Rhodium carbonyl cluster chemistry under high pressure of carbon monoxide and hydrogen. 3. Synthesis, characterization, and reactivity of HRh(CO)4

Article abstract of DOI:10.1021/ic50215a049

The fragmentation of Rh₄(CO)₁₂ in dodecane solutions under 1241-1379 atm of carbon monoxide at 5-12°C has been established by high-pressure infrared spectroscopy to give Rh₂(CO)₈. Noticeable spectral changes are caused by the introduction of small amounts of hydrogen (1542 atm, CO:H₂ = 4.5:1). Fourier subtraction of the spectra of these two species left bands at 2070 (m), 2039 (vs), and 2008 (w) cm^-1. By analogy to the spectra previously observed for HIr(CO)₄ and HCo(CO)₄ this pattern is assigned to HRh(CO)₄, a species that has eluded previous attempts of detection. The reaction of [M(CO)₄]^- (M = Co, Rh, Ir) with protonic acids in tetraglyme-toluene under high pressure of carbon monoxide resulted in the formation of HM(CO)₄ (M = Co, Rh, Ir), with the reaction having been readily detected in the case of iridium with phosphoric acid (Ir:P = 0.38:4.12), while a stronger acid such as sulfuric acid was required for cobalt (Co:S = 0.95:29.9) and rhodium (Rh:S = 0.90:29.4) for the detection of a similar reaction. These results suggest that the proton affinity of these ions varies as [Ir(CO)₄]^- > [Co(CO)₄]^- > [Rh(CO)₄]^-. The differences in the acid strength of the corresponding conjugate acids, HM(CO)₄ (M = Co, Rh, Ir), was determined under high pressures of CO-H₂ by reaction with amines of different basicities such as N-methylmorpholine (pK(water, 25°C) = 7.4) and N,N-dimethylaniline (pK(water, 25°C) = 4.8) after formation of the tetracarbonylmetal hydrides, in situ. HIr(CO)₄ is not deprotonated in a detectable fashion by N-methylmorpholine (Ir:N = 1:10), while HCo(CO)₄ is deprotonated by this amine (Co:N = 2.4:1.0) but not by N,N-dimethylaniline (Co:N = 2.4:3.0). By contrast HRh(CO)₄ readily undergoes deprotonation with this amine (Rh:N, = 2.7:1.0). These results correspond to the following trend in Br?nsted acidity: HRh(CO)₄ > HCo(CO)₄ > HIr(CO)₄.