An Investigation of the Photochemical and Spectroscopic Properties of Chromium, Molybdenum, Tungsten, and Rhodium Isocyanide Complexes

Citation

Mann, Kent Robert (1977) An Investigation of the Photochemical and Spectroscopic Properties of Chromium, Molybdenum, Tungsten, and Rhodium Isocyanide Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/s9tb-gr40. https://resolver.caltech.edu/CaltechTHESIS:08132024-203836380

Abstract

The X-ray crystal structure of Cr(CNPh)₆ has been determined. The complex crystallizes in the space group R3 with one molecule in the unit cell a = b = c = 10.628Å; α = β = γ = 111.17°; V = 861.1 ų. The metal atom has crystallographic site symmetry S₆ with the MC₆ framework forming a perfect octahedron. The Cr-C-N bond angles are all 174.7°, the Cr-C bond lengths are 1.933(2)Å, and the C-N bond length are 1.168(2)Å.

Both infrared spectra, X-ray crystal structure data, and electronic absorption spectra are consistent with the operation of a second order Jahn Teller distortion of the ground state of Cr(CNPh)₆. Data obtained on the sterically hindered systems Cr(CNDph)₆ and Cr(CNIph)₆ [Dph = 2,6-dimethylphenyl and Iph = 2,6-diisopropylphenyl] also support the operation of a Jahn Teller effect. Similar considerations apply to the Mo and W systems.

The electronic absorption spectra of M(CNAr)₆ [M = Cr(0), Mo(0), W(0), Ar = phenyl, ph; 2,6-dimethylphenyl, Dph; and 2, 6-diisopropylphenyl, Iph], Mn(CNP'n)₆Cl and [Mn(CNPh)₆](PF₆)₂ are reported. Each of the M(CNAr)₆ complexes exhibits intense allowed metal-to-ligand charge transfer (MLCT) absorption bands between 20.8 and 32.7 kK. The lowest MLCT bands are observed at 29.9 and 31.1 kl( in the electronic spectrum of Mn(CNPh)₆⁺. Low energy bands at 18.2 and 20.4 kK in [Mn(CNPh)₆]²⁺ are assigned to vibronic components of a σ(CNPh) → dπ charge transfer transition. The unique electronic structural properties of arylisocyanide complexes are apparently related to the π conjugation of aromatic ring orbitals with the out-of-plane π*(CN) function.

The emission and photochemical behavior of M(CNPh)₆ and M(CNIph)₆ complexes (M = Cr, Mo, W; Ph = phenyl, Iph = 2,6- diisopropylphenyl) has been studied. The complexes of Mo and W show emission attributable to an Lπ* → dπ process in a variety of solvents (2-methylpentane, 2-MeTHF, benzene, pyridine) at room temperature. Complexes of all three metals show emissions at 77 K in 2-MeTHF that overlap the MLCT absorption bands. The emission quantum yields for Mo(CNIph)₆ and W(CNIph)₆ in 2-MeTHF at 77 K are 0.78 ± 0.08 and 0.93 ± O.07, respectively. The emission lifetimes at 77 K ir: 2-methylpentane for the M(CNIph)₆ complexes are: τ(Cr) less than 10 nsec, τ(Mo) 40.2 ± 0.5 μsec (298 K, 43 ± 2 nsec), τ(W) 7.6 ± 0.5 μ sec (298 K, 83 ± 2 nsec). Both M(CNPh)₆ and M(CNIph)₆ undergo photosubstitution reactions in pyridine solutions. Formation of M(CNPh)₅py and M(CNIph)₅py occurs upon irradiation at 436 nm, with quantum yields decreasing according to a regular pattern [Cr(CNPh)₆] (0.23) ~ [Cr(CNIph)₆] (0.23) > [Mo(CNPh)₆] (0.055) > [Mo(CNIph)₆] (0.022) > [W(CNPh)₆] (0.011) >> [W(CNIph)₆] (0.0003). The very small quantum yield for photosubstitution in the case of W(CNIph)₆ is interpreted as an indication that the mechanism of formation of W(CNPh)₅ has associative character. Irradiation of M(CNIph)₆ at 436 nm in CHCl₃ yields the one-electron oxidation products [M(CNIph)₆]Cl. The quantum yield in each case is 0.19 ± 0.01. Similar irradiation of M(CNPh)₆ in CHCl₆ gives two-electron oxidation products. For M = Mo, W, the products are identified as the seven-coordinate species [M(CNPh)₆Cl] Cl.

The room temperature UV-VIS solution spectra of Rh(CNR)₄⁺ [R = aromatic or aliphatic] have been found not to follow Beer's law. This behavior has been attributed to complex oligomerization of the monomeric Rh(CNR)₄⁺ units to form species of the type [Rh(CNR)₄⁺]_n (n = 1, 2, 3). The extinction coefficients and formation constants using the following expressions:

M + M K₁ ⇄ D

D + M K₂ ⇄ T

have been obtained for the systems R = phenyl in acetonitrile solution; and R = t-butyl in water solution. The values for the parameters used are for R = phenyl, K₁ = 35 M^-1, ε₂ = 10,500, ε₂K₂ = 183,000 M^-1; for R = t-butyl, K₁ = 251 M^-1, ε₂ = 16,900.

The nature of the oligomerization is due to a direct metals metal interaction of the d⁸Rh atoms. The band positions for the oligomeric species were found to conform to predictions made by simple Hückel theory.

The synthesis and characterization of a dimeric Rh(I) complex containing the bridging ligand 1,3-diisocyanopropane(bridge) is reported. In methanol solution, [Rh₂(bridge)₄]²⁺ oligomerizes, and species containing four, six, and eight Rh atoms have been

identified spectroscopically. The dimer, [Rh₂(bridge)₄]²⁺, undergoes two center oxidative addition reactions with I₂, Br₂, and CH₃I. The products, [Rh₂(bridge)₄X₂]²⁺ (X = I, Br) which contain two strongly coupled Rh(II) atoms, possess trans stereochemistry. The mechanism of oxidative addition is thought to involve attack of the Rh(I) on the heavy atom of substrate molecule.

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