Selective formation of a triangulo-irondiplatinum cluster

Abstracts

The trinuclear platinum cluster Pt3(m-CNBu t)3(CNBu t)3 reacts selectively with the monodentate dppm-iron complex Fe(dppm-P)(CO)4 (1:1 or 2:1 ratios of Pt:Fe) dppm = bis(diphenylphosphino)methane, to give the mixed-metal cluster FePt2(m-dppm)(CNBu t)2(CO)4. A single-crystal X-ray diffraction study established the structure of an isosceles triangular FePt2 core with short Pt-Pt 2.5756(5) Å, Fe-Pt(1) 2.559(2) Å and Fe-Pt(2) 2.565(3) Å bond lengths, and with the two platinum atoms bridged by the dppm ligand. Cell data: monoclinic, P2(1)/n, a = 11.031(1) Å, b = 20.593(3) Å, c = 18.129(2) Å, b = 91.32(1) °, Dc (Z = 4) = 1.789 mg m-3. Least-squares refinement based on 4171 reflections converged to a final R = 0.0508 and Rw = 0.0558. The IR and NMR spectra (¹H, 13C-{¹H}, 31P-{¹H}, and 195Pt-{¹H}) of the cluster are also reported and discussed.

trinuclear irondiplatinum cluster; ligand transfer

O cluster de platina trinucleado Pt3(m-CNBu t)3(CNBu t)3 reage seletivamente com o complexo de ferro dppm-monodentado Fe(dppm-P)(CO)4, nas razões 1:1 ou 2:1 de Pt:Fe dppm = bis(difenilfosfino)metano, para formar o cluster metálico misto FePt2(m-dppm)(CNBu t)2(CO)4. Um estudo de difração raios-X de um monocristal estabeleceu a sua estrutura como sendo um triângulo isósceles FePt2 com distâncias curtas Pt-Pt 2,5756(5) Å, Fe-Pt(1) 2,559(2) Å, e Fe-Pt(2) 2,565(3) Å, e com o ligante dppm ligado em ponte aos dois átomos de platina. Dados cristalográficos: sistema monoclínico; grupo espacial P2(1)/n; a = 11,031(1) Å, b = 20,593(3) Å, c = 18,129(2) Å, b = 91,32(1)°;Dc (Z = 4) = 1,789 mg m-3. O refinamento por mínimos quadrados baseados em 4171 reflexões convergiu a R = 0,0508 e Rw = 0,0558. Os espectros IV e RMN (¹H, 13C-{¹H}, 31P-{¹H}, e 195Pt-{¹H}) do cluster são também fornecidos e discutidos.

Article

Selective Formation of a Triangulo-Irondiplatinum Cluster

Cleber V. Ursiniª, Gilson H.M. Dias^a*, Maria T.P. Gambardella^b, and Regina H. A. Santos^b

^a Instituto de Química, Universidade Estadual de Campinas, C.P. 6154, 13083-970 Campinas - SP, Brazil

^bInstituto de Química de São Carlos, Universidade de São Paulo, C.P. 780, 13560-250 São Carlos - SP, Brazil

Received: November 11, 1995

O cluster de platina trinucleado Pt₃(m-CNBu^t)₃(CNBu^t)₃ reage seletivamente com o complexo de ferro dppm-monodentado Fe(dppm-P)(CO)₄, nas razões 1:1 ou 2:1 de Pt:Fe dppm = bis(difenilfosfino)metano, para formar o cluster metálico misto FePt₂(m-dppm)(CNBu^t)₂(CO)₄. Um estudo de difração raios-X de um monocristal estabeleceu a sua estrutura como sendo um triângulo isósceles FePt₂ com distâncias curtas Pt-Pt 2,5756(5) Å, Fe-Pt(1) 2,559(2) Å, e Fe-Pt(2) 2,565(3) Å, e com o ligante dppm ligado em ponte aos dois átomos de platina. Dados cristalográficos: sistema monoclínico; grupo espacial P2₁/n; a = 11,031(1) Å, b = 20,593(3) Å, c = 18,129(2) Å, b = 91,32(1)°;^D_c (Z = 4) = 1,789 mg m^-3. O refinamento por mínimos quadrados baseados em 4171 reflexões convergiu a R = 0,0508 e R_w = 0,0558. Os espectros IV e RMN (¹H, ¹³C-{¹H}, ³¹P-{¹H}, e ¹⁹⁵Pt-{¹H}) do cluster são também fornecidos e discutidos.

The trinuclear platinum cluster Pt₃(m-CNBu^t)₃(CNBu^t)₃ reacts selectively with the monodentate dppm-iron complex Fe(dppm-P)(CO)₄ (1:1 or 2:1 ratios of Pt:Fe) dppm = bis(diphenylphosphino)methane, to give the mixed-metal cluster FePt₂(m-dppm)(CNBu^t)₂(CO)₄. A single-crystal X-ray diffraction study established the structure of an isosceles triangular FePt₂ core with short Pt-Pt 2.5756(5) Å, Fe-Pt(1) 2.559(2) Å and Fe-Pt(2) 2.565(3) Å bond lengths, and with the two platinum atoms bridged by the dppm ligand. Cell data: monoclinic, P2₁/n, a = 11.031(1) Å, b = 20.593(3) Å, c = 18.129(2) Å, b = 91.32(1) °, D_c (Z = 4) = 1.789 mg m^-3. Least-squares refinement based on 4171 reflections converged to a final R = 0.0508 and R_w = 0.0558. The IR and NMR spectra (¹H, ¹³C-{¹H}, ³¹P-{¹H}, and ¹⁹⁵Pt-{¹H}) of the cluster are also reported and discussed.

Keywords: trinuclear irondiplatinum cluster, ligand transfer

Introduction

Recent studies have attracted considerable interest in developing a rational stepwise assembly of organometallic building blocks and in understanding the factors that govern the selectivity of reactions¹. In contrast with monophosphines that do not tend to prevent cluster fragmentation², dppm as an assembling ligand greatly contributes to the stability of the resulting cluster molecule^2a. The strong propensity of the dppm ligand for bridging between two metal centers has been useful to prepare mixed-metal cluster complexes with a five-membered M(m-dppm)M unit³. The regioselective insertions of a metal carbonyl fragment towards bridging PtPd(m-dppm)₂Cl₂ and the four-membered chelating Pt(dppm-PP)Cl₂ compounds take place at the labile Pd-P and Pt-P bonds^4,5.

A good iron precursor of heterobimetallic Fe(m-dppm)Pt units is Fe(dppm-P)(CO)₄. Although it has been described as inert toward substitution by several organic substrates, the attack of the uncoordinated phosphorus on labile metallic centers occurs without the need of breaking the Fe-P bond⁶.

The results of our investigation of a selective reaction of the monodentate dppm-iron complex Fe(dppm-P)(CO)₄ with Pt₃(m-CNBu^t)₃(CNBu^t)₃ to form the FePt₂(m-dppm)(CNBu^t)₂(CO)₄ heterometallic cluster are described. However, in contrast to the usual reactions of the dppm-assisted metal-metal bond formation, a complete transfer of the dppm ligand from the iron precursor to the two Pt atoms is observed in this case.

Results and Discussion

Synthesis and Characterization of FePt₂(m-dppm) (CNBu^t)₂(CO)₄

The reactions of Pt₃(m-CNBu^t)₃(CNBu^t)₃ with Fe(dppm-P)(CO)₄(ratios Fe:Pt of 1:1 or 1:2) in THF result initially in a deep dark green solution. After a period of 1.5 h at room temperature, analytically pure orange samples of FePt₂(m-dppm)(CNBu^t)₂(CO)₄ are isolated in the 60-70% range from the orange reaction mixture. Spectroscopic data were consistent with the structure drawn for FePt₂(m-dppm)(CNBu^t)₂(CO)₄, which was further confirmed by an X-ray diffraction study. The IR spectrum of FePt₂(m-dppm)(CNBu^t)₂(CO)₄ displays n(CO) absorptions of terminal and semi-bridged CO ligands at 1976, 1909, and 1851 cm^-1.

The ¹⁹⁵Pt (I = 1/2) isotopehas a natural abundance of 33.8%, corresponding to three isotopic distributions in a binuclear Pt₂(m-dppm) arrangement: the zero-spin Pt isotopomer (43.95%), mono (44.68%) and doubly (11.35%) labelled ¹⁹⁵Pt isotopomers⁷.

Therefore, a 1:7.8:17.4:7.8:1 quintet of triplets is expected for the CH₂P group of the dppm ligand bonded to two Pt atoms in ¹H-NMR spectrum. The observed spectrum was in good agreement with this prediction, although the weakest outer triplets with unit relative intensity were obscured by the noise.

The ³¹P{¹H} NMR spectrum of FePt₂(m-dppm) (CNBu^t)₂(CO)₄ displays a sharp singlet at d - 4.18 with platinum satellites ¹J(PtP) 3671 Hz, which confirms a symmetrical A-frame Pt(m-dppm)Pt moiety⁸. Moreover, the first indication of the presence of a strong Pt-Pt bond is provided by a large negative value (75 Hz) for ²J(PtP), showing through-bond contributions of ²J(PPtPt) to be dominant⁹.

The color change of the THF solution containing Pt₃(m-CNBu^t)₃(CNBu^t)₃ and Fe(dppm-P)(CO)₄ from orange to green only occurs above -22 °C. The analytical data of the dark green solid produced in ethyl ether/petroleum ether at -5 °C, coincidentally pointed to FePt₂(dppm)(CNBu^t)₂(CO)₄ formulation as the main cluster. However, the time ³¹P-{¹H} NMR spectra exhibit the characteristic pattern of a Pt(m-dppm)Fe core d -10.5 (PPt) and 46.0 (Pfe), that quickly converts towards FePt₂(m-dppm)(CNBu^t)₂(CO)₄. On the basis of these spectroscopic and analytical data, therefore, the green intermediate can be analogous to the main cluster but with a Pt(m-dppm)Fe unit.

Curiously, the reaction that yelds FePt₂(m-dppm)(CNBu^t)₂(CO)₄ does not depend on the stoichiometry of the reagents. Even when the precursor Fe(dppm-P)(CO)₄ is used in excess, no by-product is further isolated. This contrasts, therefore, with the coupling reactions of Fe(dppm-P)(CO)₄ with Pt(PR₃)₂(C₂H₄) for which dimeric complexes of the FePt(m-CO)(m-dppm)(CO)₃(PR₃)⁶ type form selectively. It is likely that the reactions take place via the primary dimeric intermediate FePt(m-CO)(m-dppm)(CO)₃(CNBu^t) with a Fe(m-dppm)Pt moiety, although this was not spectroscopically observed. However, the iron-phosphorus bond breaking and phosphorus migration to the second platinum atom appears to occur after the formation of the green triangle-intermediate with a Fe(m-dppm)Pt unit, as illustrated in Scheme 1.

X-Ray Diffraction study of FePt₂(m-dppm)(CNBu^t)₂ (CO)₄

An ORTEP¹⁰ drawing of the molecular structure is shown in Fig. 1 with its atom numbering scheme. Selected bond distances and angles are given in Table 1. Final positional parameters are listed in Table 3.

The FePt₂(m-dppm)(CNBu^t)₂(CO)₄ complex contains an isosceles triangle of FePt₂ atoms (mean 60.0°) with a dppm ligand bridging two platinum edges to form a roughly planar FePt₂ core. The C(A1)O(A1) and C(A2)O(A2) carbonyls, the isocyanide ligands, and phosphorus atoms of the dppm ligand are approximately coplanar with the metal triangle, but no longer collinear with the metal-metal axis. The bond angles between the Fe(C)₂ and Pt(C)(P) planes C(A1)-Fe-C(A2), P(1)-Pt(1)-C(1) and P(2)-Pt(2)-C(2) have an average value of 102°. Another feature of note in the FePt₂(m-dppm)(CNBu^t)₂(CO)₄ molecular structure and one that may influence the iron coordination geometry is the angular deformation of both apical carbonyl ligands bound to the Fe atom C(A3)-Fe-C(A4) angle 143°, which are semi-bridging¹¹, but not symmetrically arranged around the corresponding Pt-Pt bond. The C(A3)...Pt(1) (2.51 Å) and C(A4)...Pt(2) (2.68 Å) distances are shorter than those of C(A3)...Pt(2) (2.75 Å) and C(A4)...Pt(1) (2.80 Å), respectively. Therefore, the geometry adopted by the iron in the Pt₂Fe(CO)₄ moiety deviates slightly from octahedral symmetry. The distortion is toward a bicapped tetrahedral disposition of the four carbonyl groups, with the platinum atoms capping two tetrahedral faces. The Pt-Pt 2.5756(5) Å and Fe-Pt (mean 2.563(2) Å) bonds lengths found in FePt₂(m-dppm)(CNBu^t)₂(CO)₄ lie around the lower limit range reported for formal single bonds in other mixed platinum-iron compounds (Pt-Pt 2.565 Å and Pt-Fe 2.530 Å)^1c,5c,12. The contraction of metal-metal separations by bridging CO and CNR groups is commonly observed^1b,13 and may attribute in part to the small radius of the bridging carbon atoms since those distances are found to depend mostly on the size of the atoms of the bridging ligands^11d. All the internal P-C-P 106.6(5)° and Pt-Pt-P angles 95.83(7)° and 90.96(7)° are close to those reported in unidentate dppm complexes¹⁴, indicating no particular strain of the five membered ring besides the usual clashing between phenyl groups in axial sites on the envelope Pt₂P₂C ring¹⁵. The terminal isocyanide groups are effectively linear C-N-C and Pt-C-N (mean 175°). The average Fe-C and C-O distances are 1.76 Å and 1.18 Å, respectively. The other bond angles and distances are as expected (Table 1). The architecture of FePt₂(m-dppm)(CNBu^t)₂ (CO)₄ bears a strong resemblance to that found for the FePt₂(m-dppm)(CO)₆^5c cluster.

Conclusion

This work shows the assisting role of the dppm ligand in building triangular FePt₂(m-dppm)(CNBu^t)₂(CO)₄ cluster when Fe(dppm-P)(CO)₄reacts with Pt₃(m-CNBu^t)₃(CNBu^t)₃.

Experimental

General considerations

All manipulations were performed using general Schlenk and highvacuum techniques under a dry, oxygenfree, argon atmosphere. Solvents were appropriately dried and distilled under argon prior to use. The Pt(COD)₂¹⁶ (COD = 1,5-cyclooctadiene), Pt₃(m-CNBu^t)₃(CNBu^t)₃¹⁷ and Fe(CO)₄(dppmP)¹⁸ complexes were prepared as described previously. Other reagents were used as obtained from commercial sources. Elemental analyses were performed using a PERKINELMER 2400 CHN microanalyser. All percentages given are the average of at least two independent determinations. IR spectra were recorded as Nujol mulls with a JASCO IR 700 spectrometer.NMR spectra were obtained with a BRUKER WH 400 (¹³C,100.589; ³¹P, 161.796; and ¹⁹⁵Pt, 85.629 MHz) and a BRUKER AW 80 (¹H, 80 MHz) instruments. The chemical shifts of ¹H and ¹³C spectra were referenced to TMS, and the ³¹P spectra were quoted relative to external 85% H₃PO₄ at 0.00 ppm. Platinum-195 chemical shifts were quoted relative to the absolute scale X (¹⁹⁵Pt) = 21.4 MHz.

Preparations of FePt₂(m-dppm)(CNBu^t)₂(CO)₄

The Pt₃(m-CNBu^t)₃(CNBu^t)₃ complex (0.162 g, 0.150 mmol) was slowly added to a stirred solution of Fe(CO)₄(dppmP) (0.247 g, 0.447 mmol) in THF (15 mL) at 5 °C. The mixture was allowed to reach room temperature over 1.5 h. The color gradually changed from deep black to orange. Filtration of the solution with a short alumina column, and evaporation to 3 mL followed by addition of petroleum ether resulted in orange microcrystals. The product was washed with petroleum ether (3 x 3 mL), and dried under reduced pressure for 3 h to give 0.149 g (60% based on Pt). Anal. Calcd for C₃₉H₄₀N₂FeP₂Pt₂: C 42.25, H 3.64, N 2.53%. Found: C 42.56, H 3.57, N 2.34%. IR (cm^-1), CH₂Cl₂: n(CN) 2136 s; n(CO) 1976 s; 1904 m; and 1851 m, br. ¹H-NMR (CDCl₃): d 1.30 (s, 18H, CH₃); 5.22 m, 2H, ³J(PtH) 68 Hz, ²J(PH) 11 Hz, CH₂; 7.00-7.60 (m, 20H, CH, Ph). ¹³C-{¹H} NMR (CD₂Cl₂): d 30.2 (s, CH₃, Bu^t); 55.3 m, PCH₂, ¹J(PC) 32 Hz, ²J(PtC) 133 Hz; 57.4 (s, CCH₃); 128.4 {m, C_m, Ph, J(PC) + J(PC) 10 Hz}; 130.1 (s, C_p, Ph); 133.0 {m, C_o, Ph, J(PC) + J(PC) 15 Hz}; 135.7 {m, C_ipso, Ph, J(PC) + J(PC) 50 Hz}; 144.1 s, CNBu^t, ¹J(PtC) 1380 Hz; 220.3 (s, CO). ³¹P-{¹H} NMR: d 4.18 {s, ¹J(PtP) 3671 Hz, ²J(PtP) 75 Hz, ²J(PP) 30 Hz, N = ¹J(PtP) + ²J(PtP) = 3596 Hz; ¹J(PtPt) = 850 Hz }. ¹⁹⁵Pt-{¹H} (CD₂Cl₂): d 457, ¹J(PtPt) = 850 Hz.

A similar reaction was conducted under the same conditions as the preceding reaction by using Pt₃(m-CNBu^t)₃(CNBu^t)₃ (0.160 g, 0,148 mmol) and Fe(CO)₄(dppmP) (0.120 g, 0.217 mmol) in THF. This reaction gave 0.176 g (73% based on Pt) of FePt₂(m-dppm)(CNBu^t)₂(CO)₄. IR and NMR data and analogous satisfactory elemental analyses, identical to those above, were obtained.

Reaction of Pt₃(m-CNBu^t)₃(CNBu^t)₃ with Fe(CO)₄ (dppmP) in petroleum ether/diethyl ether

The Pt₃(m-CNBu^t)₃(CNBu^t)₃ complex (0.073 g, 0,068 mmol) was slowly added to a stirred suspension of Fe(CO)₄(dppmP) (0.057 g, 0.103 mmol) in petroleum ether/diethyl ether (1:1, 10 mL) at -5 °C. The reaction mixture was stirred for 15 min until the formation of a black residue. The dark supernatant liquid was then removed, the residue washed several times with petroleum ether, and dried under vacuum, resulting in a dark green solid (0.031 g). More product was present in the supernatant liquid, as shown by its NMR spectrum, but the yield has not been optimized. Anal. Calcd for C₃₉H₄₀N₂FeP₂Pt₂: C 42.25, H 3.64, N 2.53%. Found: C 42.00, H 3.65, N 2.53%. IR (cm^-1), CH₂Cl₂: n(CN) 2142 s; br; n(CO) 2042 s; 1980 m, sh; 1970 s; 1945 s; 1936 s; 1840 m; and 1812m, br. ³¹P-{¹H} NMR (THF): d -10.5 s, PPt, ¹J(PtP) 3092 Hz, ²J(PtP) 107 Hz, ²J(PP) 48 Hz, 46.0 PFe, J(Pt¹P^b) and J(Pt²P^b) not discernible, ²J(PP) 48 Hz and the resonances assigned to FePt₂(m-dppm)(CNBu^t)₂(CO)₄ as the minor product.

X-ray Data Collection, Structure Determination, and Refinement for FePt₂(m-dppm)(CNBu^t)₂(CO)₄

An orange crystal grown by slow diffusion of methanol into toluene, was mounted in an Enraf-Nonius CAD 4 diffractometer. The diffraction experiment was carried out at room temperature (298 K) using graphite monochromated CuKa (d = 1.5418 Å) radiation. The cell constants were determinated from a list of 25 selected reflections by using automatic search, indexing factors, and least-squares routines. The data were corrected for Lorentz and polarization factors, and a semiempirical absorption correction was applied¹⁹. The coordinates of the iron and platinum atoms were obtained by the Patterson method. An alternating sequence of least-squares refinement and difference Fourier maps revealed the positions of all remaining atoms. The hydrogen atoms were introduced in calculated positions (d_C-H = 1.08 Å) and the coordinates were recalculated after each refinement cycle with fixed isotropic parameters (B = 6.0 Ų). Final refinement was carried out using anisotropic displacement parameters for all non-hydrogen atoms. All calculations were performed using the SHELX-76²⁰ and SHELXS²¹ packages of programs. Atomic scattering factors were taken from the usual source²². A summary of the data collection is given in Table 2.

Acknowledgment

We are very grateful to the CNPq/PADCT for financial support. We Thank Dr. Brian Mann for several suggestions and for recording some of the NMR spectra.

Supporting Information Available: Tables of hydrogen atomic fractional coordinates, interatomic distances, anisotropic thermal parameters. Ordering information is given on any current masthead page.

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FAPESP helped in meeting the publication costs of this article

Publication Dates

• Publication in this collection
  10 Sept 2010
• Date of issue
  1997

History

• Received
  11 Nov 1995