Jason D. Masuda

Reversible, Metal-Free Hydrogen Activation

Although reversible covalent activation of molecular hydrogen (H2) is a common reaction at transition metal centers, it has proven elusive in compounds of the lighter elements. We report that the compound (C6H2Me3)2PH(C6F4)BH(C6F5)2 (Me, methyl), which we derived through an unusual reaction involving dimesitylphosphine substitution at a para carbon of tris(pentafluorophenyl) borane, cleanly loses H2 at temperatures above 100°C. Preliminary kinetic studies reveal this process to be first order. Remarkably, the dehydrogenated product (C6H2Me3)2P(C6F4)B(C6F5)2 is stable and reacts with 1 atmosphere of H2 at 25°C to reform the starting complex. Deuteration studies were also carried out to probe the mechanism.

Activation of SiH, BH, and PH Bonds at a Single Nonmetal Center

For many years, it was believed that only transition-metal centers could activate small molecules and enthalpically strong bonds. However, it has recently been shown that several nonmetallic systems are capable of some of these tasks. For example, stable singlet carbenes can activate CO, H2, [3b] and P4. [3c–e] Such reactions have long been known for transition metals. However, stable singlet carbenes can also activate NH3; [3b] a much more difficult task for transition metals. The oxidative addition of hydrosilanes, hydroboranes, and hydrophosphines at vacant coordination sites of transition metals are well-exemplified and are considered as key steps in the transition-metal-catalyzed hydrosilylation, hydroboration, and hydrophosphination of multiple bonds. Herein, we report the first examples of the activation of E H bonds (E=Si, B, P) at a single nonmetal center. On the basis of our successful results with H2, [3b] we began our study with the activation of Si H bonds. Indeed, silanes are similar to H2 in that they lack both nonbonding electron pairs and p electrons. They can bind to various metal centers to form stable Si H s complexes, which undergo subsequent oxidative addition. To test the possible activation of Si H bonds with carbenes, we treated the cyclic (alkyl)(amino)carbenes (CAACs) 1a and 1b with primary, secondary, and tertiary silanes. The addition of phenylsilane to 1a and 1b occurred readily at room temperature, and the corresponding adducts 2a,b were isolated in 91 and 83% yield, respectively (Scheme 1). As expected, in the case of the enantiomerically pure CAAC 1a, two diastereomers 2a,a’ were formed (in a 2:1 ratio), as shown by two singlets at d= 36.4 and 29.3 ppm in the Si NMR spectrum. The C NMR spectrum revealed the loss of the carbene signal and a new C H peak at d= 63.2 (2a) and 65.5 ppm (2b). The H NMR spectrum of the major isomer 2a revealed a pseudotriplet at d= 4.78 ppm (SiCH) and two doublets at d= 4.29 and 4.21 ppm corresponding to the diastereotopic hydrogen atoms of the SiH2 fragment. The structure of 2a was confirmed by X-ray crystallography (Figure 1, top), whereas the presence of a triplet at d= 4.53 ppm and a doublet at d= 4.08 ppm in the H NMR spectrum confirmed the identity of adduct 2b. CAACs 1a,b also reacted with (EtO)3SiH to afford 3a (d.r. 3:1) and 3b in 64 and 73% yield, respectively. However, when Ph2SiH2 was used, only the less bulky carbene 1b underwent insertion into the Si H bond (to give 4b in 65% yield), and a reaction time of 16 hours at 80 8C was necessary for the reaction to reach completion. Surprisingly, although it has been shown that, in contrast to CAACs, N-heterocyclic carbenes (NHCs) do not react with H2, [11] we found that imidazolidin-2-ylidene 5 also reacted at room temperature with phenylsilane to afford the Si H insertion product 6 in 88% yield (Figure 1, bottom). The formation of 6 raises the question of the mechanism of the activation of Si H bonds with carbenes. Why should NHCs react with silanes although they are inert towards hydrogen? The evident difference is the presence of low-lying vacant orbitals in silanes. In other words, the observed reactivity might be due to the Lewis acid character of silanes; indeed, several NHC–SiX4 adducts are known. [13]

A Lanthanide Phosphinidene Complex: Synthesis, Structure, and Phospha-Wittig Reactivity

The first lanthanide complex featuring a phosphinidene functional group has been prepared and isolated. Preliminary reactivity studies demonstrate that the lutetium(III) phosphinidene complex, [{2-(iPr2P)-4-Me-C6H3}2NLu]2(μ-PMes)2, behaves as a phospha-Wittig reagent with aldehydes and ketones to give the corresponding phosphaalkenes. Attempts to use the bulky phosphine H2P-2,4,6-tBu3-C6H2 to kinetically stabilize a terminal phosphinidene resulted in C−H activation of an ortho-tBu group and formation of a phosphaindole.

Preparation of a diphosphine with persistent phosphinyl radical character in solution: characterization, reactivity with O2, S8, Se, Te, and P4, and electronic structure calculations.

A new, easily synthesized diphosphine based on a heterocyclic 1,3,2-diazaphospholidine framework has been prepared. Due to the large, sterically encumbering Dipp groups (Dipp = 2,6-diisopropylphenyl) on the heterocyclic ring, the diphosphine undergoes homolytic cleavage of the P-P bond in solution to form two phosphinyl radicals. The diphosphine has been reacted with O(2), S(8), Se, Te, and P(4), giving products that involve insertion of elements between the P-P bond to yield the related phosphinic acid anhydride, sulfide/disulfide, selenide, telluride, and a butterfly-type perphospha-bicyclobutadiene structure with a trans,trans-geometry. All molecules have been characterized by multinuclear NMR spectroscopy, elemental analysis, and single-crystal X-ray crystallography. Variable-temperature EPR spectroscopy was utilized to study the nature of the phosphinyl radical in solution. Electronic structure calculations were performed on a number of systems from the parent diphosphine [H(2)P](2) to amino-substituted [(H(2)N)(2)P](2) and cyclic amino-substituted [(H(2)C)(2)(NH)(2)P](2); then, bulky substituents (Ph or Dipp) were attached to the cyclic amino systems. Calculations on the isolated diphosphine at the B3LYP/6-31+G* level show that the homolytic cleavage of the P-P bond to form two phosphinyl radicals is favored over the diphosphine by ~11 kJ/mol. Furthermore, there is a significant amount of relaxation energy stored in the ligands (52.3 kJ/mol), providing a major driving force behind the homolytic cleavage of the central P-P bond.

Synthesis, Characterization, and Electrochemical Studies of PPh3–n(dipp)n (dipp = 2,6-Diisopropylphenyl): Steric and Electronic Effects on the Chemical and Electrochemical Oxidation of a Homologous Series of Triarylphosphines and the Reactivities of the Corresponding Phosphoniumyl Radical Cations

Activation barriers to the electrochemical oxidation for the series PPh3-n(dipp)n (dipp = 2,6-diisopropylphenyl) in CH2Cl2/Bu4NPF6 were measured using large amplitude FT ac voltammetry. Increasing substitution across this series, which offers the widest range of steric requirements across any analogous series of triarylphosphines reported to date, increases the energetic barrier to electron transfer; values of 18, 24, and 25 kJ mol(-1) were found for compounds with n = 1, 2, and 3, respectively. These values are significantly greater than those calculated for outer sphere activation barriers, with deviations between observed and calculated values increasing with the number of dipp ligands. This suggests that the steric congestion afforded by these bulky substituents imposes significant reorganizational energy on the electron transfer processes. This is the first investigation of the effect of sterics on the kinetics of heterogeneous electron transfer across a structurally homologous series. Increased alkyl substitution across the series also increases the chemical reversibility of the oxidations and decreases the oxidation peak potentials. As the compounds for which n = 1 and 2 are novel, the synthetic strategies employed in their preparation are described, along with their full spectroscopic, physical, and crystallographic characterization. Optimal synthesis when n = 1 is via a Grignard reagent, whereas when n = 2 an aryl copper reagent must be employed, as use of a Grignard results in reductive coupling. Chemical oxidation studies were performed to augment the electrochemical work; the O, S, and Se oxidation products for the parent triarylphosphines for which n = 1 and 2 were isolated and characterized.

Photophysical, dynamic and redox behavior of tris(2,6-diisopropylphenyl)phosphine

The title phosphine, Dipp3P, was synthesized using an aryl copper reagent and the structure determined by X-ray crystallography (R = 2.94%): d(P–C) = 1.852(1) A, ∠C–P–C = 111.88(5)°. In hexane solution, the electronic spectrum displays 3 bands [326 (9.3), 254 (8.7), 205 (11.4) nm (log|e|)] and the fluorescence spectrum has a Stokes shift of 129 kJ mol−1. NMR: (δ) 31P = −49.7 ppm in solution and −49.5 in the solid (CP-MAS). Room temperature 1H and 13C spectra reflect D3 symmetry, changing below −30 °C to C3. A variable temperature NMR study provided an activation enthalpy of 49(±1) kJ mol−1 and entropy of 24–27(±5) J mol−1K−1. An energy surface calculation using HF/3-21G theory discovered a single low-energy path describing pyramidal inversion through a transition state that is close to D3 geometry. The B3LYP/6-31G(d) calculated barrier to planarization is 37.5 kJ mol−1. Voltammetric studies employing cyclic, rotating disk, steady state and Fourier Transform ac methods confirm a fully chemically reversible one-electron oxidation of Dipp3P to Dipp3P+˙ at +0.18 (CH3CN–nBu4NPF6) and +0.09 (CH2Cl2–nBu4NPF6) V vs. Fc+/0 (Fc = ferrocene). The diffusion coefficient for Dipp3P is 1.0–1.2 × 105 cm2 s−1. The electrode process displays quasi-reversible electron transfer kinetics [ks ≈ 0.01 (CH2Cl2) to 0.08 (CH3CN) cm s−1]. Optically transparent thin layer electrolysis reversibly generates Dipp3P+˙ in CH2Cl2–nBu4NPF6 [UV-Vis: 498 (3.31), 456 (3.29), 373 (4.04), 357 (3.84), 341 (3.49), 296 (3.78), 385 (3.91), 251 (3.99) nm (log|e|)]. The EPR spectrum of Dipp3P+˙ in solution is a doublet (a(P) = 23.9 mT, g = 2.008), and in frozen solution is axial (a∥ = 42.6 mT, g∥ = 2.0045; a⊥ = 12.7 mT, g⊥ = 2.0085 mT).

An N,P-disubstituted-2-aminophosphaalkene and lithium and potassium complexes of the deprotonated “phosphaamidinate” anion

The reaction of DippPH2(Dipp = 2,6-iPr2C6H3) with DippN=C(p-CH3C6H4)Cl in refluxing xylenes affords DippP=C(p-CH3C6H4)N(H)Dipp; deprotonation with alkali metal reagents produces unique lithium and potassium complexes with the ligand in a different geometry to that of the free phosphaamidine.

Lewis Base Stabilized Oxophosphonium Ions

The rich functional group chemistry of phosphorus has been exploited over the last several decades and even as of late, there have been major advances to the field. There have been regular contributions to the area of N-heterocyclic phosphenium ions A since the first reports 30 years ago. The area has seen a recent resurgence 9–23] which may be due to the popularity of the isovalent singlet carbenes and, as such, the use of phosphenium ions as ligands in transition metal chemistry has been exploited. In the context of main group chemistry, phosphenium ions have been involved in the preparation of phosphinophosphonium systems, as well in reactions with P4, [39,41–44] and other reagents.

1,3-Bis(2,6-diisopropyl­phen­yl)imidazolidin-2-yl­idene

The title compound, C27H38N2, is the first reported free imidazolidin-2-ylidene carbene with 2,6-diisopropylphenyl groups in the 1,3-positions. The five-membered ring adopts a twisted conformation and the dihedral angle between the aromatic rings is 48.81 (6)°. Both isopropyl groups attached to one of the benzene rings are disordered over two sets of sites in 0.74 (2):0.26 (2) and 0.599 (8):0.401 (8) ratios.

Hexaaqua­gallium(III) trinitrate trihydrate

The title compound, [Ga(H2O)6](NO3)3·3H2O, is isostructural to other known M III nitrate hydrates (M = Al, Cr, Fe). The structure contains two distinct octahedral Ga(OH2)6 units (each of symmetry) which are involved in intermolecular hydrogen bonding with the three nitrate anions and three water molecules within the asymmetric unit.