PCH CH PPh Research Articles

Detection of Unusual Reaction Intermediates during the Conversion of W(N2)2(dppe)2 to W(H)4(dppe)2 and of H2O into H2

W(N(2))(2)(dppe-κ(2)P)(2) reacts with H(2) to form WH(3){Ph(C(6)H(4))PCH(2)CH(2)PPh(2)-κ(2)P}(dppe-κ(2)P) and then W(H)(4)(dppe-κ(2)P)(2). When para-hydrogen is used in this study, polarized hydride signals are seen for these two species. The reaction is complicated by the fact that trace amounts of water lead to the formation of H(2), PPh(2)CH(2)CH(2)Ph(2)P(O) and W(H)(3)(OH)(dppe-κ(2)P)(2), the latter of which reacts further via H(2)O elimination to form W(H)(4)(dppe-κ(2)P)(2) and [WH(3){Ph(C(6)H(4))PCH(2)CH(2)PPh(2)-κ(2)P}(dppe-κ(2)P)]. These studies demonstrate a role for the 14-electron intermediate W(dppe-κ(2)P)(2) in the CH activation reaction pathway leading to [WH(3){Ph(C(6)H(4))PCH(2)CH(2)PPh(2)-k(2)P}(dppe-k(2)P)]. UV irradiation of W(H)(4)(dppe-κ(2)P)(2) under H(2) led to phosphine dechelation and the formation of W(H)(6)(dppe-k(2)P)(dppe-k(1)P) rather than H(2) loss and W(H)(2)(dppe-κ(2)P)(2) as expected. Parallel DFT studies using the simplified model system W(N(2))(2)((Ph)HPCH(2)CH(2)PH(2)-κ(2)P)(H(2)PCH(2)CH(2)PH(2)-κ(2)P) confirm that ortho-metalation is viable via both W(dppe-κ(2)P)(2) and W(H)(2)(dppe-κ(2)P)(2) with explicit THF solvation being necessary to produce the electronic singlet-based reaction pathway that matches with the observation of para-hydrogen induced polarization in the hydride signals of [WH(3){Ph(C(6)H(4))PCH(2)CH(2)PPh(2)-κ(2)P}(dppe-κ(2)P)], W(H)(3)(OH)(dppe-κ(2)P)(2) and W(H)(4)(dppe-κ(2)P)(2) during this study. These studies therefore reveal the existence of differentiated and previously unsuspected thermal and photochemical reaction pathways in the chemistry of both W(N(2))(2)(dppe-κ(2)P)(2) and W(H)(4)(dppe-κ(2)P)(2) which have implications for their reported role in N(2) fixation.

Directionality of Dihydrogen Bonds: The Role of Transition Metal Atoms

A theoretical study on two series of electron-rich group 8 hydrides is carried out to evaluate involvement of the transition metal in dihydrogen bonding. To this end, the structural and electronic parameters are computed at the DFT/B3PW91 level for hydrogen-bonded adducts of [(PP(3))MH(2)] and [Cp*MH(dppe)] (M = Fe, Ru, Os; PP(3) = κ(4)-P(CH(2)CH(2)PPh(2))(3), dppe = κ(2)-Ph(2)PCH(2)CH(2)PPh(2)) with CF(3)CH(2)OH (TFE) as proton donor. The results are compared with those of adduct [Cp(2)NbH(3)]⋅TFE featuring a "pure" dihydrogen bond, and classical hydrogen bonds in pyridine⋅TFE and Me(3)N⋅TFE. Deviation of the H⋅⋅⋅H-A fragment from linearity is shown to originate from the metal participation in dihydrogen bonding. The latter is confirmed by the electronic parameters obtained by NBO and AIM analysis. Considered together, orbital interaction energies and hydrogen bond ellipticity are salient indicators of this effect and allow the MH⋅⋅⋅HA interaction to be described as a bifurcate hydrogen bond. The impact of the M⋅⋅⋅HA interaction is shown to increase on descending the group, and this explains the experimental trends in mechanisms of proton-transfer reactions via MH⋅⋅⋅HA intermediates. Strengthening of the M⋅⋅⋅H interaction in the case of electron-rich 5d metal hydrides leads to direct proton transfer to the metal atom.

Group 11 complexes containing the [C5(CN)5]− ligand; ‘coordination-analogues’ of molecular organometallic systems

The reactions of Na[C(5)(CN)(5)] (Na[1]) with group 11 phosphine complexes [(P)(n)MCl] (M = Cu, Ag, Au, P = Ph(3)P; M = Cu, P = dppe (Ph(2)PCH(2)CH(2)PPh(2))] give a range of compounds containing the pentacyanocyclopentadienide ligand, [C(5)(CN)(5)](-) (1). The new complexes [(Ph(3)P)(2)M{1}](2) [M = Cu (3); M = Ag (5)], [(Ph(3)P)(3)Ag{1}] (4), [(dppe)(3)Cu(2){1}(2)] (6) and [Au(PPh(3))(2)][1] (7) include the first complete series of group 11 complexes of any cyclopentadienide ligand to be structurally characterised.

Oxidatively Induced Reactivity of [Fe2(CO)4(κ2-dppe)(μ-pdt)]: an Electrochemical and Theoretical Study of the Structure Change and Ligand Binding Processes

The one-electron oxidation of the diiron complex [Fe(2)(CO)(4)(κ(2)-dppe)(μ-pdt)] (1) (dppe = Ph(2)PCH(2)CH(2)PPh(2); pdt = S(CH(2))(3)S) has been investigated in the absence and in the presence of P(OMe)(3), by both electrochemical and theoretical methods, to shed light on the mechanism and the location of the oxidatively induced structure change. While cyclic voltammetric experiments did not allow to discriminate between a two-step (EC) and a concerted, quasi-reversible (QR) process, density functional theory (DFT) calculations favor the first option. When P(OMe)(3) is present, the one-electron oxidation produces singly and doubly substituted cations, [Fe(2)(CO)(4-n){P(OMe)(3)}(n)(κ(2)-dppe)(μ-pdt)](+) (n = 1: 2(+); n = 2: 3(+)) following mechanisms that were investigated in detail by DFT. Although the most stable isomer of 1(+) and 2(+) (and 3(+)) show a rotated Fe(dppe) center, binding of P(OMe)(3) occurs at the neighboring iron center of both 1(+) and 2(+). The neutral compound 3 was obtained by controlled-potential reduction of the corresponding cation, while 2 was quantitatively produced by reaction of 3 with CO. The CO dependent conversion of 3 into 2 as well as the 2(+) ↔ 3(+) interconversion were examined by DFT.

Synthesis and Hydride Transfer Reactions of Cobalt and Nickel Hydride Complexes to BX3Compounds

Hydrides of numerous transition metal complexes can be generated by the heterolytic cleavage of H(2) gas such that they offer alternatives to using main group hydrides in the regeneration of ammonia borane, a compound that has been intensely studied for hydrogen storage applications. Previously, we reported that HRh(dmpe)(2) (dmpe = 1,2-bis(dimethylphosphinoethane)) was capable of reducing a variety of BX(3) compounds having a hydride affinity (HA) greater than or equal to the HA of BEt(3). This study examines the reactivity of less expensive cobalt and nickel hydride complexes, HCo(dmpe)(2) and [HNi(dmpe)(2)](+), to form B-H bonds. The hydride donor abilities (ΔG(H(-))°) of HCo(dmpe)(2) and [HNi(dmpe)(2)](+) were positioned on a previously established scale in acetonitrile that is cross-referenced with calculated HAs of BX(3) compounds. The collective data guided our selection of BX(3) compounds to investigate and aided our analysis of factors that determine favorability of hydride transfer. HCo(dmpe)(2) was observed to transfer H(-) to BX(3) compounds with X = H, OC(6)F(5), and SPh. The reaction with B(SPh)(3) is accompanied by the formation of dmpe-(BH(3))(2) and dmpe-(BH(2)(SPh))(2) products that follow from a reduction of multiple B-SPh bonds and a loss of dmpe ligands from cobalt. Reactions between HCo(dmpe)(2) and B(SPh)(3) in the presence of triethylamine result in the formation of Et(3)N-BH(2)SPh and Et(3)N-BH(3) with no loss of a dmpe ligand. Reactions of the cationic complex [HNi(dmpe)(2)](+) with B(SPh)(3) under analogous conditions give Et(3)N-BH(2)SPh as the final product along with the nickel-thiolate complex [Ni(dmpe)(2)(SPh)](+). The synthesis and characterization of HCo(dedpe)(2) (dedpe = Et(2)PCH(2)CH(2)PPh(2)) from H(2) and a base is also discussed, including the formation of an uncommon trans dihydride species, trans-[(H)(2)Co(dedpe)(2)][BF(4)].

Active-Site Models for the Nickel–Iron Hydrogenases: Effects of Ligands on Reactivity and Catalytic Properties

Described are new derivatives of the type [HNiFe(SR)(2)(diphosphine)(CO)(3)](+), which feature a Ni(diphosphine) group linked to a Fe(CO)(3) group by two bridging thiolate ligands. Previous work had described [HNiFe(pdt)(dppe)(CO)(3)](+) ([1H](+)) and its activity as a catalyst for the reduction of protons (J. Am. Chem. Soc. 2010, 132, 14877). Work described in this paper focuses on the effects on properties of NiFe model complexes of the diphosphine attached to nickel as well as the dithiolate bridge, 1,3-propanedithiolate (pdt) vs 1,2-ethanedithiolate (edt). A new synthetic route to these Ni-Fe dithiolates is described, involving reaction of Ni(SR)(2)(diphosphine) with FeI(2)(CO)(4) followed by in situ reduction with cobaltocene. Evidence is presented that this route proceeds via a metastable μ-iodo derivative. Attempted isolation of such species led to the crystallization of NiFe(Me(2)pdt)(dppe)I(2), which features tetrahedral Fe(II) and square planar Ni(II) centers (H(2)Me(2)pdt = 2,2-dimethylpropanedithiol). The new tricarbonyls prepared in this work are NiFe(pdt)(dcpe)(CO)(3) (2, dcpe = 1,2-bis(dicyclohexylphosphino)ethane), NiFe(edt)(dppe)(CO)(3) (3), and NiFe(edt)(dcpe)(CO)(3) (4). Attempted preparation of a phenylthiolate-bridged complex via the FeI(2)(CO)(4) + Ni(SPh)(2)(dppe) route gave the tetrametallic species [(CO)(2)Fe(SPh)(2)Ni(CO)](2)(μ-dppe)(2). Crystallographic analysis of the edt-dcpe compund [2H]BF(4) and the edt-dppe compound [3H]BF(4) verified their close resemblance. Each features pseudo-octahedral Fe and square pyramidal Ni centers. Starting from [3H]BF(4) we prepared the PPh(3) derivative [HNiFe(edt)(dppe)(PPh(3))(CO)(2)]BF(4) ([5H]BF(4)), which was obtained as a ∼2:1 mixture of unsymmetrical and symmetrical isomers. Acid-base measurements indicate that changing from Ni(dppe) (dppe = Ph(2)PCH(2)CH(2)PPh(2)) to Ni(dcpe) decreases the acidity of the cationic hydride complexes by 2.5 pK(a)(PhCN) units, from ∼11 to ∼13.5 (previous work showed that substitution at Fe leads to more dramatic effects). The redox potentials are more strongly affected by the change from dppe to dcpe, for example the [2](0/+) couple occurs at E(1/2) = -820 for [2](0/+) vs -574 mV (vs Fc(+/0)) for [1](0/+). Changes in the dithiolate do not affect the acidity or the reduction potentials of the hydrides. The acid-independent rate of reduction of CH(2)ClCO(2)H by [2H](+) is about 50 s(-1) (25 °C), twice that of [1H](+). The edt-dppe complex [2H](+) proved to be the most active catalyst, with an acid-independent rate of 300 s(-1).

Transformations of aryl isothiocyanates on tetraphosphine tungsten complexes and reactivity of the resulting dithiocarbonimidate ligand

Treatment of [WH(4)(κ(4)-P4)] (3: P4 = meso-o-C(6)H(4)(PPhCH(2)CH(2)PPh(2))(2)) with aryl isothiocyanate ArNCS at 50 °C afforded the dithiocarbonimidate-isocyanide complex [W(κ(2)-S(2)CNAr)(CNAr)(κ(4)-P4)] (4) in moderate yields. The reaction also produced ArNHCH(3) and a small amount of ArNH(2). The yield of the hydrodesulfurization product ArNHCH(3) increased when the reaction was conducted under H(2) (up to 0.65 equiv. to 3 for Ar = p-MeC(6)H(4) (Tol)). Complex 4 was proposed to be formed via reductive disproportionation of two ArNCS molecules on a zero-valent W species generated by dissociation of H(2) from 3. The reaction of W(0) complex [W(dppe)(κ(4)-P4)] (dppe = Ph(2)PCH(2)CH(2)PPh(2)) with ArNCS also yielded 4 accompanied by free dppe, in contrast to that of [Mo(dppe)(κ(4)-P4)], which had been previously reported to undergo sulfur-atom transfer to phosphine ligands. The dithiocarbonimidate ligands in 4a (Ar = Tol) received the addition of electrophiles [PhMe(2)NH][BF(4)], MeI, and PhCOCl selectively at the N atom to afford the cationic dithiocarbamate complexes [W(κ(2)-S(2)CNHTol)(CNTol)(κ(4)-P4)][BF(4)] (6), [W{κ(2)-S(2)CN(Me)Tol}(CNTol)(κ(4)-P4)]I (7), and [W{κ(2)-S(2)CN(COPh)Tol}(CNTol)(κ(4)-P4)]Cl (8). Complexes 4a, 6, 7, and 8 have been characterized by spectroscopic and crystallographic methods, and the donor strengths of their κ(2)-dithio ligands are discussed.

Core Conversion Reactions of the Cubane-Type Metal-Sulfido Clusters: Shape Shift, Contraction, and Expansion of the MM′Re2S4 Cubanes (M = Ir, Rh, Ru; M′ = Pt, Pd)

Treatment of incomplete cubane-type clusters [(Cp*M){Re(L)}(2)(mu(3)-S)(mu(2)-S)(3)] (M = Ir (1a), Rh (1b); Cp* = eta(5)-C(5)Me(5); L = S(2)C(2)(SiMe(3))(2)) and [{(Pmb)Ru}{Re(L)}(2)(mu(3)-S)(mu(2)-S)(3)] (Pmb = eta(6)-C(6)Me(5)H) with 1 equiv of [Pt(PPh(3))(3)] gave tetranuclear tetra(sulfido) clusters having raft-type cores, [(Cp*M){Pt(PPh(3))(2)}{Re(L)}(2)(mu(3)-S)(4)] (M = Ir (3a), Rh) and [{(Pmb)Ru}{Pt(PPh(3))(2)}{Re(L)}(2)(mu(3)-S)(4)], which presents a sharp contrast to the reactions with [Pd(PPh(3))(4)] reported previously, affording the cubane-type clusters [(Cp*M){Pd(PPh(3))}{Re(L)}(2)(mu(3)-S)(4)] (M = Ir (2a), Rh) and [{(Pmb)Ru}{Pd(PPh(3))}{Re(L)}(2)(mu(3)-S)(4)]. The reactions of 2a with diphosphines P2 resulted in the conversion of its cubane-type core into the analogous raft-type frameworks, forming [(Cp*Ir){Pd(P2)}{Re(L)}(2)(mu(3)-S)(4)] (P2 = cis-Ph(2)PCH=CHPPh(2) (6), Ph(2)PCH(2)CH(2)PPh(2), Ph(2)PCH(2)CH(2)CH(2)PPh(2)). On the other hand, when 2 was allowed to react with Ph(2)PCH(2)PPh(2) (dppm) as P2, the trinuclear tri(sulfido) cluster [(Cp*Ir){Re(L)}(2)(mu(3)-S)(2)(mu(2)-S)(mu(2)-dppm)] (9a) was obtained. Alternatively, this cluster 9a and its Rh analogue 9b were derived from the incomplete cubane-type clusters 1a and 1b by treatment with dppm. It has also been found that further treatment of the cubane-type cluster 2a with excess [Pd(PPh(3))(4)] affords the heptanuclear tetra(sulfido) cluster [(Cp*Ir){Pd(PPh(3))}(4)Re(2)(mu(3)-L)(2)(mu(3)-S)(4)] (10). The detailed structures have been determined by the X-ray analyses for 3a, 6, 9a, and 10.

Cyclopentadienyl Ruthenium(II) Complexes with Bridging Alkynylphosphine Ligands: Synthesis and Electrochemical Studies

The reaction of [CpRuCl(PPh(3))(2)] (Cp = cyclopentadienyl) and [CpRuCl(dppe)] (dppe = Ph(2)PCH(2)CH(2)PPh(2)) with bis- and tris-phosphine ligands 1,4-(Ph(2)PC[triple bond]C)(2)C(6)H(4) (1) and 1,3,5-(Ph(2)PC[triple bond]C)(3)C(6)H(3) (2), prepared by Ni-catalysed cross-coupling reactions between terminal alkynes and diphenylchlorophosphine, has been investigated. Using metal-directed self-assembly methodologies, two linear bimetallic complexes, [{CpRuCl(PPh(3))}(2)(mu-dppab)] (3) and [{CpRu(dppe)}(2)(mu-dppab)](PF(6))(2) (4), and the mononuclear complex [CpRuCl(PPh(3))(eta(1)-dppab)] (6), which contains a "dangling arm" ligand, were prepared (dppab =1,4-bis[(diphenylphosphino)ethynyl]benzene). Moreover, by using the triphosphine 1,3,5-tris[(diphenylphosphino)ethynyl]benzene (tppab), the trimetallic [{CpRuCl(PPh(3))}(3)(mu(3)-tppab)] (5) species was synthesised, which is the first example of a chiral-at-ruthenium complex containing three different stereogenic centres. Besides these open-chain complexes, the neutral cyclic species [{CpRuCl(mu-dppab)}(2)] (7) was also obtained under different experimental conditions. The coordination chemistry of such systems towards supramolecular assemblies was tested by reaction of the bimetallic precursor 3 with additional equivalents of ligand 2. Two rigid macrocycles based on cis coordination of dppab to [CpRu(PPh(3))] were obtained, that is, the dinuclear complex [{CpRu(PPh(3))(mu-dppab)}(2)](PF(6))(2) (8) and the tetranuclear square [{CpRu(PPh(3))(mu-dppab)}(4)](PF(6))(4) (9). The solid-state structures of 7 and 8 have been determined by X-ray diffraction analysis and show a different arrangement of the two parallel dppab ligands. All compounds were characterised by various methods including ESIMS, electrochemistry and by X-band ESR spectroscopy in the case of the electrogenerated paramagnetic species.

Models of the iron-only hydrogenase: Synthesis and protonation of bridge and chelate complexes [Fe2(CO)4{Ph2P(CH2)nPPh2}(μ-pdt)] (n = 2–4) – evidence for a terminal hydride intermediate

Abstract Reactions of [Fe 2 (CO) 6 (μ-pdt)] (pdt = SCH 2 CH 2 CH 2 S) and diphosphines, Ph 2 P(CH 2 ) n PPh 2 ( n = 2–4) and trans -Ph 2 PCH CHPPh 2 , have been carried out under different conditions. For all, at room temperature in MeCN with added Me 3 NO·2H 2 O the diphosphine-linked complexes [{Fe 2 (CO) 5 (μ-pdt)} 2 (μ,κ 1 ,κ 1 -diphosphine)] result. For trans -Ph 2 PCH CHPPh 2 this is the only product under all conditions. It has been crystallographically characterised revealing a C 2 symmetric structure with apical substitution at the diiron centres. In refluxing toluene, reactions with dppe and dppp lead to the formation of a mixture of diphosphine-bridged and chelate isomers [Fe 2 (CO) 4 (μ-diphosphine)(μ-pdt)] and [Fe 2 (CO) 4 (κ 2 -diphosphine)(μ-pdt)], respectively, while with dppb the bridged complex [Fe 2 (CO) 4 (μ-dppb)(μ-pdt)] is the only product. In MeCN at 60–70 °C (with added Me 3 NO·2H 2 O) similar products result although the ratios differ providing evidence for the conversion of chelate to bridge isomers. Three complexes, [Fe 2 (CO) 4 (μ-dppe)(μ-pdt)], [Fe 2 (CO) 4 (κ 2 -dppp)(μ-pdt)] and [Fe 2 (CO) 4 (μ-dppb)(μ-pdt)], have been crystallographically characterised and are compared to the previously reported dppm ( n = 1) complexes [Fe 2 (CO) 4 (μ-dppm)(μ-pdt)] and [Fe 2 (CO) 4 (κ 2 -dppm)(μ-pdt)]. Diphosphine-bridged complexes are structurally superficially similar although significant differences are noted in some key bond lengths and angles, while chelate complexes [Fe 2 (CO) 4 (κ 2 -dppp)(μ-pdt)] and [Fe 2 (CO) 4 (κ 2 -dppm)(μ-pdt)] differ in adopting basal–apical and dibasal coordination geometries, respectively, in the solid state. A number of protonation studies have been carried out. Addition of HBF 4 ·Et 2 O to [Fe 2 (CO) 4 (μ-dppe)(μ-pdt)] affords a bridging hydride complex with poor stability, while in contrast with [Fe 2 (CO) 4 (μ-dppb)(μ-pdt)] the stable hydride [(μ-H)Fe 2 (CO) 4 (μ-dppb)(μ-pdt)][BF 4 ] results. This difference is partially ascribed to the greater flexibility of the diphosphine backbone in dppb. With [Fe 2 (CO) 4 (κ 2 -dppp)(μ-pdt)] the bridging hydride complex [(μ-H)Fe 2 (CO) 4 (κ 2 -dppp)(μ-pdt)][BF 4 ] is the final product, in which the diphosphine occupies two basal sites. Monitoring by NMR at low temperature shows the initial formation of a terminal hydride, which rapidly rearranges to a bridged isomer in which the diphosphine adopts a basal–apical geometry and this in turn rearranges in a slower process to the dibasal isomer. This behavior is similar to that recently communicated for [Fe 2 (CO) 4 (κ 2 -dppe)(μ-pdt)]. [S. Ezzaher, J.-F. Capon, F. Gloaguen, F.Y. Petillon, P. Schollhammer, J. Talarmin, R. Pichon, N. Kervarec, Inorg. Chem. 46 (2007) 3426–3428.]

Synthesis of monoorganotin(IV) chloride complexes of cis-1,2-bis(diphenylphosphino)ethylene: Solution and solid state structures

Organotin(IV) complexes of the type [RSnCl 3( cis-Ph 2 PCH CHPPh 2)] [R = Me ( 1), n Bu ( 2), Ph ( 3)] were prepared by the reaction of RSnCl 3 with the rigid bisphosphine ligand, cis-1,2-bis(diphenylphosphino)ethylene in dichloromethane. The complexes have been characterized both in solution and the solid state. Low temperature 31P and 119Sn NMR studies indicate two different phosphorus environments. The crystal structures indicate a weak but chelating mode of coordination of the two phosphorus atoms to tin, leading to a distorted octahedral geometry. In solution the complexes 1– 3 undergo a redistribution reaction to form [SnCl 4( cis-Ph 2 PCH CHPPh 2)] ( 4) as one of the products. In order to confirm the redistribution, complex 4 has been prepared separately and characterized both structurally and spectrally.

Characterization of Steric and Electronic Properties of NiN2S2 Complexes as S-Donor Metallodithiolate Ligands

The physical properties and structures of a series of six complexes of the type (NiN(2)S(2))W(CO)(4) have been used to establish electronic and steric parameters for square planar NiN(2)S(2) complexes as bidentate, S-donor ligands. According to the nu(CO) stretching frequencies and associated computed Cotton-Kraihanzel force constants of the tungsten carbonyl adducts, there is little difference in donor abilities of the five neutral NiN(2)S(2) metallodithiolate ligands in the series. The dianionic Ni(ema)(2)(-) (ema = N,N'-ethylenebis(2-mercaptoacetamide)) complex transfers more electron density onto the W(CO)(4) moiety. A ranking of donor abilities and a comparison with classical bidentate ligands is as follows: Ni(ema)(=) > {[NiN(2)S(2)](0)} > bipy approximately phen > Ph(2)PCH(2)CH(2)PPh(2) > Ph(2)PCH(2)PPh(2). Electrochemical data from cyclic voltammetry find that the reduction event in the (NiN(2)S(2))W(CO)(4) derivatives is shifted to more positive potentials by ca. 0.5 V compared to the ca. -2 V Ni(II/I) redox event in the free NiN(2)S(2) ligand, consistent with the electron drain from the nickel-dithiolate ligands by the W(CO)(4) acceptor. Differences in Ni(II/I) DeltaE(1/2) values appear to have a ligand dependence which is related to a structural feature of the hinge angle imposed by the (mu-SR)(2) bridges. Thus the angle formed by the intersection of NiN(2)S(2)/WS(2)C(2) planes has been established by X-ray diffraction analyses as a unique orientational feature of the nickel-dithiolate ligands in contrast to classical diphosphine or diimine ligands and ranges in value from 136 to 107 degrees . Variable-temperature (13)C NMR studies show that the spatial orientations of the ligands remained fixed with respect to the W(CO)(4) moiety to temperatures of 100 degrees C.

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.

Is the allylpalladium structure altered between solid and solutions?

Recent EXAFS measurements on [(Ph(2)PCH(2)CH(2)PPh(2))Pd(H(2)CCHCMe(2))]O(3)SCF(3) (Tromp et al. J. Am. Chem. Soc. 2002, 124, 14814) were interpreted as evidence that, when the complex is dissolved in THF, the allyl ligand adopts an eta(2) structure with a dangling allyl CH(2) substituent. DFT calculations of the Pd complex using H(2)P-CH(2)CH(2)-PH(2) as a model for Ph(2)P-CH(2)CH(2)-PPh(2) (dppe), in the absence or the presence of the triflate counteranion, and modeling the THF solvent by explicit Me(2)O molecules or by a continuum model give always a conventional eta(3)-H(2)CCHCMe(2) structure with equal Pd-C bonds to the terminal carbon centers of the allyl. QM/MM calculations using the dppe ligand also fail to support an eta(2)-allyl structure as a global minimum. The EXAFS parameter space is shown to have multiple minima. These have very similar overall EXAFS, but have very different structural parameters. The minimum that was the basis for the previous structural conclusion gives a slightly better fit but has unrealistic Debye-Waller factors and threshold energies.

Proton transfer to nickel-thiolate complexes. 1. Protonation of [Ni(SC(6)H(4)R-4)(2)(Ph(2)PCH(2)CH(2)PPh(2))] (R = Me, MeO, H, Cl, or NO(2)).

The kinetics of the equilibrium reaction between [Ni(SC(6)H(4)R-4)(2)(dppe)] (R= MeO, Me, H, Cl, or NO(2); dppe = Ph(2)PCH(2)CH(2)PPh(2)) and mixtures of [lutH](+) and lut (lut = 2,6-dimethylpyridine) in MeCN to form [Ni(SHC(6)H(4)R-4)(SC(6)H(4)R-4)(dppe)](+) have been studied using stopped-flow spectrophotometry. The kinetics for the reactions with R = MeO, Me, H, or Cl are consistent with a single-step equilibrium reaction. Investigation of the temperature dependence of the reactions shows that DeltaG = 13.6 +/- 0.3 kcal mol(-)(1) for all the derivatives but the values of DeltaH and DeltaS vary with R (R = MeO, DeltaH() = 8.5 kcal mol(-)(1), DeltaS = -16 cal K(-)(1) mol(-)(1); R = Me, DeltaH() = 10.8 kcal mol(-)(1), DeltaS = -9.5 cal K(-)(1) mol(-)(1); R = Cl, DeltaH = 23.7 kcal mol(-)(1), DeltaS = +33 cal K(-)(1) mol(-)(1)). With [Ni(SC(6)H(4)NO(2)-4)(2)(dppe)] a more complicated rate law is observed consistent with a mechanism in which initial hydrogen-bonding of [lutH](+) to the complex precedes intramolecular proton transfer. It seems likely that all the derivatives operate by this mechanism, but only with R = NO(2) (the most electron-withdrawing substituent) does the intramolecular proton transfer step become sufficiently slow to result in the change in kinetics. Studies with [lutD](+) show that the rates of proton transfer to [Ni(SC(6)H(4)R-4)(2)(dppe)] (R = Me or Cl) are associated with negligible kinetic isotope effect. The possible reasons for this are discussed. The rates of proton transfer to [Ni(SC(6)H(4)R-4)(2)(dppe)] vary with the 4-R-substituent, and the Hammett plot is markedly nonlinear. This unusual behavior is attributable to the electronic influence of R which affects the electron density at the sulfur.

A new route to achiral and chiral 1,2-bis(phosphino)ethanes, 1-arsino-2-phosphinoethanes, and 1,3-bis(phosphino)propanes and the molecular structure and catalytic activity of some rhodium(I) complexes derived thereof.

A series of unsymmetrical 1,2-bis(phosphino)ethanes R(2)PCH(2)CH(2)PR'(2) and 1-arsino-2-phosphinoethanes R(2)AsCH(2)CH(2)PR'(2) mainly with bulky substituents R and R' were prepared from the cyclic sulfate by stepwise cleavage of the carbon-oxygen bonds by LiPR(2) and LiPR'(2) or LiAsR(2) and LiPR'(2), respectively. Analogously, racemic mixtures of R(2)PCH(2)CH(Me)PPh(2)(R =iPr, Cy ) as well as the enantiomers (R)-, (R)- and (R)-tBu(2)PCH(2)CH(Me)PPh(2)(R)- were obtained from the corresponding unsymmetrical cyclic sulfates and (S)-. On a similar route, the racemates of the 1,3-bis(phosphino)propanes R(2)PCH(2)CH(2)CH(Me)PPh(2)(R =iPr, tBu ), optically pure (R)- and (S,S)-iPr(2)PCH(Me)CH(2)CH(Me)PPh(2)(S,S)- were prepared. The reaction of [[RhCl([small eta](4)-C(8)H(12))](2)] with chelating ligands L-L, where L-L is R(2)PCH(2)P(men)(2)(R =iPr, Ph; men =(1S,2R,5S)-menthyl), Cy(2)AsCH(2)P(men)(2), or (R)-, (R)-, (R)-, (R)- and (S,S)-, in the presence of AgPF(6), gave the complexes [Rh(eta(4)-C(8)H(12))(L-L)]PF(6) which were used as pre-catalysts in the hydrogenation of the methyl ester of alpha-acetamidocinnamic acid (ACM). Depending on L-L, the solvent, the temperature and the pressure of H(2), optical yields of up to 69% ee were achieved. For two of the rhodium complexes, and, the molecular structures were determined by X-ray crystallography.

The reaction of M(CO)3(Ph2PCH2CH2PPh2) (M = Fe, Ru) with parahydrogen: probing the electronic structure of reaction intermediates and the internal rearrangement mechanism for the dihydride products

The photochemical reaction of Ru(CO)(3)(dppe) and Fe(CO)(3)(dppe)(dppe = Ph(2)PCH(2)CH(2)PPh(2)) with parahydrogen has been studied by in situ-photochemistry resulting in NMR spectra of Ru(CO)(2)(dppe)(H)(2) that show significant enhancement of the hydride resonances while normal signals are seen in Fe(CO)(2)(dppe)(H)(2). This effect is associated with a singlet electronic state for the key intermediate Ru(CO)(2)(dppe) while Fe(CO)(2)(dppe) is a triplet. DFT calculations reveal electronic ground states consistent with this picture. The fluxionality of Ru(CO)(2)(dppe)(H)(2) and Fe(CO)(2)(dppe)(H)(2) has been examined by NMR spectroscopy and rationalised by theoretical methods which show that two pathways for ligand exchange exist. In the first, the phosphorus and carbonyl centres interchange positions while the two hydride ligands are unaffected. A second pathway involving interchange of all three ligand sets was found at slightly higher energy. The H-H distances in the transition states are consistent with metal-bonded dihydrogen ligands. However, no local minimum (intermediate) was found along the rearrangement pathways.

Amidines derived from Pt(IV)-mediated nitrile-amino alcohol coupling and their Zn(II)-catalyzed conversion into oxazolines.

The reaction between the platinum(IV) complex trans-[PtCl(4)(EtCN)(2)] and the amino alcohols NH(2)CH(2)CH(2)OH, NH(2)CH(2)CH(Me)OH-(R)-(-), NH(2)CH(Ph)CH(2)OH-(R)-(-), NH(2)CH(Et)CH(2)OH-(R)-(-), NH(2)CH(Et)CH(2)OH-(S)-(+), and NH(2)CH(Pr(n)())CH(2)OH proceeds rapidly at room temperature in CH(2)Cl(2) to furnish the amidine complexes [PtCl(4)(HN=C(Et)NH(arcraise;)OH)(2)] (1-6) in good yield (70-80%). The related reaction between the platinum(II) complex trans-[PtCl(2)(EtCN)(2)] and monoethanolamine in a molar ratio of 1:2 in CH(2)Cl(2) results in the addition of 4 equiv of NH(2)CH(2)CH(2)OH per mole of complex to give [Pt(HN=C(Et)NHCH(2)CH(2)OH)(2)(NH(2)CH(2)CH(2)OH)(2)](2+) (7). Formulation of 1-6 is based upon satisfactory C, H, N elemental analyses, electrospray mass spectrometry, IR spectroscopy, and (1)H, (13)C((1)H), (15)N, and (195)Pt NMR spectroscopies, while the structures of trans-[PtCl(4)((Z)-NH=C(Et)NHCH(2)CH(2)OH)(2)] (1), trans-[PtCl(4)((Z)-NH=C(Et)NHCH(2)CH(Me)OH-(R)-(-))(2)] (2), and trans-[PtCl(4)((Z)-NH=C(Et)NHCH(Et)CH(2)OH-(R)-(-))(2)] (4) were determined by X-ray single-crystal diffraction. The Z-amidine configuration of the ligands is preserved in CDCl(3) solutions as confirmed by gradient-enhanced (15)N,(1)H-HMQC spectroscopy and NOE experiments. The amidines, formed upon Pt(IV)-mediated nitrile-amino alcohol coupling, were liberated from their platinum(IV) complexes 1, 3, and 4 by reaction with Ph(2)PCH(2)CH(2)PPh(2) (dppe) giving free NH=C(Et)NHCHRCH(2)OH (R = H 8, Et 9, Ph 10), with the substituents R of different types, and dppe oxides; the P-containing species were identified by (31)P((1)H) NMR spectroscopy. NOESY spectroscopy indicates that the liberated amidines retained the same configuration relative to the C=N double bond, i.e., syn-(H,Et)-NH=C(Et)NHCHRCH(2)OH. The liberated hydroxo-functionalized amidines 8-10 were converted into oxazolines (11-13) in the presence of a catalytic amount of ZnCl(2). A similar catalytic effect has also been reached using anhydrous MSO(4) (M = Cu, Co, Cd), CdCl(2), and AlCl(3).

Steric and electronic effects on the reactivity of rh and ir complexes containing P-S, P-P, and P-O ligands. Implications for the effects of chelate ligands in catalysis.

Kinetic studies of the reactions of [M(CO)(L-L)I] [M = Rh, Ir; L-L = Ph(2)PCH(2)P(S)Ph(2) (dppms), Ph(2)PCH(2)CH(2)PPh(2) (dppe), and Ph(2)PCH(2)P(O)Ph(2) (dppmo)] with methyl iodide have been undertaken. All the chelate ligands promote oxidative addition of methyl iodide to the square planar M(I) centers, by factors of between 30 and 50 compared to the respective [M(CO)(2)I(2)](-) complexes, due to their good donor properties. Migratory CO insertion in [Rh(CO)(L-L)I(2)Me] leads to acetyl complexes [Rh(L-L)I(2)(COMe)] for which X-ray crystal structures were obtained for L-L = dppms (3a) and dppe (3b). Against the expectations of simple bonding arguments, methyl migration is faster by a factor of ca. 1500 for [Rh(CO)(dppms)I(2)Me] (2a) than for [Rh(CO)(dppe)I(2)Me] (2b). For M = Ir, alkyl iodide oxidative addition gives stable alkyl complexes [Ir(CO)(L-L)I(2)R]. Migratory insertion (induced at high temperature by CO pressure) was faster for [Ir(CO)(dppms)I(2)Me] (5a) than for its dppe analogue (5b). Reaction of methyl triflate with [Ir(CO)(dppms)I] (4a) yielded the dimer [[Ir(CO)(dppms)(mu-I)Me](2)](2+) (7), which was characterized crystallographically along with 5a and [Ir(CO)(dppms)I(2)Et] (6). Analysis of the X-ray crystal structures showed that the dppms ligand adopts a conformation which creates a sterically crowded pocket around the alkyl ligands of 5a, 6, and 7. It is proposed that this steric strain can be relieved by migratory insertion, to give a five-coordinate acetyl product in which the sterically crowded quadrants flank a vacant coordination site, exemplified by the crystal structure of 3a. Conformational analysis indicates similarity between M(dppms) and M(2)(mu-dppm) chelate structures, which have less flexibility than M(dppe) systems and therefore generate greater steric strain with the "axial" ligands in octahedral complexes. Ab initio calculations suggest an additional electronic contribution to the migratory insertion barrier, whereby a sulfur atom trans to CO stabilizes the transition state compared to systems with phosphorus trans to CO. The results represent a rare example of the quantification of ligand effects on individual steps from catalytic cycles, and are discussed in the context of catalytic methanol carbonylation. Implications for other catalytic reactions utilizing chelating diphosphines (e.g., CO/alkene copolymerization and alkene hydroformylation) are considered.

Substitution reactions of [Ru(dppe)(CO)(H(2)O)(3)][OTf](2).

The labile nature of the coordinated water ligands in the organometallic aqua complex [Ru(dppe)(CO)(H(2)O)(3)][OTf](2) (1) (dppe = Ph(2)PCH(2)CH(2)PPh(2); OTf = OSO(2)CF(3)) has been investigated through substitution reactions with a range of incoming ligands. Dissolution of 1 in acetonitrile or dimethyl sulfoxide results in the facile displacement of all three waters to give [Ru(dppe)(CO)(CH(3)CN)(3)][OTf](2) (2) and [Ru(dppe)(CO)(DMSO)(3)][OTf](2) (3), respectively. Similarly, 1 reacts with Me(3)CNC to afford [Ru(dppe)(CO)(CNCMe(3))(3)][OTf](2) (4). Addition of 1 equiv of 2,2'-bipyridyl (bpy) or 4,4'-dimethyl-2,2'-bipyridyl (Me(2)bpy) to acetone/water solutions of 1 initially yields [Ru(dppe)(CO)(H(2)O)(bpy)][OTf](2) (5a) and [Ru(dppe)(CO)(H(2)O)(Me(2)bpy)][OTf](2) (6a), in which the coordinated water lies trans to CO. Compounds 5a and 6a rapidly rearrange to isomeric species (5b, 6b) in which the ligated water is trans to dppe. Further reactivity has been demonstrated for 6b, which, upon dissolution in CDCl(3), loses water and coordinates a triflate anion to afford [Ru(dppe)(CO)(OTf)(Me(2)bpy)][OTf] (7). Reaction of 1 with CH(3)CH(2)CH(2)SH gives the dinuclear bridging thiolate complex [[(dppe)Ru(CO)](2)(mu-SCH(2)CH(2)CH(3))(3)][OTf] (8). The reaction of 1 with CO in acetone/water is slow and yields the cationic hydride complex [Ru(dppe)(CO)(3)H][OTf] (9) via a water gas shift reaction. Moreover, the same mechanism can also be used to account for the previously reported synthesis of 1 upon reaction of Ru(dppe)(CO)(2)(OTf)(2) with water (Organometallics 1999, 18, 4068).