Ligand Electronic Effects Research Articles

Steric and electronic ligand perturbations in catalysis: asymmetric allylic substitution reactions using C2-symmetrical phosphorus-chiral (bi)ferrocenyl donors.

Three series of P-chiral diphosphines based on ferrocene (1a-f, 2a-c) and biferrocenyl skeletons (3a-c), including novel ligands 1f and 3c, were employed in palladium-catalyzed allylic substitution reactions. Steric effects imposed by the phosphine residues were studied using C2-symmetrical donors 1 (1 = 1,1'-bis(arylphenylphosphino)ferrocene with aryl groups a = 1-naphthyl, b = 2-naphthyl, c = 2-anisyl, d = 2-biphenylyl, e = 9-phenanthryl, and f = ferrocenyl), whereas para-methoxy- and/or para-trifluoromethyl substitution of the phenyl moieties in 1a enabled investigation of ligand electronic effects applying ferrocenyl diphosphines 2a-c. Ligands 3 (3 = 2,2'-bis- (arylphenylphosphino)-1,1'-biferrocenyls with aryl substituents a,c = 1-naphthyl (diastereomers) and b = 2-biphenylyl) allowed for comparison of backbone structure effects (bite angle variation) in catalysis. Linear and cyclic allylic acetates served as substrates in typical test reactions; upon attack of soft carbon and nitrogen nucleophiles on (E)-1,3-diphenylprop-2-ene-1-yl acetate the respective malonate, amine, or imide products were obtained in enantioselectivities of up to 99% ee. A crystal structure analysis of a palladium 1,3-diphenyl-eta 3-allyl complex incorporating ligand (S,S)-1a revealed a marked distortion of the allyl fragment, herewith defining the regioselectivity of nucleophile addition.

New Insights into the Mechanisms of O−O Bond Cleavage of Hydrogen Peroxide andtert-Alkyl Hydroperoxides by Iron(III) Porphyrin Complexes

The mechanisms of heterolytic versus homolytic O−O bond cleavage of H2O2, tert-butyl hydroperoxide (t-BuOOH), 2-methyl-1-phenyl-2-propyl hydroperoxide (MPPH), and m-chloroperoxybenzoic acid (m-CPBA) by iron(III) porphyrin complexes have been studied by carrying out catalytic epoxidations of cyclohexene in protic solvent. In these reactions, various iron(III) porphyrin complexes containing electron-withdrawing and -donating substituents on phenyl groups at the meso position of the porphyrin ring were employed to study the electronic effect of porphyrin ligands on the heterolytic versus homolytic O−O bond cleavage of the hydroperoxides. In addition, various imidazoles were introduced as axial ligands to investigate the electronic effect of axial ligands on the pathways of hydroperoxide O−O bond cleavage. Unlike the previous suggestions by Traylor, Bruice, and co-workers, the hydroperoxide O−O bonds were found to be cleaved both heterolytically and homolytically and partitioning between heterolysis and homolysis was significantly affected by the electronic nature of the iron porphyrin complexes (i.e., electronic properties of porphyrin and axial ligands). Electron-deficient iron porphyrin complexes show a tendency to cleave the hydroperoxide O−O bonds heterolytically, whereas electron-rich iron porphyrin complexes cleave the hydroperoxide O−O bonds homolytically. The heterolytic versus homolytic O−O bond cleavage of the hydroperoxides was also found to be significantly affected by the substituent of the hydroperoxides, ROOH (R = C(O)R‘, H, C(CH3)3, and C(CH3)2CH2Ph for m-CPBA, H2O2, t-BuOOH, and MPPH, respectively), in which the tendency of O−O bond heterolysis was in the order of m-CPBA > H2O2 > t-BuOOH > MPPH. This result indicates that the O−O bond of hydroperoxides containing electron-donating tert-alkyl groups such as t-BuOOH and MPPH tends to be cleaved homolytically, whereas electron-withdrawing substituents such as an acyl group in m-CPBA facilitates O−O bond heterolysis. Since we have observed that the homolytic O−O bond cleavage of hydroperoxides prevails in the reactions performed with electron-rich iron porphyrin complexes and with hydroperoxides containing electron-donating substituents such as the tert-alkyl group, we suggest that the homolytic O−O bond cleavage is facilitated when more electron density resides on the O−O bond of (Porp)Fe(III)-OOR intermediates. We also present convincing evidence that the previous assertion that the reactions of iron(III) porphyrin complexes with hydrogen peroxide and tert-alkyl hydroperoxides invariably proceed by heterolytic O−O bond cleavage in protic solvent and that the failure to obtain high epoxide yields in iron porphyrin complex-catalyzed epoxidation of olefins by hydroperoxides is due to the mechanism of heterolytic O−O bond cleavage followed by a fast hydroperoxide oxidation is highly unlike.

Ligand Electronic Effect in Enantioselective Palladium-Catalyzed Copolymerization of Carbon Monoxide and Propene

Highly active and stereoselective palladium catalysts for the copolymerization of carbon monoxide with propene are prepared by using Josiphos-type ligands bearing trifluoromethyl substituents. Thus, productivities of up to 1800 g polymer per gram Pd per hour were obtained with ligand 1.

Synthesis and structural characterization of uranium ansa-metallocene complexes containing organoimido functional groups; electronic effects of ancillary ligands

Complexes of U(IV) employing the chelating bis(cyclopentadienyl) ligand sets [Me2Si(η5-C5Me4)2]2− and [Me2Si(η5-C5Me4)(η5-C5H4)]2− have been prepared to examine their utility in generating organoimido complexes of uranium. The chloride complexes [Me2Si(C5Me4)2]UCl2·2LiCl·4(Et2O) (1) and [Me2Si(C5Me4)(C5H4)]-UCl2·2LiCl·4(THF) (2) may be generated at room temperature by reaction of UCl4 with the corresponding bis(cyclopentadienyl) dilithio salt in diethyl ether and tetrahydrofuran, respectively. Complex 1 may be recrystallized from toluene in the presence of TMEDA to yield the complex [Me2Si(C5Me4)2]U(μ-Cl4){Li(TMEDA)}2 (1·TMEDA). The molecular structure of this complex has been determined by single-crystal X-ray diffraction. Compounds 1 and 2 may be alkylated by Grignard reagents. While the dimethyl complex [Me2Si(C5Me4)2]UMe2 (3) may be isolated, the analogous complex and [Me2Si(C5Me4)(C5H4)]UMe2 appears to be thermally unstable. Benzyl derivatives employing both ligand sets (4, 5) have been prepared. These alkyl complexes are protonated by N,N′-diphenylhydrazine to yield phenylimido-containing products. Surprisingly, while reaction of compounds 3 or 4 with N,N′-diphenylhydrazine yields the expected U(VI) complex [Me2Si(C5Me4)2]U(NC6H5)2, reactions employing 5 generate only the U(IV) monoimido dimer [Me2Si(C5Me4)(C5H4)]U(NC6H5)]2 (8). The molecular structure of 8 has been determined; the compound exists as a dimer with asymmetric organoimido bridging ligands. Electrochemical investigations of the chloride compounds 1 and 2 suggest that the ancillary ligands have the capacity to significantly alter the redox activity of the metal center. The use of electron-rich ancillary ligands appears to be important in the isolation of high-valent organouranium complexes.

Outstanding Performances of Magnesia-Supported Platinum–Tin Catalysts for Citral Selective Hydrogenation

Magnesia-supported Sn–Pt catalysts were prepared by reaction between preformed platinum metal particles and Sn(n-C4H9)4. HRTEM and H2-chemisorption data confirm the synthesis of very small tin-decorated platinum particles (∼1.0 nm mean diameter). Preliminary studies on catalytic hydrogenation of citral in a trickle-bed reactor reveal that 97% selectivity to unsaturated alcohols at 100% conversion can be easily achieved. The electronic ligand effect produced by the basic support is supposed to be fundamental to enhance the selectivity to unsaturated alcohols and to stabilise the bimetallic particles.

Significant Electronic Effect of Porphyrin Ligand on the Reactivities of High-Valent Iron(IV) Oxo Porphyrin Cation Radical Complexes.

High-valent iron(IV) oxo porphyrin cation radical complexes containing a series of substituents at the meso position of the porphyrin ring (i.e., electron-donating and -withdrawing substituents on phenyl groups) were prepared and used in oxygen atom transfer reactions to elucidate the electronic effect of porphyrin ligands on the reactivities of iron porphyrin complexes. The reactions that we studied with the in situ generated high-valent iron oxo porphyrins were (1) the relative reactivities of the intermediates toward oxygen atom transfer and ROOH disproportionation (ROOH = hydrogen peroxide and tert-butyl hydroperoxide), (2) the mechanism of heterolytic versus homolytic O-O bond cleavage of hydroperoxides, (3) the dependence of oxidizing power of the intermediates on the electronic nature of porphyrin ligands, and (4) the relative rates between oxygen atom transfer and oxygen exchange with labeled H(2)(18)O. We found from these reactivity studies that (1) a high-valent iron oxo porphyrin complex containing electron-donating substituents reacts fast with ROOH in a competitive reaction performed with a mixture of olefin and ROOH, whereas a high-valent iron oxo porphyrin containing electron-withdrawing substituents transfers its oxygen atom to olefin to give an epoxide product at a fast rate, (2) the O-O bond of hydroperoxides is homolytically cleaved by iron porphyrin complexes in aprotic solvent, (3) a high-valent iron oxo complex of electron-deficient porphyrin ligand is a more powerful oxidizing species than that of electron-rich porphyrin ligand in alkane hydroxylation reactions, and (4) the presence of electron-donating substituents on a porphyrin ligand gives a relatively high (18)O incorporation from labeled H(2)(18)O into an oxygenated product when a mixture of olefin and H(2)(18)O is added to a reaction solution containing a high-valent iron oxo intermediate, whereas only a small amount of (18)O incorporation is observed with iron porphyrin complexes containing electron-withdrawing substituents. These results clearly demonstrate that the electronic nature of iron porphyrin complexes is an important factor in determining the reactivities of iron porphyrin complexes in oxygen atom transfer reactions.

Probing organometallic structure and reactivity by transition metal NMR spectroscopy†

Transition metal NMR chemical shifts are readily measured by modern 1D and 2D pulse techniques and serve as a probe into electronic and steric effects of ligands and substituents in metal complexes. Quantitative correlations of metal chemical shifts with reaction rates and catalytic activities, both experimental and as a result of quantum chemical calculations, give new mechanistic insights and permit reactivity predictions and a screening of homogeneous catalysts. A variety of such examples involving the spin-1/2 nuclei 57Fe, 103Rh, and 187Os as well as the quadrupolar nuclei 51V, 53Cr, 55Mn, 59Co, and 91Zr are discussed.

STERIC EFFECTS ON BINDING O2, SO2 AND CH3I TO SQUARE-PLANAR IRIDIUM(I) COMPLEXES, trans-Ir(CO)(Cl)(P(i-Pr)3)2 AND trans-Ir(CO)(Cl)(P(t-Bu)2Me)2

Steric and electronic effects of phosphine ligands on binding of molecules to iridium(I) have been probed by studying reactions of O2, SO2 and CH3I with trans-Ir(CO)(Cl)(P(i-Pr)3)2 and trans-Ir(CO)(Cl)(P(t-Bu)2Me)2 for comparison to known reactions with trans-Ir(CO)(Cl)L2 (L = PPh3, P(p-tolyl)3 and PCy3). Reaction of O2 is significantly retarded by increasing size of the ligand while reaction of SO2 is essentially unaffected by the nature of the phosphine ligand. Reaction of CH3I with trans-Ir(CO)(Cl)(P(t-Bu)2Me)2 produces two products which have been characterized by NMR and infrared spectroscopy.

Use of Intraligand CH Coupling Constants to Assess Binding in Organometallic Compounds. Insight into Electronic and Steric Properties of Monodentate P-Donor Ligands

In a series of methylcobaloximes (LCo(DH)2CH3, where DH = monoanion of dimethylglyoxime), the influence of P-donor ligands (L) on 1JCH values of the Co−CH3 moiety correlated well with the pKa of L except when L was bulky (Cy3P and i-Pr3P). In contrast, no single steric parameter of those known for the trivalent P donors examined could be used to describe the variance of 1JCH. The results indicate that the dominant property of L influencing this 1JCH is electron donation. Although poor, the best correlation with a steric parameter was found with calculated cone angles (CCA) determined by experimental methods on methanol-coordinated cobaloximes. Since the single parameter fits with pKa indicated a steric effect, multiparameter regressions were performed using various combinations of electronic and steric terms. Terms reflecting only σ-donor ability (e.g., pKa) gave better correlations than terms reflecting combinations of σ-donor and π-acceptor properties (e.g., Σχ). CCA values also resulted in better multiparameter fits of the data than other reported steric terms. Using a multiparameter approach also allowed for the simultaneous fit of data for L = phosphine and phosphite ligands. Correlations of this type were poor with single parameter fits. The sensitivity of 1JCH to trans ligand electronic and steric effects indicates that small changes in the hybridization of the Co−CH3 moiety occur. This method allows us to probe the nature of Co−C bonding in the ground state. The other experimental methods for probing this bond (e.g., X-ray crystallography, FT-Raman spectroscopy, etc.) indicate that this bond is relatively insensitive to the trans ligand in the ground state. Thus, 1JCH values may be a useful general method for examining B12 systems and also hold promise for other types of organometallic compounds.

Rh(I) and Rh(III) silyl PMe 3 complexes. Syntheses, reactions and 103Rh NMR spectroscopy

Synthetic approaches to Rh(I) silyls are described. The complexes L n RhSiR 3 (L=PMe 3; 6, n=4, R 3=(OEt) 3; 7, n=4, R 3=Me(OMe) 2; 21, n=3, R 3=Ph 3) resulted from the reactions of MeRhL 4 ( 1) with the corresponding silanes HSiR 3. Complex 21 was prepared alternatively from PhRhL 3 ( 2) and HSiPh 3, while analogous reactions of HSi(OEt) 3, HSiMe(OMe) 2 and HSi(OMe) 3 led to the bis(silyl)hydrides fac-L 3Rh(SiR 3) 2(H) ( 8, R 3=(OEt) 3; 9, R 3=Me(OMe) 2; 13, R 3=(OMe) 3). Like in analogous iridium-based systems, the outcome of these reactions largely depends on the nature of substituents at the silicon atom. Synthesis of Rh(I) silyls inaccessible by this route, namely those with alkyl substituents at the silicon, L n RhSiR 3 ( 19, n=3, R 3=PhMe 2; 22, n=4, R 3=Me 3), was achieved utilizing nucleophilic attack of the corresponding silyllithiums at [L 4Rh]Cl. The solid-state structure of 19 was determined by X-ray crystallography. C 17H 38P 3SiRh, Fw=466.38 monoclinic, C2/m, a=13.304(3) Å, b=13.814(2) Å, c=13.123(4) Å, β=110.66(3) deg, V=2257(1) Å 3, Z=4, d calcd=1.373 g cm −3, μ=1.019 mm −1. A series of di(hydrido)silyls fac-L 3Rh(H) 2(SiR 3) ( 10, R 3=(OEt) 3; 15, R 3=PhMe 2; 16, R 3=Ph 3) was synthesized using oxidative additions of HSiR 3 to HRhL 4 ( 3). Complexes 10, 15, 16 are thermodynamically stable with respect to H–H and Si–H reductive-elimination reactions at ambient conditions. Complex 8 reductively eliminates HSi(OEt) 3 reversibly at room temperature and complex 13 is capable upon heating of mediating dehydrogenative Si–Si coupling of HSi(OMe) 3 and redistribution of [(MeO) 3Si] 2. 103Rh NMR data obtained for MeRhL 4 ( 1), HRhL 4 ( 3), L 3RhSiPhMe 2 ( 19), L 3RhSiPh 3 ( 21) and for the di(hydrido) silyls ( 10, 15, 16) allowed to qualitatively evaluate steric and electronic effects of methyl, silyl, and hydride ligands on the 103Rh chemical shift.

Mössbauer and visible spectra of iron(III) complexes of para-substituted tetraphenylporphyrins—electronic effects of substituents and axial ligands—

Mössbauer and visible spectra of iron(III) complexes of para-substituted tetraphenylporphyrins—electronic effects of substituents and axial ligands—

Geometric flexibility, ligand and transition metal electronic effects on stereoselective polymerization of propylene in homogeneous catalysis

The stereotacticities of a series of polypropylene polymer chains produced with different categories of structurally characterized ansa-zirconocene and hafnocene procatalysts in homogeneous catalysis have been determined by statistical analyses of the methyl pentad signal intensity distributions of their 13C NMR spectra. The polymers microstructural characteristics have been contrasted with the intramolecular parameters of the discrete metallocene molecules obtained from single crystal X-ray diffractions and different 1H NMR measurements. For the first category, by contrasting the polymerization behavior of zirconocene-hafnocene pairs the origin of the stereospecificity differences have been discussed in terms of heavy atom electron configuration, lanthanoid shell contraction and relativistic effects operative on elements belonging to the sixth row of the periodic table. For the second category the observed changes in polymerization behavior for a pair of zirconocene based catalysts with ligand modification of irrelevant steric control have been correlated to the electron density redistribution on different aromatic carbons and the consequent change in flexibility of the catalyst framework, increase of the haptocity of MC5 bonding and variation of the frontier orbital shape and diffusivity. For the third category, pairs of syndio- and isotactic specific ansa-zirconocene and hafnocenes have been selected in which the organic ligands are constructed with the same aromatic moieties joined with different bridges. The observed catalytic changes have been correlated to combined steric and electronic factors altering orbital energies and hybridizations. Finally, by introducing a fourth category, the Hf effect is demonstrated by comparing the catalytic olefin activities of two non-bridged hafnocene-zirconocene pairs.

Comparative Study of the Visible Spectra of the Iron(III) Complexes of para-Substituted Tetraphenylporphyrins. Electronic Effects of Substituents and Axial Ligands

Abstract The visible absorption spectra of the high-spin iron(III) complexes of the tetra-para-substituted derivatives of 5,10,15,20-tetraphenylporphyrin, Fe(p-Rtpp)X (R = OMe, H, F or Cl; X = Cl, Br or I), have been compared with each other in order to clarify the electronic effects of the phenyl substituents and axial ligands. Two charge-transfer bands shift to higher energy as the electron-withdrawing ability of the substituents increases. This is attributed to a stabilization of the porphyrin a1u(π) and a2u(π) orbitals involved in the electronic transition. These bands shift to lower energy in the order X = Cl < Br < I; the energies of these bands for I are the lowest of the three. By considering the predominantly ionic character of the bond between Fe and X, it has been proved that the iron dyz and dzx levels are lowered in this order, thereby causing a red shift. The Qv porphyrin transition shifts to lower energy in the order X = Cl < Br < I, and the Q0 transition exhibits the same trend to a less extent. These results are explained by taking into consideration two kinds of effects of axial ligands: the electron–electron repulsion in the porphyrin ring and the porphyrin skeletal vibrations.

57Fe NMR Study of Ligand Effects in Cyclopentadienyliron Complexes

The first systematic 57Fe NMR study of ligand effects in cyclopentadienyliron complexes is presented. Four series encompassing a total of 35 compounds have been studied. Among the compounds are five new ring-substituted complexes with the formula (C5H4Y)Fe(CO)(PPh3)(Me), Y = Me, SiMe3, NEt2, I, and Ph. The crystal structure of (C5H4I)Fe(CO)(PPh3)(Me) (32) was determined. For the series CpFe(CO)2R (type I), 57Fe shielding was found to decrease with the bulkiness of the alkyl ligand R and to correlate with the CO-insertion rate of the complex. In type III complexes CpFe(CO)(L)(COMe), L = PR3, and CO, it is again the steric requirement of the ligand L that dominates the 57Fe chemical shift which increases with larger cone angles (θ). In contrast, a strong electronic effect was found for type II complexes CpFe(CO)(PPh3)X, X = H, Me, and I. Ligands with higher electronegativity induce a shift of the 57Fe resonance to higher frequencies. In the complexes (C5H4Y)Fe(CO)(PPh3)(Me), Y = Me, H, SiMe3, NEt2, I, Ph, COOMe, and COiPr (type IV), again electronic effects are dominant, whereby electron acceptor substituents (e.g. Y = COiPr, COOMe) cause a deshielding of the iron nucleus, relative to complexes with electron donor substituents (e.g. Y = Me, H, SiMe3). Dominant electronic ligand effects in type IV complexes are also apparent from the correlation of δ(57Fe) with the electronic substituent parameter σI. The 57Fe chemical shift is shown to be very sensitive to changes in the ligand sphere, and the effects are discussed in terms of the paramagnetic shielding constant (σpara). The 1J(57Fe,13C) coupling constants and longitudinal relaxation times T1 of 57Fe in selected complexes have been determined and ligand effects are discussed.

Comparative study of Mössbauer spectra of iron(III) complexes of para-substituted tetraphenylporphyrins. Electronic effects of substituents and axial ligands

The Mossbauer spectra of high-spin iron(III) complexes of tetra-para-substituted derivatives of 5,10,15,20-tetraphenylporphyrin [FeL(X)](R = OMe, H, F or Cl; X = Cl, Br or I), have been compared with each other to clarify the electronic effects of the phenyl substituents and the axial ligands. They show a variety of patterns depending on the temperature: asymmetric broadening of the high-velocity line at higher temperatures, a symmetric doublet at a temperature Tr, and reversal in asymmetry below Tr. The temperature dependence results from the temperature-dependent spin–spin relaxation of high-spin iron(III) ion and the off-diagonal terms of the hyperfine operator, and reflects the ionic zero-field splitting. The reversal temperature, Tr, and the quadrupole splitting, ΔEQ, show two kinds of dependence on R. This is explained by considering the σ- and π-bonding characteristics of the porphyrins under the influence of the substituents. With a given R, the values of both Tr and ΔEQ increase in the order X = Cl < Br < I. This is interpreted as due to the predominantly ionic character of the bond between the halide and iron ions, except for R = OMe.

A CoMFA Model of Steric and Electronic Effects of Phosphorus Ligands

AbstractThe method of comparative molecular field analysis (CoMFA) has been applied to a series of 6 reactions involving a transition‐metal center and a series of substituted phosphine ligands. A CoMFA analysis of parameters used in classical QSAR including the Tolman cone angle and Brown's steric parameter ER has also been performed. Provided that the dataset of compounds is carefully chosen and is sufficiently large, CoMFA can be used in place of traditional QSAR methods which have employed Tolman cone angles and carbon‐13 chemical shifts as parameters. The CoMFA results show that steric and electronic factors have comparable weight in determining chemical reactivity in the types of reactions considered.

Synthesis, characterization and redox properties of mixed phosphine nitroruthenium(II) complexes. crystal structure of trans-[Ru(NO2)(terpy)-(PMe3)(PPh3)][ClO4]·H2O

The mixed phosphine complexes trans-[Ru(NO2)(terpy)(PMe3)(PR3)][ClO4](terpy = 2,2′ : 6′,2″-terpyridine; R = Et, Pr, Bz or Ph) were synthesized in high yields by a stepwise addition of each phosphine. These syntheses demonstrate the utility of ruthenium(II) in the preparation of mixed phosphine complexes. The stereochemistry of trans-[Ru(NO2)(terpy)(PMe3)(PPh3)][ClO4]·H2O was confirmed by a single-crystal X-ray diffraction study. This species crystallizes in the monoclinic space group P21/c with a= 11.0199(15), b= 18.3888(28), c= 19.3089(25)Å, β= 112.845(9)° and Z= 4. The two phosphine ligands are mutually trans, with P–Ru–P 177.5(1)°. The relative redox stabilities of the mixed phosphine complexes and a related series of trans-[Ru(NO2)(terpy)(PR3)2][ClO4](R = Et, Pr, Bz or Ph) complexes were evaluated using cyclic voltammetric peak current ratios (ipc/ipa) for the ruthenium(III)–ruthenium(II) couples. The rate constants for nitroruthenium(III) decomposition were calculated from the ipc/ipa data and the contributions of electronic (E) and steric (S) factors to its rate of decomposition were determined using the relationship ln k=aE+bS+c. The average ratio of steric to electronic ligand effects on ln k is approximately 30 : 70.

Crystal structures of trans-[Ru(dppe) 2(CO)(Cl)](BF 4)·2(toluene) and trans-[Ru(dppm) 2(CO)(Cl)](BF 4)·CH 2Cl 2: a study of the steric and electronic ligand effects of trans-positioned diphosphine ligands

Single-crystal X-ray diffraction studies were carried out on the complexes trans-[Ru(dppe) 2(CO)(Cl)](BF 4) ·2(toluene) ( 1) (where dppe = 1,2-bis(diphenylphosphino)ethane) and trans-[Ru(dppm) 2(CO)(Cl)](BF 4) ·CH 2Cl 2 ( 2) (where dppm = bis(diphenylphosphino)methane). Complex 1 crystallizes in the orthorhombic space group P2 12 12 with a = 14.366(3), b = 17.075(3), c = 12.433(2) A ̊ , V = 3049.8(10) A ̊ 3 and Z = 2 . The structure was refined to R = 3.02% for 3233 reflections above 6σ ( R = 4.46% for all 3991 point-group independent data); the Ru-cation lies on a site of C 2 symmetry, leading to disorder of the Cl and CO ligands. Complex 2 crystallizes in the orthorhombic space group Pna2 1, with a = 22.425(7), b = 11.515(4), c = 19.511(10) A ̊ , V = 5038(3) A ̊ 3 and Z = 4 . The structure was refined to R = 4.43% for 4770 reflections above 6σ ( R = 7.09% for all 6631 point-group independent data). The crystal structure data suggest increased intramolecular interligand interactions with the trans-bis(dppe) complexes relative to the trans-bis(dppm) complexes. In order to furth assess the steric ligand effects of dppm and dppe on the redox chemistry of ruthenium complexes, the electrochemical data for complexes 1 and 2 as well as for trans-Ru(dppe) 2(Cl) 2 and trans-Ru(dppm) 2(Cl) 2 were analyzed. The E pa value for the oxidation of complex 1 was more positive than the E pa value for the oxidation of complex 2; similarly, the E 1 2 value for the oxidation of trans-Ru(dppe) 2(Cl) 2 was more positive than the E 1 2 value for the oxidation of trans-Ru(dppm) 2(Cl) 2. The increase in the redox potentials for the oxidation of the dope complexes may be due to the enhanced intramolecular interligand interactions of the dppe ligands, which is in agreement with the crystal structure data.

Ligand Electronic Effects in Asymmetric Catalysis: Enhanced Enantioselectivity in the Asymmetric Hydrocyanation of Vinylarenes

The enantioselectivity of the nickel-catalyzed, asymmetric hydrocyanation of vinylarenes using glucose- derived, chiral phosphinite ligands, L, increases dramatically when the ligands contain electron-withdrawing P-aryl substituents. The substrate and solvent also strongly influence the enantioselectivity, with the highest ee's (85-91% for 6-methoxy-2-vinylnaphthalene (MVN)) obtained for the hydrocyanation of electron-rich vinylarenes in a nonpolar solvent such as hexane. Mechanistic studies suggest the catalytic cycle consists of an initial HCN oxidative addition or vinylarene coordination to NiL, followed by insertion to form an (q3-benzy1)nickel cyanide complex, and irreversible reductive elimination of the nitrile. A kinetic analysis of the NiL.(COD) (L., P-aryl = 3,5-(CF3)2C6H3) catalyzed hydrocyanation of MVN indicates that as the HCN concentration is increased the catalyst resting state shifts from NiL,(COD) to a complex containing both MVN and HCN, presumably the (q3-benzy1)nickel cyanide intermediate NiL,(q3-CH3CHCloH60CH3)CN. A 31P NMR analysis of the intermediate NiL(MVN) shows little ground state differentiation of the MVN enantiofaces and suggests that the enantioselectivity is determined later in the mechanism. Deuterium labeling studies suggest that electron-withdrawing P-aryl substituents increase the rate of reductive elimination of the product nitrile from the (q3-benzy1)nickel cyanide intermediate and, on this basis, a rationale for the ligand electronic effect is proposed.

Correlation of the structure of chiral o,o′-bisphenol-Ti(IV) complexes with ligand electronic effects. A relationship between Lewis acidity and angle strain

The electron deficiency of Ti(IV) in complexes with chiral o,o′-bisphenols influences the C ArOTi angle which, in turn determines the topology of the most stable (or easily formed) cyclic complex.