The preparation and properties are described of two related Mn 10 clusters with the same [Mn 10 O 8 ] 14+ (10 Mn III ) core. Dissolution of [Mn 4 O 2 (O 2 CPh) 6 (MeCN) 2 (pic) 2 ] (1) in MeCONMe 2 (DMA) and CH 2 Cl 2 in the presence of picH (picolinic acid) leads to slow crystallization of [Mn 10 O 8 (O 2 CPh) 6 (pic) 8 ] (2) in 50-55% yield. The same reaction in the presence of dibenzoylmethane (dbmH) instead of picH gives [Mn 10 O 8 (O 2 CPh) 6 (pic) 6 (dbm) 2 ] (3). Complex 2‚6CH 2 Cl 2 crystallizes in the monoclinic space group P2 1 /n with (at -169°C) a ) 14.987 (3) Å, b ) 17.177(3) Å, c ) 21.250(4) Å, ) 91.48(1)°, and Z ) 2. Complex 3‚3CH 2 Cl 2 crystallizes in the triclinic space group P1 h with (at -165°C) a ) 14.892(6) Å, b ) 15.537(6) Å, c ) 14.697(6) Å, R ) 109.82(2)°, ) 97.75-(2)°, γ ) 111.66 (2)°, and Z ) 1. The two structures are very similar, differing only in the chelate identity at two positions, and resulting small perturbations in some metric parameters. Variable-temperature magnetic susceptibility data were obtained for 2‚3H 2 O with a dc SQUID in the range 2.00-320 K and in a 10 kG applied field. The effective magnetic moment (µ eff ) per molecule slowly decreases from 14.28 µ B at 320 K to 4.94 µ B at 2.00 K. Data were also collected with an ac SQUID in the range 2.00-30.0 K in a 1 G field oscillating at 250 Hz. The ac data essentially superimpose on the dc data except at temperatures e6 K. The combined results indicate a S T ) 0 ground state for 2 with a very low-lying excited state(s) that is (are) populated even at 2.00 K. The possibility of a S T ) 1 (or even S T ) 2) ground state cannot be completely ruled out, however, in the absence of data at <2 K.
Convenient, high-yield routes have been developed to [Fe 10(OMe) 20(O 2CR) 10] ( 1) "ferric wheels" involving the alcoholysis of [Fe 3O(O 2CR) 6(H 2O) 3] (+) salts in MeOH in the presence of NEt 3. Reactivity studies have established [Fe 10(OMe) 20(O 2CMe) 10] ( 1a) to undergo clean carboxylate substitution with a variety of other RCO 2H groups to the corresponding [Fe 10(OMe) 20(O 2CR) 10] product. In contrast, the reaction with phenol causes a nuclearity change to give a smaller [Fe 8(OH) 4(OPh) 8(O 2CR) 12] ( 2) wheel. Similarly, reactions of [Fe 10(OMe) 20(O 2CR) 10] with the bidentate chelate ethylenediamine (en) cause a structural change to give either [Fe 8O 5(O 2CMe) 8(en) 8](ClO 4) 6 ( 3) or [Fe 2O(O 2CBu (t))(en) 4](NO 3) 3 ( 4), depending on conditions. Complex 3 possesses a "Christmas-star" Fe 8 topology comprising a central planar [Fe 4(mu 4-O)] (10+) square subunit edge-fused to four oxide-centered [Fe 3(mu 3-O)] (7+) triangular units. Variable-temperature, solid-state dc and ac magnetization studies on complexes 1a- 4 in the 5.0-300 K range established that all the complexes possess an S = 0 ground state. The magnetic susceptibility data for 4 were fit to the theoretical chi M versus T expression derived by the use of an isotropic Heisenberg spin Hamiltonian and the Van Vleck equation, and this revealed an antiferromagnetic exchange parameter with a value of J = -107.7(5) cm (-1). This value is consistent with that predicted by a previously published magnetostructural relationship. Theoretically computed values of the exchange constants in 3 were obtained with the ZILSH method, and the pattern of spin frustration within its core and the origin of its S = 0 ground state have been analyzed in detail.
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