Versatile coordination and C-H activation of a multi-donor phosphinoferrocene carboxamide ligand in Pd(II) complexes
Abstract
A multi-donor phosphinoferrocene carboxamide, FcCONHCH(2)CH(2)PPh(2) (1, Fc = ferrocenyl), was prepared, converted into the corresponding phosphine oxide 1O and phosphine selenide 1Se and, mainly, studied as a ligand in Pd(II) complexes. In its native form, amide 1 preferentially coordinated soft Pd(II) as a simple phosphine, giving rise to mixtures of cis and trans-[PdX2(1-κP)(2)] (2; X = Cl (a), Br (b), and I (c)), wherein the isomer ratios depended on the auxiliary halide ligand or, alternatively, to the complex [(L-NC)PdCl(1-κP)] (6, L-NC = 2-[(dimethylamino)methyl-κN]phenyl- κC-1).
This coordination mode was nevertheless easily changed when creating a vacant coordination site at the palladium. Thus, treatment of 2a with NH4[PF6] in the presence of free 1 produced [PdCl(1-κP)(3)][PF6] (3), while complete halogen removal with a Ag(I) salt led to cationic complexes cis-[Pd(1-κ^2 O-2,P)(2)]X-2 (4, X = CF3SO3 (a), ClO4 (b), BF4 (c)) or [(L-NC)Pd(1-κ^2 O-2,P)]X (7a and 7b), containing seven-membered O,P-chelate rings.
In contrast, amide nitrogen deprotonation with KOt-Bu followed by spontaneous intramolecular halogen substitution resulted in the transformation of 6 into the chelate complex [(L-NC)Pd{(1 - H)-κ^2 N-2,P}] (8) featuring a five-membered N,P-chelate ring, and in the conversion of 2a and 2b into the product of C-H bond activation [Pd{Fe(η5-C5H3CONCH2CH2PPh2-κ^3 C-3,N,P)(η5-C5H5)}(1-κP)] (5), with doubly chelating deprotonated 1. Importantly, complexes 2-4-5 and 6-7-8 were mutually interconverted in triads (by protonation/deprotonation and by halide addition/abstraction), which highlights the flexible coordination and chemical stability of ligand 1.
The crystal structures of 1O.1/2H2O, trans-2a.MeCN, trans-2b.3C2H4Cl2, trans-2c.2.5C2H4Cl2, 4a.CH2Cl2, 5.3CHCl3Et(2)O, and 8 were determined by single-crystal X-ray diffraction analysis, and the representative compounds were studied by cyclic voltammetry.