Organo–platinum–iron complexes containing one bridging Ph2PCH2PPh2 ligand. Crystal structures of [(OC)3Fe(µ-dppm)(µ-CO)Pt(PPh3)], [(OC)2Fe(µ-dppm){µ-C(O)C2H2}Pt(PPh3)], and [(OC)3Fe(µ-dppm)(µ-CMeCH2)Pt(PPh3)][BF4]

Abstract

Treatment of [Fe(CO)₄(dppm-P)](dppm = Ph₂PCH₂PPh₂) with [Pt(trans-PhCHCHPh)(PPh₃)₂] gave the bimetallic [(OC)₃Fe(µ-dppm)(µ-CO)Pt(PPh₃)](1) in 75% isolated yield. Complex (1) was also prepared by the sodium tetrahydroborate reduction of [(OC)₃Fe(µ-dppm)(µ-CO)PtCl₂] in the presence of PPh₃. Its structure was established by X-ray crystallography: the crystals are triclinic, space group P, with a= 1 177.3(2), b= 1 198.4(1), c= 1 776.5(1) pm, α= 103.34(1), β= 107.81(1), γ= 91.76(1)°, and Z= 2; final R factor 0.0257 for 5 422 observed reflections. The structure shows that one of the carbonyl ligands is almost symmetrically bridging the Fe–Pt bond. Protonation of complex (1) with HBF₄·OEt₂ gave the hydride [(OC)₄Fe(µ-dppm)PtH(PPh₃)][BF₄](4), ¹H n.m.r. studies of which indicated the hydride ligand to be terminal on platinum. Ethyne reacted with (1) at 20 °C to give the complex [(OC)₂Fe(µ-dppm){µ-C(O)C₂H₂}Pt(PPh₃)](5), the structure of which was established by X-ray crystallography: the crystals are orthorhombic, space group Pbca, with a= 1 898.9(4), b= 1 828.3(3), c= 2 414.4(5) pm, and Z= 8; R 0.0381 for 4 163 observed reflections. The molecule contains a dimetallacyclopentenone ring in which the ethylenic bond is η²-bound to iron, so that the C(O)C₂H₂ moiety is σ-co-ordinated to platinum and η³-co-ordinated to iron. Unsymmetrical alkynes (MeCCH, PhCCH, 4-MeC₆H₄CCH, MeNHCH₂CCH, HOMe₂CCCH, and MeCO₂CCH) also reacted with complex (1), at 80 °C, to give complexes of type [(OC)₂Fe(µ-dppm){µ-C(O)CRCH}Pt(PPh₃)]. At 20 °C, MeCCH and 4-MeC₆H₄CCH reacted with (1) to give isolable complexes of type [(OC)₂Fe(µ-dppm){µ-C(O)CHCR}Pt(PPh₃)] which isomerised in solution, at rates dependent on the steric bulk of R, to complexes of type [(OC)₂Fe(µ-dppm){µ-C(O)CRCH}Pt(PPh₃)]. The mechanism of the alkyne-insertion reactions appears to involve an intermediate containing a monodentate dppm ligand co-ordinated to iron, formed by opening of the five-membered [graphic omitted]t ring in (1). Protonation of the alkyne complexes breaks the alkyne–CO link to give µ-vinyl cations of type [(OC)₃Fe(µ-dppm)(µ-CRCH₂)Pt(PPh₃)]⁺(R = H, Me, or C₆H₄Me-4). The structure of [(OC)₃Fe(µ-dppm)(µ-CMeCH₂)Pt(PPh₃)][BF₄](16) has also been established by X-ray crystallography: the crystals are monoclinic, space group Cc, with a= 2 321.3(3), b= 1 091.2(2), c= 1 962.6(2) pm, β= 98.48(1)°, and Z= 4, R 0.0460 for 3 615 observed reflections. The structure shows that the µ-CMeCH₂ moiety is σ-co-ordinated to platinum and η²-co-ordinated to iron. Treatment of complex (1) with MeO₂CCCCO₂Me displaced the PPh₃ ligand to give [(OC)₄Fe(µ-dppm)Pt(MeO₂CCCCO₂Me)](19). The complex [(OC)₃Fe(µ-dppm)(µ-SO₂)Pt(PPh₃)](20) was formed in high yield when SO₂ was bubbled through a dichloromethane solution of (1). In addition, analogues of many of the above complexes have been prepared with (Ph₂P)₂CCH₂.