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.     

Dihydrogen Generation from Amine/Boranes: Synthesis,FT‐ICR,and Computational Studies

A Fourier transform ion cyclotron resonance spectrometry (FT‐ICR) study of the gas‐phase protonation of ammonia‐borane and sixteen amine/boranes R¹R²R³N? BH₃ (including six compounds synthesized for the first time) has shown that, without exception, the protonation of amine/boranes leads to the formation of dihydrogen. The structural effects on the experimental energetic thresholds of this reaction were determined experimentally. The most likely intermediate and the observed final species (besides H₂) are R¹R²R³N? BH₄⁺ and R¹R²R³N? BH₂⁺, respectively. Isotopic substitution allowed the reaction mechanism to be ascertained. Computational analyses ([MP2/6‐311+G(d,p)] level) of the thermodynamic stabilities of the R¹R²R³N? BH₃ adducts, the acidities of the proton sources required for dihydrogen formation, and the structural effects on these processes were performed. It was further found that the family of R¹R²R³N? BH₄⁺ ions is characterized by a three‐center, two‐electron bond between B and a loosely bound H₂ molecule. Unexpected features of some R¹R²R³N? BH₄⁺ ions were found. This information allowed the properties of amine/boranes most suitable for dihydrogen generation and storage to be determined.     

Hydroboration of Arynes with N‐Heterocyclic Carbene Boranes

Arynes were generated in situ from ortho‐silyl aryl triflates and fluoride ions in the presence of stable N‐heterocyclic carbene boranes (NHC? BH₃). Spontaneous hydroboration ensued to provide stable B‐aryl‐substituted NHC‐boranes (NHC? BH₂Ar). The reaction shows good scope in terms of both the NHC‐borane and aryne components and provides direct access to mono‐ and disubstituted NHC‐boranes. The formation of unusual ortho regioisomers in the hydroboration of arynes with an electron‐withdrawing group supports a hydroboration process with hydride‐transfer character.     

Are Boryl Radicals from Amine–Boranes and Phosphine–Boranes the Most Stable Radicals?

The relative stability of the radicals that can be produced from amine–boranes and phosphine–boranes is investigated at the G3‐RAD level of theory. Aminyl ([RNH]^.:BH₃) and phosphinyl ([RPH]^.:BH₃) radicals are systematically more stable than the boryl analogues, [RNH₂]:BH₂^. and [RPH₂]:BH₂^.. Despite similar stability trends for [RNH]^.:BH₃ and [RPH]^.:BH₃ radicals with respect to boryl radicals, there are significant dissimilarities between amine– and phosphine–boranes. The homolytic bond dissociation energy of the N?H bond decreases upon association of the amines with BH₃, whereas that of the P?H bond for phosphines increases. The stabilization of the free amine is much smaller than that of the corresponding aminyl radical, whereas for phosphines this is the other way around. The homolytic bond dissociation energy of the B?H bond of borane decreases upon complexation with both amines and phosphines.     

SYNTHESIS AND CHARACTERIZATION OF THE FIRST BORANE ADDUCTS AND BORON CATIONS OF SOME N-ALKYL AND N-AMINOTRIPHENYLPHOSPHORANIMINES

Abstract

The first borane adducts of N-alkyl and N-aminotriphenylphosphoranimines, Ph₃P[dbnd]N—R, were prepared by two different general synthetic methods. The first method involved displacement of THF (tetrahydofuran) from THF-borane by the free imines, and the second employed the reaction of LiBH₄ with iminium bromides, Ph₃P[dbnd]N(R)HBr, in diethyl ether. Imine boranes, Ph₃P[dbnd]N(R)BH₃, were synthesized where R [dbnd] methyl, ethyl, n-propyl, isopropyl, isobutyl, t-butyl. dimethylamino, phenylamino, and methyl, phenylamino as the nitrogen attached groups. Symmetrical boron cations, (Ph₃P[dbnd]NR)₂, BH₂ ⁺, where R = methyl, ethyl, and n-propyl, were synthesized by displacement of iodide from in-situ generated iodoborane adducts, Ph₃P[dbnd]N(R)BH₂I, by the free imines. An attempt to form an unsymmetrical boron cation from (CH₃)₃ NBH₂I and Ph, P[dbnd]N(n-C₃H₇) resulted only in a mixture of the corresponding symmetrical boron cations. Physical, chemical and spectral properties of the borane adducts and boron cations, namely thermal and hydrolytic stabilities, infrared and NMR data are presented. Oxidative and reductive stabilities of the boron cations were studied. The borane adducts can be chlorinated with either HCI or Ph₃CCI. Relative base strengths of some imines were determined by following the exchange of BH₃ between borane adducts of (CH₃)₃ N or 4- (CH₃)C₅H₄ N and the imines via NMR.     

Manganese‐σ‐Borane Complexes from Dihaloboranes

The hydride complex K[(η⁵‐C₅H₅)Mn(CO)₂H] reacted with a range of dihalo(organyl)boranes X₂BR (X = Cl, Br; R = tBu,Mes, Ferrocenyl) to give the corresponding borane complexes[(η⁵‐C₅H₅)Mn(CO)₂(HB(X)R)]., The presence of a hydride in bridging position between manganese and boron was deduced from ¹¹B decoupled ¹H NMR spectra. Additionally, the structure of the tert‐butyl borane complex was confirmed by single‐crystal X‐ray diffraction.     

Do Rhodium Bis(σ‐amine‐borane) Complexes Play a Role as Intermediates in Dehydrocoupling Reactions of Amine‐boranes?

The recently synthesized rhodium complex [Rh{P(C₅H₉)₂(η²‐C₅H₇)}(Me₂HNBH₃)₂]BAr^F₄ ( 2 ), which incorporates two amine‐boranes coordinated to the rhodium center with two different binding modes, namely η¹ and η², has been used to probe whether bis(σ‐amine‐borane) motifs are important in determining the general course of amine‐boranes dehydrocoupling reactions. DFT calculations have been carried out to explore mechanistic alternatives that ultimately lead to the formation of the amine‐borane cyclic dimer [BH₂NMe₂]₂ ( A ) by hydrogen elimination. Sequential concerted, on‐ or off‐metal, intramolecular dehydrogenations provide two coordinated amine‐borane molecules. Subsequent dimerization is likely to occur off the metal in solution. In spite of the computationally confirmed presence of a BH???NH hydrogen bond between amine‐borane ligands, neither a simple intermolecular route for dehydrocoupling of complex 2 is operating, nor seems [Rh{P(C₅H₉)₂(η²‐C₅H₇)} B ]⁺ to be important for the whole dehydrocoupling process.     

Kinetic resolution of P-stereogenic phosphine boranes via deprotonation using s-butyllithium/(−)-sparteine

The attempted kinetic resolution of racemic secondary phosphine boranes [t-BuPhP(BH₃)H and t-BuMeP(BH₃)H] by P–H deprotonation using 0.5 equiv of s-BuLi and (?)-sparteine was unsuccessful and generated racemic benzyl bromide-trapped adducts in 42–49% yield. In contrast, an efficient kinetic resolution was observed with racemic tertiary phosphine boranes [t-BuPhP(BH₃)Me and t-BuEtP(BH₃)Me] by C–H deprotonation on the P–Me group using 0.5 or 0.6 equiv of s-BuLi and (?)-sparteine. For example, the use of 0.6 equiv of s-BuLi/(?)-sparteine with t-BuEtP(BH₃)Me and trapping with DMF gave the (R)-aldehyde adduct in 37% yield and 83:17 er together with recovered (R)-t-BuEtP(BH₃)Me in 44% yield and 74:26 er. These are the first examples of kinetic resolution of P-stereogenic phosphine boranes via deprotonation using s-BuLi/(?)-sparteine.     

Theoretical Investigation on the Chemistry of Entrapment of the Elusive Aminoborane (H2NBH2) Molecule

Aminoborane (H₂N?BH₂) is an elusive entity and is thought to be produced during dehydropolymerization of ammonia borane, a molecule of prime interest in the field of chemical hydrogen storage. The entrapment of H₂N?BH₂ through hydroboration of exogenous cyclohexene has emerged as a routine technique to infer if free H₂N?BH₂ is produced or not during metal‐catalyzed ammonia borane dehydrogenation reactions. But to date, the underlying mechanism of this trapping reaction remains unexplored. Herein, by using DFT calculations, we have investigated the mechanism of trapping of H₂N?BH₂ by cyclohexene. Contrary to conventional wisdom, our study revealed that the trapping of H₂N?BH₂ does not occur through direct hydroboration of H₂N?BH₂ on the double bond of cyclohexene. We found that autocatalysis by H₂N?BH₂ is crucial for the entrapment of another H₂N?BH₂ molecule by cyclohexene. Additionally, nucleophilic assistance from the solvent is also implicated for the entrapment reaction carried out in nucleophilic solvents. In THF, the rate‐determining barrier for formation of the trapping product was predicted to be 16.7 kcal mol^?1 at M06 L(CPCM) level of theory.     

Recent development of aminophosphine chalcogenides and boranes as ligands in s-block metal chemistry

The coordination chemistry of the aminophosphine chalcogenide ligands [Ph₂P(O)NHR], [Ph₂P(S)NHR], and [Ph₂P(Se)NHR] (R = 2,6-Me₂C₆H₃,^tBu, CHPh₂, CPh₃) or corresponding borane derivative [Ph₂P(BH₃)NHR] toward group 1 and 2 metals is reviewed. The structural characterization of a huge number of mono- and bis-aminophosphine chalcogenide/borane complexes with group 1 and 2 metals—in most cases lithium, sodium, potassium, magnesium, calcium, strontium, and barium complexes—reveals a poly-metallacyclic motif in each case. The coordination takes place from adjacent chalcogen/borane and nitrogen as donor atom or group of the ligand confirming the direct bond between metal and chalcogen/borane to develop homoleptic and heteroleptic complexes. The heteroleptic group 2 metal complexes were used as pre-catalysts in hydrophosphination and hydroamination reactions. Similarly, aminophosphine chalcogenide alkaline earth metal complexes were used in the catalytic ring-opening polymerization (ROP) study of ?-caprolactone.     

Metal‐Free Dehydrogenation of Amine‐Boranes by Tunable N‐Heterocyclic Iminoboranes

We report the synthesis of structurally tunable boron complexes supported by N‐heterocyclic imine ligands IPr=N?BR₂ (IPr=[(HCNDipp)₂C], Dipp=2,6‐iPr₂C₆H₃, R=Cl and/or Ph) that have the ability to abstract dihydrogen from amine‐boranes, and instigate their dehydrocoupling. In one instance, mild heating of the hydrogen addition product IPr=NH?B(Ph)HCl releases H₂ to regenerate the starting N‐heterocyclic iminoborane; accordingly IPr=N?B(Ph)Cl can be used as a metal‐free catalyst to promote the dehydrocoupling of MeNH₂ ? BH₃ to yield N‐methylaminoborane oligomers [MeNH‐BH₂]_x.     

Alkali Metal and Alkaline Earth Metal Complexes with the Bis(borane‐diphenylphosphanyl)amido Ligand – Synthesis,Structures, and Catalysis for Ring‐Opening Polymerization of ϵ‐Caprolactone

The syntheses of lithium and alkaline earth metal complexes with the bis(borane‐diphenylphosphanyl)amido ligand ( 1 ‐ H ) of molecular formulas [{κ²‐N(PPh₂(BH₃))₂}Li(THF)₂] ( 2 ) and [{κ³‐N(PPh₂(BH₃))₂}₂M(THF)₂] [(M = Ca ( 3 ), Sr ( 4 ), Ba ( 5 )] are reported. The lithium complex 2 was obtained by treatment of bis(borane‐diphenylphosphanyl)amine ( 1 ‐ H ) with lithium bis(trimethylsilyl)amide in a 1:1 molar ratio via the silylamine elimination method. The corresponding homoleptic alkaline earth metal complexes 3 – 5 were prepared by two synthetic routes – first, the treatment of metal bis(trimethylsilyl)amide and protio ligand 1 ‐ H via the elimination of silylamine, and second, through salt metathesis reaction involving respective metal diiodides and lithium salt 2 . The molecular structures of lithium complex 2 and barium complex 5 were established by single‐crystal X‐ray diffraction analysis. In the solid‐state structure of 2 , the lithium ion is ligated by amido nitrogen atoms and hydrogen atoms of the BH₃ group in κ²‐coordination of the ligand 1 resulting in a distorted tetrahedral geometry around the lithium ion. However, in complex 5 , κ³‐coordination of the ligand 1 was observed, and the barium ion adopted a distorted octahedral arrangement. The metal complex 5 was tested as catalyst for the ring opening polymerization of ?‐caprolactone. High activity for the barium complex 5 towards ring opening polymerization (ROP) of ?‐caprolactone with a narrow polydispersity index was observed. Additionally, first‐principle calculations to investigate the structure and coordination properties of alkaline earth metal complexes 3 – 5 as a comparative study between the experimental and theoretical findings were described.     

Ammonia–(Dinitramido)boranes: High‐Energy‐Density Materials

Two ammonia–(dinitramido)boranes were synthesized by the reaction of dinitroamine with ammonia–borane. These compounds are the first reported examples of (dinitramido)boranes. Ammonia–mono(dinitramido)borane is a perfectly oxygen‐balanced high‐energy‐density material (HEDM) composed of an ammonia–BH₂ fuel group and a strongly oxidizing dinitramido ligand. Although it is thermally not stable enough for practical applications, its predicted specific impulse as a solid rocket propellant would be 333 s. Its predicted performance as an explosive matches that of pentaerythtritol tetranitrate (PETN) and significantly exceeds that of trinitrotoluene (TNT). Its structure was established by X‐ray crystallography and vibrational and multinuclear NMR spectroscopy. Additionally, the over‐oxidized ammoniabis(dinitramido)borane was detected by NMR spectroscopy.     

Darstellung,Charakterisierung und Reaktionsverhalten von Natrium‐ und Kaliumhydridosilylamiden R2(H)Si—N(M)R′ (M = Na,K) — Kristallstruktur von [(Me3C)2(H)Si—N(K)SiMe3]2·THF

Preparation, Characterization and Reaction Behaviour of Sodium and Potassium Hydridosilylamides R₂(H)Si—N(M)R′ (M = Na, K) — Crystal Structure of [(Me₃C)₂(H)Si—N(K)SiMe₃]₂ · THF The alkali metal hydridosilylamides R₂(H)Si—N(M)R′ 1a‐Na — 1d—Na and 1a‐K — 1d‐K ( a : R = Me, R′ = CMe₃; b : R = Me, R′ = SiMe₃; c : R = Me, R′ = Si(H)Me₂; d : R = CMe₃, R′= SiMe₃) have been prepared by reaction of the corresponding hydridosilylamines 1a — 1d with alkali metal M (M = Na, K) in presence of styrene or with alkali metal hydrides MH (M = Na, K). With NaNH₂ in toluene Me₂(H)Si—NHCMe₃ ( 1a ) reacted not under metalation but under nucleophilic substitution of the H(Si) atom to give Me₂(NaNH)Si—NHCMe₃ ( 5 ). In the reaction of Me₂(H)Si—NHSiMe₃ ( 1b ) with NaNH₂ intoluene a mixture of Me₂(NaNH)Si—NHSiMe₃ and Me₂(H)Si—N(Na)SiMe₃ ( 1b‐Na ) was obtained. The hydridosilylamides have been characterized spectroscopically. The spectroscopic data of these amides and of the corresponding lithium derivatives are discussed. The ²⁹Si‐NMR‐chemical shifts and the ²⁹Si—¹H coupling constants of homologous alkali metal hydridosilylamides R₂(H)Si—N(M)R′ (M = Li, Na, K) are depending on the alkali metal. With increasing of the ionic character of the M—N bond M = K > Na > Li the ²⁹Si‐NMR‐signals are shifted upfield and the ²⁹Si—¹H coupling constants except for compounds (Me₃C)(H)Si—N(M)SiMe₃ are decreased. The reaction behaviour of the amides 1a‐Na — 1c‐Na and 1a‐K — 1c‐K was investigated toward chlorotrimethylsilane in tetrahydrofuran (THF) and in n‐pentane. In THF the amides produced just like the analogous lithium amides the corresponding N‐silylation products Me₂(H)Si—N(SiMe₃)R′ ( 2a — 2c ) in high yields. The reaction of the sodium amides with chlorotrimethylsilane in nonpolar solvent n‐pentane produced from 1a‐Na the cyclodisilazane [Me₂Si—NCMe₃]₂ ( 8a ), from 1b‐Na and 1‐Na mixtures of cyclodisilazane [Me₂Si—NR′]₂ ( 8b , 8c ) and N‐silylation product 2b , 2c . In contrast to 1b‐Na and 1c‐Na and to the analogous lithium amides the reaction of 1b‐K and 1c‐K with chlorotrimethylsilane afforded the N‐silylation products Me₂(H)Si—N(SiMe₃)R′ ( 2b , 2c ) in high yields. The amide [(Me₃C)₂(H)Si—N(K)SiMe₃]₂·THF ( 9 ) crystallizes in the space group C2/c with Z = 4. The central part of the molecule is a planar four‐membered K₂N₂ ring. One potassium atom is coordinated by two nitrogen atoms and the other one by two nitrogen atoms and one oxygen atom. Furthermore K···H(Si) and K···CH₃ contacts exist in 9 . The K—N distances in the K₂N₂ ring differ marginally.     

Thermodynamic modeling of SiBCN film deposition from the gas phase in the Si—B—N—C—H system

Thermodynamic modeling of the chemical vapor deposition (CVD) of films of complex composition in the Si—B—N—C—H system under reduced pressure (0.01 or 10 Torr) in a wide temperature range of 500–1500 K using various organoelement compounds was carried out. An example with mixtures of tetramethylsilane SiMe₄ and hexamethyldisilane (SiMe₃)₂ with trimethylamine borane Me₃N · BH₃ or triethylamine borane Et₃N · BH₃ illustrates a possibility to produce films of various compositions: from boron and silicon nitrides to their mixtures with carbides and/or carbon. According to the CVD diagrams, the prevailing equilibrium condensed phases are various phase complexes containing SiC, Si₃N₄, BN, and C.

     

Sandwich Compounds of Transition Metals with Cyclopolyenes and Isolobal Boron Analogues

A series of sandwich compounds of transition metals (M=Ni, Fe, Cr) with cyclic hydrocarbon (M(CH)_n) and borane (M(BH₂)_n), ligands (including mixed hydrocarbon/borane sandwiches) has been studied using density functional theory (B3LYP/6‐311+G(df,p)). Multicenter bonding between the central metal atom and basal cycloborane rings provides stabilization to planar cycloborane species. Large negative NICS values allude to aromatic character in the cycloboranes similar to the analogous cyclic hydrocarbons. The ability of cycloborane sandwiches to stabilize attached carbocations, radicals and carbanions is also assessed.     

Reaction of K[(π-C5H5)Fe(CO)(CN)2] with boranes and AlCl3: Preparation and investigation of iron(II) complexes containing isocyanoborate ligands

Iron(II) complexes containing CNBX^?₃ or CNBX₂NC^? ligands were prepared from the reaction of K[(π-C₅H₅)Fe(CO)(CN)₂] with boranes (BX₃; X = F, Cl, Br H, Ph). Stable, twelve-membered ring compounds containing Fe, C, N, and B atoms were formed involving CNBF₂NC^? and CNBBr₂NC^? ligands. The reaction of K[(π-C₅H₅)Fe(CO)(CN)₂] with AlCl₃ gave a four-center complex with two Fe and two Al atoms. The compounds were studied by infrared and mass spectroscopic methods.     

Selective α‐Monomethylation by an Amine‐Borane/N,N‐Dimethylformamide System as the Methyl Source

A new and practical α‐monomethylation strategy using an amine‐borane/N,N‐dimethylformamide (R₃N‐BH₃/DMF) system as the methyl source was developed. This protocol has been found to be effective in the α‐monomethylation of arylacetonitriles and arylacetamides. Mechanistic studies revealed that the formyl group of DMF delivered the carbon and one hydrogen atoms of the methyl group, and R₃N‐BH₃ donated the remaining two hydrogen atoms. Such a unique reaction pathway enabled controllable assemblies of CDH₂‐, CD₂H‐, and CD₃‐ units using Me₂NH‐BH₃/d₇‐DMF, Me₃N‐BD₃/DMF and Me₃N‐BD₃/d₇‐DMF systems, respectively. Further application of this method to the facile synthesis of anti‐inflammatory flurbiprofen and its varied deuterium‐labeled derivatives was demonstrated.     

Borane Derivatives: A New Class of Super‐ and Hyperhalogens

Super‐ and hyperhalogens are a class of highly electronegative species whose electron affinities far exceed those of halogen atoms and are important to the chemical industry as oxidizing agents, biocatalysts, and building blocks of salts. Using the well‐known Wade–Mingos rule for describing the stability of closo‐boranes B_nH_n^2? and state‐of‐the‐art theoretical methods, we show that a new class of super‐ and hyperhalogens, guided by this rule, can be formed by tailoring the size and composition of borane derivatives. Unlike conventional superhalogens, in which a central metal atom is surrounded by halogen atoms, the superhalogens formed according to the Wade–Mingos rule do not have to have either halogen or metal atoms. We demonstrate this by using B₁₂H₁₃ and its isoelectronic cluster CB₁₁H₁₂ as examples. We also show that while conventional superhalogens containing alkali atoms require at least two halogen atoms, a single borane‐like moiety is sufficient to give M(B₁₂H₁₂) clusters (M=Li, Na, K, Rb, Cs) superhalogen properties. In addition, hyperhalogens can be formed by using the above superhalogens as building blocks. Examples include M(B₁₂H₁₃)₂ and M(CB₁₁H₁₂)₂ (M=Li–Cs). This finding opens the door to an untapped source of superhalogens and weakly coordinating anions with potential applications.     

Fine Tuning of Metallaborane Geometries: Chemistry of Metallaboranes of Early Transition Metals Derived from Metal Halides and Monoborane Reagents

Reaction of [Cp_nMCl_4?x] (M=V: n=x=2; M=Nb: n=1, x=0) or [Cp*TaCl₄] (Cp=η⁵‐C₅H₅, Cp*=η⁵‐C₅Me₅), with [LiBH₄?thf] at ?70 °C followed by thermolysis at 85 °C in the presence of [BH₃?thf] yielded the hydrogen‐rich metallaboranes [(CpM)₂(B₂H₆)₂] ( 1 : M=V; 2 : M = Nb) and [(Cp*Ta)₂(B₂H₆)₂] ( 3 ) in modest to high yields. Complexes 1 and 3 are the first structurally characterized compounds with a metal–metal bond bridged by two hexahydroborate (B₂H₆) groups forming a symmetrical complex. Addition of [BH₃?thf] to 3 results in formation of a metallaborane [(Cp*Ta)₂B₄H₈(μ‐BH₄)] ( 4 ) containing a tetrahydroborate ligand, [BH₄]^?, bound exo to the bicapped tetrahedral cage [(Cp*Ta)₂B₄H₈] by two Ta‐H‐B bridge bonds. The interesting structural feature of 4 is the coordination of the bridging tetrahydroborate group, which has two B? H bonds coordinated to the tantalum atoms. All these new metallaboranes have been characterized by mass, ¹H, ¹¹B, and ¹³C NMR spectroscopy and elemental analysis and the structural types were established unequivocally by crystallographic analysis of 1 – 4 .