Alkene rotation in [Ru(η5-C5H5) (L2) (η2-alkene][CF3SO3] with L2 = iPr- or pTol-diazabutadiene. X-ray crystal structure of [Ru(η5-C5H5(pTol-DAB) (η2-ethene][CF3SO3]

Reaction of RuCl(η5-C5H5(pTol-DAB) with AgOTf (OTf = CF3SO3) in CH2Cl2 or THF and subsequent addition of L′ (L′ = ethene (a), dimethyl fumarate (b), fumaronitrile (c) or CO (d) led to the ionic complexes [Ru(η5-C5H5)(pTol-DAB)(L′)][OTf] 2a, 2b and 2d and [Ru(η5-C5H5)(pTol-DAB)(fumarontrile-N)][OTf] 5c. With the use of resonance Raman spectroscopy, the intense absorption bands of the complexes have been assigned to MLCT transitions to the iPr-DAB ligand. The X-ray structure determination of [Ru(η5-C5H5)(pTol-DAB)(η2-ethene)][CF3SO3] (2a) has been carried out. Crystal data for 2a: monoclinic, space group P21/n with a = 10.840(1), b = 16.639(1), c = 14.463(2) Å, β = 109.6(1)°, V = 2465.6(5) Å3, Z = 4. Complex 2a has a piano stool structure, with the Cp ring η5-bonded, the pTol-DAB ligand σN, σN′ bonded (Ru-N distances 2.052(4) and 2.055(4) Å), and the ethene η2-bonded to the ruthenium center (Ru-C distances 2.217(9) and 2.206(8) Å). The C = C bond of the ethene is almost coplanar with the plane of the Cp ring, and the angle between the plane of the Cp ring and the double of the ethene is 1.8(0.2)°. The reaction of [RuCl(η5-C5H5)(PPh)3 with AgOTf and ligands L′ = a and d led to [Ru(η5-C5H5)(PPh3)2(L′)]OTf] (3a) and (3d), respectively. By variable temperature NMR spectroscopy the rottional barrier of ethene (a), dimethyl fumarate (b and fumaronitrile (c) in complexes [Ru(η5-C5H5)(L2)(η2-alkene][OTf] with L2 = iPr-DAB (a, 1b, 1c), pTol-DAB (2a, 2b) and L = PPh3 (3a) was determined. For 1a, 1b and 2b the barrier is 41.5±0.5, 62±1 and 59±1 kJ mol−1, respectively. The intermediate exchange could not be reached for 1c, and the ΔG# was estimated to be at least 61 kJ mol−. For 2a and 3a the slow exchange could not be reached. The rotational barrier for 2a was estimated to be 40 kJ mol−. The rotational barier for methyl propiolate (HC≡CC(O)OCH3) (k) in complex [Ru(η5-C5H5)(iPr-DAB) η2-HC≡CC(O)OCH3)][OTf] (1k) is 45.3±0.2 kJ mol−1. The collected data show that the barrier of rotational of the alkene in complexes 1a, 2a, 1b, 2b and 1c does not correlate with the strength of the metal-alkene interaction in the ground state.