Bite Angle Research Articles

Structural Versatility and Supramolecular Isomerism in Redox‐Active Tetra‐ and Hexaruthenium Macrocycles

We report on six macrocyclic tetra‐ and hexaruthenium complexes formed by the self‐assembly of 2,5‐divinylthiophene‐ or 2,5‐divinylfuran‐bridged diruthenium and 2,5‐thiophene‐, ‐furan‐ or ‐pyrroledicarboxylate linkers. All complexes were scrutinized by NMR spectroscopy and UHR ESI‐MS, cyclic and square wave voltammetry and, in five cases, by X‐ray diffraction analyses. Although the utilized building blocks differ only slightly with respect to their intrinsic bite angles, the resulting macrocycles exhibit remarkable structural versatility. Electrolysis inside an optically transparent electrochemical (OTTLE) cell provided their associated di‐/tri‐ and tetra‐/hexacations, which were studied by IR, UV/Vis/NIR and EPR spectroscopy. The divinylthiophene‐furandicarboxylate complex 2‐TF provides a rare example of supramolecular isomerism in metallamacrocyclic complexes. Thus, hexanuclear 2‐TF6 is initially formed as a kinetic isomer, which subsequently transforms slowly and cleanly into tetranuclear 2‐TF4.

Mechanism and Origin of Ligand-Controlled Chemo- and Regioselectivities in Palladium-Catalyzed Methoxycarbonylation of Alkynes.

Pd-catalyzed alkoxycarbonylation of alkynes provided a redox-neutral method to selectively access branched/linear α,β-unsaturated monoesters and 1,4-dicarboxylic acid diesters. Herein, a systematic computational study was performed to elucidate the mechanism and origin of ligand-controlled chemo- and regioselectivities. It is found that the catalytic cycle, including hydrometallation, carbon monoxide insertion, and methanolysis, is more likely than that involving palladium alkoxycarbonyl intermediates. Both hydrometallation and methanolysis stages are important to determine the chemo- and regioselectivities. Hydrometallation proceeds via anti-Markovnikov-selective migratory insertion or Markovnikov-selective ligand-participated electrophilic addition. A flexible bidentate phosphine ligand slows down migratory insertion due to the stronger trans effect of the CO ligand but accelerates the ligand-participated electrophilic addition by adopting better orbital orientations. On the other hand, a ligand-participated mechanism and an unrevealed mechanism involving ketene intermediates can promote methanolysis, whereas ligands with large bite angles or bulky substituents are detrimental to methanolysis. On the basis of these mechanistic foundations, the influence of the flexibility, basicity, bite angle, and steric hindrance of ligands on chemo- and regioselectivities was clarified. The present study provided more universal and deeper mechanistic insights into Pd-catalyzed alkoxycarbonylation reactions and shed light on the superior regulation performance of the bifunctional pyridyl-containing phosphine ligands.

Construction of SCO-Active Fe(II) Mononuclear Complexes from the Thio-pybox Ligand.

The development of new spin-crossover complexes provides novel promising switching materials with significant potential at the molecular level. Ter-imine-type molecules represent one of the important classes of ligands in creating SCO-active complexes. Herein we report a family of mononuclear Fe(II) SCO-active compounds constructed from a new type of ter-imine ligand named the thio-pybox ligand (2,6-bis(4,4-dimethyl-4,5-dihydrothiazol-2-yl)pyridine, L1). Through the variation of counteranions, some cases display complete SCO and with T1/2 near ambient temperature. Among them, annealed [FeII(L1)2](ClO4)2 [1(ClO4)] shows T1/2↓ and T1/2↑ as 319 and 349 K, respectively. The wide thermal hysteresis of ΔT = 30 K originated from the weak interaction between complex cations and counteranions in the crystal lattice. Impressively, its high-spin population can be increased considerably by annealing at high temperature. The metastable high-spin phase is stable in the successive magnetic measurements and would gradually relax to its initial state with high population of low-spin configuration at ambient temperature. In acetonitrile-diluted solution, 1(ClO4) still maintains SCO with an estimated T1/2 at 240 K. Differential scanning calorimetry discloses the structural phase at around 355 K in the first heating process and the increase in the high-spin population concomitant with annealing was also corroborated by 57Fe Mössbauer measurements. Additionally, the influences on SCO by counteranion and ligand structure are investigated, which show that the fine tuning of complex structures can affect the behavior of the spin state significantly. Finally, magneto-structural correlation studies were performed on the structures of 1(ClO4) and its oxygen analogue at multiple temperatures. The analyses of some structural parameters, including terminal N···N donor separation, bite angle, patulous angle, and the root mean squared deviation indicate that the replacement of the oxygen atom with a sulfur atom can effectively improve the flexibility and release the steric strain and thus tune the SCO toward ambient temperature. Our research demonstrates the rational design of the ligand can lead to new SCO-active compounds with high performance.

Synthesis and crystal structures of tetra-meric [2-(4,4-dimethyl-2-oxazolin-2-yl)anilido]sodium and tris-[2-(4,4-dimethyl-2-oxazolin-2-yl)anilido]ytterbium(III).

Reaction of 2-(4,4-dimethyl-2-oxazolin-2-yl)aniline (H2-L1) with one equivalent of Na[N(SiMe3)2] in toluene afforded pale-yellow crystals of tetra-meric poly[bis-[μ3-2-(4,4-dimethyl-2-oxazolin-2-yl)anilinido][μ2-2-(4,4-dimethyl-2-oxa-zolin-2-yl)aniline]tetra-sodium(I)], [Na4(C11H13N2O)4] n or [Na4(H-L1)4] n (2), in excellent yield. Subsequent reaction of [Na4(H-L1)4] n (2) with 1.33 equivalents of anhydrous YbCl3 in a 50:50 mixture of toluene-THF afforded yellow crystals of tris-[2-(4,4-dimethyl-2-oxazolin-2-yl)anilinido]ytterbium(III), [Yb(C11H13N2O)3] or Yb(H-L1)3 (3) in moderate yield. Direct reaction of three equivalents of 2-(4',4'-dimethyl-2'-oxazolin-yl)aniline (H2-L1) with Yb[N(SiMe3)2]3 in toluene resulted in elimination of hexa-methyl-disilazane, HN(SiMe3)2, and produced Yb(H-L1)3 (3) in excellent yield. The structure of 2 consists of tetra-meric Na4(H-L1)4 subunits in which each Na+ cation is bound to two H-L1 bridging bidentate ligands and these subunits are connected into a polymeric chain by two of the four oxazoline O atoms bridging to Na+ cations in the adjacent tetra-mer. This results in two 4-coordinate and two 5-coordinate Na+ cations within each tetra-meric unit. The structure of 3 consists of a distorted octa-hedron where the bite angle of ligand L1 ranges between 74.72 (11) and 77.79 (11) degrees. The oxazoline (and anilide) N atoms occupy meridional sites such that for one ligand an anilide nitro-gen is trans to an oxazoline nitro-gen while for the other two oxazoline N atoms are trans to each other. This results in a significantly longer Yb-N(oxazoline) distance [2.468 (3) Å] for the bond trans to the anilide compared to those for the oxazoline N atoms trans to one another [2.376 (3), 2.390 (3) Å].

Crystal structures and Hirshfeld surface analysis of (cat)2[FeIII(η3-dipic)(NCS)3] and (cat)[FeIII(η3-dipic)(H2O)(NCS)2] (dipic = dipicolinate)

Crystal and molecular structures of black (cat)2[FeIII(η3-dipic)(NCS)3] (cat = C9H7NH; C10H9NH); (HNC10H8NH)[FeIII(dipic)(NCS)3], and (C6H7NH)[FeIII(η3-dipic)(H2O)(NCS)2] (dipic pyridine-2,6-dicarboxylate (−2); C9H7NH = quinolinium; C10H9NH = 3-methylisoquinolinium; HNC10H8NH = 4,4-bipyridinium; C6H7NH = 2-picolinium) were determinated. These complexes crystallized in monoclinic crystal class. Each Fe(III) atom is six coordinated. The octahedron is build up unsymmetrically, one half side is creates by terdentate dipic–1O,2N,3O ligand and the other half by three monodentate (NCS)3 or (NCS)2(H2O) donor ligands. The terdentate dipic–1O,2N,3O ligand forms a pair of five-membered metallocyclic rings with the common central 2N atom of pyridine (1OC22NC23O) with the mean value of O–Fe–N bite angles of 75.5°. The mean value of trans1O–Fe–3O bond angles is 150.8° in complexes with an inner coordination spheres FeO2N4 and FeO3N3. The mean value of sum of all three ((Fe–1O) + (Fe–2N) + (Fe–3O)) bond distances is 6.217 Å (FeO2N4) and 6.183 Å (FeO3N3). The opposite side in FeO2N4 is created by three monodentate NCS groups with the sum of Fe–N(x3) bond distances of 6.050 Å and in the FeO3N3 two NCS groups with water create opposite side with the sum of (Fe–N(x2) + Fe–O) bond distances of 6.183 Å. One polymorph of complex (C10H9NH)2[FeIII(dipic)(NCS)3] contains two crystallographically independent molecules differ mostly by degree of distortion in Fe–L bond distances and L–Fe–L bond angles and are examples of distortion isomerism. The data of the complexes are compare and discuses with the data of familiar iron(III) complexes. The intermolecular interactions of all compounds are studied by Hirshfeld surface analysis.

Synthesis of difluoromethylated benzylborons via rhodium(I)-catalyzed fluorine-retainable hydroboration of gem-difluoroalkenes

The synthesis of borylated organofluorines is of great interest due to their potential values as synthons in modular construction of fluorine-containing molecules. Reported herein is a rhodium-catalyzed hydroboration of aryl gem-difluoroalkenes leading to a series of α-difluoromethylated benzylborons. The use of cationic rhodium catalyst and a biphosphine ligand with large bite angle was crucial for reactivity by offering good regioselectivity and diminishing the undesired β-F elimination. Preliminary derivatizations of the products were conducted to showcase the utility of this protocol.

Redox-switchable carboranes for uranium capture and release.

The uranyl ion (UO22+; U(VI) oxidation state) is the most common form of uranium found in terrestrial and aquatic environments and is a central component in nuclear fuel processing and waste remediation efforts. Uranyl capture from either seawater or nuclear waste has been well studied and typically relies on extremely strong chelating/binding affinities to UO22+ using chelating polymers1,2, porous inorganic3-5 or carbon-based6,7 materials, as well as homogeneous8 compounds. By contrast, the controlled release of uranyl after capture is less established and can be difficult, expensive or destructive to the initial material2,9. Here we show how harnessing the redox-switchable chelating and donating properties of an ortho-substituted closo-carborane (1,2-(Ph2PO)2-1,2-C2B10H10) cluster molecule can lead to the controlled chemical or electrochemical capture and release of UO22+ in monophasic (organic) or biphasic (organic/aqueous) model solvent systems. This is achieved by taking advantage of the increase in the ligand bite angle when the closo-carborane is reduced to the nido-carborane, resulting in C-C bond rupture and cage opening. The use of electrochemical methods for uranyl capture and release may complement existing sorbent and processing systems.

Structural evolution of the Ru-bms complex to the real water oxidation catalyst of Ru-bda: the bite angle matters.

Ru-Based complexes have advanced the study of molecular water oxidation catalysts (WOCs) both in catalysis and mechanism. The electronic effect has always been considered as an essential factor for the catalyst properties while less attention has been focused on the bite angle effect on water oxidation catalysis. The Ru-bda ([Ru(bda)(pic)2]; bda2- = 2,2'-bipyridine-6,6'-dicarboxylate; pic = 4-picoline) catalyst is one of the most active WOCs and it has a largely distorted octahedral configuration with an O-Ru-O bite angle of 123°. Herein, we replaced the carboxylate (-COO-) groups of bda2- with two methylenesulfonate (-CH2SO3-) groups and prepared a negatively charged ligand, bms2- (2,2'-bipyridine-6,6'-dimethanesulfonate), and the Ru-bms complex [Ru(bms)(pic)2]. The O-Ru-O bite angle changed from 123° in Ru-bda to 84° in Ru-bms, leading to a dramatic influence on the catalytic behavior. Systematic analysis of the reaction intermediates suggested that Ru-bms transformed all the way to Ru-bdavia oxidative decomposition under CeIV-driven water oxidation conditions.

Ligand-controlled cobalt-catalyzed regiodivergent hydroboration of aryl,alkyl-disubstituted internal allenes.

We report a stereoselective regiodivergent hydroboration of aryl,alkyl-disubstituted internal allenes with pinacolborane (HBpin) in the presence of cobalt catalysts generated from bench-stable Co(acac)2 and bisphosphine ligands. An interesting correlation between the regioselectivity of this hydroboration and the bite angles of bisphosphine ligands was identified. When hydroboration was conducted with cobalt catalysts containing bisphosphines with medium bite angles (e.g. 98° for dppb and 96° for dppf), HBpin was selectively added to the alkyl-substituted double bond. However, HBpin was selectively added to the aryl-substituted double bond when the reactions were conducted with cobalt catalysts containing bisphosphines with large bite angles (e.g. 111° for xantphos and 114° for Nixantphos). A range of internal allenes underwent these Co-catalyzed hydroboration reactions in a regiodivergent manner to yield the corresponding (Z)-alkenylboronates in high isolated yields and with high regioselectivity. These reactions show good functional group compatibility and can be readily scaled up to gram scales without using a dry box. In addition, the comparison of regioselectivity between the Co-catalyzed hydrosilylation and hydroboration reactions of the same allene substrate suggests that this Co-catalyzed regiodivergent hydroboration of allenes proceeds through a Co-Bpin intermediate. Deuterium-labeling experiments suggest that the Co-Bpin intermediates react with allenes to form allylcobalt species which then react with HBpin to release (Z)-alkenylboronate products.

Structural, electrochemical and photophysical behavior of Ru(ii) complexes with large bite angle sulfur-bridged terpyridyl ligands

Sulfur-bridged terpyridyl ligands expand the bite angles in Ru(ii) species leading to geometries very close to that of a “perfect” octahedron. Altering the donor strength of substituents results in systematic tuning of the redox properties.

Origin of the ligand effect in the cobalt catalyzed regioselective hydroboration of 1,3-diene.

Hydroboration of 1,3-dienes can provide useful intermediates with multiple functionalities. However, achieving high regioselectivity is still a challenge. Recent experimental research studies indicate that this challenge could be overcome by the ligand effect. We made DFT calculations to elucidate the origin of ligand controlled regioselectivity in cobalt catalyzed hydroboration of 2-substituted 1,3-diene. The following conclusions have been reached: when using PHOX ((2-oxazolinyl)-phenyldiphenylphosphine) as the ligand, the favorable 1,4-selective oxidative hydrogen migration pathway was suggested to start with the rate-determining step of 1,4-selective oxidative hydrogen migration followed by reductive boryl migration. The unique 1,4-selectivity is proposed to be a result of the less steric hindrance between the substrate and the ligand PHOX. When dppp (1,3-bis-(diphenylphosphino)propane) is used as the ligand, the favorable pathway is proposed to be a 1,2-selective oxidative boryl migration pathway which involves 1,2-selective oxidative boryl migration and reductive hydrogen migration. Interestingly, another smaller-bite angle bisphosphine ligand dppe (1,2-bis(diphenylphosphino)ethane) favors the 1,4-selective oxidative boryl migration pathway. DFT calculations revealed that the preferred oxidative boryl migration pathway with both dppp and dppe is attributed to their electron-rich properties which accelerate the oxidative boryl migration step. The larger bite angle of dppp than that of dppe leads to bulkier steric hindrance and promotes 1,2-selective reductive hydrogen migration. On the other hand, for dppe with a smaller bite angle, the steric effect in the reductive hydrogen migration step is not dominant and 1,4-selective reductive hydrogen migration is favored. It is expected that the analysis of the ligand effect on the regioselectivity would enable further catalyst design.

Palladium catalyzed carbonylative generation of potent, pyridine-based acylating electrophiles for the functionalization of arenes to ketones.

We describe here the design of a palladium catalyzed route to generate aryl ketones via the carbonylative coupling of (hetero)arenes and aryl- or vinyl-triflates. In this, the use of the large bite angle Xantphos ligand on palladium provides a unique avenue to balance the activation of the relatively strong C(sp2)–OTf bond with the ultimate elimination of a new class of potent Friedel–Crafts acylating agent: N-acyl pyridinium salts. The latter can be exploited to modulate reactivity and selectivity in carbonylative arene functionalization chemistry, and allow the efficient synthesis of ketones with a diverse array of (hetero)arenes.

Catalytic activity of Pd dithiolate complexes with large‐bite‐angle diphosphines in Heck coupling reactions

Palladium(II) complexes of aryl dithiolates and wide‐bite‐angle diphosphines Xantphos and dppf have been developed as efficient catalysts in Suzuki and Suzuki carbonylation reactions. The catalytic activity of these highly stable, discrete and charged complexes was investigated in Heck coupling reactions of styrene and a variety of aryl bromides. Under optimized reaction conditions these palladium complexes showed excellent activity with high turnover number (6 × 106) and high turnover frequency (4 × 105 h−1). The effect of bite angle of diphosphines on the catalytic activity of the complexes [Pd2(P∩P)2(SC12H8S)]2(OTf)4 followed the trend P∩P = Xantphos > dppf > dppe as the order of their bite angles. The catalyst could be reused, and after three cycles the formation of significant amount of Pd nanoparticles was noticed, which were characterized using powder X‐ray diffraction, energy‐dispersive X‐ray analysis and transmission electron microscopy. The high catalytic activity has been attributed to the Pd nanoparticles.

Butadiene hydroformylation to adipaldehyde with Rh-based catalysts: Insights into ligand effects

Rh-catalyzed hydroformylation of butadiene to adipaldehyde is a promising alternative route for producing valuable C6 compounds such as adipic acid and hexamethylenediamine. Fundamental insights into reaction pathways, aimed at enhancing adipaldehyde yield, were obtained from temporal concentration profiles and in situ ReactIR studies of butadiene hydroformylation on Rh complexes at 80 °C and 14 bar syngas (molar CO/H2 = 1) pressure in a batch reactor. Specifically, the effects of operating conditions and eight commercially available ligands on activity and selectivity were systematically investigated. It was found that the adipaldehyde selectivity is independent of the ligand/Rh ratio, rhodium concentration, butadiene concentration and syngas pressure, but significantly dependent on the type of ligand used. For example, while the DIOP ligand provided an adipaldehyde yield of ∼40% with butadiene as a substrate, the 6-DPPon ligand gave a maximum adipaldehyde yield of ∼93% with 4-pentenal as substrate. Furthermore, the adipaldehyde selectivity correlates well with the natural bite angle of the various ligands. ReactIR studies suggest that the preferential formation of the stable rhodium η3-crotyl complex with the various Rh complexes may be the main reason for the low adipaldehyde selectivity.

μ2-Methanol-κ2 O:O-bis[(1,10-phenanthroline-κ2 N,N′)bis(2,3,4,5-tetrafluorobenzoato)-κO;κ2 O,O′-copper(II)

In the title compound, [Cu2(C7HF4O2)4(C12H8N2)2(CH3OH)], the molecule lies on a twofold rotation axis in space group C2/c. The Cu2+ ion exhibits a distorted octahedral sphere with two N atoms from the phenanthroline ligand, three O atoms from the 2,3,4,5-tetrafluorobenzoate ligands and one O atom from a methanol molecule. The distortion from an octahedral shape is a consequence of the Jahn–Teller effect of CuII and the small bite angle for the bidentate fluorobenzoate ligand [54.50 (11)°]. The methanol molecule bridges two symmetry-related CuII atoms to form the complete molecule. In the bidentate fluorobenzoate ligand, one F atom is disordered over two positions of equal occupancy. In the crystal structure, only weak intermolecular interactions are observed.

Bis-tridentate IrIII Phosphors Bearing Two Fused Five-Six-Membered Metallacycles: A Strategy to Improved Photostability of Blue Emitters.

Iridium complexes bearing chelating cyclometalates are popular choices as dopant emitters in the fabrication of organic light-emitting diodes (OLEDs). In this contribution, we report a series of blue-emitting, bis-tridentate IrIII complexes bearing chelates with two fused five-six-membered metallacycles, which are in sharp contrast to the traditional designs of tridentate chelates that form the alternative, fused five-five metallacycles. Five IrIII complexes, Px-21-23, Cz-4, and Cz-5, have been synthesized that contain a coordinated dicarbene pincer chelate incorporating a methylene spacer and a dianionic chromophoric chelate possessing either a phenoxy or carbazolyl appendage to tune the coordination arrangement. All these tridentate chelates afford peripheral ligand-metal-ligand bite angles of 166-170°, which are larger than the typical bite angle of 153-155° observed for their five-five-coordinated tridentate counterparts, thereby leading to reduced geometrical distortion in the octahedral frameworks. Photophysical measurements and TD-DFT studies verified the inherent transition characteristics that give rise to high emission efficiency, and photodegradation experiments confirmed the improved stability in comparison with the benchmark fac-[Ir(ppy)3 ] in degassed toluene at room temperature. Phosphorescent OLED devices were also fabricated, among which the carbazolyl-functionalized emitter Cz-5 exhibited the best performance among all the studied bis-tridentate phosphors, showing a maximum external quantum efficiency (EQEmax ) of 18.7 % and CIEx,y coordinates of (0.145, 0.218), with a slightly reduced EQE of 13.7 % at 100 cd m-2 due to efficiency roll-off.

Influence of the Bite Angle of Dianionic C,N,C-Pincer Ligands on the Chemical and Photophysical Properties of Iridium(III) and Osmium(IV) Hydride Complexes

The study of the reactions of IrH5(PiPr3)2 (1) with 2,6-diphenylpyridine and 2-phenoxy-6-phenylpyridine and the photophysical characterization of some of the resulting compounds, as well as their o...

Gadolinium Complexes of Highly Rigid, Open-Chain Ligands Containing a Cyclobutane Ring in the Backbone: Decreasing Ligand Denticity Might Enhance Kinetic Inertness.

In an effort to explore novel ligand scaffolds for stable and inert lanthanide complexation in magnetic resonance imaging contrast agent research, three chiral ligands containing a highly rigid (1S,2S)-1,2-cyclobutanediamine spacer and different number of acetate and picolinate groups were efficiently synthesized. Potentiometric studies show comparable thermodynamic stability for the Gd3+ complexes formed with either the octadentate (L3)4- bearing two acetate or two picolinate groups or the heptadentate (L2)4- analogue bearing one picolinate and three acetate groups (log KGdL = 17.41 and 18.00 for [Gd(L2)]- and [Gd(L3)]-, respectively). In contrast, their dissociation kinetics is revealed to be very different: the monohydrated [Gd(L3)]- is considerably more labile, as a result of the significant kinetic activity of the protonated picolinate function, as compared to the bishydrated [Gd(L2)]-. This constitutes an uncommon example in which lowering ligand denticity results in a remarkable increase in kinetic inertness. Another interesting observation is that the rigid ligand backbone induces an unusually strong contribution of the spontaneous dissociation to the overall decomplexation process. Thanks to the presence of two inner-sphere water molecules, [Gd(L2)]- is endowed with high relaxivity (r1 = 7.9 mM-1 s-1 at 20 MHz, 25 °C), which is retained in the presence of large excess of endogenous anions, excluding ternary complex formation. The water exchange rate is similar for [Gd(L3)]- and [Gd(L2)]-, while it is 1 order of magnitude higher for the trishydrated tetraacetate analogue [Gd(L1)]- (kex298 = 8.1, 10, and 127 × 106 s-1, respectively). A structural analysis via density functional theory calculations suggests that the large bite angle imposed by the rigid (1S,2S)-1,2-cyclobutanediamine spacer could allow the design of ligands based on this scaffold with suitable properties for the coordination of larger metal ions with biomedical applications.

The crystal structure of ((cyclo-hexyl-amino){(Z)-2-[(E)-5-meth-oxy-3-nitro-2-oxido-benzyl-idene-κO]hydrazin-1-yl-idene-κN 2}methane-thiol-ato-κS)(dimethyl sulfoxide-κS)platinum(II): a supra-molecular two-dimensional network.

The PtII atom in the title complex, [Pt(C15H18N4O4S)(C2H6OS)], exists within a square-planar NS2O donor set provided by the N, S, O atoms of the di-anionic tridentate thio-semicarbazo ligand and a dimethyl sulfoxide S atom. The two chelate rings are coplanar, subtending a dihedral angle of 1.51 (7)°. The maximum deviation from an ideal square-planar geometry is seen in the five-membered chelate ring with an S-Pt-S bite angle of 96.45 (2)°. In the crystal, mol-ecules are linked via N-H⋯O, C-H⋯O, C-H⋯N and C-H⋯π inter-actions into two-dimensional networks lying parallel to the ab plane. The conformations of related cyclo-hexyl-hydrazine-1-carbo-thio-amide ligands are compared to that of the title compound.

Intriguing Behavior of an Apparently Simple Coupling Promoter Ligand, PPh2(p-C6H4–C6F5), in Their Pd Complexes

Ligand PPh2(p-C6H4–C6F5), LHF, is an example of monodentate biphenyl phosphine that allows for cis coordination of two phosphines to Pd, as in complex cis-[PdPf2Pd(LHF)2] (A) (Pf = C6F5). At 25 °C, complex A undergoes easy reductive elimination to decafluorobiphenyl and, competitively, isomerizes to trans-[Pd(C6F5)2Pd(LHF)2] via functionally three-coordinate intermediates cis-[PdPf2(LHF)(S)] with the fourth position empty or weakly protected (S= THF, OH2, or π-aryl). Unexpectedly, the direct reductive C6F5–C6F5 elimination is faster from the four-coordinate complex A than from the intermediates with only one strong LHF. The reason for this is that two cis LHF ligands play the role of a chelate with a large bite angle and some tetrahedral distortion. As a matter of fact, using LHF in excess (Pd:L ≪ 1:2), a Pf–Pf coupling barrier ΔG‡Pf–Pf = 23.1 kcal·mol–1 is measured, which ranks its efficiency for coupling and formation of the corresponding Pd0 catalyst as better than XantPhos or PhPEWO-F and about the sa...