Tetranuclear manganese carboxylate complexes with a trigonal pyramidal metal topology via controlled potential electrolysis

Controlled potential electrolysis (CPE) procedures are described that provide access to complexes with a [Mn4(mu 3-O)3(mu 3-O2CR)]6+ core (3MnIII,MnIV) and a trigonal pyramidal metal topology, starting from species containing the [Mn4(mu 3-O)2]8+ core (4MnIII). [Mn4O2(O2CMe)6(py)2(dbm)2] (6): triclinic, P1, a = 10.868(3) A, b = 13.864(3) A, c = 10.625(3) A, alpha = 108.62(1) degrees, beta = 118.98(1) degrees, gamma = 89.34(2) degrees, V = 1307 A3, Z = 1, T = -131 degrees C, R (Rw) = 3.24 (3.70)%. [Mn4O2(O2CPh)6(py)(dbm)2] (8): monoclinic, P2(1)/c, a = 14.743(6) A, b = 15.536(8) A, c = 30.006(13) A, beta = 102.79(1) degrees, V = 6702 A3, Z = 4, T = -155 degrees C, R (Rw) = 4.32 (4.44)%. Both 6 and 8 contain a [Mn4O2]8+ core; 8 only has one py group, the fourth MnIII site being five-coordinate. (NBun4)[Mn4O2(O2CPh)7(dbm)2] (10) is available from two related procedures. CPE of 10 at 0.65 V vs ferocene in MeCN leads to precipitation of [Mn4O3(O2CPh)4(dbm)3] (11); similarly, CPE of 6 at 0.84 V in MeCN/CH2Cl2 (3:1 v/v) gives [Mn4O3(O2CMe)4(dbm)3] (12). Complex 11: monoclinic, P2(1)/n, a = 15.161(3) A, b = 21.577(4) A, c = 22.683(5) A, beta = 108.04(3) degrees, V = 7056 A3, Z = 4, T = -100 degrees C, R (wR2) = 8.63 (21.80)%. Complex 12: monoclinic, P2(1)/n, a = 13.549(2) A, b = 22.338(4) A, c = 16.618(2) A, beta = 103.74(1) degrees, V = 4885 A3, Z = 4, T = -171 degrees C, R (Rw) = 4.63 (4.45)%. Both 11 and 12 contain a [Mn4(mu 3-O)3(mu-O2CR)] core with a Mn4 trigonal pyramid (MnIV at the apex) and the RCO2- bridging the MnIII3 base. However, in 11, the carboxylate is eta 2,mu 3 with one O atom terminal to one MnIII and the other O atom bridging the other two MnIII ions, whereas in 12 the carboxylate is eta 1,mu 3, a single O atom bridging three MnIII ions. Variable-temperature, solid-state magnetic susceptibility studies on 11 and 12 show that, for both complexes, there are antiferromagnetic exchange interactions between MnIII/MnIV pairs, and ferromagnetic interactions between MnIII/MnIII pairs. In both cases, the resultant ground states of the complex is S = 9/2, confirmed by magnetization vs field studies in the 2.00-30.0 K and 0.50-50 kG temperature and field ranges, respectively.     

Initial example of a triangular single-molecule magnet from ligand-induced structural distortion of a [MnIII3O]7+ complex

The reaction of [Mn3O(O2CR)6(py)3](ClO4) (R = Me, Et) with methyl 2-pyridyl ketone oxime (mpkoH) in a 1:3 molar ratio in MeOH/MeCN leads to [Mn3O(O2CR)3(mpko)3](ClO4) in 80-90% isolated yield. Ferromagnetic exchange interactions between the three MnIII ions in the nonplanar [MnIII3O]7+ triangular core lead to a spin ground state of S = 6; single-crystal studies reveal the temperature and sweep rate dependent hysteresis loops expected for a single-molecule magnet.     

Trapping an elusive intermediate in manganese-oxo cubane chemistry

A new member of the Mn-oxo cubane core complex family [Mn2III,2IV4O4L6] (1), where L = (p-MeOPh)2PO2-, has been synthesized and characterized. Compound 1 possesses structurally inequivalent MnIII and MnIV with clear valence electron localization in the crystal phase, quite unlike the structurally equivalent sites, tetrahedral core symmetry, and delocalized valence of its analogue where L = Ph2PO2-. Compound 1 exhibits appreciable positive shifts (0.1-0.3 V) of both the oxidation and reduction electrochemical potentials, attributable to the remote electron donating p-MeO groups. Reduction of 1 by methanol yields a novel core complex [MnIII4O2(OMe)2(MeOH)[(p-MeOPh)2PO2]6] (2). Each MnIII of 2 is tetragonally distorted due to the Jahn-Teller effect with elongated Mn-O bonds that are directed at the two micro3-MeO bridges and neither of the two micro3-oxos. These electronically driven distortions provide a structural rationale for the greater basicity of the former sites and explain why 2 of the 4 corner oxos are preferentially reduced to water molecules.     

From pseudo to true C(3) symmetry: magnetic anisotropy enhanced by site-specific ligand substitution in two Mn(15)-carboxylate clusters

Two new mixed-valence manganese-carboxylate clusters, [MnIII9MnIV6(O2CPh)12(micro3-O)13(micro-O)4(micro-OMe)5(MeOH)4(H2O)5]2.1.5PhCO2H.MeOH.6H2O (1, PhCO2H = benzoic acid) and [MnIII9MnIV6(O2CCh)12(micro3-O)13(micro-O)4(micro-OMe)5(MeOH)3(H2O)6].0.5MeOH.2.5H2O (2, ChCO2H= cyclohexanecarboxylic acid) contain new disklike Mn15 cores. Both 1 and 2 can be synthesized by the conventional manganese redox reaction (MnO4- oxidizing Mn2+) in methanol solution. 2 can be also synthesized via the site-specific ligand substitution reaction from 1. 1 crystallizes in the triclinic space group P, whereas 2 crystallizes in the trigonal space group P. Magnetic study shows that both 1 and 2 have the same ground spin states ST = 2. Compared to the silence of the out-of-phase ac susceptibility of 1, 2 shows clearly slow magnetic relaxation behavior above 1.8 K due to the dramatically enhanced axial magnetic anisotropy (D = -0.89 and -1.58 cm-1 for 1 and 2, respectively, which was obtained by fitting the plots of M vs H/T with the program ANISOFIT 2.0).     

High-spin Mn4 and Mn10 molecules: large spin changes with structure in mixed-valence MnII4MnIII6 clusters with azide and alkoxide-based ligands

The use has been explored of both azide (N3-) and alkoxide-containing groups such as the anions of 2-(hydroxymethyl)pyridine (hmpH), 2,6-pyridinedimethanol (pdmH2), 1,1,1-tris(hydroxymethyl)ethane (thmeH3) and triethanolamine (teaH3) in Mn cluster chemistry. The 1:1:1:1 reactions of hmpH, NaN3 and NEt3 with Mn(ClO4)(2).6H 2O or Mn(NO3)2.H2O in MeCN/MeOH afford [MnII4MnIII6O4(N3)4(hmp)12](X)2 [X=ClO4- (1), N3- (2)]. The [Mn10(mu4-O) 4(mu3-N3)4]14+ core of the cation has a tetra-face-capped octahedral topology, with a central MnIII6 octahedron, whose eight faces are bridged by four mu 3-N3- and four mu 4-O2- ions, the latter also bridging to four extrinsic MnII atoms. The core has Td symmetry, but the complete [MnII4MnIII6O4(N3)4(hmp)12]2+ cation has rare T symmetry, which is crystallographically imposed. A similar reaction of Mn(ClO4) (2).6H2O with one equiv each of NaN3, thmeH3, pdmH2, and NEt3 in MeCN/MeOH led to [MnII4MnIII6O2(N3)6(pdmH)4(thme)4] (3). Complex 3 is at the same oxidation level as 1/2 but its core is structurally different, consisting of two edge-fused [MnII2MnIII4(mu4-O)]14+ octahedra. Replacement of thmeH3 with teaH3 in this reaction gave instead [MnII2MnIII2(N3)4(pdmH)2(teaH)2] (4), containing a planar Mn 4 rhombus. Variable-temperature, solid-state dc and ac magnetization studies were carried out on 1-4 in the 5.0-300 K range. Complexes 1 and 2 are completely ferromagnetically coupled with a resulting S=22 ground state, one of the highest yet reported. Fits of dc magnetization vs field (H) and temperature (T) data by matrix diagonalization gave S=22, g=2.00, and D approximately 0.0 cm(-1) (D is the axial zero-field splitting parameter). In contrast, the data for 3 revealed dominant antiferromagnetic interactions and a resulting S=0 ground state. Complex 4 contains weakly ferromagnetically coupled Mn atoms, leading to an S=9 ground-state and low-lying excited states, and exhibits out-of-phase ac susceptibility signals characteristic of a single-molecule magnet. Theoretical values of the exchange constants in 1 obtained with density functional theory and ZILSH calculations were in good agreement with experimental values. The combined work demonstrates the synthetic usefulness of alcohol-based chelates and azido ligands when used together, and the synthesis in the present work of two "isomeric" MnIII6MnII4 cores that differ in spin by a remarkable 22 units.     

Carboxylate-bridged helical chains based on an azo carboxylate oximate ligand

Two helical one-dimensional complexes [MnII(MeOH)4][MnIV(L·)2]·2MeOH (1) and [MnIII(salen)][MnIII(L)2] (2) (H2L = HON=C(Ph)N=NC6H4CO2H) contain the noninnocent ligand [Mn(L·)2]2- and innocent low-spin [Mn(L)2]-. Intrachain anti-ferromagnetic interaction between adjacent manganese ions via the syn-anti carboxylate bridges in complex 1. Alternate syn-anti and anti-anti carboxylate bridges have been found to transmit ferro- and antiferromagnetic coupling between high-spin and low-spin Mn(III) ions in complex 2.     

Spin switching via targeted structural distortion

The deliberate "stepwise" structural distortion of the [MnIII6O2(sao)6(O2CR)2L4] (S = 4, Ueff = 28 K) family of SMMs (where sao2- is the dianion of salicylaldoxime or 2-hydroxybenzaldeyhyde oxime and L = MeOH, EtOH) via the use of derivatized oxime ligands and bulky carboxylates leads to a family of single-molecule magnets with larger spin ground states and enhanced blocking temperatures. Replacing sao2- and HCO2- in the molecule [MnIII6O2(sao)6(O2CH)2(MeOH)4] (1), with Et-sao2- (Et-saoH2 = 2-hydroxypropiophenone oxime) and Me3CCO2- (pivalate), produces the complex [MnIII6O2(Et-sao)6(O2CCMe3)2(EtOH)5] (2) that displays an S = 7 ground state with Ueff = 30 K. Replacing Me3CCO2- with PhCO2- produces the complex [MnIII6O2(Et-sao)6(O2CPh)2(EtOH)4(H2O)2] (3) that displays an S = 12 ground state with Ueff = 53 K. The ligand substitution invokes a subtle structural distortion to the core of the molecule evidenced by an increased "twisting" of the oxime moiety (Mn-N-O-Mn) and a change in carboxylate ligation, which, in turn, invokes a dramatic change in the observed magnetic properties by switching weak antiferromagnetic exchange to weak ferromagnetic exchange.     

Mixed transition metal-lanthanide complexes at high oxidation states: heteronuclear CeIVMnIV clusters

The syntheses of the first mixed-metal CeIVMnIV complexes are reported. [CeMn2O3(O2CMe)(NO3)4(H2O)2(bpy)2](NO3) (1; bpy=2,2'-bipyridine) was obtained from the reaction of Mn(NO3)2.xH2O and bpy with (NH4)2Ce(NO3)6 in a 1:1:2 molar ratio in 25% aqueous acetic acid. The complexes [CeMn6O9(O2CR)9(X)(H2O)2]y+ (R=Me, X=NO3-, y=0 (2); R=Me, X=MeOH, y=+1 (3); R=Et, X=NO3-, y=0 (7)) were obtained from reactions involving a [Mn(O2CR)2].4H2O/CeIV ratio of approximately 1:1.5 in concentrated aqueous carboxylic acid. A related reaction in less-concentrated aqueous acetic acid and in the presence of L (where L=2-hydroxy-6-methylpyridine (mhpH), 2-pyrrolidinone (pyroH), or pyridine (py)) gave [Ce3Mn2O6(O2CMe)6(NO3)2(L)a(H2O)b] (L=mhpH, a=4, b=0 (4); L=pyroH, a=2, b=3 (5)) and {{(pyH)3[Ce3Mn2O6(O2CMe)7.5(NO3)3].(HO2CMe)0.5.(H2O)2}2(NO3)}n (6), respectively. Solid-state magnetic susceptibility (chiM) data for compounds 1, 4, and 5 were fit to the theoretical chiMT versus T expression for a MnIV2 complex derived using the isotropic Heisenberg spin Hamiltonian (H=-2J?1? 2) and the Van Vleck equation. The obtained fit parameters were (in the format J, g) 1, -45.7(3) cm(-1), 1.95(5); 4, -0.40(10) cm(-1), 2.0(1); and 5, -0.34(10) cm(-1), 2.0(1), where J is the exchange interaction constant between the two MnIV ions. The data for compound 3 were fit by a matrix diagonalization method that gave J1=-5.8 cm(-1), J2=-0.63 cm(-1), J3 approximately 0, and g=2.0(1), where J1 and J2 are the exchange interactions for the [MnIV2O2(Omicron2CMe)] and [MnIV2O(Omicron2CMe)2] units, respectively, and J3 for a uniform next-nearest-neighbor interaction. Theoretical estimates of the exchange constants in compounds 1 and 3 obtained with the ZILSH method were in excellent and good agreement, respectively, with the values obtained from fits of the magnetization data. The difference for 3 is assigned to the presence of the Ce4+ ion, and atomic bond indices obtained from the ZILSH calculations were used to rationalize the values of the various exchange constants based on metal-ligand bond strengths.     

Magnetic ordering and spin-glass behavior in first-row transition metal hexacyanomanganate(IV) Prussian blue analogues

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.     

High-nuclearity Ce/Mn and Th/Mn cluster chemistry: preparation of complexes with [Ce4Mn10O10(OMe)6]18+ and [Th6Mn10O22(OH)2]18+ cores

The syntheses, structures, and magnetic properties are reported of the mixed-metal complexes [Ce4Mn10O10(OMe)6(O2CPh)16(NO3)2(MeOH)2(H2O)2] (1) and [Th6Mn10O22(OH)2(O2CPh)16-(NO3)2(H2O)8] (2), which were both prepared by the reaction of (NBun4)[Mn4O2(O2CPh)9(H2O)] (3) with a source of the heterometal in MeCN/MeOH. Complexes 1 and 2 crystallize in the monoclinic space group C2/c and the triclinic space group P, respectively. Complex 1 consists of 10 MnIII, 2 CeIII, and 2 CeIV atoms and possesses a very unusual tubular [Ce4Mn10O10(OMe)6]18+ core. Complex 2 consists of 10 MnIV and 6 ThIV atoms and possesses a [Th6Mn10O22(OH)2]18+ core with the metal atoms arranged in layers with a 2:3:6:3:2 pattern. Peripheral ligation around the cores is provided by 16 bridging benzoates, 2 chelating nitrates, and either (i) 2 each of terminal H2O and MeOH groups in 1 or (ii) 8 terminal H2O groups in 2. Complex 1 is the largest mixed-metal Ce/Mn cluster and the first 3d/4f cluster with mixed-valency in its lanthanide component, while complex 2 is the first Th/Mn cluster and the largest mixed transition metal/actinide cluster to date. Solid-state dc and ac magnetic susceptibility measurements on 1 and 2 establish that they possess S = 4 and 3 ground states, respectively. Ac susceptibility studies on 1 revealed nonzero frequency-dependent out-of-phase (chiM' ') signals at temperatures below 3 K; complex 2 displays no chiM' ' signals. However, single-crystal magnetization vs dc field scans at variable temperatures and variable sweep-rates down to 0.04 K on 1 revealed no noticeable hysteresis loops, except very minor ones at 0.04 K assignable to weak intermolecular interactions propagated by hydrogen bonds involving CeIII-bound ligands. Complex 1 is thus concluded not to be a single-molecule magnet (SMM), and the combined results thus represent a caveat against taking such ac signals as sufficient proof of a SMM.     

A new class of oxo-bridged high-valent hexamanganese clusters supported by sterically hindered carboxylate ligands

A new class of oxo-bridged high-valent hexamanganese (Mn6) clusters containing a novel (Mn6O8)6+ core, [MnIV(4)MnIII2(mu-O)4(mu3-O)4(dmb)6(O2CR)2]4+ (where dmb=4,4'-dimethyl-2,2'-bipyridine, and RCO2=2,6-di(p-tolyl)benzoate (Ar(Tol)CO2-) (3) or 2,6-di(4-tert-butylphenyl)benzoate (Ar(4-tBuPh)CO2-) (4)), was synthesized using sterically hindered m-terphenyl-derived carboxylate ligands. These complexes can be synthesized by oxidizing the MnII mononuclear complexes, [Mn(dmb)2(OH2)(O2CR)]+ (where RCO2=Ar(Tol)CO2- (1) or Ar(4-tBuPh)CO2- (2)) with (n-Bu4N)MnO4, by direct Mn(II) + Mn(VII) in situ comproportionation reactions, or by ligand substitution on the dinuclear manganese (III,IV) or (IV,IV) complexes, [(Mn2(mu-O)2(dmb)4)](3+/4+). The compound [MnIV4MnIII2(mu-O)4(mu3-O)4(dmb)6(Ar(Tol)CO2)2](OTf)4 [3(OTf)4] crystallizes in the monoclinic space group P2(1)/n, with the cell parameters a=15.447(1) A, b=15.077(2) A, c=27.703(2) A, beta=91.68(2) degrees, V=6449.3(6) A3, and Z=2. The X-ray structure reveals that there are three different bridging modes for the oxo groups: mu, "pyramidal" mu3, and "T-shaped" mu3. Solid-state variable temperature magnetic susceptibility studies suggest that the Mn centers are net antiferromagnetically coupled to yield a diamagnetic ST=0 ground spin state with a large number of low-lying, thermally accessible states with ST>0. 1H NMR spectra were recorded for both Mn6 clusters and selected resonances assigned. The electronic and redox properties of these complexes along with the effect of the presence of the bulky carboxylate ligands are also described here.     

Catalysis and kinetics of hydrogen peroxide disproportionation by dinuclear manganese(III) complexes of 1,5-bis(salicylidenamino)pentan-3-ol and the 5-bromophenyl-substituted derivative

The dinuclear MnIII complex [Mn2(mu-OAc)(mu-OMe)(5-Br-salpentO)(MeOH)2]Br has been prepared and its structure and reactivity toward H2O2 studied in comparison with [Mn2(mu-OAc)(mu-OMe)(salpentO)(MeOH)2]Br (salpent-OH = 1,5-bis(salicylidenamino)pentan-3-ol and 5-Br-salpentOH = 1,5-bis(5-bromesalicylidenaminopentan-3-ol). The X-ray diffraction analysis of [Mn2(mu-OAc)(mu-OMe)(5-Br-salpentO)(MeOH)2]Br (monoclinic, P21/n, a = 13.081(2) A, b = 13.429(2) A, c = 17.375(2) A, beta = 102.31(1) degrees, V = 2982.0 A3, Z = 4) revealed a mu-alkoxo, mu-acetatodimanganese(III) core with a Mn...Mn separation of 2.932(1) A. The ligand lies in the meridional plane, and the sixth coordination position of each manganese atom is occupied by a methanol molecule providing two substitution-labile sites in the cis position. The two complexes showed catalytic activity toward disproportionation of H2O2 in methanol and dimethylformamide in the 0-25 degrees C temperature range. The initial rate of oxygen evolution in the presence of [Mn2(mu-OAc)(mu-OMe)(5-Br-salpentO)(MeOH)2]Br or [Mn2(mu-OAc)(mu-OMe)(salpentO)(MeOH)2]-Br is first order in catalyst concentration. The two complexes show saturation kinetics in methanol, with the higher kcat = 0.98 s-1 and kcat/KM = 70 M-1 s-1 observed for [Mn2(mu-OAc)(mu-OMe)(salpentO)(MeOH)2]Br.     

Tetranuclear manganese complexes with dimer-of-dimer and ladder structures from the use of a bis-bipyridyl ligand

The reaction of the bis-chelating ligand 1,2-bis(2,2'-bipyridine-6-yl)ethane (L) with the trinuclear species of formula [Mn(3)O(O(2)CR)(6)(py)(3)](ClO(4)) (R = Me (1); R = Et (2); R = Ph (3)) has afforded the new tetranuclear mixed-valent complexes [Mn(4)O(2)(O(2)CR)(4)L(2)](ClO(4))(2) (R = Me (4); R = Et (5); R = Ph (6)) and [Mn(4)O(2)(OMe)(3)(O(2)CR)(2)L(2)(MeOH)](ClO(4))(2) (R = Me (7); R = Et (8); R = Ph (9)). Complexes 4-6 were obtained in yields of 20%, 44%, and 37%, respectively. They are mixed-valent, with an average Mn oxidation state of +2.5. Complexes 7-9 were obtained in yields of 57%, 65%, and 70%, respectively. They are also mixed-valent, but with an average Mn oxidation state of +2.75. Complexes 4 x 2THF and 9 x 3MeOH x H(2)O crystallize in the triclinic space group P1 macro and contain [Mn(4)(mu(3)-O)(2)](6+) and [Mn(4)(mu(3)-O)(2)(mu-OMe)(2)](5+) cores, respectively, the latter being a new structural type in the family of Mn(4) complexes. Reactivity studies of 4-9 have shown that 4-6 can be converted into 7-9, respectively, and vice versa. The magnetic properties of 5 and 9 have been studied by dc and ac magnetic susceptibility techniques. Complex 5 displays antiferromagnetic coupling between its Mn ions resulting in a spin ground state of S = 0. Complex 9 also displays antiferromagnetic coupling, but the resulting ground state is S = (7)/(2), as confirmed by fitting magnetization versus field data collected for 9 at low temperatures, which gave S = (7)/(2), D = -0.77 cm(-1), and g = 1.79. Complex 9 exhibits a frequency-dependent out-of-phase ac susceptibility peak, indicative of the slow magnetization relaxation that is diagnostic of single-molecule magnetism behavior.     

Mixed-valence MnIIIMnIV clusters [Mn7O8(O2SePh)8(O2CMe)(H2O)] and [Mn7O8(O2SePh)9(H2O)]: single-chain magnets exhibiting quantum tunneling of magnetization

The syntheses, structures, and magnetic properties of two new Mn7 complexes containing phenylseleninate ligands are reported. [Mn7O8(O2SePh)8(O2CMe)(H2O)] (1) and [Mn7O8(O2SePh)9(H2O)] (2) were both prepared by the reaction of 18 equiv of benzeneseleninic acid (PhSeO2H) with [Mn12O12(O2CMe)16(H2O)4] in MeCN. Complex 1 x 6MeCN crystallizes in the triclinic space group P, and complex 2 x 2CH2Cl2 crystallizes in the monoclinic space group P2(1)/m. Both compounds possess an unprecedented [Mn7O8]9+ core comprising a central [MnIII3(micro3-O)4]+ unit attached to [MnIV2(micro-O)2]4+ and [MnIV2(micro-O)(micro3-O)]4+ units on either side. In each cluster, the PhSeO2- groups function as bridging ligands between adjacent Mn centers. The structure reveals strong Se.O intermolecular contacts between Mn7 units to give a one-dimensional chain structure, with weak interchain interactions. Solid-state DC magnetic susceptibility measurements of complexes 1 and 2 reveal that they have very similar properties, and detailed studies on 1 by AC susceptibility measurements confirm an S = 2 ground-state spin value. In addition, out-of-phase AC signals are observed, suggesting slow magnetization relaxation. Magnetization versus DC field sweeps down to 0.04 K reveals hysteresis loops, but the temperature dependence of the coercivity is not what is expected of a single-molecule magnet. Instead, the behavior is due to single-chain magnetism, albeit with weak antiferromagnetic interactions between the chains, with the barrier to relaxation arising from a combination of molecular anisotropy and ferromagnetic intermolecular exchange interactions mediated by the Se...O contacts. An Arrhenius plot was constructed from the magnetization versus time decay data. The thermally activated region at > 0.5 K gave an effective relaxation barrier (Ueff) of 14.2 K. Below approximately 0.1 K, the relaxation is independent of temperature, which is characteristic of magnetization quantum tunneling through the anisotropy barrier. These Mn7 compounds are thus the first single-chain magnets to comprise polynuclear metal clusters and also the first for which the temperature-independent relaxation characteristic of tunneling has been identified. The work also emphasizes that out-of-phase AC signals for ostensibly molecular compounds are not sufficient proof by themselves of a single-molecule magnet.     

Single-molecule magnets: a family of MnIII/CeIV complexes with a [Mn8CeO8]12+ core

Four heterometallic, enneanuclear Mn8Ce clusters [Mn8CeO8(O2CMe)12(H2O)4] (4), [Mn8CeO8(O2CMe)12(py)4] (5), [Mn8CeO8(O2CPh)12(MeCN)4] [Mn8CeO8(O2CPh)12(dioxane)4] (6), and [Mn8CeO8(O2CCHPh2)12(H2O)4] (7) have been prepared by various methods. Their cores are essentially isostructural and comprise a nonplanar, saddlelike [MnIII8O8]8+ loop containing a central CeIV ion attached to the eight micro3-O2- ions. Peripheral ligation around the [Mn8CeO8]12+ core is provided by eight micro- and four micro3-O2CR- groups. Terminal ligation on four MnIII atoms is provided by H2O in 4 and 7, pyridine in 5, and MeCN/dioxane in 6. Solid-state magnetic susceptibility studies, fits of dc magnetization vs field and temperature data, and in-phase ac susceptibility studies in a zero dc field have established that complexes 4, 5, and 7 possess S=16, S=4 or 5, and S=6+/-1 spin ground states, respectively, but in all cases there are very low-lying excited states. The large variation in the ground-state spins for this isostructural family is rationalized as due to a combination of weak exchange interactions between the constituent MnIII atoms, and the presence of both nearest-neighbor and next-nearest-interactions of comparable magnitudes. Magnetization vs applied dc field sweeps on single crystals of 4.4H2O and 7.4H2O.3MeCN.2CH2Cl2 down to 0.04 K have established that these two complexes are new single-molecule magnets (SMMs). The former also shows an exchange-bias, a perturbation of its single-molecule properties from very weak intermolecular interactions mediated by hydrogen-bonding interactions with lattice-water molecules of crystallization.     

A novel 9-MC-3 and 15-MC-6 onset stacked metallacrown single-molecule magnet: synthesis and crystal structure

A novel enneanuclear manganese complex, [Mn(9)O(4)(Mesao)(6)(MeO)(3)(O(2)CMe)(3)(OH)(MeOH)(2)]·2.5DMF [1; Me-saoH(2) = 2-hydroxyphenylethanone oxime], was synthesized. The structure of 1 contains an unusual [Mn(9)O(4)] core with an unprecedented defective "supertetrahedron" topology based on two parallel, onset stacked 9-MC-3 and 15-MC-6 metallacrown subunits. Magnetic studies indicate that 1 behaves as a single-molecule magnet.     

High-spin Mn wheels

The syntheses, structures, and magnetic properties of the complexes [MnIV4MnIII10MnII2O2(OCH3)12(tmp)8(O2CCH3)10].3Et2O (1.3Et2O), [MnIV2MnIII18MnII2O6(OCH3)14(O2CCH3)16(tmp)8(HIm)2].2CH3OH (2.2CH3OH), and [MnIV2MnIII18MnII2O6(OCH3)14(O2CCH3)16(Br-mp)8(HIm)2].2C6H14.5CH3OH (3.2C6H14.5CH3OH) are reported. The unusual wheel-like complexes were prepared by the treatment of [Mn3O(O2CCH3)6(HIm)3](O2CCH3) (HIm = imidazole) with 1,1,1-tris-(hydroxymethyl)propane (H3tmp) (1 and 2) or 2-(bromomethyl)-2-(hydroxymethyl)-1,3-propanediol (Br-mpH3) (3) in the presence of sodium methoxide (NaOCH3, 2, and 3) in CH3OH. Complex 1.3Et2O crystallizes in the triclinic space group P, while complexes 2.2CH3OH and 3.2C6H14.5CH3OH crystallize in the orthorhombic space group Pbca. Direct current magnetic susceptibility data, collected for 1-3 in the respective 1.8-300 K and 0.1-7 T temperature and magnetic-field ranges, afford spin ground-state values of S = 14 +/- 1 for complex 1 and S = 9 +/- 1 for complexes 2 and 3. Alternating current susceptibility measurements performed on all three complexes in the 1.8-10 K temperature range in a 3.5 G oscillating field at frequencies between 50 and 1000 Hz reveal out-of-phase chi"M signals below approximately 3 K. Single-crystal hysteresis loop and relaxation measurements confirm single-molecule magnetism behavior.     

"Switching on" the properties of single-molecule magnetism in triangular manganese(III) complexes

The reaction between oxide-centered, triangular [MnIII3O(O2CR)6(py)3](ClO4) (R = Me (1), Et (2), Ph (3)) compounds and methyl 2-pyridyl ketone oxime (mpkoH) affords a new family of Mn/carboxylato/oximato complexes, [MnIII3O(O2CR)3(mpko)3](ClO4) [R = Me (4), Et (5), and Ph (6)]. As in 1-3, the cations of 4-6 contain an [MnIII3(mu3-O)]7+ triangular core, but with each Mn2 edge now bridged by an eta1:eta1:mu-RCO2- and an eta1:eta1:eta1:mu-mpko- group. The tridentate binding mode of the latter causes a buckling of the formerly planar [MnIII3(mu3-O)]7+ core, resulting in a relative twisting of the three MnIII octahedra and the central O2- ion now lying approximately 0.3 A above the Mn3 plane. This structural distortion leads to ferromagnetic exchange interactions within the molecule and a resulting S = 6 ground state. Fits of dc magnetization data for 4-6 collected in the 1.8-10.0 K and 10-70 kG ranges confirmed S = 6 ground states, and gave the following D and g values: -0.34 cm(-1) and 1.92 for 4, -0.34 cm(-1) and 1.93 for 5, and -0.35 cm(-1) and 1.99 for 6, where D is the axial zero-field splitting (anisotropy) parameter. Complexes 4-6 all exhibit frequency-dependent out-of-phase (chi" M) ac susceptibility signals suggesting them possibly to be single-molecule magnets (SMMs). Relaxation rate vs T data for complex 4 down to 1.8 K obtained from the chi" M vs T studies were supplemented with rate vs T data measured to 0.04 K via magnetization vs time decay studies, and these were used to construct Arrhenius plots from which was obtained the effective barrier to relaxation (Ueff) of 10.9 K. Magnetization vs dc field sweeps on single-crystals of 4.3CH2Cl2 displayed hysteresis loops exhibiting steps due to quantum tunneling of magnetization (QTM). The loops were essentially temperature-independent below approximately 0.3 K, indicating only ground-state QTM between the lowest-lying Ms = +/-6 levels. Complexes 4-6 are thus confirmed as the first triangular SMMs. High-frequency EPR spectra of single crystals of 4.3CH2Cl2 have provided precise spin Hamiltonian parameters, giving D = -0.3 cm(-1), B40 = -3 x 10(-5) cm(-1), and g = 2.00. The spectra also suggest a significant transverse anisotropy of E > or = 0.015 cm(-1). The combined work demonstrates the feasibility that structural distortions of a magnetic core imposed by peripheral ligands can "switch on" the properties of an SMM.     

Manganese(III,IV) and manganese(III) oxide clusters trapped by copper(II) complexes

Reactions of quinquedentate Schiff base ligands with Mn and Cu ions afforded icosa- and hexadecanuclear mixed-metal clusters in which dinuclear CuII complexes trapped oxo-bridged [MnIII8MnIV4O12] and [MnIII6O6] cores, respectively. Maximum entropy method analysis for synchrotron X-ray diffraction data was used to determine the oxidation states of the Mn ions.     

Structural,spectroscopic, and electrochemical studies of binuclear manganese(II) complexes of bis(pentadentate) ligands derived from bis(1,4,7-triazacyclononane) macrocycles

Structural, electrochemical, ESR, and H2O2 reactivity studies are reported for [Mn(dmptacn)Cl]ClO4 (1, dmptacn = 1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane) and binuclear complexes of bis(pentadentate) ligands, generated by attaching 2-pyridylmethyl arms to each secondary nitrogen in bis(1,4,7-triazacyclononane) macrocycles and linked by ethyl (tmpdtne, [Mn2(tmpdtne)Cl2](ClO4)2.2DMF, 2), propyl (tmpdtnp, [Mn2(tmpdtnp)Cl2](ClO4)2.3H2O, 3), butyl (tmpdtnb, [Mn2(tmpdtnb)Cl2](ClO4)2.DMF.2H2O, 4), m-xylyl (tmpdtn-m-X, [Mn2(tmpdtn-m-X)-Cl2](ClO4)2, 5) and 2-propanol (tmpdtnp-OH, [Mn2(tmpdtnp-OH)Cl2](ClO4)2, 6) groups. 1 crystallizes in the orthorhombic space group P2(1)2(1)2(1) (No. 19) with a = 7.959(7) A, b = 12.30(1) A, and c = 21.72(2) A; 2, in the monoclinic space group P2(1)/c (No. 14) with a = 11.455(4) A, b = 15.037(6) A, c = 15.887(4) A, and beta = 96.48(2) degrees; 3, in the monoclinic space group P2(1)/c (No. 14) with a = 13.334(2) A, b = 19.926(2) A, c = 18.799(1) A, and beta = 104.328(8) degrees; and [Mn2(tmpdtnb)Cl2](ClO4)2.4DMF.3H2O (4'), in the monoclinic space group P2(1)/n (No. 14) with a = 13.361(3) A, b = 16.807(5) A, c = 14.339(4) A, and beta = 111.14(2) degrees. Significant distortion of the Mn(II) geometry is evident from the angle subtended by the five-membered chelate (ca. 75 degrees) and the angles spanned by trans donor atoms (< 160 degrees). The Mn geometry is intermediate between octahedral and trigonal prismatic, and for complexes 2-4, there is a systematic increase in M...M distance with the length of the alkyl chain. Cyclic and square-wave voltammetric studies indicate that 1 undergoes a 1e- oxidation from Mn(II) to Mn(III) followed by a further oxidation to MnIV at a significantly more positive potential. The binuclear Mn(II) complexes 2-5 are oxidized to the Mn(III) state in two unresolved 1e- processes [MnII2-->MnIIMnIII-->MnIII2] and then to the MnIV state [MnIII2-->MnIIIMnIV-->MnIV2]. For 2, the second oxidation process was partially resolved into two 1e- oxidation processes under the conditions of square-wave voltammetry. In the case of 6, initial oxidation to the MnIII2 state occurs in two overlapping 1e- processes as was found for 2-5, but this complex then undergoes two further clearly separated 1e- oxidation processes to the MnIIIMnIV state at +0.89 V and the MnIV2 state at +1.33 V (vs Fc/Fc+). This behavior is attributed to formation of an alkoxo-bridged complex. Complexes 1-6 were found to catalyze the disproportionation of H2O2. Addition of H2O2 to 2 generated an oxo-bridged mixed-valent MnIIIMnIV intermediate with a characteristic 16-line ESR signal.