Cu-O stretching frequency correlation with phenanthroline pKa values in mixed copper complexes

Article abstract of DOI:10.1016/S0020-1693(99)00034-1

A study of Cu-O vibration frequencies in 32 isostructural mixed copper complexes containing eight differently substituted phenanthrolines and four different oxygen donor bidentate ligands, is presented. In all cases linear correlation was found between phenanthroline pK_a values and the studied vibration frequencies, suggesting that an increase in phenanthroline basicity weakens Cu-O bonds in this type of compound.

Full text of DOI:10.1016/S0020-1693(99)00034-1

Inorganica Chimica Acta 288 (1999) 106–111
Note
Cu–O stretching frequency correlation with phenanthroline pKa
values in mixed copper complexes
Laura Gasque *, Gerardo Medina, Lena Ruiz-Ram´ırez, Rafael Moreno-Esparza
Departamento de Qu´ımica Inorga´nica, Facultad de Qu´ımica, Uni6ersidad Nacional Auto´noma de Me´xico, Mexico D.F., C.P. 04510, Mexico
Received 22 September 1998; accepted 20 January 1999
Abstract
A study of Cu–O vibration frequencies in 32 isostructural mixed copper complexes containing eight differently substituted
phenanthrolines and four different oxygen donor bidentate ligands, is presented. In all cases linear correlation was found between
phenanthroline pKa values and the studied vibration frequencies, suggesting that an increase in phenanthroline basicity weakens
Cu–O bonds in this type of compound. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Copper complexes; Phenanthroline complexes
1. Introduction
Contrary to what is expected on a statistical basis,
the equilibrium constant for this reaction is found to be
larger than for
As part of an extensive study [1–8] dealing with
copper ternary chelates of general types Cu(N–N)-
(O–O)+ and Cu(N–N)(O–O), where N–N is an aro-
matic diimine and O–O is acetylacetonate or salicy-
laldehydate in the first case, and oxalate or malonate in
the second, we have studied the frequency variations in
their Cu–O vibrations. In this paper, complexes with
substituted 1,10-phenanthrolines are discussed. These
phenanthrolines (X-phen) are: 1,10-phenanthroline
(phen), 4,7-dimethyl-1,10-phenanthroline (4,7dmphen),
5,6-dimethyl-1,10-phenanthroline (5,6dmphen), 3,4,7,8-
tetramethyl-1,10-phenanthroline (3,4,7,8tmphen), 4,7-
diphenyl-1,10-phenanthroline (4,7dfphen), 5-methyl-
1,10-phenanthroline (5mphen), 5-phenyl-1,10-phenan-
throline (5fphen) and 5-nitro-1,10-phenanthroline
(5NO2phen).
2+
(aq)
n−
(aq)
(2−n)+
(aq)
Cu +(O–O) X Cu(O–O)
whenever N–N is a good p-acceptor ligand.
Sigel et al. [9,10] explained this behaviour based on
the HSAB principle [11,12]. A p-acceptor ligand bound
to a copper ion will withdraw electron density, making
[Cu(N–N)]2+ a harder acid, which will consequently be
preferred, as opposed to the simple aqueous Cu2+ ion,
by hard bases such as oxygen donors.
Different substituents on a phenanthroline molecule
affect its basicity, or s-donor ability, in a way easily
measured by pKa values (Table 1). However, the effect
of these same substituents on the p-acceptor properties
of phenanthroline is not easily evaluated, even though
it is expected to be of significant importance and to
affect bonding parameters in the molecule. An experi-
mental parameter widely accepted to indirectly measure
these properties is the half wave potential for the N–N/
N–N− pair. Sanna et al. [13] have obtained E1/2 values
for seven of the eight phenanthrolines studied in this
paper, which have an inverse correlation with pKa
values. Solution equilibrium studies to determine the
This type of complex has been thoroughly studied in
solution since the publication of several reports on the
anomalous behaviour of the equilibrium [9,10]
2+
(aq)
n−
(aq)
(2−n)+
(aq)
Cu(N–N) +(O–O) X Cu(N–N)
* Corresponding author. Tel.: +52-5-622 3811; fax: +52-5-622
3724.
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00034-1

L. Gasque et al. / Inorganica Chimica Acta 288 (1999) 106–111
107
Table 1
2.2. Analyses and spectroscopical measurements
pKa values of substituted phenanthrolines
Complexes were characterised by standard tech-
niques. Elemental analyses were performed by Desert
Analytics, Tucson, AZ.
2.2.1. IR spectra
All measurements were carried out in a Nicolet 740
FT-IR spectrophotometer. Mid-IR spectra were ob-
tained in KBr pellets from 4000 to 400 cm−1 and in
polyethylene pellets in the 700–70 cm−1 region. Reso-
a
Phenanthrolines
pKa
lution was better than 1 cm−1
.
3,4,7,8-Tetramethyl-1,10-phenanthroline
4,7-Dimethyl-1,10-phenanthroline
5,6-Dimethyl-1,10-phenanthroline
5-Methyl-1,10-phenanthroline
1,10-Phenanthroline
5-Phenyl-1,10-phenanthroline
4,7-Diphenyl-1,10-phenanthroline
5-Nitro-1,10-phenanthroline
6.31
5.95
5.60
5.27
4.93
4.90
4.80
3.22
2.3. Synthesis of the complexes
2.3.1. [Cu(X-phen)(salal)]NO3 and [Cu(X-phen)(acac)]
NO3
Phenanthroline (0.5 mmol) was dissolved in 10 ml of
a 1:4 ethanol–water mixture and was then added to 0.5
mmol of Cu(NO3)2 · 2.5H2O previously dissolved in 5
ml of water. To this mixture, 5 ml of a 0.1 M solution
of the O–OH ligand were added, followed by dropwise
neutralisation with a 10% aqueous ammonia solution.
Dark green crystals were collected 1 day after for the
salicylaldehydate complexes and dark blue needles were
obtained for the acetylacetonate complexes.
a All values were taken from Ref. [21], except for 3,4,7,8-tetra-
methyl-1,10-phenantroline, which was taken form Ref. [22].
formation constants of metal complexes with substi-
tuted phenanthrolines are not feasible because of their
increased hydrophobic character which makes them
and their metal complexes quite insoluble. Thus, in
order to study the differences in strength interaction of
copper(II) with oxygen donors, the variations on the
Cu–O stretching frequencies have been analysed in this
work.
The results obtained in this work are consistent with
similar studies concerning Quantitative Analysis of Lig-
and Effects (QALE, [14–16]) in which it has been
suggested that the p-acceptor properties of a ligand
decrease regularly as their | basicity increases [17].
However, most of these studies became important in
organometallic chemistry, where the dual p-acceptor/s-
donor character of ligands such as phosphines is
claimed to be responsible for measurable changes in
various physicochemical properties of their complexes.
However, until now, this type of analysis has never
dealt with either copper or phenanthroline complexes.
All complexes studied here are five-coordinated with
a distorted square pyramidal geometry, with an oxygen
atom either from a water molecule or a nitrate ion on
the apical position [1–7,18–20].
2.3.2. [Cu(X-phen)(ox)]
Copper oxalate was prepared by mixing equimolar
quantities of Cu(NO3) · 2.5H2O and potassium oxalate
in water. The [Cu(ox)]n precipitate was filtered and air
dried; 0.5 mmol were suspended in water and 0.5 mmol
of the corresponding diimine dissolved in ethanol was
added with stirring. The precipitate dissolved to form a
deep blue solution, which was left to crystalise.
2.3.3. [Cu(X-phen)(mal)]
A total of 0.5 mmol of the corresponding diimine
dissolved in methanol were added to 5 ml of a 0.1 M
solution of Cu(NO3)2 · 2.5H2O. To this solution, 5 ml of
a 0.1 M malonic acid solution were added, followed by
neutralisation with 10% aqueous ammonia solution.
This formed a deep blue solution from which crystals
were collected.
3. Results and discussion
Characterisation data obtained for all the complexes
studied, which included X-ray structure determination
for several complexes, supported the assumption that
they all have the same structure as regards to the
coordination environment around the copper atom.
Assignment of Cu–O bands was thus easily achieved,
and variations in these frequency values for each family
of complexes may be solely attributed to variations in
bond strength. These frequencies are shown in Table 2.
2. Experimental
2.1. Materials
All eight differently substituted 1,10-phenanthrolines
(X-phen), acetylacetone, salicylaldehyde, potassium ox-
alate, malonic acid and Cu(NO3)2 · 2.5H2O were reagent
grade, obtained from Aldrich Chemical.

108
L. Gasque et al. / Inorganica Chimica Acta 288 (1999) 106–111
Table 2
Analysed Cu–O vibration frequencies (cm−1) in mixed copper complexesa
Complex
w1 (cm−1
)
w2 (cm−1
)
Complex
w1 (cm−1
)
w2 (cm−1
)
L1=acac−
L3=ox2−
18[Cu(phen)(acac)]NO3
[Cu(4,7dmphen)(acac)]NO3
[Cu(5,6dmphen)(acac)]NO3
[Cu(3,4,7,8tmphen)(acac)]NO3
[Cu(4,7dfphen)(acac)]NO3
[Cu(5mphen)(acac)]NO3
[Cu(5fphen)(acac)]NO3
[Cu(5NO2phen)(acac)]NO3
592
–
275
266
267
262
273
266
270
283
20[Cu(phen)(ox)]
546
529
527
528
–
542
550
570
–
413
410
412
427
417
425
436
[Cu(4,7dmphen)(ox)]
[Cu(5,6dmphen)(ox)]
[Cu(3,4,7,8tmphen)(ox)]
[Cu(4,7dfphen)(ox)]
[Cu(5mphen)(ox)]
[Cu(5fphen)(ox)]
[Cu(5NO2phen)(ox)]
590
587
593
592
593
–
L2=salal−
L4=mal2−
1[Cu(phen)(salal)]NO3
546
534
536
531
547
–
528
509
513
498
532
530
535
533
19[Cu(phen)(mal)]
366
359
360
357
359
360
366
363
331
320
325
318
320
325
331
347
3[Cu(4,7dmphen)(salal)]NO3
6[Cu(5,6dmphen)(salal)]NO3
4[Cu(3,4,7,8tmphen)(salal)]NO3
5[Cu(4,7dfphen)(salal)]NO3
[Cu(5mphen)(salal)]NO3
[Cu(5fphen)(salal)]NO3
[Cu(5NO2phen)(salal)]NO3
[Cu(4,7dmphen)(mal)]
2[Cu(5,6dmphen)(mal)]
[Cu(3,4,7,8tmphen)(mal)]
7[Cu(4,7dfphen)(mal)]
[Cu(5mphen)(mal)]
[Cu(5fphen)(mal)]
7[Cu(5NO2phen)(mal)]
–
547
a [1–7] and [18–20] are the references for crystal structure of the compounds.
Fig. 1. Cu–O vibrations vs. phenanthroline pKa in [Cu(X-phen)(acac)]NO3. ꢀ, w(Cu–O), correlation coefficient r= −0.98079; ꢁ, l(O–Cu–O),
correlation coefficient r= −0.95382.
3.1. [Cu(X-phen)(acac)]NO3
It can be seen from Fig. 1 that both of these Cu–O
related frequencies decrease linearly as the substituted
phenanthrolines s-donor strength (pKa) increases.
Assignment of Cu–O related vibrations in these
mixed complexes was based on Mikami’s [23] normal
coordinate analysis for Cu(acac)2. All mixed complexes
exhibit a band slightly below 600 cm−1 assigned to
w(CuꢀO), and another between 262 and 283 cm−1
assigned to l(O–Cu–O). These two bands could be
identified in all spectra for the mixed complexes in this
family, and their corresponding frequencies are shown
in Table 2.
3.2. [Cu(X-phen)(salal)]NO3
Salicylaldehydate complexes’ IR spectra were
studied by Percy and Thornton [24], who assigned two
bands at 552 and 539 cm−1 as w(Cu–O). The
salicylaldehydate mixed complexes in this work display
these two bands, which are listed in Table 2. It can

L. Gasque et al. / Inorganica Chimica Acta 288 (1999) 106–111
109
be seen from Fig. 2, that both frequencies have a very
good linear correlation with the substituted phenan-
throline pKa values.
plexes exclusively, the synthetic procedure for this fam-
ily of complexes excludes anionic species, needed to
stabilise binuclear complexes of the type (N–N)Cu-ox-
alato-Cu(N–N)2+. Analysis of copper–oxygen vibra-
tions in these complexes was based on Fujita’s study on
K2[Cu(ox)2] [27]. In spite of the fact that, in this com-
plex, copper–oxygen vibrations are coupled with other
vibrations that do not compromise the Cu–O bond,
their frequencies (Table 2) correlate with substituted
phenanthroline pKa values quite satisfactorily (Fig. 3).
3.3. [Cu(X-phen)(ox)]
Oxalate can easily act as a bridging ligand between
two metal atoms, and several binuclear complexes with
copper and nitrogen donor chelates have been reported
[25,26]. To ensure the formation of mononuclear com-
Fig. 2. Cu–O vibrations vs. phenanthroline pKa in [Cu(X-phen)(salal)]NO3. ꢀ, w(Cu–O)1, correlation coefficient r= −0.98793; ꢁ, w(Cu–O)2,
correlation coefficient r= −0.96584.
.
Fig. 3. Cu–O vibrations vs. phenanthroline pKa in [Cu(X-phen)(ox)]. ꢀ, w[(CuꢀO)+(CꢀC)], correlation coefficient r= −0.96767; ꢁ, w[(CuꢀO)+
ring def.], correlation coefficient r= −0.93171.

110
L. Gasque et al. / Inorganica Chimica Acta 288 (1999) 106–111
Fig. 4. Cu–O vibrations vs. phenanthroline pKa in [Cu(X-phen)(mal)]. ꢀ, w(CuꢀO)1, correlation coefficient r= −0.93236; ꢁ, w(CuꢀO)2,
correlation coefficient r= −0.981.
3.4. [Cu(X-phen)(mal)]
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