Tridentate κ3-P,P,C iridium complexes: influence of ligand saturation on intramolecular C-H bond activation

Whereas κ3-P,C,P-based donor ligands are ubiquitous across synthetic chemistry, related unsymmetric systems having κ3-P,P,C-coordination are comparatively rare. In this contribution, we expose a new κ3-P,P,C ligand system, bearing a C3-anionic linker and its coordination chemistry with iridium. The title ligand has been coordinated in saturated and unsaturated forms. The degree of ligand saturation affected an onward (and unusual) oxidative rearrangement reaction.

Ynone Co-Coordination at a Nickel Borane Complex: An Assessment of Secondary Coordination Sphere Effects

Secondary coordination sphere ligand effects can be used to direct or organize small molecule substrates at a metal center. Herein, we assess the bifunctional ambiphilic diphosphine, tri-tert-butylboranyldiphosphinoethane (ttbbpe) and its ability to influence stereoselective substrate coordination, while appended to nickel. This report takes a synthetic/computational approach to test the impacts and limitations associated with ligand-directed substrate coordination using [Ni(ttbbpe)(η2:η2-COD)] (COD = 1,5-cyclooctadiene) and ynones (alkynes having an α-carbonyl group at the propargylic position) as model substrates.

An Investigation of Allyl-Substituted Bis(Diphosphine) Iron Complexes: Towards Precursors for Cooperative CO2 Activation

In developing homogenous catalysts capable of CO2 activation, interaction with a metal center is often imperative. This work provides primary efforts towards the cooperative activation of CO2 using a Lewis acidic secondary coordination sphere (SCS) and iron via a paired theoretical/experimental approach. Specifically, this study reports efforts towards [Fe(diphosphine)2 (N2 )] as a CO2 -coordinated synthon where diphosphine=1,2-bis(di(3-cyclohexylboranyl)propylphosphino)ethane) (P2 BCy 4 ) or its precursor, 1,2-bis(diallylphosphino)ethane (tape). Initial efforts toward the {Fe(0)-N2 } complex were focused on deprotonation reactions of [Fe(diphosphine)2 (H)(NCCH3 )]+ and reduction of [Fe(tape)2 Cl2 ]. In the latter case, a mixture of intramolecularly π-bonded alkene and associated metallacyclic Fe(II)-H species were produced - heating this mixture provided the hydride as the major product. Notably, the interconversion of this pair counters that of related intermolecular reactions between [Fe(depe)2 ] (depe=1,2-bis(diethylphosphino)ethane) and ethylene, where hydride formation occurs subsequent to π-coordination; this has been probed by theoretical calculations. Finally, reactivity of the metallacyclic {Fe(II)-H} complex with CO2 was probed, resulting in a pair of isomeric ferra(II)lactones.

Late-stage Ligand Modification After Coordination Strengthens Stereoselectively Self-Assembled Hemiaminal Ether Complexes

Fragile hemiaminal ether linkages present in the backbone of koneramines (LR OR'), tridentate ligands, bound to copper(II) in stereoselectively self-assembled syn-[Cu(LR OR')X2 ] complexes were transformed into sturdy methylene linkages to make corresponding rac-[Cu(LR H)Cl2 ] complexes by late-stage ligand modification after coordination with the retention of coordination sphere. The generality of stereoselective self-assembly of koneramine complexes is shown by utilising a number of metal ions, anions, amines, alcohols and thiols with complete characterisations.

Reactions of nickel boranyl compounds with pnictogen-carbon triple bonds

The catalytic conversion of unsaturated small molecules such as nitriles into reduced products is of interest for the production of fine chemicals. In this vein, metal-ligand cooperativity has been leveraged to promote such reactivity, often conferring stability to bound substrate - a balancing act that may offer activation at the cost of turnover efficiency. This report describes the reactivity of a [(diphosphine)Ni] compound with pnictogen carbon triple bonds (R-C[triple bond, length as m-dash]E; E = N, P), where the diphosphine contains two pendant borane groups. For E = N, cooperative nitrile coordination is observed to afford {Ni}₂ complexes displaying B-N interactions, whereas for E = P, B-P interactions are absent. This work additionally outlines a structure-activity relationship that uses nitrile dihydroboration as a model reaction to unveil the effect of SCS stabilization, employing [(diphosphine)Ni] where the diphosphine contains 0, 1, or 2 pendant Lewis acid groups.

Secondary Coordination Sphere Alkylation Promotes Cyclometalation

Diphosphines have taken on a dominant role as supporting ligands in transition-metal chemistry. Here, we describe complexes of the type [Cp*Fe(diphosphine)(X)] (X = Cl, H) where for diphosphine = 1,2-bis(di-allylphosphino)ethane (tape), a Lewis-acidic secondary coordination sphere (SCS) was installed via allyl group hydroboration using dicyclohexylborane (HBCy₂). The resulting chloride complex, [Cp*Fe(P₂B^Cy₄)(Cl)] (P₂B^Cy₄ = 1,2-bis(di(3-cyclohexylboranyl)propylphosphino)ethane), was treated with n-butyllithium (1-10 equiv), resulting incyclometalation at iron. This reactivity is contrasted with [Cp*Fe(dnppe)(Cl)] (dnppe = 1,2-bis(di-n-propylphosphino)ethane), whereby addition of n-butyllithium provides a mixture of products. Overall, cyclometalation is a common elementary transformation in organometallic chemistry; here we describe how this outcome is accessed in response to Lewis acid SCS incorporation.

Cooperative Nitrile Coordination Using Nickel and a Boron-Containing Secondary Coordination Sphere

Metal-ligand cooperation has emerged as a versatile tool for substrate activation in chemical reactivity. Herein, we provide the synthesis and characterization of a monoboranyl-containing analogue of the ubiquitous bulky diphosphine ligand, 1,2-bis(di-tert-butylphosphino)ethane, whose reactivity has been examined using nickel. Together, the pairing of nickel and boron provides a platform that allows for the cooperative coordination of organonitriles, giving unusual examples of intermolecularly bound dinickelacycles.

On the Reactivity of Phosphaalumenes towards C-C Multiple Bonds

Heterocycles containing group 13 and 15 elements such as borazines are an integral part of organic, biomedical and materials chemistry. Surprisingly, heterocycles containing P and Al are rare. We have now utilized phosphaalumenes in reactions with alkynes, alkenes and conjugated double bond systems. With sterically demanding alkynes 1,2-phosphaalumetes were afforded, whereas the reaction with HCCH or HCCSiMe₃ gave 1,4-phosphaaluminabarrelenes. Using styrene saturated 1,2-phosphaalumates were formed, which reacted further with additional styrene to give different regio-isomers of 1,4-aluminaphosphorinanes. Using ethylene, a 1,4-aluminaphosphorinane is obtained, while with 1,3-butadiene a bicyclic system containing an aluminacyclopentane and a phosphirane unit was synthesized. The experimental work is supported by theoretical studies to shed light on the mechanism governing the formation of these heterocycles.

A Borane Lewis Acid in the Secondary Coordination Sphere of a Ni(II) Imido Imparts Distinct C-H Activation Selectivity

Two borane-functionalized bidentate phosphine ligands that vary in tether length have been prepared to examine cooperative metal-substrate interactions. Ni(0) complexes react with aryl azides at low temperatures to form structurally unusual κ2-(N,N)-N3Ar adducts. Warming these adducts affords products of N2 extrusion and in one case, a Ni-imido compound that is capped by the appended borane. Reactions with 1-azidoadamantane (AdN3) provide a distinct outcome, where a proposed nickel imido intermediate activates the sp2 C-H bonds of arenes, even in the presence of benzylic C-H sites. Combined experimental and computational mechanistic studies demonstrate that the unique reactivity is a consequence of Lewis-acid-induced polarization of the Ni-NR bond, potentially providing a synthetic strategy for chemoselective reaction engineering.