A Frustrated Lewis Pair Based on a Cationic Aluminum Complex and Triphenylphosphine

The highly Lewis acidic, cationic aluminum species [DIPP-nacnacAlMe]⁺[B­(C₆F₅)₄]⁻ (<b>1</b>, DIPP-nacnac = [HC­{C­(Me)­N­(2,6-^<i>i</i>Pr₂C₆H₃)}₂]⁻) has been shown to undergo reactions with a wide variety of small molecules, in both the presence and absence of an external weak phosphine base, PPh₃. Cycloaddition reactions of unsaturated C–C bonds across the aluminum diketiminate framework are reported, and the first structural confirmation of this type of cycloaddition product is presented. Addition of PPh₃ to <b>1</b> produces the cationic aluminum phosphine complex [DIPP-nacnacAl­(Me)­PPh₃]⁺[B­(C₆F₅)₄]⁻, which undergoes fluxional dissociation/coordination of the phosphine in solution. This weak Al–P interaction can be utilized in frustrated Lewis pair type reactions to activate alkenes, alkynes, CO₂, propylene oxide, and the C–Cl bonds of CH₂Cl₂. The CO₂ adduct [DIPP-nacnacAl­(Me)­OC­(PPh₃)­O]⁺[B­(C₆F₅)₄]⁻ undergoes further stoichiometric reduction with Et₃SiH to produce an aluminum formate species

Half-Sandwich Complexes of an Extremely Electron-Donating, Redox-Active η⁶‑Diborabenzene Ligand

The heteroarene 1,4-bis­(CAAC)-1,4-diborabenzene (<b>1</b>; CAAC = cyclic (alkyl)­(amino)­carbene) reacts with [(MeCN)₃M­(CO)₃] (M = Cr, Mo, W) to yield half-sandwich complexes of the form [(η⁶-diborabenzene)­M­(CO)₃] (M = Cr (<b>2</b>), Mo (<b>3</b>), W (<b>4</b>)). Investigation of the new complexes with a combination of X-ray diffraction, spectroscopic methods and DFT calculations shows that ligand <b>1</b> is a remarkably strong electron donor. In particular, [(η⁶-arene)­M­(CO)₃] complexes of this ligand display the lowest CO stretching frequencies yet observed for this class of complex. Cyclic voltammetry on complexes <b>2</b>–<b>4</b> revealed one reversible oxidation and two reversible reduction events in each case, with no evidence of ring-slippage of the arene to the η⁴ binding mode. Treatment of <b>4</b> with lithium metal in THF led to identification of the paramagnetic complex [(<b>1</b>)­W­(CO)₃]­Li·2THF (<b>5</b>). Compound <b>1</b> can also be reduced in the absence of a transition metal to its dianion <b>1</b>^2–, which possesses a quinoid-type structure

Half-Sandwich Complexes of an Extremely Electron-Donating, Redox-Active η⁶‑Diborabenzene Ligand

The heteroarene 1,4-bis­(CAAC)-1,4-diborabenzene (<b>1</b>; CAAC = cyclic (alkyl)­(amino)­carbene) reacts with [(MeCN)₃M­(CO)₃] (M = Cr, Mo, W) to yield half-sandwich complexes of the form [(η⁶-diborabenzene)­M­(CO)₃] (M = Cr (<b>2</b>), Mo (<b>3</b>), W (<b>4</b>)). Investigation of the new complexes with a combination of X-ray diffraction, spectroscopic methods and DFT calculations shows that ligand <b>1</b> is a remarkably strong electron donor. In particular, [(η⁶-arene)­M­(CO)₃] complexes of this ligand display the lowest CO stretching frequencies yet observed for this class of complex. Cyclic voltammetry on complexes <b>2</b>–<b>4</b> revealed one reversible oxidation and two reversible reduction events in each case, with no evidence of ring-slippage of the arene to the η⁴ binding mode. Treatment of <b>4</b> with lithium metal in THF led to identification of the paramagnetic complex [(<b>1</b>)­W­(CO)₃]­Li·2THF (<b>5</b>). Compound <b>1</b> can also be reduced in the absence of a transition metal to its dianion <b>1</b>^2–, which possesses a quinoid-type structure

Half-Sandwich Complexes of an Extremely Electron-Donating, Redox-Active η⁶‑Diborabenzene Ligand

The heteroarene 1,4-bis­(CAAC)-1,4-diborabenzene (<b>1</b>; CAAC = cyclic (alkyl)­(amino)­carbene) reacts with [(MeCN)₃M­(CO)₃] (M = Cr, Mo, W) to yield half-sandwich complexes of the form [(η⁶-diborabenzene)­M­(CO)₃] (M = Cr (<b>2</b>), Mo (<b>3</b>), W (<b>4</b>)). Investigation of the new complexes with a combination of X-ray diffraction, spectroscopic methods and DFT calculations shows that ligand <b>1</b> is a remarkably strong electron donor. In particular, [(η⁶-arene)­M­(CO)₃] complexes of this ligand display the lowest CO stretching frequencies yet observed for this class of complex. Cyclic voltammetry on complexes <b>2</b>–<b>4</b> revealed one reversible oxidation and two reversible reduction events in each case, with no evidence of ring-slippage of the arene to the η⁴ binding mode. Treatment of <b>4</b> with lithium metal in THF led to identification of the paramagnetic complex [(<b>1</b>)­W­(CO)₃]­Li·2THF (<b>5</b>). Compound <b>1</b> can also be reduced in the absence of a transition metal to its dianion <b>1</b>^2–, which possesses a quinoid-type structure

Half-Sandwich Complexes of an Extremely Electron-Donating, Redox-Active η⁶‑Diborabenzene Ligand

The heteroarene 1,4-bis­(CAAC)-1,4-diborabenzene (<b>1</b>; CAAC = cyclic (alkyl)­(amino)­carbene) reacts with [(MeCN)₃M­(CO)₃] (M = Cr, Mo, W) to yield half-sandwich complexes of the form [(η⁶-diborabenzene)­M­(CO)₃] (M = Cr (<b>2</b>), Mo (<b>3</b>), W (<b>4</b>)). Investigation of the new complexes with a combination of X-ray diffraction, spectroscopic methods and DFT calculations shows that ligand <b>1</b> is a remarkably strong electron donor. In particular, [(η⁶-arene)­M­(CO)₃] complexes of this ligand display the lowest CO stretching frequencies yet observed for this class of complex. Cyclic voltammetry on complexes <b>2</b>–<b>4</b> revealed one reversible oxidation and two reversible reduction events in each case, with no evidence of ring-slippage of the arene to the η⁴ binding mode. Treatment of <b>4</b> with lithium metal in THF led to identification of the paramagnetic complex [(<b>1</b>)­W­(CO)₃]­Li·2THF (<b>5</b>). Compound <b>1</b> can also be reduced in the absence of a transition metal to its dianion <b>1</b>^2–, which possesses a quinoid-type structure

Half-Sandwich Complexes of an Extremely Electron-Donating, Redox-Active η⁶‑Diborabenzene Ligand

The heteroarene 1,4-bis­(CAAC)-1,4-diborabenzene (<b>1</b>; CAAC = cyclic (alkyl)­(amino)­carbene) reacts with [(MeCN)₃M­(CO)₃] (M = Cr, Mo, W) to yield half-sandwich complexes of the form [(η⁶-diborabenzene)­M­(CO)₃] (M = Cr (<b>2</b>), Mo (<b>3</b>), W (<b>4</b>)). Investigation of the new complexes with a combination of X-ray diffraction, spectroscopic methods and DFT calculations shows that ligand <b>1</b> is a remarkably strong electron donor. In particular, [(η⁶-arene)­M­(CO)₃] complexes of this ligand display the lowest CO stretching frequencies yet observed for this class of complex. Cyclic voltammetry on complexes <b>2</b>–<b>4</b> revealed one reversible oxidation and two reversible reduction events in each case, with no evidence of ring-slippage of the arene to the η⁴ binding mode. Treatment of <b>4</b> with lithium metal in THF led to identification of the paramagnetic complex [(<b>1</b>)­W­(CO)₃]­Li·2THF (<b>5</b>). Compound <b>1</b> can also be reduced in the absence of a transition metal to its dianion <b>1</b>^2–, which possesses a quinoid-type structure

Unexpectedly High Barriers to M–P Rotation in Tertiary Phobane Complexes: PhobPR Behavior That Is Commensurate with ^tBu₂PR

The four isomers of 9-butylphosphabicyclo[3.3.1]­nonane, <i>s-</i>PhobPBu, where Bu = <i>n</i>-butyl, <i>sec</i>-butyl, isobutyl, <i>tert</i>-butyl, have been prepared. Seven isomers of 9-butylphosphabicyclo[4.2.1]­nonane (<i>a</i>₅<i>-</i>PhobPBu, where Bu = <i>n</i>-butyl, <i>sec</i>-butyl, isobutyl, <i>tert</i>-butyl; <i>a</i>₇<i>-</i>PhobPBu, where Bu = <i>n-</i>butyl, isobutyl, <i>tert</i>-butyl) have been identified in solution; isomerically pure <i>a</i>₅<i>-</i>PhobPBu and <i>a</i>₇<i>-</i>PhobPBu, where Bu = <i>n</i>-butyl, isobutyl, have been isolated. The σ-donor properties of the PhobPBu ligands have been compared using the <i>J</i>_PSe values for the PhobP­(Se)­Bu derivatives. The following complexes have been prepared: <i>trans-</i>[PtCl₂(<i>s-</i>PhobPR)₂] (R = ⁿBu (<b>1a</b>), ⁱBu (<b>1b</b>), ^sBu (<b>1c</b>), ^tBu (<b>1d</b>)); <i>trans-</i>[PtCl₂(<i>a</i>₅<i>-</i>PhobPR)₂] (R = ⁿBu (<b>2a</b>), ⁱBu (<b>2b</b>)); <i>trans-</i>[PtCl₂(<i>a</i>₇<i>-</i>PhobPR)₂] (R = ⁿBu (<b>3a</b>), ⁱBu (<b>3b</b>)); <i>trans-</i>[PdCl₂(<i>s-</i>PhobPR)₂] (R = ⁿBu (<b>4a</b>), ⁱBu (<b>4b</b>)); <i>trans-</i>[PdCl₂(<i>a</i>₅<i>-</i>PhobPR)₂] (R = ⁿBu (<b>5a</b>), ⁱBu (<b>5b</b>)); <i>trans-</i>[PdCl₂(<i>a</i>₇<i>-</i>PhobPR)₂] (R = ⁿBu (<b>6a</b>), ⁱBu (<b>6b</b>)). The crystal structures of <b>1a</b>–<b>4a</b> and <b>1b</b>–<b>6b</b> have been determined, and of the ten structures, eight show an anti conformation with respect to the position of the ligand R groups and two show a syn conformation. Solution variable-temperature ³¹P NMR studies reveal that all of the Pt and Pd complexes are fluxional on the NMR time scale. In each case, two species are present (assigned to be the syn and anti conformers) which interconvert with kinetic barriers in the range 9 to >19 kcal mol^–1. The observed trend is that, the greater the bulk, the higher the barrier. The magnitudes of the barriers to M–P bond rotation for the PhobPR complexes are of the same order as those previously reported for ^tBu₂PR complexes. Rotational profiles have been calculated for the model anionic complexes [PhobPR-PdCl₃]⁻ using DFT, and these faithfully reproduce the trends seen in the NMR studies of <i>trans-</i>[MCl₂(PhobPR)₂]. Rotational profiles have also been calculated for [^tBu₂PR-PdCl₃]⁻, and these show that the greater the bulk of the R group, the lower the rotational barrier: i.e., the opposite of the trend for [PhobPR-PdCl₃]⁻. Calculated structures for the species at the maxima and minima in the M–P rotation energy curves indicate the origin of the restricted rotation. In the case of the PhobPR complexes, it is the rigidity of the bicycle that enforces unfavorable H···Cl clashes involving the Pd–Cl groups with H atoms on the α- or β-carbon in the R substituent and H atoms in 1,3-axial sites within the phosphabicycle