Ghenwa Bouhadir

Metallaboratranes Derived from a Triphosphanyl–Borane: Intrinsic C3 Symmetry Supported by a Z‐Type Ligand

Following the pioneering contributions of Knowles, Kagan, and Noyori, C1 and C2 chiral ligands have played a prominent role in asymmetric catalysis with transition-metal complexes. Over the last few years, increasing attention has been devoted to C3-symmetric derivatives, [2] and spectacular achievements have been reported using facially coordinating tripodal ligands assembled around a remote junction point (which does not enter the coordination sphere), an L-type coordination site, or an X-type coordination site. The trisoxazoline, trisamido, and triphosphane complexes A–C are archetypal examples of these three different situations (Scheme 1). Our interest in ambiphilic ligands that combine donor and acceptor moieties prompted us to investigate the ability of s-acceptor (Z-type) coordination sites to also support such a three-fold geometry. Accordingly, an L3Z tetradentate triphosphanyl–borane (TPB) ligand is reported herein to afford gold and platinum metallaboratranes D featuring dative M!B interactions and exhibiting C3 symmetry. [11–14] Such helical geometry has been shown to result from the tendency of the PCCBM metalla-

Gold–Silane and Gold–Stannane Complexes: Saturated Molecules as σ‐Acceptor Ligands

The discovery that saturated molecules may form s complexes by side-on coordination of a s bond to a transition metal represents a major breakthrough in transition-metal chemistry. Over the years, considerable progress has been made in the understanding of this bonding situation. The coordination and activation of s bonds involving Group 14 elements (E = C, Si, Ge, Sn, Pb) is at the forefront of developments in this area. An increasing variety of complexes A and B 7] (Scheme 1) featuring side-on coordinated s(E H) and s(E E) bonds have been isolated, and the key factors governing the delicate balance between dissociation and oxidative addition have been progressively identified. The common feature and believed prerequisite for the coordination of saturated molecules free of lone pairs to transition metals is the superposition of ligand!metal donation (from a filled s orbital of the ligand to an empty d orbital of the metal) and metal!ligand back-donation (from a filled d orbital of the metal to an empty s* orbital of the ligand). Aiming at identifying new types of metal–ligand interactions, and stimulated by our work on Group 13 Lewis acids as acceptor ligands, we recently became interested in complexes of type C, in which a saturated Group 14 element could behave as an end-on, s-acceptor ligand toward a transition metal. Heavier Group 14 elements such as silicon and tin are known to readily form hypervalent compounds through donor!acceptor interactions with organic Lewis bases. A related situation is envisioned in complexes C, with a transition metal acting as a Lewis base. Such donor! acceptor interactions between transition metals and silanes or stannanes have been invoked in a few highly strained complexes on the basis of relatively short M E distances. Furthermore, the presence of a Pd!Sn dative bond was recently evidenced structurally and theoretically in a palladastannatrane cage complex supported by four methimazolyl groups. Herein, we report the straightforward synthesis and complete characterization of three gold complexes supported by diphosphino silane and stannane ligands. The presence of metal!silane and metal!stannane interactions in these complexes has been substantiated spectroscopically, structurally, and theoretically, thus providing unambiguous evidence for the existence of complexes of type C. From our previous studies on Group 13 Lewis acids, the use of two phosphine buttresses ligated by ortho-phenylene spacers was considered as a good compromise in order to support, but not impose, the coordination of the Group 14 element. We thus targeted the two complexes [o{(iPr2P)C6H4}2E(Ph)FAuCl] 2 (E = Si) and 4 (E = Sn). The Scheme 1. Complexes A–C featuring Group 14 saturated molecules coordinated to transition metals (E= C, Si, Ge, Sn, Pb).

Copper(I) complexes derived from mono- and diphosphino-boranes: Cu-->B interactions supported by arene coordination.

The monophosphino-boranes o-iPr(2)P(C(6)H(4))BR(2) (1: R = Ph and 3: R = Cy) and diphosphino-boranes [o-R(2)P(C(6)H(4))](2)BPh (5: R = Ph and 6: R = iPr) readily react with CuCl to afford the corresponding complexes {[o-iPr(2)P(C(6)H(4))BPh(2)]Cu(mu-Cl)}(2) 2, {[o-iPr(2)P(C(6)H(4))BCy(2)]Cu(mu-Cl)}(2) 4, {[o-Ph(2)P(C(6)H(4))](2)BPh}CuCl 7, and {[o-iPr(2)P(C(6)H(4))](2)BPh}CuCl 8. The presence of Cu-->B interactions supported by arene coordination within complexes 2, 7, and 8 has been unambiguously evidenced by NMR spectroscopy and X-ray diffraction studies. The unique eta(2)-BC coordination mode adopted by complexes 7 and 8 has been thoroughly analyzed by density-functional theory (DFT) calculations.

Gold(I) Complexes of Phosphanyl Gallanes: From Interconverting to Separable Coordination Isomers†

Gallant exchange: Upon coordination of phosphanyl gallane ligands to AuCl, both neutral and zwitterionic complexes coexist. NMR spectroscopy provides direct evidence for the transfer of the chloride between gold and gallium in diphosphanyl gallane. The introduction of a third phosphanyl buttress allows the separation and structural characterization of the two coordination isomers (see picture; Au yellow, P red, Cl green, Ga blue).

Reaction of Singlet Dioxygen with Phosphine–Borane Derivatives: From Transient Phosphine Peroxides to Crystalline Peroxoboronates†

The last two decades has witnessed a spectacular renaissance in main group chemistry. Of particular interest are ambiphilic compounds that result from the combination of donor and acceptor moieties, typically involving Group 13 and 15 elements. Most strikingly, the groups of D. W. Stephan and G. Erker have shown that phosphine–boranes R2P-spacerB(C6F5)2 (R = tBu or Mes (2,4,6-Me3C6H2); spacer = p-C6H4 or CH2CH2) reversibly activate dihydrogen under mild conditions, opening new avenues in metal-free catalytic hydrogenation. Recently, reversible binding of carbon dioxide has also been reported. Moreover, our group has evidenced versatile coordination properties for monoand polyphosphine–boranes featuring o-C6H4 linkers, which provide a better understanding of unusual interactions between transition metals and Lewis acids. The borane moiety ortho to phosphorus was also exploited to stabilize highly reactive adducts of phosphines. Accordingly, the reaction of R2P(o-C6H4)BMes2 with phenyl azide was shown to give a phosphazide that is dramatically stabilized by intramolecular a N!B interaction that undergoes an unusual photoisomerization process. Furthermore, treatment of the o-phenylene phosphine–borane system with diethyl azodicarboxylate and phenyl isocyanate afforded stable versions of the zwitterionic intermediates involved in the Mitsunobu reaction and cyclooligomerization of isocyanates, respectively. Our continued interest in phosphine–borane compounds prompted us to investigate their behavior toward singlet dioxygen. 9] Very little is known about peroxidic adducts between phosphines and O2. Indeed, the phosphadioxirane deriving from the tris(o-methoxyphenyl)phosphine is the only such adduct that has been characterized to date, thanks to low-temperature NMR techniques. Herein we report that the presence of a Lewis acid in close proximity to the phosphorus center dramatically influences the fate of the reaction. In particular, phosphine–boronates are shown to readily split O2. The initially formed phosphine peroxides are stabilized by intramolecular O!B interactions, but spontaneously rearrange by B!O migration to give stable peroxoboronates. We began to investigate the behavior of phosphine– borane compounds towards dioxygen with the previously described phosphine–borane 1. No sign of oxidation was observed after several hours when a toluene solution of 1 was subjected to air bubbling at room temperature. In marked contrast, compound 1 rapidly reacted with O2 (generated by irradiation of O2 in the presence of tetraphenylporphyrin as photosensitizer) to afford a new compound 2 (Scheme 1). The

A Stable but Highly Reactive Phosphine-Coordinated Borenium: Metal-free Dihydrogen Activation and Alkyne 1,2-Carboboration†

Borenium cations have been found to be valuable analogues of boranes as a result of their cationic character which imparts high electrophilicity. Herein, we report the synthesis, characterization, and reactivity of a new type of borenium cation employing a naphthyl bridge and a strong intramolecular P→B interaction. The cation reacts with H2 in the presence of PtBu3 (frustrated Lewis pair (FLP) approach) but also on its own. The mechanism of the reaction between the borenium cation and H2 in the absence of PtBu3 has been investigated using deuterium-labeling experiments and DFT calculations. Both experiments and calculations imply the side-on coordination of H2 to the B center, followed by heterolytic splitting and B-C bond cleavage. An uncommon syn 1,2-carboboration has also been observed upon reaction of the borenium ion with 3-hexyne.

A Significant but Constrained Geometry Pt→Al Interaction: Fixation of CO2 and CS2, Activation of H2 and PhCONH2.

Reaction of the geminal PAl ligand [Mes2PC(═CHPh)AltBu2] (1) with [Pt(PPh3)2(ethylene)] affords the T-shape Pt complex [(1)Pt(PPh3)] (2). X-ray diffraction analysis and DFT calculations reveal the presence of a significant Pt→Al interaction in 2, despite the strain associated with the four-membered cyclic structure. The Pt···Al distance is short [2.561(1) Å], the Al center is in a pyramidal environment [Σ(C-Al-C) = 346.6°], and the PCAl framework is strongly bent (98.3°). Release of the ring strain and formation of X→Al interactions (X = O, S, H) impart rich reactivity. Complex 2 reacts with CO2 to give the T-shape adduct 3 stabilized by an O→Al interaction, which is a rare example of a CO2 adduct of a group 10 metal and actually the first with η(1)-CO2 coordination. Reaction of 2 with CS2 affords the crystalline complex 4, in which the PPtP framework is bent, the CS2 molecule is η(2)-coordinated to Pt, and one S atom interacts with Al. The Pt complex 2 also smoothly reacts with H2 and benzamide PhCONH2 via oxidative addition of H-H and H-N bonds, respectively. The ensuing complexes 5 and 7 are stabilized by Pt-H→Al and Pt-NH-C(Ph) = O→Al bridging interactions, resulting in 5- and 7-membered metallacycles, respectively. DFT calculations have been performed in parallel with the experimental work. In particular, the mechanism of reaction of 2 with H2 has been thoroughly analyzed, and the role of the Lewis acid moiety has been delineated. These results generalize the concept of constrained geometry TM→LA interactions and demonstrate the ability of Al-based ambiphilic ligands to participate in TM/LA cooperative reactivity. They extend the scope of small molecule substrates prone to such cooperative activation and contribute to improve our knowledge of the underlying factors.

Bridging MCl Bonds with Ambiphilic Phosphine–Borane Ligands

A combined experimental/theoretical study provides insight into the bridging coordination of M-Cl bonds with the ambiphilic phosphine-borane ligands iPr(2)P-o(C(6)H(4))-BR(2) (PBCy(2): R = Cy; PBMes(2): R = Mes). Reaction of [PdCl(allyl)(PBCy(2))] (3) with HCl affords the related dinuclear complex [PdCl(mu-Cl)(PBCy(2))](2) (5). Subsequent cleavage of the chloride bridge by PPh(3) leads to the heteroleptic mononuclear complex trans-[PdCl(2)(PPh(3))(PBCy(2))] (6). The solid-state structures of complexes 5 and 6 substantiate the propensity of the PBCy(2) ligand to bridge Pd-Cl bonds via P-->Pd--Cl-->B interactions. DFT calculations carried out on both the model mononuclear complexes 3* and 6* reveal that in each system the energy of the linkage isomer with a Cl-->B interaction is very similar to that without. A comparison of the solution and solid-state (11)B NMR spectroscopic data for complexes 3 and 6 suggests the possible interconversion of the bridging and B-pendant forms in solution. Bridging coordination of the PBCy(2) ligand across a Rh-Cl bond is observed in the solid-state structure of the related complex [RhCl(nbd)(PBCy(2))] (7). Replacement of the Cy groups at boron by Mes substituents illustrates the role of steric factors on the participation of the Lewis acid upon coordination, no Cl-->B interaction being observed in the complex [PdCl(allyl)(PBMes(2))] (8).

A 1,1′-ferrocenyl phosphine-borane: synthesis, structure and evaluation in Rh-catalyzed hydroformylation

The new ambiphilic ligand Ph2P–(1,1′-ferrocenyl)–BMes2, prepared by sequential lithiation/electrophilic trapping of 1,1′-dibromoferrocene, adopts a monomeric structure free of dative P → B and Fe → B interactions. This flexible phosphine-borane and the related o-phenylene bridged system have been evaluated in Rh-catalyzed hydroformylation.

Phosphino-boryl-naphthalenes: geometrically enforced, yet Lewis acid responsive P → B interactions.

Three naphthyl-bridged phosphine-borane derivatives 2-BCy2, 2-BMes2, and 2-BFlu, differing in the steric and electronic properties of the boryl moiety, have been prepared and characterized by spectroscopic and crystallographic means. The presence and magnitude of the P → B interactions have been assessed experimentally and theoretically. The naphthyl linker was found to enforce the P → B interaction despite steric shielding, while retaining enough flexibility to respond to the Lewis acidity of boron.