Activation of M–Cl Bonds with Phosphine–Alanes: Preparation and Characterization of Zwitterionic Gold and Copper Complexes

Aluminum in Frustrated Lewis Pair Chemistry

This review article describes the development of the use of aluminum compounds in the chemistry of frustrated Lewis pairs (FLPs) over the last 14 years. It also discusses the synthesis, reactivity and catalytic applications of intermolecular, intramolecular and so-called hidden FLPs with phosphorus, nitrogen and carbon Lewis bases. The intrinsically higher acidity of aluminum compounds compared to their boron analogs opens up different reaction pathways. The results are presented in a more or less chronological order. It is shown that Al FLPs react with a variety of polar and non-polar substrates and form both stable adducts and reversibly activate bonds. Consequently, some catalytic applications of the title compounds were presented such as dimerization of alkynes, hydrogenation of tert-butyl ethylene and imines, C-F bond activation, reduction of CO2, dehydrogenation of amine borane and transfer of ammonia. In addition, various Al FLPs were used as initiators in polymerization reactions.

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.

Cooperation towards nobility: equipping first-row transition metals with an aluminium sword

The exploration for noble metals substitutes in catalysis has become a highly active area of research, driven by the pursuit of sustainable chemical processes. Although the utilization of base metals holds great potential as an alternative, their successful implementation in predictable catalytic processes necessitates the development of appropriate ligands. Such ligands must be capable of controlling their intricate redox chemistry and promote two-electron events, thus mimicking well-established organometallic processes in noble metal catalysis. While numerous approaches for infusing nobility to base metals have been explored, metal-ligand cooperation has garnered significant attention in recent years. Within this context, aluminium-based ligands offer interesting features to fine-tune the activity of metal centres, but their application in base metal catalysis remains largely unexplored. This perspective seeks to highlight the most recent breakthroughs in the reactivity of heterobimetallic aluminium-base-metal complexes, while also showcasing their potential to develop novel and predictable catalytic transformations. By turning the spotlight on such heterobimetallic species, we aim to inspire chemists to explore aluminium-base-metal species and expand the range of their applications as catalysts.

Lewis Acid-Assisted C(sp3)-C(sp3) Reductive Elimination at Gold

The phosphine-borane iPr₂P(o-C₆H₄)BFxyl₂ (Fxyl = 3,5-(F₃C)₂C₆H₃) 1-Fxyl was found to promote the reductive elimination of ethane from [AuMe₂(μ-Cl)]₂. Nuclear magnetic resonance monitoring revealed the intermediate formation of the (1-Fxyl)AuMe₂Cl complex. Density functional theory calculations identified a zwitterionic path as the lowest energy profile, with an overall activation barrier more than 10 kcal/mol lower than without borane assistance. The Lewis acid moiety first abstracts the chloride to generate a zwitterionic Au(III) complex, which then readily undergoes C(sp³)-C(sp³) coupling. The chloride is finally transferred back from boron to gold. The electronic features of this Lewis-assisted reductive elimination at gold have been deciphered by intrinsic bond orbital analyses. Sufficient Lewis acidity of boron is required for the ambiphilic ligand to trigger the C(sp³)-C(sp³) coupling, as shown by complementary studies with two other phosphine-boranes, and the addition of chlorides slows down the reductive elimination of ethane.

Tuning of Cu-Al Interactions in Complexes Derived from Tris(pyridonyl-6-methyl)aluminum Metalloligands

A series of bioinspired polar atrane Cu-Al complexes were studied with a combined experimental and computational approach to assess the range and nature of Cu-Al interactions in these novel species. The aluminum metalloligand [Na{Me₂Al(OPy-6-Me)₂}] (2) was furnished in excellent yield (92%) from the nucleophilic attack of Na(OPy-6-Me) to AlMe₃ and the subsequent alkane elimination reaction with 6-methyl-2-hydroxypyridine. At the same time, the metalloligand [Al(OPy-6-Me)₃] (3) was isolated in an also excellent yield (95%) via alkane elimination of AlMe₃ with 6-methyl-2-hydroxypyridine. The zwitterionic Cu-Al atranes [Cu{MeAl(OPy-6-Me)₃}] (5^Me) and [Cu{MesAl(OPy-6-Me)₃}] (5^Mes) were isolated (73 and 97% yields) from metalloligands 2 and 3, respectively. [(Cu{Al(OPy-6-Me)₄})₂(μ-Cu)]⁺ ([6⁺][B(Ar^CF3)₄]) was isolated via a reaction that involves alkane elimination and redistribution reacting from 5^Me with [H(OEt₂)₂][B(Ar^CF3)₄] in benzene solution. Alkane elimination in benzene of either 5^Me or 5^Mes with [HNEt₃][B(Ar^CF3)₄] renders [Cu{(Et₃N)Al(OPy-6-Me)₃}]⁺ (Et₃N-5⁺). The Lewis base-free cationic complex [Cu{Al(OPy-6-Me)₃}]⁺ (5⁺) was isolated in 68% yield upon reacting 3 with [Cu(COD)₂][B(Ar^CF3)₄] in benzene. Metalloligands and complexes were fully characterized with an array of spectroscopic and analytical techniques that include multinuclear NMR, ATR-IR, ESI-spectrometry, combustion microanalysis, cyclic voltammetry (CV), and, whenever feasible, SCXRD. X-ray and DFT parameters indicate that the strength of the Cu→Al transannular interaction follows the trend 5⁺ > Et₃N-5⁺ > [6⁺][B(Ar^CF3)₄], 5^Me, and 5^Mes in a smooth transition from zwitterionic species where the Cu-Al interaction is nonexistent to moderate Cu-Al Z-type interactions. CV, in conjunction with DFT calculations of Et₃N-5⁺ and 5⁺, hint at the generation in the electrochemical cell of the radical species 5^rad at -1.82 V and the anionic complex 5^- at -2.32 V vs Fc/Fc⁺, respectively. The proposed species 5^rad exhibits 2-center/1-electron (2c/1e) σ bonding whereas 5^- a 2-center/2-electron (2c/2e) bond.

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]⁺.

Coordination chemistry and structural rearrangements of the Me2PCH2AlMe2 ambiphilic ligand

Reaction of 2 equivalents of (Me₂PCH₂AlMe₂)₂ with [{RhCl(cod)}₂] (cod = 1,5-cyclooctadiene) afforded [{κ²P,P-(Me₃Al)ClMeAl(CH₂PMe₂)₂}Rh(cod)] (1), which features a κ²-coordinated bis(phosphino)aluminate anion. In compound 1, an Al-Cl substituent bridges to a molecule of AlMe₃, which could be removed in vacuo to provide [{κ²P,P-ClMeAl(CH₂PMe₂)₂}Rh(cod)] (2). By contrast, reaction of 1 equiv. of (Me₂PCH₂AlMe₂)₂ with [{RhCl(cod)}₂] yielded [Rh(cod)(μ-Cl)(Me₂PCH₂AlMe₂)] (3) as the major product, where the phosphine donor of an intact Me₂PCH₂AlMe₂ ligand is coordinated to rhodium and a chloride ligand bridges between Rh and Al. [Rh(cod)(μ-Cl)(Me₂PCH₂AlClMe)] (3A) and 2 were also formed as minor products. The aforementioned reactions were carried out in benzene or toluene, whereas the 1 : 1 reaction of (Me₂PCH₂AlMe₂)₂ with [{RhCl(cod)}₂] in THF afforded [{Rh(μ-CH₂PMe₂)(cod)}₂] (4). Reactions of (Me₂PCH₂AlMe₂)₂ with iridium(I), gold(I) and platinum(II) precursors were also explored. A 1 : 1 reaction of (Me₂PCH₂AlMe₂)₂ with [{IrCl(cod)}₂] afforded [{κ²P,P-Cl₂Al(CH₂PMe₂)₂}Ir(cod)] (5) as one of two major phosphine-containing products; unlike 3, this compound features two chlorine substituents on aluminium. For comparison, the rhodium analogue of 5, [{κ²P,P-Cl₂Al(CH₂PMe₂)₂}Rh(cod)] (6), was also synthesized via the 1 : 1 reaction of {ClAl(CH₂PMe₂)₂}₂ with [{RhCl(cod)}₂]. Reactions of (Me₂PCH₂AlMe₂)₂ with [AuCl(CO)] or [PtCl₂(cod)] also resulted in chloride-methyl group exchange between the transition metal and aluminium. However, these reactions generated free (Me₂PCH₂AlClMe)₂ accompanied by gold and ethane, or [PtMe₂(cod)], respectively. Reaction of 1.5 equivalents of (Me₂PCH₂AlMe₂)₂ with [PtMe₂(cod)] at 75 °C afforded zwitterionic [(PtMe{μ-κ¹P:κ²P,P-MeAl(CH₂PMe₂)₃})₂] (7) which features two tris(phosphino)aluminate anions bridging between PtMe units. Compounds 1-2, 3/3A, 4-7 and (Me₂PCH₂AlClMe)₂ were crystallographically characterized.

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.

Formation of an Ag→Al dative bond is avoided in reactions of an alane/tris(phosphine) ligand with monovalent silver

An alane/tris(phospine) ligand reacts with AgOTf by coordination of three phosphines to the Ag center and transfer of triflate to the tris(pyrrolyl) Al site. Reaction with Ag[HCB₁₁Cl₁₁] results in the coordination of two phosphines to Ag and one to Al, with no significant Ag-Al bonding in either structure.

H2 and carbon-heteroatom bond activation mediated by polarized heterobimetallic complexes

The field of heterobimetallic chemistry has rapidly expanded over the last decade. In addition to their interesting structural features, heterobimetallic structures have been found to facilitate a range of stoichiometric bond activations and catalytic processes. The accompanying review summarizes advances in this area since January of 2010. The review encompasses well-characterized heterobimetallic complexes, with a particular focus on mechanistic details surrounding their reactivity applications.

Tripodal P3XFe-N2 Complexes (X = B, Al, Ga): Effect of the Apical Atom on Bonding, Electronic Structure, and Catalytic N2-to-NH3 Conversion

Terminal dinitrogen complexes of iron ligated by tripodal, tetradentate P3X ligands (X = B, C, Si) have previously been shown to mediate catalytic N2-to-NH3 conversion (N2RR) with external proton and electron sources. From this set of compounds, the tris(phosphino)borane (P3B) system is most active under all conditions canvassed thus far. To further probe the effects of the apical Lewis acidic atom on structure, bonding, and N2RR activity, Fe-N2 complexes supported by analogous group 13 tris(phosphino)alane (P3Al) and tris(phosphino)gallane (P3Ga) ligands are synthesized. The series of P3XFe-N2[0/1-] compounds (X = B, Al, Ga) possess similar electronic structures, degrees of N2 activation, and geometric flexibility as determined from spectroscopic, structural, electrochemical, and computational (DFT) studies. However, treatment of [Na(12-crown-4)2][P3XFe-N2] (X = Al, Ga) with excess acid/reductant in the form of HBArF4/KC8 generates only 2.5 ± 0.1 and 2.7 ± 0.2 equiv of NH3 per Fe, respectively. Similarly, the use of [H2NPh2][OTf]/Cp*2Co leads to the production of 4.1 ± 0.9 (X = Al) and 3.6 ± 0.3 (X = Ga) equiv of NH3. Preliminary reactivity studies confirming P3XFe framework stability under pseudocatalytic conditions suggest that a greater selectivity for hydrogen evolution versus N2RR may be responsible for the attenuated yields of NH3 observed for P3AlFe and P3GaFe relative to P3BFe.

1,1-Phosphaboration of C[triple bond, length as m-dash]C and C[double bond, length as m-dash]C bonds at gold

The phosphine-borane iPr2P(o-C6H4)BFXyl2 (Fxyl = 3,5-(F3C)2C6H3) was found to react with gold(i) alkynyl and vinyl complexes via an original 1,1-phosphaboration process. Zwitterionic complexes resulting from Au to B transmetallation have been authenticated as key intermediates. X-ray diffraction analyses show that the alkynyl-borate moiety remains pendant while the vinyl-borate is side-on coordinated to gold. According to DFT calculations, the phosphaboration then proceeds in a trans stepwise manner via decoordination of the phosphine, followed by anti nucleophilic attack to the π-CC bond activated by gold. The boron center acts as a relay and tether for the organic group.

Siloxyaluminate and Siloxygallate Complexes as Models for Framework and Partially Hydrolyzed Framework Sites in Zeolites and Zeotypes

Anionic molecular models for nonhydrolyzed and partially hydrolyzed aluminum and gallium framework sites on silica, M[OSi(OtBu)₃ ]₄ ^- and HOM[OSi(OtBu)₃ ]₃ ^- (where M=Al or Ga), were synthesized from anionic chlorides Li{M[OSi(OtBu)₃ ]₃ Cl} in salt metathesis reactions. Sequestration of lithium cations with [12]crown-4 afforded charge-separated ion pairs composed of monomeric anions M[OSi(OtBu)₃ ]₄ ^- with outer-sphere [([12]crown-4)₂ Li]⁺ cations, and hydroxides {HOM[OSi(OtBu)₃ ]₃ } with pendant [([12]crown-4)Li]⁺ cations. These molecular models were characterized by single-crystal X-ray diffraction, vibrational spectroscopy, mass spectrometry and NMR spectroscopy. Upon treatment of monomeric [([12]crown-4)Li]{HOM[OSi(OtBu)₃ ]₃ } complexes with benzyl alcohol, benzyloxide complexes were formed, modeling a possible pathway for the formation of active sites for Meerwin-Ponndorf-Verley (MPV) transfer hydrogenations with Al/Ga-doped silica catalysts.

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.

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.

Carbeniophosphines versus Phosphoniocarbenes: The Role of the Positive Charge

The chemistry of carbeniophosphines and phosphoniocarbenes, which have general structures derived formally from the three-component "carbene/phosphine/positive charge" association, is presented. These two complementary classes of carbon-phosphorus-based ligands, defined by the presence of an inverted cationic coordinating structure (C+ ∼P: vs. P+ ∼C:) have the common purpose of positioning a positive charge in the vicinity of the metal center. Through selected examples, the synthetic methods, coordination properties, and general reactivity of both cationic species is described. Particular emphasis is placed on the influence of the positive charge on the respective chemical behavior of the two classes of compound.

Gold(I) Complexes of the Geminal Phosphinoborane tBu2PCH2BPh2

In this work, we explored the coordination properties of the geminal phosphinoborane tBu₂PCH₂BPh₂ (2) toward different gold(I) precursors. The reaction of 2 with an equimolar amount of the sulfur-based complex (Me₂S)AuCl resulted in displacement of the SMe₂ ligand and formation of linear phosphine gold(I) chloride 3. Using an excess of ligand 2, bisligated complex 4 was formed and showed dynamic behavior at room temperature. Changing the gold(I) metal precursor to the phosphorus-based complex, (Ph₃P)AuCl impacted the coordination behavior of ligand 2. Namely, the reaction of ligand 2 with (Ph₃P)AuCl led to the heterolytic cleavage of the gold-chloride bond, which is favored over PPh₃ ligand displacement. To the best of our knowledge, 2 is the first example of a P/B-ambiphilic ligand capable of cleaving the gold-chloride bond. The coordination chemistry of 2 was further analyzed by density functional theory calculations.

Facile reactions of gold(i) complexes with tri(tert-butyl)azadiboriridine

Direct structural evidence for group 11 metal-mediated B-B bond activation was obtained from reactions of tri(tert-butyl)azadiboriridine (1) with AuCl(L) complexes. The AuCl(SMe2) reaction afforded [η2-B,B-B(tBu)N(tBu)B(tBu)]AuCl (2) by ligand displacement. More donating phosphines as co-ligands led to B-B bond cleavage accompanied by either halide or L migration to form boron-gold complexes 3 (L = PPh3) and 4 (L = PMe3). A similar product 5, which is isostructural to 4, was obtained by the addition of dimethylaminopyridine (DMAP) to 2-4. Complexes 2-5 constitute rare examples of metal complexes bearing two Lewis acidic centres. The effect of the boryl ligand was demonstrated in the formation of a gold(i) complex 6 bearing a 5-membered heterocycle from 3 and tert-butylisonitrile. Plausible reaction mechanisms that led to these complexes and their bonding situation were explored computationally at the DFT level.

Attempted synthesis of ortho-phenylene phosphino-tritylium cations

With the synthesis of ortho-phenylene phosphino-tritylium cations as an objective, we generated (2-lithiophenyl)diphenylphosphine and (2-lithiophenyl)di-isopropylphosphine and allowed these organolithium reagents to react with benzophenone. The resulting phosphino-triarylcarbinols were allowed to react with HBF₄ in the presence of trifluoroacetic anhydride in order to generate the corresponding cations. Instead of the targeted ortho-phenylene phosphino-tritylium, the cations formed in these reactions were identified as the four-membered phosphonium species 7,7-bis(phenyl)-8,8-bis(phenyl)-7-phosphoniabicyclo[4.2.0]octa-1,3,5-triene (3⁺) and 7,7-bis(phenyl)-8,8-bis(isopropyl)-7-phosphoniabicyclo[4.2.0]octa-1,3,5-triene (4⁺), which were both isolated as tetrafluoroborate salts. The structure of these compounds has been confirmed by X-ray analysis. These new cations are thermally unstable and isomerize into the corresponding 5,10-dihydro-5,5-diphenyl-10-phenyl-acridophosphinium (5⁺) and 5,10-dihydro-5,5-di(isopropyl)-10-phenyl-acridophosphinium (6⁺) as tetrafluoroborate salts. These reactions suggest the involvement of ortho-phenylene phosphino-tritylium cations, which would be obtained by dissociation of the R₃P⁺-CAr₃ bonds in 4⁺ and 5⁺This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

Structure and dynamic NMR behavior of rhodium complexes supported by Lewis acidic group 13 metallatranes

Z-type complexes featuring Rh → Al and Rh → Ga interactions show distorted Rh centers and fluxionality on the NMR timescale.

Activation of an Au-Cl Bond by a Pendent SbIII Lewis Acid: Impact on Structure and Catalytic Activity

With the objective of identifying new coordination modes of ambiphilic ligands, we have investigated the bidentate Sb/P ligands (o-(Ph₂ P)C₆ H₄ )SbCl₂ (L^Cl ) and (o-(Ph₂ P)C₆ H₄ )SbPh₂ (L^Ph ). Reaction of these ligands with (tht)AuCl affords the monoligated species L^Cl AuCl (1) and L^Ph AuCl (2), respectively, in which the antimony centers are only weakly engaged with the coordinated gold atom. Treatment of 1 with PPh₃ induces an intramolecular transfer of a chloride ligand from gold to antimony to form the zwitterionic species o-(Cl₃ Sb)C₆ H₄ (Ph₂ P)Au(PPh₃ ) (3). Natural bond orbital (NBO) calculations show that the antimony and gold centers are involved in weak Sb→Au and Au→Sb interactions, the latter reflecting the Lewis acidity of the pendent antimony group. Finally, we demonstrate that the ability of the antimony center in 1 to abstract a gold-bound chloride in the presence of a Lewis basic substrate may be utilized to activate the gold center for the electrophilic cycloisomerization of propargylic amides.

Novel zwitterionic complexes arising from the coordination of an ambiphilic phosphorus-aluminum ligand to gold

Coordination of Mes2PC(=CHPh)AltBu2 to metal chlorides has been studied. Bridging P→M-Cl→Al coordinations were observed with Rh and Pd fragments, while chloride abstraction systematically occurred with gold. The resulting zwitterionic complexes have been fully characterized and analyzed by DFT calculations.

Electronic and Steric Influences of Pendant Amine Groups on the Protonation of Molybdenum Bis(dinitrogen) Complexes

The synthesis of a series of P(Et)P(NRR(')) (P(Et)P(NRR(')) = Et2PCH2CH2P(CH2NRR')2, R = H, R' = Ph or 2,4-difluorophenyl; R = R' = Ph or (i)Pr) diphosphine ligands containing mono- and disubstituted pendant amine groups and the preparation of their corresponding molybdenum bis(dinitrogen) complexes trans-Mo(N2)2(PMePh2)2(P(Et)P(NRR('))) is described. In situ IR and multinuclear NMR spectroscopic studies monitoring the stepwise addition of triflic acid (HOTf) to trans-Mo(N2)2(PMePh2)2(P(Et)P(NRR('))) complexes in tetrahydrofuran at -40 °C show that the electronic and steric properties of the R and R' groups of the pendant amines influence whether the complexes are protonated at Mo, a pendant amine, a coordinated N2 ligand, or a combination of these sites. For example, complexes containing monoaryl-substituted pendant amines are protonated at Mo and the pendant amine site to generate mono- and dicationic Mo-H species. Protonation of the complex containing less basic diphenyl-substituted pendant amines exclusively generates a monocationic hydrazido (Mo(NNH2)) product, indicating preferential protonation of an N2 ligand. Addition of HOTf to the complex featuring more basic diisopropyl amines primarily produces a monocationic product protonated at a pendant amine site, as well as a trace amount of dicationic Mo(NNH2) product that is additionally protonated at a pendant amine site. In addition, trans-Mo(N2)2(PMePh2)2(depe) (depe = Et2PCH2CH2PEt2) was synthesized to serve as a counterpart lacking pendant amines. Treatment of this complex with HOTf generated a monocationic Mo(NNH2) product. Protonolysis experiments conducted on several complexes in this study afforded trace amounts of NH4(+). Computational analysis of trans-Mo(N2)2(PMePh2)2(P(Et)P(NRR('))) complexes provides further insight into the proton affinity values of the metal center, N2 ligand, and pendant amine sites to rationalize differences in their reactivity profiles.

Dative Au→Al interactions: crystallographic characterization and computational analysis

Hitherto unknown Au→Al interactions have been evidenced upon coordination of the geminal phosphorus-aluminum Lewis pair Mes2 PC(=CHPh)AltBu2 (Mes=2,4,6-trimethylphenyl). Four different gold(I) complexes featuring alkyl (Me), aryl (Ph, C6F5), and alkynyl (C≡CPh) co-ligands have been prepared. X-ray diffraction analyses show that P→Au→Al bridging coordination induces noticeable bending of the ligand (the PCAl bond angle shrinks by 13°). This new type of transition metal→Lewis acid interaction has been analyzed by DFT calculations.