Single-molecule magnets: site-specific ligand abstraction from [Mn12O12(O2CR)16(H2O)4] and the preparation and properties of [Mn12O12(NO3)4(O2CCH2Bu(t))12(H2O)4].

Site-selective carboxylate abstraction has been achieved from [Mn(12)O(12)(O(2)CR)(16)(H(2)O)(4)] complexes by treatment with HNO(3) in MeCN. The reaction of the R = Ph or CH(2)Bu(t)() complexes with 4 equiv of HNO(3) gives [Mn(12)O(12)(NO(3))(4)(O(2)CR)(12)(H(2)O)(4)] (R = CH(2)Bu(t) (6) or Ph (7)) in analytical purity. Complex 6.MeNO(2) crystallizes in monoclinic space group C2/c with the following cell parameters at -168 degrees C: a = 21.280(5), b = 34.430(8), c = 33.023(8) A, beta = 104.61(1) degrees, V = 23413 A, and Z = 8. The four NO(3)(-) groups are not disordered and are bound in bridging modes at axial positions formerly occupied by bridging carboxylate groups. (1)H NMR spectroscopy in CD(2)Cl(2) and CDCl(3) shows retention of the solid-state structure on dissolution in these solvents. DC magnetic susceptibility (chi(M)) and magnetization (M) studies have been carried out in the 2.00-300 K and 1.0-7.0 T ranges. Fits of M/Nmu(B) versus H/T plots gave S = 10, g = 1.92, and D = -0.40 cm(-1), where D is the axial zero-field splitting parameter. AC magnetic susceptibility studies on 6 have been performed in the 1.70-10.0 K range in a 3.5 Oe field oscillating at frequencies up to 1500 Hz. Out-of-phase magnetic susceptibility (chi(M)' ') signals were observed in the 4.00-8.00 K range which were frequency-dependent. Thus, 6 displays the slow magnetization relaxation diagnostic of a single-molecule magnet (SMM). The data were fit to the Arrhenius law, and this gave the effective barrier to relaxation (U(eff)) of 50.0 cm(-1) (72.0 K) and a pre-exponential (1/tau(0)) of 1.9 x 10(8) s(-1). Complex 6 also shows hysteresis in magnetization versus DC field scans, and the hysteresis loops show steps at regular intervals of magnetic field, the diagnostic evidence of field-tuned quantum tunneling of magnetization. High-frequency EPR (HFEPR) spectroscopy on oriented crystals of complex 6 shows resonances assigned to transitions between zero-field split M(s) states of the S = 10 ground state. Fitting of the data gave S = 10, g = 1.99, D = -0.46 cm(-1), and B(4)(0) = -2.0 x 10(-5), where B(4)(0) is the quartic zero-field coefficient. The combined results demonstrate that replacement of four carboxylate groups with NO(3)(-) groups leads to insignificant perturbation of the magnetic properties of the Mn(12) complex. Complex 6 should now be a useful starting point for further reactivity studies, taking advantage of the good leaving group properties of the NO(3)(-) ligands.

Mixed-valence tetranuclear manganese single-molecule magnets.

The preparations, X-ray structures, and detailed physical characterizations are presented for two new mixed-valence tetranuclear manganese complexes that function as single-molecule magnets (SMM's): [Mn4(hmp)6Br2(H2O)2]Br2-4H2O (2) and [Mn4(6-me-hmp)6Cl4]-4H2O (3), where hmp(-) is the anion of 2-hydroxymethylpyridine and 6-me-hmp(-) is the anion of 6-methyl-2-hydroxymethylpyridine. Complex 2-4H2O crystallizes in the space group P2(1)/c, with cell dimensions at -160 degrees C of a = 10.907(0) A, b = 15.788(0) A, c = 13.941(0) A, beta = 101.21(0) degrees, and Z = 2. The cation lies on an inversion center and consists of a planar Mn4 rhombus that is mixed-valence, Mn2(III)Mn2(II). The hmp(-) ligands function as bidentate ligands and as the only bridging ligands in 2-4H2O. Complex 3-4H2O crystallizes in the monoclinic space group C2/c, with cell dimensions at -160 degrees C of a = 17.0852(4) A, b = 20.8781(5) A, c = 14.835(3) A, beta = 90.5485(8) degrees, and Z = 4. This neutral complex also has a mixed-valence Mn2(III)Mn2(II) composition and is best described as having four manganese ions arranged in a bent chain. A mu2-oxygen atom of the 6-me-hmp(-) anion bridges between the manganese ions; the Cl(-) ligands are terminal. Variable-field magnetization and high-frequency and -field EPR (HFEPR) data indicate that complex 2-4H2O has a S = 9 ground state whereas complex 3.4H(2)O has S = 0 ground state. Fine structure patterns are seen in the HFEPR spectra, and in the case of 2.4H(2)O it was possible to simulate the fine structure assuming S = 9 with the parameters g = 1.999, axial zero-field splitting of D/k(B) = -0.498 K, quartic longitudinal zero-field splitting of B4(omicron)/k(B) = 1.72 x 10(-5) K, and rhombic zero-field splitting of E/k(B) = 0.124 K. Complex 2-4H2O exhibits a frequency-dependent out-of-phase AC magnetic susceptibility signal, clearly indicating that this complex functions as a SMM. The AC susceptibility data for complex 2-4H2O were measured in the 0.05-4.0 K range and when fit to the Arrhenius law, gave an activation energy of DeltaE = 15.8 K for the reversal of magnetization. This DeltaE value is to be compared to the potential-energy barrier height of U/k(B) = absolute value DSz(2) = 40.3 K calculated for 2-4H2O.

Reactivity of [RuHCl(PiPr3)2]2 with functionalized vinyl substrates. The H2 ligand as a sensitive probe of electronic structure.

Reaction of [RuHClL2]2 (L = PiPr3) with 2-vinylpyridine gives L2ClRu(eta 2-CH=CHC5H4N) with liberation of H2. Reaction of [RuHClL2]2 with a range of olefins D(H)C=CR(EWG) substituted by electron-donating (D) and -withdrawing (EWG) groups occurs by oxidative addition of a vinyl C-H bond to give the metallacycles L2ClHnRu(eta 2-(++)C(D)=CR(EWG)). The 13C chemical shift of ++C and the fate of the "Hn" unit (decoordination, binding as H2, or binding as two hydrides) are strongly correlated, depend on the donating and withdrawing power of D and EWG, and can be used to decide whether ++C binds to Ru as a carbene or as a vinyl. These results emphasize the reducing power of Ru(II) when pi-acid ligands such as CO are absent.

A new class of electron-rich unsaturated molecules: Ru2HnX4-n(PiPr3)4, X = anion.

Synthesis, spectroscopic, and X-ray structural characterization of Ru2HnCl4-nL4 (n = 2, 3) and Ru2H2F2L4 (L = PiPr3) are reported. The structure of Ru2HCl3L4 is also reported. These are dinuclear species containing two five-coordinate, approximately square-pyramidal metal atoms. Halides, not hydrides, preferentially occupy bridging sites, and the RuXL2 terminal moiety shows limited fluxionality, but hydrides do not migrate between metals. The limited steric protection provided by PiPr3 is evident from the dimerization observed and from the fact that all these structures have rather small [symbol: see text]P-Ru-P (approximately 105 degrees). Also reported are RuHXL2 species with X = acetylacetonate, phenoxide, O3SCH3, and O3SCF3. Several examples of coordinated olefin to complexed carbene conversions are used to test the influence of anion X on reactivity.

[Mn6O2[O2C-3,5-(NO2)2-C6H3]10-(C5H5N)2[(CH3)2CO]2].2(CH3)2CO.- 2(C2H5)2O and [Mn6O2(O2CC6H5)10-(NCCH3)4].

The structures of di(acetone)tetrakis(mu 3,5-dinitrobenzoato-kappa 2 O:kappa O')hexakis(mu-3,5-dinitrobenzoato-kappa O:kappa O')-di-mu 4-oxo-di(pyridine)tetramanganese(II)dimanganese (III)-acetone-diethyl ether (1/2/2), (1), and tetrakis(acetonitrile)tetrakis(mu 3-benzoato-kappa 2 O:kappa O')hexakis(mu-benzoato-kappa O:kappa O')-di-mu 4-oxo-tetramanganese(II)dimanganese(III), (2), are reported. Both compounds contain six octahedrally coordinated Mn centres, arranged as two MnII2MnIII2 (mu 4-O) tetrahedra sharing the MnIII-MnIII edge.