Air‐Stable Platinum and Palladium Complexes Featuring Bis[2,4‐bis(trifluoromethyl)phenyl]phosphinous Acid Ligands

Abstract

Secondary phosphane oxides, R(2)P(O)H, are commonly used as preligands for transition-metal complexes of phosphinous acids, R(2)P-OH (R=alkyl, aryl), which are relevant as efficient catalysts in cross-coupling processes. In contrast to previous work by other groups, we are interested in the ligating properties of an electron-deficient phosphinous acid, (R(f))(2)P-OH, bearing the strongly electron-withdrawing and sterically demanding 2,4-bis(trifluoromethyl)phenyl group towards catalysis-relevant metals, such as palladium and platinum. The preligand bis[2,4-bis(trifluoromethyl)phenyl]phosphane oxide, (R(f))(2)P(O)H, reacts smoothly with solid platinum(II) dichloride yielding the trans-configured phosphinous acid platinum complex trans-[PtCl(2)({2,4-(CF(3))(2)C(6)H(3)}(2)POH)(2)]. The deprotonation of one phosphinous acid ligand with an appropriate base leads to the cis-configured monoanion complex cis-[PtCl(2)({2,4-(CF(3))(2)C(6)H(3)}(2)PO)(2)H](-), featuring the quasi-chelating phosphinous acid phosphinito unit, (R(f))(2)P-O-H···O=P(R(f))(2), which exhibits a strong hydrogen bridge substantiated by an O···O distance of 245.1(4) pm. The second deprotonation step is accompanied by a rearrangement to afford the trans-configured dianion trans-[PtCl(2)({2,4-(CF(3))(2)C(6)H(3)}(2)PO)(2)](2-). The reaction of (R(f))(2)P(O)H with solid palladium(II) dichloride initially yields a mononuclear palladium complex [PdCl(2)({2,4-(CF(3))(2)C(6)H(3)}(2)POH)(2)], which condenses under liberation of HCl to the neutral dinuclear palladium complex [Pd(2)(μ-Cl)(2){({2,4-(CF(3))(2)C(6)H(3)}(2)PO)(2)H}(2)]. The equilibrium between the mononuclear [PdCl(2)({2,4-(CF(3))(2)C(6)H(3)}(2)POH)(2)] and dinuclear [Pd(2)(μ-Cl)(2){({2,4-(CF(3))(2)C(6)H(3)}(2)PO)(2)H}(2)] palladium complexes is reversible and can be shifted in each direction by the addition of base or HCl, respectively. Treatment of palladium(II) hexafluoroacetylacetonate, [Pd(F(6)acac)(2)], with a slight excess of (R(f))(2)P(O)H yields the complex [Pd(F(6)acac)({2,4-(CF(3))(2)C(6)H(3)}(2)PO)(2)H]. The quasi-chelating phosphinous acid phosphinito unit, which is formed by the liberation of HF(6)acac, is characterized by a O···O distance of 244.1(3) pm. These transition metal complexes are stable towards air and moisture and can be stored for months without any evidence of decomposition.

Crystal structure of bis(μ 2-chlorido)-bis(di-p-tolylhydroxyphosphine-κP)-bis(di-p-tolylphosphite-κP)dipalladium(II), C56H58Cl2O4P4Pd2

“…The dimerized title complex consists of a di-nuclear chlorido-bridged palladium entity consisting of hydrogen bridged through the oxygen atoms of the di-p-tolylphosphite groups on both phorsphorous atoms. The average O· · · O and P-O distances are 2.4057(27) Å and 1.546(2) Å respectively and are comparable to other similar compounds found in literature [7][8][9][10][11]. The asymmetric unit contains a monomer of the title complex while the other monomer is generated through the (1−x, 2−y, 1−z) symmetry.…”

Transition Metal Complexes of Phosphinous Acids Featuring a Quasichelating Unit: Synthesis, Characterization, and Hetero-bimetallic Complexes

“…A similar compound, featuring 2,4-bis(trifluoromethyl)phenyl substituents at the phosphorus atom, was published in 2011. 9 The bond lengths are virtually identical.…”

“…The reaction of (C 2 F 5 ) 2 8a,8b Transition metal complexes of phosphinous acids with the electron-withdrawing group 2,4-(CF 3 ) 2 C 6 H 3 exhibit an impressive catalytic activity in Suzuki type reactions. 9 The present paper describes the rich coordination chemistry of bis(perfluoroalkyl)phosphinous acids. Efficient syntheses of mononuclear complexes featuring a quasichelating phosphinic acid phosphinito unit, [R f 2 PO···H···OPR f 2 ] − , will be described.…”

Coordination chemistry and catalysis with secondary phosphine oxides

“…161 The whole set of compounds was successfully applied in Suzuki-Miyaura crosscoupling reactions (Scheme 98) displaying good conversions with a wide range of electron-deficient and electron-rich aryl iodides, bromides and chlorides. 163 In order to study the influence of electron-deficient substituents on the catalytic activity of these processes, Hoge and co-workers 164,165 and their applications in this type of reactions has recently appeared. 169 Some described complexes contain a bridging-acetato ligand between the two Pd(II) centres and the optically pure t BuPhP(O)H moiety (Scheme 54).…”