Spectroscopic Detection and Theoretical Confirmation of the Role of Cr₂(CO)₅(C₅R₅)₂ and ·Cr(CO)₂(ketene)(C₅R₅) as Intermediates in Carbonylation of NNCHSiMe₃ to OCCHSiMe₃ by ·Cr(CO)₃(C₅R₅) (R = H, CH₃)

Abstract

Conversion of N=N=CHSiMe3 to O=C=CHSiMe3 by the radical complexes .Cr(CO)3C5R5 (R = H, CH3) derived from dissociation of [Cr(CO)3(C5R5)]2 have been investigated under CO, Ar, and N2 atmospheres. Under an Ar or N2 atmosphere the reaction is stoichiometric and produces the Cr[triple bond]Cr triply bonded complex [Cr(CO)2(C5R5)]2. Under a CO atmosphere regeneration of [Cr(CO)3(C5R5)]2 (R = H, CH3) occurs competitively and conversion of diazo to ketene occurs catalytically as well as stoichiometrically. Two key intermediates in the reaction, .Cr(CO)2(ketene)(C5R5) and Cr2(CO)5(C5R5)2 have been detected spectroscopically. The complex .Cr(13CO)2(O=13C=CHSiMe3)(C5Me5) has been studied by electron spin resonance spectroscopy in toluene solution: g(iso) = 2.007; A(53Cr) = 125 MHz; A(13CO) = 22.5 MHz; A(O=13C=CHSiMe3) = 12.0 MHz. The complex Cr2(CO)5(C5H5)2, generated in situ, does not show a signal in its 1H NMR and reacts relatively slowly with CO. It is proposed to be a ground-state triplet in keeping with predictions based on high level density functional theory (DFT) studies. Computed vibrational frequencies are also in good agreement with experimental data. The rates of CO loss from 3Cr2(CO)5(C5H5)2 producing 1[Cr(CO)2(C5H5)]2 and CO addition to 3Cr2(CO)5(C5H5)2 producing 1[Cr(CO)3(C5H5)]2 have been measured by kinetics and show DeltaH approximately equal 23 kcal mol(-1) for both processes. Enthalpies of reduction by Na/Hg under CO atmosphere of [Cr(CO)n(C5H5)]2 (n = 2,3) have been measured by solution calorimetry and provide data for estimation of the Cr[triple bond]Cr bond strength in [Cr(CO)2(C5H5)]2 as 72 kcal mol(-1). The complex [Cr(CO)2(C5H5)]2 does not readily undergo 13CO exchange at room temperature or 50 degrees C implying that 3Cr2(CO)5(C5H5)2 is not readily accessed from the thermodynamically stable complex [Cr(CO)2(C5H5)]2. A detailed mechanism for metalloradical based conversion of diazo and CO to ketene and N2 is proposed on the basis of a combination of experimental and theoretical data.

EPR Spectroscopy as a Tool in Homogeneous Catalysis Research

“…For high spin transition metal complexes and other (odd-electron) complexes bearing more than one unpaired electron, the situation is different, and (large) zero field splitting parameters usually makes it rather difficult to correlate the measured g-tensors to the location of the unpaired electrons. High frequency (HF) EPR spectroscopy is often beneficial to record and interpret the spectra [9,10], but it remains in general difficult to correlate the (apparent) g-tensors of S = n/2 systems to the spin density distribution over the metal and the ligands. For oddelectron S [ systems, the g-tensor components do however provide useful information about the spin states and zero-field splitting parameters.…”

Binuclear Carbonylheptalenechromium Complexes: Partition of Heptalene into a Complexed Heptafulvene Subunit and an Uncomplexed 1,3‐Diene Subunit for Coordination to a Multiply Bonded Pair of Chromium Atoms

“…2.36 Å suggests the formal triple bond required to give each chromium atom the favored 18-electron configuration. A slightly higher energy C 12 H 10 Cr 2 (CO) 5 structure by ca. 3 kcal/mol has all 12 heptalene carbon atoms bonded to an unbridged Cr 2 (CO) 5 with a Cr-Cr distance of ca.…”

“…A slightly higher energy C 12 H 10 Cr 2 (CO) 5 structure by ca. 3 kcal/mol has all 12 heptalene carbon atoms bonded to an unbridged Cr 2 (CO) 5 with a Cr-Cr distance of ca. 3.3 Å corresponding to the formal single bond required to give each chromium atom the favored 18-electron configuration.…”

“…The global minima for the more highly unsaturated C 12 H 10 Cr 2 (CO) n (n = 4, 3) complexes are predicted to have a four-electron donor bridging η 2 -μ-CO group as indicated by short Cr-O distances consistent with a direct bond and a low ν(CO) frequency consistent with a low formal C-O formal bond order. discovered [5] instability of (C 5 H 5 ) 2 Cr 2 (CO) 5 with respect to disproportionation into (C 5 H 5 ) 2 Cr 2 (CO) 6 with a Cr-Cr single bond and (C 5 H 5 ) 2 Cr 2 (CO) 4 with a CrϵCr triple bond. This reaction is an unusual example of the disproportionation of a M=M double bonded derivative into a M-M single bonded derivative and the corresponding MϵM triple bonded derivative.…”

“…Azulene is the most readily available of such hydrocarbons and thus has been studied the most extensively. [6] Both a mononuclear azulene-tricarbonylchromium [7] (η 5 -C 10 H 8 )Cr(CO) 3 and a binuclear azulene dichromium hexacarbonyl [8] (η 5 ,η 5 -C 10 H 8 )-Cr 2 (CO) 6 have been synthesized (Figure 2). X-ray diffraction shows the latter to have a Cr-Cr single bond length of 3.26 Å, which is very close to the Cr-Cr single-bond length of 3.28 Å in (η 5 -C 5 H 5 ) 2 Cr 2 (CO) 6 , discussed above.…”