Synthesis, characterization, and acidity of ruthenium dihydrogen complexes with 1,4,7-triazacyclononane, 1,4,7-trimethyl-1,4,7-triazacyclononane, and hydrotris (pyrazolyl)borato ligands

Protonation of [^RCnRuH(L)(L′)]⁺ (^RCn = 1,4,7-triazacyclononane and 1,4,7-trimethyl-1,4,7-triazacyclononane; L,L′ = (PPh₃)₂, dppe, and CO,PPh₃) produced the corresponding dicationic dihydrogen complexes [^RCnRu(H₂)(L)(L′)]²⁺. Protonation of TpRuH(dppe) (Tp = hydrotris-(pyrazolyl)borato) yielded the new monocationic dihydrogen complex [TpRu(H₂)(dppe)]⁺. The acidity of the dihydrogen complexes [^RCnRu(H₂)(L)(L′)]²⁺ and monocationic dihydrogen complexes [TpRu(H₂)(L)(L′)]⁺ (L,L′ = dppe, (PPh₃)₂, CH₃CN,PPh₃, and CO,PPh₃) has been studied. It was found that the dicationic complexes are more acidic than their monocationic Tp and Cp counterparts. [^MeCnRu(H₂)(CO)(PPh₃)]²⁺ was found to be more acidic than [^HCnRu(H₂)(CO)(PPh₃)]²⁺, probably due to the stronger H-H interaction in the latter complex. It is also noted that triazacyclononane and hydrotris(pyrazolyl)borato dihydrogen complexes with pseudo aqueous pK_a values well above that of H₃O⁺ can be deprotonated by H₂O to form the corresponding monohydride complexes in organic/aqueous mixed solvents. It is believed that deprotonation of the dihydrogen ligands in these complexes is assisted by strong solvation of H⁺ by H₂O.