Rule breaker boron clusters: a new class of hypoelectronic osmaborane clusters [(Cp*Os)2BnHn] (n = 6-10).

Since the pioneering electron counting rule for borane clusters was proposed by Wade, the structures and bonding of boron clusters and their derivatives have been elegantly rationalised. However, this rule and its modified versions faced problems explaining the electronic structures of less spherical deltahedra, unlike the core geometries of borate dianions [BnHn]2- (n = 6-12). Herein, we report the isolation of a series of osmaborane clusters [(Cp*Os)2BnHn], 1-5, (n = 6-10) by the thermolysis of an in situ generated intermediate, obtained from the rapid condensation of [Cp*OsBr2]2 and [LiBH4·THF], with [BH3·THF] or [BH3·SMe2]. Interestingly, all these clusters show unusual less spherical deltahedral shapes that can be generated from canonical [BnHn]2- (n = 8-12) shapes by doing diamond-square-diamond (DSD) rearrangements. The DSD rearrangements led to the generation of higher degree vertices, which are occupied by Os atoms and also generated Os-Os bonds in these clusters. Theoretical calculations revealed that these Cp*Os⋯OsCp* interactions in clusters 1-5 played a crucial role in their structural shape and electron count. These less spherical deltahedral clusters are rare, and most significantly, clusters 1-5 with (n-1) skeleton electron pairs (SEPs) do not follow Wade-Mingos electron counting rules and can be classified as hypoelectronic closo clusters.

Doubly Base-Stabilized Diborane(4) and Borato-Boronium Species and Their Chemistry with Chalcogens.

In an effort to isolate diborane(4) derivatives, we have developed an efficient and uncatalyzed approach using [BH3·THF] and the mercaptopyridine ligand. Thermolysis of 2-mercaptopyridine, in the presence of [BH3·THF], afforded a doubly base-stabilized diborane(4) species 1, [HB(µ-C5H4NS)]2, along with the formation of its isomeric species 2, [HB(µ-C5H4NS)]2, albeit in less yield. Based on the coordination of the boron with the mercaptopyridine ligand in 2 and its spectroscopic data, compound 2 has been designated as a borato-boronium species, in which the anionic borate and cationic boronium units are covalently bonded to each other. Furthermore, we have demonstrated the oxidative insertion of chalcogen atoms (S and Se) through the B-B bond of the base-stabilized diborane(4), 1, that yielded chalcogenido-diboron species, 3(S) and 4(Se).

Hetero-trimetallic complexes comprising bridging boryl and borylene ligands: an experimental and theoretical study.

In an effort to explore the coordination chemistry of the coordinative sulfur centers in arachno-ruthenaborane [(Cp*Ru)2(B3H8)(CS2H)] (arachno-1), we have thermolyzed arachno-1 with group-6 metal carbonyls [M(CO)5·THF] (M = Cr, Mo and W). The reaction of arachno-1 with [Cr(CO)5·THF] resulted in the formation of hetero-trimetallic triply bridging borylene [(Cp*Ru)2(µ-CO)(µ3-CH2S2-κ2S':κ2S''){Cr(CO)3}(µ3-BH)] (2), bridging boryl-borylene [(Cp*Ru)2(µ-CO){(µ3-BH(CH2S2)-κ2B:κ2S':κ1S'')}{Cr(CO)3}(µ3-BH)] (3), and sulfido bridged hetero-trimetallic complex [(Cp*Ru)2(µ-CO)3{Cr(CO)3}(µ3-S)] (4). In 2, one side of Ru2Cr-triangle features a µ3-BH ligand while the other side is quadruply bridged by a methanedithiolato ligand in an unsymmetrical fashion. Unlike 2, in complex 3, one side of the Ru2Cr-triangle has a µ3-BH ligand while the opposite side is bridged by a boryl ligand BH(CH2S2) in an unsymmetrical way (µ3-κ2:κ2:κ1) to the metal centers. Interestingly, when the similar reactions of arachno-1 were performed with heavier group-6 metal carbonyls [M(CO)5·THF] (M = Mo and W), it led to the formation of methanedithiolato bridged hetero-trimetallic chain complexes, [{Cp*Ru(CO)}2(µ-CO)2(µ3-CH2S2-κ2S':κ2S''){M(CO)2}] (5, M = Mo; 6, M = W) and sulfido-bridged hetero-trimetallic complexes [(Cp*Ru)2(µ-CO)3{M(CO)3}(µ3-S)] (7, M = Mo; 8, M = W), analogous to 4. In complexes 5 and 6, a Ru2M-chain is symmetrically bridged by a methanedithiolato ligand. On the other hand, in complexes 4, 7, and 8, a sulfido ligand coordinates to two ruthenium and one group-6 metal atoms in µ3-fashion. All the complexes have been characterized by 1H NMR, 13C NMR, UV-vis, IR spectroscopy, and mass spectrometry and their structural architectures have been unambiguously established by single crystal X-ray diffraction studies. In addition, theoretical investigations provided valuable insights into their electronic structures and bonding properties.

Transition Metal Triple-decker Sandwich Complexes Containing Group 13 Elements.

Transition metal triple-decker complexes are an interesting class of sandwich complexes that engrossed great attention due to their structures and properties. Over the decades, synthesis of triple-decker complexes featuring homocyclic, heterocyclic or π-conjugated rings as middle decks have been abundantly reported. In this regard, the chemistry of such complexes bearing boron in the middle deck are well explored due to the ability of boron-containing cycles to readily coordinate bifacially with metal atoms thereby forming triple-decker complexes. On the other hand, electron counting rules and theoretical calculations have strengthened our knowledge of the structure and bonding in these complexes. Further, these complexes can be used as synthons to generate organometallic polymers having interesting electronic, optical and magnetic properties that can be appropriately tuned to cater to a wide range of applications. In our quest for novel metallaboranes and metallaheteroboranes, we have been successful in isolating various triple-decker complexes that feature boron in the middle deck. This review explained elaborately the synthesis, structures, and bonding in such complexes reported by us and others.

B-P Coupling: Metal Stabilized Phosphinoborate Complexes.

In an effort to establish B-P coupling reactions without the use of phosphine-borane dehydrocoupling agent, we have developed a new synthetic methodology employing group 8 metal σ-borate complex [{κ3 -H,S,S'-BH2 L2 }Ru{κ3 -H,H,S-BH3 L}] (L=NC5 H4 S), 1. Treatment of 1 with chlorodiphenyl phosphine (PPh2 Cl) yielded 1,5-P,S chelated Ru-dihydridoborate species [PPh2 H{κ3 -H,H,S-BH(OH)L}Ru{κ2 -P,S-(Ph2 P)BH2 L}], 2. The insertion of phosphine moiety (PPh2 ) by the cleavage of 3c-2e σ(Ru… H-B) bonding interaction led to the formation of B-P bond. The κ2 -P,S chelated six-membered ring adopted a boat conformation in complex 2. The heterocycle is made of all different atoms, which is one of the rarest examples of heteroatomic ring systems. Theoretical outcomes demonstrated the electronic insight of B-P coupling and stabilization through transition metal. In order to explore an alternate route of B-P bond formation, we have further explored the reaction of 1 and Ru-bis(dihydridoborate) complex, 5 with secondary phosphine oxide (SPO). Although, thermolysis of 1 with diphenylphosphine oxide yielded analogous σ-borate complex 3, the similar reaction of 5 at room temperature led to the formation of novel phosphinous(III) acid incorporated Ru(σ-borate)(dihydridoborate) complex, 6. In a similar fashion, the reaction of 5 with phosphite ligand generated Ru(σ-borate)(dihydridoborate) complex, 7, which is analogous to 6.

16-Vertex oblato-hypho-titanaborane [(Cp*Ti)2B14H18].

Although Lipscomb predicted in 1977 that supra-icosahedral boron clusters would be viable, their synthesis has been impeded by the unavailability of appropriate synthetic methodologies. Herein, we report the first examples of the open 16-vertex oblato-hypho-titanaborane clusters [(Cp*Ti)2B14H17R] (1: R = H; 2: R = Me) having a non-Wadean 19-skeletal-electron-pair count. Interestingly, these clusters show a six-membered [Ti2B4] open face, which could lead to closo-19-vertex clusters.

Macropolyhedral syn-B18H22, the "Forgotten" Isomer.

The chemistry and physics of macropolyhedral B18H22 clusters have attracted significant attention due to the interesting photophysical properties of anti-B18H22 (blue emission, laser properties) and related potential applications. We have focused our attention on the "forgotten" syn-B18H22 isomer, which has received very little attention since its discovery compared to its anti-B18H22 isomer, presumably because numerous studies have reported this isomer as nonluminescent. In our study, we show that in crystalline form, syn-B18H22 exhibits blue fluorescence and becomes phosphorescent when substituted at various positions on the cluster, associated with peculiar microstructural-dependent effects. This work is a combined theoretical and experimental investigation that includes the synthesis, separation, structural characterization, and first elucidation of the photophysical properties of three different monothiol-substituted cluster isomers, [1-HS-syn-B18H21] 1, [3-HS-syn-B18H21] 3, and [4-HS-syn-B18H21] 4, of which isomers 1 and 4 have been proved to exist in two different polymorphic forms. All of these newly substituted macropolyhedral cluster derivatives (1, 3, and 4) have been fully characterized by NMR spectroscopy, mass spectrometry, single-crystal X-ray diffraction, IR spectroscopy, and luminescence spectroscopy. This study also presents the first report on the mechanochromic shift in the luminescence of a borane cluster and generally enriches the area of rather rare boron-based luminescent materials. In addition, we present the first results proving that they are useful constituents of carbon-free self-assembled monolayers.

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.

Chemistry of CS2 and CS3 Bridged Decaborane Analogues: Regular Coordination Versus Cluster Expansion.

In an effort to synthesize metallaheteroborane clusters of higher nuclearity, the reactivity of metallaheteroboranes, nido-[(Cp*M)2B6S2H4(CS3)] (Cp* = C5Me5) (1: M = Co; 2: M = Rh) with various metal carbonyls have been investigated. Photolysis of nido-1 and nido-2 with group 6 metal carbonyls, M'(CO)5.THF (M' = Mo or W) were performed that led to the formation of a series of adducts [(Cp*M)2B6S2H4(CS3){M'(CO)5}] (3: M = Co, M' = Mo; 4: M = Co, M' = W; 5: M = Rh, M' = Mo; 6: M = Rh, M' = W) instead of cluster expansion reactions. In these adducts, the S atom of C=S group of di(thioboralane)thione {B2CS3} moiety is coordinated to M'(CO)5 (M = Mo or W) in η1-fashion. On the other hand, thermolysis of nido-1 with Ru3(CO)12 yielded one fused metallaheteroborane cluster [{Ru(CO)3}3S{Ru(CO)}{Ru(CO)2}Co2B6SH4(CH2S2){Ru(CO)3}2S], 7. This 20-vertex-fused cluster is composed of two tetrahedral {Ru3S} and {Ru2B2}, a flat butterfly {Ru3S} and one octadecahedron {Co2RuB7S} core with one missing vertex, coordinated to {Ru2SCH2S2} through two boron and one ruthenium atom. On the other hand, the room temperature reaction of nido-2 with Co2(CO)8 produced one 19-vertex fused metallaheteroborane cluster [(Cp*Rh)2B6H4S4{Co(CO)}2{Co(CO)2}2(µ-CO)S{Co(CO)3}2], 8. Cluster 8 contains one nido-decaborane {Rh2B6S2}, one butterfly {Co2S2} and one bicapped square pyramidal {Co6S} unit that exhibits an intercluster fusion with two sulfur atoms in common. Clusters 3-6 have been characterized by multinuclear NMR and IR spectroscopy, mass spectrometry and structurally determined by XRD analyses. Furthermore, the DFT calculations have been carried out to gain insight into electronic, structural and bonding patterns of the synthesized clusters.

Carborane-thiol protected copper nanoclusters: stimuli-responsive materials with tunable phosphorescence.

Atomically precise nanomaterials with tunable solid-state luminescence attract global interest. In this work, we present a new class of thermally stable isostructural tetranuclear copper nanoclusters (NCs), shortly Cu4@oCBT, Cu4@mCBT and Cu4@ICBT, protected by nearly isomeric carborane thiols: ortho-carborane-9-thiol, meta-carborane-9-thiol and ortho-carborane 12-iodo 9-thiol, respectively. They have a square planar Cu4 core and a butterfly-shaped Cu4S4 staple, which is appended with four respective carboranes. For Cu4@ICBT, strain generated by the bulky iodine substituents on the carboranes makes the Cu4S4 staple flatter in comparison to other clusters. High-resolution electrospray ionization mass spectrometry (HR ESI-MS) and collision energy-dependent fragmentation, along with other spectroscopic and microscopic studies, confirm their molecular structure. Although none of these clusters show any visible luminescence in solution, bright µs-long phosphorescence is observed in their crystalline forms. The Cu4@oCBT and Cu4@mCBT NCs are green emitting with quantum yields (Φ) of 81 and 59%, respectively, whereas Cu4@ICBT is orange emitting with a Φ of 18%. Density functional theory (DFT) calculations reveal the nature of their respective electronic transitions. The green luminescence of Cu4@oCBT and Cu4@mCBT clusters gets shifted to yellow after mechanical grinding, but it is regenerated after exposure to solvent vapour, whereas the orange emission of Cu4@ICBT is not affected by mechanical grinding. Structurally flattened Cu4@ICBT didn't show mechanoresponsive luminescence in contrast to other clusters, having bent Cu4S4 structures. Cu4@oCBT and Cu4@mCBT are thermally stable up to 400 °C. Cu4@oCBT retained green emission even upon heating to 200 °C under ambient conditions, while Cu4@mCBT changed from green to yellow in the same window. This is the first report on structurally flexible carborane thiol appended Cu4 NCs having stimuli-responsive tunable solid-state phosphorescence.

Synthesis and Chemistry of Dihydridoborate Group 7 Metal Complexes with Varied N,E-Chelated Ligands (E = O, NH, or S).

Several dihydridoborate group 7 metal complexes have been synthesized and their structural aspects have been described from various N,S-, N,N-, and N,O-chelated borate species, such as Na[(H3B)mp] (mp = 2-mercaptopyridyl), Na[(H3B)amt] (amt = 2-amino-5-mercapto-1,3,4-thiadiazolyl), Na[(H3B)hp] (hp = 2-hydroxypyridyl), Na[(H2B)bap] (bap = bis(2-aminopyridyl)), and Na[(H2B)bdap] (bdap = bis(2,6-diaminopyridyl)). Room temperature photolysis of [M2(CO)10] (M = Mn or Re) with these borate species afforded dihydridoborate complexes [(CO)3M(µ-H)2BHL] 1-6 (1, M = Mn, L = mp; 2, M = Re, L = mp; 3, M = Mn, L = amt; 4, M = Mn, L = hp; 5, M = Mn, L = ap; 6, M = Mn, L = dap, ap = 2-aminopyridyl, dap = 2,6-diaminopyridyl). In complexes 1-3, the corresponding (H2BHL) units are coordinated to the metal centers through the (κ3-H,H,S) mode. However, in complexes 4 and 5 (or 6), the connection is via (κ3-H,H,O) and (κ3-H,H,N) modes of coordination, respectively. Complexes 1 and 5 underwent hydroboration reactions with terminal alkynes that yielded trans-hydroborated species [Mn(CO)3(µ-H)2(NC5H4E)B(PhC═CH2)] (7, E = S; 8, E = NH). Density functional theory (DFT) calculations have been carried out to investigate the electronic structures of these dihydridoborate species as well as the nature of bonding in them.

Trimetallic Chalcogenide Species: Synthesis, Structures, and Bonding.

In an attempt to isolate boron-containing tri-niobium polychalcogenide species, we have carried out prolonged thermolysis reactions of [Cp*NbCl4] (Cp* = ɳ5-C5Me5) with four equivalents of Li[BH2E3] (E = Se or S). In the case of the heavier chalcogen (Se), the reaction led to the isolation of the tri-niobium cubane-like cluster [(NbCp*)3(µ3-Se)3(BH)(µ-Se)3] (1) and the homocubane-like cluster [(NbCp*)3(µ3-Se)3(µ-Se)3(BH)(µ-Se)] (2). Interestingly, the tri-niobium framework of 1 stabilizes a selenaborate {Se3BH}- ligand. A selenium atom is further introduced between boron and one of the selenium atoms of 1 to yield cluster 2. On the other hand, the reaction with the sulfur-containing borate adduct [LiBH2S3] afforded the trimetallic clusters [(NbCp*)3(µ-S)4{µ-S2(BH)}] (3) and [(NbCp*)3(µ-S)4{µ-S2(S)}] (4). Both clusters 3 and 4 have an Nb3S6 core, which further stabilizes {BH} and mono-sulfur units, respectively, through bi-chalcogen coordination. All of these species were characterized by 11B{1H}, 1H, and 13C{1H} NMR spectroscopy, mass spectrometry, infrared (IR) spectroscopy, and single-crystal X-ray crystallography. Moreover, theoretical investigations revealed that the triangular Nb3 framework is aromatic in nature and plays a vital role in the stabilization of the borate, borane, and chalcogen units.

Cluster Growth Reactions: Structures and Bonding of Metal-Rich Metallaheteroboranes Containing Heavier Chalcogen Elements.

In an effort to synthesize cobalt-rich metallaheteroboranes from decaborane(14) analogues, we have studied the reaction of 10-vertex nido-[(Cp*Co)2B6H6E2] (Cp* = η5-C5Me5, 1: E = Se and 2: E = Te) with [Co2(CO)8] under thermolytic conditions. All of these reactions yielded face-fused clusters, [(Cp*Co)2B6H6E2{Co(CO)}(µ-CO){Co3(CO)6}] (3: E = Se and 4: E = Te). Further, when clusters 3 and 4 were treated with [Co2(CO)8], they underwent further cluster buildup reactions leading to the formation of 16-vertex doubly face-fused clusters [(Cp*Co)2B6H6E2{Co2(CO)2}(µ-CO)2{Co4(CO)8}] (5: E = Se and 6: E = Te). Cobaltaheteroboranes 3 and 4 comprise one icosahedron {Co4B6E2} and one square pyramidal {Co3B2} moiety, whereas 5 and 6 are made with one icosahedron {Co4B6E2} and two square pyramidal {Co3B2} cores. In an attempt to generate heterometallic metal-rich clusters, we have explored the reactivity of decaborane(14) analogue nido-[(Cp*Co)2B7TeH9] (7) with [Ru3(CO)12] at 80 °C, which afforded face-fused 13-vertex cluster [(Cp*Co)2B7H7Te{Ru3(CO)8}] (8). Cluster 8 is a rare example of a metal-rich metallaheteroborane in which one icosahedron {Co2Ru2B7Te} and a tetrahedron {Ru2B2} units are fused through a common {RuB2} triangular face. Further, the treatment of nido-[(Cp*Co)2B6S2H4(CH2S2)] (9) with [Fe2(CO)9] afforded 11-vertex nido-[(Cp*Co)2B6S2H4(CH2S2){Fe(CO)3}] (10). The core structure of 10 is similar to that of [C2B9H11]2- with a five-membered pentahapto coordinating face. All of the synthesized metal-rich metallaheteroboranes have been characterized by multinuclear nuclear magnetic resonance (NMR) spectroscopy, IR spectroscopy, ESI-MS, and structurally solved by single-crystal X-ray diffraction analysis. Furthermore, theoretical investigations gave insight into the bonding of such higher-nuclearity clusters containing heavier chalcogen atoms.

Cooperative B-H bond activation: dual site borane activation by redox active κ2-N,S-chelated complexes.

Cooperative dual site activation of boranes by redox-active 1,3-N,S-chelated ruthenium species, mer-[PR3{κ2-N,S-(L)}2Ru{κ1-S-(L)}], (mer-2a: R = Cy, mer-2b: R = Ph; L = NC7H4S2), generated from the aerial oxidation of borate complexes, [PR3{κ2-N,S-(L)}Ru{κ3-H,S,S'-BH2(L)2}] (trans-mer-1a: R = Cy, trans-mer-1b: R = Ph; L = NC7H4S2), has been investigated. Utilizing the rich electronic behaviour of these 1,3-N,S-chelated ruthenium species, we have established that a combination of redox-active ligands and metal-ligand cooperativity has a big influence on the multisite borane activation. For example, treatment of mer-2a-b with BH3·THF led to the isolation of fac-[PR3Ru{κ3-H,S,S'-(NH2BSBH2N)(S2C7H4)2}] (fac-3a: R = Cy and fac-3b: R = Ph) that captured boranes at both sites of the κ2-N,S-chelated ruthenacycles. The core structure of fac-3a and fac-3b consists of two five-membered ruthenacycles [RuBNCS] which are fused by one butterfly moiety [RuB2S]. Analogous fac-3c, [PPh3Ru{κ3-H,S,S'-(NH2BSBH2N)(SC5H4)2}], can also be synthesized from the reaction of BH3·THF with [PPh3{κ2-N,S-(SNC5H4)}{κ3-H,S,S'-BH2(SNH4C5)2}Ru], cis-fac-1c. In stark contrast, when mer-2b was treated with BH2Mes (Mes = 2,4,6-trimethyl phenyl) it led to the formation of trans- and cis-bis(dihydroborate) complexes [{κ3-S,H,H-(NH2BMes)Ru(S2C7H4)}2], (trans-4 and cis-4). Both the complexes have two five-membered [Ru-(H)2-B-NCS] ruthenacycles with κ2-H-H coordination modes. Density functional theory (DFT) calculations suggest that the activation of boranes across the dual Ru-N site is more facile than the Ru-S one.

Hexagonal Planar [B6 H6 ] within a [B6 H12 ] Borate Complex: Structure and Bonding of [(Cp*Ti)2 (µ-ɳ6 : ɳ6 -B6 H6 )(µ-H)6 ].

Isolation of planar [B6 H6 ] is a long-awaited goal in boron chemistry. Several attempts in the past to stabilize [B6 H6 ] were unsuccessful due to the domination of polyhedral geometries. Herein, we report the synthesis of a triple-decker sandwich complex of titanium [(Cp*Ti)2 (µ-η6 : η6 -B6 H6 )(µ-H)6 ] (1), which features the first-ever experimentally achieved nearly planar six-membered [B6 H6 ] ring, albeit within a [B6 H12 ] borate. The small deviation from planarity is a direct consequence of the predicted structural pattern of the middle ring in 24 Valence Electron Count (VEC) triple-decker complexes. The large ring size of [B6 H6 ] in 1 brings the metal-metal distance into the bonding range. However, significant electron delocalization from the M-M bonding orbital to the bridging hydrogen and B-B skeleton in the middle decreases its bond strength.

Cooperative B-H activation by Cp* based κ2-N,S-chelated Ru(II) and Mo(II) complexes (Cp* = η5-C5Me5).

The chemistry of the Cp* based κ2-N,S-chelated ruthenium complex, [Cp*RuPPh3(κ2-N,S-(NC7H4S2)], 1 with different boranes has been explored. The room temperature reaction of 1 with BH3·THF and bulky boranes, such as MesBH2 and H2BArF, led to the formation of different dihydridoborate complexes, [{κ3-S,H,H-(NBH2R)(S2H4C7)}RuCp*], 2-4 (2: R = H, 3: R = Mes, and 4: R = ArF; Mes = 2,4,6-trimethylphenyl, and ArF = 3,5-bistrifluoromethyl-benzene). In contrast, the Cp* based κ2-N,S-chelated molybdenum complex, [Cp*Mo(CO)2{κ2-N,S-(NC7H4S2)}], 5, yielded the agostic borate species, [Cp*Mo(CO)2{κ2-S,H-(NBH2R) (NC7H4S2)}], 6 and 7 (6: R = Mes and 7: R = ArF) at elevated temperatures.

Coordination and Hydroboration of Ru(II)-Borate Complexes: Dihydridoborate vs. Bis(dihydridoborate).

Treatment of [Cp*RuCl2 ]2 , 1, [(COD)IrCl]2 , 2 or [(p-cymene)RuCl2 ]2, 3 (Cp*=η5 -C5 Me5, COD= 1,5-cyclooctadiene and p-cymene=η6 -i PrC6 H4 Me) with heterocyclic borate ligands [Na[(H3 B)L], L1 and L2 (L1 : L=amt, L2 : L=mp; amt=2-amino-5-mercapto-1,3,4-thiadiazole, mp=2-mercaptopyridine) led to the formation of borate complexes having uncommon coordination. For example, complexes 1 and 2 on reaction with L1 and L2 afforded dihydridoborate species [LA M(µ-H)2 BHL] 4-6 (4: LA =Cp*, M=Ru, L=amt; 5: LA =Cp*, M=Ru, L=mp; 6: LA =COD, M=Ir, L=mp). On the other hand, treatment of 3 with L2 yielded cis- and trans-bis(dihydridoborate) species, [Ru{(µ-H)2 BH(mp)}2 ], cis-7 and trans-7. The isolation and structural characterization of fac- and mer-[Ru{(µ-H)2 BH(mp)}{(µ-H)BH(mp)2 }], 8 from the same reaction offered an insight into the behaviour of these dihydridoborate species in solution. Fascinatingly, despite having reduced natural charges on Ru centres both at cis-and trans-7, they underwent hydroboration reaction with alkynes that yielded both Markovnikov and anti-Markovnikov addition products, 10 a-d.

Hydroboration reactions using transition metal borane and borate complexes: an overview.

In recent years, the chemistry of transition metal borane and borate complexes has advanced fast with reasonable growth. Frequent utilisation of these complexes in hydrofunctionalisation reactions is one of the key driving forces for their development. As a result, the important role of borate complexes in the hydroboration/hydrosilylation/hydroamination of unsaturated organic species has been successfully demonstrated together with the isolation of many different boron-containing transition metal complexes such as borataallyl, vinylborane, silyl complexes featuring the known bonding modes of boron. Both the uncatalysed and catalysed hydroboration reactions using the transition metal borane/borate complexes are known, which show these complexes' huge potential. Careful investigation and fine-tuning of the electronic and steric properties of the borane/borate ligands has facilitated the synthesis of these transition metal complexes which are convenient for use in the hydroboration reactions. Furthermore, the systematic development of this field has established the connection between the structure and reactivity of these complexes and their utilisation in hydroboration reactions. This Frontier sheds light on the recent developments that have been made with hydroboration reactions using transition metal borane/borate complexes. Also, in this Frontier we have provided meaningful synthetic methods to make new boron-containing transition metal complexes together with mechanistic insights for some of these reactions.

Light-Activated Intercluster Conversion of an Atomically Precise Silver Nanocluster.

Noble metal nanoclusters protected with carboranes, a 12-vertex, nearly icosahedral boron-carbon framework system, have received immense attention due to their different physicochemical properties. We have synthesized ortho-carborane-1,2-dithiol (CBDT) and triphenylphosphine (TPP) coprotected [Ag42(CBDT)15(TPP)4]2- (shortly Ag42) using a ligand-exchange induced structural transformation reaction starting from [Ag18H16(TPP)10]2+ (shortly Ag18). The formation of Ag42 was confirmed using UV-vis absorption spectroscopy, mass spectrometry, transmission electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, and multinuclear magnetic resonance spectroscopy. Multiple UV-vis optical absorption features, which exhibit characteristic patterns, confirmed its molecular nature. Ag42 is the highest nuclearity silver nanocluster protected with carboranes reported so far. Although these clusters are thermally stable up to 200 °C in their solid state, light-irradiation of its solutions in dichloromethane results in its structural conversion to [Ag14(CBDT)6(TPP)6] (shortly Ag14). Single crystal X-ray diffraction of Ag14 exhibits Ag8-Ag6 core-shell structure of this nanocluster. Other spectroscopic and microscopic studies also confirm the formation of Ag14. Time-dependent mass spectrometry revealed that this light-activated intercluster conversion went through two sets of intermediate clusters. The first set of intermediates, [Ag37(CBDT)12(TPP)4]3- and [Ag35(CBDT)8(TPP)4]2- were formed after 8 h of light irradiation, and the second set comprised of [Ag30(CBDT)8(TPP)4]2-, [Ag26(CBDT)11(TPP)4]2-, and [Ag26(CBDT)7(TPP)7]2- were formed after 16 h of irradiation. After 24 h, the conversion to Ag14 was complete. Density functional theory calculations reveal that the kernel-centered excited state molecular orbitals of Ag42 are responsible for light-activated transformation. Interestingly, Ag42 showed near-infrared emission at 980 nm (1.26 eV) with a lifetime of >1.5 µs, indicating phosphorescence, while Ag14 shows red luminescence at 626 nm (1.98 eV) with a lifetime of 550 ps, indicating fluorescence. Femtosecond and nanosecond transient absorption showed the transitions between their electronic energy levels and associated carrier dynamics. Formation of the stable excited states of Ag42 is shown to be responsible for the core transformation.

Stabilization of dichalcogenide ligands in the coordination sphere of a ruthenium system.

The synthesis, structure and electronic properties of tetraruthenium dichalcogenide complexes displaying the exclusive coordination mode of dichalcogenide ligands have been discussed. The reactions of Li[BH2E3] (E = S or Se) with [ClRu(µ-Cl)(p-cymene)]2 (p-cymene = η6-{p-C6H4(iPr)Me}) at room temperature yielded tetrametallic dichalcogenide complexes [{Ru2Cl2(p-cymene)2}2(µ4,η2-E2)], 1-2 (E = S (1) and Se (2)). The solid-state X-ray structure of 1 shows that two {(p-cymene)RuCl}2 moieties are bridged by a S-S bond. In addition to 2, the reaction of Li[BH2Se3] with [ClRu(µ-Cl)(p-cymene)]2 also yielded a mononuclear tris-homocubane analogue [Ru(p-cymene){Se7(BH)3}] (3) which is an analogue of 1,3,3-tris-homocubane and possesses D3 symmetry. In order to isolate the Cp* analogue of 1, the reaction of [Cp*Ru(µ-Cl)Cl]2 with Li[BH2S3] was carried out, which led to the formation of bis/tris-homocubane derivatives [(Cp*Ru)2{µ-Sn(BH)2}] (n = 7 (4) and 6 (5)) along with the formation of ruthenium disulfide complexes [(RuCp*)2(µ,η2:η2-S2)(µ,η1:η1-S2)] and [(RuCp*)2(µ-SBHS-κ1B:κ2S:κ2S)(µ,η1:η1-S2)]. Complexes 1-5 have been characterized by multi-nuclear NMR, IR, UV-vis spectroscopy, and mass spectrometry and their molecular formulations (except 2) have been determined by single crystal X-ray crystallography. Furthermore, DFT calculations were performed that rationalize the stabilization of the dichalcogenide units (E22-) by the tetrametallic systems in 1-2.