C5H5 Ring Research Articles

Formation of Metallic Polyferrocene Chains under Pressure.

The effect of pressure on the structural reorganization of ferrocene, Fc = (C5H5)2Fe, is studied using density functional theory (DFT) calculations assisted by evolutionary crystal structure prediction algorithms based on USPEX code. Pressure brings the individual molecules in close contact, and above 220 GPa, the 18-electron closed-shell molecular unit undergoes polymerization through the formation of quasi-one-dimensional (1D) chains, [(C5H5)2Fe]∞, termed as polyferrocene (p-Fc). Pressure induced polymerization (PIP) of Fc causes significant deviations from the 5-fold symmetry of the cyclopentadiene (Cp, C5H5 rings) and loss of planarity due to the onset of envelope-like distortions. This triggers distortions within the multidecker sandwich structures and σ(C-C) bond formation between the otherwise weak noncovalently interacting Cp rings in Fc crystals. Pressure gradually reduces the band gap of Fc, and for p-Fc, metallic states are found due to increased electronic coupling between the covalently linked Cp rings. Polyferrocene is significantly more rigid than ferrocene as evident from the 5-fold increase in its bulk modulus. Pressure dependent Raman spectra show a clear onset of polymerization in Fc at P = 220 GPa. Higher mechanical strength coupled with its metallicity makes p-Fc an interesting candidate for high pressure synthesis.

High-pressure behavior of the crystal structure of the fullerene molecular complex with ferrocene C60·{Fe(C5H5)2}2

The crystal structure of the molecular complex C60·{Fe(C5H5)2}2 was studied by single crystal X-ray diffraction (XRD) analysis at pressures up to 5 GPa using the diamond anvil cell (DAC) technique. The XRD data and subsequent structural analysis clearly show that there is no pressure-induced polymerization in fullerene layers in pressure range studied. The reciprocal unit cell volume V0/V is a smooth and monotonous function of pressure and fits well to the Murnaghan equation of state (V0/V) B'= {1 + P·(B'/B0)}, where V0 is the volume at ambient pressure, B0=8.7 GPa and B'=10.5 are the bulk modulus and its derivative, respectively. Pressure dependence of the shortest distance between the C5H5 ring of the ferrocene molecule and the center of the nearest fullerene molecule is linear, while the slope changes at 2.2 GPa indicating on the intermolecular interactions crossover. Other relevant parameter, the Fe-C bond lengths of ferrocene, sensitive to iron charge state, gradually increases. This peculiarity can a sign of pressure-induced partial charge transfer between the donor ferrocene and acceptor fullerene molecules.

Energetics of Variable Hapticity of Carbocyclic Rings in Cyclopentadienylmetal Carbonyl Systems of the Second Row Transition Metals C5H5M(CO)nCmHm (M = Ru, Tc, Mo, Nb) Including Mechanistic Studies of Carbonyl Dissociation

Decarbonylation of the experimentally known CpRu(CO)2(η1-C5H5), CpMo(CO)2(η3-C7H7), and CpNb(CO)2(η4-C8H8) (Cp = η5-C5H5), each with uncomplexed 1,3-butadiene units in the CnHn ring, as well as the related CpTc(CO)2(η2-C6H6), to give the corresponding carbonyl-free derivatives CpM(ηn-CnHn) derivatives has been studied by density functional theory. For ruthenium, technetium, and molybdenum the coordinated CnHn ring of the intermediate monocarbonyl CpM(CO)(ηn–2-CnHn) contains an uncomplexed C═C double bond and each decarbonylation step proceeds with a significant energy barrier represented by a higher energy transition state. However, decarbonylation of CpNb(CO)2(η4-C8H8) to the monocarbonyl proceeds without an energy barrier, preserving the tetrahapto coordination of the C8H8 ring to give CpNb(CO)(η4-C8H8) in which the niobium atom has only a 16-electron configuration. All of the monocarbonyl derivatives CpM(CO)(CnHn) are predicted to be strongly energetically disfavored with respect to disproportionation to give CpM(CO)2(CnHn) + CpM(CnHn). This allows us to understand the failure to date to synthesize any of the monocarbonyl derivatives.

Metal–metal bonding in biscycloheptatrienyl dimetal compounds of the second‐row transition metals

AbstractDensity functional theory has been used to examine the dimetallocene‐like dicycloheptatrienyl dimetal compounds of the second‐row transition metals (C7H7)2M2 (M = Ru, Tc, Mo, Nb, Zr). The lowest energy (C7H7)2Mo2 structure is a coaxial structure with terminal η7C7H7 rings, whereas the lowest energy (C7H7)2M2 structures (M = Ru, Tc, Nb, Zr) are perpendicular structures with bridging η4,η4C7H7 rings except for the perpendicular (η4,η3C7H7)2Ru2 structure. The metal–metal bond orders in the (C7H7)2M2 structures (M = Ru, Tc, Mo, Nb), as determined by analysis of their frontier molecular orbitals, suggest preferred 16‐ rather than 18‐electron configurations for the central metal atoms. Thus, in the coaxial (η7C7H7)2M2 structures the formal bond orders are two for M = Tc and three for M = Mo. For the perpendicular structures both (η4,η3C7H7)2Ru2 and (η4,η4C7H7)2Tc2 have 16‐electron configurations with metal–metal single bonds owing to the different modes of bonding of the bridging C7H7 rings in the two structures. For the (C7H7)2Zr2 system the perpendicular structure has a formal ZrZr double bond and the coaxial structure has a very long (∼3.5 Å) ZrZr bond indicating only 12‐ to 14‐electron configurations for the zirconium atoms.

Stability and Destabilization Processes in the Formation of Ferrocene-Based Metal–Organic Molecule–Metal Nano-Junctions

The realization of functional metal–molecule junctions relies on our understanding and control of the fundamental interactions leading to or preventing the formation of nanojunctions. In this work, focusing on the promising double-decker organometallic compound ferrocene, deposited on a Cu(111) surface, we show that destabilization or dissociation of these compounds can occur depending on the nature of the metal atom deposited on the ferrocene. Specifically, at variance with the more common deposition of Cu atoms, Fe atoms can give rise to a disassembly of the Fe(C5H5)2 structure in a barrierless way, potentially realizing catalytic active sites. Conversely, the deposition of Co can occur only upon overcoming an activation barrier of about 0.85 eV. In this case, the ferrocene does not dissociate, but the Co atom results fully inserted in the upper C5H5 ring of the organometallic molecule, thus becoming a new six-member structure that eventually allows for a selective extraction of deposited ferrocene mole...

Spin Trapping of Carbon-Centered Ferrocenyl Radicals with Nitrosobenzene

In contrast to metal centered 17 valence electron radicals, such as [Mn(CO)5]•, ferrocenium ions [Fe(C5H5)2]+ (1+), [Fe(C5Me5)2]+ (2+), [Fe(C5H5)(C5H4Et)]+ (3+), [Fe(C5H5)(C5H4NHC(O)Me)]+ (4+), and [Fe(C5H5)(C5H4NHC(S)Me)]+ (5+) do not add to nitrosobenzene PhNO to give metal-coordinated stable nitroxyl radicals. In the presence of the strong and oxidatively stable phosphazene base tert-butylimino-tris(dimethylamino)phosphorane, the quite acidic ferrocenium ions 1+–5+ are deprotonated to give a pool of transient and persistent radicals with different deprotonation sites [1–Hx]•–[5–Hx]•. One rather persistent iron-centered radical [4–HN]•, deprotonated at the nitrogen atom, has been detected by rapid-freeze EPR spectroscopy at 77 K. This iron-centered radical [4–HN]• is also inert toward PhNO. The transient carbon-centered radicals [1–Hx]•–[5–Hx]• appear to rapidly abstract hydrogen atoms from the adjacent base or the solvent to regenerate the corresponding ferrocenes 1–5. These transient radicals are only present in trace amounts (<1%). However, some of the transient carbon-centered radicals in the radical pool can be trapped by 1–1.2 equiv of PhNO, even at room temperature. The corresponding resulting stable nitroxyl radicals [6]•–[10]• were studied by EPR spectroscopy at room temperature and at 77 K. The hyperfine coupling pattern to protons close to the spin center allows one to assign the site of PhNO attack in radicals [6]•–[10]•, namely, at the C5H5 ring in [6]•, [9Cp]•, and [10Cp]•, at a methyl group in [7]•, and at the methylene group in [81]•. These studies give a deeper insight into the stability and reactivity of radicals derived from ferrocene derivatives which might also be relevant for the biological activity of high-potent antitumor and antimalaria ferrocene-based drugs and prodrugs such as ferrocifen or ferroquine.

ChemInform Abstract: Systematic Approach for the Bonding in Ferrocene

AbstractReview: 3 refs.

Infrared matrix-isolation and theoretical studies of the reactions of ferrocene with ozone.

The reactions between ferrocene (Cp2Fe) (2a) and ozone (O3) were studied using low-temperature matrix-isolation techniques coupled with theoretical density functional theory (DFT) calculations. Co-deposition of Ar/Cp2Fe and Ar/O3 gas mixtures onto a cryogenically cooled CsI window produced a dark-green charge-transfer complex, Cp2Fe-O3, that photodecomposed upon red (λ ≥ 600 nm) and infrared (λ ≥ 1000 nm) irradiation but was stable to green or blue irradiation. Products of photodecomposition were characterized by FT-IR, oxygen-18 labeling, and DFT calculations using the B3LYP functionals and the 6-311G++(d,2p) basis set. Likely, photochemical products included four structures having the molecular formula C10H10FeO, identified by DFT calculations based on their calculated infrared spectra and (18)O isotope shifts. Each of these calculated molecules had one intact and fully coordinated η(5)-C5H5 cyclopentadienyl (Cp) ring and (1) an η(5)-C5H5O cyclic ether (pyran ring) (2b), (2) an η(4)-C5H5O linear aldehyde (2c), (3) a bidentate cyclic aldehyde with a seven-membered ring including the iron atom (2d), or (4) an Fe-O bond and an η(2)-C5H5 (Cp) ring (2e). No conclusive evidence for a gas-phase thermal reaction between ferrocene and ozone was observed under the conditions of these experiments. However, strong evidence for a surface-catalyzed thermal reaction was observed in merged-jet experiments wherein the gases were premixed before deposition. Surface-catalyzed ferrocene-ozone reaction products included a thin film of Fe2O3 observed on the walls of the merged tube as well as cyclopentadiene (C5H6), cyclopentadienone (C5H4O), and further oxidation products observed in the matrix. Possible mechanisms for both the photochemical and the thermal reactions are discussed.

Electronic structure and spectroscopy of the cycloheptatrienyl molybdenum halide complexes [MoBrL2(η-C7H7)]n+ (L2 = 2CO, n = 0; L2 = 2,2′-bipyridyl, n = 0 or 1)

DFT calculations at the B3LYP/Def2-SVP level have been conducted on the half-sandwich cycloheptatrienyl molybdenum complexes [Mo(CO)3(η-C7H7)]+, [1]+ and [MoBrL2(η-C7H7)]n+ (L2=2 CO, n=0, 2; L2=bpy, n=0, 3; L2=bpy, n=1, [3]+; bpy=2,2′-bipyridyl). In all cases, strong δ-bonding interactions operate between the e2 level of the C7H7 ring and metal dxy and dx2-y2 orbitals resulting in a metal-centred HOMO with substantial dz2 character in the 18-electron, closed shell systems. The experimental electronic UV–Vis spectra of [1]+, 2 and 3 are accurately reproduced by TD-DFT methods. For complexes 2 and 3, assignments made with the assistance of calculated spectra indicate that absorptions in the region 390–770nm originate from a series of MLCT (metal–ligand charge transfer) or ILCT (inter-ligand charge transfer) transitions in which carbonyl, C7H7 and 2,2′-bipyridyl ligands act as acceptor systems from the metal or mixed metal and bromide donor groups. The metal-centred, one-electron oxidation of 3 to 3[PF6] results in almost complete quenching of the visible region MLCT/ILCT absorptions of 3 and replacement with weak transitions probably arising from bromide to metal LMCT (ligand to metal charge transfer) processes.

The flexibility of the cycloheptatrienyl ring in cycloheptatrienylvanadium carbonyl derivatives

The cycloheptatrienylvanadium carbonyl (η7-C7H7)V(CO)3 is a stable species that can be synthesized by the reaction of V(CO)6 with cycloheptatriene. The related species of the types (C7H7)V(CO)n (n=5, 4, 3) and (C7H7)2V2(CO)n (n=5, 4, 3, 2, 1, 0), which are potentially accessible from (η7-C7H7)V(CO)3, have now been investigated by density functional theory. For the lowest energy singlet mononuclear (C7H7)V(CO)n (n=5, 4, 3) structures, the hapticity of the C7H7 ring is adjusted to give the vanadium atom the favored 18-electron configuration. Furthermore, for the lowest energy singlet binuclear (C7H7)2V2(CO)n (n=5, 4, 3) structures the hapticity of the C7H7 ring is adjusted to give a VV triple bond of ∼2.5Å length with an 18-electron configuration for each vanadium atom similar to the known binuclear cyclopentadienylvanadium carbonyl (η5-C5H5)2V2(CO)5. Evidence is presented for the presence of higher order multiple V–V bonds in the more highly unsaturated singlet (C7H7)2V2(CO)n (n=2, 1, 0) structures including a formal sextuple bond in the bent singlet (C7H7)2V2 structure. A lower energy coaxial triplet (C7H7)2V2 structure exhibiting the rare D7d point group symmetry is shown to have a σ+2π VV triple bond by analysis of its frontier molecular orbitals. The thermochemistry of the (C7H7)2V2(CO)n system suggests (η7-C7H7)2V2(μ-CO)3 to be a viable synthetic target analogous to the experimentally known and structurally characterized species (η6-C6H6)2Cr2(μ-CO)3 and (η5-C5H5)2Mn2(μ-CO)3.

Mono- and Dilithiation of [(η7-C7H7)Ti(η5-C5Me5)] (Pentamethyltroticene) for the Synthesis of Troticenyl Monophosphanes and [2]Troticenophanes with C–P and C–Si Bridges

Pentamethyltroticene, [(η7-C7H7)Ti(η5-C5Me5)] (1), can be selectively metalated at the C7H7 ring or at both the C5Me5 and C7H7 rings using pmdta/nBuLi or pmdta/tBuLi mixtures (pmdta = N,N′,N′,N″,N″-pentamethyldiethylenetriamine) in 1/1 and 1/4 ratios, respectively. The mono- and dilithiated species [(η7-C7H6Li)Ti(η5-C5Me5)]·pmdta (2) and [(η7-C7H6Li)Ti(η5-C5Me4CH2Li)]·pmdta (3) were isolated in high yield and characterized by NMR spectroscopy and elemental analysis. Compound 2 was used to synthesize the pentamethyltroticenyl monophosphane [(η7-C7H6PPh2)Ti(η5-C5Me5)] (4) by reaction with Ph2PCl, while 3 was treated with RPCl2 (R = Ph, Mes) or Me2SiCl2 to give the carbaphospha- and carbasila[2]troticenophanes [(η7-C7H6)Ti(η5-C5Me4CH2)]PR (5, R = Ph; 6, R = Mes) and [(η7-C7H6)Ti(η5-C5Me4CH2)]SiMe2 (8). The chiral phosphanes 5 and 6 are the first examples of non-iron metallocenophanes with a phosphorus atom in the bridge. The coordination ability of 4 and 6 toward transition metals was demonstrated by reaction with Mo(CO)6 and [(tht)AuCl] (tht = tetrahydrothiophene) or Me2SAuCl and formation of the bimetallic complexes [{η7-C7H6PPh2·Mo(CO)5}Ti(η5-C5Me5)] (9), [(η7-C7H6PPh2·AuCl)Ti(η5-C5Me5)] (10), and [(η7-C7H6)Ti(η5-C5Me4CH2)]PMes·AuCl (11). These compounds were structurally characterized by multinuclear 1H, 13C, 31P, and 29Si NMR spectroscopy, UV/vis spectroscopy, electron ionization mass spectrometry (EI–MS), elemental analysis, and single-crystal X-ray diffraction analysis. The molecular structures of 5, 6 and 8 reveal strained sandwich compounds with tilt angles (α) of 18.5(1)° (5), 19.7(7)° (6), and 13.4(2)° (8). Treatment of 2 with ZnCl2 afforded the pentamethyltroticenyl zinc chloride [(η7-C7H6ZnCl)Ti(η5-C5Me5)] (12), which was employed in palladium-catalyzed Negishi C–C cross-coupling reactions with phenyl iodide and iodotroticene to afford phenylpentamethyltroticene [(η7-C7H6Ph)Ti(η5-C5Me5)] (13) and the [7–5]bitroticene [(η7-C7H6)Ti{μ-η5:η7-(C5H4-C7H6)}Ti(η5-C5Me5)] (14), which bears a bridging sesquifulvalene ligand. The molecular structures of 13 and 14 in the solid state were also determined by single-crystal X-ray diffraction analysis.

Synthesis of [2]ferrocenophanes containing trivalent diphosphine units

Three (PR)2-bridged [2]ferrocenophanes (R = tBu (10a), nBu (10b), and Ph (10c)) in which the two C5H4 rings of the ferrocene unit were bridged with a P(III)—P(III) bond were synthesized via Mg-mediated reductive coupling between the phosphorus centers of corresponding [Fe{C5H4P(Cl)R}2]. The P(Cl)R groups of [Fe{C5H4P(Cl)R}2] were derived from the P(NEt2)2 groups of [Fe{C5H4P(NEt2)2}2] as follows: the P(NEt2)2 group was first converted to P(Cl)NEt2 by HCl, and then R (R = tBu, nBu, and Ph) was introduced to give P(R)NEt2, which was again allowed to react with HCl to form the P(Cl)R group. Of the three diphospha[2]ferrocenophanes, (PtBu)2-bridged [2]ferrocenophane (10a) could be isolated as free diphosphine, while because of difficulty in their isolations as free ligands, the nBu and Ph analogs (10b and 10c, respectively) were characterized as bis(pentacarbonylmetal) complexes (μ-10b)-[W(CO)5]2 (11) and (μ-10c)-[Cr(CO)5]2 (12), in which the respective phosphorus centers were coordinated to the pentacarbonylmetal fragments. X-ray analysis was performed for the three diphospha[2]ferrocenophanes, 10a, 11, and 12, demonstrating that their P—P bond lengths are 2.3093(7), 2.261(5), and 2.2502(8) Å, respectively. Because these tether P—P lengths are smaller than an ideal distance between the two parallel C5H5 rings of ferrocene, the two C5H4 rings of each of 10a, 11, and 12 are inclined from the parallel by 12.9°, 9.7°, and 13.6°, respectively. DFT calculations were performed on (PH)2-bridged [2]ferrocenophane as a model-compound, and showed that there are two conformational isomers. One, named the parallel form, features a P—P bond that is almost parallel to the axis passing through the centers of the two C5H4 rings, while the P—P bond of the other, named the twist form, leans with the two C5H4 rings twisted. The smaller inclined angle of 9.7° observed for 11 was found to result from its parallel-form conformation, while the C5H4 rings of 10a and 12 were more inclined due to adoption of the twist-form conformation.

Stereochemical analysis of ferrocene and the uncertainty of fluorescence XAFS data

Methods for the quantification of statistically valid measures of the uncertainties associated with X-ray absorption fine structure (XAFS) data obtained from dilute solutions using fluorescence measurements are developed. Experimental data obtained from 10 mM solutions of the organometallic compound ferrocene, Fe(C(5)H(5))(2), are analysed within this framework and, following correction for various electronic and geometrical factors, give robust estimates of the standard errors of the individual measurements. The reliability of the refinement statistics of standard current XAFS structure approaches that do not include propagation of experimental uncertainties to assess subtle structural distortions is assessed in terms of refinements obtained for the staggered and eclipsed conformations of the C(5)H(5) rings of ferrocene. Standard approaches (XFIT, IFEFFIT) give refinement statistics that appear to show strong, but opposite, preferences for the different conformations. Incorporation of experimental uncertainties into an IFEFFIT-like analysis yield refinement statistics for the staggered and eclipsed forms of ferrocene which show a far more realistic preference for the eclipsed form which accurately reflects the reliability of the analysis. Moreover, the more strongly founded estimates of the refined parameter uncertainties allow more direct comparison with those obtained by other techniques. These XAFS-based estimates of the bond distances have accuracies comparable with those obtained using single-crystal diffraction techniques and are superior in terms of their use in comparisons of experimental and computed structures.

Platinum(ii) and palladium(ii) complexes derived from 1-ferrocenylmethyl-3,5-diphenylpyrazole. Coordination, cyclometallation or transannulation?

The synthesis and characterization of the novel pyrazole derivative [1-(Fc–CH2)-3,5-Ph2–(C3HN2)] (2) {Fc = (η5–C5H5)Fe(η5–C5H4)–} with a ferrocenylmethyl substituent on position 1 of the heterocycle is described. The study of the reactivity of 2 with cis-[MCl2L2] (M = Pt and L = dmso or M = Pd and L = dmso or CH3CN), Pd(AcO)2 or Na2[PdCl4] under different experimental conditions, has allowed us to isolate and characterize a wide variety of platinum(II) or palladium(II) complexes: trans-[Pt{1-(Fc–CH2)-3,5-Ph2-(C3HN2)}Cl2(dmso)] (3), the cis- isomers of [M{1-(Fc–CH2)-3,5-Ph2–(C3HN2)}Cl2(dmso)] {M = Pt (4) or Pd (7)}, trans-[Pd{1-(Fc–CH2)-3,5-Ph2–(C3HN2)}2Cl2] (8), the cyclometallated compounds [M{1-(Fc–CH2)-(3–C6H4)-5-Ph–(C3HN2)}Cl(L)] {with M = Pt and L = dmso (5) or PPh3 (6) or M = Pd and L = PPh3 (9)} and the palladium(II) complex [Pd{1-[(η5–C5H4)Fe{(η5–C5H4)–CH2]-3,5-Ph2–(C3HN2)}Cl(PPh3)] (10) that arises from a transannulation process. The crystal structures of the free ligand 2 and compounds 4, 7, 9 and 10 are also reported and confirm the cis- disposition of the Cl− ligands in 4 and 7, the trans- arrangement of the phosphorous and the nitrogen atoms in 9 and 10, the mode of binding of the ligand in 4, 7, 9 and 10 and the nature of the metallated carbon atom {C(sp2, phenyl) in 9 or the C(sp2, ferrocenyl) of the C5H5 ring in 10}. In order to rationalize the different nature of the products isolated in the reactions of 2 with Pd(AcO)2 or Na2[PdCl4] and NaAcO density functional theory (DFT) calculations of the complexes have also been carried out.

Mixed Sandwich Compounds C5H5MC8H8 of the First-Row Transition Metals: Variable Hapticity of the Eight-Membered Ring

The C5H5MC8H8 complexes (M = Ti, V, Cr, Mn, Fe, Co, Ni) have been investigated by density functional theory. Only for titanium is the octahapto parallel ring sandwich (η5-C5H5)Ti(η8-C8H8) the lowest energy structure; this 17-electron complex has been synthesized and characterized structurally by X-ray crystallography. For vanadium, chromium, and manganese the (η5-C5H5)M(η6-C8H8) (M = V, Cr, Mn) structures with hexahapto C8H8 rings are the lowest energy structures. However, for the vanadium compound the octahapto parallel ring singlet spin state structure (η5-C5H5)V(η8-C8H8) lies only ∼6 kcal/mol above the hexahapto triplet spin state (η5-C5H5)V(η6-C8H8) structure. The chromium and manganese (η5-C5H5)M(η6-C8H8) complexes have been synthesized. For iron the 17-electron tetrahapto structure (η5-C5H5)Fe(η4-C8H8) with four adjacent C8H8 carbons bonded to the iron is the lowest energy structure. For cobalt, two isomeric 18-electron tetrahapto structures, namely, (η5-C5H5)Co(η4-C8H8), with four adjacent C8H8 carbons bonded to the cobalt, and (η5-C5H5)Co(η2,2-C8H8), with two nonadjacent C8H8 double bonds bonded to the cobalt, lie within 5 kcal/mol in energy. Experimental work indicates the existence of both isomers in solution, but only (η5-C5H5)Co(η2,2-C8H8) has been isolated in the crystalline state. The lowest energy structure for the nickel complex is the 17-electron complex (η5-C5H5)Ni(η2-C8H8), with a dihapto C8H8 ring containing three uncomplexed conjugated C═C double bonds.

Spectroscopic Properties and Electronic Structure of the Cycloheptatrienyl Molybdenum Alkynyl Complexes [Mo(C≡CR)(Ph2PCH2CH2PPh2)(η-C7H7)]n+(n= 0 or 1; R =But, Fc, CO2Me, or C6H4-4-X, X = NH2, OMe, Me, H, CHO, CO2Me)

A series of molybdenum alkynyl complexes [Mo(C≡CR)(dppe)(η-C7H7)] featuring a range of alkynyl substituents R with varying electron-donating and -withdrawing properties have been prepared. Oxidation of representative members of the series to the corresponding 17-electron radical cations has been achieved through both chemical oxidation and in situ spectroelectrochemical methods. The largely metal-centered character of the HOMO in this class of compounds has been established through a combination of experimental measurements (IR, UV−vis−NIR, EPR spectroscopies) and DFT-based calculations and rationalized in terms of the stabilization of the metal dxy, dyz, dxz, and dx2-y2 through π- and δ-interactions with the C7H7 ring and concomitant destablization of the dz2 orbital.

Bis(cycloheptatrienyl) Derivatives of the First‐Row Transition Metals: Variable Hapticity of the Cycloheptatrienyl Ring

AbstractExtrapolation from ferrocene and dibenzenechromium suggests the currently unknown 18‐electron sandwich compound (η7‐C7H7)2Ti as a possible stable molecule. However, density functional theory (DFT) methods using the B3LYP and BP86 functionals predict (η7‐C7H7)2Ti to distort by ring slippage to give a singlet 16‐electron complex (η7‐C7H7)Ti(η5‐C7H7) having an uncomplexed C=C double bond in one of the C7H7 rings but otherwise closely related to the known (η7‐C7H7)Ti(η5‐C5H5). The situation is similar with (C7H7)2V except that the corresponding structures have doublet spin multiplicity. However, an analogous (η7‐C7H7)Cr(η5‐C7H7) structure is not found for chromium. Instead the most stable (C7H7)2Cr structure is a singlet d0 six‐coordinate formally CrVI complex with two tridentate cyclopropyldivinyl C7H7 ligands. The latter rearrangement of the C7H7 rings bonded to chromium can involve initially a (η6‐C7H7)2Cr diradical sandwich compound followed by a bis(η5‐norcaradienyl)Cr intermediate. For the later first‐row transition metals the lowest energy (C7H7)2M structures are predicted to be doublet (η7‐C7H7)Mn(η3‐C7H7) rather than (η5‐C7H7)2Mn for manganese, singlet cis and trans structures (η5‐C7H7)2Fe for iron, doublet (η3‐C7H7)2Co for cobalt, and two singlet (η3‐C7H7)2Ni stereoisomers for nickel.(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)

Platinum(II) Complexes with 1‐Cyclohepta‐2,4,6‐trienyl‐diphenylphosphane, Ph2P(C7H7), and Alkyn‐1‐yl Ligands

Abstract[1‐Cyclohepta‐2,4,6‐trienyl‐diphenylphosphane]platinum(II) dichloride (4) was prepared by the 1:1 reaction of (cod)PtCl2 (cod = η4‐cycloocta‐1,5‐diene) with the phosphane. The reaction of 4 with di(alkyn‐1‐yl)dimethylstannanes, Me2Sn(C≡C‐R)2 [R = H (a), Me (b), Ph (c), SiMe3 (d)], in boiling THF gave selectively in high yield the monoalkynyl complexes [Ph2P(C7H7)]Pt(Cl)C≡C‐R (6b,c,d), in which the alkyn‐1‐yl group is arranged in cis position with respect to the phosphorus atom, and the C7H7 ring is η2‐coordinated to platinum through the central C=C bond. The same reaction at room temperature afforded, again selectively and in high yield, the dialkynyl complexes [Ph2P(C7H7)]Pt(C≡C‐R)2 (7a‐d). The new complexes were characterised in solution by 1H, 13C, 29Si, 31P and 195Pt NMR spectroscopy, and the molecular structures of 4 and 7d were determined by X‐ray crystallography.

Synthesis and crystal structures of dichlorocyclopentadienyl β-diketonate titanium complexes

Complexes of acetylacetonatodichlorocyclopentadienyltitanium(IV) (1) and dichlorocyclopentadienyl(2,2,6,6-tetramethyl-3,5-heptanedionato)titanium(IV) (2) have been prepared and their crystal structures determined by X-ray diffraction methods. Complex 1 crystallizes in space group P21/m with a = 7.1893(10), b = 11.7680(17), c = 7.6129(11) Å, β = 109.901(12)°; complex 2 crystallizes in space group P21/n with a = 10.065(2), b = 16.322(3), c = 12.219(2) Å, β = 110.99(3)°. The molecular structures of 1 and 2 can be described as square-based pyramidal, with the centroid of the C5H5 ring occupying the apical site and the bidentate β-diketonate and two chloride ligands occupying the basal positions. The average distances between titanium and oxygen atoms are 1.991(2) and 1.967(3) Å in compounds 1 and 2, respectively.

Microwave Spectroscopy Measurements of the Gas-Phase Structure of Cyclopentadienyltungsten Tricarbonyl Hydride

High-resolution microwave spectra for 13C isotopomers of cyclopentadienyltungsten tricarbonyl hydride were measured using a pulsed-beam Fourier transform microwave spectrometer system. The new rotational constants for the 13C isotopomers are combined with the previously obtained rotational constants for normal and deuterium isotopomers to obtain the gas-phase structure of cyclopentadienyltungsten tricarbonyl hydride. The new frequencies for the five unique 13C isotopomers were measured in the 5−7 GHz range for C5H5W(CO)3H. Kraitchman analysis and least-squares structure-fitting procedures were used to determine the structural parameters. The results from the structural fit yielded the W−H bond length, r0(W−H) = 1.79(2) Å, which agrees very well with the previously reported value, rs(W−H) = 1.79(4) Å, obtained with a much smaller data set. The present study also yielded the distance from tungsten to the C5H5 ring, r(W−Cp) = 2.03(1) Å, which corresponds to an average W−C (of Cp) bond length of 2.37(2) Å. The experimental ring radius for Cp of r(Cp) = 1.20(2) Å corresponds to an average cyclopentadienyl C−C bond length of 1.423(4) Å. Deviations of near 0.02 Å from C5 symmetry for the W−C bond lengths for the Cp ligand and smaller deviations for the C−C bond lengths were obtained from the DFT calculations, and incorporating these deviations into the least-squares fit improved the standard deviation by a factor of 5. The average bond length from tungsten to the carbonyl carbons is r(W−CO) = 1.97(2) Å. Results obtained from the structural fit are in very close agreement with the experimental, Kraitchman analysis values. Results from new DFT calculations are given, with heavy-atom coordinates in very good agreement with experimental values, and a slightly shorter calculated W−H bond length, re(W−H) = 1.73 Å.