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

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.

一系列双硅氮杂锆鎌复合物的反应活性主要由阴离子基团（氢、烷基、卤素、OTf）在锆和硅中心之间的迁移所主导。这些迁移的方向受到添加二电子供体（路易斯碱）或二电子受体（路易斯酸）的调控。阳离子非经典复合物 [Cp2ZrN(SiHMe2)2]+ ([2]+) 由 Cp2Zr{N(SiHMe2)2}H (1) 与 B(C6F5)3 或 [Ph3C][B(C6F5)4] 反应制备而成，而 B(C6F5)3 与 Cp2Zr{N(SiHMe2)2}R (R = Me (3), Et (5), n-C3H7 (7), CH2=CHSiMe3 (9)) 的反应则产生 [2]+ 和 [Cp2ZrN(SiHMe2)(SiRMe2)]+ 的混合物。后者通过 B(C6F5)3 对 β-H 和 R 基团从锆中心迁移至 β-Si 中心的提取作用而形成。对于 Cp2Zr{N(SiHMe2)2}X (X = OTf (11), Cl (13), OMe (15), O-i-C3H7 (16))，观察到相关的 β-氢提取和 X 基团迁移反应。另外，向 [2]+ 中添加 DMAP (DMAP = 4-(二甲基氨基)吡啶) 导致与 Si 中心的配位，以及氢向锆中心的迁移，从而得到阳离子复合物 [Cp2Zr{N(SiHMe2)(SiMe2DMAP)}H]+ ([19]+)。相关的氢迁移发生在 [Cp2ZrN(SiHMe2)(SiMe2OCHMe2)]+ ([18]+) 向 [Cp2Zr{N(SiMe2DMAP)(SiMe2OCHMe2)}H]+ ([22]+) 的转化过程中，而向 [Cp2ZrN(SiHMe2)(SiMe2X)]+ (X = OTf ([12]+), Cl ([14]+)) 添加 DMAP 则观察到 X 基团迁移，从而得到 [Cp2Zr{N(SiHMe2)(SiMe2DMAP)}X]+ (X = OTf ([26]+), Cl ([20]+))。参与这些转化的物种可通过共振结构描述，这些结构暗示了 β-消除。值得注意的是，此类途径在早期金属酰胺化学中尚属未知。最后，这些迁移促进了直接对羰基的 Si-H 添加，这一过程被认为是通过一种之前仅限于后过渡金属化合物的途径发生的。