Paul A. Duckworth

Substituent effects and chiral discrimination in the complexation of benzoic, 4-methylbenzoic and (RS)-2-phenylpropanoic acids and their conjugate bases by β-cyclodextrin and 6A-amino-6A-deoxy-β-cyclodextrin in aqueous solution: potentiometric titration and 1H nuclear magnetic resonance spectroscopic study

A potentiometric titration study in aqueous solution (l= 0.10 mol dm–3, KCl) of the complexation of benzoic, 4-methylbenzoic and (RS)–2-phenylpropanoic acids (HA) and their conjugate bases (A–) with β-cyclodextrin, βCD, and its substituted analogue, 6A-amino-6A-deoxy-β-cyclodextrin, βCDNH2, in which a primary hydroxy group is replaced by an amino group which may be protonated to produce a singly charged species, βCDNH+3, is reported. At 298.2 K the stability constants for the complexes have the values (in dm3 mol–1) shown in parentheses: benzoic acid ·βCD (K1HA= 590 ± 60); benzoate ·βCD (K1A= 60 ± 10); benzoic acid ·βCDNH+3(K2HA= 340 ± 30); benzoate ·βCDNH+3(K2A= 120 ± 20); benzoate ·βCDNH2(K3A= 50 ± 20); 4-methylbenzoic acid ·βCD (K1HA= 1680 ± 90); 4-methylbenzoate ·βCD (K1A= 110 ± 1); 4-methylbenzoic acid ·βCDNH+3(K2HA= 910 ± 20); 4-methylbenzoate ·βCDNH+3(K2A= 330 ± 20); and 4-methylbenzoate ·βCDNH2(K3A= 100 ± 20). These data indicate that for a given cyclodextrin the guest carboxylic acids form complexes of higher stability than do their conjugate base analogues, and that βCDNH+3 forms more stable complexes with the conjugate bases than do βCD and βCDNH2. These trends are also observed for the complexation of (RS)-2-phenylpropanoic acid and (RS)-2-phenylpropanoate where the complexes indicated are characterised by the stability constants (in dm3 mol–1) shown in parentheses: (RS)-2-phenylpropanoic acid ·βCD (K1RHA= 1090 ± 30, K1SHA= 1010 ± 40); (RS)-2-phenylpropanoate ·βCD (K1RA= 63 ± 8, K1SA= 52 ± 5); (RS)-2-phenylpropanoic acid ·βCDNH+3(K2RHA= 580 ± 20, K2SHA= 480 ± 10); (RS)-2-phenylpropanoate ·βCDNH+3(K2RA= 150 ± 8, K2SA= 110 ± 10); and (RS)-2-phenylpropanoate ·βCDNH2(K3RA= 36 ± 6, K3SA= 13 ± 7). These data also show that while K1RHA and K1SHA, and K1RA and K1SA are indistinguishable for (RS)-2-phenylpropanoic acid ·βCD and (RS)-2-phenylpropanoate ·βCD, chiral discrimination is indicated by K2RHA > K2SHA for (RS)-2-phenylpropanoic acid ·βCDNH+3, K2RA > K2SA for (RS)-2-phenylpropanoate ·βCDNH+3, and K3RA > K3SA for (RS)-2-phenylpropanoate ·βCDNH2. The 1H NMR spectra of the methyl groups of the enantiomers of (RS)-2-phenylpropanoic acid appear as two separate doublets, indicating chiral discrimination when complexed by βCD or βCDNH+3, but such chiral discrimination is not observed for (RS)-2-phenylpropanoate when complexed by βCDNH+3. The implications of these observations are discussed.

Isolation of the anions [M(CO)5(SH)]– and [M2(CO)10(µ-SH)]–(M = Cr, Mo, and W) as salts of the sodium (18-crown-6-ether) cation. X-Ray analysis of the sodium carbonyl linked ‘polymeric’ structures of the tungsten species

Ultraviolet irradiation has provided a convenient synthetic route to the anions [M(CO)5(SH)]– and [M2(CO)10(µ-SH)]–(M = Cr, Mo, and W) which have been isolated as their sodium (18-crown-6-ether) salts. X-Ray analysis of [Na(18-crown-6)][W(CO)5(SH)](1) and of [Na(18-crown-6)][W2(CO)10(µ-SH)](2) have shown them to be chain-polymeric in nature with sodium–carbonyl linkages between alternating cations and anions. The Na–O-(carbonyl) distances are in the range 2.41–2.47 A. In addition to the Na–O distances, compound (1) also shows a weaker Na–S interaction of 3.01 A. Crystals of (1) are orthorhombic, space group Pna21 with a= 16.484(3), b= 7.622(2), c= 19.220(3)A, and Z= 4. Crystals of (2) are triclinic, space group P, with a= 15.014(3), b= 11.874(2), c= 9.636(2)A, α= 102.28(2), β= 96.29(2), γ= 100.63(2)°, and Z= 2. Data were collected on a four-circle diffractometer using Mo-Kα radiation from a graphite monochromator. Blocked full-matrix refinement converged to R 0.044 for (1) and 0.031 for (2).

Group 12 complexes of N,N′,N″,N‴-tetrakis(2-aminoethyl)-1,4,8,11-tetraazacyclotetradecane (L). Crystal structures of [Zn2(OH)L][CLO4]3 and [Cd2CI(L)][CF3SO3]3 together with formation constant determinations and pKa measurements for the aquated bimetallic species

The complex-forming properties of the ligand N,N′,N″,N‴-tetrakis(2-aminoethyl)-1,4,8,11-tetraaza-cyclotetradecane (L) with zinc(II), cadmium(II) and mercury(II) have been investigated by synthesis, X-ray crystallography and potentiometric titration. With zinc(II) in aqueous or aqueous methanol solution the only complexes which could be isolated were salts of the µ-hydroxo-bridged bimetallic cation [Zn2(OH)(L)]3+, although potentiometric titration data suggest the presence of small amounts of [Zn(H3L)]5+ at low pH. Crystallographic data for [Zn2(OH)(L)]3+ indicate that each zinc(II) ion is five-co-ordinate, being bound by a pair of tertiary amines, a pair of primary amines and the bridging hydroxide. The µ-hydroxo group could not be replaced, in attempted substitution reactions, by other potentially bridging anions. The pKa for the aqua species which leads to the µ-hydroxo species is <5.3. Complexation with cadmium(II) and mercury(II) leads to both mono- and bi-metallic species. The monometallic species are the first isolated and can be recognised spectroscopically by the relatively high-field 13C NMR chemical shift for the carbon atoms β to nitrogen. In the bimetallic complexes the tendency for µ-hydroxo formation is lower than for zinc(II) and it is possible to isolate [Cd2L]4+ to which Cl– can be added forming [Cd2Cl(L)]3+. X-Ray crystallography indicates a structure for [Cd2Cl(L)]3+ which is qualitatively similar to that of [Zn2(OH)(L)]3+. In aqueous solution the species [Cd2(OH)(L)]3+ can be detected from potentiometric titration data and forms in response to a pKa value of <6.6 for the aquated precursor. The pKa values for the corresponding cobalt(II), nickel(II) and copper(II) aquated species were measured, for comparison, and found to be 7.3, 9.0 and 7.7, respectively.

Transition metal-mediated discrimination between diastereoisomers of a new linear P2N2 ligand, L1; X-ray structure analysis of rac-L1, [Pt(rac-L1)Cl]Cl·2H2O, and an unusual RhI dimer [Rh2(rac-L1–H)2(CO)2]·2H2O

The new linear P2N2 ligand 1,3-bis{(o-aminophenyl)phenylphosphino}propane, L1, as syntheisised by reaction of (o-amonopheynyl)diphenylphosphine, L2, with lithium and 1,3-dichloropropane, forms racemic and meso isomer with distinctly different stereochemical requirements so that on reaction with PtCl2Y2(Y = dimethyl sulphoxide, DMSO, or benzonitrile) they bond in tridentate and tetradentate modes respectively to give the monocationic complex [Pt(rac-L1)Cl]+, (1), and the dicationic complex [Pt(meso-L1)]2+, (2); X-ray structure analysis of rac-L1, the chloride salt of (1), and the ligand bridged dimer [Rh2(rac-L1–H)2(CO)2], (3), has shown how the stereochemical requirements of this racemic ligand lead to unusual modes of co-ordination.

Macrocyclic ligand design: Structure—function relationships involving the interaction of cobalt(II), nickel(II) and copper(II) with mixed donor macrocyclic ligands

Abstract A comparative investigation of the interaction of Co(II), Ni(II) and Cu(II) with a series of mixed-donor dibenzo-substituted macrocyclic ligands, incorporating O 2 N 3 -, O 2 N 4 -, O 3 N 2 -, O 3 N 3 - and O 4 N 2 -donor sets, has been carried out. The thermodynamic stabilities of the respective complexes in 95% methanol ( I = 0.1; Et 4 NClO 4 , 25°C) have been determined. All ligands form 1:1 (metal:ligand) species with the above metal ions although, in a few instances, species of type MLH 2+ were also detected. In an attempt to elucidate ligand structure/function relationships, an analysis of the effect of variation of ligand backbone and/or donor atom set on the respective complex stabilities has been carried out; the results are compared with those obtained previously for related mixed-donor (cyclic) systems. The X-ray structure of the nickel(II) nitrate complex of the O 3 N 3 -donor macrocycle (V) is presented. C 23 H 35 N 5 NiO 10 crystallised in the monoclinic space group P 2 1 / n , with a = 17.201(3), b = 11.341(2), c = 14.742(3) A, β = 104.34(2)° and Z = 4.

Synthesis of a new P2N2 ligand N,N′-bis[2-(diphenylphosphino)phenyl]propane-1,3-diamine, H2L2, and some of its complexes with elements of the nickel triad and rhodium: X-ray structure analyses of the neutral complex [NiL2] and the trans-spanned [Rh(CO)Cl(H2L2)]

The new multidentate phosphorus–nitrogen P2N2 hybrid ligand N,N′-bis[2-(diphenylphosphino)phenyl]propane-1,3-diamine, H2L2, has been prepared by demetallation of its neutral deprotonated nickel complex [NiL2],(1a), obtaibed from the metal-template catalysed condensation of bis(2-phenylphosphinophenylamido)nickel(II), [Ni(HL1)2], with 1,3-bis(toluene-p-sulphonyloxy)-propane in the presence of base. Under basic conditions, H2L2 forms complexes (1b) and (1c) analogous to (1a) with palladium(II) and platinum(II) in which the ligand is tetradentate around a square-planar metal ion. The complexes (1) may be protonated to form the corresponding dications [M(H2L2)]2+, (2a)–(2c). With rohodium(I), H2L2 forms both the monocation [Rh(H2L2)]+, (3), and the neutral square-planar complex [Rh(CO)Cl(H2L2)], (4), in which H2L2 behaves as a P2trans-spanning bidentate ligand. X-Ray crystal-structure analyses of both (1a) and (4) have been performed. Compound (1a) crystallises with a molecule of mesitylene as solvate in the tetragonal space group I/41/a with a= 35.246(6) and c= 13.190(2)A and Z= 16. Blocked full-matrix refinement of 2 837 unique reflections with I 3σ(I)(3.0 2.5σ(I)(1.0 < θ < 25.0°) gave R= 0.041 and R′= 0.047.

Ligand design and metal-ion recognition. The interaction of copper(II) with a range of 16- to 19-membered macrocycles incorporating oxygen, nitrogen and sulfur donor atoms

The interaction of copper(II) with a series of quinquedentate macrocycles incorporating nitrogen, oxygen, and/or sulfur donor atoms is reported. These ligands form part of a structural matrix in which a systematic variation in donor atom pattern and ring size is present (and against which structure–function relationships are more clearly seen). Stability constants for the 1 : 1 copper complexes have been determined in 95% methanol (I= 0.1 mol dm–3, NEt4ClO4) at 25 °C. The study demonstrates the extremely large stability differentials [up to 1010 for copper(II)] that may be achieved through structural variation within the ligand framework employed for this study. The contribution of the respective donor atom types to overall complex stability was found to follow the expected order of NH(aliphatic) > NH(anilino) > S > O. Based on earlier X-ray studies, the molecular mechanics modelling of two representative examples is also reported.

Macrocyclic ligand design. Effect of donor-set and ringsize variation on silver(I)/lead(II) discrimination within an extended series of dibenzo substituted rings

The effect of systematic variation of the donor-atom set within dibenzo-substituted, 17- to 20-membered macrocycles on their ability to discriminate between silver(I) and lead(II) has been investigated. The log K values for the respective complexes, determined potentiometrically in 95% methanol (l= 0.1 mol dm–3, NEt4ClO4), show that successive replacement of either nitrogen or oxygen heteroatoms by thioether donors leads to increasing affinity for silver(I) relative to lead(II), as does the incorporation of an ‘aliphatic’ NON or NSN donor sequence in the structure. For the 17-membered system incorporating a S2N2S-donor set, exceptional discrimination (of the order of 109)was observed. The characterisation of selected complexes has included the crystal structure of the silver perchlorate complex of the 17-membered ON2S2macrocycle (L17). The silver ion in [AgL17]ClO4 has a very distorted trigonal-bipyramidal geometry with all metal to donor bond lengths appearing normal. The complex crystallises in the monoclinic space group P21/n with lattice parameters a= 19.410(4), b= 9.602(2), c= 12.747(3)A, β= 103.67(2)° and Z= 4.

Ligand design and metal-ion recognition. Comparison of the interaction of cobalt(II) and nickel(II) with 16- to 19-membered mixed-donor macrocycles

The interaction of cobalt(II) and nickel(II) with a range of 16- to 19-membered ring macrocycles incorporating nitrogen, oxygen and/or sulfur heteroatoms is reported. These ligands constitute an extensive array of related macrocyclic structures in which the positions of the donor atoms, their spacing, and the macrocyclic ring size all vary in a systematic manner. Emphasis has been given to the examination of structure–function relationships in the complexation behaviour. Physical measurements confirm the 1 : 1 metal to macrocyclic ligand stoichiometry of the respective complexes. Stability constants for the metal complexes have been determined potentiometrically in 95% methanol (I= 0.1 mol dm–3, NEt4ClO4). An X-ray crystallographic study of [NiL18(H2O)][NO3]2(L18= 5,6,7,8,10,11,12,13,19,20-decahydrodibenzo[f,I][1,8,11,4,15]oxadithiadiazacycloheptadecine) confirms that the nickel ion is six-co-ordinate with the complex cation exhibiting a distorted-octahedral geometry defined by all five donor atoms of the ON2S2 macrocycle and a water molecule; the macrocyclic backbone incorporating the N–O–N donor fragment is arranged meridionally. Molecular mechanics modelling of selected nickel(II) complexes has also been undertaken. As well as their considerable intrinsic interest, the results provide a potentially useful background upon which the design of new reagents for metal-ion discrimination may be based.

Synthesis and oxidation reactions of mono- and di-nuclear rhodium carbonyl complexes of (o-diphenylphosphinophenyl)amine, H2L; X-ray structure analysis of a rhodium(II) amido-bridged dimer, [{Rh(µ-HL)(CO)Cl}2]·EtOH

Reaction of cis-[NBun4][Rh(CO)2Cl2] or [{Rh(CO)2Cl}2] with one or two molar equivalents of (o-diphenylphosphinophenyl)amine (H2L) respectively gives the mononuclear halogenocarbonylrhodium(I) complexes [Rh(H2L)(CO)X][X = Cl (2a), Br (2b), or I (2c)]. These new compounds were found to react with air to give the novel amido-bridged rhodium(II) metal–metal bonded dimers [{Rh(µ-HL)(CO)X}2][X = Cl (5a), Br (5b), or I(5c)]. The chloro species (2a) was also shown to react with base in an inert atmosphere to generate the rhodium(I) amido-bridged dimer [{Rh(µ-HL)(CO)}2](4), which reacts with one molar equivalent of halogen to produce (5) or with excess halogen to give the monomeric rhodium(III)mer-trihalogeno complexes [Rh(H2L)(CO)X3][X = Cl (7a), Br (7b), or I (7c)]. The chloro rhodium(II) dimer (5a) has been characterised by X-ray structure analysis. Crystals of (5a)·EtOH are monoclinic, space group P21/c, with a= 14.669(3), b= 14.754(3), c= 18.233(4)A, β= 92.46(4)°, and Z= 4. Full-matrix refinement of 3 498 unique data with I 3σ(I)(3 < θ < 25°) gave R= 0.045 and R′= 0.042.