Triphos (Phosphorous-centered linear ligand) | CAS 23582-02-7 | ≥97%

Bis(2-diphenylphosphinoethyl)phenylphosphine (linear triphos, etp; PhP(CH2CH2PPh2)2) is a tridentate P,P,P-phosphine ligand in which a central PhP apex is connected through two ethane-1,2-diyl linkers to two terminal -PPh2 donors, giving an open chain of three phosphorus atoms. The two -CH2CH2- arms confer conformational flexibility, so that the ligand can bind through one, two or all three phosphorus donors depending on metal, oxidation state and coligands, and in the tridentate mode can adopt either a facial or a meridional geometry at the metal centre. This contrasts with the carbon-centered tripodal congener 1,1,1-tris(diphenylphosphinomethyl)ethane (CAS 22031-12-5), whose neopentyl-type apex more strongly preorganises its three donor arms toward facial tripodal binding, and which is the ligand used in the well-known Ru/Triphos hydrogenation systems developed by the Leitner and Klankermayer groups for CO2-to-methanol catalysis, CO2-based N-methylation of amines, and the hydrogenation of carboxylic acids, esters, amides and lactones. Reported complexes of the linear ligand include Mn, Re, Fe, Co, Ni, Mo, W, Ru, Os, Rh, Ir, Pd and Pt systems, with particularly strong recent anchoring in base-metal Co and Ni homogeneous catalysis. As with all tertiary aryl phosphines, slow oxidation to phosphine oxide occurs on prolonged air exposure; for catalytic use it is best handled and stored under inert atmosphere.

Base-metal reductive amination and nitroarene hydrogenation (Co, Ni). Linear triphos is a privileged tridentate ligand for earth-abundant 3d-metal catalysed reductive amination. Co(BF4)2·6H2O / linear-triphos systems are proposed to operate via cationic [(triphos)CoH]+ intermediates and catalyse the reductive amination of aldehydes and ketones with gaseous ammonia and molecular H2 to functionalised linear and branched benzylic, heterocyclic and aliphatic primary amines. Closely related Ni(BF4)2·6H2O / linear-triphos systems are reported as the first homogeneous Ni-based catalysts for both the reductive amination of aldehydes/ketones with NH3/H2 and the hydrogenation of nitroarenes to primary anilines, and for the reductive amination of γ-keto acids such as levulinic acid and 2-acetylbenzoic acid with primary amines.

Coordination chemistry and tridentate flexibility. The two ethano linkers allow the ligand to bind through one, two or three phosphorus donors and to switch between binding modes under thermolysis or substrate exchange. Mo(II) and W(II) carbonyl iodides illustrate this stepwise behaviour: equimolar reaction with [MI2(CO)3(NCMe)2] initially affords a κ2-P,P tricarbonyl complex, which converts intramolecularly on thermolysis to the seven-coordinate κ3-P,P,P dicarbonyl complex, a pathway exploiting the conformational mobility of the ethano linkers. Well-defined Mn(I), Re(I), Fe(0/II), Co(I/II/III), Ni(0/II), Ru(II), Rh(I/III) and Ir(I/III) complexes are documented, including dihalide, dihydride, hydride-carbonyl and trihydride species, with mer- and fac-isomers providing distinct trans-influence templates for mechanistic studies of oxidative addition, reductive elimination and substrate binding.

Ruthenium hydrogenation and cobalt electrocatalysis. Ru(II)/linear-triphos complexes such as RuCl2(linear-triphos) are reported for homogeneous hydrogenation and isomerisation of olefins and for hydrogenation of alkynes, aldehydes and ketones under moderate conditions. Cationic [(CH3CN)2Co(linear-triphos)](BF4)2 electrocatalytically oxidises benzyl alcohol to benzaldehyde in the presence of a Brønsted base such as N,N-diisopropylethylamine, with kinetic and DFT studies supporting an alcohol-binding pre-equilibrium followed by deprotonation and Co(II)/Co(I) cycling.