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

Abstract

CpRuCl(PPh3)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 P21/n, with four molecules in a unit cell of dimensions: a = 16.812(4), b = 12.233(3), c = 9.938(3) Å and = 105.47(3)°. The structure was solved via the heavy atom method and refined to R = 0.060 and Rw = 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 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: -N coordinated to Ru (Ru---N(2) = 2.125(4) Å), 2-N′ bridging the Ru---Co bond and 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 2-C=N′ bonded imine group has a bond length of 1.394(7) Å indicating substantial -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 -N and 2-C=N′ coordinated imine parts of the R-DAB ligand.