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

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).

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.

On the σ-complex character of bis(gallyl)/digallane transition metal species

σ-complexes of homoatomic E-E bonds are key intermediates in catalytically relevant oxidative addition reactions, but are as yet unknown for the group 13 elements. Here, stable species best described as σ-complexes of a 1,2-dichlorodigallane derivative with Ni and Pd are reported. They are readily accessed through the combination of a 1,2-dichlorodigallane derivative, which features chelating phosphine functionalities, with Ni0 and Pd0 synthons. In-depth computational analyses of these complexes importantly reveal considerable Ga-Ga bonding interactions in both Ni and Pd complexes, despite the expected elongation of the Ga-Ga bond upon complexation, suggestive of σ-complex character as opposed to more commonly described bis(gallyl) character. Finally, the well-defined disproportion of the Ni complex is described, leading to a unique GaI-nickel complex, with concomitant expulsion of uncomplexed GaIII species.

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.

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 H₂BAr^F, 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 = Ar^F; Mes = 2,4,6-trimethylphenyl, and Ar^F = 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 = Ar^F) at elevated temperatures.

A safe and efficient synthetic method for alkali metal octahydrotriborates, unravelling a general mechanism for constructing the delta B3 unit of polyhedral boranes

A safe and efficient synthetic method for MB₃H₈ (M = Na, K, Rb and Cs) has been developed with excellent yields by directly reacting the corresponding MBH₄ with the dimethyl sulfide borane complex (DMS·BH₃). A general mechanism for constructing B₃H₈^-, a basic unit for building polyhedral boranes, has been unravelled.

Chemistry of group 5 metallaboranes with heterocyclic thiol ligands: a combined experimental and theoretical study

Thermolysis of [(Cp*Nb)₂(B₂H₆)₂], 1b (Cp* = η⁵-C₅Me₅), with 2-mercaptobenzothiazole, C₆H₄NSCSH (MBT), and 2-mercaptobenzoxazole, C₆H₄NOCSH (MBO), yielded hydrogen substituted compounds 2 and 3 with a general formula [(Cp*Nb)₂(B₂H₆)(B₂H₅L)] (2: L = C₆H₄NSCS and 3: L = C₆H₄NOCS). A similar reaction of 1b with Ph₂Se₂ yielded the monosubstituted derivative [(Cp*Nb)₂(B₂H₆){B₂H₅(PhSe)}], 4. All further efforts towards persubstitution of 1b under various drastic conditions were unfruitful. In parallel, in an effort to find a better synthetic route to the known Ta-aziridine complex [Cp*TaBH(C₇H₄NS₂)CH₂S₂NC₆H₄], Cp*TaCl₄ was treated with a 2-mercaptobenzothiazolyl-based borate ligand Na[H₂B(C₆H₄NSCS)₂]. Surprisingly, the reaction led to the formation of the half-sandwich trichloroaryltantalum(v) complex [Cp*TaCl₃{κ²-N,S-C₆H₄NSCS}], 5, containing a heterocyclic thiol ligand. Using an alternative method complex 5 was isolated in good yield when Cp*TaCl₄ was treated with the potassium salt of 2-mercaptobenzothiazole K[C₆H₄NSCS]. All the compounds were characterized by ¹H, ¹¹B{¹H}, and ¹³C{¹H} NMR spectroscopy, and their structures were unequivocally established by crystallographic analysis.

Recent Advances in the Synthesis and Reactivity of Transition Metal σ-Borane/Borate Complexes

The coordination of an element-element σ bond to a transition metal (TM) is both a fundamentally intriguing binding mode and of critical importance to metal-mediated bond activation mechanisms and catalysis, particularly the hotly contested field of C-H activation. TM σ complexes of dihydrogen (i.e., H-H) and silanes (H-SiR₃) have been extensively studied, the latter being of interest as models for the (generally unstable and unisolable) σ complexes of alkanes (i.e., H-CR₃). TM σ complexes of hydroboranes and hydroborates (i.e., H-BR₂, H-BR₃, (H-)₂BR₂) are somewhat less well studied but similarly have relevance to catalytic borylation reactions that are of high current interest to organic synthesis. Our two research groups have made significant contributions to elaborating the family of σ-borane/-borate complexes using two distinct approaches: while the Ghosh group generally starts from hydrogen-rich tetracoordinate boron species such as borates, the Braunschweig group starts from hypovalent and/or hypocoordinate boron building blocks. Through these two approaches, a wide range of species containing one or two σ-bound B-H ligands have been prepared, some with additional chelating donor sites. Over the past 2 years, the body of work on σ-borane/-borate complexes from our two research groups has significantly expanded, with a combined nine published articles in 2019-2020 alone. Very recent work from the Braunschweig group has led to the synthesis of the first bis(σ)-borane complexes of group 6 metals, as well as the synthesis of a series of novel bis(σ)-borane and bis(σ)-borate complexes of ruthenium and iridium, the former being useful precursors for pentacoordinate borylene complexes of Ru. Recent work from the Ghosh group has uncovered a remarkable diversity of structures with σ(B-H)-bound ligands from the combination of borohydrides and nitrogen/chalcogen-containing groups and heterocycles. These reactions, while in some cases producing conventional scorpionate-type chelating products, more frequently undergo fascinating rearrangements with unpredictable outcomes. This Account aims to highlight this recent acceleration of research progress in this area, particularly the distinct but related approaches of-and complexes produced by-our two research groups, in addition to relevant works from other groups where appropriate.

Cooperative B-H and Si-H Bond Activations by κ2-N,S-Chelated Ruthenium Borate Complexes

Cooperative E-H (E = B, Si) bond activations employing κ²-N,S-chelated ruthenium borate species, [PPh₃{κ²-N,S-(NS₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(NC₇H₄S₂)₂}], (1) are established. Treatment of 1 with BH₃·SMe₂ yielded the six-membered ruthenaheterocycle [PPh₃{κ²-S,H-(BH₃NS₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (2) formed by a hemilabile ring opening of a Ru-N bond and capturing of a BH₃ unit coordinated in an "end-on" fashion. On the other hand, the bulky borane H₂BMes shows different reactivity with 1 that led to the formation of the two dihydroborate complexes [{κ³-S,H,H-(NBH₂Mes)(S₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (3) and [PPh₃{κ³-S,H,H-(NBH₂Mes)(S₂C₇H₄)}Ru(κ²-N,S-C₇H₄NS₂)] (4), in which H₂BMes has been inserted into the Ru-N bond of the initial κ²-N,S-chelated ligand. In an attempt to directly activate hydrosilanes by 1, reactions were carried out with H₂SiPh₂ that yielded two isomeric five-membered ruthenium silyl complexes, namely [PPh₃{κ²-S,Si-(NSiPh₂)(S₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (5a,b), and the hydridotrisilyl complex [Ru(H){κ²-S,Si-(SiPh₂NC₇H₄S₂}₃] (6). These complexes were generated by Si-H bond activation with the release of H₂ and the formation of N-Si and Ru-Si bonds. When the reaction of 1 was carried out in the presence of PhSiH_3, the reaction only produced the analogous complexes [PPh₃{κ²-S,Si-(NSiPhH)(S₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (5a',b'). Density functional theory (DFT) calculations have been used to probe the bonding modes of boranes/silane with the ruthenium center.

Diborane(6) and Its Analogues Stabilized by Mono-, Bi-, and Trinuclear Group 7 Templates: Combined Experimental and Theoretical Studies

Irradiation of [Re₂(CO)₁₀] in the presence of BH₃·thf resulted in the formation of several rhenium diborane(6) species, for example, [{(OC)₄Re}{Re(CO)₃}₂(μ₃-η²:η²:η²-B₂H₆)(μ-H)], 1; [{(OC)₄Re}₂{Re(CO)₃}(μ₃-η²:η²:η¹-B₂H₆)(μ-H)], 2; and [{(OC)₄Re}₂(μ-η²:η²-B₂H₆)], 3, comprising diverse coordination modes of the [B₂H₆]^2- ligand. Compound 1 contains a tris(bidentate) [B₂H₆]^2- unit, whereas 2 consists of an unsymmetrically bound [μ₃-η²:η²:η¹-B₂H₆]^2- ligand. In contrast, the irradiation of [Mn₂(CO)₁₀] with BH₃·thf yielded only the Mn analogue of 1, [{(OC)₄Mn}{Mn(CO)₃}₂(μ₃-η²:η²:η²-B₂H₆)(μ-H)], 1'. In an attempt to generate the bimetallic Mn-diborane(6), we have carried out the reaction of 1' with PCy₃ under photolytic conditions. The reaction led to the formation of two single base stabilized unsymmetrical diborane(5) species, [{Mn(CO)₃}{Mn(CO)₂PCy₃}(μ-η²:η²-B₂H₅·PCy₃)(μ-H)], 4, and [{Mn(CO)₂PCy₃}(η³-B₂H₅·PCy₃)], 5. As [B₂H₆]^2- and [B₂H₅·PCy₃]^- are isoelectronic, the bondings in 4 and 5 are analogous to that of diborane(6) species 1-3. All the new species have been characterized spectroscopically, and their structures were further confirmed by single-crystal X-ray diffraction studies. DFT-type quantum chemical calculations were carried out that provided insight into the bonding interaction of [B₂H₆]^2- and [B₂H₅·PCy₃]^- with the M(CO)_n fragments.

Toward Transition-Metal-Templated Construction of Arylated B4 Chains by Dihydroborane Dehydrocoupling

The reactivity of a diruthenium tetrahydride complex towards three selected dihydroboranes was investigated. The use of [DurBH₂ ] (Dur=2,3,5,6-Me₄ C₆ H) and [(Me₃ Si)₂ NBH₂ ] led to the formation of bridging borylene complexes of the form [(Cp*RuH)₂ BR] (Cp*=C₅ Me₅ ; 1 a: R=Dur; 1 b: R=N(SiMe₃ )₂ ) through oxidative addition of the B-H bonds with concomitant hydrogen liberation. Employing the more electron-deficient dihydroborane [3,5-(CF₃ )₂ -C₆ H₃ BH₂ ] led to the formation of an anionic complex bearing a tetraarylated chain of four boron atoms, namely Li(THF)₄ [(Cp*Ru)₂ B₄ H₅ (3,5-(CF₃ )₂ C₆ H₃ )₄ ] (4), through an unusual, incomplete threefold dehydrocoupling process. A comparative theoretical investigation of the bonding in a simplified model of 4 and the analogous complex nido-[1,2(Cp*Ru)₂ (μ-H)B₄ H₉ ] (I) indicates that there appear to be no classical σ-bonds between the boron atoms in complex I, whereas in the case of 4 the B₄ chain better resembles a network of three B-B σ bonds, the central bond being significantly weaker than the other two.

Five-Membered Ruthenacycles: Ligand-Assisted Alkyne Insertion into 1,3-N,S-Chelated Ruthenium Borate Species

Building upon previous work, the chemistry of [(η⁶ -p-cymene)Ru{P(OMe)₂ OR}Cl₂ ], (R=H or Me) has been extended with [H₂ B(mbz)₂ ]^- (mbz=2-mercaptobenzothiazolyl) using different Ru precursors and borate ligands. As a result, a series of 1,3-N,S-chelated ruthenium borate complexes, for example, [(κ² -N,S-L)PR₃ Ru{κ³ -H,S,S'-H₂ B(L)₂ }], (2 a-d and 2 a'-d'; R=Ph, Cy, OMe or OPh and L=C₅ H₄ NS or C₇ H₄ NS₂ ) and [Ru{κ³ -H,S,S'-H₂ B(L)₂ }₂ ], (3: L=C₅ H₄ NS, 3': L=C₇ H₄ NS₂ ) were isolated upon treatment of [(η⁶ -p-cymene)RuCl₂ PR₃ ], 1 a-d (R=Ph, Cy, OMe or OPh) with [H₂ B(mp)₂ ]^- or [H₂ B(mbz)₂ ]^- ligands (mp=2-mercaptopyridyl). All the Ru borate complexes, 2 a-d and 2 a'-d' are stabilized by phosphine/phosphite and hemilabile N,S-chelating ligands. Treatment of these Ru borate species, 2 a'-c' with various terminal alkynes yielded two different types of five-membered ruthenacycle species, namely [PR₃ {C₇ H₄ S₂ -(E)-N-C=CH(R')}Ru{κ³ -H,S,S'-H₂ B(L)₂ }], (4-4'; R=Ph and R'=CO₂ Me or C₆ H₄ NO₂ ; L=C₇ H₄ NS₂ ) and [PR₃ {C₇ H₄ NS-(E)-S-C=CH(R')}Ru{κ³ -H,S,S'-H₂ B(L)₂ }], (5-5', 6 and 7; R=Ph, Cy or OMe and R'=CO₂ Me or C₆ H₄ NO₂ ; L=C₇ H₄ NS₂ ). All these five-membered ruthenacycle species contain an exocyclic C=C moiety, presumably formed by the insertion of a terminal alkyne into the Ru-N and Ru-S bonds. The new species have been characterized spectroscopically and the structures were further confirmed by single-crystal X-ray diffraction analysis. Theoretical studies and chemical-bonding analyses established that charge transfer occurs from phosphorus to ruthenium center following the trend PCy₃ <PPh₃ <P(OPh)₃ <P(OMe)₃ .

Adding to the Family of Copper Complexes Featuring Borohydride Ligands Based on 2-Mercaptopyridyl Units

Borohydride ligands featuring multiple pendant donor functionalities have been prevalent in the chemical literature for many decades now. More recent times has seen their development into new families of so-called soft scorpionates, for example, those featuring sulfur based donors. Despite all of these developments, those ligands containing just one pendant group are rare. This article explores one ligand family based on the 2-mercaptopyridine heterocycle. The coordination chemistry of the monosubstituted ligand, [H3B(mp)]− (mp = 2-mercaptopyridyl), has been explored. Reaction of Na[BH3(mp)] with one equivalent of Cu(I)Cl in the presence of either triphenylphosphine or tricyclohexylphosphine co-ligands leads to the formation of [Cu{H3B(mp)}(PR3)] (R = Ph, 1; Cy, 2), respectively. Structural characterization confirms a κ3-S,H,H coordination mode for the borohydride-based ligand within 1 and 2, involving a dihydroborate bridging interaction (BH2Cu) with the copper centers.

Use of Single-Metal Fragments for Cluster Building: Synthesis, Structure, and Bonding of Heterometallaboranes

The synergic property of the CO ligand, in general, can stabilize metal complexes at lower oxidation states. Utilizing this feature of the CO ligand, we have recently isolated and structurally characterized a highly fluxional molybdenum complex [{Cp*Mo(CO)₂}₂{μ-η²:η²-B₂H₄}] (2; Cp* = η⁵-C₅Me₅) comprising the diborane(4) ligand. Compound 2 represents a rare class of bimetallic diborane(4) complex corresponding to a singly bridged C _s structure. In an attempt to isolate the tungsten analogue of 2, [{Cp*W(CO)₂}₂{μ-η²:η²-B₂H₄}], we have isolated a rare vertex-fused cluster, [(Cp*W)₃WB₉H₁₈] (5). Having a structural likeness with the dimolybdenum alkyne complex [{CpMo(CO)₂}₂C₂H₂], we have further explored the chemistry of 2 with CO gas that yielded a homoleptic trimolybdenum complex, [(Cp*Mo)₃(μ-H)₂(μ₃-H)(μ-CO)₂B₄H₄] (4). In an attempt to replace the 16-electron {Cp*MoH(CO)₂} moiety in 4 with isolobal fragment {W(CO)₅}, we treated the intermediate, obtained from the reaction of Cp*MoCl₄ and LiBH₄, with the monometal carbonyl fragment {W(CO)₅·THF}. The reaction indeed yielded two bimetallic clusters, [(Cp*Mo)₂B₄H₈W(CO)₄] (7) and [(Cp*Mo)₂B₄H₆W(CO)₅] (8), that seem to have been generated by the replacement of one {BH} or {BH₃} vertex from [(Cp*Mo)₂B₅H₉], respectively. All of the compounds have been characterized by various spectroscopic analyses and single-crystal X-ray diffraction studies. Electron-counting rules and molecular orbital analyses provided further insight into the electronic structure of all of these molecules.

Synthesis of Trithia-Borinane Complexes Stabilized in Diruthenium Core: [(Cp*Ru)2(η1-S)(η1-CS){(CH2)2S3BR}] (R = H or SMe)

The thermolysis of arachno-1 [(Cp*Ru)2(B3H8)(CS2H)] in the presence of tellurium powder yielded a series of ruthenium trithia-borinane complexes: [(Cp*Ru)2(η1-S)(η1-CS){(CH2)2S3BH}] 2, [(Cp*Ru)2(η1-S)(η1-CS){(CH2)2S3B(SMe)}] 3, and [(Cp*Ru)2(η1-S)(η1-CS){(CH2)2S3BH}] 4. Compounds 2–4 were considered as ruthenium trithia-borinane complexes, where the central six-membered ring {C2BS3} adopted a boat conformation. Compounds 2–4 were similar to our recently reported ruthenium diborinane complex [(Cp*Ru){(η2-SCHS)CH2S2(BH2)2}]. Unlike diborinane, where the central six-membered ring {CB2S3} adopted a chair conformation, compounds 2–4 adopted a boat conformation. In an attempt to convert arachno-1 into a closo or nido cluster, we pyrolyzed it in toluene. Interestingly, the reaction led to the isolation of a capped butterfly cluster, [(Cp*Ru)2(B3H5)(CS2H2)] 5. All the compounds were characterized by 1H, 11B{1H}, and 13C{1H} NMR spectroscopy and mass spectrometry. The molecular structures of complexes 2, 3, and 5 were also determined by single-crystal X-ray diffraction analysis.

5-Mercaptotetrazolyl-derived metallaboratranes

The reactions of 1-tert butyl-5-thiotetrazole (HtttBu) with Na[BH4] provides Na[H2B(tttBu)2] (Na[1]) or Na[HB(tttBu)3] (Na[2]) in refluxing THF or xylene, respectively. Treating [RuCl(Ph)(CO)(PPh3)2] with Na[2] affords the triply-buttressed ruthenaboratrane [Ru(CO)(PPh3){κ4-B,S,S',S''-B(tttBu)3}] (5) whilst Na[1] with [IrCl(CO)(PPh3)2] or [RuCl(Ph)(CO)(PPh3)2] provides rare examples of doubly-buttressed metallaboratranes [IrH(CO)(PPh3){κ3-B,S,S'-BH(tttBu)2}] (7) and [Ru(CO)(PPh3)2{κ3-B,S,S'-BH(tttBu)2}] (12), the latter via the isolable thiotetrazoylborate complex [Ru(Ph)(CO)(PPh3){κ3-H,S,S'-H2B(tttBu)2}] (11).

Mercapto-benzothiazolyl based ruthenium(ii) borate complexes: synthesis and reactivity towards various phosphines

Ruthenium complexes featuring phosphinate and dual Ru⋯H–B interactions between Ru and B–H bonds of borate ligands supported by mercapto-benzothiazolyl heterocycles have been synthesized.

Synthesis and ligand substitution reactions of κ4-B,S,S′,S′′-ruthenaboratranes

Ruthenaboratranes of the form [Ru(CO)L{κ⁴-B(mt)₃}] (mt = N-methimazolyl) arise via substitution of the PPh₃ ligand in [Ru(CO)(PPh₃){κ⁴-B(mt)₃}] by L (L = PMe₂Ph, PMe₃, P(OMe)₃, P(OEt)₃, P(OPh)₃) or reactions of [RuCl(R)(CO)L_n] (R = Ph, CHCHPh; n = 2, L = PCy₃; n = 3, L = P(OMe)₃, PMe₂Ph) with Na[HB(mt)₃].

Methimazolyl based diptych bicyclo-[3.3.0]-ruthenaboratranes

The reactions of [RuCl(R)(CO)(PPh₃)₂] (R = CHCHPh, Ph) with Na[H₂B(mt)₂] (mt = N-methyl-2-mercaptoimidazolyl) transiently provide [Ru(R)(CO)(PPh₃){κ³-H,S,S′-H₂B(mt)₂}] which each evolve to the ruthenaboratrane [Ru(CO)(PPh₃)₂{κ³-B,S,S′-BH(mt)₂}](Ru→B)⁸.

Synthesis, characterization and photophysical studies of a novel polycyclic diborane

A long polycyclic diborane (4) with a dihedral angle of 21.75° between the two naphthalene rings has been successfully prepared and fully characterized.

Heterometallic boride clusters: synthesis and characterization of butterfly and square pyramidal boride clusters*

A number of heterometallic boride clusters have been synthesized and structurally characterized using various spectroscopic and crystallographic analyses. Thermolysis of [Ru3(CO)12] with [Cp*WH3(B4H8)] (1) yielded [{Cp*W(CO)2}2(μ 4-B){Ru(CO)3}2(μ-H)] (2), [{Cp*W(CO)2}2(μ 5-B){Ru(CO)3}2{Ru(CO)2}(μ-H)] (3), [{Cp*W(CO)2}(μ 5-B){Ru(CO)3}4] (4) and a ditungstaborane cluster [(Cp*W)2B4H8Ru(CO)3] (5) (Cp*=η 5-C5Me5). Compound 2 contains 62 cluster valence-electrons, in which the boron atom occupies the semi-interstitial position of a M4-butterfly core, composed of two tungsten and two ruthenium atoms. Compounds 3 and 4 can be described as hetero-metallic boride clusters that contain 74-cluster valence electrons (cve), in which the boron atom is at the basal position of the M5-square pyramidal geometry. Cluster 5 is analogous to known [(Cp*W)2B5H9] where one of the BH vertices has been replaced by isolobal {Ru(CO)3} fragment. Computational studies with density functional theory (DFT) methods at the B3LYP level have been used to analyze the bonding of the synthesized molecules. The optimized geometries and computed 11B NMR chemical shifts satisfactorily corroborate with the experimental data. All the compounds have been characterized by mass spectrometry, IR, 1H, 11B and 13C NMR spectroscopy, and the structural architectures were unequivocally established by crystallographic analyses of clusters 2–5.

An Efficient Method for the Synthesis of Boratrane Complexes of Late Transition Metals

In a quest for efficient precursors for the synthesis of boratrane complexes of late transition metals, we have developed a useful synthetic method using [L'M(μ-Cl)Cl_x ]₂ as precursors (L'=η⁶ -p-cymene, M=Ru, x=1; L'=COD, M=Rh, x=0 and L'=Cp*, M=Ir or Rh, x=1; COD=1,5-cyclooctadiene, Cp*=η⁵ -C₅ Me₅ ). For example, treatment of Na[(H₃ B)bbza] or Na[(H₂ B)mp₂ ] (bbza=bis(benzothiazol-2-yl)amine; mp=2-mercaptopyridyl) with [L'M(μ-Cl)Cl_x ]₂ yielded [(η⁶ -p-cymene)RuBH{(NCSC₆ H₄ )(NR)}₂ ] (2; R=NCSC₆ H₄ ), [{N(NCSC₆ H₄ )₂ }RhBH{(NCSC₆ H₄ )(NR)}₂ ] (3; R=NCS-C₆ H₄ ), [(η⁶ -p-cymene)RuBH(L)₂ ] (5; L=C₅ H₄ NS), and [Cp*MBH(L)₂ ] (6 and 7; L=C₅ H₄ NS, M=Ir or Rh). In order to delineate the significance of the ligands, we studied the reactivity of [(COD)Rh(μ-Cl)]₂ with Na[(H₃ B)bbza], which led to the formation of the isomeric agostic complexes [(η⁴ -COD)Rh(μ-H)BHRh(C₁₄ H₈ N₃ S₂ )₃ ], 4 a and 4 b, in parallel to the formation of 16-electron square-pyramidal rhodaboratrane complex 3. Compounds 4 a and 4 b show two different geometries, in which the Rh-B bonds are shorter than in the reported Rh agostic complexes. The new compounds have been characterized in solution by various spectroscopic analyses, and their structural arrangements have been unequivocally established by crystallographic analyses. DFT calculations provide useful insights regarding the stability of these metallaboratrane complexes as well as their M→B bonding interactions.

Design, Synthesis, and Chemistry of Bis(σ)borate and Agostic Complexes of Group 7 Metals

A series of new bis(σ)borate and agostic complexes of group 7 metals have been synthesized and structurally characterized from various borate ligands, such as trihydrobis(benzothiazol-2-yl)amideborate (Na[(H₃ B)bbza]), trihydro(2-aminobenzothiazolyl)borate (Na[(H₃ B)abz]), and dihydrobis(2-mercaptopyridyl)borate (Na[(H₂ B)mp₂ ]) (bbza=bis(benzothiazol-2-yl)amine, abz=2-aminobenzothiazolyl, and mp=2-mercaptopyridyl). Photolysis of [Mn₂ (CO)₁₀ ] with Na[(H₃ B)bbza] formed bis(σ)borate complex [Mn(CO)₃ (μ-H)₂ BHNCSC₆ H₄ (NR)] (1; R=NCSC₆ H₄ ). Octahedral complex [Re(CO)₂ (N₃ C₂ S₂ C₁₂ H₈ )₂ ] (2) was generated under similar reaction conditions with [Re₂ (CO)₁₀ ]. Similarly, when [Mn₂ (CO)₁₀ ] was treated with Na[(H₃ B)abz], bis(σ)borate complex [Mn(CO)₃ (μ-H)₂ BH(HN₂ CSC₆ H₄ )] (3) and the agostic complex [Mn(CO)₃ (μ-H)BH(HN₂ CSC₆ H₄ )₂ ] (4) were formed. To probe the potential formation of agostic complexes of the heavier group 7 metals, we carried out the photolysis of [M₂ (CO)₁₀ ] with Na[(H₂ B)mp₂ ] and found that [M(CO)₃ (μ-H)BH(C₅ H₄ NS)₂ ] (5: M=Re; 6: M=Mn) was formed in moderate yield. Complexes 1 and 3 feature a (κ³ -H,H,N) coordination mode, whereas 4, 5, and 6 display both (κ³ -H,N,N) and (κ³ -H,S,S) modes of the corresponding ligands. To investigate the lability of the CO ligands of 1 and 3, we treated the complexes with phosphine ligands that generated novel bis(σ)borate complexes [Mn(μ-H)₂ (BHNCSC₆ H₄ )(NR)(CO)₂ PL₂ L'] (R=NCSC₆ H₄ ; 7 a: L=L'=Ph; 7 b: L=Ph, L'=Me) and [Mn(μ-H)₂ BHN(NCSC₆ H₄ )R(CO)₂ PL₂ L'] (R=NCSC₆ H₄ ; 8 a: L=L'=Ph; 8 b: L=Ph, L'=Me). Complexes 7 and 8 are structural isomers with different coordination modes of the bbza ligand. In addition, DFT calculations were performed to shed some light on the bonding and electronic structures of these complexes.