Stereoisomerism of bis(σ-Zincane) Complexes: Evidence for an Intramolecular Pathway.

The first bis(σ-zincane) complexes, heterotri- metallic species [M(CO)4 (η2 -HZnBDI)2 ], have been prepared (BDI=κ2 -{2,6-(iPr)2 C6 H3 NCMe}2 CH). For M=Cr, a single stereoisomer is observed in solution and the solid-state. For M=Mo and W, cis and trans isomers were found to reversibly interconvert at 297 K. Despite the huge steric demands of the ligand on zinc, the cis isomer was found to be the most thermodynamically stable in all cases. The activation parameters for the isomerisation when M=Mo are ΔH≠ =20.8 kcal mol-1 and ΔS≠ =-12.8 cal K-1 mol-1 . In combination with DFT calculations, the negative activation entropy suggests an intramolecular rotation mechanism for isomerisation.

Trajectory of Approach of a Zinc-Hydrogen Bond to Transition Metals.

Through a dramatic advance in the coordination chemistry of the zinc-hydride bond, we describe the trajectory for the approach of this bond to transition metals. The dynamic reaction coordinate was interrogated through analysis of a series of solid state structures and is one in which the TM-H-Zn angle becomes increasingly acute as the TM-Zn distance decreases. Parallels may be drawn with the oxidative addition of boron-hydrogen and silicon-hydrogen bonds to transition metal centers.

Addition of aluminium, zinc and magnesium hydrides to rhodium(iii).

We report the addition of M-H bonds (M = Al, Zn, Mg) to a Rh(iii) intermediate generated from the reductive elimination of triethylsilane from [Cp*Rh(H)2(SiEt3)2]. A series of new heterobimetallic complexes possessing Rh-M bonds have been isolated and characterised by a number of spectroscopic (1H, 29Si, 13C, 103Rh NMR, infrared, and X-ray diffraction) and computational techniques (NBO and QTAIM analysis). Experimental and computational data are consistent with cleavage of the M-H bond upon addition to rhodium with formation of new Rh-M and Rh-H bonds. Upon photolysis the Al analogue of this series undergoes a further elimination reaction producing triethylsilane and a highly unusual Rh2Al2H4 containing cluster proposed to contain an Al(i) bridging ligand.

Reactivity of an intramolecular fluorophosphonium fluoroborate.

The reactions of the intramolecular frustrated Lewis pair-adduct Ph(2) PC(p-Tol)=C(C(6) F(5))B(C(6)F(5))2 (CNtBu) with XeF(2) gave Ph(2)P(F)C(p-Tol)=C(C(6)F(5))B(F)(C(6)F(5))(2)(3). This species reacts with two equivalents of Al(C(6)F(5))(3)⋅C(7)H(8) producing the salt, [Ph(2)P(F)C(p-Tol)=C(C(6)F(5))B(C(6)F(5))(2)][F(Al(C(6)F(5))(3))(2)] (4), whereas reaction with HSiEt(3)/B(C(6)F(5))(3) gave Ph(2) P(F)C(p-Tol)=C(H)B(C(6)F(5))(3) (5). The photolysis of 3 resulted in aromatization affording the phenanthralene derivative Ph(2) P(F)C(p-Tol(o-C(6)F(4)))=CB(F)(C(6)F(5))(2) (6).

Solid-state NMR studies for the determination of 11B electric field-gradient tensor orientations in P/B Frustrated Lewis Pairs and related systems.

A solid state NMR method is described for measuring the angle Θ specifying the orientation of the principal component of the (11)B electric field gradient tensor relative to the (11)B(…31)P internuclear vector of (11)B-(31)P spin pairs. It is based on the anisotropic dephasing of (11)B spins in the dipolar field of (31)P nuclei via (11)B{(31)P} Rotational Echo DOuble Resonance (REDOR) experiments. The method is applied to four solid borane-phosphane compounds related to Frustrated Lewis Pair (FLP) chemistry. Results determined by numerical line shape simulations are found in excellent agreement with theoretically calculated values using advanced DFT methods. The angle Θ, which can be measured with an estimated precision of ±5°, offers a clear spectroscopic distinction between classical Lewis-acid/base adducts and active Frustrated Lewis pairs (FLPs).

Reaction of strongly electrophilic alkenylboranes with phosphanylalkynes: rare examples of intermolecular 1,1-alkenylboration reactions.

Simple alkenylbis(pentafluorophenyl)boranes undergo 1,1-alkenylboration with phosphanylacetylenes to give phosphane-borane Lewis pairs with a conjugated diene backbone.

New insights into frustrated Lewis pairs: structural investigations of intramolecular phosphane-borane adducts by using modern solid-state NMR techniques and DFT calculations.

Covalent bonding interactions between the Lewis acid and Lewis base functionalities have been probed in a series of "frustrated Lewis pairs" (FLPs) (mainly substituted vinylene linked intramolecular phosphane-borane adducts), using solid-state nuclear magnetic resonance techniques and accompanying DFT calculations. Both the (11)B NMR isotropic chemical shifts and nuclear electric quadrupolar coupling parameters turn out to be extremely sensitive experimental probes for such interactions, revealing linear correlations with boron-phosphorus internuclear distances. The principal component V(zz) of the (11)B electric field gradient tensor is tilted slightly away (~20°) from the boron-phosphorus internuclear vector, leading to an improved understanding of the remarkable reactivity of the FLPs. Complementary (31)P{(1)H}-CPMAS experiments reveal significant (31)P-(11)B scalar spin-spin interactions ((1)J ≈ 50 Hz), evidencing covalent bonding interactions between the reaction centers. Finally, (11)B{(31)P} rotational echo double resonance (REDOR) experiments show systematic deviations from calculated curves based on the internuclear distances from X-ray crystallography. These deviations suggest non-zero contributions from anisotropic indirect spin-spin (J anisotropy) interactions, thereby offering additional evidence for covalent bonding.

Phosphirenium-borate zwitterion: formation in the 1,1-carboboration reaction of phosphinylalkynes.

Reaction of the acetylene Mes(2)P-C≡C-Ar with B(C(6)F(5))(3) at rt gives a zwitterionic phosphirenium product, which reacts further at >100 °C to complete the 1,1-carboboration reaction.

P-C bond activation chemistry: evidence for 1,1-carboboration reactions proceeding with phosphorus-carbon bond cleavage.

A series of diarylphosphinyl-substituted acetylenes of the type (aryl)(2)P-C≡C-R (aryl = phenyl or mesityl, R = Ph or n-propyl) react with the strongly Lewis acid reagent B(C(6)F(5))(3) in toluene at elevated temperatures (70-105 °C) to give the 1,1-carboboration products 4. Treatment of bis(diphenylphosphinyl)acetylene with B(C(6)F(5))(3) under analogous conditions proceeded with phosphinyl migration to yield the 1,1-carboboration product 4d, bearing a geminal pair of Ph(2)P substituents at one former acetylene carbon atom and a C(6)F(5) substituent and the remaining -B(C(6)F(5))(2) group at the other. Prolonged thermolysis of 4d resulted in an intramolecular aromatic substitution reaction by means of Ph(2)P attack on the adjacent C(6)F(5) ring to yield the zwitterionic phospha-indene derivative 7. The compounds 4a, 4c, 4d, and 7 were characterized by X-ray diffraction.