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 Ph(2)PfcNC(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)-κ(2)P,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)-κ(2)P,N)] converted into [PdCl(1(iPr)-κ(3)Fe,P,N)](+) species with Fe-Pd interaction, upon chloride removal, the analogous Pt(II) complex dimerised into [Pt-2(μ-Cl)(2)(1(iPr)-κ(2)P,N)(2)](2+).
Deprotonation of [PdCl2(1(iPr)-κ(2)P,N)] produced a unique, doubly chelating phosphinoguanidinate complex [PdCl{(1(iPr)-H)-κ(3)P,N,N'}], which was smoothly converted into [Pd(MeCN){(1(iPr)-H)-κ(3)P,N,N'}][SbF6]. The latter, a convenient starting material for substitution reactions, was used to prepare either [Pd(L){(1(iPr)-H)-κ(3)P,N,N'}][SbF6] (L = 4-(dimethylamino)pyridine and 2-phenylpyridine), by simple substitution, or the hydroxide and acetylacetonate (acac) complexes, [Pd-2(μ-OH)(2)(1(iPr)-κ(2)P,N)(2)][SbF6](2) and [Pd(acac)(1(iPr)-κ(2)P,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(iPr)H)Cl and (1(iPr)H)[SbF6]. Depending on the metal-to-ligand ratio, adding (1(iPr)H)[SbF6] to [PdCl2(MeCN)(2)] produced [Pd2Cl2(μ-Cl)(2)(1(iPr)H-κP)(2)][SbF6](2) or [PdCl2(1(iPr)H-κP)(2)][SbF6](2).
Analogous reactions involving (1(iPr)H)Cl were more complicated due to competing coordination of the chloride anion, leading to (in addition to other compounds) the zwitterionic complex [PdCl3(1(iPr)H-κP)], which was alternatively obtained by selective protonation of [PdCl2(1(iPr)-κ(2)P,N)] with HCl. Apparently, the protonation state of the guanidine moiety controls the coordination behaviour of phosphinoferrocene guanidines.