Synthesis, platinum(II) complexes and structural aspects of the new tetradentate phosphine cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane

www.elsevier.nl/locate/ica Inorganica Chimica Acta 290 (1999) 167 – 179

Synthesis, platinum(II) complexes and structural aspects of the new tetradentate phosphine cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane Werner Oberhauser a, Christian Bachmann a, Thomas Stampfl a, Rainer Haid a, Christoph Langes a, Holger Kopacka a, Alexander Rieder b, Peter Bru¨ggeller a,* a

Institut fu¨r Allgemeine, Anorganische und Theoretische Chemie, Uni6ersita¨t Innsbruck, Innrain 52a, 6020 Innsbruck, Austria b Institut fu¨r Organische Chemie, Uni6ersita¨t Innsbruck, Innrain 52a, 6020 Innsbruck, Austria Received 4 November 1998; accepted 2 March 1999

Abstract Several novel dimers of the composition [M2Cl4(trans-dppen)2] (M=Ni (1), Pd (2), Pt (3)) containing trans-1,2-bis(diphenylphosphino)ethene (trans-dppen) have been prepared and characterized by X-ray diffraction methods, NMR spectroscopy (195Pt{1H}, 31P{1H}), elemental analyses, and melting points. The intramolecular [2 + 2] photocycloaddition of the two diphosphine-bridges in 3 produces [Pt2Cl4(dppcb)] (4), where dppcb is the new tetradentate phosphine cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane. Neither 1 nor the free diphosphine trans-dppen shows this reaction. In the case of 2 the photocycloaddition is slower than in 3. This difference can be explained by the shorter distance between the two aliphatic double bonds in 3 than in 2, but also different transition probabilities within ground and excited states of the used metals could be involved. Furthermore, variable-temperature 31P{1H} NMR spectroscopy of 2 or 3 reveals a negative activation entropy of 2 for the [2 + 2] photocycloaddition, but a positive of 3. The removal of chloride from 4 by precipitating AgCl with AgBF4, and subsequent treatment with 2,2%-bipyridine (bipy) or 1,10-phenanthroline (phen) leads to [Pt2(dppcb)(bipy)2](BF4)4 (5) and [Pt2(dppcb)(phen)2](BF4)4 (6), respectively. In an analogous reaction of 4 with PMe2Ph or PMePh2, [Pt2(dppcb)(PMe2Ph)4](BF4)4 (7) and [Pt2(dppcb)(PMePh2)4](BF4)4 (8) are formed. Complexes 1–8 show square – planar coordinations, where the compounds 4–8 have also been characterized by the above mentioned methods together with fast atom bombardment mass spectrometry (7, 8). The crystal structure of 4 reveals two conformations, which arise from an energetic competition between the sterical demands of dppcb and an ideal square–planar environment of Pt(II). The free tetraphosphine dppcb can be obtained easily from 4 by treatment with NaCN. It has been characterized fully by the above methods including 13C{1H} and 1H NMR spectroscopy. The X-ray structure analysis shows the pure MMMP-enantiomer in the solid crystal, which is therefore optically active. This chirality is induced by a conformation of dppcb, where all four PPh2 groups are non-equivalent. Variable-temperature 31P{1H} NMR spectroscopy of dppcb confirms this explanation, since the single signal at room temperature is split into two doublets at 183 K. The goal of this article is to demonstrate the facile production of a new tetradentate phosphine from a diphosphine precursor via Pt(II) used as a template. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Crystal structures; Palladium complexes; Platinum complexes; Diphosphine complexes; Tetraphosphine complexes

1. Introduction Though tetradentate phosphines are versatile ligands with interesting structural and catalytic properties, to

* Corresponding author. Tel.: + 43-512-507 5115; fax: + 43-512507 2934. E-mail address: [email protected] (P. Bru¨ggeller)

our knowledge, only [Ph2PCH2CH2P(Ph)CH2]2 (tetraphos-1) and P(CH2CH2PPh2)3 (tetraphos-2) are available commercially, since convenient synthetic methods are involved also for an industrial production [1]. Therefore, as a consequence of the restricted industrial product spectrum, the easy synthesis of a new perphenylated tetraphosphine, which is relatively airstable, is a desirable challenge. Here it is shown, that Pd(II) and Pt(II) complexes of the series [M2Cl4(trans-

0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 9 9 ) 0 0 1 3 7 - 1

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W. Oberhauser et al. / Inorganica Chimica Acta 290 (1999) 167–179

Scheme 1. Structure types observed in the compounds 1– 8. Structure (a) occurs in [M2Cl4(trans-dppen)2] (M=Ni (1), Pd (2), Pt (3)); structure (b) in [Pt2Cl4(dppcb)] (4); structure (c) in [Pt2(dppcb)(bipy)2](BF4)4 (5); structure (d) in [Pt2(dppcb)(phen)2](BF4)4 (6); structure (e) in [Pt2(dppcb)(PMe2Ph)4](BF4)4 (7); structure (f) in [Pt2(dppcb)(PMePh2)4](BF4)4 (8).

dppen)2] (M= Ni (1), Pd (2), Pt (3); structure (a) in Scheme 1), where trans-dppen is trans-1,2-bis (diphenylphosphino)ethene and which have been prepared and characterized fully for the first time, undergo an intramolecular [2+ 2] photocycloaddition and in the case of Pt(II), [Pt2Cl4(dppcb)] (4; structure (b) in Scheme 1) is formed containing the new tetraphosphine cis,trans,cis - 1,2,3,4 - tetrakis(diphenylphosphino)cyclobutane (dppcb). Only recently a comparable formation of carbon–carbon bonds mediated by transition metal complexes has been reported [2], where also the proximity of two unsaturated ligands due to cis coordination is important. The same is true for a Pd(II) complex, which has been used successfully as the chiral template to promote an asymmetric [4+2] Diels – Alder reaction [3]. As in the case of the free diphosphine trans-dppen, in the absence of the reaction promoter, no cycloaddition reaction was observed. Similar to the production of the cis,trans,cis configuration of dppcb, the template directed synthesis has proceeded as intended with the controlled creation of stereocenters. The use of templates, producing the directed coupling of two unsaturated groups held proximate outside the coordination sphere of a transition metal, has now

become a well-established method for the synthesis of new ligands [4]. Comparable to the (triphos)Rh system, where triphos is MeC(CH2PPh2)3 and which can enter readily into catalytic cycles without dissociation or apparent destabilization of the resulting complexes retaining their overall structures [5], in 4 there is considerable steric blocking on one side of each coordination unit by the cyclobutane ring and the phenyl groups and the compounds [Pt2(dppcb)(bipy)2](BF4)4 (5), [Pt2(dppcb)(phen)2](BF4)4 (6), [Pt2(dppcb)(PMe2Ph)4](BF4)4 (7), and [Pt2(dppcb)(PMePh2)4](BF4)4 (8) corresponding to structures (c)–(f) in Scheme 1 are produced with retention of the bimetallic complexes. Similar to the use of tetradentate phosphines in [Mo2X4(tetraphos-1)] (X= Cl, Br) or rac-[Mo-4MoCl4(PEt3)(h3-tetraphos-2)] [6], dppcb especially favors (or even requires) certain geometric arrangements that can never be achieved using mono- or bidentate phosphine ligands. Furthermore, metals with more rigid stereochemical demands will have greater mechanical coupling for dppcb, where mechanical coupling refers to the energy terms associated with bond stretches, bond angle bends, torsional deformations, van der Waals, and electrostatic interac-

W. Oberhauser et al. / Inorganica Chimica Acta 290 (1999) 167–179

tions [7]. Preliminary X-ray results of Rh(I) complexes with dppcb reveal, that the effect of mechanical coupling can be present in dppcb dimers. Therefore, it is conceivable that with dppcb the oxidation of one metal may induce conformational changes which are conducive to the oxidation of the second metal. Such cooperative mechanical coupling is believed to operate in the oxygenation of hemoglobin [8].

2. Experimental

2.1. Reagents and chemicals Reagent grade chemicals were used as received unless stated otherwise. trans-1,2-Bis(diphenylphosphino)ethene (trans-dppen) was purchased from Aldrich, PMe2Ph and PMePh2 from Strem. All other chemicals and solvents were obtained from Fluka. Solvents used for NMR measurements and crystallization purposes were of purissimum grade quality. NiCl2 · 6H2O, PdCl2, and Na2PtCl4 · 4H2O were also received from Fluka.

2.2. Instrumentation Fourier-mode 195Pt{1H}, 31P{1H}, 13C{1H}, and 1H NMR spectra were obtained by use of a Bruker AC-200 spectrometer (internal deuterium lock) and were recorded at 43.02, 80.96, 50.29 and 200 MHz, respectively. Positive chemical shifts are downfield from the standards; 1.0 M Na2PtCl6 for the 195Pt{1H} resonances, 85% H3PO4 for the 31P{1H} resonances and TMS for the 13C{1H} and 1H resonances.

2.3. X-ray data collection The X-ray data collections were performed on a Siemens P4 diffractometer. A yellow crystal of [Pd2Cl4(trans-dppen)2] (2) and colorless crystals of [Pt2Cl4(trans-dppen)2] (3), [Pt2Cl4(dppcb)] (4), and dppcb were fixed on quartz pins. The lattices were found to be triclinic (2, 3), monoclinic (4), and orthorhombic (dppcb) by standard procedures using the software of the Siemens P4 diffractometer. No decay in the intensities of three standard reflections was observed during the course of data collection in each case. The data were corrected for Lorentz and polarization effects. The empirical absorption corrections were based on c-scans of nine reflections, respectively (x =78 – 102°, 360° scans in 10° steps in c) [9].

2.4. Structure solution and refinement All structure determination calculations were carried out on Pentium PCs using the PC version of SHELXTL PLUS and SHELXL93 [10]. The positions of the palla-

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dium and platinum atoms were found by the Patterson method. The positions of the phosphorus atoms in dppcb were found by direct methods. Other atom positions were located from successive difference Fourier maps. The crystal structures of 2 and 3 are isomorphous. In both cases final refinement was carried out with anisotropic thermal parameters for all non-hydrogen atoms including one molecule of DMF per asymmetric unit. Hydrogen atoms were included using a riding model with isotropic U values depending on Ueq of the adjacent carbon atoms. The final R values of 0.037 (2) and 0.051 (3) were computed for 606 parameters (2, 3) and 5696 (2) and 4039 (3) reflections. In the case of 4 all non-hydrogen atoms including 3.55 molecules of DMF per asymmetric unit were refined anisotropically. The occupancy factor of one DMF molecule has been refined. The hydrogen atoms were treated as above. The final R value of 0.047 was computed for 745 parameters and 5505 reflections. In the case of dppcb final refinement was carried out with anisotropic thermal parameters for all non-hydrogen atoms. The hydrogen atoms were refined isotropically with fixed isotropic U. The final R value of 0.039 was computed for 637 parameters and 4364 reflections. The absolute stereochemistry was determined unambiguously by both R-factor tests and by use of the Flack parameter [for MMMP-dppcb: R + =0.039, R − = 0.040, x + = 0.01(9), x − = 1.01(9)] [11]. In all cases upon convergence the last difference Fourier maps showed no significant features. The structure determinations are summarized in Table 1.

2.5. Syntheses of Ni(II), Pd(II) and Pt(II) complexes and dppcb A Schlenk apparatus and oxygen-free, dry Ar were used in the syntheses of all complexes and dppcb. Solvents were degassed by several freeze–pump–thaw cycles prior to use. All reactions were carried out at room temperature unless stated otherwise.

2.5.1. [Ni2Cl4 (trans-dppen)2] (1) To a solution of NiCl2 · 6H2O (0.2 mmol, 0.048 g) in 4 ml H2O was added trans-dppen (0.2 mmol, 0.079 g) dissolved in 4 ml CH2Cl2. Then EtOH was added with stirring and a brownish precipitate formed. The reaction mixture was stirred for 10 min and the volume of the solvent was reduced. The brownish residue was filtered off, washed with H2O, and dried in vacuo. A brownish powder was recrystallized from CH2Cl2: yield 0.071 g (67%); m.p. 190°C. Anal. Calc. for C52H44Cl4P4Ni2: C, 59.4; H, 4.2. Found: C, 59.2; H, 4.3%.

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Table 1 Structure determination data for [M2Cl4(trans-dppen)2] (M= Pd (2), Pt (3)); [Pt2Cl4(dppcb)] (4), and dppcb 4

dppcb

C52H44Cl4P4Pd2·DMF C52H44Cl4P4Pt2·DMF 1220.52 1397.90 yellow colorless triclinic triclinic P1 P1 10.839(2) 11.100(2) 13.954(3) 14.103(3) 18.773(4) 18.736(4) 76.73(3) 76.64(3) 74.43(3) 74.08(3) 71.45(3) 70.86(3) 2560.6 2632.2 213 293 2 2 0.3×0.2×0.1 0.4×0.3×0.2 1.583 1.764 1.074 5.674 Siemens P4 (Mo Ka radiation) 4.1–53.0 4.0–60.0 −15h513 −15h515 −165k517 −135k513 −235l523 −185l518

C52H44Cl4P4Pt2·3.55DMF 1584.29 colorless monoclinic P21/n 14.507(3) 19.949(4) 23.485(5)

C52H44P4 792.80 colorless orthorhombic P212121 12.744(2) 15.268(2) 21.872(2)

12139 10420 5696 606 0.037 0.037 w−1 = s 2(F 2)+(0.030P)2

2 Empirical formula Formula weight Color Crystal system Space group a (A, ) b (A, ) c (A, ) a (°) b (°) g (°) V (A, 3) T (K) Z Crystal dimensions (mm) Dcalc (Mg m−3) m (mm−1) Diffractometer 2u Range (°) Index ranges

No. reflections: Collected Independent Observed (I\3s(I)) No. parameters Final R indices (observed data) R Rw

3

6551.9 293 4 0.2×0.2×0.1 1.606 8.022

4255.8 213 4 1.2×0.4×0.4 1.237 0.213

3.6–60.0 −15h520 −15k528 −335l532

4.2–55.0 −15h514 −15k519 −15l528

12945 11468 4039 606

22081 18935 5505 745

6424 6182 4364 637

0.051 0.051 w−1 =s 2(F 2)

0.047 0.047 w−1 =s 2(F 2)

0.039 0.039 w−1 =s 2(F 2)+(0.056P)2

+ (0.043P)2

+0.66P GOF Largest difference peak (e A, −3) Rint Transmission

1.02 0.58 0.023 0.819–0.899

105.42(3)

0.97 1.33 0.028 0.424–1.000

2.5.2. [Pd2 Cl4 (trans-dppen)2] (2) To a solution of trans-dppen (0.2 mmol, 0.079 g) in 8 ml CH2Cl2 was added PdCl2 (0.2 mmol, 0.036 g) with vigorous stirring and prevention of light. The slurry was stirred for 8 h, the volume of the solvent was reduced, and n-hexane was added. The yellow precipitate was filtered off, washed with n-hexane, and dried in vacuo. A yellow powder was recrystallized from CH3NO2/DMF (vol./vol. = 1:1): yield 0.096 g (79%); m.p. 286°C. Anal. Calc. for C52H44Cl4P4Pd2·DMF: C, 54.1; H, 4.2. Found: C, 54.0; H, 4.4%. Single crystals suitable for an X-ray structure analysis with the composition [Pd2Cl4(trans-dppen)2]·DMF were obtained from a CH3NO2/DMF (vol./vol. =1:1) solution of 2. 2.5.3. [Pt2 Cl4 (trans-dppen)2] (3) Compound 3 was prepared in an analogous manner to 1, where Na2PtCl4 · 4H2O (0.2 mmol, 0.091 g) was

+(0.037P)2 0.97 1.15 0.027 0.695–0.979

+0.10P 1.03 0.27 0.019 0.950–1.000

used. A white powder was recrystallized from CH3NO2/ DMF (vol./vol. = 1:1): yield 0.112 g (80%); m.p. 250°C dec. Anal. Calc. for C52H44Cl4P4Pt2·DMF: C, 47.3; H, 3.7. Found: C, 47.1; H, 3.9%. Single crystals suitable for an X-ray structure analysis with the composition [Pt2Cl4(trans-dppen)2]·DMF were obtained from a CH3NO2/DMF (vol./vol. = 1:1) solution of 3.

2.5.4. [Pt2 Cl4 (dppcb)] (4) Compound 3 (0.7 mmol, 0.979 g) was suspended in 50 ml DMF and irradiated with a high-pressure mercury-lamp with vigorous stirring for 26 h. Then the solvent was evaporated completely and CH2Cl2 was added. The white residue was filtered off, washed with CH2Cl2, and dried in vacuo. A white powder was recrystallized from DMF: yield 0.887 g (80%); m.p.\ 310°C. Anal. Calc. for C52H44Cl4P4Pt2·3.55DMF: C, 47.5; H, 4.4. Found: C, 47.5; H, 4.6%. Single crystals

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suitable for an X-ray structure analysis with the composition [Pt2Cl4(dppcb)]·3.55DMF were obtained from a DMF solution of 4.

2.5.5. [Pt2 (dppcb)(bipy)2](BF4)4 (5) To a suspension of 4 (0.05 mmol, 0.079 g) in 6 ml DMF was added AgBF4 (0.2 mmol, 0.039 g). The reaction mixture was stirred for 6 h and the formed AgCl was filtered off. Then the solvent was evaporated completely and CH2Cl2 was added. To this solution, 2,2%-bipyridine (0.1 mmol, 0.016 g) was added. The slurry was stirred for 8 h and Et2O was added. The white precipitate was filtered off and dried in vacuo. A white powder was recrystallized from DMF: yield 0.065 g (71%); m.p.\310°C. Anal. Calc. for C72H60B4F16N4P4Pt2: C, 46.9; H, 3.3; N, 3.0. Found: C, 46.7; H, 3.5; N, 3.1%.

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2.5.9. dppcb To a suspension of 4 (0.5 mmol, 0.792 g) in 20 ml DMF was added NaCN (4 mmol, 0.196 g) with vigorous stirring. The slurry was stirred for 5 h. Then the solvent was evaporated completely, CH2Cl2 was added, and the colorless solution was filtered1. The volume of the solvent was reduced and EtOH/Et2O (vol./vol. = 1:1) was added. A white precipitate formed immediately. It was filtered off, washed with Et2O, and dried in vacuo. A white powder was recrystallized from DMF: yield 0.357 g (90%); m.p. 170–171°C; FAB mass spectrum: m/z 792.8 dppcb + . Anal. Calc. for C52H44P4: C, 78.8; H, 5.6. Found: C, 78.6; H, 5.7%. Single crystals suitable for an X-ray structure analysis consisting only of dppcb molecules were obtained from a DMF solution of dppcb.

3. Results

2.5.6. [Pt2 (dppcb)(phen)2](BF4)4 (6) Compound 6 was prepared in an analogous manner to 5, where 1,10-phenanthroline·H2O (0.1 mmol, 0.020 g) was used. A white powder was recrystallized from DMF: yield 0.066 g (70%); m.p.\ 310°C dec. Anal. Calc. for C76H60B4F16N4P4Pt2: C, 48.3; H, 3.2; N, 3.0. Found: C, 48.1; H, 3.4; N, 3.1%.

2.5.7. [Pt2 (dppcb)(PMe2 Ph)4](BF4)4 (7) To a suspension of 4 (0.05 mmol, 0.079 g) in 6 ml DMF was added AgBF4 (0.2 mmol, 0.039 g). The reaction mixture was stirred for 6 h and the formed AgCl was filtered off. The solvent was evaporated completely and CH2Cl2 was added. Then PMe2Ph (0.2 mmol, 0.028 g) was added via a syringe with vigorous stirring. The solution immediately turned yellowish and was stirred for 4 h. The solvent was evaporated completely and EtOH/Et2O (vol./vol. = 1:1) was added. A pink precipitate was filtered off, washed with EtOH, and dried in vacuo. A pink powder was recrystallized from CH2Cl2: yield 0.079 g (76%); m.p. 175°C; FAB mass spectrum: m/z 1996.0 [Pt2(dppcb)(PMe2Ph)4](BF4)3+ , 1771.1 [Pt2(dppcb)(PMe2Ph)3](BF4)2+ . Anal. Calc. for C84H88B4F16P8Pt2: C, 48.4; H, 4.3. Found: C, 48.3; H, 4.5%.

2.5.8. [Pt2 (dppcb)(PMePh2)4](BF4)4 (8) Compound 8 was prepared in an analogous manner to 7, where PMePh2 (0.2 mmol, 0.040 g) was used. A white powder was recrystallized from CH2Cl2: yield 0.070 g (60%); m.p. 198°C; FAB mass spectrum: m/z 2244.3 [Pt2(dppcb)(PMePh2)4](BF4)3+ , 1957.3 [Pt2(dppcb)(PMePh2)3](BF4)2+ . Anal. Calc. for C104H96B4F16P8Pt2: C, 53.6; H, 4.2. Found: C, 53.4; H, 4.3%.

3.1. Crystal structures of [M2Cl4(trans-dppen)2] (M= Pd (2), Pt (3)) In order to characterize complexes definitely of the type [M2Cl4(trans-dppen)2] (M =Ni (1), Pd (2), Pt (3)) corresponding to structure (a) in Scheme 1, the solid state structures of 2 and 3 were determined by X-ray crystallography. This is necessary, since it has been claimed that square–planar Pt(II) dimers containing four bridging trans-dppen ligands are possible [12], where also species with only one bridging transdppen molecule could occur [13]. The X-ray structures of 2 and 3 show isomorphous crystal lattices allowing comparison of the radii of Pd and Pt atoms [14]. Both structures contain one discrete [M2Cl4(trans-dppen)2] molecule (M=Pd (2), Pt (3)) and one DMF molecule per asymmetric unit. Views of 2 are given in Figs. 1 and 2; Table 2 contains selected bond distances and bond angles of 2 and 3. The square–planar coordinations in 2 and 3 are only distorted slightly, where the largest deviations from least-squares planes through the coordination planes are 0.085 A, for Pd(1) in 2 and 0.059 A, for Pt(2) in 3. The mean Pd–P bond length of 2.273(6) A, in 2 is longer than the mean Pt–P bond length of 2.248(3) A, in 3 confirming the trend for the covalent radii of square–planar d8 Pd and Pt due to the ‘relativistic or Lanthanide contraction’ [14]. Nevertheless,

1 Metallic platinum can be recovered by reduction of the formed Na2Pt(CN)4 with NaBH4.

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Table 2 Selected bond distances (A, ) and bond angles (°) for [M2Cl4(trans-dppen)2] (M= Pd (2), Pt (3)) Bond lengths (A, )

Bond angles (°)

Pd(1)–P(1) Pd(1)–P(2) Pd(1)–Cl(1) Pd(1)–Cl(2) Pd(2)–P(3) Pd(2)–P(4) Pd(2)–Cl(3) Pd(2)–Cl(4) P(1)–C(1) P(2)–C(3) P(3)–C(2) P(4)–C(4) C(1)–C(2) C(3)–C(4) Pt(1)–P(1) Pt(1)–P(2) Pt(1)–Cl(1) Pt(1)–Cl(2) Pt(2)–P(3) Pt(2)–P(4) Pt(2)–Cl(3) Pt(2)–Cl(4) P(1)–C(1) P(2)–C(3) P(3)–C(2) P(4)–C(4) C(1)–C(2) C(3)–C(4)

P(1)–Pd(1)–P(2) P(1)–Pd(1)–Cl(2) P(2)–Pd(1)–Cl(2) P(1)–Pd(1)–Cl(1) P(2)–Pd(1)–Cl(1) Cl(1)–Pd(1)–Cl(2) P(3)–Pd(2)–P(4) P(4)–Pd(2)–Cl(4) P(3)–Pd(2)–Cl(4) P(4)–Pd(2)–Cl(3) P(3)–Pd(2)–Cl(3) Cl(3)–Pd(2)–Cl(4) C(1)–P(1)–Pd(1) C(3)–P(2)–Pd(1) C(2)–P(3)–Pd(2) C(4)–P(4)–Pd(2) C(2)–C(1)–P(1) C(1)–C(2)–P(3) C(4)–C(3)–P(2) C(3)–C(4)–P(4) P(1)–Pt(1)–P(2) P(1)–Pt(1)–Cl(1) P(2)–Pt(1)–Cl(1) P(1)–Pt(1)–Cl(2) P(2)–Pt(1)–Cl(2) Cl(1)–Pt(1)–Cl(2) P(3)–Pt(2)–P(4) P(4)–Pt(2)–Cl(4) P(3)–Pt(2)–Cl(4) P(4)–Pt(2)–Cl(3) P(3)–Pt(2)–Cl(3) Cl(3)–Pt(2)–Cl(4) C(1)–P(1)–Pt(1) C(3)–P(2)–Pt(1) C(2)–P(3)–Pt(2) C(4)–P(4)–Pt(2) C(2)–C(1)–P(1) C(1)–C(2)–P(3) C(4)–C(3)–P(2) C(3)–C(4)–P(4)

2.271(2) 2.286(2) 2.340(2) 2.322(2) 2.278(2) 2.256(2) 2.356(2) 2.338(2) 1.805(5) 1.815(5) 1.818(5) 1.814(5) 1.326(6) 1.306(7) 2.251(4) 2.253(4) 2.331(4) 2.343(4) 2.251(4) 2.239(4) 2.356(4) 2.347(4) 1.817(14) 1.83(2) 1.846(13) 1.77(2) 1.34(2) 1.33(2)

97.34(6) 172.19(5) 90.47(6) 83.24(6) 175.69(5) 88.96(6) 96.79(6) 88.18(6) 172.43(5) 174.94(5) 82.80(6) 91.74(6) 116.5(2) 115.3(2) 120.1(2) 116.0(2) 127.5(4) 122.6(4) 126.7(4) 121.1(4) 94.77(14) 173.3(2) 91.7(2) 86.63(14) 177.25(14) 86.8(2) 95.08(14) 89.7(2) 173.50(13) 176.95(13) 85.23(14) 89.8(2) 115.0(5) 114.5(6) 117.3(5) 114.8(6) 124.8(11) 120.8(11) 127.6(14) 124.2(13)

the P–Pd–P angles of 97.34(6) and 96.79(6)° in 2 are significantly larger than the P – Pt – P angles of 94.77(14) and 95.08(14)° in 3. This is certainly a consequence of the larger stabilization energy for an ideal square–planar environment in the case of Pt [15]. This opening of the P–Pd–P angles leads to longer C(1)···C(3) and C(2)···C(4) distances in 2 of 3.115 and 3.086 A, , respectively, compared with the corresponding values of 2.837 and 2.858 A, in 3. The C(1)···C(3) projection angles between the aliphatic double bonds (Fig. 2) are 37.6° (2) and 34.8° (3), indicating slightly different orientations of the trans-dppen bridges in 2 and 3. Comparable differences in the orientations of diphosphine-

bridges have been observed in the X-ray structures of a series of dimers containing 1,2-bis(diphenylphosphino)acetylene (dppa) [16]. The angles between the coordination planes of 2 and 3 are 37.0 and 39.7°, respectively, producing a significantly smaller metal– metal separation of 6.779(1) A, in 2 than of 6.800(1) A, in 3. Both 2 and 3 undergo an intramolecular [2+2] photocycloaddition already with sunlight. This reaction is faster in the case of 3 in agreement with its closer aliphatic double bonds leading to a larger entropic aid. However, the presence of different transition probabilities within ground and excited states of the used metals cannot be ruled out. A comparable sunlight initiated intramolecular [2+ 2] photodimerization of phospholes occurs within the coordination spheres of Ru(II), Cr(0), Mo(0), and W(0) [4a,17]. All these [2+ 2] cyclodimerizations including 2 and 3 do not proceed in the absence of a suitable transition metal, since the coordination of two ligands to a transition metal provides molecular direction and electronic activation in a highly organized transition state. The exciting photons are assumed to activate the p systems of the coordinated ligands, where the conventional wisdom concerning [2 +2] photocycloadditions is that they are most productive from triplet states that are biradical in nature [18]. However, since 2 and 3 show a twisted arrangement between the reacting p bonds (Fig. 2), the mechanism of the dimerization is at least topologically in agreement with the [p2s+ p2a] pathway proposed by Woodward and Hoffmann [19]. This confirms that reaction mechanisms involving phosphaalkenes are remarkably similar to those involving alkenes. Also the mechanism of the phosphaketene dimerization corresponds to the [p2s+ p2a] pathway [20].

Fig. 1. View of [Pd2Cl4(trans-dppen)2] (2), showing the atom labelling scheme.

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Fig. 2. C(1)···C(3) projection of [Pd2Cl4(trans-dppen)2] (2). Phenyl rings omitted for clarity.

3.2. Crystal structure of dppcb In order to determine the solid state structure of the free tetraphosphine dppcb, an X-ray structure analysis has been carried out. The crystal structure of dppcb consists of four discrete dppcb molecules of the pure MMMP-enantiomer [21] per unit cell. Views of dppcb are given in Fig. 3. Table 3 contains selected bond distances and bond angles. The origin of chirality in dppcb produced by the axes of the cyclobutane ring has been described by Prelog and Helmchen [21]. Fig. 3(b) shows how the first descriptor along the C(1) – C(2) axis is obtained. The P(1) –C(1)–C(2)–P(2) torsion angle is −39.4(3)°, thus minus (M). The corresponding P(2) – C(2) – C(3) –P(3), P(3) –C(3)–C(4)–P(4), and P(4) – C(4) – C(1) – P(1) torsion angles are − 86.0(3), −23.9(3), and 156.7(2)° defining the observed dppcb-enantiomer as MMMP. Furthermore, the large difference between the absolute values of 86.0(3) and 156.7(2)° of the torsion angles along the trans axes produces one pair of equatorial diphenylphosphino groups along C(2) – C(3) and one pair of axial diphenylphosphino groups along C(4)– C(1). This can be seen clearly in Fig. 3(a). The reason for this non-equivalence of the diphenylphosphino groups is the folding of the cyclobutane ring, where the dihedral angle between the least-squares planes through C(1), C(2), C(4) and C(2), C(3), C(4), respectively, is 145.6° (see Fig. 3(b)). This value is consistent with values found in other comparable compounds which range from 145° in cyclobutane and 149° in cis-1,3-cyclobutanedicarboxylic acid to 160° in chlorocyclobutane [22]. Interestingly, the P(1) – C(1) and P(4) – C(4) bond lengths of 1.874(3) and 1.875(3) A, belonging to the axial pair, are significantly longer than the corresponding P(2) –C(2) and P(3)– C(3) bond lengths of 1.847(3) and 1.848(3) A, of the equatorial pair. However, all P–C bonds to the cyclobutane ring in dppcb are significantly longer than the P–C bonds to the monosubstituted sites of the ethene bridges of 1.815(2) and 1.813(2) A, in cis-1,2-bis(diphenylphosphino)ethene (cis-dppen) and

Fig. 3. (a) View of dppcb, showing the atom labelling scheme; (b) view of dppcb, showing how the first descriptor along the C(1)–C(2) axis is obtained. Phenyl rings and hydrogen atoms omitted for clarity.

of 1.811(5) A, in tris(diphenylphosphino)ethene [23], where both phosphines contain rigid backbones comparable to dppcb. Within statistical significance the C–C bond lengths of the cyclobutane ring in dppcb are equal, where the mean value of 1.562(2) A, is longer than the corresponding value of 1.548(7) A, in 4, but again within the usual range of 1.545–1.607 A, [4a,24]. However, in contrast to 4, where all C–C–C angles of the cyclobutane ring are 90° within statistical significance, all corresponding C–C–C angles in dppcb are significantly smaller than 90° (mean value: 87.4(2)°). This difference is of course a result of the folding of the cyclobutane ring in dppcb and in line with previous

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Table 3 Selected bond distances (A, ) and bond angles (°) for dppcb and [Pt2Cl4(dppcb)] (4) Bond lengths (A, )

Bond angles (°)

P(1)–C(1) P(2)–C(2) P(3)–C(3) P(4)–C(4) C(1)–C(2) C(1)–C(4) C(2)–C(3) C(3)–C(4) Pt(1)–P(1) Pt(1)–P(2) Pt(1)–Cl(1) Pt(1)–Cl(2) Pt(2)–P(3) Pt(2)–P(4) Pt(2)–Cl(3) Pt(2)–Cl(4) P(1)–C(1) P(2)–C(2) P(3)–C(3) P(4)–C(4) C(1)–C(2) C(1)–C(2) a C(3)–C(4) C(3)–C(4) b

C(2)–C(1)–C(4) C(2)–C(1)–P(1) C(4)–C(1)–P(1) C(1)–C(2)–C(3) C(1)–C(2)–P(2) C(3)–C(2)–P(2) C(4)–C(3)–C(2) C(4)–C(3)–P(3) C(2)–C(3)–P(3) C(1)–C(4)–C(3) C(1)–C(4)–P(4) C(3)–C(4)–P(4) P(1)–Pt(1)–P(2) P(1)–Pt(1)–Cl(1) P(2)–Pt(1)–Cl(1) P(1)–Pt(1)–Cl(2) P(2)–Pt(1)–Cl(2) Cl(1)–Pt(1)–Cl(2) P(3)–Pt(2)–P(4) P(4)–Pt(2)–Cl(3) P(3)–Pt(2)–Cl(3) P(4)–Pt(2)–Cl(4) P(3)–Pt(2)–Cl(4) Cl(3)–Pt(2)–Cl(4)

87.3(2) 110.5(2) 111.1(2) 87.7(2) 121.7(2) 122.0(2) 87.0(2) 120.4(2) 118.7(2) 87.5(2) 120.0(2) 114.1(2) 85.19(11) 92.84(11) 176.51(12) 173.21(12) 92.15(11) 89.49(11) 89.11(10) 178.51(12) 91.29(11) 90.73(11) 178.64(11) 88.91(12)

C(2)–P(2)–Pt(1) C(3)–P(3)–Pt(2) C(4)–P(4)–Pt(2) C(2) a–C(1)–C(2) C(2) a–C(1)–P(1) C(2)–C(1)–P(1) C(1) a–C(2)–C(1) C(1) a–C(2)–P(2) C(1)–C(2)–P(2) C(4) b–C(3)–C(4) C(4) b–C(3)–P(3) C(4)–C(3)–P(3) C(3) b–C(4)–C(3) C(3) b–C(4)–P(4) C(3)–C(4)–P(4)

108.2(3) 109.4(3) 110.1(3) 89.1(7) 114.4(7) 112.9(7) 90.9(7) 115.4(7) 113.1(6) 90.6(7) 118.1(7) 115.6(6) 89.4(7) 119.7(7) 114.1(7)

a

1.874(3) 1.847(3) 1.848(3) 1.875(3) 1.556(5) 1.563(5) 1.565(4) 1.564(4) 2.221(3) 2.221(3) 2.349(3) 2.360(3) 2.227(3) 2.220(3) 2.350(3) 2.368(3) 1.843(10) 1.860(10) 1.861(9) 1.866(9) 1.558(12) 1.554(13) 1.552(13) 1.526(12)

Symmetry transformations used to generate equivalent atoms: = –x+1, −y ,−z; b = −x+2, −y+1, −z.

results [22b,24,25]. The X-ray structures of 4 and dppcb indicate that dppcb is able to adopt centrosymmetric as well as chiral conformations. This flexibility of cyclobutane derivatives has been found to be important for the crystal engineering of b-nitrostyrenes [26].

molecules of conformation A, two discrete [Pt2Cl4(dppcb)] molecules of conformation B, and 14.2 molecules of DMF per unit cell. Both conformations are located on centers of symmetry. Views of the two conformations are given in Figs. 4 and 5. Table 3 contains selected bond distances and bond angles. Due to crystallographic constraints produced by the centers of symmetry, the cyclobutane rings are completely planar in both conformations, where also the bond lengths and bond angles of these rings show no differences within statistical significance (see Table 3). The mean C–C bond length for both squares of 1.548(7) A, is within the usual range of 1.545–1.607 A, [4a,24]. However, this is the only common feature of conformations A and B. Obviously, the larger ‘envelope’-folding of 153.1° for the five-membered rings in A compared with 168.3° in B (see Fig. 5), reduces the strain within the five-membered rings in A. This larger ‘envelope’-folding in A produces a significantly smaller Pt(1)···Pt(1a) separation of 6.952(1) A, in A than Pt(2)···Pt(2a) of 7.327(1) A, in B. However, a comparison of the significantly different P–Pt–P chelate angles of 85.19(11)° in A and 89.11(10)° in B indicates a better fulfilment of an ideal square–planar environment in B. Thus it seems likely, that the occurrence of two conformations in the solid state structure of 4 is the result of a subtle energetic balance between the preference of dppcb to release the strain of the five-membered rings by a larger ‘envelope’-folding in A and the preference of Pt(II) for an ideal square–planar environment in B. Like in the case of other polyphosphines [27], dppcb produces a ‘steric pressure’, which is opposed by the electronic demands of Pt(II). A comparable structural flexibility occurs in [Ni2Cl2(eHTP)]2 + , where eHTP is (Et2PCH2CH2)2PCH2P(CH2CH2PEt2)2, leading to an anion dependence of the actual conformations [28]. In [Ni2Cl2(eHTP)]2 + the rotational conformation determines the Ni···Ni separation [29] similar to the variable Pt···Pt distance in the two conformations of 4. This is a common feature of bimetallic complexes of dppcb or eHTP, respectively, despite their different dentateness and steric pressures. In the case of (Et2PCH2CH2)(Ph)PCH2P(Ph)(CH2CH2PEt2), eLTTP, this flexibility leads to cooperative bimetallic behavior and with Rh(I) to a remarkably good hydroformylation catalyst [30].

3.3. Crystal structure of [Pt2Cl4(dppcb)] (4)

3.4. In6estigation of 1 – 8 and dppcb in the solution state

In order to characterize definitely the [2+ 2] photocycloaddition product of 3, [Pt2Cl4(dppcb)] (4) with structure (b) in Scheme 1, the solid state structure of 4 was determined by X-ray crystallography. The crystal structure of 4 consists of two discrete [Pt2Cl4(dppcb)]

The 195Pt{1H} and 31P{1H} NMR parameters of compounds 1–8 are summarized in Table 4. The NMR data for 1–3 are consistent with structure (a) in Scheme 1 showing four equivalent phosphorus atoms. By analogy with 2, 3, and [M2(CN)4(trans-dppen)2] (M=Pd,

W. Oberhauser et al. / Inorganica Chimica Acta 290 (1999) 167–179 Table 4 195 Pt{1H} and Compound

175

31

P{1H} NMR data for 1–8a

d(Pt)

1 2 3 4 5 6 7

−3932t −4577t −4588t −4582t −5127pq

8

−5108pq

d(P) 20.0 24.7 6.0 57.0 60.0 60.9 68.5d − 11.8d 65.7d −2.9d

1

2

3550b 3630 3399 3431 2297 2301 2327 2350

286 286 293 293

J(Pt,P)

J(P,P)

a

J values in Hz. d, doublet; t, triplet; pq, pseudo-quintet. Spectra were run at 298 K. The following solvents were used: CH2Cl2 (1–3, 7, 8), DMF (4–6). b4 J(Pt,P)=44. Fig. 4. View of conformation A of [Pt2Cl4(dppcb)] (4), showing the atom labelling scheme.

Pt) [14c], 1 is postulated as a dinuclear species. In the case of 3 the simultaneous presence of 4J(Pt,P) of 44 Hz and a value of 1J(Pt,P) of 3550 Hz typical for trans chloride clearly indicates that 3 is also binuclear in solution. However, the X-ray structures of 2 and 3 indicate two different diphosphine-bridges in both complexes (see Figs. 1 and 2). Therefore, a 31P{1H} NMR investigation of their dynamic solution behavior has

Fig. 5. Views of the two conformations of [Pt2Cl4(dppcb)] (4) with the cyclobutane planes perpendicular to the projection planes. (a) Conformation A; (b) conformation B. Phenyl rings omitted for clarity.

been carried out at low temperatures. The corresponding thermodynamic parameters have been derived from computer line-shape analysis using an adaptation of the DNMR3 program [31]. In the 31P{1H} NMR spectra the non-equivalence of the diphosphine-bridges is resolved at 198 K in 2 and at 223 K in 3. The corresponding DG ‡ values for an equalization of the diphosphinebridges of 41.2 and 39.6 kJmol − 1 in 2 and 3, respectively, are of comparable magnitude. The values of the entropy of activation, by contrast, are − 52.2 and 104.0 JK − 1 mol − 1 in 2 and 3, respectively. The negative DS ‡ value for 2 suggests a transition state with two identical diphosphine-bridges producing a center of symmetry analogous to structure (a) in Scheme 1. However, the large positive DS ‡ value for 3 indicates that no symmetrical transition state is involved in the equalization of the diphosphine-bridges. An alternative route is via several conformations with differently twisted aliphatic double bonds in agreement with a large positive DS ‡ value. Comparable to this, the DNMR3 program has been used successfully to distinguish between associative and dissociative reaction pathways [32]. During the dynamic processes in 2 and 3, only in 3 the arrangement of the aliphatic double bonds remains twisted, which is another reason for the faster [2+2] photocycloaddition in 3 than in 2 [20]. In the syntheses of the complexes 5–8 the removal of chloride from [Pt2Cl4(dppcb)] (4) by precipitating AgCl with AgBF4 was necessary for the completion of the reactions. The 31P{1H} NMR parameters of a DMF solution of the resulting cationic intermediate (d(P) 40.5, 1J(Pt,P)= 3996 Hz) are consistent with [Pt2(dppcb)(DMF)4](BF4)4 containing oxygen-bonded DMF ligands [33]. In contrast to 1–3, the d(P) values of chelating dppcb in 4–8 show the typical downfield five-ring contribution (see Table 4 and Scheme 1) [34]. This effect is accompanied by a highfield shift of the

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d(Pt) values of 4–8 compared with 3, which has been observed previously [16a,33a,35]. Also the larger d(Pt) highfield shift in 7 and 8 containing Pt(II) centers coordinated by four phosphorus ligands instead of only two in 4–6 is in line with former results [33a]. All NMR data for 5 –8 are consistent with structures (c) –(f) in Scheme 1. The 1J(Pt,P) parameters for 5 and 6 are in the typical range for Pt(II) compounds with a phosphine coordinated trans to nitrogen heterocycles [33a,36]. The 1J(Pt,P) values for 7 and 8 are characteristic for trans positions of two phosphines. The same is true for the 2J(P,P) parameters [14c,33a,36d]. The facile production of the compounds 5 – 8 shows that dppcb, like eLTTP, provides an open environment about the metal centers for reactions to occur in bimetallic species [37]. Simultaneously, the steric constraint of dppcb is comparable to cis,cis-1,3,5-tris(diphenylphosphino)cyclohexane [27] and does not allow the dissociation of one phosphine arm while the other remains bonded to the metal. This might be important in developing catalytic systems. The 31P{1H}, 13C{1H}, and 1H NMR parameters of dppcb are summarised in Table 5. For purposes of comparison the corresponding data for the precursor phosphine trans-dppen are also given. The d(P) values of both phosphines indicate a stronger shielding of the phosphorus atoms by cyclobutane than by ethene. However, the d(P) parameters of − 25.7 and − 19.6 of the diphenylphosphino groups belonging to the monosubstituted sites of the ethene or ethane bridges in tris(diphenylphosphino)ethene and 1,1,2-tris(diphenylphosphino)ethane, respectively, are comparable to d(P) of −19.0 in dppcb [23b,38]. The d(C) and d(H) values of the cyclobutane ring in dppcb are in the same range as in trans-1,2-bis(diphenylphosphino)cyclobutane [39]. In Fig. 6 the variable-temperature 31P{1H} NMR spectra of a dppcb solution in CH2Cl2 are shown. At 193 K the signals corresponding to the axial and equaTable 5 31 P{1H},

13

C{1H}, and 1H NMR data for dppcb and trans-dppena

Compound

d(P)

d(C)

d(H)

Assignment

dppcb

−19.0

3.5 6.8 7.2 7.3

cyclobutane phenyl groups

trans-dppen

−8.0

40.4 128.4 129.4 132.8tb 134.9tb 142.4 128.6 133.2

6.6tc 7.2

ethene phenyl groups

a

t, triplet. Spectra were run at 298 K. The following solvents were used: 31P{1H} NMR spectra in CH2Cl2, 13C{1H} and 1H NMR spectra in CH2Cl2–d2. b2 J(C,H)=12 Hz. c2 J(P,H)+3J(P,H)=14 Hz.

Fig. 6. Variable-temperature solution in CH2Cl2.

31

P{1H} NMR spectra of a dppcb

torial pairs of diphenylphosphino groups, respectively, are separated clearly. In accordance with the X-ray structure of dppcb this non-equivalence of the phosphorus atoms stems from the folding of the cyclobutane ring. The signal at lower field is attributed to the axial pair in agreement with its significantly longer P–C bonds to the cyclobutane ring. At 183 K also the 3 J(P,P)cis coupling of 99 Hz between the axial and equatorial pairs of diphenylphosphino groups is completely resolved. The spin system is of AA%BB% type, where the axial and equatorial pairs of diphenylphosphino groups can clearly be seen in Fig. 3(b). The 3 J(P,P)cis value in dppcb is comparable to 3J(P,P)cis of 105.5 Hz in tris(diphenylphosphino)ethene [23b]. Like this phosphine, dppcb appears to adopt the same overall conformation in the solid and in solution. However, in the solid state structure of dppcb the two diphenylphosphino groups within each axial or

W. Oberhauser et al. / Inorganica Chimica Acta 290 (1999) 167–179

equatorial pair become nonequivalent, giving rise to the pure MMMP-enantiomer and to different relative orientations of the lone pairs on the phosphorus atoms especially within the axial pair (see Fig. 3(a)). This effect is not resolved in the 31P{1H} NMR spectrum of dppcb at 183 K, which is the lowest possible temperature for practical reasons. Using the variable-temperature 31P{1H} NMR parameters of Fig. 6 and the DNMR3 program, the DG  and DS  values of 37.4 kj mol − 1 and 57.9 JK − 1 mol − 1, respectively, for the folding of the cyclobutane ring in dppcb have been obtained. The positive entropy of activation indicates that no symmetrical transition state with a planar cyclobutane ring and a center of symmetry corresponding to the conformation of dppcb in the solid state structure of 4 occurs. It seems likely, that comparable to 3 several conformations with differently folded cyclobutane rings are involved in the equalization of the diphenylphosphino groups in dppcb. Furthermore, as in the case of cis,cis-1,3,5-tris-[(diphenylphosphino)methyl] - 1,3,5 - trimethylcyclohexane [40], the deformation of the cyclobutane ring in dppcb (see Fig. 3) leads to a fast equalization of the diphenylphosphino groups because the ground state structure has approached the transition state geometry.

4. Discussion The new tetraphosphine dppcb shows combined spirocyclic and branched properties [1a]. The photochemical production of cyclobutanes has a counterpart in nature, since cyclobutane type pyrimidine dimers are the most common product of UV irradiation of DNA [41]. Dppcb has the advantages of typical perphenylated polydentate phosphines, which are easy to synthesize, relatively air-stable, and used widely. In its bridging bimetallics, through-bond coupling could occur by the interactions of the appropriate metal orbitals with those of the bridging atoms as is described commonly for superexchange [7]. Preliminary photochemical experiments of Ru(II) complexes with dppcb show, that superexchange must be present in these compounds. Due to its capability of fivemembered chelate ring formation, dppcb is related to 1,2-bis(diphenylphosphino)ethane (dppe) and cis-1,2bis(diphenylphosphino)ethene (cis-dppen) similar to other polyphosphines introduced by McFarlane [23b,42]. Comparable to the classical diphos ligands dppe and cis-dppen, which can only act as bidentate ligands, dppcb mostly acts as bis-bidentate ligand. However, different bridging modes of dppcb are conceivable, including the coordination of two dppcb ligands to the same metal center. Like in tetraphos-1, in

177

dppcb Ph···Ph interactions could be present demonstrating that small changes in ligand geometry can have profound effects on the coordination chemistry of the complex as a consequence of mechanical coupling [43]. The compounds 4–8 show the main coordination possibility of dppcb, which is binuclear tetraligate single-bridging with two equal coordination units. However, since the X-ray structure of the free tetraphosphine dppcb reveals the pure MMMP-enantiomer [21], mechanical coupling could lead to a ‘frozen in’-chirality in complexes of dppcb, where the chirality of this system is a desirable feature for promoting stereo- and enantioselective reactions. The origin of enantioselection in catalysts containing C2symmetric chelating diphosphines corresponding to one half of dppcb, has been linked intimately to the chiral array of P-aryl substituents surrounding the metal [4b]. Comparable to the tetradentate phosphine (Et2PCH2CH2)(Ph)PCH2P(Ph)(CH2CH2PEt2), eLTTP, the chelating and bridging capabilities of dppcb are critical in allowing two metal centers to cooperate in a catalysis cycle [37]. Tight binding of dppcb likely derives from entropic factors associated with the rather rigid cyclobutane ring. In the case of dppe, by contrast, dissociation of one arm may be accompanied by rotation about the backbone C–C bond and complete loss of that phosphine moiety (entropy can overcome binding energy). The cyclobutane ring in dppcb precludes this scenario for ligand dissociation. The production of dppcb via platinum(II) as a template is comparable to the formation of macrocycles, where nickel(II) is used as a template and the metal can also be removed from the complex [44]. The stereochemical control during the synthesis of dppcb is typical for platinum(II) as a template and has already been observed in mixed-chain complexes [45]. The resulting pure MMMP-enantiomer of dppcb could be applied as its rhodium complex in asymmetric hydrogenation, where the comparable chiral diphosphine trans-1,2-bis(diphenylphosphino)cyclobutane has been used successfully for this purpose [39]. A further possible application of dppcb is the alternating copolymerization of ethene and carbon monoxide, which has attracted considerable interest from both academia and industry over the last decades [46]. The combined importance of bidentate tertiary phosphine ligands and weakly coordinating anions for high palladium(II) catalyst activity and yield, has opened the way for efficient synthesis of polyketone. Preliminary catalytic experiments of Pd(II) complexes with dppcb show, that these compounds are good copolymerization catalysts.

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5. Supplementary material Tables of atomic coordinates, bond lengths and bond angles, thermal parameters (32 pages) and structure factors (103 pages) are available from the authors on request. Further details on the structure determinations are also obtainable at the Cambridge Structural Database.

[15] [16]

[17]

Acknowledgements

[18]

We thank the Fonds zur Fo¨rderung der wissenschaftlichen Forschung, Austria, for financial support. T.S., R.H. and C.L. thank Swarovski and the Austrian Society of Industrialists for support.

[19] [20] [21] [22]

References [23] [1] (a) F.A. Cotton, B. Hong, Prog. Inorg. Chem. 40 (1992) 179. (b) C. Bachmann, W. Oberhauser, P. Bru¨ggeller, Polyhedron 15 (1996) 2223. (c) P. Bru¨ggeller, Inorg. Chem. 26 (1987) 4125. [2] C. Bohanna, B. Callejas, A.J. Edwards, M.A. Esteruelas, F.J. Lahoz, L.A. Oro, N. Ruiz, C. Valero, Organometallics 17 (1998) 373. [3] P.-H. Leung, S.-Y. Siah, A.J.P. White, D.J. Williams, J. Chem. Soc., Dalton Trans. (1998) 893. [4] (a) H.-L. Ji, J.H.. Nelson, A. DeCian, J. Fischer, C. Li, C. Wang, B. McCarty, Y. Aoki, J.W. Kenny III, L. Solujic, E.B. Milosavljevic, J. Organomet. Chem. 529, (1997) 395. (b) M.J. Burk, J.E. Feaster, W.A. Nugent, R.L. Harlow, J. Am. Chem. Soc. 115 (1993) 10125. (c) T. Bukachaisirikul, S. Arabi, F. Hartstock, N.J. Taylor, A.J. Carty, Organometallics 3 (1984) 1587. (d) R.D. Waid, D.W. Meek, Inorg. Chem. 23 (1984) (1997) 778. [5] C. Bianchini, A. Meli, F. Laschi, F. Vizza, P. Zanello, Inorg. Chem. 28 (1989) 227. [6] (a) J.-D. Chen, F.A. Cotton, B. Hong, Inorg. Chem. 32 (1993) 2343. (b) F.A. Cotton, B. Hong, Inorg. Chem. 32 (1993) 2354. [7] D.G. McCollum, G.P.A. Yap, A.L. Rheingold, B. Bosnich, J. Am. Chem. Soc. 118 (1996) 1365. [8] R.D. Jones, D.A. Summerville, F. Basolo, Chem. Rev. 79 (1979) 139. [9] A.C.T. North, D.C. Phillips, F.S. Mathews, Acta Crystallogr., Sect. A 24 (1968) 351. [10] G.M. Sheldrick, SHELXS86, in: G.M. Sheldrick, C. Kru¨ger, and R. Goddard (Eds.), Crystallographic Computing 3, Oxford University Press, London, 1985, p. 175; G.M. Sheldrick, SHELXL93, program for crystal structure determination, Universita¨t Go¨ttingen, Germany, 1993. [11] H.D. Flack, Acta Crystallogr., Sect. A 39 (1983) 876. [12] R.B. King, P.N. Kapoor, Inorg. Chem. 11 (1972) 1524. [13] (a) J.M. Vila, M. Gayoso, A. Fernandez, N.A. Bailey, H. Adams, J. Organomet. Chem. 448 (1993) 233. (b) J.M. Vila, M. Gayoso, M.T. Pereira, J.M. Ortigueira, A. Fernandez, N.A. Bailey, H. Adams, Polyhedron 12 (1993) 171. [14] (a) A. Bayler, A. Schier, G.A. Bowmaker, H. Schmidbaur, J. .

[24] [25] [26] [27] [28] [29] [30] [31]

[32] [33]

[34] [35] [36]

[37] [38] [39]

Am. Chem. Soc. 118 (1996) 7006. (b) C. Papadimitriou, P. Veltsistas, J. Novosad, R. Cea-Olivares, A. Toscano, P. Garcia y Garcia, M. Lopez-Cardosa, A.M.Z. Slawin, J.D. Woollins, Polyhedron 16 (1997) 2727. (c) W. Oberhauser, C. Bachmann, T. Stampfl, R. Haid, C. Langes, A. Rieder, P. Bru¨ggeller, Polyhedron 17 (1998) 3211. J.A. Rahn, D.J. O’Donnell, A.R. Palmer, J.H. Nelson, Inorg. Chem. 28 (1989) 2631. (a) W. Oberhauser, C. Bachmann, T. Stampfl, P. Bru¨ggeller, Inorg. Chim. Acta 256 (1997) 223. (b) G. Hogarth, T. Norman, Polyhedron 15 (1996) 2859. C.C. Santini, J. Fischer, F. Mathey, A. Mitschler, J. Am. Chem. Soc. 102 (1980) 5809. N.J. Turro, Modern Molecular Photochemistry, Benjamin/ Cummings, Menlo Park, CA, 1978, p. 419. R.B. Woodward, R. Hoffmann, Angew. Chem. 81 (1969) 797; Angew. Chem., Int. Ed. Engl. 8 (1969) 781. U. Salzner, S.M. Bachrach, J. Organomet. Chem. 529 (1997) 15. V. Prelog, G. Helmchen, Angew. Chem. 94 (1982) 614; Angew. Chem., Int. Ed. Engl. 21 (1982) 567. (a) S. Meiboom, L.C. Snyder, J. Am. Chem. Soc. 89 (1967) 1038. (b) E. Adman, T.N. Margulis, J. Phys. Chem. 73 (1969) 1480. (c) H. Kim, W.D. Gwinn, J. Chem. Phys. 44 (1966) 865. (a) P.G. Jones, M.C. Gimeno, Z. Kristallogr. 209 (1994) 688. (b) J.L. Bookham, W. McFarlane, I.J. Colquhoun, M. Thornton-Pett, J. Organomet. Chem. 354 (1988) 313. R. Gilardi, C. George, J.L. Flippen-Anderson, Acta Crystallogr., Sect. C 48 (1992) 1680. T.N. Margulis, J. Am. Chem. Soc. 93 (1971) 2193. V.R. Pedireddi, J.A.R.P. Sarma, G.R. Desiraju, J. Chem. Soc., Perkin Trans. 2 (1992) 311. H.A. Mayer, W.C. Kaska, Chem. Rev. 94 (1994) 1239. S.A. Laneman, G.G. Stanley, Inorg. Chem. 26 (1987) 1177. S.E. Saum, S.A. Laneman, G.G. Stanley, Inorg. Chem. 29 (1990) 5065. M.E. Broussard, B. Juma, S.G. Train, W.-J. Peng, S.A. Laneman, G.G. Stanley, Science 260 (1993) 1784. D.A. Klein, G. Binsch, ‘DNMR3: A Computer Program for the Calculation of Complex Exchange-Broadened NMR Spectra. Modified Version for Spin Systems Exhibiting Magnetic Equivalence or Symmetry’, Program 165, Quantum Chemistry Program Exchange, Indiana University, Bloomington, 1970. K.D. Tau, R. Uriarte, T.J. Mazanec, D.W. Meek, J. Am. Chem. Soc. 101 (1979) 6614. (a) W. Oberhauser, C. Bachmann, P. Bru¨ggeller, Inorg. Chim. Acta 238 (1995) 35. (b) F.R. Hartley, S.G. Murray, A. Wilkinson, Inorg. Chem. 28 (1989) 549. P.E. Garrou, Chem. Rev. 81 (1981) 229. P.S. Pregosin, Coord. Chem. Rev. 44 (1982) 247. (a) A. Albinati, F. Isaia, W. Kaufmann, C. Sorato, L.M. Venanzi, Inorg. Chem. 28 (1989) 1112. (b) W. Kaufmann, L.M. Venanzi, A. Albinati, Inorg. Chem. 27 (1988) 1178. (c) K. Dillinger, W. Oberhauser, C. Bachmann, P. Bru¨ggeller, Inorg. Chim. Acta 223 (1994) 13. (d) W. Oberhauser, C. Bachmann, P. Bru¨ggeller, Polyhedron 14 (1995) 787. S.A. Laneman, F.R. Fronczek, G.G. Stanley, Inorg. Chem. 28 (1989) 1872. J.L. Bookham, W. McFarlane, I.J. Colquhoun, J. Chem. Soc., Chem. Commun. (1986) 1041. T. Minami, Y. Okada, R. Nomura, S. Hirota, Y. Nagahara, K. Fukuyama, Chem. Lett. (1986) 613.

W. Oberhauser et al. / Inorganica Chimica Acta 290 (1999) 167–179 [40] H.A. Mayer, R. Fawzi, M. Steimann, Chem. Ber. 126 (1993) 1341. [41] P.F. Heelis, J. Mol. Model. 1 (1995) 18. [42] J.L. Bookham, W. McFarlane, I.J. Colquhoun, J. Chem. Soc. Dalton Trans. (1988) 503.

.

179

[43] J.M. Brown, L.R. Canning, J. Organomet. Chem. 267 (1984) 179. [44] R. Bartsch, S. Hietkamp, H. Peters, O. Stelzer, Inorg. Chem. 23 (1984) 3304. [45] L.M. Green, D.W. Meek, Polyhedron 9 (1990) 35. [46] E. Drent, P.H.M. Budzelaar, Chem. Rev. 96 (1996) 663.

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