Synthesis and Reactions of a Cyclopentadienyl-Amidinate Titanium <i>tert-</i>Butoxyimido Compound

We report the first detailed reactivity study of a group 4 alkoxyimido complex, namely Cp*Ti­{PhC­(NⁱPr)₂}­(NO^tBu) (<b>19</b>), with heterocumulenes, aldehydes, ketones, organic nitriles, Ar^F₅CCH, and B­(Ar^F₅)₃ (Ar^F₅ = C₆F₅). Compound <b>19</b> was synthesized via imide/alkoxyamine exchange from Cp*Ti­{PhC­(NⁱPr)₂}­(N^tBu) and ^tBuONH₂. Reaction of <b>19</b> with CS₂ and Ar′NCO (Ar′ = 2,6-C₆H₃ⁱPr₂) gave the [2 + 2] cycloaddition products Cp*Ti­{PhC­(NⁱPr)₂}­{SC­(S)­N­(O^tBu)} and Cp*Ti­{PhC­(NⁱPr)₂}­{N­(O^tBu)­C­(NAr′)­O}, respectively, whereas reaction with 2 equiv of TolNCO afforded Cp*Ti­{PhC­(NⁱPr)₂}­{OC­(NTol)­N­(Tol)­C­(NO^tBu)­O} following a sequence of cycloaddition–extrusion and cycloaddition–insertion steps. Net NO^tBu group transfer was observed with both ^tBuNCO and PhC­(O)­R, yielding the oxo-bridged dimer [Cp*Ti­{PhC­(NⁱPr)₂}­(μ-O)]₂ and either the alkoxycarbodiimide ^tBuNCNO^tBu or the oxime ethers PhC­(NO^tBu)­R (R = H (<b>25a</b>), Me (<b>25b</b>), Ph (<b>25c</b>)). DFT studies showed that in the reaction with PhC­(O)­R (R = H, Me) the product distribution between the <i>syn</i> and <i>anti</i> isomers of PhC­(NO^tBu)­R was under kinetic control. Reaction of <b>19</b> with ArCN gave the TiN_α insertion products Cp*Ti­{PhC­(NⁱPr)₂}­{NC­(Ar)­NO^tBu} (Ar = Ph (<b>28</b>), 2,6-C₆H₃F₂ (<b>27</b>), Ar^F₅ (<b>26</b>)) containing <i>tert</i>-butoxybenzimidamide ligands. Reaction of <b>19</b> or <b>26</b> with an excess of Ar^F₅CN gave Cp*Ti­{PhC­(NⁱPr)₂}­{NC­(Ar^F₅)­NC­(Ar^F₅)­N­(C­{Ar^F₅}­NO^tBu)} (<b>29</b>) following net head-to-tail coupling of 2 equiv of Ar^F₅CN across the TiN_α bond of <b>26</b>. Reductive N_α–O_β bond cleavage was observed with Ar^F₅CCH, forming Cp*Ti­(O^tBu)­{NC­(Ar^F₅)­C­(H)­N­(ⁱPr)­C­(Ph)­N­(ⁱPr)} (<b>30</b>). Addition of 2 equiv of [Et₃NH]­[BPh₄] to <b>19</b> in THF-<i>d</i>₈ resulted in protonolysis of the amidinate ligand, forming [PhC­(NHⁱPr)₂]­[BPh₄] and the cationic alkoxyimido complex [Cp*Ti­(NO^tBu)­(THF-<i>d</i>₈)₂]⁺. In contrast, reaction with B­(Ar^F₅)₃ resulted in elimination of isobutene and formation of Cp*Ti­{PhC­(NⁱPr)₂}­{η²-ON­(H)­B­(Ar^F₅)₃}

Synthesis and Reactivity of Titanium Hydrazido Complexes Supported by Diamido-Ether Ligands

The synthesis and reactivity of titanium diphenyl hydrazido(2−) complexes supported by the diamido-ether ligands O­(2-C₆H₄NSiMe₃)₂ (N₂^ArO) and O­(CH₂CH₂NSiMe₃)₂ (N₂O) are described. Reaction of Li₂N₂^ArO or Li₂N₂O with Ti­(NNPh₂)­Cl₂(py)₃ afforded Ti­(N₂^ArO)­(NNPh₂)­(py)₂ (<b>14</b>) or Ti­(N₂O)­(NNPh₂)­(py)₂ (<b>15</b>) with κ³-<i>mer</i>-bound diamido-ether ligands. Reaction with ^tBu-bipy (4,4′-di-<i>tert</i>-butyl-2,2′-bipyridyl) or bipy (2,2′-bipyridyl) gave a switch to κ³-<i>fac</i>-coordination. Reaction of <b>15</b> with Ar′NCO (Ar′ = 2,6-C₆H₃ⁱPr₂) gave Ti­{O­(CH₂CH₂NSiMe₃)­(CH₂CH₂NC­(O)­N­(SiMe₃)­Ar′)}-{N­(NPh₂)­C­(O)­N­(Ar′)}, in which the substrate has inserted into a Ti–N_amide bond of N₂O as well as adding to the TiN_α multiple bond. With Ar′NCS the [2+2] cycloaddition product Ti­(N₂O)­{N­(NPh₂)­C­(NAr′)­S}­(py) was obtained, and with Ar′NCSe a mixture was formed including Ti₂(N₂O)₂(μ-Se)₂. Both <b>14</b> and <b>15</b> reacted with Ar^FxCN (Ar^Fx = C₆H₃F₂ or C₆F₅) to give TiN_α bond insertion products of the type Ti­(L)­{NC­(Ar^Fx)­NNPh₂}­(py)₂ (L = N₂^ArO or N₂O) containing hydrazonamide ligands. Reaction of <b>14</b> with XylNC (Xyl = 2,6-C₆H₃Me₂) gave only the isonitrile σ-adduct Ti­(N₂^ArO)­(NNPh₂)­(py)­(CNXyl), whereas <b>15</b> underwent N_α–N_β bond reductive cleavage with ^tBuNC or XylNC forming Ti­(N₂O)­(NPh₂)­(NCN^tBu) or Ti­{O­(CH₂CH₂NSiMe₃)­(CH₂CH₂NCN­(SiMe₃)­Xyl)}­(NPh₂)­(NCNXyl) (<b>27</b>). Both contain metalated carbodiimide ligands, but in <b>27</b> an additional reaction of XylNC with the Ti–N_amide bond of N₂O has taken place. Compound <b>15</b> also reacted with a number of internal alkynes RCCR′ (R = R′ = Me or Ph; R = Me, R′ = aryl) to give N_α–N_β bond reductive cleavage products of the type Ti­{O­(CH₂CH₂NSiMe₃)­(CH₂CH₂NC­(R)­C­(R′)­NSiMe₃}­(NPh₂), again involving a reaction of a Ti–N_amide bond

Synthesis and Reactions of a Cyclopentadienyl-Amidinate Titanium <i>tert-</i>Butoxyimido Compound

We report the first detailed reactivity study of a group 4 alkoxyimido complex, namely Cp*Ti­{PhC­(NⁱPr)₂}­(NO^tBu) (<b>19</b>), with heterocumulenes, aldehydes, ketones, organic nitriles, Ar^F₅CCH, and B­(Ar^F₅)₃ (Ar^F₅ = C₆F₅). Compound <b>19</b> was synthesized via imide/alkoxyamine exchange from Cp*Ti­{PhC­(NⁱPr)₂}­(N^tBu) and ^tBuONH₂. Reaction of <b>19</b> with CS₂ and Ar′NCO (Ar′ = 2,6-C₆H₃ⁱPr₂) gave the [2 + 2] cycloaddition products Cp*Ti­{PhC­(NⁱPr)₂}­{SC­(S)­N­(O^tBu)} and Cp*Ti­{PhC­(NⁱPr)₂}­{N­(O^tBu)­C­(NAr′)­O}, respectively, whereas reaction with 2 equiv of TolNCO afforded Cp*Ti­{PhC­(NⁱPr)₂}­{OC­(NTol)­N­(Tol)­C­(NO^tBu)­O} following a sequence of cycloaddition–extrusion and cycloaddition–insertion steps. Net NO^tBu group transfer was observed with both ^tBuNCO and PhC­(O)­R, yielding the oxo-bridged dimer [Cp*Ti­{PhC­(NⁱPr)₂}­(μ-O)]₂ and either the alkoxycarbodiimide ^tBuNCNO^tBu or the oxime ethers PhC­(NO^tBu)­R (R = H (<b>25a</b>), Me (<b>25b</b>), Ph (<b>25c</b>)). DFT studies showed that in the reaction with PhC­(O)­R (R = H, Me) the product distribution between the <i>syn</i> and <i>anti</i> isomers of PhC­(NO^tBu)­R was under kinetic control. Reaction of <b>19</b> with ArCN gave the TiN_α insertion products Cp*Ti­{PhC­(NⁱPr)₂}­{NC­(Ar)­NO^tBu} (Ar = Ph (<b>28</b>), 2,6-C₆H₃F₂ (<b>27</b>), Ar^F₅ (<b>26</b>)) containing <i>tert</i>-butoxybenzimidamide ligands. Reaction of <b>19</b> or <b>26</b> with an excess of Ar^F₅CN gave Cp*Ti­{PhC­(NⁱPr)₂}­{NC­(Ar^F₅)­NC­(Ar^F₅)­N­(C­{Ar^F₅}­NO^tBu)} (<b>29</b>) following net head-to-tail coupling of 2 equiv of Ar^F₅CN across the TiN_α bond of <b>26</b>. Reductive N_α–O_β bond cleavage was observed with Ar^F₅CCH, forming Cp*Ti­(O^tBu)­{NC­(Ar^F₅)­C­(H)­N­(ⁱPr)­C­(Ph)­N­(ⁱPr)} (<b>30</b>). Addition of 2 equiv of [Et₃NH]­[BPh₄] to <b>19</b> in THF-<i>d</i>₈ resulted in protonolysis of the amidinate ligand, forming [PhC­(NHⁱPr)₂]­[BPh₄] and the cationic alkoxyimido complex [Cp*Ti­(NO^tBu)­(THF-<i>d</i>₈)₂]⁺. In contrast, reaction with B­(Ar^F₅)₃ resulted in elimination of isobutene and formation of Cp*Ti­{PhC­(NⁱPr)₂}­{η²-ON­(H)­B­(Ar^F₅)₃}

Reactivity of Boryl- and Silyl-Substituted Carbenoids toward Alkynes: Insertion and Cycloaddition Chemistry

Three modes of reactivity of phenyl-substituted alkynes toward acyclic tetrelenes are reported, with reaction pathways found to be dependent not only on the nature of the group 14 element but also on the supporting ligand set. Systems featuring Sn–B or Ge–B bonds undergo insertion chemistry, forming borane-appended (vinyl)­Sn^II and Ge^II species. With a bis­(amido)­stannylene, phenylacetylene acts as a protic acid, generating a Sn^II acetylide with a unique bridged structure. Reactivity toward a more strongly reducing Si^II system is dominated by the possibility of accessing Si^IV via [2 + 1] cycloaddition chemistry

Oxidative Bond Formation and Reductive Bond Cleavage at Main Group Metal Centers: Reactivity of Five-Valence-Electron MX₂ Radicals

Monomeric five-valence-electron bis­(boryl) complexes of gallium, indium, and thallium undergo oxidative M–C bond formation with 2,3-dimethylbutadiene, in a manner consistent with both the redox properties expected for M^II species and with metal-centered radical character. The weaker nature of the M–C bond for the heavier two elements leads to the observation of reversibility in M–C bond formation (for indium) and to the isolation of products resulting from subsequent B–C reductive elimination (for both indium and thallium)

Oxidative Bond Formation and Reductive Bond Cleavage at Main Group Metal Centers: Reactivity of Five-Valence-Electron MX₂ Radicals

Monomeric five-valence-electron bis­(boryl) complexes of gallium, indium, and thallium undergo oxidative M–C bond formation with 2,3-dimethylbutadiene, in a manner consistent with both the redox properties expected for M^II species and with metal-centered radical character. The weaker nature of the M–C bond for the heavier two elements leads to the observation of reversibility in M–C bond formation (for indium) and to the isolation of products resulting from subsequent B–C reductive elimination (for both indium and thallium)

Oxidative Bond Formation and Reductive Bond Cleavage at Main Group Metal Centers: Reactivity of Five-Valence-Electron MX₂ Radicals

Monomeric five-valence-electron bis­(boryl) complexes of gallium, indium, and thallium undergo oxidative M–C bond formation with 2,3-dimethylbutadiene, in a manner consistent with both the redox properties expected for M^II species and with metal-centered radical character. The weaker nature of the M–C bond for the heavier two elements leads to the observation of reversibility in M–C bond formation (for indium) and to the isolation of products resulting from subsequent B–C reductive elimination (for both indium and thallium)

A Stable Two-Coordinate Acyclic Silylene

Simple two-coordinate acyclic silylenes, SiR₂, have hitherto been identified only as transient intermediates or thermally labile species. By making use of the strong σ-donor properties and high steric loading of the B­(NDippCH)₂ substituent (Dipp = 2,6-^<i>i</i>Pr₂C₆H₃), an isolable monomeric species, Si­{B­(NDippCH)₂}­{N­(SiMe₃)­Dipp}, can be synthesized which is stable in the solid state up to 130 °C. This silylene species undergoes facile oxidative addition reactions with dihydrogen (at sub-ambient temperatures) and with alkyl C–H bonds, consistent with a low singlet–triplet gap (103.9 kJ mol^–1), thus demonstrating fundamental modes of reactivity more characteristic of transition metal systems

A Stable Two-Coordinate Acyclic Silylene

Simple two-coordinate acyclic silylenes, SiR₂, have hitherto been identified only as transient intermediates or thermally labile species. By making use of the strong σ-donor properties and high steric loading of the B­(NDippCH)₂ substituent (Dipp = 2,6-^<i>i</i>Pr₂C₆H₃), an isolable monomeric species, Si­{B­(NDippCH)₂}­{N­(SiMe₃)­Dipp}, can be synthesized which is stable in the solid state up to 130 °C. This silylene species undergoes facile oxidative addition reactions with dihydrogen (at sub-ambient temperatures) and with alkyl C–H bonds, consistent with a low singlet–triplet gap (103.9 kJ mol^–1), thus demonstrating fundamental modes of reactivity more characteristic of transition metal systems