Copper(I) complexes with conjugated dienes

Abstract

Three copper(I) complexes containing conjugated dienes and one containing the acetylenic analogue of isoprene, 2-methyl-1-butene-3-yne (isopropenylacetylene), have been prepared and characterised by means of crystal-structure determination. Direct reaction between isoprene (2-methylbutadiene) and copper(I) trifluoromethylsulfonate, using triphenylphosphine as a stabilising ligand, results in [Cu2(PPh3)2{μ-(H2CCHC(CH3)CH2)}(O3SCF3)2] (1), whereas reaction between copper(I) chloride and isopropenylacetylene, dimethylbutadiene, and trans-1,3-pentadiene yields the labile compounds [Cu2Cl2{μ-(H2CCHC(CH3)CH)}] (2), [Cu2Cl2{μ-(H2CC(CH3)C(CH3)CH2)}] (3) and [Cu2Cl2{μ-(trans-H2CCHCHCH(CH3))}] (4), respectively. All four compounds are polymeric. Thus, the organic ligands bridge two copper(I) centres via the alkene or alkyne bonds, the conjugated dienes all assuming the s-trans conformation. Further bridging is effected by the trifluoromethylsulfonate ligands in 1 and by the chloride ligands in 2, 3 and 4. In 1, copper(I) has distorted tetrahedral geometry, whereas in 2, 3 and 4, copper(I) exhibits tetrahedral or trigonal pyramidal geometry with the CC/CC linkage in the trigonal plane and an apical Cu–Cl bond. The nearly planar carbon skeleton of isoprene in 1 bonds to two copper(I) atoms from opposite faces of the diene. The Cu–C distances range from 2.085(6) to 2.220(6) Å [C(CH3)] and the CC bond lengths are 1.360(8) and 1.353(8) Å. Trifluoromethylsulfonate ligands bridge adjacent [Cu2(PPh3)2{μ-(H2CCHC(CH3)CH2)}(O3SCF3)2] units leading to the formation of chains, containing internal chair-shaped Cu2S2O4 rings alternating with isoprene ligands, with peripheral triphenylphosphine ligands. The carbon skeleton of the isopropenylacetylene ligand in 2 and those of dimethylbutadiene in 3 and of trans-1,3-pentadiene in 4 are also approximately planar, but, unlike the situation in 1, in compounds 2–4, two copper(I) atoms are coordinated by CC/CC from the same face of the ligand. In 2, Cu–C distances range from 2.005(11) to 2.158(9) Å [C(CH3)] and the CC and CC bond lengths are 1.373(13) and 1.200(14) Å, respectively. In 3, Cu–C distances range from 2.06(1) to 2.17(1) Å [C(CH3)] and both CC bonds are 1.35(1) Å. Dimeric [Cu2Cl2{μ-(H2CCHC(CH3)CH)}]2 and [Cu2Cl2{μ-(H2CC(CH3)C(CH3)CH2)}] units are linked by long Cu–Cl bonds leading to the formation of chains with peripheral isopropenylacetylene (2) and dimethylbutadiene (3) ligands. The crystal structure of 4 could be determined only with low precision, but can be described in terms of copper(I) chloride layers with peripheral trans-1,3-pentadiene ligands. Shifts in the infrared absorptions on coordination of the conjugated dienes (including butadiene and cis-1,3-pentadiene) to copper(I) are discussed in the light of the crystal structures of 2–4.