Pnictogen-Hydride Activation by (silox)3Ta (silox = tBu3SiO); Attempts to Circumvent the Constraints of Orbital Symmetry in N2 Activation

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• 作者：Elliott B. Hulley ; Jeffrey B. Bonanno ; Peter T. Wolczanski ; Thomas R. Cundari ; Emil B. Lobkovsky
• 刊名：Inorganic Chemistry
• 出版年：2010
• 出版时间：September 20, 2010
• 年：2010
• 卷：49
• 期：18
• 页码：8524-8544
• 全文大小：1495K
• 年卷期：v.49,no.18(September 20, 2010)
• ISSN：1520-510X

文摘

Activation of Nb>2b> by (silox)b>3b>Ta (<b>1b>, silox = ^tBub>3b>SiO) to afford (silox)b>3b>Ta═N−N═Ta(silox)b>3b> (<b>1b>b>2b>-Nb>2b>) does not occur despite ΔG°b>caldb> = −55.6 kcal/mol because of constraints of orbital symmetry, prompting efforts at an independent synthesis that included a study of REHb>2b> activation (E = N, P, As). Oxidative addition of REHb>2b> to <b>1b> afforded (silox)b>3b>HTaEHR (<b>2b>-NHR, R = H, Me, ⁿBu, Cb>6b>Hb>4b>-p-X (X = H, Me, NMeb>2b>); <b>2b>-PHR, R = H, Ph; <b>2b>-AsHR, R = H, Ph), which underwent 1,2-Hb>2b>-elimination to form (silox)b>3b>Ta═NR (<b>1b>═NR; R = H, Me, ⁿBu, Cb>6b>Hb>4b>-p-X (X = H (X-ray), Me, NMeb>2b>, CFb>3b>)), (silox)b>3b>Ta═PR (<b>1b>═PR; R = H, Ph), and (silox)b>3b>Ta═AsR (<b>1b>═AsR; R = H, Ph). Kinetics revealed NH bond-breaking as critical, and As > N > P rates for (silox)b>3b>HTaEHPh (<b>2b>-EHPh) were attributed to (1) ΔG°b>calcb>(N) < ΔG°b>calcb>(P) ΔG°b>calcb>(As); (2) similar fractional reaction coordinates (RCs), but with RC shorter for N < PAs; and (3) stronger TaE bonds for N > PAs. Calculations of the pnictidenes aided interpretation of UV−vis spectra. Addition of Hb>2b>NNHb>2b> or Hb>2b>N−N(^cNCb>2b>Hb>3b>Me) to <b>1b> afforded <b>1b>═NH, obviating these routes to <b>1b>b>2b>-Nb>2b>, and formation of (silox)b>3b>MeTaNHNH2 (<b>4b>-NHNHb>2b>) and (silox)b>3b>MeTaNH(-^cNCHMeCHb>2b>) (<b>4b>-NH(azir)) occurred upon exposure to (silox)b>3b>Ta═CHb>2b> (<b>1b>═CHb>2b>). Thermolyses of <b>4b>-NHNHb>2b> and <b>4b>-NH(azir) yielded [(silox)b>2b>TaMe](μ-Nb>αb>HNb>βb>)(μ-Nb>γb>HNb>δb>H)[Ta(silox)b>2b>] (<b>5b>) and [(silox)b>3b>MeTa](μ-η²-N,N:η¹-C-NHNHCHb>2b>CHb>2b>CHb>2b>)[Ta(κ-O,C-OSi^tBub>2b>CMeb>2b>CHb>2b>)(silox)b>2b>] (<b>7b>, X-ray), respectively. (silox)b>3b>Ta═CPPhb>3b> (<b>1b>═CPPhb>3b>, X-ray) was a byproduct from Phb>3b>PCHb>2b> treatment of <b>1b> to give <b>1b>═CHb>2b>. Addition of Na(silox) to [(THF)b>2b>Clb>3b>Ta]b>2b>(μ-Nb>2b>) led to [(silox)b>2b>ClTa](μ-Nb>2b>) (<b>8b>-Cl), and via subsequent methylation, [(silox)b>2b>MeTa]b>2b>(μ-Nb>2b>) (<b>8b>-Me); both dimers were thermally stable. Orbital symmetry requirements for Nb>2b> capture by <b>1b> and pertinent calculations are given.