Synthesis and spectroscopic properties of CpRuCl(R-DAB(4e)) and CpRuCo(CO) 3(R-DAB(6e)). Part XVII. X-ray structure determination of CpRuCo(CO) 3(t-Bu-DAB(6e))

Abstract

CpRuCl(PPh 3) 2 reacted with excess R-DAB in refluxing toluene to give CpRuCl(R-DAB(4e)) ( 1a: R = i-Pr; 1b: R = t-Bu; 1c: R = neo-Pent; 1d: R = p-Tol). 1H NMR and 13C NMR spectroscopic data indicated that in these complexes the R-DAB ligand is bonded in a chelating 4e coordination mode. Reaction of 1a and 1b with one equivalent of [Co(CO) 4] − afforded CpRuCo(CO) 3(R-DAB(6e)) ( 2a: R = i-Pr; 2b: R = t-Bu). The structure of 2b was determined by a single crystal X-ray structure determination. Crystals of 2b are monoclinic, space group P2 1/ n, with four molecules in a unit cell of dimensions: a = 16.812(4), b = 12.233(3), c = 9.938(3) Å and β = 105.47(3)°. The structure was solved via the heavy atom method and refined to R = 0.060 and R w = 0.065 for the 3706 observed reflections. The molecule contains a RuCo bond of 2.660(3) Å and a cyclopentadienyl group that is η 5-coordinated to ruthenium [RuC(cyclopentadienyl) = 2.208(3) Å (mean)]. Two carbonyls are terminally coordinated to cobalt (CoC(1) = 1.746(7) and CoC(2) = 1.715(6) Å) while the third is slightly asymmetrically bridging the RuCo bond (RuC(3) = 2.025(6) and CoC(3) = 1.912(6) Å). The RuC(3)O(3) and CoC(3)O(3) angles are 138.4(5)° and 136.5(5)°, respectively. The t-Bu-DAB ligand is in the bridging 6e coordination mode: σ-N coordinated to Ru (RuN(2) = 2.125(4) Å), μ 2-N′ bridging the RuCo bond and η 2-CN coordinated to Co (RuN(1) = 2.113(5), CoN(1) = 1.941(4) and CoC(4) = 2.084(5) Å). The η 2-CN′ bonded imine group has a bond length of 1.394(7) Å indicating substantial π-backbonding from Co into the anti-bonding orbital of this CN bond. 1H NMR spectroscopy indicated that 2a and 2b are fluxional on the NMR time scale. The fluxionality of 6e bonded R-DAB ligands is rarely observed and may be explained by the reversible interchange of the σ-N and η 2-CN′ coordinated imine parts of the R-DAB ligand.