3.2 Preparation And Characterisation
3.2.7 Preparation of [Rh2(02CCH3)2(phen)2(NCgHs) 2 ] [PF ^ ; ]
A drop o f pyridine was added to a methanolic suspension of
[Rh2(0 2 CCH3)2(phen)2Cl2], which had been prepared by the reported literature m e t h o d . A methanolic solution K[Pp6] was then added whilst stirring at room temperature. The red/orange crystalline precipitate which formed was collected by filtration, washed with methanol and dried
in vacuo.
3.2.8 Characterisation of [Rh2(02CCH3)2(phen)2(NC5H5)2][PFj2*
The infrared data confirmed the presence o f the counterion [PF^]' with a strong band
x>
(P- F) at 844 cm *. The *H NMR spectrum, Figure 3.11, containsresonances for the acetate ligands as well as phenanthroline and pyridine ligands, the signals o f which overlap in the aromatic region o f the spectrum. An off- resonance *H NMR decoupling experiment enabled the correct assignment o f the signals. The signals at Ô 7.78 ppm (integral 1) was assigned to the H3 proton.
Figure 3,10 *H NMR Decoupling Experiments on JRh,(HNOCCF,),(phen),(NCHc),irPFJ,
I
Figure 3.11 H NMR Spectrum of rRh,(0,CCH,UDhen),(NC.H,),11PF.l,
C » _ J
a>-\
Irradiating at this frequency results in the collapse of the multiplet at Ô 7.27 ppm indicating this signal to be due to H2. Irradiating at this latter frequency causes the signal at Ô 8.19 ppm to collapse, indicating this signal to be due to Hi (see Figure 3.11 for numbering scheme on the pyridine). The signal at the highest chemical shift (Ô 8.69 ppm) is assigned to the most deshielded proton of the phenanthroline, Ha, the signal at ô 8.25 ppm is assigned to the He protons which are
para
to the nitrogen atom. The multiplet at 5 7.66 ppm is due to the Hb protons since it couples to both Ha and He. The singlet at ô 7.70 ppm is therefore assigned to the Hd protons. It is obvious that coordination has taken place both to thephenanthroline and pyridine ligand, since there are changes in the chemical shifts of the proton Hb and He (by about 0.05 ppm) relative to the free ligand values. The greatest change is a shift o f 0.5 ppm upfield of the signal due to Ha. These changes are echoed in the pyridine signals with an upfield shift of 0.5 ppm, of the signal due to H i. There is a shift of approximately 0.2 ppm of the signals due to Ha, Hb, and He, and 0.1 ppm for that of Hd, on comparing the spectrum of [Rh2(0 2 CCH3)2(phen)2Cl2] to that of [Rh2(0 2 CCH3)2(phen)2(NCsH5)2][ P F j2. This is due to the increase in the net positive charge on the dirhodium unit, which occurs on changing the axial ligand from chloride ion to pyridine.
3.3 Structural Investigation o f rRh,(HNOCCF,)^(phen)^(NCÆ)^ÏÏPFJ^ and rR h,(0,C C H ,).(phen),(N C Æ ),ÏÏPF J,
Crystals o f the ether solvate [Rh2(HNOCCF3)2(phen)2(NC5H5)2][ P F j2 (C2Hg) 2 0 were obtained by recrystallisation of the compound from diethylether/dichloromethane.
Crystals of [Rli2(0 2 CCH3)2(phen)2(NC5H5)2][ P F j2 (CH3)2(CO) were obtained by the slow evaporation of an acetone/diethylether solution. The basic structures of both of the cations [Rh2(HNOCCF3)2(phen)2(NC5H5)2]^‘^, Figure 3.12, and
[Rh2(0 2 CCH3)2(phen)2(NC5H5)2]^‘^, Figure 3.13 are similar to those of
[Rh2(0 2 CCH3)2(chel)2(NMid)2]^"’ compounds which have been reported in the literature The X-ray crystal structure determination of [Rh2(HNOCCF3)2-
(phen)2(NC5Hj)2]^^ reveals a dirhodium unit bridged by two amidate ligands and chelated by two phenanthroline ligands, with the pyridine ligands in the axial sites. Each rhodium ion is surrounded by two phenanthroline nitrogens, a pyridine nitrogen, an oxygen and nitrogen atom from two different amidate ligands and the other rhodium ion. The geometry is that o f a distorted octahedron about each o f the metal centres. The angles between adjacent atoms in the metal coordination sphere range from 81.2(1)° to 97.0(1)°. The former angle is formed by the ’’bite” of the phenanthroline ligands, and the latter is that between Rh-Rh and Rh-N(phen) vectors. The Rh-N(pyridine) bonds form an average angle of 171.3(2)° with the Rh-Rh bond. This non-linearity is often observed in structures in which steric interactions between axial and bridging ligands is observed. However in this structure no such steric interactions are likely to be present. The geometries about the dirhodium unit in [Rh2(0 2 CCH3)2(phen)2(NCgHg)2]^^ are very similar. One significant difference between the two structures is in the Rh-Rh bond lengths which is much greater in [Rh2(HNOCCF3)2(phen)2(NCgH5)2]^^, 2.612(1) Â,
compared to [Rh2(0 2 CCH3)2(phen)2-(NC5H5)2]^^, 2.559(1) Â. Comparing this Rh-Rh bond length o f 2.612(1) Â to that observed in [Rh2(HNOCCF3)4(NC5H5)2], 2.472(3) Â we see an increase of 0.14 Â. This difference is observed with other tetra-
Figure 3.12 The Structure o f rRh->(HNOCCF,),(phen)->(NCcHc),12+
C 2 6 r : ; r ^ C 2 7
C30N7
Figure 3.13 The Structure of rRh,fO,CCH.),(phen),(NCÆ),l^*
C 3 2
C 3 6
C 3 8
amidato bridged compounds. A similar difference is observed when making the analogous comparison for doubly and tetra bridged acetato compounds,
ie.
comparing the Rh-Rh bond length o f 2.559(1) Â in [Rh2(0 2 CCH3)2(phen)2-
to that in [Rh2(0 2 C C H ;)/N C ;H ;)j, 2.396(1) Â; a difference o f 0.16 Â is observed. This increase in Rh-Rh bond length as the number of bridging ligands is reduced is probably due to the decrease in the constraints due to the ’bite’ experienced by the Rh-Rh bond. The Rh-Rh bond length o f [Rh2(HNOCCF3)2- (phen)2(NC3Hg)2]^^, is surprisingly similar to that observed for [Rh2(CH3CN)io]'^, 2.624(1) A, which experiences no constraints due to bridging ligands, since it has none. The Rh-Npy bond lengths in both these new compounds are similar, and the average Rh-O^^jj^^g bond lengths, 2.057(5) A, are, as expected, longer than the average Rh-N^midate bonds 2.047(5) Â although the difference is not statistically significant. The Rh-Np^en
trans
to the nitrogen, 2.040(4) Â is greater than the Rh- Nphentrans
to the oxygen, 2.020(5) A, due to the differenttrans
influences of the oxygen and nitrogen atoms. The Rh-Np^^n bond lengths, 1.999(5) Â and 2.010(4) A, in [Rh2(0 2 CCH3)2(phen)2(NCgHg)2]^^ are statistically indistinguishable, as might have been expected.In both [Rh2(HNOCCF3)2(phen)2(N Q H5)2]^+ and [Rh2(0 2 CCH3)2(phen)2(NC5H5)2]^^ the phenanthroline ligands are rotated by 15.8° and 15.4° respectively, from a totally eclipsed conformation, and torsion angles in the bridging ligands are 0 (1)- Rh(l)-Rh(2)-N(7), 14.6°, 0(2)-R h(2)-R h(l)-N (8) 13.8°, and 0 (l)-R h (l)-R h (2 )-0 (2 )
13.7°, 0(3)-R h(l)-R h(2)-0(4) 12.6°, respectively. The mean planes of the phenanthroline ligands are not parallel in either structure, with an angle of 8.7° between the two ligand planes in [Rh2(HN0 CCF3)2(phen)2(NC3Hg)2]^^ and a
dihedral angle of 9.9° for [Rh2(0 2 CCH3)2(phen)2(NC5H5)2]^^. These angles are less than those observed in the closely related compounds [Rh2(OCR)2(R’COCH- C0 R ” )2(NC5H5)2], where the pairs of chelating ligands are pushed away from coplanarity by substantial steric interactions.*'^^ The two pyridine ligands in
[Rh2(HNOCCF3)2(phen)2(NC5Hg)2]^^ are near to coplanar, 15.1°, and are situated in an area between the chelating and bridging ligands. In contrast in [Rh2(0 2 CCH3)2- (phen)2(NC3Hg)2]^^ the two pyridine ligands are almost orthogonal, 84.4° with the mean plane o f one, N(6)-C(30)-C(31)-C(32)-C(33)-C(34) almost bisecting the N(3)- Rh(2)-N(4) angle. The closest approach, of the phenanthroline ligands, ring
centroid to ring centroid, is 3.5 Â.
3.4 Removal o f The Axial Ligands.
The 1,10-phenanthroline ligand is planar and has a delocalised
n
system, and hence ought to have potential for the type o f coupling reaction seen in Chapter two. In order to promote coupling, attempts were made to remove the axial ligands.The two compounds prepared, [Rh2(0 2 CCH3)2(phen)2(py)2][ P F j2 and [Rh2(HNOC- CF3)2(phen)2(py)2][ P F j2, although structurally similar (x-ray evidence) might be considered to have very different electronic properties.3.4.1 Silica Column Method
Removal of the axial pyridine molecules was attempted by loading the two compounds previously prepared onto a silica column and eluting each with
methanol. However after each compound had progressed down their respective columns by no more than 5 cms, the compounds were seen to stick to the silica and remain in the same position on the column. This procedure was repeated using different solvents, such as dichloromethane, n-hexane, and ether. However the compounds remained fixed on the silica in each case.
This is probably due to the fact that these compounds are charged, unlike the earlier p-diketone compounds described in Chapter 2.