Multisite Coordination Ligand Research Articles

Tetranuclear CoII4O4 Cubane Complex: Effective Catalyst Toward Electrochemical Water Oxidation.

The reaction of Co(OAc)2·6H2O with 2,2'-[{(1E,1'E)-pyridine-2,6-diyl-bis(methaneylylidene)bis(azaneylylidene)}diphenol](LH2) a multisite coordination ligand and Et3N in a 1:2:3 stoichiometric ratio forms a tetranuclear complex Co4(L)2(μ-η1:η1-OAc)2(η2-OAc)2]· 1.5 CH3OH· 1.5 CHCl3 (1). Based on X-ray diffraction investigations, complex 1 comprises a distorted Co4O4 cubane core consisting of two completely deprotonated ligands [L]2- and four acetate ligands. Two distinct types of CoII centers exist in the complex, where the Co(2) center has a distorted octahedral geometry; alternatively, Co(1) has a distorted pentagonal-bipyramidal geometry. Analysis of magnetic data in 1 shows predominant antiferromagnetic coupling (J = -2.1 cm-1), while the magnetic anisotropy is the easy-plane type (D1 = 8.8, D2 = 0.76 cm-1). Furthermore, complex 1 demonstrates an electrochemical oxygen evolution reaction (OER) with an overpotential of 325 mV and Tafel slope of 85 mV dec-1, required to attain a current density of 10 mA cm-2 and moderate stability under alkaline conditions (pH = 14). Electrochemical impedance spectroscopy studies reveal that compound 1 has a charge transfer resistance (Rct) of 2.927 Ω, which is comparatively lower than standard Co3O4 (5.242 Ω), indicating rapid charge transfer kinetics between electrode and electrolyte solution that enhances higher catalytic activity toward OER kinetics.

Multisite Coordination Ligands on Cyclotriphosphazene Core for the Assembly of Metal Clusters and Porous Coordination Polymers

AbstractIn this review, we account the recent advancements on cyclotriphosphazene (N3P3Cl6) based multi‐site coordination ligands for the synthesis of multi‐metallic architectures. In case of N3P3Cl6, Phosphorous centres with two chlorides use to be bonded with nitrogen [−P(Cl2)=N−]3 at alternative positions in six membered ring to provide a robust planar structure to anchor the ligand arms with wide flexibility. Cyclic P−N robust motif is renowned as the potential support for the design of multi‐site ligands originating from phosphorus centres by viable substitution of variable number of ligand units (1 to 6) consist of one or more coordination site(s) via geminal and/or non‐geminal modes. Despite these phosphorus centres are promising to support in ligand structure, occasionally involve in coordination with transition metals as well. Three of alternative nitrogen atoms in this heterocyclic core are supposed to function as Lewis base centres while the electron releasing substituents occurred to tether on phosphorus centres, also when the resultant ring size favours adequate interaction towards the transition metal ions with respect to skeletal flexibility. The ligand modes vary with respect to numbers and orientation of coordination sites, which is imperative to enrich the ligation ability. Moreover, several examples of cyclotriphosphazene core ligands consist of the spacer oxygen atom while tethering with exocyclic ligand units to incorporate more flexibility. In accordance with coordination sites on ligand framework and choice of metal, the skeletal ring nitrogen atom(s) of the cyclotriphosphazene can be involved in coordination with metal ions. Indeed, the coordination of ligands is reported to coordinate with main group, transition and lanthanide metal ions to form variety of metal clusters and porous coordination polymers. Subsequently, the geometry of homo or heterometallic complexes with respect to versatile coordination modes of metallic complexes tends to afford inherent chemical properties for specific application. In terms of exocyclic ligands, the heterocyclic units such as pyridyloxy‐, pyrazolyl‐, pyridylalkylamino‐, bipyridyl‐, 1,10‐phenanthroline, porphyrinato, hydrazide, hydrazone, Schiff's base and spirocyclic amine units attached to cyclotriphosphazene were exploited to form series of metal complexes. Similarly, the variety of metal clusters, polymeric network (1D to 3D) and porous coordination polymers have been reported on basis of cyclic core molecule substituted with ligand units, pyridyloxy‐, multi‐carboxylic, aromatic amines substituted pyridyloxy‐ and imidazole moiety involved to form variety of products.

Heterometallic 3d-4f Complexes as Single-Molecule Magnets.

Heterometallic 3d-4f complexes are being investigated, for some time, as being useful in molecular magnetism, particularly as single-molecule magnets (SMMs). This interest is primarily because of the possibility of an increased ligand-mediated super-exchange phenomenon between the 3d and 4f metal ions. Such an interaction, apart from bestowing a favorable ground-state spin to the complex, also assists in reducing quantum tunneling of magnetization that is widely prevalent in SMMs making them to lose magnetization. However, assembling both 3d as well as 4f ions using same ligand system is challenging and involves the design of multi-site coordination ligands with specific coordination compartments for the 3d and the 4f metal ions while at the same time allowing these disparate metal ions to be linked to each other through a bridging ligating atom. This review presents a summary of the 3d-4f complexes primarily derived from the author's work while alluding to important examples from the literature. We also provide an outlook for the future design of such complexes.

Heterometallic Heptanuclear [Cu5Ln2] (Ln = Tb, Dy, and Ho) Single-Molecule Magnets Organized in One-Dimensional Coordination Polymeric Network.

The reaction of a multisite coordination ligand, LH3, with Cu(II) salts and Ln(NO3)3·nH2O in a 1:2:1 stoichiometric ratio in the presence of triethylamine was found to afford a series of one-dimensional heterometallic [{Cu5Ln2(L)2(μ3-OH)4(ClO4)(NO3)3(OH2)5}(ClO4)2(H2O)x]∞ [Ln = Tb(1), Dy(2) and Ho(3), x = 4.25, 5.5, and 5 for 1-3, respectively] coordination polymers. Complexes 1-3 have been characterized by single crystal X-ray crystallography. The detailed study of the magnetic properties has also been performed and compared with the parent [Cu5Ln2] molecular analogues. The ac susceptibility measurements for complexes 1-3 confirm their SMM behavior with the following characteristics: Δeff/kB = 23.4 K, τ0 = 1.1 × 10-6 s and Δeff/kB = 27.9 K, τ0 = 6.6 × 10-7 s under 0 and 1200 Oe dc fields, respectively for 1; Δeff/kB = 8.3 K, τ0 = 3.1 × 10-6 s for 2 under 0 dc field. For 3, the fast QTM below 4 K prevents the estimation of the SMM energy barrier. Remarkably, the magnetic and SMM properties of the previously reported molecular [Cu5Ln2] analogues are preserved after their assembly in these coordination networks.

Syntheses and structures of a family of heterometallic pentanuclear [MnIII3LnNa] (Ln = Dy, Tb, Gd and Nd) complexes: H-bonding reduces the nuclearity from nine to five

The reaction of a multisite coordination ligand (LH3) with LnCl3·6H2O, followed by the addition of Mn(OAc)2·4H2O, sodium azide and pivalic acid in the presence of triethylamine in a 2 : 1 : 1 : 4 : 4 : 6 (LH3 : LnCl3 : Mn(OAc)2 : NaN3 : pivalic acid : Et3N) stoichiometric ratio afforded a series of heterometallic pentanuclear neutral complexes containing a [MnIII3LnNa] core {Ln = Dy (1), Tb (2), Gd (3) and Nd (4)}. The oxidation states of the MnIII ions were confirmed by BVS calculation. All the complexes were characterized by X-ray crystallography. The pentametallic aggregates form a trigonal-bipyramidal geometry in which the three MnIII ions are in the equatorial plane whereas the sodium and lanthanide ions are in the apical positions. Three different types of MnIII ions are present in the complexes and all are in a distorted octahedral geometry. An all-oxygen environment is found to be present around the lanthanides as well as the sodium ions. The lanthanide ion is eight-coordinated and in distorted dodecahedral geometry, while the sodium ion is six-coordinate in a distorted trigonal prismatic geometry in 1–4. The pentanuclear core [MnIII3LnNa] is a subset of one-half of the nonanuclear ensemble, [MnIII6LnNa2]+, reported in literature. We propose that the reduction in the nuclearity of the heterometallic metal ensemble occurs as a result of the influence of hydrogen bonding interactions present in the system.

Linear, Edge‐Sharing Heterometallic Trinuclear [CoII–LnIII–CoII] (LnIII = GdIII, DyIII, TbIII, and HoIII) Complexes: Slow Relaxation of Magnetization in the DyIII Derivative

AbstractThe sequential reaction of a multisite coordination ligand (LH4) with lanthanide (III) nitrates followed by the addition of Co(NO3)2·6H2O in a 4:1:2 ratio in the presence of triethylamine afforded a series of heterometallic, linear trinuclear complexes [Co2Ln(LH3)4]·3NO3 [Ln = DyIII (1), GdIII (2), TbIII (3), and HoIII (4)]. These tricationic complexes contain nitrate counteranions. The cationic portion of these complexes consists of three metal ions that are arranged in a linear fashion; a central LnIII is present between the two terminal CoII ions and is attached to the latter through two phenoxide bridging groups. This arrangement generates two contiguous four‐membered CoLnO2 rings. The CoII ions are six‐coordinate (2N, 4O) in a distorted octahedral geometry, whereas the central lanthanide ion is eight‐coordinate (8O) in a distorted square‐antiprismatic geometry. Magnetic studies of these complexes have been performed and indicate a slow relaxation of the magnetization for the DyIII derivative 1.

Slow Magnetic Relaxation in Co(III)–Co(II) Mixed-Valence Dinuclear Complexes with a CoIIO5X (X = Cl, Br, NO3) Distorted-Octahedral Coordination Sphere

The reaction of the multisite coordination ligand (LH4) with CoX2·nH2O in the presence of tetrabutylammonium hydroxide affords a series of homometallic dinuclear mixed-valence complexes, [Co(III)Co(II)(LH2)2(X)(H2O)](H2O)m (1, X = Cl and m = 4; 2, X = Br and m = 4; 3, X = NO3 and m = 3). All of the complexes have been structurally characterized by X-ray crystallography. Both cobalt ions in these dinuclear complexes are present in a distorted-octahedral geometry. Detailed magnetic studies on 1-3 have been carried out. M vs H data at different temperatures can be fitted with S = 3/2, the best fit leading to D(3/2) = -7.4 cm(-1), |E/D| < 1 × 10(-3), and g = 2.32 for 1 and D(3/2) = -9.7 cm(-1), |E/D| <1 × 10(-4), and g = 2.52 for 2. In contrast to 1 and 2, M vs H data at different temperatures suggest that compound 3 has comparatively little magnetic anisotropy. In accordance with the large negative D values observed for compounds 1 and 2, they are single-molecule magnets (SMMs) and exhibit slow relaxation of magnetization at low temperatures under an applied magnetic field of 1000 Oe with the following energy barriers: 7.9 cm(-1) (τo = 6.1 × 10(-6) s) for 1 and 14.5 cm(-1) (τo = 1.0 × 10(-6) s) for 2. Complex 3 does not show any SMM behavior, as expected from its small magnetic anisotropy. The τo values observed for 1 and 2 are much larger than expected for a SMM, strongly suggesting that the quantum pathway of relaxation at very low temperatures is not fully suppressed by the effects of the applied field.

Syntheses, Structures, and Magnetic Properties of a Family of Heterometallic Heptanuclear [Cu5Ln2] (Ln = Y(III), Lu(III), Dy(III), Ho(III), Er(III), and Yb(III)) Complexes: Observation of SMM behavior for the Dy(III) and Ho(III) Analogues

Sequential reaction of the multisite coordination ligand (LH3) with Cu(OAc)2·H2O, followed by the addition of a rare-earth(III) nitrate salt in the presence of triethylamine, afforded a series of heterometallic heptanuclear complexes containing a [Cu5Ln2] core {Ln = Y(1), Lu(2), Dy(3), Ho(4), Er(5), and Yb(6)}. Single-crystal X-ray crystallography reveals that all the complexes are dicationic species that crystallize with two nitrate anions to compensate the charge. The heptanuclear aggregates in 1-6 are centrosymmetrical complexes, with a hexagonal-like arrangement of six peripheral metal ions (two rare-earth and four copper) around a central Cu(II) situated on a crystallographic inversion center. An all-oxygen environment is found to be present around the rare-earth metal ions, which adopt a distorted square-antiprismatic geometry. Three different Cu(II) sites are present in the heptanuclear complexes: two possess a distorted octahedral coordination sphere while the remaining one displays a distorted square-pyramidal geometry. Detailed static and dynamic magnetic properties of all the complexes have been studied and revealed the single-molecule magnet behavior of the Dy(III) and Ho(III) derivatives.

Synthesis of One- and Two-Dimensional Coordination Polymers Containing Organotin Macrocycles. Reactions of (n-Bu3Sn)2O with Pyridine Dicarboxylic Acids. Structure-Directing Role of the Ancillary 4,4′-Bipyridine Ligand

The reaction of (n-Bu3Sn)2O with pyridine-2,6-dicarboxylic acid (L1H2) in a 1:1 ratio resulted in the formation of a one-dimensional (1D) coordination polymer [(n-Bu3Sn)2(n-Bu2Sn)2(μ-L1)2(μ-OH)2]n (1). The formation of 1 is accompanied by a Sn–butyl bond cleavage reaction involving half of the organotin units. The formation of the 1D coordination polymer is facilitated by the multisite coordination capability of the dianionic ligand L1. The reaction of (n-Bu3Sn)2O with pyridine-2,5-dicarboxylic acid (L2H2) or pyridine-3,5-dicarboxylic acid (L3H2), on the other hand, results in the generation of the two-dimensional (2D) coordination polymers [(n-Bu3Sn)2(μ-L2)]n (2) and [(n-Bu3Sn)4(μ-L3)2]n (3), respectively. The formation of 2 and 3 emphasizes the importance of the relative orientation of the coordinating units in the multisite coordination ligand. Compounds 1–3 show a rich supramolecular architecture in their solid state as a result of multiple secondary interactions. Investigation of the fate of the reac...

Phosphorus-Supported Ligands for the Assembly of Multimetal Architectures

Modeled after boron-based scorpionate ligands, acyclic and cyclic phosphorus-containing compounds possessing reactive groups can serve as excellent precursors for the assembly of novel phosphorus-supported ligands that can coordinate multiple sites. In such ligands, the phosphorus atom does not have any role in coordination but is used as a structural support to assemble one or more coordination platforms. In this Account, we describe the utility of inorganic heterocyclic rings such as cyclophosphazenes and carbophosphazenes as well as acyclic phosphorus-containing compounds such as (S)PCl(3), RP(O)Cl(2), and R(2)P(O)Cl for building such multisite coordination platforms. We can modulate the number and orientation of such coordination platforms through the choice of the phosphorus-containing precursor. This methodology is quite general and modular and allows the creation of well-defined libraries of multisite coordination ligands. Phosphorus-supported pyrazolyl ligands are quite useful for building multimetallic architectures. Some of these ligands are prone to P-N bond hydrolysis upon metalation, but we have exploited the P-N bond sensitivity to generate hydrolyzed ligands in situ, which are useful to build multimetal assemblies. In addition, the intimate relationship between small molecule cyclophosphazenes and the corresponding pendant cyclophosphazene-containing polymer systems facilitated our design of polymer-supported catalysts for phosphate ester hydrolysis, plasmid DNA modification, and C-C bond formation reactions. Phosphorus hydrazides containing reactive amine groups are ideal precursors for integration into more complex ligand systems. The ligand (S)P[N(Me)N=CH-C(6)H(4)-2-OH](3) (LH(3)) contains six coordination sites, and its coordination response depends upon the oxidation state of the metal ion employed. LH(3) reacts with divalent transition metal ions to afford neutral trimetallic derivatives L(2)M(3), where the three metal ions are arranged in a perfectly linear manner in many cases. Incorporating an additional methoxy group into LH(3) affords the ligand (S)P[N(Me)N=CH-C(6)H(3)-2-OH-3-OMe](3) (L'H(3)), which contains nine coordination sites: three imino nitrogen atoms, three phenolate oxygen atoms, and three methoxy oxygen atoms. The reaction of L'H(3) with transition metal salts in 1:1 ratio leads to the in situ formation of a metalloligand (L'M), which on further treatment with lanthanide salts gives heterobimetallic trinuclear cationic complexes [L'(2)M(2)Ln](+) containing a M-Ln-M linear array (M = transition metal ion in a +2 oxidation state). Many of these 3d-4f compounds behave as single-molecule magnets at low temperatures. Although challenges remain in the development of synthetic methods and in the architectural control of the coordination platforms, we see opportunities for further research into coordination platforms supported by main group elements such as phosphorus. As we have shown in this Account, one potential disadvantage, sensitivity of P-N bonds to hydrolysis, can be used successfully to build larger assemblies.

Cyclophosphazene-Supported Tetranuclear Copper Assembly Containing 15 Contiguous Inorganic Rings

The cyclophosphazene hydrazide gem-N3P3Ph2[N(Me)NH2]4 was reacted with o-hydroxybenzaldehyde to afford the multisite coordination ligand gem-N3P3Ph2[N(Me)N=CHC6H4-2-OH]4 (LH4). The latter reacted with copper(II) salts to afford a novel tetranuclear copper assembly {N3P3Ph2[N(Me)N=CHC6H4-2-O]4Cu2}2, which contains, remarkably, 15 contiguous inorganic rings.

A phosphorus-supported multisite coordination ligand containing three imidazolyl arms and its metalation behaviour. An unprecedented co-existence of mononuclear and macrocyclic dinuclear Zn(ii) complexes in the same unit cell of a crystalline lattice

The reaction of (S)P[N(Me)NH(2)](3) with three equivalents of 4(5)-imidazolecarboxaldehyde afforded, in situ, (S)P[N(Me)N[double bond, length as m-dash]CHIm](3) (Im = imidazolyl). The reaction of the latter with metal salts afforded mononuclear complexes [{(S)P[N(Me)N[double bond, length as m-dash]CHIm](3).M}][X](2) [M = Co, X = NO(3); M = Ni, X = ClO(4); M = Cd, X = NO(3)]. In these compounds, in the cationic part, the metal ion is bound by three imino and three imidazolyl nitrogen atoms to generate a trigonal prismatic type of coordination environment. Structural analysis of the zinc complex showed the presence of a mononuclear [{(S)P[N(Me)N[double bond, length as m-dash]CHIm](3).Zn}][NO(3)](2) and a dinuclear 20-membered metallamacrocycle [{(S)P[N(Me)N[double bond, length as m-dash]CHIm](3).Zn}(2)][NO(3)](4) in the same unit cell. Solution studies of the zinc complex revealed a monomer-macrocycle equilibrium.

Synthesis, structure and metallation of spiro-N 3P 3(O 2C 12H 8)(OC 5H 4N-2) 4: A heptacoordinate Co(II) in the molecular structure of N 3P 3(O 2C 12H 8)(OC 5H 4N-2) 4 · Co(NO 3) 2

Spiro-N 3P 3(O 2C 12H 8)(OC 5H 4N-2) 4 ( L) is an efficient multi-site coordination ligand which binds with transition metal ions to afford the mononuclear complexes [LCu(NO 3) 2], [LCo(NO 3) 2] and [LZnCl 2]. In [LCu(NO 3) 2] the copper atom is six-coordinate with the cyclophosphazene ligand coordinating to the metal ion through one skeletal ring nitrogen atom and two nitrogen atoms of geminal pyridyloxy substituents in a η 3- gem-N 3 mode. The two nitrate ligands provide monodentate as well as chelating coordination. In [LCo(NO 3) 2] the coordination geometry around Co(II) is pentagonal bipyramidal with a 3N, 4O coordination environment. The cyclophosphazene ligand binds to the metal ion in a η 3- non-gem-N 3 mode. The two nitrate ligands function as chelating ligands and complete the coordination around cobalt. The coordination response of L in the complex [LZnCl 2] is also η 3- non-gem-N 3. Two chloride ligands complete the coordination environment around zinc.

Di- and Trinuclear Complexes Derived from Hexakis(2-pyridyloxy)cyclotriphosphazene. Unusual P−O Bond Cleavage in the Formation of [{(L‘CuCl)2(Co(NO3)}Cl] (L‘ = N3P3(OC5H4N)5(O))

Hexakis(2-pyridyloxy)cyclotriphosphazene (L) is an efficient multisite coordination ligand which binds with transition metal ions to produce dinuclear (homo- and heterometallic) complexes [L(CuCl)(CoCl3)], [L(CuCl)(ZnCl3)], [L(CoCl)(ZnCl3)], and [L(ZnCl2)2]. In these dinuclear derivatives the cyclophosphazene ligand utilizes from five to six nitrogen coordination sites out of the maximum of nine available sites. Further, the spacer oxygen that separates the pyridyl moiety from the cyclophosphazene ring ensures minimum steric strain to the cyclophosphazene ring upon coordination. This is reflected in the near planarity of the cyclophosphazene ring in all the dinuclear derivatives. In the dinuclear heterobimetallic derivatives one of the metal ions [Cu(II) or Co(II)] is hexacoordinate and is bound by the cyclophosphazene in a eta5-gem-N5 mode. The other metal ion in these heterobimetallic derivatives [Co(II) or Zn(II)] is tetracoordinate and is bound in an eta(1)-N(1) fashion. In the homobimetallic derivative, [L(ZnCl2)2], one of the zinc ions is five-coordinate (eta3-nongem-N3), while the other zinc ion is tetracoordinate(eta2-gem-N2). The reaction of L with CuCl2 followed by Co(NO3)2.6H2O yields a trinuclear heterobimetallic complex [{(L'CuCl)2Co(NO3)}Cl] [L' = N3P3(OC5H4N)5(O)]. In the formation of this compound an unusual P-O bond cleavage involving one of the phosphorus-pyridyloxy bonds is observed. The molecular structure of [{(L'CuCl)2Co(NO3)}Cl] [L' = N3P3(OC5H4N)5(O)] reveals that each of the two the P-O-cleaved L' ligands is involved in binding to Cu(II) to generate the motif L'CuCl. Two such units are bridged by a Co(II) ion. The coordination environment around the bridging Co(II) ion contains four oxygen (two P-O units, one chelating nitrate) and two nitrogen atoms (pyridyloxy nitrogens).

Multi-site coordination ligands assembled on organostannoxane supports

The reactions of bis(pyrazol-1-yl) acetic acid LCOOH ( 1) (L = (Pz) 2CH–) and bis(3,5-dimethylpyrazol-1-yl)acetic acid L′COOH ( 2) (L = (3,5-Me 2Pz) 2CH–) with organotin oxide (hydroxide) precursors, n-BuSn(O)(OH), n-Bu 2SnO, ( n-Bu 3Sn) 2O and (Ph 3Sn) 2O has led to the isolation of several organotin compounds containing bispyrazolyl unit(s) on the periphery of the stannoxane structure [ n-BuSn(O)O 2CL] 6 ( 3), [ n-BuSn(O)O 2CL′] 6 ( 4), [ n-Bu 3SnO 2CL] n ( 5), [ n-Bu 3SnO 2CL′] n ( 6), [Ph 3SnO 2CL] n ( 7), [Ph 3SnO 2CL′] n ( 8), [{ n-Bu 2SnO 2CL} 2O] 2 ( 9) and [{ n-Bu 2SnO 2CL′} 2O] 2 ( 10). Compounds 5, 7, 9 and 10 have been structurally characterized by X-ray crystallography. In the solid state, these compounds possess interesting 3-D and 2-D supramolecular networks as a result of intermolecular C–H⋯O, C–H⋯N, C–H⋯Cl and C–H⋯π interactions.

Phosphazenes as scaffolds for the construction of multi-site coordination ligands

Chlorocyclophosphazenes containing a reactive periphery and a robust framework offer an ideal platform for the construction of a large variety of multi-site coordination ligands. Various types of neutral or (multi)anionic multi-site coordinating ligands can be assembled containing different numbers, type and stereochemical orientations of coordination sites reflecting the ease of modulation of these ligand types. Also it is possible to construct new types of macrocyclic ligands using cyclophosphazenes as scaffolds. The interaction of these ligands with transition and main group metals leads to a number of different complex types showing the rich diversity that is present in this family of compounds. A study of these ligands in principle also allows the assembly of the corresponding polymeric ligands.