Synthesis and molecular and electronic structures of reduced bis(imino)pyridine cobalt dinitrogen complexes: Ligand versus metal reduction

Article abstract of DOI:10.1021/ja908955t

Sodium amalgam reduction of the aryl-substituted bis(imino)pyridine cobalt dihalide complexes (^ArPDI)CoCl₂ and (^iPrBPDI) CoCl₂ (^ArPDI ) 2,6-(2,6-R₂-C₆H ₃N=CMe)₂C₅H₃N (R ) ⁱPr, Et, Me); ^iPrBPDI ) 2,6-(2,6-ⁱPr₂-C ₆H₃N=CPh)₂C₅H₃N) in the presence of an N₂ atmosphere furnished the corresponding neutral cobalt dinitrogen complexes (^ArPDI)CoN₂ and ( ^iPrBPDI)CoN₂. Magnetic measurements on these compounds establish doublet ground states. Two examples, (^iPrPDI)CoN ₂ and (^iPrBPDI)CoN₂, were characterized by X-ray diffraction and exhibit metrical parameters consistent with one-electron chelate reduction and a Co(I) oxidation state. Accordingly, the toluene solution EPR spectrum of (^iPrPDI)CoN₂ at 23 °C exhibits an isotropic signal with a g value of 2.003 and hyperfine coupling constant of 8 x 10^-4 cm^-1 to the I = 7/2 ⁵⁹Co center, suggesting a principally bis(imino)pyridine-based SOMO. Additional one-electron reduction of (^iPrPDI)CoN₂ was accomplished by treatment with Na[C₁₀H₈] in THF and yielded the cobalt dinitrogen anion [(^iPrPDI)CoN₂]^-. DFT calculations on the series of cationic, neutral, and anionic bis(imino)pyridine cobalt dinitrogen compounds establish Co(I) centers in each case and a chelate-centered reduction in each of the sequential one-electron reduction steps. Frequency calculations successfully reproduce the experimentally determined N≡N infrared stretching frequencies and validate the computational methods. The electronic structures of the reduced cobalt dinitrogen complexes are evaluated in the broader context of bis(imino)pyridine base metal chemistry and the influence of the metal d electron configuration on the preference for closed-shell versus triplet diradical dianions.

Full text of DOI:10.1021/ja908955t

Published on Web 01/19/2010
Synthesis and Molecular and Electronic Structures of
Reduced Bis(imino)pyridine Cobalt Dinitrogen Complexes:
Ligand versus Metal Reduction
Amanda C. Bowman,† Carsten Milsmann,‡ Crisita Carmen Hojilla Atienza,†
Emil Lobkovsky,† 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 October 20, 2009; E-mail: pc92@cornell.edu
Abstract: Sodium amalgam reduction of the aryl-substituted bis(imino)pyridine cobalt dihalide complexes
i
(ArPDI)CoCl2 and (iPrBPDI)CoCl2 (ArPDI ) 2,6-(2,6-R2-C6H3NdCMe)2C5H3N (R ) Pr, Et, Me); iPrBPDI )
2,6-(2,6-iPr2-C6H3NdCPh)2C5H3N) in the presence of an N2 atmosphere furnished the corresponding neutral
cobalt dinitrogen complexes (ArPDI)CoN2 and (iPrBPDI)CoN2. Magnetic measurements on these compounds
establish doublet ground states. Two examples, (iPrPDI)CoN2 and (iPrBPDI)CoN2, were characterized by
X-ray diffraction and exhibit metrical parameters consistent with one-electron chelate reduction and a Co(I)
oxidation state. Accordingly, the toluene solution EPR spectrum of (iPrPDI)CoN2 at 23 °C exhibits an isotropic
signal with a g value of 2.003 and hyperfine coupling constant of 8 × 10-4 cm-1 to the I ) 7/2 59Co center,
suggesting a principally bis(imino)pyridine-based SOMO. Additional one-electron reduction of (iPrPDI)CoN2
was accomplished by treatment with Na[C10H8] in THF and yielded the cobalt dinitrogen anion [(iPrPDI)CoN2]-.
DFT calculations on the series of cationic, neutral, and anionic bis(imino)pyridine cobalt dinitrogen
compounds establish Co(I) centers in each case and a chelate-centered reduction in each of the sequential
one-electron reduction steps. Frequency calculations successfully reproduce the experimentally determined
NtN infrared stretching frequencies and validate the computational methods. The electronic structures of
the reduced cobalt dinitrogen complexes are evaluated in the broader context of bis(imino)pyridine base
metal chemistry and the influence of the metal d electron configuration on the preference for closed-shell
versus triplet diradical dianions.
Introduction
to small-molecule organic methods. Gal and co-workers have
reported that square planar, aryl-substituted bis(imino)pyridine
In base metal catalysis, aryl-substituted bis(imino)pyridines
are a privileged class of ligands.1,2 Brookhart and Gibson’s
independent discoveries of ethylene and R-olefin polymerization
(oligomerization) upon treatment of [(ArPDI)MX2] (ArPDI ) 2,6-
(2,6-R2-C6H3NdCMe)2C5H3N (R ) iPr, Et, Me, H, etc.); M )
Fe, Co; X ) Cl, Br) derivatives with excess methylaluminoxane
(MAO) renewed interest in exploring the utility of classical,
Werner-type coordination compounds of iron and cobalt as
catalyst precursors.3-6 Bis(imino)pyridines have also found
application as supporting ligands in catalytic reactions relevant
cobalt alkyl complexes (iPrPDI)CoR1 (R1 ) Me, CH2Ph,
CH2SiMe3) are precatalysts for the hydrogenation of mono- and
disubstituted olefins.7 In iron chemistry, the bis(imino)pyridine
iron bis(dinitrogen) complexes (iPrPDI)Fe(N2)2 and (iPrBPDI)-
Fe(N2)2 exhibit high activity for the hydrogenation and hydrosi-
lylation of unactivated alkenes.8 In addition, (iPrPDI)Fe(N2)2
serves as a precatalyst for the hydrogenation of functionalized
olefins,9 diene [2π + 2π] cycloisomerization,10 and hydrogen-
mediated enyne and diyne cyclization.11
One distinguishing feature of bis(imino)pyridine ligands is
their redox activity12,13sthe ability to accept one to three
electrons from the metal and stabilize reduced compounds whose
low formal oxidation state assignment may be deceiving.14
† Cornell University.
‡ Max-Planck Institute for Bioinorganic Chemistry.
(1) Gibson, V. C.; Redshaw, C.; Solan, G. A. Chem. ReV. 2007, 107, 1745.
(2) Matsugi, T.; Fujita, T. Chem. Soc. ReV. 2008, 37, 1264.
(3) Bianchini, C.; Giambastiani, G.; Rios, I. G.; Mantovani, G.; Meli, A.;
Segarra, A. M. Coord. Chem. ReV. 2006, 250, 1391.
(4) 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.
(7) 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.
(8) (a) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 13794. (b) Archer, A. M.; Bouwkamp, M. W.; Cortez, M.-P.;
Lobkovsky, E.; Chirik, P. J. Organometallics 2006, 25, 4269.
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2008, 27, 1470.
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1998, 120, 4049.
(6) 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.; Stro¨mberg, S.; White, A. J. P.; Williams, D. J. J. Am. Chem.
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(10) Bouwkamp, M. W.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J.
J. Am. Chem. Soc. 2006, 128, 13340.
(11) Sylvester, K. T.; Chirik, P. J. J. Am. Chem. Soc. 2009, 131, 8772.
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1676 J. AM. CHEM. SOC. 2010, 132, 1676–1684
10.1021/ja908955t  2010 American Chemical Society

Bis(imino)pyridine Cobalt Dinitrogen Complexes
A R T I C L E S
Scheme 1
Seminal electrochemical studies by Toma on bis(chelate)
complexes of bis(imino)pyridine iron established the participa-
tion of these ligands in the electronic structures of reduced metal
complexes.15,16 In preparative scale chemistry, Wieghardt and
co-workers synthesized bis(imino)pyridine bis(chelate) com-
plexes of a series of first-row ions and demonstrated that redox
processes occur at the ligand rather than the metal.17,18 These
studies also established many of the metrical and spectroscopic
features that signal chelate participation. It is likely that the
ability of the bis(imino)pyridine ligands to smoothly adjust to
the electronic demands of the metal center is the reason why
these ligands enable such a broad and rich catalytic chemistry.19
reagents,7 Grignard reagents,23,24 or zinc metal23 resulted in one-
electron reduction to yield the square planar, diamagnetic cobalt
monochloride (iPrPDI)CoCl. Subsequent alkylation of this spe-
cies furnished a family of the desired cobalt alkyls (iPrPDI)CoR
25,26
n
n
(R ) Me, Et, Pr, Bu) (Scheme 1).
Analogous cobalt
chloride and alkyl compounds with the phenylated bis(imi-
no)pyridines, (ArBPDI)CoCl and (ArBPDI)CoR, have also been
reported.27
Computational studies by Budzelaar and co-workers28 estab-
lished that diamagnetic (ArPDI)CoCl, (ArPDI)CoR, and
(R′PDI)CoR are best described as low-spin Co(II) complexes
(SCo ) 1/2) antiferromagnetically coupled to a bis(imino)pyri-
dine radical anion (SPDI ) 1/2). In many cases, low-lying triplet
states were found to be only a few kilocalories per mole higher
in energy than the ground states. This view of the electronic
structure has been corroborated by the observation of bis(imi-
no)pyridine chelate distortions in the solid-state structures as
well as the unusual upfield chemical shifts of the in-plane imine
methyl groups observed by solution NMR spectroscopy.7,23,29
While bis(imino)pyridine cobalt chloride and alkyl com-
pounds with one-electron-reduced chelates have been synthe-
sized and well-studied, formal two- and three-electron reduction
products of (ArPDI)CoCl2 and (iPrBPDI)CoCl2 have not been
prepared or characterized. In addition to their potential role in
catalysis, these complexes were of interest to determine the
degree of chelate participation in the electronic structure and
further explore the fundamental concept of redox-active ligands
in base metal complexes relevant to catalysis. In addition, these
synthetic targets, as in iron chemistry, may also be useful
platforms to assemble reactive cobalt-ligand multiple bonds.30-36
In this paper, we report the results of a combined experimental
Irreversible carbon-oxygen bond cleavage has been identified
as a significant catalyst deactivation pathway in alkene hydro-
genation and [2π + 2π] cycloisomerization reactions promoted
by (iPrPDI)Fe(N2)2.20 Because of this limitation and for com-
parison to established bis(imino)pyridine iron catalysts, we
sought to prepare analogous reduced bis(imino)pyridine cobalt
dinitrogen compounds. In addition to answering questions
relevant to electronic structure, catalytically active cobalt
compounds are also of interest because they offer many of the
same cost and environmental advantages of iron.
Bis(imino)pyridine cobalt complexes date to Busch’s21 and
Sacconi’s22 initial reports of the synthesis and spectroscopic
characterization of a variety of halide, nitrate, and thiocyanate
derivatives. These classical, Werner-type coordination com-
pounds have received renewed interest following Brookhart’s
and Gibson’s independent discoveries of highly active olefin
polymerization catalysts.4,5 To date, investigations into reduced
bis(imino)pyridine cobalt chemistry have been motivated by
elucidating the identity of the propagating species during olefin
polymerization. Treatment of (iPrPDI)CoCl2 with alkyllithium
(23) Gibson, V. C.; Humphries, M. J.; Tellmann, K. P.; Wass, D. F.; White,
A. J. P.; Williams, D. J. Chem. Commun. 2001, 2252.
(24) Steffen, W.; Blo¨mker, T.; Kleigrewe, N.; Kehr, G.; Fro¨hlich, R.; Erker,
G. Chem. Commun. 2004, 1188.
(12) (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.
(25) Gibson, V. C.; Tellmann, K. P.; Humphries, M. J.; Wass, D. F. Chem.
Commun. 2002, 2316.
(26) Tellmann, K. P.; Humphries, M. J.; Rzepa, H. S.; Gibson, V. C.
Organometallics 2004, 23, 5503.
(13) Butin, K. P.; Beloglazkina, E. K.; Zyk, N. V. Russ. Chem. ReV. 2005,
74, 531.
(27) Kleigrewe, N.; Steffen, W.; Blo¨mker, T.; Kehr, Fro¨hlich, R.; Wib-
beling, B.; Erker, G.; Wasilke, J.-C.; Wu, G.; Bazan, G. C. J. Am.
Chem. Soc. 2005, 127, 13955.
(14) Chaudhuri, P.; Wieghardt, K. Prog. Inorg. Chem. 2001, 50, 151.
(15) Kuwabara, I. H.; Comninos, F. C. M.; Pardini, V. L.; Viertler, H.;
Toma, H. E. Electrochim. Acta 1994, 39, 2401.
(28) Knijnenburg, Q.; Hetterscheid, D.; Kooistra, T. M.; Budzelaar, P. H. M.
Eur. J. Inorg. Chem. 2004, 1204.
(16) Toma, H. E.; Chavez-Gil, T. E. Inorg. Chim. Acta 1997, 257, 197.
(17) de Bruin, B.; Bill, E.; Bothe, E.; Weyhermu¨ller, T.; Wieghardt, K.
Inorg. Chem. 2000, 39, 2936.
(29) Humphries, M. J.; Tellmann, K. P.; Gibson, V. C.; White, A. J. P.;
Williams, D. J. Organometallics 2005, 24, 2039.
(18) Budzelaar, P. H. M.; de Bruin, B.; Gal, A. W.; Wieghardt, K.; van
Lenthe, J. H. Inorg. Chem. 2001, 40, 4649.
(30) Shay, D. T.; Yap, G. P. A.; Zakharov, L. N.; Rheingold, A. L.;
Theopold, K. H. Angew. Chem., Int. Ed. 2005, 44, 1508.
(31) Jenkins, D. M.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002,
124, 11238.
(19) For representative examples of this concept in stoichiometric and
catalytic reactions in early-transition-metal chemistry, see: (a) Zarkesh,
R. A.; Ziller, J. W.; Heyduk, A. F. Angew. Chem., Int. Ed. 2008, 47,
4715. (b) Blackmore, K. J.; Lal, N.; Ziller, J. W.; Heyduk, A. F. J. Am.
Chem. Soc. 2008, 130, 2728. (c) Stanciu, C.; Jones, M. E.; Fanwick,
P. E.; Abu-Omar, M. M. J. Am. Chem. Soc. 2007, 129, 12400.
(20) Trovitch, R. J.; Lobkovsky, E.; Bouwkamp, M. W.; Chirik, P. J.
Organometallics 2008, 27, 6264.
(32) Chomitz, W. A.; Arnold, J. Chem. Commun. 2008, 3648.
(33) Cowley, R. E.; Bontchev, R. P.; Sorrell, J.; Sarracino, O.; Feng, Y. H.;
Wang, H. B.; Smith, J. M. J. Am. Chem. Soc. 2007, 129, 2424.
(34) Hu, X. L.; Meyer, K. J. Am. Chem. Soc. 2004, 126, 16322.
(35) Dai, X. L.; Kapoor, P.; Warren, T. H. J. Am. Chem. Soc. 2004, 126,
4798.
(21) Figgins, P. E.; Busch, D. H. J. Am. Chem. Soc. 1960, 82, 820.
(22) Sacconi, L.; Morassi, R.; Midollini, S. J. Chem. Soc. A 1968, 1510.
(36) Thyagarajan, S.; Shay, D. T.; Incarvito, C. D.; Rheingold, A. L.;
Theopold, K. H. J. Am. Chem. Soc. 2003, 125, 4440.
9
J. AM. CHEM. SOC. VOL. 132, NO. 5, 2010 1677

A R T I C L E S
Bowman et al.
and computational study aimed at evaluating the electronic
structure of reduced bis(imino)pyridine cobalt dinitrogen com-
pounds. A homologous series of cationic, neutral, and anionic
bis(imino)pyridine cobalt dinitrogen compounds has been
studied where the redox states of the compounds differ by three
one-electron steps. Electronic structures unique in reduced, base
metal bis(imino)pyridine chemistry have been elucidated.
Results and Discussion
Aryl-Substituted Bis(imino)pyridine Cobalt Dinitrogen
Complexes. The synthetic methods used to prepare the bis(imi-
no)pyridine iron bis(dinitrogen) complexes (iPrPDI)Fe(N2)2 and
8
(iPrBPDI)Fe(N2)2 served as the inspiration for study of the
analogous cobalt compounds. Stirring toluene solutions of the
bis(imino)pyridine cobalt dichloride complexes (ArPDI)CoCl2
with 6 equiv of sodium amalgam under 1 atm of N2 for 6 h
followed by filtration and recrystallization furnished teal solids
identified as the neutral cobalt dinitrogen compounds (ArPDI)-
CoN2 (eq 1). These products were also prepared by addition of
2 equiv of NaBEt3H to (ArPDI)CoCl2. Reduction of the
diamagnetic bis(imino)pyridine cobalt chloride compounds
(ArPDI)CoCl7,23 with either 0.5% Na(Hg) or NaBEt3H also
yielded the (ArPDI)CoN2 derivatives.
Figure 1. Temperature-dependent SQUID magnetization data (1 T) for
samples of (iPrPDI)CoN2 plotted as a function of the magnetic moment (µeff
)
vs temperature (T). The data are corrected for underlying diamagnetism.
analytically pure samples, and the individual runs were indis-
tinguishable. (iPrPDI)CoN2 shows a nearly temperature-inde-
pendent effective magnetic moment (µeff) of 1.72 µB in the
temperature range between 50 and 300 K, which is in excellent
agreement with the spin-only value for an S ) 1/2 ground state
(1.73 µB). Below 50 K, µeff declines rapidly to 1.37 µB at 5 K
due to field saturation effects and weak intermolecular antifer-
romagnetic coupling that was accounted for by including a small
Weiss constant (Θ) of -2 K in the simulation of the data. The
isotropic g value of 1.99 that was obtained from the simulation
is in good agreement with the g value determined by room
temperature EPR spectroscopy (vide infra).
1
The benzene-d6 H NMR spectra of each of the cobalt dinitrogen
complexes at 23 °C exhibit three paramagnetically broadened peaks
between 2.9 and 6.4 ppm. While observation of only three
resonances makes definitive assignment of the individual peaks
difficult, the spectra are useful in identifying the presence of the
compound and assaying its purity, especially from diamagnetic
(ArPDI)CoCl contaminants. The presence of a dinitrogen ligand
was also confirmed by infrared spectroscopy. Toluene solution IR
spectra exhibit strong NtN bands located at 2093 cm-1 for
(iPrPDI)CoN2, at 2095 cm-1 for (EtPDI)CoN2, and at 2093 cm-1
for (MePDI)CoN2. For the phenylated compound (iPrBPDI)CoN2,
this band was observed at 2111 cm-1, consistent with a more
electron-withdrawing ligand.8b These values are lower than the
value of 2184 cm-1 reported previously for the cationic bis(imi-
no)pyridine cobalt dinitrogen complex [(iPrPDI)CoN2][MeB-
(C6F5)3],23 as is expected for more reduced cobalt compounds.
Two-electron reduction of the phenylated bis(imino)pyridine
cobalt dichloride (iPrBPDI)CoCl2 was also attempted. In iron
chemistry, (iPrBPDI)Fe(N2)2 has proven to be a more active
catalyst precursor than (iPrPDI)Fe(N2)2 for 1-hexene hydrogena-
tion, although formation of η6-arene compounds limited the
catalyst lifetime and substrate scope.8b The desired cobalt
dinitrogen complex (iPrBPDI)CoN2 was isolated as a dark green
solid in 42% yield using an identical procedure used to prepare
the (ArPDI)CoN2 compounds (eq 2).
Because NMR and IR spectroscopies do not determine the
amount of N2 present in bulk samples and identify the presence
of possible reduction products that lack dinitrogen, each of the
(ArPDI)CoN2 complexes was oxidized and the liberated dini-
trogen gas quantified with a Toepler pump. Addition of excess
PbCl2 to a diethyl ether solution of each of the (ArPDI)CoN2
compounds furnished (ArPDI)CoCl2 and free N2 gas. In the case
of (iPrPDI)CoN2, 76% of the expected noncombustible gas was
collected. For (EtPDI)CoN2 and (MePDI)CoN2, 84% and 83%
of the expected dinitrogen gas was obtained, respectively.
Two examples, (iPrPDI)CoN2 and (iPrBPDI)CoN2, were char-
acterized by single-crystal X-ray diffraction. A representation
of each solid-state structure is presented in Figure 2, and selected
bond distances are reported in Table 1. Also included in Table
The bis(imino)pyridine cobalt dinitrogen complexes (ArPDI)-
CoN2 and (iPrBPDI)CoN2 are paramagnetic with solution and
solid-state magnetic moments consistent with one unpaired
electron and a doublet ground state (see the Experimental
Section). One representative example, (iPrPDI)CoN2, was also
studied by variable-temperature SQUID magnetometry (Figure
1). Data were collected on three independently prepared,
9
1678 J. AM. CHEM. SOC. VOL. 132, NO. 5, 2010

Bis(imino)pyridine Cobalt Dinitrogen Complexes
A R T I C L E S
Figure 2. Molecular structures of (iPrPDI)CoN2 and (iPrBPDI)CoN2 at 30% probability ellipsoids. Hydrogen atoms are omitted for clarity.
1 are the metrical parameters previously reported for (iPrPDI)-
CoCl2, (iPrPDI)CoCl, and [(iPrPDI)CoN2][MeB(C6F5)3]23 for
comparison. For the first two reference compounds, it has been
established that the bis(imino)pyridine is in its neutral and
monoanionic forms, respectively.12
The solid-state structures of (iPrPDI)CoN2 and (iPrBPDI)CoN2
confirm the isolation of four-coordinate, neutral cobalt dinitrogen
complexes. The geometry about each cobalt is best described
as idealized square planar with the sum of the bond angles equal
to 360.01(13)° and 360.03(20)° for (iPrPDI)CoN2 and (iPrBPDI)-
CoN2, respectively. The N(4)-N(5) distances of the dinitrogen
ligands are 1.104(2) and 1.011(3) Å, suggesting little perturba-
tion by the cobalt center. The metrical parameters (Table 1) of
the bis(imino)pyridine ligands signal chelate reduction and
participation in the electronic structures of the compounds.
In (iPrPDI)CoN2, the Cimine-Cipso distances are contracted to
1.429(2) and 1.418(2) Å while the Cimine-Nimine bonds are
elongated to 1.331(2) and 1.341(2) Å. Slightly less pronounced
distortions are present in (iPrBPDI)CoN2, as the Cimine-Cipso
Figure 3. Toluene solution EPR spectrum of (iPrPDI)CoN2 recorded at 23
°C (microwave frequency 9.43 GHz, power 1.26 mW, modulation 1 mT/
100 kHz).
lengths are 1.433(4) and 1.434 Å and the Cimine-Nimine bonds
are 1.352(3) and 1.351(3) Å. While the C-C bond distances in
(iPrPDI)CoN2 are slightly more contracted than what is usually
observed for a monoanionic chelate,12 the corresponding bond
lengths in (iPrBPDI)CoN2 are more typical. Taken on the whole,
the ligand perturbations in both compounds are most consistent
with a bis(imino)pyridine radical anion. The metal-ligand
distances, most notably those associated with the Co-Nimine and
Co-N2 bonds, in both (iPrPDI)CoN2 and (iPrBPDI)CoN2 are
significantly contracted. In the cationic dinitrogen complex
[(iPrPDI)CoN2][MeB(C6F5)3], the Co-N distance for the N2
ligand is 1.841(3) Å, while in the neutral compound (iPrPDI)-
CoN2, the value is 1.7884(16) Å. Contractions of a similar
magnitude are observed for the cobalt-imine and -pyridine
distances in both neutral dinitrogen compounds.
spectrum recorded in fluid toluene solution at 23 °C is presented
in Figure 3. The spectrum is centered at an isotropic giso value
of 2.003 and shows a distinct separation of the positive and
negative derivative peaks, which were assigned to the presence
of eight overlapping hyperfine lines, arising from hyperfine
coupling of the ligand radical spin to the 59Co nucleus with I )
7/2. A corresponding simulation (red line, Figure 3) yields an
isotropic hyperfine coupling constant of Aiso ) 8 × 10-4 cm-1
and a line width of 30 × 10-4 cm-1. No hyperfine coupling to
hydrogen or nitrogen atoms was observed because of the
relatively large line width caused by fast relaxation of the cobalt
nucleus. The EPR spectrum of a related bis(imino)pyridine
compound, (iPrPDI)AlMe2, exhibits a ligand-centered radical
EPR Spectroscopy. The electronic structure of (iPrPDI)CoN2
was also studied by EPR spectroscopy. The X-band EPR
1
with observable hyperfine coupling to 15N and H nuclei. The
Table 1. Selected Bond Distances (Å) for (iPrPDI)CoN2, (iPrBPDI)CoN2, (iPrPDI)CoCl2,6 iPrPDI)CoCl,23 and [(iPrPDI)CoN2][MeB(C6F5)3]23
(
(
iPrPDI)CoN2
(
iPrBPDI)CoN2
(
iPrPDI)CoCl2
(
iPrPDI)CoCl
[(iPrPDI)CoN2]+
Co-Nimine
1.8773(14)
1.8719(15)
1.8084(14)
1.7884(16)
1.331(2)
1.341(2)
1.429(2)
1.418(2)
1.379(2)
1.875(2)
1.872(2)
1.809(2)
1.817(2)
1.352(3)
1.351(3)
1.433(4)
1.434(4)
1.360(3)
1.366(3)
2.211(3)
2.211(3)
2.051(3)
1.916(3)
1.912(3)
1.797(3)
1.915(3)
1.908(3)
1.812(3)
1.841(3)
1.303(5)
1.303(5)
1.465(5)
1.459(5)
1.355(4)
Co-Npyridine
Co-N2
Nimine-Cimine
1.280(6)
1.285(6)
1.483(7)
1.481(7)
1.337(5)
1.322(5)
1.317(5)
1.444(5)
1.435(5)
1.373(5)
Cimine-Cipso
Cipso-Npyridine
9
J. AM. CHEM. SOC. VOL. 132, NO. 5, 2010 1679

A R T I C L E S
Bowman et al.
magnitude of the hyperfine couplings between the ligand-
of a dinitrogen atmosphere furnished a dark brown powder
identified as (iPrPIEA)CoN2, arising from deprotonation of one
of the imine methyl groups (eq 4).
1
centered radical and the 15N and H nuclei is (1-5) × 10-4
cm-1 in this case.37 These couplings are at least 6 times smaller
than the line broadening for (iPrPDI)CoN2; therefore, it is
1
unlikely that hyperfine interactions to 15N and H would be
observed for (iPrPDI)CoN2.
Reactivity of (iPrPDI)CoN2. The synthesis, isolation, and
crystallographic characterization of neutral bis(imino)pyridine
cobalt dinitrogen complexes prompted exploration of the
synthesis of the corresponding isoelectronic carbonyl com-
pounds. Exposure of a toluene solution of (iPrPDI)CoN2 to 1
atm of carbon monoxide at -196 °C followed by warming to
23 °C furnished the red-brown carbonyl complex (iPrPDI)-
Co(CO) (eq 3). The benzene-d6 1H NMR spectrum recorded at
23 °C exhibits four paramagnetically broadened resonances,
similar to the spectrum of (ArPDI)CoN2. The infrared spectrum
of (iPrPDI)Co(CO) in toluene exhibits a single strong CtO
stretch centered at 1975 cm-1, consistent with formation of a
monocarbonyl compound. This value is similar to that of one
of the bands of the bis(imino)pyridine iron dicarbonyl compound
(iPrPDI)Fe(CO)2, which has CO stretching frequencies of 1974
The 1H NMR spectrum of diamagnetic (iPrPIEA)CoN2 exhibits
the number of peaks consistent with a Cs-symmetric molecule,
signaling imine methyl group deprotonation. Diagnostic olefin
peaks are observed at 4.05 and 4.73 ppm in benzene-d6 for the
CdCH2 positions of the chelate. The intact imine methyl group
appears as a singlet at 0.71 ppm. The solid-state (KBr) infrared
spectrum also confirmed dinitrogen coordination and imine
methyl group deprotonation. A medium-intensity CdC stretch
and a strong NtN band were observed at 1583 and 2148 cm-1
,
and 1914 cm-1
.
The relative stabilities of (iPrPDI)CoN2 and (iPrPDI)Co(CO)
were probed by in situ infrared spectroscopy. Exposure of a
toluene solution of either compound to a vacuum in solution at
-78 °C resulted in little to no erosion of either the N2 or CO
bands, demonstrating stability to a vacuum at low temperature.
However, addition of 1 atm of carbon monoxide to a toluene
solution of (iPrPDI)CoN2 at -78 °C cleanly and quantitatively
furnished (iPrPDI)Co(CO) with concomitant loss of dinitrogen.
Re-exposure of the toluene solution of (iPrPDI)Co(CO) to
dinitrogen at -78 °C produced no change, demonstrating the
increased binding affinity of carbon monoxide relative to N2.
Unlike the iron dicarbonyl complex (iPrPDI)Fe(CO)2, the
carbonyl ligand in (iPrPDI)Co(CO) is labile and subject to
substitution by dinitrogen at higher temperatures. Allowing a
red-brown benzene-d6 solution of (iPrPDI)Co(CO) to stand at
ambient temperature for 6 h in the presence of 1 atm of
dinitrogen resulted in a rapid change to green, signaling
formation of (iPrPDI)CoN2. Both 1H NMR and IR spectroscopy
corroborated formation of the cobalt dinitrogen compound. The
conversion of (iPrPDI)Co(CO) to (iPrPDI)CoN2 could be hastened
by removing the solvent from the carbonyl compound and
preparing solutions of the compound in the presence of N2. CO
displacement by N2 was only observed under conditions where
dinitrogen was in vast excess.
respectively. To compare the N2 stretching frequency of
(iPrPIEA)CoN2 to that of (iPrPDI)CoN2, the IR spectrum of the
former compound was also recorded in toluene. A strong NtN
stretch was observed at 2145 cm-1, higher in energy than the
value of 2093 cm-1 recorded for (iPrPDI)CoN2. (iPrPIEA)CoN2
is related to the dimeric variant of the compound prepared by
Gambarotta and co-workers, where the monomers are bridged
through the intact imine methyl groups.42 The solid-state infrared
spectrum (Nujol) of the dimer exhibits a single strong band at
2153 cm-1, comparable to the value of 2148 cm-1 for the
monomeric compound described here.
Because previous studies have shown that the bis(imino)py-
ridine ligand is capable of accepting up to three electrons,12a,43
additional reduction chemistry of (iPrPDI)CoN2 was explored.
Treating a THF solution of (iPrPDI)CoCl2 with excess (∼3 equiv)
sodium naphthalenide for 3 h at 23 °C followed by filtration
and recrystallization from diethyl ether furnished a mixture of
the cobalt dinitrogen anions, [(iPrPDI)CoN2][Na(solv)3] (solv )
THF, Et2O), as judged by solid-state (KBr) infrared spectroscopy
(eq 5). For the diethyl ether solvate, a strong NtN band was
observed at 2046 cm-1 in toluene solution. The diethyl ether
solvate was not isolated free of the THF solvate and was only
observed as a mixture by IR spectroscopy following handling
of the compound in Et2O. This band shifts substantially to 1981
cm-1 for the THF compound. The differences in stretching
With a small library of neutral aryl-substituted bis(imino)py-
ridine cobalt dinitrogen complexes in hand, we sought to prepare
other N2 derivatives with different formal cobalt oxidation states
for comparison. Previous studies38-41 have shown that the imine
methyl groups of the bis(imino)pyridine chelate are subject to
deprotonation and present the opportunity to prepare authentic
mono- and dianionic forms of the ligand. Treatment of a THF
solution of (iPrPDI)CoCl with 1 equiv of NaOtBu in the presence
9
1680 J. AM. CHEM. SOC. VOL. 132, NO. 5, 2010

Bis(imino)pyridine Cobalt Dinitrogen Complexes
A R T I C L E S
Table 2. Comparison of Experimentally Determined and Computed
Selected Bond Lengths (Å) of [(iPrPDI0)CoIN2]+, [(iPrPDI•-)CoIN2],
and [(iPrPDI2-)CoIN2]-
frequencies are likely a result of the different coordination modes
for the sodium atom and its interaction with the N2 ligand in
the cation. In iron chemistry with deprotonated bis(imino)py-
ridine ligands, Gambarotta and co-workers have observed side-
on coordination of a sodium cation with coordinated N2 in a
THF solvate, while end-on sodium coordination with the N2
ligand was observed in the corresponding ether solvate.44
Notably, these structural differences are reflected in the N2
stretching frequencies as the ether solvate has a NtN band at
1965 cm-1, a value that shifts to 1912 cm-1 in the related THF
compound. It is likely that a similar effect is responsible for
the differences in the cobalt dinitrogen anions described here;
however, the lack of structural evidence precludes a definitive
rationale. Importantly, in both [(iPrPDI)CoN2][Na(solv)3] com-
pounds, the N2 stretching frequency is at lower energy than that
in the corresponding neutral compound (iPrPDI)CoN2.
[(iPrPDI0)CoIN2]+
[(iPrPDI•-)CoIN2]
[(iPrPDI2-)CoIN2]-
calcd
exptla
calcd
exptl
calcd
Co(1)-N(1)
Co(1)-N(2)
Co(1)-N(3)
Co(1)-N(4)
N(1)-C(2)
N(2)-C(3)
N(2)-C(7)
N(3)-C(8)
C(2)-C(3)
C(2)-C(3)
N(4)-N(5)
1.964
1.844
1.965
1.853
1.302
1.347
1.347
1.302
1.479
1.479
1.102
1.908(3)
1.812(3)
1.915(4)
1.841(3)
1.303(5)
1.354(4)
1.356(5)
1.303(5)
1.458(5)
1.464(4)
1.112(5)
1.932
1.839
1.932
1.810
1.337
1.368
1.367
1.337
1.443
1.443
1.110
1.8719(15)
1.8084(14)
1.8773(14)
1.7884(16)
1.341(2)
1.379(2)
1.367(2)
1.331(2)
1.418(2)
1.920
1.830
1.920
1.782
1.368
1.389
1.389
1.368
1.417
1.418
1.122
1.429(2)
1.104(2)
a From ref 23.
Table 3. Metal Contributions (%) to the Frontier Orbitals of
Because of limited solubility, the benzene-d6 1H NMR
spectrum of [(iPrPDI)CoN2][Na(THF)3] was recorded in a 1:1
mixture of benzene-d6 and THF-d8. Resonances in the typical
diamagnetic region were observed. For example, the in-plane
imine methyl group appears at 1.85 ppm while the pyridine meta
and para hydrogens appear at 8.39 and 7.07 ppm, suggesting a
pure diamagnetic ground state with little to no contribution from
triplet excited states.12b,28 Treatment of a diethyl ether slurry
of [(iPrPDI)CoN2][Na(THF)3] with 1 equiv of [Cp2Fe][BPh4]
resulted in quantitative conversion to (iPrPDI)CoN2 and ferrocene
with precipitation of NaBPh4 (eq 5).
Computational Studies and Evaluation of Electronic Struc-
tures. The synthesis of the neutral and anionic cobalt dinitrogen
complexes (iPrPDI)CoN2 and [(iPrPDI)CoN2]- in conjunction with
the previously reported cationic compound [(iPrPDI)CoN2]+
provides a rare opportunity to evaluate the electronic structure
of a series of molecules that differ by three sequential oxidation
states. Holland and co-workers recently reported the synthesis
and characterization of both neutral and dianionic, dimeric,
ꢀ-diketiminate cobalt complexes with bridging end-on dinitrogen
ligands and demonstrated the influence of the alkali-metal
cations on the weakening of the NtN bond.45 In addition to
the structural and spectroscopic data described above, DFT
calculations using the B3LYP functional were also performed.
For the cationic and anionic compounds, only the [(iPrPDI)CoN2]
portion of the molecule was calculated in the gas phase. No
other truncations of the compounds were performed. The
optimized geometries for all three of the computed compounds
are in reasonable agreement with the available crystallographic
data (Table 2). The largest discrepancies are with the Co-N
bond lengths, where the B3LYP functional typically overesti-
mates the distances.
[(iPrPDI0)CoIN2]+, [(iPrPDI•-)CoIN2], and [(iPrPDI2-)CoIN2]-
[(iPrPDI0)CoIN2]+
[(iPrPDI•-)CoIN2]
[(iPrPDI2-)CoIN2]-
2
2
dx -y
73
6
73
12
11
88
80
73
77
70
12
10
89
75
72
72
PDI a2
PDI b2
11
92
82
79
75
2
dz
dyz
dxz
dxy
The geometry of the diamagnetic, cationic dinitrogen complex
[(iPrPDI0)CoIN2]+ was calculated using spin-restricted DFT (S
) 0). The alternative, spin-unrestricted broken-symmetry (BS)
(1,1) solution, corresponding to a [(iPrPDI•-)CoIIN2]+ formula-
tion, was found to be 8.2 kcal/mol higher in energy. In this
solution, the R and ꢀ orbitals have different symmetry (metal
2
dz (a1) and ligand b2). For the spin-restricted solution, the
computed C-N and C-C bond distances clearly indicate a
neutral, diamagnetic PDI0 ligand.12 Accordingly, the a2 and b2
orbitals of the bis(imino)pyridine ligand, those typically re-
sponsible for redox activity, are empty and constitute the
LUMO+1 and LUMO of the complex, respectively. Four
doubly occupied metal d orbitals (metal contributions >70% in
each case) were found, establishing a low-spin d8 electronic
configuration for the Co(I) center (Table 3). One empty cobalt-
2
2
based d orbital (dx -y ) was also identified. This electronic
structure is in agreement with that proposed by Gibson and co-
workers23 and is as expected for a Co(I) complex in a square
planar ligand field. Thus, this compound is best described as
[(iPrPDI0)CoIN2]+.
As established experimentally, the neutral cobalt dinitrogen
compound (iPrPDI)CoN2 has an S ) 1/2 ground state and was
calculated accordingly as a spin-unrestricted doublet. The
computed frontier molecular orbital scheme is similar to that
for [(iPrPDI0)CoIN2]+ with four doubly occupied metal d orbitals
(37) Scott, J.; Gambarorra, S.; Korobkov, I.; Knijnenburg, Q.; de Bruin,
B.; Budzelaar, P. H. M. J. Am. Chem. Soc. 2005, 127, 17204.
(38) Bowman, A. C.; Bart, S. C.; Heinemann, F. W.; Meyer, K.; Chirik,
P. J. Inorg. Chem. 2009, 48, 5587.
2
2
and one empty cobalt orbital assigned as dx -y , again establish-
ing a cobalt(I), d8 center (Table 3). The unpaired electron was
located in the b2 orbital of the bis(imino)pyridine ligand, while
the corresponding a2 orbital remains empty and is the LUMO
of the complex. This view of the electronic structure is
corroborated by the Mulliken spin density population analysis,
which establishes that the unpaired electron is almost exclusively
located on the PDI ligand while only a small amount of spin
density is transferred to the Co(I) ion by spin polarization (Figure
4).
(39) Bouwkamp, M. W.; Lobkovsky, E.; Chirik, P. J. Inorg. Chem. 2006,
45, 2.
(40) Sugiyama, H.; Aharonian, G.; Gambarotta, S.; Yap, G. P. A.;
Budzelaar, P. H. M. J. Am. Chem. Soc. 2002, 124, 12268.
(41) Korobkov, I.; Gambarotta, S.; Yap, G. P. A. Organometallics 2002,
21, 3088.
(42) Scott, J.; Gambarotta, S.; Korobkov, I. Can. J. Chem. 2005, 83, 279.
(43) Enright, D.; Gambarotta, S.; Yap, G. P. A.; Budzelaar, P. H. M. Angew.
Chem., Int. Ed. 2002, 41, 3873.
(44) Scott, J.; Vidyaratne, I.; Korobkov, I.; Gambarotta, S.; Budzelaar,
P. H. M. Inorg. Chem. 2008, 47, 896.
(45) Ding, K.; Pierpont, A. W.; Brennessel, W. W.; Lukat-Rodgers, G.;
Rodgers, K. R.; Cundari, T. R.; Bill, E.; Holland, P. L. J. Am. Chem.
Soc. 2009, 131, 9471.
The computed spin density distribution presented in Figure
4 is in excellent agreement with the experimentally obtained
9
J. AM. CHEM. SOC. VOL. 132, NO. 5, 2010 1681

A R T I C L E S
Bowman et al.
radicals.37 The [(iPrPDI•-)CoIN2] electronic structure description
is also consistent with the SQUID magnetization data, which
establish a plateau in the effective magnetic moment more
consistent with a bis(imino)pyridine radical rather than a cobalt-
based SOMO. One discrepancy between the computational and
experimental data is the short Cimine-Cipso distances of 1.418
and 1.429 Å determined by X-ray diffraction (Table 2). These
bond lengths are near the typical range for a dianionic
bis(imino)pyridine ligand.12,12 However, the differences in
computational and experimental bond length are not significant
within the accepted error of the calculations (0.025 Å), and no
such discrepancy was observed between the computational and
experimental Cimine-Nimine distances. A similar overestimation
of the Cimine-Cipso bond distances was observed for the cationic
cobalt dinitrogen complex [(iPrPDI)CoN2]+. To address this issue,
alternative BS(1,2) and BS(3,2) electronic structure descriptions
were also calculated to account for potential low- and high-
spin Co(II) possibilities, respectively. Both approaches con-
verged to the [(iPrPDI•-)CoIN2] solution, reaffirming the validity
of this description of the compound. Although the Cimine-Cipso
distances in the solid-state structure of the neutral compound
(iPrPDI)CoN2 are more contracted than those of most compounds
assigned as having monoreduced chelates,12 the data for
(iPrBPDI)CoN2 are well within the accepted range for a [PDI•-]
ligand, and overall the combined spectroscopic and computa-
tional data are most consistent with [(iPrPDI•-)CoIN2]. It should
also be noted that both (iPrPDI)CoCl2 and [(iPrPDI)CoN2]+ are
accepted as having neutral bis(imino)pyridine chelates, but the
Cimine-Cipso and Cimine-Nimine distances are notably different.
Thus, reduction of [(iPrPDI0)CoIN2]+ to [(iPrPDI•-)CoIN2] is
purely ligand-based (Figure 4).
DFT calculations were also performed on the cobalt dinitro-
gen anion [(iPrPDI)CoN2]- using a spin-restricted method.
Unfortunately, this compound has eluded structural characteriza-
tion, and as a result, comparison of computed and experimental
metrical parameters was not possible. As with the cationic and
neutral cobalt dinitrogen compounds, [(iPrPDI)CoN2]- is best
described as a cobalt(I) complex with four doubly occupied
orbitals and one empty metal-based orbital (Table 3). Thus, one-
electron reduction of [(iPrPDI•-)CoIN2] to [(iPrPDI2-)CoIN2]- is
ligand-based. Consequently, the electronic structure of the anion
is very similar to that of the neutral complex with the exception
of the now doubly reduced [PDI]2- ligand, which is clearly
supported by the computed structural parameters of the ligand
(Table 2).12 Although a high-spin Co(I) configuration is unlikely
in an idealized square planar geometry, an open-shell BS(2,2)
calculation was performed to exclude the alternative of a triplet
ligand coordinated to a high-spin cobalt(I) center. Reassuringly,
this computation converged back to the closed-shell singlet
solution.
More detailed analysis of the electronic structure of the
bis(imino)pyridine dianion reveals coordination of the closed-
shell form of the ligand with a doubly occupied b2 orbital. In
related reduced bis(imino)pyridine iron chemistry, previous
computational studies have established that the triplet diradical
form of the [PDI]2- dianion is favored although the singlet state
is only slightly higher in energy and accessible.12b The differ-
ences in coordination of the singlet versus triplet bis(imino)py-
ridine dianion can be traced to the d electron configuration of
the metal. In the neutral iron dinitrogen complex (iPrPDI)Fe(N2)2,
the metal is in an intermediate-spin d6 electron configuration
(SFe ) 1), well poised to antiferromagnetically couple to a
chelate triplet diradical dianion. By contrast, the anionic cobalt
Figure 4. (a) Qualitative molecular orbital diagram for [(iPrPDI•-)CoIN2]
computed from a B3LYP DFT calculation. (b) Spin density plot obtained
from a Mulliken population analysis. Positive spin density is shown in red,
and negative spin density is shown in yellow. The coordinate system is
defined as “z” perpendicular to the idealized square plane of the molecule
and “x” along the CosNtN vector.
EPR data which exhibit a very small 57Co superhyperfine
coupling and almost no g anisotropy, typical for [PDI]-
9
1682 J. AM. CHEM. SOC. VOL. 132, NO. 5, 2010

Bis(imino)pyridine Cobalt Dinitrogen Complexes
A R T I C L E S
Table 4. Comparison of Experimentally Determined and Computed
square planar complex. Once this electronic configuration is
achieved, the second bis(imino)pyridine ligand reduction, as
evidenced by the synthesis and computation of
[(iPrPDI2-)CoIN2]-, is chelate-based due to the relatively high
N2 Stretching Frequencies (cm-1
)
[(iPrPDI0)CoIN2]+
[(iPrPDI•-)CoIN2]
[(iPrPDI2-)CoIN2]-
exptl
calcd
2184
2190
2093
2114
2046
2041
2
2
energy of the dx -y orbital in a square planar ligand field.
Conclusions
dinitrogen complex [(iPrPDI2-)CoIN2]- has a d8 electronic
configuration at cobalt (SCo ) 0) and is ill-equipped to engage
in antiferromagnetic coupling, and hence, the closed-shell form
of the dianionic chelate is preferred.
Synthesis of neutral and anionic bis(imino)pyridine cobalt
dinitrogen complexes has been accomplished, providing the first
examples of two- and three-electron reduction products from
the corresponding cobalt dichloride compounds. The electronic
structure of these compounds, as well as the previously reported
cationic dinitrogen compound, was evaluated by spectroscopic
and computational methods. Within the series of dinitrogen
compounds, bis(imino)pyridine-based reductions were observed
with preservation of the Co(I), d8 configuration. For the neutral
compound (iPrPDI)CoN2, the ligand-centered radical was ob-
served by EPR spectroscopy and confirmed by SQUID mag-
netometry. In the case of the anionic cobalt dinitrogen complex,
the bis(imino)pyridine dianion is closed-shell, in contrast to the
related iron dinitrogen compound where the triplet diradical is
observed. The preference for the closed-shell versus triplet
diradical bis(imino)pyridine dianion is determined by the
electronic configuration of the metal where intermediate-spin
d6 ferrous anions are well suited for antiferromagnetic coupling
to the S ) 1 diradical while the Co(I), d8 configuration leaves
a low-lying bis(imino)pyridine molecular orbital which favors
the singlet state.
Frequency calculations were also performed at the BP86 level
of DFT for all three dinitrogen compounds. Reproducing
spectroscopic properties is key to calibrate the computational
results against experimental data. The computed and experi-
mental NtN stretching frequencies are presented in Table 4,
and there is excellent agreement with the data. The computations
reproduce the N2 stretching frequencies within 6, 21, and 5 cm-1
for the cationic, neutral, and anionic cobalt dinitrogen com-
plexes, respectively, thereby validating the computed models
of the electronic structure. For the dinitrogen anion, the diethyl
ether solvate was chosen as the experimental comparison likely
due to the less perturbing “end-on” coordination of the sodium
cation to the terminal N2 ligand.
The combined experimental and computational data clearly
establish the electronic structures for the series of bis(imino)py-
ridine cobalt dinitrogen complexes (Figure 5). The sequential
one-electron reductions from [(iPrPDI0)CoIN2]+ to [(iPrPDI•-)-
CoIN2] to [(iPrPDI2-)CoIN2]- are ligand based. Considering the
synthetic sequence used to prepare the neutral dinitrogen
compound [(iPrPDI•-)CoIN2] and the anion [(iPrPDI2-)CoIN2]-
also reveals that one-electron reduction of [(iPrPDI0)CoIICl2] to
[(iPrPDI•-)CoIICl] is ligand based and maintains the Co(II)
oxidation state although the metal ion transitions from high spin
(SCo ) 3/2) to low spin (SCo ) 1/2) due to the increased field
strength imparted by the one-electron-reduced bis(imino)pyri-
dine and the square planar geometry. The second one-electron
reduction from [(iPrPDI•-)CoIICl] to [(iPrPDI•-)CoIN2] occurs at
the metal and preserves the oxidation state of the chelate, likely
due to the ligand field stabilization energy derived from a d8
Experimental Section46
Preparation of (iPrPDI)Co(N2). A 250 mL round-bottom flask
was charged with 21.6 g (107.7 mmol) of Hg, approximately 100
mL of toluene, and a magnetic stir bar. To the flask was added
0.108 g (4.68 mmol) of Na in small pieces with vigorous stirring.
The amalgam was stirred for 20 min, and then 0.477 g (0.780 mmol)
of (iPrPDI)CoCl2 was added to the flask. A color change to purple
over 30 min was initially observed followed by a gradual color
change to dark teal. The reaction was stirred for 6 h and then filtered
through Celite and concentrated. Cooling the solution to -35 °C
afforded 0.301 g (65%) of dark teal crystals identified as (iPrPDI)-
Figure 5. Electronic structure summary for bis(imino)pyridine cobalt dinitrogen complexes and reduction from the corresponding chloride derivatives.
9
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A R T I C L E S
Bowman et al.
Co(N2). Anal. Calcd for C33H43N5Co: C, 69.70; H, 7.62; N, 12.32.
Found: C, 69.48; H, 7.81; N, 12.07. Magnetic susceptibilty
(benzene-d6, 293 K): µeff ) 1.6(2) µB. 1H NMR (benzene-d6): δ )
2.90 (124.5 Hz), 4.27 (242.9 Hz), 6.01 (54.5 Hz). IR (toluene):
the solution became green in color and homogeneous. After this
time, 0.200 g (0.327 mmol) of (iPrPDI)CoCl2 was 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, producing a bright green solution. The solution
was filtered through Celite, concentrated, and cooled to -35 °C to
yield forest green crystals identified as a mixture of [(iPrPDI)Co-
(N2)][Na(Et2O)3] and [(iPrPDI)Co(N2)][Na(THF)3]. Dissolution of
the crystals in tetrahydrofuran followed by removal of volatiles
resulted in quantitative conversion to [(iPrPDI)Co(N2)][Na(THF)3]
(0.277 g, 52%). Anal. Calcd. for C45H67N5O3NaCo: C, 66.89; H,
νNN ) 2093 cm-1
.
Alternative Preparation of (iPrPDI)Co(N2). A 250 mL round-
bottom flask was charged with 0.502 g (0.821 mmol) of (iPrPDI)-
CoCl2, approximately 100 mL of toluene, and a magnetic stir bar.
The reaction was cooled in a liquid nitrogen chilled cold well, and
NaEt3BH (820 µL of a 1.0 M solution, 0.821 mmol) was added
dropwise with stirring. The reaction was warmed to room temper-
ature accompanied by a color change to dark purple. The solution
was again chilled in the cold well, and NaEt3BH (820 µL of a 1.0
M solution, 0.821 mmol) was added dropwise. The reaction was
warmed to room temperature, resulting in a color change to dark
teal. The solution was filtered through Celite, concentrated, and
cooled to -35 °C and yielded 0.294 (63%) of dark teal crystals of
(iPrPDI)Co(N2).
1
8.36; N, 8.67. Found: C, 66.91; H, 8.60; N, 8.69. H NMR (1:1
THF-d8/benzene-d6): δ ) 1.19 (s, 24H, CH(CH3)2), 1.85 (s, 6H,
C(CH3)), 3.02 (s, 4H, CH(CH3)2), 7.07 (s, 1H, pyridine para), 7.25
(s, 4H, aryl meta), 7.36 (s, 2H, aryl para), 8.39 (s, 2H, pyridine
meta). IR (KBr): νNN ) 2046 cm-1 (solv ) Et2O), 1981 cm-1 (solv
) THF).
Spectroscopic Characterization of (iPrPDI)Co(CO). A J. Young
NMR tube was charged with 0.009 g (0.016 mmol) of (iPrPDI)-
Co(N2) in benzene-d6. The tube was degassed, and 20 equiv (50
Torr in a 100.1 mL gas bulb) of carbon monoxide was added at
-196 °C. Upon warming to room temperature, there was a rapid
color change to red-brown. 1H NMR spectroscopy confirmed
quantitative conversion to (iPrPDI)Co(CO). The infrared spectrum
of (iPrPDI)Co(CO) was obtained by exposing a 7.0 mM solution of
the compound to an atmosphere of carbon monoxide in a three-
neck flask containing the infrared probe. 1H NMR (benzene-d6): δ
) 2.63 (183.6 Hz), 3.21 (140.3 Hz), 6.03 (256.7 Hz), 10.62 (405.5
Preparation of (iPrBPDI)Co(N2). A 100 mL round-bottom flask
was charged with 16.6 g (82.8 mmol) of Hg, approximately 50
mL of toluene, and a magnetic stir bar. To the flask was added
0.083 g (3.61 mmol) of Na in small pieces with vigorous stirring.
The amalgam was stirred for 20 min, and then 0.467 g (0.635 mmol)
of (iPrBPDI)CoCl2 was added to the flask. A color change to dark
red over 30 min was initially observed followed by a gradual color
change to forest green. The reaction was stirred for 24 h and then
filtered through Celite and concentrated. Cooling the solution to
-35 °C afforded 0.185 g (42%) of forest green crystals identified
as (iPrBPDI)Co(N2). Anal. Calcd for C43H47N5Co: C, 74.55; H, 6.85;
N, 10.11. Found: C, 74.71; H, 6.73; N, 9.91. Magnetic susceptibilty
(benzene-d6, 293 K): µeff ) 1.1(3) µB. 1H NMR (benzene-d6): δ )
3.61 (at least 350 Hz), 3.78 (at least 350 Hz), 5.58 (192.65 Hz). IR
Hz). IR (toluene): νCO ) 1975 cm-1
.
Acknowledgment. We thank the U.S. National Science Foun-
dation and Deutsche Forschungsgemeinschaft for a Cooperative
Activities in Chemistry between U.S. and German Investigators
grant. We also thank Dr. Stephen Sproules and Dr. Eckhard Bill
for assistance with EPR spectroscopy and data analysis.
(toluene): νNN ) 2111 cm-1
.
Preparation of [(iPrPDI)Co(N2)][Na(solv)3]. Sodium naphtha-
lenide was prepared in situ by addition of approximately 50 mL of
tetrahydrofuran to a 100 mL round-bottom flask containing 0.026
g (1.13 mmol) of sodium metal and 0.147 g (1.14 mmol) of
naphthalene. The reaction was stirred for 3 h, during which time
Supporting Information Available: Additional experimental
procedures and crystallographic details for (iPrPDI)CoN2 and
(iPrBPDI)CoN2 (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
(46) Representative experimental procedures are reported here. General
considerations, computational details, and additional experimental
procedures and spectroscopic data are reported in the Supporting
Information.
JA908955T
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1684 J. AM. CHEM. SOC. VOL. 132, NO. 5, 2010