Theoretical and photochemical studies of some group 6 organometallic complexes
Alamiry, Mohammed Abid Hassan (2005) Theoretical and photochemical studies of some group 6 organometallic complexes. PhD thesis, Dublin City University.
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This thesis consists of six chapters. Chapter 1 reviews the bonding considerations of carbonyl compounds, pyridine, and arene ligands, and recent considerations of the photochemical techniques and theoretical calculations applied to the complexes under this study.
Chapter 2 describes the photochemistry o f M (C O )5L, M = Cr, or W; L = Pyridine, acetylpyridine, cyanopyridine, or triphenylphosphine. This chapter also presents the steady state photolysis, laser flash photolysis and the matrix isolation studies on these complexes. Generally, the steady state photolysis o f most of these complexes resulted in either ligand loss or CO loss photoproducts. In contrast with the published literature, no photochemical change was observed upon the photolysis o f either W(CO)sPy or W(CO)sAcpy in the presence o f excess o f the ligand (0.01 M ) with different broad band irradiations. Monochromatic irradiations (354.7 nm) give the tetracarbonyl complexes as precipitates from the cyclohexane solution. The photolysis o f M (C O )5PPh3; M = Cr or W under CO atmosphere, resulted the formation of the corresponding hexacarbonyl, while the photolysis in the presence o f excess PPI13 ligand resulted tetracarbonyl photoproducts trans-Cr(CO)4(PPh3)2 and cis-W(CO)4(PPh3)2 for the chromium and tungsten complexes respectively.
The photolysis of W(CO)sCNpy in toluene in the presence o f excess ligand (0.01 M ) with different broad-band irradiations and also with monochromatic light produced the cyano-bond complex W (C O )5(cyano-CNpy) along with the tetracarbonyl complex. Photolysis o f Cr(CO)sAcpy at different wavelengths resulted the formation of linkage isomer C r(C 0 )s (0 -A cp y ). Reformation o f the N-bond complex occurred at room temperature upon leaving the solution in the dark. The analysis o f the laser flash photolysis data for these complexes reveals the presence of two transient species. The first transient species is assigned to ligand loss product to form the coordinatively unsaturated M(CO)s species, while the second resulted from the CO loss process forming M (C O )4L. Matrix isolation studies on the complex Cr(CO)sPy and Cr(CO)sAcpy in an argon matrix indicated both the ligand loss and CO loss process.
Chapter 3 deals with the matrix isolation studies o f (r|6-C 6H 5-X )C r(C O )3 complexes, (X = H , N H 2, O C H 3, C HO , or COOMe) in methane, dinitrogen, 2 %, 5%, or 10 % CO-methane matrixes. The nature o f the substituent on the benzene ring has been shown to affect the photochemical properties of these complexes.
The studies were extended to the molybdenum complexes. Chapter 4 presents the matrix isolation experiments on complexes o f the type (r|6-C 6H 5-X )M o (C O )3, X = C H 3, O C H 3, or N (C H 3)2 in methane, dinitrogen, 2 %, or 5 % CO-methane mixtures at 12 K . The formation of molybdenum hexacarbonyl and (riI-C 6H 5-X )M o (C O )3(N 2)2 upon the photolysis of (r|6-C 6H 5-X )M o (C O )3 complexes in CO-methane matrix and N 2 matrix respectively provides good evidence of that haptotropic shift.
Chapter 5 deals with the theoretical studies on the complexes o f the type Cr(CO)sL, L = Py, Acpy or CNpy and the complexes o f the type (C<5H 5-X )C r(C O )3 complexes, (X = H , N H 2, O C H 3, CHO, or CO OMe). D F T calculations provides geometries, IR frequencies, and the molecular orbitals o f these complexes which compared with the experimental data available.
TDDFT calculations for the first three low lying excited states o f this set of complexes generally reveals that these excitation involve transition o f electron from the highest occupied molecular orbital (H O M O ) which carry c.a. 60 % Cr-d character to the lowest unoccupied molecular orbitals (L U M O ) which are principally located on the pyridine or CO ligands. So the low lying excited state for pyridine complex is similar to that o f acetyl- or cyano-pyridine complexes, and carries mainly a Cr-Py C T or Cr-CO C T character.
The electronic structure and the orbital composition for each these complexes were calculated. Many of the molecular orbitals o f the benzene complex are degenerate.
The degeneracy is lost upon substitution of the arene ligand. Electron-donor substituents on the arene ligand destabilise the molecular orbitals. While electrondrawing substituents stabilise the molecular orbitals. Complexes with donor substituents tend to stabilise the dxz orbital and destabilize the dyz orbital. The reverse occurs with electron withdrawing substituents. This was explained by considering geometry and substituent effects. TDDFT calculations on the three lowest excited states of these complexes reveal that the transitions occur from the highest occupied molecular orbitals (HOMO), which are natively localised on chromium d-orbitals to the lowest unoccupied molecular orbitals, which are localised on the arene or CO ligands. The excitations are a mixture of mainly Cr-arene CT, Cr-CO C T and to smaller extent LF (Cr d-d) transition).
Outlined in chapter 6 are the experimental details and suggestions for future work.
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