A Hydrido η 1-Alkynyl Diplatinum Complex Obtained from a Phosphinito Phosphanido Complex and Trimethylsilylacetylene

The reaction of (trimethylsilyl)acetylene with the phosphinito phosphanido Pt(I) complex [(PHCy2)Pt(μ-PCy2){κ2P,O-μ-P(O)Cy2}Pt(PHCy2)](Pt-Pt) (1) results in the protonation of the Pt-Pt bond with the formation of the bridging hydride complex [(PHCy2)(Me3SiC≡C)Pt(μ-PCy2)(μ-H) Pt(PHCy2){κP-P(O)Cy2}](Pt-Pt) (2), which was characterized by spectroscopic, spectrometric and XRD analyses. Complex 2 exhibits in the solid state at 77 K a long-lived, weak, orange emission assigned as metal-metal to ligand charge transfer (MMLCT) (L = alkynyl) due to the presence of a very short Pt···Pt distance [2.8209(2) Å]. Reaction of 2 with etherate HBF4 results in the selective protonation of the phosphinito ligand to afford the species [(PHCy2)(Me3SiC≡C)Pt(μPCy2)(μ-H) Pt(PHCy2){κP-P(OH)Cy2}](Pt-Pt)[BF4] ([3]BF4).

Therefore, we decided to explore the reactivity of the Pt(I) Complex 1 with substituted acetylenes possessing the C≡C-H moiety able to protonate the diplatinum complex and to form new Pt-C bonds.In this paper, we describe for the first time the reaction between a platinum(I) complex and alkynes, which led to the synthesis of the acetylide complex [(PHCy2)(Me3Si-C≡C)Pt(μ-PCy2) {κP-P(O)Cy2}Pt(PHCy2)](Pt-Pt) (2).
Complex 2 is a pale yellow solid soluble in alkanes, aromatic solvents and tetrahydrofuran, which gave, from the HR ESI-MS analysis, an intense peak at m/z 1295.5852(Figure 1) corresponding to the cation obtained by protonation of 2, plus a peak at 2591.1691 ascribable to a protonated dimer (exact mass = 2591.1657Da) [19]   The molecular structure of Complex 2 was deduced from IR, NMR and XRD analyses.In the IR spectrum (KBr disk), Complex 2 showed, beside the bands of P-H and Pt-H-Pt stretchings at 2329 and 1632 cm −1 , respectively, a very strong band at 2042 cm −1 attributable to a C≡C stretching of the (trimethylsilyl)acetylide.This value is in the range commonly observed for platinum  [21].In fact, the blue-shift of 4 cm −1 observed for the C≡C stretching on passing from free Me3SiC≡C-H (ν = 2038 cm −1 in n-hexane) to 2 indicates that the C≡C bond order in the coordinated acetylide and in the free (trimethylsilyl)acetylene is nearly the same, thus ruling out a possibleη 2 -coordination of the C≡C triple bond to Pt.
The position and the multiplicity of the 31 P, 1 H and 195 Pt NMR spectral features are similar to those reported for the analogous compounds having Cl, Br, CF3CH2O or PhO in place of SiMe3-C≡C and support the structure in which the (trimethylsilyl)acetylide is η 1 -bonded to Pt 1 and the P(O)Cy2 ligand is κ-P-bonded to Pt 2 .For Complex 2, the 31 P{ 1 H} NMR spectrum showed four mutually-coupled signals at δ 120.9, δ 77.0, δ 13.7 and δ 2.8 (Figure 2).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.As a consequence of a higher trans influence of the Me3SiC≡C − ligand compared to halogens or alkoxides [24], the value of the Pt 1 -μP 1 direct coupling constant is significantly lower for 2 ( 1 JPt,P = 1,680 Hz) than for analogous complexes having chlorine ( 1 JPt,P = 2,484 Hz), bromine ( 1 JPt,P = 2,465 Hz) or trifluoroethoxide ( 1 JPt,P = 2,520 Hz) 3 in place of trimethylsilylacetylide.In the 1 H NMR spectrum of 2, the presence of a bridging hydride is attested by a multiplet centered at δ −4.46 flanked by two sets of 195 Pt satellites (Figure 3a).The multiplicity of the central signal is due only to 1 H-31 P couplings, as indicated by the 1 H{ 31 P} spectrum shown in Figure 3b.

Photoluminescence of 2
Due to the presence of the very short Pt•••Pt distance, we decided to investigate the photoluminescent properties of Complex 2 in the solid state.The complex is not emissive at room temperature.However, it displays a long-lived, weak emission at 77 K (Figure 5), which fits to a double exponential decay (τ1 5.6 µs 55%; τ2 13.5, µs 45%).According to previous luminescence studies in alkynyl dinuclear complexes with short Pt••Pt distances [35,36], this emission is tentatively ascribed to an excited state having a metal-metal to alkynyl charge transfer character ( 3 MMLCT).
The profile of the emission band is asymmetric, and the excitation spectra monitored at two different wavelengths are somewhat different, suggesting the presence of heterogeneity in the bulk solid.

Reactivity of 2 with HBF4
Complex 2 possesses several sites susceptible to protonation.In fact, beside the bridging phosphanide, the bridging hydride, the terminal (trimethylsilyl)acetylide (that could give dihydrogen and trimethylsilylacetylene, respectively) and the dicyclohexylphosphinite represent possible protonation sites.
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.Moreover, as reported in related hydrido bridged diplatinum phosphinito species, [3,4] the protonation of the POCy2 ligand provokes a high field shift of the 1 H NMR hydride resonance (from δ −4.46 (in 2) to δ −5.83 (in [3]BF4)) and a low field shift of the phosphinito 31 P NMR resonance (from δ 77.0 (in 2) to δ 124.3 (in [3]BF4)).IR and 13 C NMR data confirmed the presence of the η 1 -acetylide bonded to Pt 1 in [3]BF4.In fact, the C≡C stretching band was found in the IR spectrum at

Experimental Section
Complex 1 was prepared as described [2].(Trimethylsilyl)acetylene and etherate fluoroboric acid were from commercial suppliers and were used without further purification.All manipulations were carried out under a pure nitrogen atmosphere, using freshly distilled and oxygen-free solvents.C, H elemental analyses were carried out on a Vario Micro-CHNSO elemental analyzer.Infrared spectra were recorded on a Jasco 4200-FTIR spectrometer.NMR spectra were recorded on a Bruker Avance 400 spectrometer; frequencies are referenced to Me4Si ( 1 H and 13 C), 85% H3PO4 ( 31 P) and H2PtCl6 ( 195 Pt).The signal attributions (see Schemes 4 and 5 for atom numbering) and coupling constant assessment was made on the basis of a multinuclear NMR analysis, including 1 H-31 P HMQC, 1 H-195 Pt HMQC, COSY and NOESY experiments.The coupling constants not directly extractable from the monodimensional spectra were obtained and attributed by the tilts of the multiplets due to the "passive" nuclei [37] in the aforementioned 2D spectra.High resolution mass spectrometry (HR-MS) analyses were performed using a time-of-flight mass spectrometer equipped with an electrospray ion source (Bruker micrOTOF-Q II).The sample solutions were introduced by continuous infusion with the aid of a syringe pump at a flow-rate of 180 μL/h.The instrument was operated at end plate offset −500 V and capillary −4500 V. Nebulizer pressure was 0.3 bar (N2) and the drying gas (N2) flow 4.0 L/min.Capillary exit was 170 V.The drying gas temperature was set at 180 °C.The software used for the simulations is Bruker Daltonics Data Analysis (version 4.0, Bruker Daltonik GmbH, Bremen, Germany).All experimental HRMS signals had an isotope pattern superimposable to that calculated for the relevant formula, on the basis of the natural abundances.Emission and excitation spectra were obtained on a Jobin-Yvon Horia Fluorog 3-11 Tau-3 spectrofluorometer (Horiba, Kyoto, Japan), with the lifetime measured in the phosphorimeter

Synthesis of 2
To a n-hexane solution of 1 (164 mg, 0.137 mmol in 4.0 mL), 21 μL of (trimethylsilyl)acetylene (14.8 mg, 0.150 mmol) were added at 298 K and stirred for 12 h.After the reaction, the mixture was taken to dryness under vacuum to eliminate the solvent and the excess (trimethylsilyl)acetylene. The residue was redissolved in 1.0 mL of n-hexane, and the procedure was repeated twice.The final pale yellow solid was dried under high vacuum overnight.
Yield: 77 mg (70%).Complex [3]BF4 is stable in the solid state, but slowly undergoes a substitution of a chloride for the trimethylsilylacetylide, when it is dissolved in chlorinated solvents.It is soluble in halogenated solvents and insoluble in diethyl ether, in alkanes and in aromatic solvents.

X-ray Structure Determination
Details of the structural analysis for Complex 2 are summarized in Table 2. Pale yellow crystals were obtained by slow evaporation at 4 °C of a concentrated solution of the complex in n-hexane.Two molecules of n-hexane and one of water were found in the asymmetric unit.Graphite-monochromatic Mo-Kα radiation was used.X-ray intensity data were collected with a NONIUS-κCCD area-detector diffractometer (CAMCOR, Eugene, OR, USA) and images processed using the DENZO (Academic Press: New York, NY, USA) and SCALEPACK suite of programs [38], carrying out the absorption correction at this point.The structure was solved by direct and Patterson methods using SIR2004 [39] and refined by full-matrix least squares on F 2 with SHELXL-97 [40].All non-hydrogen atoms were assigned anisotropic displacement parameters.The hydride ligand, H(100), has been located from difference maps and assigned isotropic parameters.The rest of the hydrogen atoms (except the H2O hydrogens) were constrained to idealized geometries fixing isotropic displacement parameters 1.2-times the Uiso value of their attached carbon.Several restraints have been used in order to model the n-hexane molecules.The hydrogen atoms of the H2O molecule have been omitted from the density map, but have been included in the empirical formula and in the molecular weight.Finally, the structure showed one residual peaks greater than 1 eA −3 in the vicinity of the Pt(2) atom, but with no chemical meaning.

Conclusions
Reaction of the phosphinito-phosphanido Pt(I) complex [(PHCy2)Pt(μ-PCy2){κ 2 P,O-μ-P(O)Cy2} Pt(PHCy2)](Pt-Pt) (1) with (trimethylsilyl)acetylene results in the C-H bond activation of the alkyne with the formation of the bridging hydride complex [(PHCy2)(η 1 -Me3SiC≡C)Pt(μ-PCy2){κP-P(O)Cy2} Pt(PHCy2)](Pt-Pt) (2), which displays a σ-bonded acetylide ligand and a bridging hydride.The reactivity of 1 with (trimethylsilyl)acetylene parallels that already found with PhOH or CF3CH2OH, two weak Brønsted acids endowed with a coordinating anion.Complex 2 is stable both in the solid state and in solution, and it is resistant to deprotonation by NaOH.It crystallizes from n-hexane as 2•2 n-hexane•H2O.The water molecule is simultaneously hydrogen bonded to two phosphinite ligands of two neighboring diplatinum complexes, giving rise to a dimer.Due to the presence of the short Pt-Pt bond, Complex 2 exhibits a long-lived weak emission at 77 K, which is ascribed to an excited state having a metal-metal to alkynyl charge transfer character ( 3 MMLCT).Despite the number of protonation sites present in Complex 2, its reaction with HBF4•Me2O in toluene gave the phosphinic acid cationic Complex 3 + selectively, still embodying the trimethylsilylacetylide ligand.
formed in the ionization chamber, presumably featuring a hydrogen ion linking two neutral molecules of 2 through the P=O moieties.The MS/MS spectrogram of the peak at m/z 2591.1691showed only two peaks at 1197.5257 and 1295.5765ascribable to [1 + H] + (exact mass = 1197.5303)and to [2 + H] + (exact mass = 1295.5855),respectively.On the other hand, the MS/MS spectrogram of the peak at m/z 1295.5852showed an intense peak at m/z 1197.5210due to the loss of (trimethylsilyl)acetylene from 2, plus a weak peak at 981.3602, due to the contemporary loss of (trimethylsilyl)acetylene and PH(O)Cy2 from 2.

Table 1 .Figure 4 .
Figure 4. View of the molecular structure of 2•2 n-hexane•H2O; (a) the asymmetric unit showing the presence of the oxygen from the H2O and (b) the dimer generated through four O•••H•••O bonds and at equivalent positions (-x, -y, -z).

Figure 5 .
Figure 5. Excitation and emission spectra of 2 in solid state at 77 K. (first line).