Isolation of the novel dirhodium(II/II) thiolate compound Rh(2)(eta(1)-C(6)H(5)S)(2)(mu-C(6)H(5)S)(2)(bpy)(2)

The reaction of the anticancer active compound [Rh(2)(mu-O(2)CCH(3))(2)(bpy)(2)(CH(3)CN)(2)][BF(4)](2) (1) (bpy = 2,2'-bipyridine) with NaC(6)H(5)S under anaerobic conditions yields Rh(2)(eta(1)-C(6)H(5)S)(2)(mu-C(6)H(5)S)(2)(bpy)(2).CH(3)OH (2), which was characterized by UV-visible, IR, and (1)H NMR spectroscopies as well as single-crystal X-ray crystallography. Compound 2 crystallizes as dark red platelets in the monoclinic space group C2/c with cell parameters a = 20.398(4) A, b = 11.861(2) A, c = 17.417(4) A, beta = 108.98 degrees, V = 3984.9(14) A(3), Z = 4. The main structural features are the presence of a [Rh(2)](4+) core with a Rh-Rh distance of 2.549(2) A bridged by two benzene thiolate ligands in a butterfly-type arrangement. The axial positions of the [Rh(2)](4+) core are occupied by two terminal benzene thiolates. Cyclic voltammetric studies of 2 reveal that the compound exhibits an irreversible oxidation at +0.046 V in CH(3)CN, which is in accord with the fact that the compound readily oxidizes in the presence of O(2). The fact that this unusual dirhodium(II/II) thiolate compound is formed under these conditions is an important first step in understanding the metabolism of dirhodium anticancer active compounds with thiol-containing peptides and proteins.     

New dinuclear catalysts Rh(2)(N-O)(2)[(C6H4)P(C6H5)2]2 with imidate ligands: synthesis and isomerization from head-to-tail to head-to-head configuration of the imidate ligands

Two new dirhodium(II) catalysts of general formula Rh(2)(N-O)(2)[(C(6)H(4))P(C(6)H(5))(2)](2) (N-O = C(4)H(4)NO(2)) are prepared, starting from Rh(2)(O(2)CCH(3))(2)(PC)(2)L(2) [PC = (C(6)H(4))P(C(6)H(5))(2) (head-to-tail arrangement); L = HO(2)CCH(3)]. The thermal reaction of Rh(2)(O(2)CCH(3))(2)(PC)(2).L(2) with the neutral succinimide stereoselectively gives one compound that according to the X-ray structure determination has the formula Rh(2)(C(4)H(4)NO(2))(2)[(C(6)H(4))P(C(6)H(5))(2)](2) (1). It corresponds to the polar isomer with two bridging imidate ligands in a head-to-head configuration. However, stepwise reaction of Rh(2)(O(2)CCH(3))(2)(PC)(2).L(2) with (CH(3))(3)SiCl and potassium succinimidate yields a mixture of 1 and one of the two possible isomers (structure B) with a head-to-tail configuration of the imidate ligands, Rh(2)(C(4)H(4)NO(2))(2)[(C(6)H(4))P(C(6)H(5))(2)](2) (2), also characterized by X-ray methods. In solution, compound 2 undergoes slow isomerization to 1; the rate of this process is enhanced by the presence of acetonitrile. Compounds 1 and 2 are obtained as pure enantiomers starting from (M)- and (P)-Rh(2)(O(2)CCH(3))(2)(PC)(2).L(2) rather than from the racemic mixture. Their enantioselectivities in cyclopropanation of 1-diazo-5-penten-2-one are similar to those reported for the dirhodium amidate catalysts.     

Dendritic arrays of [Re6(mu3-Se)8]2+ core-containing clusters: exploratory synthesis and electrochemical studies

The reaction between the previously reported site-differentiated cluster solvate [Re(6)(mu(3)-Se)(8)(PEt(3))(5)(MeCN)](SbF(6))(2) (1) with pyridyl-based ditopic ligands 4,4'-trimethylenedipyridine (2), 1,2-bis(4-pyridyl)ethane (3), and (E)-1,2-bis(4-pyridyl)ethene (4) afforded cluster complexes of the general formula [Re(6)(mu(3)-Se)(8)(PEt(3))(5)(L)](SbF(6))(2) (5-7), where L represents one of the pyridyl-based ligands. Reacting these cluster complex-based ligands with the fully solvated cluster complex [Re(6)(mu(3)-Se)(8)(MeCN)(6)](SbF(6))(2) (8) produced dendritic arrays of the general formula {Re(6)(mu(3)-Se)(8)[Re(6)(mu(3)-Se)(8)(PEt(3))(5)(L)](6)}(SbF(6))(14) (9-11), each featuring six circumjacent [Re(6)(mu(3)-Se)(8)(PEt(3))(5)](2+) units bridged to a [Re(6)(mu(3)-Se)(8)](2+) core cluster by the pyridyl-based ligands. Electrochemical studies using a thin-layer electrochemical cell revealed cluster-based redox events in these cluster arrays. For 9 (L = 2), one reversible oxidation event corresponding to the removal of 7 electrons was observed, indicating noninteraction or extremely weak interactions between the clusters. For 10 (L = 3), two poorly resolved oxidation waves were found. For 11 (L = 4), two reversible oxidation events, corresponding respectively to the removal of 1 and 6 electrons, were observed with the 1-electron oxidation event occurring at a potential 150 mV more positive than the 6-electron oxidation. These electrochemical studies suggest intercluster coupling in 11 via through-bond electronic delocalization, which is consistent with electronic spectroscopic studies of this same molecule.     

Polymer-bound dirhodium tetracarboxylate films for fluorescent detection of nitric oxide

A polymer-bound dirhodium complex, [{Rh2(O2CCH3)3(Ds-pip)}n(O2C-P)], was prepared via ligand exchange of [Rh2(O2CCH3)4] with the side chains of a methyl methacrylate/methacrylic acid copolymer (O2C-P) followed by axial coordination of the fluorophore, N-dansylpiperazine (Ds-pip). Emission from Ds-pip is quenched when coordinated to the dirhodium complex but can be restored upon displacement by analytes. Exposure of [{Rh2(O2CCH3)3(Ds-pip)}n(O2C-P)] films to aqueous nitric oxide (NO) evokes a 2.2-fold increase in integrated emission. The polymer matrix excludes potentially interfering analytes including reactive oxygen or nitrogen species, which cannot readily permeate the film.     

Spirocyclic sulfur and selenium ligands as molecular rigid rods in coordination of transition metal centers

A set of analogous chalcogen-containing spirocycles, 2,6-dithiaspiro[3.3]heptane, 2,6-diselenaspiro[3.3]heptane, and 2-thia-6-selenaspiro[3.3]heptane [E(2)C(5)H(8), E = S (1), Se (2), and S/Se (3)], has been prepared and fully characterized by spectroscopic methods and by X-ray diffraction. The structural characterization of 2 was presented by us earlier, while the crystal structures of 1 and 3 are reported here for the first time. Molecules 1-3 are built around the central tetrahedral carbon atom and therefore are nonplanar. The E...E separation ranges from 4.690(1) A in 1 to 4.906(1) A in 2. Molecule 3 has statistically mixed positions of sulfur and selenium atoms in the solid state with all geometric characteristics being intermediate between those of 1 and 2. Compounds 2 and 3 have been tested as molecular rigid rod ligands in coordination reactions with transition metal complexes such as Cu(hfac)(2) (4), cis-Co(hfac)(2).2H(2)O (5), and cis-Ni(hfac)(2).2H(2)O (6) (hfac = hexafluoroacetylacetonate). Several coordination products built of two building blocks, M(hfac)(2) (M = Cu, Co, and Ni) and Se(2)C(5)H(8) (2), have been prepared in crystalline form and structurally characterized. The copper-based product (7) is comprised of the oligomeric units {[Cu(hfac)(2)](3).2mu(2)-Se(2)C(5)H(8)-Se,Se'} built on the axial Cu...Se interactions averaged at 2.909 A. These units are further assembled into 1D polymeric chains via intermolecular Cu...F contacts at 2.829 A. The SSeC(5)H(8) (3) ligand was also used in the reaction with Cu(hfac)(2) to afford an analogue of 7, namely {[Cu(hfac)(2)](3).2mu(2)-SSeC(5)H(8)-S,Se} (8). Complex 8 exhibits statistically mixed positions of the donor sulfur and selenium atoms to give an average axial Cu...S/Se contact at 2.892 A. In contrast to the copper complexes of composition 3:2, the stoichiometries of the isolated cobalt and nickel products are 1:1, [M(hfac)(2).Se(2)C(5)H(8)] (M = Co (9) and Ni (10)). Complexes 9 and 10 exhibit 1D polymer structures having alternating metal units cis-M(hfac)(2) and ligands 2 with intermolecuar M...Se separations of 2.6046(8) and 2.5523(16) A, respectively. In all products 7-10 the initial cis or trans geometry of M(hfac)(2) complexes is preserved and the spiro[3.3]heptane ligands act as bidentate linkers bridging transition metal centers via both donor ends. The magnetic properties of this series of new Cu(II), Co(II), and Ni(II) complexes have been tested by variable-temperature magnetic susceptibility measurements.     

Alcohol addition to acetonitrile activated by the [Re6(mu3-Se)8]2+ cluster core

The addition of methanol and ethanol to the previously reported cluster solvates [Re6(mu3-Se)8(PEt3)5(MeCN)](SbF6)2 and trans-[Re6(mu3-Se)8(PEt3)4(CH3CN)2][SbF6]2 afforded three cluster complexes with imino ester ligands: {Re6(mu3-Se)8(PEt3)5[HN=C(OCH3)(CH3)]}(SbF6)2, {Re6(mu3-Se)8(PEt3)5[HN=C(OCH2CH3)(CH3)]}{SbF6}2, and trans-{Re6(mu3-Se)8(PEt3)4[HN=C(OCH3)(CH3)]2}{SbF6}2. In all cases, predominant formation of the Z isomers was observed.     

Binding of DNA purine sites to dirhodium compounds probed by mass spectrometry

The adducts formed between the antitumor active compounds [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](BF(4))(2), Rh(2)(O(2)CCH(3))(4), and Rh(2)(O(2)CCF(3))(4) with DNA oligonucleotides have been assessed by matrix-assisted laser desorption ionization (MALDI) and nanoelectrospray (nanoESI) coupled to time-of-flight mass spectrometry (TOF MS). A series of MALDI studies performed on dipurine (AA, AG, GA, and GG)-containing single-stranded oligonucleotides of different lengths (tetra- to dodecamers) led to the establishment of the relative reactivity cis-[Pt(NH(3))(2)(OH(2))(2)](2+) (activated cisplatin) approximately Rh(2)(O(2)CCF(3))(4) > cis-[Pt(NH(3))(2)Cl(2)] (cisplatin) > [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](BF(4))(2) > Rh(2)(O(2)CCH(3))(4) approximately Pt(C(6)H(6)O(4))(NH(3))(2) (carboplatin). The relative reactivity of the complexes is associated with the lability of the leaving groups. The general trend is that an increase in the length of the oligonucleotide leads to enhanced reactivity for Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](BF(4))(2) and Rh(2)(O(2)CCH(3))(4) (except for the case of [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+), which reacts faster with the GG octamers than with the dodecamers), whereas the reactivity of Rh(2)(O(2)CCF(3))(4) is independent of the oligonucleotide length. When monitored by ESI, the dodecamers containing GG react faster than the respectiveAA oligonucleotides in reactions with Rh(2)(O(2)CCF(3))(4) and Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](BF(4))(2), whereas AA oligonucleotides react faster with Rh(2)(O(2)CCH(3))(4). The mixed (AG, GA) purine sequences exhibit comparable rates of reactivity with the homopurine (AA, GG) dodecamers in reactions with Rh(2)(O(2)CCH(3))(4). The observation of initial dirhodium-DNA adducts with weak axial (ax) interactions, followed by rearrangement to more stable equatorial (eq) adducts, was achieved by electrospray ionization; the Rh-Rh bond as well as coordinated acetate or acetonitrile ligands remain intact in these dirhodium-DNA adducts. MALDI in-source decay (ISD), collision-induced dissociation (CID) MS-MS, and enzymatic digestion studies followed by MALDI and ESI MS reveal that, in the dirhodium compounds studied, the purine sites of the DNA oligonucleotides interact with the dirhodium core. Ultimately, both MALDI and ESI MS proved to be complementary, valuable tools for probing the identity and stability of dinuclear metal-DNA adducts.     

Studies of dirhodium(II) tetra(trifluoroacetate). 5. Remarkable examples of the ambidentate character of dimethyl sulfoxide

The ambidentate character of dimethyl sulfoxide, already known for dirhodium carboxylates, has been remarkably manifested in new ways. Three novel complexes of dirhodium(II) tetra(trifluoroacetate) with the DMSO ligand, namely, [Rh2(O2CCF3)4]m(DMSO)n with m:n = 7:8 (1), 1:1 (2), and 3:2 (3), have been obtained by deposition from the vapor phase, and their crystal structures have been determined by X-ray crystallography. The crystallographic parameters are as follows: for 1, monoclinic space group P2(1)/c with a = 28.261(2) A, b = 16.059(4) A, c = 17.636(2) A, beta = 92.40(4) degrees, and Z = 2; for 2, triclinic space group P1 with a = 8.915(2) A, b = 10.592(2) A, c = 11.916(2) A, alpha = 84.85(1) degrees, beta = 88.86(1) degrees, and gamma = 65.187(8) degrees, and Z = 2; and for 3, triclinic space group P1 with a = 8.876(2) A, b = 9.017(2) A, c = 19.841(3) A, alpha = 101.91(2) degrees, beta = 97.144(8) degrees, gamma = 100.206(9) degrees, and Z = 1. In the oligomeric molecule of 1, six DMSO ligands bridge seven dirhodium tetra(trifluoroacetate) units in a bidentate fashion via S and O atoms, and two additional DMSO molecules terminate the chain. In the structure of the monoadduct Rh2(O2CCF3)4(DMSO) (2), the dirhodium blocks are bridged through the O atoms of DMSO ligands, forming a one-dimensional polymeric chain. Compound 3 also has an infinite chain structure with the molecules of dimethyl sulfoxide acting in a bidentate mu-DMSO-S,O mode. Every second DMSO molecule is missing in 3, so that two of every three Rh2(O2CCF3)4 units are associated through the O atoms of carboxylate groups to give the overall composition [Rh2(O2CCF3)4]3(DMSO)2.     

Cluster carbonyls of the [Re6(mu3-Se)8]2+ core

The first [Re(6)(mu(3)-Se)(8)](2+) core-containing cluster carbonyls, [Re(6)(mu(3)-Se)(8)(PEt(3))(5)(CO)][SbF(6)](2) and trans-[Re(6)(mu(3)-Se)(8)(PEt(3))4(CO)(2)][SbF(6)](2), were produced by reacting [Re(6)(mu(3)-Se)(8)(PEt(3))(5)I]I and trans-[Re(6)(mu(3)-Se)8(PEt(3))(4)I2], respectively, with AgSbF(6) in CO-saturated dichloromethane solutions. Spectroscopic and crystallographic studies suggest significant cluster-to-CO back-donation in these novel cluster derivatives and interesting electronic structures. Thermal and photolytic studies of the mono-carbonyl complex revealed its interesting and synthetically useful reactivity in producing new cluster derivatives.     

Divalent dirhodium imido complexes: formation, structure, and alkyne cycloaddition reactivity

Dirhodium amido complexes [(Cp*Rh)2(mu2-NHPh)(mu2-X)] (X = NHPh (2), Cl (3), OMe (4); Cp* = eta5-C5Me5) were prepared by chloride displacement of [Cp*Rh(mu2-Cl)]2 (1) and have been used as precursors to a dirhodium imido species [Cp*Rh(mu2-NPh)RhCp*]. The imido species can be trapped by PMe3 to give the adduct [Cp*Rh(mu2-NPh)Rh(PMe3)Cp*] (5) and undergoes a formal [2 + 2] cycloaddition reaction with unactivated alkynes to give the azametallacycles [Cp*Rh(mu2-eta2:eta3-R1CCR2NPh)RhCp*] (R1 = R2 = Ph (6a), R1 = H, R2 = t-Bu (6b), R1 = H, R2 = p-tol (6c)). Isolation of a relevant unsaturated imido complex [Cp*Rh(mu2-NAr)RhCp*] (7) was achieved by the use of a sterically hindered LiNHAr (Ar = 2,6-diisopropylphenyl) reagent in a metathesis reaction with 1. X-ray structures of 2, 6a, 7 and the terminal isocyanide adduct [Cp*Rh(mu2-NAr)Rh(t-BuNC)Cp*] (8) are reported.     

Routes to metallodendrimers of the [Re(6)(mu(3)-Se)(8)](2+) core-containing clusters

The reaction of [Re6(mu3-Se)8(PEt3)5(MeCN)](SbF6)2 with an excess of 1,2-bis(4-pyridyl)ethane (L1) and (E)-1,2-bis(4-pyridyl)ethene (L2) produced [Re6(mu3-Se)8(PEt3)5(L1)](SbF6)2 and [Re6(mu3-Se)8(PEt3)5(L2)](SbF6)2, respectively, each bearing an accessible pyridyl N atom capable of further metal coordination. Reacting these cluster complex-based ligands with [Re6(mu3-Se)8(MeCN)6](SbF6)2 afforded two heptacluster metallodendrimers, each featuring a central [Re6(mu3-Se)8]2+ cluster core surrounded by six units of [Re6(mu3-Se)8(PEt3)5]2+ via the bridging interactions of its respective dipyridyl-based ligands. Their identity and stereochemistry have been established, with the most convincing evidence furnished by a unique 77Se NMR spectroscopic study. Electrochemical studies suggest very interesting electronic properties of these novel metallodendrimers.     

Homologous series of redox-active, dinuclear cations [M(2)(O(2)CCH(3))(2)(pynp)(2)](2+) (M = Mo, Ru, Rh) with the bridging ligand 2-(2-pyridyl)-1,8-naphthyridine (pynp)

A homologous series of dinuclear compounds with the bridging ligand 2-(2-pyridyl)-1,8-naphthyridine (pynp) has been prepared and characterized by X-ray crystallographic and spectroscopic methods. [Mo(2)(O(2)CCH(3))(2)(pynp)(2)][BF(4)](2) x 3CH(3)CN (1) crystallizes in the monoclinic space group P2(1)/c with a = 15.134(5) A, b = 14.301(6) A, c = 19.990(6) A, beta = 108.06(2) degrees, V = 4113(3) A(3), and Z = 4. [Ru(2)(O(2)CCH(3))(2)(pynp)(2)][PF(6)](2) x 2CH(3)OH (2) crystallizes in the monoclinic space group C2/c with a = 14.2228(7) A, b = 20.3204(9) A, c = 14.1022(7) A, beta = 95.144(1) degrees, V = 4059.3(3) A(3), and Z = 4. [Rh(2)(O(2)CCH(3))(2)(pynp)(2)][BF(4)](2) x C(7)H(8) (3) crystallizes in the monoclinic space group C2/c with a = 13.409(2) A, b = 21.670(3) A, c = 13.726(2) A, beta = 94.865(2) degrees, V = 3973.9(8) A(3), and Z = 4. A minor product, [Rh(2)(O(2)CCH(3))(2)(pynp)(2)(CH(3)CN)(2)][BF(4)][PF(6)] x 2CH(3)CN (4), was isolated from the mother liquor after crystals of 3 had been harvested; this compound crystallizes in the triclinic space group, P1 with a = 12.535(3) A, b = 13.116(3) A, c = 13.785(3) A, alpha = 82.52(3) degrees, beta = 77.70(3) degrees, gamma = 85.76(3) degrees, V = 2193.0(8) A(3), and Z = 2. Compounds 1-3 constitute a convenient series for probing the influence of the electronic configuration on the extent of mixing of the M-M orbitals with the pi system of the pynp ligand. Single point energy calculations performed on 1-3 at the B3LYP level of theory lend insight into the bonding in these compounds and allow for correlations to be made with electronic spectral data. Although purely qualitative in nature, the values for normalized change in orbital energies (NCOE) of the frontier orbitals before and after reduction are in agreement with the observed differences in reduction potentials as determined by cyclic voltammetry.     

Novel concentration-driven structural interconversion in shape-specific solids supported by the octahedral [Re(6)(mu3-Se)8]2+ cluster core

A complex containing the face-capped octahedral [Re(6)(mu(3)-Se)(8)](2+) cluster core, cis-[Re(6)(mu(3)-Se)(8)(PPh(3))(4)(4,4'-dipyridyl)(2)](SbF(6))(2) (1), is used as a ditopic ligand with an enforced right angle between the two 4,4'-dipyridyl moieties for the coordination of Cd(2+) ion. Two coordination polymers, [[Re(6)(mu(3)-Se)(8)(PPh(3))(4)(4,4'-dipyridyl)(2)](2)[Cd(NO(3))(2)]](SbF(6))(4).21C(4)H(10)O.21CH(2)Cl(2) (2) and [[Re(6)(mu(3)-Se)(8)(PPh(3))(4)(4,4'-dipyridyl)(2)][Cd(NO(3))(3)]](NO(3)).2C(4)H(10)O.CH(2)Cl(2) (3), are obtained. The relative concentration of Cd(2+) determines which species is isolated, and the conversion of the first structure into the second is demonstrated experimentally.     

Excited-state properties of Rh(2)(O(2)CCH(3))(4)(L)(2) (L = CH(3)OH, THF, PPh(3), py)

The photophysical properties of Rh(2)(O(2)CCH(3))(4)(L)(2) (L = CH(3)OH, THF = tetrahydrofuran, PPh(3) = triphenylphosphine, py = pyridine) were explored upon excitation with visible light. Time-resolved absorption shows that all the complexes possess a long-lived transient (3.5-5.0 micros) assigned as an electronic excited state of the molecules, and they exhibit an optical transition at approximately 760 nm whose position is independent of axial ligand. No emission from the Rh(2)(O(2)CCH(3))(4)(L)(2) (L = CH(3)OH, THF, PPh(3), py) systems was detected, but energy transfer from Rh(2)(O(2)CCH(3))(4)(PPh(3))(2) to the (3)pipi excited state of perylene is observed. Electron transfer from Rh(2)(O(2)CCH(3))(4)(PPh(3))(2) to 4,4'-dimethyl viologen (MV(2+)) and chloro-p-benzoquinone (Cl-BQ) takes place with quenching rate constants (k(q)) of 8.0 x 10(6) and 1.2 x 10(6) M(-1) s(-1) in methanol, respectively. A k(q) value of 2 x 10(8) M(-1) s(-1) was measured for the quenching of the excited state of Rh(2)(O(2)CCH(3))(4)(PPh(3))(2) by O(2) in methanol. The observations are consistent with the production of an excited state with excited-state energy, E(00), between 1.34 and 1.77 eV.     

Hydrogen-bonded extended arrays of the [Re6(mu3-Se)8]2+ core-containing clusters

Site-differentiated solvated clusters of the general formula [Re(6)(mu(3)-Se)(8)(PEt(3))(n)(MeCN)(6)(-)(n)](SbF(6))(2) (n = 4, cis and trans; n = 5) undergo ligand substitution reaction with isonicotinamide to afford the corresponding amide derivatives, [Re(6)(mu(3)-Se)(8)(PEt(3))(n)(isonicotinamide)(6)(-)(n)](2+) [1 (n = 5); 2 (n = 4, trans); 3 (n = 4, cis)]. Retention of stereochemistry in each case was confirmed by (1)H and (31)P NMR. The solid-state structures of all three compounds were established crystallographically, which revealed self-complementary hydrogen-bonding interactions between adjacent cluster units. While complex 1 exists as hydrogen-bonded dimers in the solid state, compounds 2 and 3 form one-dimensional chains of clusters bridged by paired hydrogen bonds. It is the rigid stereochemistry of the cluster, combined with the classic crystal engineering motif of complementary N-H.O amide hydrogen bonding, that affords the predictable solid-state structures and dimensionality.     

Ligand substitution in cubic clusters: surprising isolation of the cocrystallization products of Cu8(mu8-Se)[S2P(OEt)2]6 and Cu6[S2P(OEt)2]6

The cluster (Cu8(mu8-Se)[S2P(OEt)2]6)0.54(Cu6[S2P(OEt)2]6)0.46 (2) was prepared in 78% yield from the reaction of Cu8(Se)[Se2P(OPr)2]6 (1) and NH4S2P(OEt)2 in toluene. The central selenide ion in 2 was characterized by 77Se NMR at delta -976 ppm. The simulated solid-state 31P NMR spectrum shows two components with an intensity ratio close to 55:45. The peak centered at 100.7 ppm is assigned to the 31P nuclei in the hexanuclear copper cluster, and that at 101.1 ppm is due to the octanuclear copper cluster. The single-crystal X-ray diffraction analysis confirms the cocrystallization structures of Cu8(Se)[S2P(OEt)2]6 (54%) and Cu6[S2P(OEt)2]6 (46%) (2: trigonal, space group R3, a=21.0139(13) A, c=11.404(3) A, gamma=120 degrees, Z=3). While the octanuclear copper cluster possesses a 3-fold crystallographic axis which pass through the Cu2, Se, and Cu(2A) atoms, the six copper atoms having the S6 point group symmetry in Cu6[S2P(OEt)2]6 form a compressed octahedron. The Cu8(mu8-Se) cubic core in Cu8(mu8-Se)[S2P(OEt)2]6 is larger in size than the metal core in Cu8(mu8-Se)[Se2P(OPr)2]6 (1) although the bite distance of the Se-containing bridging ligand is larger than that of the S ligand. To understand the nature of the structure contraction of the metal core and metal-mu8-Se interaction, molecular orbital calculations have been carried out at the B3LYP level of density functional theory. MO calculations suggest that Cu-mu8-Se interactions are not very strong and a half bond can be formally assigned to each Cu-mu8-Se bond. Moderate Cu...Cu repulsion exists, and it is the bridging ligands that are responsible for the observed Cu...Cu contacts. Hence, the S-ligating copper clusters have greater Cu...Cu separations because each Cu carries more positive charge in the presence of the more electronegative S-containing ligands.     

Built to order: molecular tinkertoys from the [Re(6)(mu(3)-Se)(8)](2+) clusters

Ligand substitution of [Re(6)(mu(3)-Se)(8)(PEt(3))(5)(CH(3)CN)](SbF(6))(2) (1) with pyridyl-based ligands, 2,4,6-tri-4-pyridyl-1,3,5-triazine (L1) and 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (L2), produced respectively the star-shaped tricluster (T1) and tetracluster (T2) arrays, wherein three (T1) and four (T2) units of the [Re(6)(mu(3)-Se)(8)](2+) core-containing clusters are interconnected by the corresponding bridging ligands. These novel supramolecular assemblies were characterized by a combination of NMR ((1)H and (31)P) spectroscopy, ESI-MS, and microanalysis. The molecular and solid-state structures of T1 have also been established by single-crystal X-ray diffraction.     

Unprecedented head-to-head conformers of d(GpG) bound to the antitumor active compound tetrakis (mu-carboxylato)dirhodium(II,II)

The N7/O6 equatorial binding interactions of the antitumor active complex Rh(2)(OAc)(4)(H(2)O)(2) (OAc(-) = CH(3)CO(2)(-)) with the DNA fragment d(GpG) have been unambiguously determined by NMR spectroscopy. Previous X-ray crystallographic determinations of the head-to-head (HH) and head-to-tail (HT) adducts of dirhodium tetraacetate with 9-ethylguanine (9-EtGH) revealed unprecedented bridging N7/O6 guanine nucleobases that span the Rh-Rh bond. The absence of N7 protonation at low pH and the notable increase in the acidity of N1-H (pK(a) approximately 5.7 as compared to 8.5 for N7 only bound platinum adducts), suggested by the pH dependence titrations of the purine H8 (1)H NMR resonances for Rh(2)(OAc)(2)(9-EtG)(2) and Rh(2)(OAc)(2-)[d(GpG)],are consistent with bidentate N7/O6 binding of the guanine nucleobases. The pK(a) values estimated for N1-H (de)protonation, from the pH dependence studies of the C6 and C2 (13)C NMR resonances for the Rh(2)(OAc)(2)(9-EtG)(2) isomers, concur with those derived from the H8 (1)H NMR resonance titrations. Comparison of the (13)C NMR resonances of C6 and C2 for the dirhodium adducts Rh(2)(OAc)(2)(9-EtG)(2) and Rh(2)(OAc)(2)[d(GpG)] with the corresponding resonances of the unbound ligands [at pH 7.0 for 9-EtGH and pH 8.0 for d(GpG)], shows substantial downfield shifts of Deltadelta approximately 11.0 and 6.0 ppm for C6 and C2, respectively; the latter shifts reflect the effect of O6 binding to the dirhodium centers and the ensuing enhancement in the acidity of N1-H. Intense H8/H8 ROE cross-peaks in the 2D ROESY NMR spectrum of Rh(2)(OAc)(2)[d(GpG)] indicate head-to-head arrangement of the guanine bases. The Rh(2)(OAc)(2)[d(GpG)] adduct exhibits two major right-handed conformers, HH1 R and HH2 R, with HH1 R being three times more abundant than the unusual HH2 R. Complete characterization of both adducts revealed repuckering of the 5'-G sugar rings to C3'-endo (N-type), retention of C2'-endo (S-type) conformation for the 3'-G sugar rings, and anti orientation with respect to the glycosyl bonds. The structural features obtained for Rh(2)(OAc)(2))[d(GpG)] by means of NMR spectroscopy are very similar to those for cis-[Pt(NH(3))(2))[d(GpG)]] and corroborate molecular modeling studies.     

Cluster carbonyls of the [Re(6)(mu(3)-Se)(8)](2+) core: synthesis, structural characterization, and computational analysis

The reactions of the previously reported cluster complexes [Re(6)(mu(3)-Se)(8)(PEt(3))(5)I]I, trans-[Re(6)(mu(3)-Se)(8)(PEt(3))(4)I(2)], and cis-[Re(6)(mu(3)-Se)(8)(PEt(3))(4)I(2)] with the [Re(6)(mu(3)-Se)(8)](2+) core with CO in the presence of AgSbF(6) afforded the corresponding cluster carbonyls [Re(6)(mu(3)-Se)(8)(PEt(3))(5)(CO)][SbF(6)](2) (), trans-[Re(6)(mu(3)-Se)(8)(PEt(3))(4)(CO)(2)][SbF(6)](2) (), and cis-[Re(6)(mu(3)-Se)(8)(PEt(3))(4)(CO)(2)][SbF(6)](2) (). Infrared spectroscopy indicated weakening of the bond in CO, suggesting the existence of backbonding between the cluster core and the CO ligand(s). Electrochemical studies focusing on the reversible, one-electron oxidation of the cluster core revealed a large increase in the oxidation potential upon going from the acetonitrile derivatives to their carbonyl analogs, consistent with the depleted electron density of the cluster core upon CO ligation. Disparities between the IR spectra and oxidation potential between and indicate that electronic differences exist between sites trans and cis to the location of a ligand of interest. The active role played by the Se atoms in influencing the cluster-to-CO bonding interactions is suggested through this result and density functional (DF) computational analysis. The computations indicate that molecular orbitals near the HOMO account for backbonding interactions with a high percentage of participation of Se orbitals.