Extended Huckel Molecular Orbital Calculations Research Articles

Structure and bonding in a series of cyano complexes: RuCp(PPh3) 2CN, [RuCp(PPh3) 2(CNH)]CF3SO3, and H-bridged [Ru2(Cp) 2(PPh3) 4CNHNC]CF3SO3

Abstract The three title cyanoruthenium complexes have been characterized by means of X-ray diffraction analysis, IR and NMR solution spectroscopies, as well as extended Huckel molecular orbital calculations examining the properties of the cyanide fragment changing with complexation and with the co-ligands Cp and PPh3. Explanations are given for crystallographic results of the C-N bond shortening upon complexation, the supershort (2.573 A) bond length of N(H) ⋯ N in the bridged complex, as well as the Ru-C-N and C-N-H-N-C bendings. Although the crystallographically found asymmetry of coordinated Cp is not significant, the MO calculations suggest a distorted endocyclic bond-length pattern indicative of the relative importance of σ and π bonding in the metalcyclopentadienyl interactions.

Oxidation of Ruthenium(II) to Ruthenium(IV) η4-Diene Complexes: Swing Mechanism and Diene−Allyl Conversion

This paper reports on the synthesis and reactivity of Ru(IV) η4-diene complexes of the type Ru(η5-C5Me5)(η4-diene)Br2+ obtained by oxidative bromine additions to Ru(II) η4-diene complexes. Mechanistic details are derived from the reaction products varying with the oxidizing agent and the leaving ligand and are backed up by extended Huckel molecular orbital calculations. The diene fragment in Ru(IV) is prone to gauche deformation and is therefore extremely susceptible to nucleophilic attack of even weak anionic bases generating the corresponding Ru(IV) η3-allyl complexes. Accordingly, Ru(IV) η4-diene complexes are stabilized either by using 2,3-disubstituted diene ligands in which case gauche deformation is highly unfavorable or else by excluding nucleophilic agents. For the latter method, two convenient routes to affording Ru(IV) η4-diene complexes are (i) the reaction of Br+CF3SO3- with Ru(η5-C5Me5)(η4-diene)Br and (ii) the reaction of dibromine with labile Ru(η5-C5Me5)(η4-diene)(CF3SO3). The conversion ...

Reaction of dimethylamine with 4,5-dichloro-4-cyclopenten- 1,3-dione: X-ray diffraction structure and cyclic voltammetric data for {ie-1}

Dimethylamine reacts with 4,5-dichloro-4-cyclopenten-1,3-dione (1) to give the monosubstituted amine compound 4-dimethylamino-5-chloro-4-cyclopenten-1,3-dione (2) as the sole isolable product. Attempts to replace the remaining chlorine group in 2 by a second dimethylamine group have been unsuccessful. Compound 2 has been isolated and characterized in solution by IR and NMR ( 1 H and 13 C) spectroscopy, and the solid-state structure for ClC=C(NMe 2 )C(O)CH 2 C(O) has been determined by X-ray diffraction analysis. ClC=C(NMe 2 )C(O)CH 2 (O) crystallizes in the triclinic space group P1, a = 7.2224(4) A, b = 7.9008(6) A, c = 8.3284(8) A, α = 63.803(6)°, β = 67.401(6)], γ = 87.889(5)°, V = 388.59(6) A 3 , Z = 2, d calc = 1.484 g.cm -3 ; R = 0.0422, R w = 0.0450 for 1552 observed reflections with l > 3σ(l). The cyclic voltammetric behavior of ClC=C(NMe 2 )C(O)CH 2 C(O) has been explored at a platinum electrode in CH 2 Cl 2 and THF, and the orbital composition of the HOMO and LUMO in 2 was established by extended Huckel molecular orbital calculations on the model compound ClC=C(NH 2 )C(O)CH 2 C(O). The cyclic voltammetry data of 2 is contrasted with the known diphosphine compound 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd; 3) and predictions concerning the ability of ClC=C(NMe 2 )C(O)CH 2 C(O) to function as an electron reservoir in organometallic compounds is discussed.

Crystalline, microporous zirconium silicates with MFI structure

Zirconium-containing medium-pore, Al-free molecular sieves (Si/Zr molar ratios > 50) with MFI structure have been synthesized both in alkaline and HF/CH 3 NH 2 medium. The samples synthesized in HF/CH 3 NH 2 medium exhibit a high crystallinity, bigger particle size, lower surface area and lower catalytic activity in the hydroxylation of phenol as compared to those synthesized in alkaline medium. An expansion in unit cell volume (XRD), an absorption at 965 cm -1 (FTIR) and at 212 nm (UV-Visible) as well as the catalytic activity data indicate that upto 0.6 zirconium atom per unit cell (Si/Zr = 158) could be substituted isomorphously within the silicalite-I framework. The IR study of adsorbed pyridine has shown that strong Lewis acid and weak Bronsted acid sites exist on all zirconium silicate samples, synthesized in alkaline medium. The relative stability of cluster models representing various possible lattice structures and the ordering of molecular orbitals derived from extended Huckel molecular orbital calculations are discussed to understand the nature of active sites in the zirconium silicates.

The Synthesis of the Arene Clusters Ru 6 C(CO)14(η6-Arene) (Arene = C 6 H 5 CHMe 2 , C 6 H 5 C 4 H 9 , C 6 H 5 C 3 H 7 , and C 6 H 5 C 3 H 5 )

On reaction with Ru3(CO)12, isopropenylbenzene and 4-phenyl-l-butene undergo hydrogenation, to yield the clusters, Ru6C(CO)14(η6-C6H5CHMe2) 1 and Ru6C(CO)14(η6-C6H5C4H9) 2, respectively. With allylbenzene, both hydrogenation and isomerization occurs affording Ru6C(CO)14(η6-C6H5C3H7) 3 and Ru6C(CO)14(η6-C6H5C3H5) 4. The structures of 1 and 2 have been established by single crystal X-ray diffraction. One of the Ru–Ru bond lengths in 2 is unusually long and extended Huckel molecular orbital calculations have been used in an attempt to rationalize this feature.

Ruthenium(II) tris(pyrazolyl)borate complexes.† Reversible vinylidene complex formation

A convenient high-yield route to [Ru{HB(pz)3}Cl(PPh3)(CCHR)] (pz = pyrazolyl; R = Ph, SiMe3, Bun, But, CO2Et or C6H9) has been found through the intermediary [Ru{HB(pz)3}Cl(PPh3)(dmf)] (dmf = dimethylformamide) which has been crystallographically characterized. This complex is readily obtained on treatment of [Ru{HB(pz)3}Cl(cod)] (cod = cycloocta-1,5-diene) with 1 equivalent of PPh3 in boiling dmf. The vinylidene moiety in complexes of the type [Ru{HB(pz)3}Cl(PPh3)(CCHR)] is remarkably labile being easily replaced by nucleophiles L = PMe3, PPh3, MeCN, pyridine or CO to give [Ru{HB(pz)3}Cl(PPh3)L]. With the exceptions of L = PMe3 or CO, these reactions are reversible. The complex [Ru{HB(pz)3}Cl(PPh3)(CO)] has been characterized by X-ray crystallography. On treatment of [Ru{HB(pz)3}Cl(PPh3)(CCHPh)] with an excess of HCCR′ (R′ = SiMe3, Bun, But, CO2Et or C6H9) the neutral vinylidene complex [Ru{HB(pz)3}Cl(PPh3)(CCHR′)] was reversibly formed. Comparative studies and extended-Huckel molecular orbital calculations have been performed to elucidate the nature of the bonding in the various vinylidene complexes. The bonding between the metal center and the vinylidene ligand is dominated by the dyz (metal)–p (vinylidene) interaction. If this contribution is small, reconversion of the vinylidene complex into an η2-alkyne complex can be accomplished.

Reaction of a dicarbon Ru5 cluster with CO: crystal structures of [Ru5(μ4-C2)(μ-SMe)2( μ-PPh2)2(CO)13] and [Ru4(μ4-C2)(μ-SMe)2( μ-PPh2)2(CO)10

C. J. Adams, M. I. Bruce, B. W. Skelton, A. H. White, G. Frapper and J. Halet, J. Chem. Soc., Dalton Trans., 1997, 371 DOI: 10.1039/A605529J

Mono- and bi-nuclear titanium imido complexes supported by aryloxide ligands: fine control by ortho substituents

Reaction of the titanium imido complexes [Ti(NR)Cl 2 (py) 3 ] (R = Bu t , C 6 H 3 Me 2 -2,6 or C 6 H 3 Pr i 2 -2,6; py = pyridine) with 2 equivalents of lithium aryloxide Li[OC 6 H 3 R′ 2 -2,6] (R′ = Me, Pr i or Bu t ) afforded the mononuclear four- or five-co-ordinate or binuclear four-co-ordinate complexes [{Ti(NR)(OC 6 H 3 R′ 2 -2,6) 2 (py) n } m ] (m = 1, n = 1 or 2; m = 2, n = 0) depending on the identity of R and R′. The crystal structures of [Ti 2 (µ-NBu t ) 2 (OC 6 H 3 Me 2 -2,6) 4 ] 1 and [Ti(NC 6 H 3 Me 2 -2,6)(OC 6 H 3 Me 2 -2,6) 2 (py) 2 ] have been determined. Extended-Huckel molecular orbital calculations for models of 1 showed that the deviation of the µ-imido Bu t substituents from coplanarity with the Ti 2 N 2 core in this and some related species can be attributed to a second-order Jahn–Teller distortion.

Ground-State Geometry Preferences in (Tris(pyrazolyl)borato)- and Cyclopentadienylniobium Alkyne Complexes

Reaction of NbCl3(DME)(RC⋮CR‘) with KTp (DME = 1,2-dimethoxyethane, Tp = hydrotris(pyrazolyl)borate) yields TpNbCl2(RC⋮CR‘) (R = Ph, R‘ = Me (2a); R = R‘ = Me (2b), Et (2c), SiMe3 (2d), Ph (2e)), which has the alkyne in the molecular mirror plane in the solid state (X-ray crystal structure for 2a) and in solution. The barriers to alkyne rotation are low, the highest being measured for complex 2d (52 kJ mol-1 at 273 K), which contains the bulky SiMe3 group on the alkyne. In TpCpNb(Cl)(PhC⋮CMe) (3a), formed by reaction of 2a with NaCp·DME, the alkyne is parallel to the Cp plane, as observed in solution and in the crystal structure. Extended Huckel molecular orbital calculations indicate that in complexes 2 the geometry is mainly governed by steric interactions, whereas in the case of 3a orbital interactions dictate the observed geometry.

Crystalline, Microporous Zirconium Silicates with MEL Structure

Zirconium-containing medium-pore, Al-free molecular sieves (Si/Zr molar ratios > 50) with MFI structure have been synthesized both in alkaline and HF/CH3NH2 medium. The samples synthesized in HF/CH3NH2 medium exhibit a high crystallinity, bigger particle size, lower surface area and lower catalytic activity in the hydroxylation of phenol as compared to those synthesized in alkaline medium. An expansion in unit cell volume (XRD), an absorption at 965 cm-1 (FTIR) and at 212 nm (UV-Visible) as well as the catalytic activity data indicate that upto 0.6 zirconium atom per unit cell (Si/Zr=158) could be substituted isomorphously within the silicalite-1 framework. The IR study of adsorbed pyridine has shown that strong Lewis acid and weak Bronsted acid sites exist on all zirconium silicate samples, synthesized in alkaline medium. The relative stability of cluster models representing various possible lattice structures and the ordering of molecular orbitals derived from extended Huckel molecular orbital calculations are discussed to understand the nature of active sites in the zirconium silicates.

Preparation, luminescence and interactions with Li+, Na+ and U022+ of two new metallocalix[4]arene complexes of iridium(I)

Abstract Two new complexes of Ir(I) using diphosphine and diphosphinite calix[4]arene ligands have been prepared and characterized. The luminescent lowest energy excited MLCT state (metal-to-ligand charge transfer) has been addressed theoretically by extended Huckel molecular orbital calculations (EHMO), and a Franck-Condon analysis on the absorption spectra using the time dependent Heller's theory, has been made. The perturbation of re of the two complexes in ethanol at 77 K by the addition of substrates (U0 2 2+ , Li + , Na + ), is investigated. The amplitude in to change with additions of substrates varies as U0 2 2+ > Li + > Na + .

Synthesis and reactivity of η3-γ-lactonyl complexes of molybdenum; crystal structures of [Mo{η3-OC(O)CHCHCH}(CO)2(η-C5Me5)], [Mo{η-2PhCH2NHCHCHCH(CO2H)}(CO)2(η5-C9H7)] and [Mo{η3-OC(O)CHCHCHCO}(NCMe)(CO)(η-C5H5)

Reaction of 2-(trimethylsilyloxy)furan with the compounds cis-[Mo(NCMe)2(CO)2L][BF4](L =η-C5H5, η-C5Me5 or η5-C9H7) afforded the η3-γ-lactonyl complexes [Mo{η3-[graphic omitted]H}(CO)2L]. The structure of one of these species, [Mo{η3-[graphic omitted]H}(CO)2(η-C5Me5)], has been established by a single-crystal X-ray diffraction study, which confirmed that the γ-lactonyl moiety is bound to the molybdenum via three carbon atoms as an η3-allyl. Treatment of these lactonyl complexes with nucleophilic reagents (amines, methoxide) resulted in lactone ring opening and overall addition of the nucleophile to the γ-carbon of the lactone ring, rather than at the lactonyl carbonyl carbon atom as might have been expected. The product of the reaction between [Mo{η3-[graphic omitted]H}(CO)2(η5-C9H7)] and PhCH2NH2 has been structurally characterised by an X-ray diffraction study as the zwitterionic, η2-alkene complex [Mo{η2-PhCH2NHCHCHCH(CO2H}(CO)2(η5-C9H7)]. Similarly, [Mo{η3-[graphic omitted]H}(CO)2(η-C5H5)] and methoxide anion gives, after acidification, [Mo{anti-η3-(MeO)CHCHCH(CO2H)}(CO)2(η-C5H5)], in which the lactone ring has been cleaved to give an η3-allyl moiety ligating the metal centre. An extended Huckel molecular orbital calculation on [Mo{η3-[graphic omitted]H}(CO)2(η-C5Me5)] suggests that these reactions proceed via initial attack at the metal centre, followed by a rearrangement which effectively transfers the nucleophilic moiety to the γ-carbon of the lactone ring. Reaction of the η3-lactonyl complex [Mo{η3-[graphic omitted]H}(CO)2(η-C5H5)] with HBF4·Et2O resulted in a remarkable ring-enlargement reaction, in which a co-ordinated carbon monoxide formally inserts into the lactone carbon–oxygen bond to form the crystallographically characterised complex [Mo{η3-[graphic omitted]O}(NCMe)(CO)(η-C5H5)].

Conversion of [(tBuCH2)2TaN]5 to Cubic TaN: Related Syntheses, EHMO Calculations, and MAS and Spin Echo 15N NMR Spectroscopies

The conversion of pentamer [(tBuCH2)2TaN]5 (2) and ammonia to cubic TaN0.8 (5) proceeds via precipitate 1/n[TaC1.41H3.90N1.90]n (3) and amorphous TaN (4). Precursors other than (tBuCH2)3TaCHtBu (1)...

Ruthenium tris(pyrazolyl)borate complexes. Formation and characterization of acetone, dimethylformamide and vinylidene complexes containing N,N-donor co-ligands

Chloride abstraction from [Ru{HB(pz)3}(tmen)Cl](pz = pyrazolyl, tmen = Me2NCH2CH2NMe2) with NaBPh4 in the solvents acetone and dimethylformamide led to the formation of the respective cationic [Ru{HB(pz)3}(tmen)(solv)]+ complexes. In the presence of phenylacetylene, these are easily transformed into the first example of a ruthenium HB(pz)3 vinylidene complex. Extended-Huckel molecular-orbital calculations have been performed to establish the nature of the bonding involved. From [Ru{HB(pz)3}(py)2Cl](py = pyridine) the corresponding vinylidene complex could be prepared in the same way, although the intermediate solvent complexes could not be isolated. Selected crystal structures were determined.

Chemical origin of high activity in oxygenation of cyclohexane by H2O2 catalysed by dinuclear iron(III) complexes with amide-containing ligands

The crystal structures of two dinuclear iron(III) complexes containing an oxo bridge, [Fe2OCl2L2][ClO4]2·2H2O 1[L =N,N-bis(2-pyridylmethyl)glycinamide] and [Fe2OCl2L2][ClO4]22(L =N-{2-[bis(2-pyridylmethyl)amino]ethyl}morpholine) were determined. Their structural features are quite similar to those of the corresponding linear dinuclear complex [Fe2OCl2(tpa)2][ClO4]2, where tpa is tris(2-pyridylmethyl)amine; the ligands act as tetradentate tripods, and the Fe–O (amide) and average Fe–N (morpholine) distances are 2.165(6) and 2.45(1)A, respectively. Complex 1 exhibited much higher activity for the hydroxylation of cyclohexane in the presence of H2O2, while the activities of the other two complexes are negligible. In contrast, all three complexes exhibited high activity for the decomposition of H2O2. These results indicate that the active species for oxygenation of cyclohexane, which may be an iron(III)–peroxide adduct (I), should be different from that for decomposition of H2O2, adduct II, and that these two species may exist in the solution of complex 1. It is postulated that adduct I may be a dinuclear iron(III)–η1-hydroperoxide species stabilized through hydrogen bonding between the hydroperoxide ion and the oxygen atom of the amide group. Extended-Huckel molecular orbital calculations showed that the hydrogen bonding may lead to induction of high ‘oxo-like’ activity in the peroxide adduct. In the cases of the tpa and morpholine complexes the formation of a η1-hydroperoxide adduct seems unfavourable because of both steric and electronic reasons; instead a (µ-η1:η1-peroxo)diiron(III) species, adduct II, is formed which induces high catalase-like activity.

Synthesis and reactivity of η2(4e)-alkyne and η2(3e)-vinyl complexes of rhenium

Reaction of cis-/trans-[ReBr2(CO)2(η-C5H5)] with PhC2Ph and MeC2Ph in refluxing toluene afforded good yields of the η2(4e)-donor alkyne complexes [ReBr2(η2-PhC2Ph)(η-C5H5)]1 and [ReBr2(η2-MeC2Ph)(η-C5H5)]2, respectively. Treatment of 1 and 2 with either AgBF4 or TlPF6 in the presence of PPh3, PMePh2 or P(OMe)3(L) gave monocations [ReBr{η2(4e)-alkyne}L(η-C5H5)]+, whereas a similar reaction with 2 equivalents of AgBF4 and 1 equivalent of Ph2PCH2CH2PPh2(dppe) afforded dications [Re(η2-PhC2Ph)(dppe)(η-C5H5)][BF4]2 and [Re(η2-MeC2Ph)(dppe)(η-C5H5)][BF4]2. The structural identity of [ReBr(η2-PhC2Ph)(PMePh2)(η-C5H5)][PF6] was confirmed by single-crystal X-ray crystallography. The alkyne C–C vector lies parallel to the Re–Br bond and the alkyne C–C bond length [C(1)–C(2) 1.26(4)A] is relatively short. Treatment of [ReBr(η2-PhC2Ph)(PPh3)(η-C5H5)][BF4] and [ReBr(η2-PhC2Ph)(PMePh2)(η-C5H5)][PF6] with K[BHBus3] in dichloromethane at –78 °C led to neutral η2(3e)-vinyl complexes [[graphic omitted]HPh}Br(PPh3)(η-C5H5)] and [[graphic omitted]HPh}Br(PMePh2)(η-C5H5)]. The crystal structure of the latter showed that the C–C vector of the vinyl moiety lies almost parallel to the Re–Br bond. The stereochemistry of these reactions is discussed in the light of extended-Huckel molecular orbital calculations. Reaction (–78 °C) of [Re(η2-PhC2Ph)(dppe)(η-C5H5)][BF4]2 with 1 equivalent of K[BHBus3] in tetrahydrofuran afforded the X-ray crystallographically identified monocationic η2(3e)-vinyl complex [[graphic omitted]HPh}(dppe)(η-C5H5)][BF4], which reacted at room temperature with a further equivalent of K[BHBus3] to give the cis-stilbene-substituted complex [Re(η2-Z-PhCHCHPh)(dppe)(η-C5H5)]. The crystal structure of the latter showed that the alkene phenyl substituents are orientated towards the cyclopentadienyl ring. In contrast, a similar reaction between K[BHBus3] and [Re(η2-MeC2Ph)(dppe)(η-C5H5)][BF4]2 gave initially the η2(3e)-vinyl complex [[graphic omitted]HPh}(dppe)(η-C5H5)][BF4]; a further equivalent of K[BHBus3] led to deprotonation and formation of the η2-allene complex [Re{η2-CH(Ph)CCH2}(dppe)(η-C5H5)], in which the substituted allenic bond is co-ordinated to the rhenium. The dinuclear complex [Re2Br2(PPh3)2(µ-O)(η-C5H5)2][BF4]2 was also prepared and shown crystallographically to possess a single rhenium–rhenium bond [2.731(5)A].

Electrochemical and spectroelectrochemical studies on platinum complexes containing 2,2′-bipyridine

The electrochemical and spectroelectrochemical (UV/VIS/NIR and EPR) properties of a series of complexes of general formula [Pt(bipy)L2]n+(L = Cl– or CN–, n= 0; L = NH3, py, PMe3 or L2= en, n= 2; bipy = 2,2′-bipyridine, py = pyridine, en = ethylenediamine) have been investigated. The complexes undergo a reversible one-electron reduction process at potentials of ca.–1 V vs. Ag–AgCl. In each case UV/VIS/NIR spectra of the one-electron reduction products were consistent with co-ordinated bipyridyl anion-radical species rather than d9 metal centres. Electron paramagnetic resonance spectra of the chemically or electrochemically generated 17-electron species, in conjunction with the results of extended-Huckel molecular-orbital calculations, show a significant admixture of metal 5dyz and/or 6pz orbitals in the singly-occupied molecular orbital. The complex [Pt(bipy)(PMe3)2][BF4]2 has been prepared.

MBOHO calculations of phosphorus-carbon nuclear spin-spin coupling constants

The novel generalized correlation of the nuclear spin-spin coupling constants with the atomic hybrids and net charges is employed to give a new relationship for calculating the directly bonded phosphorus-carbon coupling constants by use of the maximum bond order hybrid orbital procedure together with the extended Huckel molecular orbital calculation. The calculated coupling constants of phosphorus-carbon are all in good agreement with the experimental data, which shows that the new relationship obtained in the present paper is quite satisfactory for calculation of the phosphorus-carbon coupling constants.

Synthesis, extended Hückel molecular orbital calculations, and X-ray diffraction structure of the pentamethylcyclopentadienylruthenium complex Cp*Ru(NO)(2,3-quinoxalinediolate)

The reaction between Cp*Ru(NO)Cl2 and the heterocyclic compound 2,3-quinoxalinediol in the presence of methanolic KOH leads to the formation of the new piano-stool compound Cp*Ru(NO)(2,3-quinoxalinediolate) (1). Product isolation and characterization by solution spectroscopic methods (1H and13C NMR and IR) are described. The solid-state structure of Cp*Ru(NO)(2,3-quinoxalinediolate) confirms the replacement of each chloride ligand in Cp*Ru(NO)Cl2 by an oxygen group of the 2,3-quinoxalinediol ligand. Cp*Ru(NO)(2,3-quinoxalinediolate) crystallizes in the orthorhombic space group Pbca, a=14.846(1) A,b=12.718(1) A,c=18.255(2) A,V=3446.7(5) A3,Z=8,d calc=1.643 g·cm−3;R=0.0364,R w=0.0393 for 1245 observed reflections withI>3σ(I). The observed Ru−N−O bond angle of 156.3(7)° in the X-ray structure of1 confirms the nonlinear nature of the nitrosyl linkage. This bend in the nitrosyl moiety away from the quinoxalinediolate ligand presumably results from solid-state packing forces and does not derive from electronic interactions, on the basis of extended Huckel molecular orbital calculations. The MO properties of1 are compared with other Cp*Ru(NO)-substituted complexes previously synthesized in these laboratories.

A ‘direct’ relationship between ETi–L bond angle and TiE bond length can exist in [Ti(E)L4] complexes (E = organoimido or oxo)

Evidence for a ‘direct’ relationship between ETi–L bond angle and TiE bond length in five-coordinate complexes [Ti(E) L4](E = organoimido or oxo) is presented and is supported by extended-Huckel molecular orbital calculations; the crystal structure of [Ti(tmtaa)(NC6H3Me2-2,6)] is described (H2tmtaa = tetramethyldibenzotetraaza[14]annulene).