4.2 Reactions of Pt/Pd Pincer Compounds
4.2.3 Protonation of Platinum Methyl Complexes
As mentioned in Chapter 1, the low temperature protonation of a rhodium PNP pincer complex by the Brookhart research group yielded the first stable metal- methane σ-complex able to be fully characterised by NMR spectroscopy.25 As the
[(PCP)PtMe] complexes synthesised were isoelectronic with [(PNP)RhMe] com- plexes, the protonation reactions of these platinum methyl pincer complexes were examined in an attempt to determine the extent to which the electronic character of the ligand affected the reactivity of the Pt−Me group.
O X P(C6F5)2 Pt PR2 Me X HPhC(SO2CF3)2 CH2Cl2, -80 oC - CH4 O X P(C6F5)2 Pt PR2 HBF4.Et2O X = O, R = C6F5 X = CH2, R = tBu + BF 4-
Figure 4.8 Protonation of 43 and 45 at low temperature.
The most electron-poor and least electron-poor of the platinum methyl complexes, [(POCOP)PtMe] (43) and [(POCCP)PtMe] (45) respectively, were dissolved in dichloromethane-d2, cooled to −196 ◦C, and treated with a slight excess of the
carbacid HPhC(SO2CF3)2 (Figure 4.8). Each sample was placed in the NMR in-
strument and allowed to warm to −80 ◦C, with the sample monitored continuously
by 1H and 19F NMR spectroscopy. After twenty minutes in the instrument, nei-
ther reaction had progressed at all, and samples were withdrawn and maintained at −78 ◦C. Each sample in turn was treated with an equimolar amount of tetrafluoro-
boric acid and immediately placed in the instrument at −80◦C for further analysis.
NMR spectroscopy of these reaction mixtures showed that both samples had re- acted immediately and completely with the acid, with only methane and the cationic [(PCP)Pt]+ detected in each instance. Non-coordinated BF4− was detected in the 19F NMR spectrum at δ
F = −152 ppm for each sample (coordinated BF4− has been
observed at δF = −169 ppm for a similar cationic PCP pincer species139), ruling out
CH4 displacement from the coordination sphere of the metal by BF4−, and indicat-
ing that the parent methyl complexes 43 and 45 do not interact with any liberated methane at this temperature. Similar to the decarbonylated palladium complexes 38, 39, and 40, the “vacant” coordination site on the metal centre is likely to be occupied by solvent in solution, owing to the instability and high reactivity of 14- electron species.
4.3
Concluding Remarks
A major drawback to the synthesis of electron-deficient PCP pincer complexes is the increased energy barrier to ligand metallation for electron-poor ligands. The choice of starting material has a pronounced effect on the ease of synthesis of metallated pincer complexes. Altering the ancillary ligands on [PtCl2Ln] species was observed
to increase the rate of metallation with increasing binding strength of donor ligand. Although this result appeared somewhat counterintuitive, readily displaced ancillary ligands favoured the formation of dimeric intermediates, which increased the barrier to metallation, as they underwent an energy-consuming rearrangement process to allow metallation to occur. With the use of more strongly donating diethyl sulfide ligands in place of 1,5-hexadiene, dimer formation was noticeably diminished and metallation of ligand 1 proceeded approximately twice as fast.
A significant increase in the reactivity of the starting material could be achieved by the substitution of an anionic chloride ligand for either a hexamethyldisilazane group or a methyl group. These substituents provided good proton-accepting leav- ing groups, reducing the reaction times and temperatures required for metallation. However, as these starting materials required more complicated syntheses than the simple platinum dichlorides, the increase in reactivity they provided may not offer a practical advantage in terms of time saved or overall atom economy.
The electronic nature of the ligand also played a large part in the ease of forming PCP pincer complexes. Metallation was seen to be significantly more facile for less electron-poor ligands, due to their increased ability to donate electron density into the σ*
-antibonding orbital of the C−H bond being cleaved. However, the rate of metallation was not proportional to the yields of each complex obtained; it was observed that P−O bonds were unstable for prolonged periods under metallation conditions, and the fewer P−O bonds a ligand possessed the greater the yield of the metallated complex.
With the successful synthesis of the platinum and palladium [(PCP)MCl] pincer compounds, these were subjected to halide abstraction and treatment with carbon monoxide to produce the metal carbonyl species [(PCP)M(CO)]+. Examination of
the carbonyl stretching frequency by infrared spectroscopy confirmed that POCOP complexes were the most electron-poor, and POCCP complexes the least electron- poor. It also revealed that a number of these compounds possessed C−O stretching frequencies greater than that of free carbon monoxide, and were among the most electron-deficient pincer compounds known to date. Crystallographic characterisa- tion of [(POCOP)Pt(CO)][SbF6] revealed the expected tridentate, planar coordi-
nation of the ligand, as well as a short carbonyl C−O distance arising from the electron-deficient metal centre.
Whilst the platinum carbonyl complexes proved to be indefinitely stable, the palla- dium carbonyl species underwent gradual loss of carbon monoxide over time in the solid state. This CO loss could be encouraged by the passage of inert gas though solutions of the palladium carbonyl, with the carbonyl complex regenerated upon the introduction of carbon monoxide into solution. This reversible carbonyl binding was observed be more facile with decreasing electron density on the metal centre. Methylation of the [(PCP)PtCl] species was achieved via the use of dimethyl zinc; reactions with methyl magnesium iodide led to halide exchange and the forma- tion of platinum iodide species, while methyllithium led to decomposition of the electron-poor complexes. The methylation of [(POCCP)PtCl] with methyllithium was observed to be facile and selective on an NMR scale, but upon scaling up led to nucleophilic attack on the phosphinite donor, displacing a pentafluorophenyl group which migrated to the metal to form the unusual byproduct [(PMeOCCP)Pt(C
6F5)].
In all cases treatment of the corresponding palladium pincer complexes with methy- lating agents led to decomposition.
Ligand electronic effects play a large part in dictating the synthesis and reactivity of platinum and palladium PCP pincer complexes. Species with highly electron- withdrawing ligands are significantly harder to synthesise than their more electron- rich analogues, and are more susceptible to decomposition or unwanted side reactions during reactions with nucleophiles. However, possessing a highly electron-deficient metal centre was seen to be a boon for the palladium pincer carbonyl species, as it favoured the reversible binding of carbon monoxide, and also allowed the electron- deficient platinum chlorides to react rapidly and selectively with dimethyl zinc.
Chapter 5
Synthesis and Reactivity of PNP
Pincer Complexes
In Chapters 3 and 4, it was observed that the incorporation of electron-withdrawing bis(pentafluorophenyl)phosphine substituents into the PCP pincer framework re- sulted in significant energetic barriers to ligand metallation and pincer complex formation. Metallation reactions typically required prolonged thermolysis, and pro- ceeded through a number of stable oligomeric intermediates. To circumvent the need for ligand C−H activation, the synthesis of PNP pincer complexes was investi- gated. The pyridyl backbone of PNP pincer ligands possesses a nitrogen donor with a lone pair of electrons, meaning the only bond broken in the metallation reaction is between the metal centre and the ligand being displaced by the pyridyl group. Without the requirement for scission of a strong C−H bond, tridentate pincer coor- dination should be achieved under significantly milder conditions with electron-poor PNP ligands than for the analogous PCPH ligands.
While it may assist in pincer complex formation, moving from a phenyl to a pyridyl ligand backbone will have a pronounced effect on the character of the metal complex produced. When metallated, PCP pincer ligands are nominally monoanionic, due to the deprotonated aromatic carbon, while PNP ligands carry no formal charge. This means that when identical metal complexes are prepared with PCP and PNP ligands, the complex with the PNP ligand will carry an extra +1 charge. Moreover, because the pincer coordination motif places the donor atom of the ligand backbone
trans to an available coordination site on the metal centre, PCP and PNP complexes
will possess different trans effects. Computational studies on platinum complexes have shown that the donor atom trans to the active site has a large influence on the energy barrier for C−H activation reactions, with more electronegative donor atoms
producing a lower energy barrier.149,150 Therefore, PNP pincer complexes may be
better suited for small small molecule activation reactions than PCP pincer com- plexes.
5.1
Synthesis of Platinum PNP Complexes
As the ease of PCP pincer complex formation was rigorously investigated with a number of platinum chloride starting materials, the coordination chemistry of the PNNNP and PONOP pincer ligands 10 and 11 was initially investigated with the same platinum precursors. This would enable a direct comparison of the ease of pincer complex formation between the electron-poor PCPH and PNP pincer ligands, which would also help to explore whether significant barriers to electron-poor pincer complex formation existed, other than C−H activation.
Previous reports indicated that the formation of electron-rich [(PNP)PtX]+ species
occured in a matter of hours from [PtX2(diene)] precursors at room temperature
in dichloromethane.78 Ligands 10 and 11 were reacted with [PtCl2(COD)] at room
temperature in dichloromethane-d2, with reaction mixtures monitored in situ by
NMR spectroscopy. Results showed that reactions with the PNNNP pincer ligand 10 proceeded smoothly but slowly, with the quantitative formation of the pincer complex [(PNNNP)PtCl]+ (47) observed after 22 hours at room temperature. Re-
actions with the PONOP ligand 11 proceeded extremely slowly — after 48 hours at room temperature, starting materials still comprised the greater part of the reaction mixture.
The [(PNNNP)PtCl]+ complex 47 was isolated as the chloride salt upon upscaling
of the initial exploratory reaction (Scheme 5.1). Mass spectrometry data confirmed the formulation of 47 as the cationic PNP pincer complex [(PNNNP)PtCl]+, with
the [M]+ ion detected at 1066 amu. The NMR spectra of this compound displayed
the expected three environments in the 1H and 19F NMR spectra. The 31P NMR
spectrum displayed a singlet resonance at δP= 30.6 ppm (free ligand 10 appeared at
δP = −10.8 ppm), with platinum-phosphorus coupling consistent with a mutually trans arrangement of phosphorus donors (1J
Pt-P = 3153 Hz), as expected upon
pincer coordination.
The unexpectedly low reactivity of the PONOP ligand 11 could be attributed in part to it being significantly more electron-poor than the PNNNP ligand 10. This differ-
N H N N H P(C6F5)2 P(C6F5)2 N H N N H P(C6F5)2 Pt P(C6F5)2 Cl CH2Cl2 [PtCl2(COD)] Cl r.t. 37% yield 10 47
Scheme 5.1 Synthesis of the platinum PNNNP pincer compound 47. ence in electronic character has previously been observed for PCP pincer complexes — carbonyl stretching frequencies reported for POCOP and PNCNP complexes with identical substituents have indicated that complexes of phosphoramines are signifi- cantly more electron-rich than those of analogous phosphinites.52 This observation
was also supported by the 19F NMR ∆
δm,p values for these PNP ligands (10.6 ppm
for 10, 12.2 ppm for 11), which demonstrated that the phosphorus donors of the PNNNP ligand 10 were more electron-rich than those of the PONOP ligand 11. In order to promote the pincer coordination of the more electron-poor ligand 11, the synthesis of [(PONOP)PtCl]+ was subsequently attempted in acetone, as the more
polar solvent should better stabilise the charge developed during the formation of the desired cationic PNP pincer complex (according to the Hughes-Ingold Rules).151Re-
actions were performed on an NMR scale between ligand 11 and platinum dichlorides [PtCl2(hex)] and [PtCl2(SEt2)2] in acetone-d6. Analysis of the reaction mixtures by
NMR spectroscopy revealed that both reactions produced the same product (48). However, reactions performed with [PtCl2(SEt2)2] proceeded more selectively than
reactions with [PtCl2(hex)], resulting in quantitative formation of the new species, 48, in solution after 15 hours at room temperature.
Unfortunately, the NMR data obtained for compound 48 indicated that it was not the desired PNP pincer complex. For all of the pincer complexes reported herein, the
31P NMR chemical shift of the phosphorus donors was observed downfield from that
of the free ligand, due to the displacement of an electronegative chloride ligand upon backbone coordination. In 48, the 31P resonance was shifted significantly upfield
from that of the free ligand (at δP= 29.0 ppm, compared to 70.2 ppm for ligand 11).
Together with the observation of a large platinum-phosphorus coupling constant (1J
Pt-P = 4149 Hz), this was consistent with compound 48 being the cis-bridged
oligomeric species cis-[(PONOP)PtCl2]x (Scheme 5.2). For the analogous platinum
POCOPH coordination chemistry discussed in Chapter 3, the 31P NMR resonances
of phosphorus donors of unmetallated ligands coordinated trans to a chloride ligand (in cis-[(POCOPH)2PtCl2], 18, and cis,trans-[(POCOPH)PtCl2]2, 19) were shifted
coupling of about 4500 Hz. This was consistent with the upfield shift from starting material of 41.2 ppm and platinum-phosphorus coupling of 4149 Hz observed for 48, especially as PNP complexes have been observed to display platinum-phosphorus couplings around 400–500 Hz less than their PCP analogues.∗ Analogy with the
coordination chemistry of the POCOPH ligand also suggested that the oligomer 48 is a dimer. Analysis of this oligomer by mass spectrometry gave only signals attributed to degradation of 48 inside the instrument, and so the nuclearity of 48 was not established. Reaction mixtures containing the oligomer 48 in acetone were heated to reflux for 48 hours in an attempt to promote the formation of the triden- tate PONOP pincer complex. However, compound 48 appeared stable in acetone solutions up to 60 ◦C, and no further reaction was observed.
N O O PR2 PR2 acetone [PtCl2(SEt2)2] ∆ Cl x Pt R2 P R2 P Cl O O N R = C6F5 ∆ X N O O PR2 PR2 Pt Cl Cl 11 48
Scheme 5.2 Formation of the oligomeric platinum PONOP compound 48. Syn- thesis of the corresponding PONOP pincer complex was not observed even upon prolonged thermolysis.
To facilitate the formation of the PONOP pincer complex, reactions were also un- dertaken between the PONOP ligand 11 and [PtCl2(NCMe)2] in acetonitrile-d3,
as well as between 11, [PtCl2(SEt2)2] and NaSbF6 in acetone-d6. Similar reaction
conditions have been successfully employed in the synthesis of tert-butyl-substituted PONOP pincer complexes of platinum and palladium.153However, with the electron-
poor ligand 11, neither of these reaction mixtures showed any signs of the desired pincer complex [(PONOP)PtCl]+. Analysis of both reaction mixtures by NMR spec-
troscopy revealed quantities of the oligomeric bridged species 48. The observation of 48 having formed from starting materials with differing ancillary ligands helped to confirm its formulation as [(PONOP)PtCl2]x, as it ruled out 48 possessing any
neutral ligands other than the PONOP ligand 11 (such as diethyl sulfide).
Reactions under more harsh conditions were not attempted, as the reason for the investigation of PNP pincer ligands was that they generally allowed for pincer com- plex formation under more mild conditions than for analogous PCPH pincer ligands. The coordination chemistry of ligands 10 and 11 with platinum dichloride starting
∗For example, where the phosphine donor is CH
2PtBu2, the 31P NMR data for the PCNCP
pincer complex is δP= 53.7 ppm,1JPt-P= 2403 Hz;152for the PCCCP pincer complex it is δP=
64.9,1J
materials had clearly demonstrated that pincer complex formation was significantly more facile for the more electron-rich PNNNP ligand 10 than for the electron-poor PONOP ligand 11. This low reactivity of ligand 11 was likely to be due to its inability to displace a chloride ligand from the metal centre. As the synthesis of PNP pincer complexes requires halide displacement from the metal by the nitrogen donor of the pyridyl backbone, the decreased electron density on metal centres with electron-poor ligands should lead to an increased electrostatic interaction between the metal and halide ligand, decreasing the ease of halide displacement. It may also be anticipated that cation formation for complexes of the more electron-poor PONOP ligand 11 would be less favourable than for complexes of the PNNNP lig- and 10, as there would be less electron density available for the PONOP ligand to stabilise the positive charge developed on the metal centre.