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In the final grant period the following results were accomplished:

1)    Given our original goal to study the reactivity of tripodal/imido complexes towards silanes and our previous  mechanistic studies on reactions of Cp(ArN=)Mo(H)(PMe₃) (1) with silanes and carbonyls, we turned to the isolobal complex Tp(ArN=)Mo(H)(PMe₃) (2, Tp= tris(pyrazolyl)borate). The structure of 2 was established by spectroscopic methods and X-ray diffraction. Unlike its Cp-analogue 1, which exchanges the PMe₃ ligand with free phosphine by an associative mechanism, kinetic measurements for 2 revealed a dissociative mechanism which involves dissociation of one of the three legs of the Tp-ligand to free up a coordination site. 2 was found to catalyze hydrosilylation of carbonyls. Unfortunately, like the previously studied tris(carbene) tripods, complex 2 does not react with silanes. Instead, mechanistic studies of catalytic hydrosilylation showed that 2 adds benzaldehyde to furnish the alkoxy derivative (Tp)(ArN)Mo(OCH₂Ph)(PMe₃) (3). But unlike the similar reaction of 1, this process is dissociatively activated (DS^≠ = 45.3 ± 53.45 J/(mol*K)). Complex 3 reacts further with PhSiH₃ to regenerate the hydride, thus closing the catalytic cycle.

2)    In order to understand the catalytic nitrile hydrosilylation by complex (ArN=)(Me₃P)₃Mo(H)(Cl) (4) reported in the previous grant period, we studied reactions of 4 with nitriles and PhSiH₃. The new vinylidenamido complex (ArN=)(Me₃P)₂Mo(Cl)(h¹-N=CHPh) (5) was obtained when 4 reacts with benzonitrile. Complex 5 was studied by NMR and X-ray diffraction. It reacts with PhSiH₃ to regenerate the starting hydride but this reaction requires longer times and elevated temperatures and is the slowest step in the catalytic process. We also discovered that 4 reacts with excess silane to give the new complex (ArN=)(Me₃P)₃Mo(SiH₂Ph)(Cl) (5) whose reactivity is still the subject of ongoing research.

3)     Reaction of the bis(imide) (^tBuN=)₂Mo(PMe₃)₂ with excess PhSiH₃ results in the new complex {h²- ^tBuN(PhHSi)₂}Mo(PMe₃)₂(H)(SiH₂Ph) (6, Scheme 1), which is a rare example of triple silane addition to a metal complex. This result also highlights our notion that the imido ligand can be coupled with silanes and eventually detached from the metal, which may be a step in the proposed tricomponent coupling of an amine, silane, and unsaturated organic substrate. We have reliably established the structure of 6 by NMR spectroscopy and X-ray diffraction. Detailed investigation of its reactivity will be the subject of future work. When the formation is followed by low-temperature NMR, clean formation of the double silane addition intermediate (h³-(^tBuN)(PhHSi)(SiHPh-H^…)Mo(PMe₃)₂(H) (7) was seen. Complex 7 is SiH-M agostic on the basic of a ²⁹Si NMR study. Surprisingly, when (^tBuN=)₂Mo(PMe₃)₂ reacts with three equivalents of  PhSiH₃, a 1:1 mixture of 6 and the silanimine (h²- ^tBuN=SiHPh)Mo(PMe₃)₃(H)₄ (8) forms. The latter product was characterized by NMR spectroscopy. Unfortunately, its instability precluded a preparative scale reaction.

Scheme 1.

Overall, this PRF grant generated a wealth of results and allowed me to train three students in synthetic organometallic chemistry, catalysis, and mechanistic studies. The main highlights of this research are the discovery of new Si-C and Si-N coupling and Si-N bond cleavage reactions as well as the discovery of new mechanistic pathways in the hydrosilylation of carbonyls. In some cases we proved the intermediacy of silanimine and SiH^…M agostic complexes.