INTRODUCTION:

Rhodium can access a variety of oxidation states as a coordination compound. The two most common states are Rh(I) and Rh(III), which are reversible. Rh(I) has a d⁸electron configuration and a square planar 4-coordinate or a bipyrimidal 5-coordinate conformation and the Rh(III) oxidation state has a d⁶electron configuration and an octahedral conformation. The significance of these rhodium compounds as catalysts in several organic transformations such as hydroformylation, carbonylation, and hydrogenation is well documented in literature^1-4. The medicinal properties associated with rhodium compounds is also reported^{5, 6}. A large variety of monodente and bidentate ligands N, O, S and P donor ligands have been explored to design these rhodium compounds^7-11.

We have been long been involved in the synthesis of water-soluble Rh(I)-Phosphines using a variety of polar-group functionalized phosphines and their application in catalytic hydroformylation, isomerization and also in biological studies^12-20.

Mono and dinuclear Rh(I) compounds of the type [Rh(L^L)(CO)(PR₃)] and [Rh(µ- L^L)(CO)(PR₃)]₂ (L^L = COD, or N^O, O^O, type monoionic bidentate ligands and PR₃ = carboxyl, formyl, carbonyl and hydroxyl based polar-phosphine) were explored by our group.

In addition to the Rh(I) chemistry, we are also interested to develop the Rh(III) chemistry with the polar-phosphines. It is well known that the discovery of the antitumor activity of cis-platin, cis-[Pt(NH₃)₂Cl₂], arose interest in studying the biological properties of transition metal complexes²¹. The potential of Rh(III) complexes formed with nitrogen donor ligands of pyridine, ethylenediamine, bipyridine,and 1,10-phenathroline in certain antibacterial studies was recognized^22,23.

The aim of this work is to incorporate a variety polar-group functionalized phosphines into the [Rh(III)Cl₂(1,10-Phen)]⁺ core to afford new water-soluble cationic [Rh(III)Cl₂(1,10-Phen)(PR₃)]⁺ complexes with pendant polar groups for catalytic and biological applications. However, when the rhodium precursor Rh(III)Cl₃ was treated with 1,10-phenanthroline followed by a carboxyl group functionalized phosphine in refluxing ethanol, serendipitous formation of first neutral mononuclear Rh(II) complex of 1,10-phenanthroline as [RhCl₂(1,10-Phen)₂] was identified. A detailed description on the structural features of [RhCl₂(1,10-Phen)₂] was provided by using the data obtained from elemental analysis, single crystal X-ray diffraction, IR, Mass and ESR spectroscopic techniques.

EXPERIMENTAL:

In a Schlenk flask, an ethanolic solution (30 mL) of RhCl₃.3H₂O (0.263 g, 1mmol) was allowed to react with 1,10-Phenanthroline (0.18 g, 1 mmol) followed by (3-carboxyphenyl)diphenylphopshine (0.306 g, 2 mmol). The reaction mixture was refluxed for 4 hrs. During the reaction course the solution color was changed from orange to yellow. While the reaction solution was left over night for slow cooling, an yellow colored semi crystalline solid was formed in the solution. Recrystallization of this solid material in DCM/hexane produced long needles of yellow crystals.

Elemental analysis data was collected from a Perkin Elmer 2400 CHN analyzer. Single crystal X-ray diffraction data was collected from Bruker AXS with CCD area-detector. The structure was solved by direct methods (SHELXS-97); refinement was done by full-matrix least squares on F2 using the SHELXL-97 program suite. Details of data collection and x, y and z coordinates are given in Table 1 and 2.

Table 1 Crystal data and details of structure determination

Empirical Formula	C₁₆H₁₈ClN₂Rh
Formula weight	534.32
Temperature	273
Wavelength	0.71069
Crystal system	monoclinic
Space group	C2/c
Unit cell dimensions
a, Å	15.904(9) alpha = 90 deg
b, Å	13.750(10) beta = 99.20(5) deg
g, Å	12.599(7) gama = 90 deg
Z	8
Density	1.617 mg/m³
Crystal dimensions	0.4 x 0.3 x 20
Absorption coefficient	0.863 mm^-1
F(000)	1324
θ <>	1.97 to 27.57
Index ranges	-20<=h<=20, 0<=k<=17, 0<=l<=16
Reflections collected	3455
Independent reflections	3151
Data/restraints/parameters	3150/0/161
Refinement method	Full-matrix least-square on F²
Goodness of fit on F²	1.040
R1/wR2 (I > 2s(I)	0.0635/0.1274
R1/wR2 (all data)	0.0538/0.1163

Symmetry transformations used to generate equivalent atoms: #1 –x+1, y, -z+1/2 #2 -x+1, y, -z+3/2 #3 -x+1, y, -z+1

Table 2 Atomic coordinates (x10⁴) and equivalent isotropic displacement parameters (A² x 10³). U (equation) is defined as one third of the trace of the orthogonalized Uij tensor.

	x	y	z	U(eq)
Rh	5000	3226(1)	2500	37(1)
C(1)	4158(4)	2499(4)	457(5)	42(1)
N(1)	4791(3)	3156(4)	839(4)	42(1)
C(2)	5154(5)	3676(5)	141(5)	53(2)
C(3)	4916(5)	3576(6)	-985(6)	65(2)
C(4)	4278(5)	2914(6)	-1376(6)	65(2)
C(5)	3877(4)	2344(5)	-663(5)	52(2)
C(6)	3201(5)	1644(6)	-982(6)	61(2)
C(7)	2852(5)	1146(6)	-229(6)	63(2)
C(8)	3119(4)	1282(5)	903(5)	50(2)
C(9)	2754(5)	819(6)	1712(7)	65(2)
C(10)	3029(5)	1040(6)	2776(7)	70(2)
C(11)	3693(5)	1731(5)	3054(6)	59(2)
N(2)	4061(3)	2175(4)	2297(4)	44(1)
C(12)	3778(4)	1966(4)	1240(5)	42(1)
Cl(1)	3917(1)	4430(1)	2378(1)	51(1)

EPR spectrum was recorded on a ESR spectra were recorded on JEOL-JES-FE-3X spectrometer. Mass spectrum was recorded on a MICROMASS-7070 spectrometer.

RESULTS AND DISCUSSION:

The yellow crystals were characterized by elemental analysis, Single crystal X-ray diffraction and ESR and Mass spectroscopic methods. The experimental elemental analysis data (Found: C, 53.86; H, 3.01; N, 10.41%) of these yellow crystalline product is consistent with the formula of neutral [RhCl₂(1,10-Phen)₂] (Calculated C, 53.96; H, 3.02; N, 10.49) instead of the expected cationic [RhCl₂(1,10-Phen)(PR₃)₂]⁺ compound.

The single crystal X-ray diffraction data indicates that the yellow crystals are neutral mononuclear Rh(II) compound formed as [RhCl₂(1,10-Phen)₂]. An ORTEP view of the molecular structure of this compound is depicted in Figure 1. Selected bond lengths and bond angles are given in Table 3. The coordination of Rh(II) is octahedral, which is surrounded by two N,N’- bidentate 1,10-phenanthroline and two monodentate chlorides. The Rh-N(1) bond length 2.067 and and Rh-N(2) bond length 2.064Å are comparable with literature reports^24,
25. Further, the planes the two phenanthroline rings were perpendicular, showing that the N(2)-Rh-N(1)#1 and N(2)-Rh-N(2)#2 are arranged at the angles of 91.1(3) and 95.1° respectively. The Rh-Cl(1) and Rh-Cl(1)#1 bond lengths are also normal. The two chlorides occupied cis-position in the coordination sphere and oriented perpendicularly along the Cl(1)-Rh-Cl(1)#1 axis with an angle of 91.70°. No metal-metal interaction was observed could be due to the steric crowding of the 1,10-phenanthroline around Rh(II).

Figure 1 ORTEP View of [Rh(II)Cl₂(1,10-Phen)₂]. Thermal ellipsoids are drawn at the 30% probability level.

Table 3 Important bond lengths and angles of non-hydrogen atoms on [Rh(II)Cl₂(1,10-phen)₂]

Bond	length(Å)	Bonds	Angle(°)
Rh-N(1)	2.067(5)	N(2)-Rh-N(1)#1	95.1(3)
Rh-N(2)	2.064(5)	N(1)-Rh-N(2)#2	95.1(2)
Rh-Cl(1)	2.377(2)	N(2)-Rh-N(1)	81.1(2)
Rh-Cl(1)#1	2.377(2)	N(2)-Rh-Cl(1)	88.7(2)
C(1)-N(1)	1.381(8)	N(2)-Rh-Cl(1)#1	176.6(2)
C(2)-N(1)	1.334(8)	N(1)-Rh-Cl(1)	88.25(14)
C(1)-C(12)	1.437(8)	N(1)-Rh-Cl(1)#1	95.5(2)
C(12)-N(2)	1.366(8)	Cl(1)-Rh-Cl(1)#1	91.70(10)
C(11)-N(2)	1.343(8)	C(2)-N(1)-C(1)	119.2(5)
C(2)-C(3)	1.334(8)	C(12)-N(2)-C(11)	118.9(5)
C(3)-C(4)	1.393(8)
C(4)-C(5)	1.418(10)
C(5)-C(6)	1.452(10)
C(6)-C(7)	1.359(11)
C(7)-C(8)	1.433(10)
C(8)-C(9)	1.404(10)
C(9)-C(10)	1.376(11)
C(10)-C(11)	1.423(10)

Generally, the Rh(II) compounds can be generated as short lived intermediates by the one electron reduction of Rh(III) compounds in solution. Only a limited number of reports are available on the isolation of stable mononuclear Rh(II) compounds^26-29. It is known that the stability of mononuclear compounds can be improved by sterically bulky ligands such as crown ethers, porphyrins, shiff-bases and pyridines. In this case, the mononuclear Rh(III) compounds with 1,10-phenanthroline were reported^30-33. However, with Rh(II) metal center, only the formation of dinuclear compound containing 1,10-phenanthroline was reported^{34, 35}.

We postulate that the formation of neutral mononuclear [Rh(II)Cl₂(1,10-Phen)₂] is postulated to proceed through the reduction of Rh(III) by polar-phosphine added in the reaction. The reduction of metal ions by phosphine ligands is a known phenomenon in cross-coupling reactions.

To provide further evidence for the existence of +2 oxidation state of rhodium, EPR spectroscopic experiments were monitored. The EPR spectrum recorded at 273K in acetone showed only very broad signals. However, the EPR spectrum recorded at 77K (Fig. 2) showed fine spectrum signals of g_^ = 2.286 ± 0.001 and g_ïï= 2.198 ± 0.001. The g values are normal. The trivalent rhodium has an outer electronic configuration of 4d⁶ and is known to exist in its low spin diamagnetic form. When it traps one electron from polar-phosphine successively, it goes into lower valence states Rh²⁺ (4d⁷). [Rh(II)Cl₂(1,10-Phen)₂] complex is paramagnetic and can satisfactorily explain the observed EPR parameters for Rh centre³⁶. The g-values of the Rh²⁺ centre with g_^> g_ll> 2.0 suggest that the unpaired electron is in a metal orbital having predominant (d_x2-y2) character.This suggests Rh²⁺ at an axial site with compressed to an octahedral coordination^{37, 38}.

Figure 2 EPR spectrum of [Rh(II)Cl₂(1,10-Phen)₂] at 77K.

The molecular ion peak and isotopic pattern of [Rh(II)Cl₂(1,10-Phen)₂] complex shows different m/z values with different intensities as 376(100%,M⁺), 377(19.5%), 378(34.4%), 379(6.6%). The isotope pattern calculations help to calculate the exact molecular formula and molecular weights of the complexes. These values are in good agreement with the proposed molecular formula.

CONCLUSION:

The serendipitous formation of neutral mononuclear complex of [Rh(II)Cl₂(1,10-phen)₂] was demonstrated on the basis of crystallographic and spectroscopic evidences. To our knowledge this the first neutral mononuclear Rh(II) complex with 1,10-phenanthroline. According to our observation a polar-phosphine can reduce the Rh(III) to Rh(II) depending on the synthetic reaction conditions. Preliminary results suggest that this compound possesses good catalytic activity in the hydroformylation of 1-hexene in biphasic system.

ACKNOWLEDGEMENTS:

We thank Professor C. Janaik for sending the X-ray crystal data. PG thank to GVK bioscience for supporting research facilities and SK thanks CSIR, New Delhi for the award of Research Associate.

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Received on 07.10.2012 Modified on 25.10.2013

Asian J. Research Chem. 6(8): August 2013; Page 727-730