Reduced N -alkyl substituted bis(imino)pyridine cobalt complexes: Molecular and electronic structures for compounds varying by three oxidation states

Article abstract of DOI:10.1021/ic100717w

The stepwise 1?3 electron reduction of the N-alkyl substituted bis(imino)pyridine cobalt dichloride complexes, (^RAPDI)CoCl ₂, was studied where ^RAPDI = 2,6-(RN - CMe) ₂C₅H₃N, R = C₆H₁₁ (Cy), CHMe₂ (ⁱPr). One electron reduction with either zinc metal or NaBEt₃H furnished the bis(imino)pyridine cobalt monochloride compounds, (^RAPDI)CoCl. X-ray diffraction on the ( ^iPrAPDI)CoCl derivative established a distortion from square planar geometry where the chloride ligand is lifted out of the idealized cobalt-chelate plane. Superconducting Quantum Interference Device (SQUID) magnetometry on both compounds established spin crossover behavior with an S = 1 state being predominant at room temperature. Computational studies, in combination with experimental results, establish that the triplet spin isomer arises from a high spin Co(II) center (S_Co = 3/2) antiferromagnetically coupled to a bis(imino)pyridine chelate radical anion, [PDI]^? (S_PDI = 1/2). At lower temperatures, the Co(II) ion undergoes a spin transition to the low spin form (S_Co = 1/2) and antiferromagnetic coupling gives rise to the observed diamagnetic ground state. Replacing the chloride ligand with a methyl group, namely (^RAPDI)CoCH₃, also yielded distorted compounds, albeit less pronounced, that are diamagnetic at room temperature. Two electron reduction of the (^RAPDI)CoCl₂ derivatives with excess 0.5% sodium amalgam or 2 equiv of NaBEt₃H furnished the bis(chelate)cobalt complexes, (^RAPDI)₂Co, while three electron reduction with 3 equiv of sodium naphthalenide yielded the cobalt dinitrogen anions, [Na(solv)₃][(^RAPDI)CoN₂] (solv = THF, Et₂O). Both bis(chelate) compounds were crystallographically characterized and determined to have S = 3/2 ground states by SQUID magnetometry and electron paramagnetic resonance (EPR) spectroscopy. Computational studies, in combination with metrical parameters determined from X-ray diffraction, establish a high spin (S_Co = 3/2) cobalt(II) center with two bis(imino)pyridine chelate radical anions. Antiferromagnetic coupling between the two chelate centered radicals is mediated by a doubly occupied t_2g cobalt orbital and gives rise to the observed overall quartet ground state.

Full text of DOI:10.1021/ic100717w

6110 Inorg. Chem. 2010, 49, 6110–6123
DOI: 10.1021/ic100717w
Reduced N-Alkyl Substituted Bis(imino)pyridine Cobalt Complexes: Molecular and
Electronic Structures for Compounds Varying by Three Oxidation States
†
‡
‡
†
†
€
Amanda C. Bowman, Carsten Milsmann, Eckhard Bill, Emil Lobkovsky, Thomas Weyhermuller,
Karl Wieghardt,*,‡ and Paul J. Chirik*,†
†Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853,
and ‡Max-Planck Institute for Bioinorganic Chemistry, Stiftstrasse 34-36, D-45470 Mu€lheim an der Ruhr, Germany
Received April 14, 2010
The stepwise 1-3 electron reduction of the N-alkyl substituted bis(imino)pyridine cobalt dichloride complexes,
R
(RAPDI)CoCl2, was studied where APDI = 2,6-(RNdCMe)2C5H3N, R = C6H11 (Cy), CHMe2 (iPr). One electron
reduction with either zinc metal or NaBEt3H furnished the bis(imino)pyridine cobalt monochloride compounds,
(RAPDI)CoCl. X-ray diffraction on the (iPrAPDI)CoCl derivative established a distortion from square planar geometry
where the chloride ligand is lifted out of the idealized cobalt-chelate plane. Superconducting Quantum Interference
Device (SQUID) magnetometry on both compounds established spin crossover behavior with an S = 1 state being
predominant at room temperature. Computational studies, in combination with experimental results, establish that the
triplet spin isomer arises from a high spin Co(II) center (SCo = 3/2) antiferromagnetically coupled to a bis-
(imino)pyridine chelate radical anion, [PDI]- (SPDI = 1/2). At lower temperatures, the Co(II) ion undergoes a spin
transition to the low spin form (SCo = 1/2) and antiferromagnetic coupling gives rise to the observed diamagnetic
ground state. Replacing the chloride ligand with a methyl group, namely (RAPDI)CoCH3, also yielded distorted
compounds, albeit less pronounced, that are diamagnetic at room temperature. Two electron reduction of the
(RAPDI)CoCl2 derivatives with excess 0.5% sodium amalgam or 2 equiv of NaBEt3H furnished the bis(chelate)cobalt
complexes, (RAPDI)2Co, while three electron reduction with 3 equiv of sodium naphthalenide yielded the cobalt
dinitrogen anions, [Na(solv)3][(RAPDI)CoN2] (solv = THF, Et2O). Both bis(chelate) compounds were crystal-
lographically characterized and determined to have S = 3/2 ground states by SQUID magnetometry and electron
paramagnetic resonance (EPR) spectroscopy. Computational studies, in combination with metrical parameters
determined from X-ray diffraction, establish a high spin (SCo = 3/2) cobalt(II) center with two bis(imino)pyridine chelate
radical anions. Antiferromagnetic coupling between the two chelate centered radicals is mediated by a doubly
occupied t2g cobalt orbital and gives rise to the observed overall quartet ground state.
Introduction
and the high spin forms of the d7 ion.2,3 Magnetic data on these
bis(chelate) cobalt dications also proved interesting. Depend-
ing on the specific imine substituents and counterions, mag-
netic susceptibilities consistent with the spin only values
Bis(imino)pyridine ligands, 2,6-(R1NdCR2)2C5H3N (R1 =
alkyl, aryl, amino; R2 = H, Me), have a long-standing history
in the classical Werner-type coordination chemistry of cobalt.
Because of the early successes of terpyridine ligands, attention
was devoted to preparing easily accessed and modular analo-
gues that preserved the tridendate, meridional “N3” coordina-
tion motif.1 The first examples of bis(imino)pyridine cobalt
compounds were six-coordinate, dicationic bis(chelate) deri-
vatives of cobalt(II), for example, [(PDI)2Co]X2 (X = Cl, I,
ClO4). These molecules proved to be early experimental
examples of the success of ligand field theory as the values
of ΔOct and pairing energies were measured for both the low
4
2
expected for the T1 and E extremes were observed as well
as examples with “anomalous magnetic behavior” where
temperature dependent μeff values between the pure high and
low spin limits were measured.3,4 Initially, a simple two-state
model was proposed where an equilibrium between low and
high spin forms was used to account for the observed magnetic
susceptibilities.3 More detailed studies later showed that such
an explanation was likely oversimplified as mixing of the 4T1
2
and E states near the crossing point produces a variety of
thermally accessible states where a Boltzman distribution would
account for the observed magnetic moments.4 In addition to
magnetic studies, the hexacoordinate bis(chelate) cobalt(II)
*To whom correspondence should be addressed. E-mail: pc92@
cornell.edu (P.J.C.).
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Chem. 1966, 5, 1167.
r
pubs.acs.org/IC
Published on Web 06/11/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 13, 2010 6111
Figure 1. Synthesis and electronic structures of aryl-substituted bis(imino)pyridine cobalt chloride and alkyl compounds.
dications were some of the first examples of Co(II) coordina-
tion compounds to be studied by both solid state and solution
electron paramagnetic resonance (EPR) spectroscopy.5
Following these seminal studies, manipulation of the ligand
architecture was explored and new coordination numbers and
structural types of the resulting cobalt compounds were
obtained. Introduction of larger N-imine substituents or
replacing the in-plane backbone hydrogens with methyl
groups resulted in more sterically protected ligands that
allowed isolation of so-called “1:1” bis(imino)pyridine cobalt-
(II) compounds.6-8 In each case examined, high spin Co(II),
d7 centers (SCo=3/2) were observed. Brookhart and Gibson’s
independent use of bis(imino)pyridines with sterically de-
manding N-aryl substituents, ArPDI where ArPDI = 2,6-(2,6-
R2-C6H3NdCMe)2C5H3N, R=iPr, Et, Me, H, etc., produced
a class of cobalt(II) dihalide complexes that when activated
with excess methylaluminoxane (MAO) were extremely active
catalysts for olefin polymerization.9-12 These landmark dis-
coveries renewed interest in the fundamental coordination
chemistry and electronic structure of bis(imino)pyridine cobalt
compounds, particularly in the context of defining and under-
standing the propagating species.
(Figure 1), as diamagnetic solids.16-18 Analysis of these com-
pounds by 1H NMR spectroscopy and density functional
theory (DFT) calculations established that these apparent
Co(I) derivatives were actually composed of low-spin Co(II)
(SCo=1/2) metal centers antiferromagnetically coupled to bis-
(imino)pyridine chelate radical anions (SPDI = 1/2).19 The
computational studies also established low-lying, energetically
accessible triplet excited states that were likely the origin of the
anomalous upfield 1H NMR chemical shifts observed for the
in-plane imine methyl backbones.19,20 These studies, along with
the electrochemical work of Toma21,22 and the preparative and
spectroscopic experiments of Wieghardt and co-workers,23,24
provided the foundation for establishing the redox-activity of
the bis(imino)pyridine chelate25-27 and likely the importance
of this phenomenon in base metal catalysis.28,29 Notably, the
reduction of (iPrPDI)CoCl2 to (iPrPDI)CoCl occurs at the
ligand, preserving the Co(II) oxidation state, although a change
from high spin to low spin accompanies chelate reduction
because of the increased field strength of the chelate radical
anion (Figure 1).
Given the rich chemistry associated with the bis(imino)-
pyridine iron dinitrogen complex, (iPrPDI)Fe(N2)2,30 our
collaborative effort recently explored the synthesis and spec-
troscopy of related cobalt dinitrogen complexes. Reduction
of the Brookhart-Gibson cobalt dihalide precursor, (iPrPDI)-
CoCl2, with excess 0.5% sodium amalgam under an N2
atmosphere yielded the neutral, square planar cobalt dinitro-
gen compound, (iPrPDI)CoN2.31 Performing the reduction of
Treatment of one of the prototypical olefin polymerization
precatalysts, (iPrPDI)CoCl2 (iPrPDI = 2,6-(2,6-iPr2-C6H3Nd
CMe)2C5H3N) with alkyl lithiums,13 Grignard reagents,14,15
or zinc metal14 resulted in one electron reduction to yield the
square planar, diamagnetic cobalt monochloride, (iPrPDI)CoCl.
Subsequent alkylation of this species furnished a family of
the desired cobalt alkyls, (iPrPDI)CoR (R = Me, Et, nPr, nBu)
€
€
(18) Kleigrewe, N.; Steffen, W.; Blomker, T.; Kehr; Frohlich, R.; Wibbeling,
B.; Erker, G.; Wasilke, J.-C.; Wu, G.; Bazan, G. C. J. Am. Chem. Soc. 2005, 127,
13955.
(5) Schmidt, J. G.; Brey, W. S.; Stoufer, R. C. Inorg. Chem. 1967, 6, 268.
(6) Curry, J. D.; Robinson, M. A.; Busch, D. H. Inorg. Chem. 1967, 6, 1570.
(7) Sacconi, L.; Morassi, R.; Midollini, S. J. Chem. Soc., A 1968, 1510.
(8) Davis, R. N.; Tanski, J. M.; Adrian, J. C.; Tyler, L. A. Inorg. Chim.
Acta 2007, 360, 3061.
(9) Bianchini, C.; Giambastiani, G.; Rios, I. G.; Mantovani, G.; Meli, A.;
Segarra, A. M. Coord. Chem. Rev. 2006, 250, 1391.
(10) Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B. S.; Maddox, S. J.;
Solan, G. A.; White, A. J. P.; Williams, D. J. Chem. Commun. 1998, 849.
(11) (a) Small, B. M.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 7143.
(b) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc. 1998, 120,
4049.
(19) Knijnenburg, Q.; Hetterscheid, D.; Kooistra, T. M.; Budzelaar,
P. H. M. Eur. J. Inorg. Chem. 2004, 1204.
(20) Humphries, M. J.; Tellmann, K. P.; Gibson, V. C.; White, A. J. P.;
Williams, D. J. Organometallics 2005, 24, 2039–2050.
(21) Kuwabara, I. H.; Comninos, F. C. M.; Pardini, V. L.; Viertler, H.;
Toma, H. E. Electrochim. Acta 1994, 39, 2401.
(22) Toma, H. E.; Chavez-Gil, T. E. Inorg. Chim. Acta 1997, 257, 197.
€
(23) de Bruin, B.; Bill, E.; Bothe, E.; Weyermuller, T.; Wieghardt, K.
Inorg. Chem. 2000, 39, 2936.
(24) Budzelaar, P. H. M.; de Bruin, B.; Gal, A. W.; Wieghardt, K.;
van Lenthe, J. H. Inorg. Chem. 2001, 40, 4649.
(25) Chaudhuri, P.; Wieghardt, K. Prog. Inorg. Chem. 2001, 50, 151.
(26) (a) Knijnenburg, Q.; Gambarotta, S.; Budzelaar, P. H. M. Dalton
Trans. 2006, 5442. (b) Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M. W.;
Lobkovsky, E.; Neese, F.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2006,
128, 13901.
(27) Butin, K. P.; Beloglazkina, E. K.; Zyk, N. V. Russ. Chem. Rev. 2005,
74, 531.
(28) Bouwkamp, M. W.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J.
J. Am. Chem. Soc. 2006, 128, 13340.
(29) Sylvester, K. T.; Chirik, P. J. J. Am. Chem. Soc. 2009, 131, 8772.
(30) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004, 126,
13794.
(12) Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.; Kimberely, B. S.;
Maddox, P. J.; Mastroianni, S.; McTavish, S. J.; Redshaw, C.; Solan, G. A.;
€
Stromberg, S.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1999, 121,
8728.
(13) Kooistra, T. M.; Knijnenburg, Q.; Smits, J. M. M.; Horton, A. D.;
Budzelaar, P. H. M.; Gal, A. W. Angew. Chem., Int. Ed. 2001, 40, 4719.
(14) Gibson, V. C.; Humphries, M. J.; Tellmann, K. P.; Wass, D. F.;
White, A. J. P.; Williams, D. J. Chem. Commun. 2001, 2252.
€
€
(15) Steffen, W.; Blomker, T.; Kleigrewe, N.; Kehr, G.; Frohlich, R.;
Erker, G. Chem. Commun. 2004, 1188.
(16) Gibson, V. C.; Tellmann, K. P.; Humphries, M. J.; Wass, D. F.
Chem. Commun. 2002, 2316.
(17) Tellmann, K. P.; Humphries, M. J.; Rzepa, H. S.; Gibson, V. C.
Organometallics 2004, 23, 5503.
(31) Bowman, A. C.; Milsmann, C.; Atienza, C. C. H.; Lobkovsky, E.;
Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2010, 132, 1676.

6112 Inorganic Chemistry, Vol. 49, No. 13, 2010
Bowman et al.
Figure 2. Electronic structure summary for bis(imino)pyridine cobalt dinitrogen complexes over three formal oxidation states.
(
iPrPDI)CoCl2 with 3 equiv of sodium naphthalenide furn-
cobalt dichloride complexes, (CyAPDI)CoCl28 and (iPrAPDI)-
CoCl2, to form the corresponding cobalt monochloride
compounds was initially explored. Stirring a toluene solu-
tion of either cobalt dichloride with 1 equiv of NaBEt3H in
toluene followed by filtration and recrystallization from
pentane yielded (CyAPDI)CoCl and (iPrAPDI)CoCl as red-
brown (63%) and mauve (60%) crystalline solids, respec-
tively (eq 1). Both (CyAPDI)CoCl and (iPrAPDI)CoCl were
also prepared by stirring the corresponding cobalt dichlor-
ide complexes with excess zinc metal in toluene at 45 ꢀC.
ished the three-electron reduced cobalt dinitrogen anion,
[(iPrPDI)CoN2]-. These two compounds, in conjunction with
the previously reported cobalt dinitrogen cation, [(iPrPDI)-
CoN2]þ,32 allow evaluation of the electronic structure of the
same core structure over three different formal oxidation states
(Figure 2). Our combined synthetic, spectroscopic and com-
putational studies established that the low-spin cobalt(I), d8
electronic configuration is preserved throughout the series and
that the redox-events are confined to the bis(imino)pyridine
chelate. For example, in the cation, [(iPrPDI)CoN2]þ, the che-
late is neutral while at the opposite extreme, the dinitrogen
anion, [(iPrPDI)CoN2]-, has the bis(imino)pyridine as a closed-
shell dianion.
Because of the successes of bis(imino)pyridine ligands in
supporting base metal polymerization catalysts,9 the major-
ity of recent studies on reduced versions of these compounds
have focused on compounds with N-aryl substituted chelates.
In iron chemistry, our laboratories recently explored the two
electron reduction chemistry of the N-alkyl substituted bis-
(imino)pyridine iron dibromides, (RAPDI)FeBr2 (RAPDI =
2,6-(RNdCMe)2C5H3N; R=Cy, iPr, cis-myrtanyl).33 In each
case, the neutral bis(chelate) iron complex, (RAPDI)2Fe, was
isolated. Magnetic measurements, metrical parameters from
At this point, it is useful to comment on the representa-
tion of the structures in the equations, figures, and
schemes. For the results of the synthetic work, the bis-
(imino)pyridine chelate will be drawn in its innocent,
neutral form. Once the spectroscopic, magnetic, crystal-
lographic, and computational studies are presented and
the oxidation states of both the chelate and metal defined
will the molecules be drawn with a more accurate depic-
tion of the redox activity of the bis(imino)pyridine.
In contrast to the previously reported diamagnetic aryl-
substituted bis(imino)pyridine chloride complexes,14,15
both (CyAPDI)CoCl and (iPrAPDI)CoCl are paramag-
netic at 23 ꢀC. Each compound exhibits the same number
of paramagnetically broadened and shifted 1H NMR
resonances expected for C2v symmetric molecules. The
chemical shift dispersion for (CyAPDI)CoCl is larger than
that for (iPrAPDI)CoCl. As is typical for compounds of
this type, the hydrogens on the chelate that are in the
plane of the metal experience the largest deviations from
their diamagnetic reference values. For (CyAPDI)CoCl in
benzene-d6 at 23 ꢀC, the imine methyl group was located
at -22.8 ppm while the meta and para-pyridine hydro-
gens were identified at 12.1 and 66.3 ppm, respectively.
For (iPrAPDI)CoCl, these same peaks were located at
-12.6, 9.5, and 41.6 ppm.
€
X-ray structures, Mossbauer spectroscopy, and open-shell,
broken-symmetry DFT calculations established that the ex-
perimentally observed S = 1 ground states arise from antiferro-
magnetic coupling of the high spin ferrous (SFe=2) centers with
two bis(imino)pyridine radical anions (SPDI = 1/2). In cobalt
chemistry, Budzelaar and co-workers reported studies of the
electronic structure of square planar bis(imino)pyridine cobalt
halide, alkyl, and hydride compounds, (R1APDI)CoCH2SiMe3
(R1 = n-C6H13, n-C18H37), with N-alkyl substituted chelates.19
No differences in electronic or molecular structure were noted
between these compounds and the more well-studied N-arylated
derivatives. Here we describe synthetic, spectroscopic and
computational studies into the one, two, and three-electron
reduction products of N-alkyl substituted bis(imino)pyridine
cobalt dichlorides, (RAPDI)CoCl2 (R = Cy, iPr) and discover
magnetic behavior, including spin crossover phenomena, struc-
tural distortions, and electronic structures distinct from the
previously studied N-aryl substituted compounds.
Results and Discussion
The solid state structure of one example, (iPrAPDI)-
CoCl, was determined by X-ray diffraction at 173 K. A
representation of the solid state structure is presented in
Figure 3, and selected bond distances and angles are
presented in Table 1. In contrast to rigorously square
planar aryl-substituted bis(imino)pyridine cobalt chlor-
ide compounds such as (iPrPDI)CoCl,14 the overall
Synthesis and Electronic Structure of N-Alkyl Bis(imino)-
pyridine Cobalt Chloride and Methyl Complexes. One elec-
tron reduction of the N-alkyl substituted bis(imino)pyridine
(32) Gibson, V. C.; Humphries, M. J.; Tellmann, K. P.; Wass, D. F.;
White, A. J. P.; Williams, D. J. Chem. Commun. 2001, 2252.
(33) Wile, B. M.;Trovitch, R. J.;Bart, S. C.; Tondreau, A. M.;Lobkovsky, E.;
Milsmann, C.; Bill, E.; Wieghardt, K.; Chirik, P. J. Inorg. Chem. 2009, 48, 4190.

Article
Inorganic Chemistry, Vol. 49, No. 13, 2010 6113
Figure 4. Variable temperature SQUID magnetization data for (CyAPDI)-
CoCl (red) and (iPrAPDI)CoCl (blue). Data are corrected for underlying
diamagnetism.
Figure 3. Molecular structure of (iPrAPDI)CoCl at 30% probability
ellipsoids. Hydrogen atoms omitted for clarity.
Table 1. Selected Bond Distances (A) and Angles (deg) for (iPrAPDI)CoCl,
˚
(
iPrPDI)CoCl,14 iPrAPDI)CoMe, and (iPrPDI)CoMe.14
(
(
iPrAPDI)-
CoCl
(
iPrPDI)-
CoCl
(
iPrAPDI)-
CoMe
(
iPrPDI)-
CoMe
Co-Nimine
1.9334(15)
1.9203(16)
1.7870(16)
2.2188(6)
1.314(3)
1.321(3)
1.435(3)
1.438(3)
1.371(2)
1.371(2)
1.916(3)
1.912(3)
1.797(3)
2.1807(10)
1.322(5)
1.317(5)
1.444(5)
1.435(5)
1.373(5)
1.369(6)
1.9103(16)
1.9228(16)
1.8072(16)
1.967(2)
1.326(2)
1.326(2)
1.440(3)
1.434(3)
1.373(2)
1.383(2)
1.907(3)
1.903(3)
1.834(3)
1.960(4)
1.335(4)
1.329(4)
1.443(5)
1.434(6)
1.366(4)
1.365(4)
Co-Npyridine
Co-X
Nimine-Cimine
Cimine-Cipso
Cipso-Npyridine
geometry of (iPrAPDI)CoCl is significantly distorted with
an Npyr-Co-Cl bond angle of 150.73(5)ꢀ. The isopropyl
imine substituents are oriented essentially orthogonal to
the cobalt-chelate plane with the methine hydrogens
directed toward the in-plane backbone methyl groups.
Figure 5. Molecular structure of (iPrAPDI)CoMe at 30% probability
ellipsoids. Hydrogen atoms omitted for clarity.
The magnetic data for (iPrAPDI)CoCl exhibit the same
general features as (CyAPDI)CoCl but are overall less
pronounced. The effective magnetic moment reaches a
maximum of 1.0 μB at 300 K, the highest temperature
where data were recorded. The lower maximum magnetic
moment for this compound is consistent with the NMR
spectroscopic data where a narrower chemical shift dis-
persion was observed for (iPrAPDI)CoCl as compared to
˚
The Co-Cl bond length of 2.2188(6) A is slightly elongated
from the value of 2.1807(10) A found in (iPrPDI)CoCl.14
˚
The metrical parameters for the bis(imino)pyridine chelate
are diagnostic of one electron reduction (Table 1).26 For
example, the Cimine-Nimine distances of 1.321(3) and
˚
1.314(3) A are elongated from those typically associated
with the neutral ligand,26 while the Cimine-Cipso bonds are
contracted to 1.435(3) and 1.438(3) A.
˚
(
CyAPDI)CoCl. At low temperatures, an effective mag-
The observation of paramagnetic ground states for
both (CyAPDI)CoCl and (iPrAPDI)CoCl prompted more
detailed studies of the solid state magnetic susceptibility
by variable temperature Superconducting Quantum In-
terference Device (SQUID) magnetometry (Figure 4).
Data were collected on three independently prepared,
analytically pure samples, and the individual runs were
indistinguishable. The data for (CyAPDI)CoCl exhibit a
steep decrease in effective magnetic moment as the tem-
perature is lowered, consistent with spin crossover beha-
vior. A maximum of 2.67 μB was observed at 300 K,
approaching the spin only value for two unpaired elec-
trons. Accordingly, a solution magnetic moment of
2.5(2) μB was measured in benzene-d6 at 23 ꢀC. At temp-
eratures below 50 K, the moment drops to 0.23 μB and
demonstrates greater population of the singlet state.
netic moment of 0.21 μB was observed, consistent with
population of a singlet state and the value observed for
(
CyAPDI)CoCl.
The observation of temperature dependent magnetic
behavior in both compounds prompted a series of vari-
able temperature 1H NMR experiments. Warming a
benzene-d6 solution of (iPrAPDI)CoCl to 65 ꢀC resulted
in a broader chemical shift range for the in-plane hydro-
gen resonances. At this temperature, the chemical shifts
of both the imine methyl group (δ = -16.21 ppm) and the
meta- (δ = 10.83 ppm) and para-pyridine (δ = 48.63
ppm) approach the 23 ꢀC values of (CyAPDI)CoCl. Cool-
ing a toluene-d8 sample of the compound to -60 ꢀC
resulted in a contraction of the chemical shift range
although the resonances still appeared outside the win-
dow typically observed for diamagnetic compounds such

6114 Inorganic Chemistry, Vol. 49, No. 13, 2010
Bowman et al.
Figure 6. Molecular structures of (iPrAPDI)2Co (left) and (CyAPDI)2Co (right) at 30% probability ellipsoids. For (CyAPDI)2Co, the cyclohexyl sub-
stituents are omitted for clarity. Hydrogens omitted for clarity in both structures.
as (iPrPDI)CoCl. For example, the imine methyl resonance
was observed at -6.24 ppm, while the meta- and para-
pyridine resonances were shifted upfield to 7.81 ppm and
26.10 ppm, respectively.
indistinguishable from the value of 1.960(4) A reported
for (iPrPDI)CoMe.20 As with (iPrAPDI)CoCl, elongated
˚
˚
˚
Cimine-Nimine bonds of 1.326(2) A and 1.326(2) A and
˚
˚
contractedCimine-Cipso bonds of 1.440(3) A and1.434(3) A
were observed, consistent with one electron chelate
reduction.26 It is important to note that the distortions in
the solid state structures of (iPrAPDI)CoMeand (iPrAPDI)-
CoCl are similar; however, the electronic structures are
different. The alkyl, (iPrAPDI)CoMe, is diamagnetic while
The spin crossover behavior observed with the (RAPDI)-
CoCl compounds inspired preparation of the corresponding
alkyl derivatives to determine the effect of a stronger field,
purely σ-donating ligand on the magnetic behavior and
overall electronic structure of the compound. Addition of
a slight excess of a 1.6 M diethyl ether solution of MeLi to
a toluene solution of either (RAPDI)CoCl compound at
-95 ꢀC followed by warming to room temperature, filtra-
tion, and recrystallization from pentane at -35 ꢀC yielded
the desired cobalt methyl complexes, (CyAPDI)CoMe and
(
iPrAPDI)CoCl is paramagnetic at room temperature and
exhibits spin crossover behavior. The structural similarity
between the two compounds demonstrates that the differ-
ence in magnetism and hence electronic structure is not a
result of the distortion but rather because of the inherent
field strength differences between methyl and chloride.
Two Electron Reduction. Synthesis and Spectroscopy
of (RAPDI)2Co Compounds. Having established distinct
electronic structures for the N-arylated and N-alkylated one
electron reduction products, (ArPDI)CoCl and (RAPDI)-
CoCl, the two electron reduction chemistry of (RAPDI)CoCl2
(
yields, respectively (eq 2).
iPrAPDI)CoMe, as dark red-brown solids in 52 and 81%
i
was explored. Stirring (RAPDI)CoCl2 (R = Cy, Pr) with
excess 0.5% Na(Hg) under a dinitrogen atmosphere furnished
the bis(chelate) cobalt compounds, (RAPDI)2Co (R = Cy,
iPr), in poor (<40%) yield. With the identity of the product in
hand, the synthetic procedure was modified such that an
additional equivalent of free bis(imino)pyridine ligand was
added to the reduction protocol. This modification as well as
performing the reaction under vacuum increased the yield of
the bis(chelate) cobalt products to 63% for (CyAPDI)2Co and
to 70% for (iPrAPDI)2Co (eq 3).
The benzene-d6 1H NMR spectra of the (RAPDI)CoMe
compounds are diagnostic of diamagnetic molecules. Similar
to the previously reported N-arylated compound, (iPrPDI)-
CoMe,14,19 the in-plane imine methyl groups were observed
upfield at 0.13 (R = Cy) and 0.14 (R = iPr) ppm, and the
para-pyridine resonances were located at 9.15 (R = Cy) and
9.41 (R = iPr) ppm. Similar chemical shifts were noted by
Budzelaar and co-workers in (R1APDI)CoCH2SiMe3 (R1 =
n-C6H13, n-C18H37) and were accounted for by population of
a low-lying triplet excited state.19
The solid state structure of (iPrAPDI)CoMe was deter-
mined by X-ray diffraction, and a representation of the
molecule is presented in Figure 5. Selected bond distances
and angles are reported in Table 1. As was observed in
(
iPrAPDI)CoCl, the overall geometry of the cobalt methyl
complex is best described as distorted square planar with
the alkyl group lifted out of the idealized metal-chelate
plane. The Npyr-Co-CMe angle of 147.11(8)ꢀ is slightly
smaller than the value of 150.73(5)ꢀ found in the chloro
The solid state structures of both compounds were
determined by X-ray diffraction and are presented in
Figure 6 with selected bond distances and angles reported
in Table 2. Also reported in Table 2 are the corresponding
˚
derivative. The Co-CMe bond length of 1.967(2) A is

Article
Inorganic Chemistry, Vol. 49, No. 13, 2010 6115
Table 2. Selected Bond Distances (A) and Angles (deg) for (iPrAPDI)2Co, (CyAPDI)2Co, [(CyAPDI)2Co](BF4)2,8 iPrAPDI)2Fe,33 and (CyAPDI)2Fe33 a
(
˚
(
iPrAPDI)2Co
(
CyAPDI)2Co
[(CyAPDI)2Co](BF4)2
(
iPrAPDI)2Fe
(
CyAPDI)2Fe
M(1)-N(1)
M(1)-N(2)
M(1)-N(3)
M(1)-N(4)
M(1)-N(5)
M(1)-N(6)
2.2683(9)
1.9515(9)
2.2408(10)
2.2484(10)
2.0064(9)
2.2236(9)
2.286(2)
2.221(2)
2.0079(18)
2.273(2)
2.219(2)
2.0084(19)
2.250(2)
2.252(2)
2.243(2)
2.005(2)
2.221(2)
2.209(2)
2.005(2)
2.277(3)
1.9520(19)
2.2197(19)
2.2442(19)
1.9704(18)
2.2320(18)
2.0180(17)
2.2506(19)
2.259(2)
2.0236(18)
2.1846(18)
N(1)-C(2)
N(2)-C(3)
N(2)-C(7)
N(3)-C(8)
C(2)-C(3)
C(7)-C(8)
N(4)-C(23)
N(5)-C(24)
N(5)-C(28)
N(6)-C(29)
C(23)-C(24)
C(28)-C(29)
1.2985(13)
1.3756(14)
1.3719(13)
1.3029(14)
1.4618(16)
1.4592(17)
1.3175(13)
1.3810(16)
1.3725(15)
1.3044(14)
1.4415(17)
1.4518(19)
1.285(3)
1.369(3)
1.381(3)
1.299(3)
1.458(4)
1.451(4)
1.297(3)
1.377(3)
1.384(3)
1.303(3)
1.451(3)
1.450(3)
1.285(3)
1.341(3)
1.343(3)
1.280(3)
1.496(3)
1.498(3)
1.279(3)
1.339(3)
1.341(3)
1.283(3)
1.492(4)
1.484(4)
1.316(3)
1.399(3)
1.362(3)
1.299(3)
1.431(3)
1.457(4)
1.312(3)
1.387(3)
1.358(3)
1.304(3)
1.426(3)
1.450(4)
1.293(4)
1.384(4)
1.375(3)
1.310(3)
1.437(4)
1.442(4)
1.303(3)
1.380(4)
1.386(4)
1.282(4)
1.443(4)
1.445(5)
N(1)-M(1)-N(3)
N(4)-M(1)-N(6)
N(1)-M(1)-N(4)
N(2)-M(1)-N(5)
152.51(3)
150.41(3)
93.78(3)
152.21(7)
151.21(7)
93.70(8)
152.56(7)
152.05(8)
93.27(8)
149.37(7)
149.06(7)
94.99(7)
149.64(8)
149.25(9)
93.01(8)
177.43(4)
177.13(8)
176.24(8)
176.90(8)
176.46(10)
a For purposes of the table, the numbering scheme for (CyAPDI)2Co is used for (iPrAPDI)2Co and (iPrAPDI)2Fe.
distances and angles for the related cobalt dication,
[(CyAPDI)2Co](BF4)2,8 and the neutral iron bis(chelate)
compounds, (CyAPDI)2Fe and (iPrAPDI)2Fe. For both
cobalt compounds, an idealized D2d geometry is observed
with essentially orthogonal bis(imino)pyridine chelates.
For (CyAPDI)2Co, the Nimine-Co-Nimine angles are
152.21(7)ꢀ and 151.74(7)ꢀ and the Npyr-Co-Npyr angle is
177.13(8)ꢀ, while for (iPrAPDI)2Co the Nimine-Co-Nimine
angles are 152.51(3)ꢀ and 151.21(7)ꢀ and the Npyr-Co-Npyr
angle is 177.43(4)ꢀ. The deviation of these angles from the
ideal 180ꢀ is likely caused by the constraints of the bis-
(imino)pyridine chelate.
Examination of the metrical parameters of (CyAPDI)2Co,
presented in Table 2, establishes Cimine-Cipso bonds lengths
˚
that are contracted to ∼1.45 A compared to the average dis-
˚
tance of ∼1.49 A observed for the neutral ligand bis(chelate)
cobalt dication, [(CyAPDI)2Co](BF4)2.8 The Cimine-Cipso
bonds lengths of (iPrAPDI)2Co are also contracted to an
average distance of 1.45 A. The Cimine-Nimine bond lengths
Figure 7. Variable temperature SQUID magnetization data for (iPrAPDI)2-
Co at 1 T (red) and 0.1 T (black).
˚
for both neutral bis(ligand) complexes are slightly elongated
compared to [(CyAPDI)2Co](BF4)2, as would also be ex-
pected with ligand reduction.34 Overall, the Cimine-Cipso and
Cimine-Nimine bond lengths observed for (CyAPDI)2Co and
variable temperature SQUID magnetometry (Figure 7). The
effective magnetic moment, μeff, of (iPrAPDI)2Co is nearly
temperature independent in the temperature range from 150
to 300 K. The wide plateau at 4.1 μB close to the spin-only
value of 3.9 μB indicates the presence of a well-defined S =
3/2 ground state. Below 150 K the magnetic moment
increases unexpectedly to a maximum at 20 K before the
values decrease again because of the usual influence of zero-
field splitting and magnetization saturation. Since the mag-
nitude of the increase is field dependent and more pro-
nounced at higher field, we infer mechanical torquing, where
the small particles of the powder sample are aligned in the
magnetic field, as the origin of the unusual behavior.
(
iPrAPDI)2Co are similar to the metrical parameters of the
iron analogues, (CyAPDI)2Fe and (iPrAPDI)2Fe, which are
best described as high-spin iron(II) with two monoreduced
bis(imino)pyridine chelates.33 This indicates that the neutral
bis(imino)pyridine cobalt bis(ligand) complexes contain
cobalt(II) centers where each chelate is singly reduced.
Both bis(chelate) cobalt complexes are paramagnetic.
Effective magnetic moments of 2.6(2) μB and 3.0(2) μB
were measured in benzene-d6 solution (Evans method35)
for (CyAPDI)2Co and (iPrAPDI)2Co, respectively. One
example, (iPrAPDI)2Co was studied in the solid state with
The electronic structure of the N-alkyl substituted
cobalt bis(ligand) complexes was also investigated by
EPR spectroscopy. The EPR spectrum of (iPrAPDI)2Co
recorded at 10 K with a powder sample is presented in
Figure 8. The spectrum exhibits axial symmetry, with
(34) Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky, E.;
Neese, F.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 13901.
(35) Sur, S. K. J. Magn. Reson. 1989, 82, 169.

6116 Inorganic Chemistry, Vol. 49, No. 13, 2010
Bowman et al.
instead antiferromagnetic coupling between the two li-
gand radicals must dominate over the metal-ligand
coupling (Figure 8, inset). This is even more surprising
considering the almost perfectly orthogonal arrangement
of the two ligand planes (dihedral angle of 88.8ꢀ). A more
detailed discussion of the electronic ground state of the
neutral bis(chelate) complexes will be presented in the
computational section.
Synthesis and Spectroscopy of [(RAPDI)Co(N2)]- Com-
pounds. The recent preparation of the aryl-substituted
bis(imino)pyridine cobalt dinitrogen anion, [(iPrPDI)CoN2]-,
prompted exploration of the synthesis of the corresponding
N-alkyl substituted compounds.31 These studies were also
designed to explore the feasibility of dinitrogen coordination
for an “overreduced” species. Treatment of (iPrAPDI)CoCl2
with 3 equiv of sodium naphthalenide, NaC10H8, in tetrahy-
drofuran (THF) under an N2 atmosphere at 23 ꢀC followed
by extraction into and recrystallization from Et2O furnished a
dark green solid identified as the desired N-alkyl substituted
bis(imino)pyridine cobalt dinitrogen anion, [Na(solv)3]-
[(iPrAPDI)CoN2] (solv = THF, Et2O) as a mixture of THF
and Et2O solvates (eq 4). Pure [Na(THF)3] [(iPrAPDI)CoN2]
was obtained by dissolving the isolated material in THF
followed by solvent removal.
Figure 8. Powder EPR spectrum of (iPrAPDI)2Co recorded at 10 K
(microwave frequency 9.44 GHz, power 0.63 mW, modulation 2 mT,
blacktrace) andspinHamiltoniansimulationforS = 3/2withg|| =2.255,
g^ = 2.068, E/D ∼ 0, D = 3 cm-1 and Lorentzian lineshapes (red). The
inset visualizes the alignment of Co(II) and ligand spins in the quartet
ground state.
effective g values of g^eff = 4.5 and g||eff = 2.1. The values
are typical for a spin quartet, S = 3/2, with large axial
zero-field splitting (D . hν ≈ 0.3 cm-1 at X-band) and
low rhombicity, E/D ≈ 0.36,37 At this condition, the
quartet is split by zero-field interaction into two Kramers
doublets which can be labeled by magnetic quantum
numbers “ms =(1/2” and “ms =(3/2”. The “ms =(1/2”
doublet has effective g values approximately g^eff= 4 and
g||eff = 2 (for electronic Zeeman g value g = 2), whereas
the doublet with “ms = (3/2” is virtually EPR silent
because of excessive g anisotropy, geff ≈ (0, 0, 6), and
vanishing transition probability. Accordingly, the experi-
mental spectrum of (iPrAPDI)2Co could be simulated
with g values of g^ = 2.255 and g|| = 2.068, vanishing
The N-alkyl substituted bis(imino)pyridine cobalt dini-
trogen anions were characterized by infrared and NMR
spectroscopies. The THF solvate, [Na(THF)3][(iPrAPDI)-
CoN2], exhibited a strong NtN stretch at 1991 cm-1 in
the KBr infrared spectrum. This value is at lower fre-
quency than the corresponding diethyl etherate value of
2037 cm-1. The difference in N2 stretching frequencies for
the different solvates is likely a result of the different
coordination mode or degree of interaction of the sodium
counterion with the chelate or dinitrogen ligand. Similar
effects have been noted by Gambarotta in various modi-
fied N-aryl substituted bis(imino)pyridine iron dinitrogen
anions,39 as well as with the analogous N-aryl sub-
stituted complexes [Na(solv)3][(iPrPDI)CoN2].31
rhombicity E/D ≈ 0, and any value |D| > 1.0 cm
;
-1 38 for
simplicity the D value was fixed at 3 cm-1 (red line,
Figure 8). The large line width, which results from inter-
molecular interactions in the solid, prevents observation
of hyperfine coupling. In summary the EPR measurements
support the conclusion from magnetic susceptibility that
(
iPrAPDI)2Co has a ground state withtotal spin of S = 3/2.
1
The H NMR spectrum of [Na(THF)3][(iPrAPDI)CoN2]
In comparison to the previously reported correspond-
ing iron complex, (iPrAPDI)2Fe, the quartet ground state
of (iPrAPDI)2Co is counterintuitive. In the iron complex,
antiferromagnetic coupling of the two ligand radicals to
the high-spin iron(II) center (SFe = 2) dominates radical-
radical interaction, which forces the radical spins into a
parallel alignment and gives rise to an overall S = 1
ground state. One would therefore expect an S = 1/2
ground state for the cobalt complex, if the two ligand
radicals would also couple antiferromagnetically to the
central high-spin CoII ion (SCo = 3/2). However, the
experimentally observed S = 3/2 state indicates that
was recorded in a 1:1 mixture of benzene-d6 and THF to
dissolve the ion pair, and the chemical shifts are indicative of a
pure diamagnetic molecule without contamination from ther-
mally accessible triplet excited states (as in (ArPDI)CoX and
(RAPDI)Co-alkyl compounds). For example, the singlet for
the in-plane imine methyl resonance appears at 2.24 ppm
while the para-pyridine hydrogen was located at 6.68 ppm.
One electron oxidation of [Na(THF)3][(iPrAPDI)CoN2] in
the presence of 1 atm of N2 was explored with the goal of
synthesizing the elusive neutral, four-coordinate cobalt
dinitrogen complex. Unfortunately, addition of 1 equiv of
[Cp2Fe][BPh4] to a diethyl ether solution of [Na(solv)3]-
[(iPrAPDI)CoN2] furnished a brown solid identified as the
(36) Banci, L.; Bencini, A.; Benelli, C.; Gatteschi, D.; Zanchini, C. In
Structure and Bonding; Springer Verlag: Berlin/Heidelberg, 1982; Vol. 52, p 36.
(37) Benelli, C.; Gatteschi, D. Inorg. Chem. 1982, 21, 1788.
(38) Bencini, A.; Benelli, C.; Gatteschi, D.; Zanchini, C. Inorg. Chem.
1979, 18, 2137.
(39) Scott, J.; Vidyaratne, I.; Korobkov, I.; Gambarotta, S.; Budzelaar,
P. H. M. Inorg. Chem. 2008, 47, 896.

Article
Inorganic Chemistry, Vol. 49, No. 13, 2010 6117
˚
Table 3. Comparison of Computed and Experimental Bond Distances (A) and
Angles (deg) for (iPrAPDI)CoX (X = Cl, Me) Compounds
(
iPrAPDI)CoCl
(
iPrAPDI)CoMe
exp.
calc.
exp.
calc.
Co(1)-N(1)
Co(1)-N(2)
Co(1)-N(3)
Co(1)-Cl
1.9334(15)
1.7870(16)
1.9203(16)
2.2188(6)
2.003
1.838
2.003
2.249
1.9103(16)
1.8072(16)
1.9228(16)
1.967(2)
1.985
1.869
1.983
1.990
N(1)-C(2)
N(2)-C(3)
N(2)-C(7)
N(3)-C(8)
C(2)-C(3)
C(7)-C(8)
1.321(3)
1.371(2)
1.371(2)
1.314(3)
1.435(3)
1.438(3)
1.318
1.372
1.372
1.318
1.453
1.453
1.326(2)
1.373(2)
1.383(2)
1.326(2)
1.440(3)
1.434(3)
1.326
1.369
1.369
1.326
1.452
1.452
N(2)-Co(1)-X
150.73(5)
156.34
147.11(8)
147.66
(
iPrPDI)CoCl
(
iPrPDI)CoMe
exp.
calc.
exp.
calc.
Co(1)-N(1)
Co(1)-N(2)
Co(1)-N(3)
Co(1)-Cl
1.916(3)
1.797(3)
1.912(3)
2.1807(10)
1.978
1.833
1.978
2.212
1.907(3)
1.834(3)
1.903(3)
1.960(4)
1.973
1.879
1.971
1.973
N(1)-C(2)
N(2)-C(3)
N(2)-C(7)
N(3)-C(8)
C(2)-C(3)
C(7)-C(8)
1.322(5)
1.373(5)
1.369(6)
1.317(5)
1.444(5)
1.435(5)
1.320
1.372
1.372
1.320
1.451
1.451
1.335(4)
1.366(4)
1.365(4)
1.329(4)
1.443(5)
1.434(6)
1.329
1.367
1.367
1.328
1.450
1.449
N(2)-Co(1)-X
179.15
179.85
178.93
179.28
bis(chelate) cobalt compound, (iPrAPDI)2Co, with no evi-
dence for any other cobalt species (eq 5). It is interesting that
the N-alkylated bis(imino)pyridine ligand will support an
anionic cobalt dinitrogen complex but not a neutral one.
Figure 9. (a) Qualitative molecular orbital diagram for the S = 0 state
(
iPrAPDI)CoCl computed from a broken symmetry BS(1,1) B3LYP DFT
calculation. (b) Spin density plot obtained from a Mulliken population
analysis (red, positive spin density; yellow, negative spin density). The
coordinate system is defined as “z” perpendicular to the idealized plane of
the iPrAPDI ligand and “x” along the Npyr-Co vector.
(iii) What are the electronic structures of the
(RAPDI)2Co derivatives and how do they com-
pare to related iron compounds?33
Computational Studies. The electronic structures of the
N-alkyl substituted bis(imino)pyridine cobalt halide, al-
kyl, and bis(chelate) compounds were examined by den-
sity functional theory (DFT). Previous work from our
laboratories has demonstrated the utility of the combina-
tion of DFT, spectroscopy, and magnetochemistry for
identifying chelate participation in the electronic struc-
ture of reduced bis(imino)pyridine iron26b,33 and cobalt31
compounds. Several questions guided these studies:
Previous DFT studies by Budzelaar and co-workers estab-
lished an open-shell singlet ground state for (ArPDI)CoX and
(RAPDI)CoX (X=H, Me, Cl) arising from antiferromag-
netic coupling of a low spin CoII ion (SCo = 1/2) with a
bis(imino)pyridine radical anion (SPDI = 1/2).19 All com-
plexes were predicted to be square planar independent of the
imine substituents, which is in contradiction to the experi-
mental data provided for (RAPDI)CoX (X = Me, Cl; R =
iPr, Cy) in this work. Nonplanar structures were only found
for hydride and methyl complexes optimized at the restricted
DFT level. These restricted solutions, however, were com-
puted to be 10-20 kcal/mol less stable than the planar
structures obtained from the unrestricted calculations.
(i) What is the origin of the deviation from square
planar geometry observed for (RAPDI)CoCl
and (RAPDI)CoMe compounds that is absent
in the corresponding N-aryl substituted ArPDI
compounds?
(ii) What is the origin of the observed spin crossover
behavior in (RAPDI)CoCl and why are these com-
pounds different from (RAPDI)CoMe, although the
same structural distortion is observed in the solid
state structure?
Broken symmetry (BS) DFT calculations were per-
formed at the B3LYP level to understand the electronic
structures of (RAPDI)CoMe and (RAPDI)CoCl and to

6118 Inorganic Chemistry, Vol. 49, No. 13, 2010
Bowman et al.
Figure 10. Truncated versions of the bis(imino)pyridine ligands used for DFT studies to examine the origin of the observed distortion in (RAPDI)CoCl
compounds.
compare the geometric preferences to the corresponding
N-arylated RPDI analogues (Table 3). Both (iPrAPDI)CoX
and (iPrPDI)CoX (X = Me, Cl) were chosen as represen-
tative examples of the two classes of compounds and have
also been crystallographically characterized. In each case,
the molecules were computed without truncations. In
agreement with the experimentally determined diamag-
netic, low-temperature ground states, the geometries were
optimized using a BS(1,1) approach corresponding to a
low spin CoII ion antiferromagnetically coupled to a
bis(imino)pyridine radical anion. A qualitative molecular
orbital (MO) diagram derived from these results for
(
iPrAPDI)CoCl is shown in Figure 9 and illustrates the
common open-shell singlet ground state of all four com-
plexes at low temperature. In conformity with the results
reported by Budzelaar,19 the closed-shell ground states,
for example, (APDI0)CoICl, for all complexes obtained
via the restricted DFT method were found to be signifi-
cantly higher in energy.
To investigate the steric effects of the imine substituents
on the Npyr-Co-X angle, truncated versions of the chlor-
ide complexes were investigated in silico (Figure 10). For
this purpose, the isopropyl groups on the aryl substituents
of iPrPDIwerereplaced byhydrogenatoms(HPDI), and the
isopropyl substituents of iPrAPDI were replaced by methyl
groups (MeAPDI). For both complexes, (HPDI)CoCl and
(
MeAPDI)CoCl, square planar structures were obtained
after geometry optimization independent of the initial
Npyr-Co-X angle (180ꢀ or 150ꢀ). In a second experiment,
the methyl groups on the imine backbone of iPrAPDI were
replaced by hydrogen atoms. Again, a planar structure was
obtained after optimization of the corresponding Co-Cl
complex. This indicates that the distortion from planarity
observed for diamagnetic (iPrAPDI)CoCl and (CyAPDI)-
CoCl atlowtemperaturesisa resultofthestericinfluenceof
the bulky secondary alkyl substituents on the imines in
combination with the methyl group of the ligand backbone
and is not due to electronic effects.
Finally, the origin of the spin-crossover phenomenon
experimentally observed for (iPrAPDI)CoCl was investi-
gated by DFT. It has been shown that modern hybrid
DFT functionals are slightly biased towards high spin solu-
tions because of the admixture of exact Hartree-Fock
exchange. It is therefore challenging to predict the correct
ground state multiplicities for transition metal complexes.
Nevertheless, it has been suggested that small, predicted
energy differences in favor of the high spin state may be
considered indicative of spin-crossover behavior.40
Figure 11. (a) Qualitative molecular orbital diagram for the S = 1 state
of (iPrAPDI)CoCl computed from a broken symmetry BS(3,1) B3LYP
DFT calculation. (b) Spin density plot obtained from a Mulliken popula-
tionanalysis (red: positive spin density, yellow: negative spin density). The
coordinate system is defined as “z” perpendicular to the idealized plane of
the iPrAPDI ligand and “x” along the Npyr-Co vector.
approach assuming a triplet state resulted in an electronic
structure as described by Budzelaar and co-workers.19
The CoII ion possesses a low spin d7 configuration with one
unpaired electron located in the dz2 orbital, while the second
unpaired electron occupies a bis(imino)pyridine π symmetry
orbital. In agreement with the previous calculations,19 this
state lies 3.1 kcal/mol above the open-shell singlet and retains
preference for the square planar geometry of the BS(1,1)
state. A different triplet state was obtained from a BS(3,1)
calculation. In this solution, the ligand π radical is antiferro-
To separate electronic from steric effects, calculations
were first carried out on the sterically truncated model
complex, (MeAPDI)CoCl. A simple unrestricted DFT
magnetically coupled to a high spin CoII center (d7, SCo
3/2). Surprisingly, this electronic configuration was found to
=
(40) Ye, S.; Neese, F. Inorg. Chem. 2010, 49, 772.

Article
Inorganic Chemistry, Vol. 49, No. 13, 2010 6119
˚
Table 4. Comparison of Computed and Experimental Bond Distances (A) and
Angles (deg) for (iPrAPDI)2Co
(
iPrAPDI)2Co
exp.
calc.
Co(1)-N(1)
Co(1)-N(2)
Co(1)-N(3)
Co(1)-N(4)
Co(1)-N(5)
Co(1)-N(6)
2.2683(9)
1.9515(9)
2.2408(10)
2.2236(9)
2.0064(9)
2.2484(10)
2.270
1.992
2.238
2.242
1.992
2.262
N(1)-C(2)
N(2)-C(3)
N(2)-C(7)
N(3)-C(8)
C(2)-C(3)
C(7)-C(8)
N(4)-C(23)
N(5)-C(24)
N(5)-C(28)
N(6)-C(29)
C(23)-C(24)
C(28)-C(29)
1.2985(13)
1.3756(14)
1.3719(13)
1.3029(14)
1.4618(16)
1.4592(17)
1.3044(14)
1.3725(15)
1.3810(16)
1.3175(13)
1.4518(19)
1.4415(17)
1.320
1.383
1.384
1.322
1.467
1.464
1.322
1.384
1.383
1.321
1.465
1.466
Figure 12. Spin density plot for (iPrAPDI)2Co derived from a Mulliken
spin population analysis of the BS(4,1) calculation (red, positive spin
density; yellow, negative spin density).
N(1)-Co(1)-N(3)
N(4)-Co(1)-N(6)
N(1)-Co(1)-N(4)
N(2)-Co(1)-N(5)
152.51(3)
150.41(3)
91.13(4)
151.86
151.88
93.60
177.43(4)
178.91
be only 0.1 kcal/mol less stable than the open-shell singlet
solution obtained from the BS(1,1) calculation with a low
spin CoII ion and thus is more stable than the simple unrest-
ricted triplet. More importantly, the optimized structure for
the BS(3,1) triplet is significantly bent with a Npyr-Co-Cl
angle of 153.55ꢀ. Because of the absence of bulky substituents
in (MeAPDI)CoCl this distortion must be exclusively due to
electronic effects.
For (iPrAPDI)CoCl, the BS(3,1) state is found to be
6.6 kcal/mol more stable than the BS(1,1) open-shell
singlet state. This is clearly due to a destabilization of
the singlet state by the bulky isopropyl groups enforcing a
bent structure and promoting the spin-crossover behavior. In
the bent molecule, the dyz orbital is destabilized by strong π
donation of the Cl ligand, while the dx2-y2 orbital is stabilized,
so that the energy gap between these two orbitals decreases
and favors a high spin CoII center. A qualitative MO diagram
for the S=1stateof(iPrAPDI)CoCl is presented in Figure 11.
Accordingly, the corresponding complex (iPrAPDI)CoMe
containing a purely σ donating methyl ligand exhibits no
spin-crossover behavior because of the absence of the desta-
bilization of the dyz orbital. In fact, all attempts to calculate a
BS(3,1) solution for this complex converged back to the
unrestricted triplet solution in spite of the sterically enforced
lifting of the Me ligand.
For the N-aryl substituted PDI complexes, similar
computational results were obtained. For the sterically
less demanding complex, (HPDI)CoCl, a small BS(1,1)
singlet-BS(3,1) triplet gap of 4.7 kcal/mol was calculated
in favor of the triplet state. In addition, the structure
shows a lifting of the Cl ligand with a Npyr-Co-Cl angle
of 141.20ꢀ. However, the introduction of the bulky 2,6-
isopropyl substituents in (iPrPDI)CoCl forces the molecule
into a more planar geometry in the triplet state (Npyr-Co-Cl
angle of 159.93ꢀ) and decreases the singlet-triplet gap to 0.5
kcal/mol. These computational results imply that the experi-
mentally observed spin-crossover phenomena in N-alkyl
Figure 13. Qualitative molecular orbital diagram for (iPrAPDI)2Co
from a B3LYP DFT calculation. The coordinate system is defined as
“z” perpendicular to the idealized plane of the iPrAPDI ligand and “y”
along the Npyr-Co vector.
substituted (CyAPDI)CoCl and (iPrAPDI)CoCl are due to a
high spin/low spin transition of the central CoII ion (SCo
=
3/2 f 1/2) associated with a lifting of the Cl ligand out of the
idealized metal-chelate plane. The energy gap between the
BS(1,1) singlet and the BS(3,1) triplet state is modified by the
steric influence of the imine substituents.
The electronic structure of the bis(chelate) cobalt com-
plex, (iPrAPDI)2Co, was also investigated by DFT calcu-
lations. The ground state geometry was optimized using
the experimentally determined S = 3/2 ground state with
the BP86 functional. The experimental and calculated
geometric structures are in excellent agreement (Table 4).
Single point calculations on the optimized structure
were performed at the B3LYP level of DFT. The simple
unrestricted approach assuming a quartet ground state

6120 Inorganic Chemistry, Vol. 49, No. 13, 2010
Bowman et al.
Figure 14. Electronic structure summary and comparison of reduced N-aryl versus N-alkyl substituted bis(imino)pyridine cobalt complexes.
resulted in spontaneous symmetry breaking to a BS(4,1)
solution. The obtained electronic structure is consistent
with a high-spin CoII ion (SCo = 3/2) and two ligand-
centered radicals. This is similar to the situation encoun-
tered for the corresponding iron complex (iPrAPDI)2Fe,
which contains a high spin ferrous ion (d6, SFe = 2)
and two ligand radicals, one on each bis(imino)pyridine
ligand.33 In the iron complex, the two ligand π radical
singly occupied molecular orbitals (SOMOs) are antifer-
romagnetically coupled to two degenerate t2g orbitals (dxy
and dyz) of the metal ion resulting in a parallel alignment
of the ligand spins and a total spin of S = 1. In the cobalt
complex, in contrast, the two ligand radical spins are
aligned in an antiparallel fashion. This can most clearly be
deduced from the spin density plot shown in Figure 12
where the sum of the spin densities on each ligand is close
to one, but of opposite sign.
The corresponding qualitative MO diagram obtained
from the calculation is shown in Figure 13. Compared to
the analogous iron complex, the high spin CoII ion (d7)
possesses one more electron in the t2g set, which leaves one
t2g SOMO (dyz) available for antiferromagnetic coupling
to a ligand radical, while the dxy orbital is filled. The
remaining metal SOMOs (dz2 and dx2-y2) comprise the eg
set and are orthogonal to the π radical SOMO of the
second ligand resulting in ferromagnetic coupling. This
description of the electronic structure of (iPrAPDI)2Co is,
however, a gross oversimplification of the true ground
state and is due to the failure of the BS wave function to
accurately describe the multireference character of such
radical systems. Because of the symmetry of the complex,
a second degenerate solution exists, in which the spin
densities on the ligands are inverted and one of the
unpaired electrons resides in the dxy orbital, while the
dyz orbital is doubly occupied. Consequently, the ground
state of the molecule is the superposition of both states
where the ligand radicals are antiferromagnetically
coupled to each other and form a diamagnetic pair that
carries no spin density. The antiferromagnetic coupling
between the two orthogonal ligand π systems is mediated
through a doubly occupied MO of the degenerate dyz and
dxy set via a superexchange pathway reminiscent of the
radical-radical coupling in square-planar, singlet-dira-
dical Ni complexes described by Neese and co-workers.41
This coupling must be stronger than the antiferromag-
netic coupling between the Co center and the two radicals,
which is due to the half-filled orbital of the dyz-dxy set
(Figure 8). The spin density of three unpaired electrons is
completely localized on the Co center.
A summary and comparison of the electronic structures
of both the N-aryl and N-alkyl substituted bis(imino)-
pyridine cobalt complexes is presented in Figure 14. One
salient difference is with the formally one electron (from
the corresponding (PDI)CoCl2) reduced monochloride
compounds where the N-alkyl substituted derivatives
access triplet ground states at higher temperatures while
the corresponding N-alkyl compounds are diamagnetic
over all temperatures reported. The two electron reduced
products, again using (PDI)CoCl2 as the starting point,
differ both structurally and electronically. For the N-
arylated cases, the sterically demanding chelates support
neutral dinitrogen complexes where the reduction occurs
at the cobalt. In the N-alkyl compounds, bis(chelate)
formation is observed and the cobalt(II) oxidation state
is maintained and the second reduction event is ligand
based.
Conclusions
Replacing the N-aryl substituents on the bis(imino)pyridine
chelates with alkyl groups has a profound impact on the
molecular and electronic structures of reduced cobalt halide,
alkyl, and bis(ligand) complexes. Whereas (ArPDI-)CoIICl
compounds are diamagnetic over all temperatures examined
and are essentially planar, the alkylated variants, (iPrAPDI-)-
CoIICl and (CyAPDI-)CoIICl are spin crossover compounds
with distorted geometries where the chloride ligand is lifted
out of the idealized metal-chelate plane. Computational
(41) Herebian, D.; Wieghardt, K.; Neese, F. J. Am. Chem. Soc. 2003, 125,
10997.

Article
Inorganic Chemistry, Vol. 49, No. 13, 2010 6121
studies suggest that the deviation from planarity is steric
rather than electronic in origin. The spin crossover behavior
is related to the non-planarity of the molecule and arises from
spin state changes at the Co(II), d7 ion. At low temperature,
the low-spin SCo = 1/2 configuration antiferromagnetically
couples to (RAPDI-) giving rise to an overall diamagnetic
molecule. As the temperature is increased, contributions from
high spin Co(II), SCo = 3/2 increase, and antiferromagnetic
coupling with the ligand centered radical yields a triplet state.
Note in both forms, a singly reduced bis(imino)pyridine
chelate antiferromagnetically coupled to at least one cobalt-
based unpaired electron is constant.
APEX2 diffractometer equipped with a molybdenum X-ray
˚
tube (λ = 0.71073 A). Preliminary data revealed the crystal
system. A hemisphere routine was used for data collection and
determination of lattice constants. The space group was identi-
fied and the data were processed using the Bruker SAINTþ
program and corrected for absorption using SADABS. The
structures were solved using direct methods (SHELXS) com-
pleted by subsequent Fourier synthesis and refined by full-
matrix least-squares procedures.
SQUID magnetization data of crystalline powdered samples
were recorded with a SQUID magnetometer (Quantum Design)
at 10 kOe between 5 and 300 K for all samples. Values of the
magnetic susceptibility were corrected for the underlying dia-
magnetic increment by using tabulated Pascal constants and the
effect of the blank sample holders. Samples used for magnetiza-
tion measurement were recrystallized multiple times and
Two electron reduction of (RAPDI0)CoIICl2 furnished the
bis(chelate) cobalt complexes, (RAPDI-)2CoII. Spectro-
scopic, magnetic and computational data establish quartet
1
checked for chemical composition by H NMR spectroscopy.
ground states arising from a high spin Co(II), d7 ion (SCo
=
The program julX written by E. Bill was used for (elements of)
the simulation and analysis of magnetic susceptibility data.43
Quantum-Chemical Calculations. All DFT calculations were
performed with the ORCA program package.44 The geometry
optimizations of the complexes and single-point calculations on
theoptimizedgeometrieswerecarriedoutattheB3LYPlevel45-47
of DFT. This hybrid functional often gives better results for
transition metal compounds than pure gradient-corrected func-
tionals, especially with regard to metal-ligand covalency.48 The
all-electron Gaussian basis sets were those developed by the
Ahlrichs group.49,50 Triple-ζ quality basis sets TZVP with one
set of polarization functions on the metals and on the atoms
directly coordinated to the metal center were used.50 For the
carbon and hydrogen atoms, slightly smaller polarized split-
valence SV(P) basis sets were used, that were of double-ζ quality
in the valence region and contained a polarizing set of d-func-
tions on the non-hydrogen atoms.49 Auxiliary basis sets used to
expand the electron density in the resolution-of-the-identity
(RI) approach were chosen,51-53 to match the orbital basis.
The SCF calculations were tightly converged (1 ꢀ 10-8 Eh in
energy, 1 ꢀ 10-7 Eh in the density change, and 1 ꢀ 10-7 in maxi-
mum element of the DIIS error vector). The geometry optimiza-
tions for all complexes were carried out in redundant internal
coordinates without imposing symmetry constraints. In all cases
the geometries were considered converged after the energy
change was less than 5 ꢀ 10-6 Eh, the gradient norm and maxi-
mum gradient element were smaller than 1 ꢀ 10-4 Eh Bohr-1
and 3 ꢀ 10-4 Eh Bohr-1, respectively, and the root-mean square and
maximum displacements of all atoms were smaller than 2 ꢀ 10-3
Bohr and 4 ꢀ 10-3 Bohr, respectively.
3/2) and two singly reduced bis(imino)pyridine radical anions.
In this description, the two ligand-centered radical spins are
antiparallel and antiferromagnetically coupled to each other by
mediation of filled, degenerate dyz and dxy cobalt orbitals. This
electronic structure is distinct from the related bis(chelate) iron
complexes, (RAPDI-)2FeII, where a triplet ground state is
observed arising from a high spin iron center (SFe = 2)
antiferromagnetically coupled to two chelate radical anions.
The difference in electronic structures is a result of the addi-
tional electron in a t2g-type orbital in the cobalt case that
diminishes antiferromagnetic coupling between the metal and
the chelate electrons but promotes coupling between the ligand
radicals. Thus, ligand participation in the overall electronic
structure of the metal complex is a synergistic interaction
influenced by the specific d electron configuration and orbital
occupancy.
Experimental Section
General Considerations. All air- and moisture-sensitive ma-
nipulations were carried out using standard vacuum line,
Schlenk, and cannula techniques or in an MBraun inert atmo-
sphere drybox containing an atmosphere of purified nitrogen.
Solvents for air- and moisture-sensitive manipulations were
initially dried and deoxygenated using literature procedures.42
Benzene-d6 was purchased from Cambridge Isotope Labora-
˚
tories and dried over 4 A molecular sieves.
1H NMR spectra were recorded on Varian Mercury 300, Inova
400, 500, and 600 spectrometers operating at 299.76, 399.78,
500.62, and 599.78 MHz, respectively. 13C NMR spectra were
recorded on an Inova 500 spectrometer operating at 125.893
MHz. All 1H and 13C NMR chemical shifts are reported relative
Throughout this paper we describe our computational results
by using the broken-symmetry (BS) approach by Ginsberg54
and Noodleman.55 Because several broken symmetry solutions
to the spin-unrestrictedKohn-Sham equations may beobtained,
1
to SiMe4 using the H (residual) and 13C chemical shifts of the
solvent as a secondary standard. Peak widths at half heights are
reported for paramagnetically broadened and shifted resonances.
Solution magnetic moments were determined by Evans method35
using a ferrocene standardand arethe average value of atleasttwo
independent measurements. Gouy balance measurements were
performed with a Johnson Matthey instrument that was cali-
bratedwithHgCo(SCN)4. Peakwidthsathalfheightsarereported
for paramagnetically broadened and shifted resonances. Infrared
spectra were collected on a Thermo Nicolet spectrometer. Ele-
mental analyses were performed at Robertson Microlit Labora-
tories, Inc., in Madison, NJ.
(43) http://ewww.mpi-muelheim.mpg.de/bac/logins/bill/julX_en.php.
(44) Neese, F. Orca - an ab initio, DFT and Semiempirical Electronic
Structure Package, Version 2.7, Revision 0; Institut f€ur Physikalische und
€
Theoretische Chemie, Universitat Bonn: Bonn, Germany, August 2009.
(45) Becke, A. D. J. Chem. Phys. 1986, 84, 4524.
(46) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(47) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
(48) Neese, F.; Solomon, E. I. In Magnetism: From Molecules to Materi-
als; Miller, J. S., Drillon, M., Eds.; Wiley: New York, 2002; Vol. 4, p 345.
€
(49) Schafer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571.
€
(50) Schafer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829.
(51) Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Theor. Chem.
Acc. 1997, 97, 119.
Single crystals suitable for X-ray diffraction were coated with
polyisobutylene oil in a drybox, transferred to a nylon loop, and
then quickly transferred to the goniometer head of a Bruker X8
€
€
(52) Eichkorn, K.; Treutler, O.; Ohm, H.; Haser, M.; Ahlrichs, R. Chem.
Phys. Lett. 1995, 240, 283.
€
€
(53) Eichkorn, K.; Treutler, O.; Ohm, H.; Haser, M.; Ahlrichs, R. Chem.
Phys. Lett. 1995, 242, 652.
(54) Ginsberg, A. P. J. Am. Chem. Soc. 1980, 102, 111.
(55) Noodleman, L.; Peng, C. Y.; Case, D. A.; Mouesca, J. M. Coord.
Chem. Rev. 1995, 144, 199.
(42) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.

6122 Inorganic Chemistry, Vol. 49, No. 13, 2010
Bowman et al.
the general notation BS(m,n)56,16 has been adopted, where m (n)
denotes the number of spin-up (spin-down) electrons at the two
interacting fragments.
CH), 12.07 (41.8 Hz, 2H, m-pyridine), 66.27 (76.5 Hz, 1H,
p-pyridine).
Preparation of (iPrAPDI)CoCl. This compound was prepared
in a similar manner to (CyPDI)CoCl with 0.544 g (1.45 mmol) of
Canonical and corresponding57 orbitals, as well as spin
density plots were generated with the program Molekel.58
Preparation of 2,6-(iPrNdCMe)2C5H3N (iPrAPDI). This
compound was synthesized by a modification of the literature
procedure.59 A 250 mL round-bottom flask was charged with
3.00 g (18.38 mmol) of 2,6-diacetylpyridine, approximately
100 mL of isopropyl amine, and a magnetic stir bar. p-Tolue-
nesulfonic acid (0.340 g, 1.97 mmol) was added, and the reaction
was heated to a reflux for 3 days. The solution was cooled to
ambient temperature, and excess isopropyl amine was removed
in vacuo. The resulting tan solid was extracted into dichloro-
methane, washed with 2 ꢀ 100 mL water and 100 mL saturated
sodium bicarbonate solution, and dried over MgSO4. The
solution was filtered, the volatiles were removed in vacuo, and
the resulting solid was recrystallized from methanol at -35 ꢀC to
yield 1.30 g (28%) of off-white plates identified as iPrAPDI.59
Preparation of (CyAPDI)CoCl2. This compound was synthe-
sized by a modification of the literature procedure.8 A 100 mL
round-bottom flask was charged with 0.507 g (1.56 mmol) of
CyAPDI and 0.184 g (1.42 mmol) of CoCl2. Approximately
50 mL of THF were added to the flask, and the resulting mixture
was stirred for 12 h during which time a pale green precipitate
formed. The solid was collected by filtration and washed with
pentane to yield 0.627 g (94%) of a pale green powder identified
as (CyAPDI)CoCl2. 1H NMR (CD2Cl2): δ = -59.37 (170.8 Hz,
4H, Cy CH2), -35.44 (237.9 Hz, 4H, Cy CH2), -21.44 (133.6 Hz,
6H, C(CH3)), -18.79 (149.8 Hz, 4H, Cy CH2), -9.64 (123.3 Hz,
4H, Cy CH2), -7.64 (121.3 Hz, 2H, Cy CH2), -4.93 (17.7 Hz,
1H, p-pyridine), -3.38 (150.7 Hz, 2H, Cy CH2), 77.92 (162.2 Hz,
2H, m-pyridine or Cy CH), 84.91 (151.1 Hz, 2H, m-pyridine or
Cy CH).
(
iPrAPDI)CoCl2 and NaEt3BH (1450 μL of a 1.0 M solution in
toluene, 1.45 mmol). The resulting dark mauve solid was
recrystallized from diethyl ether at -35 ꢀC yielding 0.294 g
(60%) of dark mauve crystals identified as (iPrAPDI)CoCl.
Analysis for C15H23N3CoCl: Calcd C, 53.03; H, 6.82; N,
12.37. Found C, 53.17; H, 6.79; N, 12.24. Magnetic suscept-
ibility (benzene-d6, 296 K): μeff = 1.0(2) μB. 1H NMR (benzene-
d6): δ = -12.62 (47.4 Hz, 6H, C(CH3)), 0.42 (47.1 Hz, 12H,
CH(CH3)2), 7.20 (51.8 Hz, 2H, CH(CH3)2), 9.52 (42.1 Hz, 2H,
m-pyridine), 41.6 (71.6 Hz, 1H, p-pyridine).
Preparation of (CyAPDI)CoMe. A 20 mL scintillation vial was
charged with 0.204 g (0.486 mmol) of (CyAPDI)CoCl, approxi-
mately 15 mL of toluene, and a magnetic stir bar. The solution
was frozen in a liquid nitrogen cooled cold well, and MeLi
(425 μL of a 1.6 M solution in diethyl ether, 0.680 mmol) was
added to the thawing solution. The reaction mixture was warmed
to ambient temperature with stirring and then filtered through
Celite. The volatiles wereremoved in vacuo, andthe resulting dark
purple solid was recrystallized from diethyl ether at -35 ꢀC to
yield 0.100 g (52%) of a dark red-brown solid identified as
(
CyAPDI)CoMe. Analysis for C22H34N3Co: Calcd C, 66.15; H,
1
8.58; N, 10.52. Found C, 66.04; H, 8.24; N, 10.04. H NMR
(benzene-d6): δ = 0.13 (s, 6H, C(CH3)), 1.41 (m, 2H, Cy C4H2),
1.56 (m, 4H, Cy C3H2), 1.70 (m, 2H, Cy C4H2), 1.78 (m, 4H, Cy
C3H2), 2.36 (m, 4H, Cy C2H2), 2.53 (m, 4H, CyC2H2), 3.37 (s, 3H,
Co-CH3), 4.65 (m, 2H, Cy C1H), 7.35 (d, 7.6Hz, 2H, m-pyridine),
9.41 (t, 7.6 Hz, 1H, p-pyridine). 13C {1H} NMR (benzene-d6):
δ = 21.16 (C(CH3)), 26.42 (Cy C3H2), 26.70 (Cy C4H2), 32.11
(Cy C2H2), 70.77 (Cy C1H), 111.09 (p-pyridine), 126.61 (m-pyri-
dine), 146.63 (o-pyridine), 158.87 (C(CH3)), one resonance not
located.
Preparation of (iPrAPDI)CoCl2. This compound was prepared
in a similar manner to (CyAPDI)CoCl2 with 0.489 g (1.99 mmol) of
iPrAPDI and 0.246 g (1.89 mmol) of CoCl2 yielding 0.717 g (96%)
of a gray-green solid identified as (iPrAPDI)CoCl2. Analysis for
C15H23N3CoCl2: Calcd C, 48.02; H, 6.18; N, 11.20. Found C,
47.94; H, 5.85; N, 10.94. Magnetic susceptibility (Gouy balance,
293 K): μeff = 4.2(2) μB. 1H NMR (CD2Cl2): δ = -38.23 (111.0
Hz, 12H, CH(CH3)2), -20.26 (60.5 Hz, 6H, C(CH3)), -2.30 (58.2
Hz, 1H, p-pyridine), 74.39 (4.6 Hz, 2H, m-pyridine or CH(CH3)2),
87.14 (4.6 Hz, 2H, m-pyridine or CH(CH3)2).
Preparation of (iPrAPDI)CoMe. This compound was pre-
pared using a similar method to (CyAPDI)CoMe using 0.136 g
(0.400 mmol) (iPrAPDI)CoCl and MeLi (350 μL of a 1.6 M
solution in diethyl ether, 0.560 mmol). Recrystallization from
diethyl ether at -35 ꢀC yielded 0.104 g (81%) of a dark red solid
identified as (iPrAPDI)CoMe. Analysis for C16H26N3Co: Calcd
C, 60.18; H, 8.21; N, 13.16. Found C, 60.04; H, 8.18; N, 13.18.
1H NMR (benzene-d6): δ = 0.14 (s, 6H, C(CH3)), 1.78 (d, 6.6
Hz, 12H, CH(CH3)2, 2.05 (s, 3H, Co-CH3), 5.01 (septet, 6.6 Hz,
2H, CH(CH3)2), 7.30 (d, 7.6 Hz, 2H, m-pyridine), 9.15 (t, 7.6 Hz,
1H, p-pyridine). 13C {1H} NMR (benzene-d6): δ = 20.49 (C(CH3)),
22.06 (CH(CH3)2), 60.78 (CH(CH3)2), 112.49 (p-pyridine), 123.74
(m-pyridine), 147.68 (o-pyridine), 158.43 (C(CH3)), one resonance
not located.
Preparation of (CyAPDI)CoCl. A 100 mL round-bottom flask
was charged with 0.171 g (0.376 mmol) of CyAPDICoCl2,
approximately 50 mL of toluene and a stir bar. The toluene
slurry was cooled in a liquid nitrogen chilled cold well, NaEt3BH
(376 μL of a 1.0 M solution in toluene, 0.376 mmol) was added
dropwise with stirring. The reaction was warmed to ambient
temperature with stirring. Upon warming a gradual color
change to bright purple occurred, and the resulting purple
solution was filtered through Celite and concentrated. The
solution was cooled to -35 ꢀC and yielded 0.099 g (63%) of a
dark red-brown solid identified as (CyAPDI)CoCl. Analysis for
C21H31N3Co: Calcd C, 60.07; H, 7.44; N, 10.01. Found C, 59.87;
H, 7.64; N, 9.80. Magnetic susceptibility (benzene-d6, 296 K):
Preparation of (iPrAPDI)2Co. A thick walled glass vessel
was charged with 28.2 g of Hg (140.6 mmol) and approximately
100 mL of toluene. Sodium metal (0.141 g, 6.13 mmol) was
added in small pieces, and the resulting amalgam was stirred for
15 min. The flask was cooled in a cold well at liquid nitrogen
temperature followed by addition of (iPrAPDI)CoCl2 (0.460 g,
1.23 mmol) and iPrAPDI (0.301 g, 1.23 mmol). The flask was
fitted with a 180ꢀ needle valve and quickly removed from the
drybox and submerged in liquid nitrogen. The solution was
degassed four times on a high vacuum line, and the reaction was
warmed to room temperature and stirred for 16 h, during which
time a color change to red was observed. The solution was
filtered through Celite, the volatiles were removed in vacuo, and
the remaining red-brown solid was recrystallized from pentane
at -35 ꢀC to yield 0.108 g (70%) of red crystals identified as
μ
eff = 2.5(2) μB. 1H NMR (benzene-d6): δ = -22.79 (57.6 Hz,
6H, C(CH3)), -4.38 (48.5 Hz, 4H, Cy CH2), 0.55 (50.0 Hz, 4H,
Cy CH2), 0.65 (63.5 Hz, 4H, Cy CH2), 1.50 (67.1 Hz, 6H, Cy
CH2), 2.47 (56.7 Hz, 2H, Cy CH2), 8.72 (45.9 Hz, 2H, Cy
(56) Kirchner, B.; Wennmohs, F.; Ye, S.; Neese, F. Curr. Opin. Chem.
Biol. 2007, 11, 134.
(
iPrAPDI)2Co. Analysis for C30H46N6Co: Calcd C, 65.55; H,
(57) Neese, F. J. Phys. Chem. Solids 2004, 65, 781.
(58) Molekel, Advanced Interactive 3D-Graphics for Molecular Sciences;
Swiss National Supercomputing Centre: Manno, Switzerland; available under
http://www.cscs.ch/molkel/.
8.43; N, 15.29. Found C, 65.28; H, 8.46; N, 15.22. Magnetic
susceptibility (293 K): μeff = 3.6(2) μB. 1H NMR (benzene-d6):
δ= -64.35 (193.3 Hz, 2H, p-pyridine), -4.32 (77.4 Hz, 24H,
CH(CH3)2), 46.24 (119.0 Hz, 4H, m-pyridine or CH(CH3)2),
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(59) Castro, P. M.; Lappalainen, K.; Ahlgren, M.; Leskela, M.; Repo, T.
J. Polym. Sci., Part A: Polym, Chem. 2003, 41, 1380–1389.

Article
Inorganic Chemistry, Vol. 49, No. 13, 2010 6123
64.49 (189.0 Hz, 12H, C(CH3)), 185.13 (211.0 Hz, 4H,
m-pyridine or CH(CH3)2).
of dark green crystals identified as a mixture of [Na(Et2O)3]-
[(iPrAPDI)Co(N2)] along with [Na(THF)3][(iPrAPDI)Co(N2)].
Dissolution of the dark green crystals in THF followed by
removal of volatiles gave quantitative yield of a dark green
Preparation of (CyAPDI)2Co. This compound was prepared
in a similar manner to (iPrAPDI)2Co with 25.0 g (124.6 mmol) of
Hg, 0.125 g (5.44 mmol) of sodium metal, 0.355 g (1.09 mmol) of
CyAPDI, and 0.496 g (1.09 mmol) of (CyAPDI)CoCl2. Recrys-
tallization of the resulting red-brown solid from pentane at
-35 ꢀC yielded 0.485 g (63%) of dark red-brown crystals
identified as (CyAPDI)2Co. Analysis for C42H62N6Co: Calcd
C, 71.06; H, 8.80; N, 11.84. Found C, 70.71; H, 8.42; N, 11.45.
1
powder identified as [Na(THF)3][(iPrAPDI)Co(N2)]. H NMR
(1:1 THF-d8/benzene-d6): δ = 1.89 (s, 12H, CH(CH3)2), 2.24 (s,
6H, C(CH3)), 4.70 (s, 2H, CH(CH3)2), 6.68 (s, 1H, p-pyridine),
8.04 (s, 2H, m-pyridine). IR (KBr): νNN = 2037 cm-1 (solv =
Et2O), 1991 cm-1 (solv = THF).
Oxidation of [Na(Et2O)3][(iPrAPDI)Co(N2)] with [Cp2Fe][BPh4].
A 20 mL scintillation vial was charged with 0.036 g (0.062 mmol) of
[Na(Et2O)3][(iPrAPDI)Co(N2)], approximately 5 mL of diethyl
ether, and a stir bar, yielding a bright green solution. 0.031 g
(0.061 mmol) of Cp2Fe was added by spatula, and the reaction
was stirred for 1 h. A color change to brown was observed. The
solution was filtered though Celite, and volatiles were removed to
give a dark brown solid. Formation of (iPrAPDI)2Co and Cp2Fe
was confirmed by 1H NMR spectroscopy.
1
Magnetic susceptibility (293 K): μeff = 3.0(2) μB. H NMR
(benzene-d6): δ = -62.361 (938.1 Hz, 2H, p-pyridine), -21.11
(83.2 Hz, 8H, Cy CH2), -8.67 (197.2 Hz, 8H, Cy CH2), -3.53 (31.2
Hz, 8H, Cy CH2), 0.26 (20.8 Hz, 4H, Cy CH2), 2.34 (7.1 Hz, 4H, Cy
CH2), 6.51 (30.7 Hz, 8H, Cy CH2), 47.91 (140.0 Hz, 4H, m-pyridine
or Cy CH), 61.09 (30.9 Hz, 12H, C(CH3)), 178.17 (9.1 Hz, 4H, m-
pyridine or Cy CH).
Preparation of [Na(solv)3][(iPrAPDI)Co(N2)] (solv = THF,
Et2O). Sodium naphthalenide was prepared in situ by addition
of approximately 50 mL of THF to a 100 mL round-bottom
flask containing 0.072 g (3.13 mmol) of sodium metal and
0.402 g (3.13 mmol) of naphthalene. The reaction was stirred
for 3 h during which time a green solution formed and the solids
had dissolved. Small portions totaling 0.335 g (0.893 mmol) of
Acknowledgment. We thank the U.S. National Science
Foundation and the Deutsche Forschungsgemeinschaft for a
Cooperative Activities in Chemistry between U.S. and German
Investigators grant.
(
iPrAPDI)CoCl2 were added to the flask, and the reaction was
stirred for 1 h after which time the volatiles were removed. The
residual solid was extracted into diethyl ether and stirred for 1 h
to give a bright green solution. The solution was filtered through
Celite, concentrated and cooled to -35 ꢀC to yield 0.366 g (71%)
Supporting Information Available: Crystallographic details
for (iPrAPDI)CoCl, (iPrAPDI)CoCMe, (iPrAPDI)2Co, and
(
CyAPDI)2Co in cif format. This material is available free of
charge via the Internet at http://pubs.acs.org.