Selective Formation of a Triangulo-Irondiplatinum Cluster

O cluster de platina trinucleado [Pt 3(μ-CNBu)3(CNBu)3] reage seletivamente com o complexo de ferro dppm-monodentado [Fe(dppmP)(CO)4], nas razões 1:1 ou 2:1 de Pt:Fe [dppm = bis(difenilfosfino)metano], para formar o cluster metálico misto [FePt 2(μ-dppm)(CNBu)2(CO)4]. Um estudo de difração raios-X de um monocristal estabeleceu a sua estrutura como sendo um triângulo isósceles FePt 2 com distâncias curtas 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, β = 91,32(1)°; Dc (Z = 4) = 1,789 mg m. O refinamento por mínimos quadrados baseados em 4171 reflexões convergiu a R = 0,0508 e Rw = 0,0558. Os espectros IV e RMN ( H, C-{H}, P-{H}, e Pt-{H}) do cluster são também fornecidos e discutidos.


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 reactions 1 .In contrast with monophosphines that do not tend to prevent cluster fragmentation 2 , 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(µ-dppm)M' unit 3 .The regioselective insertions of a metal carbonyl fragment towards bridging [PtPd(µdppm)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(µdppm)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 bond 6 .
The results of our investigation of a selective reaction of the monodentate dppm-iron complex [Fe(dppm-P)(CO)4] with [Pt3(µ-CNBu t )3(CNBu t )3] to form the [FePt2(µ-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.
Therefore, a 1:7.8:17.4:7.8:1quintet of triplets is expected for the CH2P group of the dppm ligand bonded to two Pt atoms in 1 H-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.

X-Ray Diffraction study of [FePt2(µ-dppm)(CNBu t )2 (CO)4]
An ORTEP 10 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(µ-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)  5) Å] and Fe-Pt (mean 2.563(2) Å) bonds lengths found in [FePt2(µ-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 complexes 14 , indicating no particular strain of the five membered ring besides the usual clashing between phenyl groups in axial sites on the envelope Pt2P2C ring 15 .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(µ-dppm)(CNBu t )2 (CO)4] bears a strong resemblance to that found for the [FePt2(µ-dppm)(CO)6] 5c cluster.

X-ray Data Collection, Structure Determination, and refinement for [FePt2(µ-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 CuKα (δ = 1.5418Å) 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 applied 19 .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 (dC-H = 1.08 Å) and the coordinates were recalculated after each refinement cycle with fixed isotropic parameters (B = 6.0 Å 2 ).Final refinement was carried out using anisotropic displacement parameters for all non-hydrogen atoms.All calculations were performed using the SHELX-76 20 and SHELXS 21 packages of programs.Atomic scattering factors were taken from the usual source 22 .A summary of the data collection is given in Table 2.