Reversible CO exchange at platinum(0). An example of similar complex properties produced by ligands with very di ﬀ erent stereoelectronic characteristics †

The ligands 1,2-C 6 H 4 (CH 2 P t Bu 2 ) 2 ( L a ) and 1,2-C 6 H 4 (P t Bu 2 )(CH 2 P t Bu 2 ) ( L b ) displace norbornene (nbe) from [Pt( η 2 -nbe) 3 ] to give [PtL( η 2 -nbe)] where L = L a ( 1a ) or L b ( 1b ); 1a is ﬂ uxional on the NMR timescale. Reaction of 1a,b with CO gives the corresponding monocarbonyls [PtL(CO)] where L = L a ( 2a ) or L b ( 2b ) which then react further, and reversibly, to give the dicarbonyls [PtL(CO) 2 ] where L = L a ( 3a ) or L b ( 3b ). The CO interchange between 2a,b and 3a,b is compared with the only other such system ( 2f and 3f) , which are complexes of (C 2 F 5 ) 2 PCH 2 CH 2 P(C 2 F 5 ) 2 ( L f ). Ethene reacts smoothly with 2a to give ( 4a ) and H 2 with 2a generates some [PtH 2 ( L a )]. Protonation of 2a gives [Pt( L a )(H)(CO)][B(C 6 F 5 ) 4 ] ( 5a ) whose crystal structure has been determined. Similarly protonation of 2b gives [Pt( L b )(H)(CO)][B(C 6 F 5 ) 4 ] as a mixture of geometric isomers 5b – 6b .


Introduction
The bulky o-xylenyl diphosphine L a (Chart 1) was first prepared in 1976 by Shaw et al. who also reported cis-[PtH 2 (L a )], the first example of a cis-dihydridoplatinum(II) complex. 1 Spencer et al. later reported that treatment of complexes of the type [Pt(L a )-(η 2 -alkene)] with a Brønsted acid produced the first examples of β-agostic C-H-Pt complexes. 2,3 More recently, L a has been used in the commercialised Pd-catalysed ethene hydromethoxycarbonylation (Lucite Process) for the production of methyl methacrylate, shown in eqn (1). 4 This application has galvanised academic interest in the Pd and Pt chemistry of L a and related ligands (e.g. ligands L b -L e shown in Chart 1) 5-10 with the aim of understanding the special qualities of the L a chelate. For example, using isotopically labelled MeOD and 13 CH 2 vCH 2 , Iggo et al. 6,7 identified by NMR the key intermediates in the carbonylation cycle with Pd-L a and Pt-L a catalysts; they determined that the Pt-catalysis It has been established that the activity and longevity of the Pd-catalyst for carbonylation can be improved by ligand modification. 6 For example, we have shown that changing the backbone from xylenediyl in L a to toluenediyl in L b led to a more active Pd-catalyst. 8 This has prompted us to investigate and compare the coordination chemistry of L a and L b , particularly that which may be relevant to the ethene hydromethoxycarbonylation catalysis (eqn (1)). 9 The dichloroplatinum(II) complexes of L a and L b have been shown to be fluxional on the NMR timescale. 11 The conformations of the chelate rings in [PtCl 2 (L)], as determined by X-ray crystallography, are depicted as A (L = L a ) and B (L = L b ) in Chart 2 and the observed fluxionality is associated with ring inversion.
Here we present the characterisation of Pt(0)-carbonyl complexes of the electron-rich, bulky ligands L a and L b , that have the property of reversible CO-interchange between three-and four-coordinate complexes, only previously observed with Pt(0) complexes of the electron-poor L f . The reactions of the carbonyl complexes of L a and L b with ethene, H 2 and H + are also described and the relevance of this chemistry to the carbonylation catalysis shown in Scheme 1 is discussed.

Results and discussion
The platinum(0) chemistry of L a and L b is summarised in Scheme 2. Addition of 1 equiv. of L a or L b to [Pt(η 2 -nbe) 3 ] (nbe = norbornene) gave the corresponding [Pt(L)(η 2 -nbe)] (1a, L = L a ; 1b, L = L b ). The 31 P NMR spectrum of the previously reported 2 1a at −60°C in toluene, showed 2 singlets in the ratio of ca. 7 : 3 (δ 49.2 ppm J PtP = 3325 Hz and δ 47.3 ppm J PtP = 3308 Hz) which coalesce at 0°C, consistent with the interchange of the diastereoisomeric rotamers 1a and 1a′ shown in eqn (2). The 1 H NMR spectrum of 1a at −40°C shows two multiplets for the diphos CH 2 which coalesce at −20°C and appear as a broad singlet (w 1/2 = 23 Hz) at room temperature; only one A 9 XX′ signal for the t Bu groups was discerned across the temperature range −40 to +20°C indicating the insensitivity of this signal to the inequivalence of these groups. Two mechanisms can be postulated for the interchange shown in eqn (2): chelate ring inversion and Pt-nbe bond rotation. Both are plausible since there is precedent for Pt-L a chelate ring inversion on the NMR timescale [8][9][10][11] and slow rotation about the Pt-nbe bonds in [Pt(L d )(η 2 -nbe)], a complex of the xylenediyl ligand L d (Chart 1), has been observed. 12 ð2Þ Scheme 1 Outline of the mechanism of Pt-catalysed hydromethoxycarbonylation of ethene 7 including two exocyclic equilibria.
Chart 2 Conformations of the chelate rings in square planar (A and B) and tetrahedral (A' and B') complexes.
In the case of 1b, its 31 P NMR spectrum shows sharp doublets over the whole temperature range of +23 to −90°C which we interpret as due to there being a thermodynamic preference for one of the rotamers analogous to those shown in eqn (2). The 1 H NMR spectrum of 1b showed 2 multiplets for the diphos CH 2 and 4 sharp A 9 XX′ multiplets for the t Bu groups, consistent with the presence of a predominant isomer.
The displacement of the nbe from 1a and 1b to give the monocarbonyls 2a and 2b was achieved by bubbling CO through their toluene solutions and removing the solvent with the displaced nbe by evaporation to dryness. The colourlessness of 1a/1b contrasts with the vivid orange-red of 2a/2b.
The labelled compounds [Pt(L)( 13 CO)] (2a*, L = L a ; 2b*, L = L b ) were prepared in order to confirm their structures and to probe further the apparent fluxionality of 2b. Treatment of 2a and 2b with 13 CO gave the labelled complexes 2a* and 2b*. The 31 P NMR spectrum of 2a* at ambient temperature is a doublet with 2 J (PC) = 45 Hz and its 13 C NMR spectrum shows a triplet at 229 ppm with 1 J (PtC) = 2096 Hz and the same 2 J (PC) of 45 Hz; these spectra support the monocarbonyl structure assigned to 2a. The 31 P NMR spectrum of 2b* at −90°C showed an ABX pattern with 195 Pt satellites (see Fig. 1) and its 13 C NMR spectrum showed a doublet of doublets at 235 ppm with 2 J (PC) = 52 and 41 Hz and 1 J (PtC) = 2136 Hz; again, these spectra are consistent with the monocarbonyl structure assigned to 2b. The ambient temperature 31 P and 13 C NMR spectra of 2b* resembled the spectra for the unlabelled 2b, being broad and unresolved. Above room temperature, the 31 P NMR spectrum of 2b* broadens progressively until at +100°C, w 1/2 ∼ 400 Hz. In the 13 C NMR spectra of 2b* measured between +40 and +100°C, no CO signal was observed at all presumably because of its broadness. When the high temperature NMR samples were cooled to ambient temperature, the 31 P and 13 C NMR spectra reassumed their original forms.
It is not obvious what the source of the line-broadening is in the 31 P and 13 C NMR spectra of 2b at ambient temperature and above. If the conformation of the 6-membered chelate in 2b is B/B′-like (see Chart 2), then 2b would exist as a pair of Scheme 2 When diphos = L a , the complex is denoted na, and when diphos = L b , the complex is denoted nb. enantiomers (with identical 31 P shifts) and therefore interchange between such conformers would not contribute to the line broadening. If intermolecular CO exchange were taking place, P-C coupling would be lost in the labelled 2b* which apparently it is, but this observation is inconclusive because the large observed NMR line-width would swamp the 2 J (PC) of 52 and 41 Hz (measured at −90°C). Nevertheless, one explanation for the NMR behaviour is that at elevated temperatures, CO exchange is taking place perhaps via an analogue of [Pt 2 (CO)(μ-CO)(L g ) 2 ] (see Chart 1 for structure of L g ) reported by Mezailles et al. 14 which is in equilibrium with 2b.
The dicarbonyl complexes, [PtL(CO) 2 ] (3a, L = L a ; 3b, L = L b ) were generated when solutions of 2a and 2b were pressurised with 2 atm of CO . The solid state IR spectrum of 3a showed ν(CO) bands at higher frequencies (1971 and 1931 cm −1 ) than the 1907 cm −1 for 2a, consistent with the two CO ligands in 3a sharing the electron density from the platinum(0). The 31 P NMR spectrum of 3a at room temperature showed no discernable resonances but at −90°C, a singlet at 35.3 ppm with 1 J (PPt) of 3107 Hz was observed. At +40°C, the only 31 P NMR signal observed was for 2a, showing that CO dissociation from 3a occurs readily and this explains the great broadness of the signal at ambient temperatures. The labelled complex [Pt(L a )-( 13 CO) 2 ] (3a*) was prepared from 3a and 13 CO and its 13 C NMR spectrum at −90°C had multiplets at 187.1 and 186.6 ppm with 1 J (CPt) of 1832 and 1900 Hz respectively which were assigned to inequivalent Pt-CO ligands. The 2 J (CP) coupling was approximately 4 Hz, leading to multiplets in the 31 P and 13 C NMR spectra. The inequivalence of the CO ligands detected in the low temperature 13 C NMR spectrum of 3a* is consistent with an A′-type conformation of the 7-membered chelate (see Chart 2) giving rise to CO ligands being syn and anti to the phenylene of the chelate.
The solid state IR spectrum of 3b showed ν(CO) bands at higher frequencies (1961 and 1915 cm −1 ) than the 1898 cm −1 in 2b. The 31 P NMR spectrum of 3b at ambient temperatures showed 2 sets of broad singlets at 44.4 and 42.7 ppm with 1 J (PPt) = 3106 and 3818 Hz respectively. The 13 C NMR spectrum of the labelled complex [Pt(L b )( 13 CO) 2 ] (3b*) had two signals at 187.1 and 186.0 ppm with 1 J (CPt) of 1930 and 1850 Hz respectively; the C-P coupling was not resolved in the 31 P or 13 C NMR spectra of 3b*. According to solid-state IR spectroscopy (see Fig. 2), when red solid monocarbonyl 2b was subjected to a CO atmosphere, it was converted to yellow 3b and this was reversed upon application of a vacuum to powdered 3b.
Dissociation of CO from the dicarbonyls 3a/3b to give 2a/2b occurred slowly when their solutions were stirred under a N 2 atmosphere and rapidly when solutions were put under vacuum, presumably due to the dissolved CO being removed under the reduced pressure. The uptake of CO by 2a/2b (eqn (3)) is notable because of its rarity 13 (3)); the bulk and bite angles of L h and L b should be similar. However there is only one previously reported example of a 16-electron [Pt(CO)(diphos)] complex, 2f, which Roddick et al. 18 reported undergoes a similar CO interchange between 2f and 3f (eqn (3)). The explanation that was given for the stability of 2f relative to 3f was that the high π-acceptor capacity of the fluorophos ligand L f efficiently delocalises the electron-density on the Pt(0); this is supported by the high ν CO values for 2f and 3f (see eqn (3)). A similar argument is not tenable for 2a/2b since L a /L b are strong σ-donors, as reflected in the low ν CO values for their carbonyl complexes (see eqn (3)). The stability of the coordinatively unsaturated 2a/2b is therefore associated with the large bulk of L a /L b . This steric argument can be extended to explain why CO dissociation from 3a appears to be more facile than from 3b, since L a is more sterically demanding than L b . 11 The complementary explanations for the equilibria shown in eqn (3) for L a,b vs. L f exemplify how ligands with very different stereoelectronic properties can produce complexes with similar properties.
When a toluene solution of 2a was stirred under an atmosphere of H 2 for 5 days, the cis-[PtH 2 (L a )] was formed in approximately 50% NMR yield (along with other uncharacterised products), as shown by the close matching of the NMR data ( 1 H: δ −3.8 ppm, J PtH = 1004 Hz; 31 P: δ 45.7 ppm, J PtP = 2109 Hz) with the literature values for this compound. 1 The CO ligands in 2a were displaced by ethene to give 4a by repeated vacuum/ethene-addition cycles (see Experimental) and this transformation was readily reversed by the application of a CO atmosphere to 4a (Scheme 2). Complex 4a was characterised by matching of the NMR data with those previously reported for this complex. 19 Under similar conditions to those used to generate 4a, 2b reacted with ethene but the product, according to the 31 P NMR spectrum at −90°C, was a mixture containing 2b (50%), 3b (30%) and a third species (20%) whose 31 P NMR parameters (60.9 with 1 J (PPt) = 3288 Hz, 2 J (PP) = 45 Hz; 57.9 1 J (PPt) = 3288 Hz) led us to tentatively assign this minor product to 4b, although we have not isolated it.
In the carbonylation catalytic cycle shown in Scheme 1, neutral species are not involved in the core cycle because the reaction is carried out in an acidic medium. For this reason, we investigated the protonation of the monocarbonyl complex 20 in chlorobenzene gave a product assigned structure 5a (Scheme 2) on the basis of its 1 H NMR spectrum which showed a signal at −4.7 ppm with J (PtH) = 736 Hz characteristic of a hydride. A band at 2092 cm −1 in its IR spectrum is typical of a cationic Pt(II)-CO. 21 The NMR spectroscopic data for 5a match well those reported by Iggo et al. 7 for the triflate salt of 5a which they characterized in solution only.
Crystals of 5a suitable for X-ray crystallography grew from its CH 2 Cl 2 solution, crystallising in the triclinic space group P1 with two [Pt(L a )(H)(CO)][B(C 6 F 5 ) 4 ] moieties in the asymmetric unit, i.e. Z′ = 2. Both of these moieties have essentially the same geometrical conformation and selected bond lengths and angles are listed below Fig. 3. The location of the hydride was not determined directly from the data, but its position was inferred from the metal geometry and located accordingly. The P1-Pt1-P2 bite angle is 104.00(12)°and the P3-Pt2-P4 bite angle is 104.74(12)°a nd all four of the half cone angles are ∼117°.
The chelate conformations for xylenediyl diphos chelates can be described in terms of the two M-P-C-C(Ph) torsion angles. There are currently 12 Pt and 26 Pd xylenediyl diphos chelates in the Cambridge Crystallographic Database (CSD) 22 , 35 of which have structures where the two M-P-C-C(Ph) torsion angles have similar absolute values and the ring conformation is half-chair-like with an essentially coplanar MP 2 C 2 component as depicted as A in Chart 2. In the remaining 3 structures (HUXCIN, 23 HUXCEJ 23 and IHETOF 24 ) the chelate rings adopt a twist-boat-like conformation, with two different Pt-P-C-C(Ph) torsion angles of approximately ±50°and ±14°. The conformation of the Pt-L a ring in 5a (see Fig. 3) matches the majority of those in the CSD, with Pt-P-C-C(Ph) torsion angles for Pt1 of −54.5(12)°and 65.0(11)°and for Pt2 of 57.9 (11)°and −57.7(11)°.
Protonation of 2b gave a mixture of geometric isomers of hydridocarbonyl complexes where the position of the H ligand relative to the P-aryl is cis (5b) or trans (6b). Evidence for the two isomers in solution comes from the 1 H NMR spectrum which showed 2 hydride signals at −2.3 and −4.3 ppm in a 1 : 4 ratio and the 31 P NMR spectrum showed two sets of signals; the NMR data for the two species are similar and therefore it was not possible to assign the major isomer (see Experimental for the data). The solid state IR spectrum of the 5b-6b mixture shows 2 ν(CO) bands at 2102 and 2094 cm −1 .

Conclusion
We have shown here that the mono-and dicarbonylplatinum(0) complexes 2a/2b and 3a/3b are readily formed and interconvert. The stability of the coordinatively unsaturated [Pt(CO)(diphos)] species 2a/2b relative to 3a/3b is associated with the steric congestion provided by the bulky diphos ligands L a and L b . The only related [Pt(CO) n (diphos)] species reported (2f/3f ) feature the electron-poor fluorinated diphos ligand L f (which has modest steric bulk) 25 where the explanation given for the stability of the monocarbonyl is electronicthe π-acceptor properties of the diphos ligand destabilises the corresponding [Pt(CO) 2 (diphos)]. This is a textbook example of ligands with very different stereoelectronic properties producing similar outcomes in terms of the properties of their complexes. The hydromethoxycarbonylation of ethene is efficiently catalysed by Pd-complexes of L a and L b and Iggo et al. 7 have shown that the analogous organoplatinum chemistry is relevant in the study of the mechanism of the catalysis. Ethene reacts with the coordinatively unsaturated 2a to give 4a presumably via the 18-electron tetrahedral intermediate X (Scheme 3). The protonation of the 2a/2b to give the cationic hydridocomplexes 5a/5b (and the geometric isomer 6b) is pertinent because the catalysis is carried out at low pH. It is possible that complexes of the type 2-5 described above are present in non-productive equilibria during the Pt-catalysed carbonylation shown in Scheme 1; their place is shown in Scheme 3.

Experimental
Unless otherwise stated, all reactions were carried out using standard Schlenk line and dry box techniques. Dry N 2 -saturated solvents were collected from a Grubbs system in flame and vacuum-dried glassware. Deuterated solvents were dried and distilled from CaH 2 . Pentane was dried over 4 Å molecular sieves and N 2 -saturated by repeated freeze, vacuum and thawing cycles. The complex [Pt(η 2 -nbe) 3 ] 26 was prepared by literature methods. Ligand L a was obtained from Lucite International. Ligand L b was prepared as previously described. 11 CO, C 2 H 4 and H 2 , were used as obtained from BOC and 13 CO was used as obtained from Aldrich. NMR spectra were recorded on a Jeol ECP (Eclipse) 300 or a Varian VNMR S500 spectrometer. Chemical shifts are referenced relative to high frequency of Si(CH 3 ) 4 ( 1 H or 13 C), 85% H 3 PO 4 ( 31 P), and CF 3 Cl ( 19 F). Infrared spectra were obtained using a Perkin Elmer 1600 series FTIR. Mass spectrometry was carried out by the Mass Spectrometry Service at the University of Bristol. Elemental analyses were carried out by the Microanalytical Laboratory at the University of Bristol.

Preparation of [Pt(L a )(η 2 -nbe)] (1a)
This was made according to the method of Spencer et al. and characterised by comparison of the NMR spectra with the reported data. 2 The spectra at low temperature have not been previously reported. 31

Preparation of [Pt(L b )(η 2 -nbe)] (1b)
A solution of L b (0.089 g, 0.26 mmol) in toluene (5 mL) was added in one portion to a solution of [Pt(η 2 -nbe) 3 ] (0.125 g, 0.262 mmol) in toluene (5 mL) at −78°C and the resulting mixture was stirred for 2 h, allowed to warm to room temperature and then stirred for a further 16 h. The volatiles were then removed under reduced pressure to yield an off-white solid that was dissolved in pentane (1 mL) and the product crystallized at −78°C.

Preparation of [Pt(L a )(CO)] (2a)
A solution of L a (0.158 g, 0.415 mmol) in toluene (5 mL) was added in one portion to a solution of [Pt(η 2 -nbe) 3 ] (0.198 g, 0.415 mmol) in toluene (5 mL) at −78°C, the resulting mixture was stirred for 2 h, allowed to warm to room temperature and then stirred overnight. CO was then bubbled through the solution for 30 min to give an orange solution which was filtered and then the volatiles removed under reduced pressure. The residue was redissolved in toluene (5 mL) and CO was bubbled through solution again. The vacuum/CO cycle was repeated twice more to ensure that all norbornene had been displaced. The solvent was then removed under reduced pressure and the solid extracted with pentane (3 × 5 mL). The volume of the solution was reduced to 3 mL and the product crystallised at −78°C.

Preparation of [Pt(L a )( 13 CO)] (2a*)
A solution of 2a (9.4 mg, 0.015 mmol) in d 6 -benzene (1.2 mL) was placed in a valved NMR tube. The solution was frozen with liquid N 2 and then the NMR tube was evacuated and sealed. The solution was then thawed and the NMR tube backfilled with 13 CO (2 bar) and the resulting mixture allowed to react for 30 min. The vacuum/CO cycle was repeated twice more to ensure that most of the 12

Preparation of [Pt(L b )(CO)] (2b)
CO was bubbled through solution a solution of 1b (0.135 g, 0.202 mmol) in toluene (10 mL) for 30 min to give a yellow solution which was filtered and then the volatiles removed under reduced pressure. The residue was redissolved in toluene (5 mL) and CO was bubbled through the solution again. The vacuum/CO cycle was repeated twice more to ensure that all of the norbornene had been displaced. The solvent was removed under reduced pressure to give a yellow solid which turned red under prolonged exposure to vacuum. The solid was extracted with pentane (3 mL) and the product crystallized at −78°C.

Preparation of [Pt(L b )( 13 CO)] (2b*)
A solution of 2b (11.2 mg, 0.0185 mmol) in benzene (1.2 mL) was placed in a valved NMR tube. The solution was frozen with liquid N 2 and then the NMR tube was evacuated and sealed. The solution was then thawed and the NMR tube backfilled with 13 CO (2 bar) and the resulting mixture allowed to react for 30 min. The vacuum/CO cycle was repeated twice more to ensure that all the 12 CO had been displaced. The volatiles were removed under reduced pressure to yield a yellow solid that turned red under prolonged exposure to vacuum. The product was identified in solution by NMR only. The 1 H NMR spectrum of 2b* was essentially the same as for 2b. 31  was placed in a valved NMR tube. The solution was frozen with liquid N 2 and then the NMR tube was evacuated and sealed. The solution was then thawed and the NMR tube backfilled with 13 CO (2 bar) and the resulting mixture allowed to react for 30 min. The vacuum/CO cycle was repeated twice more to ensure that most of the 12 CO had been displaced. The solution was then saturated with N 2 to remove the excess of 13 CO. The product was not isolated because it reverted to 2a* upon removal of solvent and was identified in solution by NMR. The 1 H NMR spectrum of 3a* was essentially the same as for 3a.

Preparation of [Pt(L b )( 13 CO) 2 ] (3b*)
A solution of 2b (9.9 mg, 0.0164 mmol) in d 8 -toluene (1.2 mL) was placed in a valved NMR tube. The solution was frozen with liquid N 2 and then the NMR tube was evacuated and sealed. The solution was then thawed and the NMR tube backfilled with 13 CO (2 bar) and then the resulting mixture allowed to react for 30 min. The vacuum/CO cycle was repeated twice more to ensure that all 12 CO had been displaced. The product was not isolated because it reverted to 2b* upon removal of solvent and was identified in solution by NMR. The 1 H NMR spectrum of 3b* was essentially the same as for 3b.

Preparation of [Pt(L a )(C 2 H 4 )] (4a)
A solution of 2a (12.2 mg, 0.0194 mmol) in d 6 -benzene (1.2 mL) was placed in a valved NMR tube. The solution was frozen with liquid N 2 and then the NMR tube was evacuated and sealed. The solution was then thawed and the NMR tube backfilled with C 2 H 4 (2 bar) and the resulting mixture allowed to react for 30 min. The vacuum/C 2 H 4 cycle was repeated twice more to ensure that most of the CO had been displaced. The product was identified by comparison of its NMR spectra with the literature data for this previously reported complex. 19

Crystal structure determination
A single crystal of 5a was mounted on a glass fibre and X-ray diffraction data were collected at 100 K on a Bruker APEX II CCD diffractometer using graphite monochromatised Mo-K α radiation (λ = 0.71073 Å). Absorption corrections were based on equivalent reflections using SADABS. 27 The structure was solved by direct methods in SHELXS and refined by full matrix least squares on F 2 in SHELXL. 28 All of the non-hydrogen atoms were refined anisotropically and all of the hydrogen atoms were located geometrically and refined using a riding model with the exception of H1D, H1E, H2D, H2E. The structure showed a small amount of disorder in the positions of the CO group and hydride attached to the Pt metal centres. The occupancy of each CO group was determined by refining them against a free variable with the sum of the occupancies for the two CO sites attached to each Pt set to equal one, prior to fixing the occupancy at the refined values. The hydrogen atoms (H1D, H1E, H2D, H2E) were located on the basis of chemical knowledge and their occupancy fixed to that of the related CO occupancy. Restraints were applied to maintain chemically sensible geometries (DFIX, SADI, DANG) for the disordered sections, while the CO thermal parameters were restrained to similar values using SIMU and the H U iso values was fixed at 1.5 × U eq (Pt). Crystal structure and refinement data are given in Table 1.