Tetranuclear manganese carboxylate complexes with a trigonal pyramidal metal topology via controlled potential electrolysis

Article abstract of DOI:10.1021/ic991068m

Controlled potential electrolysis (CPE) procedures are described that provide access to complexes with a [Mn₄(μ₃-O)₃(μ₃-O₂CR)]⁶⁺ core (3Mn(III),Mn(IV)) and a trigonal pyramidal metal topology, starting from species containing the [Mn₄(μ₃-O)₂]⁸⁺ core (4Mn(III)). [Mn₄O₂(O₂CMe)₆(py)₂(dbm)₂] (6): triclinic, P1, a = 10.868(3) A, b = 13.864(3) A, c = 10.625(3) A, α = 108.62(1)°, β = 118.98(1)°, γ = 89.34(2)°, V = 1307 A³, Z = 1, T = -131 °C, R (R(w)) = 3.24 (3.70)%. [Mn₄O₂(O₂CPh)₆(py)(dbm)₂] (8): monoclinic, P2₁/c, a = 14.743(6) A, b = 15.536(8) A, c = 30.006(13) A, β = 102.79(1)°, V = 6702 A³, Z = 4, T = -155 °C, R (R(w)) = 4.32 (4.44)%. Both 6 and 8 contain a [Mn₄O₂]⁸⁺ core; 8 only has one py group, the fourth Mn(III) site being five-coordinate. (NBu(n)₄)[Mn₄O₂(O₂CPh)₇(dbm)₂] (10) is available from two related procedures. CPE of 10 at 0.65 V vs ferrocene in MeCN leads to precipitation of [Mn₄O₃(O₂CPh)₄(dbm)₃] (11); similarly, CPE of 6 at 0.84 V in MeCN/CH₂Cl₂ (3:1 v/v) gives [Mn₄O₃(O₂CMe)₄(dbm)₃] (12). Complex 11: monoclinic, P2₁/n, a = 15.161(3) A, b = 21.577(4) A, c = 22.683(5) A, β = 108.04(3)°, V = 7056 A³, Z = 4, T = -100 °C, R (wR2) = 8.63 (21.80)%. Complex 12: monoclinic, P2₁/n, a = 13.549(2) A, b = 22.338(4) A, c = 16.618(2) A, β = 103.74(1)°, V = 4885 A³, Z = 4, T = -171 °C, R (R(w)) = 4.63 (4.45)%. Both 11 and 12 contain a [Mn₄(μ₃-O)₃(μ-O₂CR)] core with a Mn₄ trigonal pyramid (Mn(IV) at the apex) and the RCO₂^- bridging the Mn(III)₃ base. However, in 11, the carboxylate is η²,μ₃ with one O atom terminal to one Mn(III) and the other O atom bridging the other two Mn(III) ions, whereas in 12 the carboxylate is η¹,μ₃, a single O atom bridging three Mn(III) ions. Variable-temperature, solid-state magnetic susceptibility studies on 11 and 12 show that, for both complexes, there are antiferromagnetic exchange interactions between Mn(III)/Mn(IV) pairs, and ferromagnetic interactions between Mn(III)/Mn(III) pairs. In both cases, the resultant ground state of the complex is S = 9/2, confirmed by magnetization vs field studies in the 2.00-30.0 K and 0.50-50 kG temperature and field ranges, respectively.

Full text of DOI:10.1021/ic991068m

Inorg. Chem. 2000, 39, 1501-1513
1501
Tetranuclear Manganese Carboxylate Complexes with a Trigonal Pyramidal Metal
Topology via Controlled Potential Electrolysis
Sheyi Wang,1a Michael S. Wemple,1a Jae Yoo,1c Kirsten Folting,1a John C. Huffman,1a
Karl S. Hagen,1b David N. Hendrickson,*,1c and George Christou*,1a
Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington, Indiana
47405-7102, Department of Chemistry, Emory University, Atlanta, Georgia 30322, and Department of
Chemistrys0358, University of California at San Diego, La Jolla, California 92093-0358
ReceiVed September 7, 1999
Controlled potential electrolysis (CPE) procedures are described that provide access to complexes with a [Mn4-
(µ3-O)3(µ3-O2CR)]6+ core (3MnIII,MnIV) and a trigonal pyramidal metal topology, starting from species containing
the [Mn4(µ3-O)2]8+ core (4MnIII). [Mn4O2(O2CMe)6(py)2(dbm)2] (6): triclinic, P1h, a ) 10.868(3) Å, b ) 13.864(3)
Å, c ) 10.625(3) Å, R ) 108.62(1)°, â ) 118.98(1)°, γ ) 89.34(2)°, V ) 1307 Å3, Z ) 1, T ) -131 °C, R (Rw)
) 3.24 (3.70)%. [Mn4O2(O2CPh)6(py)(dbm)2] (8): monoclinic, P21/c, a ) 14.743(6) Å, b ) 15.536(8) Å, c )
30.006(13) Å, â ) 102.79(1)°, V ) 6702 Å3, Z ) 4, T ) -155 °C, R (Rw) ) 4.32 (4.44)%. Both 6 and 8 contain
a [Mn4O2]8+ core; 8 only has one py group, the fourth MnIII site being five-coordinate. (NBun )[Mn4O2(O2CPh)7-
4
(dbm)2] (10) is available from two related procedures. CPE of 10 at 0.65 V vs ferrocene in MeCN leads to
precipitation of [Mn4O3(O2CPh)4(dbm)3] (11); similarly, CPE of 6 at 0.84 V in MeCN/CH2Cl2 (3:1 v/v) gives
[Mn4O3(O2CMe)4(dbm)3] (12). Complex 11: monoclinic, P21/n, a ) 15.161(3) Å, b ) 21.577(4) Å, c ) 22.683(5)
Å, â ) 108.04(3)°, V ) 7056 Å3, Z ) 4, T ) -100 °C, R (wR2) ) 8.63 (21.80)%. Complex 12: monoclinic,
P21/n, a ) 13.549(2) Å, b ) 22.338(4) Å, c ) 16.618(2) Å, â ) 103.74(1)°, V ) 4885 Å3, Z ) 4, T ) -171
°C, R (Rw) ) 4.63 (4.45)%. Both 11 and 12 contain a [Mn4(µ3-O)3(µ-O2CR)] core with a Mn4 trigonal pyramid
(MnIV at the apex) and the RCO2- bridging the MnIII3 base. However, in 11, the carboxylate is η2,µ3 with one O
atom terminal to one MnIII and the other O atom bridging the other two MnIII ions, whereas in 12 the carboxylate
is η1,µ3, a single O atom bridging three MnIII ions. Variable-temperature, solid-state magnetic susceptibility studies
on 11 and 12 show that, for both complexes, there are antiferromagnetic exchange interactions between MnIII/
MnIV pairs, and ferromagnetic interactions between MnIII/MnIII pairs. In both cases, the resultant ground state of
the complex is S ) 9/2, confirmed by magnetization vs field studies in the 2.00-30.0 K and 0.50-50 kG temperature
and field ranges, respectively.
Introduction
photosystem II reaction center of green plants, which is
responsible for the light-driven oxidation of water to O2 gas.6-9
Manganese carboxylate cluster chemistry is of interest from
a variety of viewpoints, including magnetic materials and
bioinorganic chemistry. In the former area, it has been found
that such clusters often possess large numbers of unpaired
electrons,2 making them attractive as potential precursors to
molecule-based magnetic materials or as discrete, nanoscale
magnetic particles in their own right.2-5 In the bioinorganic
arena, a tetranuclear Mn cluster is an integral component in the
This Mn4-containing water oxidation complex (WOC) is the
site of water binding, deprotonation, and oxidative coupling to
O2, but the precise mechanism remains unestablished. EXAFS
studies have shown that a minimal structural fragment of the
WOC is a [Mn2(µ-O)2] unit with a Mn‚‚‚Mn separation of ca.
2.7 Å, and it has been suggested that the WOC contains two
such units in close proximity, although several structural
possibilities for the complete Mn4 unit still remain.7 During
function, the WOC cycles through five oxidation levels labeled
S0-S4, the highest spontaneously reductively eliminating O2
and returning to S0.6
(1) (a) Indiana University. (b) University of California at San Diego. (c)
Emory University.
(2) (a) Christou, G. In Magnetism: A Supramolecular Function: Kahn,
O., Ed.; NATO ASI Series; Kluwer: Dordrecht, The Netherlands,
1996; pp 383-410. (b) 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.
(3) (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.
In the last 15 years or so, a variety of tetranuclear clusters
have been synthesized as inorganic models of the WOC.10-12
Most of these have some structural/spectroscopic/physical
properties similar to those of the WOC, but no synthetic species
(5) Brechin, E. K.; Yoo, J.; Nakano, M.; Huffman, J. C.; Hendrickson,
D. N.; Christou, G. Chem. Commun. 1999, 783.
(4) (a) 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.
(b) Aubin, S. M. J.; Dilley, N. R.; Pardi, L.; Krzystek, J.; Wemple,
M. W.; Brunel, L.-C.; Maple, M. B.; Christou, G.; Hendrickson, D.
N. J. Am. Chem. Soc. 1998, 120, 4991. (c) 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.
(6) (a) Debus, R. J. Biochim. Biophys. Acta 1992, 1102, 269 and references
therein. (b) Manganese Redox Enzymes; Pecoraro, V. L., Ed.; Verlag
Chemie: Weinheim, Germany, 1992. (c) Penner-Hahn, J. E. Struct.
Bonding 1998, 90, 1.
(7) Yachandra, V. K.; Sauer, K.; Klein, M. P. Chem. ReV. 1996, 96, 2927.
(8) Ruttinger, W.; Dismukes, G. C. Chem. ReV. 1997, 97, 1.
(9) Tommos, C.; Babcock, G. T. Acc. Chem. Res. 1998, 31, 18.
10.1021/ic991068m CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/14/2000

1502 Inorganic Chemistry, Vol. 39, No. 7, 2000
Wang et al.
currently seems to reproduce the properties of the WOC in all
ways, and a satisfactory synthetic model of the WOC has thus
yet to be obtained.
Scheme 1
Although the mechanism of action of the WOC is uncertain,
it is clear that the binding, deprotonation, and oxidation of
substrate H2O molecules are coupled to and driven by the
sequentially increasing oxidation level of the WOC. For this
reason, we have recently been exploring the oxidation reactions
of certain synthetic Mn4 complexes in our possession, seeking
to identify the structural and reactivity consequences of oxida-
tion, particularly in the presence of H2O groups. Specifically,
we have found that electrochemical oxidation of [Mn4O2]8+
-
containing species such as [Mn4O2(O2CMe)6(py)2(dbm)2]
(4MnIII; dbm- is the anion of dibenzoylmethane) leads to a core
transformation and a resultant increase in the O2-:Mn ratio from
2:4 to 3:4, suggesting incorporation and deprotonation of a H2O
molecule.
In this paper are described the preparation and solid-state
structural and magnetochemical properties of the products of
two such oxidation products, both of formula [Mn4O3(O2CR)4-
(dbm)3]. In subsequent papers will be reported their solution
properties and site-specific ligand substitution reactions.13
CPh)8(dbm)]16 were prepared as described previously. The syntheses
below are summarized in Scheme 1: acacH ) acetylacetone (pentane-
2,4-dione); dbmH ) dibenzoylmethane; picH ) picolinic acid.
Na(dbm). A slurry of NaOH (1.06 g, 26.5 mmol) and dbmH (6.00
g, 26.8 mmol) in EtOH (45 mL) was stirred overnight to give a fine
yellow precipitate. The solvent was removed in vacuo and the yellow
solid washed with pentane (3 × 15 mL) to remove unreacted dbmH
and dried in air. The yield was essentially quantitative. Anal. Calcd
(found) for C15H11O2Na: C, 73.16 (72.89); H, 4.50 (4.51); Na, 9.34
(9.38).
[Mn6O2(O2CPh)10L4] (3). To a stirred yellow-brown solution of
[Mn3O(O2CPh)6(py)3](ClO4) (1.24 g, 1.00 mmol) in MeCN (30 mL)
was added acacH (0.31 mL, 3.0 mmol) dropwise to give a deep brown
solution. No immediate precipitate was observed, but slow formation
of a fine orange-brown solid began soon after. When precipitation was
judged complete (3 days), the orange-brown powder was collected by
filtration, washed with MeCN, and dried in air. The complex was
identified as [Mn6O2(O2CPh)10(py)2(MeCN)2] (3a) by IR spectral
comparison with authentic material. The yield was 45-60%. Use of
complex 2 also gives 3a in comparable yield.
Recrystallization of complex 3a from a DMF solution that was
carefully layered with a 1:1 (v/v) solution of Et2O/hexanes slowly gave
large brown crystals that were collected by filtration, washed with Et2O,
and dried in air. The complex was identified as [Mn6O2(O2CPh)10-
(DMF)4]‚C6H14 (3b) by IR, elemental analysis, and crystallography.
Anal. Calcd (found) for C88H92N4O26Mn6: C, 54.16 (54.15); H, 4.75
(4.5); N, 2.87 (3.15); Mn, 16.89 (16.6).
[Mn(dbm)3] (4). To a stirred solution of complex 1, [Mn3O(O2-
CMe)6(py)3](ClO4) (0.87 g, 1.0 mmol), in MeCN (25 mL) was added
solid dbmH (0.70 g, 3.1 mmol) in small portions. A brown precipitate
was rapidly formed. After several hours, the solid was collected by
filtration, washed with MeCN, and dried in air. Recrystallization from
CH2Cl2/hexanes, CHCl3/hexanes, or THF/Et2O gave well-formed black
crystals. The yield varies depending on the ratio of complex 1:dbmH
used. For example, when the ratio was 1:3, the yield was 20%; with
the ratio changed to 1:5, the yield was 53%. Anal. Calcd (found) for
C45H33O6Mn: C, 74.58 (74.8); H, 4.60 (4.6); Mn, 7.58 (7.6). Electronic
spectrum: λmax, nm (ꢀM, L mol-1 cm-1) in CH2Cl2: 368 (36 000), 284
(50 000).
Experimental Section
Syntheses. All manipulations were performed under aerobic condi-
tions, using materials as received, except where noted otherwise. Where
dried solvents were required, they were distilled from CaH2 (MeCN)
or Na/benzophenone (Et2O, THF). The complexes [Mn3O(O2CMe)6-
(py)3](ClO4) (1),14 [Mn3O(O2CEt)6(py)3](ClO4),15 [Mn3O(O2CPh)6(py)2-
(H2O)] (2),14 (NBun4)[Mn4O2(O2CPh)9(H2O)],16 and (NBun4)[Mn4O2(O2-
(10) (a) Wieghardt, K. Angew. Chem., Int. Ed. Engl. 1989, 28, 1153. (b)
Pecoraro, V. L. Photochem. Photobiol. 1988, 48, 249. (c) Christou,
G. Acc. Chem. Res. 1989, 22, 328. (d) Armstrong, W. H. In ref 6b,
pp 261-286.
(11) (a) Philouze, C.; Blondin, G.; Girerd, J.-J.; Guilhem, J.; Pascard, C.;
Lexa, D. J. Am. Chem. Soc. 1994, 116, 8557. (b) Chan, M. K.;
Armstrong, W. H. J. Am. Chem. Soc. 1990, 112, 4985. (c) Chandra,
S. K.; Chakraborty, P.; Chakravorty, A. J. Chem. Soc., Dalton Trans.
1993, 863. (d) Sakiyama, H.; Tokuyama, K.; Matsumura, Y.; Okawa,
H. J. Chem. Soc., Dalton Trans. 1993, 2329. (e) Kirk, M. L.; Chan,
M. K.; Armstrong, W. H.; Solomon, E. I. J. Am. Chem. Soc. 1992,
114, 10432. (f) Gedye, C.; Harding, C.; McKee, V.; Nelson, J.;
Patterson, J. J. Chem. Soc., Chem. Commun. 1992, 392. (g) Ruettinger,
W. F.; Campana, C.; Dismukes, G. C. J. Am. Chem. Soc. 1997, 119,
6670. (h) Mikuriya, M.; Hashimoto, Y.; Kawamori, A. Chem. Lett.
1995, 1095. (i) Kawasaki, H.; Kusunoki, M.; Hayashi, Y.; Suzuki,
M.; Munezawa, K.; Suenaga, M.; Senda, H.; Uehara, A. Bull. Chem.
Soc. Jpn 1994, 67, 1310. (j) McKee, V.; Tandon, S. S. J. Chem. Soc.,
Chem. Commun. 1988, 1334. (k) Hagen, K. S.; Westmoreland, T. D.;
Scott, M. J.; Armstrong, W. H. J. Am. Chem. Soc. 1989, 111, 1907.
(l) Dube´, C. E.; Wright, D. W.; Pal, S.; Bonitatebus, P. J.; Armstrong,
W. H. J. Am. Chem. Soc. 1998, 120, 3704.
(12) (a) Wang, S.; Tsai, H.-L.; Libby, E.; Folting, K.; Streib, W. E.;
Hendrickson, D. N.; Christou, G. Inorg. Chem. 1996, 35, 7578. (b)
Wang, S.; Folting, K.; Streib, W. E.; Schmitt, E. A.; McCusker, J.
K.; Hendrickson, D. N.; Christou, G. Angew. Chem., Int. Ed. Engl.
1991, 30, 305. (c) Libby, E.; McCusker, J. K.; Schmitt, E. A.; Folting,
K.; Hendrickson, D. N.; Christou, G. Inorg. Chem. 1991, 30, 3486.
(d) Wemple, M. W.; Tsai, H.-L.; Folting, K.; Hendrickson, D. N.;
Christou, G. Inorg. Chem. 1993, 32, 2025. (e) Wemple, M. W.; Adams,
D. M.; Folting, K.; Hendrickson, D. N.; Christou, G. J. Am. Chem.
Soc. 1995, 117, 7275. (f) Wemple, M. W.; Adams, D. M.; Hagen, K.
S.; Folting, K.; Hendrickson, D. N.; Christou, G. J. Chem. Soc., Chem.
Commun. 1995, 1591.
[Mn(dbm)2(py)2](ClO4) (5). Mn(ClO4)2‚6H2O (3.61 g, 10.0 mmol)
and dbmH (5.94 g, 27.0 mmol) were dissolved in a solvent mixture
comprising pyridine (15 mL) and EtOH (25 mL) to give an orange
solution. To this solution was added solid NBun MnO4 (1.26 g, 3.50
4
(13) Wemple, M. W.; Adams, D. M.; Folting, K.; Hagen, K. S.; Hendrick-
son, D. N.; Christou, G. Inorg. Chem., to be submitted.
(14) Vincent, J. B.; Chang, H.-R.; Folting, K.; Huffman, J. C.; Christou,
G.; Hendrickson, D. N. J. Am. Chem. Soc. 1987, 109, 5703.
(15) Vincent, J. B.; Christmas, C.; Chang, H.-R.; Li, Q.; Boyd, P. D. W.;
Huffman, J. C.; Hendrickson, D. N.; Christou, G. J. Am. Chem. Soc.
1989, 111, 2086.
mmol) in small portions to give a green-brown solution and a brown
crystalline precipitate. After several hours, the crystalline solid was
(16) Wemple, M. W.; Tsai, H.-L.; Wang, S.; Claude, J.-P.; Streib, W. E.;
Huffman, J. C.; Hendrickson, D. N.; Christou, G. Inorg. Chem. 1996,
35, 6450.

Tetranuclear Manganese Carboxylate Complexes
Inorganic Chemistry, Vol. 39, No. 7, 2000 1503
collected by filtration, washed with EtOH and then Et2O, and dried in
air. The crude material was recrystallized from CH2Cl2/Et2O. The overall
yield was 25-30%. Anal. Calcd (found) for C40H32N2O8ClMn: C, 63.3
(63.2); H, 4.3 (4.4); N, 3.7 (3.5). Electronic spectrum in MeCN: 310
(38 800), 476 (2280), 510 (1810).
Method B. A red-brown solution of complex 8 in THF was layered
with 1.5 volumes of Et2O to give red-brown crystals after several days
in 40-45% yield. The dried sample analyzed as 9‚THF. Anal. Calcd
(found) for C84H76O21Mn4: C, 61.5 (62.1); H, 4.7 (4.6); Mn, 13.4 (13.5).
Method C. A slurry of complex 6 (0.30 g, 0.20 mmol) and PhCO2H
(0.61 g, 5.0 mmol) in THF (25 mL) was stirred until all complex 6
had slowly dissolved to give a red-brown solution. The latter was then
filtered and the filtrate layered with equivolume Et2O/hexanes (1:1 v/v).
The resulting red-brown crystals were collected by filtration, washed
with hexanes, and dried in air. The yield was ∼80%. IR spectral
examination confirmed the identity of the product as complex 9.
[Mn4O2(O2CMe)6(py)2(dbm)2] (6). To a stirred brown solution of
complex 1 (0.87 g, 1.0 mmol) in MeCN (25 mL) was added solid dbmH
(0.34 g, 1.5 mmol) in small portions to give a deep brown solution.
After several minutes, a red-brown microcrystalline precipitate began
to form. Approximately 24 h later, the precipitate was collected by
filtration, washed with MeCN, and dried in vacuo; yield 75-80%.
Recrystallization from hot MeCN yielded dark red-brown crystals in
an overall yield of 60-70%. The crystallographic studies confirmed
the title formulation, but dried samples analyzed for [Mn4O2(O2CMe)6-
(py)2(dbm)2]‚H2O, suggesting absorption of H2O molecules from the
atmosphere. Anal. Calcd (found) for C52H52N2O19Mn4: C, 50.8 (50.5);
H, 4.3 (4.1); N, 2.3 (2.1); Mn, 17.9 (17.7). Electronic spectrum in CH2-
Cl2: 336 (35 660), 368 (29 630), 394 (18 360), 454 (2640), 488 (2100),
528 (1010). Selected IR data (cm-1): 1610 (s), 1597 (s), 1587 (s), 1568
(s), 1533 (s), 1522 (s), 1487 (s), 1475 (s), 1441 (s), 1400 (s), 1350 (s),
1334 (s), 1321 (s), 1307 (m, sh), 1226 (m), 1070 (m), 1026 (m), 758
(m), 721(s), 702 (m), 686 (m), 669 (s), 655 (s), 630 (s), 613 (s), 572
(m). When [Mn3O(O2CMe)6(py)3](py) was employed in place of
complex 1, the same product 6 was obtained in comparable yield,
atmospheric O2 presumably compensating for the lower oxidation state
starting material.
(NBun )[Mn4O2(O2CPh)7(dbm)2] (10). Method A. To a stirred red-
4
brown solution of (Bun N)[Mn4O2(O2CPh)9(H2O)] (0.40 g, 0.25 mmol)
4
in CH2Cl2 (15 mL) was added solid Na(dbm) (0.125 g, 0.50 mmol).
The solution color changed slowly from red-brown to brown over the
course of 4 h. A precipitate (NaO2CPh) was removed by filtration, and
the filtrate was treated with Et2O (15 mL) and hexanes (15 mL). After
2 days, a black crystalline product was isolated by filtration, washed
with Et2O, and dried in air; the yield was 60-70% of 10‚1/4CH2Cl2.
Anal. Calcd (found) for C95.25H93.5NO20Cl0.5Mn4: C, 63.22 (63.26); H,
5.21 (5.23); N, 0.77 (0.75); Mn, 12.14 (12.23). Electronic spectrum in
CH2Cl2: 452 (2390), 514 (1240). Selected IR data (Nujol): 1622 (s,
sh), 1612 (s), 1589 (s), 1572 (s), 1533 (s), 1520 (s), 1321 (s), 1304 (s),
1224 (m), 1174 (m), 1157 (w, sh), 1068 (m), 1026 (m), 688 (m), 679
(s), 659 (s), 640 (s), 613 (m), 569 (w), 532 (w), 466 (w).
Method B. A slurry of (NBun )[Mn4O2(O2CPh)8(dbm)] (0.35 g, 0.20
4
mmol) and Na(dbm) (0.050 g, 0.20 mmol) in CH2Cl2 (15 mL) was
stirred for 4 h. A precipitate (NaO2CPh) was removed by filtration,
and the brown filtrate was treated with Et2O (15 mL) and hexanes (15
mL) to give a brown precipitate, which was collected by filtration,
washed with Et2O, and dried in air; the yield was 50%. The IR spectrum
was identical to that of material prepared by method A.
[Mn4O2(O2CEt)6(py)2(dbm)2] (7). To a stirred brown solution of
[Mn3O(O2CEt)6(py)3](ClO4) (0.48 g, 0.50 mmol) in MeCN (20 mL)
was added solid dbmH (0.19 g, 0.75 mmol) in small portions to give
a deep brown solution. A red-brown microcrystalline solid began to
precipitate within minutes. The next day, the solid was collected by
filtration, washed with MeCN, and dried; yield 73-88%. The solid
analyzed as 7‚H2O. Anal. Calcd (found) for C58H62N2O18Mn4: C, 53.06
(53.2); H, 4.91 (4.8); N, 2.13 (2.1); Mn, 16.74 (16.5).
Conversion of 10 to (NBun )[Mn4O2(O2CPh)7(pic)2]. Complex 10‚
4
1/4CH2Cl2 (0.18 g, 0.10 mmol) and picolinic acid (0.028 g, 0.22 mmol)
were dissolved in CH2Cl2 (10 mL) to give a red-brown solution. After
30 min, Et2O (10 mL) and hexanes (20 mL) were added, causing the
precipitation of a red-brown microcrystalline product, which was
collected by filtration, washed with Et2O, and dried in air. The yield
[Mn4O2(O2CPh)6(py)(dbm)2] (8). Method A. A solution of complex
2 (1.10 g, 1.0 mmol) in MeCN (60 mL) was filtered, and to the stirred
filtrate was added solid dbmH (0.34 g, 1.5 mmol) in small portions to
give a homogeneous greenish-brown solution. This was allowed to stand
undisturbed for 3 days. The resulting well-formed, essentially black
crystals were collected by filtration, washed with MeCN, and dried in
vacuo. The yield was 60-65%. No recrystallization was needed. Anal.
Calcd (found) for C77H57NO18Mn4: C, 61.49 (61.65); H, 3.82 (3.85);
N, 0.93 (0.90); Mn, 14.6 (14.4). Electronic spectrum in CH2Cl2: 338
(39 950), 368 (30 770), 390 (17 530), 458 (2180), 484 (1770), 506
(1165). Selected IR data (cm-l): 1608 (s), 1589 (s), 1568 (s), 1522
(s), 1346 (s), 1321 (s), 1305 (s), 1223 (m), 1176 (s), 1070 (m), 1026
(m), 719 (s), 686 (s), 657 (s), 642 (s), 626 (m), 617 (m), 572 (m).
When [Mn3O(O2CPh)6(py)3](ClO4) was used in place of 2, the same
product was obtained in a comparable yield.
of (NBun )[Mn4O2(O2CPh)7(pic)2], identified by IR spectral comparison
4
with authentic material, was 85%. The same reaction but with Na-
(pic)‚3/4H2O in place of picH gave the same product in ∼65% isolated
yield.
[Mn4O3(O2CPh)4(dbm)3] (11). Method A. (NBun4)[Mn4O2(O2CPh)7-
(dbm)2] (0.18 g, 0.10 mmol) and NBun ClO4 (1.00 g, 2.92 mmol) were
4
dissolved in MeCN (10 mL) to give a red-brown solution. The solution
was electrolyzed at a potential of 0.65 V versus the ferrocene/
ferrocenium (Fc/Fc+) couple (1.10 V vs SCE); a brown precipitate was
slowly produced during the course of the electrolysis. After the
electrolysis was complete (60-75 min), the solid was collected by
filtration, washed with MeCN, and dried under vacuum; the yield was
30-45% (28-43 mg) based on dbm- available. Recrystallization from
CH2Cl2/hexanes gave dark red crystals susceptible to solvent loss. The
crystallographic studies confirmed the formulation of the product as
11‚3/2CH2Cl2; however, the dried solid analyzed as solvent-free. Anal.
Calcd (found) for C73H53O17Mn4: C, 61.7 (61.4); H, 3.8 (3.8); Mn,
15.5 (15.3). Selected IR data (Nujol): 1599 (s), 1589 (s), 1558 (s),
1523 (vs), 1346 (s), 1323 (s), 1228 (m), 1174 (s), 1070 (m), 1026 (m),
943 (w), 844 (w), 760 (w), 719 (s), 686 (m), 648 (m), 621 (m), 603
(m), 578 (s), 540 (m), 509 (m), 470 (w).
Method B. Solid benzoic acid (1.22 g, 10.0 mmol) was added to a
stirred solution of complex 6 (0.60 g, 0.50 mmol) in CH2Cl2 (30 mL).
The solution was stirred for a further 10 min to dissolve the acid, and
the solution was then layered carefully with Et2O (45 mL). After several
days, the resulting black/brown crystals were collected by filtration,
washed with Et2O, and dried in air. The yield was 65-75%. IR spectral
examination confirmed that the product was identical with that from
method A.
[Mn4O2(O2CPh)6(THF)2(dbm)2] (9). Method A. To a brown
solution of complex 2 (0.55 g, 0.50 mmol) in THF (25 mL) was added
solid dbmH (0.19 g, 0.75 mmol) in small portions to give a
homogeneous brown solution. The solution was filtered and the filtrate
layered with Et2O (40 mL) to give well-formed crystals of 9‚2THF
after several days. These were collected by filtration, washed with Et2O,
and dried in air. The yield was 35%. Anal. Calcd (found) for C88H84O22-
Mn4: C, 61.7 (61.1); H, 4.9 (5.1); Mn, 12.8 (12.5). Electronic spectrum
in CH2Cl2: 378 (31 730), 460 (3080), 488 (2620). Selected IR data
(Nujol): 1610 (s), 1599 (s), 1589 (s), 1572 (s), 1523 (s), 1479 (s),
1348 (s), 1305 (s), 1223 (m), 1176 (m), 1157 (m), 1068 (m), 1045
(m), 1026 (m), 758 (m), 719 (s), 684 (s), 655 (s), 638 (s), 621 (s), 571
(m), 468 (m).
Method B. The yield can be improved by adding dbmH to the
electrolysis solution. Thus, dbmH (26.2 mg, 0.117 mmol) in MeCN (5
mL, 0.2 M NBun ClO4) was added dropwise over 60 min to a solution
4
of complex 10 (0.213 g, 0.117 mmol) in MeCN (20 mL, 0.2 M
TBAClO4) that was being electrolyzed at 0.65 V. The rate of addition
of the dbmH solution was controlled to roughly parallel the rate of the
oxidation of complex 10. After the electrolysis was complete, the
precipitate was collected by filtration, washed with MeCN, and dried
under vacuum; the yield was 70-80%. The IR spectrum is identical to
that of material prepared by method A.
[Mn4O3(O2CMe)4(dbm)3] (12). A slurry of complex 6 (0.24 g, 0.20
mmol) in a solution of 0.2 M NBun ClO4 or NBun PF6 in dried MeCN/
4
4

1504 Inorganic Chemistry, Vol. 39, No. 7, 2000
Wang et al.
Table 1. Crystallographic Data for Complexes 6, 8, 11, and 12
6
8
11
12
a
formula
fw, g/mol
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
C52H50N2O18Mn4
1210.72
P1h
10.868(3)
13.864(3)
10.625(3)
108.62(1)
118.98(1)
89.34(2)
1307
C77H57NO18Mn4
1504.05
P21/c
14.743(6)
15.536(8)
30.006(13)
90
102.79(1)
90
6702
C74.5H56O17Cl3Mn4
1549.38a
P21/n
15.161(3)
21.577(4)
22.683(5)
90
108.04(3)
90
7056
C53H45O17Mn4
1173.68
P21/n
13.549(2)
22.338(4)
16.618(2)
90
103.74(1)
90
4885
Z
1
4
4
4
T, °C
-131
0.71069
1.538
-155
0.71069
1.491
-100
0.71069
1.458
-171
0.71069
1.596
radiation, Åb
F
calc, g/cm3
µ, cm-1
9.790
7.789
7.315
10.429
R (Rw or wR2)
3.24 (3.70)c,d
4.32 (4.44)c,d
8.63 (21.80)c,e
4.63 (4.45)c,d
3
2
a Including /2CH2Cl2 solvate molecules. b Graphite monochromator. c R ) 100∑||Fo| - |Fc||/∑|Fo|. d Rw ) 100[∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2
where w ) 1/σ2(|Fo|). e wR2 ) 100[∑w(Fo - Fc2)2/∑wFo ]1/2
.
2
4
CH2Cl2 (3:1) (16 mL) was electrolyzed at 0.84 V (vs Fc/Fc+) under an
argon atmosphere. During the electrolysis, a solution of dbmH (0.045
g, 0.20 mmol) in 0.2 M (NBun )(ClO4) or (NBun )(PF6) in dried MeCN
absences corresponding to the unique monoclinic space group P21/c.
Subsequent solution and refinement of the structure confirmed this
choice. The R for averaging of equivalent reflections measured more
than once was 0.053. All non-hydrogen atoms were readily located
and refined with anisotropic thermal parameters. A difference Fourier
map phased on the non-hydrogen atoms revealed the location of some,
but not all, hydrogen atoms. All hydrogen atom positions were therefore
calculated using idealized geometries and d(C-H) ) 0.95 Å. The
hydrogen atoms were included as fixed atom contributors at these
calculated positions for the final cycles of refinement. No absorption
correction was performed. The final difference Fourier map was
essentially featureless, with the largest peak being 0.53 e/Å3.
For complex 11‚3/2CH2Cl2, a suitable crystal (0.10 × 0.33 × 0.70
mm) was located while the sample was in mineral oil to prevent solvent
loss and transferred to the goniostat as above for characterization and
data collection (+h, +k, (l; 1° e 2θ e 45°) at -100 °C. A systematic
search of a limited hemisphere of reciprocal space located a set of
diffraction maxima with Laue symmetry and systematic absences
corresponding to monoclinic space group P21/n, which was confirmed
by the subsequent successful solution of the structure. The R for
averaging equivalent reflections measured more than once was 0.0623.
All non-hydrogen atoms were readily located and refined with
anisotropic thermal parameters; the phenyl rings were included as rigid
bodies with C-C bond lengths of 1.39 Å and C-C- angles of 120°.
In the final cycles of least-squares refinement on F2, the hydrogen atoms
were included with isotropic thermal parameters. The final difference
Fourier map was essentially featureless, the largest peak being 0.90
e/Å3.
4
4
(5 mL) was slowly added dropwise. When the electrolysis was complete
(60-90 min), the precipitated solid was collected by filtration under
anaerobic conditions, washed with MeCN, and dried in vacuo. The
yield was typically 45-55%. Anal. Calcd (found) for C53H45O17Mn4:
C, 54.24 (53.84); H, 3.86 (3.88). Selected IR data (Nujol): 1680 (m),
1597 (m), 1588 (m), 1568 (m), 1534 (s), 1522 (s), 1353 (s), 1339 (s),
1325 (s), 1254 (m), 1233 (m), 1076 (m), 1047 (w), 1024 (m), 945 (m),
785 (w), 772 (m), 731 (m), 704 (w), 693 (m), 648 (m), 612 (m), 590
(m), 573 (m), 544 (m), 523 (m), 473 (w). A CH2Cl2 solution of this
solid was layered with MeCN and stored at 5 °C to give X-ray quality
crystals. [Mn4O3(O2CCD3)4(dbm)3] can be prepared in an analogous
manner using [Mn4O2(O2CCD3)6(py)2(dbm)2].
X-ray Crystallography and Solution of Structures. Data were
collected on complexes 6, 8, and 12 at the IUMSC, and on complex
11‚3/2CH2Cl2 at Emory University. Details of the diffractometry, low-
temperature facilities, and computational procedures employed by the
Molecular Structure Center are available elsewhere.17 Data collection
parameters and structure solution details are listed in Table 1. The
structures were solved by standard methods (MULTAN78 or SHELXL-
92) and Fourier techniques and refined on F or F2 by a full-matrix
least-squares approach.
For complex 6, a suitable crystal (0.25 × 0.30 × 0.35 mm) was
affixed to a glass fiber using silicone grease and transferred to a
goniostat, where it was cooled to -131 °C for characterization and
data collection (+h, (k, (l; 6° e 2θ e 45°). A systematic search of
a limited hemisphere of reciprocal space revealed no Laue symmetry
or systematic absences. An initial choice of space group P1h was
subsequently confirmed by the successful solution of the structure.
Following intensity data collection, data processing gave a residual of
0.020 for 657 unique intensities which had been measured more than
once. Four standards measured every 300 data showed no significant
trends. No correction was made for absorption. After the non-hydrogen
atoms had been located and partially refined, a difference Fourier map
phased on the non-hydrogen atoms revealed most of the hydrogen atom
positions. In the final cycles of least-squares refinement, the non-
hydrogen atoms were varied with anisotropic thermal parameters and
the hydrogen atoms were varied with isotropic thermal parameters. The
final difference map was essentially featureless, the largest peak being
0.66 e/Å3.
For complex 12, a suitable crystal (0.10 × 0.24 × 0.30 mm) was
located and transferred to the goniostat as above for characterization
and data collection (+h, +k, (l; 6° e 2θ e 45°) at -171 °C. A
systematic search of a limited hemisphere of reciprocal space yielded
a set of reflections which exhibited monoclinic diffraction symmetry
(2/m). The systematic extinction of h01 for h + 1 ) 2n + 1 and of
0k0 for k ) 2n + 1 identified the space group as P21/n, confirmed by
the subsequent solution and refinement of the structure. The R was
0.042 for the averaging of 5320 equivalent reflections measured more
than once. Plots of the four standard reflections measured every 300
reflections showed no significant trends. No absorption correction was
performed. All non-hydrogen atoms were readily located. Many of the
hydrogen atoms were evident in a difference Fourier map phased on
the non-hydrogen atoms. Hydrogen atoms were introduced in fixed,
idealized positions, with a C-H distance of 0.95 Å and individual
isotropic thermal parameters equal to 1.0 plus the isotropic equivalent
of the parent atom; it should be noted that all of the methyl groups had
at least one hydrogen atom located. The final difference map was
essentially featureless, the largest peak being 0.42 e/Å3.
For complex 8, a suitable crystal (0.25 × 0.25 × 0.25 mm) was
located and transferred to the goniostat as above for characterization
and data collection (+h, +k, (l; 6° e 2θ e 45°) at -155 °C. A
systematic search of a limited hemisphere of reciprocal space located
a set of diffraction maxima with Laue symmetry and systematic
For all complexes, the values of discrepancy indices R and Rw, or R
and wR2, are included in Table 1.
(17) Chisholm, M. H.; Folting, K.; Huffman, J. C.; Kirkpatrick, C. C. Inorg.
Chem. 1984, 23, 1021.

Tetranuclear Manganese Carboxylate Complexes
Inorganic Chemistry, Vol. 39, No. 7, 2000 1505
Other Studies. Infrared and electronic spectra were recorded on
Nujol mulls between KBr plates or in solution, respectively, using
Nicolet 510P FTIR and Hewlett Packard 8452A spectrophotometers.
Cyclic voltammograms (CV) and differential pulse voltammograms
(DPV) were recorded on a BAS CV50W voltammetric analyzer:
measurements were performed at room temperature using a glassy
carbon working electrode, a Pt wire auxiliary electrode, and an SCE
reference electrode. Potentials are quoted vs the ferrocene/ferrocenium
couple under the same conditions. No IR compensation was employed.
Controlled potential electrolyses were performed with a similar three-
electrode system using a high surface area Pt basket electrode. The
supporting electrolytes were NBun ClO4 or NBun PF6. 1H NMR
4
4
spectroscopy was performed using a Varian XL-300 instrument;
chemical shifts are quoted using the δ scale (shifts downfield are
positive). Variable-temperature magnetic susceptibility data were
obtained with a Quantum Design MPMS5 SQUID susceptometer
equipped with a 5.5 T magnet. Pascal’s constants were used to estimate
the diamagnetic correction for each complex, which was subtracted
from the experimental susceptibility to give the molar magnetic
susceptibility (øM).
Figure 1. ORTEP representation at the 50% probability level of
[Mn4O2(O2CMe)6(py)2(dbm)2] (6). Primed and unprimed atoms are
related by an inversion center.
of 2 with dbmH in THF, or reaction of 6 with an excess of
PhCO2H in THF all gave [Mn4O2(O2CPh)6(THF)2(dbm)2] (9),
containing two THF groups presumably due to the greater
flexibility of THF vs rigid py groups. In contrast, reaction of 6
with PhCO2H in CH2Cl2 gives 8, again with a single py ligand.
The reaction of 8 with an excess of py (25 equiv) caused core
degradation, and the isolated product in 79% yield was the
known [Mn6O2(O2CPh)10(py)4].19
Results
Syntheses. The transformations involving complexes 1-9
summarized below and the complex numbering scheme are
presented for convenience in Scheme 1.
The reactions of the trinuclear complexes 1 and 2 with
â-diketones represent an excellent entry point into tetranuclear
Mn/O complexes with a predominantly or completely O-based
ligation environment. AcacH itself is of little use, however,
because it is too susceptible to oxidation by good oxidizing
agents such as MnIII; the oxidation of acacH by metal ions has
been well documented.18 Thus, treatment of 2 (MnII,2MnIII) with
acacH leads to 45-60% isolated yields of the 4MnII,2MnIII
complex [Mn6O2(O2CPh)10L4] (3), where 4L ) (MeCN)2(H2O)2
(3a) or (DMF)4 (3b) after recrystallization from DMF/Et2O/
hexanes. As discussed previously,19 this conversion is rational-
ized as due to initial reduction of the [Mn3O]6+ core followed
by dimerization (eq 1). Much more resistant to oxidation are
The complex (NBun )[Mn4O2(O2CPh)7(dbm)2] (10) is not
4
available from the reactions between dbmH and [Mn3O] species
explored to date, but it can be conveniently prepared in high
yield (60-70%) from the reaction of Na(dbm) with (NBun )-
4
[Mn4O2(O2CPh)9(H2O)] or (NBun )[Mn4O2(O2CPh)8(dbm)], the
4
latter itself being obtained from the former as described
elsewhere.16 This can be summarized in eq 2. Complex 10 will
undergo chelate substitution with picolinic acid to give
[Mn4O2(O2CPh)7(pic)2]- in high yield (85%) (eq 3). It was of
[Mn4O2(O2CPh)9(H2O)]- + 2Na(dbm) f
[Mn4O2(O2CPh)7(dbm)2]- + 2NaO2CPh + H2O (2)
[Mn3O]6+ + e- f [Mn3O]5+ f 1/2[Mn6O2]10+
(1)
[Mn4O2(O2CPh)7(dbm)2]- + 2picH f
[Mn4O2(O2CPh)7(pic)2]- + 2dbmH (3)
dbmH and dbm-. Thus, reactions of 1 and 2 with dbmH lead
to MnIII products. In mapping out such reactions, it was found
that dbmH:[Mn3O] ratios of 3:1 or greater give, as the main
product, the mononuclear complex Mn(dbm)3 (4), or [Mn(dbm)2-
(py)2](ClO4) (5) if an excess of pyridine is present. With a
dbmH:[Mn3O] ratio of 3:2 employing complex 1, however, the
product is [Mn4O2(O2CMe)6(py)2(dbm)2] (6), analogous to the
earlier reactions with bpy10 or picH,20 which gave [Mn4O2(O2-
CMe)7(bpy)2]+ and [Mn4O2(O2CMe)7(pic)2]- products, respec-
tively. Similarly, the reaction of [Mn3O(O2CEt)6(py)3](ClO4)
with dbmH gives [Mn4O2(O2CEt)6(py)2(dbm)2] (7). The analo-
gous reaction with complex 2 gave essentially the same type
of product, although the complex [Mn4O2(O2CPh)6(py)(dbm)2]
(8) contains only one py group, suggesting one MnIII ion to be
five-coordinate, a relatively unusual coordination number for
MnIII with such ligation; this was subsequently confirmed by a
crystallographic study (vide infra). The cause may be steric
congestion between the many Ph rings of hypothetical [Mn4O2(O2-
CPh)6(py)2(dbm)2]. Recrystallization of 8 from THF, reaction
interest to find the exchange to be so clean rather than the
picolinate substituting at a benzoate position to give products
containing both dbm- and pic-. The observed exchange is
presumably facilitated by the protonation of dbm- (pKa of dbmH
) 8.95) by picH (pKa ) 5.39).21
Structures of Complexes 6 and 8. ORTEP representations
of 6 and 8 are shown in Figures 1 and 2, respectively. Selected
interatomic distances and angles are listed in Tables 2 and 3,
respectively.
Complex 6 crystallizes in triclinic space group P1h with the
molecule lying on an inversion center. The [Mn4(µ3-O)2]8+ core
thus contains an exactly planar Mn4 parallelogram with
Mn1‚‚‚Mn2 (3.308(1) Å) slightly shorter than Mn1‚‚‚Mn2′
(3.398(1) Å), consistent with two vs one MeCO2- bridges across
these Mn2 pairs. The µ3-O2- ions O3′ and O3′ are 0.325 Å above
and below their Mn3 planes and give a central [Mn2O2] rhomb
with a short Mn2‚‚‚Mn2′ distance (2.875(1) Å). In addition to
a total of six bridging MeCO2- groups, there are two chelating
dbm- groups at the two ends and two terminal py groups on
the central Mn atoms, completing six-coordinate, near-octahedral
(18) Bhattacharjee, M. N.; Chadhuri, M. K.; Khathing, D. T. J. Chem. Soc.,
Dalton Trans. 1982, 669.
(19) Schake, A. R.; Vincent, J. B.; Li, Q.; Boyd, P. D. W.; Folting, K.;
Huffman, J. C.; Hendrickson, D. N.; Christou, G. Inorg. Chem. 1989,
28, 1915.
(20) Libby, E.; McCusker, J. K.; Schmitt, E. A.; Folting, K.; Hendrickson,
D. N.; Christou, G. Inorg. Chem. 1991, 30, 3486.
(21) Serjeant, E. P.; Dempsey, B. Ionization Constants of Organic Acids
in Aqueous Solution; Pergamon: New York, 1979.

1506 Inorganic Chemistry, Vol. 39, No. 7, 2000
Wang et al.
Table 3. Selected Interatomic Distances (Å) and Angles (deg) for
[Mn4O2(O2CPh)6(py)(dbm)2] (8)
Mn(3)‚‚‚Mn(4)
Mn(1)‚‚‚Mn(4)
Mn(1)‚‚‚Mn(2)
Mn(2)‚‚‚Mn(3)
Mn(2)‚‚‚Mn(4)
Mn(1)-O(5)
Mn(1)-O(7)
Mn(1)-O(11)
Mn(1)-O(41)
Mn(1)-O(59)
Mn(1)-O(68)
Mn(2)-O(5)
Mn(2)-O(6)
Mn(2)-O(52)
3.299(2)
3.425(2)
3.327(2)
3.360(2)
2.820(2)
1.892(4)
1.923(5)
1.942(4)
2.193(5)
1.939(4)
2.165(5)
1.859(5)
1.882(5)
1.941(5)
Mn(2)-O(61)
Mn(2)-O(70)
Mn(3)-O(6)
Mn(3)-O(24)
Mn(3)-O(28)
Mn(3)-O(50)
Mn(3)-O(77)
Mn(3)-O(86)
Mn(4)-O(5)
Mn(4)-O(6)
Mn(4)-O(43)
Mn(4)-O(79)
Mn(4)-O(88)
Mn(4)-N(95)
2.076(5)
1.941(5)
1.872(5)
1.941(5)
1.911(5)
2.200(5)
1.932(5)
2.172(5)
1.892(5)
1.891(5)
1.926(5)
2.145(4)
1.957(5)
2.404(6)
O(5)-Mn(1)-O(7)
O(5)-Mn(1)-O(11)
O(5)-Mn(1)-O(41)
O(5)-Mn(1)-O(59)
O(5)-Mn(1)-O(68)
O(7)-Mn(1)-O(11)
O(7)-Mn(1)-O(41)
O(7)-Mn(1)-O(59)
O(7)-Mn(1)-O(68)
O(11)-Mn(1)-O(41)
178.45(19) O(24)-Mn(3)-O(50)
85.03(19)
Figure 2. ORTEP representation at the 50% probability level of
[Mn4O2(O2CPh)6(py)(dbm)2] (8). For clarity, only the ipso-carbon of
each benzoate phenyl ring is shown.
92.20(18) O(24)-Mn(3)-O(77) 176.42(20)
88.51(19) O(24)-Mn(3)-O(86)
93.90(19) O(28)-Mn(3)-O(50)
88.57(19) O(28)-Mn(3)-O(77)
89.10(18) O(28)-Mn(3)-O(86)
92.47(19) O(50)-Mn(3)-O(77)
85.55(20)
87.50(19)
86.58(19)
90.45(19)
94.38(18)
Table 2. Selected Interatomic Distances (Å) and Angles (deg) for
[Mn4O2(O2CMe)6(py)2(dbm)2] (6)
84.85(19) O(50)-Mn(3)-O(86) 170.36(18)
Mn(1)‚‚‚Mn(2)
Mn(1)‚‚‚Mn(2)′
Mn(2)‚‚‚Mn(2)′
Mn(1)-O(3)
Mn(1)-O(4)
Mn(1)-O(8)
Mn(1)-O(12)
Mn(1)-O(16)
3.308(1)
3.398(1)
2.875(1)
1.877(2)
2.136(2)
1.928(2)
2.206(2)
1.931(2)
Mn(1)-O(20)
Mn(2)-O(3)
Mn(2)-O(3)′
Mn(2)-O(6)
Mn(2)-O(10)
Mn(2)-O(14)
Mn(2)-N(33)
1.925(2)
1.885(2)
1.894(2)
1.957(2)
2.187(2)
1.953(2)
2.410(2)
90.60(19) O(77)-Mn(3)-O(86)
83.07(18) O(5)-Mn(4)-O(6)
94.90(19)
82.17(20)
95.47(20)
98.64(18)
166.69(20)
83.44(20)
176.65(20)
89.94(18)
94.87(20)
86.35(20)
88.06(18)
87.96(20)
95.77(20)
94.32(19)
O(11)-Mn(1)-O(59) 172.91(19) O(5)-Mn(4)-O(43)
O(11)-Mn(1)-O(68)
O(41)-Mn(1)-O(59)
O(41)-Mn(1)-O(68) 172.46(18) O(5)-Mn(4)-N(95)
90.09(19) O(5)-Mn(4)-O(79)
93.46(19) O(5)-Mn(4)-O(88)
O(59)-Mn(1)-O(68)
O(5)-Mn(2)-O(6)
O(5)-Mn(2)-O(52)
O(5)-Mn(2)-O(61)
O(5)-Mn(2)-O(70)
O(6)-Mn(2)-O(52)
O(6)-Mn(2)-O(61)
O(6)-Mn(2)-O(70)
O(52)-Mn(2)-O(61)
O(52)-Mn(2)-O(70)
O(61)-Mn(2)-O(70)
O(6)-Mn(3)-O(24)
O(6)-Mn(3)-O(28)
O(6)-Mn(3)-O(50)
O(6)-Mn(3)-O(77)
O(6)-Mn(3)-O(86)
O(24)-Mn(3)-O(28)
93.67(19) O(6)-Mn(4)-O(43)
83.30(20) O(6)-Mn(4)-O(79)
171.29(20) O(6)-Mn(4)-O(88)
93.69(18) O(6)-Mn(4)-N(95)
94.70(20) O(43)-Mn(4)-O(79)
93.05(20) O(43)-Mn(4)-O(88)
107.14(19) O(43)-Mn(4)-N(95)
155.32(19) O(79)-Mn(4)-O(88)
O(3)-Mn(1)-O(4)
O(3)-Mn(1)-O(8)
O(3)-Mn(1)-O(12)
O(3)-Mn(1)-O(16)
O(3)-Mn(1)-O(20)
O(4)-Mn(1)-O(8)
O(4)-Mn(1)-O(12)
O(4)-Mn(1)-O(16)
O(4)-Mn(1)-O(20)
O(8)-Mn(1)-O(12)
O(8)-Mn(1)-O(16)
O(8)-Mn(1)-O(20)
O(12)-Mn(1)-O(16)
O(12)-Mn(1)-O(20)
O(16)-Mn(1)-O(20)
O(3)-Mn(2)-O(3)′
O(3)′-Mn(2)-O(6)
94.72(8) O(3)-Mn(2)-O(6)
95.01(9) O(3)′-Mn(2)-O(10)
92.88(8) O(3)-Mn(2)-O(10)
87.74(8) O(3)-Mn(2)-O(14)′
176.89(8) O(3)′-Mn(2)-O(14)′
93.37(9) O(3)′-Mn(2)-N(33)
169.81(8) O(3)-Mn(2)-N(33)
88.96(9) O(6)-Mn(2)-O(10)
86.79(9) O(6)-Mn(2)-O(14)′
92.71(9) O(6)-Mn(2)-N(33)
176.23(8) O(10)-Mn(2)-O(14)′
100.14(9)
96.97(9)
88.27(8)
175.15(9)
95.05(9)
98.52(9)
88.59(9)
85.53(9)
84.00(9)
79.09(10)
89.54(8)
94.94(19) O(79)-Mn(4)-N(95) 175.47(19)
85.31(19) O(88)-Mn(4)-N(95)
83.42(20)
97.53(19) Mn(1)-O(5)-Mn(2) 124.98(24)
88.19(19) Mn(1)-O(5)-Mn(4) 129.70(24)
176.29(21) Mn(2)-O(5)-Mn(4)
97.48(21)
89.18(19) Mn(2)-O(6)-Mn(3) 127.06(24)
87.60(9) O(10)-Mn(2)-N(33) 163.50(9)
95.34(19) Mn(2)-O(6)-Mn(4)
96.71(21)
84.57(8) O(14)′-Mn(2)-N(33)
85.30(8) Mn(1)-O(3)-Mn(2)
94.72(9)
123.17(10)
92.55(19) Mn(3)-O(6)-Mn(4) 122.49(25)
89.86(19)
89.58(9) Mn(1)-O(3)-Mn(2)′ 128.61(10)
80.93(9) Mn(2)-O(3)-Mn(2)′
99.07(9)
causes a slight shortening of Mn2-O2- distances (average 1.870
Å) compared with Mn4 (average 1.891 Å). Again, the six-
coordinate MnIII atoms show JT elongations, along the O41-
Mn1-O68, O50-Mn3-O86, and O79-Mn4-O95 axes.
177.33(9)
geometry at each metal. The symmetry is Ci. All metals show
clear evidence of Jahn-Teller (JT) distortions, as expected for
high-spin MnIII (d4) in near-octahedral geometry, taking the form
of axial elongation along the O4-Mn1-O12 and O10-Mn2-
N33 axes; the former and latter are approximately parallel and
perpendicular, respectively, to the Mn4 plane. As is expected,
the JT axes are oriented as to avoid the Mn-O2- bonds, which
are the shortest and strongest in the molecule (<1.9 Å).
Complex 8 crystallizes in monoclinic space group P21/c with
a complete molecule in the asymmetric unit. As for 6, there is
a [Mn4(µ3-O)2]8+ core, but the Mn4 unit now has a bent or
“butterfly” arrangement, with Mn1 and Mn3 the “wingtip” atoms
and Mn2 and Mn4 the “body” atoms of the butterfly. O5 and
O6 lie 0.299 and 0.399 Å below their Mn3 planes. Peripheral
ligation is again by bridging carboxylate, terminal py, and
chelating dbm- groups, but now there is only one py on the
central metals, with Mn2 consequently being five-coordinate,
an unusual coordination number for MnIII with such ligands.
The geometry at Mn2 is essentially square-pyramidal (sp) with
O61 occupying the apex and consequently yielding a Mn2-
O61 bond (2.076(5) Å) noticeably longer than the basal Mn-O
(carboxylate) bonds (1.941(5) Å). Consideration of the angles
about Mn2 gives a τ value of 0.27 (τ ) 0 and 1 for sp and tbp
geometries, respectively22), indicating significant but not severe
distortion from sp geometry. The five-coordination at Mn2
Magnetochemistry of [Mn4O2(O2CMe)6(py)2(dbm)2] (6).
Variable-temperature, solid-state magnetic susceptibility data
were collected on powdered samples of 6 in the 10.0-279 K
range. The effective magnetic moment (µeff per Mn4) decreases
gradually from 7.51 µB at 279 K to 6.08 µB at 50.1 K, and then
decreases more rapidly to 4.12 µB at 10.0 K. This is similar to
the behavior already described in detail previously for (NBun )-
4
[Mn4O2(O2CMe)7(pic)2].20 The data were fit to the same
theoretical øm vs T expression employed for the pic- complex,
where Jwb and Jbb are the wingtip/body and body/body exchange
interactions, respectively, and which includes the effects of zero-
field splitting in the ground state (responsible for the rapid
decrease in µeff at the lowest temperatures). The fitting
parameters were Jwb ) -5.6 cm-1, Jbb ) -21.9 cm-1, g )
1.88, and D ) 3.82 cm-1; temperature independent paramagnet-
ism was held constant at 800 × 10-6 cm3 mol-1. These data
indicate a S ) 3 ground state for complex 6, in agreement with
the same value previously determined for (NBun )[Mn4O2(O2-
CMe)7(pic)2]20 and [Mn4O2(O2CMe)7(bpy)2](ClO4),15 and shown
4
(22) Addison, A. W.; Rao, T. N.; Reedijk, J.; Rijn, J.; Verschoor, G. C. J.
Chem. Soc., Dalton Trans. 1984, 1349.

Tetranuclear Manganese Carboxylate Complexes
Inorganic Chemistry, Vol. 39, No. 7, 2000 1507
Figure 4. CV scan of complex 10 in MeCN showing only the first,
reversible oxidation, and a scan rate (ν) dependence in the 20-500
mV/s range; the inset is a plot of the peak current vs ν1/2 for the forward
and reverse scans.
Figure 3. CV (top) and DPV (bottom) scans of (NBun )[Mn4O2(O2-
CPh)7(dbm)2] (10) in MeCN; the scan rates were 100 and 5 mV/s,
respectively. The potentials are given vs ferrocene under identical
conditions.
4
on the electrochemical time scale. Figure 5 shows the results
for 6: an oxidation at 0.61 V is well observed (albeit broad) in
the DPV but not the CV scan where the reverse wave is not
particularly well formed. The reductions are again irreversible.
In addition, there are several small features reproducibly seen
in analytically pure material suggesting more than one species
in solution, probably those formed by py dissociation and/or
py/MeCN exchange. Additional, very irreversible oxidation
processes are evident at more positive potentials than those
shown in Figure 5. The main conclusion from the electrochemi-
cal studies is thus that the [Mn4O2]8+ complexes display
oxidation processes in MeCN but that only for 10 is reversibility
by electrochemical criteria approached.
Controlled Potential Electrolysis (CPE) Studies. In order
to probe the stability and/or reactivity of oxidized 10 on a longer
time scale, CPE studies were performed in distilled MeCN under
argon at a potential of 0.65 V. A solid was found to precipitate
during the course of the electrolysis (60-75 min), and this was
subsequently identified to be not the one-electron-oxidized
version of 10, but instead [Mn4O3(O2CPh)4(dbm)3] (11) (eq 5)
in a yield of 10%. Consideration of the formulas of 10 and 11
to be a consequence of the competing Jwb and Jbb exchange
parameters.
Electrochemical Studies. Complexes 6 and 8-10 were
investigated by cyclic voltammetry (CV) and differential pulse
voltammetry (DPV). Complex 10 displayed the most reversible
behavior; its CV and DPV traces in MeCN are shown in Figure
3 and consist of an irreversible reduction at ∼ -0.7 V and two
oxidation processes at 0.48 and 1.17 V vs ferrocene, the first
of which appears quasi-reversible in the DPV trace but not so
in the CV scan. However, if the CV switching potential is set
at a value less positive than for the second oxidation, the
resulting CV scan (Figure 4) looks much more like that expected
for a reversible process, with if/ir approaching unity (if and ir
are the forward and reverse currents), and plots of peak current
vs the square root of scan rate being linear in the 20-500 mV/s
range, suggesting a diffusion-controlled process. The observed
CV/DPV behavior suggests the electron-transfer series of eq 4,
0.48 V
[Mn4O2]10+ 71.17 V [Mn4O2]9+ y
z [Mn4O2]8+ -0.66 V8
[Mn4O2]7+ (4)
0.65 V
[Mn4O2(O2CPh)7(dbm)2]-
8
MeCN
with only the first oxidation appearing reversible on the
electrochemical time scale. In contrast, the neutral complexes
6, 8, and 9 do not display behavior that approaches reversibility
[Mn4O3(O2CPh)4(dbm)3] (5)
indicates that a one-electron oxidation from a 4MnIII to a 3MnIII,

1508 Inorganic Chemistry, Vol. 39, No. 7, 2000
Wang et al.
Figure 7. ORTEP representation at the 50% probability level of
[Mn4O3(O2CMe)4(dbm)3] (12).
of 10, the generated species reacts with H2O and undergoes a
rearrangement involving additional dbm incorporation to give
11. This allowed the procedure described in the Experimental
Section (method B) to be developed: the CPE was carried out
in air in undistilled MeCN with 1 equiv of dbmH added
dropwise over ∼ 60 min; the yield under these conditions is
reproducibly in the 70-80% range. The transformation may
now be summarized as in eq 6.
Figure 5. CV (top) and DPV (bottom) scans of [Mn4O2(O2CMe)6-
(py)2(dbm)2] (6) in MeCN at 100 and 5 mV/s, respectively. The
potentials are vs ferrocene.
[Mn4O2(O2CPh)7(dbm)2]- + H2O + dbmH -8
-e
[Mn4O3(O2CPh)4(dbm)3] + 3PhCO2H (6)
Given these results for 10, CPE was also performed on
complex 6. After preliminary experimentation, which showed
that a mixed-solvent system (MeCN:CH2Cl2 ) 3:1) was required
to cause product precipitation and that this product was sensitive
to subsequent reaction with air, the procedure under argon in
the Experimental Section was developed that leads to isolation
of [Mn4O3(O2CMe)4(dbm)3] (12) in 45-55% yield. This is
summarized in eq 7. Thus, the CPE of 6 proceeds analogously
to that of 10 with a similar (but not structurally identical) product
(vide infra).
[Mn4O2(O2CMe)6(py)2(dbm)2] + dbmH + H2O -8
-e
[Mn4O3(O2CMe)4(dbm)3] + 2MeCO2H + pyH+ + py (7)
Structures of Complexes 11 and 12. ORTEP representations
of 11 and 12 are shown in Figures 6 and 7, respectively, and
selected interatomic distances and angles are listed in Tables 4
and 5.
Figure 6. ORTEP representation at the 50% probability level of
[Mn4O3(O2CPh)4(dbm)3] (11). For clarity, only the ipso-carbon atom
of each dbm phenyl ring is shown.
Complex 11 crystallizes in monoclinic space group P21/n with
the Mn4 molecule in a general position. The structure consists
of a Mn4 trigonal pyramid with the MnIV ion Mn(1) at the apex.
-
MnIV product had indeed occurred, but that the product had
increased O2-:Mn and dbm:Mn ratios compared with 10. Thus,
the CPE was repeated under a variety of conditions and the
following observations were made: (i) use of undistilled MeCN
under air gave an increased yield of 11 of 20-30%; (ii) addition
of dbmH to the filtrate from (i) resulted in precipitation of more
11 for a total yield of 40-45%, and (iii) addition of 1 equiv of
dbmH to the solution before electrolysis gave a reduced yield
of ∼25%. These observations suggested that, upon oxidation
The three vertical faces are capped by a µ3-PhCO2 group
ligated through both its O atoms, with O(6) terminal to Mn(2)
and O(7) bridging Mn(3) and Mn(4). The latter O atom O(7)
does not bridge symmetrically, however; Mn(3)-O(7) (2.267(10)
Å) is noticeably longer than Mn(4)-O(7) (2.173(9) Å). This
asymmetry is also seen in the resulting MnIII‚‚‚MnIII separa-
tions: as expected, Mn(3)‚‚‚Mn(4) (3.197(2) Å) is the shortest,
but the others are noticeably different, with Mn(2)‚‚‚Mn(3)
(3.277(2) Å) much shorter than Mn(2)‚‚‚Mn(4) (3.432(2) Å).

Tetranuclear Manganese Carboxylate Complexes
Inorganic Chemistry, Vol. 39, No. 7, 2000 1509
Table 4. Selected Interatomic Distances (Å) and Angles (deg) for
[Mn4O3(O2CPh)4)dbm)3] (11)
Table 5. Selected Interatomic Distances (Å) and Angles (deg) for
[Mn4O3(O2CMe)4(dbm)3] (12)
Mn(1)‚‚‚Mn(2)
Mn(1)‚‚‚Mn(3)
Mn(1)‚‚‚Mn(4)
Mn(1)-O(5)
Mn(1)-O(6)
Mn(1)-O(7)
Mn(1)-O(9)
Mn(1)-O(13)
Mn(1)-O(19)
Mn(2)-O(5)
Mn(2)-O(7)
Mn(2)-O(8)
Mn(2)-O(11)
Mn(2)-O(21)
2.8113(10)
2.7910(11)
2.7941(11)
1.866(3)
1.860(3)
1.876(3)
1.923(3)
1.961(3)
1.941(3)
1.936(3)
1.954(3)
2.281(3)
2.173(3)
1.918(3)
Mn(2)-O(25)
Mn(3)-O(5)
Mn(3)-O(6)
Mn(3)-O(8)
Mn(3)-O(17)
Mn(3)-O(38)
Mn(3)-O(42)
Mn(4)-O(6)
Mn(4)-O(7)
Mn(4)-O(8)
Mn(4)-O(15)
Mn(4)-O(55)
Mn(4)-O(59)
1.914(3)
1.935(3)
1.924(3)
2.297(4)
2.141(3)
1.926(3)
1.912(3)
1.936(3)
1.914(3)
2.320(3)
2.154(3)
1.913(3)
1.932(3)
Mn(1)‚‚‚Mn(2)
Mn(1)‚‚‚Mn(3)
Mn(1)‚‚‚Mn(4)
Mn(1)-O(2)
Mn(1)-O(1)
Mn(1)-O(3)
Mn(1)-O(5)
Mn(1)-O(10)
Mn(2)-O(15)
Mn(2)-O(1)
Mn(2)-O(14)
Mn(2)-O(13)
Mn(2)-O(6)
Mn(2)-O(8)
Mn(3)-O(12)
2.802(3)
2.780(2)
2.787(3)
1.835(7)
1.856(8)
1.859(8)
1.912(8)
1.941(8)
1.905(8)
1.932(7)
1.935(8)
1.935(8)
2.164(9)
2.183(8)
1.867(8)
Mn(2)‚‚‚Mn(3)
Mn(2)‚‚‚Mn(4)
Mn(3)‚‚‚Mn(4)
Mn(3)-O(13)
Mn(3)-O(1)
Mn(3)-O(2)
Mn(3)-O(4)
Mn(3)-O(7)
Mn(4)-O(16)
Mn(4)-O(2)
Mn(4)-O(17)
Mn(4)-O(3)
Mn(4)-O(11)
Mn(4)-O(7)
3.277(2)
3.432(2)
3.197(2)
1.900(7)
1.912(7)
1.942(7)
2.169(7)
2.267(10)
1.894(7)
1.920(7)
1.924(7)
1.947(7)
2.134(8)
2.173(9)
O(5)-Mn(1)-O(6)
O(5)-Mn(1)-O(7)
O(5)-Mn(1)-O(9)
O(5)-Mn(1)-O(13)
O(5)-Mn(1)-O(19)
O(6)-Mn(1)-O(7)
O(6)-Mn(1)-O(9)
O(6)-Mn(1)-O(13)
O(6)-Mn(1)-O(19)
O(7)-Mn(1)-O(9)
O(7)-Mn(1)-O(13)
O(7)-Mn(1)-O(19)
O(9)-Mn(1)-O(13)
O(9)-Mn(1)-O(19)
O(13)-Mn(1)-O(19)
O(5)-Mn(2)-O(7)
O(5)-Mn(2)-O(8)
O(5)-Mn(2)-O(11)
O(5)-Mn(2)-O(21)
O(5)-Mn(2)-O(25)
O(7)-Mn(2)-O(8)
O(7)-Mn(2)-O(11)
O(7)-Mn(2)-O(21)
O(7)-Mn(2)-O(25)
O(8)-Mn(2)-O(11)
O(8)-Mn(2)-O(21)
O(8)-Mn(2)-O(25)
O(11)-Mn(2)-O(21)
O(11)-Mn(2)-O(25)
O(21)-Mn(2)-O(25)
O(5)-Mn(3)-O(6)
O(5)-Mn(3)-O(8)
O(5)-Mn(3)-O(17)
O(5)-Mn(3)-O(38)
O(5)-Mn(3)-O(42)
O(6)-Mn(3)-O(8)
85.45(14) O(8)-Mn(3)-O(38)
85.76(14) O(8)-Mn(3)-O(42)
94.29(14) O(17)-Mn(3)-O(38) 95.08(14)
177.36(14) O(17)-Mn(3)-O(42) 89.48(14)
93.41(15) O(38)-Mn(3)-O(42) 92.11(14)
96.10(14)
98.99(14)
O(2)-Mn(1)-O(1)
O(2)-Mn(1)-O(3)
O(1)-Mn(1)-O(3)
O(2)-Mn(1)-O(5)
O(1)-Mn(1)-O(5)
O(3)-Mn(1)-O(5)
O(2)-Mn(1)-O(9)
O(1)-Mn(1)-O(9)
O(3)-Mn(1)-O(9)
O(5)-Mn(1)-O(9)
O(2)-Mn(1)-O(10)
O(1)-Mn(1)-O(10)
O(3)-Mn(1)-O(10)
O(5)-Mn(1)-O(10)
O(9)-Mn(1)-O(10)
O(15)-Mn(2)-O(1)
O(15)-Mn(2)-O(14)
O(1)-Mn(2)-O(14)
O(15)-Mn(2)-O(3)
O(1)-Mn(2)-O(3)
O(14)-Mn(2)-O(3)
O(15)-Mn(2)-O(6)
O(1)-Mn(2)-O(6)
O(14)-Mn(2)-O(6)
O(3)-Mn(2)-O(6)
O(15)-Mn(2)-O(8)
O(1)-Mn(2)-O(8)
O(14)-Mn(2)-O(8)
O(3)-Mn(2)-O(8)
O(6)-Mn(2)-O(8)
O(12)-Mn(3)-O(13)
O(12)-Mn(3)-O(1)
O(13)-Mn(3)-O(1)
O(12)-Mn(3)-O(2)
O(13)-Mn(3)-O(2)
86.2(3) O(1)-Mn(3)-O(2)
84.7(3) O(12)-Mn(3)-O(4)
83.0(3) O(13)-Mn(3)-O(4)
92.5(3) O(1)-Mn(3)-O(4)
92.7(3) O(2)-Mn(3)-O(4)
175.0(3) O(12)-Mn(3)-O(7)
177.3(4) O(13)-Mn(3)-O(7)
96.5(3) O(1)-Mn(3)-O(7)
95.9(3) O(2)-Mn(3)-O(7)
87.1(3) O(4)-Mn(3)-O(7)
93.6(3) O(16)-Mn(4)-O(2)
178.2(3) O(16)-Mn(4)-O(17)
95.2(3) O(2)-Mn(4)-O(17)
89.0(3) O(16)-Mn(4)-O(3)
83.7(3) O(2)-Mn(4)-O(3)
172.7(4) O(17)-Mn(4)-O(3)
93.2(4) O(16)-Mn(4)-O(11)
94.0(3) O(2)-Mn(4)-O(11)
93.7(3) O(17)-Mn(4)-O(11)
79.0(3) O(3)-Mn(4)-O(11)
170.6(3) O(16)-Mn(4)-O(7)
81.5(4) O(2)-Mn(4)-O(7)
100.1(4) O(17)-Mn(4)-O(7)
83.9(4) O(3)-Mn(4)-O(7)
103.3(3) O(11)-Mn(4)-O(7)
85.6(4) Mn(1)-O(1)-Mn(3)
94.2(3) Mn(1)-O(1)-Mn(2)
81.7(3)
99.5(3)
93.9(3)
88.8(3)
86.3(3)
99.2(3)
80.3(3)
96.1(3)
75.8(3)
160.5(3)
171.3(3)
92.4(3)
95.6(3)
92.0(3)
80.1(3)
174.7(3)
90.2(3)
93.4(3)
88.5(3)
88.6(3)
97.8(3)
78.5(3)
91.9(3)
90.4(3)
171.9(3)
95.1(3)
95.4(3)
85.11(14) O(6)-Mn(4)-O(7)
178.46(14) O(6)-Mn(4)-O(8)
91.93(14) O(6)-Mn(4)-O(15)
92.51(15) O(6)-Mn(4)-O(55)
82.02(13)
79.13(13)
86.38(13)
92.08(14)
93.35(14) O(6)-Mn(4)-O(59) 175.76(14)
94.34(14) O(7)-Mn(4)-O(8)
177.54(15) O(7)-Mn(4)-O(15)
80.14(13)
91.14(13)
88.34(14) O(7)-Mn(4)-O(55) 173.67(15)
89.02(14) O(7)-Mn(4)-O(59)
94.36(14)
86.39(14) O(8)-Mn(4)-O(15) 163.95(13)
81.74(13) O(8)-Mn(4)-O(55)
80.26(13) O(8)-Mn(4)-O(59)
96.54(13)
98.13(13)
87.83(13) O(15)-Mn(4)-O(55) 90.78(14)
173.91(14) O(15)-Mn(4)-O(59) 95.93(14)
92.89(14) O(55)-Mn(4)-O(59) 91.45(14)
80.32(13) Mn(1)-O(5)-Mn(2)
86.48(13) Mn(1)-O(5)-Mn(3)
95.35(15)
94.49(14)
92.81(14) Mn(2)-O(5)-Mn(3) 111.23(16)
174.63(14) Mn(1)-O(6)-Mn(3)
163.32(12) Mn(1)-O(6)-Mn(4)
95.02(15)
94.76(14)
96.21(14) Mn(3)-O(6)-Mn(4) 112.64(15)
98.82(13) Mn(1)-O(7)-Mn(2)
94.57(14) Mn(1)-O(7)-Mn(4)
93.36(14) Mn(2)-O(7)-Mn(4)
92.56(14) Mn(2)-O(8)-Mn(3)
81.84(14) Mn(2)-O(8)-Mn(4)
79.88(13) Mn(2)-O(8)-C(72)
90.35(14) Mn(3)-O(8)-Mn(4)
94.63(14) Mn(3)-O(8)-C(72)
173.24(15) Mn(4)-O(8)-C(72)
79.94(13) Mn(1)-O(9)-C(10)
94.43(15)
95.00(14)
11.41(16)
88.49(12)
87.98(12)
122.3(3)
88.18(11)
124.2(3)
132.8(3)
125.7(3)
122.8(5)
116.1(4)
84.8(3) Mn(3)-O(1)-Mn(2) 116.9(4)
89.5(3) Mn(1)-O(2)-Mn(4)
162.3(4) Mn(1)-O(2)-Mn(3)
95.8(3)
94.7(3)
90.8(3) Mn(4)-O(2)-Mn(3) 111.7(4)
O(6)--Mn(3)-O(17) 88.39(13) O(8)-C(72)-C(73)
92.0(3) Mn(1)-O(3)-Mn(2)
175.7(3) Mn(1)-O(3)-Mn(4)
95.2(3)
94.1(3)
O(6)-Mn(3)-O(38)
O(6)-Mn(3)-O(42)
O(8)-Mn(3)-O(17)
175.08(14) O(8)-C(72)-C(74)
91.40(14) O(73)-C(72)-C(74) 121.1(5)
165.69(13)
171.4(3) Mn(2)-O(3)-Mn(4) 124.2(4)
95.1(3) Mn(4)-O(7)-Mn(3)
92.1(4)
being slightly longer than the others (2.281(3) and 2.297(4) Å).
The [Mn4O3(µ3-O2CR)] cores of 11 and 12 are conveniently
compared in Figure 8, where it can be seen that the [Mn4O3]
positions are nearly superimposable.
In contrast, the MnIII‚‚‚MnIV separations are much shorter at
2.780(2)-2.802(3) Å.The MnIII ions Mn(2-4) are Jahn-Teller
(JT) distorted, as expected for a high-spin d4 ion in near-
octahedral geometry, and the µ3-PhCO2- group occupies posi-
tions on all three JT elongation axes (which again avoid O2-
Complexes 11 and 12 thus possess structures that represent
new additions to the near-isostructural family of [Mn4O3X(O2-
CMe)3(dbm)3] (X ) Cl, Br) species,12 the only difference
between them being the identity of X. Complex 11 does differ
in the bidentate nature of its µ3-PhCO2-, which begs the question
why. It appears that the origin of the different bridging modes
in 11 vs 12 is steric: in Figure 9 are shown space-filling
stereoviews down the C3 rotation axis of the [Mn4O3] core (i.e.,
through the MnIV ion). The dbm groups form a concave cavity
in which resides the µ3-RCO2- group. For the latter to convert
to a η1:µ3-mode, its Ph ring would have to tilt by ∼60°, but
this would be unfavorable due to steric interactions with the
-
ions). Three µ-PhCO2 groups bridging each MnIII/MnIV pair
and a chelating dbm- group on each MnIII complete ligation to
the metals. The molecule has virtual Cs symmetry with the
mirror plane passing through Mn(1), Mn(2), and O(2).
Complex 12 also crystallizes in monoclinic space group P21/n
with no crystallographically imposed symmetry on the Mn4
molecule. The structure is very similar to that of 11 except that
a µ3-MeCO2- group now bridges the three MnIII ions with only
O(8). This η1:µ3-bridging mode leaves O(73) uncoordinated.
The molecule is consequently more symmetric, with virtual C3V
symmetry if free rotation about O(8)-C(72) is assumed. The
MnIII‚‚‚MnIII separations are now in a much narrower range of
3.195(1)-3.213(1) Å, although the three MnIII-O(8) bonds
show some small asymmetry, with Mn(4)-O(8) (2.320(3) Å)
-
dbm groups. In contrast, the smaller MeCO2 group can bind
η1:µ3 without steric problems. It could, of course, also bind like
-
the PhCO2 group in 11, and this suggests that, other things

1510 Inorganic Chemistry, Vol. 39, No. 7, 2000
Wang et al.
eigenvalues for this Hamiltonian are given in eq 11, where
constant terms contributing to all states have been omitted. For
E(ST) ) -J[ST(ST + 1) - SB(SB + 1)] - J33[SB(SB + 1) -
SA(SA + 1)] - J′33[SA(SA + 1)] (11)
3
complex 11, S1 ) /2, S2 ) S3 ) S4 ) 2, and the overall
degeneracy of the spin system is 500, made up of 70 individual
1
spin states ranging from ST ) /2 to 15/2.
A theoretical øM vs T expression for complex 11 was derived
using the Van Vleck equation24 and eq 11, and this expression,
in the form of µeff vs T, was used to fit the experimental data:
only data at 20.0 K and above were employed since the decrease
below 20.0 K is likely due to a combination of Zeeman effects,
zero-field splitting (ZFS), and intermolecular exchange interac-
tions, none of which is incorporated into the fitting model. An
excellent fit was obtained (solid line in Figure 10) having fitting
parameters J34 ) -28.5 cm-1, J33 ) +2.8 cm-1, J′33 ) +2.1
cm-1, and g ) 1.85, with temperature independent paramagnet-
ism (TIP) held constant at 800 × 10-6 cm3 mol-1. These
parameters indicate a ST ) 9/2 ground state for 11, with a ST )
7/2 first excited state.
Figure 8. Comparison of the cores of 11 (open lines) and 12 (black
lines) showing the near superimposability of the [Mn4(µ3-O)3] portions.
-
For 11, the PhCO2 phenyl group is omitted, while for 12 only the
-
metal-bound MeCO2 O atom is shown.
being equal, the η1:µ3-binding mode for a RCO2- group is the
favored one.
Magnetochemistry of Complexes 11 and 12. Variable-
temperature magnetic susceptibility data (øM) were collected on
powdered samples of 11 and 12 in the 2.00-320 K range.
For 11, the effective magnetic moment (µeff) per Mn4
gradually increases from 7.81 µB at 320 K to a maximum of
9.24 µB at 20.0 K and then decreases slightly to 8.63 µB at 5.01
K (Figure 10). These correspond to øMT values of 7.62, 10.67,
and 9.31 cm3 K mol-1 at three temperatures. The data thus
suggest a relatively high ground state spin value, and the
maximum µeff value of 9.24 µB at 20.0 K is comparable with
the spin-only (g ) 2.00) value of 9.95 µB expected for a S )
9/2 state.
In order to fit the experimental data, an appropriate theoretical
øM vs T and µeff vs T expression for 11 was sought. The Cs
symmetry of the core requires four exchange parameters (J), as
shown in Figure 11. The Heisenberg spin Hamiltonian describ-
ing the pairwise interactions is given in eq 8. A full-matrix
For complex 12, the µeff per Mn4 gradually decreases from
8.37 µB at 320 K to a plateau of 9.64 µB at 20.0-40.0 K, and
then decreases to 6.93 µB at 2.00 K (Figure 12). These
correspond to øMT values of 8.76, 11.62, and 6.00 cm3 K mol-1
at these three temperatures. The experimental data at temper-
atures g20.0 K were fit to the theoretical expression derived
previously12a for a C3V symmetry Mn4 trigonal pyramid with
exchange parameters J34 and J33 for the MnIII/MnIV and MnIII/
MnIII interactions, respectively. This is also that obtained from
Figure 11 and eqs 8-11 by putting J34 ) J′34 and J33 ) J′33.
The fit, shown as a solid line in Figure 12, has J34 ) -33.9
cm-1, J33 ) +5.4 cm-1, and g ) 1.94, with TIP held constant
9
at 800 × 10-6 cm3 mol-1. This fit again indicates a ST ) /2
7
ground state and a ST ) /2 first excited state.
In order to confirm the ST ) 9/2 ground states for 11 and 12,
magnetization (M) data were collected in the 2.00-30.0 K range
in magnetic fields of 0.50-50 kG. These data, shown in Figures
13 and 14 as reduced magnetization (M/NµB) vs H/T plots, were
fit by diagonalization of the spin Hamiltonian matrix incorporat-
Hˆ ) -2J34(Sˆ1Sˆ3 + Sˆ1Sˆ4) - 2J′34Sˆ1Sˆ2 - 2J33(Sˆ2Sˆ4 + Sˆ3Sˆ4) -
2J′33Sˆ3Sˆ4 (8)
2
ing axial ZFS (DSZ ) and Zeeman interactions, and assuming
only the ground state to be occupied at these temperatures. In
both cases, good fits were obtained (shown as solid lines in
Figures 13 and 14) with fitting parameters, in the format 11/
diagonalization approach for obtaining the eigenvalues of this
spin Hamiltonian would require diagonalization of a 500 × 500
matrix. Unfortunately, the more convenient equivalent operator
approach, based on the Kambe vector coupling method,23 is not
possible for this Cs symmetry system. However, a very
reasonable simplifying approximation is possible by taking J34
) J34′: these interactions are likely very similar since both
pathways involve very similar [Mn2(µ-O2-)2]3+ rhombs (vide
supra). The spin Hamiltonian now becomes that given in eq 9,
for which the Kambe method is applicable. Using the substitu-
9
12, of ST ) /2 (for both), g ) 2.01/1.96, and D ) -0.58/-
0.47 cm-1
.
The ST and J values obtained for 11 and 12 are compared in
Table 6 with those obtained previously for [Mn4O3X(O2CMe)3-
(dbm)3] complexes with the C3V-symmetry [Mn4(µ3-O)3(µ3-X)]6+
core.12 As can be seen, the four complexes have similar
properties: in all cases, the J34 and J33 parameters are antifer-
romagnetic and ferromagnetic, respectively, and although there
is some variation in absolute magnitude, they are all in the -30
( 3 cm-1 and 5 ( 3 cm-1 ranges, respectively. This is as
expected from their very similar structural parameters, and it
appears that the additional structural perturbations present as a
result of the η2-benzoate group in 11 have little consequence
to the resultant magnetic properties. This is consistent with the
conclusion that the dominant pathways for the exchange
interactions between MnIII/MnIV pairs and between MnIII/MnIII
Hˆ ) -2J34(Sˆ1Sˆ2 + Sˆ1Sˆ3 + Sˆ1Sˆ4) - 2J33(Sˆ2Sˆ4 + Sˆ3Sˆ4) -
2J′33Sˆ3Sˆ4 (9)
tions SˆA ) Sˆ3 + Sˆ4, SˆB ) SˆA + Sˆ2, and SˆT ) SˆB + Sˆ1, where ST
is the spin of the complete Mn4 unit, the spin Hamiltonian of
eq 9 can be converted to the equivalent one in eq 10. The
2
2
Hˆ ) -J34[SˆT2 - SˆB2 - Sˆ1 ] - J33[SˆB2 - SˆA2 - Sˆ2 ] -
(23) Kambe, K. J. Phys. Soc. Jpn. 1950, 5, 48.
(24) Van Vleck, J. H. The Theory of Electric and Magnetic Susceptibilities;
Oxford University Press: London, 1932.
J′33[SˆA2 - Sˆ32 - Sˆ4 ] (10)
2

Tetranuclear Manganese Carboxylate Complexes
Inorganic Chemistry, Vol. 39, No. 7, 2000 1511
Figure 9. Space-filling stereoviews approximately along the C3 virtual axes of 11 (top) and 12 (bottom) showing the µ3-O2CR group in a concave
cavity formed by the dbm groups.
Figure 11. The exchange interactions in complex 11 with Cs symmetry.
The Mn numbering corresponds to that in Figure 6: Mn1 ) MnIV, Mn2
) Mn3 ) Mn4 ) MnIII. J33 and J34 refer to MnIII/MnIII and MnIII/MnIV
interactions, respectively.
for 11 is the smallest among these compounds as a result of its
weaker J33 and J33′ interactions compared with J33 for the µ3,η1-
X- complexes. Similarly, the D values of the four complexes
are of comparable magnitude, with complex 11 having the
greatest absolute magnitude. Note that the absolute magnitudes
Figure 10. Plot of effective magnetic moment (µeff) vs temperature
of the D values are slightly larger than we reported previously;4c
for [Mn4O3(O2CPh)4(dbm)3] (11). The solid line is a fit of the data to
the appropriate theoretical expression; see the text for the fitting
parameters.
for complex 12, we previously reported that D ) -0.32 cm-1
.
The present values in Table 6 are considered more reliable
because they have been obtained with a matrix diagonalization
theoretical approach that calculates a full powder average,
whereas the earlier procedure did not involve a powder average.
Also note that fitting of magnetization data generally yields two
minima, one with D > 0 and one with D < 0, usually giving
comparable fits of the data.4c However, we consider only the D
pairs are the oxide bridges. Also compared in Table 6 are the
9
energy separations between the ST ) /2 ground states and the
ST ) 7/2 first excited states, and the D values obtained from the
7
M/NµB vs H/T fits. In all cases, the ST ) /2 first excited state
is at relatively high energy, and the complexes thus possess
well-isolated ground states. Note, however, that this separation

1512 Inorganic Chemistry, Vol. 39, No. 7, 2000
Wang et al.
Table 6. Comparison of Fitting Parameters for
[Mn4O3X(O2CR)3(dbm)3] Complexes
X
parama
O2CPh (11) O2CMe (12)
Cl
Br
J34, cm-1
-28.5
+2.8
-33.9
+5.4
-28.4 -30.1
J33, cm-1
J′33, cm-1
ground ST
+8.3
+7.4
+2.1
9
9
9
9
/
/
/
/
2
2
2
2
E(ST ) 7/2),b cm-1 114
167
1.96
-0.47
c
185
2.00
179
2.01
g
2.01
-0.58
c
D, cm-1
-0.35 -0.50
ref
d
e
a J34 ) J(MnIII/MnIV); J33 ) J(MnIII/MnIII). b Energy of the ST ) 7/2
first excited state above the ground state. c This work. d Ref 4c.
e Redetermined with a negative D (see text).
Scheme 2
Figure 12. Plot of effective magnetic moment (µeff) vs temperature
for [Mn4O3(O2CMe)4(dbm)3] (12). The solid line is a fit of the data to
the appropriate theoretical expression; see the text for the fitting
parameters.
to prove a valuable chelate in the stabilization of tetranuclear
species, as well as other nuclearities, and this has allowed
reactivity characteristics of certain species to be assessed. In
the present work, dbm- has been used to prepare new
[Mn4O2]8+-containing complexes 6-10 whose electrochemical
signatures suggest a quasi-reversible oxidation. The conse-
quences of this oxidation have been explored, and the products
of the CPE experiments, complexes 11 and 12, have been found
to have distinctly different cores from the starting materials,
with an increased O2-:Mn ratio. This and the reaction conditions
and yields of 11 and 12 suggest that these final products are
forming by reaction of the electrochemically oxidized starting
materials with H2O molecules in the solvent. The overall
transformations to give 11 and 12 are summarized in eqs 6 and
7, respectively. We envisage the sequence of events to, e.g.,
complex 11 to involve an initial oxidation of the [Mn4O2-
(O2CPh)7(dbm)2]- anion of 10 to give [Mn4O2(O2CPh)7(dbm)2]
(eq 12), which is then attacked by an H2O molecule. Although
Figure 13. Plot of reduced magnetization (M/NµB) vs H/T for complex
11 at 0.50 (b), 1.00 (0), 3.00 (4), 5.00 (1), 10.0 (]), 20.0 (9), 30.0
(O), 40.0 (2), and 50.0 kG (3). The solid lines result from a least-
squares fit of the data; see the text for the fitting procedure and the
fitting parameters.
[Mn4O2(O2CPh)7(dbm)2]- -8
-e
[Mn4O2(O2CPh)7(dbm)2] (12)
the identity of transient intermediates is not known, we suggest
that it may involve species in which the water has bound, has
been deprotonated, and is bridging a MnIII/MnIV pair as a OH-,
-
the displaced PhCO2 acting as the H+ acceptor (eq 13); the
Figure 14. Plot of reduced magnetization (M/NµB) vs H/T for complex
12 at 0.50 (b), 1.00 (0), 3.00 (4), 5.00 (1), 10.0 (]), 20.0 (9), 30.0
(O), 40.0 (2), and 50.0 kG (3). The solid lines result from a least-
squares fit of the data; see the text for the fitting procedure and the
fitting parameters.
core transformation is depicted in Scheme 2. The conversion
[Mn4O2(O2CPh)7(dbm)2] + H2O f
[Mn4O2(OH)(O2CPh)6(dbm)2] + PhCO2H (13)
< 0 fits in the present work because previous studies of the
temperature dependencies of high-field EPR signals have shown
unequivocally that D < 0 for these [Mn4O3X(O2CMe)3(dbm)3]
complexes.4b
of this [Mn4O2(OH)] core to that of final product 11 now
requires deprotonation of the OH- and its conversion to a µ3-
O2- (Scheme 2) concomitant with attachment of a third dbm-
-
Discussion
group, with the displaced PhCO2 groups again acting as
A variety of Mn carboxylate species has been obtained by a
combination of synthetic methods. The dbm- anion continues
Brønsted bases for the OH- and dbmH deprotonations (eqs 6
and 7). Note that Scheme 2 is consistent with the structural

Tetranuclear Manganese Carboxylate Complexes
Inorganic Chemistry, Vol. 39, No. 7, 2000 1513
relationship between the [Mn4O2] and [Mn4O3] cores, i.e., the
latter is a “closed-up” version of the former caused by the
presence of a third µ3-O2- ion. Also note that the spontaneous
incorporation of an extra O2- ion on oxidation of the [Mn4O2]
core is consistent with the general trend that increasing average
oxidation state favors an increase in the proportion of hard O2-
ions.25 The MnO4- ion lies at one extreme of this relationship.
The spontaneous incorporation of a third O2- ion in 11 and 12
is thus consistent with the average oxidation state increase from
+3.00 to +3.25. Alternatively, one can describe this as the third
will react with H2O to give [Mn4O3(OH)(O2CMe)3(dbm)3], a
µ3-OH- replacing the µ3-O2CMe-. In effect, this and the
conversion of 6 to 12 provide the demonstration of the sequence
of eq 14 that represents the activation of two H2O molecules
[Mn4O2] -8 [Mn4O3]
8 [Mn4O3(OH)]
H2O
(14)
-e
H2O
by binding to Lewis acidic Mnn+ centers and partial or complete
deprotonation. Fuller details will be provided in a subsequent
paper on the hydrolysis of 12. The [Mn4O2] and [Mn4O3]
complexes are not exact structural models of the WOC on the
basis of, for example, EXAFS data27 and ground state spin
differences; however, the oxidation-driven chemistry they are
exhibiting may nevertheless be providing important insights into
the means by which H2O molecules are bound and activated to
oxidation to O2 in photosynthesis, whatever the precise structure
of the native WOC may be.
O
2- ion helping to stabilize the higher oxidation state species.12a
From a biological viewpoint, the results described in this
paper offer some food for thought. The coupled oxidation/oxide
incorporation seen in going from, for example, [Mn4O2] complex
10 to [Mn4O3] complex 11 could be considered a model system
for the progressive oxidation/substrate activation processes in
the native WOC, i.e., H2O-derived O2- ions are stabilizing the
increasing WOC Sn oxidation level, and, inversely, of course,
the increasing WOC Sn oxidation level is stabilizing activated/
deprotonated forms of H2O in preparation for their oxidative
coupling to O2.
Acknowledgment. This work was supported by NIH Grant
GM 39083 to G.C. and NSF Grant CHE 9727312 to D.N.H.
Supporting Information Available: Tables of crystallographic
data, structure refinement details, atomic coordinates, interatomic
distances and angles, anisotropic thermal parameters, and calculated
hydrogen parameters. This material is available free of charge via the
Internet at http://pubs.acs.org.
The above discussion can be extended one step further when
our recently reported26 observation is noted that complex 12
(25) Christou, G.; Vincent, J. B. In Metal Clusters in Proteins; ACS Symp.
Ser. 372; Que, L., Ed.; American Chemical Society, 1988; Chapter
12, pp 238-255.
(26) Arom´ı, G.; Wemple, M. W.; Aubin, S. M. J.; Folting, K.; Hendrickson,
D. N.; Christou, G. J. Am. Chem. Soc. 1998, 120, 5850.
IC991068M
(27) Cinco, R. M.; Rompel, A.; Visser, H.; Arom´ı, G.; Christou, G.; Sauer,
K.; Yachandra, V. K.; Klein, M. P. Inorg. Chem., in press.