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The specific results of the current grant period are:

1)      The reaction of (RN)₂Mo(PMe₃)₃ with HSiCl₃ has been earlier reported to furnish (RN)₂MoCl₂ (PMe₃)₃ and the silanimine dimer (RNSiClH)₂. We carried out detailed investigation of this reaction for R = Ar (Ar=2,6-Prⁱ₂C₆H₃),  2,6-Me₂C₆H₃, Bu^t, and found that the first observable product (at -30 °C) is the Cl-bridged species (ArN)(k²-ArNSiHCl-Cl)Mo(PMe₃)₂Cl (1). Analogous compound is formed and can be isolated when the W complex (ArN)₂W(SiCl₂Me)(H)(PMe₃) reacts with PMe₃. Back to Mo, warming the reaction mixture to – 15 - 0 °C leads to a slow rearrangement of this initial product into another bis(phosphine) compound whose spectroscopic parameters are consistent with the silanimine (ArN)(η²-ArN=SiHCl)Mo(PMe₃)₃Cl₂ (2). Addition of a further equivalent of PMe₃ to the latter gives (ArN)₂MoCl₂(PMe₃)₃ and the dimer (RNSiClH)₂, thus proving the inference of a silanimine ligand. Detailed DFT calculations of the reaction between the model complex (MeN)₂Mo(PMe₃)₃ and silanes HSiMe_nCl_3-n (n=1-3) underpin our mechanistic conclusion (Scheme 1). A report on this chemistry has been accepted to Inorganic Chemistry and is selected as a cover paper for an October issue 2009.

Scheme 1.

2)      We continued investigation of the reactivity of the agostic complex (ArN=)(η³-ArN-SiHPh-H^...)Mo(SiH₂Ph)(PMe₃) (3), and its derivatives. In particular, we carried out detailed mechanistic study of the reaction of complex (ArN=)(ArN-SiHPh-CH=CH₂)Mo(CH₂CH₃)(PMe₃) (4), formed upon the reaction of 3 with ethylene, with silanes in order to determine the mechanism of olefin hydrosilylation. The formation of the first product, the hydride (ArN=)(ArN-SiHPh-CH=CH₂)Mo(H)(PMe₃) (5), is overall a dissociative reaction (DS^≠<0) but detailed kinetic measurements revealed a complex interplay of two competing mechanisms (Scheme 2).

Scheme 2.

When the exchange reaction was attempted in the presence of BPh₃, the reaction takes a different path and gives a Si-H agostic ethylene derivative (ArN=)(η³-ArN-SiEtPh-H^...)Mo(SiH₂Ph)(CH₂=CH₂) (6), in which the ethyl group on the agostic silicon results from the vinyl group hydrogenation and the olefin ligand comes from a b-H shift from the ethyl. A possible mechanism of this transformation supported by labelling experiments is shown in Scheme 3. Subsequent reaction of 6 with PMe₃ affords the agostic complex (ArN=)(η³-ArN-SiEtPh-H^...)Mo(SiH₂Ph)(PMe₃) (7),which then slowly reacts with the silane to give the starting compound 3, thus closing the cycle. The mechanism of the latter transformation is still under investigation.

Scheme 3.

We also investigated the reactivity of the hydride species (ArN=)(ArN-SiHPh-CH=CH₂)Mo(H)(PMe₃) (5) that is very prone to insertion reactions, as summarized in Scheme 4.

Scheme 4.

3)    Our initial plan to prepare silanimine complexes supported by tripodal ligands isolobal with Cp, did not yield the target compounds. However, while pursuing this goal, we came across some interesting compounds exhibiting catalytic activity. Thus, we carried out detailed mechanistic investigations on the reaction of complex Cp(ArN=)Mo(H)(PMe₃) (8) with H₃SiPh and on the mechanism of carbonyl hydrosilylation catalyzed by 8. Kinetic measurements, labelling experiments, and DFT calculations of the first reaction established a s-bond metathesis mechanism for the formation of Cp(ArN=)Mo(SiH₂Ph)(PMe₃) (9). This result is significant because s-bond metathesis reactions are not typical for d² systems, such as complex 8. The catalytic hydrosilylation mediated by 8 proceeds as a two-step reaction, starting with the unexpected for a 18e complex associatively activated  insertion of carbonyl into the Mo-H bond (for PhCH=O: DH^≠ = 59.4 kJ/mol, DS^≠ = - 93 J/(K*mol)) to give Cp(ArN=)Mo(OR)(PMe₃) (10) followed by an associatively activated  reaction of the Mo-O bond with silane (DH^≠ = 58.3 ± 4.8 kJ/mol, DS^≠ = -113 ± 15.9 J/(K*mol)) to regenerate 8. Such a reaction should be classified as a heterolytic Si-H cleavage. To the best of our knowledge, this is the first detailed mechanistic investigation of this type of mechanism.

4)      The new complex (ArN=)(Me₃P)₃Mo(H)(SiH₂Ph) (11) was accidentally prepared in the course of our research on silanimine compounds described in section 2. Complex 11 was found to be an efficient catalyst for a variety of hydrosilylation reactions (carbonyls, alkenes, alcohols, nitriles). Mechanistic investigations revealed that its reactions with ketones give the alkoxy derivatives (ArN=)(Me₃P)₂Mo(H)(OR) (12, Scheme 5). With PhCH=O, however, a low temperature NMR study detected (ArN=)(Me₃P)₂Mo(h²-PhCHO)₂ as the first observable product. This compound easily releases phosphine to give the bis(aldehyde) species (ArN=)(Me₃P)Mo(h²-PhCHO)₂ (13) which was isolated and studied by NMR and X-ray diffraction. DFT studies show that 13 adds silane to give an unusual h¹-silane s-complex (MeN=)(Me₃P)Mo(h²-MeCHO)₂(h¹-H₃SiMe) (14), which then transfers the hydride to aldehyde to give the intermediate (MeN=)(Me₃P)Mo(h²-MeCHO)(OCH₂Me)(SiH₂Ph) (15), in a step preceding the formation of the hydrosilylation product.

Scheme 5.

The current PRF grant has made a profound effect on boosting our research program in molecular catalysis, particularly in that part which involves hydrosilylation reactions mediated by silanimine and Si-H agostic compounds. New Si-C and Si-N bond formation and Si-N bond cleavage reactions have been discovered, along with new and unexpected reaction pathways. During the current grant period, the students involved in the project learned new NMR techniques (such as 1D EXSY) to study kinetics of chemical reactions and became very well trained in sophisticated mechanistic research.