperature
ꢀH (kJ mol−1
)
Observed
[MnBr2(tpp)2]
60.31
14.96
19.53
59.43
15.73
19.60
5.24a
443–497
497–681
900–985
−1.7 tpp
349
439
34.18
9.34
−0.3 tpp −0.4 Br
−1.6 Br −0.3 Mn
[FeBr2(tpp)2]
[CoBr2(tpp)2]
[NiBr2(tpp)2]
[CuBr(tpp)3]
90.30
90.06
9.94a
599–666
−2 tpp −1.8 Br
335
350
415
2.75
2.48
10.57
52.92
17.64
24.28
53.85b
15.20
25.75
490–614
614–650
650–870
−1.5 tpp
440
0.44
23.19
67.76
6.85
−0.5 tpp
−2 Br −0.35 Co
67.07
21.82
67.01b
21.02
11.97a
491–556
836–926
−1.9 tpp
337
−0.1 tpp −1.7 Br
56.39
28.20
14.28
57.52b
27.06
14.14
451–553
553–602
737–782
−2 tpp
437
451
−tpp
−Br −5/6 Cu
[ZnBr2(tpp)2]
34.98
34.98
30.04
36.14
34.22
31.75
468–497
497–595
597–746
−tpp
326
−tpp
−2 Br −Zn
a
Residue at 1233 K.
Two overlapping steps.
b

P.O. Dunstan / Thermochimica Acta 437 (2005) 100–105
103
Table 4
Band maxima and calculated ligand-field parameters for the compounds
Compound
Band maxima (×103 cm−1
)
d-d
Intraligand + charge transfer
36.3
[MnBr2(tpp)2]
19.5, 21.4
d-d
Intraligand + charge transfer
a
ν1
Dq(cm−1
5352
)
[FeBr2(tpp)1.5
]
5.35
20.7
d-d
Intraligand + charge transfer
Dq (cm−1
)
B (cm−1
)
Dq/B
β+
ν2
ν3
[CoBr2(tpp)2]
[NiBr2(tpp)2]
8.12b
8.78d
14.9c
16.9e
24.9
23.1
485
476
567
909
0.855
0.527
0.584
0.883
β+ = B/B0; B0 = 971 cm−1(Co2+); B0 = 1030 cm−1 (Ni2+) [17].
a
b
c
d
e
5
ν1 = 5T2 ← E.
ν2 = 4T1(F) ← A2.
4
ν3 = 4T1(P)) ← A2.
4
3
ν2 = 3A2 ← T1(F).
3
ν3 = 3T1 ← T1(F).
␪
Mn(II) ion. The ligand field parameters for the Fe(II) adduct
were calculated according to Bolster et al. [17]. It is con-
cluded by the position of the observed absorption band and
considering the magnitude of Dq that the Fe(II) is pseudo-
tetrahedrally surrounded by two phosphorus atoms from two
ligand molecules and two bromide ions and another Fe(II)
atom linking by a bromide bridge to the first Fe(II) atom,
is pseudo-tetrtahedrally surrounded by one phosphorus atom
from one ligand molecule and three bromide ions forming a
dimeric structure. For the adduct of Co(II), the ligand field
parameters were calculated according to Lever [19]. Accord-
ing to the number and position of the bands [17,20] and
considering the magnitude of the crystal field parameters
as compared with that of Bolster et al. [17], it is concluded
that the adduct of Co(II) is pseudo-tetrahedrally surrounded
by two phosphorus atoms from two ligand molecules and
two bromide ions. The ligand field parameters for the Ni(II)
adduct were calculated according to Reedijk et al. [21] and
Lever [19]. According to the number and position of the
observed bands and considering the magnitude of the crys-
tal field parameters as compared with that of Brown et al.
[18], it is concluded that the adduct of Ni(II) is pseudo-
tetrahedrally surrounded by two phosphorus atoms from two
ligand molecules and by two bromide ions.
n tpp(s) + solution A → solution B; ∆2H
(2)
␪
[MBr2(tpp)n](s) + calorimetric solvent → solution C; ∆3H
(3)
␪
solution B → solution C; ∆4H
(4)
The application of Hess’ law to the series of reactions
(1)–(4) gives the standard enthalpies of the acid/base reac-
␪
tions (ꢀrH ) according to reaction (5):
␪
MBr2(s) + n tpp(s) → [MBr2(tpp)n](s); ∆rH
␪
␪
␪
= ∆1H + ∆2H − ∆3H
(5)
since the final state of reactions (2) and (3) is the same
and ꢀ4H = 0. Table 5 gives the values obtained for the
enthalpies of dissolution of MBr2 (ꢀ1H ), tpp into the solu-
␪
␪
␪
␪
tion of metal(II) bromide (ꢀ2H ) and of the adducts (ꢀ3H ).
Unhappily, Cu(I) bromide and its adduct of tpp are extremely
insoluble in most solvents and enthalpies of dissolution could
not be determined. Uncertainty intervals given in Table 5 are
twice the standard deviation of the means of 4–10 replicate
measurements. Combined errors were calculated from the
square root of the sum of the square of the component errors.
From the values obtained for the standard enthalpies of
3.4. Calorimetric measurements
␪
the acid/base reactions (ꢀrH ) and by using appropriate ther-
mochemical cycles [11,22–24], the following thermochem-
ical parameters for the adducts, were determined: the stan-
The standard enthalpies of dissolution of metal(II) bro-
mides, triphenylphosphine and adducts were obtained as pre-
viously reported [11]. The standard enthalpies of dissolution
were obtained accordingly with the standard enthalpies of
reactions (1)–(4) in solution:
␪
dard enthalpies of formation (ꢀfH ), the standard enthalpies
␪
of decomposition (ꢀDH ), the standard lattice enthalpies
␪
(ꢀMH ) and the standard enthalpies of the Lewis acid/base
␪
reactions in the gaseous phase (ꢀrH (g)). These latter val-
ues can be used to calculate the standard enthalpies of the
␪
M–P bonds [22], being equal to: D(M−P) = −ꢀrHθ(g)/n.
MBr2(s) + calorimetric solvent → solution A; ꢀ1H
(1)
¯

104
P.O. Dunstan / Thermochimica Acta 437 (2005) 100–105
Table 5
Enthalpies of dissolution at 298.15 K
␪
Compound
Calorimetric solvent
Number of experiments
6
5
4
5
5
4
4
4
4
4
4
4
10
6
(i)
ꢀiH /kJ mol−1
MnBr2(s)
tpp(s)
[MnBr2(tpp)2](s)
FeBr2(s)
tpp(s)
[FeBr2(tpp)1.5
CoBr2(s)
tpp(s)
[CoBr2(tpp)2](s)
NiBr2(s)
tpp(s)
[NiBr2(tpp)2](s)
ZnBr2(s)
Methanol
2:1 MBr2–Methanol
Methanol
Methanol
1.5:1 FeBr2–Methanol
Methanol
Methanol
2:1 CoBr2–Methanol
Methanol
TEA–Methanol–CH3CNa
2:1 TEA–Methanol–CH3CN
TEA–Methanol–CH3CN
Methanol
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
−78.83 1.36
23.55 1.09
−29.70 1.81
−68.93 0.76
20.92 0.69
35.08 2.22
−107.42 3.37
20.10 0.41
22.50 0.64
−130.32 4.30
23.98 1.01
65.84 1.95
−48.30 0.64
25.02 1.17
63.01 3.10
]
(s)
tpp(s)
2:1 ZnBr2–Methanol
Methanol
[ZnBr2(tpp)2](s)
a
4
10% (v/v) triethanolamine, 40% (v/v) CH3CN and 50% (v/v) methanol.
Table 6
Summary of the thermochemical results (kJ mol−1
)
␪
␪
␪
␪
␪
␪
Compound
ꢀrH
ꢀfH
ꢀsgH
ꢀM
H
ꢀDH
ꢀrH (g)
¯
D(M−O)
MnBr2(s)
FeBr2(s)
CoBr2(s)
NiBr2(s)
ZnBr2(s)
tpp(s)
[MnBr2(tpp)2](s)
−384.9a
206b
−249.8a
204b
−220.9a
183b
170b
159.7b
96.2 17.3c
−212.1a
−328.65a
218.0 10.7c
25.5 21.6
167.1 21.6
105.3 21.8
183.4 22.0
21.1 21.8
−25.58 2.51
−19.14 2.09
−109.82 3.45
−40.50 4.83
−82.29 3.37
−423.9 34.7
−416 35
218.0 34.7
211.5 34.7
302.2 34.8
232.9 34.9
278.7 34.8
−327.7 38.8
−319 39
163.9 19.4
213 26
195 20
154 20
171.1 19.5
[FeBr2(tpp)1.5
]
(s)
[CoBr2(tpp)2](s)
[NiBr2(tpp)2](s)
[ZnBr2(tpp)2](s)
−485 35
−389 39
−403 35
−307 39
−438.4 34.8
−342.2 38.9
a
[25].
[26].
[27].
b
c
Table 6 lists the values obtained for all these thermochemical
parameters.
Table 7 lists the values obtained for these enthalpy values.
Correlationofthe ꢀfIH valueswiththemetalatomicnumber
for the adducts with stoichiometry 1:2 is presented in Fig. 1.
The values obtained depend on the electronic structure of the
central metallic ion. That relation allows determination of
the thermodynamic stabilization energy in the ligand field on
␪
␪
For the determination of ꢀrH (g) it was necessary to
assume that the molar standard enthalpies of sublimation of
the adducts were equal to the enthalpy of sublimation of one
mole of the ligand [28,29] as the melting points and/or ther-
mal studies showed that the adducts decomposed on heating
and were not found in the liquid state and probably not in the
gaseous phase.
Table 7
Auxiliary data and enthalpy changes of formation in the gaseous phase
(kJ mol−1
)
␪
Based on ꢀrH values for the adducts, the acid-
␪
␪
␪
Compound
ꢀfH
ꢀrH (g)
ꢀfIH
ity order of the salts can be obtained: CoBr2 > ZnBr2 >
−
Br(g)
−219.07a
¯
NiBr2 > MnBr2. Using the D(M−P) values, the order is
2+
Mn(g)
2522.0 0.1b
2751.6 2.3b
2841.7 3.4b
2930.5 1.5b
2781.0 0.4b
122 56
CoBr2 > ZnBr2 > MnBr2 > NiBr2.
2+
Fe(g)
The enthalpies for the process of hypothetical complex
formation in the gaseous phase from metal(II) ions, bromide
ions and tpp molecules, can be evaluated:
2+
Co(g)
2+
Ni(g)
2+
Zn(g)
[MnBr2(tpp)2](g)
−327.7 38.8
−319 39
−389 39
−307 39
−2590 69
−2679 69
−2831 69
−2842 69
M(g)2+ + 2Br(g)− + n tpp(g) → [MBr2(tpp)n](g); ∆fIH
␪
[FeBr2(tpp)1.5
]
263 56
201 56
279 56
(g)
[CoBr2(tpp)2](g)
[NiBr2(tpp)2](g)
[ZnBr2(tpp)2](g)
(6)
117.2 56.2 −342.2 38.9 −2854.1 69.4
where ꢀfIH = ꢀfH (adduct(g)) − ꢀfH (M(g)2+) − 2 ꢀfH
␪
␪
␪
␪
a
[25].
[30].
b
(Br(g)−) − n ꢀfH (tpp(g)).
␪

P.O. Dunstan / Thermochimica Acta 437 (2005) 100–105
105
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