First Bis(σ)-borane Complexes of Group 6 Transition Metals: Experimental and Theoretical Studies

A series of bis(σ)-borane complexes of Group 6 transition metals were prepared by direct dihydroborane coordination to the metal center. Reaction of [M(CO)₃(PCy₃)₂] and two dihydroboranes [DurBH₂] and [(Me₃Si)₂NBH₂] (Dur=2,3,5,6-Me₄C₆H) yielded bis(σ)-borane complexes fac-[M(CO)₃(PCy₃){η²-(H₂BR)}] (R=Dur; 1 : M=Cr, 2 : M=W; R=N(SiMe₃)₂; 3 : M=Cr, 4 : M=W). In the case of molybdenum, we have isolated an arene complex ( 5 ) with [DurBH₂] in which the Dur group acts as a η⁶-bound ligand, and with [(Me₃Si)₂NBH₂] a similar bis(σ)-borane complex was isolated, cis,trans-[Mo(CO)₂(PCy₃)₂{η²-(H₂BN(SiMe₃)₂}] ( 6 ), with a different pattern of auxiliary ligands. The complexes were investigated by multinuclear NMR spectroscopy, mass spectrometry, X-ray diffraction analysis, and computational methods. Quantum theory of atoms in molecules (QTAIM) calculations demonstrated that the borane complexes may be described as pure bis(σ)-borane complexes rather than elongated or stretched examples given that the calculations do not show the presence of a ring-critical point (RCP) at the ring formed by the interactions of the B−H with metal center.     

Fundamental Differences between Group 8 Metals: Unexpected Oxidation State Preferences and Mechanisms in Ruthenium Borylene Complex Formation

The reaction of the salts K[Ru(CO)₃(PMe₃)(SiR₃)] (R=Me, Et) with Br₂BDur or Cl₂BDur (Dur=2,3,5,6‐Me₄C₆H) leads to both boryl and borylene complexes of divalent ruthenium, the former through simple salt elimination and the latter through subsequent CO loss and 1,2‐halide shift. The balance of products can be altered by varying the reaction conditions; boryl complexes can be favored by the addition of CO, and borylene complexes by removal of CO under vacuum. All of these products are in competition with the corresponding (aryl)(halo)(trialkylsilyl)borane, a reductive elimination product. The Ru^II borylene products and the mechanisms that form them are distinctly different from the analogous reactions with iron, which lead to low‐valent borylene complexes, highlighting fundamental differences in oxidation state preferences between iron and ruthenium.     

Synthesis and Structure of the First Terminal Borylene Complexes

A nearly linear arrangement is observed for the three atoms in the central W-B-N unit of the tungsten complex [(CO)₅WBN(SiMe₃)₂] ( 1 ) in the crystal (W-B-N 177.9°; see picture). This compound along with its Cr analogue represent the first examples of terminal borylene complexes with a two-coordinate metal-bound boron atom. The geometries of the axial and equatorial CO groups in 1 are similar, and thus indicate that there is no trans effect of the borylene ligand.     

Hydroboration of Alkynes with Zwitterionic Ruthenium–Borate Complexes: Novel Vinylborane Complexes

Building upon previous studies on the synthesis of bis(sigma)borate and agostic complexes of ruthenium, the chemistry of nido‐[(Cp*Ru)₂B₃H₉] ( 1 ) with other ligand systems was explored. In this regard, mild thermolysis of nido‐ 1 with 2‐mercaptobenzothiazole (2‐mbzt), 2‐mercaptobenzoxazole (2‐mbzo) and 2‐mercaptobenzimidazole (2‐mbzi) ligands were performed which led to the isolation of bis(sigma)borate complexes [Cp*RuBH₃L] ( 2 a – c ) and β‐agostic complexes [Cp*RuBH₂L₂] ( 3 a – c ; 2 a , 3 a : L=C₇H₄NS₂; 2 b , 3 b : L=C₇H₄NSO; 2 c , 3 c : L=C₇H₅N₂S). Further, the chemistry of these novel complexes towards various diphosphine ligands was investigated. Room temperature treatment of 3 a with [PPh₂(CH₂)_nPPh₂] (n=1–3) yielded [Cp*Ru(PPh₂(CH₂)_nPPh₂)‐BH₂(L₂)] ( 4 a – c ; 4 a : n=1; 4 b : n=2; 4 c : n=3; L=C₇H₄NS₂). Mild thermolysis of 2 a with [PPh₂(CH₂)_nPPh₂] (n=1–3) led to the isolation of [Cp*Ru(PPh₂(CH₂)_nPPh₂)(L)] (L=C₇H₄NS₂ 5 a – c ; 5 a : n=1; 5 b : n=2; 5 c : n=3). Treatment of 4 a with terminal alkynes causes a hydroboration reaction to generate vinylborane complexes [Cp*Ru(R?C?CH₂)BH(L₂)] ( 6 and 7 ; 6 : R=Ph; 7 : R=COOCH₃; L=C₇H₄NS₂). Complexes 6 and 7 can also be viewed as η‐alkene complexes of ruthenium that feature a dative bond to the ruthenium centre from the vinylinic double bond. In addition, DFT computations were performed to shed light on the bonding and electronic structures of the new compounds.     

trans‐Selective Insertional Dihydroboration of a cis‐Diborene: Synthesis of Linear sp3‐sp2‐sp3‐Triboranes and Subsequent Cationization

The reaction of aryl‐ and amino(dihydro)boranes with dibora[2]ferrocenophane 1 leads to the formation 1,3‐trans‐dihydrotriboranes by formal hydrogenation and insertion of a borylene unit into the B=B bond. The aryltriborane derivatives undergo reversible photoisomerization to the cis‐1,2‐μ‐H‐3‐hydrotriboranes, while hydride abstraction affords cationic triboranes, which represent the first doubly base‐stabilized B₃H₄⁺ analogues.     

Heterometallic Triply‐Bridging Bis‐Borylene Complexes

Triply‐bridging bis‐{hydrido(borylene)} and bis‐borylene species of groups 6, 8 and 9 transition metals are reported. Mild thermolysis of [Fe₂(CO)₉] with an in situ produced intermediate, generated from the low‐temperature reaction of [Cp*WCl₄] (Cp*=η⁵‐C₅Me₅) and [LiBH₄?THF] afforded triply‐bridging bis‐{hydrido(borylene)}, [(μ₃‐BH)₂H₂{Cp*W(CO)₂}₂{Fe(CO)₂}] ( 1 ) and bis‐borylene, [(μ₃‐BH)₂{Cp*W(CO)₂}₂{Fe(CO)₃}] ( 2 ). The chemical bonding analyses of 1 show that the B?H interactions in bis‐{hydrido (borylene)} species is stronger as compared to the M?H ones. Frontier molecular orbital analysis shows a significantly larger energy gap between the HOMO‐LUMO for 2 as compared to 1 . In an attempt to synthesize the ruthenium analogue of 1 , a similar reaction has been performed with [Ru₃(CO)₁₂]. Although we failed to get the bis‐{hydrido(borylene)} species, the reaction afforded triply‐bridging bis‐borylene species [(μ₃‐BH)₂{WCp*(CO)₂}₂{Ru(CO)₃}] ( 2′ ), an analogue of 2 . In search for the isolation of bridging bis‐borylene species of Rh, we have treated [Co₂(CO)₈] with nido‐[(RhCp*)₂(B₃H₇)], which afforded triply‐bridging bis‐borylene species [(μ₃‐BH)₂(RhCp*)₂Co₂(CO)₄(μ‐CO)] ( 3 ). All the compounds have been characterized by means of single‐crystal X‐ray diffraction study; ¹H, ¹¹B, ¹³C NMR spectroscopy; IR spectroscopy and mass spectrometry.     

Solvent and substitution effects on the structure and properties of a half-sandwich complex of vanadium with a terminal borylene ligand: Theoretical study

The structures and properties of a terminal borylene vanadium complexes CpV(CO)₃(BNH₂) were explored using theoretical methods. The density of states (DOS) was beneficially used to analyze the main features of electronic structure. The influence of solvent on the structural parameters, frontier orbital energies and wavelength absorption maxima (λ_max) of this structure was studied. These calculations were performed in different solvents, i.e. cyclohexane, dichloromethane, tetrahydrofuran, chlorobenzene, and chloroform, utilizing the self-consistent reaction field (SCRF) theory. Also, substituent effect in CpV(CO)₃(BNR₂) complexes on the structure and properties was investigated.     

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)₄Mn}(η⁶‐B₂H₆){Mn(CO)₃}₂(μ‐H)] ( 1 ) and a triply bridged borylene complex [(μ₃‐BH)(Cp*Co)₂(μ‐CO)(μ‐H)₂MnH(CO)₃] ( 2 ). In a similar fashion, [Re₂(CO)₁₀] generated [(μ₃‐BH)(Cp*Co)₂(μ‐CO)(μ‐H)₂ReH(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)₃Ru₃(CO)₉B] ( 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 ¹H, ¹¹B, ¹³C 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 .     

New Heteronuclear Bridged Borylene Complexes That Were Derived from [{Cp*CoCl}2] and Mono‐Metal?Carbonyl 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)_nPPh₂] (n=1–3) yielded complexes 9 – 11 , [3,4‐(Ph₂P(CH₂)_nPPh₂)‐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 ¹H, ¹³C, and ¹¹B NMR spectroscopy and their structural types were unequivocally established by crystallographic analysis of complexes 1 , 2 , 4 , 9 , and 10 .     

Synthesis and Structures of Ruthena-octahydrotetraborane Complexes

The reactivity of κ²-N,S-chelated ruthenium borate complexes, [Ru(PPh₃)(κ²-N,S-(NC₇H₄S₂)Ru{κ³-H,S,S′-H₂B (NC₇H₄S₂)₂}] ( 1 a ) and [Ru(PPh₃)(κ²-N,S-(NC₅H₄S){κ³-H,S,S′-H₂B(NC₅H₄S)₂}] ( 1 b ) with monoboranes have been explored. The prolonged room temperature reaction of [Ru(PPh₃){κ²-N,S-(NC₇H₄S₂)}{κ³-H,S,S′-H₂B(NC₇H₄S₂)₂}], 1 a with an excess of [BH₃ ⋅ THF] led to the formation of hydrogen-rich complex, arachno-[PPh₃(κ²-B₃H₈)Ru{κ³-H,S,S′-H₂B(NC₇H₄S₂)₂] ( 2 ). In a similar fashion, the isomeric ruthenatetraborane complexes of [Ru(PPh₃)(κ²-N,S-(B₃H₈){κ³-H,S,S′-H₂B(NC₅H₄S)₂}], 4 and 5 were isolated from the room temperature reaction of 1 b with [BH₃ ⋅ THF]. In complex 2 , the borate ligand, [H₂B(NC₅H₄S)₂] and the PPh₃ occupy the endo and exo sites of the butterfly-shaped RuB₃ core, respectively. In contrast, the borate unit [H₂B(NC₅H₄S)₂] in 4 sits on the exo position of the butterfly core, while the phosphine ligand occupies the endo site. Multinuclear spectroscopic analyses were done to characterize all new complexes and the structures were further confirmed by single-crystal X-ray diffraction analysis. Density functional theory (DFT) calculations were performed to probe into the bonding modes of these complexes.     

η4‐HBCC‐σ,π‐Borataallyl Complexes of Ruthenium Comprising an Agostic Interaction

Thermolysis of [Cp*Ru(PPh₂(CH₂)PPh₂)BH₂(L₂)] 1 (Cp*=η⁵‐C₅Me₅; L=C₇H₄NS₂), with terminal alkynes led to the formation of η⁴‐σ,π‐borataallyl complexes [Cp*Ru(μ‐H)B{R‐C=CH₂}(L)₂] ( 2 a – c ) and η²‐vinylborane complexes [Cp*Ru(R‐C=CH₂)BH(L)₂] ( 3 a – c ) ( 2 a , 3 a : R=Ph; 2 b , 3 b : R=COOCH₃; 2 c , 3 c : R=p‐CH₃‐C₆H₄; L=C₇H₄NS₂) through hydroboration reaction. Ruthenium and the HBCC unit of the vinylborane moiety in 2 a – c are linked by a unique η⁴‐interaction. Conversions of 1 into 3 a – c proceed through the formation of intermediates 2 a – c . Furthermore, in an attempt to expand the library of these novel complexes, chemistry of σ‐borane complex [Cp*RuCO(μ‐H)BH₂L] 4 (L=C₇H₄NS₂) was investigated with both internal and terminal alkynes. Interestingly, under photolytic conditions, 4 reacts with methyl propiolate to generate the η⁴‐σ,π‐borataallyl complexes [Cp*Ru(μ‐H)BH{R‐C=CH₂}(L)] 5 and [Cp*Ru(μ‐H)BH{HC=CH‐R}(L)] 6 (R=COOCH₃; L=C₇H₄NS₂) by Markovnikov and anti‐Markovnikov hydroboration. In an extension, photolysis of 4 in the presence of dimethyl acetylenedicarboxylate yielded η⁴‐σ,π‐borataallyl complex [Cp*Ru(μ‐H)BH{R‐C=CH‐R}(L)] 7 (R=COOCH₃; L=C₇H₄NS₂). An agostic interaction was also found to be present in 2 a – c and 5 – 7 , which is rare among the borataallyl complexes. All the new compounds have been characterized in solution by IR, ¹H, ¹¹B, ¹³C NMR spectroscopy, mass spectrometry and the structural types were unequivocally established by crystallographic analysis of 2 b , 3 a – c and 5 – 7 . DFT calculations were performed to evaluate possible bonding and electronic structures of the new compounds.     

Novel P‐Stereogenic PCP Pincer‐Aryl Ruthenium(II) Complexes and Their Use in the Asymmetric Hydrogen Transfer Reaction of Acetophenone

Achiral P‐donor pincer‐aryl ruthenium complexes ([RuCl(PCP)(PPh₃)]) 4c , d were synthesized via transcyclometalation reactions by mixing equivalent amounts of [1,3‐phenylenebis(methylene)]bis[diisopropylphosphine] ( 2c ) or [1,3‐phenylenebis(methylene)]bis[diphenylphosphine] ( 2d ) and the N‐donor pincer‐aryl complex [RuCl{2,6‐(Me₂NCH₂)₂C₆H₃}(PPh₃)], ( 3 ; Scheme 2). The same synthetic procedure was successfully applied for the preparation of novel chiral P‐donor pincer‐aryl ruthenium complexes [RuCl(P*CP*)(PPh₃)] 4a , b by reacting P‐stereogenic pincer‐arenes (S,S)‐[1,3‐phenylenebis(methylene)]bis[(alkyl)(phenyl)phosphines] 2a , b (alkyl=ⁱPr or ^tBu, P*CHP*) and the complex [RuCl{2,6‐(Me₂NCH₂)₂C₆H₃}(PPh₃)], ( 3 ; Scheme 3). The crystal structures of achiral [RuCl(equation/tex2gif-sup-3.gifPCP)(PPh₃)] 4c and of chiral (S,S)‐[RuCl(equation/tex2gif-sup-6.gifPCP)(PPh₃)] 4a were determined by X‐ray diffraction (Fig. 3). Achiral [RuCl(PCP)(PPh₃)] complexes and chiral [RuCl(P*CP*)(PPh₃)] complexes were tested as catalyst in the H‐transfer reduction of acetophenone with propan‐2‐ol. With the chiral complexes, a modest enantioselectivity was obtained.     

Chemistry of Guanidinate‐Stabilised Diboranes: Transition‐Metal‐Catalysed Dehydrocoupling and Hydride Abstraction

Herein, we analyse the catalytic boron–boron dehydrocoupling reaction that leads from the base‐stabilised diborane(6) [H₂B(hpp)]₂ (hpp=1,3,4,6,7,8‐hexahydro‐2H‐pyrimido[1,2‐a]pyrimidinate) to the base‐stabilised diborane(4) [H₂B(hpp)]₂. A number of potential transition‐metal precatalysts was studied, including transition‐metal complexes of the product diborane(4). The synthesis and structural characterisation of two further examples of such complexes is presented. The best results for the dehydrocoupling reactions were obtained with precatalysts of Group 9 metals in the oxidation state of +I. The active catalyst is formed in situ through a multistep process that involves reduction of the precatalyst by the substrate [H₂B(hpp)]₂, and mechanistic investigations indicate that both heterogeneous and (slower) homogeneous reaction pathways play a role in the dehydrocoupling reaction. In addition, hydride abstraction from [H₂B(hpp)]₂ and related diboranes is analysed and the possibility for subsequent deprotonation is discussed by probing the protic character of the cationic boron–hydrogen compounds with NMR spectroscopic analysis.     

Hydrogen Release from Dialkylamine–Boranes Promoted by Mg and Ca Complexes: A DFT Analysis of the Reaction Mechanism

Mg and Ca β‐diketiminato silylamides [HC{(Me)CN(2,6‐iPr₂C₆H₃)}₂M(THF)_n{N(SiMe₃)₂}] (M=Mg, n=0; M=Ca, n=1) were studied as precatalysts for the dehydrogenation/dehydrocoupling of secondary amine–boranes R₂HNBH₃. By reaction with equimolar quantities of amine–boranes, the corresponding amidoborane derivatives are formed, which further react to yield dehydrogenation products such as the cyclic dimer [BH₂?NMe₂]₂. DFT was used here to explore the mechanistic alternatives proposed on the basis of the experimental findings for both Mg and Ca amidoboranes. The influence of the steric demand of amine–boranes on the course of the reaction was examined by performing calculations on the dehydrogenation of dimethylamine–borane (DMAB), pyrrolidine–borane (PB), and diisopropylamine–borane. In spite of the analogies in the catalytic activity of Mg‐ and Ca‐based complexes in the dehydrocoupling of amine–boranes, our theoretical analysis confirmed the experimentally observed lower reactivity of Ca complexes. Differences in catalytic activity of Mg‐ and Ca‐based complexes were examined and rationalized. As a consequence of the increase in ionic radius on going from Mg²⁺ to Ca²⁺, the dehydrogenation mechanism changes and formation of a key metal hydride intermediate becomes inaccessible. Dimerization is likely to occur off‐metal in solution for DMAB and PB, whereas steric hindrance of iPr₂NHBH₃ hampers formation of the cyclic dimer. The reported results are of particular interest because, although amine–borane dehydrogenation is now well established, mechanistic insight is still lacking for many systems.     

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-Me₄C₆H) and [(Me₃Si)₂NBH₂] led to the formation of bridging borylene complexes of the form [(Cp*RuH)₂BR] (Cp*=C₅Me₅; 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-C₆H₃BH₂] led to the formation of an anionic complex bearing a tetraarylated chain of four boron atoms, namely Li(THF)₄[(Cp*Ru)₂B₄H₅(3,5-(CF₃)₂C₆H₃)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)B₄H₉] ( 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.     

Syntheses and Characterization of New Vinyl‐Borylene Complexes by the Hydroboration of Alkynes with [(μ3‐BH)(Cp*RuCO)2(μ‐CO)Fe(CO)3]

Room temperature photolysis of a triply‐bridged borylene complex, [(μ₃‐BH)(Cp*RuCO)₂(μ‐CO)Fe(CO)₃] ( 1 a ; Cp*=C₅Me₅), in the presence of a series of alkynes, 1,2‐diphenylethyne, 1‐phenyl‐1‐propyne, and 2‐butyne led to the isolation of unprecedented vinyl‐borylene complexes (Z)‐[(Cp*RuCO)₂(μ‐CO)B(CR)(CHR′)] ( 2 : R, R′=Ph; 3 : R=Me, R′=Ph; 4 : R, R′=Me). This reaction permits a hydroboration of alkyne through an anti ‐ Markovnikov addition. In stark contrast, in the presence of phenylacetylene, a metallacarborane, closo‐[1,2‐(Cp*Ru)₂(μ‐CO)₂{Fe₂(CO)₅}‐4‐Ph‐4,5‐C₂BH₂] ( 5 a) , is formed. A plausible mechanism has been proposed for the formation of vinyl‐borylene complexes, which is supported by density functional theory (DFT) methods. Furthermore, the calculated ¹¹B NMR chemical shifts accurately reflect the experimentally measured shifts. All the new compounds have been characterized in solution by mass spectrometry and IR, ¹H, ¹¹B, and ¹³C NMR spectroscopies and the structural types were unequivocally established by crystallographic analysis of 2 , 5 a , and 5 b .     

Synthesis,characterization, electrochemistry,catalytic, and antimicrobial studies of ruthenium(III) complexes containing ONO donor ligands

A series of ruthenium(III) complexes [RuX(EPh₃)₂L] (where X = Cl or Br; E = P or As; L = deprotonated dibasic tridentate ligand) were prepared by the reaction of [RuX₃(EPh₃)₃] with Schiff bases (H₂L¹–H₂L⁴). The ligands were prepared by the condensation of N-4 phenyl/methyl semicarbazide with o-vanillin/o-hydroxy acetophenone. The complexes were characterized by elemental, physico-chemical, and electrochemical methods. Catalytic studies of these complexes for the oxidation of alcohols and aryl–aryl coupling were carried out. Antimicrobial experiments were also carried out.     

Well‐Defined N‐Heterocyclic Carbene‐Palladium Complexes as Efficient Catalysts for Domino Sonogashira Coupling/Cyclization Reaction and C‐H bond Arylation of Benzothiazole

Well‐defined and air‐stable PEPPSI (Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation) themed palladium bis‐N‐heterocyclic carbene complexes have been developed for the domino Sonogashira coupling/cyclization reaction of 2‐iodophenol with a variety of terminal alkynes and C‐H bond arylation of benzothiazole with aryl iodides. The PEPPSI themed palladium complexes, 2a and 2b were synthesized in good yields from the reaction of corresponding imidazolium salts with PdCl₂ and K₂CO₃ in pyridine. The new air‐stable palladium‐NHC complexes were characterized by NMR spectroscopy, X‐ray crystallography, elemental analysis, and mass spectroscopy studies. The PEPPSI themed palladium(II) bis‐N‐heterocyclic carbene complexes 2a and 2b exhibited excellent catalytic activities for domino Sonogashira coupling/cyclization reaction of 2‐iodophenol with terminal alkynes yielding benzofuran derivatives. In addition, the palladium complexes, 2a and 2b successfully catalyzed the direct C‐H bond arylation of benzothiazole with aryl iodides as coupling partners in presence of CuI as co‐catalyst.     

From Metallaborane to Borylene Complexes: Syntheses and Structures of Triply Bridged Ruthenium and Tantalum Borylene Complexes

Reaction of [1,2‐(Cp*RuH)₂B₃H₇] ( 1 ; Cp*=η⁵‐C₅Me₅) with [Mo(CO)₃(CH₃CN)₃] yielded arachno‐[(Cp*RuCO)₂B₂H₆] ( 2 ), which exhibits a butterfly structure, reminiscent of 7 sep B₄H₁₀. Compound 2 was found to be a very good precursor for the generation of bridged borylene species. Mild pyrolysis of 2 with [Fe₂(CO)₉] yielded a triply bridged heterotrinuclear borylene complex [(μ₃‐BH)(Cp*RuCO)₂(μ‐CO){Fe(CO)₃}] ( 3 ) and bis‐borylene complexes [{(μ₃‐BH)(Cp*Ru)(μ‐CO)}₂Fe₂(CO)₅] ( 4 ) and [{(μ₃‐BH)(Cp*Ru)Fe(CO)₃}₂(μ‐CO)] ( 5 ). In a similar fashion, pyrolysis of 2 with [Mn₂(CO)₁₀] permits the isolation of μ₃‐borylene complex [(μ₃‐BH)(Cp*RuCO)₂(μ‐H)(μ‐CO){Mn(CO)₃}] ( 6 ). Both compounds 3 and 6 have a trigonal‐pyramidal geometry with the μ₃‐BH ligand occupying the apical vertex, whereas 4 and 5 can be viewed as bicapped tetrahedra, with two μ₃‐borylene ligands occupying the capping position. The synthesis of tantalum borylene complex [(μ₃‐BH)(Cp*TaCO)₂(μ‐CO){Fe(CO)₃}] ( 7 ) was achieved by the reaction of [(Cp*Ta)₂B₄H₈(μ‐BH₄)] at ambient temperature with [Fe₂(CO)₉]. Compounds 2 – 7 have been isolated in modest yield as yellow to red crystalline solids. All the new compounds have been characterized in solution by mass spectrometry; IR spectroscopy; and ¹H, ¹¹B, and ¹³C NMR spectroscopy and the structural types were unequivocally established by crystallographic analysis of 2 – 6 .     

The First Boron–Tellurium Double Bond: Direct Insertion of Heavy Chalcogens into a Mn=B Double Bond

The base‐stabilized borylene [Cp(OC)₂Mn=Bt Bu(IMe)] readily reacts with elemental chalcogens in an insertion reaction that yields borachalcone complexes [Cp(OC)₂Mn‐E=Bt Bu(IMe)] (E=S, Se, Te). The tellurium example features the first double bond between boron and tellurium, making Te the heaviest main‐group element to make multiple bonds with boron. This unprecedented interaction has been fully investigated both experimentally and computationally.