Rhodium(I) Complexes of a PBP Ambiphilic Ligand: Evidence for a Metal→Borane Interaction

Theoretical Investigation on Dialumenes toward Dihydrogen Activation: Mechanism and Ligand Effect

Herein, we report a computation study based on the density functional theory calculations to understand the mechanism and ligand effect of the base-stabilized dialumenes toward dihydrogen activation. Among all of the examined modes of dihydrogen activation using the base-stabilized dialumene, we found that the concerted 1,2-hydrogenation of the Al═Al double bond is kinetically more preferable. The concerted 1,2-hydrogenation of the Al═Al double bond adopts an electron-transfer model with certain asynchrony. That is, the initial electron donation from the H-H σ bonding orbital to the empty 3p orbital of the Al1 center is followed by the backdonation from the lone pair electron of the Al2 center to the H-H σ antibonding orbital. Combined with the energy decomposition analysis on the transition states of the concerted 1,2-hydrogenation of the Al═Al double bond and the topographic steric mapping analysis on the free dialumenes, we ascribe the higher reactivity of the aryl-substituted dialumene over the silyl-substituted analogue in dihydrogen activation to the stronger electron-withdrawing effect of the aryl group, which not only increases the flexibility of the Al═Al double bond but also enhances the Lewis acidity of the Al═Al core. Consequently, the aryl-substituted dialumene fragment suffers less geometric deformation, and the orbital interactions between the dialumene and dihydrogen moieties are more attractive during the 1,2-hydrogenation process. Moreover, our calculations also predict that the Al═Al double bond has a good tolerance with the stronger electron-withdrawing group (-CF3) and the weaker σ-donating N-heterocyclic carbene (NHC) analogue (e.g., triazol carbene and NHSi). The reactivity of the dialumene in dihydrogen activation can be further improved by introducing these groups as the supporting ligand and the stabilizing base on the Al═Al core, respectively.

A first glance into mixed phosphine-stibine moieties as protecting ligands for gold clusters

Atomically precise gold clusters have attracted considerable research interest as their tunable structure-property relationships have resulted in widespread applications, from sensing and biomedicine to energetic materials and catalysis. In this article, the synthesis and optical properties of a novel [Au₆(SbP₃)₂][PF₆]₂ cluster are reported. Despite the lack of spherical symmetry in the core, the cluster shows exceptional thermal and chemical stability. Detailed structural attributes and optical properties are evaluated experimentally and theoretically. This, to the best of our knowledge, is the first report of a gold cluster protected via synergistic multidentate coordination of stibine (Sb) and phosphine moieties (P). To further show that the latter moieties give a set of unique properties that differs from monodentate phosphine-protected [Au₆(PPh₃)₆]²⁺, geometric structure, electronic structure, and optical properties are analyzed theoretically. In addition, this report also demonstrates the critical role of overall-ligand architecture in stabilizing mixed ligand-protected gold clusters.

Facile Synthesis and Utilization of Bis(o-phosphinophenyl)zinc as Isolable PZnP-pincer Ligands Enabled by Boron-Zinc Double Transmetallation

Bis(o-phosphinophenyl)zinc derivatives were successfully synthesized by the reaction of o-phosphinophenylboronates with dimethylzinc via boron-zinc double transmetallation. The transmetallation was significantly accelerated by the presence of the ortho PR2 substituent to...

A masked form of an O-borylated Breslow intermediate for the diastereoselective FLP-type activation of aldehydes

Breslow intermediates are very often elusive species whose application in Frustrated Lewis Pair chemistry is unprecedented. We describe herein the use of a masked form of an O-Borylated Breslow (OBB) intermediate that performs FLP-type activation of the carbonyl function of five different benzaldehyde derivatives with complete diastereoselectivity. The resulting compounds are characterised in solution by NMR spectroscopy (compounds 4 - 8 ) and in solid state by X-Ray diffraction analysis (compounds 4 - 6 ). A combined kinetic and theoretical investigation reveals the associative nature of the rate determining step and suggests that the OBB intermediate part is never released during the whole process.

Solid-state 11B NMR studies of coinage metal complexes containing a phosphine substituted diboraanthracene ligand

Transition metal interactions with Lewis acids (M → Z linkages) are fundamentally interesting and practically important. The most common Z-type ligands contain boron, which contains an NMR active ¹¹B nucleus. We measured solid-state ¹¹B{¹H} NMR spectra of copper, silver, and gold complexes containing a phosphine substituted 9,10-diboraanthracene ligand (B₂P₂) that contain planar boron centers and weak M → BR₃ linkages ([(B₂P₂)M][BAr^F₄] (M = Cu (1), Ag (2), Au (3)) characterized by large quadrupolar coupling (C_Q) values (4.4-4.7 MHz) and large span (Ω) values (93-139 ppm). However, the solid-state ¹¹B{¹H} NMR spectrum of K[Au(B₂P₂)]^- (4), which contains tetrahedral borons, is narrow and characterized by small C_Q and Ω values. DFT analysis of 1-4 shows that C_Q and Ω are expected to be large for planar boron environments and small for tetrahedral boron, and that the presence of a M → BR₃ linkage relates to the reduction in C_Q and ¹¹B NMR shielding properties. Thus solid-state ¹¹B NMR spectroscopy contains valuable information about M → BR₃ linkages in complexes containing the B₂P₂ ligand.

Understanding, Modulating, and Leveraging Transannular M → Z Interactions

Density functional theory calculations have been performed on metallatranes featuring a group 13 elements at the bridgehead position to understand the factors that influence the nature of the M···Z (M = Fe, Co, Ni; Z = Al, Ga, In) interaction present in these complexes and the resultant reactivity at the metal center. The strength of the M···Z interaction increases with the increase in the size and polarizability of the bridgehead group 13 elements. The calculated reaction free energies (ΔG° values) for binding of different Lewis bases to the metallatranes are found to be significantly more exergonic for the larger In(III) ions. Quantum theory of atoms in molecules calculations reveal the covalent nature of the M···Z interactions, while the EDA-NOCV analysis indicates the strong binding ability of these metallatranes not only to different σ-donor and π-acceptor ligands but also to relatively inert species, such as N₂.

[Development of Gold-catalyzed Reaction Utilizing Electron Acceptability of Z-type Ligand]

The interaction between transition metals and ligands is important for catalytic reactions. The ligands are largely dominated by the covalent X-type (hydride, alkyl and halogen) and/or dative L-type ligands (e.g., P, N, CO, olefin, etc.). Therefore, the interaction of the Z-type ligands (B, Al and Si, etc.) with transition metals is emerging as a new concept for the reactivity of the metal center. Recently, we developed the synthesis of the gold complex Au(DPB)X (DPB=diphosphine-borane) featuring the Z-type ligand, and their catalytic reaction. The gold catalysts showed a high activity compared to the general catalysts (without Z-ligand) for the various cyclization reactions due to the electron-withdrawing effect of the Z-ligand on the coordinating gold center. In this review, first the structure analysis of the synthesized Au→Z complex is introduced in detail, and second, the catalytic reactions based on the alkyne activation are described.

Synthesis and reactivity of phosphine borohydride compounds

Four lithium phosphine borohydride compounds featuring phenyl and naphthyl linkers have been synthesized. In-depth NMR analysis affords evidence for non-bonded through space P-B coupling. Reactivity towards CO2 leads to LiH transfer and to the quantitative formation of the corresponding ambiphilic phosphine-borane products.

Ambiphilic boryl groups in a neutral Ni(ii) complex: a new activation mode of H2

The concept of metal–ligand cooperation opens new avenues for the design of catalytic systems that may offer alternative reactivity patterns to the existing ones. Investigations of this concept with ligands bearing a boron center in their skeleton established mechanistic pathways for the activation of small molecules in which the boron atom usually performs as an electrophile. Here, we show how this electrophilic behavior can be modified by the ligand trans to the boron center, evincing its ambiphilic nature. Treatment of diphosphinoboryl (PBP) nickel–methyl complex 1 with bis(catecholato)diboron (B₂Cat₂) allows for the synthesis of nickel(ii) bis-boryl complex 3 that promotes the clean and reversible heterolytic cleavage of dihydrogen leading to the formation of dihydroborate nickel complex 4. Density functional theory analysis of this reaction revealed that the heterolytic activation of H₂ is facilitated by the cooperation of both boryl moieties and the metal atom in a concerted mechanism that involves a Ni(ii)/Ni(0)/Ni(ii) process. Contrary to 1, the boron atom from the PBP ligand in 3 behaves as a nucleophile, accepting a formally protic hydrogen, whereas the catecholboryl moiety acts as an electrophile, receiving the attack from the hydride-like fragment. This manifests the dramatic change in the electronic properties of a ligand by tuning the substituent trans to it and constitutes an unprecedented cooperative mechanism that involves two boryl ligands in the same molecule operating differently, one as a Lewis acid and the other one as a Lewis base, in cooperation with the metal. In addition, reactivity towards different nucleophiles such as amines or ammonia confirmed the electrophilic nature of the Bcat moiety, allowing the formation of aminoboranes.

A bis(boryl)nickel complex promotes the facile and reversible activation of H₂ through a cooperative mechanism that involves the metal and both boryl moieties in a concerted five-center process.

Coordination Flexibility of the Rh(PXP) Complex to NH3, CO, and C2H4 (PXP = Diphosphine-Based Pincer Ligand; X = B, Al, and Ga): Theoretical Insight

The recently synthesized rhodium-aluminum bimetallic complex Rh(PAlP) 1 (PAlP = pincer-type diphosphino-aluminyl ligand Al{[N(C₆H₄)]₂NMe}[CH₂P(ⁱPr)₂]₂) containing a unique Rh-Al direct bond exhibits coordination flexibility because Rh and Al can play the role of coordination site for the substrate. DFT calculations of NH₃, CO, and C₂H₄ adducts with 1 show that the Rh atom is favorable for all these substrate but the Al atom is as favorable as the Rh atom for NH₃ and unfavorable for CO and C₂H₄. NH₃ and CO prefer the coordination at the Rh-axial (Ax) site to the Rh-equatorial (Eq) site, but C₂H₄ prefers coordination at the Rh-Eq site to the Rh-Ax site. Consequently, two CO and C₂H₄ molecules coordinate with 1 at the Rh-Ax and Rh-Eq sites to afford trigonal bipyramidal complexes Rh(PAlP)(CO)₂ and Rh(PAlP)(C₂H₄)₂, which is consistent with the experimental observation of Rh(PAlP)(CO)₂. Energy decomposition analysis reveals that an electrostatic term plays an important role for NH₃ coordination with the Al atom of 1, because Al has a significantly large positive charge and NH₃ has a much negatively charged N atom and exhibits a considerably negative electrostatic potential at the Al position. In B and Ga analogues Rh(PBP) 2 and Rh(PGaP) 3, B and Ga atoms are not good for CO and C₂H₄ like the Al atom in 1. NH₃ adducts with 2 and 3 at the B and Ga sites are less stable than those adducts at the Rh-Ax site unlike the NH₃ adduct with 1 at the Al site. This difference in the NH₃ adduct between Rh(PAlP) and others (Rh(PBP) and Rh(PGaP)) arises from much less positive charges of B and Ga and a smaller atomic size of B than that of Al. These results indicate that the significantly large electropositive nature and appropriate atomic size of Al are responsible for the characteristic coordination flexibility of Rh(PAlP).

Reversible addition of ethylene to a pincer-based boryl-iridium unit with the formation of a bridging ethylidene

This report examines reactions of a series of Ir complexes supported by the diarylboryl/bis(phosphine) PBP pincer ligand with ethylene: (PBP)IrH₄ (1), (PBP)IrH₂(CO) (2), and (PBP)Ir(CO)₂ (3). The outcomes of these reactions differ from those typical for Ir complexes supported by other pincer ligands and do not give rise to simple ethylene adducts or products of insertion of Ir into the C–H bond of ethylene. Instead, the elements of ethylene are incorporated into the molecules to result in B–C bonds. In the case of 2 and 3, ethylene addition results in the formation of B/Ir bridging ethylidene complexes 5 and 6. For 6, the addition of ethylene (and the analogous addition of 1-hexene) is shown to be partially reversible. Addition of ethylene to 2 and 3 is remarkable because they are saturated at Ir and yet the net outcome is such that ethylene binds without replacing any ligands already present. A mechanistic inquiry suggests that dissociation of CO from 3 or 6 is necessary in order for the addition or loss of ethylene to proceed.

(PBP)Ir pincer complexes containing a boryl-iridium linkage reversibly bind ethylene as an ethylidene bridging B and Ir.

Catalysis using transition metal complexes featuring main group metal and metalloid compounds as supporting ligands

Recent development in catalytic application of transition metal complexes having an M-E bond (E = main group metal or metalloid element), which is stabilized by a multidentate ligand, is summarized. Main group metal and metalloid supporting ligands furnish unusual electronic and steric environments and molecular functions to transition metals, which are not easily available with standard organic supporting ligands such as phosphines and amines. These characteristics often realize remarkable catalytic activity, unique product selectivity, and new molecular transformations. This perspective demonstrates the promising utility of main group metal and metalloid compounds as a new class of supporting ligands for transition metal catalysts in synthetic chemistry.

PSb+P Ligand: Platform for a Stibenium to Transition-Metal Interaction

The coordination chemistry of cationic divalent pnictogen ligands, such as nitrenium and phosphenium, has been well-explored in recent years. However, corresponding studies of a heavier congener, stibenium ion, are rare. To better facilitate a Sb⁺-metal interaction, a tridentate P-Sb⁺-P ligand with two phosphine buttresses was designed and synthesized, and its coordination chemistry toward late transition metals was investigated. The stibenium ligand was delivered as an activated P(SbCl)P-AgOTf complex (2) that releases AgCl and the P-Sb⁺-P ligand upon the treatment with transition metals. Reacting 2 with Rh(I) and Ir(I) metals yielded the anticipated stibenium-transition-metal complexes [(Rh(COD)Cl)₂(μ-PSb⁺P)] OTf ([3][OTf]) and [(Ir(COD)Cl)₂(μ-PSb⁺P)] OTf ([4][OTf]). The M-Sb⁺-M bridging structure was confirmed by single-crystal X-ray crystallography, and the bonding situation was examined computationally. Theoretical studies revealed the presence of three-center delocalized M-Sb⁺-M bonding interactions in [3][OTf] and [4][OTf].

Platinum complexes of a boron-rich diphosphine ligand

Herein, we describe the preparation, characterization, and reactivity of two Pt^II bis-hydrocarbyl complexes containing the 1,2-bis(di(3-dicyclohexylboraneyl)propylphosphino)ethane (P₂B^Cy₄) ligand.

Semi-bridging σ-silyls as Z-type ligands

The reactions of SiHPh(NCH₂PPh₂)₂C₆H₄-1,2 with zerovalent group 10 reagents afford the homoleptic bimetallic complexes [M₂{μ-κ³-Si,P,P′-SiPh(CH₂PPh₂)₂C₆H₄}₂] (M = Ni, Pd, Pt) in which the M–M bond is unsymmetrically bridged by two σ-silyl groups.

Strong metal-borane interactions in low-valent cyclopentadienyl rhodium complexes

The first examples of half-sandwich Rh(i) complexes stabilized by borane coordination have been prepared and structurally characterized. As substantiated by NMR spectroscopy and single-crystal X-ray diffraction, the phosphine-borane ligand iPr₂P-(o-C₆H₄)-BFxyl₂ 1 [Fxyl = 3,5-(F₃C)₂C₆H₃] engages in tight η³-BCC or η¹-B coordination, depending on the metal environment.

Silver halide complexes of a borane/bis(phosphine) ligand

Silver halide complexes of a borane/bis(phosphine) ligand have been prepared and characterized. With AgF, the borane abstracts fluoride, resulting in a zwitterionic complex. With AgCl, AgBr, and AgI, the halide stays coordinated to Ag, with little to no Ag-B interaction.

Square-Planar Pt(II) and Ir(I) Complexes as the Lewis Bases: Donor-Acceptor Adducts with Group 13 Trihalides and Trihydrides

The stability and properties of donor-acceptor adducts of square-planar Pt(II) and Ir(I) complexes (designated as PtX, IrX, or generally MX complexes) with trihydrides and trihalides of group 13 elements of general formula YZ₃ (Y = B, Al, Ga; Z = H, F, Cl, Br) were studied theoretically using DFT methodology in the gas phase. MX complexes were represented by wide range of the ligand environment which included model complexes [Ir(NH₃)₃X]⁰ and cis-[Pt(NH₃)₂X₂]⁰ (X = H, CH₃, F, Cl, Br) and isoelectronic complexes [Ir(NNN)(CH₃)]⁰ and [Pt(NCN)(CH₃)]⁰ with tridentate NNN and NCN pincer ligands. MX complexes acted as the Lewis bases donating electron density from the doubly occupied 5d _z² atomic orbital of the metal M atom to the empty valence p _z orbital of Y whose evidence was clearly provided by the natural atomic orbital (NAO) analysis. This charge transfer led to the formation of pentacoordinated square pyramidal MX·(YZ₃) adducts with M·Y dative bond. Binding energies were -44.7 and -75.2 kcal/mol for interaction of GaF₃ as the strongest acid with PtNCN and IrNNN pincer ligands complexes. Only M·B bonds had covalent character although MX·BZ₃ adducts were the least stable due to large values of Pauli repulsion and deformation energies. The highest degree of covalent character was found for adducts of BH₃ in all series of structures studied. Al and Ga adducts showed remarkably similar behavior with respect to geometry and binding energies.

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).

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.

Strategies toward realization of unsupported transition metal-boron donor-acceptor complexes: an insight from theory

Pre-pyramidalization of the boron centre in the Lewis acidic groups is a prerequisite for the formation of unsupported donor-acceptor complexes with Lewis basic metal fragments such as [Pt(NHC)2].

Zero-valent isocyanides of nickel, palladium and platinum as transition metal σ-type Lewis bases

Transition metal complexes that contain metal-to-ligand retrodative σ-bonds have become the subject of increasing studies over the last decade. Lewis acidic "Z-type ligands" can modulate the electronic structure of their resultant complexes in a manner distinct from 2e^- donor ligands, and can also engage in cooperative reactivity with a Lewis basic transition metal. In this Feature article, we summarize our work with transition metal isocyanide complexes of group 10 metals that have exploited metal-based σ-type Lewis basicity. While the complexes Ni(CNAr^Mes2)₃, Pd(CNAr^Dipp2)₂ and Pt(CNAr^Dipp2)₂ were initially targeted as analogues to unstable, low-coordinate metal carbonyls, it soon became apparent that these zero-valent metal centers bore appreciable Lewis basic qualities due largely to the enhanced σ-donor/π-acid ratio of isocyanides compared to CO. Detailed spectroscopic and structural studies of metal-only Lewis pairs (MOLPs) formed from these complexes have furthered our understanding of the electronic structure perturbations effected by Z-type ligand binding. In addition, the platinum (boryl)iminomethane (BIM) complex Pt(κ²-N,B-^Cy₂BIM)(CNAr^Dipp2) has illuminated a general ligand design strategy that can engender significant reverse-dative interactions with buttressed Lewis acids, and also has expanded the known scope of cooperative reactivity that can be realized at a transition metal-borane linkage.

Functional group migrations between boron and metal centres within transition metal-borane and -boryl complexes and cleavage of H-H, E-H and E-E' bonds

This feature article examines some of the recent advances in the chemistry of Z-type transition metal-borane and X-type transition metal-boryl complexes. It focuses on the employment of these boron-based functionalities acting as stores and transfer agents for functional groups such as hydrides, alkyl groups and aryl groups which can either be abstracted or delivered to the metal centre. The review also explores the rather novel reactivity involving the cleavage of H-H, E-H and E-E' bonds (where E and E' are a range of groups) across the transition metal-boron bond in such complexes. It explores the early examples of the addition of H-H across transition metal-borane bonds and describes the new transformation in the context of other known modes of hydrogen activation including classic oxidative addition and heterolytic cleavage at transition metal centres as well as Frustrated Lewis Pair chemistry. Similar reactivity involving transition metal-boryl complexes are also described particularly those which undergo both boryl-to-borane and borane-to-borohydride transformations. The delivery of hydride to the metal centre in combination with the potential to regenerate the borohydride functional group via a recharging process is explored in the context of providing a new strategy for catalysis. Finally, a light-hearted look at the analogy of the 'stinging processes' involving Trofimenko type ligands is taken one step further to determine whether it is indeed in the nature of scorpionate ligands to repeatedly 'sting' just as the real life scorpions do.

Electrophilic activation of alkynes for enyne cycloisomerization reactions with in situ generated early/late heterobimetallic Pt-Ti catalysts

In situ formation of heterobimetallic Pt-Ti catalysts enables rapid room temperature catalysis in enyne cycloisomerization reactions. The Lewis acidic titanium atom in the ligand framework is shown to be essential for fast catalysis. A range of enyne substrates are efficiently cyclized to carbocycles and heterocycles in high yield.

Synthesis and properties of heterobimetallic rhodium complexes featuring LiI, CuI or ZnII as a Lewis acidic metalloligand

Coordination of Lewis acidic metalloligands Li^I, Cu^I and Zn^II shows a remarkable impact on the CO stretching band in heterobimetallic rhodium complexes.

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.

(Dicyclohexyl(2-(dimesitylboryl)phenyl)phosphine: en route to stable frustrated Lewis pairs-hydrogen adducts in water

The new ansa-phosphinoborane (dicyclohexyl(2-(dimesitylboryl)phenyl)phosphine was synthesized via an one-pot protocol in 67% yield. The compound has been characterized by 1H, 13C, 11B and 31P NMR, and its solid-state structure determined by a single crystal X-ray diffraction analysis. The ansa-phosphinoborane does not react with molecular hydrogen or water at room or elevated temperature. According to performed DFT studies, heterolytic splitting of water or hydrogen by the phosphinoborane are both endergonic but close in thermodynamics. In polar solvents, such as in methanol or acetonitrile, addition of hydrogen is energetically more favorable than of water.