Ligand directed self-assembly vs. metal ion coordination algorithm-when does the ligand or the metal take control?

Polyfunctional hydrazone ligands with multidentate terminal donor groups offer metal ions many donor choices, and the coordination outcome depends mainly on the identity of the metal ion. Co(ii) and Ni(ii) prefer to adopt largely undistorted, six-coordinate geometries, while Cu(ii) can easily adapt to a variety of coordination situations (e.g. CN 4-6), and will optimize its coordination number and stereochemistry based on all the available donors. Ni(ii) and Co(ii) form simple [2 x 2] [M(4)-(micro(2)-O)(4)] square grids with such ditopic hydrazone ligands, and ignore other coordination options, while Cu(ii) tries to bind to all the available donors, and forms extended and 2D structures based on linked Cu(ii) triads rather than grids. Ni(ii) is also reluctant to compromise its desire to maximize its crystal field stabilization energy (CFSE) by binding to 'weak' ligands, and with a tetratopic pyrazole bis-hydrazone ligand it ignores the oxygen donors in favour of nitrogen, forming a novel trinuclear, triangular cluster. Also, reaction of a linear Ni(ii)(3) complex of a tetratopic pyridazine bis-hydrazone ligand with NiN(6) coordination spheres with Cu(ii), leads exclusively to a square Cu(12) grid based complex, and complete displacement of nickel. Structural and magnetic properties are highlighted, and metal-ligand interactions are discussed in detail.

NC-(CF2)4-CNSSN radical containing 1,2,3,5-dithiadiazolyl radical dimer exhibiting triplet excited states at low temperature and thermal hysteresis on melting-solidification: structural, spectroscopic, and magnetic characterization.

A high yield, one-pot synthesis of the 1,2,3,5-dithiadiazolyl radical NC-(CF2)4-CNSSN radical by reduction of the corresponding 1,3,2,4-dithiadiazolium salt is reported. In the solid state, the title compound is dimerized in trans-cofacial fashion with intra-dimeric Sdelta+...N(delta-) interactions of ca. 3.2 angstroms, and the dimeric units are linked by electrostatic -C triple bond N(delta-)...Sdelta+ interactions forming an infinite chain. Magnetic susceptibility measurements performed on the solid state sample indicate a magnetic moment of 1.8 microB per dimer (1.3 microB per monomer) at 300 K and a good fit to the Bleaney-Bowers model in the temperature range 2-300 K with 2J = -1500 +/- 50 cm(-1), g = 2.02(5), rho = 0.90(3)%, and TIP = 1.25(4) x 10(-3) emu mol(-1). The [NC-(CF2)4-CNSSN radical]2 dimer is the second example of a 1,2,3,5-dithiadiazolyl radical dimer with an experimentally detected triplet excited state as probed by solid-state EPR [2J = -1730 +/- 100 cm(-1), |D| = 0.0278(5) cm(-1), |E| = 0.0047(5) cm(-1)]. The value of the singlet-triplet gap has enabled us to estimate the "in situ" dimerization energy of the radical dimer as ca. -10 kJ mol(-1). The diradical character of the dimer was calculated [CASSCF(6,6)/6-31G*] as 35%. The title radical shows magnetic bistability in the temperature range of 305-335 K as probed by the solid-state EPR presumably arising from the presence of a metastable paramagnetic supercooled phase. Bistability is accompanied by thermochromic behavior with a color change from dark green (dimeric solid) to dark brown (paramagnetic liquid).

Ligand directed self-assembly of polymetallic [n x n] grids: rational routes to large functional molecular subunits?

Poly-(n)-topic ligands with a linear arrangement of coordination pockets self-assemble, generally in high yield, to produce square [n x n] grid complexes. Oligomeric, non-grid intermediates, identified by structural studies, have shown alternative construction pathways, but have also indicated possible mechanistic routes to grid assembly. Various factors are considered critical to grid formation, including reaction pH, metal ion identity, CFSE, and metal ion redox behaviour. Ultimately the design of the ligand is pivotal to successful grid self-assembly, and the size of the [n x n] grid is largely limited only by the synthetic limitations of the ligands themselves.

Coordination "oligomers" in self-assembly reactions of some "tritopic" picolinic dihydrazone ligands-mononuclear, dinuclear, hexanuclear, heptanuclear, and nonanuclear examples.

"Tritopic" picolinic dihydrazone ligands with tridentate coordination pockets are designed to produce homoleptic [3 x 3] nonanuclear square grid complexes on reaction with transition-metal salts, and many structurally documented examples have been obtained with Mn(II), Cu(II), and Zn(II) ions. However, other oligomeric complexes with smaller nuclearities have also been discovered and identified structurally in some reactions involving Fe(II), Co(II), Ni(II), and Cu(II), with certain tritopic ligands. This illustrates the dynamic nature of the metal-ligand interaction and the conformationally flexible nature of the ligands and points to the possible involvement of some of these species as intermediates in the [3 x 3] grid formation process. Examples of mononuclear, dinuclear, hexanuclear, heptanuclear, and nonanuclear species involving Fe(II), Co(II), Ni(II), and Cu(II) salts with a series of potentially heptadentate picolinic dihydrazone ligands with pyrazine, pyrimidine, and pyridine end groups are described in the present study. Iron and cobalt complexation reactions are complicated by redox processes, which lead to mixed-oxidation-state Co(II)/Co(III) systems when starting with Co(II) salts, and reduction of Fe(III) to Fe(II) when starting with Fe(III). Magnetic exchange within the polynuclear structural frameworks is discussed and related to the structural features.

Supramolecular self-assembled polynuclear complexes from tritopic, tetratopic, and pentatopic ligands: structural, magnetic and surface studies.

Polymetallic, highly organized molecular architectures can be created by "bottom-up" self-assembly methods using ligands with appropriately programmed coordination information. Ligands based on 2,6-picolyldihydrazone (tritopic and pentatopic) and 3,6-pyridazinedihydrazone (tetratopic) cores, with tridentate coordination pockets, are highly specific and lead to the efficient self-assembly of square [3 x 3] Mn9, [4 x 4] Mn16, and [5 x 5] Mn25 nanoscale grids. Subtle changes in the tritopic ligand composition to include bulky end groups can lead to a rectangular 3 x [1 x 3] Mn9 grid, while changing the central pyridazine to a more sterically demanding pyrazole leads to simple dinuclear copper complexes, despite the potential for binding four metal ions. The creation of all bidentate sites in a tetratopic pyridazine ligand leads to a dramatically different spiral Mn4 strand. Single-crystal X-ray structural data show metallic connectivity through both mu-O and mu-NN bridges, which leads to dominant intramolecular antiferromagnetic spin exchange in all cases. Surface depositions of the Mn9, Mn16, and Mn25 square grid molecules on graphite (HOPG) have been examined using STM/CITS imagery (scanning tunneling microscopy/current imaging tunneling spectroscopy), where tunneling through the metal d-orbital-based HOMO levels reveals the metal ion positions. CITS imagery of the grids clearly shows the presence of 9, 16, and 25 manganese ions in the expected square grid arrangements, highlighting the importance and power of this technique in establishing the molecular nature of the surface adsorbed species. Nanoscale, electronically functional, polymetallic assemblies of this sort, created by such a bottom-up synthetic approach, constitute important components for advanced molecule-based materials.

An octanuclear [Co(II)2-Co(III)2]2 interlocked grid--example of an inorganic [2]catenane.

An oxalic dihydrazide based flexible polydentate amidrazone ligand undergoes spontaneous [2 x 2] grid assembly, followed by partial disassembly, and then reassembly in the presence of Co(NO3)2.6H2O and NH4(NCS) to form a unique octanuclear [Co(II)2-Co(III)2]2 interlocked, single braided 4 + 4 [2]catenane.

Copper(II) complexes of a series of alkoxy diazine ligands: mononuclear, dinuclear, and tetranuclear examples with structural, magnetic, and DFT studies.

Picolyl hydrazide ligands have two potentially bridging functional groups (micro-O, micro-N-N) and consequently can exist in different coordination conformers, both of which form spin-coupled polynuclear coordination complexes, with quite different magnetic properties. The complex [Cu(2)(POAP-H)Br(3)(H(2)O)] (1) involves a micro-N-N bridge (Cu-N-N-Cu 150.6 degrees ) and exhibits quite strong antiferromagnetic coupling (-2J = 246(1) cm(-)(1)). [Cu(2)(PZOAPZ-H)Br(3)(H(2)O)(2)] (2) has two Cu(II) centers bridged by an alkoxide group with a very large Cu-O-Cu angle of 141.7 degrees but unexpectedly exhibits quite weak antiferromagnetic exchange (-2J = 91.5 cm(-)(1)). This is much weaker than anticipated, despite direct overlap of the copper magnetic orbitals. Density functional calculations have been carried out on compound 2, yielding a similar singlet-triplet splitting energy. Structural details are reported for [Cu(2)(POAP-H)Br(3)(H(2)O)] (1), [Cu(2)(PZOAPZ-H)Br(3)(H(2)O)(2)] (2), [Cu(2)(PAOPF-2H)Br(2)(DMSO)(H(2)O)].H(2)O (3), [Cu(4)(POMP-H))(4)](NO(3))(4).2H(2)O (4), and PPOCCO (5) (a picolyl hydrazide ligand with a terminal oxime group) and its mononuclear complexes [Cu(PPOCCO-H)(NO(3))] (6) and [Cu(PPOCCO-H)Cl] (7). Compound 1 (C(12)H(13)Br(3)Cu(2)N(5)O(4)) crystallizes in the monoclinic system, space group P2(1)/c, with a = 15.1465(3) A, b = 18.1848(12) A, c = 6.8557(5) A, beta = 92.751(4) degrees, and Z = 4. Compound 2 (C(10)H(13)Br(3)Cu(2)N(7)O(4)) crystallizes in the triclinic system, space group P, with a = 9.14130(1) A, b = 10.4723(1) A, c = 10.9411(1) A, alpha = 100.769(1), beta = 106.271(1) degrees, gamma = 103.447(1) degrees, and Z = 2. Compound 3 (C(23)H(22)Br(2)Cu(2)N(7)O(5.5)S) crystallizes in the monoclinic system, space group P2(1)/c, with a = 12.406(2) A, b = 22.157(3) A, c = 10.704(2) A, beta = 106.21(1) degrees, and Z = 4. Compound 4(C(52)H(48)Cu(4)N(20)O(18)) crystallizes in the monoclinic system, space group P2(1)/n, with a = 14.4439(6) A, b = 12.8079(5) A, c = 16.4240(7) A, beta = 105.199(1) degrees, and Z = 4. Compound 5 (C(15)H(14)N(4)O(2)) crystallizes in the orthorhombic system, space group Pna2(1), with a = 7.834(3) A, b = 11.797(4) A, c = 15.281(3) A, and Z = 4. Compound 6(C(15)H(13)CuN(5)O(5)) crystallizes in the monoclinic system, space group P2(1)/c, with a = 8.2818(9) A, b = 17.886(2) A, c = 10.828(1) A, beta = 92.734(2) degrees, and Z = 4. Compound 7 (C(15)H(13)CuClN(4)O(2)) crystallizes in the orthorhombic system, space group Pna2(1), with a = 7.9487(6) A, b = 14.3336(10) A, c = 13.0014(9) A, and Z = 4. Density functional calculations on PPOCCO are examined in relation to the anti-eclipsed conformational change that occurs on coordination to copper(II).

Supramolecular Mn(II) and Mn(II)/Mn(III) grid complexes with [Mn9(mu2-O)12] core structures. Structural, magnetic, and redox properties and surface studies.

A series of [3 x 3] Mn(II)(9), antiferromagnetically coupled, alkoxide-bridged, square grid complexes, derived from a group of "tritopic" dihydrazide ligands, is described. The outer ring of eight Mn(II) centers in the grids is isolated magnetically from the central Mn(II) ion, leading to an S = 0 ground state for the ring, and an S = 5/2 ground state overall in each case. Exchange in the Mn(II)(8) ring can be represented by a 1D chain exchange model. Rich electrochemistry displayed by these systems has led to the production of Mn(II)/Mn(III) mixed-oxidation-state grids by both electrochemical and chemical means. Structures are reported for [Mn(9)(2poap)(6)](C(2)N(3))(6).10H(2)O (1), [Mn(9)(2poap)(6)](2)[Mn(NCS)(4)(H(2)O)](2)(NCS)(8).10H(2)O (2), [Mn(9)(2poapz)(6)](NO(3))(6).14.5H(2)O (3), [Mn(9)(2popp)(6)](NO(3))(6).12H(2)O (4), [Mn(9)(2pomp)(6)](MnCl(4))(2)Cl(2).2CH(3)OH.7H(2)O (5), and [Mn(9)(Cl2poap)(6)](ClO(4))(9).7H(2)O (6). Compound 1 crystallized in the tetragonal system, space group P4(2)/n, with a = 21.568(1) A, c = 16.275(1) A, and Z = 2. Compound 2 crystallized in the triclinic system, space group P, with a = 25.043(1) A, b = 27.413(1) A, c = 27.538(2) A, alpha = 91.586(2) degrees, beta = 113.9200(9) degrees, gamma = 111.9470(8) degrees, and Z = 2. Compound 3 crystallized in the triclinic system, space group P, with a = 18.1578(12) A, b = 18.2887(12) A, c = 26.764(2) A, alpha = 105.7880(12) degrees, beta = 101.547(2) degrees, gamma = 91.1250(11) degrees, and Z = 2. Compound 4 crystallized in the tetragonal system, space group P4(1)2(1)2, with a = 20.279(1) A, c = 54.873(6) A, and Z = 4. Compound 5 crystallized in the tetragonal system, space group I, with a = 18.2700(2) A, c = 26.753(2) A, and Z = 2. Compound 6 crystallized in the triclinic system, space group P, with a = 19.044(2) A, b = 19.457(2) A, c = 23.978(3) A, alpha = 84.518(3) degrees, beta = 81.227(3) degrees, gamma = 60.954(2) degrees, and Z = 2. Preliminary surface studies on Au(111), with a Mn(II) grid complex derived from a sulfur-derivatized ligand, indicate monolayer coverage via gold-sulfur interactions, and the potential for information storage at high-density levels.

Octanuclear and nonanuclear supramolecular copper(II) complexes with linear "tritopic" ligands: structural and magnetic studies.

The structures and magnetic properties of self-assembled copper(II) clusters and grids with the "tritopic" ligands 2poap (a), Cl2poap (b), m2poap (c), Cl2pomp (d), and 2pomp (e) are described [ligands derived by reaction of 4-R-2,6-pyridinedicarboxylic hydrazide (R = H, Cl, MeO) with 2-pyridinemethylimidate (a-c, respectively) or 2-acetylpyridine (d, R = Cl; e, R = H)]. Cl2poap and Cl2pomp self-assemble with Cu(NO(3))(2) to form octanuclear "pinwheel" cluster complexes [Cu(8)(Cl2poap-2H)(4)(NO(3))(8)].20H(2)O (1) and [Cu(8)(Cl2pomp-2H)(4)(NO(3))(8)].15H(2)O (2), built on a square [2 x 2] grid with four pendant copper arms, using "mild" reaction conditions. Similar reactions of Cl2pomp and 2pomp with Cu(ClO(4))(2) produce pinwheel clusters [Cu(8)(Cl2pomp-2H)(4)(H(2)O)(8)](ClO(4))(8).7H(2)O (3) and [Cu(8)(2pomp-2H)(4)(H(2)O)(8)](ClO(4))(8) (4), respectively. Heating a solution of 1 in MeOH/H(2)O produces a [3 x 3] nonanuclear square grid complex, [Cu(9)(Cl2poap-H)(3)(Cl2poap-2H)(3)](NO(3))(9).18H(2)O (5), which is also produced by direct reaction of the ligand and metal salt under similar conditions. Reaction of m2poap with Cu(NO(3))(2) produces only the [3 x 3] grid [Cu(9)(m2poap-H)(2)(m2poap-2H)(4)](NO(3))(8).17H(2)O (6) under similar conditions. Mixing the tritopic ligand 2poap with pyridine-2,6-dicarboxylic acid (picd) in the presence of Cu(NO(3))(2) produces a remarkable mixed ligand, nonanuclear grid complex [Cu(9)(2poap-H)(4)(picd-H)(3)(picd-2H)](NO(3))(9).9H(2)O (7), in which aromatic pi-stacking interactions are important in stabilizing the structure. Complexes 1-3 and 5-7 involve single oxygen atom (alkoxide) bridging connections between adjacent copper centers, while complex 4 has an unprecedented mixed micro-(N-N) and micro-O metal ion connectivity. Compound 1 (C(76)H(92)N(44)Cu(8)O(50)Cl(4)) crystallizes in the tetragonal system, space group I, with a = 21.645(1) A, c = 12.950(1) A, and Z = 2. Compound 2 (C(84)H(88)N(36)O(44)Cl(4)Cu(8)) crystallizes in the tetragonal system, space group I, with a = 21.2562(8) A, c = 12.7583(9) A, and Z = 2. Compound 4 (C(84)H(120)N(28)O(66)Cl(8)Cu(8)) crystallizes in the tetragonal system, space group I4(1)/a, with a = 20.7790(4) A, c = 32.561(1) A, and Z = 4. Compound 7(C(104)H(104)N(46)O(56)Cu(9)) crystallizes in the triclinic system, space group P, with a = 15.473(1) A, b = 19.869(2) A, c = 23.083(2) A, alpha = 88.890(2) degrees, beta = 81.511(2) degrees, gamma = 68.607(1) degrees, and Z = 2. All complexes exhibit dominant intramolecular ferromagnetic exchange coupling, resulting from an orthogonal bridging arrangement within each polynuclear structure.

Structural and magnetic properties of a self-assembled spheroidal triakonta-hexanuclear Cu36 cluster.

An extended tritopic picolinic dihydrazide ligand with terminal oxime groups (dpocco) undergoes spontaneous self-assembly in the presence of copper acetate to produce a unique Cu36 cluster exhibiting intramolecular anti-ferromagnetic exchange.