Novel biscapped and monocapped tris(dioxime) Mn(II) complexes: X-ray crystal structure of the first cationic tris(dioxime) Mn(II) complex [Mn(CDOH)3BPh]OH (CDOH2 = 1,2-cyclohexanedione dioxime)

Article abstract of DOI:10.1021/ic060216n

This report describes the synthesis and characterization of a series of novel biscapped and monocapped tris-(dioxime) Mn(II) complexes [Mn(dioxime) ₃(BR)₂] and [Mn(dioxime)₃BR]⁺ (dioxime = cyclohexanedione dioxime (CDOH₂) and 1,2-dimethylglyoxyl dioxime (DMGH₂); R = Me, n-Bu, and Ph). All tris(dioxime) Mn(II) complexes have been characterized by elemental analysis, IR, UV/vis, cyclic voltammetry, ESI-MS, and, in the cases of [Mn(CDOH)₃BPh] OH·CHCl₃ and [Mn(CDO)(CDOH)₂(BBu(OC ₂H₅))₂], X-ray crystallography. It was found that biscapped Mn(II) complexes [Mn(dioxime)₃(BR)₂] are not stable in the presence of water and readily hydrolyze to form monocapped cationic complexes [M(dioxime)₃BR]⁺. This instability is most likely caused by mismatch between the size of Mn(II) and the coordination cavity of the biscapped tris(dioxime) ligands. In contrast, monocapped cationic complexes [M(dioxime)₃BR]⁺ are very stable in aqueous solution even in the presence of PDTA (1,2-diaminopropane-N,N,N′,N′- tetraacetic acid) because of the kinetic inertness imposed by the monocapped tris(dioxime) chelators that are able to completely wrap Mn(II) into their N₆ coordination cavity. [Mn(CDO)₃BPh]OH has a distorted trigonal prismatic coordination geometry, with the Mn(II) being bonded by six imine-N donors. The hydroxyl groups from three dioxime chelating arms form very strong intramolecular hydrogen bonds with the hydroxide counterion so that the structure of [Mn(CDOH)₃BPh]OH can be considered as being the clathrochelate with the hydroxide counterion as a cap .

Full text of DOI:10.1021/ic060216n

Inorg. Chem. 2006, 45, 5034−5043
Novel Biscapped and Monocapped Tris(dioxime) Mn(II) Complexes:
X-ray Crystal Structure of the First Cationic Tris(dioxime) Mn(II)
Complex [Mn(CDOH)3BPh]OH (CDOH2
) 1,2-cyclohexanedione dioxime)
Wen-Yuan Hsieh and Shuang Liu*
Department of Industrial & Physical Pharmacy, School of Health Sciences, Purdue UniVersity,
550 Stadium Mall DriVe, West Lafayette, Indiana 47907
Received February 7, 2006
This report describes the synthesis and characterization of a series of novel biscapped and monocapped tris-
(dioxime) Mn(II) complexes [Mn(dioxime)3(BR)2] and [Mn(dioxime)3BR]+ (dioxime
cyclohexanedione dioxime
(CDOH2) and 1,2-dimethylglyoxyl dioxime (DMGH2); R Me, n-Bu, and Ph). All tris(dioxime) Mn(II) complexes
have been characterized by elemental analysis, IR, UV/vis, cyclic voltammetry, ESI-MS, and, in the cases of
[Mn(CDOH)3BPh]OH CHCl3 and [Mn(CDO)(CDOH)2(BBu(OC2H5))2], X-ray crystallography. It was found that biscapped
)
)
‚
Mn(II) complexes [Mn(dioxime)3(BR)2] are not stable in the presence of water and readily hydrolyze to form
monocapped cationic complexes [M(dioxime)3BR]+. This instability is most likely caused by mismatch between the
size of Mn(II) and the coordination cavity of the biscapped tris(dioxime) ligands. In contrast, monocapped cationic
complexes [M(dioxime)3BR]+ are very stable in aqueous solution even in the presence of PDTA (1,2-diaminopropane-
N,N,N′,N′-tetraacetic acid) because of the kinetic inertness imposed by the monocapped tris(dioxime) chelators
that are able to completely “wrap” Mn(II) into their N6 coordination cavity. [Mn(CDO)3BPh]OH has a distorted trigonal
prismatic coordination geometry, with the Mn(II) being bonded by six imine-N donors. The hydroxyl groups from
three dioxime chelating arms form very strong intramolecular hydrogen bonds with the hydroxide counterion so
that the structure of [Mn(CDOH)3BPh]OH can be considered as being the clathrochelate with the hydroxide counterion
as a “cap”.
Introduction
can be improved by using chelators, such as EDTA (ethyl-
enediaminetetraacetic acid) and DTPA (diethylenetriamine-
pentaacetic acid);6 but they often undergo rapid ligand
exchange because of their lack of kinetic inertness. This may
explain why thermodynamically stable Mn(II) complexes,
such as Mn-DPDP (DPDP ) N,N′-dipyridoxylethylenedi-
amine-N,N-diacetate-5,5-bis(phosphonate)), decompose rap-
idly to produce the “free” Mn(II) ions once they are injected
into the biological system.7-10 Despite the success of Mn-
There is a burgeoning interest in Mn(II) complexes as
magnetic resonance imaging (MRI) contrast agents.1-5 For
Mn(II) complexes to be useful as MRI contrast agents, they
must have a sufficient solution stability to withstand trans-
chelation in the blood circulation. However, most Mn(II)
complexes have a low solution stability, because of the d5
configuration of Mn(II) and a lack of ligand-field stabilization
energy. The thermodynamic stability of Mn(II) complexes
* To whom correspondence should be addressed. Address: Room 1275,
Civil Engineering Building, School of Health Sciences, Purdue University,
550 Stadium Mall Drive, West Lafayette, IN 47907. Phone: 765-494-0236.
Fax 765-496-1377. E-mail: lius@pharmacy.purdue.edu.
(6) Ogino, H. Bull. Chem. Soc. Jpn. 1965, 38, 771-778.
(7) Federle, M. P.; Chezmar, J. L.; Rubin, D. L.; Weinreb, J. C.; Freeny,
P. C.; Schmiedl, U. P.; Brown, J. J.; Borrello, J. A.; Lee, J. K. T.;
Semelka, R. C.; Mattrey, R.; Dachman, A. H.; Saini, S.; Harms, S.
E.; Mitchell, D. G.; Anderson, M. W.; Halford, H. H., III; Bennett,
W. F.; Young, S. W.; Rifkin, M.; Gay, S. B.; Ballerini, R.; Sherwin,
P. F.; Robison, R. O. J. Magn. Reson. Imaging 2000, 12, 689-701.
(8) Gallez, B.; Baudelet, C.; Adline, J.; Geurts, M.; Delzenne, N. Chem.
Res. Toxicol. 1997, 10, 360-363.
(1) Lauffer, R. B. Chem. ReV. 1987, 87, 901-927.
(2) Storey, P.; Danias, P. G.; Post, M.; Li, W.; Seoane, P. R.; Harnish, P.
P.; Edelman, R. R.; Prasad, P. V. InVest. Radiol. 2003, 38, 642-652.
(3) Kruk, D.; Kowalewski, J. J. Biol. Inorg. Chem. 2003, 8, 512-518.
(4) Aime, S.; Anelli, P. L.; Botta, M.; Brocchetta, M.; Canton, S.; Fedeli,
F.; Gianolio, E.; Terreno, E. J. Biol. Inorg. Chem. 2002, 7, 58-67.
(5) Troughton, J. S.; Greenfield, M. T.; Greenwood, J. M.; Dumas, S.;
Wiethoff, A. J.; Wang, J.; Spiller, M.; McMurry, T. J.; Caravan, P.
Inorg. Chem. 2004, 43, 6313-6323.
(9) Gallez, B.; Baudelet, C.; Geurts, M. Magn. Reson. Imaging 1998, 16,
1211-1215.
(10) Schmidt, P. P.; Toft, K. G.; Skotland, T.; Andersson, K. K. J. Biol.
Inorg. Chem. 2002, 7, 241-248.
5034 Inorganic Chemistry, Vol. 45, No. 13, 2006
10.1021/ic060216n CCC: $33.50
© 2006 American Chemical Society
Published on Web 05/25/2006

Bis- and Monocapped Tris(dioxime) Mn(II) Complexes
DPDP in the detection of liver and cardiovascular diseases,11-13
there is a continuing need for Mn(II) complex contrast agents
with improved solution stability.
One approach to achieving a high solution stability of
Mn(II) complexes is to increase their kinetic inertness. For
example, macrocyclic pentaamines, such as 1,4,7,10,13-
pentaazacyclo-pentadecane, have been used to prepare Mn(II)
complexes with a high solution stability, because these
macrocycles are able to impart kinetic inertness by wrapping
Mn(II) into their N5 coordination cavity.14-17 Mn(II) com-
plexes of C-substituted macrocyclic pentaamines have been
studied as manganese superoxide dismutase (Mn-SOD)
mimetics that are useful for the treatment of diseases, such
as myocardial ischemia reperfusion injury, inflammation, and
cerebral ischemia injury.18-23 Macrocyclic chelators, such
as 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid
(DOTA) and 1,4,7,10-tetraazacyclododecane-1,4-diacetic
acid (DO2A), have also been used to prepare Mn(II)
complexes with high thermodynamic stability and kinetic
inertness.24,25
Figure 1. Mn(II) complexes with BATO type chelators, where R can be
methyl or butyl phenyl groups.
In this report, we describe a new approach to achieving
high solution stability for Mn(II) complexes by using the
boron-capped tris(dioxime) chelators that are able to com-
pletely wrap the Mn(II) into their N6 coordination cavity.
We are particularly interested in cationic Mn(II) complexes,
[Mn(dioxime)3BR]+ (Figure 1, complex D; dioxime )
CDOH2 (cyclohexanedione dioxime) and DMGH2 (1,2-
dimethylglyoxyl dioxime); R ) Me, n-Bu, and Ph), because
of their similarity to the Tc(III) complexes [99mTcCl-
(dioxime)3BR] (Figure 1, complex B), which have been
studied as potential radiotracers for myocardial perfusion
imaging.26-31 As a matter of fact, [99mTcCl(CDOH)3BCH3]
is a radiopharmaceutical approved by the FDA (Food and
Drug Administration) for heart imaging under the tradename
of Cardiotec. It is postulated that like their 99mTc(III)
analogues, cationic complexes [Mn(dioxime)3BR]+ might be
able to selectively localize in the heart because of their
cationic nature.
As the first step in our research toward new Mn(II)-based
MRI contrast agents, we now present the synthesis and
characterization of novel tris(dioxime) Mn(II) complexes
(Figure 1, complexes A and D) as well as the X-ray crystal
structure of the cationic complex [Mn(CDOH)3BPh]OH.
Different alkyl or aryl groups in dioxime chelating arms and
boron caps were used to modify the lipophilicity and water
solubility of the boron-capped tris(dioxime) Mn(II) com-
plexes. The main objective is to determine their structures
and study their stability in aqueous solution.
(11) Federle, M. P.; Chezmar, J. L.; Rubin, D. L.; Weinreb, J. C.; Freeny,
P. C.; Semelka, R. C.; Brown, J. J.; Borrello, J. A.; Lee, J. K. T.;
Mattrey, R.; Dachman, A. H.; Saini, S.; Harmon, B.; Fenstermacher,
M.; Pelsang, R. E.; Harms, S. E.; Mitchell, D. G.; Halford, H. H., III;
Anderson, M. W.; Johnson, D.; Francis, I. R.; Bova, J. G.; Kenney,
P. J.; Klippenstein, D. L.; Foster, G. S.; Turner, D. A.; Stillman, A.
E.; Nelson, R. C.; Young, S. W.; Patt, R. H.; Rifkin, M.; Seltzer, S.
E.; Gay, S. B.; Robison, R. O.; Sherwin, P. F.; Ballerini, R. J. Magn.
Reson. Imaging 2000, 12, 186-197.
(12) Wyttenbach, R.; Saeed, M.; Wendland, M. F.; Geschwind, J.-F.;
Bremerich, J.; Arheden, H.; Higgins, C. B. J Magn. Reson. Imaging
1999, 9, 209-214.
(13) Brurok, H.; Skoglund, T.; Berg, K.; Skarra, S.; Karlsson, J. O. G.;
Jynge, P. NMR Biomed. 1999, 12, 364-372.
(14) Riley, D. P.; Henke, S. L.; Lennon, P. J.; Weiss, R. H.; Neumann, W.
L.; Rivers, W. J.; Aston, K. W.; Sample, K. R.; Rahman, H.; Ling, C.
S.; Shieh, J. J.; Busch, D. H.; Szulbinski, W. Inorg. Chem. 1996, 35,
5213-5231.
(15) Riley, D. P.; Lennon, P. J.; Neumann, W. L.; Weiss, R. H. J. Am.
Chem. Soc. 1997, 119, 6522-6528.
(16) Riley, D. P.; Henke, S. L.; Lennon, P. J.; Aston, K. Inorg. Chem.
1996, 35, 1908-1917.
(17) Aston, K.; Rath, N.; Naik, A.; Slomczynska, U.; Schall, O. F.; Riley,
D. P. Inorg. Chem. 2001, 40, 1779-1789.
(18) (a) Weiss, R. H.; Riley, D. P. Drugs Future 1996, 21, 383-389. (b)
Riley, D. P. AdV. Supramol. Chem. 1999, 6, 217-244. (c) Riley, D.
P. Chem. ReV. 1999, 99, 2573-2587. (d) Salvemini, D.; Muscoli, C.;
Riley, D.; Cuzzocrea, S. Pulm. Pharmacol. Ther. 2002, 15, 439-447.
(e) Salvemini, D.; Riley, D. P.; Cuzzocrea, S. Nature ReV. 2002, 1,
367-374.
Tris(dioxime) metal complexes [M(dioxime)3(BR)2] (Fig-
ure 1, complex A; M ) Fe, Co, Ru) have been known for
(19) Salvemini, D.; Wang, Z.-Q.; Zweier, J. L.; Samouilov, A.; MacArthur,
H.; Misko, T.; Currie, M. G.; Cuzzocreas, S.; Sikorski, J. A.; Riley,
D. P. Science 1999, 286, 304-306.
(20) Weiss, R. H.; Fretland, D. J.; Baron, D. A.; Ryan, U. S.; Riley, D. P.
J. Biol. Chem. 1996, 271, 26149-26156.
(26) Treher, E. N.; Francesconi, L. C.; Gougoutas, J. Z.; Malley, M. F.;
Nunn, A. D. Inorg. Chem. 1989, 28, 3411-3416.
(21) Cuzzocrea, S.; Muzzon, E.; Dugo, L.; Caputi, A. P.; Aston, K.; Riley,
D. P.; Salvemini, D. Brit. J. Pharmacol. 2001, 132, 19-29.
(22) Salvemini, D.; Mazzon, E.; Dugo, L.; Serraino, I.; De Sarro, A.; Caputi,
A. P.; Cuzzocrea, S. Arthritis Rheum. 2001, 44, 2909-2921.
(23) Di Filippo, C.; Cuzzocrea, S.; Marfella, R.; Fabbroni, V.; Scollo, G.;
Benino, L.; Giugliano, D.; Rossi, F.; D’Amico, M. Eur. J. Pharmacol.
2004, 497, 65-74.
(27) Jurisson, S.; Halihan, M. M.; Lydon, J. D.; Barnes, C. L.; Nowotnik,
D. P.; Nunn, A. D. Inorg. Chem. 1998, 37, 1922-1928.
(28) Linder, K. E.; Malley, M. F.; Gougoutas, J. Z.; Unger, S. E.; Nunn,
A. D. Inorg. Chem. 1990, 29, 2428-2434.
(29) Jurisson, S.; Francesconi, L.; Linder, K. E.; Treher, E.; Malley, M.
F.; Gougoutas, J. Z.; Nunn, A. D. Inorg. Chem. 1991, 30, 1820-
1827.
(24) Chaves, S.; Delgado, R.; Frausto Da Silva, J. J. R. Talanta 1992, 39,
249-254.
(30) Narra, R. K.; Nunn, A. D.; Kuczynski, B. L.; Feld, T.; Wedeking, P.;
Eckelman, W. C. J. Nucl. Med. 1989, 30, 1830-1837.
(31) Narra, R. K.; Nunn, A. D.; Kuczynski, B. L.; Di Rocco, R. J.; Feld,
T.; Silva, D. A.; Eckelman, W. C. J. Nucl. Med. 1990, 31, 1370-
1377.
(25) Bianchi, A.; Calabi, L.; Giorgi, C.; Losi, P.; Mariani, P.; Palano, D.;
Paoli, P.; Rossi, P.; Valtancoli, B. J. Chem. Soc., Dalton Trans. 2001,
917-922.
Inorganic Chemistry, Vol. 45, No. 13, 2006 5035

Hsieh and Liu
mode) 650 (calcd for [MnC30H33N6O6B2]-, 650.20); (positive mode)
566 (calcd for [MnC24H32N6O6B]+, 650.20).
many years.32-39 However, very limited information is
available on Mn(II) complexes [Mn(dioxime)3(BR)2] (Figure
1, complex A; dioxime ) CDOH2 and DMGH2; R ) Me,
n-Bu, and Ph). The structure of [Mn(CDO)(CDOH)2(BPh-
(OCH3))2] has been reported by Jurisson and co-workers,29
but it has an unusual biscapped structure (Figure 1, complex
C), in which only two of the three dioxime oxygen atoms
are covalently bonded to the capping boron atoms. The X-ray
crystal structure of [Mn(CDOH)3BPh]OH represents the first
example of structurally characterized cationic Mn(II) com-
plexes with boron-capped tris(dioxime) ligands.
[Mn(CDO)(CDOH)2(BBu(OC2H5))2]. Crystals of [Mn(CDO)-
(CDOH)2(BBu(OC2H5))2] were obtained from evaporation of the
ethanol containing [Mn(CDO)3(BBu)2]. Yield: 0.46 g (∼65%). IR
(KBr, cm-1): 3385 (s, υO-H); 1562, (s, υCdN); 1211, 1057 (s, υN-O);
1172 and 810, (s, υB-O). ESI-MS m/z: (positive mode) 546 (calcd
for [MnC22H36N6O6B]+, 546.22).
[Mn(CDO)3(BCH3)2]. The compound was recrystallized from
a mixture of acetonitrile and chloroform (50:50 v/v). Yield: 0.41
g (66%). IR (KBr, cm-1): 3434 (s, υO-H); 1513, (s, υCdN); 1211,
1043, (s, υN-O); 1172 and 810 (s, υB-O). ESI-MS m/z: (positive
mode) 504 (calcd for [MnC19H30N6O6B]+, 504.23).
Experimental Section
[Mn(DMG)3(BPh)2]. The compound was recrystallized from a
mixture of acetonitrile and methanol (50:50 v/v). Yield: 0.38 g
(66%). IR (KBr, cm-1): 1605, 1513, 1453, 1419, 1340 (s, υCdN
and υring); 1201, 1043 (s, υN-O); 1172 and 810, (s, υB-O). ESI-MS
m/z: (negative mode) 572 for [MnC24H27N6O6B]-; (positive mode)
488 (calcd for [MnC18H27N6O6B]+, 488.14).
[Mn(DMG)3(BBu)2]. The compound was recrystallized from a
mixture of acetonitrile and methanol (50:50 v/v). Yield: 0.32 g
(60%). IR (KBr, cm-1): 3410 (s, υO-H); 1513 (s, υCdN); 1201, 1043
(s, υN-O); 1172 and 810 (s, υB-O). ESI-MS m/z: (positive mode)
468 (calcd for [MnC16H29N6O6B]+, 468.17).
[Mn(DMG)3(BCH3)2]. The compound was recrystallized from
a mixture of acetonitrile and methanol (50:50 v/v). Yield: 0.28 g
(62%). IR (KBr, cm-1): 3350 (s, υO-H); 1513 (s, υCdN); 1201, 1043
(s, υN-O); 1172 and 810 (s, υB-O). ESI-MS m/z: (positive mode)
426 (calcd for [MnC13H23N6O6B]+, 426.12).
Materials and Methods. All chemicals were purchased from
Sigma Aldrich (St. Louis, MO) and used without purification.
Infrared (IR) spectra (4000-400 cm-1) were recorded on a Perkin-
Elmer FT-IR spectrometer. UV/visible spectra were recorded on a
Beckman DU-640 UV/vis spectrometer. Electrospray ionization
mass spectral (ESI-MS) data were collected on a Finnigan LCQ
classic mass spectrometer at the School of Pharmacy, Purdue
University. Elemental analysis was performed with a Perkin-Elmer
Series III analyzer at the Department of Chemistry, Purdue
University. The HPLC method used a LabAlliance semi-prep HPLC
system with a LabAlliance UV/vis detector (λ ) 265 nm) and a
Zorbax Rx-C18 column (4.6 × 150 mm, 5 µm). The flow rate was
1 mL/min, with the mobile phase starting from 100% solvent A
(10 mM NH4OAc buffer, pH 6.8) and changing to 90% solvent A
and 10% solvent B (acetonitrile) at 10 min and 50% solvent A and
50% solvent B at 20 min. Cyclic voltammograms of Mn(II)
complexes were recorded on a Bioanalytical System BAS-100A
electrochemical analyzer. A standard three-electrode cell was used
with a polished glassy-carbon working electrode, a Pt wire as the
auxiliary electrode, and Ag/AgNO3 in acetonitrile solution as the
reference electrode. All measurements were performed in aceto-
nitrile containing 0.1 M n-Bu4NPF6 at a scan rate of 100 mV/s.
The sample solution was blanketed with extra pure N2 gas during
the experiment.
General Procedure for Preparation of Biscapped [Mn-
(dioxime)3(BR)2]. To a solution containing anhydrous MnCl2 (0.255
g, 2 mmol) in 30 mL of absolute ethanol was added the dioxime
(6 mmol) in 30 mL of ethanol under a nitrogen atmosphere. After
the solution was refluxed for 60 min, alkyl- or arylboronic acid (4
mmol) in 20 mL of degassed ethanol was added. The mixture was
stirred at room temperature for 2 h. The light brown precipitate
was filtered and dried under vacuum overnight. Recrystallization
of the crude product in the appropriate solvent or solvent mixture
afforded the pure product, which was dried under vacuum for 4 h
at room temperature before being submitted for elemental analysis.
[Mn(CDO)3(BPh)2]. The compound was recrystallized from
acetonitrile and chloroform (50:50 v/v). Yield: 1.05 g (76%). IR
(KBr, cm-1): 1605, 1513, 1453, 1419 (s, υCdN and υring); 1211,
1043 (s, υN-O); 1172 and 810, (s, υB-O). ESI-MS m/z: (negative
General Procedure for Preparation of Monocapped [Mn-
(dioxime)3BR]+. Mn(OAc)2‚4H2O (0.49 g, 2 mmol) and dioxime
(6 mmol) were dissolved in 50 mL of degassed absolute ethanol
under a nitrogen atmosphere. After the solution was refluxed for 3
h, alkyl or arylboronic acid (2 mmol) in 20 mL of degassed ethanol
was added slowly in order to minimize formation of the corre-
sponding biscapped Mn(II) complex. The mixture was refluxed for
2 h, and the volume was reduced to ∼10% to give a brown
precipitate. The solid was filtered, washed with cold ethanol and
diethyl ether, and dried under vacuum for 4 h at room temperature
before being submitted for elemental analysis.
[Mn(CDOH)3BPh]Cl. Yield: 0.65 g (68%). IR (KBr, cm-1):
3401 (s, υO-H); 1636, 1605, 1562, 1445, 1421, 1340 (s, υCdN and
υring); 1211, 1057 (s, υN-O); 1172 and 810 (s, υB-O). ESI-MS m/z:
(positive mode) 566 (calcd for [MnC24H32N6O6B]+, 566.19).
[Mn(CDOH)3BBu]Cl. Yield: 0.38 g (62%). IR (KBr, cm-1):
3395 (s, υO-H); 1557 (s, υCdN); 1211, 1057 (s, υN-O); 1172 and
810 (s, υB-O). ESI-MS m/z: (positive mode) 546 (calcd for
[MnC22H36N6O6B]+, 546.22).
[Mn(CDOH)3BCH3]Cl. Yield: 0.56 g (65%). IR (KBr, cm-1):
3144 (s, υO-H); 1552 (s, υCdN); 1211, 1043 (s, υN-O); 1172 and
810 (s, υB-O). ESI-MS m/z: (positive mode) 504 (calcd for
[MnC19H30N6O6B]+, 504.23).
[Mn(DMGH)3BPh]OAc. Yield: 0.34 g (62%). IR (KBr, cm-1):
3387 (s, υO-H); 1677 (s, υCdO); 1605, 1513, 1453, 1419, 1340 (s,
υ
CdN and υring); 1201, 1043 (s, υN-O); 1172 and 810 (s, υB-O). ESI-
(32) Busch, D. H. Rec. Chem. Prog. 1964, 25, 107-126.
(33) Jackels, S. C.; Dierdorf, D. S.; Rose, N. J. J. Chem. Soc., Chem.
Commun. 1972, 1291-1292.
MS m/z: (positive mode) 488 (calcd for [MnC18H27N6O6B]+,
488.14).
(34) Boston, D. R.; Rose, N. J. J. Am. Chem. Soc. 1968, 90, 6859-6860.
(35) Boston, D. R.; Rose, N. J. J. Am. Chem. Soc. 1973, 95, 4163-4168.
(36) Johnson, J. N.; Rose, N. J. Inorg. Synth. 1982, 21, 112-114.
(37) Robbins, M. K.; Naser, D. W.; Heiland, J. L.; Grzybowski, J. J. Inorg.
Chem. 1985, 24, 3381-3387.
[Mn(DMGH)3BBu]OAc. Yield: 0.33 g (63%). IR (KBr, cm-1):
3404 (s, υO-H); 1667, 1513 (s, υCdO and υCdN); 1201, 1043 (s,
υ
N-O); 1172 and 810 (s, υB-O). ESI-MS m/z: (positive mode) 468
(calcd for [MnC16H29N6O6B]+, 468.17).
(38) Muller, J. G. Grzybowski, J. J. Takeuchi, K. J. Inorg. Chem. 1986,
25, 2665-2667.
(39) Jackels, S. C.; Rose, N. J. Inorg. Chem. 1973, 12, 1232-1237.
[Mn(DMGH)3BCH3]OAc. Yield: 0.31 g (64%). IR (KBr,
cm-1): 3405 (s, υO-H); 1675, 1510 (s, υCdO and υCdN); 1201, 1043
5036 Inorganic Chemistry, Vol. 45, No. 13, 2006

Bis- and Monocapped Tris(dioxime) Mn(II) Complexes
Table 1. Selected Crystallographic Data of [Mn(CDOH)2(CDO)-(BBu(OC2H5))2] and [Mn(CDOH)3BPh]OH‚CHCl3
[Mn(CDOH)2(CDO)-(BBu(OC2H5))2]
[Mn(CDOH)3BPh]OH‚CHCl3
formula
fw
C30H52B2MnN6O8
701.34
C25H34BCl3MnN6O7
702.69
space group
a (Å)
b (Å)
P21/n (No. 14)
17.8789(6)
10.3613(4)
20.9762(8)
112.2941(19)
3595.3(2)
4
Pbca (No. 61)
12.6712(5)
18.2899(4)
c (Å)
26.8276(9)
â (deg)
V (Å3)
Z
6217.4(3)
d
calcd (g cm3)
1.296
1.501
T (K)
150
150
crystal dimensions (mm3)
Mo KR λ (Å)
0.48 × 0.45 × 0.38
0.30 × 0.28 × 0.13
0.71073
0.718
0.863-0.914
0.048
0.71073
linear absorption coefficient (mm-1
transmission factors
R(Fo)a
)
0.403
0.80-0.86
0.065
2
Rw(Fo )b
0.169
0.123
a R ) ∑||Fo| - |Fc||/∑|Fo| for Fo > 2σ(Fo ). b Rw ) [∑w(|Fo | - |Fc |)2/∑w|Fo | ]1/2
.
2
2
2
2
2 2
Table 2. Selected Bond Distances (Å) of
[Mn(CDOH)2(CDO)(BBu(OC2H5))2] and [Mn(CDOH)3BPh]OH‚CHCl3
HPLC (λ ) 265 nm) at t ) 0, 1, 3, 5, and 8 h post-dissolution.
The chelator challenge experiment was performed by dissolving
the Mn(II) complex (1 mg) in 2 mL of a mixture of water and
acetonitrile (50:50 v/v). PDTA was added in large excess (∼100
fold), and the pH was 7.5. Samples from the mixture were analyzed
by HPLC at t ) 0, 1, 3, 5, and 8 h post-dissolution.
[Mn(CDOH)2(CDO)(BBu(OC2H5))2]
[Mn(CDOH)3BPh]OH‚CHCl3
atom 1
atom 2
distance (Å)
atom 1 atom 2 distance (Å)
Mn
Mn
Mn
Mn
Mn
Mn
O(11)
O(12)
O(21)
O(22)
O(31)
O(32)
O(11)
O(21)
O(111)
O(22)
O(31)
O(211)
N(11)
N(12)
N(21)
N(22)
N(31)
N(32)
N(11)
N(12)
N(21)
N(22)
N(31)
N(32)
B(1)
2.308(3)
2.275(3)
2.278(3)
2.237(2)
2.313(2)
2.246(2)
1.376(3)
1.381(3)
1.378(3)
1.381(3)
1.375(3)
1.382(3)
1.493(5)
1.499(5)
1.460(5)
1.492(4)
1.493(4)
1.472(4)
Mn
Mn
Mn
Mn
Mn
Mn
O(11)
O(12)
O(21)
O(22)
O(31)
O(32)
O(11)
O(21)
O(31)
O(12)
O(22)
O(32)
N(11)
N(12)
N(21)
N(22)
N(31)
N(32)
N(11)
N(12)
N(21)
N(22)
N(31)
N(32)
B(1)
2.242(3)
2.260(3)
2.262(3)
2.232(3)
2.213(3)
2.272(3)
1.383(3)
1.372(3)
1.386(3)
1.381(3)
1.387(3)
1.376(3)
1.490(4)
1.499(4)
1.505(4)
0.87(3)
X-ray Crystallographic Analysis. Selected crystallographic data
for complexes [Mn(CDOH)3BPh]OH‚CHCl3 and [Mn(CDOH)2-
(CDO)(BBu(OC2H5))2] were collected on a Nonius Kappa CCD
diffractometer and are listed in Table 1. The selected bond distance
and bond angles are listed in Tables 2 and 3, respectively. Crystals
were mounted on a glass fiber in a random orientation. Preliminary
examination and data collection were performed using graphite-
monochromated Mo KR radiation (λ ) 0.71073 Å). Cell constants
and an orientation matrix for data collection were obtained from
least-squares refinement, using setting angles in the range of 2° <
θ < 27° for [Mn(CDOH)2(CDO)(BBu(OC2H5))2] and 1° < θ <
24° for [Mn(CDOH)3BPh]OH‚CHCl3. A total of 24 681 reflec-
tions were collected and 5451 reflections were unique for
[Mn(CDOH)3BPh]OH‚CHCl3. A total of 29 952 reflections were
collected and 8440 reflections were unique for [Mn(CDOH)2(CDO)-
(BBu(OC2H5))2]. Lorentz and polarization corrections were applied
to the data. A linear absorption coefficient is 4.0 cm-1 for Mo KR
radiation in [Mn(CDOH)2(CDO)(BBu(OC2H5))2] and 7.2 cm-1 in
[Mn(CDOH)3BPh]OH‚CHCl3. An empirical correction was applied
using the program SCALEPACK.40 Both structures were solved
by the direct method using SIR200241 and refined on a Linux PC
using SHELXL97.42 Crystallographic drawings were produced using
the program ORTEP.43
B(1)
B(1)
B(2)
B(2)
B(1)
B(1)
H(12)
H(22)
H(32)
0.87(3)
0.86(3)
B(2)
(s, υN-O); 1172 and 810 (s, υB-O). ESI-MS m/z: (positive mode)
426 (calcd for [MnC13H23N6O6B]+, 426.12).
[Mn(CDOH)3BPh]OH. Crystals of [Mn(CDOH)3BPh]OH‚
CHCl3 suitable for X-ray crystallographic analysis were obtained
from slow evaporation of the chloroform solution containing
[Mn(CDO)3(BPh)2]. IR (KBr, cm-1): 3434 (s, υO-H); 1605, 1513,
1453, 1419, 1340 (s, υCdN and υring); 1211, 1043 (s, υN-O); 1172
and 810 (s, υB-O). ESI-MS m/z: (positive mode) 566 (calcd for
[MnC24H32N6O6B]+, 566.19).
Solution Stability Experiments. The Mn(II) complex (1 mg)
was dissolved in 2 mL of a mixture of methanol and acetonitrile
(50:50 v/v). Samples from the resulting solution were analyzed by
Table 3. Selected Bond Angles (deg) of [Mn(CDOH)2(CDO)(BBu(OC2H5))2] and [Mn(CDOH)3BPh]OH‚CHCl3
[Mn(CDOH)2(CDO)(BBu(OC2H5))2] [Mn(CDOH)3BPh]OH‚CHCl3
atom 1
atom 2
atom 3
angle (deg)
atom 1
atom 2
atom 3
angle (deg)
N(11)
N(21)
N(31)
N(12)
N(12)
N(22)
N(11)
N(11)
N(21)
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
N(12)
N(22)
N(32)
N(22)
N(32)
N(32)
N(21)
N(32)
N(32)
68.82(9)
70.42(9)
69.82(8)
114.40(9)
94.18(9)
133.94(10)
79.36(10)
82.34(10)
116.65(9)
N(11)
N(21)
N(31)
N(12)
N(12)
N(22)
N(11)
N(11)
N(21)
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
N(12)
N(22)
N(32)
N(22)
N(32)
N(32)
N(21)
N(31)
N(31)
70.36(10)
70.80(9)
70.61(9)
102.74(10)
99.43(10)
100.98(9)
78.03(9)
77.87(9)
77.14(9)
Inorganic Chemistry, Vol. 45, No. 13, 2006 5037

Hsieh and Liu
Table 4. Elemental Analysis Data for Bis- and Monocapped Mn(II) Complexes
compd
C anal. (found)
H anal. (found)
N anal. (found)
[Mn(CDO)3(BPh)2]‚2H2O
52.40 (52.01)
49.40 (49.75)
47.04 (47.17)
51.23 (50.98)
39.95 (39.75)
45.58 (45.23)
40.89 (40.73)
50.30 (50.27)
42.50 (42.15)
44.84 (45.01)
40.90 (41.12)
36.72 (36.43)
37.11 (37.15)
5.53 (5.35)
5.66 (5.47)
5.76 (5.54)
7.74 (7.52)
5.91 (5.49)
5.74 (6.06)
5.38 (5.46)
4.92 (4.76)
5.53 (5.64)
6.97 (7.12)
6.49 (6.55)
5.50 (5.55)
5.93 (6.02)
12.22 (12.30)
14.40 (14.02)
13.17 (12.89)
11.95 (11.92)
14.71 (14.77)
15.94 (15.57)
15.07 (14.94)
14.66 (14.56)
14.87 (14.58)
15.30 (14.95)
15.47 (15.31)
18.35 (18.22)
16.75 (16.56)
[Mn(CDOH)3BPh]OH
[Mn(CDOH)3BPh]Cl‚0.75H2O‚0.5C2H5OH
[Mn(CDOH)2(CDO)(BBu(OC2H5))2]
[Mn(CDOH)3BBu]Cl‚1.75H2O
[Mn(CDO)3(BCH3)2]
[Mn(CDOH)3BCH3]Cl‚H2O
[Mn(DMG)3(BPh)2]
[Mn(DMGH)3BPh]OAc‚H2O
[Mn(DMG)3(BBu)2]‚0.5CH3OH
[Mn(DMGH)3BBu]OAc‚0.5CH3OH
[Mn(DMG)3(BCH3)2]‚H2O
[Mn(DMGH)3BCH3]OAc‚0.5CH3OH
Scheme 1. Synthesis of Biscapped and Monocapped Mn(II)
Complexes
Table 5. Summary of Oxidation Potentials (Eox) and Extinction
Coefficients (ꢀ) for Biscapped and Monocapped Mn(II) Complexes
compd
[Mn(CDO)3(BPh)2]
[Mn(CDOH)3BPh]Cl
[Mn(CDOH)2(CDO)(BBu(OC2H5))2]
[Mn(CDOH)3BBu]Cl
[Mn(CDO)3(BCH3)2]
[Mn(CDOH)3BCH3]Cl
[Mn(DMG)3(BPh)2]
[Mn(DMGH)3BPh]OAc
[Mn(DMG)3(BBu)2]
[Mn(DMGH)3BBu]OAc
[Mn(DMG)3(BCH3)2]
[Mn(DMGH)3BCH3]OAc
E
ox (V)a
ꢀ (M-1 cm-1)*
1.35
1.35
1.31
1.31
1.26
1.26
1.25
1.25
1.22
1.22
1.20
1.20
16 250
16 250
15 550
15 550
14 350
14 350
14 750
14 750
13 950
13 950
12 950
12 950
a Both cyclic voltammograms and extinction coefficients of Mn(II)
complexes were obtained in acetonitrile containing a trace amount of water
(0.02%).
[Mn(CDOH)3BPh]OH‚CHCl3 suitable for X-ray crystal-
lographic analysis were isolated from the chloroform solution
containing [Mn(CDO)3(BPh)2] in the presence of air. Both
the biscapped and monocapped Mn(II) complexes have been
characterized by elemental analysis (Table 4), IR, UV/vis,
ESI-MS, cyclic voltammetry (Table 5), and, in the cases of
[Mn(CDOH)3BPh]OH‚CHCl3 and [Mn(CDOH)2(CDO)(BBu-
(OC2H5))2], X-ray crystallography.
Results and Discussion
Synthesis of Biscapped and Monocapped Mn(II) Com-
plexes. Biscapped Mn(II) complexes [Mn(dioxime)3(BR)2]
(Figure 1, complex A; dioxime ) CDOH2 and DMGH2; R
) Me, n-Bu, and Ph) were prepared according to Scheme 1
by reacting anhydrous Mn(II) chloride with 3 equiv of
dioximes and 2 equiv of alkyl- or arylboronic acids in
absolute ethanol under a nitrogen atmosphere. Complexes
[Mn(dioxime)3(BR)2] are stable in the solid state but
decompose rapidly in the presence of water. [Mn(CDOH)2-
(CDO)(BBu(OC2H5))2] was isolated from the ethanol solution
containing [Mn(CDO)3(BBu)2] during recrystallization. Cat-
ionic complexes [M(dioxime)3BR]+ (Figure 1, complex D;
dioxime ) CDOH and DMGH; R ) Me, n-Bu, and Ph) were
prepared in a similar fashion except that only 1 equiv of
alkyl- or arylboronic acid was used. The addition of alkyl-
or arylboronic acid had to be slow to minimize formation of
the corresponding biscapped Mn(II) complex. Cationic
complexes [M(dioxime)3BR]+ could also be prepared by
hydrolysis of biscapped complexes [Mn(dioxime)3(BR)2] in
a mixture of water and acetonitrile or chloroform with a trace
amount of water. For example, crystals of the complex
Spectroscopic Characterization. The IR spectra of both
the monocapped and biscapped complexes are similar to
those reported for [M(dioxime)3(BR)2] (M ) Fe, Co, and
Ru) and [MX(dioxime)3BR] (M ) Tc and Re; X ) Cl, Br,
NCS, and SCN; R ) alkyl and aryl).26,27,38,39 The single band
between 1650 and 1550 cm-1 is due to the CdN stretch.
Several strong bands between 950 and 1050 cm-1 and 1220-
1270 cm-1 are tentatively assigned as N-O stretches, and
the multiple absorption bands at 810-815 cm-1 and 1000-
1200 cm-1 are due to the B-O stretches. The UV/visible
spectra of both monocapped and biscapped Mn(II) complexes
in chloroform show no transitions in the visible region (400-
800 nm) because of the high-spin d5 configuration of Mn(II).
The single transition that has been observed in the UV region
is in the range of 250-270 nm with an extinction coefficient
value around 12 000-17 000 (Table 5). This transition is
likely due to the metal-to-ligand charge transfer (MLCT).
(42) Shelderick, G. M. SHELXL 97: A Program for Crystal Structure
Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(43) Johnson, C. K. ORTEPII; Report ORNL-5138; Oak Ridge National
Laboratory: Oak Ridge, TN, 1976.
(40) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307-326.
(41) Burla, M. C.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.; Giaco-
vazzo, C.; Polidori, G.; Spagna, R. J Appl. Crystallogr. 2003, 36, 1103.
5038 Inorganic Chemistry, Vol. 45, No. 13, 2006

Bis- and Monocapped Tris(dioxime) Mn(II) Complexes
Figure 2. ESI-MS spectra of [Mn(CDO)3(BPh)2] in positive mode (A) and negative mode (B), and that of [Mn(CDOH)3BPh]+ in positive mode (C), along
with the proposed molecular fragments. The ethoxy group might come from crystallization ethanol in the bulk sample.
ESI-MS data of biscapped and monocapped Mn(II)
complexes were obtained using chloroform, acetonitrile, or
methanol as the matrix, depending on the complex solubility.
Figure 4 shows typical ESI-MS spectra of [Mn(CDO)3-
(BPh)2] and [Mn(CDOH)3BPh]+ using chloroform as the
matrix. The positive mode ESI-MS spectra of both biscapped
and monocapped Mn(II) complexes show the molecular ion
[Mn(dioxime)3BR]+ (panels A and B of Figure 2). The
negative mode ESI-MS spectrum of [Mn(CDO)3(BPh)2]
shows the expected molecular ion, [M-H]-, and the
hydrolyzed molecular ion [M + H2O - H]- (Figure 2B),
which are not observed in the negative mode ESI-MS
spectrum of [Mn(CDOH)3BPh]+. This suggests that com-
plexes [Mn(dioxime)3(BR)2] are indeed synthesized, even
though they are unstable in aqueous solution. However, this
cannot completely exclude the presence of the partially
capped complex (Figure 2). The positive mode ESI-MS
spectrum of [Mn(CDOH)2(CDO)(BBu(OC2H5))2] always
Inorganic Chemistry, Vol. 45, No. 13, 2006 5039

Hsieh and Liu
the monocapped tris(dioxime) Tc(III) and Re(III) complexes,
probably because of their differences in size and coordination
number of the metal ion.28,29 Each dioxime chelating arm
forms a planar five-membered chelate ring with the Mn(II)
center. The average bidentate bite angle is 70.59(9)°, which
is smaller than those observed in other biscapped and
monocapped tris(dioxime) metal complexes,27-29 most likely
because of the large size of Mn(II). Three hydroxyl groups
from the dioxime chelating arms in [Mn(CDOH)3BPh]OH
form strong intramolecular hydrogen bonds with the hy-
droxide counterion. The average hydrogen bond distance is
0.87 Å. These hydrogen bonds in some way act as a
topological closure (B-Mn-O angle ) 179.45°). Therefore,
the structure of [Mn(CDOH)3BPh]OH can be considered as
being the clathrochelate type with the hydroxide counterion
as a cap. In addition, there are two strong intermolecular
hydrogen bonds between the hydroxide hydrogen and two
oxygen atoms of the three B-O bonds (Figure 4).
Figure 3. ORTEP diagram of [Mn(CDOH)3BPh](OH). (Ellipsoids are at
the 50% probability level). Crystallization chloroform and hydrogen atoms
are omitted for the sake of clarity.
The structure of [Mn(CDOH)3BPh]OH is different from
those of the tris(dioxime) complexes, [MCl(dioxime)3BR]
(M ) Tc and Re; R ) alkyl and aryl),26-31 in which Tc and
Re are seven-coordinated and the two uncapped dioxime
(CDOH) groups form strong intramolecular hydrogen bonds
with the deprotonated CDO (Figure 1, complex B).26-29 It
is surprising to see that the Mn(II) in [Mn(dioxime)3BR]+
is six-coordinated, whereas smaller metal ions, Tc(III) and
Re(III), in [MCl(dioxime)3BR] are seven-coordinated by
virtually the same monocapped tris(dioxime) chelating
system. We believe that this structural difference is probably
related to the charge of metal ions. For cationic complexes
[Mn(dioxime)3BR]+, the Mn(II) is highly stabilized by six
imine-N donors, and the intramolecular hydrogen bonding
between the hydroxide counterion (Figure 4) and hydroxyl
groups of the CDOH chelating arms prevents the coordina-
tion of other ligands, such as chloride, to the Mn(II). In
complexes [MCl(dioxime)3BR] (M ) Tc and Re), the metal
ion is in the +3 oxidation state. The monodentate ligand
and the intramolecular hydrogen bonding are needed to
satisfy their neutrality.
It is interesting to note that the biscapped complexes
[M(dioxime)3(BR)2] (M ) Fe and Co) are stable, whereas
the biscapped tris(dioxime) Mn(II) complexes [Mn(dioxime)3-
(BR)2] tend to hydrolyze in the presence of water. This is
probably related to the size of the metal ions. For complexes
[M(dioxime)3(BR)2] (M ) Fe and Co),38,39 the metal ions
are relatively small (ionic radii ) 0.61 and 0.65 Å for Co(II)
and Fe(II),47 respectively) and fit into the coordination cavity
of the biscapped tris(dioxime) ligand. For [Mn(dioxime)3-
(BR)2], however, Mn(II) (ionic radius ) 0.67 Å) is larger
than Co(II) and Fe(II).47 Its size may not match the cavity
of the tris(dioxime) ligand and causes significant constraints
to the ligand framework. Losing one of the two boron caps
allows the tris(dioxime) ligand to release these constraints
Figure 4. H-bonding network in [Mn(CDOH)3BPh]OH.
shows the molecular ion due to [Mn(CDOH)3BBu]+. It loses
one boron cap and one ethanol from the remaining boron
cap in the mass spectrophotometer. These findings are
completely consistent with the instability of biscapped Mn-
(II) complexes in protic solvents.
X-ray Crystal Structure of [Mn(CDOH)3BPh]OH‚
CHCl3. The ORTEP drawing of [Mn(CDOH)3BPh]OH is
illustrated in Figure 3. Figure 4 shows the H-bonding network
in [Mn(CDOH)3BPh]OH. Crystallization chloroform and
hydrogen atoms are omitted for the sake of clarity. There
are four [Mn(CDOH)3BPh]+ cations in each unit cell. In
general, [Mn(CDOH)3BPh]+ has a near C3 symmetry. The
Mn(II) is coordinated with six nitrogen atoms from three
dioxime chelating arms, which are capped by a tetrahedral
boron atom at one end through three covalent B-O bonds.
The coordination geometry is best described as trigonal
prismatic, with the two triangular planes being defined by
N11-N21-N31 and N12-N22-N32. These two triangular
planes are almost parallel, with the dihedral angle at 1.4-
(2)°. The average Mn-N distance in [Mn(CDOH)3BPh]OH
is 2.247(3) Å, which is well comparable to those observed
in the Mn(II) complexes with distorted trigonal prismatic
coordination geometry.44-46 The average Mn-N bond length
at the boron-capped end is about 0.016(3) Å shorter than
that at the uncapped end because of constraints imposed by
the boron cap. This difference is smaller than that found in
(44) Mikuriya, M.; Fukumoto, H.; Kako, T. Inorg. Chem. Commun. 1998,
1, 225-227.
(45) Mikuriya, M.; Hatano, Y.; Asato, E. Bull. Chem. Soc. Jpn. 1997, 70,
2495-2507.
(46) Mikuriya, M.; Hatano, Y.; Asato, E. Chem. Lett. 1996, 849-850.
(47) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751-767.
5040 Inorganic Chemistry, Vol. 45, No. 13, 2006

Bis- and Monocapped Tris(dioxime) Mn(II) Complexes
the ethoxyl group. In this way, the constraints imposed by
the mismatch between Mn(II) and coordination cavity of the
tris(CDOH) ligand can be released while it is still able to
wrap the Mn(II) with its six imine-N donor atoms. That may
explain why [Mn(CDOH)2(CDO)(BBu(OC2H5))2] is isolated
during recrystallization of [Mn(CDOH)3(BBu)2] from etha-
nol.
Solution Instability of Biscapped Mn(II) Complexes.
We used a reversed-phase HPLC method to monitor the
solution stability of Mn(II) complexes. It was found that both
[Mn(CDO)3(BPh)2] and [Mn(CDOH)3BPh]OH have the
same retention time at ∼16 min. To confirm this observation,
we co-injected [Mn(CDO)3(BPh)2] and [Mn(CDOH)3BPh]-
OH using the same HPLC method. Figure 6 shows a
representative HPLC chromatogram of the aqueous solution
containing [Mn(CDO)3(BPh)2] and [Mn(CDOH)3BPh]OH.
The 16 min peak was the only signal detected, suggesting
that the compounds share the same composition in the HPLC
mobile phase. The ESI-MS spectrum of the collected fraction
at ∼16 min displayed a molecular ion at m/z ) 566 that
corresponds to [Mn(CDOH)3BPh]+, clearly demonstrating
that [Mn(CDO)3(BPh)2] is not stable in aqueous solution.
As soon as [Mn(CDO)3(BPh)2] is in contact with water in
the HPLC mobile phase, one of the boron caps quickly
hydrolyzes to form [Mn(CDOH)3BPh]OH. These results
would explain why crystals of [Mn(CDOH)3BPh]OH were
isolated from the slow evaporation of the chloroform solution
containing [Mn(CDO)3(BPh)2]. Similar instability was ob-
served for [Mn(CDOH)2(CDO)(BBu(OC2H5))2] in aqueous
solution.
Figure 5. ORTEP diagram of [Mn(CDOH)2(CDO)(BBu(OC2H5))2].
(Ellipsoids are at the 50% probability level). Hydrogen atoms are omitted
for the sake of clarity.
while remaining able to completely wrap Mn(II) with its six
imine-N donor atoms. That may explain why biscapped
Mn(II) complexes [Mn(dioxime)3(BR)2] tend to hydrolyze
in the presence of water, whereas the biscapped metal
complexes [M(dioxime)3(BR)2] (M ) Fe and Co) remain
stable.
X-ray Crystal Structure of [Mn(CDOH)2(CDO)(BBu-
(OC2H5))2]. Figure 5 shows the ORTEP drawing of
[Mn(CDOH)2(CDO)(BBu(OC2H5))2], which is almost identi-
cal to that of [Mn(CDOH)2(CDO)(BPh(OCH3))2].29 Mn(II)
is coordinated by six imine-N atoms, and the coordination
geometry is distorted trigonal prism. One triangular plane is
defined by N11, N21, and N32, whereas the other is defined
by N12, N22, and N31. The dihedral angle between these
two triangular planes is 3.21(13)°. The average Mn-N
distance is 2.276(3) Å, consistent with the Mn-N (dioxime)
distances reported by Jurisson and co-workers.29 Each
dioxime group forms a planar five-membered chelate ring
with Mn(II). The average bite angle is 69.69(9)°.
Solution Stability of Monocapped Mn(II) Complexes.
The results from the stability experiment also showed that
[Mn(CDOH)3BPh]OH remained stable in solution for more
than 8 h. To further demonstrate its solution stability, we
carried out a chelator challenge experiment in a mixture of
acetonitrile and water (50:50 v/v) with a large excess of
added PDTA (∼100-fold), which forms stable Mn(II)
complex Mn(PDTA)-2 with a log K value of 15.48 It
was found that the peak intensity at ∼16 min from
[Mn(CDOH)3BPh]+ remains unchanged over 8 h (Figure 7)
and that the pH (5.0-9.0) has no significant impact on the
stability of [Mn(CDOH)3BPh]+. These data clearly dem-
In [Mn(CDOH)2(CDO)(BBu(OC2H5))2], only one of three
dioxime chelating arms is bonded to both boron atoms. The
other two are bonded to one boron atom at one end and have
a hydroxyl group at the other end. Both boron atoms are
bonded to three oxygen atoms: two from the hydroxyl
groups of two dioxime chelating arms and the third from
Figure 6. Typical HPLC chromatogram of the solution containing [Mn(CDO)3(BPh)2] and [Mn(CDOH)3BPh]OH. The presence of a PhB(OH)2 peak at
∼16.5 min is caused by the hydrolysis of [Mn(CDO)3(BPh)2].
Inorganic Chemistry, Vol. 45, No. 13, 2006 5041

Hsieh and Liu
Figure 7. HPLC chromatograms of the aqueous solution of [Mn(CDOH)3BPh]Cl at 0.5 h (top) and 8 h (bottom) after the addition of a 100-fold excess of
PDTA.
5. The fact that biscapped Mn(II) complexes [Mn(dioxime)3-
(BR)2] have oxidation potentials identical to those of their
cationic analogues provides further support for our conclu-
sion that they are not stable and form cationic complexes
[M(dioxime)3BR]+ in the presence of water. As the boron
cap becomes smaller (phenyl, n-butyl, and methyl), the
oxidation potential of [M(dioxime)3BR]+ decreases. A similar
Figure 8. Typical cyclic voltammogram of [Mn(CDOH)3BPh]Cl. The
concentrations of Mn(II) complexes were about 2 mM in acetonitrile. The
oxidation potential is given as that vs NHE, because the Ag/AgNO3 electrode
has a potential of +0.4 V vs NHE. The scan rate was 100 mV/s.
trend was also observed when CDOH was replaced by
DMGH in cationic complexes [M(dioxime)3BR]+. The high
oxidation potential for the Mn(II)/Mn(III) couple clearly
onstrated the solution stability of cationic complexes
indicates that the +2 oxidation state is highly stabilized by
[Mn(dioxime)3BR]+. It is important to note that most Mn(II)
six soft imine-N donors. The irreversibility suggests that the
complexes are not thermodynamically stable because of a
oxidation from Mn(II) to Mn(III) probably involves signifi-
lack of ligand-field stabilization energy.49 In [Mn(CDOH)3-
cant conformational changes of the coordinated dioxime
BPh]+, the Mn(II) is completely wrapped by six imine-N
ligand. Similar irreversibility was also observed in the
donor atoms so that it is very difficult for Mn(II) to become
monocapped Fe(II) clathrochelates.50,51
dissociated. Therefore, the high solution stability of cationic
complex [Mn(CDOH)3BPh]+ is most likely due to its kinetic
inertness imposed by the boron-capped tris(CDOH) chelator.
Conclusion
The key finding of this study is that the biscapped
Electrochemistry. Cyclic voltammograms were obtained
complexes [Mn(dioxime)3(BR)2] can be readily prepared,
using acetonitrile as the solvent for both biscapped and
even though they are not stable in the presence of water.
monocapped Mn(II) complexes. They all show an irreversible
One of the boron caps quickly hydrolyzes to for cationic
one-electron oxidation wave at 1.20-1.35 V vs NHE,
depending on the identity of the boron cap and dioxime
complexes [M(dioxime)3BR]+, which are stable in aqueous
solution in the presence of a strong Mn(II) chelator such as
chelating arms. Figure 8 shows a cyclic voltammogram of
PDTA. This high solution stability is most likely due to the
[Mn(CDO)3BPh]Cl. The oxidation potentials of biscapped
and monocapped Mn(II) complexes are summarized in Table
(50) Bieda, K. L.; Kranitz, A. L.; Grzybowski, J. J. Inorg. Chem. 1993,
32, 4209-4213.
(48) Ogino, H. Bull. Chem. Soc. Jpn. 1965, 38, 771-778.
(49) Ijeri, V. S.; Srivastava, A. K. Polyhedron 2003, 22, 569-574.
(51) Belinski, J. A.; Squires, M. E.; Kuchna, J. M.; Bennett, B. A.;
Grzybowski, J. J. J. Coord. Chem. 1988, 19, 159-169.
5042 Inorganic Chemistry, Vol. 45, No. 13, 2006

Bis- and Monocapped Tris(dioxime) Mn(II) Complexes
kinetic inertness imposed by the boron capped tris(dioxime)
chelators that are able to completely wrap the Mn(II) into
their N6 coordination cavity. The three hydroxyl groups from
dioxime chelating arms in [Mn(CDOH)3BPh]OH form strong
intramolecular hydrogen bonds with the hydroxide counte-
rion. Therefore, its structure can be considered as being
clathrochelate with a hydroxide as a cap.
Myers Squibb Medical Imaging Inc., and the following
research grants: AHA0555659Z (S.L.) from the Greater
Midwest Affiliate of the American Heart Association, R21
EB003419 (S.L.) from the National Institute of Biomedical
Imaging and Bioengineering (NIBIB), and BCTR0503947
(S.L.) from the Susan G. Komen Breast Cancer Foundation.
Supporting Information Available: X-ray crystallographic files
in CIF format for the reported structures of [Mn(CDOH)3BPh]OH
and [Mn(CDOH)2(CDO)(BBu(OC2H5))2]. This material is available
free of charge at http://pubs.acs.org.
Acknowledgment. Acknowledgment is made to Dr.
Phillip E. Fanwick, Department of Chemistry, Purdue
University, for X-ray diffraction analysis of [Mn(CDOH)3BPh]-
OH‚CHCl3 and [Mn(CDOH)2(CDO)(BBu(OC2H5))2]. This
work is supported, in part, by Purdue University, Bristol-
IC060216N
Inorganic Chemistry, Vol. 45, No. 13, 2006 5043