SYNTHESIS, CHARACTERIZATION AND REACTIVITY OF THE SILYLATED DIPHOSPHITES [(R1O)2PO]2SIR2R3 (R1 = ME, ET-R2, R3 = ME, PH, H, CH=CH2) - CRYSTAL-STRUCTURE OF [MNBR(CO)3(P(OME)2O)2SIME2]

A convenient, general, high yield synthesis of the novel, silylated diphosphites [(R1O)2PO]2SiR2R3 by reaction of the secondary phosphites (R1O)2P(=O)H (R1 = Me, Et) with Cl2SiR2R3 (R2 = R3 = Me; R2 = R3 = Ph; R2 = Me, R3 = Ph; R2 = Me, R3 = CH=CH2; R2 = Me, R3 = H) in the presence of NEt3 is described. The relative rates of reaction are sensitive to the nature of the substituents on both phosphorus and silicon and follow the order [SiMeH] > [SiMe2] > [SiMe(CH=CH2)] > [SiMePh] > [SiPh2] for both R1 = Me, Et and R1 = Me > R1 = Et for all pairs with the same silicon substituents. P-31 NMR spectroscopic data suggest that the nature of the silicon substituents has only a small effect on through-bond electronic induction or hybridization at the phosphorus atoms of the free diphosphites. Complexation of these diphosphites to transition metals, of the form MX(CO)3[P(OMe)2O]2SiR2R3 (M = Mn, X = Br; M = Mo, X = CO), stabilizes the ligand towards hydrolysis compared to the uncoordinated diphosphites. There is some correlation between the C-13 NMR shifts of the carbonyl ligands and the stereo-electronic character of the silicon substituents in MX(CO)3[P(OMe)2O]2SiR2R3, suggesting that the electron-releasing properties of the diphosphite ligands increase in the order [SiPh2] < [SiMePh] = [SiMe(CH=CH2)] < [SiMe2]. The crystal structure of MnBr(CO)3 [P(OMe)2O]2SiMe2 has been determined. The compound exists as discrete, distorted octahedral monomers in which the diphosphite ligand occupies cis coordination sites and the six-membered chelate ring has an unusual "chaise longue" conformation.