Coordinating Anions: (Phosphino)tetraphenylborate Ligands as New Reagents for Synthesis

Coinage Metal Complexes of a Sterically Encumbered Anionic Pyridylborate

Sterically loaded, anionic pyridine has been synthesized and utilized successfully in the stabilization of a isoleptic series of coinage metal complexes. The treatment of [4-(Ph3B)-2,6-Trip2Py]K (Trip=2,4,6-iPr3C6H2) with CuBr(PPh3), AgCl(PPh3) or AuCl(PPh3) (Py=pyridine) afforded the corresponding [4-(Ph3B)-2,6-Trip2Py]M(PPh3) (M=Au, Ag, Cu) complexes, via salt metathesis, as isolable, crystalline solids. Notably, these reactions avoid the facile single electron transfer chemistry reported with the less bulky ligand systems. The X-ray structures revealed that they are two-coordinate metal adducts. The M-N and M-P bond distances are longest in the silver and shortest in the copper adduct among the three group 11 family members. Computational analysis revealed an interesting stability dependence on steric bulk of the anionic pyridine (i. e., pyridyl borate) ligand. A comparison of structures and bonding of [4-(Ph3B)-2,6-Trip2Py]Au(PPh3) to pyridine and m-terphenyl complexes, {[2,6-Trip2Py]Au(PPh3)}[SbF6] and [2,6-Trip2Ph]Au(PPh3) are also provided. The Au(I) isocyanide complex, [4-(Ph3B)-2,6-Trip2Py]Au(CNBut) has been stabilized using the same anionic pyridylborate illustrating that it can support other gold-ligand moieties as well.

Electrostatic vs. inductive effects in phosphine ligand donor properties and reactivity

Enhanced rates and selectivity in enzymes are enabled in part by precisely tuned electric fields within active sites. Analogously, the use of charged groups to leverage electrostatics in molecular systems is a promising strategy to tune reactivity. However, separation of the through space and through bond effects of charged functional groups is a long standing challenge that limits the rational application of electric fields in molecular systems. To address this challenge we developed a method using the phosphorus selenium coupling value (J _P-Se) of anionic phosphine selenides to quantify the electrostatic contribution of the borate moiety to donor strength. In this analysis we report the synthesis of a novel anionic phosphine, PPh₂CH₂BF₃K, the corresponding tetraphenyl phosphonium and tetraethyl ammonium selenides [PPh₄][SePPh₂CH₂BF₃] and [TEA][SePPh₂CH₂BF₃], and the Rh carbonyl complex [PPh₄][Rh(acac)(CO)(PPh₂(CH₂BF₃))]. Solvent-dependent changes in J _P-Se were fit using Coulomb's law and support up to an 80% electrostatic contribution to the increase in donor strength of [PPh₄][SePPh₂CH₂BF₃] relative to SePPh₂Et, while controls with [TEA][SePPh₂CH₂BF₃] exclude convoluting ion pairing effects. Calculations using explicit solvation or point charges effectively replicate the experimental data. This J _P-Se method was extended to [PPh₄][SePPh₂(2-BF₃Ph)] and likewise estimates up to a 70% electrostatic contribution to the increase in donor strength relative to SePPh₃. The use of PPh₂CH₂BF₃K also accelerates C-F oxidative addition reactivity with Ni(COD)₂ by an order of magnitude in comparison to the comparatively donating neutral phosphines PEt₃ and PCy₃. This enhanced reactivity prompted the investigation of catalytic fluoroarene C-F borylation, with improved yields observed for less fluorinated arenes. These results demonstrate that covalently bound charged functionalities can exert a significant electrostatic influence under common solution phase reaction conditions and experimentally validate theoretical predictions regarding electrostatic effects in reactivity.

Fusing triphenylphosphine with tetraphenylborate: introducing the 9-phosphatriptycene-10-phenylborate (PTB) anion

In a fusion of two ubiquitous organometallic reagents, triphenylphosphine (PPh3) and tetraphenylborate (BPh4-), the 9-phosphatriptycene-10-phenylborate (PTB) anion has been prepared for the first time. This borato species has been fully characterized by a suite of spectroscopic methods, and initial reactivity studies introduce it as a competent ligand for transition metals, including Co(ii) and Fe(ii).

Investigation of interactions in Lewis pairs between phosphines and boranes by analyzing crystal structures from the Cambridge Structural Database

The interactions between phosphines and boranes in crystal structures have been investigated by analyzing data from the Cambridge Structural Database (CSD). The interactions between phosphines and boranes were classified into three types; two types depend on groups on the boron atom, whereas the third one involves frustrated Lewis pairs (FLPs). The data enabled geometric parameters in structures to be compared with phosphine-borane FLPs with classical Lewis pairs. Most of the crystal structures (78.1%) contain BH₃ as the borane group. In these systems, the boron-phosphorus distance is shorter than systems where the boron atom is surrounded by groups other than hydrogen atoms. The analysis of the CSD data has shown that FLPs have a tendency for the longest boron-phosphorus distance among all phosphine-borane pairs, as well as different other geometrical parameters. The results show that most of the frustrated phosphine-borane pairs found in crystal structures are bridged ones. The minority of non-bridged FLP structures contain, beside phosphorus and boron atoms, other heteroatoms (O, N, S for instance).

The reactivity patterns of low-coordinate iron-hydride complexes

We report a survey of the reactivity of the first isolable iron-hydride complexes with a coordination number less than 5. The high-spin iron(II) complexes [(beta-diketiminate)Fe(mu-H)] 2 react rapidly with representative cyanide, isocyanide, alkyne, N 2, alkene, diazene, azide, CO 2, carbodiimide, and Brønsted acid containing substrates. The reaction outcomes fall into three categories: (1) addition of Fe-H across a multiple bond of the substrate, (2) reductive elimination of H 2 to form iron(I) products, and (3) protonation of the hydride to form iron(II) products. The products include imide, isocyanide, vinyl, alkyl, azide, triazenido, benzo[ c]cinnoline, amidinate, formate, and hydroxo complexes. These results expand the range of known bond transformations at iron complexes. Additionally, they give insight into the elementary transformations that may be possible at the iron-molybdenum cofactor of nitrogenases, which may have hydride ligands on high-spin, low-coordinate metal atoms.

Bio-inspired, side-on attachment of a ruthenium photosensitizer to an iron hydrogenase active site model

The first ruthenium-diiron complex [(mu-pdt)Fe2(CO)5{PPh2(C6H4CCbpy)}Ru(bpy)2]2+ 1 (pdt = propyldithiolate, bpy = 2,2'-bipyridine) is described in which the photoactive ruthenium trisbipyridyl unit is linked to a model of the iron hydrogenase active site by a ligand directly attached to one of the iron centers. Electrochemical and photophysical studies show that the light-induced MLCT excited state of the title complex is localized towards the potential diiron acceptor unit. However, the relatively mild potential required for the reduction of the acetylenic bipyridine together with the easily oxidized diiron portion leads to a reductive quenching of the excited state, instead. This process results in a transiently oxidized diiron unit which may explain the surprisingly high light sensitivity of complex 1.

Sterically hindered and robust pnictogen ligands derived from carboranes: synthesis and X-ray structure determination of tris(1'-methyl(1,2-dicarba-closo-dodecaboran-1-yl))phosphine, tris(1'-methyl(1,2-dicarba-closo-dodecaboran-1-yl))arsine and chloro(tris(1'-methyl(1,2-dicarba-closo-dodecaboran-1-yl))phosphine)gold(I)

Sterically hindered phosphine and arsine ligands derived from ortho-carborane were synthesized and characterized by X-ray crystallography. Tris(1'-methyl(1,2-dicarba-closo-dodecaboran-1-yl))phosphine, 2 (crystal data, hexagonal, space group P6(3), a = b = 12.251(3) A, c = 11.514(4) A, alpha = beta = 90 degrees, gamma = 120 degrees, V = 1496.6(7) A(3), Z = 2, R(1) = 0.0568) and tris(1'-methyl(1,2-dicarba-closo-dodecaboran-1-yl))arsine, 3 (crystal data, hexagonal, space group P6(3), a = b = 12.330(3) A, c = 11.474(4) A, alpha = beta = 90 degrees, gamma = 120 degrees, V = 1510.7(7) A(3), Z = 2, R(1) = 0.0930) were prepared in 82% and 68% yield, respectively. The phosphine ligand is resistant to air-oxidation but was converted to corresponding oxide when heated with hydrogen peroxide. The tertiary carboranyl phosphine reacted with (Tht)AuCl to yield chloro(tris(1'-methyl(1,2-dicarba-closo-dodecaboran-1-yl))phosphine)gold(I), 4 (crystal data, monoclinic, space group P2(1)/c, a = 19.101(4) A, b = 12.167(2) A, c = 13.846(3) A, alpha = gamma = 90 degrees, beta = 91.13(3) degrees, V = 3217.2(11) A(3), Z = 4, R(1) = 0.0396) in 82% yield. From the X-ray structure of the gold complex, the cone angle of the phosphine was determined to be 213(2) degrees, which is among the largest values reported to date.