Reactivity of Cr(III) μ-Oxo Compounds: Catalyst Regeneration and Atom Transfer Processes

Oxidation of CpCr­[(XylNCMe)₂CH] (Xyl = 2,6-Me₂C₆H₃) with pyridine <i>N</i>-oxide or air generated the μ-oxo dimer, {CpCr­[(XylNCMe)₂CH]}₂(μ-O). The μ-oxo dimer was converted to paramagnetic Cr­(III) CpCr­[(XylNCMe)₂CH]­(X) complexes (X = OH, O₂CPh, Cl, OTs) via protonolysis reactions. The related Cr­(III) alkoxide complexes (X = OCMe₃, OCMe₂Ph) were prepared by salt metathesis and characterized by single crystal X-ray diffraction. The interconversion of the Cr­(III) complexes and their reduction back to Cr­(II) with Mn powder were monitored using UV–vis spectroscopy. The related CpCr­[(DepNCMe)₂CH] (Dep = 2,6-Et₂C₆H₃) Cr­(II) complex was studied for catalytic oxygen atom transfer reactions with PPh₃ using O₂ or air. Both Cr­(II) complexes reacted with pyridine <i>N</i>-oxide and γ-terpinene to give the corresponding Cr­(III) hydroxide complexes. When CpCr­[(DepNCMe)₂CH] was treated with pyridine <i>N</i>-oxide in benzene in the absence of hydrogen atom donors, a dimeric Cr­(III) hydroxide product was isolated and structurally characterized, apparently resulting from intramolecular hydrogen atom abstraction of a secondary benzylic ligand C–H bond followed by intermolecular C–C bond formation. The use of very bulky hexaisopropylterphenyl ligand substituents did not preclude the formation of the analogous μ-oxo dimer, which was characterized by X-ray diffraction. Attempts to develop a chromium-catalyzed intermolecular hydrogen atom transfer process based on these reactions were unsuccessful. The protonolysis and reduction reactions of the μ-oxo dimer were used to improve the previously reported Cr-catalyzed radical cyclization of a bromoacetal

Oxidatively Induced Reductive Elimination from a Chromium(III) Bis(aryl) Complex

The previously reported high-spin Cr­(II) compounds CpCr­(C₆H₄CH₂NMe₂) (<b>1</b>) and CpCr­[C­(Ph)­C­(Ph)­C₆H₄CH₂NMe₂] (<b>2</b>) were investigated as precursors to CpCr­(III) complexes. Single-electron oxidation of <b>1</b> was used to prepare CpCr­(C₆H₄CH₂NMe₂)­(X) for X = I (<b>3</b>), OTs (<b>4</b>), O₂CPh (<b>5</b>), OCMe₂Ph (<b>6</b>), SPh (<b>7</b>). Similarly, CpCr­[C­(Ph)­C­(Ph)­C₆H₄CH₂NMe₂]­(X) for X = I (<b>8</b>), SPh (<b>9</b>) were obtained from <b>2</b>. Reactions of <b>4</b> with PhCH₂MgCl or Mg­(C₆H₄Me)₂ reagents gave the Cr­(III) benzyl (<b>10</b>) and <i>p</i>-tolyl (<b>11</b>) complexes, respectively. The corresponding reaction of <b>4</b> with Mg­(CH₂CMe₃)₂ led to isolation of <b>12</b>, a bimetallic complex with a bridging neopentylidene group. While the reaction of <b>7</b> or <b>10</b> with AgOTs gave Cr­(III) tosylate complex <b>4</b>, the reaction of <b>11</b> and AgOTs led to the product of reductive elimination. The structures of <b>1</b>, <b>2</b>, <b>6</b>–<b>8</b>, <b>11</b>, and <b>12</b> were elucidated using single-crystal X-ray diffraction

Reactivity of Cr(III) μ-Oxo Compounds: Catalyst Regeneration and Atom Transfer Processes

Oxidation of CpCr­[(XylNCMe)₂CH] (Xyl = 2,6-Me₂C₆H₃) with pyridine <i>N</i>-oxide or air generated the μ-oxo dimer, {CpCr­[(XylNCMe)₂CH]}₂(μ-O). The μ-oxo dimer was converted to paramagnetic Cr­(III) CpCr­[(XylNCMe)₂CH]­(X) complexes (X = OH, O₂CPh, Cl, OTs) via protonolysis reactions. The related Cr­(III) alkoxide complexes (X = OCMe₃, OCMe₂Ph) were prepared by salt metathesis and characterized by single crystal X-ray diffraction. The interconversion of the Cr­(III) complexes and their reduction back to Cr­(II) with Mn powder were monitored using UV–vis spectroscopy. The related CpCr­[(DepNCMe)₂CH] (Dep = 2,6-Et₂C₆H₃) Cr­(II) complex was studied for catalytic oxygen atom transfer reactions with PPh₃ using O₂ or air. Both Cr­(II) complexes reacted with pyridine <i>N</i>-oxide and γ-terpinene to give the corresponding Cr­(III) hydroxide complexes. When CpCr­[(DepNCMe)₂CH] was treated with pyridine <i>N</i>-oxide in benzene in the absence of hydrogen atom donors, a dimeric Cr­(III) hydroxide product was isolated and structurally characterized, apparently resulting from intramolecular hydrogen atom abstraction of a secondary benzylic ligand C–H bond followed by intermolecular C–C bond formation. The use of very bulky hexaisopropylterphenyl ligand substituents did not preclude the formation of the analogous μ-oxo dimer, which was characterized by X-ray diffraction. Attempts to develop a chromium-catalyzed intermolecular hydrogen atom transfer process based on these reactions were unsuccessful. The protonolysis and reduction reactions of the μ-oxo dimer were used to improve the previously reported Cr-catalyzed radical cyclization of a bromoacetal

Controlling Secondary Alkyl Radicals: Ligand Effects in Chromium-Catalyzed C–P Bond Formation

Chromium cyclopentadienyl β-diketiminate catalysts have been used to form Ph₂PCy from Ph₂PY (Y = Cl, PPh₂, H) and CyX (X = Br, Cl) substrates. Manganese powder activated by PbX₂ or Me₃SiCl was used as the stoichiometric reductant. The Cr­(III) cyclohexyl intermediate has been synthesized and structurally characterized. The observed variations in catalytic activity have been correlated with the previously observed reactivity differences imparted by modifying the N-aryl substituents on the β-diketiminate ancillary ligand

Enhanced Hypsochromic Shifts, Quantum Yield, and π–π Interactions in a <i>meso</i>,β-Heteroaryl-Fused BODIPY

We report the synthesis and investigation of an unprecedented 8-heteroaryl-fused BODIPY <b>4</b>. This compound exhibits enhanced π–π stacking in the solid state, unusually large blue-shifts in the absorbance and emission spectra, and higher fluorescence quantum yield than its unfused precursor; DFT calculations suggest a small energy gap for <b>4</b> and strong electronic communication between the 8-OPh and the BODIPY core

Photolytic Reactivity of Organometallic Chromium Bipyridine Complexes

Known stable [Cr­(bpy)₂(Ph)₂]­(BPh₄) complexes undergo reductive elimination of biphenyl with visible-light photolysis using household incandescent or compact fluorescent light bulbs. A series of [Cr­(R-bpy)₂(Ar)₂]­(X) complexes (R = H or CMe₃; Ar = Ph, C₆H₄-CMe₃, or C₆H₄-OMe; X = I, BPh₄, or PF₆) were prepared, and the effect of varying the bipyridine and aryl ligands on the UV–visible spectra and electrochemistry of the chromium­(III) complexes was investigated. Photolysis of a mixture of two different bis­(aryl) complexes gave only the homocoupled biaryl products by ¹H NMR and gas chromatography/mass spectrometry analysis. The initial product of photoinduced reductive elimination of [Cr­(bpy)₂(Ar)₂]­(PF₆) was trapped with bipyridine to generate [Cr­(bpy)₃]­(PF₆) and with benzoyl peroxide to form [Cr­(bpy)₂(O₂CPh)₂]­(PF₆). The latter chromium­(III) bis­(benzoate) complex was also synthesized by the addition of bipyridine and PhCO₂H to Cp₂Cr, followed by air oxidation. The neutral Cr­(bpy)­(S₂CNMe₂)­Ph₂ complex also generated biphenyl upon visible-light photolysis. While the treatment of Cr­(^tBu-bpy)­(dpm)­Cl₂ [dpm = (OC^tBu)₂CH] with AgO₂CPh gave <i>trans</i>-Cr­(^tBu-bpy)­(dpm)­(O₂CPh)₂, reaction of the dichloro precursor with PhMgCl produced anionic [Cr­(^tBu-bpy)­Ph₃]⁻ with [Mg­(dpm)­(THF)₄]⁺ as the countercation, with both complexes characterized by single-crystal X-ray diffraction. Protonolysis of Cr­(bpy)­Ph₃(THF) with 8-hydroxyquinoline produced Cr­(bpy)­(quin)­Ph₂, which generated biphenyl under visible-light photolysis, and the initial product of reductive elimination was trapped by bipyridine or benzoyl peroxide. A related Cr­(bpy)­(quin)₂ complex was synthesized by protonolysis of Cr­(bpy)­[N­(SiMe₃)₂]₂ and characterized by single-crystal X-ray diffraction

Ambient-Temperature Carbon–Oxygen Bond Cleavage of an α‑Aryloxy Ketone with Cp₂Ti(BTMSA) and Selective Protonolysis of the Resulting Ti–OR Bonds

Reaction of Cp₂Ti­[η²-(CSiMe₃)₂] with an α-aryloxy ketone produces a Ti­(IV) enolate aryloxide complex. Selective protonolysis of the enolate ligand or both Ti–OR bonds can be achieved with various acids. The reaction of the enolate aryloxide with 1-phenyl-2-phenoxyethanol is catalyzed by a mixture of NEt₃ and [HNEt₃]­X (X = OTf, BPh₄)

Direct Synthesis of Ligand-Based Radicals by the Addition of Bipyridine to Chromium(II) Compounds

The reaction of 2,2′-bipyridine (bpy) with monomeric chromium­(II) precursors was used to prepare the <i>S</i> = 1 complexes Cr­(tBu-acac)₂(bpy) (<b>1</b>) and (η⁵-Cp)­(η¹-Cp)­Cr­(bpy) (<b>3</b>), as well as the <i>S</i> = 2 compound Cr­[N­(SiMe₃)₂]₂(bpy) (<b>4</b>). The crystallographically determined bond lengths indicate that the bpy ligands in <b>1</b> and <b>3</b> are best regarded as radical anions, while <b>4</b> shows no structural evidence for electron transfer from Cr^II to the neutral bpy ligand

Enhanced Hypsochromic Shifts, Quantum Yield, and π–π Interactions in a <i>meso</i>,β-Heteroaryl-Fused BODIPY

We report the synthesis and investigation of an unprecedented 8-heteroaryl-fused BODIPY <b>4</b>. This compound exhibits enhanced π–π stacking in the solid state, unusually large blue-shifts in the absorbance and emission spectra, and higher fluorescence quantum yield than its unfused precursor; DFT calculations suggest a small energy gap for <b>4</b> and strong electronic communication between the 8-OPh and the BODIPY core

Ambient-Temperature Carbon–Oxygen Bond Cleavage of an α‑Aryloxy Ketone with Cp₂Ti(BTMSA) and Selective Protonolysis of the Resulting Ti–OR Bonds

Reaction of Cp₂Ti­[η²-(CSiMe₃)₂] with an α-aryloxy ketone produces a Ti­(IV) enolate aryloxide complex. Selective protonolysis of the enolate ligand or both Ti–OR bonds can be achieved with various acids. The reaction of the enolate aryloxide with 1-phenyl-2-phenoxyethanol is catalyzed by a mixture of NEt₃ and [HNEt₃]­X (X = OTf, BPh₄)