No abstract

A series of dinuclear M(III) (M = Fe or Ga) catecholate complexes has been prepared using bisbidentate catecholate ligands (L). The products contain discrete, dinuclear M2(L)3 6- anions featuring pseudo-octahedral coordination centers. The helical nature of the dinuclear complexes has been established by CD spectroscopy and X-ray crystallography. The salt (N(CH3)4)6Ga2(L 3 )3 (L 3 = N,N‘-bis(2,3-dihydroxy-4-carbamoylbenzoyl)-1,4-phenylenediamine) has been characterized by X-ray diffraction; crystals are hexagonal, space group P3̄1c with unit cell dimensions a = 14.283(2) Å, c = 42.966(2) Å, V = 7591 Å3, and Z = 2. Variable-temperature 1H NMR experiments demonstrate that the configuration inversion of the enantiomers of K6Ga2(L 4 )3 (L 4 = N,N‘-bis(2,3-dihydroxy-4-(isopropylcarbamoyl)benzoyl)-1,4-phenylenediamine) and K6Ga2(L 5 )3 (L 5 = N-(2,3-dihydroxy-4-(isopropylcarbamoyl)benzoyl)-N‘-(2,3-dimethoxy-4-(methylcarbamoyl)benzoyl)-1,4-phenylenediamine) is facile in D2O or DMSO-d 6. The mechanism of inversion has been probed by dynamic NMR spectroscopy, using the complex K6Ga2(L 5 )3 which exists in two isomeric forms in solution, cis- and trans. The intramolecular inversion of the dinuclear helicates occurs without cis−trans isomerization and proceeds by independent trigonal twisting of each metal center, affording the heterochiral meso complex as an intermediate. The free energy of activation for the inversion of K6Ga2(L 4 )3 in D2O at p[D] = 12.1 is ΔG ⧧ 298 = 79(2) kJ mol-1, with ΔH = 75(2) kJ mol-1 and ΔS ⧧ = −12(6) J mol-1 K-1. Under slightly acidic conditions a proton-assisted pathway becomes dominant and the rate of inversion shows a second-order dependence in [D+]. The heterochiral meso complex of Ga2(L 4 )6 3- is shown to be a transient kinetic intermediate in the (Λ,Λ) ↔ (Δ,Δ) inversion process of the helicate complex.

The catechol siderophore analog K3[Ga(3)3], 4 (H2 3 = 2,3-dihydroxy-N,N‘-diisopropylterephthalamide), is D 3-symmetric in aqueous solution, and exists in two enantiomeric forms, Δ-4 and Λ-4. Variable temperature NMR experiments demonstrate that the inversion of the enantiomers of 4 in D2O is facile. The rate of inversion is independent of pH above pH 8. The mechanism is intramolecular. From line-shape analysis the free energy of activation ΔG ⧧ 298 = 67.4(9) kJ mol-1 in D2O at pD 12.1, with ΔH ⧧ = 58.5(6) kJ mol-1 and ΔS ⧧ = −0.030(9) kJ mol-1 K-1. Below pD 8 the rate of inversion for 4 is pD dependent and initially first order in [D+]. Potentiometric titrations reveal that 4 protonates in two one-proton steps with log K HML 3 = 4.66(4) and log K H 2 ML 3 = 3.99(7). In DMSO-d 6, formation of a tight contact ion pair between K+ and [Ga(3)3]3- ions increases the free energy barrier to inversion by ∼7 kJ mol-1. The complex K3[Ga(9)3], 10 (H2 9 = 2,3-dihydroxy-N-tert-butyl-N‘-benzylterephthalamide), was prepared to elucidate the mechanism of inversion by dynamic NMR spectroscopy, using the fact that 10 exists in two isomeric forms, cis-10 and trans-10, which are of C 3 and C 1 symmetry in solution. The ratio cis-10:trans-10 is 0.78(3) at ambient temperature in D2O or DMSO-d 6. Two processes are distinguishable on the NMR time scale in D2O or DMSO-d 6, cis- 10−trans- 10 isomerization and the inversion of the enantiomers of trans-10. Both processes proceed intramolecularly with T c = 295(1) K for Λ-trans-10 to Δ-trans-10 inversion and T c = 335(1) K for cis-10 to trans-10 isomerization in D2O at pD 9.5. The discrete exchange pattern of the tert-butyl resonances during inversion of trans-10 confirms that the reaction proceeds by a trigonal twist mechanism via a trigonal prismatic transition state. The free energy barriers to inversion are ΔG ⧧ 295 = 60 kJ mol-1 in D2O (pD 9.8) and ΔG ⧧ 327 = 67 kJ mol-1 in DMSO-d 6.

Metallkomplexe mit hydrophoben Bindungstaschen erfahren wegen ihrer ungewöhnlichen chemischen Reaktivität gegenwärtig ein groûes Interesse. Die meisten dieser Verbindungen sind einkernige Spezies. Sie wurden zur Stabilisierung von reaktiven Intermediaten, [1] für selektive organische Transformationen [2] oder als Katalysatoren für Reaktionen, die vom Reaktionsmedium abhängen, verwendet. [3] Es wurden auch bereits einige Verbindungen entwickelt, um hydrophobe Umgebungen von Substratbindungsstellen in Metalloproteinen nachzubilden. [4] Diese Befunde veranlassten uns, Schema 1. Strukturen der Liganden und schematische Darstellung der Strukturen der entsprechenden Metallkomplexe vom Typ A oder B (X Substratbindungsstelle der Komplexe).Zweikernkomplexe von peralkylierten Amin-Thiophenol-Makrocyclen zu untersuchen, um damit einen hydrophoben Käfig um eine freie Koordinationsstelle erzeugen zu können. Wir beschreiben hier die Synthesen und Strukturen von zweikernigen Ni II -, Co II -und Zn II -Komplexen des permethylierten Makrocyclus (L Me ) 2À (siehe Schema 1) sowie deren bemerkenswerte Eigenschaft, Kohlendioxid zu fixieren und zu transformieren.Es wurde bereits gezeigt, dass sich die Brückenliganden in Zweikernkomplexen des Typs A viel leichter ersetzen lassen, wenn anstelle von (L H ) 2À der permethylierte Ligand (L Me ) 2À eingesetzt wird (Schema 1). [5] So kann der Hydroxo-verbrückte Komplex 3 (Tabelle 1), der den Ausgangspunkt dieser Arbeit darstellt, durch Reaktion der m-Cl-Spezies 2 mit Natriumhydroxid in Methanol in hohen Ausbeuten erhalten werden.

A series of dinickel(II) complexes with the 24-membered macrocyclic hexaazadithiophenol ligand H(2)L(Me) was prepared and examined. The doubly deprotonated form (L(Me))(2-) forms complexes of the type [(L(Me))Ni2II(mu-L')](n+) with a bioctahedral N(3)Ni(II)(mu-SR)(2)(mu-L')Ni(II)N(3) core and an overall calixarene-like structure. The bridging coordination site L' is accessible for a wide range of exogenous coligands. In this study L'=NO(3)(-), NO(2)(-), N(3)(-), N(2)H(4), pyrazolate (pz), pyridazine (pydz), phthalazine (phtz), and benzoate (OBz). Crystallographic studies reveal that each substrate binds in a distinct fashion to the [(L(Me))Ni(2)](2+) portion: NO(2)(-), N(2)H(4), pz, pydz, and phtz form mu(1,2)-bridges, whereas NO(3)(-), N(3)(-), and OBz(-) are mu(1,3)-bridging. These distinctive binding motifs and the fact that some of the coligands adopt unusual conformations is discussed in terms of complementary host-guest interactions and the size and form of the binding pocket of the [(L(Me))Ni(2)](2+) fragment. UV/Vis and electrochemical studies reveal that the solid-state structures are retained in the solution state. The relative stabilities of the complexes indicate that the [(L(Me))Ni(2)](2+) fragment binds anionic coligands preferentially over neutral ones and strong-field ligands over weak-field ligands. Secondary van der Waals interactions also contribute to the stability of the complexes. Intramolecular ferromagnetic exchange interactions are present in the nitrito-, pyridazine-, and the benzoato-bridged complexes where J=+6.7, +3.5, and +5.8 cm(-1) (H=-2 JS(1)S(2), S(1)=S(2)=1) as indicated by magnetic susceptibility data taken from 300 to 2 K. In contrast, the azido bridge in [(L(Me))Ni(2)(mu(1,3)-N(3))](+) results in an antiferromagnetic exchange interaction J=-46.7 cm(-1). An explanation for this difference is qualitatively discussed in terms of bonding differences.