Reactions of a Cyclopentadienyl–Amidinate Titanium Benzimidamido Complex

We
report the first reactivity study of a transition-metal benzimidamido
complex, namely Cp*Ti­{PhC­(NiPr)2}­{NC­(ArF5)­NOtBu} (5, ArF5 = C6F5). Reaction with CO2 and tBuNCO gave the cycloaddition products Cp*Ti­{PhC­(NiPr)2}­{OC­(O)­N­(C­{ArF5}­NOtBu)} and Cp*Ti­{PhC­(NiPr)2}­{OC­(NtBu)­N­(C­{ArF5}­NOtBu)} (10), respectively, whereas with CS2 slow extrusion
of ArF5CN from 5 occurred to ultimately
form Cp*Ti­{PhC­(NiPr)2}­{SC­(S)­N­(OtBu)}.
Reaction of 5 with ArC(O)H (Ar = Ph, 4-C6H4Me, 4-C6H4tBu, 4-C6H4OMe, 4-C6H4NMe2, 4-C6H4CF3) also gave the isolable metallacyclic complexes Cp*Ti­{PhC­(NiPr)2}­{N­(C­{ArF5}­NOtBu)­C­(Ar)­(H)­O} (13) via reversible [2 + 2] cycloaddition
reactions. In contrast, reaction with HC­(O)­NMe2 formed
Me2N­{NC­(ArF5)­NOtBu}H (16) within 1 h at room temperature. Upon heating, 10 and 13 also underwent retrocyclization, forming the
organic products tBuNCNC­(ArF5)­NOtBu and ArC­{NC­(ArF5)­NOtBu}­H
(14), respectively. Selected examples of 14 and 16 were studied by DFT and UV–visible spectroscopy.
Addition of isonitriles tBuNC and XylNC (Xyl = 2,6-C6H3Me2) to Cp*Ti­{PhC­(NiPr)2}­{NC­(Ar)­NOtBu} (Ar = ArF5 (5), 2,6-C6H3F2 (ArF2)) afforded the σ adducts Cp*Ti­{PhC­(NiPr)2}­{NC­(Ar)­NOtBu}­(CNR) (Ar = ArF5, R = tBu, Xyl (19); Ar
= ArF2, R = Xyl). Subsequently, 19 formed Cp*Ti­{PhC­(NiPr)2}­{NC­(NOtBu)­C6F4N­(Xyl)­C}­(F) (20) via C–F
bond activation. Reaction of 5 with 2 equiv of B­(ArF5)3 gave Cp*Ti­{PhC­(NiPr)2}­{ON­(B­{ArF5}3)­C­(ArF5)­N­(H)­(B­{ArF5}3)} with
elimination of 2-methylpropene.

本研究首次报道了过渡金属苯甲脒基配合物的反应性研究，该配合物为五甲基环戊二烯基（Cp*，pentamethylcyclopentadienyl）Ti{Ph（苯基，phenyl）C(NiPr)₂}{NC(ArF5)NOtBu}（记为化合物5，其中ArF5 = C6F5）。该配合物与二氧化碳（CO2）及叔丁基异氰酸酯（tBuNCO）发生环加成反应，分别得到环加成产物Cp*Ti{PhC(NiPr)₂}{OC(O)N(C{ArF5}NOtBu)}与Cp*Ti{PhC(NiPr)₂}{OC(NtBu)N(C{ArF5}NOtBu)}（均记为化合物10）；而该配合物与二硫化碳（CS2）反应时，会缓慢脱除五氟苯甲腈（ArF5CN），最终生成Cp*Ti{PhC(NiPr)₂}{SC(S)N(OtBu)}。化合物5与芳醛（ArC(O)H，Ar分别为Ph、4-C6H4Me、4-C6H4tBu、4-C6H4OMe、4-C6H4NMe2、4-C6H4CF3）反应，通过可逆[2+2]环加成反应，可分离得到金属环配合物Cp*Ti{PhC(NiPr)₂}{N(C{ArF5}NOtBu)C(Ar)(H)O}（记为化合物13）。与之相反，化合物5与N,N-二甲基甲酰胺（HC(O)NMe2）在室温下反应1小时内即可生成Me2N{NC(ArF5)NOtBu}H（记为化合物16）。加热条件下，化合物10与13同样会发生逆环化反应，分别生成有机产物tBuNCNC(ArF5)NOtBu与ArC{NC(ArF5)NOtBu}H（记为化合物14）。研究人员选取部分化合物14与16的样品，通过密度泛函理论（DFT，Density Functional Theory）与紫外-可见分光光度法（UV–visible spectroscopy）进行了表征分析。将异腈类化合物（isonitriles）叔丁基异腈（tBuNC）与二甲苯基异腈（XylNC，Xyl=2,6-C6H3Me2）加入到Cp*Ti{PhC(NiPr)₂}{NC(Ar)NOtBu}（Ar分别为ArF5（即化合物5）、2,6-C6H3F2（记为ArF2））的体系中，可得到σ加合物（σ adducts）Cp*Ti{PhC(NiPr)₂}{NC(Ar)NOtBu}(CNR)，具体包括Ar=ArF5、R=tBu或Xyl（记为化合物19）以及Ar=ArF2、R=Xyl的产物。随后，化合物19通过碳氟键活化（C–F bond activation）反应，生成Cp*Ti{PhC(NiPr)₂}{NC(NOtBu)C6F4N(Xyl)C}(F)（记为化合物20）。化合物5与2当量的三(五氟苯基)硼（B(ArF5)3）发生反应，消除2-甲基丙烯（2-methylpropene）后，生成Cp*Ti{PhC(NiPr)₂}{ON(B{ArF5}3)C(ArF5)N(H)(B{ArF5}3)}。