Bulky Diphosphine Research Articles

Cooperative Nitrile Coordination Using Nickel and a Boron-Containing Secondary Coordination Sphere.

Metal-ligand cooperation has emerged as a versatile tool for substrate activation in chemical reactivity. Herein, we provide the synthesis and characterization of a monoboranyl-containing analogue of the ubiquitous bulky diphosphine ligand, 1,2-bis(di-tert-butylphosphino)ethane, whose reactivity has been examined using nickel. Together, the pairing of nickel and boron provides a platform that allows for the cooperative coordination of organonitriles, giving unusual examples of intermolecularly bound dinickelacycles.

Reaching strong absorption up to 700 nm with new benzo[g]quinoxaline-based heteroleptic copper(I) complexes for light-harvesting applications.

Heteroleptic copper(I) complexes, with a diimine as a chromophoric unit and a bulky diphosphine as an ancillary ligand, have the advantage of a reduced pseudo Jahn-Teller effect in their excited state over the corresponding homoleptic bis(diimine) complexes. Nevertheless, their lowest absorption lies generally between 350 to 500 nm. Aiming at a strong absorption in the visible by stable heteroleptic Cu(I) complexes, we designed a novel diimine based on 4-(benzo[g]quinoxal-2'-yl)-1,2,3-triazole derivatives. The large π-conjugation of the benzoquinoxaline moiety shifted bathochromically the absorption with regard to other diimine-based Cu(I) complexes. Adding another Cu(I) core broadened the absorption and extended it to considerably longer wavelengths. Moreover, by fine-tuning the structure of the dichelating ligand, we achieved a panchromatic absorption up to 700 nm with a high molar extinction coefficient of 8000 M-1 cm-1 at maximum (λ = 570 nm), making this compound attractive for light-harvesting antennae.

Iridium-catalyzed asymmetric addition of imides to alkenes.

Enantioselective addition of an imide N-H bond to alkenes was realized by use of a cationic iridium catalyst. Bulky diphosphine ligands such as DTBM-segphos, DTBM-MeO-biphep, and DTBM-binap were indispensable for the reaction. A variety of styrene derivatives, allylsilanes, and norbornene were good substrates to give the corresponding chiral adducts with high enantioselectivity.

Bulky Diphosphine Acetate Ruthenium Complexes: Synthesis and Catalytic Activity in Ketone Transfer Hydrogenation and Alkyne Dimerization

Several mononuclear ruthenium complexes containing bulky diphosphines have been easily prepared from acetate ruthenium precursors. The cyclohexyl complex [Ru(η2-OAc)2(DCyPF)] (1-cy; DCyPF = 1,1′-bi...

New Photosensitizers Based on Heteroleptic CuI Complexes and CO2 Photocatalytic Reduction with [NiII (cyclam)]Cl2.

Earth‐abundant metal complexes have been attracting increasing attention in the field of photo(redox)catalysis. In this work, the synthesis and full characterisation of four new heteroleptic CuI complexes are reported, which can work as photosensitizers. The complexes bear a bulky diphosphine (DPEPhos=bis[(2‐diphenylphosphino)phenyl] ether) and a diimine chelating ligand based on 1‐benzyl‐4‐(quinol‐2′yl)‐1,2,3‐triazole. Their absorption has a relative maximum in the visible‐light region, up to 450 nm. Thus, their use in photocatalytic systems for the reduction of CO2 with blue light in combination with the known catalyst [NiII(cyclam)]Cl2 was tested. This system produced CO as the main product through visible light (λ=420 nm) with a TON up to 8 after 4 hours. This value is in line with other photocatalytic systems using the same catalyst. Nevertheless, this system is entirely noble‐metal free.

Photoinduced Terminal Hydride of [FeFe]-Hydrogenase Biomimetic Complexes.

The active site of the [FeFe]-hydrogenase ([FeFe]-H2ase) has a bridging carbonyl ligand and a terminal hydride in the key H-cluster intermediate Hhyd. However, nearly all of the synthetic mimics reported, so far, prefer a hydride bridging the two irons, and only few mimics with a terminal hydride were achieved by tuning the steric effects of bulky diphosphine ligands. Moreover, although intermediates with either a terminal hydride or a protonated bridging thiolate ligand were proposed to exist during protonation processes or hydrogen exchange in the [FeFe]-H2ase mimic, [Fe2(μ-pdt)(μ-H)(CO)4(PMe3)2]+ (1H+), only bridging hydrides were observed by time-resolved IR spectroscopy. In this report, FTIR spectroscopy of 1H+, under CO with longer irradiation time, revealed several new photoinduced species. In addition to the CO loss species, many of the photoinduced products can be assigned to 1H+ with a terminal hydride by comparison of their CO vibrational frequencies with density functional theory calculations.

Ir(III)/Ir(V) or Ir(I)/Ir(III) Catalytic Cycle? Steric-Effect-Controlled Mechanism for the para-C–H Borylation of Arenes

Density functional theory method N12 was used to study the mechanism of the [Ir(cod)OH]2/Xyl–MeO–BIPHEP-catalyzed para-selective C–H borylation reaction. The results revealed that the use of a bulky diphosphine ligand such as Xyl–MeO–BIPHEP was unfavorable for the previously proposed iridium(III)/iridium(V) catalytic cycle because it resulted in considerable steric repulsion in the hepta-coordinated iridium(V) intermediate. Inspired by this steric effect, we have proposed a novel iridium(I)-/iridium(III)-based catalytic cycle for this transformation and shown that it can be used to account for the experimental results. The iridium(I)/iridium(III) catalytic cycle induced by this steric effect consists of several steps, including (i) the oxidative addition of the C–H bond of the substrate to an active iridium(I) boryl complex; (ii) the reductive elimination of a C–B bond; (iii) the oxidative addition of B2pin2 to an iridium(I) hydride complex; and (iv) the reductive elimination of a B–H bond. Notably, the computed regioselectivity of this reaction was consistent with the experimental observations. The high para-selectivity of this reaction was also explained using structural analysis and a 2D contour model, which revealed that the strong steric repulsion between the diphosphine ligand and the meta-substituents resulted in a higher energy barrier for meta-C–H activation.

Isomerizing Methoxycarbonylation of Alkenes to Esters Using a Bis(phosphorinone)xylene Palladium Catalyst

The synthesis and characterization of bulky diphosphine 1,2-bis(4-phosphorinone)xylene, BPX, and its palladium complexes [(BPX)PdCl2] and [(BPX)Pd(O2CCF3)2] are described. BPX was evaluated as a ligand in Pd-catalyzed isomerizing methoxycarbonylation. A broad range of alkenes, including terminal, internal, branched, and functionalized alkenes, can be converted to esters with activities and selectivities matching or surpassing the performance of the state-of-the-art palladium bis(di(tert-butyl)phosphino-o-xylene (Pd-DTBPX) catalyst. A molecular structure of the precatalyst [(BPX)Pd(O2CCF3)2] was obtained showing a square planar geometry and a bite angle of 100.11(3)°. Rhodium carbonyl complexes [(BPX)Rh(CO)Cl] and [(DTBPX)Rh(CO)Cl] were synthesized to compare the relative electronic parameters, revealing a ν(C≡O) of 1956.8 and 1948.3 cm–1, respectively, suggesting a reduced ability of BPX to donate electron density to the metal relative to DTBPX. Competitive protonation experiments between BPX and DTBPX in...

Flexible Reaction Pocket on Bulky Diphosphine–Ir Complex Controls Regioselectivity in para-Selective C–H Borylation of Arenes

We used DFT calculations to elucidate the mechanism and source of regioselectivity for Ir-catalyzed para-selective C–H borylation with the bulky Xyl-MeO-BIPHEP diphosphine ligand (L1). We found that the bulky diphosphine–Ir complex forms a flexible reaction pocket that roughly mimics the role of an enzyme active site by modulating access of the substrate to the Ir center. It is shown that the regioselectivity of the reaction arises from a complicated balance of the attractive and repulsive interactions between the substrate and ligand and their corresponding entropic penalties across the high-energy C–H activation and C–B bond formation transition states in the reaction pocket. We predict a trend of increasing para-selectivity with an increase in the size of the substrate substituent in the order SiH3 (and Me) < SiMe3 < Si(t-Bu)3, which is consistent with the experimentally observed trend. It is shown that an increase in the steric bulk of the ligand by introducing bulkier 3,5-substituents to the phenyl r...

Steric and Electronic Effects of Bidentate Phosphine Ligands on Ruthenium(II)-Catalyzed Hydrogenation of Carbon Dioxide.

The reactivity difference between the hydrogenation of CO2 catalyzed by various ruthenium bidentate phosphine complexes was explored by DFT. In addition to the ligand dmpe (Me2 PCH2 CH2 PMe2 ), which was studied experimentally previously, a more bulky diphosphine ligand, dmpp (Me2 PCH2 CH2 CH2 PMe2 ), together with a more electron-withdrawing diphosphine ligand, PN(Me) P (Me2 PCH2 N(Me) CH2 PMe2 ), have been studied theoretically to analyze the steric and electronic effects on these catalyzed reactions. Results show that all of the most favorable pathways for the hydrogenation of CO2 catalyzed by bidentate phosphine ruthenium dihydride complexes undergo three major steps: cis-trans isomerization of ruthenium dihydride complex, CO2 insertion into the Ru-H bond, and H2 insertion into the ruthenium formate ion. Of these steps, CO2 insertion into the Ru-H bond has the lowest barrier compared with the other two steps in each preferred pathway. For the hydrogenation of CO2 catalyzed by ruthenium complexes of dmpe and dmpp, cis-trans isomerization of ruthenium dihydride complex has a similar barrier to that of H2 insertion into the ruthenium formate ion. However, in the reaction catalyzed by the PN(Me) PRu complex, cis-trans isomerization of the ruthenium dihydride complex has a lower barrier than H2 insertion into the ruthenium formate ion. These results suggest that the steric effect caused by the change of the outer sphere of the diphosphine ligand on the reaction is not clear, although the electronic effect is significant to cis-trans isomerization and H2 insertion. This finding refreshes understanding of the mechanism and provides necessary insights for ligand design in transition-metal-catalyzed CO2 transformation.

Synthesis of a sterically bulky diphosphine synthon and Ru(ii) complexes of a cooperative tridentate enamide-diphosphine ligand platform.

In order to generate tridentate enamido diphosphine ligand platforms, we developed procedures for the preparation of tBu2PCH2CH2P(tBu)I, which involve low temperatures, pentane solvent and addition of 4 equiv. of tBuLi to Cl2PCH2CH2PCl2 or 2 equiv. of tBuLi to known Cl(tBu)PCH2CH2P(tBu)Cl also at low temperatures in pentane; an alternate method involves the inverse addition of Cl(tBu)PCH2CH2P(tBu)Cl to 2 equiv. of tBuLi in pentane at 0 °C; all of these methods generate good yields of the tetraphosphine dimer (tBu2PCH2CH2P(tBu))2 contaminated by small amounts of tBu2PCH2CH2PtBu2 (dtbpe), which can be conveniently separated by sublimation. Subsequent oxidative cleavage of the P-P bond with I2 or 1,2-diiodoethane results in the formation of the desired tBu2PCH2CH2P(tBu)I, which undergoes C-P bond formation when added to 1 equiv. of the lithium N-2,6-diisopropylphenylenamide of cyclopentylidene imine to generate the HNPP ligand precursor; this species exists as a tautomeric mixture of the corresponding enamine and imine, the ratio of which depends on workup conditions used. This enamine-imine mixture can be used directly to form Ru(ii) species either directly with heating to generate the five-coordinate (NPP)RuCl(CO) via loss of H2 or by inclusion of 1 equiv. of KOtBu to generate (NPP)RuH(CO). X-ray crystallographic studies confirm that the geometry in the solid state matches the solution spectroscopic data. Subsequent studies of (NPP)RuH(CO) indicate that it reacts with benzaldehyde, benzyl alcohol, and H2 in a cooperative manner to generate a series of hydride carbonyls that have been characterized fully by NMR spectroscopy and X-ray crystallography.

ChemInform Abstract: para‐C—H Borylation of Benzene Derivatives by a Bulky Iridium Catalyst.

A highly para-selective aromatic C–H borylation has been accomplished. By a new iridium catalyst bearing a bulky diphosphine ligand, Xyl-MeO-BIPHEP, the C–H borylation of monosubstituted benzenes can be affected with para-selectivity up to 91%. This catalytic system is quite different from the usual iridium catalysts that cannot distinguish meta- and para-C–H bonds of monosubstituted benzene derivatives, resulting in the preferred formation of meta-products. The para-selectivity increases with increasing bulkiness of the substituent on the arene, indicating that the regioselectivity of the present reaction is primarily controlled by steric repulsion between substrate and catalyst. Caramiphen, an anticholinergic drug used in the treatment of Parkinson’s disease, was converted into five derivatives via our para-selective borylation. The present [Ir(cod)OH]2/Xyl-MeO-BIPHEP catalyst represents a unique, sterically controlled, para-selective, aromatic C–H borylation system that should find use in streamlined, ...

Palladium Complexes with Bulky Diphosphine Ligands as Highly Selective Catalysts for the Synthesis of (Bio-) Adipic Acid from Pentenoic Acid Mixtures.

A series of sterically bulky diphosphines have been prepared, including P2 = trans-1,2-bis[(di-tert-butylphosphino)methyl]cyclohexane (4), (2-methylenepropane-1,3-diyl)bis(di-tert-butylphosphine) (5), bis[(di-tert-butylphosphino)methyl]dimethylsilane (6), and cis- and trans-11,12-bis[(di-tert-butylphosphino)methyl]-9,10-dihydro-9,10-ethanoanthracene (10 and 11). The corresponding palladium complexes of these ligands, P2Pd(CF3CO2)2, have been synthesized and characterized. The solid-state structures of [Pd(4)(CF3CO2)2], [Pd(5)(CF3CO2)2], [Pd(6)(CF3CO2)2], and [Pd(11)(CF3CO2)2] were obtained by single-crystal X-ray diffraction and confirm the bidentate binding mode of the ligand and a square-planar coordination geometry with a minor distortion from the ideal. The diphosphines in combination with Pd(OAc)2 have been applied in the hydroxycarbonylation of a mixture of pentenoic acid isomers to produce adipic acid with high selectivity (in several cases >95%). The (regio)selectivity of the hydroxycarbonylation ...

Para-C-H Borylation of Benzene Derivatives by a Bulky Iridium Catalyst.

A highly para-selective aromatic C-H borylation has been accomplished. By a new iridium catalyst bearing a bulky diphosphine ligand, Xyl-MeO-BIPHEP, the C-H borylation of monosubstituted benzenes can be affected with para-selectivity up to 91%. This catalytic system is quite different from the usual iridium catalysts that cannot distinguish meta- and para-C-H bonds of monosubstituted benzene derivatives, resulting in the preferred formation of meta-products. The para-selectivity increases with increasing bulkiness of the substituent on the arene, indicating that the regioselectivity of the present reaction is primarily controlled by steric repulsion between substrate and catalyst. Caramiphen, an anticholinergic drug used in the treatment of Parkinson's disease, was converted into five derivatives via our para-selective borylation. The present [Ir(cod)OH]2/Xyl-MeO-BIPHEP catalyst represents a unique, sterically controlled, para-selective, aromatic C-H borylation system that should find use in streamlined, predictable chemical synthesis and in the rapid discovery and optimization of pharmaceuticals and materials.

Rational synthesis and characterization of dimolybdenum(II) compounds bearing ferrocenyl-containing ligands toward modulation of electronic coupling.

Three novel cis-to-trans-converted dimolybdenum(II) complexes, trans-[Mo2(O2C-Fc)2(DPPX)2][BF4]2 {2a-2c; DPPX = DPPA [N,N-bis(diphenylphosphino)amine], DPPM [1,1-bis(diphenylphosphino)methane], and DPPE [1,2-bis(diphenylphosphino)ethane], respectively}, were synthesized through the insertion of bulky diphosphine ligands, which force a permanent trans arrangement, as evidenced by X-ray crystallography and density functional theory calculations. All compounds were characterized by means of NMR, UV-vis, and IR spectroscopy as well as thermogravimetry-mass spectrometry measurements. Interestingly, uncommon UV-vis transitions and oxidation sequences were observed compared to previously reported ones. As verified by electrochemical measurements, all synthesized complexes show two separate one-electron-redox processes assigned to subsequent oxidations of the two redox-active ferrocenecarboxylate ligands, with a split of ca. 70 mV. This behavior reveals electronic interaction between the two equatorially trans-positioned ferrocenyl units. The presented work provides new insights into the rational synthesis of electronically coupled trans-coordinated Mo2 systems, paving the way toward the design of linear multicenter redox-active oligomers.

A comprehensive mechanistic picture of the isomerizing alkoxycarbonylation of plant oils.

Theoretical studies on the overall catalytic cycle of isomerizing alkoxycarbonylation reveal the steric congestion around the diphosphine coordinated Pd-center as decisive for selectivity and productivity. The energy profile of isomerization is flat with diphosphines of variable steric bulk, but the preference for the formation of the linear Pd-alkyl species is more pronounced with sterically demanding diphosphines. CO insertion is feasible and reversible for all Pd-alkyl species studied and only little affected by the diphosphine. The overall rate-limiting step associated with the highest energetic barrier is methanolysis of the Pd-acyl species. Considering methanolysis of the linear Pd-acyl species, whose energetic barrier is lowest within all the Pd-acyl species studied, the barrier is calculated to be lower for more congesting diphosphines. Calculations indicate that energy differences of methanolysis of the linear versus branched Pd-acyls are more pronounced for more bulky diphosphines, due to involvement of different numbers of methanol molecules in the transition state. Experimental studies under pressure reactor conditions showed a faster conversion of shorter chain olefin substrates, but virtually no effect of the double bond position within the substrate. Compared to higher olefins, ethylene carbonylation under identical conditions is much faster, likely due not just to the occurrence of reactive linear acyls exclusively but also to an intrinsically favorable insertion reactivity of the olefin. The alcoholysis reaction is slowed down for higher alcohols, evidenced by pressure reactor and NMR studies. Multiple unsaturated fatty acids were observed to form a terminal Pd-allyl species upon reaction with the catalytically active Pd-hydride species. This process and further carbonylation are slow compared to isomerizing methoxycarbonylation of monounsaturated fatty acids, but selective.

Iron Phosphine Catalyzed Cross-Coupling of Tetraorganoborates and Related Group 13 Nucleophiles with Alkyl Halides

Iron phosphine complexes prove to be good precatalysts for the cross-coupling of alkyl, benzyl, and allyl halides with not only aryl triorganoborate salts but also related aluminum-, gallium-, indium-, and thallium-based nucleophiles. Mechanistic studies revealed that while Fe(I) can be accessed on catalytically relevant time scales, lower average oxidation states are not formed fast enough to be relevant to catalysis. EPR spectroscopic studies reveal the presence of bis(diphosphine)iron(I) complexes in representative catalytic reactions and related processes with a range of group 13 nucleophiles. Isolated examples were studied by Mossbauer spectroscopy and single-crystal X-ray structural analysis, while the electronic structure was probed by dispersion-corrected B3LYP DFT calculations. An EPR study on an iron system with a bulky diphosphine ligand revealed the presence of an S = 1/2 species consistent with the formation of a mono(diphosphine)iron(I) species with inequivalent phosphine donor environments....

Reversible CO exchange at platinum(0). An example of similar complex properties produced by ligands with very different stereoelectronic characteristics.

The ligands 1,2-C6H4(CH2P(t)Bu2)2 (La) and 1,2-C6H4(P(t)Bu2)(CH2P(t)Bu2) (Lb) displace norbornene (nbe) from [Pt(η(2)-nbe)3] to give [PtL(η(2)-nbe)] where L = La (1a) or Lb (1b); 1a is fluxional on the NMR timescale. Reaction of 1a,b with CO gives the corresponding monocarbonyls [PtL(CO)] where L = La (2a) or Lb (2b) which then react further, and reversibly, to give the dicarbonyls [PtL(CO)2] where L = La (3a) or Lb (3b). The CO interchange between 2a,b and 3a,b is compared with the only other such system (2f and 3f), which are complexes of (C2F5)2PCH2CH2P(C2F5)2 (Lf). Ethene reacts smoothly with 2a to give (4a) and H2 with 2a generates some [PtH2(La)]. Protonation of 2a gives [Pt(La)(H)(CO)][B(C6F5)4] (5a) whose crystal structure has been determined. Similarly protonation of 2b gives [Pt(Lb)(H)(CO)][B(C6F5)4] as a mixture of geometric isomers 5b–6b.

Solvated-Ion-Pairing-Sensitive Molecular Bistability Based on Copper(I)-Coordinated Pyrimidine Ring Rotation

We describe herein the effect of solvated ion pairing on the molecular motion of a pyrimidine ring coordinated on a copper center. We synthesized a series of heteroleptic copper(I) complex salts bearing an unsymmetrically substituted pyridylpyrimidine and a bulky diphosphine. Two rotational isomers of the complexes were found to coexist and interconvert in solution via intramolecular ligating atom exchange of the pyrimidine ring, where the notation of the inner (i-) and outer (o-) isomers describes the orientation of the pyrimidine ring relative to the copper center. The stability of the pyrimidine orientation was solvent- and counterion-sensitive in both 2·BF(4) {2(+) = [Cu(Mepypm)(dppp)](+), where Mepypm = 4-methyl-2-(2'-pyridyl)pyrimidine and dppp = 1,3-bis(diphenylphosphino)propane} and previously reported 1·BF(4), which possesses a bulky diphosphine ligand (1(+) = [Cu(Mepypm)(DPEphos)](+), where DPEphos = bis[2-(diphenylphosphino)phenyl] ether). Two rotational isomers of 2(+) were separately obtained as single crystals, and the structure of each isomer was examined in detail. Both the enthalpy and entropy values for the rotation of 2·BF(4) in CDCl(3) (ΔH = 6 kJ mol(-1); ΔS = 25 J K(-1) mol(-1)) were more positive than that tested under other conditions, such as in more polar solvents CD(2)Cl(2), acetone-d(6), and CD(3)CN. The reduced contact of the anion to the cation in a polar solvent seems to contribute to the enthalpy, entropy, and Gibbs free energy for rotational isomerization. This speculation based on solvated ion pairing was further confirmed by considering the rotational behavior of 2(+) with a bulky counterion, such as B(C(6)F(5))(4)(-). The findings are valuable for the design of molecular mechanical units that can be readily tuned via weak electrostatic interactions.

Processing of Polyacetylene from Aqueous Nanoparticle Dispersions

Printed circuits: Aqueous polyacetylene dispersions were prepared by catalytic polymerization of acetylene in an aqueous emulsion with a cationic palladium catalyst with a bulky diphosphine ligand. Facile processing is demonstrated by ink-jet printing of the conductive-polymer dispersions to produce a functioning electronic device (see picture).