Olefin coordination in copper(I) complexes of bis(2-pyridyl)amine

Abstract

Complexes of the type [Cu(H-dpa)(olefin)]BF₄ for ethylene (1), propylene (2), 1-butene (3), 1-hexene (4), 1-octene (5), cis-2-octene (6), trans-2-octene (7), cis-3-octene (8), trans-3-octene (9), 2-norbornylene (10), 1,5-cyclooctadiene (11), styrene (12), cis-stilbene (13), trans-stilbene (14), and Ph₂CCH₂ (15) have been prepared and characterized by ¹H and ¹³C NMR, FTIR, and TGA. The crystal structures have been determined for compounds 5, 6, 8, and 10–13. With the exception of compound 11, copper atoms in each complex are coordinated to the two pyridine nitrogen atoms and the appropriate olefin; consistent with a pseudo three-coordinate Cu(I) cation. Compound 11 has a second weaker π-interaction resulting in a distorted tetrahedral geometry. Steric hindrance between the olefin and H-dpa manifests as both a twisting of the olefin out of the plane of the H-dpa ligand and a concomitant folding of the H-dpa ligand. The shifts in the ν_N-H IR spectral band for H-dpa ligand are consistent with the formation of N–H⋯F hydrogen bonded interactions observed in the crystal structures. The ¹H and ¹³C NMR spectra of [Cu(H-dpa)(olefin)]BF₄ exhibit an upfield shift in the olefin signal as compared to free olefin. A comparison of the Δδ values for terminal olefins shows that the similarity of binding for H₂CCHR (R = C_nH_2n+1, n = 1–6) mitigates any preferential complexation of various terminal olefins using the H-dpa ligand. For octenes there is a significant difference in binding between a terminal and internal olefin, but there is little preference between binding for different internal olefins and only a modest difference between the cis and trans isomers of the same olefin. A good correlation exists between the ¹H NMR Δδ values and the TGA data, confirming that the shift of the olefin NMR resonances upon coordination is associated with the binding strength of the complex. Ab initio calculations using four different method/basis set combinations on the structure of [Cu(H-dpa)(1-octene)]⁺ were compared with the crystal structure.