Phenyl-Group Exchange in Triphenylphosphine Mediated by Cationic Gold–Platinum Complexes—A Gas-Phase Mimetic Approach

Abstract

The PPh 3 ligands in the heterodinuclear AuPt complex [(Ph 3 P)AuPt(PPh 3 ) 3 ][BAr 4 F ] (BAr 4 F = tetrakis[3,5bis(trifluoromethyl)phenyl]borate) exhibit a high fluxionality on the AuPt core. Fast intramolecular and slow intermolecular processes for the reversible exchange of the PPh 3 ligands have been identified. When [(Ph 3 P)AuPt(PPh 3 ) 3 ][BAr 4F ] is heated in solution, the formation of benzene is observed, and a trinuclear, cationic AuPt 2 complex is generated. This process is preceded by reversible phenyl-group exchange between the PPh 3 ligands present in the reaction mixture as elucidated by deuterium-labeling studies. Both the elimination of benzene and the preceding reversible phenyl-group exchange have originally been observed in massspectrometry-based CID experiments (CID = Collision-Induced Dissociation). While CID of mass-selected [Au,Pt,(PPh 3 ) 4 ] + results exclusively in the loss of PPh 3 , the resulting cation [Au,Pt,(PPh 3 ) 3 ] + selectively eliminates C 6 H 6 . Thus, the dissociation of a PPh 3 ligand from [Au,Pt,(PPh 3 ) 3 ] + is energetically not able to compete with processes which result in C−H-and C−P-bond cleavage. In both media, the heterobimetallic nature of the employed complexes is the key for the observed reactivity. Only the intimate interplay of the gas-phase investigations, studies in solution, and thorough DFT computations allowed for the elucidation of the mechanistic details of the reactivity of [(Ph 3 P)AuPt(PPh 3 ) 3 ][BAr 4 F ].

Investigations into the AuI and PtII Coordination Chemistry of Bidentate “cis‐Spanning” Ligands

“…While the observation of platinum satellites for both phosphorous atoms proves the Au−Pt interaction within [L PP AuPt(CNC)] + ( 24 ) also in solution, the small 2 J P−Pt coupling constant is clearly indicative of coupling through the gold atom [15,16,19,42] . Interestingly, this 2 J P−Pt coupling constant is significantly smaller than those observed in the Au I Pt 0 complexes [(Cy 3 P)Au I Pt 0 (PCy 3 ) 2 ](B(C 6 H 3 Cl 2 ) 4 ) ( 4 (B(C 6 H 3 Cl 2 ) 4 ), 1709 Hz), [(Ph 3 P)Au I Pt 0 (PPh 3 ) 3 ](BAr 4 F ) ( 5 (BAr 4 F ), 321 Hz), and [(Ph 3 P)Au I Pt 0 (nbe) 3 ](BAr 4 F ) (770 Hz, [BAr 4 F ] − =[B(C 6 H 3 (CF 3 ) 2 ) 4 ] − , nbe=norbornene) [15–17,43] . The 195 Pt‐NMR signal in the 1 H, 195 Pt‐HMBC NMR spectrum of 24 [SbF 6 ] is located at −3926 ppm ( 1 J Pt−P =3762 Hz) and thus shifted to lower field with respect to 23 (~328 ppm).…”

“…Examples for the "cis-coordination mode" of phosphine ligands in formal Au I Pt 0 and Au I Pt II complexes (top). [15][16][17][18][19] Structural motif 7 of M 1 M 2 complexes featuring a metal-metal bond with bidentate, long-range "cis-spanning" ligands together with the structures of the two "cis-spanning" ligands L PP and L PN studied in this work (bottom). toluene solution of the crude product over silica.…”

“…[15,16,19,42] Interestingly, this 2 J PÀ Pt coupling constant is significantly smaller than those observed in the Au I Pt 0 complexes [(Cy 3 P)Au I Pt 0 (PCy 3 ) 2 ](B(C 6 H 3 Cl 2 ) 4 ) ( 4 F ] À = [B(C 6 H 3 (CF 3 ) 2 ) 4 ] À , nbe = norbornene). [15][16][17]43] The 195 Pt-NMR signal in the 1 H, 195 Pt-HMBC NMR spectrum of 24[SbF 6 ] is located at À 3926 ppm ( 1 J PtÀ P = 3762 Hz) and thus shifted to lower field with respect to 23 (~328 ppm). This downfield shift must be a consequence of the donation of electron density from the ipso carbon-atom not only to the platinum atom but also to the Au I moiety.…”

“…[15] Also in the cationic complex [(Ph 3 P)Au I Pt 0 (PPh 3 ) 3 ] + (5), the strongly bound phosphine ligands coordinate in a "cis-fashion", while the platinum-bound PPh 3 ligand trans to the gold atom is only weakly bound. [16,17] This situation is a consequence of the transeffect exhibited by the gold atom. Also in the formal Au I Pt II complex [(Ph 3 P)Au I Pt II (CNC)(PPh 3 )] + (6), as investigated by Martín and co-workers, [18,19] the two PPh 3 ligands coordinate to the two metal atoms in a "cis-fashion".…”

“…For example, in the formal Au I Pt 0 complex [(Cy 3 P)Au I Pt 0 (PCy 3 ) 2 ] + ( 4 ) of Braunschweig and co‐workers, the PCy 3 ligands coordinate to the gold and the platinum atom in a “ cis ‐fashion” (Figure 2). [15] Also in the cationic complex [(Ph 3 P)Au I Pt 0 (PPh 3 ) 3 ] + ( 5 ), the strongly bound phosphine ligands coordinate in a “ cis ‐fashion”, while the platinum‐bound PPh 3 ligand trans to the gold atom is only weakly bound [16,17] . This situation is a consequence of the trans ‐effect exhibited by the gold atom.…”