Combined Experimental and Theoretical Investigations of Group 6 Dimetallaboranes [(Cp*M)₂B₄H₁₀] (M = Mo and W)

Thermolysis of mono metal carbonyl fragment, [M′(CO)₅·thf, M′ = Mo and W, thf = tetrahydrofuran] with an <i>in situ</i> generated intermediate, obtained from the reaction of [Cp*MCl₄] (M = Mo and W, Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) with [LiBH₄·thf], yielded dimetallaboranes, <b>1</b> and <b>2</b>. Isolations of [{Cp*M­(CO)}₂B₄H₆] (M = Mo (<b>1</b>) and W­(<b>2</b>)) provide direct evidence for the existence of saturated molybdaborane and tungstaborane clusters, [(Cp*M)₂B₄H₁₀]. Our extensive theoretical studies together with the experimental observation suggests that the intermediate may be a saturated cluster [(Cp^#M)₂B₄H₁₀], not unsaturated [(Cp^#M)₂B₄H₈] (Cp^# = Cp or Cp*), which was proposed earlier by Fehlner. Furthermore, in order to concrete our findings, we isolated and structurally characterized analogous clusters [(Cp*Mo)₂(CO)­(μ-Cl)­B₃H₄W­(CO)₄] (<b>3</b>) and [(Cp*WCO)₂(μ-H)₂B₃H₃W­(CO)₄] (<b>4</b>). All the compounds have been characterized by solution-state ¹H, ¹¹B, IR, and ¹³C NMR spectroscopy, mass spectrometry, and the structural architectures of <b>1</b>, <b>3</b>, and <b>4</b> were unequivocally established by X-ray crystallographic analysis. The density functional theory calculations yielded geometries that are in close agreement with the observed structures. Both the Fenske–Hall and Kohn–Sham molecular orbital analyses showed an increased thermodynamic stability for [(Cp^#M)₂B₄H₁₀] compared to [(Cp^#M)₂B₄H₈]. Furthermore, large HOMO–LUMO gap and significant cross cluster M–M bonding have been observed for clusters <b>1</b>–<b>4</b>

Combined Experimental and Theoretical Investigations of Group 6 Dimetallaboranes [(Cp*M)₂B₄H₁₀] (M = Mo and W)

Thermolysis of mono metal carbonyl fragment, [M′(CO)₅·thf, M′ = Mo and W, thf = tetrahydrofuran] with an <i>in situ</i> generated intermediate, obtained from the reaction of [Cp*MCl₄] (M = Mo and W, Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) with [LiBH₄·thf], yielded dimetallaboranes, <b>1</b> and <b>2</b>. Isolations of [{Cp*M­(CO)}₂B₄H₆] (M = Mo (<b>1</b>) and W­(<b>2</b>)) provide direct evidence for the existence of saturated molybdaborane and tungstaborane clusters, [(Cp*M)₂B₄H₁₀]. Our extensive theoretical studies together with the experimental observation suggests that the intermediate may be a saturated cluster [(Cp^#M)₂B₄H₁₀], not unsaturated [(Cp^#M)₂B₄H₈] (Cp^# = Cp or Cp*), which was proposed earlier by Fehlner. Furthermore, in order to concrete our findings, we isolated and structurally characterized analogous clusters [(Cp*Mo)₂(CO)­(μ-Cl)­B₃H₄W­(CO)₄] (<b>3</b>) and [(Cp*WCO)₂(μ-H)₂B₃H₃W­(CO)₄] (<b>4</b>). All the compounds have been characterized by solution-state ¹H, ¹¹B, IR, and ¹³C NMR spectroscopy, mass spectrometry, and the structural architectures of <b>1</b>, <b>3</b>, and <b>4</b> were unequivocally established by X-ray crystallographic analysis. The density functional theory calculations yielded geometries that are in close agreement with the observed structures. Both the Fenske–Hall and Kohn–Sham molecular orbital analyses showed an increased thermodynamic stability for [(Cp^#M)₂B₄H₁₀] compared to [(Cp^#M)₂B₄H₈]. Furthermore, large HOMO–LUMO gap and significant cross cluster M–M bonding have been observed for clusters <b>1</b>–<b>4</b>

ICT–Isomerization-Induced Turn-On Fluorescence Probe with a Large Emission Shift for Mercury Ion: Application in Combinational Molecular Logic

A unique turn-on fluorescent device based on a ferrocene–aminonaphtholate derivative specific for Hg²⁺ cation was developed. Upon binding with Hg²⁺ ion, the probe shows a dramatic fluorescence enhancement (the fluorescence quantum yield increases 58-fold) along with a large red shift of 68 nm in the emission spectrum. The fluorescence enhancement with a red shift may be ascribed to the combinational effect of CN isomerization and an extended intramolecular charge transfer (ICT) mechanism. The response was instantaneous with a detection limit of 2.7 × 10^–9 M. Upon Hg²⁺ recognition, the ferrocene/ferrocenium redox peak was anodically shifted by Δ<i>E</i>_1/2 = 72 mV along with a “naked eye” color change from faint yellow to pale orange for this metal cation. Further, upon protonation of the imine nitrogen, the present probe displays a high fluorescence output due to suppression of the CN isomerization process. Upon deprotonation using strong base, the fluorescence steadily decreases, which indicates that H⁺ and OH^– can be used to regulate the off–on–off fluorescence switching of the present probe. Density functional theory studies revealed that the addition of acid leads to protonation of the imine N (according to natural bond orbital analysis), and the resulting iminium proton forms a strong H-bond (2.307 Å) with one of the triazole N atoms to form a five-membered ring, which makes the molecule rigid; hence, enhancement of the ICT process takes place, thereby leading to a fluorescence enhancement with a red shift. The unprecedented combination of H⁺, OH^–, and Hg²⁺ ions has been used to generate a molecular system exhibiting the INHIBIT–OR combinational logic operation

Hypoelectronic Dimetallaheteroboranes of Group 6 Transition Metals Containing Heavier Chalcogen Elements

We have synthesized and structurally characterized several dimetallaheteroborane clusters, namely, <i>nido</i>-[(Cp*Mo)₂B₄SH₆], <b>1</b>; <i>nido</i>-[(Cp*Mo)₂B₄SeH₆], <b>2</b>; <i>nido</i>-[(Cp*Mo)₂B₄TeClH₅], <b>3</b>; [(Cp*Mo)₂B₅SeH₇], <b>4</b>; [(Cp*Mo)₂B₆SeH₈], <b>5</b>; and [(CpW)₂B₅Te₂H₅], <b>6</b> (Cp* = η⁵-C₅Me₅, Cp = η⁵-C₅H₅). In parallel to the formation of <b>1</b>–<b>6</b>, known [(CpM)₂B₅H₉], [(Cp*M)₂B₅H₉], (M = Mo, W) and <i>nido</i>-[(Cp*M)₂B₄E₂H₄] compounds (when M = Mo; E = S, Se, Te; M = W, E = S) were isolated as major products. Cluster <b>6</b> is the first example of tungstaborane containing a heavier chalcogen (Te) atom. A combined theoretical and experimental study shows that clusters <b>1</b>–<b>3</b> with their open face are excellent precursors for cluster growth reactions. As a result, the reaction of <b>1</b> and <b>2</b> with [Co₂(CO)₈] yielded clusters [(Cp*Mo)₂B₄H₄E­(μ₃-CO)­Co₂(CO)₄], <b>7</b>–<b>8</b> (<b>7</b>: E = S, <b>8</b>: E = Se) and [(Cp*Mo)₂B₃H₃E­(μ-CO)₃Co₂(CO)₃], <b>9</b>–<b>10</b> (<b>9</b>: E = S, <b>10</b>: E = Se). In contrast, compound <b>3</b> under the similar reaction conditions yielded a novel 24-valence electron triple-decker sandwich complex, [(Cp*Mo)₂{μ-η⁶:η⁶-B₃H₃TeCo₂(CO)₅}], <b>11</b>. Cluster <b>11</b> represents an unprecedented metal sandwich cluster in which the middle deck is composed of B, Co, and Te. All the new compounds have been characterized by elemental analysis, IR, ¹H, ¹¹B, ¹³C NMR spectroscopy, and the geometric structures were unequivocally established by X-ray diffraction analysis of <b>1</b>, <b>2</b>, <b>4</b>–<b>7</b>, and <b>9</b>–<b>11</b>. Furthermore, geometries obtained from the electronic structure calculations employing density functional theory (DFT) are in close agreement with the solid state structure determinations. We have analyzed the discrepancy in reactivity of the chalcogenato metallaborane clusters in comparison to their parent metallaboranes with the help of a density functional theory (DFT) study

Novel Neutral Zirconaborane [(Cp₂Zr)₂B₅H₁₁]: An <i>arachno</i>-B₃H₉ Analogue (Cp = η⁵‑C₅H₅)

The first example of a homometallic neutral zirconaborane, [(Cp₂Zr)₂B₅H₁₁], <b>1</b>, has been prepared through the thermolysis of [Cp₂Zr­(BH₄)₂], generated from the fast metathesis reaction of [Cp₂ZrCl₂] and LiBH₄·thf with BH₃·thf. The solid-state structure of <b>1</b> shows an open geometry with a planar B₃ ring. The bonding between the Zr center and the central B₃ ring was studied computationally by DFT methods, and based on the combined experimental and computational results compound <b>1</b> can be defined as a metal-stabilized <i>arachno</i>-B₃H₉. Further, in an attempt to synthesize a hybrid analogue of <b>1</b> by introducing two electron fragments into <i>arachno</i>-[(Cp₂Zr)­(Cp*Ir)­B₄H₁₀], <b>2</b>, we have performed the reaction of <b>2</b> with [Ru₃(CO)₁₂]. However, the reaction led to the formation of a hybrid metallaborane, [(Cp*Ir)­{Ru₃(CO)₈}­B₄H₁₀], <b>3</b>

Homometallic Cubane Clusters: Participation of Three-Coordinated Hydrogen in 60-Valence Electron Cubane Core

This work describes the synthesis, structural characterizations, and electronic structures of a series of novel homometallic cubane clusters [(Cp*Ru)₂{Ru­(CO)₂}₂BH­(μ₃-E)­(μ-H)­B­(μ-H)₃M], (<b>2</b>, M = Cp*Ru, E = CO; <b>3</b>, M = Ru­(Cp*Ru)₂(μ-CO)₃­(μ-H)­BH), E = BH), [(Cp*Ru)₃(μ₃-CO)­(BH)₃(μ₃-H)₃], <b>4</b>, and [(Cp*Ru)₂(μ₃-CO)­{Ru­(CO)₃}₂(BH)₂(μ-H)­B], <b>5</b> (Cp* = η⁵-C₅Me₅). These cubane clusters have been isolated from a thermally driven reaction of diruthenium analogue of pentaborane(9) [(Cp*RuH)₂B₃H₇], <b>1</b>, and [Ru₃(CO)₁₂]. Structural and spectroscopic studies revealed the existence of triply bridged hydrogen (μ₃-H) atoms that participate as a vertex in the cubane core formation for compounds <b>2</b>, <b>3</b>, and <b>4</b>. In addition, the crystal structure of these clusters clearly confirms the presence of an electron precise borane ligand (borylene fragment) which is triply bridged to the trimetallic units. Bonding of these novel complexes has been studied computationally by DFT methods, and the studies demonstrate that the cubane clusters <b>2</b> and <b>3</b> possess 60 cluster valence electrons (cves) with six metal–metal bonds. All the new compounds have been characterized in solution by mass spectrometry; IR; and ¹H, ¹¹B, and ¹³C NMR studies, and the structural types were unequivocally established by crystallographic analysis of compounds <b>2</b>–<b>5</b>

A Novel Heterometallic μ₉‑Boride Cluster: Synthesis and Structural Characterization of [(η⁵‑C₅Me₅Rh)₂{Co₆(CO)₁₂}(μ-H)(BH)B]

The preparation, characterization, and electronic structure of the first heterometallic μ₉-boride cluster [(Cp*Rh)₂{Co₆(CO)₁₂}­(μ-H)­(BH)­B)] has been reported. The interstitial boron atom in the title cluster is within the bonding contact of eight metal and one boron atom in a unique tricapped trigonal prism geometry

A Novel Heterometallic μ₉‑Boride Cluster: Synthesis and Structural Characterization of [(η⁵‑C₅Me₅Rh)₂{Co₆(CO)₁₂}(μ-H)(BH)B]

The preparation, characterization, and electronic structure of the first heterometallic μ₉-boride cluster [(Cp*Rh)₂{Co₆(CO)₁₂}­(μ-H)­(BH)­B)] has been reported. The interstitial boron atom in the title cluster is within the bonding contact of eight metal and one boron atom in a unique tricapped trigonal prism geometry

An Early–Late Transition Metal Hybrid Analogue of Hexaborane(12)

Metal assisted borane condensation method has been employed for the preparation of novel metallaborane cluster containing group 4 metal. Pyrolysis of [Cp*IrB₃H₉], <b>1</b>, with in situ generated zirconium bisborohydrate [Cp₂Zr­(BH₄)₂], produced from the fast metathesis reaction of [Cp₂ZrCl₂] and LiBH₄, and excess of BH₃·THF yielded <i>arachno</i>-[(Cp₂Zr)­(Cp*Ir)­B₄H₁₀], <b>2</b>, in 56% yield. Compound <b>2</b> constitutes the first crystallographic structure determination of hexaborane(12) metal analogue containing zirconium. The observed geometry of <b>2</b> can be generated from a dodecahedron by removing two vertices and an edge. Anticipating a straight metal fragment substitution/addition reaction, mild thermolysis of <b>2</b> with [Fe₂(CO)₉] was carried out. However, the reaction led to the formation of known trimetallic [Cp*IrFe₂(CO)₉] cluster via cluster degradation. In addition, density functional theory (DFT) calculations have been carried out for <b>2</b> and other hypothetical early–late combinations of hexaborane(12) metal analogues to reveal the electronic structures

An Early–Late Transition Metal Hybrid Analogue of Hexaborane(12)

Metal assisted borane condensation method has been employed for the preparation of novel metallaborane cluster containing group 4 metal. Pyrolysis of [Cp*IrB₃H₉], <b>1</b>, with in situ generated zirconium bisborohydrate [Cp₂Zr­(BH₄)₂], produced from the fast metathesis reaction of [Cp₂ZrCl₂] and LiBH₄, and excess of BH₃·THF yielded <i>arachno</i>-[(Cp₂Zr)­(Cp*Ir)­B₄H₁₀], <b>2</b>, in 56% yield. Compound <b>2</b> constitutes the first crystallographic structure determination of hexaborane(12) metal analogue containing zirconium. The observed geometry of <b>2</b> can be generated from a dodecahedron by removing two vertices and an edge. Anticipating a straight metal fragment substitution/addition reaction, mild thermolysis of <b>2</b> with [Fe₂(CO)₉] was carried out. However, the reaction led to the formation of known trimetallic [Cp*IrFe₂(CO)₉] cluster via cluster degradation. In addition, density functional theory (DFT) calculations have been carried out for <b>2</b> and other hypothetical early–late combinations of hexaborane(12) metal analogues to reveal the electronic structures