Hydrido Phosphanido Bridged Polynuclear Complexes Obtained by Protonation of a Phosphinito Bridged Pt(I) Complex with HBF4 and HF

The protonation of the phosphinito-bridged Pt(I) complex [(PHCy2)Pt(μ-PCy2){κ2P,O-μ-P(O)Cy2}Pt(PHCy2)](Pt−Pt) (1) by aqueous HBF4 or hydrofluoric acid leads selectively to the hydrido-bridged solvento species syn-[(PHCy2)(H2O)Pt(μ-PCy2)(μ-H)Pt(PHCy2){κP-P(OH)Cy2}](Y)2(Pt−Pt) ([2-H2O]Y2) {Y = BF4, F(HF)n} when an excess of acid was used. On standing in halogenated solvents, complex [2-H2O](BF4)2 undergoes a slow but complete isomerization to [(PHCy2)2Pt(μ-PCy2)(μ-H)Pt{κP-P(OH)Cy2}(H2O)](BF4)2(Pt−Pt) ([4-H2O][BF4]2) having the P(OH)Cy2 ligand trans to the hydride. The water molecule coordinated to platinum in [2-H2O][BF4]2 is readily replaced by halides, nitriles, and triphenylphosphane, and the acetonitrile complex [2-CH3CN][BF4]2 was characterized by XRD analysis. Solvento species other than aqua complexes, such as [2-acetone-d6]2+ or [2-CD2Cl2]2+ were obtained in solution by the reaction of excess etherate HBF4 with 1 in the relevant solvent. The complex [2-H2O](Y)2 [Y = F(HF)n] spontaneously isomerizes into the terminal hydrido complexes [(PHCy2)Pt(μ-PCy2){κ2P,O-μ-P(O)Cy2}Pt(H)(PHCy2)](Y)(Pt−Pt) ([6](Y)). In the presence of HF, complex [6](Y) transforms into the bis-phosphanido-bridged Pt(II) dinuclear complex [(PHCy2)(H)Pt(μ-PCy2)2Pt{κP-P(OH)Cy2}](Y)(Pt−Pt) ([7](Y)). When the reaction of 1 with HF was carried out with diluted hydrofluoric acid by allowing the HF to slowly diffuse into the dichloromethane solution, the main product was the linear 60e tetranuclear complex [(PHCy2){κP-P(O)Cy2}Pt1(μ-PCy2)(μ-H)Pt2(μ-PCy2)]2(Pt1−Pt2) (8). Insoluble compound 8 is readily protonated by HBF4 in dichloromethane, forming the more soluble species [(PHCy2){κP-P(OH)Cy2}Pt1(μ-PCy2)(μ-H)Pt2(μ-PCy2)]2(BF4)2(Pt1−Pt2) {[9][BF4]2}. XRD analysis of [9][BF4]2·2CH2Cl2 shows that [9]2+ is comprised of four coplanar Pt atoms held together by four phosphanido and two hydrido bridges. Both XRD and NMR analyses indicate alternate intermetal distances with peripheral Pt−Pt bonds and a longer central Pt···Pt separation. DFT calculations allow tracing of the mechanistic pathways for the protonation of 1 by HBF4 and HF and evaluation of their energetic aspects. Our results indicate that in both cases the protonation occurs through an initial proton transfer from the acid to the phosphinito oxygen, which then shuttles the incoming proton to the Pt−Pt bond. The different evolution of the reaction with HF, leading also to [6](Y) or 8, has been explained in terms of the peculiar behavior of the F(HF)n− anions and their strong basicity for n = 0 or 1.