NMR spectroscopy of the solid-state isomerization of nitrito- and nitro-pentamminecobalt(III) chloride

Article abstract of DOI:10.1139/V06-001

Cobalt-59 and nitrogen-15 NMR spectra of the nitritopentamminecobalt(III) chloride, [(NH₃)₅Co-ONO]Cl₂, and nitropentamminecobalt(III) chloride, [(NH₃)₅Co-NO ₂]Cl₂, isomers in the solid state have been obtained at several applied magnetic field strengths. The ⁵⁹C_Q NMR line shapes indicate that both the cobalt nuclear quadrupolar coupling constant (C_Q) and the span of the chemical shift tensor (Ω) decrease when the complex isomerizes from [(NH₃)₅Co-ONO] ²⁺ to [(NH₃)₅Co-NO₂]²⁺; C_Q decreases from 23 to 10.3 MHz and Ω changes from 1650 to 260 ppm. The ¹⁵N NMR line shapes also show a significant change in the nitrogen magnetic shielding tensor upon isomerization, with Ω decreasing from 710 to 547 ppm; also, an indirect spin-spin coupling, ¹J( ⁵⁹Co,¹⁵N) = 63 Hz, is observed in the ¹⁵N NMR spectra of the nitro isomer. The NMR parameters are rationalized based on differences in the molecular structure of the two isomers. NMR spectra have also been recorded as the isomerization progresses with time and demonstrate the practicality of the technique for the study of solid-state isomerizations.

Full text of DOI:10.1139/V06-001

3
00
NMR spectroscopy of the solid-state isomerization
of nitrito- and nitro-pentamminecobalt(III)
1
chloride
Kristopher J. Ooms and Roderick E. Wasylishen
Abstract: Cobalt-59 and nitrogen-15 NMR spectra of the nitritopentamminecobalt(III) chloride, [(NH ) Co-ONO]Cl ,
3
5
2
and nitropentamminecobalt(III) chloride, [(NH ) Co-NO ]Cl , isomers in the solid state have been obtained at several
3
5
2
2
applied magnetic field strengths. The 59Co NMR line shapes indicate that both the cobalt nuclear quadrupolar coupling
constant (C ) and the span of the chemical shift tensor (Ω) decrease when the complex isomerizes from [(NH ) Co-
Q
3 5
15
ONO]2 to [(NH ) Co-NO ] ; C decreases from 23 to 10.3 MHz and Ω changes from 1650 to 260 ppm. The
+
2+
N
3
5
2
Q
NMR line shapes also show a significant change in the nitrogen magnetic shielding tensor upon isomerization, with Ω
decreasing from 710 to 547 ppm; also, an indirect spin-spin coupling, J( Co, N) = 63 Hz, is observed in the
1
59
15
15
N
NMR spectra of the nitro isomer. The NMR parameters are rationalized based on differences in the molecular structure
of the two isomers. NMR spectra have also been recorded as the isomerization progresses with time and demonstrate
the practicality of the technique for the study of solid-state isomerizations.
Key words: 15N, 59Co, solid-state NMR, linkage isomerization, chemical shift tensor, electric field gradient tensor.
Résumé : Opérant en phase solide et à plusieurs forces de champ magnétique appliqué différentes, on a déterminé les
spectres RMN du 59Co et du N d’isomères du chlorure de nitritopentamminecobalt(III), [(NH ) Co-ONO]Cl et du
15
3
5
2
5
9
chlorure de nitropentamminecobalt(III), [(NH ) Co-NO ]Cl . Les formes des raies des spectres RMN du Co indiquent
3
5
2
2
que la constante de couplage quadripolaire nucléaire du cobalt (C ) ainsi que l’envergure du tenseur du déplacement
Q
2
+
2+
chimique (Ω) diminuent toutes les deux lorsque le complexe s’isomérise de [(NH ) Co-ONO] à [(NH ) Co-NO ] ; la
3
5
3 5
2
valeur de CQ diminue de 23 à 10,3 MHz alors que la valeur de Ω passe de 1650 à 260 ppm. Lors de l’isomérisation,
1
5
les formes des raies en RMN du N présentent aussi un changement significatif dans le tenseur de blindage magné-
1
59
15
tique de l’azote, avec Ω, qui passe de 710 à 547 ppm; de plus, on observe un couplage spin-spin indirect, J( Co, N) =
6
3 Hz, dans le spectre RMN du 15N de l’isomère nitro. Les différences dans les paramètres RMN sont rationalisées sur
la base de différences dans la structure moléculaire de chacun des isomères. On a aussi enregistré les spectres RMN
alors qu’ils s’isomérisent en fonction du temps et on démontrent que cette technique peut être pratique pour l’étude
d’isomérisations à l’état solide.
Mots clés : 15N, 59Co, RMN à l’état solide, isomérisation de liaison, tenseur du déplacement chimique, tenseur du gra-
dient du champ électrique.
[
Traduit par la Rédaction] Ooms and Wasylishen 308
Introduction
for over 100 years (7). This reaction has been studied in
depth by numerous techniques, including solution NMR
spectroscopy (8), IR spectroscopy (9–14), single-crystal and
powder X-ray diffraction (15–17), and recently, DFT quan-
tum chemical methods (18, 19).
Several studies, primarily employing IR spectroscopy,
have determined the rate constants, equilibrium values, and
activation energies of the reaction in the solid state (11, 15,
20–23). Typically the isomerization half-life is on the order
of hours to days depending on the temperature. One problem
with the solid-state investigations to date is that the tempera-
tures at which the kinetic data are measured are usually
Reactions that occur in the solid state are of interest in
many areas of research such as pharmaceutical chemistry (1,
2
6
), organometallic chemistry (3), and organic synthesis (4–
). Linkage isomerizations are one class of solid-state reac-
tions that involve a change in the bond(s) between a ligand
and a metal centre. The linkage isomerization involving the
–
1
nitrite anion (NO2 ) in nitritopentamminecobalt(III) chlo-
ride (nitrito), which isomerizes to form the more thermody-
namically stable nitropentamminecobalt(III) chloride (nitro),
2
+
2+
[
(NH ) Co-ONO] → [(NH ) Co-NO ] , has been known
3 5 3 5 2
Received 21 September 2005. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 21 March 2006.
Dedicated to Dr. Arthur J. Carty for his contributions to science in Canada.
2
K.J. Ooms and R.E. Wasylishen. Department of Chemistry, University of Alberta, Edmonton, AB T6G 2G2, Canada.
1
This article is part of a Special Issue dedicated to Professor Arthur Carty.
Corresponding author (e-mail: roderick.wasylishen@ualberta.ca).
2
Can. J. Chem. 84: 300–308 (2006)
doi:10.1139/V06-001
© 2006 NRC Canada

Ooms and Wasylishen
301
above room temperature, while the X-ray diffraction data
has been obtained well below room temperature. This makes
it difficult to draw connections between the kinetic data and
the crystal structures. In addition, there are significant dis-
crepancies in the rate constants reported by different investi-
gators. In principle, solid-state NMR spectroscopy can
provide a valuable link between the structural and thermody-
namic properties of this system at the same temperature;
however, only the solid-state 59Co NMR of the more stable
nitro isomer has been reported (24).
ting the ammonium peak of labelled, solid 15NH NO to
4
3
23.8 ppm (32). Magic angle spinning (MAS) rates were con-
trolled using automated MAS controllers to within ±2 Hz.
5
9
The Co nonselective π/2 pulse lengths were determined for
the setup sample (1.0 mol/L K Co(CN) ) and were 4.5, 2.0,
3
6
and 1.9 µs at 4.70, 7.05, and 11.75 T, respectively. A one-
pulse or echo-pulse sequence was used for acquiring 59Co
NMR spectra with pulse lengths (τp(sel)), that selectively ex-
cited the central transition (τ
= τp(nonsel)/(I + 1/2)) (33).
p(sel)
Nitrogen-15 NMR spectra were acquired using a cross polar-
In the current study we have determined the 59Co and 15
N
1
ization (CP) pulse sequence (π/2( H) = 4.5 µs) and a contact
time of 8.0 ms, which was initially setup on solid 15NH NO
NMR parameters of both the nitrito and nitro complexes of
pentamminecobalt(III) chloride using solid-state NMR.
Cobalt-59 is a spin I = 7/2 quadrupolar nucleus and has one
of the largest chemical shift ranges in the periodic table,
4
3
and then optimized for the samples. High-power proton de-
coupling was performed during the acquisition of all spectra
using the TPPM method (34). The sample temperature was
determined using a Chemagnetics temperature controller that
>
18 000 ppm (25), making it ideally suited for solid-state
2
07
NMR investigations of cobalt complexes because of its sen-
sitivity to subtle changes in molecular structure (25–29).
had been calibrated using the Pb NMR peak of solid lead
nitrate (32, 35).
Typically, Gaussian line-broadening functions of 20–
50 Hz and 500–3000 Hz were applied when processing the
spectra of MAS and stationary samples, respectively. The
calculated and experimental spectra were fit by visual in-
spection using the program WSOLIDS (36).
–
also monitored the thermal isomerization reaction using
Ab initio calculations of the nitrogen magnetic shielding
tensors were performed using Gaussian 98W (revision A.7)
(37) employing the B3LYP method (38, 39) to determine the
orientation of the tensor in the molecular frame of reference.
Either the 6-311++G(d,p) basis set, or the LANL2DZ basis
set with an effective core potential, was used on the cobalt
(40); 6-311++G(d,p) basis sets were used on all other atoms.
Calculations with both basis sets yielded the same orienta-
tion of the magnetic shielding tensor. The calculated mag-
netic shielding values (σ) were converted to chemical shifts
5
9
15
solid-state Co and N NMR spectroscopy in an effort to
determine whether this technique has the potential to provide
the relative amounts of each isomer in the solid state.
Experimental
The pink nitrito, [(NH ) Co-ONO]Cl , and yellow nitro,
3
5
2
[
(NH ) Co-NO ]Cl , isomers were synthesized according to
3 5 2 2
literature methods, and the product purity was verified using
relative to the reference compound, NH (l, 20 °C), by apply-
3
1
5
IR spectroscopy (10, 12). The isotopically enriched N sam-
ples were prepared using 99% 15N-labelled sodium nitrite
obtained from C/D/N Isotopes Inc., Pointe-Claire, Quebec.
The precursor compound for the synthesis (chloropenta-
mminecobalt(III) chloride) was synthesized by the addition
of 15 mL of concentrated aqueous hydrochloric acid to 5 g
of cobalt(II) carbonate in 35 mL of distilled water. The pre-
cipitate was filtered twice and 5.05 g of ammonium chloride
and 50 mL of concentrated aqueous ammonia were added.
The solution was cooled and 80 mL of 6% hydrogen perox-
ide was slowly added. When bubbling ceased, nitrogen was
bubbled through the solution for 1 h. The solution was neu-
tralized with hydrochloric acid, after which an additional
ing the formula δ = σ – σ, where σ = 244.6 ppm (41).
ref
ref
The atomic coordinates of the cations used were calculated
from the X-ray crystal structures determined by Boldyreva et
al. (15) and Grenthe and Nordin (17); chloride ions were not
included in the DFT calculations.
Results and discussion
Solid-state NMR results
Nitritopentamminecobalt(III) chloride
5
9
Figure 1 shows the Co NMR spectra for the central tran-
sition (m = 1/2 ↔ –1/2) of stationary samples of the nitrito
I
2
+
2
0 mL of concentrated hydrochloric acid were added and the
isomer, [(NH ) Co-ONO] , acquired at 4.70, 7.05, and
3 5
solution was set in a hot water bath for 2 h. The purple pre-
cipitate was filtered and washed with water and ethanol,
yielding approximately 9 g of sample.
Solid-state NMR spectroscopy was performed on three
spectrometers with field strengths of 4.70 T (Chemagnetics
CMX Infinity 200), 7.05 T (Bruker Avance 300), and
11.75 T. The parameters used to calculate the spectra are
presented in Table 1 and are defined in the table footnote.
The spectra in Fig. 1 are influenced by both the 59Co
quadrupolar interaction and anisotropic magnetic shielding
at all three fields. At 4.70 T the second-order quadrupolar
broadening (i.e., the line width without the addition of
anisotropic magnetic shielding to the simulation) accounts
for approximately 90% of the total breadth, while at 11.75 T
it accounts for approximately 33%. The spectra are all be-
tween 180 and 220 kHz broad, making uniform excitation of
the powder pattern difficult and magic angle spinning im-
1
1.75 T (Bruker Avance 500). Bruker double or triple reso-
nance probes (4 mm o.d. MAS) were used on the Bruker
systems, while Chemagnetics double resonance probes
(
4 mm o.d. MAS and 5 mm o.d. solenoid) were used on the
5
9
Chemagnetics system. All Co NMR chemical shifts were
referenced with respect to a 1.0 mol/L aqueous solution of
K Co(CN) at 0.0 ppm, while the N NMR chemical shifts
practical.
1
5
59
The Co isotropic chemical shift (δ ) of the nitrito iso-
3
6
iso
were referenced with respect to liquid NH (20 °C) by set-
mer in the solid state is 8650 ± 50 ppm, the span is 1650 ±
3
©
2006 NRC Canada

3
02
Can. J. Chem. Vol. 84, 2006
Table 1. The experimental chemical shift and EFG parameters determined by fitting the 15N and 59Co NMR spectra for nitrito- and ni-
tro-pentamminecobalt(III) chloride.
59Co NMR parameters
Isomer
Structure
δiso (ppm)
Ω (ppm)
κ
CQ (MHz)
η
α (°)
0±5
β (°)
0±5
γ (°)
2
+
Nitrito
Nitro
[(NH ) Co-ONO]
8650±50
1650±100
0.8±0.1
±23±1
0.20±0.05
0.35 0.05
0±2
3±1
3
5
2
+
[(NH ) Co-NO ]
7505±5
260±20
–0.20±0.05
+10.3±0.1
90±5
90±5
3
5
2
15N NMR parameters
1
59
15
Isomer
Nitrito
Nitro
Structure
[(NH ) Co-ONO]
δiso (ppm)
611±1
Ω (ppm)
710±6
κ
J( Co, N) (Hz) α (°)
β (°)
2
+
–0.95±0.05
0.5±0.1
3
5
2
+
[(NH ) Co-NO ]
470±1
547±6
±63±2 90±2
90±2
3
5
2
Note: The chemical shift parameters are defined as follows: δ ≤ δ ≤ δ , δ = (δ + δ + δ )/3, Ω = δ – δ , κ = 3(δ – δ )/Ω (62). The compo-
33
22
11
iso
11
22
33
11
33
22
iso
nents of the chemical shift tensor are δ = (ν – ν )/ν (ii = 11, 22, 33). The EFG parameters are C = eQV /h and η = (V – V )/V , where |V | ≥
ii
ii
ref
ref
Q
ZZ
Q
XX
YY
ZZ
ZZ
|
V | ≥ |VXX|. The Euler angles α, β, and γ describe the rotations illustrated in Fig. 2.
YY
Fig. 1. Calculated and experimental 59Co NMR spectra of sta-
Fig. 2. (a) The counter-clockwise rotations defined by the Euler
angles α, β, and γ required to bring the PAS of the EFG tensor
(VXX, V , V ) into coincidence with the PAS of the chemical
tionary samples of [(NH ) Co-ONO]Cl acquired at 4.70, 7.05,
3
5
2
and 11.75 T. Each experimental spectrum is the sum of 10 000 –
0 000 scans collected with a 0.1 s pulse delay. The spectrum
YY
ZZ
5
9
15
5
shift tensor (δ , δ , δ ). (b) Relative orientation of the Co- N
11 22 33
acquired at 7.05 T has a small amount of nitro isomer present.
internuclear vector (r) with respect to a tensor.
VZZ
(a)
δ33
(b)
Z
Calcd.
11.75 T
β
r
β
δ22
Exp.
VYY
Y
1
200
1100
700
500
1000
900
500
300
800
α
VXX
α
γ
δ11
X
Calcd.
Exp.
7.05 T
59Co NMR spectra may not be of the pure nitrito form. Any
nitro isomer present in the sample would cause a distortion
of the low frequency discontinuity, leading to some uncer-
tainty in the analysis. The Euler angles, which describe the
relative orientations of the Co EFG and chemical shift ten-
sors, are also obtained from the experiments and are all 0°,
8
00
600
400
59
placing δ along VXX and δ along VZZ (Fig. 2a shows how
1
1
33
the angles are defined, ref. 43).
Calcd.
Exp.
4.70 T
Figure 3 shows the experimental and calculated 15N NMR
spectra of nitritopentamminecobalt(III) chloride, with 99%
N-labelled nitrite, obtained at 4.70 T with MAS rates of
.0 and 4.0 kHz. The observed spectra were fit with calcu-
lated spectra based on the procedure developed by Herzfeld
and Berger (44) as implemented in the WSOLIDS program.
The experimental and calculated spectra of a stationary sam-
ple acquired at 4.70 T are also shown in Fig. 3. Analysis of
the spectra yields the experimental NMR parameters listed
in Table 1. The tensor is approximately axially symmetric
κ = –0.95 ± 0.05), has a span of 710 ± 6 ppm, and the δ
611 ± 1 ppm) is only slightly less than that of the nitrogen
in sodium nitrite (625.4 ppm) (45). The near axial symmetry
is probably fortuitous given the lack of rotational symmetry
at the nitrogen of this compound.
15
2
6
00
200
kHz
1
00 ppm, and the skew of the tensor is 0.8 ± 0.1. Since 59Co
has a large chemical shift range, the large span is not unex-
pected. In solution, the chemical shift of the nitrito isomer is
(
(
iso
7
532 ppm (26), corresponding to an 1118 ppm solution-to-
solid shift. This is a large change in δ , and highlights the
iso
sensitivity of the cobalt chemical shift to changes in the lo-
cal structure (25, 42).
The cobalt electric field gradient (EFG) tensor is nearly
axially symmetric (η = 0.20 ± 0.05). The fact that a devia-
Nitropentamminecobalt(III) chloride
Q
5
9
tion from axial symmetry is observed can be explained by
The Co central transition NMR spectra of freshly syn-
2
+
the deviation from C symmetry in the immediate coordina-
thesized nitro isomer, [(NH ) Co-NO ] , collected at three
4
3 5 2
tion sphere. The quadrupolar coupling constant (C ) is 23 ±
fields (4.70, 7.05, and 11.75 T) are shown in Fig. 4; the pa-
rameters derived from the calculated spectra are given in Ta-
Q
1
MHz. While the error is large, it reflects the fact that the
©
2006 NRC Canada

Ooms and Wasylishen
303
Fig. 3. Calculated and experimental 15N NMR spectra of MAS
spinning side bands (not shown), are split by indirect spin-
5
9
1
59
15
and stationary samples of [(NH ) Co-ONO]Cl acquired at
spin coupling to the Co, | J( Co, N)|, which is 63 ±
3
5
2
1
59
15
4
.70 T using CP. Each experimental spectrum was acquired using
2 Hz. Previous solid-state J( Co, N) values have been re-
a pulse delay of 5.0 s and is the sum of 612 and 6144 scans for
the MAS and stationary samples, respectively.
ported for a series of cobaloxime complexes with either
1
5
15
pyridine- N or aniline- N ligands; the magnitudes range
from 36 to 75 Hz (46). The splitting pattern in the 15N NMR
spectrum is influenced by the cobalt quadrupolar coupling
5
9
15
parameters because the Co– N dipolar coupling can not
5
9
15
be completely averaged by MAS (46–48). For the Co– N
1
5
Calcd.
pair, the peaks in the N spectra are split into eight peaks,
5
9
corresponding to the eight possible spin states of the Co
m = –7/2 to +7/2). The peak intensities and splittings are
(
I
Stationary
1
59
15
59
15
dictated by J( Co, N), the Co– N dipolar coupling con-
stant, R , the Co C , η , and the orientation of the Co
EFG tensor with respect to the Co– N dipolar vector.
With the magnitude of the Co quadrupolar coupling and ηQ
determined from the Co NMR spectra and the value of
Exp.
59
59
eff
Q
Q
5
9
15
5
9
Calcd.
59
Reff = –402 Hz, calculated based on the Co—N distance ob-
tained from the crystal structure (assuming ∆J = 0), the ex-
perimental spectrum in Fig. 5 can be best fit when the sign
ν
r
= 4 kHz
Exp.
of C is positive and the Euler angles that orient the Co-N
Q
5
9
dipolar vector in the PAS of the Co EFG are both 0°, plac-
ing VZZ along the dipolar vector (Fig. 2b shows how the an-
gles are defined for a vector with respect to a tensor).
Analysis of the N NMR spectrum of a stationary sample
of the nitro isomer (Fig. 6) indicates that the span of the ni-
trogen chemical shift tensor is 547 ± 6 ppm and that the
skew is 0.5 ± 0.1 (Table 1). This is a significant change from
Calcd.
Exp.
1
5
ν
r
= 2 kHz
200
1
200 1000
800
600
400
the nitrito isomer and is accompanied by a change in δ
iso
1
5
ppm
from 611 ± 1 to 470 ± 1 ppm. The N NMR isotropic
chemical shift value of the nitrite ligand in the nitro isomer
is significantly less than that reported for the nitrite ion in
NaNO (δ = 625.4 ppm) (45). Also, the span of the nitro
ble 1. The spectra suffered from a general broadening,
which obscured some of the features, most likely because of
direct and indirect spin-spin coupling to the six directly
2
iso
nitrogen shielding tensor in the cobalt complex is less than
that of the nitrite ion in NaNO (Ω = 951 ppm, κ = 0.37)
2
1
4
bonded N nuclei. From the spectrum of an MAS sample,
(
45).
ν = 13 kHz, collected at 7.05 T (Fig. 4b), it is straightfor-
ward to determine the quadrupolar parameters because the
chemical shift anisotropy is averaged by spinning at the
magic angle. The values obtained are C = 10.3 ± 0.1 MHz
and η = 0.35 ± 0.05. This corresponds to a significant de-
crease in the Co quadrupolar coupling upon conversion
15
r
The N NMR line shape of a stationary sample of an iso-
15
59
lated spin pair, such as N coupled to Co, depends on the
15
1
59
15
N chemical shift tensor, R , J( Co, N), and the orienta-
eff
Q
tion of the dipolar vector in the principal axis system (PAS)
15
Q
of the N chemical shift tensor (46, 49). The angles that re-
5
9
15
late the Co-N dipolar vector to the N chemical shift tensor,
from the nitrito isomer (C = 23 ± 1 MHz). δ also changes
15
Q
iso
determined from the analysis of the N NMR spectrum in
upon isomerization from 8650 ± 50 to 7505 ± 5 ppm.
Fig. 6 are α = 90° and β = 90°; the δ component of the ni-
With the C , η , and δ determined from the 5 Co NMR
9
22
Q
Q
iso
trogen chemical shift tensor is along the Co—N bond, while
spectra obtained with MAS, the spectra of the stationary
samples (Figs. 4a and 4c) were analysed, and the chemical
shift tensor and its orientation relative to the EFG tensor
were determined. The span of the chemical shift tensor is
δ33 is perpendicular to the Co-NO plane.
2
Relationship between molecular structure and the EFG
and chemical shift tensors
2
60 ± 20 ppm and the skew is –0.20 ± 0.05. The Euler an-
gles are 90 ± 5°, 90 ± 5°, and 3 ± 1° for α, β, and γ, respec-
tively, making the EFG and chemical shift tensors nearly
coincident with δ parallel to VZZ and δ parallel to VYY.
To examine the relationship between the molecular struc-
tures and the cobalt and nitrogen NMR parameters of the
two isomers, it is important to look at the three principal
components of the tensors and their orientation in the molec-
ular frame. As mentioned, the magnitude of the cobalt CQ
decreases when the nitrito isomer changes to the nitro form,
from 23 to 10.3 MHz. This change indicates that the elec-
tronic distribution about the cobalt is more symmetric in the
nitro isomer, where the cobalt atom is bonded to six nitrogen
atoms, compared with the nitrito isomer, where the cobalt is
bonded to five nitrogen atoms and one oxygen atom.
The orientation of the EFG tensors at the cobalt nuclei can
1
1
33
5
9
These Co EFG and chemical shift parameters differ from
those previously reported (24): C = 11.25 MHz, η = 0.55,
Q
Q
Ω = 270 ppm, κ = 0.519, and α = 7°, β = 88°, and γ = 7°.
The differences can be related primarily to the different C
Q
values; in the previous study, measurements of samples un-
der MAS were not carried out.
Figure 5 shows the centre band region of the 15N NMR
spectrum acquired for a sample of the nitro isomer under
MAS conditions at 4.70 T. The centre band, as well as the
©
2006 NRC Canada

3
04
Can. J. Chem. Vol. 84, 2006
Fig. 4. Cobalt-59 NMR spectra of [(NH ) Co-NO ]Cl acquired at (a) 11.75 T, (b) 7.05 T, and (c) 4.70 T. The spectrum acquired at
3
5
2
2
5
9
7
.05 T is of a MAS sample (νrot = 13 kHz), while those acquired at the other two fields are Co NMR spectra of stationary samples.
The spectra are the sum of 359, 869, and 6227 scans for the three fields, respectively, with a 0.1 s pulse delay. The distortions in the
baseline of the spectrum obtained with MAS are a result of not being able to spin rapidly enough to completely average the second-
order quadrupolar interaction and chemical shift anisotropy.
(a) Stationary, B
0
= 11.75 T
(b) MAS, B
0
= 7.05 T
0
(c) Stationary, B = 4.70 T
Calcd.
Exp.
9
30
900
870
550 540 530 520 510 390
360
330
300
kHz
kHz
kHz
Fig. 5. Calculated and experimental 15N NMR spectra of a MAS
sample of [(NH ) Co-NO ]Cl spinning at 8 kHz obtained using
Fig. 6. Calculated and experimental 15N NMR spectra of a sta-
tionary sample of [(NH ) Co-NO ]Cl acquired using CP at
3
5
2
2
3
5
2
2
1
59
15
CP acquired at 4.70 T showing the J( Co, N) coupling to the
4
.70 T. The spectrum is the sum of 17 692 scans using a 5.0 s
5
9
Co. The intensities of the spinning side bands were added to
the centre band.
pulse delay.
Calcd.
Calcd.
Exp.
Exp.
1000
5
00
450
400
800
600
400
200
0
ppm
ppm
be understood based on symmetry arguments. While it was
not possible to obtain the orientation of the 59Co EFG in the
molecular frame from the experimental data of the nitrito
“unique” component of the cobalt chemical shift tensor
along the Co—ONO bond, as would be expected. The com-
ponent of the cobalt chemical shift along the bond is 7605 ±
50 ppm, while the two components of the chemical shift
isomer, the pseudo-C symmetry of the molecule suggests
4
that the unique component of the EFG (V ) is approxi-
tensor that lie in the Co(NH ) plane, perpendicular to the
ZZ
3 4
mately along the Co—ONO bond, and the two components
Co—ONO bond, are 9255 ± 50 and 9099 ± 50 ppm. The
Co chemical shift tensor of the nitro isomer is oriented
5
9
that are nearly equal in magnitude are perpendicular to this
1
5
bond. For the nitro isomer, the N spectrum for the sample
under MAS indicates that the 59Co EFG tensor orientation is
the same: the VZZ component is parallel to the Co—NO2
bond. The lack of change in the orientation of the EFG
tensor and the asymmetry parameter indicates that the
isomerization simply results in a scaling of the EFG at the
cobalt nucleus.
such that the direction of least shielding is along the Co—
NO bond; δ has a value of 7643 ± 10 ppm and the two
2
11
components perpendicular to the bond have values of
7487 ± 10 and 7383 ± 10 ppm. By comparing the values of
the components for the two isomers, it is clear that the co-
balt shielding along the Co—X bond (δ = 7605 ppm, X =
ONO and δ = 7643 ppm, X = NO ) does not change signifi-
2
The orientations of the 59Co chemical shift tensors in both
isomers can be fixed in the molecular frame based on their
relative orientation to the EFG tensor. The Euler angles de-
termined from the 59Co stationary line shapes of the nitrito
cantly when the compound isomerizes. In contrast, the
shielding perpendicular to the bond increases by approxi-
mately 1750 ppm in the nitro isomer.
Correlation of cobalt chemical shifts to octahedral crystal
isomer indicate that δ11 is along VXX and δ is along VZZ.
field splitting (∆ ) has been used for many years to explain
33
O
Since the tensor is close to axially symmetric, this places the
the isotropic chemical shifts of octahedral cobalt complexes
©
2006 NRC Canada

Ooms and Wasylishen
305
Fig. 7. Orientation of the nitrogen CS tensor in the molecular
frame of reference for: (a) the nitrito isomer, obtained from DFT
calculations, and (b) the nitro isomer, obtained from DFT calcu-
lations and experiment.
in solution and in the solid state (50–53). The paramagnetic
shielding term, as described by Ramsey (54) in his original
formalism, depends on the energy difference between elec-
tronic states that undergo symmetry-allowed mixing in the
presence of a magnetic field and is generally deshielding.
The good correlation between the cobalt chemical shifts and
(
a)
(b)
1
1
the excitation wavelength of the A1g → T electronic tran-
1g
sition in octahedral cobalt(III) complexes was one of the
early confirmations of Ramsey’s formalism. In the solid
state, the correlation can be used for octahedral complexes
of high symmetry. Equations describing the dependence of
shielding on the transition energies have been derived for
numerous symmetries including D , C , D , C , etc. (55,
3
4v
4v
2v
5
6). In the present study, however, neither complex has high
symmetry at the cobalt atom, which limits the application of
the derived equations for determining the individual compo-
nents or isotropic values of the chemical shift tensor. De-
spite this, an approximate correlation can be made between
the overall shielding that occurs as the complex isomerizes
the nitro isomer as determined from the analysis of the
N NMR line shape of the stationary sample indicates that
15
and an increase in the ∆ . The change in colour, from the
O
δ
is perpendicular to the Co-NO plane and δ is along the
3
3
2
22
pink nitrito isomer to the yellow nitro isomer, indicates an
Co—N bond (Fig. 7b). The calculations with both the 6-
311++G(d,p) and LANL2DZ basis sets on the Co yielded
values for δ , δ , and δ that were in excellent agreement
increase in ∆ . The increase in ∆ leads to a smaller para-
O
O
magnetic shielding of the cobalt in the nitro isomer and re-
sults in the smaller chemical shift. This change is noticed
primarily in the components of the shielding tensor perpen-
dicular to the Co—X bond.
1
1
22
33
with the experimental values, within 11, 16, and 62 ppm of
the experimental values, respectively. The process of coordi-
nation to the cobalt metal has the most profound effect on
δ , which decreases by 412 ppm when the isomer changes
The orientation of the nitrogen chemical shift tensor in the
nitrito isomer is unavailable from the NMR experiments. In
other two-coordinate nitrogen compounds, such as sodium
nitrite and bent nitrosyl compounds (45, 57), the nitrogen
chemical shift tensor is oriented such that δ22 is approxi-
mately along the formal nitrogen lone pair and δ33 is perpen-
dicular to the ONO plane. Quantum chemical calculations
were performed and predict the same orientation for the
nitrito isomer (Fig. 7a). The calculations reproduced the rel-
1
1
from the nitrito to the nitro form. This is a result of the
changes in the nonbonding molecular orbitals of the nitrito
isomer as they coordinate to the cobalt centre to form the
Co—N σ bond in the nitro isomer. The chemical shift
component along the Co—N bond (δ ) in the nitro isomer
2
2
changes relatively little as a consequence of the coordina-
tion, increasing by 103 ppm with respect to the nitrito
isomer. This is expected because the δ component does
2
2
ative magnitudes of the δ11 and δ tensor components to
22
not involve mixing with the orbitals that form the
within approximately 100 and 33 ppm, respectively, using
both the 6-311++G(d,p) and LANL2DZ basis sets on the co-
balt. Given that the chemical shift range of nitrogen is
Co—N σ bond. Finally, δ in the nitro isomer decreases by
3
3
approximately 169 ppm with respect to the nitrito isomer.
around 1200 ppm, the errors in δ11 and δ are reasonable
22
Monitoring the isomerization reaction
and lend confidence to the orientations calculated for these
two components. The δ33 component was underestimated by
approximately 250 ppm, which is quite a large discrepancy.
The origin of this difference between theory and experiment
most likely arises from the neglect of intermolecular effects
in the calculations.
The isomerization from the nitrito to the nitro isomer was
1
5
59
monitored directly using both N and Co solid-state NMR.
The spectra acquired at 4.70 T as the reaction progressed are
1
5
59
shown in Figs. 8 and 9 for N and Co, respectively. For
1
5
the N MAS spectra a heating scheme was used. The tem-
perature was raised from room temperature (293 K) to
353 K in 5 min, where it was held for 8 min then cooled to
room temperature before the spectra were acquired (Fig. 8a).
In an attempt to drive the reaction to completion, the sample
that yielded the top spectrum in Fig. 8a was heated at 413 K
for 40 min yielding the spectrum in Fig. 8b; the sample was
allowed to equilibrate at room temperature for 2 h prior to
data collection.
The individual components of the nitrogen chemical shift
tensor can be related to the electronic states that are respon-
sible for the paramagnetic term in δ. For certain nitrosyl
compounds (57), as well as in this case, mixing of the nitro-
gen n and π* electronic states is expected to be responsible
for the paramagnetic shielding term associated with the δ
1
1
component of the chemical shift tensor (30). It has also been
reported that the orientation of the intermediate component
of the nitrogen chemical shift tensor (δ ) is in most cases
The results show the benefit of using NMR to monitor the
1
5
2
2
extent of the reaction. The N peak from the nitrito isomer
slowly decreases in intensity while a second peak appears at
470 ± 3 ppm, corresponding to the formation of the nitro
isomer (Fig. 8a). It is clear that in the initial preparation
there is very little of the nitro isomer present. Due to the
breadth of the peak from the nitro isomer there could be as
much as 10% of this form that is not detected because of the
parallel to the formal nitrogen lone pair for nitrogen atoms
bonded to two other atoms such as nitrosyl compounds, ox-
imes, and azides (58–61). The direction of greatest shielding
is nearly perpendicular to the ONO plane and arises from the
mixing of the in-plane σ and σ* states (57).
The orientation of the nitrogen chemical shift tensor of
©
2006 NRC Canada

3
06
Can. J. Chem. Vol. 84, 2006
Fig. 8. Nitrogen-15 NMR spectra of a MAS sample of the com-
pound, acquired at 4.70 T. The spectra in (a) were obtained at
room temperature after successive applications of the heating
scheme described (see text). The sample that yielded the top
spectrum in (a) was then heated at 413 K for an additional
Fig. 9. The 59Co NMR spectra of a stationary sample of the
compound, acquired at 4.70 T, collected at room temperature
without heating and with 2.75 h between each spectrum; 10 000
scans were added using a 0.1 s pulse delay.
4
0 min to drive the reaction to equilibrium and the NMR spec-
trum in (b) was acquired. Each spectrum in (a) is the sum of
Nitro
6
5
12 scans and (b) is the sum of 10 050 scans, both using a
.0 s pulse delay.
(a)
Nitrito
Nitrito
Nitro
400
1
2000
9000
6000
7
00
00
600
600
500
ppm
ppm
16.7 min. The 59Co NMR spectra show a decrease in the
broad nitrito peak, while a new peak from the nitro isomer
appears at 7505 ± 10 ppm. Since the line width of the nitro
peak is much smaller than that of the nitrito isomer, the 59Co
NMR spectra show the opposite sensitivities to those of the
N NMR spectra. Whereas the N NMR spectra are sensi-
tive to small amounts of the nitrito isomer and less so to the
presence of the nitro isomer, the Co spectra are more sen-
sitive to the presence of the nitro isomer. These two nuclei
therefore offer complementary methods of analyzing the rel-
ative quantities of the two isomers.
(b)
7
500
400
ppm
15
15
5
9
baseline noise. This illustrates one drawback to the 15N
NMR of this system; since the peak from the nitro isomer is
much broader than that from the nitrito isomer under MAS
conditions, when only small amounts of nitro are present it
is difficult to get an accurate estimate of the relative
amounts of each isomer. However, this is offset by the abil-
ity to measure very small quantities of nitrito isomer in a
mixture.
While the use of CP generally does not result in com-
pletely quantitative peak intensities, the optimal CP parame-
ters were almost identical for the two isomers; hence, it is
assumed that the comparison of the integrated peak intensi-
ties is a good approximation of the relative amounts of the
two isomers. The analysis of the intensities for the spectrum
in Fig. 8b shows that a final nitro/nitrito equilibrium ratio of
Conclusions
The 59Co and 15N NMR parameters of nitritopentammine-
cobalt(III) chloride and nitropentamminecobalt(III) chloride
have been determined in the solid state. There is a change in
Co quadrupolar coupling constants (23 to
10 MHz), and the spans of the chemical shift tensor (1650 to
260 ppm), for the nitrito and nitro isomers, respectively. The
principal components of the Co chemical shift tensor that
are perpendicular to the Co—X (X = ONO or NO ) bond de-
crease by approximately 1750 ppm upon isomerization,
while the components parallel to the bond are similar in both
isomers. The nitrogen chemical shift tensor also shows sig-
nificant variations; δ moves from 611 to 470 ppm and Ω
decreases from 710 to 547 ppm as the ligand changes from
the nitrito to nitro form. The orientations of the N and Co
chemical shift tensors and the Co EFG tensor in the molec-
ular frame have been related to the molecular structure. The
isomerization reaction was monitored directly using both
Co and N NMR and the results suggest that solid-state
NMR can be used to measure isomer ratios during the reac-
5
9
both the
5
9
2
5
.0 is reached after the sample was heated at 413 K. This is
in excellent agreement with a previous study that measured
the equilibrium values based on the reaction rates (11). From
the spectra in Fig. 8a, it is apparent that the reaction is close
to equilibrium after the second heating step.
The Co NMR spectra (Fig. 9) were collected on a sec-
ond sample preparation without any heating to investigate
the sensitivity of the experiment to the isomerization over
times consistent with the room temperature rate of reaction.
Every 2.75 h a spectrum was acquired over a time of
iso
1
5
59
5
9
59
5
9
15
©
2006 NRC Canada

Ooms and Wasylishen
307
24. C.W. Kirby and W.P. Power. Can. J. Chem. 79, 296 (2001).
tion. These results highlight the value of solid-state NMR
studies and the potential to use NMR to monitor reactions
that occur in the solid state.
25. R. Goodfellow. In Multinuclear NMR. Edited by J. Mason.
Plenum, New York. 1989. pp. 521.
2
2
6. J.C.C. Chan and S.C.F. Au-Yeung. J. Chem. Soc. Faraday
Trans. 92, 1121 (1996).
7. J.C.C. Chan and S.C.F. Au-Yeung. Annu. Rep. NMR
Acknowledgments
Spectrosc. 41, 1 (2000).
8. A. Medek and L. Frydman. J. Am. Chem. Soc. 122, 648 (2000).
9. W.P. Power, C.W. Kirby, and N.J. Taylor. J. Am. Chem. Soc.
The authors wish to thank members of the solid-state
NMR group at the University of Alberta, Edmonton, Al-
berta, as well as Dr. Klaus Eichele for helpful discussions.
Dr. Guy Bernard, in particular, is thanked for his comments.
REW is a Canada Research Chair in Physical Chemistry at
the University of Alberta and thanks the Natural Sciences
and Engineering Research Council of Canada (NSERC),
Alberta Ingenuity, and the University of Alberta for research
grants. KJO thanks NSERC, the Alberta Ingenuity Fund, and
the University of Alberta for post-graduate scholarships.
Finally, REW thanks Arthur Carty for several fruitful sci-
entific collaborations, and personal encouragement dating
back to the late 1960s at the University of Waterloo.
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2
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Dordrecht. 1993. pp. 449.
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