Stoichiometric and catalytic reactivity of the N-heterocyclic carbene ruthenium hydride complexes [Ru(NHC)(L)(CO)HCl] and [Ru(NHC)(L)(CO)H(eta2-BH4)] (L=NHC, PPh3)

Thermolysis of [Ru(AsPh3)3(CO)H2] with the N-aryl heterocyclic carbenes (NHCs) IMes (1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) or the adduct SIPr.(C6F5)H (SIPr=1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene), followed by addition of CH2Cl2, affords the coordinatively unsaturated ruthenium hydride chloride complexes [Ru(NHC)2(CO)HCl] (NHC=IMes , IPr , SIPr ). These react with CO at room temperature to yield the corresponding 18-electron dicarbonyl complexes . Reduction of and [Ru(IMes)(PPh3)(CO)HCl] () with NaBH4 yields the isolable borohydride complexes [Ru(NHC)(L)(CO)H(eta2-BH4)] (, L=NHC, PPh3). Both the bis-IMes complex and the IMes-PPh3 species react with CO at low temperature to give the eta1-borohydride species [Ru(IMes)(L)(CO)2H(eta1-BH4)] (L=IMes , PPh3), which can be spectroscopically characterised. Upon warming to room temperature, further reaction with CO takes place to afford initially [Ru(IMes)(L)(CO)2H2] (L=IMes, L=PPh3) and, ultimately, [Ru(IMes)(L)(CO)3] (L=IMes , L=PPh3). Both and lose BH3 on addition of PMe2Ph to give [Ru(IMes)(L)(L')(CO)H2](L=L'=PMe2Ph; L=PPh3, L'=PMe2Ph). Compounds and have been tested as catalysts for the hydrogenation of aromatic ketones in the presence of (i)PrOH and H2. For the reduction of acetophenone, catalytic activity varies with the NHC present, decreasing in the order IPr>IMes>SIMes.     

Divalent carbon(0) chemistry, part 2: Protonation and complexes with main group and transition metal Lewis acids.

Quantum-chemical calculations with DFT (BP86) and ab initio methods (MP2, SCS-MP2) were carried out for protonated and diprotonated compounds N-H(+) and N-(H(+))(2) and for the complexes N-BH(3), N-(BH(3))(2), N-CO(2), N-(CO(2))(2), N-W(CO)(5), N-Ni(CO)(3) and N-Ni(CO)(2) where N=C(PH(3))(2) (1), C(PMe(3))(2) (2), C(PPh(3))(2) (3), C(PPh(3))(CO) (4), C(CO)(2) (5), C(NHC(H))(2) (6), C(NHC(Me))(2) (7) (Me(2)N)(2)C==C==C(NMe(2))(2) (8) and NHC (9) (NHC(H)=N-heterocyclic carbene, NHC(Me)=N-substituted N-heterocyclic carbene). Compounds 1-4 and 6-9 are very strong electron donors, and this is manifested in calculated protonation energies that reach values of up to 300 kcal mol(-1) for 7 and in very high bond strengths of the donor-acceptor complexes. The electronic structure of the compounds was analyzed with charge- and energy-partitioning methods. The calculations show that the experimentally known compounds 2-5 and 8 chemically behave like molecules L(2)C which have two L-->C donor-acceptor bonds and a carbon atom with two electron lone pairs. The behavior is not directly obvious when the linear structures of carbon suboxide and tetraaminoallenes are considered. They only come to the fore on reaction with strong electron-pair acceptors. The calculations predict that single and double protonation of 5 and 8 take place at the central carbon atom, where the negative charge increases upon subsequent protonation. The hitherto experimentally unknown carbodicarbenes 6 and 7 are predicted to be even stronger Lewis bases than the carbodiphosphoranes 1-3.     

Thermodynamic hydride donor abilities of [HW(CO)4L]- complexes (L = PPh3, P(OMe)3, CO) and their reactions with [C5Me5Re(PMe3)(NO)(CO)]+

The thermodynamic hydride donor abilities of [HW(CO)(5)](-) (40 kcal/mol), [HW(CO)(4)P(OMe(3))](-) (37 kcal/mol), and [HW(CO)(4)(PPh(3))](-) (36 kcal/mol) have been measured in acetonitrile by either equilibrium or calorimetric methods. The hydride donor abilities of these complexes are compared with other complexes for which similar thermodynamic measurements have been made. [HW(CO)(5)](-), [HW(CO)(4)P(OMe(3))](-), and [HW(CO)(4)(PPh(3))](-) all react rapidly with [CpRe(PMe(3))(NO)(CO)](+) to form dinuclear intermediates with bridging formyl ligands. These intermediates slowly form [CpRe(PMe(3))(NO)(CHO)] and [W(CO)(4)(L)(CH(3)CN)]. The structure of cis-[HW(CO)(4)(PPh(3))](-) has been determined and has the expected octahedral structure. The hydride ligand bends away from the CO ligand trans to PPh(3) and toward PPh(3).     

Stabilisation of iridium(III) fluoride complexes with NHCs

The first neutral, [IrClF(2)(NHC)(COD)] and [IrClF(2)(CO)(2)(NHC)] (NHC = IMes, IPr), and cationic, [IrF(2)py(IMes)(COD)][BF(4)] and [IrF(2)L(CO)(2)(NHC)][BF(4)] (NHC = IMes, L = PPh(2)Et, PPh(2)CCPh, py; NHC = IPr, L = py), NHC iridium(III) fluoride complexes, have been synthesised by the xenon difluoride oxidation of iridium(I) substrates. The stereochemistries of these iridium(III) complexes have been confirmed by multinuclear NMR spectroscopy in solution and no examples of fluoride-trans-NHC arrangements were observed. Throughout, CO was found to be a better co-ligand for the stabilisation of the iridium(III) fluoride complexes than COD. Attempts to generate neutral trifluoroiridium(III) complexes, [IrF(3)(CO)(NHC)], via the ligand substitution reaction of [IrF(3)(CO)(3)] with the free NHCs were unsuccessful.     

Carbodiphosphorane analogues E(PPh3)2 with E=C-Pb: a theoretical study with implications for ligand design

Quantum-chemical calculations at the BP86/TZVPP level have been carried out for the heavy Group 14 homologues of carbodiphosphorane E(PPh(3))(2), where E=Si, Ge, Sn, Pb, which are experimentally unknown so far. The results of the theoretical investigation suggest that the tetrelediphosphoranes E(PPh(3))(2) (1E) are stable compounds that could become isolated in a condensed phase. The molecules possess donor-acceptor bonds Ph(3)P→E←PPh(3) to a bare tetrele atom E, which retains its four valence electrons as two electron lone pairs. The analysis of the bonding situation and the calculation of the chemical reactivity indicate that the molecules 1E belong to the class of divalent E(0) compounds (ylidones). All molecules 1C-1Pb have very large first but also very large second proton affinities, which distinguishes them from the N-heterocyclic carbene homologues, in which the donor atom is a divalent E(II) species that possesses only one electron lone pair. Compounds 1E are powerful double donors that strongly bind Lewis acids such as BH(3) and AuCl in the complexes 1E(BH(3))(n) and 1E(AuCl)(n) (n=1, 2). The bond dissociation energies (BDEs) of the second BH(3) and AuCl molecules are only slightly less than the BDE of the first BH(3) and AuCl. The results of this work are a challenge for experimentalists.     

Electronic and steric effects on molecular dihydrogen activation in [CpOsH4(L)]+ (L = PPh3, AsPh3, and PCy3)

Single-crystal neutron diffraction, inelastic neutron scattering, and density functional calculations provide experimental and theoretical analyses of the nature of the osmium-bound, "elongated" dihydrogen ligands in [Cp*OsH(4)(L)][BF(4)] complexes (L = PPh(3), AsPh(3), or PCy(3)). The PPh(3) and AsPh(3) complexes clearly contain one dihydrogen ligand and two terminal hydrides; the H(2) ligand is transoid to the Lewis base, and the H-H vector connecting the central two hydrogen atoms lies parallel to the Ct-Os-L plane (Ct = centroid of Cp* ring). In contrast, in the PCy(3) complex the H-H vector is perpendicular to the Ct-Os-L plane. Not only the orientation of the central two hydrogen atoms but also the H-H bond length between them depends significantly on the nature of L: the H...H distance determined from neutron diffraction is 1.01(1) and 1.08(1) A for L = PPh(3) and AsPh(3), respectively, but 1.31(3) A for L = PCy(3). Density functional calculations show that there is a delicate balance of electronic and steric influences created by the L ligand that change the molecular geometry (steric interactions between the Cp* and L groups most importantly change the Ct-Os-L angle), changing the relative energy of the Os 5d orbitals, which in turn govern the H-H distance, preferred H-H orientation, and rotational dynamics of the elongated dihydrogen ligand. The geometry of the dihydrogen ligand is further tuned by interactions with the BF(4)(-) counterion. The rotational barrier of the bound H(2) ligand in [Cp*OsH(4)(PPh(3))](+), determined experimentally (3.1 kcal mol(-)(1)) from inelastic neutron scattering experiments, is in reasonable agreement with the B3LYP calculated H(2) rotational barrier (2.5 kcal mol(-)(1)).     

The first acetimine gold(I) and gold(III) complexes and the first acetonine complexes

Ketimino(phosphino)gold(I) complexes of the type [Au[NR=C(Me)R']L]X (X = ClO4, R = H, L = PPh3, R'=Me (la), Et (2a); L=PAr3 (Ar=C6H4OMe-4), R'=Me (1b), Et (2b); L=PPh3, R=R'=Me (3); X= CF3SO3 (OTf), L=PPh3, R=R'=Me (3'); R=Ar, R'=Me (4)) have been prepared from [Au(acac)L] (acac = acetyl acetonate) and ammonium salts [RNH3]X dissolved in the appropriate ketone MeC(O)R'. Complexes [Au(NH=CMe2)2]X (X = C1O4 (6), OTf (6')) were obtained from solutions of [Au(NH3)2]X in acetone. The reaction of 6 with PPN[AuCl2] or with PhICl2 gave [AuCl(NH=CMe2)] (7) or [AuCI2(NH=CMe2)2]ClO4 (8), respectively. Complex 7 was oxidized with PhICl2 to give [AuCl3(NH=CMe2)] (9). The reaction of [AuCl(tht)] (tht = tetrahydrothiophene), NaClO4, and ammonia in acetone gave [Au(acetonine)2]ClO4 (10) (acetonine = 2,2,4,4,6-pentamethyl-2,3,4,5-tetrahydropyrimidine) which reacted with PPh3 or with PPN[AuCl2] to give [Au(PPh3)(acetonine)]ClO4 (11) or [AuCl(acetonine)] (12), respectively. Complex 11 reacts with [Au(PPh3)(Me2CO)]ClO4 to give [(AuPPh3)2(mu-acetonine)](ClO4)2 (13). The reaction of AgClO4 with acetonine gave [Ag(acetonine)(OClO3)] (14). The crystal structures of [Au(NH2Ar)(PPh3)]OTf (5), 6' and 10 have been determined.     

Distinguishing carbones from allenes by complexation to AuCl

Quantum chemical calculations have been performed for the dicoordinated carbon compounds C(PPh(3))(2), C(NHC(Me))(2), R(2) C=C=CR(2) (R = H, F, NMe(2)), C(3)O(2), C(CN)(2)(-) and N-methyl-substituted N-heterocyclic carbene (NHC(Me)). The geometries of the complexes in which the dicoordinated carbon molecules bind as ligands to one and two AuCl moieties have been optimized and the strength and nature of the metal-ligand interactions in the mono- and diaurated complexes were investigated by means of energy decomposition analysis. The goal of the study is to elucidate the differences in the chemical behavior between carbones, allenes and carbenes. The results show that carbones bind one and two AuCl species in η(1) fashion, whereas allenes bind them in η(2) fashion. Compounds with latent divalent carbon(0) character can coordinate in more than one way, with the dominant mode indicating the degree of carbone or allene character. The calculated structures of the mono- and diaurated tetraaminoallenes (TAAs) reveal that TAAs exhibit a chameleon-like behavior: The bonding situation in the equilibrium structure is best described as allene [(R(2)N)(2)]C=C=C[(NR(2))(2)] in which the central carbon atom is a tetravalent C(IV) species, but the reactivity suggests that TAAs should be considered as divalent C(0) compounds C{C[(NR(2))(2)]}(2), that is, as "hidden" carbones. Carbon suboxide binds one AuCl preferentially in the η(1) mode, whereas the equilibrium structures of the η(1)- and η(2)-bonded diaurated complex are energetically nearly degenerate. The doubly negatively charged isoelectronic carbone C(CN)(2)(2-) binds one and two AuCl very strongly in characteristic η(1) fashion. The N-heterocyclic carbene complex, [NHC(Me)(AuCl)], possesses a high bond dissociation energy (BDE) for the splitting off of AuCl. The diaurated NHC adduct, [NHC(Me)(AuCl)(2)], has two η(1)-bonded AuCl moieties that exhibit aurophilic attraction, which yield a moderate bond strength that might be large enough for synthesizing the complex. The BDE for the second AuCl in [NHC(Me)(AuCl)(2)] is clearly smaller than the values for the second AuCl in doubly aurated carbone complexes.     

Experimental and computational investigation of C-N bond activation in ruthenium N-heterocyclic carbene complexes

A combination of experimental studies and density functional theory calculations is used to study C-N bond activation in a series of ruthenium N-alkyl-substituted heterocyclic carbene (NHC) complexes. These show that prior C-H activation of the NHC ligand renders the system susceptible to irreversible C-N activation. In the presence of a source of HCl, C-H activated Ru(I(i)Pr(2)Me(2))'(PPh(3))(2)(CO)H (1, I(i)Pr(2)Me(2) = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) reacts to give Ru(I(i)PrHMe(2))(PPh(3))(2)(CO)HCl (2, I(i)PrHMe(2) = 1-isopropyl-4,5-dimethylimidazol-2-ylidene) and propene. The mechanism involves (i) isomerization to a trans-phosphine isomer, 1c, in which hydride is trans to the metalated alkyl arm, (ii) C-N cleavage to give an intermediate propene complex with a C2-metalated imidazole ligand, and (iii) N-protonation and propene/Cl(-) substitution to give 2. The overall computed activation barrier (ΔE(++)(calcd)) corresponds to the isomerization/C-N cleavage process and has a value of +24.4 kcal/mol. C-N activation in 1c is promoted by the relief of electronic strain arising from the trans disposition of the high-trans-influence hydride and alkyl ligands. Experimental studies on analogues of 1 with different C4/C5 carbene backbone substituents (Ru(I(i)Pr(2)Ph(2))'(PPh(3))(2)(CO)H, Ru(I(i)Pr(2))'(PPh(3))(2)(CO)H) or different N-substituents (Ru(IEt(2)Me(2))'(PPh(3))(2)(CO)H) reveal that Ph substituents promote C-N activation. Calculations confirm that Ru(I(i)Pr(2)Ph(2))'(PPh(3))(2)(CO)H undergoes isomerization/C-N bond cleavage with a low barrier of only +21.4 kcal/mol. Larger N-alkyl groups also facilitate C-N bond activation (Ru(I(t)Bu(2)Me(2))'(PPh(3))(2)(CO)H, ΔE(++)(calcd) = +21.3 kcal/mol), and in this case the reaction is promoted by the formation of the more highly substituted 2-methylpropene.     

High-spin and low-spin iron(II) complexes with facially-coordinated borohydride ligands

Rare examples of monometallic high-spin and low-spin L3Fe(H3BH) complexes have been characterized, where the two L3 ligands are [TpPh2] and [PhBP3] ([TpPh2] = [HB(3,5-Ph2pz)3]- and [PhBP3] = [PhB(CH2PPh2)3]-). The structures are reported wherein the borohydride ligand is facially coordinated to the iron center in each complex. Density functional methods have been employed to explain the bonding in these unusual iron(II) centers. Despite the differences in spin states, short Fe-B distances are observed in both complexes and there is significant theoretical evidence to support a substantial bonding interaction between the iron and boron nuclei. In light of this interaction, we suggest that these complexes can be described as (L3)Fe(eta4-H3BH) complexes.     

Iridium(III) complexes formed by O-H and/Or C-H activation of 2-(arylazo)phenols

Reaction of 2-(arylazo)phenols with [Ir(PPh(3))(3)Cl] in refluxing ethanol in the presence of a base (NEt(3)) affords complexes of three different types, viz. [Ir(PPh(3))(2)(NO-R)(H)Cl] (R = OCH(3), CH(3), H, Cl and NO(2)), [Ir(PPh(3))(2)(NO-R)(H)(2)] and [Ir(PPh(3))(2)(CNO-R)(H)]. Structures of the [Ir(PPh(3))(2)(NO-Cl)(H)Cl], [Ir(PPh(3))(2)(NO-Cl)(H)(2)] and [Ir(PPh(3))(2)(CNO-Cl)(H)] complexes have been determined by X-ray crystallography. In the [Ir(PPh(3))(2)(NO-R)(H)Cl] and [Ir(PPh(3))(2)(NO-R)(H)(2)] complexes, the 2-(arylazo)phenolate ligands are coordinated to the metal center as monoanionic bidentate N,O-donors, whereas in the [Ir(PPh(3))(2)(CNO-R)(H)] complexes, they are coordinated to iridium as dianionic tridentate C,N,O-donors. In all three products formed in ethanol, the two PPh(3) ligands are trans. Reaction of 2-(arylazo)phenols with [Ir(PPh(3))(3)Cl] in refluxing toluene in the presence of NEt(3) affords complexes of two types, viz. [Ir(PPh(3))(2)(CNO-R)(H)] and [Ir(PPh(3))(2)(CNO-R)Cl]. Structure of the [Ir(PPh(3))(2)(CNO-Cl)Cl] complex has been determined by X-ray crystallography, and the 2-(arylazo)phenolate ligand is coordinated to the metal center as a dianionic tridentate C,N,O-donor and the two PPh(3) ligands are cis. All of the iridium(III) complexes show intense MLCT transitions in the visible region. Cyclic voltammetry shows an Ir(III)-Ir(IV) oxidation on the positive side of SCE and an Ir(III)-Ir(II) reduction on the negative side for all of the products.     

Synthesis of acetimino complexes of Pt(II) and Pt(IV) and the first heteronuclear mu-acetimido complexes

The reactions of [Ag(NH=CMe2)2]ClO4 with cis-[PtCl2L2] in a 1:1 molar ratio give cis-[PtCl(NH=CMe2)(PPh3)2]ClO4 (1cis) or cis-[PtCl(NH=CMe2)2(dmso)]ClO4 (2), and in 2:1 molar ratio, they produce [Pt(NH=CMe2)2L2](ClO4)2 [L = PPh3 (3), L2= tbbpy = 4,4'-di-tert-butyl-2,2'-dipyridyl (4)]. Complex 2 reacts with PPh3 (1:2) to give trans-[PtCl(NH=CMe2)(PPh3)2]ClO(4) (1trans). The two-step reaction of cis-[PtCl2(dmso)2], [Au(NH=CMe2)(PPh3)]ClO4, and PPh3 (1:1:1) gives [SP-4-3]-[PtCl(NH=CMe2)(dmso)(PPh3)]ClO4 (5). The reactions of complexes 2 and 4 with PhICl2 give the Pt(IV) derivatives [OC-6-13]-[PtCl3(NH=CMe2)(2)(dmso)]ClO4 (6) and [OC-6-13]-[PtCl2(NH=CMe2)2(dtbbpy)](ClO4)2 (7), respectively. Complexes 1cis and 1trans react with NaH and [AuCl(PPh3)] (1:10:1.2) to give cis- and trans-[PtCl{mu-N(AuPPh3)=CMe2}(PPh3)2]ClO4 (8cis and 8trans), respectively. The crystal structures of 4.0.5Et2O.0.5Me2CO and 6 have been determined; both exhibit pseudosymmetry.     

Gold(I) Complexes with the nido-Diphosphino Ligand [7,8-(Ph(2)P)(2)-7,8-C(2)B(9)H(10)](-). Preparation of the First Metallocarborane Complex of this Ligand. Crystal Structures of [Au{(PPh(2))(2)C(2)B(9)H(10)}(PPh(3))].CH(2)Cl(2) and [Au(2){(PPh(2))(2)C(2)B(9)H(10)}(2){(PPh(2))(2)(CH(2))(3)}].3Me(2)CO

The reaction of [AuCl(PR(3))] with [1,2-(Ph(2)P)(2)-1,2-C(2)B(10)H(10)] in refluxing ethanol proceeds with partial degradation (removal of a boron atom adjacent to carbon) of the closo species to give [Au{(PPh(2))(2)C(2)B(9)H(10)}(PR(3))] [PR(3) = PPh(3) (1), PPh(2)Me (2), PPh(2)(4-Me-C(6)H(4)) (3), P(4-Me-C(6)H(4))(3) (4), P(4-OMe-C(6)H(4))(3) (5)]. Similarly, the treatment of [Au(2)Cl(2)(&mgr;-P-P)] with [1,2-(Ph(2)P)(2)-1,2-C(2)B(10)H(10)] under the same conditions leads to the complexes [Au(2){(PPh(2))(2)C(2)B(9)H(10)}(2)(&mgr;-P-P)] [P-P = dppe = 1,2-bis(diphenylphosphino)ethane (6), dppp = 1,3-bis(diphenylphosphino)propane (7)], where the dppe or dppp ligands bridge two gold nido-diphosphine units. The reaction of 1 with NaH leads to removal of one proton, and further reaction with [Au(PPh(3))(tht)]ClO(4) gives the novel metallocarborane compound [Au(2){(PPh(2))(2)C(2)B(9)H(9)}(PPh(3))(2)] (8). The structure of complexes 1 and 7 have been established by X-ray diffraction. [Au{(PPh(2))(2)C(2)B(9)H(10)}(PPh(3))] (1) (dichloromethane solvate) crystallizes in the monoclinic space group P2(1)/c, with a = 17.326(3) ?, b = 20.688(3) ?, c = 13.442(2) ?, beta = 104.710(12) degrees, Z = 4, and T = -100 degrees C. [Au(2){(PPh(2))(2)C(2)B(9)H(10)}(2)(&mgr;-dppp)] (7) (acetone solvate) is triclinic, space group P&onemacr;, a = 13.432(3) ?, b = 18.888(3) ?, c = 20.021(3) ?, alpha = 78.56(2) degrees, beta = 72.02(2) degrees, gamma = 73.31(2) degrees, Z = 2, and T = -100 degrees C. In both complexes the gold atom exhibits trigonal planar geometry with the 7,8-bis(diphenylphosphino)-7,8-dicarba-nido-undecaborate(1-) acting as a chelating ligand.     

Stable gold(III) complexes with thiosemicarbazone derivatives

Novel thiosemicarbazonato complexes of gold(III) have been prepared from reactions of [Au(damp-C1,N)Cl2(damp- = 2-(N,N-dimethylaminomethyl)phenyl) or [NBu4][AuCl4] with 2-pyridineformamide thiosemicarbazones (HL). The thiosemicarbazones deprotonate and coordinate as mononegative, tridentate NNS ligands to gold to give [Au(Hdamp-C1)(L)]Cl2 or [AuCl(L)]Cl complexes. The organometallic damp- ligand is protonated during the reactions and the Au-N bond is cleaved. The [AuCl(L)]+ cations represent the first gold(III) complexes with thiourea derivatives which are not stabilised by an additional organometallic ligand. Reactions of [NBu4][AuX4](X = Cl, Br) with diphenylthiocarbazone (dithizone) result in reduction of the metal and the formation of gold(I) complexes of the composition [AuX(SCN4-3,4-Ph2)] where SCN4-3,4-Ph2 is 3,4-diphenyltetrazole thione which is formed from cyclisation of dithizone.     

Rhenium diazenide ternary complexes with dithiocarbamate ligands: towards new rhenium radiopharmaceuticals

The reaction of the mono-diazenide core, [ReCl2(NNC6H(4)-4-OCH3)(NCCH3)(PPh3)2], with four equivalents of the sodium or potassium salts of dithiocarbamate (dtc) ligands gives neutral complexes of the formula [Re(NNC6H(4)-4-OCH3)(dtc)2(PPh3)]. It is possible to use a wide range of dithiocarbamate ligands (S2CNRR') with a variety of R groups (R=R'=methyl, phenyl or ethyl; R=methyl, R'=phenyl and R=R'=morpholino). Substitution reactions with dtc ligands on the mono-diazenide derived from 2-hydrazinopyridine, [ReCl2(NNC5H4N)(PPh3)2, give the analogous complexes, [Re(NNC6H5N)(dtc)2(PPh3)]. The new complexes have been characterised by a combination of NMR spectroscopy, mass spectrometry and X-ray crystallography. Cyclic voltammetry measurements in dimethyl formamide show that the rhenium diazenido-dtc complexes undergo a quasi-reversible oxidation tentatively assigned to a ReIII/ReIV oxidation. Since the parent complex, [ReCl2(NNC6H(4)-4-R)(NCCH3)(PPh3)2] can be prepared directly from perrhenate and readily derivatised with dtc ligands these complexes have potential relevance to the development of new therapeutic rhenium radiopharmaceuticals.     

Syntheses and structures of pyrazolylmethane complexes of rhenium(III), (IV) and (V)

Reaction of [ReOCl3(PPh3)(2)] with HCpz(3) (pz = pyrazole) in dichloromethane leads to the formation of a new Re(iv) complex [ReCl3(HCpz3)]X (X=Cl, [ReO4]) with loss of the rhenium-oxo group. We also report a convenient, high-yield synthetic route to complexes of the type [ReOXn(L)](3-n)+ (X=Cl, Br, n = 2, 3) by the reaction of bis(pyrazolylmethane) and bis(pyrazolylacetate) ligands with [ReOCl3(PPh3)2]. Dinuclear complexes containing the O=Re-O-Re=O group were also isolated and structurally characterised. We have also investigated the reactions of these ligands with diazenide precursors and isolated and characterised complexes of the type [ReClx(N2Ph) (L)(PPh3)] (x = 1,2). The potential applications of these complexes as radiopharmaeuticals is discussed.     

Neutral gold complexes with tridentate SNS thiosemicarbazide ligands

Na[AuCl(4)]·2H(2)O reacts with tridentate thiosemicarbazide ligands, H(2)L1, derived from N-[N',N'-dialkylamino(thiocarbonyl)]benzimidoyl chloride and thiosemicarbazides under formation of air-stable, green [AuCl(L1)] complexes. The organic ligands coordinate in a planar SNS coordination mode. Small amounts of gold(I) complexes of the composition [AuCl(L3)] are formed as side-products, where L3 is an S-bonded 5-diethylamino-3-phenyl-1-thiocarbamoyl-1,2,4-triazole. The formation of the triazole L3 can be explained by the oxidation of H(2)L1 to an intermediate thiatriazine L2 by Au(3+), followed by a desulfurization reaction with ring contraction. The chloro ligands in the [AuCl(L1)] complexes can readily be replaced by other monoanionic ligands such as SCN(-) or CN(-) giving [Au(SCN)(L1)] or [Au(CN)(L1)] complexes. The complexes described in this paper represent the first examples of fully characterized neutral Gold(III) thiosemicarbazone complexes. All the [AuCl(L1)] compounds present a remarkable cell growth inhibition against human MCF-7 breast cancer cells. However, systematic variation of the alkyl groups in the N(4)-position of the thiosemicarbazone building blocks as well as the replacement of the chloride by thiocyanate ligands do not considerably influence the biological activity. On the other hand, the reduction of Au(III) to Au(I) leads to a considerable decrease of the cytotoxicity.     

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.     

Investigation of the stability of the M-H-B bond in borane sigma complexes [M(CO)5(eta1-BH2R.L)] and [CpMn(CO)2(eta1-BH2R.L)] (M=Cr, W; L=tertiary amine or phosphine): substituent and Lewis base effects

We investigated the influence of a substituent and a Lewis base on boron upon the thermodynamic stability of metal complexes of borane-Lewis base adducts, [M(CO)5(eta1-BH(2)R.L)] (M=Cr, W) and [CpMn(CO)2(eta1-BH2R.L)], where R=Cl, I, m-C6H4F, Ph, H, Me, Et; L=PMe3, PPh3, NMe3, quinuclidine. In these compounds, the stability of the metal-borane linkage was enhanced by increasing the electron-releasing ability of the substituent on boron. A stronger base L additionally stabilized the complexes. The strength of the borane-metal interaction is thus mainly ascribed to the electron donation from the BH sigma orbital to metal rather than the back-donation into the BH sigma* orbital. This result supports the bonding model for the B-H-M linkage in the borane complexes suggested by MO calculations, where the borane-to-metal electron donation is predominant while the metal back-donation into the BH sigma* orbital is negligible. Such a stability trend of the borane complexes makes a sharp contrast to that of many silane and dihydrogen complexes.     

Synthesis and dehydrocoupling reactivity of iron and ruthenium phosphine-borane complexes

The Fe and Ru phosphine-borane complexes CpM(CO)2PPh2 x BH3 (1, M = Fe, 4, M = Ru) were synthesized utilizing the reaction of the phosphine-borane anion Li[PPh2 x BH3] with the iodo complexes CpM(CO)2I. The Fe complex 1 reacted with PMe3 to yield CpFe(CO)(PMe3)(PPh2 x BH3) (2) and CpFe(PMe3)2(PPh2 x BH3) (3) whereas the Ru species 4 gave only CpRu(CO)(PMe3)(PPh2 x BH3) (5). The complexes 1-5 were characterized by 1H, 11B, 13C and 31P NMR spectroscopy, MS, IR and X-ray crystallography for 1 to 4, and EA for 1, 2 and 4. The reactivity of 1 and 4 towards PPh2H x BH3 was explored. Although no stoichiometric reactions were detected under mild conditions, both 1 and 4 were found to function as dehydrocoupling catalysts to afford Ph2PH x BH2 x PPh2 x BH3 in the melt at elevated temperature (120 degrees C). The carbonyl Fe2(CO)9 also functioned as a dehydrocoupling catalyst under similar conditions. Complex 1 and Fe2(CO)9 represent the first reported active Fe complexes for the catalytic dehydrocoupling of phosphine-borane adducts.