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 i Pr, 7a) and the ONO hydrogen bonded species ROH···ONW(CO)(H)(PMe3)3 (R = C6H2Me3,3b and i Pr, 7b) which were studied in hexane and d 8-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 (ΔH 1 = −5.8 ± 0.4 kcal/mol and ΔS 1 = −15.3 ± 1.4 e.u.) and hydrogen bonding to give 3b (ΔH 2 = −2.8 ± 0.1 kcal/mol and ΔS 2 = −5.8 ± 0.3 e.u.). Additional data for the hydrogen bonded complexes 3a,b and 7a,b were determined via NMR titrations in d 8-toluene from the equilibrium constants K (Δδ ) and K(ΔR1)K(ΔR1) measuring either changes in the chemical shifts of H W(Δδ ) or the excess relaxation rates of H W (ΔR 1) (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.