{Bis[2-(diphenyl-phosphino)eth-yl]phenyl-phosphine-κP,P',P''}chloridopalladium(II) hexa-fluoridophosphate.

In the title compound, [PdCl(C(34)H(33)P(3))]PF(6), the Pd(II) atom adopts a distorted PdP(3)Cl square-planar geometry arising from the P,P',P''-tridentate triphos ligand and a chloride ion.

The role of the second coordination sphere of [Ni(PCy2NBz2)2](BF4)2 in reversible carbon monoxide binding.

The complex [Ni(PCy2NBz2)2](BF4)2, 1, reacts rapidly and reversibly with carbon monoxide (1 atm) at 25 degrees C to form [Ni(CO)(PCy2NBz2)2](BF4)2, 2, which has been characterized by spectroscopic data and by an X-ray diffraction study. In contrast, analogous Ni(II) carbonyl adducts were not observed in studies of several other related nickel(II) diphosphine complexes. The unusual reactivity of 1 is attributed to a complex interplay of electronic and structural factors, with an important contribution being the ability of two positioned amines in the second coordination sphere to act in concert to stabilize the CO adduct. The proposed interaction is supported by X-ray diffraction data for 2 which shows that all of the chelate rings of the cyclic ligands are in boat conformations, placing two pendant amines close (3.30 and 3.38 A) to the carbonyl carbon. Similar close C-N interactions are observed in the crystal structure of the more sterically demanding isocyanide adduct, [Ni(CNCy)(PCy2NBz2)2]2(BF4)2, 4. The data suggest a weak electrostatic interaction between the lone pairs of the nitrogen atoms and the positively charged carbon atom of the carbonyl or isocyanide ligand, and illustrate a novel (non-hydrogen bonding) second coordination sphere effect in controlling reactivity.

4-amino- and 4-nitrodipicolinatovanadium(V) complexes and their hydroxylamido derivatives: synthesis, aqueous, and solid-state properties.

A number of 4-substituted, dipicolinatodioxovanadium(V) complexes and their hydroxylamido derivatives were synthesized to characterize the solid state and solution properties of five- and seven-coordinate vanadium(V) complexes. The X-ray crystal structures of Na[VO2dipic-NH2].2H2O (2) and K[VO2dipic-NO2] (3) show the vanadium adopting a distorted, trigonal-bipyramidal coordination environment similar to the parent coordination complex, [VO2dipic]- (1), reported previously as the Cs+ salt. The observed differences in the chemical shifts of the complexes both in the 1H (ca. 0.7-1.4 ppm) and 51V (ca. 1-11 ppm) NMR spectra were consistent with the electron-donating or electron-withdrawing properties of the substituent groups, respectively. Stoichiometric addition of a series of hydroxylamine ligands (H2NOH, MeHNOH, Me2NOH, and Et2NOH) to complexes 1-3 led to the formation of seven-coordinate vanadium(V) complexes. The X-ray crystal structure of [VO(dipic)(Me2NO)(H2O)].0.5H2O (1c) was found to be similar to the previously characterized complexes [VO(dipic)(H2NO)(H2O)] (1a) and [VO(dipic)(OO-tBu)(H2O)]. While only slight differences in the 1H NMR spectra were observed upon addition of the hydroxylamido ligand, the signals in the 51V NMR spectra change by up to 100 ppm. The addition of the hydroxylamido ligand increased the complex stability of complexes 2 and 3. Evidence for a nonstoichiometric redox reaction was found for the monoalkyl hydroxylamine ligand. The reaction of an unsaturated five-coordinate species with a hydroxylamine to form a seven-coordinate vanadium complex will, in general, dramatically increase the amounts of the vanadium compound that remain intact at pH values near neutral.

Synthesis and structure of Ag(1-Me-12-SiPh3-CB11F10): selective F12 substitution in 1-Me-CB11F11- and the first Ag(arene)4+ tetrahedron.

The selective substitution of the antipodal F atom in 1-Me-CB(11)F(11)- with a SiPh(3) moiety led to the isolation and structure determination of the cesium(I) and silver(I) salts of the 1-Me-12-SiPh(3)-CB(11)F(10)- anion. The silver salt contains both a nearly trigonal-planar Ag(arene)(3)+ cation and the first example of a Ag(arene)(4)+ cation.

X-ray structure and DFT study of C1-C60(CF3)12. A high-energy, kinetically-stable isomer prepared at 500 degrees C.

The title compound, prepared from C(60) and CF(3)I at 500 degrees C, exhibits an unusual fullerene(X)12 addition pattern that is 40 kJ mol(-1) less stable than the previously reported C(60)(CF(3))12 isomer.

Thermally stable perfluoroalkylfullerenes with the skew-pentagonal-pyramid pattern: C60(C2F5)4O, C60(CF3)4O, and C60(CF3)6.

Reaction of C(60) with CF(3)I at 550 degrees C, which is known to produce a single isomer of C(60)(CF(3))(2,4,6) and multiple isomers of C(60)(CF(3))(8,10), has now been found to produce an isomer of C(60)(CF(3))(6) with the C(s)-C(60)X(6) skew-pentagonal-pyramid (SPP) addition pattern and an epoxide with the C(s)-C(60)X(4)O variation of the SPP addition pattern, C(s)-C(60)(CF(3))(4)O. The structurally similar epoxide C(s)-C(60)(C(2)F(5))(4)O is one of the products of the reaction of C(60) with C(2)F(5)I at 430 degrees C. The three compounds have been characterized by mass spectrometry, DFT quantum chemical calculations, Raman, visible, and (19)F NMR spectroscopy, and, in the case of the two epoxides, single-crystal X-ray diffraction. The compound C(s)-C(60)(CF(3))(6) is the first [60]fullerene derivative with adjacent R(f) groups that are sufficiently sterically hindered to cause the (DFT-predicted) lengthening of the cage (CF(3))C-C(CF(3)) bond to 1.60 A as well as to give rise to a rare, non-fast-exchange-limit (19)F NMR spectrum at 20 degrees C. The compounds C(s)-C(60)(CF(3))(4)O and C(s)-C(60)(C(2)F(5))(4)O are the first poly(perfluoroalkyl)fullerene derivatives with a non-fluorine-containing exohedral substituent and the first fullerene epoxides known to be stable at elevated temperatures. All three compounds demonstrate that the SPP addition pattern is at least kinetically stable, if not thermodynamically stable, at temperatures exceeding 400 degrees C. The high-temperature synthesis of the two epoxides also indicates that perfluoroalkyl substituents can enhance the thermal stability of fullerene derivatives with other substituents.

Synthesis and structures of poly(perfluoroethyl)[60]fullerenes: 1,7,16,36,46,49-C60(C2F5)6 and 1,6,11,18,24,27,32,35-C60(C2F5)8.

The high-temperature reaction of C60 and C2F5I produced poly(perfluoroethyl)fullerenes with unprecedented addition patterns.

Cd(II), Zn(II), and Pd(II) complexes of an isoindoline pincer ligand: consequences of steric crowding.

The sterically crowded isoindoline pincer ligand, 6'-MeLH, prepared by condensation of 4-methyl-2-aminopyridine and phthalonitrile, exhibits very different reaction chemistry with Cd2+, Zn2+, and Pd2+. Three different ligand coordination modes are reported, each dependent upon choice of metal ion. This isoindoline binds to Cd2+ as a charge-neutral, zwitterionic, bidentate ligand using imine and pyridine nitrogen atoms to form the eight-coordinate fluxional complex, Cd(6'-MeLH)2(NO3)2. In the presence of Zn2+, however, loss of a pyridine arm occurs through solvolysis and tetrahedrally coordinated complexes are formed with coordination of pyrrole and pyridine nitrogen atoms. Reaction with Pd2+ produces the highly distorted, square planar complex Pd(6'-MeL)Cl in which a deprotonated isoindoline anion coordinates as a tridentate pyridinium NNC pincer ligand.

Synthesis, structure, and 19F NMR spectra of 1,3,7,10,14,17,23,28,31,40-C60(CF3)10.

A significant improvement in the selectivity of fullerene trifluoromethylation reactions was achieved. Reaction of trifluoroiodomethane with [60]fullerene at 460 degrees C and [70]fullerene at 470 degrees C in a flow reactor led to isolation of cold-zone-condensed mixtures of C60(CF3)n and C70(CF3)n compounds with narrow composition ranges: 6 < or = n < or = 12 for C(60)(CF3)n and 8 < or = n < or = 14 for C70(CF3)n. The predominant products in the C(60) reaction, an estimated 40+ mol % of the cold-zone condensate, were three isomers of C60(CF3)10. Two of these were purified by two-stage HPLC to 80+% isomeric purity. The third isomer was purified by three-stage HPLC to 95% isomeric purity. Thirteen milligrams of this orange-brown compound was isolated (5% overall yield based on C60, and its C1-symmetric structure was determined to be 1,3,7,10,14,17,23,28,31,40-C60(CF3)10 by X-ray crystallography. The CF3 groups are either meta or para to one another on a p-m-p-p-p-m-p-m-p ribbon of edge-sharing C6(CF3)2 hexagons (each pair of adjacent hexagons shares a common CF3 group). The selectivity of the C70 reaction was even higher. The predominant product was a single C70(CF3)10 isomer representing >40 mol % of the cold-zone condensate. Single-stage HPLC led to the isolation of 12 mg of this brown compound in 95% isomeric purity (27% overall yield based on converted C70. The new compounds were characterized by EI or S(8)-MALDI mass spectrometry and 2D-COSY 19F NMR spectroscopy. The NMR data demonstrate that through-space coupling via direct overlap of fluorine orbitals is the predominant contribution to J(FF) values in these and most other fullerene(CF3)n compounds.

Inhibition of yeast growth by molybdenum-hydroxylamido complexes correlates with their presence in media at differing pH values.

The effects of Mo-hydroxylamido complexes on cell growth were determined in Saccharomyces cerevisiae to investigate the biological effects of four different Mo complexes as a function of pH. Studies with yeast, an eukaryotic cell, are particularly suited to examine growth at different pH values because this organism grows well from pH 3 to 6.5. Studies can therefore be performed both in the presence of intact complexes and when the complexes have hydrolyzed to ligand and free metal ion. One of the complexes we examined was structurally characterized by X-ray crystallography. Yeast growth was inhibited in media solutions containing added Mo-dialkylhydroxylamido complexes at pH 3-7. When combining the yeast growth studies with a systematic study of the Mo-hydroxylamido complexes' stability as a function of pH and an examination of their speciation in yeast media, the effects of intact complexes can be distinguished from that of ligand and metal. This is possible because different effects are observed with complex present than when ligand or metal alone is present. At pH 3, the growth inhibition is attributed to the forms of molybdate ion that exist in solution because most of the complexes have hydrolyzed to oxomolybdate and ligand. The monoalkylhydroxylamine ligand inhibited yeast growth at pH 5, 6 and 7, while the dialkylhydroxylamine ligands had little effect on yeast growth. Growth inhibition of the Mo-dialkylhydroxylamido complexes is observed when a complex exists in the media. A complex that is inert to ligand exchange is not effective even at pH 3 where other Mo-hydroxylamido complexes show growth inhibition as molybdate. These results show that the formation of some Mo complexes can protect yeast from the growth inhibition observed when either the ligand or Mo salt alone are present.

Using ligand bite angles to control the hydricity of palladium diphosphine complexes.

A series of [Pd(diphosphine)(2)](BF(4))(2) and Pd(diphosphine)(2) complexes have been prepared for which the natural bite angle of the diphosphine ligand varies from 78 degrees to 111 degrees. Structural studies have been completed for 7 of the 10 new complexes described. These structural studies indicate that the dihedral angle between the two planes formed by the two phosphorus atoms of the diphosphine ligands and palladium increases by over 50 degrees as the natural bite angle increases for the [Pd(diphosphine)(2)](BF(4))(2) complexes. The dihedral angle for the Pd(diphosphine)(2) complexes varies less than 10 degrees for the same range of natural bite angles. Equilibrium reactions of the Pd(diphosphine)(2) complexes with protonated bases to form the corresponding [HPd(diphosphine)(2)](+) complexes were used to determine the pK(a) values of the corresponding hydrides. Cyclic voltammetry studies of the [Pd(diphosphine)(2)](BF(4))(2) complexes were used to determine the half-wave potentials of the Pd(II/I) and Pd(I/0) couples. Thermochemical cycles, half-wave potentials, and measured pK(a) values were used to determine both the homolytic ([HPd(diphosphine)(2)](+) --> [Pd(diphosphine)(2)](+) + H*) and the heterolytic ([HPd(diphosphine)(2)](+) --> [Pd(diphosphine)(2)](2+) + H(-)) bond-dissociation free energies, Delta G(H*)* and Delta G(H-)*, respectively. Linear free-energy relationships are observed between pK(a) and the Pd(I/0) couple and between Delta G(H-)* and the Pd(II/I) couple. The measured values for Delta G(H*)* were all 57 kcal/mol, whereas the values of Delta G(H-)* ranged from 43 kcal/mol for [HPd(depe)(2)](+) (where depe is bis(diethylphosphino)ethane) to 70 kcal/mol for [HPd(EtXantphos)(2)](+) (where EtXantphos is 9,9-dimethyl-4,5-bis(diethylphosphino)xanthene). It is estimated that the natural bite angle of the ligand contributes approximately 20 kcal/mol to the observed difference of 27 kcal/mol for Delta G(H-)*.

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).

Synthesis and characterization of ammonioundecafluoro-closo-dodecaborates(1-). New superweak anions.

The ammonioborane monoanion H(3)NB(12)H(11)(-) was per-B-fluorinated with elemental fluorine in liquid hydrogen fluoride to yield the first member of a new class of weakly coordinating anions, H(3)NB(12)F(11)(-) (isolated as [N(n-Bu)(4)](2)[H(2)NB(12)F(11)] in 41% yield). The pK(a) of the H(3)NB(12)F(11)(-) anion is 9.6. Several salts of the tri-N-alkylated anions Me(3)NB(12)F(11)(-) and Dd(3)NB(12)F(11)(-) (Dd = n-C(12)H(25)) were also prepared. The structure of [CPh(3)][Me(3)NB(12)F(11)] was determined by single-crystal X-ray diffraction: monoclinic, space group P2(1)/c, a = 18.053(3) A, b = 33.139(5) A, c = 9.600(2) A, beta = 91.459(4) degrees, V = 5742(2) A(3), Z = 8, T = 173(2) K, R(1) = 0.045. It revealed that the only direct interactions between the undecafluoroammonioborate monoanions and the trityl cations in the two independent ion pairs were long and weak BF...CPh(3) interactions of 2.992(6) and 2.942(6) A. Salts of the new anions were chemically, electrochemically, and thermally stable. The conductivity of Li(Me(3)NB(12)F(11)) in dimethoxyethane was comparable to that of LiPF(6) but less than half the value of Li(1-Me-CB(11)F(11)).

Synthesis and stability of reactive salts of dodecafluoro-closo-dodecaborate(2-).

The synthesis and characterization of several salts of the B(12)F(12)(2-) anion are reported. The potassium salt was prepared in 72% recrystallized yield by treating K(2)B(12)H(12) with liquid HF at 70 degrees C for 14 h and 20% F(2)/N(2) in liquid HF at 25 degrees C for 72 h. The CPh(3)(+), N(n-Bu)(4)(+), NH(n-C(12)H(25))(3)(+), NH(4)(+), and Li(+) salts were prepared by metathesis reactions. The [NH(n-C(12)H(25))(3)](2)[B(12)F(12)] salt is soluble in aromatic hydrocarbon solvents. The B(12)F(12)(2-) anion is remarkably stable. The salts Li(2)B(12)F(12) and [NH(4)](2)[B(12)F(12)] were stable when heated to 450 and 480 degrees C, respectively. The B(12)F(12)(2-) anion did not react with 98% H(2)SO(4), 70% HNO(3), 3 M KOH, a 10-fold excess of Ce(NH(4))(2)(NO(3))(6) in aqueous solution, or metallic sodium in THF. In addition, B(12)F(12)(2-) did not react with metallic lithium in a mixture of ethylene carbonate and dimethyl carbonate, was not reduced at 0 V versus Li(+/0) in that solvent, and underwent a quasi-reversible oxidation at 4.9 V versus Li(+/0). The structure of [CPh(3)](2)[B(12)F(12)] was determined by single-crystal X-ray diffraction: tetragonal, space group I4(1)/acd, a = 19.102(2), b = 19.102(2), c = 20.535(3) A, V = 7492.2(2) A(3), Z = 8, T = 173(2) K, R(1) = 0.064. The B(12)F(12)(2-) anion weakly interacts with the two symmetry related CPh(3)(+) cations via F.C contacts of 3.087(2) A, which are very close to the 3.17 A sum of van der Waals radii for these two atoms. Taken together, the data suggest that B(12)F(12)(2-) may be useful as a very robust weakly coordinating anion.

Cobalt(II) and cobalt(III) dipicolinate complexes: solid state, solution, and in vivo insulin-like properties.

The synthesis and characterization of Co(II) and Co(III) 2,6-pyridinedicarboxylate (dipic(2-)) complexes are reported. Solid-state X-ray characterizations were performed on [Co(H(2)dipic)(dipic)].3H(2)O and [Co(dipic)(mu-dipic)Co(H(2)O)(5)].2H(2)O. Two coordination modes not previously observed in dipicolinate transition metal complexes were observed in these complexes; one involves metal coordination to the short C-O (C=O) bond, and the other involves metal coordination to a protonated oxygen atom. Solution studies, including paramagnetic NMR and UV-vis spectroscopy, were done showing the high stability and low lability of the Co(III) complex, whereas the Co(II) complexes exhibited ligand exchange in the presence of excess ligand. The [Co(dipic)(2)](2-) complex has pH dependent lability and in this regard is most similar to the [VO(2)dipic](-) complex. The [Co(dipic)(2)](2-) was found to be effective in reducing the hyperlipidemia of diabetes using oral administration in drinking water in rats with STZ-induced diabetes. Oral administration of VOSO(4) was used as a positive control for metal efficacy against diabetes. In addition to providing a framework to evaluate structure-function relationships of various transition metal complexes in alleviating the symptoms of diabetes, this work describes novel aspects of structural and solution cobalt chemistry.

Weakly coordinating anions: crystallographic and NQR studies of halogen-metal bonding in silver, thallium, sodium, and potassium halomethanesulfonates.

35Cl, (79,81)Br, and (127)I NQR (nuclear quadrupole resonance) spectroscopy in conjunction with X-ray crystallography is potentially one of the best ways of characterizing secondary bonding of metal cations such as Ag(+) to halogen donor atoms on the surfaces of very weakly coordinating anions. We have determined the X-ray crystal structure of Ag(O(3)SCH(2)Cl) (a = 13.241(3) A; b = 7.544(2) A; c = 4.925(2) A; orthorhombic; space group Pnma; Z = 4) and compared it with the known structure of Ag(O(3)SCH(2)Br) (Charbonnier, F.; Faure, R.; Loiseleur, H. Acta Crystallogr., Sect. B 1978, 34, 3598-3601). The halogen atom in each is apical (three-coordinate), being weakly coordinated to two silver ions. (127)I NQR studies on Ag(O(3)SCH(2)I) show the expected NQR consequences of three-coordination of iodine: substantially reduced NQR frequencies nu(1) and nu(2) and a fairly small NQR asymmetry parameter eta. The reduction of the halogen NQR frequency of the coordinating halogen atom in Ag(O(3)SCH(2)X) becomes more substantial in the series X = Cl < Br < I, indicating that the coordination to Ag(+) strengthens in this series, as expected from hard-soft acid-base principles. The numbers of electrons donated by the organic iodine atom to Ag(+) have been estimated; these indicate that the bonding to the cation is weak but not insignificant. We have not found any evidence for the bonding of these organohalogen atoms to another soft-acid metal ion, thallium. A scheme for recycling of thallium halide wastes is included.

Stepwise Cluster Assembly Using VO(2)(acac) as a Precursor: cis-[VO(OCH(CH(3))(2))(acac)(2)], [V(2)O(2)(&mgr;-OCH(3))(2)(acac)(2)(OCH(3))(2)], [V(3)O(3){&mgr;,&mgr;-(OCH(2))(3)CCH(3)}(2)(acac)(2)(OC(2)H(5))], and [V(4)O(4)(&mgr;-O)(2)(&mgr;-OCH(3))(2)(&mgr;(3)-OCH(3))(2)(acac)(2)(OCH(3))(2)].2CH(3)CN(1).

The studies of an underexplored synthetic reagent, VO(2)(acac) (Hacac = acetylacetone) and semirational strategies for the formation of a complete series of simple vanadium(V) alkoxide clusters in alcohol-containing solvents. The neutral mono-, di-, tri-, and tetranuclear oxovanadium(V) complexes [V(2)O(2)(&mgr;-OCH(3))(2)(acac)(2)(OCH(3))(2)] (1), [V(4)O(4)(&mgr;-O)(2)(&mgr;-OCH(3))(2)(&mgr;(3)-OCH(3))(2)(acac)(2)(OCH(3))(2)].2CH(3)CN (2), [V(4)O(4)(&mgr;-O)(2)(&mgr;-OCH(3))(2)(&mgr;(3)-OCH(3))(2)(acac)(2)(OCH(3))(2)] (3), [V(3)O(3){&mgr;,&mgr;-(OCH(2))(3)CCH(3)}(2)(acac)(2)(OR)] (R = CH(3) (4), C(2)H(5) (5)), and cis-[VO(OCH(CH(3))(2))(acac)(2)] (6) with alkoxide and acac(-) ligands were obtained by reaction of VO(2)(acac) with a monoalcohol and/or a tridentate alcohol. The structures of complexes 1-3, 5, and 6 were determined by X-ray diffraction methods. Complex 1 crystallized in the monoclinic system, P2(1)/n, with a = 7.8668(5) Å, b = 15.1037(9) Å, c = 8.5879(5) Å, beta = 106.150(1) degrees, V = 980.1(1) Å(3), Z = 2, and R (wR2) = 0.040 (0.121). Complex 2 crystallized in the monoclinic system, P2(1)/n, with a = 8.531(2) Å, b = 14.703(3) Å, c = 12.574(2) Å, beta = 95.95(2) degrees, V = 1568.7(5) Å(3), Z = 2, and R (wR2) = 0.052 (0.127). Complex 3 crystallized in the triclinic system, P&onemacr;, with a = 8.5100(8) Å, b = 8.9714(8) Å, c = 10.3708(10) Å, alpha = 110.761(1) degrees, beta = 103.104(1) degrees, gamma = 100.155(1) degrees, V = 691.85(11) Å(3), Z = 1, and R (wR2) = 0.040 (0.105). Complex 5 crystallized in the monoclinic system, P2(1)/n, with a = 14.019(2) Å, b = 11.171(2) Å, c = 19.447(3) Å, beta = 109.18(1) degrees, V = 2876.5(8) Å(3), Z = 4, and R (wR2) = 0.062 (0.157). Complex 6 crystallized in the monoclinic system, P2(1)/n, with a = 15.0023(8) Å, b = 8.1368(1) Å, c = 26.5598(2) Å, beta = 95.744(1) degrees, V = 3225.89(8) Å(3), Z = 8, and R (wR2) = 0.060 (0.154). Complex 1 is a discrete, centrosymmetric dimer in which two vanadium atoms are bridged by two methoxide ligands. Compound 2 contains a V(4)O(4) eight-membered ring with both &mgr;-oxo and &mgr;-alkoxo bridging ligands; the ring is capped above and below by two triply bridging methoxo ligands. Compound 3 has the same structure as 2. The three vanadium atoms in complex 5 are linked by four bridging oxygen atoms from two tridentate thme(3)(-) ligands to form a V(3)O(4) chain in which V-O bonds alternate in length. The V-O(isopropoxo) bond in 6 is cis to V=O, and the V-O(acac) bond trans to the oxo group is relatively long. The V(2)O(2) rings of complex 1 and the mononuclear 1:2 complex can be considered to be the basic building block of the trinuclear complexes 4 and 5 and the tetranuclear complex 2, acting to extend the vanadium-oxide framework. (51)V and (1)H NMR spectroscopic studies of the solution state of complexes 1-6 revealed dramatic differences in structural and hydrolytic stability of these complexes. Compounds 1 and 3 only remained intact at low temperature in CDCl(3) solution, whereas the mononuclear compound 6 could remain at ambient temperature for approximately 10 h. Compound 4 only maintained its solid-state structure at low temperature in CDCl(3) solution, whereas compound 5 was significantly more stable. The structural integrity of oligomeric vanadium-oxygen frameworks increased significantly when the coordinating alkoxide group showed more resistance to exchange reactions than the methoxide group. The solid state and solution properties of this new group of complexes not only testify to the versatility of VO(2)(acac) as a vanadium(V) precursor but also raise questions relating to solution structure and properties of related vanadium complexes with insulin-mimetic properties and catalytic properties.

Dinuclear Oxovanadium(IV) N-(Phosphonomethyl)iminodiacetate Complexes: Na(4)[V(2)O(2){(O)(2)P(O)CH(2)N(CH(2)COO)(2)}(2)].10H(2)O and Na(8)[V(2)O(2){(O)(2)P(O)CH(2)N(CH(2)COO)(2)}(2)](2).16H(2)O(1).

The dioxovanadium(IV) complexes with pida(4)(-) ligands (H(4)pida) = N-(phosphonomethyl)iminodiacetic acid), Na(4)[V(2)O(2){(O)(2)P(O)CH(2)N(CH(2)COO)(2)}(2)].10H(2)O (1) and Na(8)[V(2)O(2){(O)(2)P(O)CH(2)N(CH(2)COO)(2)}(2)](2).16H(2)O (2), were isolated from reactions of H(4)pida with either oxovanadium(V) (i.e., NaVO(3)) or oxovanadium(IV) precursors within the pH range of 2-8. The structures of complexes 1 and 2 were investigated by X-ray diffraction methods and in contrast to expectation were both found to be dinuclear. Complex 1 crystallized in the monoclinic system: P2(1)/n, a = 10.5632(1) Å, b = 11.1868(1) Å, c = 12.6921(1) Å, beta = 106.45 degrees, V = 1438.44(2) Å(3), Z = 4, and R (wR2) = 0.0781 (0.2017). Complex 2 crystallized in the monoclinic system P2(1)/c: a = 13.9822(2) Å, b = 11.1888(2) Å, c = 18.6519(3) Å, beta = 100.88 degrees, V = 2865.51(8) Å(3), Z = 4, and R (wR2) = 0.046 (0.125). Both complexes 1 and 2 have similar dimeric frameworks where two vanadium centers are linked by two phosphonate groups of two pida(4)(-) ligands (quadridentate binucleating), bridging through their four oxygen atoms to form a V(2)O(4)P(2) eight-membered ring which possesses a crystallographic inversion center. In contrast to their solid-state features, in aqueous solution both dinuclear crystalline compounds immediately dissociate to monomeric species, as observed by EPR and UV/vis spectroscopy. Both solution-state EPR and NMR spectroscopy confirmed that redox chemistry is involved in the reaction between vanadate and H(4)pida. Studies in mixed solvent systems showed that the dinuclear complex would remain intact in the presence of sufficient organic solvent. In the absence of oxygen the mononuclear and the dinuclear complexes will reversibly interconvert, whereas, in the presence of oxygen, the complexes will oxidize. These studies document the existence of higher oligomeric vanadium compounds and surprisingly, in general, lend credibility to several emerging mechanistic proposals involving oligomeric species of vanadium compounds in catalytic processes.