Hydrogen bonds A-H…AA, where A and AA are electronegative atoms have been widely discussed. Weak hydrogen bonds involving such different arrangements as X-H…A, where X can be C; X-H…p, with phenyl rings, C•C bonds; X-H…M, where M is a transition metal; X-H…H-M and X-H…H-B, have also been described in recent years. While the first types are typical of organic and inorganic compounds, as well as biological molecules, those involving transition metal atoms are special to organometallic chemistry. Theoretical calculations of different kinds and at several levels have been performed for many systems, revealing that a similar geometrical arrangement can hide another type of interaction. This happens for N-H…M close contacts which can be agostic interactions or hydrogen bonds, not so easily distinguishable for 16-electron complexes. M-H…H-X interactions also exhibit a different behavior, depending on whether the complexes are neutral or ionic. The AIM approach, by analysing the topological properties of the charge density with the determination of critical points, provides another way of looking for bonds, as discussed in several examples. Computational methodsMany different approaches have been used to study weak hydrogen bonds, ranging from semiempirical calculations (extended Hückel, 4a,9 MNDO 10 ) to ab initio studies (HF/ MP2, 5c B3LYP, 6,11 DFT 12 ). These methods allow either a qualitative interpretation of the bond (extended Hückel), or geometry optimization, determination of binding energies, and calculation of charges and other relevant parameters. MP2 and B3LYP approaches are compared in a detailed study of formation of hydrogen bonded complexes of small molecules with water, where water can behave as donor or acceptor,
A complete catalytic cycle for the cyclotrimerization of acetylene with the CpRuCl fragment has been proposed and discussed based on DFT/B3LYP calculations, which revealed a couple of uncommon intermediates. The first is a metallacyclopentatriene complex RuCp(Cl)(C(4)H(4)) (B), generated through oxidative coupling of two alkyne ligands. It adds another alkyne in eta(2) fashion to give an alkyne complex (C). No less than three successive intermediates could be located for the subsequent arene formation. The first, an unusual five- and four-membered bicyclic ring system (D), rearranges to a very unsymmetrical metallaheptatetraene complex (E), which in turn provides CpRuCl(eta(2)-C(6)H(6)) (F) via a reductive elimination step. The asymmetry of E, including Cp ring slippage, removes the symmetry-forbidden character from this final step. Completion of the cycle is achieved by an exothermic displacement (21.4 kcal mol(-)(1)) of the arene by two acetylene molecules regenerating A. In addition to acetylene, the reaction of B with ethylene and carbon disulfide, the latter taken as a model for a molecule lacking hydrogen atoms, has also been investigated, and several parallels noted. In the case of the coordinated alkene, facile C-C coupling to the alpha carbon of the metallacycle is feasible due to an agostic assistance, which tends to counterbalance the reduced degree of unsaturation. Carbon disulfide, on the other hand, does not coordinate to ruthenium, but a C=S bond adds instead directly to the Ru=C bond. The final products of the reactions of B with acetylene, ethylene, and carbon disulfide are, respectively, benzene, cyclohexadiene, and thiopyrane-2-thione, the activation energies being lower for acetylene.
Complexes of the general formula [MoO2X2L2] (X=Cl, Br, Me; L2=bipy, bpym) have been prepared and fully characterized, including X‐ray crystallographic investigations of all six compounds. Additionally, the highly soluble complex [MoO2Cl2(4,4′‐bis(hexyl)‐2,2′‐bipyridine)] has been synthesized. The reaction of the complexes with tert‐butyl hydroperoxide (TBHP) is an equilibrium reaction, and leads to MoVI η1‐alkylperoxo complexes that selectively catalyze the epoxidation of olefins. Neither the Mo−X bonds nor the Mo−N bonds are cleaved during this reaction. These experimental results are supported by theoretical calculations, which show that the attack of TBHP at the Mo center through the X‐O‐N face is energetically favored and the TBHP hydrogen atom is transferred to a terminal oxygen of the Mo=O moiety. After the attack of the olefin on the Mo‐bound peroxo oxygen atom, epoxide and tert‐butyl alcohol are formed. The latter compound acts as a competitive inhibitor for the TBHP attack, and leads to a significant reduction in the catalytic activity with increasing reaction time.
Treatment of 1 equiv of the 1,3-bis(trimethylsilylethynyl)benzene with 2.3 equiv of [Fe(Cp*)(dppe)Cl] [1, Cp* = η5-C5Me5, dppe = η2-bis(diphenylphosphino)ethane], KF and KPF6 salts in a methanol/THF mixture (10:1) produced the bis(iron) alkyne complex [{Fe(Cp*)(η2-dppe)(C⋮C−)}2(1,3-C6H4)] (2) in 80% yield. Similarly, the trinuclear iron derivative [{Fe(Cp*)(η2-dppe)(C⋮C−)}3(1,3,5-C6H3)] (3) was obtained from reaction of 1 with 1,3,5-tris(trimethylsilylethynyl)benzene (80% yield with respect to the organic ligand). The X-ray crystal structure of 3 shows that it crystallizes in the triclinic space group with unit-cell parameters of a = 18.142(6) Å, b = 18.652(8) Å, c = 20.108(6) Å, α = 113.72(3), β = 89.87(3), γ = 116.07(3)°, and Z = 2. The structure was solved and refined (6922 reflections) to the final values R = 0.081 and R w = 0.069. Cyclic voltammetric analysis of complexes 2 and 3 from −1.0 to +0.5 V displays two and three one-electron reversible oxidation waves, respectively. The redox processes are all separated by 0.130 ± 0.010 V, indicating significant electronic communication between the metal centers. A theoretical treatment using density functional molecular orbital calculations has been made on compounds 2, 3, and the related compound [{Fe(Cp*)(η2-dppe)(C⋮C−)}2(1,4-C6H4)]. These results verify the experimental structure of 3 and allow an interpretation of its electronic structure.
We describe the synthesis of the new Zn–N‐heterocyclic carbene (NHC) alkoxide complexes [(S,CNHC)ZnCl(OBn)]2 (5) and [(O,CNHC)ZnCl(OBn)]2 (6) for use as ring‐opening polymerization (ROP) initiators for lactide polymerization. Complexes 5 and 6 are readily available through an alcoholysis reaction between BnOH and the corresponding Zn–NHC ethyl species [(S,CNHC)ZnCl(Et)] (3) and [(O,CNHC)ZnCl(Et)] (4), and species 3 and 4 were obtained from the reaction of ZnEt2 with the N‐methyl‐N'‐ethylphenylsulfide (1⋅HCl) and N‐methyl‐N'‐ethylmethylether (2⋅HCl) imidazolium salts, respectively. Both solution and solid‐state structural data for Zn benzyloxide species 5 and 6 agree with dimeric structures under the studied conditions (reaction conditions: CH2Cl2 or THF, room temperature). A computational analysis of species 3 and 4 supports a dimeric structure in solution. The ZnII alkoxide species 5 and 6 were found to mediate either the ROP of lactide (in an effective and controlled manner) to produce chain length‐controlled polylactide (PLA) or, in the presence of an alcohol source such as MeOH, the controlled degradation of PLA through extensive transesterification reactions to afford methyl lactate as the major product. A thorough DFT computational analysis of the ROP of lactide initiated by complex 5 was performed, which revealed that the operating coordination–insertion mechanism was assisted by the second Zn center, leading to a lower‐energy ROP process; this result may be of interest for the future design of well‐defined and high‐performance metal‐based catalysts.
Several new iron(II) complexes of the types [Fe(PNP)X 2 ] (X = Cl, Br) containing tridentate PNP pincer-type ligands based on 2,6-diaminopyridine and 2,6-diaminopyrimidine have been prepared. They all exhibit intermolecular Fe-X 3 3 3 H-N hydrogen bonds, forming supramolecular networks in the solid state. In the case of X = Cl these compounds react readily with gaseous CO both in the solid state and in solution to give selectively the octahedral complexes cis-and trans-[Fe(PNP)(CO)-(Cl) 2 ], respectively, whereas with X = Br mixtures of cis and trans isomers are obtained. These transformations are accompanied by color and spin-state changes. CO binding is fully reversible in all cases, and heating solid samples of either cis-or trans-[Fe(PNP)(CO)(X) 2 ] leads to complete regeneration of analytically pure [Fe(PNP)(X) 2 ]. M€ ossbauer spectroscopy confirmed the high-spin nature of the parent five-coordinate Fe(II) complex (δ = 0.80(1) mm s -1 ) and the shift to two different low-spin octahedral species after reaction with CO in the solid (δ = 0.13(1) mm s -1 ) or in solution (δ = 0.15(1) mm s -1 ). Magnetization studies led to a magnetic moment close to 4.9 μ B , reflecting the expected four unpaired d-electrons in [Fe(PNP)Cl 2 ], which were confirmed by DFT calculations. The DFT study of the reaction pathway for CO capture led to low energy barriers (e3.4 kcal mol -1 ). The cis-trans isomerization reaction was found to take place with a low energy barrier (10.8 kcal mol -1 ), after initial loss of chloride, and involves also spin-state changes.
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