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).     

Mixed-valence nickel-iron dithiolate models of the [NiFe]-hydrogenase active site

A series of mixed-valence nickel-iron dithiolates is described. Oxidation of (diphosphine)Ni(dithiolate)Fe(CO)(3) complexes 1, 2, and 3 with ferrocenium salts affords the corresponding tricarbonyl cations [(dppe)Ni(pdt)Fe(CO)(3)](+) ([1](+)), [(dppe)Ni(edt)Fe(CO)(3)](+) ([2](+)) and [(dcpe)Ni(pdt)Fe(CO)(3)](+) ([3](+)), respectively, where dppe = Ph(2)PCH(2)CH(2)PPh(2), dcpe = Cy(2)PCH(2)CH(2)PCy(2), (Cy = cyclohexyl), pdtH(2) = HSCH(2)CH(2)CH(2)SH, and edtH(2) = HSCH(2)CH(2)SH. The cation [2](+) proved unstable, but the propanedithiolates are robust. IR and EPR spectroscopic measurements indicate that these species exist as C(s)-symmetric species. Crystallographic characterization of [3]BF(4) shows that Ni is square planar. Interaction of [1]BF(4) with P-donor ligands (L) afforded a series of substituted derivatives of type [(dppe)Ni(pdt)Fe(CO)(2)L]BF(4) for L = P(OPh)(3) ([4a]BF(4)), P(p-C(6)H(4)Cl)(3) ([4b]BF(4)), PPh(2)(2-py) ([4c]BF(4)), PPh(2)(OEt) ([4d]BF(4)), PPh(3) ([4e]BF(4)), PPh(2)(o-C(6)H(4)OMe) ([4f]BF(4)), PPh(2)(o-C(6)H(4)OCH(2)OMe) ([4g]BF(4)), P(p-tol)(3) ([4h]BF(4)), P(p-C(6)H(4)OMe)(3) ([4i]BF(4)), and PMePh(2) ([4j]BF(4)). EPR analysis indicates that ethanedithiolate [2](+) exists as a single species at 110 K, whereas the propanedithiolate cations exist as a mixture of two conformers, which are proposed to be related through a flip of the chelate ring. M?ssbauer spectra of 1 and oxidized S = 1/2 [4e]BF(4) are both consistent with a low-spin Fe(I) state. The hyperfine coupling tensor of [4e]BF(4) has a small isotropic component and significant anisotropy. DFT calculations using the BP86, B3LYP, and PBE0 exchange-correlation functionals agree with the structural and spectroscopic data, suggesting that the SOMOs in complexes of the present type are localized in an Fe(I)-centered d(z(2)) orbital. The DFT calculations allow an assignment of oxidation states of the metals and rationalization of the conformers detected by EPR spectroscopy. Treatment of [1](+) with CN(-) and compact basic phosphines results in complex reactions. With dppe, [1](+) undergoes quasi-disproportionation to give 1 and the diamagnetic complex [(dppe)Ni(pdt)Fe(CO)(2)(dppe)](2+) ([5](2+)), which features square-planar Ni linked to an octahedral Fe center.     

Synthesis and structural characterisation of group 10 metal(II) gallyl complexes: analogies with platinum diboration catalysts?

Reactions of the anionic gallium(i) heterocycle, [:Ga{[N(Ar)C(H)](2)}](-) (Ar = C(6)H(3)Pr(i)(2)-2,6), with a variety of mono- and bidentate phosphine, tmeda and 1,5-cyclooctadiene (COD) complexes of group 10 metal dichlorides are reported. In most cases, salt elimination occurs, affording either mono(gallyl) complexes, trans-[MCl{Ga{[N(Ar)C(H)](2)}}(PEt(3))(2)] (M = Ni or Pd) and cis-[PtCl{Ga{[N(Ar)C(H)](2)}}(L)] (L = R(2)PCH(2)CH(2)PR(2), R = Ph (dppe) or cyclohexyl (dcpe)), or bis(gallyl) complexes, trans-[M{Ga{[N(Ar)C(H)](2)}}(2)(PEt(3))(2)] (M = Ni, Pd or Pt), cis-[Pt{Ga{[N(Ar)C(H)](2)}}(2)(PEt(3))(2)], cis-[M{Ga{[N(Ar)C(H)](2)}}(2)(L)] (M = Ni, Pd or Pt; L = dppe, Ph(2)CH(2)PPh(2) (dppm), tmeda or COD). The crystallographic and spectroscopic data for the complexes show that the trans-influence of the gallium(i) heterocycle lies in the series, B(OR)(2) > H(-) > PR(3) approximately [:Ga{[N(Ar)C(H)](2)}](-) > Cl(-). Comparisons between the reactivity of one complex, [Pt{Ga{[N(Ar)C(H)](2)}}(2)(dppe)], with that of closely related platinum bis(boryl) complexes indicate that the gallyl complex is not effective for the catalytic or stoichiometric gallylation of alkenes or alkynes. The phosphaalkyne, Bu(t)C[triple bond, length as m-dash]P, does, however, insert into one gallyl ligand of the complex, leading to the novel, crystallographically characterised P,N-gallyl complex, [Pt{Ga{[N(Ar)C(H)](2)}}{Ga{PC(Bu(t))C(H)[N(Ar)]C(H)N(Ar)}}(dppe)]. An investigation into the mechanism of this insertion reaction has been undertaken.     

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.     

Connecting [NiFe]- and [FeFe]-Hydrogenases: Mixed-Valence Nickel-Iron Dithiolates with Rotated Structures

New mixed-valence iron-nickel dithiolates are described that exhibit structures similar to those of mixed-valence diiron dithiolates. The interaction of tricarbonyl salt [(dppe)Ni(pdt)Fe(CO)(3)]BF(4) ([1]BF(4), where dppe = Ph(2)PCH(2)CH(2)PPh(2) and pdt(2-) = -SCH(2)CH(2)CH(2)S-) with P-donor ligands (L) afforded the substituted derivatives [(dppe)Ni(pdt)Fe(CO)(2)L]BF(4) incorporating L = PHCy(2) ([1a]BF(4)), PPh(NEt(2))(2) ([1b]BF(4)), P(NMe(2))(3) ([1c]BF(4)), P(i-Pr)(3) ([1d]BF(4)), and PCy(3) ([1e]BF(4)). The related precursor [(dcpe)Ni(pdt)Fe(CO)(3)]BF(4) ([2]BF(4), where dcpe = Cy(2)PCH(2)CH(2)PCy(2)) gave the more electron-rich family of compounds [(dcpe)Ni(pdt)Fe(CO)(2)L]BF(4) for L = PPh(2)(2-pyridyl) ([2a]BF(4)), PPh(3) ([2b]BF(4)), and PCy(3) ([2c]BF(4)). For bulky and strongly basic monophosphorus ligands, the salts feature distorted coordination geometries at iron: crystallographic analyses of [1e]BF(4) and [2c]BF(4) showed that they adopt "rotated" Fe(I) centers, in which PCy(3) occupies a basal site and one CO ligand partially bridges the Ni and Fe centers. Like the undistorted mixed-valence derivatives, members of the new class of complexes are described as Ni(II)Fe(I) (S = (1)/(2)) systems according to electron paramagnetic resonance spectroscopy, although with attenuated (31)P hyperfine interactions. Density functional theory calculations using the BP86, B3LYP, and PBE0 exchange-correlation functionals agree with the structural and spectroscopic data, suggesting that the spin for [1e](+) is mostly localized in a Fe(I)-centered d(z(2)) orbital, orthogonal to the Fe-P bond. The PCy(3) complexes, rare examples of species featuring "rotated" Fe centers, both structurally and spectroscopically incorporate features from homobimetallic mixed-valence diiron dithiolates. Also, when the NiS(2)Fe core of the [NiFe]-hydrogenase active site is reproduced, the "hybrid models" incorporate key features of the two major classes of hydrogenase. Furthermore, cyclic voltammetry experiments suggest that the highly basic phosphine ligands enable a second oxidation corresponding to the couple [(dxpe)Ni(pdt)Fe(CO)(2)L](+/2+). The resulting unsaturated 32e(-) dications represent the closest approach to modeling the highly electrophilic Ni-SI(a) state. In the case of L = PPh(2) (2-pyridyl), chelation of this ligand accompanies the second oxidation.     

Evidence for the formation of terminal hydrides by protonation of an asymmetric iron hydrogenase active site mimic

Treatment of [Fe2(mu-pdt)(CO)6] [pdt=S(CH2)3S] with dppe (Ph2PCH2CH2PPh2) in refluxing toluene affords the asymmetric complex [Fe2(mu-pdt)(CO)4(dppe)] (1). Protonation of 1 with HBF4-Et2O in CH2Cl2 gives at room temperature the mu-hydrido derivative [Fe2(mu-pdt)(CO)4(dppe)(mu-H)](BF4) (2). Monitoring the reaction by 1H, 31P, and 13C NMR at low temperature reveals unambiguously that the process of the protonation of 1 implies terminal hydride intermediates.     

Pd2L2 metallacycles as molecular containers for small molecules

M(2)L(2) type metallacyclic complexes, [Pd(2)(L1)(2)Cl(4)]·1.5CH(2)Cl(2) (1), [Pd(2)(L1)(2)Cl(4)]·2CHCl(3) (2), [Pd(2)(L2)(2)Cl(4)]·2CH(2)Cl(2)·2CH(3)CN (3), [Pd(2)(L2)(2)Cl(4)]·2CHCl(3)·2CH(3)CN (4) and [Pd(2)(L3)(2)Cl(4)]·CH(2)Cl(2)·2CH(3)CN (5), have been prepared from three semi-rigid benzimidazol or benzotriazol ligands, 1,4-bis(benzimidazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene (L1), 1,4-bis(5,6-dimethylbenzimidazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene (L2) and 1,4-bis(benzotriazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene (L3). All the complexes were structurally characterized by single-crystal X-ray diffraction and the phase purity was confirmed by powder X-ray diffraction (PXRD) measurements. The solution structure of representative complex 1 was studied by (1)H NMR titration and ESI mass spectroscopy. The thermal stability and guest-exchange properties of 1, 3 and 4 were investigated, revealing that the Pd(2)L(2) metallacycles can act as a selective receptor for CH(2)Cl(2) or CHCl(3) guest molecules. The catalytic activity of 1 in Suzuki-Miyaura coupling reaction was also studied and 1 could be recycled at least 5 times under heterogeneous conditions, indicative of a potential self-supported catalyst.     

Infrared and laser Raman studies of [Ni(II)(dppe)Cl2] and [Co(III)(dppe)2Cl2]PF6 (dppe = 1,2-bis(diphenylphosphino)ethane)

Infrared and laser Raman spectral investigations of [Ni(II)(dppe)Cl2] and [Co(III)(dppe)2Cl2]PF6 have been made to determine the conformation and nature of bonding in Ni(II) and Co(III) dppe complexes. The stereochemistry of the two forms of these complexes has been confirmed. The role of steric interferences in cis-Planar [Ni(II)(dppe)Cl2] complex is interpreted in terms of reduction in symmetry upon coordination. The strong trans influence of the chelating dppe ligand is observed in the [Co(III)(dppe)2Cl2]PF6 complex. Both complexes exhibit the effect of crystalline field on molecular vibrations. The Fermi resonance overtone is also observed in these complexes.     

Preparation of benzyne complexes of group 10 metals by intramolecular suzuki coupling of ortho-metalated phenylboronic esters: molecular structure of the first benzyne-palladium(0) complex

A series of nickel(II) and palladium(II) aryl complexes substituted in the ortho position of the aromatic ring by a (pinacolato)boronic ester group, [MBr[o-C(6)H(4)B(pin)]L(2)] (M = Ni, L(2) = 2PPh(3) (2a), 2PCy(3) (2b), 2PEt(3) (2c), dcpe (2d), dppe (2e), and dppb (2f); M = Pd, L(2) = 2PPh(3) (3a), 2PCy(3) (3b), and dcpe (3d)), has been prepared. Many of these complexes react readily with KO(t)Bu to form the corresponding benzyne complexes [M(eta(2)-C(6)H(4))L(2)] (M = Ni, L(2) = 2PPh(3) (4a), 2PCy(3) (4b), 2PEt(3) (4c), dcpe (4d); M = Pd, L(2) = 2PCy(3) (5b)). This reaction can be regarded as an intramolecular version of a Suzuki cross-coupling reaction, the driving force for which may be the steric interaction between the boronic ester group and the phosphine ligands present in the precursors 2 and 3. Complex 3d also reacts with KO(t)Bu, but in this case disproportionation of the initially formed eta(2)-C(6)H(4) complex (5d) leads to a 1:1 mixture of a novel dinuclear palladium(I) complex, [(dcpe)Pd(mu(2)-C(6)H(4))Pd(dcpe)] (6), and a 2,2'-biphenyldiyl complex, [Pd(2,2'-C(6)H(4)C(6)H(4))(dcpe)] (7d). Complexes 2a, 3b, 3d, 4b, 5b, 6, and 7d have been structurally characterized by X-ray diffraction; complex 5b is the first example of an isolated benzyne-palladium(0) species.     

Activation of 1, 2‐Dihydrodisilanes at Platinum(0) Phosphine Complexes: Syntheses and Structures of Bissilyl Platinum(II) Complexes

The bissilyl complexes 3 – 6 were synthesized by reactions of the platinum(0) complexes [Pt(η²‐C₂H₄)(diphos)] ( 1 : diphos = dppe; 2 : diphos = dcpe) with the disilanes 1, 1,2, 2‐tetramethyldisilane and 1, 1,2, 2‐tetraphenyldisilane via Si–Si bond activation. The molecular structures of 4 and 5 in the solid state are reported. The reaction of 2 with HPh₂SiSiPh₂H led to the immediate formation of the hydrido disilanyl complex [Pt(H)(SiPh₂SiPh₂H)(dcpe)] ( 7 ), which converts slowly into the bissilyl complex [Pt(SiHPh₂)₂(dcpe)] ( 6 ). The latter was reported before to be a η²‐disilene complex.     

Palladium catalysed alkyne hydrogenation and oligomerisation: a parahydrogen based NMR investigation

The role phosphine ligands play in the palladium(ii)-bis-phosphine-hydride cation catalysed hydrogenation of diphenylacetylene is explored through a PHIP (parahydrogen induced polarization) NMR study. The precursors Pd(LL')(OTf)(2) () [LL' = dcpe (PCy(2)CH(2)CH(2)PCy(2)), dppe, dppm, dppp, cppe (PCy(2)CH(2)CH(2)PPh(2))] are used. Alkyl palladium intermediates of the type [Pd(LL')(CHPhCH(2)Ph)](OTf) ( and ) are detected and demonstrated to play an active role in hydrogenation catalysis. Magnetization transfer experiments reveal chemical exchange from the alpha-H of the alkyl ligand of (LL' = dcpe) and linkage isomer ' (LL' = cppe) into trans-stilbene on the NMR timescale. Activation parameters (DeltaH( not equal) and DeltaS( not equal)) for this transformation have been determined. These experiments, coupled with GC/MS data, indicate that the catalytic activity is greater in methanol, where it follows the order: dcpe > cppe > dppp > dppe > dppm, than in CD(2)Cl(2). All five of the phosphine systems described are less active than those based on bcope [where bcope is (C(8)H(14))PCH(2)-CH(2)P(C(8)H(14))] and (t)bucope [where (t)bucope is (C(8)H(14))PC(6)H(4)CH(2)P((t)Bu)(2)]. cis, cis-1,2,3,4-Tetraphenyl-buta-1,3-diene is detected as a minor reaction product with Pd(LL')(PhCH-CHPh-CPh[double bond, length as m-dash]CHPh)(+) () also being shown to play a role in the alkyne dimerisation step.     

Mononuclear nickel(III) complexes [Ni(III)(OR)(P(C6H3-3-SiMe3-2-S)3)](-) (R = Me, Ph) containing the terminal alkoxide ligand: relevance to the nickel site of oxidized-form [NiFe] hydrogenases

The unprecedented nickel(III) thiolate [Ni (III)(OR)(P(C 6H 3-3-SiMe 3-2-S) 3)] (-) [R = Ph ( 1), Me ( 3)] containing the terminal Ni (III)-OR bond, characterized by UV-vis, electron paramagnetic resonance, cyclic voltammetry, and single-crystal X-ray diffraction, were isolated from the reaction of [Ni (III)(Cl)(P(C 6H 3-3-SiMe 3-2-S) 3)] (-) with 3 equiv of [Na][OPh] in tetrahydrofuran (THF)-CH 3CN and the reaction of complex 1 with 1 equiv of [Bu 4N][OMe] in THF-CH 3OH, respectively. Interestingly, the addition of complex 1 into the THF-CH 3OH solution of [Me 4N][OH] also yielded complex 3. In contrast to the inertness of complex [Ni (III)(Cl)(P(C 6H 3-3-SiMe 3-2-S) 3)] (-) toward 1 equiv of [Na][OPh], the addition of 1 equiv of [Na][OMe] into a THF-CH 3CN solution of [Ni (III)(Cl)(P(C 6H 3-3-SiMe 3-2-S) 3)] (-) yielded the known [Ni (III)(CH 2CN)(P(C 6H 3-3-SiMe 3-2-S) 3)] (-) ( 4). At 77 K, complexes 1 and 3 exhibit a rhombic signal with g values of 2.31, 2.09, and 2.00 and of 2.28, 2.04, and 2.00, respectively, the characteristic g values of the known trigonal-bipyramidal Ni (III) [Ni (III)(L)(P(C 6H 3-3-SiMe 3-2-S) 3)] (-) (L = SePh, SEt, Cl) complexes. Compared to complexes [Ni (III)(EPh)(P(C 6H 3-3-SiMe 3-2-S) 3)] (-) [E = S ( 2), Se] dominated by one intense absorption band at 592 and 590 nm, respectively, the electronic spectrum of complex 1 coordinated by the less electron-donating phenoxide ligand displays a red shift to 603 nm. In a comparison of the Ni (III)-OMe bond length of 1.885(2) A found in complex 3, the longer Ni (III)-OPh bond distance of 1.910(3) A found in complex 1 may be attributed to the absence of sigma and pi donation from the [OPh]-coordinated ligand to the Ni (III) center.     

Diphosphines possessing electronically different donor groups: synthesis and coordination chemistry of the unsymmetrical Di(N-pyrrolyl)phosphino-functionalized dppm analogue Ph2PCH2P(NC4H4)2

The unsymmetrical diphosphinomethane ligand Ph(2)PCH(2)P(NC(4)H(4))(2) L has been prepared from the reaction of Ph(2)PCH(2)Li with PCl(NC(4)H(4))(2). The diphenylphosphino group can be selectively oxidized with sulfur to give Ph(2)P(S)CH(2)P(NC(4)H(4))(2) 1. The reaction of L with [MCl(2)(cod)] (M = Pd, Pt) gives the chelate complexes [MCl(2)(L-kappa(2)P,P')] (2, M = Pd; 3, M = Pt) in which the M-P bond to the di(N-pyrrolyl)phosphino group is shorter than that to the corresponding diphenylphosphino group. However, the shorter Pd-P bond is cleaved on reaction of 2 with an additional 1 equiv of L to give [PdCl(2)(L-kappa(1)P)(2)] 4. Complex 4 reacts with [PdCl(2)(cod)] to regenerate 2, and with [Pd(2)(dba)(3)].CHCl(3) to give the palladium(I) dimer [Pd(2)Cl(2)(mu-L)(2)] 5, which exists in solution and the solid state as a 1:1 mixture of head-to-head (HH) and head-to-tail (HT) isomers. The palladium(II) dimer [Pd(2)Cl(2)(CH(3))(2)(mu-L)(2)] 6, formed by the reaction of [PdCl(CH(3))(cod)] with L, also exists in solution as a mixture of HH and HT isomers, although in this case the HT isomer prevails at low temperature and crystallizes preferentially. Complex 6 reacts with TlPF(6) to give the A-frame complex [Pd(2)(CH(3))(2)(mu-Cl)(mu-L)(2)]PF(6) 7. The reaction of L with [RuCp*(mu(3)-Cl)](4) leads to the dimer [Ru(2)Cp*(2)(mu-Cl)(2)(mu-L)] 8, for which the enthalpy of reaction has been measured. The reaction of L with [Rh(mu-Cl)(cod)](2) gives a mixture of compounds from which the dimer [Rh(2)(mu-Cl)(cod)(2)(mu-L)]PF(6) 9 can be isolated. The crystal structures of 2.CHCl(3), 3.CH(2)Cl(2), 4, 5.(1)/(4)CH(2)Cl(2), 6, 7.2CH(2)Cl(2), 8, and 9.CH(2)Cl(2) are reported.     

Novel access to carbodiphosphoranes in the coordination sphere of group 10 metals: template synthesis and protonation of PCP pincer carbodiphosphorane complexes of C(dppm)2

In a novel template synthesis of carbodiphosphoranes (CDPs), the phosphine functionalized CDP ligand C(dppm)(2) (dppm = Ph(2)PCH(2)PPh(2)) is formed in the coordination sphere of group 10 metals from CS(2) and 4 equivalents of dppm. The products are the PCP pincer complexes [M(Cl)(C(dppm)(2)-κ3P,C,P)]Cl (M = Ni, Pd, Pt) and 2 equivalents of dppmS. The compound C(dppm)(2), which is composed of a divalent carbon atom and two dppm subunits, represents a new PCP-type pincer ligand with the formally neutral carbon Lewis base of the CDP functionality as the central carbon. Treatment of [M(Cl)(C(dppm)(2)-κ3P,C,P)]Cl (M = Pd, Pt) with hydrochloric acid results in protonation at the CDP carbon atom and the formation of the PCP pincer complexes [M(Cl)(CH(dppm)(2)-κ3P,C,P)]Cl(2) (M = Pd, Pt). The PCP pincer ligand [CH(dppm)(2)](+) involves a formally cationic central carbon donor. The reaction of [Ni(Cl)(C(dppm)(2)-κ3P,C,P)]Cl with HCl leads to the extrusion of NiCl(2) and formation of the diprotonated CDP compound [CH(2)(dppm)(2)]Cl(2), from which the monoprotonated conjugate base [CH(dppm)(2)]Cl is obtained upon addition of bases, such as NH(3). The crystal structures of [M(Cl)(C(dppm)(2)-κ3P,C,P)]Cl (M = Ni, Pd, Pt), [Ni(Cl)(C(dppm)(2)-κ3P,C,P)](2)[NiCl(4)], [M(Cl)(CH(dppm)(2)-κ3P,C,P)]Cl(2) (M = Pd, Pt) as well as [CH(2)(dppm)(2)]Cl(2) and [CH(dppm)(2)]Cl are presented. A comparison of the solid state structures reveals interesting features, e.g. infinite supramolecular networks mediated by C-H···Cl hydrogen bond interactions and an unexpected loss of molecular symmetry upon protonation in the complexes [M(CH(dppm)(2)-κ3P,C,P)(Cl)]Cl(2) (M = Pd, Pt) as a result of the flexible ligand backbone. Additionally the new compounds were characterized comprehensively in solution by multinuclear (31)P, (13)C and (1)H NMR spectroscopy: Several spectroscopic parameters show a striking variability in particular regarding the carbodiphosphorane functionality. Furthermore the compound [Ni(Cl)(C(dppm)(2)-κ3P,C,P)]Cl was examined by cyclic voltammetry (CV) and could be shown to display quasi-reversible oxidative as well as reductive behaviour.     

Influence of the terminal ligands on the redox properties of the {Pt2(mu-S)2} core in [Pt2(Ph2X(CH2)2XPh2)2(mu-S)2] (X = P or As) complexes and on their reactivity towards metal centres, protic acids and organic electrophiles

In order to explore possible ways for modulating the unusually rich chemistry shown by complexes of formula [L2Pt(mu-S)2PtL2] we have studied the influence of the nature of the terminal ligand L on the chemical properties of the {Pt2(mu-S)2} core. The systematic study we now report allows comparison of the behaviour of [Pt2(dpae)2(mu-S)2](dpae = Ph2As(CH2)2AsPh2) (1) with the already reported analogue [Pt2(dppe)2(mu-S)2](dppe = Ph2P(CH2)2PPh2). Complex 1 as well as the corresponding multimetallic derivatives [Pt(dpae){Pt2(dpae)2(mu-S)2}](BPh4)2 2, [M{Pt2(dpae)2(mu-S)2}2]X2 (M = Cu(II), X = BF4 3; M = Zn(II), X = BPh4 4; M = Cd(II), X = ClO4 5; M = Hg(II), X = Cl 6 or X2 = Cl(1.5)[HCl2](0.5) 6') have been characterized in the solid phase and in solution. Comparison of structural parameters of 1 and 3-6' with those of the corresponding phosphine analogues, together with the results of the electrochemical study for 1, allow us to conclude that replacement of dppe by dpae causes a decrease in basicity of the {Pt2(mu-S)2} core. The study of the reactivity of 1 towards CH2Cl2 and protic acids has led to the structural characterization of [Pt(dpae)(S2CH2)] 9 and [PtCl2(dpae)] 10. Moreover, comparison with the reactivity of [Pt2(dppe)2(mu-S)2] indicates that the stability of the intermediate species as well as the nature of the final products in both multistep reactions are sensitive to the nature of the terminal ligand.     

Nickel(II) complexes of bidentate N-heterocyclic carbene/phosphine ligands: efficient catalysts for Suzuki coupling of aryl chlorides

Nickel(II) complexes of bidentate N-heterocyclic carbene (NHC)/phosphane ligand L were prepared and structurally characterized. Unlike palladium, which forms [PdCl(2)(L)], the stable nickel product isolated is the ionic [Ni(L)(2)]Cl(2). These Ni(II) complexes are highly robust in air. Among different N-substituents on the ligand framework, the nickel complex of ligand L bearing N-1-naphthylmethyl groups (2 a) is a highly effective catalyst for Suzuki cross-coupling between phenylboronic acid and a range of aryl halides, including unreactive aryl chlorides. The activities of 2 a are largely superior to those of other reported nickel NHC complexes and their palladium counterparts. Unlike the previously reported [NiCl(2)(dppe)] (dppe=1,2-bis(diphenylphosphino)ethane), 2 a can effectively catalyze the cross-coupling reaction without the need for a catalytic amount of PPh(3), and this suggests that the PPh(2) functionality of hybrid NHC ligand L can partially take on the role of free PPh(3). However, for unreactive aryl chlorides at low catalyst loading, the presence of PPh(3) accelerates the reaction.     

Synthesis of Ge2NiS4 clusters and the thermal transformation to a Ge4Ni6S12 cluster

The reaction of Ni(dppe)Cl2 and syn-[DmpGe(SLi)(mu-S)2Ge(SLi)Dmp] prepared in situ from syn-[DmpGe(SH)(mu-S)2Ge(SH)Dmp] (1) and n-BuLi (2 equiv) afforded the Ge2NiS4 cluster, [DmpGe(mu-S)]2(mu-S)2Ni(dppe) (2) (Dmp = 2,6-dimesitylphenyl). The nickel in 2 assumes a slightly distorted square planar geometry. However, another Ge2NiS4 cluster, [DmpGe(mu-S)]2(mu-S)2Ni(PPh3)2 (3) obtained from a similar reaction with Ni(PPh3)2Cl2, contains the nickel in a tetrahedron. When 3 was heated to 120 degrees C in toluene, a novel Ge4Ni6S12 cluster [DmpGe(mu-S)3]4Ni6 (5) was obtained. In cluster 5, six nickels form an octahedron with the nickels occupying its vertexes, and four DmpGeS3 units cap half of the trigonal faces.     

Synthesis and structure of CpMo(CO)(dppe)H and its oxidation by Ph3C+

The reaction of CpMo(CO)(dppe)Cl (dppe = Ph2PCH2CH2PPh2) with Na+[AlH2(OCH2CH2OCH3)2]- gives the molybdenum hydride complex CpMo(CO)(dppe)H, the structure of which was determined by X-ray crystallography. Electrochemical oxidation of CpMo(CO)(dppe)H in CH3CN is quasi-reversible, with the peak potential at -0.15 V (vs Fc/Fc+). The reaction of CpMo(CO)(dppe)H with 1 equiv of Ph3C+BF4- in CD3CN gives [CpMo(CO)(dppe)(NCCD3)]+ as the organometallic product, along with dihydrogen and Gomberg's dimer (which is formed by dimerization of Ph3C.). The proposed mechanism involves one-electron oxidation of CpMo(CO)(dppe)H by Ph3C+ to give the radical-cation complex [CpMo(CO)(dppe)H].+. Proton transfer from [CpMo(CO)(dppe)H].+ to CpMo(CO)(dppe)H, loss of dihydrogen from [CpMo(CO)(dppe)(H)2]+, and oxidation of Cp(CO)(dppe)Mo. by Ph3C+ lead to the observed products. In the presence of an amine base, the stoichiometry changes, with 2 equiv of Ph3C+ being required for each 1 equiv of CpMo(CO)(dppe)H because of deprotonation of [CpMo(CO)(dppe)H].+ by the amine. Protonation of CpMo(CO)(dppe)H by HOTf provides the dihydride complex [CpMo(CO)(dppe)(H)2]+OTf-, which loses dihydrogen to generate CpMo(CO)(dppe)(OTf).     

Nickel complexes of bidentate [S2] and [SN] borato ligands

Nickel(II) complexes of the monoanionic borato ligands [Ph2B(CH2SCH3)2] (abbreviated Ph2Bt), [Ph2B(CH2S(t)Bu)2] (Ph2Bt(tBu)), [Ph2B(1-pyrazolyl)(CH2SCH3)], and [Ph2B(1-pyrazolyl)(CH2S(t)Bu)] have been prepared and characterized. While [Ph2Bt] formed the square planar homoleptic complex, [Ph2Bt]2Ni, the larger [S2] ligand with tert-butyl substituents, [Ph2BttBu], yielded an unexpected organometallic derivative, [Ph2Bt(tBu)]Ni(eta2-CH2SBut), resulting from B-C bond rupture. The analogous thiametallacycle derived from the [S3] ligand, [PhB(CH2S(t)Bu)3] (PhTt(tBu)), has been structurally authenticated (Schebler, P. J.; Mandimutsira, B. S.; Riordan, C. G.; Liable-Sands, L.; Incarvito, C. D.; Rheingold, A. L. J. Am. Chem. Soc. 2001, 123, 331). The [SN] borato ligands formed exclusively the cis stereoisomers upon reaction with Ni(II) sources, [Ph2B(1-pyrazolyl)(CH2SR)]2Ni. Analysis of the Ni(II/I) reduction potentials by cyclic voltammetry revealed a approximately 600 mV anodic shift upon replacement of two thioether donors ([Ph2Bt]2Ni) with two pyrazolyl donors ([Ph2B(1-pyrazolyl)(CH2SCH3)]2Ni) consistent with the all thioether environment stabilizing the lower oxidation state of nickel.     

Diverse evolution of [[Ph(2)P(CH(2))(n)PPh(2)]Pt(mu-S)(2)Pt[Ph(2)P(CH(2))(n)PPh(2)]] (n = 2, 3) metalloligands in CH(2)Cl(2)

The nucleophilicity of the [Pt(2)S(2)] core in [[Ph(2)P(CH(2))(n)PPh(2)]Pt(mu-S)(2)Pt[Ph(2)P(CH(2))(n)PPh(2)]] (n = 3, dppp (1); n = 2, dppe (2)) metalloligands toward the CH(2)Cl(2) solvent has been thoroughly studied. Complex 1, which has been obtained and characterized by X-ray diffraction, is structurally related to 2 and consists of dinuclear molecules with a hinged [Pt(2)S(2)] central ring. The reaction of 1 and 2 with CH(2)Cl(2) has been followed by means of (31)P, (1)H, and (13)C NMR, electrospray ionization mass spectrometry, and X-ray data. Although both reactions proceed at different rates, the first steps are common and lead to a mixture of the corresponding mononuclear complexes [Pt[Ph(2)P(CH(2))(n)PPh(2)](S(2)CH(2))], n = 3 (7), 2 (8), and [Pt[Ph(2)P(CH(2))(n)PPh(2)]Cl(2)], n = 3 (9), 2 (10). Theoretical calculations give support to the proposed pathway for the disintegration process of the [Pt(2)S(2)] ring. Only in the case of 1, the reaction proceeds further yielding [Pt(2)(dppp)(2)[mu-(SCH(2)SCH(2)S)-S,S']]Cl(2) (11). To confirm the sequence of the reactions leading from 1 and 2 to the final products 9 and 11 or 8 and 10, respectively, complexes 7, 8, and 11 have been synthesized and structurally characterized. Additional experiments have allowed elucidation of the reaction mechanism involved from 7 to 11, and thus, the origin of the CH(2) groups that participate in the expansion of the (SCH(2)S)(2-) ligand in 7 to afford the bridging (SCH(2)SCH(2)S)(2-) ligand in 11 has been established. The X-ray structure of 11 is totally unprecedented and consists of a hinged [(dppp)Pt(mu-S)(2)Pt(dppp)] core capped by a CH(2)SCH(2) fragment.