Inorganic Fluorine Chemistry - American Chemical Society

Chapter 27 (Fluoroalkyl)phosphine Coordination

Chemistry

Dean M. Roddick and Richard C. Schnabel

Downloaded by PENNSYLVANIA STATE UNIV on May 2, 2013 | http://pubs.acs.org Publication Date: April 29, 1994 | doi: 10.1021/bk-1994-0555.ch027

Department of Chemistry, Box 3838, University of Wyoming, Laramie, WY 82071

A general review of (fluoroalkyl)phosphine coordination chemistry and recent results concerning the synthesis and reactivity patterns of dimeric rhodium and iridium complexes incorporating the chelate (C F ) PCH CH P(C F ) ("dfepe") are presented. [(dfepe)M(μ-Cl)] (M = Rh (1), Ir (2)) and [(dfepe)M(μ-O CCF )] (M = Rh (3), Ir (4)) are obtained in high yield from [(cod)M(μ-Cl)] and [(cod)M(μ-O2CCF )] , respectively. While 1, 2 and 3 are inert toward oxidative addition reactions, 4 reacts readily with H to form (dfepe)Ir(H)(μ-H)2(μ-OCCF3) (5). Thermolysis of 4 in neat cyclopentane at 150 °C affords a mixture of 5 and the alkane dehydrogenation product CpIr(dfepe) (6). Hydrogenolysis of (dfepe)Ir(η -C H5) (9) affords the dimeric tetrahydride (dfepe) Ir2(Η)(μ-H) (10), which in the presence of (r-butyl)ethylene also reacts with cyclopentane at 120 °C to give 6. 2

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One of the principal goals in coordination chemistry and homogeneous catalysis is to control metal reactivity through systematic variation of ligand properties. To date, most research has focused on structure/reactivity relationships involving sigma and pi-donor ligands such as phosphines (7,2), alkoxides (3-6), thiolates (7), amides (8-11), phosphides (12-14), and alkyls (75,76). In contrast, the possibilities inherent in similar π-acceptor ligand modification remain largely unexplored. This imbalance in approach is particularly significant in light of the important role π-acceptor ligands play in the chemistry of low valent metal complexes. The development of stable low coordinate metal systems incorporating bulky donor ligands has steadily expanded over the years and has no counterpart in acceptor ligand coordination chemistry. A graphical illustration of the steric range of typical acceptor and donor phosphine ligands is depicted in Figure 1. In comparison to the wide range of steric and electronic properties available to common donor phosphines (1), strong acceptor phosphine ligands are essentially limited to the sterically undemanding fluorophosphine, PF . This marked differ3

0097-6156/94/0555-0421$08.00/0 © 1994 American Chemical Society In Inorganic Fluorine Chemistry; Thrasher, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

422

INORGANIC FLUORINE CHEMISTRY: TOWARD THE 21ST CENTURY

ence between donor and acceptor phosphine size ranges is reflected more generally by the uniformly small size of the common π-acceptor ligands CO, NO , and CNR. While isonitriles can in principle incorporate a range of R group functionalities, direct steric influence on the coordination sphere is minimized by the linear nature of the M-C-N-R moiety. In addition, isonitriles have a tendency to undergo coup ling (77), insertion (18,19), and nucleophilic addition (79) side reactions that severely restrict their potential use as stable ancillary ligands. Because of these considerations, most efforts in acceptor ligand design have focused on modifying the basic PF framework (20). +


3

Figure 1.

Qualitative range of steric and electronic properties for some common phosphine ligands

A sterically and electronically tunable CO analogue would have a profound impact on low valent coordination chemistry. Recognizing this challenge, numerous workers have attempted to prepare new types of acceptor ligands and establish systems which complement or parallel known metal carbonyl chemistry. Nixon (20), King (27,22), and others (23) have noted in particular the lack of polydentate acceptor ligands and have investigated the coordination chemistry of RN(PF2)2 and relatedfluorophosphinechelates (24) in a variety of metal systems. Although these ligands in several cases do substitute completely for CO in a manner similar to PF , they suffer from a number of limitations due to P-F and P-N cleavage side reactions and a preference for binuclear-bridging coordination modes. Examples of other substituted fluorophosphines that have been reported include R PF _ (R = CF (25,26), OCH (25^7), (CF ) CHO (2829), (CF ) C(CN)0 (30), Me N (31), Ph (32)) as well as the chelates F PCH CH PF , F PCF CF PF , and F PC H PF (33 34). It is significant that the coordination chemistry of these ligands remains largely undeveloped. 3

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In Inorganic Fluorine Chemistry; Thrasher, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

2

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27.

(Fluoroalkyl)phosphine Coordination Chemistry 4

RODDICK & SCHNABEL

A promising class of ligands that has thus far received little attention is the fluorocarbon-substituted phosphines, (R ) PR . (35). Substitution of inductively similar fluorocarbon groups for fluorine in PF retains to a large degree the acceptor ability of the phosphorus center while at the same time providing the opportunity for steric manipulation of coordination properties. The greatest f

x

3

x

3


Acceptor Ability:

advantage (fluoroalkyl)phosphines have over fluorophosphines is that P-C and C-F bonds are considerably more inert than P-F bonds, and thus are much better suited for applications in coordination chemistry where highly reactive metal centers are involved. Although the prototypical fluorinated trialkylphosphine (CF ) P has been the subject of a number of theoretical (36-39) and spectroscopic studies (40-43), the only transition metal complexes that have been reported are ((CF ) P) Ni(CO)4_ (X = 1 - 3) (44), ((CF ) P)Ni(NO)(CO) (45), ((CF P) P) Fe(CO) . (X = 1 - 3) (46), and ((CF ) P)((CF ) P(0))PtCl (47). Several reports of secondary and primary (CF ) PX . (n = 1,2; X = H, halide) coordination chem istry have also appeared (48-50). A number of years ago, Burg reported the synthesis of the trifluoromethyl-substituted phosphine chelates (CF ) P(B)P(CF ) (B = O, S, NR) (51-53) and (CF ) PCX CX P(CF ) (X = H, F) (25,54). These ligands were demonstrated to be π-acceptors comparable to CO in strength, with binding affinities superior to that of (CF ) P. However, practical difficulties in the synthesis of the common precursor (CF ) PP(CF ) have forestalled the develop ment of trifluoromethylated phosphine coordination chemistry (55). We have recently developed an efficient synthetic route to the perfluoroethyl-substituted diphosphine (C F ) PCH CH P(C F ) (abbreviated "dfepe") (Scheme 1) (56). Using the readily available dichlorophosphine chelate C1 PCH CH PC1 , 60 gram quantities of dfepe are conveniently prepared in 3

3

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3 3

5 x

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PCI

^

—

3

•

(C F )3P 2

5

n-BuLi

C F C1 2

5

-95

°c

•

X

n

3

2

3

2

3

3

3

[Cy^Li] ( C ^ P C H ^ H ^ C ^ C1 P 2

PC1

2

dfepe" Scheme 1.


3

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greater than 85% yield; ( C y ^ P has been prepared analogouslyfromPC1 in 40% yield (unpublished results). An alternative synthetic procedure utilizing Schlenk techniques has recently been reported (57). Because of the greater thermal stability of the higher perfluoroalkyllithium homologues (58), this direct alkylation methodology may be easily extended to the synthesis of a broad array of (fluoroalkyl)phosphine ligands. 3


Physical and Coordination Properties of (Fluoroalkyl)phosphines. Perfluoroalkyl substituents induce profound changes in the chemical properties of the phosphorus center. In comparison to conventional donor phosphines, the proton affinity of the central Ρ atom is greatly reduced. Angelici has reported enthalpies of protonation (ΔΗρ) for a range of monodentate and bidentate phosphines with 0.1 M CF S0 H in 1,2-dichloroethane as a measure of phosphine basicity (59). A good linear correlation between the pK of the conjugate phosphonium ion R PH CF S0 ' and ΔΗ was found with pK 's varying in a pre dictable fashion based on the electron-donor ability of the phosphine substituent between (p-CF C H ) P (-1.32) and (*Bu) P (+11.4). For all phosphines examined, the protonation by CF S0 H was found to be rapid and quantitative. In contrast, no reaction is observed between excess CF S0 H and dfepe or ( C y ^ P in CH C1 as judged by *H and P NMR; extrapolation of a thermoneutral ΔΗ yields an estimated pK value of -9.0 as an upper limit for these phosphines. The low basicity of the (perfluoroaryl)phosphine (C F ) P, which is not protonated in neat H S0 , has previously been noted (60). The drastically-reduced basicity of the phosphorus lone pair in fluoroalkylphosphines is in accord with (CF ) P(CH ) . PES data, which show a 0.7 - 1.2 eV increase in lone pair ionization energy upon stepwise replacement of C H by CF (40). The low donor ability of (fluoroalkyl)phosphines suggested by PES data and protonation experiments is similarly indicated by comparative infrared studies of isostructural c«-(R P) Mo(CO) complexes. On the basis of IR data for group VI (L)M(CO) and cw-(R P) M(CO) (Table I) systems, the π-acceptor ligand ordering (CF ) P > F P > CO > C1 P > Me(CF ) P > Me (CF )P » Me P has been proposed (42). A recent ligand effect study of dfepe by Brookhart places this ligand between PCl Ph and PC1 in acceptor ability (68). Two features relevant to the coordination chemistry of fluorinated phosphine ligands are evident from the data in Table I. First, although the electronegativity of CF is considered to be less than that of fluorine (69), there is little variation in electronic properties between (CF ) PF _ ligands. This equivalence has been ascribed to a compensating interaction of the fluorine ρπ lone pairs with the phosphorus acceptor orbitals (61). Second, a comparison between (CF ) PCH CH P(CF ) and ( C y ^ P CH CH P(C F ) shows that substitution of C F by C F does not affect the acceptor ability of the phosphorus center. Thus, for the full steric range of fluo rine-modified phosphines (R ) PF . (R = CF , C F , C F ), the electronic properties should remain essentially constant. An additional measure of the electronic properties of dfepe is provided by comparative infrared and electrochemical studies of piano-stool complexes (n -C Rn)M(L) , summarized in Table Π. In general, substitution of dfepe for dppe or 3

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27. RODDICK & SCHNABEL


Table I. Infrared and Cone Angle Data for ris-(R P) Mo(CO)4 Complexes 3

phosphine

v(CO)(A?),cm-

ref

2P(CF )F 2P(CF ) F 2PF 2P(CF )

2094 2093 2091 2086 2082 2074 2066 2066 2064 2063 2044 2041 2033 2020 2012

61 61 61 62 55 34 63 62 56 55 62 64 65 66 66

3

2

3

2

3

3

3

EtN(PF2) Downloaded by PENNSYLVANIA STATE UNIV on May 2, 2013 | http://pubs.acs.org Publication Date: April 29, 1994 | doi: 10.1021/bk-1994-0555.ch027

2

(MeO) PCH CH P Ph PCH CH PPh Et PCH CH PEt 2

2

2

2

2

2

2

2

2

2

2

l

2

cone angle, deg* 115 126 104 137 120 98 94 131 129 120 124 151 100 125 115

b

b

Values taken from ref 1, except where noted. Value taken from ref 67.

two Ph P ligands results in roughly a one volt anodic shift in oxidation potential and an increase of v(CO) for monocarbonyls of about 100 cm . For (R P) M(CO) (M = Cr, Mo) compounds, substitution of R PCH CH PR (R = alkyl) by the perfluoroaryl chelate (C F ) PCH CH P(C F ) induces a 0.7 V anodic shift in potential (64). It is not surprising in view of these large redox shifts that (perfluoroalkyl)phosphine complexes generally exhibit increased air stability and enhanced electrophilic chemical properties (vide infra). Apart from their unique electronic and steric properties, fluorinated ligand complexes also exhibit unusually high volatilities due to reduced intermolecular Van der Waals attractive forces. Despite the significant increase in molecular weight relative to alkylphosphines, mixed perfluoroalkylphosphine carbonyl systems are typically liquids or readily sublimable low-melting solids (46,62,79). This trend is paralleled by the lower boiling points of the free phosphines (CF ) P (17 °C) and ( C F ^ F ^ P (70 °C) relative to their alkyl counterparts Me P (38 °C) and Et P (128 °C). The volatility imparted to metal complexes by fluorinated phosphine ligands is particularly intriguing and should prove advantageous in the design of CVD precursors for transition metal deposition (80). An additional interesting physical property associated with perfluorinated phosphine transition metal complexes is fluorocarbon solubility. In contrast to the generally poor solubility of complexes with hydrocarbon ligand substituents in fluorocarbon media, (dfepe)Cr(CO) (68) and ((CF ) P) Fe(CO) _ (46) are moderately soluble in perfluoroalkane solvents. While cw-(dfepe) RuH (81) is only sparingly soluble in most polar and nonpolar solvents, it is substantially more 3

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soluble in perfluorohexane (0.03M) than hexane (< 0.002M). The ability of fluoroalkylphosphine ligands to promotefluorocarbonsolubility offers a distinct advantage for the study of weak ligand interactions in non-coordinating media (68). Table Π. Comparison of Infrared and Electrochemical Data for (îl -arene)Mo(L)3, CpMn(L) , and CpRu(L)3 Complexes 6

+

3


complex

v(CO), c m

1

(C H Me)Mo(dfepe)(CO) (C H Me)Mo(dfepe)(py) (C H PPh2)Mo(dppe)(CO) (C H Me)Mo(py)

1922

CpMn(dfepe)(CO) CpMn(Me2NPF2) (CO) CpMn((PhO) P)(CO) CpMn(Ph P)(CO) CpMn(Ph P) (CO)

1933 1920 1961,1893 1931,1863 1824

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1817

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CpRu(dfepe)(CO) CpRu(MeCN) (CO) CpRu(Ph P) (CO) CpRu(Me P) (CO)

+

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Ει(οχ), V (vs. SCE)

ref

+1.06 +0.37 +0.15 -1.09

67 67 70 71

+1.09

72 73 74 74 74

+0.98 +0.65 -0.21

2075 2000 1980 1961

75 76 77 78

To date, much of our fluoroalkylphosphine work has focused on the coordination chemistry of the dfepe chelate. Substitution of sterically demanding dfepe for small monodentate carbonyl ligands should maintain the electron-poor nature of the metal center and provide unprecedented access to stable low coordinate electrophilic metal systems with unusual structural and electronic properties. As a useful qualitative guideline, (dfepe)ML complexes are conceptually viewed as chelating electronic mimics to (CO) ML complexes with steric properties equal to or exceeding that of (dppe)ML donor phosphine analogues. n

2

n

n

Group IX Reactivity Studies. The coordination chemistry of the cobalt triad has received considerable attention in the areas of organometallic reaction mechanisms and homogeneous catalysis (82-84). Of particular interest have been processes such as hydrogénation, carbonylation, and hydroformylation, where the oxidative addition of substrate C-X or H-X bonds to coordinatively-unsaturated M(I) centers plays a critical role. Systems which have received the most attention thus far have been electron-rich


27.


RODDICK & SCHNABEL

'n

versus

or

(Rf) P 2


+

alkylphosphine or mixed phosphine-carbonyl complexes such as (R P) M(solv) , (R P) MX, or (R P) M(CO)X (X = H or halide). In contrast, comparatively little is known regarding the chemistry of the related electron-poor carbonyls, (CO) MX (n = 2-4). In comparison to well-studied (CO) CoH, which has marginal stability, the heavier metal homologues (CO) MH (M = Rh, Ir) rapidly aggregate to form binary carbonyl clusters M (CO) in the absence of high CO pressures (85,86). Although simple carbonyl halides such as [(00) Μ(μ-Ο)] and (CO) IrCl are known, much of the chemistry reported for these compounds concerns their use as synthetic precursors to carbonyl clusters and ligand addition products (87). Our interest in the development of (fluoroalkyl)phosphine analogues to Group IX carbonyl systems stems from our work on the dehydrogenation of cycloalkanes by (dfepe) RuH (81). Since the extreme conditions (200 °C, days) required for dehydrogenation by this complex are attributable to its substitutionally-inert octahedral d 18 e" configuration, preparative routes to coordinatively-unsaturated 16 e" square-planar rhodium and iridium complexes have been examined. Alkane dehydrogenation is well precedented for Group IX systems (88-94). For hydrogen transfer systems reported by Crabtree (90) and Goldman (93) (equations 1 and 2), 14 e- reactive intermediates have been proposed. Accordingly, initial efforts have been directed toward the synthesis of rhodium and iridium dfepe derivatives which could readily access similar 14 e" species. 3

3

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12

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+ (R P) IrH (0 CCF ) 3

Ο Α +

2

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3

Ο*A «


428


[(οοά)Μ(μ-0)] (M = Rh, Ir) compounds provide a convenient entry to dfepe systems (95). Direct substitution of cyclooctadiene by dfepe occurs under mild conditions to afford the μ-chloro dimers [(dfepe)M^-Cl)] (M = Rh, 1; Ir, 2) in high yield as air-stable crystalline solids (equation 3). X-ray diffraction studies reveal that 1 and 2 have isostractural hinged Μ (μ-0) core geometries. Unlike donor phosphine derivatives, the chloride bridges are quite robust and do not cleave in the presence of excess dfepe to form (dfepe) MCl. In keeping with the expected electron-poor nature of these compounds, no oxidative additions of substrates such as CH I, 0 , or H to the Ir(I) center of 2 have been observed. Although the thermal stability of Ίι(ΐ) phosphine complexes is often limited by 2

2

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2

2


3

2

2

,C1 x^Cl^/t ^ 2 F)5 )^P-"'7^ p..^M^ ) (C C""pΡ(0>F 5>2

2dfepe

[(cod)M(μ-Cl)]

•^

—

2

ρ

C

(M = Rh,Ir)

( C 2

Vp

5

2

(3)

p

Inorganic Fluorine Chemistry - American Chemical Society

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