Bridging Allyl Research Articles

ChemInform Abstract: Dinuclear PdI Complexes with Bridging Allyl and Related Ligands

AbstractReview: synthesis, structure, and reactivity of dinuclear PdI complexes containing bridging allyl or related bridging cyclopentadienyl and/or indenyl ligands;134 refs.

Dinuclear Pd(I) complexes with bridging allyl and related ligands.

There are many important synthetic methods that utilize palladium catalysts. In most of these reactions, the palladium species are proposed to exist exclusively in either the Pd(0) or Pd(II) oxidation states. However, in the last decade, dinuclear Pd(I) complexes have repeatedly been isolated from reaction mixtures previously suggested to involve only species in the Pd(0) and Pd(II) oxidation states. As a consequence, in order to design improved catalysts there is considerable interest in understanding the chemistry of dinuclear Pd(I) complexes. A significant proportion of the known dinuclear Pd(I) complexes are supported by bridging allyl or related ligands such as cyclopentadienyl or indenyl ligands. This review provides a detailed account of the synthesis, electronic structure and stoichiometric reactivity of dinuclear Pd(I) complexes with bridging allyl and related ligands. Additionally, it describes recent work where dinuclear Pd(I) complexes with bridging allyl ligands have been detected in catalytic reactions, such as cross-coupling, and discusses the potential implications for catalysis.

Synthesis and Properties of NHC-Supported Palladium(I) Dimers with Bridging Allyl, Cyclopentadienyl, and Indenyl Ligands

The synthesis of a family of new Pd(I) dimers, (μ-All)(μ-Cp){Pd(IPr)}2 (All = C3H5, Cp = C5H5, IPr = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene), (μ-All)(μ-Ind){Pd(IPr)}2 (Ind = C7H9), (μ-Cp)(μ-Cp){Pd(IPr)}2, (μ-Cp)(μ-Ind){Pd(IPr)}2, and (μ-Ind)(μ-Ind){Pd(IPr)}2, which contain a combination of bridging allyl, Cp, and indenyl ligands and are all supported by IPr as the ancillary ligand, is reported. All of these compounds are thermally stable at room temperature, and the solid-state geometries, electronic structures, reactivity, and redox chemistry of these new compounds have been compared with those of the dimer (μ-All)2{Pd(IPr)}2, which was previously reported. This work provides further evidence that bridging allyl, Cp, and indenyl ligands bind in a similar manner to Pd(I). However, it is demonstrated that there are notable differences between the IPr-supported species and related Pd(I) dimers with triethylphosphine ancillary ligands, which have been previously described.

Synthesis, Electronic Structure, and Reactivity of Palladium(I) Dimers with Bridging Allyl, Cyclopentadienyl, and Indenyl Ligands

The synthesis of three new Pd(I) dimers, (μ-All)(μ-Cp){Pd(PEt3)}2 (All = C3H5, Cp = C5H5), (μ-All)(μ-Ind){Pd(PEt3)}2 (Ind = C7H9), and (μ-Cp)(μ-Ind){Pd(PEt3)}2, which contain a combination of bridging allyl, Cp, or indenyl ligands and are all supported by triethylphosphine as the ancillary ligand, is reported. The solid-state geometries, electronic structures, and reactivity of these new compounds have been compared with those of the dimers (μ-All)2{Pd(PEt3)}2 and (μ-Cp)2{Pd(PEt3)}2, which have previously been reported. This work establishes that there are many similarities in the solid-state and electronic structures of complexes containing bridging allyl, Cp, or indenyl ligands. For example, in all cases the bridging ligands bind through three carbon atoms to the two Pd atoms, with only the central carbon atom of the bridging group bound to both metal centers. However, there are also important differences based on the identity of the bridging ligand. As a result of different overlap between the metal ce...

Rhodium and Iridium Complexes with a New Scorpionate Phosphane Ligand

A straightforward synthesis of a new hybrid scorpionate ligand [(allyl)2B(CH2PPh2)(Pz)](-) ([A2BPN](-)) is reported. Coordination to rhodium resulted in square-planar complexes [Rh(κ(2)-A2BPN)(L)(L')] [L = L' = (1)/2cod (1,5-cyclooctadiene), CN(t)Bu, CO (6); L = CO, L' = NH3, pyridine, PPh3, PMe3] for which spectroscopic data and the molecular structure of [Rh(κ(2)-A2BPN)(CO)PPh3] (11) indicate the ligand to be κN,κP-bound to rhodium with two dangling free allyl groups. Studies in solution point out that the six-membered Rh-N-N-B-C- P metallacycle undergoes a fast inversion in all of them. The bis(carbonyl) complex 6 easily loses a CO group to give [{Rh(A2BPN)(CO)}2], a dinuclear compound in which two mononuclear subunits are brought together by two bridging allyl groups. Coordination to iridium is dominated by a tripodal κN,κP,η(2)-C═C binding mode in the TBPY-5 complexes [Ir(κ(3)-A2BPN)(L)(L')] [L = L' = (1)/2cod (3), CN(t)Bu (5), CO (7); L = CO, L' = PPh3 (13), PMe3 (14), H2C═CH2, (17), MeO2CC≡CCO2Me (dmad, 18)], as confirmed by the single-crystal structure determination of complexes 3 and 18. A fast exchange between the two allyl arms is observed for complexes having L = L' (3, 5, and 7), while those having CO and L ligands (14, 17, and 18) were found to be nonfluxional species. An exception is complex 13, which establishes an equilibrium with the SP-4 configuration. Protonation reactions on complexes 13 and 14 with HCl yielded the hydride complex [Ir(κ(2)-A2BPN)(CO)(Cl)(H)PPh3] (15) and the C-alkyl compound [ Ir{κ(3)-(allyl)B(CH2 CHCH3)(CH2PPh2)(Pz)}(Cl)(CO)PMe3] (16), respectively. The bis(isocyanide) complex 5 reacts with dmad to form [Ir(κ(2)-A2BPN)(CN(t)Bu)2(dmad)]. On the whole, the electronic density provided to the metal by the [A2BPN](-) ligand is very sensitive to the coordination mode. The basicity of the new ligand is similar to that of the Tp(Me2) ligand in the κN,κP mode but comparable to Tp if coordinated in the κN,κP,η(2)-C═C mode.

Photoelectron Spectroscopy of Palladium(I) Dimers with Bridging Allyl Ligands

The dianionic PdI dimers [TBA]2[(TPPMS)2Pd2(μ-C3H5)2] (1) [TBA = tetrabutylammonium, TPPMS = PPh2(3-C6H4SO3)−] and [TBA]2[(TPPMS)2Pd2(μ-C3H5)(μ-Cl)] (2), containing two bridging allyl ligands and one bridging allyl ligand and one bridging chloride ligand, respectively, were synthesized. The electronic structures of these complexes were investigated by combining electrospray mass spectrometry with gas phase photodetachment photoelectron spectroscopy. The major difference between the photoelectron spectra of the anions of 1 and 2 is the presence of a low-energy detachment band with an adiabatic electron detachment energy of 2.44(6) eV in 1, which is not present in 2. The latter has a much higher adiabatic electron detachment energy of 3.24(6) eV. Density functional theory calculations suggest that this band is present in 1 due to electron detachment from the out-of-phase combination of the π2 orbitals, which are localized on the terminal carbon atoms of the bridging allyl ligands. In 2, the Pd centers stabi...

Binuclear allyliron carbonyls: Fragile dimers and diverse types of allyl groups

The known (η3-C3H5)2Fe2(CO)6 is an interesting unbridged “fragile dimer” having an unusually long ∼3.1Å Fe–Fe bond, which readily breaks to form paramagnetic (C3H5)Fe(CO)3 monomers. Theoretical studies confirm this experimental (η3-C3H5)2Fe2(CO)6 structure but also predict higher energy (η3-C3H5)2Fe2(CO)4(μ-CO)2 structures with two bridging carbonyl groups and Fe2(η3-μ-C3H5)2(CO)6 structures with two bridging allyl groups. For the pentacarbonyl (C3H5)2Fe2(CO)5 low energy structures with four-electron donor bridging CO groups and structures with FeFe formal double bonds are both found. Both types of structures satisfy the 18-electron rule. The lowest energy structures for the tetracarbonyl (C3H5)2Fe2(CO)4 have Fe–Fe single bonds and obvious gaps in the iron coordination spheres consistent with an iron 16-electron configuration. However, a higher energy (C3H5)2Fe2(CO)4 structure is found with the short (∼2.3Å) formal FeFe triple bond predicted by the 18-electron rule. Carbonyl-rich (C3H5)2Fe2(CO)n (n=7, 8) structures are found having monohapto allyl ligands. However, such structures have relatively low carbonyl dissociation energies to give the experimentally known (η3-C3H5)2Fe2(CO)6 and thus are not likely to be viable candidates for experimental realization under normal conditions.

ChemInform Abstract: Pd(I)‐Bridging Allyl Dimers: A New System for the Catalytic Functionalization of Carbon Dioxide

AbstractReview: 41 refs.

Pd(I)-Bridging Allyl Dimers: A New System for the Catalytic Functionalization of Carbon Dioxide

The transition-metal-mediated functionalization of CO2 is currently an area attracting considerable interest due to the poten- tial benefits of utilizing CO2 as a C1 feedstock. Here, recent advanc- es in CO2 activation and functionalization using monomeric Pd h 1 - allyls are summarized. Subsequently, we discuss the chemistry of unusual Pd(I)-bridging allyl dimers, which may be masks for Pd h 1 - allyl complexes, and are stable catalysts for CO2 functionalization.

Bridging allyl ligands upon allene insertion into electron-deficient triosmium-hydride clusters [Os 3(CO) 9(μ 3-NSC 7H 3R)(μ-H)] (R = H, Me)

Allene reacts with the benzothiazolide clusters [Os 3(CO) 9(μ 3-NSC 7H 3R)(μ-H)] (R = H, Me) to afford the bridging allyl complexes [Os 3(CO) 7(μ-CO) 2(μ-NSC 7H 3R)(μ-η 1:η 2:η 1-CH 2CHCH 2)] resulting from insertion of allene into the metal–hydride. Both have been crystallographically characterized and differ with respect to the relative arrangement of allyl and benzothiazolide at the triosmium centre.

DFT studies on the reaction of CO 2 with allyl-bridged dinuclear palladium(I) complexes

Density functional theory (DFT) calculations have been carried out to study the reactions of dinuclear palladium(I) complexes containing two allyl bridging ligands with CO 2. The mechanism of the transformations of the first and second bridging allyl ligand to the bridging carboxylate ligands has been studied. It is found that the transformation of the first bridging allyl ligand is kinetically and thermodynamically favorable, while the transformation of the second ligand is thermodynamically and kinetically unfavorable. The energy decomposition and frontier molecular orbital analyses suggest that in the dinuclear palladium(I) complexes containing two allyl bridging ligands the strong trans influencing properties of the allyl ligands weaken the metal–allyl bonding interactions, facilitating the first CO 2 insertion. The dinuclear palladium(I) complexes obtained from the first CO 2 insertion contain only one bridging allyl ligand and show remarkably strong metal–allyl bonding interactions, impeding the second CO 2 insertion.

Palladium(I)-Bridging Allyl Dimers for the Catalytic Functionalization of CO2

In general, the chemistry of both η(1)-allyl and η(3)-allyl Pd complexes is extremely well understood; η(1)-allyls are nucleophilic and react with electrophiles, whereas η(3)-allyls are electrophilic and react with nucleophiles. In contrast, relatively little is known about the chemistry of metal complexes with bridging allyl ligands. In this work, we describe a more efficient synthetic methodology for the preparation of Pd(I)-bridging allyl dimers and report the first studies of their stoichiometric reactivity. Furthermore, we show that these compounds can activate CO(2) and that an N-heterocyclic carbene-supported dimer is one of the most active and stable catalysts reported to date for the carboxylation of allylstannanes and allylboranes with CO(2).

Allyl complexes of the heavy alkaline-earth metals: molecular structure and catalytic behavior

Reaction of two equivalents of K[1,3-(SiMe3)2C3H3] (= KA′) with SrI2 in thf produces the allyl complex SrA′2(thf)2. In the solid state, the monomeric complex is isostructural with its calcium counterpart; the allyl ligands are bound in a symmetrical trihapto manner to the metals, with an average Sr–C distance of 2.801(5) A. The reaction of KA′ and BaI2 does not result in the expected bis(allyl)barium complex, but rather in a heterometallic barium/potassium species, K(thf)Ba2A′5. The allyls are in fast exchange in solution, and in the solid state, the complex forms a polymeric chiral chain. Each barium center is coordinated by one terminal and two bridging allyl ligands; the potassium center is coordinated by two bridging allyl ligands and one thf molecule. On reaction with iodine, C–C coupling occurs with CaA′2(thf)2, SrA′2(thf)2 and K(thf)Ba2A′5 to give the 1,5-hexadiene, [(SiMe3)2C3H3]2. CaA′2(thf)2, SrA′2(thf)2 and K(thf)Ba2A′5 are initiators for methyl methacrylate polymerization; the analogous magnesium complex is not active.

From mononuclear to multinuclear complexes of palladium containing unsaturated hydrocarbon ligands

This review summarizes the author’s contributions to the field of chemistry of group 10 metal complexes containing unsaturated hydrocarbon ligands, with a brief introduction showing how his research subject has shifted from mononuclear type to multinuclear type complexes. New structure and reactivity trends in the multipalladium complexes with bridging allyl and allenyl/propargyl ligands, as well as bridging conjugated polyene molecules are discussed in terms of some unique bonding features of these complexes.

The tungsten-tungsten triple bond18. Bridging and terminal allyl ligands in complexes of the formula W 2(R) 2(NMe 2) 4, where R  C 3H 5 and C 4H 7

The reaction between RMgCl (two equivalents) and 1,2-W 2Cl 2(NMe 2) 4 in hydrocarbon solvents affords the compounds W 2R 2(NMe 2) 4, where R = allyl and 1− and 2-methyl-allyl. In the solid state the molecular structure of W 2(C 3H 5) 2(NMe 2) 4 has C 2 symmetry with bridging allyl ligands and terminal WNMe 2 ligands. The WW distance 2.480(1) Å and the CC distances, 1.47(1) Å, imply an extensive mixing of the allyl π-MOs with the WW π-MOs, and this is supported by an MO calculation on the molecule W 2(C 3H 5) 2(NH 2) 4 employing the method of Fenske and Hall. The most notable interaction is the ability of the (WW) 6+ centre to donate to the allyl π *-MO (π 3). This interaction is largely responsible for the long WW distance, as well as the long CC distances, in the allyl ligand. The structure of the 2-methyl-allyl derivative W 2(C 4H 7) 2(NMe 2) 4 in the solid state reveals a gauche-W 2C 2N 4 core with WW = 2.286(1) Å and WC = 2.18(1) Å, typical of WW and WC triple and single bonds, respectively. In solution (toluene-d 8) 1H and 13C NMR spectra over a temperature range −80°C to +60°C indicate that both anti- and gauche- W 2C 2N 4 rotamers are present for the 2-methyl-allyl derivative. In addition, there is a facile fluxional process that equilibrates both ends of the 2-methyl-allyl ligand on the NMR time-scale. This process leads to a coalescence at 100°C and is believed to take place via an η 3-bound intermediate. The 1-methyl-allyl derivative also binds in an η 1 fashion in solution and temperature-dependent rotations about the WN, WC and CC bonds are frozen out at low temperatures. The spectra of the allyl compound W 2(C 3H 5) 2(NMe 2) 4 revealed the presence of two isomers in solution—one of which can be readily reconciled with the presence of the bridging isomer found in the solid state while the other is proposed to be W 2(η 3-C 3H 5) 2(NMe 2) 4. The compound W 2R 2(NMe 2) 4 where R = 2,4-dimethyl- pentadiene was similarly prepared and displayed dynamic NMR behaviour explainable in terms of facile η 1 = η 3 interconversions.

Preparation of (Me 3P) 2Pd 2(μ-η 3-C 3H 5(μ-SPH) by reaction of Pd(O) complex with allyl phenyl sulfide

Pd(CH 2CHPh)(PME 3) 2, prepared in situ by reaction of PdEt 2(PMe 3) 2 with styrene, reacts with allyl phenyl sulfide to give (Me 3P) 2Pd 2(μ-η 3-C 3H 5)(μ-SPh) ( 1). Complex 1 has been characterized by means of NMR ( 1H, 13C, and 31P) spectroscopy as well as X-ray crystallography. Space group P2 1/ n, a = 11.662(10, b = 17.898(2), C = 9.849(2) Å, β = 91.22(1)°, Z = 4, R = 0.049, and Rw = 0.050. The molecule has a bridging allyl ligand which coordinates symmetrically to two palladium centers. The distance between the palladium atoms (2.613(1) Å) indicates the presence of PdPd bonding in the molecule. Reaction of 1 with CO gives allyl phenyl sulfide.

Fluxional behaviour in η3-allyl complexes of cr, mo and w as shown by magnetisation transfer difference spectroscopy (MTDS)

(η3-allyl)4M2 (M  Cr, I;M  Mo, II) exist in solution as mixtures of two isomers (Ia/Ib  60/40; IIa/IIb  90/10). Each isomer has two inequivalent allyl groups which are trans to one another and bridge the two metal atoms. The other allyl groups are bonded to each one of the metals. In the major isomer these two allyl groups are cis, whereas in the minor isomer they are trans. At temperatures above 30°C for I (8O°C for II), interconversion of the isomers Ia and Ib (IIa and IIb) can be detected by magnetisation transfer difference spectroscopy (MTDS) (1H NMR 400 MHz). This interconversion proceeds by exchange of syn (anti) protons of the non-bridging allyl groups of Ia (IIa) with syn (anti) protons of the non-bridging allyl groups of Ib (IIb). In addition, in Ia and Ib syn—syn (anti—anti) exchange of the protons of the bridging allyl groups can be detected. (η3-allyl)4M (M  Mo, III; M  W, IV) shows fluxional behaviour at temperatures above 70°C for III (90°C for IV). Syn—anti and also syn—syn (anti—anti) exchange takes place. Each of these processes can be monitored separately by MTDS.

Transition metal–carbon bonds. Part XXVI. Allylic and olefin complexes of platinum(II)

The complexes [Pt(all)2], (all = allyl or 2-methylallyl) are shown to be mononuclear and to exist as cis-trans-mixtures with symmetrically bonded π-allylic groups. With dry hydrogen chloride [Pt(allyl)2] gives the tetranuclear [PtCl(allyl)]4 with bridging (or µ-bonded) allyl groups and bridging chlorines. With thallous acetylacetonate, [PtCl(allyl)]4 gives the binuclear [Pt(acac)(allyl)]2 with bridging allyl ligands. In contrast, [Pt(2-methylallyl)2] reacts with dry hydrogen chloride to give the binuclear [PtCl(2-methylallyl)]2 which has bridging chlorines and symmetrically bonded π-2-methylallyl groups. This complex exists as a mixture having both cis- and trans-arrangements of the 2-methylallyl ligands with respect to the Pt2Cl2-bridging system (as shown by low-temperature 1H n.m.r. spectroscopy and double resonance). Complexes of type [PtCl(all)]x react with neutral ligands, L (isoquinoline, pyridine, tertiary phosphines or tertiary arsines), to give mononuclear complexes [PtCl(all)L] having asymmetrically bonded π-allylic ligands, or cationic species [Pt(all)L2]+. [Pt(2-methylallyl)2] reacts with EPh3(E = P or As) to give [Pt(σ-2-methylallyl)2(EPh3)2] with σ-bonded 2-methylallyl ligands. Several complexes of the type [PtCl(all)]2(all = alkyl-substituted allylic ligand) were made by heating olefin complexes of the type [PtCl2(olefin)]2 with 50% aqueous acetic acid. Complexes of the type [PtX(all)]2, [Pt(all)(acac)] and [Pt(all)(C5H5)] are also described. Mononuclear complexes show sharp satellites in their 1H n.m.r. spectra due to coupling with platinum-195 but the satellites in some of the spectra of the binuclear complexes, [PtX(all)]2, are broadened or not observable. With [PtCl(2-methylallyl)]2 the satellite widths show complex behaviour with temperature. Double resonance experiments on [PtCl(2-methylallyl)]2 show that the platinum-195 relaxation time must be short compared with the relaxation time of the syn-protons. Some rate processes which occur in these allylic-platinum complexes were studied qualitatively by 1H n.m.r. spectroscopy.