Hydridic reactivity of W(CO)(H)(NO)(PMe3)3 – Dihydrogen bonding and H2 formation with protic donors

The hydridic reactivity of the complex W(CO)(H)(NO)(PMe3)3 (1) was investigated applying a variety of protic donors. Formation of organyloxide complexes W(CO)(NO)(PMe3)3(OR) (R = C6H5 (2), 3,4,5-Me3C6H2 (3), CF3CH2 (4), C6H5CH2 (5), Me (6) and iPr (7)) and H2 evolution was observed. The reactions of 1 accelerated with increasing acidity of the protic donor: Me2CHOH (pKa = 17) < MeOH (pKa = 15.5) < C6H5CH2OH (pKa = 15) < CF3CH2OH (pKa = 12.4) < C6H2Me3OH (pKa = 10.6) < C6H5OH (pKa = 10).Regioselective hydrogen bonding of 1 was probed with two of the protic donors furnishing equilibrium formation of the dihydrogen bonded complexes ROH···HW(CO)(NO)(PMe3)3 (R = 3,4,5-Me3C6H2,3a and iPr, 7a) and the ONO hydrogen bonded species ROH···ONW(CO)(H)(PMe3)3 (R = C6H2Me3,3b and iPr, 7b) which were studied in hexane and d8-toluene solutions using variable temperature IR and NMR spectroscopy. Quantitative IR experiments at low temperatures using 3,4,5-trimethylphenol (TMP) confirmed the two types of competitive equilibria: dihydrogen bonding to give 3a (ΔH1 = −5.8 ± 0.4 kcal/mol and ΔS1 = −15.3 ± 1.4 e.u.) and hydrogen bonding to give 3b (ΔH2 = −2.8 ± 0.1 kcal/mol and ΔS2 = −5.8 ± 0.3 e.u.). Additional data for the hydrogen bonded complexes 3a,b and 7a,b were determined via NMR titrations in d8-toluene from the equilibrium constants K(Δδ) and K(ΔR1)K(ΔR1) measuring either changes in the chemical shifts of HW(Δδ) or the excess relaxation rates of HW (ΔR1) (3a,b: ΔH(Δδ) = −0.8 ± 0.1 kcal/mol; ΔS(Δδ) = −1.4 ± 0.3 e.u. and ΔH(ΔR1)ΔH(ΔR1) = −5.8 ± 0.4 kcal/mol; ΔS(ΔR1)ΔS(ΔR1) = −22.9 ± 1.9 e.u) (7a,b: ΔH(Δδ) = −2.3 ± 0.2 kcal/mol; ΔS(Δδ) = −11.7 ± 0.9 e.u. and ΔH(ΔR1)ΔH(ΔR1) = −2.9 ± 0.2 kcal/mol; ΔS(ΔR1)ΔS(ΔR1) = −14.6 ± 1.0 e.u). Dihydrogen bonding distances of 1.9 Å and 2.1 Å were derived for 3a and 7a from the NMR excess relaxation rate measurements of HW in d8-toluene. An X-ray diffraction study was carried out on compound 2.

Protonation of W(CO)(H)(NO)(PMe3)3 (1) with various ROH protic donors produced organyloxy complexes with evolution of H2. Dihydrogen bonding and hydrogen bonding equilibria to HW and ONO of 1 preceding this step were detected and analyzed in quantitative VT IR and NMR studies. The dihydrogen bonding enthalpy with 3,4,5-Me3C6H2OH amounts to −5.8 kcal/mol and the dihydrogen bonding H···H distance was determined to 1.9 Å.Figure optionsDownload as PowerPoint slide