VINYLIDENE COMPLEXES OF THE MOLYBDENUM AUXILIARY MO(PH(2)PCH(2)CH(2)PPH(2))(C7H7) - STRUCTURAL AND SPECTROSCOPIC INVESTIGATIONS ON VINYLIDENE LIGAND ORIENTATION

The new vinylidene complexes [Mo{C=C(H)R}(L-L)(eta-C7H7)](+)[L-L=Ph(2)PCH(2)CH(2)PPh(2)(dppe), R = Bu(t), Bu(n) or H; L-L = Ph(2)PCH(2)PPh(2) (dppm), R = Bu(t); L-L = Me(2)PCH(2)CH(2)PMe(2) (dmpe). R = Bu(t)] have been synthesised by reaction of an excess: of alk-1-yne, RC=CH, either with [Mo(Me(2)CO)(L-L)(eta-C7H7)](+)(L-L=dppe or dmpe) {formed in situ from [Mo(eta(6)-C(6)H(5)Me)(eta-C7H7)](+) and dppe or dmpe in acetone) or with [MoCl(L-L)(eta-C7H7)](L-L = dppe or dppm) in methanol in the presence of [NH4][PF6]. The formation of [Mo{C=C(H)Bu(t)}(dmpe)(eta-C7H7)](+) is accompanied by conversion of the excess of alkyne into polymeric (BUC)-C-t=CH with respective M(n) and M(w) values of 29 x 10(3) and 63 x 10(3) as determined by gel permeation chromatography. Deprotonation of [Mo{C=C(H)Bu(n)}(dppe)(eta-C7H7)](+) with KOBu(t) yields the alkynyl [Mo(C=CBu(n))(dppe)(eta-C7H7)]. The disubstituted vinylidenes [Mo(C=C(Me)R)(dppe)(eta-C7H7)](+) (R = Bu(t) or Bu(n)) were obtained by C-beta methylation of [Mo(C=CR)(dppe)(eta-C7H7)] with Mel. One-electron oxidation of the alkynyls [Mo(C=CR)(dppe)(eta-C7H7)](R = Bu(n) or Ph) with [Fe(eta-C5H5)(2)](+) yields the 17-electron radicals [Mo(C=CR)(dppe)(eta-C7H7)](+) (R = Bu(n) or Ph) which undergo coupling at C-beta to yield dimeric, divinylidene-bridged [Mo-2(dppe)(2)(eta-C7H7)(2)(mu-C(4)R(2))](2+) (R = Bu(n) or Ph). The crystal structures of [Mo(C=C(H)Ph}(dppe)(eta-C7H7)][BF4] and [Mo-2(dppe)(2)(eta-C7H7)(2)(mu C(4)Ph(2))][PF6](2), have been determined. In the former the vinylidene substituents lie approximately in the pseudo-mirror plane of the Mo(dppe)(eta-C7H7) moiety (a vertical orientation) with the phenyl substituent directed 'up' towards the cycloheptatrienyl ring. In the latter centrosymmetric dimer the vinylidene ligand is rotated by 19.1 degrees away from an exact vertical orientation and the phenyl substituents are located 'down' into a pocket enclosed by two phenyl groups of the dppe ligand. Variable-temperature H-1 NMR investigations on [Mo(C=CH2)(dppe) (eta-C7H7)](+) are consistent with a preferred vertical orientation of the vinylidene ligand in solution at low temperature and the barrier to vinylidene rotation has been estimated as 51.9 +/- kJ mol(-1). The H-1 and P-31 NMR solution spectra of [Mo{C=C(R')Bu(n)}(dppe)(eta-C7H7)](+) (R' = H or Me) are also consistent;with a vertically orientated vinylidene ligand and reveal the existence of two isomeric forms of each complex which differ in the location of the n-butyl substituent in the 'up' or 'down' position.