Influence of ligand substitution pattern on structure in cobalt(II) complexes of bulky N,N'-diarylformamidinate N-oxides

Article abstract of DOI:10.1002/ejic.201402895

Cobalt(II) complexes of bulky N,N'-diarylformamidinate N-oxide ligands were synthesized and structurally characterized. The cobalt(II) bis(chelates) are square-planar (low spin) in the solid state, according to XRD and magnetic measurements (μ_eff

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DOI:10.1002/ejic.201402895
Influence of Ligand Substitution Pattern on Structure in
Cobalt(II) Complexes of Bulky N,NЈ-Diarylformamidinate
N-Oxides
Mihaela Cibian,[a] Sophie Langis-Barsetti,[a]
Fernanda Gomes De Mendonça,[a] Sammy Touaibia,[a]
Sofia Derossi,[a] Denis Spasyuk,[a] and Garry S. Hanan*[a]
Keywords: Cobalt / N,O ligands / Substituent effects / Structure elucidation / Stereochemistry
Cobalt(II) complexes of bulky N,NЈ-diarylformamidinate N-
oxide ligands were synthesized and structurally charac-
terized. The cobalt(II) bis(chelates) are square-planar (low
spin) in the solid state, according to XRD and magnetic mea-
surements (μeff = 1.8 to 2.1 μB), but show square-planar (low
spin) to tetrahedral (high spin: μeff = 3.5 to 4.7 μB) isomeriza-
tion in solutions of noncoordinating solvents, as demon-
strated by different spectroscopic techniques. The isomeriza-
tion equilibrium is highly sensitive to the substitution pattern
on the ligand due to a combination of steric and electronic
influences.
to tetrahedral isomerisation can be observed. For a tetraco-
ordinate cobalt(II) system this is particularly interesting,
because the structural change is also accompanied by a
change in spin.[4] Investigation of such systems affords de-
tailed information concerning their coordination number
and geometry, which is needed to predict and control the
architecture and properties of the transition-metal com-
plexes for application purposes.[5]
Introduction
Several reports related to amidine N-oxide (AMOX, also
called α-aminonitrones and N-hydroxyamidines) ligands
and their transition metal complexes exist in the literature.[1]
However, investigation of their properties from a coordina-
tion and supramolecular chemistry perspective has received
little attention. Amidinate ligands are closely related to the
N-hydroxyamidinate ligands and have been intensively ex-
plored in organometallic and coordination chemistry over
the past decades, resulting in applications in catalysis and
materials.[2] AMOX ligands are good chelators for metal
ions and exhibit good electronic delocalization in the amid-
ine backbone. Furthermore, they offer the possibility for
modulation and fine-tuning of their electronic and steric
properties by varying the substitution pattern on the three
atoms of the amidine moiety.[1a,1d] Our goal is to fine-tune
the properties of their corresponding metal complexes for
applications in catalysis and magnetic materials.
Our preliminary results regarding the CoIIL2 complex
(2a) (Scheme 1), where HL = N-hydroxy-N,NЈ-bis(2,6-di-
methylphenyl)formamidine (1a), highlight an interesting
consequence of the metalligand interaction: steric and elec-
tronic effects combine to offer a rare square-planar geome-
try around the d7 CoII centre in a bis(bidentate) environ-
ment, while permitting facile oxidation of the metal ion.[3]
In solutions of noncoordinating solvents, a square-planar
Scheme 1. Synthesis of Co(AMOX)2 complexes 2a–c.
The square-planar to tetrahedral isomerisation of a series
of bis(chelate) metal(II) compounds was previously studied
by Holm.[5a,6] In cobalt-based systems, this process was in-
vestigated for organometallic complexes,[7] and for cobalt
complexes of triazene 1-oxides,[8] tropocoronand macro-
cycles[9] and iminopyrrolyl ligands.[10] These studies re-
vealed the importance of the electronic and steric properties
of the ligand on the structural preference of the complex.
Recently, the same type of processes was also reported for
cobalt complexes of α-imino alkoxide ligands.[11] However,
examples of square-planar to tetrahedral isomerisation in
[a] Département de Chimie, Université de Montréal,
2900 Edouard-Montpetit, Montréal, Québec, H3T-1J4, Canada
E-mail: garry.hanan@umontreal.ca
http://www.groupeenergieverte.com/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402895.
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five-membered ring metal complexes of N,O-bidentate li- stable for months without decomposition and readily dis-
gands are scarce.[5a,8,11,12]
solve in chlorinated solvents to give yellow-orange solu-
The cobalt bis(chelates) of sterically demanding AMOX tions, but are poorly soluble in polar solvents. The com-
ligands offer a new opportunity to explore the effect of li- pounds undergo oxidation over a period of days when left
gand–metal interactions on their structural, spectroscopic in solution. Complexes 2a–c were characterised by X-ray
1
and electrochemical properties. Homoleptic complexes of crystallography, H NMR and UV/Vis spectroscopy, mass
cobalt(II) (2b–c; Scheme 1) with the following AMOX spectrometry and elemental analysis.
ligands: N-hydroxy-N,NЈ-bis(2,6-diisopropylphenyl)form-
amidine (1b) and N-hydroxy-N,NЈ-bis(2-isopropylphenyl)-
formamidine (1c) were synthesised and characterised.
X-ray Diffraction
Green X-ray quality crystals were obtained by recrystalli-
sation of 2a and 2c from DCM/hexane (1:1 v/v) at –20 °C
and/or room temperature and from AcOEt/hexane at room
temperature for 2b. X-ray structures and data are presented
in Figures 1 and 2 and Tables 1 and 2. As is the case for
2a,[3] the metal ions in 2b and 2c display a square-planar
Results and Discussion
Synthesis
The syntheses of ligands 1a–c and complex 2a were pre- geometry (Figures 1 and 2). In all cases, the metal ion is
viously reported.[3,13] Complexes 2b and 2c were also sitting on an inversion centre, and the two five-membered
straightforward to prepare by treating 2 equiv. of the corre- chelate rings are coplanar with no axially coordinated sol-
sponding ligand with cobalt(II) acetate in aqueous ethanol vent molecule. There are few examples of square-planar co-
at room temperature. A green precipitate formed in each ordination geometries reported for cobalt(II) complexes of
case, and the products were isolated in good yields (81– bidentate ligands,[3,6a–6c,10,11,12c,14] since a tetrahedral geom-
90%) as air-stable powders (Scheme 1). The solids are air- etry is favoured on steric grounds in bis(bidentate) four-
Figure 1. ORTEP view of 2b with Ci symmetry. Ellipsoids are shown at the 50% probability level. Hydrogen atoms have been removed
for clarity.
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Figure 2. ORTEP view of 2c with Ci symmetry. Ellipsoids are shown at the 50% probability level. Hydrogen atoms have been removed
for clarity.
Table 1. Solid-state structure and refinement data for compounds 2a–c.
2a[a]
2b
2c
Formula
C34H38CoN4O2
593.61
200
0.71073
monoclinic
8.8879(12)
8.3485(11)
20.363(3)
90
93.057(2)
90
1508.8(3)
P21/c
C50H70CoN4O2
818.03
150
1.54178
triclinic
C38H46CoN4O2
649.72
150
1.54178
monoclinic
7.5524(2)
24.9130(7)
9.3964(3)
90
104.449(1)
90
1712.04(9)
P21/c
Mw [gmol–1]
Temperature [K]
Wavelength [Å]
Crystal system
a [Å]
10.6229(3)
10.6858(2)
11.8393(2)
109.052(1)
105.705(1)
99.860(1)
1172.06(4)
b [Å]
c [Å]
α [°]
β [°]
γ [°]
V [Å3]
¯
P1
1
1.159
3.177
Space group
Z
2
2
dcalcd. [gcm–3]
1.307
0.606
1.260
4.223
μ [mm–1]
F(000)
626
36660
4749
0.975
441
690
34923
3218
1.060
Reflections collected
Independent reflections
GoF
18441
18441
1.046
R1(F) [I Ͼ 2σ(I)]
wR(F2) [I Ͼ 2σ(I)]
R1(F) (all data)
wR(F2) (all data)
Largest difference peak/hole [eÅ–3]
0.0367
0.1008
0.0512
0.1066
0.858/–0.336
0.0327
0.0829
0.0341
0.0841
0.153/–0.293
0.0298
0.0860
0.0313
0.0869
0.272/–0.478
[a] From ref.[3]
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Table 2. Selected bond lengths [Å] and angles [°] for 2a–c and chosen examples of cobalt complexes from the literature: 3–8.
Compound
τ4[i]
Bond lengths [Å]
Angle [°]
O1–Co–N2
Tilt angle[k] [°]
Co–O1
Co–N2
C1–N1
C1–N2
O1–N1
Ar(NO)
ArN
2a[a]
0.00
0.00
0.00
0.00
0.02
0.00
0.02
0.00
0.00
0.00
1.834(1)
1.841(2)
1.833(1)
1.805(3)
1.823(3)[j]
1.828(1)
1.848(3)[j]
1.848(1)[j]
1.846(1)
1.839
1.886(1)
1.883(1)
1.898(1)
1.856(3)
1.836(3)[j]
1.840(1)
1.841(4)[j]
1.967(1)[j]
1.938(1)
1.890
1.319(2)
1.319(2)
1.316(2)
1.312(2)
1.306(3)
1.316(2)
1.392(1)
1.389(2)
1.387(1)
1.339(4)
84.5(1)
84.9(1)
84.6(1)
81.9(1)
85.7(1)
85.7(1)
94.1(1)
84.3(1)
84.5(1)
84.4
59(1)
84(1)
60(1)
–
–
–
–
–
–
79(1)
87(1)
60(1)
39(1)
90(1)
69(1)
–
–
–
89
2b
2c
3[b]
–
–
–
–
–
–
–
–
–
–
4[c]
–
–
–
–
–
5[d]
6[e]
7[f]
8[g]
–
–
2a-sqpl[h]
1.324
1.317
1.398
74
(DFT-optimized CH2Cl2)
[a] From ref.[3] [b] 3: Co(L)2, L = triazene 1-oxide type.[12c] [c] 4: Co(L)2, L = o-aminophenol-type.[14d] [d] 5: Co(L)2, L = o-aminophenol-
type.[14b] [e] 6: Co(L)2, L = [N2O2] Schiff base type.[21] [f] 8: Co(L)2, L = α-imino alkoxide type.[11] [g] 9: Co(L)2, L = carbohydrazide-
type.[14e] [h] Theory level: uB3LYP/LANL2DZ; CPCM: CH2Cl2. [i] As defined by Houser.[22] [j] Average value. [k] Angle between the
plane of the aromatic ring and the plane of the –N–C=N– moiety.
coordinate environments. In some cases, axial coordination degree of delocalisation in the molecule. Among 2a–c, the
of solvent molecules is present, which offers the preferred largest ArN and Ar(NO) dihedral angles are observed for
octahedral geometry around the metal centre, as for Co- 2b. This fact is in accordance with 1b being the sterically
(acac)2.[15] Stabilization of the square-planar geometry can most demanding ligand.
usually be achieved in the presence of macrocyclic li-
Examination of the packing in the three structures shows
gands[16] or by a special combination of steric and elec- weak C(π)–H···O (d = 2.45 Å, θ = 162°) and C(π)–H···π
tronic factors.[3,10,12c,14b,14d] Square-planar cobalt(II) com- intermolecular interactions for 2a and 2c, respectively.
plexes with bidentate N,O-ligands that form five-membered
chelate rings are rare, and only ten (including 2a) crystal
structures of this type are reported in the CCDC.[17]
Five of these structures correspond to cobalt complexes
of redox-active o-aminophenol-type ligands and can be
1H NMR Spectroscopy
1
The H NMR spectroscopic signals of 2a–c are shifted
considered as a special case due to the “noninnocent” na- due to paramagnetic effects (from δ = 69 ppm to δ =
ture of the ligand.[14b,14d,18] In one other case, the stabiliza- –200 ppm; Table 3 and Figure S1, Supporting Information),
tion of the square-planar geometry at the Co centre is as- which is consistent with the oxidation state of the metal ion
sisted by two intramolecular interactions of the X–H···Co [cobalt(II), d7]. They exhibit characteristically broadened
type.[19] This leaves only three other types of N,O-ligands, signals, without coupling constants, albeit with accurate in-
except the AMOXs, able to stabilise the square-planar ge- tegrations. Thus, it was possible to assign the 1H NMR res-
ometry of cobalt(II) bis(chelates) with five-membered che- onances based on the integration ratio, 2D COSY and T1
late rings: α-imino alkoxide,[11] carbohydrazide[14e] and tri- relaxation time measurement experiments, and to correlate
azene 1-oxide derivatives.[12c]
these results with the proximity of the protons to the metal
The Co–O bond lengths in 2a–c are similar, and their centre. The aryl ring on the N-Co is closer to the metal
values are closer to those observed in cobalt complexes of centre and thus more influenced than the aryl ring, which
Schiff base type, α-imino alkoxide type and carbohydrazide- sits on the NO-Co.[20] As a consequence, the para H4Ј reso-
type ligands (compounds 6, 7 and 8; Table 2) rather than nances are paramagnetically more shifted than the para H4
the triazene 1-oxide analogues (3; Table 2) or the cobalt(II) resonances (see Scheme 1 for the notation of H). In the case
complexes of redox-active ligands (4, 5; Table 2). The of 2a this is also supported by the T1 relaxation time values
Co–N bonds are also similar within the 2a–c series, but they of 22.7 ms for H4 vs. 34.4 ms for H4Ј. For 2a (substituted
differ from those in compounds 3–8 (Table 2) as result of with methyl groups in the 2,6-positions), the two broad sig-
the differences in the five-membered chelate rings (type of nals at δ = +37.0 and –40.1 ppm integrate to 6 H, corre-
atoms, substituents, electronic delocalisation).
sponding to the –CH3 protons. They are positioned in close
All three structures 2a–c show electron density delocal- proximity to the cobalt centre due to the tilt of the aryl
ised over the amidine bridge, resulting in equal (within 3σ) rings with respect to the amidine oxide plane. Their short
C1–N1 and C1–N2 bond lengths. However, only partial or- T1 relaxation time values of 1.3 and 1.7 ms are also in line
bital overlap exists between the aryl and the amidine π-sys- with their position close to the paramagnetic centre. In the
tems, since the aryl rings are twisted from the mean plane case of 2b, the –CH–CH3 protons are the ones close to the
of the –N–C=N– moiety in order to accommodate the ste- metal centre, and their resonances are thus the most shifted
ric bulk generated by the 2,6-substituents. The values of the due to paramagnetism, being observed as broad signals at
tilt angles are shown in Table 2 and are indicative of the δ = +18.1 and –26.9 ppm. Their resonances are observed as
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1
Table 3. Chemical shift values (ppm) for H NMR spectra[a] of complexes 2a–c.[b]
Compound
1
3Ј
–0.7 –1.6 30.8
0.5 8.8 14.8
0.1 –9.5 30.3
3
4Ј
–34.3 19.6 –1.6 30.8
– 3.7 11.5 8.8 14.8
–34.1 20.1 11.6 33.3
4
5Ј
5
6Ј
6
–CHЈ–CH3 –CH–CH3
–CH3Ј
–CH3
2a
2b
2c
–
–
–
–
–
–26.9
–22.7
–
18.1
51.1
–40.1
–2.5, –16.5
–29.0
37.0
8.0, 0.5
8.9
–200 69.2
[a] See Scheme 1 for the notation of the H atoms. [b] In CD2Cl2.
broad signals at δ = +18.1 and –26.9 ppm. For compound Table 5. Thermodynamic parameters related to the isomerisation
equilibrium: tetrahedral high-spin (HS) to square-planar low spin
2c (monosubstituted in position 2), protons H6, H6Ј (in the
ortho position) and H4Ј (in the para position) are the most
influenced by the metal centre, and thus their signals appear
at δ = 69.2, –200 and –34.1 ppm, respectively.
(LS) for 2a–c in noncoordinating solvents.[a]
Compound
ΔG298
ΔH
ΔS
[JK–1 mol–1
]
Nt[b]
[kJmol–1
]
[kJmol–1
]
2a
2b
2c
–5Ϯ0.8
1Ϯ0.2 11Ϯ2.0
–1Ϯ0.2
5Ϯ0.9
34Ϯ6.1 0.9Ϯ0.16
32Ϯ5.8 0.4Ϯ0.07
18Ϯ3.2 0.6Ϯ0.11
5Ϯ0.9
1–15
0–12
[c]
Square-Planar (LS) to Tetrahedral (HS) Isomerisation
Co(RN3Ar)2
Co(L)2
–
–
5–30
6–32
–
–
[d]
In four-coordinate cobalt(II) systems, the square-planar
to tetrahedral structural change is also accompanied by a
change in spin.[4] The solution and solid-state effective mag-
netic moments for 2a–c are shown in Table 4. The solid-
state magnetic moments have the characteristic low spin
(LS, S = 1/2) values (1.8–2.2 μB) expected for cobalt(II) in
square-planar geometry.[4,7] These values are in line with
the square-planar geometry of the compounds as shown by
XRD structural characterisation. For 2a, the solution-
phase (CH2Cl2) magnetic moment (4.7 μB) corresponds to
a tetrahedral configuration of the cobalt(II) d7 centre: high-
spin, S = 3/2. This value is higher than the spin-only one
(3.89 μB) due to spin-orbit coupling.[23] The values of the
magnetic moment determined in solution for 2b (3.5 μB)
and 2c (4.0 μB) are between the accepted high-spin (4.2–
5.2 μB) and low-spin (1.8–2.2 μB) values[4] for cobalt(II)
complexes, suggesting an equilibrium between the LS
square-planar and HS tetrahedral forms.[6a,6c,8,20a]
[a] Derived from magnetic susceptibility measurements performed
by Evans’ method in CD2Cl2, at variable temperatures (193–
298 K); corrections for the change in solvent density with tempera-
ture were applied,[25,26] errors are estimated as follows: Ϯ4% for
the magnetic moments; Ϯ15% for ΔG values (considering an error
of 10% on the magnetic moment assumed for the pure tetrahedral
form); Ϯ18% for ΔH, ΔS and Nt values.[27] [b] Nt: mole fraction of
the tetrahedral form at 298 K. [c] Cobalt(II) complexes of triazene
1-oxide type ligands.[14a] [d] L is a β-ketoamine- or β-iminoamine-
type ligand.[5a]
1/2
μeff = 2.83(χM
)
(1)
T
the free energy changes (ΔG) were evaluated from the tem-
perature dependence of the magnetic susceptibility (χM: mo-
lar magnetic susceptibility) and hence the corresponding ef-
fective magnetic moments:
χMt – χMobs
χMobs – χMp
μ2t – μ2obs
μ2obs – μ2p
ΔG = RT ln
= RT ln
(2)
Table 4. Effective magnetic moments μeff [μB] for complexes 2a–c,
measured in solution[a] and the solid[b] state.
Expression 2 was derived using Equations (3)–(7):
ΔG = –RTlnK
Compound
μeff (solution)[c]
[μB]
μeff (solid state)[c]
[μB]
(3)
(4)
χM = NtχM + NpχM
p
2a
2b
2c
4.67Ϯ0.16
3.54Ϯ0.14
4.03Ϯ0.06
1.84Ϯ0.21
1.86Ϯ0.16
2.10Ϯ0.09
obs
t
Nt
K =
(5)
[a] Measured by Evans’ method in CD2Cl2 (10vol.-% TMS) at
298 K. [b] Measured by using a magnetic susceptibility balance at
295 K. [c] The measurements were done at least in triplicate, and
the reported error represents the confidence interval at 95% prob-
ability.
Np
Nt + Np = 1
Nt = (1 + eΔG/RT –1
(6)
(7)
)
The thermodynamic parameters for this equilibrium for
The equilibrium constant (K) of the square-planar to tet-
2a–c are presented in Table 5. They were derived based on rahedral isomerisation process is defined as in (5), with Nt
variable-temperature (193–298 K) magnetic susceptibility and Np representing the mole fractions of the tetrahedral
measurements. This method was previously reported for and square-planar forms, respectively. The magnetic mo-
similar systems.[6a,6c,8,24] The magnetic moment increased ments determined for 2a–c in the solid form (100% square-
with increasing temperature for 2a–c, with the highest vari- planar) were used as magnetic moments of the pure square-
ation in the 193–298 K temperature range being observed planar form (μp). For the pure tetrahedral form (μt), mag-
for 2b, followed by 2c and 2a (Figure S3, Supporting Infor- netic moments of 4.85 μB were assumed, based on literature
mation). By using:
reports of similar systems.[4,8]
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The ΔG values for 2a–c varied linearly with the tempera- (doublet to quartet) is the same for all three compounds, it
ture (Figure S2, Supporting Information). By using:
is difficult to distinguish between the specific contributions
in terms of solvation and vibrational entropy due to the
bulky substituents on the aryl rings. Complexes 2a and 2b
are ortho-disubstituted on the aryl rings and present similar
values for the entropy change vs. ortho-monosubstituted 2c,
for which a decrease in ΔS can be observed.
ΔG = ΔH – TΔS
(8)
the changes in enthalpy (ΔH) and entropy (ΔS) were ob-
tained from the least-square fit of ΔG vs. T (Table 5, Figure
S2, Supporting Information).
The values of the thermodynamic parameters obtained
for the bis(AMOX) cobalt chelates are in the same order of
magnitude as those previously reported for cobalt com-
plexes of triazene 1-oxide type[8] and β-ketoamine-type[5a]
ligands. For 2a and 2c, the values of Nt at 298 K (0.9 and
0.6) indicate a preference for tetrahedral geometry in
CH2Cl2 solution. Interestingly, in the case of 2b, the square-
planar form is slightly stabilised (Nt = 0.4). Thus, the
thermodynamic data suggest a high sensitivity of the iso-
merisation equilibrium to the substitution pattern on the
ligand. The higher proportion of the square-planar form
(LS) in 2b vs. 2a and 2c also correlates with the UV/Vis
spectroscopic data (vide infra) and the chemical shifts in 1H
NMR spectrum, which are the paramagnetically least
shifted in the case of 2b. The preference of 2a for the tetra-
hedral form is also supported by DFT calculations (vide
infra). To eliminate the possibility of five- or six-coordinate
low-spin species, and hence their influence on the observed
magnetic moments, the adherence to the Beer–Lambert law
was confirmed in the range of concentrations over which
the magnetic and spectroscopic properties were measured
(10–5–10–3 m).[6b] This would not be the case if dimerisation
promoted octahedral coordination at higher concentrations.
As for the other cases of square-planar to tetrahedral
isomerisation, positive values are obtained for ΔH, which
can be attributed to the endothermic weakening of the
metal–ligand bonds in the isomerisation process.[5a,8] The
highest value of ΔH is observed for 2b, indicating a higher
degree of stabilisation for the square-planar form in this
case. The high steric bulk of the 2,6-di-iPr substituents on
the aryl rings pushes the phenyl rings almost perpendicular
to the plane of the amidine backbone (tilt angles of 84° and
87°; see Table 2). Therefore, the electronic density of the
anionic –O–N–C=N– AMOX linkage is less delocalised on
the aromatic moieties of the ligand thereby generating a
stronger ligand field for the metal centre.
UV/Vis Spectroscopic Properties
The spectroscopic properties of 2a–c are presented in
Table 6. Their UV/Vis spectra in CH2Cl2 show low-intensity
bands in the visible and NIR region, with intense bands
centred at higher energies than the solvent cut-off point
(Figure 3 and Figure S4, Supporting Information). The ab-
sorption bands in the visible and NIR region correspond
to spin-allowed and Laporte-forbidden dd transitions. They
present characteristic low molar absorptivity values.[23,28]
The bands at 540–550 nm, 940–980 nm and 1230 nm are
characteristic of the tetrahedral form.[5a,8] Similar features
were identified for tetrahedral cobalt(II) complexes of triaz-
ene 1-oxides[8] and β-ketoamine[5a] ligands. These three
transitions could be tentatively assigned, based on Orgel dia-
grams for tetrahedral metal complexes, to A2(F)Ǟ4T1(P),
4
4
4A2(F)Ǟ4T1(F) and A2(F)Ǟ4T2(F), respectively.[23,28] The
analysis of the electronic spectra of 2a and 2c shows that
UV/Vis spectroscopy also supports the existence of tetrahe-
dral geometry in solutions of noncoordinating solvents for
these complexes at room temperature.
The total entropy change ΔS is considered to have three
contributions: the change in spin multiplicity, the difference
in solvation and the change in vibrational entropy between
Figure 3. Electronic spectra of 2a–c in CH2Cl2 at room tempera-
the two isomers.[16] While the change in spin multiplicity ture.
Table 6. Spectroscopic data for complexes 2a–c and their corresponding ligands 1a–c.[a]
Compound
λmax [nm] (εϫ102 [m–1 cm–1])
1a
1b
1c
2a
2b
2c
286 (130)
280 (110)
293sh (120), 313 (154)
247sh (239), 265sh (209), 350 (32), 540 (0.97), 622 (0.32), 942 (0.29), 1233 (0.27)
260sh (253), 340 (55), 550 (0.40), 635 (0.24), 945 (0.15), 1000sh (0.11), sh1310 (0.11)
290 (196), 305 (176), 540 (0.57), 644sh (0.18), 987 (0.16), 1247 (0.20)
[a] Measured in dichloromethane, at room temperature; sh = shoulder.
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Figure 4 presents the comparison of the electronic spec-
tra of 2a recorded in CH2Cl2 at 298 K and 188 K. The band
at 620–640 nm is indicative of the square-planar form. A
second band characteristic of the square-planar isomer ap-
pears at 900–1000 nm, but overlaps with the signal iden-
tified for the tetrahedral form.[6c,8] The square-planar form
is stabilised with decreasing temperature.
Figure 6. Electronic spectra of 2b in CH2Cl2 at 298 K (red), 188 K
(black) and in the solid state (green).
hedral (S
=
3/2, quadruplet) (2a-tetra)] in di-
chloromethane (spin-unrestricted uB3LYP/LANL2DZ
theory level; CPCM: CH2Cl2) are illustrated in Figure 7a.
The optimised calculated structure of 2a-sqpl is in good
agreement with the XRD data (Table 2). The comparison
of the energy values of the two isomers 2a-sqpl and 2a-tetra
(Table S1, Supporting Information) shows a slight stabilisa-
tion (4.3 kcalmol–1) of the tetrahedral form, which is in ac-
cordance with the experimental observations and the values
of the thermodynamic parameters. In an effort to account
for the preference for the square-planar form in the case of
2b, DFT calculations using the same theory level were also
run for its two isomers 2b-sqpl (S = 1/2, multiplicity dou-
blet) and 2b-tetra (S = 3/2, multiplicity quadruplet)
(Tables S1–S3, S6, S7, Supporting Information). Noncon-
vergence of the 2b-sqpl frequency calculation does not al-
low us to conclude if this structure is at its minimum and,
therefore, the energy comparison is inconclusive in this case.
Figure 4. Electronic spectra of 2a in CH2Cl2 at 298 K (red) and
188 K (black).
The absorption spectrum of 2b confirms that the square-
planar form is present in a higher proportion compared
with 2a and 2c at room temperature. The variable-tempera-
ture UV/Vis analysis for 2b further indicates an increase in
the square-planar form with decreasing temperature (Fig-
ure 5). Its comparison with the absorption spectrum of 2b
in the solid state (Figure 6) confirms the assignment of the
band at 620–640 nm to the square-planar form.
Figure 5. Electronic spectra of 2b in CH2Cl2 at variable tempera-
ture.
For the AMOX ligands 1a–c the electronic spectra dis-
play the characteristic ligand-centred (LC) π–π* transitions
in the UV region, with high molar absorptivity coefficients
(Figure S4, Supporting Information).[23,28]
Figure 7. (a) DFT-optimised structures (uB3LYP/LANL2DZ;
CPCM: CH2Cl2) for the square-planar (S = 1/2, doublet) and the
tetrahedral (S = 3/2, quadruplet) form of 2a; (b) electron-spin den-
sity plots for the two isomers of 2a.
DFT Calculations
Figure 7b displays the spin-density plots for each of the
The optimised structures of 2a for the two forms [square- two isomers of 2a. The corresponding values are illustrated
planar (S 1/2, doublet) (2a-sqpl) and tetra- in Table S2, Supporting Information. The unpaired elec-
=
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trons are concentrated on the metal centre (dyz orbital for mined. They suggest a major influence of the ligand substi-
the square-planar form; dxy, dxz and dyz orbitals for the tet- tution pattern on the isomerisation equilibrium by a subtle
rahedral form). The same observation stands for 2b.
interplay of steric and electronic factors. Very bulky substit-
uents in 2,6-positions of the aryl rings of the ligand favour
the square-planar (LS) form in the solid state, and in solu-
tions of noncoordinating solvents. An extended delocalis-
ation of the electron density of the amidine N-oxide moiety
is hindered in this case by steric factors (tilt angles formed
by the aryl planes and the five-membered chelate ring plane
are close to 90°). As observed, slightly less bulky substitu-
ents (2-iPrPh and 2,6-Me2Ph) still stabilise the square-
planar form in the solid state. In solutions of noncoordina-
ting solvents at room temperature, however, they allow tilt
angles more favourable to an enhanced delocalisation of the
electron density of the amidine N-oxide moiety, reducing
the field strength of the corresponding ligands and stabilis-
ing the tetrahedral form. This is also confirmed by DFT
calculations, in the case of 2,6-Me2C6H3 substituents. The
fact that the square-planar form stabilises with decreasing
solution temperature is in line with the explanations given
above.
The extra substitution position available on the central
C atom in the AMOX ligand system can be seen as an ad-
vantage in terms of steric modularity and electronic tuning
(e.g. compared with the triazene 1-oxide analogues). We are
extending the family of AMOX-based metal complexes in
order to better assess and understand the interplay of steric
and electronic factors and their influence on the properties
in these systems.
Electrochemistry
Table 7 presents the electrochemical data for 2a–c in
DCM and DMF. The CVs. (Figure S5, Supporting Infor-
mation) display a one-electron reversible process, assigned
to the CoII/III couple, followed by irreversible ligand-based
oxidation processes. The easier oxidation of 2a–c in DMF
(0.09–0.17 V) than in DCM (0.41–0.47 V) can be seen as a
consequence of the difference in coordination capacity and
polarity of the two solvents. It is worth mentioning that no
reduction wave can be observed for 2a–c in the electrochem-
ical window of DCM (down to –1.8 V). The same observa-
tion was reported for the cobalt(II) complexes of triazene
1-oxides.[8] The difficult reduction is in line with the anionic
character of the ligand, enhanced by the effect of the elec-
tron-donating substituents. However, an irreversible re-
duction process can be observed in DMF, in the –2.16 to
–2.34 V potential range. This process is absent in the CV of
the free ligand (Figure S5, Supporting Information), and it
is therefore tentatively assigned to the CoII/I reduction.
Table 7. Electrochemical data for compounds 2a–c in dry DCM
and DMF.[a,b]
Compound
E [V] vs. SCE
Epa(irr)
[c]
E1/2
Epc(irr)
DCM DMF DCM
DMF
DCM DMF
[e]
2a
2b
2c
0.47
(70)
0.41
(80)
0.46
(82)
0.14 1.70[d] 1.06, 1.22,
–
–2.27
–2.34
–2.16
Experimental Section
(62)
0.17
(76)
0.09
(86)
1.57
1.88 1.15, 1.38,
1.56
[e]
–
Materials and Instrumentation: The metal salts, anilines and triethyl
orthoformate were purchased from Aldrich, and m-CPBA (77%)
was purchased from Acros. All were used without further purifica-
tion. ACS grade solvents were purchased from VRW and Fisher
and were removed under reduced pressure using a rotary evapora-
tor, unless otherwise stated. Nuclear magnetic resonance (NMR)
spectra were recorded in CD2Cl2 and CDCl3 at 25 °C, with the
following spectrometers: Bruker AV-400, AV-500 and DRX-400.
Chemical shifts (δ) are reported in parts per million (ppm) relative
to TMS, and are referenced to the residual solvent signal (δ =
5.35 ppm for CD2Cl2 and 7.26 ppm for CDCl3). Variable-tempera-
ture NMR spectra and T1 relaxation-time measurements were per-
formed at the NMR Spectroscopy Service of the Université de
Montréal. Absorption spectra were measured in dichloromethane
(previously distilled), between 230 and 1400 nm, at room tempera-
ture (room temp.), with a Cary 500i UV/Vis/NIR spectrophotome-
[e]
1.56 0.92, 1.21
–
[a] [nBu4N]PF6 (0.1 m), compound concentration about 1 mm,
glassy carbon electrode, scan rate 100 mV/s, room temperature, Ar,
ferrocene used as internal reference. [b] All potentials are reported
in V, vs. SCE (Fc/Fc+ vs. SCE was considered 0.46 V in DCM
and 0.45 V in DMF.[29]). [c] The difference Epc – Epa is given in
parentheses. [d] From ref.[3] [e] No process is observed.
Conclusions
New bis(AMOX) cobalt(II) complexes were synthesised
and characterised. They are a new family of compounds
that isomerise from square-planar LS in the solid state to ter. Absorption spectra at variable temperature were measured with
a Cary 6000i UV/Vis/NIR spectrophotometer by using an Oxford
Instrument cryostat and temperature controller. Solution samples
were prepared in the concentration range of 10–5 to 10–3 m. The
absorption spectrum of 2b in the solid state was measured with the
latter instrument, by using a microcrystal mounted on a solid sam-
ple holder. Electrochemical measurements were carried out in ar-
gon-purged dry DCM, at room temperature, with a BAS CV50W
multipurpose instrument interfaced with a PC. The working elec-
trode was a glassy carbon electrode. The counter electrode was a
Pt wire, and the pseudo-reference electrode was a silver wire. The
tetrahedral HS in solutions of noncoordinating solvents.
This process was probed by the crystal structures of the
compounds showing square-planar geometry at the metal
centre, and the magnetic moments in the solid state with
values characteristic of the LS form. NMR spectroscopic
data, values of solution magnetic moments and UV/Vis
data are in agreement with the existence of an equilibrium
between the square-planar and tetrahedral forms in solu-
tions of noncoordinating solvents. The thermodynamic pa-
rameters for the isomerisation equilibrium were deter- reference was set using an internal 1 mm ferrocene/ferrocenium
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sample at 460 mV and 450 mV vs. SCE in DCM and DMF, respec-
tively.[29] The concentration of the compounds was about 1 mm.
Tetrabutylammonium hexafluorophosphate (TBAP) was used as
the supporting electrolyte, and its concentration was 0.10 m. Cyclic
voltammograms were obtained at scan rates of 50, 100, 200 and
500 mVs–1. For irreversible oxidation processes, the anodic peak
was used as E. Experimental uncertainties are as follows: absorp-
tion maxima, Ϯ2 nm; molar absorption coefficient, 10%; redox po-
(1 equiv.) in water. The formation of a precipitate was observed
almost instantly. The reaction mixture was stirred at room tempera-
ture before water was added (reaction times are specified below for
each compound) and was kept at 4 °C for 1–2 h before being fil-
tered. The resultant solid was washed with hot water and aqueous
ethanol (50vol.-%) and was taken up in DCM and dried with
MgSO4. A second filtration and solvent evaporation yielded the
desired products as solids. When necessary, further purification by
tentials, Ϯ10 mV. The microanalyses and the mass spectrometry recrystallisation was performed.
analyses were performed at the Elemental Analysis Service and the
Bis[N,NЈ-bis(2,6-diisopropylphenyl)-N-oxidoformamidinate] of Co-
Regional Mass Spectrometry Centre of the Université de Montréal.
Magnetic susceptibility measurements in solution were performed
by Evans’ method,[24] in CD2Cl2, by using 10% vol.-% TMS at
298 K. The sample concentration was in the order of 10–3 m. In
the solid state, the measurements were carried out on powder or
microcrystalline samples (10–20 mg), at 295 K, by using a John-
son–Matthey magnetic susceptibility balance and HgCo(NCS)4 as
calibrant.[30] The measurements were performed at least in
triplicate, and the confidence interval at 95% probability is given
as the error. The standard diamagnetic corrections using Pascal’s
constants were applied.[31] Magnetic moments were calculated by
using:
balt(II) (2b): The following reagents were combined according to
the general procedure: N-hydroxy-N,NЈ-bis(2,6-diisopropylphenyl)-
formamidine (1b) (0.40 g, 1.1 mmol, 2 equiv.) and cobalt(II) acetate
tetrahydrate (0.13 g, 0.51 mmol, 1 equiv.). Reaction time: 90 min.
Yield 0.33 g, 83%. Green crystals (X-ray quality) were obtained
after recrystallisation in AcOEt/hexane (1:1). 1H NMR (CD2Cl2,
400 MHz): δ = 18.13 (s, 2 H, -CH-CH3), 14.79 (s, 2 H, -m-C6H3),
11.49 (s, 1 H, -p-C6H3), 8.83 (s, 2 H, -m-C6H3), 8.00 (s, 6 H, -CH-
CH3), 0.54 (s, 7 H, -CH-CH3 and -N-CH=N-), –2.50 (s, 6 H, -CH-
CH3), –3.71 (s, 1 H, -p-C6H3), –16.45 (s, 6 H, -CH-CH3), –29.91
(s, 2 H, -CH-CH3) ppm. C50H70CoN4O2 (818.06): calcd. C 73.41,
H 8.62, N 6.85; found C 73.41, H 8.90, N 6.87. MS (ESI, DCM):
1/2
μeff = 2.83(χM
)
(9)
m/z = 817.4 [M]+. IR (ATR, solid sample): ν = 3066, 3028, 2959,
T
˜
2927, 2864, 1608, 1586, 1461, 1442, 1404, 1380, 1360, 1327, 1306,
1270, 1254, 1214, 1191, 1178, 1099, 1060, 1043, 992, 930, 886, 820,
801, 770, 756, 731, 689, 642, 620, 599, 546, 534, 497, 485, 427 cm–1.
As previously reported[6a,6c,8,24] and succinctly presented in the Re-
sults and Discussion section, magnetic susceptibility measurements
in solution (Evans’ method) were performed at variable tempera-
ture (193–298 K) in order to derive thermodynamic parameters for
the isomerisation equilibrium discussed in this paper.
Bis[N,NЈ-bis(2-isopropylphenyl)-N-oxidoformamidinate] of Co-
balt(II) (2c): The following reagents were combined according to
the general procedure: N-hydroxy-N,NЈ-bis(2-isopropylphenyl)for-
mamidine (1c) (0.29 g, 1.0 mmol, 2 equiv.) and cobalt(II) acetate
tetrahydrate (0.13 g, 0.51 mmol, 1 equiv.). Reaction time: 90 min.
Product obtained as green powder. Yield 0.29 g, 87%. Recrystalli-
sation in DCM/hexane (1:1) at room temperature afforded X-ray
X-ray Structure Determination: Crystal structure determination and
refinement data for 2a–c are given in Tables 1 and 2 and Figures 1
and 2. Details are provided in the Supporting Information. CCDC-
963737 (for 2b) and -963738 (for 2c) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
1
quality green crystals. H NMR (CD2Cl2, 400 MHz): δ = 69.21 (s,
1 H, -o-C6H4), 51.14 (s, 1 H, -CH-CH3), 33.29 (s, 1 H, -m-C6H4),
30.32 (s, 1 H, -m-C6H4), 20.10 (s, 1 H, -p-C6H4), 11.66 (s, 1 H, -m-
C6H4), 8.87 (s, 6 H, -CH-CH3), 0.12 (s, 1 H, -N-CH=N-), –9.54 (s,
1 H, -m-C6H4), –28.99 (s, 6 H, -CH-CH3), –32.67 (s, 1 H, -CH-
CH3), –34.12 (s, 1 H, -p-C6H4), –200.6 (s, 1 H, -o-C6H4) ppm.
C38H46CoN4O2 (649.74): calcd. C 70.25, H 7.14, N 8.62; found C
70.43, H 7.13, N 8.60. MS (ESI, DCM): m/z = 650.2 [M + H]+.
Computational Details: Gaussian 09, Revision D.01[32] was used for
all theoretical calculations discussed herein, with the spin-unrestric-
ted B3LYP[33] DFT method, LANL2DZ ECP[34] basis set and
CPCM (CH2Cl2) solvation model. Initial atom coordinates for ge-
ometry optimisation were taken from XRD data (cif) of the corre-
sponding square-planar structures. For the tetrahedral isomers,
XDR data of the Zn analogues (of tetrahedral geometry) were
used, with the Zn atom changed for a Co atom. No symmetry
constraints were used for the geometry optimisation. Details of op-
timised structures are given in Tables S4–S7, Supporting Infor-
mation. No imaginary frequencies were obtained when frequency
calculations on optimised geometries were performed, except for
2b-sqpl (see DFT calculations part in this paper and the Support-
ing Information). The values of ϽS2Ͼ were monitored and did not
show major spin contamination (Table S3, Supporting Infor-
mation). The total energy values used for calculating energy differ-
ences between isomers were obtained from single-point calculations
on the optimised structures using a mixed basis set: LANL2DZ
ECP for the Co atom and 6-311g(d,p) for the N, O, C and H atoms.
GaussView 3.0.9[35] and Chemissian 2.200[36] software were used
for data analysis, visualisation and surface plots.
IR (ATR, solid sample): ν = 3066, 3027, 2955, 2927, 2859, 1605,
˜
1586, 1572, 1495, 1484, 1446, 1407, 1379, 1360, 1295, 1276, 1243,
1227, 1198, 1162, 1113, 1093, 1085, 1035, 997, 938, 888, 863, 823,
775, 755, 679, 638, 628, 538, 507, 468, 436 cm–1.
Acknowledgments
We are grateful to the Natural Sciences and Engineering Research
Council (NSERC) of Canada, the Fonds québécois de la recherche
sur la nature et les technologies (FQRNT), the Centre for Self As-
sembled Chemical Structures (CSACS), and the Université de
Montréal for financial support. M. C. thanks NSERC for a
Canada Graduate Scholarship and FQRNT for a Doctoral Schol-
arship (A7). The authors thank Alexandre Rodrigue-Witchel, PhD,
Andréanne Bolduc, PhD, Nicolas Bélanger Desmarais, and Pro-
fessor Christian Reber for their assistance with the absorption mea-
surements. M. C. thanks Daniel Chartrand and Amlan Pal for use-
ful scientific discussions. The authors are also grateful to Compute
Canada and to UdeM NMR, EA, XRD, and MS services and
personnel for their help.
Synthesis: The syntheses of ligands 1a–c and complex 2a were pre-
viously reported.[3,13]
Synthesis of Complexes
General Procedure:[1c,3] A solution of the ligand (2 equiv.) in aque-
ous ethanol (90vol.-%) was added to a solution of metal salt
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Published Online: ᭿
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Job/Unit: I42895
/KAP1
Date: 09-12-14 12:45:51
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FULL PAPER
Amidinate N-Oxide Complexes
M. Cibian, S. Langis-Barsetti,
F. G. De Mendonça, S. Touaibia,
S. Derossi, D. Spasyuk,
G. S. Hanan* .................................. 1–11
Influence of Ligand Substitution Pattern
on Structure in Cobalt(II) Complexes of
Bulky N,NЈ-Diarylformamidinate N-Ox-
ides
Keywords: Cobalt / N,O ligands / Substitu-
ent effects / Structure elucidation / Stereo-
chemistry
Cobalt(II) bis(chelates) of bulky N,NЈ-di-
arylformamidinate N-oxide ligands show
isomerization from square-planar low-spin
(in the solid state) to tetrahedral high-spin
(in solutions of noncoordinating solvents).
Eur. J. Inorg. Chem. 0000, 0–0
11
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim