Bijan Mondal

Boron Beyond the Icosahedral Barrier: A 16‐Vertex Metallaborane

A neutral metallaborane comprising a Rh4B12 polyhedron with icosioctahedron geometry with 16 vertices and 28 triangular faces was prepared (see structure; Rh: red, B: green). The cage has the shape of a 12-membered truncated tetrahedron with four capped hexagonal faces.

Reactivity of Diruthenium and Dirhodium Analogues of Pentaborane(9): Agostic versus Boratrane Complexes

A series of novel Cp*-based (Cp*=η(5)-C5Me5) agostic, bis(σ-borate), and boratrane complexes have been synthesized from diruthenium and dirhodium analogues of pentaborane(9). The synthesis and structural characterization of the first neutral ruthenadiborane(6) analogue are also reported. This new route offers a very efficient method for the isolation of bis(σ-borate) and agostic complexes from diruthenapentaborane(9).

Supraicosahedral polyhedra in metallaboranes: synthesis and structural characterization of 12-, 15-, and 16-vertex rhodaboranes.

Syntheses and structural characterization of supraicosahedral rhodaborane clusters are reported. Reaction of [(Cp*RhCl2)2], (Cp* = η(5)-C5Me5) with [LiBH4·thf] followed by thermolysis with excess of [BH3·thf] afforded 16-vertex closo-[(Cp*Rh)3B12H12Rh{Cp*RhB4H9}], 1, 15-vertex [(Cp*Rh)2B13H13], 2, 12-vertex [(Cp*Rh)2B10Hn(OH)m], (3a: n = 12, m = 0; 3b: n = 9, m = 1; 3c: n = 8, m = 2) and 10-vertex [(Cp*Rh)3B7H7], 4, and [(Cp*Rh)4B6H6], 5. Cluster 1 is the unprecedented 16-vertex cluster, consists of a sixteen-vertex {Rh4B12} with an exo-polyhedral {RhB4} moiety. Cluster 2 is the first example of a carbon free 15-vertex supraicosahedral metallaborane, exhibits icosihexahedron geometry (26 triangular faces) with three degree-six vertices. Clusters 3a-c have 12-vertex isocloso geometry, different from that of icosahedral one. Clusters 4 and 5 are attributed to the 10-vertex isocloso geometry based on 10-vertex bicapped square antiprism structure. In addition, quantum-chemical calculations with DFT methods at the BP86 level of theory have been used to provide further insight into the electronic structure and stability of the optimized structures which are in satisfactory agreement with the structure determinations. All the compounds have been characterized by IR, (1)H, (11)B, (13)C NMR spectroscopy in solution, and the solid state structures were established by crystallographic analysis of compounds 1-5.

Chemistry of Diruthenium and Dirhodium Analogues of Pentaborane(9): Synthesis and Characterization of Metal N,S‐Heterocyclic Carbene and B‐Agostic Complexes

Building upon our earlier results on the synthesis of electron-precise transition-metal-boron complexes, we continue to investigate the reactivity of pentaborane(9) and tetraborane(10) analogues of ruthenium and rhodium towards thiazolyl and oxazolyl ligands. Thus, mild thermolysis of nido-[(Cp*RuH)2B3H7] (1) with 2-mercaptobenzothiazole (2-mbtz) and 2-mercaptobenzoxazole (2-mboz) led to the isolation of Cp*-based (Cp* = η(5)-C5Me5) borate complexes 5 a,b [Cp*RuBH3L] (5 a: L = C7H4NS2; 5 b: L = C7H4NOS)) and agostic complexes 7 a,b [Cp*RuBH2(L)2], (7 a: L = C7H4NS2; 7 b: L = C7H4NOS). In a similar fashion, a rhodium analogue of pentaborane(9), nido-[(Cp*Rh)2B3H7] (2) yielded rhodaboratrane [Cp*RhBH(L)2], 10 (L = C7H4NS2). Interestingly, when the reaction was performed with an excess of 2-mbtz, it led to the formation of the first structurally characterized N,S-heterocyclic rhodium-carbene complex [(Cp*Rh)(L2)(1-benzothiazol-2-ylidene)] (11) (L = C7H4NS2). Furthermore, to evaluate the scope of this new route, we extended this chemistry towards the diruthenium analogue of tetraborane(10), arachno-[(Cp*RuCO)2B2H6] (3), in which the metal center possesses different ancillary ligands.

Synthesis of triazole linked fluorescent amino acid and carbohydrate bio-conjugates: a highly sensitive and skeleton selective multi-responsive chemosensor for Cu(II) and Pb(II)/Hg(II) ions

A set of triazole-based chromogenic and fluorescent chemosensors with amino acid/carbohydrate–fluorophore conjugates have been designed and synthesized. The metal cation-sensing properties of glycine–anthracene, C24H24O4N4 (3), glycine–pyrene C26H24O4N4 (4), glucose–anthracene, C32H33O10N3 (5) and glucose–pyrene, C34H33O10N3 (6) bio-conjugates have been studied systematically. The significant changes in their absorption spectra are accompanied by a strong color change from light yellow to brown for 3 and 4 and colorless to greenish blue for 5 and 6. Receptors 3 and 4 have potential in the “naked eye” detection of Cu2+ and 5 and 6 for Pb2+/Hg2+ ion. The receptors 3 and 4 show fluorescence diminution following Cu2+ coordination within the limit of detection at 0.89 parts per billion (ppb) and this is unprecedented, whereas the receptors 5 and 6 present drastic fluorescence quenching upon addition of Hg2+ and Pb2+ within the limit of detection at 4 and 2 ppb respectively. Interestingly, their fluorescence and colorimetric responses are preserved in the presence of water that can be used for the selective colorimetric detection of these ions in aqueous environments. Along with the spectroscopic data, combined 1H NMR titration of the complexes and the DFT studies suggest the proposed coordination modes.

Chemistry of diruthenium analogue of pentaborane(9) with heterocumulenes: toward novel trimetallic cubane-type clusters.

Reactions of the CS2 and CO2 heterocumulene ligands with nido-ruthenaborane cluster [1,2-(Cp*Ru)2(μ-H)2B3H7], 1, were explored (Cp* = pentamethylcyclopentadienyl). Compound 1 when treated with CS2 underwent metal-assisted hydroboration to yield arachno-ruthenaborane [(Cp*Ru)2(B3H8)(CS2H)], 2, with a dithioformato ligand attached to it. The chemistry of 2 was then explored with various transition metal carbonyl compounds under photolytic and thermolytic conditions. Thermolysis of 2 with [Mn2(CO)10] resulted in the formation of an unprecedented cubane-type cluster [(Cp*Ru)2Mn(CO)3(CS2H2)B3H4], 3, with a rare [M3E5] formulation (E = B, S). On the other hand, when compound 2 was photolyzed in the presence of [Mn2(CO)10], it yielded an incomplete cubane-type cluster [(Cp*Ru)2Mn(CO)3BH2(CS2H2)], 4. The room-temperature reaction of 2 with [Fe2(CO)9] yielded heterometallic arachno clusters [(Cp*Ru)(CO)2{Fe(CO)3}2S2CH3], 6 and [(Cp*Ru)2(B3H8)(CO){Fe(CO)3}2(CS2H)], 7. In contrast, photolysis of 2 with [Fe2(CO)9] yielded a tetrahedral cluster [(Cp*Ru)(CO)2S(μ-H){Fe(CO)3}3], 8, tethered to an exo-polyhedral moiety [(Cp*Ru)(CO)2]. Compound 6 provides an unusual bonding pattern by means of fusing the wing-tip vertex (S) of the [Fe2S2] butterfly core by an exo-polyhedral [(Cp*Ru)(CO)2] unit. Density functional theory calculations were carried out to provide insight into the mechanistic pathway, electronic structure, and bonding properties.

First‐Row Transition‐Metal–Diborane and –Borylene Complexes

A combined experimental and quantum chemical study of Group 7 borane, trimetallic triply bridged borylene and boride complexes has been undertaken. Treatment of [{Cp*CoCl}2 ] (Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) with LiBH4 ⋅thf at -78 °C, followed by room-temperature reaction with three equivalents of [Mn2 (CO)10 ] yielded a manganese hexahydridodiborate compound [{(OC)4 Mn}(η(6) -B2 H6 ){Mn(CO)3 }2 (μ-H)] (1) and a triply bridged borylene complex [(μ3 -BH)(Cp*Co)2 (μ-CO)(μ-H)2 MnH(CO)3 ] (2). In a similar fashion, [Re2 (CO)10 ] generated [(μ3 -BH)(Cp*Co)2 (μ-CO)(μ-H)2 ReH(CO)3 ] (3) and [(μ3 -BH)(Cp*Co)2 (μ-CO)2 (μ-H)Co(CO)3 ] (4) in modest yields. In contrast, [Ru3 (CO)12 ] under similar reaction conditions yielded a heterometallic semi-interstitial boride cluster [(Cp*Co)(μ-H)3 Ru3 (CO)9 B] (5). The solid-state X-ray structure of compound 1 shows a significantly shorter boron-boron bond length. The detailed spectroscopic data of 1 and the unusual structural and bonding features have been described. All the complexes have been characterized by using (1) H, (11) B, (13) C NMR spectroscopy, mass spectrometry, and X-ray diffraction analysis. The DFT computations were used to shed light on the bonding and electronic structures of these new compounds. The study reveals a dominant B-H-Mn, a weak B-B-Mn interaction, and an enhanced B-B bonding in 1.

Synthesis and sensing properties of 1,1′-disubstituted unsymmetrical ferrocene-triazole derivatives: a multichannel probe for Hg(II) ion

Triazole-based unsymmetrically 1,1′-disubstituted ferrocene derivatives 4 and 5 have been synthesized using “click chemistry” with sequential functionalization at the cyclopentadienyl rings. The cation complexation properties of these ferrocene derivatives have been studied using electrochemical and spectroscopic techniques. The exceptional structural features present in these ligands are the presence of one cholesterol moiety, connected to the 1-position of the ferrocene center through a 1,2,3-triazole ring, and a fluorescent moiety, linked to the 1′-position of such a core through another triazole-ether linkage. Both the receptors experience perturbation of the CV in the presence of Hg2+ with a change of electrochemical potential (the large anodic shift ΔE1/2 = 129 mV for 4 and ΔE1/2 = 140 mV for 5). The changes in the absorption spectra are accompanied by the appearance of a new high energy (HE) peak at ca. 206 nm for 4 and 202 nm for 5 (4: e = 3123 × 102 M−1 cm−1, 5: e = 3140 × 102 M−1 cm−1) in the presence of Hg2+ ion. In addition, receptors 4 and 5 show colorimetric response and act as a selective “turn off” fluorescent receptor for Hg2+ and Cu2+ ions. On the basis of the 1H NMR titration, ESI-MS and DFT study, probable binding modes of 1,1′-disubstituted ferrocene derivatives 4 and 5 with Hg2+ have been proposed.

New Heteronuclear Bridged Borylene Complexes That Were Derived from [{Cp*CoCl}2] and Mono-MetalCarbonyl Fragments†

The synthesis, structural characterization, and reactivity of new bridged borylene complexes are reported. The reaction of [{Cp*CoCl}2] with LiBH4·THF at -70 °C, followed by treatment with [M(CO)3(MeCN)3] (M=W, Mo, and Cr) under mild conditions, yielded heteronuclear triply bridged borylene complexes, [(μ3-BH)(Cp*Co)2(μ-CO)M(CO)5] (1-3; 1: M=W, 2: M=Mo, 3: M=Cr). During the syntheses of complexes 1-3, capped-octahedral cluster [(Cp*Co)2(μ-H)(BH)4{Co(CO)2}] (4) was also isolated in good yield. Complexes 1-3 are isoelectronic and isostructural to [(μ3-BH)(Cp*RuCO)2(μ-CO){Fe(CO)3}] (5) and [(μ3-BH)(Cp*RuCO)2(μ-H)(μ-CO){Mn(CO)3}] (6), with a trigonal-pyramidal geometry in which the μ3-BH ligand occupies the apical vertex. To test the reactivity of these borylene complexes towards bis-phosphine ligands, the room-temperature photolysis of complexes 1-3, 5, 6, and [{(μ3-BH)(Cp*Ru)Fe(CO)3}2(μ-CO)] (7) was carried out. Most of these complexes led to decomposition, although photolysis of complex 7 with [Ph2P(CH2)(n)PPh2] (n=1-3) yielded complexes 9-11, [3,4-(Ph2P(CH2)(n)PPh2)-closo-1,2,3,4-Ru2Fe2(BH)2] (9: n=1, 10: n=2, 11: n=3). Quantum-chemical calculations by using DFT methods were carried out on compounds 1-3 and 9-11 and showed reasonable agreement with the experimentally obtained structural parameters, that is, large HOMO-LUMO gaps, in accordance with the high stabilities of these complexes, and NMR chemical shifts that accurately reflected the experimentally observed resonances. All of the new compounds were characterized in solution by using mass spectrometry, IR spectroscopy, and (1)H, (13)C, and (11)B NMR spectroscopy and their structural types were unequivocally established by crystallographic analysis of complexes 1, 2, 4, 9, and 10.

Novel triple-decker sandwich complex with a six-membered [B3Co2(μ4-Te)] ring as the middle deck.

Thermolysis of nido-[(Cp*Mo)2B4TeClH5], with an excess of Co2(CO)8 at room temperature, afforded a triple-decker sandwich complex [(Cp*Mo)2{μ-η(6):η(6)-B3H3TeCo2(CO)5}] (4), which represents an unsaturated 24-valence-electron sandwich cluster in which the middle deck is composed of B, Co, and a heavy group 16 element.