THEORETICAL STUDIES ON TUNGSTEN-CARBONYL-HALIDE COMPLEXES

Róbert K. Szilágyi, Lajos Bencze and Gábor Szalai

Müller Laboratory,
Department of Organic Chemistry,
University of Veszprém,
POB 158, Veszprém, H8201, HUNGARY
Fax: +36 88 427492

The C,O bond stretching frequencies of carbonyl complexes (1900-2400 cm-1 region of IR spectra) can be used to characterise non-isolated intermediates, reaction products and transition states, as well. Reaction mechanism and equilibrium or kinetics can be understood by evaluating of changes in the νCO frequencies. Several applications can be found in literature related to oxosynthesis by Co, Rh and Ru complexes and for olefin metathes by tungsten carbonyls.

The W(CO)nXm type tungsten-carbonyl-halide complexes (X=Cl, Br, I) are known as an active, unicomponent catalysts. They can catalyse metathesis reaction of linear olefins and cycloolefins. To understand their mechanism of action, their structure as a basic information should be known. The aim of our work is to elucide an accurate structure via assignment of carbonyl stretching frequencies for these compounds.

This contribution presents a case study for the W(CO)4Cl2 (1) complex. The structure, energy and vibrational properties were studied by ab initio (RHF+MP2+ECP level of theory) and semi-empirical (PM3(tm) Hamiltonian) quantum chemical methods. The computations were performed by the Gaussian92 and Spartan4.0 packages. Since it is an electron deficient complex, the single and triplet states were investigated and compared energetically and structurally. The vibrational spectra simulated were scaled to experimental data. The scaling factors derived by the comparison of calculated and experimental νCO frequencies for W(CO)6.

Calculated relative energies for the cis and trans isomers in two multiplicity states are given in Table 1.

Table 1 Energetical comparison of isomers of the W(CO)4Cl2 complex

Level of
Theory* cis isomer 		trans isomer
singlet triplet singlet triplet
HF 29.88 19.92 26.57 0
HF+MP2 5.13 17.76 6.70 0
PM3(tm) 0 20.83 28.90 16.77

*relative energies (kcal mol-1) to isomer of the lowest energy for each level of theory

The calculations underline that the monomeric form of 1 has two isomers corresponding to cis and trans arrangements of the chlorine atoms, respectively (Figures 1 and 2).

Cis isomer of W(CO)4Cl2 Trans isomer of W(CO)4Cl2
Figure 1
Structure of the cis W(CO)4Cl2
(triplet state) 		Figure 2
Structure of the trans W(CO)4Cl2
(triplet state)

Solution spectra of W(CO)4Cl2 in apolar solvents shows four adsorption peaks. Two of them could be identificated as those of cis W(CO)4Cl2 and trans W(CO)4Cl2. However, the rest two peaks remain still unassigned and this leads us to suppose that probably the dimeric or higher oligomeric forms can exist.

Acknowledgement

This work was supported by the Hungarian Science Foundation under Grant No. OTKA T016326.