Syntheses and 31 P NMR Studies of Transition Metal Complexes Containing Derivatives of Dioxaphospholane and Dioxaphosphorinane

A síntese de vários complexos de Pt(II), Pt(0) e Pd(II) com 2-cloro-1,3,2-dioxafosfolana, 2-fluoro1,3,2-dioxafosfolana, 2-cloro-4,5-benzo-1,3,2-dioxafosfolana, 2-fluoro-4,5-benzo-1,3,2-dioxafosfolana, 2-cloro-1,3,2-dioxafosforinana e 2-fluoro-1,3,2-dioxafosforinana é descrita. Estudos de RMN de 31P{1H} destes complexos revelam alguns aspectos inexplorados em relação às constantes de acoplamento 1J(PtP), os quais chamam a atenção para o conceito de ácidos e bases duros e macios. Este trabalho mostra também as diferenças espectroscópicas de RMN de 31P{1H} entre isômeros cis e trans.


Introduction
A variety of complexes containing phosphorus ligands have been reported over the past years and the interest on their chemistry is growing due to their possible catalytic activity.Our main interests have been to prepare new complexes containing unusual phosphorus ligands and investigate their chemical properties by the use of NMR spectroscopy.
The reactivity of dioxaphospholane and related compounds towards several organic compounds is well documented 1 .As a result of those studies several applications for this class of compound have been found.Some examples are the use of dioxaphospholanes in the development of specific imunoassays for the detection of pesticides 2 , polymerisation reactions 3,4 , syntheses of naturally ocuring lipids 5 and to analyse labile hydrogens functional on coal materials 6 .However few publications dealing with complexation of dioxaphospholanes have appeared in the literature.
Recent reports of the new facile preparation methods of dioxaphospholane derivatives 7 and their ruthenium (II) complexes 8 prompted us to report some new platinum and palladium complexes.

Results and Discussion
The 31 P{ 1 H} NMR spectra of the three complexes show a similar pattern of lines, which consists of an AB 2 X spin system (A,B = 31 P, 100%; X = 195 Pt, 33.8%) for the two first compounds and of an AB 2 XY spin system (A,B = 31 P, 100%; X = 195 Pt, 33.8%; Y = 19 F, 100%;) for the latter.Complexes 7 and 8 show a triplet with the platinum satellites at δ 60 and δ 38, respectively, relative to P A whereas P B is seen as a doublet with the platinum satellites at δ 22 and δ 27.A doublet of triplet at δ 113 and a doublet of doublets at δ 16 with the platinum satellites are observed in the 31 P{ 1 H} NMR spectrum of 9.These results suggest a trigonal planar geometry around the platinum centre and due to the fact that both PPh 3 ligands are equivalent in the 31 P{ 1 H} NMR spectra it can be said that either the chloro atom at the dioxaphospholane ligand lies on a perpendicular plane to the one containing the Pt atom and the two PPh 3 or there is a rapid rotation around the Pt-dioxaphospholane bond.Identical conclusion has been reached about a similar platinum(0) complex, [Pt(PPh 3 ) 2 (R 2 C=P-N(R)-PR)] 9 .The 31 P{ 1 H} NMR data are shown in Table 1.
It is interesting to compare the coupling constants 1 J(PtP A ) for complexes 8 and 9.As would be expected, the PtP A coupling constant in 9 is larger than in 8, reflecting the increase of s character on the Pt-P bond, caused by the presence of the more electronegative fluorine, bonded through the P A , in the first complex 10 .The 31 P{ 1 H} NMR studies also suggest an increase in the magnitude of 1 J(PtP A ) on changing the group bonded to the oxygen atom in the phosphorus from -CH 2 CH 2 CH 2 -to 1,2-benzo.A very small change is observed in the 1 JP A F coupling constant of the ligand 6 (1304 Hz) upon coordination to platinum(0) to form complex 9 (vide Table 1).A long range 3 J(P B F) coupling constant is also observed in the 31 P{ 1 H} NMR spectrum of 9.The 31 P{ 1 H} NMR studies of complexes 10 -14 showed that they posses a cis arrangement around the platinum centre since the values for the 2 J(P A P B ) coupling constants lie between 23 and 41 Hz and those for the 1 J(PtP) are typical for this kind of complexes 10 .

Syntheses and characterisation of platinum(II) complexes
Some of these complexes -10, 11 and 13 -have been obtained as a mixture of two conformers, a and b, as confirmed by the 31 P{ 1 H} NMR spectra, which consist of two sub spectra with the pattern of lines corresponding to an ABX spin system (A, B = 31 P, 100%; X = 195 Pt, 33,8%).In most of the spectra some lines, corresponding to one of the conformers, are superimposed to the ones of the other conformer.This is exemplified by the 31 P{ 1 H} NMR spectrum of 10 in which the signal corresponding to P A appears as a "false triplet" instead of two doublets.However, two doublets, in the P B region of the spectrum, are easily assigned.As little difference is seen in the 31 P{ 1 H} NMR data for both conformers a and b of 10, 11 and 12, their formation can be understood as a result of the coexistence of two conformers for the free ligands 11,12 , as depicted in Scheme 1.In this case, the metal fragment [PtCl 2 (PEt 3 )] can be bonded through the phosphorus lone pair in an axial or equatorial position (Scheme 2).The 31 P{ 1 H} NMR spectra of complexes 11 and 13 show a decrease in the 1 J(PF) coupling constants (556 and 590 Hz, respectively) compared to those found for the free ligands (1302 and 1171 Hz, respectively).This large decrease in the 1 J(PF) coupling constants, upon complexation, suggests a fluxional behaviour involving a change between the fluorine bonded through the phosphorus atom with the chlorine bonded through the platinum atom.It is interesting to note that an additional long range 3 J(PF) coupling constant has also been observed for both complexes and its value is ~26 Hz.Due to the small 2 J(PP) coupling constants measured in the 31 P{ 1 H} NMR spectra, a cis arrangement around the platinum centre can be suggested for both complexes.The 31 P{ 1 H} NMR data for all the Pt(II) complexes, herein reported, have been compiled in Table 2.
Unlike for platinum(0) complexes, a change of chlorine for fluorine on the phosphorus of the dioxaphospholane does not produce an increase in the 1 J(PtP A ) coupling constant.The 1 J(PtP A ) observed for the chlorodioxaphospholane complexes 11 and 13 are in fact considerably larger than those found for their analogues with the fluorodioxaphospholanes 10 and 12 (vide Table 2).Similar results, though with smaller differences between the coupling constants, have been obtained by Nixon and coworkers 13 for complexes of the type [RhCl(P B Ph 3 ) (η 1 -XP A =CR 2 )], i.e. 1 J(RhP A ) = 264 Hz when X = Cl and 258 Hz when X = F.These results can be understood in terms of the hard and soft acid-base concept 14 .It seems that the fluoro ligands 2 and 4 are a lot softer than platinum(II).Then, complexes 11 and 13 can be regarded as a pair of hard acid-soft base, and in this case they will be less stable than their analogues with the chloro ligands, complexes 10 and 12.One would therefore expect the Pt-P A bond length to be longer in complexes 11 and 13 than in the analogues 10 and 12. Longer bond lengths in a soft acid-hard base pair than in soft acid-not so hard base pair have been evidenced, very recently, by Durig and co-workers 15 .The B-P bond length in the H 3 BPF 3 pair is 1.836Å, whereas in H 3 BPH 3 it is 1.943Å.So, if the bond lengths are longer, the 1 J(MP) must be smaller because of the smaller contact between the bonding orbitals of the metal and phosphorus nuclei, even if the s character in the phosphorus lone pair is increased.In fact, a less soft metal might simply not react with a ligand in which the s character of the phosphorus lone pair has been increased.Very simple 31 P{ 1 H} NMRspectra have been obtained for complexes 15 -17.Patterns of lines corresponding to AB or ABX spin systems (A, B = 31 P, 100%; X = 19 F, 100%) are observed for complexes derived from chloro ligands 15 and 16, and for that derived from the fluoro ligand, 17, respectively.Similarly to the platinum(II) complexes, palladium(II) complexes are also formed as a mixture of two conformers a and b (vide Table 3), with the exception to the fluoro derivative, which might be due to the small size of fluorine, compared to chlorine.It is interesting that the chloro derivative ligands 1 and 3 yield trans complexes 15 and 16, as shown by the 2 J(P A P B ) coupling constants measured in the 31 P{ 1 H} NMR spectra (Table 3), whereas ligand 4 affords complex 17 with cis configuration around the metallic centre 10 .Again, an explanation in terms of the combined sizes of palladium atom and the halogen bonded through the phosphorus can be given.Since platinum is bigger than palladium, it allows a cis configuration of the phosphorus ligand, showing a better trans influence of the chlorine ligand.In the palladium complexes 15 and 16 it seems that the major role is played by the steric hindrance of the chlorine in the dioxaphospholane.On the other hand, the trans influence of the chlorine is evidenced by the formation of a cis complex 17 because fluorine is smaller.The 2 J(P A P B ) coupling constants (Table 3) are in accordance with the data found in the literature 9,10 .

Conclusions
The 31 P{ 1 H} NMR results for the platinum(0) complexes show that changing the dioxaphospholane P bonded chlorine for fluorine leads to an increase in the s character of the phosphorus lone-pair and, as a consequence, to a increase in the Pt-P coupling constant.On the other hand, 1 J(PtP) coupling constants of platinum(II) complexes tend to decrease upon changing the chlorine for fluorine, thus revealing a relationship between the Pearson soft and hard acid-base concept and Pt-P coupling constants.Although a larger number of systems would need to be studied, the present work is a good starting point and dioxaphospholane and dioxaphosphorinane derivatives are good systems for the study of this relationship.

Experimental
All reactions were carried out either under dry dinitrogen in Schlenk tubes or by use of high-vacuum techniques.Glassware was flame-dried in vacuum and solvents were dried, freshly distilled under dinitrogen, and degassed prior to use.The NMR spectra were recorded on a Bruker DRX400 spectrometer at 400.13 MHz for 1 H and 161.98 MHz for 31 P.All chemical shift data were recorded at 25 o C and are quoted in ppm, with positive values to high frequency of the indicated reference (external 85% H 3 PO 4 for 31 P and SiMe 4 for 1 H) and corrected with respect to the appropriate deuterium frequency.Coupling constants are quoted in Hertz.