A new macrocyclic ligand with a pendant naphthalene group, N-[2-(l-naphthyl)ethyl]-1-aza-4,8-dithia (L), has been synthesized and characterized. The copper(I)-acetonitrile complex [LCu(CH3CN)](PF6) (1)was synthesized from L and [Cu(CH3CN)(4)](PF6). The acetonitrile ligand from 1 was easily removed to give [LCu](PF6) (2). Complexes 1 and 2 have been crystallographically characterized. 1: C21H28N2CuF6PS2, triclinic, P (1) over bar, a = 11.1901(10) Angstrom, b=11.2735(12) Angstrom, c=12.1350(10) Angstrom, alpha 98.996(8)degrees, beta = 117.188(6)degrees, gamma = 105.354(7)degrees, Z`= 2, R1 = 0.0505 (wR2 = 0.1418), 2 . 0.5hexane: C22H31NCuF6PS2, inonoclinic, P2(1)/c, a = 15.7318(15) a, b = 8.9164(10) Angstrom, c = 17.205(5) Angstrom, beta = 102.431(6)degrees, Z = 4, R1 = 0.0587 (wR2 = 0.1545). In addition, a cocrystallized mixture of both complexes was crystallographically characterized. 1&2 hexane: C46H61N3Cu2F12P2S4, triclinic, P (1) over bar, a 10.8308(9) Angstrom, b 12.6320(8) Angstrom, c = 19.9412(13) Angstrom, alpha = 80.445(5)degrees, beta = 76.405(6)degrees gamma = 78.825(5)degrees, Z = 2, R1 = 0.0661 (wR2 = 0.1871). The solid-state structure of 2 features the pendant naphthalene group bound in an eta(2)-fashion, which is highly unusual for copper complexes. In CDCl3, 2 exhibits fluxional behavior with the barrier to the process estimated, Delta G(double dagger) = 12-13 kcal. Variable temperature NMR spectroscopy gave compelling evidence for solution binding of the naphthalene group in 2, apparently the first example for copper(I). The fluxional process seen for 1 is best described as interconversion of the two enantiomers via a species with an unbound naphthalene group. Consistent with the weak binding of the naphthalene group, it is readily replaced with other ligands, such as triphenylphosphine to form [LCu(PPh3)](PF6) (3). Complex 3 has also been structurally characterized: C37H40NCuF6P2S2, monoclinic, P2(1)/c, a 11.462(2) Angstrom, b = 15.972(2) Angstrom, c = 19.835(9) Angstrom, beta = 94.50('3)degrees, Z= 4, R1 = 0.0906 (wR2 = 0.1889).