Early/late heterobimetallic complexes: syntheses and spectral and structural studies of thiolato-bridged titanium/copper and vanadium/copper complexes

Abstract

to a direct interference of the handle upon the bond formation step when the chain is located at a short distance from the iron atom. In spite of similar AG¡ values, Table I shows that the activation energy remains close to 4 kJ-moU1 for 02 whereas it is increased by nearly 20 kJ-mol"1 for CO with 5. On the contrary, A changes by more than 2 orders of magnitude for 02, but exhibits little variation for CO along the 3-5 series. Accordingly, the origin of the destabilization of transition state I is almost purely entropic for oxygen and predominantly enthalpic for carbon monoxide. This can be understood by considering the structural modifications that must take place during the binding process. A linear orientation of carbon monoxide should be accompanied at first by an increased interaction of this ligand with the chain. At the same time the porphyrin becomes more ruffled, leaving the chain farther away from the iron in order to accommodate CO linearly. Presumably, the shorter the handle length, the more pronounced the increase of distortion required in the binding process. Using the crystallographic data relative to the penta-and hexacoordinated complexes of 5, we calculated that the distance of the two central C atoms from the iron is increased from about 4.8 to 6.9 Á and that the ruffling distortion increases from 6 to 14°d uring the binding process. The energy that must be expended appears as a larger activation enthalpy in compounds 4 and 5 as compared to 3. Binding oxygen, on the contrary, implies a bent conformation.Although crystallographic data for the 02 complexes are not available, it does not seem necessary to assume an increase of ruffling in this case since the bent orientation would tend to release the steric constraints with the chain. The invariant activation enthalpy supports this view. The destabilization appears as entropy terms. The orientational freedom of oxygen is known to be strongly limited due to H-bonding with the amide protons. However the effect has been shown to be significant only in the bound state.8 Orientational constraint of a different nature, e.g. the avoiding of transient lateral interactions with the protons of the methylene groups, may play a specific role in the transition state.Thus, in addition to confirming that hybrid models also react via two sequential energy barriers, the present results indicate that steric discrimination against CO does not occur in either transition state since both are destabilized to the same extent for CO and 02. This is partly in agreement with the findings of Traylor et al.21 that steric effects are seen mainly in association rates and that those are changed equally for CO and 02. However, a previous discussion of the linear free energy relationships observed in various porphyrin series8 has shown that this behavior is not general and can only be expected for a strong central steric hindrance. Moreover, only a detailed thermodynamic study is able to reveal that the AG changes, although very close for 02 and CO, are brought about by the compl...

Rhodium induced titanium–sulfur bond cleavage: crystal and molecular structure of ((COD)Rh(μ-SMe))2

“…Our interest in early-late heterobimetallics (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) has included an extensive investigation of synthetic routes to thiolato-bridged mixed metal species (2-7). We have recently described the utilization of titanocene dithiolates as metalloligands for Cu (1) ( 5 ) and Ni(0) (6).…”

Syntheses, Structures and Theoretical Investigations of [Li(thf)4]2[Ti2Cu8S4(SPh)10] and [Ti2Ag6S6Cl2(PPhiPr2)6]

“…A very similar picture is found for sulfur, where a formal oxidation state -II is connected with occupations around 3s These examples show again that formal oxidation states have no direct connection to actual atomic orbital occupations or atomic charges. [29] We come back to the idea of dative d 10 Ǟd 0 bonds, which was proposed on the basis of the relatively short titaniumcopper distances (280-302.4 pm) [12,30,31] The idea has been supported by Fenske- + , which resulted in a small positive TiCu overlap population. [32] Although the authors admit that "bonding cannot be quantitatively assessed with accuracy by FHMO calculations" they consider it as a "qualitative and comparative guide".…”

The crystal and molecular structure of the coordinatively unsaturated Ni(0) species Ni(PPh3)3

“…The stability of this complex was attributed to the cage of cyclohexyl rings that surrounds the bridging nitrogen molecule. Other coordinatively unsaturated Ni-phosphine complexes have been known and studied spectroscopically for years (15); however, there have been no reports of structural studies of complexes of the form Ni(PR3)3 or Ni(PR3), During the course of our investigation of the chemistry of heterobimetallic complexes (16)(17)(18)(19)(20), we isolated the complex Ni(PPh3)3, 1. This red-brown complex was previously prepared by Tolman et al (15) via the Zn reduction of NiCl, in the presence of PPh3; however, it was reportedly extremely unstable and proved to be most difficult to characterize.…”