Facile Insertion of Rh and Ir into a Boron-Phenyl Bond, Leading to Boryl/Bis(phosphine) PBP Pincer Complexes

Dimeric Rh Complexes Supported by a Bridging Phosphido/Bis(Phosphine) PPP Ligand

Rh complexes of a tridentate PPP ligand bearing 1,2-pyrrolediyl linkers have been prepared, including examples with the central P donor being either a phosphine or a phosphide. Three bimetallic Rh complexes containing the diamandoid Rh2P2 core (P = phosphido) have been structurally and spectroscopically characterized. The Rh-Rh interaction in these three dimers was examined by way of structural comparisons and DFT investigations.

Borataalkenes, boraalkenes, and the η2-B,C coordination mode in coordination chemistry and catalysis

Borataalkenes and boraalkenes are the boron-containing isoelectronic analogues of alkenes and vinyl cations respectively. Compared with alkenes, the borataalkene and boraalkene ligand motifs in transition metal coordination chemistry are relatively underexplored. In this review, the synthesis of borataalkene and boraalkene complexes and other transition metal complexes featuring the η2-B,C coordination mode is described. The diversity of coordination modes and geometry in these complexes, and the spectroscopic and structural evidence supporting their assignments is disclosed as well as computational analysis of bonding. The applications of the borataalkene ligand motif in synthetic organic homogeneous catalysis, especially those involving geminal bis(pinacolatoboronates) and 1,4-azaborines, are discussed.

Synthesis, Structure, and Reactivity of Molybdenum- and Tungsten-Indane Complexes with Tris(pyrazolyl)borate Ligand

The reaction of molybdenum complexes with a tris(pyrazolyl)borate ligand (Et4N[TpMo(CO)3] and Et4N[Tp*Mo(CO)3] (Tp = hydridotris(pyrazolyl)borate, Tp* = hydridotris(3,5-dimethylpyrazolyl)borate)) and InBr3 at a 1:1 molar ratio afforded molybdenum-indane complexes (Et4N[TpMo(CO)3(InBr3)] 1 and Et4N[Tp*Mo(CO)3(InBr3)] 2). In addition, tungsten-indane complexes, Et4N[TpW(CO)3(InBr3)] 3 and Et4N[Tp*W(CO)3(InBr3)] 4, were obtained by the reaction of corresponding tungsten complexes. Complex 4 reacted with H2O to form the hydrido complex Tp*W(CO)3H, in which the W-In bond was cleaved. On the other hand, 4 reacted with three equiv. of AgNO3 to form Et4N[Tp*W(CO)3{In(ONO2)}] 5, in which three substituents on the In were exchanged while retaining the W-In dative bond. Complexes 1-5 were fully characterized using NMR measurements and elemental analyses, and the structures of 1-5 and Et4N[Tp*W(CO)3] were determined via X-ray crystallography. These are the first examples of mononuclear molybdenum- and tungsten-indane complexes with Mo-In and W-In dative bonds.

Migration of Hydride, Methyl, and Chloride Ligands between Al and M in (PAlP)M Pincer Complexes (M = Rh or Ir)

Protolysis of AlMe3 or AlBui3 with 2-diisopropylphosphinopyrrole (1) yields molecules containing two flanking phosphines and a central Al-Me (2-Me), Al-iBu (2-iBu), or Al-H (2-H) unit. The reactions of 2-Me with [L2MCl]2 (L = cyclooctene or 1/2 1,5-cyclooctadiene and M = Rh or Ir) in the presence of pyridine produces PAlClP pincer complexes (3-Rh and 3-Ir) with Al-Cl and M-Me bonds. The analogous reaction of a mixture of 2-iBu and 2-H with [L2MCl]2 and pyridine resulted in the formation of analogous Rh-H (4-Rh) and Ir-H (4-Ir) complexes. Treatment of 3-Rh with NaBEt3H produced compound 5-Rh with an Al-Me and a Rh-H bond; the analogous reaction of 3-Ir did not result in a clean product. 4-Ir accepted an equivalent of H2 to produce 6-Ir with two terminal Ir-H bonds and one bridging Al-H-Ir moiety, whereas 4-Rh did not react with H2. The density functional theoretical treatment is in accord with this finding, highlights the likely mechanism for the H2 addition, and supports the bonding picture in 6-Ir arising from NMR and X-ray diffraction (XRD) observations. Spectroscopic data and XRD studies are consistent with distorted square-pyramidal structures (about Rh or Ir) for compounds 3-5, with an alane occupying the apical position. Complexes 3 and 4 possess some of the shortest known Rh-Al or Ir-Al distances.

Synthesis, Reactivity and Coordination Chemistry of Group 9 PBP Boryl Pincer Complexes: [(PBP)M(PMe3)n] (M = Co, Rh, Ir; n = 1, 2)

The unsymmetrical diborane(4) derivative [(d(CH2P(iPr)2)abB)-Bpin] (1) proved to be a versatile PBP boryl pincer ligand precursor for Co(I) (2a, 4a), Rh(I) (2-3b) and Ir(I/III) (2-3c, 5-6c) complexes, in particular of the types [(d(CH2P(iPr)2)abB)M(PMe3)2] (2a-c) and [(d(CH2P(iPr)2)abB)M-PMe3] (2b-c). Whilst similar complexes have been obtained before, for the first time, the coordination chemistry of a homologous series of PBP pincer complexes, in particular the interconversion of the five- and four-coordinate complexes 2a-c/3a-c, was studied in detail. For Co, instead of the mono phosphine complex 2a, the dinitrogen complex [(d(CH2P(iPr)2)abB)Co(N2)(PMe3)] (4a) is formed spontaneously upon PMe3 abstraction from 2a in the presence of N2. All complexes were comprehensively characterised spectroscopically in solution via multinuclear (VT-)NMR spectroscopy and structurally in the solid state through single-crystal X-ray diffraction. The unique properties of the PBP ligand with respect to its coordination chemical properties are addressed.

A hampered oxidative addition of pre-coordinated pincer ligands can favour alternative pathways of activation

Pre-coordination to a transition metal by the terminal donor groups of a tri-dentate ligand is a common strategy to stabilise elusive groups, to achieve unprecedented bond activation and to develop novel modes of metal-ligand-cooperation for catalysis. In the current manuscript, we demonstrate that the oxidative addition of a central E-H-bond after pre-coordination to the metal centre is disfavoured for metals with d¹⁰ electron configuration. For exemplary pincer ligands and metals with d¹⁰ electron configuration, quantum chemical calculations suggest a second barrier, which is associated with the rearrangement of the saw-horse structure, obtained after oxidative addition, to the expected square planar geometry for the resulting d⁸ electron configuration. In the case of PBP-type ligands with a central L₂BH₂-group (L = R₃P) the reaction with Pt⁰ precursors proceeds via an alternative pathway of activation, which involves the backside attack of a nucleophile to the boron atom, which facilitates the nucleophilic attack of the Pt⁰ centre and formation of a boryl complex (LBH₂). As the corresponding reaction with a Pt^II precursor leads to B-H- instead of B-L-activation and formation of complex 2 with a L₂BH donor, our results show that ligand-stabilized borylenes (L₂BH) can in principle be converted to boryls (LBH₂) via boronium salts (L₂BH₂⁺).

Experimental and Computational Studies of Ruthenium Complexes Bearing Z-Acceptor Aluminum-Based Phosphine Pincer Ligands

Reaction of [Ru(C₆H₄PPh₂)₂(Ph₂PC₆H₄AlMe(THF))H] with CO results in clean conversion to the Ru-Al heterobimetallic complex [Ru(AlMePhos)(CO)₃] (1), where AlMePhos is the novel P-Al(Me)-P pincer ligand (o-Ph₂PC₆H₄)₂AlMe. Under photolytic conditions, 1 reacts with H₂ to give [Ru(AlMePhos)(CO)₂(μ-H)H] (2) that is characterized by multinuclear NMR and IR spectroscopies. DFT calculations indicate that 2 features one terminal and one bridging hydride that are respectively anti and syn to the AlMe group. Calculations also define a mechanism for H₂ addition to 1 and predict facile hydride exchange in 2 that is also observed experimentally. Reaction of 1 with B(C₆F₅)₃ results in Me abstraction to form the ion pair [Ru(AlPhos)(CO)₃][MeB(C₆F₅)₃] (4) featuring a cationic [(o-Ph₂PC₆H₄)₂Al]⁺ ligand, [AlPhos]⁺. The Ru-Al distance in 4 (2.5334(16) Å) is significantly shorter than that in 1 (2.6578(6) Å), consistent with an enhanced Lewis acidity of the [AlPhos]⁺ ligand. This is corroborated by a blue shift in both the observed and computed ν_CO stretching frequencies upon Me abstraction. Electronic structure analyses (QTAIM and EDA-ETS) comparing 1, 4, and the previously reported [Ru(ZnPhos)(CO)₃] analogue (ZnPhos = (o-Ph₂PC₆H₄)₂Zn) indicate that the Lewis acidity of these pincer ligands increases along the series ZnPhos < AlMePhos < [AlPhos]⁺.

Toggling the Z-type interaction off-on in nickel-boron dihydrogen and anionic hydride complexes

Completing a series of nickel-group 13 complexes, a coordinatively unsaturated nickel-boron complex and its derivatives with a H₂, N₂, or hydride ligand were synthesized and characterized. The toggling "on" of a Ni(0)-B(III) inverse-dative bond enabled the stabilization of a nickel-bound anionic hydride with a remarkably low thermodynamic hydricity of kcal mol^-1 in THF. The flexible topology of the boron metalloligand confers both favorable hydrogen binding affinity and strong hydride donicity, albeit at the cost of high H₂ basicity during deprotonation to form the hydride.

Synthesis and reactivity of PC(sp3)P-pincer iridium complexes bearing a diborylmethyl anion

Novel PCP-pincer iridium complexes bearing a diborylmethyl anion were synthesized. Strong σ-electron-donation to the metal and significant π-backdonation from the metal to boron atoms at the β-position were observed both experimentally and computationally. H/D exchange of the aromatic C-H bond proceeded smoothly and, in addition, the α-methine-hydrogen between boron atoms was found to be replaced with deuterium in benzene-d₆ solution possibly through diborylcarbene metal complexes as intermediates.

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...

Pincer-supported metal/main-group bonds as platforms for cooperative transformations

Electron-rich late metals and electropositive main-group elements (metals and metalloids) can be combined to provide an ambiphilic façade for exploring metal-ligand cooperation, yet the instability of the metal/main-group bond frequently limits the study and application of such units. Incorporating main-group donors into pincer frameworks, where they are stabilized and held in proximity to the transition-metal partner, can allow discovery of new modes of reactivity and incorporation into catalytic processes. This Perspective summarizes common modes of cooperativity that have been demonstrated for pincer frameworks featuring metal/main-group bonds, highlighting similarities among boron, aluminium, and silicon donors and identifying directions for further development.

Bimetallic Ru-Pd and Trimetallic Ru-Pd-Cu Assemblies on the Carborane Cluster Surface

The synthesis of well-defined heterometallic complexes remains a frontier challenge in inorganic chemistry. We report an approach that relies on the sequential insertion of electrophilic metal fragments into electron-rich Ru-B bonds of the η²-BB-carboryne complex (POBBOP)Ru(CO)₂ [POBBOP = 1,7-OP(ⁱPr)₂-m-2,6-dehydrocarborane]. Utilizing this synthetic strategy, bimetallic (POBBOP)(Ru)(CO)₂[Pd(P^tBu₃)] and trimetallic (POBBOP)(Ru)(CO)₂[Pd(P^tBu₃)](CuBr) complexes were selectively prepared. Structural and theoretical analysis of the features of chemical bonding within Ru-B-B-Cu and Ru-B-B-Pd fragments is presented.

Cooperative C-H activation of pyridine by PBP complexes of Rh and Ir can lead to bridging 2-pyridyls with different connectivity to the B-M unit

Pyridine and quinoline undergo selective C–H activation in the 2-position with Rh and Ir complexes of a boryl/bis(phosphine) PBP pincer ligand, resulting in a 2-pyridyl bridging the transition metal and the boron center. Examination of this reactivity with Rh and Ir complexes carrying different non-pincer ligands on the transition metal led to the realization of the possible isomerism derived from the 2-pyridyl fragment connecting either via B–N/C–M bonds or via B–C/N–M bonds. This M–C/M–N isomerism was systematically examined for four structural types. Each of these types has a defined set of ligands on Rh/Ir besides 2-pyridyl and PBP. A pair of M–C/M–N isomers for each type was computationally examined for Rh and for Ir, totaling 16 compounds. Several of these compounds were isolated or observed in solution by experimental methods, in addition to a few 2-quinolyl variants. The DFT predictions concerning the thermodynamic preference within each M–C/M–N isomeric match the experimental findings very well. In two cases where DFT predicts <2 kcal mol⁻¹ difference in free energy, both isomers were experimentally observed in solution. Analysis of the structural data, of the relevant Wiberg bond indices, and of the ETS-NOCV partitioning of the interaction of the 2-pyridyl fragment with the rest of the molecule points to the strength of the M–C(pyridyl) bond as the dominant parameter determining the relative M–C/M–N isomer favorability. This M–C bond is always stronger for the analogous Ir vs. Rh compounds, but the nature of the ligand trans to it has a significant influence, as well. DFT calculations were used to evaluate the mechanism of isomerization for one of the molecule types.

The thermodynamic preference between two isomeric products of C–H activation of pyridine, with 2-pyridyl bridging boron and iridium or rhodium, primarily depends on the M–C bond strength.

Pd/Ni-Catalyzed Germa-Suzuki coupling via dual Ge–F bond activation

Pd/Ni → Ge–F interactions supported by phosphine-chelation were found to trigger dual activation of Ge–F bonds under mild conditions.

Nucleophilic reactivity of the gold atom in a diarylborylgold(i) complex toward polar multiple bonds

A di(o-tolyl)borylgold complex was synthesized via the metathesis reaction of a gold alkoxide with tetra(o-tolyl)diborane(4). The resulting diarylborylgold complex exhibited a Lewis acidic boron center and a characteristic visible absorption that arises from its HOMO–LUMO excitation, which is narrower than that of a previously reported dioxyborylgold complex. The diarylborylgold complex reacted with isocyanide in a stepwise fashion to afford single- and double-insertion products and a C–C coupled product. Reactions of this diarylborylgold complex with C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O/N double bond species furnished addition products under concomitant formation of Au–C and B–O/N bonds, which suggests nucleophilic reactivity of the gold metal center. DFT calculations provided details of the underlying reaction mechanism, which involves an initial coordination of the CO/N bond to the boron vacant p-orbital of the diarylboryl ligand followed by a migration of the gold atom from the tetracoordinate sp³-hybridized boron center, which is analogous to the reactivity of the conventional sp³-hybridized borate species. The DFT calculations also suggested a stepwise mechanism for the reaction of this diarylborylgold complex with isocyanide, which afforded three different reaction products depending on the applied reaction conditions.

A di(o-tolyl)borylgold complex added to CO/N double bond to form Au–C and B–O/N bonds. DFT calculations revealed a two-step mechanism consisting of the coordination of O/N atom to B atom followed by nucleophilic migration of Au atom.

Impact of the Novel Z-Acceptor Ligand Bis{(ortho-diphenylphosphino)phenyl}zinc (ZnPhos) on the Formation and Reactivity of Low-Coordinate Ru(0) Centers

The preparation and reactivity with H₂ of two Ru complexes of the novel ZnPhos ligand (ZnPhos = Zn(o-C₆H₄PPh₂)₂) are described. Ru(ZnPhos)(CO)₃ (2) and Ru(ZnPhos)(IMe₄)₂ (4; IMe₄ = 1,3,4,5-tetramethylimidazol-2-ylidene) are formed directly from the reaction of Ru(PPh₃)(C₆H₄PPh₂)₂(ZnMe)₂ (1) or Ru(PPh₃)₃HCl/LiCH₂TMS/ZnMe₂ with CO and IMe₄, respectively. Structural and electronic structure analyses characterize both 2 and 4 as Ru(0) species in which Ru donates to the Z-type Zn center of the ZnPhos ligand; in 2, Ru adopts an octahedral coordination, while 4 displays square-pyramidal coordination with Zn in the axial position. Under photolytic conditions, 2 loses CO to give Ru(ZnPhos)(CO)₂ that then adds H₂ over the Ru-Zn bond to form Ru(ZnPhos)(CO)₂(μ-H)₂ (3). In contrast, 4 reacts directly with H₂ to set up an equilibrium with Ru(ZnPhos)(IMe₄)₂H₂ (5), the product of oxidative addition at the Ru center. DFT calculations rationalize these different outcomes in terms of the energies of the square-pyramidal Ru(ZnPhos)L₂ intermediates in which Zn sits in a basal site: for L = CO, this is readily accessed and allows H₂ to add across the Ru-Zn bond, but for L = IMe₄, this species is kinetically inaccessible and reaction can only occur at the Ru center. This difference is related to the strong π-acceptor ability of CO compared to IMe₄. Steric effects associated with the larger IMe₄ ligands are not significant. Species 4 can be considered as a Ru(0)L₄ species that is stabilized by the Ru→Zn interaction. As such, it is a rare example of a stable Ru(0)L₄ species devoid of strong π-acceptor ligands.

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).

Evaluation of Pd→B Interactions in Diphosphinoborane Complexes and Impact on Inner-Sphere Reductive Elimination

The dative Pd→B interaction in a series of ^RDPB^R’ Pd⁰ and Pd^II complexes (^RDPB^R’=(o‐PR₂C₆H₄)₂BR’, diphosphinoborane) was analyzed using XRD, ¹¹B NMR spectroscopy and NBO/NLMO calculations. The borane acceptor discriminates between the oxidation state Pd^II and Pd⁰, stabilizing the latter. Reaction of lithium amides with [(^RDPB^R’)Pd^II(4‐NO₂C₆H₄)I] chemoselectively yields the C−N coupling product. DFT modelling indicates no significant impact of Pd^II→B coordination on the inner‐sphere reductive elimination rate.

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.

Palladium(ii) complexes supported by PBP and POCOP pincer ligands: a comparison of their structure, properties and catalytic activity

A Pd(ii) chloride complex supported by a Yamashita-Nozaki PBP pincer ligand, [C6H4-1,2-(NCH2PtBu2)2B]PdCl (1a), was synthesized. The structure, properties and catalytic activity of complex 1a were compared with those of the corresponding POCOP pincer complex [C6H3-2,6-(OPtBu2)2]PdCl (2a). It was found that the Pd centre in complex 1a is more electron rich and easier to be oxidized than that in complex 2a; complex 1a is a much better catalyst for Suzuki-Miyaura cross-coupling reactions than complex 2a. Starting from complexes 1a and 2a, two series of Pd(ii) pincer complexes bearing a SH, BH4, N[combining low line]CS, N[combining low line]CSe or N3 covalent ligand, [C6H4-1,2-(NCH2PtBu2)2B]PdY (Y = SH, 1b; BH4; 1c; N[combining low line]CS, 1d; N[combining low line]CSe, 1e; and N3, 1f) and [C6H3-2,6-(OPtBu2)2]PdY (Y = SH, 2b; BH4, 2c; N[combining low line]CS, 2d; N[combining low line]CSe, 2e; and N3, 2f), were synthesized and fully characterized. Single crystal X-ray diffraction analysis indicated that the Pd centre is less tightly chelated in PBP pincer complexes. The strong σ-donor ability of the PBP pincer ligand has little influence on the structure of the covalent ligand possessing both σ-donor and π-acceptor properties. However, the stretching vibrational frequencies of N[combining low line]CS, N[combining low line]CSe and N3 ligands and the coordination mode of the BH4 ligand are significantly different in these two types of palladium pincer complexes.

A PAlP Pincer Ligand Bearing a 2-Diphenylphosphinophenoxy Backbone

A PAlP pincer ligand derived from 2-diphenylphosphino-6-isopropylphenol was synthesized. The Lewis acidity of the Al center of the ligand was evaluated with coordination of (O)PEt3. A zwitterionic rhodium-aluminum heterobimetallic complex bearing the PAlP ligand was synthesized through its complexation with [RhCl(nbd)]2. Moreover, reduction of the zwitterionic rhodium-aluminum complex with KC8 afforded heterobimetallic complexes bearing an X-type PAlP pincer ligand.

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.

Characterization of Rh-Al Bond in Rh(PAlP) (PAlP = Pincer-type Diphosphino-Aluminyl Ligand) in Comparison with Rh(L)(PMe3)2 (L = AlMe2, Al(NMe2)2, BR2, SiR3, CH3, Cl, or OCH3): Theoretical Insight

The unique Rh-Al bond in recently synthesized Rh(PAlP) 1 {PAlP = pincer-type diphosphino-aluminyl ligand Al[NCH₂(P ⁱPr₂)]₂(C₆H₄)₂NMe} was investigated using the DFT method. Complex 1 has four doubly occupied nonbonding d orbitals on the Rh atom and one Rh d orbital largely participating in the Rh-Al bond which exhibits considerably large bonding overlap between Rh and Al atoms like in a covalent bond. Interestingly, Rh^δ--Al^δ+ polarization is observed in the bonding MO of 1, which is reverse to Rh^δ+-E^δ- (E = coordinating atom) polarization found in a usual coordinate bond. This unusual polarization arises from the presence of the Al valence orbital at significantly higher energy than the Rh valence orbital energy. Characteristic features of 1 are further unveiled by comparing 1 with similar Rh complexes RhL(PMe₃)₂ (2 for L = AlMe₂, 3 for L = Al(NMe₂)₂, 4 for L = BMe₂, 5 for L = SiMe₃, 6 for L = SiH₃, 7 for L = CH₃, 8 for L = OMe, and 9 for L = Cl). As expected, 7, 8, and 9 exhibit usual Rh^δ+-E^δ- polarization (E = coordinating atom) in the Rh-E bonding MO. On the other hand, the reverse Rh^δ--E^δ+ polarization is observed in the Rh-E bonding MOs of 2-5 like in 1, while the Rh-Si bond is polarized little in 6. These results are clearly understood in terms of the valence orbital energy of the ligand. Because the LUMO of 1 mainly consists of the Rh 4d_σ, 5s, and 5p orbitals and the Al 3s and 3p orbitals, both Rh and Al atoms play the role of coordinating site for a substrate bearing a lone pair orbital. For instance, NH₃ and pyridine coordinate to both Al and Rh atoms with considerably large binding energy. PAlP exhibits significantly strong trans influence, which is as strong as that of SiMe₃ but moderately weaker than that of BMe₂. The trans influence of these ligands is mainly determined by the valence orbital energy of the ligand and the covalent bond radius of the coordinating E atom.

Selective Double Addition Reaction of an E‒H Bond (E = Si, B) to a C≡N Triple Bond of Organonitriles

The catalytic double hydrometalation such as hydrosilylation and hydroborylation of organonitriles has attracted considerable attention because the obtained products are widely used in organic synthesis and it is thought to be one of the effective methods for reduction of organonitriles. However, the examples of these reactions are quite limited to date. This paper summarizes the development of selective double hydrosilylation, double hydroborylation, and dihydroborylsilylation of organonitriles, including their reaction mechanisms and the role of the metal species in the catalytic cycle.