Organometallics 1995, 14, 3570-3573
3570
A New Chiral Tridentate Ferrocenyl Ligand. Synthesis and Characterization of Its Palladium(I1)and Nickel(I1) Complexes Pierluigi Barbarot and Antonio Togni* Laboratory of Znorganic Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zurich, Switzerland Received March 21, 1995@ Summary: A novel chiral tridentate phosphine ligand based on ferrocene (5) can be readily obtained from cyclohexylphosphine and N,N-dimethyl-(S)-l-[(R)-2(diphenylphosphino)ferrocenyllethylamine(4) in acetic acid. Ligand 5 adopts a meridional coordination geometry with d8-metal centers. The crystal structure of [Pd(5)ClIPF6 (6) has been determined.
Scheme 1
*&+
AcOH, 65°C
IipPCy
&$$ ‘ Me
47 Yo
PPhP
Introduction
ax PPh2
I
a
Me
PPh2 p C
I
Fe I
y
2
Me
a
;
N
\
It
I Fe I
5
/
Chiral, enantiomerically pure ferrocenyl ligands have been very successful in asymmetric catalysis. Hayashi, Ito, and co-workers have demonstrated that bidentate ligands of type 1, bearing the phosphino substituents
- e%
1
Ph2P
fe
4s
’ l W n
’ phzdl
Phz L
6 (M = Pd; L = CI‘; (X)” = PF);
1
2
3
in the 1,l’-positions,can be adapted to the specific needs of a series of catalytic reactions by virtue of the variable side chain X.’ We reported more recently a new generation of easily accessible and highly effective chelating ligands based on ferrocene, compounds 2 and 3 being representative examples.2 The main feature of such new P,P and P,N ligands is their highly tunable character, with respect to both steric and electronic properties. The key reaction in their synthesis is the introduction of the ligating fragment attached to the stereogenic center. This is carried out in acetic acid using either a secondary phosphine or a pyrazole as the nucleophile. We extended this methodology to primary phosphines and now report the preparation and char+ Present address: ISSECC-CNR, via J . Nardi, 39,I-50132 Firenze, Italy. Abstract published in Advance ACS Abstracts, June 15, 1995. ( 1 ) For reviews, see: (a) Hayashi, T. In Ferrocenes. Homogeneous Catalysis, Organic Synthesis, Materials Science; Togni, A,, Hayashi, T., Eds.; VCH: Weinheim, 1995; pp 105-142. (b) Sawamura, M.; Ito, Y. Chem. Rev. 1992,92, 857-871. ( 2 )(a) Togni, A,; Breutel, C.; Schnyder, A.; Spindler, F.; Landert, H.; Tijani, A. J . Am. Chem. SOC.1994, 116, 4062-4066. (b) Breutel, C.; Pregosin, P. S.;Salzmann, R.; Togni, A. J . Am. Chem. SOC.1994, 116, 4067-4068. (c) Togni, A.; Breutel, C.; Soares, M. C.; Zanetti, N.; Gerfin, T.; Gramlich, V.; Spindler, F.; Rihs, G. Inorg. Chim. Acta 1994, 222, 213-224. (d) Abbenhuis, H. C. L.; Burckhardt, U.; Gramlich, V.; Togni, A.; Albinati, A.; Muller, B. Organometallics 1994, 13, 44814493. (e) Abbenhuis, H. C. L.; Burckhardt, U.; Gramlich, V.; Kollner, C.; Pregosin, P. S.; Salzmann, R.; Togni, A. Organometallics 1995,14, 759-766. (0 Schnyder, A.; Hintermann, L.; Togni, A. Angew. Chem. 1995,107,996-998;Angew. Chem., Int. Ed.Engl. 1995,34,931-933.
7a (M = Pd; L = MeCN; (XIn= (PF;I2 7b (M = Pd; L = MeCN; ( X ) n = (BF4‘)2 8 (M = Ni; L = MeCN; (X-)n = (C104‘)2
acterization of a novel tridentate phosphine that can potentially adopt both meridional or facial coordination geometries. This ligand is still a quite rare example of a chiral tridentate p h ~ s p h i n e .We ~ envisage applications in asymmetric Lewis-acid catalysis using late transition metal centers (see below).
Results and Discussion Synthesis. The new chiral bis(ferroceny1)chelating ligand “Pigiphos” (SI4 has been prepared as shown in Scheme 1. The reaction of the starting (S)-(R)-PPFA (4)5 with cyclohexylphosphine in acetic acid at 65 “C proceeds with retention of configuration on the side
@
( 3 )For a general review on polydentate phosphines, see: Cotton, F. A.; Hong, B. Prog. Inorg. Chem. 1992,40, 179-289. For examples of chiral-tripodal phosphines, see, e.g.: (a) Burk, M. J.; Harlow, R. L. Angew. Chem. 1990,102, 1511-1513. (b) Burk, M. J.; Feaster, J. E.; Harlow, R. L. Tetrahedron: Asymmetry 1991,2, 569-592. (c) Ward, T. R.; Venanzi, L. M.; Albinati, A,; Lianza, F.; Gerfin, T.; Gramlich, V.; Ramos Tombo, G. M. Helu. Chim. Acta 1991, 74, 983-988. ( 4 )In order to avoid its long systematic name (Bis{(S)-l-[(R)-2~diphenylphosphino)fe~ocenyllethyl}cyclohexylphosphine~, compound 5 is called “Pigiphos” in our laboratory, after the nickname of one of the authors (P.B.). ( 5 )Hayashi, T.; Mise, T.; Fukushima, M.; Kagotani, M.; Nagashima, N.; Hamada, Y.; Matsumoto, A,; Kawakami, S.; Konishi, M.; Yamamoto, K.; Kumada, M. Bull. Chem. SOC.Jpn. 1980,53, 1138-1151.
0276-7333/95/2314-3570$09.QQIQ 0 1995 American Chemical Society
Notes
Organometallics, Vol. 14, No. 7, 1995 3571
Cilli chain stereocenters, leading to an (S)-(R)configuration CllOl on both the ferrocenyl units. The final product could be isolated in satisfactory yield d e r purification by column chromatography and recrystallization. Noteworthy, the use of phenylphosphine as the nucleophile under the same reaction conditions gave only a small amount of the elimination product l-(diphenylphosphino)-2-vinylferrocene,along with unidentified species. The two ferrocenyl moieties of “Pigiphos”are diastereotopic, as demonstrated by the 31P and lH NMR spectra. The 31PNMR spectrum of “Pigiphos” can be seen as the superimposition of two AX spin systems (A = CyP, X = PhzP) in which the two PhzP phosphorus atoms have the same chemical shift but very different coupling constants to the central CyP nucleus C4Jpp = 28.1 and 4Jpp = 9.9 Hz, respectively). The larger coupling is attributed to a 1,3-bis(phosphino)fragment Figure 1. ORTEP view of the cation [(Pigiphos)PdCll+(6) assuming a conformation similar to that encountered and atom numbering scheme. Selected bond lengths (A) and angles (deg): Pd(l)-C1(1) = 2.339(5), Pd(1)-P(1a) = for the ligand Josiphos (2, for which 4 J ~ = p 30 HZ).~* 2.333(4), Pd(1)-P(1b) = 2.331(4), Pd(l)-P(2) = 2.255(4); The other subunit obviously adopts a different conforCl(l)-Pd(l)-P(la) = 84.8(1), Cl(l)-Pd(l)-P(lb) = 89.0mation, probably because of steric reasons. (11, Cl(l)-Pd(l)-P(2) = 177.4(2), P(1a)-Pd(1)-P(1b) = The observed ‘H NMR parameters are rather routine 168.2(2),P(la)-Pd(l)-P(2) = 95.4(1),P(lb)-Pd(l)-P(2) = and are in the range observed for the parent ligand 91.4(1). Josiphos.2a However, the lH NMR spectrum shows separated sets of resonances for each of the two ferrothere are no drastic changes in the overall structure of cenyl fragments, thus indicating the lack of any s y m the complexes on going from Pd to Ni. metry element relating them. Solid State Structure of 6. Compound 6 was Orange crystals of the Pd complex 6 were afforded characterized by X-ray crystal structural analysis and by treatment of [PdClz(NCCH3)21 with TlPF6 in the an ORTEP view of the cation [PdCl(Pigiphos)l+ is presence of the ligand “Pigiphos”. The compound is airdepicted in Figure 1. The coordination around the metal stable both in the solid state and in solution. The 31P center approximates a square planar geometry in which NMR spectrum of 6 consists of an ABX pattern, the two the chlorine atom and the P(2) atom (CyP) assume a low-frequency resonances being attributed to the PhzP mutual trans position. A significant distortion from the atoms showing a mutual zJpp coupling constant of 411.5 idealized geometry is indicated by the displacement of Hz, consistent with their trans position. The observed the Cl-Pd vector from the plane containing the three moderate line broadening of the 31Psignals is indicative phosphorus atoms, such that the chlorine atom is ca. of a possible dynamic behavior of the molecule. 0.47 A above this plane. The nonideal square planar The dicationic derivative 7 is obtained either from geometry is also reflected by the distance of the Pd atom complex 6 utilizing TlPF6 as a chloride scavenger in from the same plane (0.18 A), and by the angles P(2)CH3CN solution (7a)or by treatment of the acetonitrile Pd-P(1a) and P(a)-Pd-P(lb) of 95.4(1) and 91.4(1)’, complex [Pd(NCCH&1(BF4)z6with the ligand “Pigiphos” respectively. (7b). Compounds 7a,b are stable under a nitrogen Nevertheless, considering the best idealized coordinaatmosphere in the solid state but decompose very slowly tion plane of Pd (maximum deviations of the donor in solution. The IR spectrum shows bands a t 2318 and atoms -0.12 to +0.14 A), it is instructive to analyze the 2289 cm-l, assigned to v(C=N) of a coordinated acetoposition of different parts of the molecule with respect nitrile. The 31PNMR spectrum of 7 consists of an ABX pattern similar to that observed for 6 , trans 2 J ~ to~this plane. Thus one observes that (a) one of the unsubstituted Cp rings (Cp,t) as well as the two methyl coupling constant being 336.9 Hz, thus showing that the groups lie below this plane, while the other Cp ring two complexes share the same primary structure. On (CPb’) and the cyclohexyl group are above, and (b) the the other hand, the CyP chemical shift falls ca. 14 ppm four phenyl rings are pairwise above and below the downfield compared to the corresponding resonance of plane, overall assuming an approximately CZsymmetric 6, indicating that this parameter is rather sensitive to arrangement. Furthermore, important differences in the nature of the trans ligand. the conformation of the two 1,3-bis(phosphino)ferrocenyl The nickel complex 8 was prepared by treatment with moieties are evident: (a) the phenyl rings bonded to the ligand “Pigiphos” of a Ni2+-CH3CN adduct generP(1b) are closer to an axidequatorial arrangement with ated in situ. The compound is stable under a nitrogen respect to the coordination plane of Pd than those atmosphere in the solid state but decomposes very bonded t o P(1a) [Cl(l)-Pd(l)-P(Ib)-C(2Ob) = 82.4(3), slowly in solution, as was the case for derivatives 7. The Cl(l)-Pd(l)-P(lb)-C(14b) = -33.1(3), Cl(l)-Pd(l)observed IR bands at 2319 and 2285 cm-l v(C=N) fall P(la)-C(2Oa) = 64.8(3), Cl(l)-Pd(l)-P(la)-C(l4a) = in the same range as those of 7. The 31PNMR spectrum -53.1(4)”1, and (b) the side chains of the two moieties of 8 shows the expected ABX pattern with a trans z J p ~ also adopt different conformations, as illustrated by the coupling constant of 235.9 Hz. Comparison of the 31P two torsion angles C(lb)-C(2b)-C(Sb)-P(2) = -28.3and ‘H NMR spectra of 8 with those of 7 suggests that (2) and C(la)-C(2a)-C(Ga)-P(2) = 60.8(3Y. The differing conformational features Of the side chains are (6) Wayland, B. B.;Schramm, R. F. Inorg. Chem. 1969, 8,971also reflected by the relative positions of the two methyl 976.
3572 Organometallics, Vol. 14, No. 7, 1995
Figure 2. Schematic representation of the cation 6 , projected along the CI(l)-Pd(l) axis.
groups (see the torsion angles C(lb)-C(2b)-C(Gb)= 11.8C(7b) = 83.7(4) and C(la)-C(2a)-C(Ga)-C(7a) (4)”). These structural characteristics are clearly visible in the schematic representation of Figure 2.7
Conclusions The readily prepared ligand “Pigiphos” is suited for the formation of cationic d8-metal complexes in which the metal center is held in a rigid coordination environment. The system is thus suited for reactions involving the singular fowth coordination site. Preliminary experiments show that the dicationic Ni(I1) complex 8 behaves as a Lewis acid and readily catalyzes the hetero Diels-Alder reaction of aldehydes with activated dienes? These investigations, as well as studies concerning the behavior of “Pigiphos” in an octahedral coordination geometry are under way and will be reported in due course. Experimental Section All manipulations were performed under an N2 atmosphere using standard Schlenk techniques. ‘H and 31PNMR spectra were recorded at 250.13 and 101.3 MHz, respectively, on a Bruker 250-ACP spectrometer (unless otherwise stated), and IR spectra on a Perkin-Elmer Paragon 1000 FT-IR spectrometer using KBr pellets. Merck silica gel 60 (70-230 mesh) was used for column chromatography. Optical rotations were measured with a Perkin-Elmer 341 polarimeter using 10 cm cells. Elemental analyses were performed by the “Mikroelementar-analytisches Laboratorium der ETH”. Pigiphos4 (5). Cyclohexylphosphine (360 pL, 2.7 mmol) was added to a solution of (23)-(R)-PPFA(4,2.22 g, 5.03 mmol) in acetic acid (50 mL) and the mixture stirred at 65 “C for 7 h. The solvent was then evaporated at 65 “C under reduced pressure and the residue chromatographed over silica using n-hexane:EtOAc = 3:l as eluent (Rf=0.68). Recrystallization from CH2ClfltOH under a stream of nitrogen gave 855 mg of pure product (47% yield). [ a I 2 O ~= +398 (c = 1.0, CHC13). Anal. Calcd for C54H55Fe2P3: C, 71.38; H, 6.10. Found: C, 70.99; H, 6.30. 31PNMR (CDC13): 6 15.60 (dd, lP, J p p = 28.1, J p p = 9.91, -26.04 (2d, 2P). ‘H NMR (CDC13): 6 0.55-1.60 (m, l l H ) , 1.53 (dd, 3H, J = 7.21, 1.64 (t, 3H, J = 7.4), 2.81 (dquint, lH, J = 4.6, J = 7.4), 3.15 (qd, lH, J 7.4, J = 2-71, (7) Preliminary 500 MHz lH NOESY experiments on complex 6 indicate that the conformational features found in the solid state are essentially retained in solution. (8)For an example of a chiral transition metal Lewis acid as catalyst for hetero Diels-Alder reactions, see, e.g.: (a) Togni, A. Organometallics 1990,9,3106-3113.(b)Togni, A.;Rist, G.; Rihs, G.; Schweiger, A. J. Am. Chem. SOC.1993, 115, 1908-1915 and references quoted therein.
Notes 3.82 (bs, 6H), 3.86 (s,5H), 3.96 (bm, 2H), 4.07 (bm, lH), 4.22 (t, lH), 4.26 (bm, lH), 7.12-7.70 (m, 20H). [(Pigiphos)PdCl]PF~ (6). “Pigiphos”(300 mg, 0.33 mmol) was slowly dissolved in acetone (40 mL), and solid LPdClz(CH3was then added. After the mixture CN)2] (85 mg, 0.33 “01) was stirred for 10 min, solid TlPFs (115 mg, 0.33 mmol) was added. TlCl was then filtered off and the solution evaporated to ca. 10 mL under a stream of nitrogen. Addition of diethyl ether (30 mL) gave the product as an orange solid. Crystals were obtained by recrystallization from CHzClfltOH (252 mg, 64%). Anal, Calcd for C54H55C1FsFe2P4Pd:C, 54.26; H, 4.60. Found: C, 54.12; H, 4.56. 31PNMR (202.5 MHz, CDzC12): (ABXspin system) ~ ( P z80.70 ) (Jp2p1. = 12.0, JP~P,, = 29.01, 6(P1,)15.15 ( J p l p l a = 411.5 Hz), 6(Plb)9.77. ‘H NhfR (500.138 MHz, CD2C12): 6 0.66 (br q, lH), 0.80 (qt, lH), 1.03 (qt lH), 1.19 (9, lH), 1.62 (m, lH), 1.64 (dd, 3H), 1.67 (m, lH), 1.85 (m, 2H), 2.05 (dd, 3H), 2.08 (m, lH), 2.36 (br d, lH), 3.02 (br q, 1H), 3.34 (m, lH), 3.64 (dq, lH), 3.73 (9, 5H), 3.95 (s, 5H), 4.28 (br s, lH), 4.60 (m, lH), 4.66 (t, 1H), 4.73 (9, W , 4.77 (m, lH), 4.92 (m, lH), 7.10-7.90 (several m, 20H). [(Pigiphos)Pd(CHsCN)lY2Cy = PFs Ua), BF4 (7b)). 7a. Solid TlPF6 (26 mg, 0.08 mmol) was added to a stirred solution of 6 (90 mg, 0.08 mmol) in CH&N (10 mL). After 1 h the mixture was filtered over Celite and the solvent removed under vacuum. Recrystallization from CH&N/diethyl ether gave 7a as purple microcrystals in 62% yield. Anal. Calcd for C56H58NF12Fe2P5Pd: C, 49.97; H, 4.34; N, 1.04. Found: C, 49.86; H, 4.36; N, 0.98. 7b. Solid Pigiphos (82 mg, 0.09 mmol) was added to a solution of [Pd(CH&N)dBF4)2 (40 mg, 0.09 mmol) in CH&N (10 mL). After the mixture was stirred until no ligand was detectable (1h), the solvent was evaporated to ca. half under vacuum. Slow addition of diethyl ether (20 mL) gave 7b as a purple solid in 85%yield. Anal. Calcd for C~dhNBzFsFezP3Pd: C, 54.69; H, 4.75; N, 1.14. Found: C, 54.55; H, 4.90; N, 1.20. 31P NMR (101.3 MHz, cD~C12):(ABX spin system) 6(CyP) 94.57 (Jpp. = 28.0, J p y = 1.6), 6(F)7.43 ( J p y = 336.91, 6 ( P )16.34. ‘H NMR (500.138 MHz, CD2C12): 6 0.30 (qt, lH), 0.45 (bm, lH), 0.98 (qt, lH), 1.08 (bm, lH), 1.4-1.6 (m, 2H), 1.48(~,3H),1.7-1.9(m,3H),1.82(dd,3H,J=6.9,J=16.7), 2.13 (dd, 3H, J = 14.21, 2.44 (bd, lH), 2.71 (bq lH), 3.33 (dq, l H , J = 9.6, J = 7.6), 3.88 (s, 5H), 3.98 (bm, l H , J = 9.5),4.17 (s 5H), 4.29 (bs, lH), 4.70 (bs, lH), 4.76 (t, lH), 4.82 (9, lH), 4.99 (bs, lH), 5.08 (bs, lH), 6.85 (m, 2H), 7.30-8.00 (m, 18H). IR: 2318, 2289 (v(C=N)) cm-’. [(Pigiphos)Ni(NCCHs)](ClOr)z (8). A solution of Ni(C104)26H20 (30 mg, 0.08 mmol) in n-BuOH (20 mL) was gently concentrated by heating a t boiling temperature to ca. 1 mL. The procedure was repeated twice; then 1 mL of CH3CN was added. The resulting pale blue-green solution turned brown-red by treatment with Pigiphos (74 mg, 0.08 mmol) in CHzCl2 (10 mL). Addition of diethyl ether (10 mL) followed by the slow flow of ether vapors through the solution under a stream of nitrogen gave deep purple crystals in a 70% yield. c,E55.71; Anal. Calcd for C ~ & & , ~ N C ~ ~ F ~ Z O P ~ N ~H,: 4.84; N, 1.16. Found: C, 55.34; H, 4.92; N, 1.03. 31PNMR (CDzClz): (ABX spin system) S(CyP) 83.42 (Jpp = 24.8, J p y = 10.31, 6(P”)9.23 ( J p = 235.9), 6(P”) 18.10. ’H NMR (CDzClz): 6 0.50 (bm, 2H), 0.97 (q, lH), 1.06 (bm, lH), 1.4-1.6 (m, 3H), 1.51 (s, 3H), 1.7-1.8 (m, 2H), 1.91 (dd, 3H, J = 6.9, J = 16.71, 2.05 (dd, 3H, J = 14.2),2.38 (bd, lH), 2.77 (bq, lH), 3.31 (quint, lH,J=9.6,J=7.6),3.71(bm,lH,J=9.5),3.88(~,5H),4.17 (s, 5H), 4.21 (bs, lH), 4.64 (bs, lH), 4.75 (bs, lH), 4.79 (bm, lH), 4.96 (bs, lH), 5.05 (bs, lH), 6.88 (m, 2H), 7.40-8.10 (m, 18H). IR: 2319, 2285 cm-’ (v(C=N)). X-ray Crystallographic Study of 6. Red crystals of 6 suitable for X-ray crystallography were obtained by recrystallization from CH2Cl&tOH. Crystal data for C54H55ClFsP4Pd cfw 1195.4): orthorhombic, space group P212121, with cell dimensions a t 293 K of a = 13.344(7) A, b 19.516(10) A, c = 19.717(11)A, and V = 513545) A3 with 2 = 4 and Dcaled = 1.546 Mg/m3, p(Mo Ka)= 11.39 cm-’ (graphite monochromated), A
Organometallics, Vol. 14, No. 7, 1995 3573
Notes = 0.710 73 A, F(OO0) = 2432. The data were collected on a Syntex P21 diffractometer using the w scan mode (28 range = 3.0-40.0”). Data were measured with variable scan speeds (l.O-4.Oo/min in w ) to ensure constant statistical precision on the collected intensities. No absorption correction was applied. The structure was solved using direct methods. Of the 2718 independent reflections, 2117 with F =- 3.OdF) were used in the refinement. A total of 343 parameters were refined by fullmatrix least squares using SHELXTL PLUS (data-to-parameter ratio 6.2:1, quantity minimized Zw(F, - FA2). Due to the relatively low number of reflections, all carbon atoms were refined using isotropicdisplacement parameters. The contribution of the hydrogen atoms in their idealized positions (riding model with fured isotropic U = 0.080 Az)was taken into account but not refined. The final residuals (on all data) were R = 0.059 and R, = 0.046 (weighting scheme w - l = u2(F) 0.0005F; GOF = 1.01).
+
Acknowledgment. This research was supported by the Swiss National Science Foundation (Special Program “CHiral2”,Grant No. 21-36700.92). We are grateful to U. Burckhardt and V. Gramlich for the X-ray structural analysis of compound 6. Supporting Information Available: Tables of crystal data and refinement details, atomic coordinates, bond distances and angles, anisotropic displacement coefficients for non-carbon atoms, and coordinates of hydrogen atoms for compound 6 (11pages). Ordering information is given on any current masthead page. A table of calculated and observed structure factors (9 pages) may be obtained from the authors upon request. OM950210Y
