Bonding and energetics of phosphorus (III) ligands in transition metal complexes

Phosphine versus Carbene Metal Interactions: Bond Energies

A variety of different ground-state structures of carbene and phosphine groups 1 and 2 cationic, group 11 cationic, and group 10 neutral complexes were studied using density functional theory (DFT) and correlated molecular orbital theory (CCSD(T)) methods. Geometries of complexes with phosphines were studied and compared to available experimental data. Among the three analyzed phosphine ligands, PH3, PMe3, and PPh3, PH3 was found to have noticeably smaller ligand binding energies (LBEs, ΔH298 K). PPh3 has the greatest LBEs with group 2 dications. The difference in LBEs for PMe3 and PPh3 in complexes with group 1 monocations and transition-metal (TM) complexes was significantly less pronounced. The stability and reactivity of phosphine complexes were analyzed and compared with those of previously studied N-heterocyclic carbenes (NHC). PH3 has smaller LBEs compared to NHC carbenes. The lower LBEs correlate with the hardness for M(11)+ complexes and correlate with both the hardness and ionic radii for the M(1)+ and M(2)2+ complexes. The presence of additional PH3 substituents on the metal center makes the LBE smaller compared to their unsubstituted or less substituted analogs. The presence of NH3 in a structure causes a smaller effect on binding, and, except for carbene-PtNH3, an increase in LBE was observed. Composite-correlated molecular orbital theory (G3MP2) was used to predict the LBE of various Lewis acidic ligands with PH3 and NHCs to contrast their binding behavior. Binding either phosphine or carbene to metal diamine complexes caused ligand exchange and transfer of NH3 to an outer coordination sphere.

From phosphanylphosphaalkenes to coordination copper and silver polymers containing P-P bonds

This study was focused on the activation of the CP bond via reactions of Ph2CP-PtBu2 (1) with 1,6-hexanediol and selected dithiols (1,4-butanedithiol, 1,4-benzenedithiol and 1,4-benzenedimethanethiol). These reactions proceed according to a 1,2-addition mechanism, providing new compounds with formulas {(Ph)2(H)C-P-PtBu2}{μ2-(O-(CH2)6-O)}{tBu2P-P-C(H)(Ph)2} (2), {(Ph)2(H)C-P-PtBu2}{μ2-(S-(CH2)4-S)}{tBu2P-P-C(H)(Ph)2} (3a), {(Ph)2(H)C-P-PtBu2}{μ2-(S-C6H4-S)}{tBu2P-P-C(H)(Ph)2} (3b), and {(Ph)2(H)C-P-PtBu2}{μ2-(S-CH2-C6H4-CH2-S)}{tBu2P-P-C(H)(Ph)2} (3c). Next, the reactions of 3a and 3c with metal chlorides led to the growth of desired coordination polymers of copper(I) and silver(I). All the obtained compounds remained stable under atmospheric conditions.

Probing Homogeneous Catalysts and Precatalysts in Solution by Exchange-Mediated Overhauser Dynamic Nuclear Polarization NMR

Triphenylphosphine (PPh3) is a ubiquitous ligand in organometallic chemistry that has been shown to give enhanced 31P NMR signals at high magnetic field via a scalar-dominated Overhauser effect dynamic nuclear polarization (OE DNP). However, PPh3 can only be polarized via DNP in the free form, while the coordinated form is DNP-inactive. Here, we demonstrate the possibility of enhancing the 31P NMR signals of coordinated PPh3 in metal complexes in solution at room temperature by combining Overhauser effect DNP and chemical exchange between the free and coordinated PPh3 forms. With this method, we successfully obtain 31P DNP enhancements of up to 2 orders of magnitude for the PPh3 ligands in Rh(I), Ru(II), Pd(II), and Pt(II) complexes, and we show that the DNP enhancements can be used to determine the activation energy of the ligand exchange reaction.

Triphenylphosphine─Closed-Shell Metal Cation Interactions

The interactions between group 1 and 11 monocations and group 2 dications with triphenylphosphine were studied by using a combination of correlated molecular orbital theory and density functional theory. Two binding modes were found: the front side (phosphorus lone pair) and back side (phenyl rings). Group 1 and 2 cations prefer binding to the π system rather than to the lone pair of the phosphorus atom, and their ligand binding energies (LBEs) correlate with the atomic ionic radii as well as the hardness of the atomic ion. Group 11 monocations prefer binding to the lone pair of the phosphorus atom, and their LBEs are correlated with the hardness of the cation but exhibit a different trend than for the groups 1 and 2 cations. The LBEs of the cations with C2H4, C6H6, and C6H5PH2 are also reported to aid in the analysis of the cation-π interactions and the influence of the PH2 substituent on the energy of this interaction. The LBEs for binding to C2H4 and C6H6 are the most complete and reliable set of values for these species.

Palladium(0)-Catalyzed Anti-Selective Addition-Cyclizations of Alkynyl Electrophiles

Palladium(0)/monophosphine complexes catalyze anti-selective alkylative, arylative, and alkynylative cyclizations of alkynyl electrophiles with organometallic reagents. The remarkable anti-selectivity results from novel oxidative addition, that is, the nucleophilic attack of electron-rich palladium(0) on the electrophile across the alkyne followed by transmetalation and reductive elimination ("anti-Wacker"-type cyclization). With regard to 5-alkynals, triphenylphosphine (PPh3 )-ligated palladium(0) catalyzes the cyclization of terminal alkynes and conjugated alkenyl- or alkynyl-substituted ones to afford 2-cyclohexen-1-ol and 2-alkylidene-cyclopentanol derivatives, respectively. For 6-alkyl- or 6-aryl-5-alkynals, the cyclization does not proceed with the palladium/PPh3 catalyst; however, it does proceed with palladium/tricyclohexylphosphine (PCy3 ), to yield the former products predominantly. Remarkably, the latter catalyst completely switches the regioselectivity in the cyclization of the conjugated diyne-aldehydes. Notably, palladium/PPh3 -catalyzed cyclizations also proceed with other organometallics or even without them.

Bismuth species in the coordination sphere of transition metals: synthesis, bonding, coordination chemistry, and reactivity of molecular complexes

This contribution is focused on bismuth species in the coordination sphere of transition metals. In molecular transition metal complexes, three types of Bi-M bonding are considered, namely dative Bi→M interactions (with Bi acting as a donor), dative Bi←M interactions (with Bi acting as an acceptor) and covalent Bi-M interactions (M = transition metal). Synthetic routes to all three classes of compounds are outlined, the Bi-M bonding situation is discussed, trends in the geometric parameters and in the coordination chemistry of the compounds are addressed, and common spectroscopic properties are summarized. As an important part of this contribution, the reactivity of bismuth species in the coordination sphere of transition metal complexes in stoichiometric and catalytic reactions is highlighted.

Rhenium(V)-oxo complexes [ReOX2(N∩O)(EPh3)] (X=Cl, Br, I; E=P, As) – synthesis, structure, spectroscopy, and catalytic properties

The present review aims to give a comprehensive survey about the chemistry of rhenium(V)-oxo complexes of general formula [ReOX2(N∩O)(EPh3)], where X=Cl, Br, I, E=P, As, and N∩O stands for uninegative chelating N∩O-ligand, carried out within the last four decades. In addition to the synthesis aspects, the available structural data as well as the results issued from techniques such as infrared and ultraviolet-visible spectroscopies are collected and discussed. Furthermore, a brief description of the applications of these compounds in catalysis is included.

All-zinc coordinated nickel-complexes as molecular mimics for NiZn catalyst surfaces, a density functional theory study

A prospective connection between Hume-Rothery inspired TM/E (TM = transition metal; E = Al, Ga, Zn) complexes and clusters with the related solid-state intermetallic TM/E compounds is presented with respect to the industrially relevant catalytic semihydrogenation of acetylene. The theoretical study dealing with [Ni(ER)_n(C₂H_x)_4-n] (x = 2, 4; R = CH₃, C₅Me₅,) calculated on the DFT level of theory shows intriguing structural and electronic properties of the examined complexes. Different Ni-E complexes show preferred binding of C₂H₂ over C₂H₄ in bridging positions between Ni and E depending on the [Ni(ER)_n] fragment. These findings render molecular TM/E systems, such as Ni/Zn, promising candidates to mimic key intermediates of intermetallic catalysts applied in heterogeneous hydrogenation reactions. We put these findings into the context of existing synthetic results and illustrate different experimental approaches to obtain compounds of the general formula [TM_aE_b](Cp*)_c(UHC)_d (UHC = unsaturated hydrocarbon ligands) as potential surface models.

Suppressed Phosphine Dissociation by Polarization Effects on the Donor-Acceptor Bonds in [Ni(PEt3)4- n(ECp*) n] (E = Al, Ga)

A series of heteroleptic complexes [Ni(PEt₃)_{4- n}(ECp*) _n] (E = Al, Ga, Cp* = pentamethylcyclopentadienyl, n = 0-4) was prepared and characterized by experimental as well as computational means. The series of compounds was studied with respect to ligand dissociation processes which are fundamental for reactivity. In contrast to the homoleptic complexes [Ni(PR₃) _n] phosphine dissociation is remarkably suppressed in the heteroleptic title complexes. Single crystal X-ray structures as well as density functional theory calculations reveal a gradual decrease of the Ni-PEt₃ distances with increasing number of coordinated group-13 ligands ECp*. Accordingly, variable-temperature UV-vis studies of [Ni(PEt₃)_{4- n}(AlCp*) _n] in solution indicate no ligand dissociation equilibrium for n ≥ 2. Energy decomposition analysis with the natural orbital for chemical valence extension shows higher Ni-P interaction energies for [Ni(PEt₃)_{4- n}(AlCp*) _n] than for [Ni(PEt₃)₄] which is mainly a result of an increase in columbic attraction forces induced by Ni-PEt₃ bond polarization upon ECp* coordination.

Electronic and bite angle effects in catalytic C–O bond cleavage of a lignin model compound using ruthenium Xantphos complexes

Bite angle and electronic effects on the ruthenium–diphosphine catalysed ether bond cleavage of the lignin β-O-4 model compound 2-phenoxy-1-phenylethanol were tested.

Development of Chiral Bis-hydrazone Ligands for the Enantioselective Cross-Coupling Reactions of Aryldimethylsilanolates

A palladium-catalyzed, enantioselective, aryl-aryl cross-coupling reaction using 1-naphthyldimethylsilanolates and chiral bis-hydrazone ligands has been developed. A family of glyoxal bis-hydrazone ligands containing various 2,5-diarylpyrrolidine groups was prepared to evaluate the influence of ligand structure on the rate and enantioselectivity of the cross-coupling. New synthetic routes to the 1-amino-2,5-diarylpyrrolidines were developed to enable the structure/reactivity-selectivity studies. Role reversal experiments of aryldimethylsilanolates and aryl bromides result in biaryl products with the same configuration and similar enantioselectivities implying that reductive elimination is the stereodetermining step. The origin of stereoselectivity is rationalized through computational modeling of diarylpalldium(II) complex which occurs through a conrotatory motion for the two aryl groups undergoing C-C bond formation.

Phosphine substitution reactions of (η5-cyclopentadienyl)ruthenium bis(triarylphosphine) chloride, CpRu(PAr3)2Cl {PAr3 = PPh3, P(p-CH3C6H4)3, P(p-FC6H4)3, P(p-CH3OC6H4)3, and PPh2(p-CH3C6H4)}: a tale of two mechanisms

The kinetics of phosphine substitution in CpRu(PAr3)2Cl by PMePh2 under pseudo-first order conditions in CDCl3 have been measured for PAr3 = PPh3, 1a, PPh2(p-tol), 1b, P(p-tol)3, 1c, P(p-CH3OC6H4)3, 1d, and P(p-FC6H4)3), 1e. Activation parameters characteristic of a dissociative pathway (ΔH(†) = 110-124 ± 2 kJ mol(-1), ΔS(†) = 16-44 ± 5-12 J mol(-1) K(-1)) are observed for all five compounds. The rate of substitution in CpRu(PAr3)2Cl (1a) and CpRu[P(p-FC6H4)3]2Cl (1e) is independent of added chloride ion and decreases in the presence of excess PAr3, however, the rate of substitution in CpRu[P(p-CH3OC6H4)3]2Cl (1d) is first order in added chloride ion and is less dependent on added PAr3. A mechanism involving [CpRu(PAr3)2(PMePh2)](+)[Cl](-) intermediates contributes to the substitution in 1b-d.

On the coordination chemistry of phosphinecarboxamide: assessing ligand basicity

We describe the coordination chemistry of the primary phosphine PH₂C(O)NH₂ (phosphinecarboxamide) to form the novel complexes W(CO)₅(PH₂C(O)NH₂) (1) and cis-Mo(CO)₄(PH₂C(O)NH₂)₂ (2).

Gas-phase behaviour of Ru(II) cyclopentadienyl-derived complexes with N-coordinated ligands by electrospray ionization mass spectrometry: fragmentation pathways and energetics

RATIONALE

The gas-phase behaviour of six Ru(II) cyclopentadienyl-derived complexes with N-coordinated ligands, compounds with antitumor activities against several cancer lines, was studied. This was performed with the intent of establishing fragmentation pathways and to determine the Ru-L(N) and Ru-L(P) ligand bond dissociation energies. Such knowledge can be an important tool for the postulation of the mechanisms of action of these anticancer drugs.

METHODS

Two types of instruments equipped with electrospray ionisation were used (ion trap and a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer). The dissociation energies were determined using energy-variable collision-induced dissociation measurements in the ion trap. The FTICR instrument was used to perform MS(n) experiments on one of the compounds and to obtain accurate mass measurements. Theoretical calculations were performed at the density functional theory (DFT) level using two different functionals (B3LYP and M06L) to estimate the dissociation energies of the complexes under study.

RESULTS

The influence of the L(N) on the bond dissociation energy (D) of RuCp compounds with different nitrogen ligands was studied. The lability order of L(N) was: imidazole<1-butylimidazole<5-phenyl-1H-tetrazole<1-benzylimidazole. Both the functionals used gave the following ligand lability order: imidazole<1-benzylimidazole<5-phenyl-1H-tetrazole<1-butylimidazole. It is clear that there is an inversion between 1-benzylimidazole and 1-butylimidazole for the experimental and theoretical lability orders. The M06L functional afforded values of D closer to the experimental values. The type of phosphane (L(P) ) influenced the dissociation energies, with values of D being higher for Ru-L(N) with 1-butylimidazole when the phosphane was 1,2-bis(diphenylphosphino)ethane. The Ru-L(P) bond dissociation energy for triphenylphosphane was independent of the type of complex.

CONCLUSIONS

The D values of Ru-L(N) and Ru-L(P) were determined for all six compounds and compared with the values calculated by the DFT method. For the imidazole-derived ligands the energy trend was rationalized in terms of the increasing extension of the σ-donation/π-backdonation effect. The bond dissociation energy of Ru-PPh(3) was independent of the fragmentations.

A Study of the Heck Reaction in Non-Polar Hydrocarbon Solvents and in Supercritical Carbon Dioxide

The effects of electronic and steric properties of phosphorus ligands on Heck reactions in supercritical CO2 and non-polar hydrocarbon solvents were studied. In Heck reactions between iodobenzene and butyl acrylate, higher yields were obtained with less electron-rich phosphine ligands. This trend was also observed with the electron-poor triphenyl phosphite. A range of sterically demanding phosphites were then investigated. Biphenyl-containing phosphites 8 and 13 were found to be highly effective. In the Heck reaction between the less-reactive bromobenzene and butyl acrylate, the bulky, electron-rich tri-tert-butylphosphine ligand produced the best yield. These results emphasize the importance of both electronic and steric properties of phosphorus ligands in the Heck reaction in non-polar solvents.