Showing papers by "Christopher C. Cummins published in 1996"

Dinitrogen cleavage by three-coordinate molybdenum(III) complexes: Mechanistic and structural data

Abstract: The synthesis and characterization of the complexes Mo[N(R)Ar]3 (R = C(CD3)2CH3, Ar = 3,5-C6H3Me2), (μ-N2){Mo[N(R)Ar]3}2, (μ-15N2){Mo[N(R)Ar]3}2, NMo[N(R)Ar]3, 15NMo[N(R)Ar]3, Mo[N(t-Bu)Ph]3, (μ-N2){Mo[N(t-Bu)Ph]3}2, and NMo[N(t-Bu)Ph]3 are described. Temperature-dependent magnetic susceptibility data indicate a quartet ground state for Mo[N(R)Ar]3. Single-crystal X-ray diffraction studies for Mo[N(R)Ar]3 and NMo[N(t-Bu)Ph]3 are described. Extended X-ray absorption fine structure (EXAFS) structural studies for Mo[N(R)Ar]3, (μ-N2){Mo[N(R)Ar]3}2, and NMo[N(R)Ar]3 are reported. Temperature-dependent kinetic data are given for the unimolecular fragmentation of (μ-N2){Mo[N(R)Ar]3}2 to 2 equiv of NMo[N(R)Ar]3 and for the fragmentation of (μ-15N2){Mo[N(R)Ar]3}2 to 2 equiv of 15NMo[N(R)Ar]3. The temperature dependence of the 15N2 isotope effect for the latter N2 cleavage process was fitted to a simple harmonic model, leading to a prediction for the difference in NN stretching frequencies for the two isotopomers. ...

Unusual 31P Chemical Shielding Tensors in Terminal Phosphido Complexes Containing a Phosphorus−Metal Triple Bond

Abstract: Phosphorus-31 NMR data for one-coordinate phosphorus compounds are extremely scarce in the literature.1,2 Until recently, the only stable compounds with a terminal phosphorus atom involved in a triple bond were phosphaalkynes, RCtP,3,4 and the isoelectronic iminophosphenium cations, [RNtP]+.5 The isotropic 31P chemical shift range for these compounds extends from 96 to -207 ppm. In general, phosphaalkynes and iminophosphenium ions are found to have their 31P chemical shifts at lower frequency (more shielded) compared to phosphaalkenes and iminophosphines, respectively. This trend is analogous to the relationship between 13C chemical shifts in alkynes and alkenes. Recently, the first terminal phosphido (P3-) complexes containing a phosphorus-metal triple bond were successfully synthesized.6,7 Surprisingly, 31P NMR signals in these complexes were found above 1000 ppm. Observation of such unusual 31P chemical shieldings suggests that phosphorusmetal triple-bonding schemes may differ substantially when compared to phosphorus-carbon and phosphorus-nitrogen triple-bonding schemes. Here we report the 31P chemical shielding tensors for the terminal phosphido (MtP) complexes Mo(P)[N(Bu)Ar]3 (1, Ar ) C6H5; 2, Ar ) 3,5-C6H3Me2) and M(P)(NN3) (NN3 ) [Me3SiNCH2CH2)3N]; 3, M ) Mo; 4, M ) W). The anisotropies are all in excess of 2000 ppm, which is by far the largest 31P chemical shielding anisotropy (CSA) ever observed. A typical solid-state 31P NMR spectrum8 of the terminal phosphido complexes obtained with magic-angle-spinning (MAS) is shown in Figure 1. In addition to the isotropic peak, a large number of rotational sidebands extend over a range of ca. 2000 ppm. From the rotational sideband intensities, it is possible to compute the principal components of the chemical shielding tensor.9,10 For complexes 1-4, orientations of the principal components of the 31P chemical shielding tensor are constrained by crystallographic symmetry. For example, complex 1 crystallizes in the space group I4h3d with its MotP bond lying along the crystallographic C3 axis.11 Therefore, the 31P chemical shielding tensor in 1 must be axially symmetric with the unique axis lying along the MotP direction. Similarly, complexes 2 and 4 crystallize in the space groups P63 and Pa3h, respectively; each complex has its MtP vector lying along a cystallographic C3 axis.6,7 The structure of 3, although not yet determined, is expected to be identical to 4. Our NMR results also confirmed that the 31P chemical shielding tensor in 3 is axially symmetric, suggesting the presence of an axis with C3 or greater symmetry.

Assembly of Molybdenum/Titanium μ-Oxo Complexes via Radical Alkoxide C−O Cleavage

Abstract: Three-coordinate Ti(NRAr)3 [R = C(CD3)2(CH3), Ar = C6H3Me2] was prepared in 73% yield by sodium amalgam reduction of ClTi(NRAr)3 and in 83% yield upon treatment of TiCl3(THF)3 with 3 equiv of Li(NRAr)(OEt2) in the presence of TMEDA. Ti(tBuNPh)3 was prepared similarly in 75% yield by treatment of TiCl3(THF)3 with 3 equiv of Li(tBuNPh)(OEt2) in the presence of TMEDA. Reaction of Ti(NRAr)3 with NMo(OtBu)3 in hydrocarbon solvents at −35 °C generates a thermally unstable intermediate formulated as (tBuO)3Mo[μ-N]Ti(NRAr)3, which readily loses a tert-butyl radical and isomerizes at 25 °C. Kinetics of the latter process were obtained over the temperature range 20−60 °C; the process exhibits clean first-order behavior. The following activation parameters were obtained: ΔH⧧ = 21.4 ± 0.2 kcal mol-1 and ΔS⧧ = −3.7 ± 0.6 cal mol-1 K-1. The oxo-bridged product (tBuO)2(N)Mo[μ-O]Ti(NRAr)3 was isolated in 83% yield from this reaction. Full characterization of the latter diamagnetic complex included an X-ray crystal struc...