Intriguing Behavior of an Apparently Simple Coupling Promoter Ligand, PPh₂(p-C₆H₄–C₆F₅), in Their Pd Complexes

Abstract

is an example of monodentate biphenyl phosphine that allows for cis coordination of two phosphines to Pd, as in complex cis-[Pd(C 6 F 5) 2 Pd(L HF) 2 ] (A). At 25 °C complex A undergoes easy reductive elimination to decafluorobiphenyl and, competitively, isomerizes to trans-[Pd(C 6 F 5) 2 Pd(L HF) 2 ] via a 3-coordinated intermediate [Pd(C 6 F 5) 2 Pd(L HF)]. Unexpectedly the direct reductive C 6 F 5-C 6 F 5 elimination is faster from the 4-coordinated complex A than from the 3-coordinated intermediate. The reason of this is that two cis L HF ligands play the role of a chelate with a large bite angle with tetrahedral distortion. As a matter of fact, using L HF in excess gives ΔG ‡ Pf-Pf = 23.1 kcal.mol-1 , which ranks its efficiency for coupling and formation of the corresponding Pd 0 catalyst as better than XantPhos or PhPEWO-F, and about the same as t BuBrettPhos. On the other hand, complex (µ-Cl) 2 [Pd 2 (C 6 F 5) 2 (L HF) 2 ] (B), obtained by reaction of (µ-Cl) 2 [Pd 2 (C 6 F 5) 2 (tht) 2 ] with L HF , presents in the 19 F NMR COSY spectrum a very intriguing through space coupling pattern of the F ortho atoms of the C 6 F 5 group in L HF and the C 6 F 3 Cl 2-3,5 group on Pd. The intermitent coupling mechanism proposed is based on the switching of π-π stacking of C 6 F 5 with one or another Ph group of L HF , which gives rise to enantiomers at the chiral P atom. Rotation around the P-biphenyl bond under hindrance of rotation around the CC 6 F 5 bond produces the intriguing selective coupling observed.

Ranking Ligands by Their Ability to Ease (C6F5)2NiIIL → Ni0L + (C6F5)2 Coupling versus Hydrolysis: Outstanding Activity of PEWO Ligands

“…This suggests that the PR 2 (biaryl) Ni complexes are behaving in cis -[Ni­(C 6 F 5 ) 2 {PR 2 (biaryl)}] or cis -[Ni­(C 6 F 5 ) 2 {PR 2 (biaryl)}­(THF)] as bulky monodentate ligands, allowing for easy coordination of water and hydrolysis. As an exception, fluorinated biaryl phosphine 13 in solution forms a mixture of cis -[Ni­(C 6 F 5 ) 2 {PR 2 (biaryl)}­(THF)] and some trans -[Ni­(C 6 F 5 ) 2 {PR 2 (biaryl)} 2 ] (entries 12 and 13), the latter with two P-coordinated phosphines …”

“…As an exception, fluorinated biaryl phosphine 13 in solution forms a mixture of cis-[Ni(C 6 F 5 ) 2 {PR 2 (biaryl)}(THF)] and some trans-[Ni(C 6 F 5 ) 2 {PR 2 (biaryl)} 2 ] (entries 12 and 13), the latter with two P-coordinated phosphines. 28 In the lack of access to other experimental information, we performed DFT calculations on the stabilization of the potential Pd and Ni complexes formed by reaction of one molecule of the ligand JohnPhos (10) to complex [M-(C 6 F 5 ) 2 (THF) 2 ] (M = Pd and Ni), taking as the starting complex in each case zero energy. The results in Table 2 show that in dichloromethane the replacement of one or the two THF ligands in Pd produces more stable complexes, supporting plausibility of chelation along the coupling evolution.…”

“…The recently reported fluorinated biaryl ligand 13, 28 where the fluorinated aryl might somehow resemble an electron withdrawing olefin (see later), was also tested. Its reactivity was slow and produced somewhat better but still very unsatisfactory coupling/hydrolysis ratio (entries 12-13).…”

Fluorinated vs Nonfluorinated PR2(biaryl) Ligands and Their [AuCl(L)] Complexes: Synthesis, X-ray Structures, and Computational Study of Weak Interactions. Bond, No Bond, and Beyond

“…The new ligands L 1 – L 6 were obtained by lithiation of the corresponding biaryl bromide with Li­( n -Bu) followed by reaction with PR 2 Cl (R = Ph, Cy), as shown in Scheme . The addition of the L n ligand to [AuCl­(tht)] (tht = tetrahydrothiophene) affords the colorless complexes [AuCl­( L n )] ( 1 – 6 ), labeled according to the n number of the ligand.…”

Experimental study of the [ZnCl2(THF)2] catalyzed cis/trans-isomerization of [Pd(C6Cl2F3)Me(PPh3)2] and of the transmetalation of trans-[PdCl(C6Cl2F3)(PPh3)2] with [ZnMeCl(THF)2]

“…This mechanism seems to be quite general for palladium(II) and platinum(II) organometallics. [34][35][36][37][38] However the catalysis is often carried out under an excess of phosphine in order to prevent the decomposition of Pd(0) intermediates, so this pathway becomes inaccessible under catalytic conditions.…”