Preparation and redox properties of the complexes trans-[ReL2(dppe)2]BF4 (L = CO or isocyanide). Estimate of the oxidation potential of octahedral 18-electron complexes with 14-electron square planar metal centres and of related electrochemical parameters for derived 16-electron sites

The complexes trans-[ReL₂(dppe)₂]BF₄ (I, L = CO, CNR with R = Me,Bu^t, C₆H₄OMe-4, C₆H₄Me-4, C₆H₄Cl-4 and C₆H₃Cl₂-2,6) were prepared from the reaction of the parent dinitrogen complex trans-[ReCl(N₂)(dppe)₂] with the appropriate ligand, in thf and in the presence of TlBF₄.Their redox properties were studied by cyclic voltammetry at a Pt electrode in [Bu₄N][BF₄]/thf or acetonitrile; they undergo a one-electron reversible oxidation and the observed E^OX_{$\text{1}\text{2}$} values were applied to test the validity of a proposed general expression —derived from the electrochemical ligand (P_L) and 16-electron metal site (E_s, β) parameters [6]— to estimate E^OX_{$\text{1}\text{2}$} for 18-electron octahedral complexes of the type [M′_sL₂] with a square planar 14-electron metal centre {M′_s}. E_s and β parameters were also estimated for the auxilliary {ReL(dppe)₂}⁺ (L = CO or CNR) centres.     

Evidence for σ → η3 rearrangement reaction in gaseous ions from Fe,Mn and Re allylic derivatives

The fragmentation pathways of (η³-C₃H₄X)FeCO)₂NO, (σ-C₃H₄X)Fe(CO)₂(NO)L, (η³-C₃H₄X)Fe(CO)(NO)L, (σ-C₃H₄X)Fe(CO)₂(NO)L′, (η³- C₃H₄X)Fe(CO)(NO)L′, (σ-C₃H₅)M(CO)₅, (η³-C₃H₅)M(CO)₄, (σ-CH₂CHC(Me)₂)Mn(CO)₅, (η³-CH₂ CHC(Me)₂)Mn(CO)₄, (X=2-Cl; L=PPh₃; L′= P(OMe)₃; M=Mn, Re) have been investigated by mass spectrometry. In the σ derivatives the molecular ion loses CO or the allylic ligand, while in the η³ derivative loss of a CO group is the only fragmentation mode of the molecular ion. Electron impact as well as methane chemical ionization mass spectra have been reported. Kinetic energy release of selected metastable ions indicates that a σ → η³ rearrangement reaction occurs.     

Transition metal complexes with sulfur ligands. XXVIII. Ruthenium complexes of the pentadentate thioether thiolate ligands dpttd (2,3,11,12-dibenzo-1,4,7,10,13-pentathiatridecane(−2)) and the sterically demanding derivative bu4-dpttd (14,16,18,20-tetra (t-butyl)-2,3,11,12-dibenzo-1,4,7,10,13-pentathiatridecane(−2))

The template alkylation of Li₂[Ru(CO)₂(S₂C₆H₄)₂] (S₂C₆H₄²⁻ = 1,2-benzenedithiolate(−2)) by S(C₂H₄Br)₂ yields [Ru(CO)₂(dpttd)] (dpttd²⁻ = 3,11,12-dibenzo-1,4,7,10,13-pentathiatridecane(−2)) which is thermally converted into the monocarbonyl complex [Ru(CO)(dpttd)]. The reactions of dpttd-H₂ or dpttd²⁻ with [RuCl₂(PPh₃)₃], [RuCl₂(DMSO)₄], [RuCl₃(PhSCH₃)₃] and RuCl₃(NO)·xH₂O lead to [Ru(L)(dpttd)] and [Ru(L)(dpttd)]Cl (L = PPh₃, DMSO, PhSCH₃, NO), respectively, which are practically insoluble in all common solvents. Better soluble complexes are obtained with the new sterically demanding ligand ^tbu₄-dpttd²⁻ = 14,16,18,20-tetra(t-butyl)-2,3,11,12-dibenzo-1,4,7, 10,13-pentathiatridecane(−2); it is obtained in isomerically pure form by the reaction of tetrabuthylammonium-3,5-di (t-butyl)-1,2-benzenethiolthiolate, NBu₄[^tbu₂-C₆H₂S(SH), with S(C₂H₄Br)₂ and yields on reaction with [RuCl₂(PPh₃)₃] the very soluble [Ru(PPh₃)₂(^tbu₄-dpttd)] as well as [Ru(PPh₃(^tbu₄-dpttd)]. The ¹H NMR and ³¹P NMR spectra indicate that in solution [Ru(PPh₃)₂(^tbu₄-dpttd)] exists as a mixture of diastereomers, whereas [Ru(PPh₃)(^tbu₄-dpttd)] forms one pair of enantiomers only. This was confirmed by an X-ray structure determination of a single crystal. [Ru(PPh₃)(^tbu₄-dpttd)] crystallizes in space group P2₁/n with a = 10.496(4), b = 14.888(6), c = 32.382(12) Å, β = 98.04(3)°, Z = 4 and D_calc. = 1.27 g/cm³, R = 4.84; R_W = 5.06%; the ruthenium center is coordinated pseudooctahedrally by one phosphorus, two thiolate and three thiother S atoms.     

Substitution reactions of tetrahydrofuran in [Cr(thf)3Cl3] with mono and bidentate N-donor ligands: X-ray crystal structures of [Cr(bipy)(OH2)Cl3] and [HpyNH2][Cr(bipy)Cl4]

Substitution of thf ligands in [Cr(thf)₃Cl₃] and [Cr(thf)₂(OH₂)Cl₃] was investigated. 2,2′-Bipyridine (bipy) was reacted with [Cr(thf)₃Cl₃] to form [Cr(bipy)(thf)Cl₃] (1), which was subsequently reacted with water to give [Cr(bipy)(OH₂)Cl₃] (2). Reaction of 1 with acetonitrile (CH₃CN), pyridine (py) and pyridine derivatives to form [Cr(bipy)(L)Cl₃] (L = CH₃CN 3, py 4 and 4-pyR with R = NH₂5, Bu^t6 and Ph 7). In addition, the substitution of bipy in [Cr(thf)₃Cl₃] was followed by ¹H NMR spectroscopy at room temperature, which showed completion of the reaction in ca. 100 min. Complex 2 was characterised by single crystal X-ray diffraction. The theoretical powder diffraction pattern of 2 was compared to the experimentally obtained powder X-ray diffraction pattern, and shows excellent agreement. The dimer [Cr₂(bipy)₂Cl₄(μ-Cl)₂] was cleaved asymmetrically to give the anionic complex [Cr(bipy)Cl₄]⁻ (8) and [Cr(bipy)₂Cl₂]⁺ (9). Complexes 8 and 9 were characterised by single crystal X-ray diffraction.     

Preparation of benzophenone imine complexes of transition metals

Benzophenone imine [M(η¹-NHCPh₂)(CO)_nP_5-n]BPh₄ [M = Mn, Re; n = 2, 3; P = P(OEt)₃, PPh(OEt)₂, PPh₂OEt, PPh₃] complexes were prepared by allowing triflate M(κ¹-OTf)(CO)_nP_5-n compounds to react with an excess of the imine. Hydride-imine [MH(η¹-NHCPh₂)P₄]BPh₄ (M = Ru, Os), triflate-imine [Os(κ¹-OTf)(η¹-NHCPh₂)P₄]BPh₄ and bis(imine) [Ru(η¹-NHCPh₂)₂P₄](BPh₄)₂ [P = P(OEt)₃] derivatives were also prepared. The complexes were characterized spectroscopically (IR, ¹H, ³¹P, ¹³C NMR) and a geometry in solution was also established. Hydride-benzophenone imine [IrHCl(η¹-NHCPh₂)L(PPh₃)₂]BPh₄and [IrHCl(η¹-NHCPh₂)L(AsPh₃)₂]BPh₄ [L = P(OEt)₃ and PPh(OEt)₂] complexes were prepared by reacting hydride IrHCl₂L(PPh₃)₂ and IrHCl₂L(AsPh₃)₂ precursors with an excess of imine. Dihydride IrH₂(η¹-NHCPh₂)(PPh₃)₃ complex was also obtained and a geometry in solution was proposed.     

SS Bond-activation of diorganyl disulfide by anionic [Mn(CO)5]: crystal structures of [Mn(SC5H4NO)3] and [(CO)3Mn(μ-SR)3Co(μ-SR)3Mn(CO)3] (R=C6H4NHCOPh)

The SS bond-activation of diorganyl disulfide by the anionic metal carbonyl fragment [Mn(CO)₅]⁻ gives rise to an extensive chemistry. Oxidative decarbonylation addition of 2,2′-dithiobis(pyridine-N-oxide) to [Mn(CO)₅]⁻, followed by chelation and metal-center oxidation, led to the formation of [Mn^II(SC₅H₄NO)₃]⁻ (1). The effective magnetic moment in solid state by SQUID magnetometer was 5.88 μ_B for complex 1, which is consistent with the Mn^II having a high-spin d⁵ electronic configuration in an octahedral ligand field. The average Mn(II)S, SC and NO bond lengths of 2.581(1), 1.692(4) and 1.326(4) Å, respectively, indicate that the negative charge of the bidentate 1-oxo-2-thiopyridinato [SC₅H₄NO]⁻ ligand in complex 1 is mainly localized on the oxygen atom. The results are consistent with thiolate-donor [SC₅H₄NO]⁻ stabilization of the lower oxidation state of manganese (Mn(I)), while the O,S-chelating [SC₅H₄NO]⁻ ligand enhances the stability of manganese in the higher oxidation state (Mn(II)). Activation of SS bond as well as OH bond of 2,2′-dithiosalicylic acid by [Mn(CO)₅]⁻ yielded [(CO)₃Mn(μ-SC₆H₄C(O)O)₂Mn(CO)₃]²⁻ (4). Oxidative addition of bis(o-benzamidophenyl) disulfide to [Mn(CO)₅]⁻ resulted in the formation of cis-[Mn(CO)₄(SR)₂]⁻ (R=C₆H₄NHCOPh) which was employed as a chelating metallo ligand to synthesize heterotrinuclear [(CO)₃Mn(μ-SR)₃Co(μ-SR)₃Mn(CO)₃]⁻ (8) possessing a homoleptic hexathiolatocobalt(III) core.     

Silver derivatives of tris(pyrazol-1-yl)methanes. A silver(I) nitrate complex containing a tris(pyrazolyl)methane coordinated in a bridging mode

The reaction of AgX (X=ClO₄, NO₃ or SO₃CH₃) acceptors with excesses of tris(pyrazol-1-yl)methane ligands L (L=CH(pz)₃, CH(4-Mepz)₃, CH(3,5-Me₂pz)₃, CH(3,4,5-Me₃pz)₃ or CH(3-Mepz)₂(5-Mepz)) yields 1:1 [AgX(L)], 2:1 [Ag(L)₂]X or 3:2 [(AgX)₂(L)₃] complexes. The ligand to metal ratio in all complexes is dependent on the number and disposition of the Me substituents on the azole ring of the neutral ligand and on the nature of the Ag(I) acceptor. All complexes have been characterized in the solid state as well as in solution (medium- and far-IR, ¹H and ¹³C NMR and conductivity determinations) and the solid-state structures of [Ag(NO₃){(pz)₃CH}]_(∞/∞) and [Ag{(3,5-Me₂pz)₃CH}₂]NO₃ determined by single crystal X-ray studies.     

Preparation of trihydridostannyl complexes of rhenium stabilised by isocyanide ligands

Mixed-ligand complexes [ReBr(CO)₂(CNR)_nL_3−n] (1-4) [R = 4-CH₃OC₆H₄, 4-CH₃C₆H₄, C(CH₃)₃; L = P(OEt)₃, PPh(OEt)₂; n = 1, 2] were prepared by allowing carbonyl compounds [ReBr(CO)₄L] and [ReBr(CO)₃L₂] to react with an excess of isocyanide. Treatment of these bromocomplexes [ReBr(CO)₂(CNR)_nL_3−n] with SnCl₂ · 2H₂O yielded the trichlorostannyl derivatives [Re(SnCl₃)(CO)₂(CNR)_nL_3−n] (5-8). Trihydridestannyl complexes [Re(SnH₃)(CO)₂(CNR)_nL_3−n] (9-12) were prepared by allowing trichlorostannyl compounds 5-8 to react with NaBH₄ in ethanol. The trimethylstannyl derivative [Re(SnMe₃)(CO)₂(CNC₆H₄-4-CH₃){PPh(OEt)₂}₂] (13b) was also prepared by treating [Re(SnCl₃)(CO)₂(CNC₆H₄-4-CH₃){PPh(OEt)₂}₂] with an excess of MgBrMe in diethylether. Reaction of the tin trihydride complexes [Re(SnH₃)(CO)₂(CNR)_nL_3−n] (9-12) with CO₂ (1 atm) led to dinuclear OH-bridging bis(formate) derivatives [Re{Sn(OC(H)O)₂(μ-OH)}(CO)₂(CNR)_nL_3−n]₂ (14, 15). The complexes were characterised spectroscopically (IR, ¹H, ³¹P, ¹³C, ¹¹⁹Sn NMR) and by X-ray crystal structure determination of [Re(SnH₃)(CO)₂{CNC(CH₃)₃}{PPh(OEt)₂}₂] (10b).     

Receptor interactions of the position 4 side chains of angiotensin II analogues: Importance of aromatic ring quadrupole

A triad of interacting group (TyrOH? His$ \underline\ominus$O₂C) in angiotensin II (ANG II) has been postulated to create the tyrosinate anion pharmacophore (tyanophore) responsible for receptor activation/triggering (Biochim. Biophys. Acta 1991, 1065, 21). In the present study we investigated the effects on bioactivity of substituting the Tyr⁴ residue in [Sar¹]ANG II with other anionic or electronegative amino acids, and with a number of aromatic amino acids lacking a hydroxyl group. [Sar¹ Nva(δ-OH)⁴]ANG II, [Sar¹ Nva(δ-OCH₃)⁴]ANG II, [Sar¹ Met⁴]ANG II, [Sar¹ Gln⁴]ANG II, [Sar¹ Glu⁴]ANG II and [Sar¹ DL -Alg⁴]ANG II had agonist activities in the rat isolated uterus assay of 4, 3, 19, 10, > 0.1 and > 0.1%, respectively, of that of ANG II. [Sar¹ Nal⁴]ANG II, [Sar¹ Pal⁴]ANG II, [Sar¹ DL -Phg(4′-F)⁴]ANG II, [Sar¹ Phe(4′-F)⁴]ANG II, [Sar¹ Phe(F₅)⁴]ANG II and [Sar¹ His⁴]ANG II had agonist activities of 4.5, 7, < 0.1, 0.2, 1 and 0.6%, respectively. All peptides investigated were devoid of measurable antagonist activity except [Sar¹] Phe(4′-F)⁴ ANG II (pA₂ = 7.7). These findings illustrate that anionic or electronegative aliphatic side chains replacing tyrosinate at position 4 can partially activate the angiotension receptor. For ANG II analogues containing an aromatic amino acid other than Tyr at position 4, ligand binding and agonist activity are not dependent on the electronegativity or dipole moment of the aromatic ring, or on the ability of the 4′ ring substituent to accept a proton. Modelling based on ab initio calculations of aromatic ring multipoles illustrate that the apparent binding affinity (PA₂) of ANG II analogues is associated with a perpendicular electrostatic interaction of the position 4 aromatic ring with a receptor-based group. In addition, intramolecular interactions providing for the conformation of the ligand as it approaches its receptor appear to have a role in determining agonist vs antagonist activity.     

Titanium, zirconium and hafnium halide complexes of trithioether and triselenoether ligands

The reaction of TiX₄(X=Cl or Br) with the tripodal ligands MeC(CH₂SMe)₃ or MeC(CH₂SeMe)₃, (L³) in anhydrous n-hexane or CH₂Cl₂ produced the extremely moisture sensitive complexes [TiX₄(L³)]. These were characterised by microanalysis, IR, UV-Vis and variable temperature ¹H,¹³C{¹H} and ⁷⁷Se NMR spectroscopy. The NMR studies showed that in solution in CH₂Cl₂ the complexes contain L³ bound as bidentates, and that pyramidal inversion and exchange between the free and coordinated chalcogen donors is rapid at room temperature. Ligand dissociation/exchange increases TiCl₄<TiBr₄ and MeC(CH₂SMe)₃<MeC(CH₂SeMe)₃ and attempts to isolate TiI₄ analogues were unsuccessful. The reactions of [MCl₄(Me₂S)₂] (M=Zr or Hf) with (L³) in anhydrous CH₂Cl₂ produces white or cream 7-coordinate [MCl₄(L³)], which are insoluble in chlorocarbon solvents. The reactions of TiX₄ (X=Cl, Br or I) with the trithia-macrocycles [9]aneS₃ and [10]aneS₃ produced [TiX₃([n]aneS₃)]X, whilst reaction of TiCl_4, SbCl₅ and [9]aneS₃ in anhydrous CH₂Cl₂ gave [TiCl₃([9]aneS₃)]SbCl₆. Spectroscopic studies suggest these macrocyclic compounds contain 6-coordinate cations, [TiX₃([n]aneS₃)]⁺ (n=9 or 10) but with Zr and Hf the complexes [MCl₄([n]aneS₃)] are 7-coordinate and neutral.     

Synthesis, spectroscopy and structural characterization of silver(I) complexes containing unidentate N-donor azole-type ligands

From the interaction between azole-type ligands L and AgX (X = NO₃ or ClO₄) or [AgX(PPh₃)_n] (X = Cl, n = 3; X = MeSO₃, n = 2), new ionic mononuclear [Ag(L)₂]X and [Ag(PPh₃)₃L][X] or neutral mono-([Ag(PPh₃)_nL(X)]) or di-nuclear ([{Ag(PPh₃)(L)(μ-X)}₂]) complexes have been obtained which have been characterized through elemental analysis, conductivity measurements, IR, ¹H NMR and, in some cases, also by ³¹P{¹H} NMR spectroscopy, and single-crystal X-ray studies. Stoichiometries and molecular structures are dependent on the nature of the azole (steric hindrance and basicity), of the counter ion, and on the number of the P-donor ligands in the starting reactants. Solution data are consistent with partial dissociation of the complexes, occurring through breaking of both Ag-N and Ag-P bonds.     

A bicarbonate bridged diruthenium(I) complex: Key evidence for the decarboxylation step in the base-assisted reduction of Ru2Cl4(CO)6

Base-assisted reduction of [Ru(CO)₃Cl₂]₂ in the presence of NP-Me₂ (2,7-dimethyl-1,8-naphthyridine) in thf provides an unsupported diruthenium(I) complex [Ru₂(CO)₄Cl₂(NP-Me₂)₂] (1). Two NP-Me₂ and four carbonyls bind at equatorial positions and two chlorides occupy sites trans to the Ru-Ru single bond. Reaction of [Ru(CO)₃Cl₂]₂, TlOTf, KOH and NP-Me₂ in acetonitrile, in a sealed container, affords a bicarbonate bridged diruthenium(I) complex [Ru₂(CO)₂(μ-CO)₂(μ-O₂COH)(NP-Me₂)₂](OTf) (2). The in situ generated CO₂ is the source for bicarbonate under basic reaction medium. Isolation of 2 validates the decarboxylation step in the base-assisted reduction of [Ru^II(CO)₃Cl₂]₂ → [Ru^I₂(CO)₄]²⁺.     

An exploratory scandium-45 NMR study into the complexation of alanine and oligopeptides

The 87.5 MHz ⁴⁵Sc NMR spectrum of 0.025 M aqueous Sc(NO₃)₃ exhibits two resonance signals, separated by ca. 25 ppm, attributable to [Sc(H₂O)₆]³⁺ and [Sc(H₂O)₅OH]²⁺. Acidification leads to a single, comparatively sharp line (W_1/2 = 160 Hz) for the hexaqua complex, the temperature dependence (temperature gradient = 0.076 ppm/deg) of which indicates that relaxation is dominated by the quadrupole mechanism. Addition of α-alanine gives rise to an additional broad signal at ca. +70 ppm (relative to [Sc(H₂O)₆]³⁺), which is assigned to a carboxylato complex [Sc(H₂O)_6−n(ala)_n]³⁺ or [Sc(H₂O)_5−nOH- (ala)_n]²⁺ (1 < n < 2). At ambient temperatures, these species are in slow exchange with the hexaqua and pentaqua-hydroxo complex, progressing through medium towards fast exchange as the temperature increases, and giving rise to an exchange contribution to relaxation. W_1/2 becomes a measure for the stability of the complexes, which increases in the order ala < (ala)₄ ∼ (ala)₂ < ala-val-leu. The pronounced stability of the latter is due to the formation of a chelate-five ring structure (participation of the NH- function of the peptide bond in coordination to Sc³⁺). 1 M aqueous ScCl₃ probably contains the two species [Sc(H₂O)₆]³⁺ and [Sc(H₂O)₅Cl]²⁺, separated by 33 ppm.     

Adducts of titanium tetrahalides with neutral Lewis bases. Part I. Structure and stability: a vibrational and multinuclear NMR study

Raman, infra-red and multinuclear NMR spectroscopy were used to establish the structure of several TiX₄·2L adducts (X=F, Cl, Br; L=Lewis base) in inert solvents. In contrast to the analogous SnX₄·2L adducts where a cis-trans equilibrium prevails, most of the TiX₄·2L adducts studied were found to have only the cis configuration. Trans isomers were observed but their formation was dependent on the donor ability of the ligand. In dichloromethane solution, the adducts with L=Me₂O, Me₂S, (MeOCH₂-)₂, Et₂S, THT, Me₂Se, MeCN, Me₂CO, Cl(MeO)₂PO, Cl₂(MeO)PO, Cl₃PO and Cl₂(Me₂N)PO were found to have the cis configuration only. For the adducts with L=THF, Cl(Me₂N)₂PO and TMPA, a cis-trans equilibrium was observed. The thermodynamic parameters were measured for cis-trans isomerization for TiCl₄·2TMPA in CHCl₃; these parameters are: K_iso²⁷⁷=[trans] / [cis]=0.36, ΔH°_iso=− 1.3 ± 1.3 kJ/mol, ΔS°_iso=−13.1 + 7.5 J/mol K, and ΔV°_iso= − 1.3+0.8 cm³/mol. A complex equilibrium involving cis and trans isomers and the ionic complex [TiCl₃·3HMPA]Cl was found to occur for the TiCl₄ adduct with L=HMPA. ¹H NMR was used to establish the relative stabilities of the cis adducts and the following sequence was obtained: Me₂O ∼ MeCN < Me₂CO < Me₂S < Me₂Se < Cl(MeO)₂PO < TMPA < CI(Me₂N)₂PO.     

Metal cluster topology. 4. Rhodium carbonyl clusters having fused polyhedra

Previously discussed topological models of metal cluster bonding are now extended to the treatment of anionic rhodium carbonyl clusters having structures consisting of fused polyhedra. Examples of such rhodium carbonyl clusters built from fused octahedra include the ‘biphenyl analogue’ [Rh₁₂(CO)₃₀]⁻², the ‘face-sharing naphthalene analogue’ [Rh₉- (CO)₁₉]³⁻, and the ‘perinaphthene analogue’, [Rh₁₁- (CO)₂₃]³⁻. More complicated anionic rhodium carbonyl clusters treated in this paper include the [Rh₁₃(CO)₂₄H_5−q]^q− anions (q = 2, 3, 4) having an Rh₁₃ centered cuboctahedron, the [Rh₁₄(CO)₂₅- H_4−q]^q− (q = 3,4) and [Rh₁₄(CO)₂₆]²⁻ anions based on a centered pentacapped cube, the [Rh₁₅- (CO)₃₀]³⁻ anion having an Rh₁₅ centered 14-vertex deltahedron, the [Rh₁₅(CO)₂₇]³⁻ anion having a tricapped centered 11-vertex polyhedron, the [Rh₁₇- (CO)₃₀]³⁻ anion having a tetracapped centered cuboctahedron, and the [Rh₂₂(CO)₃₇]⁴⁻ anion having a hexacapped centered cuboctahedron fused to an octahedron so that the octahedron and the cuboctahedron share a triangular face. Analyses of the bonding topologies in [Rh₉(CO)₁₉]³⁻, [Rh₁₇- (CO)₃₀]³⁻, and [Rh₂₂(CO)₃₇]⁴⁻ indicate that a polyhedral network containing several fused globally delocalized polyhedral chambers will not necessarily have a multicenter core bond in the center of each such polyhedral chamber. This observation is of potential importance in extending topological models of metal cluster bonding to bulk metals.     

Variation of the electron population by four units in the cluster series [(η-Cp′)3Mo3S4Co(L)] (L = I, CO, PPh3, NO; n = 0, 1)

The versatility of cuboidal Mo₃S₄Co clusters for the preparation of complexes with different numbers of valence shell electrons (VSE) in the cluster is described. The reaction of the geometrically incomplete cuboidal cluster salt [(η⁵-Cp′)₃Mo₃S₄][pts] (pts = p-toluenesulfonate) with one molar equivalent of [Co₂(CO)₈] afforded almost quantitatively the electroneutral 60 VSE cluster [(η⁵-Cp′)₃Mo₃S₄Co(CO)] (1), which previously has been prepared in low yield by Curtis et al. in autoclave syntheses [M.D. Curtis, U. Riaz, O.J. Curnow, J.W. Kampf, Organometallics 14 (1995) 5337]. Cluster 1 was also obtained in high yield by reaction of [(η⁵-Cp′)₃Mo₃S₄][pts] with [(η⁵-Cp^∗)Co(CO)₂]. Reaction of [(η⁵-Cp′)₃Mo₃S₄][pts] with two molar equivalents of [Co(I)(CO)₃(PPh₃)] led to a complex mixture of products, of which the electron deficient 58 VSE cluster salt [(η⁵-Cp′)₃Mo₃S₄Co(I)][Co(I)₃(thf)] ([2][Co(I)₃(thf)]) was isolated as single crystals. In the crystal structures of 1 and [2][Co(I)₃(thf)], the Co-Mo bond lengths are almost identical, indicating a delocalization of the electron deficiency in [2]⁺. The reduced form of [2]⁺, [(η⁵-Cp′)₃Mo₃S₄Co(I)] (2), was prepared by oxidative substitution of the carbonyl ligand in 1 by I₂. Further reactions of 1 with PPh₃ and NO leading to the 60 and 61 VSE cluster complexes [(η⁵-Cp′)₃Mo₃S₄Co(PPh₃)] (3) and [(η⁵-Cp′)₃Mo₃S₄Co(NO)] (4), respectively, enabled the preparation of Mo₃S₄Co clusters in altogether four different oxidation states.     

Synthesis of some mixed seven-coordinate complexes of the type [MI2(CO)3LL′] (M = Mo or W;L = pyridine or substituted pyridines; L′ = PPh3, AsPh3 or SbPh3)

The seven-coordinate bisacetonitrile complexes [MI₂(CO)₃(NCMe)₂] (M = Mo or W) react with L′ (L′ = PPh₃, AsPh₃ or SbPh₃) in CH₂Cl₂ at room temperature to give [MI₂(CO)₃(NCMe)L′] which when reacted in situ with L (L = pyridine or substituted pyridines) affords good yields of 28 mixed seven-coordinate complexes [MI₂(CO)₃LL′]. It is likely these reactions occur via successive dissociative displacements of two acetonitrile ligands.     

Catalytic effect of cis-diammineplatinum α-pyrrolidone tan adsorbed on a platinum electrode in electrochemical oxidation of water into molecular oxygen

Cyclic voltammograms of cis-diammineplatinum α-pyrrolidone-blue and -tan, [Pt₄(NH₃)₈(C₄H₆NO)₄]ⁿ⁺ (n = 5 and 6, respectively) show for either complex only one redox peak at 0.53 V (average potential of the anodic and cathodic peak potentials). Coulometry and UVVis spectra of bulk- electrolyzed solution indicated that the redox peak corresponds to the reaction [Pt₄(NH₃)₈(C₄H₆NO)₄]⁸⁺ + 4e ⇄ 2[Pt₂(NH₃)₄(C₄H₆NO)₂]²⁺. When cyclic voltammetry is carried out in a solution of [Pt₄(NH₃)₈(C₄H₆NO)₄]⁶⁺ or a platinum electrode adsorbed with [Pt₄(NH₃)₈(C₄H₆NO)₄]⁶⁺ is used in the presence of oxidizing agent in the solution, O₂ gas generates from the electrode surface with large catalytic cathodic current at potentials below ca. 0.8 V. The O₂ gas was confirmed to generate from water by GC-MS analysis. This abnormal O₂ generation phenomenon is explained with cyclic reactions of chemical surface oxide formation on the electrode by the oxidizing agent and electrochemical reduction of the surface oxide. Oxygen gas generates from the reaction of [Pt₄(NH₃)₈(C₄H₆NO)₄]⁸⁺ or [Pt₄(NH₃)₈(C₄H₆NO)₄]⁶⁺ with OH⁻ produced in the course of the electrochemical reduction of the electrode surface oxide. The ability of [Pt₄(NH₃)₈(C₄H₆NO)₄]⁸⁺ and [Pt₄(NH₃)₈(C₄H₆NO)₄]⁶⁺ to oxidize OH⁻ into O₂ has been reported previously.     

Reactions of copper(II) and nickel(II) compounds of 6-methyl-2-picolylamine with acetone,including and x-ray structural analysis of [Ni(C20H28N4)(NO3)]NO3

The reaction with acetone of nickel(II) and copper(II) bis-chelated compounds of 6-methyl-2-pyridylmethylamine gives compounds of the quadridentate [N₄] ligand 2,6-diaza-1,7-bis-(6′-methyl-2′-pyridyl)-3,5,5-trimethyl-hept-2-ene(Q). In the nickel series also, a bis-chelated perchlorate of the terdentate ligand 2-aza-1-(6′-methyl-2′-pyridyl)-3-methyl-hex-2-ene-5-one was obtained. In the copper series, five-coordinate species [Cu(Q)X]X (X = Br, I, NCS) and [Cu(QX]ClO₄ (X = Cl) were isolated. If left in acetone, these undergo further reaction, with increasing ease in the order Cl < Br < I. An intermediate formation of a transient brown colour suggests the possible involvement of a copper(I) intermediate. The nature of the products was established by an X-ray analysis of the structure of [Ni(Q)NO₃]NO₃. Crystals are orthorhombic, a = 20.36(2), b = 13.38(1), c = 8.226(5) Å, space group Pna2₁. Using two-circle diffractometer data (1598 reflections), the structure was solved by Patterson and Fourier methods, and refined by block diagonal least-squares methods to a final R of 0.030. The expected quadridentate ligand was found in the cis-β configuration about the metal, with coordination sphere completed by a bidentate nitrate. Bond-lengths and angles within the molecular cation were unexceptionable considering the small ‘bite’ of the chelated nitrato group of only 59°.     

Oxidative addition of methyl iodide to [Rh(CO)2I]2: synthesis, structure and reactivity of neutral rhodium acetyl complexes, [Rh(CO)(NCR)(COMe)I2]2

Reaction of [Rh(CO)₂I]₂ (1) with MeI in nitrile solvents gives the neutral acetyl complexes, [Rh(CO)(NCR)(COMe)I₂]₂ (R=Me, 3a; ^tBu, 3b; vinyl, 3c; allyl, 3d). Dimeric, iodide-bridged structures have been confirmed by X-ray crystallography for 3a and 3b. The complexes are centrosymmetric with approximate octahedral geometry about each Rh centre. The iodide bridges are asymmetric, with Rh-(μ-I) trans to acetyl longer than Rh-(μ-I) trans to terminal iodide. In coordinating solvents, 3a forms mononuclear complexes, [Rh(CO)(sol)₂(COMe)I₂] (sol=MeCN, MeOH). Complex 3a reacts with pyridine to give [Rh(CO)(py)(COMe)I₂]₂ and [Rh(CO)(py)₂(COMe)I₂] and with chelating diphosphines to give [Rh(Ph₂P(CH₂)_nPPh₂)(COMe)I₂] (n=2, 3, 4). Addition of MeI to [Ir(CO)₂(NCMe)I] is two orders of magnitude slower than to [Ir(CO)₂I₂]⁻. A mechanism for the reaction of 1 with MeI in MeCN is proposed, involving initial bridge cleavage by solvent to give [Rh(CO)₂(NCMe)I] and participation of the anion [Rh(CO)₂I₂]⁻ as a reactive intermediate. The possible role of neutral Rh(III) species in the mechanism of Rh-catalysed methanol carbonylation is discussed.