From dihydrogen complexes to polyhydrides

We investigated the anion dependence of the exchange coupling constant in the ansa tungstenocene VI complex [(eta-C5H 4)2CMe2WH3]+. The data presented in this chapter suggest that exchange coupling may be used as a reporter for weak interactions due to the counterion. The formation of ion pairs leads to smaller coupling constants in comparison to better solvated and separated charged molecules. The more interactions there are between cation and anion, the smaller the coupling constant is.; Protonation of [(eta-C5H4)2CMe 2MoH2] affords the thermally labile cationic dihydrogen/hydride complex [(eta-C5H4)2CMe2MoH(H 2)]+ (1). Low temperature 1H NMR spectra give a single line for which the T1 measured at 175 K (23 ms, 750 MHz,) is consistent with the presence of an H2 ligand. This structure is confirmed by the partial deuteration of the hydride sites. Variable temperature 1H NMR spectra of 1-d1 and 1- d2 allows observation of JH-D = 11.9 Hz in 1-d1 and 9.9 Hz in 1-d2 (245 K). Low temperature 1 H NMR spectra of 1-d2 allow the observation of decoalescence at 180 K into two resonances. The bound dihydrogen ligand exhibits hindered rotation with DeltaG≠150 = 7.4 kcal/mol, but hydrogen atom exchange is still rapid at all accessible temperatures (down to 130 K).; Complexes of the [Cp*M(LL)H2][B(C6F5) 4]2 (LL = dmpm, dppm; M = Ir, Rh (LL = dmpm)) type were prepared by treating methylene chloride solutions of [Cp*Ir(LL)Cl]Cl under H2 gas with two equivalents of [Et3Si][B(C6F 5)4]. In contrast to [Cp*Ru(dmpm)(H2)] +, the H-H distance in [Cp*Ir(dmpm)H2]2+ is temperature dependent; it decreases as temperature increases. The d1 isotopomer exhibits a temperature dependent HD coupling; JHD increases from 7.1 Hz at 223 K to 9.0 Hz at 303 K. The measurement of T1min of the hydride resonance (219 ms at 750 MHz) is consistent with rHH = 1.49 A. The revisited nonlinear correlation between J HD and rHH allows us to estimate rHH = 1.46 A at the temperature of T 1min. We anticipate this novel temperature dependence of r HH to be a common property of dicationic iridium complexes of the [Cp*Ir(LL)H 2]2+ type since the dppm complex exhibits this behavior as well. On the other hand, the rhodium complex [Cp*Rh(dmpm)(H2)] 2+ is characterized as a dihydrogen complex with J HD = 31 Hz.