Lewis Base Mediated β-Elimination and Lewis Acid Mediated Insertion Reactions of Disilazido Zirconium Compounds

Abstract

The reactivity of a series of disilazido zirconocene complexes is dominated by the migration of anionic groups (hydrogen, alkyl, halide, OTf) between the zirconium and silicon centers. The direction of these migrations is controlled by the addition of two-electron donors (Lewis bases) or two-electron acceptors (Lewis acids). The cationic nonclassical [Cp2ZrN(SiHMe2)2](+) ([2](+)) is prepared from Cp2Zr{N(SiHMe2)2}H (1) and B(C6F5)3 or [Ph3C][B(C6F5)4], while reactions of B(C6F5)3 and Cp2Zr{N(SiHMe2)2}R (R = Me (3), Et (5), n-C3H7 (7), CH═CHSiMe3 (9)) provide a mixture of [2](+) and [Cp2ZrN(SiHMe2)(SiRMe2)](+). The latter products are formed through B(C6F5)3 abstraction of a β-H and R group migration from Zr to the β-Si center. Related β-hydrogen abstraction and X group migration reactions are observed for Cp2Zr{N(SiHMe2)2}X (X = OTf (11), Cl (13), OMe (15), O-i-C3H7 (16)). Alternatively, addition of DMAP (DMAP = 4-(dimethylamino)pyridine) to [2](+) results in coordination to a Si center and hydrogen migration to zirconium, giving the cationic complex [Cp2Zr{N(SiHMe2)(SiMe2DMAP)}H](+) ([19](+)). Related hydrogen migration occurs from [Cp2ZrN(SiHMe2)(SiMe2OCHMe2)](+) ([18](+)) to give [Cp2Zr{N(SiMe2DMAP)(SiMe2OCHMe2)}H](+) ([22](+)), whereas X group migration is observed upon addition of DMAP to [Cp2ZrN(SiHMe2)(SiMe2X)](+) (X = OTf ([12](+)), Cl ([14](+))) to give [Cp2Zr{N(SiHMe2)(SiMe2DMAP)}X](+) (X = OTf ([26](+)), Cl ([20](+))). The species involved in these transformations are described by resonance structures that suggest β-elimination. Notably, such pathways are previously unknown in early metal amide chemistry. Finally, these migrations facilitate direct Si-H addition to carbonyls, which is proposed to occur through a pathway that previously had been reserved for later transition metal compounds.

Electrophilic Activation of Silicon–Hydrogen Bonds in Catalytic Hydrosilations

“…For example, it was recently reported that a cationic zirconium complex 21 activates a β-agostic Si-H bond to undergo facile heterolytic cleavage upon attack of 4-(dimethylamino)pyridine at silicon (Scheme 24). [63] This process was not incorporated into a catalytic reaction, but this work clearly demonstrates that metals as early as group 4 are able to engage in this type of Si-H activation. It is interesting that the ionic hydrosilation mechanism may be possible for d 0 early metal catalysts since these catalysts are usually believed to operate by a variation of the Chalk-Harrod mechanism involving Si-C bond formation via σ-bond metathesis.…”

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

“…[198,218] Amore recent and well-characterized example of hydride abstraction chemistry is the reaction of Cp 2 Zr(H)-(N(SiHMe 2 ) 2 )( Cp = cyclopentadienyl) with B(C 6 F 5 ) 3 to give the cationic and nonclassically bonded [Cp 2 Zr(N(SiHMe 2 ) 2 )] + (Scheme 3). [219] Ar ecent example of methyl abstraction in transitionmetal chemistry is as eries of complexes of the type [Cp*Ir-(NHC)(solv)Me][MeB(C 6 F 5 ) 3 ](where solv = solvent;NHC = 1,3,4,5-tetramethylimidazol-2-ylidene;1 ,3-dimethylimidazol-2-ylidene;1 ,3-benzimidazol-2-ylidene) formed from their corresponding neutral Cp*Ir(NHC)Me 2 precursors and B-(C 6 F 5 ) 3 ,which were investigated as potential catalysts for CÀ Hbond activation. [220] An extensive review of p-block cations has been reported and for detailed examples featuring hydride/alkyl abstraction we refer to this review; however, selected exemplary complexes are mentioned in this section.…”

Direct hydrosilylation by a zirconacycle with β-hydrogen

“…In contrast, the cationic compound [Cp 2 ZrN(SiHMe 2 ) 2 ][HB(C 6 F 5 ) 3 ] ([2] + ) is characterized by unusual spectroscopic and geometric features supporting the presence of two non-classical Zr↼H-Si structures. 7 Related diagostic structures were previously reported for rare earth tetramethyldisilazido compounds, [11][12][13][14] but before that only molybdenum, niobium, tantalum and zirconium tetramethyldisilazido compounds were reported. [15][16][17] Moreover, studies of these two zirconium compounds reveal unusual reactivity.…”

“…7 The reactions with excess carbonyl molecules under these conditions do not result in further conversion of the N(SiHMe 2 ) 2 group. 7 Similarly, hydridozirconium silazide containing one β-SiH group Cp 2 Zr{N(SiHMe 2 )t-Bu}H also undergoes insertion with paraformaldehyde exclusively into the Zr-H bond to give Cp 2 Zr{N(SiHMe 2 )t-Bu}OMe (4). This compound is isolated and fully characterized by spectroscopic methods, and its connectivity is confirmed by a single crystal X-ray diffraction study (Figure 1).…”