RESUMEN
The synthesis and structural characterization of a mixed-valent uranium(V/VI) oxo-imido complex are reported. Reaction of the uranyl chloride complex [K(18-crown-6)](2)[UO(2)Cl(4)] (1) with the triamidoamine ligand Li(3)[N(CH(2)CH(2)NSiBu(t)Me(2))(3)] yields oxo-imido [K(18-crown-6)(Et(2)O)][UO(mu(2)-NuCH(2)CH(2)N(CH(2)CH(2)NSiBu(t)Me(2))(2))](2) (2) as the major isolated uranium product in moderate yield. The reaction that forms 2 involves activation of both the triamidoamine ligand and the uranyl dioxo unit of 1. An X-ray crystal structure determination of 2 reveals a dimeric complex in which the coordination geometry at each uranium center is that of a capped trigonal bipyramid. The multidentate triamidoamine ligand coordinates to uranium through the capping amine and two of the three pendant amido ligands, while the third pendant amido donor has been activated to generate a bridging imido ligand by loss of the silyl substituent. One of the uranyl oxo groups is retained as a terminal ligand to complete the coordination sphere for each uranium center. The oxo and imido nitrogen may be regarded as the axial ligands of the trigonal bipyramid, while the two amido ligands and the other imido donor occupy equatorial coordination sites. The central amine of the tripodal set serves as the capping ligand. Distortion of the axial O-U-N angle from 180 degrees emanates from the proximity of the capping amine and the bridging interaction to the other uranium center. The structure and bonding in 2 are assessed in the context of metal-ligand multiple bonding in high-valent actinide complexes. The possibility of valence averaging [5.5/5.5 vs 5.0/6.0] via delocalization or rapid intramolecular electron-transfer dynamics of the unpaired electron is also discussed in the context of crystallographic, spectroscopic (NMR, IR, Raman, and EPR), and electrochemical data. Crystal data for 2: triclinic space group P1 macro, a = 12.1144(6) A, b = 12.6084(6) A, c = 14.5072(7) A, alpha = 101.374(1) degrees, beta = 103.757(1) degrees, gamma = 109.340(1) degrees, z = 1, R1 = 0.0523, wR2 = 0.1359.
RESUMEN
The first known paramagnetic, tetrahedral cyanide complex, [Mn(II)(CN)(4)](2)(-), is formed by the photoinduced decomposition of [Mn(IV)(CN)(6)](2)(-) in nonaqueous solutions or by thermal decomposition in the solid state. In acetonitrile or dichloromethane, photoexcitation into the ligand-to-metal charge transfer band (lambda(max) = 25 700 cm(-1), epsilon = 3700 cm(-1) M(-1)) causes the homolytic cleavage of cyanide radicals and reduction of Mn(IV). Free cyanide in dichloromethane leads to the isolation of polycyanide oligomers such as [C(12)N(12)](2)(-) and [C(4)N(4)](-), which was crystallographically characterized as the PPN(+) salt C(40)H(30)N(5)P(2): monoclinic space group = I2/a, a = 18.6314(2) A, b = 9.1926(1) A, c = 20.8006(1), beta =106.176(2) degrees, Z = 4]. In the solid state Mn(IV)-CN bond homolysis is thermally activated above 122 degrees C, according to differential scanning calorimetry measurements, leading to the reductive elimination of cyanogen. The [Mn(II)(CN)(4)](2-) ion has a dynamic solution behavior, as evidenced by its concentration-dependent electronic and electron paramagnetic spectra, that can be attributed to aggregation of the coordinatively and electronically unsaturated (four-coordinate, 13-electron) metal center. Due to dynamics and lability of [Mn(II)(CN)(4)](2-) in solution, its reaction with divalent first-row transition metal cations leads to the formation of lattice compounds with both tetrahedral and square planar local coordination geometries of the metal ions and multiple structural and cyano-linkage isomers. alpha-Mn(II)[Mn(II)(CN)(4)] has an interpenetrating sphalerite- or diamond-like network structure with a unit cell parameter of a = 6.123 A (P43m space group) while a beta-phase of this material has a noninterpenetrating disordered lattice containing tetrahedral [Mn(II)(CN)(4)](2-). Linkage isomerization or cyanide abstraction during formation results in alpha-Mn(II)[Co(II)(CN)(4)] and Mn(II)[Ni(II)(CN)(4)] lattice compounds, both containing square planar tetracyanometalate centers. alpha-Mn(II)[Co(II)(CN)(4)] is irreversibly transformed to its beta-phase in the solid state by heating to 135 degrees C, which causes a geometric isomerization of [Co(II)(CN)(4)](2)(-) from square planar (nu(CN) = 2114 cm(-1), S = (1)/(2)) to tetrahedral (nu(CN) = 2158 cm(-1), S = (3)/(2)) as evidenced by infrared and magnetic susceptibility measurements. Mn(II)[Ni(II)(CN)(4)] is the only phase formed with Ni(II) due to the high thermodynamic stability of square planar [Ni(II)(CN)(4)](2)(-).
RESUMEN
Magnetically ordered Prussian blue analogues with the general formulation of M[Mn(CN)6] (M = V, Cr, Mn, Co, Ni) were made in aprotic media utilizing [MnIV(CN)6]2-. These analogs are valence-ambiguous, as they can be formulated as MII[MnIV(CN)6] or MIII[MnIII(CN)6]. The X-ray powder diffraction of each member of this family can be indexed to the face-centered cubic (fcc) Prussian blue structure type, with atypically reduced unit cell parameters (a approximately 9.25 +/- 0.25 A) with respect to hydrated Prussian blue structured materials (a > or = 10.1 A). The reduced a-values are attributed to a contraction of the lattice in the absence of water or coordinating solvent molecule (i.e., MeCN) that is necessary to help stabilize the structure during lattice formation. Based on vCN IR absorptions, X-ray photoelectron spectra, and magnetic data, the following oxidation state assignments are made: MII[MnIV(CN)6] (M = Co, Ni) and MIII[MnIII(CN)6] (M = V, Cr, Mn). Formation of MnIII[MnIII(CN)6] is in contrast to MnII[MnIV(CN)6] prepared from aqueous media. Above 250 K, the magnetic susceptibilities of M[Mn(CN)6] (M = V, Cr, Mn, Co, Ni) can be fit to the Curie-Weiss equation with theta = -370, -140, -105, -55, and -120 K, respectively, suggesting strong antiferromagnetic coupling. The room temperature effective moments, respectively, are 3.71, 4.62, 5.66, 4.54, and 4.91 microB, consistent with the above oxidation state assignments. All compounds do not exhibit magnetic saturation at 50 kOe, and exhibit frequency-dependent chi'(T) and chi"(T) responses characteristic of spin-glass-like behavior. M[Mn(CN)6] order as ferrimagnets, with Tc's taken from the peak in the 10 Hz chi'(T) data, of 19, 16, 27.1, < 1.75, and 4.8 K for M = V, Cr, Mn, Co, and Ni, respectively. The structural and magnetic disorder prevents NiII[MnIV(CN)6] from ordering as a ferromagnet as anticipated, and structural inhomogeneities allow CoII[MnIV(CN)6] and VIII[MnIII(CN)6] to unexpectedly order as ferrimagnets. Also, MnIII[MnIII(CN)6] behaves as a reentrant spin glass showing two transitions at 20 and 27.1 K, and similar behavior is evident for CrIII[MnIII(CN)6]. Hysteresis with coercive fields of 340, 130, 8, 9, and 220 Oe and remanent magnetizations of 40, 80, 1500, 4, and 250 emuOe/mol are observed for M = V, Cr, Mn, Co, and Ni, respectively.
