| Treatment of 2,2-dimethyl-1,3-propanediol (neopenyl glycol) with excess phosphorus trichloride affords 2-chloro-5,5-dimethyl-1,3,2-dioxaphosphorinane, (OCH2CMe2CH2O)PCl. This product is made in higher yield than its ethylene glycol analogue due to the formation of a six-membered ring, which accommodates the coordination geometry about the phosphorus atom. (OCH2CMe2CH2O)PCl reacts readily with Me 2PCH2Li at -78 °C to afford (OCH2CMe 2CH2O)PCH2PMe2, which can be treated with a slight excess of methyllithium or methyl Grignard to generate bis(dimethylphosphino)methane, Me2PCH2PMe2, or with a slight excess of ethyl Grignard to generate the unsymmetric phosphine (diethylphosphino)-(dimethylphosphino)methane, Et2PCH2PMe2. This synthetic route to bis(dimethylphosphino)methane (dmpm) improves upon the yield from and time required by previously reported syntheses, while its modularity allows a library of unsymmetric diphosphines to be synthesized.;Treatment of [(cod)OsBr2]n with NaC5H5, NaC5H4Me, LiC5H 4SiMe3, or KC5H3(SiMe3) 2 affords CpROs(cod)Br, where CpR is the cyclopentadienide derivative. Substitution of the cod ligand with dmpm affords CpROs(dmpm)Br, which can then be methylated with Me2Mg or MeMgBr to generate osmium methyl compounds of formula CpROs(dmpm)Me. Protonation of (C5H5)Os(dmpm)Me and (C5H 4Me)Os(dmpm)Me at -130 °C with the carbon acid H2C(SO 2CF3)2 in CDFCl2 yields the cisoid methyl-hydride compounds [(C5H 5)Os(dmpm)H(Me)][HC(SO2CF3)2] and [(C 5H4Me)Os(dmpm)H(Me)]-[HC(SO2CF3) 2]. In both compounds, the methyl and hydridic hydrogen atoms undergo rapid exchange observable on the NMR timescale, a process observed previously in the synthetic analogues [(C5Me5)Os(dmpm)H(CH 3)+] and [(C5Me4)Os(dmpm)H(CH 3)+] This exchange, which we propose to occur through a coordinated methane intermediate, is implicated by the temperature-dependent broadening of resonances in the 1H NMR spectrum and confirmed by spin saturation transfer experiments. The barrier to hydrogen exchange, DeltaG ‡, was calculated to range from 7.92 to 7.98 kcal/mol for the C5H5 compound and from 8.42 to 8.51 kcal/mol for the C5H4Me compound. Comparison of DeltaG‡ values measured at -110 °C for the series C5Me 5→C5Me4H→C5H4Me→C 5H5 reveals an unexpectedly non-linear trend, as the series represents a gradiented decrease in the electron-donating nature of the ligand.;To understand these variances, the molecular geometries of several osmium(II) and osmium(IV) complexes were investigated. (C5H5)Os(dmpm)Br reacts with NaOMe in refluxing methanol to afford the monohydride compound (C5H5)Os(dmpm)H, which can be protonated with HBF 4·Et2O to afford [(C5H5)Os(dmpm)H 2][BF4]. Comparison of the X-ray crystal structures of (C 5H5)Os(dmpm)H and (C5Me5)Os(dmpm)H revealed no significant differences. In contrast, comparison of the structures of [(C5H5)Os(dmpm)H2+] and [(C5Me5)Os(dmpm)H2+] showed that C5Me5 is more sterically demanding than C 5H5, as evidenced by the 7--8° decrease in the dihedral angle between the P-Os-P and C5 ring planes for the C5H 5 analogue, suggesting that movement of the phosphine toward the less sterically bulky C5 ring may lead to greater distances between cisoid ligands, effectively raising the barrier to hydrogen exchange in the dihydride and the methyl hydride systems. |
