3760 Inorganic Chemistry, Vol. 37, No. 15, 1998
Abbati et al.
Synthesis. All procedures were performed under aerobic conditions
with strict exclusion of moisture. All reagents and solvents were used
as received, with exception of methanol, which was distilled over Mg-
(OMe)2 shortly before use.15 The complex Mn(dbm)3 (4) was
synthesized by a literature procedure.16b
oxygen cores often exhibit closest-packing layered structures
which are typical of bulk metal oxides. Surprisingly, manganese
chemistry with alkoxide ligands remains a quite unexplored area.
Recently, the synthesis of MnII complexes with cuboidal [Mn4-
(OR)4]4+ cores, namely [Mn4(OMe)4(L)4(MeOH)4] (HL )
Hdbm, Hdpm) and [Mn4(OEt)4(dpm)4(EtOH)2],11,12 has been
reported. Their extreme sensitivity to air oxidation has been
ascribed to the hard-donor character of alkoxide ligands, which
stabilize high-valent metal states.11 We therefore investigated
the possibility of exploiting this tendency to address the
synthesis of high-nuclearity, preferably high-valent manganese
clusters which may behave as single-molecule magnets.3 The
synthesis, solid-state structure, and magnetic properties of the
complex [NaMnIII6(OMe)12(dbm)6]BPh4‚xCHCl3 (1) have been
already reported.12,13 The cluster comprises a ring of six MnIII
ions, with a sodium ion in the center. The coupling between
the Jahn-Teller distorted ions is ferromagnetic, yielding a
ground S ) 12 state. Substitution of the sodium ion with a
magnetic ion appeared to be exciting for tuning the magnetic
properties of the clusters and also for introducing valence
fluctuation, exploring the possibility of synthesizing either
trapped or delocalized mixed-valence species. We report here
the synthesis and characterization of the novel heptanuclear
alkoxomanganese cluster [Mn7(OMe)12(dbm)6]‚CHCl3‚14MeOH
(2), which has noninteger average valence of the manganese
ions. It is a high-spin molecule whose layered structure mimics
a fragment of the manganese oxide mineral lithiophorite. We
report also the structure and solid-state magnetic behavior of
the dimeric complex [MnIII2(OMe)2(dbm)4] (3), which was
obtained as a byproduct of the synthetic procedure used for 2.
[Mn7(OMe)12(dbm)6]‚CHCl3‚14MeOH (2). To 10 mmol of MnCl2
dissolved in 30 mL of anhydrous methanol was added dropwise with
vigorous stirring a solution containing 40 mmol of NaOMe and 10
mmol of Hdbm in 120 mL of anhydrous methanol, yielding a brown
precipitate which was rapidly collected by filtration and dried under
vacuum. A 0.25-g amount of freshly prepared precipitate was dissolved
in chloroform (10 mL) and the resulting solution was layered with a
double volume of anhydrous methanol in a sealed tube. Dark-brown
cubes of 2 formed in 1-2 weeks. The overall yield was about 53%
(referred to Mn). The crystals underwent extremely rapid solvent loss
when removed from their mother liquid and immediately became a
brown microcrystalline powder. Analyses were performed on crystals
washed with an MeOH-CHCl3 1:1 (v/v) mixture and dried under
vacuum. Anal. Calcd for [Mn7(OMe)12(dbm)6]: C, 58.44; H, 4.90;
Mn, 18.43. Found: C, 58.04; H, 4.94; Mn, 18.92. UV-vis (solid):
10 000, 17 000, 21 000 cm-1. IR (KBr pellets): 3430 (s), 3060 (m),
2918 (m), 2811 (m), 1618 (m), 1605 (s), 1555 (s), 1520 (s), 1480 (s),
1453 (s), 1442 (m), 1390 (s), 1308 (s), 1290 (s), 1226 (s), 1182 (m),
1096 (w), 1069 (s), 1060 (s), 1023 (s), 1000 (w), 940 (w), 756 (m),
720 (s), 688(s), 624 (s), 551 (s) cm-1
.
[Mn2(OMe)2(dbm)4] (3). Air-stable dark-brown crystals of 3 were
obtained in a manner analogous to that for 2, replacing the layering of
liquid methanol with slow diffusion of methanol vapors to induce
crystallization. Anal. Calcd for 3: C, 69.93; H, 4.73. Found on a
vacuum-dried sample: C, 69.83; H, 4.76. UV-vis (solid): 9000,
18 000, 21 000 cm-1. IR (KBr pellets): 1594 (s), 1509 (s), 1476 (s),
1453 (m), 1441 (m), 1388 (s), 1341 (m), 1306 (s), 1285 (s), 1220 (m),
1173 (m), 1121 (w), 1050 (m), 1015 (m), 988 (m), 925 (m), 764 (w),
726 (s), 688 (s), 623 (m), 570 (w), 514 (w) cm-1
.
Experimental Section
[Mn(dbm)2(H2O)2] (6). A variation of the method of Fernelius and
Bryant was used.17 Under a dinitrogen atmosphere, 4 mmol of solid
MnBr2‚4H2O was quickly added to a solution of Hdbm (8 mmol) and
triethylamine (20 mmol) in 40 mL of dioxygen-free MeCN. Upon
stirring, a yellow precipitate formed in good yield (about 70%) in few
minutes. The mixture was stirred for an additional 30 min; then the
precipitate was collected by filtration and dried under a dinitrogen
stream. Compound 6 underwent extremely rapid air oxidation if wet
or if in solution. The dry material was air-stable. Anal. Calcd for 6:
C, 67.04; H, 4.88. Found: C, 66.57; H, 4.92.
X-ray Crystallography. X-ray-quality samples of 2 and 3 were
obtained directly as described above. Structures were solved using
direct methods (SIR9218) and refined using the SHELXL-93 package.19
Details of the crystal data and structure refinement are reported in Table
1. All non-hydrogen atoms, with the exception of solvent atoms, were
refined with anisotropic thermal parameters. Unless otherwise stated,
hydrogen atoms were treated as fixed contributors in calculated positions
and refined isotropically with B(H) ) 1.2Beq(C). Final atomic
coordinates for 2 and 3 are given in Tables 2 and 3, respectively.
Selected interatomic distances and angles can be found in Tables 4
and 5, respectively. CIF files for 2 and 3 are available as Supporting
Information.
Analytical Procedures. 1H NMR spectra were recorded on a Bruker
Advance DPX spectrometer operating at 200 MHz. IR and UV-vis
spectra were recorded on FTIR Bruker IFS 113v and Perkin-Elmer
Lambda 9 spectrometers. Conductivity measurements were carried out
by using an Amel 134 conductometer. C, H, N elemental analyses
were performed using a Carlo Erba 1106 automatic analyzer. Mn was
determined by an ICP (inductively coupled plasma) spectrometer,
SPECTRO D, after decomposition of the sample in a Kjeldahl flask
by treatment with sulfuric acid.14 Major calculations were performed
on an Alpha 3000/800S computer and an RISC workstation.
(7) (a) Taft, K. L.; Caneschi, A.; Pence, L.; Delfs, C. D.; Papaefthymiou,
G. C.; Lippard, S. J. J. Am. Chem. Soc. 1993, 115, 11753 and
references therein. (b) Khan, M. I.; Chen, Q.; Ho¨pe, H.; Parkin, S.;
O’Connor, C. J.; Zubieta, J. Inorg. Chem. 1993, 32, 2929 and
references therein.
(8) (a) Caneschi, A.; Cornia, A.; Lippard, S. J. Angew. Chem., Int. Ed.
Engl. 1995, 34, 467. (b) Caneschi, A.; Cornia, A.; Fabretti, A. C.;
Foner, S.; Gatteschi, D.; Grandi, R.; Schenetti, L. Chem. Eur. J. 1996,
2, 1379. (c) Abbati, G. L.; Caneschi, A.; Cornia, A.; Fabretti, A. C.;
Gatteschi, D.; Malavasi, W.; Schenetti, L. Inorg. Chem. 1997, 36, 6443.
(9) (a) Caneschi, A.; Cornia, A.; Fabretti, A. C.; Gatteschi, D.; Malavasi,
W. Inorg. Chem. 1995, 34, 4660. (b) Caneschi, A.; Cornia, A.; Fabretti,
A. C.; Gatteschi, D. Angew. Chem., Int. Ed. Engl. 1995, 34, 2716. (c)
Cornia, A.; Gatteschi, D.; Hegetschweiler, K. Inorg. Chem. 1994, 33,
1559. (d) Taft, K. L.; Papaefthymiou, G. C.; Lippard, S. J. Inorg.
Chem. 1994, 33, 1510. (e) Caneschi, A.; Cornia, A.; Lippard, S. J.;
Papaefthymiou, G. C.; Sessoli, R. Inorg. Chim. Acta 1996, 243, 295.
(10) Le Gall, F.; Fabrizi de Biani, F.; Caneschi, A.; Cinelli, P.; Cornia, A.;
Fabretti, A. C.; Gatteschi, D. Inorg. Chim. Acta 1997, 262, 123.
(11) Caneschi, A.; Pence, L.; Lippard, S. J. Inorg. Chem. 1996, 35, 3069.
(12) Abbreviations used in the text: Hdbm ) dibenzoylmethane; Hdpm
) dipivaloylmethane; Hhmp ) 2-(hydroxymethyl)pyridine; tren ) tris-
(aminoethyl)amine; Htphpn ) N,N,N′,N′-tetrakis(2-pyridylmethyl)-2-
hydroxypropane-1,3-diamine.
[Mn7(OMe)12(dbm)6]‚CHCl3‚14MeOH (2). A dark-brown cube
(0.38 × 0.72 × 0.66 mm) was removed from the mother liquid and
mounted under a cold dinitrogen stream on the top of a quartz glass
(15) Vogel, A. I. Practical Organic Chemistry, 3rd ed.; Longmans: London,
1959; p 169.
(16) (a) Zaitseva, E. G.; Baidina, I. A.; Stabnikov, P. A.; Borisov, S. V.;
Igumenov, I. K. Zh. Strukt. Khim. 1990, 31, 184. See also Chem. Abstr.
1990, 113, 88602h. (b) 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.
(17) Fernelius, W. C.; Bryant, B. E. Inorg. Synth. 1957, 5, 105.
(18) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo,
C.; Guagliardi, A.; Polidori, G. J. Appl. Crystallogr. 1994, 27, 435.
(19) Sheldrick, G. M. SHELXL-93: Program for Crystal Structure Refine-
ment; University of Goettingen: Goettingen, Germany, 1993.
(13) Abbati, G. L.; Cornia, A.; Fabretti, A. C.; Caneschi, A.; Gatteschi, D.
Inorg. Chem. 1998, 37, 1430.
(14) Ingram, G. Methods of Organic Elemental Microanalysis; Chapman
& Hall: London, 1962; p 274.