Para-quinone-containing bis(pyrazol-1-yl)methane ligands: Coordination behavior toward CoII and a C-H activation reaction with Ce IV

Article abstract of DOI:10.1021/ic100754k

Two series of sterically demanding para-dimethoxyphenyl-substituted bis(pyrazol-1-yl)methane ligands have been synthesized, i.e., L ²R,R′ = ((MeO)₂C₆H₃)C(H) (pz^R,R′)₂ and L³R,R′ = ((MeO) ₂C₆H₃)C(Me)(pz^R,R′) ₂ (R,R′ = 3-Me,5-Me; 3-Ph,5-H; 3-tBu,5-H). In the solid state, already the sterically least encumbered derivative L2^Me2 is able to stabilize Co^II complexes of the form [X₂Co(L2 ^Me2)] (X = Cl, NO₃); in solution, these complexes are at an equilibrium with the 1:2 species [Co(L2^Me2)₂] ²⁺. Oxidative demethylation of L3^Ph and L ³t^Bu with [Ce(NH₄)₂(NO ₃)₆] leads to the corresponding para-benzoquinonyl- substituted bis(pyrazol-1-yl)methane ligands L4^Ph and L ⁴t^Bu. Contrary to that, the methyl derivative L3 ^Me2 is transformed into the ortho-benzoquinone species L5 ^Me2, which still contains one methoxy substituent while one oxygen atom has been newly introduced. The formation of L5^Me2 requires (i) the admission of air, (ii) the presence of both methoxy substituents of L3 ^Me2, and (iii) the presence of (methyl) substituents both at the exocyclic carbon atom and at the 5-positions of the pyrazolyl rings. The parent ortho-hydroquinonyl-substituted bis(pyrazol-1-yl)methane ligand ((HO) ₂C₆H₃)C(H)(pz^H,H)₂ is readily available from 3,4-dihydroxybenzaldehyde and (pz^H,H) ₂SO/pyridine.

Full text of DOI:10.1021/ic100754k

Inorg. Chem. 2010, 49, 7435–7445 7435
DOI: 10.1021/ic100754k
Para-Quinone-Containing Bis(pyrazol-1-yl)methane Ligands:
Coordination Behavior Toward CoII and a C-H Activation Reaction with CeIV
Florian Blasberg,† Jan W. Bats,‡ Michael Bolte,† Hans-Wolfram Lerner,† and Matthias Wagner*,†
†
€
Institut fu€r Anorganische und Analytische Chemie, Goethe-Universitat Frankfurt, Max-von-Laue-Str. 7,
‡
€
D-60438 Frankfurt (Main), Germany, and Institut fu€r Organische Chemie, Goethe-Universitat Frankfurt,
Max-von-Laue-Str. 7, D-60438 Frankfurt (Main), Germany
Received April 20, 2010
Two series of sterically demanding para-dimethoxyphenyl-substituted bis(pyrazol-1-yl)methane ligands have been
0
0
0
0
synthesized, i.e., L2R,R =((MeO)2C6H3)C(H)(pzR,R )2 and L3R,R =((MeO)2C6H3)C(Me)(pzR,R )2 (R,R0 =3-Me,5-Me;
3-Ph,5-H; 3-tBu,5-H). In the solid state, already the sterically least encumbered derivative L2Me2 is able to stabilize CoII
complexes of the form [X2Co(L2Me2)] (X = Cl, NO3); in solution, these complexes are at an equilibrium with the 1:2
species [Co(L2Me2)2]2þ. Oxidative demethylation of L3Ph and L3tBu with [Ce(NH4)2(NO3)6] leads to the correspond-
ing para-benzoquinonyl-substituted bis(pyrazol-1-yl)methane ligands L4Ph and L4tBu. Contrary to that, the methyl derivative
L3Me2 is transformed into the ortho-benzoquinone species L5Me2, which still contains one methoxy substituent while
one oxygen atom has been newly introduced. The formation of L5Me2 requires (i) the admission of air, (ii) the presence
of both methoxy substituents of L3Me2, and (iii) the presence of (methyl) substituents both at the exocyclic carbon atom
and at the 5- positions of the pyrazolyl rings. The parent ortho-hydroquinonyl-substituted bis(pyrazol-1-yl)methane
ligand ((HO)2C6H3)C(H)(pzH,H)2 is readily available from 3,4-dihydroxybenzaldehyde and (pzH,H)2SO/pyridine.
Introduction
that stoichiometric quantities of BQ are consumed,6 because
the reoxidation of para-hydroquinone (HQ) to BQ by O2 is
usually slow under the reaction conditions applied.2 In order
to regenerate BQ with O2 as the final oxidizing agent, a second
redox catalyst is therefore necessary. Among other com-
pounds, transition metal complexes with macrocyclic ligands
([MLm]) have proven to be efficient electron transfer reagents
for this purpose (e.g., [MLm] = iron phthalocyanine, cobalt
tetraphenylporphyrin, cobalt salenes).7-9
Further developments with the aim to increase the overall
rate of electron transfer from Pd0 to O2 focused on hybrid
redox catalysts, {Q-[MLm]}, in which para-quinone moieties
are covalently tethered to cobalt salene or cobalt porphyrine
complexes (note: in this paper, the general term “quinone (Q)”
will be used whenever the oxidation state of any derivative of
hydroquinone/benzoquinone needs not to be specified).10-12
The importance of transition metal ions in homogeneous
catalysis rests to a large extent on their ability to adopt diffe-
rent oxidation states. However, in several cases, like in some
catalytic oxidation reactions of organic compounds with
dioxygen,1,2 the presence of an additional redox cocatalyst
is required for efficient substrate conversion. One prominent
example is the PdII-mediated oxidation of ethylene to acet-
aldehyde (Wacker oxidation), which employs CuCl2 in order
to promote the aerobic reoxidation of intermediately formed
Pd0 and to close the catalytic cycle.3
Later developments led to para-benzoquinone (BQ) deri-
vatives as oxidizing agents in Wacker-related syntheses2 as
well as in Ru-catalyzed dehydrogenation reactions of alcohols4
and amines.5 A drawback, however, results from the fact
€
(7) Backvall, J.-E.; Hopkins, R. B.; Grennberg, H.; Mader, M. M.;
*To whomcorrespondence should beaddressed. E-mail: Matthias.Wagner@
chemie.uni-frankfurt.de.
(1) van Leeuwen, P. W. N. M. Homogeneous Catalysis. Understanding the
Awasthi, A. K. J. Am. Chem. Soc. 1990, 112, 5160–5166.
€
(8) Backvall, J.-E.; Chowdhury, R. L.; Karlsson, U. J. Chem. Soc., Chem.
Commun. 1991, 473–475.
Art; Kluwer Academic Publishers: Dordrecht, Boston, London, 2004.
ꢀ
€
(9) Csjernyik, G.; Ell, A. H.; Fadini, L.; Pugin, B.; Backvall, J.-E. J. Org.
€
(2) Piera, J.; Backvall, J.-E. Angew. Chem., Int. Ed. 2008, 47, 3506–3523.
€
(3) Smidt, J.; Hafner, W.; Jira, R.; Sedlmeier, J.; Sieber, R.; Ruttinger, R.;
Chem. 2002, 67, 1657–1662.
˚
€
(10) Bystrom, S. E.; Larsson, E. M.; Akermark, B. J. Org. Chem. 1990, 55,
5674–5675.
Kojer, H. Angew. Chem. 1959, 71, 176–182.
€
(4) Johnson, J. B.; Backvall, J.-E. J. Org. Chem. 2003, 68, 7681–7684.
ꢀ
€
€
(5) Ell, A. H.; Samec, J. S. M.; Brasse, C.; Backvall, J.-E. Chem. Commun.
(11) Grennberg, H.; Faizon, S.; Backvall, J.-E. Angew. Chem., Int. Ed.
2002, 1144–1145.
Engl. 1993, 32, 263–264.
˚
€
(6) Hegedus, L. S.; Allen, G. F.; Bozell, J. J.; Waterman, E. L. J. Am.
Chem. Soc. 1978, 100, 5800–5807.
(12) Purse, B. W.; Tran, L.-H.; Piera, J.; Akermark, B.; Backvall, J.-E.
Chem.;Eur. J. 2008, 14, 7500–7503.
r
2010 American Chemical Society
Published on Web 07/21/2010
pubs.acs.org/IC

7436 Inorganic Chemistry, Vol. 49, No. 16, 2010
Blasberg et al.
Scheme 1. Synthesis of the CoII Complex [Co(L1)2]a
Figure 1. The para-naphthoquinone-substituted bis(pyrazol-1-yl)methane
complex A and the para-hydroquinone-based bis(pyrazol-1-yl)methane
ligands B and C; pz = pyrazol-1-yl.
In a complementary approach, our group has recently
reported on the para-naphthoquinone-substituted bis-
(pyrazol-1-yl)methane13 complex A (Figure 1).14 Here, the
substrate-selective catalyst is connected to the redox mediator
({[PdLn]-Q}). Using A as model system, we have monitored
the formation of a para-naphthosemiquinonate radical after
selective PdII f Pd0 reduction.14 This result clearly indicates
that A is properly designed to allow for a single-electron trans-
fer from the metal atom to the para-naphthoquinone moiety,
which is an important proof-of-concept on the way to func-
tional monomolecular oxidation catalysts {[PdLn]-Q-[MLm]}.
The question thus arises whether bis(pyrazol-1-yl)methane
ligands with para-quinone substituents are also suitable for
the preparation of alternative hybrid redox systems {Q-[MLm]}.
If this is the case, ditopic ligands like B15 (Figure 1) would
have promising potential as linkers between the two metal com-
plex parts [PdLn] and [MLm] (after synthesis protocols lead-
ing to heterobimetallic compounds have been worked out).
Given this background, we decided to explore the coordi-
nation properties of derivatives of the monotopic bis(pyrazol-
1-yl)methane C15 (Figure 1) toward CoII. The CoII ion was
chosen, because it has already been found to be an efficient redox
mediator in other combined redox systems {Q-[MLm]}.7-9
Moreover, tris(pyrazol-1-yl)borate complexes of the form
[CoTptBu,Me(η2-O2)]16 are known, and dioxygen binding to
cobalt complexes in general has been extensively reviewed.17-19
The purpose of this paper is to report on CoII complexes
with selected C-type ligands. Special emphasis is put on the
influence of the secondary coordination sphere (i.e., the
substitution pattern of the pyrazolyl rings) on the mole-
cular structures of the compounds. Since we are interested
in the redox behavior of the new compounds, it was neces-
sary to prepare each ligand both in its reduced and in its
oxidized state. In one case, oxidation with [Ce(NH4)2-
(NO3)6] led to an unexpected C-H activation reaction with
the formation of an ortho- rather than a para-benzoquinone
moiety; key factors governing this unusual transformation
will be unveiled.
a Reagents and conditions: (a) þ NaOMe, MeOH, rt.
Results and Discussion
Synthesis and Characterization of the Hexacoordinate
Complex [Co(L1)2]. For a start, we chose the known
ligand [L1]- (Scheme 1),15 because it contains a solubilizing
Si(iPr)3 group. Moreover, the molecule carries a tert-butyl
substituent and thus possesses similar steric bulk as 2,6-
di-tert-butylbenzoquinone, which has been shown to be
superior to parent para-benzoquinone in triple catalytic
systems (Ru-catalyst/2,6-tBu-BQ/Co-macrocycle) for the
aerobic oxidation of secondary alcohols.20
[Co(L1)2] was prepared from Co(OAc)2 and HL1 in
MeOH and in the presence of NaOMe as a base (Scheme 1).
Single crystals of [Co(L1)2] suitable for X-ray diffraction
were grown from a CH2Cl2 solution at -20 °C (Table 1).
The compound crystallizes with two crystallographically
independent molecules, [Co(L1)2]A (C1 symmetry) and
[Co(L1)2]B (Ci symmetry), in the asymmetric unit. Since
all key structural parameters of the two molecules are
very similar, only the centrosymmetric [Co(L1)2]B will be
discussed further (Figure 2).
The CoII ion of [Co(L1)2]B is located in an octahedral
coordination environment made up by two tripodal, tri-
dentate [L1]- ligands. The Co2-O5 bond length amounts
˚
to 2.020(2) A; the average Co-pyrazolyl bond length is
˚
2.130(2) A. These values are close to the corresponding
bond lengths in a related complex, which also adopts a
trans configuration but features methyl groups instead of
i
˚
OSi( Pr)3 substituents (Co-O=1.998(2) A; av. Co-N=
2.145(2) A).
(13) Pettinari, C.; Pettinari, R. Coord. Chem. Rev. 2005, 249, 663–691.
(14) Scheuermann, S.; Sarkar, B.; Bolte, M.; Bats, J. W.; Lerner, H.-W.;
Wagner, M. Inorg. Chem. 2009, 48, 9385–9392.
(15) Scheuermann, S.; Kretz, T.; Vitze, H.; Bats, J. W.; Bolte, M.; Lerner,
H.-W.; Wagner, M. Chem.;Eur. J. 2008, 14, 2590–2601.
(16) Egan, J. W.; Haggerty, B. S.; Rheingold, A. L.; Sendlinger, S. C.;
Theopold, K. H. J. Am. Chem. Soc. 1990, 112, 2445–2446.
(17) Jones, R. D.; Summerville, D. A.; Basolo, F. Chem. Rev. 1979, 79,
139–179.
21
˚
An important piece of information gained from the solid-
state structure of [Co(L1)2] is that the tert-butyl group on
the HQ fragment does not provide sufficient steric hin-
drance to prevent the formation of homoleptic [M(L)2]
€
(18) Niederhoffer, E. C.; Timmons, J. H.; Martell, A. E. Chem. Rev. 1984,
84, 137–203.
(20) Wang, G.-Z.; Andreasson, U.; Backvall, J.-E. J. Chem. Soc., Chem.
Commun. 1994, 1037–1038.
(19) Tolman, W. B. Activation of Small Molecules - Organometallic and
Bioinorganic Perspectives; Wiley-VCH: Weinheim, Germany, 2006.
(21) Higgs, T. C.; Ji, D.; Czernuscewicz, R. S.; Carrano, C. J. Inorg. Chim.
Acta 1999, 286, 80–92.

Article
Inorganic Chemistry, Vol. 49, No. 16, 2010 7437
Table 1. Selected Crystallographic Data and Structure Refinement Details for [Co(L1)2], L2tBu, and L3Me2
compound
[Co(L1)2]
L2tBu
L3Me2
4
formula
fw
color, shape
temp (K)
radiation
C52H78CoN8O4Si2 ꢀ /3 CH2Cl2
C23H32N4O2
396.53
colorless, block
165(2)
Mo KR, 0.71073 A
triclinic
P1
9.5910(18)
11.048(2)
12.917(4)
71.396(11)
71.334(16)
65.837(13)
1154.8(4)
2
C20H26N4O2
354.45
colorless, plate
173(2)
Mo KR, 0.71073 A
monoclinic
P21/n
8.0829(4)
30.7570(17)
8.7926(4)
90
1107.57
brown, block
160(2)
˚
˚
˚
Mo KR, 0.71073 A
triclinic
P1
cryst syst
space group
˚
a (A)
9.4947(10)
18.288(3)
26.640(5)
94.370(9)
93.364(9)
104.378(11)
4453.4(11)
3
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
116.165(3)
90
3
˚
V (A )
Z
1961.90(17)
4
1.200
760
0.079
Dcalcd (g cm-3
F(000)
)
1.239
1767
1.140
428
μ (mm-1
)
0.498
0.72 ꢀ 0.60 ꢀ 0.30
0.074
0.60 ꢀ 0.44 ꢀ 0.26
cryst size (mm)
no of reflns coll
no of ind reflns (Rint
data/restr/params
GOF on F2
0.41 ꢀ 0.37 ꢀ 0.15
72853
16982
19156
)
24618 (0.0559)
24618/5/959
1.392
5069 (0.0590)
5069/0/264
1.069
3676 (0.0474)
3676/0/243
1.045
R1, wR2 (I>2 σ(I))
R1, wR2 (all data)
˚
largest diff peak and hole (e A
0.0841, 0.2320
0.1389, 0.2528
2.457, -1.875
0.0514, 0.1001
0.0865, 0.1143
0.245, -0.197
0.0386, 0.1035
0.0493, 0.1090
0.227, -0.207
-3
)
(L2Me, L2Me2) or met with failure). It has to be mentioned
that in the case of L2Me an isomeric mixture of products
was obtained: the 3,30-isomer and the 3,50-isomer. In the
crude product, the stoichiometric ratio between both
isomers is 4:1, respectively. Repeated recrystallizations
from hexane led to samples containing 90% of the 3,30-
isomer. However, due to these difficulties in obtaining
isomerically pure L2Me, the compound was not conside-
red further (cf. the Supporting Information for details of
the X-ray crystal structure analysis of L2Me).
Figure 2. Molecular structure and numbering scheme of compound
[Co(L1)2]B (50% displacement ellipsoids; H atoms omitted for clarity).
Selected bond lengths [A] and bond angles [deg]: Co2-O5=2.020(2), Co2-
N10 = 2.137(3), Co2-N12 = 2.121(3), O5-C53 = 1.301(4), O6-C56 =
1.394(4); O5-Co2-N10 = 87.7(1), O5-Co2-N12 = 90.0(1), N10-
Co2-N12=86.2(1).
In the next step, the acidic methine protons of L2Me2
-
L2tBu were replaced by inert methyl groups in order to
avoid future unwanted side reactions;especially when
the ligands will be in their BQ states (cf. previous negative
experiences with the base-lability of BQ-substituted bis-
(pyrazol-1-yl)methanes14,15). To this end, the methine protons
were first abstracted with 1 equiv of nBuLi, which is why
the synthesis sequence had to start with para-dimethoxy-
benzaldehyde instead of para-dihydroxybenzaldehyde.
Subsequent addition of MeI (1 equiv) to the anionic inter-
mediate gave the corresponding ligands L3Me2-L3tBu. In
˚
complexes. In conclusion, we need to increase the steric
bulk on the pyrazolyl rings in order to be able to prepare
more reactive heteroleptic species of the form [XnM(L)].
Synthesis and Characterization of Sterically Demanding
Quinone-Substituted Bis(pyrazol-1-yl)methane Ligands. The
ligand syntheses were performed as outlined in Scheme 2.
the case of the sterically encumbered derivative, L3tBu
,
First, commercially available para-dimethoxybenzal-
0
dehyde was treated with the appropriate (pzR,R )2SO deri-
increasing the bulk by methylation results in partial
isomerization of the bis(pyrazol-1-yl)methane fragment
(3,30-isomer/3,50-isomer ≈ 8:1; note: upon prolonged
storage, the 3,50-isomer of L3tBu slowly converts back to
the 3,30-isomer).
0
vative (prepared in situ from HpzR,R , NaH, and SOCl2)
in a pyridine-catalyzed reaction.22,23 Thereby, we obta-
ined the bis(pyrazol-1-yl)methane ligands L2Me-L2tBu in
yields between 52% and 80% (note: 0 in our hands, the
alternative approach24-26 using (pzR,R )2CO and catalytic
amounts of CoCl2 gave either prohibitively low yields
Oxidative O-demethylation of L3Ph and L3tBu to their
BQ state (L4Ph, L4tBu) was achieved by treatment with
2-3 equiv of [Ce(NH4)2(NO3)6] in H2O/MeCN. Much to
our surprise, however, L3Me2 was not transformed into a
para-benzoquinone under identical reaction conditions.
Instead, we observed C-H activation with the formation
of the ortho-benzoquinone derivative L5Me2 (Scheme 2;
results of an investigation into mechanistic details of this
reaction are presented in a subsequent paragraph).
CDCl3 solutions of all ligands of the L2 series are stable
toward hydrolysis over a period of several days, even when
(22) Reger, D. L.; Grattan, T. C.; Brown, K. J.; Little, C. A.; Lamba,
J. J. S.; Rheingold, A. L.; Sommer, R. D. J. Organomet. Chem. 2000, 607,
120–128.
(23) Elflein, J.; Platzmann, F.; Burzlaff, N. Eur. J. Inorg. Chem. 2007,
5173–5176.
ꢀ
(24) The, K. I.; Peterson, L. K. Can. J. Chem. 1973, 51, 422–426.
(25) The, K. I.; Peterson, L. K.; Kiehlmann, E. Can. J. Chem. 1973, 51,
2448–2451.
ꢀ
(26) Peterson, L. K.; Kiehlmann, E.; Sanger, A. R.; The, K. I. Can. J.
Chem. 1974, 52, 2367–2374.

7438 Inorganic Chemistry, Vol. 49, No. 16, 2010
Blasberg et al.
Scheme 2. Synthesis of the Sterically Demanding Ligand Systems
at 5.83 ppm (s, 2 H), and the aromatic protons on the
para-dimethoxyphenyl fragment give rise to signals at
6.35 ppm (d, 1 H; H6) and 6.83 ppm (m, 2 H; H3,4). The
resonance of the methine proton appears at 7.72 ppm.
tBu
tBu
tBu
L2Me/L2Me2/L2Ph/L2 , L3Me2/L3Ph/L3 , L4Ph/L4 , and L5Me2a
1
In the H NMR spectrum (CDCl3) of L3Me2, this signal
has vanished, and a new resonance is visible at 2.70 ppm
(s, 3 H; CH3).
The 3-phenylpyrazolyl derivative L2Ph (d6-DMSO)
shows the typical proton resonance pattern of monosub-
stituted pyrazolyl rings, i.e., δ(1H)=6.83 (d, 2 H; pz-H4)
and 7.85 (d, 2 H; pz-H5). Upon going to L3Ph, successful
H/Me exchange is evidenced by the absence of a methine
proton resonance at 8.02 ppm and the presence of a new
methyl signal at 2.75 ppm in the 1H NMR spectrum of this
compound (d6-DMSO).
The tert-butyl group of L2tBu resonates at δ(1H)=1.32
(C6D6) and the methine proton at 8.18 ppm. The cor-
responding tert-butyl signal of L3tBu (C6D6) appears at
1.37 ppm (18 H); the signal for the newly introduced methyl
group is observed at 3.05 ppm. Apart from the signals for
the 3,30-isomer, another set of resonances with smaller
intensities is detectable in the 1H NMR spectrum of freshly
prepared L3tBu. The number of these signals indicates a less
symmetrical molecular structure. Especially, two tert-butyl
resonances at 1.35 ppm (9 H) and 1.39 ppm (9 H) lead to the
conclusion that the deprotonation/methylation sequence
has led to partial 3,30 f 3,50 isomerization.
Oxidation of L3Ph with [Ce(NH4)2(NO3)6] results in a
compound which shows no methoxy 1H/13C NMR signals
but two 13C NMR resonances in the typical range of benzo-
quinone carbonyl moieties (184.7 ppm, 187.2 ppm; d6-
DMSO). These features, together with all other NMR data,
are in accord with the proposed product L4Ph. Com-
pound L4tBu reveals corresponding CO resonances for
the 3,30-isomer at δ(13C) = 184.7 and 186.9 (C6D6). The
tBu
ratio of the 3,30- vs the 3,50-isomer was the same for L4
and L3tBu (note: upon prolonged storage, the 3,50-isomer
of L4tBu also slowly converts back to the 3,30-isomer).
In contrast to L3Ph and L3tBu, oxidation of L3Me2 re-
producibly led to a product that still contained one methoxy
group (δ(1H)=2.74; δ(13C)=57.1). Moreover, one of the
three proton resonances of the quinone core went miss-
ing, and the two remaining resonances then appeared as
singlets (5.06 ppm, 5.45 ppm). The 13C NMR spectrum of
the oxidation product revealed two low-field signals at
178.0 ppm and 180.8 ppm, which suggested the presence
of a benzoquinone moiety. All of these NMR data are con-
sistent with the molecular structure of the ortho-benzoquinone
derivative L5Me2; this structure proposal was further cor-
roborated by X-ray crystallography (see below).
0
a (a) þ 1 (pzR,R )2SO, þ 1 pyridine, THF, reflux, 12 h. (b) 1: þ 1 nBuLi,
THF/hexane, 0 °C, 30 min; 2: þ 1 MeI, THF/hexane, rt, 30 min. (c) þ
exc. [Ce(NH4)2(NO3)6], H2O/MeCN, 0 °C to rt, 3 h.
small amounts of aqueous HCl are added (NMR spectro-
scopic control). L3Ph and L3tBu are still fairly inert toward
water, but decompose slowly in the presence of (aqueous)
HCl. L3Me2 is the most sensitive compound within the L3
series. The addition of 1 equiv of oxalyl chloride to L3Me2
in aqueous d6-DMSO led to the immediate formation of a
1:1 mixture of ((MeO)2C6H3)C(pzMe2)dCH2 and HpzMe2
(NMR spectroscopic control); in the presence of excess
HCl, decomposition to the ketone ((MeO)2C6H3)C(O)CH3
takes place.
In order to get an estimate of the degree of steric crowding
in our bis(pyrazol-1-yl)methane ligands, X-ray crystal struc-
ture analyses of L2tBu and L3Me2 were performed (Figures 3
and 4, Table 1; cf. the Supporting Information for details of
the X-ray crystal structure analysis of L2Me2, L2Ph, andL3Ph).
L2tBu has both tert-butyl substituents in the 3- positions
The H NMR spectrum (CDCl3) of L2Me2 is characte-
of its pyrazolyl rings. The C11-N11=1.459(2) A, C11-
1
˚
˚
˚
rized by one set of pyrazolyl resonances and one set of para-
dimethoxyphenyl signals in an overall integral ratio of 2:1
(the numbering scheme is given in Scheme 2). We observe
two singlets at 2.12/2.20 ppm for the methyl groups on the
pyrazolyl rings and further singlets at 3.65/3.67 ppm for the
two methoxy substituents. The pz-H4 resonance appears
N21=1.459(2) A, and C1-C11=1.525(2) A bond lengths
are almost the same as in the parent system15 without tert-
butyl groups. Since this also applies for the bond angles
about C11 and for the dihedral angles between the ring
planes, the tert-butyl substituents apparently have no
significant influence on the ligand skeleton.

Article
Inorganic Chemistry, Vol. 49, No. 16, 2010 7439
Figure 5. Molecular structure and numbering scheme of compound L4Ph
(50% displacement ellipsoids; H atoms on the pyrazolyl moieties omitted for
˚
clarity). Selected bond lengths [A], bond angles [deg], torsion angles [deg],
Figure 3. Molecular structure and numbering scheme of compound
L2tBu (50% displacement ellipsoids; H atoms omitted for clarity). Selected
˚
bond lengths [A], bond angles [deg], torsion angles [deg], and dihedral
angles [deg]: C11-N11 = 1.459(2), C11-N21 = 1.459(2), C1-C11 =
1.525(2), C2-O1=1.373(2),C5-O2=1.381(2);C1-C11-N11=114.4(1),
C1-C11-N21=110.5(1), N11-C11-N21=110.1(1); C1-C11-N21-
N22=149.0(1), C2-C1-C11-H11A=35; HQ//pz(N11)=56.9, HQ//
pz(N21)=78.6, pz(N11)//pz(N21)=80.8.
and dihedral angles [deg]: C11-N11=1.476(2), C11-N21=1.459(2), C1-
C11=1.529(3), C2-O1=1.218(2), C5-O2=1.226(2), C1-C2=1.497(2),
C1-C6 = 1.336(2), C2-C3 = 1.474(3), C3-C4 = 1.326(3), C4-C5 =
1.459(3), C5-C6=1.470(3); C1-C11-N11=109.1(1), C1-C11-N21=
109.2(1), N11-C11-N21=107.4(1); C1-C11-N21-N22=37.7(2), C2-
C1-C11-C12 = -66.4(2); BQ//pz(N11) = 86.7, BQ//pz(N21) = 74.7,
pz(N11)//pz(N21)=63.4.
Figure 4. Molecular structure and numbering scheme of compound
L3Me2 (50% displacement ellipsoids; H atoms omitted for clarity). Selec-
Figure 6. Molecular structure and numbering scheme of compound
L5Me2 (50% displacement ellipsoids; H atoms on the pyrazolyl moieties
˚
ted bond lengths [A], bond angles [deg], torsion angles [deg], and dihedral
˚
omitted for clarity). Selected bond lengths [A], bond angles [deg], torsion
angles [deg]: C11-N11 = 1.476(2), C11-N21 = 1.472(2), C1-C11 =
1.536(2), C2-O1=1.371(2),C5-O2=1.370(2);C1-C11-N11=110.1(1),
C1-C11-N21=110.6(1), N11-C11-N21=107.3(1); C1-C11-N21-
N22 = -7.9(2), C2-C1-C11-C12 = 58.3(2); HQ//pz(N11) = 87.8,
HQ//pz(N21)=67.5, pz(N11)//pz(N21)=85.9.
angles [deg], and dihedral angles [deg]: C11-N11=1.485(2), C11-N21=
1.463(2), C1-C11=1.541(3), C2-O1=1.336(2), C4-O3=1.228(2), C5-
O2=1.213(3), C1-C2=1.497(3), C1-C6=1.340(3), C2-C3=1.356(3),
C3-C4 = 1.442(3), C4-C5 = 1.539(3), C5-C6 = 1.463(3); C1-C11-
N11=110.0(2), C1-C11-N21=108.2(1), N11-C11-N21=108.7(2);
C1-C11-N21-N22 = 10.9(2), C2-C1-C11-C12 = -58.1(2); BQ//
pz(N11)=83.0, BQ//pz(N21)=69.2, pz(N11)//pz(N21)=88.4.
The X-ray crystal structure analysis of L3Me2 (Figure 4)
provides evidence for a higher degree of steric crowding
about the carbon atoms C2 and C11 than is the case in
t
L2 Bu: (i) The C-N bonds (1.472(2) A, 1.476(2) A) and the
In order to corroborate the molecular structures of L4Ph
on the one hand and L5Me2 on the other, single crystals of
these compounds have been investigated by X-ray crystal-
lography (Figures 5 and 6; Table 2).
˚
˚
˚
C1-C11 bond (1.536(2) A) are slightly elongated com-
pared to the corresponding bonds in the tert-butyl deri-
vative. (ii) In L3Me2, there are two short contacts between
the ipso carbon atom C2 and atoms belonging to substi-
In line with the NMR spectroscopic results, L4Ph still
features both phenyl groups in the 3- positions of its pyra-
zolyl substituents; no 3,30 f 3,50 isomerization has occurred
during the oxidation process. The successful generation of a
para-benzoquinone fragment is evidenced by (i) the missing
OMe groups and (ii) a characteristic bond length alternation
˚
tuents on C11 (C2 N22=2.985(2) A and C2 C12=
3 3 3
3 3 3
tBu
˚
3.077(2) A), while there is only one such contact in L2
˚
(C2 N21 = 3.173(2) A).
3 3 3
None of the C/N-C11-C/N angles in L3Me2 deviates by
more than 3.4(1)° from the ideal tetrahedral angle of 109.5°.

7440 Inorganic Chemistry, Vol. 49, No. 16, 2010
Blasberg et al.
Table 2. Selected Crystallographic Data and Structure Refinement Details for L4Ph, L5Me2, and [Cl2Co(L2Me2)]
compound
L4Ph
L5Me2
[Cl2Co(L2Me2)]
formula
fw
color, shape
temp (K)
radiation
C26H20N4O2
420.46
orange-red, block
173(2)
Mo KR, 0.71073 A
monoclinic
P21/n
13.5748(10)
8.6772(4)
18.9711(15)
90
C19H22N4O3 ꢀEt2O
428.53
red, rod
C19H24Cl2CoN4O2 ꢀ1/2 MeCN
490.78
blue, needle
173(2)
165(2)
Mo KR, 0.71073 A
˚
˚
˚
Mo KR, 0.71073 A
cryst syst
space group
orthorhombic
P212121
8.4840(12)
14.384(3)
19.216(2)
90
monoclinic
C2/c
˚
a (A)
32.053(2)
8.6594(8)
16.5253(11)
90
104.463(5)
90
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
100.306(6)
90
90
90
3
˚
V (A )
Z
2198.6(3)
4
1.270
880
0.083
2345.0(6)
4
1.214
920
0.084
4441.4(6)
8
1.468
2032
1.038
Dcalcd (g cm-3
F(000)
)
μ (mm-1
)
cryst size (mm)
no of reflns coll
no of ind reflns (Rint
data/restr/params
GOF on F2
0.35ꢀ0.23ꢀ0.22
0.60ꢀ0.28ꢀ0.25
0.31ꢀ0.11ꢀ0.11
20929
34676
11201
)
4024 (0.0898)
4024/0/290
0.914
3129 (0.0561)
3129/0/287
1.049
4140 (0.0691)
4140/0/283
0.906
R1, wR2 (I>2 σ(I))
R1, wR2 (all data)
˚
largest diff peak and hole (e A
0.0418, 0.0887
0.0714, 0.0977
0.177, -0.235
0.0386, 0.0783
0.0627, 0.0862
0.176, -0.137
0.0396, 0.0905
0.0613, 0.0957
0.483, -0.770
-3
)
Scheme 3. Synthesis of the CoII Complexes [X2Co(L2Me2)]a
already provide sufficient steric bulk to prevent the for-
mation of homoleptic complexes like [Co(L1)2], CoCl2
and Co(NO3)2 were treated with 1 equiv of L2Me2 in
MeOH and EtOH, respectively (Scheme 3).
Single crystals suitable for X-ray crystallography were
grown from MeCN ([Cl2Co(L2Me2)]; Figure 7, Table 2)
and EtOH ([(NO3)2Co(L2Me2)]; cf. the Supporting Infor-
mation for details of the X-ray crystal structure analysis).
Complex [Cl2Co(L2Me2)] features four-coordinate CoII
ions in a distorted tetrahedral environment that is composed
of one chelating L2Me2 ligand and two chloride ions. The
oxygen atom O1 does not bind to the metal center. As can be
˚
anticipated, the average Co-N bond length (2.019(2) A)
is considerably shorter than the corresponding bond length in
˚
the hexa-coordinate compound [Co(L1)2]B (2.130(3) A),
but it fits well to the value in [Cl2Co(dmpzm)] (dmpzm=
bis(3,5-dimethylpyrazol-1-yl)methane; Co-N = 2.034(7)
A ). A similar good agreement is found between the ave-
27
˚
a (a) X = Cl: MeOH, rt; X = NO3: EtOH, rt.
rage Co-Cl bond lengths of [Cl2Co(L2Me2)] (2.229(1) A)
˚
27
˚
and [Cl2Co(dmpzm)] (2.229(2) A ). The bond angles about
within the benzoquinone ring (C1-C6 and C3-C4 are
significantly shorter than the other four C-C bonds).
The solid-state structure of L5Me2 reveals a methoxy-
substituted ortho-benzoquinone molecule (C2-O1 =
the CoII center deviate considerably from the ideal value of
109.5°, the smallest angle being N12-Co1-N22=93.0(1)°
and the largest being Cl1-Co1-N22= 120.0(1)°.
The solid-state structure of [(NO3)2Co(L2Me2)] is simi-
lar to that of [Cl2Co(L2Me2)], apart from the fact that
the former complex contains a five-coordinate central
metal atom because one of the two nitrate ions acts as
a bidentate ligand. According to the τ5 parameter28 of
0.27, the geometry of [(NO3)2Co(L2Me2)] is best described
as distorted square-pyramidal with the η1-coordinating
˚
˚
˚
1.336(2) A vs C4-O3 = 1.228(2) A/C5-O2 = 1.213(3) A).
The newly introduced O3 atom occupies the sterically
least shielded position, while the methoxy substituent that
resisted oxidative demethylation is the one next to the bis-
(pyrazol-1-yl)methyl group. As is expected for an ortho-
benzoquinone derivative, the six-membered ring contains
˚
˚
two short (C1-C6 = 1.340(3) A, C2-C3 = 1.356(3) A)
˚
and four long C-C bonds (C1-C2=1.497(3) A, C3-C4=
˚
˚
˚
1.442(3) A, C4-C5=1.539(3) A, C5-C6=1.463(3) A).
The bond lengths, bond angles, and dihedral angles with-
in the bis(pyrazol-1-yl)methyl fragment are similar to those
in L3Me2 and therefore do not merit further discussion.
Synthesis and Characterization of the Heteroleptic Com-
plexes [X2Co(L2Me2)] (X=Cl, NO3). To find out whether
methyl groups in the 3- positions of the pyrazolyl rings do
(27) Li, Q.-Y.; Zhang, W.-H.; Li, H.-X.; Tang, X.-Y.; Lang, J.-P.; Zhang,
Y.; Wang, X.-Y.; Gao, S. Chin. J. Chem. 2006, 24, 1716–1720.
(28) For five-coordinate complexes, the geometry index τ5 =(θ - j)/60°
provides a quantitative measure of whether the ligand sphere more closely
approaches a square-pyramidal (τ5=0) or a trigonal-bipyramidal geometry
(τ5=1; θ, j are the two largest bond angles and θ g j): Addison, A. W.; Rao,
T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C. J. Chem. Soc., Dalton Trans.
1984, 1349–1356.

Article
Inorganic Chemistry, Vol. 49, No. 16, 2010 7441
semiquinone intermediate of the first CeIV-induced one-
electron oxidation step reacts with triplet oxygen, which is
the entry to ortho-quinone formation.
In order to elucidate what effect the steric demand of
the bis(pyrazol-1-yl) substituent has on the outcome of
the oxidation reaction, we treated also the derivative
L2Me2 with [Ce(NH4)2(NO3)6] in H2O/MeCN (in this
case and in all following experiments, air was admitted
to the reaction mixture). As shown in Scheme 4, the reac-
tion of L2Me2 took the usual course and gave the para-
benzoquinone derivative L6Me2 in 64% yield.34
Given that CeIV oxidation of L3Ph and L3tBu also leads
to para-benzoquinones (L4Ph/L4tBu; Scheme 2), it is safe
to conclude that both the methyl substituent at the alipha-
tic bridge and the methyl groups in the 5- positions of the
pyrazolyl rings are required to enforce ortho-quinone
formation.
In the next step, we studied the influence of the π-electron
density within the six-membered ring on the reaction
outcome with the help of two model systems: the ortho-
isomer M134 and the meta-isomer M234 (Scheme 4). Both
compounds underwent almost quantitative hydrolysis to
the corresponding methoxyphenylketone upon the addi-
tion of [Ce(NH4)2(NO3)6] in H2O/MeCN. This observa-
tion is revealing in two respects: (i) it becomes clear that
both MeO substituents are required for the CeIV-induced
oxidation to take place, even though only one of the
methoxy groups is finally transformed and incorporated
into the ortho-benzoquinone moiety; (ii) hydrolysis of our
bis(pyrazol-1-yl)methane ligands is obviously retarded
when the redox-active unit is in its BQ state. The latter
Figure 7. Molecular structure and numbering scheme of compound
[Cl2Co(L2Me2)] (50% displacement ellipsoids; H atoms omitted for clarity).
˚
Selected bond lengths [A], bond angles [deg], and dihedral angles [deg]:
Co1-Cl1=2.218(1), Co1-Cl2=2.240(1), Co1-N12=2.015(2), Co1-
N22=2.023(2), C11-N11=1.468(3), C11-N21=1.456(3), C1-C11=
1.509(4), C2-O1 = 1.372(3), C5-O2 = 1.384(3); Cl1-Co1-Cl2 =
113.8(1), Cl1-Co1-N12=114.2(1), Cl1-Co1-N22=120.0(1), Cl2-
Co1-N12 = 108.2(1), Cl2-Co1-N22 = 105.3(1), N12-Co1-N22 =
93.0(1), N11-C11-N21=110.1(2); HQ//pz(N11)=76.5, HQ//pz(N21)=
89.7, pz(N11)//pz(N21)=43.4.
nitrate donor occupying the axial position (for more
details, see the Supporting Information).
Thus, the solid-state structures of [Cl2Co(L2Me2)]
and [(NO3)2Co(L2Me2)] show the aimed-for 1:1 ratio
between the CoII ions and the respective bis(3,5-di-
methylpyrazol-1-yl)methane ligand. However, electro-
spray mass spectra of MeCN solutions of these com-
plexes reveal peaks at m/z = 370 ([Co(L2Me2)2]2þ), which
indicates that in solution also the homoleptic 1:2 com-
plexes are apparent.
result can be rationalized by taking into account that a
]
0
cationic hydrolysis intermediate [(HQ)C(H,Me)pzR,R þ
should be mesomerically stabilized, whereas the oxidized
0
congener [(BQ)C(H,Me)pzR,R ]þ is not. We therefore con-
clude that the oxidative demethylation of ligands L3Me2
,
L3Ph, and L3tBu must be significantly faster than the
competing hydrolysis reaction.
Molecular modeling studies on the various bis(pyrazol-
1-yl)methane ligands indicate that the presence of the
methyl substituents in L3Me2 has a pronounced effect on the
preferred conformation of the molecule. If we assume
that coordination of CeIV to the pyrazolyl rings precedes
the metal-mediated oxidation event(s), a unique orientation
of the metal ion with respect to the para-dimethoxy-
benzene ring in the [CeIV(L3Me2)] fragment may well
explain the unique product selectivity. Given this back-
ground, we prepared the noncoordinating model systems
M334 and M534 (Scheme 4) in order to evaluate whether
ortho-benzoquinone formation just requires a sterically
demanding diarylmethyl substituent or whether CeIV pre-
coordination is also likely to play a role.
In both cases, para-benzoquinone derivatives (i.e.,
M434 and M634) were isolated after the oxidation reaction
was completed. It has, however, to be taken into account
that a realistic noncoordinating mimic of L3Me2 would
have to bear one methyl substituent (as in M3) and at the
same time two para-xylyl groups (as in M5) on the exo-
cyclic carbon atom. Unfortunately, all attempts to methy-
late M5 at its alkyl bridge resulted in methylation of the
Investigations into the Formation of the ortho-Quinone
Ligand L5Me2. Only scattered examples are known for the
transformation of phenol or para-hydroquinone deriva-
ꢀ
tives into ortho-quinones. In most cases, Fremy’s salt has
been employed as an oxidizing agent.29-33 Thus, the
unexpected CeIV-induced formation of the ortho-quinone
ligand L5Me2 posed a number of questions regarding the
factors governing this reaction.
At first, we investigated whether the source of the newly
introduced oxygen atom is dioxygen or water. To this
end, we repeated the reaction between L3Me2 and [Ce-
(NH4)2(NO3)6], which had previously been carried out in
the presence of air, under an atmosphere of nitrogen in
deaerated solvents (H2O/MeCN). As a consequence, the
isolated yield of L5Me2 dropped from previously 59% to
now <10%. We take this result as an indication that the
(29) Zimmer, H.; Lankin, D. C.; Horgan, S. W. Chem. Rev. 1971, 71, 229–
246.
(30) Barton, D. H. R.; Finet, J.-P.; Thomas, M. Tetrahedron 1988, 44,
6397–6406.
(31) Krohn, K.; Rieger, H.; Khanbabaee, K. Chem. Ber. 1989, 122,
2323–2330.
€
(32) Krohn, K.; Rieger, H.; Bruggmann, K. Synthesis 1990, 1141–1143.
(33) Nebois, P.; Cherkaoui, O.; Benameur, L.; Fillion, H.; Fenet, B.
(34) For details of the synthesis procedure(s) and the characterization of
this (these) compound(s), see the Supporting Information.
Tetrahedron 1994, 50, 8457–8464.

7442 Inorganic Chemistry, Vol. 49, No. 16, 2010
Blasberg et al.
Scheme 4. Reactivities of L2Me2 and the Model Systems M1, M2, M3,
M5, and M7 toward [Ce(NH4)2(NO3)6]a
Moreover, we want to emphasize that the reduced parent
system ((HO)2C6H3)C(H)(pzH,H)2 of L5Me2 is readily avail-
able from 3,4-dihydroxybenzaldehyde and (pzH,H)2SO/
pyridine.34
Conclusion
This work aimed at the preparation of para-quinone-
containing redox-active bis(pyrazol-1-yl)methane ligands L
for applications in bioinorganic chemistry and homogeneous
catalysis. One major focus was on the development of deri-
vatives bearing bulky substituents at the 3- positions of their
pyrazolyl rings in order to stabilize 1:1 transition metal com-
plexes [M(L)]nþ and to avoid the formation of less reactive
[M(L)2]nþ species.
To this end, two series of sterically demanding para-
dimethoxyphenyl-substituted bis(pyrazol-1-yl)methane ligands
0
have been synthesized, i.e., L2R,R = ((MeO)2C06H3)C(H)-
0
0
(pzR,R )2 and L3R,R =((MeO)2C6H3)C(Me)(pzR,R )2 (R,R0 =
3-Me,5-Me; 3-Ph,5-H; 3-tBu,5-H).
Complexation studies with CoII as the transition metal ion
showed that already the derivative L2Me2 is able to stabilize
complexes of the form [X2Co(L2Me2)] (X = Cl, NO3) in the
solid state. In MeCN solution, however, these complexes are
at an equilibrium with the 1:2 species [Co(L2Me2)2]2þ
.
Oxidative demethylation of L3Ph and L3tBu with[Ce(NH4)2-
(NO3)6] leads to the corresponding para-benzoquinonyl-
substituted bis(pyrazol-1-yl)methanes L4Ph and L4tBu so
that these ligands can now operate at their full redox
capacity.
Under similar reaction conditions, the methyl derivative
L3Me2 is transformed into the ortho-benzoquinone species
L5Me2, which still contains one methoxy substituent while
one carbonyl oxygen atom has been newly introduced. We
have repeated the reaction under an inert gas atmosphere in
deaerated solvents, and we have investigated numerous model
systems with different substitution patterns. These experi-
ments led to the conclusion that L5Me2 formation requires
(i) the admission of O2, (ii) the presence of both methoxy
substituents of L3Me2, and (iii) the presence of (methyl)
substituents both at the exocyclic carbon atom and at the
5- positions of the pyrazolyl rings.
We have also prepared a selection of noncoordinating
analogs of L2 and L3 in which the pyrazolyl rings are
replaced by phenyl or para-xylyl groups. Our motivation
was to find out whether CeIV coordination is important in the
product-determining step. In all cases, the CeIV-induced oxi-
dation went the usual course and gave the respective para-
benzoquinone derivatives. It has, however, to be mentioned
that the most realistic analog of L3Me2 (i.e., ((MeO)2C6H3)-
C(Me)(C6H3Me2)2) was synthetically not accessible. Thus,
at the present stage, it is not possible to fully exclude that
CeIV precoordination may have an impact on the formation
of L5Me2
.
a (a) þ exc. [Ce(NH4)2(NO3)6], H2O/MeCN, rt, 3 h.
The redox-active fragment in L5Me2 resembles the enzyme
cofactor topaquinone, which makes L5Me2 an interesting
ligand for the development of Cu-dependent amine oxidase
model systems. L5Me2 (in its hydroquinone form) can also act
as a ditopic bridging unit in (hetero)oligonuclear complexes.
Here, coordination of transition metal ions to the ortho-
hydroquinone unit will not only modify the redox behavior of
the entire system but also decouple electron transfer from
proton transfer.
para-dimethoxybenzene fragment (cf. M7;34 Scheme 4).
At the present stage, it is therefore not possible to fully
evaluate the impact that CeIV precoordination may have
on the formation of L5Me2
.
We note in passing that we have also treated M7 with
[Ce(NH4)2(NO3)6] in H2O/MeCN and again obtained a
para-benzoquinone derivative M834 (Scheme 4).

Article
Inorganic Chemistry, Vol. 49, No. 16, 2010 7443
Experimental Section
dissolved in EtOAc and the resulting solution evaporated to
dryness under a vacuum. The solid residue was recrystallized
from Et2O to obtain colorless pure L2Me2. Yield: 14.20 g (80%).
Single crystals were obtained by gas-phase diffusion of pentane
into a dilute Et2O solution of L2Me2. Rf = 0.32 (silica gel;
hexane/EtOAc 1:1). 1H NMR (400.1 MHz, CDCl3): δ 2.12,
2.20 (2 ꢀ s, 2 ꢀ 6 H; pz-CH3), 3.65, 3.67 (2 ꢀ s, 2 ꢀ 3 H; OCH3),
5.83 (s, 2 H; pz-H4), 6.35 (d, 4JHH =2.1 Hz, 1 H; HQ-H6), 6.83
(m, 2 H; HQ-H3,4), 7.72 (s, 1 H; CH). 13C NMR (100.6 MHz,
CDCl3): δ 11.4, 13.7 (pz-CH3), 55.6, 56.5 (OCH3), 70.0 (Cpz2),
106.5 (pz-C4), 112.0, 114.0 (HQ-C3,4), 115.2 (HQ-C6), 126.2
(HQ-C1), 140.4, 147.8 (pz-C3,5), 151.1, 153.6 (HQ-C2,5). ESI-
MS: m/z (%) 245 [M - pzMe2]þ (78), 341 [M þ H]þ (100). Anal.
Calcd for C19H24N4O2 [340.42]: C, 67.04; H, 7.11; N, 16.46.
Found: C, 67.20; H, 7.21; N, 16.46.
General. All reactions and manipulations of air-sensitive
compounds were carried out under dry, oxygen-free nitrogen
by using standard Schlenk ware. Where appropriate, THF was
freshly distilled under argon from Na/benzophenone. NaH was
used as a 60% per weight suspension in mineral oil (commer-
cially available); however, throughout the Experimental Section,
all weight-out quantities refer to neat NaH.
NMR spectra were recorded with Bruker AM-250, Bruker
Avance-300, and Bruker Avance-400 spectrometers. Chemical
shifts are referenced to residual solvent signals (1H/13C{1H},
C6D6: 7.16/128.00; CDCl3: 7.26/77.16; CD2Cl2: 5.32/53.8; d6-
DMSO: 2.50/39.52). Abbreviations: s = singlet; d = doublet;
dd=doublet of doublets; m=multiplet; br=signal broadened;
1
n.r. = multiplet expected in the H NMR spectrum, but not
resolved; pz = pyrazol-1-yl; HQ = hydroquinone core; BQ =
benzoquinone core.
Synthesis of L2Ph. The compound was prepared as described
for L2Me from phenylpyrazole (1.00 g, 6.9 mmol), NaH (0.17 g,
7.1 mmol), SOCl2 (0.25 mL, 0.41 g, 3.5 mmol), para-dimethoxy-
benzaldehyde (0.57 g, 3.4 mmol), and pyridine (0.28 mL, 0.27 g,
3.5 mmol). Recrystallization of the crude product from MeCN
gave a colorless solid. Yield: 0.79 g (52%). To grow single
crystals of L2Ph, a MeCN solution was layered with hexane.
1H NMR (300.0 MHz, d6-DMSO): δ 3.65, 3.72 (2 ꢀ s, 2 ꢀ 3 H;
Elemental analyses were performed by the Microanalytical
Laboratory of the University of Frankfurt. Mass spectra were
recorded with a VG PLATFORM II mass spectrometer.
Synthesis of [Co(L1)2]. A solution of HL1 (189 mg, 0.40 mmol)
in MeOH (15 mL) was treated at room temperature with a solution
of NaOMe in MeOH (0.5 M, 1 mL). Neat Co(OAc)2 (35 mg, 0.20
mmol) was added, and the reaction mixture was stirred at room
temperature for 12 h. All volatiles were removed under reduced
pressure, and the solid residue was recrystallized from CH2Cl2.
Yield: 158 mg (73%). ESI-MS: m/z 995 [M]þ. Anal. Calcd for
C52H78CoN8O4Si2 [994.34] ꢀ CH2Cl2 [84.93]: C, 58.98; H, 7.47; N,
10.38. Found: C, 58.86; H, 7.49; N, 10.65.
OCH3), 6.71 (d, 4JHH = 2.9 Hz, 1 H; HQ-H6), 6.83 (d, 3JHH
=
2.5 Hz, 2 H; pz-H4), 7.02 (dd, 3JHH=8.9 Hz, 4JHH=2.9 Hz, 1 H;
HQ-H4), 7.09 (d, JHH = 8.9 Hz, 1 H; HQ-H3), 7.31 (m, 2 H;
3
Ph-Hp), 7.40 (m, 4 H; Ph-Hm), 7.80 (m, 4 H; Ph-Ho), 7.85 (d,
3JHH =2.5 Hz, 2 H; pz-H5), 8.02 (s, 1 H; CH). 13C NMR (75.5
MHz, d6-DMSO): δ 55.3, 56.3 (OCH3), 72.2 (Cpz2), 103.3 (pz-
C4), 112.7 (HQ-C3), 114.4, 114.5 (HQ-C4,6), 124.6 (HQ-C1),
125.2 (Ph-Co), 127.8 (Ph-Cp), 128.6 (Ph-Cm), 131.8 (pz-C5),
132.6 (Ph-Ci), 150.4, 151.2, 152.9 (pz-C3, HQ-C2,5). ESI-MS:
m/z (%) 293 [M - pzPh]þ (100), 437 [M þ H]þ (41). Anal. Calcd
for C27H24N4O2 [436.51]: C, 74.29; H, 5.54; N, 12.84. Found: C,
74.24; H, 5.61; N, 13.05.
Synthesis of L2Me. In a representative procedure, neat methyl-
pyrazole (0.98 mL, 1.00 g, 12.2 mmol) was added at 0 °Ctoastirred
suspension of NaH (0.29 g, 12.1 mmol) in THF (75 mL). Stirring
was continued for 30 min. Neat SOCl2 (0.44 mL, 0.72 g, 6.1 mmol)
was added in one portion via syringe, and the resulting mixture was
allowed to warm to room temperature. After treatment with para-
dimethoxybenzaldehyde (1.01 g, 6.1 mmol) and pyridine (0.49 mL,
0.48 g, 6.1 mmol), the reaction mixture was kept at the reflux
temperature for 12 h. H2O (40 mL) was added and the aqueous
phase extracted into CH2Cl2 (3 ꢀ 100 mL). The combined organic
extracts were dried over MgSO4 and filtered, and the filtrate was
evaporated to dryness under a vacuum. The crude product of L2Me
was purified by column chromatography (silica gel; hexane/EtOAc
1:1). Yield: 1.07 g (56%). The pure product was obtained as a
colorless oil, which solidified upon standing. It was composed of a
mixture of the 3,30- and the rac-3,50-isomers in a stoichiometric
ratio of 4:1. The isomeric ratio could be raised to 10:1 by repeated
recrystallizations from hexane. Rf=0.40 (silica gel; hexane/EtOAc
1:1). 3,30-Isomer. 1H NMR (250.1 MHz, C6D6): δ 2.16 (s, 6 H; pz-
CH3), 3.09, 3.28 (2 ꢀ s, 2 ꢀ 3 H; OCH3), 5.83 (d, 3JHH =2.3 Hz,
Synthesis of L2tBu. The compound was prepared as described
for L2Me from tert-butylpyrazole (0.50 g, 4.0 mmol), NaH (0.10 g,
4.1 mmol), SOCl2 (0.15 mL, 0.25 g, 2.0 mmol), para-dimethoxy-
benzaldehyde (0.33 g, 2.0 mmol), and pyridine (0.16 mL, 0.16 g,
2.0 mmol). L2tBu, which is a colorless solid, was purified by
column chromatography (silica gel, hexane/EtOAc 1:2). How-
ever, recrystallization from hexane is more advisable for a large-
scale synthesis. Yield: 0.60 g (76%). Single crystals were grown
by slow evaporation of a dilute hexane solution of L2tBu. Rf =
0.83 (silica gel; hexane/EtOAc 1:2). 1H NMR (400.1 MHz, C6D6):
δ 1.32 (s, 18 H; CCH3), 3.04, 3.31 (2 ꢀ s, 2 ꢀ 3 H; OCH3), 6.00 (d,
3JHH = 2.4 Hz, 2 H; pz-H4), 6.34 (d, 3JHH = 8.9 Hz, 1 H; HQ-
H3), 6.68 (dd, 3JHH=8.9 Hz, 4JHH=3.0 Hz, 1 H; HQ-H4), 6.88
(d, 4JHH=3.0 Hz, 1 H; HQ-H6), 7.18 (d, 3JHH=2.4 Hz, 2 H; pz-
H5), 8.18 (s, 1 H; CH). 13C NMR (100.6 MHz, C6D6): δ 30.8
(CCH3), 32.5 (CCH3), 55.2, 55.7 (OCH3), 73.5 (Cpz2), 102.3 (pz-
C4), 112.4 (HQ-C3), 114.5 (HQ-C6), 115.4 (HQ-C4), 127.0
(HQ-C1), 129.6 (pz-C5), 151.3, 154.2 (HQ-C2,5), 162.9 (pz-
C3). ESI-MS: m/z (%) 273 [M - pztBu]þ (100), 397 [M þ H]þ
(13). Anal. Calcd for C23H32N4O2 [396.53]: C, 69.67; H, 8.13; N,
14.13. Found: C, 69.81; H, 8.17; N, 14.17.
Synthesis of L3Me2. In a representative procedure, nBuLi
(1.52 M in hexane; 1.93 mL, 2.9 mmol) was added at 0 °C via
syringe to a stirred solution of L2Me2 (1.00 g, 2.9 mmol) in THF
(40 mL). After 30 min, the bright red reaction mixture was allowed
to warm to room temperature, and neat MeI (0.18 mL, 0.41 g,
2.9 mmol) was added via syringe, whereupon the red color
vanished rapidly. Stirring was continued for another 30 min.
The reaction mixture was poured into H2O (30 mL), and L3Me2
was extracted into CH2Cl2 (3 ꢀ 20 mL). The combined extracts
were dried over MgSO4 and filtered, and the filtrate was evapo-
rated to dryness. The crude product was triturated with hexane
and recrystallized from hot hexane to obtain colorless crystals.
Yield: 0.73 g (71%). Rf = 0.69 (silica gel; hexane/EtOAc 1:1).
2H;pz-H4), 6.36(d, 3JHH=8.9 Hz, 1 H; HQ-H3), 6.67 (dd, 3JHH
=
8.9 Hz, 4JHH=3.1 Hz, 1 H; HQ-H4), 7.08 (d, 4JHH=3.1 Hz, 1 H;
HQ-H6), 7.21 (d, 3JHH = 2.3 Hz, 2 H; pz-H5), 8.14 (s, 1 H; CH).
13C NMR (62.9 MHz, C6D6): δ 13.8 (pz-CH3), 55.2, 55.7 (OCH3),
73.2 (Cpz2), 105.7 (pz-C4), 112.1, 115.0, 115.2 (HQ-C3,4,6), 127.9
(HQ-C1), 130.1 (pz-C5), 149.6 (pz-C3), 151.3, 154.4 (HQ-C2,5).
3,50-Isomer. 1H NMR (300.0 MHz, C6D6): δ 2.07, 2.18 (2 ꢀ s, 2 ꢀ
3 H; pz-CH3), 3.13, 3.30 (2 ꢀ s, 2 ꢀ 3 H; OCH3), 5.79, 5.88 (n.r., d,
3JHH = 2.3 Hz, 2 ꢀ 1 H; pz-H4,40), 6.40 (d, 3JHH = 8.9 Hz, 1 H;
HQ-H3), 6.70 (dd, 3JHH =8.9 Hz, 4JHH =3.1 Hz, 1 H; HQ-H4),
7.17 (n.r., 1 H; HQ-H6), 7.42, 7.62 (n.r., d, 3JHH=2.3 Hz, 2 ꢀ 1 H;
pz-H30,5), 8.14 (s, 1 H; CH). ESI-MS: m/z (%) 231 [M - pzMe]þ
(100), 313[M þ H]þ (26). Anal. Calcd for C17H20N4O2 [312.37]: C,
65.37; H, 6.45; N, 17.94. Found: C, 65.29; H, 6.40; N, 18.05.
Synthesis of L2Me2. The compound was prepared as described
for L2Me from 3,5-dimethylpyrazole (10.00 g, 104.0 mmol),
NaH (2.50 g, 104.2 mmol), SOCl2 (3.80 mL, 6.22 g, 52.3 mmol),
para-dimethoxybenzaldehyde (8.64 g, 52.0 mmol), and pyridine
(4.20 mL, 4.12 g, 52.0 mmol). The yellow oily crude product was

7444 Inorganic Chemistry, Vol. 49, No. 16, 2010
Blasberg et al.
1H NMR (400.1 MHz, CDCl3): δ 1.68, 2.19 (2 ꢀ s, 2 ꢀ 6 H; pz-
CH3), 2.70 (s, 3 H; CH3), 3.60, 3.72 (2 ꢀ s, 2 ꢀ 3 H; OCH3), 5.42
(d, 4JHH =3.0 Hz, 1 H; HQ-H6), 5.86 (s, 2 H; pz-H4), 6.81 (dd,
3JHH=8.8 Hz, 4JHH=3.0 Hz, 1 H; HQ-H4), 6.91 (d, 3JHH=8.8
Hz, 1 H; HQ-H3). 13C NMR (100.6 MHz, CDCl3): δ 11.9, 13.6
(pz-CH3), 28.2 (CH3), 55.3, 56.6 (OCH3), 83.0 (Cpz2), 108.3 (pz-
C4), 112.9 (HQ-C6), 113.3, 114.1 (HQ-C3,4), 132.6, 141.2, 145.9
(HQ-C1, pz-C3,5), 152.1, 153.5 (HQ-C2,5). ESI-MS: m/z (%)
259 [M - pzMe2]þ (100). Anal. Calcd for C20H26N4O2 [354.45]:
C, 67.77; H, 7.39; N, 15.81. Found: C, 68.00; H, 7.43; N, 15.81.
Synthesis of L3Ph. The compound was prepared as described
for L3Me2 from L2Ph (0.44 g, 1.0 mmol), nBuLi (1.60 M in hexane;
0.63 mL, 1.0 mmol), and MeI (0.06 mL, 0.14 g, 1.0 mmol). The
crude product was purified by column chromatography (silica
gel; hexane/EtOAc 3:1). In order to obtain X-ray quality
crystals, a sample of L3Ph was dissolved in boiling hexane and
the clear solution stored in an oven at þ50 °C overnight. Yield:
0.37 g (82%). Rf=0.55 (silica gel; hexane/EtOAc 3:1). 1H NMR
(400.1 MHz, d6-DMSO): δ 2.75 (s, 3 H; CH3), 3.55, 3.58 (2 ꢀ s,
2.3 Hz, 2 H; pz-H4), 6.98 (dd, 3JHH=8.9 Hz, 4JHH=3.0 Hz, 1 H;
HQ-H4), 7.09 (d, 3JHH=8.9 Hz, 1 H; HQ-H3), 7.31 (m, 2 H; Ph-
Hp), 7.40 (m, 4H; Ph-Hm), 7.52 (d, 3JHH =2.3 Hz, 2 H; pz-H5),
7.81 (m, 4 H; Ph-Ho). 13C NMR (100.6 MHz, d6-DMSO): δ 24.5
(CH3), 55.1, 56.3 (OCH3), 81.5 (Cpz2), 103.4 (pz-C4), 113.5,
114.0, 114.2 (HQ-C3,4,6), 125.3 (Ph-Co), 127.8 (Ph-Cp), 128.7
(Ph-Cm), 131.0 (pz-C5), 131.4, 132.9 (Ph-Ci, HQ-C1), 150.3,
150.4, 152.8 (pz-C3, HQ-C2,5). ESI-MS: m/z (%) 307 [M -
pzPh]þ (58). Anal. Calcd for C28H26N4O2 [450.55]: C, 74.65; H,
5.82; N, 12.44. Found: C, 74.62; H, 5.96; N, 12.40.
δ 24.3 (CH3), 79.8 (Cpz2), 104.4 (pz-C4), 125.4 (Ph-Co), 128.1,
128.3 (Ph-Cm,Cp), 128.7 (Ph-Ci), 131.1 (pz-C5), 132.5 (BQ-C6),
136.0, 137.8 (BQ-C3,4), 146.9 (BQ-C1), 150.9 (pz-C3), 184.7,
187.2 (BQ-C2,5). ESI-MS: m/z (%) 277 [M - pzPh]þ (100), 421
[M þ H]þ (13). Anal. Calcd for C26H20N4O2 [420.46]: C, 74.27;
H, 4.79; N, 13.33. Found: C, 74.36; H, 4.84; N, 13.44.
Synthesis of L4tBu. The compound was prepared as described
for L4Ph from L3tBu (1.04 g, 2.5 mmol; 8:1 mixture of isomers)
and [Ce(NH4)2(NO3)6] (2.77 g, 5.1 mmol). L4tBu was purified by
column chromatography (silica gel; hexane/EtOAc 2:1) and
obtained as a red oil (8:1 mixture of regioisomers). Yield: 0.86 g
(90%). Rf=0.68 (silica gel; hexane/EtOAc 2:1). 3,30-Isomer. 1H
NMR (400.1 MHz, C6D6): δ 1.32 (s, 18 H; CCH3), 2.54 (s, 3 H;
CH3), 5.59 (d, 4JHH=2.0 Hz, 1 H; BQ-H6), 6.00 (m, 4 H; pz-H4,
BQ-H3,4), 7.00 (d, 3JHH =2.5 Hz, 2 H; pz-H5). 13C NMR (62.9
MHz, C6D6): δ 24.7 (CH3), 30.6 (CCH3), 32.4 (CCH3), 79.9
(Cpz2), 103.4 (pz-C4), 128.6 (pz-C5), 132.5, 135.4, 137.0 (BQ-
C3,4,6), 149.0 (BQ-C1), 162.5 (pz-C3), 184.7, 186.9 (BQ-C2,5).
ESI-MS: m/z (%) 257 [M - pztBu]þ (100), 381 [M þ H]þ (67).
Anal. Calcd for C22H28N4O2 [380.48]: C, 69.45; H, 7.42; N,
14.73. Found: C, 69.25; H, 7.44; N, 14.74.
2 ꢀ 3 H; OCH3), 5.39 (d, 4JHH=3.0 Hz; HQ-H6), 6.84 (d, 3JHH
=
Synthesis of L5Me2. [Ce(NH4)2(NO3)6] (1.55 g, 2.8 mmol) in
H2O (20 mL) was added at 0 °C to a stirred solution of L3Me2
(0.50 g, 1.4 mmol) in MeCN (20 mL). The mixture was warmed
to room temperature, and stirring was continued for 3 h. The
product was extracted into CH2Cl2 (3 ꢀ 50 mL). The combined
organic extracts were dried over MgSO4 and filtered, and the
filtrate was evaporated to dryness under reduced pressure. L5Me2
was purified by column chromatography (silica gel; hexane/
EtOAc 1:3; note: the column should be shorter than 5 cm,
because prolonged contact with silica gel leads to product de-
composition). L5Me2 was obtained as a red solid. Yield: (0.29 g,
59%). Single crystals of L5Me2 were obtained by storing a Et2O
solution at -20 °C. Rf=0.56 (silica gel; hexane/EtOAc 1:3). 1H
NMR (300.0 MHz, C6D6): δ 1.51, 2.10 (2 ꢀ s, 2 ꢀ 6 H; pz-CH3),
2.60 (s, 3 H; CH3), 2.74 (s, 3 H; OCH3), 5.06, 5.45 (2 ꢀ s, 2 ꢀ 1 H;
BQ-H3,6), 5.56 (s, 2 H; pz-H4). 13C NMR (75.5 MHz, d6-
DMSO): δ 10.8 (very br), 13.3 (pz-CH3), 27.9 (CH3), 57.1
(OCH3), 81.4 (Cpz2), 103.3 (BQ-C3 or 6), 108.7 (pz-C4), 124.2
(BQ-C3 or 6), 150.5, 168.8, n.o., n.o. (BQ-C1,2, pz-C3,5), 178.0,
180.8 (BQ-C4,5). ESI-MS: m/z (%) 259 [M - pzMe2]þ (26), 355
[M þ H]þ (100). IR (KBr, cm-1): ν~ 1688 (m), 1648 (s), 1627 (m),
1570 (s), 1561 (s), 1449 (m), 1414 (m), 1371 (s), 1343 (m), 1251 (s).
Anal. Calcd for C19H22N4O3 [354.40]: C, 64.39; H, 6.26; N,
15.81. Found: C, 64.46; H, 6.18; N, 15.81.
Synthesis of [Cl2Co(L2Me2)]. CoCl2 (23 mg, 0.18 mmol) was
added at room temperature to a stirred solution of L2Me2 (60 mg,
0.18 mmol) in MeOH (10 mL). Stirring was continued for 30 min.
The crude product was isolated by filtration, repeatedly washed
with small portions of MeOH, and then recrystallized from
MeCN. Yield: 80 mg (95%). Blue single crystals were obtained
by slow evaporation of a dilute MeCN solution. ESI-MS: m/z
(%) 245 [L2Me2 - pzMe2]þ (50), 341 [L2Me2 þ H]þ (49), 370 [Co-
(L2Me2)2]2þ (100). Anal. Calcd for C19H24Cl2CoN4O2 [470.25]:
C, 48.53; H, 5.14; N, 11.91. Found: C, 46.11; H, 4.87; N, 11.76.
Synthesis of [(NO3)2Co(L2Me2)]. Co(NO3)2 ꢀ 6H2O (171 mg,
0.59 mmol) was added at room temperature to a stirred solution
of L2Me2 (200 mg, 0.59 mmol) in EtOH (10 mL). Stirring was
continued overnight. The crude product was collected on a frit,
washed with EtOH, and dried under reduced pressure. Yield: 168
mg (54%). Purple single crystals were grown by slow evapora-
Synthesis of L3tBu. The compound was prepared as described
for L3Me2 from L2tBu (1.00 g, 2.5 mmol), nBuLi (1.52 M in hexane;
1.66 mL, 2.5 mmol), and MeI (0.16 mL, 0.36 g, 2.5 mmol). The
crude product was purified by column chromatography (silica
gel; hexane/EtOAc 2:1). L3tBu was obtained as a colorless oil and
a mixture of isomers (3,30-isomer/3,50-isomer ≈ 8:1). Yield: 0.82 g
(80%). Rf=0.75 (silica gel; hexane/EtOAc 2:1). 3,30-Isomer. 1H
NMR (300.0 MHz, C6D6): δ 1.37 (s, 18 H; CCH3), 3.05 (s, 3 H;
CH3), 3.06, 3.28 (2 ꢀ s, 2 ꢀ 3 H; OCH3), 5.80 (d, 4JHH=3.1 Hz,
1 H; HQ-H6), 6.03 (d, 3JHH=2.4 Hz, 2 H; pz-H4), 6.45 (d, 3JHH
=
8.9 Hz, 1 H; HQ-H3), 6.72 (dd, 3JHH =8.9 Hz, 4JHH =3.1 Hz,
1 H; HQ-H4), 7.01 (d, 3JHH = 2.4 Hz, 2 H; pz-H5). 13C NMR
(75.5 MHz, C6D6): δ 25.1 (CH3), 30.8 (CCH3), 32.5 (CCH3),
55.0, 55.8 (OCH3), 82.5 (Cpz2), 102.1 (pz-C4), 113.4, 114.1,
115.3 (HQ-C3,4,6), 129.6 (pz-C5), 133.0 (HQ-C1), 151.2, 154.2
1
(HQ-C2,5), 162.1 (pz-C3). 3,50-Isomer. H NMR (300.0 MHz,
C6D6): δ 1.35, 1.39 (2 ꢀ s, 2 ꢀ 9 H; CCH3), 3.08, 3.11, 3.29 (3 ꢀ s,
3 ꢀ 3 H; CH3, OCH3), 5.84 (n.r., 1 H; HQ-H6), 5.90, 6.01 (2 ꢀ d,
3JHH =3.1 Hz, 2.4 Hz, 2 ꢀ 1 H; pz-H4,40), 6.17 (n.r., 2 H; HQ-
H3,4), 6.97 (d, 3JHH =2.4 Hz, 1 H; pz-H5 or 50), n.o. (pz-H5 or
50). ESI-MS: m/z (%) 287 [M - pztBu]þ (100), 411 [M þ H]þ (9).
Anal. Calcd for C24H34N4O2 [410.55]: C, 70.21; H, 8.35; N,
13.65. Found: C, 70.26; H, 8.61; N, 13.58.
Synthesis of L4Ph. In a representative procedure, [Ce(NH4)2-
(NO3)6] (2.63 g, 4.8 mmol) in H2O (20 mL) was added at 0 °C to a
stirred solution of L3Ph (0.72 g, 1.6 mmol) in MeCN (20 mL).
The mixture was allowed to warm to room temperature, and
stirring was continued for 3 h. The product was extracted into
CH2Cl2 (3 ꢀ 50 mL). The combined extracts were dried over
MgSO4 and filtered, and the filtrate was evaporated to dryness
under reduced pressure. L4Ph was purified by column chromato-
graphy (silica gel; hexane/EtOAc 3:1) and obtained as a yellow
solid. Yield: (0.55 g, 82%). X-ray quality crystals of the compound
were obtained from a saturated solution in Et2O at -20 °C. Rf=
0.39 (silica gel; hexane/EtOAc 3:1). 1H NMR (250.1 MHz,
tion of a dilute EtOH solution. ESI-MS: m/z (%) 245 [L2Me2
-
pzMe2]þ (83), 341 [L2Me2 þ H]þ (7), 370 [Co(L2Me2)2]2þ (100), 461
[(NO3)Co(L2Me2)]þ (21). Anal. Calcd for C19H24CoN6O8 [523.37]:
C, 43.60; H, 4.62; N, 16.06. Found: C, 43.84; H, 4.62; N, 16.26.
CD2Cl2): δ 2.76 (s, 3 H; CH3), 5.68 (d, JHH = 1.9 Hz, 1 H;
X-Ray Crystallography of L2Me, L2Me2, L2Ph, L2tBu, L3Me2
,
4
BQ-H6), 6.70 (d, 3JHH=2.6 Hz, 2 H; pz-H4), 6.78 (m, 2 H; BQ-
H3,4), 7.38 (m, 6 H; Ph-Hp,Hm), 7.52 (d, JHH = 2.6 Hz, 2 H;
L3Ph, L4Ph, L5Me2, S4, M3, M7, [Co(L1)2], [Cl2Co(L2Me2)], and
[(NO3)2Co(L2Me2)]. Data collections were performed on a Stoe-
3
pz-H5), 7.81 (m, 4 H; Ph-Ho). 13C NMR (62.9 MHz, d6-DMSO):
IPDS-II two-circle diffractometer (L2Me, L2Me2, L3Me2, L3Ph
,

Article
Inorganic Chemistry, Vol. 49, No. 16, 2010 7445
L4Ph, M3, [Cl2Co(L2Me2)], [(NO3)2Co(L2Me2)]) or on a SIE-
MENS SMART CCD diffractometer (L2Ph, L2tBu, L5Me2, S4,
M7, [Co(L1)2]) with graphite-monochromated Mo KR radia-
tion. In all cases, repeatedly, measured reflections remained
stable. Empirical absorption corrections were performed using
either the MULABS35 option in PLATON36 ([Cl2Co(L2Me2)]
and [(NO3)2Co(L2Me2)]) or the program SADABS37 (L2Ph, S4,
M7, and [Co(L1)2]). Equivalent reflections were averaged. The
structures were solved by direct methods using the program
SHELXS38 and refined with full-matrix least-squares on F2
using the program SHELXL-97.39 Hydrogen atoms were gene-
rally placed on ideal positions and refined with fixed isotropic
displacement parameters using a riding model.
very large displacement parameters, showing this group to be
disordered. It was not attempted to resolve this disorder.
The crystal lattice of [Cl2Co(L2Me2)] contains 0.5 equiv of
MeCN solvate molecules. Each of these molecules is disordered
over two equally occupied positions about a 2-fold rotation axis.
CCDC reference numbers: 773297 (L2Me), 773293 (L2Me2),
773305 (L2Ph), 773302 (L2tBu), 773295 (L3Me2), 773300 (L3Ph),
773298 (L4Ph), 773303 (L5Me2), 773306 (S4), 773299 (M3), 773307
(M7), 773301 ([Co(L1)2]), 773294 ([Cl2Co(L2Me2)]), 773296
([(NO3)2Co(L2Me2)]).
Acknowledgment. This research was supported by the
Deutsche Forschungsgemeinschaft (DFG). F.B. is grateful to
the Fonds der Chemischen Industrie (FCI) for a Ph.D. grant.
One iPr group of compound [Co(L1)2] was found to be seri-
ously disordered and was refined with split atoms. One of the
two independent CH2Cl2 solvate molecules in this structure has
Supporting Information Available: Crystallographic data of
L2Me, L2Me2, L2Ph, L2tBu, L3Me2, L3Ph, L4Ph, L5Me2, S4, M3, M7,
[Co(L1)2], [Cl2Co(L2Me2)], and [(NO3)2Co(L2Me2)] in CIF format;
synthesis and analytical data of L6Me2, ((HO)2C6H3)C(H)(pzH,H)2,
and the model systems M1-M8; structure plots and selected
(35) Blessing, R. H. Acta Crystallogr. 1995, A51, 33–38.
(36) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.
€
€
(37) Sheldrick, G. M. SADABS; University of Gottingen: Gottingen,
Germany, 2000.
crystallographic data/structure parameters of L2Me, L2Me2, L2Ph
,
(38) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467–473.
L3Ph, [(NO3)2Co(L2Me2)], M3, S4, and M7. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
€
€
€
(39) Sheldrick, G. M. SHELXL-97; Universitat Gottingen: Gottingen,
Germany, 1997.