Acetonitrile hydration and ethyl acetate hydrolysis by pyrazolate-bridged cobalt(II) dimers containing hydrogen-bond donors

Article abstract of DOI:10.1021/ic700685g

The preparation of new Co^II-μ-OH-Co^II dimers with the binucleating ligands 3,5-bis{bis[(N′-R-ureaylato)-N-ethyl]- aminomethyl}-1H-pyrazolate ([H₄P^Rbuam]^5-, R = ^tBu, ⁱPr) is described. The molecular structure of the isopropyl derivative reveals that each Co^II center has a trigonal-bipyramidial coordination geometry, with a Co...Co separation of 3.5857(5) A. Structural and spectroscopic studies show that there are four hydrogen-bond (H-bond) donors near the Co^II-μ-OH-Co^II moiety; however, they are too far away to be form intramolecular H-bonds with the bridging hydroxo ligand. Treating [Co^II₂H ₄P^Rbuam(μ-OH)]^2- with acetonitrile led to the formation of bridging acetamidato complexes, [Co^II ₂H₄P_Rbuam(μ-1,3-OC(NH)CH₃)] ^2-; in addition, these Co^II-μ-OH-Co^II dimers hydrolyze ethyl acetate to form Co^II complexes with bridging acetato ligands. The Co^II-1,3-μ-X′-Co^II complexes (X′ = OAc^-, [OC(NH)CH₃]^-) were prepared independently by reacting [Co^II₂H₃P ^Rbuam]^2- with acetamide or [Co^II ₂H₄P^Rbuam]^- with acetate. X-ray diffraction studies show that the orientation of the acetate ligand within the H-bonding cavity depends on the size of the R substituent appended from the urea groups. The tetradentate ligand 3-{bis[(N′-tert-butylureaylato)-N-ethyl] aminomethyl}-5-tert-butyl-1H-pyrazolato ([H₂P^tBuuam] ^3-) was also developed and its Co^II-OH complex prepared. In the crystalline state, [Co^IIH₂P^tBuuam(OH)] ^2- contains two intramolecular H-bonds between the urea groups of [H₂P^tBuuam]^3- and the terminal hydroxo ligand. [ⁿPr₄N]₂[Co^IIH₂P ^tBuuam(OH)] does not hydrate acetonitrile or hydrolyze ethyl acetate. In contrast, K₂[Co^IIH₂P^tBuuam(OH)] does react with ethyl acetate to produce KOAc; this enhanced reactivity is attributed to the presence of the K⁺ ions, which can possibly interact with the Co^II-OH unit and ester substrate to assist in hydrolysis. However, K₂[Co^IIH₂P ^tBuuam(OH)] was still unable to hydrate acetonitrile.

Full text of DOI:10.1021/ic700685g

Inorg. Chem. 2007, 46, 10120−10132
Acetonitrile Hydration and Ethyl Acetate Hydrolysis by
Pyrazolate-Bridged Cobalt(II) Dimers Containing Hydrogen-Bond Donors
Paul J. Zinn,† Thomas N. Sorrell,‡ Douglas R. Powell,† Victor W. Day,† and A. S. Borovik*,†,§
Department of Chemistry, UniVersity of Kansas, 2010 Malott Hall, 1251 Wescoe Hall DriVe,
Lawrence, Kansas 66045, and Department of Chemistry, UniVersity of North Carolina,
Chapel Hill, North Carolina 27599
Received April 10, 2007
The preparation of new CoII−µ-OH
aminomethyl
−
CoII dimers with the binucleating ligands 3,5-bis
{
bis[(N
′
-R-ureaylato)-N-ethyl]-
i
}
-1H-pyrazolate ([H4PRbuam]5-, R
)
tBu, Pr) is described. The molecular structure of the isopropyl
derivative reveals that each CoII center has a trigonal-bipyramidial coordination geometry, with a Co‚‚‚Co separation
of 3.5857(5) Å. Structural and spectroscopic studies show that there are four hydrogen-bond (H-bond) donors near
the CoII−µ-OH
−
CoII moiety; however, they are too far away to be form intramolecular H-bonds with the bridging
-
hydroxo ligand. Treating [CoII2H4PRbuam(
µ
-OH)]2 with acetonitrile led to the formation of bridging acetamidato
complexes, [CoII H4PRbuam(
µ
-1,3-OC(NH)CH3)]2-; in addition, these CoII−µ-OH
−
CoII dimers hydrolyze ethyl acetate
2
to form CoII complexes with bridging acetato ligands. The CoII
−1,3-µ
-X′−CoII complexes (X′ ) OAc-, [OC(NH)CH3]-)
were prepared independently by reacting [CoII2H3PRbuam]2 with acetamide or [CoII2H4PRbuam]- with acetate.
-
X-ray diffraction studies show that the orientation of the acetate ligand within the H-bonding cavity depends on the
size of the R substituent appended from the urea groups. The tetradentate ligand 3-
{
bis[(N
′
-tert-butylureaylato)-
OH complex
N-ethyl]aminomethyl
}
-5-tert-butyl-1H-pyrazolato ([H2PtBuuam]3-) was also developed and its CoII
−
t
-
prepared. In the crystalline state, [CoIIH2P Buuam(OH)]2 contains two intramolecular H-bonds between the urea
t
t
groups of [H2P Buuam]3 and the terminal hydroxo ligand. [nPr4N]2[CoIIH2P Buuam(OH)] does not hydrate acetonitrile
-
or hydrolyze ethyl acetate. In contrast, K2[CoIIH2PtBuuam(OH)] does react with ethyl acetate to produce KOAc; this
enhanced reactivity is attributed to the presence of the K+ ions, which can possibly interact with the CoII
−OH unit
and ester substrate to assist in hydrolysis. However, K2[CoIIH2PtBuuam(OH)] was still unable to hydrate acetonitrile.
Introduction
through the hydration of nitriles can be overcome through
the utilization of metal ions. In spite of numerous reports
on the metal-mediated processes,1 the use of synthetic CoII
complexes in investigating these types of reactivities remains
relatively unexplored.3
The hydration and hydrolysis of unactivated molecules is
an area of synthetic interest for the preparation of industrially
and pharmacologically important compounds.1 For instance,
the hydration of nitriles can be applied for more efficient
synthesis of amides such as acrylamide or nicotinamide.
However, the preparation of amides through standard base-
or acid-catalyzed reactions has several limitations and/or
disadvantages.2 The difficulties in the preparation of amides
In most metal-based systems that perform this chemistry,
a metal-hydroxo unit is proposed to be the active species.
For example, the active sites of multinuclear metallohydro-
(2) (a) Parkins, A. W. Platinum Met. ReV. 1996, 40, 169-174 and
references cited therein. (b) Ghaffar, T.; Parkins, A. W. J. Mol. Catal.
A: Chem. 2000, 160, 249-261. (c) Murahashi, S.-I.; Takaya, H. Acc.
Chem. Res. 2000, 33, 225-233.
* To whom correspondence should be addressed. E-mail: aborovik@
uci.edu.
† University of Kansas.
(3) Only CoII metal salts have been used: (a) Clarke, C. R.; Hay, R. W.
Dalton Trans. 1974, 2148-2152. (b) Segl’a, P.; Jamnicky, M.; Koman,
M.; Sima, J.; Glowiak, T. Polyhedron 1998, 17, 4525-4533. (c)
Kopylovich, M. N.; Kukushkin, V. Y.; Guedes da Silva, M. F. C.;
Haukka, M.; Frau´sto da Silva, J. J. R.; Pombeiro, A. J. L. J. Chem.
Soc., Perkin Trans. 1 2001, 1569-1573.
‡ University of North Carolina.
§ Current address: Department of Chemistry, University of California,
Irvine, CA 92697.
(1) Kukushkin, V. Y.; Pombeiro, A. J. L. Inorg. Chim. Acta 2005, 358,
1-21 and references cited therein.
10120 Inorganic Chemistry, Vol. 46, No. 24, 2007
10.1021/ic700685g CCC: $37.00
© 2007 American Chemical Society
Published on Web 11/01/2007

Pyrazolate-Bridged CoII Dimers with H-Bond Donors
Figure 1. Dinucleating ligand and general dinuclear complex with an exogenous ligand (X) accepting H-bonds donated by the unsymmetrical urea chelates.
lases, metal-containing enzymes that efficiently catalyze the
hydrolysis of unactivated substrates such as DNA, peptides,
and urea, are proposed to contain two kinds of active metal-
hydroxo species: (i) a M(M-OH) species where the hydroxo
ligand is coordinated to one metal site of the dinuclear center
in a terminal fashion and (ii) a M-µ-OH-M moiety
containing a µ-hydroxo ligand bridged between the two metal
ions.4 X-ray diffraction and spectroscopic studies on many
of these enzymes suggest that basic functionalities such as
carboxylates are used to scavenge protons liberated during
the formation of M-OH moieties. In addition, intramolecular
hydrogen bonds (H-bonds) between amino acid residues of
the protein backbone and external ligands coordinated to the
active-site metal ions are found to effectively control metal-
mediated hydrolysis while adding structural stability.5 These
studies illustrate the need to control both primary and
secondary coordination spheres about the M-OH unit to
achieve efficient hydration/hydrolysis of unactivated com-
pounds.
metal-ion-mediated processes.10,11 Recently, we have devel-
oped dinucleating ligands (H9PRbuam; R ) tBu or iPr) that
are capable of forming cavities comprised of four H-bond
donors upon chelating two metal ions within close proximity
(Figure 1).12,13 Deprotonation of the pyrazole and the urea
RNH groups provides a primary coordination sphere for each
metal ion comprised of two urea RN- donors, one pyrazolate
N atom, an apical amine N atom, and an exogenous bridging
ligand arranged in a distorted trigonal-bipyramidal geometry.
The urea arms are sufficiently rigid to position four R′NH
groups proximal to the metal centers, enforcing intramo-
lecular H-bonds with external species (X) that bind within
the cavity. Further control of the secondary coordination
sphere originates in the steric demand exhibited by the
different R groups appended to the ends of the unsymmetrical
urea arms. These effects were illustrated in the molecular
structures of cobalt dimers with CoII-µ-Cl-CoII motifs; the
chloro ligands were involved in four additional intramolecu-
lar H-bonds.12 These noncovalent interactions appear to
influence the length of the CoII-Cl bonds, which were
unusually long at distances greater than 2.6 Å.
Synthetic systems that contain many of these structural
attributes have been prepared, and some structure-function
studies have been reported.6 Notable examples include the
works of Berreau and Marques-Rivas, who have shown that
intramolecular H-bonds affect the physical and chemical
properties of ZnII-OH units.7-9 We have also developed
systems that utilize intramolecular H-bonds to influence
We now report the preparation of new cobalt dimers with
CoII-µ-OH-CoII moieties. During the course of this study,
(8) Mononuclear complexes: (a) Ko¨va´ri, E.; Kra¨mer, R. J. Am. Chem.
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Andersen, J. F.; Anslyn, E. V. Angew. Chem., Int. Ed. 2002, 41, 4014-
4016. (f) Livieri, M.; Mancin, F.; Tonellato, U.; Chin, J. Chem.
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Martin de Rosales, R. T.; Williams, N. H. J. Am. Chem. Soc. 2005,
127, 13470-13471.
(4) (a) Stra¨ter, N.; Lipscomb, W. N.; Klabunde, T.; Krebs, B. Angew.
Chem., Int. Ed. Engl. 1996, 35, 2024-2055. (b) Wilcox, D. E. Chem.
ReV. 1996, 96, 2435-2458.
(5) (a) Vobeda, A.; Lahm, A.; Sakiyama, F.; Suck, D. EMBO J. 1991,
10, 1607-1618. (b) Chevrier, B.; Schalk, C.; D’Orchymont, H.;
Rondeau, J.-M.; Moras, D.; Tarnus, C. Structure 1994, 2, 283-291.
(c) Romier, C.; Dominguez, R.; Lahm, A.; Dahl, O.; Suck, D. Proteins
1998, 32, 414-424. (d) Desmarais, W. T.; Bienvenue, D. L.; Bzymek,
K. P.; Holz, R. C.; Petsko, G. A.; Ringe, D. Structure 2002, 10, 1063-
1072. (e) Bzymek, K. P.; Moulin, A.; Swierczek, S. I.; Ringe, D.;
Petsko, G. A.; Bennett, B.; Holz, R. C. Biochemistry 2005, 44, 12030-
12040. (f) Bzymek, K. P.; Swierczek, S. I.; Bennett, B.; Holz, R. C.
Inorg. Chem. 2005, 44, 8574-8580.
(9) Feng, G.; Mareque-Rivas, J. C.; Williams, N. H. Chem. Commun. 2006,
1845-1847.
(10) (a) Hammes, B. S.; Young, V. G., Jr.; Borovik, A. S. Angew. Chem.,
Int. Ed. 1999, 38, 666-669. (b) MacBeth, C. E.; Golombek, A. P.;
Young, V. G., Jr.; Yang, C.; Kuczera, K.; Hendrich, M. P.; Borovik,
A. S. Science 2000, 289, 938-941. (c) Larsen, P. L.; Gupta, R.; Powell,
D. R.; Borovik, A. S. J. Am. Chem. Soc. 2004, 126, 6522-6523. (d)
MacBeth, C. E.; Gupta, R.; Mitchell-Koch, K. R.; Young, V. G., Jr.;
Lushington, G. H.; Thompson, W. H.; Hendrich, M. P.; Borovik, A.
S. J. Am. Chem. Soc. 2004, 126, 2556-2567. (e) Borovik, A. S. Acc.
Chem. Res. 2005, 38, 54-61. (f) Lucas, R. L.; Powell, D. R.; Borovik,
A. S. J. Am. Chem. Soc. 2005, 127, 11596-11597.
(6) For recent examples, see: (a) Wada, A.; Harata, M.; Hasegawa, K.;
Jitsukawa, K.; Masuda, H.; Mukai, M.; Kitagawa, T.; Einaga, H.
Angew. Chem., Int. Ed. 1998, 37, 798-799. (b) Matsu-ura, M.; Tani,
F.; Nakayama, S.; Nakamura, N.; Naruta, Y. Angew. Chem., Int. Ed.
2000, 39, 1989-1991. (c) Grotjahn, D. B.; Incarvito, C. D.; Rheingold,
A. L. Angew. Chem., Int. Ed. 2001, 40, 3884-3887. (d) Garner, D.
K.; Fitch, S. B.; McAlexander, L. H.; Bezold, L. M.; Arif, A. M.;
Berreau, L. M. J. Am. Chem. Soc. 2002, 124, 9970-9971. (e) Chang,
C. J.; Chng, L. L.; Nocera, D. G. J. Am. Chem. Soc. 2003, 125, 1866-
1876. (f) Rivera, G.; Crabtree, R. H. J. Mol. Catal. A: Chem. 2004,
222, 59-73.
(11) MacBeth, C. E.; Hammes, B. S.; Young, V. G., Jr.; Borovik, A. S.
Inorg. Chem. 2001, 40, 4733-4741.
(12) Zinn, P. J.; Powell, D. R.; Day, V. W.; Hendrich, M. P.; Sorrell, T.
N.; Borovik, A. S. Inorg. Chem. 2006, 45, 3484-3486.
(13) Another dinucleating ligand containing H-bond donors has been
reported: Arii, H.; Funahashi, Y.; Jitsukawa, K.; Masuda, H. Dalton
Trans. 2003, 2115-2116.
(7) Dinuclear complex: Allred, R. A.; Doyle, K.; Arif, A. M.; Berreau,
L. M. Inorg. Chem. 2006, 45, 4097-4108.
Inorganic Chemistry, Vol. 46, No. 24, 2007 10121

Zinn et al.
solid. The oily solid was placed under vacuum for 2 h to remove
as many volatile components as possible. Next, thionyl chloride
(24.7 g, 208 mmol) was introduced to the dark-orange residue and
stirred for 17 h. SOCl2 was removed under reduced pressure to
produce a brown, sticky solid. Following a similar procedure
developed by Meyer,17 the resulting crude 5-tert-butyl-3-(chloro-
methyl)pyrazole hydrochloride was dissolved in CH2Cl2 and 1,2-
dihydropyran (6.12 g, 72.8 mmol) was added. The dark-brown
solution was stirred for 27 h. The mixture was then washed with a
solution of 16.6 g of sodium bicarbonate in 270 mL of water, and
the organic phase was dried over magnesium sulfate. The organic
phase was filtered, and the filtrate was concentrated under reduced
pressure to afford a dark-brown oil, which was eluted through a
column of silica gel using 10% Et2O/hexanes. The product was
isolated (Rf ) 0.25; 10% Et2O/hexanes and silica gel) as a light-
orange oil, which was then dissolved in a minimal amount of
hexanes and placed in a freezer overnight, resulting in an off-white,
microcrystalline solid. This microcrystalline solid was filtered,
washed with cold hexanes, and placed under vacuum to dry.
Recrystallization was repeated with the filtrate several times to
obtain a total of 4.77 g (57%). Mp: 39-41 °C. HRMS (ESI, m/z):
257.1427 [M+]. Exact mass calcd for C13H22N2O1Cl1 [M + H]:
Figure 2. Mononucleating ligand, H5PtBuuam, and general mononuclear
complex with an additional ligand (X).
we have found that these complexes can hydrate unactivated
nitriles and hydrolyze unactivated esters, such as acetonitrile
and ethyl acetate, at room temperature. Isolation and
structural characterization of the hydration/hydrolysis prod-
ucts confirm that bridging acetamidate or acetate ligands are
present. In a comparative study, we have developed a related
monomeric CoII complex with a terminal hydroxo ligand
(Figure 2) and investigated its hydrolytic properties. The
results of these studies have provided insights into the role
that the metal centers and H-bond networks have on the
observed reactivity.
1
257.1421. H NMR (CDCl3): δ 6.18 (s, 1H, Pz-H4), 5.43 (dd, J
) 2.7 and 9.5 Hz, 1H, H2-thp), 4.71 (pseudo-d, J ) 12.6 Hz, 1H,
Pz-C3-CH2), 4.65 (pseudo-d, J ) 12.6 Hz, 1H, Pz-C3-CH2),
4.01 (m, 1H, H6-thp), 3.68 (m, 1H, H6-thp), 2.43 (m, 1H, H3-
thp), 2.10 (m, 1H, H3-thp), 2.00 (m, 1H, H4-thp), 1.73-1.57 (m,
3H, H4/H5-thp), 1.28 (s, 9H, (C(CH3)). 13C NMR (CDCl3): δ
160.94 (Pz-C5), 138.23 (Pz-C3), 104.86 (Pz-C4), 85.55 (C2-
thp), 67.72 (C6-thp), 35.28 (Pz-C3-CH2Cl), 32.27 (C(CH3)),
30.54 (C(CH3)3), 29.52 (C3-thp), 25.13 (C5-thp), 22.66 (C4-thp).
3-{Bis[(N′-tert-butylureayl)-N-ethyl]aminomethyl}-5-tert-bu-
tyl-1-(tetrahydropyran-2-yl)-1H-pyrazole (2). A mixture of 1
Experimental Section
General Methods. All reagents were purchased from commercial
sources and used as received, unless otherwise noted. Solvents were
purified according to standard procedures.14 Anhydrous solvents
were purchased from Aldrich. Anhydrous N,N-dimethylacetamide
(DMA), CH2Cl2, and CH3CN were eluted through a column of
alternating bands of silica gel and neutral alumina and stored over
molecular sieves before use. Potassium hydride (KH) as a 30%
dispersion in mineral oil was filtered with a medium-porosity glass
frit and washed five times each with pentane and Et2O. The solid
KH was dried under vacuum and stored under an inert atmosphere.
The syntheses of metal complexes were conducted in a Vacuum
Atmospheres Co. drybox under an argon atmosphere. Elemental
analyses were accomplished at Desert Analytics (Tucson, AZ). The
binucleating ligands 3,5-bis{bis[(N′-R-ureayl)-N-ethyl]amino-
methyl}-1H-pyrazole (H9PRbuam, R ) tBu, iPr),12 6-[(tetrahydro-
2H-pyran-2-yl)oxy]-2,2-dimethylpent-4-yn-3-one,15 and bis[(N′-tert-
butylureayl)-N-ethyl]amine (tBuN5)16 were prepared by literature
methods.
Ligand Synthesis. 3-(Chloromethyl)-5-tert-butyl-1-(tetrahy-
dropyran-2-yl)-1H-pyrazole (1). Following a modified procedure
based on a synthetic route reported by Grotjahn et al.,15 crude
6-[(tetrahydro-2H-pyran-2-yl)oxy]-2,2-dimethylpent-4-yn-3-one (7.29
g, 32.5 mmol) was dissolved in MeOH (33 mL) in a 100 mL round-
bottomed flask. Hydrazine monohydrate (1.64 g, 32.8 mmol) was
added very slowly to the solution (exothermic reaction). The
reaction mixture became hot and started to reflux. The resulting
room temperature mixture was allowed to stir for 3 h. Concentrated
HCl was then added until the solution reached pH 2, and it was
subsequently stirred for 19 h. The solvent was removed under
reduced pressure in the same flask to obtain a dark-orange, oily
tBu
(3.77 g, 14.7 mmol), N5 (4.64 g, 15.4 mmol), oven-dried sodium
carbonate (7.79 g, 73.5 mmol), and anhydrous acetonitrile (150 mL)
was refluxed under nitrogen for 24 h. Thin-layer chromatography
(TLC) indicated that 1 was not present in the reaction mixture (Rf
) 0.34; silica gel TLC and 10% Et2O/hexanes) after 24 h. The
boiling reaction mixture was filtered, and the white solid was
washed with boiling acetonitrile. The solvent was then removed
from the combined filtrate under reduced pressure, resulting in a
yellow-orange solid. Next, the solid was dissolved in boiling
acetone, concentrated to saturation, and allowed to cool to room
temperature. The solution was placed in a freezer (∼-20 °C)
overnight to produce an off-white, microcrystalline solid. The
recrystallized product was filtered, washed with cold diethyl ether,
and dried under vacuum. Additional product was obtained by
repeating the recrystallization with the filtrate, affording a total of
6.49 g (85%). Mp: 106-108 °C. LRMS (FAB+; m/z): 522.5 [M+].
IR (KBr, cm-1): ν(NH) 3377, ν(NH) 3291, ν(CO) 1645, ν(NH)
1563. 1H NMR (CDCl3): δ 6.00 (s, 1H, Pz-H4), 5.37 (dd, J ) 2.6
and 9.7, 1H, H2-thp), 5.04 (t, J ) 4.9 Hz, 2H, CH2NH-C(O)),
4.71 (s, 2H, NH-C(CH3)3), 4.05 (m, 1H, H6-thp), 3.66 (m, 1H,
H6-thp), 3.62 (pseudo-d, J ) 13.9 Hz, 1H, Pz-C3-CH2), 3.54
(pseudo-d, J ) 13.9 Hz, 1H, Pz-C3-CH2), 3.29 (m, 2H, CH2CH2-
NH), 3.09 (m, 2H, CH2CH2NH), 2.53 (t, J ) 4.9 Hz, 4H, NCH2-
CH2), 2.51 (m, 1H, H3-thp), 2.08 (m, 1H, H4-thp), 1.86 (m, 1H,
H3-thp), 1.69-1.56 (m, 3H, H4/H5-thp), 1.32 (s, 18H, NH-
C(CH3)3), 1.27 (s, 9H, Pz-C5-C(CH3)3). 13C NMR (CDCl3): δ
160.91 (Pz-C5), 158.25 (C(O)), 140.21 (Pz-C3), 104.85 (Pz-C4),
84.23 (C2-thp), 67.56 (C6-thp), 54.81 (NCH2CH2), 50.30
(14) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon Press: Elmsford, NY, 1988.
(15) Grotjahn, D. B.; Van, S.; Combs, D.; Lev, D. A.; Schneider, C.;
Rideout, M.; Meyer, C.; Hernandez, G.; Mejorado, L. J. Org. Chem.
2002, 67, 9200-9209.
(16) Lucas, R. L. Ph.D. Dissertation, University of Kansas, Lawrence, KS,
2005.
(17) Ro¨der, J. C.; Meyer, F.; Pritzkow, H. Organometallics 2001, 20, 811-
817.
10122 Inorganic Chemistry, Vol. 46, No. 24, 2007

Pyrazolate-Bridged CoII Dimers with H-Bond Donors
(Pz-C3-CH2N), 49.95 (NH-C(CH3)3), 38.19 (CH2CH2NH), 32.28
(Pz-C5-C(CH3)3), 30.67 (Pz-C5-C(CH3)3), 29.76 (NH-C(CH3)3),
29.65 (C3-thp), 25.17 (C5-thp), 22.81 (C4-thp).
pyrazole (H9PiPrbuam; 0.100 g, 0.157 mmol), KH (0.038 g, 0.94
mmol), CoCl2 (0.041 g, 0.31 mmol), H2O (0.0028 g, 0.16 mmol),
and tetrapropylammonium chloride (0.070 g, 0.31 mmol) to yield
a purple solid, 0.17 g (94%). X-ray-quality crystals were obtained
by vapor diffusion of diethyl ether in a DMA solution of the isolated
3-{Bis[(N′-tert-butylureayl)-N-ethyl]aminomethyl}-5-tert-bu-
tyl-1H-pyrazole (H5PtBuuam). To a solution of 2 (6.49 g, 12.4
mmol) in absolute ethanol (118 mL) was added 67 mL of freshly
prepared ethanolic HCl, and the reaction mixture was stirred at room
temperature for 18 h. The solvent was removed under reduced
pressure to obtain an off-white solid. Next, the solid was dissolved
in water (100 mL), and 100 mL of a saturated sodium carbonate
solution was added, resulting in a white precipitate. The pH of the
aqueous solution was g8. The precipitated product was then
extracted with three 200 mL portions of dichloromethane, and the
combined organic layer was dried with sodium sulfate. The mixture
was filtered, and the filtrate was concentrated under reduced
pressure to yield an off-white solid, 5.34 g (98%). X-ray-quality
crystals were obtained by slow evaporation of a CHCl3 solution of
the product. Mp: 115-117 °C. HRMS (ESI, m/z): 438.3564 [M+].
Exact mass calcd for C22H44N7O2 [M + H]: 438.3556. IR (KBr,
cm-1): ν(NH) 3369, ν(NH) 3296, ν(NH) 3214, ν(CO) 1640,
ν(NH) 1561. 1H NMR (CDCl3): δ 5.95 (s, 1H, Pz-H4), 5.72 (bs,
2H, CH2NH-C(O)), 5.16 (s, 2H, NH-C(CH3)3), 3.61 (s, 2H,
Pz-C3-CH2), 3.17 (bs, 4H, CH2CH2NH), 2.52 (t, J ) 5.0 Hz,
4H, NCH2CH2), 1.33 (s, 18H, NH-C(CH3)3), 1.30 (s, 9H,
Pz-C5-C(CH3)3). 13C NMR (CDCl3): δ 159.18 (C(O)), 101.60
(Pz-C4), 55.06 (NCH2CH2), 51.11 (NH-C(CH3)3), 50.46
(Pz-C3-CH2N), 38.17 (CH2CH2NH), 31.94 (Pz-C5-C(CH3)3),
30.88 (Pz-C5-C(CH3)3), 30.01 (NH-C(CH3)3). Note: Quaternary
carbons Pz-C3 and Pz-C5 are not observed by 13C NMR.
Complex Synthesis. Tetrapropylammonium or Tetraethyl-
ammonium µ-Hydroxo-µ-{3,5-bis{bis[(N′-tert-butylureaylato)-
N-ethyl]aminatomethyl}-1H-pyrazolato}dicobaltate(II) ([nPr4N]2-
[CoII H4PtBubuam(µ-OH)]) or ([Et4N]2[CoII H4PtBubuam(µ-OH)]).
metal salt. Anal. Calcd (found) for [nPr4N]2[CoII H4PiPrbuam(µ-
2
OH)]‚0.1CH2Cl2, C53.1H110.2Cl0.2Co2N14O5: C, 55.46 (55.98); H,
9.66 (9.24); N, 17.05 (16.60). IR (Nujol, cm-1): ν(OH) 3612,
ν(NH) 3348, ν(CO) 1637, 1582. UV/vis (DMA): λmax, nm (ꢀ, M-1
cm-1) 508 (sh), 560 (250), 847 (37).
Tetrapropylammonium or Tetraethylammonium µ-1,3-Acet-
amidato-µ-{3,5-bis{bis[(N′-tert-butylureaylato)-N-ethyl]aminato-
methyl}-1H-pyrazolato}dicobaltate(II) ([nPr4N]2[CoII H4PtBubuam-
2
(µ-1,3-OC(NH)CH3)]) or ([Et4N]2[CoII2H4PtBubuam(µ-1,3-OC-
(NH)CH3)]). A solution of H9PtBubuam (0.100 g, 0.144 mmol) in
anhydrous DMA (5 mL) was treated with solid KH (0.035 g, 0.86
mmol) under an argon atmosphere. After gas evolution ceased, solid
CoCl2 (0.037 g, 0.29 mmol) was added. The resulting turquoise-
blue solution was stirred for 30 min. Next, acetamide (0.009 g,
0.14 mmol) was added, resulting in a violet solution while being
stirred for an additional 30 min. Tetrapropylammonium chloride
(0.064 g, 0.29 mmol) or tetraethylammonium chloride (0.048 g,
0.29 mmol) was then added and stirred for 1 h. The solvent was
removed under reduced pressure at room temperature, and the
resulting solid residue was washed with diethyl ether and dried
under vacuum to afford a violet powder. The powder was then
dissolved in anhydrous CH2Cl2, and the insoluble KCl was fil-
tered using a fine-porosity glass frit. The solvent was evaporated
from the filtrate under reduced pressure to afford a violet solid,
0.19 g. X-ray-quality crystals were obtained by vapor diffusion of
diethyl ether in a DMA solution of the isolated metal salt. Anal.
Calcd (found) for [Et4N]2[CoII H4PtBubuam(µ-1,3-OC(NH)CH3)],
2
C51H105Co2N15O5: C, 54.38 (54.57); H, 9.40 (9.22); N, 18.65
2
2
(18.34). [nPr4N]2[CoII H4PtBubuam(µ-1,3-OC(NH)CH3)]. IR (Nujol,
A solution of 3,5-bis{bis[(N′-tert-butylureayl)-N-ethyl]amino-
methyl}-1H-pyrazole (H9PtBubuam; 0.100 g, 0.144 mmol) in
anhydrous DMA (5 mL) was treated with solid KH (0.035 g, 0.86
mmol) under an argon atmosphere. After gas evolution ceased, solid
CoCl2 (0.037 g, 0.29 mmol) was added. The resulting turquoise-
blue solution was stirred for 30 min. Ultrapure H2O (0.0026 g, 0.14
mmol) was added by microsyringe, resulting in a purple solution,
with stirring continued for an additional 30 min. Tetrapropylam-
monium chloride (0.064 g, 0.29 mmol) or tetraethylammonium
chloride (0.048 g, 0.29 mmol) was then added and stirred for 1 h.
The solvent was removed under reduced pressure at room temper-
ature, and the resulting solid residue was washed with diethyl ether
and dried under vacuum to afford a purple powder. The powder
was then dissolved in anhydrous CH2Cl2, and the insoluble KCl
was filtered using a fine-porosity glass frit. The solvent was
evaporated from the filtrate under reduced pressure to afford a
purple solid, 0.18 g (99+%). X-ray-quality crystals were obtained
by vapor diffusion of diethyl ether in a DMA solution of the isolated
2
cm-1): ν(NH) 3406, 3304, ν(CO) 1639, 1583, ν(NCO) 1539, 1516.
UV/vis (DMA): λmax, nm (ꢀ, M-1 cm-1) 484 (sh), 542 (240), 718
(40).
Tetrapropylammonium µ-1,3-Acetamidato-µ-{3,5-bis{bis[(N′-
isopropylureaylato)-N-ethyl]aminatomethyl}-1H-pyrazolato}-
dicobaltate(II) ([nPr4N]2[CoII2H4PiPrbuam(µ-1,3-OC(NH)CH3)]).
This compound was prepared by the same route as [nPr4N]2-
[CoII H4PtBubuam(µ-1,3-OC(NH)CH3)] using H9PiPrbuam (0.100
2
g, 0.157 mmol), KH (0.038 g, 0.94 mmol), CoCl2 (0.041 g, 0.31
mmol), acetamide (0.009 g, 0.16 mmol), and tetrapropylammon-
ium chloride (0.070 g, 0.31 mmol) to yield a violet solid, 0.19 g
(99+%). X-ray-quality crystals were obtained by vapor diffusion
of diethyl ether in a DMA solution of the isolated metal salt. Anal.
Calcd (found) for [nPr4N]2[CoII H4PiPrbuam(µ-1,3-OC(NH)CH3)],
2
C55H113Co2N15O5: C, 55.87 (56.40); H, 9.63 (9.38); N, 17.77
(17.46). IR (Nujol, cm-1): ν(NH) 3415, 3289, ν(CO) 1577, ν(NCO)
1538, 1494. UV/vis (DMA): λmax, nm (ꢀ, M-1 cm-1) 480 (sh),
542 (220), 720 (34).
metal salt. Anal. Calcd (found) for [Et4N]2[CoII H4PtBubuam(µ-
2
OH)]‚0.5CH2Cl2, C49.5H103ClCo2N14O5: C, 52.72 (53.03); H, 9.21
Tetrapropylammonium µ-1,3-Acetato-µ-{3,5-bis{bis[(N′-tert-
butylureaylato)-N-ethyl]aminatomethyl}-1H-pyrazolato}di-
cobaltate(II) ([nPr4N]2[CoII2H4PtBubuam(µ-1,3-OAc)]). A solution
of H9PtBubuam (0.100 g, 0.144 mmol) in anhydrous DMA (5 mL)
was treated with solid KH (0.029 g, 0.72 mmol) under an argon
atmosphere. After gas evolution ceased, solid Co(OAc)2 (0.051 g,
0.29 mmol) was added. The resulting violet solution was stirred
for 30 min. Tetrapropylammonium chloride (0.064 g, 0.29 mmol)
was then added and stirred for 1 h. The solvent was removed under
reduced pressure at room temperature, and the resulting solid residue
was washed with diethyl ether and dried under vacuum to afford a
(8.73); N, 17.39 (17.40). Anal. Calcd (found) for [nPr4N]2-
[CoII H4PtBubuam(µ-OH)], C57H118Co2N14O5: C, 57.17 (57.26); H,
2
9.93 (9.76); N, 16.38 (15.90). IR (Nujol, cm-1): ν(OH) 3620,
ν(NH) 3352, 3294, ν(CO) 1637, 1587. UV/vis (DMA): λmax, nm
(ꢀ, M-1 cm-1) 503 (sh), 562 (260), 930 (36).
Tetrapropylammonium µ-Hydroxo-µ-{3,5-bis{bis[(N′-isopro-
pylureaylato)-N-ethyl]aminatomethyl}-1H-pyrazolato}dicobaltate-
(II) ([nPr4N]2[CoII2H4PiPrbuam(µ-OH)]). This compound was
prepared by the same route as [nPr4N]2[CoII H4PtBubuam(µ-OH)]
2
using 3,5-bis{bis[(N′-isopropylureayl)-N-ethyl]aminomethyl}-1H-
Inorganic Chemistry, Vol. 46, No. 24, 2007 10123

Zinn et al.
purple powder. The powder was then dissolved in anhydrous
CH2Cl2, and the insoluble KCl and KOAc were filtered using a
fine-porosity glass frit. The solvent was evaporated from the fil-
trate under reduced pressure to afford 0.18 g of a violet solid.
X-ray-quality crystals were obtained by vapor diffusion of di-
ethyl ether in a DMA solution of the isolated metal salt. Anal.
1642, 1587. EPR (X-band, DMA, 77 K): g ) 4.51. UV/vis
(DMA): λmax, nm (ꢀ, M-1 cm-1) 459 (sh), 537 (sh), 563 (93), 612
(sh), 751 (13).
Hydrolysis Reactions. [nPr4N]2[CoII2H4PtBubuam(µ-OH)],
[Et4N]2[CoII2H4PtBubuam(µ-OH)], or [nPr4N]2[CoII2H4PiPrbuam-
(µ-OH)] with CH3CN. The appropriate isolated µ-hydroxo complex
was dissolved in anhydrous acetonitrile and stirred for 2 h at room
temperature under an argon atmosphere. The solvent was then
removed under reduced pressure, and the resulting solid residue
was washed with diethyl ether and dried under vacuum to obtain a
violet powder. Fourier transform IR (FTIR) and UV/vis spec-
troscopies confirmed the hydrolysis of acetonitrile by displaying
the same features as those observed for the corresponding µ-1,3-
acetamidato complex synthesized by the direct route. Alternatively,
X-ray-quality crystals of the hydrolysis product were obtained by
vapor diffusion of diethyl ether in an acetonitrile solution of the
appropriate isolated µ-hydroxo complex. For instance, [nPr4N]2-
Calcd (found) for [nPr4N]2[CoII H4PtBubuam(µ-1,3-OAc)]‚DMA,
2
C63H129Co2N15O7: C, 57.04 (57.24); H, 9.80 (9.53); N, 15.84
(15.49). IR (Nujol, cm-1): ν(NH) 3344, ν(CO) 1580, 1553, 1407.
UV/vis (DMA): λmax, nm (ꢀ, M-1 cm-1) 485 (sh), 502 (sh), 542
(170), 734 (31).
Tetrapropylammonium µ-1,3-Acetato-µ-{3,5-bis{bis[(N′-iso-
propylureaylato)-N-ethyl]aminatomethyl}-1H-pyrazolato}-
dicobaltate(II) ([nPr4N]2[CoII2H4PiPrbuam(µ-1,3-OAc)]). This
compound was prepared by the same route as [nPr4N]2 [CoII H4PtBu
-
2
buam(µ-1,3-OAc)] using H9PiPrbuam (0.100 g, 0.157 mmol), KH
(0.031 g, 0.79 mmol), Co(OAc)2 (0.056 g, 0.31 mmol), and
tetrapropylammonium chloride (0.070 g, 0.31 mmol) to yield 0.19
g of a violet solid. X-ray-quality crystals were obtained by vapor
diffusion of diethyl ether in a DMA solution of the isolated metal
[CoII H4PtBubuam(µ-OH)] (0.050 g, 0.042 mmol) was dissolved
2
in 3-4 mL of anhydrous acetonitrile and crystallized by the diffu-
sion of diethyl ether to afford crystals of [nPr4N]2[CoII H4PtBubuam(µ-
2
1,3-OC(NH)CH3)], 0.031 g (60%). [nPr4N]2[CoII H4PiPrbuam(µ-
salt. Anal. Calcd (found) for [nPr4N]2 [CoII H4PiPrbuam(µ-1,3-
2
2
OH)] (0.050 g, 0.044 mmol) was used to produce crystals of
OAc)]‚DMA, C59H121Co2N15O7: C, 55.77 (55.10); H, 9.60 (9.74);
N, 16.54 (16.26). IR (Nujol, cm-1): ν(NH) 3344, ν(CO) 1640,
1574, 1561, 1411. UV/vis (DMA): λmax, nm (ꢀ, M-1 cm-1) 484
(sh), 544 (180), 746 (29).
[nPr4N]2[CoII H4PiPrbuam(µ-1,3-OC(NH)CH3)] (0.040 g, 77%).
2
[nPr4N]2[CoII H4PtBubuam(µ-OH)] or [nPr4N]2[CoII H4PiPrbuam-
2
2
(µ-OH)] with EtOAc. The appropriate isolated µ-hydroxo complex
was dissolved in a 1:1 mixture of anhydrous ethyl acetate and
anhydrous DMA and stirred for 2 h at room temperature under an
argon atmosphere. The solvent was then removed under reduced
pressure, and the resulting solid residue was washed with diethyl
ether and dried under vacuum to obtain a violet powder. FTIR and
UV/vis spectroscopies confirmed the hydrolysis of ethyl acetate
by displaying the same features as those observed for the corre-
sponding µ-1,3-OAc complex synthesized by the direct route.
Alternatively, X-ray-quality crystals of the hydrolysis product were
obtained by vapor diffusion of diethyl ether in a 1:1 ethyl acetate/
DMA solution of the appropriate isolated µ-hydroxo complex. For
Potassium 3-{Bis[(N′-tert-butylureaylato)-N-ethyl]aminato-
methyl}-5-tert-butyl-1H-pyrazolato(hydroxo)cobaltate(II)
(K2[CoIIH2PtBuuam(OH)]). A solution of H5PtBuuam (0.200 g,
0.457 mmol) in anhydrous DMA (5 mL) was treated with solid
KH (0.073 g, 1.8 mmol) under an argon atmosphere. After gas
evolution ceased, solid Co(OAc)2 (0.081 g, 0.46 mmol) was added.
The resulting purple solution was stirred for 30 min and then filtered
to separate the precipitated KOAc (0.088 g, 98%) from the reaction
mixture. Next, ultrapure H2O (0.0082 g, 0.46 mmol) was added to
the filtrate by microsyringe, resulting in a violet solution, which
was stirred for an additional 30 min. The solvent was removed
under reduced pressure at room temperature, and the resulting solid
residue was washed with diethyl ether, removing a purple, diethyl
ether soluble impurity. The solid was subsequently dried under
vacuum to afford a violet powder. X-ray-quality crystals were
obtained by vapor diffusion of diethyl ether in a DMA solution of
the isolated metal salt, 0.29 g (83%). Anal. Calcd (found) for
K2[CoIIH2PtBuuam(OH)]‚2DMA, C30H59CoK2N9O5: C, 47.23 (47.04);
H, 7.79 (7.72); N, 16.52 (16.24). IR (Nujol, cm-1): ν(OH) 3620,
ν(NH) 3230, 3140, ν(CO) 1645, 1595. EPR (X-band, DMA, 4 K):
g ) 5.13, 4.54, 3.42, 2.75, 1.97. UV/vis (DMA): λmax, nm (ꢀ, M-1
cm-1) 465 (sh), 486 (sh), 522 (150), 751 (35). µeff ) 4.76 µB (solid,
294 K, ødMiam ) -420.85 × 10-6).
instance, [nPr4N]2[CoII H4PtBubuam(µ-OH)] (0.050 g, 0.042 mmol)
2
was dissolved in 3-4 mL of 1:1 ethyl acetate/DMA and crystallized
by the diffusion of diethyl ether to afford crystals of [nPr4N]2-
[CoII H4PtBubuam(µ-1,3-OAc)]‚DMA, 0.038 g (68%). [nPr4N]2-
2
[CoII H4PiPrbuam(µ-OH)] (0.050 g, 0.044 mmol) was used to
2
produce crystals of [nPr4N]2[CoII H4PiPrbuam(µ-1,3-OAc)]‚DMA
2
(0.044 g, 79%).
K2[CoIIH2PtBuuam(OH)] with EtOAc. A crystalline compound
of K2[CoIIH2PtBuuam(OH)]‚DMA (0.073 g, 0.096 mmol) was
dissolved in 2 mL of anhydrous DMA under an argon atmos-
phere, and 2 mL of anhydrous EtOAc was added. The solution
was then set up for crystallization using diethyl ether vapor
diffusion. A white, microcrystalline solid immediately appeared.
After 48 h, the purple mother liquor was filtered, and the white
solid was washed with DMA and then with diethyl ether and
dried under vacuum. This yielded 0.009 g of KOAc (100%), which
was verified by FTIR (Nujol). The solvent was also removed from
the mother liquor. The resulting residue was washed with diethyl
ether and dried under vacuum to yield a purple powder. FTIR and
UV/vis verified that this purple product was not the hydroxo starting
complex.
Tetrapropylammonium 3-{Bis[(N′-tert-butylureaylato)-N-
ethyl]aminatomethyl}-5-tert-butyl-1H-pyrazolato(hydroxo)co-
baltate(II) ([nPr4N]2[CoIIH2PtBuuam(OH)]). This compound was
prepared by the same route as K2[CoIIH2PtBuuam(OH)] with the
following exceptions. After the addition of H2O and stirring for 30
min, tetrapropylammonium chloride (0.203 g, 0.914 mmol) was
added and subsequently stirred for 1 h. The isolated purple powder
was then dissolved in anhydrous CH2Cl2, and the insoluble KCl
was filtered using a fine-porosity glass frit. The solvent was
evaporated from the filtrate under reduced pressure to afford a
sticky purple solid. This solid was washed with 1:1 Et2O/pentane
and dried under vacuum to afford a purple powder, 0.41 g (95%).
Anal. Calcd (found) for [nPr4N]2[CoIIH2PtBuuam(OH)]‚0.8CH2Cl2,
Physical Methods. NMR spectra were recorded on a Bruker
Avance 400 or 500 MHz spectrometer equipped with a Silicon
Graphics workstation. Chemical shifts are reported in ppm relative
to the residual solvent. Most assignments are based on a series of
two-dimensional experiments. Fast atom bombardment mass spectra
(FAB-MS) were recorded on a Hewlett-Packard 5989A spectrom-
C
46.8H98.6Cl1.6CoN9O3: C, 59.09 (58.80); H, 10.45 (11.13); N, 13.25
(12.95). IR (Nujol, cm-1): ν(OH) 3620, ν(NH) 3268, 3175, ν(CO)
10124 Inorganic Chemistry, Vol. 46, No. 24, 2007

Pyrazolate-Bridged CoII Dimers with H-Bond Donors
Table 1. Crystallographic Data for [nPr4N]2[CoII2H4PiPrbuam(µ-OH)0.82(µ-Cl)0.18],a [nPr4N]2[CoII2H4PiPrbuam(µ-1,3-OC(NH)CH3)0.90(Cl)0.10]‚DMA,b
[nPr4N]2[CoII2H4PtBubuam(µ-1,3-OC(NH)CH3)]‚DMA, [nPr4N]2[CoII2H4PiPrbuam(µ-1,3-OAc)]‚DMA, [nPr4N]2[CoII2H4PtBubuam(µ-1,3-OAc)]‚DMA, and
K2[CoIIH2PtBuuam(OH)]‚2DMA
salt
[nPr4N]2[CoII2H4-
[nPr4N]2[CoII2H4-
PiPrbuam(µ-1,3-
[nPr4N]2[CoII2H4-
PtBubuam(µ-1,3-
OC(NH)CH3)]‚DMA
[nPr4N]2[CoII2H4-
PiPrbuam(µ-1,3-
OAc)]‚DMA
[nPr4N]2 [CoII2H4-
PtBubuam(µ-1,3-
OAc)]‚DMA
K2[CoIIH2-
PtBuuam(OH)]‚
2DMA
PiPrbuam(µ-OH)0.82
-
OC(NH)CH3)0.90-
a
(µ-Cl)0.18
]
(Cl)0.10]‚DMAb
molecular
formula
fw
T (K)
space
group
a (Å)
b (Å)
c (Å)
C53H109.82Co2N14-
4.82Cl0.18
C
58.8H121.6Co2N15.9
-
C63H130Co2N16O6
C59H121Co2N15O7
C63H129Co2N15O7
C30H59CoK2-
N9O5
762.99
O
O5.9Cl0.10
1141.41
100(2)
1269.59
100(2)
P21/n
1325.69
100(2)
1270.57
100(2)
P21/n
1326.67
100(2)
P21/n
100(2)
P21/c-C2h
P1h-Ci1
Cc-Cs4
5
(No. 2)
(No. 14)
11.137(4)
16.708(6)
21.762(8)
90
100.683(6)
90
4 (Z′ ) 1)
3979(2)
1.274
11.647(1)
12.429(1)
23.566(2)
96.208(4)
93.471(4)
107.709(4)
2 (Z′ ) 1)
3215.1(5)
1.182
11.707(3)
26.956(8)
22.901(6)
90
101.976(7)
90
4 (Z′ ) 1)
7070(3)
1.191
13.562(1)
19.052(2)
29.098(2)
90
92.625(3)
90
4 (Z′ ) 0.5)
7511(1)
1.158
11.310(4)
28.068(9)
22.372(7)
90
92.779(9)
90
4 (Z′ ) 1)
7094(4)
1.190
13.5403(12)
18.9521(16)
29.080(2)
90
92.144(4)
90
4 (Z′ ) 1)
7457.2(10)
1.104
R (deg)
â (deg)
γ (deg)
Z
V (Å3)
δcalcd
(Mg/m3)
Rc
0.054
0.139
0.978
0.047
0.126
1.050
0.062
0.179
1.036
0.055
0.155
1.030
0.047
0.137
1.073
0.0426
0.1203
1.003
d
Rw
GOFe
a This salt cocrystallized with 18.2% of the isodimensional anion [CoII2H4PiPrbuam(µ-Cl)]2-
.
.
b This salt cocrystallized with 10% of the isodimensional
anion [CoII2H4PiPrbuam(Cl)]2-
.
c R ) [∑|∆F|/∑|Fo|]. d Rw ) {∑[w(Fo - Fc )2]/∑[w(Fo )2]}1/2
e Goodness of fit on F2.
2
2
2
Scheme 1. Preparative Route for [CoII2H4PRbuam(µ-OH)]2- a
eter with a high-performance liquid chromatography (HPLC; HP
1050) HP particle-beam interface mass system and a phasor FAB
gun. FAB experiments were carried out in a thioglycerol/glycerol
matrix, and a xenon fast atom beam was used. Electrospray
ionization (ESI) mass spectra were recorded on a Micromass LCT
Premier spectrometer operating in W mode (high- and low-
resolution modes). Melting points were obtained with a Laboratory
Devices MEL-TEMP apparatus and are uncorrected. FTIR spectra
were collected on a Mattson Genesis series FTIR instrument with
values reported in wavenumbers. Electronic spectra were collected
on a Cary 50 spectrophotometer using 1.00 mm or 1.00 cm quartz
cuvettes. Room temperature magnetic susceptibility measurements
of solid samples were obtained using a MSB-1 magnetic suscep-
tibility balance (Johnson Matthey). Perpendicular-mode X-band
electron paramagnetic resonance (EPR) spectra were collected using
a Bruker EMX spectrometer equipped with an ER041XG micro-
wave bridge. A quartz liquid-nitrogen finger Dewar (Wilmad Glass)
was used to record spectra at 77 K, and 4 K spectra were obtained
using an Oxford Instrument liquid-helium quartz cryostat. Spectra
for EPR-active samples were collected using the following spec-
trometer settings: attenuation ) 15 dB, microwave power ) 6.300
mW, frequency ) 9.26 GHz, sweep width ) 5000 G, modula-
tion amplitude ) 10.02 G, gain ) 1.00 × 103, conversion time )
81.920 ms, time constant ) 655.36 ms, and resolution ) 1024
points.
a Conditions: (a) 6 equiv of KH, DMA, rt, Ar; (b) 2 equiv of CoCl2,
DMA, rt, Ar; (c) 1 equiv of H2O, DMA, rt, Ar; (d) 2 equiv of [nPr4N]Cl,
DMA, rt, Ar.
Results and Discussion
Preparation and Spectroscopic Properties of the [nPr4N]2-
[CoII H4PRbuam(µ-OH)] Complexes. Preparation of the
2
hydroxo-bridged dimers was accomplished by following
the method outlined in Scheme 1. Treating the binucle-
ating ligands with 6 equiv of KH affords [H3PRbuam]6-
in DMA, which readily binds two Co ions to produce
[CoII H3PRbuam]2-. Without isolation, [CoII H3PRbuam]2-
2
2
was treated with 1 equiv of H2O to form the [CoII H4PRbuam-
2
(µ-OH)]2- complexes, which were metathesized with 2 equiv
of [nPr4N]Cl to afford [nPr4N]2[CoII H4PRbuam(µ-OH)] in
2
Crystallographic Methods. Partial crystallographic data for
[nPr4N]2[CoII2H4PiPrbuam(µ-OH)], [nPr4N]2[CoIIH4PRbuam(µ-
1,3-OC(NH)CH3)]‚DMA, [nPr4N]2[CoIIH4PRbuam(µ-1,3-OAc)]‚
DMA, and K2[CoIIH4PtBuuam(OH)]‚2DMA are presented in Table
1. Note that [nPr4N]2[CoII2H4PiPrbuam(µ-OH)] and [nPr4N]2-
yields greater than 94%. Normally, 5 equiv of base are used
to bind two metal ions to these ligands; the extra 1 equiv of
base deprotonates one of the R′NH groups to provide com-
plexes with an additional basic site within the cavity. This
is needed to act as an internal base to scavenge the proton
produced when the hydroxide ion is formed from water. A
similar synthetic method has been used to prepare M-OH
complexes with tripodal urea ligands.11 Analytical and spec-
[CoII H4PiPrbuam(µ-1,3-OC(NH)CH3)]‚DMA cocrystallized with
2
18.2% and 10.0% of the isodimensional anion [CoII H4PiPrbuam(µ-
2
Cl)]2-. Details of the data collection and refinement for each
structure, plus full refinement parameters in CIF format, are found
in the Supporting Information.
troscopic measurements of the [nPr4N]2[CoII H4PRbuam(µ-
2
Inorganic Chemistry, Vol. 46, No. 24, 2007 10125

Zinn et al.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
[CoII2H4PiPrbuam(µ-OH)]2-
Co1-N1
2.372(2)
N3-Co1-N1
N4-Co1-N1
N11-Co1-N1
N5-Co2-N6
N5-Co2-N12
N6-Co2-N12
N5-Co2-N2
N6-Co2-N2
N12-Co2-N2
N3-Co1-O5
N4-Co1-O5
N11-Co1-O5
N1-Co1-O5
N5-Co2-O5
N6-Co2-O5
N12-Co2-O5
N2-Co2-O5
Co1-O5-Co2
80.8(1)
79.1(1)
73.5(1)
113.3(1)
118.2(1)
115.8(1)
78.9(1)
Co1-N3
2.002(2)
1.988(2)
1.992(2)
2.394(2)
1.980(2)
1.984(2)
1.970(2)
2.128(4)
2.133(4)
3.5857(5)
3.213(5)
3.201(5)
3.327(5)
3.116(5)
Co1-N4
Co1-N11
Co2-N2
Co2-N5
Co2-N6
Co2-N12
Co1-O5
81.2(1)
73.8(1)
Co2-O5
111.8(1)
107.1(1)
87.2(1)
160.1(1)
107.1(1)
111.0(1)
88.2(1)
Co1‚‚‚Co2
N7‚‚‚O5
N8‚‚‚O5
N9‚‚‚O5
Figure 3. Molecular structure of [CoII2H4PiPrbuam(µ-OH)]2-. The ellip-
soids are drawn at the 50% probability level, and C-bonded H atoms are
omitted for clarity.
N10‚‚‚O5
N3-Co1-N4
N3-Co1-N11
N4-Co1-N11
116.0(1)
112.5(1)
118.3(1)
161.6(1)
114.6(2)
OH)] salts indicated that the products isolated directly from
the reaction mixtures were of sufficient purity to be used
for further studies.
reduced from the normally observed angle of 180°. In
addition, the Co1-N1 and Co2-N2 distances of 2.372(2)
and 2.394(2) Å are 0.16 Å longer from those observed in
The FTIR spectra of both the iPr and tBu derivatives show
weak peaks at 3612 and 3620 cm-1 that are assigned to the
M(O-H) vibrations (Figure S1A in the Supporting Informa-
tion). The NH region indicates that the iPr analogue contains
more of a symmetric arrangement of H-bond donors, giving
[CoII H4PiPrbuam(µ-Cl)]2-, the related CoII-µ-Cl-CoII com-
2
plex.
The molecular structure of [CoII H4PiPrbuam(µ-OH)]2-
2
also reveals that the four R′NH groups are positioned within
the cavity toward the bridging HO- ligand. There may be
some dipolar effect resulting from the presence of four
H-bond donors proximal to the CoII-µ-OH-CoII unit, which
could contribute to the relatively long Co-O5 bonds.
However, all of the R′N‚‚‚O5 distances are greater than 3.11
Å; these distances are too long for H-bonds, which are
generally less than 3.0 Å.19 The placement of the hydroxo
ligand within the cavity is such that intramolecular H-bonds
to the rigid urea group are diminished. Note that in other
t
a single sharp peak. In contrast, the Bu derivative has two
separate N-H peaks, suggesting a less symmetrical arrange-
ment of H-bond donors. Similar vibrational properties were
observed for the previously reported chloro analogues,
i
[CoII H4PRbuam(µ-Cl)]2-, in which the Pr derivative pos-
2
sessed a more symmetric H-bonding network about the
chloro ligand compared to the tBu analogue.12 Both
[CoII H4PRbuam(µ-OH)]2- complexes exhibit similar optical
2
complexes of [H4PiPrbuam]5-, such as [CoII H4PiPrbuam(µ-
properties in solution (Figure S1B in the Supporting Infor-
2
mation).
Cl)]2-, multiple intramolecular H-bonds are observed be-
tween the R′NH groups and the chloro bridge. A comparison
of the molecular structures of the CoII-µ-X-CoII (X ) OH-,
Cl-) complexes reveals that the chloro ligand is nearly 0.5
Å farther from the Co-Co axis than O5, placing the bridging
Cl ion in an optimal position to form H-bonds.
Solid-State Structure of [nPr4N]2[CoII H4PiPrbuam-
2
(µ-OH)]. X-ray-quality single crystals of [nPr4N]2-
[CoII H4PiPrbuam(µ-OH)] were obtained by vapor diffusion
2
of diethyl ether into a DMA solution of the isolated metal
salt, which were stable for weeks under a dry, anaerobic
environment. Crystals of [nPr4N]2[CoII H4PtBubuam(µ-OH)]
2
Hydration of Acetonitrile and Hydrolysis of Ethyl
were obtained in the same manner but were not suitable to
obtain a molecular structure by X-ray diffraction. The molec-
ular structure of the isopropyl derivative determined from
X-ray diffraction data is presented in Figure 3, with selected
distances and angles shown in Table 2. The structure reveals
the CoII centers as five-coordinate with distorted trigonal-
bipyramidal geometry. A single hydroxo ligand that bridges
between the two CoII ions is present in the complex,
producing a metal-metal separation of 3.5857(5) Å. The
CoII-µ-OH-CoII unit is statistically symmetrical, having
Co1-O5 and Co2-O5 distances of 2.128(4) and 2.133(4)
Å. These CoII-OH distances are significantly longer com-
pared to the average distance observed for other reported
synthetic CoII-µ-OH-CoII compounds [avg CoII-O(H) )
2.004 Å].18 Other distortions away from trigonal-bipyramidal
coordination geometry presumably occur to accommodate
the CoII-µ-OH-CoII unit. For instance, the N1-Co1-O5
and N2-Co2-O5 angles are less than 162°, significantly
Acetate with [CoII H4PRbuam(µ-OH)]2-. During the course
2
of our studies on [CoII H4PRbuam(µ-OH)]2-, we dis-
2
covered that it hydrates acetonitrile to form complexes with
bridging µ-1,3-acetamidato ligands (µ-1,3-OC(NH)CH3;
Scheme 2). The reactions were completed within 2 h at
room temperature, and crystalline products were obtained
by vapor diffusion of diethyl ether into acetonitrile solutions
of the [CoII H4PRbuam(µ-OH)]2- complexes: [nPr4N]2-
2
[CoII H4PtBubuam(µ-1,3-OC(NH)CH3)]‚DMA and [nPr4N]2-
2
[CoII H4PiPrbuam(µ-1,3-OC(NH)CH3)]‚DMA were obtained
2
in 60% and 77% crystalline yields, respectively.
(18) (a) Kitajima, N.; Hikichi, S.; Taniaka, M.; Moro-oka, Y. J. Am. Chem.
Soc. 1993, 115, 5496-5508. (b) He, C.; Lippard, S. J. J. Am. Chem.
Soc. 1998, 120, 105-113. (c) Singh, U. P.; Babbar, P.; Sharma, A.
K. Inorg. Chim. Acta 2005, 358, 271-278.
(19) (a) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford
University Press, Inc.: New York, 1997. (b) Desiraju, G. R.; Steiner,
T. The Weak Hydrogen Bond In Structural Chemistry and Biology;
Oxford University Press Inc.: New York, 1999.
10126 Inorganic Chemistry, Vol. 46, No. 24, 2007

Pyrazolate-Bridged CoII Dimers with H-Bond Donors
Scheme 2. Hydration/Hydrolysis Reactions with [CoII2H4PRbuam(µ-OH)]2-
The vibrational properties of the isolated salts are consis-
tent with the formation of acetamidato ligands.20 The
signature M(O-H) stretching for the bridging hydroxo
complexes at ν ∼ 3620 cm-1 was absent in the solid-state
FTIR spectra of the crystallized products. Two new carbonyl-
cm-1 and νa(COO) ) 1411 cm-1 for the iPr derivative (Figure
S3B in the Supporting Information). The ∆ν values (∆ν )
146 cm-1 for R ) tBu and ∆ν ) 150 cm-1 for R ) iPr) are
in agreement with the literature values found for other
complexes with µ-1,3 acetate bridges.21
i
type peaks appeared at ν ) 1538 and 1494 cm-1 (R ) Pr)
Direct Preparations of Hydration/Hydrolysis Products.
To confirm the formulation of the hydration/hydrolysis prod-
ucts, direct synthetic methods were developed for complexes
with CoII-(µ-1,3-X′)-CoII (X′ ) OAc-, [OC(NH)CH3]-)
cores. The procedures were modeled after that used to
t
and at ν ) 1539 and 1516 cm-1 (R ) Bu). In addition, a
new weak, but relatively sharp, ν(NH) band was present at
3415 cm-1 for R ) iPr and at 3406 cm-1 for R ) tBu (Figure
S2 in the Supporting Information).
While exploring the scope of hydrolysis performed by
prepare the [CoII H4PRbuam(µ-OH)]2- complexes and are
2
[CoII H4PRbuam(µ-OH)]2-, it was also found that inactivated
outlined in Scheme 3. Tetrapropylammonium salts of the
2
esters such as ethyl acetate can be cleaved (Scheme 2).
[CoII H4PRbuam(µ-1,3-X′)]2- complexes were isolated in
2
Dissolving [nPr4N]2[CoII H4PRbuam(µ-OH)] in a 1:1 solution
nearly quantitative yields and displayed spectral properties
identical with those of the crystallized products obtained from
the hydration/hydrolysis reactions.
2
of DMA and ethyl acetate and stirring the mixture for 2 h at
room temperature under argon produced complexes with
µ-1,3-acetate ligands. The isolated salts were crystallized in
a 1:1 mixture of DMA and ethyl acetate using diethyl ether
Molecular Structures of [CoII H4PRbuam(µ-1,3-X′)]2-.
2
[CoII H4PRbuam(µ-1,3-OC(NH)CH3)]2-
Complexes.
2
vapor diffusion, affording [nPr4N]2[CoII H4PtBubuam(µ-1,3-
X-ray-quality crystals of [nPr4N]2[CoII H4PRbuam(µ-1,3-OC-
2
2
OAc)]‚DMA and [nPr4N]2[CoII H4PiPrbuam(µ-1,3-OAc)]‚
(NH)CH3)] and [nPr4N]2[CoII H4PRbuam(µ-1,3-OAc)] were
2
2
DMA in 68% and 79% crystalline yield, respectively.
The isolated complexes had predictable changes in the their
spectral properties that indicated the presence of acetate
bridges. The FTIR spectra again showed the absence of peaks
associated with an M(O-H) vibration from the starting
µ-hydroxo compounds, and the signals associated with NH
vibrations changed from a cluster of medium-intensity, broad
peaks to a single, strong, and sharp peak, indicating a
different H-bonding network within the cavities (Figure S3A
in the Supporting Information). The solid-state FTIR spectra
of the crystalline products show distinctive symmetric and
asymmetric stretching vibrations consistent with a µ-1,3-OAc
moiety at νa(COO) ) 1553 cm-1 and νs(COO) ) 1407 cm-1
obtained by vapor diffusion of diethyl ether into DMA
solutions of the appropriate isolated metal salt. These salts
were stable for weeks under a dry, anaerobic environment.
Single-crystal X-ray diffraction measurements corroborate
the similarities in the solid-state structures for both
[CoII H4PRbuam(µ-1,3-OC(NH)CH3)]2- complexes. The mo-
2
i
lecular structure of the Pr derivative is presented in Fig-
ure 4, with selected metrical parameters for both com-
plexes found in Table 3. The acetamidato H atom in
[CoII H4PiPrbuam(µ-1,3-OC(NH)CH3)]2- was located and
2
refined, and the different O5-C30 and N13-C30 bond
lengths of 1.281(2) and 1.310(3) Å confirms the presence
t
of an acetamidato ligand. The Bu analogue contained a
for [CoII H4PtBubuam(µ-1,3-OAc)]2- and at νa(COO) ) 1561
disordered acetamidato ligand with nearly equally occupied
2
(20) Meyer, F.; Kaifer, E.; Kircher, P.; Heinze, K.; Pritzkow, H. Chem.s
Eur. J. 1999, 5, 1617-1630.
(21) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, 5th ed.; John Wiley: New York, 1997.
Inorganic Chemistry, Vol. 46, No. 24, 2007 10127

Zinn et al.
Scheme 3. Preparative Routes for [CoII2H4PRbuam(µ-1,3-X′)]2- (X′ ) OAc-, [OC(NH)CH3]-)a
a Conditions: (a) 6 equiv of KH, DMA, rt, Ar; (b) 2 equiv of CoCl2, DMA, rt, Ar; (c) 1 equiv of acetamide, DMA, rt, Ar; (d) 2 equiv of [nPr4N]Cl, DMA,
rt, Ar; (e) 5 equiv of KH, DMA, rt, Ar; (f) 2 equiv of Co(OAc)2, DMA, rt, Ar; (g) 2 equiv of [nPr4N]Cl, DMA, rt, Ar.
Table 3. Selected Bond Distances (Å) and Angles (deg) for
i
t
[CoII2H4PRbuam(µ-1,3-OC(NH)CH3)]2- (R ) Pr and Bu)
R
iPr
tBu
Co1-N1
Co1-N3
Co1-N4
Co1-N11
Co2-N2
Co2-N5
Co2-N6
Co2-N12
Co2-O5
Co1-N13
Co1‚‚‚Co2
N9‚‚‚O5
2.204(1)
2.022(1)
2.018(1)
2.128(1)
2.181(1)
2.028(2)
2.001(1)
2.088(1)
2.128(1)
2.105(2)
4.482(1)
2.770(2)
2.830(2)
3.048(2)
3.020(2)
2.191(4)
1.996(5)
2.018(5)
2.165(5)
2.173(5)
2.034(5)
2.006(5)
2.120(5)
2.103(5)
2.096(5)
4.6697(5)
2.916(7)
2.897(7)
2.875(8)
2.913(8)
Figure 4. Molecular structure of [CoII2H4PiPrbuam(µ-1,3-OC(NH)CH3)]2-
The ellipsoids are drawn at the 50% probability level, and C-bonded H
atoms are omitted for clarity.
.
N10‚‚‚O5
N7‚‚‚N13
N8‚‚‚N13
(52/48) orientations about the pseudo-C2 axis of the dianion
(Figure S6 in the Supporting Information). In addition, the
N3-Co1-N4
N3-Co1-N11
N4-Co1-N11
N3-Co1-N1
N4-Co1-N1
N11-Co1-N1
N5-Co2-N6
N5-Co2-N12
N6-Co2-N12
N5-Co2-N2
N6-Co2-N2
N12-Co2-N2
N5-Co2-O5
N6-Co2-O5
N12-Co2-O5
N2-Co2-O5
N3-Co1-N13
N4-Co1-N13
N11-Co1-N13
N1-Co1-N13
3.048(2)
110.88(6)
119.30(5)
120.67(5)
81.13(5)
114.6(2)
H atom on N13 of the acetamidate in [CoII H4PtBubuam(µ-
2
116.21(19)
120.60(18)
81.94(17)
80.80(17)
77.95(17)
113.2(2)
1,3-OC(NH)CH3)]2- could not be located in the Fourier
difference map. Nevertheless, large differences in bond
lengths were found within the bridging ligand to support the
presence of both C-O and C-N bonds [i.e., 1.171(10) and
1.448(10) Å].
77.08(5)
120.02(6)
117.96(5)
113.20(5)
80.61(6)
81.46(6)
77.94(5)
121.7(2)
117.0(2)
Each complex contains Co centers that are five-coordinate
with distorted trigonal-bipyramidal geometry containing a
deprotonated acetamidato as a bridge between the two metal
centers. The Co1‚‚‚Co2 separations are 4.482(1) Å for the
81.27(19)
80.88(18)
79.25(18)
98.9(2)
104.60(19)
121.7(2)
173.8(2)
104.6(2)
99.60(19)
95.5(2)
172.41(19)
97.89(5)
101.96(5)
100.16(5)
176.56(5)
105.73(6)
102.75(6)
92.51(6)
t
iPr analogue and 4.6697(5) Å for the Bu analogue; these
distances are nearly 1.0 Å longer than those in the µ-hydroxo
complex, illustrating the flexibility within the [H4PRbuam]5-
iPr
ligands about the pyrazolate core. In [CoII H4P buam(µ-
2
169.41(6)
1,3-OC(NH)CH3)]2-, the H-bond donors of the urea groups
form intramolecular H-bonds with the heteroatoms of the
acetamidato bridge (Table 3). The average R′N‚‚‚OC(NH)-
CH3 and R′N‚‚‚NHC(O)CH3 distances of 2.800(2) and 3.034-
(2) Å are within those normally observed for H-bonds; in
addition, N-H‚‚‚X (X ) O, N) angles greater than 140°
are observed, which are consistent with intramolecular
H-bonds.19
atoms Co2, O5, N13, and Co1 were used to define the
reference mean plane. The fold angle was then determined,
which is the angle between the reference mean plane and
the plane containing atoms of the bridging ligand: in the
iPr complex, the ligand plane contains the atoms of the
acetamidato bridge, O5, N13, C30, and C31. Therefore,
the fold angle observed for the bridging acetamidato is 63°
Note the unusual binding of the bridging acetamidato, in
which the C-C bond is nearly perpendicular to the pyra-
zolate plane (Figure 4). To compare this structural feature
to other bridging systems, a reference mean plane was
defined by the two metal centers and the atom(s) of the
bridging ligand bound directly to the metal centers. For
in [CoII H4PiPrbuam(µ-1,3-OC(NH)CH3)]2- and 84° in
2
[CoII H4PtBubuam(µ-1,3-OC(NH)CH3)]2-. Other bridging
2
amidate complexes have fold angles that are usually less than
20°.20,22,23
The larger fold angles in the [CoII H4PRbuam(µ-1,3-OC-
2
(NH)CH3)]2- complexes undoubtedly arise from secondary
instance, in [CoII H4PiPrbuam(µ-1,3-OC(NH)CH3)]2-, the
coordination sphere effects created by the [H4PRbuam]5-
2
10128 Inorganic Chemistry, Vol. 46, No. 24, 2007

Pyrazolate-Bridged CoII Dimers with H-Bond Donors
Table 4. Selected Bond Distances (Å) and Angles (deg) for
i
t
[CoII2H4PRbuam(µ-1,3-OAc)]2- (R ) Pr, Bu)
R
iPr
tBu
Co1-N1
Co1-N3
Co1-N4
Co1-N11
Co2-N2
Co2-N5
Co2-N6
Co2-N12
Co1-O5
Co2-O6
Co1‚‚‚Co2
N7‚‚‚O5
N8‚‚‚O5
N9‚‚‚O6
N10‚‚‚O6
2.221(2)
2.017(2)
2.016(2)
2.066(2)
2.260(2)
2.024(2)
1.982(2)
2.038(2)
2.126(1)
2.112(2)
4.311(1)
2.824(2)
2.961(2)
3.077(2)
2.915(3)
2.178(16)
2.016(18)
2.008(17)
2.131(17)
2.163(16)
1.997(17)
2.027(17)
2.111(16)
2.139(14)
2.130(14)
4.6507(5)
2.915(3)
2.937(2)
2.920(2)
2.943(3)
N3-Co1-N4
N3-Co1-N11
N4-Co1-N11
N3-Co1-N1
N4-Co1-N1
N11-Co1-N1
N5-Co2-N6
N5-Co2-N12
N6-Co2-N12
N5-Co2-N2
N6-Co2-N2
N12-Co2-N2
N3-Co1-O5
N4-Co1-O5
N11-Co1-O5
N1-Co1-O5
N5-Co2-O6
N6-Co2-O6
N12-Co2-O6
N2-Co2-O6
111.55(7)
113.54(7)
125.44(7)
81.28(7)
80.52(7)
77.58(7)
82.47(7)
124.83(7)
112.65(7)
80.18(7)
82.47(7)
79.70(7)
100.28(7)
102.47(7)
98.16(6)
175.72(6)
99.26(7)
100.10(7)
98.58(7)
177.33(6)
113.74(7)
119.96(7)
118.92(7)
81.95(7)
81.39(7)
81.82(7)
115.34(7)
117.11(7)
119.50(7)
81.40(7)
80.84(7)
79.23(6)
99.67(6)
103.44(6)
94.46(6)
173.12(6)
103.77(6)
100.59(6)
94.35(6)
173.18(6)
Figure 5. Molecular structures of [CoII2H4PRbuam(µ-1,3-OAc)]2-: R )
iPr (A) and tBu (B). The ellipsoids are drawn at the 50% probability level,
and non-urea H atoms are omitted for clarity.
ligand. Optimal intramolecular H-bonds occur when elec-
tronic repulsions are minimized between the N-H bond of
the deprotonated acetamidate and the H-bond donors within
the cavity; this occurs when the fold angle is large.
Additionally, steric repulsions with the bulky R groups
appended to the urea arms can influence the orientation of
t
the bridging ligand, particularly in the Bu derivative.
lecular H-bonds between the [H4PRbuam]5- ligands and the
bridged acetate moieties.
Molecular Structures of [CoII H4PRbuam(µ-1,3-OAc)]2-.
2
The solid-state molecular structures of [CoII H4PRbuam(µ-
2
The molecular structures also reveal H-bonding networks
surrounding the CoII-µ-1,3-OAc-CoII unit, involving the
R′NH groups of [H4PRbuam]5- and the O atoms of the
1,3-OAc)]2- are shown in Figure 5, with selected bond
distances and angles presented in Table 4. The complexes
also have Co centers with distorted trigonal-bipyramidal
geometries containing an acetate ion as the bridging, ex-
ternal ligand. The Co1‚‚‚Co2 separations are 4.311(1) and
bridging acetate. For [CoII H4PtBubuam(µ-1,3-OAc)]2-, all
2
of the R′N‚‚‚O distances are nearly equivalent at distances
of less than 3.0 Å and N-H‚‚‚O angles greater than 160°.
A less symmetrical H-bonding network is found in the Pr
4.6507(5) Å for [CoII H4PiPrbuam(µ-1,3-OAc)]2- and
2
i
[CoII H4PtBubuam(µ-1,3-OAc)]2-, respectively. The metal-
2
derivative, yet the R′N‚‚‚O distances and angles also suggest
the presence of four intramolecular H-bonds. The most
noticeable difference between the structures of the two
complexes is the orientation of the acetate bridge. In
t
metal separation for the Bu complex is comparable to that
observed in the analogous bridging acetamidato complex;
i
however, the Pr complex exhibits a significantly shorter
[CoII H4PiPrbuam(µ-1,3-OAc)]2-, the C30-C31 vector is
Co1‚‚‚Co2 distance, which may be caused by the orienta-
tion of the bridging acetate (vide infra). The CoII-O bond
lengths for each complex are approximately 0.07 Å longer
than the average CoII-O bond distance reported for other
CoII dimers with bridging acetate ligands.24 The lengthening
of the CoII-O bond is partially attributed to the intramo-
2
nearly parallel to the pyrazolate plane, whereas the com-
parable bond in the Bu analogue, the C34-C35 vector,
is virtually perpendicular to the pyrazolate plane. This
t
difference is seen in their fold angles, which are 34°
i
t
(R ) Pr) and 82° (R ) Bu). The small fold angle in
[CoII H4PiPrbuam(µ-1,3-OAc)]2- is similar to those found
2
(22) (a) Curtis, N. J.; Hagen, K. S.; Sargeson, A. M. J. Chem. Soc., Chem.
Commun. 1984, 1571-1573. (b) Alcock, N. W.; Creaser, I. I.; Curtis,
N. J.; Roecker, L.; Sargeson, A. M.; Willis, A. C. Aust. J. Chem. 1990,
43, 643-654.
(23) Bauer, C. B.; Concolino, T. E.; Eglin, J. L.; Rogers, R. D.; Staples,
R. J. J. Chem. Soc., Dalton Trans. 1998, 2813-2817.
(24) Brown, D. A.; Errington, W.; Glass, W. K.; Haase, W.; Kemp, T. J.;
Nimir, H.; Ostrovsky, S. M.; Werner, R. Inorg. Chem. 2001, 40, 5962-
5971.
in other complexes with bridging acetates and occurs because
the appended iPr groups create a relatively open cavity that
can accommodate the C30-C31 vector. In contrast, the more
sterically demanding tBu groups in [CoII H4PtBubuam(µ-1,3-
2
OAc)]2- form a constrained structure, without enough space
to house the methyl group of the bridging acetate. Conse-
Inorganic Chemistry, Vol. 46, No. 24, 2007 10129

Zinn et al.
Figure 6. Proposed mechanism for the hydration of acetonitrile by [CoII2H4PRbuam(µ-OH)]2-
.
quently, the C34-C35 vector is positioned out of the cavity.
These structural differences demonstrate that the R groups
appended to the ends of the urea arms allow additional
control of the secondary coordination sphere by regulating
the size and shape of the H-bonding pocket.
be expected. This type of shift in the binding mode has been
suggested by studies performed on aminopepidase, a di-
nuclear metallohydrolase.28 In addition, secondary-sphere
H-bonds are proposed to play an essential role in the
enzymatic activity of many hydrolases.5 We have shown that
[H4PRbuam]5- provides H-bond donors capable of forming
intramolecular H-bonds with species bound within the
cavity;12 thus, it is reasonable to suggest that the H-bond
donors are playing a role in regulating the reactivity of
Mechanism of Hydration/Hydrolysis. Outlined in Figure
6 is a proposed mechanism for the hydration of acetonitrile
by [CoII H4PRbuam(µ-OH)]2-; this mechanism is based on
2
other reported dinuclear systems that perform similar
reactions.20,22,23,25-27 Displacement of the hydroxo ligand from
one of the Co centers by an N-coordinated nitrile results in
a change in the coordination of the hydroxo ligand from a
bridging mode to a terminal mode. This process may be facile
and energetically favorable because of the H-bond donors,
which can provide stabilization to the resulting terminal
hydroxo intermediate. Binding the nitrile substrate places the
now activated, electrophilic carbon atom in a position that
allows intramolecular nucleophilic attack by the adjacent
hydroxo ligand. A final proton shift produces the stable
µ-1,3-acetamidato bridge. A similar mechanism would apply
to the hydrolysis of ethyl acetate, producing the bridging
acetate and ethanol.
[CoII H4PRbuam(µ-OH)]2- while adding structural stability
2
to the proposed reactive intermediate and final product.
Synthesis and Structure of a Monomeric CoII-OH
Complex. The mononuclear complex, [CoIIH2PtBuuam(OH)]2-,
was prepared in order to explore the cooperation of two metal
ions in the hydration/hydrolysis of substrates. We reasoned
that the mononuclear CoII-OH complex was a good system
for comparison because it possesses properties similar to
those of [CoII H4PRbuam(µ-OH)]2- but can prevent interac-
2
tions between two [CoIIH2PtBuuam(OH)]2- complexes. Note
that similar strategies were used by Meyer and Mareque-
Rivas in probing metal-assisted hydrolytic transformations.9,20
The ligand [H2PtBuuam]3- was designed to represent half of
the dinucleating ligand [H4PtBubuam]5-, in which a tBu group
replaces one set of urea arms (Figure 2). The syntheses of
H5PtBuuam and its corresponding CoII-OH complex are
outlined in Scheme 4.
A hydroxide or water spanning two metal ions has been
detected crystallographically in many biological hydrolases
and is normally considered the active nucleophile.4 A tightly
bound, bridging hydroxide would yet be expected to exhibit
relatively low nucleophilic character. Thus, a shift in the
binding mode of the bridging hydroxo ligand to a terminal
position prior to attack on the coordinated substrate would
The molecular structure of [CoIIH2PtBuuam(OH)]2-, de-
termined from single-crystal X-ray diffraction data, is
presented in Figure 7, and metrical parameters are found in
Table 5. The Co center is five-coordinate and has nearly
perfect trigonal-bipyramidal geometry based on the angular
structural parameter introduced by Addison et al.29 The
primary coordination sphere contains the same ligation as
that observed for the Co ions in the dinuclear derivatives.
(25) (a) Hazell, A.; Jensen, K. B.; McKenzie, C. J.; Toftlund, H. Inorg.
Chem. 1994, 33, 3127-3134. (b) Wilkinson, E. C.; Dong, Y.; Que,
L., Jr. J. Am. Chem. Soc. 1994, 116, 8394-8395. (c) Duboc-Toia, C.;
Menage, S.; Vincent, J.-M.; Averbuch-Pouchot, M. T.; Fontecave, M.
Inorg. Chem. 1997, 36, 6148-6149.
(26) Frey, S. T.; Murthy, N. N.; Weintraub, S. T.; Thompson, L. K.; Karlin,
K. D. Inorg. Chem. 1997, 36, 956-957.
(27) (a) McKenzie, C. J.; Robson, R. J. Chem. Soc., Chem. Commun. 1988,
112-114. (b) Hoskins, B. F.; McKenzie, C. J.; MacDonald, I. A. S.;
Robson, R. J. Chem. Soc., Dalton Trans. 1996, 2227-2232.
(28) Chen, G.; Edwards, T.; D’souza, V. M.; Holz, R. C. Biochemistry
1997, 36, 4278-4286.
(29) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijin, J.; Verschoor, G.
C. J. Chem. Soc., Dalton Trans. 1984, 1349-1356.
10130 Inorganic Chemistry, Vol. 46, No. 24, 2007

Pyrazolate-Bridged CoII Dimers with H-Bond Donors
Scheme 4. Preparative Route for [CoIIH2PtBuuam(OH)]2- a
structure also shows that there are no intermolecular interac-
tions between [CoIIH2PtBuuam(OH)]2- anions, suggesting that
dimerization was prevented in the solid state.
Reactivity of [CoIIH2PtBuuam(OH)]2- with Acetonitrile
and Ethyl Acetate. K2[CoIIH2PtBuuam(OH)] was metathe-
sized with 2 equiv of [nPr4N]Cl to increase its solubility in
acetonitrile; this was necessary to compare its reactivity to
that of its dimeric [CoII H4PRbuam(µ-OH)]2-. The spectro-
2
scopic properties of [nPr4N]2[CoIIH2PtBuuam(OH)] were the
same as those of the potassium salt, including the same
frequency for the O-H vibrations [ν(OH) ) 3620 cm-1].
[nPr4N]2[CoIIH2PtBuuam(OH)] does not hydrate acetoni-
trile; stirring the salt in acetonitrile for 2 h at room
temperature under an argon atmosphere did not produce any
spectroscopic changes in solution or in the isolated complex.
Prolonged exposure to acetonitrile also does not produce
evidence for hydration to an acetamide species. Similar
observations were found when [nPr4N]2[CoIIH2PtBuuam(OH)]
was treated with a 1:1 mixture of EtOAc and DMA. This
salt was not able to hydrolyze ethyl acetate, and the isolated
metal complex was determined as [CoIIH2PtBuuam(OH)]2-.
Taken together, these results suggest that a second metal ion
in close proximity to the CoII-OH unit is necessary for
hydration/hydrolysis to occur.
a Conditions: (a) H2NNH2‚H2O, MeOH, rt/HCl, MeOH, rt/SOCl2,
tBu
rt/1,2-dihyropyran, CH2Cl2, rt (57%); (b) N5, Na2CO3, CH3CN, ∆ (85%);
(c) ethanolic HCl/NaCO3(aq) (98%); (d) 4 equiv of KH, DMA, rt, Ar; (e)
1 equiv of Co(OAc)2, DMA, rt, Ar; (f) 1 equiv of H2O, DMA, rt, Ar.
In contrast, the potassium salt of [CoIIH2PtBuuam(OH)]2-
does react with ethyl acetate; stirring K2[CoIIH2PtBuuam(OH)]
in a 1:1 mixture of DMA and ethyl acetate for 2 h at room
temperature under an argon atmosphere produced changes
in the spectroscopy of the isolated complex. The most
obvious differences were the absence of the peak associated
with the O-H vibration and the appearance of a strong
ν(NH) signal at 3361 cm-1, suggesting a change in the
H-bonding network. In addition, a white solid precipitated
from the pink-purple reaction mixture, which was identified
by FTIR spectroscopy as KOAc. Moreover, attempts to
recrystallize K2[CoIIH2PtBuuam(OH)] in a 1:1 DMA/ethyl
acetate solution using vapor diffusion of diethyl ether
produced a white solid product, which analyzed as KOAc
in nearly quantitative yield.
This difference in reactivity between the two salts of
[CoIIH2PtBuuam(OH)]2- can be accounted for by assuming
that the K+ ion(s) function(s) as the additional metal
ion during hydrolysis. The solid-state structure of
K2[CoIIH2PtBuuam(OH)] indicates that the H-bond cavity
surrounding the CoII-OH unit is large enough to allow K+
ions to interact with the hydroxo ligand. Similar interactions
could occur in solution, which would help facilitate the
hydrolysis of ethyl acetate. When no additional metal ions
are present, as in the tetrapropylammonium salt, reactivity
is not possible because the [H2PtBuuam]3- ligand prevents
intermolecular interactions between two CoII complexes.
Although K2[CoIIH2PtBuuam(OH)] hydrolyzes esters, it does
not react with acetonitrile: no IR or optical changes were
observed when the salt was dissolved in a 1:1 mixture of
DMA/acetonitrile and stirred for at least 2 h. Contributing
factors to this lack of reactivity is that K+ ions have a higher
affinity for O atoms over N atoms and carboxy esters are
more labile toward hydrolysis than nitriles.
Figure 7. Molecular structure of [CoIIH2PtBuuam(OH)]2-. The ellipsoids
are drawn at the 50% probability level, and C-bonded H atoms are omitted
for clarity.
Table 5. Selected Bond Distances (Å) and Angles (deg) for
K2[CoIIH2PtBuuam(OH)]‚2DMA
Co1-N1
Co1-N2
Co1-N3
Co1-N6
Co1-O3
2.235(2)
N4‚‚‚O3
N5‚‚‚O3
K1‚‚‚O3
K2‚‚‚O3
2.774(3)
2.783(3)
2.826(2)
3.117(2)
2.0325(19)
2.0156(19)
2.044(2)
2.0441(18)
N2-Co1-N3
N2-Co1-N6
N3-Co1-N6
N2-Co1-N1
N3-Co1-N1
121.16(8)
N6-Co1-N1
N2-Co1-O3
N3-Co1-O3
N6-Co1-O3
N1-Co1-O3
77.46(8)
99.66(7)
102.98(7)
97.32(8)
174.49(7)
117.07(7)
112.84(7)
81.41(7)
80.87(7)
The hydroxo ligand is terminal with a Co1-O3 bond
length of 2.0441(18) Å, a distance significantly shorter than
those found in [CoII H4PiPrbuam(µ-OH)]2- but comp-
2
arable to that found in other monomeric five-coordinate
CoII-OH complexes possessing intramolecular H-bonds to
the hydroxo O atom.11 The N1-Co1-O3 angle is nearly
linear at 174.49(7)°. Unlike in the molecular structure of the
dimeric [CoII H4PiPrbuam(µ-OH)]2-, there are two intramo-
2
lecular H-bonds involving the hydroxo O atom O3 in
[CoIIH2PtBuuam(OH)]2-: this is indicated by N4‚‚‚O3 and
N5‚‚‚O3 distances of 2.774(3) and 2.783(3) Å and the
average N-H‚‚‚O angle of 167(3)°. In addition, the cavity
structure in [CoIIH2PtBuuam(OH)]2- is less constrained,
leaving the CoII-OH unit more exposed; in the crystal phase,
this is evident by O3 interacting with the two K+ counterions
[K1‚‚‚O3, 2.826(2) Å; K2‚‚‚O3, 3.117(2) Å]. The crystal
Inorganic Chemistry, Vol. 46, No. 24, 2007 10131

Zinn et al.
Summary and Conclusions
The proposed mechanism of hydration/hydrolysis suggests
that the H-bonding network within the synthetic “active site”
may function to orient the substrate, stabilizing a reactive
terminal hydroxo intermediate and stabilizing the hydrated/
hydrolyzed product. Similar shifts in the binding mode of
the hydroxo ligand have been suggested in studies done on
binuclear metallohydrolases such as aminopepidases.28 The
mononuclear CoII-OH complex [CoIIH2PtBuuam(OH)]2- was
also prepared, and its reactivity is consistent with the need
for two metal ions to accomplish the hydration/hydrolysis
reactions. Hydrolysis of ethyl acetate to acetate was only
observed with K2[CoIIH2PtBuuam(OH)], suggesting that the
K+ counterions may act as the second metal ion to assist in
activating the substrate. The open-cavity structure in
[CoIIH2PtBuuam(OH)]2- supports the view that K+ ions can
interact with the CoII-OH unit.
This work described synthetic methods for preparing
dimeric CoII-µ-X′-CoII complexes using the pyrazolate-
based ligands H9PRbuam, where X′ ) OH-, [OC(NH)CH3]-,
and OAc-. The CoII-µ-OH-CoII complex was obtained
through the addition of water and the presence of an
intramolecular base positioned within the cavity through
deprotonation of one of the R′NH groups.11 The dual function
of the synthetic cavity in these complexes resembles active
sites found in metalloproteins by utilizing H-bonds and
general basic sites to control functional properties.
[CoII H4PRbuam(µ-OH)]2- was found to react stoichio-
2
metrically with unactivated nitriles and esters under ambient
conditions: this was illustrated with the hydration of
acetonitrile and the hydrolysis of ethyl acetate at room
temperature to afford [CoII H4PRbuam(µ-1,3-OC(NH)-
2
CH3)]2- and [CoII H4PRbuam(µ-1,3-OAc)]2-. X-ray diffrac-
2
Acknowledgment. We thank the NIH (Grant GM50781)
for financial support and the NSF (Grant CHE-0079282) for
funding of the X-ray diffraction instrumentation.
tion studies on the CoII-µ-1,3-X′-CoII products (X′ )
[OC(NH)CH3]- and -OAc) reveal that the superstructures
of the cavities provide four Η-bond donors that are directed
intramolecularly toward the bridging ligands. This results
in the formation of intramolecular H-bonds with the hydrated/
hydrolyzed products bound within the cavity, illustrating
some control within the secondary coordination sphere.
Additional control of the secondary sphere is provided by
the alkyl groups appended from the urea arms, as seen in
Supporting Information Available: Spectral data (Figures
S1-S3) for [CoII H4PRbuam(µ-X)]2- (R ) tBu, iPr and X ) OH-,
2
[OC(NH)CH3]-, and OAc-) and completed details for the X-ray
diffraction experiments, including CIFs, with thermal ellipsoid plots
of [CoII H4PiPrbuam(µ-OH)0.82(µ-Cl)0.18
]
2-, [CoII H4PiPrbuam(µ-
2
2
2
1,3-OC(NH)CH3)0.90(Cl)0.10
]
2-, and [CoII H4PtBubuam(µ-1,3-OC-
(NH)CH3)]2- (Figures S4-S6). This material is available free of
the molecular structures of [CoII H4PRbuam(µ-1,3-OAc)].
The large differences in fold angles of the bound acetate
bridge demonstrate how the alkyl groups affect the size of
the H-bonding pocket.
2
charge via the Internet at http://pubs.acs.org.
IC700685G
10132 Inorganic Chemistry, Vol. 46, No. 24, 2007