Spectroscopic Elucidation of - the Coordinative Form of the DPPM Ligand in [ ~ o ( C 0 ) 4 ( ~ ~ - d p p m and )] f a c - [ ~ o ( ~ ~ ) ~ ( ~ ~ - p h e n ) ( ~ ' - Complexes dppm)] A Challenging Organometallic Experiment for Undergraduate Students Mercedes ~ a n o , 'Jos5 A. Campo, Paloma Ovejero, and Jose V. Heras Departamento de Quirnica Inorganica, Facuitad de Ciencias Quimicas, Universidad Complutense,28040-Madrid (SPAIN) There is increasing evidence that the transition metal carbonyl complexes have played an important role in the systematic development of coordination chemistry ( I ) . One of the most common ways used for the synthesis of metal carbonyl complexes is the replacement of carbonyl groups on the group 6 metal hexacarbonyl by different Zdonor ligands (Z = P, N, 5.. . .) (2). I n t h e P-donor ligand class, bis(dipheny1phosphine)methane (dppm), synthesized by Issleib and Muller in 1959 (3),is currently playing a key mle as a small-bite PPdonor ligand. It also is remarkably versatile in its properties, being coordinated as monodentate, bridge or less frequently chelate (Fig. 1)(41, and allowing the stabilization of homonuclear and heteronuclear metal-metal bonded
Figure 2. Sequence of the reactions proposed in the experiment. ligand in the complexes (1)and (2) by 31P-NMR spectrosCOPY. This experiment could be performed by a small group of students (2 or 4) and one week should be used a t least. Experimental Caution The metal carbonyl compounds should be considered as taxic and should be handled with care. However, the amounts of reagents used m a k e these experiments less dangerous. The solvents used are flammable. Gloves should be worn during all transfers, and the solvents should be handled with care.
AU reactions must be carried out in a eff~cientventilation hood, because carbon monoride is evolved.
Figure 1 . Differentcoordination modes of the dpprn ligand. and characterization of molybdenum carbonyl compounds containing dppm in two different coordinative forms (monodentate or hidentate-chelate). The complexes [ ~ o ( ~ ~ ) ~ ( q ~(1) - and d~~ m)l [M~(CO)~(q~-phen)(q'-dppm)l (2) (phen= lJ0-phenanthroline) have been chosen as representatives of both dppm coordinative forms. Group 6 carbony1 complexes containing bridged dppm have been preferentially described in heterobimetallic systems ( 5 ) and this kind of complex is not considered in this work. The complexes studied here were prepared by substitution of carbonyl groups of Mo(C0)e by PP- or NN- and PPdonor ligands using thermolytic mutes. Figure 2 summarizes the reactions carried out. The aim ofthis experiment deals principally with the determination of the coordinative form of the PP-donor 'Author to whom correspondence should be addressed
600
Journal of Chemical Education
General All manipulations were performed under oxygen-free dry nitrogen, using freshly distilled, dried and degassed solvents. Molybdenum hexacarbonyl, 1,lO-phenanthmline and bis(dipheny1phosphine)methane were use as purchased. The complex [M~(CO)~(q~-phen)l was prepared as previously described (6). Preparation of Compounds Preparation of [ ~ o f ~ ~ ) d q ~ - d p(Refs. p m j l(71) A mixture of molybdenum hexacarbonyl (1.0 mmol; 0.264 g)dppm (1.0 mmol; 0.385 g) and 15 mL of diethyl glycol dimethyl ether (diglyme)was heated a t 115 'C for 10 h. After cooling and adding methanol (10 mL), the crystals that had formed were removed by filtration. The crude product was recrystallized three times from chlorofodmethanol. (Yield: 75%.) Other synthetic procedure for the above mmpound have been described in the literature, but more safety . precau. tions should be used in this case(8). Preparation o f ~ o ( ~ 0 ) 3 ( q ~ - p h e n ) f q ~ -(Ref: d p p 19)). m~ To a solution of [ ~ o ( C ~ ) ~ ( q ~ - (1.0 ~ h emmol; n ) l 0.388 g) in acetone (15 mL) was added dppm (1.0 mmol; 0.385 g). The
T coq>N
metry) are consistent with those characteristic of cisDlo(C0)dL)~lcompounds (12). The same behavior is obtained \O c /? when the spectrum was registered in solid state (KBr pellets). On the other hand, the IR spectrum of 2 shows three strong bands in the same region (Fig. 3). N This pattern is consistent with the presence of a tricarbonyl complex, but it does not determine the establishment of a fac or mer distribution of the three carbonyl groups around the metal. The distinctionhetween the fac and mer isomers in [MO(CO)~L~I compounds often is made on the basis of the number of carbonyl stretching bands, due to the fact that a fac configuration for the Mo(CO)~group (C3" symmetry) should give two active modes (A1+E) and a mer Mo(COI3 arrangement (C* point group) should afford three active modes (2A1+B)(12). However some ambiguities are observed in the literature data related with this m IRM) 46% fact (13). I II In addition if the fac-M0(C0)~ moiety is considered in the whole of molecules of the type [Mo(CO)&YI (X = NN-bidentate donor ligand; Y = P, N, S. . . .Figure 3. IR spectra in the carbonyl region and representation of the molecular rearrangement of donor ligand) C3" symmetry may ( l e n ) [ ~ o ( ~ ~ ) h ~ - d( p m )(right) l 1 )pand [ ~ o ( ~ ~ ) ~ h ~ - ~ h e n ) h '(2). -d~pm)l be involved only ifX and Y are indistinguishable in their bonding properties, but C, molecular mixture was refluxed for 30 min. A change in the color (red symmetry is applicable when X and Y differ appreciably in to violet) was observed. The solution was cooled to room their bonding properties, then the E mode (in C3,. symmetemperature, and the precipitate formed was filtered off, try) split into A'+ A" modes (in C, symmetry). Both modes washed with hexane (2 x 10 mL) and dried in vacuo. (Yield: together with the remaining A' mode (A1in C3, symmetry) 8O%J are consistent with three vCO bands observed in some of fac mentioned complexes (14). Results and Discussion Because IR spectra may not alwa s conclusively distinguish between fac and mer isomers, 'c-NMR spectroscopy P-NMR Study could be considered more appropriate to establish this difference. The 31P-NMR spectra of both compounds, (1)and (2), were registered using 85%H3P04as an external reference. %NMR Study The snectrum of 1shows a sinelet at 1.5 n ~ m (10). consistent bith the equivalence of tce two phosiho&s atoms. The 13C-NMRspectrum of 2 was registered in CDCl3 soThis signal is shifted 23.17 ppm with respect to the free lution, using TMS as an internal reference. dppm ligand (-21.67 ppm (Ill), indicating the coordination The signal at 217.15 ppm, a doublet ('JGp.= 39.3 HZ), of this ligand to the metal. Thus, a PP-bidentate chelate was assigned to the carbon atom of the carbonyl group coordination form is deduced for this complex (Fig. 3). trans to the phosphorus atom (9, 15). Two additional sigThe 31P-NMR spectrum of 2 shows two resonances at nals appear very close at 228.85 and 228.97 ppm as sin28.7 and -27.5 ppm. Both signals are split into doublets glets, and they were assigned to the other carbonyl groups due to the phosphorus-phosphorus coupling (2Jp,pb= 66.1 NN-bidentate ligand. Thus, a fac configuration trans to the Hz). These signals are assigned to the coordinated (Pa)and for this compound is concluded. uncoordinated (Pa) phosphorus atoms, respectively (9). The crystal structure of 2, desnibed in the literature (9), This observation is consistent with the presence of a is in agreement with the above deduction. monodentate dppm ligand (Fig. 3).
7 ,f -
IV
I_
"
IR Study
The IR spectrum (in 12-dichloroethane or cyclohexane solution) of 1shows four bands in the stretching carbonyl region (vCO) (7, 8) (Fig. 3). The pattern and the assignment of these bands as the 2A1, Bz, and B1modes (C* sym-
H-NMR Study
The 'H-NMR spectra of both complexes, 1 and 2, were recorded in CDC13 solution with tetramethylsilane as the internal reference. They show the characteristic resonances of the coordinated ligands. Volume 70 Number 7 July 1993
601
Spectroscopic Data for the Complexes 1 and 2 2
1
IR
v(C0) ( ~ m - ' ) ~2020s 1905vs 1925vs 181Ovs 1900vs 1760vs ... 186UVS -
The table summarizes the infrared and NMR spedmscopic data of We compounds 1and 2. Conclusion The above considerations, along with the literature references, should allow the students to establish the followinrr ---m wanlta.
.The distinction between a bidentate-~helateand monodentate dppm ligand by 31~-NMR. .This fact could be supported by 'H-NMR spectroscopy. The use of the group theory to assign We vCO frequencies in order to establish the differences between fac and mer isorners. T h e unambiguous assignation of the fac isomer by %NMR
2020s 1915vs 1920sh 1819s 1 9 0 7 ~ 5 1790s 1879s
'
speetrascopy. 1900' 1H-NMR
'
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s(c&) (pprn)@ 4.48(0
z2.48(dd)
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2 ~ ( ~(HZ) - ~ ) 9.0
4.8 (H-Pa)2.7(H-Pb)
2.
2 ~ ( ~ o -(HZ) ~ , )-
39.3
Wm2) ( P P ~ )~
27.82(dd) 32.7 Pd (C-Pa) 21.4(C-
2 ~ ( m z -(HZ) ~)
-
3. 4. 5.
6. 7. 8. 9.
lo. 2 ~ ( ~ r (HZ) ~b) s v s v e ~ s t m n gs , .nrong. 4 shoulder KBr pellets, ref. this work. 'In KBr pellefs. ref. 9 1,2dichloroethanesolution, ref. 7. 'In dichlomrnethane solution, ref. 9. 'In cyclohexane solution, ref. 8. %triplet. 6 doublet, d& doublet of d o u b l e t s %fs. 8 a n d 10.
66.1
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s= singlet.
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11. 12. 13.
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Journal of Chemical Education
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