Competition between triborane as a ligand and a hydride donor at platinum centres containing chelating phosphines: molecular structures of [{Ph2P(CH2)2PPh2}PtB3H7], [{Ph2P(CH2)4PPh2}PtB3H7] and [Pt2H3{(Ph2PC5H4)2Fe}2]Cl

Abstract

The reaction of [PtCl₂(L–L)][L–L = Ph₂P(CH₂)_nPPh₂, n= 2, 3 or 4; or Fe(η⁵-C₅H₄PPh₂)₂] with the octahydrotriborate(1–) anion, [B₃H₈]^–, leads to both [(L–L)PtB₃H₇] in which the borane unit acts as a pseudo-bidentate ligand and [Pt₂H₃(L–L)₂]Cl which results from the borane anion functioning as a hydride donor. For each of the complexes containing a bis(diphenylphosphino) alkane ligand the major product is [{Ph₂P(CH₂)_nPPh₂}PtB₃H₇], whereas for the reaction of [B₃H₈]^– with [Fe(η⁵-C₅H₄PPh₂)₂], [Pt₂H₃{(Ph₂PC₅H₄)₂Fe}₂]Cl is the predominant species formed. Three products have been structurally characterised: [{Ph₂P(CH₂)₂PPh₂}PtB₃H₇]1, space group P2₁/c, a= 16.049(4), b= 15.395(4), c= 21.075(5)Å, β= 90.53(2)°, Z= 8, R= 0.039; [{Ph₂P(CH₂)₄PPh₂}PtB₃H₇]3, space group P2₁, a= 8.8826(18), b= 17.7329(32), c= 9.5520(17)Å, β= 114.115(15)°, Z= 2, R= 0.032; [Pt₂H₃{(Ph₂PC₅H₄)₂Fe}₂]Cl·3CH₂Cl₂6, space group p2₁/c, a= 12.862(2), b= 38.732(6), c= 14.271(2)Å, β= 99.027(15)°, Z= 4, R= 0.067. The structure of the cation of 6 resembles that of [Pt₂(µ-H)₂H{Ph₂P(CH₂)₂PPh₂}₂]⁺ rather than that of [Pt₂(µ-H)H₂{Bu^t₂P(CH₂)₃PBu^t₂}₂]⁺ thereby indicating that it is the steric bulk of the phosphorus substituents rather than the P–Pt–P bite angle that controls the geometry of the {Pt₂P₄} framework and thus the hydride locations. Fenske–Hall molecular orbital calculations have been used to probe the mode of bonding of the hydride ligands to the diplatinum centre.