J. Phys. Chem. 1981, 85, 417-426
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Tertiary Phosphine Oxide Complexes with Lanthanide 0-Dlketonates. NMR-Relaxation and Pseudocontact-Shift Studies J. P. Quaegebeur,” S. Belald, C. Chachaty, and H. Le Ball Departement de Physicochimie, Section de Chimie Moieculaire, C.E.N. de Saclay, 91 191 Gif sur Yvette, Cedex, France (Received: July 3, 1980; In Final Form: August 26, 1980)
The conformations and dynamical behavior of several tertiary phosphine oxides with phenyl and/or alkyl groups of different lengths, coordinated to Ln(DPM)3,have been investigated by 31Pand 13Cparamagnetic relaxation and pseudocontact shifts. The 31P relaxation shows the formation of 2:l complexes with Ln-0 = 2.9 f 0.1 A and LLn-O-P = 127 f 2 O . The conformations of the substituents have been derived from the analysis of the 13C relaxation and pseudocontact data by assuming that alkyl chains undergo a gauche-trans isomerism whereas the reorientation of phenyl group occurs by 180’ jumps. The populations and interconversion rates of the multiple conformers have been obtained by the association of these two methods. An attempt has been made to correlate the kinetic parameters of the ligand exchange between the solution and complexes with the relative donor properties of the phosphine oxides under study.
Introduction Organophosphorus compounds such as phosphine oxides have found wide application in the solvent extraction of lanthanides and actinides (see, for example, ref 1)resulting from adduct formation with metal c h e l a t e ~ . ~ -Most ~ of the data published in the literature deal with the formation of the complexes between the lanthanide salts and the phosphine oxides5as well as their stability constants which are related to the magnitude of donor-acceptor interactiom6 The interactions are generally rationalized in terms of flexibility of the bases.’ However, because of the lack of structural data for these complexes, the metal-organophosphorus ligand interactions and therefore the steric requirements have not been very well elucidated up to now. In order to gain a better understanding of those interactions, we have investigated by NMR and nuclear relaxation the complexation of (R1)2R2P-0ligands with lanthanide P-diketonates. The NMR data have been correlated to the donor properties of the phosphoryl group and more precisely to the nature of R1 and R2 substituents (phenyl or alkyl groups). Paramagnetic relaxation reagents are extensively used in conformational studies of organic molecules. Though most studies are concerned with rigid systems or ligands having a limited internal reorientational freedom, we have considered in a recent paper the more general situation of the relaxation induced by the dipolar interaction between two spins separated by several bonds about which jump motions occur among three sites.s This model is suitable for the nuclear longitudinal relaxation enhanced by the Gd3+ion, which has relatively long electronic relaxation times. Thus the Gd3+complexes provide significant information about the transient conformations and (1) Y. Marcus and S. Kertes, “Ion Exchange and Solvent Extraction of Metal Complexes”, Wiley-Interscience, London, 1969. (2) T. Shigematsu and T. Honjyo, Bull. Chem. Soc. Jpn., 43, 796 (1970).
(3) T. Aoki, E. Deguchi, M. Matsui, and T. Shigematsu, Bull. Inst. Chem. Res., Kyoto Uniu., 49, 307 (1971). (4) T. Taketatsu and C. V. Banks, Anal. Chem., 38,1524 (1966). (5) V. K. Manchanda, K. Chander, N. P. Singh, and G . M. Nair, J . Inorg. Nucl. Chem., 39, 1039 (1977). (6) A. M. Rozen, Z. I. Nikolotova, and N. A. Kartasheva, Russ J.Inorg. Chem. (Engl. Transl.) 24, 909 (1979). (7) R. J. Niedzielski, R. S. Drago, and R. L. Middaugh, J. Am. Chem. Soc., 86, 1694 (1964). (8) A. Tsutsumi, J. P. Quaegebeur, and C. Chachaty, Mol. Phys., 38, 1717 (1979).
0022-3654/81/2085-0417$01.00/0
interconversion rates of R1 and Rz substituents around the
P-0 bond. This method has been extended to the more specific example of longer chain substituted ligands such as trin-octylphosphine oxide (TOPO) and dibutylmethoxyoctylphosphine oxide (POX2). The appearance probabilities of different conformations of the three chains, together with some other useful information concerning their equilibrium positions, were confirmed by pseudocontactshift measurements induced by the lanthanide 0-diketonates possessing a large anisotropy of the magnetic susceptibility tensor.
Experimental Section The phosphine oxides investigated are phenyldimethylphosphine oxide (PDMPO), methyldiphenylphosphine oxide (MDPPO), ethyldiphenylphosphine oxide (EDPPO), tri-n-octylphosphine oxide (TOPO), and dibutylmethoxyoctylphosphine oxide (POX2). They were purchased from I.R.C.H.A. (France) and used without further purification. The absence of any organophosphorus impurity in the samples was checked by 31PNMR spectroscopy. The 31Pand 13C paramagnetic shifts and relaxation measurements were performed at constant ligand concentration (1 M) in CDC13 and for ratios p = [metal]/[ligand] varying from to in order to maintain a linear dependence of these parameters upon p . The NMR experiments were carried out with Varian CFT2O (Bo = 1.87 T, V l 3 c = 20 MHz), Varian XL 100 (Bo = 2.35 T, ~ 1 %= 25.2 MHz, V31p = 40 MHz, VIH = 100 MHz), and Bruker WH90 (Bo= 2.11 T, V l 3 c = 22.63 MHz, V 3 1 p = 36.45 MHz) spectrometers. Some subsidiary experiments of 13Cpseudocontact shifts in the complexes of TOPO and POX2 with Ln(DPM)3 (DPM = dipivaloylmethane) were performed on a Cameca TSN 250 spectrometer (Bo= 5.87 T, v i 3 c = 62.86 MHz). Since the paramagnetic shifts in ppm were the same at 20 and 62.86 MHz with respect to an internal reference of Me4Si, the conditions of fast exchange between the free substrate and the complexes were verified for all carbons. 13CNMR spectra of MDPPO and EDPPO are exhibited in Figure 1with the assignment of resonances. Assignment of NMR signals for POX2 substrate was made easier by studying the 13C relaxation enhancement induced by Gd(DPM)3and by comparison of spectra recorded a t 22.63 and 100.8 MHz with a Bruker WH400 spectrometer without 31Pdecoupling (Figure 2). @ 1981 American Chemical Society
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100
60
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0 pp,
MDPPO
io2
io
iz
3
EDPPO
98
9
11.5
2.6
Flgure 1. 13C NMR spectra of MDPPO (a) and EDPPO (b) (1 M In CDC13, T = 300 K, v and Jp-c coupling constants.
74 73.3
5.2
= 22.63 MHz) together with the assignment of resonances
c05 I
Flgure 2. I3C NMR spectra of dibutylmethoxyoctylphosphine oxide (POX2) recorded at v = 22.63 and 100.8 MHz (a and b, respectively). The resonances are assigned with respect to an internal reference of Me,Si.
The spin-lattice relaxation times T1were obtained by progressive saturation (go", T, 90" sequences) or by inversion recovery (lSO", 7,90" sequences) in the Fourier transform mode. The transverse relaxation times T2were obtained from line-width measurements after correction for magnetic field inhomogeneities and exchange rate (see below). The temperature of the samples was checked before and after each measurement. The contribution of the outer sphere relaxation for nuclei remote from the metal ion was estimated from the T1and line width of the solvent.
Results and Discussion 31PRelaxation Data in Phosphine Oxide-Gd(DPiW3 Complexes. Among the isomorphous series of lanthanide chelates, the Gd3+ ion ground state) coordinated to organic ligands induces isotropic paramagnetic shifts arising solely from contact interactions. Moreover the electronic relaxation time of this ion is comparatively long (Tie s at 300 K) and it behaves as an efficient
-
relaxation reagent. The hyperfine coupling constant, Ak, under the conditions of fast exchange of the substrate between the solution and the complex is related to the contact shift induced on nucleus k in the metal coordination sphere by eq 1: Ak being expressed in hertz. Y N
is the nuclear magnetogyric ratio; S = 7/2, the total electron spin of Gd3+;and ge 72, ita spectroscopic splitting factor. q is the number of ligand molecules coordinated to Gd(DPM), and p the metal-to-ligand ratio. The longitudinal and transverse relaxation rates in a rigid paramagnetic complex undergoing isotropic reorientation are given by modified formdo of Bloembergen and (9) N. Bloembergen, J. Chem. Phys., 27,595 (1957). (10)J. Reuben, G.H. Reed, and M. Cohn, J. Chem. Phys., 62, 161 (1970).
The Journal of Physical Chemistry, Vol. 85, No. 4, 1981 419
Tertiary Phosphine Oxide Complexes
lo4
.
10'
2.5
I
1
I
I
3.0
3.5
4.0
4.5
10
roy1 (0
~~
2.5
3.0
3.5
4 .O
4.5
103/1
('
K-1)
K-l
Flgure 3. Temperature dependence of 31Plongitudinal (solid lines) and transverse (dotted lines) relaxation rates in the phosphine oxide-Gd(DPM)3 complexes (Bo = 2.11 T): (A) 1.2, and 3: PDMPO, MDPPO, and EDPPO, respectively. (B) 1 and 2: TOP0 and POX2, respectlvely.
Solomon equations" when Tle and Tzeare different (eq 2 and 3). Uk and osare the nuclear and electron Larmor
temperature dependence can be interpreted by using the Eyring relation kT AH* AS* 7h-'= - erp[ -(4) h RT R A" and AS*being the activation enthalpy and entropy, respectively. In the general case where the exchange rate, q,-', is of the same order of magnitude as the contact shift, AUM,or the relaxation rates, ( Tl,2-1)M, in the coordination sphere of Gd3+,the observed values Aw0M and (T1,2)0M are related to ?h by'2
+-I
+ -1 +137c2 ws27,;
]
'[
+ %S(S + 1)Ak2
+
le,
1
(3)
frequencies, respectively, and Ak is the scalar hyperfine coupling constant. The different correlation times are defined by
TR is the reorientational correlation time of the electron spin-nuclear spin vector rk. 7h is the mean lifetime of the phosphine oxide in the metal coordination sphere, and its ~~
(11) 1. Solomon, Phys. Reu., 99, 559 (1955).
and by13
(Tl-')ob%d = TiF'(1 - Pq) Pq(TiM + 7h)-l (7) ( A u ] / and ~ ) ~Tlfare the line width and the relaxation time, respectively, corresponding to the free substrate. The determination of 7 h is achieved by considering the low-temperature linear portion of the semilog plot of Avl12 and Tc' vs. T1(Figures 3 and 4).14 From these figures (12) T. J. Swift and R. E. Connick, J. Chem. Phys.,37,307 (1962). (13) 2.Luz and S. Meiboom, J. Chem. Phys.,40, 2686 (1964).
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TABLE I : 31PNMR Data and Thermodynamic Parameters of Exchange in Phosphine Oxide-Gd(DPM), ComplexesC PDMPO 31P
7.6 x 103 3.4 x 104 7 . o x 105 1.65 x 105 6.9 -12.0 10.5 9.7 x lo-"
TIM-',s-'
T 2 M ' I , s-1 A k , rd s'l 7h-1, s-I
AH*, kcal mol-' A S * , eu AG*,kcal mol-' 7R,b s a
MDPPO
EDPPO
POX 2
TOPO
8.6 X l o 3
8.3 x 103
8.7 x 103
8.0 x 103
a a
a a
a
a
a
3.8 x 103 3.7 - 30.6 12.9 1.16 X l o - ' '
3.3 x 103 6.8 -- 20.7 13.0 1.26 X l o - ' '
a
1.8 X lo4 4.5 - 25.5 12.2 1.1 x 10-1°
Undetermined values owing to the too large contribution of exchange rates.
Calculated from e q 9.
3.16 x 103 4.1 -29.2 12.9 1.28 X l o - ' '
T = 300 K ; q = 2.
1
The reorientational correlation time of the phosphine oxide complexes investigated is estimated as previously reportede from the longitudinal relaxation time of the methine carbon of the P-diketonate chelates in the diamagnetic analogue complexes with La(DPM)3 (eq 9). T~(c-H) vmng
AUy/Hr
6000
/
-
(T
