Conformation-sensitive molecular pendulums: variable temperature NMR study of dimeric palladium(I) bisphosphine complexes

Inorganica Chimica Acta 357 (2004) 4413–4422 www.elsevier.com/locate/ica

Conformation-sensitive molecular pendulums: variable temperature NMR study of dimeric palladium(I) bisphosphine complexes Ga´bor Szalontai a

a,*

, Ga´bor Besenyei

b

NMR Laboratory, University of Veszpre´m, Egyetem utca 10, H-8200, Veszpre´m Pf.158, Hungary b Institute of Chemistry, Chemical Research Center, MTA, H-1525 Budapest Pf.17., Hungary Received 18 February 2004; accepted 26 June 2004 Available online 6 August 2004

Abstract Solution and solid state 31P NMR studies were carried out on a series of [Pd2X2(dppm)2] (X = Cl (1a), Br (1b), I (1c)), or [Pd2XY(dppm)2] (X = Cl, Y ¼ SnCl3
(1d)) complexes and on methyl substituted derivatives such as [Pd2Cl2(dppm)(dppmMe)] (2), syn-[Pd2Cl2(dppmMe)2] (3), and anti-[Pd2Cl2(dppmMe)2] (4) (dppmMe = 1,1-bis(diphenylphosphino)ethane) in order to study and understand the conformational behaviour of the eight-membered Pd2P4C2 rings depending on the substituents and their stereochemistry. These complexes with metal–metal bonds and mutually trans-dppm ligands act as molecular pendulums. On the basis of temperature dependent spectra qualitative correlations have been found between the molecular conformations and the rate of a specific intramolecular motion called ‘‘swinging’’. While for the extended-boat conformers (2 and 3) this exchange process is of intermediate energy (41–45 kJ mol
1), the barrier is definitely higher (54 kJ mol
1) for the extended-chair conformer 4. Changes of symmetry relations are reflected very vividly in the 31P NMR spectra. The observed different chemical shifts, ‘‘swinging’’ rates and activation free energies obtained for the boat and chair conformers are explained by the steric effects and low-temperature conformations of the axial phenyl groups. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Molecular pendulums; Dimeric Pd(I) complexes; Dynamic NMR spectroscopy; Conformation dependent internal motion

1. Introduction Identification and study of spontaneous mechanicallike molecular motions such as ‘‘propellers’’, ‘‘turnstiles’’, ‘‘rotors’’, ‘‘pendulums’’, etc., became intense and fashionable in the last few years [1]. Homo- and heterodinuclear complexes possessing the [M2(dppm)2] framework have been intensively studied in the past decades. The diversity of their chemical properties and the variety of their structures have been reviewed [2,3]. NMR spectroscopy turned out to be a * Corresponding author. Tel.: +36 88 422 022 4356; fax: +36 88 421 869. E-mail addresses: [email protected] (G. Szalontai), [email protected] (G. Besenyei).

0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2004.06.046

versatile and powerful, though still not fully exploited, tool to elucidate problems related to internal molecular motions [4,5] and conformational properties [6,7] of these compounds. We have conducted solution and solid state 31P NMR studies on a series of [Pd2X2(dppm)2] (X = Cl (1a), Br (1b), I (1c)), or [Pd2XY(dppm)2] (X = Cl, Y ¼ SnCl3
(1d)) complexes and on methyl substituted derivatives such as [Pd2Cl2(dppm)(dppmMe)] (2), syn-[Pd2Cl2(dppmMe)2] (3), and anti-[Pd2Cl2(dppmMe)2] (4) (dppmMe = 1,1-bis(diphenylphosphino)ethane) in order to study and understand the conformational behaviour of the eight-membered rings depending on the substituents and their stereochemistry (see Scheme 1). The conformational equilibriums involved may mimic a pendulum-like motion of the frame around the

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G. Szalontai, G. Besenyei / Inorganica Chimica Acta 357 (2004) 4413–4422 CH 3

Ph2P

Ph 2P

PPh2

PPh 2 Ph 2P

X

Pd

Ph2P

Pd

Y

Cl

Cl

Ph 2P

Pd

Pd

Ph 2P

PPh 2

PPh2

X = Y = Cl X = Y = Br X=Y=I X = Cl, Y = SnCl3

PPh 2

Pd Cl

Pd

Cl

PPh 2

CH 3

CH3

1a 1b 1c 1d

CH3

2

3 (syn)

Ph2 P

Cl

PPh2

Pd

Pd

Ph2 P

Cl

PPh2

CH3

4 (anti)

Scheme 1.

Pd–Pd axis, but a separate flipping of the bridging methylene groups can also be envisaged. The methyl substitution is expected to change the ‘‘swinging time’’ of the pendulums. We know from earlier studies [6,8] that in these molecules the coordination geometries around the Pd atoms in solid state are not planar, but they show tetrahedral distortions in which the P atoms are displaced above and below the coordination planes. The dihedral angles of these planes incorporating the Pd atoms vary between 37° and 50°. Since presumably equal quantities of clockwise and anticlockwise distorted forms are present a racemic mixture of enantiomeric forms should exist (see Scheme 2). In a very recent report of James and coworkers [9] on similar dipalladium(I) bis(dialkylphosphine) complexes reversible coalescence was found in low-temperature 1 H spectra and explained by a ‘‘ring-flipping’’ mechanism which seems to be identical to what we call ‘‘swinging’’ process. These structures may be of interest also from purely NMR point of view especially in the solid state since the dipolar coupled P atoms form tightly J-coupled homonuclear four-spin systems and so, are capable to show complicated rotational resonance effects [10].

Pd

Pd

Pd

Pd

Scheme 2.

2. Results and discussion 2.1. Unsubstituted complexes: [Pd2XY(dppm)2] (X = Cl) (1a), (X = Br) (1b), (X = I) (1c) and (X = Cl, Y = SnCl3) (1d) Solution phase NMR: In these bridged complexes the rapid conformational changes of the rings render the axial and equatorial methylene protons and also the phenyl rings equivalent in the 1H spectrum [11]. In agreement the 13C spectra in CD2Cl2 at room temperature indicate the presence of one kind of phenyl ring (i.e. all the eight phenyls are identical). Binomial-like quintets (the A part of an AXX 0 X00 X000 system, A = 13C, X = X 0 = X00 = X000 = 31P) appear for all carbon atom coupled at least to one phosphorus. 31 P NMR: At 7.04 T, the spectrum of 1a consists of a singlet at
1.1 ppm (
2.5 ppm has been reported earlier in the literature for this compound [12]), that does not show any broadening when cooling down the sample to 189 K in CD2Cl2. Likewise, singlets have been observed in the 31P{1H} spectra for the bromo 1b and iodo 1c derivatives at
4.7 and
10.5 ppm, respectively [12]. Earlier papers on analogous [Pt2X2(dppm)2] complexes also reported a single P-environment at room temperature [13]. X-ray diffraction and solid phase NMR: In solid state, however, the symmetry is completely lost. The crystal structures of 1a [6] and 1b [15a,b] have already been reported, both are monoclinic with space group P21/c, the Pd2P4C2 ring has a skewed ‘‘chair-like’’ conformation (called extended-chair). The coordination planes of the two Pd-centres are twisted about the Pd–Pd bond by about 39° in these cases (see below the crystal structure of 1a). In agreement the solid state 31P MAS spectra of all compounds studied so far (1–4) show four different phosphorus environments [16]. As pointed out already the restricted motion in solutions should allow the formation of biaryl-like enantiomers in equal quantities. However, the interconversion

Compound

1

13

C NMR data (chemical shiftsa and fine structures) of 1a, 1d, 2, 3 and 4 relevant to the exchange processes studied 13

H NMR (CD2Cl2)

C NMR (CD2Cl2)

293 K daCH splitting

P 193–183 K dCH2 splitting

dCH3 splitting

Phenyl rings

dCH splitting

dCH2 splitting

References

293 K dCH3 splitting

Phenyl rings

CH

CH2

CH3

Phenyl rings

1a (Cl)

4.14 v.q.

One set

n.a.

n.a.

39.88 v.q.

One set

1d Cl, SnCl
3

4.24 b.s. 4H

One set (broadened)

4.7 b.m. 2H 3.72 b.m. 2H

Four sets

41.8 b.m.

15.5 vt

One set (Four sets at 183 K) Four sets

2 (Cl) H, Me

4.95 m.

3 (syn) (Cl) Me, Me

4.84 v.m. 4.84 (v.s.)

1.08 v.h. 1.09 (v.s)

4 (anti) (Cl) Me, Me

4.92 v.s.

1.02 v.s.

a

4.58 ax v.dt 3.66 eqv v.dt

1.02 dt

Four sets: 7.8–7.4 axialb 7.4–6.8 equatorialb Two sets: 7.68–7.46 axialb 7.38–7.12 equatorialb Two sets: 7.82–7.42 axialb 7.38–6.80 equatorialb

5.0 b.m. axial

4.82 b.m.

4.54 bm. ax 3.54 bm. eqv.

0.88 b.m. eqv.

0.84 b.m.

39.5 vt.

Spectr. [25] Synth. [14] Synth. [18]

Eight sets

42.8 vt.

Spectr. [8] Synth. [8]

Two sets [6]

30.65

15.16 qn

Two sets

Spectr. [6] Synth. [19]

Four sets

42.2 [6]

15.02 [6]

Two sets

Spectr. [6,8] Synth. [8]

In ppm relative to external TMS. The assignment is based on NOE experiments, n.a.=not available, v.q.=virtual quintet, m.=multiplet, b.m.=broad multiplet, b.s.=broad singlet, v.dt.=virtual double triplet, v.s.=virtual sextet. ax=axial. b

G. Szalontai, G. Besenyei / Inorganica Chimica Acta 357 (2004) 4413–4422

Table 1 Summary of the available room and low temperature 1H and

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Crystal structure: It was reported long ago by Balch et al. [18], the structure is again monoclinic with space group P21/c, and, like the chloro complex the Pd2P4C2 ring has chair-like conformation. The twisting angle is 41.3° in this case. Solid state 31P NMR MAS spectra show an AB-like (at about 1 and 3 ppm) and an AX-like (at about
1 and
18.3 ppm) system with splittings of 385 and 467 Hz, respectively. 119,117Sn satellites are also seen, but are mostly overlapped by other signals. No doubt the exchange process detected should involve the flipping of the chelate rings, this inversion is promoted by the association and dissociation of the SnCl3
anion. At low temperatures the bound state exists long enough to see the scalar coupling between the 31 P and 119Sn nuclei. In the bound state the cis related P atoms are not equivalent and form a tightly coupled AA 0 BB 0 spin-system (for spectra see the supporting information). Fig. 1. X-ray structure of the unsubstituted chloro derivative 1a [6]. Twisted (39°) extended-chair conformation. The extreme positions of the ‘‘swinging’’ represent energy minima (local) on the conformational energy surface.

of the enantiomeric species must be fast on the NMR time scale. [Pd2ClSnCl3(dppm)2] 1d: The structure is analogous to 1a, 1b and 1c in the sense that it is unsubstituted on the dppm ligands, however a coordination–dissociation equilibrium of the SnCl3
anion may be present in solution what can make the exchange phenomenon more complicated. 31 P NMR: At ambient temperature the signal is slightly exchange-broadened and asymmetric. Satellites due to the different two- and three-bond 119,117Sn–31P couplings could not be identified. At 193 K, a symmetric, highly second-order multiplet is seen. Broad satellite lines with estimated coupling constants of about 160 and 610 Hz, presumably due to the 119,117Sn isotopomers, are also present. Coupling to the 119Sn nucleus could be confirmed by recording the 119Sn spectrum at 183 K. It shows a triplet of triplets at
123 ppm with coupling values of 625 and 170 Hz, most likely these should correspond to the two- and three-bond coupling values, respectively [17]. 1 H NMR: At room temperature the signals are heavily broadened, the methylene protons are close to coalescence. At 183 K, however, the aromatic region is well resolved, four different phenyl groups could be identified, and also the methylene protons exhibit different chemical shifts (see Table 1). In 1D steady state NOE experiments, irradiation of the equatorial and axial protons at 4.7 and 3.72 ppm (the assignments are not known) caused enhancements of the same aromatic protons and proved slow exchange among the assumed axial and equatorial positions.

2.2. Methyl-substituted (dppmMe) complexes (2–4) By replacing one methylene hydrogen on one of the dppm ligands by a methyl group we get the unsymmetrical complex 2 that miss plane of symmetry or C2 axis through the Pd–Pd bond even in solutions. By replacing one methylene H-atom on both dppm ligands by a methyl group two isomers could be isolated [19], in the syn form 3 both methyl groups take identical positions with respect to the Pd–C–Pd plane (two planes of symmetry exist), whereas in the anti-isomer 4 their positions are different (only one plane of symmetry is present). The methylation is expected to change the conformational barriers and therefore should have an impact on the dynamic behaviour of the pendulums too (see Fig. 1). 2.3. [Pd2Cl2(dppm)(dppmMe)] (2) 31

P NMR: As expected phosphorus pairs of the dppm and dppmMe ligands are not identical, the 31P spin system observed at room temperature in CD2Cl2 is an AA 0 XX 0 or AA 0 BB 0 system (Fig. 2). The spectrum is dominated by large 2J(P. . .P) couplings (486 and 448 Hz) confirming the non-equivalence of the trans related P atoms [8,20], the signals are slightly broadened unsymmetrical multiplets. However, low-temperature spectra (vide infra) have allowed for the first time to determine all isotropic J-coupling values among the four P atoms. Upon cooling to 193 K, the spectrum changes gradually, signals of the multiplets broaden and double at about 242 K, (Tc = 242 K) and sharpen thereafter into eight double-doublets (an almost first order four-spin system) at 195 K and below (see Fig. 2). A 31P–31P COSY spectrum (see supporting information) recorded at 193 K confirmed that all phosphorous atoms are coupled to all others, i.e. not only the two-bond cis coupling

G. Szalontai, G. Besenyei / Inorganica Chimica Acta 357 (2004) 4413–4422

4417

Cl

~ AA’BB’

CH3 35

30

25

20

15

10

5

0

-5

-10

-15

ppm

Cl

* TC = 242

35

30

25

20

15

10

5

0

-5

-10

-15

ppm

* 213 K

35

30

25

20

A1

15

10

5

0

B1

A2

-5

30

-15

ppm

CH3

B2 193 K

AA’BB’ → AMXY

35

-10

CH3

25

20

15

10

5

0

-5

-10

-15

ppm

MAS 293

35

30

25

20

15

10

5

0

-5

-10

-15

ppm

31

Fig. 2. Temperature dependence of the P NMR spectra (121.4 MHz) of [Pd2Cl2(dppm)(dppmMe)] (2) recorded in CD2Cl2. Upper traces: 293, 242, 213 and 193 K (kexch. = 2500 s
1 at 242 K, DGà = 45.2 kJ mol
1), chemical shifts, AB(1) (A1 = 22.8 ppm, B1 = 4.0 ppm) AB(2) (A2 = 13.2 ppm, B2 =
6.1 ppm), coupling constants, JA1B1 = 486 Hz, JA2B2 = 448 Hz, JA1A2 = 48 Hz, JA1B2 = 38 Hz, JA2B1 = 34 Hz, JB1B2 = 48 Hz. Bottom: 31P MAS spectrum (rotation rate 8200 Hz, isotropic region only), Tc=coalescence temperature, *impurity=traces of (1a).

(48 Hz), but also the three-bond through-metal coupling values involving the Pd–Pd bonds are significant (they are 38 and 34 Hz between the A 0 X and X 0 A nuclei or vice versa, absolute values) as expected. Note, however, that in this case the observed splitting is the sum of the two-bond through-backbone and the three-bond through-metal couplings which are often of different signs [13]. 125 Hz was found for the 2J(P,P) coupling in the free dppm in solution [21]. The spectroscopic values obtained are reported in Table 1 and in the caption of Fig. 2. An exchange rate constant of k = 2500 s
1 was obtained at the coalescence temperature which corresponds to an activation free energy of about 45.2 kJ/mol. 13 C NMR: Four phenyl pairs are present in the spectrum at room temperature. All carbon signal but the para-phenyl ones show triplet splittings due to couplings between the chemically identical (cis related) P atoms. The methine, methylene and methyl carbons of the rings also exhibit these virtual triplets [22] (see Table 1 and supporting information).

At 293 K, the 1H spectrum also reveals four different phenyl environments. COSY-45 experiments recorded at this temperature enabled us to identify them. On the basis of 1D difference NOE experiments the high-frequency group between 7.4 and 7.8 ppm (the signals heavily overlap) and the low-frequency group between 6.8 and 7.36 ppm could be assigned to the equatorial and axial phenyls, respectively. This time almost unambiguous assignments of the axial and equatorial methine and methylene protons were possible (vide infra). Upon cooling the spectrum changes drastically and shows coalescence at about 223 K, however, even at 183 K it is too complicated to allow the complete assignment of the signals. 1D NOE experiments (see supporting information) were also used to reveal long distance spatial relationships among protons. Selective irradiation of one of the methylene protons (this should be the axial one) of the bridging dppm moiety at 4.6 ppm resulted in surprisingly significant enhancement (over 10%) of the methineÕs signal of the dppmMe moiety at 5 ppm. We can estimate from the crystal structure of analogous compounds (e.g. 3) that these nuclei are about

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Fig. 3. Temperature dependence of the 31P NMR spectra (121.4 MHz) of syn-[Pd2Cl2(dppmMe)2] (3) recorded in CD2Cl2. Upper traces: 273, 233, 213 and 183 K, Tc=coalescence temperature (kexch. = 924 s
1 at 213 K, DGà = 41.4 kJ mol
1), X-ray structure (ORTEP drawing), bottom: simulation of an AA 0 BB 0 spin system, JAB = 450 Hz, JAA 0 = 42 Hz, JA 0 B = 42 Hz, JAB 0 = 42 Hz, JBB 0 = 42 Hz, JA 0 B 0 = 450 Hz, DmAB = 360 Hz (cis related P nuclei have identical chemical shifts).

˚ (on time average) away of each other. This 5.5–5.8 A suggests a relatively rigid extended-boat-like conformation for the eight-membered Pd2P4C2 ring since the magnitude of the observed enhancement makes a ring flipping process very unlikely. In full conformity with this observation, the methylene protons show NOE enhancements to the ortho hydrogens belonging to different pairs of phenyl groups. The result is somewhat unexpected because the boat-like arrangement of the Pd2P4C2 ring was considered energetically unfavoured.

Concerning the exchange processes involved we assume that the motion of the pendulum becomes slow below 242 K, and, therefore, not even the cis related P atoms are equivalent anymore. Note that in the case of fast equilibrium around the Pd–Pd bond the flipping of the individual five-membered rings would not lead to this cis non-equivalence. The 31P MAS spectrum obtained in the solid state (see Fig. 2 bottom) exhibits four different phosphorus environments at about 24 and 3 ppm (A1B1) and at

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Tc= 291 K

35

293 K

30

25

20

15

10

5

ppm

253 K

35

30

25

20

15

10

5

ppm

233 K

35

30

25

20

15

10

5

ppm

213 K

35

30

25

20

25.0

20.0

15

10

5

ppm

Simulation: AA’XX’

ppm (t1) 35.0

30.0

15.0

Cl A H3C

X

10.0

5.0

CH3 Pd Pd

A' X'

Cl

Fig. 4. Temperature dependence of the 31P NMR spectra (121.4 MHz) of anti-[Pd2Cl2(dppmMe)2] (4) recorded in CD2Cl2. Upper traces: 293, 253, 233 and 213 K, Tc=coalescence temperature (kexch. = 2940 s
1 at 291 K, DGà = 54.4 kJ mol
1), bottom: simulation of an AA 0 XX 0 (A2X2) spin system, JAA 0 = 450 Hz, JAX = 42 Hz, JA 0 X = JAX 0 = 42 Hz, JA 0 X 0 = 42 Hz, JX 0 X 0 = 450 Hz, DmAX = 810 Hz (trans related P nuclei have identical chemical shifts).

about 13 and
8 ppm (A2B2) (tentative assignments), i.e. rather close to the values obtained in solution at low temperatures for the A1B1 (22.8 and 4 ppm) and A2B2 (13.2 and
6.1 ppm) pairs. On this basis it is reasonable to assume that at very low temperatures the solution state structure is approaching that of the solid state. 2.4. Syn-[Pd2Cl2(dppmMe)2] (3) 31

P NMR: as noted already in this compound the Pd2P4C2 ring itself has two planes of symmetry, at least on the time-average, consequently much simpler 31P spectrum is expected. Indeed, like compound 1a, the phosphorus spectrum displays a singlet at room temper-

4419

ature, however by decreasing the temperature, the signal broadens and splits into a doublet at 220 K, which transforms into a second-order multiplet, thereafter. The coalescence temperature is 213 K. Also the 1H spectrum (Table 1) suggests two planes of symmetry at room temperature, since it reveals only two slightly different phenyl groups (axial, equatorial), one methine and one methyl signal (virtually coupled multiplets). In the solid state the 31P MAS spectrum exhibits an AB-like system (see supporting information). It is again noteworthy that the estimated isotropic chemical shift values (though they are not very reliable due to the presence of dipolar homonuclear P–P couplings [23]) are rather close to those of observed below 190 K in solution. Crystal structure data have been reported very recently. The structure is triclinic with space group P
1 [6]. The Pd2P4C2 ring has an extended-boat conformation with equatorial methyl groups, short Pd–Pd bond ˚ ) and with a dihedral angle of the coordination (2.569 A planes of 49.1°. The observed temperature dependence of the spectra can be explained by the slow-down of the ‘‘swinging’’ process around the Cl–Pd–Pd–Cl axis. The process is of intermediate energy (Tc = 213 K, kexch. = 924 s
1 and DGà = 41.4 kJ mol
1) and results in the transformation of the A4 spin system into an AA 0 BB 0 system. As the motion slows down the trans P atoms become pair-wise non-equivalent since the symmetry plane among them vanishes. This is supported by the appearance of large trans 2JPP couplings and was confirmed by spin-simulations too (see Fig. 3). 2.5. Anti-[Pd2Cl2(dppmMe)2] (4) 31

P NMR: unlike the syn form 3 in this compound the Pd2P4C2 ring itself has only one plane of symmetry, the other plane, through the metal–metal bond, is transformed into a C2 axis. Temperature-dependent spectra of the analogous bis(dialkylphosphino)methane complex have been reported earlier and were related to the fluxionality of the rings [9]. In CD2Cl2 we could go down to 213 K (see Fig. 4), the broad signal observed at ambient temperature (293 K) shows coalescence at 291 K (kexch. = 2940 s
1 at 291 K, DGà = 54.4 kJ mol
1) and exhibits a deceptively simple A2X2-like system (JAX = 42 Hz) already below 238 K (Fig. 4), which does not change upon further cooling. The lack of the large two-bond trans couplings from the spectrum indicates the equivalence of the trans related P atoms i.e. the ’’swinging’’ is still fast at 213 K. Simulations of an AA 0 XX 0 system (see Fig. 4 bottom) confirmed that in the case of equivalent trans P

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atoms the spectrum mimics a simple A2X2 system provided the three-bond couplings (P–Pd–Pd–P) between AX 0 or A 0 X and the couplings of the cis related P atoms are similar. 1 H NMR: in agreement with a recent publication [6] the 1H spectrum displays two non-overlapping regions for the equatorial (high-frequency) and axial (low-frequency) phenyl rings, respectively. Upon cooling the sample to 183 K most of the aromatic signals doubled and became narrower. The COSY-45 spectrum recorded at 183 K revealed several correlations and suggested the presence of four different phenyl groups. Note that slow phenyl rotation would cause nonequivalent ortho and meta protons and therefore more complicated patterns. At the same time the symmetry relations of the Pd2P4C2 ring did not change, i.e. only one methine and one methyl signal was observed in the whole temperature range studied (for spectra see the supporting information). At ambient temperature selective irradiation of the methine signal at 4.9 ppm resulted in significant enhancement (4–8%) of the ortho protons at 7.8 ppm whereas irradiation of the methyl group at 1 ppm resulted enhancements of the ortho proton of both the equatorial and axial phenyl groups. Irradiation of any of the axial phenyl protons e.g. that of the ortho protons at 6.95 ppm did not result polarization transfer to the equatorial phenyl protons and vice versa. Consequently, significant exchange of the axial and equatorial phenyls can be excluded at this temperature. In case of extended-chair conformation in solution, the whole structure retains a C2 symmetry axis (it goes through the Pd–Pd bond) whereas in extended-boat form this axis is missing. Therefore the observed identity of the trans related P atoms, the methine protons, the methyl groups, etc., cannot be explained by the boat form. If so the two phosphorus environments must be assigned to cis related P atoms, however, it remains to understand what makes the cis P atoms different? Please note that so far we have ignored the impact the phenyl rings may have on the symmetry relations, their free rotation was readily assumed since non-equivalence of ortho, meta, etc., protons have not been noticed or

proved unambiguously in the temperature range studied. However, this is not necessarily so, especially not for the axial phenyl groups at lower temperatures since these groups are very close to each other. Concerning the observed transformation of the 31P spectrum it must be related to a process what makes all P atoms equivalent at higher temperatures, but retains a C2 axis even at 213 K. As was pointed out above, the only change that runs parallel with the formation of the AA 0 XX 0 spin system is the rearrangement and doubling of the aromatic 1H and 13C signals. Therefore the most likely explanation for the observed A2X2-like spin-system is that at low temperature the hindered rotation of the axial phenyls make the cis P atoms inequivalent, but have almost no or only negligible effect on the chemical shifts of the trans related P atoms. In solid state the single crystal data also indicate an extended-chair structure with equatorial methyl groups, ˚ ) and with relatively large Pd–Pd bond length (2.664 A with relatively small dihedral angle (37.4°) [8]. However it still remains to explain what makes the dynamic behaviour of the syn form so different from the anti isomer since both are constructed from the same building blocks (the bridging dppmMe ligands). The only difference is that in the syn form (extended-boat) these are mirror images whereas in the anti form (extended-chair) they are related by a C2 axis.

3. Summary The complexes studied represent examples of molecular pendulums with different dynamic behaviour. While interconversion of the clockwise and anti-clockwise distorted forms is rapid for the unsubstituted derivatives (1a–d) even at the limiting temperature in CD2Cl2, it becomes ring-conformation-dependent for the methylsubstituted ones (2–4). The extent of dependence is, however, not easily predictable. While in 2 (boat conformation with one equatorial methyl substituent) and 3 (boat conformation with two syn related equatorial methyl substituents) the ‘‘swingings’’ are of similar energy processes, in 4 (chair conformation with two anti

Ph Ph Cl A H 3C

XP

Pd

d

CH3 A' X' Cl Ph Ph

Cl

H3C

Pd B A' Pd B' Ph Ph Cl Ph Ph A

Scheme 3. The equatorial phenyls are omitted for clarity.

CH3

G. Szalontai, G. Besenyei / Inorganica Chimica Acta 357 (2004) 4413–4422

related equatorial methyl substituent) the energy barrier is definitely higher. Concerning the open question of the different dynamic behaviour of the syn and anti forms we assume that the driving force behind the swinging is the steric repulsion between the axial phenyl rings. The methyl substitution will change this and influence thereby indirectly the motions. The phenomenon can also be related to the substantial difference found in their twist angles (37.4° and 49.1° in solid state for the anti and syn forms, respectively [6]). The less crowded chair conformer with two axial phenyl groups on both sides of the molecule perhaps makes possible a ‘‘face-edge’’ or similar arrangement of the phenyls (as observed in the X-ray structures). In such stereochemical situation the substantial anisotropy of the phenyl ring can well explain the sizeable chemical shift difference between the two cis sites. In the boat forms with all axial phenyls on the same side of the molecule this arrangement is either not possible or lasts for much shorter time due to the stronger repulsion between the four phenyl rings. Consequently, the ‘‘swinging’’ slows down and becomes an observable process with an activation free energy of about 41–46 kJ mol
1. The five-membered rings undergo flipping in the unsubstituted derivatives (1a–d) only. In 1d the simultaneous coordination equilibrium of the bulky SnCl3
anion increases the barrier for the ‘‘swinging’’ and makes the interconversion among the extended-chair and extended-boat forms visible on the NMR time scale (see Scheme 3). As demonstrated changes of symmetry relations are reflected very vividly in their 31P NMR spectra. Easy distinction can be made between the syn and anti isomers (or perhaps rather between the boat and chair conformers) solely on the basis of the observed temperature dependence and symmetry of the signals. Solid state 31P MAS spectra are sensitive reporters of the asymmetric crystal structures.

4421

1

H and 13C nuclei, respectively. The chemical shift data were measured by the replacement methods and are given relative to external H3PO4 and TMS. The 119Sn spectra were referenced to external SnMe4. The liquid phase spectra were recorded in CD2Cl2, the solute concentration varied between 5 and 25 mg ml
1. The estimated accuracy of the reported solution phase 31P chemical shift and coupling data was about ±0.1 ppm and ±2 Hz, respectively. In the case of 31P NMR spectra the number of scans varied between 32 and 128. The usual digital resolution was better then 0.4 Hz, spectral widths for the 31P spectra were about 20,000–30,000 Hz, data points P 40k. The raw data were processed by the standard Varian software (Vnmr 6.1C), Gaussian weighting function were applied occasionally to improve resolution. Pulse conditions: the 90° 31P pulse was about 10 ls at a rf field strength of about 150 W. Probe: Varian Broad Band 5 mm. VT experiments: The accuracy of the temperature settings was ±0.6 °C. The exchange rate constants, kexch. reported refer to data obtained at the coalescence temperature (kexch. = pdm/2
1/2). In the case of 3 where we have only estimates of chemical shift difference of the exchanging nuclei, dm the error of the calculation can be as large as ± 5 kJ mol
1. Spectral simulations were carried out using the Mestre-C 3.3.2 freeware package. The solid state NMR: Spectra were recorded with a Doty XC5 probe at room temperature without regulation. Spectral width of 25,000–30,000 Hz were applied, the acquisition time was 50 ms with a 90° 31P pulse length of about 3.8 ls. The decoupler rf field strength was about 120 W. The relaxation delays varied between 4 and 6 s for the CP/MAS experiments, whereas in the MAS experiments 20–30 s delays were applied. Thin wall 5 mm Zirkonia rotors and Kel-F caps were used, the rotation speeds varied between 3000 and 12,000 Hz and were not regulated. The accuracy of the solid phase 31P chemical shift and coupling data was about ±0.5 ppm and ±8 Hz, respectively.

4. Experimental 4.1. Synthesis

Acknowledgement

The complexes were synthesized according to previously reported procedures, 1a [14,25], 1b [25], 1c [25], 1d [18], 2 [8], (3) [19] and (4) [8].

The authors thank the Hungarian Scientific Research Fund (OTKA T34335) for financial support.

4.2. Spectroscopy

Appendix A. Supplementary material

All NMR experiments were performed on a Varian UNITY 300 NMR spectrometer. The resonance frequencies were 121.4, 300 and 75.4 MHz for the 31P,

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.ica.2004.06.046 (http:/sparc4.mars.vein.hu).

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