High-spin molecules: Hexanuclear Mn(III) clusters with [Mn6O4X4]6+ (X = Cl-, Br-) face-capped octahedral cores and S = 12 ground states

Article abstract of DOI:10.1021/ja983446c

Reaction of R₂dbmH (R = H, Me, Et; dbmH is dibenzoylmethane) with MnCl₃ (generated in situ from MnCl₂ and MnO₄^-) in MeCN yields the five- coordinate, square-pyramidal complexes [MnCl(R₂dbm)₂] (R = H, 1; Me, 4; Et, 7). The same reaction with MnBr₂ yields [MnBr(R₂dbm)₂] (R = H, 2; Me, 5; Et, 8). Slow hydrolysis of 4 or 7 in CH₂Cl₂/MeCN over two weeks gives [Mn₆O₄Cl₄(R₂dbm)₆] (R = Me, 9; Et, 11); the crystal structure of 9 reveals a novel [Mn₆(μ₃-O)₄(μ₃-Cl)₄]⁶⁺ core comprising an Mn₆/(III) octahedron whose faces are capped by O^2- or Cl^- ions. Similar slow hydrolysis of 5 or 8 gives [Mn₆O₄Br₄(R₂dbm)₆] (R = Me, 10; Et, 12); the crystal structure of 10 is identical with that of 9 except for the Br^--for- Cl^- substitution. The ¹H NMR spectra of 1-12 in CDCl₃ show paramagnetically shifted and broadened R₂dbm resonances. The spectra are as expected for retention of the solid-state structures, the Mn₆ complexes exhibiting effective T(d) solution symmetry with no evidence of fragmentation to [MnX(R₂dbm)], [Mn(R₂dbm)₃], or any other species. The effective magnetic moment (μ(eff)) per Mn₆ for 9/12 slowly increases from 16.01/16.73 μ(B) at 320/300 K to a maximum of 24.37/24.60 μ(B) at 30.1/40.0 K, and then decreases to 13.69/13.86 μ(B) at 2.00 K. Fitting of the data to the theoretical equation derived for a Mn₆/(III) octahedron gives (for 9/12) J(cis) = 8.6/8.5 cm^-1 and g = 1.97/1.98 with J(trans) and TIP held fixed at 0 cm^-1 and 1200 x 10^-6 cm³ mol^-1, respectively. These values indicate a S = 12 ground state. Fitting of magnetization vs field data collected in the 2.00-15.0 K and 0.500-50.0 KG ranges confirmed S = 12 ground states with D ? 0 cm^-1.

Full text of DOI:10.1021/ja983446c

J. Am. Chem. Soc. 1999, 121, 5489-5499
5489
High-Spin Molecules: Hexanuclear MnIII Clusters with [Mn6O4X4]6+
(X ) Cl-, Br-) Face-Capped Octahedral Cores and S ) 12 Ground
States
Guillem Arom´ı,1a Michael J. Knapp,1b Juan-Pablo Claude,1a,c John C. Huffman,1a
David N. Hendrickson,*,1b and George Christou*,1a
Contribution from the Department of Chemistry and Molecular Structure Center,
Indiana UniVersity, Bloomington, Indiana 47405-4001, and Department of Chemistry-0358,
UniVersity of California at San Diego, La Jolla, California 93093-0358
ReceiVed September 28, 1998
Abstract: Reaction of R2dbmH (R ) H, Me, Et; dbmH is dibenzoylmethane) with MnCl3 (generated in situ
from MnCl2 and MnO4-) in MeCN yields the five-coordinate, square-pyramidal complexes [MnCl(R2dbm)2]
(R ) H, 1; Me, 4; Et, 7). The same reaction with MnBr2 yields [MnBr(R2dbm)2] (R ) H, 2; Me, 5; Et, 8).
Slow hydrolysis of 4 or 7 in CH2Cl2/MeCN over two weeks gives [Mn6O4Cl4(R2dbm)6] (R ) Me, 9; Et, 11);
the crystal structure of 9 reveals a novel [Mn6(µ3-O)4(µ3-Cl)4]6+ core comprising an Mn6III octahedron whose
faces are capped by O2- or Cl- ions. Similar slow hydrolysis of 5 or 8 gives [Mn6O4Br4(R2dbm)6] (R ) Me,
10; Et, 12); the crystal structure of 10 is identical with that of 9 except for the Br--for-Cl- substitution. The
1H NMR spectra of 1-12 in CDCl3 show paramagnetically shifted and broadened R2dbm resonances. The
spectra are as expected for retention of the solid-state structures, the Mn6 complexes exhibiting effective Td
solution symmetry with no evidence of fragmentation to [MnX(R2dbm)], [Mn(R2dbm)3], or any other species.
The effective magnetic moment (µeff) per Mn6 for 9/12 slowly increases from 16.01/16.73 µB at 320/300 K to
a maximum of 24.37/24.60 µB at 30.1/40.0 K, and then decreases to 13.69/13.86 µB at 2.00 K. Fitting of the
data to the theoretical equation derived for a MnI6II octahedron gives (for 9/12) Jcis ) 8.6/8.5 cm-1 and g )
1.97/1.98 with Jtrans and TIP held fixed at 0 cm-1 and 1200 × 10-6 cm3 mol-1, respectively. These values
indicate a S ) 12 ground state. Fitting of magnetization vs field data collected in the 2.00-15.0 K and 0.500-
50.0 kG ranges confirmed S ) 12 ground states with D ≈ 0 cm-1
.
Introduction
ations, including the desire to elucidate the various factors
responsible for yielding high-spin molecules. These include the
topological arrangement of the paramagnetic ions, the nature
(ferromagnetic or antiferromagnetic) of the inter-ion exchange
interactions, and the presence of spin frustration effects from
the presence of competing interactions of comparable magnitude.
In addition, it has recently become apparent that a relatively
large ground-state S value is one of the necessary requirements
for molecules to be able to exhibit the new phenomenon of
single-molecule magnetism.6-12 Another requirement is a
significant magnetoanisotropy as reflected in the zero-field
splitting (ZFS) parameter D, which needs also to be negative.
The study of molecules possessing unusually large spin (S)
values in their ground state is an area of intense current
research.2-5 These efforts are driven by a number of consider-
(1) (a) Indiana University. (b) University of California. (c) Present
address: Department of Chemistry, University of Alabama at Birmingham,
Birmingham, AL 35249.
(2) (a) Eppley, H. J.; Tsai, H.-L.; de Vries, N.; Folting, K.; Christou,
G.; Hendrickson, D. N. J. Am. Chem. Soc. 1995, 117, 301. (b) Tsai, H.-L.;
Wang, S.; Folting, K.; Streib, W. E.; Hendrickson, D. N.; Christou, G. J.
Am. Chem. Soc. 1995, 117, 2503. (c) Eppley, H. J.; Wang, S.; Tsai, H.-L.;
Aubin, S. M.; Folting, K.; Streib, W. E.; Hendrickson, D. N.; Christou, G.
Mol. Cryst. Liq. Cryst. 1995, 274, 159.
(3) Powell, A. K.; Heath, S. L.; Gatteschi, D.; Pardi, L.; Sessoli, R.;
Spina, G.; Del Giallo, F.; Pieralli, F. J. Am. Chem. Soc. 1995, 117, 2491.
(4) (a) Scuiller, A.; Mallah, T.; Verdaguer, M.; Nivorozkhin, A.;
Tholence, J.-L.; Veillet, P. New J. Chem. 1996, 20, 1. (b) Delfs, C. D.;
Gatteschi, D.; Pardi, L.; Sessoli, R.; Wieghardt, K.; Hanke, D. Inorg. Chem.
1993, 32, 3099. (c) Caneschi, A.; Gatteschi, D.; Laugier, J.; Rey, P.; Sessoli,
R.; Zanchini, C. J. Am. Chem. Soc. 1988, 110, 2795. (d) Goldberg, D. P.;
Caneschi, A.; Delfs, C. D.; Sessoli, R.; Lippard, S. J. J. Am. Chem. Soc.
1995, 117, 5789. (e) Blake, A. J.; Grant, C. M.; Parsons, S.; Rawson, J.
M.; Winpenny, R. E. P. J. Chem. Soc., Chem. Commun. 1994, 2363. (f)
Sun, Z.; Gantzel, P. K.; Hendrickson, D. N. 1996, 35, 6640. (g) Benelli,
C.; Parsons, S.; Solan, G. A.; Winpenny, R. E. P. Angew. Chem., Int. Ed.
Engl. 1996, 35, 1825. (h) Abbati, G. L.; Cornia, A.; Fabretti, A. C.; Caneschi,
A.; Gatteschi, D. Inorg. Chem. 1998, 37, 1430.
(6) (a) Sessoli, R.; Tsai, H.-L.; Schake, A. R.; Wang, S.; Vincent, J. B.;
Folting, K.; Gatteschi, D.; Christou, G.; Hendrickson, D. N. J. Am. Chem.
Soc. 1993, 115, 1804. (b) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak,
M. A. Nature 1993, 365, 141.
(7) (a) Arom´ı, G.; Aubin, S. M. J.; Bolcar, M. A.; Christou, G.; Eppley,
H. J.; Folting, K.; Hendrickson, D. N.; Huffman, J. C.; Squire, R. C.; Tsai,
H.-L.; Wang, S.; Wemple, M. W. Polyhedron 1998, 17, 3005. (b) Tsai,
H.-L.; Eppley, H. J.; De Vries, N.; Folting, K.; Christou, G.; Hendrickson,
D. N. Mol. Cryst. Liq. Cryst. 1995, 274, 167.
(8) (a) Aubin, S. M. J.; Wemple, M. W.; Adams, D. M.; Tsai, H.-L.;
Christou, G.; Hendrickson, D. N. J. Am. Chem. Soc. 1996, 118, 7746. (b)
Aubin, S. M. J.; Dilley, N. R.; Wemple, M. W.; Maple, M. B.; Christou,
G.; Hendrickson, D. N. J. Am. Chem. Soc. 1998, 120, 839.
(9) (a) Friedman, J. R.; Sarachik, M. P.; Tejada, J.; Maciejewski, J.; Ziolo,
R. J. Appl. Phys. 1996, 79, 6031. (b) Friedman, J. R.; Sarachik, M. P.;
Tejada, J.; Ziolo, R. Phys. ReV. Lett. 1996, 76, 3830.
(10) (a) Tejeda, J.; Ziolo, R. F.; Zhang, X. X. Chem. Mater. 1996, 8,
1784. (b) Hernandez, J. M.; Zhang, X. X.; Luis, F.; Bartolome´, J.; Tejada,
J.; Ziolo, R. Europhys. Lett. 1996, 35, 301.
(5) (a) Bolcar, M. A.; Aubin, S. M. J.; Folting, K.; Hendrickson, D. N.;
Christou, G. Chem. Commun. 1997, 1485. (b) Tsai, H.-L.; Wang, S.; Folting,
K.; Streib, W. E.; Hendrickson, D. N.; Christou, G. J. Am. Chem. Soc.
1995, 117, 2503. (c) Christou, G. In Magnetism: A Supramolecular
Function; Kahn, O., Ed.; NATO ASI Series; Kluwer: Dordrecht, 1996.
10.1021/ja983446c CCC: $18.00 © 1999 American Chemical Society
Published on Web 05/27/1999

5490 J. Am. Chem. Soc., Vol. 121, No. 23, 1999
Arom´ı et al.
minutes, and the resulting brown microcrystalline solid was collected
by filtration, washed with Et2O, and dried in vacuo. The yield was
47% based on Mn. Crystals suitable for X-ray crystallography were
obtained from a layering of a solution of 4 (40 mg) in CH2Cl2 (4 mL)
with equivolume Et2O. Anal. Calcd (Found) for 4‚0.2CH2Cl2: C, 67.34
(67.25); H, 5.02 (5.06); N, 0.00 (0.00).
The development of synthetic procedures to new examples of
high-spin molecules is thus of continuing interest.
One area in which high-spin molecules are encountered on a
relatively frequent basis is manganese cluster chemistry.4,5,7 Spin
values of S e 12 have been observed; for calibration, the largest
S value observed to date in a molecular species is S ≈ 33/2 in a
cocrystallized mixture of Fe17 and Fe19 clusters.3 We and others
have reported several Mnx clusters with S values g 4,4,5,7,8,11
and some of these have also proven to be single-molecule
magnets. In this paper, we describe two new Mn6 clusters of
formula [Mn6O4X4(R2dbm)6] (X ) Cl-, Br-). Not only do they
possess very aesthetically pleasing structures comprising the
initial examples of the M6A4B4 near-tetrahedral core, but they
have been found to possess S ) 12 ground states. Portions of
this work have been previously communicated.13
MnBr(Me2dbm)2 (5). Solid MnBr2‚4H2O (0.16 g, 0.56 mmol) was
dissolved with stirring in a yellow solution of Me2dbmH (0.35 g, 1.4
mmol) in MeCN (11 mL), which had been slightly warmed to
completely dissolve the solid. The solution was then treated with a
purple solution of NBun MnO4 (51 mg, 0.14 mmol) in MeCN (6 mL).
4
After a few minutes, the microcrystalline solid that formed was collected
by filtration and washed with Et2O. The solid was slurried for a few
minutes in a 1/1 mixture of MeCN/CH2Cl2 (10 mL), refiltered, washed
with Et2O, and dried in vacuo. The yield was 25% based on Mn. Anal.
Calcd (Found) for 5‚0.1MeCN‚0.55CH2Cl2; C, 60.64 (60.59); H, 4.60
(4.27); N, 0.20 (0.22).
Mn(Me2dbm)3 (6). To a light pink stirred solution of Mn(O2CMe)2‚
4H2O (55 mg, 0.22 mmol) in MeOH (10 mL) was added a purple
Experimental Section
solution of NBun MnO4 (20 mg, 0.056 mmol) in MeOH (5 mL) to
All manipulations were performed under aerobic conditions using
4
materials as received, except where indicated otherwise. NBun MnO4
give a red-brown solution to which was added a solution of Me2dbmH
(212 mg, 0.84 mmol) in MeCN (20 mL). After a few minutes, the
green precipitate that rapidly formed was collected by filtration, washed
with Et2O, and dried in vacuo. The yield was 68% based on Mn. Anal.
Calcd (Found): C, 75.73 (75.83); H, 5.61 (5.64); N, 0.00 (0.00).
MnCl(Et2dbm)2 (7). To a solution of MnCl2‚4H2O (110.8 mg, 0.56
mmol) in MeOH (3 mL) was added a purple solution of Bu4NMnO4
(50.6 mg, 0.14 mmol) in MeCN (6 mL). To the resulting red-brown
solution was immediately added solid Et2dbmH (392 mg, 1.4 mmol),
the mixture stirred for about 45 min, and the solvents were evaporated
to dryness. The residue was thoroughly washed with Et2O, and the
remaining solid was redissolved in CH2Cl2. A small amount of a white
precipitate was removed by filtration, and the solvent was removed to
dryness. The residue was washed with Et2O and dried in vacuo. The
yield was 56% based on Mn. Anal. Calcd (Found) for 7‚0.4 MeCN:
C, 70.02 (69.96); H, 5.94 (6.00); N, 0.84 (0.86).
4
was prepared as described elsewhere.14 dbmH ) dibenzoylmethane )
1,3-diphenyl-1,3-propanedione; Me2dbmH ) 4,4′-dimethyldibenzoyl-
methane; Et2dbmH ) 4,4′-diethyldibenzoylmethane.
MnCl(dbm)2 (1). A stirred solution of MnCl2‚4H2O (0.11 g, 0.56
mmol) in MeOH (3 mL) was treated with a purple solution of
NBun MnO4 (51 mg, 0.14 mmol) in MeCN (6 mL), and to the resulting
4
red-brown solution was immediately added solid dbmH (0.31 g, 1.4
mmol). The mixture was stirred for about 45 min, and the solvents
were evaporated to dryness. The residue was washed with Et2O, and
the remaining solid was redissolved in CH2Cl2. A small amount of a
white powder was removed by filtration, and the solvent was re-
moved to dryness. The solid was washed with Et2O and dried in
vacuo. The yield was 61% based on Mn. Anal. Calcd (Found) for
1‚0.08CH2Cl2: C, 66.45 (66.43); H, 4.11 (4.22); N, 0.00 (0.00). Crystals
suitable for X-ray crystallography were obtained from layering a
solution of 1 (25 mg) in CH2Cl2 (5 mL) with Et2O.
MnBr(Et2dbm)2 (8). Solid MnBr2‚4H2O (0.15 g, 0.56 mmol) was
dissolved in a stirred yellow solution of Et2dbmH (0.39 g, 1.4 mmol)
in MeCN (10 mL), followed by addition of a purple solution of
MnBr(dbm)2 (2). Solid MnBr2‚4H2O (0.17 g, 0.56 mmol) was
dissolved in a stirred yellow solution of dbmH (0.31 g, 1.4 mmol) in
MeCN (20 mL), and the resulting solution was treated with a purple
NBun MnO4 (51 mg, 0.14 mmol) in MeCN (5 mL). A microcrystalline
4
solution of NBun MnO4 (51 mg, 0.14 mmol) in MeCN (10 mL). The
solid soon started to precipitate, and after several more minutes, it was
collected by filtration, washed with Et2O, and dried in vacuo. The yield
was 30% based on Mn. Anal. Calcd (Found) for 8‚0.1MeCN: C, 65.77
(65.63); H, 5.53 (5.53); N, 0.20 (0.16).
4
mixture was stirred for 2 h, and the brown precipitate was collected
by filtration, washed with Et2O, and dried in vacuo. The yield was
38% based on Mn. Anal. Calcd (Found) for 2‚0.1MeCN: C, 61.96
(61.66); H, 3.84 (3.83); N, 0.24 (0.20).
Mn6O4Cl4(Me2dbm)6 (9). MnCl(Me2dbm)2 (4) (0.25 g, 0.42 mmol)
was dissolved with stirring in a 1:1 mixture of MeCN/CH2Cl2 (32 mL).
The solution was filtered and the filtrate allowed to concentrate by
slow evaporation over a period of two weeks to give black, well-formed
crystals (suitable for X-ray crystallography). These were collected by
filtration, washed with EtOH to remove some white solid, and dried in
vacuo. The yield was ∼25% based on Mn. Anal. Calcd (Found) for
9‚0.4CH2Cl2: C, 59.21 (59.28); H, 4.41 (4.49); N, 0.00 (0.00). Crystals
kept in contact with the mother liquor were identified crystallographi-
cally as 9‚3CH2Cl2.
Mn(dbm)3 (3). To a stirred light pink solution of Mn(O2CMe)2‚
4H2O (55 mg, 0.22 mmol) in MeOH (10 mL) was added a purple
solution of NBun MnO4 (20 mg, 0.056 mmol) in MeOH (5 mL) to
4
give a red-brown solution. This was treated with a solution of dbmH
(0.19 g, 0.84 mmol) in MeCN (20 mL), and a brown microcrystalline
solid started to precipitate within minutes. After 1 h, the solid was
collected by filtration, washed with Et2O, and dried in vacuo. The yield
was 84% based on Mn. Anal. Calcd (Found) for 3: C, 74.58 (74.40);
H, 4.59 (4.69); N, 0.00 (0.00).
Mn6O4Br4(Me2dbm)6 (10). MnBr(Me2dbm)2 (5) (169 mg, 0.26
mmol) was dissolved with stirring in a 1:2 mixture of MeCN/CH2Cl2
(100 mL). The solution was filtered and the filtrate allowed to slowly
concentrate by evaporation. After a few days, black, well-formed
crystals (suitable for X-ray crystallography) had formed. They were
isolated as for complex 9. The yield was 12% based on Mn. Anal.
Calcd (Found): C, 55.16 (55.06); H, 4.08 (4.11); N, 0.00 (0.00).
Mn6O4Cl4(Et2dbm)6 (11). MnCl(Et2dbm)2 (7) (0.45 g, 0.64 mmol)
was dissolved with stirring in MeCN (130 mL). The solution was
filtered and the filtrate allowed to slowly concentrate by evaporation.
After a few days, black, well-formed crystals had deposited. These were
collected by filtration, washed with EtOH, and dried in vacuo. The
yield was 25% based on Mn. Anal. Calcd (Found) for 11‚0.8MeCN:
C, 61.86 (61.95); H, 5.23 (5.14); N, 0.50 (0.50).
MnCl(Me2dbm)2 (4). To a stirred solution of MnCl2‚4H2O (0.11 g,
0.56 mmol) in MeOH (3 mL) was added a purple solution of
NBun MnO4 (51 mg, 0.14 mmol) in MeCN (6 mL). To the resulting
4
red-brown solution was immediately added a solution of Me2dbmH
(0.35 g, 1.4 mmol) in MeCN (11 mL) which had been warmed slightly
to completely dissolve the solid. The mixture was stirred for a few
(11) (a) Aubin, S. M. J.; Spagna, S.; Eppley, H. J.; Sager, R. E.; Folting,
K.; Christou, G.; Hendrickson, D. N. Mol. Cryst. Liq. Cryst. 1997, 305,
181. (b) Eppley, H. J.; Aubin, S. M. J.; Wemple, M. W.; Adams, D. M.;
Tsai, H.-L.; Grillo, V. A.; Castro, S. L.; Sun, Z.; Folting, K.; Huffman, J.
C.; Hendrickson, D. N.; Christou, G. Mol. Cryst. Liq. Cryst. 1997, 305,
167.
(12) Barra, A. L.; Debrunner, P.; Gatteschi, D.; Schultz, C. E.; Sessolli,
R. Europhys. Lett. 1996, 35, 133.
(13) Aromi, G.; Wemple, M. W.; Aubin, S. M. J.; Folting, K.;
Hendrickson, D. N.; Christou, G. J. Am. Chem. Soc. 1998, 120, 2977.
(14) Vincent, J. B.; Folting, K.; Huffman, J. C.; Christou, G. Inorg. Chem.
1986, 25, 996.
Mn6O4Br4(Et2dbm)6 (12). MnBr(Et2dbm)2 (8) (0.936 g, 1.35 mmol)
was dissolved with stirring in MeCN (370 mL). The solution was
filtered and the filtrate allowed to slowly concentrate by evaporation

Hexanuclear MnIII Clusters
J. Am. Chem. Soc., Vol. 121, No. 23, 1999 5491
Table 1. Crystallographic Data for Complexes 1‚CH2Cl2, 4‚CH2Cl2, 9‚3CH2Cl2, and 10‚CH2Cl2‚MeCN
1
4
9
10
formulaa
formula wt, g/mol
space group
a, Å
b, Å
c, Å
C31H24O4Cl3Mn
621.83
P21/n
13.888(3)
8.348(1)
24.702(4)
102.69(1)
2794
C35H32O4Cl3Mn
677.93
C2/c
24.802(5)
14.814(2)
17.458(3)
94.24(1)
6397
C105H96O16Cl10Mn6
2298.06
P21/c
17.172(2)
18.302(2)
34.534(4)
100.36(1)
10677
C105H95NO16Cl2Br4Mn6
2347.06
P21/c
17.180(6)
18.454(7)
34.636(14)
100.88(2)
10784
â, deg
V, Å3
Z
8
8
4
4
T, °C
-168
0.71069
2.957
15.548
5.33 (4.56)
-122
0.71069
1.408
6.850
5.61 (4.32)
-171
0.71069
1.430
9.726
5.80 (5.73)
-168
0.71069
1.446
22.775
9.72 (6.25)
radiation, Åb
F
calc, g/cm3
µ, cm-1
R(Rw), %
2
a Including solvate molecules. b Mo KR, Graphite monochromator. c R ) 100 ∑||Fo| - |Fc||/∑|Fo|. d Rw ) 100 [∑w(|Fo| - |Fc |)2/∑w|Fo| ]1/2
where w ) 1/σ2(|Fo|).
over 10 days. At the end of this period, black crystals suitable for
X-ray crystallography had deposited and were collected by filtration,
washed with EtOH, and dried in vacuo. The yield was 15% based on
Mn. Anal. Calcd (Found): C, 57.31 (57.02); H, 4.81 (4.75); N, 0.00
(0.00).
essentially featureless, the largest peak being 0.91 e/Å3 near a disordered
solvent molecule.
For complex 10‚CH2Cl2‚MeCN (6° e 2θ e 45°), a systematic search
of a limited hemisphere of reciprocal space revealed monoclinic
symmetry and systematic absences, corresponding to the unique space
group P21/c. Subsequent solution and refinement of the structure
confirmed this choice. All non-hydrogen atoms of the Mn6 molecules
were readily located and refined anisotropically; one MeCN and a badly
disordered CH2Cl2 molecule were also located; a total of seven closely
spaced peaks (C136-C142) suggested additional solvent molecules that
were too badly disordered to be identified. Hydrogen atoms on the
Mn6 molecule were included in calculated positions as fixed-atom
contributors. The final difference Fourier map was essentially feature-
less, the largest peaks being 0.8-1.3 e/Å3 near the disordered solvent
molecules.
X-ray Crystallography. Crystals suitable for crystallography were
grown from CH2Cl2/Et2O layerings (1‚CH2Cl2 and 4‚CH2Cl2) or ob-
tained directly from the preparative solution (9‚3CH2Cl2 and 10‚
CH2Cl2‚MeCN). Suitable crystals were selected from the mother liquors,
affixed to the tips of glass fibers with silicone grease, and transferred
to the goniostat and cooled for characterization and data collection.
The equipment employed was a Picker four-circle diffractometer; details
of the diffractometry, low-temperature facilities, and computational
procedures are available elsewhere.15 After data collection (+h, +k,
(l), correction for Lorentz and polarization effects and averaging of
equivalent reflections, the structures were solved by direct methods
(MULTAN or SHELXTL) and standard Fourier techniques and refined
on F by full matrix, least-squares methods.
For complex 1‚CH2Cl2 (6° e 2θ e 50°), a systematic search of a
limited hemisphere of reciprocal space revealed monoclinic symmetry
and systematic absences, indicating the unique monoclinic space group
P21/n. Subsequent solution and refinement of the structure confirmed
this choice. All non-hydrogen atoms were readily located and refined
anisotropically; all hydrogen atoms were located in difference Fourier
maps and refined isotropically in the final refinement cycles. The final
difference Fourier map was essentially featureless, the largest peak being
1.03 e/Å3 near the solvent molecule.
For complex 4‚CH2Cl2 (6° e 2θ e 50°), a systematic search of a
limited hemisphere of reciprocal space revealed monoclinic symmetry
and systematic absences, corresponding to one of the monoclinic space
groups, C2/c or Cc. Subsequent solution and refinement of the structure
confirmed centrosymmetric C2/c to be the correct space group. All
non-hydrogen atoms were readily located and refined anisotropically;
hydrogen atoms were included in calculated positions as fixed atom
contributors in the final refinement cycles. The structure contains
molecules packed in sheets parallel to the [404] plane; the [404]
reflection is the most intense by nearly a factor of 10. The final
difference Fourier map was essentially featureless, the largest peak being
0.24 e/Å3.
For complex 9‚3CH2Cl2 (6° e 2θ e 45°), a systematic search of a
limited hemisphere of reciprocal space revealed monoclinic symmetry
and systematic absences corresponding to the unique space group
P21/c. Subsequent solution and refinement of the structure confirmed
this choice. All non-hydrogen atoms of the Mn6 molecules were
readily located and refined anisotropically; a total of three CH2Cl2
molecules were also located, two slightly disordered and one seriously
disordered, involving several peaks with partial occupancy. Hydrogen
atoms were included in calculated positions as fixed-atom contributors
in the final refinement cycles. The final difference Fourier map was
Crystallographic data and final values of discrepancy indices R(F)
and Rw(F) are listed in Table 1.
Physical Measurements. 1H NMR spectra were collected on a 300
MHz Varian Gemini 2000 spectrometer and a 500 MHz Varian Inova
spectrometer with the protio-solvent signal used as reference. IR and
electronic spectra were recorded on KBr pellets and solutions, respec-
tively, on Nicolet model 510P and Hewlett Packard model 8452A
spectrophotometers, respectively. DC magnetic susceptibility data were
collected on powdered, microcrystalline samples (restrained in eicosane
to prevent torquing) on a Quantum Design MPMS5 SQUID magne-
tometer equipped with a 5.5 T (55 kG) magnet. A diamagnetic correc-
tion to the observed susceptibilities was applied using Pascal’s constants.
AC magnetic susceptibility measurements were made on a Quantum
Design MPMS2 SQUID magnetometer equipped with a 1 T (10 kG)
magnet and capable of achieving temperatures of 1.7 to 400 K. The
AC field range is 1 × 10-4 to 5 G oscillating at a frequency in the
range 5 × 10-4 to 1512 Hz. AC susceptibility data were collected on
powdered, microcrystalline samples in an AC field of 1.0 G oscillating
in the 0.74-1512 Hz range and in an applied DC field of 0-6.0 kG.
Results and Discussion
Syntheses. Entry into the new family of Mn6 clusters is via
the mononuclear, five-coordinate complexes MnX(R2dbm)2 (X
) Cl-, Br-; R ) Me, Et). The Cl- monomers were originally
obtained from the reactions of [Mn4O2(O2CMe)6(py)2(R2dbm)2]
with 6 equiv. of the carboxylate-abstracting reagent Me3SiCl,
but more direct procedures were subsequently developed to the
R ) H (1), Me (4), and Et (7) species.
In the absence of a simple source of MnCl3 stable in the solid
state, MnIII was generated in situ in MeCN/MeOH mixtures by
oxidation of MnCl2 with MnO4- in a 4:1 ratio (eq 1). This gave
4MnII + MnVII f 5 MnIII
a brown solution from which deposited the desired [MnCl-
(1)
(15) Chisholm, M. H.; Folting, K.; Huffman, J. C.; Kirkpatrick, C. C.
Inorg. Chem. 1984, 23, 1021.

5492 J. Am. Chem. Soc., Vol. 121, No. 23, 1999
Arom´ı et al.
(R2dbm)2] species on addition of R2dbmH (eq 2). This one-pot
4 MnCl2 + MnO4- + 10 R2dbmH f
5 MnCl(R2dbm)2 + 4 H2O + 3 Cl- + 2 H+ (2)
procedure can be readily extended to the preparation of the
[MnBr(R2dbm)2] complexes 2, 5, and 8 by use of MnBr2 in
place of MnCl2.
The six-coordinate, tris-chelate complexes [Mn(dbm)3] (3)
and [Mn(Me2dbm)3] (6) were required to assist with spectro-
scopic assignments in the formation of the Mn6 species, and
they were prepared by a procedure similar to that employed for
-
the [MnX(R2dbm)2] complexes, i.e., using MnII/MnO4 to
generate MnIII in situ followed by the addition of 3 equiv of
R2dbmH (eq 3).
4 Mn(O2CMe)2 + MnO4- + 15 R2dbmH f
-
5 Mn(R2dbm)3 + 4 H2O + 7 MeCO2H + MeCO2 (3)
The conversion of [MnCl(Me2dbm)2] (4) to [Mn6O4Cl4-
(Me2dbm)6] (9) was first realized in a solution of 4 in CH2Cl2/
MeCN (1:1), which slowly deposited black crystals of 9 over
the course of two weeks. Yields of e25% have been obtained.
The source of the oxide ions in 9 is undoubtedly H2O, suggesting
that the formation of the hexanuclear product involves hydrolysis
of the mononuclear species, facilitated by the Brønsted basicity
of Me2dbm- (eq 4). The overall transformation is not as simple
Figure 1. ORTEP representation (50% probability ellipsoids) of
[MnCl(dbm)2 (1, top) and [MnCl(Me2dbm)2] (4, bottom).
Table 2. Selected Bond Distances (Å) and Angles (deg) for
[MnCl(dbm)2] (1)
Mn(1)-Cl(2)
Mn(1)-O(3)
Mn(1)-O(7)
2.374(2)
1.896(3)
1.896(3)
Mn(1)-O(20)
Mn(1)-O(24)
1.895(3)
1.901(3)
6 MnCl(Me2dbm)2 + 4 H2O f
[Mn6O4Cl4(Me2dbm)6] + 6 Me2dbmH + 2 HCl (4)
Cl(2)-Mn(1)-O(3) 98.64(12) O(3)-Mn(1)-O(20) 164.81(15)
as this equation suggests, however: IR spectral examination of
solid obtained from the filtrate by solvent removal shows the
presence of Mn(Me2dbm)3 (6) as well as other undefined
species. Thus, the reaction solution probably contains several
species in equilibrium, and the isolation of pure 9 is probably
due to its low solubility in this solvent mixture. Analogous slow
hydrolysis reactions of [MnCl(Et2dbm)2] (7) were subsequently
found to yield the corresponding complex [Mn6O4Cl4(Et2dbm)6]
(11). Interestingly, however, analogous solutions of [MnCl-
(dbm)2] (1) do not produce [Mn6O4Cl4(dbm)6] on standing or
any other related material. If this complex is among the species
present in solution, it must be too soluble to crystallize. Attempts
to prepare it by altering reaction conditions have not been
pursued.
Cl(2)-Mn(1)-O(7) 98.96(11) O(3)-Mn(1)-O(24)
Cl(2)-Mn(1)-O(20) 96.53(11) O(7)-Mn(1)-O(20)
85.91(14)
88.00(14)
Cl(2)-Mn(1)-O(24) 98.92(11) O(7)-Mn(1)-O(24) 162.08(15)
O(3)-Mn(1)-O(7) 90.28(14) O(20)-Mn(1)-O(24) 91.11(14)
Table 3. Selected Bond Distances (Å) and Angles (deg) for
[MnCl(Me2dbm)2] (4)
Mn(1)-Cl(2)
Mn(1)-O(3)
Mn(1)-O(7)
2.380(2)
1.897(3)
1.895(3)
Mn(1)-O(22)
Mn(1)-O(26)
1.887(3)
1.890(3)
Cl(2)-Mn(1)-O(3) 102.65(11) O(3)-Mn(1)-O(22)
Cl(2)-Mn(1)-O(7)
Cl(2)-Mn(1)-O(22) 100.58(11) O(7)-Mn(1)-O(22) 163.51(15)
Cl(2)-Mn(1)-O(26) 97.14(11) O(7)-Mn(1)-O(26) 86.92(12)
O(3)-Mn(1)-O(7) 90.22(13) O(22)-Mn(1)-O(26) 90.85(13)
86.35(12)
95.90(11) O(3)-Mn(1)-O(26) 160.19(15)
The identification of a novel [Mn6(µ3-O)4(µ3-Cl)4]6+ core in
9 (vide infra) raised the question of whether the corresponding
Br- complexes might be attainable, and this was subsequently
shown to be the case. The [MnBr(R2dbm)2] complexes 2, 5,
and 8 can be prepared in a fashion very similar to that for the
Cl- species by using MnBr2, and subsequent slow hydrolysis
in MeCN/CH2Cl2 (1:2) or MeCN gave [Mn6O4Br4(R2dbm)6]
(R ) Me, 10; R ) Et, 12). Again, the dbm complex 2 did not
yield a Mn6 crystalline solid. Attempts are in progress to extend
the [Mn6O4X4]6+ family to F- and other similar anionic ligands
(RO-, N3-, etc.).
tively)16 with the Cl- ion in the apical position. The complexes
possess virtual C2V symmetry. Inspection of packing diagrams
shows interesting supramolecular architecture: for both 1 and
4, the molecules align face-to-face in a zigzag fashion (side view
shown)
Description of Structures. Labeled ORTEP plots of com-
plexes 1 and 4 are presented in Figure 1, and selected interatomic
distances and angles are listed in Tables 2 and 3. The two
complexes are very similar and comprise a five-coordinate MnIII
ligated to two chelate groups (Mn-O ≈ 1.89 Å) and a Cl- ion
(Mn-Cl ≈ 2.37 Å. The geometry is square-pyramidal (τ is 0.05
and 0.06 for 1 and 4, respectively, where τ ) 0 and 1 for perfect
square-pyramidal and trigonal bipyramidal geometries, respec-
to give a one-dimensional double ribbon held together by
π-stacking interactions between the R2dbm groups. These double
ribbons are stacked on top of each other to form a sheet, which
then stacks to give the 3D structure. In 4, these sheets all have
the ribbon directions parallel, whereas, in 1, adjacent sheets have
(16) Addison, A. W.; Rao, T. N.; Reedijk, J.; Rijn, J.; Verschoor, G. C.
J. Chem. Soc., Dalton Trans. 1984, 1349.

Hexanuclear MnIII Clusters
J. Am. Chem. Soc., Vol. 121, No. 23, 1999 5493
Figure 3. ORTEP representation (50% probability ellipsoids) of
[Mn6O4Br4(Me2dbm)6] (10); for clarity, only the ipso C atom of each
aromatic ring is shown.
Figure 2. ORTEP representation (50% probability ellipsoids) and
stereopair of [Mn6O4Cl4(Me2dbm)6] (9) from a viewpoint that empha-
sizes the near tetrahedron; for clarity, only the ipso C atom of each
aromatic ring is shown.
perpendicular ribbon directions. Packing diagrams are available
as Supporting Information.
The structures of 9 and 10 are shown in Figures 2-4, and
selected interatomic distances and angles are listed in Tables 4
and 5, with the most important parameters compared in Table
6. The structure of 9 consists of a MnI6II octahedron (Figure 4)
with four nonadjacent faces bridged by µ3-O2- ions and the
other four faces bridged by µ3-Cl- ions. Six-coordinate, ap-
proximately octahedral geometry at each metal is completed
by a chelating Me2dbm group. The Cl- ions possess trigonal
pyramidal coordination geometry with a sum-of-angles (soa)
of ∼223°, but the O2- ions are almost trigonal planar with a
soa of ∼349°, and they lie only ∼0.37 Å above the Mn3 plane
which they bridge. As expected for high-spin MnIII (d4) in near-
octahedral geometry, there is a Jahn-Teller (JT) distortion, and
it takes the form of an axial elongation of the two trans
Mn-Cl bonds, making them unusually long (2.618(3)-
2.692(3) Å). In contrast, the Mn-O2- (1.876(4)-1.899(5) Å)
and Mn-O(Me2dbm) (1.903(5)-1.925(5) Å) bond lengths
are as expected. As a result of (i), the long Mn-Cl vs short
Mn-O2- bonds and (ii) trigonal planar vs trigonal pyrami-
dal geometry at the O2- and Cl- ions, respectively, the
[Mn6O4Cl4]6+ core is a near tetrahedron with a Cl- at each
vertex, a Mn at the midpoint of each edge, and an O2- bridging
each face. The cluster has virtual Td symmetry.
Figure 4. ORTEP representation (50% probability ellipsoids) of
[Mn6O4Cl4(Me2dbm)6] (9) with Mn-Mn vectors shown to emphasize
the Mn6 octahedron.
Br-, and O2- and the location of the JT axes are the same. In
Table 6 are collected the metric parameters for the [Mn6O4X4]6+
cores of 9 and 10, and it can be seen that only small differences
exist. The main difference is in the Mn-Br (av 2.789 Å) vs
Mn-Cl (av 2.653 Å) bond lengths, the difference of 0.136 Å
being comparable to the 0.15 Å difference in ionic (and
covalent) radii for six-coordinate Br-/Cl-.17 In [Mn4O3X(O2-
CMe)3(dbm)3] (X ) Cl-, Br-),18 where the X- ions are µ3-
bridging MnI3II triangles as in 9 and 10, the Mn-Br (2.803 Å)
vs Mn-Cl (2.650 Å) average bond difference is 0.15 Å. The
longer Mn-Br vs Mn-Cl bonds in 9/10 lead to expected
changes in core angles, i.e., the Mn-Br-Mn angles are smaller
(av 70.3°) than Mn-Cl-Mn angles (74.3°), and Br-Mn-Br
angles (167.2°) are larger than Cl-Mn-Cl angles (162.1°). It
(17) Shannon, R. D. Acta Crystallogr., Sect. A 1976, A32, 751.
(18) Wang, S.; Tsai, H.-L.; Libby, E.; Folting, K.; Streib, W. E.;
Hendrickson, D. N.; Christou, G. Inorg. Chem. 1996, 35, 7578.
Complex 10 is isostructural with 9, the only difference being
the Br--for-Cl- substitution; the coordination geometries at Mn,

5494 J. Am. Chem. Soc., Vol. 121, No. 23, 1999
Arom´ı et al.
Table 4. Selected Interatomic Distances (Å) and Angles (deg) for [Mn6O4Cl4(Me2dbm)6] (9)
Mn(1)‚‚Mn(2)
Mn(1)‚‚Mn(3)
Mn(4)‚‚Mn(4)
Mn(1)‚‚Mn(6)
Mn(2)‚‚Mn(3)
Mn(2)‚‚Mn(5)
Mn(2)‚‚Mn(6)
Mn(3)‚‚Mn(4)
Mn(3)‚‚Mn(5)
Mn(1)-Cl(7)
Mn(4)-Cl(7)
Mn(6)-Cl(7)
Mn(1)-Cl(8)
3.195(2)
3.207(2)
3.203(2)
3.193(2)
3.199(2)
3.210(2)
3.219(2)
3.204(2)
3.189(2)
2.662(3)
2.675(3)
2.652(3)
2.652(3)
Mn(4)‚‚Mn(5)
Mn(4)‚‚Mn(6)
Mn(5)‚‚Mn(6)
Mn(1)‚‚Mn(5)
Mn(2)‚‚Mn(4)
Mn(3)‚‚Mn(6)
Mn(3)-Cl(8)
Mn(3)-Cl(9)
Mn(4)-Cl(9)
Mn(5)-Cl(9)
Mn(2)-Cl(10)
Mn(5)-Cl(10)
Mn(6)-Cl(10)
3.205(2)
3.199(2)
3.221(2)
4.526(2)
4.534(2)
4.532(2)
2.692(3)
2.651(3)
2.638(3)
2.623(3)
2.618(3)
2.666(3)
2.658(3)
Mn(2)-Cl(8)
Mn(1)-O(11)
Mn(1)-O(12)
Mn(1)-O(34)
Mn(1)-O(38)
Mn(2)-O(12)
Mn(2)-O(14)
Mn(2)-O(53)
Mn(4)-O(15)
Mn(4)-O(19)
Mn(5)-O(13)
Mn(5)-O(14)
Mn(5)-O(72)
2.644(3)
1.880(4)
1.876(4)
1.927(5)
1.918(5)
1.899(5)
1.886(4)
1.903(5)
1.921(5)
1.922(5)
1.887(4)
1.885(5)
1.909(5)
Mn(2)-O(57)
Mn(3)-O(11)
Mn(3)-O(14)
Mn(3)-O(110)
Mn(3)-O(114)
Mn(4)-O(11)
Mn(4)-O(13)
Mn(5)-O(76)
Mn(6)-O(12)
Mn(6)-O(13)
Mn(6)-O(91)
Mn(6)-O(95)
1.910(5)
1.885(4)
1.877(4)
1.910(5)
1.918(5)
1.881(5)
1.895(4)
1.905(5)
1.894(5)
1.890(5)
1.922(5)
1.915(5)
O(11)-Mn(1)-O(12)
O(11)-Mn(1)-O(34)
O(11)-Mn(1)-O(38)
O(12)-Mn(1)-O(34)
O(12)-Mn(1)-O(38)
O(34)-Mn(1)-O(38)
O(12)-Mn(2)-O(14)
O(12)-Mn(2)-O(53)
O(12)-Mn(2)-O(57)
O(14)-Mn(2)-O(53)
O(14)-Mn(2)-O(57)
O(53)-Mn(2)-O(57)
O(11)-Mn(3)-O(14)
O(11)-Mn(3)-O(110)
93.24(19) O(110)-Mn(3)-O(114)
175.84(21) O(11)-Mn(4)-O(13)
87.14(19) O(11)-Mn(4)-O(15)
90.34(20) O(11)-Mn(4)-O(19)
176.19(21) O(13)-Mn(4)-O(15)
89.43(20) O(13)-Mn(4)-O(19)
93.64(19) O(15)-Mn(4)-O(19)
175.17(21) O(13)-Mn(5)-O(14)
89.25(20) O(13)-Mn(5)-O(72)
88.18(20) O(13)-Mn(5)-O(76)
174.43(20) O(14)-Mn(5)-O(72)
89.33(21) O(14)-Mn(5)-O(76)
92.28(19) O(72)-Mn(5)-O(76)
89.82(20) O(12)-Mn(6)-O(13)
89.00(20) O(13)-Mn(6)-O(95) 177.24(21) O(91)-Mn(6)-O(95)
89.49(21)
74.57(20)
74.42(20)
75.05(20)
74.80(20)
75.19(20)
74.45(20)
92.39(19) Mn(1)-Cl(7)-Mn(4)
176.38(19) Mn(1)-Cl(7)-Mn(6)
87.91(20) Mn(4)-Cl(7)-Mn(6)
90.82(19) Mn(1)-Cl(8)-Mn(2)
175.85(21) Mn(1)-Cl(8)-Mn(3)
89.02(20) Mn(2)-Cl(8)-Mn(3)
73.77(20) Mn(3)-Cl(9)-Mn(4)
73.87(20) Mn(3)-Cl(9)-Mn(5)
73.82(20) Mn(4)-Cl(9)-Mn(5)
74.21(20) Mn(2)-Cl(10)-Mn(5)
73.75(20) Mn(2)-Cl(10)-Mn(6)
73.68(20) Mn(5)-Cl(10)-Mn(6)
93.28(19) Mn(1)-O(11)-Mn(3) 116.79(23) Mn(4)-O(13)-Mn(5) 115.89(23)
177.94(20) Mn(1)-O(11)-Mn(4) 116.79(23) Mn(4)-O(13)-Mn(6) 115.39(23)
88.85(19) Mn(3)-O(11)-Mn(4) 116.61(22) Mn(5)-O(13)-Mn(6) 117.04(23)
88.63(20) Mn(1)-O(12)-Mn(2) 115.61(23) Mn(2)-O(14)-Mn(3) 116.46(23)
176.99(20) Mn(1)-O(12)-Mn(6) 115.74(23) Mn(2)-O(14)-Mn(5) 116.66(23)
89.28(20) Mn(2)-O(12)-Mn(6) 116.10(22) Mn(3)-O(14)-Mn(5) 115.96(23)
93.91(19) Cl(7)-Mn(1)-Cl(8)
163.23(23) Cl(7)-Mn(4)-Cl(9)
162.06(23)
161.29(23)
161.85(23)
O(11)-Mn(3)-O(114) 175.63(21) O(12)-Mn(6)-O(91)
O(14)-Mn(3)-O(110) 172.42(21) O(12)-Mn(6)-O(95)
177.32(20) Cl(8)-Mn(2)-Cl(10) 162.77(23) Cl(9)-Mn(5)-Cl(10)
87.98(20) Cl(8)-Mn(3)-Cl(9)
88.65(20)
161.35(23) Cl(7)-Mn(6)-Cl(10)
O(14)-Mn(3)-O(114)
89.43(20) O(13)-Mn(6)-O(91)
Table 5. Selected Interatomic Distances (Å) and Angles (deg) for [Mn6O4Br4(Me2dbm)6] (10)
Mn(1)‚‚Mn(2)
Mn(1)‚‚Mn(3)
Mn(1)‚‚Mn(4)
Mn(1)‚‚Mn(6)
Mn(2)‚‚Mn(3)
Mn(2)‚‚Mn(5)
Mn(2)‚‚Mn(6)
Mn(3)‚‚Mn(4)
Mn(3)‚‚Mn(5)
Mn(4)‚‚Mn(5)
Mn(4)‚‚Mn(6)
Mn(5)‚‚Mn(6)
Mn(1)‚‚Mn(5)
3.200(4)
3.217(4)
3.223(4)
3.231(4)
3.215(4)
3.216(4)
3.195(4)
3.218(4)
3.207(4)
3.197(4)
3.223(4)
3.198(4)
4.535(4)
Mn3‚‚Mn6
4.546(4)
2.791(2)
2.809(2)
2.799(2)
2.794(2)
2.828(2)
2.762(2)
2.770(2)
2.749(2)
2.786(2)
2.789(2)
2.824(2)
2.770(2)
Mn(2)‚‚Mn(4)
Mn(1)-O(11)
Mn(1)-O(12)
Mn(1)-O(15)
Mn(1)-O(19)
Mn(2)-O(12)
Mn(2)-O(14)
Mn(2)-O(34)
Mn(4)-O(72)
Mn(4)-O(76)
Mn(5)-O(13)
Mn(5)-O(14)
Mn(5)-O(91)
4.541(4)
Mn(1)-Br(7)
Mn(2)-Br(7)
Mn(3)-Br(7)
Mn(1)-Br(8)
Mn(4)-Br(8)
Mn(6)-Br(8)
Mn(3)-Br(9)
Mn(4)-Br(9)
Mn(5)-Br(9)
Mn(2)-Br(10)
Mn(5)-Br(10)
Mn(6)-Br(10)
1.892(9)
1.896(8)
1.915(8)
1.921(8)
1.866(8)
1.880(8)
1.912(8)
1.903(9)
1.929(8)
1.860(8)
1.889(8)
1.914(8)
Mn(2)-O(38)
Mn(3)-O(11)
Mn(3)-O(14)
Mn(3)-O(53)
Mn(3)-O(57)
Mn(4)-O(11)
Mn(4)-O(13)
Mn(5)-O(95)
Mn(6)-O(12)
Mn(6)-O(13)
Mn(6)-O(110)
Mn(6)-O(114)
1.930(8)
1.871(8)
1.892(8)
1.911(9)
1.901(9)
1.908(8)
1.874(8)
1.909(9)
1.920(8)
1.922(8)
1.921(8)
1.906(9)
O(11)-Mn(1)-O(12)
O(11)-Mn(1)-O(15)
93.3(3) O(11)-Mn(4)-O(13)
89.2(3) O(11)-Mn(4)-O(72)
92.2(3) Mn(1)-O(11)-Mn(3) 116.9(4)
88.9(4) Mn(1)-O(11)-Mn(4) 116.0(4)
178.1(4) Mn(3)-O(11)-Mn(4) 116.7(4)
178.7(4) Mn(1)-O(12)-Mn(2) 116.6(4)
89.2(3) Mn(1)-O(12)-Mn(6) 115.7(4)
89.6(4) Mn(2)-O(12)-Mn(6) 115.1(4)
Mn(4)-O(13)-Mn(5) 117.8(4)
Mn(4)-O(13)-Mn(6) 116.2(4)
Mn(5)-O(13)-Mn(6) 115.5(4)
Mn(2)-O(14)-Mn(3) 116.9(4)
Mn(2)-O(14)-Mn(5) 117.1(4)
Mn(3)-O(14)-Mn(5) 116.1(4)
69.70(4) Mn(4)-Br(9)-Mn(5) 70.57(4)
O(11)-Mn(1)-O(19) 176.5(4) O(11)-Mn(4)-O(76)
O(12)-Mn(1)-O(15) 177.4(4) O(13)-Mn(4)-O(72)
O(12)-Mn(1)-O(19)
O(15)-Mn(1)-O(19)
O(12)-Mn(2)-O(14)
88.2(3) O(13)-Mn(4)-O(76)
89.2(3) O(72)-Mn(4)-O(76)
93.3(3) O(13)-Mn(5)-O(14)
92.3(3) Mn(1)-Br(7)-Mn2
89.8(3) Mn(1)-Br(7)-Mn(3)
173.9(4) Mn(2)-Br(7)-Mn(3)
176.0(4) Mn(1)-Br(8)-Mn(4)
89.4(4) Mn(1)-Br(8)-Mn(6)
88.8(4) Mn(4)-Br(8)-Mn(6)
93.1(3) Mn(3)-Br(9)-Mn(4)
175.7(4) Mn(3)-Br(9)-Mn(5)
88.8(3) Br(7)-Mn(1)-Br(8)
O(12)-Mn(2)-O(34) 176.5(4) O(13)-Mn(5)-O(91)
70.02(4) Mn(2)-Br(10)-Mn(5)
69.89(4)
70.15(4)
69.72(4)
166.1(4)
O(12)-Mn(2)-O(38)
O(14)-Mn(2)-O(34)
89.5(3) O(13)-Mn(5)-O(95)
87.7(4) O(14)-Mn(5)-O(91)
69.96(4) Mn(2)-Br(10)-Mn(6)
69.95(4) Mn(5)-Br(10)-Mn(6)
71.11(4) Br(8)-Mn(4)-Br(9)
O(14)-Mn(2)-O(38) 176.7(4) O(14)-Mn(5)-O(95)
O(34)-Mn(2)-O(38)
O(11)-Mn(3)-O(14)
O(11)-Mn(3)-O(53)
89.6(4) O(91)-Mn(5)-O(95)
91.7(4) O(12)-Mn(6)-O(13)
91.4(4) O(12)-Mn(6)-O(110)
70.41(4) Br(9)-Mn(5)-Br(10) 166.4(4)
71.33(4) Br(8)-Mn(6)-Br(10) 168.3(4)
70.53(4)
167.3(4)
O(11)-Mn(3)-O(57) 176.0(4) O(12)-Mn(6)-O(114)
O(14)-Mn(3)-O(53) 176.4(4) O(13)-Mn(6)-O(110)
O(14)-Mn(3)-O(57)
O(53)-Mn(3)-O(57)
89.1(3) Br(7)-Mn(2)-Br(10) 167.9(4)
87.4(4) O(13)-Mn(6)-O(114)
89.6(4) O(110)-Mn(6)-O(114)
175.6(4) Br(7)-Mn(3)-Br(9)
167.2(4)
89.2(4)
is interesting to note, however, that other bond distances and
angles in the core are (essentially) unaffected by the Br--for-
Cl- substitution. This will be of relevance to the magnetochemi-
cal discussion (vide infra).
Only the [Ti6(µ3-O)4(µ3-Cl)4] core of [(C5H5Me)6Ti6O4Cl4]20
contains, as 9, four O2- and four Cl- bridges, but the structure
(19) Lee, S. L.; Holm, R. H. Angew. Chem., Int. Ed. Engl. 1990, 29,
840 and references therein.
(20) (a) Roth, A.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J. Am. Chem.
Soc. 1986, 108, 6823. (b) Carofiglio, T.; Floriani, C.; Roth, A.; Sgamellotti,
A.; Rosi, M.; Chiesi-Villa, A.; Rizzoli, C. J. Organomet. Chem. 1995, 488,
141.
Although many [M6(µ3-X)8] face-capped metal octahedra are
known,19 only a relative few contain two types of X group, e.g.,
the [Ti6O4Cl4],20 [Ti6O6Cl2],20 [Ti6Te6O2],21 and [Re6YxZ8-x
]
(x ) 5, Y ) S or Se, Z ) Cl; x ) 6, Y ) S, Z ) Cl)22 cores.
(21) Gindelberger, D. E. Acta Crystallogr., Sect. C 1996, 52, 2493.

Hexanuclear MnIII Clusters
J. Am. Chem. Soc., Vol. 121, No. 23, 1999 5495
Table 6. Comparison of the [Mn6O4X4]6+ Core Parameters (Å,
deg) of 9 (X ) Cl) and 10 (X ) Br)
Cla
Bra
Mn‚‚‚Mnb
Mn‚‚Mnc
Mn-X
3.204(9)
4.531(3)
2.653(20)
1.886(7)
3.212(11)
4.541(4)
2.789(23)
1.889(2)
Mn-Od
X-Mn-X
O-Mn-O
Mn-O-Mn
Mn-X-Mn
162.1(7)
167.2(8)
93.1(6)
116.3(5)
74.3(5)
92.7(6)
116.4(7)
70.3(5)
a Averaged using virtual Td symmetry; numbers in parentheses are
the standard deviation from the mean. b Adjacent atoms. c Opposite
atoms. d µ3-O atoms.
Table 7. 1H NMR Spectral Dataa for Complexes 1-12
complex
chemical shift/ppmb,c
[MnCl(dbm)2] (1)
5.4 (m), 2.08 (p), 54.7 (CH)
[MnBr(dbm)2] (2)
5.6 (m), 2.04 (p), 58.6 (CH)
[Mn(dbm)3] (3)
5.8 (o), 8.00 (m), 3.63 (p), 23.4 (CH)
5.1 (m), 57.6 (CH), 1037 (Me)
5.3 (m), 59.9 (CH), 10.88 (Me)
5.5 (o), 7.76 (m), 24.0 (CH), 7.47 (Me)
5.1 (m), 60 (CH), 9.09 (CH2), 1.36 (Me)
5.4 (m), 63 (CH), 9.48 (CH2), 1.49 (Me)
3.9 (m), 80-85 (CH),d 18.15 (Me)
[MnCl(Me2dbm)2] (4)
[MnBr(Me2dbm)2] (5)
[Mn(Me2dbm)3] (6)
[MnCl(Et2dbm)2] (7)
[MnBr(Et2dbm)2] (8)
[Mn6O4Cl4(Me2dbm)6] (9)
[Mn6O4Br4(Me2dbm)6] (10) ∼4 (m),e 80-85 (CH),d 18.18 (Me)
[Mn6O4Cl4(Et2dbm)6] (11)
[Mn6O4Br4(Et2dbm)6] (12)
4.0 (m), ∼83 (CH), 15.07 (CH2), 1.15 (Me)
4.3 (m), ∼83 (CH), 14.97 (CH2), 1.27 (Me)
a In CDCl3 at ∼23 °C. b On the δ scale; shifts downfield are positive.
c Very broad signals are given to 1 decimal place ((0.2 ppm) or to the
nearest integer ((1 ppm); other signals are to two decimal places and
are (0.05 ppm. d Poorly defined. e Poorly defined; in CD2Cl2, δ ) 4.1
ppm.
1
Figure 5. 500 MHz H NMR spectra in CDCl3 of [MnBr(Et2dbm)2]
(8, top) and [Mn6O4Br4(Et2dbm)6] (12, bottom); the insets are expansion
of the indicated regions.
does not approximate to a tetrahedron. [MnIIIX8] octahedral
6
species have been unknown to date, although a [Mn6(µ3-O)4-
(µ3-X)4]4+ unit as found in 9 and 10 but at the 2MnII, 4MnIII
level is also a recognizable fragment within the higher nuclearity
clusters [Mn10O4X12(biphen)4]4- (biphen ) 2,2′-biphenoxide).4d
In the latter, four additional MnII atoms are attached to the core
O2- ions, making them µ4.
paramagnetic shifts for the R2dbm ligands from their diamag-
netic positions are all greater for the MnX(R2dbm)2 complexes
than for the Mn(R2dbm)3 complexes (e.g., compare 1/2 with 3,
and 4/5 with 6) even though they both contain S ) 2 MnIII
ions. This is possibly due to greater spin delocalization onto
each of two rather than three R2dbm ligands in the five-
coordinate complexes and perhaps greater dipolar contributions
to the observed shifts in the lower symmetry five-coordinate
complexes.
The Mn6 complexes 9-12 all show very similar NMR spec-
tra. The spectrum for 12 in Figure 5 (bottom) displays a CH
resonance even more broadened and shifted to ∼83 ppm, and
the other peaks also show slightly greater line widths than the
mononuclear species. The o-H resonances are again broadened
beyond detection. The appearance of only one set of R2dbm
resonances is consistent with effective Td symmetry in solution
and thus supports retention of the solid-state structure on
dissolution. It had been suspected that any fragmentation of the
Mn6 clusters would likely yield MnX(R2dbm)2 and/or Mn-
(R2dbm)3 as at least some of the products, but no evidence of
these or other species was observed in the NMR spectra. The
NMR results thus suggest that structural integrity is retained
on dissolution, and this in turn suggests the feasibility of further
reactivity investigations on the [Mn6O4X4] cores. The results
on the Mn6 complexes are as found for many other [MnxOy]
clusters2,18 where the oxide bridges serve to help retain the
structure on dissolution, at least in MeCN and chlorinated hy-
drocarbon solvents.
1
1H NMR Spectroscopy. H NMR spectroscopy is a conve-
nient and useful method to probe the solution behavior of
paramagnetic species. In the present case, an important question
to be addressed was whether the Mn6 clusters retain their
structure on dissolution. Complexes 1-12 are all soluble to a
sufficient extent in CH2Cl2 and CHCl3, and NMR spectra were
recorded both in CD2Cl2 and CDCl3. The obtained data were
very similar in the two solvents, and only the CDCl3 data will
1
be detailed. The H NMR data are collected in Table 7, and
representative spectra for the bromo complexes 8 and 12 are
presented in Figure 5. Peak assignments were made by
consideration of peak integrations, the relative broadnesses, and
the effect of Me or Et substitution at the para position of the
dbm- phenyl rings.
The complexes are all highly paramagnetic, and the NMR
resonances are thus greatly shifted and broadened compared with
those of a diamagnetic system. The spectrum for 8 in Figure 5
(top) shows a very broadened CH resonance at ∼63 ppm,
(broadness r-6, where r is the Mn-H distance). The ortho
hydrogens on the Et2dbm ligands are broadened beyond
detection and could not be safely located in any of the MnX-
(R2dbm)2 complexes; only in the Mn(R2dbm)3 complexes could
very broad o-H resonances be detected at ∼5.8 ppm. The
DC Magnetic Susceptibility Studies. Variable-tempera-
(22) Dolbecq, A.; Boubekeur, K.; Batail, P.; Canadell, E.; Auban-Senzier,
P.; Coulon, C.; Lerstrup, K.; Bechgaard, K. J. Mater. Chem. 1995, 5, 1707.
ture magnetic susceptibility studies were performed on po-

5496 J. Am. Chem. Soc., Vol. 121, No. 23, 1999
Arom´ı et al.
wdered samples of complexes 1‚0.08CH2Cl2, 4‚0.2CH2Cl2, 9‚
0.4CH2Cl2, and 12.
Complexes 1 and 4 exhibit behavior expected for high-spin
MnIII (S ) 2) centers exhibiting zero-field splitting (ZFS). To
characterize the ZFS parameter D further, magnetization vs field
data were collected in the 2.00-4.00 K range in fields of 10.0-
50.0 kG. Fits of the data (vide infra for method) gave S ) 2, g
) 1.96, and D ) -1.9 cm-1 for 1, and S ) 2, g ) 1.93, and
D ) -1.9 cm-1 for 4.
For complexes 9 and 12, magnetic susceptibility data were
collected in the range 2.00-320 K in an applied field of 10.0
kG (1 T). For complex 9, the µeff/µB (ømT/cm3 K mol-1) values
slowly increase from 16.01 (32.04) at 320 K to a maximum of
24.27 (73.63) at 30.1 K and then decrease to 13.69 (23.43) at
2.00 K (Figure 6). The data for complex 12 were very similar
to those for 9, with values of 16.73 (34.94), 24.60 (75.65), and
13.86 (24.01) at 300, 40.0, and 2.00 K, respectively. The
maxima are similar to the 24.99 (78.0) values of a S ) 12 spin
system with g ) 2.00, suggesting the presence of ferromagnetic
coupling in 9 and 12; the drop at the lowest temperatures is
likely due to zero-field splitting, Zeeman effects, and intermo-
lecular interactions.
Figure 6. Plot of effective magnetic moment (µeff) per molecule vs T
for [Mn6O4Cl4(Me2dbm)6] in a 10.0 kG DC field. The solid line is a fit
of the 40.0-300 K data to the appropriate theoretical equation; see the
text for the fitting parameters. The inset compares the 2.00-40.0 K
DC data (b) with the in-phase (øm′) AC data (2).
The Heisenberg exchange Hamiltonian appropriate for a Mn6
octahedron is given in eq 5,
Hˆ ) -2Jcis(Sˆ1Sˆ2 + Sˆ1Sˆ3 + Sˆ1Sˆ4 + Sˆ1Sˆ6 + Sˆ2Sˆ3 + Sˆ2Sˆ5 +
Sˆ2Sˆ6 + Sˆ3Sˆ4 + Sˆ3Sˆ5 + Sˆ4Sˆ5 + Sˆ4Sˆ6 + Sˆ5Sˆ6) -
2Jtrans(Sˆ1Sˆ5 + Sˆ2Sˆ4 + Sˆ3Sˆ6) (5)
temperatures. Good fits were obtained for both 9 and 12 with
fitting parameters (in the format 9/12) of Jcis ) +8.6/+8.5 cm-1
,
Jtrans ) +0.33/-0.31 cm-1, and g ) 1.96/1.98, with temperature
independent paramagnetism (TIP) held constant at 1200 × 10-6
cm3mol-1 (solid line in Figure 6). Owing to the small value of
where Si refers to the spin of metal Mni, and Jcis and Jtrans are
the pairwise exchange parameters for adjacent and opposite
metals of the octahedron, respectively; the Mn labeling scheme
of Figures 2 and 3 is employed. This Hamiltonian can be
transformed into an equivalent form (eq 6)
J
trans, a fit with Jtrans fixed at zero was also attempted and gave
an excellent fit with Jcis ) +8.6/+8.5 cm-1 and g ) 1.97/1.98.
The latter fits with Jtrans ) 0 are preferred, since they are
superimposable to the former fits and involve fewer fitting
parameters. Note that Jtrans is not necessarily zero but it is very
small and therefore difficult to quantify reliably.
2
2
Hˆ ) -Jcis(SˆT2 - SˆA2 - SˆB2 - SˆC ) - Jtrans(SˆA2 + SˆB
+
2
SˆC2 - Sˆ12 - Sˆ22 - Sˆ32 - Sˆ42 - Sˆ52 - Sˆ6 ) (6)
The fits indicate that the Mn6 complexes 9 and 12 have ST )
12 ground states. In the notation (ST, SA, SB, SC), this is the
(12, 4, 4, 4) state in which all six MnIII spins are aligned parallel.
The first excited state is a triply degenerate set of ST ) 11 states
comprising the (11, 3, 4, 4), (11, 4, 3, 4), and (11, 4, 4, 3) states
at 138 and 136 cm-1 above the ground state for 9 and 12,
respectively. The S ) 12 ground state in these Mn6 complexes
is thus well-isolated from the nearest excited state.
by use of the Kambe vector coupling method23a,b and the
substitutions SˆA ) Sˆ1 + Sˆ5, SˆB ) Sˆ2 + Sˆ4, SˆC ) Sˆ3 + Sˆ6 and SˆT
) SˆA + SˆB + SˆC where ST is the resultant spin of the complete
molecule. There are a total of 1751 ST states for a Mn6III
octahedron (Si ) 2) ranging from a single ST ) 12 state to 65
different ST ) 0 states. From eq 6 can be obtained the energy
expression (eq 7) for the
To confirm the S ) 12 ground states and to obtain values of
the ZFS parameter D, magnetization data for 9 and 12 were
collected in the 2.00-15.0 K range with applied fields in the
0.500-50.0 kG range. Plots of reduced magnetization (M/NµB)
vs H/T for 9 are given in Figure 7. The M/NµB values for 9 and
12 saturate at values of 23.02 and 23.62, respectively, with each
isofield data set virtually superimposable for each complex. The
expected saturation value for an S ) 12 state with g ) 2.00
and no ZFS is 24.00, which is close to the observed values.
This and the virtually superimposable isofield lines indicate that
the ground states of these Mn6 complexes are characterized by
very small values of the ZFS parameter D; this would be
consistent with the virtual Td symmetry of these complexes.
The M/NµB vs H/T data for 9 and 12 were fit by assuming
only the ground state is populated in the 2.00-15.0 K range. A
spin Hamiltonian was constructed which incorporated isotropic
E(ST) ) -Jcis[ST(ST + 1) - SA(SA + 1) - SB(SB + 1) -
SC(SC + 1)] - Jtrans[SA(SA + 1) + SB(SB + 1) +
SC(SC + 1)] (7)
energies, E(ST), of each ST state; constant terms contributing
equally to all states have been omitted from eq 7. Using eq 7
and the Van Vleck equation,23c a theoretical øm vs T equation
was derived,23d and this was used to fit23e the experimental data
for 9 and 12. Only data for the 30.0-300 K range were used,
since the model does not incorporate ZFS and other minor
effects and therefore cannot reproduce the decrease at lower
(23) (a) Kambe, K. J. Phys. Soc. Jpn. 1950, 5, 48. (b) Cornia, A.;
Gatteschi, D.; Hegetschweiler, K. Inorg. Chem. 1994, 33, 1559. (c) Carlin,
R. L. Magnetochemistry, Springer-Verlag: New York, 1986. (d) The
equation contains ∼1750 terms in each of the numerator and denominator
and is thus too long to be presented. See Supporting Information. (e) For
the fitting program, see: Schmitt, E. A. Ph.D. Thesis, University of Illinois
at Urbana-Champaign, 1995.
2
Zeeman splitting and axial ZFS terms (DSˆz ). This was used to
construct the energy matrix for the ST ) 12 ground state, which
has (2S + 1)2 ) 625 matrix elements. An iterative fitting
program2a,23e which diagonalizes the matrix on each cycle was

Hexanuclear MnIII Clusters
J. Am. Chem. Soc., Vol. 121, No. 23, 1999 5497
vectorial addition of single ion anisotropies should sum to zero.
This is borne out by the experimental observation of D ≈ 0 for
9 and 12.
One might ask why the exchange interactions in these Mn6
complexes are ferromagnetic. Consideration of this question
benefits from comparison with the [Mn4O3X(O2CR)3(dbm)3] (X
) Cl-, Br- and various others) complexes,18 which contain a
[Mn4(µ3-O)3(µ3-X)]6+ (3 MnIII, MnIV) distorted-cubane core of
C
3V symmetry: the MnIII/MnIV and MnIII/MnIII interactions are
antiferromagnetic (J34 ≈ -28 to -30 cm-1) and ferromagnetic
(J33 ≈ 8 cm-1), respectively, with the MnIII pairs bridged by
one X- and one O2- ion, as shown, i.e., the same as in the Mn6
complexes.
Unfortunately, no dinuclear complexes with the [Mn2(µ-O)-
(µ-X)] core are known for comparison, but this unit within both
the Mn4 and Mn6 clusters leads to ferromagnetic interactions.
In both species, the MnIII JT elongation axes intersect at X-
ions: taking this as the local z axis means the singly occupied
dz2 orbitals will intersect at the X- ion. Regardless of whether
this σ-superexchange pathway leads to ferro- or antiferromag-
netic interactions, the very long Mn-X bonds likely make this
a minor contributor to the overall interaction. The dominant
superexchange pathways will likely be through the O2- bridges,
which also rationalizes why Br-for-Cl substitution has essentially
no effect on the J33 parameter in the Mn4 complexes or Jcis in
9 vs 12. A σ pathway through the O2- bridges, which would
normally give strong antiferromagnetic coupling at these Mn-
O-Mn angles (115-117°) if the metal dσ orbitals were half-
occupied, is nonoperative in these Mn4 and Mn6 species because
Figure 7. Plot of reduced magnetization (M/NµB) vs H/T for
[Mn6O4Cl4(Me2dbm)6] (9) showing data collected in the 2.00-4.00 K
ranges and fields up to 50 kG. The solid line is a fit of the data with
fitting parameters given in the text.
used to least-squares fit the magnetization data by use of the
expression in eq 8,
M ) ø H ) N[ (-δE/δH)exp(-E/kT)]/ exp(-E/kT)]
∑
∑
m
(8)
where δE/δH is the change in the energy of a given matrix
element in response to the magnetic field. The program
functioned by stepping through various values g and D for a
given spin state in order to find the minimum error between
the experimental and calculated susceptibilities. Excellent fits
were obtained for 9/12 with ST ) 12, g ) 1.94/1.98 and D )
+0.007/-0.011 cm-1. Given the very small values of D, the
latter was fixed at D ) 0 cm-1 and excellent fits were again
obtained with S ) 12 and g ) 1.94/1.98. The two sets of fits
were essentially superimposable, and we thus conclude that |D|
< 0.01 cm-1 for both 9 and 12. High-frequency EPR experi-
ments are in progress to determine D values more precisely.
The ZFS of the ground state of a polynuclear MnIII complex
is largely a consequence of the vectorial addition of the single-
ion ZFS tensors. A large Jahn-Teller (JT) distortion is typically
seen in an octahedral MnIII ion, in which a unique axis is formed
by two noticeably longer Mn-L bond distances. This axis often
defines the unique axis of the magnetic structure of the MnIII
ion; for example, the g and ZFS tensors will typically have their
principal axis along the JT axis. Single-ion ZFS in MnIII ions
can be very large; for example, high-frequency EPR experiments
have shown that D ) -4.35 cm-1 in Mn(dbm)3.24 When all JT
axes are oriented parallel, the resultant ZFS of a MnIxII complex
will be quite large; for example, in [Mn12O12(O2CR)16(H2O)4]
complexes (8 MnIII, 4 MnIV), the 8 MnIII ions have their JT
axes essentially parallel, and the net ZFS in the molecule is
fairly large (D ) -0.5 cm-1).2,9-11 On the other hand, when
the JT axes do not all point in the same direction, the resultant
ZFS will be small, as the contributions from the individual sites
partially or completely cancel out. In the Mn6 complexes 9 and
12, the high symmetry of the molecule (∼Td) means that the
the dσ (dx -y ) orbitals pointing at the O2- ions are empty. The
overall coupling between any two MnIII ions thus becomes the
balance of several weaker ferromagnetic (JF) and antiferromag-
netic (JAF) contributions, all of which are likely to be of
comparable magnitude; the overall Jobs should therefore be weak
and either side of zero, as was found. Possible contributions
2
2
include d1 - Oπ - d1 π-overlaps (JAF) and dz12 - Oσ - dx -y
2
2
π
π
2
2
2
σ-overlaps (JF) between empty dx -y and occupied dz (via
“doughnuts” in the xy plane). In both the Mn4 and Mn6 cases,
the JF contributions win out to give a positive overall J.
AC Magnetic Susceptibility Studies. Since a large ground-
state spin value of S ) 12 had been characterized for the Mn6
complexes, their AC magnetic susceptibilities were investigated
to check for the presence of slow magnetic relaxation. Other
molecules with large spin ground states, such as the [Mn12O12-
(O2CR)16(H2O)4] complex with S ) 10, have been shown to
act as superparamagnets at near liquid He temperatures, as
shown by the presence of an out-of-phase AC susceptibility
signal (øm′′) for fields oscillating at frequencies e1500 Hz.2,6
A superparamagnet is a subtype of a magnet in which the
number of interacting spin carriers is too small to form more
than one domain, and thus it is a single-domain magnetic particle
consisting of up to several thousand interacting metal ions/spin
carriers yielding a large net spin. A superparamagnet exhibits
slow magnetic relaxation (reorientation of its magnetization
vector), resulting in an out-of-phase signal when examined by
AC susceptibility. The Mn12 complex is thus one of the smallest
superparamagnets, consisting of only 12 interacting spin carriers
and yielding a net spin of S ) 10; similarly certain Mn4,8 V4,25
(24) Barra, A.-L.; Gatteschi, D.; Sessoli, R.; Abbati, G. L.; Cornia, A.;
Fabretti, A. C.; Uytterhoeven, M. G. Angew. Chem., Int. Ed. Engl. 1997,
36, 2329.
5c,11b
Fe8,12 and Mn11
clusters are other examples of molecular
superparamagnets which have been termed single-molecule

5498 J. Am. Chem. Soc., Vol. 121, No. 23, 1999
Arom´ı et al.
magnets8a to reflect their unusual properties. The slow relaxation
is caused by magnetic anisotropy resulting from a combination
of a large ground-state spin (S) value and a large and negative
zero-field splitting (ZFS) parameter D. This results in a potential
energy barrier to relaxation of S2|D|, or ∼50 cm-1 for S ) 10
and D ≈ -0.5 cm-1 as found in the Mn12 complexes.2,6,26 The
Mn6 complexes 9 and 12 have a large spin of S ) 12 but
virtually no magnetic anisotropy as reflected in D values of
e0.01 cm-1, corresponding to an energy barrier for magnetiza-
tion reversal of e1.44 cm-1; the latter is much too small to
cause slow magnetic relaxation and the appearance of øm′′
signals in AC susceptibility studies. Nevertheless, it was hoped
that, as for the V4 complexes [V4O2(O2CR)7(L-L)2]z (L-L )
bpy, z ) +1; pic, z ) -1),25 application of a DC field would
cause the rate of magnetization reversal to decrease sufficiently
to permit observation of øm′′ signals. The AC susceptibilities
of complexes 9 and 12 were collected initially in zero DC field
over the 2.00 to 40.0 K temperature range in a 1 G field
oscillating at 997 Hz. Both complexes gave essentially the same
behavior, and only the results for 9 will thus be detailed. The
insert to Figure 6 contains the AC susceptibility data for 9, in
which the in-phase susceptibility (øm′) is plotted as µeff/µB and
compared with the DC effective magnetic moment in a 10.0
kG DC field. The out-of-phase signal (øm′′) was zero at all
temperatures down to 2.00 K. The AC µeff value was constant
at 24.2 µB in the 2.00-40.0 K range in contrast to the DC µeff
value which decreased with decreasing temperature. The
expected spin-only (g ) 2.0) µeff value for a S ) 12 state is
25.0 µB, very close to the observed AC value. As no øm′′ signal
was observed, the difference between the AC and DC data sets
arises not from relaxation effects but from the effects of the
10.0 kG DC field.
An interesting question arises, however. What is the AC
susceptibility response in the presence of a modest DC field?
The AC susceptibilities of 9 and 12 were measured over a wide
range of applied DC fields in the 0.500-7.00 kG range between
1.71 and 10.0 K. Out-of-phase signals were observed in all
isofield data sets, indicating slow magnetic relaxation; one set
of experimental data for complex 9 at 2.00 K is shown in Figure
8. The data are presented in the form of normalized Cole-
Cole (or Argand) plots, in which the out-of-phase (øm′′) and
in-phase (øm′) susceptibilities obtained at a given AC frequency
are plotted as a single point, with øm′′ plotted as a function of
øm′. For each AC frequency used (0.74-1512 Hz), a different
datum point is plotted. At 2.00 K, each Argand plot is clearly
the sum of two overlapping arcs that are only partially defined
by the experimental data. The Argand plots at 4.50 K (not
shown) are also clearly composed of two arcs. For a single
relaxation process, an Argand diagram has the form of a single
arc in which the point at the arc’s maximum indicates the
frequency for which the condition ωτ ) 1 is met,27 where ω is
the angular frequency of the AC field (ω ) 2πν) and τ is the
characteristic lifetime for that relaxation process. When the
relaxation process is characterized by a single lifetime, the arc
will form a semicircle that has its origin on the øm′ axis. The
relaxation process more typically will be characterized by a
Figure 8. Argand plot for [Mn6O4Cl4(Me2dbm)6] (9) at 2.00 K at AC
frequencies in the 0.74-1512 Hz range and applied DC fields of 500
(0), 1000 (b), 2000 (3), 3000 (9) and 6000 (]) G.
distribution of lifetimes, and the semicircle has its origin below
the øm′ axis. In such instances, as for Mn6, it is more appropriate
to speak of an average τ for the relaxation process. What is
immediately apparent is that there are two relaxation processes
for Mn6 in the presence of an applied DC field, each of which
has an average relaxation rate constant (k ) 1/τ) near that of
the oscillating AC field (0.74-1512 Hz).
There are three basic relaxation mechanisms: the direct,
Raman, and Orbach processes.27b,28 Each mechanism involves
the absorption of lattice vibrational (phonon) energy to induce
changes in the populations of different energy levels, with
different dependencies upon field (H) and temperature (T). In
the simplest picture of magnetic relaxation, a molecule converts
between two states (|a and |b ) that differ in energy by ∆. The
direct process involves the absorption of one phonon whose
energy matches ∆, which results in a transition between the
levels |a and |b . The Raman process involves an excited virtual
energy level (|c ) and operates by the spontaneous absorption
and emission of two phonons whose energy difference matches
∆. The Orbach process is similar to the Raman process, but the
excited energy level |c is in thermal equilibrium with the levels
|a and |b . Direct processes often dominate relaxation mech-
anisms at low temperatures but tend to have a limiting
dependence upon H and T (k H2T).23c Raman mechanisms
have a large dependence upon temperature (k T7-9),23c and
therefore are less likely to dominate relaxation mechanisms at
the low temperatures studied here. The most likely origin of
the two observed relaxation processes is a thermally activated
(Orbach) relaxation process.
Conclusions
Convenient procedures have been developed to the five-
coordinate complexes [MnX(R2dbm)2] (X ) Cl, Br; R ) H,
Me, Et), containing high-spin MnIII in square-pyramidal coor-
dination geometry. The R ) Me and Et derivatives have proved
to be excellent starting points for the synthesis of a new class
of Mn cluster of formulation [Mn6O4X4(R2dbm)6] (X ) Cl, Br).
These clusters contain a MnI6II octahedron with a combination
(25) Castro, S. L.; Sun, Z.; Grant, C. M.; Bollinger, J. C.; Hendrickson,
D. N.; Christou, G. J. Am. Chem. Soc. 1998, 120, 2365.
(26) (a) Aubin, S. M. J.; Spagna, S.; Eppley, H. J.; Sager, R. E.; Christou,
G.; Hendrickson, D. N. Chem. Commun. 1998, 803. (b) Aubin, S. M. J.;
Sun, Z.; Gruzei, I. A.; Rheingold, A. L.; Christou, G.; Hendrickson, D. N.
Chem. Commun. 1997, 2239.
(27) (a) van Duyneveldt, A. J., Lakeshore Cryotonocs, Inc., application
note for model 7000 AC susceptometer, Lakeshore Cryotonics, Westerville,
Ohio. (b) van Duyneveldt, A. J. In Magnetic Molecular Materials; Gatteschi,
D., Kahn, O., Miller, J., Palacio, F., Eds., Kluwer Academic Publishers:
London, 1991.
(28) Abragam, A.; Bleaney, B. Electron Paramagnetic Resonance of
Transition Ions; Dover Press: Mineola, New York, 1986.

Hexanuclear MnIII Clusters
J. Am. Chem. Soc., Vol. 121, No. 23, 1999 5499
of µ3-O2- and µ3-X- bridges on the faces. In addition to its
structural aesthetics, this cluster type has been found to be a
rare example of a completely ferromagnetically coupled multi-
nuclear species, and as a result, these Mn6 species have S ) 12
ground states, the joint-highest yet observed for Mn and one of
the highest for any metal. Three other S ) 12 Mn clusters are
also known, and two of these also happen to be hexanuclear,4c,h
the third being decanuclear.4d The present Mn6 clusters thus
join a still small but continually growing family of high-spin
(S g 4) clusters, the majority of which are in Mn chemistry.
Unfortunately, however, the present Mn6 clusters are not new
additions to the small group of species exhibiting the new
magnetic phenomenon of single-molecule magnetism. This can
be explained by their high (Td) symmetry, which averages out
to zero the magnetic anisotropy resulting from single-ion zero-
field splitting effects in Jahn-Teller distorted MnIII. Conse-
quently, although one requirement for a single-molecule magnet
(SMM) is fulfilled, i.e., a high-spin ground state, the second
requirement of a large, negative ZFS parameter D is not fulfilled.
The Mn6 species have D values e0.01 cm-1, too small to yield
a sufficiently large barrier S2|D| to magnetization reversal. As
a result, they do not exhibit the slow magnetic relaxation that
would yield, for example, magnetization hysteresis curves.
Nevertheless, the Mn6 complexes represent an important step
forward in the overall objective of developing synthetic
procedures to high-spin clusters and understanding both (i) the
factors that yield large ground-state spin values and reason-
ably large magnetic anisotropies and (ii) the behavior of such
species in DC and/or AC magnetic fields. This understanding
is important to the future progress of the new field of single-
molecule magnetism.
Acknowledgment. This work was supported by the National
Science Foundation (D.N.H. and G.C.). The AC measurements
were performed with a MPMS2 SQUID magnetometer provided
by the Center for Interface and Material Science, funded by
the W. M. Keck Foundation.
Supporting Information Available: Tables of crystal data,
structure solution and refinement, atomic coordinates, aniso-
tropic thermal parameters, and bond distances and angles for
complexes 1, 4, 9 and 10 and unit cell packing diagrams for
1 and 4; listing of the ST states for an MnIII octahedron (PDF)
6
and a crystallographic file in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA983446C