Exchange Processes in Complexes with Two Ruthenium (η<SUP>2</SUP>-Silane) Linkages:  Role of the Secondary Interactions between Silicon and Hydrogen Atoms

NMR studies and DFT calculations (B3LYP) are used to elucidate the mechanisms for the hydride/(η²-Si-H) hydrogen exchange observed in the bis(silane) complexes RuH<INF>2</INF>[(η²-HSiMe<INF>2</INF>)<INF>2</INF>(CH<INF>2</INF>)<INF>2</INF>](PCy<INF>3</INF>)<INF>2</INF> (<BO>2</BO>) and RuH<INF>2</INF>[(η²-HSiPh<INF>2</INF>)<INF>2</INF>O](PCy<INF>3</INF>)<INF>2</INF> (<BO>4</BO>) obtained from the reactions of the bis(dihydrogen) ruthenium complex RuH<INF>2</INF>(H<INF>2</INF>)<INF>2</INF>(PCy<INF>3</INF>)<INF>2</INF> (<BO>1</BO>) with the corresponding disilane HMe<INF>2</INF>Si(CH<INF>2</INF>)<INF>2</INF>SiMe<INF>2</INF>H or disiloxane HSiPh<INF>2</INF>OSiPh<INF>2</INF>H. Evidence is presented that exchange in <BO>2 </BO>occurs via formation of isomers with dihydrogen ligands. Conversion of the most stable isomer <BO>A</BO> with C<INF>2</INF><INF>v</INF><INF></INF> symmetry to the asymmetric Ru(η²-SiH)<INF>2</INF> isomer <BO>B</BO> is the prelude to formation of the Ru(η²-SiH)(η²-H<INF>2</INF>) isomer <BO>C</BO> and the Ru(η²-H<INF>2</INF>)<INF>2</INF> isomer <BO>D</BO>. Exchange occurs via rotation of the dihydrogen ligands and reversal of the isomerization process. The transformation of <BO>A</BO> into <BO>B</BO> via <BO>TS</BO><BO><INF>AB</INF></BO><INF></INF> is the most energetically demanding individual step. This corresponds to the breaking of the SISHA (secondary interactions between silicon and hydrogen atoms) interactions between the silicons and the classical hydrides Hb and Hc. The calculated barrier of 43 kJ/mol for the complex with H<INF>3</INF>SiCH<INF>2</INF>CH<INF>2</INF>SiH<INF>3</INF> and PH<INF>3</INF> ligands compares to the experimental enthalpy of activation of 77 ± 11 kJ/mol for <BO>2</BO>. Whereas complex <BO>2</BO> adopts a geometry with C<INF>2</INF><INF>v</INF><INF></INF> symmetry, low-temperature NMR spectroscopy shows that complex <BO>4</BO> is present as two isomers, an asymmetric isomer with C<INF>1</INF> symmetry and the symmetric isomer in proportions of ca. 1:2. The pathways for interconversion of the isomers and for exchange between the hydride and SiH hydrogen atoms have been delineated by DFT calculations and resemble those for complex <BO>2</BO>. Reaction of <BO>4</BO> with excess triphenylphosphine yields RuH<INF>2</INF>{(η²-H-SiPh<INF>2</INF>)O(SiHPh<INF>2</INF>)}(PPh<INF>3</INF>)<INF>3</INF> (<BO>5</BO>), which is shown by NMR spectroscopy to contain one coordinated and one uncoordinated SiH group. A crystal structure of the partially hydrolyzed analogue RuH<INF>2</INF>{(η²-H-SiPh<INF>2</INF>)O(Si(OH)Ph<INF>2</INF>)}(PPh<INF>3</INF>)<INF>3</INF> (<BO>6</BO>) confirms the assignment. A ruthenium(II) formulation with one (η²-SiH) bond and two SISHA interactions is favored over a ruthenium(IV) structure with three hydrides and a silyl ligand.