Platinum and palladium imido and oxo complexes with small natural bite angle diphosphine ligands

Role of Phosphine Sterics in Strained Aminophosphine Chelate Formation

The preparation of four-membered aminophosphine (PN) chelates from common metal precursors has largely evaded realization because of the ring strain associated with these species. We report a straightforward approach to the synthesis of such PN metallacycles using simple α-PN ligands analogous to the popular class of small-bite-angle diphosphinomethane ligands. It is demonstrated that bulky phosphine substituents are important to the formation of these chelates.

[2,2-Bis(diphenyl-phosphan-yl)propane-κP,P']tetra-carbonyl-chromium(0) dichloro-methane monosolvate

The title compound, [Cr(C₂₇H₂₆P₂)(CO)₄]·CH₂Cl₂, was obtained by the reaction of Ph₂PCMe₂PPh₂ with Cr(CO)₆ in refluxing toluene by substitution of two carbonyl ligands. The CrC₄P₂ coordination geometry at the Cr atom is distorted octa­hedral, with a P—Cr—P bite angle of 70.27 (2)°.

Three-coordinate terminal imidoiron(III) complexes: structure, spectroscopy, and mechanism of formation

Reaction of 1-adamantyl azide with iron(I) diketiminate precursors gives metastable but isolable imidoiron(III) complexes LFe=NAd (L = bulky beta-diketiminate ligand; Ad = 1-adamantyl). This paper addresses (1) the spectroscopic and structural characterization of the Fe=N multiple bond in these interesting three-coordinate iron imido complexes, and (2) the mechanism through which the imido complexes form. The iron(III) imido complexes have been examined by (1)H NMR and electron paramagnetic resonance (EPR) spectroscopies and temperature-dependent magnetic susceptibility (SQUID), and structurally characterized by crystallography and/or extended X-ray absorption fine structure (EXAFS) measurements. These data show that the imido complexes have quartet ground states and short (1.68 +/- 0.01 A) iron-nitrogen bonds. The formation of the imido complexes proceeds through unobserved iron-N(3)R intermediates, which are indicated by QM/MM computations to be best described as iron(II) with an N(3)R radical anion. The radical character on the organoazide bends its NNN linkage to enable easy N(2) loss and imido complex formation. The product distribution between imidoiron(III) products and hexazene-bridged diiron(II) products is solvent-dependent, and the solvent dependence can be explained by coordination of certain solvents to the iron(I) precursor prior to interaction with the organoazide.

Matrix metalloprotease selective peptide substrates cleavage within hydrogel matrices for cancer chemotherapy activation

To utilize biologic mechanisms to elicit controlled release in response to disease, protease-sensitive devices have been created. Hydrogels were created with pendant peptide-drug complexes. For the matrix metalloproteases (MMPs) examined, a length of six amino acids greatly improved the specificity of the peptide (k(cat)/K(m) approximately 2.4+/-0.1x10(4)M(-1)s(-1)) over shorter sequences (k(cat)/K(m) approximately 4.4+/-0.2x10(2)M(-1)s(-1)). The peptides did not exhibit anti-proliferative effects upon cancer cells, and peptide-platinum complexes showed similar anti-proliferative effects upon the cancer cells compared to the free platinum drugs. Once the peptide-drug complex was incorporated into the hydrogels, the release was dependent upon the presence of MMP in the solution with approximately 35% of platinum released from hydrogels in the presence of MMP and only 10% without MMP in the week examined. The released drug exhibited the expected anti-proliferative activity over several days of incubation. The MMP selective drug delivery holds much potential for treatment of cancer and other diseases.

Intermolecular amidation of unactivated sp2 and sp2 C-H bonds via palladium-catalyzed cascade C-H activation/nitrene insertion

This communication describes the Pd(OAc)2-catalyzed intermolecular amidation reactions of unactivated sp2 and sp3 C-H bonds using primary amides and potassium persulfate. The substrates containing a pendent oxime or pyridine group were amidated with excellent chemo- and regioselectivities. It is noteworthy that reactive C-X bonds were well-tolerated and a variety of primary amides can be effective nucleophiles for the Pd-catalyzed C-H amidation reactions. For the reaction of unactivated sp3 C-H bonds, beta-amidation of 1 degrees sp3 C-H bonds versus 2 degrees C-H bonds is preferred. The catalytic reaction is initiated by chelation-assisted cyclopalladation involving C-H bond activation. Preliminary mechanistic study suggested that the persulfate oxidation of primary amides should generate reactive nitrene species, which then reacted with the cyclopalladated complex.

Elucidating the Mechanism of the Asymmetric Aza-Michael Reaction

The mechanism of the palladium-catalysed asymmetric aza-Michael addition of aniline to alpha,beta-unsaturated N-imide was examined from several aspects using a combination of techniques, including X-ray crystallography, mass spectrometry, NMR, UV/Vis spectroscopy, and kinetic studies. The binding of aniline to the dicationic palladium(II) metal centre was found to occur in two consecutive steps: The binding of the first aniline is fast and reversible, whereas the binding of the second aniline is slower and irreversible. This occurs in competition with the binding of the N-imide, which forms a planar six-membered chelate ring with the metal centre; coordinating through the 1,3-dicarbonyl moiety. Isotopic labelling revealed that the addition of N-H occurs in a highly stereoselective manner, allowing the synthesis of optically active beta(2)- and beta(2,3)-amino acid derivatives. The stereochemistry of the addition is postulated to be syn. In situ kinetic studies provided evidence for product inhibition. The binding of the N-imide to the catalyst was found to be the rate-limiting step. Aniline was found to be an inhibitor of the pre-catalyst. The study culminated in the design of a new reaction protocol. By maintaining a low concentration of the aniline substrate during the course of the reaction, significant enhancement of yield and enantioselectivity can be achieved.

A para-Hydrogen Investigation of Palladium-Catalyzed Alkyne Hydrogenation

The complexes [Pd(bcope)(OTf)2] (1a), where bcope is (C8H14)PCH2-CH2P(C8H14), and [Pd(tbucope)(OTf)2] (1b), where tbucope is (C8H14)PC6H4CH2P(tBu)2, catalyze the conversion of diphenylacetylene to cis- and trans-stilbene and 1,2-diphenylethane. When this reaction was studied with para-hydrogen, the characterization of [Pd(bcope)(CHPhCH2Ph)](OTf) (2a) and [Pd(tbucope)(CHPhCH2Ph)](OTf) (2b) was achieved. Magnetization transfer from the alpha-H of the CHPhCH2Ph ligands in these species proceeds into trans-stilbene. This process has a rate constant of 0.53 s-1 at 300 K in methanol-d4 for 2a, where DeltaH = 42 +/- 9 kJ mol-1 and DeltaS = -107 +/- 31 J mol-1 K-1, but in CD2Cl2 the corresponding rate constant is 0.18 s-1, with DeltaH = 79 +/- 7 kJ mol-1 and DeltaS = 5 +/- 24 J mol-1 K-1. The analogous process for 2b was too fast to monitor in methanol, but in CD2Cl2 the rate constant for trans-stilbene formation is 1.04 s-1 at 300 K, with DeltaH = 94 +/- 6 kJ mol-1 and DeltaS = 69 +/- 22 J mol-1 K-1. Magnetization transfer from one of the two inequivalent beta-H sites of the CHPhCH2Ph moiety proceeds into trans-stilbene, while the other site shows transfer into H2 or, to a lesser extent, cis-stilbene in CD2Cl2, but in methanol it proceeds into the vinyl cations [Pd(bcope)(CPh=CHPh)(MeOD)](OTf) (3a) and [Pd(tbucope)(CPh=CHPh)(MeOD)](OTf) (3b). When the same magnetization transfer processes are monitored for 1a in methanol-d4 containing 5 microL of pyridine, transfer into trans-stilbene is observed for two sites of the alkyl, but the third proton now becomes a hydride ligand in [Pd(bcope)(H)(pyridine)](OTf) (5a) or a vinyl proton in [Pd(bcope)(CPh=CHPh)(pyridine)](OTf) (4a). For 1b, under the same conditions, two isomers of [Pd(tbucope)(H)(pyridine)](OTf) (5b and 5b') and the neutral dihydride [Pd(tbucope)(H)2] (7) are detected. The single vinylic CH proton in 3 and the hydride ligands in 4 and 5 appear as strong emission signals in the corresponding 1H NMR spectra.

Synthesis of Ru-Pt and Ru-Pd mixed-metal imido clusters from a diruthenium imido-methylene scaffold [(Cp*Ru)2(mu2-NPh)(mu2-CH2)]

The diruthenium mu2-imido mu2-methylene complex [(Cp*Ru)2(mu2-NPh)(mu2-CH2)] serves as a bifunctional scaffold for cluster synthesis, producing a mu3-imido Ru2Pt cluster [(Cp*Ru)2(mu3-NPh)(mu2-CH2)Pt(PMe3)2] on treatment with [Pt(eta2-C2H4)(PMe3)2] and a mu3-methylidyne Ru4Pd2 cluster [(Cp*Ru)2(mu2-NPh)(mu3-CH)PdCl]2 with [PdMeCl(cod)].

Neutral Pyridyl-Functionalized C,N-ortho-Chelated Aminoaryl Platinum(II) Corner Building Blocks for Application in Coordination Reactions

Two homoleptic pyridyl-functionalized C,N-ortho-chelating aminoaryl platinum(II) complexes, cis-[Pt(eta(2)-C,N)] (3a,b), were prepared via an unconventional method involving the initial synthesis of a bromide-functionalized C,N-chelating aminoaryl platinum(II) precursor complex 8, to which subsequently pyridyl groups were attached via a Suzuki-Miyaura C-C coupling reaction. The electron-donating properties of the pyridyl nitrogen atoms of the resulting complexes (3a,b) were used in complexation reactions with monocationic NCN-pincer (NCN = [C6H3(CH2NMe2)(2-)2,6]-) platinum(II) (11a) and palladium(II) (12a) nitrate complexes [M(NCN)(NO3)], thereby obtaining four trimetallic coordination complexes 16-19. The difference in the pyridine-metal coordination behavior between platinum and palladium was studied by varying the ratios of the reagents and by variable-temperature NMR experiments. IR and Raman analyses of 11a and 12a were performed to determine the coordination behavior of the nitrate counteranion, and it was found that both NO3- and H2O coordinate to the metal centers. The crystal structure determinations of free pyridyl complex 3a, [Pt(NCN)(NO3)] (11a), and [Pt(NCN)(NO3)].(H2O) (11b), as well as the crystal structure of trisplatinum coordination complex 16, are reported.

Palladium–gold oxo complexes

Treatment of L2PdCl2(L2= 4,4-di-tert-butyl-2,2-bipyridine) with [(L'Au)3(mu3-O)]BF4(L'= PPh3) has yielded cationic oxo complexes with core formulas Pd2Au2O2 and Pd4AuO3 thereby doubling the number of structurally characterized Pd oxo complexes.

Oligodentate P,N ligands: N,N,N′,N′-tetrakis(diphenylphosphanyl)-1,3-diaminobenzene complexes of rhodium, nickel and palladium

The oligodentate P,N ligand N,N,N',N'-tetrakis(diphenylphosphanyl)-1,3-diaminobenzene reacts with two equivalents of [{Rh(mu-Cl)(COD)}(2)], [NiBr(2)(DME)] or [PdCl(2)(NCMe)(2)](COD = 1,5-cyclooctadiene, DME = dimethoxyethane) in dichloromethane to give the tetranuclear complex [1,3-{cis-Rh(COD)(mu-Cl)(2)Rh(PPh(2))(2)N}(2)C(6)H(4)](1) or the dinuclear complexes [1,3-{cis-NiBr(2)(PPh(2))(2)N}(2)C(6)H(4)](2) and [1,3-{cis-PdCl(2)(PPh(2))(2)N}(2)C(6)H(4)](3), respectively. Compounds 1-3 were characterised by NMR ((1)H, (13)C, (31)P) and IR spectroscopy. The molecular structure of 2 and 3 shows the formation of a bis-chelate complex with M-P-N-P four-membered rings (M = Pd, Ni). An N,N,N',N'-tetrakis(diphenylphosphanyl)-1,3-diaminobenzene/Pd(OAc)(2) mixture was used for the copolymerisation of carbon monoxide with ethene or ethylidenenorbornene. Compound 1 was employed as catalyst in the hydrogenation of styrene.

Acidities of platinum(II) mu-hydroxo complexes bearing diphosphine ligands

The pK(a) values in DMSO of the monoprotic complexes [(L(2)Pt)(2)(mu-OH)(mu-NMePh)](2+) (4) (L(2) = Ph(2)PCH(2)CH(2)PPh(2) (dppe), Ph(2)PCMe(2)PPh(2) (dppip)) are 11.9 +/- 0.1 (L(2) = dppe) and 13.5 +/- 0.2 (L(2) = dppip) as determined by (31)P NMR equilibrium titration with bases of known pK(a). Complexes 4 were prepared by treatment of [L(2)Pt(mu-OH)](2)(2+) (1) with N-methylaniline. The oxo complexes [(L(2)Pt)(2)(mu-O)(mu-NMePh)](+), formed in the equilibrium titration reactions, were independently synthesized in THF by deprotonation of [(L(2)Pt)(2)(mu-OH)(mu-NMePh)](2+) with NaN(SiMe(3))(2) and characterized as NaBF(4) adducts. Similar experiments with diprotic [L(2)Pt(mu-OH)](2)(2+) (L(2) = dppe, Ph(2)PCH(2)CH(2)CH(2)PPh(2) (dppp)) were complicated by exchange processes and were less conclusive, giving pK(a1) < 18 and pK(a2) > 18 in DMSO.

N-benzoylimido complexes of palladium. Synthesis, structural characterisation and structure–reactivity relationship

Benzoyl azides, ArC(O)N3, 2, (Ar = phenyl or substituted phenyl), react with [Pd2Cl2(dppm)2], 1, [dppm = bis(diphenylphosphino)methane] with the formation of novel [Pd2Cl2(mu-NC(O)Ar)(dppm)2], 3, benzoylnitrene complexes that were structurally characterised by multinuclear magnetic resonance and IR spectroscopy and, in several instances, by single crystal X-ray diffraction. As shown by crystallographic studies, the C2P4Pd2 rings adopt extended twist-boat conformations with methylene groups bending towards the bridging benzoylimido moieties. X-ray diffraction studies have revealed the chiral nature of the imido complexes, the chiral element being the propeller-like C2P4Pd2 ring. Structural data accumulated on complexes 3 such as short C-N distances (1.32 A), elongated C=O bonds (1.30 A) as well as the outstandingly high barrier to internal rotation around the N-C(O) linkage (88.3 kJ mol(-1)) are in line with extensive ppi-ppi interaction between the bridging nitrogen and the carbonyl carbon atoms. Theoretical calculations indicate an electron shift from the dimer towards the apical nitrogen atom, which, in turn, facilitates the donation of electrons towards the carbonyl moiety. To elucidate the structure-reactivity relationship of benzoyl azides towards 1, crystallographic and solution IR spectroscopic studies were carried out on a series of para-substituted benzoyl azides. The reaction obeys the Hammett equation. The large positive value of the reaction constant indicates that the azides act as electrophiles in the reaction studied. The enhanced reactivity of 2-nitrobenzoyl azide has been attributed to a decreased conjugation of the phenyl and carbonyl moieties in this reagent.

Effect of chelating vs. bridging coordination of chiral short-bite P–X–P (X = C, N, O) ligands in enantioselective palladium-catalysed allylic substitution reactions

The chiral short-bite ligands (Ra,Ra)-bis(dinaphthylphosphonito)methane, (Ra,Ra)-1, (Ra,Ra)-bis-dinaphthylpyrophosphite, (Ra,Ra)-2, (Sc)-bis(diphenylphosphino)-sec-butylamine, (Sc)-3, (Ra,Ra)-bis(dinaphthylphosphonito)phenylamine, (Ra,Ra)-4a, (Ra,Ra,Sc)-bis(dinaphthylphosphonito)-sec-butylamine, (Ra,Ra,Sc)-4b, and (Ra,Sc)-(dinaphthylphosphonito)(diphenylphosphino)-sec-butylamine, (Ra,Sc)-5, have been synthesised. The cationic palladium-allyl mononuclear chelate, [Pd(eta3-PhCHCHCHPh)(mu-L-Lshort-bite)]PF6 [L-Lshort-bite=(Sc)-3, (Ra,Ra)-4a, (Ra,Ra,Sc)-4b and (Ra,Sc)-5 for complexes, and, respectively] and binuclear bridged [Pd(eta3-PhCHCHCHPh)(mu-Ra,Ra-2)]2(PF6)2, 12, have been isolated. The short-bite chiral ligands synthesised have been tested in the palladium-allyl catalysed substitution reaction of 1,3-diphenylallyl acetate with dimethyl malonate. The catalytic system was studied, in solution, by a multinuclear NMR technique. In the catalytically active species formed with (Ra,Ra)-2 ligand, [Pd(eta3-PhCHCHCHPh)(Ra,Ra-2)]2(PF6)2, 12, the palladium(II) centres are bridged by two ligands which are forced to adopt a nearly cis-coordination to allow coordination of the allyl-moiety. Semiempirical calculations on a biphenyl-model molecule, similar to the species 12, indicate that this situation induces a strain and rigid conformation in the chiral ligands, which produce differences in the terminal allyl carbon atoms. As consequence, the catalytic product was obtained with an enantiomeric excess of 57.1% in the S form. A low e.e. value was obtained when the (Ra,Ra)-1, (Sc)-3, (Ra,Ra)-4a, (Ra,Ra,Sc)-4b and (Ra,Sc)-5 ligands have been tested in the same palladium-catalysed reaction.