Margarida S. Salema

Enthalpies of formation of Ti(η-C5H5)2(OR)2 complexes (R C6H5, 2-CH3C6H4, 3-CH3CH6H4 4-CH3C6H4, and 2-ClC6H4)

Abstract The standard enthalpies of formation of the title crystalline complexes at 298.15 K have been determined by reaction-solution calorimetry. The results give Δ H o f [Ti(η-C 5 H 5 ) 2 (OC 6 H 5 ) 2 ,c]  −379.2 ± 8.0, Δ H o f [Ti(η-C 5 H 5 ) 2 (2-CH 3 -C 6 H 4 O) 2 ,c]  −416.7 ± 8.1, Δ H o f [Ti(η-C 5 H 5 ) 2 (3-CH 3 C 6 H 4 O) 2 ,c]  −393.6 ± 8.1, Δ H o f [Ti(ηC 5 H 5 ) 2 (4-CH 3 C 6 H 4 O) 2 ,c]  −416.5 ± 7.8 and Δ H o f [Ti(η-C 5 H 5 ) 2 (2-ClC 6 H 4 O 2 ,c]  −407.6 ± 21.5 kJ mol-1. The metal—oxygen bond strenghts have been evaluated as mean bond-disruption enthalpies ( D ) and as mean bond-enthalpy terms ( E ). The method of calculation of these values is analysed and earlier relevant thermochemical data are reviewed.

Synthesis and characterization of [W(NC4Me4)2Cl2] and [W(NC4Me4)2(CH3)2], the first azametallocene tungsten complexes with pyrrolyl ligands. Electronic structure and bonding of tungsten bispyrrolyl complexes

Abstract The synthesis of the first tungsten azametallocene complexes with pyrrolyl ligands, [W(NC4Me4)2L2], is reported. The dichloro complex (L=Cl) was obtained from the reaction of WCl4DME (DME=1,2-dimethoxyethane) with Li(NC4Me4) in poor yield (6.5%). A small amount (ca. 0.063 g, yield: 15.8%) of the dimethyl complex, [W(NC4Me4)2(CH3)2], resulted from the reaction of the dichloro species (L=Cl) with LiCH3. Both complexes were characterised by 1H, 13C{1H} NMR and mass spectroscopy. Molecular orbital studies based on B3LYP/DFT calculations were performed in order to study the complexes electronic structure, focusing the pyrrolyl coordination mode. Equivalent bond strengths were found for the two extreme coordination modes of that ligand: π coordination (η5) through the pyrrolyl π system, and N-σ coordination established by the nitrogen lone pair. The relative stability of the isomers originated by the different pyrrolyl coordination modes, results from a delicate balance between electronic and stereochemical factors, on each case. The calculations suggest the existence of a fluxional process in solution, involving the π/π and the σ/π isomers of [W(NC4Me4)2(CH3)2].

Synthesis, characterisation, crystal structure, reactivity and bondingin titanium complexes containing2,3,4,5-tetramethylpyrrolyl

The complex [Ti(η 5 -NC 4 Me 4 )Cl 3 ] 1 has been synthesized, characterised and used as a precursor for the preparation of the bis(pyrrolyl) complex [Ti(η 5 -NC 4 Me 4 ) 2 Cl 2 ] 2 and the thiolate complexes [Ti(η 5 -NC 4 Me 4 )(SPh) 3 ] 3 and [Ti(η 5 -NC 4 Me 4 )(SPh)Cl 2 ] 4. The η 5 co-ordination of the heterocyclic ring was predicted by the 13 C NMR data and confirmed by the structure determination of complexes 1 and 3, which present a piano-stool conformation and a slippage of the pyrrolyl towards the titanium. The mixed-ring complex [Ti(η 5 -C 5 H 5 )(η 5 -NC 4 Me 4 )Cl 2 ] 5 has also been synthesized and its structure determined, revealing a distorted pseudo-tetrahedral geometry and slippage of the heterocyclic ring.

Uracil and thiouracil complexes of dicyclopentadienyl molybdenum and tungsten: Preparation and electrochemistry. The structures of [M(η5-C5H5)2(2-SN2OC4H3)][PF6], [M(η5-C5H5)2{2-S(CH3)N2OC4H2}][PF6], [Mo(η5-C5H5)2 (4-SN2OC4H3)][PF6] and [Mo(η5-C5H5)2{4-S(CH3)N2OC4H2}][PF6] (M Mo and W)

Abstract Cationic complexes [MoCp2(U)][PF6] (1), [MCp2(2TU)][PF6] (2, 2′), [MCp2(4TU)][PF6] (3, 3′), [MoCp2(2,4-DTU)]Cl (4), [MCp2(2MTU)][PF6] (5, 5′) and [MCp2(4MTU)][PF6] (6, 6′) (M  Mo, W; Cp = η5-C5H5; U = uracilato; 2TU = 2-thiouracilato; 4TU = 4-thiouracilato; 2,4-DTU = 2,4-dithiouracilato; 2MTU = 2-methylthiouracilato and 4MTU = 4-methylthiouracilato) have been prepared from the reaction of [MCp2Cl2] with the corresponding uracil in the presence of triethylamine. All the uracilato derivatives act as bidentate ligands and experimental evidence (X-ray and IR) shows the following preference for binding atoms in this system: N(3) > N(1), = S > = O, O(2) > O(4); although there is no clear evidence it is also likely that S(2) > S(4). Electrochemical studies, by cyclic voltammetry carried out in acetonitrile or dimethylformamide, showed that compounds where ligands coordinate through N,S atoms are more stable upon oxidation than compounds where ligands coordinate through N,O donor atoms. The molecular structures of [MoCp2(2TU)][PF6](2), [WCp2(2TU)][PF6](2′), [MoCp2(4TU)][PF6](3), [MoCp2(2MTU)][PF6](5), [WCp2(2MTU)][PF6](5′) and [MoCp2(4MTU)][PF6](6) have been determined by X-ray crystallography.

Mercaptopyridine complexes of dicyclopentadienylmolybdenum and -tungsten: preparation and electrochemistry. The structure of [Mo(η5-C5H5)2(2-SNC5H4)][PF6]

Abstract Neutral complexes of the type [MCp2(2-Spy)2] and [MCp2(4-Spy)2] and cationic complexes [MCp2(2-Spy)][PF6] (Cp = η5-C5H5; M = Mo, W; py = pyridine), where 2-mercaptopyridine is acting as a chelating ligand, have been prepared from the reaction of [MCp2Cl2] and the corresponding mercaptopyridine in the presence of an excess of triethylamine. The protonated derivatives [MCp2(2-SpyH)2][PF6]2 and [MCp2(4-SpyH)2][PF6]2 have also been prepared by reacting the parent neutral compound with concentrated hydrochloric acid. Electrochemical studies, by cyclic voltammetry and coulometry, carried out in acetonitrile and dichloromethane, showed that these compounds follow the general trend of oxidation of the bis-thiolates MCp2(SR)2 (R = aryl or alkyl). The molecular structure of [MoCp2(2-SNC5H4)][PF6] has been determined. The crystals are triclinic, space group P 1 ¯ , a = 10.577(3), b = 7.644(2), c = 10.783(1) A, α = 99.26(2), β = 96.29(2), γ = 87.95(2)°, V = 855.26 A3, Z = 2.

Thermochemistry of the complexes Ti(η5-C5Me5)(C5Me4CH2) and Ti(η5-C5Me5)[C5Me3(CH2)2]

Abstract Reaction-solution calorimetric studies of the crystalline complexes Ti(n5-C5Me5)L L = C5Me4CH2 or C5Me5(CH2)2) led to relative values of their standard enthalpies of formation. These data, together with early results obtained for several analogous TiIII complexes, enalbed to discuss the energetics of two reactions of synthetic interest. The results are also used to analyse the possible bonding modes of ligands L to the titanium atom.

Enthalpies of formation of [M(η-C5H5)2(OOCR)2] complexes; M = Mo, W, or Ti and R = C6H5 or CF3

The standard enthalpies of formation of the title crystalline solids at 298.15 K have been determined by reaction–solution calorimetry. The results gave ΔHf⊖{[Mo(η-C5H5)2(OOCC6H5)2], c}=–486.2 ± 3.4, ΔHf⊖{[Mo(η-C5H5)2(OOCCF3)2], c}=–1 952.0 ± 3.9, ΔHf⊖{[W(η-C5H5)2(OOCC6H5)2], c}=–448.9 ± 3.5, ΔHf⊖{[W(η-C5H5)2(OOCCF3)2], c}=–1 914.5 ± 3.8, ΔHf⊖{[Ti(η-C5H5)2(OOCC6H5)2], c}=–775.2 ± 8.1, and ΔHf⊖{[Ti(η-C5H5)2(OOCCF3)2], c}=–2 219.0 ± 8.0 kJ mol–1. The metal–oxygen mean bond-enthalpy contributions have been derived as (Mo–OOCC6H5)≈ 305, (W–OOCC6H5)≈ 342, (Ti–OOCC6H5)≈ 432, (Mo–OOCCF3)≈ 300, (W–OOCCF3)≈ 337, and (Ti–OOCCF3)≈ 417 kJ mol–1.

Enthalpies of formation of Ti(η-C5H5)2L2 complexes (L = 3-CH3C6H4, 4-CH3C6H4, 4-CF3C6H4, and 4-CH3OC6H4)

The standard enthalpies of formation of the Ti(η-C5H5)2L2 crystalline complexes at 298.15 K have been determined by reaction-solution calorimetry. The results gave ΔH°f[Ti(η-C5H5)2(3-CH3C6H4)2,c] 199.0±9.3 kJ mol−1, ΔH°f[Ti(η-C5H5)2(4-CH3C6H4)2, c] 201.8 ± 9.5 kJ mol−1, ΔH°f[Ti(η-C5H5)2(4-CF3C6H4)2,c] - 1110.4± 8.2 kJ mol−1, and ΔH°f[Ti(η-C5H5)2(4-CH3OC6H4)2,c] −59.6 ± 8.4 kJ mol−1. The titanium-carbon mean bond dissociation enthalpies, D(TiC), and bond enthalpy terms, E(TiC), were also evaluated.

Stepwise Ti-Cl, Ti-CH3, and Ti-C6H5 bond dissociation enthalpies in bis(pentamethylcyclopentadienyl)titanium complexes

Abstract Reaction-solution calorimetric studies involving the complexes Ti[η5-C5(CH3)5]2-(CH3)2, Ti[η5-C5(CH3)5]2(CH3), Ti[η5-C5(CH3)5]2(C6H5), Ti[η5-C5(CH3)5]2Cl2, and Ti[η5-C5(CH3)5]2Cl, have enabled derivation of titaniumcarbon and titaniumchlorine stepwise bond dissociation enthalpies in these species.