Electron-withdrawing Ligands Research Articles

Activity of Rhodium-Catalyzed Hydroformylation: Added Insight and Predictions from Theory

We have performed a density functional theory investigation of hydroformylation of ethylene for monosubstituted rhodium-carbonyl catalysts, HRh(CO)3L, where the modifying ligand, L, is a phosphite (L = P(OMe)3, P(OPh)3, or P(OCH2CF3)3), a phosphine (L = PMe3, PEt3, PiPr3, or PPh3), or a N-heterocyclic carbene (NHC) based on the tetrahydropyrimidine, imidazol, or tetrazol ring, respectively. The study follows the Heck and Breslow mechanism. Excellent correspondence between our calculations and existing experimental information is found, and the present results constitute the first example of a realistic quantum chemical description of the catalytic cycle of hydroformylation using ligand-modified rhodium carbonyl catalysts. This description explains the mechanistic and kinetic basis of the contemporary understanding of this class of reaction and offers unprecedented insight into the electronic and steric factors governing catalytic activity. The insight has been turned into structure-activity relationships and used as guidelines when also subjecting to calculation phosphite and NHC complexes that have yet to be reported experimentally. The latter calculations illustrate that it is possible to increase the electron-withdrawing capacity of both phosphite and NHC ligands compared to contemporary ligands through directed substitution. Rhodium complexes of such very electron-withdrawing ligands are predicted to be more active than contemporary catalysts for hydroformylation.

Electronic Ligand Effects on the Regioselectivity of the Rhodium−Diphosphine-Catalyzed Hydroformylation of Propene

Electronic effects induced by diphosphine bidentate ligands on the regioselectivity of the rhodium-catalyzed hydroformylation of propene were investigated using density functional theory based calculations (B3LYP). To this end, the key hydride migration step was evaluated for HRh(propene)(CO)L2 (L2 = PF3, PF3; PH3, PH3; PMe3, PMe3; PH3, PF3; PH3, PMe3) incorporating either two identical or two electronically distinct phosphorus moieties. The phosphorus moieties span a wide range of ligand basicities. While the electronic properties of the ligands do not influence the regioselectivity of the hydride migration reaction directly, they do govern the amount of back-donation from the metal to the alkene substrate. As a result, important differences in transition-state geometries are obtained for different ligand systems. For electron-withdrawing ligands low activation energies and trigonal-bipyramidal transition-state geometries are observed. Increasing the basicity of the diphosphine ligand leads to higher act...

Pd-Mediated Activation of Molecular Oxygen in a Nonpolar Medium

The mechanism for direct insertion of O(2) in a toluene-solvated palladium-hydride bond (avoiding palladium zero) has been elucidated using quantum mechanics (B3LYP/LACVP** with the PBF polarizable continuum solvent model) for Pd(II)(-)-sparteine)(Cl)(H) and the model compound Pd(II)(bipyridine)(Cl)(H). We find that the process involves (1) the abstraction of the hydrogen atom by triplet oxygen, (2) the formation of a stable L(2)XPd(I)OOH triplet species, (3) a spin transition resulting in a stable L(2)XPd(II)OOH singlet species, and (4) the loss of H(2)O(2)and completion of the catalytic cycle upon the addition of HX. The limitations involved in the spin transition, the formation of the triplet Pd(I)-OOH species and the stability of that triplet species are all dependent on the presence of an H-bond acceptor cis to the hydride and the electronic characteristics of the other ligands which may or may not stabilize the Pd(I) species. Without this cis H-bond acceptor and/or electron-withdrawing ligands that can stabilize Pd(I), the reaction will not proceed via the palladium hydride insertion mechanism in a nonpolar environment.

Investigation of transition metal-imido bonding in M(NBut)2(dpma).

A complete series down group 6 of the formula M(NBu(t))(2)(dpma) has been synthesized, where dpma is N,N-di(pyrrolyl-alpha-methyl)-N-methylamine. A fourth complex, Mo(NAr)(2)(dpma) (4), was also prepared, where Ar is 2,6-diisopropylphenyl. All four of these complexes display geometries in the solid state best described as square pyramidal with one imido ligand occupying the axial position and the other an equatorial site. In all cases, the axial imido ligand has a significantly smaller M-N(imido)-C bond angle with respect to the equatorial multiple-bond substituent. From the (1)H, (13)C, and (14)N NMR spectra, the axial (bent) imido appears to be more electron-rich than the equatorial and linear imido, with the differences becoming less pronounced down the column. The angular deformation energies for the axial imido ligands were studied by DFT in order to discern if and to what extent imido bond angles were important energetically. The electronic energies associated with straightening the axial imido ligand, while holding the remainder of the molecule at the ground-state geometry, for the Cr, Mo, and W derivatives were calculated as 4.5, 2.7, and 2.0 kcal/mol, respectively. A straight-line plot is found for deformation energies versus estimated electronegativity of the group 6 metals in the +6 oxidation state. The study suggests that the electronic differences between metal imido ligands of different angles are quite small; however, the effects may be more pronounced for metal centers with higher electronegativity, e.g. Cr(VI) with electron-withdrawing ligands.

Synthesis and Structural Characterization of Trivalent Amino Acid Derived Chiral Phosphorus Compounds

Reaction of the N-toluenesulfonyl derivatives of (S)-alanine, phenylalanine, and valine (4-6) with PhPCl(2) gave in high yield the 4-methyl, benzyl, and isopropyl derivatives (7-9) of 2-phenyl-1-p-toluenesulfonyl-1,3,2-oxazaphospholidin-5-one. The ratios of the (2S,4S)/(2R,4S) diastereomers (cis/trans isomers) were 1:1, 2:1, and 10:1 for the methyl, benzyl, and isopropyl derivatives 7a,b, 8a,b, and 9a,b, respectively. For 7a,b, both isomers could be crystallized, but for the others only the major isomers were isolable. The X-ray crystal structure of 9a shows that the isopropyl and phenyl groups are mutually cis and that the tolyl moiety is oriented s-trans to both the isopropyl and phenyl groups. Reaction of 6 with Cl(2)PCH(2)CH(2)PCl(2) (10) gave a 56:38:7 mixture of the cis/cis, cis/trans, and trans/trans diphosphorus heterocycles 11a-c. The major isomer could be crystallized and isolated free of the other diastereomers. Reaction of 6 with EtPCl(2) gave a 6:1 mixture of cis/trans isomers of the ethyl-substituted heterocycles 12a,b as an inseparable oil but allowed confirmation of the structure of 11a. Slow epimerization at phosphorus may occur by inversion but more likely by ring opening/closure, since 7b, 9a, and 11a give rise upon standing in solution to mixtures containing starting material and 7a, 9b, and 11b, respectively, along with the free amino acid derivatives 4 and 6. The NMR spectra, and in particular the coupling constants between the alpha-hydrogen atom of the amino acid moiety and phosphorus, were used to establish the identities of the cis and trans isomers. Reaction of 9a with (THF)W(CO)(5) gave the phosphorus-ligated adduct (9a)W(CO)(5) (13), and the IR spectrum of this complex shows that 9a is a strongly electron-withdrawing ligand. The geometry of the sulfonamide moiety is discussed in detail, as are the (1)H NMR coupling constants. The data are consistent with the presence of little steric interaction between the cis isopropyl and phosphorus substituent in 9a, 11a, and 12a and orientation of the tolyl moiety s-cis to the isopropyl group in 9b, 12b, and 13.

Synthesis and characterization of cyclometalated Ir(III) complexes with pyrazolyl ancillary ligands

The syntheses and structures of a series of ( N, C 2′-(2- para-tolylpyridine)) 2Ir(LX) [( tpy) 2Ir(LX)] and ( N, C 2′-(4 ′,6 ′-difluorophenylpyridine)) 2Ir(LX) [( dfppy) 2Ir(LX)] are reported, where LX are pyrazolyl and pyrazolyl-borate ligands. Reaction of the dichloro-bridged dimer [( tpy) 2Ir(μ-Cl)] 2 with excess pyrazolyl-borate ligands forms a protonated-dipyrazolyl Ir complex, ( tpy) 2Ir(pz 2H). The Ir bound chloride in [( tpy) 2Ir(μ-Cl)] 2 presumably facilitates the hydrolysis of pyrazolyl-borate. Thus, the syntheses of Ir complexes with bis(pyrazolyl)borates, e.g., ( tpy) 2Ir(pz 2Bpz 2) and ( tpy) 2Ir(pz 2BEt 2) require the chloride abstraction by CF 3SO 3Ag before pyrazolyl-borate is added to the reaction solution. ( tpy) 2Ir(pz 2H), ( tpy) 2Ir(pzH) 2(CF 3SO 3), ( tpy) 2Ir(pz 2Bpz 2) and ( tpy) 2Ir(pz 2BEt 2) have been structurally characterized by X-ray crystallography. The two pyrazolyl rings of ( tpy) 2Ir(pz 2H) are nearly coplanar (dihedral angle, 12°), due to the presence of N–H⋯N hydrogen bond (N⋯N distance of 2.56 Å), while the two pyrazolyl rings of ( tpy) 2Ir(pzH) 2(CF 3SO 3) are not coplanar (pz–pz dihedral angle, 47°). Unlike other η 2-pyrazolyl-borate complexes, the ( tpy) 2Ir(pz 2BEt 2) exhibits a rare quasi-chair conformation adopted by ▪ the ring, rather than the more common boat conformations which have been observed previously. Similarly, the ▪ cycle of ( tpy) 2Ir(pz 2Bpz 2) adopts a half-chair conformation, more flattened than the typical boat conformation. While the choice of pyrazolyl-borate ligand does not change the structural properties of the “( tpy) 2Ir” fragment, it markedly affects the photophysical properties of both ( tpy) 2Ir(pz 2BR 2) and ( dfppy) 2Ir(pz 2BR 2) complexes. The pyrazolyl-borate ancillary ligands affect the absorption and emission energies of these complexes by tuning the HOMO (highest occupied molecular orbital) energies. The electron-withdrawing ancillary ligands, e.g., pz 2Bpz 2 − , decrease the electron density on the iridium, stabilizing the metal-centered HOMOs.

Sterically hindered electron-withdrawing ligands: the reactions of N-carbazolyl phosphines with rhodium and palladium centres.

The series of N-carbazolyl phosphines PPh(3-n)(NC(12)H(8))(n)(n= 1, L1; n= 2, L2; n= 3, L3) has been synthesised using BuLi to generate the N-carbazolyl lithium salt, followed by reaction with the appropriate chlorophosphine. The reactions between [Rh(mu-Cl)(CO)(2)](2) and four equivalents of L1 or L2 gave [RhCl(CO)(L1)(2)] 1 and [RhCl(CO)(L2)(2)] 2, though attempts to synthesise the analogous complex using L3 resulted in the formation of [Rh(mu-Cl)(CO)(L3)](2) 3 instead. The inability of L3 to cleave the chloride bridges can be related to its considerable steric requirements. The electronic properties of L1-3 were assessed by comparison of the nu(CO) values of the [Rh(acac)(CO)(L1-3)] complexes 4-6. The increase in number of N-carbazolyl substituents at the phosphorus atom results in a decrease of the sigma-donor and increase in the pi-acceptor character in the order L1 < L2 < L3. In the reactions of L1-3 with [PdCl(2)(cod)] only L1 was able to displace cod from the metal centre and form [PdCl(2)(L1)(2)] 7. The use of [PdCl(2)(NCMe)(2)] instead of [PdCl(2)(cod)] resulted in the formation of the complexes [PdCl(2)(L1)(2)] 7 from L1, the cyclometallated complex [Pd(mu-Cl)[P(NC(12)H(8))(2)(NC(12)H(7))-kappa(2)P,C]](2) 8 from L3 , and a mixture of [PdCl(2)(L2)(2)] 9 and [Pd(mu-Cl)[PPh(NC(12)H(8))(NC(12)H(7))-kappa(2)P,C]](2) 10 from L2 . The reaction of L3 with [Pd(OAc)(2)] produced the cyclometallated complex [Pd(mu-O(2)CCH(3))[P(NC(12)H(8))(2)(NC(12)H(7))-kappa(2)P,C]](2) 11. The reaction of L3 with [Pd(2)(dba)(3)].CHCl(3) produced the 14-electron complex [Pd(L3)(2)] 12. The X-ray crystal structures of six complexes are reported, all of which show the presence of C-H...Pd hydrogen bonding.

Addition Polymerization of Norbornene-Type Monomers Using Neutral Nickel Complexes Containing Fluorinated Aryl Ligands

The strong Lewis acid B(C6F5)3 was found to activate complexes of nickel toward the polymerization of norbornene-type monomers. The active species in this reaction is created by the transfer of C6F5 from boron to nickel. As a result, a class of neutral, single-component nickel complexes was developed containing two electron-withdrawing aryl ligands that polymerize norbornene and norbornenes with functional pendant groups. Active complexes include Ni(C6F5)2(PPh2CH2C(O)Ph), (η6-toluene)Ni(C6F5)2, and Ni(2,4,6-tris(trifluoromethyl)phenyl)2(1,2-dimethoxyethane). In the case of (η6-toluene)Ni(C6F5)2, isolation and characterization of low molecular weight norbornene polymers, using ethylene, indicated that each polymer chain contained a C6F5 headgroup. This points to the initiation step as being the insertion of norbornene into the Ni−C6F5 bond. The polymer microstructure as revealed by 1H and 13C NMR spectrometry is entirely different from that produced using the cationic nickel catalyst, [(η3-crotyl)Ni(1,4-CO...

Can the ebselen derivatives catalyze the isomerization of peroxynitrite to nitrate?

The reaction of ebselen and its derivatives (1-7) with peroxynitrite anion (ONOO(-); PN) has been studied in gas phase and in aqueous, dichloromethane, benzene, and cyclohexane solutions using B3LYP/6-311+G(d,p)//B3LYP/6-311G(d,p) and PCM-B3LYP/6-311+G(d,p)//B3LYP/6-311G(d,p) approaches, respectively. It was shown that the reaction of 2 (R=H) with PN proceeds via 2 + PN --> 2-PN --> 2-TS1 (O-O activation) --> 2-O(NO(2)(-)()) --> 2-SeO + NO(2)(-) pathway with a rate-determining barrier of 25.3 (14.8) kcal/mol at the NO(2)(-) dissociation step (numbers presented without parentheses are enthalpies, and those in parentheses are Gibbs free energies). The NO(3)(-) formation process, starting from the complex 2-O(NO(2)(-)()), requires by (7.9) kcal/mol more energy than the NO(2)(-) dissociation process and is unlikely to compete with the latter. Thus, in the gas phase, the peroxynitrite --> nitrate isomerization catalyzed by complex 2 is unlikely to occur. It is shown that the NO(3)(-) formation process is slightly more favorably than the NO(2)(-) dissociation process for complex 4, with a strongest electron-withdrawing ligand R=CF(3). Therefore, complex 4 (as well as complex 6 with R=OH) is predicted to be a good catalyst for peroxynitrite <--> nitrite isomerization in the gas phase. Solvent effects (a) change the rate-determining step of the reaction 2 + PN from NO(2)(-) dissociation in the gas phase to O-O activation, which occurs with barriers of (13.9), (8.4), (8.4), and (8.2) kcal/mol in water, dichloromethane, benzene, and cyclohexane, respectively, and (b) significantly reduce the NO(2)(-) dissociation energy, while only slightly destabilizing the NO(3)(-) formation barrier, and make the peroxynitrite <--> nitrate isomerization process practically impossible, even for complex 4.

Nitrogen NMR spectroscopy of metal nitrosyls and related compounds.

The chemistry of transition-metal-NO complexes, or metal nitrosyls, has taken on added significance in recent years because of the important role that nitric oxide has been found to play as a signaling molecule in biological systems. Recent work has also established that the characteristic chemistry of nitrosyl complexes is often markedly different from that exhibited by their isoelectronic carbonyl analogues. It has been almost 10 years since these complexes were last reviewed in the book Metal Nitrosyls,1 and so it seemed to us that an update of this rapidly expanding field would be appropriate at the present time. Consequently, this article picks up where Metal Nitrosyls left off and summarizes comprehensively the literature dealing with nitrosyl complexes from 1991 to mid-2001. Some particularly relevant work reported in late 2001 has been included where appropriate during the final stages of revising and polishing the text. As outlined in the table of contents, the pertinent information concerning the compounds themselves is first partitioned in terms of the position of the central transition metal in the periodic table and then in terms of the ligands other than NO present in the complexes. In the interests of keeping the article to a manageable size, most of the routine preparative and characterization data for individual compounds are simply referenced rather than being presented and discussed in detail. The review concludes with a general section that applies, in principle, to all classes of transition-metal-NO complexes, namely the characteristic reactivities of bound NO groups. Whenever possible, emphasis is placed on the unique physical and chemical properties imparted to nitrosyl complexes by the presence of the strongly electron-withdrawing NO ligands. If not specified otherwise, the abbreviations employed throughout are those summarized in the ACS Style Guide.

Electron-Withdrawing Phosphine Compounds in Hydroformylation Reactions. 1. Syntheses and Reactions Using Mono- and Bis(p-toluenesulfonylamino) Phosphines

The rhodium-catalyzed hydroformylation of 1-hexene has been examined in the presence of members of a new class of electron-withdrawing phosphorus ligands, the N-sulfonylphosphoramides. All of the phosphorus compounds in this initial study contain one or two p-toluenesulfonylamino (TsN) groups attached to the phosphorus atom, including three compounds that have been described previously, TosL (1), a monophosphorus compound with two TsN groups, diTosL (2), a diphosphorus compound with one TsN group on each phosphorus atom, and 3, a chiral amino acid-derived ligand with one TsN and one O-acyl group on phosphorus. In addition, two new chelating analogues of 1 containing two- and four-carbon bridges between the phosphorus atoms (5, 7), an analogue of 1 with an ethyl instead of a phenyl group on phosphorus (8), a nonchelating monophosphorus analogue of 2 (10), and a monophosphorus adduct of the ditosylate of o-phenylenediamine (12) have been synthesized and used in hydroformylations, and comparison reactions with PPh3 in THF, toluene, and CH2Cl2 have been run. The 13C NMR spectra of 5 and 7 and related diphosphorus compounds have been examined for evidence of false AA‘X spectra in which the chemical shifts of the nominally equivalent phosphorus atoms are split by the presence of a single 13C atom. The chelating compound 2 is by far the most effective hydroformylation ligand, giving high turnover frequencies (TOF) and linear to branched (n:i) ratios of the aldehyde product. Reactions of 2 run at a 1000:10:1 ratio of 1-hexene:2:Rh(acac)(CO)2 at 84 psi CO/H2 at 60 °C in THF gave TOF = 440 mol aldehyde/mol Rh/h and an n:i ratio of 10, and at 80 °C gave TOF = 760 and an n:i ratio of 15.8. Reactions with 2 were also run in toluene, giving similar results, and in CH2Cl2, giving rise to higher n:i ratios (up to 28.5) but also to faster catalyst deactivation. In the absence of chelation, 10 gave lower turnover frequencies (TOF) and linear-to-branched ratios (n:i), and 1 and 3 also gave lower TOF values and low n:i ratios similar to those of PPh3 and 10. The chelating analogues of 1, 5 and 7, were very poor ligands and gave n:i ratios characteristic of monophosphorus ligands. Compounds 8 and 12 inhibit all reaction.

Mechanism of Cyclopropanation Reactions Mediated by (5,10,15,20-Tetra-p-tolylporphyrinato)osmium(II) Complexes

Catalytic systems derived from [Os(TTP)]2 or Fe(TTP) (TTP = 5,10,15,20-tetra-p-tolylporphyrinato) are extremely efficient at converting styrenes and diazo reagents to cyclopropanes in high yields and high stereoselectivity. A number of mechanistic studies have been undertaken to elucidate the catalytic pathway. A mono(carbene) complex, (TTP)OsCHCO2Et, has been isolated but is not the catalytically active species. An electron-withdrawing ligand trans to the carbene in (TTP)OsCHCO2Et activates the carbon fragment toward transfer to an olefin. Labeling studies with (TTP)OsCHX and N2CHY and substrate reactivity profiles are consistent with a trans-osmium(II) bis(carbene) species as the active catalyst.

Kinetics and substitution mechanism in the reaction of the seven-coordinate complexes bis-μ-diphenylphosphido- and bis-μ-dimethylphosphido-octacarbonyldimolybdenum (MoMo) with tri-n-butyl-phosphine

The dinuclear phosphido-bridged molybdenum complexes [(CO) 4 Mo(μ-PR 2) 2M o(CO) 4] (R=Ph or Me) react with P(n-Bu) 3 to give the corresponding mono and bis-phosphine derivatives. A kinetic study of the first substitution in decalin indicates a CO-dissociative mechanism involving the coordinatively unsaturated intermediate [(CO 4) Mo(μ-PR 2) 2M o(CO) 3]. The overall substitution rate depends on the rate of CO dissociation, k 1, and on the rate of bimolecular attack by CO, k −1, and by P(n-Bu) 3, k 2, on the reactive intermediate. The nature of the substituents at the phosphido bridge markedly affects the value of k 1, which is higher for the phenyl compared with the methyl group. This is mainly due to a lower activation enthalpy (Δ H 1*=125 (Ph) versus 141 (Me) kJ mol −1), which may reflect a weakening of the MoCO bond in the presence of a more electron-withdrawing ligand in trans position. The values of the competition rate ratio k −1/ k 2, always largely greater than unity, show that attack of the small CO is favoured with respect to the large P(n-Bu) 3; this suggests that the steric crowding observed on the molecular structure of the starting seven-coordinate complexes should play an important role also on the reactivity of their six-coordinate intermediates.

Synthesis and Characterization of Soluble Chloro- and Aryl(naphthalocyaninato)indium(III) Complexes and Their Precursors

The syntheses of highly soluble chloro(naphthalocyaninato)indium(III) compounds (R2)4(R4)4NcInCl (5a: R2 = H, R4 = tert-butyl; 6a: R2 = R4 = tert-butyl; 7a: R2 = 2-ethylhexyloxy, R4 = tert-butyl; 8a: R2 = R4 = 2-ethylhexyloxy), and their reactions with R′MgBr [b: R′ = p-(trifluoromethyl)phenyl; c: R′ = pentafluorophenyl] producing the axially substituted aryl(naphthalocyaninato)indium(III) compounds 5b, 5c, 6b, 6c, 7b, 7c, 8b, and 8c are described. All compounds were characterized by IR, UV/Vis, MS, and 1H, 13C, and 19F NMR, as well as by elemental analysis. The high solubilities of the octasubstituted naphthalocyanines 6−8 and their low tendencies to form aggregates are due to steric crowding arising from the new unsymmetrical peripheral substitution pattern and the bulky, electron-withdrawing axial ligands. These properties make the (naphthalocyaninato)indium compounds very interesting candidates for further investigations concerning their nonlinear optical properties, particularly for optical limiting applications. We also report on the complete syntheses and characterization of the corresponding diiminoisoindoline precursors 2 (R2 = R4 = tert-butyl), 3 (R2 = 2-ethylhexyloxy; R4 = tert-butyl) and 4 (R2 = R4 = 2-ethylhexyloxy).

Synthesis of highly ethynylated mono and dinuclear Pt(II) tethers bearing the 4,4′-bis(ethynyl)biphenyl (debp) unit as central core

Mono and bisubstituted bis(ethynyl)biphenyl Pt(II) complexes, i.e. trans-[(RCC)Pt(PPh 3) 2(CC p-C 6H 4 p-C 6H 4CCH)], R= p-NO 2C 6H 4 ( 3b), (η 5-C 5H 5)Fe(η 5-C 5H 4) ( 3c), and trans-[(RCC)Pt(PPh 3) 2(CC p-C 6H 4 p-C 6H 4CC)Pt(PPh 3) 2(CCR)], R=C 6H 5 ( 4a) , p-NO 2C 6H 4 ( 4b), (η 5-C 5H 5)Fe(η 5-C 5H 4) ( 4c), have been synthesized by the dehydrohalogenation reaction from the appropriate Pt monochloro acetylides trans-[(RCC)Pt(PPh 3) 2Cl], R=C 6H 5 ( 1a), R= p-NO 2C 6H 4 ( 1b), R=(η 5-C 5H 4)Fe(η 5-C 5H 5) ( 1c) and HCC p-C 6H 5 p-C 6H 5CCH, (4,4′-bis-ethynylbiphenyl), (DEBP) ( 2). In order to make possible a direct evaluation of the role of Pt centers on the properties of these highly ethynylated complexes, the corresponding species without Pt were prepared, i.e. C 6H 5CCCCC 6H 4C 6H 4CCCCC 6H 5, (4,4′-(bis-phenylethynyl)-diethynylbiphenyl) ( 7), and (η 5-C 5H 5)Fe(η 5-C 5H 4)(CCCCC 6H 4C 6H 4CCCC)(η 5-C 5H 4)Fe(η 5-C 5H 5), (4,4′-(bis-ferrocenylethynyl)-diethynylbiphenyl) ( 8). Preparation of the latter compounds was only achieved by the use of the palladium catalyzed Stille coupling reaction, since the dehydrohalogenation route afforded only homocoupled and polymeric products. The electron-donor or electron-withdrawing ligands coordinated to Pt were chosen for the purpose of tuning the optical properties of the complexes through the different charge distribution along the π-electron conjugation. Characteristic spectroscopic features (UV–vis, FT-IR and NMR) of these complexes are discussed.

A First-Principles Quantum Chemical Analysis of the Factors Controlling Ruffling Deformations of Porphyrins: Insights from the Molecular Structures and Potential Energy Surfaces of Silicon, Phosphorus, Germanium, and Arsenic Porphyrins and of a Peroxidase Compound I Model

Using nonlocal density functional theory calculations, we have examined several factors influencing ruffling deformations of porphyrin and porphyrazine complexes. Because a ruffling distortion is often a direct result of a small macrocycle core size, which, in turn, is brought about by complexation of a central ion with a small ionic radius, this study focuses on the conformations and potential energy surfaces of porphyrin complexes with small central ions (herafter symbolized M) such as SiIV, PV, GeIV, and AsV. The optimized geometries exhibit ruffling torsion angles ranging from 0° for (P)GeIVF2 and (P)SiIV(C⋮CPh)2 to about 55° for [(P)PVF2]+. For relatively substantial ruffling distortions, a good linear correlation has been found between the ruffling torsion angle and the M−N distance for a wide variety of central ions including transition metals, for sterically unhindered porphyrins, and for a database including both experimental and optimized structures. The threshold between ruffling and planar structures is at M−N bond distances of 2.00−2.02 Å for sterically unhindered porphyrins and at 1.85−1.87 Å for porphyrazines. The calculations confirm an experimental observation that electron-withdrawing axial ligands lead to increased ruffling, especially for phosophorous and silicon porphyrins. The ortho hydrogens of axial phenyl ligands and the 2- and 6- hydrogens of axial pyridine ligands can sterically interfere with the porphyrin and contribute to ruffling. In addition to the M−N distance, a number of other geometrical parameters also vary systematically with the ruffling distortion. Thus, the Cα−Cmeso−Cα angle decreases with ruffling and the CβCβ distance and Cα−N−Cα angle increase with ruffling. These structural variations are reflected in a number of ruffling-sensitive vibrational frequencies.. The ruffled D2d geometries of [(P)PVF2]+ and [(P)PVCl2]+ are stabilized by 9.25 and 5.26 kcal/mol, respectively, relative to planar D4h symmetry-constrained optimized geometries. In contrast, the ruffled D2d geometries of [(Pz)PVF2]+ and of all the silicon complexes studied are more stable than the corresponding D4h symmetry-constrained optimized geometries by less than 0.01 kcal/mol. This underscores the extreme softness of ruffling deformations and shows that even fairly large distortions, where the ruffling torsion angle changes by up to 25°, can occur with almost no expenditure of energy. Finally, through a reinvestigation of a recent study of a peroxidase compound I model, we have uncovered a heretofore unsuspected role of Fe(3dxy)-porphyrin(atu) orbital interactions in macrocycle ruffling. Also seen for certain low-spin ferrihemes, this orbital interaction is not important for manganese porphyrins.

Quadruple Metal−Metal Bonds with Strong Donor Ligands. Ultraviolet Photoelectron Spectroscopy of M2(form)4 (M = Cr, Mo, W; form = N,N‘-diphenylformamidinate)

The He I photoelectron spectra of M2(form)4 (M = Cr, Mo, W; form = N,N‘-diphenylformamidinate) and Mo2(cyform)4 (cyform = N,N‘−dicyclohexylformamidinate) are presented. For comparison, the Ne I, He I, and He II photoelectron spectra of Mo2(p-CH3-form)4 have also been obtained. The valence ionization features of these molecules are interpreted based on (1) the changes that occur with the metal and ligand substitutions, (2) the changes in photoelectron cross sections with excitation source, and (3) the changes from previously studied dimetal complexes. These photoelectron spectra are useful for revealing the effects that better electron donor ligands have on the valence electronic structure of M2(L⌒L)4 systems. Comparison with the He I spectra of the isoelectronic M2(O2CCH3)4 compounds is particularly revealing. Unlike with the more electron-withdrawing acetate ligand, several formamidinate-based ionizations derived from the nitrogen pπ orbitals occur among the metal−metal σ, π, and δ ionization bands. Alth...

Theoretical Study of Oxidative Addition and Reductive Elimination of 14-Electron d10ML2Complexes: A ML2+ CH4(M = Pd, Pt; L = CO, PH3, L2= PH‘2CH2CH2PH2) Case Study

We have chosen six ML2 complexes, with a systematic variation in the ligands and metals, to investigate oxidative additions as well as reductive eliminations by using the MP2/LANL2DZ and the MP4SDTQ//MP2/LANL2DZ levels of theory. A qualitative model based on the theory of Pross and Shaik (Su, M.-D. Inorg. Chem. 1995, 34, 3829) has been used to develop an explanation for the barrier heights. Considering the geometrical effect, the substituent effect, and the nature of the metal center, the following conclusions emerge: for 14-electron ML2 complexes, a smaller L−M−L angle and a better electron-donating ligand as well as a heavier transition metal center (such as Pt) should be a potential model for the oxidative addition of saturated C−H bonds. Conversely, a linear structure and a better electron-withdrawing ligand as well as a lighter transition metal center (such as Pd) would be a good candidate for reductive coupling of C−H bonds. The results obtained are in good agreement with the available experimental...

Mechanistic aspects of the thermal mer-to- fac isomerization of mer-[Mn(X)(CO) 3( α-diimine)] (X=Cl, Br, I)

The thermal mer-to- fac isomerization of the photochemically generated complexes mer-[Mn(X)(CO) 3( α-diimine)] (X=Cl, Br, I, α-diimine= N,N′-diisopropyl-1,4-diazabutadiene ( iPr-DAB); X=Br, α-diimine=4,4′-dimethyl-2,2′-bipyridine (bpy′), pyridine-2-carbaldehyde- N-isopropylimine ( iPr-PyCa), 2,3-dipyrid-2′-ylpyrazine (dpp), and R-DAB (R= cPr, iPr, tBu)) was investigated by real-time (RT) FTIR spectroscopy at various temperatures and in different solvents. In addition, the fac- and mer-isomers were also studied by UV–vis spectroscopy. The intricate isomerization process has been characterized by determination of (i) (pseudo) first-order rate constants from linear ln(A oo–A t) versus time plots, and (ii) the corresponding thermodynamic activation parameters ΔS ‡ and ΔH ‡ from Eyring plots. For mer-[Mn(Br)(CO) 3(bpy′)] in THF the kinetic and thermodynamic data are consistent with a mechanism which probably involves association of THF and partial dissociation of the Br − ligand. In contrast with this, the rapid isomerization of the complexes mer-[Mn(X)(CO) 3(R-DAB)] (X=Cl, Br; R= iPr, and X=Br; R= cPr, iPr, tBu) with the highly flexible electron-withdrawing R-DAB ligands follows a dissociative course probably involving cleavage of a Mn–N bond. These major reaction pathways have been employed to explain the isomerization of the other complexes under study, viz. mer-[Mn(X)(CO) 3( α-diimine)] (X=Br; α-diimine= iPr-PyCa, dpp, or X=I; α-diimine= iPr-DAB).

A Chiral N-Sulfonylphosphoramide: Synthesis and X-ray Crystal Structure of a 1,3,2-Oxazaphospholidin-5-one, a Trivalent Electron-Withdrawing Amino Acid-Derived Phosphorus Compound, and Synthesis of Its W(CO)(5) Adduct.

Reaction of the amino acid derivative (S)-N-tolylsulfonylvaline with PhPCl2 and NEt3 gives in >90% yield a novel chiral phosphorus heterocycle as a greater than 7:1 mixture of diastereomers. The X-ray crystal structure of the major isomer 7a shows that the isopropyl and phenyl groups are mutually cis. The IR spectrum of the W(CO)5 adduct of 7a shows that the N-sulfonyl and carboxylate moieties make it a strongly electron-withdrawing ligand.