Synthesis, structure and hapticity interconversion reactions of pentahapto-bonded cyclooctadienyl complexes of molybdenum

The pentahapto-bonded cyclooctadienylmolybdenum complexes [Mo(CO)2(L2)(1–3:5,6-eta-C8H11)][BF4](L2= Ph2P (CH2)nPPh2, n= 1, (dppm), 1a; n= 2, (dppe), 2a) are synthesised by reaction of [MoBr(CO)2(L2)(1–3-:5,6-C8H11)] with Ag[BF4] in CH2Cl2; [Mo(CO)2(dppm)(1–5-eta-C8H11)][BF4], 1b, is obtained from [MoBr(CO)2(dppm)(1–3-4,5C8H11)] by an identical procedure. Related syntheses afford [Mo(CO)2{P(OMe)3}2(1–5-eta-C8H11)][BF4], 4b, and [Mo(CO)2(CNBut)2(1–3:5,6-eta-C8H11)][BF4], 5a. The complexes [Mo(CO)2(NCMe)3(3-C8H11)]+(3-C8H11= 1–3-:5,6C8H11, 1–3-:4,5-C8H11) are precursors to [Mo(CO)(CNBut)3(1–3:5,6-eta-C8H11)][BF4], 6a, [Mo(CO)(CNBut)3(1–5-etaC8H11)][BF4], 6b and [Mo(CO)2(norborna-2,5-diene)(1–3:5,6-eta-C8H11)][BF4], 7a. The X-ray crystal structures of complexes 1a and 6b have been determined. NMR spectroscopic investigations on the 1–5-eta-C8H11 complexes 1b, 4b and 6b, indicate a high barrier to rotation of the metal group with respect to the cyclooctadienyl ring. Complexes 1a, 1b and 2a undergo ligand addition reactions with accompanying 53 hapticity conversion at the cyclooctadienyl ligand to yield adducts [Mo(CO)2(L)(L2)(3-C8H11)]+(L= NCMe or CO, L2= dppm or dppe, 3C8H11= 1–3-:5,6-C8H11 or 1–3-:4,5-C8H11, not all combinations). The facility of these processes is strongly dependent upon the identity of the variables L2 and C8H11.