Organometallics 2010, 29, 3593–3601 DOI: 10.1021/om100510w
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Probing Cyanocuprates by Electrospray Ionization Mass Spectrometry Aliaksei Putau and Konrad Koszinowski* Department Chemie, Ludwig-Maximilians-Universit€ at M€ unchen, Butenandtstrasse 5-13, 81377 M€ unchen, Germany Received May 25, 2010
Solutions of CuCN/(RLi)n (n =0.5, 0.8, 1.0, and 2.0 and R = Me, Et, nBu, sBu, tBu, and Ph) in tetrahydrofuran are analyzed by electrospray ionization mass spectrometry. In all cases, organocuprate anions are observed, whose exact nature depends on the reagent stoichiometry. While cyanide-free Lin-1CunR2n- anions completely predominate for CuCN/(RLi)2 solutions, cyanidecontaining Lin-1CunRn(CN)n- complexes prevail for CuCN/(RLi)n reagents with n e 1. Mixing studies with CuCN/MeLi/RLi as well as gas-phase fragmentation experiments of the mass-selected anions provide further insight into the relative stability and the likely structural composition of polynuclear cuprates. The thus obtained information not only agrees quite well with previous results from spectroscopic and crystallographic work but moreover reveals clear trends of how different organyl substituents influence the aggregation and ion-pairing behavior of cyanocuprates. In addition, the observation of Cu(þ2)-containing cyanocuprate complexes for CuCN/(RLi)n solutions, with n e 1 and R = nBu and sBu, indicates the intrinsic stability of these unprecedented species in the diluted gas phase.
1. Introduction Lithium organocuprates are known as highly valuable and versatile reagents in synthetic organic chemistry.1 Among the different variants of organocuprates, the cyanocuprates2 are arguably the most popular ones. These reagents easily form by transmetalation of CuCN with organolithium compounds RLi, eq 1, and find application in conjugate additions,3-5 carbocuprations of alkynes,3a,6 epoxide-opening reactions,3 and nucleophilic substitutions of alkyl halides3,4,7 and sulfonates.3a RLi
RLi
CuCN sf LiCuRðCNÞ sf LiCuR2 3 LiCN
ð1Þ
The high reactivity of cyanocuprates has provoked numerous mechanistic and structural investigations.8-10 In particular, the binding site of the cyanide anion has been discussed controversially. Originally, Lipshutz and co-workers postulated the formation of so-called higherorder diorganocuprates Li2CuR2(CN), in which the CNions coordinate to the Cu centers.3,11 On the basis of 13C NMR and X-ray absorption spectroscopic measurements as well as on theoretical calculations, respectively, Bertz12 and others13 challenged this view and instead proposed the existence of lower-order diorganocuprates LiCuR2 3 LiCN. These species resemble traditional Gilman-type cuprates, with CN- bound to Liþ. X-ray crystallographic studies
*To whom correspondence should be addressed. E-mail: konrad.
[email protected]. (1) The Chemistry of Organocopper Compounds; Rappoport, Z.; Marek, I., Eds.; Wiley: Hoboken, 2009. (2) (a) Gorlier, J.-P.; Hamon, L.; Levisalles, J.; Wagnon, J. J. Chem. Soc., Chem. Commun. 1973, 88. (b) Mandeville, W. H.; Whitesides, G. M. J. Org. Chem. 1974, 39, 400–405. (c) Koosha, K.; Berlan, J.; Capmau, M.-L.; Chodkiewicz, W. Bull. Soc. Chim. Fr. 1975, 1284–1290. (d) Acker, R.-D. Tetrahedron Lett. 1977, 18, 3407–3410. (e) Four, P.; Riviere, H.; Tang, P. W. Tetrahedron Lett. 1977, 18, 3879–3882. (3) (a) Lipshutz, B. H.; Wilhelm, R. S.; Kozlowski, J. A. Tetrahedron 1984, 40, 5005–5038. (b) Lipshutz, B. H. Synthesis 1987, 325–341. (4) Krause, N.; Gerold, A. Angew. Chem. 1997, 109, 194–213; Angew. Chem., Int. Ed. Engl. 1997, 36, 186-204. (5) (a) Polet, D.; Alexakis, A. In The Chemistry of Organocopper Compounds; Rappoport, Z.; Marek, I., Eds.; Wiley: Hoboken, 2009, pp 693€ In The Chemistry of Organocopper 730. (b) Krause, N.; Aksin-Artok, O. Compounds; Rappoport, Z.; Marek, I., Eds.; Wiley: Hoboken, 2009, pp 857-879. (6) Chemla, F.; Ferreira, F. In The Chemistry of Organocopper Compounds; Rappoport, Z.; Marek, I., Eds.; Wiley: Hoboken, 2009; pp 527584. (7) Spino, C. In The Chemistry of Organocopper Compounds; Rappoport, Z.; Marek, I., Eds.; Wiley: Hoboken, 2009; pp 603-691.
(8) (a) Nakamura, E.; Mori, S. Angew. Chem. 2000, 112, 3902–3924; Angew. Chem., Int. Ed. 2000, 39, 3750-3771. (b) Nakamura, E.; Yoshikai. In The Chemistry of Organocopper Compounds; Rappoport, Z.; Marek, I., Eds.; Wiley: Hoboken, 2009; pp 1-21. (9) (a) Gschwind, R. M. Chem. Rev. 2008, 108, 3029–3053. (b) G€artner, T.; Gschwind, R. M. In The Chemistry of Organocopper Compounds; Rappoport, Z.; Marek, I., Eds.; Wiley: Hoboken, 2009; pp 163-215. (10) Van Koten, G.; Jastrzebski, J. T. B. H. In The Chemistry of Organocopper Compounds; Rappoport, Z.; Marek, I., Eds.; Wiley: Hoboken, 2009; pp 23-143. (11) (a) Lipshutz, B. H.; Wilhelm, R. S.; Floyd, D. M. J. Am. Chem. Soc. 1981, 103, 7672–7674. (b) Lipshutz, B. Synlett 1990, 119–128. (c) Lipshutz, B. H.; Sharma, S.; Ellsworth, E. L. J. Am. Chem. Soc. 1990, 112, 4032–4034. (d) Lipshutz, B. H.; James, B. J. Org. Chem. 1994, 59, 7585–7587. (12) (a) Bertz, S. H. J. Am. Chem. Soc. 1990, 112, 4031–4032. (b) Snyder, J. P.; Bertz, S. H. J. Org. Chem. 1995, 60, 4312–4313. (c) Bertz, S. H.; Miao, G.; Eriksson, M. Chem. Commun. 1996, 815–816. (13) (a) Stemmler, T.; Penner-Hahn, J. E.; Knochel, P. J. Am. Chem. Soc. 1993, 115, 348–350. (b) Snyder, J. P.; Spangler, D. P.; Behling, J. R. J. Org. Chem. 1994, 59, 2665–2667. (c) Barnhart, T. M.; Huang, H.; PennerHahn, J. E. J. Org. Chem. 1995, 60, 4310. (d) Stemmler, T. L.; Barnhart, T. M.; Penner-Hahn, J. E.; Tucker, C. E.; Knochel, P.; B€ohme, M.; Frenking, G. J. Am. Chem. Soc. 1995, 117, 12489–12497. (e) Mobley, T. A.; M€uller, F.; Berger, S. J. Am. Chem. Soc. 1998, 120, 1333–1334.
r 2010 American Chemical Society
Published on Web 07/21/2010
pubs.acs.org/Organometallics
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Scheme 1. Proposed Structures of Organocopper Species Present in Solutions of Cyanocuprates LiCuR2 3 LiCN in Ethereal Solventsa
a
For 1 and 2, coordinating solvent molecules are omitted for clarity.
confirmed the lower-order nature of cyanocuprates,14,15 which since then has been generally accepted.16 After the end of this dispute, the question of the aggregation state of cyanocuprates received increasing attention. For solutions of LiCuR2 3 LiCN in diethyl ether, extensive NMR spectroscopic experiments by Gschwind and collaborators point to the predominance of dimeric contact ion pairs 1 (Scheme 1),17 which presumably form even larger, chainlike oligomers.9,18 In contrast, the situation is less clear for solutions of LiCuR2 3 LiCN in tetrahydrofuran (THF). IR19 and X-ray absorption20 spectroscopic experiments indicate the presence of the contact ion pair 2 in THF (R = Me), which is also consistent with the results of cryoscopic measurements.21 Moreover, the predominance of 2 was inferred from 15N NMR spectroscopic studies of LiCunBu2 3 LiCN in THF.22 However, Gschwind, Boche, and co-workers detected only very small 1H,6Li HOESY NMR cross signals for LiCuR2 3 LiCN in THF (R = Me, CH2SiMe3) and thus concluded that these cyanocuprates preferentially form solvent-separated ion pairs, i.e., CuR2-/Li(THF)4þ (3), in this relatively strongly coordinating solvent; the small cross signals observed were assigned to minor equilibrium populations of the dimeric contact ion pair 1.9,17a,b The assumed preponderance of solvent-separated ion pairs in THF seems to be in line with X-ray crystallographic data9,17a,b and can also rationalize the relative rates of conjugate addition reactions,12c,23 for which the participation of lithium centers is considered essential.24 Yet, (14) Boche, G.; Bosold, F.; Marsch, M.; Harms, K. Angew. Chem. 1998, 110, 1779–1781; Angew. Chem., Int. Ed. 1998, 37, 1684-1686. (15) Kronenburg, C. M. P.; Jastrzebski, J. T. B. H.; Spek, A. L.; van Koten, G. J. Am. Chem. Soc. 1998, 120, 9688–9689. (16) Krause, N. Angew. Chem. 1999, 111, 83–85; Angew. Chem., Int. Ed. 1999, 38, 79-81. (17) (a) Gschwind, R. M.; Rajamohanan, P. R.; John, M.; Boche, G. Organometallics 2000, 19, 2868–2873. (b) John, M.; Auel, C.; Behrens, C.; Marsch, M.; Harms, K.; Bosold, F.; Gschwind, R. M.; Rajamohanan, P. R.; Boche, G. Chem.—Eur. J. 2000, 6, 3060–3068. (c) Gschwind, R. M.; Xie, X.; Rajamohanan, P. R.; Auel, C.; Boche, G. J. Am. Chem. Soc. 2001, 123, 7299–7304. (18) (a) Xie, X.; Auel, C.; Henze, W.; Gschwind, R. M. J. Am. Chem. Soc. 2003, 125, 1595–1601. (b) Henze, W.; Vyater, A.; Krause, N.; Gschwind, R. M. J. Am. Chem. Soc. 2005, 127, 17335–17342. (19) Huang, H.; Alvarez, K.; Cui, Q.; Barnhart, T. M.; Snyder, J. P.; Penner-Hahn, J. E. J. Am. Chem. Soc. 1996, 118, 8808-8816; correction: J. Am. Chem. Soc. 1996, 118, 12252. (20) Huang, H.; Liang, C. H.; Penner-Hahn, J. E. Angew. Chem. 1998, 110, 1628–1630; Angew. Chem., Int. Ed. 1998, 37, 1564-1566. (21) Gerold, A.; Jastrzebski, J. T. B. H.; Kronenburg, C. M. P.; Krause, N.; van Koten, G. Angew. Chem. 1997, 109, 778–780; Angew. Chem., Int. Ed. Engl. 1997, 36, 755-757. € Snyder, J. P. Angew. (22) Bertz, S. H.; Nilsson, K.; Davidsson, O.; Chem. 1998, 110, 327–331; Angew. Chem., Int. Ed. 1998, 37, 314-317. (23) (a) Bertz, S. H.; Eriksson, M.; Miao, G.; Snyder, J. P. J. Am. Chem. Soc. 1996, 118, 10906–10907. (b) Bertz, S. H.; Chopra, A.; Ogle, C. A.; Seagle, P. Chem.—Eur. J. 1999, 5, 2680–2691. (24) (a) Krauss, S. R.; Smith, S. G. J. Am. Chem. Soc. 1981, 103, 141– 148. (b) Nakamura, E.; Mori, S.; Morokuma, K. J. Am. Chem. Soc. 1997, 119, 4900–4910. (c) Mori, S.; Nakamura, E. Chem.—Eur. J. 1999, 5, 1534–1543.
Putau and Koszinowski
it apparently is in conflict with the results of the earlier IR, X-ray absorption, cryoscopic, and 15N NMR spectroscopic experiments. The discrepancies in the literature reflect the inherent difficulties of determining the aggregation state of cyanocuprates in solution by spectroscopic methods, which probe this quantity only in a rather indirect manner. As an alternative and possibly more direct approach to identify the nuclearity of cuprate anions, Lipshutz et al. therefore employed electrospray ionization (ESI) mass spectrometry.25 This method26,27 feeds the diluted sample solution through a capillary, from which small charged droplets are produced upon action of a high voltage and a stream of heated nitrogen gas. While these charged droplets are accelerated toward the entrance of the mass spectrometer, they shrink by the evaporation of solvent molecules until the concentration of excess charges reaches a critical value. At this point, some of the ions carrying the excess charge are released from the nanodroplets and propelled into the high-vacuum system of the mass spectrometer, thus becoming available for detection (ion evaporation mechanism).28,29 With this method, Lipshutz et al. succeeded in the observation of a multitude of inorganic25a and organometallic cuprate anions, the latter bearing 2-thiophenyl, alkynyl, and (trimethylsilyl)methyl substituents.25b However, analogous experiments probing the more sensitive methyl- and n-butylcuprate anions were reported to be unsuccessful.25b Because of the apparent difficulties in producing such nonstabilized organocuprates by direct ESI, O’Hair and co-workers chose to prepare these species from gaseous precursor ions.30 In this way, these authors generated many different mononuclear diorganocuprate anions CuR1(R2)- and investigated the gas-phase reactivity of selected examples.30 These studies offer detailed insight into the intrinsic reactivity of organocuprate anions. In contrast to the direct ESI approach pursued by Lipshutz and co-workers, O’Hair’s gas-phase preparation does not provide any polynuclear organocuprate ions and thus cannot be used for investigating aggregation effects. Encouraged by the recent observation of organozincate31 and organoindate anions32 by direct ESI mass spectrometry, we have revisited the application of this method to the analysis of cyanocuprates in THF. We have found that under (25) (a) Lipshutz, B. H.; Stevens, K. L.; James, B.; Pavlovich, J. G.; Snyder, J. P. J. Am. Chem. Soc. 1996, 118, 6796–6797. (b) Lipshutz, B. H.; Keith, J.; Buzard, D. J. Organometallics 1999, 18, 1571–1574. (26) (a) Yamashita, M.; Fenn, J. B. J. Phys. Chem. 1984, 88, 4451– 4459. (b) Yamashita, M.; Fenn, J. B. J. Phys. Chem. 1984, 88, 4671–4675. (27) For selected reviews on the application of ESI mass spectrometry to the analysis of organometallic species, see: (a) Plattner, D. A. Int. J. Mass Spectrom. 2001, 207, 125–144. (b) Chen, P. Angew. Chem. 2003, 115, 2938–2954; Angew. Chem., Int. Ed. 2003, 42, 2832-2847. (c) Henderson, W.; McIndoe, J. S. Mass Spectrometry of Inorganic, Coordination and Organometallic Compounds: Tools, Techniques, Tips; Wiley: Chichester, 2005; pp 175-219. (d) M€uller, C. A.; Markert, C.; Teichert, A. M.; Pfaltz, A. Chem. Commun. 2009, 1607–1618. (28) (a) Iribarne, J. V.; Thomson, B. A. J. Chem. Phys. 1976, 64, 2287–2294. (b) Thomson, B. A.; Iribarne, J. V. J. Chem. Phys. 1979, 71, 4451–4463. (29) (a) Cole, R. B. J. Mass Spectrom. 2000, 35, 763–772. (b) Kebarle, P. J. Mass Spectrom. 2000, 35, 804–817. (30) (a) James, P. F.; O’Hair, R. A. J. Org. Lett. 2004, 6, 2761–2764. (b) Rijs, N.; Khairallah, G. N.; Waters, T.; O'Hair, R. A. J. J. Am. Chem. Soc. 2008, 130, 1069–1079. (c) Rijs, N. J.; Yates, B. F.; O'Hair, R. A. J. Chem.— Eur. J. 2010, 16, 2674–2678. (31) (a) Koszinowski, K.; B€ ohrer, P. Organometallics 2009, 28, 100– 110. (b) Koszinowski, K.; B€ohrer, P. Organometallics 2009, 28, 771–779. (c) Fleckenstein, J. E.; Koszinowski, K. Chem.—Eur. J. 2009, 15, 12745– 12753. (32) Koszinowski, K. J. Am. Chem. Soc. 2010, 132, 6032–6040.
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carefully optimized experimental conditions (see below) nonstabilized cuprate anions can indeed be observed, and we have systematically studied solutions of CuCN/(RLi)n, with n = 0.5, 0.8, 1.0, and 2.0 and R = Me, Et, nBu, sBu, tBu, and Ph. From the results of these experiments, we draw conclusions on the ionic constituents of the sampled solutions and compare these with the hypotheses proposed in the literature. In doing so, we are fully aware of the fact that the detected ions do not directly stem from the sampled solution but rather from the nanodroplets formed during the ESI process. Compared to the former, the increased concentration, the higher surface-to-volume ratio, and the deviating temperature of the nanodroplets may lead to a readjustment of the relevant equilibria. Therefore, we do not claim that the observed aggregation states are necessarily identical to those of the cyanocuprates in solution, but instead we focus on the trends derived from the comparison of the different systems probed under constant experimental conditions. In addition, we also investigate the fragmentation of the mass-selected cuprate anions in the gas phase. These experiments help to clarify the constitution of the detected aggregates and provide indirect information on the relative stability of the neutral organocopper fragments.
2. Experimental Section 2.1. Sample Preparation. Standard Schlenk techniques were used in all cases. THF was distilled from sodium/benzophenone. CuCN was dried by repeated heating under high vacuum at 350 °C. Solutions of organolithium compounds RLi were used as purchased: MeLi (1.49 M) in Et2O, EtLi (0.42 M) in benzene/ cyclohexane (90:10), nBuLi (2.37 M) in hexane, sBuLi (1.58 M) in cyclohexane, tBuLi (1.88 M) in pentane, and PhLi (1.74 M) in n Bu2O. Their exact concentrations were determined by titration of 1,3-diphenyl-2-propanone tosylhydrazone.33 Solutions of CuCN/(RLi)n were prepared by adding RLi to suspensions of CuCN in THF under argon at -78 °C. After stirring at low temperature for 1 h, the CuCN completely dissolved for CuCN/(RLi)n, n = 1 and 2, which indicated the formation of LiCuR(CN) and LiCuR2 3 LiCN, respectively. In contrast, some solid CuCN remained if n = 0.5 or 0.8 equiv of RLi was employed. Aliquots of the resulting solutions (of typical concentrations c = 25 mM) were then transferred into a gastight syringe and introduced into the ESI source of a mass spectrometer. Sample solutions of LiCuR2 3 LiCN showed relatively high macroscopic stabilities in the syringe held at room temperature. In contrast, solutions of LinCuRn(CN), n e 1, decomposed in e10 min and produced black or greenish precipitates, which then caused clogging of the inlet line connecting the syringe with the ESI source. To avoid this problem, sample solutions of LinCuRn(CN), n e 1, had to be analyzed as quickly as possible. 2.2. ESI Mass Spectrometry. Preliminary experiments compared the performance of a TSQ 7000 multistage mass spectrometer (Thermo-Finnigan)31a with that of an HCT quadrupole ion trap (Bruker Daltonik).31c,32 While the latter showed clear differences between solutions of LiCuR2 3 LiCN and LiCuR(CN), R = nBu and Ph (see below), the former did not and invariantly produced ions with R/Cu ratios e 1 (Figures S1-S4, Supporting Information). Apparently, the experiments with the TSQ 7000 instrument suffered from the occurrence of hydrolysis and/or oxidation reactions, which presumably resulted from an imperfect insulation of the spray from the ambient atmosphere (the predominance of Cu2R2(CN)- and the deficiency of ions in higher aggregation states furthermore point to fragmentation (33) Lipton, M. F.; Sorensen, C. M.; Sadler, A. C.; Shapiro, R. H. J. Organomet. Chem. 1980, 186, 155–158.
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during the ESI process). Therefore, all further experiments employed the HCT ion trap and probed samples of c = 25 mM at flow rates of 1-4 mL h-1. With these settings, hydrolysis and/or oxidation reactions could be suppressed almost completely, whereas products of such degradation reactions were observed for samples of lower concentrations administered at lower flow rates. The improved performance of the HCT ion trap presumably largely results from the design of its (commercial) ESI source, which tightly insulates the spray from the ambient atmosphere. To prevent moisture from entering the source, we also kept it closed between experiments and constantly purged it with dry N2 gas heated to 300 °C. The ESI source of the HCT ion trap was operated with N2 as sheath gas (10 L min-1), an ESI voltage of (3 kV, and N2 as drying gas (5 L min-1). While the latter usually is heated to θ g 200 °C to facilitate desolvation during the ESI process, in the present study we decreased its temperature to 60 °C and still obtained sufficiently high signal intensities. The lower temperature was chosen in order to minimize the risk of thermal degradation of the analyte. Note that solutions of cyanocuprates are typically held below room temperature to avoid thermal decomposition. Higher temperatures are tolerated during the short time of the ESI process (τ , 1 s) because excess energy is readily released by solvent evaporation. Control experiments for LiCunBu2 3 LiCN sample solutions indeed showed the absence of thermal degradation reactions for dryinggas temperatures up to 300 °C. The thus produced ions then passed a capillary, a skimmer, and two transfer octopoles before entering the quadrupole ion trap. Varying the voltage offsets of the capillary exit (Figure S5) and the two transfer octopoles (Figures S6 and S7) had significant effects. For higher absolute voltages, the ratio I(Li2Cu3nBu6-)/I(CunBu2-) strongly decreased because of fragmentation reactions due to energetic collisions with residual gas, as was proven by deliberate fragmentation of mass-selected Li2Cu3nBu6- (see below). To avoid these unwanted decomposition reactions, low absolute voltages (V(capillary exit) = (20 V, V(skimmer) = (20 V, V(Oct 1 DC) = (5 V, V(Oct 2 DC) = (1.7 V) were applied consistently. Very similar conditions had been successfully used for the detection of sensitive allyl indium ions.32 The quadrupole ion trap itself was filled with helium (Air Liquide, 99.999% purity, estimated pressure p(He) ≈ 2 mTorr) and operated at a trap drive level of 20. This low value was chosen on purpose to avoid unwanted fragmentation reactions resulting from too high a kinetic excitation of the trapped ions. At the same time, the trap drive level also affects the relative efficiency of ion ejection toward the detector and thereby discriminates against either light or heavy ions (Figure S8). While the constant trap drive level applied in all experiments ensures the comparability of relative signal intensities for different experiments, it is obvious that no rigorous quantitation independent of mass discrimination is possible. To avoid the impression of unrealistic accuracy, we therefore deliberately do not give numerical values for the observed signal intensities. The ions observed were identified on the basis of their m/z ratios, their isotope patterns (see, e.g., Figures S9 and S10), and their fragmentation behavior (see below). In the case of LiCuMe2 3 LiCN and LiCuMe(CN), we also employed deuterium labeling as an additional test. Typically, m/z ranges of 50-1000 were scanned. 2.3. Gas-Phase Fragmentation Reactions. For gas-phase fragmentation experiments, ions were mass-selected with mass windows of 1-2 amu, subjected to excitation voltages of amplitudes Vexc, and allowed to collide with He gas. Note that the low-mass cutoff of the ion trap prohibits the detection of fragment ions whose m/z ratio is e27% of the parent ion. In the particular case of Li2Cu3sBu6-, the probed ion resulted from a sample solution of LiCusBu2 3 LiCN in diethyl ether, which was prepared in a way analogous to the LiCuR2 3 LiCN samples in THF.
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Putau and Koszinowski Table 2. Organocuprate Anions Observed upon ESI of THF Solutions of LiCu(Me)R 3 LiCN in High (þþ) and Medium (þ) Relative Abundance R= entry 1 2 3 4 5 6 7 8 9
Lin-1CunMe2n-xRx-
n
x
1 2
2 3 4 1 2 3 4 5 6
3
Et
n
Bu
þþ
þþ þþ þ
þ þþ þþ þ
s
t
Bu
Ph
þþ
þþ þ þ
þþ
Bu
þ þ þ
þ þþ þ þ
Figure 1. Anion-mode ESI mass spectrum of a 25 mM solution of LiCuMe2 3 LiCN in THF, a = Li3Cu4Me8-x(OH)x-, b = Li5Cu6Me12-x(OH)x-, x = 1-3. Table 1. Organocuprate Anions Observed upon ESI of THF Solutions of LiCuR2 3 LiCN in High (þþ) and Medium (þ) Relative Abundance R= entry 1 2 3
Lin-1CunR2n-
n
Me
1 2 3
þþ þþ
Et
þþ
n
Bu
s
t
Bu
Ph
þ
þþ
þþ
þþ
þþ
þþ þ
Bu
þþ
3. Results 3.1. Anion-Mode ESI Mass Spectra of LiCuR2 3 LiCN Solutions. The anion-mode ESI mass spectra obtained for solutions of LiCuR2 3 LiCN in THF are almost completely dominated by organocuprate anions of the homologous series Lin-1CunR2n-, n = 1-3, as illustrated for R = Me (Figure 1). In this case, the di- and trimeric members of the series are both observed in high relative abundance, whereas monomeric CuMe2- is absent. Ions of smaller signal intensities centered at m/z = 397 and 603 are assigned to higher aggregates Lin-1CunMe2n-, n = 4 and 6, respectively, in which the methyl substituents are partially exchanged for hydroxy groups. Here, hydrolysis reactions are apparently not suppressed completely. With the notable exception of the tBu system, all of the other LiCuR2 3 LiCN solutions also show the trimeric complex in high relative signal intensity (Table 1 and Figures S11-S15). In contrast, the dimeric cuprate is considerably less abundant. For R = nBu and, in particular, R = sBu, tBu, and Ph, the CuR2- monomer is also observed in high signal intensity. This finding proves that the relative depletion of the dimeric complex does not result from a mass discrimination effect (see Experimental Section) but rather reflects its intrinsically lower tendency of formation. The absence of any hydroxyl-containing ions indicates the complete exclusion of hydrolysis reactions. 3.2. Anion-Mode ESI Mass Spectra of LiCu(Me)R 3 LiCN Solutions. Besides homoleptic cuprates, we also probed mixed cuprates LiCu(Me)R 3 LiCN prepared by transmetalation of CuCN with a 1:1 mixture of MeLi and RLi. The anions observed all belong to the Lin-1CunMe2n-xRx-
Figure 2. Anion-mode ESI mass spectrum of a 25 mM solution of LiCu(Me)nBu 3 LiCN in THF, a = Li2Cu3Me4nBu2-, b = Li2Cu3Me3nBu3-.
homologous series (Table 2). With the exception of R = t Bu, no dimeric but only mono- and trimeric complexes exhibit significant abundance. This trend matches the behavior of the homoleptic Lin-1CunR2n- anions (R 6¼ Me), thus indicating that the larger organyl group R and not the methyl substituent controls the aggregation state of the complexes. The apparently different influence of the methyl and the other organyl substituents on the aggregation state is paralleled by their asymmetric distribution in the detected cuprate anions. As illustrated for the case of R = nBu, the observed complexes are enriched in the larger organyl and depleted in the methyl substituent relative to a completely statistical partitioning (Figure 2). This trend holds for all other detected species (Figures S16-S19) except for Li2Cu3Me6-xPhx-, for which the methyl-rich anions predominate (Figure S20). Virtually identical results (Figures S21-S26) are obtained when the mixed samples are prepared by combining two separate solutions of LiCuMe2 3 LiCN and LiCuR2 3 LiCN immediately (Δt ≈ 2 min) before the measurement. This result implies that the exchange and equilibration of the different organyl groups occurs relatively fast. 3.3. Anion-Mode ESI Mass Spectra of LinCuRn(CN) Solutions, n e 1. Anion-mode ESI of solutions of LiCuR(CN) in THF produces richer mass spectra than found for their LiCuR2 3 LiCN counterparts, as illustrated for the case of R=Me (Figure 3). The majority of observed species can be assigned to the homologous series Lin-1CunMen(CN)n-,
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Table 3. Organocuprate Anions Observed upon ESI of THF Solutions of LiCuR(CN) in High (þþ) and Medium (þ) Relative Abundance R= entry 1 2 3 4 5 6 7 8 9 10
Lin-1CunRn(CN)n
-
Cu2R2(CN)Lin-1CunR2nLin-3CunRn(CN)n-1-
n
Me
Et
2 3 4 5
þþ þ þþ þ
þþ þ þ
1 2 3 3 4
þ
n
Bu þ þ
þ þ
s
Bu þ
þ þþ þ þ
þþ þ þþ
t
Bu þ þ
þþ þ
Ph þþ þ
þ
Figure 3. Anion-mode ESI mass spectrum of a 25 mM solution of LiCuMe(CN) in THF, a = Li2Cu3Me6-, b = Li2Cu3Me3(CN)3-.
Figure 5. Cation-mode ESI mass spectrum of a 25 mM solution of LiCuMe2 3 LiCN in THF, a = Li3(CN)2(THF)2þ.
Figure 4. Anion-mode ESI mass spectrum of a 25 mM solution of LiCusBu(CN) in THF, a = LiCu2sBu2(CN)2-. Cu(þ2)containing ions are shown in red.
n = 2-6. Note that the stoichiometry of these complexes reflects the nominal overall composition of the sampled solution. The only anion of significant abundance that does not fit into this series corresponds to Li2Cu3Me6-. This species belongs to the Lin-1CunR2n- series already known from the LiCuR2 3 LiCN samples (see above). For all of the other LiCuR(CN) solutions, members of the Lin-1CunRn(CN)n- series are detected upon anion-mode ESI mass spectrometric analysis (Table 3, Figure 4, and Figures S27-S30). In no case is the monomer CuR(CN)observed, while the relative abundances of the higher aggregates depend on the organyl groups R. In addition, the mass spectra again show the presence of anions of the Lin-1CunR2n- series (see above). The remaining species of significant intensity correspond to Cu2Et2(CN)- and Lin-3CunRn(CN)n-1-, with R = nBu and sBu and n = 3 and 4. Being particularly prominent in the case of R = sBu (Figure 4), the Lin-3CunRn(CN)n-1- complexes stand out from the other cuprate anions in that they contain not only Cu(þ1) atoms but also one Cu(þ2) center (unless the presence of unbound organyl radicals R 3 is postulated). We have consistently observed these species in significant abundance and for
different batches of RLi reagents used in the preparation of the sample solutions. We also considered the possibility that the Cu(þ2) centers might arise from oxidation by residual traces of air. However, the brief exposure of a solution of LiCusBu2 3 LiCN to air led only to the detection of Cu2sBu2(CN)-, Cu3sBu2(CN)2-, and Cu4sBu2(CN)3-, thus indicating the occurrence of hydrolysis but not oxidation of the Cu(þ1) atoms (Figure S33). We have also performed analogous experiments on THF solutions of LinCuRn(CN), n = 0.5 and 0.8. Members of the Lin-1CunRn(CN)n- series and the Cu(2þ)-containing species Lin-3CunRn(CN)n-1- (R = nBu and sBu) remain virtually unaffected. In contrast, the cyanide-free Lin-1CunR2n- anions are reduced in signal intensity with decreasing RLi/ CuCN ratios, but in some cases remain visible for n = 0.8 and 0.5 (Figures S31 and S32, respectively, for R = Me). Complexes showing the opposite behavior and increasing in relative abundance are Cu2R2(CN)- (observed for R=Me, Et, nBu, sBu, tBu), LiCu3R3(CN)2- (R = nBu and sBu), and Li2Cu5R5(CN)3- (R = nBu and sBu). These species have in common R/CN ratios > 1. In no case are complexes with R/CN ratios
