Copper-63 NMR line width study of the copper(I)-acetonitrile system

Article abstract of DOI:10.1021/ic030020c

The principal focus of this study is the ⁶³Cu NMR line widths in Cu(I)-acetonitrile (AN) solutions. The variations with the concentrations of Cu(I) salts (trifluoromethanesulfonate and perchlorate), added salts, water, chloride ion, and temperature have been studied. A quantitative analysis shows that the anomalous temperature dependence of the line widths is not due to ion pairing or anion complexation but results primarily from formation of a species with a different coordination number or less symmetrical arrangement of AN ligands than in the normal tetrahedral Cu(AN)₄⁺ ion. Solvent viscosity and ion pairing (with triflate) also are identified as factors having the expected effects on the line widths. The results of earlier studies also are discussed and analyzed by the current model where possible.

Full text of DOI:10.1021/ic030020c

Inorg. Chem. 2003, 42, 3934−3942
Copper-63 NMR Line width Study of the Copper(I)−Acetonitrile System
Japhet K. Irangu and R. B. Jordan*
Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G2
Received January 15, 2003
The principal focus of this study is the 63Cu NMR line widths in Cu(I)−acetonitrile (AN) solutions. The variations
with the concentrations of Cu(I) salts (trifluoromethanesulfonate and perchlorate), added salts, water, chloride ion,
and temperature have been studied. A quantitative analysis shows that the anomalous temperature dependence
of the line widths is not due to ion pairing or anion complexation but results primarily from formation of a species
with a different coordination number or less symmetrical arrangement of AN ligands than in the normal tetrahedral
Cu(AN)4+ ion. Solvent viscosity and ion pairing (with triflate) also are identified as factors having the expected
effects on the line widths. The results of earlier studies also are discussed and analyzed by the current model
where possible.
Introduction
temperature is expected for quadrupolar relaxation, as
described in a following paragraph, but the cause of the
increasing line width above -5 °C remains unexplained. The
earlier studies suggested that this effect might be due to a
coordination change1 or to ion pairing.2 One goal of this study
was to seek a quantitative explanation for this anomaly, with
the hope that it might then be possible to find conditions to
minimize it and thereby improve the sensitivity, and also to
add to the understanding of the coordination chemistry of
the Cu(I)AN system and provide speciation information that
might be critical for electron-exchange studies.
Various salts of tetrakis(acetonitrile)copper(I) are widely
used as starting materials in Cu(I) chemistry. The same salts
dissolved in acetonitrile (AN) also are often used as line
width benchmarks and shift standards in 63/65Cu NMR studies.
The present study began as an exploration of the possibility
of using 63Cu(I) NMR to measure the Cu(I)/Cu(II) electron-
exchange rate in acetonitrile. Preliminary studies found that
the trifluoromethanesulfonate (triflate) salts of Cu(I) and
Cu(II) have advantageous solubilities in acetonitrile that
would permit the widest concentration range for both in the
electron-exchange studies. As a consequence, a major con-
cern here is the NMR behavior of Cu(AN)4(triflate) which
had not been studied previously. The perchlorate salt also
has been studied for comparison purposes. In addition, the
effects of chloride ion and water on the 63Cu(I) line widths
have been examined.
A major problem with 63Cu(I) NMR in AN is that the line
is unusually broad so that the sensitivity is seriously affected.
Two early studies1,2 have shown that the temperature
dependence of the line width is anomalous in that it shows
a minimum at about -5 °C and then broadens as the
temperature increases through the ambient temperature
region. As a result, the line width at ∼25 °C is much greater
than would be predicted from extrapolation of low-temper-
ature line widths. The decrease in line width with increasing
Results that may be relevant to the AN system have been
reported by Marker and Gunter.3 They studied the complexes
of Cu(I) with trimethyl and triethyl phosphite in CD2Cl2 and
CDCl3 and found a temperature dependence of the 63Cu line
widths (100-700 Hz) analogous to that obseved in the AN
system, with a minimum in the 0-20 °C range. The
phosphite systems have the advantage that both 63Cu and
31P NMR can be done and Cu-P coupling is observed in
the 63Cu and 31P spectra. The authors interpreted the higher
temperature behavior as due to ligand exchange with
activation energies in the range of ∼4 kcal mol-1.
Since Cu(AN)4+ is such a common standard in 63Cu NMR
studies, there have been a number of reports of line widths,
and they are summarized in Table 1. Aside from a few
-
strange results, such as the most recent on BF4 , and the
-
earliest on PF6 , the reported values commonly fall in the
480-550 Hz range. However, there are several, including
those from this study, in the 400-440 Hz range. Taken
overall, the range of ∼150 Hz seems unexpected for materials
* To whom correspondence should be addressed. E-mail: bob.jordan@
ualberta.ca.
(1) Ochsenbein, U.; Schla¨pfer, C. W. HelV. Chim. Acta 1980, 63, 1926.
(2) Kroneck, P.; Kodweiss, J.; Lutz, O.; Nolle, A.; Zepf D. Z. Naturforsch.
1982, 37a, 186.
(3) Marker, A.; Gunter, M. J. J. Magn. Reson. 1982, 47, 118.
3934 Inorganic Chemistry, Vol. 42, No. 12, 2003
10.1021/ic030020c CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/22/2003

63Cu NMR of Cu-AN System
Table 1. 63Cu Line Widths of Cu(AN)4X Salts in Acetonitrile
X-
W
1/2, Hz
date
conditionsa
ref
BF4
540
1978 0.1 M, Bruker SXP, 10 mm
1980 0.1 M, Varian XL-100, 10 mm
1982 0.087 M, Bruker SXP, 10 mm
1987 1.0 M, Varian FX-270, 30 °C
1988 saturated (?), Bruker WH-200
c
d
e
f
BF4
BF4
BF4
BF4
490
540
600
8570
g
PF6
PF6
800
400
1980 0.1 M, Varian XL-100, 10 mm
1983 saturated (?), Nicolet NT-360
d
h
ClO4
ClO4
ClO4
ClO4
ClO4
ClO4
ClO4
ClO4
ClO4
ClO4
ClO4
ClO4
522 (494, T1)b 1980 0.1 M, Varian XL-100, 10 mm
d
e
e
i
j
k
l
m
n
o
p
p
550
580
500
1982 0.061 M, Bruker SXP, 10 mm
1982 0.10 M, Bruker SXP, 10 mm
1984 0.05 M, JEOL FX-200, 10 mm
505 (481, T1)b 1990 0.02-0.06 M, Bruker AM-400
480
480
480
440
520
443
423
1995 0.05-0.065 M, Bruker AM-400
1998 0.064 M, Bruker-500, 10 mm
1999 0.064 M, Bruker-500, 10 mm
1999 0.1 M, Bruker MSL-300
2001 0.064 M, Bruker-300, 10 mm
2002 0.062 M, Varian S-400
Figure 1. Temperature dependence of the 63Cu(I) line width for Cu(AN)4X
salts in AN: X ) ClO4- (]) 0.10 M, ref 2, (9) 0.0621 M, this work; X )
BF4- (+) 0.10 M, ref 1; X ) triflate (b) 4.94 × 10-3 M, (O) 0.117 M, this
work.
2002 0.0062 M, Varian S-400
F3CSO3 417
F3CSO3 463
F3CSO3 460
2002 0.00494 M, Varian S-400
2002 0.062 M, Varian S-400
2002 0.061 M, Bruker AM-200
p
p
p
p
F3CSO3 488(492, T1)b 2002 0.117 M, Varian S-400
a Temperatures are 25 °C unless stated otherwise, although sometimes
not given. Sample tube diameter is 5 mm unless otherwise indicated. b A
measured T1 converted to the equivalent in Hz. c Reference 4. d Reference
1. e Reference 2. f Endo, K.; Yamamoto, K.; Deguchi, K.; Matsushita K.
Bull. Chem. Soc. Jpn. 1987, 60, 2803. g Connor, J. A.; Kennedy, R. J.
Polyhedron 1988, 7, 161. h Reference 10. i Kitagawa, S.; Munakata, M.
Inorg. Chem. 1984, 23, 4388. j Reference 6. k Reference 7. l Reference 8.
m Gill, D. S.; Singh, L. J.; Singh, R.; Zamir, T.; Quickenden, L. Ind. J.
Chem. 1999, 38A, 913. n Szlyk, E.; Szymanska, I. Polyhedron 1999, 18,
2941. o Reference 9. p Present study.
that are relatively easy to prepare and handle and are
considered as standards. The range of line widths does not
seem to be a function of advancements or differences in
instrumentation. In seeking an explanation for this variation,
the effects of sample size and spinning, and of possible
common contaminants, also have been examined in this
study.
Figure 2. Concentration dependence of the 63Cu(I) line width for Cu-
(AN)4X salts in AN: X ) ClO4- (]) ref 2, (0) this work; X ) BF4- (+)
-
ref 1; X ) triflate (b) this work; X ) PF6 (.) ref 1.
, 1. For 63Cu on a 400 MHz 1H spectrometer, πν0 ) 3.3 ×
+
108 s-1 and τQ e 5 × 10-11 s for species such as Cu(AN)4
in a normal fluid, so that the extreme narrowing condition
should be applicable. Then, the quadrupolar relaxation rate
is given by eq 1
Results
There have been several systematic studies of concentra-
tion and temperature effects on the line widths of 63Cu(I)
salts in acetonitrile by Ochsenbein and Schla¨pfer1 and
Kroneck et al.2 Some representative results of temperature
and concentration effects from this and the earlier studies
are shown in Figures 1 and 2. The present results with the
triflate and perchlorate salts are qualitatively similar to those
obtained previously in that the line width shows a minimum
at about -5 °C and increases with increasing concentration
of the Cu(I) salt. The triflate salt gives slightly broader lines
than the perchlorate. Although the trends are similar, there
are considerable differences in the line widths of the
perchlorate salt between the present and an earlier study.2
For 63Cu with nuclear spin I ) 3/2 and magnetogyric ratio
γ ) 7.097 × 107 s-1 T-1, nuclear relaxation is expected to
be dominated by the quadrupolar relaxation mechanism and
to be in the extreme narrowing limit. The latter requires that
the nuclear precession rate times the correlation time for
molecular tumbling (τQ) should satisfy the condition (πν0τQ)2
2
3(2I + 3)
1
T2
η2 2πe2qQ
) πW1/2
)
1 +
(
τ
(1)
)(
)
Q
40{I2(2I - 1)}
3
h
where W1/2 is the full-width at half-height, η is the asymmetry
parameter, and τQ is the correlation time. It is common
practice to group the last two terms in brackets in the
equation together as an effective quadrupole coupling
constant (QCC), and eq 1 simplifies to eq 2.
3π2(2I + 3)
10{I2(2I - 1)}
1
T2
) πW1/2
)
(QCC)2τQ ) 3.9478(QCC)2τQ
(2)
Quadrupolar relaxation is consistent with the line widths
of 63Cu(I) and 65Cu(I) determined by Lutz and co-workers.4
The ratio of the line widths (540/470 ) 1.149) is in
reasonable agreement with the square of the nuclear qua-
(4) Lutz, O.; Oehler, H.; Kroneck, P. Z. Phys. A: Hadrons Nucl. 1978,
288, 17.
Inorganic Chemistry, Vol. 42, No. 12, 2003 3935

Irangu and Jordan
drupolar coupling constants (1.1686).5 In the present study,
similar measurements on 0.062 M Cu(I) perchlorate and
triflate gave values of 443/380 ) 1.166, and 465/399 )
1.165, respectively. The extreme narrowing condition is
consistent with the observations of Ochsenbein and Schla¨pfer,1
Gill and co-workers,6 and the present study that T1 and T2
for Cu(I) in acetonitrile are essentially equal (see Table 1).
However, the temperature dependence above -5 °C is
anomalous because quadrupolar relaxation should show a
simple decrease in line width with increasing temperature
due to the shortening of the correlation time (τQ). It has been
suggested that this effect might be due to a coordination
change1 or to ion pairing2 in the higher temperature region.
In a series of studies, Gill and co-workers have examined
the 63Cu(I) line width at 25 °C as cosolvents are added to
AN. The lines inevitably broaden with the addition of another
solvent, and this is attributed primarily to the formation of
mixed complexes, but the temperature dependence was not
studied, and the anomaly was ignored. In 1995, Gill et al.7
suggested that the 63Cu(I) line widths are so large because
of the Sternheimer antishielding factor (γ∞) which has a value
of -17 and contributes a factor of (1 - γ∞) to the electric
field gradient. More recently, Gill, Byrne, and Quickenden8
used the Stokes-Einstein equation to calculate a correlation
time of ∼4.5 × 10-11 s, and a QCC ≈ 3.0 MHz for 0.064
M CuClO4 in AN. Most recently, Gill and co-workers9 have
calculated a correlation time of 1.39 × 10-11 s in AN and
used this to estimate that QCC ) 5.46 MHz.
concentration limit, all of the salts should give the same line
width. However, extrapolated values from the earlier studies
(Figure 2) on the BF4- and ClO4- salts are ∼470 and ∼490
Hz, respectively, while the present study gives much smaller
-
values of ∼415 and 420 Hz for the triflate and ClO4 ,
respectively. Second, below -5 °C where the temperature
dependence is that expected for quadrupolar relaxation, all
of the curves in Figure 1 should converge to that expected
for the Cu(NCCH3)4+ ion. This expectation does seem to be
realized in Figure 1 for the ∼0.1 M triflate, perchlorate
(present study), and tetrafluoroborate salts,1 and the other
deviations might be due to concentration differences.
Viscosity Effects. The Stokes-Einstein relationship sug-
gests that the correlation time τQ
η/T, where η is the
viscosity of the solvent, and many studies have shown that
viscosity increases with salt concentration.11 Then, the
simplest explanation for the concentration effects would be
that they are due to the increase in viscosity with salt
concentration which causes τQ and hence W1/2 to increase.
Gill and co-workers12 studied the viscosity of CuClO4 in
AN and found that the variation of viscosity with concentra-
tion of CuClO4 is given by η ) η0(1 + A[CuClO4]1/2
+
B[CuClO4]) with A ) 1.76 × 10-2 and B ) 0.77 at 25 °C.
The concentration dependence of the viscosity is similar to
that of tetra(n-propyl)ammonium bromide studied by Nikam
and Sawant13 and by Saha and Das,14 who also studied the
temperature dependence of the viscosity for n-Pr4NBr in AN.
The data for n-Pr4NBr suggest that the temperature depen-
dence of B (15-45 °C) can be represented by B ) 0.77(1 +
4.0 × 10-3(298 - T)) and A has no discernible temperature
dependence, possibly because it makes a small contribution
to the overall effect. The temperature dependence of the
viscosity of pure AN was determined between -40 and 20
°C by Kanes15 and is given by η0 ) 0.152 exp(924.7/T).
The concentration dependence for CuClO4 from Gill and co-
workers, and the temperature dependencies from Saha and
Das, and Kanes, can be combined to estimate the viscosity
of CuClO4 in AN, with the assumption that CuClO4 and
n-Pr4NBr have the same temperature dependence.
Of the results in Figure 2, the observations of Ochsenbein
-
and Schla¨pfer1 on the PF6 salt are clearly unique. The
concentration dependence is much more dramatic with an
overall change in line width of ∼400 Hz compared to ∼50
Hz for the other anions. The temperature dependence (not
shown) also is anomalous with a minimum at ∼10 °C versus
-5 °C for the others, and with a much larger line width in
the low-temperature limit. Caulton et al.10 have reported a
line width of 400 Hz for a “nearly saturated” solution of
Cu(NCCH3)4PF6 at 25 °C, compared to ∼850 Hz at 0.22 M
reported by Ochsenbein and Schla¨pfer. In the present study,
0.0613 M CuClO4 containing 0.050 M (Me4N)PF6 was found
to have a line width of 447 Hz, entirely consistent with other
samples at the same total salt concentration. These anomalies
suggest that the sample of Ochsenbein and Schla¨pfer was
contaminated in some way, and these results will not be
included in the further analysis.
It is noteworthy that both the line widths (Figure 2) and
the viscosity have an essentially linear dependence on
concentration. Since all of the concentration studies have
been done at 25 °C, it is convenient for purposes of
comparison to analyze the data in terms of the reduced
viscosity, η/η0, where η0 is the viscosity of the pure solvent
(3.41 mP (millipoise)) and η is the viscosity of the salt
solution. If W1/2 τQ and τQ η/T, then
If the Cu(I)/AN system is to be described by a two species
+
model, Cu(NCCH3)4 and either an ion pair or another
species of different coordination or symmetry, then there are
some simple expectations for these results. First, in the low
η0
Cv η
W
)
0 ) (W1/2)0,T
(3)
( )
1/2
η
T
(5) Raghavan, P. At. Data Nucl. Data Tables 1989, 42, 189.
(6) Gill, D. S.; Rodehu¨ser, L.; Delpuech, J.-J. J. Chem. Soc., Faraday
Trans. 1990, 86, 2847.
(7) Gill, D. S.; Rodehu¨ser, L.; Rubini, P.; Delpuech, J.-J. J. Chem. Soc.,
Faraday Trans. 1995, 91, 2307.
where Cv is a proportionality constant and (W1/2)0,T is the
line width in the pure solvent at temperature T. If viscosity
(8) Gill, D. S.; Byrne, L.; Quickenden, T. L. Z. Naturforsch. 1998, 53a,
1004.
(9) Gill, D. S.; Kamp, U.; Doelle, A.; Zeidler, M. D. Ind. J. Chem, 2001,
40A, 693.
(10) Geerts, R. L.; Huffman, J. C.; Folting, K.; Lemmen, T. H.; Caulton,
K. G. J. Am. Chem. Soc. 1983, 105, 3503.
(11) Jenkins, H. D. B.; Marcus, Y. Chem. ReV. 1995, 95, 2695.
(12) Gill, D. S.; Cheema, J. S. Z. Phys. Chemie (N. F.) 1983, 134, 205.
Gill, D. S.; Chauhan, J. S. Z. Phys. Chemie (N. F.) 1984, 140, 149.
(13) Nikam, P. S.; Sawant, A. B. J. Chem. Eng. Data 1997, 42, 1151.
(14) Saha, N.; Das, B. J. Chem. Eng. Data 2000, 45, 1125.
(15) Kanes, J. P. Diss. Abstr. 1967, 26, 1367.
3936 Inorganic Chemistry, Vol. 42, No. 12, 2003

63Cu NMR of Cu-AN System
+
Table 2. Water Dependence of 63Cu(I) Line Width for Cu(AN)4
in AN
[Cu(I)], AN, [H2O],
vol %
η,b
cP
[Cu(II)],c W1/2
mM Hz
,
W
1/2(η0/η),
Hz
anion
M
M
a
BF4
0.087 100
0
0.363
0
540
560
620
805
850
900
1215
443
508
522
573
699
716
777
941
422
525
a
a
a
a
a
a
BF4
BF4
BF4
BF4
BF4
BF4
0.10
0.10
0.10
0.10
0.10
0.10
99
98
95.25
95.25
95
0.51 0.365
1.08 0.370
2.64 0.393
2.64 0.393
2.78 0.395
5.61 0.440
0.099
0.196
0
0.476
0
0
0
0
0
0.18
0
90
ClO4
ClO4
ClO4
ClO4
ClO4
0.062 100
0.061 100
0
0
0.356
0.356
a
a
a
d
550
0.089
0.062
91
91
5.04 0.428
5.04 0.420
0
2700
4800
520
2152
3887
(491)
0.064 100
0.403e
(0.36)f
2.69 0.423e
5.36 0.440e
5.61 0.429b
0
0
0
0
0
0
0
0
0
0
0
997
1221
1235
2850
4300
5160
6400
450
805
948
982
1552
1725
1800
1940
433
d
d
g
d
d
d
d
ClO4
ClO4
ClO4
ClO4
ClO4
ClO4
ClO4
F3CSO3
F3CSO3
F3CSO3
F3CSO3
0.064
0.064
Figure 3. Concentration dependence of the 63Cu(I) line width times
-
reduced viscosity for Cu(AN)4X salts in AN: X ) ClO4 (]) ref 2, (0)
0.061
0.064
0.064
0.064
0.064
90
-
0.627e
0.849e
0.977e
1.124e
0.350
this work; X ) BF4 (+) ref 1; X ) triflate (b) this work. Dashed lines
14.5
(- - -) are eyeguides only.
23.2
28.3
32.9
0
0.20 0.350
0.53 0.351
1.00 0.355
change is the only factor affecting the line width, then a plot
of the factor (η0/η)W1/2 versus salt concentration should be
a constant, independent of the salt concentration.
g
g
g
g
0.045 100
0.045
0.045
460
500
640
447
484
615
0.045
The appropriate plots are shown in Figure 3. The results
suggest that (η0/η)W1/2 is essentially constant for the per-
chlorate and tetrafluoroborate salts, and that the concentration
effect with these salts is primarily due to the viscosity change.
The anomaly of different (W1/2)0,T values from different salts
and sources remains. There appears to be a significant
remaining effect of salt concentration on line widths for the
triflate salt.
a Reference 2. b Calculated from densities and viscosities compiled in
ref 13 for AN/H2O solutions. c The source of Cu(II) is not specified.
d Reference 9 in AN/D2O solutions. e Viscosities measured for the Cu(I)/
AN/D2O solutions. f Calculated value consistent with earlier CuClO4/AN
viscosity vs concentration results in ref 12. g Determined in the present study
in AN/H2O solutions.
the viscosity.13 This also might account for some of the
variability between reported line widths noted in Table 1.
The apparently different behavior of triflate might be
ascribed to quite different viscosity effects of the perchlorate
and triflate anions, suggesting a much larger B value for the
latter. But a rather large value of B ≈ 1.4 is required to
linearize the Cu(trif) (trif- ) triflate) data. This seems
unlikely from a comparison of the viscosities of sodium
perchlorate and triflate in DMSO.16 The two salts show
essentially the same concentration dependence in DMSO,
and B values in AN and DMSO are generally of similar
magnitude.11 Results described later will show that there
actually is a triflate ion effect on the line width which can
be explained by ion pairing.
The effect of water has been examined in this study by
preparing samples under vacuum and adding known amounts
of water by dehydrating weighed amounts of [Co(NH3)5OH2]-
Br3 at ∼100 °C and condensing the water in the Cu(I)-AN
solution in a dry ice slush. In addition, through a series of
determinations of the density of various volume percentages
of AN/H2O, it has been possible to convert the earlier
observations of Kroneck et al.2 to water molarities for
comparison to the present study. As part of the same study,
Kroneck et al. also added Cu(II), at the submillimolar level
to the solutions. These results are summarized in Table 2. It
appears that the viscosity effect is not sufficient to account
for the line width increases beyond [H2O] > 0.5 M.
The viscosity/temperature dependence has been tested by
comparing the temperature dependence of the 14N line widths
in pure acetonitrile17 to those of a 0.045 M Cu(trif) solution.
The results (Figure S1 in the Supporting Information) show
that the viscosity corrected line widths of the Cu(trif) solution
agree with those of the pure solvent, and both have the
temperature dependence expected from the viscosity.
The results in Table 2 for the BF4- and F3CSO3- salts are
in agreement with regard to the trend with increasing [H2O]
and indicate a marked increase in line width for [H2O] g
0.5 M, with an essentially linear increase with [H2O]. The
somewhat larger line widths observed by Kroneck et al.2 at
lower [H2O] might be due to water already present from the
solvent and suggest that the [H2O] ≈ 0.5 M, i.e., 99 vol %
Water and Cu(II) Effects. Another possible explanation
for the concentration dependence, and the different line
widths with zero salt, is the presence of Cu(II) and/or
adventitious water. The latter might be present in the solvent,
or be introduced along with the salt. Water could act as a
ligand in a lower symmetry complex and also would increase
-
AN, in Kroneck’s samples. The effect of Cu(II) with BF4
-
seems to be minor, but the ClO4 samples of Kroneck are
anomalous in showing an apparently large Cu(II) effect, and
more than twice the line width expected in 5.04 M water,
-
compared to BF4 . This anomaly is further shown by the
fact that Gill’s results for ClO4- are in good agreement with
(16) Yao, N.-P.; Benion, D. N J. Phys. Chem. 1971, 75, 1727.
(17) Wakai, C.; Hirokazu, S.; Matubayasi, N.; Nakahara, M. J. Chem. Phys.
2000, 112, 1462. Yuan, P.; Schwartz, M. J. Chem. Soc., Faraday
Trans. 1990, 86, 593.
-
-
those of Kroneck for BF4 , but not with the latter’s ClO4
data.
Inorganic Chemistry, Vol. 42, No. 12, 2003 3937

Irangu and Jordan
substitution into eq 5 and rearrangement gives eq 6.
In the study of Kroneck et al.,2 the temperature dependence
of the line width for the ClO4 salt in 91% AN also is
-
T-2 1
CA
T
CB
T
anomalous in showing a maximum at ∼30 °C, while in pure
AN there is a minimum at about -5 °C. The authors2 admit
to having no explanation for this behavior. In the present
study, the temperature dependence of 0.0613 M CuClO4 in
90% AN/H2O was found to have a line width at 25 °C in
agreement with Gill et al.9 (see Table 2), and the temperature
dependence was analogous to that in pure AN (see Figure
S2 in Supporting Information). It appears that the sample of
Kroneck et al.2 was contaminated in some way.
It was noted previously that the PF6- sample of Schla¨pfer
had anomalously large line widths. If this was due to water
in the sample, the sample would need to have been ∼5 M
in H2O, or ∼95% AN. This would seem to be unlikely. It
has been our observation that samples prepared in air from
freshly opened acetonitrile as supplied by the manufacturer,
and simply sealed with a plastic cap and Parafilm, show no
effects of water in their NMR line widths.
πW1/2
[A]
[B]
)
)
+
(6)
(
)
η
η
[A + B]
[A + B]
The temperature dependence of the right-hand side of this
expression depends only on the thermodynamic parameters,
∆H° and ∆S°, which characterize the A to B equilibrium.
These parameters and the total concentration of Cu(I) and
X can be used to calculate [A] and [B] and hence to fit the
relaxation rate data as a function of temperature and
concentration.
Concentration Dependence: Ion-Pair Model. The ion-
pair model has been tested first because it seems the most
natural way to account for the concentration dependence with
-
F3CSO3 . The lack of real concentration dependence with
-
ClO4- and BF4 , beyond the viscosity effect already noted,
could be ascribed to the low charge density and polarity of
-
these ions compared to F3CSO3 . Gejji et al. have considered
+
the theoretical aspects of ion pairing of Li+, Na+, and NH4
Chloride Ion Effect. The variation in line widths observed
by various workers might be caused by the presence of traces
of potential ligands for Cu(I). Chloride ion is one of the more
ubiquitous of such species, and its effect on the line widths
has been investigated by adding (Et4N)Cl to 0.0613 M
CuClO4 in AN. For chloride ion concentrations of 0.0015,
0.0031, 0.015, and 0.031 M, it was found that the 63Cu line
widths are 1089, 1874, 4370, and 7557 Hz, respectively, at
25 °C. Clearly, even millimolar amounts of chloride ion have
a large effect on the line width. These observations empha-
size that traces of complexing anions can have very
significant effects.
with F3CSO3- in the gas phase.18 The Eigen-Fuoss19 model
predicts that Ki might be in the range 1-5 M-1 for outer-
sphere ion association between unit charged species in
acetonitrile at 25 °C.
For the Cu(I)-triflate solutions, a preliminary examination
revealed that the line width increased nearly linearly with
the concentration up to ∼0.05 M. This suggests that the
concentration of the ion pair is small relative to the total
Cu(I) for such solutions, so that K e 3 M-1. A visual
analysis, with Ki ) 2, produced a good fit of the data at 298
K for all the Cu(trif) solutions. It also was found that four
solutions with (Bu4N)(trif)/Cu(X) concentrations of 0.05/
0.0613, 0.10/0.0613 (X ) ClO4), and 0.20/0.0624, 0.401/
0.135 (X ) trif) have line widths consistent with this model.
However, solutions with Li(trif) added to Cu(ClO4) or Cu-
(trif) are not fitted as well, with line widths consistently
smaller than predicted, and the Cu(trif) solutions have
consistently larger line widths than corresponding solutions
of Cu(ClO4). This suggests that the total triflate concentration
does affect the line width, but the effective triflate ion
concentration in Li+ solutions is smaller than the expected
value. One possible explanation for this is through ion pairing
of Li+ and F3CSO3-. There have not been any studies relating
to ion pairing of lithium triflate in AN, but Salomon and
co-workers20 have interpreted conductivity measurements to
indicate Ki ) 18.4 for Li(ClO4) in AN. Lithium triflate has
been studied in butyrolactone,21 which has a dielectric
constant similar to AN, and found to have Ki ≈ 20.
In the final analysis, the model included the coupled
equilibria for triflate ion pairing with Cu+ and Li+, and the
contributions of Cu+ and the ion-pair Cu‚(trif) to the line
Line Width Temperature and Concentration Depen-
dence Models. A simple two species model has been
developed to model the Cu(I)/AN system. The species, A
and B, are assumed to be in rapid equilibrium, as shown in
eq 4.
A {
\
X} B
(4)
Species A is expected to be Cu(AN)4+ and is dominant at
low temperatures, while B is a lower symmetry species with
a larger line width and is favored at higher temperatures and
concentrations. The reaction could be a simple structural
change or involve the anion (X) to form an ion pair. The
model also should give a rationalization for the observation
that only triflate seems to show an appreciable concentration
effect. In any case, if quadrupolar relaxation is dominant for
both species, then the line width (W1/2) and relaxation rate
(T2-1) will be related to the species concentrations by eq 5
[A]
A [A + B]
[B]
B [A + B]
πW1/2 ) T-2 1 ) C°
τQA + C°
τQB (5)
(18) Gejji, S. P.; Suresh, C. H.; Bartolotti, L. J.; Gadre, S. R. J. Phys.
Chem. A 1997, 101, 5678.
(19) Eigen, M. Z. Elektrochem. 1960, 64, 115. Fuoss, R. M. J. Am. Chem.
Soc. 1958, 80, 5059.
where C° and C° are temperature independent constants,
A
B
(20) D’Aprano, A.; Sesta, B.; Mauro, V.; Salomon, M. J. Inclusion Phenom.
Macrocyclic Chem. 1999, 35, 451; J. Sol. Chem. 2000, 29, 1075.
(21) Caillon-Caravanier, M.; Bosser, G.; Claude-Montigny, B.; Lemordant,
D. J. Electrochem. Soc. 2002, 149, E340.
and τQ is the correlation time governing the quadrupolar
relaxation. If the Stokes-Einstein relationship is assumed,
then τQ η/T, where η is the viscosity of the solvent, and
3938 Inorganic Chemistry, Vol. 42, No. 12, 2003

63Cu NMR of Cu-AN System
Table 3. Triflate Ion Dependence of 63Cu(I) Line Widths in AN
[Cu(I)]tot added salt [trif]tot
,
W
1/2, Hza
obsd
W
1/2, Hza
calcd
Cu(I) source
mM
mM
mM
Cu(trif)
Cu(trif)
Cu(trif)
Cu(trif)
Cu(trif)
Cu(trif)
Cu(trif)
Cu(trif)
Cu(trif)
Cu(trif)
Cu(trif)
Cu(trif)
Cu(trif)
Cu(ClO4)
Cu(ClO4)
Cu(ClO4)
Cu(trif)
Cu(trif)
Cu(trif)
Cu(trif)
Cu(ClO4)
Cu(trif)
Cu(ClO4)
Cu(ClO4)
Cu(ClO4)
4.94
9.63
9.88
0
0
0
0
0
0
0
0
0
0
0
0
0
4.94
9.63
9.88
24.7
26.0
49.4
50.6
50.6
62.0
98.8
417
416
420b
437
440b
449
456
449b,f
460b
488
488
482b
579
477
475
513
603
748
482
497
484
527
513
513
576
418
422
422
434
435
452
453
453
461
484
495
495
562
473
473
510
600
758
481
498
484
524
515
515
576
24.7
26.0
49.4
50.6
50.6
62.0
98.8
117.0
117.0
247.0
61.3
61.3
61.3
62.4
135.0
62.4
62.4
61.3
62.4
61.3
61.3
61.3
117.0
117.0
247.0
50.0
50.0c
50.0c
100.0c
200.0c
401.0c
50.0d
100.0d
100.0d
201.0d
200.0d
200.0d,e
500.0d
Figure 4. Temperature and concentration dependence of 63Cu(I) line widths
for Cu(AN)4(triflate): (+) 4.94 × 10-3 M; (0) 4.5 × 10-2 M; (O) 0.117
M. Curve (s) calculated from the ion-pair model for 4.94 × 10-3 M, Ki )
1.5 (25 °C), ∆H° ) 6.0 kcal mol-1, and ∆S° ) 20.93 cal mol-1 K-1. Curve
(- - -) calculated for 9.88 × 10-3 M and the same ∆H° and ∆S°.
50.0
100.0
262.4
536.0
112.4
162.4
100.0
263.4
200.0
200.0
500.0
widths at higher temperature. The ∆H° for the ion-pair
formation constant (Ki) will be determined essentially by the
observed temperature dependence of the line widths.
To test the ion-pair model, the temperature dependences
of 4.94 × 10-3, 4.50 × 10-2, and 0.117 M solutions of Cu-
(trif) in AN were studied. A qualitative examination of the
line widths (Figure 4) reveals that there is not a marked
sensitivity to concentration at higher temperature, and it
would appear that this model may not work, unless the ∆H°
is sufficiently small so that Ki does not change too much
between 25 and 70 °C. Small Ki and ∆H° values also are
suggested by the fact that the temperature of the minimum
line width does not seem to shift much with concentration.
Otherwise, one would expect the higher temperature arm to
become more important at higher concentrations, and the
minimum would shift to lower temperature.
a Measured on a Varian S-400 MHz spectrometer in 5 mm tubes unless
otherwise indicated. b Measured on Bruker AM-200 MHz spectrometer.
c Added as Bu4N(trif). d Added as Li(trif). e Salt oven-dried immediately
before sample preparation. f In 10 mm tube on Bruker AM-200.
width, while taking viscosity effects into account, using the
parameters of Gill et al.12 It was assumed that the viscosity
B coefficient was the same for Cu+, Li+, and Bu4N+, since
the values are known to be quite similar.11 The contributions
of the ion pairs Cu‚(trif) and Li‚(trif) to the viscosity are
problematic, and it was assumed that these neutral species
did not make a significant contribution. Then, the line widths
were fitted by least-squares to eq 7
[Cu+]
[Cu‚trif]
[Cu(I)]tot
For the analysis, the value of Ki ) 1.5 at 25 °C was taken
from the previous analysis, various ∆H° values (1-10 kcal
mol-1) were assumed, and the ∆S° was calculated. It was
found that a ∆H° in the range 5-6 kcal mol-1 is required to
reproduce the observed temperature dependence of the line
widths. Such ∆H° values are clearly in conflict with the
implication of the data that Ki does not change much from
25 to 70 °C. This was confirmed by model calculations based
on fitting the lowest concentration data with Ki ) 1.5 (25
°C), ∆H° ) 6.0 kcal mol-1, and ∆S° ) 20.93 cal mol-1
K-1 (solid curve in Figure 4). If the same parameters are
used to calculate the curve for twice the total Cu(trif)
concentration (9.88 × 10-3 M) (dashed curve in Figure 4),
the predicted line widths are much larger than those observed
even in the 0.117 M solution. The calculated curve follows
the expectations of increasing line width change at higher
temperature and shift of the minimum to lower temperature,
but it simply cannot reproduce the experimental observations.
CA
CB
W1/2 ) η
+
(7)
( )
( )
{
}
T
T
[Cu(I)]tot
where η is the viscosity in millipoise. The best fit gave Ki
values for Cu+ and Li+ of 1.51 ( 0.48 and 9.0 ( 1.3 M-1,
respectively, with (CA/T) ) 1.213 ( 0.007 × 103 and (CB/
T) ) 2.16 ( 0.15 × 103.
The solution compositions and results for the ion-pair
model study are summarized in Table 3. The excellent
agreement between the observed and calculated line widths,
and the quite reasonable magnitudes of the Ki’s seem to
vindicate the model as an explanation for the concentration
effects with triflate. It should be noted that the latter depend
on the details of the viscosity correction, but similar values
are obtained with any model assuming a normal dependence
of viscosity on salt concentration.
Temperature Dependence. (a) Ion-Pair Model. The
ability to explain the temperature dependence also is a critical
requirement of any successful model. The Cu+-F3CSO3
ion-pair model assumes that there is more ion pairing at
higher temperature in order to explain the increasing line
widths above -5 °C. This implies in turn that there should
be a more pronounced concentration dependence of the line
-
The conclusion from this analysis is that the ion-pair model
alone cannot account for the increase in line width with
temperature. It can account for the small concentration
effects, but the ion-pair formation constant must have a much
smaller temperature dependence than the line widths.
Inorganic Chemistry, Vol. 42, No. 12, 2003 3939

Irangu and Jordan
5 is calculated on the basis of the parameters given and shows
that the model does fit the data.
Further justification for the model is difficult to provide
because of the unknown nature of the lower symmetry
species B. The ∆H° is similar to that found by Marker and
Gunter3 for the Cu(I)-phosphite systems. The relative values
of CA and CB should depend on the values of (QCC)2 for
the two species, assuming that they have very similar
correlation times. The ratio CB/CA ) 158 suggests that B
has a 12.6 times larger QCC than A.
The value of CA can be used to estimate that W1/2 ≈ 128
Hz for species A at 25 °C, and then, the most recent value
of τQ ≈ 1.4 × 10-11 s, from Gill and co-workers,9 yields
+
Figure 5. Temperature dependence of the 63Cu(I) line width divided by
QCCA ) 2.7 MHz for Cu(AN)4 . The only homoleptic
viscosity for Cu(AN)4ClO4 in AN: (+) 6.21 × 10-2 M; (0) 6.21 × 10-3
tetrahedral Cu(I) species with a known QCC seems to be
Cu(CN)43-, for which QCC ) 1.125 MHz has been
determined by Kroeker and Wasylishen.22
M.
(b) Coordination Change Model. An alternative model
assumes that A and B are related by some change in the
coordinated ligands such that B is less symmetrical than A
and therefore provides more effective quadrupolar relaxation.
It should be noted that the minimal temperature dependence
of the triflate concentration effect, already noted, shows that
the coordination change does not involve formation of Cu-
(AN)3(trif). The concentration dependence is due in part to
viscosity changes and to ion pairing but is relatively
temperature independent compared to the coordination
change equilibrium.
The value of QCCA and the ratio CB/CA gives QCCB ≈
34 MHz for species B. Lucken and co-workers23 have
reported a number of NQR measurements for Cu(I) com-
plexes, but they have more complex ligand systems than AN.
For 3-coordinate (2,6-lutidine)2Cu(halide) complexes, the
NQR frequencies are 33-38 MHz, while analogous (2,6-
lutidine)3Cu+ species are in the range 43-46 MHz. For
several compounds of the general coordination type P2CuS2
and PCuS3, Asaro et al.24 have determined quadrupole
coupling constants from solid-state NMR in the range 17-
27 MHz. Two linear bis(tribenzylphosphine)Cu(I) salts have
QCC in the range of 80 MHz.25 All that can be said is that
a value of ∼34 MHz for species B appears to be in the range
of possible values.
The perchlorate samples studied here present a good initial
test for this model. The concentration dependence is con-
sistent with viscosity effects, so that there are no complica-
tions with the magnitude or temperature dependence of ion-
pair formation. Minor modification of eq 7 gives eq 8
-
The BF4 system previously studied by Ochsenbein and
Schla¨pfer1 and Kronek et al.2 is analogous to the perchlorate
in that there is no indication of ion pairing, so that the same
W1/2
[A]
A [A + B]
[B]
1
T
-
parameters for KC should describe the BF4 system. Plots
)
C
+ CB
(8)
{
}
η
[A + B]
of W1/2/η versus 1/T for these data are shown in Figure 6.
The results of the two studies generally are in agreement
between about -15 and 20 °C, but Kroneck et al. reported
larger line widths at higher temperatures and somewhat
smaller values at lower temperatures. The curve through the
data of Ochsenbein and Schla¨pfer in Figure 6 is drawn with
essentially the same parameters as those for the perchlorate
system, CA ) 1.15 × 104 and CB ) 17.9 × 105. However,
the data of Kroneck et al. require CA ) 0.90 × 104 and CB
) 21.6 × 105, which are ∼20% lower and higher, respec-
tively, than the others. These observations, combined with
those for the triflate system described in following para-
graphs, suggest that there is some systematic error in the
observations of Kroneck et al.
where A and B are the symmetrical and less symmetrical
species, respectively, and CA and CB are temperature
independent constants dependent on the QCC for the
respective species. The concentration and temperature de-
pendence of the viscosity, η, can be calculated as previously
described. In the low-temperature limit where [A] . [B], a
plot of W1/2/η versus 1/T should be linear with a slope of
CA. In the opposite limit, the slope would be CB, but the
data do not indicate that this limit is ever really reached for
the accessible temperature range.
The appropriate plots for two samples of 6.21 × 10-3 and
6.21 × 10-2 M Cu(ClO4) are shown in Figure 5. The low-
temperature limit is clearly observed, and the viscosity
change adequately corrects for the concentration dependence.
A nonlinear least-squares analysis gave an excellent fit of
the data with ∆H° ) 5.4 kcal mol-1, ∆S° ) 9.78 cal mol-1
K-1, CA ) 1.125 × 104, and CB ) 17.86 × 105. However,
because the latter value is not defined by the data, it was
necessary to hold ∆H° constant during the fitting, and the
values given produce the minimum standard error of the fit
for ∆H° in the range 4.5-8.0 kcal mol-1. The curve in Figure
The temperature dependence for Cu(trif) is more complex
to analyze because the ion-pairing equilibrium is coupled to
(22) Kroeker, S.; Wasylishen, R. E. Can. J. Chem. 1999, 77, 1962.
(23) Lucken, E. A. C. Z. Naturforsch. 1994, 49a 155. Ramaprabhu, S.;
Lucken, E. A. C.; Bernardinelli, G. J. Chem. Soc., Dalton Trans. 1995,
115 and references therein.
(24) Asaro, F.; Camus, A.; Gobetto, R.; Olivieri, A. C.; Pellizer, G. Solid
State Nucl. Magn. Reson. 1997, 8, 81.
(25) Kroeker, S.; Hanna, J. V.; Wasylishen, R. E.; Ainscough, E. W.;
Brodie, A. M. J. Magn. Reson. 1998, 135, 208.
3940 Inorganic Chemistry, Vol. 42, No. 12, 2003

63Cu NMR of Cu-AN System
+
that for Cu(AN)n . Then, the contributions from the (AN)4
species and from the (AN)n species are given by F4 and Fn,
respectively.
F4 ) CA([Cu(AN) +4 ] + 1.8[Cu(AN)4‚(trif)])
Fn ) CB([Cu(AN) +n ] + 1.8[Cu(AN)n‚(trif)])
(9)
(9)
The predicted line width divided by viscosity is given by
eq 11
W1/2 F4 + Fn
)
(11)
η
T[Cu]tot
Figure 6. Temperature dependence of the 63Cu(I) line width divided by
viscosity for Cu(AN)4BF4 in AN: (b) 0.10 M, ref 1; (O) 0.09 M, ref 2.
where [Cu]tot is the total Cu(I) concentration.
The analysis of the results initially indicated that the ∆H°
for Ki is <1 kcal mol-1, and a value of 0.50 kcal mol-1 has
been used, along with ∆S° ) 2.48 cal mol-1 K-1, to give Ki
) 1.5 at 25 °C. For KC, the thermodymamic parameters from
the perchlorate system were used. The curves in Figure 7,
which reproduce the data very well, were calculated with
CA ) 1.06 × 104 and CB ) 17.9 × 105. The latter value is
identical to that found to fit the perchlorate system, and the
former is 6% smaller than for the perchlorate salt.
In theory, if the viscosity/concentration effect in both
systems and the ion-pair effect with triflate have been
correctly evaluated, then the two systems should give the
same values for CA and CB. The agreement found gives
substantial support for the generic, coordination-change
model to explain the large 63Cu line widths and their
anomalous temperature dependence.
Figure 7. Temperature dependence of the 63Cu(I) line width divided by
viscosity for Cu(AN)4(trif) in AN: (+) 4.94 × 10-3 M; (0) 9.88 × 10-3
M; (O) 0.117 M. The calculated curves for the first two solutions are
indistinguishable.
+
The question remains: what really is Cu(AN)n ? The
present study has established that it is not an ion pair, Cu-
(AN)n‚(X), and it is not the stoichiometrically equivalent
inner-sphere complex, Cu(AN)3(X). On the basis of the
general coordination chemistry of Cu(I), a coordination
number greater than 4 is very improbable. Then, one is left
Scheme 1
+
with Cu(AN)3 as the most likely possibility.
Experimental Section
Materials. Acetonitrile (Caledon, Fisher, or BDH) was dried over
4 Å molecular sieves for several days before use. Doubly distilled
water was used whenever required. Diethyl ether (Fisher), perchloric
acid (Fisher), trifluoromethanesulfonic (triflic) acid (Aldrich),
cuprous oxide (Matheson, Coleman and Bell), cupric oxide (Fisher),
and copper foil (Matheson, Coleman and Bell) were used as
supplied. Tetraethylammonium chloride (Sigma), tetra(n-butyl)-
ammonium triflate (Aldrich), tetra(n-butyl)ammonium perchlorate
(Aldrich), tetramethylammonium tetrafluoroborate (Aldrich), and
tetramethylammonium hexafluorophosphate (Aldrich) were dried
in an oven at 90 °C. Lithium triflate was prepared by adding ∼5
M triflic acid to an aqueous slurry of the required amount of lithium
carbonate (Fisher), concentrating the resulting clear colorless
solution to dryness, and then washing the solid with diethyl ether
and drying in vacuo.
that for the coordination change. The plot of W1/2/η versus
1/T, shown in Figure 7, is quite similar to that for the
perchlorate system, aside from the slight changes with
concentration. The results have been analyzed by solving
the coupled equilibria in Scheme 1 for the four species, Cu-
(AN)4+, Cu(AN)4‚(trif), Cu(AN)n+, and Cu(AN)n‚(trif), where
the latter two result from the coordination change.
The analysis of the perchlorate system has given KC and
its temperature dependence, and some indication of the ratio
+
of the QCC values for Cu(AN)4+ and Cu(AN)n . The triflate
system has shown that Ki ) 1.5 at 25 °C with a small
temperature dependence, and it indicates that the ion pair
causes ∼1.8 times more broadening than its parent. However,
nothing is known about KCi or the magnitude of QCC for
Cu(AN)n‚(trif). Therefore, the simplest assumptions have
been made in analyzing this system, namely that KCi ) KC
and that the broadening due to Cu(AN)n‚(trif) is 1.8 times
Cu(OH2)4(O3SCF3)2. A 1% excess of 5 M triflic acid was added
to an aqueous slurry of cupric oxide (3.58 g, 25 mmol) at 60 °C
and stirred until a clear blue solution remained. The solution was
concentrated to dryness on a steam bath, and the remaining solid
was dissolved in a minimum amount of acetonitrile, recrystalized
Inorganic Chemistry, Vol. 42, No. 12, 2003 3941

Irangu and Jordan
with ether, filtered, washed with ether, and dried in air. Anal. Calcd
for CuC2F6H8O10S2: Cu, 14.65; C, 5.54; H, 1.86. Found: Cu, 14.50;
C, 5.20; H, 1.67.
Cu(O3SCF3)2. The anhydrous material was obtained as a white,
hygroscopic powder by heating the tetrahydrate in an oven at 100
°C for 3 days. Anal. Calcd for CuC2F6O6S2; C, 6.64. Found: C,
6.30.
Crystals obtained from a solution of the hydrated salt in dried
acetonitrile are Cu(H2O)2(CH3CN)2(O3SCF3)2. Water-free crystals,
Cu(CH3CN)4(O3SCF3)2, were only obtained when the anhydrous
salt was used.
was then capped and covered with Parafilm. Aliquots of freshly
prepared stock were diluted to obtain the required concentration.
Samples of 0.6 mL (5 mm o.d. tubes) or 4 mL (10 mm o.d. tubes)
were then transferred to the NMR tube which had been dearated
by flushing with argon for 5 min. The argon was bubbled through
the solution for a further 1 min before placing a septum cap and
sealing with Parafilm. Other salts were added similarly to the Cu(I)
solutions. These samples were routinely stored in a refrigerator.
Samples also were prepared on a vacuum line by distilling CH3-
CN/CD3CN onto a weighed amount of Cu(NCCH3)4(O3SCF3) in a
weighed NMR tube. Then, the tube was sealed and reweighed to
determine the final concentration. Samples containing small amounts
of water were prepared on the vacuum line by collecting the water
released from weighed amounts of (NH3)5Co(OH2)Br3 (heated at
100-110 °C)28 onto the Cu(I)-acetonitrile sample in a dry ice
slush. The tube was then sealed under vacuum and the composition
determined from the mass as described.
63,65Cu NMR and 14N NMR Measurements. The63Cu NMR
spectra were recorded using broad band probe heads on Varian S
400 and Bruker AM 200 spectrometers. The latter also was used
for the 14N spectra. Samples were in 5 mm o.d. tubes, except for
one 63Cu measurement at 25 °C in a 10 mm o.d. tube on the Bruker
system. Typical NMR parameters were the following: 45° pulse,
delay ∼5T1, spectroscopic window 50 kHz, exponential filter (line
broadening) 10 Hz. For the 63Cu spectra, about 9500 scans (Bruker)
or 1000 scans (Varian) were collected, while 400 scans were used
for 14N. The line widths were determined by fitting to a Lorentzian
line shape using the spectrometer’s software, as well as locally
developed programs. Both methods were in good agreement. The
variable temperature 63Cu and a few 65Cu (25 °C) NMR spectra
were recorded on the Varian S 400. The T1 was determined by a
(π-τ-π/2-Acq-D)n (n ) 1500 transients at each of 11τ values)
sequence on 0.117 M Cu(NCCH3)4(O3SCF3) and analyzed using
the Varian software. In all line width measurements, it was
observed, in agreement with Marker and Gunter3, that when an
exponential filter was applied to the FID, the resultant line
broadening needed to be subtracted to obtain the correct line width.
Cu(NCCH3)4(O3SCF3). In a modification of a published pro-
cedure,26 a mixture of acetonitrile (20 mL), Cu(H2O)4(O3SCF3)2
(2 g, 4.6 mmol), and copper foil (0.5 g, 7.9 mmol) was stirred in
a capped parafilm-sealed Erlenmeyer flask until the solution turned
colorless. Filtration on a 4-8 µm sintered glass frit gave a clear
solution. The product was recovered as a white powder in two ways.
The first method involved removal of the solvent by vacuum
distillation. An ice/water slush was used to trap the solvent, and
pumping was stopped just when the solid appeared to be reasonably
dry. The use of a liquid nitrogen trap or prolonged pumping seemed
to give a product that was more sensitive to air and moisture. The
product was then transferred to a glovebag and stored under
prepurified N2. In the second method, anhydrous diethyl ether was
added to the solution and the white solid allowed to settle before
collecting by filtration under argon. The product was dried and
stored under argon. This product seemed more air-resistant than
that from the vacuum distillation. A portion of product was
2-
dissolved in 0.1 M HClO4 and analyzed for copper by the KI/S2O3
method. Anal. Calcd for CuC9F3H12N4O3S: Cu, 16.86; N, 14.87;
C, 28.69; H, 3.21. Found: Cu, 17.10; N, 14.18; C, 27.64; H 2.98.
Cu(NCCH3)4(ClO4). A published procedure27 was modified by
adding a 1% excess of 70% (∼11.6 M) perchloric acid to an
acetonitrile slurry of cuprous oxide (1.43 g, 10 mmol) at 40 °C
and stirring the mixture for 1 h. To the resulting colorless solution,
a similar volume of diethyl ether was added, and the resulting white
crystals were allowed to settle before collecting by filtration and
washing with 80%/20% v/v ether/acetonitrile. This salt is stable in
air for several months with no visible oxidation if moisture is
excluded.
(NH3)5Co(OH2)Br3. A 1.0 g sample of [(NH3)5CoCO3]NO3 was
added to a solution of 20 mL of water and 20 mL of concentrated
HBr in an ice bath. The mixture was stirred for 10 min, and then,
the solid product was allowed to settle before it was collected by
filtration. The product was washed with ethanol and ether and air-
dried.
Acknowledgment. The authors are pleased to acknowl-
edge the assistance of G. Bigam, L. Kong, and G. Aarts with
the NMR measurements and the financial support of the
Natural Sciences and Engineering Research Council of
Canada.
Supporting Information Available: Figure S1 comparing the
temperature dependence of the 14N line widths of acetonitrile and
a 0.045 M solution of Cu(trif) and Figure S2 showing the
temperature dependence of the 63Cu(I) line widths of 0.0613 M
Cu(ClO4) in 10% water, 90% acetonitrile. This material is available
free of charge via the Internet at http://pubs.acs.org.
NMR Samples. On the benchtop, stock solutions were made
by quickly weighing and transferring the triflate or perchlorate salt
to a measured volume of a CH3CN/CD3CN mixture in a vial which
IC030020C
(26) Ogura, T. Transition Met. Chem. 1976, 1, 179.
(27) Hemmerich, P.; Sigwart, C. Experientia 1963, 19, 488.
(28) Jordan, R. B.; Taube, H. J. Am. Chem. Soc. 1966, 88, 4406.
3942 Inorganic Chemistry, Vol. 42, No. 12, 2003