Borohydride Ligand Research Articles

Approaching the free-ion limit in magnetically isotropic gadolinium(III) via borohydride ligands.

Three new crystalline phases of the borohydride complex, [n-Bu4N][RE(BH4)4] (RE = Gd (1), Gd0.097Y0.903 (2) and Gd0.017Y0.983 (3)), were obtained in pure form and characterised using EPR spectroscopy and AC/DC magnetometry. In all 1-3, unusually slow field-induced magnetic relaxation was observed up to high temperatures, which above 10 K is dominated by power law thermal dependence identified as the two-phonon Raman process. Therefore, weak-field ligands such as borohydride seem to be the perfect candidates for triggering slow magnetic relaxation of magnetically isotropic gadolinium(III) ions.

Synthesis of New Series of Transition Metal Complexes with Poly (Pyrazolyl) Borates

In structuring catalysis enzyme and chemistry, tridentate ligands and Scorpionate ligands are of significant worth. This study presents the synthesis of a tris(pyrazolyl)borate ligand to be utilized in transition metal complexes as possible redox shuttles. Complexes of general formula [AgTp], [MIIITp (Cl2)] (M = Fe, Co), Tp = tri (1-pyrazolyl) borohydride and [AgTp*], [FeIIITp*(Cl2)], Tp* = tris (3, 5-dimethyl-1-pyrazolyl) borohydride were synthesized and characterized in solid state. The Tp ligands were considered triply coordinated with the metal center with two bounded chloride atoms as per the information gathered from spectroscopic information. Entire preparations and operations were performed under argon using common Schlenk procedures. Elemental analysis was performed using (the EURO EA instrument). Thermolysis shows that the Tp ligand decomposes around 100oC and above 300oC for some complexes. The composites were simple to compose, yielded high yields, and were reasonably air sensitive. This study has examined the synthesis of a tris(pyrazolyl)borohydride ligand to develop an iron complex. Further studies conducting electrochemical tests should be carried out to demonstrate the effectiveness of this likely redox mediator.

Modulation effects of Cu modification and ligands (oxalate and borohydride) functionalization on Pt d-band center, upper d-band edge, and alloyed PtCe support acidity on semihydrogenation of acetylene

The modulating effects of Cu modification and oxalate or borohydride ligands functionalization on the structure, catalyst d-band center (εd), upper d-band edge (εu), and acetylene partial hydrogenation of expediently synthesized Ce alloyed Pt supported catalysts were investigated. Firstly, a 5 wt% Pt alloyed Ce was synthesized via flame spray pyrolysis. The PtCe sample was further supported on zeolite Y, ZY, (PtCe/ZY) and copper modified ZY (PtCe/Cu-ZY). Furthermore, the PtCe was supported on two other oxalate and borohydride ligands functionalized copper modified ZY (PtCe/CuX-ZY and PtCe/CuB-ZY, respectively). The high-angle annular darkfield scanning transmission electron microscopy (HAADF/STEM) data showed a reduction in the PtO average particle size from 2.65 nm in PtCeO2 to average 1.73, 0.64, and 0.30 nm in PtCe/Cu-ZY, PtCe/CuX-ZY, and PtCe/CuB-ZY, which was corroborated by the electron energy-loss spectroscopy (EELS) results wherein nonhomogeneous mixing of elements was seen with segregated Pt clusters in the non-functionalized samples. Conversely, both PtCe/CuX-ZY and PtCe/CuB-ZY samples showed near-perfect homogeneity with no distinct Pt signals. The measured εu values for PtCe, PtCe/Cu-ZY, PtCe/CuX-ZY, and PtCe/CuB-ZY are +1.85, +0.40, −0.15, and −0.19 eV, respectively. The positive values indicated strong metal-adsorbate bonding typical of large Pt sizes while the negative values indicated weak metal-adsorbate bonding due to highly downsized Pt sizes. The ethylene yield (YC2H4) over the PtCe sample showed depletion as the reaction temperature increased, while it reflected maxima at 120 °C with 55.3% YC2H4 over PtCe/ZY. The maxima shifted to 180 °C with enhanced YC2H4 of 71.4% in PtCe/Cu-ZY. On the contrary, both PtCe/CuX-ZY and PtCe/CuB-ZY exhibited a monotonous increase in YC2H4 up to the maximum C2H2 conversion with YC2H4 of 81.9% and 92.1% at 180 and 160 °C, respectively. These results showed that both the Cu modification and ligands functionalization were highly invaluable to enhance the properties and activities of the semihydrogenation of acetylene (SHA) catalysts.

Rational design of zeolite Y supported oxalate and borohydride ligands functionalized Cu catalysts for CO2 conversion to specialty chemicals

This study investigates the effect of oxalate and borohydride ligands functionalization on the properties and activity of zeolite Y (ZY) supported copper nanoparticles in CO2 hydrogenation to specialty chemicals. The ZY-supported copper oxalate (CuX-ZY) and copper borohydride (CuB-ZY) catalysts exhibited a drastic difference in physicochemical properties compared to a benchmark copper supported on ZY (Cu-ZY) catalyst without any functionalization. The average Cu particle sizes are 7.74, 0.69, and 0.57 nm for Cu-ZY, CuX-ZY, and CuB-ZY, respectively. The smaller the copper oxide particle sizes, the higher the reduction temperatures to the metallic state due to strong-metal-support-interactions. The CO2 conversions (XCO2) at 270 °C are 28.3%, 26.4%, and 14.6%, for CuX-ZY, CuB-ZY, and Cu-ZY which aligns with the trend of basic site-strength, Cu particle sizes, and Cu dispersion of the catalysts. While methanol yield was maximum at 70.5 gproduct gcat−1 ∙ min−1 over CuX-ZY, the dimethyl ether was maximum at 48.9 gproduct gcat−1 ∙ min−1 over CuB-ZY. Furthermore, the oxalate and borohydride functionalization were effective in minimizing CO formation.

Rational Design of Zeolite Y Supported Oxalate and Borohydride Ligands Functionalized Cu Catalysts for Co2 Conversion to Specialty Chemicals

Rational Design of Zeolite Y Supported Oxalate and Borohydride Ligands Functionalized Cu Catalysts for Co2 Conversion to Specialty Chemicals

Cyclopentadienyl lanthanide borohydrides derived from the unsubstituted cyclopentadienyl ligand. Unprecedented structural diversity and ε-caprolactone polymerization

A series of mono-, bis- and tris(cyclopentadienyl) complexes of La, Nd and Tb with borohydride ligand were synthesized and structurally characterized. Various structural types of complex anions and cations containing cyclopentadienyl and borohydride ligands in the coordination sphere of Ln3+ have been studied by single-crystal X-ray diffraction. Five different structural types of Cp-lanthanide borohydrides were found for neodymium: {CpNd(BH4)2} in [(CpNd(BH4)2(THF)2] (2Nd); {Cp2Nd(BH4)2}− in [Cp2Nd(BH4)2][Na(18-crown-6)(THF)2] (3Nd); {Cp2Nd(BH4)} in [Cp2Nd(BH4)(THF)] (4Nd); {CpNd(18-crown-6)(BH4)}+ in [Cp2Nd(BH4)2]2[CpNd(18-crown-6)(BH4)][Na(18-crown-6)(THF)2] (6Nd); {Cp3Nd(BH4)}− in [Cp3Nd(BH4)][Na(18-crown-6)(THF)2] (8Nd). Complex anions {Cp2Nd(BH4)2}− and {Cp3Nd(BH4)}− are able to replace each other in the crystal lattice. Mono- and bis(cyclopentadienyl)borohydride complexes of neodymium effectively initiate ε-caprolactone polymerization.

Rationalizing the Reactivity of Mixed Allyl Rare-Earth Borohydride Complexes with DFT Studies

The reactivity of rare-earth complexes RE(BH4)2(C3H5)(THF)x (RE = La, Nd, Sm, Y, Sc) toward the Ring-Opening Polymerization (ROP) of ε-caprolactone (ε-CL) was rationalized by Density Functional Theory (DFT) calculations. Even if the polymerization reaction can be initiated by both RE-(BH4) and RE-allyl bonds, experimental investigations have shown that the initiation via the borohydride ligand was favored, as no allyl group could be detected at the chain-end of the resulting polymers. DFT studies could confirm these observations, as it was highlighted that even if the activation barriers are both accessible, the allyl group is not active for the ROP of ε-CL due to the formation of a highly stable intermediate that disfavors the subsequent ring-opening.

Transformation of a Norbornadiene Unit to Ethylenylcyclopentene Requiring Cooperation between Boron and Rhodium Centers

The synthesis of the lithium salt of a bis-substituted borohydride anion containing a phenyl substituent and a 2-mercaptopyridyl unit (mp) is reported herein. This salt has been used as a pro-ligand for the synthesis of rhodium(I) complex, [Rh{κ3-H,H,S-H2B(Ph)(mp)}(NBD)] (1) (where NBD = 2,5-norbornadiene). The new boron-based ligand coordinates to the rhodium center via the thione donor and the two B–H bonds of the BPhH2 unit, with a dihydroborate motif. Reaction of complex 1 with two equivalents of triphenylphosphine leads to an unprecedented rearrangement and transfer of the former norbornadiene ligand to the boron center. The transformation occurs via an initial hydride migration from boron to rhodium center followed by a hydroboration of one of the double bonds. Finally, a ring-opening process occurs involving both boron and rhodium centers leading to an unusual boron-bound ethylenylcyclopentene unit. The product of this reaction was confirmed as [Rh{η1-S,η2-B,C-B(Ph)(CHCH2(C5H7)(mp)}(PPh3)2] (2). The net result of these transformations is the incorporation of the two hydrogen atoms from the secondary borohydride ligand [BPhH2(mp)]− into the former norbornadiene unit. The end point positions of these hydrogen atoms were confirmed by deuterium labeling experiments. Complex 2 was further reacted with carbon monoxide to generate [Rh{η1-S,η2-B,C-B(Ph)(CHCH2(C5H7)(mp)}(CO)(PPh3)] (3) via ligand substitution. The new ligand and the three complexes, 1, 2, and 3, have been characterized by spectroscopic techniques as well as by X-ray crystallography. Detailed characterization of 2 and 3 revealed an unusual η2-B,C coordination mode within these complexes. Further studies have demonstrated that complexes 2 and 3 react with hydrogen gas (or dimethylamine borane as a source of H2) to generate the hydrogen addition products involving the unprecedented activation of the Rh−η2-B,C motif. Complexes 2 and 3 were further found to be active catalysts for the hydrogenation of olefins and the dehydrogenation of dimethylamine borane.

Fulvalene as a platform for the synthesis of a dimetallic dysprosocenium single-molecule magnet.

The dinucleating fulvalenyl ligand [1,1',3,3'-(C5 t Bu2H2)2]2- (Fvtttt) was used to synthesize the dimetallic dysprosocenium cation [{Dy(η5-Cp*)}2(μ-BH4)(η5:η5-Fvtttt)]+ (3) as the salt of [B(C6F5)4]- (Cp* = C5Me5). Compound [3][B(C6F5)4] was obtained using a method in which the double half-sandwich complex [{Dy(BH4)2(THF)}2(Fvtttt)] (1) was reacted with KCp* to give the double metallocene [{Dy(Cp*)(μ-BH4)}2(Fvtttt)] (2), followed by removal of a bridging borohydride ligand upon addition of [(Et3Si)2(μ-H)][B(C6F5)4]. The dimetallic fulvalenyl complexes 1-3 give rise to single-molecule magnet (SMM) behaviour in zero applied field, with the effective energy barriers of 154(15) cm-1, 252(4) cm-1 and 384(18) cm-1, respectively, revealing a significant improvement in performance across the series. The magnetic properties are interpreted with the aid of ab initio calculations, which show substantial increases in the axiality of the crystal field from 1 to 2 to 3 as a consequence of the increasingly dominant role of the Fvtttt and Cp* ligands, with the barrier height and hysteresis properties being attenuated by the equatorial borohydride ligands. The experimental and theoretical results described in this study furnish a blueprint for the design and synthesis of poly-cationic dysprosocenium SMMs with properties that may surpass those of benchmark systems.

Improved hydrogen storage properties of Mg/MgH2 thanks to the addition of nickel hydride complex precursors

In order to improve the hydrogenation/dehydrogenation properties of the Mg/MgH2 system, the nickel hydride complex NiHCl(P(C6H11)3)2 has been added in different amounts to MgH2 by planetary ball milling. The hydrogen storage properties of the formed composites were studied by different thermal analyses methods (temperature programmed desorption, calorimetric and pressure-composition-temperature analyses). The optimal amount of the nickel complex precursor was found to be of 20 wt%. It allows to homogeneously disperse 1.8 wt% of nickel active species at the surface of the Mg/MgH2 particles. After the decomposition of the complex during MgH2 dehydrogenation, the formed composite is stable upon cycling at low temperature. It can release hydrogen at 200 °C and absorb 6.3 wt% of H2 at 100 °C in less than 1 h. The significantly enhanced H2 storage properties are due to the impact of the highly dispersed nickel on both the kinetics and thermodynamics of the Mg/MgH2 system. The hydrogenation and dehydrogenation enthalpies were found to be of −65 and 63 kJ/mol H2 respectively (±75 kJ/mol H2 for pure Mg/MgH2) and the calculated apparent activation energies of the hydrogen uptake and release processes are of 22 and 127 kJ/mol H2 respectively (88 and 176 kJ/mol H2 for pure Mg/MgH2). The change in the thermodynamics observed in the formed composite is likely to be due to the formation of a Mg0.992Ni0.008 phase during dehydrogenation/hydrogenation cycling. The impact of another hydride nickel precursor in which chloride has been replaced by a borohydride ligand, namely NiH(BH4)(P(C6H11)3)2, is also reported.

Chelating Borohydrides for Lanthanides and Actinides: Structures, Mechanochemistry, and Case Studies with Phosphinodiboranates.

In this Forum Article, we review the development of chelating borohydride ligands called aminodiboranates (H3BNR2BH3-) and phosphinodiboranates (H3BPR2BH3-) for the synthesis of trivalent f-element complexes. The advantages and history of using mechanochemistry to prepare molecular borohydride complexes are described along with new results demonstrating the mechanochemical synthesis of M2(H3BPtBu2BH3)6, where M = U, Nd, Tb, Er, and Lu (1-5). Multinuclear NMR, IR, and single-crystal X-ray diffraction data are reported for 1-5 alongside complementary density functional theory calculations to reveal differences in their structure and reactivity with and without tetrahydrofuran. The results demonstrate how mechanochemistry can be used to access f-element complexes with chelating borohydrides in improved and reproducible yields, which is an important step toward investigating the properties of lanthanide and actinide phosphinodiboranate complexes with different phosphorus substituents. The relevance of these results is contextualized by a discussion of structural factors known to influence the volatility of f-element borohydrides and applications that require the development of volatile f-element complexes.

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.

Amidinate bisborohydride complexes of rare-earth metals [6-Me-C5H3N-2-CH2C(NPri)2]Ln(BH4)2THF2 (Ln = Y, Nd): synthesis, structure, and catalytic activity in isoprene polymerization

Reactions of equimolar amounts of RN=C=NR (R = Pri, Cy) and 6-Me-C5H3N-2-CH2Li prepared in situ by metallation of 2,6-dimethylpyridine with n-butyllithium afforded corresponding lithium amidinates [Li{6-Me-C5H3N-2-CH2C(NPri)2}]•1/3THF (1) and [Li{6-MeC5H3N-2-CH2C(NCy)2}]4 (2) containing new tridentate amidinate ligands. The salt metathesis reactions of Ln(BH4)3(THF)3 (Ln = Y, Nd) with 1 (1: 1 molar ratio, THF) result in neutral amidinate bisborohydride complexes [Ln{6-Me-C5H3N-2-CH2C(NPri)2}(BH4)2(THF)2} (Ln = Y (3), Nd (4)). According to X-ray data, both compounds are monomeric, terminal borohydride ligands being coordinated to the rare earth metal atom in η3-fashion. Nitrogen atoms of the pyridine fragments of amidinate ligands are not involved in complexation with metal cations. Complexes 3 and 4 in combination with [Ph3C][B(C6F5)4] and AlBu 3 i (1: 1: 10 molar ratio) exhibit catalytic activity in isoprene polymerization.

Investigating the decomposition pathways and hydrogen storage capacity of V, Cr, and Fe amino borohydrides

ABSTRACTCr, V, and Fe amino borohydride complexes were synthesized using a solution based approach. Thermogravimetric Analysis with simultaneous Differential Scanning Calorimetry was used to investigate their decomposition behavior. The synthesized Cr and Fe complexes exhibited significant hydrogen release around 100 °C. The synthesized V complex showed a large mass loss at temperatures between 50 °C and 100 °C and release of amine byproducts. FTIR of decomposed intermediates showed the decomposition of Cr amino borohydride occurs through the simultaneous loss of hydrogen from both the borohydride and amino ligands, while the Fe complex displays preferential dehydrogenation of the borohydride over the amino ligand. The decomposed products take on a BN type structure when heated to 400 °C.

Bisborohydride yttrium complexes containing amidinate ligands [o-Me2NC6H4CH2C(NR)2]Y(BH4)2L n (R = Pri, L = DME, n = 1; R = Cy, L = THF, n = 2). Synthesis, structure, and catalytic activity in polymerization of rac-lactide and isoprene

A reaction of o-N,N-dimethyltoluidine lithium derivative (o-Me2NC6H4CH2Li) with carbodiimides (RN=C=NR, where R = Pri, Cy) in THF at room temperature leads to lithium complex of the general formula [{o-Me2NC6H4CH2C(NR)2}Li(THF)2]2 (R = Pri (1), Cy (2)) containing a new tridentate amidinate ligand. X-ray diffraction studies showed that complexes 1 and 2 are dimeric due to the coordination of the nitrogen atoms of the amidinate group with different lithium atoms. The nitrogen atoms of the aminobenzyl fragment are not involved in the complexation. The exchange reactions of Y(BH4)3(THF)3 with lithium amidinates 1 and 2 carried out with the equimolar ratio of reagents in solution in THF proceeded with the formation of neutral amidinatebisborohydride complexes of the general formula [o-Me2NC6H4CH2C(NR)2]Y(BH4)2Ln (3: R = Pri, L = DME (with added DME), n = 1; 4: R = Cy, L = THF, n = 2). According to the X-ray diffraction data, compounds 3 and 4 are monomeric, borohydride ligands are coordinated to the rare-earth metal atom by the η3- and η2-type. The donor NMe2 groups of the amidinate ligands are not involved in the metal—ligand interaction. Yttrium complexes 3 and 4 exhibit catalytic activity in polymerization of rac-lactide and isoprene.

Synthesis, Structure, and Reactivity of Co(II) and Ni(II) PCP Pincer Borohydride Complexes.

The 15e square-planar complexes [Co(PCPMe-iPr)Cl] (2a) and [Co(PCP-tBu)Cl] (2b), respectively, react readily with NaBH4 to afford complexes [Co(PCPMe-iPr)(η2-BH4)] (4a) and [Co(PCP-tBu)(η2-BH4)] (4b) in high yields, as confirmed by IR spectroscopy, X-ray crystallography, and elemental analysis. The borohydride ligand is symmetrically bound to the cobalt center in η2-fashion. These compounds are paramagnetic with effective magnetic moments of 2.0(1) and 2.1(1) μB consistent with a d7 low-spin system corresponding to one unpaired electron. None of these complexes reacted with CO2 to give formate complexes. For structural and reactivity comparisons, we prepared the analogous Ni(II) borohydride complex [Ni(PCPMe-iPr)(η2-BH4)] (5) via two different synthetic routes. One utilizes [Ni(PCPMe-iPr)Cl] (3) and NaBH4, the second one makes use of the hydride complex [Ni(PCPMe-iPr)H] (6) and BH3·THF. In both cases, 5 is obtained in high yields. In contrast to 4a and 4b, the borohydride ligand is asymmetrically bound to the nickel center but still in an η2-mode. [Ni(PCPMe-iPr)(η2-BH4)] (5) loses readily BH3 at elevated temperatures in the presence of NEt3 to form 6. Complexes 5 and 6 are both diamagnetic and were characterized by a combination of 1H, 13C{1H}, and 31P{1H} NMR, IR spectroscopy, and elemental analysis. Additionally, the structure of these compounds was established by X-ray crystallography. Complexes 5 and 6 react with CO2 to give the formate complex [Ni(PCPMe-iPr)(OC(C=O)H] (7). The extrusion of BH3 from [Co(PCPMe-iPr)(η2-BH4)] (4a) and [Ni(PCPMe-iPr)(η2-BH4)] (5) with the aid of NH3 to yield the respective hydride complexes [Co(PCPMe-iPr)H] and [Ni(PCPMe-iPr)H] (6) and BH3NH3 was investigated by DFT calculations showing that formation of the Ni hydride is thermodynamically favorable, whereas the formation of the Co(II) hydride, in agreement with the experiment, is unfavorable. The electronic structures and the bonding of the borohydride ligand in [Co(PCPMe-iPr)(η2-BH4)] (4a) and [Ni(PCPMe-iPr)(η2-BH4)] (5) were established by DFT computations.

Copper(I), copper(II), and heterovalent copper(I,II) complexes with 1,10-phenanthroline and the closo-decaborate anion

The reactions proceeding in air between CuI/CuII and phen and influence of the closo-decaborate anion on the reaction path are studied. In the absence of [B10H10]2−, a rapid redox reaction between CuCl and phen yields [CuII2(phen)4(μ-CO3)]Cl2 (2). The [B10H10]2− anion produces a decelerating effect on the redox reaction. It stabilizes the +1 oxidation state of copper and coordinates two metal atoms by apical faces to form [CuI2(phen)2[B10H10]] (3). Heating of the reaction mixture results in [CuI(phen)2]2[B10H10] (4) and [CuI(phen)2]2[CuII(phen)3][B10H10]2 (5) indicating that a part of copper atoms is oxidized and the borohydride ligand is replaced by phen molecules in the inner sphere of the metal. Compounds [CuII2(phen)4(μ-CO3)][B10H10] (1) and [Cu(phen)2][B10H10] (6) are prepared starting from CuII-phen complexes. Compounds 1–6 are identified by IR spectroscopy, elemental analysis, and X-ray diffraction.

Synthesis and Structure of Six-Coordinate Iron Borohydride Complexes Supported by PNP Ligands

The preparation of a number of iron complexes supported by ligands of the type HN{CH2CH2(PR2)}2 [R = isopropyl (((i)Pr)PNP) or cyclohexyl ((Cy)PNP)] is reported. This is the first time this important bifunctional ligand has been coordinated to iron. The iron(II) complexes (((i)Pr)PNP)FeCl2(CO) (1a) and ((Cy)PNP)FeCl2(CO) (1b) were synthesized through the reaction of the appropriate free ligand and FeCl2 in the presence of CO. The iron(0) complex (((i)Pr)PNP)Fe(CO)2 (2a) was prepared through the reaction of Fe(CO)5 with ((i)Pr)PNP, while irradiating with UV light. Compound 2a is unstable in CH2Cl2 and is oxidized to 1a via the intermediate iron(II) complex [(((i)Pr)PNP)FeCl(CO)2]Cl (3a). The reaction of 2a with HCl generated the related complex [(((i)Pr)PNP)FeH(CO)2]Cl (4a), while the neutral iron hydrides (((i)Pr)PNP)FeHCl(CO) (5a) and ((Cy)PNP)FeHCl(CO) (5b) were synthesized through the reaction of 1a or 1b with 1 equiv of (n)Bu4NBH4. The related reaction between 1a and excess NaBH4 generated the unusual η(1)-HBH3 complex (((i)Pr)PNP)FeH(η(1)-HBH3)(CO) (6a). This complex features a bifurcated intramolecular dihydrogen bond between two of the hydrogen atoms associated with the η(1)-HBH3 ligand and the N-H proton of the pincer ligand, as well as intermolecular dihydrogen bonding. The protonation of 6a with 2,6-lutidinium tetraphenylborate resulted in the formation of the dimeric complex [{(((i)Pr)PNP)FeH(CO)}2(μ2,η(1):η(1)-H2BH2)][BPh4] (7a), which features a rare example of a μ2,η(1):η(1)-H2BH2 ligand. Unlike all previous examples of complexes with a μ2,η(1):η(1)-H2BH2 ligand, there is no metal-metal bond and additional bridging ligand supporting the borohydride ligand in 7a; however, it is proposed that two dihydrogen-bonding interactions stabilize the complex. Complexes 1a, 2a, 3a, 4a, 5a, 6a, and 7a were characterized by X-ray crystallography.

Imido‐Supported Borohydrides of Titanium, Vanadium and Molybdenum

AbstractNew imido bis(borohydride) complexes [(RN=)M(PMe3)2(η2‐BH4)2] [R = Ar, M = Ti (3); R = Ar, M = V (5); R = Ar, M = Mo (6); R = Ar′, M = Mo (7); Ar = 2,6‐iPr2C6H3, Ar′ = 2,6‐Me2C6H3] were prepared by the reaction of dichlorides [(RN=)MCl2(PMe3)n] (M = Ti, n = 2; M = V, n = 2, M = Mo, n = 3) with LiBH4 (2 equiv) in THF. Compounds 3, 5, 6 and 7 were studied by IR spectroscopy and X‐ray diffraction, and an EPR spectroscopy study was performed for paramagnetic compound 5. In 3, the two borohydride units are orthogonal to each other as a result of the overlap of an antiphase combination of B–H orbitals with the empty dxy orbital of Ti. In contrast, the dxy orbital of the metal atom is singly occupied in 5 and fully occupied in 6 and 7, and both borohydride ligands are oriented in the same way, with the B(μ‐H2)M moieties orthogonal to the P–M–P vector.

Synthesis and structural characterization of bis and tris(2-mercapto-1-methylbenzimidazolyl)hydroborato complexes: benzannulation promotes κ3-coordination

The benzannulated bis and tris(mercaptoimidazolyl)borohydride compounds, [BmMeBenz]Na and [TmMeBenz]Na, have been synthesized via the reactions of NaBH4 with two and three equivalents of 1-methyl-1,3-dihydro-2H-benzimidazole-2-thione, respectively. X-ray diffraction studies on the THF adducts, {μ-[BmMeBenz]Na(THF)₂}₂ and {[TmMeBenz]Na}₂(μ-THF)₃, indicate that both compounds are dinuclear but differ according to the nature of the bridging ligand. Specifically, {μ-[BmMeBenz]Na(THF)₂}₂ possesses bridging [BmMeBenz] ligands and terminal THF ligands, while {[TmMeBenz]Na}₂(μ-THF)₃ possesses terminal [TmMeBenz] ligands and bridging THF ligands. The tris(mercaptoimidazolyl)borohydride ligand of {[TmMeBenz]Na}₂(μ-THF)₃ coordinates in a κ³-manner, which is in marked contrast to the κ²-, κ¹- and κ⁰-modes that have been reported for various [TmMe]Na derivatives. Density functional theory (DFT) geometry optimization calculations of the anions [TmMeBenz]⁻ and [TmMe]⁻ in the gas phase indicate that the conformation required for κ³-S₃ coordination, i.e. one in which the three sulfur donors point away from the B-H group, is relatively more stable for [TmMeBenz]⁻ than for [TmMe]⁻, and thus provides a rationalization for the observation that benzannulation enables κ³-coordination of tris(mercaptoimidazolyl)borohydride ligand in {[TmMeBenz]Na}₂(μ-THF)₃. Furthermore, comparison of the molecular structure and IR spectroscopic properties of [TmMeBenz]Re(CO)₃ with those of [TmMe]Re(CO)₃ indicates that benzannulation reduces the electron donating properties of the ligand, but has little effect on its steric properties. {μ-[BmMeBenz]Na(THF)₂}₂ and {[TmMeBenz]Na}₂(μ-THF)₃ react with [Me₃PCuCl]₄ to give [BmMeBenz]CuPMe₃ and [TmMeBenz]CuPMe₃, the first pair of structurally related bis and tris(mercaptoimidazolyl)hydroborato copper(I) compounds.