The protonation state governs the coordination of phosphinoferrocene guanidines

Abstract

Compared to phosphines with guanidinium tags, studied as polar ligands for aqueous catalysis, their counterparts bearing guanidine substituents received only limited attention. This contribution focuses on the coordination of phosphinoferrocene guanidine Ph₂PfcNC(NHiPr)₂ (1^iPr, fc = ferrocene-1,1′-diyl) as a hybrid, P,N-donor ligand to Group 10 metals. In its native state, 1^iPr coordinated as a P,N-chelating ligand, affording [M(X)(Y)(1^iPr-κ²P,N)] (M/X/Y = Pd/Cl/Cl, Pd/Br/4-C₆H₄CN, Pt/Cl/Cl; the corresponding Ni(II) complex was not isolated). While [PdCl₂(1^iPr-κ²P,N)] converted into [PdCl(1^iPr-κ³Fe,P,N)]⁺ species with Fe–Pd interaction, upon chloride removal, the analogous Pt(II) complex dimerised into [Pt₂(μ-Cl)₂(1^iPr-κ²P,N)₂]²⁺. Deprotonation of [PdCl₂(1^iPr-κ²P,N)] produced a unique, doubly chelating phosphinoguanidinate complex [PdCl{(1^iPr–H)-κ³P,N,N′}], which was smoothly converted into [Pd(MeCN){(1^iPr–H)-κ³P,N,N′}][SbF₆]. The latter, a convenient starting material for substitution reactions, was used to prepare either [Pd(L){(1^iPr–H)-κ³P,N,N′}][SbF₆] (L = 4-(dimethylamino)pyridine and 2-phenylpyridine), by simple substitution, or the hydroxide and acetylacetonate (acac) complexes, [Pd₂(μ-OH)₂(1^iPr-κ²P,N)₂][SbF₆]₂ and [Pd(acac)(1^iPr-κ²P,N)][SbF₆], by substitution with concomitant proton transfer. In contrast, protonation of the guanidine moiety prevented its coordination, as shown in reactions of the salts (1^iPrH)Cl and (1^iPrH)[SbF₆]. Depending on the metal-to-ligand ratio, adding (1^iPrH)[SbF₆] to [PdCl₂(MeCN)₂] produced [Pd₂Cl₂(μ-Cl)₂(1^iPrH-κP)₂][SbF₆]₂ or [PdCl₂(1^iPrH-κP)₂][SbF₆]₂. Analogous reactions involving (1^iPrH)Cl were more complicated due to competing coordination of the chloride anion, leading to (in addition to other compounds) the zwitterionic complex [PdCl₃(1^iPrH-κP)], which was alternatively obtained by selective protonation of [PdCl₂(1^iPr-κ²P,N)] with HCl. Apparently, the protonation state of the guanidine moiety controls the coordination behaviour of phosphinoferrocene guanidines.