Redox-Dependent Isomerization of Organometallic Ru^II/Ru^IIICompounds Containing the Hydrotris(methimazolyl)borate Ligand:  An Electrochemical Square Scheme Mechanism

Abstract

Cp*Ru III {HB(mt) 3 }]X (1A(X); X ) Cl, PF 6 ) and [Cp*Ru II {HB(mt) 3 }] (2A) (Cp* ) η 5 -C 5 -Me 5 , mt ) N-methyl-2-mercaptoimidazol-1-yl) were synthesized by the reactions of K[HB-(mt) 3 ] with [Cp*Ru III Cl 2 ] 2 and [Cp*Ru II (OMe)] 2 , respectively. 1A and 2A exist in the solid state in κ 3 -S,S′,S′′ coordination, so that the sulfur atom in each mt group coordinates to the central Ru ion, producing the normal tripodal geometry of the [HB(mt) 3 ] ligand. However, both compounds undergo an isomerization reaction in solution, where the sulfur on one mt group is displaced in favor of coordination to the hydrogen bonded to the boron (an agostic B-H-Ru interaction), resulting in κ 3 -H,S,S′ coordination about the Ru (κ 3 -H,S,S′ forms of [Cp*Ru III {HB(mt) 3 }] and [Cp*Ru II {HB(mt) 3 }] are designated 1B and 2B, respectively). The rate and equilibrium constants associated with the reactions 1A h 2A h 2B h 1B h 1A have been determined by theoretical digital simulation comparisons of experimental 1 H NMR spectroscopic and cyclic voltammetric data over a range of temperatures.

HMB and Cp* ruthenium(II) complexes containing bis- and tris-(mercaptomethimazolyl)borate ligands: Synthetic, X-ray structural and electrochemical studies (HMB=η6-C6Me6, Cp*=η5-C5Me5)

“…There was no sign of the solvent-dependent equilibrium observed in the solution of C between j 3 -S, S 0 , S 00 (form I) and j 3 -H, S, S 0 (form II) isomers, which were found to be present in relative concentration of 1:3 in a non-polar solvent like C 6 D 6 and 0:1 in a polar solvent like CD 2 Cl 2 (Scheme 6) [15].…”

“…Using a synthetic methodology similar to that for [(pcymene)Ru II (Tm)]Cl (A) (Scheme 1) [8a], and [Cp*Ru III -(Tm)]Cl (D) (Scheme 2) [15], [(HMB)Ru II (Tm)]Cl (1) was obtained from the reaction of [(HMB)RuCl 2 ] 2 with KTm in 66% yield as a bright red crystalline solid (Scheme 3).…”

“…As in the reaction of [Cp*RuOMe] 2 with KTm to yield Cp*Ru II (Tm) C [15], it was found that [Cp*RuOMe] 2 was a good precursor to Cp*Ru II (Bm) (3), which was obtained as orange crystals in 85% yield from a 2 h reaction with NaBm at room temperature (Scheme 5). …”

“…These reports are indicative of possible equilibria between j 3 -S, S 0 , S 00 and j 3 -H, S, S 0 coordination of Tm complexes in solution. We have since synthesised the allied complexes [Cp*Ru II (Tm)] (C) and [Cp*Ru III (Tm)]Cl (D) (Scheme 2) for such a study; indeed a combination of VT-1 H NMR spectral and cyclic voltammetric studies revealed that the tripodal Tm ligand underwent a facile j 3 -S, S 0 , S 00 and j 3 -H, S, S 0 coordination exchange depending on the oxidation state of Ru, resulting in an electrochemical square scheme mechanism [15]. (See discussions below).…”

Poly(methimazolyl)borate Alkyne Complexes of Molybdenum and Tungsten

“…[11b, [12][13][14][15] Both of these features are manifest in the chemistry to be described, which concerns the synthesis of the first alkyne complexes ligated by poly(methimazolyl)borates. 16] appeared to be suitable substrates for the introduction of poly(methimazolyl)borate ligands (Scheme 2).…”