Organometallics 2010, 29, 4579–4583 DOI: 10.1021/om1007049
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Solution Calorimetric Study of Ligand Exchange Reactions in the [Au(L)Cl] System (L = Phosphine and Phosphite) George C. Fortman and Steven P. Nolan* EastCHEM School of Chemistry, University of St. Andrews, St Andrews, KY16 9ST, U.K. Received July 16, 2010
The relative L-Au bond dissociation enthalpies of [Au(L)Cl] (L = tetrahydrothiophene (tht), PCynPh(3-n), [1,10 -biphenyl]-2-yldicyclohexylphosphine (Cy-JohnPhos), dicyclohexyl(20 ,60 -dimethoxy[1,10 -biphenyl]-2-yl)phosphine (SPhos), dicyclohexyl[20 ,40 ,60 -tris(1-methylethyl)[1,10 -biphenyl]-2-yl]phosphine (XPhos), P(OPh)3, P(O-2,4-tBu2Ph)3, P(OiPr)3, and the two enantiomers of 1,10 ,100 -[phosphinidynetris[oxy-(1-S/R)-[1,10 -binaphthalene]-20 ,2-diyl]] tricyclo[3.3.1.13,7]decane-1-carboxylic acid ester) are examined. These enthalpic values are examined in terms of the intrinsic steric and electronic properties of the ancillary ligand. Estimates of the absolute BDE for the investigated ligands are presented.
Introduction Homogenous catalysis using gold(I) complexes is an area of chemistry that has greatly matured over the past decade.1-7 Chemists have changed their view of gold and its catalytic potential and, as Hashmi so elegantly stated,8 no longer investigate the question of “how to make gold”, but “what to make with gold”. Indeed, gold catalysts have been reported to efficiently promote the carboxylation of acidic protons,9 the cycloisomerizations of enynes, the hydrogenation of C-C multiple bonds, the activation of sp, sp2, and sp3 C-H bonds, and the oxidation of alcohols and amines to name a selection of examples.1-5 Specifically, the 14-electron dicoordinate linear complexes of the form [Au(PR3)X] (X = halide, OH, NTf2 (Tf = trifluoromethanesulfonate)) have been found to be suitable catalytic precursors. Several studies have proposed a cationic species [Au(PR3)]þ as an active entity in catalytic reactions.10-14 These *To whom correspondence should be addressed. E-mail: snolan@ st-andrews.ac.uk. (1) Li, Z.; Brouwer, C.; He, C. Chem. Rev. 2008, 108, 3239–3265. ~ ez, E.; Echavarren, A. M. Chem. Rev. 2008, 108, (2) Jimenez-N un 3326–3350. (3) Gorin, D. J.; Sherry, B. D.; Toste, F. D. Chem. Rev. 2008, 108, 3351–3378. (4) Arcadi, A. Chem. Rev. 2008, 108, 3266–3325. (5) Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180–3211. (6) Belmont, P.; Parker, E. Eur. J. Org. Chem. 2009, 6075–6089. (7) F€ urstner, A. Chem. Soc. Rev. 2009, 38, 3208–21. (8) Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180–3211. (9) Boogaerts, I. I. F.; Nolan, S. P. J. Am. Chem. Soc. 2010, 132, 8858– 8859. (10) Teles, J. H.; Brode, S.; Chabanas, M. Angew. Chem., Int. Ed. 1998, 37, 1415–1418. (11) Nieto-Oberhuber, C.; Perez-Galan, P.; Herrero-G omez, E.; Lauterbach, T.; Rodrı´ guez, C.; Lopez, S.; Bour, C.; Rosellon, A.; C ardenas, D. J.; Echavarren, A. M. J. Am. Chem, Soc. 2007, 130, 269– 279. (12) Nieto-Oberhuber, C.; Mu~ noz, M. P.; Bu~ nuel, E.; Nevado, C.; C ardenas, D. J.; Echavarren, A. M. Angew. Chem., Int. Ed. 2004, 43, 2402–2406. (13) Mizushima, E.; Sato, K.; Hayashi, T.; Tanaka, M. Angew. Chem., Int. Ed. 2002, 41, 4563–4565. (14) Mizushima, E.; Hayashi, T.; Tanaka, M. Org. Lett. 2003, 5, 3349–3352. r 2010 American Chemical Society
observations have spearheaded our current efforts to better understand both the nature of the Au-P bond and the π-bonding interactions between the Au metal center and pendant biaryl ring associated with Buchwald phosphines that have been observed in the solid state.15 Unfortunately the cationic nature of the Au(I) active catalysts would result in difficulties in measuring the Au-P bond enthalpies of these complexes. As such, the authors were limited to probing the nature of the Au-P bond in neutral Au(I) complexes. Although a myriad of organic transformations utilizing gold(I) catalysts have been reported, there are scarce experimental data highlighting the thermodynamic aspects of the Au-L (L=two-electron donor) bond.16,17 Fundamental knowledge of this type can potentially lead to improvements in catalysis. Herein we report findings resulting from the measurement via solution calorimetry of the enthalpy of reaction (ΔHrxn) of several phosphorus donors with [Au(tht)Cl] (1) as shown in eq 1. The study includes a range of phosphines and phosphites (see Figure 1) that have been shown to lead to promising catalysis. CH2 Cl2
AuðthtÞCl þ PR3 s 30°C
AuðPR3 ÞCI þ tht þ ΔHrxn
ð1Þ
Finally, using previously reported estimations of the AuPPh3 bond dissociation enthalpy (BDE)17 allowed for the estimation of absolute Au-PR3 bond dissociation enthalpies of the phosphines and phosphites examined.
Results Calorimetric Measurements of the Reaction of [Au(tht)Cl] with Phosphorus Donors. The initial screening for a labile (15) Partyka, D. V.; Robilotto, T. J.; Zeller, M.; Hunter, A. D.; Gray, T. G. Organometallics 2007, 27, 28–32. (16) Ahrland, S.; Balzamo, S. Inorg. Chim. Acta 1988, 142, 285–289. (17) Borissova, A. O.; Korlyukov, A. A.; Antipin, M. Y.; Lyssenko, K. A. J. Phys. Chem. A 2008, 112, 11519–11522. Published on Web 10/01/2010
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Fortman and Nolan Table 1. Enthalpies (ΔHrxn) for the Reaction of [Au(tht)Cl] with PR3 and P(OR)3a ΔHrxnb (kcal/mol)
ligand tht PCy3 PCy2Ph PCyPh2 PPh3 Cy-JohnPhos SPhos XPhos
2a 2b 2c 2d 2e 2f 2g
0 -21.8 -20.1 -16.3 -14.6 -18.0 -17.6 -17.5
a Reactions were carried out in CH2Cl2 at 30 °C. b Uncertainties for all enthalpies of reaction are (0.5 kcal/mol.
Table 2. Enthalpies of Reaction (ΔHrxn) for Substitution of tht on [Au(tht)Cl] with P(OR)3a ΔHrxnb (kcal/mol)
ligand 4a 4b 4c 4d 4e
P(OPh)3 P(O-2,4-tBu2Ph)3 P(OiPr)3 PReetz (R) PReetz (S)
-10.3 -10.4 -15.8 -9.0 -8.9
a Reactions were carried out in CH2Cl2 at 30 °C. b Uncertainties for all enthalpies of reaction are (0.5 kcal/mol.
only the more classical phosphites but also a small sampling of more unique phosphites (i.e., PReetz (4d and 4e)). Interest in these phosphites buds first from their shear size and second due to the fact that they are enantiomers and have led to some asymmetric catalysis.20,21 Similar to the measurements of ΔHrxn of the phosphines, the phosphites cleanly formed the desire product [Au(P(OR)3Cl] (5) and tht. Quantitative product formation involving tertiary phosphines and phosphites was confirmed by 1H and 31P NMR taken after each calorimetric run.
Discussion Figure 1. Tertiary phosphine and phosphite ligands examined.
L-Au bond suitable for facile phosphine substitution was made using [Au(DMS)Cl] (DMS = dimethylsulfide). Unfortunately, the compound decomposes to elemental gold in solutions of CH2Cl2 at 30 °C. We therefore turned our attention to the tetrahydrothiophene (tht) derivative. The compound is much better behaved and is stable in solution for several hours. The reaction of [Au(tht)Cl] (1) with phosphines 2a-g in CH2Cl2 proceeds cleanly to yield [Au(L)Cl] (3) and tht.18 Standard reactions were carried out at 30 °C with the gold complex in excess.19 Reactions between 1 and P(OR)3 (4) were also investigated. To the best of our knowledge, no other thermodynamic data for reactions of phosphites with Au complexes of relevance exist. Once again we decided upon examining not (18) There were concerns as to the possible interaction of the free tht with the products, [AuLCl] (L = PR3). Calormetric measurements however of the reaction of [Au(PPh3)Cl] with tht resulted in enthalpies that were less than 0.4 kcal/mol. Therefore if any interaction does exist, it is expected to be negligeable but fall within the experimental error for measured enthalpies of reaction. (19) In the presence of excess phosphine several of the complexes were observed to coordinate two and sometimes three phosphines. Phosphine purity was examined by 1H and 31P NMR and were determined to be of a high purity. See Experimental Section for details.
Sterics and ΔHrxn for Tetrahydrothiophene (tht) Substitution. The relative enthalpies of reaction (ΔHrxn) are quite informative about the phosphine gold interactions when considered with the individual steric parameter of each phosphorus donor. Two separate methods were utilized for the steric parameters: the classical Tolman cone angle22 and the percent buried volume (%Vbur) model recently disclosed.23,24 The ΔHrxn, however, showed that sterics play only a fleeting role in the nature of the Au-P bond regardless of the method in which sterics are quantified. For example, XPhos, being the most sterically demanding of the phosphine ligands (cone angle = 256°; %Vbur = 53.1%), resulted in ΔHrxn = -17.5 ( 0.5 kcal/mol, while triphenylphosphine (cone angle = 145°; % Vbur = 29.9%) resulted in ΔHrxn = -14.6 ( 0.5 kcal/mol. Apparently, the dicoordination of gold provides ample room for the ligand to reside. In fact, Tolman in his seminal work in which he quantified electronic donor ability of phosphourus (20) Gonzalez, A. Z.; Toste, F. D. Org. Lett. 2010, 12, 200–203. (21) Reetz, M. T.; Guo, H.; Ma, J.-A.; Goddard, R.; Mynott, R. J. J. Am. Chem. Soc. 2009, 131, 4136–4142. (22) Tolman, C. A. Chem. Rev. 1977, 77, 313–348. (23) Poater, A.; Cosenza, B.; Correa, A.; Giudice, S.; Ragone, F.; Scarano, V.; Cavallo, L. Eur. J. Inorg. Chem. 2009, 2009, 1759–1766. (24) %Vbur values for 4c and 4e were calculated using ref 4 and the cif file from refs 20 and 32.
Article
ligands chose the unencumbered LNi(CO)3 (L = phosphorus ligand of interest) for its ability to accommodate sterically demanding ligands while only showing minimal steric interactions between L and Ni(CO)3. In the present case, the gold complexes have a coordination number two less than that of the nickel complexes and should be even less affected by ligand sterics. Recently, claims have been made regarding a stabilizing π-interaction between the Au metal center and the biphenyl ring of the Buchwald phosphines.15 The nature of the interaction was based on solid-state data. In solution, these interactions appear to be minimal at best. Comparison between enthalpies of reaction of PCy2Ph and those of the Buchwald phosphines show that the Buchwald phosphine complexes lead to enthalpies 2.1-2.6 kcal/mol less exothermic. As expected, both R and S enantiomers of the PReetz phosphite produced similar enthalpies of reaction. Reactions of 1 and P(OiPr)3 resulted in the most exothermic reaction among phosphites. Nevertheless, the enthalpies of reaction with the phosphites pale in comparison to those with the phosphines. The phosphite ligands also display no clear trend that relates the steric parameters with that of the determined ΔHrxn. While the PReetz phosphites are much larger than ligands 4a-c, they result in only slightly lower enthalpies of reactions. Conversely, P(OiPr)3, which is only marginally larger than P(OPh)3, results in ΔHrxn 5.4 kcal/mol more exothermic than triphenyl phosphite. Electronics and the Enthalpy of Tetrahydrothiophene Substitution. In contrast to the sterics of the phosphorus donors, the ΔHrxn seemed heavily dependent upon the Tolman electronic parameter (TEP) (see Table 3). TEP quantifies the amount of σ basicity in conjunction with π acidity of the ligand.22 As the TEP of the phosphorus donor decreases, the exothermicity of the substitution reaction increases. This trend can clearly be seen when comparing the series of PPhnCy(3-n) (n = 0-3). As the number of cyclohexyl rings increases and phenyl rings decrease, the phosphine becomes more electron donating and thusly results in a more exothermic ΔHrxn. This same correlation can also be seen for the phosphites. P(OiPr)3 being much more electron donating (TEP = 2075.9 cm-1) than P(OPh)3 (TEP = 2085.3 cm-1) results in a complex that is approximately 5.4 kcal/mol lower in energy (more strongly binding). Utilization of the observed trend that the ΔHrxn is dependent primarily on the electron-donating ability and not on the sterics of the ligand permits some insight into the electrondonating ability of the phosphorus donor ligands that have no determined TEP. Thusly, the Buchwald phosphines, XPhos, SPhos, and Cy-JohnPhos, appear to be between, in terms of electronics, the TEP of PCy2Ph and PCyPh2. They may in fact be very similar to that of PCy2Ph, but slight steric effects may interfere with the Au-P bond, decreasing ΔHrxn. This also suggests that the difference in reactivity11 of the Buchwald phosphines may possibly be the result of steric effects and not electronic ones, as the three examples here all have enthalpies of reaction that fall within 2 kcal/mol. Granted this is a very limited sampling, but it is most likely the case that changing the Cy groups here to, for example, iPr groups will have a more significant effect on the electronic nature of the Buchwald phosphine than changing the biaryl group. Using the same rationale for the phosphites, the Reetz phosphites and P(O-2,4-tBu2Ph)3 appear to be electronically similar to P(OPh)3 (TEP = 2085.3). All three of these phosphites result in ΔHrxn that are within ca. 1.5 kca/mol of one
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Table 3. Enthalpies of Reaction and Steric and Electronic Parameters of Phosphorus Donor Ligandsa Tertiary Phosphines
2a 2b 2c 2d 2e 2f 2g
ligand
ΔHrxnb
PCy3 PCy2Ph PCyPh2 PPh3 Cy-JohnPhos SPhos XPhos
-21.8 -20.1 -16.3 -14.6 -18 -17.6 -17.5
%Vbur (%)c
cone angle (deg)d
TEP (cm-1)d
33.4 32.7
170 159 153 145 226c 240c 256c
2056.4 2060.6 2064.8 2068.9
29.9 46.7 49.7 53.1
Tertiary Phosphites ligand
ΔHrxnb
%Vbur (%)c
cone angle (deg)d
TEP (cm-1)d
P(OPh)3 -10.3 31.9 128 2085.3 -10.4 32.3e P(O-2,4-tBu2Ph)3 i P(O Pr)3 -15.8 131 2075.9 PReetz (R) -9.0 PReetz (S) -8.9 47.8e a Reactions were carried out in CH2Cl2 at 30 °C. b All values (0.5 kcal/mol. c Obtained from ref 25. d Values obtained from ref 22. e Values calculated using ref 23 (see SI for details). 4a 4b 4c 4d 4e
another. Again sterics may play a small but relatively insignificant role in determining the nature of the Au-P(OR)3 bond. Derivation of Absolute Au-P Bond Dissociation Enthalpies. Lyssenko et al. have estimated17 the Au-PPh3 BDE to be 57.9 kcal/mol in the solid state via Espinosa’s correlation.26,27 Utilizing this information allows for the estimation of the absolute bond dissociation enthalpy of not only the phosphines and phosphites of interest but also the Au-S in complex 1. Scheme 1 shows the thermodynamic cycle used to derive a BDE of þ43 ( 8.0 kcal/mol for the Au-tht bond in complex 1. It should be noted that the exact nature of [Au(tht)Cl] in solution is unknown. In the solid state it has been shown to be an infinite chain with Au-Au interactions as shown in Figure 2.28b The distance between adjacent Au metals in 1 was determined to be 3.32 A˚. Also included in this work Ahrland et al. refer to experiments performed by Scmidbaur and coworkers in which they estimate the strength of Au-Au interactions in the dinuclear complex 5 (Figure 2), in which the Au-Au distance is 3.00 A˚, to be 7-8 kcal/mol. Ahrland further states that the longer Au-Au distances in 1 would suggest even weaker metal-metal interactions in [Au(tht)Cl] than those found in complex 5.28b As a result of the uncertainty associated with these Au-Au interactions, the BDE for S-Au in 1 may in fact be slightly lower than the value reported; also a larger uncertainty has been assigned to this value compared to those of the phosphorus donor complexes. (25) Clavier, H.; Nolan, S. P. Chem. Commun. 2010, 46, 841–866. (26) Espinosa, E.; Alkorta, I.; Rozas, I.; Elguero, J.; Molins, E. Chem. Phys. Lett. 2001, 336, 457–461. (27) Espinosa, E.; Molins, E.; Lecomte, C. Chem. Phys. Lett. 1998, 285, 170–173. (28) (a) The exact nature of [Au(THT)Cl] in solution is unknown. In the solid state, the complex is arranged in a maner in which there is an infinite chain of Au-Au interactions (see ref 28b.) The extent of Au-Au interactions in solution is expected to be much less, and the enthalpy of dissociation to the monomeric form is estimated as e8 kcal/mol. (b) Ahrland, S.; Dreisch, K.; Noren, B.; Oskarsson, A˚ Mater. Chem. Phys. 1993, 35, 281–289. (29) Schmidbaur, H.; Graf, W.; Muller, G. Angew. Chem., Int. Ed. Engl. 1988, 27, 417–419.
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Scheme 1. Thermodynamic Cycle Used to Derive the Au-tht BDE in [Au(tht)Cl]a
Table 4. Calculated Au-P Bond Dissociation Enthalpies Tertiary Phosphines ligand
a
Value for Ph3P-AuCl BDE obtained from ref 28.
2a 2b 2c 2d 2e 2f 2g
tht PCy3 PCy2Ph PCyPh2 PPh3 Cy-JohnPhos SPhos XPhos
ΔHrxna
derived Au-P BDE (kcal/mol)b
0 -21.8 -20.1 -16.3 -14.6 -18.0 -17.6 -17.5
43c 65 63 60 58d 61 61 61
Tertiary Phosphites ligand
ΔHrxna
derived Au-P BDE (kcal/mol)b
P(OPh)3 -10.3 54 -10.4 54 P(O-2,4-tBu2Ph)3 -15.8 59 P(OiPr)3 PReetz (R) -9.0 52 PReetz (S) -8.9 52 a All values (0.5 kcal/mol. b All values (5 kcal/mol. c Uncertainty of the measurement assigned as (8 kcal/mol.28 d Value obtained from ref 17 and used to calculate all other BDEs.
4a 4b 4c 4d 4e
Figure 2. Depiction of Au-Au interatctions of complexes 128b and 529 as determined by XRD.
Values of the BDE Au-PR3 estimations as determined using the reported BDE of Au-PPh3 in 2d17 are shown in Table 4. To our knowledge, no previous experimental Au-P BDE estimations (aside from that of the Ph3P-Au BDE determined by Leyssenko) are available for direct comparison. For simple comparison with literature data, Skinner et al. have estimated the Ph3P-Pd average BDE as 48.5 ( 2.4 kcal/mol in [Pd(PPh3)2Cl2] using microcalorimetric techniques.30 Presumably the significantly lower P-M BDE of [Pt(PPh3)2Cl2] as compared to [Au(PPh3)Cl] (58 ( 5.0 kcal/mol) can be attributed to [Pt(PPh3)2Cl2] being coordinatively more saturated in addition to the Pt being a more electron-rich metal center. Schwerdtfeger et al. have calculated the dissociation energy of PH3 from AuCl for [Au(PH3)Cl] as 53.2 kcal/mol.31 Tolman reports the TEP of PH3 as 2083.2 cm-1, which is much higher (i.e., less donating) than that of ligands 2a-g, which have much higher bond dissociation enthalpies. In fact, the TEP for PH3 is only approximately 2 cm-1 from that of P(OPh)3 and the P-Au BDE for [Au(P(OPh)3)Cl] is estimated to be very similar (54 ( 5 kcal/mol) to that of the calculated value for PH3.
Conclusions The enthalpies of reaction for a series of tertiary phosphines and phosphites with [Au(tht)Cl] have been experimentally measured by classical solution calorimetry. The (30) Takhin, G. A.; Skinner, H. A.; Zaki, A. A. J. Chem. Soc., Dalton Trans. 1984, 2323–2328. (31) Schwerdtfeger, P.; Hermann, H. L.; Schmidbaur, H. Inorg. Chem. 2003, 42, 1334–1342.
nature of the P-Au bond formed was found to be largely dependent upon the electronics parameters of the phosphorus donor, while the sterics of the L seem to play only a minor role in affecting enthalpy and BDE. In general, the phosphite ligands bind more weakly to the Au metal center than the phosphines. These measurements also suggest that, in solution, the biaryl ring of the Buchwald phosphines shows no significant sign of any stabilizing interactions with the neutral gold metal center of complexes 2e-g. The absolute BDE for the Au-P interaction was also estimated to be 58 to 65 ( 5 kcal/mol for the series of tertiary phosphines and 52 to 59 ( 5 kcal/mol for the series of tertiary phosphites studied. The BDE of the S-Au bond in [Au(tht)Cl] is estimated to be 43 ( 8 kcal/mol.
Experimental Section General Considerations. Dichloromethane was distilled over CaCl2 and degassed before use. [Au(tht)Cl] (1)32 and 1,10 ,100 [phosphinidynetris[oxy-(1S/R)-[1,10 -binaphthalene]-20 ,2-diyl]] tricyclo[3.3.1.13,7]decane-1-carboxylic acid ester (Reetz (R/S), 4d-e)21 were prepared using literature procedures. Other phosphines and phosphites were purchased from Sigma Aldrich. Their purity was determined by 1H and 31P NMR, and the phosphorus donors were recrystallized or distilled as needed to ensure a purity of >99%. [AuLCl] (L = PCynPh(3-n), [1,10 -biphenyl]2-yldicyclohexylphosphine (Cy-JohnPhos),11 dicyclohexyl(20 ,60 dimethoxy[1,10 -biphenyl]-2-yl)phosphine (SPhos), dicyclohexyl[20 ,40 ,60 -tris(1-methylethyl)[1,10 -biphenyl]-2-yl]phosphine (XPhos), P(OPh)3, P(O-2,4-tBu2Ph)3, P(OiPr)3, and the two diastereomers of 1,10 ,100 -[phosphinidynetris[oxy-(1-S/R)-[1,10 -binaphthalene]-20 , 2-diyl]] tricyclo[3.3.1.13,7]decane-1-carboxylic acid ester) complex formation was confirmed by comparison with reported literature NMR data.11,20,33,34 Solution calorimetric measurements were made using a Setaram C-80 calorimeter as previously described.35 1 H and 31P NMR were recorded using either a 300 or 400 MHz Bruker NMR instrument. (32) Uson, R.; Laguna, A.; Laguna, M.; Briggs, D. A.; Murray, H. H.; Fackler, J. P. Inorg. Synth. 1989, 26, 85–91. (33) Isab, A. A.; Fettouhi, M.; Ahmad, S.; Ouahab, L. Polyhedron 2003, 22, 1349–1354. (34) Maule on, P.; Zeldin, R. M.; Gonzalez, A. Z.; Toste, F. D. J. Am. Chem. Soc. 2009, 131, 6348–6349.
Article Solution Calorimetric Measurement of the Enthalpy of Reaction (ΔHrxn) of [Au(tht)Cl] with Phosphorus Donors. All measurements were performed in a similar manner and were based on highly pure ligands in the presence of excess [Au(tht)Cl]. A typical measurement was made as follows: in the glovebox 36.0 mg of [Au(tht)Cl] was dissolved in 1.5 mL of CH2Cl2. In addition, 1.0 mg of PPh3 was added to the solution to scavenge any potential reactive species. Then 0.75 mL of the solution (5.61 10-2 mmol [Au(tht)Cl]) was transferred to the bottom compartment of a Setaram C-80 reverse mixing cell via a Hamilton gastight syringe. In a different vial, 12.5 mg of PPh3 was dissolved in 1.0 mL of CH2Cl2. Then 300 μL of this solution (1.43 10-2 mmol of PPh3) was loaded into the top sample compartment of the same calorimeter cell. The cell was sealed, removed from the glovebox, and loaded into the calorimeter. After thermal equilibration, the reaction was initiated by inverting the calorimeter and monitored to completion at 30 °C. Following return to the baseline, the cell was returned to the (35) For examples of thermochemical studies using solution calorimetry see: (a) Nolan, S. P.; de la Vega, R.; Hoff, C. D. Organometallics 1986, 5, 2529–2537. (b) Rablen, P. R.; Hartwig, J. F.; Nolan, S. P. J. Am. Chem. Soc. 1994, 116, 4121–4122. (c) Luo, L.; Nolan, S. P. Organometallics 1994, 13, 4781–4786. (d) Huang, J.; Schanz, H.-J.; Stevens, E. D.; Nolan, S. P. Organometallics 1999, 18, 2370–2375.
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glovebox and its contents were examined by 1H and 31P NMR to confirm quantitative conversion to the desired product. The enthalpy of reaction (ΔHrxn) with all species in solution was measured as -14.6 ( 0.5 kcal/mol and is based on the average of four independent determinations. ΔHrxn for the other phosphorus donors were determined to be as follows: PCy3 = -21.8, PCy2Ph = -20.1, Cy-Johnphos = -18.0, SPhos = -17.6, XPhos = 17.5, PCyPh2 = -16.3, Reetz (R) = -9.0, Reetz (S) = -8.9, P(O2,4-tBu2Ph)3 = -10.4, and P(OiPr)3 = -15.8 kcal/mol. Standard deviations for repetitive measurements were typically less than 0.3 kcal/mol; however the largest was determined to be 0.5 kcal/ mol for PPhCy2. As such, all uncertainties for the measurements of ΔHrxn were assigned as (0.5 kcal/mol.
Acknowledgment. S.P.N. would like to thank the EPSRC and the ERC (Advanced Investigator Award, FUNCAT) for support of this work. The authors would also like to thank Dr. Xavier Bantreil for helpful discussions. S.P.N. is a Royal Society Wolfson Research Merit Award holder. Supporting Information Available: Details on the calculation of %Vbur. This material is available free of charge via the Internet at http://pubs.acs.org.
