John P. Maher

Metal–metal interactions across symmetrical bipyridyl bridging ligands in binuclear seventeen-electron molybdenum complexes

A series of 17-electron mononuclear complexes [Mo(NO)L(Cl)X] and their binuclear counterparts [{Mo-(NO)LCl}2(µ-X)][L = tris(3,5-dimethylpyrazolyl) hydroborate; X = 3,3′-dimethyl-4,4′-bipyridine (3,3′-dmbipy), 1,2-bis(4-pyridyl)acetylene (bpac), 4,4′-azopyridine (azpy), 1,4-bis[2-(4-pyridyl)ethenyl]-benzene (bpeb) or 1,4-bis(4-pyridyl) benzene (bpb)] have been prepared. Electrochemical studies show that the reduction potentials of the mononuclear complexes are sensitive to the degree of unsaturation in the monodentate ligand X, whereas the oxidation potentials are virtually constant. This suggests that the redox orbital involved in the reductions have considerable ligand-based character whereas the oxidations are more strongly metal-centred. This is supported by the electrochemical properties of the binuclear complexes, where the oxidation potentials are in every case coincident but the splitting between the reduction potentials of the equivalent molybdenum centres varies from 0.16 V (X =bpeb) to 0.56 V (X =bpac). By contrast the splitting of the redox potentials of pentaammineruthenium(II) fragments at either end of ‘extended’ 4,4′-bipyridine analogues of this type is an order of magnitude smaller. This strong interaction between [Mo(NO)LCl] moieties is in part due to a planar conformation of the bridging ligands, even when they are in principle capable of free rotation, since changing the bridging ligand from 4,4′-bipyridine to 3,3′-dmbipy (which cannot be planar due to the steric effects of the methyl groups) results in a decrease in the splitting of the reduction potentials from 0.77 to 0.38 V. The EPR spectra of the binuclear complexes all show that the two unpaired electrons (one at each 17-electron molybdenum centre) are in fast exchange across the bridging ligand at room temperature.

Copper(II) complexes of new potentially hexadentate N3S3- or N6-donor podand ligands based on the tris(pyrazolyl)borate or tris(pyrazolyl)methane core

The mononuclear copper(II) complexes of the following potentially hexadentate podand ligands have been prepared and crystallographically characterised: tris[3-{2-(methylsulfanyl)phenyl}pyrazol-1-yl]hydroborate [L1]-, phenyltris[3-(2-pyridyl)pyrazol-1-yl]methane (L2), and tris[3-{(6-methyl)-pyrid-2-yl}pyrazol-1-yl]hydroborate [L3]-. Of these, [L1]- [a potentially N3S3 donor with three pyrazolyl and three thioether groups, based on a tris(pyrazolyl)borate core] and L2 [a potentially N6 donor with three pyrazolyl and three pyridyl groups, based on a tris(pyrazolyl)methane core] have been prepared for the first time. In [Cu(L1)][PF6] the Cu(II) centre is in a five-coordinate N3S2 coordination environment which is approximately square pyramidal; the pendant thioether group has a weak, long-range interaction with the sixth coordination site of Cu(II). [Cu(L1)][PF6] undergoes a reversible Cu(I)–Cu(II) redox conversion. In [Cu(L2)(MeOH)][PF6]2 the ligand is four-coordinate via two bidentate pyridyl/pyrazolyl arms, with the third arm pendant; an axial methanol ligand completes the square-pyramidal coordination. In [Cu(L3)(H2O)][PF6], which is five coordinate and approximately trigonal bipyramidal, [L3]- acts as an N4 donor via all three pyrazolyl groups but only one pyridyl group; the two pendant pyridyl groups are involved in O–H···N hydrogen-bonding interactions with the coordinated water molecule. This ‘second-sphere’ stabilisation of a coordinated ligand is strongly reminiscent of the cooperative interactions by which substrates are bound to the active sites of metalloproteins. The EPR spectra of this complex are solvent-dependent, showing a change from a dz2 ground state in non-donor solvents to a dx2-y2 ground state in donor solvents.

Synthesis of the potentially pentadentate ligand 6,6″-bis(2-hydroxyphenyl)-2,2′ :6′,2″- terpyridine (H2L) and the crystal structure and magnetic properties of [{Cu(HL)}2][PF6]2·5MeCN

The new potentially pentadentate (ONNNO donor) ligand 6,6″-bis(2-hydroxyphenyl)-2,2′ : 6′,2″-terpyridine (H2L) reacted with CuII to form the complex [{Cu(HL)}2][PF6]2. X-Ray structural analysis of [{Cu(HL)}2][PF6]2·5MeCN revealed that the complex exists as centrosymmetric dimers. The monomeric unit is four-co-ordinate Cu(HL)+, in which the copper(II) ion is co-ordinated by one phenolate oxygen and three pyridyl nitrogen atoms of HL, with the remaining phenol group protonated and not co-ordinated but involved in hydrogen-bonding interactions with [PF6]– ions or lattice MeCN molecules. The geometry is best approximated as square planar within the constraints imposed by the ligand. Two of these units are stacked such that the copper(II) centre of one monomeric unit is co-ordinated axially by the phenolate ligand of the other, forming a Cu2(µ-O)2 core with (approximately) elongated square-pyramidal copper(II) centres. This results in aromatic stacking between the two ligands. Low-temperature magnetic susceptibility measurements indicate a weak antiferromagnetic coupling between the metals. The EPR spectrum (CH2Cl2-dimethylformamide glass at 77 K) is a typical triplet with a well resolved double-quantum transition and a septet hyperfine coupling pattern, showing that the dimer remains intact in the solvent mixture used. However in the presence of pyridine the dimeric units break up via axial ligation of pyridine, resulting in an EPR spectrum characteristic of mononuclear copper (II) species.

Electrochemical and EPR spectral studies of mono- and bimetallic (trispyrazolylborato)molybdenum and tungsten complexes with extended di-phenol bridging ligands; evidence for electron exchange below the fast limit

Abstract Mononuclear diphenolato complexes [M(NO)L*X(OQOH)] and binuclear complexes [{M(NOL)L*X}2(μ-OQO)] {M = Mo, W; L* = [HB(3,5-Me2C3HN2)3]−; X = Cl, I; Q = C6H4, 4,4′-C6H4C(O)C6H4, 4,4′-C6H4S(O)2C6H4 or C6H4C(O)(C6H4)2C(O)C6H4} were prepared. Some have been reported before; the new complexes were thoroughly characterized. Electrochemical interactions between the metal centers in the binuclear complex with the three longer bridging ligands are small. The binuclear molybdenum complexes (which contain two 16-electron metal centres) with 4,4′-OC6H4C(O)C6H4O or 4,4′-OC6H4S(O)2C6H4O as bridging ligands were reduced to dianionic species (two 17-electron centres), whose EPR spectra indicate pairwise electron exchange at or near the fast limit; with the longer bridging ligand OC6H4C(O)(C6H4)2C(O)C6H4O the more complex EPR spectrum of the reduced dianionic complex indicates that electron exchange is no longer “fast”, but occurs at an intermediate rate. With the shortest bridging ligand OC6H4O the electrochemical interaction is sufficiently large to allow selective preparation of the mixed-valence 16-electron/17-electron species, whose EPR spectrum indicates that it is valence trapped; fast exchange is only initiated when both metal centres have a 17-electron configuration. The EPR spectra of the corresponding bimetallic tungsten dianions cannot be used to examine electron exchange processes since they show no resolved hyperfine signals.

DINUCLEAR MOLYBDENUM COMPLEXES DERIVED FROM DIPHENOLS : ELECTROCHEMICAL INTERACTIONS AND REDUCED SPECIES

Abstract The new diphenolato complexes [{Mo(NO){HB(dmpz)3}Cl}2Q] where dmpz = 3,5-dimethylpyrazolyl and Q = OC6H4(C6H4O (n = 1 or 2), OC6H4CRCRC6H4O (R = H or Et), and OC6H4CHCHC6H4CHCHC6H4O have been prepared and their electrochemical properties (cyclic and differential pulse voltammetry) compared with previously reported analogues where Q = OC6H4O, OC6H4EC6H4O (E = SO2, CO and S), OC6H4 (CO)C6H4 C6H4(CO)C6H4O and 1,5- and 2,7-O2C10H6. The electrochemical interaction between the redox centres in the new complexes is very weak, in contrast to that in the 1,4-benzenediolato and naphthalendiolato species. The EPR spectra of the reduced mixed-valence species [{Mo(NO){HB(dmpz)3}Cl}2Q]− where Q = 1,3- and 1,4-OC6H4O and OC6H4SC6H4O shows that they are valence-trapped at room temperature, whereas those of the dianions [{Mo(NO){HB(dmpz)3}Cl}2Q]2− where Q = 1,4-OC6H4O, OC6H4EC6H4O (E = CO or S) and OC6H4CHCHC6H4CHCHC6H4O shows that the unpaired spins on each molybdenum centre are strongly correlated (J, the spin exchange integral ⪢AMo, the metal-hyperfine coupling constant). The electrochemical properties and the comproportionation constants for the reaction [{Mo(NO){HB(dmpz)3} Cl}2Q] + [{ Mo ( NO ){ HB ( dmpz ) 3 } Cl } O ] 2 ] 2− ↩2[{ Mo ( NO ) { HB ( dmpz ) 3 } Cl } 2 Q ] − where Q = diphenolato bridge, are compared with related compounds containing benzenediamido and dianilido bridges.

Coordination chemistry of mixed pyridine-phenol ligands; mononuclear palladium(II) and dinuclear copper(II) complexes of derivatives of bidentate N,O-chelating ligands based on 2-(2-hydroxyphenyl)pyridine

Abstract The new ligands 2-(2-hydroxyphenyl)-4-tbutyl-pyridine (HL1), 2-(2-hydroxyphenyl)-6-methylpyridine (HL2) and 2-(2-hydroxyphenyl)-6-[(2-phenyl)ethyl]pyridine (HL3) were prepared. They are all substituted derivatives of the simple N,O-bidentate chelating ligand 2-(2-hydroxyphenyl)pyridine. HL1 is solubilised derivative bearing a tBu substituent in a position which will not sterically interfere with metal-ion coordination; HL2 and HL3 contain substituents at C6 of the pyridyl ring, adjacent to the N atom, which will therefore sterically hinder metal-ion coordination. The complexes [PdL2] (PdL2] (L = L1, L2 and L3) were prepared and structurally characterised to determine the effects (if any) of the substituents on the structures of the metal complexes. All are four-coordinate complexes with square planar coordination geometry about the Pd(II) ion, but the sterically hindered ligands L2 and L3 have to adopt a different conformation from L1 to allow planar coordination, resulting in “bowl-like” structures. [Cu2(L1)4] was also prepared and found to be a phenolate-bridged dinuclear complex in the solid state, both by X-ray crystallography and EPR measurements. In solution, however, it dissociates to give [Cu(L1)2] monomeric units. Cu(II) complexes of L2 and L3 could not be isolated.

Heteropolymetallic copper(II)-gold(III) dithiocarbamate [2]catenanes via magic ring synthesis

A rare class of mixed-metal [2]catenane has been assembled via magic ring synthesis of dinuclear copper(II) and gold(III) dithiocarbamate macrocycles.

On intramolecular dyotropy: structural effects on reaction rates, crystal structure–molecular mechanics correlations and primary deuterium kinetic isotope effects. (Parameters for intramolecular recognition)

Previous attempts to prepare the pentacyclic triene 17 for comparison of the rate of intramolecular dyotropy with the kinetics of similar irreversible rearrangements of norbornene ring-substituted analogues had given only dyotropomer 18 with an estimated minimum ratio K1(17)/K1(5)∼ 2 × 105 at 36 °C. In the following it is shown that the steric proximity, dCH, of transferring H atoms to receptor sp2 carbons in the reaction zone cavity together with M M-calculated π-energy differences between dyotropomers can rationalise the large rate enhancement observed for the triene 17 compared with 5 and its analogues. For a series of compounds modelled on 5, in which dCH variations are quite small, observed differences in dyotropic rate are identified as arising from the interplay of molecular geometry changes and small changes in π-energy at the receptor alkene site occasioned by proximate polar groups, the electronic changes associated with aromatisation of the appended donor-site ring remaining essentially constant across the series. When the electronic energy changes associated with dyotropy for a pair of analogous structures are very closely similar, a rate-spread of ca. 104 can be identified with a change in dCH of 0.1–0.17 A. Similar kinetic effects concomitant on small parallel structural variations, virtually identical in relative-rate terms to those in the triene series, are seen in the irreversible dyotropy of a series of analogous pyrazolines modelled on compound 36 and may be likewise rationalised. Kinetic comparisons for a group of aryl-ring substituted analogues of pyrazoline 36 reveal quite modest substituent effects, consistent with reactant-like transition-states for these quantitative, exothermic rearrangements. Inter-series comparison of alicyclic trienes with pyrazolines indicate that when dCH values are essentially identical in representative examples, a rate-differential of 102–103 between the two series can be identified principally with the differing electronic requirements for triene and (slower) pyrazoline rearrangements. Primary deuterium kinetic isotope effects (K12H/K12D, dln[K12H/K12D]/dt and especially A2H/A2D) reveal strong evidence for non-classical behaviour especially for pyrazoline 38.

Co-ordination chemistry of mixed pyridine–phenol and phenanthroline–phenol ligands; a variable-temperature electron paramagnetic resonance and magnetic susceptibility study on two binuclear copper(II) complexes with Cu2(µ-O)2(µ-1,3,-O2CMe) cores

The EPR spectra (characteristic of triplet species) of the complexes [Cu2L12(µ-1,3-O2CMe)][PF6]1[HL1= 6-(2-hydroxyphenyl)-2,2′-bipyridine] and [Cu2L22(µ-1,3-O2CMe)][PF6]2[HL2= 2-(2-hydroxyphenyl)-1,10-phenanthroline], previously structurally characterised as containing Cu2(µ-O)2 cores in which the phenolate oxygen atoms of L1 and L2 bridge both copper(II) centres, have been successfully modelled using a perturbation-theory approach. The Cu ⋯ Cu distances calculated from the spectra are in reasonable agreement with the values given by the crystal structures. By measuring the variation in intensity of the spectrum of 1 with temperature it was established that 1 is antiferromagnetic with J= 52 ± 8 cm–1. Variable-temperature magnetic susceptibility measurements on 1 and 2 confirm the presence of moderate antiferromagnetic couplings, although the presence of a mononuclear impurity in each case precludes use of the Bleaney–Bowers equation to obtain more accurate values of J.

Investigations of the interactions between a novel polysaccharide controlled release matrix and model compounds using ESR

Interactions between TIMERx, a novel polysaccharide based controlled release system, and model compounds are studied using electron spin resonance (ESR). Mimicking the incorporation of a low dose drug into the matrix it is initially found that mixing the crystalline compound into the components of the system provides insufficiently even mixing, with poor dispersion of the agent. A similar situation pertains on the mixing of compounds with pre-formed granules of the TIMERx base. Wet granulation of the system, where the compound was dissolved in the granulating fluid prior to granulation provides, ideal distribution throughout the matrix, with no interaction between spin labels at the molecular level, of all the model compounds throughout the matrix. It is found that two of the four model compounds interact with residual micellar water in the matrix structure and two, despite having very similar structures and solubilities, do not. It is proposed that the two non-dissolved compounds, which have a greater capacity for hydrogen bonding, are binding more strongly to the polysaccharide components of the matrix. Evidence also suggests that this interaction can protect the compounds from reduction by dextrose. This interaction is unaffected by the drying method of the granules and does not occur during the tabletting process. The results suggest that ESR is a useful technique for measuring interactions between low molecular weight compounds and excipients.