High-Spin Molecules: (NBu II 4 )[Mn 8 O 6 Cl 6 (O 2 CPh) 7 (H 2 O) 2 ] (S = 11) and [Mn 9 Na 2 O 7 (O 2 CPh) 15 (MeCN) 2 ] (S = 4)

Hui-Lien Tsai, David N. Hendrickson, Sheyi Wang, Kirsten Folting, William E. Streib, George Christou

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Abstract

The syntheses and electrochemical and magnetochemical properties of (NBu n 4 )[Mn 8 O 6 Cl 6 (O 2 CPh) 7 -(H 2 O) 2 ]·XCH 2 C1 2 (2·xCH 2 Cl 2 ) and [Mn 9 Na 2 O 7 (O 2 CPh) 15 (MeCN) 2 ]·3MeCN (3·3MeCN) are reported. Both complexes were prepared from reactions involving (NBu n 4 )[Mn 4 O 2 (H 2 O)(O 2 CPh) 9 ] (1). Complex 2 was obtained in 45–60% yield on treatment of complex 1 with 4 equiv of Me 3 SiCl in CH 2 C1 2 . Complex 2· 3 / 2 CH 2 Cl 2 ·2H 2 O crystallizes in triclinic space group P1 with the following cell parameters at –169 °C: a = 16.104(4) Å, b = 21.501(6) Å, c = 14.843(4) Å, a = 94.24(1)°, β = 105.96(1)°, y = 89.07(1)°, V = 4927.79 Å 3 , and Z = 2. The structure was refined by employing 10 108 unique reflections with F > 3.0σ(F) to give R = 0.0912 and R w = 0.0944. Complex 2 contains a [Mn 8 O 6 Cl 4 ] 8+ core (8Mn III ) that may be conveniently described as resulting from the fusion of two [Mn 4 O 2 ] butterfly units by sharing one “body” or “hinge” Mn. An eighth Mn III ion is connected to the resultant [Mn 7 O4] unit by two additional bridging O 2– ions. Complex 3 was obtained in 31% yield from the treatment of complex 1 with 1 equiv of benzoyl peroxide in MeCN, followed by addition of NaClO 4 . Complex 3·3MeCN crystallizes in triclinic space group P1 with the following cell parameters at –170 °C: a = 15.116(2) Å b = 27.903(4) Å, c = 15.007(2) Å, α = 102.40(1)°, β = 112.36(1)°, γ = 84.17(1)°, V = 5715.26 Å 3 , and Z = 2. The structure was refined by employing 12 020 unique reflections with F > 3σ(F) to give R = 0.0514 and R w = 0.0525. Complex 3 possesses a mixed-metal undecanuclear [Mn 9 Na 2 O7] 15+ core (9Mn III ); the Mn 9 O 7 subcore again comprises a [Mn 7 O4] unit constructed from the fusion, in the same manner as for complex 2, of two [Mn 4 O 2 ] butterfly units. There are now two additional Mn III ions connected to the [Mn 7 O 4 ] unit, by three additional bridging O 2– ions. The Na + ions are bound to bridging O 2– ions of the [Mn 9 O 7 ] core, supporting a heterometallic-aggregate description rather than an ion-pairing description. Complex 2 displays reversible redox couples when examined by cyclic voltammetry in CH 2 C1 2 ; an oxidation and a reduction are observed at 0.91 and 0.12 V, respectively, vs ferrocene. Complex 3 displays no reversible processes. Variable-temperature and variable-field dc magnetic susceptibility data were collected for polycrystalline samples of complexes 2 and 3. In a 10.0 kG field, χ M T for complex 2 increases with decreasing temperature from 23.7 cm 3 K mol –1eff = 13.8 μ B ) at 300.0 K to a maximum of 53.2 cm 3 K mol –1eff = 20.6 μ B ) at 15.0 K, whereupon there is a decrease to 39.3 cm 3 K mol –1eff =17.7 μ B ) at 5.01 K. The shape of this χ M T vs temperature curve reflects the population of a S = 11 ground state at low temperature. Least-squares fitting of reduced magnetization vs HIT data for complex 2 in the 2.0—30.0 K and 5.00—50.0 kG field range confirms that complex 2 has a S = 11 ground state with g = 1.92 and axial zero-field splitting of D = —0.04 cm –1 . The ac susceptibility data measured in zero external field for complex 2 in the 2.0—30.0 K range also indicate a S = 11 ground state. No out-of-phase ac magnetic susceptibility signal was observed for complex 2 in the 2.0—30.0 K range in spite of its large-spin ground state. The absence of an out-of-phase ac signal is attributable to a very small zero-field splitting in the S = 11 ground state, and the small D-value results from a near cancellation of single-ion zero-field interactions at the eight Mn III ions in complex 2. Complex 3 exhibits dc and ac magnetic susceptibility data consistent with a S = 4 ground state. In a 10.0 kG field μ eff decreases gradually from 12.23 μ B at 320 K to 6.94 μ B at 5.01 K. Even though there are some similarities in molecular structure between complexes 2 and 3, differences in the nature of spin frustration result in a S = 11 ground state for complex 2 and a S = 4 ground state for complex 3.

Original languageEnglish
Pages (from-to)2503-2514
Number of pages12
JournalJournal of the American Chemical Society
Volume117
Issue number9
DOIs
Publication statusPublished - 1995 Jan 1

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

Fingerprint Dive into the research topics of 'High-Spin Molecules: (NBu <sup>II</sup> <sub>4</sub> )[Mn <sub>8</sub> O <sub>6</sub> Cl <sub>6</sub> (O <sub>2</sub> CPh) <sub>7</sub> (H <sub>2</sub> O) <sub>2</sub> ] (S = 11) and [Mn <sub>9</sub> Na <sub>2</sub> O <sub>7</sub> (O <sub>2</sub> CPh) <sub>15</sub> (MeCN) <sub>2</sub> ] (S = 4)'. Together they form a unique fingerprint.

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