Howard J. Clase

Synthesis and X-ray crystal structures of iron(II) and manganese(II) complexes of unsubstituted and benzyl substituted cross-bridged tetraazamacrocycles

Abstract The Mn2+ and Fe2+ complexes of the cross-bridged tetraazamacrocyclic ligands, 4,11-dibenzyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (1), 4,10-dibenzyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (2), 1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (3), and 1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (4) provide new compounds of these elements for fundamental studies and applications. These unsubstituted and benzyl substituted derivatives were prepared for comparison of their structures and properties with the known catalytically active dimethyl cross-bridged ligand complexes, which are especially notable for their exceptional kinetic stabilities and redox activity. The X-ray crystal structures of five complexes demonstrate that the ligands enforce a distorted octahedral geometry on the metals with two cis sites occupied by labile ligands. The Fe2+ complexes of the unsubstitued ligands form μ-oxo dimers upon exposure to air, which have also been structurally characterized. Cyclic voltammetry of the monomeric complexes shows reversible redox processes for the M3+/M2+ couples, which are sensitive to solvent, ring size, and ring substitution.

Synthesis and application to asymmetric allylic amination of substituted monodonor diazaphospholidine ligands

The synthesis of a series of substituted monodonor diazaphospholidine ligands is described. A regioselective lithiation process is a key step in one of these syntheses. The compounds are designed to be incorporated into soluble polymer and other solid phase supports. Enantiomeric excesses of up to 88% were observed when these compounds were employed in palladium-catalysed asymmetric amination reactions.

Synthesis, characterization, and X-ray crystal structures of cobalt(II) and cobalt(III) complexes of four topologically constrained tetraazamacrocycles

The high spin Co2+ complexes of 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (1), 4,10-dimethyl-1,4,7,10-tetraazabicyclo[6.5.2]pentadecane (2), 4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (3), and racemic-4,5,7,7,11,12,14,14-octamethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (4) have been synthesized and characterized by X-ray crystallography. The Co(III) complexes of 1-3 were also prepared by the chemical oxidation of the Co(II) complexes. The X-ray crystal structures of Co(1)Cl-2, Co(3)Cl-2, and [Co(3)Cl-2]PF6 demonstrate that the ligands enforce a distorted octahedral geometry on Co(II) and Co(III) with two cis sites occupied by chloro ligands. In contrast, the Co(II) complex of 4 is five-coordinate with trigonal bipyramidal geometry. The methyl groups substituted on the carbon atoms of ligand 4 define a shallow cavity, allowing only one chloride ligand to bind to the chelated metal ion. This difference in coordination geometry causes Co(4)Cl+ to be much more difficult to oxidize (E-1/2 = 1.176 V vs. SHE) than the octahedral Co(II) complexes of 1-3 (E-1/2 from -0.157 to 0.173 V vs. SHE). The Co(III) complexes of 1-3 have three absorbances in their electronic spectra, while typical cis-Co(III)N4X2 complexes have only two. This additional splitting of energy states is attributed to the increased distortion from octahedral resulting from the short ethylene cross-bridge on the macrobicyclic ligands

An octameric titanium oxo metallacycle with host–guest interactions

Treatment of TiCl4 with carboxylic acids RCO2H (R = C6F5, CH2C6F5, CH2OC6F5) yields a series of titanium oxo compounds; the X-ray molecular structure of [TiO(O2CC6F5)2] reveals an octameric unit and a unique 16-membered Ti8O8 ring, with two toluene molecules encapsulated above and below the ring in the molecular cavity.

Potentiometric Titrations and Nickel(II) Complexes of Four Topologically Constrained Tetraazamacrocycles

Abstract The novel high spin Ni2+ complexes of the topologically constrained tetraazamacrocycles (1–4) [4,11-dimethyl-1,4,8,11 - tetraazabicyclo[6.6.2]hexadecane (1); 4,10-dimethyl-1,4,7,10-tetraazabicyclo[6.5.2]pentadecane (2); 4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (3); racemic-4,5,7,7,11,12,14,14-octamethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (4)] show striking properties. Potentiometric titrations of the ligands 2 and 4 revealed them to be proton sponges, as reported earlier for 1 [1]. Ligand 3 is less basic, losing its last proton with a pK = 11.3(2). Despite high proton affinities, complexation reactions in the absence of protons successfully yielded Ni2+ complexes in all cases. The X-ray crystal structures of Ni(1)(acac)+, Ni(3)(acac)+ and Ni(1)(OH2)2 2+ demonstrate that the ligands enforce a distorted octahedral geometry on Ni2+ with two cis sites occupied by other ligands. Magnetic measurements and electronic spectroscopy on the corresponding Ni(L)Cl2 (L = 1–3) complexes reveal that all are high spin and six-coordinate with typical magnetic moments. In contrast, [Ni(4)Cl+] is five-coordinate with a slightly higher magnetic moment and its own characteristic electronic spectrum. The extra methyl groups on ligand 4 define a shallow cavity, sterically allowing only one chloride ligand to bind to the nickel(II) ion.

Synthesis and co-ordination chemistry of the pyridyl pendant-arm azamacrocycles 1-(2-pyridylmethyl)-1,5,9-triazacyclododecane L1 and 1-(2-pyridyl-2′-ethyl)-1,5,9-triazacyclododecane L2, with nickel (II), copper(II) and zinc(II). Crystal structures of [Ni(L1)(O2NO)]NO3 and [ZnL2][Zn(NO3)3.67Cl0.33]

The azamacrocyclic ligands 1-(2-pyridylmethyl)-1,5,9-triazacyclododecane L1 and 1-(2-pyridyl-2′-ethyl)-1,5,9-triazacyclododecane L2 have been prepared, and their complexes with hydrated nickel(II), copper(II) and zinc(II) nitrates have been isolated. The nickel(II) complexes are high spin and six-co-ordinate, whilst 13C NMR spectroscopy shows that [ZnL1(OH2)]2+ exists as a symmetric trigonal-bipyramidal isomer in solution, and [ZnL2]2+ exists as a 2:1 mixture of tetrahedral and asymmetric trigonal-bipyramidal isomers. X-Ray crystallography has been used to determine the solid-state structures of the octahedral [NiL1(O2NO)]+, and the tetrahedral isomer of [ZnL2]2+.

Synthesis crystal structure of [GdCl2dibenzo-18-crown-6-MeCN][SbCl6]·2MeCN dibenzo-18-crown-6-2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxacyclooctadeca-2,11-diene

Treatment of GdCl3(THF)2 with an acetonitrile solution of antimony(V) chloride (as halide abstractor) in the presence of dibenzo-18-crown-6 provides the yellow crystalline salt [GdCl2(dibenzo-18-crown-6)(MeCN)][SbCl6]·2MeCN whose structure has been determined by single-crystal X-ray diffraction. The gadolinium atom is situated within the ring cavity bonded to all six oxygen atoms of the oxacrown Gd-O 2.545(4)–2.683(4) A. Further bonding to two chlorine atoms, located on opposite sides of the ring plane, GdCl 2.632(2) 2.679(2) A one acetonitrile molecule GdN 2.579(5) A, gives a nine-coordinate cation geometry approximating a planar hexagon with two capping atoms above one below the plane.

Synthesis of racemic chiral-at-metal complexes of the Group 4 metals

Abstract A protocol for asymmetric synthesis of chiral-at-metal complexes is described, but enantiomerically-enriched products can not be isolated due to formation of complexes between the Group 4 metallocene products and the borane by-products. An efficient method for synthesis of racemic chiral-at-metal metallocenes, through lithium chloride catalysed ligand redistribution reactions, is described. Sterically-hindered racemic chiral-at-metal complexes are prepared by nucleophilic substitution of prochiral dichlorides.

Reactions of titanium tetrachloride with carboxylic acids. Crystal and molecular structure of the dinuclear titanium oxo compound [{TiCl2(O2CBut)(ButCO2H)}2O]

Reaction of TiCl4(1 mol) with 2,2-dimethylpropanoic acid (2.5 mol) at room temperature yields the dinuclear oxo-bridged species [{TiCl2(O2CBut)(ButCO2H)}2O]1 which has been fully characterised by an X-ray crystal structure analysis. The two metal atoms are bridged by the oxo and two carboxylate groups, which with a co-ordinated acid molecule and two chlorine atoms on each titanium gives a slightly distorted octahedral environment around each metal centre. At 40 °C compound 1 decomposes to the trinuclear species [Ti3Cl3(O2CBut)5O2], and at higher temperatures (100–120 °C) to another dinuclear oxo derivative [{TiCl(O2CBut)2}2O] which can also be obtained from the action of 2,2-dimethylpropanoic anhydride on TiCl4 in refluxing toluene. Acetic acid produces a similar, but less soluble, species to 1. Aromatic acids react differently; para-substituted aryl acids generally yield trinuclear compounds [Ti3Cl3(O2CC6H4X-p)5O2](X = Cl or Br) although the p-But acid forms [{TiCl(O2CC6H4But-p)2}2O]. In contrast the ortho-and meta-substituted acids react to give either [Ti3Cl4(O2CR)4O2](R = C6H4Cl-o or -m) or [Ti4Cl5(O2CR)7O2](R = C6H4Me-o) derivatives.

Proton control of the conformation of a macrobicyclic ligand

Crystal structure determinations for two salts show that the structures of metal-free cyclidene ligand salts are critically dependent on the degree of protonation, showing either a structure close to those found for corresponding metal complexes, or an entirely different conformation stabilised by N–H ⋯ Cl hydrogen bonds in an ‘ion quartet’, a cluster formed by the [LH5]5+ ion interacting with 3 Cl– ions.