Isolation of Free Phenylide-like Carbanions with N-Heterocyclic Carbene Frameworks

A series of 1,3-bis(2,6-diisopropylphenyl)-5-methyl-1,3-diaza-4,6-diborabenzenes with methyl, phenyl, and dimethylamino substituents on the ring boron atoms were prepared using the cyclocondensation reaction between N,N′-bis(2,6-diisopropylphenyl)trimethylsilylformamidine and the appropriately substituted 1,1-bis(organochloroboryl)ethane, followed by deprotonation of the cationic ring intermediate. The planar, heterocyclic benzene analogues could be further deprotonated at the other ring carbon using an additional equivalent of potassium hexamethyldisilazide to yield organometallic derivatives akin to the potassium phenylide. The potassium cations could be efficiently sequestered in both so…

Synthesis and Structures of Aluminum and Magnesium Complexes of Tetraimidophosphates and Trisamidothiophosphates: EPR and DFT Investigations of the Persistent Neutral Radicals {Me2Al[(μ-NR)(μ-NtBu)P(μ-NtBu)2]Li(THF)2}• (R = SiMe3, tBu)

Reactions of (RNH)3PNSiMe3 (3a, R = tBu; 3b, R = Cy) with trimethylaluminum result in the formation of {Me2Al(μ-NtBu)(μ-NSiMe3)P(NHtBu)2]} (4) and the dimeric trisimidometaphosphate {Me2Al[(μ-NCy)(μ-NSiMe3)P(μ-NCy)2P(μ-NCy)(μ-NSiMe3)]AlMe2} (5a), respectively. The reaction of SP(NHtBu)3 (2a) with 1 or 2 equiv of AlMe3 yields {Me2Al[(μ-S)(μ-NtBu)P(NHtBu)2]} (7) and {Me2Al[(μ-S)(μ-NtBu)P(μ-NHtBu)(μ-NtBu)]AlMe2} (8), respectively. Metalation of 4 with nBuLi produces the heterobimetallic species {Me2Al[(μ-NtBu)(μ-NSiMe3)P(μ-NHtBu)(μ-NtBu)]Li(THF)2} (9a) and {[Me2Al][Li]2[P(NtBu)3(NSiMe3)]} (10) sequentially; in THF solutions, solvation of 10 yields an ion pair containing a spirocyclic tetraimid…

A sigma-donor with a planar six-pi-electron B2N2C2 framework: anionic N-heterocyclic carbene or heterocyclic terphenyl anion?

Isolation of Free Phenylide-like Carbanions with N-Heterocyclic Carbene Frameworks

A series of 1,3-bis(2,6-diisopropylphenyl)-5-methyl-1,3-diaza-4,6-diborabenzenes with methyl, phenyl, and dimethylamino substituents on the ring boron atoms were prepared using the cyclocondensation reaction between N,N'-bis(2,6-diisopropylphenyl)trimethylsilylformamidine and the appropriately substituted 1,1-bis(organochloroboryl)ethane, followed by deprotonation of the cationic ring intermediate. The planar, heterocyclic benzene analogues could be further deprotonated at the other ring carbon using an additional equivalent of potassium hexamethyldisilazide to yield organometallic derivatives akin to the potassium phenylide. The potassium cations could be efficiently sequestered in both so…

More electron rich than cyclopentadienyl: 1,2-diaza-3,5-diborolyl as a ligand in ferrocene and ruthenocene analogs

Ruthenium and iron sandwich complexes incorporating cyclopentadienyl analogs with CB(2)N(2)(-) skeletons were characterized. Electrochemical measurements supported by computational studies revealed that in combination with larger metal ions such as Ru the CB(2)N(2)(-) ligand can be more electron-rich than its organic counterpart.

Characterization of β-B-Agostic Isomers in Zirconocene Amidoborane Complexes

The reaction of Cpx2ZrCl2 (Cpx = Cp, Cp*) with ammonia borane in presence of n-butyllithium yielded Cp2Zr(Cl)NH2BH3 and Cpx2Zr(H)NH2BH3. These derivatives are isoelectronic with the ethyl zirconocene chloride and hydride, respectively, and feature a chelating amidoborane ligand coordinating through a Zr−N bond and a Zr−H−B bridge. In solution, each of the complexes consists of an equilibrium mixture of two isomers differing in the orientation of the amidoborane ligand with respect to the Zr−X bond (X = H, Cl), while in the solid state, only one isomer was observed. Such isomers have not been characterized for any metal complexes containing the isoelectronic β-agostic ethyl ligand or any oth…

Cubic and Spirocyclic Radicals Containing a Tetraimidophosphate Dianion [P(NR)3(NR‘)]•2-

The reaction of Cl3PNSiMe3 with 3 equiv of LiHNR (R = iPr, Cy, tBu, Ad) in diethyl ether produces the corresponding tris(amino)(imino)phosphoranes (RNH)3PNSiMe3 (1a, R = iPr; 1b, R = Cy; 1c, R = tBu; 1d, R = Ad); subsequent reactions of 1b−d with nBuLi yield the trilithiated tetraimidophosphates {Li3[P(NR)3(NSiMe3)]} (2a, R = Cy; 2b, R = tBu; 2c, R = Ad). The reaction of [(tBuNH)4P]Cl with 1 equiv of nBuLi results in the isolation of (tBuNH)3PNtBu (1e); treatment of 1e with additional nBuLi generates the symmetrical tetraimidophosphate {Li3[P(NtBu)4]} (2d). Compounds 1 and 2 have been characterized by multinuclear (1H, 13C, and 31P) NMR spectroscopy; X-ray structures of 1b,c were also obtai…

A σ-Donor with a Planar Six-π-Electron B2N2C2 Framework: Anionic N-Heterocyclic Carbene or Heterocyclic Terphenyl Anion?

NB! The anionic ligand 2 was synthesized through deprotonation of a planar, formally zwitterionic diazadiborine precursor, isolated as a lithium salt, and structurally characterized. According to experimental evidence and theoretical calculations, 2 can be considered as an intermediate between two classical classes of ligands: N-heterocyclic carbenes 1 and terphenyls 3. peerReviewed

Ammonia Activation by a Nickel NCN-Pincer Complex featuring a Non-Innocent N-Heterocyclic Carbene: Ammine and Amido Complexes in Equilibrium

A Ni0-NCN pincer complex featuring a six-membered N-heterocyclic carbene (NHC) central platform and amidine pendant arms was synthesized by deprotonation of its NiII precursor. It retained chloride in the square-planar coordination sphere of nickel and was expected to be highly susceptible to oxidative addition reactions. The Ni0 complex rapidly activated ammonia at room temperature, in a ligand-assisted process where the carbene carbon atom played the unprecedented role of proton acceptor. For the first time, the coordinated (ammine) and activated (amido) species were observed together in solution, in a solvent-dependent equilibrium. A structural analysis of the Ni complexes provided insig…

Unusual B4N2C2 Ligand in a Ruthenium Pseudo-Triple-Decker Sandwich Complex Displaying Three Reversible Electron-Transfer Steps

Open, sesame: The reaction of a heterobicyclic pentalenediyl-like Me2Ph4B4N2C2 dianion with [{(C5Me5)RuCl}4] cleaves the N[BOND]N bond of the ligand and affords a pseudo-triple-decker sandwich complex containing a B4N2C2 middle deck (see picture). This eight-membered ring features nearly linear B-N-B moieties and brings the ruthenium centers unusually close. Cyclic voltammetry indicates efficient electron delocalization over the framework. peerReviewed

Characterization of β-B-Agostic Isomers in Zirconocene Amidoborane Complexes

The reaction of Cp(x)(2)ZrCl(2) (Cp(x) = Cp, Cp*) with ammonia borane in presence of n-butyllithium yielded Cp(2)Zr(Cl)NH(2)BH(3) and Cp(x)(2)Zr(H)NH(2)BH(3). These derivatives are isoelectronic with the ethyl zirconocene chloride and hydride, respectively, and feature a chelating amidoborane ligand coordinating through a Zr-N bond and a Zr-H-B bridge. In solution, each of the complexes consists of an equilibrium mixture of two isomers differing in the orientation of the amidoborane ligand with respect to the Zr-X bond (X = H, Cl), while in the solid state, only one isomer was observed. Such isomers have not been characterized for any metal complexes containing the isoelectronic beta-agosti…

Assembly of a planar, tricyclic B4N8 framework with s-indacene structure.

A neutral, formally 16pi-electron, tricyclic tetrahydrazidotetraborane was obtained in a two-step procedure involving self-assembly of a dilithiodiborate with B(4)N(8) framework and subsequent oxidation of the phenylborate moieties to boranes and biphenyl using Fe(II) as an oxidant.

Dihydrogen Activation by Antiaromatic Pentaarylboroles

Facile metal-free splitting of molecular hydrogen (H2) is crucial for the utilization of H2 without the need for toxic transition-metal-based catalysts. Frustrated Lewis pairs (FLPs) are a new class of hydrogen activators wherein interactions with both a Lewis acid and a Lewis base heterolytically disrupt the hydrogen−hydrogen bond. Here we describe the activation of hydrogen exclusively by a boron-based Lewis acid, perfluoropentaphenylborole. This antiaromatic compound reacts extremely rapidly with H2 in both solution and the solid state to yield boracyclopent-3-ene products resulting from addition of hydrogen atoms to the carbons α to boron in the starting borole. The disruption of antiar…

Assembly of a planar, tricyclic B4N8 framework with s-indacene structure

A neutral, formally 16π-electron, tricyclic tetrahydrazidotetraborane was obtained in a two-step procedure involving self-assembly of a dilithiodiborate with B4N8 framework and subsequent oxidation of the phenylborate moieties to boranes and biphenyl using Fe(II) as an oxidant. peerReviewed

Unusual B4N2C2 Ligand in a Ruthenium Pseudo-Triple-Decker Sandwich Complex Displaying Three Reversible Electron-Transfer Steps

Dihydrogen Activation by Antiaromatic Pentaarylboroles

Facile metal-free splitting of molecular hydrogen (H(2)) is crucial for the utilization of H(2) without the need for toxic transition-metal-based catalysts. Frustrated Lewis pairs (FLPs) are a new class of hydrogen activators wherein interactions with both a Lewis acid and a Lewis base heterolytically disrupt the hydrogen-hydrogen bond. Here we describe the activation of hydrogen exclusively by a boron-based Lewis acid, perfluoropentaphenylborole. This antiaromatic compound reacts extremely rapidly with H(2) in both solution and the solid state to yield boracyclopent-3-ene products resulting from addition of hydrogen atoms to the carbons alpha to boron in the starting borole. The disruption…

More electron rich than cyclopentadienyl: 1,2-diaza-3,5-diborolyl as a ligand in ferrocene and ruthenocene analogs

Ruthenium and iron sandwich complexes incorporating cyclopentadienyl analogs with CB2N2− skeletons were characterized. Electrochemical measurements supported by computational studies revealed that in combination with larger metal ions such as Ru the CB2N2− ligand can be more electron-rich than its organic counterpart. peerReviewed

CCDC 1405505: Experimental Crystal Structure Determination

Related Article: Rudy M. Brown, Javier Borau Garcia, Juuso Valjus, Christopher J. Roberts, Heikki M. Tuononen, Masood Parvez, and Roland Roesler|2015|Angew.Chem.,Int.Ed.|54|6274|doi:10.1002/anie.201500453

CCDC 1405504: Experimental Crystal Structure Determination

Related Article: Rudy M. Brown, Javier Borau Garcia, Juuso Valjus, Christopher J. Roberts, Heikki M. Tuononen, Masood Parvez, and Roland Roesler|2015|Angew.Chem.,Int.Ed.|54|6274|doi:10.1002/anie.201500453