Molecular and electronic structures of bis(pyridine-2,6-diimine)metal complexes [ML2](PF6)(n) (n = 0, 1, 2, 3; M = Mn, Fe, Co, Ni, Cu, Zn)

Article abstract of DOI:10.1021/ic000113j

A series of mononuclear, octahedral first-row transition metal ion complexes mer-[M(II)L⁰₂](PF₆)₂ containing the tridentate neutral ligand 2,6-bis[1-(4-methoxyphenylimino)ethyl]pyridine (L⁰) and a Mn(II), Fe(II), Co(II), Ni(II), Cu(II), or Zn(II) ion have been synthesized and characterized by X-ray crystallography. Cyclic voltammetry and controlled potential coulometry show that each dication (except those of Cu(II) and Zn(II)) can be reversibly one-electron-oxidized, yielding the respective trications [M(III)L⁰₂]³⁺, and in addition, they can be reversibly reduced to the corresponding monocations [ML₂]⁺ and the neutral species [ML₂]⁰ by two successive one-electron processes. [MnL₂]PF₆ and [CoL₂]PF₆ have been isolated and characterized by X-ray crystallography; their electronic structures are described as [Mn(III)L¹₂]PF₆ and [Co(I)L⁰₂]PF₆ where (L¹)^1- represents the one-electron-reduced radical form of L⁰. The electronic structures of the tri-, di-, and monocations and of the neutral species have been elucidated in detail by a combination of spectroscopies: UV-vis, NMR, X-band EPR, Mossbauer, temperature-dependent magnetochemistry. It is shown that pyridine-2,6-diimine ligands are noninnocent ligands that can be coordinated to transition metal ions as neutral L⁰ or, alternatively, as monoanionic radical (L¹)^1-. All trications are of the type [M(III)L⁰₂]³⁺, and the dications are [M(II)L⁰₂]²⁺. The monocations are described as [Mn(III)L¹₂]⁺ (S = 0), [Fe(II)L⁰L¹]⁺ (S = 1/2 ), [Co(I)L⁰₂]⁺ (S = 1), [Ni(I)L⁰₂]⁺ (S = 1/2 ), [Cu(I)L⁰₂]⁺ (S = 0), [Zn(II)L¹L⁰]⁺ (S = 1/2 ) where the Mn(III) and Fe(II) ions are low-spin-configurated. The neutral species are described as [Mn(II)L¹₂]⁰, [Fe(II)L¹₂]⁰, [Co(I)L⁰L¹]⁰, [Ni(I)L⁰L¹]⁰, and [Zn(II)L¹₂]⁰; their electronic ground states have not been determined.

Full text of DOI:10.1021/ic000113j

2936
Inorg. Chem. 2000, 39, 2936-2947
Molecular and Electronic Structures of Bis(pyridine-2,6-diimine)metal Complexes
[ML2](PF6)n (n ) 0, 1, 2, 3; M ) Mn, Fe, Co, Ni, Cu, Zn)†
Bas de Bruin, Eckhard Bill, Eberhard Bothe, Thomas Weyhermu1ller, and Karl Wieghardt*
Max-Planck-Institut fu¨r Strahlenchemie, Stiftstrasse 34-36, D-45470 Mu¨lheim an der Ruhr, Germany
ReceiVed February 3, 2000
A series of mononuclear, octahedral first-row transition metal ion complexes mer-[MIIL0 ](PF6)2 containing the
2
tridentate neutral ligand 2,6-bis[1-(4-methoxyphenylimino)ethyl]pyridine (L0) and a MnII, FeII, CoII, NiII, CuII, or
ZnII ion have been synthesized and characterized by X-ray crystallography. Cyclic voltammetry and controlled
potential coulometry show that each dication (except those of CuII and ZnII) can be reversibly one-electron-
oxidized, yielding the respective trications [MIIIL0 ]3+, and in addition, they can be reversibly reduced to the
2
corresponding monocations [ML2]+ and the neutral species [ML2]0 by two successive one-electron processes.
[MnL2]PF6 and [CoL2]PF6 have been isolated and characterized by X-ray crystallography; their electronic structures
are described as [MnIIIL1 ]PF6 and [CoIL0 ]PF6 where (L1)1- represents the one-electron-reduced radical form of
2
2
L0. The electronic structures of the tri-, di-, and monocations and of the neutral species have been elucidated in
detail by a combination of spectroscopies: UV-vis, NMR, X-band EPR, Mo¨ssbauer, temperature-dependent
magnetochemistry. It is shown that pyridine-2,6-diimine ligands are noninnocent ligands that can be coordinated
to transition metal ions as neutral L0 or, alternatively, as monoanionic radical (L1)1-. All trications are of the type
[MIIIL0 ]3+, and the dications are [MIIL0 ]2+. The monocations are described as [MnIIIL1 ]+ (S ) 0), [FeIIL0L1]+
2
2
2
(S ) 1/2), [CoIL0 ]+ (S ) 1), [NiIL0 ]+ (S ) 1/2), [CuIL0 ]+ (S ) 0), [ZnIIL1L0]+ (S ) 1/2) where the MnIII and FeII
2
2
2
ions are low-spin-configurated. The neutral species are described as [MnIIL1 ]0, [FeIIL1 ]0, [CoIL0L1]0, [NiIL0L1]0,
2
2
and [ZnIIL1 ]0; their electronic ground states have not been determined.
2
Introduction
Most complexes prepared to date are either five-coordinate
[MIIX2(L0)] (M ) Mn, Fe, Co, Ni, Cu, Zn, Cd; X ) Cl, Br) or
six-coordinate species [MIIL0 ]X2 containing one or two neutral,
Werner-type coordination chemistry of classic tridentate
nitrogen donor ligands with transition metal ions has in recent
years been revitalized because of the rather unexpected discov-
ery that complexes of this type are very active olefin poly-
merization catalysts. In particular, the pyridine-2,6-diimine (L)
ligands as 2,2′:6,2′′-terpyridine analogues and their iron and
cobalt complexes have shown spectacular results in this respect.1
Brookhart et al. also reported recently (pyridine-2,6-diimine)-
ruthenium complexes that catalyze the epoxidation of olefins,2
whereas Vrieze et al.3 reported an interesting stoichiometric
reaction of an R-chlorotolyl(pyridine-2,6-diimine)rhodium com-
plex with H2O and O2 to give benzylaldehyde and H2O2.
Electrocatalytic reduction of CO2 with [CoL2](PF6)2 and [NiL2]-
2
tridentate meridionally coordinated L0 ligands.5-17 In all cases
these neutral, coordinated ligands have been described to be
redox-innocent and the oxidation state of the central metal ion
has been calculated accordingly. This approach is shown here
to be somewhat simplistic and may yield an incorrect description
of the true electronic structure of a given complex in view of
the observation that all [ML0 ]2+ complexes display a rich redox
2
chemistry where at least two reversible one-electron reductions
and, in addition, quite often a reversible one-electron oxidation
are observed. In earlier work Toma et al.14,15 have employed
spectroelectrochemical methods to characterize the reduced and
(PF6)2 complexes as catalysts has also been reported.4
L
(5) Alyea, E. C.; Merrell, P. H. Synth. React. Inorg. Met.sOrg. Chem.
1974, 4, 535.
(6) Alyea, E. C.; Ferguson, G.; Restivo, R. J. Inorg. Chem. 1975, 14,
2491.
(7) Alyea, E. C.; Ferguson, G.; Restivo, R. J.; Merrell, P. H. J. Chem.
Soc., Chem. Commun. 1975, 269.
(8) Alyea, E. C.; Merrell, P. H. Inorg. Chim. Acta 1978, 28, 91.
(9) Merrell, P. H.; Alyea, E. C.; Ecott, L. Inorg. Chim. Acta 1982, 59,
25.
(10) Alyea, E. C. Inorg. Chim. Acta 1983, 76, L239.
(11) Restivo, R. J.; Ferguson, G. J. Chem. Soc., Dalton Trans. 1976, 518.
(12) Edwards, D. A.; Mahon, M. F.; Martin, W. R.; Molloy, K. C.; Fanwick,
P. E.; Walton, R. A. J. Chem. Soc., Dalton Trans. 1990, 3161.
(13) Edwards, D. A.; Edwards, S. D.; Martin, W. R.; Pringle, T. J.;
Thornton, P. Polyhedron 1992, 11, 1569.
(14) Kuwabara, I. H.; Comninos, F. C. M.; Pardini, V. L.; Viertler, H.;
Toma, H. E. Electrochim. Acta 1994, 39, 2401 and references therein.
(15) Toma, H. E.; Chavez-Gil, T. E. Inorg. Chim. Acta 1997, 257, 197
and references therein.
(16) Stor, G. J.; van der Vis, M.; Stufkens, D. J.; Oskam, A. J. Organomet.
Chem. 1994, 482, 15.
(17) Marcos, D.; Folgado, J.-V.; Beltran-Porter, D.; do Prado-Gambardella,
M. T.; Pulcinelli, S. H.; deAlmeida-Santos, R. H. Polyhedron 1990,
9, 2699.
represents in this instance 2,6-bis[1-(phenylimino)ethyl]pyridine.
The “reduced forms” of complexes are assumed to be the ones
that are active in the electrocatalytic process. These forms had
not been further characterized.
† Dedicated to Professor Heinrich Vahrenkamp on the occasion of his
60th birthday.
(1) (a) Small, B. L.; Brookhart, M. Macromolecules 1999, 32, 2120. (b)
Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.; Kimberley, B. S.;
Maddox, P. J.; Mastroianni, S.; McTavish, S. J.; Redshaw, C.; Solan,
G. A.; Stro¨mberg, S.; White, A. J. P.; Williams, D. J. J. Am. Chem.
Soc. 1999, 121, 8728. (c) Small, B. L.; Brookhart, M.; Bennett, A.
M. A. J. Am. Chem. Soc. 1998, 120, 4049. (d) Britovsek, G. J. P.;
Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.; McTavish, S. J.; Solan,
G. A.; White, A. J. P.; Williams, D. J. Chem. Commun. 1998, 849.
(2) Cetinkaya, B.; Cetinkaya, E.; Brookhart, M.; White, P. S. J. Mol. Catal.
A 1999, 142, 101.
(3) Haarman, H. F.; Bregman, F. R.; van Leeuwen, P. W. N. M.; Vrieze,
K. Organometallics 1997, 16, 979.
(4) Arana, C.; Yau, S.; Keshavarz, K. M.; Potts, K. T.; Abruna, H. D.
Inorg. Chem. 1992, 31, 3680.
10.1021/ic000113j CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/03/2000

Bis(pyridine-2,6-diimine)metal Complexes
Inorganic Chemistry, Vol. 39, No. 13, 2000 2937
Scheme 1. Resonance Structures of the L0 and (L1)1-
Ligands
Scheme 2. Possible Oxidation Level Distribution of
Ligands and Metal Ions in Tri-, Di-, and Monocations and in
the Neutral Species [ML2]n+ (n ) 3, 2, 1, 0)
oxidized forms of [FeIIL0 ]2+ complexes but have not provided
2
characterized by X-ray crystallography.22 The present work
clearly demonstrates that these pyridine-2,6-diimine ligands are
noninnocent.
definitive evidence for the electronic structures of these species.
In other words, metal vs ligand-centered reductions (or oxida-
tions) must be carefully considered. For example, this problem
has been addressed in the chemistry of [M(bpy)3]n+ 19
,
[M-
The following species have been synthesized and spectro-
scopically partially characterized previously: [MnIIL0 ](ClO4)2
12
20
(terpy)2]n+
,
and [M(diimine)3]n+ 21 complexes (n ) 3, 2, 1,
2
and [CoIIL0 ](ClO4)2.13 Note that in the following we will use
2
0). Recently, an example for bidentate coordination of a
pyridine-2,6-bis(imine) ligand has been provided in fac-[ReCl-
(CO)3L0].18
the designation L for the ligand C23H23N3O2 with unspecified
overall charge or oxidation level, where L0 represents the
diamagnetic, neutral ligand [C23H23N3O2]0 and (L1)1- is its
monoanionic radical [C23H23N3O2]1- (Scheme 1).
In this work we show that octahedral complexes [ML2]n+ of
the ligand 2,6-bis[1-(4-methoxyphenylimino)ethyl]pyridine5,22
and a first-row transition metal ion (M ) Mn, Fe, Co, Ni, Cu,
Zn) can exist as tri-, di-, or monocations and even as neutral
complexes [ML2]0. For each complex we will carefully address
the question of the actual oxidation level of the ligands that
can be coordinated as a neutral ligand L0 or as radical
monoanion (L1)1-. Scheme 1 gives a number of resonance
structures for both forms, which are taken as a first rough
indication of structural changes within the ligand that might be
expected when L0 is one-electron-reduced to (L1)1-. In principle,
it is conceivable that the tri-, di-, and monocations and the
neutral species [ML2]n+ (n ) 3, 2, 1, 0) can each adopt one of
three electronic structures shown in Scheme 2 depending on
the oxidation level of the two ligands and the central metal ion.
We have chosen the p-methoxy derivative of the pyridine-
2,6-diimines because it is synthetically readily accessible5 and
because the crystallinity of [ML2]n+ complexes (n ) 2, 1) was
found to be superior in comparison to other aryl derivatives. In
addition, the uncoordinated L0 ligand has been structurally
Synthesis and Characterization
The reaction of 2 equiv of the neutral, tridentate ligand 2,6-
bis[1-(4-methoxyphenylimino)ethyl]pyridine (L0) with 1 equiv
of an appropriate metal(II) salt in methanol yields octahedral
dications [ML2]2+ (M ) Mn, Fe, Co, Ni, Cu, Zn) of which the
hexafluorophosphate salts have been isolated as crystalline solids
by addition of excess [NH4]PF6.
The corresponding monocationic species [ML2]PF6 (M ) Mn,
Co) have been generated and isolated as crystalline solids by
one-electron reduction of the corresponding dication with 1
equiv of cobaltocene in acetonitrile solution under an argon
blanketing atmosphere. These are the first isolated [ML2]X salts.
From temperature-dependent susceptibility measurements (2-
298 K) using a superconducting quantum interference device
(SQUID) magnetometer we have determined the magnetic
moments of complexes. Complexes [ML2](PF6)2 (M ) Fe, Zn)
and [MnL2]PF6 are diamagnetic; they possess a singlet ground
state (S ) 0). All other species are paramagnetic. Their magnetic
moments as a function of the temperature are shown in Figure
1. Orange-red [MnL2](PF6)2 exhibits a nearly temperature-
independent magnetic moment of 5.90 µB at 10 K and 6.02 µB
at 290 K, close to the expected value of 5.92 µB for an S ) 5/2
system. From the susceptibility data we derive g ) 2.005, |D|
) 0.12 cm-1 and a temperature-independent paramagnetism,
(18) Granifo, J.; Bird, S. J.; Orrell, K. G.; Osborne, A. G.; Sik, V. Inorg
Chim. Acta 1999, 295, 56.
(19) Perez-Cordero, E.; Buigas, R.; Brady, N.; Eschegoyen, L.; Arana, C.;
Lehn, J.-M. HelV. Chim. Acta 1994, 77, 1222. (b) Eschegoyen, L.;
De Cian, A.; Fischer, J.; Lehn, J.-M. Angew. Chem., Int. Ed. Engl.
1991, 30, 838. (c) Perez-Cordero, E. E.; Campana, C.; Eschegoyen,
L. Angew. Chem., Int. Ed. Engl. 1997, 36, 137.
(20) (a) Berger, R. M.; McMillin, D. R. Inorg. Chem. 1988, 27, 4245. (b)
Morris, D. E.; Hanck, K. W.; DeArmond, M. K. J. Electroanal. Chem.
1983, 149, 115. (c) Pyo, S.; Perez-Cordero, E.; Bott, S. G.; Eschegoyen,
L. Inorg. Chem. 1999, 38, 3337.
(21) (a) Gagne, R. R.; Ingle, D. M. Inorg. Chem. 1981, 20, 420. (b) Vrieze,
K.; van Koten, G. Inorg. Chim. Acta 1985, 100, 79. (c) van Koten,
G.; Jastrzebski, J. T. B. H.; Vrieze, K. J. Organomet. Chem. 1983,
250, 49. (d) Dieck, H. T.; Franz, K.-D.; Hohmann, F. Chem. Ber.
1975, 108 (1), 163.
ø
TIP, of 463 × 10-6 cm3 mol-1 indicative of a high-spin
manganese(II) ion. Similar results have been reported previously
for the perchlorate salt.12 Thus, this species contains two neutral
L0 ligands: [MnIIL0 ](PF6)2.
2
The nearly temperature-independent magnetic moments of
1.81 µB at 4 K and 1.91 µB at 298 K indicate an S ) 1/2 ground
state for [CoL2](PF6)2. In contrast, previous authors13 have
(22) Meehan, P. R.; Alyea, E. C.; Ferguson, G. Acta Crystallogr., Sect. C:
Cryst. Struct. Commun. 1997, 53, 888.
3
reported an S )
/
ground state for the corresponding
2

2938 Inorganic Chemistry, Vol. 39, No. 13, 2000
de Bruin et al.
Table 1. NMR Data for the Diamagnetic Compoundsa
L0 b
[ZnIIL0 ](PF6)2 [MnIIIL1 ]PF6 [FeIIL0 ](PF6)2
c
c
c
2
2
2
1H NMR
Py-H3
(3J(H,H))
Py-H4
8.29 (7.8) 8.20 (7.3)
7.84 (7.6)
7.41 (7.6)
8.05-8.15
8.05-8.15
7.94 (7.8) 8.40 (7.3)
(3J(H,H))
-OCH3
3.80
2.40
3.71
2.41
3.60
2.47
6.45 (8.1)
3.68
2.54
6.66 (8.8)
-CH3
Ph-H2
6.95 (8.8) 6.72 (8.9)
(3J(H,H))
Ph-H3
6.82 (8.8) 6.41 (8.9)
5.64 (8.1)
6.08 (8.8)
(3J(H,H))
13C NMR
Figure 1. Temperature-dependent magnetic moments, µeff, µB, of
complexes. The solid lines represent best fits using parameters given
in the text.
Py-C2
Py-C3
Py-C4
CdN
155.7
122.1
136.7
167.4
16.1
159.4
128.5
138.0
168.2
17.8
158.2
121.6
124.0
167.2
17.4
160.3
127.7
136.6
179.2
19.4
-CH3
-OCH3
Ph-C1
Ph-C2
Ph-C3
Ph-C4
55.5
56.2
56.1
56.3
144.4
120.9
114.2
156.3
145.8
122.8
115.6
148.1
140.7
122.0
114.3
150.9
137.7
121.8
115.6
159.7
a The signals (δ (ppm), J (Hz)) are assigned using the numbering in
Scheme 1. b 1H NMR spectrum recorded in CD3CN, 13C NMR spectrum
recorded in CDCl3. c 1H NMR and 13C NMR spectra recorded in
CD3CN.
likely because of the unfavorable orientation of the eg1 and π*1
orbitals for ferromagnetic coupling. Furthermore, the observed
zero-field splitting |D| ) 3.4 for [CoIL0 ]PF6 is very similar to
2
|D| ) 4.5 cm-1 for the isoelectronic [NiIIL0 ](PF6)2.
2
Similarly, a nearly temperature-independent magnetic moment
of 2.87 µB of [NiL2](PF6)2 is indicative of an octahedral nickel-
(II) ion (S ) 1). From the temperature-dependent susceptibility
data we derive the parameters g ) 2.11, |D| ) 4.5 cm-1, and
øTIP ) 175 × 10-6 cm3 mol-1, in agreement with the electronic
structure of [NiIIL0 ](PF6)2. Note that the cations in [CoIL0 ]-
2
2
Figure 2. Top: X-band EPR spectrum of [CoIIL0 ](PF6)2 in CH3CN
at 10 K. Conditions are the following: frequency, 9.6463 GHz;
PF6 and [NiIIL0 ](PF6)2 are isostructural and isoelectronic, both
2
2
containing a metal ion with a high-spin d8 electron configuration.
microwave power, 1 µW; modulation amplitude, 1.25 mT. Bottom:
The complex [CuIIL0 ](PF6)2 displays a nearly temperature-
2
X-band EPR spectrum of [CuIIL0 ](PF6)2 in CH3CN at 10 K. Conditions
2
independent magnetic moment of 1.82 µB at 10 K and 1.88 µB
at 298 K. From the susceptibility data we derived the parameters
g ) 2.097 and øTIP ) 100 × 10-6 cm3 mol-1, indicative of an
octahedral CuII (d9) complex (Figure S1 of Supporting Informa-
tion). From its X-band EPR spectrum in CH3CN solution at 10
K (Figure 2, bottom) the following parameters were derived:
are the following: frequency, 9.4346 GHz; microwave power, 1 µW;
modulation amplitude, 1.25 mT. The simulations were obtained with
parameters given in the text.
perchlorate salt [CoL2](ClO4)2 (µeff (25 °C) ) 4.62 µB). We
1
have reexamined this salt and find an S ) /2 ground state in
the solid state (Figure 1). From the susceptibility data we derive
g ) 2.10, øTIP ) 235 × 10-6 cm3 mol-1 for the PF6 salt and g
) 2.08, øTIP ) 165 × 10-6 cm3 mol-1 for the ClO4 salt. The
X-band EPR spectrum of [CoL2](PF6)2 in CH3CN at 10 K shown
in Figure 2 also confirms the S ) 1/2 ground state of this dication
in frozen solution. The rhombic spectrum was successfully
simulated with gx ) 2.221, gy ) 2.203, gz ) 1.973 and hyperfine
g
) 2.087, g| ) 2.245, A| ) 145 × 10-4 cm-1, A
)
unresolved.
Surprisingly, the dark-blue complex [MnL2]PF6 possesses an
S ) 0 ground state. From temperature-dependent susceptibility
measurements small effective magnetic moments of 1.29 µB at
290 K and 0.88 µB at 8 K were measured. We obtained a
satisfactory fit of the magnetic susceptibility data with øTIP
)
coupling to the 59Co (I ) /2) nucleus with Axx ) 24.9 × 10-4
390 × 10-6 cm3 mol-1 and 3% of a paramagnetic impurity (S
7
5
cm-1, Ayy ) 85.3 × 10-4 cm-1, and Azz ) unresolved. No
resolved superhyperfine coupling to the 14N nuclei has been
observed. These data are fully consistent with an electronic
) /2).
The diamagnetic complexes [ZnL2](PF6)2, [MnL2]PF6, and
[FeL2](PF6)2 dissolved in CD3CN solution at ambient temper-
ature display 1H and 13C NMR signals in the normal range for
diamagnetic compounds. Table 1 summarizes these data. The
spectra reveal that the two ligands of the [ML2]n+ cations are
equivalent on the NMR time scale and, furthermore, that a given
LM fragment is bisected by a (noncrystallographic) mirror plane
where the metal ion, the pyridine nitrogen, and a carbon atom
in the para position lie on this plane. Although significant shifts
structure of [CoIIL0 ]X2 containing a low-spin cobalt(II) ion (d7).
2
Deep-green [CoL2]PF6 exhibits a nearly temperature-inde-
pendent magnetic moment of 2.99-2.96 µB in the temperature
range 300-20 K. This value is very close to the expected value
of 2.83 µB for an S ) 1 system and is indicative of an octahedral
cobalt(I) ion in [CoIL0 ]PF6. From the temperature-dependent
2
susceptibility data we derive the following parameters (Figure
1
1): g ) 2.09; |D| ) 3.4 cm-1. Thus, an electronic structure
of some signals are observed in the H and 13C NMR spectra
[CoIL0 ]PF6 appears to prevail. The description of [CoIIL1L0]-
on going from the free L0 ligand to [ZnL2](PF6)2 and to [MnL2]-
PF6, it is difficult to distinguish anisotropic shielding/deshielding
because of ring current effects from shifts caused by charge
2
PF6 containing a low-spin CoII (S ) 1/2) strongly ferromagneti-
1
cally coupled to a ligand radical (L1)1- (S ) /2) is much less

Bis(pyridine-2,6-diimine)metal Complexes
Inorganic Chemistry, Vol. 39, No. 13, 2000 2939
[ML2]n+ cation are, within experimental error, identical and,
furthermore, that, as noted above, the LM fragment is bisected
by a noncrystallographic mirror plane, i.e., the two halves of a
coordinated L ligand are identical. For the following discussion
it therefore suffices to compare the averaged bond distances
Cpy-Npy, Cpy-Cimine, Cimine-Nimine as summarized in Table 3.
The dimensions of the p-methoxyphenyl groups do not vary
across the series within experimental error.
The compound [ZnIIL0 ](PF6)2 serves as a benchmark for a
2
redox innocent case where the well-defined oxidation state of
the central zinc ion (+II, d10) unequivocally renders the two
ligands as L0 (neutral molecules). Typically, the average Cimine
-
Nimine distance at 1.285(2) Å is short and indicates a CdN
double bond whereas the average Cpy-Cimine bond length at
1.502(2) Å corresponds to that of a typical C-C single bond.
The Cpy-Npy bonds at 1.336(2) Å and the Cpy-Cpy distances
are equidistant at 1.390(2) Å and indicative of an unperturbed
pyridine ring.
Figure 3. Structure of the monocation in crystals of [MnIIIL1 ]PF6.
2
As can be seen from the data compiled in Table 3, exactly
the same oxidation level of the two ligands in [MnL0 ](PF6)2
2
accumulation. In general, ring current shielding/deshielding
effects are less prominent for 13C NMR signals. Thus, the
observed upfield shifts of some of the 13C NMR pyridine signals
in [MnL2]PF6 relative to [ZnL2](PF6)2 are indicative of an
increased negative charge on this pyridine ring.
prevails, namely, L0. Thus, the electronic structure of this
compound must be described as [MnIIL0 ](PF6)2 comprising an
2
octahedral high-spin MnII ion and two neutral L0 ligands.
Similarly, the metrical details of the coordinated ligands in
[CoIIL0 ](PF6)2 are in good agreement with its formulation as
2
The diamagnetic complex [FeL2](PF6)2 has been studied by
zero-field Mo¨ssbauer spectroscopy. At 80 K a symmetrical
quadrupole doublet with an isomer shift, δ, of 0.235(8) mm
s-1 and a quadrupole splitting, ∆EQ, of 1.081(5) mm s-1 has
been observed (Figure S2). In comparison, these parameters are
δ ) 0.238 and ∆EQ ) 1.14 mm s-1 for low-spin [FeII(terpy)2]-
(ClO4)2,23 whereas for low-spin [FeIII(terpy)2](ClO4)3 the values
neutral L0 ligands rendering the central metal ion low-spin CoII.
However, barely significantly the average Cimine-Nimine bond
at 1.295(2) Å is longer by 0.01 Å than in [ZnL0 ](PF6)2 and
2
[MnIIL0 ](PF6), the average Cpy-Cimine bond at 1.480(3) Å is
2
shorter by 0.022 Å, and the average Cpy-Npy distance is again
longer by 0.012 Å than in the two former cases. This trend is
augmented on going to [FeIIL0 ](PF6)2, which, according to the
are 0.07 and 3.43 mm s-1 24
. Clearly, the electronic structure of
2
Mo¨ssbauer spectrum and magnetic susceptibility measurements,
contains a low-spin ferrous ion. The average Cimine-Nimine is
again slightly elongated, whereas the average Cpy-Cimine bond
is shortened and the Cpy-Npy distances are elongated in
comparison with the [ZnIIL0 ](PF6)2 and [MnIIL0 ](PF6)2 species.
the above complex must be described as [FeIIL0 ](PF6)2. On
2
the other hand, for low-spin [FeII(bpy)3](ClO4)2 Mo¨ssbauer
parameters δ ) 0.325, ∆EQ ) 0.39 mm s-1 25 have been
reported. The lower isomer shifts of [FeIIL0 ](PF6)2 and [FeII-
2
(terpy)2](ClO4)2 relative to [FeII(bpy)3](ClO4)2 indicate signifi-
cant charge delocalization from the metal t2g orbitals into the
ligand π* orbitals via π back-donation. Consistent with this
interpretation is the increasing intensity of the metal-to-ligand
2
2
The metrical details of the ligands in [CoL2]PF6, [NiL2](PF6)2
and [CuL2](PF6)2 are between those of [FeIIL0 ](PF6)2 and
2
[ZnL0 ](PF6)2. Consequently, we assign the following oxidation
2
levels: [CoIL0 ]PF6 containing an octahedral cobalt(I) ion (d8),
charge-transfer bands15 in the visible on going from [ZnL0 ]2+
2
2
[NiIIL0 ](PF6)2 with an isoelectronic nickel(II) ion, and [CuIIL0 ]-
to [MnIIL0 ]2+ to [CoIIL0 ]2+ and, finally, to [FeIIL0 ]2+ (see
2
2
2
2
2
(PF6)2 containing an octahedral copper(II) ion (d9).
Figures 5 and 10).
It is now significant and quite remarkable that in [MnL2]PF6
the longest average Cimine-Nimine bond length at 1.321(2) Å,
the shortest average Cpy-Cimine bond at 1.442(2) Å, the longest
Cpy-Npy bonds at 1.373(2) Å of the whole series are observed.
The extremes are depicted in Scheme 3 where the ligand
X-ray Structures. Single crystals suitable for X-ray crystal-
lography of orange-red [MnL2](PF6)2, dark-blue [MnL2]PF6,
purple-black [FeL2](PF6)2, brown-black [CoL2](PF6)2, green-
black [CoL2]PF6, brown [NiL2](PF6)2, brown [CuL2](PF6)2, and
yellow [ZnL2](PF6)2 were grown from acetonitrile solutions top-
layered with diethyl ether.
dimensions in [MnIIL0 ](PF6)2 and its one-electron-reduced form,
2
All structures consist of mono- or dications, [ML2]+/2+, and
well-separated hexafluorophosphate anions. The metal ions are
in a distorted octahedral environment comprising two meridi-
onally coordinated L ligands that orientate the pyridine nitrogen
donors in a mutually trans position. As a representative example,
the molecular structure of [MnL2]+ is shown in Figure 3. Atom
labels used here are the same for all structures. Table 2 gives
bond distances and angles.
[MnL2]PF6, are shown. We take this as an indication that in
[MnL2]PF6 the ligands are coordinated as monoanionic radicals
(L1)1- that would render the central metal ion a low-spin MnIII
ion (S ) 1).
Interestingly, the structure of the octahedral diamagnetic
complex [MnI(dapa)(CO)2Br] has been reported16 where dapa
is 2,6-diacetylpyridine-bis(anil) which is an analogue of our L0
ligand without the p-methoxy groups. According to the reported
dimensions, the ligand is of the L0 type with short CdNimine
bonds (av 1.288(6) Å), which resembles in this respect the
A common feature of all structures is the observation that
the metrical details of both coordinated ligands in a given
situation in [MnIIL0 ](PF6)2 but not [MnL2]PF6. Thus, we cannot
2
describe the electronic structure as [MnIL0 ]PF6 containing a
(23) Epstein, L. M. J. Chem. Phys. 1964, 40, 435.
(24) Reif, W. M.; Baker, W. A., Jr.,; Erickson, N. E. J. Am. Chem. Soc.
1968, 90, 4794.
(25) Collins, R. L.; Pettit, R.; Baker, W. A., Jr. J. Inorg. Nucl. Chem. 1966,
28, 1001.
2
low-spin MnI ion.
Now we turn to the MN6 octahedra of complexes. For
[ZnIIL0 ](PF6)2 and [MnIIL0 ](PF6)2 containing a spherical ZnII
2
2

2940 Inorganic Chemistry, Vol. 39, No. 13, 2000
de Bruin et al.
Table 2. Selected Bond Distances (Å) and Bond Angles (deg) of Complexes
Bond Distance
[MnIIL0 ]2+
[MnIIIL1 ]+
[FeIIL0 ]2+
[CoIIL0 ]2+
[CoIL0 ]+
[NiIIL0 ]2+
[CuIIL0 ]2+
[ZnIIL0 ]2+
2
2
2
2
2
2
2
2
M-NPy
1.852(2)
1.991(2)
N2-M1
N5-M1
2.177(2)
2.171(2)
1.8867(12)
1.8832(12)
1.8707(14)
1.8655(14)
1.986(5)
1.993(5)
1.973(2)
1.965(2)
1.9419(11)
1.9561(11)
2.0369(10)
2.0464(10)
Mn-Nimine
2.009(2)
2.021(2)
2.145(2)
2.165(2)
N1-M1
N3-M1
N4-M1
N6-M1
2.277(2)
2.249(2)
2.274(2)
2.253(3)
1.9831(13)
1.9720(13)
1.9988(12)
1.9831(12)
1.974(2)
1.999(2)
1.9817(14)
1.9922(14)
2.153(5)
2.158(4)
2.136(5)
2.134(5)
2.113(2)
2.134(2)
2.129(2)
2.134(2)
2.1588(11)
2.1626(11)
2.2331(11)
2.2133(11)
2.1997(11)
2.2051(10)
2.1893(11)
2.2003(12)
Cimine-Nimine
1.298(3)
1.305(3)
1.289(3)
1.289(3)
N1-C8
1.281(4)
1.283(4)
1.284(4)
1.283(4)
1.319(2)
1.323(2)
1.323(2)
1.320(2)
1.306(2)
1.309(2)
1.311(2)
1.311(2)
1.294(7)
1.300(7)
1.307(7)
1.310(7)
1.286(3)
1.295(3)
1.288(3)
1.291(3)
1.284(2)
1.292(2)
1.283(2)
1.284(2)
1.282(2)
1.283(2)
1.284(2)
1.290(2)
N3-C15
N4-C38
N6-C45
CPy-Cimine
1.472(4)
1.476(3)
1.487(3)
1.485(3)
C8-C10
1.496(5)
1.500(4)
1.499(4)
1.497(4)
1.442(2)
1.441(2)
1.440(2)
1.443(2)
1.466(2)
1.466(2)
1.466(2)
1.470(2)
1.477(7)
1.456(8)
1.455(8)
1.470(9)
1.492(3)
1.495(3)
1.488(3)
1.493(3)
1.490(2)
1.496(2)
1.501(2)
1.497(2)
1.502(2)
1.505(2)
1.499(2)
1.500(2)
C14-C15
C38-C40
C44-C45
NPy-CPy
1.347(3)
1.350(3)
1.347(3)
1.350(3)
N2-C10
N2-C14
N5-C40
N5-C44
1.335(4)
1.338(4)
1.339(4)
1.341(4)
1.373(2)
1.373(2)
1.371(2)
1.375(2)
1.356(2)
1.351(2)
1.355(2)
1.351(2)
1.353(7)
1.363(7)
1.348(8)
1.364(7)
1.339(3)
1.341(3)
1.343(3)
1.341(3)
1.338(2)
1.344(2)
1.342(2)
1.347(2)
1.339(2)
1.336(2)
1.336(2)
1.334(2)
CPy-CPy
1.387(3)
1.389(4)
1.385(4)
1.393(3)
1.381(3)
1.389(4)
1.387(4)
1.393(3)
C10-C11
C11-C12
C12-C13
C13-C14
C40-C41
C41-C42
C42-C43
C43-C44
1.392(4)
1.384(5)
1.385(5)
1.392(4)
1.386(4)
1.381(5)
1.382(5)
1.396(4)
1.390(2)
1.391(2)
1.396(2)
1.390(2)
1.394(2)
1.390(2)
1.390(2)
1.392(2)
1.388(2)
1.386(3)
1.398(3)
1.386(2)
1.383(2)
1.386(3)
1.389(3)
1.397(2)
1.384(8)
1.377(8)
1.396(8)
1.391(8)
1.399(8)
1.377(8)
1.379(8)
1.385(8)
1.385(3)
1.387(3)
1.387(3)
1.389(3)
1.385(3)
1.383(3)
1.386(3)
1.390(3)
1.393(2)
1.391(2)
1.392(2)
1.392(2)
1.394(2)
1.386(2)
1.383(2)
1.389(2)
1.384(2)
1.387(2)
1.385(2)
1.393(2)
1.393(2)
1.388(2)
1.392(2)
1.394(2)
Bond Angle
[MnIIL0 ]2+
[MnIIIL1 ]+
[FeIIL0 ]2+
[CoIIL0 ]2+
[CoIL0 ]+
[NiIIL0 ]2+
[CuIIL0 ]2+
[ZnIIL0 ]2+
2
2
2
2
2
2
2
2
N1-M1-N2
N1-M1-N3
N1-M1-N4
N1-M1-N5
N1-M1-N6
N2-M1-N3
N2-M1-N4
N2-M1-N5
N2-M1-N6
N3-M1-N4
N3-M1-N5
N3-M1-N6
N4-M1-N5
N4-M1-N6
N5-M1-N6
71.74(9)
143.25(9)
89.17(9)
102.47(9)
100.14(9)
72.06(9)
99.42(9)
169.56(9)
116.74(9)
102.76(9)
114.27(9)
90.48(9)
71.44(9)
143.81(9)
72.41(9)
77.92(5)
156.31(5)
92.12(5)
98.19(5)
95.27(5)
78.53(5)
106.20(5)
173.78(5)
97.37(5)
91.94(5)
105.49(5)
90.27(5)
78.64(5)
156.30(5)
78.03(5)
79.81(6)
159.69(6)
88.59(6)
99.05(6)
97.79(6)
79.89(6)
99.70(6)
178.80(6)
101.03(6)
94.75(6)
101.25(6)
86.16(6)
79.86(6)
159.09(6)
79.47(6)
80.17(8)
160.56(8)
87.60(8)
98.52(8)
97.79(8)
80.42(8)
100.97(8)
178.48(8)
103.46(8)
97.11(8)
100.91(8)
85.73(8)
78.16(8)
155.54(8)
77.45(8)
74.9(2)
151.0(2)
98.2(2)
97.7(2)
87.3(2)
76.2(2)
110.7(2)
170.7(2)
98.4(2)
89.8(2)
111.3(2)
99.2(2)
75.4(2)
150.8(2)
75.4(2)
77.57(7)
155.26(7)
88.07(7)
101.32(7)
98.73(7)
77.73(7)
101.44(7)
178.55(7)
103.86(7)
98.26(7)
103.40(7)
85.75(6)
77.54(7)
154.65(7)
77.18(7)
77.65(4)
155.62(4)
98.01(4)
100.78(4)
87.43(4)
78.16(4)
104.50(4)
178.28(5)
101.93(4)
85.11(4)
103.45(4)
100.53(4)
76.36(4)
153.58(4)
77.22(4)
75.69(4)
150.92(4)
88.49(4)
103.30(4)
101.01(4)
75.24(4)
103.25(4)
178.50(4)
105.62(4)
99.13(4)
105.78(4)
85.82(4)
75.56(4)
151.02(4)
75.61(4)
Table 3. Averaged Bond Distances (Å) in [ML2]n+ Cations
Scheme 3. Average Cimine-Nimine, Cimine-Cpy, and Cpy-Npy
Bond Lengths in [MnIIL0 ](PF6)2 (Left) and [MnIIIL1 ]PF6
2
2
Cimine-Nimine
CPy-Cimine
NPy-CPy
(Right)
[MnIIL0 ]2+
1.283(2)
1.321(1)
1.309(1)
1.295(2)
1.303(4)
1.290(2)
1.286(1)
1.285(1)
1.498(2)
1.442(1)
1.467(1)
1.480(2)
1.465(4)
1.492(2)
1.496(1)
1.502(1)
1.338(2)
1.373(1)
1.353(1)
1.349(2)
1.357(4)
1.341(2)
1.343(1)
1.336(1)
2
[MnIIIL1 ]+
2
[FeIIL0 ]2+
2
[CoIIL0 ]2+
2
[CoIL0 ]+
2
[NiIIL0 ]2+
2
[CuIIL0 ]2+
2
[ZnIIL0 ]2+
2
(d10) and a high-spin MnII (d5) ion, respectively, four nearly
equidistant M-Nimine bonds and two relatively shorter M-Npy
bond distances are observed. This compression of the Npy-
M-Npy axis is observed in all structures.
(1) Å), they compare well with those reported for low-spin [FeII-
(terpy)2]2+ (av Fe-Npy-central 1.891(4) Å; av Fe-Npy-distal
1.988(3) Å).26
Although the Fe-N bond lengths in [FeIIL0 ](PF6)2 appear
2
to be rather short (av Fe-Npy 1.868(1) Å; av Fe-Nimine 1.987-
In [CoIIL0 ](PF6)2 there are two longer Co-Nimine bonds at
2

Bis(pyridine-2,6-diimine)metal Complexes
Inorganic Chemistry, Vol. 39, No. 13, 2000 2941
Table 4. Electrochemical Half-Wave Potentialsa
[ML2]3+/[ML2]2+ [ML2]2+/[ML2]+ [ML2]+/[ML2]0
∼2.15 Å and two shorter ones at ∼2.01 Å; the Co-Npy bonds
are not equivalent but are still the shortest at 1.852(2) and 1.991-
(2) Å. This highly irregular CoN6 polyhedron results from Jahn-
Teller distortions of a low-spin CoII ion (d7) with an unsym-
metrical occupancy of the eg orbitals. Similar structures have
been reported for low-spin CoII complexes containing a mac-
rocyclic bis(pyridine-2,6-diimine) ligand27 or two terpyridine
ligands.28 Very similar Jahn-Teller distortions are also observed
[ZnL0 ]2+
-1.39
-1.67
-1.66
-1.92
-1.82
-1.21c
-1.87
2
[FeL0 ]2+
+0.86
+0.01
+1.19
+1.20c
+0.97
-1.31
2
[CoL0 ]2+
-0.93
2
[NiL0 ]2+ b
-1.30
2
[CuL0 ]2+
-0.57
2
[MnL0 ]2+
Eanodic: -1.21
2
E
cathodic: -0.62
for the copper(II) complex (d9) [CuIIL0 ](PF6)2.
2
a E1/2 in volts vs Fc+/Fc (CH3CN (0.1 M [N(n-Bu)4]PF6); scan rate,
200 mV/s). b CH2Cl2 (0.1 M [N(n-Bu)4]PF6; 200 mV/s). c Irreversible;
In [CoIL0 ]PF6, the one-electron-reduced form of [CoIIL0 ]-
2
2
(PF6)2, the t2g6eg electron configuration of the central CoI ion
yields again four equivalent Co-Nimine bond lengths at ∼2.14
Å and two equivalent Co-Npy bonds at 1.99 Å. Crystallo-
graphically characterized CoI (d8) octahedral complexes (S )
1) with a CoIN6 octahedron are extremely rare. [CoI(bpy)3]Cl‚
H2O is an example29 where the average Co-N distance has
been determined to be 2.115(20) Å, in excellent agreement with
the present structure where the average Co-N distance is 2.093-
2
red
peak potentials Epox, Ep are given.
MnII complex (S ) 1/2) containing an MnN6 octahedron.31 The
nitrogens are imine-type donors, and the polyhedron around the
metal ion was described as a Jahn-Teller compressed octahe-
dron with two short Mn-N distances at 1.89(1) Å in trans
position with respect to each other and four longer equatorial
Mn-Nimine bonds at an average of 1.985 Å. These values
correspond exactly to those in [MnL2]PF6, and therefore, the
description as a low-spin MnII coupling antiferromagnetically
to one ligand radical (L1)1- as in [MnIIL0L1]PF6 is a possibility,
taking into consideration only the MnN6 polyhedron. It would
require the two ligands to be distinctly different, which they
are not (see above). Thus, we favor the description as low-spin
(2) Å (in [CoIIL0 ](PF6)2 this average Co-N length is shorter
2
at 2.031(2) Å). Similar results have been reported for the cobalt-
(I) complex [CoI(Lpy)2]+ containing two neutral tripodal ligands
tris(2-pyridyl)methane (Lpy).30 The same is true for isoelectronic
[NiIIL0 ](PF6)2.
2
The most remarkable structural changes are observed when
the high-spin [MnIIL0 ](PF6)2 species is one-electron-reduced
[MnIIIL1 ]PF6, which appears to accommodate the observed
2
2
to the [MnL2]PF6 species. If one concurs with the notion from
above that effectively both L0 ligands are reduced to the radical
anion (L1)1-, then the central Mn ion must have an oxidation
state of +III. Since [MnL2]PF6 possesses an S ) 0 ground state,
it must then be a low-spin MnIII ion (S ) 1) that is strongly
antiferromagnetically coupled to the two radical anions: [MnIIIL12]-
structural features of both the MnN6 octahedron and the
dimensions of the ligands (L1)1-. The Cimine-Nimine and Npy-
Cpy distances in the monocation [MnL2]+ are consistently 0.03-
0.04 Å longer than in the dication [MnIIL0 ]2+, and the Cpy-
2
C
imine distances are ∼0.06 Å shorter. All other C-C and C-N
distances are unaffected on going from [MnL2]2+ to [MnL2]+.
We also note that all corresponding C-C and C-N bond
distances in [ZnL0 ]2+ and [MnIIL0 ]2+ are, within experimental
PF6. In comparison to the MnN6 octahedron in [MnIIL0 ](PF6)2
2
the corresponding polyhedron in [MnIIIL1 ]PF6 has Mn-N
2
2
2
error, the same as those in the uncoordinated, free L0 ligand.22
Thus, one-electron reduction of L0 to (L1)1- induces exactly
the structural changes one expects, considering the resonance
structures shown in Scheme 1.
distances that are on average 0.29 Å (!) shorter. This is a clear
indication that whatever the true oxidation state of the central
Mn ion is, namely, MnI (d6), MnII (d5), or MnIII (d4), it must
possess a low-spin configuration with an empty eg* orbital
subset. In [MnI(dapa)Br(CO)2]16 the Mn-Npy bond length is
1.944(4) Å and the two Mn-Nimine distances are at 2.004(4)
and 1.988(4) Å. (As noted above, the dapa ligand corresponds
to an oxidation level of L0.) The average Mn-Npy and Mn-
Nimine bond lengths in [MnL2]PF6 at 1.885(1) and 1.984(1) Å
are significantly shorter by 0.059 and 0.012 Å, respectively.
Electro- and Spectroelectrochemical Investigations
Cyclic voltammograms (CV) of all complexes have been
recorded in CH3CN or CH2Cl2 solutions containing 0.10 M
[N(n-Bu)4]PF6 as supporting electrolyte at a glassy carbon
working electrode and a Ag/AgNO3 reference electrode. Fer-
rocene was used as an internal standard, and potentials are
referenced versus the ferrocenium/ferrocene couple (Fc+/Fc).
Table 4 summarizes the results.
Therefore, [MnIL0 ]PF6 can safely be ruled out.
2
Figure S3 shows a histogram where we have plotted all Mn-
Npy distances found in the Cambridge Crystallographic Struc-
tural Database. A total of 1106 complexes have been found
containing the Mn-pyridine entity. The two maxima of Mn-
Npy bond lengths at ∼2.28 and ∼2.07 Å correspond to high-
spin MnIINpy and high-spin MnIIINpy (or MnIVNpy). The Mn-
Npy distance of 1.885(1) Å in [MnL2]PF6 is the shortest bond
of this type reported to date. On the other hand, we have
previously reported the crystal structure of a genuine low-spin
Figure 4 (top) shows the CV of [ZnL0 ](PF6) in the potential
2
range +1.3 to -2.2 V. Two reversible one-electron-transfer
waves are observed at quite negative potentials. Coulometric
measurements establish that both processes correspond to a one-
electron reduction that must be ligand-centered as depicted in
eq 1.
-e
[ZnIIL0 ]
w\
x [ZnIIL0L1]+ w-+ee
\
x [ZnIIL1 ]0
(1)
2+
2
2
+e
(26) Baker, A. T.; Goodwin, H. A. Aust. J. Chem. 1985, 38, 207.
(27) Drew, M. G. B.; McCann, M.; Nelson, S. M. Inorg. Chim. Acta 1980,
41, 213.
(28) (a) Figgis, B. N.; Kucharski, E. S.; White, A. H. Aust. J. Chem. 1983,
36, 1527. (b) Figgis, B. N.; Kucharski, E. S.; White, A. H. Aust. J.
Chem. 1978, 31, 737.
(29) Szalda, D. J.; Creutz, C.; Mahajan, D.; Sutin, N. Inorg. Chem. 1983,
22, 2372.
(30) (a) Adam, K. R.; Anderson, P. A.; Astley, T.; Atkinson, I. M.;
Charnock, J. M.; Garner, C. D.; Gulbis, J. M.; Hambley, T. W.;
Hitchman, M. A.; Keene, F. R.; Tiekink, E. R. T. J. Chem. Soc., Dalton
Trans. 1997, 519. (b) Hafeli, T. A.; Keene, F. R. Austr. J. Chem. 1988,
41, 1379.
Thus, electrochemistry establishes that the coordinated L0 ligand
can be reduced to its monoanionic radical (L1)1-. Since both
reduced forms are stable on the time scale of a coulometric
experiment, we have recorded their electronic spectra, shown
in Figure 5. We have also recorded the X-band EPR spectrum
of [ZnIIL0L1]+, which displays a featureless isotropic signal at
(31) Knof, U.; Weyhermu¨ller, T.; Wolter, T.; Wieghardt, K. J. Chem. Soc.,
Chem. Commun. 1993, 726.

2942 Inorganic Chemistry, Vol. 39, No. 13, 2000
de Bruin et al.
mV is predominantly due to differences of solvation energies
for a di- and monocationic pair and the monocation/neutral pair.
Thus, the mixed-valent form [ZnIIL0L1]+ does not experience a
significant electronic stabilization via delocalization of the
excess electron over two ligands. Consequently, in agreement
with the additivity of the band in the visible region for
[ZnIIL0L1]+ and [ZnIIL1 ]0, we suggest that L0 and (L1)1- in
2
[ZnL0L1]+ have localized oxidation levels.
Figure 4 shows the CV of [FeIIL0 ](PF6)2. Three reversible
2
one-electron waves are observed. Controlled potential coulom-
etry at appropriately fixed potentials reveals that the process at
E1/2 ) +0.86 V corresponds to a one-electron oxidation that
must be metal-centered, whereas the waves at E1/2 ) -1.31
and -1.66 V correspond to ligand-centered successive one-
electron reductions:
Figure 4. Cyclic voltammograms of [ZnL2]PF6 (CH3CN), [FeL2](PF6)2
(CH2Cl2), [CoL2](PF6)2 (CH3CN), and [NiL2](PF6)2 (CH2Cl2). Condi-
tions are the following: 0.10 M [N(n-Bu)4]PF6 supporting electrolyte;
glassy carbon working electrode; scan rate, 200 mV s-1; 22 °C.
-e
-e
2+
[FeIIIL0 ]
w
x [FeIIL0 ] x [FeIIL1 ]0
w\ \
x [FeIIL0L1]+ w-+ee
2 2
+e
3+
2
+e
(2)
The latter potentials are very similar to those observed for
[ZnIIL0 ]2+, and the same arguments concerning the nature of
2
[FeIIL0L1]+ apply. Since the oxidized32 and two reduced forms
of [FeIIL0 ]2+ are stable at -20 °C on the time scale of
2
coulometric experiments, we have recorded their electronic
spectra, which are displayed in Figure 5. In general, the electro-
and spectroelectrochemistry of [FeIIL0 ]2+ is very similar to that
2
reported by Toma et al.14,15 for bis(2,6-diacetylfurfuryliminepy-
ridine)iron(II) complexes, a notable exception being the spec-
trum of [FeIIL1 ]0, which is clearly different from the analoguous
2
species in Toma’s work. The spectrum of [FeIIL0 ]2+ is
2
interesting with respect to that of [ZnL0 ]2+, since it displays a
2
series of intense metal-to-ligand charge-transfer (MLCT) bands
in the range 500-650 nm that are absent in the spectrum of
[ZnL0 ]2+. These MLCT transitions are characteristic of iron-
2
(II) diimine complexes.14,15
Since [FeIIL0L1]+ is stable in CH3CN solution at -20 °C,
we have recorded its X-band EPR spectrum shown in Figure
S4. The slightly rhombic, almost isotropic signal could be
satisfactorily simulated with the parameters gx ) 1.996, gy )
1.975, gz ) 1.961 (gav ) 1.977). Evidently, the spectrum
indicates formation of [FeIIL0L1]+ containing the ligand radical
(L1)1-. Anion radical complexes of transition metal fragments
usually display rather small deviations of ge (∆g e (0.02)
because the spin is delocalized mainly over light atoms with
small spin-orbit coupling factors.33 The low g values of
[FeIIL0L1]+ may be taken as a strong indication of at least one
excited state close to the MLCT excited state of the nonreduced
precursor complex [FeIIL0 ]2+ 33
.
2
The CV of [CoIIL0 ](PF6)2 also displays three reversible one-
2
electron-transfer waves (Figure 4). Coulometry established that
at E1/2 ) 0.01 V this species undergoes a one-electron oxidation
that must be metal-centered, whereas the processes at -0.93
and -1.92 V correspond to two successive one-electron
reductions. Note that the first reduction, [CoL2]2+ to [CoL2]+,
occurs at a less negative potential than the ligand-centered,
corresponding reductions in [ZnIIL0 ]2+ and [FeIIL0 ]2+. There-
Figure 5. Electronic spectra of electrochemically generated [ML2]3+
,
[ML2]+, and [ML2]0 species. The spectra of [ML2]2+ complexes were
recorded from acetonitrile solutions of the corresponding hexafluoro-
phosphate salts. From top to bottom, M ) Zn, Fe, Co, and Ni,
respectively.
g ) 2.0008 at 298 K without resolved hyperfine splitting and
a sharp isotropic signal at g ) 2.0045 at 10 K in frozen CH3-
CN solution, typical of an organic radical (S ) 1/2). The observed
UV/vis spectral changes upon two successive one-electron
2
2
(32) The zero-field Mo¨ssbauer spectrum of electrochemically generated
[FeIIIL0 ]3+ in frozen CH3CN at 200 K shows a dominant quadrupole
2
doublet (68%) for low-spin Fe(III) with δ ) 0.056 and ∆EQ ) 3.105
mm s-1, very similar to the values reported for [FeIII(terpy)2]3+ (δ )
0.07, ∆EQ ) 3.43 mm s-1; ref 24). However, a minor quadrupole
doublet (32%) from a side product was also observed, indicative of
reductions of [ZnIIL0 ]2+ are accompanied by the generation of
2
a new charge transfer (CT) band of presumably π-π* character
in the visible region (500-600 nm). Upon formation of
high-spin Fe(III) with δ ) 0.447 and ∆EQ ) 0.718 mm s-1
.
[ZnIIL1 ]0 this band doubles in intensity and is a little red-shifted.
2
(33) (a) Kaim, W. Coord. Chem. ReV. 1987, 76, 187. (b) Kaim, W. Inorg.
Chem. 1984, 23, 3365.
The small difference between the two redox potentials of 280

Bis(pyridine-2,6-diimine)metal Complexes
Inorganic Chemistry, Vol. 39, No. 13, 2000 2943
Figure 7. Electronic spectra of [CuIIL0 ](PF6)2 and electrochemically
2
generated [CuIL0 ]+ in CH2Cl2 solution.
2
nuclei (I ) 1). Simulation reveals the following parameters:
g
) 2.129, g| ) 2.022, AN ) 60 × 10-4 cm-1, A
)
|
unresolved.
Interestingly, the EPR spectrum of [NiIL0 ]+ (Figure 6,
2
bottom) also shows an axial spectrum but with g| > g . No
resolved hyperfine couplings were observed. The simulation
gives g| ) 2.213, g ) 2.115.
Figure 6. X-band EPR spectra of electrochemically generated [NiI-
IIL0 ]3+ (top) and [NiIL0 ]+ (bottom) in frozen CH2Cl2 at 10 K. For
2
2
[NiIIIL0 ]3+, conditions are the following: frequency, 9.4386 GHz;
2
microwave power, 200.7 nW; modulation amplitude, 1.25 mT. For
Remarkably, the situation g| < g holds for both the
[NiIL0 ]+, conditions are the following: frequency, 9.4343 GHz;
(isoelectronic) low-spin d7 species [NiIIIL0 ]3+ and [CoIIL0 ]2+
,
2
2
2
microwave power, 200.7 nW; modulation amplitude, 1.25 mT. The
simulations were obtained using parameters given in the text.
whereas g| > g is observed for each of the d9 species [NiIL0 ]+
2
and [CuIIL0 ]2+ (vide supra).
2
We define the N-M-N axis with the longest M-N distances
2
as the molecular z axis (Table 2). Thus, the dz orbital points in
fore, and in agreement with the crystal structure, we assign this
the direction of two Nimine donors in both [CoIIL0 ]2+ (half-filled
2
process to a metal-centered reduction CoII f CoI as in
II
0
2+
2
2
2
2
dz ) and [Cu L ] (filled dz , half filled dx -y ), as derived from
2
the observed Jahn-Teller distortions in the X-ray structures.
In good agreement, g| < g indicates an unpaired electron in
-e
-e
2+
[CoIIIL0 ]
w
x [CoIIL0 ] x [CoIL0L1]0
w\ \
x [CoIL0 ]+ w-e
2 2
+e +e
3+
2
+e
dz for [NiIIIL0 ]3+ and [CoIIL0 ]2+, whereas g| > g suggests
2
(3)
2
2
an unpaired electron in dx -y for [NiIL0 ]+ and [CuIIL0 ]2+ 34
.
2
2
2
2
The electronic spectra of [CoIIIL0 ]3+, [CoIIL0 ]2+, [CoIL0 ]+,
The observed hyperfine coupling with two equivalent N nuclei
2
2
2
and [CoIL0L1]0 have been recorded and are shown in Figure 5.
of g| in the spectrum of [NiIIIL0 ]3+ supports our idea that the
2
The spectra of [CoIL0 ]+ and [CoIL0L1]0 both exhibit an
2
unpaired electron is localized in the dz orbital.
2
The CV of [CuIIL0 ](PF6)2 displays one fully reversible one-
intense absorption at ∼1000 nm with identical intensity. This
2
transition is absent in the spectra of [CoIIIL0]3+ and [CoIIL0 ]2+
,
electron-transfer wave at E1/2 ) -0.57 V in the potential range
2
ox
and therefore, we assign it to a d-π* MLCT band involving
CoI.30 In addition, the spectrum of [CoIL0L1]0 displays an intense
CT band in the range 500-600 nm, which indicates the presence
of the radical (L1)1-. The intense broad band at 400-550 nm
+1.4 to -2.2 V. An irreversible oxidation occurs at Ep
)
+1.20 V, and a further irreversible reduction is observed at
-1.21 V. Both processes were not further investigated. The one-
electron reduction is most likely metal-centered, as is deduced
from the electronic spectrum shown in Figure 7 of the reduced
in the spectrum of [CoIIIL0 ]3+ may be assigned to a LMCT
2
band due to presence of CoIII (low-spin d6). Note the similarity
form, which does not display the radical bands of (L1)1-
.
of this transition to that of isoelectronic [FeIIL0 ]2+
.
2
-e
The CV of [NiIIL0 ](PF6)2 in CH2Cl2 (0.10 M [N(n-Bu)4]PF6)
2+
[CuIIL0 ]
w\
x [CuIL0 ]+
(5)
2
2
2
+e
displays three reversible one-electron-transfer processes at E1/2
) +1.19, -1.30, -1.82 V (Figure 4) of which the first has
been established by controlled potential coulometry to be a one-
electron oxidation whereas the last two correspond to two
successive one-electron reductions. In conjunction with the EPR
Figure 8 shows the remarkable CV of [MnIIL0 ](PF6)2 in CH2-
2
Cl2, which is the same in CH3CN (0.10 M [N(n-Bu)4]PF6). It
ox
displays at a very positive potential, E1/2 ) +0.97 V, and at
a very negative potential, E1/2red2 ) -1.87 V, two fully reversible
spectrum of [NiIL0 ]+ we assign these processes as in eq 4.
2
one-electron-transfer waves, butsin betweensat medium po-
red
tential an apparently irreversible couple of a reduction (Ep
)
-e
-e
2+
-1.21 V) and an oxidation wave (Epox ) -0.62 V) are observed.
At a scan rate of 200 mV s-1 the two processes are separated
by 0.59 V (!). Amazingly, this CV does not change upon
repetitive scanning, which indicates that both “irreversible”
processes do not lead to chemical decomposition of the complex.
[NiIIIL0 ]
w
x [NiIIL0 ] x [NiIL0L1]0 (4)
w\ \
x [NiIL0 ]+ w-e
2 2
+e +e
3+
2
+e
The electronic spectra of [NiL2]n+ (n ) 3, 2, 1, 0) are shown in
Figure 5 (bottom).
We have recorded the X-band EPR spectra of electrochemi-
The CV of [MnIIIL1 ]PF6 is almost identical except that in the
2
cally generated [NiIIIL0 ]3+ and [NiIL0 ]+ in frozen CH2Cl2
2
2
first cycle the reduction wave at -1.21 V is not observed when
the scan is started between the couple even after allowing for
solutions at 10 K. These are shown in Figure 6. The EPR
spectrum of [NiIIIL0 ]3+ (Figure 6, top) shows an axial signal
2
with g| < g , of which g| is split into a well resolved quintet as
a result of hyperfine coupling with two equivalent nitrogen
(34) Goodman, B. A.; Raynor, J. B. AdV. Inorg. Chem. Radiochem. 1970,
13, 135-361.

2944 Inorganic Chemistry, Vol. 39, No. 13, 2000
de Bruin et al.
Figure 8. Cyclic voltammogram of [MnIIL0 ](PF6)2 in CH3CN (0.10
2
M [N(n-bu)4]PF6) at a glassy carbon electrode; scan rate, 200 mV s-1
.
an equilibration period of 15 s at the starting potential. In the
second and all following cycles after having passed through
the oxidation wave at -0.62 V the missing peak appears at the
same potential as observed for [MnIIL0 ](PF6)2. This is good
2
evidence that the two separated peaks, Eox and Ered, correspond
to the chemically reversible interconversion of [MnIIL0 ]2+ and
2
[MnIIIL1 ]+, as is supported by the spectroelectrochemical
2
investigations (see below).
There are two mechanistically different origins conceivable
for this behavior of the couple [MnIIL0 ]2+/[MnIIIL1 ]+: (i) slow
2
2
electrode kinetics due to a substantial reorganization energy
(large Franck-Condon barrier) of the reduced or oxidized
complex; (ii) the involvement of two reaction intermediates on
going from [MnIIL0 ]2+ to [MnIIIL1 ]+ and vice versa.35 Both
2
2
mechanisms differ in their response to the scan rate; a 10-fold
increase of the scan rate gives rise to an increase in peak
ox
red
separation, |Ep - Ep |, of 120 mV, if mechanism i is
operative, whereas a homogeneous first-order followup reaction
gives rise to a peak separation of only 60 mV per 10-fold
increase of the scan rate. As shown in Figure 9, a 10-fold
Figure 9. Top: cyclic voltammograms of [MnIIL0 ]2+ at two different
2
scan rates and their simulations (Scheme 4). Parameters are given in
ox
red
the text. Bottom: dependence of peak separation Ep - Ep on the
red
increase in scan rate shifts the peak position Ep by 31 mV
scan rate.
ox
toward more negative potentials whereas Ep is shifted by 33
Scheme 4. Mechanistic Proposal for an EC-EC
mV to more positive potentials. The resulting increase in peak
separation of 64 mV strongly supports a mechanism invoking
two short-lived intermediates (EC-EC mechanism).
Mechanism for the Reversible Interconversion [MnIIIL1 ]+
H
2
[MnIIL0 ]2+ + e
2
We have also performed CV experiments of [MnIIL0 ]2+ at
2
scan rates up to 5 × 104 V s-1 in a 10 µm diameter
ultramicroelectrode cell. The two well-separated peaks Epred and
Epox are still observed at these very fast scan rates. This clearly
demonstrates that both putative intermediates must have life-
times shorter than 10-5 s; i.e., their forward rate constants of
decay, kf, is >105 s-1. Otherwise, reversible one-electron-transfer
waves at E1/2 ≈ -1.5 V (starting the scan at 0.0 V) or at about
-0.5 V (starting at -1.7 V) would have been observed. Along
the same lines, the back reactions must be slow (kb < 0.1 s-1),
because otherwise, reversible one-electron-transfer peaks would
have been observed at slow scan rates.
Scheme 4 gives a mechanistic proposal invoking an EC-
EC mechanism that accommodates the electrochemical results.
We suggest a cycle where the high-spin complex [MnIIL0 ]2+
2
(starting material) accepts an electron into one of the L0 ligands
forming intermediate 1, [MnIIL0L1]+, which rapidly rearranges
intramolecularly to the product [MnIIIL1 ]+. Upon one-electron
2
oxidation of this form the second intermediate, 2, namely,
[MnIIIL0L1]2+, is formed, which again rapidly isomerizes
intramolecularly to the product [MnIIL0 ]2+. The two back
2
reactions [MnIIIL1 ]+ f [MnIIL0L1]+ (intermediate 1) and
2
[MnIIL0 ]2+ f [MnIIIL0L1]2+ (intermediate 2) are slow.
2
This EC-EC mechanistic scheme in conjunction with the
above limits for the rate constants kf and kb allows one to
satisfactorily simulate the CV’s at all scan rates of the
(35) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals
and Applications; John Wiley & Sons: New York, 1980; Chapters 6
and 11 and references therein.

Bis(pyridine-2,6-diimine)metal Complexes
Inorganic Chemistry, Vol. 39, No. 13, 2000 2945
interconversion [MnIIL0 ]2+ + e- H [MnIIIL1 ]+. Examples of
2
2
two scan rates are shown in Figure 9.
red
Since the actual peak positions Epox, Ep in a given cyclic
voltammetric experiment are governed independently from each
other by the intrinsic redox potentials E1 and E2 for the two
couples [MnIIL0 ]2+/[MnIIL0L1]+ and [MnIIIL0L1]2+/[MnIIIL1 ]+
2
2
and by the rates of the intramolecular followup reactions, kf
and kf′, there is not only one set of parameters that gives a
satisfactory fit to the available experimental data. Thus, a change
in the reaction rates by a factor of 10 has the same effect as a
shift in E1 or E2 by 30 mV. Since the rate constants kf and kf′
are in the range 105-1013 s-1, the redox potential for the couple
[MnIIL0 ]2+/[MnIIL0L1]+ can be estimated to be in the range
2
Figure 10. Electronic spectra of electrochemically generated [MnL2]n+
(n ) 3, 2, 1, 0) species (CH3CN).
-1.31 to -1.55 V, and at the same time, the redox potential of
the couple [MnIIIL0L1]2+/[MnIIIL1 ]+ is in the range -0.52 to
2
a central divalent metal ion: M ) MnII (d5 high-spin, S ) 5/2);
M ) FeII (d6 low-spin, S ) 0); M ) CoII (d7 low-spin, S ) 1/2),
M ) NiII (d8, S ) 1); M ) ZnII (d10, S ) 0).
-0.28 V. All other parameters are of only minor importance
for the peak positions Epox, Epred. Interestingly, the estimated
redox potential for the [MnIIL0 ]2+/[MnIIL0L1]+ couple at -1.31
2
to -1.55 V is in the vicinity of the couples [ZnL0 ]2+/[ZnL0L1]+
For the monocations [ML2]+ the situation is more diverse:
the zinc complex due to the stability of a ZnII (d10) configuration
2
at -1.39 V and [FeIIL0 ]2+/[FeIIL0L1]+ at -1.31 V. In all these
2
cases an L0 ligand coordinated to a divalent metal ion is
reversibly reduced to a monoanionic ligand radical (L1)1-. We
feel that this observation is strong evidence that the interpretation
given in Scheme 4 is essentially correct.
contains one coordinated L0 and one radical anion (L1)1-
,
[ZnIIL0L1]+ (S ) 1/2), whereas [MnL2]+ is described as
[MnIIIL1 ]+. However, we cannot directly discriminate between
2
the following discriptions on the basis of the experimental
data: (1) electrons unpairedsa low-spin manganese(III) ion (S
) 1) antiferromagnetically coupled to two ligand radicals (L1)1-
The reversible one-electron oxidation at E1/2 ) +0.97 V is
proposed to be metal-centered,
1
(S ) /2), yielding the observed singlet ground state (S ) 0);
[MnIIL0 ]2+ H [MnIIIL0 ]3+ + e
(6)
(2) electrons pairedsa delocalized model in which all six
electrons are paired but delocalized over the manganese t2g and
ligand π* orbitals (π-back-bonding model).
2
2
whereas the reversible one-electron transfer at very negative
potentials, e.g., E1/2 ) -1.87 V, may correspond to a metal-
In principle, model 2 might be regarded as manganese(I) with
strong π-back-bonding to the ligand π* orbitals. However, to
explain the experimental data, this interaction should be (nearly)
covalent, leading to a net charge distribution in which each
ligand π* orbital is filled with one electron. As such, the
description [MnIIIL1 ]+, and not [MnIL0 ]+, is still valid.
centered one-electron reduction of [MnIIIL1 ]+,
2
[MnIIIL1 ]+ + e- H [MnIIL1 ]0
(7)
2
2
2
2
The spectral changes observed upon coulometric one-electron
reduction of [MnIIL0 ]2+ to [MnIIIL1 ]+ at a fixed potential of
We are aware of only two precedents36,37 in which (one-
electron) reduction of a transition metal complex results in a
net increase of the metal oxidation state through charge
redistribution within the ligands, i.e., eqs 8 and 9 where TAAB
represents a tetraaza[16]annulene macrocycle, Cl4SQ is the
tetrachlorosemiquinonate anion, and Cl4Cat is the tetrachloro-
catecholate dianion.
2
2
-1.40 V and subsequent reoxidation at +0.05 V are shown in
Figure S5. Both processes proceed smoothly; three well-defined
isosbestic points are observed at 244, 387, and 499 nm. The
final spectra of these electrochemically generated species are
identical to those of CH3CN solutions of solid materials
[MnIIL0 ](PF6)2 and [MnIIIL1 ]PF6. The inset in Figure S5 shows
2
2
the time dependence of the absorption maxima at 231 and 633
nm during electrolytic reduction and reoxidation. Thus, although
the complicated waveform and the peak separation in the CV
+e
2+
[CoII(TAAB)]
w\
x [CoIII(TAAB2-)]+
(8)
-e
of [MnIIL0 ]2+ apparently indicated otherwise, the [MnIIL0 ]2+
/
and
2
2
[MnIIIL1 ]+ redox couple is fully reversible on the time scale
2
[VIII(Cl4SQ)3]0 w+e
\
x [VV(Cl4Cat)3]-
-e
(9)
of a coulometric experiment (40 s).
Figure 10 shows the spectra of electrochemically generated
[MnIIIL0 ]3+, [MnIIL0 ]2+, and [MnIIIL1 ]+ and of the fully
[FeL2]+ (S ) 1/2) must be described as a low-spin ferrous species
(d6 low-spin; S ) 0) containing one ligand radical (L1)1- (S )
1/2) and one L0: [FeIIL0L1]+. In contrast, [CoL2]+ (S ) 1)
2
2
2
reduced [MnL2]0 species. Upon reduction of [MnIIIL1 ]+ to
2
[MnIIL1 ]0 the characteristic MLCT bands at 633 and 815 nm
2
contains a cobalt(I) ion (d8, S ) 1) and two L0: [CoIL0 ]+.
disappear and a new band between 500 and 600 nm grows in.
2
This band is similar to that of [ZnIIL1 ]0 (Figure 5) and indicates
Similarly, [NiL2]+ (S ) 1/2) contains a NiI (d9, S ) 1/2) ion and
2
the presence of (L1)1- ligands. Thus, this reduction is metal-
two L0: [NiIL0 ]+. [CuL2]+ (S ) 0) contains two L0 ligands
2
centered MnIII f MnII yielding [MnIIL1 ]0.
and a CuI (d10, S ) 0) ion. It is conceivable that the two L0
ligands in this species are only bidentate, giving rise to a CuIN4
polyhedron and two “dangling” uncoordinated imine nitrogens.
2
Discussion
All characterized tricationic species [ML2]3+ contain two
neutral L0 ligands and a trivalent metal ion: M ) MnIII (d4
low-spin (?), S ) 1 (?); M ) FeIII (d5 low-spin, S ) 1/2); M )
(36) Cass, M. E.; Gordon, N. R.; Pierpont, C. G. Inorg. Chem. 1986, 25,
3962.
(37) Takvoryan, N.; Faramery, K.; Katovic, V.; Lovecchio, F. V.; Gore,
E. S.; Anderson, L. B.; Busch, D. H. J. Am. Chem. Soc. 1974, 96,
731.
1
CoIII (d6 low-spin, S ) 0); M ) NiIII (d7 low-spin, S ) /2)
Similarly, all dications also contain two neutral L0 ligands and

2946 Inorganic Chemistry, Vol. 39, No. 13, 2000
de Bruin et al.
Intensity data were collected at 100(2) K. Semiempirical absorption
corrections using the program SADABS38 were performed on data sets
collected with the SMART system. Crystallographic data of the
compounds and diffractometer types used are listed in Table 5. The
Siemens ShelXTL39 software package was used for solution, refinement,
and artwork of the structures. All structures were readily solved and
refined by direct methods and difference Fourier techniques. All non-
hydrogen atoms were refined anisotropically except the minor split
Because of their extraordinarily strong reducing power, the
neutral species [ML2]0 are the least well-characterized complexes
of the series. None has been isolated as a crystalline solid. It is
safe to assume that [ZnL2] actually is [ZnIIL1 ]0, but its electronic
2
ground state is not known. S ) 0 or S ) 1 is possible if
intramolecular anti- or ferromagnetic coupling of the two (L1)1-
radicals prevails. [MnL2] contains most likely two (L1)1- ligands
1
and a low-spin MnII ion, i.e., [MnIIL1 ] (S ) /2), and [FeL2]0
component of a disordered PF6-anion in [MnIIL0 ](PF6)2. A split atom
2
2
probaby contains a low-spin FeII ion and two (L1)1-, i.e.,
model was used here with occupancies of 0.7 and 0.3. All hydrogen
atoms bound to carbon were placed at calculated positions and refined
as riding atoms with isotropic displacement parameters. The hydrogen
[FeIIL1 ]0. [CoL2]0 is probaby [CoIL0L1]0, and [NiL2]0 is then
2
[NiIL0L1] on the basis of the argument that Co0 and Ni0 in an
octahedral N6 environment have not been observed in a
coordination compound and that the +I oxidation state of the
two metal ions is accessible already in the dications.
In summary, we have shown that pyridine-2,6-diimine ligands
are noninnocent.
atoms of a water molecule in [FeIIL0 ](PF6)2‚H2O could not be located
2
from the difference map and were therefore not included in the
refinement.
Synthesis of Complexes. 1. [ZnIIL02](PF6)2. Amounts of 0.30 g (1.01
mmol) of Zn(NO3)2‚6H2O and 0.33 g (0.88 mmol) of 2,6-bis[1-(4-
methoxyphenylimino)ethyl]pyridine (L0) were dissolved in 18 mL of
MeOH, and the mixture was stirred at 20 °C for 30 min. Subsequently,
1.00 g (6.1 mmol) of NH4PF6 was added to the yellow solution, causing
Experimental Section
the precipitation of [ZnIIL0 ](PF6)2 as yellow platelike crystals. The solid
General Procedures. All reactions were performed under an argon
atmosphere using standard Schlenk techniques unless indicated other-
wise. Solvents (p.a.) were deoxygenated by passing a stream of argon
through solutions or by the freeze-pump-thaw method. The ligand
2,6-bis[1-(4-methoxyphenylimino)ethyl]pyridine (L0) was prepared as
described previously.5 All other chemicals are commercially available
and were used without further purification.
Physical Measurements. Electronic spectra of the complexes and
spectra of the spectroelectrochemical investigations were recorded on
a HP 8452A diode array spectrophotometer (range: 221-1100 nm).
Cyclic voltammograms, square-wave voltammograms and coulometric
experiments were performed using an EG&G potentiostat/galvanostat.
Simulations of the cyclic voltammograms were obtained using the
program DigiSim 3.0. (Bioanalytical Systems, Inc., West Lafayette,
IN). Temperature-dependent (2-298 K) magnetization data were
recorded on a SQUID magnetometer (MPMS Quantum design) in an
external magnetic field of 1.0 T. The experimental susceptibility data
were corrected for underlying diamagnetism by the use of tabulated
Pascal’s constants.
2
was collected by filtration, washed three times (10 mL) with ice-cold
MeOH, air-dried, and recrystallized from CH3CN/Et2O. Yield: 436 mg
(90% based on L0 ligand) as yellow crystals. FT-IR (KBr, cm-1): 1630,
1601, 1587 (ν(CdN) + aryl- and pyridyl-ring deformations), 841, 558
(PF6-). ESI-MS, m/z: 955 [M - PF6]+, 405 [M - 2PF6]2+/2. Anal.
Calcd for C46H46F12N6O4P2Zn: C, 50.13; H, 4.21; N, 7.62; Zn, 5.93.
Found: C, 50.00; H, 4.18; N, 7.69, Zn, 5.79.
2. [CoIIL0 ](PF6)2. Amounts of 0.23 g (1.76 mmol) of CoCl2 and
2
0.33 g (0.88 mmol) of 2,6-bis[1-(4-methoxyphenylimino)ethyl]pyridine
(L1) were dissolved in 25 mL of MeOH, and the mixture was stirred at
20 °C for 30 min. Subsequently, 1.00 g (6.1 mmol) of NH4PF6 was
added to the dark-brown solution, causing the precipitation of [CoIIL0 ]-
2
(PF6)2 as a brown solid. The solid was collected by filtration, washed
three times (10 mL) with ice-cold MeOH, air-dried, and recrystallized
from CH3CN/Et2O. Yield: 455 mg (94% based on L0 ligand) as brown-
black crystals. FT-IR (KBr, cm-1): 1603, 1582 (ν(CdN) + py- and
aryl-ring deformations), 841, 558 (PF6-). ESI-MS, m/z: 950 [M -
PF6]+, 402.5 [M - 2PF6]2+/2. Anal. Calcd for C46CoH46F12N6O4P2: C,
50.42; H, 4.23; N, 7.67; Co, 5.38. Found: C, 50.31; H, 4.18; N, 7.75,
Co, 5.45.
IR spectra were recorded on a Perkin-Elmer FT-IR spectrometer
2000. X-band EPR spectra were recorded on a Bruker ESP 300. The
spectra were simulated by iteration of the (an)isotropic g values,
hyperfine coupling constants, and line widths. We thank Dr. F. Neese
(Abteilung Biologie der Universita¨t Konstanz) for a copy of his EPR
simulation program. NMR experiments were carried out on a Bruker
3. [CoIL0 ]PF6. Treatment of a brown solution of 200 mg (0.18
2
mmol) of [CoIIL0 ](PF6)2 in 8 mL of CH3CN with 52 mg (0.27 mmol)
2
of cobaltocene resulted in a dark-green solution. This solution was
stirred for 30 min and subsequently top-layered with Et2O. After a few
1
ARX250 (250 and 63 MHz for H and 13C NMR, respectively) and a
days, a mixture of dark-green/black crystals of [CoIL0 ]PF6 and bright-
2
Bruker WM400 (400 and 100 MHz for 1H and 13C NMR, respectively).
yellow crystals [Cp2Co]PF6 were collected. The yellow [Cp2Co]PF6
1
Internal shift reference for H NMR, with CHD2CN, is δH ) 1.94.
material was removed by washing the mixture with hot water (50 °C)
Internal shift reference for 13C NMR, with CD3CN, is δC ) 118.3.
Abbreviations used are the following: s ) singlet, d ) doublet, t )
triplet. Mo¨ssbauer data were recorded on an alternating constant-
acceleration spectrometer. The minimum experimental line width was
0.24 mm s-1 (full width at half-height). A constant sample temperature
was maintained with an Oxford Instruments Variox or an Oxford
Instruments Mo¨ssbauer-Spectromag 2000 Cryostat. The latter is a split-
pair superconducting magnet system for applied fields up to 8 T in
which the sample temperature can be varied between 1.5 and 250 K
with the field of the sample oriented perpendicular to the γ-beam. The
57Co/Rh source (1.8 GBq) was positioned at room-temperature inside
the gap of the magnet system at zero-field positions. Reported isomer
shifts (δ) are referenced versus iron metal at 300 K.
until the water came off colorless. Yield: 109 mg (64%) of [CoIL0 ]-
2
PF6 as green/black crystals. FT-IR (KBr, cm-1): 1622, 1603, 1592,
1580 (ν(CdN) + py- and aryl-ring deformations), 841, 558 (PF6-).
Anal. Calcd for C46CoH46F6N6O4P: C, 58.11; H, 4.88; N, 8.84; Co,
6.20. Found: C, 57.80; H, 4.86; N, 8.65, Co, 5.47.
4. [MnIIL0 ](PF6)2. Amounts of 1.10 g (4.38 mmol) of Mn(NO3)2‚
2
4H2O and 1.0 g (2.68 mmol) of 2,6-bis[1-(4-methoxyphenylimino)-
ethyl]pyridine (L0) were dissolved in 50 mL of MeOH, and the mixture
was stirred at 20 °C for 30 min. Subsequently, 3.00 g (18.4 mmol) of
NH4PF6 was added to the orange-red solution, causing the precipitation
of [MnIIL0 ](PF6)2 as an orange microcrystalline solid. The solid was
2
collected by filtration, washed three times (10 mL) with ice-cold MeOH,
and air-dried. Yield: 1.40 g (96% based on L0 ligand). FT-IR (KBr,
cm-1): 1624, 1603, 1584 (ν(CdN) + py- and aryl-ring deformations),
841, 558 (PF6-). ESI-MS, m/z: 946 [M - PF6]+. Anal. Calcd for
C46H46F12MnN6O4P2: C, 50.61; H, 4.25; N, 7.70; Mn, 5.03. Found:
C, 49.96; H, 3.79; N, 7.73, Mn, 4.50.
X-ray Structure Determinations. Single crystals of orange-red
[MnIIL0 ](PF6)2, deep-blue [MnIIIL1 ]PF6, purple-black [FeIIL0 ](PF6)2,
2
2
2
brown-black [CoIIL0 ](PF6)2, green-black [CoIL0 ]PF6, brown [NiIIL0 ]-
2
2
2
(PF6)2, brown [CuIIL0 ](PF6)2, and yellow [ZnIIL0 ](PF6)2 were mounted
2
2
5. [MnIIIL1 ]PF6. Treatment of an orange-red solution of 504 mg
in glass capillaries sealed under argon. Graphite monochromated Mo
KR radiation (λ ) 0.710 73 Å) was used throughout. Final cell constants
were obtained from a least-squares fit of a subset of several thousand
strong reflections. Data collection was performed by hemisphere runs
taking frames between 0.3° and 1.0° in ω. The CCD detector system
diffractometers were equipped with a cryogenic nitrogen cold stream.
2
(0.46 mmol) of [MnIIL0 ](PF6)2 in 10 mL of CH3CN with 100 mg (0.53
2
(38) Sheldrick, G. M. Universita¨t Go¨ttingen, Germany, 1994.
(39) ShelXTL, version 5; Siemens Analytical X-ray Instruments, Inc.:
Madison, WI, 1994.

Bis(pyridine-2,6-diimine)metal Complexes
Inorganic Chemistry, Vol. 39, No. 13, 2000 2947
mmol) of cobaltocene resulted in immediate formation of a dark-blue
solution. This solution was stirred for 30 min and subsequently top-
layered with Et2O. After a few days, a mixture of dark-blue crystals of
[MnIIIL12]PF6, bright-yellow crystals of [Cp2Co]PF6, and a minor amount
of bright-red crystals [MnIIL0 ](PF6)2 were collected. The yellow [Cp2-
2
Co]PF6 material was removed by washing the mixture with hot water
(50 °C) until the water came off colorless. The red crystals of [MnIIL0 ]-
2
(PF6)2 were removed by hand under a microscope. Yield: 260 mg (59%)
of [MnIIIL1 ]PF6 as dark-blue/black crystals. FT-IR (KBr, cm-1): 1629,
2
1605, 1582 (ν(CdN) + py- and aryl-ring deformations), 841, 558
(PF6-). Anal. Calcd for C46H46F6MnN6O4P: C, 58.35; H, 4.90; N, 8.88;
Mn, 5.80. Found: C, 58.19; H, 4.81; N, 8.74, Mn, 5.89.
6. [FeIIL0 ](PF6)2. Amounts of 0.20 g (1.57 mmol) of FeCl2 and
2
0.60 g (1.61 mmol) of 2,6-bis[1-(4-methoxyphenylimino)ethyl]pyridine
(L0) were dissolved in 25 mL of MeOH, and the mixture was stirred at
20 °C for 30 min. Subsequently, 1.00 g (6.1 mmol) of NH4PF6 was
added to the deep-purple solution, causing the precipitation of [FeIIL0 ]-
2
(PF6)2 as a purple solid. The solid was collected by filtration, washed
three times (10 mL) with ice-cold MeOH, air-dried, and recrystallized
from CH3CN/Et2O. Yield: 808 mg (92% based on L0 ligand) as purple/
black crystals. FT-IR (KBr, cm-1): 1604, 1582 (ν(CdN) + py- and
aryl-ring deformations), 841, 558 (PF6-). ESI-MS, m/z: 947 [M -
PF6]+,]+, 401 [M - 2PF6]2+/2. Anal. Calcd for C46H46F12FeN6O4P2:
C, 50.56; H, 4.24; N, 7.69; Fe, 5.11. Found: C, 50.42; H, 4.20; N,
7.73, Fe, 4.95.
7. [NiIIL0 ](PF6)2. Amounts of 0.24 g (0.82 mmol) of Ni(NO3)2‚
2
6H2O and 0.51 g (1.37 mmol) of 2,6-bis[1-(4-methoxyphenylimino)-
ethyl]pyridine (L0) were dissolved in 20 mL of MeOH, and the mixture
was stirred at 20 °C for 30 min. Subsequently, 1.00 g (6.1 mmol) of
NH4PF6 was added to the brown solution, causing the precipitation of
[NiIIL0 ](PF6)2 as a light-brown microcrystalline solid. The solid was
2
collected by filtrataion, washed three times (10 mL) with ice-cold
MeOH, air-dried, and recrystallized from CH3CN/Et2O. Yield: 560 mg
(74% based on L0 ligand), as brown-black crystals. FT-IR (KBr, cm-1):
1602, 1583 (ν(CdN) + py- and aryl-ring deformations), 841, 558
(PF6-). ESI-MS, m/z: 949 [M - PF6]+, 402 [M - 2PF6]2+/2. Anal.
Calcd for C46H46F12N6NiO4P2: C, 50.43; H, 4.23; N, 7.67; Ni, 5.36.
Found: C, 50.11; H, 4.15; N, 7.62; Ni 5.35.
8. [CuIIL0 ](PF6)2. Amounts of 0.20 g (0.83 mmol) of Cu(NO3)2‚
2
3H2O and 0.30 g (0.80 mmol) of 2,6-bis[1-(4-methoxyphenylimino)-
ethyl]pyridine (L0) were dissolved in 20 mL of MeOH, and the mixture
was stirred at 20 °C for 30 min. Subsequently, 1.00 g (6.1 mmol) of
NH4PF6 was added to the brown solution, causing the precipitation of
[CuIIL0 ](PF6)2 as a light-brown microcrystalline solid. The solid was
2
collected by filtrataion, washed three times (10 mL) with ice-cold
MeOH, air-dried, and recrystallized from CH3CN/Et2O. Yield: 320 mg
(72% based on L0 ligand), as brown-black crystals. FT-IR (KBr, cm-1):
1619, 1599, 1584(ν(CdN) + py- and aryl-ring deformations), 841,
558 (PF6-). ESI-MS, m/z: 954 [M - PF6]+, 404.5 [M - 2PF6]2+/2.
Anal. Calcd for C46H46F12N6NiO4P2: C, 50.21; H, 4.21; N, 7.64; Cu,
5.77. Found: C, 50.06; H, 4.28; N, 7.67; Cu 5.85.
Acknowledgment. B. de Bruin thanks the Alexander von
Humboldt Foundation for a research fellowship. We thank the
Fonds der Chemischen Industrie for financial support. We thank
Dr. P. Budzelaar (University of Nijmegen) for helpful discus-
sions on the basis of some preliminary DFT calculations.
Supporting Information Available: Temperature-dependent mag-
netic susceptibility data (and fit) for [CuIIL0 ](PF6)2 (Figure S1),
2
Mo¨ssbauer spectrum of [FeIIL0 ](PF6)2 at 80 K (Figure S2), the
2
histogram of Mn-pyridine distances as compiled from the Cambridge
Crystallographic Data Centre (Figure S3), and X-band EPR spectrum
of [FeIIL0L1]+ (Figure S4), tables of crystallographic and structure
refinement data, atom coordinates and Ueq values, bond lengths and
angles, anisotropic thermal parameters, and calculated and refined
positional parameters of hydrogen atoms for complexes [ML2](PF6)2
(M ) Mn, Fe, Co, Ni, Cu, Zn) and [ML2]PF6 (M ) Mn, Co), and
eight X-ray crystallographic files in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC000113J