Reactions of Scorpionate-Anchored Yttrium and Lutetium Dialkyls with

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Organometallics 2010, 29, 4950–4965 DOI: 10.1021/om100393w

Reactions of Scorpionate-Anchored Yttrium and Lutetium Dialkyls with Terminal Alkynes: From Bimetallic Complexes with Bridging Enynediyl Ligands to Monomeric Terminal Dialkynyl Complexes† Kuburat O. Saliu, Jianhua Cheng, Robert McDonald, Michael J. Ferguson, and Josef Takats* Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada Received April 30, 2010

The reactions of scorpionate-anchored yttrium and lutetium dialkyls with terminal alkynes were investigated, and the nature of the complexes was found to depend on both the ancillary scorpionate ligand and the alkyne substituent. Reaction of (TpMe2)Ln(CH2SiMe3)2(THF) (1; Ln = Y, Lu) with the terminal alkynes HCtCR (R = Ph, SiMe3, tBu, adamantyl (Ad)) afforded dimeric complexes. The solid-state structures, determined by single-crystal X-ray diffraction, showed the two metal centers bridged by two asymmetric alkynyl ligands and a coupled dialkynyl ligand, the latter exhibiting the unusual enyne bonding motif: [{(TpMe2)Ln( μ-CtCR)}2(μ-RC4R)] (Ln = Y, Lu, R = Ph (3), SiMe3 (4), tBu (5); Ln = Y, R = Ad (6-Y)). Protonolysis of complexes 3-Y and 4-Y with 2,4,6-trimethylphenol gave a 2:1 mixture of free alkyne and (Z)-enyne; however, with 5-Y and 6-Y both enyne and butatriene were obtained in addition to the free alkyne. Dissolution of dimeric 3-Lu and 4-Y in THF resulted in cleavage of the bridging units to give (TpMe2)Ln(CtCR)2(THF) (Ln = Lu, R = Ph (7); Ln = Y, R = SiMe3 (8)); however, the putative monomers could not be isolated, as they were only stable in THF solution. In the presence of 1 equiv of 2,20 -bipyridine, 1-Lu reacted with HCtCtBu to give the monomeric bipyridine adduct (TpMe2)Lu(CtCtBu)2(bipy) (9-Lu). Compound 9-Lu is seven-coordinate in the solid state with the metal center coordinated to a κ3-TpMe2 ligand, two terminal alkynyl ligands, and a κ2-bipyridine ligand. Reaction of (TptBu,Me)Ln(CH2SiMe3)2 (2; Ln = Y, Lu) with 2 equiv of HCtCPh yielded the terminal dialkynyl complexes (TptBu,Me)Ln(CtCPh)2 (10), on the basis of characteristic NMR spectroscopic data. Complexes 1-Y and 1-Lu reacted with HCtCTrit* (Trit* = tris(3,5-di-tert-butylphenyl)methyl) to give the corresponding monomeric complexes (TpMe2)Ln(CtCTrit*)2(THF) (11; Ln =Y, Lu). The monomeric nature and the presence of terminal alkynyl ligands in 11-Lu was verified by X-ray crystallography. The complex adopts a six-coordinate distorted-octahedral structure, with the Lu center coordinated to a κ3-TpMe2 ligand, two terminal alkynyl ligands, and a THF molecule. NMR-tube reactions of the dimeric complexes 3-Y and 4-Y and the dialkyl complexes 1-Y and 1-Lu with excess terminal alkynes showed that the complexes are able to dimerize terminal alkynes, albeit with low activity.

Introduction The study of organolanthanide complexes continues to receive a great deal of attention, due to their applications in both stoichiometric and catalytic reactions.1 In this vast field, alkynyl complexes occupy a special place, because they † Part of the Dietmar Seyferth Festschrift. Dedicated to Professor Dietmar Seyferth, with much appreciation, for his outstanding scientific contributions to organometallic chemistry and his visionary leadership of Organometallics. *To whom correspondence should be addressed. E-mail: joe.takats@ ualberta.ca. (1) (a) Edelmann, F. T. In Comprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: Oxford, U.K., 2007; Vol. 4.01, pp 1-190. (b) Edelmann, F. T. Coord. Chem. Rev. 2009, 253, 2515. (c) Weiss, C. J.; Marks, T. J. Dalton Trans. 2010, 39, 6576. (d) Nishiura, M.; Hou, Z. Nat. Chem. 2010, 2, 257. (e) Rodrigues, A.-S.; Kirillov, E.; Carpentier, J.-F. Coord. Chem. Rev. 2008, 252, 2115. (f) Zeimentz, P. M.; Arndt, S.; Elvidge, B. R.; Okuda, J. Chem. Rev. 2006, 106, 2404. (g) Evans, W. J.; Davis, B. L. Chem. Rev. 2002, 102, 2119. (h) Molander, G. A.; Romero, J. A. C. Chem. Rev. 2002, 102, 2161.

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are known to adopt a variety of bonding modes ranging from terminal alkynyls (a) to asymmetric alkynyl (b) and coupled butatrienediyl (c) bridging ligands in dimeric complexes (Scheme 1). The particular bonding mode adopted is the result of a subtle interplay of steric crowding around the lanthanide center and the electronic properties of the alkynyl substituent.2-4 In addition, some of the complexes have been shown to be active catalysts for the dimerization of terminal alkynes to enynes.4-7

(2) Evans, W. J.; Keyer, R. A.; Ziller, J. W. Organometallics 1990, 9, 2628. (3) Forsyth, C. M.; Nolan, S. P.; Stern, C. L.; Marks, T. J.; Rheingold, A. L. Organometallics 1993, 12, 3618. (4) Evans, W. J.; Keyer, R. A.; Ziller, J. W. Organometallics 1993, 12, 2618. (5) Heeres, H. J.; Teuben, J. H. Organometallics 1991, 10, 1980. (6) Nishiura, M.; Hou, Z.; Wakatsuki, Y.; Yamaki, T.; Miyamoto, T. J. Am. Chem. Soc. 2003, 125, 1184. (7) Ge, S.; Meetsma, A.; Hessen, B. Organometallics 2009, 28, 719. r 2010 American Chemical Society

Article Scheme 1. Commonly Observed Bonding Modes of Alkynyl Ligands in Lanthanide Complexes

Lanthanide alkynyl complexes have a long history, their isolation and studies dating back to the early 1970s,8 but they were mostly limited to those based on the lanthanocene systems [(C5R5)2Ln(CtCR)]n.8-10 Later works identified the asymmetric bridging structure (type b) as the most common bonding motif, with examples including [(C5H4Me)2Sm( μ-CtCtBu)]2,11 [(C5H5)2Er( μ-CtCtBu)]2,12 [(C5H4tBu)2Sm( μ-CtCPh)]2,13 [(DAC)Y( μ-CtCPh)]2 (DAC = deprotonated 4,13-diaza18-crown-6),14 [{PhC(NSiMe3)2}2Y( μ-CtCH)]2,15 and [(C5H4tBu)2Ln( μ-CtCPh)]2 (Ln = Nd, Gd).16 The investigations also uncovered fascinating bridging butatrienediyl ligands (type c), the result of head-to-head alkyne coupling, as found in [{(C5Me5)2Ln}2( μ-η2:η2-PhC4Ph)] (Ln = Ce, Sm, Nd),2,4 (8) Tsutsui, M.; Ely, N. J. Am. Chem. Soc. 1974, 96, 4042. (9) Evans, W. J.; Wayda, A. L. J. Organomet. Chem. 1980, 202, C6. (10) Nast, R. Coord. Chem. Rev. 1982, 47, 89–124. (11) Evans, W. J.; Bloom, I.; Hunter, W. E.; Atwood, J. L. Organometallics 1983, 2, 709. (12) Atwood, J. L.; Hunter, W. E.; Wayda, A. L.; Evans, W. J. Inorg. Chem. 1981, 20, 4115. (13) Shen, Q.; Zheng, D.; Lin, L.; Lin, Y. J. Organomet. Chem. 1990, 391, 307. (14) Lee, L.; Berg, D. J.; Bushnell, G. W. Organometallics 1995, 14, 5021. (15) Duchateau, R.; van Wee, C. T.; Meetsma, A.; Teuben, J. H. J. Am. Chem. Soc. 1993, 115, 4931. (16) Ren, J.; Hu, J.; Lin, Y.; Xing, Y.; Shen, Q. Polyhedron 1996, 15, 2165. (17) Heeres, H. J.; Nijhoff, J.; Teuben, J. H.; Rogers, R. D. Organometallics 1993, 12, 2609. (18) Evans, W. J.; Ulibarri, T. A.; Chamberlain, L. R.; Ziller, J. W.; Alvarez, D., Jr. Organometallics 1990, 9, 2124. (19) Duchateau, R.; Brussee, E. A. C.; Meetsma, A.; Teuben, J. H. Organometallics 1997, 16, 5506. (20) Lin, G.; McDonald, R.; Takats, J. Organometallics 2000, 19, 1814. (21) den Haan, K. H.; Wielstra, Y.; Teuben, J. H. Organometallics 1987, 6, 2053. (22) Duchateau, R.; van Wee, C. T.; Teuben, J. H. Organometallics 1996, 15, 2291. (23) Emslie, D. J. H.; Piers, W. E.; Parvez, M.; McDonald, R. Organometallics 2002, 21, 4226. (24) The synthesis and reactivity of LLnR2 complexes are very active current areas of research, and both bulky cyclopentadienyls25and various noncyclopentadienyl26 ancillaries have been used. (25) (a) Cameron, T. M.; Gordon, J. C.; Scott, B. L. Organometallics 2004, 23, 2995 and references therein. (b) Hou, Z.; Luo, L.; Li, X. J. Organomet. Chem. 2006, 691, 3114 and references therein. (c) Sun, J.; Berg, D. J.; Twamley, B. Organometallics 2008, 27, 683. (26) (a) Lyubov, D. M.; Fukin, G. K.; Trifonov, A. A. Inorg. Chem. 2007, 46, 11450 and references therein. (b) Trambitas, A. G.; Panda, T. K.; Jenter, J.; Roeskey, P. W.; Daniliuc, C.; Hrib, C. G.; Jones, P. G.; Tamm, M. Inorg. Chem. 2010, 49, 2435 and ref 33 therein. (c) Jantunen, K. C.; Scott, B. L.; Hay, P. J.; Gordon, J. C.; Kiplinger, J. L. J. Am. Chem. Soc. 2006, 128, 6322. (d) Masuda, J. D.; Jantunen, K. C.; Ozerov, O. V.; Noonan, K. J. T.; Gates, D. P.; Scott, B. L.; Kiplinger, J. L. J. Am. Chem. Soc. 2008, 130, 2408. (e) Doring, C.; Kretschmer, W. P.; Kempe, R. Eur. J. Inorg. Chem. 2010, 2853. (f) Zhang, Z.; Cui, D.; Trifonov, A. A. Eur. J. Inorg. Chem. 2010, 2861. (g) Otero, A.; Lara-Sanchez, A.; Fernandez-Baeza, J.; MartinezCaballero, E.; Marquez-Segovia, I.; Alonso-Moreno, C.; Sanchez-Barba, L. F.; Rodriguez, A. M.; Lopez-Solera, I. Dalton Trans. 2010, 39, 930.

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[{(C5Me5)2Ln}2( μ-η2:η2-RC4R)] (Ln = La, R = Me; Ln = Ce, R = Me, tBu),17 and [{(C5Me5)2La}2( μ-η2:η2-RC4R)] (R=Ph, tBu).3 Also, although there are several examples of complexes with the alkynyl ligands adopting the terminal bonding mode4,18-20 (type a), their presence has often been inferred from NMR data.5,14,21-23 The potentially more interesting dialkynyl complexes of the type “(Ligand)Ln(CtCR)2” are still rather rare, and their paucity, until recently, was mainly due to the lack of appropriate starting materials, “(Ligand)LnR2”.24-26 Hessen et al. described a series of dimeric lanthanide dialkynyl complexes with terminal and bridging alkynyl ligands: [(L)Ln(CtCPh)(μ-CtCPh)]2 (L = η3:η1-Me2TACN(CH2)2NtBu (TACN = triazacyclononane), Ln = La;27 L = N-(2-pyrrolidin-1-ylethyl)1,4-diazepan-6-amido ligand, Ln=Y, La7). Not surprisingly, in the scandium analogue with the latter ligand, both alkynyl ligands occupy terminal positions, [(L)Sc(CtCPh)2].7 Terminal and bridging bonding motifs were also observed by Berg and coworkers in the solid-state structure of the dimeric yttrium complex [(PCp*)Y(CtCSiMe3)(THF)(μ-CtCSiMe3)]2 (PCp*H = 1,2,3-trimethyl-1H-cyclopenta[l]phenanthrene),25c whereas Cameron and co-workers identified terminal and coupled alkynyl ligands in the pentamethylcyclopentadienyl lutetium complex [{(C5Me5)Lu(CtCPh)(bipy)}2( μ-η2:η2-PhC4Ph)].25a In another recent report, Cui and co-workers detailed the synthesis of a lutetium complex, supported by an anilidophosphinimine ligand, in which both alkynyl ligands are terminal: [(L)Lu(CtCPh)2(DME)] (L = o-(2,6-C6H3-iPr2)NC6H4P(C6H5)2N(2,4,6-C6H2Me3)).28 We have recently reported the synthesis and characterization of a series of scorpionate-anchored lanthanide dialkyl complexes, (TpMe2)Ln(CH2SiMe3)2(THF) (1; Ln = Nd, Sm, Y, Yb, Lu) and (TptBu,Me)Ln(CH2SiMe3)2 (2; Ln = Y, Yb, Lu).29 As part of our continuing investigation of the reactivity of these complexes, we examined their reactions with terminal alkynes. The effects of changing the size of the alkyne substituents as well as the ancillary scorpionate ligand on the structure of the resulting complexes are described. The ability of selected complexes to effect the catalytic dimerization of terminal alkynes was also briefly examined.

Results and Discussion Bimetallic Complexes with Bridging and Coupled Alkynyl Ligands. Reaction of the lanthanide dialkyl complexes (TpMe2)Ln(CH2SiMe3)2(THF) (Ln = Y (1-Y), Lu (1-Lu)) with 2 equiv of the terminal alkynes HCtCR (R = Ph, SiMe3, tBu, Ad) proceeded readily in excellent yields, with elimination of tetramethylsilane and incorporation of two alkynyl ligands per metal center (EA analysis, Scheme 2). The compounds are air- and moisture-sensitive, but under an inert atmosphere, they are stable at room temperature for an extended period. Those with SiMe3- and tBu-substituted alkynyls (4 and 5) are soluble in both aliphatic and aromatic solvents, while the Ph-substituted compounds 3 are soluble only in aromatic solvents and the Ad-capped alkynyl complex (6-Y) is only sparingly soluble in the latter. The color of the solid products varies from intense red (3 and 4) to yellow (5 and 6-Y). (27) Tazelaar, C. G. J.; Bambirra, S.; van Leusen, D.; Meetsma, A.; Hessen, B.; Teuben, J. H. Organometallics 2004, 23, 936. (28) Liu, B.; Liu, X.; Cui, D.; Liu, L. Organometallics 2009, 28, 1453. (29) Cheng, J.; Saliu, K.; Kiel, G. Y.; Ferguson, M. J.; McDonald, R.; Takats, J. Angew. Chem., Int. Ed. 2008, 47, 4910.

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Figure 1. (a) ORTEP view of the molecular structure of 3-Lu. Primed atoms are related to unprimed atoms by the 2-fold rotation axis at (0, y, 1/4). Only one part of the disordered coupled alkynyl group (PhC4Ph) and one orientation of the disordered phenyl ring of the bridging alkynyl group are shown. (b) ORTEP view of 3-Y in which the carbon atoms of the pyrazole rings are omitted for clarity. Here and elsewhere, non-hydrogen atoms are represented by Gaussian ellipsoids at the 20% probability level. The hydrogen atoms on boron are shown with arbitrarily small thermal parameters; the remaining hydrogen atoms are not shown. Scheme 2. Reaction of (TpMe2)Ln(CH2SiMe3)2(THF) Complexes with Terminal Alkynes

The intense red color of complexes 3 and 4 is unusual for Ln(III) complexes and is more suggestive of the formation of binuclear complexes bridged by a coupled dialkynyl unit, instead of mononuclear complexes with terminal or binuclear complexes with terminal and bridging alkynyl ligands. The color of complexes 3 is similar to that of the deep red lutetium complex of Cameron et al., [{(C5Me5)Lu(CtCPh)(bipy)}2( μ-η2:η2-PhC4Ph)].25a Deep red colors were also observed in other compounds containing the coupled butatrienediyl unit,2-4,17 as opposed to the cream-colored lanthanum complex of Hessen et al., [{η3:η1Me2TACN(CH2)2NtBu}La(CtCPh)(μ-CtCPh)]2,27 with terminal and bridging alkynyl ligands. To verify the above conclusion and to determine the finer details of the bonding between the coupled dialkynyl unit and the metal centers, the solid-state structures of complexes 3, 4-Y, and 6-Y were determined by single-crystal X-ray analysis. (i). Solid-State Structures of Complexes 3, 4-Y, and 6-Y. The solid-state structures of compounds 3, 4-Y, and 6-Y confirmed that the compounds are dimeric and their structures are similar. ORTEP drawings of 3-Lu and 3-Y are shown in Figure 1 and those of 4-Y and 6-Y in Figures 2 and 3, respectively. Selected distances and angles are given in Table 1, while the metrical parameters concerning the alkynyl units are collected in Tables 2 and 3. Each structure features two metal centers bridged by two μ2-alkynyl ligands and a coupled dialkynyl unit, and the coordination sphere around each metal atom is completed by a κ3-pyrazolylborate ligand. The metal centers are both formally seven-coordinate, each bonded to a carbon atom of the two μ2 bridging alkynyl ligands and to two carbons of the coupled dialkynyl unit; however, it is convenient to consider the latter as occupying one coordination position on each metal for an

Figure 2. Perspective view of the molecular structure of 4-Y, showing atom labeling.

easier description of the coordination geometry and for structural comparison with other complexes supported by the TpMe2 ligand. Under this formalism of “six-coordinate” metals, the coordination geometry is a distorted octahedron with the three N atoms of the tripodal κ3-TpMe2 ligand occupying one triangular face and the two carbon atoms of the μ2-bridging alkynyls (C5 and C7) and one from the coupled dialkynyl unit (C2) forming the other triangular face. As a result of both metals sharing the three-carbon triangular face, the pyrazolyl rings of

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Table 2. Selected Distances (A˚) and Angles (deg) within the Coupled Alkynyl Units of 3-Y, 3-Lu, 4-Y, and 6-Y 3-Lua

3-Y

4-Y

6-Y

1.339(6) 1.428(7) 1.213(7) 2.295(5) 2.465(5) 2.560(5) 2.649(5) 3.035(5)

1.305(6) 1.430(8) 1.241(8) 2.336(5) 2.478(5) 2.497(5) 2.732(6) 3.310(7)

126.5(4) 127.6(5) 166.9(6) 159.2(5)

129.3(5) 123.6(5) 171.9(6) 166.9(6)

Distances C1-C2 C2-C3 C3-C4 Ln1-C1 Ln1-C2 Ln2-C2 Ln2-C3 Ln2-C4

1.518(8) 1.217(12) 1.23(3) 2.172(7) 2.467(5) 2.467(5) 2.830(10) 3.38(2)

1.348(5) 1.413(5) 1.209(5) 2.283(4) 2.480(4) 2.531(4) 2.691(4) 3.167(4) Angles

b

R-C1-C2 C1-C2-C3 C2-C3-C4 C3-C4-R

Figure 3. Perspective view of the molecular structure of 6-Y, showing atom labeling. Table 1. Selected Distances (A˚) and Angles (deg) in [{(TpMe2)Ln( μ-CtCR)}2( μ-RC4R)] Complexes (3, 4-Y, and 6-Y) 3-Lua

3-Y

4-Y

6-Y

3.5017(7) 2.471(4) 2.503(4) 2.444(4) 2.389(4) 2.518(4) 2.428(4)

3.3476(8) 2.441(4) 2.502(5) 2.439(4) 2.394(4) 2.516(4) 2.403(4)

76.87(13) 81.60(14) 72.08(14) 74.57(13) 80.41(14) 76.81(13)

73.01(15) 83.85(15) 73.34(16) 77.17(14) 81.24(14) 75.82(14)

Distances Ln1-Ln2b Ln1-N12 Ln1-N22 Ln1-N32 Ln2-N42 Ln2-N52 Ln2-N62

3.3094(5) 2.345(5) 2.435(5) 2.378(5)

3.4103(5) 2.429(3) 2.485(3) 2.455(3) 2.406(3) 2.472(3) 2.394(3) Angles

N12-Ln1-N22 N12-Ln1-N32 N22-Ln1-N32 N42-Ln2-N52 N42-Ln2-N62 N52-Ln2-N62

76.32(17) 86.19(16) 75.62(18)

74.55(10) 84.18(11) 73.33(11) 75.84(11) 82.48(11) 76.17(11)

a Disordered 3-Lu contains a C2 axis perpendicular to the Lu (Ln1) and Lu0 (Ln2) vector, rendering the halves of the dimeric molecule equivalent, resulting in only one set of bond distances and angles. b Nonbonded distance.

the “top” and “bottom” pyrazolylborate ligands are eclipsed. The substituents of the coupled dialkynyl unit are nestled between two “top” and “bottom” pyrazolyl rings. The substituents of the μ2-alkynyl bridges are also positioned between two pyrazolyl rings: in 3-Lu, 3-Y, and 4-Y, the orientation is between alternating “top” and “bottom” rings, whereas in 6-Y both adamantyl substituents are in clefts formed by a pair of “bottom” pyrazolyl rings not occupied by the adamantyl groups of the coupled dialkynyl unit. Predictably, the intercalation of the bulky, coupled dialkynyl moiety between two pyrazolyl rings results in the opening of the intraligand N12-Ln1-N32 and N42-Ln2-N62 angles compared to the others (Table 1). The “top” angle, N12-Ln1-N32, is somewhat larger than N42-Ln2-N62, and the reason for this difference will be addressed in the discussion on the nature of the bonding between the coupled dialkynyl ligand and the metal centers. The exception to the aforementioned difference is 3-Lu, in which the coupled alkynyl ligand is disordered and the crystallographically imposed C2 symmetry renders “top” and “bottom” halves of the molecule equivalent.

130.4(6) 122.3(9) 165.8(14) 172.4(16)

127.4(4) 128.3(4) 169.7(4) 164.9(4)

a The large esd values on the distances and angles for 3-Lu and the seemingly “abnormal” C1-C2 and C2-C3 distances are the result of the C2 type disorder, with carbon C2 of the enynediyl ligand sitting on the 2-fold rotational axis; Ln1 = Lu and Ln2 = Lu0 in Figure 1a. b R denotes the carbon atom of the alkynyl ligand substituent.

The average Ln-N bond distances to the pyrazolylborate ligands are 2.39(3), 2.44(2), 2.46(2), and 2.45(2) A˚ in 3-Lu, 3-Y, 4-Y, and 6-Y, respectively. These values are comparable to 2.42(2) and 2.47(2) A˚ in the precursor dialkyl complexes 1-Lu and 1-Y, respectively,29 and require no further comments. The overall bonding motif between the metal centers and alkynyl ligands is the same for all four complexes; however, there are subtle variations within each fragment depending on the metal and the alkynyl substituents. The salient features within the two different bonding fragments are presented below. Coupled Dialkynyl, Ln2(μ-RC4R), Core. Undoubtedly the most interesting feature of the dimeric alkynyl complexes is the presence of the bridging coupled dialkynyl unit, in particular since it is different from previously structurally characterized coupled dialkynyl ligands. Scheme 3. Diagrams of Coupled Dialkynyl Cores: (a, b14) Previously Observed Bonding Motifs and (c) the Bonding Motif in 3, 4-Y, and 6-Y

Thus, unlike the commonly observed fully conjugated (E)butatrienediyl bridge2-4,17,25a or Berg’s unique Z conformation,14 the bridging unit in the present case is best described as an enynediyl ligand (parts a-c in Scheme 3, respectively). This can be clearly seen in the C-C bond distances and C-C-C bond angles within the coupled dialkynyl units (Table 2). For example, in compound 3-Y, the C1-C2 bond distance of 1.348(5) A˚ is typical for a double bond, and the C2-C3 and C3-C4 bond distances of 1.413(5) and 1.209(5) A˚ are as expected for single (sp2-sp) and triple bonds, respectively.30 (30) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1.

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Table 3. Selected Distances (A˚) and Angles (deg) within the Bridging Alkynyl Units of 3-Y, 3-Lu, 4-Y, and 6-Y 3-Lua

Δb

3-Y

2.524(5) 2.468(5)

0.056

2.617(4) 2.506(4) 2.581(4) 2.531(4) 3.095(4) 3.662(4) 3.708(4) 3.399(4) 1.206(5) 1.202(5)

Δ

4-Y

Δ

6-Y

Δ

2.658(5) 2.492(5) 2.532(5) 2.569(5) 2.996(5) 3.731(6) 3.704(5) 3.021(5) 1.213(7) 1.209(6)

0.166

2.552(6) 2.507(6) 2.565(5) 2.523(6) 3.666(6) 3.687(5) 3.253(6) 3.312(7) 1.175(7) 1.195(6)

0.045

93.9(4) 176.7(4) 171.3(4) 99.9(4) 85.62(15) 86.70(15) 75.98(15) 78.29(16) 167.3(5) 169.4(5)

82.8

157.5(5) 119.6(5) 155.7(5) 121.8(4) 82.85(18) 82.28(16) 78.36(17) 80.00(18) 172.9(6) 175.6(6)

37.9

Distances Ln1-C5 Ln2-C5 Ln1-C7 Ln2-C7 Ln1-C6 Ln1-C8 Ln2-C6 Ln2-C8 C5-C6 C7-C8

3.124(5) 3.628(6) 1.181(7)

0.111 0.050

0.037

0.042

Angles Ln1-C5-C6 Ln2-C5-C6 Ln1-C7-C8 Ln2-C7-C8 Ln1-C5-Ln2 Ln1-C7-Ln2 C5-Ln1-C7 C2-Ln2-C7 C5-C6-Rc C7-C8-Rc c

109.5(4) 167.1(4) 83.04(13) 77.62(17) 178.4(7)

57.6

101.7(3) 173.9(3) 148.7(3) 127.5(3) 83.42(12) 83.67(12) 73.42(12) 79.35(12) 175.8(4) 178.9(4)

72.2 21.2

71.4

33.9

a The C2 disorder in 3-Lu results in one set of data; Ln1 = Lu and Ln2 = Lu0 in Figure 1a. b Δ = differences between distances (Δdist) or angles (Δang). R denotes the carbon atom of the alkynyl ligand substituent.

The seemingly abnormal values of these distances in the lutetium compound 3-Lu is the result of the disordered coupled alkynyl unit, with C2 occupying the special position on a 2-fold rotational axis. The bonding of the enynediyl ligand to the metal centers is rather asymmetric, as shown by the trend in the observed Ln-C distances; Ln1-C1 < Ln1-C2 < Ln2-C2 < Ln2-C3, with the same implied trend in bond strengths. The Ln2-C3 distances are long (2.65-2.83 A˚), and there is virtually no bonding interaction between Ln2 and C4, except in 4-Y (Ln2-C4 = 3.035(5) A˚); the distances in the other complexes range from 3.167(4) to 3.38(2) A˚ (Table 2). It is worth noting that the longest Ln2-C3 distance is that in the smaller lutetium complex 3-Lu, implying that steric factors also have an important bearing on the bonding and geometry of the present complexes. Further evidence is the opening of the N12-Ln1-N32 and N42-Ln2N62 angles as a result of the closer approach of the enynediyl ligand to the metals compared to the μ2-bridging alkynyl ligands, and the somewhat larger former angle reflects the stronger bonding of the enynediyl ligand to Ln1. The stronger bonding to Ln1 also has predictable consequences on the

bonding between the metal centers and the bridging alkynyl unit (vide infra). The fact that the commonly observed (E)-butatrienediyl linkage is not seen in the present complexes is clearly due to the presence of the bulky and eclipsed pyrazolylborate ligands and the two μ2-bridging alkynyls, which restrict the coupled dialkynyl ligand to nestle between two “top” and “bottom” pyrazolyl rings. The formation of enyne as opposed to the (Z)butatrienediyl bonding motif in [{(DAC)Y}2( μ-(Z)-PhC4Ph)]14 may be the result of sterics, as the latter would necessarily bring both coupled alkynyl substituents into close proximity of the pyrazolyl ring substituents, thereby causing more unfavorable steric crowding than with the observed enynediyl ligand. Bridging Alkynyl [Ln(μ-CtCR)]2 Core. Views of the core structures are shown in Scheme 4. As already mentioned, but more clearly seen in the scheme, the bridging alkynyl ligands in 3-Lu, 3-Y, and 4-Y point toward opposite scorpionate ligands, whereas in the adamantyl-substituted 6-Y, the orientation is toward the same ancillary. It is also apparent that, typical of alkynyl groups bridging two lanthanide centers, the bonding is asymmetric.7,11-14,16,25c,31

Scheme 4. Bridging Alkynyl Cores in 3-Lu, 3-Y, 4-Y, and 6-Y

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Figure 4. 13C{1H} NMR spectrum (100 MHz, C6D6) of [{(TpMe2)Y(μ-CtCPh)}2(μ-PhC4Ph)] (3-Y). The carbons of the bridging alkynyl ligands are labeled with § (CR and Cβ), those of the enynediyl ligands with # (terminal, Ct and internal, Ci), and, for simplicity, the phenyl carbons are labeled with ‡ (see the Experimental Section for assignments); the residual solvent peak is labeled with *.

The magnitude of the asymmetry can be gauged from the differences in Ln-CR distances (i.e., C5 and C7), Δdist, and the corresponding Ln-CR-Cβ angles (i.e., Ln-C5-C6 and Ln-C7-C8), Δang; these values are collected in Table 3. Inspection of the Ln-CR distances (Ln-C5 and Ln-C7) reveals that those to Ln2 are shorter than those to Ln1, with the exception of one in the 4-Y complex (Y2-C7). The reason for this can be traced to the highly asymmetric disposition of the enynediyl ligand, which, as discussed above, furnishes less electron density to Ln2 than to Ln1. The shorter (stronger) Ln2-CR distances compensate for this electron deficiency and implicate electronic factors as being at least partially responsible for the asymmetric coordination of the bridging alkynyl ligands. However, the different asymmetry exhibited by the two bridging alkynyl ligands in complexes 3-Y and 4-Y and the same orientation of the bulky adamantyl-substituted bridging alkynyl ligands in 6-Y, indicate that steric effects play a role also; the asymmetry in 3-Lu is the same due to the C2-symmetric disorder. Further evidence of steric crowding in these molecules comes from inspection of the CtC-R angles. This angle is close to being linear in 3-Lu (178.4(7)°) and 3-Y (average 177.4°), whereas in 4-Y the average value has been reduced to 168.4° due to the bending away of the bulky SiMe3 substituent to avoid unfavorable steric interaction with the TpMe2 ligand. On the whole, the asymmetry in the alkynyl bridges increases in the order 3-Lu < 3-Y < 4-Y. The average values of Δdist = 0.10 A˚ and Δang = 77.1° in the last complex are indications of significant asymmetry, and the asymmetry is greater than in previously reported alkynyl-bridged yttrium complexes. For comparison, the respective values in structurally characterized yttrium dimers are as follows: [(L)Y(CtCPh)(μ-Ct CPh)]2 (L = 1,4,6-trimethyl-N-(2-pyrrolidin-1-ylethyl)-1,4diazepan-6-amido),7 0.171 A˚ and 20.0°; [(C5Me4CH2SiMe2NPh-κN)Y(μ-CtCtBu)]2,31 0.006 A˚ and 49.4°; [(DAC)Y(μ-

CtCPh)]2,14 0.024 A˚ and 44.7°; [(PCp*)Y(CtCSiMe3)(THF)(μ-CtCSiMe3)]2,25c 0.010 A˚ and 52.7°; [{PhC(NSiMe3)2}2Y(μ-CtCH)]2,15 0.047 A˚ and 61.4°. The presence of significantly asymmetric alkynyl bridges has been taken as an indication of weak π-interaction between the electron-deficient lanthanide metal and the alkynyl triple bond, although the view has also been expressed that “solid-sate packing forces probably play a major role in bridge angle asymmetry”.25c In the present context, the Y-C-C angles of 171.3(4) and 176.7(4)° in 4-Y are close to linearity and the β-carbon atoms are in relatively close contact with the yttrium centers; the Y1-C6 and Y2-C8 distances are 2.996(5) and 3.021(5) A˚, respectively, and are indeed somewhat shorter than the distance between yttrium and the end carbon of the enynediyl ligand’s triple bond, Y2-C4 (3.035(5) A˚). The values of the Y-C-C angles are significantly closer to linearity, and the Y- - -β-carbon distances are comparable to those in [(C5Me4CH2SiMe2NPh-κN)Y(μ-CtCtBu)]231 (156.1(2)° and 3.061(2) A˚) and [{PhC(NSiMe3)2}2Y(μ-CtCH)]215 (159.8(5)° and 2.959 A˚), where π-interaction was also invoked. Hence, complex 4-Y may represent another example of such a bonding interaction. The strength of the interaction is not strong since, and not unlike observations made on other asymmetrically bridged lanthanide alkynyl complexes,12-15,25c,31 the CtC bond length remains short (average of C5-C6 and C7-C8, 1.211(7) A˚) and is not different from the 1.208 A˚ reported for free PhCtCH.32 In addition, and contrary to expectation should π-interaction dictate orientation of the alkynyl bridge, in 3-Lu, with the smaller and more polarizing lutetium, the Lu-C5-C6 angle (167.1(4)°) deviates more from linearity and the Lu-Cβ distance (3.124(5) A˚) is longer than the corresponding distances in 4-Y. Clearly increased crowding around the smaller lutetium center appears to be sufficient to offset the benefit that increased π-interaction would entail.

(31) Robert, D.; Voth, P.; Spaniol, T. P.; Okuda, J. Eur. J. Inorg. Chem. 2008, 2810.

(32) Cox, A. P.; Ewart, I. C.; Stigliani, W. M. J. Chem. Soc., Faraday Trans. 2 1975, 71, 504.

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Scheme 5. Dynamic Exchange of the Enynediyl and Bridging Alkynyl Ligands between Ln1 and Ln2

Steric factors are also most likely responsible for the bonding of the bridging alkynyl ligands in 6-Y being significantly different from that in the other complexes. The inability of the bulky adamantyl group to fit between the pyrazolyl rings results in less asymmetric bridges, Δdist = 0.044 A˚ and Δang = 35.9°, and the bonding may be described as mainly a σ-interaction between the yttrium centers and CR carbons (C5 and C7) of the bridging alkynyls with very little, if any, additional π-component. This is seen in the long Y2-Cβ distances of 3.253(6) and 3.312(6) A˚, as well as the very short CtC bond lengths of 1.175(7) and 1.195(6) A˚ for C5-C6 and C7-C8, respectively. The orientation of the bridging alkynyl ligands is also different, with both β-carbon atoms and the attached adamantyl substituents pointing in the same direction toward Y2, this being favored by the somewhat less crowded nature of the TpMe2 ligand bonded to Y2 as a result of the triple bond of the enynediyl ligand bending away from this scorpionate ligand. (ii). Solution Structure of the [{(TpMe2)Ln(μ-CtCR)}2(μRC4R)] Complexes. The room-temperature 1H and 13C{1H} NMR spectra of complexes 3-5 showed one set of resonances for the pyrazolyl rings in a 2:1 ratio and two sets of resonances for the alkynyl ligands. The 1H NMR spectrum of 6-Y is similar, but its very low solubility in aromatic solvents prevented the recording of a good-quality 13C{1H) NMR spectrum; the compound is soluble in THF but, as will be shown later, it reacts with this solvent. The 13C{1H} NMR spectrum of 3-Y is shown in Figure 4, as a representative example. Both the 13C chemical shifts and, in the case of yttrium, the values of the 89Y-13C coupling constants give valuable information on the nature of the alkynyl ligands. Table 4 summarizes the typical chemical shifts, as well as the 89 Y-13C coupling constants, for the different types of alkynyl bonding modes, while Table 5 gives the relevant 13C{1H} NMR data for complexes 3-5. The simple NMR spectra are not in accord with the lowsymmetry solid-state structure of the complexes and indicate a more symmetrical solution structure, the possible reasons for which will be addressed below. However, and in particular, the 13C NMR features confirm the persistence of the dimeric form with bridging and coupled alkynyl ligands. Diagnostic of the coupled alkynyl ligand is the low-field 13C signal for the terminal carbon atoms (Tables 4 and 5). The chemical shift in 3-Y, 181.12 ppm, is at somewhat higher field

than that observed in [{(DAC)Y}2(μ-(Z)-PhCdCdCdCPh)]14 and [{(C5Me5)2La}2(μ-(E)-MeCdCdCdCMe)]17 (195.6 and 208.6 ppm, respectively) and the 1JYC of 17.0 Hz is less than 50% of what is seen in the former complex, 38.4 Hz. The internal carbons of the coupled alkynyl ligand in 3-Y are observed as a doublet at 133.91 ppm (1JYC = 5.7 Hz). Consistent with the presence of alkynyl ligands bridging two metal centers are two triplets in 3-Y at 143.84 ppm (1JYC= 22.3 Hz) and at 120.76 ppm (2JYC = 3.9 Hz), belonging to the R- and β-carbons, respectively. Similar features are seen in the analogous 4-Y and 5-Y, although in these complexes the β-carbon resonances could only be assigned by 2D-correlation (HMBC) experiments and, of course, the signals in the corresponding lutetium complexes appear as singlets. The chemical shifts of the CR and Cβ carbons and especially the triplet appearance and 1JYC value of the CR carbon of the yttrium complexes are similar to those in related alkynyl-bridged lanthanide complexes (Table 4). An exception is the complex [(C5Me4CH2SiMe2NPh-κN)Y(μ-CtCtBu)]2,31 which shows two doublets in the 13C{1H} NMR spectrum for CR and Cβ carbons of the alkynyl ligands (d, 129.5 ppm, 1JYC = 53.5 Hz and 116.3 ppm, 2 JYC = 10.7 Hz, respectively), and this observation was attributed to asymmetric coordination of the alkynyl ligands and weak π-bonding of the triple bond to yttrium. As mentioned above, the simple solution NMR spectra of the complexes, although corroborating the presence of coupled and bridging alkynyl ligands, are not in accord with the solid-state structures but are indicative of a symmetrical solution structure, real or time-averaged by fluxionality. The fluxionality may occur by rapid change in the enyne bonding mode of the coupled alkynyl ligand between metal centers, accompanied by oscillation of the μ2-CtCR alkynyl bridges, as shown in Scheme 5. The intermediate in the exchange process, also shown in the scheme, is the C2v-symmetric molecule with symmetrically bridging alkynyl and butatrienediyl ligands, which produces the simple NMR features: two sets of signals for the pyrazolyl rings in a 2:1 ratio, two triplets for the equivalent bridging alkynyl ligands, and a signal each for the equivalent terminal (Ct) and internal (Ci) carbons of the coupled alkynyl ligand. The dynamic exchange of the enynediyl ligand provides a rationale for the smaller 1JYC value of 17.0 Hz in 3-Y, compared to the 38.4 Hz reported for [{(DAC)Y}2(μ-(Z)-PhCdCdCdCPh)],14 as it averages the strong one-bond coupling between Y1-C1 and

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Table 4. 13C{1H} NMR Data (δ and JYC) for the Different Types of Alkynyl Bonding Modes δa

multb

JYCc

assignt

Table 5. 13C{1H} NMR Data of the Alkynyl Fragments in Complexes 3-5

ref

coupled unit δ

Terminal Alkynyl (C5Me5)2Y(CtCPh)(OEt2) (C5Me5)2Y(CtCSiMe3)(OEt2) (PCp*)Y(CtCSiMe3)2(THF)d (C5Me5)Lu(CtCPh)2(bipy)(THF) f (N-O)2Y(CtCSiMe3)g

146.95 109.59 134.21 114.19 171.23 noe 159.8 108.6 163.12 108.91

d d d d d s s d d

70.9 12.9 72.0 6.1 53.6

CR Cβ CR Cβ CR Cβ CR Cβ CR Cβ

21

14

53.5 10.7

CR Cβ CR Cβ CR Cβ CR Cβ

17

38.4 4.0

C tj Cik Ct Ci

72 12.5

21 25c 25a 23

μ2-Bridging Alkynyl [(DAC)Y(μ-CtCPh)]2h [{PhC(NSiMe3)2}2Y(μ-CtCH)]2 [{PhC(NSiMe3)2}2Y(μ-CtCPh)]2 [(C5Me4CH2SiMe2NPh-κN)Y(μ-CtCtBu)]2i

150.1 no 146.7 115.4 141.8 131.1 129.5 116.3

br t s t s d d

20 21

22

208.6 153.9 196.5 169.5

s s d d

a

181.12 133.91 3-Lu 184.88 134.72 4-Y 204.00 158.28 4-Lu 204.99 155.61 5-Y 195.50 126.66 5-Lu 198.80 130.21

3-Y

mult d d s s d d s s d d s s

JYC

b

17.0 5.7 14.9 4.6 17.2 8.5, 1.5

bridging unit assignt

δ

mult

JYC

assignt

Ctc Cid

143.84 120.76 148.60 122.71 173.07

t t s s t

22.3 3.9

CR Cβ CR Cβ CR Cβ CR Cβ CR Cβ CR Cβ

Ct Ci Ct Ci Ct Ci Ct Ci Ct Ci

21.7

e

177.67

s

e

127.02

t

132.47 128.52

s s

e

23.6

a δ is the chemical shift in ppm. b JYC is the coupling constant in Hz; JYC (Ct, Ci, or CR), 2JYC (Cβ). c Ct denotes a terminal carbon. d Ci denotes an internal carbon. e The Cβ resonance could not be located; it is hidden under solvent peaks at ca. 128 ppm. The peaks were assigned by 2D-correlation experiments (HMBC). 1

22 31

Coupled Alkynyl [{(C5Me5)2La}2(μ-MeC=CdC=CMe)] [{(DAC)Y}2(μ-PhC=CdC=CPh)]

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a δ is the chemical shift in ppm. b Multiplicity: d, doublet; s, singlet; br, broad; t, triplet. c JYC is the coupling constant in Hz; 1JYC (CR) and 2JYC (Cβ). d PCp* = phenanthrene-fused trimethylcyclopentadienyl. e no = not observed. f bipy=2,20 -bipyridine. g N-O=salicylaldiminate. h DAC = diaza-18-crown-6. i See text. j Ct denotes a terminal carbon. k Ci denotes an internal carbon.

the virtually zero coupling between Y2 and the distant C4 atom. The higher field chemical shift of the terminal carbon atoms of this unit is another possible consequence of this exchange. The appearance of the internal carbon resonance is more difficult to predict, as it depends on the relative magnitude of the coupling between internal carbons (C2 and C3) and the yttrium centers (Y1 and Y2). In 3-Y, the signal is a doublet with a JYC value of 5.7 Hz, a slightly larger value than the 4.0 Hz in [{(DAC)Y}2(μ-(Z)-PhCdCdCdCPh)].14 The subtle influence of bonding on coupling pattern is evidenced by the appearance of this resonance in the tBu-substituted compound (5-Y) as a doublet of doublets with 89Y-13C couplings of 8.5 and 1.5 Hz. To resolve the ambiguity between real or time-averaged symmetrical solution structure, a low-temperature 13C NMR study was conducted on the highly soluble SiMe3-substituted complex 4-Y. Unfortunately, lowering the temperature to -80 °C caused only a slight broadening of the resonances, accompanied by slight temperature-dependent chemical shift changes, leaving the question of solution structure ambiguity unresolved. Nevertheless, we are more inclined to attribute the simple NMR spectra to maintenance of the solid-state structural motif and to facile bonding mode changes, as depicted in Scheme 5. Indeed, we have noted that it may be sterics which favor the enyne bonding of the coupled alkynyl ligand, and this should prevail in solution as well. Furthermore, the activation energy for the exchange process is expected to be very low, as it involves only movement of two bonds, Y1-C2 to make a C2-C3 double bond and a C3-C4 triple bond to form a

Y2-C4 bond, to achieve the intermediate with a bridging butatrienediyl ligand. Thus, it is not surprising that the static solid-state type structure is not frozen out at -80 °C. (iii). Protonolysis of Complexes 3-Y, 4-Y, 5-Y, and 6-Y. Adoption of the enyne bonding mode by the coupled alkynyl moiety prompted us to investigate whether this unit could be liberated intact, and with the (Z)-ene conformation. Thus, protonolysis of compounds 3-Y, 4-Y, 5-Y, and 6-Y was examined. Treatment of 3-Y, 4-Y, 5-Y, and 6-Y with 2,4,6-trimethylphenol (HOMes) in C6D6 resulted in quantitative formation of the corresponding lanthanide aryloxides, (TpMe2)Y(OMes)2 (Mes = 2,4,6-Me3-C6H2). With complexes 3-Y and 4-Y, the other products of the reaction were a 2:1 mixture of free alkyne and (Z)-1,4-R2-1-butene-3-yne, whereas in the case of 5-Y and 6-Y, the protonolysis reaction is less selective and gave 2 mol of free alkynes and two coupled products, (Z)-1,4-R2-1-butene-3yne (80%) and 1,4-R2-butatriene (20%), presumably the Z isomer (Scheme 6). Formation of the observed products can be rationalized by the reaction sequence given in Scheme 7, similar to previous postulates made by Teuben et al.17 in their study of the protonolysis of [{(C5Me5)2Ln}2(μ-η2:η2-RC4R)] (Ln = Ce, R = Me, tBu; Ln = La, R = Me). Protonation of C4 of the bridging enyne unit gives the metalated butatriene A as the first product of protonolysis. This species may either undergo a second protonolysis reaction to give the corresponding 1,4-disubstituted butatriene or rearrange via a 1,3-metal shift to give the metalated enyne C through intermediate B. Further reaction of the metalated enyne with another proton gives the corresponding 1,4-disubstituted enyne. Teuben et al.17 and Berg et al.14,33 observed such rearrangement in their systems and formation of 1,4-disubstituted enyne. The rearrangement is not unexpected, since theoretical studies have shown that 1,4-R2-butatrienes are less stable than the corresponding 1,4-R2-enynes by about 80 kJ/mol.34 As was previously pointed out by Teuben,17 the relative rates of the second protonolysis and 1,3-metal shift determine the enyne/butatriene ratio. In the case of compounds (33) Lee, L. Deprotonated Aza-Crown as Simple and Effective Alternatives to C5Me5 in Group 3, 4 and Lanthanide Chemistry. Ph. D. Thesis, University of Victoria, British Columbia, Canada, 1997. (34) Wakatsuki, Y.; Yamazaki, H.; Kumegawa, N.; Satoh, T.; Satoh, J. Y. J. Am. Chem. Soc. 1991, 113, 9604.

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Scheme 6. Products of the Protonolysis of Dimeric [{(TpMe2)Y(μ-CtCPh)}2(μ-RC4R)] Complexes

Scheme 7. Plausible Scenario for the Formation of (Z)-Enynes and (Z)-Butatrienes in the Protonolysis of Complexes 3-Y, 4-Y, 5-Y, and 6-Y

3-Y and 4-Y, the results obtained suggest that rearrangement is much faster than protonolysis, thus accounting for the exclusive formation of enynes. The formation of a mixture of enynes and butatrienes in the case of 5-Y and 6-Y can be explained on the basis of relief of steric crowding. In the metalated butatriene A, there will be unfavorable steric interaction between the bulky tBu or Ad substituent on C1 and the TpMe2 pyrazolyl substituents. Thus, in order to relieve this unfavorable steric interaction, intermediate A can either undergo a second protonolysis reaction or undergo a 1,3-metal shift to give the metalated enyne B. Given that the rate of rearrangement is slower with bulky substituents,17 with complexes 5-Y and 6-Y the second protonolysis is more competitive with the 1,3-metal shift and hence formation of (Z)-butatriene products is also observed. (iv). Catalytic Dimerization of Terminal Alkynes. Lanthanide alkynyl complexes are known to be important catalysts or (35) Nishiura, M.; Hou, Z. J. Mol. Catal. A: Chem. 2004, 213, 101.

catalyst precursors for the dimerization of terminal alkynes to enynes.4-7,35 Previous studies mostly involved the use of monoalkynyl complexes, but there have been recent reports employing dialkyl and dialkynyl complexes.7,36 It was therefore of interest to examine whether the dimeric [{(TpMe2)Ln(μCtCR)}2(μ-RC4R)] complexes, with the enyne bonding motif, were competent to selectively dimerize terminal alkynes and, for comparison, the dialkyls (TpMe2)Ln(CH2SiMe3)2(THF) (1; Ln = Y, Lu) were also briefly examined. The reaction of [{(TpMe2)Y(μ-CtCPh)}2(μ-PhC4Ph)] (3-Y) with a 50-fold excess of phenylacetylene, followed by 1 H NMR spectroscopy in benzene-d6 at 80 °C, resulted in the stereo- and regioselective formation of the head-to-head Z isomer, 1,4-diphenyl-1-butene-3-yne, and trace amounts of trimers, albeit in low yields and slow conversion; after 72 h only 40% of the alkyne was consumed. The performance (36) Ge, S.; Norambuena, V. F. Q.; Hessen, B. Organometallics 2007, 26, 6508.

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of the trimethylsilyl-substituted complex [{(TpMe2)Y(μ-Ct CSiMe3)}2(μ-Me3SiC4SiMe3)] (4-Y) toward HCtCSiMe3 was marginally better (50% conversion after 72 h) but curiously gave the (E)-enyne (E)-Me3SiCHdCHCtCSiMe3, identified by comparison with known NMR spectroscopic data, in particular the large trans 3JH-H coupling constant of 19.2 Hz.37 Stereo- and regioselective formation of the head-to-head Z isomer, 1,4-diphenyl-1-butene-3-yne, and trace amounts of trimers was also seen when the dialkyls (TpMe2)Ln(CH2SiMe3)2(THF) (1-Y, 1-Lu) were treated with a 50-fold excess of phenylacetylene. However, the rate of conversion was slower than with preformed 3-Y, and 1-Y was slower than 1-Lu. These observations, not unlike those made before, indicate that it is the alkynyl complexes that are the catalysts in these reactions. The poor catalyst performance of the complexes did not warrant more extensive investigation. Road to Monomeric Complexes with Terminal Alkynyl Ligands. A major goal of the present study was the isolation of further examples of the rare monoligand Ln(III) complexes with two terminal alkynyl ligands. Since the reaction of (TpMe2)Ln(CH2SiMe3)2(THF) with terminal alkynes, even those with bulky substituents (SiMe3, tBu, and Ad), gave dimeric complexes, several alternative strategies were explored for the isolation of the desired monomeric compounds. (i). Solution Characterization of (Tp Me2 )Ln(CtCR)2 (THF-d8) Complexes (Ln = Lu, R = Ph (7); Ln = Y, R = SiMe3 (8)). Splitting of alkynyl bridges and decoupling of coupled alkynyl ligands with THF are well-documented processes.14,22,25a Thus, this was the first approach investigated in our attempts to isolate complexes with terminal alkynyl ligands. Dissolution of complexes 3-Lu and 4-Y in THF resulted in a gradual loss of the intense red coloration of these complexes and the development of very pale, almost colorless solutions, indicative of the formation of (TpMe2)Ln(CtCR)2(THF-d8) (Ln = Lu, R = Ph (7); Ln = Y, R = SiMe3 (8)), However, unlike in the above reports, attempts to isolate the putative terminal alkynyl complexes were not successful, as crystallization or solvent removal gave back the intensely colored dimeric starting materials. A similar observation was made on the phenanthrene-fused cyclopentadienyl-anchored yttrium dialkynyl complex. The compound [(PCp*)Y(CtCSiMe3)(THF)(μ-CtCSiMe3)]2 is dimeric in the solid state but, upon dissolution in benzene, gives monomeric (PCp*)Y(CtCSiMe3)2(THF) with terminal alkynyl ligands.25c,38 Therefore, complexes 7 and 8 were characterized only in solution by NMR spectroscopy. The 1H and 13C{1H} NMR spectra of the THF-d8 solution are consistent with the formation of monomeric complexes bearing terminal alkynyl ligands. The room-temperature 1H NMR spectra show one set of resonances each for the pyrazolyl rings and the SiMe3 and phenyl protons. In the 13C{1H} NMR spectrum of 8, the R- and β-carbons of the alkynide ligands appear as doublets at 170.26 ppm (1JYC = 53.5 Hz) and 107.0 ppm (2JYC = 10.2 Hz), respectively. The R-carbon chemical shift and 1JYC coupling constant are very close to those in (PCp*)Y(CtCSiMe3)2(THF) (171.23 ppm and 1JYC = 53.6 Hz), in which the β-carbon was not observed.25c The values are also (37) Yi, C. S.; Liu, N. Organometallics 1996, 15, 3968. (38) Sun, J. The Synthesis and Characterization of Lanthanide Complexes with Phenalenide and Aromatic-Fused Cyclopentadienyl Ligands. Ph.D. Thesis, University of Victoria, British Columbia, Canada, 2007.


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similar to those in other monomeric yttrium complexes with terminal alkynyl ligands (Table 4). (ii). Capture of a Monomeric Complex with Terminal Alkynyl Ligand: (TpMe2)Lu(CtCtBu)2(2,20 -bipy) (9-Lu). From the above observations, it was clear that ligands stronger than THF were needed to secure isolation of the monomeric dialkynyl species. Thus, we turned to 2,20 -bipyridine, a ligand used previously by other workers.25a,39,40 An attempt to effect simple splitting of dimeric 3-Lu with 2 equiv of 2,20 -bipyridine was unsuccessful and gave a mixture of products and free 2,20 bipyridine, as shown by 1H NMR spectroscopy. However, reaction of the dialkyl complex 1-Lu with 1 equiv of 2,20 bipyridine, followed immediately by 2 equiv of tert-butylacetylene, led to (TpMe2)Lu(CtCtBu)2(2,20 -bipy) (9-Lu) in excellent yield. The 1H NMR spectrum in C6D6 at room temperature shows two sets of signals in a 2:1 ratio for the pyrazolyl ring protons and distinct signals for the protons of the coordinated bipyridine ligand. The 13C{1H} NMR spectrum exhibited similar features: in particular, 10 singlets for the 10 different 2,20 -bipy carbons. The appearance of the spectra is indicative of a Cs-symmetric solution structure. The solid-state structure of 9-Lu was determined by singlecrystal X-ray diffraction. The molecular structure is shown in Figure 5, and selected metrical parameters are listed in the figure caption. X-ray analysis confirmed the monomeric nature of the complex, in which the lutetium center is seven-coordinate and is bonded to a κ3-TpMe2 ligand, two alkynyl carbon atoms, and the two nitrogen atoms of the chelating bipyridyl ligand. The monomeric nature of this complex is in contrast to the behavior of the related C5Me5 complex [{(C5Me5)Lu(CtCPh)(bipy)}2(μ-η2:η2-PhC4Ph)], which is dimeric and contains a bridging (E)-butatrienediyl ligand in the solid state,25a and further illustrates the sterically more demanding nature of the pyrazolylborate ligands compared to their cyclopentadienyl analogues.41 The geometry around the metal center is a distorted capped octahedron, with either N1 or N2 capping one of the triangular faces. The former might be preferable, as the Lu-N1 distance is ca. 0.07 A˚ longer than Lu-N2. Under this view, the triangular faces, formed by C1, C2, N12 and N2, N22, N32 (expanded face), are inclined by 14.5° and the Lu-N1 bond vector makes an 83.0° angle with the latter plane. In the alternate view, the N12, N22, N33 and C1, C3, N1 planes are less inclined (9.4°), but the Lu-N2 bond vector and plane angle is 80.2° and, as mentioned, the Lu-N2 distance is shorter. The Lu-N(TpMe2) distances ranges from 2.404(3) to 2.454(2) A˚, with an average value of 2.41(2) A˚. This is similar to those found in other seven-coordinate TpMe2 lanthanide complexes: 2.600(4) A˚ in (TpMe2)LaCl2(bipy),39 2.55(3) A˚ in (TpMe2)NdCl2(L) (L = 4,40 -di-tert-butyl-2,20 -bipyridyl),40 and 2.52(7) A˚ in (TpMe2)2Nd(O3SCF3),42 after accounting for the difference in ionic radii of the respective lanthanides.43 The average Lu-C bond length of 2.381(3) A˚ is comparable to 2.376(3) A˚ in 1-Lu and 2.390(7) A˚ found in (C5Me5)Lu(Ct CPh)2(bipy)(py).25a Although the Ln-C(alkynyl) bond is expected to be shorter than the Ln-C(alkyl) bond distance, the (39) Roitershtein, D.; Domingos, A.; Pereira, L. C. J.; Ascenso, J. R.; Marques, N. Inorg. Chem. 2003, 42, 7666. (40) Sun, C.-D.; Wong, W. T. Inorg. Chim. Acta 1997, 255, 355. (41) Trofimenko, S. Chem. Rev. 1993, 93, 943. (42) Liu, S.-Y.; Maunder, G. H.; Sella, A.; Stevenson, M.; Tocher, D. A. Inorg. Chem. 1996, 35, 76. (43) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.

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Figure 5. Perspective view of the molecular structure of 9-Lu, showing atom labeling. Selected distances (A˚) and angles (deg): Lu-N12, 2.433(2); Lu-N22, 2.404(3); Lu-N32, 2.454(2); Lu-C1, 2.378(3); Lu-C3, 2.384(3); Lu-N1, 2.529(3); Lu-N2, 2.460(3); C1-C2, 1.210(5); C3-C4, 1.199(5); N1-Lu-N2, 63.90(9); N1Lu-N12, 143.81(8); N1-Lu-N22, 76.42(9); N1-Lu-N32, 74.67(8); N1-Lu-C1, 111.79(11); N1-Lu-C3, 117.97(11); C1Lu-C3, 105.54(12); N2-Lu-N12, 152.28(9); N2-Lu-N22, 121.62(9); N2-Lu-N32, 125.65(9); N2-Lu-C1, 77.21(11); N2Lu-C3, 79.09(12); N12-Lu-N22, 76.87(9); N12-Lu-N32, 75.94(8); N22-Lu-N32, 77.19(8); Lu-C1-C2, 162.9(3); LuC3-C4, 169.3(3); C1-C2-C40, 177.2(4); C3-C4-C50, 173.2(4).

similarity between 9-Lu and 1-Lu can be accounted for by the higher coordination number and thus more congested lutetium center in 9-Lu. Curiously, this is not reflected in the Lu-N(TpMe2) distances, as their average (2.41(2) A˚) is the same as that in the starting dialkyl complex, (TpMe2)Lu(CH2SiMe3)2(THF) (2.42(2) A˚).29 As noted, the bipyridyl ligand is bonded to the lutetium center in an asymmetric fashion, which is not surprising on the basis of the capped-octahedral geometry, but the average Lu-N(bipy) distance of 2.50(4) A˚ is comparable to the 2.699(5) and 2.63(5) A˚ found in (TpMe2)LaCl2(bipy)39 and (TpMe2)NdCl2(L),42 respectively, again after taking into account the differences in ionic radii.43 (iii). Bulking up the Scorpionate: (TptBu,Me)Lu(CtCPh)2 (Ln = Y (10-Y), Lu (10-Lu)). It seemed clear from the above results that, although the TpMe2 ligand is able to stabilize terminal dialkynyl complexes, it is not bulky enough to do so in the absence of additional coordinating ligands. For this reason, we employed the extremely bulky TptBu,Me ligand, which is known for its ability to stabilize even lanthanide complexes with unusually low coordination numbers.44 The reaction of the dialkyl complexes (TptBu,Me)Ln(CH2SiMe3)2 (2; Ln = Y, Lu) with 2 equiv of phenylacetylene gave pale yellow solids in good yields. Unfortunately, all attempts to grow single crystals of these compounds were unsuccessful, and hence they were characterized by NMR spectroscopy. On the basis of their NMR signatures, they can be formulated as the monomeric terminal dialkynyl species (TptBu,Me)Lu(CtCPh)2 (10; Ln = Y, Lu), free of any other coordinated ligand. The 1H and 13 C{1H} NMR spectra consist of a single set of signals for the pyrazolylborate as well as the alkynyl ligands. For the yttrium complex (TptBu,Me)Y(CtC-Ph)2 (10-Y) the R and β (44) Marques, N.; Sella, A.; Takats, J. Chem. Rev. 2002, 102, 2137.

Saliu et al.

Figure 6. 13C{1H} NMR spectrum (100 MHz, C6D6) of (TptBu,Me)Y(CtCPh)2 (10-Y). The asterisk (*) denotes a residual solvent peak.

C atoms both show coupling to the yttrium center: 59.3 and 12.2 Hz, respectively (Figure 6). These values are similar to those observed in (TpMe2)Y(CtCSiMe3)2(THF-d8) (8). (iv). Bulking up the Terminal Alkynyl Substituent: (TpMe2)Ln(CtCTrit*)2(THF) (11; Ln = Y, Lu). The NMR signatures of compounds 10 are consistent with their formulation as the terminal dialkynyl species; however, the lack of corroborating solid-state X-ray structural support prompted us to seek yet another route in pursuit of such complexes. Logically, the alternative approach involved using an alkynyl ligand with a substituent even bulkier than those giving dimeric complexes. Reaction of (TpMe2)Ln(CH2SiMe3)2(THF) (1; Ln = Y, Lu) with 2 equiv of the extremely bulky terminal alkyne (tris(3,5di-tert-butylphenyl)methyl)acetylene (HCtCTrit*) gave the colorless compounds (TpMe2)Ln(CtCTrit*)2(THF) (11; Ln= Y, Lu), with terminal alkynyl ligands and a coordinated THF molecule. The room-temperature 1H and 13C{1H} NMR spectra of the lutetium complex 11-Lu in C6D6 show the expected signals for a six-coordinate octahedral complex with a facially coordinated tripodal pyrazolylborate ligand exhibiting Cs symmetry: two sets of sharp signals for the pyrazolyl rings in a 2:1 ratio, one set for the alkynyl ligands, and coordinated THF resonances. On the other hand, the signals in the room-temperature NMR spectra of the yttrium compound 11-Y are somewhat broadened; in particular, the 13C{1H} NMR spectrum displays one set of rather broad signals for the pyrazolylborate ligand, an indication of dynamic solution behavior. However, the chemical shifts of the R and β C atoms of the alkynyl ligands and the 89Y-13C coupling constants (136.89 ppm, 1JYC = 58.9 Hz and 106.87 ppm, 2JYC = 12.0 Hz, respectively) are as expected for terminal alkynyl species (Table 4). X-ray analysis of 11-Lu confirmed the monomeric nature, in which the lutetium center is coordinated by three nitrogen atoms of the κ3-pyrazolylborate, one carbon atom from each of the alkynyl ligands, and the oxygen atom of the coordinated THF molecule in a distorted-octahedral arrangement (Figure 7); selected bond lengths and angles are given in the figure caption. The molecular symmetry approaches Cs, with the THF O, Lu, B, and pyrazole (N11, N12) almost in a plane; thus, very little

Article


Figure 7. Perspective view of the molecular structure of 11-Lu, showing atom labeling. Only the major orientation of the disordered tert-butyl groups is shown for clarity. Selected distances (A˚) and angles (deg): Lu-O, 2.283(3); Lu-N12, 2.443(5); Lu-N22, 2.401(4); Lu-N32, 2.362(4); Lu-C1, 2.358(6); Lu-C4, 2.328(6); C1-C2, 1.183(7); C4-C5, 1.216(7); O-Lu-N12, 86.07(14); O-Lu-N22, 81.68(14); O-Lu-N32, 159.13(15); O-Lu-C1, 89.60(16); O-Lu-C4, 96.68(17); C1-Lu-C4, 101.50(19); N12-Lu-N22, 78.09(16); N22-Lu-N32, 77.09(16); N22-LuN32, 82.84(15); Lu-C1-C2, 169.9(5); Lu-C4-C5, 159.4(5); C1-C2-C3, 174.7(6); C4-C5-C6, 172.6(6).

movement is required to render the two alkynyl moieties and the other two pyrazole rings solution equivalent. The Lu-N distances (Lu-N12, 2.443(5); Lu-N22, 2.401(4); Lu-N32, 2.326(4) A˚) and Lu-C bond lengths (Lu-C1, 2.358(6); Lu-C4 2.328(6) A˚) in 11-Lu are, as expected, slightly shorter than those in the seven-coordinate 9-Lu. The C1-Lu-C4 angle of 101.50° is larger than 90°, and this opening up may be attributed to the large steric bulk of the “super trityl” substituents on the alkynyl ligands. The Lu-CR-Cβ angles of 169.5(5) and 159.4(5)° are comparable to the corresponding values of 162.9(3) and 169.3(3)° in 9-Lu, and again, the deviation from linearity must be a consequence of the bulky alkynyl substituents, tBu and Trit*, respectively.

Conclusions The lanthanide dialkyl complexes (TpMe2)Ln(CH2SiMe3)2(THF) (1) and (TptBu,Me)Ln(CH2SiMe3)2(THF) (2) provided attractive entries into the synthesis of a series of lanthanide dialkynyl complexes whose structures vary as a function of both the size of the ancillary scorpionate ligand and that of the terminal alkyne substituents. With the TpMe2 ligand and HCtCR (R = Ph, SiMe3, tBu, Ad) dimeric complexes, [{(TpMe2)Ln(μ-CtCR)}2(μ-RC4R)], were obtained, which feature the heretofore not observed enynediyl bonding motif of the coupled dialkynyl ligand. Protonolysis of the dimeric complexes with 2,4,6-trimethylphenol cleanly released the coupled alkynide unit. With R = Ph, SiMe3, 1,4-disubstituted

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(Z)-enynes were obtained; however, with R = tBu, Ad, formation of small amounts of 1,4-disubstituted butatriene was also observed. The dimeric complexes as well as their dialkyl precursors were able to effect the catalytic dimerization of terminal alkynes to exclusively give head-to-head dimers, the stereoselectivity of the reaction depending on the alkyne substituent. In the presence of coordinating ligands, monomeric complexes with terminal alkynyl ligands, (TpMe2)Ln(CtCR)2(L) (L = THF, 2,20 -bipy), were obtained, although in the case of L = THF the compounds are only stable in solution. In a search for terminal dialkynyl complexes, the bulky TptBu,Me ligand was enlisted. Reaction of the dialkyl complexes (TptBu,Me)Ln(CH2SiMe3)2 (2) with HCtCPh gave the desired complexes, (TptBu,Me)Ln(CtCPh)2, in which the bulky TptBu,Me ligand prevented further solvent coordination. Attempts to grow crystals of these complexes were not successful; however, their 13C{1H} NMR signatures support their formulation as complexes with two terminal alkynyl ligands. Increasing the bulk of the alkyne substituent by using the extremely bulky terminal alkyne HCtC Trit* finally gave X-ray characterized compounds with two terminal alkynyl ligands, (TpMe2)Ln(CtCTrit*)2(THF) (11; Ln = Y, Lu). However, despite the very bulky Trit* alkyne substituent, compounds 11 retained a THF molecule coordinated to the lanthanide center, even in the case of the smallest lanthanide, lutetium. Thus, even though the very bulky Trit* substituent on the alkynyl Cβ carbon successfully prevents dimer formation, this does not translate into increased steric bulk in the vicinity of the electron-deficient lanthanide center to prevent THF solvent coordination.

Experimental Section General Considerations. The preparation and handling of the compounds were carried out under an inert atmosphere of purified nitrogen or argon using standard Schlenk techniques in conjunction with a double manifold or in an argon-filled Vacuum Atmospheres HE-553-2 DRILAB. Solvents were distilled from Na/K alloy (toluene), Na/K alloy-benzophenone ketyl (THF), or CaH2 (pentane and hexane) under nitrogen and degassed by three freeze-pump-thaw cycles prior to use. Deuterated solvents (C6D6, C7D8, and THF-d8; Cambridge Isotope Laboratories) were dried over Na/K alloy-benzophenone ketyl, degassed by three freeze-pump-thaw cycles, and vacuumtransferred prior to use. (TpMe2)Ln(CH2SiMe3)2(THF) (1) and (TptBu,Me)Ln(CH2SiMe3)2 (2) (Ln = Y, Lu) were prepared as previously reported.29 The alkynes, phenylacetylene (PhCtCH), tert-butylacetylene (tBuCtCH), and (trimethylsilyl)acetylene (Me3SiCtCH) were obtained from Aldrich and used without further purification. Adamantylacetylene (AdCtCH)45,46 and (tris(3,5-di-tert-butylphenyl)methyl)acetylene (Trit*CtCH),47,48 prepared by modification of literature procedures, were received as gifts from Wes Chalifoux of the Department of Chemistry, University of Alberta. 2,4,6-Trimethylphenol and 2,20 -bipyridine were purchased from Aldrich and used as received. Elemental microanalysis was performed on a Carlo Erba (Thermo Fisher Scientific) CHNS-O EA1108 elemental analyzer by the Analytical and Instrumentation Laboratory, University of Alberta. NMR spectra were recorded on Varian Inova (45) Stetter, H.; Goebel, P. Chem. Ber. 1962, 95, 1039. (46) Broxterman, Q. B.; Hogeveen, H.; Kingma, R. F. Tetrahedron Lett. 1986, 27, 1055. (47) Khuong, T. V.; Zepeda, G.; Ruiz, R.; Khan, S. I.; GarciaGaribay, M. Cryst. Growth Des. 2004, 4, 15. (48) Chalifoux, W. A.; Tykwinski, R. R. Nat. Chem. 2010, doi: 10.1038/nchem.828.

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400 and 500 MHz, instruments, with shifts reported relative to residual solvent signals. Products of the protonolysis and catalytic reactions were analyzed by NMR spectroscopy and GC-MS on a Hewlett-Packard 5890 GC-MS instrument. [{(TpMe2)Y(μ-CtCPh)}2(μ-PhC4Ph)] (3-Y). To a colorless toluene solution (3 mL) of (TpMe2)Y(CH2SiMe3)2(THF) (1-Y) (0.100 g, 0.158 mmol) was added 2 equiv of PhCtCH (0.032 g, 0.316 mmol). The solution immediately turned an intense red. The solution was kept at room temperature for ca. 24 h, layered with pentane, and then cooled to -40 °C overnight. Bright red microcrystalline material formed, the supernatant was decanted, and the crystals were dried under vacuum to obtain 0.085 g (0.067 mmol) of 3-Y as the toluene solvate in 84% isolated yield. Single crystals suitable for X-ray crystallographic studies were obtained by layering a toluene solution with pentane and cooling to -30 °C. Anal. Calcd for C69H72B2N12Y2 (3-Y 3 C7H8): C, 65.32; H, 5.72; N, 13.25. Found: C, 64.97; H, 5.93; N, 13.19. 1 H NMR (400 MHz, C6D6, 27 °C): δ 6.95 (4H, apparent t, 3 JHH = 8.0 Hz, coupled m-H), 6.90 (4H, dd, 3JHH = 8.0 Hz, 4 JHH = 2.0 Hz, bridging o-H), 6.85 (2H, tt 3JHH = 8.0 Hz, 4 JHH = 2.0 Hz, coupled p-H), 6.79 (4H, dd, 3JHH = 8.0 Hz, 2.0 Hz, coupled o-H), 6.68-6.63 (6H, overlapped multiplet bridging m- and p-H), 5.63 (2H, s, 4-H Pz), 5.43 (4H, s, 4-H Pz), 2.99 (6H, s, 3-Me Pz), 2.45 (12H, s, 3-Me Pz), 2.25 (18H, s, 5-Me Pz). 13C{1H} NMR (100 MHz, C6D6, 27 °C): δ 181.12 (d, 1 JYC = 17.0 Hz, Ct coupled), 151.48 (3-C Pz), 150.89 (3-C Pz), 144.88 (5-C Pz), 144.40 (5-C Pz), 143.84 (t, 1JYC = 22.3 Hz, CR bridging), 136.50 (ipso-C coupled), 133.91 (d, 1JYC = 5.7 Hz, Ci coupled), 132.34 (ipso-C bridging), 131.11 (o-C bridging), 128.84 (o-C coupled), 127.88 (m-C bridging), 127.60 (m-C coupled), 127.11 (p-C bridging), 125.92 (p-C coupled) 120.76 (t, 2JYC = 3.9 Hz, Cβ bridging), 106.11 (4-C Pz), 105.67 (4-C Pz), 15.87 (3-Me Pz), 13.77 (3-Me Pz), 13.28 (5-Me Pz), 13.01 (5-Me Pz). 11B{1H} NMR (128 MHz, C6D6, 27 °C): δ -8.93. Note on NMR assignments: here and for all the dimeric complexes, the signals for the coupled and bridging alkynyls were assigned by 2D-correlation experiments (HMBC and HMQC); ipso, o, m, and p represent resonances due to the phenyl substituents. [{(TpMe2)Lu(μ-CtCPh)}2(μ-PhC4Ph)] (3-Lu). Following a procedure analogous to that for 3-Y using (TpMe2)Lu(CH2SiMe3)2(THF) (1-Lu; 0.100 g, 0.139 mmol) and PhCtCH (0.028 g, 0.278 mmol), 3-Lu was obtained as a bright red microcrystalline solid (0.090 g, 0.063 mmol) in 90% isolated yield. X-ray-quality crystals were obtained as described for 3-Y. Anal. Calcd for C69H72B2N12Lu2 (3-Lu 3 C7H8): C, 57.51; H, 5.04; N, 11.60. Found: C, 57.49; H, 5.02; N, 11.30. 1 H NMR (400 MHz, C6D6, 27 °C): δ 6.96 (4H, apparent t, 3 JHH = 8.0 Hz, coupled m-H), 6.85 (2H, t, 3JHH = 8.0 Hz, coupled p-H), 6.83 (4H, d, 3JHH = 8.0 Hz bridging o-H), 6.78 (4H, d, 3JHH = 8.0 Hz, coupled o-H), 6.70-6.62 (6H, overlapped multiplet coupled m- and p-H), 5.64 (2H, s, 4-H Pz), 5.46 (4H, s, 4-H Pz), 2.99 (6H, s, 3-Me Pz), 2.49 (12H, s, 3-Me Pz), 2.26 (18H, s, 5-Me Pz). 13C{1H} NMR (100 MHz, C6D6, 27 °C): δ 184.88 (Ct coupled), 151.94 (3-C Pz), 151.33 (3-C Pz), 148.60 (CR bridging), 144.76 (5-C Pz), 144.25 (5-C Pz), 137.19 (ipso-C coupled), 134.72 (Ci coupled), 132.34 (ipso-C bridging), 131.45 (o-C bridging), 128.97 (o-C coupled), 128.75 (p-C bridging), 128.50 (m-C coupled), 127.50 (m-C bridging), 125.73 (p-C bridging), 122.71 (Cβ bridging), 106.35 (4-C Pz), 105.85 (4-C Pz), 16.11 (3-Me Pz), 14.03 (3-Me Pz), 13.25 (5-Me Pz), 12.99 (5-Me Pz). 11B{1H} NMR (160 MHz, C6D6, 27 °C): δ -8.41. [{(TpMe2)Y2(μ-CtCSiMe3)}2(μ-Me3SiC4SiMe3)] (4-Y). To a colorless toluene solution (3 mL) of (TpMe2)Y(CH2SiMe3)2(THF) (1-Y; 0.100 g, 0.158 mmol) was added 2 equiv of Me3SiCtCH (0.031 g, 0.316 mmol). The solution, which slowly turned to an intense red, was kept at room temperature for ca. 24 h. Removal of solvent under reduced pressure resulted in a red oily residue. Addition of hexane followed by removal of

Saliu et al. solvent under vacuum afforded a red solid. The solid was redissolved in 2 mL of pentane, concentrated to about 1 mL, and kept at -30 °C to give 0.074 g (0.064 mmol) of 4-Y (80% isolated yield) as a red solid. X-ray-quality crystals were grown by cooling a concentrated hexane solution of 4-Y to -30 °C for several days to afford 4-Y 3 1/2C6H14. Anal. Calcd for C53H87B2N12Si4Y2 (4-Y 3 1/2C6H14): C, 52.87; H, 7.28; N, 13.96. Found: C, 52.93; H, 7.32; N, 13.94. 1 H NMR (400 MHz, C6D6, 27 °C): δ 5.93 (2H, s, 4-H Pz), 5.59 (4H, s, 4-H Pz), 3.10 (6H, s, 3-Me Pz), 2.48 (12H, s, 3-Me Pz), 2.27 (12H, s, 5-Me Pz), 2.22 (6H, s, 5-Me Pz), -0.05 (18H, s, SiMe3 coupled), -0.22 (18H, s, SiMe3 bridging). 13C{1H} NMR (100 MHz, C6D6, 27 °C): δ 204.00 (d, 1JYC = 14.9 Hz, Ct coupled), 173.07 (t, 1 JYC = 21.7 Hz, CR bridging), 158.28 (d, 1JYC = 4.6 Hz, Ci coupled), 150.66 (3-C Pz), 149.95 (3-C Pz), 144.60 (5-C Pz), 144.33 (5-C Pz) (Cβ bridging could not be located, it is hidden under C6D6 peaks at ca. 128 ppm, as shown by HMBC), 105.83 (4-C Pz), 105.50 (4-C Pz), 16.82 (3-Me Pz), 14.63 (3-Me Pz), 13.14 (5-Me Pz), 12.96 (5-Me Pz), -0.42 (SiMe3 coupled), -0.59 (SiMe3 bridging). 11B{1H} NMR (160 MHz, C6D6, 27 °C): δ -8.92. 1 H NMR (400 MHz, C7D8, 27 °C): δ 5.65 (2H, s, 4-H Pz), 5.57 (4H, s, 4-H Pz), 3.05 (6H, s, 3-Me Pz), 2.43 (12H, s, 3-Me Pz), 2.30 (12H, s, 5-Me Pz), 2.24 (6H, s, 5-Me Pz), -0.10 (18H, s, SiMe3 coupled), -0.25 (18H, s, SiMe3 bridging). 13C{1H} NMR (100 MHz, C6D6, 27 °C): δ 204.22 (d, 1JYC = 14.9 Hz, Ct coupled), 173.14 (t, 1JYC = 21.4 Hz, CR bridging), 158.38 (d, 1 JYC = 4.3 Hz, Ci coupled), 150.60 (3-C Pz), 149.94 (3-C Pz), 144.55 (5-C Pz), 144.29 (5-C Pz) (Cβ bridging could not be located, it is hidden under C7D8 peaks at ca. 128 ppm, as shown by HMBC), 105.84 (4-C Pz), 105.48 (4-C Pz), 16.81 (3-Me Pz), 14.60 (3-Me Pz), 13.13 (5-Me Pz), 12.95 (5-Me Pz), -0.43 (SiMe3 coupled), -0.59 (SiMe3 bridging). 1 H NMR (400 MHz, C7D8, -80 °C): δ 5.68 (2H, s, 4-H Pz), 5.50 (4H, s, 4-H Pz), 3.17 (6H, s, 3-Me Pz), 2.56 (12H, s, 3-Me Pz), 2.25 (12H, s, 5-Me Pz), 2.20 (6H, s, 5-Me Pz), -0.03 (18H, s, SiMe3 coupled), -0.16 (18H, s, SiMe3 bridging). 13C{1H} NMR (100 MHz, C7D8, -80 °C): δ 202.07 (d, 1JYC = 13.7 Hz, Ct coupled), 1743.61 (t, 1JYC = 20.5 Hz, CR bridging), 155.97 (br, Ci coupled), 150.22 (3-C Pz), 149.37 (3-C Pz), 144.24 (5-C Pz), 143.80 (5-C Pz) (Cβ bridging could not be located, hidden under C7D8 peaks at ca. 128 ppm, as shown by HMBC), 105.50 (4-C Pz), 105.25 (4-C Pz), 16.50 (3-Me Pz), 14.63 (3-Me Pz), 13.37 (5-Me Pz), 12.15 (5-Me Pz), -0.60 (SiMe3 coupled), -0.82 (SiMe3 bridging). [{(Tp Me2 )Lu(μ-CtCSiMe 3 )}2 (μ-Me 3 SiC 4 SiMe 3 )] (4-Lu). Following a procedure analogous to that for 4-Y using (TpMe2)Lu(CH2SiMe3)2(THF) (1-Lu; 0.137 g, 0.190 mmol) and Me3SiCtCH (0.037 g, 0.380 mmol) afforded 0.124 g (0.093 mmol) of 4-Lu (98% isolated yield) as a red solid. Attempts to grow X-ray-quality crystals by cooling a concentrated hexane solution of 4-Lu to -30 °C for several days gave poor-quality crystals. Anal. Calcd for C53H87B2N12Si4Lu2 (4-Lu 3 1/2C6H14): C, 46.25; H, 6.37; N, 12.21. Found: C, 46.24; H, 6.21; N, 11.85. 1 H NMR (400 MHz, C6D6, 27 °C): δ 5.68 (2H, s, 4-H Pz), 5.61 (4H, s, 4-H Pz), 3.11 (6H, s, 3-Me Pz), 2.54 (12H, s, 3-Me Pz), 2.27 (12H, s, 5-Me Pz), 2.22 (6H, s, 5-Me Pz), -0.04 (18H, s, SiMe3 coupled), -0.26 (18H, s, SiMe3 bridging). 13C{1H} NMR (100 MHz, C6D6, 27 °C): δ 204.99 (Ct coupled), 177.67 (CR bridging), 155.61 (Ci coupled), 151.13 (3-C Pz), 150.49 (3-C Pz), 144.55 (5-C Pz), 144.25 (5-C Pz) (Cβ bridging could not be located, it is hidden under C6D6 peaks at ca. 128 ppm, as shown by HMBC), 105.98 (4-C Pz), 105.77 (4-C Pz), 16.87 (3-Me Pz), 14.78 (3-Me Pz), 13.08 (5-Me Pz), 12.94 (5-Me Pz), -0.63 (SiMe3 coupled), -0.66 (SiMe3 bridging). 11B{1H} NMR (160 MHz, C6D6, 27 °C): δ -8.95. [{(TpMe2)Y(μ-CtCtBu)}2(μ-tBuC4tBu)] (5-Y). To a colorless toluene solution (3 mL) of (TpMe2)Y(CH2SiMe3)2(THF) (1-Y; 0.058 g, 0.092 mmol) was added 2 equiv of tBuCtCH (0.015 g, 0.184 mmol). The resulting yellow solution was kept at room temperature for ca. 24 h. Solvent was stripped under vacuum to give a pale yellow residue, which was extracted with ca. 2 mL of hexane, concentrated to ca. 1 mL, and then cooled to -30 °C

Article overnight to obtain a pale yellow solid which was dried under vacuum to give 0.038 g (0.035 mmol) of 5-Y as a yellow powder in 76% isolated yield. Anal. Calcd for C54H80B2N12Y2: C, 59.14; H, 7.35; N, 15.33. Found: C, 58.04; H, 7.50; N, 15.04. 1 H NMR (400 MHz, C6D6, 27 °C): δ 5.76 (2H, s, 4-H Pz), 5.67 (4H, s, 4-H Pz), 3.13 (6H, s, 3-Me Pz), 2.59 (12H, s, 3-Me Pz), 2.25 (12H, s, 5-Me Pz), 2.19 (6H, s, 5-Me Pz), 0.94 (18H, s, C(CH3)3 coupled), 0.88 (18H, s C(CH3)3 bridging). 13C{1H} NMR (100 MHz, C6D6, 27 °C): δ 195.50 (d, 1JYC = 17.2 Hz, Ct coupled), 150.60 (3-C Pz), 149.90 (3-C Pz), 144.45 (5-C Pz), 144.38 (5-C Pz) (Cβ bridging could not be located, it is hidden under C6D6 peaks at ca. 128 ppm, as shown by HMBC), 127.02 (t, 1JYC = 23.6 Hz, CR bridging), 126.66 (dd, JYC = 8.5 Hz, 1.5 Hz, Ci coupled), 105.72 (4-C Pz), 105.21 (4-C Pz), 36.02 (C(CH3)3 coupled), 30.81 (C(CH3)3 bridging), 29.83 (C(CH3)3 coupled) 28.54 (C(CH3)3 bridging), 17.04 (3-Me Pz), 14.97 (3-Me Pz), 13.22 (5-Me Pz), 13.00 (5-Me Pz). 11B{1H} (128 MHz, C6D6, 27 °C): δ -8.53. [{(TpMe2)Lu(μ-CtCtBu)}2(μ-tBuC4tBu)] (5-Lu). Following a procedure analogous to that for 5-Y using (TpMe2)Lu(CH2SiMe3)2(THF) (1-Lu; 0.120 g, 0.167 mmol) and tBuCtCH (0.027 g, 0.334 mmol) gave 0.093 g (0.073 mmol) of 5-Lu as a yellow powder in 88% isolated yield. Anal. Calcd for C61H88B2N12Lu2 (5-Lu 3 C7H8): C, 53.83; H, 6.52; N, 12.35. Found: C, 53.35; H, 6.34; N, 12.18. 1 H NMR (400 MHz, C6D6, 27 °C): δ 5.76 (2H, s, 4-H Pz), 5.69 (4H, s, 4-H Pz), 3.14 (6H, s, 3-Me Pz), 2.64 (12H, s, 3-Me Pz), 2.26 (12H, s, 5-Me Pz), 2.20 (6H, s, 5-Me Pz), 0.94 (18H, s, coupled C(CH3)3), 0.22 (18H, s, bridging C(CH3)3). 13C{1H} NMR (100 MHz, C6D6, 27 °C): δ 198.80 (Ct coupled), 150.99 (3-C Pz), 150.36 (3-C Pz), 144.36 (5-C-Pz), 144.22 (5-C Pz), 132.47 (CR bridging), 130.21 (Ci coupled), 128.52 (Cβ bridging), 105.89 (4-C Pz), 105.37 (4-C Pz), 36.05 (C(CH3)3 coupled), 30.70 (C(CH3)3 bridging), 29.66 (C(CH3)3 coupled), 28.50 (C(CH3)3 bridging), 17.34 (3-Me Pz), 15.26 (3-Me Pz), 13.20 (5-Me Pz), 12.99 (5-Me Pz). 11B{1H} NMR (128 MHz, C6D6, 27 °C): δ -9.10. [{(TpMe2)Y(μ-CtCAd)}2(μ-AdC4Ad)] (6-Y). Adamantylacetylene (0.051 g, 0.316 mmol) was dissolved in 1 mL of toluene and added to a colorless toluene solution (3 mL) of (TpMe2)Y(CH2SiMe3)2(THF) (1-Y; 0.100 g, 0.158 mmol). The solution slowly turned yellow. The solution was left to stand at room temperature for ca. 4 h, during which time crystalline solid was deposited at the bottom of the vial, the supernatant was decanted, and the solid washed with pentane and dried under vacuum to give 6-Y as a bright yellow crystalline solid (0.035 g). Solvent was stripped under vacuum from the decanted supernatant to obtain a yellow oily residue which upon trituration with pentane gave 0.057 g of 6-Y as a yellow powder: 84% combined yield (0.092 g, 0.065 mmol). X-ray-quality crystals were grown by allowing a dilute toluene solution of AdCtCH to slowly diffuse into a dilute toluene solution of (TpMe2)Y(CH2SiMe3)2(THF) in an NMR tube kept at room temperature for about 24 h and then cooling the resultant pale yellow solution to -30 °C for several days. Anal. Calcd for C92H120B2N12Y2 (6-Y 3 2C7H8): C, 70.93; H, 7.59; N, 10.55. Found: C, 70.11; H, 7.92; N, 10.65. 1 H NMR (400 MHz, C6D6, 27 °C): δ 5.83 (2H, s, 4-H Pz), 5.74 (4H, s, 4-H Pz), 3.25 (6H, s, 3-Me Pz), 2.69 (12H, s, 3-Me Pz), 2.26 (12H, s, 5-Me Pz), 2.20 (6H, s, 5-Me Pz), 1.90-1.38 complex overlap of signals integrating for 60H and consistent with four Ad substituents. 13C{1H} NMR (100 MHz, C6D6, 60 °C): δ 150.81 (3-C Pz), 150.25 (3-C Pz), 144.51 (5-C Pz), 105.74 (4-C Pz), 105.22 (4-C Pz), 43.29, 42.89, 41.55, 38.73, 37.72, 36.72, 29.82, 28.52 (Ad carbon atoms), 17.49 (3-Me Pz), 15.46 (3-Me Pz), 13.10 (5-Me Pz), 12.85 (5-Me Pz); the alkynyl carbon resonances could not be located. 11B{1H} (C6D6, 27 °C, 160 MHz): -8.92. (TpMe2)Lu(CtCPh)2(THF-d8) (7). Dissolution of 3-Lu in THF-d8 led to a gradual change in color from bright red to yellow-orange and finally to very pale yellow (almost colorless). 1 H NMR monitoring showed gradual disappearance of the signals due to 3-Lu and the appearance of those due to the


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monomeric product (TpMe2)Lu(CtCPh)2(THF-d8) (7). After about 2 h at room temperature, 3-Lu had completely converted to (TpMe2)Lu(CtCPh)2(THF-d8). 1 H NMR (400 MHz, THF-d8, 27 °C): δ 7.28 (4H, dd, 3JHH = 7.2 Hz, 4JHH = 1.6 Hz, o-H), 7.11 (4H, tt, 3JHH = 7.2 Hz, 4JHH = 1.6 Hz, m-H), 7.04 (2H, tt, 3JHH = 7.2 Hz, 4JHH = 1.6 Hz, p-H), 5.77 (3H, s, 4-H Pz), 2.66 (9H, s, 3-Me Pz), 2.40 (9H, s, 5-Me Pz). 13 C{1H} (100 MHz, THF-d8, 27 °C): δ 154.51 (CtCPh), 151.56 (3-C Pz), 145.84 (5-C Pz), 132.73 (ipso C), 132.27 (o-C), 128.31 (m-C), 125.97 (p-C), 106.39 (4-C Pz), 105.85 (CtCPh), 14.34 (3-Me Pz), 12.96 (5-Me Pz). 11B{1H} NMR (160 MHz, THF-d8, 27 °C): δ -10.77. (TpMe2)Y(CtCSiMe3)2(THF-d8) (8). Dissolution of 4-Y in THF-d8 led gradually to a change in color from deep red to pale red to orange and finally to very pale, almost colorless. 1H NMR monitoring showed gradual disappearance of the signals due to 4-Y and the appearance of those due to the monomeric product (TpMe2)Y(CtCSiMe3)2(THF-d8) (8). After about 2 h at room temperature, 4-Y had completely converted to (TpMe2)Y(CtCSiMe3)2(THF-d8). 1 H NMR (500 MHz, THF-d8, 27 °C): δ 5.70 (3H, s, 4-H Pz), 2.56 (9H, s, 3-Me Pz), 2.37 (9H, s, 5-Me Pz). 0.02 (18H, s, SiMe3). 13C{1H} NMR (125 MHz, THF-d8, 27 °C): δ 170.26 (d, 1 JYC = 53.5 Hz, CtC-SiMe3), 150.95 (3-C Pz), 145.64 (5-C Pz), 106.98 (d, 2JYC = 10.2 Hz, CtC-SiMe3), 106.19 (4-C Pz), 14.73 (3-Me Pz), 12.95 (5-Me Pz), 1.29 (SiMe3). 11B{1H} NMR (160 MHz, THF-d8, 27 °C): δ -9.30. (Tp Me2 )Lu(CtCtBu)2 (2,2 0 -Bipy) (9-Lu). 2,20 -Bipyridine (0.030 g; 0.200 mmol) was dissolved in 1 mL of toluene and added to a colorless toluene solution (3 mL) of (TpMe2)Lu(CH2SiMe3)2(THF) (1-Lu; 0.143 g, 0.200 mmol); the solution turned red immediately. Immediately, tBuCtCH (0.033 g, 0.400 mmol) was added to the red solution. The solution slowly turned orange-yellow, and the resulting solution was kept at room temperature for ca. 24 h. Solvent was stripped under vacuum to obtain an oily residue which was triturated with pentane to give 0.120 g (0.152 mmol) of 9-Lu as very pale, almost colorless solid in 76 % isolated yield. Cooling a dilute toluene solution to -30 °C for several days afforded pale crystals suitable for X-ray studies, albeit as the benzene solvate. Anal. Calcd for C37H48BN8Lu: C, 56.21; H, 6.12; N, 14.17. Found: C, 56.42; H, 6.34; N, 13.35. 1 H NMR (400 MHz, C6D6, 40 °C): δ 10.77 (1H, s, bipy), 7.27 (1H, ddd, JHH = 8.0 Hz, 2.4 Hz, 0.8 Hz, bipy), 7.12-6.99 (4H, overlapping multiplets, bipy), 6.78 (1H. dt, JHH = 8.0 Hz, 1.6 Hz, bipy), 6.03 (1H, dt, JHH = 8.0 Hz, 1.2 Hz, bipy), 5.78 (1H, s, 4-H Pz), 5.56 (2H, s, 4-H Pz), 3.57 (3H, s, 3-Me Pz), 2.27 (6H, s, 5-Me Pz), 2.24 (3H, s, 5-Me Pz). 2.07 (6H, s, 3-Me Pz), 1.22 (18H, s, C(CH3)3). 13C{1H} NMR (100 MHz, C6D6, 40 °C): δ 157.18 (bipy), 155.45 (bipy), 152.08 (3-C Pz), 151.95 (bipy), 149.34 (3-C Pz), 143.65 (CtCC(CH3)3), 142.58 (5-C Pz), 141.97 (5-C Pz), 138.83 (bipy), 138.09 (bipy), 125.64 (bipy), 124.23 (bipy) 123.90 (bipy), 119.75 (bipy), 119.32 (bipy), 113.32 (CtCC(CH3)3), 106.05 (4-C Pz), 105.82 (4-C Pz), 33.09 (C(CH3)3, 32.94 C(CH3)3, 16.76 (3-Me Pz), 14.68 (3-Me Pz), 14.33 (5-Me Pz), 13.09 (5-Me Pz). 11B{1H} NMR (160 MHz, C6D6, 27 °C): δ -8.92. (TptBu,Me)Y(CtCPh)2 (10-Y). To a colorless THF solution (2 mL) of (TptBu,Me)Y(CH2SiMe3)2 (2-Y; 0.100 g, 0.145 mmol) was added 2 equiv of PhCtCH (0.030 g, 0.290 mmol). The solution gradually changed color from colorless to pale yellow then to deep orange-yellow and finally to a light brown. The solution was kept at room temperature for ca. 24 h. Solvent was stripped under reduced pressure to obtain an oily residue which was triturated with hexane to give a yellow-orange solid with a greenish tint. The solid was washed several times with hexane until the supernatant was colorless. The solid was dried under vacuum to obtain 0.073 g (0.102 mmol) of 10-Y (70% isolated yield) as a yellow solid. Attempts to grow X-ray-quality crystals proved unsuccessful. Anal. Calcd for C40H50BN6Y: C, 67.23; H, 7.05; N, 11.76. Found: C, 66.53; H, 7.06; N, 11.24.

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H NMR (500 MHz, C6D6, 27 °C): δ 7.57 (4H, d, 3JHH = 7.6 Hz, o-H), 7.01 (4H, apparent t, 3JHH = 7.6 Hz, m-H), 6.92 (2H, t, 3 JHH = 7.6 Hz, p-H), 5.61 (3H, s, 4-H Pz), 2.00 (9H, s, 5-Me Pz), 1.70 (27H, s, 3-C(CH3)3 Pz). 13C{1H} NMR (125 MHz, C6D6, 27 °C): δ 165.25 (3-C Pz), 146.55 (5-C Pz), 143.77 (d, 1JYC = 59.3 Hz, CtCPh), 132.09 (o-C), 128.29 (m-C) 127.36 (ipso C), 126.15 (p-C), 102.22 (d, 2JYC = 12.2 Hz, CtCPh), 103.39 (4-C Pz), 32.99 (C(CH3)3 Pz), 31.69 (C(CH3)3 Pz), 13.05 (5-Me Pz). 11B{1H} NMR (160 MHz, C6D6, 27 °C): δ -8.26. (TptBu,Me)Lu(CtCPh)2 (10-Lu). Following a procedure analogous to that for 10-Y using (TptBu,Me)Lu(CH2SiMe3)2 (2-Lu; 0.100 g, 0.130 mmol) and PhCtCH (0.027 g, 0.260 mmol) afforded 0.086 g (0.107 mmol) of 10-Lu as a yellow solid in 80% isolated yield. Attempts to grow X-ray-quality crystals proved unsuccessful. Anal. Calcd for C40H50BN6Lu: C, 60.01; H, 6.29; N, 10.50. Found: C, 60.35; H, 5.69; N, 9.71. 1 H NMR (500 MHz, C6D6, 27 °C): δ 7.57 (4H, d, 3JHH = 8.0 Hz, o-H), 7.01 (4H, apparent t, 3JHH = 8.0, Hz, m-H), 6.91 (2H, t, 3 JHH = 8.0 Hz, p-H), 5.64 (3H, s, 4-H Pz), 1.97 (9H, s, 5-Me Pz), 1.74 (27H, s, 3-C(CH3)3 Pz). 13C{1H} NMR (125 MHz, C6D6, 27 °C): δ 165.99 (3-C Pz), 155.81 (CtCPh), 146.57 (5-C Pz), 132.10 (ipso C), 132.27 (o-C), 126.75 (p-C), 126.17 (m-C), 108.20 (CtCPh), 103.84 (4-C Pz), 32.93 (C(CH3)3 Pz), 31.51 (C(CH3)3 Pz), 13.03 (5Me Pz). 11B{1H} NMR (160 MHz, C6D6, 27 °C): δ -8.33. (TpMe2)Y(CtCTrit*)2(THF) (11-Y). To a colorless toluene solution (2 mL) of (TpMe2)Y(CH2SiMe3)2(THF) (1-Y; 0.063 g, 0.100 mmol) was added a solution of Trit*CtCH (0.120 g, 0.200 mmol) in the same solvent. The solution was left to stand at room temperature overnight (ca. 19 h). Solvent was stripped under vacuum to obtain an oily residue (with a tinge of pink) which upon trituration with pentane gave a pale solid. Recrystallization of the solid from pentane afforded 11-Y as a white powder (0.128 g, 0.077 mmol) in 77% recrystallized yield. Anal. Calcd for C109H156BN6YO: C, 78.57; H, 9.44; N, 5.04. Found: C, 78.43; H, 9.45; N, 4.32. 1 H NMR (500 MHz, C6D6, 27 °C): δ 7.57 (12H, d, 4JHH = 1.6 Hz, o-H), 7.39 (6H, t, 4JHH = 1.6 Hz, p-H), 5.47 (3H, s, 4-H Pz), 4.07 (4H, br, THF), 3.10 (3H, s, 3-Me Pz), 2.26 (6H, s, 3-Me Pz), 2.19 (9H, s, 5-Me Pz), 1.43 (4H, br, THF), 1.27 (108H, s, C(CH3)3). 13C{1H} NMR (125 MHz, C6D6, 27 °C): δ 150.36 (3C Pz), 149.26 (m-C), 148.22 (ipso-C), 144.84 (5-C Pz), 136.89 (d, 1 JYC = 58.9 Hz, CtCTrit*), 124.87 (o-C), 119.19 (p-C), 106.87 (d, 2JYC =12.0 Hz, CtCTrit*), 106.01 (4-C Pz), 70.79 (THF), 58.25 (CtCCAr300 ), 35.00 (C(CH3)3), 31.78 (C(CH3)3), 25.49 (THF), 14.61 (3-Me Pz),12.93 (5-Me Pz). 11B{1H} NMR (160 MHz, C6D6, 27 °C): δ -9.15. (TpMe2)Lu(CtCTrit*)2(THF) (11-Lu). Following a procedure analogous to that for 11-Y using (TpMe2)Lu(CH2SiMe3)2(THF) (0.072 g, 0.100 mmol) and Trit*CtCH (0.120 g, 0.200 mmol), 0.134 g (0.077 mmol) of 11-Lu was obtained as a white powder in 77% isolated yield. X-ray-quality crystals were obtained by keeping a concentrated pentane solution at room temperature for several days. Anal. Calcd for C109H156BN6LuO: C, 74.72; H, 8.97; N, 4.80. Found: C, 74.36; H, 9.12; N, 4.49. 1 H NMR (500 MHz, C6D6, 27 °C): δ 7.56 (12H, d, 4JHH = 2.0 Hz, o-H), 7.39 (6H, t, 4JHH = 2.0 Hz, p-H), 5.49 (1H, s, 4-H Pz), 5.47 (2H, s, 4-H Pz), 4.13 (4H, br, THF), 3.13 (3H, s, 3-Me Pz), 2.26 (6H, s, 3-Me Pz), 2.22 (3H, s, 5-Me Pz), 2.17 (6H, s, 5-Me Pz), 1.45 (4H, br THF), 1.27 (108H, s, C(CH3)3). 13C{1H} NMR (100 MHz, C6D6, 27 °C): δ 151.66 (3-C Pz), 150.88 (3-C Pz), 149.25 (m-C), 148.19 (ipso-C), 147.11 (CtCTrit*), 144.67 (5-C Pz), 144.47 (5-C Pz), 124.90 (o-C), 119.21 (p-C), 109.83 (CtCTrit*), 106.16 (4-C Pz), 105.87 (4-C Pz), 71.33 (THF), 58.29 (CtCCAr300 ), 35.00 (C(CH3)3), 31.78 (C(CH3)3), 25.63 (THF), 15.17 (3-Me Pz), 14.65 (3-Me Pz), 12.88 (5-Me Pz), 12.65 (5-Me Pz). 11B{1H} NMR (128 MHz, C6D6, 27 °C) -9.14. General Procedure: Protonolysis of Dimeric Complexes 3-Y, 4-Y, 5-Y, and 6-Y with 2,4,6-Trimethylphenol. The protonolysis was carried out in NMR tubes, and the products were identified by NMR spectroscopy. A benzene-d6 solution of the 1

Saliu et al. appropriate dimeric complex was treated with 4 equiv of 2,4,6trimethylphenol in the same solvent. The 1H NMR spectrum of the solution taken after ca. 0.5 h showed quantitative formation of products. The identity of the dimeric alkyne products was ascertained by comparison of the NMR data with known literature values.17,37,49 Protonolysis of 3-Y and 5-Y are given below as representative examples. (i). [{(Tp Me2 )Y(μ-CtCPh)}2 (μ-PhC 4 Ph)] (3-Y). 2,4,6-Trimethylphenol (5.0 mg, 34 μmol) dissolved in ca. 0.3 mL of C6D6 was added to an NMR tube containing 10.0 mg (8.4 μmol) of 3-Y in the same solvent. The red solution changed immediately to pale pink and then to colorless. 1H NMR of the solution after about 0.5 h showed formation of 2 equiv of the aryloxide, (TpMe2)Y(OMes)2 and 1 mol of (Z)-1,4-diphenylbutenyne, as well as 2 mol of free HCtCPh. 1 H and 13C{1H} NMR data of (TpMe2)Y(OMes)2 are given below. The spectral data for the coupled alkyne products are given in the Supporting Information, Table S1. 1 H NMR (400 MHz, C6D6, 27 °C): δ 6.90 (4H, s, m-H), 5.44 (3H, s, 4-H Pz), 2.27 (6H, s, p-Me), 2.24 (12H, s, o-Me), 2.08 (9H, s, 3-Me Pz), 2.06, (9H, s, 5-Me Pz). 13C{1H} NMR (100 MHz, C6D6, 27 °C): δ 159.53 (d, 2JYC = 5.0 Hz, ipso-C), 151.16 (3-C Pz), 145.93 (5-C Pz), 129.09 (m-C), 125.42 (o-C) 125.13 (p-C), 106.03 (4-C Pz), 20.89 (p-Me), 17.84 (o-Me), 13.17 (3-Me Pz), 12.87 (5-Me Pz). (ii). [{(TpMe2)Y(μ-CtCtBu)}2(μ-tBuC4tBu)] (5-Y). The same approach was used as for 3-Y, with 5.0 mg (36.4 μmol) of 2,4,6trimethylphenol and 10.0 mg (9.1 μmol) of 5-Y. The initially yellow solution changed to colorless after sitting at room temperature for ca. 0.5 h. The 1H NMR spectrum taken at this time showed formation of 2 equiv of the aryloxide (TpMe2)Y(OMes)2 and 2 equiv of free HCtCtBu, as well as a mixture of (Z)-1,4-tBu2butenyne (80%) and 1,4-tBu2-butatriene (20%). General Procedure for Catalytic Dimerization of Terminal Alkynes. All reactions were carried out on an NMR scale. Into a C6D6 solution of the alkynyl or dialkyl complex in an NMR tube was syringed a ca. 50-fold excess of the corresponding terminal alkyne. The NMR tube was flame-sealed and heated in an oil bath at 80 °C for ca. 72 h, with periodic monitoring of the reaction progress by 1H NMR spectroscopy. The identity of the dimeric alkyne products was ascertained by comparison of the NMR data with known literature values.17,37,49 (i). [{(TpMe2)Y(μ-CtCPh)}2(μ-PhC4Ph)] (3-Y) and HCt CPh. In an NMR tube, HCtCPh (43.0 mg, 420.0 μmol) was added with a syringe to a C6D6 solution of complex 3-Y (10.0 mg, 8.4 μmol). The tube was flame-sealed and heated in an oil bath at 80 °C for 72 h while periodically monitoring the reaction progress by NMR spectroscopy. After 72 h of heating, about 33% of the alkyne was converted to (Z)-PhCHdCHCtCPh, as seen in the 1 H NMR spectrum. Analysis of the reaction mixture by GC-MS confirmed formation of (Z)-PhCHdCHCtCPh as well as trace amounts of trimers. (ii). [{(TpMe2)Y(μ-CtCSiMe3)}2(μ-Me3SiC4SiMe3)] (4-Y) and HCtCSiMe3. The above procedure was followed; HCt CSiMe3 (50.0 mg, 505.0 μmol) and 4-Y (12.0 mg, 10.0 μmol) at 80 °C for 72 h resulted in 50% of the alkyne being converted to (E)Me3SiCHdCHCtCSiMe3 and trace amounts of trimers, as shown by GC-MS . (iii). (TpMe2)Y(CH2SiMe3)2(THF) (1-Y) and HCtCPh. To a C6D6 solution of 1-Y (11.6 mg, 18.4 μmol) in an NMR tube was added HCtCPh (94.0 mg, 920.0 μmol) via a syringe. The mixture turned red immediately, and the 1H NMR spectrum after addition showed a mixture of 1-Y, HCtCPh, SiMe4, THF, and 3-Y. The mixture was then heated for ca. 18 h at 80 °C. The 1 H NMR spectrum showed the disappearance of 1-Y and the formation of (Z)-PhCHdCHCtCPh in addition to the other components of the mixture. Heating the mixture for a total of 90 h showed that ca. 25% of the alkyne was converted to (49) Haskel, A.; Straub, T.; Dash, A. K.; Eisen, M. S. J. Am. Chem. Soc. 1999, 121, 3014.

Article (Z)-PhCHdCHCtCPh and trace amounts of trimers, as shown by 1H NMR spectroscopy and GC-MS. (iv). (TpMe2)Lu(CH2SiMe3)2(THF) (1-Lu) and HCtCPh. The same procedure was followed; HCtCPh (94.0 mg, 920.0 μmol) and 1-Lu (13.2 mg, 18.4 μmol) at 80 °C for a total of 84 h resulted in 40% of the alkyne being converted to give (Z)-PhCHd CHCtCPh and trace amounts of trimers, as shown by 1H NMR spectroscopy and GC-MS. X-ray Crystallographic Studies. Crystals suitable for singlecrystal X-ray diffraction were obtained as described in the Experimental Section. The crystals were manipulated in the glovebox, coated with Paratone-N oil, and transferred to a cold gas stream on the diffractometer. Data were collected on a Bruker PLATFORM/ SMART 1000 CCD diffractometer50 using Mo KR radiation at -80 °C for compounds 3, 4, 6-Y, and 9-Lu and at -100 °C for the compound 11-Lu. The data for 3-Lu, 3-Y, 6-Y, and 9-Lu were corrected for absorption through use of the SADABS procedure;51 for 4-Y and 11-Lu, the data were corrected for absorption by the Gaussian integration (face-indexed) method. See Tables S1 and S2 in the Supporting Information for summaries of crystal data and X-ray data collection information. The structures of the compounds were solved using direct methods (SHELXS-86 and SHELXS-97) for 3-Lu and 3-Y and for 4-Y and 11-Lu, respectively;52 SIR97 was used for 6-Y53 and (50) Programs for diffractometer operation, data collection, data reduction, and absorption correction were those supplied by Bruker. (51) Sheldrick, G. M. SADABS 2.10: Program for Detector Scalin and Absorption Correction; University of G€ottingen, G€ottingen, Germany, 2003.


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a Patterson search/structure expansion (DIRDIF-99) for 9-Lu.54 Refinement was completed by full-matrix least squares on F2 using the program SHELXL-93 for 3-Lu and 3-Y and SHELXL-97 for 4-Y, 6-Y, 9-Lu, and 11-Lu.55

Acknowledgment. We are grateful to the National Sciences and Engineering Research Council of Canada and the University of Alberta for financial support. We thank the staffs of the Department of Chemistry’s HighField NMR and Analytical and Instrumentation Laboratories, and the reviewers for their constructive comments and helpful suggestions. Supporting Information Available: Spectroscopic data of dimeric alkynes obtained from protonolysis and catalytic dimerization (Table S1) and summary of data collection (Tables S2 and S3) and CIF files giving crystallographic data, including bond lengths and angles, of compounds 3-Y, 3-Lu, 4-Y, 6-Y, 9-Lu, and 11-Lu. This material is available free of charge via the Internet at http://pubs.acs.org. (52) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467. (53) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115. (54) Beurskens, P. T.; Beurskens, G.; de Gelder, R.; Garcia-Granda, S.; Israel, R.; Gould, R. O.; Smits, J. M. M. The DIRDIF-99 program system; Crystallography Laboratory, University of Nijmegen, Nijmegen, The Netherlands, 1999. (55) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.

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