Site Selectivity in the Protonation of a Phosphinito Bridged Pt^I−Pt^I Complex: a Combined NMR and Density-Functional Theory Mechanistic Study

Abstract

The protonation of the dinuclear phosphinito bridged complex [(PHCy2)Pt(mu-PCy2){kappa(2)P,O-mu-P(O)Cy2}Pt(PHCy2)] (Pt-Pt) (1) by Brønsted acids affords hydrido bridged Pt-Pt species the structure of which depends on the nature and on the amount of the acid used. The addition of 1 equiv of HX (X = Cl, Br, I) gives products of formal protonation of the Pt-Pt bond of formula syn-[(PHCy2)(X)Pt(mu-PCy2)(mu-H)Pt(PHCy2){kappaP-P(O)Cy2}] (Pt-Pt) (5, X = Cl; 6, X = Br; 8, X = I), containing a Pt-X bond and a dangling kappa P-P(O)Cy2 ligand. Uptake of a second equivalent of HX results in the protonation of the P(O)Cy2 ligand with formation of the complexes [(PHCy2)(X)Pt(mu-PCy2)(mu-H)Pt(PHCy2){kappaP-P(OH)Cy2}]X (Pt-Pt) (3, X = Cl; 4, X = Br; 9, X = I). Each step of protonation is reversible, thus reactions of 3, 4, with NaOH give, first, the corresponding neutral complexes 5, 6, and then the parent compound 1. While the complexes 3 and 4 are indefinitely stable, the iodine analogue 9 transforms into anti-[(PHCy2)(I)Pt(mu-PCy2)(mu-H)Pt(PHCy2)(I)] (Pt-Pt) (7) deriving from substitution of an iodo group for the P(OH)Cy2 ligand. Complexes 3 and 4 are isomorphous crystallizing in the triclinic space group P1 and show an intramolecular hydrogen bond and an interaction between the halide counteranion and the POH hydrogen. The occurrence of such an interaction also in solution was ascertained for 3 by (35)Cl NMR. Multinuclear NMR spectroscopy (including (31)P-(1)H HOESY) and density-functional theory calculations indicate that the mechanism of the reaction starts with a prior protonation of the oxygen with formation of an intermediate (12) endowed with a six membered Pt(1)-X...H-O-P-Pt(2) ring that evolves into thermodynamically stable products featuring the hydride ligand bridging the Pt atoms. Energy profiles calculated for the various steps of the reaction between 1 and HCl showed very low barriers for the proton transfer and the subsequent rearrangement to 12, while a barrier of 29 kcal mol(-1) was found for the transformation of 12 into 5.

Uncovering Intramolecular π‐Type Hydrogen Bonds in Solution by NMR Spectroscopy and DFT Calculations

“…The existence of a π‐type hydrogen bond between the PO−H and the C≡C triple bond in solution is also suggested by the sharpness of the 31 P NMR signal of the P 4 HCy 2 ligand. In fact, for systems analogous to 2 , the 31 P NMR signal of the P 4 HCy 2 ligand is expected to be quite broad when the P(OH)Cy 2 in the cis position can freely rotate about the Pt−P bond, and relatively sharp when the rotation of the P(OH)Cy 2 ligand is hindered due to the formation of the PO⋅⋅⋅H⋅⋅⋅X bridge (X=Lewis base) …”

“…The existence of a p-type hydrogen bond between the POÀ Ha nd the CCt riple bond in solution is also suggestedb yt he sharpnesso ft he 31 PNMR signalo ft he P 4 HCy 2 ligand.I nf act, for systems analogoust o2,t he 31 PNMR signal of the P 4 HCy 2 ligand is expected to be quite broad when the P(OH)Cy 2 in the cis positionc an freely rotate about the PtÀPb ond, [21] and rela-tively sharpw hen the rotationo ft he P(OH)Cy 2 ligand is hindered due to the formationo ft he PO···H···X bridge (X = Lewis base). [22][23][24][25][26] Another clue to the presence of the p-type hydrogen bond derives from the analysis of the 1 H-195 Pt HMQC spectrum of 2 ( Figure 1) displaying, besides the correlation between the Cy 2 PH protons and the 195 Pt atoms to which the phosphanes are bonded, intense cross peaks between the POH protona nd Pt 1 (but not Pt 2 ).…”

A Hydrido η1-Alkynyl Diplatinum Complex Obtained from a Phosphinito Phosphanido Complex and Trimethylsilylacetylene

“…In order to study the site selectivity of the protonation reaction, Complex 2 was treated with HBF4·Me2O in n-hexane (Scheme 3). The reaction gave smoothly the corresponding dicyclohexylphosphinic acid complex [3]BF4 as the only product, indicating that only the POCy2 ligand is protonated. The most striking evidence revealing the transformation of the POCy2 ligand into P(OH)Cy2 is the presence, in the 1 H NMR spectrum of [3]BF4, of a broad peak at δ 6.71 ascribable to the POH proton.…”

“…The signal at δ 120.9 is ascribable to a bridging phosphanide subtending a Pt-Pt bond [22], while the signal at δ 77.0 is diagnostic of a dicyclohexylphosphinite P(O)Cy2 ligand bonded to Pt through the P atom. In fact, Pt-P(O)Cy2 31 P NMR resonances fall in the range of 75-90 ppm, while their protonated analogues Pt-P(OH)Cy2 give 31 P NMR signals in the range of 115-135 ppm [3,4,23]. The remaining signals (δ 13.7 and δ 2.8) are ascribed to terminal PHCy2 ligands.…”

“…It is now well established that the mixed bridge phosphinito-phosphanido Pt(I) complex [(PHCy2)Pt 1 (μ-PCy2){κP,O-μ-P(O)Cy2}Pt 2 (PHCy2)](Pt-Pt) (1) [1] exhibits an amphiphilic behavior, in that it reacts with both nucleophiles (with initial attack at the Pt 1 center) [2] and electrophiles (with initial attack at the O atom) [3,4]. In particular, reaction of 1 with HX Brønsted acids affords, with the exception of HF, [4] dinuclear compounds endowed with a bridging hydride, a bridging dicyclohexylphosphanide and a covalent bond between Pt 1 and the X residue (Scheme 1).…”