R.R. Andréa

A Cryogenic Cell for Liquid Noble Gases. Vibrational Spectroscopy and Photochemistry of Transition Metal Carbonyls in Liquid Xenon

The construction of a special cryogenic cell for spectroscopic and photochemical measurements in liquefied noble gases under pressure is described. The inner (sample) cell, withstanding a pressure of at least 700 psi, has no high-vacuum around it. It has two crossed IR and UV-visible optical pathways of 30 mm. The usefulness of these noble gases in vibrational spectroscopy is demonstrated for the following transition metal carbonyls, dissolved in liquid xenon (=LXe, pressure <150 psi, 170 < T < 240 K): [W(CO)6], [Mn2(CO)10l, and [Co2(CO)8]. The great advantage of LXe is its complete transparency over a wide spectral range. The limited solubility of many complexes in LXe in comparison with “normal” solvents is often compensated by the long optical pathway of the cell. Because of its complete inertness, reactive intermediates and products of photochemical reactions can be stabilized in LXe, even at moderate temperatures. The photochemical reaction is described of [W(CO)6] with a l,4-diaza-l,3-butadiene (=R-DAB; RN=CHCH=NR) ligand. During this reaction a photoproduct is identified as a stable complex which is so unstable in normal solvents that it can only be observed with rapid-scan FT-IR spectroscopy.

He(I) and He(II) Photoelectron Spectra of Some Mixed Sandwich Compounds of Titanium, Zirconium and Hafnium

Abstract The He(I) and He(II) photoelectron spectra are reported for two series of transition metal mixed sandwich complexes of general formula L a ML b (M = Ti, L a = η 5 -C 5 H 5 , η 5 -CH 3 C 5 H 4 , η 5 -C 5 (CH 3 ) 5 ; M = Zr, Hf, L a = η 5 -C 5 (CH 3 ) 5 ; L b = η 7 -C 7 H 7 (series I); M = Ti, L a = η 5 -CH 3 C 5 H 4 , η 5 -C 5 (CH 3 ) 5 ; M = Zr, L a = η 5 -C 5 (CH 3 ) 5 ; L b = η 8 -C 8 H 8 (series II)). Assignments were made of the metal d , cyclopentadienyl and carbocyclic π orbitals on the basis of He(I)/He(II) intensity ratios and shift effects and by comparison with UP data for related compounds. For series I no influence of the central metal upon the IEs of the highest occupied molecular orbital e 2 was observed. The IE of the non-bonding metal d z 2 orbital of Ti or Zr (5.28 and 4.70 eV, respectively) in the complexes of series II (L a = η 5 -C 5 (CH 3 ) 5 ) is very low.

Spectroscopic (IR, UVVis, 1H and 119Sn NMR, resonance raman) properties of [(C6H5)3EM(CO)3(α-diimine)] (M = Mn, E = Ge, Sn, Pb; M = Re, E = Sn) complexes and x-ray structure of [(C6H5)3SnMn(CO)3(t-Bu-DAB)]

Abstract The X-ray structure of the title compound has been determined by the heavy-atom method and refined by means of block-diagonal least-squares calculations from 4598 independent reflections. The crystals are monoclinic, space group P 2 1 / c with unit cell dimensions a = 20.025(1), b = 18.917(1), c = 17.117(1) A, β = 112.120(0)° and Z = 8. The complex has a nearly octahedral geometry around the Mn atom. The MnSn bond length is 2.70(1) A and the two MnN bond lengths are 2.10(2) A. Infrared (IR), electronic absorption, 1 H and 119 Sn NMR and resonance Raman (rR) spectra are reported for the complexes [(C 6 H 5 ) 3 EM(CO) 3 (α-diimine)] (M = Mn, E = Ge, Sn, Pb; M = Re, E = Sn; α-diimine = 1,4-diaza-1,3-butadiene, pyridine-2-carbaldehyde-imine and 2,2′-bipyridine). The IR bands in the CO stretching region are assigned and the rR effect observed for one of these vibrations (ν(CO) ax ) in the case of the Mn complexes is explained with a delocalization of the lowest MLCT state over this axial-carbonyl ligand.

He(I) and He(II) photoelectron spectra of the four metal-metal bonded complexes [(CO)5M-M′(CO)3(1,4-i-Pr2-1,4-diaza-13-butediene)] (M = M′ = Mn or Re; M = Mn, M′ = Re; M = Re, M′ = Mn)

Abstract The ultraviolet He(I) and He(II) photoelectron (UP) spectra have been recorded for the dinuclear d 7 -metal carbonyl complexes [(CO) 5 M-M′(CO) 3 (i-PrNCHCHN-i-Pr)], where M = Mn, M′ = Re; M = Re, M′ = Mn; M = M′ = Mn; M = M′ = Re. The observed vertical ionization energies (IEs) of these four complexes are assigned to metal d orbitals, the ligand orbitals, the metal-metal σ b orbital with the aid of semi-empirical molecular orbital (MO) calculations, consideration of He(I)/He(II) intensity ratios, and comparison with data for related complexes. The IEs of the σ b (M = M′) orbital of these complexes are 6.95 eV (M = M′ = Mn), 7.19 eV (M = Re, M′ = Mn), 7.15 eV (M = Mn, M′ = Re) and 7.03 eV (M = M′ = Re), respectively. These data and other UP results are of importance for the interpretation of the photochemical behaviour of these complexes.

He(I) and He(II) photoelectron spectra of mononuclear transition metal carbonyl complexes containing a 1,4-diaza-1,3-butadiene ligand

Abstract He(I) and He(II) photoelectron spectra have been recorded for the following mononuclear transition metal carbonyl complexes containing symmetrically 1,4-disubstituted 1,4-diaza-1,3-butadienes, RNC(R′)C(R″)NR, d6 [M(CO)4(RNCHCHNR)] (M = Cr, W, R = i-Pr; M = Mo, R = t-Bu) and [ReCl(CO)3(i-Pr-NCHCHN-i-Pr)], d8 [M(CO)3(RNCR′CR″NR)] (M = Fe, R = t-Bu, n-Bu, c-Hex, R′ = R″ = H; M = Fe, R = n-Bu, R′ = R″ = Me; M = Ru, R = i-Pr2CH, R′= R″ = H) and [Fe(t-BuNCHCHN-t-Bu)2], and d10 [Ni(CO)2(t-BuNCHCHN-t-Bu)] and [Ni(t-BuNCHCHN-t-Bu)2]. The observed vertical ionization energies (IEs) are tabulated and assigned to metal d orbitals and ligand systems on the basis of the results of semi-empirical molecular orbital (MO) calculations, He(I)/He(II) intensity ratios, and comparisons with related molecules (trend effects). The weighted average IE of the metal d orbitals in each R-DAB complex ( IE ′d) is lower than that of the corresponding unsubstituted metal carbonyl complex ( IE d). The values of ΔIEd (= IE ′d - IE d) are −1.54 eV for the d6 complexes, −1.80 eV for the d8 system and −2.09 eV for the d10 compound. This means that ΔIEd increases by the same amount on going from d6 to d8 complexes (0.26 eV) as it does on going from d8 to d10 complexes (0.29 eV). Replacement of two CO groups by a second RNCHCHNR ligand leads to a further lowering of the weighted average IE of the d orbitals (−0.5 eV) which is significantly less than that found for the first substitution.

Reexamination of the photochemical oxidative decarbonylation of Cr(CO)6 by ortho-quinones: Low-temperature photolysis of Cr(CO)6 with ortho- and para-quinone isomers

Abstract In order to stabilize and identified intermediates in the photochemical formation of CrIII(SQ)3 (SQ = ortho-semiquinone radical-anion) out of Cr(CO)6 and ortho-quinones (o-Q), this reaction has been studied at lower temperatures (T Warming the solution of Cr0(CO)5(o-Q) in the absence of CO afforded CrIII(SQ)3, with no evidence for any carbonyl-containing intermediate. This is explained in terms of a concerted mechanism involving chelate ring formation accompanied by electron transfer to the o-Q ligand.

He(I) and He(II) photoelectron spectra of Hyprrole-2-CHN′t-Bu and the biscarbonyl Rh(I) and Ir(I) complexes of its anion

Abstract The He(I) and He(II) photoelectron spectra of a series of [(LL)M(CO) 2 ] (LL = pyrrole-2-CHN′ R; R = t-Bu; M = Rh, Ir) complexes are reported. Assignments are proposed based on He(I)/He(II) intensity differences, on molecular orbital calculations of related complexes and of free ligands, and by comparison with the spectra of the free ligands Hpyrrole-2-CHN′t-Bu, Hpyrrole-2-carbaldehyde and Hpyrrole. The electronic structure of the complexes is discussed and conclusions are drawn about the metal-ligand interaction.

He(I) and He(II) photoelectron spectra of 1-aza-1,3-butadiene-tricarbonyliron complexes: [Fe(CO)3(R1NCHCHCHR2)]

Abstract He(I) and He(II) photoelectron spectra are reported for the 1-aza-1,3-butadienes (R 1 NCHCHCHR 2 denoted by R 1 ,R 2 -ABD) t-Bu,Me-ABD and i-Pr,Ph-ABD and their tricarbonyliron complexes [Fe(CO) 3 (R 1 ,R 2 -ABD)]. Assignments of ionizations from the iron d and ligand orbitals have been made with the aid of He(I)/He(II) intensity ratios and some semi-empirical molecular orbital calculations on the model ligand Me,H-ABD (MNDO) and on the model complex [Fe(CO) 3 (H,H-ABD)] (CNDO/S). A remarkable feature is the lowering of the ionization energy from the Fe d xz/yz 2 orbital with respect to the other d orbitals ( d xy / d x 2 – y 2 / d z 2 ) 6 by about 0.9 eV, an effect which has not been found for the related [Fe(CO) 3 (1,3-butadiene)] complexes. The involvement of the nitrogen lone pair in the bonding between the R 1 ,R 2 -ABD and Fe(CO) 3 moieties is discussed.

Photochemistry of metalmetal bonded complexes.: IV. Breaking of the metalmetal bond and decoupling of the two 1,4-diaza-1,3-butadiene (= R-DAB) fragments of bis(1-t-butylimino-2-t-butylaminoethane) (= t-Bu-IAE) during the photochemical conversion of Mo2(CO)6(t-Bu-IAE) into Mo2(CO)6(t-Bu-DAB)2

Abstract The complex Mo2(CO)6(t-Bu-IAE) (I) (bis(1-t-butylimino-2-t-butylaminoethane) which contains a MoMo bond and a tetradentate 10e donor ligand t-Bu-IAE, consisting of two CC coupled t-Bu-DAB (t-BuNCHCHNBu-t) ligands has been irradiated into its σ → σ* transition. The photolysis yields Mo2(CO)6(t-Bu-DAB)2 (II), a complex in which the MoMo bond is broken and the uncoupled t-Bu-DAB ligands act as 6e (σ-N,μ2-N′,η2-CN′) donors.

Identification of H2-, D2-, N2- Bonded Intermediates in the Cr(CO)6 Photocatalyzed Hydrogenation Reactions

The mechanisms of photochemical reactions involving transition metal complexes have been the subject in several recent papers. Many attempts have been made to identify intermediates. This can be performed by studying parent molecules and primary photoproducts in inert matrices or solid glasses. Usually these intermediates can be characterized by IR and UV-visible spectroscopy. It is also possible to use flash photolysis in combination with fast resolved spec- troscopy. This technique has the great advantage that reactions can be followed under normal conditions. This time-resolved techniques however have special restrictions as time and money consuming methods. Although matrix isolation spectroscopy is a very valuble technique for the identification and conformational analysis of primary photoproducts it has severe limitations in studying ongoing reactions because of lack of diffusion and mobility of the components.