Self-assemblies of new rigid angular ligands and metal centers toward the rational construction and modification of novel coordination polymer networks

Self-assemblies of rigid angular ligands with 120 degrees molecular angle and metal centers have been investigated with the aim of achieving the rational construction and modification of coordination polymer structures. The reactions of Co(NCS)(2) with 1,3-bis(trans-4-styrylpyridyl)benzene (L(1)()), 2,6-bis(trans-4-styrylpyridyl)pyridine (L(2)()), 1,3-bis(trans-4-styrylpyrimidyl)benzene (L(3)()), and 1,3-bis(trans-4-styrylquinoly)benzene (L(4)()) afford complexes [Co(L(1)())(2)(NCS)(2)]( infinity ) (1), [Co(L(2)())(2)(NCS)(2)]( infinity ) (2), Co(L(3)())(2)(NCS)(2)(CH(3)OH)(2) (3), and [Co(L(4)())(NCS)(2)]( infinity ) (4), respectively. The resulting complexes exhibit open framework, stairlike hydrogen-bonded chain and single-stranded helical coil structures, which are controlled by the variation of the geometry around the coordination site in ligands. Moreover, the coordination of L(1)() and L(2)() to Mn(hfac)(2) (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate) yields single-stranded helical coordination polymers of [Mn(L(1)())(hfac)(2)]( infinity ) (5) and [Mn(L(2)())(hfac)(2)]( infinity ) (6), respectively.     

Plutonium(IV) sequestration: structural and thermodynamic evaluation of the extraordinarily stable cerium(IV) hydroxypyridinonate complexes

Ligands containing the 1-methyl-3-hydroxy-2(1H)-pyridinone group (Me-3,2-HOPO) are powerful plutonium(IV) sequestering agents. The Ce(IV) complexes of bidentate and tetradentate HOPO ligands have been quantitatively studied as models for this sequestration. The complexes Ce(L1)4, Ce(L2)4, Ce(L3)2, and Ce(L4)2 (L1 = Me-3,2-HOPO; L2 = PR-Me-3,2-HOPO; L3 = 5LI-Me-3,2-HOPO; L4 = 5LIO-Me-3,2-HOPO) were prepared in THF solution from Ce(acac)4 and the corresponding ligand. The complex Ce(L4)2 was also prepared in aqueous solution by air oxidation of the Ce(III) complex [Ce(L4)2]-. Single-crystal X-ray diffraction analyses are reported for Ce(L1)(4)x2CHCl3 [P1 (no. 2), Z = 2, a = 9.2604(2) A, b = 12.1992(2) A, c = 15.9400(2) A, alpha = 73.732(1) degrees, beta = 85.041(1) degrees, gamma = 74.454(1) degrees], Ce(L3)2x2CH3OH [P2(1)/c (no. 14), Z = 4, a = 11.7002(2) A, b = 23.0033(4) A, c = 15.7155(2) A, beta = 96.149(1) degrees], Ce(L4)(2).2CH3OH [P1 (no. 2), Z = 2, a = 11.4347(2) A, b = 13.8008(2) A, c = 15.2844(3) A, alpha = 101.554(1) degrees, beta = 105.691(1) degrees, gamma = 106.746(1) degrees], and Ce(L4)2x4H2O [P2(1)/c (no. 14), Z = 4, a = 11.8782(1) A, b = 22.6860(3) A, c = 15.2638(1) A, beta = 96.956(1) degrees]. A new criterion, the shape measure S, has been introduced to describe and compare the geometry of such complexes. It is defined as [formula: see text], where m is the number of edges, delta i is the observed dihedral angle along the ith edge of delta (angle between normals of adjacent faces), theta i is the same angle of the corresponding ideal polytopal shape theta, and min is the minimum of all possible values. For these complexes the shape measure shows that the coordination geometry is strongly influenced by small changes in the ligand backbone or solvent. Solution thermodynamic studies determined overall formation constants (log beta) for Ce(L2)4, Ce(L3)2, and Ce(L4)2 of 40.9, 41.9, and 41.6, respectively. A thermodynamic cycle has been used to calculate these values from the corresponding formation constants of Ce(III) complexes and standard electrode potentials. From the formation constants and from the protonation constants of the ligands, extraordinarily high pM values for Ce(IV) are generated by these tetradentate ligands (37.5 for Ce(L3)2 and 37.0 for Ce(L4)2). The corresponding constants for Pu(IV) are expected to be substantially the same.     

Hexadentate hydroxypyridonate iron chelators based on TREN-Me-3,2-HOPO: variation of cap size

TREN-Me-3,2-HOPO, TR322-Me-3,2-HOPO, TR332-Me-3,2-HOPO, and TRPN-Me-3,2-HOPO correspond to stepwise replacement of ethylene by propylene bridges. A series of tripodal, hexadentate hydroxypyridinone ligands are reported. These incorporate 1-methyl-3,2-hydroxypyridinone (Me-3,2-HOPO) bidentate chelating units for metal binding. They are varied by systematic enlargement of the capping scaffold which connects the binding units. The series of ligands and their iron complexes are reported. Single crystal X-ray structures are reported for the ferric complexes of all four tripodal ligands: FeTREN-Me-3,2-HOPO.0.375C(4)H(10)O.0.5CH(2)Cl(2) [P2(1)/n (No. 14), Z = 8, a = 20.478(3) A, b = 12.353(2) A, c = 27.360(3) A; beta = 91.60(1) degrees ]; FeTR322-Me-3,2-HOPO.CHCl(3).0.5C(6)H(14).CH(3)OH.0.5H(2)O [P2(1)/n (No. 14), Z = 4, a = 12.520(3) A, b = 22.577(5) A, c = 16.525(3) A; beta = 111.37(3) degrees ]; FeTR332-Me-3,2-HOPO.3.5CH(3)OH [C2/c (No. 15), Z = 8, a = 13.5294(3) A, b = 19.7831(4) A, c = 27.2439(4) A; beta = 101.15(3) degrees ]; FeTRPN-Me-3,2-HOPO.C(3)H(7)NO.2C(4)H(10)O [P1 (No. 2), Z = 2, a = 11.4891(2) A, b = 12.3583(2) A, c = 15.0473(2) A; alpha = 86.857(1) degrees, beta = 88.414(1) degrees, gamma = 70.124(1) degrees ]. The structures show the importance of intermolecular hydrogen bonds and the effect of cap enlargement to the stability and geometry of the metal complexes throughout the series. All protonation and iron complex formation constants have been determined from solution thermodynamic studies. The TREN-capped derivative is the most acidic, with a cumulative protonation constant, log beta(014), of 25.95. Corresponding values of 26.35, 26.93, and 27.53 were obtained for the TR322, TR332, and TRPN derivatives, respectively. The protonation constants and NMR spectroscopic data are interpreted as being due to the influence of specific hydrogen-bond interactions. The incremental enlargement of ligand size results in a decrease in iron-chelate stability, as reflected in the log beta(110) values of 26.8, 26.2, 26.42, and 24.48 for the TREN, TR322, TR332, and TRPN derivatives, respectively. The metal complex formation constants are also affected by the acidity of a proximal (non-metal-binding) amine in the complexes, a trend consistent with the effects of internal hydrogen bonding. The ferric complexes display reversible reduction potentials (measured relative to the normal hydrogen electrode (NHE)) between -0.170 and -0.223 V.     

Synthesis of a ligand based upon a new entry into the 3-hydroxy-N-alkyl-2(1H)-pyridinone ring system and thermodynamic evaluation of its gadolinium complex

The synthesis of a new, more water soluble derivative of TREN-Me-3,2-HOPO (tris[(3-hydroxy-1-methyl-2-oxo-1,2- didehydropyridine-4-carboxamido)ethyl]amine) is presented. The synthesis starts with the condensation reaction of (N-methoxyethylamino)acetonitrile hydrochloride and oxalyl chloride to give 3,5-dichloro-N-(methoxyethyl)-2(1H)-pyrazinone. The 3-position is readily substituted with a benzyloxy group, and the pyrazinone is converted to ethyl 3-(benzyloxy)-N-(methoxyethyl)-2(1H)-pyridinone-4-carboxylate by a Diels-Alder cycloaddition with ethyl propiolate. Basic deprotection of the ester followed by activation, coupling to tren, and acidic deprotection of the benzyl groups gives the ligand TREN-MOE-3,2-HOPO (tris[(3-hydroxy-1-(methoxyethyl)- 2-oxo-1,2-didehydropyridine-4-carboxamido)ethyl]amine). The gadolinium complex of TREN-MOE-3,2-HOPO was prepared by metathesis, starting from gadolinium chloride. The solubility of the new metal complex is significantly enhanced. The four protonation constants (determined by potentiometry) for TREN-MOE-3,2-HOPO (log Ka1 = 8.08, log Ka2 = 6.85, log Ka3 = 5.81, log Ka4 = 4.98) are virtually identical to those reported for the parent ligand. The stability constants for the gadolinium complex of TREN-MOE-3,2-HOPO determined by potentiometry (log beta 110 = 19.69(2), log beta 111 = 22.80(2)) and by spectrophotometry (log beta 110 = 19.80(1), log beta 111 = 22.88(1), log beta 112 = 25.88(1)) differ slightly from those for the parent ligand; this follows from a change in the complexation model in which a new diprotonated species, [Gd(TREN-MOE-3,2-HOPO)(H)2]2+, was included. The presence of this extra species was demonstrated by factor analysis, comparison of spectral data, and nonlinear least-squares refinement. Significant formation of this species is observed between pH 3 and pH 1.5.     

Synthesis of homochiral tris(2-alkyl-2-aminoethyl)amine derivatives from chiral alpha-amino aldehydes and their application in the synthesis of water soluble chelators

A novel synthesis of 3-fold symmetric, homochiral tris(2-alkyl-2-aminoethyl)amine (TREN) derivatives is presented. The synthesis is general in scope, starting from readily prepared chiral alpha-amino aldehydes. The optical purity of the N-BOC protected derivatives of tris(2-methyl-2-aminoethyl)amine and tris(2-hydroxymethyl-2-aminoethyl)amine has been ascertained by polarimetry and chiral NMR chemical shift experiments. An X-ray diffraction study of the L-alanine derivative (tris(2-methyl-2-aminoethyl)amine.3 HCl, L-Ala(3)-TREN) is presented: crystals grown from ether diffusion into methanol are cubic, space group P2(1)3 with unit cell dimensions a = 11.4807(2) A, V = 1513.23(4) A(3), and Z = 4. Attachment of the triserine derived backbone tris(2-hydroxymethyl-2-aminoethyl)amine (L-Ser(3)-TREN) to three 3-hydroxy-1-methyl-2(1H)-pyridinonate (3,2-HOPO) moieties, followed by complexation with Gd(III) gives the complex Gd(L-Ser(3)-TREN-Me-3,2-HOPO)(H(2)O)(2), which is more water soluble than the parent Gd(TREN-Me-3,2-HOPO)(H(2)O)(2) and a promising candidate for magnetic resonance imaging (MRI) applications. Crystals of the chiral ferric complex Fe(L-Ser(3)-TREN-Me-3,2-HOPO) grown from ether/methanol are orthorhombic, space group P2(1)2(1)2(1), with unit cell dimensions a = 13.6290(2) A, b = 18.6117(3) A, c = 30.6789(3) A, V = 7782.0(2) A(3), and Z = 8. The solution conformation of the ferric complex has been investigated by circular dichroism spectroscopy. The coordination chemistry of this new ligand and its iron(III) and gadolinium(III) complexes has been studied by potentiometric and spectrophotometric methods. Compared to the protonation constants of previously studied polydentate 3,2-HOPO-4-carboxamide ligands, the sum of protonation constants (log beta(014)) of L-Ser(3)-TREN-Me-3,2-HOPO (24.78) is more acidic by 1.13 log units than the parent TREN-Me-3,2-HOPO. The formation constants for the iron(III) and gadolinium(III) complexes have been evaluated by spectrophotometric pH titration to be (log K) 26.3(1) and 17.2(2), respectively.     

Syntheses and relaxation properties of mixed gadolinium hydroxypyridinonate MRI contrast agents

The tripodal ligand tris[(3-hydroxy-1-methyl-2-oxo-1,2- didehydropyridine-4-carboxamido)ethyl]amine (TREN-Me-3,2-HOPO) forms a stable Gd3+ complex that is a promising candidate as a magnetic resonance imaging (MRI) contrast agent. However, its low water solubility prevents detailed magnetic characterization and practical applicability. Presented here are a series of novel mixed ligand systems that are based on the TREN-Me-3,2-HOPO platform. These new ligands possess two hydroxypyridinone (HOPO) chelators and one other chelator, the latter of which can be easily functionalized. The ligands described use salicylamide, 2-hydroxyisophthalamide, 2,3-dihydroxyterephthalamide, and bis(acetate) as the derivatizable chelators. The solution thermodynamics and relaxivity properties of these new systems are presented. Three of the four complexes (salicylamide-, 2-hydroxyisophthalamide-, and 2,3-dihydroxyterephthalamide-based ligands) possess sufficient thermodynamic stability for in vivo applications. The relaxivities of the three corresponding Gd3+ complexes range from 7.2 to 8.8 mM-1 s-1 at 20 MHz, 25 degrees C, and pH 8.5, significantly higher than the values for the clinically employed polyaminocarboxylate complexes (3.5-4.8 mM-1 s-1). The high relaxivities of these complexes are consistent with their faster rates of water exchange (< 100 ns), higher molecular weights (> 700), and greater numbers of inner-sphere coordinated water molecules (q = 2) relative to those of polyaminocarboxylate complexes. A mechanism for the rapid rates of water exchange is proposed involving a low energy barrier between the 8- and 9-coordinate geometries for lanthanide complexes of HOPO-based ligands. The pathway is supported by the crystal structure of La[TREN-Me-3,2-HOPO] (triclinic P1: Z = 4, a = 15.6963(2) A, b = 16.9978(1) A, c = 17.1578(2) A, alpha = 61.981(1) degrees, beta = 75.680(1) degrees, gamma = 71.600(1) degrees), which shows both 8- and 9-coordinate metal centers in the asymmetric unit, demonstrating that these structures are very close in energy. These properties make heteropodate Gd3+ complexes promising candidates for use in macromolecular contrast media, particularly at higher magnetic field strengths.     

Terephthalamide-containing analogues of TREN-Me-3,2-HOPO

A series of terephthalamide-containing analogues based on TREN-Me-3,2-HOPO have been prepared. These analogues contain one, two, or three bidentate 2,3-dihydroxyterephthalamide (TAM) units in place of the 3,2-hydroxypyridinone (HOPO) units on the parent hexadentate ligand. One representative ligand in the series, TRENHOPOTAM2, and its gallium complex have been structurally characterized by X-ray diffraction. TRENHOPOTAM2 crystallizes in the monoclinic space group P2(1)/c with cell parameters a = 16.0340(17) A, b = 17.0609(18) A, c = 16.0695(17) A, beta = 113.837(2) degrees, and Z = 4. Ga[TRENHOPOTAM2] also crystallizes in the monoclinic space group P2(1)/c, with cell parameters a = 16.3379(14) A, b = 15.2722(13) A, c = 19.4397(17) A, beta = 91.656(2) degrees, and Z = 4. The conformation of the TRENHOPOTAM2 ligand structure suggests that the ligand is predisposed for metal ion binding. The aqueous protonation and ferric ion coordination chemistry of all ligands in the series were examined using potentiometric and spectrophotometric methods, giving log formation constants of 34.6(2) (beta110) and 38.8(2) (beta111) for the ferric TRENHOPO2TAM complexes, 41.0(3) (beta110) and 45.4(3) (beta111) for the ferric TRENHOPOTAM2 complexes, and 45.2(2) (beta110) and 50.9(2) (beta111) for the ferric TRENTAM3 complexes. These thermodynamic data confirm that adding terephthalamide units to a hydroxypyridinone-containing ligand tends to increase the stability of the resulting iron complex. The ferric TRENTAM3 complex is one of the most stable iron complexes yet reported.     

Synthesis of Polysiloxane-Bound (Ether-phosphine)palladium Complexes. Stoichiometric and Catalytic Reactions in Interphases(1)

The reaction of two equiv of the monomeric ether-phosphine O,P ligand (MeO)(3)Si(CH(2))(3)(Ph)PCH(2)-Do [1a(T(0)()), 1b(T(0)())] {Do = CH(2)OCH(3) [1a(T(0)())], CHCH(2)CH(2)CH(2)O [1b(T(0)())]} with PdCl(2)(COD) yields the monomeric palladium(II) complexes Cl(2)Pd(P approximately O)(2) [2a(T(0)())(2)(), 2b(T(0)())(2)()]. The compounds 2a(T(0)())(2)() and 2b(T(0)())(2)() are sol-gel processed with variable amounts (y) of Si(OEt)(4) (Q(0)()) to give the polysiloxane-bound complexes 2a(T(n)())(2)()(Q(k)())(y)(), 2b(T(n)())(2)()(Q(k)())(y)() (Table 1) {P approximately O = eta(1)-P-coordinated ether-phosphine ligand; for T(n)() and Q(k)(), y = number of condensed T type (three oxygen neighbors), Q type (four oxygen neighbors) silicon atoms; n and k = number of Si-O-Si bonds; n = 0-3; k = 0-4; 2a(T(n)())(2)()(Q(k)())(y)(), 2b(T(n)())(2)()(Q(k)())(y)() = {[M]-SiO(n)()(/2)(OX)(3)(-)(n)()}(2)[SiO(k)()(/2)(OX)(4)(-)(k)()](y)(), [M] = (Cl(2)Pd)(1/2)(Ph)P(CH(2)Do)(CH(2))(3)-, X = H, Me, Et}. The complexes 2b(T(n)())(2)()(Q(k)())(y)() (y = 4, 12, 36) show high activity and selectivity in the hydrogenation of 1-hexyne and tolan. The dicationic complexes [Pd(P&arcraise;O)(2)][SbF(6)](2) [3a(T(0)())(2)(), 3b(T(0)())(2)()] are formed by reacting Cl(2)Pd(P approximately O)(2) with 2 equiv of a silver salt {P&arcraise;O = eta(2)-O&arcraise;P-coordinated ether-phosphine ligand; 3a(T(0)())(2)(), 3b(T(0)())(2)() = [M]-SiOMe(3); [M] = {[Pd(2+)](1/)(2)P(Ph)(CH(2)CH(2)OCH(3))(CH(2))(3)-}{SbF(6)} (a), {[Pd(2+)](1/)(2)P(Ph)(CH(2)CHCH(2)CH(2)CH(2)O)(CH(2))(3)-}{SbF(6)} (b)}. Their polysiloxane-bound congeners 3a(T(n)())(2)(), 3b(T(n)())(2)() {[M]-SiO(n)()(/2)(OX)(3)(-)(n)} are obtained if a volatile, reversible bound ligand like acetonitrile is employed during the sol-gel process. The bis(chelate)palladium(II) complexes 3a(T(n)())(2)(), 3b(T(n)())(2)() are catalytic active in the solvent-free CO-ethene copolymerization, producing polyketones with chain lengths comparable to those obtained with chelating diphosphine ligands. The polysiloxane-bound palladium(0) complexes 5a(T(n)())(2)()(Q(k)())(4)(), 5b(T(n)())(2)()(Q(k)())(4)() {[M]-SiO(n)()(/)(2)(OX)(3)(-)(n)}(2)[SiO(k)()(/2)(OX)(4)(-)(k)](4), [M] = [(dba)Pd](1/)(2)P(Ph)(CH(2)Do)(CH(2))(3)-} undergo an oxidative addition reaction with iodobenzene in an interphase with formation of the complexes PhPd(I)(P approximately O)(2).4SiO(2) [6a(T(n)())(2)()(Q(k)())(4)(), 6b(T(n)())(2)()(Q(k)())(4)()] {[M]-SiO(n)()(/)(2)(OX)(3)(-)(n)](2)[SiO(k)()(/2)(OX)(4)(-)(k)](4), [M] = [PhPd(I)](1/2)P(Ph)(CH(2)Do)(CH(2))(3)-}, which insert carbon monoxide into the palladium-aryl bond even in the solid state.     

Toward optimized high-relaxivity MRI agents: thermodynamic selectivity of hydroxypyridonate/catecholate ligands

The thermodynamic selectivity for Gd(3+) relative to Ca(2+), Zn(2+), and Fe(3+) of two ligands of potential interest as magnetic resonance imaging (MRI) contrast agents has been determined by NMR spectroscopy and potentiometric and spectrophotometric titration. The two hexadentate ligands TREN-6-Me-3,2-HOPO (H(3)L2) and TREN-bisHOPO-TAM-EA (H(4)L3) incorporate 2,3-dihydroxypyridonate and 2,3-dihydroxyterephthalamide moieties. They were chosen to span a range of basicity while maintaining a structural motif similar to that of the parent ligand, TREN-1-Me-3,2-HOPO (H(3)L1), in order to investigate the effect of the ligand basicity on its selectivity. The 1:1 stability constants (beta(110)) at 25 degrees C and 0.1 M KCl are as follows. L2: Gd(3+), 20.3; Ca(2+), 7.4; Zn(2+), 11.9; Fe(3+), 27.9. L3: Gd(3+), 24.3; Ca(2+), 5.2; Zn(2+), 14.6; Fe(3+), 35.1. At physiological pH, the selectivity of the ligand for Gd(3+) over Ca(2+) increases with the basicity of the ligand and decreases for Gd(3+) over Fe(3+). These trends are consistent with the relative acidities of the various metal ions;- more basic ligands favor harder metals with a higher charge-to-radius ratio. The stabilities of the Zn(2+) complexes do not correlate with basicity and are thought to be more influenced by geometric factors. The selectivities of these ligands are superior to those of the octadentate poly(aminocarboxylate) ligands that are currently used as MRI contrast agents in diagnostic medicine.     

The influence of linker geometry in bis(3-hydroxy-N-methyl-pyridin-2-one) ligands on solution phase uranyl affinity

Seven water-soluble, tetradentate bis(3-hydroxy-N-methyl-pyridin-2-one) (bis-Me-3,2-HOPO) ligands were synthesized that vary only in linker geometry and rigidity. Solution-phase thermodynamic measurements were conducted between pH 1.6 and pH 9.0 to determine the effects of these variations on proton and uranyl cation affinity. Proton affinity decreases by introduction of the solubilizing triethylene glycol group as compared to unsubstituted reference ligands. Uranyl affinity was found to follow no discernable trends with incremental geometric modification. The butyl-linked 4 li-Me-3,2-HOPO ligand exhibited the highest uranyl affinity, consistent with prior in vivo decorporation results. Of the rigidly-linked ligands, the o-phenylene linker imparted the best uranyl affinity to the bis-Me-3,2-HOPO ligand platform.     

Poly(2-thienyl)borates: An Investigation into the Coordination of Thiophene and Its Derivatives

The synthesis, characterization, and reactivity of a new sulfur-rich tridentate ligand, tetrakis(2-thienyl)borate (1(-)()), are reported along with a molecular orbital analysis of its coordination to a metal center. Unlike the analogous tetrakis((methylthio)methyl)borate (2(-)()), 1(-)() does not coordinate Mo(CO)(3) when reacted with (C(7)H(8))Mo(CO)(3). The sulfur atoms in both ligands are oriented to coordinate the metal in a pyramidal eta(1) sulfur-bound mode. Approximate molecular orbital calculations are used to compare the metal-ligand interactions in these related species, and the results indicate that the magnitude and polarizability of the electronic charge density of the lone pairs on the sulfur atoms dictate the coordination strength of the ligands. Simple Mulliken atomic charges and orbital occupation numbers are used to determine the extent of charge delocalization. While the conjugation of the sulfur lone pair electrons with adjacent pi bonds in the ligands decreases the corresponding Lewis basicity, the contribution from the aromaticity in the thienyl groups is negligible. During the course of these studies, the structure of K[1] was determined by X-ray diffraction. K[1]: monoclinic space group C2/c, with a = 16.00(2) ?, b = 7.680(7) ?, c = 16.22(2) ?, beta = 118.520(7) degrees, V = 1750(3) ?(3), Z = 4, R(F) = 0.0494, and R(w)(F(2)()) = 0.122. The crystal lattice contains one-dimensional chains of 1(-)() bridged by K ions.     

Solid state and solution studies of lanthanide(III) complexes of cyclohexanetriols, models of the coordination sites found in sugars

This report covers studies in trivalent lanthanide complexation by two simple cyclohexanetriols that are models of the two coordination sites found in sugars and derivatives. Several complexes of trivalent lanthanide ions with cis,cis-1,3,5-trihydroxycyclohexane (L(1)()) and cis,cis-1,2,3-trihydroxycyclohexane (L(2)()) have been characterized in the solid state, and some of them have been studied in organic solutions. With L(1)(), Ln(L)(2) complexes are obtained when crystallization is performed from acetonitrile solutions whatever the nature of the salt (nitrate or triflate) [Ln(L(1)())(2)(NO(3))(2)](NO(3)) (Ln = Pr, Nd); [Ln(L(1)())(2)(NO(3))H(2)O](NO(3))(2) (Ln = Eu, Ho, Yb); [Ln(L(1)())(2)(OTf)(2)(H(2)O)](OTf) (Ln = Nd, Eu). Lanthanum nitrate itself gives a mixed complex [La(L(1)())(2)(NO(3))(2)][LaL(1)()(NO(3))(4)] from acetonitrile solution while [La(L(1)())(2)(NO(3))(2)](NO(3)) is obtained using dimethoxyethane as reaction solvent and crystallization medium. With L(2)(), Ln(L)(2) complexes have also been crystallized from methanol solution [Ln(L(2)())(2)(NO(3))(2)]NO(3), (Ln = Pr, Nd, Eu). Single-crystal X-ray diffraction analyses are reported for these complexes. Complex formation in solution has been studied for several triflate salts (La, Pr, Nd, Eu, and Yb) with L(1 )()and L(2)(), respectively in acetonitrile and in methanol. In contrast to the solid state, both structures Ln(L) and Ln(L)(2) equilibrate in solution, as was demonstrated by low-temperature (1)H NMR and electrospray ionization mass spectrometry experiments. Competing experiments in complexing abilities of L(1)() and L(2)() with trivalent lanthanide cations have shown that only L(2)() exhibits a small selectivity (Nd > Pr > Yb > La > Eu) in methanol.     

The effect of ligand scaffold size on the stability of tripodal hydroxypyridonate gadolinium complexes

The variation of the size of the capping scaffold which connects the hydroxypyridonate (HOPO) binding units in a series of tripodal chelators for gadolinium (Gd) complexes has been investigated. A new analogue of TREN-1-Me-3,2-HOPO (1) (TREN = tri(ethylamine)amine) was synthesized: TREN-Gly-1-Me-3,2-HOPO (2) features a glycine spacer between the TREN cap and HOPO binding unit. TRPN-1-Me-3,2-HOPO (3) has a propylene-bridged cap, as compared to the ethylene bridges within the TREN cap of the parent complex. Thermodynamic equilibrium constants for the acid-base properties of 2 and the Gd(3+) complexation strength of 2 and 3 were measured and are compared with that of the parent ligand. The most basic ligand is 2 while 3 is the most acidic. Both 2 and 3 form Gd(3+) complexes of similar stability (pGd = 16.7 and 15.6, respectively) and are less stable than the parent complex Gd-1 (pGd = 19.2). Two of the three complexes are more stable than the bis(methylamide)diethylenetriamine pentaacetate complex Gd(DTPA-BMA) (pGd = 15.7) while the other is of comparable stability. Enlargement of the ligand scaffold decreases the stability of the Gd(3+) complexes and indicates that the TREN scaffold is superior to the TRPN and TREN-Gly scaffolds. The proton relaxivity of Gd-2 is 6.6 mM(-)(1) s(-)(1) (20 MHz, 25 degrees C, pH 7.3), somewhat lower than the parent Gd-1 but higher than that of the MRI contrast agents in clinical practice. The pH-independent relaxivity of Gd-2 is uncharacteristic of this family of complexes and is discussed.     

Examples of reductive azo cleavage and oxidative azo bond formation on Re2(CO)10 template: isolation and characterization of Re(III) complexes of new azo-aromatic ligands

A new example of simultaneous reductive azo bond cleavage and oxidative azo bond formation in an azo-aromatic ligand is introduced. The chemical transformation is achieved by the reaction of Re(2)(CO)(10) with the ligand 2-[(2-N-Arylamino)phenylazo]pyridine (HL(1)). A new and unexpected mononuclear rhenium complex [Re(L(1))(L(3))] (1) was isolated from the above reaction. The new azo-aromatic ligand, H(2)L(3) (H(2)L(3) = 2, 2'-dianilinoazobenzene) is formed in situ from HL(1). A similar reaction of Re(2)(CO)(10) and a closely related azo-ligand, 2,4-ditert-butyl-6-(pyridin-2-ylazo)-phenol (HL(2)), resulted in a seven coordinated compound [Re(L(2)){(L(4))(?-)}(2)] (2; HL(4) = 2-amino-4,6-ditert-butyl-phenol) via reductive cleavage of the azo bond. The complexes have been characterized by using a host of physical methods: X-ray crystallography, nuclear magnetic resonance (NMR), cyclic voltammetry, ultraviolet-visible (UV-vis), electron paramagnetic resonance (EPR) spectroscopy, and density functional theory (DFT). The experimental structures are well reproduced by density functional theory calculations and support the overall electronic structures of the above compounds. Complex 1 is a closed shell singlet, while complex 2 exemplifies a singlet diradical complex where the two partially oxidized aminophenoleto ligands are coupled to each other, yielding the observed diamagnetic ground state. Complexes 1 and 2 showed two successive one-electron redox responses. EPR spectral studies in corroboration with DFT results indicated that all of the redox processes occur at the ligand center without affecting the trivalent state of the metal ion.     

1,2-hydroxypyridonates as contrast agents for magnetic resonance imaging: TREN-1,2-HOPO

1,2-Hydroxypyridinones (1,2-HOPO) form very stable lanthanide complexes that may be useful as contrast agents for magnetic resonance imaging (MRI). X-ray diffraction of single crystals established that the solid-state structures of the Eu(III) and the previously reported [Inorg. Chem. 2004, 43, 5452] Gd(III) complex are identical. The recently discovered sensitizing properties of 1,2-HOPO chelates for Eu(III) luminescence [J. Am. Chem. Soc. 2006, 128, 10 067] allow for direct measurement of the number of water molecules coordinated to the metal center. Fluorescence measurements of the Eu(III) complex corroborate that, in solution, two water molecules coordinate the lanthanide (q = 2) as proposed from the analysis of NMRD profiles. In addition, fluorescence measurements have verified the anion binding interactions of lanthanide TREN-1,2-HOPO complexes in solution, studied by relaxivity, revealing only very weak oxalate binding (KA = 82.7 +/- 6.5 M-1). Solution thermodynamic studies of the metal complex and free ligand have been carried out using potentiometry, spectrophotometry, and fluorescence spectroscopy. The metal ion selectivity of TREN-1,2-HOPO supports the feasibility of using 1,2-HOPO ligands for selective lanthanide binding [pGd = 19.3 (2), pZn = 15.2 (2), pCa = 8.8 (3)].     

Faster oxygen atom transfer catalysis with a tungsten dioxo complex than with its molybdenum analog

The synthesis and characterization of a series of molybdenum ([MoO(2)Cl(L(n))]; L(1) (1), L(2) (3)) and tungsten ([WO(2)Cl(L(n))]; L(1) (2), L(2) (4)) dioxo complexes (L(1) = 1-methyl-4-(2-hydroxybenzyl)-1,4-diazepane and L(2) = 1-methyl-4-(2-hydroxy-3,5-di-tert-butylbenzyl)-1,4-diazepane) of tridentate aminomonophenolate ligands HL(1) and HL(2) are reported. The ligands were obtained by reductive amination of 1-methyl-1,4-diazepane with the corresponding aldehyde. Complexes 3 and 4 were obtained by the reaction of [MO(2)Cl(2)(dme)(n)] (M = Mo, n = 0; W, n = 1) with the corresponding ligand in presence of a base, whereas for the preparation of 1 and 2 the ligands were deprotonated by KH prior to the addition to the metal. They were characterized by NMR and IR spectroscopy, by cyclic voltammetry, mass spectrometry, elemental analysis and by single-crystal X-ray diffraction analysis. Solid-state structures of the molybdenum and tungsten cis-dioxo complexes reveal hexa-coordinate metal centers surrounded by two oxo groups, a chloride ligand and by the tridentate monophenolate ligand which coordinates meridionally through its [ONN] donor set. In the series of compounds 1-4, complexes 3 and 4 have been used as catalysts for the oxygen atom transfer reaction between dimethyl sulfoxide (DMSO) and trimethyl phosphine (PMe(3)). Surprisingly, faster oxygen atom transfer (OAT) reactivity has been observed for the tungsten complex [WO(2)Cl(L(2))] (4) in comparison to its molybdenum analog [MoO(2)Cl(L(2))] (3) at room temperature. The kinetic results are discussed and compared in terms of their reactivity.     

Adjusting the frameworks of silver(I) complexes with new pyridyl thioethers by varying the chain lengths of ligand spacers, solvents, and counteranions

In our efforts to systematically investigate the effects of the linker units of flexible ligands and other factors on the structures of Ag(I) complexes with thioethers, five new flexible pyridyl thioether ligands, bis(2-pyridylthio)methane (L(1)()), 1,3-bis(2-pyridylthio)propane (L(3)()), 1,4-bis(2-pyridylthio)butane (L(4)), 1,5-bis(2-pyridylthio)pentane (L(5)), and 1,6-bis(2-pyridylthio)hexane (L(6)), have been designed and synthesized, and the reactions of these ligands with Ag(I) salts under varied conditions (varying the solvents and counteranions) lead to the formation of eight novel metal-organic coordination architectures from di- and trinuclear species to two-dimensional networks: [Ag(3)(L(1)())(2)(ClO(4))(2)](ClO(4)) (1), [[AgL(3)](ClO(4))]( infinity ) (2), [[Ag(2)(L(4))(2)](ClO(4))(2)(CHCl(3))]( infinity ) (3), [[AgL(4)](ClO(4))(C(3)H(6)O)]( infinity ) (4), [[Ag(2)L(4)](NO(3))(2)]( infinity ) (5), [Ag(2)L(4)()(CF(3)SO(3))(2)]( infinity ) (6), [[AgL(5)](ClO(4))(CHCl(3))](2) (7), and [[AgL(6)()](ClO(4))]( infinity ) (8). All the structures were established by single-crystal X-ray diffraction analysis. The coordination modes of these ligands were found to vary from N,N-bidentate to N,N,S-tridentate to N,N,S,S-tetradentate modes, while the Ag(I) centers adopt two-, three-, or four-coordination geometries with different coordination environments. The structural differences of 1, 2, 3, 7, and 8 indicate that the subtle variations on the spacer units can greatly affect the coordination modes of the terminal pyridylsulfanyl groups and the coordination geometries of Ag(I) ions. The structural differences of 3 and 4 indicate that solvents also have great influence on the structures of Ag(I) complexes, and the differences between 3, 5, and 6 show counteranion effects in polymerization of Ag(I) complexes. The influences of counterions and solvents on the frameworks of these complexes are probably based upon the flexibility of ligands and the wide coordination geometries of Ag(I) ions. The results of this study indicate that the frameworks of the Ag(I) complexes with pyridyl dithioethers could be adjusted by ligand modifications and variations of the complex formation conditions.     

Chromium complexes of an isomeric N-donor ligand, 2-[(N-arylamino)phenylazo]pyridine: amination reactions,X-ray structure,and redox properties

The chromium chemistry of two positional isomers of the ligand 2-[(N-arylamino)phenylazo]pyridine (HL(1)and HL(2)) are described. While the ligand HL(1) coordinates as a bischelating tridentate N,N,N-donor, [L(1)](-), with deprotonation of the amine nitrogen, its isomer HL(2) coordinates as a neutral bidentate N,N-donor. The amine nitrogen in this case remains protonated. Thus the reaction of CrCl(3).nH(2)O with HL(1) produced the brown cationic complex, [Cr(L(1))(2)](+), [1](+). The representative X-ray structure of [1a](ClO(4)) is reported. The two azo nitrogens of the anioinc tridentate ligand approach the metal center closest with Cr(1)-N(azo) av 1.862(6) A. There is a significant degree of ligand backbone conjugation in the coordinated ligands, which resulted in shortening of the C-N distances and also in lengthening of the diazo (N=N) distances. Two synthetic approaches for the synthesis of chromium complexes of HL(2) are investigated. The first approach is based on the substitution reaction, wherein all the coordinated CO ligands of Cr(CO)(6) were completely substituted by the three bidentate HL(2) ligands to produce a violet complex [Cr(HL(2))(3)]. The second approach is based on para-amination reaction of coordinated 2-(phenylazo)pyridine (pap). Thus the reaction of an inert complex, [CrCl(2)(pap)(2)], with ArNH(2) yields a mixed ligand complex, [CrCl(2)(pap)(HL(2))], 3. In this reaction one of the two coordinated pap ligands in [CrCl(2)(pap)(2)] undergoes amination at the para carbon (with respect to the diazo function) to yield HL(2) in situ. This metal-promoted transformation is authenticated by the X-ray structure determination of a representative complex, [CrCl(2)(pap)(HL(2a))], 3a. Notable differences in bond distances along the ligand backbones of the two coordinated ligands in 3a indicate different levels of metal-ligand overlap in this complex. All the chromium complexes of HL(2) are characterized by their intense blue-violet color. The frequencies of the visible range transitions in these complexes linearly correlate with the Hammett's substitution constant. Intraligand charge-transfer transitions in the visible region are believed to be responsible for the intense color. Redox properties of all these complexes are reported.     

Use of YbIII-centered near-infrared (NIR) luminescence to determine the hydration state of a 3,2-HOPO-based MRI contrast agent

The synthesis, structure, and characterization of a [Yb(Tren-Me-3,2-HOPO)(H 2O) 2] complex are reported. As a result of its Yb (III) emission in the near-infrared region, sensitized by the Me-3,2-HOPO chromophore, this complex can be utilized for the first time to determine the hydration state, q, via the luminescence lifetimes and hence the solution structure of these Me-3,2-HOPO-type ligands, which have attracted significant interest in complex with Gd (III) as possible next-generation magnetic resonance imaging contrast agents.