Coordination of Imines on Os₃Clusters: Effect of the Solvent in Addressing Isomer Formation

Abstract

Imine ligands terminally bound to triosmium clusters have been prepared by the reaction of Os 3 (µ-H) 2 -(CO) 10 with NH 3 and a keto or aldhehydic substrate. However reactions in nonpolar solvents yield only one of the two possible isomers. In contrast, reactions in polar solvents, such as methanol, yield both isomers. Experimental methods are described that clearly demonstrate that the overall stereochemistry of the resulting complexes is dependent upon the competition between intraand intermolecular hydrogen-bonding interactions. The isomers have been unambiguously assigned using 15 N and 13 C labeling as well as T 1 measurements.

Catalytic Hydrogenation by Triruthenium Clusters: A Mechanistic Study with Parahydrogen‐Induced Polarization

“…The effect of the solvent therefore arises as a consequence of solvent polarity controlling the equilibrium between the active hydride species and the unreacted precursor. In the case of 1-PPh 3 , the active species 2-PPh 3 and 3-PPh 3 are formed to a greater extent in higher polarity solvents; [58] this leads to the apparently faster ™visible∫ rate. Based on previous work, this equilibrium has been rationalised in terms of the promotion of the RuÀRu bond heterolysis step that allows access to the active H 2 addition products.…”

Comparison between hydrogen and dihydrogen bonds among H3BNH3, H2BNH2, and NH3

“…A similar DHB bond is slightly longer in other amine complexes. 29,47 They also observed that the dihydrogen bond distance strongly depends on the polarity of the solvent. 29 They concluded that, when typical H bonds are not present, weaker unconventional dihydrogen bonds become important in driving the stereochemistry of the complexes.…”

“…29,47 They also observed that the dihydrogen bond distance strongly depends on the polarity of the solvent. 29 They concluded that, when typical H bonds are not present, weaker unconventional dihydrogen bonds become important in driving the stereochemistry of the complexes. Other examples 26,48 of dihydrogen bonds such as Ir-H¯H-N also seems to have sp 2 nitrogens because of planarity at N due to delocalization of lone pairs.…”

“…Transition-metal (M ) complexes involving M -H¯H-B types of interaction are already in the front line of theoretical and experimental investigations. 18 -25 Such dihydrogen bonds were identified in several x-ray crystal structures, 17,26,27 in solution, 28,29 and the gas phase. [30][31][32] Like conventional H bonds, the dihydrogen bond is gaining attention because of its role in the synthesis of supermolecules, reactivity, and selectivity in solution, gas phase, and in solid state, and in designing catalysts for asymmetric hydrogenation.…”

“…Most of the systems involving M -H¯H-N bonds considered so far contain sp 3 hybridized N and M (AϭB, Al, etc.͒. Very recently, Aime and co-workers 29,45,46 reported Os-H¯H-N bonds where imine ligands ͑such as HNvCPh 2 and HNvCHCH 3 ) are coordinated with an osmium complex. Different H¯H distances have been reported by these authors, such as 1.79 Å in the crystal structure ͑x-ray data͒ and 2.00 Å in solution ͑NMR data͒.…”