Nido Species Research Articles

Diborane and Triborane Species in the Coordination Sphere of Group-8 Transition Metals.

The synthesis and structural elucidation of a series of ruthenium diborane and triborane compounds are described. Treatment of [Cp*Ru(PPh3)2Cl] (1) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; PPh3 = triphenylphosphine) with [BH3·THF] (THF = tetrahydrofuran) at 60 °C led to the formation of the hydrogen-rich ruthena-octahydrotetraborane arachno-[2-{Cp*Ru(PPh3)B3H8}] (2). The chemistry of 2 is explored with [Fe2(CO)9] at room temperature, which resulted in the formation of a metal-stabilized triborane species, [{Cp*Ru(PPh3)}(μ3-η1:η2:η2-B3H6){Fe2(CO)7}] (3). Compound 3 can be considered as a triborane analogue [B3H6]3- that stabilizes in the coordination sphere of two iron and one ruthenium atoms. Further, the photolysis of nido-[1,2-(Cp*Ru)2(μ-H)2(B3H7)] (4) with [M(CO)5·THF] (M = Mo and W) afforded an arachno-[1,2-(Cp*Ru)(Cp*RuCO)(μ-H)(B3H8)] (5), in which the [M(CO)5·THF] acted as a CO source. In an attempt to convert arachno-5 into a closo or nido species, we have pyrolyzed arachno-5 in toluene at 90 °C for 20 h that afforded nido-[2,3-(Cp*Ru)2(μ-CO)(μ3-H)(B3H6)] (6) having two ruthenium atoms at the basal position. Irradiation of arachno-5 with an intermediate generated from CS2 and [LiBH4·THF] in THF afforded the diborane(5) species [(Cp*RuCO)(Cp*Ru)(μ-H)(μ-η1:η2-B2H4)(CS2H)] (7) in which a dithioformato ligand (SHC═S) is attached to one Ru-B bond. Compound 7 can be considered as a diborane(5) species, which is stabilized by a dithioformato ligand. All the synthesized compounds have been characterized by employing electrospray ionization-mass spectrometry, multinuclear NMR, and IR spectroscopy techniques. The single-crystal X-ray diffraction studies of compounds 2, 3, 6, and 7 helped to establish the structural integrity of these compounds. Further, density functional theory studies were performed to provide insight into the bonding of these metal-stabilized diborane and triborane species.

Synthesis and Characterization of New Fluorescent Styrene‐Containing Carborane Derivatives: The Singular Quenching Role of a Phenyl Substituent

A set of neutral and anionic carborane derivatives in which the styrenyl fragment is introduced as a fluorophore group has been successfully synthesized and characterized. The reaction of the monolithium salts of 1-Ph-1,2-C(2)B(10)H(11), 1-Me-1,2-C(2)B(10)H(11) and 1,2-C(2)B(10)H(12) with one equivalent of 4-vinylbenzyl chloride leads to the formation of compounds 1-3, whereas the reaction of the dilithium salt of 1,2-C(2)B(10)H(12) with two equivalents of 4-vinylbenzyl chloride gives disubstituted compound 4. The closo clusters were degraded using the classical method, KOH in EtOH, to afford the corresponding nido species, which were isolated as tetramethylammonium salts. The crystal structure of the four closo compounds 1-4 were analyzed by X-ray diffraction. All compounds, except 1, display emission properties, with quantum yields dependent on the nature of the cluster (closo or nido) and the substituent on the second C(cluster) atom. In general, closo compounds 2-4 exhibit high fluorescence emission, whereas the presence of a nido cluster produces a decrease of the emission intensity. The presence of a phenyl group bonded to the C(cluster) results in an excellent electron-acceptor unit that produces a quenching of the fluorescence. DFT calculations have confirmed the charge-separation state in 1 to explain the quenching of the fluorescence and the key role of the carboranyl fragment in this luminescent process.

Synthesis and fluorescence emission of neutral and anionic di- and tetra-carboranyl compounds

A new family of photoluminescent neutral and anionic di-carboranyl and tetra-carboranyl derivatives have been synthesized and characterized. The reaction of α,α'-bis(3,5-bis(bromomethyl)phenoxy-m-xylene with 4 equiv. of the monolithium salt of 1-Ph-1,2-C(2)B(10)H(11) or 1-Me-1,2-C(2)B(10)H(11) gives the neutral tetracarboranyl-functionalized aryl ether derivatives closo-1 and closo-2, respectively. The addition of the monolithium salt of 1-Ph-1,2-closo-C(2)B(10)H(11) to α,α,'-dibromo-m-xylene or 2,6-dibromomethyl-pyridine gives the corresponding di-carboranyl derivatives closo-3 and closo-4. These compounds, which contain four or two closo clusters, were degraded using the classical method, KOH in EtOH, affording the corresponding nido species, which were isolated as potassium or tetramethylammonium salts. All the compounds were characterized by IR, (1)H, (11)B and (13)C NMR spectroscopy, and the crystal structure of closo-3 was analysed by X-ray diffraction. The carboranyl fragments are bonded through CH(2) units to different organic moieties, and their influence on the photoluminescent properties of the final molecules has been studied. All the closo- and nido-carborane derivatives exhibit a blue emission under ultraviolet excitation at room temperature in different solvents. The fluorescence properties of these closo and nido-derivatives depend on the substituent (Ph or Me) bonded to the C(cluster), the solvent polarity, and the organic unit bearing the carborane clusters (benzene or pyridine). In the case of nido-derivatives, an important effect of the cation is also observed.

High boron content carboranyl-functionalized aryl ether derivatives displaying photoluminescent properties

The reaction of alpha,alpha'-bis(3,5-bis(bromomethyl)phenoxy-p-xylene (3) with 4 equiv of the monolithium salt of 1-Ph-1,2-C2B10H11 or 1-Me-1,2-C2B10H11 gave the corresponding neutral carboranyl-functionalized aryl ether derivatives closo-4 and closo-5, respectively. These compounds contain four closo clusters that were degraded using basic conditions with KOH in EtOH, affording the corresponding nido-6 and nido-7 as potassium salts. Nido species were also isolated with tetramethylammonium as cation giving compounds nido-8 and nido-9 in good yield. The potassium salts showed good solubility in water and polar solvents. All these compounds were characterized by 1H, 11B and 13C NMR spectroscopy and UV-vis. The electronic data in different solvents indicated a solvatochromic shift for all compounds and a red shift of the absorption maxima for the nido species with respect to the closo derivatives. These neutral and anionic carboranyl-functionalized aryl ether derivatives represent a new family of high boron content luminescent compounds that show strong fluorescence emission in different solvents at room temperature. This phenomenon is very interesting considering the fact that none of the precursors have such a property. The fluorescence emission depends on the cluster substituent (Ph or Me) and the solvent polarity. Additionally, the fluorescence emission intensity was clearly dependent on the solvent polarity; the closo species showed strongest fluorescence intensities in the non-polar solvents, while anionic species were highly emissive in polar solvents.

Prediction of pKaValues ofnido-Carboranes by Density Functional Theory Methods

A great parallel exists between metal complexes of cyclopentadienyl and arene ligands on one side and metal complexes of the nido derivatives of the icosahedral o-carborane clusters. With few exceptions, the metal complexation in the cluster can be viewed as the substitution of one or more bridging hydrogen atoms by the metal. Therefore, a necessary requirement for the complexation is the deprotonation of the nido cluster to generate a coordination site for that metal. The reaction to remove these protons, which most probably is one of the most commonly done processes in boron and metallaborane chemistry, is barely known, and no quantitative data are available on the magnitude of their pKa values. With the purpose of determining the acidity of nido-carboranes, a procedure to calculate the pKa values of nido boron clusters is presented in this paper for the first time. To this objective, some nido clusters have been selected and their geometry and NMR-spectroscopic properties have been studied, giving a good correlation between the theoretical and experimental data in both geometry distances and 11B NMR spectroscopy. Of notice is the result that proves that the singular carbon atom in the thermodynamic isomer of [C2B10H13]- is definitely part of the cluster and that its connection with the C2B3 face would be better defined by adding additional interactions with the two boron atoms nearest to the second cluster carbon. The pKa values of the nido species have been calculated by correlating experimental pK(a) values and calculated reaction Gibbs energies DeltaG(s). Some pKa values of importance are -4.6 and +13.5 for 7,8-[C2B9H13] (1) and 7,8-[C2B9H12]- (2), respectively.

Phosphane‐Substituted Selenido‐(carbonyl)iron and ‐(carbonyl)ruthenium Clusters: Structures, Fluxionality and Reactivity

AbstractFor Abstract see ChemInform Abstract in Full Text.

Selective preparation of the [3,5-tBu2-1,2,4-C2P3]- ion and synthesis and structure of the cationic species nido-[3,5-tBu2-1,2,4-C2P3]+, isoelectronic with [C5R5]+.

Building pyramids: The first P–C cluster, namely, the novel nido species 2, is readily prepared from 1 (•=CtBu), which is also an excellent precursor for the selective synthesis of the anion 3. DFT calculations show that there is a switch in the global minimum, on the introduction of three P atoms (CH) into the [C5H5]+ ion, from cyclic-unsaturated to saturated cage species, such as 2.

Phosphane‐Substituted Selenido−(carbonyl)iron and −(carbonyl)ruthenium Clusters: Structures, Fluxionality and Reactivity

AbstractSelenium‐transfer reactions from tertiary phosphane and diphosphane selenides to the carbonyl clusters [M3(CO)12] (M = Fe or Ru) provide a simple, stepwise synthetic route to phosphane‐substituted, mono‐ and diselenido−(carbonyl)iron and −(carbonyl)ruthenium clusters. Four families of clusters are generally obtained: (i) the M2Se2L6 tetrahedral species (only for iron), (ii) the M3SeL10 capped triangular species, (iii) the M3Se2L9 square‐pyramidal nido species (for iron and ruthenium), and (iv) the M4Se2L11 octahedral closo species (only for ruthenium), with L being a phosphane or carbonyl ligand. When the tertiary phosphane selenide contains heterocyclic fragments, the reaction is not only limited to the transfer of the selenium atom, but proceeds by P−C and, sometimes, C−H bond cleavages, affording new selenido‐phosphido carbonyl clusters bearing coordinated heterocyclic fragments. Many clusters show fluxional behaviour in solution because of either ligand mobility or M−M bond migration. Furthermore, dimetallic substituted carbonyl clusters have been prepared by two different synthetic approaches: (1) MM′ + PSe (reactions between preformed dimetallic clusters and tertiary phosphane selenides) and (2) MSeP + M′ [cluster growth reactions from preformed chalcogenido− (carbonyl)metal clusters and suitable organometallic fragments]. In particular, from the tetrahedral clusters [HMCo3(CO)12] (M = Fe or Ru) monoselenido clusters [MCo2(μ3‐Se)] with a tetrahedral core have been obtained. The second type of approach has been applied to the open‐triangular nido clusters [Ru3(μ3‐Se)2(CO)9−nLn] (L = tertiary phosphane, n = 1 or 2), which can add zero‐s.e.p.’s organometallic fragments, such as M(CO)3 (M = Mo or W), giving the corresponding closo clusters [MRu3(µ4‐Se)2(CO)12−nLn]. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

New NS2 and S′S2 ligands derivatives of pyridine and thiophene incorporating icosahedral carboranes

New ligands incorporating carboranyl units in organic fragments NS2 (pyridine-dithia) or S′S2 (thiophene-dithia) have been synthesized. The closo species of these ligands were obtained by the reaction of sodium thiolate-o-carborane with the corresponding difunctional electrophile derivatives of pyridine or thiophene. Partial degradation of the closo species derivatives of pyridine was successfully achieved with KOH in ethanol at r.t. All these compounds have been characterized by chemical analyses and NMR techniques. The 1H-NMR spectrum of the nido species shows that these nido molecules are produced in stoichiometric purity but not in isomeric purity. The crystal structure of 2,6-bis(((1-methyl-1,2-dicarba-closo-dodecabo-ranyl)thio)methyl)pyridine is reported and confirms that the carborane cages are closo and connected to both sulfur atoms. These, through methylene bridges, are connected to the pyridine at the 2 and 6 positions.

Coupling reactions of ynamines. Syntheses, structural characterization and electrochemistry of some nido and closo ferracyclopentadiene complexes

The reaction of [Fe 2(CO) 7C(Ph)C(NEt 2)] ( 1) with ynamines RCCNEt 2 (R  Me, C 3H 5, or CH 2Ph) at 20°C yielded dinuclear ferracyclopentadiene complexes [Fe 2(CO) 6C(Ph)C(NEt 2)C-(NEt 2)C(R)] (R  Me ( 2), R  CH 2Ph ( 3), R  C 3H 5 ( 4)). Complex 5, [Fe 2(CO) 6C(C 3H 5)C(NEt 2)C-(NEt 2)C(C 3H 5)], was obtained directly from the reaction of C 3H 5CCNEt 2 with [Fe 2(CO) 9]. All complexes were characterized by a combination of microanalytical and spectroscopic techniques. The molecular structure of 4 was determined X-ray crystallographically: triclinic, P 1 , a = 8.820(2) Å, b = 9.261(2) Å, c = 18.470(2) Å, α = 89.40(1)°, β = 81.81(1)°, γ = 70.23(2)°, R w = 0.0406 for 2580 reflections. The nido complexes 4 and 5 transformed to the closo tripledecker compounds [Fe 3(CO) 8 C(Ph)C(NEt 2)C(NEt 2)C(R)] (R  Ph ( 6); R  C 3H 5 (7)) by an excess of [Fe 2(CO) 9] in refluxing hexane. Both complexes were characterized by IR, 1H NMR, mass spectra and chemical analysis. The X-ray structure of 6 was carried out: monoclinic, P2 1 / c, a = 14.900(3), b = 11.149(2), c = 18.453(4) Å, β = 91.14(2)°, R w = 0.0414 for 2304 reflections. Only one isomer could be isolated for the nido compounds. The nido-closo transformation did not modify the coupling within the butadiene moiety. Electrochemistry of the nido and closo complexes is reported. The closo compounds showed two reversible one-electron reduction waves whereas the nido species were completely irreversibly reduced.

Synthesis and in vitro evaluation of boronated uridine and glucose derivatives for boron neutron capture therapy.

The following boron-containing nucleoside and glucose derivatives have been synthesized as potential boron delivery agents for boron neutron capture therapy (BNCT): 2'-O-(o-carboran-1-ylmethyl)uridine (4a), 3'-O-(o-carboran-1-ylmethyl)uridine (4b), sodium 7-(uridin-2'-ylmethyl)dodecahydro-7,8-dicarba-++ +nido-undecaborate (5), 5'-O-(o-carboran-1-ylmethyl)uridine (9), and 3'-O-(o-carboran-1-ylmethyl)-D-glucose (13). In vitro cellular uptake studies were performed with F98 rat glioma cells. Following 16 h incubation, cellular boron concentrations were determined by direct current plasma atomic emission spectroscopy (DCP-AES). Boron concentrations ranged from 65 to 103 micrograms/g of cells for the neutral closo structures compared with 1.5 micrograms/g of cells for the charged nido species. Cellular uptake of sodium mercaptoundecahydro-closo-dodecaborate (BSH), the compound currently being used in Japan for the treatment of malignant brain tumors by BNCT, was 2 micrograms/g of cells.

A “vertex electron pair” scheme (VEPS) for describing the skeletal electron distribution in borane-type clusters

Abstract A predominantly localized electron pair scheme is outlined for describing the electron distribution and bonding in closo borane anions B n H n 2− and related electron deficient deltahedral clusters, in which a skeletal electron pair is assigned to each vertex, one pair being regarded as delocalized just inside the roughly spherical surface on which the skeletal atoms lie. The scheme gives a clearer picture of the electron distribution than is conveyed by resonating 2- and 3-centre bonds in the polyhedron edges and faces, and allows the bond orders of the polyhedron edge links to be calculated readily. The consequence of formal removal of BH 2+ units from closo species B n H n 2− to generate nido species B n −1 H n −1 4− and arachno species B n −2 H n −2 6− is explored, and seen to allow rationalization of two features of such deltahedral-fragment clusters: (i) why a high-connectivity vertex is left vacant and (ii) why the frontier orbitals of such species concentrate electronic charge around their open faces. Moreover, in the case of D 4 `h B 4 H 4 6− (cf. C 4 H 4 2− ) and D 5 h B 5 H 5 6− (cf. C 5 H 5 − ), the approach leads directly to the familiar picture for aromatic ring systems in which the highest filled, doubly degenerate π-bonding molecular orbital concentrates electronic charge in rings above and below the polygon on which the skeletal nuclei lie. It also leads to the expectation that arachno clusters with non-adjacent vacant vertices will be more stable than those with adjacent vacant vertices.

The intermediate dicarba-nido-boranes

Abstract