Preparation and characterization of the argentates: [Ag[P(CF(3))(2)](2)](-), [Ag[(micro-P(CF(3))(2))M(CO)(5)](2)](-) (M = Cr, W), and [Ag[(micro-P(C(6)F(5))(2))W(CO)(5)](2)](-): X-ray crystal structure of [K(18-crown-6)][Ag[(micro-P(CF(3))(2))Cr(CO)(5)](2)

The first example of a mononuclear diphosphanidoargentate, bis[bis(trifluoromethyl)phosphanido]argentate, [Ag[P(CF(3))(2)](2)](-), is obtained via the reaction of HP(CF(3))(2) with [Ag(CN)(2)](-) and isolated as its [K(18-crown-6)] salt. When the cyclic phosphane (PCF(3))(4) is reacted with a slight excess of [K(18-crown-6)][Ag[P(CF(3))(2)](2)], selective insertion of one PCF(3) unit into each silver phosphorus bond is observed, which on the basis of NMR spectroscopic evidence suggests the [Ag[P(CF(3))P(CF(3))(2)](2)](-) ion. On treatment of the phosphane complexes [M(CO)(5)PH(CF(3))(2)] (M = Cr, W) with [K(18-crown-6)][Ag(CN)(2)], the analogous trinuclear argentates, [Ag[(micro-P(CF(3))(2))M(CO)(5)](2)](-), are formed. The chromium compound [K(18-crown-6)][Ag[(micro-P(CF(3))(2))Cr(CO)(5)](2)] crystallizes in a noncentrosymmetric space group Fdd2 (No. 43), a = 2970.2(6) pm, b = 1584.5(3) pm, c = 1787.0(4), V = 8.410(3) nm(3), Z = 8. The C(2) symmetric anion, [Ag[(micro-P(CF(3))(2))Cr(CO)(5)](2)](-), shows a nearly linear arrangement of the P-Ag-P unit. Although the bis(pentafluorophenyl)phosphanido compound [Ag[P(C(6)F(5))(2)](2)](-) has not been obtained so far, the synthesis of its trinuclear counterpart, [K(18-crown-6)][Ag[(micro-P(C(6)F(5))(2))W(CO)(5)](2)], was successful.     

Kinetics and mechanism of platinum-thallium bond formation: the binuclear [(CN)5Pt-Tl(CN)](-) and the trinuclear [(CN)5Pt-Tl-Pt(CN)5](3-) complex

Formation kinetics of the metal-metal bonded binuclear [(CN)(5)Pt-Tl(CN)](-) (1) and the trinuclear [(CN)(5)Pt-Tl-Pt(CN)(5)](3-) (2) complexes is studied, using the standard mix-and-measure spectrophotometric method. The overall reactions are Pt(CN)(4)(2-) + Tl(CN)(2)(+) <==> 1 and Pt(CN)(4)(2-) + [(CN)(5)Pt-Tl(CN)](-) <==> 2. The corresponding expressions for the pseudo-first-order rate constants are k(obs) = (k(1)[Tl(CN)(2)(+)] + k(-1))[Tl(CN)(2)(+)] (at Tl(CN)(2)(+) excess) and k(obs) = (k(2b)[Pt(CN)(4)(2-)] + k(-2b))[HCN] (at Pt(CN)(4)(2-) excess), and the computed parameters are k(1) = 1.04 +/- 0.02 M(-2) s(-1), k(-1) = k(1)/K(1) = 7 x 10(-5) M(-1) s(-1) and k(2b) = 0.45 +/- 0.04 M(-2) s(-1), K(2b) = 26 +/- 6 M(-1), k(-2b) = k(2b)/K(2b) = 0.017 M(-1) s(-1), respectively. Detailed kinetic models are proposed to rationalize the rate laws. Two important steps need to occur during the complex formation in both cases: (i) metal-metal bond formation and (ii) the coordination of the fifth cyanide to the platinum site in a nucleophilic addition. The main difference in the formation kinetics of the complexes is the nature of the cyanide donor in step ii. In the formation of [(CN)(5)Pt-Tl(CN)](-), Tl(CN)(2)(+) is the source of the cyanide ligand, while HCN is the cyanide donating agent in the formation of the trinuclear species. The combination of the results with previous data predict the following reactivity order for the nucleophilic agents: CN(-) > Tl(CN)(2)(+) > HCN.     

Protonolysis of [((i)PrO)TiMo5O18]3-: access to a family of TiMo5 Lindqvist type polyoxometalates

The tetra-n-butylammonium (TBA) salts of [((i)PrO)TiMo(5)O(18)](3-) 1 and [((i)BuO)TiMo(5)O(18)](3-) 2 were prepared by hydrolysis of mixtures of (TBA)(2)[Mo(2)O(7)], (TBA)(4)α-[Mo(8)O(26)] and Ti(OR)(4) (R = (i)Pr or (i)Bu) in acetonitrile. Treatment of (TBA)(3)1 with alcohols ROH afforded primary and tertiary alkoxide derivatives [(RO)TiMo(5)O(18)](3-) (R = Me 3, (t)Bu 4), whilst aryloxides [(ArO)TiMo(5)O(18)](3-) were prepared by reacting 1 with phenols ArOH (Ar = C(6)H(4)Me-4 5, and C(6)H(4)CHO-2 6). Oxo-bridged [(μ-O)(TiMo(5)O(18))(2)](6-)7 rather than the hydroxo derivative [(HO)TiMo(5)O(18)](3-) was obtained upon hydrolysis of 1. X-Ray crystal structures of TBA salts of anions 3-7 show that titanium is six-coordinate in all cases, although titanium sites are disordered over two trans positions in 3. Mo-O bond length alternation is observed in the Mo(4)O(4) planes of 4 and 7 and in one of the two independent anions in the structure of 3. In solution, (17)O NMR spectra are consistent with the higher anionic charge compared to [Mo(6)O(19)](2-) and reveal an order of basicity for the anions [LM'Mo(5)O(18)](3-) associated with the ability of {LM'}(3+) to donate/withdraw electron density from {Mo(5)O(18)}(6-). Protonolysis reactions of 1 and 3 were slower than for tungstate analogues and the possibility of initial protonation at TiOM (M = Mo) rather than TiOR (M = W) in a proton-assisted S(N)1 mechanism for ligand exchange in [(RO)TiM(5)O(18)](3-) is discussed.     

A theoretical study of the NMR spin-spin coupling constants of the complexes [(NC)(5)Pt-Tl(CN)(n)](n-) (n = 0-3) and [(NC)(5)Pt-Tl-Pt(CN)(5)](3-): a lesson on environmental effects

The molecular geometries and the nuclear spin-spin coupling constants of the complexes [(NC)(5)Pt-Tl(CN)(n)](n-), n = 0-3, and the related system [(NC)(5)Pt-Tl-Pt(CN)(5)](3-) are studied. These complexes have received considerable interest since the first characterization of the n = 1 system by Glaser and co-workers in 1995 [J. Am. Chem. Soc. 1995, 117, 7550-7551]. For instance, these systems exhibit outstanding NMR properties, such as extremely large Pt-Tl spin-spin coupling constants. For the present work, all nuclear spin-spin coupling constants J(Pt-Tl), J(Pt-C), and J(Tl-C) have been computed by means of a two-component relativistic density functional approach. It is demonstrated by the application of increasingly accurate computational models that both the huge J(Pt-Tl) for the complex (NC)(5)Pt-Tl and the whole experimental trend among the series are entirely due to solvent effects. An approximate inclusion of the bulk solvent effects by means of a continuum model, in addition to the direct coordination, proves to be crucial. Similarly drastic effects are reported for the coupling constants between the heavy atoms and the carbon nuclei. A computational model employing the statistical average of orbital-dependent model potentials (SAOP) in addition to the solvent effects allows to accurately reproduce the experimental coupling constants within reasonable limits.     

Synthesis and Investigation of [B(20)H(17)O(CH(2))(5)](3-), a Novel Solvent Complex of the [B(20)H(18)](4-) Ion

Reaction of the [trans-B(20)H(18)](2-) ion with the n-butoxide ion, formed in situ from reaction of n-butanol and NaH, in tetrahydropyran (THP) produces in good yield an unexpected and isolable solvent-coordinated polyhedral borane anion, [ae-B(20)H(17)O(CH(2))(5)](3-). The anticipated product of nucleophilic attack, [ae-B(20)H(17)On-Bu](4-), is not observed under the reaction conditions. The solvent-coordinated product is also formed in the presence of either ethoxide or carbamate ion but is not observed if the ethoxide or carbamate ion is not present in stoichiometric amounts. In the presence of the n-butanethiol anion, the coordinated THP ring undergoes a ring-opening reaction, yielding the [ae-B(20)H(17)O(CH(2))(5)Sn-Bu](4-) anion. Ring opening is also observed in the presence of the ethoxide ion in refluxing THP. Isolation of the previously proposed analogous solvent-coordinated tetrahydrofuran (THF) product, [ae-B(20)H(17)O(CH(2))(4)](3-), was unsuccessful; however, the product resulting from ring opening of THF by the n-butanethiol anion is reported.     

Kinetics and mechanism of formation of the platinum-thallium bond: the [(CN)(5)Pt-Tl(CN)(3)](3)(-) complex

Formation kinetics of the metal-metal bonded [(CN)(5)PtTl(CN)(3)](3)(-) complex from Pt(CN)(4)(2)(-) and Tl(CN)(4)(-) has been studied in the pH range of 5-10, using standard mix-and-measure spectrophotometric technique at pH 5-8 and stopped-flow method at pH > 8. The overall order of the reaction, Pt(CN)(4)(2)(-) + Tl(CN)(4)(-) right harpoon over left harpoon [(CN)(5)PtTl(CN)(3)](3)(-), is 2 in the slightly acidic region and 3 in the alkaline region, which means first order for the two reactants in both cases and also for CN(-) at high pH. The two-term rate law corresponds to two different pathways via the Tl(CN)(3) and Tl(CN)(4)(-) complexes in acidic and alkaline solution, respectively. The two complexes are in fast equilibrium, and their actual concentration ratio is controlled by the concentration of free cyanide ion. The following expression was derived for the pseudo-first-order rate constant of the overall reaction: k(obs) = (k(1)(a)[Tl(CN)(4)(-) + (k(1)(a)/K(f)))(1/(1 + K(p)[H(+)]))[CN(-)](free) + k(1)(b)[Tl(CN)(4)(-)] + (k(1)(b)/K(f)), where k(1)(a) and k(1)(b) are the forward rate constants for the alkaline and slightly acidic paths, K(f) is the stability constant of [(CN)(5)PtTl(CN)(3)](3)(-), and K(p) is the protonation constant of cyanide ion. k(1)(a) = 143 +/- 13 M(-)(2) s(-)(1), k(1)(b) = 0.056 +/- 0.004 M(-)(1) s(-)(1), K(f) = 250 +/- 54 M(-)(1), and log K(p) = 9.15 +/- 0.05 (I = 1 M NaClO(4), T = 298 K). Two possible mechanisms were postulated for the overall reaction in both pH regions, which include a metal-metal bond formation step and the coordination of the axial cyanide ion to the platinum center. The alternative mechanisms are different in the sequence of these steps.     

Pentamethylcyclopentadienyl-rhodium and -iridium complexes : XXVIII. Properties and x-ray crystal structures of [(RhC5Me5)3(H)3O]+ and [(RhC5Me5)2(H)2OAc]+ prepaerd from reactions of [RhC5Me5)2(OH)3]+ with alcohols

The triμ-hydroxo-dirhodium complexes [(RhC₅Me₅)₂(OH)₃]X (X  Cl. PF₆, BF₄) react in isopropanol to give the tri-μ-hydrido-trirhodium complexes [(RhC₅Me₅)₂(H)₃O]X (X  PF₆, BF₄, BPh₄). A combination of X-ray crystal structure determination and ¹H and ¹³CNMR spectroscopy of [RhC₅Me₅)₃-(H)₃O][PF₆] showed it to contain an equilateral triangle of rhodium, each η⁵-bonded to a C₅Me₅, capped on one sid by an oxygen and with each pair of rhodiums bridged on the other side by a hydride (RhH mean 1.7(1) Å). The molecule is quite rigid and the barrier to movement of the hydrides, ΔG3, is at least 21 kcal mol^-1 at +100°C. Reasons for this rigidity are considered. The known tetrahydride complex[(RhC₅Me₅)₄]²⁺ is obtained from [(RhC₅Me₅)₂(OH)₃]Cl in isopropanol using longer reaction times. Reaction of [RhC₅Me₅)₂(HO)₃]PF₆ with primary alcohols (RCH₂OH) gave mixtures of [(RhC₅Me₅)₂H(O₂CR)₂PF₆ and [(RhC₅Me₅)₂(H)₂(O₂CR)₂PF₆, but only the latter could be easily isolated. A single crystal X-ray structure of [(RhC₅Me₅)₂-(H)₂(O₂CMe)]PF₆ showed it to be dinuclear with two rhodiums each η⁵-binded to C₅Me₅ and bridged by two hydrides (mean RhH, 1.72(10) Å) and one acetate.     

Alkylation reactions of [WS(4)](2)(-) and [(eta(5)-C(5)Me(5))WS(3)](-) with 2,6-bis(bromomethyl)pyridine: the first isolation of bisalkylated tetrathiometalate WS(2)[2,6-(SCH(2))(2)(C(5)H(3)N)

The trithio and tetrathio complexes of tungsten (PPh(4))[CpWS(3)] (Cp = eta(5)-C(5)Me(5)) and (PPh(4))(2)[WS(4)] undergo alkylation reactions with 2,6-bis(bromomethyl)pyridine to yield [(CpWS(2))(2)[2,6-(SCH(2))(2)(C(5)H(3)N)]].CH(3)CN (1.CH3CN) (73.1% yield) and WS(2)[2,6-(SCH(2))(2)(C(5)H(3)N)] (2) (76.0% yield), respectively. In the dinuclear complex 1, two CpWS(3) units are linked by a 2,6-dimethylenepyridine bridge, and the pyridine nitrogen is not coordinated at tungsten. Complex 2 is the first example of bisalkylated tetrathiometalates, the mononuclear structure of which is stabilized by coordination of the pyridine nitrogen.     

A practical and improved synthesis of (3S,5S)-3-[(tert-butyloxycarbonyl)methyl]- 5-[(methanesulfonyloxy)methyl]-2- pyrrolidinone

A practical and improved synthesis of (3S,5S)-3-[(tert-butyloxycarbonyl)methyl]-5-[(methanesulfonyloxy)methyl]-2-pyrrolidinone (1) is described. The key transformations involve a highly efficient reaction sequence consisting of ethoxycarbonylation, alkylation, hydrolysis, and decarboxylation to produce compound 10. The process described herein is practical, robust, and cost-effective, and it has been successfully implemented in a pilot plant to produce a multikilogram quantity of mesylate 1.     

Electrochemical, spectroscopic, structural, and magnetic characterization of the reduced and protonated alpha-Dawson anions in [Fe(eta(5)-C(5)Me(5))(2)](5)[HS(2)Mo(18)O(62)].3HCONMe(2).2Et(2)O and [NBu(4)](5)[HS(2)Mo(18)O(62)].2H(2)O(1)

Reaction of excess Fe(cp)(2) (cp = eta(5)-C(5)Me(5)) dissolved in Et(2)O with [NHex(4)](4)[S(2)Mo(18)O(62)] in acetonitrile, followed by recrystallization of the precipitated solid from N,N'-dimethylformamide (DMF), leads to isolation of the complex [Fe(cp)(2)](5)[HS(2)Mo(18)O(62)].3DMF.2Et(2)O. The solid has been characterized by microanalysis, by voltammetric analysis, by (1)H NMR, diffuse reflectance infrared, EPR, and M?ssbauer spectroscopies, and by temperature-dependent magnetic susceptibility measurements. The data are consistent with the presence of a paramagnetic [Fe(cp)(2)](+) cation and a diamagnetic two-electron-reduced [HS(2)Mo(18)O(62)](5-) anion. The related salt [NBu(4)](5)[HS(2)Mo(18)O(62)].2H(2)O crystallizes in space group C2/c with a = 25.1255(3) A, b = 15.4110(2) A, c = 35.8646(4) A, beta = 105.9381(4), V = 13353.3(3) A(3), and Z = 4. The (2 e(-), 1 H(+))-reduced anion exists as the alpha-Dawson isomer, and its structure may be compared with those of the oxidized and (4 e(-), 3 H(+))-reduced anions as they exist in [NEt(4)](4)[S(2)Mo(18)O(62)].MeCN and [NBu(4)](5)[H(3)S(2)Mo(18)O(62)].4MeCN, respectively. Overall, the anion expands significantly upon the addition of two and then four electrons. However, the Mo...Mo distances along the bonds which connect the two equatorial belts decrease in the order 3.801, 3.780, and 3.736 A, making these distances the shortest for the three inequivalent sets of corner-sharing octahedra in each anion. This is consistent with the two or four added electrons localizing essentially in molecular orbitals which are bondiing with respect to interactions between the belts.     

The bis-phenyltin-substituted, lone-pair-containing tungstoarsenate [(C(6)h(5)Sn)(2)as(2)W(19)O(67)(H(2)O)](8)(-)

The bis-phenyltin-substituted, lone-pair-containing tungstoarsenate [(C(6)H(5)Sn)(2)As(2)W(19)O(67)(H(2)O)](8)(-) (1) has been synthesized and characterized by multinuclear NMR, IR, and elemental analysis. Single-crystal X-ray analysis was carried out on (NH(4))(7)Na[(C(6)H(5)Sn)(2)As(2)W(19)O(67)(H(2)O)].17.5H(2)O (NH(4)(-1), which crystallizes in the monoclinic system, space group P2(1)/c, with a = 18.3127(17) A, b = 24.403(2) A, c = 22.965(2) A, beta = 106.223(2) degrees, and Z = 4. Polyanion 1 consists of two B-alpha-(As(III)W(9)O(33)) Keggin moieties linked via a WO(H(2)O) fragment and two SnC(6)H(5) groups leading to a sandwich-type structure with nominal C(2)(v) symmetry. Polyanion 1 is stable in solution as indicated by the expected 6-line pattern (4:4:4:4:2:1) in (183)W NMR and the expected (119)Sn, (13)C, and (1)H NMR spectra. Synthesis of 1 was accomplished by reaction of C(6)H(5)SnCl(3) and K(14)[As(2)W(19)O(67)(H(2)O)] in a 2:1 molar ratio in aqueous acidic medium (pH 2). In the solid-state structure of NH(4)(-1, neighboring polyanions are weakly bound via W-O-Na bonds leading to chains which interact with each other via the phenyl rings resulting in a 2-D assembly.     

Experimental and theoretical investigations of the sulfite-based polyoxometalate cluster redox series: alpha- and beta-[Mo(18)O(54)(SO(3))(2)](4-/5-/6-)

The synthesis, isolation and structural characterization of the sulfite polyoxomolybdate clusters alpha-(D(3h))(C(20)H(44)N)(4){alpha-[Mo(18)O(54)(SO(3))(2)]}CH(3)CN and beta-(D(3d))(C(20)H(44)N)(4){beta-[Mo(18)O(54)(SO(3))(2)]}CH(3)CN is presented. Voltammetric studies in acetonitrile (0.1 M Hx(4)NClO(4), Hx(4)N=tetra-n-hexylammonium) reveal the presence of an extensive series of six one-electron reduction processes for both isomers. Under conditions of bulk electrolysis, the initial [Mo(18)O(54)(SO(3))(2)](4-/5-) and [Mo(18)O(54)(SO(3))(2)](5-/6-) processes produce stable [Mo(18)O(54)(SO(3))(2)](5-) and [Mo(18)O(54)(SO(3))(2)](6-) species, respectively, and the same reduced species may be produced by photochemical reduction. Spectroelectrochemical data imply that retention of structural form results upon reduction, so that both alpha and beta isomers are available at each of the 4-, 5-, and 6-redox levels. However, the alpha isomer is the thermodynamically favored species in both the one- and two-electron-reduced states, with beta-->alpha isomerization being detected in both cases on long time scales (days). EPR spectra also imply that increasing localization of the unpaired electron occurs over the alpha- and beta-[Mo(18)O(54)(SO(3))(2)](5-) frameworks as the temperature approaches 2 K where the EPR spectra show orthorhombic symmetry with different g and hyperfine values for the alpha and beta isomers. Theoretical studies support the observation that it is easier to reduce the alpha cluster than the beta form and also provide insight into the driving force for beta-->alpha isomerization in the reduced state. Data are compared with that obtained for the well studied alpha-[Mo(18)O(54)(SO(4))(2))](4-) sulfate cluster.     

Thermoanalytical Study of O,O'-Dibenzoyl-(2R,3R)-Tartaric Acid Supramolecular Compounds. Part I. Investigation of compounds with water, achiral alcohols and phenols

Thermal behaviour of O,O'-dibenzoyl-(2R,3R)-tartaric acid (DBTA), its monohydrate, and its potential supramolecular compounds with achiral alcohols and phenols were investigated with TG, DSC, EGD. The structural differences among the anhydrous DBTA, its monohydrate, and the supramolecular derivatives were investigated with X-ray powder diffraction. The thermal behaviour of DBTA-supramoleculars with isopropyl, tert-butyl, and 5-cyclohexyl alcohols is found to be similar to each other but essentially different from that of both DBTA and its monohydrate. On heating they melt and decompose between 60–180°C while they loose in one or two steps the bound alcohol. The thermal stability of the supramolecular compounds increases with the boiling point of the alcohol component. According to the X-ray powder diffraction patterns each supramolecular substance has different structure, that may also result in the different thermal stability of the compounds. The molar ratio of DBTA:achiral alcohol samples is 1:1.01–1:1.57 estimated from the corresponding mass losses. The XRD patterns of the prepared two DBTA-phenol materials are different from those of DBTA-achiral alcohol samples. The phenol compounds melt with slow mass losses and give an endothermic peak between 73–83°C but the melting point of the anhydrous DBTA cannot be observed. DBTA:phenol molar ratio is estimated to be 1:0.41 and 1:0.65 for phenol and 2-methylphenol, respectively. This revised version was published online in July 2006 with corrections to the Cover Date.     

Tetranuclear and Pentanuclear Vanadium(IV/V) Carboxylate Complexes: [V(4)O(8)(NO(3))(O(2)CR)(4)](2-) and [V(5)O(9)X(O(2)CR)(4)](2-) (X = Cl(-), Br(-)) Salts

The syntheses and properties of tetra- and pentanuclear vanadium(IV,V) carboxylate complexes are reported. Reaction of (NBzEt(3))(2)[VOCl(4)] (1a) with NaO(2)CPh and atmospheric H(2)O/O(2) in MeCN leads to formation of (NBzEt(3))(2)[V(5)O(9)Cl(O(2)CPh)(4)] 4a; a similar reaction employing (NEt(4))(2)[VOCl(4)] (1b) gives (NEt(4))(2)[V(5)O(9)Cl(O(2)CPh)(4)] (4b). Complex 4a.MeCN crystallizes in space group P2(1)2(1)2(1) with the following unit cell dimensions at -148 degrees C: a = 13.863(13) ?, b = 34.009(43) ?, c = 12.773(11) ?, and Z = 4. The reaction between (NEt(4))(2)[VOBr(4)] (2a) and NaO(2)CPh under similar conditions gives (NEt(4))(2)[V(5)O(9)Br(O(2)CPh)(4)] (6a), and the use of (PPh(4))(2)[VOBr(4)] (2b) likewise gives (PPh(4))(2)[V(5)O(9)Br(O(2)CPh)(4)] (6b). Complex 6b crystallizes in space group P2(1)2(1)2(1) with the following unit cell dimensions at -139 degrees C: a = 18.638(3) ?, b = 23.557(4) ?, c = 12.731(2) ?, and Z = 4. The anions of 4a and 6b consist of a V(5) square pyramid with each vertical face bridged by a &mgr;(3)-O(2)(-) ion, the basal face bridged by a &mgr;(4)-X(-) (X = Cl, Br) ion, and a terminal, multiply-bonded O(2)(-) ion on each metal. The RCO(2)(-) groups bridge each basal edge to give C(4)(v)() virtual symmetry. The apical and basal metals are V(V) and V(IV), respectively (i.e., the anions are trapped-valence). The reaction of 1b with AgNO(3) and Na(tca) (tca = thiophene-2-carboxylate) in MeCN under anaerobic conditions gives (NEt(4))(2)[V(4)O(8)(NO(3))(tca)(4)] (7). Complex 7.H(2)O crystallizes in space group C2/c with the following unit cell dimensions at -170 degrees C: a = 23.606(4) ?, b = 15.211(3) ?, c = 23.999(5) ?, and Z = 4. The anion of 7 is similar to those of 4a and 6b except that the apical [VO] unit is absent, leaving a V(4) square unit, and the &mgr;(4)-X(-) ion is replaced with a &mgr;(4),eta(1)-NO(3)(-) ion. The four metal centers are now at the V(IV), 3V(V) oxidation level, but the structure indicates four equivalent V centers, suggesting an electronically delocalized system. Variable-temperature magnetic susceptibility data were collected on powdered samples of 4b, 6a, and 7 in the 2.00-300 K range in a 10 kG applied field. 4b and 6a both show a slow increase in effective magnetic moment (&mgr;(eff)) from approximately 3.6-3.7 &mgr;(B) at 320 K to approximately 4.5-4.6 &mgr;(B) at 11.0 K and then a slight decrease to approximately 4.2 &mgr;(B) at 2.00 K. The data were fit to the theoretical expression for a V(IV)(4) square with two exchange parameters J = J(cis)() and J' = J(trans)() (H = -2JS(i)()S(j)()): fitting of the data gave, in the format 4b/6a, J= +39.7/+46.4 cm(-)(1), J' = -11.1/-18.2 cm(-)(1) and g = 1.83/1.90, with the complexes possessing S(T) = 2 ground states. The latter were confirmed by magnetization vs field studies in the 2.00-30.0 K and 0.500-50.0 kG ranges: fitting of the data gave S(T) = 2 and D = 0.00 cm(-)(1) for both complexes, where D is the axial zero-field splitting parameter. Complex 7 shows a nearly temperature-independent &mgr;(eff) (1.6-2.0 &mgr;(B)) consistent with a single d electron per V(4) unit. The (1)H NMR spectra of 4b and 6a in CD(3)CN are consistent with retention of their pentanuclear structure on dissolution. The EPR spectrum of 7 in a toluene/MeCN (1:2) solution at approximately 25 degrees C yields an isotropic signal with a 29-line hyperfine pattern assignable to hyperfine interactions with four equivalent I = (7)/(2) (51)V nuclei.     

Rational synthesis and characterization of the mixed-metal organometallic polyoxometalates [Cp*Mo(x)W(6-x)O18]- (x = 0, 1, 5, 6)

The reaction between the oxometallic complexes Cp*(2)M(2)O(5) and Na(2)M'O(4) (M, M' = Mo, W) in a 1:10 molar ratio in an acidic aqueous medium constitutes a mild and selective entry into the anionic Lindqvist-type hexametallic organometallic mixed oxides [Cp*Mo(x)W(6-x)O(18)](-) [x = 6 (1), 5 (2), 1 (3), 0 (4)]. All of these compounds have been isolated as salts of nBu(4)N(+) (a), nBu(4)P(+) (b), and Ph(4)P(+) (c) cations and two of them (1 and 3) also with the n-butylpyridinium (nBuPyr(+), d) cation. The compounds have been characterized by elemental analyses, thermogravimetric analyses, electrospray mass spectrometry, and IR spectroscopy. The molecular identity and geometry of compounds 1c, 2a, and 2c have been confirmed by single-crystal X-ray diffraction. Density functional theory calculations on models obtained by replacing Cp* with Cp (I-IV) have provided information on the assignment of the terminal M═O and bridging M-O-M vibrations.