The Backbone of Success of P,N-Hybrid Ligands: Some Recent Developments.

Organophosphorus ligands are an invaluable family of compounds that continue to underpin important roles in disciplines such as coordination chemistry and catalysis. Their success can routinely be traced back to facile tuneability thus enabling a high degree of control over, for example, electronic and steric properties. Diphosphines, phosphorus compounds bearing two separated PIII donor atoms, are also highly valued and impart their own unique features, for example excellent chelating properties upon metal complexation. In many classical ligands of this type, the backbone connectivity has been based on all carbon spacers only but there is growing interest in embedding other donor atoms such as additional nitrogen (-NH-, -NR-) sites. This review will collate some important examples of ligands in this field, illustrate their role as ligands in coordination chemistry and highlight some of their reactivities and applications. It will be shown that incorporation of a nitrogen-based group can impart unusual reactivities and important catalytic applications.

Reversible P-P bond cleavage at an iridium(III) metal centre.

Treatment of a κ1-P-monodentate bicyclic diphosphane iridium(III) complex with a labile gold(I) precursor afforded an unusual IrIII/AuI complex in which the P-P single bond has been cleaved. This reaction was cleanly reversed upon addition of tertiary phosphine. Carbon-carbon bond activation, across neighbouring P2C2N rings of the coordinated bicyclic diphosphane, occurred upon thermolysis of the IrIII/AuI complex.

Low-Dimensional Architectures in Isomeric cis-PtCl2{Ph2PCH2N(Ar)CH2PPh2} Complexes Using Regioselective-N(Aryl)-Group Manipulation.

The solid-state behaviour of two series of isomeric, phenol-substituted, aminomethylphosphines, as the free ligands and bound to PtII, have been extensively studied using single crystal X-ray crystallography. In the first library, isomeric diphosphines of the type Ph2PCH2N(Ar)CH2PPh2 [1a-e; Ar = C6H3(Me)(OH)] and, in the second library, amide-functionalised, isomeric ligands Ph2PCH2N{CH2C(O)NH(Ar)}CH2PPh2 [2a-e; Ar = C6H3(Me)(OH)], were synthesised by reaction of Ph2PCH2OH and the appropriate amine in CH3OH, and isolated as colourless solids or oils in good yield. The non-methyl, substituted diphosphines Ph2PCH2N{CH2C(O)NH(Ar)}CH2PPh2 [2f, Ar = 3-C6H4(OH); 2g, Ar = 4-C6H4(OH)] and Ph2PCH2N(Ar)CH2PPh2 [3, Ar = 3-C6H4(OH)] were also prepared for comparative purposes. Reactions of 1a-e, 2a-g, or 3 with PtCl2(η4-cod) afforded the corresponding square-planar complexes 4a-e, 5a-g, and 6 in good to high isolated yields. All new compounds were characterised using a range of spectroscopic (1H, 31P{1H}, FT-IR) and analytical techniques. Single crystal X-ray structures have been determined for 1a, 1b∙CH3OH, 2f∙CH3OH, 2g, 3, 4b∙(CH3)2SO, 4c∙CHCl3, 4d∙½Et2O, 4e∙½CHCl3∙½CH3OH, 5a∙½Et2O, 5b, 5c∙»H2O, 5d∙Et2O, and 6∙(CH3)2SO. The free phenolic group in 1b∙CH3OH, 2f∙CH3OH,2g, 4b∙(CH3)2SO, 5a∙½Et2O, 5c∙»H2O, and 6∙(CH3)2SO exhibits various intra- or intermolecular O-H∙∙∙X (X = O, N, P, Cl) hydrogen contacts leading to different packing arrangements.

Ethylene oligomerisation chromium catalysts with unsymmetrical PCNP ligands.

Chromium(iii) complexes of chelating diphosphines, with PNP or PCNCP backbones, are excellent catalysts for ethylene tetra- and/or trimerisations. A missing link within this ligand series are unsymmetric chelating diphosphines based on a PCNP scaffold. New bidentate PCNP ligands of the type Ph2PCH2N(R)PPh2 (R = 1-naphthyl or 5-quinoline groups, 2a-d) have been synthesised and shown to be extremely effective ligands for ethylene tri-/tetramerisations. Three representative tetracarbonyl Cr0 complexes bearing a single PN(R)P (5), PCN(R)P (6), or PCN(R)CP (7) diphosphine (R = 1-naphthyl) have been prepared from Cr(CO)4(η4-nbd) (nbd = norbornadiene). Furthermore we report a single crystal X-ray diffraction study of these compounds and discuss their structural parameters.

A Monte Carlo simulation study for the gamma-ray/neutron dual-particle imager using rotational modulation collimator (RMC).

The aim of this work is to develop a gamma-ray/neutron dual-particle imager, based on rotational modulation collimators (RMCs) and pulse shape discrimination (PSD)-capable scintillators, for possible applications for radioactivity monitoring as well as nuclear security and safeguards. A Monte Carlo simulation study was performed to design an RMC system for the dual-particle imaging, and modulation patterns were obtained for gamma-ray and neutron sources in various configurations. We applied an image reconstruction algorithm utilizing the maximum-likelihood expectation-maximization method based on the analytical modeling of source-detector configurations, to the Monte Carlo simulation results. Both gamma-ray and neutron source distributions were reconstructed and evaluated in terms of signal-to-noise ratio, showing the viability of developing an RMC-based gamma-ray/neutron dual-particle imager using PSD-capable scintillators.

Flexible kappa4-PNN'O-tetradentate ligands: synthesis, complexation and structural studies.

The three-step synthesis of new, air-stable, PNN'O-tetradentate ligands 3a x HH-3c x HH by Schiff base condensation of the primary amines 2a-2c with 2-Ph2PC6H4(CHO) in refluxing EtOH is described. The racemic ligand 3d x HH, isolated in 79% yield, was successfully prepared from 2-C6H4(OH){C(O)NHCH2CH(Me)NH2} 2d and 2-Ph2PC6H4(CHO) in absolute EtOH. Upon careful choice of metal precursor, ligands 3a x HH-3d x HH display various coordination modes. Reaction of 3a x HH with AuCl(tht) (1:1 molar ratio) affords AuCl(3a x HH), 4a, in which kappa(1)-P-complexation of the functionalised ligand is observed. In contrast, reaction of 3a x HH (or 3d x HH) with MCl2(cod) (M = Pt, Pd) affords MCl2(3a x HH) (M = Pt, 5a; M = Pd, 5b) or MCl2(3d x HH) (M = Pt, 5c; M = Pd, 5d) in which ligand chelation is achieved using both P and imine N donor atoms. Moreover kappa(2)-P,N-chelation was also observed when 3a x HH-3c x HH were separately allowed to react with [PdCl(eta(3)-C3H5)]2 in CH2Cl2 affording new cationic eta(3)-allyl complexes [Pd(eta(3)-C3H5)(3a x HH-3c x HH)]Cl, 6a-6c. Two neutral (methyl)chloropalladium(II) complexes, 7a/7c, were isolated in high yields from 3a x HH or 3c x HH and Pd(CH3)Cl(cod). Elimination of cod and single methyl protonation from Pt(CH3)2(cod) with 1 equiv. of 3a x HH, 3b x HH or 3d x HH in toluene, at room temperature, afforded square-planar complexes Pt(CH3)(kappa(3)-3a x H/3b x H/3d x H), 8a/8b/8d, containing monoanionic kappa(3)-PNN'-tridentate ligands. The kappa(3)-PNN'-tridentate mode was likewise observed for Pd(CH3)(3a x H/3c x H), 10a/10c, upon treatment of a methanolic solution of Pd(CH3)Cl(3a x HH/3c x HH) with tBuOK. Similarly the monohapto (allyl)Pd(II) compounds Pd(CH2CHCH2)(3a x HH-3c x HH), 9a-9c, were obtained cleanly from 6a-6c and tBuOK via an eta(3)-->eta(1) allyl isomerisation. Both amide and phenolic protons in 5a-5d were smoothly deprotonated, with base, to give the kappa(4)-PNN'O-tetradentate complexes 11a/11b and 11d/11e containing the dianionic ligands 3a(2-)/3d(2-) respectively. The Ni(II) complexes 11c and 11f were synthesised directly from NiCl2 x 6 H2O, 3a x HH (or 3d x HH) and tBuOK in CH3OH. All new compounds were characterised by multinuclear NMR, FT-IR, mass spectrometry and microanalysis. Single crystal X-ray studies have been undertaken on the compounds 3a x HH, 3c x HH, 4a, 7c, 8a, 8b, 8d and 11a-11d.

Versatile routes to selenoether functionalised tertiary phosphines.

New selenoether functionalised tertiary phosphines, based on aryl (2a, 2b) or alkyl (4) backbones, have been synthesised and characterised. P,Se-chelation has been achieved upon complexation to square-planar Pt(II) (3a) or Pd(II) (3b) metal centres. For 3a and 3b, weak non-covalent M...Se contacts were established using single crystal X-ray crystallography.

Unusual metal-coordinated zwitterionic P-C-N-C-N-C phosphido adducts.

Rare examples of homodinuclear zwitterionic Pd(II) and Pt(II) complexes with bridging, two-coordinate P{cyclo-CH(2)N(R)CHN(R)CH(2)} ligands (R = 4-FC(6)H(4)CH(2), C(6)H(5)CH(2)) have been characterized by single-crystal X-ray diffraction using synchrotron radiation. Short N-C distances and enlarged N-C-N bond angles support electron delocalization in the central N-C-N backbone.

Intramolecular hydrogen-bonded tertiary phosphines as 1,3,5-triaza-7-phosphaadamantane (PTA) analogues.

New cationic trialkylphosphines [P(CH(2)NH(2)R){CH(2)N(R)CH(2)N(R)CH(2)}](+) (R = C(6)H(5)CH(2), a; 4-FC(6)H(4)CH(2), b), as their Cl(-) (1a, 1b), SbF(6)(-) (2a, 2b), and PF(6)(-) (3a, 3b) salts, are described. The phosphine framework is conformationally locked, in the solid state, through pairs of intramolecular N-H...N hydrogen bonds which are maintained in the Ru(II) and Rh(III) complexes 4 and 5. Phosphines 1a-3b can be considered as charged variants of the well-known PTA ligand.

A novel fluorene-containing kappa(4)-P(2)N(2)-tetradentate platinum(II) complex.

Base induced P,N-chelation, C-C coupling and methylene C-H deprotonation affords an unusual fluorene containing square-planar Pt(II) complex Pt(kappa4-P2N2-Ph2PCH=NNCC12H8)2 which has been isolated and structurally characterised.

µ-Pyrazine-2,5-dicarboxyl-ato-bis-[chlorido(η-p-cymene)ruthenium(II)] tert-butanol disolvate.

A new tert-butanol solvate of [{((i)PrC(6)H(4)Me)RuCl}(2){µ-2,5-pyz(COO)(2)}] (pyz = pyrazine) has been crystallized and structurally characterized. The solvate, [Ru(2)(C(10)H(14))(2)(C(6)H(2)N(2)O(4))Cl(2)]·2C(4)H(10)O, contains one half-mol-ecule of the ruthenium(II) complex and one mol-ecule of tert-butanol in the asymmetric unit. The complex mol-ecule lies on an inversion centre with the two chlorides trans. In contrast, the previously reported structure was solvent-free. Similar metric parameters are found between the butanol solvate and the solvent-free form and an inter-molecular O-H⋯O hydrogen bond exists between µ-pyrazine-2,5-dicarboxyl-ato-bis-[chlorido(η(6)-p-cymene)-ruthenium(II)] and the tert-butanol mol-ecule.

Coordination studies of a new unsymmetrical kappa4-PNN'N"-tetradentate ligand: stepwise formation and structural characterization.

The two-step synthesis of a new unsymmetrical ligand 2-[Ph2PC6H4C(H)=N]C6H4[N(H)COCH2N(H)CO2Bz], 2.HH, via acid-catalyzed Schiff base condensation of 2-(H2N)C6H4[N(H)COCH2N(H)CO2Bz], 1, with 2-Ph2PC6H4(CHO) in refluxing EtOH is reported. The multidentate ligand 2.HH, isolated in ca. 60% yield, exhibits an array of ligation modes, as exemplified by coordination studies with NiII, PdII, PtII, and AuI mononuclear metal precursors. Hence, reaction of 2.HH with AuCl(tht) (1:1 molar ratio, tht = tetrahydrothiophene) affords AuCl(2.HH), 3, in which the ligand behaves as a classic, neutral two-electron phosphorus donor. In contrast, reaction with MCl2(cod) (M = Pt, Pd; cod = cycloocta-1,5-diene) affords the corresponding dichloro complexes MCl2(2.HH) (4a M = Pt; 4b M = Pd) in which kappa2-P/N-chelation through both P and imino N-donor atoms is observed. Likewise, treatment of Pd(CH3)Cl(cod) with 2.HH gave Pd(CH3)Cl(2.HH), 4c, in which the imino nitrogen is trans to the methyl ligand. Cycloocta-1,5-diene elimination from, and single methyl protonation of, Pt(CH3)2(cod) with 1 equiv of 2.HH in toluene at ambient temperature affords the neutral complex Pt(CH3)(2.H-), 5a, in which 2.H- functions effectively in a kappa3-PNN' coordination mode. The dichloro compounds 4a or 4b undergo smooth N(H) deprotonation with tBuOK to give 6a\6a' and 6b\6b' in which 22- acts as a dianionic kappa4-PNN'N' ' ligand. The corresponding square-planar, diamagnetic, nickel(II) complex 6c\6c' was prepared in excellent yield from NiCl2.6H2O, 2.HH, and tBuOK. Variable-temperature NMR experiments confirm 6a\6a' and 6b\6b' exist, in solution, as a pair of conformational (anti and syn) isomers due to restricted rotation about the N-CO2Bz group. This feature is also borne out by single-crystal X-ray studies of anti-6a.CHCl3, syn-6a'.H2O, anti-6b.CHCl3, and anti-6c.CH2Cl2. To the best of our knowledge, we believe these constitute the first examples of crystallographically characterized conformers of a tetradentate ligand incorporating a P-donor center. All new compounds reported have been fully characterized by a combination of spectroscopic (NMR, FT-IR, ES-MS) and analytical methods. Furthermore, single-crystal X-ray studies have also been undertaken on compounds 2.HH, 3, 4a, and 5a.Et2O.

Two square-planar palladium(II) complexes with P,O-bidentate hybrid ligands.

In the two square-planar palladium(II) complexes chloro[(diphenylphosphinoamino)diphenylphosphine oxide]methylpalladium(II) dimethyl sulfoxide solvate, [Pd(CH3)Cl(C24H21NOP2)].C2H6OS, (I), and chloro{[2-(diphenylphosphino)phenyl]diethoxymethane}methylpalladium(II), [Pd(CH3)Cl(C23H25O2P)], (II), a trans disposition of the diphenylphosphino and chloro groups is observed. The Pd atom in both complexes displays a distorted square-planar configuration formed by the four unique donor atoms (P, Cl, C and O). In compound (I), the five-membered Pd-P-N-P-O metallacycle is best described as having an envelope conformation, whereas in (II) the six-membered Pd-P-C-C-C-O metallacycle adopts a skewed boat conformation. Furthermore, within the P-N-P-O backbone in (I), the P-N distances are consistent with single-bond character [1.659 (3) and 1.692 (3) A], whilst the P=O bond shows appreciable double-bond character [1.509 (2) A].

Neutral molecular Pd6 hexagons using kappa3-P2O-terdentate ligands.

The one-step synthesis of three new P2O-terdentate carboxylic acid ditertiary phosphines 2-{(Ph2PCH2)2N}-3-(X)C6H3CO2H (X = OCH3, L1; X = OH, L2) and 2-{(Ph2PCH2)2N}-5-(OH)C6H3CO2H (L3) by a phosphorus-based Mannich condensation reaction using Ph2PCH2OH and the appropriate amine in CH3OH is reported. Compounds L1-L3 function as typical kappa2-P2-didentate ligands upon complexation to Pd(CH3)Cl(cod) (cod = cycloocta-1,5-diene), affording the neutral, mononuclear complexes Pd(CH3)Cl(L1-L3) (1-3). Metathesis of 1 with NaX (X = Br, I) gave the corresponding (methyl)bromopalladium(II) (4) and (methyl)iodopalladium(II) (5) complexes, respectively. When chloroform or chloroform/methanol solutions of 1-3 (or 5) were allowed to stand, at ambient temperatures, yellow crystalline solids were isolated in very high yields (71-88%) and were analyzed for the novel hexameric palladium(II) compounds 6-9. All new compounds reported have been fully characterized by a combination of spectroscopic (multinuclear NMR, Fourier transform IR, electrospray mass spectrometry, matrix-assisted laser desorption ionization time-of-flight mass spectrometry) and analytical methods. The self-assembly reactions are remarkably clean as monitored by 31P{1H} and 1H NMR spectroscopy. Single-crystal X-ray structures have been determined for L1, 4, 7.17CDCl3.2Et2O, 8.6CHCl3.8CH3OH, and 9.17CDCl3. In hexamers 7-9, all six square-planar palladium(II) metal centers comprise a kappa2-P2-chelating diphosphine, a kappa1-O-monodentate carboxylate, and either a chloride or iodide ligand, leading to 48-membered metallomacrocycles (with outside diameters of ca. 2.5 nm). Whereas only intramolecular O-H...N hydrogen bonding between the hydroxy group and tertiary amine has been observed in 7, strong intermolecular O-H...O hydrogen bonding of the type CO...HO(CH3)...HO, involving a methanol solvate, has been found in 8, leading to an unprecedented three-dimensional network motif.