Ruthenium derivatives of NiS2N2 complexes as analogues of bioorganometallic reaction centers

Article abstract of DOI:10.1021/om020724e

Recent results from structural biology demonstrate the catalytic significance of Ni(SR)₂L₂ centers attached to a second metal that binds CO, in particular the NiFe hydrogenases and acetyl CoA synthase. In experiments aimed at developing bimetallic derivatives exhibiting an affinity for CO, we have studied Ru(II) derivatives of nickel diaminodithiolates and the reactivity of these complexes toward CO and other small molecules. The reaction of [Cp*Ru(NCMe)₃]OTf and NiS₂N₂ (S₂N₂ = N,N′-bis(2-mercaptoethyl)-N,N′-dimethyl-1,3-diaminoethane) gives [Cp*Ru(NiS₂N₂)]₂(OTf)₂ ([1]₂(OTf)₂), which exists as a monomer-dimer equilibrium in MeCN solution. Crystallographic analysis of [1]₂(OTf)₂ reveals a centrosymmetric dication with the Ru being quasi-octahedral and the NiS₂N₂ coordination sphere being relatively planar, the metal centers being linked via pairs of μ₂-SR and μ₃-SR units. Complex [1]₂²⁺ oxygenates and sulfidizes with O₂ and S₈, respectively, to give [Cp*Ru(NiS₂N₂)(η²- E₂)]⁺ (E = O, S), which were characterized spectroscopically and crystallographically. Solutions of [1]₂ (OTf)₂ also react with CO and MeNC to give the corresponding adducts [Cp*Ru(NiS₂N₂)L]OTf, where L = CO and MeNC. The v_CO = 1901 cm^-1 for the CO adduct indicates the excellent donating power of the NiS₂N₂ ligand. The Cp*Ru⁺ derivative of the bulkier version of the NiS₂N₂ species, Ni(bme*-daco) (bme*-daco = [1,5-bis(2-mercapto-2-methylpropyl)-1,5-diazacyclooctane]), is the monomeric analogue of 1⁺, [Cp*Ru(NCMe)(Ni-(bme*-daco))]⁺, whose structure was confirmed spectroscopically and crystallographically. In this species the thiolato ligands are doubly bridging and the Cp*Ru subunit adopts the usual piano-stool geometry with a terminal MeCN ligand. The MeCN is readily displaced by CO and O₂ to give the corresponding adducts. The reaction of CpRu(PPh₃)₂Cl and NiS₂N₂ produced the PPh₃ adduct [CpRu(NiS₂N₂)(PPh₃)]Cl, wherein the PPh₃ ligand is nonlabile. The corresponding reaction of NiS₂N₂ with sources of (arene)RuCl⁺ gave the expected adducts [(arene)Ru(Cl)(NiS₂N₂)]⁺, isolated as their OTf^- salts.