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Photochemistry of Osmium-Carbyne Complexes

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Photochemistry of Osmium-Carbyne Complexes Photochemistry of Osmium-Carbyne Complexes Arnd Vogler*, Josef Kisslinger Institut für Anorganische Chemie, Universität Regensburg, Universitätsstraße 31, D-8400 Regensburg and Warren R. Roper Department of Chemistry, University of Auckland, Auckland, New Zealand Dedicated to Prof. Dr. Dr. h. c. mult. E. 0. Fischer, on the occasion of his 65th birthday Z. Naturforsch. 38b, 1506-1509 (1983); received July 5, 1983 Photochemistry, Carbyne Complexes, Osmium Complexes Upon charge transfer (Os to carbyne) excitation the carbyne complexes + Os(CPh)(CO)(PPh3)2Cl and [Os(CPh)(CO)2(PPh3)2] with Ph = C6H5 are converted to the carbene complex Os(CHPh)(CO)(PPli3)2Cl2 in solutions containing HCl. It is suggested that the relaxed CT state can be described as square-pyramidal Os(II) complex containing a bent carbyne ligand which carries a lone pair at the coordinating carbon atom. Product formation occurs by the addition of a proton to the carbyne ligand and by attaching a chloride to the osmium completing an octahedral coordination. The cationic carbene complex thus formed is apparently not stable but undergoes a substitution of a CO ligand by chloride. Introduction according to ref. [10] and [11]. Solvents were of spectro-grade quality and saturated with nitrogen The first report on carbyne complexes of the type or argon. The solvent mixtures of CH2CI2, C2H5OH, X(CO)4M = C-R (with M = Cr, Mo, W and X = and aqueous HCl were prepared by adding con• halide) containing a metal-carbon triple bond was centrated hydrochloric acid to a large excess of published by E. O. Fischer and his group in 1973 equal volumes of CH2CI2 and C2H5OH. The overall concentrations of HCl are specified for every meas• [1]. This discovery stimulated a rapid expansion urement. Solutions of the complexes were prepared of the chemistry of carbyne complexes [2, 3]. The under a nitrogen or argon atmosphere using stand• synthesis of a large number of these compounds ard procedures for handling air-sensitive compounds. was accompanied by the investigation of their re• The light sources were an Osram HBO 100 W/2 activity. This experimental work was followed by and a Hanovia Xe/Hg 977 B-l (1000 W) lamp. The Schott cut-off filters WG 305/2 and WG 320/2 were theoretical studies of the electronic structure of used for irradiations at wavelengths longer than 305 carbyne complexes [4-7]. While there is an exten• and 320 nm. The Schott interference filters PIL 313, sive body of observations on thermal reactions the 333, and 366 were used for the selection of the photochemistry of carbyne complexes has been mercury lines at 313, 333, and 366 nm. The photo- almost unexplored. Although a light-induced re• lyses were performed in 1-cm spectrophotometer cells at room temperature. For quantum-yield action of a carbyne complex was reported by E. 0. determinations the complex concentrations were Fischer and P. Friedrich, the mechanism of this such as to have essentially complete light absorp• reaction is not known [8]. This lack of knowledge tion. The total amount of photolysis was limited to of the reactivity of carbyne complexes in their less than 5% to avoid light absorption by photo- products. Absorbed light intensities were determined electronically excited states prompted our present by a Polytec pyroelectric radiometer which was study as an extension of our work on the photo• calibrated and equipped with a detector RkP-345. chemistry of a carbene complex [9], The experimental error of the quantum yields were ±30%. Experimental Progress of the photolyses were monitored by UV-visible spectral measurements with a Varian- The compounds [Os(CPh)(CO)2(PPh3)2]C104 and Techtron Super Scan 3 recording spectrophotometer Os(CPh)(CO)(PPh3)2Cl (Ph = CeHs) were prepared and a Zeiss PMQ II spectrometer for measurements at single wavelengths. Changes of absorbance at * Reprints requests to Prof. Dr. A. Vogler. selected wavelengths were used to calculate 0340-5087/83/1100-1506/$ 01.00/0 the amount of photolysis. At 380 nm the molar extinction coefficients are e = 1240 for in Fig. 2. At the beginning the photolysis was a + [Os(CPh)(CO)2(PPh3)2] and e = 6460 for smooth reaction as indicated by an isosbestic point Os(CHPh)(CO)(PPh ) CI in mixtures of CH C1 , 3 2 2 2 2 at 344 nm. The first photoproduct was identified ethanol, and aqueous HCl. The extinction coefficient as II by its absorption spectrum (Amax — 365 nm, of Os(CHPh)(CO)(PPh3)2Cl2 was e = 280 at 535 nm in benzene. The neutral carbyne complex e = 7000 in solutions of CH2C12, C2H5OH, and Os(CPh)(CO)(PPh3)2Cl does not absorb at this aqueous HCl). At the isosbestic point II and III wavelength. have the same extinction coefficient (A = 344 nm, e = 6800). In this solvent II was apparently light Results sensitive and underwent a secondary photolysis. When solutions of Os(CPh)(CO)(PPh3)2Cl (I) in After continued irradiation the isosbestic point at benzene were saturated with gaseous HCl the com• 344 nm disappeared. It is assumed that II was plex I was converted to Os(CHPh)(CO)(PPh3)2Cl2 photodecomposed since a decrease of extinction (II) in a thermal reaction [11]. At low concentra• was observed over a large wavelength region tions of HCl (c < 10"4 M) this conversion was slow (A > 300 nm). The primary photochemical reaction, but could be accelerated by irradiation (A > 320 the conversion of III to II, proceeds with a quan• 3 nm). The photolysis was a smooth reaction as tum yield of <p = 0.2 (Airr = 313 nm, 10~ M HCl) indicated by the spectral changes (Fig. 1). After upon irradiation of the absorption band in the near longer irradiation times I was almost completely UV. converted to II (Amax = 376 nm, s = 9800). The spectrum f of Fig. 1 corresponds to a conversion of about 95%. In benzene solutions containing 5 x 10~5 M HCl the photolysis proceeds with a quantum yield of approximately 9? = 0.1 (Airr = 365 nm). A precise determination was not possible due to the interference of the thermal reaction. Fig. 2. Spectral changes during the irradiation of 4 1.3 X IO- M III in mixture of CH2C12, C2H5OH, and aqueous HCl (6 X 10"4 M HCl) at (a) 0 and (e) 60 min irradiation time, Airr > 305 nm, and 1 cm cell. Fig. 1. Spectral changes during the irradiation of Discussion 4 X 10"4 MI in benzene solutions containing HCl (~10~4M) at (a) 0 and (f) 30 min irradiation time, The electronic absorption spectra of the carbyne Airr = 366 nm, and 1 cm cell. complexes I (Fig. 1) and III (Fig. 2) exhibit fairly intense long-wavelength absorption maxima at The cation [Os(CPh)(CO)2(PPh3)2]+ (III) did not 318 (e = 13500) and 366 nm (e = 4700) which we react thermally in mixtures of CH2CI2, ethanol, and assign to charge transfer metal to ligand (CTML) hydrochloric acid (c < 10~3 M HCl). However, upon transitions from osmium to the carbyne ligand. irradiation (A > 305 nm) III underwent a photo• This assignment is based on several considerations. lysis. The accompanying spectral changes are shown A trigonal-bipyramidal structure of I was confirmed by X-ray analysis [11]. The cation III has probably energy of the antibonding n*-orbitals of NO or the the same structure which is characteristic of many carbyne ligand and thus stabilizes the CT excited low-valent metal complexes with a d8 electron con• state. In a limiting description the CT transition figuration. This requires formally as Os(0) metal induces a two-electron transfer from Os(0) to the center and a carbyne cation IC-Ph+ as a ligand. carbyne ligand. As a result a square-pyramidal Carbyne cations seem to be among the best rc-acid Os(II) complex is obtained with a bent carbyne in ligands due to their energetically rather low-lying an axial position. The bent carbyne ligand carries empty n* orbitals which can accept a large portion now a lone pair at the coordinating carbon atom. of charge by the interaction with occupied dn- orbitals of the metal [5-7]. The empty 7r*-orbitals Ph which are formed by the antibonding interaction may be localized to large extent at the carbyne i + ligand. A calculation on [Fe(CPh)(CO)2(PH3)2] which may be used as a model for III has shown [7], that the LUMO is indeed localized on the —/ 0s°-V- CT - / Os11 / carbyne ligand to a large degree (63%). In analogy to many other carbyne complexes the HOMO of III is assumed to be essentially a metal d-orbital. Consequently, it is quite reasonable to assign the In another description the relaxed CT state could low-energy absorption band of the cation III to be envisaged as a coordinatively unsaturated octa• the CTML (Os to carbyne) transition. The long- hedral Os(II) complex containing a deprotonated wavelength absorption of the neutral complex I phenyl carbene ligand. In this situation the complex may be of the same origin. The photochemical can be easily attacked by electrophiles such as a results are also consistent with these assignments. proton to generate a carbene ligand. At the same time a sixth ligand such as chloride should be With regard to the photochemistry we suggest attached to the metal to complete the octahedral that the carbyne ligand has much in common with coordination. Hydrochloric acid provides both the nitrosyl ligand. Both cations, CR+ [5-7] and functions. It adds apparently to the CT excited NO+ [12] are very good ^-acceptor ligands due to state of I or III to form the photoproducts II or their low-energy empty jr*-orbitals.
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