RHODIUM HALIDES-TRIFLUOROPHOSPHINES

Rhodium (I) chloride bis(trifluorophosphine) dimer [RhCl(PF3)2]2

[RhCl(PF3)2]2 SYNTHESIS AND PROPERTIES

Table: Properties of [RhCl(PF3)2]2

     [RhCl(PF3)2]2 was synthesized and characterized by a procedure reported in the literature [16]; it is a red solid at room temperature, having vapor pressure is 5.5·10-2 Torr at 23°C,  stable in high vacuum and sufficiently stable in air.[i].

[i] T. Ohta, F. Cicoira, P. Doppelt, L. Beitone, P. Hoffmann, Adv. Mater. 13 (2001) 33. 

[RhCl(PF3)2]2 MOLECULAR STRUCTURE

Fig. Molecular structure of [RhCl(PF3)2]2

  The molecular structure of [RhCl(PF3)2]2  was determined by single crystal-X-Ray diffraction at -150°C. The molecule consists of two RhCl2(PF3)2 planes with a dihedral angle of 113.54°, with Rh-Rh distance (2.9709 Å) shorter than in the iso-structural [RhCl(CO)2]2. The P-F contacts are minimized by PF3 ligands orientation towards the central axis of the dimer. The Rh atom is 0.14 Å out of the main plane formed by the four ligands as a consequence of this.  Strong back bonding to the PF3 ligand is causing Rh-P distance 2.12 Å - shorter than in other Rh complexes containing phosphine ligands. [RhCl(PF3)2]2 has square planar geometry of Rh in the complex which is typical for d8 configuration metal ions. Such complexs obey the 16 electron rule (eight electron from the metal, eight from the ligands) - an exception to the 18 electron rule.[i]

[i] Fabio CICOIRA, PhD thesis,  EPFL, http://infoscience.epfl.ch/record/33004/files/EPFL_TH2528.pdf

[RhCl(PF3)2]2 VAPOR PRESSURE

Fig. Vapor pressure of [RhCl(PF3)2]2

The vapor pressure of [RhCl(PF3)2]2 dependence on temperature (in the temperature range 0.5°C-23ºC) was measured. (Fig. ): for example, at room temperature it has vapor 7.5 Pa (5.5·10-2 Torr)/ 23°C). The obtained vapor pressure values are higher than determined by IR transmission detection method (assuming that the optical absorption cross sections for both free PF3 and PF3 ligands are the same). The enthalpy of sublimation of [RhCl(PF3)2]2 is 90.8 kJ/mole (as calculated from the Clausius-Clapeyron equation).

[RhCl(PF3)2]2 ENTHALPY OF SUBLIMATION

Fig. [RhCl(PF3)2]2 - enthalpy of sublimation

The enthalpy of sublimation of [RhCl(PF3)2]2  of 90.8 kJ/mole was determined by fitting the vapor pressure values based on Clausius-Clapeyron equation. This values is higher than that previously determined by IR transmission detection method / assuming same optical absorption cross sections for both free PF3 and PF3 ligands. No pressure increase with time have been observed during the measurements.

RhCl(PF3)2]2 for Rh metal by STM-assisted CVD

    Local decomposition of gaseous molecules of [Rh(PF3)2Cl]2 under a scanning tunneling microscope (STM) tip, by application of a voltage of a few volts on the sample (STM assisted chemical vapour deposition) occurred, allowing to deposit rhodium lines and dots on Au or Si surfaces. The gaseous molecules of [Rh(PF3)2Cl]2 were dissociated by the high electric field (about 107 V/cm) within the tip–sample gap; the study of sample voltage variation was performed, the resolution limit of the technique was investigated. The target of deposition of Rh low capacitance dots is preparation of single electron devices for their possible replacement of transistors in memories (allows to avoid lithography step at nanometric scale).[i]

[i] F. Marchi, D. Tonneau, H. Dallaporta, R. Pierrisnard, V. Bouchiat, V.I. Safarov,  P. Doppelt, R. Even, Microelec. Eng., 2000, Vol. 50, Iss.1–4, p. 59–65, « Nanometer scale patterning by scanning tunelling microscope assisted chemical vapour deposition »,  http://www.sciencedirect.com/science/article/pii/S0167931799002658 

[RhCl(PF3)2]2 MOLECULE GEOMETRY AND FRAGMENTATION (THEORETICAL CALCULATIONS BY DENSITY FUNCTIONAL THEORY)

   The fragmentation of the [RhCl(PF3)2]2 molecule in the gas phase calculated theoretically (the enthalpies of several simple decomposition reactions were estimated by the density functional theory) and for comparison was studied experimentally (by mass-spectroscopic studies – see below).

   The geometry of the [RhCl(PF3)2]2 molecule was calculated theoretically (by the density functional theory), it was in a good agreement with the available X-ray crystallographic data. The [RhCl(PF3)2]2 molecule appeared to be not rigid: the PF3 groups can rotate with a relatively low barrier (0.6 kcal/mol); the barrier for the butterfly-like motion of (RhCl)2 moiety is only 3.5 kcal/mol. The lowest energy decomposition pathway corresponds to a consecutive loss of PF3 ligands, resulting in a (RhCl)2 moiety (without P). (same conclusion valid for the ionised precursor). Thus, electron induced dissociation of the precursor cannot be seen as a simple one-step decomposition process, according to the experimental data and theoretical calculations of the energetics of the simple decomposition processes.[i]

[i] P. Seuret,   F. Cicoira,   T. Ohta,   P. Doppelt,   P. Hoffmann,   J. Weber,   T. A. Wesolowski, Phys. Chem. Chem. Phys., 2003,5, 268-274, DOI: 10.1039/B206731E, « An experimental and theoretical study of [RhCl(PF3)2]2 fragmentation », http://pubs.rsc.org/en/content/articlelanding/2003/cp/b206731e#!divAbstract

[RhCl(PF3)2]2 FRAGMENTATION (MASS SPECTROSCOPIC STUDY)

Fig. Mass spectrum of [RhCl(PF3)2]2 (at 70kV electron energy, 1.5*10-6 Torr precursor pressure)

   [RhCl(PF3)2]2 fragmentation was investigated by mass spectroscopy (see Fig.)  using electron energies of 10, 30 and 70 keV, electron current 4.4 mA, at 1.0 x 10-4 Pa (8 x 10-7 Torr) and 2 x 10-4 Pa (1.5 x 10-6 Torr) pressures. The highest fragmentation efficiency [RhCl(PF3)2]2 was observed at 70 keV, however the same relative peak intensity was observed for the spectra acquired with different electron energy, also increasing precursor pressure had no effect on the fragmentation. The mass numbers of 50, 69, 88 and 103 (corresponding to the ions P+, PF2 +, PF3 + and Rh+) have constituted the principal peaks in the mass-spectra; the ratios between the main peaks (f.e.: PF2 + : PF3+ ≈ 0.5; PF2 +: P+ ≈ 0.065 and PF2 + : Rh+ ≈ 0.02) were roughly the same in the spectra measured at different energy; the intensities of the peaks were normalized at the intensity of the 69 amu peak. The mass spectrum of [RhCl(PF3)2]2 with estimated fragmentation structure is shown in Fig. (ionization energy 70 eV, precursor pressure 1.5x10-6 Torr,. 3-4 for m/z > 100). Below 100 amu, the peaks of the ions P+ (31), Cl+ (35), F2+ (38) were clearly detected. The highest peaks were measured at 69 amu (PF2+) and 88 amu (PF3+), same as for the spectrum of free PF3. The single Rh+ peak at mass 103 with a large yield was also obtained. The mother peak at the mass 628 together with its isotopic peaks 630 and 632  (due to the presence of two Cl atoms) was clearly observed. The peaks with consecutive loss of PF3 from the molecular peak  ( (PF3)3Rh2Cl2+ < (PF3)2Rh2Cl2+ < (PF3)Rh2Cl2+ < Rh2Cl2+ ) were observed (with increasing intensity in the noted order). The peaks with the loss of one F atom ( (PF2)(PF3)2Rh2Cl2 + and (PF2)(PF3)Rh2Cl2+ ) were observed as well. Molecualr ions (PF2)Rh2Cl2 + or (PF2)3RhCl+ could be responsible for the peak at 345 amu. Most of the high intensity peaks at m/z > 200, contained two Rh atoms connected by either one or two bridging Cl atoms. The monomer of the molecule, (PF3)2RhCl+ was observed as a minor peak at mass 314.

It was proved by mass-sprectroscopy, that the decomposition of [RhCl(PF3)2]2 started with loss of a PF3 group. A weak Rh-P bond was indicated by the possible further loss of another PF3. The strength of the Rh-Cl bond was indicated by the low intensity of the monomer peak. The peaks, (PF2)3Rh2Cl2 +, (PF2)2Rh2Cl2+ and (PF2)Rh2Cl2 + indicated loss of atomic F from PF3 groups; it seemed to occur after loss of one or two PF3. The suitability of the precursor for the preparation of high content Rh layers by EBID was indicated by the presence of an intense Rh+ peak.

   The obtained mass spectrum allowed to predict that the deposit by EBID should be a high Rh content layer having some Cl contamination. The decomposition path of [RhCl(PF3)2]2 was deduced from the mass spectra was confirmed by Density Functional Theory (DFT) calculations. The calculations predicted that the removal of a PF3 group from the molecule required 45-50 Kcal/mol and that the number of previously removed PF3 groups has no effect on the energy needed for the further PF3 groups elimination, what was confirmed by the comparable intensity of the peaks of the species generated by successive loss of PF3 groups. There is no alternative path at lower energy existing for decomposition of the molecule, as was shown by the calculations; however, the processes involved in EBID are more complicated than simple gas phase ionization, thus can lead to different decomposition paths and make possible re-arrangements of the ionized or decomposed species at the surface.

The composition of deposit obtained by electron beam induced deposition (EBID) of Rh was investigated by AES (Auger electron spectroscopy). Mass-spectroscopic data indicated the presence of free P and Rh ions - in agreement with the EBID deposit composition (60% Rh, 12–25% P, 2–13% Cl, no F, 3–20% O and N).

[RhCl(PF3)2]2 as Rh PRECURSOR FOR MOCVD

     [RhCl(PF3)2]2 was successfully used for Rh deposition in thermal chemical vapor deposition (CVD) [[i]] and in scanning tunneling microscopy (STM) [[ii]] direct writing. Rh deposits with a Cl contamination of a few atomic percent were obtained by thermal CVD at 200°C; patterning of electrically conductive nano-structures was achieved by STM direct writing.

[i] P. Doppelt, L. Ricard, V. Weigel, Inorg. Chem. 32 (1993) 1039.

[ii]F. Marchi, D. Tonneau, H. Dallaporta, V. Safarof, V. Bouchiat, P. Doppelt, R. Even, L. Beitone, J. Vac. Sci. Technol. B 18 (2000) 1171.

Rhodium bis (chloride) bis(trifluorophosphine) dimer [RhCl2(PF3)2]2

[RhCl2(PF3)2]2 was mentioned as potential CVD precursor for Rh-containing thin films.[i]

[i]H. Yang, D. Gealy, G.S. Sandhu, H. Rhodes, M. Visokay, US 6518610 B2 Rhodium-rich oxygen barriers, http://www.google.com/patents/US6518610

[RhCl(PF3)2]2 for Rh metal by electron beam-assisted deposition (EBID)

      [RhCl(PF3)2]2 was applied as precursor for Rh metal deposition by electron beam-assisted deposition (EBID). Residence time measurements showed that [RhCl(PF3)2]2 does not decompose on the surface of stainless steel : residence time of 2 ms was measured, allowing to estimate ~ 0.6 e activation energy for desorption of [RhCl(PF3)2]2 on stainless steel , meaning that precursor molecules can travel distances in the micrometer range before they are desorbed.

      Rh-containig deposits obtained from [RhCl(PF3)2]2 by EBID were characterized by a wide range of techniques including TEM and SEM for the deposit morphology. [i]

[i] F. Cicoira, K.Leifer, P. Hoffmann, I. Utke, B. Dwir, D. Laub, P.A. Buffat, E. Kapon, P. Doppelt, J. Cryst. Growth, 2004, Vol 265, Iss.3–4, p.619–626, « Electron beam induced deposition of rhodium from the precursor [RhCl(PF3)2]2: morphology, structure and chemical composition »,www.researchgate.net/publication/222839787_Electron_beam_induced_deposition_of_rhodium_from_the_precursor_RhCl(PF3)22_morphology_structure_and_chemical_composition/file/9fcfd50eac3cd34ce8.pdf

Rhodium tetrakis(trimethylphosphine) chloride [Rh(PMe3)4]Cl

Rhodium (chloride) tris(trimethylphosphine) [RhCl(PMe3)3]

[RhCl(PMe3)3] AND [Rh(PMe3)4]Cl SYNTHESIS AND STRUCTURE BY SINGLE CRYSTAL XRD

    Several rhodium trimethylphosphine complexes, namely [Rh(PMe3)4]Cl, (A), RhCl(PMe3)3, (B), as well as [Rh(PMe3)3]X, (X = PF6, BPh4), [RhH2(PMe3)4]Cl, RhCl(CO)(PMe3)2, [Rh(PMe3)3(CH2Cl2)]Cl were synthesized and characterized. The crystal structures of [Rh(PMe3)4]Cl, (A), RhCl(PMe3)3 (B) were determined by single-crystal XRD. [[i]]

     [Rh(PMe3)4]Cl (A) is orthorhombic, space group Ccmm with a= 12.366, b= 13.584, c= 12.554 Å, and Z= 4;  whereas RhCl(PMe3)3 (B) is triclinic, space group P1 with a= 8.842, b= 8.982, c= 11.825 Å, α= 98.74, β= 92.38, γ= 116.09°, Z= 2.

    The geometries of both the RhP4+ cation and RhP3Cl molecule can be described as square planar, with considerable tetrahedral distortion as a result of steric crowding. Rh–P distances in [Rh(PMe3)4]Cl (A) are 2.295 and 2.299(1)Å, whilst in RhCl(PMe3)3  (B) Rh–P bonds trans to each other are 2.295 and 2.296(1)Å, but trans to Cl 2.203 Å; Rh–Cl is 2.410 Å.

The complexes can be potentially applied as Rh precursor for MOCVD.

[i] R.A. Jones, F. Mayor Real, G. Wilkinson, A.M. R. Galas, M.B. Hursthouse, K.M. Abdul Malik, J. Chem. Soc., Dalton Trans.,1980,511-518,DOI:10.1039/DT9800000511,« Synthesis of trimethylphosphine complexes of rhodium and ruthenium. X-Ray crystal structures of tetrakis(trimethylphosphine)rhodium(I) chloride and chlorotris(trimethylphosphine)rhodium(I) », pubs.rsc.org/en/content/articlelanding/1980/dt/dt9800000511#!divAbstract

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