Phosphorus(III) Ligands Based On The P-N-P Skeleton And Their Transition Metal Complexes

Abstract

There is considerable current interest in the design and synthesis of new phosphorus ligands and their transition metal complexes in view of their potential applications in homogeneous catalysis. The present study is concerned with the synthesis of new chiral and achiral “ diphosphazanes”, which constitute a class of versatile short-bite bidentate phosphine ligands, and studying their reactivity towards late transition metals (Ag, Pd and Ru). Symmetrical diphosphazane ligands, MeN{P(OR)2}2 (R = (1R, 2S, 5R)-menthyl) and MeN{P(SR)2}2, (R = C6H5) and unsymmetrical diphosphazane ligands, Ph2PN(Pri)PPhY, (Y =OC6H3Me2-2,6 or NMePh) have been synthesized and structurally characterized. The reactivity of these ligands towards the transition metal precursors viz., [PdCl2(COD)] and [CpRu(PPh3)2Cl] has been investigated. The reaction of [Ru(bipy)2Cl2] with the diphosphazane, PriN(PPh2)2 in the presence of AgOTf to synthesize [Ru(bipy)2{PriN(PPh2)2}2]OTf led to an unexpected entry into the Ag(I) chemistry of this ligand. By optimizing the reaction conditions, several mononuclear, dinuclear and trinuclear complexes such as [Ag(K2-PriN(PPh2)2)2]X, [Ag(µ-PriN(PPh2)2)X]2 and [Ag3(µ-(Cl)2(µ-PriN(PPh2)2)3]X (X = NO3, OTf or PF6) have been synthesized. A polymeric complex, [Ag2(µ-PriN(PPh2)2)( µ-NO3)2]n in which the ligand adopts a unique ‘Cs’ geometry has also beenstructurally characterized. This polymeric complex is used to synthesize a helical polymer,[Ag2{µ-PriN(PPh2)2}(DABCO)(NO3)2]n and π- π stacked supramolecular assemblies such as [Ag2(NO3)2(µ-Ph2PN(Pri)PPh2)(2,2'-bipy)2] and [Ag2{µ-PriN(PPh2)2}(1,10-phen)2](NO3)2]. The reaction of a sterically bulky diphosphazane ligand, EtN{P(OC6H3(Pri)2-2,6)2}2 (L) with[(η3-1-R,R’-C3H3)Pd(µ-Cl)]2 in the presence of NH4PF6 gives the cationic complex, [(η 3-1-R,R’-C3H3)Pd(L)]PF6 (R = H; R’= H or Me) as the sole product. In the absence of NH4PF6, theinitially formed cationic complex, [(η 3-C3H5)Pd(L)]PF6 is transformed into a mixture of chlorobridged complexes over a period of 96 h. An octa-palladium complex [(η3-C3H5)(2-Cl- η3-C3H4)Pd4(µ-Cl)4(µ-L)]2 is formed as a result of nucleophilic substitution by a chloride ligand at the central allyl carbon atom. The reaction of L with [(η3-C3H5)Pd(µ-Cl)]2 in the presence of K2CO3 yields a dinuclear complex, [(η3-C3H5)Pd2(µ-L)Cl] containing a coordinatively unsaturated T-shaped palladium center. This complex exhibits high catalytic activity and large“turn-over numbers” in the catalytic hydrophenylation of norbornene. Reactions of diphosphazanes with cyclometalated palladium complexes of the general formula [Pd( k2-(C,N)-Me2NCHMe(C6H4))(solvent)2]PF6 derived from a chiral amine, (S)-N,N-dimethyl-1-phenethylamine give chelate complexes of the type [Pd{ k2-(C,N)-Me2NCHMe(C6H4)}(LL)] PF6, (L-L = diphosphazane). Chiral racemic diphosphazanes give a mixture of diastereomeric(S,R and S,S) complexes which could not be separated. These cyclometalated complexes show moderate catalytic activity in C−C bond forming reactions (hydrophenylation /Suzuki coupling).