Synthesis and structural characterization of Groups 10 and 11 mononuclear fluoroaryloxide complexes

Polyhedron 24 (2005) 1803–1812 www.elsevier.com/locate/poly

Synthesis and structural characterization of Groups 10 and 11 mononuclear fluoroaryloxide complexes Miki Kim a, Lev N. Zakharov b, Arnold L. Rheingold b, Linda H. Doerrer

a,*

a

b

Barnard College, Chemistry Department, 3009 Broadway, New York, NY 10027, United States Department of Chemistry and Biochemistry, MC 0358, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093, United States Received 8 September 2004; accepted 14 June 2005 Available online 27 July 2005 Dedicated to Prof. Malcolm L.H. Green

Abstract A study of four late-transition metal fluoroaryloxide (OArF or OAr 0 ) complexes is presented including X-ray crystallography, polynuclear solution NMR spectroscopy, UV–Vis spectroscopy and elemental analyses. The study includes three new compounds: [(Ph3P)2Ni(OArF)2] (1a), [(Ph3P)2Ni(OAr 0 )2] (1b), [(COD)Pt(OArF)2] (2) and one compound whose synthesis and elemental analysis were reported previously: [(Ph3P)Au(OArF)] (3). These compounds represent the common L2MX2 (1a, 1b, 2) and LMX (3) ligand classes in Groups 10 and 11, respectively, but with an uncommon ligand type, the monodentate phenoxide. In the solid state, compounds 1a and 1b exhibit square-planar geometry at nickel with trans phosphines in each case. In solution, these nickel compounds slowly decompose in CH2Cl2. Compound 2 is quite stable in solution at room temperature with the two phenoxide ligands cis to one another in the solid state. Compound 3 has a virtually linear geometry at the gold center and is stable in the solid state in the dark but decomposes slowly in solution in the light. Comparison of these four fluoroaryloxide compounds with the protio analogs (or attempts to make such compounds) demonstrate the greater stability to reduction of late-metal aryloxide complexes with highly electron-withdrawing substituents on the phenoxide rings. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Phenoxide; Nickel; Platinum; Gold; Fluorinated aryloxide

1. Introduction Phenoxide and aryloxide ligands on transition metal centers are highly important in many disparate areas of science. In biology, metal–phenoxide linkages are formed between a metal (most often a first-row transition metal) and the phenolic side chain of tyrosine. Metal sequestration units often have multiple phenoxide donor atoms for chelation such as transferrin [1] or enterobactin [2]. Metal centers also bind to phenoxide ligands during oxidation of phenols to ke*

Corresponding author. Tel.: +21 28 54 2 074; fax: +21 28 542 310. E-mail address: [email protected] (L.H. Doerrer).

0277-5387/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.06.012

tones as in catecholase [3,4] or in aromatic hydroxylation as in tyrosinase [3,4]. Metal phenoxides, metal alkoxides, and mixed-metal combinations of phenoxides and alkoxides are valuable precursors for metal oxide ceramics [5]. Many important chelating ligands in chemistry include phenolate units such as macrocyclic calixarenes [6] and salen systems [7]. Metal-based homogeneous catalysts also make use of phenoxide ligands such as the alkyne metathesis catalysts prepared by the Schrock group [8], hydrogenation systems from the Rothwell group [9], and the zinc [10] and cadmium [11] compounds used in CO2/epoxide copolymerization developed by the D. Darensbourg group. Late metal aryloxides had been less well studied than

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early transition metal derivatives [12], but more recent work [13], including this paper, addresses the disparity [14]. Phenoxides are anionic oxygen-donor ligands that often bridge metal centers and typically bind hard metal centers in relatively high oxidation states. In transition metal compounds requiring monodentate phenoxides, one method to prevent bridging by the electron-rich oxygen donors, is to furnish the ligand with bulky substituents at the 2- and 6-positions. Our laboratory has demonstrated an alternative approach, namely that the bridging propensity may be reduced by extensive fluorination of the phenyl ring as in OC6F5 (OArF) or OC6H3(CF3)2 (OAr 0 ) [13]. This modification is an electronic approach, instead of a steric one, and thus results in metal centers that may still bind to other ligands for catalysis or more complex structure development. All monodentate phenoxide ligands are simple X-type ligands in the MLXZ classification system [15], but their steric and electronic properties differ significantly. Herein we describe four more compounds with monodentate fluoroaryloxides whose simple OC6H5 analogs have not been reported in all cases. These compounds serve to develop further comparisons of OPh and OArF as ligands as well as increase the number of known and structurally characterized monodentate M–OArF and M–OAr 0 linkages.

2. Experimental 2.1. General considerations All studies were carried out at room temperature on a Schlenk line or in a nitrogen- or argon-filled glovebox. For solvents dried in stills, methylene chloride and deuterated methylene chloride were refluxed over CaH2. THF and hexane were refluxed over potassium and toluene was refluxed over sodium. All solvents were refluxed under nitrogen or argon. Some work was also done with solvents (hexanes, CH2Cl2, and toluene) dried in a nitrogen-filled MBraun SPS (solvent purification system) using Al2O3. Celite was dried overnight in vacuo while heated to 125 °C with an oil bath. TlOC6F5 (TlOArF) and TlOC6H3(CF3)2 (TlOAr 0 ) were prepared according to our previously reported procedures [13]. [(Ph3P)Au(OPh)] was prepared from [(Ph3P)AuCl] and KOPh [16]. All other reagents were obtained commercially and were not purified further. NMR spectra were measured on a Varian 300 MHz, Bruker 300 or 400 MHz spectrometer. UV–Vis data were recorded with a Varian Cary 50 spectrometer. 1H and 13C chemical shifts were referenced to (CH3)4Si via the resonance of residual protiosolvent (1H) or the 13C resonance of the solvent. 19F shifts were referenced to external CFCl3. 31 P shifts were referenced to internal (MeO)3PO4 or

external 85% H3PO4. Microanalyses were performed by H. Kolbe Microanalytisches Laboratorium, Mu¨lheim an der Ruhr, BRD. 2.2. Synthesis of [(Ph3P)2Ni(OArF)2] (1a) In a 25 ml round bottom flask a portion of TlOArF (0.3014 g, 0.7780 mmol) was dissolved in
10 ml CH2Cl2. (PPh3)2NiCl2 (0.2529 g, 0.3866 mmol) was added to the reaction mixture. The reaction mixture turned a dark red-brown color and a white precipitate also evolved. The reaction was left to stir overnight at room temperature, and was filtered through Celite to remove TlCl. The mixture was concentrated in vacuo and recrystallized from CH2Cl2 and hexanes (1:1). Dark brown crystals were collected in 56% yield (0.2039 g). 1 H NMR, (d, ppm, CD2Cl2) 6.780 (t, 6 H, para, 3J = 7.38 Hz), 7.347 (t, 12H, ortho, 3J = 7.83 Hz), 7.696 (t, 12H, meta, 3J = 7.38 Hz). 19F {1H} NMR, (d, ppm, CD2Cl2) 153.99 (o, m), 171.04 (m, m), 177.47 (p, m). UV–Vis (CH2Cl2) [kmax, nm (eM, cm1 M1)] 456.0 (3698). Anal. Calc. for C48H30O2F10P2Ni: C, 60.73; H, 3.19. Found: C, 60.68; H, 3.21%. 2.3. Synthesis of [(PPh3)2Ni(OAr 0 )2] (1b) In a 25 ml round bottom flask a portion of TlOAr 0 (0.3003 g, 0.7744 mmol) was dissolved in
10 ml CH2Cl2. (PPh3)2NiCl2 (0.2535 g, 0.3872 mmol) was added to the reaction mixture. The reaction mixture turned a dark red-brown color and a white precipitate also evolved. The reaction was left to stir overnight at room temperature, and was filtered through Celite to remove TlCl. The mixture was concentrated in vacuo and recrystallized from CH2Cl2 and hexanes (1:1). Dark brown crystals (0.3629 g) were collected in 90% yield. 1 H NMR, (d, ppm, CD2Cl2) 6.350 (br, 2H, para-OAr 0 ), 7.211 (br, 4H, ortho-OAr 0 ), 7.260 (t, 12H, meta, 3 J = 2.85 Hz), 7.680 (m, 18H, ortho + para). 19F {1H} NMR, (d, ppm, CD2Cl2) 63.27 (CF3, s). UV–Vis (CH2Cl2) [kmax, nm (eM, cm1 M1)] 446.0 (2905), Anal. Calc. for C52H36O2F12P2Ni: C, 59.97; H, 3.48. Found: C, 59.77; H, 3.41%. 2.4. Synthesis of [(COD)Pt(OArF)2] (2) A portion of [(COD)2PtCl2] (151.5 mg, 0.4049 mmol) was dissolved in 5 mL of CH2Cl2 and 2 equivalents of KOArF (179.9 mg, 0.8098 mmol) in 5 mL of THF were added. The solution immediately turned pale yellow and a white precipitate (presumably KCl) was observed. The solution was allowed to stir overnight and then the precipitate was removed via filtration through Celite. The solution was concentrated to dryness in vacuo, and the crude very pale yellow product was recrystallized from CH2Cl2 and hexanes. X-ray quality crystals

M. Kim et al. / Polyhedron 24 (2005) 1803–1812

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o-CF), 129.82 (d, m, CH, 1JCP = 12.1 Hz), 132.63 (d, p, CH, 1JCP = 12.1 Hz), 134.72 (d, o, CH, 1JCP = 13.61 Hz). 19F {1H} NMR, (d, ppm, CD2Cl2) 164.65 (o, m,), 168.89 (m, m,), 180.10 (p, m). 31P (d, ppm, CD2Cl2) 24.61. UV–Vis (CH2Cl2) [kmax, nm (eM, cm1 M1)] 236 (30,200), 268 (4460), 276 (3540). Anal. Calc. for C24H15OF6PAu: C, 44.89; H, 2.35; F, 14.79. Found: C, 44.97; H, 2.28; F, 14.67%.

were grown from the same mixture. Recrystallized yield 167 mg (61.6%). 1H NMR (d, ppm, CD2Cl2) 2.714 (q, CH2, 4H, 3JHH = 8.0 Hz), 3.262 (m, CH2, 4H), 5.759 (t, CH, 4H, 3JHH = 28.8 Hz). 13C {1H} NMR, (d, ppm, CD2Cl2) 31.29 (s, CH2), 100.97 (t, CH, 1JCPt = 76.7 Hz). 19F {1H} NMR, (d, ppm, CD2Cl2) 158.25 (o, d, 3JFF = 21.2 Hz), 164.55 (m, t, 3JFF = 21.5 Hz), 158.25 (p, t of t, 3JFF = 15.2, 6.2 Hz). UV–Vis (CH2Cl2) [kmax, nm (eM, cm1 M1)] 233 (15 200), 280 (8660). Anal. Calc. for C20H10O2F10Pt: C, 35.89; H, 1.81; F, 28.38. Found: C, 35.91; H, 1.79; F, 28.42%.

2.6. X-ray crystallography A summary of crystal data, details of data collections and refinement parameters for all compounds are given in Table 1. X-ray diffraction data were collected using ˚ ) on a Bruker SMART Mo Ka radiation (k = 0.71073 A APEX CCD (1a, 1b, 2) and a Siemens P4/CCD (3) diffractometers [17]. Space groups were determined through inspection of systematic absences (1a, 1b) and intensity statistics (2, 3). All data were corrected for absorption by SADABS [18] The structures were solved using direct methods or the Patterson function and difference map techniques, and were refined by full-matrix least squares procedures on F2. All non-hydrogen atoms were refined with anisotropic displacement parameters. The H atoms in 1a and 2 were found from the F-map and refined with isotropic thermal parameters. In 1b and 3 the H atoms were treated as idealized contributions. All software and sources of scattering factors are contained in the SHELXTL program package [19].

2.5. Synthesis of [(Ph3P)Au(OArF)] (3) A portion of [(Ph3P)AuCl] (100.0 mg, 0.2021 mmol) was dissolved in 5 mL of CH2Cl2. One equivalent of KOArF (44.9 mg, 0.2021 mmol) was dissolved in 2 mL of THF and the solutions mixed together. No obvious color changes were observed. After stirring for several hours, the solvents were removed in vacuo and the crude product dissolved in 5 mL of CH2Cl2 and filtered through Celite to remove presumed KCl. A saturated solution of the product in CH2Cl2 (
1 mL) was prepared and layered with an equal volume of hexanes to yield colorless crystalline material, including crystals suitable for X-ray diffraction. Recrystallized yield 60 mg (46.2%). 1H NMR (d, ppm, CD2Cl2) 7.53 (br, Ph). 13 C {1H} NMR, (d, ppm, CD2Cl2) 120.14 (s, ipsoOC6F5), 128.54 (s, m-CF), 128.99 (s, p-CF), 129.41 (s,

Table 1 Summary of X-ray crystallographic data 1a

1b

2

C52H36F12 NiO2P2 C20H12F10O2Pt Formula C48H30F10NiO2P2 Formula weight 949.37 1041.46 669.39 Space group P21/c P21/n P
1 ˚) a (A 11.4462(8) 10.5517(5) 7.4721(4) ˚) b (A 15.5033(10) 14.8822(7) 10.2036(6) ˚) c (A 11.1220(7) 29.7183(13) 13.1568(7) a (°) 71.170(1) b (°) 91.699(1) 92.829(1) 80.831(1) c (°) 82.371(1) ˚ 3) V (A 1972.8(2) 4661.1(4) 933.73(9) Z,Z 0 2, 0.5 4, 1 2, 1 Crystal color, habit brown, plate brown, block colorless, block qcalc (g cm3) 1.598 1.484 2.381 l(Mo Ka) (mm1) 0.663 0.574 7.629 Temperature (K) 100(2) 120(2) 100(2) Reflections measured 12614 29786 5919 Reflections independent [Rint] 4675 [0.0492] 10 948 [0.0493] 4110 [0.0156] Absorption correction SADABS SADABS SADABS (Tmin/Tmax) (0.900) (0.883) (0.676) R(F) (%)a 0.0510 0.0650 0.0195 R(wF2) (%)b 0.1204 0.1579 0.0483 P P a R ¼ jjF o j  jF c jj= jF o j. nP o1=2 P b RðxF 2 Þ ¼ ½xðF 2o  F 2c Þ2 = ½xðF 2o Þ2  ; x ¼ 1=½r2 ðF 2o Þ þ ðaP Þ2 þ bP ; P ¼ ½2F 2c þ maxðF o ; 0Þ=3.

3 C24H15AuF5OP 642.30 P
1 8.9435(7) 10.8180(8) 11.0364(9) 84.628(1) 81.113(1) 89.126(2) 1050.33(14) 2, 1 colorless, block 2.031 7.138 218(2) 7731 4802 [0.0241] SADABS

(0.652) 0.0299 0.0730

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ent strategies have been used to prevent polymerization through bridging aryloxides or reduction of the metal center. The most common approach is incorporation of the aryloxide into a chelate ring (e.g., salen, salicylate, quinolate) or a macrocycle (calixarene). Table 2 contains the results of several searches in the Cambridge Structural Database V5.25 (CSD) [20] to put into context the compounds discussed herein. These values represent the crystallographically characterized subset of known compounds and are therefore not an exhaustive survey of all known compounds. They provide a quantitative picture of the relative distribution of different types of compounds in the literature.

3. Results and discussion The four compounds whose crystal structures are reported herein are shown in Scheme 1. The compounds add to the library of monodentate fluorinated aryloxide compounds reported by our group [13] and others. Extensive structural and spectroscopic studies of the [Co(OArF)4]2 and [Cu(OArF)4]2 anions in comparison with the well-known [CoCl4]2 and [CuCl4]2 anions demonstrated that the pentafluorophenoxide ligand generates a similar but slightly stronger ligand field than Cl, consistent with qualitative expectations for the more electronegative oxygen ligand. Attempts to make [M(OPh)4]2 by analogous routes with perhydro OC6H5 led to exclusive formation of [(PhO)2M(l-OPh)2M(OPh)2]2 (M = Co, Cu) [13] anions with bridging phenoxide ligands. These results suggested that highly electronegative substituents on the aryloxide ring reduce the electron density at the metal center and decrease the bridging propensity of the aryloxide oxygen atom. Described herein are further investigations of the late transition metals with these highly fluorinated aryloxide ligands and the effects of reduced electron density on compound stability.

3.1.1. Nickel Nickel aryloxide bonds are typically prepared with chelating ligands, e.g., salen, as shown in Table 2, or additional soft and somewhat p-accepting ligands in the coordination sphere to balance the powerful donation from the aryloxide, with phosphines being the most common. Interestingly, a survey [20] of structurally characterized [(Ph3P)2NiX2] compounds (27 examples) reveals no compounds with strongly Lewis basic X ligands (F, CH3  ) or oxygen donor atoms. Instead softer ligands with p-acceptor character and/or sulfur donor atoms are found with such electron rich ligands. Nickel phosphine complexes with hard ligands have more electron donating phosphines as in [(Me3P)2Ni(o-OC6H4-Cl)2] [21], [(Me3P)2Ni(OPh)(Me)] [22], [(Et3P)2Ni(C5F4N)(OPh)] [23], [(Me3P)2NiMe(OArF)]

3.1. Synthesis Successful synthesis of isolable metal–phenoxide compounds results from control of the strongly nucleophilic character of the oxygen donor atom. Many differF

F

F

CF3

F F3C

F

F3 C

O P

Ni

P

O

O P

F

F

Ni O

P

F

1a F

CF3

1b

F F

F

F F F

Pt

O

F

O

O

F F F

F

F

P

F F

F

F

Au

2 Scheme 1.

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Table 2 Comparison of structurally characterizeda nickel, platinum, and gold aryloxide compounds Category

Aryloxide linkage

Ni

Pt

Au

A B C D E F G H

M–OAr A and acyclic OAr M–(l2-OAr)–M 0 P–M–OAr (O,N)-M–OArb A and D and E A not D or E G and acyclic OAr

850 14 325 56 752 21 63 3

76 9 4 38 29 5 14 5

17 7 0 10 9 3 1 0

a b

Cambridge Structural Database V5.25 [20]. Includes five- and six-membered chelate rings.

Table 3 Selected interatomic distances, bond lengths and anglesa ˚) Compound Distances (A

Angles (°)

1a

Ni1–O1 Ni1–P1 O1–C1 P1–C7 P1–C13 P1–C19

1.852(2) 2.2862(7) 1.334(3) 1.827(3) 1.819(3) 1.822(3)

O1–Ni1–P1 O1–Ni1–P1_3 O1–Ni1–O1 P1–Ni1–P1 Ni1–O1–C1 Ni1–P1–C7 Ni1–P1–C13 Ni1–P1–C19 C7–P1–C13 C7–P1–C19 C13–P1–C19

88.88(7) 91.12(7) 180.00(8) 180.00(4) 126.71(19) 112.12(10) 112.13(10) 117.11(9) 105.09(12) 104.21(13) 105.14(13)

1b

Ni1–O1 Ni1–O2 Ni1–P1 Ni1–P2 O1–C1 O2–C9 P1–C17 P1–C23 P1–C29 P2–C35 P2–C41 P2–C47

1.858(3) 1.856(2) 2.2383(10) 2.2422(10) 1.281(5) 1.322(4) 1.814(4) 1.827(4) 1.817(4) 1.822(4) 1.815(4) 1.810(4)

O2–Ni1–O1 O1–Ni1–P1 O1–Ni1–P2 O2–Ni1–P1 O2–Ni1–P2 P1–Ni1–P2 Ni1–O1–C1 Ni1–O2–C9 Ni1–P1–C17 Ni1–P1–C23 Ni1–P1–C29 Ni1–P2–C35 Ni1–P2–C41 Ni1–P2–C47

178.49(12) 94.50(8) 86.32(8) 84.97(8) 93.94(9) 169.35(4) 122.4(2) 121.0(2) 111.91(12) 111.44(12) 118.04(12) 111.17(12) 117.50(13) 112.23(12)

2

Pt1–O1 Pt1–O2 Pt1–C1 Pt1–C2 Pt1–C5 Pt1–C6 C1–C2 C5–C6 O1–C9 O2–C15

2.014(2) 2.019(2) 2.153(3) 2.141(3) 2.146(3) 2.134(3) 1.409(5) 1.397(5) 1.328(4) 1.336(3)

Pt1–O1–C9 Pt1–O2–C15 O1–Pt1–O2

122.63(19) 122.59(19) 94.95(9)

3

Au1–O1 Au1–P1 O1–C1 P1–C11 P1–C21 P1–C31

2.028(3) 2.1978(11) 1.291(6) 1.796(4) 1.802(4) 1.803(4)

Au1–O1–C1 P1–Au1–O1 Au1–P1–C11 Au1–P1–C21 Au1–P1–C31

128.5(3) 172.76(9) 114.78(15) 112.96(14) 109.50(14)

a

Numbers in parentheses are estimated deviations of the last significant.

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[24], [(Me3P)2Ni(OAr)2] [25], and trans-[(Bz3P)2NiH(OPh)] Æ HOPh [26]. More rarely nickel–aryloxide linkages are found with other strongly electron donating ligands as observed in [(Cp*)(Et3P)Ni(p-OC6H4Me)] [27] and [(py)3Ni(2,4,6-OC6Cl3H2)2] [28]. Nickel–aryloxide linkages have also been prepared with chelating O,Pligands that possess both hard and soft donor atoms [29–31]. Usually the phenoxide is a monodentate ligand but a few examples of bridging phenoxides have been studied with allyl ligands such as [Ni(g3-allyl)(l2-OAr)]2 in diene polymerization [32–34] and bridging phenoxide compounds have also been stabilized by the multidentate Klaui ligand [35]. The present report addresses the effect of fluorination in comparisons of OArF and OAr 0 versus OPh and O-mC6H3(CH3)2 complexes. Toward that end only a handful of nickel compounds with fluoroaryloxide ligands have been reported previously including the anion [36] [Ni2(C6F5)4(l2-OArF)2]2 and the neutral species 0 [Ni(g3-allyl)(l2-OArF)]2 and [Ni(g3-allyl)(l2-OAr )]2 which were studied as diene polymerization activators [32]. The polymeric [Ni(OArF)2]1 has also been reported [37,38]. Simple reaction of [(Ph3P)2NiCl2] with the appropriate TlOAr reagent yielded compounds 1a and 1b as brown crystalline compounds. Under the same conditions, reaction between [(Ph3P)2NiBr2] and TlOAr led to insoluble materials. It has been previously reported that [(Ph3P)2NiCl2] compounds are sensitive to reduction to Ni(I) with strong nucleophiles such as [NR2] and it may be that Ni(II) compounds with less electronegative halides are more susceptible to such reduction under nucleophilic attack [39]. Too strongly basic ligands may lead to Ph3P dissociation and subsequent decomposition or reduction of Ni(II) to Ni(I) or Ni(0). A similar sensitivity for 1a and 1b to that previously observed for [(Ph3P)2NiCl2] in CH2Cl2 [40–42] and CHCl3 [43] was noted. Attempts to synthesize these compounds in THF, however, by methods analogous to those described below for CH2Cl2 were unsuccessful and led to colorless solutions with orange-brown precipitates. [(Ph3P)2NiCl2] has been isolated in both tetrahedral [44–47] (blue-green) and square-planar [46,48,49] (deep red) forms but no evidence for a tetrahedral form of 1a or 1b has been observed. The square planar form of [(Ph3P)2NiCl2] has been characterized only in the presence of CH2Cl2 or ClCH2CH2Cl whereas crystals of the tetrahedral isomer were prepared from neat ethanol. Some Ni(II) compounds exhibit a tetrahedral form in non-coordinating solvents and an octahedral form in donor solvents such as DMF [50] or DMSO [43] in which solvent molecules are bound in two of six coordination sites. Compounds 1a and 1b are infinitely stable in the solid state but the orange-brown CH2Cl2 or acetone solutions turn pale-yellow over time, the latter much more rapidly. A time-dependent UV–Vis study

of 1b in CH2Cl2 showed only decay of the 1b absorbances and no new colored species (Supplementary Figure 1). Attempts to prepare the non-fluorinated analogs of 1a and 1b were unsuccessful. Reaction of [(Ph3P)2NiCl2] with either KOPh or TlOPh in THF led to pale yellow or yellow-orange solutions similar to the results of decomposition of 1a. These results could indicate reduction of Ni to Ni(I) ([(Ph3P)3NiCl] is yellow, for example) or Ni(0), but no clean products have been isolated. Reaction of [(Ph3P)2NiCl2] with TlOPh in CH2Cl2 led to insoluble orange powders. The insoluble orange powders could be a reduced Ni species or a polymeric Ni(II) compound formed by loss of phosphine. Two equivalents of KO–m-C6H3(CH3)2 when reacted with [(Ph3P)2NiCl2] produced copious amounts of an insoluble brown/tan powder and yellow solution. These observations suggest that the formation of Ni–OAr bonds in 1a and 1b relies on the electron-withdrawing properties of OArF and OAr 0 to be stable on a [(Ph3P)2Ni]2+ center. The formation of 1a and 1b, themselves somewhat unstable in solution, is clearly made possible by the fluorination of the aryloxide rings, in the absence of which, soluble Ni(II) compounds are not obtained. 3.1.2. Platinum Reaction of [(COD)PtCl2] with two equivalents of KOArF in THF cleanly afforded [(COD)Pt(OArF)2] with no sign of Pt metal formation. (Use of thallium reagents was avoided in the platinum and gold chemistry because heterometallic M–Tl species sometimes have resulted instead of thallium halide precipitation [51].) Platinum bis(aryloxide) complexes of the form [L2Pt(OAr)2] are relatively rare [52], with mono(aryloxides) [L2Pt(OAr)X] being more common. Platinum(II) aryloxide compounds summarized in Table 2 follow some of the same trends as Ni(II) species, although there are far fewer of the heavier congener because Pt(II) preferentially binds heavier chalcogenides and this type of compound has been studied more frequently. Consistent with this preference for softer ligands is the much greater fraction of phosphine-bearing compounds for Pt versus Ni. Of the 14 Pt–OAr compounds that have neither a phosphine nor a O,N chelate ring, only 5 do not have the phenoxide involved in any chelate ring. There are two L2PtX2 cases, [(bpy)Pt(OPh)Me] [53] and [(g3-Me2N-C6H4CHCH2)Pt(OArF)Cl] [54] and one example each of [L3PtX]+, [(terpy)Pt(3,4-OC6H3Me2)]BF4, [55] and [LPtX3], (Et4N)[Pt(C6F5)2(CO){p-OC6H4(NO2)}] [56], and one Pt(IV) species, [(bpy)Pt(Me)2I(OPh)] [53]. Conspicuously absent are terminal platinum aryloxides with p-accepting ligands as demonstrated in 2. The most similar known compound to 2 is the catecholate derivative [(COD)Pt(O2C6H4)] [57]. No report existed of [(COD)Pt(OPh)2] prior to our work and attempts to make [(COD)Pt(OPh)2] by a route analogous to 2 led

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to black-brown precipitates, suggestive of Pt reduction. These observations suggest that an electron-withdrawing aryloxide such as OArF is required to stabilize Pt–OAr linkages in the absence of phosphines or chelates and prevent ligand loss (COD) and subsequent metal reduction. Platinum compounds with OArF as a ligand are also exceedingly rare [54,56,58,59]. 3.1.3. Gold Synthesis of [(Ph3P)Au(OArF)] (3) was achieved via the straightforward metathesis of [(Ph3P)AuCl] with KOArF at room temperature. No evidence of gold reduction was observed. Compounds with gold(I)–oxygen bonds are relatively fewer in the literature compared to gold(I)–phosphorous or gold(I)–sulfur because of the relative hardness of oxygen ligands and the demonstrated preference of gold for softer chalcogenide ligands [60]. Only a few gold complexes with phenoxide ligands have been reported in the literature [12,61], and the data in Table 2 show only seven crystallographically characterized species with acyclic Au–O–Ar linkages, six containing Au(I) [62–66] and one with Au(III) [67]. Each of these Au(I) examples has a tertiary phosphine trans to the aryloxide ligand. The perhydro derivative [(Ph3P)Au(OPh)] has been reported [16] but not characterized with X-ray diffraction or UV–Vis spectroscopy.

Fig. 1. ORTEP diagram of [(Ph3P)2Ni(OArF)2] (1a). Hydrogen atoms ˚ ) and angles (°): have been omitted for clarity. Selected distances (A Ni1–O1 1.852(2), Ni1–P1 2.2862(7), O1–C1 1.334(3); O1–Ni1–P1, 88.88(7), O1–Ni1–P1_3, 91.12(7), Ni1–O1–C1, 126.71(19). Ellipsoids are shown at the 50% probability level.

3.2. Structural characterization There are only six crystallographically characterized compounds [20] with [P2NiO2] coordination sphere. Among these, five compounds have chelating ligands and one is an octahedral compound with two bidentate acetylacetonate-type ligands. The nickel compounds [(Ph3P)2Ni(OArF)2] (1a) and [(Ph3P)2Ni(OAr 0 )2] (1b) both contain square-planar nickel centers with trans Ph3P ligands. Square planar [(Ph3P)2NiX2] complexes are more rare than tetrahedral isomers, but not unknown. The nickel atom in 1a, shown in Fig. 1, sits on a center of inversion such that one half of the molecule is in the asymmetric unit. Selected distances and angles are listed in Table 3. The Ni–PPh3 distance is well within the range of other four-coordinate Ni(II)–PPh3 bond lengths. In a search of the Cambridge structural database (CSD) V5.25 [20], 85 such compounds were found with an average ˚ . The angles around Ni Ni–P bond length of 2.22(5) A (Table 3) are virtually 90/180°. The only crystallographically characterized Ni–OArF species both have bridging phenoxide ligands with necessarily longer average Ni–O ˚ compared bond lengths of 1.933 [36] and 2.058(3) [35] A to 1a. The structure of 1b, shown in Fig. 2, is largely similar to that of 1a. The most noticeable difference is that both phenyl groups are on the same side of the NiO2P2 plane, an artifact of crystal packing, whereas in solution free

Fig. 2. ORTEP diagram of [(Ph3P)2Ni(OAr 0 )2] (1b). Hydrogen and ˚) fluorine atoms have been omitted for clarity. Selected distances (A and angles (°): Ni1–O1 1.858(3), Ni1–O2 1.856(2), Ni1–P1, 2.2383(10), Ni1–P2, 2.2422(10), O1–C1, 1.281(5), O2–C9, 1.322(4); O2–Ni1–O1, 178.49(12), O1–Ni1–P1, 94.50(8), O1–Ni1–P2, 86.32(8), O2–Ni1–P1, 84.97(8), O2–Ni1–P2, 93.94(9), P1–Ni1–P2, 169.35(4), Ni1–O1–C1, 122.4(2), Ni1–O2–C9, 121.0(2). Ellipsoids are shown at the 50% probability level.

rotation about the C–O bonds is expected. At this writing, except for 1b, crystallographically characterized OAr 0 linkages have been reported only in [M(OAr 0 )4]2 compounds [13] and the [(Ar 0 O)2H] anion [68]. Again the distances and angles at the nickel center are within the range of related complexes in the literature.

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Fig. 3. ORTEP diagram of [(COD)Pt(OArF)2] (2). Hydrogen atoms ˚ ) and angles (°): have been omitted for clarity. Selected distances (A Pt1–O1 2.014(2), Pt1–O2 2.019(2), Pt1–C1 2.153(3), Pt1–C2 2.141(3), Pt1–C5 2.146(3), Pt1–C6 2.134(3), C1–C2 1.409(5), C5–C6 1.397(5), O1–C9 1.328(4), O2–C15 1.336(3); Pt1–O1–C9 122.63(19), Pt1–O2– C15 122.59(19), O1–Pt1–O2 94.95(9). Ellipsoids are shown at the 50% probability level.

The crystal structure of 2 is shown with an ORTEP in Fig. 3, clearly demonstrating that this compound is a member of the L2MX2 ligand class [15]. The cyclooctadiene ligand is bound in a g4-mode with the C@C bonds perpendicular to the PtO2 plane. A plethora of structurally characterized [(COD)Pt]2+ compounds exist, but relatively few with hard oxygen donor ligands, consistent with the soft character of Pt(II) [69,70], and only nine with a [(COD)Pt(O)2] coordination sphere, according to a CSD search. The most relevant structural comparison is that with [(COD)Pt(catecholate)] in which the doubly-deprotonated catechol ligand forms a fivemembered chelate ring with platinum [57]. The Pt–O ˚ ) of 1.972 and 1.978 in contrast bonds have lengths (A to the slightly longer average bond length in 2 of 2.017(2). The less basic OC6F5 ligand has been shown previously to form slightly longer bonds [13]. The aver˚ ) are virtually the same in 2, age Pt–C bond lengths (A 2.144(8), and the catecholate compound, 2.156(12). Slightly shorter Pt–C bonds in 2 are consistent with less donating fluorinated arlyoxide ligands that result in more electron donation by the COD ligand. The coordinated COD bonds in 2 have lengths consistent with dou˚ in 205 ble bonds and within the range of 1.37(4) A crystallographically characterized [20] [(COD)Pt]2+ units. To our knowledge, only one prior report exists of a crystallographically characterized Pt–OArF bond, namely that in [(o-Me2NC6H4-CHCH2)Pt(OArF)Cl] [54] which has Pt–O and O–C bond lengths of 2.017(5) ˚ , respectively. The former bond is virtuand 1.341(9) A ally identical with that in 2 and the latter is somewhat longer.

Fig. 4. ORTEP diagram of [(Ph3P)Au(OArF)] (3). Hydrogen atoms ˚ ) and angles (°): have been omitted for clarity. Selected distances (A Au1–P1, 2.1978(11), Au1–O1 2.028(3), O1–C1 1.291(6); Au1–O1–C1 128.5(3), P1–Au1–O1, 172.76(9). Ellipsoids are shown at the 50% probability level.

Fig. 4 shows an ORTEP of compound 3 with a simple LMX [15] motif. The geometry at Au is virtually linear, as are the vast majority of Au(I) complexes [69,70]. The ˚ in length and the gold phenoxide bond is 2.028(3) A Au–O–C angle is 128.5(3)°. The parameters are quite similar to those of the five crystallographically characterized Au(I) phenoxide compounds mentioned above ˚ and that have average Au–O distances of 2.03(1) A Au–O–C angles of 126(4)°. The intraligand distances and angles are unexceptional. 3.3. Spectroscopy The 1H NMR chemical shifts in 1a and 1b are unexceptional. Both phosphine and aryloxide ligands are observed, though the signals decay over several hours. When the solution colors have become pale or colorless, only resonances for Ph3P are visible in the 1H and 31P NMR spectra. No 31P NMR signals for coordinated phosphine were observed, perhaps due to some paramagnetic character of the decomposition products. In the 19F NMR spectrum of 1a, three signals for the ortho, meta, and para fluorine atoms are observed but the 3JFF coupling is not resolved, in contrast to the spectrum of compound 2 (vide infra). A singlet is observed for the CF3 groups in 1b. The 1H, 13C, and 19F NMR spectra of 2 show bound COD and OArF ligands and are unexceptional compared to other [(COD)PtX2] complexes with oxygen-donor ligands in the literature [57,71]. The NMR spectra for 3 are consistent with the solidstate structure shown in Fig. 4 and otherwise unremarkable. UV–Vis spectra for [(COD)PtCl2] and 2 are shown in Supplementary Figure 2. Consistent with previous comparisons of Cl versus OArF as ligands [13], the extinction coefficients increase significantly upon changing halide to aryloxide, but the spectra are largely similar. Increased absorbance above 300 nm is consistent with

M. Kim et al. / Polyhedron 24 (2005) 1803–1812

the pale yellow color of 2 versus the colorless starting material. UV–Vis spectra for 3 are compared with [(Ph3P)AuOPh] and [(Ph3P)AuCl] in Supplementary Figure 3. Again the aryloxide derivatives absorb with greater intensity than the halide, but no significant shifts of the kmax values are observed, suggesting these transitions do not involve exclusively the X ligand, but are largely p–p* transitions in the phosphine phenyl rings.

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data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2005.06.012.

References 4. Summary The late-transition metal complexes [(Ph3P)2Ni(OArF)2] (1a), [(Ph3P)2Ni(OAr 0 )2] (1b), and [(COD)Pt(OArF)2] (2) have been prepared and spectroscopically as well as structurally characterized. The compound [(Ph3P)Au(OArF)] (3) for which a preliminary synthetic report exists, was also structurally and spectroscopically characterized. These four compounds exemplify the conditions under which electronic modification of aryloxide ligands enables less common metal aryloxide bonds to be formed. For late transition metals that prefer softer, less donating ligands such as phosphorous or sulfur donors, these monodentate aryloxide compounds are unusual species. Among the four compounds, a perhydro analog is known only for [(Ph3P)Au(OArF)]. Attempts to make non-fluorinated analogs of the nickel and platinum compounds resulted in metal precipitation, insoluble products, or other evidence of metal reduction instead of complex formation. The balance of electronic effects from anionic X-type ligands and neutral L-type ligands in this study suggests that highly basic aryloxides promote ligand dissociation for weaker L donors, which can lead to metal reduction and complex decomposition.

Acknowledgments This work was supported by an NSF-CAREER award (CHE-0134817) and the donors of the Petroleum Research Fund administered by the American Chemical Society (PRF #38007-GB3). The Barnard College NMR facility is supported by the National Science Foundation (DUE-9952633). L.H.D. thanks C.C. Cummins for hospitality during a sabbatical when some of these data were collected.

Appendix A. Supplementary data Supplementary Figures 1–3 with UV–Vis spectra and crystallographic information files for 1a, 1b, 2, and 3 have been deposited with the Cambridge Crystallographic Data Centre, CCDC Nos. 249765, 249766, 249767, and 249768, respectively. Crystallographic data can be obtained free of charge via www.ccdc.cam.ac.uk/

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